Which cells in the retina modify synapses between photoreceptors and bipolar cells via inhibitory feedback?

• A. Muller cells
• B. Astrocytes
• C. Horizontal cells
• D. Amacrine cells

Answer: (C)

Horizontal cells are responsible for inhibitory feedback that modulates the connection between photoreceptors and bipolar cells. They extend processes into the outer plexiform layer and form contacts with photoreceptor terminals, receiving input from nearby photoreceptors and providing inhibitory signals back to them. This feedback, primarily through GABAergic transmission, reduces glutamate release from photoreceptors onto adjacent bipolar cells, effectively tuning the strength of the signal that bipolar cells receive. This lateral inhibition enhances contrast and sharpens visual signaling right at the photoreceptor–bipolar synapse.

Other retinal glial cells like Muller cells and astrocytes support the tissue and maintain the extracellular environment but don’t mediate this synaptic feedback. Amacrine cells modulate synapses later in the circuit, at the inner retina between bipolar and ganglion cells, not at the outer photoreceptor–bipolar junction.