Neuronal pathways to the rat conjunctiva revealed by retrograde tracing and immunocytochemistry

The origin and neuropeptide content of nerve fibres in the rat conjunctiva were studied by retrograde tracing and denervations in combination with immunocytochemistry. Immunocytochemistry revealed nerve fibres containing neuropeptide Y (NPY), vasoactive intestinal peptide (VIP), peptide histidine isoleucine (PHI), helospectin, substance P (SP), and calcitonin gene-related peptide (CGRP) around blood vessels, smooth muscle bundles and glands. Removal of the sphenopalatine ganglion eliminated the majority of the VIP-, PHI- and helospectin-containing fibres. Sympathectomy eliminated the majority of the NPY-containing fibres in the conjunctiva. Removal of both the sphenopalatine ganglion and the superior cervical ganglion led to further reduction of the NPY fibres. Injection of the retrograde tracer True Blue into the conjunctiva resulted in the appearance of labelled cell bodies in the trigeminal ganglion, the sphenopalatine ganglion, the superior cervical ganglion, and the ciliary ganglion. Judging from the number of labelled nerve cell bodies, the superior cervical ganglion contributes most to the innervation. These results indicate that the majority of NPY-containing nerve fibres in the conjunctiva derives from the superior cervical ganglion (sympathetic nerve supply). Most of the VIP- and a minor population of the NPY-containing fibres in the conjunctiva originate in the sphenopalatine ganglion (parasympathetic nerve supply). A sparse amount of VIP- and NPY-containing fibres derives from the ciliary ganglion. Nerve fibres containing SP and/or CGRP emanate from the trigeminal ganglia (sensory nerve supply). Together the findings indicate that several ganglia project to the conjunctiva and that many neuropeptides may be involved in the control of conjunctival activity.