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* To whom correspondence should be addressed. E-mail: ed.dudek{at}colostate.edu.
The persistent (i.e., slowly inactivating) fraction of the Na current (INa,P) regulates excitability of CNS neurons. In isolated rat suprachiasmatic nucleus (SCN) neurons with a ramp-type voltage-clamp protocol, we have studied the properties of a robust current (INa,S) that has the general properties of INa,P, with a slow inactivation. The time dependence of the development of the inactivation was also studied by clamping of the membrane potential at different levels: time constants ranging from ~ 50 to ~ 700 ms, depending on the voltage level, were revealed. The INa,S (50-150 pA) was present in both spontaneously active and silent neurons. The neurons exhibited INa,S without visible rundown during ~1-h recordings. INa,S had a threshold between -65 and -60 mV, and was maximal at about -45 mV. Tetrodotoxin (TTX) (1 µM) completely and reversibly blocked INa,S. Riluzole, an effective blocker of INa,P, inhibited reversibly INa,S with an EC50 of 1-2 µM. Microapplication of 10 µM riluzole during either extracellular or intracellular recording suppressed spontaneous activity in isolated SCN neurons. In the slice preparation, bath application of 20 µM riluzole resulted in decreased firing rate or complete suppression of spontaneous activity in some neurons, but had no effect on other neurons. In riluzole-resistant neurons in cell-attached experiments, low-amplitude current spikes were present in 1 µM TTX. We concluded that INa,S is ubiquitously expressed by all SCN neurons, and that this current is a necessary but not sufficient depolarizing component of the mechanism for spontaneous firing.
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