Abstract

We investigated the role of caffeine-sensitive intracellular stores in regulating intracellular calcium ([Ca²⁺]_i) and glutamatergic synaptic transmission from rod photoreceptors. Caffeine transiently elevated and then markedly depressed [Ca²⁺]_i to below prestimulus levels in rod inner segments and synaptic terminals. Concomitant with the depression was a reduction of glutamate release and a hyperpolarization of horizontal cells, neurons postsynaptic to rods. Caffeine did not affect the rods’ membrane potentials indicating that caffeine likely acted via some mechanism(s) other than a voltage-dependent deactivation of the calcium channels. Most of caffeine’s depressive action on [Ca²⁺]_i, on glutamate release, and on I_Ca in rods can be attributed to calcium release from stores: (1) caffeine’s actions on [Ca²⁺]_i andI_Ca were reduced by intracellular BAPTA and barium substitution for calcium, (2) other nonxanthine store-releasing compounds, such as thymol and chlorocresol, also depressed [Ca²⁺]_i, and (3) the magnitude of [Ca²⁺]_i depression depended on basal [Ca²⁺]_i before caffeine. We propose that caffeine-released calcium reducesI_Ca in rods by an as yet unidentified intracellular signaling mechanism. To account for the depression of [Ca²⁺]_i below rest levels and the increased fall rate of [Ca²⁺]_i with higher basal calcium, we also propose that caffeine-evoked calcium release from stores activates a calcium transporter that, via sequestration into stores or extrusion, lowers [Ca²⁺]_i and suppresses glutamate release. The effects of store-released calcium reported here operate at physiological calcium concentrations, supporting a role in regulating synaptic signaling in vivo.