Galanin Reduces Carbachol Stimulation of Phosphoinositide Turnover in Rat Ventral Hippocampus by Lowering Ca2+ Influx Through Voltage‐Sensitive Ca2+ Channels

Abstract: The 29‐amino‐acid peptide galanin (GAL) caused concentration‐dependent inhibition of the accumulation of 3H‐inositol phosphates (3H‐InsPs) induced by the muscarinic agonist carbachol (CARB; 10‐3‐10‐5M) in the presence of 5 mM lithium, specifically in tissue miniprisms from rat ventral hippocampus. The inhibitory effect of GAL involved the mono‐, bis‐, tris‐, and tetrakisphosphates formed during activation for 2 min of phospholipase C by CARB (1 mM) in the absence of lithium. GAL (1 μM) did not affect α‐adrenergic or serotonergic type 2 receptor‐mediated phosphoinositide (PI) breakdown in the same tissue. GAL by itself neither acted on basal levels of 3H‐InsPs nor affected muscarinic receptors in binding studies. Blockade of the T‐, N‐, and L‐types of voltage‐sensitive calcium channel (VSCC) with 200 μM Cd2+ reduced muscarinic receptor‐mediated PI breakdown by 50% and abolished the inhibitory effect of GAL (1 μM). Reduction of the extracellular Ca2+ concentration from 1.3 mM to 0.49 μM abolished the GAL inhibition of CARB‐stimulated PI hydrolysis. Ca2+ influx promoted by 18 mM K+ depolarization or by 1 μM Bay K 8644, a selective agonist of the L‐type VSCC, prevented the inhibitory effect of GAL. Blockade of the L‐type VSCC with nifedipine (1 μM) potentiated the inhibitory effects of GAL without affecting muscarinic stimulation of PI breakdown. The neurotoxin ω‐conotoxin (2 μM), a blocker of both L‐ and N‐types of VSCC, by itself reduced CARB‐mediated breakdown of PIs by ∼25%, and when it was added before GAL (1 μM) there was no summation of the two individual inhibitory effects, a result suggesting a common site of action for GAL and ω‐conotoxin. The data presented thus indicate that GAL modulation of muscarinic stimulation of the phospholipase C activity is mediated by a reduction of Ca2+ entry through VSCCs, presumably of the N type.