Oxytocin, but not vassopressin, modulates nociceptive responses in dorsal horn neurons

Oxytocin (OT) and vasopressin (VP) are synthesized and secreted by the paraventricular hypothalamic nucleus (PVN), and both peptides have been implicated in the pain modulatory system. In the spinal cord, activation of OT-containing axons modulates nociceptive neuronal responses in dorsal horn neurons; however, it is not known whether the direct VPergic descending projection participates. Here, we show that both PVN electrical stimulation and topical application of OT in the vicinity of identified and recorded dorsal horn WDR selectively inhibit Aδ and C-fiber responses. In contrast, the topical administration of VP on the same neurons did not affect the nociceptive responses. In addition, the reduction in nociceptive responses caused by PVN stimulation or OT administration was blocked with a selective OT antagonist. The results suggest that the VP descending projection does not modulate the antinociceptive effects mediated by the PVN on dorsal horn neurons; instead, it is the hypothalamic-spinal OT projection that regulates nociceptive information.