Final answer:
Photoreceptors release glutamate in the retina and exhibit graded potentials in response to different light levels.
Step-by-step explanation:
Photoreceptors release glutamate in the retina and exhibit graded potentials in response to different light levels. The release of glutamate by photoreceptors is a crucial step in visual transduction, which involves the conversion of light energy into electrical signals that can be interpreted by the brain.