Calcium Dependence of Somatostatin (SRIF) Release and Cyclic AMP Levels in Cultured Diencephalic Neurons

Abstract

Calmodulin has been reported to be involved in the Ca²⁺-dependent hypothalamus in vitro. The present experiments were undertaken to determine whether at an early stage of development (in diencephalic primary cultures secreting SRIF, on the 11^th day) the activation of a Ca^2+-calmodulin kinase system is also involved in the release of the peptide. Since a calmodulin-dependent adenylate cyclase activity has been detected in the brain, we measured intracellular cyclic AMP accumulation as an additional parameter of calmodulin activity. SRIF release and cyclic AMP accumulation were stimulated by K⁺ (56 m M) and by the Ca²⁺ ionophores ionomycin (0.5 µ M) and A 23187 (in a dose-dependent manner). Incubation of cells in Ca²⁺-free Locke medium or in the presence of Co²⁺ (1 m M) completely blocked ionophore-induced SRIF release and cyclic AMP accumulation. Three calmodulin antagonists (calmidazolium, W-7, and chlorpromazine) and two blockers of calmodulin-dependent kinase (phenytoin and diazepam) were tested on evoked-SRIF release and cyclic AMP formation. Neither W-7 nor calmidazolium modified A 23187-induced SRIF release at any dose tested, although they inhibited, in a dose-dependent manner, the stimulatory effect of the Ca²⁺ ionophore on cyclic AMP accumulation. High concentrations of chlorpromazine (100 µ M) were needed to decrease A 23187-induced SRIF release by 45% whereas the same concentration completely blocked the cyclic AMP accumulation evoked by the ionophore (1 µ M). K⁺-induced SRIF release was not modified by 0.5 µ M calmidazolium but was inhibited by rather high concentrations of W-7 (100 µ M); both antagonists significantly decreased K⁺-evoked cyclic AMP accumulation. Phenytoin (1 µ M to 100 µ M) and diazepam (20 µ M) failed to block the stimulatory effect of A 23187 (0.5 µ M) on either parameter. The present results: (1) confirm the Ca²⁺-dependence of SRIF release in fetal diencephalic cells but without the participation of calmodulin-dependent mechanisms; (2) show that agents which increase cytosolic calcium also stimulate cyclic AMP formation, probably by stimulating a calmodulin-dependent adenylate cyclase, and (3) suggest that a relationship between these two events is unlikely in the model used here.