Pharmacological characterization of medullary serotonin neurons.

In the present study we investigated the characteristics of medullary raphe serotonergic neurons. Specifically, we sought to examine further the similarities between medullospinal 5-HT neurons and the more extensively studied neurons of the dorsal raphe. Intravenous administration of 5-methoxy-dimethyltryptamine (5-MeODMT) produced a dose-related inhibition of the firing of midline medullary 5-HT neurons. Microiontophoretically applied 5-MeODMT also inhibited medullary 5-HT neurons. The inhibitory potency of 5-MeODMT was nearly identical to that observed for dorsal raphe 5-HT neurons. Microiontophoretic or intravenous administration of the 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) did not alter the firing rate of medullary 5-HT neurons. Intravenous administration of the alpha 1-receptor antagonist prazosin resulted in an inhibition of the medullary 5-HT neuronal firing. The discharge of medullary 5-HT neurons increased during iontophoresis of norepinephrine. These data are discussed in relation to the identification and characterization of medullary 5-HT neurons. In addition, the data suggest that the firing rate of medullary 5-HT neurons is regulated in part by a tonic excitatory noradrenergic input.