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1 Physiology & Biophysics, University of Washington School of Medicine, Seattle, Washington, United States; University of Washington
2 Physiology & Biophysics, University of Washington School of Medicine, Seattle, Washington, United States
3 Dept. of Physiology & Biophysics, Univ Washington, Seattle, Washington, United States
* To whom correspondence should be addressed. E-mail: mdbinder{at}u.washington.edu.
Voltage-dependent persistent inward currents (PICs) make an important contribution to the input-output properties of alpha motoneurons. PICs are thought to be mediated by membrane channels located primarily on the dendrites, as evidenced by prolonged tail currents following the termination of a voltage step and by a clockwise hysteresis in the whole-cell inward currents recorded in response to depolarizing then repolarizing voltage ramp commands. We report here, however, that voltage-clamp currents with these same features can be generated in isolated somatic membrane patches from rat hypoglossal motoneurons. Long-lasting (200-800 ms) tail currents after 1-s voltage-clamp pulses were observed in nucleated patches from 16 out of 23 cells. Further, these somatic PICs display facilitation in response to conditioning depolarization as previously observed in whole-cell recordings from intact neurons. Pharmacological tests suggest that the PICs were primarily mediated by Cav1 channels. Our results show that many of the features of persistent calcium currents recorded from intact motoneurons do not necessarily reflect a remote dendritic origin, but can also be ascribed to the intrinsic properties of their Cav1 channels.
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