Vol. 302, Issue 3, 1176-1183, September 2002

Methyl-Laudanosine: A New Pharmacological Tool to Investigate the Function of Small-Conductance Ca²⁺-Activated K⁺ Channels

Jacqueline Scuvee-Moreau, Jean-François Liegeois, Laurent Massotte and Vincent Seutin

Research Center for Cellular and Molecular Neurobiology, Laboratory of Pharmacology (J.S.-M., L.M., V.S.), and Natural and Synthetic Drugs Research Center, Laboratory of Medicinal Chemistry (J.-F.L.), University of Liège, Liège, Belgium

Small-conductance Ca²⁺-activated K⁺ channels (SK channels) underlie the prolonged postspike afterhyperpolarization (AHP) observed in many central neurons and play an important role in modulating neuronal activity. However, a lack of specific and reversible blockers of these channels hampers their study in various experimental conditions. Because previous work has shown that bicuculline salts block these channels, we examined whether related alkaloids, namely laudanosine quaternary derivatives, would produce similar effects. Intracellular recordings were performed on rat midbrain dopaminergic neurons and hippocampus CA1 pyramidal cells. Binding experiments were performed on rat cerebral cortex membranes. Laudanosine, methyl-laudanosine, and ethyl-laudanosine blocked the apamin-sensitive AHP of dopaminergic neurons with mean IC₅₀ values of 152, 15, and 47 µM, respectively. The benzyl and butyl derivatives were less potent. Methyl-laudanosine had no effect on the I_h current, action potential parameters, or membrane resistance of dopaminergic cells, or on the decrease in input resistance induced by muscimol, indicating a lack of antagonism at GABA_A receptors. Interestingly, 100 µM methyl-laudanosine induced a significant increase in spiking frequency of dopaminergic neurons but not of CA1 pyramidal cells, suggesting the possibility of regional selectivity. Binding experiments on laudanosine derivatives were in good agreement with electrophysiological data. Moreover, methyl-laudanosine has no affinity for voltage-gated potassium channels, and its affinity for SK channels (IC₅₀ 4 µM) is superior to its affinity for muscarinic (IC₅₀ 114 µM) and neuronal nicotinic (IC₅₀ 367 µM) receptors . Methyl-laudanosine may be a valuable pharmacological tool to investigate the role of SK channels in various experimental models.