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 r hythm in spontaneous firing rate. In this whole-cell patch-clamp study in slices, we examined the possibility that H-current (I-H) contribut es to the spontaneous firing rate of SCN neurons. Most of our experime nts 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 exper iments 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 activat ion to make a significant contribution to the spontaneous firing rate. Both results suggest that I-H does not significantly contribute to th e spontaneous firing of SCN neurons. In addition, we investigated whet her the kinetics and voltage dependence of I-H were modulated in a cir cadian manner. However, no substantial day-night differences in I-H we re found. We conclude that I-H, as recorded in whole-cell mode, does n ot 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 depolarizin g the membrane. (C) 1997 Elsevier Science B.V.