Purpose: To determine the effect of diabetes mellitus on the levels of substance P (SP), calcitonin gene-related peptide (CGRP), and vasoactive intestinal polypeptide (VIP) in the rat cornea and iris.
Methods: Corneas and irides from control and diabetic rats were processed for neuropeptide radioimmunoassay 3 months after induction of diabetes with streptozotocin. Corneas and irides also were processed for SP and CGRP immunohistochemistry and were evaluated qualitatively.
Results: The radioimmunoassay data revealed no significant differences in either the content or concentration of SP, CGRP, and VIP between control and diabetic corneas. In contrast, iridial levels of CGRP and SP were significantly increased by 38% and 256%, respectively, in the diabetic animals. Iridial VIP levels were unchanged in the diabetic versus control groups. Immunohistochemical demonstrations of corneal and iridial SP- and CGRP-immunoreactive fiber plexuses were indistinguishable on the basis of purely qualitative criteria.
Conclusions: The results of this study have demonstrated a target- and peptide-specific effect of short-term diabetes on SP and CGRP expression in ocular nerves of the anterior eye segment. The absence of demonstrable changes in corneal neuropeptide levels argue against the theory that corneal abnormalities seen in clinical diabetes are caused, in part, by deficits in synthesis or axonal transport of "trophic" peptides in corneal sensory nerves. In contrast, elevated iridial SP and CGRP levels may be responsible for reported clinical deficits in pupillary diameter regulation.