Photoreceptors in the retina, namely rods and cones, are specialized cells that transduce light into neural signals. These signals are conveyed to the brain by modulating neurotransmitter release at synaptic junctions with bipolar cells. Understanding which neurotransmitter’s release diminishes when rods are illuminated is crucial in vision science.

Neurotransmitter Release by Rods in Darkness and Light

• In darkness, rod photoreceptors are relatively depolarized and continuously release the excitatory neurotransmitter glutamate.

• Glutamate acts on downstream bipolar and horizontal cells, influencing their activity via excitatory or inhibitory receptors.

• When rods are exposed to light, they hyperpolarize, reducing voltage-gated calcium channel opening and thus decreasing glutamate release.pmc.ncbi.nlm.nih+1

Why Glutamate?

• Glutamate is the principal excitatory neurotransmitter in central nervous system synapses, including photoreceptor synapses.

• It mediates information transfer from photoreceptors to bipolar cells—the first stage of visual processing.wikipedia

• The reduction in glutamate signals the presence of light, initiating complex neural circuits that lead to visual perception.

Other Neurotransmitters Are Not Released by Rods

• Acetylcholine, GABA, and Glycine are important neurotransmitters elsewhere in the retina but are not released by rod photoreceptors at their synaptic terminals.

• Instead, these neurotransmitters are released by other retinal neurons such as amacrine and ganglion cells and participate in modulation and lateral inhibition.wikipedia

Conclusion

The neurotransmitter whose release from rods is reduced when light strikes their outer segment is:

(1) Glutamate