Oxytocin prevents neuronal network pain-related changes on spinal cord dorsal horn in vitro.

Recently, oxytocin (OT) has been studied as a potential modulator of endogenous analgesia by acting upon pain circuits at the spinal cord and supraspinal levels. Yet the detailed action mechanisms of OT are still undetermined. The present study aimed to evaluate the action of OT in the spinal cord dorsal horn network under nociceptive-like conditions induced by the activation of the N-methyl-d-aspartate (NMDA) receptor and formalin injection, using calcium imaging techniques. Results demonstrate that the spontaneous Ca2+-dependent activity of the dorsal horn cells was scarce, and the coactivity of cells was mainly absent. When NMDA was applied, high rates of activity and coactivity occurred in the dorsal horn cells; these rates of high activity mimicked the activity dynamics evoked by a neuropathic pain condition. In addition, although OT treatment increased activity rates, it was also capable of disrupting the conformation of coordinated activity previously consolidated by NMDA treatment, without showing any effect by itself. Altogether, our results suggest that OT globally prevents the formation of coordinated patterns previously generated by nociceptive-like conditions on dorsal horn cells by NMDA application, which supports previous evidence showing that OT represents a potential therapeutic alternative for the treatment of chronic neuropathic pain.

Peripheral oxytocin receptors inhibit the nociceptive input signal to spinal dorsal horn wide-dynamic-range neurons.

Oxytocin (OT) has emerged as a mediator of endogenous analgesia in behavioral and electrophysiological experiments. In fact, OT receptors (OTRs) in the spinal dorsal horn participate in a selective inhibition of the neuronal activity mediated by Aδ and C fibers but not Aß fibers. This study shows that OTRs are expressed in the terminal nerve endings and are able to inhibit nociceptive neuronal firing. Indeed, local peripheral OT blocked the first sensorial activity of Aδ and C fibers recorded in the spinal cord neurons. Furthermore, using the formalin behavioral nociceptive test, we demonstrated that only ipsilateral OTR activation inhibits pain behavior. Our data are reinforced by the fact that the OTR protein is expressed in the sciatic nerve. Consistent with this, immunofluorescence of primary afferent fibers suggest that OTRs could be located in nociceptive-specific terminals of the skin. Taken together, our results suggest that OTRs could be found in nociceptive terminals and that on activation they are able to inhibit nociceptive input.