Relationships between intracellular calcium and afterhyperpolarizations in neocortical pyramidal neurons

We examined the effects of recent discharge activity on [Ca2+](i) in neocortical pyramidal cells. Our data confirm and extend the observation that there is a linear relationship between plateau [Ca2+](i) and firing frequency in soma and proximal apical dendrites. The rise in [Ca2+] activates K+ channels underlying the afterhyperpolarization (AHP), which consists of 2 Ca2+-dependent components: the medium AHP (mAHP) and the slow AHP (sAHP). The mAHP is blocked by apamin, indicating involvement of SK-type Ca2+-dependent K+ channels. The identity of the apamin-insensitive sAHP channel is unknown. We compared the sAHP and the mAHP with regard to: 1) number and frequency of spikes versus AHP amplitude; 2) number and frequency of spikes versus [Ca2+](i); 3) I-AHP versus [Ca2+](i). Our data suggest that sAHP channels require an elevation of [Ca2+](i) in the cytoplasm, rather than at the membrane, consistent with a role for a cytoplasmic intermediate between Ca2(+) and the K+ channels. The mAHP channels appear to respond to a restricted Ca2+ domain.