Discriminative stimulus effects of mu and kappa opioids in the pigeon: analysis of the effects of full and partial mu and kappa agonists.
Pigeons were trained to discriminate a dose of either 0.01 mg/kg of bremazocine or 0.05 mg/kg of fentanyl from water using a two-key drug discrimination procedure. During tests of substitution, the selective kappa-opioid agonists bremazocine, U50, 488 and tifluadom substituted for the bremazocine stimulus, whereas the less selective kappa-opioid agonists ethylketocyclazocine, levallorphan, proxorphan and nalorphine substituted for the fentanyl stimulus. The full mu-opioid agonists fentanyl, morphine, I-methadone and levorphanol, as well as the partial agonists nalbuphine, butorphanol and buprenorphine, substituted for the fentanyl stimulus. Compounds with partial-opioid agonist effects, namely nalbuphine, butorphanol, buprenorphine, proxorphan, levallorphan and nalorphine, produced 50% fentanyl-appropriate responding at doses 25 to 369.2 times smaller than the doses required to decrease response rates to 50% of control values. In contrast, the full mu-opioid agonists fentanyl, morphine, I-methadone and levorphanol produced 50% fentanyl-appropriate responding at doses only 1.3 to 10.9 times smaller than those required to decrease response rates by 50%. During tests of antagonism, both naloxone and Mr2266 produced a dose-dependent attenuation of the stimulus effects of bremazocine and fentanyl, whereas beta-funaltrexamine antagonized the stimulus effects of fentanyl but not bremazocine. Although bremazocine has been reported to have mu-opioid antagonist effects, it failed to antagonize the stimulus effects of the training dose of fentanyl. The present investigation establishes further that pigeons can discriminate selective kappa-opioid agonists from mu-opioid agonists and that in pigeons the classification of numerous opioid compounds on the basis of their kappa-like or mu-like stimulus effects differ from those in rat and monkey. In addition, under the drug discrimination procedure the actions of compounds classified as partial-opioid agonists can be differentiated from those of full mu-opioid agonists on the basis of the ratio of the dose required to engender fentanyl-like stimulus effects to the dose required to reduce response rates.(ABSTRACT TRUNCATED AT 250 WORDS)
Picker
M J
MJ
Department of Psychology, University of North Carolina, Chapel Hill.
Dykstra
L A
LA
D019900:3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; D000818:Animals; D001569:Benzodiazepines / Q000494:pharmacology; D001575:Benzomorphans / Q000494:pharmacology; D010856:Columbidae; D003496:Cyclazocine / Q000031:analogs & derivatives / Q000494:pharmacology; D004193:Discrimination Learning / Q000187:drug effects*; D016314:Ethylketocyclazocine; D005283:Fentanyl / Q000494:pharmacology; D009019:Morphinans / Q000031:analogs & derivatives / Q000494:pharmacology; D009270:Naloxone / Q000494:pharmacology; D009294:Narcotics / Q000494:pharmacology; D011759:Pyrrolidines / Q000494:pharmacology; D011957:Receptors, Opioid / Q000187:drug effects*; D017473:Receptors, Opioid, kappa; D017450:Receptors, Opioid, mu
D001575:Benzomorphans; D009019:Morphinans; D009294:Narcotics; D011759:Pyrrolidines; D011957:Receptors, Opioid; D017473:Receptors, Opioid, kappa; D017450:Receptors, Opioid, mu; D001569:Benzodiazepines; D009270:Naloxone; C027247:MR 2266; D016314:Ethylketocyclazocine; D019900:3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; C008732:oxilorphan; C027248:bremazocine; D003496:Cyclazocine; C035470:tifluadom; D005283:Fentanyl