IMMUNOHISTOCHEMICAL LOCALIZATION OF TYROSINE-HYDROXYLASE IN CORNEAL NERVES

The sympathetic innervation of the mammalian cornea is thought to play an important role in the regulation of epithelial ion transport, mito genesis, and wound healing following corneal injuries. Anatomically, t he three-dimensional organization and relative density of corneal symp athetic innervation in many species remains inadequately described. In the present study, the sympathetic innervation of five different mamm als (guinea pig, rat, mouse, hamster, and human) was studied in cornea s sectioned parallel to the main axis of fiber orientation by labeling the fibers immunohistochemically with antiserum against tyrosine hydr oxylase and an avidin-biotin-diaminobenzidine technique. The results s howed that each species displayed a distinctive pattern and density of tyrosine hydroxylase immunoreactive (TH-IR) corneal innervation that was unique to that species. The overall level of TH-IR innervation was highest in the guinea pig, moderate in the human, hamster, and rat, a nd lowest in the mouse. In all species examined, TH-IR nerves were mos t numerous in the corneoscleral limbus where they either formed intima te associations with blood vessels or coursed through the connective t issue matrix apparently unrelated to vascular elements. Other TH-IR ne rves entered the cornea proper in radially directed stromal nerve bund les to give rise to subepithelial plexuses of varying complexity. Occa sional intraepithelial penetrations were observed in the guinea pig, h uman, and rat. Removal of the superior cervical ganglion resulted in t he total loss of TH-IR staining from guinea pig and hamster corneas an d in the substantial but incomplete loss of TH-IR staining from rat an d mouse corneas, thus demonstrating their predominantly sympathetic or igin. Combined sympathetic and sensory ocular denervation in the rat e liminated almost all corneal and limbal TH-IR immunostaining, thus sug gesting a minor TH-IR sensory component in this species. In agreement with this conclusion, small numbers of TH-IR sensory neurons and an ab undance of TH-IR fibers were observed in the trigeminal ganglia of the rat and guinea pig. Removal of the rat main ciliary ganglion resulted in the loss of additional TH-IR fibers from the chamber angle and iri s, thereby confirming a partial parasympathetic contribution to the ra t iridial TH-IR innervation. Following unilateral removal of the super ior cervical ganglion in rats and guinea pigs, the contralateral corne a contained increased numbers of TH-IR nerves, suggesting an upregulat ion of tyrosine hydroxylase (TH) expression in some contralateral axon s. In summary, these data have shown pronounced interspecies differenc es in the organization and density of the mammalian corneal TH-IR inne rvation that may reflect species-specific variation in the level of sy mpathetic control over select aspects of corneal physiology. (C) 1993 Wiley-Liss, Inc.