Projections from ventral hippocampus to nucleus accumbens' cholinergic neurons are altered in depression

The cholinergic interneurons (ChIs) of the nucleus accumbens (NAc) have a critical role in the activity of this region, specifically in the context of major depressive disorder. To understand the circuitry regulating this behavior, we sought to determine the areas that directly project to these interneurons by utilizing the monosynaptic cell-specific tracing technique. Mapping showed monosynaptic projections that are exclusive to NAc ChIs. To determine if some of these projections are altered in a depression mouse model, we used mice that do not express the calcium-binding protein p11 specifically in ChIs (ChAT-p11 cKO) and display a depressive-like phenotype. Our data demonstrated that while the overall projection areas remain similar between wild type and ChAT-p11 cKO mice, the number of projections from the ventral hippocampus (vHIP) is significantly reduced in the ChAT-p11 cKO mice. Furthermore, using optogenetics and electrophysiology we showed that glutamatergic projections from vHIP to NAc ChIs are severely altered in mutant mice. These results show that specific alterations in the circuitry of the accumbal ChIs could play an important role in the regulation of depressive-like behavior, reward-seeking behavior in addictions, or psychiatric symptoms in neurodegenerative diseases.