C-fos expression in central neurons mediating the arterial baroreceptor reflex

Subregional differences in GABAA receptor subunit expression in the rostral ventrolateral medulla of sedentary versus physically active rats

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.

Development of manganese-enhanced magnetic resonance imaging of the rostral ventrolateral medulla of conscious rats: Importance of normalization and comparison with other regions of interest

Spinally projecting neurons in the rostral ventrolateral medulla (RVLM) are believed to contribute to pathophysiological alterations in sympathetic nerve activity and the development of cardiovascular disease. The ability to identify changes in the activity of RVLM neurons in conscious animals and humans, especially longitudinally, would represent a clinically important advancement in our understanding of the contribution of the RVLM to cardiovascular disease. To this end, we describe the initial development of manganese-enhanced magnetic resonance imaging (MEMRI) for the rat RVLM. Manganese (Mn²⁺ ) has been used to estimate in vivo neuronal activity in other brain regions because of both its paramagnetic properties and its entry into and accumulation in active neurons. In this initial study, our three goals were as follows: (1) to validate that Mn²⁺ enhancement occurs in functionally and anatomically localized images of the rat RVLM; (2) to quantify the dose and time course dependence of Mn²⁺ enhancement in the RVLM after one systemic injection in conscious rats (66 or 33 mg/kg, intraperitoneally); and (3) to compare Mn²⁺ enhancement in the RVLM with other regions to determine an appropriate method of normalization of T₁ -weighted images. In our proof-of-concept and proof-of-principle studies, Mn²⁺ was identified by MRI in the rat RVLM after direct microinjection or via retrograde transport following spinal cord injections, respectively. Systemic injections in conscious rats produced significant Mn²⁺ enhancement at 24 h (p < 0.05). Injections of 66 mg/kg produced greater enhancement than 33 mg/kg in the RVLM and paraventricular nucleus of the hypothalamus (p < 0.05 for both), but only when normalized to baseline scans without Mn²⁺ injection. Consistent with findings from our previous functional and anatomical studies demonstrating subregional neuroplasticity, Mn²⁺ enhancement was higher in the rostral regions of the RVLM (p < 0.05). Together with important technical considerations, our studies support the development of MEMRI as a potential method to examine RVLM activity over time in conscious animal subjects.

CGRP infusion in unanesthetized rats increases expression of c-Fos in the nucleus tractus solitarius and caudal ventrolateral medulla, but not in the trigeminal nucleus caudalis

BACKGROUND AND AIMS

Calcitonin gene-related peptide (CGRP) and glyceryl trinitrate (GTN) infusion in migraineurs provokes headache resembling spontaneous migraine, and CGRP receptor antagonists are effective in the treatment of acute migraine. We hypothesized that CGRP infusion would increase molecular markers of neuronal activation in migraine-relevant tissues of the rat.

METHODS

CGRP was infused intravenously (i.v.) in freely moving rats to circumvent factors like anesthesia, acute surgery and severe hypotension, the three confounding factors for c-Fos expression. The trigeminal nucleus caudalis (TNC) was isolated at different time points after CGRP infusion. The level of c-Fos mRNA and protein expression in TNC were analyzed by qPCR and immunohistochemistry. c-Fos-stained nuclei were also counted in the nucleus tractus solitarius (NTS) and caudal ventrolateral medulla (CVLM), integrative sites in the brain stem for processing cardiovascular signals. We also investigated Zif268 protein expression (another immediate early gene) in TNC. The protein expression of p-ERK, p-CREB and c-Fos was analyzed in dura mater, trigeminal ganglion (TG) and TNC samples using Western blot.

RESULTS

CGRP infusion caused a significant dose-dependent fall in mean arterial blood pressure. No significant activation of c-Fos in the TNC at mRNA and protein levels was observed after CGRP infusion. A significant increase in c-Fos protein was observed in the NTS and CVLM in the brain stem. Zif268 expression in the TNC was also not changed after CGRP infusion. p-ERK was increased in the dura mater 30 minutes after CGRP infusion.

CONCLUSION

CGRP infusion increased the early expression of p-ERK in the dura mater but did not increase c-Fos and Zif268 expression in the TNC. The rats may, thus, differ from migraine patients, in whom infusion of CGRP caused headache and a delayed migraine attack. The rat CGRP infusion model with c-Fos or Zif268 as neuronal pain markers in TNC is unsuitable for antimigraine drug testing.

Baroreflex and chemoreflex controls of sympathetic activity following intermittent hypoxia

There is a large amount of evidence linking obstructive sleep apnea (OSA), and the associated intermittent hypoxia that accompanies it, with the development of hypertension. For example, cross-sectional studies demonstrate that the prevalence of hypertension increases with the severity of OSA (Bixler et al., 2000; Grote et al., 2001) and an initial determination of OSA is associated with a three-fold increase for future hypertension (Peppard et al., 2000). Interestingly, bouts of intermittent hypoxia have also been shown to affect sympathetic output associated with the baroreflex and chemoreflex, important mechanisms in the regulation of arterial blood pressure. As such, the possibility exists that changes in the baroreflex and chemoreflex may contribute to the development of chronic hypertension observed in OSA patients. The aim of the current article is to briefly review the response of the baroreflex and chemoreflex to intermittent hypoxic exposure and to evaluate evidence for the hypothesis that modification of these autonomic reflexes may, at least in part, support the comorbidity observed between chronic hypertension and OSA.

Identification of sites of sympathetic outflow during concurrent recordings of sympathetic nerve activity and fMRI

The sympathetic division of the nervous system is critical for maintaining both resting arterial pressure and for producing changes in regional perfusion required by behavioral state changes. A primary determinant of arterial pressure is the level of vasoconstriction within skeletal muscle. It is well established that there is a tight relationship between dynamic changes in arterial pressure and muscle sympathetic nerve activity (MSNA) through the workings of the baroreflex. While the central circuitry underlying the baroreflex has been extensively investigated in anesthetized experimental animals, few studies have investigated the central circuitry responsible for the baroreflex in awake human subjects. Recently we were the first to record concurrently MSNA (using microneurography) and brain activity (using functional magnetic resonance imaging) in awake humans in a series of experiments designed to determine the central circuitry underlying the baroreflex in humans. We confirmed that the baroreflex involves activity changes within the nucleus tractus solitarius, caudal ventrolateral, and rostral ventrolateral medulla. Because conditions such as essential hypertension, obesity, and obstructive sleep apnea are all characterized by significant increases in resting MSNA, it is important to understand both brainstem and cortical sites involved in regulating resting levels of MSNA. Future investigations which define cortical sites involved in generating and modulating MSNA are important if we are to understand the underlying mechanisms of many conditions characterized by hypertension.

Dysfunctional nucleus tractus solitarius: its crucial role in promoting neuropathogenetic cascade of Alzheimer's dementia--a novel hypothesis

The pathophysiological mechanism(s) underlying Alzheimer's disease (AD) still remain unclear, and no disease-modifying or prophylactic therapies are currently available. Unraveling the fundamental neuropathogenesis of AD is an important challenge. Several studies on AD have suggested lesions in a number of CNS areas including the basal forebrain, hippocampus, entorhinal cortex, amygdale/insula, and the locus coeruleus. However, plausible unifying studies on the upstream factors that involve these heterogeneous regions and herald the onset of AD pathogenesis are not available. The current article presents a novel nucleus tractus solitarius (NTS) vector hypothesis that underpins several disparate biological mechanisms and neural circuits, and identifies relevant hallmarks of major presumptive causative factor(s) linked to the NTS, in older/aging individuals. Aging, obesity, infection, sleep apnea, smoking, neuropsychological states, and hypothermia-all activate inflammatory cytokines and oxidative stress. The synergistic impact of systemic proinflammatory mediators activates microglia and promotes neuroinflammation. Acutely, the innate immune response is protective defending against pathogens/toxins; however, when chronic, it causes neuroinflammation and neuronal dysfunction, particularly in brainstem and neocortex. The NTS in the brainstem is an essential multiple signaling hub, and an extremely important central integration site of baroreceptor, chemoreceptor, and a multitude of sensory afferents from gustatory, gastrointestinal, cardiac, pulmonary, and upper airway systems. Owing to persistent neuroinflammation, the dysfunctional NTS exerts deleterious impact on nucleus ambiguus, dorsal motor nucleus of vagus, hypoglossal, parabrachial, locus coeruleus and many key nuclei in the brainstem, and the hippocampus, entorhinal cortex, prefrontal cortex, amygdala, insula, and basal forebrain in the neocortex. The neuronal and synaptic dysfunction emanating from the inflamed NTS may affect its interconnected pathways impacting almost the entire CNS--which is already primed by neuroinflammation, thus promoting cognitive and neuropsychiatric symptoms. The upstream factors discussed here may underpin the neuropathopgenesis of AD. AD pathology is multifactorial; the current perspective underscores the value of attenuating disparate upstream factors--in conjunction with anticholinesterase, anti-inflammatory, immunosuppressive, and anti-oxidant pharmacotherapy. Amelioration of the NTS pathology may be of central importance in countering the neuropathological cascade of AD. The NTS, therefore, may be a potential target of novel therapeutic strategies.

Real-time imaging of the medullary circuitry involved in the generation of spontaneous muscle sympathetic nerve activity in awake subjects

To understand the central neural processes involved in blood pressure regulation we recorded muscle sympathetic nerve activity (MSNA) via a tungsten microelectrode in the common peroneal nerve while performing functional magnetic resonance imaging (fMRI) of the brainstem at 3T. Blood oxygen level dependent (BOLD) changes in signal intensity were measured over 4 s every 8 s (200) volumes; MSNA was recorded during the previous 4 s epoch, which takes into account peripheral conduction delays along unmyelinated axons and neurovascular coupling delays. Analysis of temporal coupling between BOLD signal intensity and nerve signal intensity revealed sites in which the two signals covaried, but only in the medulla. Because scans were conducted in a caudorostral direction, we could constrain the analysis to the medulla by only examining the first 1 s of the fMRI and nerve signals. Increases in MSNA were associated with robust bilateral increases in signal intensity in the dorsolateral region of the medulla that corresponds to the human equivalent of the rostal ventrolateral medulla (RVLM). Reciprocal decreases in signal intensity occurred in the regions of the nucleus tractus solitarius (NTS) and caudal ventrolateral medulla (CVLM). Group analysis also revealed increases in signal intensity in the caudal pressor area (CPA), medullary raphé (MR), and dorsal motor nucleus of the vagus (DMX). We have shown for the first time that this combined approach of recording sympathetic neural activity and fMRI provides real-time imaging of the neural processes responsible for the generation of sympathetic nerve activity in awake human subjects.

Expression of Group I metabotropic glutamate receptors on phenotypically different cells within the nucleus of the solitary tract in the rat

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.

Identification of active central nervous system sites in renal wrap hypertensive rats

To identify central neurons participating in cardiovascular regulation in hypertension, we studied Fos staining, a marker for synaptically activated neurons, in adult male normotensive and hypertensive (HT) rats. At 1 and 4 weeks after induction of unilateral nephrectomy, renal wrap hypertension mean arterial pressure was 138+/-4 mm Hg (n=6) in 1-week HT rats and 159+/-6 mm Hg (n=6) in 4-week HT rats. Mean arterial pressure was 103+/-2 mm Hg (n=6) in sham-operated, normotensive rats. Mean arterial pressure was greater in both HT groups compared with normotensive rats, and the mean arterial pressure in 4-week HT rats was greater than that in 1-week HT rats. Rats were anesthetized and perfused, brains sectioned and processed using a Fos antibody, and the number of Fos immunoreactive neurons counted in sections through various brain regions. Hypertension of 1 or 4 weeks did not alter the number of Fos immunoreactive neurons in the area postrema, the supraoptic nucleus, and the median preoptic nucleus. The number of Fos immunoreactive neurons was increased after 1 and 4 weeks in the nucleus of the solitary tract, both the caudal and ventral lateral medulla, and the organum vasculosum of the lamina terminalis. In addition, after 4 weeks of HT, the number of Fos immunoreactive neurons was increased in the parabrachial nucleus and the paraventricular nucleus of the hypothalamus. The results indicate central regions active in acute and chronic HT rats and suggest certain areas that may be differentially activated depending on the duration of the hypertension.

Intraventricular injections of tachykinin NK3 receptor agonist reduce the gain of the baroreflex in unrestrained rats

The tachykinin neuropeptides acting at NK3 receptors affect mean arterial pressure (MAP) through both neuroendocrine and neural mechanisms. NK3 receptors are found in brainstem nuclei that mediate the baroreflex, but the effects of NK3 receptor stimulation on baroreflex function are unknown. The present study tests the effects of intraventricular injections of senktide, a selective NK3 receptor agonist, on the sensitivity of the baroreflex in three stains of rats: Charles River Laboratory, Long-Evans, and Brattleboro rats, which lack the ability to synthesize vasopressin. Rats with lateral ventricle cannulas were administered injections of isotonic saline, 100 ng, or 200 ng senktide, and 5 min later arterial baroreceptor-heart rate (HR) function was examined by constructing full-range blood pressure-HR curves using alternating doses (5-20 microg kg min) of phenylephrine and nitroprusside to raise and decrease blood pressure approximately 50 mm Hg over a period of 1 min, respectively. Intraventricular injections of 200 ng senktide had no significant effect on baseline MAP, but significantly decreased the gain of the baroreflex in all three rat strains whereas the 100 ng dose had no effect on the baroreflex. These results show that NK3 receptor stimulation modulates the baroreflex that is independent of any action of vasopressin.

Central nucleus of amygdala projections to rostral ventrolateral medulla neurones activated by decreased blood pressure

The central nucleus of amygdala (CeA) participates in cardiovascular regulation during emotional behaviour but it has not been established whether any of these effects are mediated through its direct connections to blood pressure-regulating neurones in the rostral ventrolateral medulla (RVLM). The RVLM contains barosensitive neurones that maintain resting blood pressure via their projections to sympathetic preganglionic neurones in the thoracic spinal cord. In this study on rats, we used combined anterograde neuronal tracing of CeA projections with confocal and electron microscopic immunohistochemical detection of phenylethanolamine-N-methyltransferase, the adrenaline-synthesizing enzyme present in C1 catecholamine neurones of the RVLM, and Fos, the protein product of the c-fos proto-oncogene. Fos expression in barosensitive neurones was stimulated by an intravenous infusion of the hypotensive agent sodium nitroprusside. Injection of the tracer biotin dextran amine (10-kDa form) into the CeA resulted in anterograde labelling of axons and varicosities throughout the RVLM without retrograde labelling of somata in any brain area. With confocal microscopy, presumptive CeA terminals were found in close apposition to adrenergic (phenylethanolamine-N-methyltransferase-immunoreactive) and non-adrenergic neurones that displayed Fos-immunoreactive nuclei in response to decreased blood pressure. Electron microscopic analysis confirmed that some labelled terminals of CeA axons made synaptic contact with c-fos-activated adrenergic neurones. The results provide evidence that cardiovascular influences elicited from the CeA during stressful events may be mediated, at least in part, via monosynaptic neural projections to barosensitive sympathetic blood pressure-regulating neurones in the RVLM.

Vagal control of the heart: central serotonergic (5-HT) mechanisms

Cardiac vagal preganglionic neurones (CVPNs) are located within the dorsal vagal nucleus (DVN) and the nucleus ambiguus (nA). In mammals, CVPNs within the nA have small myelinated axons and mediate major chronotropic effects, those in the DVN have non-myelinated axons and mediate smaller chronotropic, dromotropic and inotropic effects. Numerous studies demonstrate important influences of serotonin (5-HT) at multiple sites controlling autonomic outflows including the nucleus tractus solitarius (NTS) where cardiorespiratory afferent fibres terminate, and the CVPNs and rostral ventrolateral medulla (RVLM), the location of sympathetic premotor neurones. We have demonstrated roles for some of the numerous 5-HT receptor subtypes (5-HT1, 5-HT2, 5-HT3, 5-HT4 and 5-HT7) in brainstem regions involved in cardiac control. Intracisternal application of selective ligands was used to study the effect of 5-HT receptors on heart rate and its reflex control. Further electrophysiological studies were also carried out to delineate their location and the mechanisms of action of these ligands. Blocking 5-HT1A receptors attenuated bradycardias evoked by stimulating baroreceptor and cardiopulmonary afferents but not arterial chemoreceptors, whereas antagonizing 5-HT7 receptors markedly attenuated all these reflex bradycardias. Within the DVN, nA and NTS, activation of 5-HT1A receptors could excite or inhibit neurones. In the NTS 5-HT2 receptors also had variable effects; 5-HT2B receptors excite and 5-HT2C receptors inhibit. Antagonism of 5-HT3 receptors attenuated upper airway and cardiopulmonary reflex bradycardias; this is compatible with data showing that 5-HT3 receptors excite DVN and NTS neurones by a glutamate-dependent mechanism. The origin of the glutamate (neuronal or glial) remains unresolved but glia are a possibility as baroreceptor-sensitive NTS neurones receive few direct 5-HT-containing synaptic contacts. Thus, 5-HT plays a critical role in the control of vagal outflow to the heart; however, why so many different receptors are involved, and their relative functional roles, remains unresolved.

Medial prefrontal cortical integration of psychological stress in rats

The present study aimed to determine whether the medial prefrontal cortex (mPFC) (prelimbic and infralimbic regions) is implicated in the integration of a stress response. Sprague-Dawely rats were implanted with telemetry probes and guide cannulae so that either muscimol or vehicle could be administered locally within the mPFC or dorsomedial hypothalamus (DMH). The heart rate and blood pressure of rats was continuously recorded as either muscimol or vehicle was administered centrally and rats were either exposed to restraint stress or left alone in their home cages. After the stress challenge, or equivalent period, rats that had received intra-mPFC injections were processed for immunohistochemical detection of Fos throughout the neuraxis. Bilateral microinjection of muscimol into the mPFC had no effect upon either baseline cardiovascular parameters or restraint stress-induced tachycardia or pressor responses whereas, in the DMH, pretreatment with muscimol attenuated the cardiovascular stress response. Analysis of Fos expression throughout the CNS of nonstressed rats showed no effect of muscimol injections into the mPFC on baseline expression in the nuclei examined. In contrast, rats that had received muscimol injections into their mPFC and were subsequently restrained exhibited an increase in the number of Fos-positive cells in the DMH, medial amygdala, and medial nucleus tractus solitarius as compared to vehicle-injected rats that experienced restraint stress. These results indicate that, during acute psychological stress, the mPFC does not modulate the cardiovascular system in rats but does inhibit specific subcortical nuclei to exert control over aspects of an integrated response to a stressor.

Cocaine- and amphetamine-regulated transcript in catecholamine and noncatecholamine presympathetic vasomotor neurons of rat rostral ventrolateral medulla

Presympathetic vasomotor adrenergic (C1) and nonadrenergic (non-C1) neurons in the rostral ventrolateral medulla (RVLM) provide the main excitatory drive to cardiovascular sympathetic preganglionic neurons in the spinal cord. C1 and non-C1 neurons contain cocaine- and amphetamine-regulated transcript (CART), suggesting that CART may be a common marker for RVLM presympathetic neurons. To test this hypothesis, we first used double-immunofluorescence staining for CART and tyrosine hydroxylase (TH) to quantify CART-immunoreactive (-IR) catecholamine and noncatecholamine neurons in the C1 region. Next, we quantified the proportion of CART-IR RVLM neurons that expressed Fos in response to a hypotensive stimulus, using peroxidase immunohistochemistry for Fos and dual immunofluorescence for CART and TH. Finally, we fluorescently detected CART immunoreactivity in electrophysiologically identified, juxtacellularly labeled RVLM presympathetic neurons. In the RVLM, 97% of TH-IR neurons were CART-IR, and 74% of CART-IR neurons were TH-IR. Nitroprusside infusion significantly increased the number of Fos-IR RVLM neurons compared with saline controls. In nitroprusside-treated rats, virtually all Fos/TH neurons in the RVLM were immunoreactive for CART (98% +/- 1.3%, SD; n = 7), whereas 29% +/- 8.3% of CART-positive, TH-negative neurons showed Fos immunoreactivity. Six fast (2.8-5.8 m/second, noncatecholamine)-, two intermediate (2.1 and 2.2 m/second)-, and five slow (<1 m/second, catecholamine)-conducting RVLM presympathetic vasomotor neurons were juxtacellularly labeled. After fluorescent detection of CART and biotinamide, all 13 neurons were found to be CART-IR. These results suggest that, in rat RVLM, all catecholamine and noncatecholamine presympathetic vasomotor neurons contain CART.

Neurokinin-1 receptor-expressing cells regulate depressor region of rat ventrolateral medulla

Intraparenchymal injection of the saporin conjugate [Sar9, Met (O2)11] substance P-saporin (SSP-SAP) into the ventrolateral medulla (VLM) destroys neurokinin-1 receptor-immunoreactivity (NK1R-ir) neurons selectively. This treatment attenuates the hypotension caused by injection of DL-homocysteic acid (DLH) into the caudal VLM (CVLM). Here we ask whether SSP-SAP creates this deficit by destroying the CVLM GABAergic interneurons that mediate the sympathetic baroreflex (baroactivated depressor neurons) or by destroying other VLM neurons. Two weeks after unilateral SSP-SAP treatment (97% loss of VLM NK1R-ir neurons) DLH-induced hypotension and sympathetic tone inhibition were blunted on the lesioned side. Unlesioned or unilaterally lesioned rats received phenylephrine (PE) while awake to identify CVLM baroactivated depressor neurons by the presence of Fos-ir nuclei. Although CVLM Fos-ir cells were not NK1R-ir, their number was reduced approximately 60-70% on the SSP-SAP-injected side. SSP-SAP spared VLM neurons devoid of NK1R-ir, such as the catecholaminergic cells and the precerebellar glutamatergic neurons. In the pre-Bötzinger region of the VLM the toxin killed glutamatergic neurons while sparing glycinergic and GABAergic inhibitory neurons. In the CVLM region approximately 26% of the inhibitory cells were destroyed. In conclusion, the baroactivated depressor neurons of the CVLM do not appear to express NK1Rs but their activity is probably modulated by a population of excitatory NK1R-ir cells located in the VLM. The results also suggest that a region located below the CVLM (subCVLM) may contain an unrelated population of GABAergic depressor neurons that are NK1R-ir but are either not barosensitive or do not express Fos during baroreceptor stimulation.

Serotonin(3) receptor stimulation in the nucleus tractus solitarii activates non-catecholaminergic neurons in the rat ventrolateral medulla

The present study was performed to determine whether or not the increased arterial pressure triggered by 5-HT(3) receptor stimulation in the nucleus tractus solitarii and underlain by a sympathoexcitation is associated with the activation of ventromedullary cells known to be involved in vascular regulation, i.e. the C1 and A1 catecholaminergic cells. For this purpose, double immunohistochemical labeling for tyrosine hydroxylase and c-fos protein was performed all along the ventrolateral medulla after microinjection of 1-(m-chlorophenyl)-biguanide, a selective and potent 5-HT(3) receptor agonist, into the nucleus tractus solitarii of alpha-chloralose/urethane-anaesthetized rats. This treatment produced a significant elevation of arterial pressure ( approximately +35 mm Hg). Concomitantly, a significant increase in the number of c-fos expressing neurons was observed in the rostral ventrolateral medulla (+63%), in particular in its most anterior part (+78%), and in the medullary region surrounding the caudal part of the facial nucleus (+91%). Retrograde labeling with gold-horseradish peroxidase complex showed that at least some of these activated c-fos expressing cells project to the spinal cord. However, the number of double-stained neurons, i.e. c-fos and tyrosine hydroxylase positive neurons, did not increase at any level of the ventrolateral medulla. In contrast, under the same alpha-chloralose/urethane anesthesia, systemic infusion of sodium nitroprusside appeared to produce a hypotension and a marked increase in the density of such double c-fos and tyrosine hydroxylase expressing cells in the rostral ventrolateral medulla and the caudal medullary region surrounding the caudal part of the facial nucleus. These data indicate that medullary catecholaminergic C1 and A1 neurons are not involved in the pressor effect elicited by 5-HT(3) receptor stimulation in the nucleus tractus solitarii. However, this 5-HT(3) receptor-mediated effect is clearly associated with the excitation of (non-catecholaminergic) neurons within the pressor region of the ventral medulla.

Baroreceptor reflex pathways and neurotransmitters: 10 years on

The central nervous system plays a critical role in the management of blood flow to the tissues and its return to the heart and lungs. This is achieved by a complex interplay of neural efferent pathways, humoral mechanisms and afferent pathways. In this review, we focus on recent progress (within the past 10 years) that has been made in the sympathetic control of arterial blood pressure with a special emphasis on the role of baroreceptor mechanisms and central neurotransmitters. In particular, we focus on new features since 1991, such as neurotransmission in the nucleus tractus solitarius, the role of neurons in the most caudal part of the ventrolateral medulla oblongata and the increasing understanding of the exquisite control of different sympathetic pathways by different neurotransmitter systems.

Neurokinin-1 receptor immunoreactivity in hypotension sensitive sympathetic preganglionic neurons

Substance P activation of neurokinin-1 (NK1) receptors on spinal sympathetic preganglionic neurons (SPN) influences blood pressure. We identified SPN likely to subserve the baroreceptor reflex and established if these neurons showed NK1 receptor-immunoreactivity. Nitroprusside (NP) infusion or inferior vena cava (IVC) constriction activated similar numbers of SPN. Of these, about 40% were NK1 receptor-immunoreactive after NP infusion, but only about 20% were NK1 receptor-immunoreactive after IVC constriction. The distribution of Fos/NK1 receptor SPN suggested that substance P may preferentially target sympathoadrenal SPN.

Plasma osmolality dipsogenic thresholds and c-fos expression in the near-term ovine fetus

In ovine and human pregnancy, fetal swallowing contributes importantly to amniotic fluid homeostasis. Fetal dipsogenic responsiveness to short-term plasma hyperosmolality develops in late gestation, although fetal swallowing is not stimulated in response to long-term plasma osmolality increases (2 to 3%), which typically stimulate adult drinking behavior. To explore the near-term fetal plasma osmolality threshold for swallowing stimulation, we examined the effects of i.v. hypertonic saline-induced subacute increases in plasma hypertonicity on fetal swallowing behavior. Central sites of activation were examined by c-fos expression in putative dipsogenic nuclei. The results demonstrate that subacute 2 to 3% plasma osmolality increases do not stimulate near-term ovine fetal swallowing. However, fetal swallowing activity significantly increased (3 times) after plasma osmolality increased >6% above basal values. Consistent with a specific dipsogenic response, i.v. hypertonic saline induced c-fos expression in the anterior third ventricle region, a putative dipsogenic center, as well as in the fetal hindbrain. The stimulation of fetal swallowing under conditions of higher osmotic stimulation and the correlation with forebrain c-fos expression indicates that near-term fetal osmoregulation mechanisms are functional, although not completely mature. Reduced fetal dipsogenic responsiveness may result from altered osmoreceptor sensitivity, downstream neuronal or synaptic immaturity, or potentially inhibitory actions of stimulated hindbrain nuclei.

Sensitivity to naloxone of the behavioral signs of morphine withdrawal and c-Fos expression in the rat CNS: a quantitative dose-response analysis

Several studies have used c-Fos expression to delineate the neural substrate underlying naloxone-precipitated morphine withdrawal (MW). However, because behavioral manifestations of MW depend on both the degree of dependence and the doses of naloxone (NAL), a comprehensive study would require examining c-Fos expression in relation with the degree of MW. Here, changes in behavior and in c-Fos-like immunoreactivity (FLI) were studied in the same rats after injection of three doses of NAL to precipitate various degrees of MW. Fifteen established signs of MW were examined for 1 hour after NAL injection, and FLI was quantified in 52 regions of the brain and in the lumbosacral spinal cord. Linear regression analyses were used to examine changes in numbers of signs and FLI neurons with the doses of NAL, and data were considered dose-related for a statistical level of significance of P < 0.05. In summary, autonomic signs of MW increased in a dose-related manner, whereas somatomotor signs did not. After MW, 33 central nervous system regions exhibited significant increases in FLI and were, thus, considered as important neural correlates of MW. Twenty of them displayed dose-related increases in c-Fos expression and correspond to regions related to autonomic functions. Low c-Fos expression was detected in some regions involved in motor control or in reward, suggesting either their minor role in MW or a limitation of the technique. This dose-response analysis suggests that the increase in the severity of autonomic manifestations of MW is associated with a gradual activation of major structures of the autonomic nervous system.

Relative importance of rostral ventrolateral medulla in sympathoinhibitory action of rilmenidine in conscious and anesthetized rabbits

The pressor region of the rostral ventrolateral medulla (RVLM) is a critical site in the sympathoinhibitory action of imidazoline receptor agonists as shown by studies in anesthetized animals. The aim of this study was to compare the importance of the RVLM in mediating the inhibitory action of rilmenidine on renal sympathetic nerve activity (RSNA) and arterial pressure in urethane-anesthetized rabbits (n = 11) and in conscious, chronically instrumented rabbits (n = 6). Bilateral microinjection of rilmenidine (4 nmol in 100 nl) into the RVLM caused a greater decrease in resting arterial pressure in anesthetized animals (-19 mm Hg) than in conscious animals (-8 mm Hg). By contrast, the decrease in resting RSNA evoked by rilmenidine was similar in conscious (-27%) and anesthetized (-36%) rabbits. Furthermore, rilmenidine microinjection into the RVLM was equally effective in inhibiting the RSNA baroreflex in both groups of animals. The upper plateau of the RSNA baroreflex decreased by 37% and 42%, and gain decreased by 41% and 44% after rilmenidine treatments in conscious and anesthetized rabbits, respectively. We conclude that the RVLM plays an equally important role in the inhibitory action of rilmenidine on RSNA in conscious and anesthetized rabbits either at rest or during baroreflex responses. A relatively moderate effect of rilmenidine on arterial pressure in conscious, chronically instrumented rabbits may relate to a lower level of sympathetic drive compared with anesthetized animals.

Influence of rostral ventrolateral medulla on renal sympathetic baroreflex in conscious rabbits

Previous studies with anesthetized animals have shown that the pressor region of the rostral ventrolateral medulla (RVLM) is a critical site in vasomotor control. The aim of this study was to develop, in conscious rabbits, a technique for microinjecting into the RVLM and to determine the influence of this area on renal sympathetic nerve activity (RSNA) and arterial pressure (AP) using local injections of glutamate, rilmenidine, ANG II and sarile. Rabbits were implanted with guide cannulas for bilateral microinjections into the RVLM (n = 7) or into the intermediate ventrolateral medulla (IVLM, n = 6) and an electrode for measuring RSNA. After 7 days of recovery, injections of glutamate (10 and 20 nmol) into the RVLM increased RSNA by 81 and 88% and AP by 17 and 25 mmHg, respectively. Infusion of glutamate (2 nmol/min) into the RVLM increased AP by 15 mmHg and the RSNA baroreflex range by 38%. By contrast, injection of the imidazoline receptor agonist rilmenidine (4 nmol) into the RVLM decreased AP by 8 mmHg and the RSNA baroreflex range by 37%. Injections of rilmenidine into the IVLM did not alter AP or RSNA. Surprisingly, treatments with ANG II (4 pmol/min) or the ANG II receptor antagonist sarile (500 pmol) into the RVLM did not affect the resting or baroreflex parameters. Infusion of ANG II (4 pmol/min) into the fourth ventricle increased AP and facilitated the RSNA baroreflex. Our results show that agents administered via a novel microinjecting system for conscious rabbits can selectively modulate neuronal activity in circumscribed regions of the ventrolateral medulla. We conclude that the RVLM plays a key role in circulatory control in conscious rabbits. However, we find no evidence for the role of ANG II receptors in the RVLM in the moment-to-moment regulation of AP and RSNA.

Nitric oxide producing neurones in the rat medulla oblongata that project to nucleus tractus solitarii

The production of nitric oxide in neurones of the rat medulla oblongata that project to the nucleus tractus solitarii (NTS) was examined by simultaneous immunohistochemical detection of nitric oxide synthase (NOS) and of cholera toxin B-subunit (CTb), which was injected into the caudal zone of the NTS. Neurones immunoreactive for CTb and neurones immunoreactive for NOS were widely co-distributed and found in almost all the anatomical divisions of the medulla. Dual-labelled cells, containing both CTb and NOS immunoreactivities were more numerous ipsilaterally to the injection sites. They were concentrated principally in the more rostral zone of the NTS, raphé nuclei, dorsal, intermediate and lateral reticular areas, spinal trigeminal and paratrigeminal nuclei and the external cuneate and medial vestibular nuclei. Isolated dual-labelled neurones were also scattered throughout most of the divisions of the reticular formation. These observations indicate that many areas of the medulla that are known to relay somatosensory and viscerosensory inputs contain NOS immunoreactive neurones that project to the NTS, and may, therefore, contribute to the dense NOS-immunoreactive innervation of the NTS. The release of nitric oxide from the axon terminals of these neurones may modulate autonomic responses generated by NTS neurones in relation to peripheral sensory stimuli, and thus ultimately regulate sympathetic and/or parasympathetic outflow.

Distribution of neurons in the anterior hypothalamic nucleus activated by blood pressure changes in the rat

Electrophysiological and Fos-like protein immunocytochemical methods were used to identify the number and distribution of anterior hypothalamic neurons that are activated by changes in arterial pressure. First, in anesthetized, male Sprague-Dawley rats, arterial pressure increases and decreases led to differential activation of neurons in the anterior hypothalamic nucleus. Most of the units that responded to a rise in arterial pressure with a decrease in activity (pressor units) were located in the central part of the anterior hypothalamic nucleus, whereas units that increased firing when arterial pressure rose (the depressor units) were found throughout the nucleus. Second, in awake, male Sprague-Dawley rats, Fos-like protein immunoreactivity was mapped following sustained arterial pressure changes. Within the anterior hypothalamus, reduction in arterial pressure increased the number of Fos-labeled neurons primarily in the paraventricular nucleus and to a lesser extent in the anterior half of the anterior hypothalamic nucleus. In contrast, elevation in arterial pressure increased Fos labeling throughout the anterior hypothalamic nucleus and to a lesser extent in the paraventricular nucleus.