1
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Baertsch NA, Marciante AB, Mitchell GS, Baker TL. Inactivity-induced phrenic motor facilitation requires PKCζ activity within phrenic motor neurons. J Neurophysiol 2024; 131:1188-1199. [PMID: 38691529 DOI: 10.1152/jn.00138.2024] [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: 04/03/2024] [Revised: 04/29/2024] [Accepted: 04/30/2024] [Indexed: 05/03/2024] Open
Abstract
Prolonged inhibition of respiratory neural activity elicits a long-lasting increase in phrenic nerve amplitude once respiratory neural activity is restored. Such long-lasting facilitation represents a form of respiratory motor plasticity known as inactivity-induced phrenic motor facilitation (iPMF). Although facilitation also occurs in inspiratory intercostal nerve activity after diminished respiratory neural activity (iIMF), it is of shorter duration. Atypical PKC activity in the cervical spinal cord is necessary for iPMF and iIMF, but the site and specific isoform of the relevant atypical PKC are unknown. Here, we used RNA interference to test the hypothesis that the zeta atypical PKC isoform (PKCζ) within phrenic motor neurons is necessary for iPMF but PKCζ within intercostal motor neurons is unnecessary for transient iIMF. Intrapleural injections of siRNAs targeting PKCζ (siPKCζ) to knock down PKCζ mRNA within phrenic and intercostal motor neurons were made in rats. Control rats received a nontargeting siRNA (NTsi) or an active siRNA pool targeting a novel PKC isoform, PKCθ (siPKCθ), which is required for other forms of respiratory motor plasticity. Phrenic nerve burst amplitude and external intercostal (T2) electromyographic (EMG) activity were measured in anesthetized and mechanically ventilated rats exposed to 30 min of respiratory neural inactivity (i.e., neural apnea) created by modest hypocapnia (20 min) or a similar recording duration without neural apnea (time control). Phrenic burst amplitude was increased in rats treated with NTsi (68 ± 10% baseline) and siPKCθ (57 ± 8% baseline) 60 min after neural apnea vs. time control rats (-3 ± 3% baseline), demonstrating iPMF. In contrast, intrapleural siPKCζ virtually abolished iPMF (5 ± 4% baseline). iIMF was transient in all groups exposed to neural apnea; however, intrapleural siPKCζ attenuated iIMF 5 min after neural apnea (50 ± 21% baseline) vs. NTsi (97 ± 22% baseline) and siPKCθ (103 ± 20% baseline). Neural inactivity elevated the phrenic, but not intercostal, responses to hypercapnia, an effect that was blocked by siPKCζ. We conclude that PKCζ within phrenic motor neurons is necessary for long-lasting iPMF, whereas intercostal motor neuron PKCζ contributes to, but is not necessary for, transient iIMF.NEW & NOTEWORTHY We report important new findings concerning the mechanisms regulating a form of spinal neuroplasticity elicited by prolonged inhibition of respiratory neural activity, inactivity-induced phrenic motor facilitation (iPMF). We demonstrate that the atypical PKC isoform PKCζ within phrenic motor neurons is necessary for long-lasting iPMF, whereas intercostal motor neuron PKCζ contributes to, but is not necessary for, transient inspiratory intercostal facilitation. Our findings are novel and advance our understanding of mechanisms contributing to phrenic motor plasticity.
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Affiliation(s)
- Nathan A Baertsch
- Department of Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin, Madison, Wisconsin, United States
| | - Alexandria B Marciante
- Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida, Gainesville, Florida, United States
| | - Gordon S Mitchell
- Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida, Gainesville, Florida, United States
| | - Tracy L Baker
- Department of Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin, Madison, Wisconsin, United States
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2
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Mitchell GS, Baker TL. Respiratory neuroplasticity: Mechanisms and translational implications of phrenic motor plasticity. HANDBOOK OF CLINICAL NEUROLOGY 2022; 188:409-432. [PMID: 35965036 DOI: 10.1016/b978-0-323-91534-2.00016-3] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/15/2023]
Abstract
Widespread appreciation that neuroplasticity is an essential feature of the neural system controlling breathing has emerged only in recent years. In this chapter, we focus on respiratory motor plasticity, with emphasis on the phrenic motor system. First, we define related but distinct concepts: neuromodulation and neuroplasticity. We then focus on mechanisms underlying two well-studied models of phrenic motor plasticity: (1) phrenic long-term facilitation following brief exposure to acute intermittent hypoxia; and (2) phrenic motor facilitation after prolonged or recurrent bouts of diminished respiratory neural activity. Advances in our understanding of these novel and important forms of plasticity have been rapid and have already inspired translation in multiple respects: (1) development of novel therapeutic strategies to preserve/restore breathing function in humans with severe neurological disorders, such as spinal cord injury and amyotrophic lateral sclerosis; and (2) the discovery that similar plasticity also occurs in nonrespiratory motor systems. Indeed, the realization that similar plasticity occurs in respiratory and nonrespiratory motor neurons inspired clinical trials to restore leg/walking and hand/arm function in people living with chronic, incomplete spinal cord injury. Similar application may be possible to other clinical disorders that compromise respiratory and non-respiratory movements.
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Affiliation(s)
- Gordon S Mitchell
- Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida, Gainesville, FL, United States.
| | - Tracy L Baker
- Department of Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI, United States
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3
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Lee KZ, Liou LM, Vinit S. Diaphragm Motor-Evoked Potential Induced by Cervical Magnetic Stimulation following Cervical Spinal Cord Contusion in the Rat. J Neurotrauma 2021; 38:2122-2140. [PMID: 33899506 DOI: 10.1089/neu.2021.0080] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022] Open
Abstract
Cervical spinal injury is typically associated with respiratory impairments due to damage to bulbospinal respiratory pathways and phrenic motoneurons. Magnetic stimulation is a non-invasive approach for the evaluation and modulation of the nervous system. The present study was designed to examine whether cervical magnetic stimulation can be applied to evaluate diaphragmatic motor outputs in a pre-clinical rat model of cervical spinal injury. The bilateral diaphragm was monitored in anesthetized rats using electromyogram at the acute, subchronic, and chronic stages following left mid-cervical contusion. The center of a figure-of-eight coil was placed 20 mm caudal to bregma to stimulate the cervical spinal cord. The results demonstrated that a single magnetic stimulation can evoke significant motor-evoked potentials in the diaphragms of uninjured animals when the animal's head was placed 30 mm right or left from the center of the coil. The spontaneous bursting of the diaphragm was significantly attenuated by contusion injury at all-time-points post-injury. However, the threshold of the diaphragmatic motor-evoked potential was reduced, and the amplitude of the diaphragmatic motor-evoked potential was enhanced in response to cervical magnetic stimulation at the acute injury stage. Moreover, the motor-evoked potentials of the bilateral diaphragm in animals with contusions were generally larger when the coil was placed at the left spinal cord at the subchronic and chronic injury stages. These results suggested that cervical magnetic stimulation can be used to examine the excitability of phrenic motor outputs post-injury, and magnetic stimulation applied more laterally may be more effective for triggering diaphragmatic motor-evoked potentials.
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Affiliation(s)
- Kun-Ze Lee
- Department of Biological Sciences, National Sun Yat-sen University, Kaohsiung, Taiwan
- Department of Biomedical Science and Environmental Biology, School of Medicine, College of Medicine, Kaohsiung Medical University, Kaohsiung, Taiwan
| | - Li-Min Liou
- Department of Neurology, Kaohsiung Medical University Hospital, School of Medicine, College of Medicine, Kaohsiung Medical University, Kaohsiung, Taiwan
- Department of Neurology, School of Medicine, College of Medicine, Kaohsiung Medical University, Kaohsiung, Taiwan
| | - Stéphane Vinit
- Université Paris-Saclay, UVSQ, Inserm, END-ICAP, Versailles, France
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4
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Perim RR, Sunshine MD, Welch JF, Santiago J, Holland A, Ross A, Mitchell GS, Gonzalez-Rothi EJ. Daily acute intermittent hypoxia enhances phrenic motor output and stimulus-evoked phrenic responses in rats. J Neurophysiol 2021; 126:777-790. [PMID: 34260289 DOI: 10.1152/jn.00112.2021] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Plasticity is a hallmark of the respiratory neural control system. Phrenic long-term facilitation (pLTF) is one form of respiratory plasticity characterized by persistent increases in phrenic nerve activity following acute intermittent hypoxia (AIH). Although there is evidence that key steps in the cellular pathway giving rise to pLTF are localized within phrenic motor neurons (PMNs), the impact of AIH on the strength of breathing-related synaptic inputs to PMNs remains unclear. Further, the functional impact of AIH is enhanced by repeated/daily exposure to AIH (dAIH). Here, we explored the effects of AIH vs. 2 weeks of dAIH preconditioning on spontaneous and evoked responses recorded in anesthetized, paralyzed (with pancuronium bromide) and mechanically ventilated rats. Evoked phrenic potentials were elicited by respiratory cycle-triggered lateral funiculus stimulation at C2 delivered prior to- and 60 min post-AIH (or an equivalent time in controls). Charge-balanced biphasic pulses (100 µs/phase) of progressively increasing intensity (100 to 700 µA) were delivered during the inspiratory and expiratory phases of the respiratory cycle. Although robust pLTF (~60% from baseline) was observed after a single exposure to moderate AIH (3 x 5 min; 5 min intervals), there was no effect on evoked phrenic responses, contrary to our initial hypothesis. However, in rats preconditioned with dAIH, baseline phrenic nerve activity and evoked responses were increased, suggesting that repeated exposure to AIH enhances functional synaptic strength when assessed using this technique. The impact of daily AIH preconditioning on synaptic inputs to PMNs raises interesting questions that require further exploration.
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Affiliation(s)
- Raphael Rodrigues Perim
- Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida, Gainesville, FL, United States
| | - Michael D Sunshine
- Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida, Gainesville, FL, United States
| | - Joseph F Welch
- Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida, Gainesville, FL, United States
| | - Juliet Santiago
- Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida, Gainesville, FL, United States
| | - Ashley Holland
- Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida, Gainesville, FL, United States
| | - Ashley Ross
- Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida, Gainesville, FL, United States
| | - Gordon S Mitchell
- Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida, Gainesville, FL, United States
| | - Elisa J Gonzalez-Rothi
- Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida, Gainesville, FL, United States
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5
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Braegelmann KM, Meza A, Agbeh AE, Fields DP, Baker TL. Retinoic acid receptor alpha activation is necessary and sufficient for plasticity induced by recurrent central apnea. J Appl Physiol (1985) 2021; 130:836-845. [PMID: 33411644 DOI: 10.1152/japplphysiol.00287.2020] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Reductions in respiratory-related synaptic inputs to inspiratory motor neurons initiate a form of plasticity that proportionally enhances inspiratory motor output, even in the absence of changing blood gases. This form of plasticity is known as inactivity-induced inspiratory motor facilitation (iMF). iMF triggered by brief, recurrent reductions in respiratory neural activity requires local retinoic acid (RA) synthesis, but receptor subtypes activated by RA are unknown. To test the hypothesis that retinoic acid receptor alpha (RARα) is necessary for iMF, RAR subtype-specific inhibitors were delivered intrathecally above the phrenic motor pool in urethane-anesthetized, ventilated rats before 5, ∼1 min central apneas (without hypoxia; separated by 5 min) while monitoring phrenic inspiratory output. Pretreatment with a spinal RARα inhibitor impaired the capacity for recurrent central apnea to trigger long-lasting increases in phrenic inspiratory output, but plasticity was expressed in rats pretreated with an RARβ/γ inhibitor. Intrathecal RA application in the absence of reduced respiratory neural activity elicited an increase in phrenic inspiratory output, which was prevented by pretreatment with an RARα inhibitor. These data indicate that spinal RARα activation is necessary for iMF triggered by recurrent reductions in respiratory neural activity, and that RARα activation in/near the phrenic motor pool in the absence of respiratory neural activity deprivation is sufficient to elicit phrenic inspiratory motor facilitation. Understanding cellular cascades underlying plasticity induced by reductions in respiratory neural activity may define avenues for pharmacological intervention in disorders in which endogenous compensatory mechanisms that defend ongoing inspiratory motor output are impaired.NEW & NOTEWORTHY Local mechanisms near phrenic motor neurons respond to reductions in respiratory-related synaptic inputs by triggering a chemoreflex-independent, proportional enhancement in inspiratory output, a form of plasticity called inactivity-induced inspiratory motor facilitation (iMF). Here, we show that activation of spinal retinoic acid receptor alpha (RARα) is necessary to trigger phrenic iMF, and that spinal RARα activation in the absence of respiratory neural activity deprivation is sufficient to elicit phrenic inspiratory facilitation.
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Affiliation(s)
- Kendra M Braegelmann
- Department of Comparative Biosciences, University of Wisconsin-Madison, Madison, Wisconsin
| | - Armand Meza
- Department of Comparative Biosciences, University of Wisconsin-Madison, Madison, Wisconsin
| | - Abiye E Agbeh
- Department of Comparative Biosciences, University of Wisconsin-Madison, Madison, Wisconsin
| | - Daryl P Fields
- Department of Comparative Biosciences, University of Wisconsin-Madison, Madison, Wisconsin
| | - Tracy L Baker
- Department of Comparative Biosciences, University of Wisconsin-Madison, Madison, Wisconsin
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6
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Fields DP, Braegelmann KM, Meza AL, Mickelson CR, Gumnit MG, Baker TL. Competing mechanisms of plasticity impair compensatory responses to repetitive apnoea. J Physiol 2019; 597:3951-3967. [PMID: 31280489 PMCID: PMC6716600 DOI: 10.1113/jp277676] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2019] [Accepted: 06/03/2019] [Indexed: 12/21/2022] Open
Abstract
KEY POINTS Intermittent reductions in respiratory neural activity, a characteristic of many ventilatory disorders, leads to inadequate ventilation and arterial hypoxia. Both intermittent reductions in respiratory neural activity and intermittent hypoxia trigger compensatory enhancements in inspiratory output when experienced separately, forms of plasticity called inactivity-induced inspiratory motor facilitation (iMF) and long-term facilitation (LTF), respectively. Reductions in respiratory neural activity that lead to moderate, but not mild, arterial hypoxia occludes plasticity expression, indicating that concurrent induction of iMF and LTF impairs plasticity through cross-talk inhibition of their respective signalling pathways. Moderate hypoxia undermines iMF by enhancing NR2B-containing NMDA receptor signalling, which can be rescued by exogenous retinoic acid, a molecule necessary for iMF. These data suggest that in ventilatory disorders characterized by reduced inspiratory motor output, such as sleep apnoea, endogenous mechanisms of compensatory plasticity may be impaired, and that exogenously activating respiratory plasticity may be a novel strategy to improve breathing. ABSTRACT Many forms of sleep apnoea are characterized by recurrent reductions in respiratory neural activity, which leads to inadequate ventilation and arterial hypoxia. Both recurrent reductions in respiratory neural activity and hypoxia activate mechanisms of compensatory plasticity that augment inspiratory output and lower the threshold for apnoea, inactivity-induced inspiratory motor facilitation (iMF) and long-term facilitation (LTF), respectively. However, despite frequent concurrence of reduced respiratory neural activity and hypoxia, mechanisms that induce and regulate iMF and LTF have only been studied separately. Here, we demonstrate that recurrent reductions in respiratory neural activity ('neural apnoea') accompanied by cessations in ventilation that result in moderate (but not mild) hypoxaemia do not elicit increased inspiratory output, suggesting that concurrent induction of iMF and LTF occludes plasticity. A key role for NMDA receptor activation in impairing plasticity following concurrent neural apnoea and hypoxia is indicated since recurrent hypoxic neural apnoeas triggered increased phrenic inspiratory output in rats in which spinal NR2B-containing NMDA receptors were inhibited. Spinal application of retinoic acid, a key molecule necessary for iMF, bypasses NMDA receptor-mediated constraints, thereby rescuing plasticity following hypoxic neural apnoeas. These studies raise the intriguing possibility that endogenous mechanisms of compensatory plasticity may be impaired in some individuals with sleep apnoea, and that exogenously activating pathways giving rise to respiratory plasticity may be a novel pharmacological strategy to improve breathing.
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Affiliation(s)
- Daryl P Fields
- Department of Comparative Biosciences, University of Wisconsin, Madison, WI, USA
| | - Kendra M Braegelmann
- Department of Comparative Biosciences, University of Wisconsin, Madison, WI, USA
| | - Armand L Meza
- Department of Comparative Biosciences, University of Wisconsin, Madison, WI, USA
| | - Carly R Mickelson
- Department of Comparative Biosciences, University of Wisconsin, Madison, WI, USA
| | - Maia G Gumnit
- Department of Comparative Biosciences, University of Wisconsin, Madison, WI, USA
| | - Tracy L Baker
- Department of Comparative Biosciences, University of Wisconsin, Madison, WI, USA
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7
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Nair J, Streeter KA, Turner SMF, Sunshine MD, Bolser DC, Fox EJ, Davenport PW, Fuller DD. Anatomy and physiology of phrenic afferent neurons. J Neurophysiol 2017; 118:2975-2990. [PMID: 28835527 DOI: 10.1152/jn.00484.2017] [Citation(s) in RCA: 28] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/28/2017] [Revised: 08/17/2017] [Accepted: 08/17/2017] [Indexed: 12/23/2022] Open
Abstract
Large-diameter myelinated phrenic afferents discharge in phase with diaphragm contraction, and smaller diameter fibers discharge across the respiratory cycle. In this article, we review the phrenic afferent literature and highlight areas in need of further study. We conclude that 1) activation of both myelinated and nonmyelinated phrenic sensory afferents can influence respiratory motor output on a breath-by-breath basis; 2) the relative impact of phrenic afferents substantially increases with diaphragm work and fatigue; 3) activation of phrenic afferents has a powerful impact on sympathetic motor outflow, and 4) phrenic afferents contribute to diaphragm somatosensation and the conscious perception of breathing. Much remains to be learned regarding the spinal and supraspinal distribution and synaptic contacts of myelinated and nonmyelinated phrenic afferents. Similarly, very little is known regarding the potential role of phrenic afferent neurons in triggering or modulating expression of respiratory neuroplasticity.
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Affiliation(s)
- Jayakrishnan Nair
- Department of Physical Therapy, College of Public Health and Health Professions, University of Florida, Gainesville, Florida.,Center for Respiratory Research and Rehabilitation, University of Florida, Gainesville, Florida; and
| | - Kristi A Streeter
- Department of Physical Therapy, College of Public Health and Health Professions, University of Florida, Gainesville, Florida.,Center for Respiratory Research and Rehabilitation, University of Florida, Gainesville, Florida; and
| | - Sara M F Turner
- Department of Physical Therapy, College of Public Health and Health Professions, University of Florida, Gainesville, Florida.,Center for Respiratory Research and Rehabilitation, University of Florida, Gainesville, Florida; and
| | - Michael D Sunshine
- Department of Physical Therapy, College of Public Health and Health Professions, University of Florida, Gainesville, Florida.,Center for Respiratory Research and Rehabilitation, University of Florida, Gainesville, Florida; and
| | - Donald C Bolser
- Department of Physiological Sciences, College of Veterinary Medicine, University of Florida, Gainesville, Florida.,Center for Respiratory Research and Rehabilitation, University of Florida, Gainesville, Florida; and
| | - Emily J Fox
- Department of Physical Therapy, College of Public Health and Health Professions, University of Florida, Gainesville, Florida.,McKnight Brain Institute, University of Florida, Gainesville, Florida.,Center for Respiratory Research and Rehabilitation, University of Florida, Gainesville, Florida; and.,Brooks Rehabilitation, Jacksonville, Florida
| | - Paul W Davenport
- Department of Physiological Sciences, College of Veterinary Medicine, University of Florida, Gainesville, Florida.,Center for Respiratory Research and Rehabilitation, University of Florida, Gainesville, Florida; and
| | - David D Fuller
- Department of Physical Therapy, College of Public Health and Health Professions, University of Florida, Gainesville, Florida; .,McKnight Brain Institute, University of Florida, Gainesville, Florida.,Center for Respiratory Research and Rehabilitation, University of Florida, Gainesville, Florida; and
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8
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Baertsch NA, Baker TL. Intermittent apnea elicits inactivity-induced phrenic motor facilitation via a retinoic acid- and protein synthesis-dependent pathway. J Neurophysiol 2017; 118:2702-2710. [PMID: 28814632 DOI: 10.1152/jn.00212.2017] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/22/2017] [Revised: 08/10/2017] [Accepted: 08/16/2017] [Indexed: 12/13/2022] Open
Abstract
Respiratory motoneuron pools must provide rhythmic inspiratory drive that is robust and reliable, yet dynamic enough to respond to respiratory challenges. One form of plasticity that is hypothesized to contribute to motor output stability by sensing and responding to inadequate respiratory neural activity is inactivity-induced phrenic motor facilitation (iPMF), an increase in inspiratory output triggered by a reduction in phrenic synaptic inputs. Evidence suggests that mechanisms giving rise to iPMF differ depending on the pattern of reduced respiratory neural activity (i.e., neural apnea). A prolonged neural apnea elicits iPMF via a spinal TNF-α-induced increase in atypical PKC activity, but little is known regarding mechanisms that elicit iPMF following intermittent neural apnea. We tested the hypothesis that iPMF triggered by intermittent neural apnea requires retinoic acid and protein synthesis. Phrenic nerve activity was recorded in urethane-anesthetized and -ventilated rats treated intrathecally with an inhibitor of retinoic acid synthesis (4-diethlyaminobenzaldehyde, DEAB), a protein synthesis inhibitor (emetine), or vehicle (artificial cerebrospinal fluid) before intermittent (5 episodes, ~1.25 min each) or prolonged (30 min) neural apnea. Both DEAB and emetine abolished iPMF elicited by intermittent neural apnea but had no effect on iPMF elicited by a prolonged neural apnea. Thus different patterns of reduced respiratory neural activity elicit phenotypically similar iPMF via distinct spinal mechanisms. Understanding mechanisms that allow respiratory motoneurons to dynamically tune their output may have important implications in the context of respiratory control disorders that involve varied patterns of reduced respiratory neural activity, such as central sleep apnea and spinal cord injury.NEW & NOTEWORTHY We identify spinal retinoic acid and protein synthesis as critical components in the cellular cascade whereby repetitive reductions in respiratory neural activity elicit rebound increases in phrenic inspiratory activity.
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Affiliation(s)
- Nathan A Baertsch
- Department of Comparative Biosciences, University of Wisconsin-Madison, Madison, Wisconsin
| | - Tracy L Baker
- Department of Comparative Biosciences, University of Wisconsin-Madison, Madison, Wisconsin
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9
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Respiratory neuroplasticity – Overview, significance and future directions. Exp Neurol 2017; 287:144-152. [DOI: 10.1016/j.expneurol.2016.05.022] [Citation(s) in RCA: 44] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/01/2016] [Accepted: 05/17/2016] [Indexed: 01/10/2023]
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10
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Reduced respiratory neural activity elicits a long-lasting decrease in the CO 2 threshold for apnea in anesthetized rats. Exp Neurol 2016; 287:235-242. [PMID: 27474512 DOI: 10.1016/j.expneurol.2016.07.020] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/26/2016] [Revised: 07/21/2016] [Accepted: 07/25/2016] [Indexed: 12/23/2022]
Abstract
Two critical parameters that influence breathing stability are the levels of arterial pCO2 at which breathing ceases and subsequently resumes - termed the apneic and recruitment thresholds (AT and RT, respectively). Reduced respiratory neural activity elicits a chemoreflex-independent, long-lasting increase in phrenic burst amplitude, a form of plasticity known as inactivity-induced phrenic motor facilitation (iPMF). The physiological significance of iPMF is unknown. To determine if iPMF and neural apnea have long-lasting physiological effects on breathing, we tested the hypothesis that patterns of neural apnea that induce iPMF also elicit changes in the AT and RT. Phrenic nerve activity and end-tidal CO2 were recorded in urethane-anesthetized, ventilated rats to quantify phrenic nerve burst amplitude and the AT and RT before and after three patterns of neural apnea that differed in their duration and ability to elicit iPMF: brief intermittent neural apneas, a single brief "massed" neural apnea, or a prolonged neural apnea. Consistent with our hypothesis, we found that patterns of neural apnea that elicited iPMF also resulted in changes in the AT and RT. Specifically, intermittent neural apneas progressively decreased the AT with each subsequent neural apnea, which persisted for at least 60min. Similarly, a prolonged neural apnea elicited a long-lasting decrease in the AT. In both cases, the magnitude of the AT decrease was proportional to iPMF. In contrast, the RT was transiently decreased following prolonged neural apnea, and was not proportional to iPMF. No changes in the AT or RT were observed following a single brief neural apnea. Our results indicate that the AT and RT are differentially altered by neural apnea and suggest that specific patterns of neural apnea that elicit plasticity may stabilize breathing via a decrease in the AT.
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11
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Braegelmann KM, Streeter KA, Fields DP, Baker TL. Plasticity in respiratory motor neurons in response to reduced synaptic inputs: A form of homeostatic plasticity in respiratory control? Exp Neurol 2016; 287:225-234. [PMID: 27456270 DOI: 10.1016/j.expneurol.2016.07.012] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/01/2016] [Revised: 06/16/2016] [Accepted: 07/20/2016] [Indexed: 12/31/2022]
Abstract
For most individuals, the respiratory control system produces a remarkably stable and coordinated motor output-recognizable as a breath-from birth until death. Very little is understood regarding the processes by which the respiratory control system maintains network stability in the presence of changing physiological demands and network properties that occur throughout life. An emerging principle of neuroscience is that neural activity is sensed and adjusted locally to assure that neurons continue to operate in an optimal range, yet to date, it is unknown whether such homeostatic plasticity is a feature of the neurons controlling breathing. Here, we review the evidence that local mechanisms sense and respond to perturbations in respiratory neural activity, with a focus on plasticity in respiratory motor neurons. We discuss whether these forms of plasticity represent homeostatic plasticity in respiratory control. We present new analyses demonstrating that reductions in synaptic inputs to phrenic motor neurons elicit a compensatory enhancement of phrenic inspiratory motor output, a form of plasticity termed inactivity-induced phrenic motor facilitation (iPMF), that is proportional to the magnitude of activity deprivation. Although the physiological role of iPMF is not understood, we hypothesize that it has an important role in protecting the drive to breathe during conditions of prolonged or intermittent reductions in respiratory neural activity, such as following spinal cord injury or during central sleep apnea.
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Affiliation(s)
- K M Braegelmann
- Department of Comparative Biosciences, University of Wisconsin-Madison, 2015 Linden Drive, Madison, WI 53706, United States
| | - K A Streeter
- Department of Comparative Biosciences, University of Wisconsin-Madison, 2015 Linden Drive, Madison, WI 53706, United States
| | - D P Fields
- Department of Comparative Biosciences, University of Wisconsin-Madison, 2015 Linden Drive, Madison, WI 53706, United States
| | - T L Baker
- Department of Comparative Biosciences, University of Wisconsin-Madison, 2015 Linden Drive, Madison, WI 53706, United States.
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12
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Fields DP, Mitchell GS. Spinal metaplasticity in respiratory motor control. Front Neural Circuits 2015; 9:2. [PMID: 25717292 PMCID: PMC4324138 DOI: 10.3389/fncir.2015.00002] [Citation(s) in RCA: 47] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2014] [Accepted: 01/07/2015] [Indexed: 12/25/2022] Open
Abstract
A hallmark feature of the neural system controlling breathing is its ability to exhibit plasticity. Less appreciated is the ability to exhibit metaplasticity, a change in the capacity to express plasticity (i.e., “plastic plasticity”). Recent advances in our understanding of cellular mechanisms giving rise to respiratory motor plasticity lay the groundwork for (ongoing) investigations of metaplasticity. This detailed understanding of respiratory metaplasticity will be essential as we harness metaplasticity to restore breathing capacity in clinical disorders that compromise breathing, such as cervical spinal injury, motor neuron disease and other neuromuscular diseases. In this brief review, we discuss key examples of metaplasticity in respiratory motor control, and our current understanding of mechanisms giving rise to spinal plasticity and metaplasticity in phrenic motor output; particularly after pre-conditioning with intermittent hypoxia. Progress in this area has led to the realization that similar mechanisms are operative in other spinal motor networks, including those governing limb movement. Further, these mechanisms can be harnessed to restore respiratory and non-respiratory motor function after spinal injury.
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Affiliation(s)
- Daryl P Fields
- Department of Comparative Biosciences, University of Wisconsin-Madison Madison, WI, USA
| | - Gordon S Mitchell
- Department of Comparative Biosciences, University of Wisconsin-Madison Madison, WI, USA
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Baertsch NA, Baker-Herman TL. Intermittent reductions in respiratory neural activity elicit spinal TNF-α-independent, atypical PKC-dependent inactivity-induced phrenic motor facilitation. Am J Physiol Regul Integr Comp Physiol 2015; 308:R700-7. [PMID: 25673781 DOI: 10.1152/ajpregu.00359.2014] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/26/2014] [Accepted: 02/07/2015] [Indexed: 12/19/2022]
Abstract
In many neural networks, mechanisms of compensatory plasticity respond to prolonged reductions in neural activity by increasing cellular excitability or synaptic strength. In the respiratory control system, a prolonged reduction in synaptic inputs to the phrenic motor pool elicits a TNF-α- and atypical PKC-dependent form of spinal plasticity known as inactivity-induced phrenic motor facilitation (iPMF). Although iPMF may be elicited by a prolonged reduction in respiratory neural activity, iPMF is more efficiently induced when reduced respiratory neural activity (neural apnea) occurs intermittently. Mechanisms giving rise to iPMF following intermittent neural apnea are unknown. The purpose of this study was to test the hypothesis that iPMF following intermittent reductions in respiratory neural activity requires spinal TNF-α and aPKC. Phrenic motor output was recorded in anesthetized and ventilated rats exposed to brief intermittent (5, ∼1.25 min), brief sustained (∼6.25 min), or prolonged sustained (30 min) neural apnea. iPMF was elicited following brief intermittent and prolonged sustained neural apnea, but not following brief sustained neural apnea. Unlike iPMF following prolonged neural apnea, spinal TNF-α was not required to initiate iPMF during intermittent neural apnea; however, aPKC was still required for its stabilization. These results suggest that different patterns of respiratory neural activity induce iPMF through distinct cellular mechanisms but ultimately converge on a similar downstream pathway. Understanding the diverse cellular mechanisms that give rise to inactivity-induced respiratory plasticity may lead to development of novel therapeutic strategies to treat devastating respiratory control disorders when endogenous compensatory mechanisms fail.
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Affiliation(s)
- Nathan A Baertsch
- Department of Comparative Biosciences, University of Wisconsin-Madison, Madison, Wisconsin
| | - Tracy L Baker-Herman
- Department of Comparative Biosciences, University of Wisconsin-Madison, Madison, Wisconsin
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Streeter KA, Baker-Herman TL. Spinal NMDA receptor activation constrains inactivity-induced phrenic motor facilitation in Charles River Sprague-Dawley rats. J Appl Physiol (1985) 2014; 117:682-93. [PMID: 25103979 DOI: 10.1152/japplphysiol.00342.2014] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
Reduced spinal synaptic inputs to phrenic motor neurons elicit a unique form of spinal plasticity known as inactivity-induced phrenic motor facilitation (iPMF). iPMF requires tumor necrosis factor-α (TNF-α) and atypical protein kinase C (aPKC) activity within spinal segments containing the phrenic motor nucleus to stabilize early, transient increases in phrenic burst amplitude into long-lasting iPMF. Here we tested the hypothesis that spinal N-methyl-d-aspartate receptor (NMDAR) activation constrains long-lasting iPMF in some rat substrains. Phrenic motor output was recorded in anesthetized, ventilated Harlan (HSD) and Charles River (CRSD) Sprague-Dawley rats exposed to a 30-min central neural apnea. HSD rats expressed a robust, long-lasting (>60 min) increase in phrenic burst amplitude (i.e., long-lasting iPMF) when respiratory neural activity was restored. By contrast, CRSD rats expressed an attenuated, transient (∼15 min) iPMF. Spinal NMDAR inhibition with DL-2-amino-5-phosphonopentanoic acid (APV) before neural apnea or shortly (4 min) prior to the resumption of respiratory neural activity revealed long-lasting iPMF in CRSD rats that was phenotypically similar to that in HSD rats. By contrast, APV did not alter iPMF expression in HSD rats. Spinal TNF-α or aPKC inhibition impaired long-lasting iPMF enabled by NMDAR inhibition in CRSD rats, suggesting that similar mechanisms give rise to long-lasting iPMF in CRSD rats with NMDAR inhibition as those giving rise to long-lasting iPMF in HSD rats. These results suggest that NMDAR activation can impose constraints on TNF-α-induced aPKC activation after neural apnea, impairing stabilization of transient iPMF into long-lasting iPMF. These data may have important implications for understanding differential responses to reduced respiratory neural activity in a heterogeneous human population.
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Affiliation(s)
- K A Streeter
- Department of Comparative Biosciences, University of Wisconsin-Madison, Madison, Wisconsin
| | - T L Baker-Herman
- Department of Comparative Biosciences, University of Wisconsin-Madison, Madison, Wisconsin
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Streeter KA, Baker-Herman TL. Decreased spinal synaptic inputs to phrenic motor neurons elicit localized inactivity-induced phrenic motor facilitation. Exp Neurol 2014; 256:46-56. [PMID: 24681155 DOI: 10.1016/j.expneurol.2014.03.007] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2013] [Revised: 03/06/2014] [Accepted: 03/11/2014] [Indexed: 12/15/2022]
Abstract
Phrenic motor neurons receive rhythmic synaptic inputs throughout life. Since even brief disruption in phrenic neural activity is detrimental to life, on-going neural activity may play a key role in shaping phrenic motor output. To test the hypothesis that spinal mechanisms sense and respond to reduced phrenic activity, anesthetized, ventilated rats received micro-injections of procaine in the C2 ventrolateral funiculus (VLF) to transiently (~30min) block axon conduction in bulbospinal axons from medullary respiratory neurons that innervate one phrenic motor pool; during procaine injections, contralateral phrenic neural activity was maintained. Once axon conduction resumed, a prolonged increase in phrenic burst amplitude was observed in the ipsilateral phrenic nerve, demonstrating inactivity-induced phrenic motor facilitation (iPMF). Inhibition of tumor necrosis factor alpha (TNFα) and atypical PKC (aPKC) activity in spinal segments containing the phrenic motor nucleus impaired ipsilateral iPMF, suggesting a key role for spinal TNFα and aPKC in iPMF following unilateral axon conduction block. A small phrenic burst amplitude facilitation was also observed contralateral to axon conduction block, indicating crossed spinal phrenic motor facilitation (csPMF). csPMF was independent of spinal TNFα and aPKC. Ipsilateral iPMF and csPMF following unilateral withdrawal of phrenic synaptic inputs were associated with proportional increases in phrenic responses to chemoreceptor stimulation (hypercapnia), suggesting iPMF and csPMF increase phrenic dynamic range. These data suggest that local, spinal mechanisms sense and respond to reduced synaptic inputs to phrenic motor neurons. We hypothesize that iPMF and csPMF may represent compensatory mechanisms that assure adequate motor output is maintained in a physiological system in which prolonged inactivity ends life.
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Affiliation(s)
- K A Streeter
- Department of Comparative Biosciences, University of Wisconsin-Madison, 2015 Linden Drive, Madison, WI 53706, USA
| | - T L Baker-Herman
- Department of Comparative Biosciences, University of Wisconsin-Madison, 2015 Linden Drive, Madison, WI 53706, USA.
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Clinical challenges to ventilatory control. Respir Physiol Neurobiol 2013; 189:211-2. [PMID: 24056024 DOI: 10.1016/j.resp.2013.09.006] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
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Broytman O, Baertsch NA, Baker-Herman TL. Spinal TNF is necessary for inactivity-induced phrenic motor facilitation. J Physiol 2013; 591:5585-98. [PMID: 23878370 DOI: 10.1113/jphysiol.2013.256644] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022] Open
Abstract
A prolonged reduction in central neural respiratory activity elicits a form of plasticity known as inactivity-induced phrenic motor facilitation (iPMF), a 'rebound' increase in phrenic burst amplitude apparent once respiratory neural activity is restored. iPMF requires atypical protein kinase C (aPKC) activity within spinal segments containing the phrenic motor nucleus to stabilize an early transient increase in phrenic burst amplitude and to form long-lasting iPMF following reduced respiratory neural activity. Upstream signal(s) leading to spinal aPKC activation are unknown. We tested the hypothesis that spinal tumour necrosis factor-α (TNFα) is necessary for iPMF via an aPKC-dependent mechanism. Anaesthetized, ventilated rats were exposed to a 30 min neural apnoea; upon resumption of respiratory neural activity, a prolonged increase in phrenic burst amplitude (42 ± 9% baseline; P < 0.05) was apparent, indicating long-lasting iPMF. Pretreatment with recombinant human soluble TNF receptor 1 (sTNFR1) in the intrathecal space at the level of the phrenic motor nucleus prior to neural apnoea blocked long-lasting iPMF (2 ± 8% baseline; P > 0.05). Intrathecal TNFα without neural apnoea was sufficient to elicit long-lasting phrenic motor facilitation (pMF; 62 ± 7% baseline; P < 0.05). Similar to iPMF, TNFα-induced pMF required spinal aPKC activity, as intrathecal delivery of a ζ-pseudosubstrate inhibitory peptide (PKCζ-PS) 35 min following intrathecal TNFα arrested TNFα-induced pMF (28 ± 8% baseline; P < 0.05). These data demonstrate that: (1) spinal TNFα is necessary for iPMF; and (2) spinal TNFα is sufficient to elicit pMF via a similar aPKC-dependent mechanism. These data are consistent with the hypothesis that reduced respiratory neural activity elicits iPMF via a TNFα-dependent increase in spinal aPKC activity.
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Affiliation(s)
- Oleg Broytman
- T. Baker-Herman: Department of Comparative Biosciences, University of Wisconsin, 2015 Linden Drive, Madison, WI, USA.
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