Behavioral effects of opioid peptides selective for mu or delta receptors. I. Morphine-like discriminative stimulus effects.
The morphine-like discriminative stimulus effects of opioid peptides with selectivity for the mu- or delta-opioid receptors were examined in rats trained to discriminate 3.0 mg/kg of morphine (s.c.) from saline in a two-choice discrete-trial avoidance paradigm. The mu-selective peptides D-Ala2-NMePhe4-Gly5(ol)enkephalin, FK 33,824 and morphiceptin, the delta-selective peptides D-Ala2-D-Leu5enkephalin and metkephamid and beta-endorphin (mu- and delta-selective) produced morphine-like stimulus effects after administration into the lateral ventricle. Generalization with the morphine cue was dose-dependent and occurred over a wide range of doses (0.01-30 micrograms), depending upon peptide. On a molar basis, the order of relative potency of the peptides as morphine-like discriminative stimuli was: D-Ala2-NMePhe4-Gly5(ol)enkephalin = FK 33,824 greater than beta-endorphin greater than D-Ala2-D-Leu5enkephalin = metkephamid greater than morphiceptin. The discriminative effects of D-Ala2-NMePhe4-Gly5(ol)enkephalin, D-Ala2-D-Leu5enkephalin and beta-endorphin were antagonized by low doses of s.c. naltrexone (0.01-1.0 mg/kg). Furthermore, the stimulus effects of s.c. morphine were antagonized by 24-hr pretreatment of rats with the irreversible mu-antagonist beta-funaltrexamine (5.0 micrograms i.c.v.). Based upon the order of relative potency of the peptides and the relative potency for antagonism of their discriminative effects by naltrexone and beta-funaltrexamine, mu-opioid receptors in the brain appear to be an important element in the genesis of morphine-like discriminative effects by opioid peptides.
D000818:Animals; D005405:D-Ala(2),MePhe(4),Met(0)-ol-enkephalin / Q000494:pharmacology; D004193:Discrimination Learning / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D004723:Endorphins / Q000494:pharmacology*; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D004743:Enkephalin, Leucine / Q000031:analogs & derivatives / Q000494:pharmacology; D016308:Enkephalin, Leucine-2-Alanine; D004745:Enkephalins / Q000494:pharmacology; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D009271:Naltrexone / Q000031:analogs & derivatives / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D011957:Receptors, Opioid / Q000187:drug effects*; D017465:Receptors, Opioid, delta; D017450:Receptors, Opioid, mu; D001615:beta-Endorphin
D004723:Endorphins; D004745:Enkephalins; D011957:Receptors, Opioid; D017465:Receptors, Opioid, delta; D017450:Receptors, Opioid, mu; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D004743:Enkephalin, Leucine; D009271:Naltrexone; D001615:beta-Endorphin; D016308:Enkephalin, Leucine-2-Alanine; D005405:D-Ala(2),MePhe(4),Met(0)-ol-enkephalin; C024524:beta-funaltrexamine; D009020:Morphine