Control of action potential-driven calcium influx in GT1 neurons by the activation status of sodium and calcium channels

An analysis of the relationship between electrical membrane activity and Ca 2+ influx in differentiated GnRH-secreting (GT1) neurons revealed that most cells exhibited spontaneous, extracellular Ca2+-dependent action potential s (APs). Spiking was initiated by a slow pacemaker depolarization from a ba seline potential between -75 and -50 mV, and AP frequency increased with me mbrane depolarization. More hyperpolarized cells fired sharp APs with limit ed capacity to promote Ca2+ influx, whereas more depolarized cells fired br oad APs with enhanced capacity for Ca2+ influx. Characterization of the inw ard currents in GT1 cells revealed the presence of tetrodotoxin-sensitive N a+, Ni2+-sensitive T-type Ca2+, and dihydropyridine-sensitive L-type Ca2+ c omponents. The availability of Na+ and T-type Ca2+ channels was dependent o n the baseline potential, which determined the activation/inactivation stat us of these channels. Whereas all three channels were involved in the gener ation of sharp APs, L-type channels were solely responsible for the spike d epolarization in cells exhibiting broad APs. Activation of GnRH receptors l ed to biphasic changes in cytosolic Ca2+ concentration ([Ca2+](i)), with an early, extracellular Ca2+-independent peak and a sustained, extracellular Ca2+-dependent phase. During the peak [Ca2+](i) response, electrical activi ty was abolished due to transient hyperpolarization. This was followed by s ustained depolarization of cells and resumption of firing of increased freq uency with a shift from sharp to broad APs. The GnRH-induced change in firi ng pattern accounted for about 50% of the elevated Ca2+ influx, the remaind er being independent of spiking. Basal [Ca2+](i) was also dependent on Ca2 influx through AP-driven and voltage-insensitive pathways. Thus, in both r esting and agonist-stimulated GT1 cells, membrane depolarization limits the participation of Na+ and T-type channels in firing, but facilitates AP-dri ven Ca2+ influx.