Transient changes in the endocannabinoid system after acute and chronic ethanol exposure and abstinence in the rat: a combined PET and microdialysis study

Abstract

Purpose Recent biochemical and post-mortem evidence suggests involvement of the endocannabinoid system in alcohol drinking behaviour and dependence. Using [ 18 F]MK-9470 small-animal PET imaging, our primary objective was to evaluate in vivo type 1 cannabinoid receptor (CB1R) binding changes in rats subjected to several ethanol conditions: (1) at baseline, (2) after acute intraperitoneal administration of ethanol (4 g/kg) or saline, (3) after 7 days of forced chronic ethanol consumption, and (4) after abstinence for 7 and 14 days. Secondly, levels of anandamide (AEA) in the nucleus accumbens (NAcc) were investigated in the same animals using in vivo microdialysis and correlated with the changes in CB1R binding. Methods In total, 28 male Wistar rats were investigated. Small-animal PET was done on a FOCUS-220 tomograph with [18 F]MK-9470. Parametric images of [18 F]MK-9470 binding based on standard uptake values (SUV, as a measure of CB1R binding) were generated. Images were normalized to Paxinos space and analysed voxel-wise using SPM8 (p height =0.005; k ext =200). The AEA content was quantified using HPLC with tandem mass spectrometry detection. Results Acute ethanol administration increased relative CB1R binding in the NAcc that was positively correlated with the change in AEA levels of that region. In contrast, compared to rats at baseline, AEA levels in the NAcc were not significantly different in rats after chronic ethanol consumption or after a 14-day abstinence period. Chronic ethanol consumption decreased relative CB1R binding in the hippocampus and caudate-putamen, whereas same regions showed increased relative CB1R binding after 7 and 14 days of abstinence compared to the baseline condition. After 7 and 14 days of abstinence, relative CB1R binding additionally decreased in the orbitofrontal cortex. The magnitude of the hippocampal and frontal changes was highly correlated with daily ethanol intake. Conclusion This study provides in vivo evidence that acute ethanol consumption is associated with enhanced endocannabinoid signalling in the NAcc, indicated by an increased CB1R binding and AEA content. In addition, chronic ethanol exposure leads to regional dysfunctions in CB1R levels, involving the hippocampus and caudateputamen that are reversible within 2 weeks in this animal model. J.C. and C.C. contributed equally to this work.

Endocannabinoid signalling in reward and addiction

“…In contrast, extracellular AEA levels in the NAc are unaltered by alcohol self-administration and decreased by non-contingent alcohol administration. Alcohol also appears to induce region-specific changes in brain tissue EC levels, with alcohol-induced disruptions consistently observed in striatal regions 30, 45, 46 but not frontal cortical areas 28 consistent with evidence that alcohol consumption is reduced by CB 1 R antagonism in the VTA and NAc, but not PFC 28, 30, 47 .…”

Blockade of alcohol escalation and “relapse” drinking by pharmacological FAAH inhibition in male and female C57BL/6J mice

“…As a possible allostatic process, the AEA surges may be resultant from the absence of an AEA release during 1-day acute withdrawal. Indeed, a recent study has found that the AEA release in the rat nucleus accumbens is observed only after acute alcohol, but not chronic alcohol or chronic withdrawal [Ceccarini et al 2013], though other reports showed that acute alcohol decreases [Ferrer et al 2007] or did not change AEA release in this region [Alvarez-Jaimes et al 2009]. AEA and other NAE elevations were detected at 1-day withdrawal, whereas behavioral responses to the FAAH inhibitor URB597 (inhibition of alcohol drinking) were apparent at both 1-day and 1-week withdrawal time points.…”

Acute and chronic ethanol exposure differentially regulate CB1 receptor function at glutamatergic synapses in the rat basolateral amygdala

“…With these limitations in mind, a number of observations support the proposed indirect activation of CB1 by ethanol. First, acute ethanol can facilitate eCB levels both in vitro [34] and in vivo [22, 23]. Notably, acute ethanol also inhibits BLA glutamatergic projections to the NAcc and cortico-striatal excitatory transmission via a similarly proposed mechanism [49, 50].…”

“…Behavioral cross-tolerance between ethanol and CB1 agonists has been observed for decades [15–17] and the eCB system is thought to participate in innate ethanol preference in both rats [18, 19] and mice [20, 21]. Further, both acute and chronic ethanol exposure alter expression levels of the CB1 receptor as well as the endogenous ligands and their respective metabolic and catabolic enzymes [22–34]. Similar alterations have been shown in human alcoholics, indicating ethanol-induced changes in this system may be conserved across species [35, 36].…”