Fish Endocrinology

In recent years the G&R Group has generated new knowledge neuroendocrine mechanisms in fish concerned with light perception, circadian biology, the control of reproduction (brain-pituitary-gonadal axis), growth (somatropic axis), smoltification and welfare. The group used various RIA, ELISA and other enzymatic assays to monitor changes in hormones involved in these key physiological functions in response to changes in the environment in a range of species. Furthermore, a large effort has been put toward the development of molecular tools (QPCR) to study gene expression of key genes along these axes like melatonin, clock genes, kisspeptin or gonadotropins. The Group is also developing a simple, real time, on farm maturation detection kit initially developed for cod and salmon based on lateral flow technology already used for human pregnancy tests (immunochromatographic assays). It is foreseen that such a technology could be used for diagnostic purposes (e.g. smoltification).

Circadian biology in fish

Central to understanding how the environment entrains physiological processes like reproduction is understanding how the environment is perceived and interpreted into neuroendocrine signals. The G&R group is thus actively studying key components of the circadian system (photoreceptors involved in the light reception, clock mechanisms that regulate the rhythms and neuroendocrine regulation of physiological functions). Central to this work is the indoleamine melatonin which is known to be a key biological time keeping hormone or “zeitgeber” which is entrained by light and displays circadian (daily) and circannual (seasonal) rhythms in all vertebrates studied to date. These rhythms can also be self-sustained and are under the control of endogenous circadian clocks. As such melatonin seems to play a major role in synchronising many behavioural (locomotor, feeding, shoaling, and migration activities) and physiological (growth, reproduction, immunity) processes across the animal kingdom. We use melatonin as a tool to interpret how the environment is perceived and hence can influence physiology

Light sensitivities

One of the major questions of circadian biology in fish is what levels of illumination are perceived as day and what are perceived as night? As plasma melatonin may have to be suppressed below a certain threshold before artificial photoperiods are capable of altering physiological functions, not only the photoperiod but also the intensity, spectral content and orientation of submerged lamps should be considered when designing effective artificial lighting systems for fish.
Through in vivo and in vitro studies, the group has shown large differences between species in light sensitivity with cod being the most sensitive teleosts studied to date. Furthermore, fish have been shown to be more sensitive to short wavelengths (blue end of the visible spectrum). Work is on going. These results are of course of prime importance for the aquaculture industry as they provide intensity threshold levels that can help the industry to adjust the lighting system/design applied in cages for a variety of economically important species in which early puberty is a major problem.

Circadian organization

At the core of any circadian rhythm is a network of autonomous endogenous oscillators, also called biological clocks or circadian pacemakers, which in the case of mammals feed information to a master clock found in the Suprachiasmatic Nucleus (SCN), synchronizing their physiology to the photic conditions. Results to date in teleosts have suggested a more decentralized organization in fish compared to that found in higher vertebrates, where the pineal gland is light sensitive and independent of the SCN (or similar structure still to be found) or eyes (retina) and may contain, depending on the species, an endogenous oscillator that can sustain in vitro melatonin rhythms. However, recent studies performed by the group have shown a different type of circadian organisation in Nile tilapia and African catfish, characterised by a pineal gland which is not light sensitive or far less sensitive than previously studied teleost species with the melatonin synthesis being entrained by the retina.. The Group is now trying to better characterise these differential circadian systems in fish.

Clock controlled circadian rhythm

Molecular clocks are a central component of circadian biology. Studies performed by the group have demonstrated the presence of circadian melatonin oscillators which can anticipate daily photic changes and maintain strong circadian rhythmic melatonin production under darkness in fish, they have also shown evidence of circannual (calendar) systems which entrain reproduction in fish. Furthermore, research has shown that short photocycles appear to disrupt endogenous melatonin rhythms, possibly by affecting the transcriptional-translational feedback loops of the circadian clock directly. Future work will aim to confirm these hypotheses in an attempt to better characterise the endogenous clock system in fish.
In higher vertebrates, the molecular basis of the circadian clock has been shown to consist of feedback loop mechanisms involving a number of clock genes (mainly BMAL, Clock, Per's, Cry's) entrained by light which maintain and synchronise self-sustained rhythms. Understanding these endogenous rhythms in fish is still in its infancy. The group has been developing qRT-PCR assays to measure mRNA expression of key components of this system. Preliminary results confirm the clocks presence and cycling but importantly also elude to a circannual element to their function in salmonids. Isolation and characterisation of this system is key to linking circadian biology to the entrainment of downstream physiology.


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