μ-, δ- and κ-opioid receptor-mediated inhibition of neurotransmitter release and adenylate cyclase activity in rat brain slices: studies with fentanyl isothiocyanate

Abstract

We investigated the effects of [D-Ala 2, D-Leu 5]enkephalin (DADLE). [D-Ala 2, MePhe 4,Gly-ol 5]enkephalin (DAGO), [D-Pen 2,D-Pen 5]enkephalin (DPDPE) (0.01–1 μM) and bremazocine (0.001–03 μM) on the electrically evoked release of radiolabelled neurotransmitters and on the dopamine (DA)-stimulated cyclic AMP efflux from superfused rat brain slices. The differential inhibitory effects of these agonists on the evoked neurotransmitter release indicate that the opioid receptors mediating presynaptic inhibition of [ 3H]noradrenaline (NA, cortex), [ 14C]acethylcholine (ACh, striatum) and [ 3H]DA (striatum) release represent μ, δ and κ receptors, respectively. In agreement with this classification, preincubation (60 min) of the slices with the δ-opioid receptor-selective irreversibel ligand, fentanyl isothiocyanate (FIT, 0.01–1 μM), antagonized the inhibitory effects of DADLE and DPDPE on striatal [ 14C]ACh release only. On the other hand, the D-1 DA receptor-stimulated cyclic AMP efflux from striatal slices appeared to be inhibited by activation of μ as well as of δ receptors. In this case, the reversible μ antagonist, naloxone (0.1 μM), fully antagonized the inhibitory effect of the μ agonist, DAGO, without changing the effect of the δ agonist DPDPE but was ineffective as an antagonist in slices pretreated with FIT (1 μM). The inhibitory effect of DAGO on the electrically evoked [ 3H]NA release was antagonized by naloxone whether the receptors were irreversibly blocked by FIT or not. These data not only further support the existence of independent presynaptic μ-, δ- and κ-opioid receptors in rat brain but also evidence strongly that μ and δ receptors mediating the inhibition of DA-sensitive adenylate cyclase could share a common binding site (for naloxone and FIT) and, therefore, may represent constituents of a functional opioid receptor complex.