Cyclic nucleotides in the cone-dominant ground squirrel retina

Dark-adapted cone-dominant ground squirrel retina has an average concentration of guanosine 3′,5′-monophosphate (cyclic GMP) of 9·0 μmol/kg dry wt. The average concentration of adenosine 3′,5′-monophosphate (cyclic AMP) is 6·6 μmol/kg dry wt. Measurement of cyclic nucleotides in individual layers of the ground squirrel retina demonstrates that the highest concentrations of cyclic GMP are found in layers containing photoreceptor cells. These levels are 10–40 times those found in the inner portions of the retina. Cyclic AMP, on the other hand, is more evenly distributed and similar to that found in other regions of the CNS. The levels of cyclic GMP in the outer segment layer of the ground squirrel are 10 times that of cyclic AMP. In contrast, in the inner retinal layers cyclic AMP concentrations are 7–8 times greater than those of cyclic GMP. Light-adaptation does not influence the average levels of either cyclic GMP or cyclic AMP or their concentrations in any layer of the ground squirrel retina. These data indicate that cyclic GMP is the predominant cyclic nucleotide in the cone photoreceptor and that its distribution is similar to the reported previously for roddominant retinas. The failure of long periods of light-adaptation to modify the levels of either cyclic GMP or cyclic AMP in the ground squirrel retina indicates either a fundamental difference in the way these cyclic nucleotides are metabolized in cone-dominant retinas, or simply a quantitative difference in the rate at which their concentrations are altered.