Light-dependent regulation of dopamine receptors in mammalian retina

The specific binding of [3H]spiperone, a D-2 dopamine receptor ligand, in in retinas from rabbits kept one week in constant light was significantly lower than in retinas from rabbits exposed to constant dark. Constant light did not alter the binding of [3H]spiperone in the striatum, where melatonin does not inhibit dopamine release. The decrease in [3H]spiperone binding induced by constant light in retina appears to be associated with the activation of inhibitory melatonin receptors on dopaminergic neurons. In support of this hypothesis, treatments that elevate melatonin concentrations, such as dark or melatonin administration, reversed the light-induced down-regulation of D-2 dopamine binding sites in retina. It is concluded that the decrease in melatonin levels in constant light disinhibits the dopamine-containing retinal neurons in vivo leading to elevated dopamine release and subsequent D-2 dopamine receptor down-regulation.