The effects of microinjection of morphine into thalamic nucleus submedius on formalin-evoked nociceptive responses of neurons in the rat spinal dorsal horn.

Previous studies have indicated that the thalamic nucleus submedius (Sm), as an ascending component, is involved in an endogenous analgesic system consisting of spinal cord-Sm-ventrolateral orbital cortex (VLO)-periaqueductal gray (PAG)-spinal cord loop. To investigate the action of opioid in this antinociception pathway, the effects of microinjection of morphine and naloxone into the Sm on the formalin-induced nociceptive responses of neurons in the spinal dorsal horn were determined in the anesthetized rat. Formalin (5%, 50 microl) subcutaneously injected into unilateral hindpaw produced a biphasic nociceptive response which was similar to that obtained from assessing the nociceptive behavior either in the relative magnitude of response or the time course. A unilateral microinjection of morphine (5 microg, 0.5 microl) into the Sm 15 min after formalin injection significantly depressed the second phasic responses of neurons induced by formalin, and this effect was significantly attenuated by pre-microinjection of opioid receptor antagonist naloxone (1 microg, 0.5 microl) into the same site. The results suggest that the Sm is involved in opioid receptor-mediated antinociceptive effect on the persistent nociception through depression of the nociceptive transmission at the spinal cord level.