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Why do vertebrate photoreceptors release more neurotransmitter in darkness?

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details of the mammalian eye — 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 more neurotransmitter in darkness is a result of the cell's electrical state, which is maintained by the continuous influx of ions.

In dark conditions, the second messenger cGMP binds to sodium channels in the photoreceptor's cell membrane, keeping them open. This allows a constant influx of positive sodium ions, which maintains the cell in a depolarized (weakly negative) state. Because the cell is depolarized, it continuously releases vesicles of neurotransmitter (glutamate) at its synapse.1

Conversely, when light is present, the biochemical cascade reduces cGMP levels, causing these sodium channels to close. This stops the inward flow of sodium ions, leading to hyperpolarization (a strongly negative state) of the cell. This hyperpolarization inhibits the release of the neurotransmitter. Thus, the photoreceptor is uniquely tuned to signal the absence of light through active neurotransmitter release, while the presence of light suppresses this activity.1