The cytosolic Ca concentration in acutely dissociated subfornical organ (SFO) neurons of rats: Spontaneous Ca oscillations and Ca oscillations induced by picomolar concentrations of angiotensin II.

Characteristics of subfornical organ (SFO) neurons were examined by measuring the cytosolic Ca concentration ([Ca]) in acutely dissociated neurons of the rat. SFO neurons, defined by the responsiveness to 50 mM K (n = 67) responded to glutamate (86%), angiotensin II (AII) (50%), arginine vasopressin (AVP) (66%) and/or carbachol (CCh) (61%), at their maximal concentrations, with marked increases in [Ca]. More than a half (174/307) of SFO neurons examined exhibited spontaneous Ca oscillations, while the remainder showed a relatively stable baseline under unstimulated conditions. Spontaneous Ca oscillations were suppressed when extracellular Ca was removed and were inhibited when extracellular Na was replaced with equimolar N-methyl-D-glucamine. Ca oscillations were unaffected by the inhibitor of Ca-dependent ATPases cyclopiazonic acid, the N-type Ca channel blocker ω-conotoxin GVIA and the P/Q-type Ca channel blocker ω-agatoxin IVA, but significantly inhibited by the high-voltage-activated Ca channel blocker Cd and the L-type Ca channel blocker nicardipine. Ca oscillations were also completely arrested by the voltage-gated Na channel blocker tetrodotoxin in 50% of SFO neurons but only partially in the remaining neurons. These results suggest that SFO neurons exhibit spontaneous membrane Ca oscillations that are dependent in part on Ca entry through L-type Ca channels, whose activation may result from burst firing. Moreover, AII at picomolar concentrations induced Ca oscillations in neurons showing no spontaneous Ca oscillations, while spontaneous Ca oscillations were arrested by gamma-aminobutyric acid (10 μM), suggesting that rises in [Ca] during Ca oscillations may play an important role in the modulation of SFO neuron function.