The dopamine D3 receptor (D3R), in the nucleus accumbens (NAc), plays an important role in alcohol reward mechanisms. The major neuronal type within the NAc is the GABAergic medium spiny neuron (MSN), whose activity is regulated by dopaminergic inputs. We previously reported that genetic deletion or pharmacological blockade of D3R increases GABAA α6 subunit in the ventral striatum. Here we tested the hypothesis that D3R-dependent changes in GABAA α6 subunit in the NAc affect voluntary alcohol intake, by influencing the inhibitory transmission of MSNs. We performed in vivo and ex vivo experiments in D3 knockout (D3R-/-) mice and wild type littermates (D3R +/+). Ro 15-4513, a high affinity α6-GABAA ligand was used to study α6 activity. At baseline, NAc α6 expression was negligible in D3R+/+, whereas it was robust in D3R−/−; other relevant GABAA subunits were not changed. In situ hybridization and qPCR confirmed α6 subunit mRNA expression especially in the NAc. In the drinking-in-the-dark paradigm, systemic administration of Ro 15-4513 inhibited alcohol intake in D3R+/+, but increased it in D3R−/−; this was confirmed by intra-NAc administration of Ro 15-4513 and furosemide, a selective α6-GABAA antagonist. Whole-cell patch-clamp showed peak amplitudes of miniature inhibitory postsynaptic currents in NAc medium spiny neurons higher in D3R-/-compared to D3R+/+; Ro 15-4513 reduced the peak amplitude in the NAc of D3R-/-, but not in D3R+/ +. We conclude that D3R-dependent enhanced expression of α6 GABAA subunit inhibits voluntary alcohol intake by increasing GABA inhibition in the NAc.

Gene-targeted mice with deficient AMPA receptor GluA1 subunits ( Gria1-/- mice) show robust hyperlocomotion in a novel environment, suggesting them to constitute a model for hyperactivity disorders such as mania, schizophrenia and attention deficit hyperactivity disorder. This behavioral alteration has been associated with increased neuronal activation in the hippocampus, and it can be attenuated by chronic treatment with antimanic drugs, such as lithium, valproic acid, and lamotrigine. Now we found that systemic cannabidiol strongly blunted the hyperactivity and the hippocampal c-Fos expression of the Gria1-/- mice, while not affecting the wild-type littermate controls. Acute bilateral intra-dorsal hippocampal infusion of cannabidiol partially blocked the hyperactivity of the Gria1-/- mice, but had no effect on wild-types. The activation of the inhibitory DREADD receptor hM4Gi in the dorsal hippocampus by clozapine- N -oxide robustly inhibited the hyperactivity of the Gria1-/- mice, but had no effect on the locomotion of wild-type mice. Our results show that enhanced neuronal excitability in the hippocampus is associated with pronounced novelty-induced hyperactivity of GluA1 subunit-deficient mice. When this enhanced response of hippocampal neurons to novel stimuli is specifically reduced in the hippocampus by pharmacological treatment or by chemogenetic inhibition, Gria1-/- mice recover from behavioral hyperactivity, suggesting a hippocampal dysfunction in hyperactive behaviors that can be treated with cannabidiol.

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