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J Neurophysiol 85: 580-593, 2001;
0022-3077/01 $5.00
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The Journal of Neurophysiology Vol. 85 No. 2 February 2001, pp. 580-593
Copyright ©2001 by the American Physiological Society

5-HT Modulation of Multiple Inward Rectifiers in Motoneurons in Intact Preparations of the Neonatal Rat Spinal Cord

Ole Kjaerulff and Ole Kiehn

Division of Neurophysiology, Department of Medical Physiology, The Panum Institute, University of Copenhagen, DK-2200 Copenhagen, Denmark

Kjaerulff, Ole and Ole Kiehn. 5-HT Modulation of Multiple Inward Rectifiers in Motoneurons in Intact Preparations of the Neonatal Rat Spinal Cord. J. Neurophysiol. 85: 580-593, 2001. This study introduces novel aspects of inward rectification in neonatal rat spinal motoneurons (MNs) and its modulation by serotonin (5-HT). Whole cell tight-seal recordings were made from MNs in an isolated lumbar spinal cord preparation from rats 1-2 days of age. In voltage clamp, hyperpolarizing step commands were generated from holding potentials of -50 to -40 mV. Discordant with previous reports involving slice preparations, fast inward rectification was commonly expressed and in 44% of the MNs co-existed with a slow inward rectification related to activation of Ih. The fast inward rectification is likely caused by an IKir. Thus it appeared around EK and was sensitive to low concentrations (100-300 µM) of Ba2+ but not to ZD 7288, which blocked Ih. Both IKir and Ih were inhibited by Cs2+ (0.3-1.5 mM). Extracellular addition of 5-HT (10 µM) reduced the instantaneous conductance, most strongly at membrane potentials above EK. Low [Ba2+] prevented the 5-HT-induced instantaneous conductance reduction below, but not that above, EK. This suggests that 5-HT inhibits IKir, but also other instantaneous conductances. The biophysical parameters of Ih were evaluated before and under 5-HT. The maximal Ih conductance, Gmax, was 12 nS, much higher than observed in slice preparations. Gmax was unaffected by 5-HT. In contrast, 5-HT caused a 7-mV depolarizing shift in the activation curve of Ih. Double-exponential fits were generally needed to describe Ih 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 constant to the largest extent. We conclude that spinal neonatal MNs possess multiple forms of inward rectification. Ih 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 IKir, and enhancement of Ih. The quantitative characterization of these effects should be useful for further studies seeking to understand how neuromodulation prepares vertebrate MNs for concerted behaviors such as locomotor activity.




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