A model to study NMDA receptors in early nervous system development

Abstract

NMDA receptors (NMDARs) are glutamate-gated ion channels that play critical roles in neuronal development and nervous system function. Pharmacological antagonism is an invaluable tool to study NMDARs, but is experimentally limited. Here, we developed a model to study NMDARs in early development in zebrafish, by generating CRISPR-mediated lesions in the NMDAR genes, grin1a and grin1b, which encode the obligatory GluN1 subunits. While receptors containing grin1a or grin1b show high Ca2+ permeability, like their mammalian counterpart, grin1a is expressed earlier and more broadly in development than grin1b. Both grin1a-/- and grin1b-/- zebrafish are viable as adults. Unlike in rodents, where the grin1 knockout is embryonic lethal, grin1 double mutant fish (grin1a-/-; grin1b-/-), which lack all NMDAR-mediated synaptic transmission, survive until about 10 days post fertilization, providing a unique opportunity to explore NMDAR function during development and in generating behaviors. Many behavioral defects in the grin1 double mutant larvae, including abnormal evoked responses to light and acoustic stimuli, prey capture deficits and a failure to habituate to acoustic stimuli, are replicated by short-term treatment with the NMDAR antagonist MK-801, suggesting they arise from acute effects of compromised NMDAR-mediated transmission. Other defects, however, such as periods of hyperactivity and alterations in place preference, are not phenocopied by MK-801, suggesting a developmental origin. Taken together, we have developed a unique model to study NMDARs in the developing vertebrate nervous system.