Pulsed infrared releases Ca2+ from the endoplasmic reticulum of cultured spiral ganglion neurons

We investigated the effects of pulsed infrared radiation (IR, 1863 nm) stimulation on cytosolic [Ca2+] in inner ear spiral ganglion neurons cultured from day 4 postnatal mice and loaded with a fluorescent Ca2+ indicator (fluo-4, -5F or -5N). IR pulse trains (200 µs, 200-250 Hz, 2-5 s) delivered via an optical fiber coupled to IR source produced a rapid, transient temperature increase of 6-11oC (above a baseline of 24-30 oC) and evoked transient increases in both nuclear and cytosolic [Ca2+] of 0.20 - 1.4 µM, with a simultaneous reduction of [Ca2+] in regions containing endoplasmic reticulum (ER). IR-induced increases in cytosolic [Ca2+] continued in medium containing no added Ca2+ ({plus minus} Ca2+ buffers) and low [Na+], indicating that the [Ca2+] increase was mediated by release from intracellular stores. Consistent with this hypothesis, the IR-induced [Ca2+] response was prolonged and eventually blocked by inhibition of ER Ca-ATPase with cyclopiazonic acid, and was also inhibited by a high concentrati...