DEVELOPMENTAL EXPRESSION AND FUNCTIONAL-CHARACTERIZATION OF THE POTASSIUM-CHANNEL SUBUNIT KV3.1B IN PARVALBUMIN-CONTAINING INTERNEURONS OF THE RAT HIPPOCAMPUS

The expression of the voltage-gated K+-channel subunit Kv3.1b in the d eveloping hippocampus was determined by immunoblot and immunohistochem ical techniques. Kv3.1b protein was detected first at postnatal day (P ) 8. The Kv3.1b-immunopositive cell number per tissue section reached a maximum at P14 and was maintained through P40, In contrast, the Kv3. 1b protein content of isolated membrane vesicles in immunoblots progre ssively increased through P40, suggesting an increase in Kv3.1b conten t per cell throughout this time period. Kv3.1b protein was expressed s electively in the somata, proximal dendrites, and axons of cells lying within or near the pyramidal cell layer, consistent with their being GABAergic inhibitory interneurons. Kv3.1b was present in similar to 80 % of parvalbumin-positive interneurons. The developmental onset of Kv3 .1b and parvalbumin immunoreactivity was identical. In contrast, Kv3.1 b was mostly absent from the subset of somatostatin-positive inhibitor y interneurons. Electrophysiological recordings were made from stratum pyramidale interneurons in which morphology and Kv3.1b-positive immun oreactivity were confirmed post hoc. Outward currents had voltage-depe ndent and biophysical properties resembling those of channels formed b y Kv3.1b, The current blocked by low concentrations of 4-aminopyridine (4-AP) showed marked inactivation, suggesting that Kv3.1b may coassem ble with other members of the Kv3 subfamily, In current-clamp recordin gs, concentrations of 4-AP that blocked the current through Kv3.1b cha nnels allowed us tentatively to assign a role to Kv3.1b-containing cha nnels in action-potential repolarization. These data demonstrate that Kv3.1b is regulated developmentally in a specific subpopulation of hip pocampal interneurons and that channels containing this subunit may be a major determinant in imparting ''fast-spiking'' characteristics to these and other cells throughout the central nervous system containing the Kv3.1b subunit.