Autonomic neurons supplying the rat eye and the intraorbital distribution of vasoactive intestinal polypeptide (VIP)-like immunoreactivity.
We traced the origin and path of autonomic nerves to the rat eye using, as an aid to dissection, a modified thiocholine method for the histochemical demonstration of cholinesterase. When applied to whole nerves and ganglia supplying the rat eye, this procedure is not specific for cholinergic neurons; instead it stains both sympathetic and parasympathetic nerves, many of which are otherwise too fine to identify in dissection. We found that nerves from the superior cervical and pterygopalatine ganglia form a plexus at the orbital apex corresponding to the retro-orbital plexus found in rabbit, monkey and man. In the rat, nerves from the retro-orbital plexus travel peripherally to the superior surface of the optic-nerve sheath. Here, they fuse with long ciliary nerves and the post-ganglionic nerves from the ciliary ganglion to form another dense nerve-fiber plexus that ultimately supplies the eye. Importantly, the plexus on the optic nerve contains many isolated or aggregated ganglion cells. These are comparable in number to those in the ciliary ganglion itself and are assumed to be accessory ciliary neurons. Using immunohistochemistry, we also sought evidence for vasoactive intestinal polypeptide in these ganglia and nerves. As previously known, many pterygopalatine ganglion cells are immunoreactive for this peptide. Vasoactive intestinal polypeptide (VIP)-like immunoreactive nerve fibers are present in nerves from the retro-orbital plexus to the optic-nerve sheath plexus, in most nerves of the latter plexus, and in most nerves entering the eye. Furthermore, a small proportion of nerve cells in the main and accessory ciliary ganglia also are immunoreactive for VIP. We conclude that in addition to the pterygopalatine ganglion, the ciliary ganglion and its accessory ganglia are sources of VIP-like immunoreactive nerves in the rat eye.