THE EFFECTS OF METHIONINE-ENKEPHALIN AND ITS RELATED METABOLITES UPONTHE DURATION OF THE DORSAL IMMOBILITY RESPONSE IN RATS

The effects of SC injections of methionine-enkephalin (Met1-5-Enk) and its N-terminal and C-terminal fragments upon the duration of the dors al immobility response (DIR) over a 60-min time course were investigat ed. Experiment 1 analyzed the effects of various dosages (0.00-100.0 m ug/kg) on DIR resulting in a potentiation of the duration in a dose-ti me course function. The effects of various fragments of Met1-5-Enk (10 .0 mug/kg) from the N-terminal in Experiment 2 and from the C-terminal in Experiment 3 on the DIR resulted in the potentiation of the durati on with the Met2-5-Enk and Met1-3-Enk fragments. All other fragments w ere not significant. The results were discussed in reference to the pr ocessing and metabolism of Met1-5-Enk.