OECD Series on Adverse Outcome Pathways

An Adverse Outcome Pathway (AOP) describes a logical sequence of causally linked events at different levels of biological organisation, which follows exposure to a chemical and leads to an adverse health effect in humans or wildlife. AOPs are the central element of a toxicological knowledge framework, promoted by member countries through OECD, built to support chemical risk assessment based on mechanistic reasoning. These AOPs are available in the AOP Wiki, an interactive and virtual encyclopaedia for AOP development. Following their development and review, the endorsed AOPs are published the OECD Series on Adverse Outcome Pathways. As scientific knowledge progresses, the publication of an AOP in this series does not preclude the regular update or new contributions to a given AOP in the AOP Wiki. While the AOP Wiki is a dynamic tool, only impactful changes to the AOP will be reflected in subsequent updates of the published AOP. The number 1 in the OECD Series on Adverse Outcome Pathways is the Users’ Handbook, which is a supplement to the Guidance Document for developing and assessing AOPs. This handbook contains an updated template for AOP development and provides focused and practical instructions for both AOP developers and reviewers.


Adverse Outcome Pathway on chronic binding of antagonist to N-methyl-D-aspartate receptors (NMDARs) during brain development induces impairment of learning and memory abilities

It is well documented and accepted that learning and memory processes rely on physiological functioning of the glutamate receptor N-methyl-D-aspartate (NMDAR). Both animal and human studies investigating NMDA itself, experiments with NMDAR antagonists and mutant mice lacking NMDAR subunits strongly support this statement (Rezvani, 2006). Activation of NMDARs results in long-term potentiation (LTP), which is related to increased synaptic strength, plasticity and memory formation in the hippocampus (Johnston et al., 2009). LTP induced by activation of NMDA receptors has been found to be elevated in the developing rodent brain compared to the mature brain, partially due to 'developmental switch' of the NMDAR 2A and 2B subunits (Johnston et al., 2009). Activation of the NMDAR also enhances brain derived neurotrophic factor (BDNF) release, which promotes neuronal survival, differentiation and synaptogenesis (Tyler et al., 2002; Johnston et al., 2009). Consequently, the blockage of NMDAR by chemical substances during synaptogenesis disrupts neuronal network formation resulting in the impairment of learning and memory processes (Toscano and Guilarte, 2005). This AOP is relevant to developmental neurotoxicity (DNT). The molecular initiating event (MIE) is described as the chronic binding of antagonist to NMDAR in neurons during synaptogenesis (development) in hippocampus (one of the critical brain structures for learning and memory formation). One of the chemicals that blocks NMDAR after chronic exposure is lead (Pb2+), a well-known developmental neurotoxicant.


Keywords: developmental neurotoxicity, N-methyl-D-aspartate receptor (NMDAR), impairment of learning and memory
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