Neuropeptides in the retina: evidence for differential topographical localization.

The retina has been examined for the presence of vasoactive intestinal polypeptide (VIP), substance P, somatostatin, and gastrin/cholecystokinin immunoreactive structures by immunocytochemistry. The results indicate that all four peptides reside in neuron-like processes of the inner plexiform layer. Interestingly, the immunoreactive processes display distinctive distributional patterns in different strata of this layer, suggesting that they may serve distinct functions, possibly related to integration and modification of visual input. This supposition is strengthened by the observation that the immunoreactivity of the VIP cell bodies and processes varies extensively with the amount of light entering the eye.