Pharmacological profile of various kappa-agonists at kappa-, mu- and delta-opioid receptors mediating presynaptic inhibition of neurotransmitter release in the rat brain.
1. The potency, relative efficacy and selectivity of a series of kappa-opioid receptor agonists at the mu-, delta- and kappa-opioid receptors mediating inhibition of electrically-induced (radiolabelled) neurotransmitter release from superfused rat brain slices was determined. 2. With regard to their potencies at kappa-receptors mediating inhibition of striatal [3H]-dopamine release, the highest pD2 value (8.7) was found for bremazocine and the lowest (7.1) for U50488; the pD2 values for ethylketocyclazocine (EKC), tifluadom, U69593 and PD117302 were between 8.0 and 8.3. There were no marked differences between the relative efficacies of the kappa-agonists (maximum inhibition being 60-70%). In contrast to the other kappa-agonists, at a concentration of 1 microM, PD117302 caused a significant (25-40%) increase of the spontaneous efflux of tritium. 3. None of the kappa-agonists significantly affected striatal [14C]-acetylcholine (ACh) release, with the exception of a slight inhibitory effect of EKC. The delta-receptor-mediated inhibitory effect of [D-Ala2, D-Leu5]enkephalin (DADLE) on [14C]-ACh release was antagonized in a concentration-dependent manner by bremazocine (0.1 and 1.0 microM) and also partially by EKC (1 microM), but not by the other kappa-agonists. The pA2 value for bremazocine as an antagonist at the delta-receptors involved was 8.0, compared to 7.6 for naloxone. 4. None of the kappa-agonists significantly affected cortical [3H]-noradrenaline (NA) release, with the notable exception of tifluadom, which strongly inhibited release by activating mu-receptors. The mu-receptor-mediated inhibitory effect of Tyr-D-Ala-Gly-(NMe)Phe-Gly-ol (DAMGO) on [3H]-NA release was antagonized in a concentration-dependent manner by bremazocine and EKC, but not by the other K-agonists. The pA2 value for bremazocine as an antagonist at the mu-receptors involved was 8.2, compared to 8.6 for naloxone. 5. Thus, whereas U69593 and PD1 17302 display high potency and selectivity towards K-opioid receptors, the potent benzomorphan K-agonists bremazocine and EKC also appear to be strong mu-opioid receptor antagonists.
Mulder
A H
AH
Department of Pharmacology, Free University Medical Faculty, Amsterdam, The Netherlands.
Burger
D M
DM
Wardeh
G
G
Hogenboom
F
F
Frankhuyzen
A L
AL
D000109:Acetylcholine / Q000378:metabolism; D000818:Animals; D001575:Benzomorphans / Q000494:pharmacology; D001921:Brain / Q000187:drug effects* / Q000378:metabolism; D003342:Corpus Striatum / Q000187:drug effects / Q000378:metabolism; D004298:Dopamine / Q000378:metabolism; D016314:Ethylketocyclazocine / Q000494:pharmacology; D066298:In Vitro Techniques; D008297:Male; D018377:Neurotransmitter Agents / Q000378:metabolism; D009638:Norepinephrine / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D011957:Receptors, Opioid / Q000187:drug effects* / Q000502:physiology; D017465:Receptors, Opioid, delta; D017473:Receptors, Opioid, kappa; D017450:Receptors, Opioid, mu
D001575:Benzomorphans; D018377:Neurotransmitter Agents; D011957:Receptors, Opioid; D017465:Receptors, Opioid, delta; D017473:Receptors, Opioid, kappa; D017450:Receptors, Opioid, mu; D016314:Ethylketocyclazocine; C027248:bremazocine; D000109:Acetylcholine; D004298:Dopamine; D009638:Norepinephrine