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PE Malkki, RH Kroeger; Circadian Rhythms in Retina and Behavior of the Cichlid Fish Aequidens Pulcher . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1353.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose:In many fishes, photoreceptors and melanosomes in retinal pigment epithelium cells move during light and dark adaptation. Such retinomotor movements (RMM) are controlled by light and endogenous, circadian signals. We investigated the maturation of circadian rhythms in rods and cones, as well as in behavioral activity in Aequidens pulcher. Methods:Experiments were performed with juvenile and adult fish (4 and 14 months, respectively). All animals were entrained to a 12:12h light:dark cycle. Each age group consisted of three experimental groups: light at daytime (LD), darkness at nighttime (DN), and darkness at daytime (DD). The eyes of the LD and DD groups were excised, opened, and fixated at noon, eyes from DN groups at midnight. Dissections in darkness were done with infrared viewing equipment. Prior to dissection, the DD groups were subjected to a five-day behavioral experiment. Three days of normal light:dark cycles were followed by 1 1/2 days of darkness. Activity of the fishes was monitored with active, infrared photodiodes. At the second day of darkness, retinae from DD groups were processed for histology. Photoreceptor positions were measured as the distance between the outer limiting membrane and the insertion of the outer segment at the inner segment. Results:Adult animals showed marked differences in activity between day and night. Those differences persisted in darkness during expected day. In juvenile animals, the cycles were less obvious. The positions of 100 rods and cones each were determined in 5 fish from each experimental group. In adults, cones in the DD group were fully contracted and rods elongated to about 70% of the value measured in the LD group. In juveniles, cones were also fully contracted in the DD group. The rods, however, were elongated only to the extend necessary to make room for the cones at the outer limiting membrane (about 40% of the value found in LD animals). Conclusion:Schooling behavior of juveniles may be the reason for inconclusive behavioral results from this group. Rod elongation in the adult DD group and cone contraction in both DD groups indicate the influence of a circadian oscillator. In the juvenile DD group, contracting cones may have displaced passive rods from the outer limiting membrane. Our results are consistent with the hypothesis that the sensitivity of rods to circadian signals or the generation of such signals by the rods themselves matures after such mechanisms are fully functional in cones.
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