Effect of electrical stimulation of the hippocampus upon corticosteroid levels in the freely-behaving, non-stressed rat.

The influence of the hippocampus upon pituitary release of adrenocorticotropic hormone (ACTH) was studied in male Sprague-Dawley rats which had concentric bipolar stainless stell electrodes chemically implanted in the dorsal and ventral hippocampus. Blood samples were withdrawn at half-hour intervals from a cannula in the right atrium. Plasma corticosterone levels reflected ACTH release. 30 min of 25 or 250-cps electrical stimulation of the hippocampus in the freely-behaving, non-stressed rat resulted in significant increases in basal plasma corticosterone levels evident at the end of stimulation. In afternoon experiments, after stimulation at either hippocampal site with 25 cps only, the increase was followed by a delayed inhibitroy phase and temporary inhibition of the circadian rise in corticosterone levels. The inhibitory effect was more pronounced with ventral than with dorsal hippocampal stimulation. The basis for these differential effects is discussed in terms of the anatomical organization of the hippocampus.