What is the primary neurotransmitter released by photoreceptors?

• A. GABA
• B. Acetylcholine
• C. Dopamine
• D. Glutamate

Answer: (D)

Photoreceptors release glutamate, the main neurotransmitter used at their synapses with bipolar and horizontal cells. In darkness, they continuously release glutamate, providing a baseline signal to the retinal circuit. When light activates photoreceptors, they hyperpolarize and reduce their glutamate release, and this change is read by the downstream neurons to encode light. ON and OFF pathways handle glutamate differently due to distinct receptor types: OFF bipolar cells depolarize in response to glutamate, while ON bipolar cells are activated when glutamate levels drop. Other substances listed (GABA, acetylcholine, dopamine) are used by different retinal neurons for various modulatory or inhibitory roles, not as the primary transmitter from photoreceptors.