Modeling the developing nervous system: a neuroscience perspective on the use of NAMs in DNT testing

There is widespread concern that environmental exposures constitute an underappreciated but significant contribution to rising rates of neurodevelopmental disorders (NDDs). There is also international consensus that regulatory frameworks for developmental neurotoxicity (DNT) testing are woefully inadequate, prompting reappraisal of DNT testing methods. One approach aims to make testing more efficient, less animal-intensive, and higher throughput, through in vitro evaluation of DNT. These new approach methodologies (NAMs) promise to accelerate and standardize DNT testing through interrogation of fundamental mechanisms of neurodevelopment. While in the early stages of development, they have significant, well-publicized shortcomings, including little to no accounting for cellular or genetic diversity, cell extrinsic signaling molecules, sex as a biological variable, developmental stage, or relevance to NDDs. One of the most advanced NAM platforms is a collection of 17 in vitro assays termed the DNT in vitro battery (IVB). While it models some aspects of neurodevelopmental processes, it fails to capture others. Proper brain ontogeny, and consequently normal behavior and cognition, relies on the integrity of fundamental mechanisms, their temporal/spatial fidelity, and the magnitude of their expression. These fundamental mechanisms are regulated by factors not considered by the DNT IVB including diverse cell types and neurotransmitters. While the DNT IVB could prove to be an important tool in DNT hazard detection, we identify key areas, including cell-extrinsic neurotransmitter signaling, diversity of neural progenitors, interneurons, and biological sex, that should be prioritized for development and inclusion in future refinements to meaningfully enhance biological coverage and relevance to human cognition and behavior.