On the immunohistochemical distribution of ionotropic P2X receptors in the nucleus tractus solitarius of the rat

Abstract

ATP has been shown to excite neurones of the nucleus tractus solitarius (NTS) via the activation of P2X receptors. In the present study, the distribution of six P2X receptors (P2X 1–P2X 6) within the rat NTS was investigated by peroxidase immunohistochemistry. Immunopositive neurones for P2X receptor subtypes were detected in all divisions of the NTS, although the staining densities differed according to receptor subtype and sub-nuclei. P2X 1-immunopositive cells were distributed throughout the rostro-caudal extent of the NTS, while varicose fibres were mainly located along the postremal border. P2X 2 immunoreactivity was present in neurones and fibres located throughout the NTS. In the commissural NTS intense staining was observed medial of the solitary tract while in the sub-postremal NTS neurones were observed along the postremal border. A high density of P2X 3-positive neurones and fibres was observed in the sub-postremal NTS along the border of the area postrema and in the rostral NTS in the medial subdivision. In comparison to the staining observed with the other receptor antibodies, there was considerably reduced P2X 4 receptor immunoreactivity. P2X 4-positive neurones tended to be more sparsely distributed, and found mainly in the intermediate portion of the commissural NTS, and along the postremal border. In contrast, we observed dense staining for the P2X 5 receptor subtype in a majority of regions within the NTS. The most striking staining was observed in the intermediate subdivision at the level of the sub-postremal NTS and the medial portion of the rostral NTS. P2X 6 immunoreactive neurones were observed in the medial commissural NTS, along the postremal border and in the dorso-medial and medial subdivisions of the rostral NTS. Taken together, our findings confirm the presence of six P2X receptor subtypes within the NTS of the rat, consistent with a neurotransmitter role for ATP in the rat NTS. These results indicate the need for more extensive functional studies to elucidate the roles of the individual and heterodimeric assemblies of P2X receptor subtypes within the NTS.