FIG. 1.
Persistent inward currents (PICs) in motoneurons are facilitated by serotonin (5-HT) and are supersensitive to 5-HT after chronic spinal transection. A: in motoneuron from acute spinal rat, a slow voltage ramp (3.5 mV/s) from −80 to −40 mV (top) was used to measure the PIC (bottom) before and after 5-HT application. PIC was quantified as the difference between the leak current (thin line) and the recorded current (downward arrow). Note that PIC was increased by 10 µM 5-HT (black trace) compared with control (gray trace). B: motoneuron from chronic spinal rat. Same format and scale as in A. 5-HT again increased the PIC amplitude and lowered onset voltage of the PIC (VSTART; black arrowhead: control; white arrowhead: 5-HT). Also, note in A and B the decrease in input conductance and depolarization of resting potential (at a given current) with 5-HT. C–F: filtered data from voltage ramps, with leak current subtracted, plotted against membrane potential. Typical motoneurons before (gray traces) and after (black traces) 5-HT application of varying doses. Drug-induced PIC (ΔPIC) was calculated as the difference in maximum leak-subtracted currents (downward arrow). Low-dose (1 µM) 5-HT did not change PIC (ΔPIC = 0 nA) in acute spinal rat (C), but did increase it in chronic spinal rat (D). High-dose 5-HT (≥ 10 µM) increased PIC in both acute (E) and chronic (F) spinal rats, but the effect was greater in chronic spinal rats. G: group data showing average change in PIC amplitude with various doses of 5-HT. Open bars: acute spinal; shaded bars: chronic spinal. Asterisk (*) indicates effect significantly greater than zero (P< 0.05). PIC amplitude significantly increased at low doses (≤1 µM) of 5-HT in motoneurons from chronic spinal rats, whereas high doses (≥ 10 µM) were required to facilitate PICs in acute spinal rats.