Photoreceptor coupling is controlled by connexin 35 phosphorylation in zebrafish retina

Electrical coupling of neurons is widespread throughout the CNS and is observed among retinal photoreceptors from essentially all vertebrates. Coupling dampens voltage noise in photoreceptors and rod-cone coupling provides a means for rod signals to enter the cone pathway, extending the dynamic range of rod-mediated vision. This coupling is dynamically regulated by a circadian rhythm and light adaptation. We examined the molecular mechanism that controls photoreceptor coupling in zebrafish retina. Connexin 35 (homologous to Cx36 of mammals) was found at both cone-cone and rod-cone gap junctions. Photoreceptors showed strong Neurobiotin tracer coupling at night, extensively labeling the network of cones. Tracer coupling was significantly reduced in the daytime, showing a 20-fold lower diffusion coefficient for Neurobiotin transfer. The phosphorylation state of Cx35 at two regulatory phosphorylation sites, Ser110 and Ser276, was directly related to tracer coupling. Phosphorylation was high at night and low during the day. Protein kinase A (PKA) activity directly controlled both phosphorylation state and tracer coupling. Both were significantly increased in the day by pharmacological activation of PKA and significantly reduced at night by inhibition of PKA. The data are consistent with direct phosphorylation of Cx35 by PKA. We conclude that the magnitude of photoreceptor coupling is controlled by the dynamic phosphorylation and dephosphorylation of Cx35. Furthermore, the nighttime state is characterized by extensive coupling that results in a well connected cone network.