oxytocin--tri-gly- and Pain

Oxytocin (OT) and arginine vasopressin (AVP) are 2 neuropeptides that display well-known effects on the reproductive system. Although still controversial, oxytocin and vasopressin were demonstrated to exert potent effects on the nociceptive system when administered directly in various central nervous structures. On the other hand, little is known about their peripheral (hormonal) actions on nociception and pain responses. The aim of the present work was to characterize the effects of physiological blood concentrations of OT and AVP on spinal nociception and on pain responses. To do so, growing doses of OT or AVP were administered intravenously and the nociceptive processing by spinal cord neurons was analyzed in anesthetized male rats in vivo. We observed that the action potentials mediated by C-type nociceptive fibers was strongly reduced (antinociception) after intravenous injections of low doses of OT (<5 μg) or AVP (<500 pg), whereas an increase (pronociception) was observed at higher doses. Interestingly, antinociceptive and pronociceptive effects were fully abolished in the presence of the OT receptor antagonist and the AVP receptor antagonist type 1A (V1A), respectively. We confirmed this result with a behavioral model of forced swim stress-induced analgesia associated with plasmatic release of OT (and not vasopressin). Stress-induced analgesia was transiently lost after i.v. administration of OTR antagonist. Together, the present work provides straightforward evidence that blood levels of OT and AVP modulate nociception, windup plasticity and pain responses. The final target structures explaining these effects remains to be identified but are likely to be C-type nociceptors.

Topics: Action Potentials; Analgesics; Animals; Arginine Vasopressin; Disease Models, Animal; Dose-Response Relationship, Drug; Drug Interactions; Hyperalgesia; Male; Nerve Fibers, Unmyelinated; Neurons; Oxytocin; Pain; Pain Threshold; Rats; Rats, Sprague-Dawley; Receptors, Vasopressin; Spinal Cord; Swimming

Hypothalamospinal control of spinal pain processing by oxytocin (OT) has received a lot of attention in recent years because of its potency to reduce pain symptoms in inflammatory and neuropathic conditions. However, cellular and molecular mechanisms underlying OT spinal antinociception are still poorly understood. In this study, we used biochemical, electrophysiological, and behavioral approaches to demonstrate that OT levels are elevated in the spinal cord of rats exhibiting pain symptoms, 24 h after the induction of inflammation with an intraplantar injection of λ-carrageenan. Using a selective OT receptor antagonist, we demonstrate that this elevated OT content is responsible for a tonic analgesia exerted on both mechanical and thermal modalities. This phenomenon appeared to be mediated by an OT receptor-mediated stimulation of neurosteroidogenesis, which leads to an increase in GABA(A) receptor-mediated synaptic inhibition in lamina II spinal cord neurons. We also provide evidence that this novel mechanism of OT-mediated spinal antinociception may be controlled by extracellular signal-related protein kinases, ERK1/2, after OT receptor activation. The oxytocinergic inhibitory control of spinal pain processing is emerging as an interesting target for future therapies since it recruits several molecular mechanisms, which are likely to exert a long-lasting analgesia through nongenomic and possibly genomic effects.

Topics: Analgesia; Animals; Carrageenan; Hormone Antagonists; Inhibitory Postsynaptic Potentials; Male; Miniature Postsynaptic Potentials; Neurons; Oxytocin; Pain; Pregnanolone; Rats; Rats, Sprague-Dawley; Receptors, GABA-A; Receptors, Oxytocin; Spinal Cord; Synaptic Transmission