Regional Influence of Cannabinoid CB1 Receptors in the Regulation of Ethanol Self-Administration by Wistar Rats

Abstract

Substantial evidence suggests a facilitatory influence of cannabinoid CB₁ receptors in the modulation of ethanol consumption by rodents. Studies performed in rats selectively bred for high alcohol preference point to an involvement of CB₁ receptors in the nucleus accumbens (NAC), ventral tegmental area (VTA) and medial prefrontal cortex (mPFC) in the modulation ethanol self-administration. However, the neural mechanisms through which CB₁ receptors regulate ethanol intake in out-bred Wistar rats have not been investigated. The present study evaluated alterations in ethanol self-administration induced by localized infusions of the CB₁ receptor antagonist SR141716A (0, 1 and 3 μg/side) into the NAC, anterior and posterior VTA and mPFC. Separate groups of Wistar rats were trained to operantly respond for an oral ethanol solution and prepared with bilateral injection cannulae aimed at each brain region. Results revealed significant decreases in ethanol intake following intra-NAC SR141716A administration, consistent with our prior observation of ethanol-induced increases extracellular 2-arachidonoyl glycerol (2-AG) in this brain region. We also observed a significant dose-dependent reduction in ethanol intake following SR141716A administration into the posterior, but not anterior VTA, consistent with evidence of a specific involvement of the posterior VTA in the regulation of ethanol intake. Ethanol consumption was unaltered following intra-mPFC SR141716A administration and ethanol self-administration did not induce robust changes in anandamide or 2-AG levels in mPFC microdialysates. These findings implicate an involvement of CB₁ receptors in the NAC and posterior VTA, but not anterior VTA and mPFC in the regulation of ethanol self-administration behavior by outbred Wistar rats.

Fig. 1

Effect of intra-NAC microinfusion of the CB₁ receptor antagonist SR141716A on EtOH self-administration by Wistar rats. Bilateral SR141716A (1 and 3 μg/side) infusions into the NAC significantly reduced EtOH self-administration as compared with EtOH intake following vehicle infusion (panel A; n = 12). Data are presented as the mean of reinforcers ± SEM. Asterisks denote significant differences, *p < 0.05. Placements of the microinjector tips in the NAC are depicted in panel B (coronal brain slice 3.00 mm anterior to Bregma [39]).

Fig. 2

Effect of intra-VTA microinfusion of the CB₁ receptor antagonist SR141716A on EtOH self-administration by Wistar rats. Infusions of SR141716A (1 & 3 μg/side) into the posterior-VTA dose dependently reduced EtOH self-administration (Panel A; n = 8) while infusion of these same antagonist doses into the anterior VTA did not alter EtOH intake (Panel C; n = 7). Data are presented as the mean of reinforcers ± SEM. Asterisks denote significant differences, *p < 0.05. Localizations of the drug injector tips into the posterior (−5.6 to −5.8 mm from Bregma) and anterior (−4.8 to 5.2 mm from Bregma) VTA are shown in Panels B and D, respectively [39].

Fig. 3

Effect of intra-mPFC microinfusion of the CB₁ receptor antagonist SR141716A on EtOH self-administration by Wistar rats. Neither 1 nor 3 μg/side infusion of SR141716A into the mPFC altered EtOH self-administration (Panel A; n = 8). Data are presented as the mean of reinforcers ± SEM. Panel B depicts coronal slices (3.00 and 2.76 mm anterior to bregma, [39]) showing the location of each microinjector tip within the mPFC.

Fig. 4

Effect of EtOH self-administration on EC levels in mPFC microdialysates. EtOH self-administration induced subtle, non-significant alterations in microdialysate EC levels within the mPFC (Panel A; n = 8). Although modest changes in dialysate AEA and 2-AG content were observed during the 90-minute post-session sampling period, there was no significant change in dialysate levels as compared with pre-EtOH baseline. Dialysate AEA and 2-AG levels are expressed as the percentage of change from baseline ± SEM. The cumulative EtOH intake (g/kg; mean ± SEM) during the self-administration session is shown in Panel B. Schematic representation of the active dialysis membrane placements within the mPFC is shown in Panel C (3.00 and 2.76 mm anterior to bregma, [39]).