Which neurotransmitter is the primary excitatory signal at photoreceptor synapses in the retina?

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

Answer: (C)

Glutamate is the main excitatory transmitter released by photoreceptors at their synapses with bipolar and horizontal cells. In darkness, photoreceptors are depolarized and continuously release glutamate, sending signals forward. When light hits, photoreceptors hyperpolarize and reduce glutamate release, altering the activity of downstream cells. OFF bipolar cells respond to glutamate with excitation via ionotropic receptors, so they depolarize when glutamate is present (in darkness) and become less active in light. ON bipolar cells use a metabotropic receptor (mGluR6) that is activated by glutamate to produce a hyperpolarizing effect; thus, when glutamate falls in light, these cells disinhibit and depolarize, signaling a light increment. Other neurotransmitters like GABA and glycine are mainly inhibitory and used by horizontal and amacrine cells to shape the signal, not the primary excitatory signal from photoreceptors. Dopamine is a modulatory neurotransmitter, not the key fast excitatory transmitter at photoreceptor synapses.