The corneas of rat, mouse and guinea pig were investigated using
light and electron microscopy as well as immunohistochemistry. We,
here, examined the cholinergic structures in the cornea by immunohistochemistry
for choline acetyltransferase of the two forms, the alternative
splice variant from choline acetyltransferase (ChAT) cDNA preferentially
located in peripheral nerves (pChAT) and the common type (cChAT).
Positive staining for cChAT was found only in non-neuronal epithelial
cells, while pChAT positive staining occurred exclusively in nerve
fibers being distributed throughout the cornea. Such pChAT positive
nerve axons were frequently smooth with few varicosities, except for
their terminal swellings. The distribution pattern of pChAT positive
nerves was compared with that of three neuropeptides known to be
found in corneal nerves, including protein gene product 9.5 (PGP 9.5), substance P (SP) and calcitonin gene-related peptide (CGRP).
The co-localization of pChAT with either of these neuropeptides has
been assumed on the base of their great similarity in distribution
pattern and closeness in their routes inside the cornea. From these
results it is concluded that rat, mouse and guinea pig corneas have
little morphological differences and have a remarkable non-neuronal
cholinergic activity in their epithelium together with what can be
suggested to be cholinergic nerves represented in the pChAT positive
nerve fibers. The function of acetyl choline in the corneal epithelium
and the possible origin and function of corneal cholinergic nerves has
been discussed.