Effect of medial hypothalamic stimulation inducing both escape and approach on unit activity in rat mesencephalon

The activity of central gray (CG) neurons was previously shown to be affected by medial hypothalamic (MH) stimulation, and the discharge rate alterations were found to be often closely correlated with the vigor of the stimulation-induced escape responses. Since MH stimulation induces not only escape responses but also approach responses, this allowed a further investigation of the degree of specificity of the correlation between stimulation-induced neuronal activity and escape responses. Out of 129 CG units studied (in 39 rats), 26 clearly responded to the MH stimulation. When the stimulation parameters were varied, the mean discharge rate alterations of 9 units located in the dorsal part of the mesencephalic CG proved to be closely correlated exclusively with the alterations that affected the stimulation-induced escape responses. Furthermore, the variation of the most frequent interspike interval (modal ISI) proved to be closely correlated with the variation of the escape response in 3 instances in which the mean discharge rate showed no such correlation. In a few instances (2 units located much more ventrally, the spike rate alterations were closely correlated only with the alterations that affected the approach responses. It is suggested that neuronal activity in the dorsal part of the CG is rather specifically implicated in the generation of MH stimulation-induced aversive effects and escape responses.