Amplitude-dependent spike-broadening and enhanced Ca2+ signaling in GnRH-secreting neurons

In GnRH-secreting (GT1) neurons, activation of Ca2+-mobilizing receptors induces a sustained membrane depolarization that shifts the profile of the action potential (AP) waveform from sharp, high-amplitude to broad, low-amplitude spikes. Here we characterize this shift in the firing pattern and its impact on Ca2+ influx experimentally by using prerecorded sharp and broad APs as the voltage-clamp command pulse. As a quantitative test of the experimental data, a mathematical model based on the membrane and ionic current properties of GT1 neurons was also used. Both experimental and modeling results indicated that inactivation of the tetrodotoxin-sensitive Na+ channels by sustained depolarization accounted for a reduction in the amplitude of the spike upstroke. The ensuing decrease in tetraethylammonium-sensitive K+ current activation slowed membrane repolarization, leading to AP broadening. This change in firing pattern increased the total L-type Ca2+ current and facilitated AP-driven Ca2+ entry. The leftward shift in the current-voltage relation of the L-type Ca2+ channels expressed in GT1 cells allowed the depolarization-induced AP broadening to facilitate Ca2+ entry despite a decrease in spike amplitude. Thus the gating properties of the L-type Ca2+ channels expressed in GT1 neurons are suitable for promoting AP-driven Ca2+ influx in receptor- and non-receptor-depolarized cells.