Environmental Cycles, Melatonin, and Circadian Control of Stress Response in Fish

Fish have evolved a biological clock to cope with environmental cycles, so they display circadian rhythms in most physiological functions including stress response. Photoperiodic information is transduced by the pineal organ into a rhythmic secretion of melatonin, which is released into the blood circulation with high concentrations at night and low during the day. The melatonin rhythmic profile is under the control of circadian clocks in most fish (except salmonids), and it is considered as an important output of the circadian system, thus modulating most daily behavioral and physiological rhythms. Lighting conditions (intensity and spectrum) change in the underwater environment and affect fish embryo and larvae development: constant light/darkness or red lights can lead to increased malformations and mortality, whereas blue light usually results in best hatching rates and growth performance in marine fish. Many factors display daily rhythms along the hypothalamus-pituitary-interrenal (HPI) axis that controls stress response in fish, including corticotropin-releasing hormone (Crh) and its binding protein (Crhbp), proopiomelanocortin A and B (Pomca and Pomcb), and plasma cortisol, glucose, and lactate. Many of these circadian rhythms are under the control of endogenous molecular clocks, which consist of self-sustained transcriptional-translational feedback loops involving the cyclic expression of circadian clock genes (clock, bmal, per, and cry) which persists under constant light or darkness. Exposing fish to a stressor can result in altered rhythms of most stress indicators, such as cortisol, glucose, and lactate among others, as well as daily rhythms of most behavioral and physiological functions. In addition, crh and pomca expression profiles can be affected by other factors such as light spectrum, which strongly influence the expression profile of growth-related (igf1a, igf2a) genes. Additionally, the daily cycle of water temperature (warmer at day and cooler at night) is another factor that has to be considered. The response to any acute stressor is not only species dependent, but also depends on the time of the day when the stress occurs: nocturnal species show higher responses when stressed during day time, whereas diurnal fish respond stronger at night. Melatonin administration in fish has sedative effects with a reduction in locomotor activity and cortisol levels, as well as reduced liver glycogen and dopaminergic and serotonergic activities within the hypothalamus. In this paper, we are reviewing the role of environmental cycles and biological clocks on the entrainment of daily rhythms in the HPI axis and stress responses in fish

Long photoperiod impairs learning in male but not female medaka

Day length in conjunction with seasonal cycles affects many aspects of animal biology. We have studied photoperiod-dependent alterations of complex behavior in the teleost, medaka (Oryzias latipes), a photoperiodic breeder, in a learning paradigm whereby fish have to activate a sensor to obtain a food reward.Medaka were tested under a long (14:10 LD) and short (10:14 LD) photoperiod in three different groups:mixed-sex, all-males, and all-females. Under long photoperiod, medaka mixed-sex groups learned rapidly with a stable response. Unexpectedly, males-only groups showed a strong learning deficit, whereas females- only groups performed efficiently. In mixed-sex groups, female individuals drove group learning, whereas males apparently prioritized mating over feeding behavior resulting in strongly reduced learning performance. Under short photoperiod, wheremedaka do notmate,male performance improved to a level similar to that of females. Thus, photoperiod has sex-specific effects on the learning performance of a seasonal vertebrate

Daily rhythms of the expression of key genes involved in steroidogenesis and gonadal function in zebrafish

Fish present daily and seasonal rhythms in spawning and plasmatic levels of steroids that control reproduction. However, the existence of the rhythms of expression of the genes that underlie the endocrine mechanisms responsible for processes such as steroidogenesis and reproduction in fish have still been poorly explored to date. Here we investigated the daily pattern of the expression of key genes involved in sex steroid production that ultimately set the sex ratio in fish. Adult zebrafish were maintained under a 12:12 h light-dark cycle at a constant temperature of 27°C and were sampled every 4 h during a 24-hour cycle. The expression of key genes in the gonads and brains of female and male individuals were analyzed. In gonads, the expression of aromatase (cyp19a1a, ovarian aromatase) and the antimüllerian hormone (amh, testis) was rhythmic, with almost opposite acrophases: ZT 5:13 h (in the light phase) and ZT 15:39 h (at night), respectively. The expression of foxl2 (forkhead box L2) was also rhythmic in the ovary (acrophase located at ZT 5:02 h) and the expression of dmrt1 (doublesex and mab-3-related transcription factor 1) was rhythmic in testes (acrophase at ZT 18:36 h). In the brain, cyp19a1b (brain aromatase) and cyp11b (11beta-hydroxylase) presented daily differences, especially in males, where the expression peaked at night. These results provide the first evidence for marked time-of-the-day-dependent differences in the expression of the genes involved in sex ratio control, which should be considered when investigating processes such as reproduction, sex differentiation and steroidogenesis in fish

Daily rhythms in the morphometric parameters of hepatocytes and intestine of the European sea bass (Dicentrarchus labrax): influence of feeding time and hepatic zonation

The digestive system presents daily rhythms at both physiological and histological levels. Although cell morphology rhythms in mammals have been reported, they have scarcely been investigated in fish. The aim of the present research was to investigate the existence of daily rhythms in the morphology of cells in the liver and intestine of a teleost fish, the European sea bass (Dicentrarchus labrax), and how feeding time influences them. Regarding liver, we also focused on differences between the two metabolic zones: perivenous and periportal. For this purpose, fish were divided into two groups: fish fed once a day in the mid-light phase (ML) or the mid-dark phase (MD). After one month under each feeding regime, liver and intestine samples were collected every 4 h during a 24-hour cycle, and different parameters were studied by image analysis and light microscopy. Daily rhythms occurred in most of the parameters evaluated in the liver. The effect of feeding time depended on the metabolic zone: the rhythms in the periportal zone were synchronized mainly by the light/dark cycle regardless of feeding time, whereas in the perivenous zone, rhythms were influenced more by feeding time. In the intestine, a daily rhythm in villi height was found with acrophases coinciding with feeding time in each group. These findings show for the first time the existence of cellular morphological rhythms in fish liver and intestine, and highlight the interactions between light and feeding cycles in the different metabolic zones of the liver

Relative gene expression of <i>cyp19a1a</i> (A) and <i>foxl2</i> (B) in ovary.

<p>Circles indicate the relative gene expression (mean±SEM) at each sampled time point. Different letters indicate the statistically significant differences between time points (one-way ANOVA, p<0.05). The sinusoidal curve calculated by Cosinor (p<0.05) is indicated by the continuous line. The white and black bars above the graphs represent the light and dark phase, respectively.</p

Relative gene expression of <i>amh</i> (A) and <i>dmrt1</i> (B) in testes.

<p>Squares indicate the relative gene expression (mean±SEM) at each sampled time point. Different capital letters indicate the statistically significant differences between time points (one-way ANOVA, p<0.05). The sinusoidal curve calculated by Cosinor (p<0.05) is indicated by the continuous line. The white and black bars above the graphs represent the light and dark phase, respectively.</p

Map of acrophases.

<p>Only the genes that displayed a statistically significant daily rhythm (Cosinor, p<0.05) are included in the graph. The acrophase is indicated by symbols and the fiducial limits (set at 95%) are indicated by lateral bars. The pink circles and blue squares indicate the genes analyzed in the ovary and testes, respectively. The white and black bars above the graph represent the light and dark phase, respectively.</p

Representative actograms and mean waveforms of locomotor activity of zebrafish males (A, B) and females (C, D).

<p>Zebrafish were subjected to 12:12 LD and fed daily at ZT 4 h. Actograms were double-plotted (time scale 48 h). The height of each point represents the number of infrared light-beam interruptions/10 min. Each horizontal line shows one experimental day on the vertical axis, and the hours of the day are represented on the X-axis. The black bar at the top of each actogram represents the dark phase, and the white one represents the light phase of the LD conditions. In the meanwaves, each point was calculated as the mean±S.D from the 10-minute binned data across all the experimental days. Each waveform is represented as single-plotted.</p