Pii: S0304-3940(01)01926-7

This study tested the time-of-day dependence of the intrinsic postsynaptic properties of hippocampal CA3 pyramidal neurons. High-voltage-activated Ca currents and the Caand voltage-dependent afterhyperpolarizations were examined in slices of rat brains obtained at four distinct time periods. Just after onset of the dark phase, the steady-state amplitude of the Ca current (21.24 ^ 0.11 nA) was signi®cantly greater (P , 0:03) than that of the light phase (20.84 ^ 0.06 nA). Over the entire time range, the amplitude of the Ca current correlated with plasma corticosterone levels in a U-shaped function. Furthermore, depolarization-induced excitability during the dark phase exhibited an increased spike after depolarization (3.1 ^ 0.1 mV) and a slower adaptation of the ®ring frequency (146 ^ 18%). These ®ndings point to a dynamic time-of-day dependence of the CA3 neuronal properties and postsynaptic Ca currents. q 2001 Published by Elsevier Science Ireland Ltd.