Mechanisms Underlying Signal Filtering at a Multisynapse Contact

Abstract

Visual information must be relayed through the lateral geniculate nucleus before it reaches the visual cortex. However, not all spikes created in the retina lead to postsynaptic spikes and properties of the retinogeniculate synapse contribute to this filtering. To understand the mechanisms underlying this filtering process, we conducted electrophysiology to assess the properties of signal transmission in the Long-Evans rat. We also performed SDS-digested freeze-fracture replica labeling to quantify the receptor and transporter distribution, as well as EM reconstruction to describe the 3D structure. To analyze the impact of transmitter diffusion on the activity of the receptors, simulations were integrated. We identified that a large contributor to the filtering is the marked paired-pulse depression at this synapse, which was intensified by the morphological characteristics of the contacts. The broad presynaptic and postsynaptic contact area restricts transmitter diffusion two dimensionally. Additionally, the presence of multiple closely arranged release sites invites intersynaptic spillover, which causes desensitization of AMPA receptors. The presence of AMPA receptors that slowly recover from desensitization along with the high presynaptic release probability and multivesicular release at each synapse also contribute to the depression. These features contrast with many other synapses where spatiotemporal spread of transmitter is limited by rapid transmitter clearance allowing synapses to operate more independently. We propose that the micrometer-order structure can ultimately affect the visual information processing.

The ubiquitous nature of multivesicular release

“…Transmitter spillover most likely occurs when the transmitter concentration is high, e.g. after MVR, when release sites are closely spaced, or in the absence of glial diffusion barriers and uptake machinery [48–51]. A clear distinction between spillover from a neighboring release site and MVR is often difficult because both phenomena can result in prolonged presence of transmitter in the synaptic cleft and at extrasynaptic sites [52–54].…”

“…Hence, desensitization occurs at some but not all MVR synapses. For example, there is minimal desensitization of AMPARs at synapses where glutamate is rapidly cleared by glial and neuronal EAATs (CF�Purkinje cell (CF�PC) synapses) [69, 70], whereas MVR results in profound desensitization at retinogeniculate contacts [51]. In conclusion, MVR can exacerbate desensitization, but synapse tortuosity, glial coverage, receptor composition and spacing of release sites are likely the main factors [71, 72].…”

An evolving view of retinogeniculate transmission

“…Glutamate is the excitatory neurotransmitter packed into numerous round vesicles contained in large synaptic terminals along the axon (Montero & Wenthold, 1989). A single retinal axon terminal can span ~1–4 microns in diameter and contain multiple spatially distinct neurotransmitter release sites (cat and mouse studies: Famiglietti & Peters, 1972; Rafols & Valverde, 1973; Sur & Sherman, 1982; Hamos et al, 1987; Robson, 1993; Bickford et al, 2010; Budisantoso et al, 2012; Morgan et al, 2016). RGC boutons contact a TC neuron near its cell body, synapsing directly onto the dendritic shaft or dendritic appendages that protrude from the proximal shaft or primary dendritic branch points (Rafols & Valverde, 1973; Robson & Mason, 1979; Wilson et al, 1984; Hamos et al, 1987; Bickford et al, 2010; Morgan et al, 2016).…”

“…Large simple boutons in the rat contain an average of 27 independent release sites, whereas each of the boutons in a glomerulus has approximately 6 (Hamos et al, 1987; Budisantoso et al, 2012; Hammer et al, 2015). Notably, multiple RGCs may contribute to the same cluster or glomerulus of boutons (Hammer et al, 2015; Morgan et al, 2016).…”

“…Fig. 1D shows an example of two such close retinogeniculate boutons along one TC neuron dendrite, highlighting that glutamate released from one bouton can diffuse to postsynaptic release sites of the neighboring bouton (Budisantoso et al, 2012). At a developmental phase when boutons are less clustered (Sur et al, 1984; Hong et al, 2014) and glomeruli have not yet formed, retinogeniculate transmission exhibits extensive gluta-mate spillover between neighboring boutons.…”