Figure 6.
Prolonged membrane hyperpolarization lowers the voltage threshold for action potential initiation and shifts resting conductances, but VGSCs provide the same magnitude of just-subthreshold EPSP amplification as in resting conditions. A, Somatic recordings from a single neuron (P16; Vrest = −62 mV). Responses to a series of EPSCs delivered at rest (upper traces, 0–2.8 nA, 0.4 nA steps) or delivered 80 ms into a 100 ms hyperpolarizing step pulse (lower traces, 0–3.6 nA, 0.4 nA steps). Bath application of 1 μm TTX revealed similar amplitudes of just-subthreshold EPSP amplification in each condition. B, Plot of EPSP amplitude measured from rest versus EPSC amplitude. Square and circle symbols indicate responses before and after the application of 1 μm TTX, respectively. Closed symbols, from rest. Open symbols, from hyperpolarization. Dotted lines indicate AP threshold in the control condition (normal ACSF). C, The reduction of EPSP amplitude by TTX determined at the current producing a just-subthreshold response is similar from rest and from a hyperpolarizing pulse (left; n = 9; p > 0.1). The TTX-induced increase in current required to reach the amplitude of a just-subthreshold EPSP is similar in each condition (right; n = 9; p > 0.1). D, Hyperpolarization decreased action potential voltage threshold (left; n = 9, p < 5 × 10−5). Hyperpolarization increased the current threshold for action potential generation (right; n = 9, p < 0.05). Mean, closed symbols, and individual responses, open symbols. *Statistically significant difference (paired t test), with a criterion level of 0.05.