UC San Diego

Neurotransmitter switching is associated with behavioral changes in every case that a behavior change has been sought, including levels of anxiety (Dulcis et al., 2013) and motor coordination (Li et al., 2016) in adult rodents (see Spitzer, 2017 for review). To preserve synaptic function, a newly expressed neurotransmitter requires the expression of matching receptors, but the mechanisms by which this is achieved remain unclear. To investigate neurotransmitter-receptor matching in vivo, Xenopus laevis embryos were implanted with drug-loaded agarose beads at the time of closure of the neural tube. Glutamate, a major excitatory neurotransmitter, and the NMDA receptor, an ionotropic glutamate receptor, were chosen as the neurotransmitter-receptor pair in this study. Immunohistochemical analysis revealed that an NMDA receptor subunit GluN1 increased in expression in the trunk muscle plasma membrane in response to glutamate-loaded beads compared to control beads. The upregulation of GluN1 was reproduced with agonists AMPA and NMDA in combination, and suppressed with AP5 (NMDAR antagonist) and NBQX (AMPAR antagonist) in combination, establishing specificity of this receptor upregulation. We established the role of mitogen-activated protein kinases (MAPKs) p38 and jun-N-kinase (JNK) as downstream effectors of glutamate signaling, as well as the role of transcription factor MEF2C as a p38 target, by pharmacological knock-down with morpholinos. We propose a model in which glutamate acts on low numbers of ionotropic glutamate receptors initially expressed on embryonic Xenopus muscle (Borodinsky & Spitzer, 2007), promoting calcium entry that activates MAP kinases p38, JNK and MEF2, leading to upregulation of NMDAR expression.