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The Journal of Neurophysiology Vol. 83 No. 6 June 2000, pp. 3497-3508
Copyright ©2000 by the American Physiological Society
Department of Physiology, Division of Neuroscience, University of Alberta, Edmonton, Alberta T6G 2S2, Canada
Martin-Caraballo, Miguel and
John J. Greer.
Development of Potassium Conductances in Perinatal Rat Phrenic
Motoneurons. J. Neurophysiol. 83: 3497-3508, 2000. Prior to the inception of inspiratory synaptic
drive transmission from medullary respiratory centers, rat phrenic
motoneurons (PMNs) have action potential and repetitive firing
characteristics typical of immature embryonic motoneurons. During the
period spanning from when respiratory bulbospinal and segmental
afferent synaptic connections are formed at embryonic day
17 (E17) through to birth (gestational period is
~21 days), a pronounced transformation of PMN electrophysiological
properties occurs. In this study, we test the hypothesis that the
elaboration of action potential afterpotentials and the resulting
changes in repetitive firing properties are due in large part to
developmental changes in PMN potassium conductances. Ionic conductances
were measured via whole cell patch recordings using a cervical
slice-phrenic nerve preparation isolated from perinatal rats. Voltage-
and current-clamp recordings revealed that PMNs expressed outward
rectifier (IKV) and A-type potassium
currents that regulated PMN action potential and repetitive firing
properties throughout the perinatal period. There was an age-dependent
leftward shift in the activation voltage and a decrease in the
time-to-peak of IKV during the period from
E16 through to birth. The most dramatic change during
the perinatal period was the increase in calcium-activated potassium
currents after the inception of inspiratory drive transmission at
E17. Block of the maxi-type calcium-dependent potassium
conductance caused a significant increase in action potential duration
and a suppression of the fast afterhyperpolarizing potential. Block of
the small conductance calcium-dependent potassium channels resulted in
a marked suppression of the medium afterhyperpolarizing potential and
an increase in the repetitive firing frequency. In conclusion, the
increase in calcium-mediated potassium conductances are in large part
responsible for the marked transformation in action potential shape and
firing properties of PMNs from the time between the inception of fetal
respiratory drive transmission and birth.
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