Visualization of serotonin neurones in the nodose ganglia of the cat. An autoradiographic study

Localization and coexistence of calcium-binding proteins and neuropeptides in the vagal ganglia of the goat

This study was performed to investigate the neurochemical characteristics of the vagal ganglia of the goat by immunohistochemical methods using calbindin D-28k (CB), calretinin (CR). parvalbumin (PA), substance P (SP). calcitonin generelated peptide (CGRP) and galanin (GAL) antibodies. In the proximal vagal ganglia (jugular ganglia), CGRP- (57.1%), SP- (48.2%), GAL- (8.6%), PA- (8.7%), CB- (8.5%) and CR-like (5.3%) immunoreactive cells were observed. In the distal vagal ganglia (nodose ganglia), CGRP- (40.5%), SP- (30.20%), CB- (22.0%) and CR-like (18.10%) immunoreactive cells were present. The double immunohistochemical study showed, that in the proximal vagal ganglia, CGRP immunoreactivity was co-localized in SP- (84.8%), GAL-(100%), CB- (5.6%) and CR- (5.7%) immunoreactive cells: SP immunoreactivity was co-localized in the CGRP- (80.0%), GAL- (100%). CB- (5.3%) and CR- (5.6%) immunoreactive cells; GAL immunoreactivity coexisted in the CGRP- (4.4%) and SP- (19.8%) immunoreactive cells, but not in calcium-binding proteins (CBP)-immunoreactive cells; PA immunoreactivity was absent in the CGRP- and SP-immunoreactive cells; CB and CR immunoreactivities were seen in the CGRP-(0.8%) and SP-immunoreactive (0.9%) cells. On the other hand, in the distal vagal ganglia, CGRP immunoreactivity appeared in SP- (66.6%), CB- (1.0%) and CR- (1.2%) immunoreactive cells; SP immunoreactivities were observed in the CGRP- (44.1%), CB- (1.0%) and CR- (1.2%) immunoreactive cells; CB immunoreactivities were present in the CGRP- (0.5%) and SP- (0.8%) immunoreactive cells; CR immunoreactivities were contained in the CGRP- (0.5%) and SP- (0.8%) immunoreactive cells. These findings indicate that the goat is distinct from other mammalian species in the distribution and localization of neurochemical substances in the vagal ganglia. and suggest that these differences may be related to physiological characteristics, particular those of the ruminant digestive system.

Brain stem control of swallowing: neuronal network and cellular mechanisms

Swallowing movements are produced by a central pattern generator located in the medulla oblongata. It has been established on the basis of microelectrode recordings that the swallowing network includes two main groups of neurons. One group is located within the dorsal medulla and contains the generator neurons involved in triggering, shaping, and timing the sequential or rhythmic swallowing pattern. Interestingly, these generator neurons are situated within a primary sensory relay, that is, the nucleus tractus solitarii. The second group is located in the ventrolateral medulla and contains switching neurons, which distribute the swallowing drive to the various pools of motoneurons involved in swallowing. This review focuses on the brain stem mechanisms underlying the generation of sequential and rhythmic swallowing movements. It analyzes the neuronal circuitry, the cellular properties of neurons, and the neurotransmitters possibly involved, as well as the peripheral and central inputs which shape the output of the network appropriately so that the swallowing movements correspond to the bolus to be swallowed. The mechanisms possibly involved in pattern generation and the possible flexibility of the swallowing central pattern generator are discussed.

Serotonin unmasks functional NK-2 receptors in vagal sensory neurones of the guinea-pig

1. The regulation of substance P (SP) responsiveness in acutely isolated nodose neurones from adult guinea-pigs was investigated using standard intracellular recording techniques. 2. In control neurones, SP produced no measurable electrophysiological effects. However, following incubation with serotonin (5-HT, 10 microM), 64% of neurones were depolarized by 10 +/- 0.6 mV (n = 84 of 132 neurones) by SP (100 nM). 5-HT-induced SP responses were inhibited by SR48968 (100 nM, n = 6), a neurokinin 2 (NK-2) receptor antagonist, but were unaffected by CP99,994 and SR142801, NK-1 and NK-3 receptor antagonists (n = 3 each), respectively. 3. 5-HT-induced unmasking of SP responses was maximal within 5 min. Increasing the 5-HT incubation time up to 120 min did not increase the mean response amplitude or the percentage of SP responsive neurones (P = 0.611 and 0.867, respectively). 4. 5-HT-induced unmasking of SP responses was dose dependent (EC50 = 14 nM). A 5-HT3 receptor agonist CPBG (1 microM), mimicked the unmasking effects of 5-HT (n = 10 of 19 neurones), while 5-CT (10 microM), a non-selective 5-HT agonist devoid of action at 5-HT3 receptors, did not (n = 18). ICS205-930 (1 microM), a 5-HT3 receptor antagonist, completely blocked the 5-HT-induced unmasking of SP responses (n = 10 of 10 neurones). 5. In 68% of the neurones tested, bath-applied 5-HT (10 microM) evoked a 178 +/- 29.5 nM increase in [Ca2+]i (n = 16), which was blocked by nominally zero [Ca2+]o (n = 4) or by ICS205-930 (1 microM, n = 4). Nodose neurones incubated with 5-HT in the presence of nominally zero [Ca2+]o did not respond to SP (n = 12 of 13 neurones) in Locke solution containing normal [Ca2+]o, indicating that the 5-HT-mediated elevation of [Ca2+]i is required for unmasking of SP responses. Calmidazolium (100 nM), a calmodulin inhibitor, inhibited the unmasking effects of 5-HT (n = 5 of 5 neurones). 6. Incubating neurones with the nitric oxide (NO) donors papaNONOate (1 mM, 15-30 min) or SNAP (50 microM, 30-60 min) unmasked depolarizing SP responses in 71% and 45% of the neurones studied, respectively. L-NMMA (30 microM), a NO synthase inhibitor, blocked 5-HT-induced unmasking of SP responses (n = 10 of 10 neurones). 7. In sum, these results suggest that stimulation of 5-HT3 receptors activates an intracellular signalling cascade that couples calcium-calmodulin and NO activation to NK-2 receptor unmasking in sensory neurones.

Effect of 5-HT1A receptor agonist, 8-OH-DPAT, on cough responses in the conscious guinea pig

We have studied the role for 5-hydroxytryptamine (5-HT) in the modulation of the cough reflex by examining the effect of a selective 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) on cough and respiratory rate in conscious guinea pigs. Animals were placed in a box and exposed to the tussive agent citric acid (0.5 M) for 10 min, 3 min after terbutaline (0.05 mg/kg i.p.) was administered to prevent bronchoconstriction. 8-OH-DPAT inhibited at low doses (0.008 and 0.016 mg/kg) but potentiated at high doses (0.25 and 0.5 mg/kg) the citric acid-induced number of coughs, but dose-dependently increased respiratory rate. Methysergide (0.05-5 mg/kg), a 5HT1 and 5HT2 receptor antagonist, and ketanserin (0.005 mg/kg), a 5HT2 receptor antagonist, had no effects on cough or respiratory rate. Methysergide inhibited the increased cough responses and respiratory rate induced by 8-OH-DPAT at high doses, while ketanserin was without effect. These results suggest that 8-OH-DPAT may induce both in inhibition and activation of the cough reflex, the latter involving central 5HT1-receptor activation.

Tyrosine-hydroxylase-containing vagal afferent neurons in the rat nodose ganglion are independent from neuropeptide-Y-containing populations and project to esophagus and stomach

Immunoreactivity to the rate limiting enzyme of catecholamine synthesis, tyrosine hydroxylase, has been described in the inferior sensory (= nodose) ganglion of the vagal nerve in the rat. The aim of the present study was to characterize further this neuronal population. The neurons do not represent displaced autonomic efferent neurons, since they do not receive synaptic input, as indicated by the absence of synaptophysin-immunoreactive terminals. In addition to the immunoreactivity to tyrosine hydroxylase, a tyrosine hydroxylase cRNA probe hybridizes with nodose ganglion neurons as demonstrated by in situ hybridization and Northern blotting. Many but not all of the tyrosine hydroxylase-immunoreactive neurons are also immunoreactive to the dopamine synthesizing enzyme, aromatic-L-amino-acid-decarboxylase, but lack the noradrenaline-synthesizing enzyme, dopamine-beta-hydroxylase, thus favoring synthesis of dopamine. Neuropeptide Y, which is often colocalized with catecholamines, is also present in a subset of nodose ganglion neurons, as indicated by immunohistochemistry, in situ hybridization and Northern blotting. However, double-labeling immunofluorescence has revealed that these two antigens are localized in different cell populations. Retrograde neuronal tracing utilizing fluorescent dyes (Fast blue, Fluoro-gold) combined with tyrosine hydroxylase immunohistochemistry has demonstrated that the esophagus and stomach are peripheral targets of tyrosine-hydroxylase-containing vagal viscero-afferent neurons.

Ultrastructural organization of the interstitial subnucleus of the nucleus of the tractus solitarius in the cat: identification of vagal afferents

This electron microscopic study, based on serial section analysis, describes the synaptic organization of the interstitial subnucleus of the nucleus of the solitary tract and identifies the terminals of the vagal primary afferents utilizing degeneration and HRP transport. The interstitial subnucleus contains sparsely scattered cell bodies, numerous dendrites and axon terminals, and bundles of unmyelinated and myelinated axons. The cell bodies which are small in diameter have an organelle poor cytoplasm and a large invaginated nucleus. Axon terminals can be classified into two main types according to their vesicular shape. The first type contains clear, round vesicles and can be further subdivided into two subgroups on the basis of their morphology and the size of their vesicles. In the first subgroup the terminals are small, contain a few mitochondria and their vesicles are densely packed with an homogeneous size. In the second subgroup the terminals which vary from small to large, contain many mitochondria and contain round vesicles which are heterogeneous in size. The second main terminal type consists of axon terminals containing pleomorphic vesicles which are associated with asymmetrical or symmetrical synaptic contacts on dendrites. Axo-axonic contacts are present in the interstitial subnucleus. In general, the presynaptic axon terminals contain pleomorphic vesicles and the postsynaptic elements contain round vesicles of varying size. In some dendrites, identified by the presence of ribosomes, groups of round and/or pleomorphic vesicles are found associated with synaptic contacts. These dendrites are presynaptic to conventional dendrites and postsynaptic to axon terminals. After removal of the nodose ganglion, degenerative alterations are seen only at the caudal and middle levels of the interstitial subnucleus. Degeneration occurs in a few myelinated axons and in axon terminals which usually contain a mixture of small and larger round, clear vesicles. After HRP injection into the vagus nerve, the HRP reaction product is visible in axon terminals filled with clear, round vesicles which are heterogeneous in size. The labelled axon terminals establish single or multiple synaptic contacts. This study demonstrates that terminals of vagal primary afferents consist principally of terminals of the second subgroup. The morphology of these terminals are compared to primary afferents in the brainstem and spinal cord.

Localization of beta pre-protachykinin mRNA in nodose ganglion

The neurons which synthesize tachykinins in the capsaicin-sensitive afferent nerves of the respiratory tract are largely localized to the nodose ganglia. Using a radiolabelled antisense cRNA probe constructed from cDNA for the major precursor of substance P and neurokinin A (beta-preprotachykinin: beta-PPT), we have localized specific mRNA for this peptide in neurons of the nodose ganglion of rat using in situ hybridization. 26% of neurons gave a positive hybridization signal, which was in agreement with the same proportion of cell bodies showing substance P-like immunoreactivity. The specificity of the hybridization was confirmed by the absence of labelling using RNase pre-treatment and a sense probe having the same sequence as beta-PPT mRNA. This approach may now allow the investigation of factors which regulate synthesis of tachykinins at a gene transcriptional level.

Effects of 8-hydroxy-2-(di-n-propylamino)tetralin, a selective agonist of 5-HT1A receptors, on the cough reflex in rats

The effects of the agonist of 5-HT1A receptors, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), on the capsaicin-induced cough reflex in rats were studied. I.p. injection of 8-OH-DPAT, at doses of 0.1 and 0.3 mg/kg, significantly decreased the number of coughs in a dose-dependent manner. The antitussive effect of 8-OH-DPAT (0.3 mg/kg) was blocked by prior injection of methysergide (3 mg/kg i.p.) and spiperone (0.3 mg/kg i.p.), whereas ketanserin (3 mg/kg i.p.) had no effect on the antitussive effect of 8-OH-DPAT. The antitussive effects of dihydrocodeine (1 mg/kg i.p.) and dextromethorphan (3 mg/kg i.p.) were also antagonized by methysergide and spiperone. However, these cough-depressant effects were not reduced by ketanserin. These results suggest that the antitussive action of 8-OH-DPAT may be related to the enhancement of the function of 5-HT1A receptors, and that antitussives interact with the 5-HT1A 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)

5-Hydroxytryptamine, substance P, and thyrotropin-releasing hormone in the adult cat spinal cord segment L7: immunohistochemical and chemical studies

The terminal projections of the descending 5-hydroxytryptamine (5-HT) bulbospinal pathway and the coexistence among 5-HT-, substance P (SP)-, and thyrotropin-releasing hormone (TRH)-like immunoreactivities (LI) in fibers innervating the L7 segment in the cat spinal cord were studied quantitatively and semiquantitatively by use of the indirect double-staining immunofluorescence technique. The content of 5-HT, SP, and TRH in different parts of the spinal cord was determined by use of radioimmunoassay (RIA) (SP and TRH) and high-performance liquid chromatography with electrochemical detection (HPLC-ECD) (5-HT). For all three substances studied, immunoreactive (IR) axon terminals were found in all parts of the gray matter, but with clear regional variation in the density of innervation. Thus, all three substances showed a dense innervation in the motor nucleus, particularly in the ventral part of the nucleus, while the superficial dorsal horn was very densely innervated by SP-IR fibers (laminae I and II) and TRH-IR fibers (laminae II and III). In the motor nucleus, the studied substances coexisted to a very high degree, but some 5-HT-IR fibers (about 10%) lacked peptide-LI and some SP-IR fibers (about 10%) lacked 5-HT-LI while virtually all TRH-IR fibers also contained 5-HT-LI. In the superficial dorsal horn (laminae I-III), no coexistence was detected, while other parts of the gray matter displayed various degrees of coexistence in between those found in the motor nucleus and laminae I-III. The quantitative analysis of IR varicosities in the motor nucleus suggested that the unilateral L7 motor nucleus is innervated by about 55-110 x 10(6) 5-HT-IR nerve terminals, which may indicate as many as 4,000 boutons per descending 5-HT cell body in the brain stem only with this restricted projection. When combing these results with the biochemical data, it could be calculated that the concentration of 5-HT in IR varicosities is about 3-6 x 10(-3) M, while the corresponding figures for SP and TRH was 0.3-0.5 x 10(-3) M and 0.1-0.2 x 10(-3) M, respectively. In cats subjected to spinal cord transection at the lower thoracic level, all 5-HT-IR fibers in the L7 segment had disappeared 44 days after the lesion, indicating a strict suprasegmental origin of 5-HT-IR fibers in this segment.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of serotonin release and autoreceptors in the dorsalis raphe nucleus in the control of serotonin release in the cat caudate nucleus

Using a push-pull cannula technique and an isotopic method for estimating [3H]serotonin continuously synthesized from [3H]tryptophan, the effects of changes in the release of serotonin in the dorsalis raphe nucleus on in vivo release of [3H]serotonin in the cat caudate nucleus were investigated. The increase in the release of serotonin in the dorsalis raphe nucleus caused by local application of parachlorophenylethylamine (10(-6) M) reduced striatal [3H]serotonin release. This inhibition in serotonin release in the striatum was blocked by the prior and continuous local superfusion of the dorsal raphe with methiothepin (10(-6) M), a serotonin autoreceptor antagonist. GABA (5 x 10(-5) M) applied to the dorsalis raphe reduced both local and striatal release of [3H]serotonin. However, picrotoxin (10(-5) M), a GABA A receptor antagonist applied locally in the dorsalis raphe nucleus increased [3H]serotonin release while decreasing striatal [3H]serotonin release. This decrease in serotonin release in the striatum was again blocked by continuous superfusion of the raphe with methiothepin. Furthermore, superfusion of serotonergic cell bodies of the dorsalis raphe nucleus with methiothepin alone never altered local release or striatal release of [3H]serotonin. These data strongly suggest that the release of serotonin from the cell body in the dorsalis raphe nucleus phasically controls release of the amine at the axonal nerve ending through serotonergic autoreceptors located on serotonergic nerve cell bodies in the dorsalis raphe nucleus. The origin of the serotonin released in the dorsalis raphe nucleus and the possibility that this type of regulation could be related to changes in nerve impulse conduction of the serotonergic raphe-striatal system are discussed.

Presence of cholinergic neurons in the vagal afferent system: biochemical and immunohistochemical approaches

The presence of cholinergic fibers in the afferent vagal system of various species was shown using biochemical and immunohistochemical methods. Biochemical activity of choline acetyl transferase, the synthesizing enzyme for acetylcholine, was detected in the nodose ganglion of cat, rabbit, dog and sheep. Immunohistochemistry, using a monoclonal antibody raised against choline acetyl transferase, revealed labelled cell bodies in the nodose ganglion of the rabbit. Acetylcholine endogenous content, measured in nodose ganglia devoid of efferent fibers, was twice as high in the right ganglion as compared to the left. Enzyme transport and choline acetyl transferase activity analysis were each determined on separate peripheral vagus nerves. These results are discussed in terms of functional properties of the vagal afferent neurons, including the modulation of vagal afferent messages at the level of the nodose ganglion and the eventual control of peripheral intrinsic neurons by sensory vagal terminals.

Substance P and serotonin mutually reverse their excitatory effects in the rat nucleus tractus solitarius

Actions and interactions of serotonin and substance P are described in the nucleus tractus solitarius using coronal brainstem slices and intracellular recordings. Substance P (10-100 nM) and serotonin (10-100 microM) applied alone were excitatory, causing depolarization and increasing the input resistance. Reversing effect was obtained using a protocol of long (greater than 5 min) conditioning applications of substance P and shorter (20-60 s) test applications of serotonin: serotonin, which was excitatory by itself during controls, became inhibitory for the steady action potential discharges induced by conditioning substance P applications. In the reverse situation, inhibition was also obtained using prolonged conditioning exposures to serotonin and test applications of substance P. Prolonged conditioning applications (greater than 5 min) were required in these experiments since addition or potentiation, but not inhibition, was found when combining 20-60 s substance P and serotonin applications. In addition to their excitatory effects, substance P and serotonin, applied alone, had another mechanism of action. They reduced the duration of tetraethylammonium-prolonged action potentials. This mechanism was also reversed using conditioning applications of substance P or serotonin. Thus, reversing effects appeared simultaneously on multiple ionic mechanisms. Furthermore, the reversing effect was unaffected in tetrodotoxin-treated preparations, which indicates a postsynaptic phenomenon. Consequently, the control of two different postsynaptic ionic mechanisms during substance P and serotonin interaction suggests that the underlying mechanisms take place at a common level, possibly in relation to second messenger processes. From a functional point of view, these results support the idea that in the nucleus tractus solitarius the effects of either neurotransmitter, serotonin or substance P, can be completely reversed by a previous release of the other one.

Effect of thalamic parafascicularis nucleus stimulation in regulation of serotoninergic transmission in the cat caudate nucleus: involvement of autoreceptors in the dorsalis raphe nucleus

The mechanisms involved in parafascicularis nucleus control on serotoninergic neurons projecting into the caudate nucleus were investigated in "encéphale-isole" cats. The effects of unilateral stimulation of the parafascicularis nucleus on the release of newly synthesized [3H]serotonin were simultaneously determined in the ipsilateral caudate nucleus and the dorsalis raphe nucleus using push-pull cannulae. The actions of various pharmacological treatments performed either in the caudate nucleus or in the dorsalis raphe nucleus were also examined. The electrical or chemical stimulation of the parafascicularis nucleus induced a decrease in striatal [3H]serotonin release and an increase in [3H]serotonin release in the dorsalis raphe nucleus. The blockade of cholinergic (mecamylamine) and glutamatergic (PK 26124) transmissions at the striatal level did not modify the thalamic stimulation-induced effect on serotonin release in the caudate nucleus or in the dorsalis raphe nucleus. However, a decrease induced by parafascicularis nucleus stimulation in serotonin release in the caudate nucleus could not be observed when the autoreceptors present on serotoninergic nerve cell bodies localized in the dorsalis raphe nucleus were blocked by a methiothepin perfusion within the nucleus. These results indicate that the parafascicularis nucleus controls striatal serotonin transmission by inducing changes in the nerve activity of serotoninergic neurons in the dorsalis raphe nucleus via somatodendritic serotonin release and autoreceptors.

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.

Serotonin-like immunoreactivity in the cat trigeminal ganglion

The immunohistochemical distribution of serotonin-like immunoreactivity (SER-LI) has been established in networks of fine nerve fibers which arborize and wind profusely between non-immunoreactive sensory neurons in the cat trigeminal ganglion. Some of the varicose nerve fibers surround occasional non-immunoreactive sensory neurons like a woven basket. None of the sensory neurons display SER-LI. An extrinsic origin of intraganglionic fine nerve fibers has been suggested.

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.

Development of the monoaminergic innervation of the avian gut: transient and permanent expression of phenotypic markers

Specific cellular accumulation of [3H]5-hydroxytryptamine ([3H]5-HT) occurs during development of the avian gut. This accumulation is transient in extraganglionic mesenchymal cells (TES cells) but is a permanent characteristic of enteric serotonergic neurons (ESN). Species-specific differences were found in the location of TES cells and ESN. In chicks TES cells surrounded myenteric ganglia and ESN were restricted to the myenteric plexus. In quails TES cells surrounded submucosal ganglia and [3H]5-HT-labeled submucosal as well as myenteric neurons. [3H]Norepinephrine accumulated only in noradrenergic terminals and not in TES cells or ESN. The origins of TES cells and ESN were studied in chimeras, in which neuraxis from appropriate or inappropriate axial levels was grafted from quail to chick. Both types of chimeric bowel contained TES cells and ESN. Most TES cells in chimeras were chick in origin and distributed as in chicks (around myenteric ganglia); however, some TES cells and all ESN were quail cells. To test whether crest cells are required for development of TES cells and ESN, aneuronal chick hindgut was explanted and grown alone, or with quail neuraxis, as chorioallantoic membrane (CAM) grafts. TES cells appeared in CAM grafts whether or not crest cells were present; however ESN only appeared in explants when quail neuraxis was included. In addition, an ectopic [3H]5-HT-labeled chromaffin-like cell, also of quail origin, was found in enteric plexuses in these combined explants of crest and gut. Most TES cells, therefore, are neither derived from nor dependent on the presence of crest cells in the gut wall. Since even an inappropriate axial level of crest was found to produce ESN when it was experimentally induced to colonize the bowel the enteric microenvironment probably plays a critical role in serotonergic neural development. The species-specific location of TES cells and ESN is consistent with the hypothesis that TES cells constitute an important component of this microenvironment.

Cardio-respiratory actions of substance P, TRH and 5-HT in the nucleus tractus solitarius of rats: evidence for functional interactions of neuropeptides and amine neurotransmitters

The cardiovascular and respiratory effects of Substance P (SP) and Thyrotrophin-Releasing Hormone (TRH) microinjections into the nucleus tractus solitarius (NTS) of urethane anaesthetized rats have been investigated. Dual injections of the peptides with serotonin (5-HT) were given to investigate possible functional interactions. In addition, SP and TRH were injected into rats in which 5-HT in the NTS area had been depleted by prior treatment with 5,7-Dihydroxytryptamine (5,7-DHT). SP (65pmol) did not elicit significant effects on blood pressure (BP) or heart rate (HR), but produced a marked, acute reduction in respiration rate (RR). TRH (110pmol) elicited a small but significant reduction in mean arterial pressure (MAP), whereas 5-HT (15nmol) caused a rise in MAP. Neither TRH nor 5-HT modified RR when given alone. A dual injection of SP (6.5pmol, ineffective alone) and 5-HT (15nmol) resulted in a rise in MAP which was insignificantly different from the effect of 5-HT alone. However, a prolonged fall in RR, unlike the effect of SP alone was also observed. A dual injection of TRH (11pmol, ineffective alone) and 5-HT (15nmol) resulted in a profound fall of RR but no significant changes in MAP or HR. SP elicited similar effects in 5,7-DHT lesioned animals as in sham operated controls. In contrast, TRH microinjections in lesioned rats were associated with a profound fall in RR, and a blood pressure response significantly different to that observed in the corresponding sham group. The results are discussed in relation to other evidence suggesting functional interactions between neuropeptides and amine neurotransmitters in the mammalian brainstem.

Ultrastructure and stereological analysis of nucleoli of rat nodose ganglion neuron during a 24-h period: a comparison with sympathetic neurons of rat superior cervical ganglion

The nucleoli of rat nodose ganglion were investigated during a 24-h span (light span 07.00-19.00 h). The mean volume of nucleoli and that of their components, especially fibrillar centers considered to be the interphasic counterpart of nucleolus-organizing regions, were determined by stereological analysis. The quantitative data showed that (1) nucleoli volumes of rat nodose ganglion neurons did not oscillate diurnally but that (2) there were diurnal dimensional changes in the volume of their fibrillar centers which strongly suggest an ultradian rhythmicity. These results are different from those obtained in studies of superior cervical ganglion neurons, in which nucleoli and nucleolar components followed a circadian rhythm with peak values during daily periods of darkness. Although the nucleoli of these 2 kinds of neurons involved in autonomic nervous system reactions do not show the same behavioural patterns, the present data bring to light a new example of circadian fluctuation in nucleoli and describes their organization in this respect.

Cytological evidence for serotonin-containing fibers in an abdominal neurohemal organ in a hemipteran

The abdominal neurohemal organs of the hemipteran Rhodnius prolixus contain an extensive serotonin containing arborization. Endogenous serotonin within fibers and terminals in the neurohemal area were detected with histochemical and immunocytological techniques. The abdominal nerves which contain the neurohemal organs selectively sequester exogenous serotonin. Serotonin and its metabolites are biochemically detected within the mesothoracic ganglion, which is a known source of projections into the neurohemal organ. However, the source of the cell bodies which might send serotonergic fibers to the neurohemal organ remains undetermined because no correspondence was found between immunocytological maps of serotonin-containing cells in the ganglion, and projection maps into neurohemal organ (determined by cobalt back-filling).

Modulation of cardiovascular and electrocortical activity through serotonergic mechanisms in the nucleus tractus solitarius of the rat

The nucleus tractus solitarius (NTS) is an integral part of the baroreceptor reflex arc. Thus, stimulation of the NTS elicits changes in arterial pressure and heart rate as well as in numerous other physiologic parameters including electrocortical activity. Serotonin (5-HT), which has been implicated in cardiovascular and electrocortical control, is present in nerve terminals within the NTS. Therefore, this study was designed to determine whether 5-HT may effect that control within the NTS. Serotonin injected into the NTS of anesthetized rats produced marked changes in the EEG, arterial pressure, and heart rate. EEG activity changed from irregular 1-5 Hz, 350-500 microV waves with an overlying 13-15 Hz, low voltage rhythm to a regular, 5 Hz, 250-300 microV rhythm. The dose-dependent cardiovascular changes were maximal at a dose of 400 pmol which produced a fall of mean arterial pressure of 48 +/- 2 mm Hg from a baseline of 96 +/- 4 mm Hg and of heart rate of 90 +/- 9 bpm from a baseline of 400 +/- 18 bpm (n = 6; P less than 0.001). Both the cardiovascular and EEG effects of 5-HT injected into the NTS were blocked by the prior injection of the 5-HT antagonist metergoline at the same site. However, the bilateral microinjection of metergoline into the NTS did not affect the baroreceptor reflex. Thus, although serotonergic mechanisms in the NTS may be involved in the modulation of electrocortical and cardiovascular activity, they are not integral to the baroreceptor reflex arc.

Release of serotonin from perikarya in cat nodose ganglia

Newly synthesized serotonin (5-HT) can be released in the nodose ganglion from the nerve cell bodies of vago-aortic serotoninergic neurones. Free-calcium led to a decrease of spontaneous and potassium-evoked release. The veratridine-stimulating response was abolished by TTX. The concept that 5-HT released from perikarya in the extracellular space could be involved in the self-regulation of the activity of the vago-aortic pathway is discussed.

Serotoninergic terminals: ultrastructure and synaptic interaction with catecholamine-containing neurons in the medial nuclei of the solitary tracts

The ultrastructural morphology of serotoninergic terminals and their synaptic relation with catecholaminergic neurons were examined in the medial nuclei of the solitary tracts (m-NTS) using combined autoradiographic and immunocytochemical methods. Adult rats were pretreated with a monoamine oxidase inhibitor and subjected to a 2-hour intraventricular infusion of 50 nM tritiated 5-hydroxytryptamine (3H-5HT). At the termination of the infusion, the brains were fixed by aortic arch perfusion with a mixture of 4% paraformaldehyde and 0.5% glutaraldehyde. Coronal Vibratome sections through the NTS and more rostral raphe nuclei were immunocytochemically labeled with specific antiserum to serotonin or tyrosine hydroxylase and then processed for autoradiography. By light microscopy, concentrations of reduced silver grains indicating uptake of 3H-5HT usually paralleled the localization of peroxidase immunoreactivity for serotonin in neuronal perikarya of the rostral raphe nuclei and in varicosities in the brainstem. The 3H-5HT-containing varicosities were found throughout the medial and commissural portions of the NTS, where they were frequently associated with processes showing immunoreactivity for the catecholamine-synthesizing enzyme tyrosine hydroxylase. Ultrastructural examination of the m-NTS revealed that the silver grains for 3H-5HT were accumulated over axon terminals. The 5HT-labeled terminals contained a heterogeneous population of vesicles and formed both symmetric and asymmetric synapses with dendrites. The recipient dendrites were either, unlabeled or showed immunoreactivity for tyrosine hydroxylase. These findings support a direct serotoninergic modulation of catecholaminergic neurons within the rat m-NTS.

Biochemical and immunohistochemical determination of 5-hydroxytryptamine located in mast cells in the trigeminal ganglion of the rat and guinea pig

The presence of 5-hydroxytryptamine (5-HT), as well as its precursor (5-HTP) and metabolite (5-HIAA), were biochemically determinated in the trigeminal ganglion of the guinea pig and rat. The distribution of 5-HT in the ganglion and in its posterior root was studied using both indirect immunofluorescence and the peroxidase-antiperoxidase method. In order to increase the possible 5-HT content of primary sensory neurons for subsequent immunohistochemical visualization, animals were first treated with nialamide, an inhibitor of monoamine oxidase, and then loaded with L-tryptophan. Another group of animals received colchicine to inhibit intra-axonal transport of transmitter substances. However, even combined use of loading and colchicine treatment did not reveal 5-HT immunoreactivity in ganglion cells. The only source of 5-HT immunoreactivity in the trigeminal ganglion and its posterior root was mast cells. These cells were located around the ganglion in adjacent leptomeningeal and connective tissues, as well as between the ganglion cells and nerve fibers. Only occasionally were mast cells found in the posterior root of the ganglion.

Quantitative autoradiographic studies of 5-HT-accumulating neurones in the nodose ganglia of the cat after perikaryal or terminal uptake

The distributions of ganglionar cell bodies, specifically labeled with 3H-5-HT, were studied in light microscope autoradiographs of the cat nodose ganglion. The size, number and localization of labeled cells were examined under different experimental conditions: after in vitro incubation of the ganglion with 3H-5-HT and after retrograde transport of 3H-5-HT injected into the nucleus of the solitary tract (NST) which receives primary visceral sensory projections from nodose ganglia cells of the vagal nerve. Following incubation of the nodose ganglia with a low concentration of 3H-5-HT (10(-6) M), some ganglionar cell bodies took up and retained the tracer. In both the right and left ganglia, they were significantly smaller in size than the unreactive neurones. The mean diameter of their perikaryon was 36.97 +/- 0.52 microns, compared with 45.76 +/- 0.87 microns in unreactive neurones. About 600 labeled cell bodies were counted in each ganglion, corresponding to 2-3% of the total nodose ganglion cell population. These reactive neurones were not localized in one particular area of the ganglia, but scattered throughout both of them. Following bilateral or unilateral microinjections of 3H-5-HT in the NST, retrogradely labeled cell bodies were observed, 24 h later, in the nodose ganglia. Their mean diameter was estimated to be 36.14 +/- 0.69 microns and they represented approximately 2% of the total ganglion cell population. As in in vitro experiments, the labeled cells were not grouped in any particular region of the ganglion. These experiments show that the distribution of both populations of labeled cells, observed under these conditions, are comparable. On an anatomical and quantitative basis one may reasonably suppose that the perikaryal or terminal uptakes concern the same neuronal population.

Serotonin-like immunoreactivity in paraffin-sections of the nodose ganglia of the cat

The presence of serotonin (5-HT)-like immunoreactivity has been detected in ganglionar cell bodies of the nodose ganglia of the cat, using the peroxidase-antiperoxidase immunohistochemical method, applied to sections of ganglia previously embedded in paraffin. The antibodies were raised in rabbits following injection of 5-HT conjugated to bovine serum albumin, with formaldehyde as the condensation reagent. Immunoreactive cell bodies are morphologically and topographically described. The ubiquitous distribution and non-uniform labelling suggest that these 5-HT-containing vagal neurones may give rise to several functionally different 5-HT types in vagal afferent fibres.

Selective retrograde transport of 3H-serotonin in vagal afferents

A new serotonergic afferent vagal component has been demonstrated in the cat by radioautography. Twenty-four hours after a bilateral injection of tritiated serotonin (3H-5-HT) into the area of the nucleus of the solitary tract (NST), heavily and lightly labelled cell bodies were observed in the nodose ganglia. After unilateral injections of 3H-5-HT into the same area, labelled ganglionar cell bodies were found in the ipsilateral nodose ganglion. Some were also found in the contralateral one, suggesting a serotonergic crossed fibers component. Dense clusters of silver grains, depicting typical labelling of neuronal varicosities, were observed in the NST. After destruction of the serotonergic terminals with 5,7-dihydroxytryptamine, followed by injection of 3H-5-HT, the number of labelled cell bodies decreased dramatically in the ipsilateral nodose ganglia and the clusters of silver grains disappeared in the NST. After ligature or section of the supranodose vagal nerve, following injection of 3H-5-HT into the NST, no radioautographic reaction was observed in the homolateral nodose ganglia. The present study demonstrates the existence of a peripheral serotonergic system in vagal afferents. The physiological implications of this new serotonergic visceral pathway remain to be studied.