In the dark, are ON-center bipolar cells depolarized or hyperpolarized?

In the dark, ON-center bipolar cells are hyperpolarized. This occurs due to the absence of light, which leads to decreased activity of photoreceptors (rods and cones) that are stimulated by light. When light is not present, these photoreceptors release more of the neurotransmitter glutamate.

In the case of ON-center bipolar cells, which respond to increases in light, the action of glutamate in the dark causes hyperpolarization. This is because ON-center bipolar cells have glutamate receptors that generate a hyperpolarizing response when activated. Thus, in low-light conditions, the increased glutamate release from the photoreceptors results in a state where the ON-center bipolar cells become hyperpolarized and less likely to release neurotransmitters.

In contrast, when light is available, the photoreceptors decrease glutamate release, leading to depolarization of the ON-center bipolar cells and subsequent signaling to the ganglion cells. Therefore, during darkness, these cells are hyperpolarized due to the high levels of glutamate released by the inactive photoreceptors.