Synaptic ribbons

Below the inner segment is the soma and nucleus, which connects at its base to the axon and synaptic terminal. Photoreceptors release glutamate at ribbon synapses (Heidelberger et al., 2005). Synaptic ribbons are specialized for sustained release of neurotransmitter and are also found in the terminals of retinal bipolar cells, as well as vestibular and cochlear hair cells. Synaptic ribbons receive their name because of their planar strnctnre in photoreceptor terminals, although bipolar and hair cell ribbons are more spherical in shape. 

Which of the following proteins is a major constituent of synaptic ribbons ... 

Fig. 4. Electron microscopic immunocytochemical localizi.Uion of GAD in the inner plexiform layer of rat retina. A rat was perfused with 600 niL of 4% paraformaldehyde and 0.2% glutnraldehyde mixture in O.IM phosphate buffer, pH 7.4, containing 8.5% sucrose and 0.01% CaCl2- A 50-pm section obtained from the vibratome sectioning was stained with antiserum against GAD. An unstained bipolar terminal (B ) (at the center) makes synapses to three stained amacrine (Ai) terminals, one unstained possible bipolar terminal (Bj.,), and one unstained gan-gbon cell dendrite (G). A typical bipolar dyad containing the synaptic ribbon (arrow head) and its postsynaptic, stained, amacrine terminal (A ), and unstained ganglion cell dendrite (G) is clearly discernible. Those three stained amacrine terminals (A,) also make direct contact to each other although there are no visible synapses. Other stained (A ) and unstained (A2) amacrine processes are also noted. Reaction product in each stained terminal is seen to be assiKiated mainly with synaptic t esicles and synaptic membrane (counterstained with lead citrate), (x 34,000). Bar = 1 ixm.

Schmitz, F., Konigstorfer, A., and Sudhof, T. C. (2000). RIBEYE, a component of synaptic ribbons A protein s journey through evolution provides insight into synaptic ribbon function. Neuron 28, 857-872. 

Figure 30-10 (A) Schematic drawing of a synapse. (B) Electron micrograph showing the synaptic junctions in the basal part (pedicle) of a retinal cone cell of a monkey.403 Each pedicle contains synaptic contacts with 12 triads, each made up of processes from a bipolar cell center that carries the principal output signal and processes from two horizontal cells that also synapse with other cones. A ribbon structure within the pedicle is characteristic of these synapses. Note the numerous synaptic vesicles in the pedicle, some arranged around the ribbon, the synaptic clefts, and the characteristic thickening of the membranes surrounding the cleft (below the ribbons). Micrograph courtesy of John Dowling.

A synaptic vesicle cycle. The number of synaptic vesicles in a single synapse in the brain varies from fewer than 100 to several hundred. In specialized synapses there may be thousands. However, at any moment only a fraction of the total are in the "active zone," often aligned along the presynaptic membrane (Fig. 30-20A) or in specialized ribbons such as those in Fig. 30-10B. The vesicles are normally reused repeatedly, undergoing a cycle of filling with neurotransmitter, translocation to the active zone, ATP-dependent priming, exocytosis with release of the neurotransmitter into the synaptic cleft, coating with clathrin, endocytosis, and acidification as outlined in Fig. 30-20B.554-557 The entire cycle may be completed within 40-60 s to avoid depletion of active vesicles.558 559 A key event in the cycle is the arrival of an action potential at the presynaptic neuron end. 

Heidelberger R, Thoreson WB, Witkovsky P (2005) Synaptic transmission at refinal ribbon syrrapses. Prog Ret Eye Res 24 682—720. 

The ribbon is composed mainly of the structural protein, Ribeye, but also includes a kinesin motor protein, KIF3A, and Rab3-interacting protein, RIM. Ribbons are attached to the synaptic active zone by bassoon, and its structural relative, piccolo. Although the ribbon appears to anchor a readily releasable pool of vesicles, molecular motors do not appear to be involved in vesicle movements near the active zone. RDVt protein mutations have been implicated in an autosomal dominant rod-cone dystrophy (Johnson et al., 2003). 

Glutamate release from synaptic terminals of photoreceptor and bipolar cells is regulated by calcium influx through L-type calcium channels (Heidelberger et al., 2005). The use of F-type channels at ribbon synapses contrasts with the reliance on N, P, and Q type channels for neurotransmission at conventional synapses of spiking neurons. A retina-specific L-type channel, alpha IF (CaVl.4), is localized to rod terminals. Mutations in this channel produce a congenital stationary night blindness (Bech-Hansen et al., 1998). 

Safieddine S, Wenthold RJ (1999) SNARE complex at the ribbon synapses of cochlear hair cells analysis of synaptic vesicle- and synaptic membrane-associated proteins. Eur J Neurosci // 803-812. 

---