Evidence for projections from the rostral medullary raphe onto medullary catecholamine neurons in the rat

Afferent and efferent connections of C1 cells with spinal cord or hypothalamic projections in mice

The axonal projections and synaptic input of the C1 adrenergic neurons of the rostral ventrolateral medulla (VLM) were examined using transgenic dopamine-beta hydroxylase Cre mice and modified rabies virus. Cre-dependent viral vectors expressing TVA (receptor for envelopeA) and rabies glycoprotein were injected into the left VLM. EnvelopeA-pseudotyped rabies-EGFP glycoprotein-deficient virus (rabies-EGFP) was injected 4-6 weeks later in either thoracic spinal cord (SC) or hypothalamus. TVA immunoreactivity was detected almost exclusively (95 %) in VLM C1 neurons. In mice with SC injections of rabies-EGFP, starter cells (expressing TVA + EGFP) were found at the rostral end of the VLM; in mice with hypothalamic injections starter C1 cells were located more caudally. C1 neurons innervating SC or hypothalamus had other terminal fields in common (e.g., dorsal vagal complex, locus coeruleus, raphe pallidus and periaqueductal gray matter). Putative inputs to C1 cells with SC or hypothalamic projections originated from the same brain regions, especially the lower brainstem reticular core from spinomedullary border to rostral pons. Putative input neurons to C1 cells were also observed in the nucleus of the solitary tract, caudal VLM, caudal spinal trigeminal nucleus, cerebellum, periaqueductal gray matter and inferior and superior colliculi. In sum, regardless of whether they innervate SC or hypothalamus, VLM C1 neurons receive input from the same general brain regions. One interpretation is that many types of somatic or internal stimuli recruit these neurons en bloc to produce a stereotyped acute stress response with sympathetic, parasympathetic, vigilance and neuroendocrine components.

Role of the red nucleus in suppressing the jaw-opening reflex following stimulation of the raphe magnus nucleus

In a previous study, we found that electrical and chemical stimulation of the red nucleus (RN) suppressed the high-threshold afferent-evoked jaw-opening reflex (JOR). It has been reported that the RN receives bilaterally projection fibers from the raphe magnus nucleus (RMg), and that stimulation of the RMg inhibits the tooth pulp-evoked nociceptive JOR. These facts imply that RMg-induced inhibition of the JOR could be mediated via the RN. The present study first examines whether stimulation of the RMg suppresses the high-threshold afferent-evoked JOR. The JOR was evoked by electrical stimulation of the inferior alveolar nerve (IAN), and was recorded as the electromyographic response of the anterior belly of the digastric muscle. The stimulus intensity was 4.0 (high-threshold) times the threshold. Conditioning electrical stimulation of the RMg significantly suppressed the JOR. A further study then examined whether electrically induced lesions of the RN or microinjection of muscimol into the RN affects RMg-induced suppression of the JOR. Electrically induced lesions of the bilateral RN and microinjection of muscimol into the bilateral RN both reduced the RMg-induced suppression of the JOR. These results suggest that RMg-induced suppression of the high-threshold afferent-evoked JOR is mediated by a relay in the RN.

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.

Anatomic relationships of the human nucleus paragigantocellularis lateralis: a DiI labeling study

The nucleus paragigantocellularis lateralis (PGL) is located in the rostral ventrolateral medulla (RVLM), a brainstem region that regulates homeostatic functions, such as blood pressure and cardiovascular reflexes, respiration. central chemosensitivity and pain. In the present study, we examined anatomic relationships of the human nucleus paragigantocellularis lateralis using a bidirectional lipophilic fluorescent tracer, 1,1'-dioctadecyl-3,3.3',3'-tetramethylindocarbocyanine perchlorate (DiI), in nine postmortem human fetal midgestational brainstems. The areas which were labeled by diffusion of DiI from the nucleus paragigantocellularis lateralis included the arcuate nucleus (ARC) of the medulla, caudal raphe (nucleus raphe obscurus and pallidus), hilum and amiculum of the inferior olive, bilateral "reticular formation" (including the nucleus paragigantocellularis lateralis, nucleus gigantocellular-is and the intermediate reticular zone (IRZ)). vestibular and cochlear nuclei, cells and fibers at the floor of the fourth ventricle with morphologic features of tanycytes, parabrachial nuclei (PBN), medial lemniscus, lateral lemniscus, inferior cerebellar peduncle and cerebellar white matter, central tegmental tract, and the capsule of the red nucleus. This pattern of DiI labeling bears many similarities with the pattern of connections of the nucleus paragigantocellularis lateralis previously demonstrated by tract-tracing methods in experimental animals, and is consistent with the role of the nucleus paragigantocellularis lateralis in central regulation of homeostatic functions. In contrast to the animal studies, however, we did not demonstrate connections of the nucleus paragigantocellularis lateralis with the nucleus of the tractus solitarius (nTS) (only connections with the rostral subdivision were examined), locus coeruleus, or the periaqueductal gray (PAG) in the human midgestational brainstem. In our previous studies, six medullary areas showed reduced serotonin receptor binding in a subset of victims of sudden infant death syndrome (SIDS). The present study demonstrated DiI labeling in all of these six areas, suggesting that they are interconnected.

Sympathoinhibition from ventrolateral periaqueductal gray mediated by 5-HT(1A) receptors in the RVLM

The role of 5-hydroxytryptamine 1A (5-HT(1A)) receptors located in the rostral ventrolateral medulla (RVLM) in the mediation of a sympathoinhibitory and depressor response elicited from the ventrolateral periaqueductal gray (vlPAG) matter of the midbrain was examined in pentobarbital sodium-anesthetized rats. Activation of neurons in the vlPAG evoked a decrease in renal and lumbar sympathetic nerve activities and a decrease in arterial blood pressure. After microinjection of the specific 5-HT(1A)-receptor antagonist WAY-100635 into the pressor area of the RVLM, the vlPAG-evoked sympathoinhibition and hypotension was attenuated to control levels (7 of 15 animals) or converted into a sympathoexcitation and pressor response (8 of 15 animals). Baroreflex inhibition of sympathetic nerve activity was not impaired by microinjection of WAY into the sympathoexcitatory region of the RVLM. These data suggest that sympathoinhibition and hypotension elicited by activation of neurons in the vlPAG are mediated by 5-HT(1A) receptors in the RVLM.

Central actions of nitric oxide in regulation of autonomic functions

The identification of nitric oxide (NO) as a gaseous, nonconventional neurotransmitter in the central nervous system has led to an explosion of studies aimed at learning about the roles of NO, not only at a cellular level, but also in regulating the activity of specific physiological systems that are coordinated by the brain. In the 1980s, publications began to appear which pointed to a role for NO in regulating peripheral autonomic function. In the 1990s, it became apparent that NO also acts centrally to affect autonomic responses. In this review, I will discuss the state of the current knowledge about the central role of NO in physiological functions which are related specifically to the control of sympathetic output. Studies which do not differentiate a central from a peripheral role for NO in these functions have not been included. After a brief discussion about the cellular events in which NO is involved, the distribution of NO-producing neurons in central autonomic areas of the brain will be presented. The more general actions of central NO in regulating sympathetic activity, as assessed with i.c.v. injections of pharmacological agents, will be followed by more specific sites of action achieved with microinjections into discrete brain areas. The review will be concluded with discussions about central NO in two physiological states of sympathetic imbalance, hypertension and stress.

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.

Periaqueductal gray matter input to cardiac-related sympathetic premotor neurons

The periaqueductal gray matter (PAG) serves as the midbrain link between forebrain emotional processing systems and motor pathways used in the defense reaction. Part of this response depends upon PAG efferent pathways that modulate cardiovascular-related sympathetic outflow systems, including those that regulate the heart. While it is known that the PAG projects to vagal preganglionic neurons, including possibly cardiovagal motoneurons, no information exists on the PAG circuits that may affect sympathetically mediated cardiac functions and, thus, the purpose of this study was to use neuroanatomical methods to identify these pathways. First, viral transneuronal retrograde tracing experiments were performed in which pseudorabies virus (PRV) was injected into the stellate ganglion of rats. After 4 days survival, five PAG regions contained transynaptically infected neurons; these included the dorsomedial, lateral and ventrolateral PAG columns as well as the Edinger-Westphal and precommissural nuclei. Second, the descending efferent PAG projections were studied with the anterograde axonal marker Phaseolus vulgaris leuco-agglutinin (PHA-L) with a particular focus on determining whether the PAG projects to the intermediolateral cell column (IML). Almost no axonal labeling was found throughout the thoracic IML suggesting that the PAG modulates sympathetic functions by indirect pathways involving synaptic relays through sympathetic premotor cell groups, especially those found in the medulla oblongata. This possibility was examined by a double tracing study. PHA-L was first injected into either the lateral or ventrolateral PAG and after 6 days, PRV was injected into the ipsilateral stellate ganglion. After an additional 4 days survival, a double immunohistochemical procedure for co-visualization of PRV and PHA-L was used to identify the sympathetic premotor regions that receive an input from the PAG. The PAG innervated specific groups of sympathetic premotor neurons in the hypothalamus, pons, and medulla as well as providing reciprocal intercolumnar connections within the PAG itself (Jansen et al., Brain Res. 784 (1998) 329-336). The major route terminates in the ventral medulla, especially within the medial region which contains sympathetic premotor neurons lying within the raphe magnus and gigantocellular reticular nucleus, pars alpha. Both serotonergic and non-serotonergic sympathetic premotor neurons in these two regions receive inputs from the PAG. Weak PAG projections to sympathetic premotor neurons were found in the rostral ventrolateral medulla (including to C1 adrenergic neurons), locus coeruleus, A5 cell group, paraventricular and lateral hypothalamic nuclei. In summary, both the lateral and ventrolateral PAG columns appear to be capable of modulating cardiac sympathetic functions via a series of indirect pathways involving sympathetic premotor neurons found in selected sites in the hypothalamus, midbrain, pons, and medulla oblongata, with the major outflow terminating in bulbospinal regions of the rostral ventromedial medulla.

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.

Internal connections in the rostral ventromedial medulla of the rat

Physiological and pharmacological data suggest that the rostral ventromedial medulla (RVM) is an important site where integration between somatic and visceral functions might occur. The aim of the present study was to describe the interconnections between various nuclei of the rostral ventromedial medulla and thus reveal the possible anatomical basis for such functional interactions. The topography of anterogradely labelled internal projections was examined following iontophoretic microinjections of Phaseolus vulgaris leucoagglutinin (PHA-L). The results revealed that the nuclei of the rostral ventromedial medulla have strong interconnections and, to varying degrees, they also have bilateral projections into the rostral ventrolateral medulla. A particularly dense projection to widespread regions of the ventral medulla was traced from the raphe obscurus. Terminals, originating from the raphe pallidus were similarly dispersed but very low density in comparison. The focus of the projections of the gigantocellular nucleus pars ventralis and pars alpha shifted from the lateral paragigantocellular nucleus towards the RVM in rostral direction. Connections from the raphe magnus were altogether restricted to the RVM and the medial aspects of the lateral paragigantocellular nucleus. The diffuse and dense intramedullary connections of the raphe obscurus suggest that it might have an important role in coordinating the activity of rostral ventral medullary cells. The raphe pallidus and the ventral gigantocellular nuclei, areas that were innervated from widespread regions of the rostral ventral medulla but gave only limited projections there, are more likely to be involved in the direct descending control of spinal activities.

Raphe magnus and reticulospinal actions on primary afferent depolarization of group I muscle afferents in the cat

1. In the anaesthetized cat, electrical stimulation of the bulbar reticular formation produced a short latency (2.1 +/- 0.3 ms) positive potential in the cord dorsum. In contrast, stimulation of the nucleus raphe magnus with strengths below 50 microA evoked a slow negative potential with a mean latency of 5.5 +/- 0.6 ms that persisted after sectioning the contralateral pyramid and was abolished by sectioning the ipsilateral dorsolateral funiculus. 2. The field potentials evoked by stimulation of the bulbar reticular formation and of the nucleus raphe magnus had a different intraspinal distribution, suggesting activation of different sets of segmental interneurones. 3. Stimulation of these two supraspinal nuclei produced primary afferent depolarization (PAD) in single Ib fibres and inhibited the PAD elicited by group I volleys in single Ia fibres. The inhibition of the PAD of Ia fibres produced by reticulospinal and raphespinal inputs appears to be exerted on different interneurones along the PAD pathway. 4. It is concluded that, although reticulospinal and raphespinal pathways have similar inhibitory effects on PAD of Ia fibres, and similar excitatory effects on the PAD of Ib fibres, their actions are conveyed by partly independent pathways. This would allow their separate involvement in the control of posture and movement.

Decreased hypothalamic epinephrine concentration by quipazine and other serotonin agonists in rats

Epinephrine concentrations in rat hypothalamus were decreased after the injection of quipazine, a direct-acting serotonin (5-HT) agonist. The decrease was statistically significant and dose-dependent from 0.1 to 10 mg/kg, s.c., was apparent within 1 hr, and persisted for 8 hr but not 24 hr. There was no decrease in epinephrine concentrations in rat medulla oblongata, a region containing epinephrine cell bodies. Epinephrine concentrations in rat hypothalamus were also decreased by 1-(m-trifluoromethylphenyl)-piperazine (TFMPP) and by 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), other direct-acting 5-HT agonists, and by d-fenfluramine, a 5-HT-releasing drug. The decrease evoked by quipazine was prevented by pretreatment with metergoline, ketanserin or LY53857 (6-methyl-1-[methylethyl]-ergoline-8-carboxylic acid 2-hydroxy-1-methyl-propyl ester), centrally acting 5-HT antagonists. The lowering of rat hypothalamic epinephrine concentrations by 8-OH-DPAT was prevented by pretreatment with pindolol, a centrally acting 5-HT1A receptor antagonist. These data suggest that serotonergic drugs affect epinephrine concentrations in rat hypothalamus.

A1 catecholamine cell group: fine structure and synaptic input from the nucleus of the solitary tract

Preembedding immunoperoxidase staining methods were used to characterize tyrosine hydroxylase-immunoreactive (TH-ir) elements in the caudal ventrolateral medulla, and to determine the extent to which neurons of the A1 cell group are directly innervated by projections of the nucleus of the solitary tract (NTS). TH-ir neurons in the A1 region were medium-sized and multipolar. They possessed rounded nuclei with infrequent invaginations, well-developed Golgi apparati, high cytoplasmic densities of mitochondria, and a low to moderate tendency for rough endoplasmic reticulum (RER) to align in parallel stacks. A1 cell bodies were commonly juxtaposed to TH-positive and TH-negative neurons, myelinated profiles, glia and/or vascular elements, but close membrane appositions were only seen with glial elements. Synaptic input to A1 neurons was predominantly asymmetric, provided virtually exclusively by non-TH-ir terminals, and directed principally to dendritic shafts; A1 somata are relatively sparsely innervated. In a second experiment, silver-intensified immunogold localization of TH-ir was combined with immunoperoxidase labeling for anterogradely transported Phaseolus vulgaris-leucoagglutinin (PHA-L), following tracer injections in the caudal aspect of the medial division of the NTS. These experiments revealed a small proportion of PHA-L-labeled axon terminals that made asymmetric contacts with dendritic shafts of TH-ir neurons. These results suggest that the fine structure and synaptic input of A1 neurons are somewhat distinct from that of rostrally situated C1 catecholamine cells. In addition, while they document a direct NTS-A1 projection that may participate in the interoceptive control of vasopressin secretion, the bulk of ventrolaterally directed projections from the caudomedial NTS contact noncatecholaminergic elements in the A1 region, some of which may correspond to so-called depressor neurons implicated in the baroreflex control of sympathetic outflow and vasopressin secretion.

Immunolocalization of putative neurotransmitters innervating autonomic regulating neurons (correction of neurones) of cat ventral medulla

This study investigated possible sites of contact of nerve fibers containing a range of putative neurotransmitter substances onto neurons in the cat ventral medulla oblongata concerned with autonomic, particularly cardiovascular, regulation. The parasympathetic preganglionic neurons of the nucleus ambiguous (correction of ambiguus) were identified by retrograde horseradish peroxidase tracing from the vagus nerve, and the groups of neurons in the A1 and C1 cell areas and the raphe nucleus by catecholamine enzyme or 5-hydroxytryptamine (5-HT) immunohistochemistry, respectively. Immunoreactive (-ir)nerve fibers and terminals in the vicinity if these neurons were visualized by subjecting the sections to a dual-staining technique using a brown peroxidase-diaminobenzidine reaction product and a blue alkaline phosphatase-Fast blue reaction product. By employing monochrome photography with combinations of blue and orange-red filters, it was possible to discriminate neural elements displaying one or the other reaction product, or colocalization of reaction products. The results revealed the presence of calcitonin gene-related peptide (CGRP) and galanin (GAL)-ir in some motoneurons of the nucleus ambiguus, but not in those innervating the heart via the cardiac vagus nerve. The latter group of parasympathetic efferent neurons were found to be densely innervated by fibers immunoreactive for dopamine beta-hydroxylase (DBH, indicating noradrenaline), glycine (GLY), gamma-aminobutyric acid (GABA), 5-HT, enkephalin (ENK), neuropeptide Y (NPY), substance P (SP), and thyrotropin-releasing hormone (TRH), and, to a lesser extent, by other neuropeptide-ir fibers. The catecholamine cells of the rostral C1 and caudal A1 groups showed a broadly similar pattern of innervation, most noticeably by fibers immunoreactive for DBH, GABA, 5-HT, cholecystokinin (CCK), CGRP, ENK, GAL, NPY, and SP. The 5-HT-ir neurons of the raphe nucleus, some also containing SP, TRH, ENK, or corticotropin-releasing factor (CRF)-ir, were most prominently innervated by terminals containing DBH, GABA, CCK, ENK, NPY, TRH, somatostatin (SRIF), and vasoactive intestinal polypeptide (VIP)-ir. Although the proof that these groups of neurons receive functional synaptic contacts from the immunoreactive fibers awaits further ultrastructural studies, the results do suggest that a wide range of putative transmitters may influence the activity of efferent neurons in the cat medulla controlling autonomic functions.

Caudal raphe-dorsal vagal complex peptidergic projections: role in gastric vagal control

Subpopulations of raphe pallidus (Rpa) and raphe obscurus (Rob) neurons containing TRH, serotonin (5-HT), and substance P contribute projections to the dorsal vagal complex (DVC). Activation of Rpa and Rob neurons induces a vagal cholinergic-dependent stimulation of gastric secretory and motor function and modulates resistance of the gastric mucosa to gastric injury in rats and cats. The caudal raphe nuclei-DVC pathways containing TRH/5-HT are involved in mediating cold-induced vagal stimulation of gastric function and erosion formation. These results suggest that Rpa/Rob-DVC projections containing TRH/5-HT may be an important pathways in the medullary regulation of vagal activity to the viscera.

Neurones in the medullary raphe nuclei attenuate the cardiovascular responses evoked from the dorsolateral periaqueductal grey matter

In rats anaesthetised with alphaxalone/alphadolone, electrical stimulation in the dorsolateral part of the periaqueductal grey matter (PAG; 10 s trains of 1 ms pulses at 80 Hz, 40-80 microA) evoked a pressor response accompanied by tachycardia. Both components of the response were attenuated following microinjection of 200 nl 0.1 M D,L-homocysteic acid into the caudal pole of the nucleus raphe magnus (NRM; n = 12) and into the nucleus raphe obscurus (NRO; n = 22) to selectively activate neuronal perikarya. Microinjection of 200 nl 165 mM NaCl into the same region (n = 15) had no effect. The attenuation of the midbrain-evoked cardiovascular responses lasted for 10-20 min and was independent of changes in resting blood pressure and heart rate. The maximum reduction in the pressor component of the midbrain-evoked responses was similar following stimulation in NRM (-35.4%) and NRO (-36.7%). However, the reduction in the midbrain-evoked tachycardia was greater following stimulation in NRM (-62.8%) compared to NRO (-27.2%). These results indicate that neurones in NRM and NRO may be involved in modulating the level of excitability of the midbrain defence area in the PAG and/or its efferent pathway.

Electrophysiological effects of serotonin in the solitary tract nucleus of the rat

The nucleus tractus solitarius (nTS) is an important site for the integration of visceral information and its modification by afferent neural systems. One such afferent system arises from the raphe nuclei. This study investigated the electrophysiological effects of the primary transmitter of the raphe nuclei, serotonin (5-HT), on neurones in the nTS of the rat. Extracellular single unit recordings were made of the spontaneous activity of nTS neurones in isolated, superfused brainstem slices during bath-application of 5-HT (50, 100, 250, 500, 1000 nM). Twenty-seven of 46 neurones studied (approximately 59%) showed concentration-dependent decreases of firing rate, with a calculated EC50 of 261 nM. An additional 3 neurones displayed excitatory responses, while the remaining 16 were unaffected. The broad-spectrum 5-HT1 antagonist methysergide (200 nM) was highly effective in producing blockade of 5-HT-evoked depressions of neuronal activity (4 of 4), whereas the 5-HT1A receptor-selective antagonist spiroxatrine (5 nM) and the 5-HT2-selective antagonist mianserin (200 nM) were considerably less effective (1 of 5 and 2 of 5, respectively). Seven additional neurones were examined during exposure to the 5-HT1-selective agonist 5-carboxamidotryptamine (5-CT) or the 5-HT2-selective agonist DOI. 5-CT depressed the activity of three of the four neurones tested, the remaining neurone being unresponsive at concentrations up to 50 nM. DOI at concentrations up to 100 nM failed to affect the activity of two of the three neurones tested, and depressed the activity of the third at a concentration of 50 nM.(ABSTRACT TRUNCATED AT 250 WORDS)

Periaqueductal gray area and cardiovascular function

The periaqueductal gray (PAG) area seems to play an important role in modulating several biological functions such as the triggering of stereotyped defence and reproductive behaviour, pain, anxiety and cardiovascular and respiratory activities. Anatomically this midbrain area is made up of symmetric neuronal columns arranged along the long axis of the aqueduct. In this paper we review the most important findings of the last 10-15 years about the interaction between the PAG area and the cardiovascular function. It is shown that these neuronal columns within the PAG area exhibit a viscerotropic organization which elicits both hypertensive and hypotensive responses. In particular, the stimulation of the ventral neuronal column evokes a hypotensive response associated with a regional decrease in the vascular resistance. On the contrary, the stimulation of the dorsal and lateral neuronal columns evokes arterial hypertension associated with specific changes of the vascular resistance. Recently the authors demonstrated that the glutamergic system in the PAG area (prevalently through NMDA subtype receptor) may also be involved in the control of cardiovascular system. Moreover, the involvement of the arginine vasopressin neuropeptide in the hypertension induced by administration of excitatory amino acids into the PAG area has been demonstrated.

Role of norepinephrine in the interaction between the lateral reticular nucleus and the nucleus raphe magnus: an electrophysiological and behavioral study

We have previously demonstrated that the nucleus raphe magnus (NRM) sends a predominantly inhibitory projection to the lateral reticular nucleus (LRN); however, the pharmacology of this pathway is not known. The purpose of this study was to examine the role of norepinephrine in the NRM-LRN system using both electrophysiological and behavioral techniques. Sixty-nine LRN cells were recorded extracellularly. Cells were tested for their response to noxious and innocuous peripheral stimulation applied to the dorsal body surface. The majority of cells were classified as wide dynamic range, with inhibition being the predominant response; receptive fields were located primarily on the tail and hind limbs. The effect of excitatory amino acid glutamate (GLU) administration into NRM (GLU-NRM) was tested on all 69 cells. GLU-NRM inhibited 55 of 69 LRN cells tested; 7 cells were excited and 7 cells did not respond. Thirty-nine LRN cells were tested for their response to norepinephrine (NE) iontophoretically applied in LRN (NE-LRN). Two distinct types of effects were noted. In 9 cells, both NE-LRN and GLU-NRM produced a strong inhibition, with the magnitude of effect between the 2 drugs significantly correlated. In a second group of cells (n = 12), GLU-NRM produced an inhibitory effect while NE-LRN had no effect on the cells' baseline firing rate. However, when the 2 drugs were applied simultaneously, NE-LRN blocked the inhibitory effects of NRM stimulation. The effect of the alpha 2-receptor antagonist yohimbine (YOH) on NRM-evoked responses was tested in 30 LRN cells. The majority of these cells were inhibited by GLU-NRM. Similar to the dichotomous effect noted by NE-LRN, YOH applied iontophoretically in LRN (YOH-LRN) had two predominant effects on NRM-produced inhibition. In 14 of 27 cells, YOH-LRN significantly potentiated the inhibitory effects of NRM stimulation by increasing the duration of the inhibitory epoch an average of 100 sec. In 7 of 27 cells, YOH directly applied in LRN partially antagonized NRM-evoked inhibition. In a second series of experiments, microinjection cannulas were placed within NRM and LRN in order to determine the effect of blocking alpha 2-receptor activity within LRN on NRM stimulation-produced analgesia in an intact animal. Administration of D,L-homocysteic acid in NRM resulted in a significant increase in baseline tail-flick latency of approximately 140%. Pretreatment with YOH (3 micrograms in 0.5 microliter) in LRN resulted in a significant potentiation of this analgesic effect.(ABSTRACT TRUNCATED AT 400 WORDS)

Innervation of serotonergic medullary raphe neurons from cells of the rostral ventrolateral medulla in rats

The rostral ventral medulla has been shown to consist of three distinct subregions: the midline or raphé region, the lateral paragigantocellular-gigantocellular region and the rostro-ventrolateral reticular nucleus. All three regions have been shown to contribute to central vaso-regulation and to project towards sympathetic preganglionic neurons of the thoracic spinal cord. Therefore it is of particular interest to describe the interconnections between the three regions and to see if local afferents reach cells which have been implicated in the regulation of descending inputs. Following injections of the anterograde tract tracer Phaseolus vulgaris leucoagglutinin into the lateral paragigantocellular nucleus or the rostroventrolateral reticular nucleus, labelled axons were traced into the medullary raphé nuclei and the contralateral rostral ventrolateral medulla. Efferents originating from both regions innervated the raphé pallidus, raphé obscurus and raphé magnus. However the distribution of terminals originating from the two regions was different in the contralateral ventrolateral medulla oblongata. The data indicate that the connection between the ipsi- and contralateral equivalents of both the lateral paragigantocellular-gigantocellular region and the rostroventrolateral reticular nucleus are stronger than the cross-connection between the ipsi- and contralateral parts of the two different regions. In the second part of the study, the existence of direct projections from the rostroventrolateral reticular nucleus and the lateral paragigantocellular-gigantocellular region onto serotonin-immunogold-labelled cells of the ventromedial medulla were investigated. The correlated light and electron microscopic analysis revealed direct synaptic contacts between axons originating from both the lateral paragigantocellular-gigantocellular region and the rostroventrolateral reticular nucleus, and serotonin-immunoreactive cells of the raphé obscurus and raphé pallidus. The results of the present light microscopic tract-tracing study revealed a different pattern of the intramedullary projection of the lateral paragigantocellular-gigantocellular region and the rostroventrolateral reticular nucleus. These data are in support of the proposed parcellation of the two cytoarchitectonically different areas of the rostral ventrolateral medulla into two functionally distinct subdivisions. Furthermore, the direct anatomical connection revealed in the present study between cells of the rostral ventrolateral and ventromedial medulla oblongata indicates the possibility that vasoregulatory effects of some cells of the rostral ventrolateral medulla oblongata might be executed via direct projections onto serotonin-immunoreactive cells of the medullary raphé nuclei.

Electrophysiological characterization of the projection from the nucleus raphe magnus to the lateral reticular nucleus: possible role of an excitatory amino acid in synaptic activation

Numerous studies have shown that the lateral reticular nucleus (LRN), located in the caudal ventrolateral medulla, is an important nuclear region in the descending analgesia system. Activation of this brainstem region, either electrically or chemically, results in a reduction in nociceptive threshold. In addition, destruction of LRN abolishes the tonic descending inhibition present on dorsal horn neurons. Recent neuroanatomical tracing studies have shown that the nucleus raphe magnus (NRM), long implicated in nociception, sends direct projections to LRN; however, no information exists regarding the physiological characteristics of this pathway, nor its role in the endogenous descending analgesia system. The purpose of this study was to physiologically characterize the synaptic influence(s) of projections from the NRM to the LRN using electrophysiological recording, electrical and chemical stimulation, and iontophoretic techniques. Sixty-one percent of LRN neurons responded to single pulse stimulation of NRM; 52% of the responsive cells were excited and 48% were inhibited. The mean latency to onset of excitation was 4.9 +/- 1.2 ms. High frequency (100 Hz) electrical stimulation of NRM influenced 69/102 neurons; 52% (36/69) were excited, while 48% (33/69) were inhibited. Microinjection of glutamate into NRM significantly modified the discharge of 83% (93/112) of LRN cells tested; of these, 71% were inhibited, while 29% were excited. In 35 cells the effects of the excitatory amino acid antagonist kynurenic acid (KYN) were studied. In 75% of the cells excited by glutamate administration into the NRM (18/24), KYN partially antagonized this response. In 11 LRN cells inhibited by NRM chemical stimulation, KYN had no effect on this inhibition. Overall, 95% of the LRN cells responsive to NRM stimulation were also responsive to noxious peripheral stimulation, indicating that these cells are receiving ascending information from the spinal cord regarding somatosensory stimulation as well as receiving descending input from the NRM. It is concluded that LRN neurons are highly responsive to both noxious peripheral stimulation and NRM efferent activation, and that this region plays a significant role as an integrator for both ascending and descending information.

Inhibitory modulation of the cardiovascular defence response by the ventrolateral periaqueductal grey matter in rats

In rats anaesthetised with alphaxalone/alphadolone, electrical stimulation in the dorsal periaqueductal grey matter (PAG) evoked a pressor response with tachycardia, vasodilatation in the hindlimb and hyperpnoea: a pattern of response known as the defence reaction. Microinjection of the synaptic excitant, D,L-homocysteic acid (DLH), but not saline, into the ventrolateral PAG at the level of the decussation of the superior cerebellar peduncle (approximately 7.3-8.3 mm caudal to bregma) produced a reduction in the size of the cardiovascular components of the defence reaction evoked by electrical stimulation in the dorsal PAG. Injections of DLH made outside this region had no effect on the defence response. Injection of DLH into the "defence inhibition area" had no effect on the pressor response evoked distally in the efferent pathway for the defence reaction, by electrical stimulation in the rostral ventrolateral medulla (RVLM). Activation of neurones in a restricted portion of the caudal ventrolateral PAG appears to modulate activity in the descending pathway for the defence response evoked from the dorsal PAG. It is argued that the inhibitory interaction probably occurs at the level of synapses in the RVLM.

Projections from the rostral ventrolateral medulla to brainstem monoamine neurons in the rat

Following the iontophoretic deposition of Phaseolus vulgaris leucoagglutinin (PHA-L) into the rostral ventrolateral medulla (RVL), two-color immunoperoxidase staining was employed to demonstrate contiguity between PHA-L-immunoreactive (PHA-LI) varicose fibers and boutons and brainstem monoaminergic cells. Black-stained PHA-LI cells in the deposition site were found to be located among amber-stained phenylethanolamine N-methyl transferase-immunoreactive (PNMT-I) neurons of the C1 cell group. RVL projections were contiguous with PNMT-I neurons of the C1, C2 and C3 cell groups, with tyrosine hydroxylase-immunoreactive (TH-I) neurons of the A1, A2 and A5 cell groups, and with serotonin-immunoreactive (5-HT-I) neurons of the B1, B2 and B3 cell groups. Preliminary findings of this study have been presented previously (Soc. Neurosci. Abstr., 15 (1989) 451).