INVITRO PHOSPHORYLATION IN ISOLATED HORIZONTAL CELLS OF THE FISH RETINA - EFFECTS OF THE STATE OF LIGHT ADAPTATION

Horizontal cells, which are second-order neurons of the vertebrate ret ina, exhibit synaptic plasticity governed by light and dark adaptation . We have investigated the alterations in the protein phosphorylation patterns of isolated carp (Cyprinus carpio) horizontal cells in relati on to their state of light adaptation by using an in vitro phosphoryla tion assay and compared the resulting data with protein synthesis patt erns of the whole retina. Phosphoproteins and [S-35]methionine-labelle d proteins were analysed by one- and two-dimensional gel electrophores is followed by autoradiography. The state of light adaptation signific antly affected the in vitro phosphorylation of horizontal cell protein s with molecular weights of 68, 56/58, 47, 28 and 15 kDa, but had no e ffect on the protein synthesis of retinal proteins. In the light the m ost prominent increase of P-32 incorporation was observed in the 47 kD a protein. The biochemical properties of this protein closely resemble d those of the growth-associated GAP-48, found in the fish retina. In addition, the phosphorylation of horizontal cell homogenates in the pr esence of protein kinase activators such as cyclic AMP, calcium, calmo dulin and phospholipids revealed that horizontal cells of the fish ret ina contain cyclic AMP-, calcium/calmodulin- and calcium/phospholipid- dependent protein kinase activity resulting in the phosphorylation of several horizontal cell proteins, including the phosphoproteins which were affected by the state of light adaptation.