Effects of methylphenidate on the aggressive behavior, serotonin and dopamine levels, and dopamine-related gene transcription in brain of male Nile tilapia (Oreochromis niloticus)

Elevated temperature impairs gonadal development by suppressing the expression of the genes for kisspeptin, GnRH1 and GTH subunits in Nile tilapia Oreochromis niloticus

Temperature is a preeminent factor in the regulation of fish reproduction and hinders gonadal development beyond a specific threshold. To comprehend the molecular mechanism responsible for reproductive suppression at different temperature, expression of the genes encoding kisspeptin (kiss2), gonadotropin-releasing hormone (gnrh1) and their receptors (gpr54, gnrh1r) in the brain, and the gonadotropin (GTH) subunits (fshb and lhb) in the pituitary were studied in juvenile Nile tilapia (Oreochromis niloticus) along with gonadal histology. Fish were acclimatized to three distinct temperatures, including 31 °C, 34 °C and 37 °C for 14 days. The mRNA levels of kiss2, gpr54, gnrh1, and gnrh1r were significantly decreased at 37 °C compared to 31 °C and 34 °C in the both sexes. In parallel, the expression level of fshb in the both sexes and lhb in the female were significantly lower at 37 °C in the pituitary. Histologically, the gonads of both sexes had normal growth of gametes at control temperature (31 °C), whereas the spermatogenesis and oocyte maturation were slowed down and atretic oocytes were found in the ovary at 37 °C acclimation temperature. Taken together, the results imply that elevated temperature beyond the specific threshold may have a negative impact on reproduction by suppressing the gene expressions of kisspeptin/GnRH1/GTH system and eventually restrains normal growth and maturation of gametes in the both sexes of Nile tilapia.

The kisspeptin-GnIH signaling pathway in the role of zebrafish courtship and aggressive behavior induced by azoxystrobin

Azoxystrobin, a strobilurin widely used to control rice diseases, has raised concerns about possible adverse effects on aquatic ecosystems. At present, very little is known about the effects of azoxystrobin on courtship and aggressive behavior and the potential underlying mechanisms. In the present study, after exposing adult male and female zebrafish to worst-case scenario concentrations of azoxystrobin (0, 2 μg/L, 20 μg/L, and 200 μg/L) for 42 d, we observed a decrease in courtship behavior and an increase in aggressive behavior in both male and female zebrafish. In addition, to elucidate the molecular mechanism of the behavioral effects of azoxystrobin, we quantified the changes in the concentrations of kisspeptin, 5-HT, GnIH, and their corresponding receptor mRNA expression in the brain. The results showed that 200 μg/L azoxystrobin decreased the concentrations of kisspeptin and increased the concentration of GnIH in both male and female zebrafish brain. In addition, azoxystrobin also significantly reduced 5-HT concentration in female zebrafish brain. Further investigation revealed that altered courtship and aggressive behavior were associated with the expression levels of genes (kiss1, kiss2, gnrh3, gnrhr3, 5ht1a, and 5ht2a) involved in kisspeptin-GnIH signaling pathway. In conclusion, our study suggested that azoxystrobin may impair courtship and aggressive behavior in zebrafish by interfering with the kisspeptin-GnIH signaling pathway, which may have more profound effects on natural zebrafish populations.

Effects of food quantity on aggression and monoamine levels of juvenile pufferfish (Takifugu rubripes)

Aggressive behavior is important for animals to obtain limited resources. Understanding fish behavior and physiological response is of great significance to evaluate aquaculture production and fish welfare. Food is an important trigger of aggressive behavior in juvenile fish under high-density aquaculture conditions. The aim of this study was to investigate the aggressive behavior and monoamine levels of juvenile pufferfish (mean body mass of 6.29 ± 0.33 g) under normal feeding and restricted feeding. Our main results included the following: (1) The mortality and fin damage were higher and aggression was more intense of juvenile pufferfish at the 1% ration than those of the 3% ration; (2) during feeding, the velocity, body contact, and activity at the 1% ration were significantly higher than that of the 3% ration; (3) the concentrations of brain 5-hydroxyindoleacetic acid (5-HIAA) and monoamine oxidase A (MAOA) at the 1% ration were significantly lower, and dopamine (DA) concentrations were significantly higher. These results suggest that juvenile pufferfish shows serious aggressive behavior at the low ration, which may be related to the decrease of 5-HIAA and MAOA concentrations, and the increase of DA concentrations.

Chronic exposure to environmentally realistic levels of diuron impacts the behaviour of adult marine medaka (Oryzias melastigma)

Diuron, a commonly used herbicide and antifouling biocide, has been frequently detected in seawater. The effects of diuron on fish behaviour are currently poorly understood. Herein, the marine medaka (Oryzias melastigma) was continuously exposed to environmentally realistic levels of diuron from the fertilised egg stage to the adult stage. Behavioural evaluation of adult marine medaka indicated that exposure to diuron increased anxiety in the light-dark test and increased predator avoidance. In addition, diuron exposure significantly reduced aggression, social interaction, shoaling, and learning and memory ability. However, only negligible variations in foraging behaviour and in behaviour in the novel tank test were observed. Marine medaka chronically exposed to diuron also showed decreased levels of dopamine in the brain, and changes were observed in the transcription of genes related to dopamine synthesis, degradation and receptors. Exposure to 5000 ng/L diuron caused significant downregulation of the expression of the genes of tyrosine hydroxylase and monoamine oxidase and significantly upregulated the expression of the genes of the D5 dopaminergic receptor. The relative expression of the D4 dopaminergic receptor was significantly upregulated in the 50, 500 and 5000 ng/L diuron-treated groups. These findings highlight the significant neurotoxic effects of diuron and the extent to which this may involve the dopaminergic system of the brain. More broadly, this study reveals the ecological risk associated with environmentally realistic levels of diuron in marine animals.

Oxidative stress, biotransformation enzymes and histopathological alterations in Nile tilapia (Oreochromis niloticus) exposed to new and used automotive lubricant oil

Lubricant oils are among oil-based products that are not fully consumed during its use, thereby producing non-biodegradable residues which can cause contamination of natural systems. This study evaluated the toxicity of new and used lubricating oil (0.01 and 0.1 mL L^-1) in adult Nile tilapia (Oreochromis niloticus), by assessing the effects on oxidative stress, biotransformation enzymes (liver and gills), and histopathological alterations on hepatic and pancreatic tissues after 3 and 7 days of exposure. Results showed that 3-days exposure to 0.1 mL L^-1 of used and new lubricating oil increased the activity of superoxide dismutase (SOD) and malondialdehyde (MDA) levels in liver of O. niloticus, respectively. In gills, catalase (CAT) was decreased in fish exposed to 0.1 mL L^-1 of non-used oil after 3 days, but pronounced increases in CAT was detected after 7 days-exposure to both new and used oil. Shorter exposure to both concentrations of new and used oil also raised glutathione-S-transferase activity (GST) in gills. Ethoxyresorufin-O-deethylase (EROD) was induced in liver of fish exposed to 0.1 mL L^-1of used oil after 3 and 7 days, however a reduced response of this enzyme was detected in gills of animals from both oil treatments. In vitro analysis showed that hepatic EROD was inhibited by lubricating oil exposures, with more pronounced responses in treatments containing used oil. Hepatic lesions, such as cytoplasmic vacuolization, nuclei abnormally, changes in hepatocytes shape, steatosis, cholestasis, eosinophilic inclusions and necrosis were mainly increased by 7 days exposure to used lubricating oil at higher concentration.

Effects of sub-chronic methylphenidate on risk-taking and sociability in zebrafish (Danio rerio)

Attention deficit hyperactive disorder (ADHD) is the most common psychiatric disorder in children affecting around 11% of children 4-17 years of age (CDC 2019). Children with ADHD are widely treated with stimulant medications such as methylphenidate (Ritalin^®). However, there has been little research on the developmental effects of methylphenidate on risk-taking and sociability. We investigated in zebrafish the potential developmental neurobehavioral toxicity of methylphenidate on these behavioral functions. We chose zebrafish because they provide a model with extensive genetic tools for future mechanistic studies. We studied whether sub-chronic methylphenidate exposure during juvenile development causes neurobehavioral impairments in zebrafish. Methylphenidate diminished responses to environmental stimuli after both acute and sub-chronic dosing. In adult zebrafish, acute methylphenidate impaired avoidance of an approaching visual stimulus modeling a predator and decreased locomotor response to the social visual stimulus of conspecifics. Adult zebrafish dosed acutely with methylphenidate demonstrated behaviors of less retreat from threatening visual stimuli and less approach to conspecifics compared with controls. In a sub-chronic dosing paradigm during development, methylphenidate caused less robust exploration of a novel tank. In the predator avoidance paradigm, sub-chronic dosing that began at an older age (28 dpf) decreased activity levels more than sub-chronic dosing that began at earlier ages (14 dpf and 21 dpf). In the social shoaling task, sub-chronic methylphenidate attenuated reaction to the social stimulus. Acute and developmental methylphenidate exposure decreased response to environmental cues. Additional research is needed to determine critical mechanisms for these effects and to see how these results may be translatable to neurobehavioral toxicity of prescribing Ritalin^® to children and adolescents.