Depolarization-induced changes in lamina I/II neuronal excitability. Current-clamp recordings from intact tonic-spiking lamina I/II neurons in spinal cord slices from P6 to P9 Wistar rats. Traces and analyses were from an IV protocol consisting of 1 s transient hyperpolaring/depolarizing current injections between -20 and +75 pA at +5 pA increments. A) Left, representative voltage traces demonstrating a lamina II neuron's response to 1 s transient current injections of +40 pA (top), 0 pA (middle), and -20 pA (bottom). The neuron had a resting membrane potential of approximately -62 mV. Right, in the presence of tonic depolarization (+15 pA), the same neuron had a resultant resting membrane potential of approximately -50 mV and altered electrical responses to 1 s transient current injections of + 25 pA (top), 0 pA (middle), and -20 pA (bottom). Scale bar x axes = 500 ms, y axes = 25 mV. Top scale bar corresponds to top traces, while bottom scale bar corresponds to middle and bottom traces. B) Bar graph demonstrating a significant (* p < 0.05, n = 6, Student's paired t test) reduction in average number of spikes (> 20 mV amplitude) elicited by 1 s transient depolarizations (46 +/- 5 pA, n = 6) for tonically depolarized neurons compared to the same neurons in their resting state (left grey bars). Significant (* p < 0.05, n = 6, Student's paired t test) increases in spontaneous activity (middle black bars) and rebound activity (right dark grey bars) over a 2 s period were observed for tonically depolarized neurons compared to the same neurons in their resting state. For tonically depolarized neurons, rebound spiking following transient -10 pA current injection was significantly (* p < 0.05, n = 6, Student's paired t test) greater than spontaneous spiking over the same 2 s period. Resting Vm = -68 +/- 2 mV, n = 6; Depolarized Vm = -52 +/- 2 mV, n = 6. LJP = 14.6 mV, corrected.