Analysis of serotonergic system in the respiratory and circulatory regulation of the medullary raphe nuclei.

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Respiratory and circulatory regulation mechanisms of the medullary raphe nuclei, that is, nucleus raphe pallidus,obscurus and magnus, were investigated, especially concerning their serotonergic and GABA ergic mechanisms, using microiontophoretic application methods.1. Electrical stimulation of medullary raphe nuclei of the cat produced almost parallel depressions of respiration (phrenic nerve), renal nerve and blood pressure, but, when the stimulating electrode advanced in the more ventral part of the nuclei, these responses moved gradually to the excitatory ones.These responses were analyzed on the averaged potential of the phrenic nerve elicited by raphe stimulation. This averaged potential was composed of the initial slight excitatory potential and following long-lusting depressant one, the former becoming larger as the stimulating site was more ventral, and producing excitatory response by repetitive stimulation. The former potential was depressed by methysergide (5-HT_2 antagonist … More ) and the latter was reduced by propranolol (recognized as possessing 5-HT_1 antagonistic effect).2. Medullary respiratory neurons (inspiratory and expiratory neurons) and vasomotor neurons in the rostral ventrolateral medulla were depressed by raphe stimulation with ghort latency, and these depressant responses were antagonized by intravenous or microiontophoretic application of picrotoxin, bicuculin (GABA antagonists) and propranolol, suggesting projection of serotonergic and GABAnergic systems of raphe nuclei to these neurons.3. These respiratory and vasomotor neurons were depressed dose-dependently by iontophoretic application of serotonin and GABA, and these depressant effects were antagonized by simultaneous application of 5-HT_1 - and GABA antagonists. But, application of methysergide rather augmented the depressant effect of serotonin, suggesting co-localization of 5-HT_2 receptor in addition to 5-HT_1 receptor.4. These effects produced by raphe stimulation and serotonergic and GABAergic drugs were observed similarly in the medullary non-respiratory and non-vasomotorneurons, too, suggesting pan-depressant control of raphe nuclei to the medullary neurons. Less