Effects of endomorphin-1 on open-field behavior and on the hypothalamic-pituitary-adrenal system.

The effects of endomorphin-1 (EM1) on behavioral responses and on the hypothalamic-pituitary-adrenal system were investigated in mice. Locomotor activity was measured in an "open-field" apparatus, with parallel recording of the numbers of rearings and groomings. Different doses of the peptide (250 ng to 5 microg) were administered to the animals intracerebroventricularly 30 min before the tests. EM1 caused significant increases in the locomotor activity and the number of rearings. The effect of EM1 on the basal corticosterone secretion was also investigated. At a dose of 5 microg, the peptide significantly increased plasma corticosterone level. The corticotropin-releasing hormone (CRH) antagonist alpha-helical CRH9-41, applied 30 min prior to EM1 administration, completely abolished the increases in both locomotion and the number of rearings and attenuated the corticosterone release evoked by EM1. These results suggest that the EM1 -induced increases in locomotion and rearing activity as well as the pituitary-adrenal activation are mediated by CRH.