Signals from cone photoreceptors to L-type horizontal cells are differentially modulated by low calcium in carp retina

Ca2+ plays crucial roles in both phototransduction and calcium-dependent gl utamate release from the photoreceptor terminal. Modulation, by lowering ex tracellular Ca2+, of red-sensitive (R-) and short wavelength-sensitive (S-) cone-driven light responses of L-type horizontal cells (LHCs) was studied in the isolated superfused carp retina using intracellular recording techni ques. Low Ca2+ (nominally Ca2+-free) Ringer's reduced responses of LHCs to both green (500 nm) and red (680 nm) flashes in darkness, with the former b eing suppressed more substantially than the latter. This differential suppr ession became more significant when contribution of R-cones to the green-li ght-induced responses was diminished by a moderate red (680 nm) background light. Application of IBMX, an inhibitor of phosphodiesterase (PDE), increa sed LHC responses to both red and green flashes equally, resembling the eff ect of low Ca2+ on phototransduction. In addition, photopic electroretinogr aphic P III responses, reflecting the activity of cones, to red flashes wer e more potentiated by low Ca2+, compared to those to green flashes, whilst they were both equally potentiated by IBMX. Furthermore, low Ca2+ caused a more pronounced suppression of LHC responses to red flashes than those to g reen flashes in the presence of IBMX. It is postulated that reduction of LH C responses in low Ca2+ may be due to the 'saturation suppression' caused b y the increased glutamate release from the photoreceptor terminal and the d ifferential modulation may reflect a consequence of the dual action of low Ca2+ on the PDE activity in the photoreceptor outer segment and the synapti c strength between cones and LHCs.