Low μ-Opioid Receptor Status in Alcohol Dependence Identified by Combined Positron Emission Tomography and Post-Mortem Brain Analysis

Abstract

Blockade of the μ-opioid receptor (MOR) by naltrexone reduces relapse risk in a subpopulation of alcohol-dependent patients. Previous positron-emission-tomography (PET) studies using the MOR ligand [¹¹C]carfentanil have found increased MOR availability in abstinent alcoholics, which may reflect either increased MOR expression or lower endogenous ligand concentration. To differentiate between both effects, we investigated two cohorts of alcoholic subjects using either post-mortem or clinical PET analysis. Post-mortem brain tissue of alcohol-dependent subjects and controls (N=43/group) was quantitatively analyzed for MOR ([³H]DAMGO)-binding sites and OPRM1 mRNA in striatal regions. [¹¹C]carfentanil PET was performed in detoxified, medication free alcohol-dependent patients (N=38), followed by a randomized controlled study of naltrexone versus placebo and follow-up for 1 year (clinical trial number: NCT00317031). Because the functional OPRM1 variant rs1799971:A>G affects the ligand binding, allele carrier status was considered in the analyses. MOR-binding sites were reduced by 23-51% in post-mortem striatal tissue of alcoholics. In the PET study, a significant interaction of OPRM1 genotype, binding potential (BP_ND) for [¹¹C]carfentanil in the ventral striatum, and relapse risk was found. Particularly in G-allele carriers, lower striatal BP_ND was associated with a higher relapse risk. Interestingly, this effect was more pronounced in the naltrexone treatment group. Reduced MOR is interpreted as a neuroadaptation to an alcohol-induced release of endogenous ligands in patients with severe alcoholism. Low MOR availability may explain the ineffectiveness of naltrexone treatment in this subpopulation. Finally, low MOR-binding sites are proposed as a molecular marker for a negative disease course.

Figure 1

Post-mortem study: μ-opioid receptor (MOR)-binding sites and OPRM1 mRNA expression in the nucleus caudatus and VS. Controls (white bar) are compared with non-intoxicated (dashed bar) and intoxicated (black bar) alcoholic subjects. (a and b) Data show protein and mRNA expression of μ-opioid receptor on post-mortem brain sections by means of saturated [³H]DAMGO receptor autoradiography (a) and qRT-PCR (b). Data are expressed as mean±SEM (receptor autoradiography: fmol/mg±SEM, n=14–40/group, OPRM1 mRNA: mean ddCT±SEM, n=12–35/group). Statistical analysis for both binding sites and mRNA was performed by analysis of covariance followed by Fisher's LSD post hoc test, *p<0.05, **p<0.01, ***p<0.001 non-intoxicated/intoxicated alcoholics vs controls, ^#p<0.05 intoxicated alcoholics vs non-intoxicated alcoholics. (c) Schematic overview of the nucleus caudatus (NC) and ventral striatum (VS) is shown on a coronal human brain section. (d) Representative [³H]DAMGO receptor autoradiography on a frontal lobe section (upper panel) showing a specific pattern of MOR similar to Hiller and Fan (Hiller and Fan, 1996) and Mathieu-Kia et al. (2001). Non-specific signal was determined by [³H]DAMGO binding in presence of the MOR antagonist CTOP (lower panel).

Figure 2

Molecular model of alcohol-mediated dynamic alterations in the striatal opioid system during the addiction. Initial and repeated alcohol consumption results in the release of endogenous opioids by progressively enhancing the frequency and strength of opioidergic neurotransmission. This neurochemical event is accompanied by reward and hedonia. As an adaptive mechanism, MOR expression is reduced and a subsequent reduction in MOR-binding sites ensures maintained homeostasis. In alcohol withdrawal and early abstinence, alcohol-induced release of endogenous opioids is suddenly stopped, whereas MOR expression is strongly diminished. These molecular events may contribute to anhedonia in early abstinence. During protracted abstinence, the responsiveness of the opioidergic system is still diminished and must adapt to less intense natural rewards in comparison to alcohol in order to recover. [¹¹C]carfentanil PET assesses MOR availability, which depends on the absolute number of MOR-binding sites and endogenous opioid levels. Therefore, PET data can only be correctly be interpreted if additional data on either one of these measures is available. The binding potential (BP_ND, solid black arrows) is the specific to non-specific equilibrium partition coefficient that is determined by the concentration of the endogenous opioids and the absolute number of MOR-binding sites. Saturated [³H]DAMGO autoradiographies in post-mortem tissue are measures of number of available MOR-binding sites (BS), which are reduced in alcoholics at different times of abstinence in relation to controls.