Functional analysis of the nonvisual opsins melanopsin and vertebrate ancient opsin (VA opsin) in Atlantic salmon (Salmo salar)
Abstract
The role of nonvisual photoreceptors is yet to be elucidated regarding the link to biological
function. This study aims to characterise the expression pattern of melanopsin and vertebrate
ancient opsin during early developmental stages of Atlantic salmon, and to relate the function
of these genes to hatching.
Fertilised salmon’s eggs were subjected to different light qualities using LED technology
where intensity and spectrum was manipulated. Two light regimes, 24 hours of continuous
light (LL) and 14 hours of light:10 hours of darkness (LD) of white light of different intensities,
high, medium, and low, beside different light spectrum of the same intensity, deep red, amber,
green, blue, royal blue, and ultra violet were used. Continuous dark was used as a control. Eggs
were monitored during the study period and hatched eggs recorded. In situ hybridization
technique was used to characterise the expression of two nonvisual opsin, the vertebrate ancient
opsin (VA) and melanopsin.
The results from hatching experiment show that, while continues white light and LD cycles of
white light of the medium intensity increase the hatching period (span), LD cycles of the low
intensity white light decease it significantly. However, the time to 50% of hatching is
significantly increased by LD cycles of green light blue light, and low intensity of white light.
The results from the expression experiments has shown that both melanopsin and VA opsin are
expressed in the brain of salmon during the early developmental stages. Both were found in the
left habenula, thalamus, hindbrain and spinal cord. Moreover, they have been found to be colocalised
in several regions in the brain. Furthermore, regional specific neural activation was
found in the habenula and hindbrain, where melanopsin and VA opsin are co-localized, upon
light stimulation. This indicate direct photoreception in these brain regions already around
hatching. The result indicates that, apparently, the hatching process in salmon may be affected by light
to some degree, but there is no strong inhibition of hatching by light such reported for Atlantic
halibut. Other factors like the temperature and low levels of oxygen might be other
environmental cues that are used by Atlantic salmon to regulate the time of hatching. The
nonvisual system is clearly developed and functional prior to hatching and may be part of the
regulation of hatching. The specific nonvisual hindbrain cluster found to regulate hatching in
Atlantic halibut are not apparent in salmon. Our data clearly shows the important of nonvisual
photoreception in the brain at early developmental stages of fish, prior to development of
functional eyes. There seems to be species-specific patterning of the nonvisual photoreceptors
in the brain, which indicates species specific tailoring of biological function.
Publisher
The University of BergenCollections
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