Ethanol reversal of cellular tolerance to morphine in rat locus coeruleus neurons

Consumption of ethanol is a considerable risk factor for death in heroin overdose. We sought to determine whether a mildly intoxicating concentration of ethanol could alter morphine tolerance at the cellular level. In rat locus coeruleus (LC) neurons, tolerance to morphine was reversed by acute exposure of the brain slice to ethanol (20 mM). Tolerance to the opioid peptide [d-Ala(2),N-MePhe(4),Gly-ol]-enkephalin was not reversed by ethanol. Previous studies in LC neurons have revealed a role for protein kinase C (PKC)α in μ-opioid receptor (MOPr) desensitization by morphine and in the induction and maintenance of morphine tolerance, but we have been unable to demonstrate that 20 mM ethanol produces significant inhibition of PKCα. The ability of ethanol to reverse cellular tolerance to morphine in LC neurons was absent in the presence of the phosphatase inhibitor okadaic acid, indicating that dephosphorylation is involved. In human embryonic kidney 293 cells expressing the MOPr, ethanol reduced the level of MOPr phosphorylation induced by morphine. Ethanol reversal of tolerance did not appear to result from a direct effect on MOPr since acute exposure to ethanol (20 mM) did not modify the affinity of binding of morphine to the MOPr or the efficacy of morphine for G-protein activation as measured by guanosine 5'-O-(3-[(35)S]thio)triphosphate binding. Similarly, ethanol did not affect MOPr trafficking. We conclude that acute exposure to ethanol enhances the effects of morphine by reversing the processes underlying morphine cellular tolerance.

Manipulation of fatty acid amide hydrolase functional activity alters sensitivity and dependence to ethanol

The aim of this study was to examine the role of fatty acid amide hydrolase (FAAH) on ethanol sensitivity, preference, and dependence. The deletion of FAAH gene or the inhibition of FAAH by carbamoyl-biphenyl-3-yl-cyclohexylcarbamate (URB597) (0.1 mg/kg) markedly increased the preference for ethanol. The study further reveals that URB597 specifically acts through FAAH and that cannabinoid-1 (CB(1)) receptor is critical for N-arachidonoyl ethanolamide (AEA) mediated ethanol-reinforced behavior as revealed by lack of URB597 effect in both FAAH and CB(1)-/- mice compared with vehicle-treated -/- mice. The FAAH -/- mice displayed a lower sensitivity to hypothermic and sedative effects to acute ethanol challenge. The FAAH -/- mice also exhibited a reduction in the severity of handling-induced convulsions following withdrawal from chronic ethanol exposure. The CB(1) receptor and proenkephalin gene expressions, and CB(1) receptor and mu-opioid (MO) receptor-mediated G-protein activation were found to be significantly lower in the caudate-putamen, nucleus accumbens core and shell of FAAH -/- than +/+ mice. Interestingly, the MO receptor-stimulated G-protein signaling was greater in the striatum of FAAH -/- than +/+ mice following voluntary ethanol consumption. These findings suggest that an elevation in the AEA content and its action on the limbic CB(1) receptor and MO receptor might contribute to ethanol-reinforced behavior. Treatment with drugs that decrease AEA tone might prove useful in reducing excessive ethanol consumption.

Comparison of peptidic and nonpeptidic delta-opioid agonists on guanosine 5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS) binding in brain slices from Sprague-Dawley rats

Previous studies have demonstrated that peptidic and nonpeptidic delta-opioid receptor agonists have different effects depending on the measure. For example, nonpeptidic delta-opioid agonists, but not peptidic agonists, produce convulsions in rats, and in vitro studies suggested that peptidic and nonpeptidic delta-opioid agonists might have differential mechanisms of receptor downregulation. The present study evaluated potential differences between peptidic and nonpeptidic delta-opioid agonists in their ability to activate G proteins using guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) autoradiography experiments in rat brain slices. The peptidic agonist [d-Pen(2),d-Pen(5)]-enkephalin and the nonpeptidic agonist (+)BW373U86 [(+)-4-[alpha(R)-alpha-[(2S,5R)-2,5-dimethyl-4-(2-propenyl)-1-piperazinyl]-(3-hydroxyphenyl)methyl]-N,N-diethylbenzamide] demonstrated concentration-dependent increases in [(35)S]GTPgammaS binding that were attenuated by the delta-opioid antagonist naltrindole. (+)BW373U86 was more potent and efficacious than the peptidic agonist, and this difference remained consistent across brain regions where significant stimulation was observed. In addition, multiple delta-opioid compounds were evaluated for their agonist activity in this assay. These data suggested that differences between peptidic and nonpeptidic delta-opioid agonists in behavioral studies were most likely caused by differences in agonist efficacy. Finally, these data also revealed that [(35)S]GTPgammaS autoradiography could be used to compare efficacy differences among agonists across various brain regions in rat brain slices.

INTERACTIONS BETWEEN CANNABINOID AND OPIOID RECEPTOR SYSTEMS IN THE MEDIATION OF ETHANOL EFFECTS

Over the past few years, advances in the investigation of the neurochemical circuits involved in the development and treatment of alcohol dependence have identified peptides and receptors as potential key targets in the treatment of problems related to alcohol consumption. The endogenous opioid system is modified by alcohol intake in areas of the brain related to reward systems, and differential basal levels of opioid gene expression are found in rodents with a high preference for ethanol. This suggests a greater vulnerability to alcohol consumption in relation to differences in genetic background. Further evidence of the involvement of opioid peptides in alcohol dependence is the ability of the opioid antagonist naltrexone to reduce alcohol intake in animal models of dependence and in alcohol-dependent patients. Abundant evidence indicates that the activation of cannabinoid receptors stimulates the release of opioid peptides, therefore the cannabinoid receptor antagonists may presumably alter opioid peptide release, thus facilitating the reduction of ethanol consumption. However, little is known about the effects of ethanol on the endogenous cannabinoid system, the vulnerability of cannabinoid receptors to alcohol intake or their neurochemical implications in reducing consumption of alcohol. In this paper, we review the role of opioid and cannabinoid receptor systems, their vulnerability to alcohol intake and the development of dependence, and the targeting of these systems in the treatment of alcoholism.