Selective attenuation of passive avoidance behaviour by microinjection of beta-LPH62-77 and beta-LPH66-77 into the nucleus accumbens in rats.

Various fragments of beta-lipotropin were micro-injected into the nucleus accumbens of free-moving rats and the effects on one-trial passive avoidance behaviour were studied. The following peptides were used: beta-endorphin (beta-LPH61-91), met-enkephalin (beta-LPH61-65), des-tyrosine-gamma-endorphin (DT gamma E, beta-LPH62-77), des-enkephalin-gamma-endorphin (DE gamma E, beta-LPH66-77), des-tyrosine-alpha-endorphin (DT alpha E, beta-LPH 62-76) and beta-LPH47-53(ACTH4-10). The gamma-type endorphins (DT gamma E and DE gamma E) were the only peptides which significantly attenuated passive avoidance behaviour when given in a 20 pg dose 1 hr before the 24-hr retention test. All the other peptides were ineffective. Microinjection of DT gamma E into the dentate gyrus of the dorsal hippocampus, on the other hand, was without effect on passive avoidance. The accumbens nucleus, therefore, is a preferential highly sensitive brain site for gamma-type endorphins. The attenuation of passive avoidance behaviour following systemic administration of these peptides might be the result of an interaction with this mesolimbic structure.