title: Blockade of D-2 dopamine receptors strongly enhances the potency of enkephalins to inhibit dopamine-sensitive adenylate cyclase in rat neostriatum: involvement of delta- and mu-opioid receptors.
abstract: The interactions between dopamine receptors and opioid receptors coupled to adenylate cyclase in rat neostriatum were investigated. cAMP efflux from neostriatal slices induced by simultaneous activation of (stimulatory) D-1 and (inhibitory) D-2 dopamine receptors with 30 microM dopamine was inhibited by the preferential delta-opioid receptor agonist [D-Ala2-D-Leu5] enkephalin (DADLE) and the mu-opioid receptor agonist morphine with an EC50 of 100 and 800 nM, respectively. On selective D-1 receptor activation (i.e., with D-2 receptors blocked by 10 microM (-)sulpiride), the EC50 of DADLE was strongly reduced to 3 nM, whereas that of morphine was unaffected. When D-1 and D-2 receptors were activated simultaneously, the inhibitory effects of DADLE (0.3 microM) and morphine (3 microM) on cAMP efflux were antagonized equally well by naloxone, a mu-opioid receptor antagonist. In contrast, on selective D-1 receptor activation, naloxone was about 20 times more potent in antagonizing the inhibitory effect of morphine than DADLE. Moreover, the delta-opioid receptor antagonist ICI 174864 (0.75 microM) did not affect the inhibitory effect of morphine but antagonized that of DADLE, provided that D-2 receptors were blocked. The highly selective delta-opioid receptor agonist [D-Pen2-D-Pen5] enkephalin (DPDPE) inhibited dopamine-stimulated cAMP efflux only when D-2 receptors were blocked. Similar results were obtained when the agonists SKF 38393 and LY 141865 were used to activate D-1 and D-2 receptors, respectively. These data indicate that blockade of D-2 receptors in the neostriatum elicits the coupling of delta-opioid receptors to dopamine-sensitive adenylate cyclase, thereby making it considerably more sensitive to inhibition by the enkephalins.
mesh_terms: D015647:2,3,4,5-Tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine; D000067956:Adenylyl Cyclase Inhibitors; D000818:Animals; D001552:Benzazepines / Q000494:pharmacology; D003342:Corpus Striatum / Q000187:drug effects / Q000201:enzymology*; D000242:Cyclic AMP / Q000378:metabolism; D004298:Dopamine / Q000494:pharmacology*; D020881:Enkephalin, D-Penicillamine (2,5)-; D004743:Enkephalin, Leucine / Q000031:analogs & derivatives / Q000494:pharmacology; D016308:Enkephalin, Leucine-2-Alanine; D004745:Enkephalins / Q000494:pharmacology*; D004873:Ergolines / Q000494:pharmacology; D008297:Male; D009020:Morphine / Q000494:pharmacology; D009270:Naloxone / Q000494:pharmacology; D019257:Quinpirole; D051381:Rats; D011919:Rats, Inbred Strains; D011954:Receptors, Dopamine / Q000187:drug effects*; D011957:Receptors, Opioid / Q000378:metabolism*; D017465:Receptors, Opioid, delta; D017450:Receptors, Opioid, mu
chemical_list: D000067956:Adenylyl Cyclase Inhibitors; D001552:Benzazepines; D004745:Enkephalins; D004873:Ergolines; D011954:Receptors, Dopamine; D011957:Receptors, Opioid; D017465:Receptors, Opioid, delta; D017450:Receptors, Opioid, mu; D019257:Quinpirole; D009270:Naloxone; D004743:Enkephalin, Leucine; D016308:Enkephalin, Leucine-2-Alanine; D015647:2,3,4,5-Tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine; D009020:Morphine; D020881:Enkephalin, D-Penicillamine (2,5)-; C040643:N,N-diallyl-tyrosyl-alpha-aminoisobutyric acid-phenylalanyl-leucine; D000242:Cyclic AMP; D004298:Dopamine