3-Isobutyl-1-methylxanthine inhibits basal mu-opioid receptor phosphorylation and reverses acute morphine tolerance and dependence in mice

L-cysteine ethyl ester prevents and reverses acquired physical dependence on morphine in male Sprague Dawley rats

The molecular mechanisms underlying the acquisition of addiction/dependence on morphine may result from the ability of the opioid to diminish the transport of L-cysteine into neurons via inhibition of excitatory amino acid transporter 3 (EAA3). The objective of this study was to determine whether the co-administration of the cell-penetrant L-thiol ester, L-cysteine ethyl ester (L-CYSee), would reduce physical dependence on morphine in male Sprague Dawley rats. Injection of the opioid-receptor antagonist, naloxone HCl (NLX; 1.5 mg/kg, IP), elicited pronounced withdrawal phenomena in rats which received a subcutaneous depot of morphine (150 mg/kg) for 36 h and were receiving a continuous infusion of saline (20 μL/h, IV) via osmotic minipumps for the same 36 h period. The withdrawal phenomena included wet-dog shakes, jumping, rearing, fore-paw licking, 360° circling, writhing, apneas, cardiovascular (pressor and tachycardia) responses, hypothermia, and body weight loss. NLX elicited substantially reduced withdrawal syndrome in rats that received an infusion of L-CYSee (20.8 μmol/kg/h, IV) for 36 h. NLX precipitated a marked withdrawal syndrome in rats that had received subcutaneous depots of morphine (150 mg/kg) for 48 h) and a co-infusion of vehicle. However, the NLX-precipitated withdrawal signs were markedly reduced in morphine (150 mg/kg for 48 h)-treated rats that began receiving an infusion of L-CYSee (20.8 μmol/kg/h, IV) at 36 h. In similar studies to those described previously, neither L-cysteine nor L-serine ethyl ester (both at 20.8 μmol/kg/h, IV) mimicked the effects of L-CYSee. This study demonstrates that 1) L-CYSee attenuates the development of physical dependence on morphine in male rats and 2) prior administration of L-CYSee reverses morphine dependence, most likely by intracellular actions within the brain. The lack of the effect of L-serine ethyl ester (oxygen atom instead of sulfur atom) strongly implicates thiol biochemistry in the efficacy of L-CYSee. Accordingly, L-CYSee and analogs may be a novel class of therapeutics that ameliorate the development of physical dependence on opioids in humans.

Mice with high FGF21 serum levels had a reduced preference for morphine and an attenuated development of acute antinociceptive tolerance and physical dependence

Because of increased opioid misuse, there is a need to identify new targets for minimizing opioid tolerance, and physical and psychological dependence. Previous studies showed that fibroblast growth factor 21 (FGF21) decreased alcohol and sweet preference in mice. In this study, FGF21-transgenic (FGF21-Tg) mice, expressing high FGF21 serum levels, and wildtype (WT) C57BL/6J littermates were treated with morphine and saline to determine if differences exist in their physiological and behavioral responses to opioids. FGF21-Tg mice displayed reduced preference for morphine in the conditioned place preference assay compared to WT littermates. Similarly, FGF21-Tg mice had an attenuation of the magnitude and rate of acute morphine antinociceptive tolerance development, and acute and chronic morphine physical dependence, but exhibited no change in chronic morphine antinociceptive tolerance. The ED50 values for morphine-induced antinociception in the 55 °C hot plate and the 55 °C warm-water tail withdrawal assays were similar in both strains of mice. Likewise, FGF21-Tg and WT littermates had comparable responses to morphine-induced respiratory depression. Overall, FGF21-Tg mice had a decrease in the development of acute analgesic tolerance, and the development of physical dependence, and morphine preference. FGF21 and its receptor have therapeutic potential for reducing opioid withdrawal symptoms and craving, and augmenting opioid therapeutics for acute pain patients to minimize tolerance development.

AMPA receptor positive allosteric modulators attenuate morphine tolerance and dependence

Development of opioid tolerance and dependence hinders the use of opioids for the treatment of chronic pain. In searching for the mechanism and potential intervention for opioid tolerance and dependence, we studied the action of two positive allosteric modulators of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR PAMs). In mice treated with morphine (100 mg/kg, s.c.), acute morphine tolerance and dependence developed in 4-6 h. Treatment with aniracetam, a well-established AMPAR PAM, was able to completely prevent and reverse the development of acute antinociceptive tolerance to morphine. Partial, but significant, effects of aniracetam on acute morphine induced-physical dependence were also observed. Moreover, aniracetam significantly reversed the established morphine tolerance and dependence in a chronic model of morphine tolerance and dependence produced by intermittent morphine (10 mg/kg, s.c. for 5d). In addition, HJC0122, a new AMPAR PAM was found to have similar effects as aniracetam but with a higher potency. These previously undisclosed actions of AMPAR PAMs are intriguing and may shed lights on understanding the APMA signaling pathway in opioid addiction. Moreover, these data suggest that AMPAR PAMs may have utility in preventing and treating morphine tolerance and dependence.

Attenuation of morphine-induced dependence and tolerance by ceftriaxone and amitriptyline in mice

INTRODUCTION

Tolerance to and dependence on the analgesic effect of opioids is a pharmacological phenomenon that occurs after their prolonged administration.

OBJECTIVE

The aim of this study was to evaluate the protective effects of ceftriaxone and amitriptyline on the development of morphine-induced tolerance and dependence.

METHODS

In this study, 18 groups (9 groups each for tolerance and dependency tests) of mice (n = 8) received saline [10 mL/kg, intraperitoneally (i.p.)], morphine (50 mg/kg, i.p.), ceftriaxone (50 mg/kg, i.p., 100 mg/kg, i.p., and 200 mg/kg, i.p.), amitriptyline (5 mg/kg, i.p., 10 mg/kg, i.p., and 15 mg/kg, i.p.), or a combination of ceftriaxone (50 mg/kg, i.p.) and amitriptyline (5 mg/kg, i.p.) once per day for 4 days for investigation and comparison of the effects of ceftriaxone and amitriptyline on the prevention of dependency and tolerance to morphine. Tolerance was assessed with administration of morphine (9 mg/kg, i.p.) and using the hot plate test on the 5(th) day. In dependency tests, withdrawal symptoms were assessed on the 4(th) day for each animal 30 minutes after the administration of naloxone (4 mg/kg, i.p.; 2 hours after the last dose of morphine).

RESULTS

It was found that treatment with ceftriaxone or amitriptyline attenuated the development of tolerance to the antinociceptive effect of morphine and also reduced naloxone-precipitated withdrawal jumping and standing on feet. Furthermore, coadministration of ceftriaxone and amitriptyline at low doses (50 mg/kg, i.p. and 5 mg/kg, i.p., respectively) prior to morphine injection also decreased both morphine-induced tolerance and dependence.

CONCLUSION

Results indicate that the treatment with ceftriaxone and amitriptyline, alone or in combination, could attenuate the development of morphine-induced tolerance and dependence.

Agonist-dependent mu-opioid receptor signaling can lead to heterologous desensitization

Desensitization of the micro-opioid receptor (MOR) has been implicated as an important regulatory process in the development of tolerance to opiates. Monitoring the release of intracellular Ca(2+) ([Ca(2+)](i)), we reported that [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO)-induced receptor desensitization requires receptor phosphorylation and recruitment of beta-arrestins (betaArrs), while morphine-induced receptor desensitization does not. In current studies, we established that morphine-induced MOR desensitization is protein kinase C (PKC)-dependent. By using RNA interference techniques and subtype specific inhibitors, PKCepsilon was shown to be the PKC subtype activated by morphine and the subtype responsible for morphine-induced desensitization. In contrast, DAMGO did not increase PKCepsilon activity and DAMGO-induced MOR desensitization was not affected by modulating PKCepsilon activity. Among the various proteins within the receptor signaling complex, Galphai2 was phosphorylated by morphine-activated PKCepsilon. Moreover, mutating three putative PKC phosphorylation sites, Ser(44), Ser(144) and Ser(302) on Galphai2 to Ala attenuated morphine-induced, but not DAMGO-induced desensitization. In addition, pretreatment with morphine desensitized cannabinoid receptor CB1 agonist WIN 55212-2-induced [Ca(2+)](i) release, and this desensitization could be reversed by pretreating the cells with PKCepsilon inhibitor or overexpressing Galphai2 with the putative PKC phosphorylation sites mutated. Thus, depending on the agonist, activation of MOR could lead to heterologous desensitization and probable crosstalk between MOR and other Galphai-coupled receptors, such as the CB1.

A novel Gbetagamma-subunit inhibitor selectively modulates mu-opioid-dependent antinociception and attenuates acute morphine-induced antinociceptive tolerance and dependence

The Gbetagamma subunit has been implicated in many downstream signaling events associated with opioids. We previously demonstrated that a small molecule inhibitor of Gbetagamma-subunit-dependent phospholipase (PLC) activation potentiated morphine-induced analgesia (Bonacci et al., 2006). Here, we demonstrate that this inhibitor, M119 (cyclohexanecarboxylic acid [2-(4,5,6-trihydroxy-3-oxo-3H-xanthen-9-yl)-(9Cl)]), is selective for mu-opioid receptor-dependent analgesia and has additional efficacy in mouse models of acute tolerance and dependence. When administered by an intracerebroventricular injection in mice, M119 caused 10-fold and sevenfold increases in the potencies of morphine and the mu-selective peptide, DAMGO, respectively. M119 had little or no effect on analgesia induced by the kappa agonist U50,488 or delta agonists DPDPE or Deltorphin II. Similar results were obtained in vitro, as only activation of the mu-opioid receptor stimulated PLC activation, whereas no effect was seen with the kappa- and delta-opioid receptors. M119 inhibited mu-receptor-dependent PLC activation. In studies to further explore the in vivo efficacy of M119, systemic administration M119 also resulted in a fourfold shift increase in potency of systemically administered morphine. Of particular interest, M119 was also able to attenuate acute, antinociceptive tolerance and dependence in mice treated concomitantly with both M119 and morphine. These studies suggest that small organic molecules, such as M119, that specifically regulate Gbetagamma subunit signaling may have important therapeutic applications in enhancing opioid analgesia, while attenuating the development of tolerance and dependence.

Neutral antagonist activity of naltrexone and 6beta-naltrexol in naïve and opioid-dependent C6 cells expressing a mu-opioid receptor

BACKGROUND AND PURPOSE

Adenylyl cyclase sensitization occurs on chronic agonist activation of mu-opioid receptors and is manifested by an increase in cAMP levels (overshoot) on challenge with antagonist. It has been proposed that a long lasting constitutively active receptor is formed on chronic mu-opioid exposure and that antagonists with inverse agonist activity rapidly return the receptor to a basal state causing a cAMP overshoot and a more severe withdrawal response in vivo. This hypothesis depends on an accurate characterization of neutral and inverse agonist properties of opioid antagonists.

EXPERIMENTAL APPROACH

C6 glioma and HEK293 cells expressing mu-opioid receptors were used. Opioid antagonists were examined for their ability to induce a cAMP overshoot following chronic treatment with the agonist DAMGO ([D-Ala(2),N-Me-Phe(4),Glyol(5)]-enkephalin). The compounds were also characterized as agonists, inverse agonists or neutral antagonists by using assays for competitive binding, [(35)S]GTPgammaS (guanosine-5'-O-(3-[(35)S]thio)triphosphate) binding and changes in cell surface receptor expression.

KEY RESULTS

Naltrexone, 6beta-naltrexol and naloxone were indistinguishable to the mu-opioid receptor in the opioid-naïve or dependent state and acted as neutral antagonists. The delta-opioid receptor inverse agonist RTI-5989-25 [(+)-N-[trans-4'-(2-methylphenyl)-2'-butenyl]-(3R,4R)-dimethyl-4-(3-hydroxyphenyl)piperidine], a 3,4-dimethyl-4-(3-hydroxyphenyl)-piperidine, was an inverse agonist at the mu-opioid receptor, and the peptide antagonist CTAP (H-D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2)) showed variable, assay-dependent properties. All the antagonists precipitated the same degree of cAMP overshoot in opioid-dependent cells.

CONCLUSIONS AND IMPLICATIONS

Antagonists at the mu-opioid receptor may be neutral or show inverse agonist activity. Formation of a constitutively active mu-opioid receptor is not a requirement for the development or expression of adenylyl cyclase sensitization.

Enkephalin release promotes homeostatic increases in constitutively active mu opioid receptors during morphine withdrawal

We previously demonstrated that naloxone administration produces a robust conditioned place aversion (CPA) in opiate-naive rodents by blocking the action of enkephalins at mu opioid receptors (MORs). The aversive response to naloxone is potentiated by prior exposure to morphine. Morphine-induced MOR constitutive activity is hypothesized to underlie this enhanced effect of naloxone, an inverse agonist at the MOR. We sought additional evidence for the role of constitutively active MORs in this morphine-induced enhancement using the pro-enkephalin knockout (pENK(-)/(-)) mouse, which is devoid of naloxone CPA in the morphine-naive state. Naloxone, but not the neutral antagonist, 6-beta-naloxol, produced CPA and physical withdrawal signs in pENK(-)/(-) mice when administered 2 h, but not 20 h, after morphine administration. Naloxone-precipitated physical withdrawal signs were attenuated in the pENK(-)/(-) mice relative to wild-type (WT) animals. In both WT and pENK(-)/(-) mice, naloxone-precipitated withdrawal jumping was greatest when naloxone was administered 2 h after morphine treatment and diminished at 3 h, in agreement with previous estimates of the time course for morphine-induced MOR constitutive activity in vitro. However, naloxone regained an ability to precipitate physical withdrawal in the WT, but not the pENK(-)/(-) mice when administered 4.5 h after morphine administration. Taken together, the data suggest that a compensatory increase in enkephalin release during spontaneous morphine withdrawal promotes a second period of MOR constitutive activity in WT mice that is responsible for the enhanced naloxone aversion observed in such animals even when naloxone is administered 20 h after morphine. The endogenous enkephalin system and MOR constitutive activity may therefore play vital roles in hedonic homeostatic dysregulation following chronic opiate administration.

Phosphodiesterase 4 inhibitors, rolipram and diazepam block the adaptive changes observed during morphine withdrawal in the heart

In this study, we investigated whether morphine dependence was inhibited by phosphodiesterase (PDE) 4 inhibitors rolipram and diazepam, since a role for the cyclic AMP systems in the development of morphine dependence was reported. Dependence of morphine was induced by a 7-day s.c. implantation of morphine pellets. Morphine withdrawal was precipitated on day 8 by an injection of naloxone. In order to determine the effect of rolipram or diazepam the animals were injected with these drugs for seven days and 30 min before the administration of naloxone. When opioid withdrawal was precipitated, enhancement of noradrenaline (NA) turnover in the heart was observed 30 min after naloxone administration. Moreover, morphine withdrawal induces Fos expression, increase in cyclic AMP and cyclic GMP levels. Co-administration of rolipram or diazepam with morphine during the pre-treatment period significantly reduces the signs of withdrawal symptoms, the enhancement of NA turnover, the increase in cyclic AMP and the Fos expression. However, these inhibitors did not modify the levels of cyclic GMP. These findings demonstrated that co-administration of rolipram or diazepam with morphine abolish the development of morphine dependence and suggest that these compounds prevent the up-regulation of the cyclic AMP pathway and the associated increase in cyclic AMP level after naloxone administration.

Constitutively active micro opioid receptors mediate the enhanced conditioned aversive effect of naloxone in morphine-dependent mice

Naloxone administration produces a robust conditioned place aversion (CPA) in opiate-naive rodents by blocking the action of enkephalins at mu opioid receptors. This aversive response is potentiated by prior exposure to morphine. In vitro studies indicate that morphine treatment may promote constitutive activity of mu opioid receptors. We hypothesized that such enhanced constitutive activity in vivo may underlie the increased aversive property of naloxone by uncovering the inverse agonist property of this drug. The CPA produced by naloxone was compared with that produced by the neutral antagonists 6-alpha- and 6-beta-naloxol in mice with and without prior morphine exposure. While all three drugs produced CPA, only naloxone CPA was enhanced by morphine given 20 h prior to each naloxone injection. Furthermore, only naloxone produced withdrawal jumping when given 20 h after morphine, even though 6-alpha-naloxol was able to produce jumping when given 4 h after morphine. These data suggest that morphine may enhance naloxone CPA by increasing levels of constitutively active mu receptors and further support the role of such constitutive activity in mediating naloxone-precipitated physical withdrawal. Such long-term changes in constitutive activity of the mu receptor induced by exogenous opiate exposure may thus be an important factor in hedonic homeostatic dysregulation proposed to underlie the addictive process.

Opioid antagonists differ according to negative intrinsic efficacy in a mouse model of acute dependence

The purpose of the present study is to compare the capacity of opioid antagonists to elicit withdrawal jumping in mice following two acute pretreatment doses of the opioid agonist morphine. Antagonists that precipitate vigorous withdrawal jumping across both morphine treatment doses are hypothesized to be strong inverse agonists at the mu-opioid receptor, whereas antagonists that elicit withdrawal jumping in mice treated with the high but not the low dose of morphine are hypothesized to be weak inverse agonists. Male, Swiss-Webster mice (15-30 g) were acutely treated with 56 or 180 mg kg(-1) morphine 4 h prior to injection with naloxone, naltrexone, diprenorphine, nalorphine, or naloxonazine. Vertical jumping, paw tremors, and weight loss were recorded. Naloxone, naltrexone, and diprenorphine produced withdrawal jumping after 56 and 180 mg kg(-1)morphine pretreatment. Nalorphine and naloxonazine produced moderate withdrawal jumping after 180 mg kg(-1) morphine pretreatment, but failed to elicit significant withdrawal jumping after 56 mg kg(-1) morphine pretreatment. Nalorphine and naloxonazine blocked the withdrawal jumping produced by naloxone. All antagonists produced paw tremors and weight loss although these effects were generally not dose-dependent. Taken together, these findings reveal a rank order of negative intrinsic efficacy for these opioid antagonists as follows: naloxone=naltrexone> or =diprenorphine>nalorphine=naloxonazine. Furthermore, the observation that nalorphine and naloxonazine blocked the naloxone-induced withdrawal jumping provides additional evidence that nalorphine and naloxonazine are weaker inverse agonists than naloxone.

ERK1/2 activation in rat ventral tegmental area by the mu-opioid agonist fentanyl: an in vitro study

Opioid receptors in the ventral tegmental area, predominantly the mu-opioid receptors, have been suggested to modulate reinforcement sensitivity for both opioid and non-opioid drugs of abuse. The present study was conducted to study signal transduction proteins, which may mediate the functioning of mu-opioid receptors in the neurons of the ventral tegmental area. Therefore, brain slices of the ventral tegmental area were exposed in vitro to the specific mu-opioid agonist fentanyl and immunohistochemically stained for four different activated proteins using phospho-specific antibodies. Fentanyl dose-dependently activated extracellular signal-regulated protein in brain slices of the ventral tegmental area. This activation was reversible with naloxone. Furthermore, naloxone itself also activated extracellular signal-regulated protein kinase. Under the present conditions fentanyl did not affect extracellular signal-regulated protein kinase 1 and 2, Stat and cyclic AMP-response element-binding protein activity. The direct activation of extracellular signal-regulated protein kinase in ventral tegmental area slices by the mu-opioid agonist fentanyl may suggest a role of extracellular signal-regulated protein kinase in reward processes.

Single nucleotide polymorphisms in the human mu opioid receptor gene alter basal G protein coupling and calmodulin binding

The mu opioid receptor (MOR) plays a central role in mediating acute and chronic effects of narcotic drugs. Three rare single nucleotide polymorphisms in the hMOR gene have been identified that cause amino acid substitutions in the third intracellular (i3) loop of MOR (R260H, R265H, and S268P). Genotyping 252 individuals of the Coriell collection identified one allele encoding the R265H-MOR variant and a new variant encoding D274N-MOR. Variants R260H-, R265H-, and S268P-MOR were constructed and transfected into HEK293 cells. Morphine stimulated G protein coupling of the three receptor variants to a maximal level approaching that of wild type MOR. In contrast, spontaneous, agonist-independent (basal) MOR signaling, proposed to play a role in opioid tolerance and dependence, was significantly reduced for R260H- and R265H-MOR. Moreover, domains within the i3 loop of MOR have been shown to interact with both G proteins and calmodulin (CaM). CaM binding was deficient for variants R265H- and S268P-MOR, suggesting that domains for G protein coupling and CaM binding overlap partially. Morphine pretreatment significantly enhanced basal G protein coupling of wild type MOR, which is thought to result from release of CaM. In contrast basal G protein coupling activity of the three variants was unaffected by morphine pretreatment consistent with diminished CaM regulation, low basal activity, or both. In conclusion, each of the three single nucleotide polymorphisms mapping to the i3 loop of MOR caused substantial changes in basal G protein coupling, CaM binding, or both. Carriers of the mutant alleles might display altered responses to narcotic analgesics.

A selective phosphodiesterase IV inhibitor, rolipram blocks both withdrawal behavioral manifestations, and c-Fos protein expression in morphine dependent mice

We investigated the effect of rolipram, a selective phosphodiesterase IV inhibitor, on morphine dependence in mice. The withdrawal manifestations were significantly reduced in mice that were treated with rolipram in combination with morphine repeatedly, compared to the mice treated with morphine and saline. Immunohistochemical study of c-Fos protein revealed a significant increase in the protein expression, 1 h after naloxone induced withdrawal manifestations. A combination of rolipram and morphine treatment for 5 days prevented the increase of c-Fos protein expression. Acute rolipram treatment prior to the naloxone challenge had no effect. Repeated treatment with rolipram itself had no effect either on behavior, or on c-Fos protein expression. These results suggest that chronic rolipram treatment in combination with morphine in mice will abolish the development of morphine dependence and the expression of c-Fos protein induced by naloxone challenge.

Endogenous opiates: 1999

This paper is the twenty-second installment of the annual review of research concerning the opiate system. It summarizes papers published during 1999 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunologic responses.

Pharmacological examination of contractile responses of the guinea-pig isolated ileum produced by mu-opioid receptor antagonists in the presence of, and following exposure to, morphine

We have assessed the potential of several mu-opioid receptor antagonists to elicit a response in the guinea-pig isolated ileum in the presence of, and following overnight exposure to, morphine. Naloxone, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP), (-)-5, 9alpha-diethyl-2-(3-furyl-methyl)-2'-hydroxy-6,7-benzomorphan (MR2266), but not D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP), produced a transient inhibition of electrically-evoked contractions of the guinea-pig ileum. The effect of 1 microM CTOP, but not that to MR2266, was inhibited by 1 microM somatostatin. Naloxone (0.3 microM), CTOP (3 microM), CTAP (3 microM) and MR2266 (0.3 microM) antagonized the inhibitory effect of morphine on electrically-evoked contractions of the guinea-pig to a similar degree and, following 60 min exposure to morphine, produced non-sustained contractions. The response to 3 microM CTOP was significantly smaller than that to 3 microM CTAP. None of the antagonists produced a response in the absence of morphine. Following overnight exposure of the ileum to 0.3 microM morphine (4 degrees C), and repeated washing to remove the agonist, all four antagonists elicited non-sustained contractions. However, the responses to 3 microM CTOP and 0.3 microM MR2266 were significantly smaller than those elicited by 0.3 microM naloxone and 3 microM CTAP. Somatostatin (1 microM) significantly reduced naloxone-induced contractions, but not those to CTAP. While all four mu-opioid antagonists elicited contractions in the presence of, and following prolonged exposure to, morphine, differences between them were noted which may be a consequence of non-opioid actions.

Inverse agonism at heptahelical receptors: concept, experimental approach and therapeutic potential

Inverse agonists (negative antagonists) are ligands that stabilize the inactive conformation (R) of receptors according to the two-state receptor model. The active conformation (R*) of heptahelical receptors, i.e. G protein-coupled receptors, has high affinity for G proteins. According to ternary complex models of receptor activation, the R*G complex is in equilibrium with R + G, with spontaneous activity in the absence of agonist. Inverse agonists, having a higher affinity for R, shift R*G towards R + G, decreasing the spontaneous activity of receptors. Agonists have the opposite effect, with a higher affinity for R*. Neutral antagonists have the same affinity for R and R* and compete for both agonists and inverse agonists. Inverse agonists have been recently proposed for a variety of heptahelical receptors. Methods to detect inverse agonists among antagonists are based on the determination of ligand affinity at R and R* with binding experiments, and on the modulation of G protein activity (GTP binding and hydrolysis) or of effector activity. Receptor inverse agonists, but also G protein antagonists and GTPase inhibitors, decrease spontaneous G protein activity corresponding to R*G. Receptor agonists, G protein agonists and GTPase inhibitors increase effector basal activity, but receptor inverse agonists decrease it. The therapeutic potential of inverse agonists is proposed in human diseases ascribed to constitutively active mutant receptors and may be extended to diseases related to wild-type receptor over-expression leading to the increase of R*. Some of the therapeutic effects of presently used receptor antagonists may be related to their inverse agonist properties. Inverse agonists lead to receptor upregulation, offering new approaches to tolerance and dependence to drugs.

Tolerance to morphine at the mu-opioid receptor differentially induced by cAMP-dependent protein kinase activation and morphine

Human neuroblastoma SH-SY5Y cells express endogenous mu-opioid receptor and develop cellular tolerance to morphine after prolonged (>/=4 h) treatment with morphine. Treatment with forskolin (25 microM, 12 h), an adenylyl cyclase activator, also desensitized mu-opioid receptor response to morphine (10 microM) by 38% (P<0. 001), which was reversed by the cyclic AMP (cAMP) dependent kinase inhibitor N-(2-aminoethyl)-5-isoquinolinesulfonamide (H8) (100 microM). Treatment with both morphine and forskolin appeared to cause an additive effect in desensitizing mu-opioid receptor. In mu-opioid receptor stably transfected human embryonic kidney 293 (HEK-mu) cells, morphine treatment produced cAMP upregulation, yet failed to induce mu-opioid receptor tolerance. However, treatment with forskolin (25 microM) or 8-bromo-cAMP (1mM) led to profound mu-opioid receptor tolerance, which was reversed by H8. These results demonstrate that cAMP-dependent kinase activation causes mu-opioid receptor tolerance. However, morphine-induced mu-opioid receptor tolerance in SH-SY5Y cells is not mediated by cAMP-dependent kinase activation. In addition, our results indicate that cAMP-upregulation does not necessarily lead to mu-opioid receptor tolerance.