Light-induced trigeminal sensitization without central visual pathways: another mechanism for photophobia.

PURPOSE The authors investigated whether trigeminal sensitization occurs in response to bright light with the retina disconnected from the rest of the central nervous system by optic nerve section. METHODS In urethane-anesthetized rats, trigeminal reflex blinks were evoked with air puff stimuli directed at the cornea in darkness and at three different light intensities. After normative data were collected, the optic nerve was lesioned and the rats were retested. In an alert rat, reflex blinks were evoked by stimulation of the supraorbital branch of the trigeminal nerve in the dark and in the light. RESULTS A 9.1 × 10(3) μW/cm(2) and a 15.1 × 10(3) μW/cm(2) light significantly enhanced the magnitude of reflex blinks relative to blinks evoked by the same trigeminal stimulus when the rats were in the dark. In addition, rats exhibited a significant increase in spontaneous blinking in the light relative to the blink rate in darkness. After lesioning of the optic nerve, the 15.1 × 10(3) μW/cm(2) light still significantly increased the magnitude of trigeminal reflex blinks. CONCLUSIONS Bright lights increase trigeminal reflex blink amplitude and the rate of spontaneous blinking in rodents. Light can modify trigeminal activity without involving the central visual system.