α4-containing GABAA receptors on DRD2-neurons of the nucleus accumbens mediate instrumental responding for conditioned reinforcers, and its potentiation by cocaine

Extrasynaptic GABAA receptors (GABAARs) composed of α4, β, and δ subunits mediate GABAergic tonic inhibition and are potential molecular targets in the modulation of behavioural responses to natural and drug rewards. These GABAARs are highly expressed within the nucleus accumbens (NAc) where they influence the excitability of the medium spiny neurons. Here we explore their role in modulating behavioural responses to food-conditioned cues and the behaviour-potentiating effects of cocaine. α4-subunit constitutive knockout mice (α4-/-) showed higher rates of instrumental responding for reward-paired stimuli in a test of conditioned reinforcement (CRf). A similar effect was seen following viral knockdown of GABAAR α4 subunits within the NAc. Local infusion of the α4βδ -GABAAR-preferring agonist, THIP, into the NAc had no effect on responding when given alone, but reduced cocaine potentiation of responding for conditioned reinforcers in wildtype but not α4-/- mice. Finally, specific deletion of α4-subunits from dopamine D2, but not D1, receptor-expressing neurons (DRD2- and DRD1-neurons), mimicked the phenotype of the constitutive knockout, potentiating CRf responding and blocking intra-accumbal THIP attenuation of cocaine-potentiated CRf responding. These data demonstrate that α4-GABAAR mediated inhibition of DRD2-neurons reduces instrumental-responding for a conditioned reinforcer, and its potentiation by cocaine, and emphasise the importance of GABAergic signalling within the NAc in mediating cocaine's effects.Significance StatementThis manuscript combines genetic and pharmacological interventions to uncover a critical role for α4-containing GABAA receptors in the nucleus accumbens in instrumental responding for conditioned reinforcers and its potentiation by cocaine, behavioural phenomenon thought to contribute to reward-seeking behaviour. These findings represent an important advancement in our understanding of the neural mechanisms underlying the reinforcing effects of conditioned stimuli and the role of the GABAergic system in this process

Early-life adversity selectively impairs a2-GABAA receptor expression in the mouse nucleus accumbens and influences the behavioral effects of cocaine

Haplotypes of the Gabra2 gene encoding the a2-subunit of the GABAA receptor (GABAAR) are associated with drug abuse, suggesting that a2-GABAARs may play an important role in the circuitry underlying drug misuse. The genetic association of Gabra2 haplotypes with cocaine addiction appears to be evident primarily in individuals who had experienced childhood trauma. Given this association of childhood trauma, cocaine abuse and the Gabra2 haplotypes, we have explored in a mouse model of early life adversity (ELA) whether such events influence the behavioral effects of cocaine and if, as suggested by the human studies, a2-GABAARs in the nucleus accumbens (NAc) are involved in these perturbed behaviors. In adult mice prior ELA caused a selective decrease of accumbal a2-subunit mRNA, resulting in a selective decrease in the number and size of the a2-subunit (but not the a1-subunit) immunoreactive clusters in NAc core medium spiny neurons (MSNs). Functionally, in adult MSNs ELA decreased the amplitude and frequency of GABAAR-mediated miniature inhibitory postsynaptic currents (mIPSCs), a profile similar to that of a2 “knock-out” (a2-/-) mice. Behaviourally, adult male ELA and a2-/- mice exhibited an enhanced locomotor response to acute cocaine and blunted sensitisation upon repeated cocaine administration, when compared to their appropriate controls. Collectively, these findings reveal a neurobiological mechanism which may relate to the clinical observation that early trauma increases the risk for substance abuse disorder (SAD) in individuals harbouring haplotypic variations in the Gabra2 gene

Cocaine effects on mouse incentive-learning and human addiction are linked to a2 subunit-containing GABAA receptors

Because GABAA receptors containing a2 subunits are highly represented in areas of the brain, such as nucleus accumbens (NAcc), frontal cortex, and amygdala, regions intimately involved in signaling motivation and reward, we hypothesized that manipulations of this receptor subtype would influence processing of rewards. Voltage-clamp recordings from NAcc medium spiny neurons of mice with a2 gene deletion showed reduced synaptic GABAA receptor-mediated responses. Behaviorally, the deletion abolished cocaine’s ability to potentiate behaviors conditioned to rewards (conditioned reinforcement), and to support behavioral sensitization. In mice with a point mutation in the benzodiazepine binding pocket of a2-GABAA receptors (a2H101R), GABAergic neurotransmission in medium spiny neurons was identical to that of WT (i.e., the mutation was silent), but importantly, receptor function was now facilitated by the atypical benzodiazepine Ro 15-4513 (ethyl 8-amido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo [1,5-a] [1,4] benzodiazepine-3-carboxylate). In a2H101R, but not WT mice, Ro 15-4513 administered directly into the NAcc-stimulated locomotor activity, and when given systemically and repeatedly, induced behavioral sensitization. These data indicate that activation of a2-GABAA receptors (most likely in NAcc) is both necessary and sufficient for behavioral sensitization. Consistent with a role of these receptors in addiction, we found specific markers and haplotypes of the GABRA2 gene to be associated with human cocaine addiction