Effect of dantrolene on KCl- or NMDA-induced intracellular Ca²⁺ changes and spontaneous Ca²⁺ oscillation in cultured rat frontal cortical neurons

Dantrolene has been known to affect intracellular Ca²⁺ concentration ([Ca²⁺](i)) by inhibiting Ca²⁺ release from intracellular stores in cultured neurons. We were interested in examining this property of dantrolene in influencing the [Ca²⁺](i) affected by the NMDA receptor ligands, KCl, L-type Ca²⁺ channel blocker nifedipine, and two other intracellular Ca²⁺-mobilizing agents caffeine and bradykinin. Effect of dantrolene on the spontaneous oscillation of [Ca²⁺](i) was also examined. Dantrolene in μM concentrations dose-dependently inhibited the increase in [Ca²⁺](i) elicited by NMDA and KCl. AP-5, MK-801 (NMDA antagonists), and nifedipine respectively reduced the NMDA and KCl-induced increase in [Ca²⁺](i). Dantrolene, added to the buffer solution together with the antagonists or nifedipine, caused a further reduction in [Ca²⁺](i) to a degree similar to that seen with dantrolene alone inhibiting the increase in [Ca²⁺](i) caused by NMDA or KCl. At 30 μM, dantrolene partially inhibited caffeine-induced increase in [Ca²⁺](i) whereas it has no effect on the bradykinin-induced change in [Ca²⁺](i). The spontaneous oscillation of [Ca²⁺](i) in frontal cortical neurons was reduced both in amplitude and in base line concentration in the presence of 10 μM dantrolene. Our results indicate that dantrolene's mobilizing effects on intracellular Ca²⁺ stores operate independently from the influxed Ca²⁺ and that a component of the apparent increase in [Ca²⁺](i) elicited by NMDA or KCl represents a dantrolene-sensitive Ca²⁺ release from intracellular stores. Results also suggest that dantrolene does not affect the IP₃-gated release of intracellular Ca²⁺ and that the spontaneous Ca²⁺ oscillation is, at least partially, under the control of Ca²⁺ mobilization from internal stores.