In vitro adrenocortical responsiveness to ACTH in rats subjected to different emotional situations

Corticosterone production in isolated adrenocortical cells from adult male Wistar rats subjected to different emotional situations and the possible relationship between adrenocortical and behavioral responses were studied. On the basis of an inhibitory avoidance paradigm (IAP), in which the innate preference of rats for a dark environment is suppressed by application of an aversive electric footshock (acquisition trial, AT), the following set of conditions was studied: Rats trained in the IAP (a) without application of electric footshock; (b) with application of electric footshock; (c) rats trained as in (b) but subjected to forced extinction (by staying for 5 min in the dark compartment 24 h after the AT, the animals overcome their fear for the dark box). Rats trained in conditions (a) and (c) displayed significantly shorter retention latencies, and concomitantly showed a significantly lower adrenocortical responsiveness than rats trained under condition (b). However, no significant differences in circulating ACTH or corticosterone levels among the various experimental groups were found. Furthermore, animals tested for retention 24, 48, 72 or 168 h after the AT showed an increased loss of the behavioural response so that the greater the time lapse, the more the retention behavior diminished concomitantly with the adrenocortical responsiveness to ACTH. Our results suggest that a good retention of the newly acquired behavior is related to an increased adrenocortical responsiveness to ACTH. Forced or innate extinction of the same behavior is accompanied by a diminished adrenocortical responsiveness. Taken together, these results also emphasize that altered adrenal responsiveness to ACTH is a better and more accurate index of the degree of emotionality associated with different behavioral performances than 'punctual' ACTH or corticosterone plasma levels.