Distinct Roles of CaMKII in Synaptic Vesicle Dynamics at Zebrafish Retinal Rod Bipolar Ribbon Synapses

Calcium (Ca²⁺) not only serves as a fundamental trigger for neurotransmitter release but also participates in shaping neurotransmitter release (NTR) during prolonged presynaptic stimulation via multiple Ca²⁺-dependent processes. The Ca²⁺/calmodulin (CaM)-dependent protein kinase II (CaMKII) is enriched at various presynaptic terminals, including ribbon synapses, where it associates with synaptic ribbons and thus may contribute to the modulation of Ca²⁺-dependent NTR. This could arise via its ability to influence one or more steps that control either the Ca²⁺ signal, the release process, or synaptic vesicle dynamics, affecting available pools. Yet, recent studies have yielded conflicting results regarding the ability of CaMKII to influence NTR at rod bipolar cell (RBC) ribbon synapses. To address this, we acutely manipulated CaMKII activity in synaptic terminals of zebrafish RBCs from both sexes by infusion of either inhibitory peptides targeting CaMKII or CaM, or a constitutively active CaMKII, while using a combination of imaging and electrophysiological approaches. Neither inhibiting nor enhancing CaMKII activity affects presynaptic Ca²⁺ channel activity. However, capacitance measurements revealed that inhibition of either CaMKII or CaM reduces exocytosis. CaMKII inhibition also reduces synaptic vesicle replenishment. Surprisingly, elevation of CaMKII activity also diminished vesicle fusion, similar to the effect of CaMKII inhibition, suggesting that CaMKII activity naturally exists at optimal levels to support neurotransmitter release. In contrast to CaMKII inhibition, CaMKII activity elevation did not impair vesicle replenishment. Collectively, these data suggest that distinct synaptic vesicle populations are differentially reliant on the level of CaMKII activity.