Abstract

Background:

The excitotoxic molecule, domoic acid (DA), is a marine algal toxin known to induce overt hippocampal neurotoxicity. Recent experimental and epidemiological studies suggest adverse neurological effects at exposure levels near the current regulatory limit (20 ppm, $\sim 0.075–0.1\mathrm{mg}/\mathrm{kg}$). At these levels, cognitive effects occur in the absence of acute symptoms or evidence of neuronal death.

Objectives:

This study aimed to identify adverse effects on the nervous system from prolonged, dietary DA exposure in adult, female Macaca fascicularis monkeys.

Methods:

Monkeys were orally exposed to 0, 0.075, and $0.15\mathrm{mg}/\mathrm{kg}\text{\hspace{0.17em}per day}$ for an average of 14 months. Clinical blood counts, chemistry, and cytokine levels were analyzed in the blood. In-life magnetic resonance (MR) imaging assessed volumetric and tractography differences in and between the hippocampus and thalamus. Histology of neurons and glia in the fornix, fimbria, internal capsule, thalamus, and hippocampus was evaluated. Hippocampal RNA sequencing was used to identify differentially expressed genes. Enrichment of gene networks for neuronal health, excitotoxicity, inflammation/glia, and myelin were assessed with Gene Set Enrichment Analysis.

Results:

Clinical blood counts, chemistry, and cytokine levels were not altered with DA exposure in nonhuman primates. Transcriptome analysis of the hippocampus yielded 748 differentially expressed genes ($\text{fold change}\ge 1.5$; $p\le 0.05$), reflecting differences in a broad molecular profile of intermediate early genes (e.g., FOS, EGR) and genes related to myelin networks in DA animals. Between exposed and control animals, MR imaging showed comparable connectivity of the hippocampus and thalamus and histology showed no evidence of hypomyelination. Histological examination of the thalamus showed a larger microglia soma size and an extension of cell processes, but suggestions of a ${\text{GFAP}}^{+}\text{astrocyte}$ response showed no indication of astrocyte hypertrophy.

Discussion:

In the absence of overt hippocampal excitotoxicity, chronic exposure of Macaca fascicularis monkeys to environmentally relevant levels of DA suggested a subtle shift in the molecular profile of the hippocampus and the microglia phenotype in the thalamus that was possibly reflective of an adaptive response due to prolonged DA exposure. https://doi.org/10.1289/EHP10923

中文翻译：

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长期低水平接触海洋毒素、软骨藻酸和非人灵长类动物的神经毒性测量

摘要

背景：

兴奋性毒性分子软骨藻酸 (DA) 是一种海洋藻类毒素，已知会引起明显的海马神经毒性。最近的实验和流行病学研究表明，在接近当前监管限值（20 ppm，$～0.075–0.1\mathrm{毫克}/\mathrm{公斤}$). 在这些水平上，认知效应在没有急性症状或神经元死亡证据的情况下发生。

目标：

本研究旨在确定成年雌性食蟹猴长期饮食 DA 暴露对神经系统的不利影响。

方法：

猴子经口暴露于 0、0.075 和$0.15\mathrm{毫克}/\mathrm{公斤}\text{\hspace{0.17em}每天}$平均 14 个月。在血液中分析了临床血细胞计数、化学和细胞因子水平。活体磁共振 (MR) 成像评估了海马体和丘脑内部和之间的体积和纤维束成像差异。对穹窿、伞状结构、内囊、丘脑和海马中的神经元和胶质细胞的组织学进行了评估。海马 RNA 测序用于鉴定差异表达的基因。使用基因集富集分析评估神经元健康、兴奋性毒性、炎症/神经胶质细胞和髓磷脂的基因网络富集。

结果：

在非人灵长类动物中，临床血细胞计数、化学和细胞因子水平没有随着 DA 暴露而改变。海马体的转录组分析产生了 748 个差异表达基因（$\text{折叠变化}\ge 1.5$;$p\le 0.05$)，反映了 DA 动物中早期中间基因（例如，FOS、EGR）和与髓鞘网络相关基因的广泛分子谱的差异。在暴露动物和对照动物之间，MR 成像显示海马体和丘脑的连接性相当，组织学显示没有髓鞘形成不足的证据。丘脑的组织学检查显示小胶质细胞体积较大，细胞突起延长，但提示${\text{玻璃纤维蛋白粉}}^{+}\text{星形胶质细胞}$反应显示没有星形胶质细胞肥大的迹象。

讨论：

在没有明显的海马兴奋性毒性的情况下，食蟹猴长期暴露于环境相关水平的 DA 表明海马的分子特征和丘脑中的小胶质细胞表型发生了微妙的变化，这可能反映了由于长时间的 DA 引起的适应性反应接触。https://doi.org/10.1289/EHP10923