Ž . Brain Research 767 1997 72–80 Research report 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 Marcel T.G. de Jeu ) , Cyriel M.A. Pennartz Netherlands Institute for Brain Research, Meibergdreef 33, 1105 AZ Amsterdam, Netherlands Accepted 6 May 1997 Abstract Ž . Neurons of the rat suprachiasmatic nucleus SCN exhibit a circadian rhythm in spontaneous firing rate. In this whole-cell patch-clamp Ž . study in slices, we examined the possibility that H-current I contributes to the spontaneous firing rate of SCN neurons. Most of our H experiments were performed during the subjective day, because this is the time epoch during which one would expect the largest q Ž excitatory effect of I if it were to fluctuate in a circadian rhythm. Current-clamp experiments showed that blockade of I by Cs 1 H H . mM did not influence the spontaneous firing rate and resting membrane potential. Voltage-clamp experiments revealed that I , when H 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 does not significantly contribute to the spontaneous firing of SCN H neurons. In addition, we investigated whether the kinetics and voltage dependence of I were modulated in a circadian manner. However, H no substantial day–night differences in I were found. We conclude that I , as recorded in whole-cell mode, does not contribute H H 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. q 1997 Elsevier Science B.V. Keywords: Suprachiasmatic nucleus; Circadian rhythm; Brain slice; Whole-cell patch-clamp; H-current; Spontaneous firing; Spike afterhyperpolarization 1. Introduction It has been repeatedly demonstrated that the suprachias- Ž . matic nucleus SCN of the hypothalamus in mammals w x generates a circadian rhythm 19,21,26,27,34 . Electro- physiological studies have revealed the existence of circa- dian rhythmicity in spontaneous firing rate of SCN neu- w x rons 4,5,12,13,15 . Blocking the neuronal firing of SCN neurons does not prevent the circadian clock from running w x 28,29,35 , indicating that these neurons have an intrinsic mechanism for circadian rhythmogenesis. The intracellular mechanism that is responsible for the expression of circa- dian rhythms in spontaneous firing rate is not yet known. One prominent feature of many SCN neurons is the presence of a time- and voltage-dependent inward current, w x q q I 1,16 . I is a mixed Na rK current, which is H H w x activated by membrane hyperpolarization 7–9,20,24,31 . Upon hyperpolarization of the cell, the net influx of posi- ) Ž . Corresponding author. Fax: q31 20 6961006; E-mail: M.de.Jeu@nih.knaw.nl tively charged ions makes a slow excitatory contribution to the membrane potential and will rectify it back towards w x rest 10,14,20,31 . It has been suggested by several authors w x 14,20,22,31 that the physiological role of this current is to counterbalance transient or prolonged membrane hyper- polarizations in order to keep the membrane potential near the firing threshold. Since the reversal potential of I is H w x estimated to lie between y50 and y20 mV 24 , there is a distinct possibility that I contributes positively to the H spontaneous firing rate. Results of McCormick and Pape wx 7 indicate that I indeed promotes spontaneous firing in H thalamic relay neurons by keeping episodes of hyperpolar- ization limited in duration. wx Akasu and coworkers 1 reported that I may con- H tribute to the spontaneous firing mechanism in SCN neu- rons by shortening the duration of the spike afterhyperpo- Ž . larization AHP , an effect which also reduces the inter- spike interval. This contribution of I to the spontaneous H firing rate may vary in a circadian manner and thereby generate the circadian rhythm of spontaneous firing rate in SCN neurons. In this study we reassessed the contribution of I to the spontaneous firing rate of SCN neurons and H 0006-8993r97r$17.00 q 1997 Elsevier Science B.V. All rights reserved.