Kynurenate treatment of autaptic hippocampal microcultures affect localized voltage-dependent calcium diffusion in the dendrites

It is not clear how different spatial compartments in the neuron are affected during epileptiform activity. In the present study we have examined the spatial and temporal profiles of depolarization induced changes in the intracellular Ca²⁺ concentration in the dendrites of cultured autaptic hippocampal pyramidal neurons rendered epileptic experimentally by treatment with kynurenate (2 mM) and Mg²⁺ (11.3 mM) in culture (treated neurons). This was examined with simultaneous somatic patch-pipette recording and Ca²⁺ imaging experiments using the Ca²⁺ indicator Oregon Green 488 BAPTA-1. Neurons stimulated by depolarization under whole-cell voltage clamp conditions revealed Ca²⁺ entry at localized sites in the dendrites. Ca²⁺ transients were observed even in the presence of NMDA and AMPA receptor antagonists suggesting that the opening of voltage gated calcium channels primarily triggered the local Ca²⁺ changes. Peak Ca²⁺ transients in the dendrites of treated neurons were larger compared to the signals recorded from the control neurons. Dendritic Ca²⁺ transients in treated neurons showed a distance dependent scaling. Estimation of dendritic local Ca²⁺ diffusion coefficients indicated higher values in the treated neurons and a higher availability of free Ca²⁺. Simulation studies of Ca²⁺ dynamics in these localized dendritic compartments indicate that local Ca²⁺ buffering and removal mechanisms may be affected in treated neurons. Our studies indicate that small dendritic compartments are rendered more vulnerable to changes in intracellular Ca²⁺ following induction of epileptiform activity. This can have important cellular consequences including local membrane excitability through mechanisms that remain to be elucidated.