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How does hyperpolarization of a photoreceptor change neurotransmitter release?
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Photoreception
The mammalian eye has a cornea and a lens and functions as a dioptric system, in which light rays are refracted to focus on the retina.
In vertebrate photoreceptors, the release of the neurotransmitter glutamate is directly linked to the cell's membrane potential.
In the dark, photoreceptors are in a depolarized state, which facilitates the continuous release of glutamate at the synapse. When light is absorbed, the resulting biochemical cascade leads to the closure of sodium channels, causing the cell to become hyperpolarized (more negative). This hyperpolarization inhibits the release of neurotransmitters. Consequently, the amount of glutamate released is proportional to the voltage of the receptor, meaning that as the cell becomes more hyperpolarized due to increased light intensity, the release of neurotransmitter is further reduced.12