Cyclic nucleotides vary by area in the retina and pigmented epithelium of the human and monkey.

Cyclic GMP and cyclic AMP are present in lower concentrations in the central (macular) region of the neural retina of the human and monkey than in other areas. This pattern approximates the distribution of rod photoreceptor cells. Surprisingly, an inverse gradient of cyclic GMP concentration is observed in the pigmented epithelium. Levels in the central region are over fourfold higher than in cells in the periphery, offering the first evidence of biochemical differences in this embryologically uniform cell type.