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 Ca2+ influx in differentiated GnRH-secreting (GT1) neurons revealed that most cells exhibited spontaneous, extracellular Ca(2+)-dependent action potentials (APs). Spiking was initiated by a slow pacemaker depolarization from a baseline potential between -75 and -50 mV, and AP frequency increased with membrane depolarization. More hyperpolarized cells fired sharp APs with limited capacity to promote Ca2+ influx, whereas more depolarized cells fired broad APs with enhanced capacity for Ca2+ influx. Characterization of the inward currents in GT1 cells revealed the presence of tetrodotoxin-sensitive Na+, Ni(2+)-sensitive T-type Ca2+, and dihydropyridine-sensitive L-type Ca2+ components. The availability of Na+ and T-type Ca2+ channels was dependent on the baseline potential, which determined the activation/inactivation status of these channels. Whereas all three channels were involved in the generation of sharp APs, L-type channels were solely responsible for the spike depolarization in cells exhibiting broad APs. Activation of GnRH receptors led to biphasic changes in cytosolic Ca2+ concentration ([Ca2+]i), with an early, extracellular Ca(2+)-independent peak and a sustained, extracellular Ca(2+)-dependent phase. During the peak [Ca2+]i response, electrical activity was abolished due to transient hyperpolarization. This was followed by sustained depolarization of cells and resumption of firing of increased frequency with a shift from sharp to broad APs. The GnRH-induced change in firing pattern accounted for about 50% of the elevated Ca2+ influx, the remainder being independent of spiking. Basal [Ca2+]i was also dependent on Ca2+ influx through AP-driven and voltage-insensitive pathways. Thus, in both resting and agonist-stimulated GT1 cells, membrane depolarization limits the participation of Na+ and T-type channels in firing, but facilitates AP-driven Ca2+ influx.
Van Goor
F
F
Endocrinology and Reproduction Research Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892, USA.
Krsmanovic
L Z
LZ
Catt
K J
KJ
Stojilkovic
S S
SS
D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology; D000200:Action Potentials / Q000187:drug effects / Q000502:physiology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002120:Calcium Channel Agonists / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D020013:Calcium Signaling; D002462:Cell Membrane / Q000378:metabolism; D004594:Electrophysiology; D007987:Gonadotropin-Releasing Hormone / Q000378:metabolism; D007031:Hypothalamus / Q000378:metabolism; D051379:Mice; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D009543:Nifedipine / Q000494:pharmacology; D015222:Sodium Channels / Q000378:metabolism*; D019789:Tetraethylammonium / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology
D002120:Calcium Channel Agonists; D002121:Calcium Channel Blockers; D015220:Calcium Channels; D015222:Sodium Channels; D007987:Gonadotropin-Releasing Hormone; D013779:Tetrodotoxin; D019789:Tetraethylammonium; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; D009543:Nifedipine; D002118:Calcium