Effect of stress exposure on the activation pattern of enkephalin-containing perikarya in the rat ventral medulla

We examined the effects of acute and chronic psychogenic stress on the acti vation pattern of enkephalin-containing perikarya in the rat ventrolateral medulla. Rats allocated to the chronic stress groups were subjected to 90 m in of immobilization for 10 days. On the 11th day, the chronically stressed rats were exposed to homotypic (90-min immobilization) or to heterotypic b ut still psychogenic (90-min immobilization coupled to air jet stress) stre ss. The acute stress group was subjected once to an acute 90-min immobiliza tion. For each group, the rats were anesthetized either before stress (time 0) or 90, 180, and 270 min after the onset of stress. Brain sections were then processed using immunocytochemistry (Fos protein) followed by radioact ive in situ hybridization histochemistry (enkephalin mRNA). Following immob ilization, the acute group displayed a marked increase in the number of act ivated en kephalin-containing perikarya within the paragigantocellularis an d lateral reticular nuclei. This level of activation was sustained up to 18 0 min following the onset of the immobilization stress and had returned to baseline levels by 270 min from the initiation of the stress. However, this stress-induced activation of enkephalin-containing perikarya of the ventro lateral medulla was not seen following either homotypic or heterotypic stre ss in the chronically stressed group. These results provide evidence that e nkephalin-containing perikarya of the ventrolateral medulla may constitute a potential circuit through which they regulate some aspect of the stress r esponses. Conversely, this enkephalinergic influence from the ventrolateral medulla was shown to be absent following chronic stress exposure. This wou ld suggest a decrease in enkephalin inhibitory input originating from the v entrolateral medulla, thereby allowing a neuroendocrine and/or autonomic re sponse to chronic stress.