The effects of endorphins on cardiac responses during an emotional stress.

Male rats were punished (0.6 mA/2 sec) for entering the shock compartment of a passive avoidance chamber. Twenty four hours later they were returned to the apparatus and confined to the grid floor of the shock compartment for 5 min (forced exposure). The bradycardic response found in control rats during forced exposure was not altered by either alpha-endorphin (beta-LPH 61-76) or (des-tyrl)-alpha-endorphin (beta-LPH 62-76) (1.5 micrograms/rat) injected subcutaneously 60 min prior to forced exposure. In contrast (des-tyrl)-gamma-endorphin (beta-LPH 62-77) (1.5 micrograms/rat/SC) enhanced the bradycardic response of shocked rats during forced exposure without altering heart rate in non-shocked rats. (Des-tyrl)-gamma-endorphin did not affect the rats' bradycardic response to a sudden reduction in background noise. The results conform to a pattern whereby C-terminal beta-LPH fragments may be distinguished in a number of behavioural tests and show that (des-tyrl)-gamma-endorphin treatment facilitates the rat's cardiac response to aversive but not novel experiences.