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Short-term ingestion of sublethal microcystin levels disrupts stress response in male mice
Salan Ghaju1, Evance Pakuwal2, Rafaela S C Takeshita1,3
1School of Biomedical Science, Kent State University, Kent, OH, United States.
Abstract:
Cyanobacterial harmful algal blooms (cHABs) produce various cyanotoxins, primarily microcystin leucine arginine (MC-LR). Studies demonstrated that intraperitoneal delivery of high-dose MC-LR affects the rat stress response, which is regulated by the hypothalamus-pituitary-adrenal (HPA) axis. In general, during stress parvocellular neurons in the hypothalamic paraventricular nucleus (PVN) secrete corticotropin-releasing hormone (CRH), triggering adrenocorticotropic hormone (ACTH) release from the anterior pituitary, which leads to increased cortisol in humans and corticosterone in rats and mice. Here, we tested the hypothesis that short-term sublethal ingestion of MC-LR activates the peripheral and central components of the HPA axis. First, we found that young adult male mice gavaged with MC-LR (50 μg/kg bw, p.o.) every 2 days for 21 days had elevated plasma corticosterone levels when compared to H2O (vehicle) mice. Ingestion of contaminated fresh water is a likely natural route of MC-LR exposure for animals, including humans. Second, Crh and arginine vasopressin (Avp) mRNA expressions were elevated in the hypothalamus of MC-LR-dosed mice. Third, ΔFOSB (i.e., long-term cell activity marker) immunofluorescence in the PVN and hippocampal dentate gyrus (DG) of MC-LR mice was significantly elevated compared to vehicle mice, but not in cornu ammonis (CA) 1, 2 and 3. In contrast, MC-LR mice had reduced hypothalamic glucocorticoid receptor (Gr) mRNA expression. Fourth, no significant changes were found in the mRNA expression of the inflammatory markers: tumor necrosis factor α (Tnf-α) and interleukin-1β (IL-1β) in the hypothalamus, liver, and spleen and C-reactive protein (Crp) in the liver and spleen. These data indicate that short-term ingestion of sublethal levels of MC-LR resulted in increased peripheral and central HPA axis activity.
Insights
Short-term exposure to microcystin-LR (MC-LR) from harmful algal blooms activates the hypothalamus-pituitary-adrenal (HPA) axis in mice, increasing stress hormones and neural activity without causing inflammation.
Area of Science:
- Environmental Toxicology
- Neuroendocrinology
- Physiology
Background:
- Cyanobacterial harmful algal blooms (cHABs) release cyanotoxins, notably microcystin leucine arginine (MC-LR).
- High-dose MC-LR exposure impacts the rat stress response via the hypothalamus-pituitary-adrenal (HPA) axis.
- Sublethal, short-term MC-LR exposure via ingestion is a potential environmental risk.
Purpose of the Study:
- To investigate the effects of short-term, sublethal oral MC-LR ingestion on the HPA axis.
- To determine if MC-LR exposure activates both central and peripheral components of the stress response.
- To assess changes in neural activity markers and inflammatory responses following MC-LR exposure.
Main Methods:
- Young adult male mice were gavaged with MC-LR (50 μg/kg bw) or vehicle (H2O) every two days for 21 days.
- Plasma corticosterone levels were measured.
- Hypothalamic gene expression of CRH, AVP, and GR was analyzed using RT-qPCR.
- ΔFOSB immunofluorescence was performed in the PVN and hippocampus.
- mRNA expression of inflammatory markers (TNF-α, IL-1β, CRP) was quantified.
Main Results:
- MC-LR ingestion significantly elevated plasma corticosterone levels compared to controls.
- Hypothalamic expression of Crh and Avp mRNA increased, while Gr mRNA decreased.
- Elevated ΔFOSB immunofluorescence was observed in the PVN and dentate gyrus, indicating increased neuronal activity.
- No significant changes in hypothalamic, liver, or spleen expression of inflammatory markers (TNF-α, IL-1β, CRP) were detected.
Conclusions:
- Short-term, sublethal oral exposure to MC-LR activates the peripheral HPA axis, leading to elevated corticosterone.
- Central HPA axis components, including CRH and AVP expression and PVN neuronal activity, are stimulated by MC-LR.
- MC-LR exposure increases long-term neuronal activity markers in specific brain regions without inducing a systemic inflammatory response.
