Collybistin (CB) is a rho guanine exchange factor found at GABAergic and glycinergic postsynapses that interacts with the inhibitory scaffold protein, gephyrin, and induces accumulation of gephyrin and GABA A receptors (GABA A Rs) to the postsynapse. We have previously reported that the isoform without the src homology 3 (SH3) domain, CBSH3-, is particularly active in enhancing the GABAergic postsynapse in both cultured hippocampal neurons as well as in cortical pyramidal neurons after chronic in vivo expression in in utero electroporated (IUE) rats. Deficiency of CB in knockout mice results in absence of gephyrin and gephyrin-dependent GABA A Rs at postsynaptic sites in several brain regions, including hippocampus. In the present study, we have generated an adeno-associated virus (AAV) that expresses CBSH3-in a cre-dependent manner. Using male and female VGLUT1-IRES-cre or VGAT-IRES-cre mice, we explore the effect of overexpression of CBSH3-in hippocampal pyramidal cells or hippocampal interneurons. The results show that: 1) the accumulation of gephyrin and GABA A Rs at inhibitory postsynapses in hippocampal pyramidal neurons or interneurons can be enhanced by CBSH3-overexpression, 2) overexpression of CBSH3-in hippocampal pyramidal cells can enhance the strength of inhibitory neurotransmission, and 3) these enhanced inhibitory synapses provide protection against PTZ-induced seizures. The results indicate that this AAV vector carrying CBSH3-can be used for in vivo enhancement of GABAergic synaptic transmission in selected target neurons in the brain.Significance statement: Excessive or imbalanced excitation in the hippocampus can result in acute or chronic pathological conditions, such as seizures, epilepsy, and learning impairments. It is therefore important to uncover target genes that can be manipulated to restore or prevent this imbalance. This study uses a novel adeno-associated virus to express collybistin SH3-in select cells of the hippocampus. We have found that overexpression of this protein enhances GABAergic inhibitory synaptic transmission. The results also bring attention to collybistin as a possible target for therapeutic intervention aimed to restore the balance between excitation and inhibition.