Opioid receptor-coupled second messenger systems

A comprehensive review of naloxone for the emergency physician

Naloxone has enjoyed long-standing success as a safe and effective opioid antagonist and has been invaluable in defining the role of endogenous opioid pathways in the response to pathological states such as sepsis and hypovolemia. We look forward to exciting research to further elucidate these pathways and to improve outcome by modulating the patient's physiological response to these stresses.

Evaluation of delayed treatment of focal cerebral ischemia with three selective kappa-opioid agonists in cats

BACKGROUND AND PURPOSE

The purpose of this study was to determine the therapeutic efficacy of three kappa-opioid agonists used for delayed treatment of experimental focal cerebral ischemia.

METHODS

Forty halothane-anesthetized cats underwent permanent occlusion of the right intracranial internal carotid, middle cerebral, and anterior cerebral arteries via a transorbital, microsurgical approach. Six hours after occlusion, animals received a blinded bolus injection, and a subcutaneous osmotic pump was implanted to provide continuous release for 7 days. The injection and pump contained either saline or one of three kappa-agonists: dynorphin (1-13), U-50,488, or DuP E3800. Survival, neurological function, tissue damage, and brain weight were assessed.

RESULTS

As a group, kappa-agonist-treated animals had higher survival (P < .02), less tissue damage (P < .02), and lower brain weight (P < .05) than saline controls. U-50,488 more effectively improved survival (P < .03) than dynorphin (P < .07) or E3800 (P < .07). Each of the three kappa compounds improved tissue damage (dynorphin, P < .02; U-50,488, P < .05; E3800, P < .05). Greater improvement in neurological function was seen after treatment with dynorphin (P < .05) than with U-50,488 (P < .6) or E3800 (P < .7). The only significant reduction in brain weight was seen after dynorphin treatment (P < .01).

CONCLUSIONS

Compounds that act at the kappa subclass of opiate receptors are effective in increasing survival, improving neurological function, and decreasing tissue damage and edema in a cat model of focal cerebral ischemia. The current study provides support for the benefits of treatment of acute cerebrovascular ischemia with kappa-opioid agonists. The agents may prove to be of superior clinical utility because of efficacy even when administered 6 hours after the onset of stroke.

Roles of second-messenger systems and neuronal activity in the regulation of lordosis by neurotransmitters, neuropeptides, and estrogen: a review

Many neurotransmitters and neuropeptides can affect the rodent feminine sexual behavior, lordosis, when administered in the ventromedial hypothalamus (VMH), midbrain central gray (MCG), or other brain regions. A survey of the electrophysiological and biochemical actions of these neural agents revealed that there is a very consistent association between lordosis facilitation with both the activation of the phosphoinositide (PI) pathway and the excitation of VMH and MCG neurons. In contrast, lordosis inhibition is associated, less consistently, with alterations of the adenylate cyclase (AC) system and the inhibition of neuronal activity. The findings that lordosis could be facilitated by going beyond membrane receptors and directly activating the PI pathway, suggest that this second-messenger pathway is a common mediator for the lordosis-facilitating agents. Furthermore, as in the case of stimulating membrane receptors, direct activation of this common mediator also requires estrogen priming for lordosis facilitation. Therefore, it is likely that the PI pathway is modulated by estrogen in the permissive action of estrogen priming. Indeed, a literature review shows that estrogen can affect selective isozymes of key enzyme families of the PI pathway at various levels. Such selective modulations, at several levels, could easily alter the course of a PI cascade; thence, the eventual functional outcome. These findings prompt us to propose that estrogen enables lordosis to be facilitated by a selective modulation of the PI pathway.

ORL1, a novel member of the opioid receptor family. Cloning, functional expression and localization

Selective PCR amplification of human and mouse genomic DNAs with oligonucleotides encoding highly conserved regions of the delta-opioid and somatostatin receptors generated a human DNA probe (hOP01, 761 bp) and its murine counterpart (mOP86, 447 bp). hOP01 was used to screen a cDNA library from human brainstem. A clone (named hORL1) was isolated, sequenced and found to encode a protein of 370 amino acids whose primary structure displays the seven putative membrane-spanning domains of a G protein-coupled membrane receptor. The hORL1 receptor is most closely related to opioid receptors not only on structural (sequence) but also on functional grounds: hORL1 is 49-50% identical to the murine mu-, delta- and kappa-opioid receptors and, in CHO-K1 cells stably transfected with a pRc/CMV:hORL1 construct, ORL1 mediates inhibition of adenylyl cyclase by etorphine, a 'universal' (nonselective) opiate agonist. Yet, hORL1 appears not to be a typical opioid receptor. Neither is it a somatostatin or sigma (N-allylnormetazocine) receptor. mRNAs hybridizing with synthetic oligonucleotides complementary to mOP86 are present in many regions of the mouse brain and spinal cord, particularly in limbic (amygdala, hippocampus, septum, habenula, ...) and hypothalamic structures. We conclude that the hORL1 receptor is a new member of the opioid receptor family with a potential role in modulating a number of brain functions, including instinctive behaviours and emotions.

mu-Opioid receptor stimulation of inositol (1,4,5)trisphosphate formation via a pertussis toxin-sensitive G protein

The cellular mechanisms underlying opioid action remain to be fully determined, although there is now growing indirect evidence that some opioid receptors may be coupled to phospholipase C. Using SH-SY5Y human neuroblastoma cells (expressing both mu- and delta-opioid receptors), we demonstrated that fentanyl, a mu-preferring opioid, caused a dose-dependent (EC50 = 16 nM) monophasic increase in inositol (1,4,5)trisphosphate mass formation that peaked at 15 s and returned to basal within 1-2 min. This response was of similar magnitude (25.4 +/- 0.8 pmol/mg of protein for 0.1 microM fentanyl) to that found in the plateau phase (5 min) following stimulation with 1 mM carbachol (18.3 +/- 1.4 pmol/mg of protein), and was naloxone-, but not naltrindole- (a delta antagonist), reversible. Further studies using [D-Ala2, MePhe4, Gly(ol)5]enkephalin and [D-Pen2,5]enkephalin confirmed that the response was specific for the mu receptor. Incubation with Ni2+ (2.5 mM) or in Ca(2+)-free buffer abolished the response, as did pretreatment (100 ng/ml for 24 h) with pertussis toxin (control plus 0.1 microM fentanyl, 26.9 +/- 1.5 pmol/mg of protein; pertussis-treated plus 0.1 microM fentanyl, 5.1 +/- 1.3 pmol/mg of protein). In summary, we have demonstrated a mu-opioid receptor-mediated activation of phospholipase C, via a pertussis toxin-sensitive G protein, that is Ca(2+)-dependent. This stimulatory effect of opioids on phospholipase C, and the potential inositol (1,4,5)trisphosphate-mediated rises in intracellular Ca2+, could play a part in the cellular mechanisms of opioid action.

The activity of opioid analgesics in seizure models utilizing N-methyl-DL-aspartic acid, kainic acid, bicuculline and pentylenetetrazole

Morphine, fentanyl and pethidine exhibited a biphasic dose response relationship with respect to their effects on seizure thresholds to bicuculline, pentylenetetrazole, N-methyl-DL-aspartate (NMDLA) and kainic acid in mice. The usual pattern was for low doses to be anticonvulsant and higher doses to be proconvulsant. However this pattern was reversed for fentanyl and pethidine when NMDLA was used to induce seizures. The low dose effects of all three opioid drugs was sensitive to 1 mg kg-1 naloxone in all seizure models. The responses to high doses of pethidine were unaffected or enhanced by this dose of naloxone. Naloxone reversed the effects of the higher doses of morphine and fentanyl in all models except bicuculline induced seizures.

Modulation of substance P-ergic system in the rat spinal cord by an opioid antagonist

Substance P- and opioid peptide-immunoreactive nerve terminals functionally interact in the spinal cord as two opposing systems in the regulation of the nociceptive pathway. In order to determine how SP-ergic system adapts to chronic opioid receptor blockade, the effects of naltrexone on SP level, SP receptor and the second messenger system coupled to the SP receptor were examined in the rat spinal cord. Male Sprague-Dawley rats were treated with naltrexone or vehicle for seven days by constant minipump infusion. Animals were sacrificed on day 8, spinal cords rapidly removed, segmentally sectioned and used to determine SP and inositol 1,4,5-trisphosphate [ins(1,4,5)P3] tissue contents, and to examine the regulation of their respective receptors in in vitro receptor binding assays. Following chronic naltrexone treatment, SP content in the lumbosacral segment of the spinal cord was increased by 53% over matched control values. The binding capacity (Bmax) of SP receptors, determined using [125I]BHSP, in lumbosacral synaptosomal membranes was significantly increased by 92%, but the binding affinity (Kd) remained unchanged. In addition, the concentration of [Sar9, Met(O2)11]SP, an NK-1 receptor-specific agonist, required to inhibit half of [125I]BHSP binding (IC50) in lumbosacral synaptosomal membranes was significantly decreased, but the IC50s for SP, the endogenous ligand for the SP receptor, and [Pro7]NK B, an NK-3 receptor-specific agonist, were unaltered by chronic blockade of opioid receptors. The data suggest that although naltrexone does not directly interact with tachykinin receptors, it acts indirectly on SP-ergic neurons to cause a change in the apparent affinity of NK-1 receptor (as reflected by a change in IC50 value). Formation of cellular ins(1,4,5)P3 in the lumbosacral cord, quantified by a highly sensitive and selective radioreceptor assay, was significantly increased by 34% relative to matched controls. A time course study indicated that increases in ins(1,4,5)P3 contents over the time studied corresponded qualitatively with increases in SP level in the lumbosacral cord. With [3H]ins(1,4,5)P3 as a ligand, Scatchard analyses of the concentration dependent saturation curves showed that the density of intracellular ins(1,4,5)P3 receptors was also increased by 119%, with no change in binding affinity. The data suggest that ins(1,4,5)P3 formation, possibly coupled to functional SP receptor activation, and ins(1,4,5)P3 receptors, which mediate ins(1,4,5)P3-induced alterations in intracellular Ca2+ flux, are increased in the lumbosacral cord by chronic blockade of opioid receptors. Taken together, the data support the concept of a role for endogenous opioids in the regulation of SP receptor activity in the spinal cord.

Differential effects of methionine enkephalin on the growth of brain tumor cells

The effect of methionine enkephalin on the growth of human brain tumor cells was investigated. The results show that this endogenous opioid has dual effects on tumor cell growth. This peptide exerted an inhibitory effect in SK-N-MC human neuroblastoma cell line; in contrast, in U-373 MG human astrocytoma cell line, the peptide showed a stimulatory effect. Treatment with naltrexone, an opioid receptor antagonist, also resulted in a similar alteration of tumor cell growth implicating that its action may be unrelated to opioid receptor blockade. These results suggest that in these tumor cell lines endogenous opioid systems may be involved in cell proliferation. Furthermore, these tumor cell lines may be useful model systems for the study of the signal transduction mechanisms of endogenous opioids.

Morphine: new aspects in the study of an ancient compound

Morphine is the most widely used compound among narcotic analgesics and remains the gold standard when the effects of other analgetic drugs are compared. Apart from its presence in the poppy plant Papaver somniferum, morphine has been shown to be present in milk, cerebrospinal fluid and also in nervous tissue extracts. Recent evidence suggests that biosynthetic pathways for morphine exist in animal and even human tissues such as liver, blood and brain. The most characteristic effect of morphine is the modulation of pain perception resulting in an increase in the threshold of noxious stimuli. Antinociception induced by morphine is mediated via opioid receptors and therefore can be inhibited by opioid antagonists, e.g., naloxone. Nevertheless, consideration of morphine as endogenous ligand for opioid receptors seems to be speculative. Recently, the primary receptor for morphine-type drugs called the mu-opioid receptor has been cloned from rat brain. There is accumulating evidence that morphine actions are, at least partly, due to one of its major metabolite morphine-6-glucuronide in man. It is concluded that further investigations are necessary to elucidate the mechanisms, whereby multiple actions of morphine are expressed in the nervous system.

Molecular biology of opioid receptors

Opium and its derivatives are potent analgesics that also have many other pharmacological effects in the nervous system. These agents and the endogenous opioid peptides exert their effects by interacting with high-affinity receptors. Complementary DNAs encoding the delta, kappa and mu opioid receptors have been isolated and characterized. These receptors, which are members of the superfamily of seven-transmembrane spanning receptors, share a high degree of amino acid sequence similarity with approximately 50% of the residues being identical. The cloned opioid receptors mediate agonist inhibition of cyclic AMP formation and have pharmacological properties similar to the endogenous proteins. The cloning of these receptors will facilitate the development of new clinically useful compounds as well as studies of the molecular basis of tolerance and drug addiction.

Synergistic interaction of muscarinic and opioid receptors with GS-linked neurotransmitter receptors to stimulate adenylyl cyclase activity of rat olfactory bulb

We reported previously that in homogenates of rat olfactory bulb muscarinic and opioid receptor agonists stimulate adenylyl cyclase activity. In the present study we show that carbachol (CCh) and Leu-Enkephalin act synergistically with vasoactive intestinal peptide (VIP) and corticotropin-releasing hormone (CRH), but not with l-isoproterenol, in increasing cyclic AMP formation. The synergistic interaction consists of an increase in the maximal adenylyl cyclase activation without a significant change in the potency of each agonist. CCh also fails to affect 125I-CRH binding to olfactory bulb membranes. The synergism requires micromolar concentrations of GTP. Substitution of the stable GTP analog guanosine 5'-O-(3'-thiotriphosphate) for GTP allows the CRH stimulation, but abolishes the CCh enhancement of both basal and CRH-stimulated enzyme activities. Moreover, in vivo treatment of olfactory bulbs with pertussis toxin completely prevents the muscarinic and opioid effects. Thus, the synergistic interaction appears to result from opioid- and muscarinic-induced activation of a pertussis toxin-sensitive GTP-binding protein which may potentiate the adenylyl cyclase stimulation by the stimulatory GTP-binding protein activated by either VIP or CRH receptors.

Pertussis toxin attenuates intracranial morphine self-administration

Mu and delta opioid receptor subtypes are thought to mediate the reinforcing actions of opioids. Since these opioid receptors use pertussis toxin (PTX)-sensitive inhibitory G-proteins for signal transduction, we determined whether PTX would block the opioid reinforcement signals produced by intrahippocampal or intraventral tegmental area (VTA) injections of morphine in rats. Hippocampal PTX pretreatment prevented the acquisition of intrahippocampal morphine self-administration. Similarly, in rats previously trained to self-administer morphine in the VTA, PTX injections in the VTA abolished morphine self-administration behavior, while sparing behavior reinforced by food pellets. This result suggested that the toxin did not interfere generally with motor capacity but rather acted selectively to block morphine reinforcement. Inactivated PTX did not reduce VTA morphine self-administration, thus demonstrating that PTX blockade of opioid reinforcement is primarily due to enzymatic inactivation of inhibitory G-proteins. All these findings are consistent with the hypothesis that inhibitory G-proteins in the hippocampus and VTA mediate the reinforcing effects of opioid drugs.

The delta-opioid signal transduction on the gonadotropin-releasing hormone release is eicosanoid dependent

In static incubations, the K+ induced release of gonadotropin-releasing hormone (GnRH) and of prostaglandin (PG) E2, was 2-3 times higher in the isolated median eminence (ME) compared to the hypothalamic area containing the arcuate nucleus (ARN) plus the ME. The delta-opioid agonist DTLET, induced a parallel, dose-dependent reduction of GnRH and PGE2 release in the ARN plus ME. Both effects of DTLET were blocked by the delta-opioid antagonist Diallyl-G. In the isolated ME, DTLET reduced the secretion of PGE2 but enhanced the release of GnRH. In this area Diallyl-G had no effect on the PGE2 release but blocked the GnRH secretion. When the PGE2 production was blocked by indomethacin in the ARN plus ME preparation, DTLET increased the release of GnRH and induced the production of leukotrienes (LTs). On the other hand, DTLET decreased the release of both GnRH and PGE2 in the presence of nordihydroguaiaretic acid (NDGA), an inhibitor of the production of LTs. The above results suggest that: (a) the delta-opioid agonist DTLET modulates GnRH release differentially in the hypothalamic areas examined; and (b) the arachidonic acid metabolites are involved in the mode of action of DTLET on the release of GnRH in the ARN plus ME hypothalamic fragment.

Effects of morphine and its metabolites on opiate receptor binding, cAMP formation and [3H]noradrenaline release from SH-SY5Y cells

Opiate receptor occupation leads to a variety of intracellular events including inhibition of adenylyl cyclase and cAMP formation. We have examined the opiate binding characteristics, effects on cAMP formation and [3H]noradrenaline release of morphine, morphine-6 (M6G) and -3 (M3G)-glucuronides, and fentanyl in SH-SY5Y human neuroblastoma cells. M6G and M3G are the major metabolites of morphine formed in vivo whose cellular action remains to be fully elucidated. In binding experiments morphine (affinity, K50 = 96 nM) and fentanyl (K50 = 99 nM) were more potent than M6G (K50 = 393 nM), while M3G was inactive. However, for cAMP inhibition morphine (half maximum inhibition, IC50 = 193 nM) and M6G (IC50 = 113 nM) were roughly equipotent, with fentanyl (IC50 = 27 nM) being more potent and producing a greater maximum inhibition (56%). M3G was inactive. These in vitro data are in general agreement with the in vivo effects of these glucuronides. Moreover, all of the opiates tested failed to inhibit K(+)-evoked release of [3H]noradrenaline. Whilst these data do not support a role for cAMP in neurotransmitter release, alterations in cAMP formation may still have a role to play in the mechanism of analgesia.

Regulation of substance P receptor system in rat striatum by chronic naltrexone treatment

Chronic blockade of opioid receptors by naltrexone increases opioid peptides in the striatum, and up-regulates brain opioid receptors resulting in functional supersensitivity. Striatal SP content was increased 3.5-fold after 8 days of naltrexone treatment relative to control animals. The present study was undertaken to determine whether SP receptors in the striatum and SP receptor-coupled second messenger system are modulated by increased striatal SP content induced by chronic opioid receptor blockade. The binding affinity and capacity of SP receptors, determined using [125I]Bolton-Hunter SP ([125I]BHSP) labeled at Lys3, in striatal synaptosomal membranes were not significantly altered by chronic blockade of opioid receptors. Although the concentrations of [Sar9,Met (O2)11]SP, a NK-1 receptor-specific agonist, and SP(1-7), an aminoterminal major metabolite of SP, required to inhibit half of [125I]BHSP binding (IC50) in striatal synaptosomal membranes were significantly decreased, the IC50s for SP and an NK-2 receptor-specific agonist, [Nle10]NK A (4-10), remained unchanged by chronic naltrexone treatment. The data suggest that naltrexone which has no SP receptor antagonistic action, not only indirectly acts on SP-ergic neurons but also causes a change in the apparent affinity of NK-1 receptor (as reflected by changes in IC50 values) in the striatum. Cellular inositol-1,4,5-trisphosphate [Ins(1,4,5)P3], quantified by a highly sensitive and selective radioreceptor mass assay, was increased in the striatum by 28% relative to control levels. With [3H]Ins(1,4,5)P3 as a ligand, Scatchard analyses of the concentration-dependent saturation curves showed that the density of striatal intracellular Ins(1,4,5)P3 receptors was increased by 53%. The levels of SP and cellular Ins(1,4,5)P3, and the density of Ins(1,4,5)P3 receptors in the cerebellum, used as a positive control, were unchanged by chronic naltrexone treatment. The findings of opiate antagonist-induced increases in SP striatal content and Ins(1,4,5)P3 receptor densities, appear to support the concept of a role of endogenous opioids in the regulation of SP receptor activity. The data also suggest that inter-regulatory mechanisms exist between phospholipase C/phosphoinositide-coupled receptors such as SP receptors, and adenylate cyclase-coupled inhibitory receptors, such as opioid receptors.

Long-term observations in gerbil brain following transient cerebral ischemia: autoradiographic and histological study

We investigated the long-term changes that occur in the gerbil brain following transient cerebral ischemia using histology and receptor autoradiography. Transient ischemia was induced for 3 and 10 min, and animals were allowed to survive for 8 months. A histological study showed that 3-min ischemia caused neuronal damage and mild atrophy only in the hippocampal CA1 sector, and that 10-min ischemia produced severe neuronal damage and marked shrinkage in the hippocampal CA1 and CA3 sectors. Furthermore, severe neuronal damage was seen in the striatum after 10-min ischemia. Autoradiography study revealed that 3-min ischemia caused a significant reduction in [3H] naloxone binding in the frontal cortex, striatum, dentate gyrus, and thalamus, whereas [3H]SCH 23390 and [3H] forskolin binding was not significantly altered in all regions. In contrast, 10-min ischemia produced marked alteration in these binding sites in the striatum, hippocampus, thalamus, and substantia nigra. The alteration was especially notable in the hippocampal region and substantia nigra. These results indicate that hippocampal damage after transient ischemia, compared with that in other regions, is not static, but particularly progressive. Furthermore, they demonstrate a reduction in adenylate cyclase system in the striatum and substantia nigra after transient ischemia. Moreover, our results suggest that long-term survival after ischemia may induce synaptic modification of neurotransmitter and adenylate cyclase system in the hippocampus.

Effect of pretreatment with pertussis toxin on the development of physical dependence on morphine

The effect of intracerebroventricular (i.c.v.) pretreatment with pertussis toxin (PTX) on the development of physical dependence on morphine was investigated in mice. Twenty four hours after PTX (0.5 microgram, i.c.v.) or vehicle pretreatment, the mice were chronically treated with morphine (8-45 mg/kg, s.c.) for 5 days. Several withdrawal signs were observed following naloxone challenge in morphine-dependent mice which had been pretreated with vehicle. In addition, 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) and noradrenaline (NA) turnover (MHPG/NA) levels in the cerebral cortex were increased following naloxone challenge in morphine-dependent mice. These findings indicate that activation of the central noradrenergic system may mediate the expression of some withdrawal signs. In contrast, pretreatment with PTX attenuated the naloxone-precipitated withdrawal signs in morphine-dependent mice. The incidence of withdrawal signs such as jumping, "wet dog" shakes, and rearing was significantly reduced by PTX pretreatment. PTX pretreatment also prevented the naloxone-precipitated increases in MHPG concentration and NA ratio (MHPG/NA) in the cerebral cortex, suggesting that central PTX-sensitive GTP-binding proteins (G-proteins) may be involved in the elevation of NA transmission in the cortex which projects from the locus coeruleus (LC) during morphine withdrawal. The blocking effects of PTX on the behavioral and biochemical changes after withdrawal suggest that central PTX-sensitive G-proteins (Gi/Go) may play an important role in the development of physical dependence on morphine.

Effect of delta-opioid antagonists on the functional coupling between opioid receptors and G-proteins in rat brain membranes

It is currently accepted that occupancy of opioid receptors by agonists, but not antagonists, promotes the association of the receptors to guanine nucleotide binding proteins (G-proteins) and stimulates a high affinity GTPase as part of the mechanism that links the receptor-ligand complex to adenylate cyclase inhibition. In this work we report that in rat brain membranes selective delta-opioid antagonists, the peptides N,N-Diallyl-Tyr-D-Leu-Gly-Tyr-Leu-OH (Diallyl-G) and N-N-Diallyl-Tyr-Aib-Aib-Phe-Leu-OH (ICI174,864), inhibit the low Km GTPase activity in a concentration dependent way. On the other hand the delta-opioid agonists D-Ala2-D-Leu5-enkephalin (DADLE) and D-Ser2-Leu5-Thr6-enkephalin stimulate dose-dependently the low Km GTPase activity in rat brain membranes. This stimulation was blocked in the presence of Diallyl-G, and reciprocally the inhibition induced by Diallyl-G was reversed by DADLE. The inhibitory effect of Diallyl-G as well as the stimulation induced by DADLE were abolished when membranes were exposed to low concentrations of N-ethylmaleimide or by ADP ribosylation with pertussis toxin which interferes with the ability of the receptor to couple to G-proteins. These observations indicate that the inhibitory effect of Diallyl-G on GTPase requires a functional G-protein and suggest that certain delta-opioid antagonists exhibit negative intrinsic activity and may have the ability to inhibit the receptor-mediated activation of G-proteins.

Go mediates the coupling of the mu opioid receptor to adenylyl cyclase in cloned neural cells and brain

In membranes from SH-SY5Y human neuroblastoma cells differentiated with retinoic acid, the mu-selective agonist Tyr-D-Ala-Gly-N-Me-Phe-Gly-ol (DAMGO) inhibited cAMP formation with an IC50 of 26 nM. Two separate antibodies raised against distinct regions of the Go alpha sequence attenuated the effect of DAMGO by 50-60%, whereas antibodies to Gi alpha 1,2 or Gi alpha 3 reduced the mu-opioid signal insignificantly or to a lesser extent. In contrast, inhibition of adenylyl cyclase by the delta-opioid agonist Tyr-D-Pen-Gly-Phe-D-Pen-OH (DPDPE; Pen = penicillamine) was very sensitive to the Gi alpha 1,2 antibody. In membranes from rat brain striatum, coupling of the mu opioid receptor to adenylyl cyclase was also maximally blocked by antibodies to Go alpha. After long-term treatment of the cells with DAMGO, the content of Go alpha was reduced by 26%, whereas the levels of Gi alpha 1,2, Gi alpha 3, and Gs alpha were unaltered. Addition of Go, purified from bovine brain, to membranes from pertussis toxin-treated SH-SY5Y cells restored the inhibition of adenylyl cyclase by DAMGO to 70% of that in toxin-untreated cells. To comparably restore the effect of DPDPE, much higher concentrations of Go were required. By demonstrating mediation of cAMP-dependent signal transduction by Go, these results describe (i) an additional role for this G protein present at a high concentration in brain, (ii) preferential, although not exclusive, interaction of mu and delta opioid receptors with different G protein subtypes in coupling to adenylyl cyclase, and (iii) reduced levels of Go following chronic opioid treatment of SH-SY5Y cells with mu opioids.

In vitro study of the interaction of salmon calcitonin with mu, delta and kappa opioid agonists

A possible interaction of salmon-calcitonin with opioid systems was studied in isolated tissues. Neurogenic contractions were elicited by electrical stimulation in guinea-pig ileum myenteric plexus-longitudinal muscle strips, rabbit vas deferens and mouse vas deferens. Bremazocine inhibited neurogenic contractions in all three tissues (presumably through kappa-receptors) [D-Pen2, D-Pen5]enkephalin and [Met5]enkephalin inhibited contractions in mouse vas deferens (presumably through delta-receptors), and [D-Ala2, N-Me-Phe4, Gly5-ol]enkephalin (DAMGO) inhibited contractions in guinea-pig ileum and mouse vas deferens (presumably through mu-receptors). All inhibitory effects were concentration-dependent. Salmon-calcitonin 0.1 IU/ml increased the effect of bremazocine in guinea-pig ileum and rabbit vas deferens and also increased the effects of [D-Pen2, D-Pen5]enkephalin and [Met5]enkephalin in mouse vas deferens. In contrast, salmon-calcitonin up to 0.4 IU/ml did not change the effect of bremazocine in mouse vas deferens and the effect of DAMGO in guinea-pig ileum and mouse vas deferens. It is concluded that salmon-calcitonin enhances agonist effects at opioid kappa- and delta- but not at opioid mu-receptors. The level of this interaction remains to be elucidated. The interaction may be the basis of the analgesic effect of salmon-calcitonin in vivo.

mu and delta opioid receptors differentially couple to G protein subtypes in membranes of human neuroblastoma SH-SY5Y cells

Opioids are regarded to act via receptors interacting with heterotrimeric pertussis toxin (PTX)-sensitive G proteins. In membranes of SH-SY5Y cells, the mu-selective agonist [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAGO) and the delta-selective agonist [D-Pen2,Pen5]-enkephalin (DPDPE) stimulated incorporation of the photoreactive GTP analog [alpha-32P]GTP azidoanilide into proteins comigrating with the alpha subunits of G(i1), G(i2), G(i3), G(o1), and another form of G(o), presumably G(o2). In membranes of PTX-treated cells, both agonists were ineffective. Subtype-specific immunoprecipitation of G protein alpha subunits photolabeled in the absence or presence of agonists revealed profound differences between mu and delta opioid receptors in coupling to PTX-sensitive G proteins. Whereas activated delta opioid receptors preferentially coupled to G(i1), activated mu opioid receptors more effectively coupled to G(i3). Additionally, we provide evidence that G(o) subtypes are also differentially activated by the two receptors. Thus, mu and delta opioid receptors appear to discriminate between PTX-sensitive G proteins and lead to activation of distinct G protein subtypes.

Effects of pertussis toxin on behavioural responses of guinea-pigs to centrally administered substance P, quinpirole, carbachol, U-50,488H, morphine and morphine withdrawal

The effects of pretreatment with pertussis toxin (PTX) on the sedative effect of morphine administered i.c.v. (200 nmol), and on the locomotor and behavioural activation precipitated by naloxone (15 mg/kg s.c.) following treatment with a single dose of morphine (i.c.v., 200 nmol), were investigated in guinea-pigs. Responses to i.c.v. administration of substance P (50 nmol), quinpirole (200 nmol), U50,488H (100 nmol) and carbachol (2 nmol) following PTX pretreatment were also investigated. Following PTX pretreatment, morphine induced mild agitation and the onset of sedation was delayed. Pretreatment with PTX also attenuated the locomotor and some components of behavioural activation induced by substance P, U50,488H, quinpirole and naloxone-precipitated morphine withdrawal, but failed to attenuate the effects induced by carbachol. These results suggest the involvement of PTX-sensitive G-protein-mediated mechanisms in the sedative effect of morphine in guinea-pigs and in the central stimulating actions of acute morphine withdrawal, U50,488H, substance P, and quinpirole.

Unlike morphine the endogenous enkephalins protected by RB101 are unable to establish a conditioned place preference in mice

The mixed inhibitor prodrug, RB101, was used to study the motivational properties of the endogenous opioid peptides, the enkephalins. In the conditioned place preference test, which is commonly used to investigate the reinforcing properties of drugs, mice alternately treated with morphine (3 mg/kg i.p.) on the initially non-preferred compartment and with saline on the preferred one, for four place pairings, spent more time in the drug-associated compartment. This shift in place preference after the conditioning procedure was not found after treatment with RB101 (80 mg/kg i.p.). Administration of naloxone (1 mg/kg s.c.) after the conditioning phase increased the preference for the drug-associated compartment of mice treated with 6 mg/kg (i.p.) of morphine. This illustrates the negative motivational properties of morphine withdrawal or the establishment of psychic dependence on the drug. In contrast, no modification of preference was observed after injection of naloxone in animals treated with a high dose of RB101 (160 mg/kg i.p.). The failure to establish conditioned place preference by inhibiting endogenous enkephalin metabolism, and the lack of development of psychic dependence after RB101 administration demonstrate for the first time the interest of mixed inhibitors of enkephalin-degrading enzymes as potent new non-addictive analgesics.

Second messenger and protein phosphorylation mechanisms underlying opiate addiction: studies in the rat locus coeruleus

We have studied the role of second messenger and protein phosphorylation pathways in mediating changes in neuronal function associated with opiate addiction in the rat locus coeruleus. We have found that chronic opiates increase levels of the G-protein subunits Gi alpha and Go alpha, adenylate cyclase, cyclic AMP-dependent protein kinase, and a number of phosphoproteins (including tyrosine hydroxylase) in this brain region. Electrophysiological data have provided direct support for the view that this up-regulation of the cyclic AMP system contributes to opiate tolerance, dependence, and withdrawal exhibited by these neurons. As the adaptations in G-proteins and the cyclic AMP system appear to occur at least in part at the level of gene expression, current efforts are aimed at identifying the mechanisms, at the molecular level, by which opiates regulate the expression of these intracellular messenger proteins in the locus coeruleus. These studies will lead to an improved understanding of the biochemical basis of opiate addiction.

The kappa-opioid receptor: evidence for the different subtypes

Classification of drugs acting on the kappa-opioid receptors seems to be difficult, since some of these ligands are also sigma agonists and/or display non-opioid actions as well. Furthermore, certain benzomorphans having kappa-agonistic character, are shown to be mu-antagonists too. Therefore the classification of the kappa-opioid receptor has to be presently restricted to two subclasses that also have physiological meaning. Dynorphin and Met-enkephalin-Arg6-Phe7 are proposed as endogenous peptide ligands for kappa-receptors. Nonpeptide agonists are benzeneacetamides interacting with the kappa1 receptor. Benzomorphans bind to both subtypes of kappa-receptors. No selective nonpeptide ligand for the kappa2 receptor exists as yet. Nor-binaltorphimine, a specific kappa-antagonist also inhibits both kappa-subtypes. Further research for kappa2 selective drugs is necessary for clear distinction between the two kappa-opioid binding sites. Molecular cloning of opioid receptors including their subtypes are expected to provide direct proof of their existence.

The modulatory effects of mu and kappa opioid agonists on 5-HT release from hippocampal and hypothalamic slices of euthermic and hibernating ground squirrels

To elucidate the role of opioids in regulating hibernation, the modulatory effects of different opioids on 35 mM K(+)-stimulated [3H]-5-HT release from brain slices were examined in the Richardson's ground squirrels. DAGO ([D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin), a specific mu agonist, evoked a significant dose-dependent (10(-7)-10(-5) M) inhibition of K(+)-stimulated 5-HT release from hippocampal slices of the non-hibernating squirrels. The inhibitory effect of DAGO was attenuated by either the opioid antagonist naloxone (10(-6) M) or the voltage dependent sodium channel blocker tetrodotoxin (TTX, 10(-6) M). The inhibitory effect of DAGO persisted in the hibernating squirrels; however, a ten fold higher concentration of DAGO (10(-6)-10(-5) M) was required to elicit a significant inhibition. In contrast, kappa agonist U50488 (10(-5) M) exerted a significant enhancement of K(+)-stimulated 5-HT release from hippocampal slices of the non-hibernating squirrels. This enhancement was blocked by either the specific kappa antagonist nor-binaltorphimine (10(-6) M) or TTX (10(-6) M). However, in the hibernating squirrels, the stimulatory effect of U50488 (10(-5) M) on 5-HT release was absent. DAGO and U50488 had no modulatory effects on K(+)-stimulated 5-HT release from the hypothalamic slices of either the non-hibernating or hibernating squirrels. These results demonstrate that the modulatory effects of opioids on 5-HT release are receptor-specific and state-dependent, indicating the complex nature of the roles of different opioids in regulating hibernation.

The modulation of placental lactogen release by opioids: a role for extracellular calcium

We previously reported that kappa opioids stimulated the release of human placental lactogen (hPL) from trophoblastic cells and that this effect was prevented by co-incubation with naloxone. We also reported that adenylate cyclase was not directly involved in this process. In order to understand the post-receptor events mediating hPL release by opioids in the human placenta, we studied the role of extracellular calcium. Human trophoblastic cells obtained by trypsin digestion were cultured for 48 h in Ham's F-10 medium supplemented with 10% fetal bovine serum (FBS), 200 U/ml penicillin, and 200 micrograms/ml streptomycin. 45Ca2+ influx was then measured by filtration on glass-fiber filters. We observed a time- and dose-dependent stimulation of 45Ca2+ influx by ethylketocyclazocine (EKC) with an EC50 of 0.5 nM and a maximal stimulation of 196% over control. This effect was completely blocked by naloxone, a non-specific opioid antagonist, and by nor-binaltorphimine, a specific kappa antagonist. We also demonstrated that U-50,488 (kappa agonist) had the same stimulatory effect as EKC (221 +/- 25% of control). D-Ala2,NMe-Phe4,Gly-ol5)-enkephalin (DAGO) (mu agonist) slightly stimulated Ca2+ influx (128 +/- 5% of control, p > 0.05) whereas D-Ser2,Leu,Thr6)-enkephalin (DSLET) (delta agonist) had no effect. Pre-incubation of trophoblastic cells with pertussis toxin (PTX) did not affect the EKC-induced 45Ca2+ influx, suggesting that this placental opiate effect is not coupled with PTX-sensitive G proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Kappa opioid binding sites on the R1.1 murine lymphoma cell line: sensitivity to cations and guanine nucleotides

The present study describes the characterization of an opioid binding site on membranes prepared from the R1.1 cell line, a murine thymoma. Specific (-)[3H]bremazocine binding was saturable, stereoselective, and limited to a single high affinity binding site with a Kd value of 15.2 +/- 1.6 pM and a Bmax value of 54.8 +/- 6.0 fmol/mg of protein. The kappa-selective alkaloids and dynorphin peptides inhibited (-)[3H]bremazocine binding with Ki values of less than 1 nM, in contrast to mu- and delta-selective ligands. The high affinity of this site for alpha-neo-endorphin and U50,488 suggests that this kappa opioid binding site resembles the kappa 1b subtype. NaCl, as well as other mono- and divalent cations, inhibited (-)[3H]bremazocine binding. In the presence of NaCl, the nucleotides GTP, GDP, and the nonhydrolyzable analog guanylyl-5'-imidodiphosphate (Gpp(NH)p) also decreased (-)[3H]bremazocine binding, suggesting that this kappa opioid binding site is coupled to a G-protein. In summary, R1.1 cells possess a single high affinity kappa opioid receptor that shares many properties with brain kappa 1b opioid receptors.

Opioid-inhibited adenylyl cyclase in rat brain membranes: lack of correlation with high-affinity opioid receptor binding sites

Opioid agonists bind to GTP-binding (G-protein)-coupled receptors to inhibit adenylyl cyclase. To explore the relationship between opioid receptor binding sites and opioid-inhibited adenylyl cyclase, membranes from rat striatum were incubated with agents that block opioid receptor binding. These agents included irreversible opioid agonists (oxymorphone-p-nitrophenylhydrazone), irreversible antagonists [naloxonazine, beta-funaltrexamine, and beta-chlornaltrexamine (beta-CNA)], and phospholipase A2. After preincubation with these agents, the same membranes were assayed for high-affinity opioid receptor binding [3H-labeled D-alanine-4-N-methylphenylalanine-5-glycine-ol-enkephalin (mu), 3H-labeled 2-D-serine-5-L-leucine-6-L-threonine enkephalin (delta), and [3H]ethylketocylazocine (EKC) sites] and opioid-inhibited adenylyl cyclase. Although most agents produced persistent blockade in binding of ligands to high-affinity mu, delta, and EKC sites, no change in opioid-inhibited adenylyl cyclase was detected. In most treated membranes, both the IC50 and the maximal inhibition of adenylyl cyclase by opioid agonists were identical to values in untreated membranes. Only beta-CNA blocked opioid-inhibited adenylyl cyclase by decreasing maximal inhibition and increasing the IC50 of opioid agonists. This effect of beta-CNA was not due to nonspecific interactions with G(i), Gs, or the catalytic unit of adenylyl cyclase, as neither guanylylimidodiphosphate-inhibited, NaF-stimulated, nor forskolin-stimulated activity was altered by beta-CNA pretreatment. Phospholipase A2 decreased opioid-inhibited adenylyl cyclase only when the enzyme was incubated with brain membranes in the presence of NaCl and GTP. These results confirm that the receptors that inhibit adenylyl cyclase in brain do not correspond to the high-affinity mu, delta, or EKC sites identified in brain by traditional binding studies.

Analgesic effect of salmon-calcitonin administered by two routes. Effect on morphine analgesia

We compared the analgesia induced by intraperitoneally (i.p.) and intracerebroventricularly (i.c.v.) administered salmon-calcitonin (S-CT), using the hot-plate test and the writhing test. The influence of the route of administration on the analgesia induced by morphine was also studied. After i.p. administration the analgesic effect was observed only in the writhing test. When S-CT was administered i.c.v., analgesia was observed in both tests, although it was greater in the writhing test than in the hot-plate test. I.c.v. injected S-CT increased the analgesia of i.p. injected morphine. Our results provide new information about the analgesic effect of S-CT and suggest that central mechanisms are involved.

Repeated systemic administration of the mixed inhibitor of enkephalin-degrading enzymes, RB101, does not induce either antinociceptive tolerance or cross-tolerance with morphine

The potent analgesic responses elicited by systemic administration of RB101, N-[(R,S)-2-benzyl-3[(S)(2-amino-4-methylthio)butyldithio]-1-oxopro pyl]- 1-oxopropyl]-L-phenylalanine benzyl ester, a prodrug able to inhibit enkephalin-degrading enzymes completely after in vivo bioactivation, has made it possible to investigate the development of antinociceptive tolerance after chronic potentiation of endogenous enkephalins. The ED50 values of RB101 obtained 10 min after i.v. injection were not significantly different in mice treated for 4 days with i.p. administered vehicle (ED50 = 9.50 (6.37-14.15) mg/kg), or with 80 mg/kg of RB101 twice daily (ED50 = 9.50 (5.86-15.39) mg/kg). In contrast, a parallel rightwards shift of the dose-response curves, corresponding to a significant 1.92 (1.49-2.52)-fold decrease in analgesic potency, was observed after i.v. administration of morphine in mice chronically treated with morphine (3 mg/kg, twice daily for 4 days) (ED50 = 3.10 (2.52-3.81) mg/kg) vs. saline (ED50 = 1.60 (1.22-2.09) mg/kg). No tolerance to RB101 was observed even after a longer period (8 days) of chronic treatment with the prodrug. Moreover, no cross-tolerance between morphine and RB101 appeared to occur since the ED50 values obtained after i.v. administration of RB101 were not significantly different in mice chronically pretreated with vehicle (ED50 = 9.50 (6.37-14.15) mg/kg) or with morphine (ED50 = 10.00 (6.62-15.10) mg/kg). The analgesic effect of RB101 observed in morphine-tolerant mice was antagonized by prior injection of naloxone, but not naltrindole (delta-selective antagonist), supporting a preferential involvement of mu-opioid receptors in the antinociceptive effect of RB101, at least in mice in the hot-plate test.(ABSTRACT TRUNCATED AT 250 WORDS)

Opioid and cannabinoid receptor inhibition of adenylyl cyclase in brain

Both opioids and cannabinoids bind to G-protein-coupled receptors to inhibit adenylyl cyclase in neurons. These reactions were assayed in brain membranes, where maximal inhibitory activity occurred in the following regions: mu-opioid inhibition in rat thalamus, delta-opioid inhibition in rat striatum, kappa-opioid inhibition in guinea pig cerebellum, and cannabinoid inhibition in cerebellum. The inhibition of adenylyl cyclase by both cannabinoid and opioid agonists was typical of G-protein-linked receptors: they required GTP, they were not supported by non-hydrolyzable GTP analogs, and they were abolished (in primary neuronal cell culture) by pertussis toxin treatment. The immediate targets of this system were determined by assaying protein phosphorylation in the presence of receptor agonists and App(NH)p, a substrate for adenylyl cyclase. In striatal membranes, opioid agonists inhibited the phosphorylation of at least two bands of MW 85 and 63 kDa, which may be synapsins I and II, respectively. Other experiments determined the long-term effects of this second messenger system. In primary neuronal cultures, opioid-inhibited adenylyl cyclase attenuated forskolin-stimulated pro-enkephalin mRNA levels, thus providing a feedback regulation of opioid synthesis. Finally, in cerebellar granule cells, both cannabinoid and opioid receptors may exist on the same cells. In these cells, agonists which bind to different receptor types may produce similar biological responses.

Opioid receptor-mediated inhibition of 3H-dopamine and 14C-acetylcholine release from rat nucleus accumbens slices. A study on the possible involvement of K+ channels and adenylate cyclase

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.

Neurobiology of opiate abuse

Opiates interact with cell surface receptors on neurons involved in the transmission of information along neural pathways that are related to behaviours essential for the life of the self and of the species. Opiates are provided with powerful and multifaceted rewarding properties that are fundamental for the acquisition, maintenance and relapse of opiate addiction. Gaetano Di Chiara and Alan North argue that both dopaminergic and non-dopaminergic systems are involved in opiate reward, and that opiate addiction results from adaptive and learning processes involving both positive reinforcing mechanisms related to the rewarding properties of opiates and negative reinforcing mechanisms related to the aversive properties of withdrawal in dependent subjects.

Expression cloning of cDNA encoding a seven-helix receptor from human placenta with affinity for opioid ligands

Here we report the expression cloning of cDNA encoding a putative opioid receptor from a human placenta cDNA library. Placental opioid receptors are of the kappa type. As the dynorphin opioid peptides are kappa-selective, a dynorphin ligand was used in an affinity-enrichment (panning) procedure to select transiently transfected COS-7 cells expressing kappa receptor binding sites. The cloned cDNA encodes a 440-residue protein of the seven-helix guanine nucleotide-binding protein (G-protein)-coupled receptor family. Ligand binding reveals a stereospecific site with typical opioid properties, which binds peptide and nonpeptide opioids with moderate affinity (Kd approximately 100 nM) and which lacks the expected kappa selectivity. The deduced transmembrane domain is 93% identical to the homologous region of the human neuromedin K (neurokinin B) receptor, but the N-terminal and C-terminal sequences have many dissimilarities. The expressed receptor binds opioid ligands but not tachykinins; and under the same conditions, a cloned rat neuromedin K receptor binds tachykinins but not opioids.

Autoradiographic mapping of neurotransmitter system receptors in mammalian brain

Regional localization of neurotransmitter system receptors was visualized in the gerbil grain and in the rat brain using receptor autoradiography. [3H]Quinuclidinyl benzilate (QNB), [3H]cyclohexyladenosine (CHA), [3H]muscimol, [3H]MK-801, [3H]SCH 23390, [3H]PN200-110, [3H]spiperone, and [3H]naloxone were label muscarinic receptors, adenosine A1 receptors, GABAA receptors, N-methyl-D-aspartate (NMDA) receptors, dopamine D1 receptors, L-type calcium channels, spirodecanone receptors, and opioid receptors, respectively. Regional localization of [3H]QNB, [3H]muscimol, [3H]MK-801, [3H]SCH 23390, and [3H]PN200-110 binding sites in the gerbil brain was relatively similar to that in the rat brain. In contrast, the autoradiographic distribution of [3H]spiperone and [3H]naloxone binding sites in the gerbil was quite different from that in the rat. This phenomenon was found especially in the hippocampus and the cerebellum. The results suggest that the gerbil differs from the rat with respect to spirodecanone and opioid binding sites in the hippocampus and the cerebellum. This finding may help to further elucidate the species differences and relationships for brain function and behavioral pharmacology.

Opioid agonists binding and responses in SH-SY5Y cells

SH-SY5Y (human neuroblastoma) cultured cells, known to have mu-opioid receptors, have been used to assess and compare the ability of eight representative mu-selective compounds from diverse opioid families to recognize and activate these receptors. A wide range of receptor affinities spanning a factor of 10,000 was found between the highest affinity fentanyl analogs (Ki = 0.1nM) and the lowest affinity analog, meperidine (Ki = 1 microM). A similar range was found for inhibition of PGE1-stimulated cAMP accumulation with a rank order of activities that closely paralleled binding affinities. Maximum inhibition of cAMP accumulation by each compound was about 80%. Maximum stimulation of GTPase activity (approximately 50%) was also similar for all compounds except the lowest affinity meperidine. Both effects were naloxone reversible. These results provide further evidence that mu-receptors are coupled to inhibition of adenylate cyclase and that the SH-SY5Y cell line is a good system for assessment of mu-agonists functional responses.

Pertussis toxin abolishes mu- and delta-opioid agonist-induced place preference

The effects of i.c.v. treatment with pertussis toxin (PTX) on the motivational effect of opioid agonists were examined in mice. Morphine (0.1-10 nmol, i.c.v.), [D-Ala2, N-MePhe4, Gly-ol5]enkephalin (DAGO, 0.001-0.1 nmol, i.c.v.), a selective mu-opioid receptor agonist, and [D-Pen2, D-Pen5]enkephalin (DPDPE, 1-15 nmol, i.c.v.), a selective delta-opioid receptor agonist, produced a dose-related place preference in mice. Administration of PTX (0.5 micrograms, i.c.v.) to mice resulted in no preference for either the drug- or vehicle-associated place. Pretreatment with PTX abolished the place preferences induced by DAGO (0.1 nmol), morphine (10 nmol) and DPDPE (15 nmol). These findings demonstrate that the appetitive effects of opioids result from the activation of central mu- and delta-receptors, and suggest that PTX-sensitive GTP-binding proteins in the central nervous system may be involved in the motivational effects of mu- and delta-opioid agonists.