The present study addressed the question as to whether or not' interacting mu and delta opioid receptors, which may constitute an opioid receptor complex-inhibitory coupled to adenylate cyclase in rat neostriatum, display different antagonistic properties than the classical (noncomplexed) mu and delta receptors. In concentrations that antagonized the presynaptic inhibitory effect of [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAMGO) on [3H]norepinephrine release from rat neocortical slices, the cyclic somatostatin-related mu opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 did not affect the inhibition of dopamine-sensitive adenylate cyclase caused by DAMGO in neostriatal slices. The delta opioid receptor antagonist naltrindole appeared to be about 200-fold more effective as an antagonist against inhibitory effect of [D-Ser2(O-tert-butyl),Leu5]enkephalyl-Thr6 on [14C]acetylcholine release from neostriatal slices than against the inhibitory effect of DAMGO on [3H]norepinephrine release from neocortical slices, in agreement with the involvement of presynaptic delta and mu receptors, respectively. However, regarding the inhibitory effect of DAMGO and [D-Ser2(O-tert-butyl),Leu5] enkephalyl-Thr6 on adenylate cyclase activity in neostriatal slices, naltrindole not only displayed a very low affinity but also only 10-fold delta-selectivity. In striking contrast to D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 and naltrindole, naloxone did not discriminate between the neurotransmitter release-and adenylate cyclase-inhibitory effects of DAMGO and [D-Ser2(O-tert-butyl), Leu5]enkephalyl-Thr6.(ABSTRACT TRUNCATED AT 250 WORDS)