Bright light produces Fos-positive neurons in caudal trigeminal brainstem.
Excessive discomfort after exposure to bright light often occurs after ocular injury and during headache. Although the trigeminal nerve is necessary for light-evoked discomfort, the mechanisms underlying this phenomenon, often referred to generally as photophobia, are not well defined. Quantitative Fos-like immunoreactivity (Fos-LI) was used to determine the pattern of neuronal activation in the caudal brainstem after bright light stimulation and, secondly, whether a neurovascular mechanism within the eye contributes to this response. Under barbiturate anesthesia, male rats were exposed to low (1 x 10(4) lx) or high intensity (2 x 10(4) lx) light delivered from a thermal neutral source for 30 min (30 s ON, 30 s OFF) and allowed to survive for 90 min. Intensity-dependent increases in Fos-LI were seen in laminae I-II at the trigeminal caudalis/cervical cord junction region (Vc/C1) and nucleus tractus solitarius (NTS). Fos-LI also increased at the trigeminal interpolaris/caudalis transition (Vi/Vc(vl)) and dorsal paratrigeminal (dPa5) regions independent of intensity. Intravitreal injection of norepinephrine greatly reduced light-evoked Fos-LI at the Vc/C1, dPa5 and NTS, but not at the Vi/Vc transition. Lidocaine applied to the ocular surface had no effect on Fos-LI produced in trigeminal brainstem regions. These results suggested that multiple regions of the caudal trigeminal brainstem complex integrate light-related sensory information. Fos-LI produced at the dPa5 and NTS, coupled with norepinephrine-induced inhibition, was consistent with the hypothesis that light-evoked activation of trigeminal brainstem neurons involves an intraocular neurovascular mechanism with little contribution from neurons that supply the ocular surface.
Okamoto
K
K
Department of Diagnostic and Biological Sciences, University of Minnesota School of Dentistry, 515 Delaware St. SE, Minneapolis, MN 55455, USA. okamo007@umn.edu
Thompson
R
R
Tashiro
A
A
Chang
Z
Z
Bereiter
D A
DA
D000818:Animals; D015415:Biomarkers / Q000032:analysis / Q000378:metabolism; D006261:Headache / Q000209:etiology / Q000378:metabolism / Q000503:physiopathology*; D007150:Immunohistochemistry; D008297:Male; D009474:Neurons / Q000378:metabolism / Q000528:radiation effects*; D009638:Norepinephrine / Q000494:pharmacology; D020795:Photophobia / Q000378:metabolism / Q000503:physiopathology*; D020671:Posterior Horn Cells / Q000378:metabolism / Q000528:radiation effects; D016760:Proto-Oncogene Proteins c-fos / Q000378:metabolism / Q000528:radiation effects*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012161:Retinal Artery / Q000187:drug effects / Q000502:physiology; D017552:Solitary Nucleus / Q000378:metabolism / Q000503:physiopathology; D013376:Substantia Gelatinosa / Q000378:metabolism / Q000503:physiopathology; D014275:Trigeminal Caudal Nucleus / Q000378:metabolism / Q000503:physiopathology*
D015415:Biomarkers; D016760:Proto-Oncogene Proteins c-fos; D009638:Norepinephrine