CELLULAR MECHANISMS UNDERLYING SPONTANEOUS FIRING IN RAT SUPRACHIASMATIC
NUCLEUS: INVOLVEMENT OF A SLOWLY INACTIVATING COMPONENT OF SODIUM CURRENT
C. M. A. Pennartz, M. A. Bierlaagh and A. M. S. Geurtsen
Graduate School Neurosciences Amsterdam,
Netherlands Institute for Brain Research,
Meibergdreef 33, 1105 AZ, Amsterdam,
the Netherlands
APStracts 4:0087N, 1997.
ABSTRACT
Neurons constituting the pacemaker of circadian rhythms, located in the
suprachiasmatic nucleus, generate spontaneous firing patterns that change
across the day/night cycle. Their average spontaneous firing rate is
considered an important functional marker of clock activity, since it is
highest during daytime and low at night. In this study we investigated the
ionic mechanisms underlying spontaneous firing in acutely prepared slices and
dissociated neurons of the suprachiasmatic nucleus.
In current clamp mode, spontaneous action potentials were consistently
preceded by depolarizing ramps. These ramps were Na+ dependent, sensitive to
tetrodotoxin and disappeared upon hyperpolarization. Ramps and associated
spikes were not abolished by blockers of the H-current (1 mM cesium) or
calcium currents (50 mM nickel or 200 æM cadmium). In voltage-clamped neurons
in slices or dissociated neurons, TTX-sensitive and Na+ dependent inward
current was observed to activate well below firing threshold (-60 to -50 mV).
The low-threshold component of Na+ current inactivated slowly as compared to
the fast component that mediates action potentials. However, its inactivation
proceeded more rapidly than has been reported for the persistent Na+ current
in cortical structures. Persistent Na+ current was generally absent or small
in amplitude. The voltage dependence and kinetics of the slowly inactivating
component of Na+ current are consistent with the hypothesis that it is
partially deinactivated during spike afterhyperpolarizations and contributes
significantly to subsequent depolarizing ramps. These observations implicate
the slowly inactivating component of Na+ current in ionic mechanisms governing spontaneous firing in suprachiasmatic nucleus neurons.
Received 4 March 1997; accepted in final form 6 June 1997.
APS Manuscript Number J189-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 15 July 1997