Different classes of spinal lamina I neurons express different subthreshold currents.

<p>(A) Traces show responses to 60 pA, 20-ms-long depolarizing pulses (black) and to equivalent hyperpolarizing pulses (gray); the latter are inverted to facilitate comparison with former. In class 1 (tonic-spiking) neurons, depolarization was amplified and prolonged relative to hyperpolarization, consistent with effects of an inward current activated by perithreshold depolarization. Class 3 (single-spiking) neurons exhibited the opposite pattern, consistent with effects of a subthreshold outward current, which is also evident from outward rectification (arrow) in the <i>I–V</i> curve. Depolarizing and hyperpolarizing responses were symmetrical in class 2 (phasic-spiking) neurons, consistent with negligible net subthreshold current. (B) Membrane current activated by voltage-clamp steps from −70 mV to −60, −50, −40, and −30 mV. For a given command potential, class 3 neurons exhibited the largest persistent outward current and class 1 neurons exhibited the smallest outward current. Red line highlights difference in current activated by step to −40 mV. (C) Steady-state <i>I–V</i> curves for voltage clamp protocols in (B). Because recordings were performed in TTX to prevent unclamped spiking, the persistent Na⁺ current (<i>I</i>_Na,p), which is expressed exclusively in tonic-spiking neurons, was blocked; to correct for this, <i>I</i>_Na,p measured in separate voltage clamp ramp protocols <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1000198#pcbi.1000198-Prescott2" target="_blank">[26]</a> was added to give a corrected <i>I–V</i> curve (dotted line). Compare with <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1000198#pcbi-1000198-g004" target="_blank">Figure 4</a>. (D) 4-AP-sensitive current determined by subtraction of response before and during application of 5 mM 4-AP to a single-spiking neuron. Protocol included prepulse to −100 mV, which revealed a small persistent outward current active at −70 mV that was deactivated by hyperpolarization to −100 mV (*). Although depolarization also activates a large transient outward current, we are concerned with the persistent component (arrowhead); effects of the transient outward current are beyond the scope of this study and were minimized by adjusting pre-stimulus membrane potential to −60 mV for all current clamp protocols. Gray line shows baseline current.