The effects of neuropeptides on discrete-trial conditioned avoidance responding.

The effects of intracerebroventricular administration of several peptides on discrete-trial, conditioned avoidance responding were assessed in the rat. Three peptides (neurotensin, bombesin and beta-endorphin) produced a neuroleptic-like effect (i.e. a decrease in avoidance responding with no effect on escape responding). A low dose (0.6 nmol) of each peptide elicited a significant effect. Neurotensin and bombesin produced a significant but partial decrease in avoidance responding; larger doses of these peptides did not produce a greater effect. beta-Endorphin elicited dose-related decrements in avoidance responding. In addition, the effect of neurotensin, but not bombesin or beta-endorphin, was antagonized by simultaneous administration of an equimolar dose of thyrotropin-releasing hormone. Hence, the 3 peptides do not appear to produce decreases in avoidance responding by the same mechanism. Thyrotropin-releasing hormone, luteinizing hormone-releasing hormone, bradykinin, substance P, des-Tyr1-gamma-endorphin and melanotropin inhibiting factor did not significantly affect avoidance responding. These findings, taken together with previous findings, suggest that intracerebroventricular administration of certain endogenous peptides (neurotensin, bombesin and beta-endorphin) may exert neuroleptic-like effects.