Paradoxical Effect of QX-314 on Persistent Inward Currents and Bistable Behavior in Spinal Motoneurons In Vivo

Abstract

Spinal motoneurons can exhibit bistable behavior, which consists of stable self-sustained firing that is initiated by a brief excitatory input and terminated by brief inhibitory input. This bistable behavior is generated by a persistent inward current ( I PIC). In cat motoneurons with low input conductances and slow axonal conduction velocities, I PIC exhibits little decay with time and thus self-sustained firing is long-lasting. In contrast, in cells that have high input conductances and fast conduction velocities, I PIC decays with time, and these cells cannot maintain long duration self-sustained firing. An alternative way to measure bistable behavior is to assess plateau potentials after the action potential has been blocked by intracellular injection of QX-314 to block sodium (Na+) currents. However, QX-314 also blocks calcium (Ca2+) currents and, because I PIC may be generated by a mixture of Ca2+ and Na+ currents, a reduction in amplitude of I PIC was expected. We therefore systematically compared the properties of I PIC in a sample of cells recorded with QX-314 to a control sample of cells without QX-314, which was obtained in a previous study. Single-electrode voltage-clamp techniques were applied in spinal motoneurons in the decerebrate cat preparation following administration of a standardized dose of the noradrenergic α1 agonist methoxamine. In the sample with QX-314, the average value of I PIC was only about half that in the control sample. However, the reduction of I PIC was much greater in cells with slow as compared with fast conduction velocities. Because a substantial portion of I PIC originates in dendritic regions and because conduction velocity covaries with the extent of the dendritic tree, this result suggests that QX-314 may fail to diffuse very far into the dendrites of the largest motoneurons. The analysis of the decay of I PIC and plateau potentials in cells with QX-314 also produced an unexpected result: QX-314 virtually eliminated time-dependent decay in both I PICand plateau potentials. Consequently, I PICbecame equally persistent in high and low input conductance cells. Therefore the decay in I PIC in high input conductance cells in the absence of QX-314 is not due to an intrinsic tendency of the underlying inward current to decay. Instead it is possible that the decay may result from activation of a slow outward current. Overall, these results show that QX-314 has a profound effect on I PIC and thus plateau potentials obtained using QX-314 do not accurately reflect the properties of I PIC in normal cells without QX-314.