Early life exposure to cortisol in zebrafish (Danio rerio): similarities and differences in behaviour and physiology between larvae of the AB and TL strains

Abstract

Maternal stress and early life stress affect development. Zebrafish (Danio rerio) are ideally suited to study this, as embryos develop externally into free-feeding larvae. The objective of this study was therefore to assess the effects of increased levels of cortisol, mimicking thereby maternal stress, on larval physiology and behaviour. We studied the effects in two common zebrafish strains, that is, AB and Tupfel long-fin (TL), to assess strain dependency of effects. Fertilized eggs were exposed to a cortisol-containing medium (1.1 μmol/l) or control medium from 0 to 6 h following fertilization, after which at 5-day following fertilization, larval behaviour and baseline hypothalamus–pituitary–interrenal cells axis functioning were measured. The data confirmed earlier observed differences between AB larvae and TL larvae: a lower hypothalamus–pituitary–interrenal axis activity in TL larvae than AB larvae, and slower habituation to repeated acoustic/vibrational stimuli in TL larvae than AB larvae. Following cortisol treatment, increased baseline levels of cortisol were found in AB larvae but not TL larvae. At the behavioural level, increased thigmotaxis or ‘wall hugging’ was found in AB larvae, but decreased thigmotaxis in TL larvae; however, both AB larvae and TL larvae showed decreased habituation to repeated acoustic/vibrational stimuli. The data emphasize that strain is a critical factor in zebrafish research. The habituation data suggest a robust effect of cortisol exposure, which is likely an adaptive response to increase the likelihood of detecting or responding to potentially threatening stimuli. This may enhance early life survival. Along with other studies, our study underlines the notion that zebrafish may be a powerful model animal to study the effects of maternal and early life stress on life history.

Early Life Glucocorticoid Exposure Modulates Immune Function in Zebrafish (Danio rerio) Larvae

“…These studies showed that, as a result of the cortisol exposure during early embryonic stages, larval baseline levels of cortisol were increased (10,13,14,21). For example, in a recent study, we have demonstrated that cortisol exposure between 0 and 6 hpf increased baseline cortisol levels 5 days post fertilization (dpf) of larvae from the AB strain (10).…”

“…Cortisol has been shown to signal through the GR already during the very early stages of embryonic development; in oocytes maternally deposited cortisol and GR mRNA are present (7)(8)(9). These cortisol levels may reflect the allostatic load that the mothers experience in their environment and it has therefore been hypothesized that these deposited cortisol levels are important for preparing the offspring for the expected allostatic load that larvae will encounter in the prevailing environment, thereby programming their cortisol secretion and the functioning of physiological systems to meet expected demands (4,10). While in previous studies we focussed on the effects of cortisol exposure [between 0 and 6 h post fertilization (hpf)] on vigilance-related behavior, baseline cortisol levels and metabolism in larvae (10,11) in the present study we focussed on the functioning of the immune system, in particular the innate immune system on which zebrafish larvae rely (12).…”

“…While in previous studies we focussed on the effects of cortisol exposure [between 0 and 6 h post fertilization (hpf)] on vigilance-related behavior, baseline cortisol levels and metabolism in larvae (10,11) in the present study we focussed on the functioning of the immune system, in particular the innate immune system on which zebrafish larvae rely (12). Cortisol exposure may be considered to be a model or proxy of maternal stress (10,(13)(14)(15).…”

“…In these studies cortisol levels were increased by injection of cortisol into the yolk of one-cell stage embryos [e.g., (13)(14)(15)] or through addition of cortisol to the medium (10,21). These studies showed that, as a result of the cortisol exposure during early embryonic stages, larval baseline levels of cortisol were increased (10,13,14,21). For example, in a recent study, we have demonstrated that cortisol exposure between 0 and 6 hpf increased baseline cortisol levels 5 days post fertilization (dpf) of larvae from the AB strain (10).…”

“…Exposure to cortisol (0-5 dpf) has been shown to lead to an enhanced expression of immune-related genes in zebrafish larvae at 5 dpf (21), suggesting that early cortisol exposure increases the activity of the immune system. In the present study, we have first measured the expression of a selected number of these up-regulated genes at 5 dpf (socs3a, mpeg1.1, mpeg1.2, irg1l) following our 0-6 hpf exposure to cortisol (10) in zebrafish larvae of the AB strain to assess whether our method produces similar effects. Tüpfel long-fin (TL) is another widely used zebrafish strain next to AB and is characterized by spots rather than stripes as well as long fins rather than short fins.…”