Benzo[a]pyrene (B[a]P) is commonly associated with oxidative stress-induced neurotoxicity. Retinoic acid (RA) has been shown to exhibit neuroprotection in brain, and disruption of RA signaling via excess or deficient RA can lead to oxidative stress. B[a]P contamination in aquatic environment has been shown to lower the internal RA level. Thus, the present study was conducted in wild-type zebrafish to ameliorate the neurotoxic effect of B[a]P by waterborne RA co-supplementation. Findings showed that B[a]P induced anxiolytic-like behavioral response, and altered antioxidant activity in zebrafish is attenuated by RA. Our study also advocated the neurotoxic potential of RA treatment alone in control condition. Previous findings showed that periventricular gray zone (PGZ) of optic tectum (TeO) in zebrafish brain regulates anxiety-like behavior. The augmented pyknotic neuronal counts in PGZ following B[a]P treatment was ameliorated by RA co-supplementation. Further, presence of B[a]P in the cell milieu is known to induce oxidative stress through increase expression of cytochrome P450 1A1 (CYP1A1), an enzyme necessary for metabolic breakdown of both B[a]P and RA. Any deviation from the required concentration of RA leads to production of reactive oxygen species. Further, low availability of RA in cell milieu is known to decrease the expression of Nrf2, a transcription factor necessary for the expression of several antioxidants and antioxidant enzymes. Recent studies also showed that RA increases glutathione synthesis and exhibits neuroprotective properties in brain cells. The findings of the present study address the potential role of exogenous RA co-supplementation as a therapeutic intervention against B[a]P-induced depletion of RA, causing neurotoxicity in zebrafish.
The aquatic environment provides a sink for the environmental pollutants that have potential to induce oxidative stress by altering neurobehavioral response of aquatic animals. Benzo[a]pyrene (B[a]P), a polycyclic aromatic hydrocarbon is known to induce oxidative stress in the brain. Withania somnifera has been used traditionally for its neuroprotective effect in experimental models of neurological disorders. The present study is aimed to evaluate the neuroprotective potential of Withania somnifera leaf extract (WSLE) following exposure to waterborne B[a]P. Wild-type zebrafish (Danio rerio) were designated as naive, control (dimethyl sulfoxide), WSLE, B[a]P, and B[a]P + WSLE groups. Behavioral studies showed reversal in scototaxis (anxiety-like) behavior in B[a]P group and was restored by WSLE cosupplementation in B[a]P + WSLE group. B[a]P-induced altered antioxidant status was ameliorated by WSLE in the B[a]P + WSLE group. Previous studies showed that the periventricular gray zone (PGZ) of the optic tectum in zebrafish brain regulates scototaxis (anxiety-like) behavior. Our histopathological observation showed a significant increase in the pyknotic neuronal counts in PGZ of the B[a]P group and was ameliorated by WSLE cosupplementation. The study showed that the reversal in scototaxis behavior following exposure to waterborne B[a]P might be associated with neuromorphological alterations in PGZ, whereas a pioneer ethnopharmacological approach of WSLE cosupplementation showed its neuroprotective role to restore normal scototaxis of zebrafish. Future research directing toward understanding the role of visual circuit involved with impaired scototaxis behavior in zebrafish might provide new pathological outcomes following exposure to B[a]P.
Environmental neurotoxicants like benzo[a]pyrene (B[a]P) have been well documented regarding their potential to induce oxidative stress. However, neonatal exposure to B[a]P and its subsequent effect on anti-oxidant defence system and hippocampal cytomorphometry leading to behavioral changes have not been fully elucidated. We investigated the effect of acute exposure of B[a]P on five days old male Wistar pups administered with single dose of B[a]P (0.2 μg/kg BW) through intracisternal mode. Control group was administered with vehicle i.e., DMSO and a separate group of rats without any treatment was taken as naive group. Behavioral analysis showed anxiolytic-like behavior with significant increase in time spent in open arm in elevated plus maze. Further, significant reduction in fall off time during rotarod test showing B[a]P induced locomotor hyperactivity and impaired motor co-ordination in adolescent rats. B[a]P induced behavioral changes were further associated with altered anti-oxidant defence system involving significant reduction in the total ATPase, Na(+) K(+) ATPase, Mg(2+) ATPase, GR and GPx activity with a significant elevation in the activity of catalase and GST as compared to naive and control groups. Cytomorphometry of hippocampus showed that the number of neurons and glia in B[a]P treated group were significantly reduced as compared to naive and control. Subsequent observation showed that the area and perimeter of hippocampus, hippocampal neurons and neuronal nucleus were significantly reduced in B[a]P treated group as compared to naive and control. The findings of the present study suggest that the alteration in hippocampal cytomorphometry and neuronal population associated with impaired antioxidant signaling and mood in B[a]P treated group could be an outcome of neuromorphological alteration leading to pyknotic cell death or impaired differential migration of neurons during early postnatal brain development.
Graphical abstractSchematic diagram for the mechanism B[a]P metabolism by activation that occurs through cytochrome P450 enzymes. During cellular metabolism large amounts of reactive oxygen species (ROS) are generated. The resulting oxidative stress induces the alteration of antioxidant enzymes (GST, GPx, GR and total glutathione) leading towards behavioral changes with altered neuronal morphology in the hippocampus of adult wistar rat brain. Early exposure to B[a]P correlates with impaired learning and memory in adults, and reduction in dendrite population in CA1 regions of hippocampus during development.
Exposure to persistent genotoxicants like benzo[a]pyrene (B[a]P) during postnatal days causes neurobehavioral changes in animal models. However, neurotoxic potential of B[a]P and its association with 6-hydroxydopamine (6-OHDA) induced neurobehavioral changes are yet to be explored. The growth of rat brain peaks at the first week of birth and continues up to one month with the attainment of adolescence. Hence, the present study was conducted on male Wistar rats at postnatal day 5 (PND 5) following single intracisternal administration of B[a]P to compare with neurobehavioral and neurotransmitter changes induced by 6-OHDA at PND 30. Spontaneous motor activity was significantly increased by 6-OHDA showing similar trend following B[a]P administration. Total distance travelled in novel open field arena and elevated plus maze was significantly increased following B[a]P and 6-OHDA administration. Neurotransmitter estimation showed significant alleviation of dopamine in striatum following B[a]P and 6-OHDA administration. Histopathological studies of striatum by hematoxylin and eosin (H&E) staining revealed the neurodegenerative potential of B[a]P and 6-OHDA. Our results indicate that B[a]P-induced spontaneous motor hyperactivity in rats showed symptomatic similarities with 6-OHDA. In conclusion, early postnatal exposure to B[a]P in rats causing neurobehavioral changes may lead to serious neurodegenerative consequences during adolescence.
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