Spontaneous action potentials in retinal horizontal cells of goldfish (Carassius auratus) are dependent upon L-type Ca2+ channels and ryanodine receptors

Horizontal cells (HCs) are interneurons of the outer retina that undergo graded changes in membrane potential during the light response and provide feedback to photoreceptors. We characterized spontaneous Ca 2+ -based action potentials (APs) in isolated goldfish ( Carassius auratus) HCs with electrophysiological and intracellular imaging techniques. Transient changes in intracellular Ca 2+ concentration ([Ca 2+ ] i ) were observed with fura-2 and were abolished by removal of extracellular Ca 2+ or by inhibition of Ca 2+ channels by 50 µM Cd 2+ or 100 µM nifedipine. Inhibition of Ca 2+ release from stores with 20 µM ryanodine or 50 µM dantrolene abolished Ca 2+ transients and increased baseline [Ca 2+ ] i . This increased baseline was prevented by blocking L-type Ca 2+ channels with nifedipine, suggesting that Ca 2+ -induced Ca 2+ release from stores may be needed to inactivate membrane Ca 2+ channels. Caffeine (3 mM) increased the frequency of Ca 2+ transients, and the store-operated channel antagonist 2-aminoethyldiphenylborinate (100 μM) counteracted this effect. APs were detected with voltage-sensitive dye imaging (FluoVolt) and current-clamp electrophysiology. In current-clamp recordings, regenerative APs were abolished by removal of extracellular Ca 2+ or in the presence of 5 mM Co 2+ or 100 µM nifedipine, and APs were amplified with 15 mM Ba 2+ . Collectively, our data suggest that during APs Ca 2+ enters through L-type Ca 2+ channels and that Ca 2+ stores (gated by ryanodine receptors) contribute to the rise in [Ca 2+ ] i . This work may lead to further understanding of the possible role APs have in vision, such as transitioning from light to darkness or modulating feedback from HCs to photoreceptors. NEW & NOTEWORTHY Horizontal cells (HCs) are interneurons of the outer retina that provide inhibitory feedback onto photoreceptors. HCs respond to light via graded changes in membrane potential. We characterized spontaneous action potentials in HCs from goldfish and linked action potential generation to a rise in intracellular Ca 2+ via plasma membrane channels and ryanodine receptors. Action potentials may play a role in vision, such as transitioning from light to darkness, or in modulating feedback from HCs to photoreceptors.