GABAA receptors (GABAARs) play a central role in mediating most of the neuronal inhibition in central nervous system. For efficient synaptic neurotransmission, it is essential that GABAARs are recruited and clustered at the synapses. However, the mechanism for this process is still unclear. Previous work done in our laboratory indicated that in a yeast-two-hybrid assay (Y2H) there is an interaction between the intracellular domain of protocadherin-Î3C5 (Pcdh-Î3C5) and the large intracellular loop of the GABAAR Î32 subunit (Î32IL). I have now shown that this interaction occurs in cultured neurons and in the brain. I have also studied the expression and localization of Pcdh-Î3C5 in the rat brain. For these studies, we have developed several antibodies to Pcdh-Î3C5. With a novel anti-Pcdh-Î3C5 specific antibody, I have found that in the entire rat brain, Pcdh-Î3C5 is highly expressed in the olfactory bulb, corpus striatum, dentate gyms, CA1 region of the hippocampus, cerebral cortex and the molecular layer of the cerebellum. The expression of Pcdh-Î3C5 started after the second postnatal week, coinciding with the time frame of synapse development. A significant percentage of synapses, more GABAergic than glutamatergic, have associated Pcdh-Î3C5 clusters. Nevertheless there is also non-synaptic Pcdh-Î3C5. Pcdh-Î3C5 is expressed in both neurons and astrocytes. In the intact rat brain, a significant number of Pcdh-Î3C5 clusters are located at non-synaptic contact points between neurons and astrocytes. An anti-Pcdh-Î3C5 antiserum coprecipitated assembled GABAARs from brain extracts. An anti-Î32 or α1 GABA AR subunit antiserum coprecipitated Pcdh-Î3C5. In cultured hippocampal neurons, an anti-Î32 or an anti-Pcdh-Î3C5 antibody induced capping increasing both the Î32 and Pcdh-Î3C5 cluster densities, indicating that there is interaction between Î32 and Pcdh-Î3C5 in cultured neurons. In cultured HEK293 cells, overexpression of Pcdh-Î3C5 helped the trafficking of GABAARs to the cell surface. Overexpression of Pcdh-Î3C5 increases the number of Î32 clusters in hippocampal neurons. Knocking down the Pcdh-Î3C5 expression level in hippocampal neurons with Pcdh-Î3C5 shRNAs, both the postsynaptic Î32 cluster density and the presynaptic GAD innervation were dramatically reduced, indicating that Pcdh-Î3C5 is essential for the stability of GABAergic synapses. In contrast, the glutamatergic synapses were not affected by the shRNAs. These results indicate that Pcdh-Î3C5 (i) plays synaptic and non-synaptic function; (ii) interacts with GABA ARs in cultured hippocampal neurons and in brain; (iii) helps GABA ARs trafficking to the cell surface via direct interaction with Î32 subunit; (iv) this interaction plays role in GABAergic synapses formation or maintenance.