Short daylight photoperiod alleviated alarm substance-stimulated fear response of zebrafish.

Photoperiod has been well-documented to be involved in regulating many activities of animals. However, whether photoperiod takes part in mood control, such as fear response in fish and the underlying mode(s) of action remain unclear. In this study, adult zebrafish males and females (Danio rerio) were exposed to different photoperiods, Blank (12 h light: 12 h dark), Control (12 h light: 12 h dark), Short daylight (SD, 6 h light: 18 h dark) and Long daylight (LD, 18 h light: 6 h dark) for 28 days. After exposure, fear response of the fish was investigated using a novel tank diving test. After alarm substance administration, the onset to higher half, total duration in lower half and duration of freezing in SD-fish were significantly decreased, suggesting that short daylight photoperiod is capable of alleviating fear response in zebrafish. In contrast, comparing with the Control, LD didn't show significant effect on fear response of the fish. Further investigation revealed that SD increased the levels of melatonin (MT), serotonin (5-HT) and dopamine (DA) in the brain while decreased the plasma level of cortisol comparing to the Control. Moreover, the expressions of genes in MT, 5-HT and DA pathways and HPI axis were also altered consistently. Our data indicated that short daylight photoperiod might alleviate fear response of zebrafish probably through interfering with MT/5-HT/DA pathways and HPI axis.

Chronic exposure to environmentally relevant concentrations of carbamazepine interferes with anxiety response of adult female zebrafish through GABA /5-HT pathway and HPI axis.

Carbamazepine (CBZ) is one of the widely distributed pharmaceutical residues in aquatic environments, yet few researches have addressed its chronic effect on the anxiety of fish, and the mechanisms possibly involved remained elusive. In this study, adult female zebrafish (Danio rerio) were exposed to environmental relevant concentrations of CBZ (CBZ-low, 10 µg/L; CBZ-high, 100 µg/L) for 28 days. After exposure, CBZ-high didn't affect the anxiety of fish. However, the onset time to the higher half of the tank was delayed and the total duration in the lower half of the tank was increased in CBZ-low fish, suggesting an increased anxiety. Further investigation indicated that CBZ-low significantly decreased the gamma-aminobutyric acid (GABA) level in the brain, while increased the serotonin (5-HT) level in the brain and cortisol level in plasma. Accordingly, the mRNA levels of genes in GABA (gad2, abat, gabrb2, gabrg2, gria1a and slc12a2) pathway and HPI (crha, actha, pc1 and pc2) axis were also altered. Despite the upregulation of tph2 was consistent with increased 5-HT level in the brain, significantly downregulated htr1aa and htr1b may indicate attenuated 5-HT potency. Although CBZ-high significantly reduced GABA level in the brain and increased cortisol level in plasma, the effects were dramatically alleviated than that of CBZ-low. Consistently, the expression of genes in HPI (crha, actha, pc1 and pc2) axis and GABA (gad2 and abat) pathway were also altered by CBZ-high, probably due to inconspicuous anxiety response of CBZ-high. Briefly, our data suggested that low concentration of CBZ disrupted zebrafish anxiety by interfering with neurotransmission and endocrine system, thereby bringing about adverse ecological consequences.