Slow inactivation of Na⁺ current and slow cumulative spike adaptation in mouse and guinea-pig neocortical neurones in slices

1. Spike adaptation of neocortical pyramidal neurones was studied with sharp electrode recordings in slices of guinea-pig parietal cortex and whole-cell patch recordings of mouse somatosensory cortex. Repetitive intracellular stimulation with 1 s depolarizing pulses delivered at intervals of < 5 s caused slow, cumulative adaptation of spike firing, which was not associated with a change in resting conductance, and which persisted when Co²⁺ replaced Ca²⁺ in the bathing medium. 2. Development of slow cumulative adaptation was associated with a gradual decrease in maximal rates of rise of action potentials, a slowing in the post-spike depolarization towards threshold, and a positive shift in the threshold voltage for the next spike in the train; maximal spike repolarization rates and after-hyperpolarizations were unchanged. 3. The data suggested that slow adaptation reflects use-dependent removal of Na⁺ channels from the available pool by an inactivation process which is much slower than fast, Hodgkin-Huxley-type inactivation. 4. We therefore studied the properties of Na⁺ channels in layer II-III mouse neocortical cells using the cell-attached configuration of the patch-in-slice technique. These had a slope conductance of 18 ± 1 pS and an extrapolated reversal potential of 127 ± 6 mV above resting potential (V(r)) (mean ± S.E.M.; n = 5). V(r) was estimated at -72 ± 3 mV (n = 8), based on the voltage dependence of the steady-state inactivation (h(∞)) curve. 5. Slow inactivation (SI) of Na⁺ channels had a mono-exponential onset with τ(on) between 0.86 and 2.33 s (n = 3). Steady-state SI was half-maximal at -43.8 mV and had a slope of 14.4 mV (e-fold)^-1. Recovery from a 2 s conditioning pulse was bi-exponential and voltage dependent; the slow time constant ranged between 0.45 and 2.5 s at voltages between -128 and -68 mV. 6. The experimentally determined parameters of SI were adequate to simulate slow cumulative adaptation of spike firing in a single-compartment computer model. 7. Persistent Na⁺ current, which was recorded in whole-cell configuration during slow voltage ramps (35 mV s^-1), also underwent pronounced SI, which was apparent when the ramp was preceded by a prolonged depolarizing pulse.