Functional and Structural Development of Mouse Cone Photoreceptor Ribbon Synapses

Abstract

Purpose Cone photoreceptors of the retina use a sophisticated ribbon-containing synapse to convert light-dependent changes in membrane potential into release of synaptic vesicles (SVs). We aimed to study the functional and structural maturation of mouse cone photoreceptor ribbon synapses during postnatal development and to investigate the role of the synaptic ribbon in SV release. Methods We performed patch-clamp recordings from cone photoreceptors and their postsynaptic partners, the horizontal cells during postnatal retinal development to reveal the functional parameters of the synapses. To investigate the occurring structural changes, we applied immunocytochemistry and electron microscopy. Results We found that immature cone photoreceptor terminals were smaller, they had fewer active zones (AZs) and AZ-anchored synaptic ribbons, and they produced a smaller Ca 2+ current than mature photoreceptors. The number of postsynaptic horizontal cell contacts to synaptic terminals increased with age. However, tonic and spontaneous SV release at synaptic terminals stayed similar during postnatal development. Multiquantal SV release was present in all age groups, but mature synapses produced larger multiquantal events than immature ones. Remarkably, at single AZs, tonic SV release was attenuated during maturation and showed an inverse relationship with the appearance of anchored synaptic ribbons. Conclusions Our developmental study suggests that the presence of synaptic ribbons at the AZs attenuates tonic SV release and amplifies multiquantal SV release. However, spontaneous SV release may not depend on the presence of synaptic ribbons or voltage-sensitive Ca 2+ channels at the AZs.

Synaptic vesicle release during ribbon synapse formation of cone photoreceptors

“…For conventional electron microscopy, specimens were treated as described elsewhere (Davison et al, 2022b). Retinas of C56BL/6J mice at P8, P10, P12, and P30 (n = 3 for each age group) were sequentially fixed in 4% PFA for 1 h, followed by fixation in 2.5% glutaraldehyde for 2 h at room temperature.…”

“…Ultrathin sections were examined and photographed with an EM10 electron microscope (Carl Zeiss) equipped with a SC1000 Orius TM CCD camera (GATAN, Pleasanton, CA) in combination with the DigitalMicrograph 3.1 software (GATAN). For analysis, we used the dataset from our previous publication (Davison et al, 2022b), consisting of approximately 150 randomly photographed cone photoreceptor terminals per timepoint. For the quantification of SVs, we placed boxes of 300 nm × 200 nm in the cytosol at ribbon-occupied and at ribbon-free AZs and counted all SVs within this region of interest.…”

“…We have recently conducted a study on the functional and structural maturation of mouse cone photoreceptor ribbon synapses (Davison et al, 2022b). We found that during postnatal development synaptic ribbons gradually occupy the ribbon-free AZs, mimicking the transition from a ribbon-free synapse to a ribbon-type synapse.…”

“…Mouse cone photoreceptor ribbon synapse development primarily occurs in the first two postnatal weeks (Blanks et al, 1974;Davison et al, 2022b). During this time, synaptic protein constituents, such as ribeye, picollino, bassoon, ubMunc13-2, CAST, RIM, and the L-type Ca 2+ channel α1 subunit arrive to the AZ (Hermes et al, 1992;Regus-Leidig et al, 2009).…”

“…Mice open their eyes at ∼P12-13 but by this time, the synapses of the photoreceptors are not yet fully developed. For example, in cone photoreceptors, only 30% of all ribbons are anchored to the AZs at this age (Davison et al, 2022b). Consequently, synapse development continues until maturity, which is completed at ∼P30 (Vinberg et al, 2014;Bonezzi et al, 2018).…”