Somatostatin modulates voltage-gated K+ and Ca2+ currents in rod and cone photoreceptors of the salamander retina

We investigated the cellular localization in the salamander retina of one o f the somatostatin [or somatotropin release-inhibiting factor (SRIF)] recep tors, sst(2A), and studied the modulatory action of SRIF on voltage-gated K + and Ca2+ currents in rod and cone photoreceptors. SRIF immunostaining was observed in widely spaced amacrine cells, whose perikarya are at the borde r of the inner nuclear layer and inner plexiform layer. sst2A immunostainin g was seen in the inner segments and terminals of rod and cone photorecepto rs. Additional sst(2A) immunoreactivity was expressed by presumed bipolar a nd amacrine cells. SRIF, at concentrations of 100-500 nM, enhanced a delaye d outwardly rectifying K+ current (I-K) in both rod and cone photoreceptors . SRIF action was blocked in cells pretreated with pertussis toxin (PTX) an d was substantially reduced by intracellular GDP(beta)S. Voltage-gated L-ty pe Ca2+ currents in rods and cones were differently modulated by SRIF. SRIF reduced Ca2+ current in rods by 33% but increased it in cones by 40%, on a verage. Both effects were mediated via G-protein activation and blocked by PTX. Ca2+-imaging experiments supported these results by showing that 500 n M SRIF reduced a K+-induced increase in intracellular Ca2+ in rod photorece ptor terminals but increased it in those of cones. Our results suggest that SRIF may play a role in the regulation of glutamate transmitter release fr om photoreceptors via modulation of voltage-gated K+ and Ca2+ currents.