Ghrelin modulates dopaminergic neuron formation and attention deficit hyperactivity disorder-like behaviors: From animals to human models

The exploration of neuroinflammatory mechanism by which CRHR2 deficiency induced anxiety disorder

Inflammation stimulates the hypothalamic-pituitary adrenal (HPA) axis and triggers glial neuroinflammatory phenotypes, which reduces monoamine neurotransmitters by activating indoleamine 2,3-dioxygenase enzyme. These changes can induce psychiatric diseases, including anxiety. Corticotropin releasing hormone receptor 2 (CRHR2) in the HPA axis is involved in the etiology of anxiety. Omega(n)-3 polyunsaturated fatty acids (PUFAs) can attenuate anxiety through anti-inflammation and HPA axis modulation. However, the underlying molecular mechanism by CRHR2 modulates anxiety and its correlation with neuroinflammation remain unclear. Here, we first constructed a crhr2 zebrafish mutant line, and evaluated anxiety-like behaviors, gene expression associated with the HPA axis, neuroinflammatory response, neurotransmitters, and PUFAs profile in crhr2+/+ and crhr2-/- zebrafish. The crhr2 deficiency decreased cortisol levels and up-regulated crhr1 and down-regulated crhb, crhbp, ucn3l and proopiomelanocortin a (pomc a) in zebrafish. Interestingly, a significant increase in the neuroinflammatory markers, translocator protein (TSPO) and the activation of microglia M1 phenotype (CD11b) were found in crhr2-/- zebrafish. As a consequence, the expression of granulocyte-macrophage colony-stimulating factor, pro-inflammatory cytokines vascular endothelial growth factor, and astrocyte A1 phenotype c3 were up-regulated. While microglia anti-inflammatory phenotype (CD206), central anti-inflammatory cytokine interleukin-4, arginase-1, and transforming growth factor-β were downregulated. In parallel, crhr2-deficient zebrafish showed an upregulation of vdac1 protein, a TSPO ligand, and its downstream caspase-3. Furthermore, 5-HT/5-HIAA ratio was decreased and n-3 PUFAs deficiency was identified in crhr2-/- zebrafish. In conclusion, anxiety-like behavior displayed by crhr2-deficient zebrafish may be caused by the HPA axis dysfunction and enhanced neuroinflammation, which resulted in n-3 PUFAs and monoamine neurotransmitter reductions.

Valbenazine promotes body growth via growth hormone signaling during zebrafish embryonic development

Vesicular monoamine transporter 2 (VMAT-2) functions by uptake of cytoplasmic monoamines into vesicles for storage. Valbenazine (VBZ) is a newly FDA-approved oral VMAT-2 inhibitor used for the treatment of movement disorders such as tardive dyskinesia (TD), and Tourette syndrome (TS). Clinical data shows that VBZ is a relatively safe drug with no cardiotoxicity or hepatotoxicity. However, the effect of VBZ on embryonic development remains unknown. Here, we use zebrafish larvae as an animal model to demonstrate that VBZ exposure causes premature hatching and increased body size and hyperactivity-like behaviors in zebrafish larvae. In addition, VBZ exposure leads to increased dopamine (DA) and Glutamate (Glu) levels. Moreover, an increase of growth hormone (gh) and enriched PI3K/AKT signaling were found in VBZ-exposed zebrafish larvae, which may explain their accelerated development. In summary, VBZ exposure may be developmentally toxic in zebrafish larvae.

Developmental and behavioral toxicity assessment of opicapone in zebrafish embryos

The use of clinical psychoactive drugs often poses unpredictable threats to fetal development. Catechol-O-methyltransferase (COMT) is a key enzyme that regulates dopamine metabolism and a promising target for modulation of cognitive functions. Opicapone, a newly effective third-generation peripheral COMT inhibitor, is used for the treatment of Parkinson's disease (PD) and possibly to improve other dopamine-related disorders such as alcohol use disorder (AUD) and obsessive-compulsive disorder (OCD). The widespread use of opicapone will inevitably lead to biological exposure and damage to the human body, such as affecting fetal development. However, the effect of opicapone on embryonic development remains unknown. Here, zebrafish larvae were used as an animal model and demonstrated that a high concentration (30 μM) of opicapone exposure was teratogenic and lethal, while a low concentration also caused developmental delay such as a shortened body size, a smaller head, and reduced locomotor behaviors in zebrafish larvae. Meanwhile, opicapone treatment specifically increased the level of dopamine (DA) in zebrafish larvae. The depletion response of the total glutathione level (including oxidized and reduced forms of glutathione) and changed antioxidant enzymes activities in zebrafish larvae suggest oxidative damage caused by opicapone. In addition, enhanced glutathione metabolism and cytokine-cytokine receptor interaction were found in zebrafish larvae treated with opicapone, indicating that opicapone treatment caused an oxidation process and immune responses. Our results provide a new insight into the significant developmental toxicity of opicapone in zebrafish larvae.

Ghrelin regulates hyperactivity-like behaviors via growth hormone signaling pathway in zebrafish (Danio rerio)

INTRODUCTION

Ghrelin is originally identified as the endogenous ligand for the growth hormone secretagogue receptor (GHSR) and partially acts by stimulating growth hormone (GH) release. Our previous studies have identified GHRELIN as a novel susceptibility gene for human attention-deficit hyperactivity disorder (ADHD), and ghrelin-depleted zebrafish (Danio rerio) display ADHD-like behaviors. However, the underlying molecular mechanism how ghrelin regulates hyperactivity-like behaviors is not yet known.

RESULTS

Here, we performed RNA-sequencing analysis using adult ghrelin ^Δ/Δ zebrafish brains to investigate the underlying molecular mechanisms. We found that gh1 mRNA and genes related to the gh signaling pathway were significantly reduced at transcriptional expression levels. Quantitative polymerase chain reaction (qPCR) was performed and confirmed the downregulation of gh signaling pathway-related genes in ghrelin ^Δ/Δ zebrafish larvae and the brain of adult ghrelin ^Δ/Δ zebrafish. In addition, ghrelin ^Δ/Δ zebrafish displayed hyperactive and hyperreactive phenotypes, such as an increase in motor activity in swimming test and a hyperreactive phenotype under light/dark cycle stimulation, mimicking human ADHD symptoms. Intraperitoneal injection of recombinant human growth hormone (rhGH) partially rescued the hyperactivity and hyperreactive-like behaviors in ghrelin mutant zebrafish.

CONCLUSION

Our results indicated that ghrelin may regulate hyperactivity-like behaviors by mediating gh signaling pathway in zebrafish. And the protective effect of rhGH on ghrelin ^Δ/Δ zebrafish hyperactivity behavior provides new therapeutic clues for ADHD patients.

Wnt/β-catenin signaling promotes neurogenesis in the diencephalospinal dopaminergic system of embryonic zebrafish

Wnt/β-catenin signaling contributes to patterning, proliferation, and differentiation throughout vertebrate neural development. Wnt/β-catenin signaling is important for mammalian midbrain dopaminergic neurogenesis, while little is known about its role in ventral forebrain dopaminergic development. Here, we focus on the A11-like, Otp-dependent diencephalospinal dopaminergic system in zebrafish. We show that Wnt ligands, receptors and extracellular antagonist genes are expressed in the vicinity of developing Otp-dependent dopaminergic neurons. Using transgenic Wnt/β-catenin-reporters, we found that Wnt/β-catenin signaling activity is absent from these dopaminergic neurons, but detected Wnt/β-catenin activity in cells adjacent to the caudal DC5/6 clusters of Otp-dependent dopaminergic neurons. Pharmacological manipulations of Wnt/β-catenin signaling activity, as well as heat-shock driven overexpression of Wnt agonists and antagonists, interfere with the development of DC5/6 dopaminergic neurons, such that Wnt/β-catenin activity positively correlates with their number. Wnt/β-catenin activity promoted dopaminergic development specifically at stages when DC5/6 dopaminergic progenitors are in a proliferative state. Our data suggest that Wnt/β-catenin signaling acts in a spatially and temporally restricted manner on proliferative dopaminergic progenitors in the hypothalamus to positively regulate the size of the dopaminergic neuron groups DC5 and DC6.