Quantification and immunocytochemical characteristics of trigeminal ganglion neurons projecting to the cornea: effect of corneal wounding.

The number and immunocytochemical characteristics of trigeminal ganglion neurons providing sensory innervation to the cornea were studied in the mouse. Corneal neurons were retrogradely labelled with fluorogold placed on the cornea after removal of the epithelium with n-heptanol. Corneal neurons were counted, sized and characterized immunocytochemically with antisera against substance P (SP), calcitonin gene-related peptide (CGRP), calbindin, calretinin, and with a monoclonal antibody (RT97) against neurofilament proteins. A total of 258 corneal neurons were counted, most of them located in the ophthalmic division of the trigeminal ganglion. They represent only a small fraction (1.3%) of the population of trigeminal ganglion neurons. More than 70% of corneal neurons were classified as 'small dark' according to their cell body area and the absence of immunoreactivity to RT97. A low percentage of corneal neurons, usually large in size, contained calcium binding proteins. Fifty-eight percent of the corneal neurons were immunoreactive to CGRP, and 20% to SP. Corneal wounding with NaOH, which affects stromal nerve trunk, did not modify the total number of corneal neurons or their neuropeptide content. However, this increased the total number of calbindin-positive and decreased the RT97-positive neurons. Thus, unlike in other nociceptive neurons, peripheral axotomy did not modify the SP/CGRP content of corneal neurons.Trigeminal ganglion neurons projecting to the cornea are similar in size and neuropeptide content to nociceptive neurons of other territories. Their number is high in relation to the corneal surface, thus confirming that the cornea has a large nociceptive representation in the trigeminal ganglion. Copyright 1999 European Federation of Chapters of the International Association for the Study of Pain.