title: Trigeminal interpolaris/caudalis transition neurons mediate reflex lacrimation evoked by bright light in the rat.
abstract: Abnormal sensitivity to bright light can cause discomfort or pain and evoke protective reflexes such as lacrimation. Although the trigeminal nerve is probably involved, the mechanism linking luminance to somatic sensory nerve activity remains uncertain. This study determined the effect of bright light on second-order ocular neurons at the ventral trigeminal interpolaris/caudalis transition (Vi/Vc) region, a major termination zone for trigeminal sensory fibers that innervate the eye. Most Vi/Vc neurons (80.9%) identified by responses to mechanical stimulation of the ocular surface also encoded bright light intensity. Light-evoked neural activity displayed a long latency to activation (> 10 s) and required transmission through the trigeminal root ganglion. Light-evoked neural activity was inhibited by intravitreal injection of phenylephrine or l-N(G) -nitro-arginine methyl ester (L-NAME), suggesting a mechanism coupled to vascular events within the eye. Laser Doppler flowmetry revealed rapid light-evoked increases in ocular blood flow that occurred prior to the increase in Vi/Vc neural activity. Synaptic blockade of the Vi/Vc region by cobalt chloride prevented light-evoked increases in tear volume, whereas blockade at the more caudal spinomedullary junction (Vc/C1) had no effect. In summary, Vi/Vc neurons encoded bright light intensity and were inhibited by drugs that alter blood flow to the eye. These results support the hypothesis that light-responsive neurons at the Vi/Vc transition region are critical for ocular-specific functions such as reflex lacrimation, whereas neurons at the caudal Vc/C1 junction region probably serve other aspects of ocular nociception.
authors.affiliation: Department of Diagnostic and Biological Sciences, University of Minnesota School of Dentistry, Moos Tower 18-214, 515 Delaware St. SE, Minneapolis, MN 55455, USA. okamo007@umn.edu
authors.forename: Keiichiro
authors.initials: K
authors.lastname: Okamoto
authors.forename: Akimasa
authors.initials: A
authors.lastname: Tashiro
authors.forename: Randall
authors.initials: R
authors.lastname: Thompson
authors.forename: Yasuhiro
authors.initials: Y
authors.lastname: Nishida
authors.forename: David A
authors.initials: DA
authors.lastname: Bereiter
mesh_terms: D000818:Animals; D003035:Cobalt / Q000494:pharmacology; D005074:Evoked Potentials, Visual; D005123:Eye / Q000098:blood supply / Q000294:innervation; D019466:Glare; D017078:Laser-Doppler Flowmetry; D008297:Male; D019331:NG-Nitroarginine Methyl Ester / Q000494:pharmacology; D009474:Neurons / Q000502:physiology*; D059225:Nociception; D010775:Photic Stimulation; D020795:Photophobia; D051381:Rats; D017207:Rats, Sprague-Dawley; D011930:Reaction Time; D012018:Reflex; D012039:Regional Blood Flow; D009435:Synaptic Transmission / Q000187:drug effects; D013666:Tears / Q000378:metabolism*; D014275:Trigeminal Caudal Nucleus / Q000502:physiology*; D014276:Trigeminal Nerve / Q000502:physiology*
chemical_list: D003035:Cobalt; C018021:cobaltous chloride; D019331:NG-Nitroarginine Methyl Ester