Which statement about ON-bipolar and OFF-bipolar cells best describes their activation in relation to photoreceptor transmitter release?

Which statement about ON-bipolar and OFF-bipolar cells best describes their activation in relation to photoreceptor transmitter release?

• A. ON-bipolar cells are activated when photoreceptor transmitter release decreases; OFF-bipolar cells are activated when transmitter release increases
• B. ON-bipolar cells are activated when transmitter release increases; OFF-bipolar cells are activated when transmitter release decreases
• C. Both ON- and OFF-bipolar cells are activated when transmitter release increases
• D. Neither is affected by transmitter release

Answer: (A)

The key idea is how ON and OFF bipolar cells translate photoreceptor transmitter release into opposite responses. In darkness, photoreceptors release glutamate. OFF bipolar cells have ionotropic glutamate receptors (AMPA/kainate) that depolarize when glutamate binds, so they are activated by high transmitter release. ON bipolar cells use a metabotropic receptor (mGluR6) that, when activated by glutamate, closes cation channels and keeps the cell hyperpolarized; when light reduces transmitter release, this inhibition is lifted and the ON bipolars depolarize and become activated. So ON cells respond to a decrease in transmitter release, while OFF cells respond to an increase. This wiring explains why that statement is the best description of their activation in relation to photoreceptor transmitter release.

The key idea is how ON and OFF bipolar cells translate photoreceptor transmitter release into opposite responses. In darkness, photoreceptors release glutamate. OFF bipolar cells have ionotropic glutamate receptors (AMPA/kainate) that depolarize when glutamate binds, so they are activated by high transmitter release. ON bipolar cells use a metabotropic receptor (mGluR6) that, when activated by glutamate, closes cation channels and keeps the cell hyperpolarized; when light reduces transmitter release, this inhibition is lifted and the ON bipolars depolarize and become activated. So ON cells respond to a decrease in transmitter release, while OFF cells respond to an increase. This wiring explains why that statement is the best description of their activation in relation to photoreceptor transmitter release.