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1 Centre for Neuroscience, University of Alberta, Edmonton, AB, Canada
* To whom correspondence should be addressed. E-mail: bennettd{at}ualberta.ca.
Months after sacral spinal transection in rats (chronic spinal rats), motoneurons below the injury exhibit large, low-threshold persistent inward currents (PICs), composed of persistent sodium currents (Na PICs) and persistent calcium currents (Ca PICs). Here, we investigated whether motoneurons of normal adult rats also exhibited Na and Ca PICs when the spinal cord was acutely transected at the sacral level (acute spinal rats) and examined the role of the Na PIC in firing behaviour. Intracellular recordings were obtained from motoneurons of acute and chronic spinal rats while the whole sacrocaudal spinal cord was maintained in vitro. Compared to chronic spinal rats, motoneurons of acute spinal rats were more difficult to activate because the input resistance was 22% lower and resting membrane potential was hyperpolarized 4.1 mV further below firing threshold (-50.9 ± 6.2 mV). In acute spinal rats, during a slow voltage ramp, a PIC was activated subthreshold to the spike (at -57.2 ± 5.0 mV) and reached a peak current of 1.11 ± 1.21 nA. This PIC was less than half the size of that in chronic spinal rats (2.79 ± 0.94 nA), and usually was not large enough to produce bistable behaviour (plateau potentials and self-sustained firing not present), unlike in chronic spinal rats. The PIC was composed of two components: a TTX-sensitive Na PIC (0.44 ± 0.36 nA) and a nimodipine-sensitive Ca PIC (0.78 ± 0.82 nA), both smaller than in chronic spinal rats (but with similar Na/Ca ratio). The presence of the Na PIC was critical for normal repetitive firing, because no detectable Na PIC was found in the few motoneurons that could not fire repetitively during a slow ramp current injection, and motoneurons that had large Na PICs more readily produced repetitive firing and had lower minimum firing rates, compared to neurons with small Na PICs. Furthermore, when the Na PIC was selectively blocked with riluzole, steady repetitive firing was eliminated, even though transient firing could be evoked on a rapid current step and the spike itself was unaffected. In summary, only small Ca and Na PICs occur in acute spinal motoneurons, and the Na PIC is essential for steady repetitive firing. We discuss how availability of monoamines may explain the variability in Na PICs and firing in the normal and spinal animals.
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