BRAIN RESEARCH ELSEVIER Brain Research 7 16 (I 996) 2 13-2 I8 Short communication Fos expression in rat visual cortex induced by ocular input of ultraviolet light Shimon Amir * , Barry Robinson Accepted 27 December 1995 Abstract We used immunostammg for the cellular transcription factor Fos to assess patterns of neuronal activation in rat visual cortex during exposure to ultraviolet light. Exposure to monochromatic ultraviolet light (A,,,, 360 nm: half-bandwidth 8.8 nm. IO pW/cm’ at eye level) induced strong expression of Fos immunoreactivity in the primary visual cortex and associated cortical visual areas of dark-adapted rats. The stimulatory effect of ultraviolet light on Fos expression was related to exposure duration, was independent of stimulus novelty or phase of the circadian cycle in which exposure occurred, and it was mediated by a mechanism located in the eye. These results demonstrate that ocular input of ultraviolet light is capable of altering neuronal activity in cortical structures involved in visual processing and are consistent with the hypothesis that rodents may use ultraviolet light for vision. Ke>+co!-ds: Ultraviolet light: Visual cortex: Fos immunohirrochemistry: Ultraviolet vision: Rat It is often assumed that mammals cannot discriminate the presence of ultraviolet light [ 12,261. Recent evidence in rodents indicates, however, that rats, mice, gophers and gerbils have retinal photoreceptors that are preferentially tuned to ultraviolet wavelengths [9,11,15,16]. This finding, combined with reports that some rodents can use ultravio- let information for visual discrimination [ 15,161. suggests that contrary to the common view rodents may use ultravi- olet light for vision. We have reported previously that ocular input of ultraviolet light induces the expression of the cellular transcription factor Fos, a marker of neuronal activation in the suprachiasmatic nucleus (SCN) circadian pacemaker in dark-adapted rats [ 1 I. Here we report that in the same animals ultraviolet light stimulated the induction of Fos expression in the visual cortex. Such an observation is consistent with a role for ultraviolet light in vision. As previously described [l], adult male Wistar rats (300-325 g) were housed in a temperature-controlled room under 12 h: 12 h light-dark cycle (lights on 20.00-0X.00 h) with free access to food and water for at least two weeks. They were then transferred to test chambers painted flat black (75 X 65 X 8.5 cm) and placed individually in plastic cages also painted flat black (36 X 24 X 20 cm) where they were kept in complete darkness for 24 h. The animals were tested on the morning of the second day in darkness. The Correspondingauthor. Fax: (1) (5 14) 848-28 17. 0006.8993/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved PII SOOOS-X993(96)00025-X test stimulus consisted of a monochromatic ultraviolet light pulse (A,,, 360 nm; half-bandwidth 8.8 nm). The light source consisted of an ultraviolet lamp (320-400 nm, h,,, 365 nm; Mineralight) equipped with a narrow band-pass interference filter (A,,,, 360 nm; half-bandwidth, 8.8 nm: Oriel Co., Stratford, CT: model no. 58650). The light source was fixed above the cage to provide irradiance of 10 pW/cm’ at eye level (measured with an Oriel Goldilux ultraviolet meter). Animals were anaesthetized with an overdose of sodium pentobarbital (100 mg/kg) 60 min after the onset of exposure and perfused transcardially with 200 ml of cold physiological saline (0.9% NaCl) followed by 400 ml of cold, fresh 4% paraformaldehyde in a 0.1 M phosphate buffer (pH 7.3). Rats that were kept in constant darkness and perfused without prior exposure to ultraviolet light served as controls. Brains were removed, post-fixed in 4% paraformaldehyde overnight (4”C), and 50 pm thick coronal sections were processed for Fos immunohisto- chemistry using a mouse monoclonal antibody raised against the N-terminal sequence of Fos (corresponding to N-terminal residues 4- 17 of human Fos protein; NCI/BCB Repository, Quality Biotech. Camden, NJ; lot #41 l- 081887). Fos immunoreactivity (Fos-IR) was detected with a Vectastain Elite ABC Kit (Dimension Labs, Missis- sauga, Ont.) using diaminobenzidine as chromogen. Exposure of dark-adapted rats to ultraviolet light for 30 min induced extensive expression of Fos-like immuno- reactivity throughout the rostro-caudal extent of the visual