NEURONAL NICOTINIC ACETYLCHOLINE-RECEPTORS ARE BLOCKED BY INTRACELLULAR SPERMINE IN A VOLTAGE-DEPENDENT MANNER

A common feature of neuronal nicotinic acetylcholine receptors (nAChRs ) is that they conduct inward current at negative membrane potentials but little outward current at positive membrane potentials, a property referred to as inward rectification. Physiologically, inward rectific ation serves important functions, and the main goal of our study was t o investigate the mechanisms underlying the rectification of these rec eptors. We examined recombinant alpha(3) beta(4), and alpha(4) beta(2) neuronal nAChR subtypes expressed in Xenopus oocytes and native nAChR s expressed on superior cervical ganglion (SCG) neurons. Whole-cell AC h-evoked currents recorded from these receptors exhibited strong inwar d rectification. In contrast, we showed that single-channel currents f rom these neuronal nAChRs measured in outside-out patches outwardly re ctify. On the basis of recent findings that spermine, a ubiquitous int racellular polyamine, confers rectification to glutamate receptors and inwardly rectifying potassium channels, we investigated whether sperm ine causes neuronal nAChRs to inwardly rectify. When spermine was adde d to the patch electrode in outside-out recordings, it caused a concen tration-and voltage-dependent block of ACh-evoked single-channel curre nts. Using these single-channel data and physiological concentrations of intracellular spermine, we could account for the inward rectificati on of macroscopic whole-cell ACh-evoked conductance-voltage relationsh ips. Therefore, we conclude that the voltage-dependent block by intrac ellular spermine underlies inward rectification of neuronal nAChRs. We also found that extracellular spermine blocks both alpha(3) beta(4) a nd alpha(4) beta(2) receptors; this finding points to a mechanism wher eby increases in extracellular spermine, perhaps during pathological c onditions, could selectively block these receptors.