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

Abstract

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.

Keywords: HPI axis; daily rhythm; fish welfare; light; temperature; thermocycles; wavelength.

Figure 1

Schematic diagram of the hypothalamus-pituitary-interrenal (HPI) axis (A). Corticotropin-releasing hormone (Crh) is synthesized in the hypothalamus and stimulates, at the pituitary, the synthesis and release of adrenocorticotropic hormone (Acth), which is formed from Proopiomelanocortin (Pomc). Acth stimulates the production and release of cortisol in the interrenal cells. In fish, the HPI axis presents daily rhythms at all of its levels. To the right of the figure, representative examples of the rhythms of crh expression (B), pomca expression (C), and plasma cortisol (D) from Senegalese sole are shown. Mean ± S.E.M. are represented by the bars and errors, the continuous curve represents the cosine function calculated from a significant Cosinor analysis (p < 0.05). White and black bars above the graphs represent the light and dark period, respectively. Modified with the permission of authors from López-Olmeda et al. (83).

Figure 2

Fitness diagrams of (A) zebrafish exposed to different light spectrum (violet, blue, green, yellow, and red), and (B) Senegalese sole larvae at 30 DPH raised under constant temperature (21.5°C), or two daily thermocycles: TC (22°C-day:19°C-night) or CT (19°C-day:22°C-night). In (A), lines represent relative values for malformations (vertical, downwards arrow), survival rate (horizontal, left arrow), gut content (vertical, upwards arrow), and expression of igf2 (rigth-up) and crh (right-down) genes. Modified from Villamizar et al. (143). In (B), vertical upwards arrows represent relative values for total length, while downwards arrows represent malformation rates. Different letters indicate significant differences. Modified with the permission of authors from Blanco-Vives et al. (141).

Figure 3

Daily rhythms of locomotor activity (A) and differences in the cortisol response depending on the time of the day (B) in the gilthead sea bream and Senegalese sole. The black area in the waveforms represents the mean values of activity and the continuous line the S.D. White and black bars above the waveforms represent the light and dark period, respectively. A stress challenge was applied to both species, consisting of air exposure during 30 s, at different time points of the LD cycle: ZT2 and 14 h for sea bream, and ZT1 and 13 h for sole. Fish were sampled 1 h after the stress and cortisol was evaluated. Unstressed control groups were sampled at all-time points. Different letters indicated significant differences between groups (ANOVA, p < 0.05) (small case letter for sea bream and upper case letters for sole). Modified with the permission of authors from López-Olmeda et al. (83) and Vera et al. (88).

Figure 4

Daily variations of mortality of zebrafish exposed to different MS-222 (A) and eugenol (B) concentrations after 15 min exposure at mid-light (ML; white circles) or mid-dark (MD; black circles) [with the permission of authors from Sánchez-Vázquez et al. (171)]. Sea bream mortality after 15 min exposure to different MS-222 (C) concentrations at ML or MD [with the permission of authors from Vera et al. (172)]. A logistic curve (dotted lines) was fitted to mortality rate (six independent replicates with n = 8). (D) Daily rhythm of mortality of zebrafish larvae exposed to 5% ethanol for 1 h. Different letters indicate significant differences (ANOVA I, p < 0.05), while the dotted black line represents the sinusoidal function fit (Cosinor analysis, p < 0.05).