Ionic currents and current-clamp depolarisations of type I and type II hair cells from the developing rat utricle

Ionic currents and the voltage response to injected currents were studied i n an acutely dissected preparation of the rat utricle between birth and pos tnatal day 12 (PN12). Based upon morphological criteria, the sensory cells examined were divided into two classes, "type I" and "type 2 category," the latter of which may include some immature type I cells. The former group c omprises a clearly defined electrophysiological population, with one large outwardly rectifying potassium conductance that is sensitive to 4-aminopyri dine (4-AP), insensitive to tetraethylammonium (TEA) and displays voltage-d ependent activation kinetics. In the absence of enzymatic dissociation proc edures, and with the epithelium left largely intact, the mean half activati on of this conductance was -30.3 mV at PN3, and -37.5 mV at PN12. At both s tages it was almost entirely turned off at -74 mV. Omission of ATP from the intracellular solution appeared to prevent rundown of this conductance. Ty pe II category hair cells formed a more heterogeneous population, exhibitin g a distinct TEA-sensitive delayed rectifier potassium conductance; the rap idly activating and inactivating I-A; an inward rectifier; and inward sodiu m currents at around PN3. Both cell types depolarised strongly in response to injected currents, with time courses reflecting the activation kinetics of their major outward conductances.