Immunofluorescence staining revealed that the injected iN cells had dispersed throughout the striatum, and formed extensive dendritic arborizations (Fig. 6A). Numerous EGFP-positive processes were found throughout the striatum and extending through the corpus callosum into the non-transplanted hemisphere. The human iN cells were selectively labeled by antibodies to human nuclei (Fig. 6B), human NCAM (Fig. 6C), and NeuN (Fig. 6D). Electrophysiological recordings from acute slices in current-clamp mode showed that the transplanted iN cells exhibited a resting potential of ~−60 mV, fired trains of action potentials when injected with current, and displayed a near physiological action-potential firing threshold and action potential amplitude (Figs. 6E and 6F). Moreover, recordings in voltage-clamp mode demonstrated that the transplanted neurons received highly active spontaneous inhibitory synaptic inputs as documented by the blockade of the synaptic events by picrotoxin (Fig. 6G). Inhibitory synaptic events would be expected given the preponderance of inhibitory medium spiny neurons in the striatum. Accordingly, inhibitory postsynaptic currents could also be elicited by extracellular stimulation, confirming that the transplanted neurons received abundant inhibitory synaptic inputs from the surrounding neurons in the striatum (Fig. 6H).
