Nodose ganglionectomy selectively reduces muscarinic cholinergic and delta opioid binding sites in the dorsal vagal complex of the cat

Vasopressin and sensory circumventricular organs

The subfornical organ, the area postrema and the organum vasculosum of the lamina terminalis are considered to be sensory circumventricular organs as they contain neuronal somata which are located outside the blood-brain barrier and are thus capable of serving as 'sensors' for blood-borne humoral messengers. The endocrine hormone, vasopressin (VP), not only causes strong antidiuresis by acting on the kidney, but also exerts centrally mediated effects as a neuromodulator. Several lines of evidence suggest that VP can influence regulatory functions mediated by the sensory circumventricular organs, since vasopressinergic somata and terminals as well as VP receptors have been reposted to be present in these structures. These biochemical prerequisites offer the possibility that blood-borne VP might on the one hand act as a feedback signal from the periphery and, on the other hand, synaptically released or locally produced VP could modulate the known functions of sensory circumventricular organs, such as thirst, fever or cardiovascular regulation. This review focuses on the possible physiological relevance of VP acting on sensory circumventricular organs in view of recent evidence obtained from biochemical and electrophysiological studies at the cellular level.

The dorsal vagal complex of the ferret: anatomical and immunohistochemical studies

To further the understanding of gastrointestinal function in this species, and in particular to advance our own work concerning central emetic pathways, the cytoarchitecture and the distribution of eight neurochemicals were studied in the ferret dorsal vagal complex (DVC; area postrema, nucleus of the solitary tract [nTS] and dorsal motor nucleus of the vagus). The cytoarchitectural features of this region in the ferret were similar to those seen in other species; however, the ferret possesses a particularly large and distinct subnucleus gelatinosus of the nTS. Dense calcitonin gene-related peptide-immunoreactivity was found in the gelatinous, interstitial and commissural subnuclei of the nTS, with lesser amounts in other regions of the DVC. Enkephalin-immunoreactivity of varying densities was found throughout the DVC. Moderate to dense galanin-immunoreactivity was observed throughout the DVC, with the exception of the subnucleus gelatinosus of the nTS, from which it was virtually absent. Dense neuropeptide Y-immunoreactivity was observed in the subnucleus gelatinosus and interstitial subnucleus, with moderate staining in other regions of the DVC. Neurotensin immunoreactivity was very sparse or absent. Immunoreactivity for serotonin was sparsely distributed throughout the DVC. Moderate somatostatin-immunoreactivity was observed over a large portion of the DVC, but was virtually absent from the gelatinosus and interstitial subnuclei. Substance P immunoreactivity was observed throughout the DVC and was particularly dense in the dorsal/dorsolateral subnucleus and the dorsal aspects of the medial and commissural subnuclei. In terms of its cytoarchitecture the DVC of the ferret is more similar to the cat than the rat, especially with regard to the area postrema and the subnucleus gelatinosus of the nTS. The distribution of neuroactive substances was largely similar to other species; however, differences were present particularly in patterns of immunoreactivity for enkephalin, serotonin, neuropeptide Y and somatostatin.

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.

Functional dopamine D2 receptors on rat vagal afferent neurones

1. In the present study in vitro electrophysiology and receptor autoradiography were used to determine whether rat vagal afferent neurones possess dopamine D2 receptors. 2. Dopamine (10-300 microM) elicited a temperature- and concentration-dependent depolarization of the rat isolated nodose ganglion preparation. When applied to the tissue 15 min prior to agonist, raclopride (10 microM), clozapine (10 microM) or a mixture of raclopride and clozapine (10 microM each) all produced a threefold parallel shift to the right of the dopamine concentration-response curve. In contrast, SCH 23390 (100 nM), phentolamine and propranolol (1 microM each) failed to antagonize the dopamine-mediated depolarization. 3. [125I]-NCQ 298 (0.5 nM), a D2 selective radioligand, bound topographically to sections of rat brainstem. Densitometric quantification of autoradiograms revealed 93.8 +/- 0.5% specific binding of this salicylamide radioligand, as determined by raclopride (10 microM, n = 10 animals). Binding was highest in the nucleus tractus solitarius (NTS), particularly the medial and gelatinous subnuclei. In addition, specific binding was also observed in the interpolar spinal trigeminal nucleus and the inferior olive. 4. Unilateral nodose ganglionectomy caused a 36.6 +/- 3.0% reduction in specific binding in the denervated NTS compared to the contralateral NTS. Furthermore, the loss of binding was confined to the dorsal aspect of the medial subnucleus of the NTS. Sham surgery had no effect on the binding of [125I]-NCQ 298 in rat brainstem. 5. The present data provide evidence for the presence of functionally relevant dopamine D2 receptors on both the soma and central terminals of rat vagal afferent neurones. In addition, the majority of D2 receptors in the rat NTS appear to be located postsynaptically with respect to vagal terminals, and are presumably located either on ascending glossopharyngeal terminals, descending terminals from higher brain regions or on neuronal cell bodies within the NTS.

Presynaptic adenosine A2a receptors on soma and central terminals of rat vagal afferent neurons

The dorsal vagal complex of the medulla oblongata is a key centre involved in the regulation of numerous autonomic functions, including cardiovascular control. Adenosine has been implicated as a potential neuromodulator of the baroreceptor reflex, and therefore the current study has investigated the presence and characteristics of adenosine receptors on rat vagal afferent neurons. In the nodose-vagal grease gap preparation, the adenosine A2a agonist CGS-21680 evoked a depolarisation only in the presence of the selective adenosine A1 antagonist PACPX. Autoradiography using [3H]NECA (4 nM) with suppression of A1 binding enabled the first visualisation of high affinity adenosine A2 receptors in the nucleus tractus solitarius (NTS). Unilateral nodose ganglionectomy resulted in over 90% reduction in binding in the lesioned (ipsilateral) NTS compared to a sham control. Furthermore, local administration of CGS-21680 increased evoked glutamate release in the NTS, as measured by in vivo microdialysis. These data suggest the presence of presynaptic adenosine A2a receptors on both the soma and central terminals of rat vagal afferent neurons, and thereby support the hypothesis that adenosine may have a modulatory role in the baroreceptor reflex.

The central connections of the vagus nerve in the ferret

The vagus nerve mediates emesis due to gastric irritation. The central representation of the vagus in the ferret was studied to establish how the nerve is connected to areas important in the regulation of emesis. In a series of 10 ferrets, WGA-HRP injections (10 microliters) were made into the nodose ganglion. After 24-48 h, animals were reanesthetized and perfused transcardially. A block extending from the pons to upper cervical spinal cord was cut at 50 microns and sections reacted. Nodose ganglion injections of WGA-HRP produced labeling of vagal preterminal segments in the ipsilateral dorsal vagal complex including all subnuclei of the solitary complex where the medial and subgelatinous subnuclei received the densest input, the area postrema (AP), which contained a modest amount of terminal label, and the dorsal motor nucleus of the vagus (DMX). Contralateral terminal label, quantitatively much less, was similarly distributed except that within the solitary complex it was limited to the medial and subgelatinous subnuclei. Retrogradely labeled cells formed ipsilateral dorsomedial and ventrolateral columns, corresponding, respectively, to the DMX and the nucleus ambiguus (including retrofacial and retroambiguus).

Effects of vagotomy on Leu-enkephalin-induced changes in memory storage processes

When given before or after training, Leu-enkephalin impairs later retention of learning. To replicate these findings, Experiment 1 determined if Leu-enkephalin impairs retention when administered after inhibitory avoidance training. Posttraining injection of 100.0 micrograms/kg of Leu-enkephalin impaired retention relative to saline controls or those receiving a lower dose of this peptide. Since Leu-enkephalin does not cross freely from the blood stream into the brain, this peptide may exert its influence on mnemonic processes by activating peripheral receptors that transmit neural messages to the brain via the vagus nerve. In Experiment 2, 100.0 micrograms/kg Leu-enkephalin impaired retention in unoperated and sham-operated animals although vagotomized animals did not differ significantly from these groups or saline controls. These results suggest that subdiaphragmatic vagotomy may not be sufficient to disrupt the mnemonic effects of Leu-enkephalin. Alternatively, the vagus nerve may be one pathway but not the only pathway by which the activation of peripheral systems may influence the memory storage process.

The neuropharmacology of loperamide-induced emesis in the ferret: the role of the area postrema, vagus, opiate and 5-HT3 receptors

Loperamide, an opiate receptor agonist, commonly used in the treatment of diarrhoea, reliably induced emesis in the ferret, when given subcutaneously. The response latency was short (less than 10 min) and the emesis lasted for approx 70 min. The dose-response curve for the emetic response was "bell-shaped" and all animals responded at 0.5 mg/kg but none at 5 mg/kg (s.c.). The response was unaffected by dopamine D2 receptor antagonism (domperidone 1.0 mg/kg, s.c.) or 5-HT3 receptor antagonism (granisetron or ondansetron 1.0 mg/kg, s.c.). The onset of the response was delayed for about 60 min by naloxone or naloxone methiodide (1.0 mg/kg, s.c.) and abolished by naloxanazine (1.0 mg/kg, s.c.), reported to be relatively selective for mu receptors. The results implicate mu receptors (possibly mu 1) in the induction of emesis by loperamide and provide some support for activation of opiate receptors also having anti-emetic effects, as suggested in previous studies. The emetic response to loperamide was unaffected by abdominal vagotomy but was abolished by ablation of the area postrema, indicating that loperamide-induced emesis may be used as a test for ablation of the area postrema in studies of the emetic mechanism in the ferret.

Morphological and electrophysiological evidence for postsynaptic localization of functional oxytocin receptors in the rat dorsal motor nucleus of the vagus nerve

The vagal complex is innervated by oxytocin immunoreactive axons of hypothalamic origin. The presence of oxytocin binding sites in the dorsal motor nucleus of the vagus nerve of the rat was evidenced by autoradiography with a radioiodinated oxytocin antagonist as ligand. Two weeks following a unilateral vagotomy, distal to the nodose ganglion, binding sites were reduced below the level of detection in the ipsilateral dorsal motor nucleus of the vagus nerve. Choline acetyltransferase immunoreactivity was also markedly reduced in the vagal motoneurons whose axons had been transected. Electrophysiological studies were performed in vitro in brainstem slices from control rats. In antidromically identified vagal motoneurones, oxytocin applied at 0.1-1.0 microM either caused a reversible depolarization or generated, under voltage-clamp conditions, a transient inward current. These responses persisted under the condition of synaptic uncoupling. Taken together these observations favour the notion that oxytocin of hypothalamic origin acts directly on rat vagal motoneurones.

[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)

Chronic opioid treatment attenuates carbachol-mediated polyphosphoinositide hydrolysis in chick embryo neuronal cultures

Opiate binding sites on cultured neurons derived from 6-day-old (E6) chick embryo cerebral hemispheres (CH), shown to be cholinergic by choline acetyltransferase immunostaining, were labeled with [3H]etorphine (mu and delta opiate receptors expression) and [3H]morphine (mostly mu). When examined by light microscope autoradiography, opiate receptors were found to be expressed by most neurons, and were distributed predominantly on neuronal perikarya. Muscarinic and opiate receptors in E6CH cultured neurons were found to be functionally coupled when the effects of opiate receptor occupancy on the inositol phosphate-linked muscarinic receptors was studied. Carbachol stimulated the release of [3H]inositol phosphates (InsP) from cultures preincubated with [3H]inositol and LiCl, in a dose-dependent manner, and the functional expression of muscarinic receptors peaked in number at day 7 in culture, declining thereafter. Short-term (less than 1 h) treatment of E6 neuronal cultures with 1 microM opioid peptides such as morphiceptin or D-Ala2-D-Leu5-enkephalin (DADLE) did not inhibit the release of inositol phosphates in response to 1 mM carbachol whereas forskolin, which also activates adenylate cyclase and raises cAMP levels, inhibited InsP release by about 25%. In contrast, long-term (48 h) opioid treatment with either morphiceptin or DADLE (1-10 microM) inhibited the carbachol-stimulated inositol phosphate release by greater than or equal to 50%. Prolonged treatment with morphiceptin also inhibited the bradykinin-mediated release of InsP from E6CH cells. In both cases, the inhibition was partially blocked by the continuous presence of naloxone, suggesting that the inhibition was mediated through opiate receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for presynaptic 5-hydroxytryptamine3 recognition sites on vagal afferent terminals in the brainstem of the ferret

Antagonists acting at the 5-hydroxytryptamine3 receptor are potent anti-emetic agents in cases of cytotoxic- and radiation-induced vomiting, and binding sites for these compounds have been described in brainstem areas known to be involved in mediation of nausea and vomiting. We have used autoradiography to examine the distribution of one of these antagonists, [3H]granisetron in the caudal brainstem of the ferret, a commonly used animal model for physiological investigations of emesis. The highest density of binding sites was found to be in the dorsomedial region of the nucleus of the solitary tract, the principal terminus for gastric vagal afferent fibres. Lower levels of binding were observed in the area postrema and the dorsal motor nucleus of the vagus. Following unilateral nodose ganglion excision, displaceable binding of [3H]granisetron in the nucleus of the solitary tract was attenuated on the ipsilateral side by 65%. Bilateral subdiaphragmatic vagotomy abolished binding of [3H]granisetron in the entire dorsal vagal complex. These results provide strong circumstantial evidence that 5-hydroxytryptamine3 receptors are located on vagal afferent terminals in the ferret brainstem.