Figure 6.

Model of how the ambient light level, by altering dopamine D₁ receptor activation, modulates the GABA_A receptor-mediated receptive field surround of ON-cBCs.The model accounts for how changes in ambient (background) light level affect the GABA feedforward signal from HCs to ON-cBC dendrites that mediates the strength of ON-cBC surround responses . As ambient illumination slowly increases during the morning, reaching a peak at midday, D₁R activation increases, which in turn augments intracellular PKA, so that the expression and activity of GABA_ARs on ON-cBC dendrites-including dendritic tips-are enhanced. As a result, the effectiveness of the GABA_AR-mediated feedforward signal from HC dendrites to ON-cBC dendrites increases, enhancing the strength of surround antagonism and activation. Conversely, as background illumination slowly decreases during the afternoon and evening, reaching darkness at night, D₁R activation is reduced. This in turn decreases intracellular PKA, substantially lowering GABA_AR expression and activity so that ON-cBC surround strength is minimal.