The properties of subthreshold sodium current suggest that it can influence the kinetics and amplitude of small EPSPs near typical resting potentials, a prediction that is confirmed using two-photon glutamate uncaging to probe the contribution of sodium currents to single synapse responses. To examine the voltage dependence and gating kinetics of subthreshold sodium current with good voltage control and high time resolution, we used acutely dissociated neurons. To

approximate physiological conditions as nearly as possible, we made MDV3100 in vivo recordings at 37°C and used the same potassium methanesulfonate-based internal solution as in previous current-clamp recordings from the neurons (Carter and Bean, 2009, 2011). Using these conditions to record from mouse cerebellar Purkinje neurons, depolarization by a slow (10mV/s) ramp evoked TTX-sensitive current that was first evident near −80mV and increased steeply with

voltage to reach a maximum near −50mV (black trace, Figure 1A). The TTX-sensitive current evoked by this slow ramp was similar to steady-state “persistent” sodium current previously recorded in Purkinje neurons but activated at considerably more negative voltages than in recordings with less physiological conditions (Raman and Bean, 1997; Kay Selleckchem Anti-cancer Compound Library et al., 1998). In recordings from 26 cells, the TTX-sensitive steady-state current was −16 ± 2 pA at −80mV, −81 ± 16 pA at −70mV, and −254 ± 23 pA at −60mV and reached a maximum of −393 ± 31 pA at −48mV ± 1mV. When converted to a conductance, the voltage dependence of steady-state current could be fit well by a Boltzmann function (Figure 1C) with average next midpoint of −62mV ± 1mV and an average slope factor of 4.9mV ± 0.1mV (n = 26). Slow ramps define the voltage dependence of the steady-state sodium current but do not provide kinetic information about channel activation. Because activation kinetics are important for determining the timing with which sodium current can be engaged by transient

synaptic potentials, we assayed kinetics by applying successive 5mV step depolarizations at the same overall rate as the ramp depolarization (10mV/s, Figure 1A, red traces). As expected, the current at the end of each voltage step reached steady-state and closely matched the ramp-evoked current at that voltage. Unexpectedly, however, there was also a prominent transient phase of sodium current for depolarizations positive to about −70mV. For example, a step from −73mV to −68mV activated a component of transient current nearly as large as the change in steady-state current (Figure 1B). The relative magnitude of the transient current evoked by 5mV steps increased at more depolarized voltages. For a step from −63mV to −58mV, transient current was on average more than three times the size of the change in steady current (−238 ± 62 pA versus −64 ± 6 pA, n = 10).