[Avoidance conditioned-reflex behavior in rats chronically administered leucine enkephalin and its synthetic tetrapeptide analog into the neostriatum].

The three-week daily administration of 5 and 15 micrograms leu-enkephalin and its synthetic tetrapeptide analogue into the rat neostriatum led to an inhibition of avoidance conditioning in shuttlebox. Starting on the 10th day of the synthetic analogue injections, a motor excitation and a stereotyped behaviour were observed. No withdrawal effects were found. The tetrapeptide analogue was more effective than the natural leu-enkephalin in the same dose. A possible dopaminergic mechanism involved in the enkephalin long-term treatment, is discussed.