The light responses in bipolar cells are initiated by synapses with photoreceptors, the photoreceptors releases only glutamate.
The photoreceptors cells are specialized neuroepithelial cells found in retina and capable of phototransduction, they convert light into signals that can stimulate biological process, the release of glutamate neurotransmitter occurs by the hiperpolarization of these cells, and they decrease with upon expusure to light, being "the lights off" the trigger to the release and "the light on" the signal to stop the release of glutamate in bipolar photoreceptor cells.