Glucocorticoid actions on synaptic plasma membranes: Modulation of dihydropyridine-sensitive calcium channels

Abstract

We have previously shown that glucocorticoids accelerate depolarization-induced 45Ca 2+ influx in synaptosomes isolated from rat cerebral cortex, indicating that the steroids may modulate voltage-dependent Ca 2+ channels. The present study was undertaken to characterize the biochemical action of glucocorticoids on dihydropyridine-sensitive voltage-dependent Ca 2+ channels known to be present in brain synaptosomes. The [ 3H]dihydropyridine labeled site was used as a marker to determine the levels of functional Ca 2+ channels. No effect on equilibrium binding of [ 3H]PN 200-110 was found when membranes from disrupted synaptosomes were incubated with corticosterone as high as 1 μM. However, when intact synaptosomes were first incubated with corticosterone at 37°C and then disrupted, a significant increase in [ 3H]PN 200-110 binding was found. Steroid incubation of synaptosomes at 0°C was ineffective. It appears that metabolic processes requiring intracellular factors were involved in the steroid action. In examining this possibility, [ 3H]PN 200-110 binding was activated in disrupted membranes by MgATP and Ca 2+-calmodulin, and corticosterone was found to enhance the activation in a concentration-dependent manner. [ 3H]PN 200-110 binding to membranes was also activated by incubation with MgATP and cAMP-dependent protein kinase, but this activation was not enhanced by the steroid. These findings are consistent with the interpretation that the steroid promotes Ca 2+ channel activity by enhancing calmodulin-dependent activation of the channels. The action on voltage-dependent Ca 2+ channels in synaptic terminals may well be a mechanism whereby glucocorticoids modulate neuronal activity.