Functional characterization of the H-current in SCN neurons in subjective day and night: a whole-cell patch-clamp study in acutely prepared brain slices

Neurons of the rat suprachiasmatic nucleus (SCN) exhibit a circadian rhythm in spontaneous firing rate. In this whole-cell patch-clamp study in slices, we examined the possibility that H-current (I-H) contributes to the spontaneous firing rate of SCN neurons. Most of our experiments were performed during the subjective day, because this is the time epoch during which one would expect the largest excitatory effect of I-H if it were to fluctuate in a circadian rhythm. Current-clamp experiments showed that blockade of I-H by Cs+ (1 mM) did not influence the spontaneous firing rate and resting membrane potential. Voltage-clamp experiments revealed that I-H, when activated at the resting membrane potential, is probably too small in magnitude and too slow in activation to make a significant contribution to the spontaneous firing rate. Both results suggest that I-H does not significantly contribute to the spontaneous firing of SCN neurons. In addition, we investigated whether the kinetics and voltage dependence of I-H were modulated in a circadian manner. However, no substantial day-night differences in I-H were found. We conclude that I-H, as recorded in whole-cell mode, does not contribute significantly to spontaneous firing in most SCN neurons and that this current, is more likely to be involved in 'rescuing' SCN neurons from large and long-lasting hyperpolarizations by depolarizing the membrane. (C) 1997 Elsevier Science B.V.