The effect of thalamic nucleus submedius lesions on nociceptive responding in rats.

Previous studies have shown that the thalamic nucleus submedius (SM) contains nociceptive neurons and is interconnected with spinal, brain-stem and cortical regions associated with nociception. The present study was performed to examine the role of the SM in nociceptive-related behaviors. The effect of SM lesions on nociceptive responding in rats was assessed using both the radiant-heat tail-flick (TF) and the tail-shock 'pain-induced' vocalization (PIV) tests. The results of Exp. 1 indicated that the intensity of electrical shock required for vocalization responses was significantly decreased following SM lesions. No changes in vocalization responses were present in the sham-lesion group. In contrast, both the sham- and SM-lesion groups exhibited a significant post-lesion increase in TF latencies. A second experiment was performed to determine whether the effects of SM lesion on the tail flick may have been masked by conditioned antinociception associated with noxious electrical stimulation of the tail to produce PIV. The results indicated that there was no post-lesion change in TF latencies in either the SM- or sham-lesion group when the antecedent PIV test was omitted. The results suggest that the SM may play a role in supraspinally mediated inhibition of nociceptive input but not in spinally mediated responses to noxious stimuli.