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.