Effects of morphine and two enzyme resistant enkephalins on schedule-controlled responding in the rat.

Water-deprived rats to respond for access to a water-filled dipper under a 20 response fixed-ratio schedule. After training, the rats were each implanted with a lateral ventricle cannula and the effects of intraventricular injection of morphine, (N-methyl-tyrosyl)1-des COOH-Norleucyl5-Enkephalin, and (D-Ala)2 -(des COOH-Norleucyl)5-Enkephalin were determined. All three compounds produced dose-related decreases in FR20 responding. Morphine was about 5 times more potent than either of the two peptides. Naloxone (1.0 mg/kg, SC) antagonized the response-rate decreasing effects of intraventricular morphine and the two opiate peptides.