Internal connections in the rostral ventromedial medulla of the rat

Hypoxia-induced increases in serotonin-immunoreactive nerve fibers in the medulla oblongata of the rat

Hypoxia induces respiratory responses in mammals and serotonergic neurons in the medulla oblongata participate in respiratory control. However, the morphological changes in serotonergic neurons induced by hypoxia have not yet been examined and respiratory controls of serotonergic neurons have not been clarified. We herein investigated the distribution of immunoreactivity for serotonin (5-hydroxytryptamine; 5-HT) in the medulla oblongata of control rats and rats exposed to 1-6h of hypoxia (10% O₂). We also examined the medulla oblongata by multiple immunofluorescence labeling for 5-HT, neurokinin 1 receptors (NK1R), a marker for some respiratory neurons in the pre-Bötzinger complex (PBC), and dopamine β-hydroxylase (DBH), a marker for catecholaminergic neurons. The number of 5-HT-immunoreactive nerve cell bodies in the raphe nuclei was higher in rats exposed to hypoxia than in control rats. The number of 5-HT-immunoreactive nerve fibers significantly increased in the rostral ventrolateral medulla of rats exposed to 1-6h of hypoxia, caudal ventrolateral medulla of rats exposed to 2-6h of hypoxia, and lateral part of the nucleus of the solitary tract and dorsal motor nucleus of the vagus nerve of rats exposed to 1-2h of hypoxia. Multiple immunofluorescence labeling showed that 5-HT-immunoreactive nerve fibers were close to NK1R-immunoreactive neurons in ventrolateral medulla and to DBH-immunoreactive neurons in the medulla. These results suggest that serotonergic neurons partly regulate respiratory control under hypoxic conditions by modulating the activity of NK1R-expressing and catecholaminergic neurons.

Brain stem activity changes associated with restored sympathetic drive following CPAP treatment in OSA subjects: a longitudinal investigation

Obstructive sleep apnea (OSA) is associated with significantly elevated muscle sympathetic nerve activity (MSNA), leading to hypertension and increased cardiovascular morbidity. Although little is known about the mechanisms responsible for the sympathoexcitation, we have recently shown that the elevated MSNA in OSA is associated with altered neural processing in various brain stem sites, including the dorsolateral pons, rostral ventrolateral medulla, medullary raphe, and midbrain. Given the risk associated with elevated MSNA, we aimed to determine if treatment of OSA with continuous positive airway pressure (CPAP) would reduce the elevated MSNA and reverse the brain stem functional changes associated with the elevated MSNA. We performed concurrent recordings of MSNA and blood oxygen level-dependent (BOLD) signal intensity of the brain stem, using high-resolution functional magnetic resonance imaging, in 15 controls and 13 subjects with OSA, before and after 6 mo CPAP treatment. As expected, 6 mo of CPAP treatment significantly reduced MSNA in subjects with OSA, from 54 ± 4 to 23 ± 3 bursts/min and from 77 ± 7 to 36 ± 3 bursts/100 heart beats. Importantly, we found that MSNA-coupled changes in BOLD signal intensity within the dorsolateral pons, medullary raphe, and rostral ventrolateral medulla returned to control levels. That is, CPAP treatment completely reversed brain stem functional changes associated with elevated MSNA in untreated OSA subjects. These data highlight the effectiveness of CPAP treatment in reducing one of the most significant health issues associated with OSA, that is, elevated MSNA and its associated elevated morbidity.

Ultrastructural analysis of rat ventrolateral periaqueductal gray projections to the A5 cell group

Stimulation of neurons in the ventrolateral periaqueductal gray (PAG) produces antinociception as well as cardiovascular depressor responses that are mediated in part by pontine noradrenergic neurons. A previous report using light microscopy has described a pathway from neurons in the ventrolateral PAG to noradrenergic neurons in the A5 cell group that may mediate these effects. The present study used anterograde tracing and electron microscopic analysis to provide more definitive evidence that neurons in the ventrolateral PAG form synapses with noradrenergic and non-catecholaminergic A5 neurons in Sasco Sprague-Dawley rats. Deposits of anterograde tracer, biotinylated dextran amine, into the rat ventrolateral PAG labeled a significant number of axons in the region of the rostral subdivision of the A5 cell group, and a relatively lower number in the caudal A5 cell group. Electron microscopic analysis of anterogradely-labeled terminals in both rostral (n=127) and caudal (n=70) regions of the A5 cell group indicated that approximately 10% of these form synapses with noradrenergic dendrites. In rostral sections, about 31% of these were symmetric synapses, 19% were asymmetric synapses, and 50% were membrane appositions without clear synaptic specializations. In caudal sections, about 22% were symmetric synapses, and the remaining 78% were appositions. In both rostral and caudal subdivisions of the A5, nearly 40% of the anterogradely-labeled terminals formed synapses with non-catecholaminergic dendrites, and about 45% formed axoaxonic synapses. These results provide direct evidence for a monosynaptic pathway from neurons in the ventrolateral PAG to noradrenergic and non-catecholaminergic neurons in the A5 cell group. Further studies should evaluate if this established monosynaptic pathway may contribute to the cardiovascular depressor effects or the analgesia produced by the activation of neurons in the ventrolateral PAG.

Exercise-induced pain requires NMDA receptor activation in the medullary raphe nuclei

PURPOSE

Pain in response to physical activity is common in people with chronic musculoskeletal pain and is likely a barrier to regular exercise, which would lead to a sedentary lifestyle. We recently developed a model of exercise-induced pain that is associated with increased activation of neurons in the medullary raphe nuclei, i.e., the nucleus raphe obscurus (NRO) and nucleus raphe pallidus (NRP). Because the NRO and NRP not only modulate motor output but also respond to noxious stimuli, we hypothesized that the NRO and NRP were key nuclei in the interaction between pain and exercise. We tested whether exercise enhances hyperalgesia through activation of N-methyl D-aspartate (NMDA) receptors in the NRO/NRP.

METHODS

Muscle insult was induced by two injections of pH 5.0 saline 5 d apart into one gastrocnemius muscle. We initially tested whether hyperalgesia developed in mice injected with acidic saline (pH 5.0) into the gastrocnemius muscle immediately after a 30-min or 2-h exercise task or 2 h after a 2-h exercise task. Next, we tested whether blockade of NMDA receptors in the NRO/NRP during the exercise task prevented the development of exercise-induced hyperalgesia. Finally, we evaluated changes in phosphorylation of the NR1 subunit of the NMDA receptor (pNR1) after the exercise task at times in which muscle insult was given in behavioral experiments, i.e., immediately after a 30-min or 2-h exercise task or 2 h after the 2-h exercise task.

RESULTS

All exercise conditions enhanced nociception (hyperalgesia) after combining with two injections of pH 5.0 saline. Microinjection of AP5 (1.0-0.1 nmol; 2-amino-5-phophonopenanoate) dose-dependently prevented the development of exercise-induced hyperalgesia. All exercise conditions increased pNR1 in the NRO and NRP.

CONCLUSIONS

Thus, exercise-induced pain in sedentary mice is associated with increased phosphorylation and activation of NMDA receptors in the NRO/NRP, suggesting that changes in central excitability mediate an interaction between unaccustomed exercise and pain.

Descending facilitatory pathways from the RVM initiate and maintain bilateral hyperalgesia after muscle insult

The rostral ventromedial medulla (RVM) is involved in facilitation of spinal nociceptive processing and generation of hyperalgesia in inflammatory and neuropathic pain models. We hypothesized that the bilateral hyperalgesia that develops after repeated intramuscular injections of acidic saline is initiated and maintained by activation of descending facilitatory pathways from the RVM. Male Sprague-Dawley rats were implanted with intracerebral guide cannulae into the nucleus raphe magnus (NRM) or the nucleus gigantocellularis (Gi). Two injections of acidic saline into one gastrocnemius muscle 5 days apart lead to robust hyperalgesia after the second injection. Either ropivacaine (local anesthetic) or vehicle (control) was microinjected into the RVM prior to the first intramuscular acid injection, prior to the second injection, or 24h after the second injection. Mechanical withdrawal thresholds of the paw (von Frey filaments) and the muscle (tweezer) were measured before and 24h after induction of hyperalgesia. The withdrawal thresholds for both the paw (cutaneous secondary hyperalgesia) and muscle (primary hyperalgesia) were decreased 24h after the second intramuscular acid injection in the vehicle control groups. Administration of ropivacaine prior to the first intramuscular acid injection had no effect on development of either cutaneous or muscle hyperalgesia that develops after the second injection. However, neither cutaneous nor muscle hyperalgesia developed in the group treated with ropivacaine prior to the second intramuscular injection. Ropivacaine also significantly reversed the hyperalgesia in the group treated 24h after the second intramuscular acid injection. Thus, the RVM is critical for both the development and maintenance of hyperalgesia after muscle insult.

Pain facilitation and activity-dependent plasticity in pain modulatory circuitry: role of BDNF-TrkB signaling and NMDA receptors

Pain modulatory circuitry in the brainstem exhibits considerable synaptic plasticity. The increased peripheral neuronal barrage after injury activates spinal projection neurons that then activate multiple chemical mediators including glutamatergic neurons at the brainstem level, leading to an increased synaptic strength and facilitatory output. It is not surprising that a well-established regulator of synaptic plasticity, brain-derived neurotrophic factor (BDNF), contributes to the mechanisms of descending pain facilitation. After tissue injury, BDNF and TrkB signaling in the brainstem circuitry is rapidly activated. Through the intracellular signaling cascade that involves phospholipase C, inositol trisphosphate, protein kinase C, and nonreceptor protein tyrosine kinases; N-methyl-D-aspartate (NMDA) receptors are phosphorylated, descending facilitatory drive is initiated, and behavioral hyperalgesia follows. The synaptic plasticity observed in the pain pathways shares much similarity with more extensively studied forms of synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD), which typically express NMDA receptor dependency and regulation by trophic factors. However, LTP and LTD are experimental phenomena whose relationship to functional states of learning and memory has been difficult to prove. Although mechanisms of synaptic plasticity in pain pathways have typically not been related to LTP and LTD, pain pathways have an advantage as a model system for synaptic modifications as there are many well-established models of persistent pain with clear measures of the behavioral phenotype. Further studies will elucidate cellular and molecular mechanisms of pain sensitization and further our understanding of principles of central nervous system plasticity and responsiveness to environmental challenge.

Collateral axonal projections from rostral ventromedial medullary nitric oxide synthase containing neurons to brainstem autonomic sites

The magnocellular reticular nucleus and adjacent lateral paragigantocellular nucleus have been shown to contain a large population of nitric oxide synthase (NOS) immunoreactive neurons. However, little is known about the projections of these neurons within the central nervous system. Retrograde tract-tracing techniques combined with immunohistochemistry were used in this study to investigate whether NOS neurons in this rostral ventromedial medullary (RVMM) region send collateral axonal projections to autonomic sites in the nucleus of the solitary tract (NTS) and in the nucleus ambiguus (Amb). Fluorogold and/or rhodamine labeled latex microspheres were microinjected into the NTS and Amb at sites that elicited bardycardia and/or depressor responses (l-glutamate; 0.25 M; 10 nl). After a survival period of 10-14 days, the rats were sacrificed and tissue sections of the brainstem were processed immunohistochemically for the identification of NOS containing neuronal perikarya. After unilateral injection of the tract-tracers into the NTS and Amb, retrogradely labeled neurons were observed bilaterally throughout the RVMM region. Of the number of RVMM neurons retrogradely labeled from the NTS (684+/-143), 9% were found to be immunoreactive to NOS. Similarly, of those RVMM neurons retrogradely labeled from the Amb (963+/-207), 7% also contained NOS immunoreactivity. Neurons with collateral axonal projections to NTS and Amb (14% and 10%, respectively) were observed predominantly within a region of RVMM that extended co-extensively with approximately the rostrocaudal extent of the facial nucleus. Of these double labeled neurons, 36.4+/-20 (39%) were also found to be immunoreactive to NOS. These data indicate that the RVMM contains at least three population of NOS neurons that send axons to innervate functionally similar cardiovascular responsive sites in the NTS and Amb. Although the function of these NOS containing medullary pathways in cardiovascular control is not known, it is likely that those with collateral axonal projections represent the anatomical substrate by which the RVMM may simultaneously coordinate cardiovascular responses during physiological changes associated with respiration and/or motor movements.

Trigeminal transition zone/rostral ventromedial medulla connections and facilitation of orofacial hyperalgesia after masseter inflammation in rats

Recent studies have implicated a role for the trigeminal interpolaris/caudalis (Vi/Vc) transition zone in response to orofacial injury. Using combined neuronal tracing and Fos protein immunocytochemistry, we investigated functional connections between the Vi/Vc transition zone and rostral ventromedial medulla (RVM), a key structure in descending pain modulation. Rats were injected with a retrograde tracer, FluoroGold, into the RVM 7 days before injection of an inflammatory agent, complete Freund's adjuvant, into the masseter muscle and perfused at 2 hours postinflammation. A population of neurons in the ventral Vi/Vc overlapping with caudal ventrolateral medulla, and lamina V of the trigeminal subnucleus caudalis (Vc), exhibited FluoroGold/Fos double staining, suggesting the activation of the trigeminal-RVM pathway after inflammation. No double-labeled neurons were found in the dorsal Vi/Vc and laminae I-IV of Vc. Injection of an anterograde tracer, Phaseolus vulgaris leucoagglutinin, into the RVM resulted in labeling profiles overlapped with the region that showed FluoroGold/Fos double labeling, suggesting reciprocal connections between RVM and Vi/Vc. Lesions of Vc with a soma-selective neurotoxin, ibotenic acid, significantly reduced inflammation-induced Fos expression as well as the number of FluoroGold/Fos double-labeled neurons in the ventral Vi/Vc (P<0.05). Compared with control rats, lesions of the RVM (n=6) or Vi/Vc (n=6) with ibotenic acid led to the elimination or attenuation of masseter hyperalgesia/allodynia developed after masseter inflammation (P<0.05-0.01). The present study demonstrates reciprocal connections between the ventral Vi/Vc transition zone and RVM. The Vi/Vc-RVM pathway is activated after orofacial deep tissue injury and plays a critical role in facilitating orofacial hyperalgesia.

Comparative anatomical assessment of the piglet as a model for the developing human medullary serotonergic system

Because the piglet is frequently used as a model for developmental disorders of the medullary serotonergic (5-HT) system in the human infant, this review compares the topography and developmental profile of selected 5-HT markers between humans in the first year of life and piglets in the first 60 days of life. The distribution of tryptophan hydroxylase-immunoreactive 5-HT neurons in the human infant medulla is very similar, but not identical, to that in the piglet. One notable difference is the presence of compact clusters of 5-HT neurons at the ventral surface of the piglet medulla. While it lacks these distinctive clusters, the human infant medulla contains potentially homologous 5-HT neurons scattered along the ventral surface embedded in the arcuate nucleus. Each species shows evidence of age-related changes in the 5-HT system, but the changes are different in nature; in the human infant, statistically significant age-related changes are observed in the proportional distribution of medullary 5-HT cells, while in the piglet, statistically significant age-related changes are observed in the levels of 5-HT receptor binding in certain medullary nuclei. Analyses of 5-HT receptor binding profiles in selected nuclei in the two species suggest that the equivalent postnatal ages for 5-HT development in piglets and human infants are, respectively, 4 days and 1 month, 12 days and 4 months, 30 days and 6 months, and 60 days and 12 months. Collectively, when certain species differences are considered, these data support the use of the piglet as a model for the human infant medullary 5-HT system.

The hypotension evoked by visceral nociception is mediated by delta opioid receptors in the periaqueductal gray

This study tested the hypothesis that the ventrolateral column of the midbrain periaqueductal gray (vlPAG) region mediates the hypotension and bradycardia evoked by visceral nociception. To test this, the local anesthetic lidocaine (2%; 0.5 microl) was microinjected into the vlPAG of halothane-anesthetized rats bilaterally and visceral nociception was induced 2 min later by injecting 5% acetic acid (0.5 ml) intraperitoneally. Acetic acid injection caused an abrupt fall in arterial pressure (-12.2+/-2.1 mm Hg) and heart rate (-37+/-93 bpm) lasting approximately 15 min. Lidocaine injection into the vlPAG prevented the fall in arterial pressure and heart rate completely. Cobalt chloride (5 mM; 0.2 or 0.5 microl) injection into the vlPAG also prevented nociceptive hypotension but it did not affect the fall in heart rate significantly. Lidocaine pretreatment also inhibited the depressor response caused by intramuscular formalin (5%; 0.2 ml) administration, a model of deep somatic nociception, although it did not prevent the response completely. To determine if opioid receptors mediate the response, selective mu, delta or kappa opioid receptor antagonists were microinjected into the vlPAG 5 min before intraperitoneal (ip) acetic acid administration. Naltrindole, a delta receptor antagonist, inhibited the response significantly but mu and kappa antagonists were completely ineffective. Lidocaine and naltrindole had no effect when injected into the dorsolateral PAG and did not influence cardiovascular function when injected into the vlPAG of saline treated control animals. These data support the hypothesis that the vlPAG mediates the depressor response evoked by visceral nociception and indicate that delta opioid receptors participate in the response.

Medullary and spinal cord projections from cardiovascular responsive sites in the rostral ventromedial medulla

The rostral ventromedial medulla (RVMM) is a sympathoexcitatory area. However, little is known about its efferent projections. In this study, biotinylated dextran amine (BDA) or Phaseolus vulgaris leucoagglutinin (PHA-L) were used to investigate the medullary and spinal cord projections from pressor sites in RVMM. Initially, RVMM was systematically explored in urethane-anesthetized rats using microinjection of L-glutamate for sites that elicited increases in arterial pressure. A pressor area was identified that included the rostral magnocellular reticular and rostral lateral paragigantocellular reticular nuclei. In the second series of experiments, BDA or PHA-L was iontophoretically injected into RVMM pressor sites. Anterograde labeling was observed throughout the brainstem and spinal cord, bilaterally, but with an ipsilateral predominance. Dense labeling was observed within the nucleus of the solitary tract (NTS); the greatest density of labeling was observed in the caudal dorsolateral, medial, and ventrolateral subnuclei. Additionally, light to moderately dense labeling was found within the nucleus substantia gelatinosus and commissural nucleus. In the nucleus ambiguus/ventrolateral medullary (Amb/VLM) region, the density of labeling was greatest in caudal regions. Within Amb, most of the labeling was localized to its external formation. Anterograde labeling was also found throughout the spinal cord. In the thoracolumbar segments, dense axonal labeling was observed within the dorsolateral funiculus. These labeled axons innervated the intermediolateral nucleus and the central autonomic area. Taken together, these data suggest that RVMM neurons elicit increases in sympathetic activity by likely providing a direct excitatory input to spinal sympathetic preganglionic neurons, and by a direct inhibitory input to medullary cardioinhibitory and depressor areas.

Human cardiovascular and gustatory brainstem sites observed by functional magnetic resonance imaging

The reflex control and relay to higher brain sites of visceral sensory information within the central nervous system is mediated via discrete sites in the brainstem. Anatomical characterization of these sites in humans has been limited due to the invasive nature of such research. The present study employed 4 Tesla functional magnetic resonance imaging (fMRI) to characterize brainstem sites involved in autonomic control in the human. Eight subjects performed tasks that activate the general visceral (the isometric hand-grip, maximal inspiration, Valsalva maneuver) or special visceral sensory systems (sucrose administration to the tongue). Activation of the nucleus of the solitary tract and parabrachial nucleus was consistently observed with all general visceral tasks. Periaqueductal gray area activation was observed during the maximal inspiration and Valsalva maneuver conditions and raphe activation was present in response to isometric hand-grip and maximal inspiration tasks. The activation in the nucleus of the solitary tract was consistently more rostral in the medulla during sucrose administration than during performance of the other experimental tasks. This finding is consistent with what has been previously demonstrated in animals. This is the first study to image the human brainstem with respect to visceral control and demonstrates the feasibility of using high-resolution fMRI to study the functional organization of the human brainstem.

Sympathoinhibitory pathway from caudal midline medulla to RVLM is independent of baroreceptor reflex pathway

Glutamate stimulation of the caudal midline medulla (CMM) causes profound sympathoinhibition due to GABAergic inhibition of presympathetic neurons in the rostral ventrolateral medulla (RVLM). We investigated whether the sympathoinhibitory pathway from CMM to RVLM, like the central baroreceptor reflex pathway, includes a glutamatergic synapse in the caudal ventrolateral medulla (CVLM). In pentobarbital sodium-anesthetized rats, the RVLM on one side was inhibited by a muscimol microinjection. Then the response evoked by glutamate microinjections into the CMM or by baroreceptor stimulation was determined before and after 1) microinjection of the GABA receptor antagonist bicuculline into the RVLM on the other side or 2) microinjections of the glutamate receptor antagonist kynurenate bilaterally into the CVLM. Bicuculline in the RVLM greatly reduced both CMM- and baroreceptor-evoked sympathoinhibition. Compared with the effect of vehicle solution, kynurenate in the CVLM greatly reduced baroreceptor-evoked sympathoinhibition, whereas its effect on CMM-evoked sympathoinhibition was not different from that of the vehicle solution. These findings indicate that the output pathway from CMM sympathoinhibitory neurons, unlike the baroreceptor and other reflex sympathoinhibitory pathways, does not include a glutamatergic synapse in the CVLM.

Serotonergic projections to the rostroventrolateral medulla from midbrain and raphe nuclei

Double-label fluoresence immunohistochemistry was performed to define serotonergic projections from the raphe and midbrain to the sympathoexcitatory region of the rostroventrolateral medulla (RVLM). Immunolabelling of cholera toxin B subunit retrogradely transported from the pressor region of the RVLM was combined with serotonin (5-HT) immunohistochemistry. Major sources of serotonergic input to the RVLM were shown to include the raphe obscurus, raphe pallidus and raphe magnus with a minor contribution from the ventrolateral, lateral and ventral regions of the periaqueductal gray matter, and the dorsal raphe nucleus. Serotonergic modulation of sympathoexcitatory neurons may establish patterns of sympathetic nerve activity evident in many aspects of cardiovascular regulation.

Extracellular serotonin levels in the medullary reticular formation during normal sleep and after REM sleep deprivation

Rapid eye movement (REM) sleep is hypothesized to result from the activity of REM sleep-generating and REM sleep-inhibiting neurons. The serotoninergic (5-HT) neurons of the dorsal raphe nucleus (DRN) represents one such population of REM-sleep inhibiting neurons since they are silent during REM sleep. Consistent with the decrease in activity of 5-HT neurons, the brain extracellular levels of 5-HT are lower during REM sleep compared to wakefulness. It is not known whether serotonin release is also reduced as a consequence of REM sleep rebound. Using microdialysis sampling coupled to HPLC-ECD, we measured the extracellular levels of 5-HT and its metabolite (5-HIAA) in the medial medullary reticular formation (mMRF) of freely behaving rats during normal sleep, REM sleep deprivation as well as during REM sleep rebound. We found that the levels 5-HT and 5-HIAA were significantly decreased by REM sleep deprivation. The reduction of 5-HT release was maintained during REM sleep rebound but the extracellular level of its main metabolite was increased. In addition, even during REM sleep rebound, 5-HT release during sleep was low compared to wakefulness. Taken together these data support the permissive role of 5-HT neurotransmission for REM sleep expression.

Microinjection of bombesin into the ventrolateral reticular formation inhibits peripherally stimulated gastric acid secretion through spinal pathways in rats

Bombesin injected into the cisterna magna potently inhibits gastric acid secretion stimulated by intravenous infusion of pentagastrin. Sites in the medulla oblongata where bombesin acts to suppress gastric acid secretion were investigated in urethane-anesthetized rats with gastric cannula. Bombesin or vehicle was injected into the medullary parenchyma or intracisternally (i.c.) 60 min after the start of an intravenous pentagastrin infusion; gastric acid secretion was monitored every 10 min for 20 min before and 150 min after the start of pentagastrin. Bombesin (0.2, 0.6 or 6.2 pmol) microinjected into the ventrolateral reticular formation (VLRF) inhibited dose-dependently the net acid response to pentagastrin by 40.8+/-11.1, 75.4+/-12.8 and 96.7+/-19.4%, respectively, at the 40-50 min period after microinjection compared with the vehicle group. Bombesin action in the VLRF was long lasting (96% inhibition still observed at 90 min after 6.2 pmol), and completely abolished by cervical spinal cord transection at the C6 level. By contrast, bombesin injected i.c. at 0.2 or 0.6 pmol had no effect while at 6.2 pmol, there was a 79.0+/-3.9% peak inhibition of pentagastrin-stimulated acid secretion. Bombesin (6.2 pmol) injected into the dorsal motor nucleus reduced the acid response to pentagastrin by 29%. The parvicellular and gigantocellular reticular nuclei were not responsive to bombesin. These results indicate that bombesin acts in the VLRF to inhibit pentagastrin-stimulated gastric acid secretion through spinal pathways, suggesting a potential role of medullary VLRF area in the sympathetic control of gastric acid secretion.

Evidence that hemorrhagic hypotension is mediated by the ventrolateral periaqueductal gray region

Severe hemorrhage lowers arterial pressure by suppressing sympathetic activity. This study tested the hypothesis that the decompensatory phase of hemorrhage is mediated by the ventrolateral periaqueductal gray (vlPAG), a region importantly involved in the autonomic and behavioral responses to stress and trauma. Neuronal activity in the vlPAG was inhibited with either lidocaine or cobalt chloride 5 min before hemorrhage (2.5 ml/100 g body wt) was initiated in conscious, unrestrained rats. Bilateral injection of lidocaine (0.5 microl of a 2% or 1 microl of a 5% solution) into the caudal vlPAG delayed the onset and reduced the magnitude of the hypotension produced by hemorrhage significantly. In contrast, inactivation of the dorsolateral PAG with lidocaine was ineffective. Cobalt chloride (5 mM; 0.5 microl), which inhibits synaptic transmission but not axonal conductance, also attenuated hemorrhagic hypotension significantly. Microinjection of lidocaine or cobalt chloride into the vlPAG of normotensive, nonhemorrhaged rats did not influence cardiovascular function. These data indicate that the vlPAG plays an important role in the response to hemorrhage.

Behavioral correlates of the activity of serotonergic and non-serotonergic neurons in caudal raphe nuclei

We investigated the behavioral correlates of the activity of serotonergic and non-serotonergic neurons in the nucleus raphe pallidus (NRP) and nucleus raphe obscurus (NRO) of unanesthetized and unrestrained cats. The animals were implanted with electrodes for recording single unit activity, parietal oscillographic activity, and splenius, digastric and masseter electromyographic activities. They were tested along the waking-sleep cycle, during sensory stimulation and during drinking behavior. The discharge of the serotonergic neurons decreased progressively from quiet waking to slow wave sleep and to fast wave sleep. Ten different patterns of relative discharge across the three states were observed for the non-serotonergic neurons. Several non-serotonergic neurons showed cyclic discharge fluctuations related to respiration during one, two or all three states. While serotonergic neurons were usually unresponsive to the sensory stimuli used, many non-serotonergic neurons responded to these stimuli. Several non-serotonergic neurons showed a phasic relationship with splenius muscle activity during auditory stimulation. One serotonergic neuron showed a tonic relationship with digastric muscle activity during drinking behavior. A few non-serotonergic neurons exhibited a tonic relationship with digastric and/or masseter muscle activity during this behavior. Many non-serotonergic neurons exhibited a phasic relationship with these muscle activities, also during this behavior. These results suggest that the serotonergic neurons in the NRP and NRO constitute a relatively homogeneous population from a functional point of view, while the non-serotonergic neurons form groups with considerable functional specificity. The data support the idea that the NRP and NRO are implicated in the control of somatic motor output.

A brainstem area mediating cerebrovascular and EEG responses to hypoxic excitation of rostral ventrolateral medulla in rat

We sought to identify the medullary relay area mediating the elevations of regional cerebral blood flow (rCBF) and synchronization of the electroencephalogram (EEG) in the rat cerebral cortex elicited by hypoxic excitation of reticulospinal sympathoexcitatory neurons of the rostral ventrolateral medulla (RVLM ). In anaesthetized spinalized rats electrical stimulation of RVLM elevated rCBF (laser-Doppler flowmetry) by 31 +/- 6 %, reduced cerebrovascular resistance (CVR) by 26 +/- 8 %, and synchronized the EEG, increasing the power of the 5-6 Hz band by 98 +/- 25 %. Stimulation of a contiguous caudal region, the medullary cerebral vasodilator area (MCVA), had comparable effects which, like responses of RVLM, were replicated by microinjection of L-glutamate (5 nmol, 20 nl). Microinjection of NaCN (300 pmol in 20 nl) elevated rCBF (17 +/- 5 %) and synchronized the EEG from RVLM, but not MCVA, while nicotine (1.2 nmol in 40 nl) increased rCBF by 13 +/- 5 % and synchronized the EEG from MCVA. In intact rats nicotine lowered arterial pressure only from MCVA (101 +/- 3 to 52 +/- 9 mmHg). Bilateral electrolytic lesions of MCVA significantly reduced, by over 59 %, elevations in rCBF and, by 78 %, changes in EEG evoked from RVLM. Bilateral electrolytic lesions of RVLM did not affect responses from MCVA. Anterograde tracing with BDA demonstrated that RVLM and MCVA are interconnected. The MCVA is a nicotine-sensitive region of the medulla that relays signals elicited by excitation of oxygen-sensitive reticulospinal neurons in RVLM to reflexively elevate rCBF and slow the EEG as part of the oxygen-conserving (diving) reflex initiated in these neurons by hypoxia or ischaemia.

Caudal midline medulla mediates behaviourally-coupled but not baroreceptor-mediated vasodepression

Within the caudal medulla there are two regions whose activation leads to vasodepression and bradycardia, the caudal ventrolateral medulla and a discrete region of the caudal midline medulla. This study investigated, in the halothane anaesthetized rat, the contribution of these two vasodepressor regions to "homeostatic" and "behaviourally-coupled" cardiovascular regulation. In an initial set of experiments the contribution of each of these two regions to the hypotension and bradycardia evoked by acute hypovolaemia (15% haemorrhage) was investigated. It was found that inactivation of the caudal midline medulla significantly attenuated (cobalt chloride) or completely blunted (lignocaine) the hypotension and bradycardia evoked by acute hypovolaemia. In contrast, inactivation of the caudal ventrolateral medulla using cobalt chloride, although attenuating the magnitude of the hypotension and completely blocking the bradycardia, did not delay the onset of the hypotension evoked by acute hypovolaemia. The caudal ventrolateral medulla is known to be critical in homeostatic cardiovascular control through the expression of the "baroreceptor reflex" and the hypotension and bradycardia evoked by activation of cardiopulmonary afferents. In a second series of experiments we found inactivation of the caudal midline medulla played no role in baroreflex-evoked bradycardia (i.v. phenylephrine) or the hypotension and bradycardia evoked by cardiopulmonary afferent activation (i.v. 5-hydroxytryptamine). These data suggest that the caudal midline medulla and caudal ventrolateral medulla play different roles in cardiovascular control. The caudal ventrolateral medulla is involved in mediating cardiovascular changes associated with a variety of stimuli including "homeostatic" and "behaviourally-coupled" cardiovascular changes, whereas the caudal midline medulla is critical for mediating "behaviourally-coupled" changes in arterial pressure and heart rate.

Synaptically released 5-HT modulates the activity of tonically discharging neuronal populations in the rostral ventral medulla (RVM)

There is substantial evidence for an important modulating role of monoamines (catecholamines and serotonin, 5-HT) in the rostral ventral medulla (RVM), a region which plays an important role in cardiovascular and nociceptive functions. We investigated in slices the role of endogenous monoamines in the synaptic control of the activity of rat RVM neuronal populations using intracellular recordings in the lateral RVM plus lateral aspect of nucleus paragigantocellularis lateralis. A triple-labelling protocol allowed us to identify the location of impaled neurons and their eventual monoaminergic phenotype within the serotonergic and catecholaminergic populations of the RVM. Focal electrical stimulation revealed the existence of a functional monoaminergic input onto RVM neurons which was mediated by endogenous 5-HT acting at inhibitory 5-HT1A receptors but did not involve noradrenergic neurotransmission. The slow 5-HT-mediated inhibitory postsynaptic potential (IPSP) was only observed in the regularly discharging neurons, which were found to be neither catecholaminergic nor serotonergic. The synaptic release of 5-HT was, itself, under an inhibitory control involving GABAA (gamma-aminobutyric acid) receptors. Moreover, we characterized the effect of the 5-HT-releasing agent fenfluramine on this functional 5-HT-mediated synaptic transmission. Our results show that the effect of fenfluramine is biphasic consisting of an initial prolongation of the serotonergic IPSP followed by a decrease in amplitude. Our data provide a basis for the previously reported inhibitory effects of exogenously applied serotonin agonists/antagonists on the autonomic functions controlled by the RVM. This 5-HT pathway, which functionally links the serotonergic and catecholaminergic regions, might play an important role in cardiovascular and nociceptive functions.

Activation of cardiac vagal afferents facilitates late vagal inhibition in neurones of the rostral ventrolateral medulla oblongata bilaterally

A reduced activity of cardiac vagal afferent fibres is considered as one of the pathophysiological causes of post-infarction complications [A. Head, Baroreflexes and cardiovascular regulation in hypertension. J. Cardiovasc. Pharmacol. 26 (1995) S7-S16]. The mechanism of how a reduction of cardiac vagal activity leads to enhanced sympathetic drive and systemic hypertension is however not yet clear. Experimental data have shown that the rostral ventrolateral medulla oblongata (RVLM) plays an important role in tonic blood pressure regulation, the control of sympathetic vasoconstriction and cardiac performance. The aim of the study was to determine whether activation of cardiac vagal afferents contributes to eliciting the long-lasting late inhibition that we have previously shown to occur in neurones of the RVLM [A. Zagon, K. Ishizuka, I. Rocha, K.M. Spyer, Late vagal inhibition in neurones of the ventrolateral medulla oblongata in the rat. Neurosci. 92 (1999) 877-888]. The experiments were carried out in terminally anaesthetised and artificially ventilated rats using in vivo intracellular recordings. The data confirmed that late vagal inhibition is elicited by cumulative activation of functionally different vagal afferents, including those that originate from cardiac receptors. It was also demonstrated that activation of cardiac afferents could lead to a significant increase in the duration of this long-lasting late response component. Facilitation of late vagal inhibition was observed in RVLM neurones both ipsi- and contralateral to the stimulated nerve. It is suggested that such facilitation of late vagal inhibition may be a mechanism of how pulse-synchronous activation of cardiac afferents leads to a tonic modulation of the activity of RVLM neurones. An attenuation of late vagal inhibition during reduced activity of cardiac vagal afferents could lead to enhanced excitability in these neurones which in turn can lead to an increase in medullary sympathetic outflows towards the heart and peripheral blood vessels.

Late vagal inhibition in neurons of the ventrolateral medulla oblongata in the rat

Stimulation of cervical vagal afferents elicits long-lasting inhibitory effects in a variety of neuronal populations, although little is known concerning the cellular mechanisms that are involved in these effects. In the present study, the electrophysiological characteristics of responses elicited by cumulative activation of vagal afferents were examined in neurons of the rostral ventrolateral medulla oblongata, which play an important role in the coordination of cardiovascular and other visceral activities. The study has focused on the late-onset, slow inhibitory component of vagal responses, which is likely to affect the temporal modulation of postsynaptic effects. Vagal stimulation elicited four distinct response patterns in intracellularly penetrated neurons (n = 78): excitation, inhibition, excitation-inhibition and inhibition-inhibition. The late inhibitory component was encountered in 43 (55%) of the cells, including five putative medullospinal neurons. It was due to a postsynaptic hyperpolarization which reversed at potentials more negative than -83 mV. The voltage dependency, as well as the average onset latency (93+/-3.0 ms), duration (270+/-16.5 ms) and amplitude (1.3+/-0.2 mV as measured at resting membrane potentials), of late inhibition were clearly different from those of the short-latency inhibitory response. The differences in the voltage dependency and time-course of the short-latency responses and the late inhibition indicate that they are mediated by different central relays. In the majority of neurons, late inhibition could be elicited by stimulating only myelinated vagal afferents. The magnitude of the response was, however, significantly enhanced in 63% of the examined cells when the intensity of stimulation was raised to recruit further myelinated and non-myelinated fibres. This indicates that late vagal inhibition is often elicited by a cumulative activation of convergent afferent inputs. The intracellularly labelled vagally responsive neurons were present at all rostrocaudal levels of the rostral ventrolateral medulla, with an accumulation in the region of the lateral paragigantocellular nucleus. Neurons that exhibited late vagal inhibition were dominant in the juxtafacial region of this nucleus. Due to its slow time-course, late vagal inhibition may contribute to a tonic modulation of the activity of neurons in the rostral ventrolateral medulla oblongata. It is proposed that late vagal inhibition plays an important role in the temporal integration of sensory inputs in neurons of the rostral ventrolateral medulla oblongata. The time-course and strength of this modulatory effect are related to the level of activity in those visceral sensory inputs that converge onto the inhibitory interneurons that mediate late inhibition to rostral ventrolateral medulla oblongata neurons.

Midline medullary depressor responses are mediated by inhibition of RVLM sympathoexcitatory neurons in rats

Mechanisms underlying the depressor and sympathoinhibitory responses evoked from the caudal medullary raphe (MR) region were investigated in pentobarbital sodium-anesthetized, paralyzed rats. Intermittent electrical stimulation (0.5 Hz, 0.5-ms pulses, 200 microA) of the MR elicited a mixed sympathetic response that consisted of a long-latency sympathoexcitatory (SE) peak (onset = 146 +/- 7 ms) superimposed on an inhibitory phase (onset = 59 +/- 10 ms). Chemical stimulation of the MR (glutamate; Glu) most frequently elicited depressor responses accompanied by inhibition of sympathetic nerve discharge. Occasionally, these responses were preceded by transient pressor and SE responses. We examined the influence of intermittent electrical stimulation (0.5 Hz, 0.5-ms pulses, 25-200 microA) and Glu stimulation of the MR on the discharge of rostral ventrolateral medulla (RVLM) premotor SE neurons. Peristimulus-time histograms of RVLM unit discharge featured a prominent inhibitory phase in response to MR stimulation (onset = 20 +/- 2 ms; duration = 42 +/- 4 ms; n = 12 units). Glu stimulation of the MR reduced blood pressure (-37 +/- 2 mmHg, n = 19) and inhibited the discharge of RVLM SE neurons (15 of 19 neurons). Depressor and sympathoinhibitory responses elicited by chemical and electrical stimulation of the MR region are mediated by inhibition of RVLM premotor SE neurons and withdrawal of sympathetic vasomotor discharge.

The ventrolateral periaqueductal gray projects to caudal brainstem depressor regions: a functional-anatomical and physiological study

The reaction of shock, a precipitous, life-threatening fall in arterial pressure and heart rate, is evoked often by the combination of deep pain and blood loss following traumatic injury. A similar "shock-like" pattern of response can be evoked by excitation of the ventrolateral midbrain periaqueductal gray. Further, ventrolateral periaqueductal gray neurons are selectively activated by deep somatic or visceral pain and haemorrhage. The pathways mediating ventrolateral periaqueductal gray evoked hypotension and bradycardia are not known. In this study, the projections from the ventrolateral periaqueductal gray to "cardiovascular" regions in the caudal medulla of the rat were examined. Injections of the anterograde tracer, biotinylated dextran amine at physiologically-defined, ventrolateral periaqueductal gray depressor sites, revealed strong projections to the caudal midline medulla and to the depressor region of the caudal ventrolateral medulla. Injections of excitatory amino acids established that substantial falls in arterial pressure could be evoked from the ventrolateral periaqueductal gray-recipient parts of the caudal midline medulla. Injections of the retrograde tracer, cholera toxin subunit B at physiologically-defined, depressor sites in the caudal midline medulla and the caudal ventrolateral medulla confirmed the existence of substantial projections from the ventrolateral periaqueductal gray. Although previous studies have emphasized the importance of projections from the ventrolateral periaqueductal gray to the pressor region of the rostral ventrolateral medulla, this study has revealed the existence of strong ventrolateral periaqueductal gray projections to depressor regions within the caudal medulla (caudal midline medulla and caudal ventrolateral medulla) which likely contribute to ventrolateral periaqueductal gray-mediated hypotension and bradycardia.

Intrinsic membrane characteristics distinguish two subsets of nociceptive modulatory neurons in rat RVM

Pain modulating neurons of the rostral ventromedial medulla (RVM) include three physiologically distinct classes of neurons in intact, anesthetized animals: and cells that change their activity before the onset of withdrawal reflexes and cells, which have activity unrelated to withdrawal reflexes. A previous in vitro intracellular study demonstrated that the RVM contains two types of neurons that are distinguished by their action-potential characteristics. The present in vivo intracellular study examined whether these intracellularly recorded action-potential characteristics are correlated with the physiological response properties of RVM neurons recorded. RVM neurons exhibited two distinct types of action potentials in vivo. Fast-spike (FS) neurons (n = 30) had short-duration action potentials (0.27 +/- 0.02 (SE) ms at half amplitude) and biphasic afterhyperpolarizations with a characteristic rapid overshooting spike repolarization. Slow-spike (SS) neurons (n = 25) had longer duration action potentials (0.44 +/- 0.02 ms at half-amplitude) due to a slower-spike repolarization rate and monophasic afterhyperpolarization. and cell classes included both FS and SS neurons. FS and neurons had an early onset response to noxious heat stimulation. SS and cells showed a delayed onset response to noxious heat. cells (n = 13) were all SS cells. Among the SS neurons, only cells had action potentials longer than 0. 45 ms (n = 9). FS and SS neurons were intermingled throughout the RVM. The majority of intracellularly labeled cells (n = 15) had fusiform somata with two to five fine caliber primary dendrites and a predominantly mediolateral orientation of the long axis of their dendritic tree. All labeled FS cells (n = 5) had large, multipolar somata with four to nine large caliber primary dendrites. The present study defines in vivo membrane and morphological characteristics of RVM neurons that correlate with physiological differences and may be used for identification of nociceptive modulatory RVM neurons in slice preparations.

Expression of c-fos in the rat brainstem after exposure to hypoxia and to normoxic and hyperoxic hypercapnia

In this study, Fos immunohistochemistry was used to map brainstem neuronal pathways activated during hypercapnia and hypoxia. Conscious rats were exposed to six different gas mixtures: (a) air; (b) 8% CO2 in air; (c) 10% CO2 in air; (d) 15% CO2 in air; (e) 15% CO2 + 60% O2, balance N2; (f) 9% O2, balance N2. Double-staining was performed to show the presence of tyrosine hydroxylase. Hypercapnia, in a dose-dependent way caused Fos expression in the following areas: caudal nucleus tractus solitarius (NTS), with few labeled A2 noradrenergic neurons; noradrenergic A1 cells and noncatecholaminergic neurons in the caudal ventrolateral medulla; raphe magnus and gigantocellular nucleus pars alpha (GiA); many noncatecholaminergic (and relatively few C1) neurons in the lateral paragigantocellular nucleus (PGCl), and in the retrotrapezoid nucleus (RTN); locus coeruleus (LC), external lateral parabrachial and Kölliker-Fuse nuclei, and A5 noradrenergic neurons at pontine level; and in caudal mesencephalon, the ventrolateral column of the periaqueductal gray (vlPAG). In most of these nuclei, hypoxia also induced Fos expression, albeit generally less than after hypercapnia. However, hypoxia did not cause labeling in RTN, juxtafacial PGCl, GiA, LC, or vlPAG. After normoxic hypercapnia, more labeled cells were present in NTS and PGCl than after hyperoxic hypercapnia. Part of the observed labeling could be caused by stress- or cardiovascular-related sequelae of hypoxia and hypercapnia. Possible implications for the neural control of breathing are also discussed, particularly with regard to the finding that several nuclei, not belonging to the classical brainstem respiratory centres, contained labeled cells.

RTN TRH causes prolonged respiratory stimulation

We injected thyrotropin-releasing hormone (TRH; 10 nl; 0.25, 0.5, 1.0, or 10 mM), its inactive free acid form (TRHOH; 1 mM), or a metabolite with low TRH-receptor binding affinity, histidine-proline diketopiperazine (cHP; 1 mM), into the retrotrapezoid nucleus of anesthetized rats. Injection location was verified by anatomic analysis. Lower doses (0.25-0.5 mM) significantly increased both the product of integrated phrenic amplitude and frequency (Phr . f) and f for 20-30 min compared with artificial cerebrospinal fluid control injections. Higher doses (1. 0-10 mM) produced greater and long-lasting stimulation of Phr . f, Phr, and f and of blood pressure. This stimulation reached values 150% of baseline and durations of 270 min after a single injection. TRHOH (1 mM ) or cHP (1 mM) had no effect on Phr but increased f, as did 1 mM TRH. We conclude that TRH has a very powerful stimulatory effect in the retrotrapezoid nucleus region on Phr . f, with the Phr response seemingly specific for TRH receptors. Similar responses of f to TRHOH and cHP suggest it may be nonspecific.

Nucleus raphe obscurus (nRO) regulation of anorectal motility in rats

Previous research has demonstrated that anorectal contractions in the rat are modulated by activation of spinal autonomic circuits. In the present study, anterograde tracing of descending pathways originating from the caudal nucleus raphe obscurus (nRO) revealed that this nucleus projects to cells within the intermediolateral (IML) cell column of the thoracic cord and the sacral parasympathetic nucleus (SPN). These anatomical studies suggested that the nRO may influence the regulation of spinal reflexes of the pelvic floor. In a second set of experiments, acute rat preparations were used to investigate changes in anorectal motility during electrical stimulation of the nRO. Anorectal contractions were measured by a fluid-filled manometer. Electrical stimulation of the nRO significantly reduced spontaneous anorectal activity when compared to baseline contractions recorded for 1 min prior to stimulation. Stimulation sites outside the nRO did not affect anorectal contractions when compared to either (a) the 1-min pre-stimulation baseline for that site or (b) the 1-min stimulation period for sites within the nRO. Stimulation of caudal portions of the nRO were more likely than the rostral nRO to reduce anorectal contractions. Given that the SPN contains preganglionic neurons which may be involved in control of anorectal contractions (mediated via the pelvic nerve), the studies presented here suggest a functional role for nRO regulation of preganglionic motoneurons innervating the distal gut of the rat.

Adrenergic responses in silent and putative inhibitory pacemaker-like neurons of the rat rostral ventrolateral medulla in vitro

Noradrenaline and adrenergic agonists were tested on pacemaker-like and silent neurons of the rat rostral ventrolateral medulla using intracellular recording in coronal brainstem slices as well as in punches containing only the rostral ventrolateral medullary region. Noradrenaline (1-100 microM) depolarized or increased the frequency of discharge of all cells tested in a dose-dependent manner. The noradrenaline-induced depolarization was associated with an apparent increase in cell input resistance at low concentrations and a decrease or no significant change at higher concentrations. Moreover, it was voltage dependent and its amplitude decreased with membrane potential hyperpolarization. Noradrenaline caused a dose-related increase in the frequency and amplitude of spontaneous inhibitory postsynaptic potentials. The alpha 1-adrenoceptor antagonist prazosin (0.5 microM) abolished the noradrenaline depolarizing response as well as-the noradrenaline-evoked increase in synaptic activity and unmasked an underlying noradrenaline dose-dependent hyperpolarizing response associated with a decrease in cell input resistance and sensitive to the alpha 2-adrenoceptor/antagonist yohimbine (0.5 microM). The alpha 1-adrenoceptor agonist phenylephrine (10 microM) mimicked the noradrenaline depolarizing response associated with an increase in membrane resistance as well as the noradrenaline-induced increase in synaptic activity. The alpha 2-adrenoceptor agonists UK-14,304 (1-3 microM) and clonidine (10-30 microM) produced only a small hyperpolarizing response, whereas the beta-adrenoceptor agonist isoproterenol (10-30 microM) had no effect. Baseline spontaneous postsynaptic potentials were abolished by strychnine (1 microM), bicuculline (30 microM) or both. However, only the strychnine-sensitive postsynaptic potentials had their frequency increased by noradrenaline or phenylephrine and they usually occurred with a regular pattern. Tetrodotoxin (1 microM) eliminated 80-95% of baseline spontaneous postsynaptic potentials and prevented the increase in synaptic activity evoked by noradrenaline and phenylephrine. Similar results were obtained in rostral ventrolateral medulla neurons impaled in both coronal slices and punches of the rostral ventrolateral medulla. It is concluded that noradrenaline could play an important inhibitory role in the rostral ventrolateral medulla via at least two mechanisms: an alpha 2-adrenoceptor-mediated hyperpolarization and an enhancement of inhibitory synaptic transmission through activation of alpha 1-adrenoceptors located on the somatic membrane of glycinergic interneurons. Some of these interneurons exhibit a regular discharge similar to the pacemaker-like neurons and might, at least in part, constitute a central inhibitory link in the baroreceptor-vasomotor reflex pathway.

Stress-induced activation of nitric oxide-producing neurons in the rat brain

Nitric oxide (NO) is a gaseous neurotransmitter that may mediate a decrease in sympathetic output to the periphery. This implication predicts that NO-producing neurons in the brain are activated in animals experiencing increased levels of sympathetic activity. To test this prediction, we subjected three groups of experimental rats to differing levels of environmental stimulation for 1 hour: minimal stimulation, moderate stimulation, and restraint stress. NO-producing neurons were histochemically visualized in sections of the brain, and activation of these neurons was assessed according to the neuronal expression of the immediate early gene c-fos. Constitutive activation of NO-producing neurons was found in the hypothalamus (paraventricular and supraoptic nuclei), dorsal raphe nuclei, and spinal nucleus of the trigeminal nerve of minimally stimulated rats. When animals were subjected to a novel environment (moderate stimulation), additional NO-producing neurons were activated in the medial septum, medial amygdala, hypothalamic nuclei (lateral, periventricular, and posterior), colliculi, nucleus raphe obscurus, medial vestibular nucleus, nucleus of the tractus solitarius, and several components of the ventrolateral medulla. Restraint stress caused the activation of NO-producing neurons in all of these areas, often in increasing numbers, and the activation of additional NO-producing neurons in the diagonal band of Broca, lateral and medial preoptic areas, basomedial and basolateral amygdalar nuclei, hypothalamic nuclei (dorsomedial, retrochiasmatic supraoptic, and circularis), nucleus raphe pontus, lateral parabrachial nucleus, and pontine nuclei. Expressed as a proportion of NO-producing neurons per section, the largest percentages (>20%) of double-stained neurons were found in the basolateral amygdala (46%), hypothalamic paraventricular nucleus (35%), corpora quadrigemina (estimated at 40%), dorsal raphe (45%), nuclei raphe pontus (33%) and obscurus (63%), lateral parabrachial nucleus (22%), medial vestibular nucleus (25%), lateral division of the nucleus paragigantocellularis (26%), and lateral reticular nucleus (35%). Evidence from other studies increasingly supports the concept that NO plays a generalized role in autonomic regulation by decreasing sympathetic output. Our results show that more NO-producing neurons were activated during stress than during minimal or moderate levels of stimulation. Together, the evidence suggests that NO is a neurochemical messenger that is utilized by individual autonomic neurons as the organism responds to increased levels of sympathetic activity.