Ca(2+)-induced light adaptation in retinal ON-bipolar cells.

Retinal ON-bipolar cells possess a metabotropic glutamate receptor linked to the control of a cGMP cascade, which functions to generate high synaptic amplification of rod signals under dark-adapted conditions. We report that a major component of light adaptation of the rod visual system results from a reduction in gain in synaptic transmission from rods to ON-bipolar cells, initiated by Ca(2+)-loading when the cGMP-activated channels of the postsynaptic cell open with light. When intracellular Ca(2+)-buffering was reduced adaptation was induced in ON-bipolar cells by background light too weak to significantly desensitize rods, a property absent in OFF-bipolar cells. Desensitization of ON-bipolar cell flash responses was induced by raising intracellular free Ca2+.