Selective suppression of cocaine- versus food-maintained responding by monoamine releasers in rhesus monkeys: benzylpiperazine, (+)phenmetrazine, and 4-benzylpiperidine.

Monoamine releasers constitute one class of drugs currently under investigation as potential agonist medications for the treatment of cocaine dependence. The efficacy and safety of monoamine releasers as candidate medications may be influenced in part by their relative potency to release dopamine and serotonin, and we reported previously that releasers with approximately 30-fold selectivity for dopamine versus serotonin release may be especially promising. The present study examined the effects of the releasers benzylpiperazine, (+)phenmetrazine, and 4-benzylpiperidine, which have 20- to 48-fold selectivity in vitro for releasing dopamine versus serotonin. In an assay of cocaine discrimination, rhesus monkeys were trained to discriminate 0.4 mg/kg i.m. cocaine from saline in a two-key, food-reinforced procedure. Each of the releasers produced a dose- and time-dependent substitution for cocaine. 4-Benzylpiperidine had the most rapid onset and shortest duration of action. Phenmetrazine and benzylpiperazine had slower onsets and longer durations of action. In an assay of cocaine self-administration, rhesus monkeys were trained to respond for cocaine injections and food pellets under a second order schedule. Treatment for 7 days with each of the releasers produced a dose-dependent and selective reduction in self-administration of cocaine (0.01 mg/kg/injection). The most selective effects were produced by phenmetrazine. Phenmetrazine also produced a downward shift in the cocaine self-administration dose effect curve, virtually eliminating responding maintained by a 30-fold range of cocaine doses (0.0032-0.1 mg/kg/injection) while having only small and transient effects on food-maintained responding. These findings support the potential utility of dopamine-selective releasers as candidate treatments for cocaine dependence.

Tolerance to 3,4-methylenedioxymethamphetamine in rats exposed to single high-dose binges.

3,4-Methylenedioxymethamphetamine (MDMA or ecstasy) stimulates the transporter-mediated release of monoamines, including 5-HT. High-dose exposure to MDMA causes persistent 5-HT deficits (e.g. depletion of brain 5-HT) in animals, yet the functional and clinical relevance of such deficits are poorly defined. Here we examine functional consequences of MDMA-induced 5-HT depletions in rats. Male rats received binges of three i.p. injections of MDMA or saline, one injection every 2 h; MDMA was given at a threshold pharmacological dose (1.5 mg/kgx3, low dose) or at a fivefold higher amount (7.5 mg/kgx3, high dose). One week later, jugular catheters and intracerebral guide cannulae were implanted. Two weeks after binges, rats received acute i.v. challenge injections of 1 and 3 mg/kg MDMA. Neuroendocrine effects evoked by i.v. MDMA (prolactin and corticosterone secretion) were assessed via serial blood sampling, while neurochemical effects (5-HT and dopamine release) were assessed via microdialysis in brain. MDMA binges elevated core temperatures only in the high-dose group, with these same rats exhibiting approximately 50% loss of forebrain 5-HT 2 weeks later. Prior exposure to MDMA did not alter baseline plasma hormones or dialysate monoamines, and effects of i.v. MDMA were similar in saline and low-dose groups. By contrast, rats pretreated with high-dose MDMA displayed significant reductions in evoked hormone secretion and 5-HT release when challenged with i.v. MDMA. As tolerance developed only in rats exposed to high-dose binges, hyperthermia and 5-HT depletion are implicated in this phenomenon. Our results suggest that MDMA tolerance in humans may reflect 5-HT deficits which could contribute to further dose escalation.

Restoration of 3,4-methylenedioxymethamphetamine-induced 5-HT depletion by the administration of L-5-hydroxytryptophan.

BACKGROUND: 3,4-Methylenedioxymethamphetamine (MDMA) causes persistent decreases in brain 5-HT content and 5-HT transporter (SERT) binding, with no detectable changes in SERT protein. Such data suggest that MDMA impairs 5-HT transmission but leaves 5-HT nerve terminals intact. To further test this hypothesis, we carried out two types of experiments in rats exposed to high-dose MDMA. First, we examined the effects of MDMA on SERT binding and function using different in vitro assay conditions. Next, we treated rats with the 5-HT precursor, l-5-hydroxytryptophan (5-HTP), in an attempt to restore MDMA-induced depletions of 5-HT. METHODS: Rats received three i.p. injections of saline or MDMA (7.5 mg/kg), one injection every 2 h. Rats in one group were decapitated, and brain tissue was assayed for SERT binding and [(3)H]5-HT uptake under conditions of normal (100 or 126 mM) and low (20 mM) NaCl concentration. Rats from another group received saline or 5-hydroxytryptophan/benserazide (5-HTP-B), each drug at 50 mg/kg i.p., and were killed 2 h later. RESULTS: MDMA reduced SERT binding to 10% of control when assayed in 100 mM NaCl, but this reduction was only 55% of control in 20 mM NaCl. MDMA decreased immunoreactive 5-HT in caudate and hippocampus to about 35% of control. Administration of 5-HTP-B to MDMA-pretreated rats significantly increased the 5-HT signal toward normal levels in caudate (85% of control) and hippocampus (66% of control). CONCLUSION: 1) Following high-dose MDMA treatment sufficient to reduce SERT binding by 90%, a significant number of functionally intact 5-HT nerve terminals survive. 2) The degree of MDMA-induced decreases in SERT binding depends on the in vitro assay conditions. 3) 5-HTP-B restores brain 5-HT depleted by MDMA, suggesting that this approach might be clinically useful in abstinent MDMA users.

Aminorex, fenfluramine, and chlorphentermine are serotonin transporter substrates. Implications for primary pulmonary hypertension.

BACKGROUND: Coadministration of phentermine and fenfluramine (phen/fen) effectively treats obesity and possibly addictive disorders. The association of fenfluramine and certain other anorexic agents with serious side effects, such as cardiac valvulopathy and primary pulmonary hypertension (PPH), limits the clinical utility of these drugs. Development of new medications that produce neurochemical effects like phen/fen without causing unwanted side effects would be a significant therapeutic breakthrough. METHODS AND RESULTS: We tested the hypothesis that fenfluramine (and other anorexic agents) might increase the risk of PPH through interactions with serotonin (5-HT) transporters. Because 5-HT transporter proteins in the lung and brain are identical, we examined, in rat brain, the effects of selected drugs on 5-HT efflux in vivo and monoamine transporters in vitro as a generalized index of transporter function. Our data show that drugs known or suspected to increase the risk of PPH (eg, aminorex, fenfluramine, and chlorphentermine) are 5-HT transporter substrates, whereas drugs that have not been shown to increase the risk of PPH are less potent in this regard. CONCLUSIONS: We speculate that medications that are 5-HT transporter substrates get translocated into pulmonary cells where, depending on the degree of drug retention, their intrinsic drug toxicity, and individual susceptibility, PPH could develop as a response to high levels of these drugs or metabolites. Emerging evidence suggests that it is possible to develop transporter substrates devoid of adverse side effects. Such medications could have therapeutic application in the management of obesity, drug dependence, depression, and other disorders.

Evidence for possible involvement of 5-HT(2B) receptors in the cardiac valvulopathy associated with fenfluramine and other serotonergic medications.

BACKGROUND: Serotonergic medications with various mechanisms of action are used to treat psychiatric disorders and are being investigated as treatments for drug dependence. The occurrence of fenfluramine-associated valvular heart disease (VHD) has raised concerns that other serotonergic medications might also increase the risk of developing VHD. We hypothesized that fenfluramine or its metabolite norfenfluramine and other medications known to produce VHD have preferentially high affinities for a particular serotonin receptor subtype capable of stimulating mitogenesis. METHODS AND RESULTS: Medications known or suspected to cause VHD (positive controls) and medications not associated with VHD (negative controls) were screened for activity at 11 cloned serotonin receptor subtypes by use of ligand-binding methods and functional assays. The positive control drugs were (+/-)-fenfluramine; (+)-fenfluramine; (-)-fenfluramine; its metabolites (+/-)-norfenfluramine, (+)-norfenfluramine, and (-)-norfenfluramine; ergotamine; and methysergide and its metabolite methylergonovine. The negative control drugs were phentermine, fluoxetine, its metabolite norfluoxetine, and trazodone and its active metabolite m-chlorophenylpiperazine. (+/-)-, (+)-, and (-)-Norfenfluramine, ergotamine, and methylergonovine all had preferentially high affinities for the cloned human serotonin 5-HT(2B) receptor and were partial to full agonists at the 5-HT(2B) receptor. CONCLUSIONS: Our data imply that activation of 5-HT(2B) receptors is necessary to produce VHD and that serotonergic medications that do not activate 5-HT(2B) receptors are unlikely to produce VHD. We suggest that all clinically available medications with serotonergic activity and their active metabolites be screened for agonist activity at 5-HT(2B) receptors and that clinicians should consider suspending their use of medications with significant activity at 5-HT(2B) receptors.

An examination of the opiate receptor subtypes labeled by [3H]cycloFOXY: an opiate antagonist suitable for positron emission tomography.

17-Cyclopropylmethyl-3,14-dihydroxy-4,5-alpha-epoxy-6-beta-fluoromorp hinan (cycloFOXY) is a fluorinated derivative of naltrexone suitable for labeling opiate receptors using positron emission transaxial tomography. Using the quantitative ligand binding method "binding surface analysis," in vitro autoradiography, and site-directed alkylating agents, [3H]cycloFOXY is shown to label mu and kappa opiate binding sites in vitro. Similar results were obtained using [3H]naloxone. Additional experiments demonstrate that [3H]cycloFOXY administered in vivo also labels mu and kappa binding sites. The relevance of these findings are discussed from clinical and basic science perspectives.

Alterations in serotonergic responsiveness during cocaine withdrawal in rats: similarities to major depression in humans.

BACKGROUND: Withdrawal from long-term cocaine use is accompanied by symptoms resembling major depression. Because acute cocaine affects serotonin (5-HT) neurons, and 5-HT dysfunction is implicated in the pathophysiology of depression, we evaluated the effects to 5-HT agonists in rats withdrawn from repeated injections of cocaine (15 mg/kg i.p., b.i.d., 7 days) or saline. METHODS: In the first study, prolactin (PRL) responses elicited by the 5-HT-releasing agent fenfluramine, the 5-HT1A agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), and the 5-HT2A/2C agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI) were examined as indices of postsynaptic 5-HT receptor function. In a second study, specific responses induced by 8-OH-DPAT, namely inhibition of brain 5-HT synthesis and stimulation of feeding, were examined as correlates of 5-HT1A autoreceptor function. RESULTS: Prior treatment with cocaine did not modify fenfluramine-evoked PRL release; however, the PRL secretory response to 8-OH-DPAT was blunted and the PRL response to DOI was potentiated after chronic cocaine treatment. Cocaine exposure did not alter the inhibitory effect of 8-OH-DPAT on 5-HT synthesis. 8-OH-DPAT-induced feeding was influenced by prior cocaine, but this effect was secondary to pronounced baseline hyperphagia in the cocaine-treated group. CONCLUSIONS: These data indicate that withdrawal from chronic cocaine renders specific subpopulations of postsynaptic 5-HT1A receptors subsensitive and 5-HT2A/2C receptors supersensitive. No evidence for cocaine-induced changes in 5-HT1A autoreceptor responsiveness was found. A survey of the literature reveals similarities in the profile of 5-HT dysfunction between rats withdrawn from cocaine and humans diagnosed with depression. We propose that withdrawal from chronic cocaine in rats may serve as a useful animal model of depressive disorders.

Effects of intravenous cocaine on plasma cortisol and prolactin in human cocaine abusers.

The aim of the present work was to examine the cortisol and prolactin responses to acute cocaine administration in human cocaine users. Each subject served as his own control during intravenous saline placebo and cocaine (40 mg) infusion sessions. Cocaine significantly elevated plasma cortisol but did not affect prolactin. The rise in cortisol coincided with an increase in heart rate and blood pressure after cocaine. In agreement with studies in animals, our data suggest that cocaine activates the hypothalamic-pituitary-adrenal axis in humans. However, based on the well-known importance of dopamine as a prolactin-inhibiting factor, the failure of cocaine to suppress prolactin in the present study raises questions concerning the role of dopamine in the mechanism of acute cocaine action in humans.

A review of the effects of dopaminergic agents on humans, animals, and drug-seeking behavior, and its implications for medication development. Focus on GBR 12909.

Medication development for cocaine abuse has focused on potential mechanisms of action related to the abuse of cocaine. The hypothesis that mesolimbic dopamine (DA) is the key neurochemical mediator of cocaine's addictive and reinforcing effects is well supported by a wide variety of data from animal studies. On the other hand, medications that increase DA or block its action in humans can produce effects that appear incompatible with this hypothesis. This article reviews these incompatibilities between animal and human data with a focus on the DAergic actions of drugs, including DA reuptake inhibitors, direct DA agonists, DA increasers, and DA antagonists. Possible reasons for these discrepancies are discussed, and the potential role of high-affinity DA uptake inhibitors, such as GBR12909, for pharmacotherapies for treating cocaine addiction in humans is likely to come from understanding its mechanisms of action, it is clear that further research on the effects of cocaine in humans and animals will be critical to the medication development effort.

Neurotoxicity, drugs and abuse, and the CuZn-superoxide dismutase transgenic mice.

Administration of methamphetamine (METH) to animals causes loss of DA terminals in the brain. The manner by which METH causes these changes in neurotoxicity is not known. We have tested the effects of this drug in copper/zinc (CuZn)-superoxide dismutase transgenic (SOD Tg) mice, which express the human CuZnSOD gene. In nontransgenic (non-Tg) mice, acute METH administration causes significant decreases in DA and dihydroxyphenylacetic acid (DOPAC) in the striata of non-Tg mice. In contrast, there were on significant decreases in striatal DA in the METH administration caused decreases in striatal DA and DOPAC in the non-Tg mice, but not in the SOD-Tg mice. Similar studies were carried out with 1-methyl-1,2,3,6-tetrahydropyridine (MPTP), which also causes striatal DA and DOPAC depletion. As in the case of METH, MPTP causes marked depletion of DA and DOPAC in the non-Tg mice, but not in the SOD Tg mice. These results suggest that the mechanisms of toxicity of both METH and MPTP involved superoxide radical formation.

Synthesis and absolute configuration of optically pure enantiomers of a kappa-opioid receptor selective agonist.

The enantiomers of U50,488, ligands highly selective for kappa-opioid receptors, have been prepared by a refined procedure and their optical purity demonstrated. The absolute configuration of (+)-trans-2-pyrrolidinyl-N-methylcyclohexylamine, a chemically versatile intermediate for synthesis of analogs of kappa-opioid receptor ligands with defined chirality, has been determined to be 1S,2S by X-ray crystallographic analysis. This intermediate has been used to synthesize the optically pure U50,488 enantiomers with known absolute configuration.

Tight binding dopamine reuptake inhibitors as cocaine antagonists. A strategy for drug development.

The experiments reported in this study tested the hypothesis that tight binding dopamine (DA) reuptake inhibitors might act as cocaine antagonists. Binding studies demonstrated that the high affinity dopamine reuptake inhibitor, 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-[3-phenylpropyl]piperazine (GBR12909) produced a wash-resistant inhibition of the DA transporter in rat striatal membranes as labeled by [3H]cocaine or [3H]1-[2-(diphenyl- methoxy)ethyl]-4-(3-phenylpropyl)piperazine [( 3H]GBR12935), indicative of tight binding. In vivo microdialysis experiments showed that administration of 25 mg/kg GBR12909 to rats produced a modest, but not statistically significant, increase in the extracellular levels of striatal DA, while this same dose of GBR12909 inhibited the ability of cocaine to elevate extracellular DA levels by 64%. These data suggest that tight binding DA reuptake blockers may provide a fruitful approach for developing a cocaine antagonist.

Synthesis of an affinity ligand ('UPHIT') for in vivo acylation of the kappa-opioid receptor.

The isothiocyanate analog (1S,2S-trans-2-isothiocyanato-4,5-dichloro-N- methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide, 3a) of the highly selective kappa-opioid receptor agonist, U50,488, was prepared as a potential site-directed affinity ligand for acylation of kappa-opioid receptors in vivo. The isothiocyanate (3a) which we have designated UPHIT and its enantiomer (3b) were synthesized in 3 steps starting from optically pure (1S,2S)-(+)-trans-2-pyrrolidinyl-N-methyl-cyclohexylamine (4a) and its enantiomer (4b), respectively, thus defining their absolute stereochemistry. Binding in vitro of the 1S,2S enantiomer 3a to kappa receptors labelled by [3H]U69,593 was shown to occur with an IC50 value of 25.92 +/- 0.36 nM, whereas 827.42 +/- 5.88 and 115.10 +/- 1.23 nM were obtained for the IC50 value of the 1R,2R enantiomer (3b) and (+/-)-3 respectively. Intracerebroventricular (ICV) injection of 100 micrograms of (+/-)-3 into guinea-pig brain followed by analysis of remaining kappa-binding sites 24 h later revealed that (+/-)-3 depleted 98% of the kappa receptors that bind [3H]U69,593 and 40% of those that bind [3H]bremazocine. These preliminary data suggest exciting uses for these compounds in furthering our knowledge of the kappa-opioid receptor.

Distribution of opiate receptor subtypes and enkephalin and dynorphin immunoreactivity in the hippocampus of squirrel, guinea pig, rat, and hamster.

The distribution of enkephalin and dynorphin immunoreactivity in the hippocampus of four rodent species (gray squirrel, guinea pig, rat, and hamster) is compared with the pattern of opiate receptor subtypes (mu, delta, and kappa). The distribution of opioid peptides is fairly consistent in the anterior hippocampus of these four species. Intense immunoreactivity for dynorphin and enkephalin is found in the hilus of the dentate gyrus and in the mossy fiber system. Occasional immunoreactive processes are seen in the dentate molecular layer and scattered throughout the CA1 and CA3 fields. In the rat and hamster, an additional plexus of enkephalinergic fibers straddles both sides of the hippocampal fissure. Cells immunoreactive for both opioid peptides are located in and just superficial to the dentate granule cell layer. Opiate receptors are variably distributed in these rodent species. In the squirrel, guinea pig, and hamster, mu and kappa binding is dense in the stratum lucidum of CA3 and the molecular layer of the dentate gyrus. In the rat, dense mu and kappa binding is localized within and adjacent to the pyramidal and granule cell layers. Delta receptor patterns show additional species differences. In the rat, the delta distribution is similar to the mu and kappa patterns. In the other species, the delta binding pattern is generally the inverse of the mu/kappa pattern: most areas of the hippocampus are enriched in delta sites, whereas the stratum lucidum and the pyramidal cell layer are receptor-sparse. Thus, the stratum lucidum--site of dense terminations of mossy fibers containing opioid peptides--is characterized by selectively sparse delta receptors in four species and by selectively dense kappa receptors in three species. The three receptor subtypes, taken either individually or together and compared to the peptides, are more variably and more widely distributed throughout the hippocampus and fail to show a correspondence with opioid-peptide-containing terminals. The mismatches suggest that receptor locations and densities are organized without relation to the sites of relevant transmitter release.

Evaluation of phentermine and fenfluramine, alone and in combination, in normal, healthy volunteers.

Recent clinical reports indicate that combined administration of phentermine and fenfluramine may have useful effects in the treatment of drug abuse. The present study was designed to evaluate the subjective and mood-altering effects of these drugs, alone and in combination, in normal healthy volunteers. Seven male and five female volunteers participated in an eight-session, double-blind study in which each subject received each of the following drug conditions: d-amphetamine (10 and 20 mg), phentermine (30 mg), fenfluramine (40 and 80 mg), phentermine (30 mg) with fenfluramine (40 mg), phentermine (30 mg) with fenfluramine (80 mg), and placebo. Sessions were conducted in a laboratory setting two or three days a week. Subjects completed standardized self-report questionnaires and psychomotor tests before and at regular intervals after each drug administration. Phentermine produced effects that were similar to those of d-amphetamine, whereas fenfluramine produced different and apparently aversive effects (e.g., it increased measures of anxiety and confusion). Phentermine reduced the apparently aversive effects of fenfluramine when the two drugs were given together. These results suggest that the combination of phentermine and fenfluramine would have a low potential for abuse.

1-(m-chlorophenyl)piperazine (mCPP) dissociates in vivo serotonin release from long-term serotonin depletion in rat brain.

Serotonin (5-HT) releasing agents such as d-fenfluramine are known to cause long-term depletion of forebrain 5-HT in animals, but the mechanism of this effect is unknown. In the present study, we examined the relationship between drug-induced 5-HT release and long-term 5-HT depletion in rat brain. The 5-HT-releasing actions of d-fenfluramine and a non-amphetamine 5-HT drug, 1-(m-chlorophenyl)piperazine (mCPP), were compared using in vivo microdialysis in the nucleus accumbens. The ability of d-fenfluramine and mCPP to interact with 5-HT transporters was tested using in vitro assays for [3H]5-HT uptake and radioligand binding. Local infusion of d-fenfluramine or mCPP (1-100 microM) increased extracellular 5-HT, with elevations in dopamine occurring at high doses. Intravenous injection of either drug (1-10 micromol/kg) produced dose-related increases in 5-HT without affecting dopamine. d-Fenfluramine and mCPP exhibited similar potency in their ability to stimulate 5-HT efflux in vivo and interact with 5-HT transporters in vitro. When rats received high-dose d-fenfluramine or mCPP (10 or 30 micromol/kg, i.p., every 2 h, 4 doses), only d-fenfluramine-treated rats displayed long-term 5-HT depletions. Thus, mCPP is a 5-HT releaser that does not appear to cause 5-HT depletion. Our data support the notion that 5-HT release per se may not be sufficient to produce the long-term 5-HT deficits associated with d-fenfluramine and other amphetamines.

Chronic cocaine exposure potentiates prolactin and head shake responses to 5-HT2 receptor stimulation in rats.

The effect of repeated cocaine administration on serotonin2 (5-HT2) receptor function was examined in male rats. Rats were fitted with indwelling jugular catheters and subsequently received cocaine (15 mg/kg, i.p., b.i.d.) or saline for 7 days. Rats were challenged with the 5-HT2 agonist DOI (25, 100, 400 micrograms/kg, i.v.) or saline 42 hr and 8 days after cessation of chronic treatment. Serial blood samples were collected at various times after DOI challenge and analyzed for prolactin levels. DOI-induced head shakes and skin jerks were examined concurrently in the same subjects. After 42 hr of withdrawal, the stimulatory effects of DOI on prolactin release and shaking behavior were significantly enhanced in cocaine-treated rats. Conversely, the skin jerk response to DOI was not altered by prior cocaine exposure. After 8 days of withdrawal, the prolactin and head shake responses to DOI were still potentiated in cocaine-treated rats, but this effect was no longer statistically significant. The data indicate that chronic cocaine enhances the sensitivity of 5-HT2 receptor mechanisms. Our findings further suggest the possibility that altered 5-HT2 receptor function may be involved in the mood disturbances experienced by abstinent cocaine addicts.

A study of the interaction of the alkylating agent, NIH10236, with opioid receptors in vitro and in vivo.

The series of experiments reported in this paper examined the spectrum of subtypes of opioid receptors alkylated in vitro by N-cyclopropylmethyl-7 alpha-methylfumaramido-6,14- endoethenotetrahydronororipavine (NIH10236) and four optical isomers of the methylfumaramidophenethyl derivatives of 3-methylfentanyl. Pretreatment of membranes with NIH10236 resulted in a wash-resistant inhibition of the binding of [3H]6 beta-fluoro-6-desoxyoxymorphone (mu binding sites), the binding of [3H][D-ala2,D-leu5]-enkephalin (both the higher and lower affinity delta binding sites) and was without effect on kappa binding sites labelled with [3H]bremazocine. All four potential alkylating derivatives of 3-methylfentanyl were inactive. Pretreatment of membranes with 1 microM of the reversible ligands, (+)-cis-3-methylfentanyl, but not its enantiomer, inhibited the binding of [3H]6 beta-fluoro-6-desoxyoxymorphone and the binding of [3H][D-ala2,D-leu5]enkephalin to the lower affinity binding sites by over 90%. This phenomenon is termed "pseudo-irreversible inhibition." Incubation of pretreated membranes for 60 min at 37 degrees C, in the presence of 200 mM NaCl and 50 microM GppNHp, only partially reversed the masking of opioid receptors by (+)-cis-3-methylfentanyl. For in vivo experiments, membranes were prepared 18-24 hr after the intracerebroventricular administration of 80 and 50 micrograms of NIH10236. This resulted in decreased labelling of mu binding sites, lower affinity [3H][D-ala2,D-leu5]enkephalin binding sites, as well as kappa binding sites, labelled by [3H]U69,593 and [3H]bremazocine. There was no apparent alteration in the higher affinity [3H][D-ala2,D-leu5]enkephalin binding site.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic administration of morphine and naltrexone up-regulate[3H][D-Ala2,D-leu5]enkephalin binding sites by different mechanisms.

Previous studies have demonstrated that chronic administration of morphine up-regulated the lower affinity binding site for [3H][D-ala2,D-leu5]enkephalin, without producing a detectable alteration in the higher affinity binding site for [3H][D-ala2,D-leu5]enkephalin (Rothman et al., Eur. J. Pharmac. 124: 113-119, 1986). The experiments reported in this paper tested the hypothesis that chronic administration of morphine and naltrexone up-regulated the binding sites for [3H][D-ala2,D-leu5]enkephalin by different mechanisms. Rats were given either morphine or naltrexone chronically. Chronic administration of morphine up-regulated the lower affinity site, while chronic administration of naltrexone up-regulated both the higher and lower affinity binding sites for [3H][D-ala2,D-leu5]enkephalin. Unlike the lower affinity binding site for [3H][D-ala2,D-leu5]enkephalin present in membranes prepared from rats treated with placebo pellets, the lower affinity binding sites which were up-regulated by naltrexone and morphine were partially (naltrexone) or completely (morphine) labile to preincubation for 60 min at 25 degrees C in 50 mM Tris-HCl, pH 7.4, containing 0.4 M NaCl. These data suggest that chronic administration of morphine and naltrexone up-regulate binding sites for [3H][D-ala2,D-leu5]enkephalin through different mechanisms, and that the lower affinity binding sites for [3H][D-ala2, D-leu5]enkephalin which are up-regulated by chronic administration of morphine and naltrexone might differ biochemically from the lower affinity binding sites present in membranes treated with placebo.