μ-Opioid Receptors Mediate the Inhibitory Effect of Opioids on Dopamine-Sensitive Adenylate Cyclase in Primary Cultures of Rat Neostriatal Neurons

The receptors mediating the inhibition of D1 dopamine receptor-stimulated adenylate cyclase by opioids were examined in primary cultures of rat neostriatal neurons. Adenylate cyclase activity was dose-dependently increased by the selective D1 dopamine receptor agonist SKF 38393 (EC50 = 0.05 microM). This stimulation was fully antagonized by the selective D1 dopamine receptor antagonist SCH 23390 (1 microM). SKF 38393 (1 microM)-stimulated adenylate cyclase activity was strongly reduced (by almost 60%) by the highly selective mu-agonist [D-Ala2, MePhe4, Gly-ol5]-enkephalin (DAGO; EC50 = 0.006 microM) and high concentrations of the selective delta-agonist [D-Ser2(O-tert-butyl), Leu5]-enkephalyl-Thr6 (DSTBU-LET; EC50 = 0.13 microM) but not by the selective delta-agonist [D-penicillamine2, D-penicillamine5]enkephalin (DPDPE). D1 dopamine receptor-stimulated adenylate cyclase activity was also slightly reduced (by approximately 20%) by high concentrations of the kappa-agonist U50,488 (EC50 = 0.63 microM). The inhibitory effects of submaximally effective concentrations of DAGO, DSTBULET, and U50,488 were equally well antagonized by the mu-opioid receptor-selective antagonist naloxone (EC50 of approximately 0.1 microM). Neither the irreversible delta-ligand fentanyl isothiocyanate (1 microM) nor the reversible delta-antagonist ICI 174864 (1 microM) reversed the inhibitory effects of DSTBULET. The inhibitory effects of DAGO and U50,488 were equally well reversed by high concentrations (greater than 0.1 microM) of the kappa-opioid receptor-selective antagonist norbinaltorphimine. The effect of DAGO (1 microM) was already detectable after 1 day in culture, whereas DPDPE (1 microM) had no effect even after 28 days in culture. These data indicate that an homogeneous population of mu-opioid receptors coupled as inhibitors to D1 dopamine receptor-stimulated adenylate cyclase is expressed in rat neostriatal neurons in primary culture.     

Morphine and Enkephalins Potently Inhibit [3H]Noradrenaline Release from Rat Brain Cortex Synaptosomes: Further Evidence for a Presynaptic Localization of μ-Opioid Receptors

Synaptosomes prepared from rat cerebral cortex and labeled with [3H]noradrenaline (NA) were superfused with calcium-free Krebs-Ringer-bicarbonate medium and exposed to 10 mM K+ plus 0.1 mM Ca2+ so that [3H]NA release was induced. 6,7-Dihydroxy-N,N-dimethyl-2-aminotetralin (TL-99) strongly inhibited synaptosomal K+-induced [3H]NA release (EC50 = 5-10 nM) by activating alpha 2-adrenoceptors. Release was also inhibited (maximally by 40-50%) by morphine (EC50 = 5-10 nM), [Leu5]enkephalin (EC50 = approximately 300 nM), [D-Ala2,D-Leu5]enkephalin (DADLE), and Tyr-D-Ala-Gly-(NMe)Phe-Gly-ol (DAGO) (EC50 values = approximately 30 nM). In contrast to the mu-selective opioid receptor agonists morphine and DAGO, the highly delta-selective agonist [D-Pen2,D-Pen5]enkephalin (1 microM) did not affect [3H]-NA release. Furthermore, the inhibitory effect of DADLE, an agonist with affinity for both delta- and mu-opioid receptors, was antagonized by low concentrations of naloxone. The findings strongly support the view that, like alpha 2-adrenoceptors, mu-opioid receptors mediating inhibition of NA release in the rat cerebral cortex are localized on noradrenergic nerve terminals.     

Regulation of Spontaneous Activity of the δ-Opioid Receptor: Studies of Inverse Agonism in Intact Cells

Abstract: Adenylyl cyclase activity was measured following labelling of the cellular ATP pool with [³H]adenine in intact Rat-1 fibroblasts that had been stably transfected to express the murine δ-opioid receptor (clone D2). Basal [³H]cyclic AMP accumulation was low and was increased substantially by the addition of the diterpene forskolin. The synthetic enkephalin d -Ala², d -Leu⁵ enkephalin (DADLE) produced strong inhibition of forskolin-amplified [³H]cyclic AMP production, whereas the δ-opioid ligand ICI174864 augmented forskolin-amplified adenylyl cyclase activity. Naloxone was unable to mimic the effects of ICI174864, and coincubation of the cells with these two ligands attenuated the effect of ICI174864. The EC₅₀ (9.4 ± 0.6 × 10⁻⁸ M ) for ICI174864 augmentation of forskolin-stimulated adenylyl cyclase was equal to its estimated K _i. Pertussis toxin pretreatment of clone D2 cells prevented both this effect of ICI174864 and the inhibition produced by DADLE. Use of a Cytosensor microphysiometer demonstrated that treatment of clone D2 cells with DADLE increased and that with ICI174864 decreased the basal rate of cellular proton extrusion. By using these two distinct experimental strategies, ICI174864 was shown to function in a manner anticipated for an inverse agonist, demonstrating that such effects can be observed in intact cells and are not restricted to assays performed on membrane preparations.     

Depression of potassium-evoked striatal acetylcholine release by delta-receptor activation: inhibition by cholinoactive agents

To examine the role of delta-opioid receptors in the modulation of striatal acetylcholine (ACh) release, the action of D-Pen2,L-Pen5-enkephalin, a selective delta-opioid receptor agonist, was tested on [3H]ACh release from slices of the rat caudate-putamen. Slices, incubated with [3H]choline, were superfused with a physiological buffer and stimulated twice by exposure to a high potassium (K+) concentration. In the absence of a cholinesterase inhibitor, 1 microM D-Pen2,L-Pen5-enkephalin produced a 46 and 35% decrease in the release of [3H]ACh evoked by 15 and 25 mM K+, respectively. The depressant action of the enkephalin analogue was concentration dependent, with a maximal effect on K+-evoked [3H]ACh release occurring at 1.0 microM, and was completely blocked in the presence of the delta-opioid receptor selective antagonist, ICI 174864 (1 microM). In the presence of the cholinesterase inhibitors physostigmine (10 microM) and neostigmine (10 microM), or the muscarinic receptor agonist oxotremorine (10 microM), D-Pen2,L-Pen5-enkephalin did not depress the K+-evoked release of [3H]ACh. Atropine (1 microM) blocked the inhibitory effect of physostigmine on the depressant action of D-Pen2,L-Pen5-enkephalin. The results of this study indicate that delta-opioid receptor activation is associated with an inhibition of striatal ACh release, but this opioid-cholinergic interaction is not apparent under conditions of presynaptic muscarinic receptor activation.     

Differential Effect of Intrastriatal Kainic Acid on the Modulation of Dopamine Release by μ- and δ-Opioid Peptides: A Microdialysis Study

The present study investigated the effects of a striatal lesion induced by kainic acid on the striatal modulation of dopamine (DA) release by mu- and delta-opioid peptides. The effects of [D-Pen2,D-Pen5]-enkephalin (DPDPE) and [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAGO), two highly selective delta- and mu-opioid agonists, respectively, were studied by microdialysis in anesthetized rats. In control animals both opioid peptides, administered locally, significantly increased extracellular DA levels. The effects of DPDPE were also observed in animals whose striatum had been previously lesioned with kainic acid. In contrast to the effects of the delta agonist, the significant increase induced by DAGO was no longer observed in lesioned animals. These results suggest that delta-opioid receptors modulating the striatal DA release, in contrast to mu receptors, are not located on neurons that may be lesioned by kainic acid.     

Modulation of Histamine Release in the Rat Brain by K-Opioid Receptors

The opioid modulation of histamine release was studied in rat brain slices labeled with L-[3H]histidine. The K(+)-induced [3H]histamine release from cortical slices was progressively inhibited by the preferential kappa-agonists ketocyclazocine, dynorphin A (1-13), Cambridge 20, spiradoline, U50,488H, and U69,593 in increasing concentrations. In contrast, the mu-agonists morphine, morphiceptin, and Tyr-D-Ala-Gly-(NMe)Phe-Gly-ol (DAGO) were ineffective as were the preferential delta-agonists [D-Ala2,D-Leu5]enkephalin (DA-DLE) and [D-Pen2,D-Pen5]enkephalin (DPDPE). Nor-binaltorphimine (nor-BNI) and MR 2266, two preferential kappa-antagonists, reversed the inhibitory effect of the various kappa-agonists more potently than did naloxone, with mean Ki values of 4 nM and 25 nM, respectively. The effects of ketocyclazocine and naloxone also were seen in slices of rat striatum, another brain region known to contain histaminergic nerve endings. We conclude that kappa-opioid receptors, presumably located on histaminergic axons, control histamine release in the brain. However, nor-BNI and naloxone failed, when added alone, to enhance significantly [3H]histamine release from cerebral cortex or striatum, and bestatin, an aminopeptidase inhibitor, failed to decrease K(+)-evoked [3H]histamine release. These two findings suggest that under basal conditions these kappa-opioid receptors are not tonically activated by endogenous dynorphin peptides. The inhibition of cerebral histamine release by kappa-agonists may mediate the sedative actions of these agents in vivo.     

Regional release of [3H]dopamine from rat brain in vitro: effects of opioids on release induced by potassium, nicotine, and L-glutamic acid

Previous studies have suggested that the release of dopamine (DA) in the rat brain may be sensitive to modulation by opioid agents, including the endogenous opioid peptides (enkephalins and endorphins). The present study examined the effects of morphine and the enkephalin analogue D-Ala2-Met5-enkephalinamide (DALA) on the release of radiolabeled DA from superfused slices of rat brain regions. The release of preloaded [3H]DA was evoked from slices of the caudate-putamen (CP) by application of potassium (K+), nicotine (NIC), or L-glutamic acid (L-GLU). The release of [3H]DA from slices of the nucleus accumbens (NA), olfactory tubercle (OT), and substantia nigra (SN) was evoked by L-GLU. Both K+ and NIC evoked a concentration-related release of [3H]DA from CP slices. K+-induced release was only partially dependent on calcium (Ca2+), while NIC-evoked release was completely Ca2+ independent. Neither morphine nor DALA influenced the release of [3H]DA evoked by K+ or NIC. L-GLU produced a concentration-dependent release of [3H]DA from slices of CP, NA, OT, and SN. In all four brain regions, this release was (a) Ca2+-dependent, (b) strongly inhibited by low concentrations of magnesium (Mg2+), (c) greater than the release evoked by D-GLU, (d) attenuated by the putative L-GLU receptor antagonist glutamic acid diethylester (GDEE), and (e) insensitive to tetrodotoxin (TTX) except in the SN. Morphine produced a significant inhibition of L-GLU-evoked [3H]DA release from all four regions. Naloxone, which by itself had no significant effect on the L-GLU-evoked release of [3H]DA, blocked the inhibitory effect of morphine on this release in the CP but not in the other regions. Levorphanol and dextrorphan were equipotent in reducing the glutamate-stimulated release of [3H]DA from CP slices. DALA had no effect on L-GLU-induced release in any of the brain regions examined. The results indicate that L-GLU provokes regional release of DA by acting at a Mg2+-sensitive glutamate receptor. This release is selectively modified by morphine through a mechanism which is insensitive to naloxone.     

Opioid receptors in rat cardiac sarcolemma: effect of phenylephrine and isoproterenol

The present study demonstrates the presence of opioid receptors in the rat cardiac sarcolemma isolated by the hypotonic LiBr-shock procedure. Opioid binding was measured by using [3H]U69 593, [3H](2-D-penicillamine,5-D-penicillamine)enkephalin ([3H]DPDPE) or [3H][D-Ala2,MePhe4,Gly-(ol)5]enkephalin ([3H]DAGO) as selective radioligands for K, delta and mu opioid receptors, respectively. Both the K- and delta-selective ligands exhibited highly specific (75-86%) binding, saturable at a concentration of about 20 nM. No specific binding for the selective agonist DAGO was observed. A marked increase in both [3H]U69 593 and [3H]DPDPE binding was observed after incubation of the sarcolemma with the alpha-adrenoceptor agonist phenylephrine or with the beta-adrenoceptor agonist isoproterenol. These stimulatory effects were associated with an increase in the Bmax values, a decrease in the Kd values, and were completely antagonized by the respective antagonists phentolamine and propranolol.     

Selective delta-opioid receptor antagonism by ICI 174,864 in the central nervous system

The effects of the novel gamma-opioid receptor antagonist ICI 174,864 (N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH: Aib = alpha-aminoisobutyric acid) have been examined in the CNS in vivo using spontaneous reflex contractions of the rat urinary bladder as an index of activity. Bladder contractions were inhibited by equipotent intracerebroventricular (ICV) doses of the selective mu-agonist DAGO [D-Ala2, MePhe4,Gly-(ol)5]enkephalin and the delta-agonist DPDPE[D-Pen2, D-Pen5]enkephalin. ICI 174,864 (1-3 micrograms) administered by the same route produce a selective and reversible antagonism of DPDPE effects. At higher doses (6-15 micrograms, ICV) ICI 174,864 exhibited marked agonistic activity, producing inhibition of bladder contractions that were resistant to ICV naloxone (1-2 micrograms). Thus ICI 174,864 was considered a selective central delta-opioid receptor antagonist but its usefulness was limited by additional agonistic properties.     

Characterization of Enkephalin Release from Rat Striatum

Abstract: Using antisera specific for methionine- and leucine-enkephalin, we studied the characteristics of the release of these peptides from rat striatal slices. Only 2–3% of the total tissue stores of enkephalin could be released by potassium depolarization; similar percentages were released from globus pallidus, thalamus, and nucleus accumbens. Enkephalin release from hippocampus could not be detected. The striatal release of both enkephalins was affected similarly by changes in potassium and calcium levels in the superfusion medium. Lithium has no effect on either basal or potassium-stimulated release; tyr-arg did not affect basal release of either peptide. Striatal enkephalin levels were stable during the short-term incubation periods used in these experiments.     

The effects of opiate agonists on growth hormone and prolactin release in rats

Various opioid receptor agonists, including Met5-enkephalin amide, Leu5-enkephalin amide, [D-Ala]2-Met5-enkephalin amide, [D-Ala]2-Leu5-enkephalin amide, morphine sulfate, d-methadone hydrochloride, and l-methadone hydrochloride were administered to adult male rats by subcutaneous injection. All opioid receptor agonists except Leu5-enkephalin amide significantly stimulated growth hormone and prolactin release. Naloxone and naltrexone blocked the hormone stimulatory effects of the opioids and both naloxone and naltrexone, when administered alone, significantly reduced serum growth hormone and prolactin concentrations. The dopaminergic agonist apomorphine, but not the alpha-adrenergic agonist clonidine, blocked opiate stimulation of prolactin. Morphine sulfate caused growth hormone release in rats pretreated with alpha-methyl-p-tryosine, a catecholamine synthesis inhibitor. Cholinergic agonists, physostigmine and pilocarpine, antagonized the growth hormone and prolactin release induced by morphine sulfate. The data suggest that the opiates stimulate prolactin via an interaction with catecholaminergic neurons controlling prolactin release and stimulate growth hormone via a mechanism independent of alpha-adrenergic or general catecholaminergic influence. The mechanism through which cholinergic agonists act to inhibit opiate agonist stimulation of growth hormone is presently unknown.     

Anticonvulsant effects of mu (DAGO) and delta (DPDPE) enkephalins in rats

The effects of highly selective mu and delta opioid peptide agonists were determined in two rat models of experimentally-induced convulsions, the flurothyl threshold test and the maximal electroshock test. Intracerebroventricular injections of the mu selective enkephalin DAGO (0.3-2.2 nmol) resulted in a dose-related protection in both seizure models. Pretreatment with a low dose of naloxone (29 nmol) or the irreversible mu antagonist beta-FNA (21 nmol), but not the delta opioid antagonist ICI 154,129 (50 nmol), antagonized the anticonvulsant actions of DAGO. Intracerebroventricular injections of the delta selective enkephalin DPDPE (70-140 nmol) also resulted in seizure protection. These effects were selectively antagonized by the delta antagonist ICI 174,864 (2.8 nmol), but not by pretreatment with beta-FNA. Thus, using agonists and antagonists highly selective for mu and delta opioid receptors, anticonvulsant actions of enkephalin have been described against chemically- and electrically-induced convulsions in rats.     

The in vitro release of endogenousm-tyramine,p-tyramine and dopamine from rat striatum

Administration of phenelzine (100 mg/kg, i.p., 18 hr) increased rat striatal concentrations of pTA, mTA and DA by 30, 6.7 and 1.5 fold, respectively. Lesions of the medial forebrain bundle prevented these increase, permitting the conclusion that the phenelzine-induced amine increases were localized in the synaptic terminals. The release of endogenous pTA, mTA and DA from striatal slices obtained from phenelzine-treated rats was investigated. 50 mM KCl elicited releases of pTA, mTA and DA which were significantly greater than their respective basal releases. These K⁺-stimulated releases were antagonized significantly by 15 mM MgCl₂, suggesting that they are calcium-dependent in nature. We have concluded, therefore, that mTA and pTA, as well as DA, are released from striatal nerve terminals in vivo. The total amounts of mTA and DA, but not pTA, released in the release experiments were greater than those found in the nonincubated tissue. It appears, therefore, that the biosynthesis of mTA and DA was stimulated during the incubation of the striatal slices.     

The stimulation of glycogenolysis and gluconeogenesis in isolated hepatocytes by opioid peptides.

The opioid agonists [leucine]enkephalin, [D-Ala2,D-Leu5]enkephalin and dynorphin-(1-13)-peptide, but not morphine, stimulated the conversion of [2-14C]pyruvate into glucose and glycogenolysis when added directly to isolated hepatocytes. Naloxone produced a small but significant inhibition of both the basal and stimulated rate of incorporation of label into glucose but had no effect on the total glucose output by the cells. The effects of the opioid peptides were mediated by a cyclic AMP-independent mechanism.     

Dopaminergic Regulation of Enkephalin Release

The effects of dopamine receptor stimulation on enkephalin release were evaluated in vitro and in vivo by measuring the changes in the levels of [Met5]enkephalin (YGGFM) and Tyr-Gly-Gly (YGG), a characteristic extracellular enkephalin metabolite produced under the action of enkephalinase. In rat striatal slices, D1-receptor agonists or antagonists did not modify enkephalin release. By contrast, D2-receptor agonists enhanced the potassium-induced release of YGGFM and YGG without affecting spontaneous release from nondepolarized slices. This response was prevented by the D2-receptor antagonists haloperidol and RIV 2093, the latter compound being more potent, which suggested the involvement of a putative D2-receptor subtype. Acute administration of apomorphine or selective D2-receptor agonists, but not that of a D1-receptor agonist, enhanced the steady-state level of YGG without affecting the YGGFM level in rat striatum. The effect was blocked selectively by D2-receptor antagonists which, administered alone, had no effect. These observations indicate that D2-receptor stimulation in vitro or in vivo facilitates enkephalin release from striatal neurons, but that endogenous dopamine does not exert any tonic influence upon the opioid peptide neuron activity under basal conditions. However, chronic administration of haloperidol resulted in increases in striatal YGGFM and YGG, an effect presumably reflecting a long-term adaptive process.     

Characterization of Opioid Receptors Modulating Noradrenaline Release in the Hippocampus of the Rabbit

Noradrenaline (NA) release and its modulation via presynaptic opioid receptors were studied in rabbit hippocampal slices, which were preincubated with [3H]NA, continuously superfused in the presence of 30 microM cocaine and stimulated electrically. The evoked release of [3H]NA was strongly reduced by the preferential kappa-agonists ethylketocyclazocine, dynorphin A1-13, dynorphin A, trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl] -benzeneacetamide (U-50,488), and (-)-5,9-dimethyl-2'-OH-2-tetrahydrofurfuryl-6,7-benzomorphan [(-)-MR 2034], whereas (+)-MR 2035 [the (+)-enantiomer of (-)-MR 2034] was ineffective. In contrast, the preferential delta-agonists Leu-enkephalin, Met-enkephalin, and D-Ala2-D-Leu5-enkephalin (DADLE) as well as the mu-agonists morphine, normorphine, D-Ala2-Gly-ol5-enkephalin (DAGO), and beta-casomorphin 1-4 amide (morphiceptin) were much less potent. However, in similar experiments on rat hippocampal slices DAGO (1 microM) was much more potent than ethylketocyclazocine (1 microM) or DADLE (1 microM). (-)-N-(3-furylmethyl)-alpha-noretazocine [(-)-MR 2266], 1 microM, a preferential kappa-antagonist, antagonized the effect of ethylketocyclazocine more potently than (-)-naloxone or (+)-MR 2267 [the (+)-enantiomer of (-)-MR 2266]. Given alone, (-)-MR 2266 slightly and (+)-MR 2267 (1 microM each) greatly enhanced NA release, apparently due to alpha 2-adrenoceptor blockade since their effects were completely abolished in the presence of yohimbine (0.1 microM).(ABSTRACT TRUNCATED AT 250 WORDS)     

Phencyclidine-induced inhibition of striatal acetylcholine release: comparisons with mu, kappa, and sigma opiate agonists

The effects of phencyclidine (PCP) on ACh release were compared to those of morphine, ethylketocyclazocine (EKC), and N-allylnormetazocine (SKF10047) in a superfused striatal slice preparation. The (+)-isomer of the prototypic sigma opiate agonist, SKF10047, and the prototypic kappa opiate agonist, EKC, had essentially the same pharmacological profile as did PCP. That is, they each inhibited ACh release in a concentration dependent manner (with EKC being the most potent) and this effect was antagonized by 0.1 microM naloxone. Since morphine was without effect on ACh release, it is unlikely that these drugs inhibit ACh release by acting at mu receptors. In addition, we observed that the inhibitory effect of PCP, (+) SKF10047, and EKC on ACh release was reversed by 0.1 microM haloperidol. Given that PCP has been shown to stimulate basal DA release in this preparation, it is possible that PCP, EKC and (+) SKF10047 inhibit ACh release indirectly by stimulating DA release. The naloxone-induced blockade of the effect of PCP and these benzomorphans is discussed in relation to the effects of naloxone on other systems known to influence ACh release.     

The involvement of opioid peptides in stress-induced analgesia in the slug Arion ater

Application of tail-pinch stress to the terrestrial slug, Arion ater, produced a significant increase in the response time when tested on the hot-plate for foot-lifting response. The analgesia was completely reversed by injections of the opiate antagonists, naltrexone and ICI 174864, in a dose-dependent manner. Analgesia could also be elicited by the injection into the foot of beta-endorphin and the enkephalin analogues, DAGO and DADLE. No effect was seen with dynorphin A (1-8) or dynorphin A (1-17). The stress-induced analgesia disappeared after 30 minutes but could be maintained for 100 min following the injection of a mixture of bestatin and the enkephalinase inhibitor, N-carboxymethyl-L-phenylalanyl-L-leucine. This work suggests that in the slug, a physical stressor produces an analgesia which may be due to the release of endogenous opiates.     

Sustained potentiation of NMDA receptor-mediated glutamate responses through activation of protein kinase C by a mu opioid.

mu opioids, such as morphine and certain enkephalin analogs, are known to modulate glutamate-evoked activity in dorsal horn neurons in the spinal cord and caudal brain stem. Yet the molecular mechanism by which this modulation occurs is not understood. We examined the interactions between glutamate and a selective mu opioid receptor agonist, D-Ala2-MePhe4-Gly-ol5-enkephalin (DAGO), in spinal trigeminal neurons in thin medullary slices of rats. DAGO caused a sustained increase in glutamate-activated currents that are mediated by N-methyl-D-aspartate receptors. Intracellularly applied protein kinase C (PKC) mimics the effect of DAGO, and a specific PKC inhibitor interrupts the sustained potentiation produced by DAGO. Thus, PKC plays a key role in mediating the action of mu opioid peptides.     

Human μ—opioid receptor overexpressed in Sf9 insect cells functionally coupled to endogenous Gi／0 proteins

Human mu-opioid receptor (HmuOR) with a tag of six consecutive histidines at its carboxyl terminus had been expressed in recombinant baculovirus infected Sf9 insect cells. The maximal binding capacity for the [3H] diprenorphine and [3H]ohmefentanyl (Ohm) were 9.1 +/- 0.7 and 6.52 +/- 0.23 nmol/g protein, respectively. The [3H] diprenorphine or [3H] Ohm binding to the receptor expressed in Sf9 cells was strongly inhibited by mu-selective agonists [D-Ala2, N-methyl-Phe4, glyol5]enkephalin (DAGO), Ohm, and morphine, but neither by delta nor by kappa selective agonist. Na+ (100 mM) and GTP (50 microM) could reduce HmuOR agonists etorphine and Ohm affinity binding to the overexpressed HmuOR. mu-selective agonists DAGO and Ohm effectively stimulated [35S]GTP-gammaS binding (EC50 = 2.7 nM and 6.9 nM) and inhibited forskolin- stimulated cAMP accumulation (IC50 = 0.9 nM and 0.3 nM). The agonist-dependent effects could be blocked by opioid antagonist naloxone or by pretreatment of cells with pertussis toxin (PTX). These results demonstrated that HmuOR overexpressed in Sf9 insect cells functionally coupled to endogenous G(i/o) proteins.