Abstract

Electrical stimulation of the nucleus tractus solitarius(NTS) produces opioid-mediated antinociception. The NTS has extensive reciprocal connections with higher brain structures. Notable among these are connections with nucleus raphe magnus (NRM). This study sought to determine whether NTS stimulation-produced antinociception (SPA) relies on cennections with higher brain structures and/or the NRM. Male Sparague-Dawley rats were anestheuized with pentobarbital (55mg/kg) and mounted stereotaxic apparatus. Pain sensitivity was masureed using the tail-flick test. In the experiment 1, a stimulating electrode was positioned in the NTS and holes were drilled so that a guide cannular could be lowered into the hole Through this cannula a wire was inserted allowing coronal brain section to be made. Thresholds for SPA were determined before and after coronal sections. Sections were made rostral to the midbrain and between the NTS and midbrain. In the experiment 2, similar procedures were followed except selective electrolytic lesion (1mA, DC, 20 sec) of the NRM were made. Transections between the NTS and midbrain significantly increased the threshold for SPA in the NTS. Cuts made rostral to the midbrain had no effect on SPA threshold, NRM lesions also increased SPA threshold. In this experiment, baseline tail-flick latencies were always increased for approximately 2 to 5 minutes after the lesion. Taken together, these results suggest that stimulation of the NTS produces antinociception via not any brain nuclei rostral to midbrain but a descending system arising in the NRM at least.