Evidence that Melanocortin Receptor Agonist Melanotan-II Synergistically Augments the Ability of Naltrexone to Blunt Binge-Like Ethanol Intake in Male C57BL/6J Mice.

BACKGROUND: The nonselective opioid receptor antagonist, naltrexone (NAL), reduces alcohol (ethanol [EtOH]) consumption in animals and humans and is an approved medication for treating alcohol abuse disorders. Proopiomelanocortin (POMC)-derived melanocortin (MC) and opioid peptides are produced in the same neurons in the brain, and recent preclinical evidence shows that MC receptor (MCR) agonists reduce excessive EtOH drinking in animal models. Interestingly, there is a growing body of literature revealing interactions between the MC and the opioid systems in the modulation of pain, drug tolerance, and food intake. METHODS: In the present report, a mouse model of binge EtOH drinking was employed to determine whether the MCR agonist, melanotan-II (MTII), would improve the effectiveness of NAL in reducing excessive binge-like EtOH drinking when these drugs were co-administered prior to EtOH access. RESULTS: Both NAL and MTII blunt binge-like EtOH drinking and associated blood EtOH levels, and when administered together, a low dose of MTII (0.26 mg/kg) produces a 7.6-fold increase in the effectiveness of NAL in reducing binge-like EtOH drinking. Using isobolographic analysis, it is demonstrated that MTII increases the effectiveness of NAL in a synergistic manner. CONCLUSIONS: The current observations suggest that activators of MC signaling may represent a new approach to treating alcohol abuse disorders and a way to potentially improve existing NAL-based therapies.

Effects of alterations in cannabinoid signaling, alone and in combination with morphine, on pain-elicited and pain-suppressed behavior in mice.

Inhibitors of fatty acid amide hydrolase (FAAH) and anandamide (AEA) uptake, which limit the degradation of endogenous cannabinoids, have received interest as potential therapeutics for pain. There is also evidence that endogenous cannabinoids mediate the antinociceptive effects of opioids. Assays of pain-elicited and pain-suppressed behavior have been used to differentiate the effects of drugs that specifically alter nociception from drugs that alter nociception caused by nonspecific effects such as catalepsy or a general suppression of activity. Using such procedures, this study examines the effects of the direct cannabinoid type 1 (CB1) agonist (-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol (CP55940), the FAAH inhibitor cyclohexylcarbamic acid 3'-carbamoylbiphenyl-3-yl ester (URB597), and the AEA uptake inhibitor N-(4-hydroxyphenyl) arachidonylamide (AM404). Additional experiments examined these compounds in combination with morphine. CP55940 produced antinociception in assays of pain-elicited, but not pain-suppressed, behavior and disrupted responding in an assay of schedule-controlled behavior. URB597 and AM404 produced antinociception in assays of pain-elicited and pain-suppressed behavior in which acetic acid was the noxious stimulus, but had no effect on the hotplate and schedule-controlled responding. CP55940 in combination with morphine resulted in effects greater than those of morphine alone in assays of pain-elicited and scheduled-controlled behavior but not pain-suppressed behavior. URB597 in combination with morphine resulted in enhanced morphine effects in assays of pain-elicited and pain-suppressed behavior in which diluted acetic acid was the noxious stimulus, but did not alter morphine's effects on the hotplate or schedule-controlled responding. These studies suggest that, compared with direct CB1 agonists, manipulations of endogenous cannabinoid signaling have enhanced clinical potential; however, their effects depend on the type of noxious stimulus.

Metabotropic glutamate antagonists alone and in combination with morphine: comparison across two models of acute pain and a model of persistent, inflammatory pain.

The present study examined the effects of the mGluR1 antagonist JNJ16259685 (JNJ) and the mGluR5 antagonist 2-methyl-6-phenylethynylpyridine (MPEP) alone and in combination with morphine in two acute pain models (hotplate, warm water tail-withdrawal), and a persistent, inflammatory pain model (capsaicin). In the hotplate and warm water tail-withdrawal procedures, JNJ and MPEP were ineffective when administered alone. In both procedures, JNJ potentiated morphine antinociception. In the hotplate procedure, MPEP potentiated morphine antinociception at the highest dose examined, whereas in the warm water tail-withdrawal procedure MPEP attenuated morphine antinociception at a moderate dose and potentiated morphine antinociception at a high dose. For both JNJ and MPEP, the magnitude of this morphine potentiation was considerably greater in the hotplate procedure. In the capsaicin procedure, the highest dose of MPEP produced intermediate levels of antihyperalgesia and also attenuated the effects of a dose of morphine that produced intermediate levels of antihyperalgesia. In contrast, JNJ had no effect when administered alone in the capsaicin procedure and did not alter morphine-induced antihyperalgesia. The present findings suggest that the effects produced by mGluR1 and mGluR5 antagonists alone and in combination with morphine can be differentiated in models of both acute and persistent pain.

Effects of morphine on pain-elicited and pain-suppressed behavior in CB1 knockout and wildtype mice.

RATIONALE: Pharmacological manipulations of the type 1 cannabinoid receptor (CB1) suggest a role for CB1 in morphine-induced antinociception, but studies utilizing CB1 knockout (KO) mice do not support this conclusion. Since studies using CB1 KO mice to study morphine's antinociceptive effects have only examined thermal nociception, this study examines these interactions in models that employ a chemical stimulus. OBJECTIVES: To determine whether the findings obtained with thermal pain models extend to other models, the effects of morphine on acetic acid-induced writhing were examined in CB1 KO and wildtype (WT) mice. Behaviors that decrease in response to acid injection, feeding and wheel running, were also examined, and investigations were carried out in the thermal hotplate assay. The CB1 antagonist SR141716A was also examined in these assays. RESULTS: Morphine completely blocked acid-induced writhing (1.0-10.0 mg/kg) and increased response latencies in the hotplate (10.0-32.0 mg/kg) in both genotypes. Morphine (3.2 mg/kg) significantly attenuated the suppression of wheel running but did not completely prevent this effect in either genotype. Morphine did not alter pain-suppressed feeding. In each of these assays, morphine's effects were not altered in CB1 KO mice compared with WT mice; however, SR141716A attenuated morphine's effects in C57BL/6 mice. CONCLUSIONS: The effects of morphine do not differ in CB1 KO and WT mice in preclinical pain models using thermal and chemical stimuli. Since SR141716A did attenuate the effects of morphine, it is possible that CB1 KO mice undergo developmental changes that mask the role of CB1 receptors in morphine's antinociceptive effects.

Increased efficacy of micro-opioid agonist-induced antinociception by metabotropic glutamate receptor antagonists in C57BL/6 mice: comparison with (-)-6-phosphonomethyl-deca-hydroisoquinoline-3-carboxylic acid (LY235959).

RATIONALE: Recent experimental data suggest that metabotropic glutamate receptor (mGluR) antagonists with selectivity for mGluR1 and mGluR2/3 enhance morphine-induced antinociception. OBJECTIVES: The present study addressed the hypothesis that mGluR antagonists enhance opioid antinociception by increasing opioid efficacy. MATERIALS AND METHODS: The antinociceptive effects of the partial mu-opioid receptor agonists buprenorphine and dezocine were first assessed in a hot-plate procedure under conditions of low (53 degrees C) and high (56 degrees C) stimulus intensity. Under conditions in which buprenorphine and dezocine produced submaximal antinociceptive effects, these drugs were assessed after pretreatment with the mGluR1 antagonist JNJ16259685, the mGluR5 antagonist MPEP, the mGluR2/3 antagonist LY341495, and for comparison, the N-methyl-D-aspartate (NMDA) receptor antagonist LY235959. RESULTS: Buprenorphine (0.032-3.2 mg/kg) and dezocine (0.1-10 mg/kg) were fully efficacious at 53 degrees C and produced submaximal antinociceptive effects at 56 degrees C (i.e., their effects did not exceed 50% of the maximum possible effect). Pretreatment with JNJ16259685 (1.0-3.2 mg/kg), LY341495 (1.0-3.2 mg/kg), and LY235959 (0.32-1.0 mg/kg) enhanced the antinociceptive effects of buprenorphine and dezocine at 56 degrees C, as revealed by significant increases in the peak effects of both drugs to approximately 100% maximum possible effect. In contrast, pretreatment with MPEP (1.0-3.2 mg/kg) did not modulate the antinociceptive effects of buprenorphine and dezocine. CONCLUSIONS: These results suggest that, similar to the NMDA receptor antagonist LY235959, the mGluR1 antagonist JNJ16259685 and the mGluR2/3 antagonist LY341495 increase the antinociceptive efficacy of buprenorphine and dezocine.

Sex differences in NMDA antagonist enhancement of morphine antihyperalgesia in a capsaicin model of persistent pain: comparisons to two models of acute pain.

In acute pain models, N-methyl-D-aspartate (NMDA) antagonists enhance the antinociceptive effects of morphine to a greater extent in males than females. The purpose of this investigation was to extend these findings to a persistent pain model which could be distinguished from acute pain models on the basis of the nociceptive fibers activated, neurochemical substrates, and duration of the nociceptive stimulus. To this end, persistent hyperalgesia was induced by administration of capsaicin in the tail of gonadally intact F344 rats, following which the tail was immersed in a mildly noxious thermal stimulus, and tail-withdrawal latencies measured. For comparison, tests were conducted in two acute pain models, the hotplate and warm water tail-withdrawal procedures. In males, the non-competitive NMDA antagonist dextromethorphan enhanced the antihyperalgesic effect of low to moderate doses of morphine in a dose-and time-dependent manner. Across the doses and pretreatment times examined, enhancement was not observed in females. Enhancement of morphine antinociception by dextromethorphan was seen in both males and females in the acute pain models, with the magnitude of this effect being greater in males. These findings demonstrate a sexually-dimorphic interaction between NMDA antagonists and morphine in a persistent pain model that can be distinguished from those observed in acute pain models.

Morphine in combination with metabotropic glutamate receptor antagonists on schedule-controlled responding and thermal nociception.

The present study examined the interactive effects of morphine in combination with metabotropic glutamate (mGlu) receptor antagonists on schedule-controlled responding and thermal nociception. Drug interaction data were examined with isobolographic and dose-addition analysis. Morphine, the mGlu1 receptor antagonist JNJ16259685 [(3,4-dihydro-2H-pyrano-[2,3-b]quinolin-7-yl)-(cis-4-methoxycyclohexyl)-methanone], the mGlu5 receptor antagonist MPEP [2-methyl-6-(phenylethynyl)pyridine hydrochloride], and the mGlu2/3 receptor antagonist LY341495 [(2S)-2-amino-2-[(1S,2S-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid] all decreased rates of schedule-controlled responding. JNJ16259685/morphine, MPEP/morphine, and LY341495/morphine mixtures produced additive effects on this endpoint. Morphine also produced dose-dependent antinociception in the assay of thermal nociception, whereas JNJ16259685, MPEP, and LY341495 failed to produce an effect. In this assay, JNJ16259685 and LY341495 potentiated the antinociceptive effects of morphine, whereas MPEP/morphine mixtures produced additive effects. These results suggest that an mGlu1 and an mGlu2/3 receptor antagonist, but not an mGlu5 receptor antagonist, selectively enhance the antinociceptive effects of morphine. In addition, these data confirm that the behavioral effects of drug mixtures depend on the endpoint under study.

Sex differences in the potency of kappa opioids and mixed-action opioids administered systemically and at the site of inflammation against capsaicin-induced hyperalgesia in rats.

RATIONALE: Sex differences in the potency of the antinociceptive effects of kappa opioids have been reported in various acute pain models with evidence suggesting that these sex differences are mediated by activity in the N-methyl-D: -aspartate (NMDA) system. OBJECTIVES: The purpose of the present study was to evaluate sex differences in the antihyperalgesic actions of selected kappa and mixed-action opioids in a persistent pain model and determine if the NMDA system modulates these effects in a sexually dimorphic manner. METHODS: Using gonadally intact male and female F344 rats, hyperalgesia was induced by local administration of capsaicin in the tail, after which the tail was immersed in a mildly noxious thermal stimulus (45 degrees C water), and tail-withdrawal latency measured. Opioids were then administered systemically (s.c.) and locally (in the tail) alone, and in selected combinations with the noncompetitive NMDA antagonist dextromethorphan. RESULTS: When administered systemically and locally, the kappa opioids spiradoline, U69,593 and U50,488, and the mixed-action opioids butorphanol and nalbuphine, produced dose-dependent antihyperalgesic effects. Whereas the kappa opioids were generally more potent in males, sex differences were not observed with the mixed-action opioids. Peripheral receptor activity was confirmed for local administration of kappa opioids by the antagonism observed after local, but not intracerebroventricular (i.c.v.), administration of the kappa antagonist nor-binaltorphamine (nor-BNI). Dextromethorphan was equally potent in attenuating the antihyperalgesia induced by kappa opioids in both males and females. CONCLUSIONS: These findings demonstrate sex differences in kappa opioid activity in a persistent pain model. Although an NMDA antagonist blocked the effects of kappa opioids in this model, these effects were not sexually dimorphic as reported in most acute pain models.

Influence of low doses of naltrexone on morphine antinociception and morphine tolerance in male and female rats of four strains.

In a recently proposed bimodal opioid receptor model, the inhibitory actions of opioids on action potential duration in dorsal root ganglion neurons have been proposed to produce antinociception, and the excitatory actions of hyperalgesia. Recent studies indicate that selectively blocking these excitatory actions with low doses of opioid antagonists enhances opioid antinociception and attenuates the development of opioid tolerance. To determine if the excitatory actions of opioids contribute to sex as well as strain differences in opioid sensitivity, the effects of morphine alone and in combination with low doses of naltrexone were examined in male and female rats of four strains. The strains examined differed in their sensitivity to opioid antinociception and magnitude of sex differences in opioid sensitivity. All testing was conducted using a thermal tail-flick procedure with the nociceptive stimulus intensity adjusted so that baseline latencies were comparable across strains/sexes. In chronic studies, the morphine dosing regimen was adjusted in each strain/sex to produce comparable levels of tolerance. In each of the strains tested, morphine produced dose-dependent increases in antinociception, with differences in morphine potency observed across strains and sexes. In male and female Sprague-Dawley and Long-Evans rats, naltrexone enhanced morphine antinociception and attenuated the development of morphine tolerance. These effects were not observed in F344 and Lewis rats, even when tests were conducted across a range of morphine and naltrexone doses. These results suggest that the ability of low doses of naltrexone to enhance opioid antinociception does not contribute to sex or rat strain differences in opioid sensitivity.

Dissociation between sex differences in the immunological, behavioral, and physiological effects of kappa- and delta-opioids in Fischer rats.

RATIONALE: The sex of the individual can have a profound effect on sensitivity to the effects of opioids. Recently, our laboratory provided the first evidence that females may be more sensitive to the immune-altering effects of mu-opioids than males. However, it remains unknown whether kappa- and delta-opioids produce sexually dimorphic effects on immune responses. OBJECTIVE: The present study sought to determine whether kappa- and delta-opioids produce differential immunological effects in males and females using the memory-T-cell-dependent in vivo inflammatory response contact hypersensitivity (CHS). As sex differences in the magnitude of opioid effects can be outcome-specific, additional experiments were conducted to compare the immunological effects of kappa- and delta-opioids with other behavioral and physiological effects. MATERIALS AND METHODS: Contact hypersensitivity was induced in male and female Fischer rats. Prior to elicitation of CHS, animals were administered selected doses of the kappa-opioid spiradoline (0.2-20 mg/kg), delta-opioid SNC80 (1-10 mg/kg), or vehicle. The antinociceptive and diuretic effects of spiradoline were also assessed in males and females, as were the locomotor effects of SNC80. RESULTS: Spiradoline produced significantly greater enhancement of CHS in females than males, but produced comparable antinociceptive and diuretic effects in both sexes. By contrast, SNC80 did not significantly affect the course of CHS in either sex, but females were significantly more sensitive to its locomotor stimulatory effects. CONCLUSIONS: These results demonstrate that females are more sensitive than males to the CHS-altering effects of spiradoline and that sex differences in the magnitude and direction of opioid-induced sex differences are outcome dependent.

Effects of the long-lasting kappa opioid 2-(3,4-dichlorophenyl)-N-methyl-N-[(1S)-1-(3-isothiocyanatophenyl)-2-(1-pyrrolidinyl) ethyl] acetamide in a drug discrimination and warm water tail-withdrawal procedure.

Although studies suggest that 2-(3,4-dichlorophenyl-N-methyl-N-[(1S)-1-(3-isothiocyanatophenyl)-2-(1-pyrrolidinyl) ethyl] acetamide (DIPPA) has transient kappa-opioid-mediated agonist effects followed by long-lasting kappa-antagonist effects, its behavioral and pharmacological actions have not been systematically examined and there is evidence suggesting that some of its effects are species dependent. The purpose of this investigation was to examine the actions of DIPPA in different behavioral procedures and in three species. In a pigeon drug discrimination procedure, DIPPA and the kappa-opioids U50,488 and ICI-199441 substituted fully for the stimulus effects produced by spiradoline. For DIPPA, this effect was observed between 0.25 and 4 h after administration. In a warm water tail-withdrawal procedure, DIPPA failed to produce antinociception in rats or mice even when relatively high doses were tested using pretreatment intervals ranging from 0.25 to 24 h. In this procedure, DIPPA antagonized the effects of spiradoline and U50,488 in mice. In rats, DIPPA antagonized the effects of U50,488 but not those of spiradoline. Taken together, these results suggest that DIPPA may function as a low-efficacy kappa-opioid and have a long duration of action, and there may be some species differences in its behavioral profile. This profile of action, however, differs from other low-efficacy kappa-opioids.

Influence of estrous cycle and gonadal hormone depletion on nociception and opioid antinociception in female rats of four strains.

UNLABELLED: Evidence suggests that gonadal hormones can modulate sensitivity to nociceptive stimuli and opioid antinociception. However, cross-study comparisons addressing the nature of this modulation have been complicated by a number of methodologic factors, including the use of different rodent strains and opioids. The present study examined the influence of estrous cycle and gonadal hormone depletion (ovariectomy) on thermal nociception and opioid antinociception in female F344, Lewis, Long Evans, and Wistar rats. Estrous cycle-dependent differences in nociceptive sensitivity were not observed in any of the strains. Ovariectomy decreased nociceptive sensitivity relative to their intact female counterparts. In normal cycling females, morphine and buprenorphine were generally most potent in metestrus and proestrus and least potent in estrus. The magnitude of these differences was consistently larger with buprenorphine. Ovariectomy increased the antinociceptive potency of morphine and buprenorphine, with this effect also being larger with buprenorphine. These data suggest that in adult females of a number of rat strains, estrous cycle and gonadal hormone depletion modulate the antinociceptive potency of opioids, with the magnitude of this effect being dependent on the type of opioid. In contrast, depletion of gonadal hormones, but not estrous cycle, modulates thermal nociceptive sensitivity in adult female rats. PERSPECTIVE: Gonadal hormones influence opioid antinociception, and this effect is apparent across different genetic backgrounds. These results suggest that the phase of the menstrual cycle might alter the effectiveness of certain opioids administered to relieve pain in women.

Behavioral assessment of temporal summation in the rat: sensitivity to sex, opioids and modulation by NMDA receptor antagonists.

RATIONALE: Temporal summation of pain reflects a perceived increase in nociceptive sensitivity following repeated noxious stimulation that can last for approximately 15 s-2 min. This short-lasting change in nociceptive sensitivity has been used as a model to examine factors that influence the central processing of pain and the mechanisms underlying some chronic pain conditions. OBJECTIVE: The purpose of this study was to develop a behavioral procedure to induce temporal summation in rats and determine the sensitivity of temporal summation (i.e., decrease tail-withdrawal latency following repeated presentations of a nociceptive thermal stimulus) to various parametric manipulations, sex, modulation by the N-methyl-D-aspartate (NMDA) receptor system, and sensitivity to reversal by opioids. RESULTS: The magnitude of temporal summation generally decreased with increases in the inter-nociceptive stimulus interval, and increased with increases in both the nociceptive stimulus intensity and the number of nociceptive stimulus presentations. Temporal summation was short-lived, evident 3.0 s after the final nociceptive stimulus presentation, but not after 30 s. Males displayed slightly higher levels of temporal summation than females. The non-competitive NMDA antagonists ketamine (3.0-30 mg/kg), dizocilpine (0.03-0.1 mg/kg) and dextromethorphan (10-30 mg/kg) attenuated the level of temporal summation at doses that failed to produce antinociceptive effects (warm water tail-withdrawal procedure). In an antinociception procedure, the opioids morphine (3.0-10 mg/kg), buprenorphine (0.3-3.0 mg/kg), butorphanol (3.0-30 mg/kg) and spiradoline (10-30 mg/kg) were more potent in males, whereas these opioids were equally potent and effective in reducing the level of temporal summation in males and females. CONCLUSIONS: These findings suggest a number of similarities in the characteristics and receptor modulation of temporal summation in humans and rats, and that in this model of chronic pain there are no sex differences in opioid potency.

Predictors of high ethanol consumption in RIIbeta knock-out mice: assessment of anxiety and ethanol-induced sedation.

BACKGROUND: Genetic and pharmacological evidence suggests that the cyclic adenosine monophosphate-dependent protein kinase A pathway modulates neurobiological responses to ethanol. Mutant mice lacking the RIIbeta subunit of protein kinase A (RIIbeta(-/-)) are resistant to ethanol-induced sedation and drink significantly more ethanol than littermate wild-type mice (RIIbeta(+/+)). We determined whether high ethanol intake by the RIIbeta(-/-) mice on alternate genetic backgrounds is reliably predicted by high basal levels of anxiety or resistance to the sedative effects of ethanol. METHODS: Two-bottle choice procedures and a battery of behavioral tests (elevated plus maze, open-field activity, and zero maze) were used to assess voluntary ethanol consumption and basal levels of anxiety in RIIbeta(-/-) and RIIbeta(+/+) mice on either a C57BL/6J or a 129/SvEv x C57BL/6J genetic background. Additionally, ethanol-induced sedation and blood ethanol levels were determined in RIIbeta(-/-) and RIIbeta(+/+) mice after intraperitoneal injection of ethanol (3.8 g/kg). RESULTS: RIIbeta(-/-) mice on both genetic backgrounds consumed more ethanol and had a greater preference for ethanol relative to RIIbeta(+/+) mice. However, RIIbeta(-/-) mice showed reduced basal levels of anxiety when maintained on the C57BL/6J background but showed increased anxiety when maintained on the 129/SvEv x C57BL/6J background. Consistent with prior research, RIIbeta(-/-) mice were resistant to the sedative effects of ethanol, regardless of the genetic background. Finally, RIIbeta(-/-) and RIIbeta(+/+) mice showed similar blood ethanol levels. CONCLUSIONS: These results indicate that high ethanol consumption is associated with resistance to the sedative effects of ethanol but that basal levels of anxiety, as well as ethanol metabolism, do not reliably predict high ethanol drinking by RIIbeta(-/-) mice.

Effects of opioids in morphine-treated pigeons trained to discriminate among morphine, the low-efficacy agonist nalbuphine, and saline.

In opioid-dependent subjects, the low-efficacy mu agonist nalbuphine generally precipitates withdrawal or withdrawal-like stimulus effects. To provide a more complete characterization of the discriminative stimulus effects of nalbuphine in opioid-treated subjects, seven White Carneux pigeons were treated daily with 10 mg/kg morphine i.m. and trained 6 h later to discriminate among 10 mg/kg morphine, 1.0 mg/kg nalbuphine, and saline by responding on one of three different keys. When tested, morphine produced morphine-key responding and nalbuphine produced nalbuphine-key responding. Replacing the daily morphine injection with saline produced nalbuphine-key responding, and this effect was reversed by the administration of morphine. In substitution tests with other compounds, the antagonists naltrexone (i.m.) and CTAP (D-Phe-Cys-Tyr-D-Tryp-Lys-Thr-Pen-Thr-NH2) (i.c.v.) produced nalbuphine-key responding. High-efficacy agonists fentanyl and etorphine produced morphine-key responding. The intermediate-efficacy agonists buprenorphine, dezocine, and butorphanol produced a pattern of morphine-, saline-, and/or nalbuphine-key responding that differed across individual pigeons. The lower efficacy agonists nalorphine and levallorphan produced predominantly nalbuphine-key responding. The kappa agonists spiradoline and U50,488 [trans-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]cyclohexyl)benzeneacetamide methanesulfonate], the nonopioid d-amphetamine, and saline produced predominantly saline-key responding. Naltrexone and nalbuphine dose dependently reversed the morphine-key responding produced by the training dose of morphine. Together, these data suggest that the discriminative-stimulus effects of the low-efficacy micro agonist nalbuphine in morphine-treated pigeons are similar to those of other low-efficacy agonists, naltrexone, and the termination of daily morphine treatment.

Pharmacogenetic analysis of sex differences in opioid antinociception in rats.

Sex differences in opioid antinociception have been reported in rodents and monkeys, with opioids being more potent in males than females. In the present study, the influence of rat strain on sex differences in opioid antinociception was examined in a warm water tail-withdrawal procedure. Antinociceptive tests were conducted with the high-efficacy micro-opioid morphine, and the less efficacious opioids buprenorphine, butorphanol and nalbuphine. Baseline nociceptive latencies were consistently higher in males than their female counterparts. Sex differences in opioid antinociception were observed in all strains tested, with the opioids being more potent and/or effective in males. The magnitude of the sex differences was related to the relative efficacy of the opioid, with morphine, buprenorphine, butorphanol and nalbuphine being on average 2.2-, 2.6-, 15.9- and 11.9-fold more potent in males. Sex differences also varied markedly across strains, with large differences consistently obtained in the F344 and F344-Sasco strains, moderate differences in the ACI, DA, Lewis, Sprague Dawley, Wistar and Wistar-Kyoto strains, and small differences in the Long Evans-Blue Spruce, Long Evans, Brown Norway and Holtzman strains. When compared across strains, there was no relationship between sex differences in nociceptive sensitivity and opioid sensitivity. These findings provide strong support for the role of genetic factors in determining sex differences in opioid antinociception, and suggest that the use of low-efficacy opioids, coupled with the use of rat strains that display small and large sex differences in opioid antinociception, may provide a sensitive tool to investigate the mechanisms underlying sex differences in opioid antinociception.

Capsaicin-induced hyperalgesia and mu-opioid-induced antihyperalgesia in male and female Fischer 344 rats.

The influence of sex in determining responses to opioid analgesics has been well established in rodents and monkeys in assays of short-lasting, phasic pain. The purpose of this investigation was to use a capsaicin model of tonic pain to evaluate sex differences in hyperalgesia and mu-opioid-induced antihyperalgesia in Fischer 344 (F344) rats. Capsaicin injected into the tail produced a dose-dependent thermal hyperalgesia in males and females, with the dose required to produce a comparable level of hyperalgesia being 3.0-fold higher in males than in females. These sex differences were modulated by gonadal hormones, inasmuch as gonadectomy increased the potency of capsaicin in males and decreased its potency in females. Morphine, buprenorphine, and dezocine administered by various routes [systemic (s.c.), local (in the tail), and central (i.c.v.)] generally produced marked antihyperalgesic effects in males and females. Although in most instances these opioids were equally potent and effective in males and females, selected doses of local and i.c.v. administered buprenorphine produced greater effects in females. When administered locally, the antihyperalgesic effects of morphine were mediated by peripheral opioid receptors in both males and females, since this effect was not reversed by i.c.v. naloxone methiodide. These data contrast with the finding that mu-opioids are more potent in male rodents in assays of phasic pain, thus suggesting that distinct mechanisms underlie male and female sensitivity to opioid antinociception in phasic and tonic pain models. These findings emphasize the need to test male and female rodents in tonic pain assays that may have greater relevance for human pain conditions.

Effect of strain and sex on mu opioid receptor-mediated G-protein activation in rat brain.

Strain and sex differences in mu opioid-mediated antinociception have been reported in rodents. The present studies evaluated mu opioid receptor-mediated G-protein activation in Lewis and Fischer 344 (F344) male and female rats using agonist-stimulated [35S]GTPgammaS binding. Compared to Lewis rats, F344 rats exhibited a 35% higher level of net DAMGO-stimulated [35S]GTPgammaS binding in striatum. Basal [35S]GTPgammaS binding was approximately 30% lower in thalamus of Lewis than F344 rats. Female Lewis rats also exhibited slightly ( approximately 15%) lower basal [35S]GTPgammaS binding in cingulate cortex relative to F344 rats of either sex. The relative efficacies of the mu partial agonists, morphine and buprenorphine, were also examined. Buprenorphine exhibited approximately 40% lower relative efficacy in the periaqueductal gray in Lewis compared to F344 rats, but no other relative efficacy differences were found between strains or sexes. Moreover, regional differences in the relative efficacy of buprenorphine were also detected in Lewis but not F344 rats. In contrast to these results, the only difference found between sexes was the 13% lower basal [35S]GTPgammaS binding in the cingulate cortex of female compared to male Lewis rats. These results suggest that differences in mu opioid receptor-mediated G-protein activation may contribute to strain differences in opioid antinociception, whereas sex differences may result predominantly from other mechanisms.

Use of irreversible antagonists to determine the relative efficacy of mu-opioids in a pigeon drug discrimination procedure: comparison of beta-funaltrexamine and clocinnamox.

The use of irreversible antagonists to assess opioid efficacy has proven fruitful for classifying opioids on the basis of high or low efficacy, but few studies have provided quantitative estimates of efficacy. The purpose of this study was to use beta-funaltrexamine (beta-FNA) and clocinnamox (C-CAM) in a drug discrimination procedure to examine the efficacy of fentanyl, morphine, l-methadone, sufentanil, and etorphine. In pigeons trained to discriminate 0.12 mg/kg fentanyl from water, dose-effect curves were determined for each opioid alone and after pretreatment with beta-FNA and C-CAM. Using quantitative analyses according to an extended model of Black and Leff (1983), apparent efficacy (tau) and affinity (KA) of each opioid was determined, as well as the degree of receptor inactivation (q) produced by each dose of each antagonist. beta-FNA and C-CAM produced dose- and time-dependent, rightward shifts in the dose-effect curves of each opioid, and analyses based on dose-ratios and tau values suggest a rank order of efficacy of etorphine > sufentanil = l-methadone > fentanyl = morphine. Marked differences in the profiles of antagonism produced by beta-FNA and C-CAM were also apparent, as C-CAM, but not beta-FNA, produced insurmountable antagonism. The q values for each antagonist were consistent with these data in indicating that C-CAM and beta-FNA can inactivate nearly 100 and 75% of the receptor population, respectively. In tests conducted in pigeons chronically treated with morphine, doses of beta-FNA that produced parallel, rightward shifts in untreated pigeons flattened the morphine dose-effect curve in morphine-treated pigeons. These results indicate that beta-FNA and C-CAM can differentiate opioids with high relative efficacy and yield comparable estimates of efficacy for various opioids. There are, however, limitations in the proportion of the receptor population that can by eliminated by beta-FNA.

Sex differences in opioid-induced enhancement of contact hypersensitivity.

Previous research has demonstrated that, in male rats, the magnitude of contact hypersensitivity (CHS) can be enhanced by morphine treatment. The present experiments test the hypothesis that the mu-opioids morphine, etorphine, and buprenorphine would produce significant sex differences in the magnitude of 2,4-dinitrofluorobenzene-induced CHS. During tests conducted over a 192-h period, morphine, etorphine, and buprenorphine administered before elicitation of CHS on the external surface of the ear (pinna) potentiated the CHS response, and the magnitude of this enhancement was significantly greater in females than males. By contrast, morphine had no effect on croton oil-induced irritant contact dermatitis, indicating that morphine's effects on CHS do not generalize to immunologically nonspecific forms of contact dermatitis. Activation of brain mu-opioid receptors is responsible for the effects of morphine on CHS, because intracerebroventricular treatment with the mu-opioid receptor antagonist beta-funaltrexamine blocked morphine potentiation of CHS in females and males. The sex differences in morphine potentiation of CHS appear to be a result of the gonadal hormonal milieu, because castration enhanced the CHS response following vehicle and morphine treatment, whereas ovariectomy significantly attenuated the enhancement of CHS by morphine. Because ovariectomy had no effect on the CHS response following vehicle treatment, the presence of female gonadal hormones may underlie the sex differences in morphine potentiation of CHS in gonadally intact animals. Overall, these results support an increased sensitivity to the modulatory effects of opioids on the CHS response in females that depends on the interaction between gonadal hormones and the central mu-opioid system.