Calcineurin enhances L-type Ca²⁺ channel activity in hippocampal neurons: Increased effect with age in culture

The Ca²⁺/calmodulin-dependent protein phosphatase, calcineurin, modulates a number of key Ca²⁺ signaling pathways in neurons, and has been implicated in Ca²⁺-dependent negative feedback inactivation of N-methyl-D-aspartate receptors and voltage-sensitive Ca²⁺ channels. In contrast, we report here that three mechanistically disparate calcineurin inhibitors, FK-506, cyclosporin A, and the calcineurin autoinhibitory peptide, inhibited high-voltage-activated Ca²⁺ channel currents by up to 40% in cultured hippocampal neurons, suggesting that calcineurin acts to enhance Ca²⁺ currents. This effect occurred with Ba²⁺ or Ca²⁺ as charge carrier, and with or without intracellular Ca²⁺ buffered by EGTA. Ca²⁺-dependent inactivation of Ca²⁺ channels was not affected by FK-506. The immunosuppressant, rapamycin, and the protein phosphatase 1/2A inhibitor, okadaic acid, did not decrease Ca²⁺ channel current, showing specificity for effects on calcineurin. Blockade of L-type Ca²⁺ channels with nimodipine fully negated the effect of FK-506 on Ca²⁺ channel current, while blockade of N-, and P-/Q-type Ca²⁺ channels enhanced FK-506-mediated inhibition of the remaining L-type-enriched current. FK-506 also inhibited substantially more Ca²⁺ channel current in 4-week-old vs. 2-week-old cultures, an effect paralleled by an increase in calcineurin A mRNA levels. These studies provide the first evidence that calcineurin selectively enhances L-type Ca²⁺ channel activity in neurons. Moreover, this action appears to be increased concomitantly with the well-characterized increase in L-type Ca²⁺ channel availability in hippocampal neurons with age-in-culture.