GALANIN REDUCES CARBACHOL STIMULATION OF PHOSPHOINOSITIDE TURNOVER IN RAT VENTRAL HIPPOCAMPUS BY LOWERING CA2+ INFLUX THROUGH VOLTAGE-SENSITIVE CA2+ CHANNELS

The 29-amino-acid peptide galanin (GAL) caused concentration-dependent inhibition of the accumulation of H-3-inositol phosphates (H-3-InsPs) induced by the muscarinic agonist carbachol (CARB; 10(-3)-10(-5) M) 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-mu-M) did not affect alpha-adrenergic or serotonergic type 2 receptor-mediated phosphoinositide (PI) breakdown in the same tissue. GAL by itself neither acted on basal levels of H-3-InsPs nor affected muscarinic receptors in binding studies. Blockade of the T-, N-, and L-types of voltage-sensitive calcium channel (VSCC) with 200-mu-M Cd2+ reduced muscarinic receptor-mediated PI breakdown by 50% and abolished the inhibitory effect of GAL (1-muM). Reduction of the extracellular Ca2+ concentration from 1.3 mM to 0.49-mu-M abolished the GAL inhibition of CARB-stimulated PI hydrolysis. Ca2+ influx promoted by 18 mM K+ depolarization or by 1-mu-M 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-mu-M) potentiated the inhibitory effects of GAL without affecting muscarinic stimulation of PI breakdown. The neurotoxin omega-conotoxin (2-mu-M), a blocker of both L- and N-types of VSCC, by itself reduced CARB-mediated breakdown of PIs by approximately 25%, and when it was added before GAL (1-mu-M) there was no summation of the two individual inhibitory effects, a result suggesting a common site of action for GAL and omega-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.