Are ON-center bipolar cells excited or inhibited by glutamate?

ON-center bipolar cells are indeed inhibited by glutamate. This characteristic is largely due to the presence of metabotropic glutamate receptors (mGluRs) on the dendrites of these bipolar cells. When glutamate, released from photoreceptors in the dark, binds to these receptors, it activates a signaling cascade that leads to hyperpolarization of the ON-center bipolar cells, resulting in inhibition.

In contrast, OFF-center bipolar cells express ionotropic glutamate receptors (such as AMPA receptors), which lead to depolarization (excitation) in response to the same neurotransmitter. Therefore, while OFF-center bipolar cells are excited by glutamate, ON-center bipolar cells respond oppositely, demonstrating the complex interactions in the retinal signaling pathway. This functional dichotomy between ON-center and OFF-center pathways forms a crucial part of how visual information is processed in the retina, facilitating contrast detection in the visual field.