text: Photoreceptor cells are unusual cells in that they depolarize in response to absence of stimuli or scotopic conditions (darkness). In photopic conditions (light), photoreceptors hyperpolarize to a potential of −60 mV.
  In the dark, cGMP levels are high and keep cGMP-gated sodium channels open allowing a steady inward current, called the dark current.  This dark current keeps the cell depolarized at about −40 mV, leading to glutamate release which inhibits excitation of neurons.
  The depolarization of the cell membrane in scotopic conditions opens voltage-gated calcium channels.  An increased intracellular concentration of Ca causes vesicles containing glutamate, a neurotransmitter, to merge with the cell membrane, therefore releasing glutamate into the synaptic cleft, an area between the end of one cell and the beginning of another neuron. Glutamate, though usually excitatory, functions here as an inhibitory neurotransmitter.
  In the cone pathway, glutamate:
  * Hyperpolarizes on-center bipolar cells.  Glutamate that is released from the photoreceptors in the dark binds to metabotropic glutamate receptors (mGluR6), which, through a G-protein coupling mechanism, causes non-specific cation channels in the cells to close, thus hyperpolarizing the bipolar cell.
  * Depolarizes off-center bipolar cells. Binding of glutamate to ionotropic glutamate receptors results in an inward cation current that depolarizes the bipolar cell.