Serotonergic projections to the ventral respiratory column from raphe nuclei in rats.

The ventral respiratory column (VRC) generates rhythmical respiration and is divided into four compartments: the Bötzinger complex (BC), pre-Bötzinger complex (PBC), rostral ventral respiratory group (rVRG), and caudal ventral respiratory group (cVRG). Serotonergic nerve fibers are densely distributed in the rostral to caudal VRC and serotonin would be one of the important modulators for the respiratory control in the VRC. In the present study, to elucidate detailed distribution of serotonergic neurons in raphe nuclei projecting to the various rostrocaudal levels of VRC, we performed combination of retrograde tracing technique by cholera toxin B subunit (CTB) with immunohistochemistry for tryptophan hydroxylase 2 (TPH2). The double-immunoreactive neurons with CTB and TPH2 were distributed in the both rostral and caudal raphe nuclei, i.e. dorsal raphe nucleus, raphe magnus nucleus, gigantocellular reticular nucleus alpha and ventral parts, lateral paragigantocellular nucleus, parapyramidal area, raphe obscurus nucleus, and raphe pallidus nucleus. The distributions of double-immunoreactive neurons were similar among injection groups of BC, PBC, anterior rVRG, and posterior rVRG/cVRG. In conclusion, serotonergic neurons in both rostral and caudal raphe nuclei projected throughout the VRC and these serotonergic projections may contribute to respiratory responses to various environmental and vital changes.