Distribution, properties, and inhibitor sensitivity of zebrafish catechol-O-methyl transferases (COMT)

Trace amine signaling in zebrafish models: CNS pharmacology, behavioral regulation and translational relevance

Tyramine, β-phenylethylamine, octopamine and other trace amines are endogenous substances recently recognized as important novel neurotransmitters in the brain. Trace amines act via multiple selective trace amine-associated receptors (TAARs) of the G protein-coupled receptor family. TAARs are expressed in various brain regions and modulate neurotransmission, neuronal excitability, adult neurogenesis, cognition, mood, locomotor activity and olfaction. Disrupted trace amine circuits have been implicated in various clinical neuropsychiatric disorders, including schizophrenia, Parkinson's disease, addiction, depression and anxiety. Dysregulated TAAR signaling has been linked in rodents to altered dopamine and serotonin neurotransmission, known to be associated with these psychiatric conditions. Complementing rodent genetic and pharmacological evidence, zebrafish (Danio rerio) are rapidly becoming a novel powerful model system in translational neuropharmacology research. Here, we review trace amine/TAAR neurobiology in zebrafish and discuss their developing translational utility as pharmacological and genetic models for unraveling the role of trace amines in CNS processes and brain disorders.

An activity-based sensing fluorogenic probe for monitoring O-methyltransferase in plants

Activity-based sensing probes are powerful tools for monitoring enzymatic activities in complex biological samples such as cellular and live animals; however, their application in plants remains challenging. Herein, fourteen activity-based fluorescent probes were assayed against Arabidopsis O-methyltransferases (AtOMTs). One probe, 3-BTD, displayed a high selectivity, reactivity, and fluorescence response toward AtOMTs especially the isoform AtCCoAOMT. We further characterized the features of this probe and explored whether it could be used to detect OMT activities in living plant cells. Our results show that 3-BTD can be used to visualize OMT activity in Arabidopsis, and no fluorescent signal was observed in the comt/ccoaomt double mutant, indicating that it has good specificity. Interestingly, in contrast to the observation that AtCCoAOMT-YFP accumulated in both cytoplasm and nucleus, OMT enzymatic activity tracked by 3-BTD probe was found only in the cytoplasm. This underscores the importance of activity-based sensing in studying protein function. Moreover, 3-BTD can be successfully applied in OMT visualization of different plants. This study indicates that 3-BTD can serve as a potential probe for in situ monitoring the real activity of OMT in multiple plants and provides a strategy for visualizing the activity of other enzymes in plants.

Chronic environmental inorganic arsenic exposure causes social behavioral changes in juvenile zebrafish (Danio rerio)

Arsenic (As) is a metalloid commonly found worldwide. Environmental As exposure may cause potential health hazards and behavioral changes in humans and animals. However, the effects of environmental As concentrations on social behavior, especially during the juvenile stage, are unclear. In this study, we observed behavioral changes in juvenile zebrafish after 28 days of exposure to inorganic As (NaAsO₂ 100 and 500 ppb) in water, especially anxiety and social deficits. Additionally, the level of oxidative stress in the zebrafish brain after As treatment increased, the content of dopamine (DA) decreased, and the transcription level of genes involved in DA metabolism with the activity of monoamine oxidase (MAO) increased. Oxidative stress is a recognized mechanism of nerve damage induced by As exposure. The zebrafish were exposed to N-acetylcysteine (NAC) to reduce As exposure-induced oxidative stress. The results showed improvements in social behavior, DA content, MAO activity, and gene transcription in zebrafish. In conclusion, environmental As exposure can induce behavioral abnormalities, such as anxiety and social deficits in zebrafish, which may be caused by As-induced oxidative stress altering gene transcription levels, causing an increase in MAO activity and a decrease in DA.

Effect of temperature and dietary pesticide exposure on neuroendocrine and olfactory responses in juvenile Chinook salmon (Oncorhynchus tshawytscha)

Projected water temperature increases based on predicted climate change scenarios and concomitant pesticide exposure raises concern about the responses of aquatic organisms. To better understand the effect of pesticide mixtures and influence of water temperature to fish, juvenile Chinook salmon (Oncorhynchus tshawytscha) were dietarily exposed to a mixture of legacy and current use pesticides (p,p'-DDE, bifenthrin, chlorpyrifos, esfenvalerate, and fipronil) at concentrations detected from field-collected prey items in the Sacramento-San Joaquin Delta, California (Delta) and exposed under current and predicted future water temperature scenarios, 11, 14, or 17 °C, for 14 days. The expression of a subset of genes (deiodinase 2-dio2, gonadotropin releasing hormone 2-gnrh2, and catechol-o-methyltransferase-comt) involved in neuroendocrine, dopaminergic, and olfactory function previously shown to be altered by individual pesticide exposures germane to this study were determined and olfactory function assessed using a Y-maze behavioral assay. When total body burdens of pesticides were measured, a significant decrease in dio2 expression was observed in Chinook salmon exposed at 14 °C compared to fish kept at 11 °C. Increases in gnrh2 expression were also observed in fish exposed to 14 °C. Similarly, increases in comt expression was noted at 14 and 17 °C. Additionally, altered expression of all transcripts was observed, showing interactions between temperature and individual pesticide concentrations. Chinook salmon spent significantly more time actively avoiding the odorant arm at baseline conditions of 11 °C in the Y-maze. At higher temperatures, Chinook spent significantly more time not making a choice between the odorant or clean arm following exposure to the low pesticide mixture, relative to 11 °C. These results suggest that dietary exposure to pesticide mixtures can potentially induce neuroendocrine effects and behavior. Impaired olfactory responses exhibited by Chinook salmon could have implications for predator avoidance in the wild under increased temperature scenarios and impact populations in the future.

Mild hypoxia exposure impacts peripheral serotonin uptake and degradation in Gulf toadfish, Opsanus beta

Plasma 5-HT homeostasis is maintained through the combined processes of uptake (via the 5-HT transporter SERT, and others), degradation (via monoamine oxidase, MAO), and excretion. Previous studies have shown that inhibiting SERT, which would inhibit 5-HT uptake and degradation, attenuates parts of the cardiovascular hypoxia reflex in Gulf toadfish (Opsanus beta), suggesting that these 5-HT clearance processes may be important during hypoxia exposure. Therefore, the goal of this experiment was to determine the effects of mild hypoxia on 5-HT uptake and degradation in the peripheral tissues of toadfish. We hypothesized that 5-HT uptake and degradation would be upregulated during hypoxia resulting in lower plasma 5-HT, with uptake occurring in the gill, heart, liver, and kidney. Fish were exposed to normoxia (97.6% O2 saturation, 155.6 torr), or 2-min, 40-min or 24 h mild hypoxia (50% O2 saturation, ∼80 torr), injected with radiolabeled [3H]5-HT and blood, urine, bile and tissues taken. Plasma 5-HT levels were reduced by 40% after 40 min of hypoxia exposure and persisted through 24 h. 5-HT uptake by the gill was upregulated following 2 min of hypoxia exposure, and degradation in the gill was upregulated at 40 min and 24 h. Interestingly, there was no change in 5-HT uptake by the heart and degradation in the heart decreased by 58% within 2 min of hypoxia exposure and by 85% at 24 h. These results suggest that 5-HT clearance is upregulated during hypoxia and is likely driven, in part, by mechanisms within the gill and not the heart.

Zebrafish as a potential non-traditional model organism in translational bipolar disorder research: Genetic and behavioral insights

Bipolar disorder (BD) is a severe and debilitating illness that affects 1-2% of the population worldwide. BD is characterized by recurrent and extreme mood swings, including mania/hypomania and depression. Animal experimental models have been used to elucidate the mechanisms underlying BD and different strategies have been proposed to assess BD-like symptoms. The zebrafish (Danio rerio) has been considered a suitable vertebrate system for modeling BD-like responses, due to the genetic tractability, molecular/physiological conservation, and well-characterized behavioral responses. In this review, we discuss how zebrafish-based models can be successfully used to understand molecular, biochemical, and behavioral alterations paralleling those found in BD. We also outline some advantages and limitations of this aquatic species to examine BD-like phenotypes in translational neurobehavioral research. Overall, we reinforce the use of zebrafish as a promising tool to investigate the neural basis associated with BD-like behaviors, which may foster the discovery of novel pharmacological therapies.

Protective Effect of Quercetin 3-O-Glucuronide against Cisplatin Cytotoxicity in Renal Tubular Cells

Quercetin, a flavonoid with promising therapeutic potential, has been shown to protect from cisplatin nephrotoxicity in rats following intraperitoneal injection, but its low bioavailability curtails its prospective clinical utility in oral therapy. We recently developed a micellar formulation (P-quercetin) with enhanced solubility and bioavailability, and identical nephroprotective properties. As a first aim, we herein evaluated the oral treatment with P-quercetin in rats, which displayed no nephroprotection. In order to unravel this discrepancy, quercetin and its main metabolites were measured by HPLC in the blood and urine after intraperitoneal and oral administrations. Whilst quercetin was absorbed similarly, the profile of its metabolites was different, which led us to hypothesize that nephroprotection might be exerted in vivo by a metabolic derivate. Consequently, we then aimed to evaluate the cytoprotective capacity of quercetin and its main metabolites (quercetin 3-O-glucoside, rutin, tamarixetin, isorhamnetin and quercetin 3-O-glucuronide) against cisplatin toxicity, in HK-2 and NRK-52E tubular cell lines. Cells were incubated for 6 h with quercetin, its metabolites or vehicle (pretreatment), and subsequently 18 h in cotreatment with 10-300 μM cisplatin. Immediately after treatment, cell cultures were subject to the MTT technique as an index of cytotoxicity and photographed under light microscopy for phenotypic assessment. Quercetin afforded no direct cytoprotection and quercetin-3-O-glucuronide was the only metabolite partially preventing the effect of cisplatin in cultured tubule cells. Our results identify a metabolic derivative of quercetin contributing to its nephroprotection and prompt to further explore exogenous quercetin-3-O-glucuronide in the prophylaxis of tubular nephrotoxicity.

Transit and Metabolic Pathways of Quercetin in Tubular Cells: Involvement of Its Antioxidant Properties in the Kidney

Quercetin is a flavonoid with antioxidant, antiviral, antimicrobial, and anti-inflammatory properties. Therefore, it has been postulated as a molecule with great therapeutic potential. The renoprotective capacity of quercetin against various toxins that produce oxidative stress, in both in vivo and in vitro models, has been shown. However, it is not clear whether quercetin itself or any of its metabolites are responsible for the protective effects on the kidney. Although the pharmacokinetics of quercetin have been widely studied and the complexity of its transit throughout the body is well known, the metabolic processes that occur in the kidney are less known. Because of that, the objective of this review was to delve into the molecular and cellular events triggered by quercetin and/or its metabolites in the tubular cells, which could explain some of the protective properties of this flavonoid against oxidative stress produced by toxin administration. Thus, the following are analyzed: (1) the transit of quercetin to the kidney; (2) the uptake mechanisms of quercetin and its metabolites from plasma to the tubular cells; (3) the metabolic processes triggered in those cells, which affect the accumulation of metabolites in the intracellular space; and (4) the efflux mechanisms of these compounds and their subsequent elimination through urine. Finally, it is discussed whether those processes that are mediated in the tubular cells and that give rise to different metabolites are related to the antioxidant and renoprotective properties observed after the administration of quercetin.

Chemical and Genetic Zebrafish Models to Define Mechanisms of and Treatments for Dopaminergic Neurodegeneration

The zebrafish (Danio rerio) is routinely used in biological studies as a vertebrate model system that provides unique strengths allowing applications in studies of neurodevelopmental and neurodegenerative diseases. One specific advantage is that the neurotransmitter systems are highly conserved throughout vertebrate evolution, including between zebrafish and humans. Disruption of the dopaminergic signaling pathway is linked to multiple neurological disorders. One of the most common is Parkinson’s disease, a neurodegenerative disease associated with the loss of dopaminergic neurons, among other neuropathological characteristics. In this review, the development of the zebrafish’s dopaminergic system, focusing on genetic control of the dopaminergic system, is detailed. Second, neurotoxicant models used to study dopaminergic neuronal loss, including 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), the pesticides paraquat and rotenone, and 6-hydroxydopamine (6-OHDA), are described. Next, zebrafish genetic knockdown models of dj1, pink1, and prkn established for investigating mechanisms of Parkinson’s disease are discussed. Chemical modulators of the dopaminergic system are also highlighted to showcase the applicability of the zebrafish to identify mechanisms and treatments for neurodegenerative diseases such as Parkinson’s disease associated with the dopaminergic system.

Rasagiline, an inhibitor of MAO-B, decreases colonic motility through elevating colonic dopamine content

BACKGROUND

Dopamine (DA) is a negative modulator of gut motility. Monoamine oxidase-B (MAO-B) is an important metabolic enzyme degrading DA. Rasagiline, an irreversible MAO-B inhibitor, is used to treat Parkinson's disease because of its neuroprotective effect and increasing central DA. However, it is unclear whether MAO-B exists in the colon and rasagiline increases colonic DA, thereby affecting colonic motility.

METHODS

Immunohistochemistry, western blotting, enzyme activity assay, colonic motility recording, gut transit test, and high-performance liquid chromatography-electrochemical detection were employed in this study.

KEY RESULTS

Monoamine oxidase-B was distributed in the colonic muscular layers including neurons and glias of rat and human. When oral treatment of rats with rasagiline for 4 weeks, in vitro colonic motility was significantly reduced, but it was greatly reversed by SCH-23390, an antagonist of DA D₁ receptor. The rasagiline-treated rats also manifested decreased MAO-B activity and increased DA content in the colonic muscular layer, but no alterations were detected in the protein expressions of D₁ and D₂ receptors, and MAO-A and MAO-B, as well as in the content of 5-hydroxytryptamine and noradrenaline. Moreover, acute administration of rasagiline did not affect the colonic motility in vitro and the colonic DA level in rats, although MAO-B activity was significantly inhibited.

CONCLUSIONS & INFERENCES

Monoamine oxidase-B is abundant in the colonic muscular layer including myenteric plexus of rat and human. Long-term administration of rasagiline can increase colonic DA thereby inhibiting colonic motility, suggesting that colonic MAO-B could be a potential drug target for colonic dysmotility.