Benzo[a]pyrene exposure and overcrowding stress impacts anxiety-like behavior and impairs learning and memory in adult zebrafish, Danio rerio.

Benzo[a]pyrene (B[a]P), a prototype of polycyclic aromatic hydrocarbons and ubiquitous environmental pollutant, alters neurobehavioral responses in aquatic organisms like zebrafish. Increasing organic load on water bodies causes population explosion leading to overcrowding (OC) stress. The effect of OC stress on neurobehavioral alterations remains unclear. The objective of our study is to elucidate the impact of OC stress on behavioral alterations and neurodegenerative phenotypes on exposure to B[a]P in zebrafish. We demonstrate the effects of OC stress (12 fish/L) on acute waterborne exposure to B[a]P (0.2 mg L ) in adult wild zebrafish. Anxiety-like behavior, learning, and memory impairment were assayed by novel tank diving test, light/dark preference test, and T-maze test. Oxidative stress bio-markers were assayed along with histopathological changes in zebrafish brain. OC stress significantly impaired the learning ability and mood behavior by increasing the number of transition and time spent in the alter zones. Increased lipid peroxidation and protein carbonyl formation with significant decreased catalase activity and reduced glutathione level showed oxidative stress on exposure to OC stress and B[a]P. Pyknotic neuronal counts dramatically increased in periventricular grey zone of optic tectum brain region of zebrafish. Our findings showed that OC stress modulates the B[a]P-induced behavioral alterations causing learning and memory deficiency with histopathological changes in adult zebrafish brain. OC stress may act as an early risk factor for the eventual development of cognitive impairments and B[a]P exposure plays a key role in mediating both the facilitating and impairing actions of OC stress in memory processes.