Microanatomy of the postsynaptic triads at the ribbon synapse of rod spherules in the mouse retina

Abstract

Purpose: The ribbon synapse at the photoreceptor terminal is the first synapse in the retina. Recent studies have revealed the molecular structure of the synaptic ribbon but there are few studies concerning the postsynaptic elements at ribbon synapses. Therefore, we examined the microanatomy of the postsynaptic triad invaginating the rod spherule by Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) combined with 3D image analysis.

Methods: C57BL/6J mouse retinas were dissected and fixed in 2% parformaldehyde and 2.5% glutaraldehyde. The tissues were stained en bloc and embedded in Epon. Retinal pieces were cut horizontally through the outer plexiform layer with 30-nm thickness and we automatically scanned the cutting surface by FIB-SEM. The scanned images were added one by one and reconstructed with Mimics software.

Results: Synaptic triads: The postsynaptic structure was composed of one or two bipolar dendrites as the lower central element with two horizontal cell axon terminals providing lateral elements closest to the synaptic ribbon. All the processes entered the rod spherule together as a thin bundle at one side of the synaptic ribbon plate. Afterwards, the two horizontal elements ran parallel on either side of the ribbon in an arc, giving a ‘hook’ or ‘question mark’ appearance when viewed from the side. When one bipolar dendrite formed the central element, it expanded horizontally like a fan with a concave top opposed to the vertical synaptic ribbon. However, when two bipolar dendrites entered the rod spherule, they expanded vertically and parallel to each other.

Conclusions: These results demonstrate that the organization of postsynaptic triads at rod ribbon synapses is variable with one or two low but central bipolar elements flanked by two horizontal cell processes in close apposition to the synaptic ribbon.

Keywords: 693 retinal connections, networks, circuitry • 728 synapse • 689 retina: distal (photoreceptors, horizontal cells, bipolar cells)