Beta-endorphin-(10-16) antagonizes behavioral responses elicited by melatonin following injection into the nucleus accumbens of rats.

The behavioral changes induced by low doses of melatonin bilaterally injected into the nucleus accumbens of rats (decrease of locomotor activity and rearing and increase of grooming and sniffing behavior) were not affected by local pretreatment with beta-endorphin, but could be completely antagonized by alpha-type and gamma-type endorphins. Structure activity relationship studies revealed that the peptide beta-endorphin-(10-16) contains the essential information in this respect. The lowest effective dose of this peptide was 10 pg. The peptide, in contrast to gamma-type endorphins, did not interfere with the decrease of locomotor activity and rearing induced by injection of low doses of apomorphine into the nucleus accumbens. It is concluded that the described action of beta-endorphin-(10-16) resembles that of serotonin and various antidepressant drugs.