Co-expression of vasoactive intestinal peptide and protein gene product 9.5 surrounding the lumen of human Schlemm's canal

Previous studies of aqueous humor outflow have focused primarily on resistance at the trabecular meshwork (TM), and little is known about the function of Schlemm's canal (SC). Here, we aimed to investigate whether SC is innervated by the peripheral nervous system. Ten eye specimens from eight donors were processed for histological analysis. CD31 was used to identify SC, after which we used protein gene product (PGP) 9.5 as a marker to detect nerve fibers around SC. We then characterized the nerves by double staining for PGP9.5 and sympathetic nerve markers, such as tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DβH), or the parasympathetic marker vasoactive intestinal peptide (VIP), as well as sensory nerve marker calcitonin gene-related peptide (CGRP) and vesicular glutamate transporter 2 (VGLUT2). Immunohistochemistry and immunofluorescence were also performed to detect the expression of γ-epithelial Na+ channel (γ-ENaC) in SC. We found that different markers were expressed in the anterior chamber angle, with the luminal surface of SC were only positive stained for PGP9.5, VIP, and γ-ENaC. CGRP and VGLUT2 were expressed in TM and scleral spur (SS), whereas TH and DβH were absent in both TM and SC. Furthermore, PGP9.5 was co-expressed with VIP and γ-ENaC in the region surrounding the SC as well as in SS. Our findings indicate that the peripheral nerves anatomically spread in the tissues around the SC and the local nerve fibers may be parasympathetic or sensory rather than sympathetic.