LOW-THRESHOLD CA2+ CURRENT AND ITS ROLE IN SPONTANEOUS ELEVATIONS OF INTRACELLULAR CA2+ IN DEVELOPING XENOPUS NEURONS

Amphibian spinal neurons exhibit spontaneous elevations of intracellul ar calcium at early stages of development. The underlying calcium infl ux involves high-voltage-activated (HVA) currents. To begin to underst and how they are triggered, we have studied the biophysical properties and developmental function of low-voltage-activated (LVA) T-type calc ium current of neurons cultured from the embryonic neural plate. T cur rent was recorded from young neurons (6-9 hr in vitro) and from mature neurons (1 8-48 hr in vitro) using whole-cell voltage clamp. For both young and mature neurons, T current has a low threshold and is activa ted at membrane potentials positive to -60 mV in 2 mm extracellular ca lcium. The current is maximal at -35 mV with a mean peak amplitude of approximately 50 pA. Nickel blocks both LVA and HVA currents, but the former are 20-fold more sensitive. Amiloride also blocks T current sel ectively. T current is recorded in 87% of young neurons. This percenta ge drops to 67% in mature neurons after 1 d in culture and to 35% in m ature neurons after 2 d in culture. There are no significant developme ntal changes in T current threshold, peak density, time course of acti vation and inactivation, and pharmacological sensitivity to blockers f rom 6 to 48 hr in culture. Spontaneous transient calcium elevations in young neurons assayed by fluo-3 fluorescence are blocked by nickel or amiloride at concentrations that specifically block T current. T curr ent has the lowest threshold among other inward currents in young neur ons. Moreover, mathematical simulations show that T current lowers the threshold of the action potential by 15 mV. We conclude that T curren t can depolarize cells and trigger action potentials, and constitutes part of the cascade of events leading to spontaneous elevations of int racellular calcium in cultured neurons at early stages of differentiat ion.