Paraventricular hypothalamic influences on spinal nociceptive processing.

Oxytocin properties have been studied in different experimental models in order to obtain evidence for its analgesic properties. The analgesic effect of an oxytocinergic pathway descending from the hypothalamus reaching the dorsal horn of the spinal cord has been studied. In anesthetized rats, we recorded single units at the L4-L5 spinal dorsal horn level and stimulated the peripheral receptive field. The evoked responses were classified according to their latencies in A-beta, A-delta, C fibers, and postdischarge. We used these responses to evaluate the effects of electrical stimulation of the paraventricular nucleus (PV) of the hypothalamus. We observed a selective blockage of A-delta and C fibers related to the duration of the train stimulus duration. Similar effects were observed when oxytocin (OT) was applied directly on the spinal cord. The effects of OT and of PV electrical stimulation were reversed in a dose-dependent manner by application of the specific OT antagonist (OTA). These effects were observed in cells with reduced wind-up and cells displaying a clear wind-up response to peripheral stimulation. Superficial and deeper cells in the dorsal spinal cord were involved. The recorded cells were marked by pontamine blue iontophoretic injection after each cell recording, and their histological locations were specified. In order to obtain a behavioral correlation, we used rats with a loose ligature of the sciatic nerve and a chronic intrathecal catheter reaching the L4-L5 spinal cord level. We tested the hyperalgesia and allodynia of these animals using von Frey filaments and the application of acetone to the hind paws. Our results show a significant reduction in the mechanical and thermal test after the administration of 15 microl of 10(-6) M OT. Our electrophysiological, pharmacological, and behavioral results point out a clear OT antialgesic effect. The results are discussed on the basis of a previous work showing an OT blockage of glutamate activation. The paraventricular hypothalamic descending OT pathway is proposed as an interesting mechanism producing analgesia.