title: Corneal dry-responsive neurons in the spinal trigeminal nucleus respond to innocuous cooling in the rat.
abstract: Corneal primary afferent neurons that respond to drying of the ocular surface have been previously characterized and found to respond to innocuous cooling, menthol, and hyperosmotic stimuli. The purpose of the present study was to examine the receptive field properties of second-order neurons in the trigeminal nucleus that respond to drying of the ocular surface. Single-unit electrophysiological recordings were performed in anesthetized rats, and dry-responsive corneal units were isolated in the brain stem at the transition zone between the spinal trigeminal subnucleus caudalis and subnucleus interpolaris. Corneal units were characterized according to their responses to changes in temperature (cooling and heating), hyperosmotic artificial tears, menthol, and low pH. All dry-responsive neurons (n = 18) responded to cooling of the ocular surface. In addition, these neurons responded to hyperosmotic stimuli and menthol application to the cornea. One-half of the neurons were activated by low pH, and these acid-sensitive neurons were also activated by noxious heat. Furthermore, neurons that were activated by low pH had a significantly lower response to cooling and menthol. These results indicate that dry-responsive neurons recorded in the trigeminal nucleus receive input from cold, sensitive primary afferent neurons, with a subset of these neurons receiving input from corneal primary afferent neurons sensitive to acid and noxious heat. It is proposed that acid-insensitive corneal neurons represent a labeled line for lacrimation in response to evaporation of tears from the ocular surface, whereas acid-sensitive neurons are involved in tearing, elicited by damaging or potentially damaging stimuli.
authors: Division of Oral Physiology, Department of Oral Biological Sciences, Niigata University, Graduate School of Medical and Dental Sciences, Niigata, Japan.
authors: Masayuki
authors: M
authors: Kurose
authors: Ian D
authors: ID
authors: Meng
mesh_terms: D000143:Acids / Q000494:pharmacology; D000200:Action Potentials; D000818:Animals; D001933:Brain Stem / Q000166:cytology / Q000502:physiology; D003080:Cold Temperature; D003315:Cornea / Q000166:cytology / Q000187:drug effects / Q000294:innervation / Q000502:physiology*; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D008297:Male; D008610:Menthol / Q000494:pharmacology; D009475:Neurons, Afferent / Q000145:classification / Q000502:physiology*; D059225:Nociception; D009883:Ophthalmic Solutions / Q000494:pharmacology; D009995:Osmosis; D051381:Rats; D017207:Rats, Sprague-Dawley; D014279:Trigeminal Nucleus, Spinal / Q000166:cytology / Q000502:physiology*
chemical_list: D000143:Acids; D009883:Ophthalmic Solutions; D008610:Menthol
keywords: cornea; eye; nociception; trigeminal