PROLYL-LEUCYL-GLYCINAMIDE, THYROTROPIN-RELEASING-HORMONE AND BETA-ENDORPHIN-(10-16), LIKE ANTIDEPRESSANTS, ANTAGONIZE MELATONIN-INDUCED BEHAVIORAL-CHANGES IN RATS

beta-Endorphin-(10-16), as well as a variety of antidepressants, has b een reported to block the behavioural changes induced by injecting mel atonin into the nucleus accumbens. In the present study the influence of subcutaneously administered prolyl-leucyl-glycinamide (PLG) and thy rotropin-releasing hormone (TRH) on the behavioural changes induced by melatonin administration in the nucleus accumbens were investigated a nd compared with that of beta-endorphin-(10-16). PLG and TRH were foun d to be as effective as beta-endorphin-(10-16) in counteracting the me latonin-induced behavioural changes. The data suggest that these pepti des may serve as a starting point for the development of a new class o f antidepressants.