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Hofmann GC, Gama de Barcellos Filho P, Khodadadi F, Ostrowski D, Kline DD, Hasser EM. Vagotomy blunts cardiorespiratory responses to vagal afferent stimulation via pre- and postsynaptic effects in the nucleus tractus solitarii. J Physiol 2024; 602:1147-1174. [PMID: 38377124 DOI: 10.1113/jp285854] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/26/2023] [Accepted: 01/29/2024] [Indexed: 02/22/2024] Open
Abstract
Viscerosensory information travels to the brain via vagal afferents, where it is first integrated within the brainstem nucleus tractus solitarii (nTS), a critical contributor to cardiorespiratory function and site of neuroplasticity. We have shown that decreasing input to the nTS via unilateral vagus nerve transection (vagotomy) induces morphological changes in nTS glia and reduces sighs during hypoxia. The mechanisms behind post-vagotomy changes are not well understood. We hypothesized that chronic vagotomy alters cardiorespiratory responses to vagal afferent stimulation via blunted nTS neuronal activity. Male Sprague-Dawley rats (6 weeks old) underwent right cervical vagotomy caudal to the nodose ganglion, or sham surgery. After 1 week, rats were anaesthetized, ventilated and instrumented to measure mean arterial pressure (MAP), heart rate (HR), and splanchnic sympathetic and phrenic nerve activity (SSNA and PhrNA, respectively). Vagal afferent stimulation (2-50 Hz) decreased cardiorespiratory parameters and increased neuronal Ca2+ measured by in vivo photometry and in vitro slice imaging of nTS GCaMP8m. Vagotomy attenuated both these reflex and neuronal Ca2+ responses compared to shams. Vagotomy also reduced presynaptic Ca2+ responses to stimulation (Cal-520 imaging) in the nTS slice. The decrease in HR, SSNA and PhrNA due to nTS nanoinjection of exogenous glutamate also was tempered following vagotomy. This effect was not restored by blocking excitatory amino acid transporters. However, the blunted responses were mimicked by NMDA, not AMPA, nanoinjection and were associated with reduced NR1 subunits in the nTS. Altogether, these results demonstrate that vagotomy induces multiple changes within the nTS tripartite synapse that influence cardiorespiratory reflex responses to afferent stimulation. KEY POINTS: Multiple mechanisms within the nucleus tractus solitarii (nTS) contribute to functional changes following vagal nerve transection. Vagotomy results in reduced cardiorespiratory reflex responses to vagal afferent stimulation and nTS glutamate nanoinjection. Blunted responses occur via reduced presynaptic Ca2+ activation and attenuated NMDA receptor expression and function, leading to a reduction in nTS neuronal activation. These results provide insight into the control of autonomic and respiratory function, as well as the plasticity that can occur in response to nerve damage and cardiorespiratory disease.
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Affiliation(s)
- Gabrielle C Hofmann
- Comparative Medicine, University of Missouri, Columbia, Missouri, USA
- Area Pathobiology, University of Missouri, Columbia, Missouri, USA
- Department of Biomedical Sciences, University of Missouri, Columbia, Missouri, USA
- Dalton Cardiovascular Research Center, University of Missouri, Columbia, Missouri, USA
| | - Procopio Gama de Barcellos Filho
- Department of Biomedical Sciences, University of Missouri, Columbia, Missouri, USA
- Dalton Cardiovascular Research Center, University of Missouri, Columbia, Missouri, USA
| | - Fateme Khodadadi
- Department of Biomedical Sciences, University of Missouri, Columbia, Missouri, USA
- Dalton Cardiovascular Research Center, University of Missouri, Columbia, Missouri, USA
| | - Daniela Ostrowski
- Department of Pharmacology, A.T. Still University, Kirksville, Missouri, USA
| | - David D Kline
- Area Pathobiology, University of Missouri, Columbia, Missouri, USA
- Department of Biomedical Sciences, University of Missouri, Columbia, Missouri, USA
- Dalton Cardiovascular Research Center, University of Missouri, Columbia, Missouri, USA
- Medical Pharmacology and Physiology, University of Missouri, Columbia, Missouri, USA
| | - Eileen M Hasser
- Area Pathobiology, University of Missouri, Columbia, Missouri, USA
- Department of Biomedical Sciences, University of Missouri, Columbia, Missouri, USA
- Dalton Cardiovascular Research Center, University of Missouri, Columbia, Missouri, USA
- Medical Pharmacology and Physiology, University of Missouri, Columbia, Missouri, USA
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Lima-Silveira L, Hasser EM, Kline DD. Cardiovascular deconditioning increases GABA signaling in the nucleus tractus solitarii. J Neurophysiol 2022; 128:28-39. [PMID: 35642806 PMCID: PMC9236861 DOI: 10.1152/jn.00102.2022] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The nucleus tractus solitarii (nTS) is the major integrative brainstem region for autonomic modulation and processing of cardiovascular reflexes. GABA and glutamate are the main inhibitory and excitatory neurotransmitters, respectively, within this nucleus. Alterations in the GABA-glutamate regulation in the nTS are related to numerous cardiovascular comorbidities. Bedridden individuals and people exposed to microgravity exhibit dysautonomia and cardiovascular deconditioning that are mimicked in the hindlimb unloading (HU) rat model. We have previously shown in the nTS that HU increases glutamatergic neurotransmission yet decreases neuronal excitability. In this study, we investigated the effects of HU on nTS GABAergic neurotransmission. We hypothesized that HU potentiates GABA signaling via increased GABAergic release and postsynaptic GABA receptor expression. Following HU or control postural exposure, GABAergic neurotransmission was assessed using whole cell patch clamp whereas the magnitude of GABA release was evaluated via an intensity-based GABA sensing fluorescence reporter (iGABASnFR). In response to GABA interneuron stimulation, the evoked inhibitory postsynaptic current (nTS-IPSC) amplitude and area, as well as iGABASnFR fluorescence, were greater in HU than in control. HU also elevated the frequency but not the amplitude of spontaneous miniature IPSCs. Picoapplication of GABA produced similar postsynaptic current responses in nTS neurons of HU and control. Moreover, HU did not alter GABAA receptor α1 subunit expression, indicating minimal alterations in postsynaptic membrane receptor expression. These results indicate that HU increases GABAergic signaling in the nTS likely via augmented release of GABA from presynaptic terminals. Altogether, our data indicate GABA plasticity contributes to the autonomic and cardiovascular alterations following cardiovascular deconditioning (CVD).NEW & NOTEWORTHY Gravity influences distribution of blood volume and autonomic function. Microgravity and prolonged bed rest induce cardiovascular deconditioning (CVD). We used hindlimb unloading (HU), a rat analog for bed rest, to investigate CVD-induced neuroplasticity in the brainstem. Our data demonstrate that HU increases GABA modulation of nucleus tractus solitarii (nTS) neurons via presynaptic plasticity. Given the importance of nTS in integrating cardiovascular reflexes, this study provides new evidence on the central mechanisms behind CVD following HU.
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Affiliation(s)
- Ludmila Lima-Silveira
- 1Department of Biomedical Sciences, University of Missouri, Columbia, Missouri,3Dalton Cardiovascular Research Center, University of Missouri, Columbia, Missouri
| | - Eileen M. Hasser
- 1Department of Biomedical Sciences, University of Missouri, Columbia, Missouri,2Department of Medical Pharmacology and Physiology, University of Missouri, Columbia, Missouri,3Dalton Cardiovascular Research Center, University of Missouri, Columbia, Missouri
| | - David D. Kline
- 1Department of Biomedical Sciences, University of Missouri, Columbia, Missouri,2Department of Medical Pharmacology and Physiology, University of Missouri, Columbia, Missouri,3Dalton Cardiovascular Research Center, University of Missouri, Columbia, Missouri
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Lima-Silveira L, Martinez D, Hasser EM, Kline DD. Mechanisms Underlying Neuroplasticity in the Nucleus Tractus Solitarii Following Hindlimb Unloading in Rats. Neuroscience 2020; 449:214-227. [PMID: 33039526 DOI: 10.1016/j.neuroscience.2020.09.043] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/17/2020] [Revised: 09/15/2020] [Accepted: 09/20/2020] [Indexed: 12/11/2022]
Abstract
Hindlimb unloading (HU) in rats induces cardiovascular deconditioning (CVD) analogous to that observed in individuals exposed to microgravity or bed rest. Among other physiological changes, HU rats exhibit autonomic imbalance and altered baroreflex function. Lack of change in visceral afferent activity that projects to the brainstem in HU rats suggests that neuronal plasticity within central nuclei processing cardiovascular afferents may be responsible for these changes in CVD and HU. The nucleus tractus solitarii (nTS) is a critical brainstem region for autonomic control and integration of cardiovascular reflexes. In this study, we used patch electrophysiology, live-cell calcium imaging and molecular methods to investigate the effects of HU on glutamatergic synaptic transmission and intrinsic properties of nTS neurons. HU increased the amplitude of monosynaptic excitatory postsynaptic currents and presynaptic calcium entry evoked by afferent tractus solitarii stimulus (TS-EPSC); spontaneous (s) EPSCs were unaffected. The addition of a NMDA receptor antagonist (AP5) reduced TS-EPSC amplitude and sEPSC frequency in HU but not control. Despite the increase in glutamatergic inputs, HU neurons were more hyperpolarized and exhibited intrinsic decreased excitability compared to controls. After block of ionotropic glutamatergic and GABAergic synaptic transmission (NBQX, AP5, Gabazine), HU neuronal membrane potential depolarized and neuronal excitability was comparable to controls. These data demonstrate that HU increases presynaptic release and TS-EPSC amplitude, which includes a NMDA receptor component. Furthermore, the decreased excitability and hyperpolarized membrane after HU are associated with enhanced GABAergic modulation. This functional neuroplasticity in the nTS may underly the CVD induced by HU.
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Affiliation(s)
- Ludmila Lima-Silveira
- Department of Biomedical Sciences, University of Missouri, 134 Research Park Dr., Columbia, MO 65211, USA; Dalton Cardiovascular Research Center, University of Missouri, 134 Research Park Dr., Columbia, MO 65211, USA.
| | - Diana Martinez
- Department of Biomedical Sciences, University of Missouri, 134 Research Park Dr., Columbia, MO 65211, USA; Dalton Cardiovascular Research Center, University of Missouri, 134 Research Park Dr., Columbia, MO 65211, USA.
| | - Eileen M Hasser
- Department of Biomedical Sciences, University of Missouri, 134 Research Park Dr., Columbia, MO 65211, USA; Department of Medical Pharmacology and Physiology, University of Missouri, 134 Research Park Dr., Columbia, MO 65211, USA; Dalton Cardiovascular Research Center, University of Missouri, 134 Research Park Dr., Columbia, MO 65211, USA.
| | - David D Kline
- Department of Biomedical Sciences, University of Missouri, 134 Research Park Dr., Columbia, MO 65211, USA; Dalton Cardiovascular Research Center, University of Missouri, 134 Research Park Dr., Columbia, MO 65211, USA.
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Approaching Gravity as a Continuum Using the Rat Partial Weight-Bearing Model. Life (Basel) 2020; 10:life10100235. [PMID: 33049988 PMCID: PMC7599661 DOI: 10.3390/life10100235] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/10/2020] [Revised: 09/30/2020] [Accepted: 10/03/2020] [Indexed: 12/12/2022] Open
Abstract
For decades, scientists have relied on animals to understand the risks and consequences of space travel. Animals remain key to study the physiological alterations during spaceflight and provide crucial information about microgravity-induced changes. While spaceflights may appear common, they remain costly and, coupled with limited cargo areas, do not allow for large sample sizes onboard. In 1979, a model of hindlimb unloading (HU) was successfully created to mimic microgravity and has been used extensively since its creation. Four decades later, the first model of mouse partial weight-bearing (PWB) was developed, aiming at mimicking partial gravity environments. Return to the Lunar surface for astronauts is now imminent and prompted the need for an animal model closer to human physiology; hence in 2018, our laboratory created a new model of PWB for adult rats. In this review, we will focus on the rat model of PWB, from its conception to the current state of knowledge. Additionally, we will address how this new model, used in conjunction with HU, will help implement new paradigms allowing scientists to anticipate the physiological alterations and needs of astronauts. Finally, we will discuss the outstanding questions and future perspectives in space research and propose potential solutions using the rat PWB model.
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Mueller PJ, Fyk-Kolodziej BE, Azar TA, Llewellyn-Smith IJ. Subregional differences in GABA A receptor subunit expression in the rostral ventrolateral medulla of sedentary versus physically active rats. J Comp Neurol 2020; 528:1053-1075. [PMID: 31642070 PMCID: PMC7046220 DOI: 10.1002/cne.24798] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/28/2018] [Revised: 09/09/2019] [Accepted: 09/10/2019] [Indexed: 12/19/2022]
Abstract
Neurons in the rostral ventrolateral medulla (RVLM) regulate blood pressure through direct projections to spinal sympathetic preganglionic neurons. Only some RVLM neurons are active under resting conditions due to significant, tonic inhibition by gamma-aminobutyric acid (GABA). Withdrawal of GABAA receptor-mediated inhibition of the RVLM increases sympathetic outflow and blood pressure substantially, providing a mechanism by which the RVLM could contribute chronically to cardiovascular disease (CVD). Here, we tested the hypothesis that sedentary conditions, a major risk factor for CVD, increase GABAA receptors in RVLM, including its rostral extension (RVLMRE ), both of which contain bulbospinal catecholamine (C1) and non-C1 neurons. We examined GABAA receptor subunits GABAAα1 and GABAAα2 in the RVLM/RVLMRE of sedentary or physically active (10-12 weeks of wheel running) rats. Western blot analyses indicated that sedentary rats had lower expression of GABAAα1 and GABAAα2 subunits in RVLM but only GABAAα2 was lower in the RVLMRE of sedentary rats. Sedentary rats had significantly reduced expression of the chloride transporter, KCC2, suggesting less effective GABA-mediated inhibition compared to active rats. Retrograde tracing plus triple-label immunofluorescence identified fewer bulbospinal non-C1 neurons immunoreactive for GABAAα1 but a higher percentage of bulbospinal C1 neurons immunoreactive for GABAAα1 in sedentary animals. Sedentary conditions did not significantly affect the number of bulbospinal C1 or non-C1 neurons immunoreactive for GABAAα2 . These results suggest a complex interplay between GABAA receptor expression by spinally projecting C1 and non-C1 neurons and sedentary versus physically active conditions. They also provide plausible mechanisms for both enhanced sympathoexcitatory and sympathoinhibitory responses following sedentary conditions.
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Affiliation(s)
- Patrick J. Mueller
- Department of Physiology, Wayne State University School of
Medicine, Detroit MI
| | | | - Toni A. Azar
- Department of Physiology, Wayne State University School of
Medicine, Detroit MI
| | - Ida J. Llewellyn-Smith
- Department of Physiology, Wayne State University School of
Medicine, Detroit MI
- Cardiovascular Medicine, Human Physiology and Centre for
Neuroscience, College of Medicine and Public Health, Flinders University, Bedford
Park SA, AUSTRALIA
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Milanez MIO, Silva AM, Perry JC, Faber J, Nishi EE, Bergamaschi CT, Campos RR. Pattern of sympathetic vasomotor activity induced by GABAergic inhibition in the brain and spinal cord. Pharmacol Rep 2020; 72:67-79. [PMID: 32016845 DOI: 10.1007/s43440-019-00025-w] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/17/2019] [Revised: 09/05/2019] [Accepted: 10/11/2019] [Indexed: 10/25/2022]
Abstract
BACKGROUND Knowledge of the central areas involved in the control of sympathetic vasomotor activity has advanced in the last few decades. γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the mammal nervous system, and a microinjection of bicuculline, an antagonist of GABA type A (GABA-A) receptors, into the paraventricular nucleus of the hypothalamus (PVN) alters the pattern of sympathetic activity to the renal, splanchnic and lumbar territories. However, studies are needed to clarify the role of GABAergic inputs in other central areas involved in the sympathetic vasomotor activity. The present work studied the cardiovascular effects evoked by GABAergic antagonism in the PVN, RVLM and spinal cord. METHODS AND RESULTS Bicuculline microinjections (400 pMol in 100 nL) into the PVN and rostral ventrolateral medulla (RVLM) as well as intrathecal administration (1.6 nmol in 2 µL) evoked an increase in blood pressure, heart rate, and renal and splanchnic sympathetic nerve activity (rSNA and sSNA, respectively), inducing a higher coherence between rSNA and sSNA patterns. However, some of these responses were more intense when the GABA-A antagonism was performed in the RVLM than when the GABA-A antagonism was performed in other regions. CONCLUSIONS Administration of bicuculline into the RVLM, PVN and SC induced a similar pattern of renal and splanchnic sympathetic vasomotor burst discharge, characterized by a low-frequency (0.5 Hz) and high-amplitude pattern, despite different blood pressure responses. Thus, the differential control of sympathetic drive to different targets by each region is dependent, in part, on tonic GABAergic inputs.
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Affiliation(s)
- Maycon I O Milanez
- Cardiovascular Division, Department of Physiology, Universidade Federal de São Paulo, Escola Paulista de Medicina, Rua Botucatu, 862, Ground Floor, CEP 04023-060, São Paulo, SP, Brazil
| | - Adilson M Silva
- Cardiovascular Division, Department of Physiology, Universidade Federal de São Paulo, Escola Paulista de Medicina, Rua Botucatu, 862, Ground Floor, CEP 04023-060, São Paulo, SP, Brazil
| | - Juliana C Perry
- Cardiovascular Division, Department of Physiology, Universidade Federal de São Paulo, Escola Paulista de Medicina, Rua Botucatu, 862, Ground Floor, CEP 04023-060, São Paulo, SP, Brazil
| | - Jean Faber
- Department of Neurology and Neurosurgery, Universidade Federal de São Paulo, Escola Paulista de Medicina, São Paulo, Brazil
| | - Erika E Nishi
- Cardiovascular Division, Department of Physiology, Universidade Federal de São Paulo, Escola Paulista de Medicina, Rua Botucatu, 862, Ground Floor, CEP 04023-060, São Paulo, SP, Brazil
| | - Cássia T Bergamaschi
- Cardiovascular Division, Department of Physiology, Universidade Federal de São Paulo, Escola Paulista de Medicina, Rua Botucatu, 862, Ground Floor, CEP 04023-060, São Paulo, SP, Brazil
| | - Ruy R Campos
- Cardiovascular Division, Department of Physiology, Universidade Federal de São Paulo, Escola Paulista de Medicina, Rua Botucatu, 862, Ground Floor, CEP 04023-060, São Paulo, SP, Brazil.
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Mueller PJ, Clifford PS, Crandall CG, Smith SA, Fadel PJ. Integration of Central and Peripheral Regulation of the Circulation during Exercise: Acute and Chronic Adaptations. Compr Physiol 2017; 8:103-151. [DOI: 10.1002/cphy.c160040] [Citation(s) in RCA: 24] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
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Dombrowski MD, Mueller PJ. Sedentary conditions and enhanced responses to GABA in the RVLM: role of the contralateral RVLM. Am J Physiol Regul Integr Comp Physiol 2017; 313:R158-R168. [PMID: 28490450 DOI: 10.1152/ajpregu.00366.2016] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/26/2016] [Revised: 05/08/2017] [Accepted: 05/08/2017] [Indexed: 02/07/2023]
Abstract
A sedentary lifestyle is a major risk factor for cardiovascular disease, and both conditions are associated with overactivity of the sympathetic nervous system. Ongoing discharge of sympathetic nerves is regulated by the rostral ventrolateral medulla (RVLM), which in turn is modulated by the primary excitatory and inhibitory neurotransmitters glutamate and γ-amino-butyric acid (GABA), respectively. We reported previously that sedentary conditions enhance GABAergic modulation of sympathoexcitation in the RVLM, despite overall increased sympathoexcitation. Thus the purpose of this study was to test the hypothesis that sedentary conditions increase responsiveness to GABA in RVLM. Male Sprague-Dawley rats performed either chronic wheeling running or remained sedentary for 12-15 wk. Animals were instrumented to perform RVLM microinjections under Inactin anesthesia while mean arterial pressure (MAP) and splanchnic sympathetic nerve activity (SSNA) were recorded. Unilateral microinjections of GABA (30 nl, 0.3-600 mM) into the RVLM produced dose-dependent decreases in MAP and SSNA; however, no group differences were observed. Inhibition of the contralateral RVLM (muscimol, 2 mM, 90 nl) caused decreases in MAP and SSNA that were not different between groups but enhanced decreases in SSNA to GABA in sedentary rats only. In sinoaortic denervated rats, GABA microinjections before or after inhibition of the contralateral RVLM caused decreases in MAP and SSNA that were not different between groups. Our results suggest that the contralateral RVLM plays an important role in buffering responses to inhibition of the ipsilateral RVLM under sedentary but not physically active conditions. Based on these studies and others, sedentary conditions appear to enhance both sympathoinhibitory and sympathoexcitatory mechanisms in the RVLM. Enhanced sympathoinhibition may act to reduce already elevated sympathetic nervous system activity following sedentary conditions.
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Affiliation(s)
- Maryetta D Dombrowski
- Department of Physiology, Wayne State University School of Medicine, Detroit, Michigan
| | - Patrick J Mueller
- Department of Physiology, Wayne State University School of Medicine, Detroit, Michigan
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Subramanian M, Mueller PJ. Altered Differential Control of Sympathetic Outflow Following Sedentary Conditions: Role of Subregional Neuroplasticity in the RVLM. Front Physiol 2016; 7:290. [PMID: 27486405 PMCID: PMC4949265 DOI: 10.3389/fphys.2016.00290] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/20/2016] [Accepted: 06/27/2016] [Indexed: 01/25/2023] Open
Abstract
Despite the classically held belief of an “all-or-none” activation of the sympathetic nervous system, differential responses in sympathetic nerve activity (SNA) can occur acutely at varying magnitudes and in opposing directions. Sympathetic nerves also appear to contribute differentially to various disease states including hypertension and heart failure. Previously we have reported that sedentary conditions enhanced responses of splanchnic SNA (SSNA) but not lumbar SNA (LSNA) to activation of the rostral ventrolateral medulla (RVLM) in rats. Bulbospinal RVLM neurons from sedentary rats also exhibit increased dendritic branching in rostral regions of the RVLM. We hypothesized that regionally specific structural neuroplasticity would manifest as enhanced SSNA but not LSNA following activation of the rostral RVLM. To test this hypothesis, groups of physically active (10–12 weeks on running wheels) or sedentary, male Sprague-Dawley rats were instrumented to record mean arterial pressure, LSNA and SSNA under Inactin anesthesia and during microinjections of glutamate (30 nl, 10 mM) into multiple sites within the RVLM. Sedentary conditions enhanced SSNA but not LSNA responses and SSNA responses were enhanced at more central and rostral sites. Results suggest that enhanced SSNA responses in rostral RVLM coincide with enhanced dendritic branching in rostral RVLM observed previously. Identifying structural and functional neuroplasticity in specific populations of RVLM neurons may help identify new treatments for cardiovascular diseases, known to be more prevalent in sedentary individuals.
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Affiliation(s)
- Madhan Subramanian
- Department of Physiology, Wayne State University School of Medicine Detroit, MI, USA
| | - Patrick J Mueller
- Department of Physiology, Wayne State University School of Medicine Detroit, MI, USA
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Mischel NA, Subramanian M, Dombrowski MD, Llewellyn-Smith IJ, Mueller PJ. (In)activity-related neuroplasticity in brainstem control of sympathetic outflow: unraveling underlying molecular, cellular, and anatomical mechanisms. Am J Physiol Heart Circ Physiol 2015; 309:H235-43. [PMID: 25957223 DOI: 10.1152/ajpheart.00929.2014] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/29/2014] [Accepted: 05/08/2015] [Indexed: 02/07/2023]
Abstract
More people die as a result of physical inactivity than any other preventable risk factor including smoking, high cholesterol, and obesity. Cardiovascular disease, the number one cause of death in the United States, tops the list of inactivity-related diseases. Nevertheless, the vast majority of Americans continue to make lifestyle choices that are creating a rapidly growing burden of epidemic size and impact on the United States healthcare system. It is imperative that we improve our understanding of the mechanisms by which physical inactivity increases the incidence of cardiovascular disease and how exercise can prevent or rescue the inactivity phenotype. The current review summarizes research on changes in the brain that contribute to inactivity-related cardiovascular disease. Specifically, we focus on changes in the rostral ventrolateral medulla (RVLM), a critical brain region for basal and reflex control of sympathetic activity. The RVLM is implicated in elevated sympathetic outflow associated with several cardiovascular diseases including hypertension and heart failure. We hypothesize that changes in the RVLM contribute to chronic cardiovascular disease related to physical inactivity. Data obtained from our translational rodent models of chronic, voluntary exercise and inactivity suggest that functional, anatomical, and molecular neuroplasticity enhances glutamatergic neurotransmission in the RVLM of sedentary animals. Collectively, the evidence presented here suggests that changes in the RVLM resulting from sedentary conditions are deleterious and contribute to cardiovascular diseases that have an increased prevalence in sedentary individuals. The mechanisms by which these changes occur over time and their impact are important areas for future study.
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Affiliation(s)
- Nicholas A Mischel
- Department of Physiology, Wayne State University School of Medicine, Detroit, Michigan; and
| | - Madhan Subramanian
- Department of Physiology, Wayne State University School of Medicine, Detroit, Michigan; and
| | - Maryetta D Dombrowski
- Department of Physiology, Wayne State University School of Medicine, Detroit, Michigan; and
| | - Ida J Llewellyn-Smith
- Department of Physiology, Wayne State University School of Medicine, Detroit, Michigan; and Cardiovascular Medicine, Physiology and Centre for Neuroscience, School of Medicine, Flinders University, Bedford Park, South Australia, Australia
| | - Patrick J Mueller
- Department of Physiology, Wayne State University School of Medicine, Detroit, Michigan; and
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Minic Z, O'Leary DS, Scislo TJ. NTS adenosine A2a receptors inhibit the cardiopulmonary chemoreflex control of regional sympathetic outputs via a GABAergic mechanism. Am J Physiol Heart Circ Physiol 2015; 309:H185-97. [PMID: 25910812 DOI: 10.1152/ajpheart.00838.2014] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 11/19/2014] [Accepted: 04/21/2015] [Indexed: 11/22/2022]
Abstract
Adenosine is a powerful central neuromodulator acting via opposing A1 (inhibitor) and A2a (activator) receptors. However, in the nucleus of the solitary tract (NTS), both adenosine receptor subtypes attenuate cardiopulmonary chemoreflex (CCR) sympathoinhibition of renal, adrenal, and lumbar sympathetic nerve activity and attenuate reflex decreases in arterial pressure and heart rate. Adenosine A1 receptors inhibit glutamatergic transmission in the CCR pathway, whereas adenosine A2a receptors most likely facilitate release of an unknown inhibitory neurotransmitter, which, in turn, inhibits the CCR. We hypothesized that adenosine A2a receptors inhibit the CCR via facilitation of GABA release in the NTS. In urethane-chloralose-anesthetized rats (n = 51), we compared regional sympathetic responses evoked by stimulation of the CCR with right atrial injections of the 5-HT3 receptor agonist phenylbiguanide (1-8 μg/kg) before and after selective stimulation of NTS adenosine A2a receptors [microinjections into the NTS of CGS-21680 (20 pmol/50 nl)] preceded by blockade of GABAA or GABAB receptors in the NTS [bicuculline (10 pmol/100 nl) or SCH-50911 (1 nmol/100 nl)]. Blockade of GABAA receptors virtually abolished adenosine A2a receptor-mediated inhibition of the CCR. GABAB receptors had much weaker but significant effects. These effects were similar for the different sympathetic outputs. We conclude that stimulation of NTS adenosine A2a receptors inhibits CCR-evoked hemodynamic and regional sympathetic reflex responses via a GABA-ergic mechanism.
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Affiliation(s)
- Zeljka Minic
- Department of Physiology and Cardiovascular Research Institute, Wayne State University School of Medicine, Detroit, Michigan
| | - Donal S O'Leary
- Department of Physiology and Cardiovascular Research Institute, Wayne State University School of Medicine, Detroit, Michigan
| | - Tadeusz J Scislo
- Department of Physiology and Cardiovascular Research Institute, Wayne State University School of Medicine, Detroit, Michigan
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Mandsager KT, Robertson D, Diedrich A. The function of the autonomic nervous system during spaceflight. Clin Auton Res 2015; 25:141-51. [PMID: 25820827 DOI: 10.1007/s10286-015-0285-y] [Citation(s) in RCA: 30] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/08/2014] [Accepted: 01/08/2015] [Indexed: 11/24/2022]
Abstract
INTRODUCTION Despite decades of study, a clear understanding of autonomic nervous system activity in space remains elusive. Differential interpretation of fundamental data has driven divergent theories of sympathetic activation and vasorelaxation. METHODS This paper will review the available in-flight autonomic and hemodynamic data in an effort to resolve these discrepancies. The NASA NEUROLAB mission, the most comprehensive assessment of autonomic function in microgravity to date, will be highlighted. The mechanisms responsible for altered autonomic activity during spaceflight, which include the effects of hypovolemia, cardiovascular deconditioning, and altered central processing, will be presented. RESULTS The NEUROLAB experiments demonstrated increased sympathetic activity and impairment of vagal baroreflex function during short-duration spaceflight. Subsequent non-invasive studies of autonomic function during spaceflight have largely reinforced these findings, and provide strong evidence that sympathetic activity is increased in space relative to the supine position on Earth. Others have suggested that microgravity induces a state of relative vasorelaxation and increased vagal activity when compared to upright posture on Earth. These ostensibly disparate theories are not mutually exclusive, but rather directly reflect different pre-flight postural controls. CONCLUSION When these results are taken together, they demonstrate that the effectual autonomic challenge of spaceflight is small, and represents an orthostatic stress less than that of upright posture on Earth. In-flight countermeasures, including aerobic and resistance exercise, as well short-arm centrifugation, have been successfully deployed to counteract these mechanisms. Despite subtle changes in autonomic activity during spaceflight, underlying neurohumoral mechanisms of the autonomic nervous system remain intact and cardiovascular function remains stable during long-duration flight.
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Affiliation(s)
- Kyle Timothy Mandsager
- Division of Clinical Pharmacology, Department of Medicine, Autonomic Dysfunction Center, Vanderbilt University School of Medicine, 1161 21st Avenue South, Suite AA3228 MCN, Nashville, TN, 37232-2195, USA
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Subramanian M, Holt AG, Mueller PJ. Physical activity correlates with glutamate receptor gene expression in spinally-projecting RVLM neurons: a laser capture microdissection study. Brain Res 2014; 1585:51-62. [PMID: 25173073 PMCID: PMC5828155 DOI: 10.1016/j.brainres.2014.06.021] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/13/2014] [Revised: 05/31/2014] [Accepted: 06/19/2014] [Indexed: 02/07/2023]
Abstract
Physical inactivity is an important risk factor in the development of cardiovascular disease. The rostral ventrolateral portion of the medulla (RVLM) is composed of heterogeneous populations of neurons that are involved in the regulation of the cardiovascular system. Because of functional heterogeneity, studying the changes in the gene expression of this specific population of neurons within the RVLM is challenging. In the present study, a fluorescent retrograde tracer was injected into the spinal cord to specifically label bulbospinal RVLM neurons in sedentary and active rats. Laser capture microdissection (LCM) was then employed to collect the fluorescently labeled neurons from sections encompassing the rostrocaudal extent of the RVLM. RNA extracted from the neurons was used in qRT-PCR analysis. Changes in gene expression levels of glutamate and GABA receptor subunits were compared between sedentary and physically active rats. GLUR3 subunit showed a significant negative correlation between total running distance and its relative gene expression in active rats. There were no significant difference in the gene expression of NMDA (NR1, NR2A, NR2B, NR2C and NR2D), AMPA (GLUR1, GLUR2 and GLUR3) and GABAA (GABAA1 and GABAA2) receptor subunits. Overall, the present study demonstrates the feasibility of utilizing LCM to investigate the gene expression changes in a specific population of neurons in the RVLM. Correlation studies suggest that physical activity could contribute to neuroplasticity in the RVLM.
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Affiliation(s)
- Madhan Subramanian
- Department of Physiology, Wayne State University School of Medicine, Detroit, MI 48201, USA
| | - Avril G Holt
- Department of Anatomy and Cell Biology, Wayne State University School of Medicine, Detroit, MI 48201, USA
| | - Patrick J Mueller
- Department of Physiology, Wayne State University School of Medicine, Detroit, MI 48201, USA.
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Medullary regulation of visceral sympathetic nerve discharge at peak hyperthermia in aged F344 rats. Auton Neurosci 2014; 186:32-7. [PMID: 25262958 DOI: 10.1016/j.autneu.2014.09.003] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/06/2014] [Revised: 08/07/2014] [Accepted: 09/09/2014] [Indexed: 11/21/2022]
Abstract
Heat stress provides a potent stimulus for the activation of visceral sympathetic nerve discharge (SND) in young but not aged rats. Central mechanisms mediating attenuated SND responses to heating in aged rats have not been investigated. Because the GABAergic system in the rostral ventral lateral medulla (RVLM) is tonically inhibitory to SND, it is plausible to hypothesize that the withdrawal of RVLM GABA tone as a strategy to activate renal SND to heating is not engaged to the same degree in aged compared with young rats. The effect of bilateral RVLM disinhibition produced by bicuculline (BIC, GABA(A) receptor antagonist, 100 pmol) microinjections on renal SND in anesthetized young (3-6 months old) and aged (22-24 months old) Fischer 344 rats was determined after core body temperature (Tc) had been increased to 41.5 °C. Renal SND at 41.5 °C was significantly increased from control levels in young but not aged rats, whereas RVLM BIC microinjections at 41.5 °C produced marked renal sympathoexcitation in both groups. RVLM BIC microinjections at 38 °C in young and aged rats increased renal SND to similar levels as produced by RVLM BIC microinjections at 41.5 °C. The enhanced heating-induced renal sympathoactivation in young compared with aged rats; coupled with marked RVLM BIC-induced SND excitation under hyperthermic and normothermic conditions in both young and aged rats, suggests age-dependent changes in the withdrawal of RVLM GABA tone as a strategy to activate renal SND in response to acute heating.
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Kenney MJ, Ganta CK, Fels RJ. Disinhibition of RVLM neural circuits and regulation of sympathetic nerve discharge at peak hyperthermia. J Appl Physiol (1985) 2013; 115:1297-303. [PMID: 23990239 DOI: 10.1152/japplphysiol.00494.2013] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Hyperthermia is a potent activator of visceral sympathetic nerve discharge (SND), and the functional integrity of the rostral ventral lateral medulla (RVLM) is critically important for sustaining sympathoexcitation at peak hyperthermia. However, RVLM mechanisms mediating SND activation to acute heat stress are not well understood. Because RVLM GABA is tonically inhibitory to sympathetic nerve outflow, it is plausible to hypothesize that disinhibition of RVLM sympathetic neural circuits, via withdrawal of GABAergic tone, may affect SND regulation at peak hyperthermia. The effect of RVLM bicuculline (BIC; GABAA receptor antagonist, 100-200 pmol) microinjections on the level of renal SND in anesthetized rats was determined after internal body temperature (Tc) had been increased to 41.5°C. Temperature-control experiments involved RVLM BIC (100-200 pmol) microinjections, with Tc maintained at 38°C. As expected, acute heating significantly increased renal SND from control levels. Bilateral RVLM BIC microinjections at 41.5°C produced immediate and significant increases in renal SND above heating-induced levels of activation. Bilateral RVLM BIC microinjections at 38°C increased renal SND to similar levels as produced by RVLM BIC microinjections after Tc had been increased to 41.5°C (heating + RVLM BIC). These results demonstrate that a considerable level of RVLM GABAergic inhibition is sustained at peak hyperthermia, an interesting physiological response profile based on the significance of SND activation to cardiovascular regulation during heat stress.
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Affiliation(s)
- Michael J Kenney
- Department of Anatomy and Physiology, Kansas State University, Manhattan, Kansas
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16
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Zhang LF. Region-specific vascular remodeling and its prevention by artificial gravity in weightless environment. Eur J Appl Physiol 2013; 113:2873-95. [DOI: 10.1007/s00421-013-2597-8] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/30/2012] [Accepted: 01/20/2013] [Indexed: 10/27/2022]
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Mueller PJ, Mischel NA. Selective enhancement of glutamate-mediated pressor responses after GABA(A) receptor blockade in the RVLM of sedentary versus spontaneous wheel running rats. Front Physiol 2012; 3:447. [PMID: 23189062 PMCID: PMC3505845 DOI: 10.3389/fphys.2012.00447] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/03/2012] [Accepted: 11/07/2012] [Indexed: 11/26/2022] Open
Abstract
Overactivity of the sympathetic nervous system (SNS) is a hallmark of many cardiovascular diseases. It is also well-known that physical inactivity independently contributes to cardiovascular diseases, likely in part via increased SNS activity. Recent work from our laboratory has demonstrated increased SNS responses in sedentary animals following either direct activation or disinhibition of the rostral ventrolateral medulla (RVLM), an integral cardiovascular brainstem region. These data led us to hypothesize that the interaction between excitation and inhibition of the RVLM is altered in sedentary versus physically active animals. To test this hypothesis, we recorded mean arterial pressure (MAP) and lumbar sympathetic nerve activity (LSNA) in Inactin anesthetized rats that were housed for 8-12 weeks with or without access to a running wheel. Pressor responses to direct activation of the RVLM with glutamate were similar between groups under intact conditions. However, blockade of γ-aminobutyric acid (GABA)(A) receptors with bicuculline selectively enhanced pressor responses to glutamate in sedentary animals. Interestingly, LSNA responses to glutamate were not enhanced in sedentary versus active animals in the presence or absence of tonic GABAergic tone. These results suggest that sedentary compared to active conditions enhance GABAergic inhibition of glutamate-sensitive neurons in the RVLM that are involved in blood pressure regulation, and by mechanisms that do not involve LSNA. We also speculate that regular physical activity has differential effects on SNS activity to specific vascular beds and may reduce the risk of developing cardiovascular diseases via changes occurring in the RVLM.
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Affiliation(s)
- Patrick J. Mueller
- Department of Physiology, Wayne State University School of MedicineDetroit, MI, USA
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18
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King TL, Heesch CM, Clark CG, Kline DD, Hasser EM. Hypoxia activates nucleus tractus solitarii neurons projecting to the paraventricular nucleus of the hypothalamus. Am J Physiol Regul Integr Comp Physiol 2012; 302:R1219-32. [PMID: 22403798 DOI: 10.1152/ajpregu.00028.2012] [Citation(s) in RCA: 67] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Peripheral chemoreceptor afferent information is sent to the nucleus tractus solitarii (nTS), integrated, and relayed to other brain regions to alter cardiorespiratory function. The nTS projects to the hypothalamic paraventricular nucleus (PVN), but activation and phenotype of these projections during chemoreflex stimulation is unknown. We hypothesized that activation of PVN-projecting nTS neurons occurs primarily at high intensities of hypoxia. We assessed ventilation and cardiovascular parameters in response to increasing severities of hypoxia. Retrograde tracers were used to label nTS PVN-projecting neurons and, in some rats, rostral ventrolateral medulla (RVLM)-projecting neurons. Immunohistochemistry was performed to identify nTS cells that were activated (Fos-immunoreactive, Fos-IR), catecholaminergic, and GABAergic following hypoxia. Conscious rats underwent 3 h normoxia (n = 4, 21% O(2)) or acute hypoxia (12, 10, or 8% O(2); n = 5 each). Hypoxia increased ventilation and the number of Fos-IR nTS cells (21%, 13 ± 2; 12%, 58 ± 4; 10%, 166 ± 22; 8%, 186 ± 6). Fos expression after 10% O(2) was similar whether arterial pressure was allowed to decrease (-13 ± 1 mmHg) or was held constant. The percentage of PVN-projecting cells activated was intensity dependent, but contrary to our hypothesis, PVN-projecting nTS cells exhibiting Fos-IR were found at all hypoxic intensities. Notably, at all intensities of hypoxia, ∼75% of the activated PVN-projecting nTS neurons were catecholaminergic. Compared with RVLM-projecting cells, a greater percentage of PVN-projecting nTS cells was activated by 10% O(2). Data suggest that increasing hypoxic intensity activates nTS PVN-projecting cells, especially catecholaminergic, PVN-projecting neurons. The nTS to PVN catecholaminergic pathway may be critical even at lower levels of chemoreflex activation and more important to cardiorespiratory responses than previously considered.
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Affiliation(s)
- T Luise King
- Department of Biomedical Sciences, University of Missouri, Columbia, Missouri 65211, USA
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19
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Mischel NA, Mueller PJ. (In)activity-dependent alterations in resting and reflex control of splanchnic sympathetic nerve activity. J Appl Physiol (1985) 2011; 111:1854-62. [PMID: 21979802 PMCID: PMC3233897 DOI: 10.1152/japplphysiol.00961.2011] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/29/2011] [Accepted: 09/30/2011] [Indexed: 02/07/2023] Open
Abstract
The negative effects of sympathetic overactivity on long-term cardiovascular health are becoming increasingly clear. Moreover, recent work done in animal models of cardiovascular disease suggests that sympathetic tone to the splanchnic vasculature may play an important role in the development and maintenance of these disease states. Work from our laboratory and others led us to hypothesize that a lack of chronic physical activity increases resting and reflex-mediated splanchnic sympathetic nerve activity, possibly through changes occurring in a key brain stem center involved in sympathetic regulation, the rostral ventrolateral medulla (RVLM). To address this hypothesis, we recorded mean arterial pressure (MAP) and splanchnic sympathetic nerve activity (SSNA) in a group of active and sedentary animals that had been housed for 10-13 wk with or without running wheels, respectively. In experiments performed under Inactin anesthesia, we tested responses to RVLM microinjections of glutamate, responses to baroreceptor unloading, and vascular reactivity, the latter of which was performed under conditions of autonomic blockade. Sedentary animals exhibited enhanced resting SSNA and MAP, augmented increases in SSNA to RVLM activation and baroreceptor unloading, and enhanced vascular reactivity to α(1)-receptor mediated vasoconstriction. Our results suggest that a sedentary lifestyle increases the risk of cardiovascular disease by augmenting resting and reflex-mediated sympathetic output to the splanchnic circulation and also by increasing vascular sensitivity to adrenergic stimulation. We speculate that regular physical exercise offsets or reverses the progression of these disease processes via similar or disparate mechanisms and warrant further examination into physical (in)activity-induced sympathetic nervous system plasticity.
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Affiliation(s)
- Nicholas A Mischel
- Dept. of Physiology, Wayne State Univ. School of Medicine, Detroit MI 48201, USA
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20
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Kenney MJ, Meyer CN, Hosking KG, Fels RJ. Is visceral sympathoexcitation to heat stress dependent on activation of ionotropic excitatory amino acid receptors in the rostral ventrolateral medulla? Am J Physiol Regul Integr Comp Physiol 2011; 301:R548-57. [PMID: 21632850 DOI: 10.1152/ajpregu.00113.2011] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Acute heat stress activates visceral sympathetic nerve discharge (SND) in young rats, and the functional integrity of the rostral ventrolateral medulla (RVLM) is required for sustaining visceral sympathoexcitation during peak increases in internal body temperature (T(c)). However, RVLM mechanisms mediating SND activation to hyperthermia remain unknown. In the present study, we investigated the role of RVLM ionotropic excitatory amino acid receptors in mediating visceral SND activation to heat stress in anesthetized, young rats. The effects of bilateral RVLM kynurenic acid (Kyn; 2.7 and 5.4 nmol), saline, or muscimol (400-800 pmol) microinjections on renal SND and splenic SND responses to heat stress were determined at peak hyperthermia (T(c) 41.5°C), during progressive hyperthermia (T(c) 40°C), and at the initiation of heating (T(c) increased from 38 to 38.5°C). RVLM Kyn microinjections did not reduce renal and splenic SND recorded during progressive or peak hyperthermia and did not attenuate SND activation at the initiation of heating. In fact, renal and splenic SND tended to be or were significantly increased following RVLM Kyn microinjections at the initiation of heating and during hyperthermia (40 and 41.5°C). RVLM muscimol microinjections at 39, 40, and 41.5°C resulted in immediate reductions in SND. These data indicate that RVLM ionotropic glutamate receptors are required for mediating visceral sympathoexcitation to acute heating and suggest that acute heating activates an RVLM ionotropic excitatory amino acid receptor dependent inhibitory input, which reduces the level of visceral SND to heating.
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Affiliation(s)
- M J Kenney
- Department of Anatomy and Physiology, Kansas State University, Manhattan, Kansas 66506, USA.
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21
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Mueller PJ, Mischel NA, Scislo TJ. Differential activation of adrenal, renal, and lumbar sympathetic nerves following stimulation of the rostral ventrolateral medulla of the rat. Am J Physiol Regul Integr Comp Physiol 2011; 300:R1230-40. [PMID: 21346240 DOI: 10.1152/ajpregu.00713.2010] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Under acute and chronic conditions, the sympathetic nervous system can be activated in a differential and even selective manner. Activation of the rostral ventrolateral medulla (RVLM) has been implicated in differential control of sympathetic outputs based on evidence primarily in the cat. Although several studies indicate that differential control of sympathetic outflow occurs in other species, only a few studies have addressed whether the RVLM is capable of producing varying patterns of sympathetic activation in the rat. Therefore, the purpose of the present study was to determine whether activation of the RVLM results in simultaneous and differential increases in preganglionic adrenal (pre-ASNA), renal (RSNA), and lumbar (LSNA) sympathetic nerve activities. In urethane-chloralose anesthetized rats, pre-ASNA, RSNA, and LSNA were recorded simultaneously in all animals. Microinjections of selected concentrations and volumes of glutamate increased pre-ASNA, RSNA, and LSNA concurrently and differentially. Pre-ASNA and RSNA (in most cases) exhibited greater increases compared with LSNA on a percentage basis. By varying the volume or location of the glutamate microinjections, we also identified individual examples of differential and selective activation of these nerves. Decreases in arterial pressure or bilateral blockade of RVLM GABA(A) receptors also revealed differential activation, with the latter having a 3- to 4-fold greater effect on sympathetic activity. Our data provide evidence that activation of the rat RVLM increases renal, lumbar, and preganglionic adrenal sympathetic nerve activities concurrently, differentially, and, in some cases, selectively.
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Affiliation(s)
- Patrick J Mueller
- Dept. of Physiology, Wayne State University School of Medicine, 540 E. Canfield, Detroit, MI 48201, USA.
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22
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Brooks VL, Dampney RAL, Heesch CM. Pregnancy and the endocrine regulation of the baroreceptor reflex. Am J Physiol Regul Integr Comp Physiol 2010; 299:R439-51. [PMID: 20504907 PMCID: PMC2928618 DOI: 10.1152/ajpregu.00059.2010] [Citation(s) in RCA: 42] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/25/2010] [Accepted: 05/19/2010] [Indexed: 02/07/2023]
Abstract
The purpose of this review is to delineate the general features of endocrine regulation of the baroreceptor reflex, as well as specific contributions during pregnancy. In contrast to the programmed changes in baroreflex function that occur in situations initiated by central command (e.g., exercise or stress), the complex endocrine milieu often associated with physiological and pathophysiological states can influence the central baroreflex neuronal circuitry via multiple sites and mechanisms, thereby producing varied changes in baroreflex function. During pregnancy, baroreflex gain is markedly attenuated, and at least two hormonal mechanisms contribute, each at different brain sites: increased levels of the neurosteroid 3alpha-hydroxy-dihydroprogesterone (3alpha-OH-DHP), acting in the rostral ventrolateral medulla (RVLM), and reduced actions of insulin in the forebrain. 3alpha-OH-DHP appears to potentiate baroreflex-independent GABAergic inhibition of premotor neurons in the RVLM, which decreases the range of sympathetic nerve activity that can be elicited by changes in arterial pressure. In contrast, reductions in the levels or actions of insulin in the brain blunt baroreflex efferent responses to increments or decrements in arterial pressure. Although plasma levels of angiotensin II are increased in pregnancy, this is not responsible for the reduction in baroreflex gain, although it may contribute to the increased level of sympathetic nerve activity in this condition. How these different hormonal effects are integrated within the brain, as well as possible interactions with additional potential neuromodulators that influence baroreflex function during pregnancy and other physiological and pathophysiological states, remains to be clearly delineated.
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Affiliation(s)
- Virginia L Brooks
- Dept. of Physiology and Pharmacology, L-334, Oregon Health & Science Univ., 3181 SW Sam Jackson Park Road, Portland, OR 97239, USA.
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Kline DD, King TL, Austgen JR, Heesch CM, Hasser EM. Sensory afferent and hypoxia-mediated activation of nucleus tractus solitarius neurons that project to the rostral ventrolateral medulla. Neuroscience 2010; 167:510-27. [PMID: 20153814 PMCID: PMC2849863 DOI: 10.1016/j.neuroscience.2010.02.012] [Citation(s) in RCA: 49] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/06/2009] [Revised: 01/19/2010] [Accepted: 02/05/2010] [Indexed: 02/07/2023]
Abstract
The nucleus tractus solitarius (nTS) of the brainstem receives sensory afferent inputs, processes that information, and sends projections to a variety of brain regions responsible for influencing autonomic and respiratory output. The nTS sends direct projections to the rostral ventrolateral medulla (RVLM), an area important for cardiorespiratory reflexes and homeostasis. Since the net reflex effect of nTS processing ultimately depends on the properties of output neurons, we determined the characteristics of these RVLM-projecting nTS neurons using electrophysiological and immunohistochemical techniques. RVLM-projecting nTS neurons were identified by retrograde tracers. Patch clamp analysis in the horizontal brainstem nTS slice demonstrated that RVLM-projecting nTS cells exhibit constant latency solitary tract evoked excitatory postsynaptic currents (EPSCs), suggesting they receive strong monosynaptic contacts from visceral afferents. Three distinct patterns of action potential firing, associated with different underlying potassium currents, were observed in RVLM-projecting cells. Following activation of the chemoreflex in conscious animals by 3 h of acute hypoxia, 11.2+/-1.9% of the RVLM-projecting nTS neurons were activated, as indicated by positive Fos-immunoreactivity. Very few RVLM-projecting nTS cells were catecholaminergic. Taken together, these data suggest that RVLM projecting nTS neurons receive strong monosynaptic inputs from sensory afferents and a subpopulation participates in the chemoreflex pathway.
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Affiliation(s)
- D D Kline
- Department of Biomedical Sciences, University of Missouri, Columbia, MO 65211, USA.
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Malpas SC. Sympathetic nervous system overactivity and its role in the development of cardiovascular disease. Physiol Rev 2010; 90:513-57. [PMID: 20393193 DOI: 10.1152/physrev.00007.2009] [Citation(s) in RCA: 431] [Impact Index Per Article: 30.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
This review examines how the sympathetic nervous system plays a major role in the regulation of cardiovascular function over multiple time scales. This is achieved through differential regulation of sympathetic outflow to a variety of organs. This differential control is a product of the topographical organization of the central nervous system and a myriad of afferent inputs. Together this organization produces sympathetic responses tailored to match stimuli. The long-term control of sympathetic nerve activity (SNA) is an area of considerable interest and involves a variety of mediators acting in a quite distinct fashion. These mediators include arterial baroreflexes, angiotensin II, blood volume and osmolarity, and a host of humoral factors. A key feature of many cardiovascular diseases is increased SNA. However, rather than there being a generalized increase in SNA, it is organ specific, in particular to the heart and kidneys. These increases in regional SNA are associated with increased mortality. Understanding the regulation of organ-specific SNA is likely to offer new targets for drug therapy. There is a need for the research community to develop better animal models and technologies that reflect the disease progression seen in humans. A particular focus is required on models in which SNA is chronically elevated.
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Affiliation(s)
- Simon C Malpas
- Department of Physiology and the Auckland Bioengineering Institute, University of Auckland and Telemetry Research Ltd., Auckland, New Zealand.
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Mueller PJ. Physical (in)activity-dependent alterations at the rostral ventrolateral medulla: influence on sympathetic nervous system regulation. Am J Physiol Regul Integr Comp Physiol 2010; 298:R1468-74. [PMID: 20357021 DOI: 10.1152/ajpregu.00101.2010] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
A sedentary lifestyle is a major risk factor for cardiovascular disease, and rates of inactivity and cardiovascular disease are highly prevalent in our society. Cardiovascular disease is often associated with overactivity of the sympathetic nervous system, which has both direct and indirect effects on multiple organ systems. Although it has been known for some time that exercise positively affects the brain in terms of memory and cognition, only recently have changes in how the brain regulates the cardiovascular system been examined in terms of physical activity and inactivity. This brief review will discuss the evidence for physical activity-dependent neuroplasticity related to control of sympathetic outflow. It will focus particularly on recent studies from our laboratory and others that have examined changes that occur in the rostral ventrolateral medulla (RVLM), considered one of the primary brain regions involved in the regulation and generation of sympathetic nervous system activity.
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Affiliation(s)
- Patrick J Mueller
- Department of Physiology, Wayne State University School of Medicine, Detroit, Michigan 48201, USA.
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Austgen JR, Fong AY, Foley CM, Mueller PJ, Kline DD, Heesch CM, Hasser EM. Expression of Group I metabotropic glutamate receptors on phenotypically different cells within the nucleus of the solitary tract in the rat. Neuroscience 2008; 159:701-16. [PMID: 19013221 DOI: 10.1016/j.neuroscience.2008.09.060] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/23/2007] [Revised: 09/09/2008] [Accepted: 09/30/2008] [Indexed: 02/07/2023]
Abstract
Group I metabotropic glutamate receptors (mGluRs) are G-coupled receptors that modulate synaptic activity. Previous studies have shown that Group I mGluRs are present in the nucleus of the solitary tract (NTS), in which many visceral afferents terminate. Microinjection of selective Group I mGluR agonists into the NTS results in a depressor response and decrease in sympathetic nerve activity. There is, however, little evidence detailing which phenotypes of neurons within the NTS express Group I mGluRs. In brainstem slices, we performed immunohistochemical localization of Group I mGluRs and either glutamic acid decarboxylase 67 kDa isoform (GAD67), neuronal nitric oxide synthase (nNOS) or tyrosine hydroxylase (TH). Fluoro-Gold (FG, 2%; 15 nl) was microinjected in the caudal ventrolateral medulla (CVLM) of the rat to retrogradely label NTS neurons that project to CVLM. Group I mGluRs were distributed throughout the rostral-caudal extent of the NTS and were found within most NTS subregions. The relative percentages of Group I mGluR expressing neurons colabeled with the different markers were FG (6.9+/-0.7) nNOS (5.6+/-0.9), TH (3.9+/-1.0), and GAD67 (3.1+/-1.4). The percentage of FG containing cells colabeled with Group I mGluR (13.6+/-2.0) was greater than the percent colabeled with GAD67 (3.1+/-0.5), nNOS (4.7+/-0.5), and TH (0.1+/-0.08). Cells triple labeled for FG, nNOS, and Group I mGluRs were identified in the NTS. Thus, these data provide an anatomical substrate by which Group I mGluRs could modulate activity of CVLM projecting neurons in the NTS.
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Affiliation(s)
- J R Austgen
- Department of Medical Pharmacology and Physiology, University of Missouri, Columbia, MO 65211, USA
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Moffitt JA, Grippo AJ, Beltz TG, Johnson AK. Hindlimb unloading elicits anhedonia and sympathovagal imbalance. J Appl Physiol (1985) 2008; 105:1049-59. [PMID: 18635876 DOI: 10.1152/japplphysiol.90535.2008] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The hindlimb-unloaded (HU) rat model elicits cardiovascular deconditioning and simulates the physiological adaptations to microgravity or prolonged bed rest in humans. Although psychological deficits have been documented following bed rest and spaceflight in humans, few studies have explored the psychological effects of cardiovascular deconditioning in animal models. Given the bidirectional link established between cardiac autonomic imbalance and psychological depression in both humans and in animal models, we hypothesized that hindlimb unloading would elicit an alteration in sympathovagal tone and behavioral indexes of psychological depression. Male, Sprague-Dawley rats confined to 14 days of HU displayed anhedonia (a core feature of human depression) compared with casted control (CC) animals evidenced by reduced sucrose preference (CC: 81 +/- 2.9% baseline vs. HU: 58 +/- 4.5% baseline) and reduced (rightward shift) operant responding for rewarding electrical brain stimulation (CC: 4.4 +/- 0.3 muA vs. 7.3 +/- 1.0 muA). Cardiac autonomic blockade revealed elevated sympathetic [CC: -54 +/- 14.1 change in (Delta) beats/min vs. HU: -118 +/- 7.6 Delta beats/min] and reduced parasympathetic (CC: 45 +/- 11.8 Delta beats/min vs. HU: 8 +/- 7.3 Delta beats/min) cardiac tone in HU rats. Heart rate variability was reduced (CC: 10 +/- 1.4 ms vs. HU: 7 +/- 0.7 ms), and spectral analysis of blood pressure indicated loss of total, low-, and high-frequency power, consistent with attenuated baroreflex function. These data indicate that cardiovascular deconditioning results in sympathovagal imbalance and behavioral signs consistent with psychological depression. These findings further elucidate the pathophysiological link between cardiovascular diseases and affective disorders.
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Affiliation(s)
- Julia A Moffitt
- Department of Psychology, The University of Iowa, Iowa City, IA 52242-1407, USA.
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Tjen-A-Looi SC, Li P, Longhurst JC. Role of medullary GABA, opioids, and nociceptin in prolonged inhibition of cardiovascular sympathoexcitatory reflexes during electroacupuncture in cats. Am J Physiol Heart Circ Physiol 2007; 293:H3627-35. [PMID: 17890425 DOI: 10.1152/ajpheart.00842.2007] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Electroacupuncture (EA) causes prolonged suppression of reflex elevations in blood pressure for 1-2 h in anesthetized preparations. A long-loop pathway involving the arcuate nucleus (ARC), ventrolateral periaqueductal gray, and rostral ventrolateral medulla (rVLM) is involved in sympathoinhibitory cardiovascular EA effects. However, the mechanisms and locations of the prolonged EA inhibition are unknown. We hypothesized that this effect is mediated through a long-loop pathway involving opioid, nociceptin, and gamma-aminobutyric acid (GABA) receptor activation in the rVLM. In anesthetized, ventilated cats application of bradykinin to the gallbladder every 10 min induced consistent reflex increases in blood pressure. Bilateral EA stimulation at the cardiovascular acupoints P5-6 overlying the median nerves reduced the reflex responses for at least 80 min. Bilateral blockade with kynurenic acid in the ARC 60 min after onset of EA inhibition reversed the cardiovascular response, suggesting a role for the ARC in the long-loop pathway during the prolonged inhibitory response. Unilateral microinjection with either an opioid or a GABA(A) antagonist in rVLM 50-60 min after the beginning of the EA response reversed EA inhibition of the cardiovascular excitatory reflex. Gabazine also reversed EA inhibition of cardiovascular premotor sympathetic rVLM neurons. Conversely, microinjection of a nociceptin/orphanin FQ peptide antagonist did not affect the prolonged inhibitory effect. Thus the ARC, an important component in the long-loop pathway in the EA cardiovascular response, is required for prolonged suppression of reflex cardiovascular excitatory responses by EA. Furthermore, in the rVLM, opioids and GABA, but not nociceptin, participate in the long-term EA-related inhibition of sympathoexcitatory cardiovascular responses.
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Affiliation(s)
- Stephanie C Tjen-A-Looi
- Department of Medicine, Susan Samueli Center for Integrative Medicine, School of Medicine, University of California-Irvine, CA 92697-4075, USA.
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Booth LC, Bennet L, Barrett CJ, Guild SJ, Wassink G, Gunn AJ, Malpas SC. Cardiac-related rhythms in sympathetic nerve activity in preterm fetal sheep. Am J Physiol Regul Integr Comp Physiol 2007; 293:R185-90. [PMID: 17379843 DOI: 10.1152/ajpregu.00891.2006] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Extensive studies in the adult have demonstrated that the sympathetic nervous system plays a central role in cardiovascular control. The maturation of the sympathetic nervous system before birth is poorly understood. In the present study, we directly recorded renal sympathetic nerve activity (renal SNA) in five preterm fetal sheep (99 ± 1 days gestation; term is 147 days). Recordings were performed in utero using a telemetry-based technique to alleviate movement artifact without anesthesia or paralysis. The preterm fetuses exhibited a coordinated discharge pattern in renal SNA, indicating many individual neurons active at approximately the same time. This is consistent with that observed previously in adult animals, although the frequency of the bursts was relatively low (0.5 ± 0.1 Hz). The discharges in renal SNA were entrained to the cardiac cycle (average delay between diastolic pressure and maximum renal SNA 319 ± 1 ms). The entrainment of the sympathetic discharges to the cardiac cycle indicates phasic baroreceptor input and that the underlying circuits controlling SNA within the central nervous system are active in premature fetuses.
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Affiliation(s)
- Lindsea C Booth
- Fetal Physiology and Neuroscience Group, and Circulatory Control Laboratory, Department of Physiology, University of Auckland Medical School, Private Bag 92019, Auckland, New Zealand
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Mueller PJ. Exercise training attenuates increases in lumbar sympathetic nerve activity produced by stimulation of the rostral ventrolateral medulla. J Appl Physiol (1985) 2006; 102:803-13. [PMID: 17053106 DOI: 10.1152/japplphysiol.00498.2006] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Exercise training (ExTr) has been associated with blunted activation of the sympathetic nervous system in several animal models and in some human studies. Although these data are consistent with the hypothesis that ExTr reduces the incidence of cardiovascular diseases via reduced sympathoexcitation, the mechanisms are unknown. The rostral ventrolateral medulla (RVLM) is important in control of sympathetic nervous system activity in both physiological and pathophysiological states. The purpose of the present study was to test the hypothesis that ExTr results in reduced sympathoexcitation mediated at the level of the RVLM. Male Sprague-Dawley rats were treadmill trained or remained sedentary for 8-10 wk. RVLM microinjections were performed under Inactin anesthesia while mean arterial pressure, heart rate, and lumbar sympathetic nerve activity (LSNA) were recorded. Bilateral microinjections of the GABA(A) antagonist bicuculline (5 mM, 90 nl) into the RVLM increased LSNA in sedentary animals (169 +/- 33%), which was blunted in ExTr animals (100 +/- 22%, P < 0.05). Activation of the RVLM with unilateral microinjections of glutamate (10 mM, 30 nl) increased LSNA in sedentary animals (76 +/- 13%), which was also attenuated by training (26 +/- 2%, P < 0.05). Bilateral microinjections of the ionotropic glutamate receptor antagonist kynurenate (40 mM, 90 nl) produced small increases in mean arterial pressure and LSNA that were similar between groups. Results suggest that ExTr may reduce increases in LSNA due to reduced activation of the RVLM. Conversely, we speculate that the relatively enhanced activation of LSNA in sedentary animals may be related to the increased incidence of cardiovascular disease associated with a sedentary lifestyle.
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Affiliation(s)
- Patrick J Mueller
- Dalton Cardiovascular Research Center and Dept. of Biomedical Sciences, University of Missouri-Columbia, 65211-3300, USA.
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Mueller PJ, Foley CM, Heesch CM, Cunningham JT, Zheng H, Patel KP, Hasser EM. Increased nitric oxide synthase activity and expression in the hypothalamus of hindlimb unloaded rats. Brain Res 2006; 1115:65-74. [PMID: 16938283 DOI: 10.1016/j.brainres.2006.07.078] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/10/2005] [Revised: 07/05/2006] [Accepted: 07/24/2006] [Indexed: 10/24/2022]
Abstract
Upon return from spaceflight or resumption of normal posture after bed rest, individuals often exhibit cardiovascular deconditioning. Although the mechanisms responsible for cardiovascular deconditioning have yet to be fully elucidated, alterations within the central nervous system have been postulated to be involved. The paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus are important brain regions in control of sympathetic outflow and body fluid homeostasis. Nitric oxide (NO) modulates the activity of PVN and SON neurons, and alterations in NO transmission within these brain regions may contribute to symptoms of cardiovascular deconditioning. The purpose of the present study was to examine nitric oxide synthase (NOS) activity and expression in the PVN and SON of control and hindlimb unloaded (HU) rats, an animal model of cardiovascular deconditioning. The number of neurons exhibiting NOS activity as assessed by NADPH-diaphorase staining was significantly greater in the PVN but not SON of HU rats. Western blot analysis revealed that neuronal NOS (nNOS) but not endothelial NOS (eNOS) protein expression was higher in the PVN of HU rats. In the SON, there was a strong trend for an increase in nNOS (p=0.052) and a significant increase in eNOS expression in HU rats. Our results suggest that increased nNOS in the PVN contributes to autonomic and humoral alterations following cardiovascular deconditioning. In contrast, the functional significance of increases in nNOS and eNOS protein in the SON may be related to alterations in vasopressin release observed previously in HU rats.
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Affiliation(s)
- Patrick J Mueller
- Department of Biomedical Sciences, University of Missouri, Columbia, MO 65211-3300, USA.
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Malpas SC, Ramchandra R, Guild SJ, Budgett DM, Barrett CJ. Baroreflex mechanisms regulating mean level of SNA differ from those regulating the timing and entrainment of the sympathetic discharges in rabbits. Am J Physiol Regul Integr Comp Physiol 2006; 291:R400-9. [PMID: 16914425 DOI: 10.1152/ajpregu.00204.2005] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The arterial baroreflex pathway provides the fundamental basis for the short-term control of blood pressure via the rapid regulation of the mean level of sympathetic nerve activity (SNA) in response to changes in blood pressure. A central tenet in the generation and regulation of bursts of SNA is that input from the arterial baroreceptors also regulates the timing of the bursts of sympathetic activity. With the use of an implantable telemetry-based amplifier, renal SNA was recorded in intact and arterial baroreceptor-denervated (SAD) conscious rabbits. Data were collected continuously while animals were in their home cage. Mean levels of SNA were not different between SAD and baroreceptor-intact animals. Whereas SNA was unresponsive to changes in blood pressure in SAD rabbits, the timing of the bursts of SNA relative to the arterial pulse wave was maintained (time between the diastolic pressure and the next maximum SNA voltage averaged 107 ± 12 ms SAD vs. 105 ± 7 ms intact). Transfer function analysis between blood pressure and SNA indicates the average gain at the heart rate frequency was not altered by SAD, indicating strong coupling between the cardiac cycle and SNA bursts in SAD animals. Further experiments in anesthetized rabbits showed that this entrainment is lost immediately after performing baroreceptor denervation surgery and remained absent while the animal was under anesthesia but returned within 20 min of turning off the anesthesia. We propose that this finding indicates the regulation of the mean level of SNA requires the majority of input from baroreceptors to be functional; however, the regulation of the timing of the bursts in the conscious animal requires only minimal input, such as a sensitive trigger mechanism. This observation has important implications for understanding the origin and regulation of SNA.
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Affiliation(s)
- Simon C Malpas
- Circulatory Control Laboratory, Department of Physiology, University of Auckland Medical School, Private Bag 92019, Auckland, New Zealand.
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Foley CM, Mueller PJ, Hasser EM, Heesch CM. Hindlimb unloading and female gender attenuate baroreflex-mediated sympathoexcitation. Am J Physiol Regul Integr Comp Physiol 2005; 289:R1440-7. [PMID: 16051718 DOI: 10.1152/ajpregu.00356.2005] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Exposure to a period of microgravity or bed rest produces several physiological adaptations. These changes, which include an increased incidence of orthostatic intolerance, have an impact when people return to a 1G environment or resume an upright posture. Compared with males, females appear more susceptible to orthostatic intolerance after exposure to real or simulated microgravity. Decreased arterial baroreflex compensation may contribute to orthostatic intolerance. We hypothesized that female rats would exhibit a greater reduction in arterial baroreflex function after hindlimb unloading (HU) compared with male rats. Mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) were recorded in conscious animals after 13–15 days of HU. Baseline HR was elevated in female rats, and HU increased HR in both genders. Consistent with previous results in males, baroreflex-mediated activation of RSNA was blunted by HU in both genders. Maximum RSNA in response to decreases in MAP was reduced by HU (male control 513 ± 42%, n = 11; male HU 346 ± 38%, n = 13; female control 359 ± 44%, n = 10; female HU 260 ± 43%, n = 10). Maximum baroreflex increase in RSNA was lower in females compared with males in both control and HU rats. Both female gender and HU attenuated baroreflex-mediated increases in sympathetic activity. The combined effects of HU and gender resulted in reduced baroreflex sympathetic reserve in females compared with males and could contribute to the greater incidence of orthostatic intolerance in females after exposure to spaceflight or bed rest.
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Affiliation(s)
- C Michael Foley
- Department of Biomedical Sciences, Dalton Cardiovascular Research Center, University of Missouri-Columbia, 134 Research Park Drive, Columbia, Missouri 65211, USA.
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Moffitt JA, Johnson AK. Short-term fluoxetine treatment enhances baroreflex control of sympathetic nervous system activity after hindlimb unloading. Am J Physiol Regul Integr Comp Physiol 2004; 286:R584-90. [PMID: 14761868 DOI: 10.1152/ajpregu.00223.2002] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Data in humans indicate that individuals with orthostatic hypotension that are refractory to other traditional forms of therapy are responsive to selective serotonin reuptake inhibitor (SSRI) treatment. We tested the hypothesis that SSRI administration would help correct the attenuated baroreflex control of sympathetic nervous system activity in the hindlimb-unloaded (HU) rat model of cardiovascular deconditioning. An initial study was conducted to determine the time course of effects of fluoxetine (Flu) administration on baroreflex control of lumbar sympathetic nerve activity (LSNA) in conscious, chronically instrumented rats. Animals received either vehicle (Veh, sterile water) or 10 mg/kg Flu for 1, 4, or 16 days of treatment. Data indicate that while 1-day and 16-day Flu administration did not affect baroreflex function, baroreflex control of LSNA was enhanced after 4-day (short term) Flu administration. HU rats were then treated with Flu for 4 days and compared with HU rats receiving Veh and to casted control rats maintained in the normal posture that received either Veh or short-term Flu treatment. Similar to pilot data, short-term Flu treatment enhanced baroreflex control of LSNA in both HU rats and control rats. These data taken together indicate that baroreflex control of sympathetic nervous system activity is a possible mechanism responsible for the successful treatment of orthostatic intolerance with Flu.
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Affiliation(s)
- Julia A Moffitt
- The Cardiovascular Center, University of Iowa, Iowa City, Iowa 52242, USA
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Malpas SC. What sets the long-term level of sympathetic nerve activity: is there a role for arterial baroreceptors? Am J Physiol Regul Integr Comp Physiol 2004; 286:R1-R12. [PMID: 14660468 DOI: 10.1152/ajpregu.00496.2003] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Much of our knowledge of the influence of the sympathetic nervous system on the control of blood pressure is built on experimental approaches that focus very much on time scales <24 h. Although direct recordings of sympathetic nerve activity (SNA) over short time scales provide important information, it is difficult to place their relevance over the longer term where the development of chronic changes in blood pressure are likely to be a mixture of hormonal, renal, and neural influences. Recently new experimental approaches are now revealing a possible role for arterial baroreceptors in the chronic regulation of SNA. These studies reveal that chronic increases in blood pressure are associated with chronic changes in SNA that may be due to nonresetting of the blood pressure-SNA baroreflex relationship. This review discusses the implications of such information, highlighting new technologies for long-term recording of SNA that appear to hold much promise for revealing the role of SNA to the kidney for the long-term control of blood pressure.
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Affiliation(s)
- Simon C Malpas
- Circulatory Control Laboratory, Department of Physiology, University of Auckland, Auckland, New Zealand.
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Meck JV, Waters WW, Ziegler MG, deBlock HF, Mills PJ, Robertson D, Huang PL. Mechanisms of postspaceflight orthostatic hypotension: low alpha1-adrenergic receptor responses before flight and central autonomic dysregulation postflight. Am J Physiol Heart Circ Physiol 2003; 286:H1486-95. [PMID: 14670816 DOI: 10.1152/ajpheart.00740.2003] [Citation(s) in RCA: 65] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Although all astronauts experience symptoms of orthostatic intolerance after short-duration spaceflight, only approximately 20% actually experience presyncope during upright posture on landing day. The presyncopal group is characterized by low vascular resistance before and after flight and low norepinephrine release during orthostatic stress on landing day. Our purpose was to determine the mechanisms of the differences between presyncopal and nonpresyncopal groups. We studied 23 astronauts 10 days before launch, on landing day, and 3 days after landing. We measured pressor responses to phenylephrine injections; norepinephrine release with tyramine injections; plasma volumes; resting plasma levels of chromogranin A (a marker of sympathetic nerve terminal release), endothelin, dihydroxyphenylglycol (DHPG, an intracellular metabolite of norepinephrine); and lymphocyte beta(2)-adrenergic receptors. We then measured hemodynamic and neurohumoral responses to upright tilt. Astronauts were separated into two groups according to their ability to complete 10 min of upright tilt on landing day. Compared with astronauts who were not presyncopal on landing day, presyncopal astronauts had 1). significantly smaller pressor responses to phenylephrine both before and after flight; 2). significantly smaller baseline norepinephrine, but significantly greater DHPG levels, on landing day; 3). significantly greater norepinephrine release with tyramine on landing day; and 4). significantly smaller norepinephrine release, but significantly greater epinephrine and arginine vasopressin release, with upright tilt on landing day. These data suggest that the etiology of orthostatic hypotension and presyncope after spaceflight includes low alpha(1)-adrenergic receptor responsiveness before flight and a remodeling of the central nervous system during spaceflight such that sympathetic responses to baroreceptor input become impaired.
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Affiliation(s)
- Janice V Meck
- Human Adaptation and Countermeasures Office, Space and Life Sciences Directorate, National Aeronautics and Space Administrattion Johnson Space Center, Houston, TX 77058, USA.
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Kenney MJ, Fels RJ. Forebrain and brain stem neural circuits contribute to altered sympathetic responses to heating in senescent rats. J Appl Physiol (1985) 2003; 95:1986-93. [PMID: 12882996 DOI: 10.1152/japplphysiol.00438.2003] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Acute heating in young rats increases visceral sympathetic nerve discharge (SND); however, renal and splanchnic SND responses to hyperthermia are attenuated in senescent compared with young Fischer 344 (F344) rats (Kenney MJ and Fels RJ. Am J Physiol Regul Integr Comp Physiol 283: R513-R520, 2002). Central mechanisms by which aging alters visceral SND responses to heating are unknown. We tested the hypothesis that forebrain neural circuits are involved in suppressing sympathoexcitatory responses to heating in chloralose-anesthetized, senescent F344 rats. Renal and splanchnic SND responses to increased (38 degrees C-41 degrees C) internal temperature were determined in midbrain-transected (MT) and sham-MT young (3-mo-old), mature (12-mo-old), and senescent (24-mo-old) F344 rats and in cervical-transected (CT) and sham-CT senescent rats. Renal SND remained unchanged during heating in MT and sham-MT senescent rats but was increased in CT senescent rats. Splanchnic SND responses to heating were higher in MT vs. sham-MT senescent rats and in CT vs. MT senescent rats. SND responses to heating were similar in MT and sham-MT young and mature rats. Mean arterial pressure (MAP) was increased during heating in MT but not in sham-MT senescent rats, whereas heating-induced increases in MAP were higher in sham-MT vs. MT young rats. These data suggest that in senescent rats suppression of splanchnic SND to heating involves forebrain and brain stem neural circuits, whereas renal suppression is mediated solely by brain stem neural circuits. These results support the concept that aging alters the functional organization of pathways regulating SND and arterial blood pressure responses to acute heating.
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Affiliation(s)
- Michael J Kenney
- Department of Anatomy and Physiology, Kansas State University, Manhattan, Kansas 66506, USA.
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Mueller PJ, Hasser EM. Enhanced sympathoinhibitory response to volume expansion in conscious hindlimb-unloaded rats. J Appl Physiol (1985) 2003; 94:1806-12. [PMID: 12533501 DOI: 10.1152/japplphysiol.00979.2002] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Prolonged exposure to microgravity or bed rest produces cardiovascular deconditioning, which is characterized by reductions in plasma volume, alterations in autonomic function, and a predisposition toward orthostatic intolerance. Although the precise mechanisms have not been fully elucidated, it is possible that augmented cardiopulmonary reflexes contribute to some of these effects. The purpose of the present study was to test the hypothesis that sympathoinhibitory responses to volume expansion are enhanced in the hindlimb-unloaded (HU) rat, a model of cardiovascular deconditioning. Mean arterial blood pressure, heart rate, and renal sympathetic nerve activity (RSNA) responses to isotonic volume expansion (0.9% saline iv, 15% of plasma volume over 5 min) were examined in conscious HU (14 days) and control animals. Volume expansion produced decreases in RSNA in both groups; however, this effect was significantly greater in HU rats (-46 +/- 7 vs. -25 +/- 4% in controls). Animals instrumented for central venous pressure (CVP) did not exhibit differences in CVP responses to volume expansion. These data suggest that enhanced cardiopulmonary reflexes may be involved in the maintenance of reduced plasma volume and contribute to attenuated baroreflex-mediated sympathoexcitation after spaceflight or bed rest.
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Affiliation(s)
- Patrick J Mueller
- Department of Biomedical Sciences, Dalton Cardiovascular Research Center, University of Missouri, Columbia, Missouri 65211, USA.
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Mueller PJ, Cunningham JT, Patel KP, Hasser EM. Proposed role of the paraventricular nucleus in cardiovascular deconditioning. ACTA PHYSIOLOGICA SCANDINAVICA 2003; 177:27-35. [PMID: 12492776 DOI: 10.1046/j.1365-201x.2003.01044.x] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
AIM Cardiovascular deconditioning occurs in individuals exposed to prolonged spaceflight or bedrest and is associated with the development of orthostatic intolerance. Although the precise mechanisms remain to be fully elucidated, astronauts returning from space or bedrest patients returning to normal upright posture present with decreases in plasma volume and alterations in autonomic function. The hindlimb unloaded (HU) rat has been a useful model to study the effects of cardiovascular deconditioning as it mimics many of the changes that occur after spaceflight and bedrest. RESULTS Experiments performed in HU rats suggest that cardiovascular deconditioning attenuates baroreflex mediated sympathoexcitation and enhances cardiopulmonary receptor mediated sympathoinhibition. These alterations appear to be due to changes in the central nervous system and may contribute to the pre disposition towards orthostatic intolerance associated with cardiovascular deconditioning. The paraventricular nucleus (PVN) of the hypothalamus is important in basal and reflex control of sympathetic outflow. Recent evidence suggests that nitric oxide (NO) is an important inhibitory neurotransmitter in the PVN and that alterations in nitroxidergic transmission in the PVN may be involved in elevated sympathetic tone in certain disease states. CONCLUSION Based on evidence from other laboratories and published and preliminary data from our own laboratories, this review proposes a role for the PVN in cardiovascular deconditioning. In particular, we discuss the hypothesis that increased NO in the PVN contributes to the altered cardiovascular reflexes observed following deconditioning and how these reflexes may be related to the orthostatic intolerance observed after prolonged spaceflight or bedrest.
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Affiliation(s)
- P J Mueller
- Department of Biomedical Sciences, Dalton Cardiovascular Research Center, University of Missouri-Columbia, 134 Research Park Drive, Columbia, MO 65211-3300, USA
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Moffitt JA, Grippo AJ, Holmes PV, Johnson AK. Olfactory bulbectomy attenuates cardiovascular sympathoexcitatory reflexes in rats. Am J Physiol Heart Circ Physiol 2002; 283:H2575-83. [PMID: 12388291 DOI: 10.1152/ajpheart.00164.2002] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Bilateral removal of the olfactory lobes in rats produces a number of behavioral, endocrine, and neurochemical alterations in the brain. Little is known, however, regarding the effects of this treatment on cardiovascular function and autonomic reflexes. Male Sprague-Dawley rats underwent bilateral surgical ablation of the olfactory bulbs (n = 10) or were sham operated (n = 8). After 3 wk of recovery, animals were instrumented with femoral catheters and a lumbar sympathetic nerve recording electrode. After 24 h of recovery, cardiovascular responses to arterial baroreflex manipulation, air jet stress, and smoke exposure were recorded. Olfactory bulbectomized rats demonstrated attenuated sympathoexcitatory responses to hypotension, air jet stress, and smoke exposure, as well as elevated basal blood pressure, compared with sham-operated rats. These data indicate that the integrity of the olfactory bulbs in rats is important for the elicitation of normal cardiovascular and autonomic responses to a number of evocative stimuli.
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