NMDA receptors on zebrafish Mauthner cells require CaMKII-α for normal development

Calcium/calmodulin dependent protein kinase 2 (CaMKII) is a multifunctional protein that is highly enriched in the synapse. It plays important roles in neuronal functions such as synaptic plasticity, synaptogenesis, and neural development. Gene duplication in zebrafish has resulted in the occurrence of seven CaMKII genes (camk2a, camk2b1, camk2b2, camk2g1, camk2g2, camk2d1, and camk2d2) that are developmentally expressed. In this study, we used single cell, real-time quantitative PCR to investigate the expression of CaMKII genes in individual Mauthner cells (M-cells) of 2 days post fertilization (dpf) zebrafish embryos. We found that out of seven different CaMKII genes, only the mRNA for CaMKII-α was expressed in the M-cell at detectable levels, while all other isoforms were undetectable. Morpholino knockdown of CaMKII-α had no significant effect on AMPA synaptic currents (mEPSCs) but decreased the amplitude of NMDA mEPSCs. NMDA events exhibited a biexponential decay with τfast ≈ 30 ms and τslow ≈ 300 ms. Knockdown of CaMKII-α specifically reduced the amplitude of the slow component of the NMDA-mediated currents (mEPSCs), without affecting the fast component, the frequency, or the kinetics of the mEPSCs. Immunolabelling of the M-cell showed increased dendritic arborizations in the morphants compared with controls, and knockdown of CaMKII-α altered locomotor behaviors of touch responses. These results suggest that CaMKII-α is present in embryonic M-cells and that it plays a role in the normal development of excitatory synapses. Our findings pave the way for determining the function of specific CaMKII isoforms during the early stages of M-cell development.