PHORBOL ESTER BINDING-SITES IN THE FISH RETINA - CORRELATION WITH STIMULATION OF ENDOGENOUS PHOSPHORYLATION AND PROTEIN-KINASE-C ACTIVATION

The injection of phorbol esters into the eyes of dark-adapted teleost fish can mimic light effects in the retina and induces corresponding s ynaptic plasticity of horizontal cells (HCs). It is therefore very lik ely that protein kinase C (PKC) mediates light-induced synaptic plasti city. In the present study, we investigated the distribution of PKC, t he phorbol ester receptor, in isolated HCs and in the whole retina by using tritiated phorbol 12,13-dibutyrate ([H-3]PDBU). The binding char acteristics analyzed for HC homogenates and retinal homogenates reveal ed that [H-3]PDBu binding is time dependent, specific, saturable, and reversible. Binding sites in HCs displayed a dissociation constant of 11.5 nM and a total number of 2.8 pmol/mg of protein. Autoradiography revealed that [H-3]PDBU labeling is present in all retinal layers, inc luding HCs, where it is associated with the somata. Furthermore, the t reatment with PDBu strongly affected the endogenous phosphorylation of several membrane, cytosolic, and HC proteins and led to PKC activatio n as measured by H1 histone phosphorylation. In HCs, the treatment wit h PDBU in particular affected the amount of P-32 incorporated into a g roup of phosphoproteins (68, 56/58, 47, 28, and 15 kDa) that were rece ntly shown to be affected by light adaptation. These proteins might th erefore be considered as important components of the observed morpholo gical and physiological synaptic plasticity of HCs in the course of li ght adaptation.