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 hypot halamic-pituitary-adrenal system were investigated in mice. Locomotor activ ity was measured in an "open-field" apparatus, with parallel recording of t he numbers of rearings and groomings, Different doses of the peptide (250 n g to 5 mug) were administered to the animals intracerebroventricularly 30 m in before the tests. EM1 caused significant increases in the locomotor acti vity and the number of rearings, The effect of EM1 on the basal corticoster one secretion was also investigated. At a dose of 5 mug, the peptide signif icantly 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 pituitaryadrenal activation are mediated by CRH.