IA current compared to low threshold calcium current in cranial sensory neurons.

Whole-cell recordings of cranial sensory neurons were obtained with CsCl- and tetraethylammonium (TEA)-loaded cells as used for ICa studies. When replacing calcium (Ca) with potassium (K) in the external medium, a transient inward current developed at low threshold, if the holding potential was below -100 mV. Its activation started above -60 mV and reached its maximum at -20 mV. Its reversal potential followed the predicted value of the K equilibrium potential (EK) characteristic of a K current. Pharmacologically, it resembles the classical outward A current since it is insensitive to TEA up to 20 mM and blocked by 4-aminopyridine (4-AP) and 3,4-AP in the millimolar range. This inward A current rises rapidly in less than 5 ms and decays monoexponentially with a time constant of about 60 ms at all potentials studied. Its steady-state inactivation occurred between -100 and -60 mV with a half-inactivation at -86 mV. All these characteristics hold for a voltage-dependent A current devoid of any Ca-dependent component. The activation and inactivation of this A inward current are compared to those of ICa,t, the other low threshold current encountered on those cells. The contribution of the two currents in the control of discharge frequency is discussed.