Influence of Ca2+-binding proteins and the cytoskeleton on Ca2+-dependent inactivation of high-voltage activated Ca2+ currents in thalamocortical relay neurons.

Ca2+-dependent inactivation (CDI) of high-voltage activated (HVA) Ca2+ channels was investigated in acutely isolated and identified thalamocortical relay neurons of the dorsal lateral geniculate nucleus (dLGN) by combining electrophysiological and immunological techniques. The influence of Ca2+-binding proteins, calmodulin and the cytoskeleton on CDI was monitored using double-pulse protocols (a constant post-pulse applied shortly after the end of conditioning pre-pulses of increasing magnitude). Under control conditions the degree of inactivation (34+/-9%) revealed a U-shaped and a sigmoid dependency of the post-pulse current amplitude on pre-pulse voltage and charge influx, respectively. In contrast to a high concentration (5.5 mM) of EGTA (31+/-3%), a low concentration (3 microM) of parvalbumin (20+/-2%) and calbindin(D28K) (24+/-4%) significantly reduced CDI. Subtype-specific Ca2+ channel blockers indicated that L-type, but not N-type Ca2+ channels are governed by CDI and modulated by Ca2+-binding proteins. These results point to the possibility that activity-dependent changes in the intracellular Ca2+-binding capacity can influence CDI substantially. Furthermore, calmodulin antagonists (phenoxybenzamine, 22+/-2%; calmodulin binding domain, 17+/-1%) and cytoskeleton stabilizers (taxol, 23+/-5%; phalloidin, 15+/-3%) reduced CDI. Taken together, these findings indicate the concurrent occurrence of different CDI mechanisms in a specific neuronal cell type, thereby supporting an integrated model of this feedback mechanism and adding further to the elucidation of the role of HVA Ca2+ channels in thalamic physiology.