Effect of stimulation and lesion of the thalamic nucleus submedius on formalin-evoked nociceptive behavior in rats

The aim of the present study was to examine whether the thalamic nucleus submedius (Sm) was involved in the modulation of persistent nociception. Using an automated movement detection system to measure nociceptive behavior (agitation) induced by subcutaneous injection of formalin into the hind paw pad, the effects of electrical stimulation or electrolytic lesion of the Sm on the agitation response were examined in conscious rats. Unilateral stimulation (100 μA, 5 min) of the Sm ipsilateral or contralateral to the formalin-injected paw produced a significant inhibition of agitation response in the second phase, while stimulation of thalamic structures more than 0.5 mm away from the Sm had no effect on agitation response. Bilaterally electrolytic lesion of the Sm did not significantly influence the number of agitation events induced by formalin injection in the first phase or the second phase. These results suggest that the Sm is not only involved in the modulation of phase nociception, as reported previously, but also of persistent nociception. The present study provides novel evidence for the participation of the Sm in descending modulation of pain.