Atypical indolamine-immunoreactive cell groups in the dorsal myelencephalon of the rat

Neuropharmacologic correlates of deglutition: lessons from fictive swallowing

Pharmacologic investigations into the transmission processes underlying fictive swallowing in the rat have disclosed the potential diversity of chemical signals used in central deglutitive pathways. Monoaminergic mechanisms appear to serve as links between subcortical structures and the medullary pattern generator of swallowing (PGS), and may play a critical role in maintaining internal facilitatory drive, required by the PGS for optimal responsivity to peripheral sensory input. Cholinergic bulbar interneurons form an integral component of the PGS subnetwork controlling esophageal peristalsis. Local GABA neurons exert a tonic inhibition of the buccopharyngeal stage, may regulate buccopharyngeal-esophageal coupling, and may contribute to peristaltic rhythmic generation at both the premotoneuronal and motoneuronal level. Receptor subtypes for excitatory amino acids (glutamate, aspartate) are differentially associated with deglutitive premotoneurons for both the buccopharyngeal and esophageal stage, as well as with ambiguus motoneurons. Preliminary evidence suggests the existence of excitatory peptidergic mechanisms involving thyrotropin-releasing hormone, vasopressin, oxytocin, and somatostatin, a probable candidate for excitatory transmitter in the solitarioambigual internuncial projection to motoneurons innervating esophageal striated musculature. Further validation of this experimental model may ultimately help to establish a framework for the clinical recognition, management, and exploitation of drug actions on central deglutitive neuroeffectors.

[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)

Excitatory action of 5-HT on deglutitive substrates in the rat solitary complex

The excitatory effect of serotonin (5-HT) on the pharyngeal stage of swallowing was investigated in urethane anaesthetised rats with respect to the involvement of neural substrates located in the central and intermediolateral regions of the nucleus tractus solitarii (NTS). Micropneumophoretic ejection of 5-HT 5-50 pmol either produced deglutitory responses or selectively facilitated the S-glutamate-evoked pharyngeal responses when applied in 1-10 pmol prepulses. The excitatory/facilitatory effect of 5-HT was enhanced by intravenous threshold doses of the 5-HT-mimetic, quipazine (0.3-1 mumol/kg) and reversibly blocked by the 5-HT2-receptor antagonists, methysergide, metergoline and ketanserin. 5-HT doses exceeding 10-60 pmol gave rise to a non-selective reversible inhibition of glutamate- and acetylcholine (ACh)-evoked pharyngeal or oesophageal responses which was not prevented or reversed by 5-HT2-receptor antagonists, but was readily overcome by increasing the amount of glutamate or ACh ejected. Non-selective deglutitive inhibition after high doses of 5-HT could, therefore, result from neuronal desensitization secondary to excessive stimulation or activation of a different type of 5-HT receptor. These results corroborate an excitatory role of 5-HT in both reflex and automatic swallowing and demonstrate that the NTS is a major site of serotoninergic facilitation of swallowing.

Suppression of penicillin-induced epileptiform activity by noxious stimulation: mediation by 5-hydroxytryptamine

Noxious stimulation has a well known suppressant effect on epileptiform activity in both laboratory animals and man. To study this phenomenon in an animal model, focal epileptiform activity was induced in anaesthetized rats by applying penicillin from one barrel of a micropipette while recording intracortical electrical activity from another barrel. Penicillin produced either focal epileptiform activity or focal penicillin spikes, depending somewhat on the rate of penicillin release. Focal epileptiform activity was suppressed by noxious stimulation, both somatic and olfactory, whereas non-noxious stimulation was ineffective in this regard. Focal penicillin spikes were only rarely suppressed by noxious stimulation. Reserpine blocked the suppressant effect of noxious stimulation as did p-chlorophenylalanine, a more selective depletor of 5-hydroxytryptamine. L-5-Hydroxytryptophan, a 5-hydroxytryptamine precursor, restored the suppressant effect of noxious stimulation blocked by reserpine and p-chlorophenylalanine. These results suggest that the suppression of SW by noxious stimulation is mediated by 5-hydroxytryptamine. Data from experiments employing pharmacological antagonists suggest the suppression of spike and wave activity by noxious stimulation not to be mediated by activation of dopamine receptors, alpha 2 or beta adrenoceptors, or muscarinic cholinoceptors. Prazosin, a selective alpha 1 adrenoceptor antagonist, did block the suppressant effect of noxious stimulation but only at a very high dose (2.4 mumols/kg). This likely reflects a known analgesic action of prazosin or weak binding to the 5-HT receptor. Methysergide, a 5-hydroxytryptamine antagonist, failed to antagonize the suppressant effect of noxious stimulation, however, many inhibitory actions of 5-hydroxytryptamine are not blocked by methysergide. It is concluded that suppression of focal epileptiform activity by noxious stimulation is mediated, at least in part, by 5-hydroxytryptamine.

Suppression of penicillin-induced focal epileptiform activity by locus ceruleus stimulation: mediation by an alpha 1-adrenoceptor

Application of penicillin to the cerebral cortex of anesthetized rats by pressure ejection from a micropipette resulted in the appearance of focal epileptiform activity with low rates of penicillin release and focal penicillin spikes with higher rates. Electrical stimulation of the locus ceruleus (LC), a major norepinephrine-containing nucleus in the brainstem, or of its axons projecting to the forebrain, the dorsal noradrenergic bundle, suppressed penicillin-induced focal epileptiform activity but was less effective in suppressing focal penicillin spikes. Depletion of monoamines with reserpine blocked the suppressant effect of LC stimulation. Neither the selective depletion of 5-hydroxytryptamine with p-chlorophenylalanine nor administration of methysergide reduced the effectiveness of LC stimulation, suggesting that 5-hydroxytryptamine probably does not mediate the suppression. Pimozide partially antagonized the suppression of focal epileptiform activity induced by LC stimulation, which is consistent with antagonism of alpha-adrenoceptors but not dopamine receptors. beta-Receptor antagonists did not block the suppression of focal epileptiform activity by LC stimulation, suggesting that beta-receptors are not important in the observed suppression. Prazosin, a selective alpha 1-antagonist, at low doses blocked the suppression of focal epileptiform activity by LC stimulation whereas yohimbine, an alpha 2-antagonist enhanced the stimulation-induced suppression. Taken together, the data are consistent with LC and dorsal bundle stimulation releasing norepinephrine, which in turn suppresses focal epileptiform activity by an action mediated by an alpha 1-adrenoceptor.