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This paper describes research methods to investigate the development of synaptic connections between transplanted GABAergic interneurons and endogenous neurons in the adult mouse hippocampus. Our protocol highlights methods for retroviral labeling adult-born GCs, one of the few cell types in the adult brain to be continuously renewed throughout life. By precise targeting of the retrovirus, labeling of adult-born GCs can be combined with optogenetic stimulation of the transplanted cells and electrophysiology in brain slices, to test whether the GABAergic interneurons integrate and establish inhibitory synaptic connections with host brain neurons. Modifications to adult neurogenesis are an important contributing factor in the development and severity of TLE and seizures. When combined with retroviral labeling, the approaches we describe in this chapter can be used to determine whether transplantation modifies the process of adult neurogenesis or other properties of the hippocampus. These approaches are helping to define parameters for potential cell replacement therapies to be used in patients with intractable seizure disorders. 

Adult male zebra finches (Taeniopygia guttata) continually incorporate adult‐born neurons into HVC, a telencephalic brain region necessary for the production of learned song. These neurons express activity‐dependent immediate early genes (e.g.,zenkandc‐fos) following song production, suggesting that these neurons are active during song production. Half of these adult‐born HVC neurons (HVC NNs) can be backfilled from the robust nucleus of the arcopallium (RA) and are a part of the vocal motor pathway underlying learned song production, but the other half do not backfill from RA, and they remain to be characterized. Here, we used cell birth‐dating, retrograde tract tracing, and immunofluorescence to demonstrate that half of all HVC NNs express the phosphoprotein DARPP‐32, a protein associated with dopamine receptor expression. We also demonstrate that DARPP‐32+ HVC NNs are contacted by tyrosine hydroxylase immunoreactive fibers, suggesting that they receive catecholaminergic input, have transiently larger nuclei than DARPP‐32‐neg HVC NNs, and do not backfill from RA. Taken together, these findings help characterize a group of HVC NNs that have no apparent projections to RA and so far have eluded positive identification other than HVC NN status.