Peirson, Stuart Neil; (2001) The photopigment content of the teleost pineal organ. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Numerous studies have addressed the issue of the photopigment complement of the retinae of different vertebrate species. However, in most non-mammalian vertebrates, extraretinal photoreceptive sites also exist, of which the pineal complex is the most evident. The pineal organ of lower vertebrates contains photoreceptive cells, similar to the cones of the retina, and is thought to play an important role in the temporal organization of the animal, through both a neural output to the brain and the synthesis of the photoperiod-mediating hormone melatonin. The visual system of teleost fishes has been studied extensively, particularly in regard to its adaptations to different photic environments. Although several studies have addressed extraretinal photoreception in teleosts, little is known as to the exact nature of the photopigments found in the teleost pineal organ, in terms of the number of photopigments present, their spectral sensitivity and their relationship to the visual pigments of the retina. Microspectrophotometry (MSP) was used to determine the absorbance characteristics of the pineal photoreceptors of the goldfish (Carassius auratus), revealing a single photoreceptor population with a λmax of 511 nm. Because of the variable A₁/A₂ chromophore content in these photoreceptors, the pineal photopigment was also bleached and regenerated with a single artificial chromophore, 9-cis retinal. Comparison with 9-cis data from goldfish retina suggested the opsin present in the pineal was most similar to the retinal rod opsin, although slightly blue-shifted. MSP was also conducted on the native pineal photoreceptors of another cyprinid, the golden orfe (Leuciscus idus) and a characid, the Mexican tetra (Astyanax fasciatis) in which similar pineal-specific photopigments were identified. The goldfish pineal was screened for retinal opsins, and during this process a novel rod-like opsin, exo-rod opsin, was identified, which has since been demonstrated to occur uniquely within the teleost pineal organ. The full coding sequence of the goldfish exorod opsin was obtained, including gene structure.

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