Previous anatomical and physiological studies of neostriatal grafts have suggested that transplanted neurons do not develop beyond an early postnatal stage. We have tested whether this hypothesis can be generalized by characterizing the developmentally regulated Ca-independent potassium currents in graft neurons. These currents were studied using a combination of the whole-cell voltage-clamp technique with acutely-dissociated neurons and intracellular recording in slices. In all of the graft neurons examined with voltage-clamp techniques (n = 13), evidence was found for a slowly-inactivating potassium current that is seen only beyond the third or fourth postnatal week in normal rats. A current resembling the delayed rectifier was also seen in all sample neurons. The rapidly inactivating A-current which dominates recordings from nearly all immature neurons was seen in only about half (54%, 7 13) of the graft neurons; in a sample of normal adult striatal neurons, the A-current was detected in a similar percentage of neurons (41%, 25 62). Recordings of graft neurons in slices corroborated the voltage-clamp findings in revealing a slowly inactivating outward current that acts in the subthreshold potential range. These findings suggest that graft neurons express the normal complement of depolarization-activated potassium channel proteins seen in adult neurons.
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