Dopamine D(3) receptor knock-out mice exhibit increased behavioral sensitivity to the anxiolytic drug diazepam

Dopamine D3 receptors (DRsD3) seem to have a pivotal role in mood disorders. Using the elevated plus maze (EPM) and the novelty-induced grooming test (NGT), we assessed the responses of DRD3-deficient (D3−/−) mice to the acute treatment (different testing time) with the anxiolytic drug, diazepam. D3−/− mice treated with diazepam (0.1 or 0.5 mg/kg) exhibited a better behavioral response in the EPM than their wild type (WT). Furthermore, in D3−/− mice, but not in WT, 1 mg/kg diazepam induced anxiolytic effects at all testing times. The contribution of DRsD3 in the anxiolytic effects of diazepam was confirmed by similar results obtained in EPM by using the selective DRD3 antagonist U99194A (10 mg/kg) in combination with diazepam, in WT animals. D3−/− mice treated with diazepam (all doses), also showed a decrease in grooming behavior. However, the [3H]flunitrazepam autoradiographic analysis revealed no significant changes in D3−/− mice compared to WT, suggesting that if γ-aminobutyric acid receptor GABAA changes are involved, they do not occur at the level of binding to benzodiazepine site. These data suggest that D3−/− mice exhibit low baseline anxiety levels and provide the evidence that the DRD3 is involved in the modulation of benzodiazepine anxiolytic effects.