Electrophysiological properties of identified trigeminal ganglion neurons innervating the cornea of the mouse

The cornea is innervated by three functional types of neurons: mechanosenso ry, polymodal and cold-sensitive neurons, all of which are presumed to be n ociceptive. To explore if corneal neurons constitute a heterogeneous popula tion according to their electrophysiological properties, intracellular reco rdings were made in vitro from trigeminal ganglion neurons innervating the cornea of the mouse. Corneal neurons were labelled with FluoroGold applied after a corneal epithelial wound. Five days later, the trigeminal ganglion attached to the eye by its nerves was removed and placed in a superfusion c hamber. FluoroGold-positive cells that also responded to electrical stimula tion of the cornea were considered corneal neurons. Non-corneal neurons wer e also studied. Based on their conduction velocity at room temperature, cor neal neurons were classified as myelinated A (>1.5 m/s) or non-myelinated C (less than or equal to1.5 m/s) neurons. A and C neurons differed significa ntly in their passive and active electrical properties. Virtually all corne al C neurons and about two-thirds of A neurons exhibited a hump in the fall ing phase of the action potential (S neurons), while the remaining A neuron s (F neurons) showed faster and narrower action potentials without a hump. Among noncorneal neurons, A neurons of the F type were found in a proportio n of about 50%. Based on their ability to produce somatic action potentials in tetrodotoxin (0.1 muM), non-corneal neurons were classified as fully or partially tetrodotoxin sensitive, which were mainly of the A delta type, a nd tetrodotoxin resistant, which were C neurons. Among the corneal neurons, those with a faster action potential, possibly associated to the expressio n of tetrodotoxin-sensitive Na+ channels, may be pure corneal mechanosensor y neurons, all of which are known to belong to the A delta type. Neurons wi th a slower action potential showing a hump in the repolarization phase are both corneal A delta and C polymodal nociceptive neurons, a type of cell i n which tetrodotoxin-resistant Na+ channels have been identified. The possi bility is raised that the small population of neurons with a very high inpu t resistance are cold-sensitive neurons. From the present results, we suggest that the electrophysiological properti es of primary sensory neurons innervating the cornea are attributable not o nly to their conduction velocities, but also to the functional characterist ics of their peripheral nerve terminals. (C) 2000 IBRO. Published by Elsevi er Science Ltd. All rights reserved.