Complex sentence: When light is shown onto a photoreceptor, the photoreceptor hyperpolarizes and reduces the release of glutamate, when this happens, horizontal cells reduce the release of GABA, which has an inhibitory affect on the photoreceptors.
Split into simpler sentences: When light is shone onto a photoreceptor, the photoreceptor hyperpolarizes and reduces the release of glutamate. When this happens, horizontal cells reduce the release of GABA, which has an inhibitory affect on the photoreceptors.