Citation:

Christopher, F., J. Brown, K. Wallace, J. Wambaugh, I. Shah, AND Tim Shafer. Defining toxicological tipping points in neuronal network development. TOXICOLOGY AND APPLIED PHARMACOLOGY. Academic Press Incorporated, Orlando, FL, 354:81-93, (2018). https://doi.org/10.1016/j.taap.2018.01.017

Impact/Purpose:

The prevalence of neurodevelopmental disorders is increasing worldwide and environmental chemical exposure is strongly implicated as a contributing factor. Recognizing this problem, information on the potential developmental neurotoxicity of chemicals is a major need of the Agency. However, information on developmental neurotoxicity is known for only a few chemicals, and current testing strategies are both too slow and too costly to adequately address this need. Efficient and predictive methods are needed to screen environmental chemicals and prioritize them for further assessment. Working under the auspices of Chemical Safety for Sustainability project 16.01, development of medium- and high-throughput tests for developmental neurotoxicity, we have developed an in vitro assay that examines chemical effects on the function of developing neural networks. In a previous manuscript (Frank et al., Toxicol Sci 2017. 160, 121-35), we examined effects of 86 compounds in this assay, 60 of which cause developmental neurotoxicity in vivo, and demonstrate that 82% of those compounds disrupted development of neural networks. Compounds that are not known to cause developmental neurotoxicity in vivo were without effects. In the present study, we determined toxicological “tipping points” that define the critical concentration of compound at which network development transitioned from a homeostatic response to chemical exposure to an adverse effect on network function. Of the 60 compounds that altered network function in Frank et al., 2017, 42 were found to have “tipping points”. Overall, these tipping points occurred at lower concentrations than EC50 values for effects on network function or viability. For 6 compounds for which sufficient data existed, these tipping point values were compared to plasma concentrations associated with adverse neurodevelopmental outcomes following in vivo exposure using high-throughput toxicokinetic approaches; 5/6 compounds had tipping points that were comparable to in vivo exposure levels. This result suggests that tipping points are relevant to in vivo dosing levels associated with developmental neurotoxicity.

Description:

Measuring electrical activity of neural networks by microelectrode array (MEA) has recently shown promise for screening level assessments of chemical toxicity on network development and function. Important aspects of interneuronal communication can be quantified from a single MEA recording, including individual firing rates, coordinated bursting, and measures of network synchrony, providing rich datasets to evaluate chemical effects. Further, multiple recordings can be made from the same network, including during the formation of these networks in vitro. The ability to perform multiple recording sessions over the in vitro development of network activity may provide further insight into developmental effects of neurotoxicants. In the current study, a recently described MEA-based screen of 86 compounds in primary rat cortical cultures over 12 days in vitro was revisited to establish a framework that integrates all available primary measures of electrical activity from MEA recordings into a composite metric for deviation from normal activity (total scalar perturbation). Examining scalar perturbations over time and increasing concentration of compound allowed for definition of critical concentrations or “tipping points” at which the neural networks switched from recovery to non-recovery trajectories for 42 compounds. These tipping point concentrations occurred at predominantly lower concentrations than those causing overt cell viability loss or disrupting individual network parameters, suggesting tipping points may be a more sensitive measure of network functional loss. Comparing tipping points for six compounds with plasma concentrations known to cause developmental neurotoxicity in vivo demonstrated strong concordance and suggests there is potential for using tipping points for chemical prioritization.

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