Prolonged, Low-Level Exposure to the Marine Toxin, Domoic Acid, and Measures of Neurotoxicity in Nonhuman Primates

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 $0.15\mathrm{mg}/\mathrm{kg}\text{per day}$ 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.