To form neuronal networks as described above, neurons need to be able to communicate with each other. This communication is based on the transmission of signals between one neuron and
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its partner and can either be electric or through chemical molecules called neurotransmitters.
While electric synapses enable bidirectional communication (i.e., there is no “sender” and no
Figure 4: Schematic overview of the input to the amygdala. Input comes from all over the brain.
Reprinted from Neuropsychopharmacology, 36, R. Elliott, R. Zahn, J. F. W. Deakin, I. M. Anderson,
‘Affective Cognition and its Disruption in Mood Disorders’, page 160, 2011, with permission from Springer Nature.
“addressee” per se, both sides can fulfil both functions), chemical synapses usually function only in one direction. They can be highly adaptable, which is called plasticity. In this study, chemical synapses will be in the spotlight. Upon the arrival of a stimulus – typically an action potential – transmitter molecules will be released from the presynaptic terminal, diffuse into the synaptic cleft, and reach the membrane of the receiving neuron, where they can interact with receptor molecules, which in turn start a downstream reaction in the postsynaptic neuron.
The neurotransmitter molecules are packed in vesicles, which are about 40nm in diameter (Qu et al., 2009). They are arranged into different pools in the presynaptic terminal (Alabi & Tsien, 2012), which I will now describe in more detail.
1.3.1 Synaptic vesicle pools and release
Typically, three different pools of SVs can be found in the presynaptic compartment: the resting pool (RP), the recycling pool and the readily releasable pool (RRP; Rizzoli & Betz, 2005). Which SV belongs to which pool depends on the definition of the pool: either based on the spatial location of the SV in the presynaptic terminal, or based on functional aspects, such as release probability of the SV (Alabi & Tsien, 2012). SVs that belong to the RP are located furthest from the presynaptic membrane, and have the lowest release probability, as mobilization of this pool
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takes time. SVs in the recycling pool and RRP have higher release probabilities. The RRP consists of SVs that are already docked to the presynaptic membrane (Imig et al., 2014). This docking is mediated by the SNARE proteins (Soluble N-ethylmaleimide sensitive factor attachment protein receptor): Vesicle-associated membrane protein (VAMP, also known as Synaptobrevin, Trimble, Cowan, & Scheller, 1988), Syntaxin (Bennett et al., 1992) and Synaptosomal-associated protein of 25kDA (SNAP25, Oyler et al., 1989). VAMP is associated with the SV membrane and forms a lose core complex with Syntaxin and SNAP25, which are attached to the presynaptic membrane.
The core SNARE complex, together with other proteins, including Munc13 and Munc18, brings the SV membrane and the presynaptic cell membrane into close proximity. Munc18 binds to Syntaxin, thereby starting the process of SV fusion (Ma et al., 2012). Munc13 on the other hand is involved in rendering SVs release-ready (before fusion can happen), which is called “priming”
(Varoqueaux et al., 2002). Both Munc-isoforms are required for neurotransmission, as deletion of either results in a total loss of SV fusion and transmitter release (Varoqueaux et al., 2002;
Verhage et al., 2000). The docked SVs are the ones that are released (and depleted) first upon the arrival of a stimulus (Rosenmund & Stevens, 1996; Schneggenburger et al., 2002; Von Gersdorff et al., 1996), thereby contributing most to the strength of the synapse (Dobrunz &
Stevens, 1997; Waters & Smith, 2002). The recycling pool replenishes the RRP after stimulus onset, which requires additional transitional processes (i.e., docking and priming). The replenishment rate generally is the limiting factor during persistent synaptic activity and greatly influences neuronal plasticity (Alabi & Tsien, 2012).
When an action potential reaches the synaptic bouton, the depolarization of the terminal leads to the opening of voltage-gated Ca2+-channels (VGCCs, Dolphin, 2009) and influx of Ca2+ into the presynaptic terminal. The elevated Ca2+-concentration causes a tightening of the SNARE-complex, which exerts tension on the two membranes, and creates a fusion pore. Through this pore, the neurotransmitter molecules can diffuse into the synaptic cleft and interact with the neurotransmitter receptors located in the postsynaptic membrane. Ca2+-channels are not localized randomly in the presynaptic membrane, but are tethered to the membrane by Rab3-interacting molecules (RIM; Kaeser et al., 2011). These specialized sites in the presynaptic membrane, where SV fusion is observed, are called active zones (AZ).
1.3.2 The active zone
At the AZ, a plethora of molecules tightly regulates the SV cycle, from docking, priming, fusion to re-uptake and refilling of the SVs. This electron-dense mesh of proteins is called the cytomatrix of the active zone (CAZ). Among the proteins forming the CAZ are Munc13, Piccolo (also called
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Aczonin), RIM and RIM-binding proteins (RIM-BPs), ELKS/CAST and Bassoon. They are arranged in a precise manner, allowing for the localization of an AZ exactly opposite the postsynaptic density.
While all CAZ-proteins have unique functions, their interplay is important for the AZ to fulfil its function: mediating the fusion of SVs and releasing neurotransmitter into the synaptic cleft. The organization of the different proteins enables SVs to be brought into close proximity to Ca2+ -channels (Ackermann et al., 2015), allowing SV fusion to happen shortly after opening of the channels. The regions where SVs and Ca2+-channels are clustered are also called microdomains (Chad & Eckert, 1984; Neher, 1998; Simon & Llinás, 1985). In some synapses, they are clustered in such close proximity that they are even called nanodomains (Bucurenciu et al., 2008). This concept of clustering is one explanation for the different release probabilities of SVs: The closer an SV is located to a Ca2+-channel, the less calcium influx is needed to induce fusion of the SV.
This means that SVs that are closest to a Ca2+-channel have the highest chance of being released, and thus the highest release probability. Other factors play a role as well, such as the intrinsic Ca2+-sensitivity of the sensor, which will be discussed in the context of superpriming later on.