Melatonin Receptors in Chick Ocular Tissues: Implications for a Role of Melatonin in Ocular Growth Regulation

Abstract

Results of this study show that all three melatonin receptor subtypes are expressed in retinal and extraretinal ocular tissues of the chick eye. The finding that administration of melatonin alters the growth of several ocular tissues in both control and form-deprived eyes suggests that melatonin, acting through specific melatonin receptors in ocular tissues, plays a role in ocular growth and development. This conclusion suggests that the action of melatonin, combined with expression of melatonin receptors, is involved in the regulation of the diurnal rhythm of ocular growth.

Melatonin Receptor Expression in the Zebra Finch Brain and Peripheral Tissues

“…Chicken retinal Mel-1C mRNA was also found to be rhythmic in LD with high expression levels during the day but opposite phasing in DD while Mel-1B was highly variable and arrhythmic (Natesan and Cassone, 2002). A comparison of our mRNA data with melatonin receptor protein levels in the chicken retina revealed certain differences in peak expression times; while the Mel-1A receptor was found to peak at ZT12 at the transition from light to dark, Mel-1B peaked at ZT20, and Mel-1C at ZT0 in the chicken retina (Rada & Wiechmann, 2006) we found mRNA in the retina to peak at ZT18 (Mel-1A), ZT10 (Mel1B), and Mel1C mRNA to be arrhythmic. These differences could be caused by differential expression of melatonin receptors in the different layers of the retina as both studies used whole tissue samples or by species-specific differences between the role the retina plays in galliform and passeriform circadian organisation (Brandstaetter, 2002).…”

“…Studies on avian melatonin receptors mainly focused on the retina (Rada and Wiechmann, 2006), gonads (Aste et al, 2001) and song-control nuclei in the brain (Cassone et al, 2008;Whitfield-Rucker and Cassone, 1996) photoreception takes place in the retina only and there has been an evolutionary loss of two retinal cone classes (Barrett et al, 2003) and of certain opsins which are found in nonmammalian vertebrates including birds and fishes (Bellingham et al, 2003). These differences in circadian organisation are believed to be the consequence of a "nocturnal bottleneck" during early evolution of mammals (Foster et al, 1993).…”

“…Rada and Wiechmann (2006) identified all three melatonin receptor types in the chicken retina-RPE-choroid and found Mel-1A and Mel-1B to have similar expression patterns with low levels occurring in the early morning and highest levels in the evening (Mel-1A peaked at ZT12 (lights off) and Mel-1B peaked at ZT20 (mid-late dark period) while Mel-1C showed out-of-phase oscillation to Mel-1A and Mel-1B with high levels in the early morning and low levels during the early-mid dark phase (ZT16). Chicken retinal Mel-1A mRNA has been found to be rhythmic under light/dark (LD) cycles and in constant darkness (DD), peaking at midday and mid-subjective day (Natesan and Cassone 2002).…”

“…The biological effects of melatonin are produced through the activation of three melatonin receptor types, while other effects are due to its role as a pervasive and powerful antioxidant with a particular role in the protection of nuclear and mitochondrial DNA (Reiter et al 2003;Rada and Wiechmann 2006;Dufourny et al, 2008). In 1994, the first membrane receptor was cloned from Xenopus laevis immortalized melanophores, which was sensitive to guanine nucleotides and its activation lead to an inhibition of adenylyl cyclase through a pertussis toxin-sensitive mechanism (Ebisawa et al, 1994).…”

The Influence of Light Exposure and Seasonal Changes on Short-Term and Longer-Term Changes in Axial Length of the Human Eye

“…Although the melatonin rhythm can be affected by many factors, light appears to have a major influence. Melanopsin, present in the ipRGCs in the retina, provides information about ambient environmental light to the SCN and hence melatonin levels appear to be influenced by retinal melanopsin signalling (Rada & Wiechmann, 2006). Melanopsin photopigment is sensitive to blue light (short wavelength) and insensitive to long wavelengths (Panda et al, 2005).…”