Voltage-sensitive calcium currents and their role in regulating phrenic motoneuron electrical excitability during the perinatal period

This study examined the ontogeny of voltage-sensitive calcium conductances in rat phrenic motoneurons (PMNs) and their role in regulating electrical e xcitability during the perinatal period. Specifically, we studied the perio d spanning from embryonic day (E)16 through postnatal day (P)1, when PMNs u ndergo fundamental transformation in their morphology, passive properties, ionic channel composition, synaptic inputs, and electrical excitability. Lo w voltage-activated (LVA) and high voltage-activated (HVA) conductances wer e measured using whole cell patch recordings utilizing a cervical slice-phr enic nerve preparation from perinatal rats. Changes between E16 and P0-1 in cluded the following: an approximate to2-fold increase in the density of to tal calcium conductances, an approximate to2-fold decrease in the density o f LVA calcium conductances, and an approximate to3-fold increase in the den sity of HVA conductances. The elevated expression of T-type calcium channel s during the embryonic period lengthened the action potential and enhanced electrical excitability as evidenced by a hyperpolarization-evoked rebound depolarization. The reduction of LVA current density coupled to the presenc e of a hyperpolarizing outward A-type potassium current had a critical effe ct in diminishing the rebound depolarization in neonatal PMNs. The increase in HVA current density was concomitant with the emergence of a calcium-dep endent "hump-like" afterdepolarization (ADP) and burst-like firing. Neonata l PMNs develop a prominent medium-duration afterhyperpolarization (mAHP) as the result of coupling between N-type calcium channels and small conductan ce, calcium-activated potassium channels. These data demonstrate that chang es in calcium channel expression contribute to the maturation of PMN electr ophysiological properties during the time from the commencement of fetal in spiratory drive to the onset of continuous breathing at birth, (C) 2001 Joh n Wiley & Sons, Inc.