Fig. 4.

Blockers of I_NaPsuppress intrinsic bursting in low [Ca²⁺]_o. Recordings were made from four grade II LTBs in Ca²⁺-free saline. Burst firing was evoked by 3 msec depolarizing stimuli.A, The responses of the neuron are shown before (a) and 5 and 20 min after (b andc, respectively) adding 0.1 μm TTX to the Ca²⁺-free saline, which caused a gradual suppression of intrinsic bursting. B, The responses of the neuron are shown before (a), 30 min after adding 50 μm phenytoin to the Ca²⁺-free saline (b), and 30 min after wash of phenytoin (c). C, The responses of the neuron are shown before (a) and 30 min after (b) adding 5 μm PDB to the Ca²⁺-free saline (b). Both phenytoin and PDB blocked the burst response completely but affected the first spike very little. D, The responses of the neuron are shown before (a) and 45 min after (b) adding 50 μm PDC to the Ca²⁺-free saline. No effect of this drug was noted.