Endorphins do not affect behavioral stress responses in mice

Effects of endorphins on behavioral stress responses were investigated in mice. For this purpose, we used environment-induced conditioned suppression of motility and forced swimming-induced immobility. The cerebral ventricular administration of α-endorphin (2.5-10 nmol), β-endorphin (0.38-1.5 nmol), or γ-endorphin (2.5-10 nmol) failed to affect either the environment-induced conditioned suppression of motility or the forced swimming-induced immobility. We have indicated previously that enkephalins attenuate both stress responses and, in contrast, dynorphin potentiates them. These findings indicate that the endorphinergic systems are not responsible for behavioral stress responses and that the role played by endorphins in the present stressful situations may be different from that of enkephalin and dynorphin.