Calbindin-D^28K (CaBP) levels and calcium currents in acutely dissociated epileptic neurons

Summary

Nerve cells that lack the cytoplasmic Ca²⁺ binding protein Calbindin-D_28K (CaBP) appear to be selectively vulnerable to Ca²⁺-related injury consistent with a postulated intraneuronal Ca²⁺-buffering role of CaBP. We have confirmed the selective loss of CaBP from the dentate gyrus during kindling-induced epilepsy in acutely dissociated granule cells (GCs) from kindled rats. Immunohistochemically stained kindled neurons showed a significant loss of CaBP when compared to controls (p < 0.001; ANOVA). The Ca²⁺-buffering role of CaBP was assessed in acutely dissociated control and kindled GCs by examining a physiological process highly sensitive to intracellular Ca²⁺-buffering: the Ca²⁺ -dependent inactivation of high-voltage activated (HVA or L-type) Ca²⁺ currents in the absence (or presence) of exogenous Ca²⁺-chelators. Whole-cell patch clamp recordings in kindled GCs demonstrated a markedly enhanced Ca²⁺-dependent inactivation of Ca²⁺-currents. After brief conditioning Ca²⁺ currents, in the absence of an exogenous intraneuronal Ca²⁺-chelator, subsequent test Ca²⁺ currents were inactivated by 58.3% in kindled GCs, a significant increase from the 37.4% inactivation observed in control GCs (p< 0.005; ANOVA). The differential Ca²⁺ current decay and Ca²⁺-dependent inactivation were prevented in both control and kindled GCs upon loading the neurons with the exogenous Ca²⁺-chelator BAPTA. These experiments demonstrate a high correlation between the loss of CaBP and changes in Ca²⁺ current inactivation and are consistent with the hypothesis that CaBP contributes to the physiological Ca²⁺-buffering in mammalian neurons.