Which neurotransmitters act as inhibitory interneurons in retinal signaling?

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

Answer: (D)

Inhibitory signaling in the retina is carried mainly by GABA and glycine. Interneurons such as amacrine and horizontal cells release these neurotransmitters to suppress activity in neighboring neurons, sharpening contrast and timing in the visual signal. GABAergic inhibition is prominent in the inner retina, while glycinergic inhibition contributes to several circuits that shape transmission from bipolar to ganglion cells. These transmitters work by opening receptor channels that hyperpolarize the postsynaptic cell, reducing its likelihood of firing. By contrast, glutamate is the primary excitatory transmitter in the retina, released by photoreceptors and many subsequent neurons to promote depolarization, while acetylcholine and dopamine mainly play modulatory roles rather than serving as the main fast inhibitory signals.