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We investigated the effects of [D-Ala2,D-Leu5]enkephalin (DADLE). [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO), [D-Pen2,D-Pen5]enkephalin (DPDPE) (0.01-1 microM) and bremazocine (0.001-0.3 microM) 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]acetylcholine (ACh, striatum) and [3H]DA (striatum) release represent mu, delta and kappa receptors, respectively. In agreement with this classification, preincubation (60 min) of the slices with the delta-opioid receptor-selective irreversible ligand, fentanyl isothiocyanate (FIT, 0.01-1 microM), 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 mu as well as of delta receptors. In this case, the reversible mu antagonist, naloxone (0.1 microM), fully antagonized the inhibitory effect of the mu agonist, DAGO, without changing the effect of the delta agonist DPDPE but was ineffective as an antagonist in slices pretreated with FIT (1 microM). 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 mu-, delta- and kappa-opioid receptors in rat brain but also evidence strongly that mu and delta 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.

The release of 14C-ACh from rat nucleus accumbens slices, induced by 15 mM [K+], was inhibited by the mu- and delta-opioid agonists DAMGO and DPDPE, respectively, whereas only the kappa agonist U50,488 reduced the release of 3H-DA. The opioid receptors involved appear to be localized on nerve terminals, since blockade of action potential propagation by 1 microM TTX did not diminish the inhibitory effects of DAMGO, DPDPE or U50,488. Enhancement of the potassium concentration in the superfusion medium to 56 mM with simultaneous reduction of the Ca2+ concentration from 1.2 mM to 0.12 mM induced a release similar to that caused by 15 mM K+ and 1.2 mM Ca+. Under this conditions, the inhibitory effects of both DAMGO and DPDPE on stimulated 14C-ACh release were reduced, whereas the inhibition of evoked 3H-DA release caused by U50,488 was not affected. Activation of mu- as well as delta-opioid receptors by DAMGO and DPDPE, respectively, inhibited forskolin-stimulated adenylate cyclase activity. However, increasing the intracellular cAMP levels with 0.3 mM 8-bromo-cAMP affected neither the depolarization-induced release of 14C-ACh or 3H-DA from accumbens slices nor the inhibitory effects of opioid receptor activation thereon. The results indicate that the mechanism by which functional mu and delta receptors presynaptically inhibit the depolarization-induced 14C-ACh release from nucleus accumbens slices is likely to involve an increase of potassium channel conductance. In contrast, activation of kappa-opioid receptors, which inhibits depolarization-evoked 3H-DA release, apparently does not result in a hyperpolarization of (dopaminergic) nerve terminals. In none of these inhibitory effects presynaptic adenylate cyclase appears to be involved.

The modulation of the electrically evoked release of [³H]dopamine (DA) and [³H]noradrenaline (NA) by opioid receptor activation was examined in superfused slices of rat mediobasal hypothalamus (MBH). [³H]DA release was inhibited (maximally by 30–35%) by both the selective ĸ-agonist U 50,488 (1 nM to 1 µM) and the selective µ-agonist DAGO (0.01–1 µM) but not by the δ-selective agonist DPDPE (1 µM). Naloxone partly antagonized the inhibitory effect of U 50,488 and completely that of DAGO, whereas the selective K-antagonist norbinaltorphimine (nor-BNI) only antagonized the inhibition caused by U 50,488. The dopamine D₂ receptor agonist quinpirole as well as the α₂-adrenoceptor agonist oxymetazoline both decreased (by 25–30%) the evoked overflow of [³H]DA. The evoked release of [³H]NA was not modulated by any of the opioid agonists nor by quinpirole. However, the α₂-adrenoceptor agonist oxymetazoline inhibited the release of [³H]NA by 30–40%. Activation of α₂-adrenoceptors by oxymetazoline prevented the inhibitory effect of U 50,488, but not DAGO, on evoked [³H]DA release, whereas the selective K-antagonist nor-BNI antagonized the inhibition by oxymetazoline of [³H]DA, but not [³H]NA, release. In conclusion, activation of both ĸ- and µ-opioid receptors results in an inhibition of evoked DA release from MBH slices but does not modulate NA release. Therefore, several of the reported effects of opioids on hormone secretion may be an (indirect) consequence of a reduction of DA release. Moreover, the results suggest that activation of α₂-adrenoceptors inhibits DA release indirectly by stimulating release of endogenous opioids (probably dynorphins) which in turn inhibit evoked DA release by activation of K receptors.

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