Spinule formulation in the fish retina: is there an involvement of actin and tubulin? An electronmicroscopic immunogold study.

During light-adaptation dendrites of teleost horizontal cells form finger-like processes, called 'spinules', which are characterized by synaptic membrane densities. To investigate the involvement of cytoskeletal elements in the formation and retraction of spinules, effects of the microtubule and actin inhibitors colchicine and cytochalasin D were examined by injection into the vitreous. Both substances inhibited the light-induced spinule formation. The ultrastructural immunolocalization of tubulin revealed labelling of dendrites only in their proximal parts. The distal parts of dendrites which invaginate into cone pedicles were free of label. Treatment with anti-actin revealed immunoreactivity along the entire length of dendrites up to the dendritic terminals. The spinules, however, showed no labelling. This finding does not support the hypothesis that spinules are protruded by actin polymerization. After cytochalasin D treatment the density of label in the dendritic terminals was enhanced by a factor of three, which suggests an accumulation of actin. Thus, spinule inhibition by cytochalasin D is probably caused by distortion of a functional actin network in the dendritic terminals.