Morphine microinjections into the rat nucleus submedius depress nociceptive behavior in the formalin test.

Our previous studies have indicated that the thalamic nucleus submedius (Sm) is involved in modulation of nociception and plays an important role in an endogenous analgesic system (a feedback loop) consisting of spinal cord-Sm-ventrolateral orbital cortex-periaqueductal gray-spinal cord. To investigate whether opioids are involved in this antinociception pathway, the effects of microinjection of morphine and naloxone into the Sm on the nociceptive behavior (agitation) evoked in the formalin test were investigated in the awake rat using an automated movement detection system. The results indicate that a unilateral microinjection of morphine (5 micro g, 0.5 microl) into the Sm suppresses the formalin-induced agitation response, but does not influence spontaneous motor activity, and that the morphine-induced depression can be reversed by microinjection of the opioid receptor antagonist naloxone (1.0 micro g, 0.5 microl) into the same Sm site. The results suggest that opioid receptors in the Sm may be involved in the Sm-mediated depression of persistent inflammatory pain.