Relationships Between Intracellular Calcium and Afterhyperpolarizations in Neocortical Pyramidal Neurons

Abstract

We examined the effects of recent discharge activity on [Ca²⁺]_iin neocortical pyramidal cells. Our data confirm and extend the observation that there is a linear relationship between plateau [Ca²⁺]_iand 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_AHPversus [Ca²⁺]_i. Our data suggest that sAHP channels require an elevation of [Ca²⁺]_iin 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.