Identification of oxytocin receptor in the dorsal horn and nociceptive dorsal root ganglion neurons.

Oxytocin (OT) secreted by the hypothalamo-spinal projection exerts antinociceptive effects in the dorsal horn. Electrophysiological evidence indicates that OT could exert these effects by activating OT receptors (OTR) directly on dorsal horn neurons and/or primary nociceptive afferents in the dorsal root ganglia (DRG). However, little is known about the identity of the dorsal horn and DRG neurons that express the OTR. In the dorsal horn, we found that the OTR is expressed principally in neurons cell bodies. However, neither spino-thalamic dorsal horn neurons projecting to the contralateral thalamic ventral posterolateral nucleus (VPL) and posterior nuclear group (Po) nor GABaergic dorsal horn neurons express the OTR. The OTR is not expressed in skin nociceptive terminals or in dorsal horn nociceptive fibers. In the DRG, however, the OTR is expressed predominantly in non-peptidergic C-fiber cell bodies, but not in peptidergic or mechanoreceptor afferents or in skin nociceptive terminals. Our results suggest that the antinociceptive effects of OT are mediated by direct activation of dorsal horn neurons and peripheral actions on nociceptive, non-peptidergic C-afferents in the DRG.