Tetraethylammonium (TEA) increases the inactivation time constant of the transient K+ current in suprachiasmatic nucleus neurons

Identifying the mechanisms that drive suprachiasmatic nucleus (SCN) neurons to fire action potentials with a higher frequency during the day than during the night is an important goal of circadian neurobiology. Selective chemical tools with defined mechanisms of action on specific ion channels are required for successful completion of these studies. The transient K+ current (IA) plays an active role in neuronal action potential firing and may contribute to modulating the circadian firing frequency. Tetraethylammonium (TEA) is frequently used to inhibit delayed rectifier K+ currents (IDR) during electrophysiological recordings of IA. Depolarizing voltage-clamped hamster SCN neurons from a hyperpolarized holding potential activated both IA and IDR. Holding the membrane potential at depolarized values inactivated IA and only the IDR was activated during a voltage step. The identity of IA was confirmed by applying 4-aminopyridine (5 mM), which significantly inhibited IA. Reducing the TEA concentration from 40 mM to 1 mM significantly decreased the IA inactivation time constant (τinact) from 9.8 ± 3.0 ms to 4.9 ± 1.2 ms. The changes in IAτinact were unlikely to be due to a surface charge effect. The IA amplitude was not affected by TEA at any concentration or membrane potential. The isosmotic replacement of NaCl with choline chloride had no effect in IA kinetics demonstrating that the TEA effects were not due to a reduction of extracellular NaCl. We conclude that TEA modulates, in a concentration dependent manner, τinact but not IA amplitude in hamster SCN neurons.