T-type calcium channels mediate the transition between tonic and phasic firing in thalamic neurons

Thalamic neurons undergo a shift from tonic to phasic (burst) firing upon hyperpolarization. This state transition results from deinactivation of a regenerative depolarizing event referred to as the low-threshold spike. Isolated adult guinea pig thalamic (dorsal lateral geniculate) neurons exhibited low-threshold spikes that could be blocked by low concentrations of nickel but were unaffected by the dihydropyridine nimodipine. Whole-cell voltage-clamp recordings from these cells demonstrated a low-threshold, rapidly inactivating (T) Ca²⁺ current that manifested similar voltage dependency and time course as the low-threshold spike. Like low-threshold spikes, the T-type Ca²⁺ current was eliminated by nickel but was unaffected by nimodipine. In thalamic neurons, T-type Ca²⁺ channels underlie the low-threshold spike and, therefore, play a critical role in regulating the firing pattern of these cells.