Figure 1: Extracellular Ca²⁺ induces AHP and spike adaptation in TC neurons.

(a) A depolarizing current of 200 pA was injected for 1 s to induce tonic firing in TC neurons in 2.4 mM Ca²⁺ buffer followed by replacement with Ca²⁺-free buffer. (b) AHP potential after current injection averaged in both 2.4 mM Ca²⁺ buffer and Ca²⁺-free buffer (n=10 from five mice). A significant decrease in post-current injection AHP was observed in Ca²⁺-free buffer compared with 2.4 mM Ca²⁺ buffer (***P=0.0002, paired t-test). (c) The number of spikes during current injection increased with a change from 2.4 mM Ca²⁺ buffer to Ca²⁺-free buffer (***P=0.0001, paired t-test), and (d) was accompanied by an increase in the adaptation index (n=10 from five mice; ***P=0.0005, paired t-test). (e) The input–output (I–O) curve showed the number of spikes as measured under various current injections. Firing rates were increased in Ca²⁺-free buffer compared with 2.4 mM Ca²⁺ buffer (n=10 from five mice). (f,g) Representative traces of tail currents after 100 ms of a 0 mV depolarizing step protocol demonstrated a net inward current (f) at a −60 mV holding potential, which became an outward current at a −30 mV holding potential (g). (h) Ca²⁺-activated tail currents were recorded in TC neurons in 2.4 mM Ca²⁺ buffer followed by replacement with Ca²⁺-free buffer in the presence of extracellular TTX. Under voltage clamp, the holding potential was −60 mV, and 100 ms voltage steps ranging from −50 to 0 mV (+10 mV per step) were administered. Tail currents were almost completely abolished in Ca²⁺-free buffer compared with 2.4 mM Ca²⁺ buffer. (i,j) The tail currents following voltage steps to 0 mV for durations of 50, 100, 200, 500 and 1,000 ms were measured in the presence of extracellular TTX. (i) The tail currents obtained from 50, 200 and 1,000 ms are shown. (j) The current decay constant obtained from each step was normalized to the current decay constant recorded over a 50 ms prepulse (n=9 from four mice). Longer prepulses produced longer current decay times. Error bars represent mean±s.e.