Mammalian rod terminal: Architecture of a binary synapse

AbstractThe mammalian rod synapse transmits a binary signal (one photon or none) using tonic, rapid exocytosis. We constructed a quantitative, physical model of the synapse. Presynaptically, a single, linear active zone provides docking sites for ∼130 vesicles, and a “ribbon” anchored to the active zone provides a depot for ∼640 vesicles. Postsynaptically, 4 processes invaginate the terminal: 2 (known to have low affinity glutamate receptors) lie near the active zone (16 nm), and 2 (known to have high affinity glutamate receptors) lie at a distance (130–640 nm). The presynaptic structure seems designed to minimize fluctuations in tonic rate owing to empty docking sites, whereas the postsynaptic geometry may permit 1 vesicle to evoke an all-or-none response at all 4 postsynaptic processes

Active zone proteins are dynamically associated with synaptic ribbons in rat pinealocytes

Synaptic ribbons (SRs) are prominent organelles that are abundant in the ribbon synapses of sensory neurons where they represent a specialization of the cytomatrix at the active zone (CAZ). SRs occur not only in neurons, but also in neuroendocrine pinealocytes where their function is still obscure. In this study, we report that pinealocyte SRs are associated with CAZ proteins such as Bassoon, Piccolo, CtBP1, Munc13–1, and the motorprotein KIF3A and, therefore, consist of a protein complex that resembles the ribbon complex of retinal and other sensory ribbon synapses. The pinealocyte ribbon complex is biochemically dynamic. Its protein composition changes in favor of Bassoon, Piccolo, and Munc13–1 at night and in favor of KIF3A during the day, whereas CtBP1 is equally present during the night and day. The diurnal dynamics of the ribbon complex persist under constant darkness and decrease after stimulus deprivation of the pineal gland by constant light. Our findings indicate that neuroendocrine pinealocytes possess a protein complex that resembles the CAZ of ribbon synapses in sensory organs and whose dynamics are under circadian regulation

Transmitter Concentration at a Three-Dimensional Synapse

this article were defrayed in part by the 100 vesicles / active zone / s. This approximates the rate mea- payment of page charges. The article must therefore be hereby marked sured for the salamander rod for calcium concentrations cor- "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. responding to the dark level (see Rieke and Schwartz 1996; 3163 0022-3077/98 $5.00 Copyright q 1998 The American Physiological Society FIG.1. Top : key architectural features of mammalian rod synapse. A and B : orthogonal views based on 3-dimensional reconstruction (Rao-Mirotznik et al. 1995) . A : synaptic vesicles at the basal edge of the ribbon (dark circles outlined in white) contact release sites at the active zone. Single vesicle discharges glutamate near to low-affinity receptors on horizontal cells (hz 1 ,hz 2 ) but far (130 -- 640 nm) from high-affinity receptors on bipolar cells (b 1 ,b 2 ). B: 1-dimensional active zone (denoted by row of white outlined vesicles) arches over the bipolar dendrites, providing 130 release sites. Two-dimensional ribbon is extensive, tethering 640 additional vesicles. C : invagination includes a large extracellular volume of 0.21 mm that empties through a short neck the extracellular space of which forms a disk of 0.12 mm radius. Bottom : alternative hypotheses of function. D : dendritic integration of unitary postsynaptic potentials (PSPs) : standing concentration of glutamate in the cleft is low; glutamate pulse due to vesicle discharge raises this concentration sharply, evoking a PSP. The bipolar cell integrates PSPs from 20 rods to produce a fluctuating hyperpolarization. One photon suppresses release long enough that the bipolar cell depolarizes and releases transmitter at its own terminal (Ra..