Immunocytochemical changes in hypothalamic and pituitary hormones after acute and prolonged stressful stimuli in the anestrous ewe.

We studied the effect of short (acute (20 min/h, for 4 h) and intermittent, long-term (20 min/h for 9 h on 3 consecutive days) electric foot shocks on the immunocytochemical localization of CRH and SRIH in the hypothalamus and of ACTH, beta-endorphin, GH and PRL in the pituitary of the anestrous ewe. Acute stress greatly reduced immunoreactive (ir) CRH in the median eminence and cellular irACTH, beta-endorphin and PRL, as well as the proportion of these cell types in the pituitary. A slight reduction of irSRIH in the median eminence was also observed. After long-term stress, reduction of irCRH in the median eminence was still observed. However, ACTH/beta-endorphin cells in the pituitary gland displayed increased secretory activity, manifested by hypertrophy and hyperplasia. A marked depletion of irSRIH in the nerve terminals of the median eminence was observed. The proportion of PRL cells but not their ir content returned to control levels. No effects were observed on the features of the GH cells. This study indicates that there are differences in the effect of short- and long-term stressful stimuli on the activity of hormonal systems in the anestrous ewe. Short-term stress immediately activates the CRH/ACTH/beta-endorphin axis. Prolonged stress appears to augment the activation of the SRIH hypothalamic system and probably has a restraining effect on ACTH/beta-endorphin release.