Figure 3. Ribbonless synapses contain clusters of presynaptic voltage-gated Ca²⁺channels juxtaposed to postsynaptic glutamate receptors.

(A–D) Maximum intensity projections of airyscan z-sections of IHCs in apical cochlea turns of P35 KO and WT littermates, immunolabeled for CtBP2/RIBEYE (A), GluA3 (B), Ca_V1.3 (C) and overlay between calcium channels and glutamate receptors (D). IHC afferent synapses from WT have a synaptic ribbon: a cytoplasmic presynaptic density comprised mainly of RIBEYE protein, recognized by anti-CtBP2 (green). The closely associated presynaptic voltage-gated Ca²⁺ channels (Ca_V1.3, gray) and postsynaptic AMPA receptors (GluA3, magenta) are co-labelled. IHC afferent synapses from KO lacked synaptic ribbons. The AMPA receptors in KO mice exhibited a ring-like morphology, and the Ca_V1.3 channels clustered in the form of round spots or elongated stripes, both similar to morphological features of WT synapses. Scale bar: 2 µm.