CYCLIC-NUCLEOTIDE REGULATION OF TELEOST ROD PHOTORECEPTOR INNER SEGMENT LENGTH

Retinal rod photoreceptors of teleost fish elongate in the light and s horten in the dark. Rod cell elongation and shortening are both mediat ed by actin-dependent mechanisms that occur in the inner segment myoid and ellipsoid. The intracellular signaling pathways by which light an d dark regulate the actin cytoskeleton in the inner segment are unknow n. To investigate the intracellular signals that regulate teleost rod motility, we have been using mechanically isolated rod inner/outer seg ments (RIS-ROS) obtained from the retinas of green sunfish, Lepomis cy anellus. In culture, RIS-ROS retain the ability to elongate in respons e to light; myoids elongate 15-20 mum in length during 45 min of light culture. A pharmacological approach was taken to investigate the role of cyclic nucleotides, cyclic nucleotide-dependent kinases, and prote in phosphatases in the regulation of RIS-ROS motility. Millimolar conc entrations of cAMP and cGMP analogues were both found to inhibit light -induced myoid elongation and two cyclic nucleotide analogues, SpCAMPS and 8BrcGMP, promoted myoid shortening after RIS-ROS had elongated in response to light. The cyclic nucleotide-dependent kinase inhibitor, H8, mimicked light by promoting myoid elongation in the dark. The effe cts of H8 were dose dependent, with maximal elongation occurring at co ncentrations of approximately 100 muM. Similar to the effects of cycli c nucleotide analogues, the phosphatase inhibitor, okadaic acid (0.1-1 0 muM), inhibited light-induced elongation and promoted shortening. Th e results presented here suggest that RIS-ROS motility is regulated by protein phosphorylation: phosphorylation in the dark by cyclic nucleo tide-dependent protein kinases promotes rod shortening, while dephosph orylation in the light promotes rod elongation.