Abstract
We examined the possible involvement of the putative neurotransmitters, GABA and serotonin (5-HT), in raphe-induced facilitatory or inhibitory effects on the respiratory activity of rats. We observed that an intravenous (i.v.) injection of (+)-bicuculline, a GABA(A) receptor antagonist, significantly reduced respiratory inhibition induced by electrical stimulation of the raphe magnus (RM) or the raphe obscurus (RO). On the other hand, an i.v. injection of methysergide, a broad-spectrum 5-HT receptor antagonist, significantly reduced the respiratory facilitation induced by stimulation of the raphe pallidus (RP) or RO. We further investigated the spinal projection of GABAergic and serotonergicneurons from the caudal raphe nuclei to the phrenic motor nucleus (PMN) using a combined method of retrograde labeling of raphespinal neurons utilizing Texas Red injected into the PMN region and immunohistochemical labeling of GAD, a GABA synthesizing enzyme, or 5-HT positive cells. We observed that GAD and Texas Red double-labeled neurons were predominantly localized in the RM, and additionally in the RO. However 5-HT and Texas Red double-labeled neurons were predominantly localized in the RP, and additionally in the RO and RM. These findings suggest that RM-, or RO-induced inhibitory effects are transmitted (at least in part) to the PMN via a direct GABAergic descending pathway. The RP-, or RO- induced facilitatory effects however, are transmitted via a serotonergic descending pathway. [J Physiol Sci. 2007;57 Suppl:S238]
