Inhibition of the Ventral Hippocampus Increases Alcohol Drinking

The function of the ventral hippocampus (vHC) supports many behaviors, including those related to reward seeking behaviors and drug addiction. We used a mouse model of alcohol dependence and relapse to investigate the role of the vHC in alcohol (ethanol) drinking. One experiment used a chemogenetic approach to inhibit vHC function while ethanol dependent and non-dependent mice had access to ethanol. Interestingly, the non-dependent mice expressing an inhibitory DREADD in the VHC showed a significant increase in ethanol drinking ([~]30%) after hM4Di DREADD activation with clozapine-n-oxide (CNO; 3 mg/kg, ip.) compared to vehicle. On the other hand, ethanol dependent mice, which were already drinking significantly more ethanol than non-dependent mice, only had a slight, non-significant increase in drinking after CNO challenge. In a separate group of dependent and non-dependent mice, GCaMP6f calcium-dependent activity was recorded in the vHC while mice were actively drinking ethanol. These data showed that once mice were rendered ethanol dependent and were drinking more ethanol than the non-dependent mice, calcium signaling in the ventral hippocampus decreased ([~]45%) in the ethanol dependent mice compared to the non-dependent mice. Together, these findings suggest that ethanol dependence reduces activity of the ventral hippocampus and that reduced function of this brain region contributes to increased ethanol drinking.