Distribution of neuroactive substances in the dorsal vagal complex of the medulla oblongata

Dedicated C-fiber vagal sensory afferent pathways to the paraventricular nucleus of the hypothalamus

The nucleus of the solitary tract (NTS) receives viscerosensory information from the vagus nerve to regulate diverse homeostatic reflex functions. The NTS projects to a wide network of other brain regions, including the paraventricular nucleus of the hypothalamus (PVN). Here we examined the synaptic characteristics of primary afferent pathways to PVN-projecting NTS neurons in rat brainstem slices.Expression of the Transient Receptor Potential Vanilloid receptor (TRPV1+ ) distinguishes C-fiber afferents within the solitary tract (ST) from A-fibers (TRPV1-). We used resiniferatoxin (RTX), a TRPV1 agonist, to differentiate the two. The variability in the latency (jitter) of evoked excitatory postsynaptic currents (ST-EPSCs) distinguished monosynaptic from polysynaptic ST-EPSCs. Rhodamine injected into PVN was retrogradely transported to identify PVN-projecting NTS neurons within brainstem slices. Graded shocks to the ST elicited all-or-none EPSCs in rhodamine-positive NTS neurons with latencies that had either low jitter (<200 µs - monosynaptic), high jitter (>200 µs - polysynaptic inputs) or both. RTX blocked ST-evoked TRPV1 + EPSCs whether mono- or polysynaptic. Most PVN-projecting NTS neurons (17/21 neurons) had at least one input polysynaptically connected to the ST. Compared to unlabeled NTS neurons, PVN-projecting NTS neurons were more likely to receive indirect inputs and be higher order. Surprisingly, sEPSC rates for PVN-projecting neurons were double that of unlabeled NTS neurons. The ST synaptic responses for PVN-projecting NTS neurons were either all TRPV1+ or all TRPV1-, including neurons that received both direct and indirect inputs. Overall, PVN-projecting NTS neurons received direct and indirect vagal afferent information with strict segregation regarding TRPV1 expression.

Distinct Calcium Sources Define Compartmentalized Synaptic Signaling Domains

Nervous system communication relies on neurotransmitter release for synaptic transmission between neurons. Neurotransmitter is contained within vesicles in presynaptic terminals and intraterminal calcium governs the fundamental step of their release into the synaptic cleft. Despite a common dependence on calcium, synaptic transmission and its modulation varies highly across the nervous system. The precise mechanisms that underlie this heterogeneity, however, remain unclear. The present review highlights recent data that reveal vesicles sourced from separate pools define discrete modes of release. A rich diversity of regulatory machinery may further distinguish the different forms of vesicle release, including presynaptic proteins involved in trafficking, alignment, and exocytosis. These multiple vesicle release mechanisms and vesicle pools likely depend on the arrangement of vesicles in relation to specific calcium entry pathways that create compartmentalized spheres of calcium influence (i.e., domains). This diversity permits release specialization. This review details examples of how individual neurons rely on multiple calcium sources and unique regulatory schemes to provide differential release and discrete modulation of neurotransmitter release from specific vesicle pools-as part of network signal integration.

5-HT3R-sourced calcium enhances glutamate release from a distinct vesicle pool

The serotonin 3 receptor (5-HT₃R) is a calcium-permeant channel heterogeneously expressed in solitary tract (ST) afferents. ST afferents synapse in the nucleus of the solitary tract (NTS) and rely on a mix of voltage-dependent calcium channels (CaVs) to control synchronous glutamate release (ST-EPSCs). CaV activation triggers additional, delayed release of glutamate (asynchronous EPSCs) that trails after the ST-EPSCs but only from afferents expressing the calcium-permeable, transient receptor potential vanilloid type 1 receptor (TRPV1). Most afferents express TRPV1 and have high rates of spontaneous glutamate release (sEPSCs) that is independent of CaVs. Here, we tested whether 5-HT₃R-sourced calcium contributes to these different forms of glutamate release in horizontal NTS slices from rats. The 5-HT₃R selective agonist, m-chlorophenyl biguanide hydrochloride (PBG), enhanced sEPSCs and/or delayed the arrival times of ST-EPSCs (i.e. increased latency). The specific 5-HT₃R antagonist, ondansetron, attenuated these effects consistent with direct activation of 5-HT₃Rs. PBG did not alter ST-EPSC amplitude or asynchronous EPSCs. These independent actions suggest two distinct 5-HT₃R locations; axonal expression that impedes conduction and terminal expression that mobilizes a spontaneous vesicle pool. Calcium chelation with EGTA-AM attenuated the frequency of 5-HT₃R-activated sEPSCs by half. The mixture of chelation-sensitive and resistant sEPSCs suggests that 5-HT₃R-activated vesicles span calcium diffusion distances that are both distal (micro-) and proximal (nanodomains) to the channel. Our results demonstrate that the calcium domains of 5-HT₃Rs do not overlap other calcium sources or their respective vesicle pools. 5-HT₃Rs add a unique calcium source on ST afferents as part of multiple independent synaptic signaling mechanisms.

Haemorrhage-evoked compensation and decompensation are mediated by distinct caudal midline medullary regions in the urethane-anaesthetised rat

Previous research using microinjections of excitatory amino acids suggested that the caudal midline medulla (including nucleus raphe obscurus and nucleus raphe pallidus) contained a mixed population of sympathoexcitatory and sympathoinhibitory neurones. The results of this study indicate that different anaesthetic regimes (urethane versus halothane) determine whether sympathoexcitatory (urethane only) or sympathoinhibitory (halothane only) responses are evoked by stimulation within distinct caudal midline medullary regions. In addition, anaesthetic regimes also affect the caudal midline medullary-mediated response to haemorrhage. Specifically, under conditions of urethane anaesthesia, inactivation (lignocaine) of the midline medullary region immediately caudal to the obex, prematurely triggered and dramatically potentiated the hypotension and bradycardia evoked by 15% haemorrhage; whereas under halothane anaesthesia, inactivation of the same region had no effect. In contrast, under urethane anaesthesia, inactivation of the midline medullary region immediately rostral to the obex, delayed the onset of the hypotension and bradycardia to 15% haemorrhage; inactivation of the same region under halothane anaesthesia blocked haemorrhage-evoked hypotension and bradycardia. Our findings indicate that topographically distinct parts of the caudal midline medulla contain neurones (i) that differentially regulate the timing and magnitude of the compensatory (normotensive) versus decompensatory (hypotensive) phases of the response to haemorrhage; and (ii) whose activity is altered by urethane versus halothane anaesthesia.

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

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

Opioid receptor involvement in the adaptation to motion sickness in Suncus murinus

The aim of the present study was to investigate an opioid receptor involvement in the adaptation response to motion sickness in Suncus murinus. Different groups of animals were treated intraperitoneally with either saline, morphine (0.1 and 1.0 mg/kg), naloxone (1.0, 10.0 and 5.0 mg/kg) or a combination of naloxone plus morphine in the absence or 30 min prior to a horizontal motion stimulus of 1 Hz and 40 mm amplitude. For the study of adaptation, different groups received saline on the first trial, and in subsequent trials (every 2 days) they received either saline, naloxone (1.0 and 10.0 mg/kg, i.p.) or morphine (0.1 mg/kg, i.p.) 30 min prior to the motion stimulus. Pretreatment with morphine caused a dose-related reduction in emesis induced by a single challenge to a motion stimulus. Pretreatment with naloxone alone did not induce emesis in its own right nor did it modify emesis induced by a single challenge to a motion stimulus. However, pretreatment with naloxone (5.0 mg/kg, i.p.) revealed an emetic response to morphine (P<.001) (1.0 mg/kg, i.p.) and antagonised the reduction of motion sickness induced by morphine. In animals that received saline or naloxone (1.0 mg/kg), a motion stimulus inducing emesis decreased the responsiveness of animals to a second and subsequent motion stimulus challenge when applied every 2 days for 11 trials. However, the animals receiving naloxone 10.0 mg/kg prior to the second and subsequent challenges showed no significant reduction in the intensity of emesis compared to the first trial. The data are revealing of an emetic potential of morphine when administered in the presence of a naloxone pretreatment. The administration of naloxone is also revealing of an additional inhibitory opioid system whose activation by endogenous opioid(s) may play a role in the adaptation to motion sickness on repeated challenge in S. murinus.

Glutamate stimulation of raphe pallidus attenuates the cardiopulmonary reflex in anaesthetised rats

Serotonin has been implicated as having a modulatory action on NTS neurones mediating vagal inputs controlling the cardiorespiratory system. Since raphe pallidus and raphe obscurus both send serotonergic projections to the NTS, we have investigated a neuromodulatory role for these structures on the cardiopulmonary reflex. A multibarrelled microelectrode positioned around the level of the area postrema was placed at varying depths into mid-line brainstem structures and the effect of glutamate stimulation on the cardiopulmonary reflex tested. Excitatory chemical stimulation in the region of raphe pallidus, but not raphe obscurus, attenuated significantly the respiratory and bradycardic components of the cardiopulmonary reflex. This attenuation was reversed by an NTS microinjection of RS-39604, a selective 5-HT, receptor antagonist. We propose that neurones in raphe pallidus that project to the NTS can release serotonin which acts via 5-HT4 receptors to attenuate the reflex phrenic nerve activity and heart rate components of the cardiopulmonary reflex.

Substance P in the hypoglossal nucleus of the rat

The distribution of substance P (SP)-containing synaptic terminals in the hypoglossal nucleus (XII) of adult rats was examined by retrograde peroxidase labelling and immunocytochemistry. From the location of peroxidase injections into the tongue and of labelled neurones in the ventral lamina of XII, motor neurones that supply intrinsic vertical, longitudinal and transverse fibres as well as the extrinsic muscle genioglossus appear to have been labelled. SP-containing terminals were found making contact, and sometimes dual synapses, with unlabelled neuronal dendrites but not with retrogradely labelled somata or dendrites. These findings suggest that SP terminals may contact dendrites of interneurones or of neurones supplying other extrinsic muscles located in the anterior part of the tongue. Dual SP-containing synapses between XII motor neurones may be the means by which tongue muscle fibres are recruited and their function synchronized.

PE-11, a peptide derived from chromogranin B, in the human brain

This study was performed to investigate the distribution of chromogranin B in the human central nervous system. We used an antiserum raised against a synthetic peptide (PE-11) present in the chromogranin B molecule. PE-11-like immunoreactivity was characterized by molecular size exclusion and reversed-phase high-performance liquid chromatography. Its localization was studied using immunocytochemistry. Only the free peptide and an N-terminally elongated peptide were detected by molecular size exclusion high-performance liquid chromatography, indicating that proteolytic processing of chromogranin B is quite extensive. PE-11-like immunoreactivity was present in differently shaped fibers, varicosities and neurons, but not in glial cells. Its density varied throughout the brain. An especially high density was observed in the bed nucleus of the stria terminalis, the central and cortical nuclei of the amygdala, the hypothalamus, the hippocampus, the raphe complex, the nucleus interpeduncularis, the nucleus of the solitary tract, and laminae I and II of the spinal cord. This study demonstrates a significant processing of chromogranin B and indicates that chromogranin B constitutes a precursor for smaller peptides which are derived by endoproteolytic processing. It provides the neuroanatomical basis to investigate the chromogranin B molecule as a widespread component of large dense-core vesicles in the human central nervous system.

Direct synaptic projections to esophageal motoneurons in the nucleus ambiguus from the nucleus of the solitary tract of the rat

Neurons of the nucleus of the solitary tract (NTS) serve as interneurons in swallowing. We investigated the synaptology of the terminals of these neurons and whether they project directly to the esophageal motoneurons in the compact formation of the nucleus ambiguus (AmC). Following wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) injection into the NTS, many anterogradely labeled axodendritic terminals were found in the neuropil of the AmC. The majority of labeled axodendritic terminals (89%) contained round vesicles and made asymmetric synaptic contacts (Gray's type I), but a few (11%) contained pleomorphic vesicles and made symmetric synaptic contacts (Gray's type II). More than half of the labeled terminals contacted intermediate dendrites (1-2 microm diameter). There were no retrogradely labeled medium-sized motoneurons, but there were many retrogradely labeled small neurons having anterogradely labeled axosomatic terminals. A combined retrograde and anterograde transport technique was developed to verify the direct projection from the NTS to the esophageal motoneurons. After the esophageal motoneurons were retrogradely labeled by cholera toxin subunit B conjugated HRP, the injection of WGA-HRP into the NTS permitted ultrastructural recognition of anterogradely labeled axosomatic terminals contacting directly labeled esophageal motoneurons. Serial sections showed that less than 20% of the axosomatic terminals were labeled in the esophageal motoneurons. They were mostly Gray's type I, but a few were Gray's type II. In the small neurons, more than 30% of axosomatic terminals were labeled, which were exclusively Gray's type I. These results indicate that NTS neurons project directly not only to the esophageal motoneurons, but also to the small neurons which have bidirectional connections with the NTS.

Clonidine-induced emesis: a multitransmitter pathway concept

The emetic effect of clonidine injected into the cerebral ventricles through chronically implanted cannulae was investigated in unanaesthetized cats. Clonidine (0.1-300 micrograms) induced dose-dependent and shortlasting emesis. The emesis induced by the supramaximal dose of clonidine (100 micrograms) was not abolished after the ablation of area postrema. Both the alpha 2 adrenoceptor blocking agent idazoxan and the mixed alpha 1 and alpha 2 adrenoceptor antagonist phenoxybenzamine, injected intracerebroventricularly, attenuated or abolished the emesis induced by clonidine (100 micrograms). On the other hand, the alpha 2 adrenoceptor blocking agent yohimbine, the alpha 1 adrenoceptor blocking drug prazosin and the non-selective beta-adrenoceptor antagonist propranolol, injected into the cerebral ventricles, had no significant effect on clonidine-induced emesis. The antimuscarinic drug atropine injected into the cerebral ventricles prevented the clonidine-induced emesis in a dose-dependent manner. The dopamine antagonist chlorpromazine, the 5-hydroxytryptamine blocking agent methysergide and the histamine H1 and H2 receptor antagonists, antazoline and cimetidine, injected intracerebroventricularly reduced or abolished the emesis produced by clonidine. The ganglionic blocking substance mecamylamine and the opioid antagonist naloxone, all injected into the cerebral ventricles, had no significant effect on clonidine-induced emesis. In cats pretreated with the intracerebroventricular competitive inhibitor of the synthesis of catecholamines, alpha-methyl-p-tyrosine, as well as with the inhibitor of acetylcholine synthesis hemicholinium-3, the emesis caused by clonidine was depressed or abolished. The clonidine-induced emesis was also abolished when catecholamine stores were depleted by intracerebroventricular reserpine. However, the clonidine-induced emesis was not significantly changed when 5-hydroxylryptaminergic nerve terminals were damaged by 5,6-dihydroxytryptamine. It follows, therefore, that cholinergic and noradrenergic mechanisms are of basic importance for the emetic action of clonidine. With regard to receptors, the emesis induced by clonidine injected into the cerebral ventricles, is mediated at least in part through alpha-adrenoceptors, muscarinic cholinoceptors, 5-hydroxytryptamine receptors and H1 and H2 histamine receptors. These receptors appear to be located mostly presynaptically and they transmit emetic impulses to neurones integrating them into emesis. However, the direct effect of clonidine on postsynaptic receptors cannot be excluded, particularly when muscarinic and 5-hydroxytryptamine receptors are implicated. Taken together, these results point to the existence of a multitransmitter pathway/s outside the area postrema, subserving the central regulation of emesis.

Cardiovascular effects induced by the stimulation of neuropeptide FF receptors in the dorsal vagal complex: an autoradiographic and pharmacological study in the rat

Previous studies have suggested that central administration of neuropeptide FF-related peptides may modulate cardiovascular parameters in the rat. In the present study, we investigated the role of dorsal vagal complex neuropeptide FF receptors in the central regulation of cardiovascular parameters. The fate of neuropeptide FF receptors in normal and nodose ganglionectomized rats was investigated using an autoradiographic approach with 125I-[DTyr1, (NMe)Phe3]NPFF as ligand for these receptors. We showed that neuropeptide FF binding sites are preferentially located postsynaptically with respect to the vagal afferent fibers in the nucleus tractus solitarius. Thus, ganglionectomy reduced by only 30% and 17% the density of peptide binding sites in the rostral and caudal regions of this nucleus, respectively. Bilateral microinjection of neuropeptide FF (1 nmol) into the commissural nucleus tractus solitarius produced an increase in blood pressure (+13.8 +/- 0.8 mmHg, n = 6), bradycardia (-29.0 +/- 3.2 bpm) and a significant inhibition (-47.6 +/- 3.1%) of the cardiac component of the baroreceptor reflex. Further studies with doses below 1 nmol indicate that NTS microinjections of the neuropeptide produced a dose-dependent decrease in heart rate. Similar cardiovascular effects were observed after bilateral NTS microinjections of one analog neuropeptide FF receptor agonist, [DTyr1, (NMe)Phe3]NPFF (1 nmol). Pretreatment with prazosin (100 micrograms/kg), an alpha 1-adrenoreceptor antagonist, inhibited the neuropeptide FF-evoked blood pressure effect. In addition, the neuropeptide FF-induced heart rate decrease was abolished by pretreatment with atropine (30 micrograms/kg), a muscarinic receptor antagonist. Taken together, these anatomical and pharmacological data suggest that neuropeptide FF receptors within the nucleus tractus solitarius, preferentially located on the postsynaptic component, are involved in the central reflex regulation of cardiovascular parameters.

Neurochemical modulation of cardiovascular control in the nucleus tractus solitarius

The central control of cardiovascular function has been keenly studied for a number of decades. Of particular interest are the homeostatic control mechanisms, such as the baroreceptor heart-rate reflex, the chemoreceptor reflex, the Bezold-Jarisch reflex and the Breuer-Hering reflex. These neurally-mediated reflexes share a common termination point for their respective centrally-projecting sensory afferents, namely the nucleus tractus solitarius (NTS). Thus, the NTS clearly plays a critical role in the integration of peripherally initiated sensory information regarding the status of blood pressure, heart rate and respiratory function. Many endogenous neurochemicals, from simple amino acids through biogenic amines to complex peptides have the ability to modulate blood pressure and heart rate at the level of the NTS. This review will attempt to collate the current knowledge regarding the roles of neuromodulators in the NTS, the receptor types involved in mediating observed responses and the degree of importance of such neurochemicals in the tonic regulation of the cardiovascular system. The neural pathway that controls the baroreceptor heart-rate reflex will be the main focus of attention, including discussion of the identity of the neurotransmitter(s) thought to act at baroafferent terminals within the NTS. In addition, this review will provide a timely update on the use of recently developed molecular biological techniques that have been employed in the study of the NTS, complementing more classical research.

Neurotransmitter mechanisms of rat vagal afferent neurons

1. The present study has employed anatomical and neurochemical techniques to assess whether the amino acid, L-glutamate, may be considered as a potential neurotransmitter at rat vagal afferent neurons, with particular reference to baroreceptor afferents. 2. Slide-mounted sections of rat nodose ganglia were incubated with a high-titre antibody to glutamate, and visualization of the resulting immunoreactivity indicated glutamate-positive staining in a population of vagal afferent perikarya. In contrast, interstitial cells were devoid of immunostaining. 3. Release of endogenous glutamate was measured by in vivo microdialysis in the nucleus tractus solitarius, the site of central vagal afferent terminals, and could be evoked with a depolarizing stimulus of KCl in a calcium-dependent fashion. In addition, baroreceptor loading with an intravenous infusion of phenylephrine (30 micrograms/kg per min) increased the spontaneous efflux of glutamate to 148 +/- 28% of basal levels, which was paralleled by an increase in mean arterial pressure (approximately 40 mmHg). Release of glutamate was also elevated two-fold by intracerebral administration of S-nitroso-N-acetylpenicillamine (30 mumol/L), an effect that could be prevented by coadministration of methylene blue (10 mumol/L). 4. These data suggest that neuronal glutamate may be formed in a population of vagal afferent cell bodies, presumably to act on soma membrane receptors. Furthermore, the excitatory amino acid is released in a neurotransmitter-like fashion at the terminal region of vagal afferent neurons, where glutamate release is increased as a consequence of baroreceptor loading and also following activation of soluble guanylate cyclase. Thus, glutamate may be considered a candidate neurotransmitter of vagal baroreceptor afferent neurons, which may be modulated by nitric oxide or an endogenous nitrosothiol.

Increased sympathetic nerve discharge without alteration in the sympathetic baroreflex response by serotonin3 receptor stimulation in the nucleus tractus solitarius of the rat

Previous studies have shown that serotonin3 receptor activation in the nucleus tractus solitarius (NTS) increased mean arterial pressure (MAP) and inhibited the cardiac component of the baroreceptor reflex [9]. We have examined the effects of such stimulation upon spontaneous and evoked sympathetic nerve activity. Microinjection of serotonin (10 nmol) into the NTS of halothane-anaesthetized, paralyzed and artificially ventilated rats produced an increase in MAP and lumbar sympathetic nerve discharge which could be completely prevented by prior local microinjection of zacopride, a potent serotonin3 antagonist (200 pmol). In addition, 1-(m-chlorophenyl)-biguanide, a selective serotonin3 receptor agonist, mimicked the sympathoexcitatory effect of serotonin. Since the gain of the sympathetic component of the baroreflex was unaltered after intra-NTS microinjection of serotonin, it could be concluded that serotonin3 receptors activation in the NTS induces a sympathetic activation which is not mediated through an inhibition of the sympathetic baroreceptor reflex arc.

Effects of microinjection of N-methyl-D-aspartic acid into the nucleus tractus solitarii on cerebral blood flow in anesthetized rats

N-Methyl-D-aspartic acid (NMDA) (10 pmol in 100 nl of 0.9% sodium chloride solution) was microinjected into the nucleus tractus solitarii (NTS) of urethane-anesthetized, paralyzed and artificially ventilated rats, and cerebral blood flow (CBF) was determined using a combination of labeled microspheres. Moderate hypertension within the upper limit of cerebral autoregulation was induced by blood transfusion in order to measure CBF at normotension. Arterial blood pressure (ABP) was decreased by unilateral microinjection into the NTS in these rats but remained within normotensive range. The CBF in the cerebral cortex ipsilateral to the stimulated NTS significantly (P < 0.01) decreased from 38 +/- 4 (mean +/- S.E.M) to 27 +/- 4 ml.min-1.(100 g)-1(n = 9). The cerebrovascular resistance (CVR) in the cerebral cortex ipsilateral to the stimulated NTS significantly (P < 0.01) increased from 2.6 +/- 0.3 to 4.1 +/- 0.7 mmHg per [ml.min-1.(100 g)-1]. Blockade of NMDA receptors in the NTS with D,L-2-amino-5-phosphonovalerate (AP5, 500 pmol) abolished the CBF decrease and CVR increase responses elicited by microinjection of NMDA into the NTS (n = 9). Blockade of non-NMDA receptors in the NTS with 6,7-dinitro-quinoxaline-2,3-dione (DNQX, 100 pmol) had little effect on the CBF decrease and CVR increase responses elicited by microinjection of NMDA into the NTS (n = 10). Microinjection of the vehicle solution into the NTS had no effects on cerebral circulation (n = 7). Cerebral autoregulation was well maintained at moderate hypertension induced by blood transfusion and at normotension returned from moderate hypertension following controlled hemorrhage (n = 8). These results suggest that the NMDA receptors in the NTS may be involved in the control of cerebral circulation.

Distribution of secretoneurin-like immunoreactivity in comparison with that of substance P in the human brain stem

Secretoneurin is a peptide of 33 amino acids generated in the brain by proteolytic processing of secretogranin II which is a member of the chromogranin/secretogranin family. The distribution of this newly characterized peptide was investigated by immunocytochemistry in the human brain stem. The staining pattern of secretoneurin-like immunoreactivity was compared with that of substance P in adjacent sections. Secretoneurin-like immunoreactivity appeared mainly in dot- and fiber-like structures with densities varying from low to very high. Only a low number of secretoneurin-immunoreactive perikarya was found. Pericellular staining of both secretoneurin-immunopositive and immunonegative cells was frequently observed in the area of the central gray, in the reticular formation and in the solitary nuclear complex. The medial part of the substantia nigra pars reticulata, the nucleus interpeduncularis, the area of the central gray, the raphe complex and the inferior olive displayed a high density of secretoneurin-like immunoreactivity. Furthermore, a very prominent staining was found in the medial, dorsal and gelatinous subnuclei of the solitary tract and the dorsal motor nucleus of vagus. The substantia gelatinosa of the caudal trigeminal nucleus and spinal cord were also very strongly secretoneurin-immunopositive. The staining patterns of secretoneurin- and substance P-like immunoreactivities were to a certain extent overlapping in several areas. The highest degree of coincidence was found in the substantia gelatinosa. This study demonstrated that secretoneurin is distinctly distributed in the human brain stem. Its distributional pattern indicates a role particularly in the modulation of afferent pain transmission and in the regulation of autonomic functions.

Neuropeptides in the human dorsal vagal complex: an immunohistochemical study

The distribution of twelve biologically active neuropeptides, i.e., thyrotropin-releasing hormone, corticotropin-releasing factor, pro-opiomelanocortin-derived peptides (adrenocorticotropic hormone, beta-endorphin, alpha-melanocyte-stimulating hormone), leucine-enkephalin, dynorphin A, dynorphin B, cholecystokinin, substance P, galanin and calcitonin gene-related peptide, was examined by immunohistochemistry in the human dorsal vagal complex including the nucleus of the solitary tract, the dorsal motor nucleus of the vagus and the area postrema. Immunoreactivity of all the twelve neuropeptides was found widely distributed in the various subdivisions of the nucleus of the solitary tract, showing a unique distribution for every peptide. Neuronal cell bodies immunostained with leucine-enkephalin, galanin and dynorphin B were found in this region. There were no immunopositive perikarya for any of the peptides in the other structures studied. Fibers containing galanin, corticotropin-releasing factor, substance P, dynorphin B, thyrotropin-releasing hormone and calcitonin gene-related peptide were observed at a relatively high density in the nucleus of the solitary tract. In the same structure, a moderately dense network of fibers immunostained with dynorphin A, cholecystokinin and leucine-enkephalin, but only solitary pro-opiomelanocortin-derived peptides-containing fiber fragments were observed. In the dorsal motor nucleus of the vagus the most prominent network of fibers was found to contain thyrotropin-releasing hormone, galanin and substance P. In contrast to these, no beta-endorphin immunoreactivity was detected. The area postrema contained only moderate to low densities of galanin-, substance P-, calcitonin gene-related peptide-, dynorphin B- and cholecystokinin-immunoreactive fibers.

Effect of atropine injected into the nucleus tractus solitarius on the regulation of blood pressure

Previous experiments have demonstrated that stimulation of muscarinic cholinergic receptors in the nucleus tractus solitarius (NTS) of the rat decreases arterial blood pressure and heart rate. The present studies were designed to examine the role of cholinergic mechanisms in the NTS in the tonic regulation of arterial pressure and the baroreceptor reflex. Atropine injected into the NTS of chloralose-anesthetized rats produced a dose-dependent inhibition of cardiovascular responses elicited by injection of acetylcholine into the same site; 240 pmol atropine eliminated acetylcholine-evoked responses. Atropine also increased arterial blood pressure but only at higher doses. Even larger doses of atropine were required to alter cardiovascular responses elicited by electrical stimulation of the aortic depressor nerve. Methylatropine injected into the NTS also blocked acetylcholine-evoked responses but, in contrast to the actions of atropine, did not increase arterial pressure in the dose range required to block acetylcholine-evoked responses. Furthermore, a dose of methylatropine (1 nmol) capable of blocking acetylcholine-evoked cardiovascular responses did not alter aortic depressor nerve-evoked cardiovascular responses. This lack of an effect of methylatropine on arterial pressure and aortic depressor nerve-evoked responses was not due to limited diffusion of the drug within the NTS since 1 nmol methylatropine completely blocked acetylcholine-evoked responses even when injected 0.5 mm distant from the site of acetylcholine injection. These results suggest that cholinergic mechanisms in the NTS are not involved in the tonic regulation of cardiovascular function or the baroreceptor reflex. Furthermore, these results highlight the importance of characterizing doses of drugs used in microinjection studies.

Central distribution of substance P, calcitonin gene-related peptide and 5-hydroxytryptamine in vagal sensory afferents in the rat dorsal medulla

The central distribution of vagal afferents in the medulla containing either substance P, calcitonin gene-related peptide or 5-hydroxytryptamine was examined using a double-labelling technique and laser scanning confocal microscopy. Areas of the nucleus tractus solitarii, dorsal motonucleus of the vagus nerve and area postrema were scanned for double-labelled axon profiles. Analysis of this material revealed that all three neurochemicals were contained within the central terminals of vagal nerve sensory neurons. However, the distribution of vagal nerve afferents containing each of these putative transmitters differed. Afferents containing 5-hydroxytryptamine were detected mainly in the areas postrema and the adjacent nucleus tractus solitarii, with a smaller number in the ventral subnuclei of the solitary tract. In contrast afferents containing calcitonin gene-related peptide were found primarily in the medial and commissural regions of the nucleus tractus solitarii. Afferents containing substance P-immunoreactivity were surprisingly few in number and did not appear to be associated with any particular region. These results establish the presence of 5-hydroxytryptamine, substance P and calcitonin gene-related peptide in the central axons of vagal sensory afferents. Furthermore, the differential distribution of afferents immunoreactive for these neurochemicals seen in this study, together with previous demonstrations of the viscerotopic organization of vagal sensory afferents suggests a possible "chemical coding" for individual end organs.

Distribution of secretoneurin immunoreactivity in the spinal cord and lower brainstem in comparison with that of substance P and calcitonin gene-related peptide

Secretoneurin is a peptide of 33 amino acids generated in brain by proteolytic processing of secretogranin II. The distribution of this newly characterized peptide was investigated by means of immunocytochemistry and in situ hybridization in the spinal cord and lower brainstem of the rat. The staining pattern of secretoneurin immunoreactivity (IR) was compared to that of substance P (SP) and calcitonin gene-related peptide (CGRP) in adjacent sections. A high density of secretoneurin-IR fibers and terminals was found in lamina I and outer lamina II of the caudal trigeminal nucleus and of the spinal cord at all levels, around the central canal, and in the sympathetic and parasympathetic areas of the lateral cell columns. The ventral horn displayed a low to moderate density of secretoneurin-IR. The highest number of secretogranin II mRNA-containing cells was found in lamina II of the dorsal horn and in neurons of the dorsal root ganglia. In the white matter, secretoneurin-IR was most prominent in the dorsolateral part of the lateral funiculus and in the tract of Lissauer. The distributions of secretoneurin-IR and SP-IR were strikingly similar. CGRP-IR and secretoneurin-IR overlapped in the outer laminae of the dorsal horn, in the lateral cell column, and probably in some motoneurons. This study establishes that, like SP and CGRP, secretoneurin is a peptide highly concentrated in the terminal field of primary afferents and in sympathetic and parasympathetic areas. Thus secretoneurin might be involved in the modulation of afferent transmission.

L-glutamate as a neurotransmitter at baroreceptor afferents: evidence from in vivo microdialysis

In vivo microdialysis was employed to measure release of endogenous L-glutamate and L-aspartate in the region of the dorsomedial medulla oblongata including the medial nucleus tractus solitarius of the anaesthetized rat. Basal extracellular levels of these amino acids were stable and increased over two-fold when the perfusate was changed to a high KCl (80 mM) artificial cerebrospinal fluid. This high K(+)-evoked release was calcium-dependent, while basal levels were insensitive to removal of calcium ions from the perfusate. An intravenous infusion of phenylephrine, which elevated blood pressure, caused a marked increase of both spontaneous and evoked release of glutamate. In contrast aspartate efflux was not significantly altered. The present data provide evidence that the excitatory amino acids, glutamate and aspartate, serve a neurotransmitter function in the nucleus tractus solitarius of the rat. Furthermore, the increase in glutamate release following baroreceptor activation with phenylephrine suggests that glutamate may be a neurotransmitter at baroreceptor afferent nerve terminals within the nucleus tractus solitarius. On the other hand, aspartate appears to be possibly an inter-neuronal transmitter in this brain region.

Efferent microvascular responses to electrical stimulation of the area postrema in rats

As part of its role to transduce blood-borne and afferent neural stimuli to the brain, the area postrema conducts efferent projections monosynaptically to individual nuclei of the medulla oblongata and pons. We hypothesized that electrical activation of the area postrema would mimic this transduction process and couple microvascular responses in efferent sites to local increases in tissue metabolism reported previously. We used quantitative autoradiographic techniques and image analysis to measure capillary transfer constants for [14C]alpha-aminoisobutyric acid (AIB, a small, neutral amino acid) and blood flow (iodo[14C]antipyrine) in individual brainstem structures of anesthetized rats. The area postrema was stimulated electrically by means of a monopolar microelectrode positioned stereotaxically 100 microns deep in the dorsocentral aspect of the organ. There were no significant effects of stimulation on [14C]AIB influx or blood flow in control hindbrain structures where postremal projections are sparse or absent--the spinal trigeminal nucleus, reticular formation, or cerebellar vermis. Stimulation of the area postrema produced equivalent increases in transcapillary influx of [14C]AIB and capillary blood flow in the nucleus of the solitary tract, dorsal motor nuclei of the vagus nerves, ventrolateral medullary C1 region, locus coeruleus, dorsal tegmental nuclei, and lateral parabrachial nuclei. Formation of ratios interrelating rates of [14C]AIB influx and blood flow with previously assessed values of tissue glucose metabolism indicated that these measures increased proportionately during postremal stimulation. Such proportional increases in capillary [14C]AIB transfer and blood flow during tissue activation by area postrema stimulation are consistent with interpretation that the increase in blood flow resulted from recruitment of unused surface area in the capillary networks of individual efferent nuclei.(ABSTRACT TRUNCATED AT 250 WORDS)

5-HT2 receptors in the nucleus tractus solitarius: characterisation and role in cardiovascular regulation in the rat

The effects of the local application of drugs acting on 5-HT2 receptors in the nucleus tractus solitarius (NTS) on the heart rate and blood pressure were investigated in normal and nodose ganglionectomized anaesthetized rats. The unilateral micro-injection of an agonist such as 2,5-dimethoxy-3-bromo-amphetamine (DOB) (0.1-0.5 pmol) or 2,5-dimethoxy-3-nitroamphetamine (DON) (0.1-0.5 pmol) produced a dose-dependent hypotension and bradycardia in both intact and ganglionectomized animals. These cardiovascular effects were similar to those observed after the unilateral micro-injection of low doses (pmol) of 5-HT, and could be prevented by the prior micro-injections of the 5-HT2 antagonists ketanserin, ritanserin and piremperone. These findings support the hypothesis that 5-HT2 receptors within the NTS play a role in the reflex regulation of blood pressure. In addition, it was also observed that the micro-injection of subthreshold doses of 5-HT or DOB significantly enhanced the hypotension and bradycardia produced by the unilateral micro-injection of N-methyl-D-aspartate (NMDA). The potentiation of NMDA depressor effects by 5-HT or DOB could be totally prevented by ketanserin or piremperone, suggesting that 5-HT acting upon 5-HT2 receptors in the NTS may intervene in the reflex control of blood pressure by modulating the glutamatergic transmission.

The actions of fentanyl to inhibit drug-induced emesis

The ability of fentanyl to inhibit drug-induced emesis was investigated in the ferret. Initial studies established that morphine, in small doses (0.025-0.5 mg/kg s.c.), induced emesis in the ferret that decreased at the larger doses of 1 and 2 mg/kg (s.c.). Fentanyl (10-80 micrograms/kg s.c.) failed to induce emesis but in this dose range prevented the emesis induced by morphine (0.5 mg/kg s.c.), apomorphine (0.25 mg/kg s.c.), copper sulphate (100 mg/kg intragastric) and cisplatin (10 mg/kg i.v.). The antiemetic effects could be obtained in the absence of sedation or motor impairment. The antagonism by fentanyl of apomorphine-, copper sulphate- and cisplatin-induced emesis was inhibited by naloxone (0.1 or 0.5 mg/kg s.c.). It is concluded that fentanyl exerts a broad spectrum of actions to inhibit drug-induced emesis. An autoradiographic study of the binding of [3H]DAGO to the brainstem of the ferret indicated high densities of mu recognition sites in the area postrema, nucleus tractus solitarius, dorsal motor nucleus of the vagus, reticular medulla and other sites. The results are discussed in terms of balanced facilitatory and inhibitory opioid systems, regulating emesis and that the antiemetic actions of fentanyl reflect an important, although not necessarily an exclusive, action at mu opioid receptors.

[The nucleus tractus solitarius: neuroanatomic, neurochemical and functional aspects]

The nucleus tractus solitarii (NTS) has long been considered as the first central relay for gustatory and visceral afferent informations only. However, data obtained during the past ten years, with neuroanatomical, biochemical and electrophysiological techniques, clearly demonstrate that the NTS is a structure with a high degree of complexity, which plays, at the medullary level, a key role in several integrative processes. The NTS, located in the dorsomedial medulla, is a structure of small size containing a limited number of neurons scattered in a more or less dense fibrillar plexus. The distribution and the organization of both the cells and the fibrillar network are not homogeneous within the nucleus and the NTS has been divided cytoarchitectonically into various subnuclei, which are partly correlated with the areas of projection of peripheral afferent endings. At the ultrastructural level, the NTS shows several complex synaptic arrangements in form of glomeruli. These arrangements provide morphological substrates for complex mechanisms of intercellular communication within the NTS. The NTS is not only the site of vagal and glossopharyngeal afferent projections, it receives also endings from facial and trigeminal nerves as well as from some renal afferents. Gustatory and somatic afferents from the oropharyngeal region project with a crude somatotopy within the rostral part of the NTS and visceral afferents from cardiovascular, digestive, respiratory and renal systems terminate viscero-topically within its caudal part. Moreover the NTS is extensively connected with several central structures. It projects directly to multiple brain regions by means of short connections to bulbo-ponto-mesencephalic structures (parabrachial nucleus, motor nuclei of several cranial nerves, ventro-lateral reticular formation, raphe nuclei...) and long connections to the spinal cord and diencephalic and telencephalic structures, in particular the hypothalamus and some limbic structures. The NTS is also the recipient of several central afferent inputs. It is worth to note that most of the structures that receive a direct projection from the NTS project back to the nucleus. Direct projections from the cerebral cortex to the NTS have also been identified. These extensive connections indicate that the NTS is a key structure for autonomic and neuroendocrine functions as well as for integration of somatic and autonomic responses in certain behaviors. The NTS contains a great diversity of neuroactive substances. Indeed, most of the substances identified within the central nervous system have also been detected in the NTS and may act, at this level, as classical transmitters and/or neuromodulators.(ABSTRACT TRUNCATED AT 400 WORDS)

Cardiovascular effects of the local injection of 5,7-dihydroxytryptamine into the nodose ganglia and nucleus tractus solitarius in awake freely moving rats

The role of the nucleus tractus solitarius (NTS) serotonergic afferents in cardiovascular (CV) regulation is yet to be established. However, several findings suggest that in this nucleus the serotonergic endings coming from the nodose ganglia (NG) are involved in the control of blood pressure (BP). The purpose of the present study was to identify the CV effects of the destruction of this NG-NTS serotonergic pathway. For that, the BP, BP variability (BPV) and heart rate (HR) effects of the local microinjection of 5,7-dihydroxytryptamine (5,7-DHT), into the NG and NTS were investigated in awake freely moving rats. The local degeneration of serotonergic elements was associated with a significant decrease in the 5-HT and 5-hydroxyindole acetic acid levels within the NG and NTS in 5,7-DHT treated rats. In addition, the microinjection of the neurotoxin in both structures produced a transient and significant increase in BP. This effect was of greater amplitude and associated with an increase in BPV in NG lesioned rats. These results may indicate that the NG-NTS serotonergic pathway participates in the transfer of the messages arising from the aortic baroreceptors. However, the vagal component of the baroreflex assessed with the phenylephrine test was not significantly modified in NG lesioned animals as compared to controls. Consequently, if the present data suggest that the NG-NTS serotonergic pathway plays a depressor role in BP regulation, its involvement in the reflex CV responses triggered by the stimulation of the aortic baroreceptors has yet to be established.

Microinjection of oxytocin into the dorsal vagal complex decreases pancreatic insulin secretion

Microinjections of oxytocin and of an oxytocin antagonist into the dorsal vagal complex of the medulla oblongata were performed in order to study the possible role of the oxytocin containing axons that innervate this region in the regulation of pancreatic insulin secretion. No significant effect was produced by the intramedullary injection of the oxytocin vehicle alone or of 0.04 pM oxytocin. Injections of 4 and 20 pM oxytocin produced a reversible decrease of plasmatic insulin levels which fall to 59% of basal levels 15 min after the injection. Such an effect was abolished when 4 pM oxytocin was injected to animals which have been previously bilaterally vagotomized. In contrast to oxytocin, intramedullary injection of a specific antagonist of oxytocin to intact animals induced a marked increase of plasmatic insulin levels which raised 131% of basal levels 15 min after the injection. In animals receiving such an injection of oxytocin antagonist, a secondary injection of 4 pM oxytocin produced a slight but not significant decrease of plasmatic insulin levels. These data strongly suggest that the hypothalamic neurons producing oxytocin that densely project to the dorsal vagal complex may be involved in an inhibitory control of the vagal preganglionic neurons that innervate the pancreas.

Subregional topography of capillaries in the dorsal vagal complex of rats: I. Morphometric properties

Cytoarchitectonic and neurochemical studies of the dorsal vagal complex in the caudal medulla oblongata of rats indicate the existence of distinct anatomical and functional compartments within its components. We applied morphometric methods to discern whether capillary networks differed quantitatively between subregions and zones of area postrema, nucleus tractus solitarii (NTS), and dorsal motor nucleus of the vagus nerve (DMN) of rats. Analysis of 11 subdivisions of area postrema identified both "true" (range in luminal diameter of 3-7.5 microns) and sinusoidal (luminal diameter greater than 7.5 microns) capillaries that, together, made the capillary density for most of area postrema 75% greater than that found in NTS and DMN (526/mm2 vs about 300/mm2). The rank order of true capillary density in area postrema along its rostracaudal axis was caudal greater than central greater than rostral, whereas the reverse order was true for sinusoidal capillaries. Dorsal (periventricular) and medial zones of area postrema throughout its rostrocaudal axis tended to have higher values for capillary density, volume, surface area, luminal diameter, and pericapillary space volume than lateral or ventral zones bordering NTS. Within 200 microns of obex, the ventral zone of rostral area postrema was distinct, having a relatively sparse capillary density that may indicate morphological specializations limiting blood-tissue communication in this subregion. There were no quantitative differences in capillary dimensions between DMN and three subnuclei of NTS. These studies add to extant evidence that the dorsal vagal complex is differentiated for specific functions. Area postrema, especially, has topographical diversity in its capillary organization that likely corresponds to complex roles in neuroendocrine, autonomic, and chemosensory mechanisms.

Serotonergic projections from the nodose ganglia to the nucleus tractus solitarius: an immunohistochemical and double labeling study in the rat

Possible projections of serotonin (5-HT)-immunoreactive neurons in the nodose ganglia (NG) to the nucleus tractus solitarius (NTS) were investigated in the rat using a double labeling method combining retrograde transport and 5-HT immunohistochemistry. After injection of a complex of colloidal gold-apo-horseradish peroxidase into the medio-caudal and commissural parts of the NTS, most of the 5-HT-immunoreactive neurons were found to be labelled by the gold complex. The present study provides direct evidence for the existence, in the rat, of a serotonergic NG-NTS system. This system may be involved in the regulation of blood pressure and vigilance states.

Cholecystokinin-, galanin-, and corticotropin-releasing factor-like immunoreactive projections from the nucleus of the solitary tract to the parabrachial nucleus in the rat

The parabrachial nucleus (PB) is the main relay for ascending visceral afferent information from the nucleus of the solitary tract (NTS) to the forebrain. We examined the chemical organization of solitary-parabrachial afferents by using combined retrograde transport of fluorescent tracers and immunohistochemistry for galanin (GAL), cholecystokinin (CCK), and corticotropin-releasing factor (CRF). Each peptide demonstrated a unique pattern of immunoreactive staining. GAL-like immunoreactive (-ir) fibers were most prominent in the "waist" area, the inner portion of external lateral PB, and the central and dorsal lateral PB subnuclei. Additional GAL-ir innervation was seen in the medial and external medial PB subnuclei. GAL-ir perikarya were observed mainly rostrally in the dorsal lateral, superior lateral, and extreme lateral PB. CCK-ir fibers and terminals were most prominent in the outer portion of the external lateral PB; some weaker labeling was also present in the central lateral PB. CCK-ir cell bodies were almost exclusively confined to the superior lateral PB and the "waist" area, although a few cells were seen in the Kölliker-Fuse nucleus. The distribution of CRF-ir terminal fibers in general resembled that of GAL, but showed considerably less terminal labeling in the lateral parts of the dorsal and central lateral PB, and the external medial and Kölliker-Fuse subnuclei. The CRF-ir cells were most numerous in the dorsal lateral PB and the outer portion of the external lateral PB; rostrally, scattered CRF-ir neurons were seen mainly in the central lateral PB. After injecting the fluorescent tracer Fast Blue into the PB, the distribution of double-labeled neurons in the NTS was mapped. GAL-ir cells were mainly located in the medial NTS subnucleus; 34% of GAL-ir cells were double-labeled ipsilaterally and 7% contralaterally. Conversely, 17% of the retrogradely labeled cells ipsilaterally and 16% contralaterally were GAL-ir. CCK-ir neurons were most numerous in the dorsomedial subnucleus of the NTS and the outer rim of the area postrema. Of the CCK-ir cells, 68% in the ipsilateral and 10% in the contralateral NTS were double-labeled, whereas 15% and 10%, respectively, of retrogradely labeled cells were CCK-ir. In the area postrema, 36% of the CCK-ir cells and 9% of the Fast Blue cells were double-labeled. CRF-ir neurons were more widely distributed in the medial, dorsomedial, and ventrolateral NTS subnuclei, but double-labeled cells were mainly seen in the medial NTS.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuropeptides and catecholamines in efferent projections of the nuclei of the solitary tract in the rat

This study focuses on the involvement of catecholamines and nine different peptides in efferents of the nucleus of the solitary tract to the central nucleus of the amygdala, the bed nucleus of the stria terminalis, and different parabrachial and hypothalamic nuclei in the rat. A double-labeling technique was used that combines a protein-gold complex as the retrograde tracer with immunohistochemistry. Catecholaminergic projection neurons were the most numerous type observed and projected mainly ipsilaterally to all targets studied. Most projections arose from areas overlying the dorsal motor nucleus, mainly the medial nucleus. Neurons synthesizing somatostatin, met-enkephalin-Arg-Gly-Leu, dynorphin B, neuropeptide Y, and neurotensin projected to all structures examined. Somatostatin and enkephalin immunoreactive projection cells were the most numerous. They were located in close proximity to each other, including all subnuclei immediately surrounding the solitary tract, bilaterally. Most dynorphin and neuropeptide Y immunoreactive projection cells were found rostral to that of enkephalinergic and somatostatinergic projections, and mainly in the ipsilateral medial nucleus. Neurotensinergic projections were sparse and from dorsal and dorsolateral nuclei. Substance P and cholecystokinin contribute to parabrachial afferents. The location of substance P immunoreactive projection cells closely resembled that of enkephalinergic and somatostatinergic projections. Projecting cholecystokinin immunoreactive cells were observed in dorsolateral nucleus. Bombesin immunoreactive cells in dorsal nucleus projected to either the parabrachial or hypothalamic nuclei. No vasoactive intestinal polypeptide-containing cells were detected. Thus, most catecholaminergic and neuropeptidergic efferents originated from different populations of cells. It is proposed that catecholaminergic neurons constitute the bulk of solitary efferents and that they may contribute to autonomic neurotransmission. Peptidergic neurons mainly form other subgroups of projections and may play a role in modulating the physiological state of the target nuclei.

Central serotonergic projections to the nucleus tractus solitarii: evidence from a double labeling study in the rat

Projections from several brainstem serotonergic nuclei to the nucleus tractus solitarii were investigated in the rat. Experiments were performed using a double labeling method combining retrograde radioautographic tracing and serotonin immunohistochemistry. After injection of the radioactive tracer ([3H] wheat germ agglutinin) into the lateral nucleus tractus solitarii, nerve cell bodies exhibiting both radioautographic labeling and immunostaining were detected in all the serotonergic nuclei investigated, namely the nucleus raphe magnus, the ventromedial paragigantocellular nucleus, the nuclei raphe pontis, medianus and dorsalis, the medial lemniscus and the reticulotegmental nucleus of the pons. Most of the double labeled perikarya observed were in the nucleus raphe magnus, the adjacent part of the paragigantocellular nucleus and the nucleus raphe dorsalis. Nerve cell bodies retrogradely labeled but devoid of immunostaining were also observed, together with the double labeled perikarya, within serotonergic nuclei. These results provide direct evidence that brainstem serotonergic neurons contribute to the innervation of the nucleus tractus solitarii. They indicate that the nucleus raphe magnus and the nucleus raphe dorsalis constitute two major sources of central serotonergic projections to the nucleus tractus solitarii.

The neurophysiology of vomiting

Nausea and vomiting can be induced by a wide variety of stimuli such as pregnancy, space travel, raised intracranial pressure, radiation and cytotoxic drugs. The mechanisms by which all these diverse stimuli culminate in a final common act is unknown. From studies in the 1950s a model of the emetic reflex emerged consisting of a chemoreceptor trigger zone in the area postrema and a vomiting centre in the brain stem. This concept has been reviewed and revised in the light of recent studies. Many discussions of emesis involve detailed descriptions of the gastrointestinal events associated with the act of vomiting only-nausea and retching receiving little attention. Here we have tried to give a broader view by considering the neurophysiology of such events and have included nausea and retching, phenomena that are usually inseparable from vomiting. The possible biological function of these events is also discussed. The involvement of visceral systems (such as the heart, airways and gut) is included, and particular attention is paid to vagal mechanisms underlying the changes in gut motor activity. Emesis has long been thought to be organized by a 'vomiting centre'; the possibility that this vomiting centre could be the parvocellular reticular formation is reviewed, as is the concept that the 'centre' is larger than an anatomically defined single group of cells. The mechanism of action of two clinically relevant emetic stimuli--radiation and cytotoxic drugs-is considered in detail. Recent studies of the antiemetic properties of novel 5-HT-3 receptor antagonists against radiation and cytotoxic drug-induced vomiting are discussed; these studies suggest that important advances will be made in the treatment of emesis induced by these and other related agents.

Pro-opiomelanocortin-derived peptides (ACTH/beta-endorphin/alpha-MSH) in brainstem baroreceptor areas of the rat

Relatively high concentrations of adrenocorticotropic hormone (ACTH), beta-endorphin and alpha-melanocyte-stimulating hormone (alpha-MSH) were determined by radioimmunoassay in the nucleus of the solitary tract (NTS) of rats. Dense networks of immunoreactive fibers for these peptides were most prominent in the commissural part of the nucleus, where immunostained perikarya (8-15 per section) were also seen in colchicine-treated rats. Moderate peptide levels and moderately dense immunoreactive networks of these peptides were found in the lateral reticular nucleus (including the A1 and A5-C1 catecholaminergic cell groups) and the nucleus ambiguus. Ten different types of surgical lesions or transections were performed in the hypothalamus and the lower brainstem to determine the origin of ACTH, beta-endorphin and alpha-MSH in the brainstem baroreceptor centers. Except the commissural part of the NTS, the baroreceptor areas receive ACTH, beta-endorphin and alpha-MSH innervations from both the hypothalamic arcuate cells and local neurons in the NTS. Fibers in the commissural part of the NTS seem to be of local origin. Hypothalamic fibers to the rostral part of the NTS and the vasomotor A5-C1 cell groups descend in both a medial (through the periaqueductal central gray) and a lateral (ventrolateral tegmental fibers) pathway, whereas fibers to the caudal lateral reticular nucleus (A1 cell group) and the nucleus ambiguus may run only in the lateral pathway. The descending fibers may decussate somewhere in the caudal hypothalamus-rostral midbrain, but caudal to that level they run and terminate ipsilaterally. Fibers from the ACTH-, beta-endorphin- and alpha-MSH-containing cells in the NTS form a bundle arching between the NTS and the ventrolateral medulla and partially (40-55%) innervate the vasomotor and the vasodepressor areas, as well as the nucleus ambiguus.

Increase in paradoxical sleep after destruction of serotoninergic innervation in the nucleus tractus solitarius of the rat

The evolution of paradoxical sleep, slow-wave sleep and arterial pressure was studied following microinjection of 5,7-dihydroxytryptamine in the nucleus tractus solitarius in rats. The extent of the lesions was assessed using immunohistochemistry for serotonin. Global lesions of serotoninergic nerve terminals of the intermediate and commissural regions of the nucleus produced an important and long-lasting increase in paradoxical sleep (+50-70%), a decrease in slow-wave sleep (-20%) and a moderate increase of arterial pressure during all states of the sleep-wake cycle. In addition, more discrete lesions indicated that only the lesion of the area near the obex produced the longer term increase of paradoxical sleep whereas only the lesion of the commissural region of the nucleus produced the long-term decrease of slow-wave sleep. These data demonstrate that serotoninergic projections to the nucleus tractus solitarius exert a regulatory influence upon the specific mechanisms responsible for paradoxical sleep and slow-wave sleep in rats. Furthermore, they suggest that serotonin within the nucleus tractus solitarius plays an important role in the homeostatic cardiovascular and sleep-wake-cycle regulation in rats.

Effect of angiotensin-converting enzyme inhibitors captopril and enalapril on cAMP content of specific brain areas in spontaneously hypertensive rats

1. The influence of two angiotensin-converting enzyme inhibitors, captopril and enalapril, on the cAMP content of microdissected brain areas was examined in spontaneously hypertensive rats. Both drugs depleted systolic arterial blood pressure significantly. 2. Captopril and enalapril increased the level of cAMP in catecholaminergic cell groups in the lower brain-stem. Captopril was more effective in the substantia nigra, while enalapril treatment resulted in high cAMP levels in the ventrolateral medulla oblongata (A1 catecholaminergic cell group). 3. Both drugs, especially captopril, depleted cAMP content in the cingulate cortex. 4. No changes in cAMP levels were measured in the primary baroreceptor centre (nucleus of the solitary tract) following either captopril or enalapril treatment.

Spinal and trigeminal projections to the nucleus of the solitary tract: a possible substrate for somatovisceral and viscerovisceral reflex activation

This study used the retrograde transport of a protein-gold complex to examine the distribution of spinal cord and trigeminal nucleus caudalis neurons that project to the nucleus of the solitary tract (NST) in the rat. In the spinal grey matter, retrogradely labeled cells were common in the marginal zone (lamina I), in the lateral spinal nucleus of the dorsolateral funiculus, in the reticular part of the neck of the dorsal horn (lamina V), around the central canal (lamina X), and in the region of the thoracic and sacral autonomic cell columns. The pattern of labeling closely resembled that seen for the cells at the origin of the spinomesencephalic tract and shared some features with that of the spinoreticular and spinothalamic tracts. Labeled cells in lamina IV of the dorsal horn were only observed when injections spread dorsally, into the dorsal column nuclei, and are thus not considered to be at the origin of the spinosolitary tract. They are probably neurons of the postsynaptic fibers of the dorsal column. Retrogradely labeled cells were also numerous in the superficial laminae of the trigeminal nucleus caudalis, through its rostrocaudal extent. The pattern of marginal cell labeling appeared to be continuous with that of labeled neurons in the paratrigeminal nucleus, located in the descending tract of trigeminal nerve. Since the NST is an important relay for visceral afferents from both the glossopharyngeal and vagus nerves, we suggest that the spinal and trigeminal neurons that project to the NST may be part of a larger system that integrates somatic and visceral afferent inputs from wide areas of the body. The projections may underlie somatovisceral and/or viscerovisceral reflexes, perhaps with a significant afferent nociceptive component.

Serotonin receptors in the rat nucleus tractus solitarii and cardiovascular regulation

The effects of local application in the nucleus tractus solitarii of different serotonin receptor agonists and antagonists were investigated in anesthesized rats. Unilateral microinjection of serotonin produced an acute and transient hypotension and bradycardia which could be totally blocked by antagonists acting preferentially upon 5-HT2 receptor binding sites: ketanserin and ritanserin. 5-HT1 receptor agonists such as RU-24969 and 8-hydroxy-2-(di-n-propylamino)tetralin failed to reproduce the acute cardiovascular effects of serotonin. Bilateral microinjection of 5-HT2 receptor antagonists produced an increase in the level and in the variability of arterial pressure but did not block the baroreceptor reflex arc. The data suggest that serotonin acting upon 5-HT2 receptors in the nucleus tractus solitarii plays an important role in the modulation of arterial pressure.