5-HT modulation of multiple inward rectifiers in motoneurons in intact preparations of the neonatal rat spinal cord

This study introduces novel aspects of inward rectification in neonatal rat spinal motoneurons (MNs) and its modulation by serotonin (5-HT). Whole cel l tight-seal recordings were made from MNs in an isolated lumbar spinal cor d preparation from rats 1-2 days of age. In voltage clamp, hyperpolarizing step commands were generated from holding potentials of -50 to -40 mV. Disc ordant with previous reports involving slice preparations, fast inward rect ification was commonly expressed and in 44% of the MNs co-existed with a sl ow inward rectification related to activation of I-h. The fast inward recti fication is likely caused by an I-Kir. Thus it appeared around E-K and was sensitive to low concentrations (100-300 muM) of Ba2+ but not to ZD 7288, w hich blocked I-h. Both I-Kir and I-h were inhibited by Cs2+ (0.3-1.5 mM). E xtracellular addition of 5-HT (10 mM) reduced the instantaneous conductance , most strongly at membrane potentials above E-K. Low [Ba2+] prevented the 5-HT-induced instantaneous conductance reduction below, but not that above, E-K. This suggests that 5-HT inhibits I-Kir, but also other instantaneous conductances. The biophysical parameters of I-h were evaluated before and u nder 5-HT. The maximal I-h conductance, G(max), was 12 nS, much higher than observed in slice preparations. G(max) was unaffected by 5-HT. In contrast , 5-HT caused a 7-mV depolarizing shift in the activation curve of I-h. Dou ble-exponential fits were generally needed to describe I-h activation. The fast and slow time constants obtained by these fits differed by an order of magnitude. Both time constants were accelerated by 5-HT, the slow time con stant to the largest extent. We conclude that spinal neonatal MNs possess m ultiple forms of inward rectification. I-h may be carried by two spatially segregated channel populations, which differ in kinetics and sensitivity to 5-HT. 5-HT increases MN excitability in several ways, including inhibition of a barium-insensitive leak conductance, inhibition of I-Kir, and enhance ment of I-h. The quantitative characterization of these effects should be u seful for further studies seeking to understand how neuromodulation prepare s vertebrate MNs for concerted behaviors such as locomotor activity.