Cyclic nucleotide-gated (CNG) channels link intracellular cyclic nucleotides to changes in membrane ionic conductance in a variety of physiological contexts. In the retina, in addition to their central role in phototransduction, CNG channels may be involved in nitric oxide signaling in bipolar neurons or in the hyperpolarizing synaptic response to glutamate inon-type (depolarizing) bipolar cells. Despite their potential physiological significance, however, expression of CNG channels has not yet been demonstrated in bipolar cells. To identify CNG channel subtypes in retinal bipolar neurons, we used single-cell molecular biological techniques in morphologically distinctiveon bipolar cells from goldfish retina. Both single-cell in situ hybridization and single-cell RT-PCR demonstrated inon bipolar cells the presence of mRNA for the CNG channel subtype that is also found in cone photoreceptors. Other bipolar cells, which likely represent off cells, did not express the cone CNG channel. Thus the CNG channel of cone photoreceptors is expressed in on bipolar cells, where it may be involved in physiological responses to nitric oxide, or in the sign-inverting glutamatergic synapse that gives rise to the on visual pathway.
