The goal of the present study was to elucidate the role of DARPP-32 (dopamine- and cyclic adenosine 3'-5'-monophosphate-regulated phosphoprotein, 32 kDa) in retinal function. We examined mouse and rat retinas for the presence of DARPP-32 by immunocytochemistry. In both rodent retinas DARPP-32 immunoreactivity was localized to horizontal and AII amacrine neurons and to the Mueller glial cells, using immuno-double labelling. Additional unidentified neurons in the amacrine cell layer also showed DARPP-32 immunoreactivity. Using mice entrained to a 12-12 h light-dark cycle, we found that exposure to light presented during the dark phase significantly enhanced phosphorylation of DARPP-32 at threonine (Thr) 34 and phosphorylation of the ionotropic glutamate receptor subunit GluR1 at serine (Ser) 845, as measured by immunoblots. However, light also increased Ser 845-GluR1 phosphorylation in DARPP-32-knockout mice. When a dopamine D1 receptor antagonist was injected into the eye prior to light exposure, phosphorylation of both Thr 34-DARPP-32 and Ser 845-GluR1 was significantly reduced. These data indicate that DARPP-32 participates in dopamine-mediated modifications of retinal function. We also tested for a possible circadian rhythm of Thr 34- and Thr 75-DARPP-32 and Ser 845-GluR1 expression. No significant circadian rhythm of either DARPP-32 or GluR1 phosphorylation was found.