Specific alterations of extracellular endocannabinoid levels in the nucleus accumbens by ethanol, heroin, and cocaine self-administration

Abstract

Ethanol and opiate self-administration are sensitive to manipulations of cannabinoid CB1 receptor function and, from this, a role for the endogenous cannabinoid system in the modulation of drug reward has been hypothesized. However, direct in vivo evidence of drug-induced alterations in brain endocannabinoid (eCB) formation has been lacking. To address this issue, we explored the effect of drug self-administration on interstitial eCB levels in the nucleus accumbens (NAc) shell using in vivo microdialysis. Ethanol, heroin, and cocaine were compared because the rewarding properties of ethanol and heroin are reduced by CB1 receptor inactivation, whereas cocaine reward is less sensitive to these manipulations. Ethanol self-administration significantly increased dialysate 2-arachidonoylglycerol (2-AG) levels with no concomitant change in dialysate anandamide (AEA) concentrations. Conversely, heroin self-administration significantly increased dialysate AEA levels, and induced a subtle but significant decrease in dialysate 2-AG levels. In each case, the relative change in dialysate eCB content was significantly correlated with the amount of drug consumed. In contrast, cocaine self-administration did not alter dialysate levels of either AEA or 2-AG. Local infusion of the CB1 antagonist SR 141716A into the NAc significantly reduced ethanol, but not cocaine, self-administration. Together with our previous observation that intra-NAc SR 141716A reduces heroin self-administration, these data provide novel in vivo support for an eCB involvement in the motivational properties of ethanol and heroin but not cocaine. Furthermore, the selective effects of ethanol and heroin on interstitial 2-AG and AEA provide new insight into the distinct neurochemical profiles produced by these two abused substances.

Figure 1.

Selective effects of ethanol, heroin, and cocaine self-administration on eCB levels in NAc shell microdialysates. Top, The effects of ethanol (EtOH; a), heroin (c), and cocaine (e) self-administration on dialysate AEA (open circles) and 2-AG (filled circles) levels expressed as the percentage of change from predrug baseline. Bottom, Cumulative drug intake during the self-administration sessions (b, ethanol 10% (w/v) oral intake; n = 9) (d, heroin 20 μg per infusion, i.v.; n = 7) (f, cocaine 0.25 mg per infusion, i.v.; n = 8). Error bars indicate SEM. *p < 0.05; relative to baseline.

Figure 2.

Correlation between total drug intake and the relative change in dialysate AEA (open circles) and 2-AG (filled circles) during self-administration. Relative change in eCB levels calculated as the total AUC of the percentage of baseline data shown in Figure 1a, c, and e. A, Relative increases in dialysate 2-AG were significantly correlated with ethanol intake (r² = 0.7683; p < 0.005) whereas changes in dialysate AEA were not (r² = 0.2499). B, Relative increases in dialysate AEA (r² = 0.7870; p < 0.01) and decreases in dialysate 2-AG (r² = 0.6317; p < 0.05) were significantly correlated with heroin intake. C, There were no correlations between changes in either AEA (r² = 0.0042) or 2-AG (r² = 0.1166) and cocaine intake.

Figure 3.

Schematic representation of the dialysis probe placements for each of the drug self-administration tests shown in Figure 1. Each section represents a coronal brain slice 1.6 mm anterior to bregma (Paxinos and Watson, 1998), and each vertical bar corresponds to the location of the active microdialysis membrane area within the medial NAc shell. Separate sections are used to display the probe placements in the ethanol (EtOH; n = 9), heroin (HER; n = 7), and cocaine (COC; n = 8) self-administration groups, respectively.

Figure 4.

Effect of intra-NAc infusion of the CB₁ receptor antagonist SR 141716A on ethanol and cocaine self-administration. A, Bilateral SR 141716A infusions into the NAc significantly reduced ethanol self-administration relative to vehicle infusions (F_(2,20) = 6.409; p < 0.01), and significant reductions were produced by both 1 and 3 μg per side SR 141716A (n = 11). These results are comparable to the previously reported reduction in heroin self-administration induced by intra-NAc SR 141716A (Caille and Parsons, 2006). Error bars indicate SEM. *p < 0.05. B, In contrast, intra-NAc administration of these same SR 141716A doses produced no effect on cocaine self-administration (F_(2,18) = 1.314; n = 10). Error bars indicate SEM. C, Placements of each NAc drug microinjector in the NAc. Each brain section represents a coronal slice 1.6 mm anterior to bregma (Paxinos and Watson, 1998), and each circle corresponds to the location of the microinjector tip within the NAc. Separate slices are shown for placements in the ethanol and cocaine (COC) self-administration experiments, respectively.