Morphological and immunohistochemical characterization of the trigeminal ganglion neurons innervating the cornea and upper eyelid of the rat

The cornea is sensitive to nociceptive stimuli and receives dense sensory innervations from the trigeminal ganglion, which also innervates the upper eyelid. We investigated the morphological and immunohistochemical characterization of the trigeminal ganglion neurons innervating the cornea and upper eyelid. We injected the retrograde tracer Fluoro-Gold (FG) into the cornea and the retrograde tracer cholera toxin subunit b (CTb) into the upper eyelid of the same animal. Less than 10% of the FG-labeled neurons were also labeled with CTb. The FG-labeled neurons were small (29.6 ± 0.6 μm), while the CTb-labeled neurons were large (36.1 ± 0.5 μm). We also characterized the neurons in the trigeminal ganglion with the retrograde tracer FG following its injection into the cornea or the upper eyelid, and immunohistochemical double-labeling with nociception-related neuronal markers, such as calcitonin gene-related peptides (CGRP), transient receptor potentiated vanilloid 1 (TRPV1), and substance P (SP). About 27% of the neurons innervating the cornea were double-labeled with CGRP, about 23% with TRPV1, and about 8% with SP. About 4% of the neurons innervating the upper eyelid were double-labeled for CGRP, about 11% for TRPV1, and 3% for SP. Thus, the percentages of double-labeled neurons for the neurons innervating the cornea were higher than those for the neurons innervating the upper eyelid. These results indicate that the cornea and the upper eyelid receive innervations mainly from different neurons of the trigeminal ganglia. The cornea is innervated by many characteristic sensory neurons containing nociception-related neuronal markers.