Temporal exposure to chronic unpredictable stress induces precocious neurobehavioral deficits by distorting neuromorphology and glutathione biosynthesis in zebrafish brain

Concerted monoamine oxidase activity following exposure to di-2-ethylhexyl phthalate is associated with aggressive neurobehavioral response and neurodegeneration in zebrafish brain

Di-2-ethylhexyl phthalate (DEHP) is the most commonly preferred synthetic organic chemical in plastics and its products for making them ductile, flexible and durable. As DEHP is not chemically bound to the macromolecular polymer of plastics, it can be easily leached out to accumulate in food and environment. Our recent report advocated that exposure to DEHP significantly transformed the innate bottom-dwelling and scototaxis behaviour of zebrafish. Our present study aimed to understand the possible role of DEHP exposure pertaining towards the development of aggressive behaviour and its association with amplified monoamine oxidase activity and neurodegeneration in the zebrafish brain. As heightened monoamine oxidase (MAO) is linked with genesis of aggressive behaviour, our observation also coincides with DEHP-persuaded aggressive neurobehavioral transformation in zebrafish. Our preliminary findings also showed that DEHP epitomized as a prime factor in transforming native explorative behaviour and genesis of aggressive behaviour through oxidative stress induction and changes in the neuromorphology in the periventricular grey zone (PGZ) of the zebrafish brain. With the finding demarcating towards heightened chromatin condensation in the PGZ of zebrafish brain, our further observation by immunohistochemistry showed a profound augmentation in apoptotic cell death marker cleaved caspase 3 (CC3) expression following exposure to DEHP. Our further observation by immunoblotting study also demarcated a temporal augmentation in CC3 and tyrosine hydroxylase expression in the zebrafish brain. Therefore, the gross findings of the present study delineate the idea that chronic exposure to DEHP is associated with MAO-instigated aggressive neurobehavioral transformation and neurodegeneration in the zebrafish brain.

Di-2-ethylhexyl phthalate-induced neurobehavioural transformation is associated with altered glutathione biosynthesis and neurodegeneration in zebrafish brain

The contamination of life-sustaining environments with synthetic pollutants such as plastic-derived compounds has increased at an alarming rate in recent decades. Among such contaminants, di-2-ethylhexyl phthalate (DEHP) is an extensively used compound in plastics and plastic products to make them flexible. DEHP causes several adverse effects such as reproductive toxicity leading to infertility, miscarriage and litter size reduction, disruption of the thyroid endocrine system, oxidative stress, neurodevelopmental defect and cognitive impairment. An aquatic environment is a fragile site, where the accumulation of DEHP poses a significant threat to living organisms. In this context, the present study focused on whether the neurobehavioural transformation following exposure to DEHP is an outcome of augmented oxidative stress and neuromorphological alteration in the zebrafish brain. Our preliminary findings advocate that DEHP acts as a typical neurotoxicant in inducing neurobehavioural transformation in zebrafish. Furthermore, our study also supports the idea that DEHP itself acts as a potent neurotoxicant by altering the glutathione biosynthetic pathway through the induction of oxidative stress in the zebrafish brain. Similarly, our findings also link the abovementioned neurobehavioural transformation and oxidative stress with augmented neuronal pyknosis and chromatin condensation in the periventricular grey zone of the zebrafish brain following chronic exposure to DEHP. Therefore, the overall conclusion of the present study advocates the potential role of DEHP in inducing neuropathological manifestation in the zebrafish brain. Future research directed towards elucidating the neuroprotective efficacy of natural compounds against DEHP-induced neurotoxicity may provide a new line of intervention.

Early life exposure to chronic unpredictable stress induces anxiety-like behaviors and increases the excitability of cerebellar neurons in zebrafish

Anxiety is a common emotional disorder in children. To understand its underlying mechanisms, chronic unpredictable stress (CUS) has been established as a stress model in zebrafish. By using the tall tank test, the stress response reliability could be improved in adult fish which has not been confirmed in larvae. In addition, the increasing evidences have shown that cerebellum plays important roles in anxiety. Whether CUS will affect cerebellar neuronal activity remains unknown. We found that CUS exposure to larvae (from 10 to 17 days post fertilization) induced anxiety-like behaviors and social cohesion impairments within 1-2 d after CUS, including a prolonged freezing time, an increased time spent at the bottom of tank, an increased thigmotaxis index, and an increased interindividual distance. Our results showed that the four behavioral tests were homogeneous, especially the tall tank test either anxiety-like behaviors or the basal locomotion. Furthermore, we found that CUS enhanced the excitability of cerebellar neurons, as the amplitude, frequency, time to peak and half-width of spontaneous firing significantly decreased, as well as the amplitude of excitatory post-synaptic current when compared with the control group. CUS also activated hyperpolarization-activated cyclic nucleotide-gated and potassium channels of cerebellar neurons. Multiple linear regression analysis showed that the total distance in bottom (tall tank test) was correlated positively with outward Na⁺-K⁺ currents (r = 0.848, P = 0.016), and the thigmotaxis index (open field test) correlated with action potential amplitude (r = 0.854, P = 0.030). Altogether, early life CUS transiently induced an anxiety-like behavior which could be more accurately assessed by combining the tall tank test with other behavior tests in young zebrafish. CUS increased the excitability of cerebellar neurons might provide new targets to treat emotional diseases such as anxiety.

Suppression of Chronic Unpredictable Stress-Persuaded Increased Monoamine Oxidase Activity by Taurine Promotes Significant Neuroprotection in Zebrafish Brain

Neuropsychiatric upshots following chronic exposure to unpredictable adverse stressors have been well documented in the literature. Considering the significant impact of chronic unpredictable stress (CUS), the literature is elusive regarding the neuroprotective efficacy of taurine against CUS-induced oxidative stress and chromatin condensation in the zebrafish brain. In this study, to ameliorate CUS-persuaded neurological outcomes, waterborne treatment of taurine as a prophylactic intervention was undertaken. Further, our approach also focused on the gross neurobehavioral response of zebrafish, oxidative stress indices and neuromorphology of the zebrafish brain following CUS exposure with taurine treatment. Because taurine provides significant neuroprotection against oxidative insult, the cytosolic level of monoamine oxidase (MAO) in the zebrafish brain following CUS exposure is worth investigating. Further, as heightened MAO activity is associated with augmented oxidative and chromatin condensation, the focus of this study was on whether taurine provides neuroprotection by downregulating MAO levels in the brain. Our findings show that CUS-persuaded altered neurobehavioral response was significantly rescued by taurine. Moreover, our findings firmly support the hypothesis that taurine acts as a radical neuroprotector by restoring glutathione biosynthesis in the zebrafish brain subsequent to CUS exposure. Additionally, the rising level of brain MAO following chronic exposure to CUS is ameliorated by taurine treatment. These findings strongly advocate the role of taurine as a natural MAO inhibitor through the neuroprotection it provides against CUS-instigated oxidative stress in zebrafish. However, the fundamental neuroprotective mechanism of such natural compounds needs to be elucidated to determine their neuroprotective efficacy against stress regimens.