Premotoneuronal organisation of swallowing in the rat

Hashim, Mir Ali (1990) Premotoneuronal organisation of swallowing in the rat. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The overall aim of this research was to examine the role of the neuronal solitarial network in the neural organization of swallowing. The specific objectives were: 1) to map deglutitive premotor loci within the nucleus tractus solitarii (NTS), 2) to examine their pharmacological characteristics and 3) trace their intrabulbar connections. Experiments were done on anaesthetised rats utilising techniques which included micropneumophoresis, neuronal tracing and immunocytochemistry. Deglutitive loci were mapped by local applications of glutamate; the pharmacology of glutamate receptor subtypes involved in the various components of swallowing were examined; and both anterograde and retrograde tracing techniques were employed. -- The results indicate that, within the NTS, pharyngeal loci extend from the level of the obex to 900 pm rostrally overlapping the subnuclei ventralis and intermedialis; both cervical and distal oesophageal loci are coextensive with the subnucleus centralis of the NTS. -- Based on a comparison of excitatory amino acid agonist potency and the effects of Af-methyl-D-aspartate (NMDA)-selective and non-selective antagonists on glutamate-evoked responses, kainate (KA) receptors were found linked to pharyngeal sites whereas NMDA receptors were preferentially associated with the subnucleus centralis. Under physiological conditions, the response to NMDA receptor-activation in the subnucleus centralis appears to be dependent in part on an intact cholinergic input directed at muscarinic cholinoceptors. In contrast, the presumptive KA receptor-mediated responses at solitarial deglutitive loci appear independent of an excitatory serotoninergic input insofar as the involvement of excitatory 5-HT2 and/or 5-HT1c receptors is concerned. The 5-HT input to the pharyngeal territory of the solitary complex originates, at least in part, from cells in the raphe obscurus, magnus and pontis nuclei. -- Anterograde fibre tracing by the use of Phaseolus vulgaris leucoagglutinin revealed dense projections from the deglutitive loci to the pharyngo- and oesophagomotor portions of the nucleus ambiguus. In addition, solitarial efferents displayed a distinct pattern of projection to other motoneuronal pools controlling the muscles of deglutition viz., the hypoglossal, the facial and its accessory nucleus, the motor trigeminal and its accessory nucleus and the dorsal vagal motor nucleus. Thus, the pharmacological data together with the neuroanatomical evidence of direct neural projections from deglutitive loci in the solitary complex to the motor nuclei involved in swallowing lend strong credence to the hypothesized role of solitarial interneurons as generators of the deglutitive motor pattern. -- KEY WORDS: Swallowing Deglutition Nucleus tractus solitarii Glutamate NMDA receptors Acetylcholine Serotonin Nucleus ambiguus Premotor neurons

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