Mechanisms of spontaneous calcium oscillations and action potentials in immortalized hypothalamic (GT1-7) neurons.

1. Individual immortalized gonadotropin-releasing hormone (GnRH)-secreting hypothalamic (GT1-7) neurons in semiconfluent cultures showed spontaneous oscillations in intracellular Ca2+ concentration ([Ca2+]i) as measured by video fluorescence microscopy and fura-2. In parallel experiments, GT1-7 neurons also showed spontaneous bursts of action potentials that were recorded as action currents from intact cells. The bursts of action currents occurred in characteristic patterns, suggesting an underlying rhythmic oscillation in membrane potential. 2. Depolarization with increased extracellular K+ evoked a concentration-dependent increase in the frequency of Ca2+ oscillations or a sustained plateau of increased [Ca2+]i in GT1-7 neurons. Increased extracellular K+ (30 mM) caused an initial increase in the frequency of action currents, after which they were reversibly abolished. 3. The Ca2+ channel blockers Ni2+ and nimodipine abolished Ca2+ oscillations, whereas nifedipine, gadolinium, omega-conotoxin and omega-agatoxin had no effect on Ca2+ oscillations. These results indicate that Ca2+ oscillations are generated by influx of Ca2+ through voltage-gated Ca2+ channels that are not sensitive to nifedipine and are not N-type or P-type channels. 4. Thapsigargin caused a small, transient rise in baseline [Ca2+]i but had no effect on Ca2+ oscillations. Caffeine and ryanodine had no effect on baseline [Ca2+]i or Ca2+ oscillations. These results indicate that the release of Ca2+ from inositol 1,4,5-trisphosphate (IP-3)-sensitive or caffeine sensitive intracellular stores does not play a major role in Ca2+ oscillations in GT1-7 neurons.(ABSTRACT TRUNCATED AT 250 WORDS)