The role of melanocortin system in the regulation of circadian rhythms in fish

The melanocortin complex system is composed of agonists, antagonists, several receptors, and accessory proteins. They are expressed in different tissues and participate in several physiological functions. The endogenous antagonist, agouti-signaling protein (ASIP), binds MC1R and MC4R decreasing their constitutive activities to regulate skin pigmentation and energy expenditure, respectively. Overexpression of ASIP1 in zebrafish (Danio rerio) disrupts the dorsoventral pigmentation pattern and enhances growth yet no obesity. Melanocortin system is also involved in the regulation of fish behavior. ASIP1 transgenic fish show higher levels of anxiety-related behaviour, are sight, display reduced exploratory behaviour, and are less aggressive than wild-type fish yet no less active. For that reason, we were interested in evaluating circadian rhythms (LCRs). During a month, we measure the locomotor activity of both wild type and ASIP1 transgenic fish using a custom system composed of photosensitive cells connected to Automata software. Data were analyzed with “El Temps” software that also allows creating chronograms and periodograms. Chronograms showed heightened locomotor activity in ASIP1 fish during the dark phase, and periodograms revealed the absence of circadian periodicity in locomotor activity in these fish. The brain and liver of 10 animals were dissected every 4 hours for a period of 24 hours on order to evaluate the daily expression patterns of clock related genes including: cry1a, per1a, clock1a, bmal1 and nr1d1. All three per1a, clock1a and nr1d1 showed enhanced expression levels during the dark phase of the photoperiod in ASIP1 fish. On the contrary, bmal1 expression ASIP1 fish raised coinciding with the lights turned on and during the light phase. These findings provide evidence for the role of the melanocortin system in the regulation of circadian rhythms