Synaptic bouton

O Sensory receptor Cell body (perikaryon or soma) Synaptic bouton/button... 

When the action potential reaches the synaptic bouton, depolarisation triggers the opening of voltage-operated calcium channels in the membrane (Figure 2.5). The concentration gradient for Ca2+ favours the passive movement of this ion into the neuron. The subsequent rise in cytoplasmic Ca2+ ion concentration stimulates the release of neurotransmitter into the synaptic cleft, which diffuses across this narrow gap and binds to receptors located on the postsynaptic neuronal membrane (Figure 2.5). 

Figure 2.5. The synapse. GPCR = guanine nucleotide-binding protein-coupled receptor, LGICR = ligand-gated ion channel receptor, SB = synaptic bouton, T = neurotransmitter, YOC = voltage-operated ion channel protein, YOCC = voltage-operated calcium channel protein, Ast = astrocyte, AA = axoaxonal synapse, ASD = axosomatic or axodendritic synapse. GPCR 1 = receptor protein, 2 = G-protein, 3 = enzyme, 4 = ion channel protein.

Psychoactive drugs can influence neurotransmission at its five different stages (Chapter 2). First, they may modify the biosynthesis of a neurotransmitter. Second, they can increase or decrease their storage within the presynaptic neuron. Third, they may stimulate or inhibit neurotransmitter release from the synaptic bouton. Fourth, they may affect the binding of the neurotransmitters to its receptor. Finally, they can retard the neurotransmitter s inactivation. Some examples of each of these stages will be given below, but it should be noted that many drugs affect several of these processes. 

Action potential, or nerve impulse The wave of electrical activity that passes from the dendrites of the neuronal cell body, down the axon to the synaptic bouton. 

Neuromodulator A neuropeptide that co-exists with an amine neurotransmitter in the synaptic bouton and has a modulatory (regulatory) effect on the release of the amine. 

Neurotransmitter A chemical stored in the synaptic bouton of the neuron that is involved in cell-to-cell signalling. 

The consequences of nicotinic signalling are dictated by the subcellular localisation of nAChRs (Fig. 2). It was clear from early studies that a significant proportion of nAChRs in the brain are presynaptic (Wonnacott 1997). Presynaptic nAChRs can directly influence transmitter release. Preterminal nAChRs located at the neck of a synaptic bouton can promote transmitter release by initiating action potentials, and are therefore sensitive to tetrodotoxin (Lena et al. 1993 Dani and Bertrand 2007). 

Emptage NJ, Reid CA, Fine A (2001) Calcium stores in hippocampal synaptic boutons mediate short-term plasticity, store-operated Ca + entry, and spontaneous transmitter release. Neuron 29 197-208... 

Figure 4.2 The neuron consists of several parts dendrite, soma (body), axon, and terminal bouton (or synaptic bouton). A gap, or synapse, separates one neuron from another. A chemical messenger, or neurotransmitter, must passively diffuse across the synapse in order to transmit information from one neuron to the next. Information travels from the soma, along the axon, and to the terminal bouton, stimulating the Ca -mediated release of a neurotransmitter.

In the wri mutant, development of the Purkinje cell dentritic trees is impaired synaptic connections of parallel fibers on dendritic spines of Pukinje cells are decreased at two weeks of age, but by nine weeks malformed synaptic boutons lacking synaptic membrane specializations are significantly increased (Inoue et al.,... 

Fig. 15A-C A Double-staining for BrdU and/1111-tubulin (/1111-tub) on day 79 in DGL. Note the double-stained nucleus (arrow) as confirmed by channel separation with orthogonal projections (right panel). Furthermore, note the extension of cellular processes (arrowheads) of the BrdU+//3III-tubulin+ cells toward neighboring BrdU-//3III-tubulin+ cells. B BrdU immunoelectron microscopy in SGZ showing a positive cell extending process. Compare with Fig. 12D. C A putative neuronal progenitor cells forming in the vicinity of which a synaptic bouton is seen (arrow magnified in the inset). Scale bars = 20 pm (A) 1 pm (C)...

Zhu PJ, Lovinger DM (2005) Retrograde endocannabinoid signaling in a postsynaptic neu-ron/synaptic bouton preparation from basolateral amygdala. J Neurosci 25(26) 6199-6207... 

Grady SR, Murphy KL, Cao J, Marks MJ, McIntosh JM, Collins AC (2002) Characterization of nicotinic agonist-induced [(3)H]dopamine release from synaptosomes prepared from four mouse brain regions. J Pharmacol Exp Ther 301 651-60 Graham B, Redman S (1994) A simulation of action potentials in synaptic boutons during presynaptic inhibition. J Neurophysiol 71 538 49... 

The neurotransmitter phenotype, (i.e., what type of neurotransmitter is stored and ultimately will be released from the synaptic bouton) is determined by the identity of the neurotransmitter transporter that resides on the synaptic vesicle membrane. Although some exceptions to the rule may exist all synaptic vesicles of a given neuron normally will express only one transporter type and thus will have a dehned neurotransmitter phenotype (this concept is enveloped in what is known as Dale s principle see also Reference 19). To date, four major vesicular transporter systems have been characterized that support synaptic vesicle uptake of glutamate (VGLUT 1-3), GABA and glycine (VGAT), acetylcholine (VAChT), and monoamines such as dopamine, norepinephrine, and serotonin (VMAT 1 and 2). Vesicles that store and release neuropeptides do not have specific transporters to load and concentrate the peptides but, instead, are formed with the peptides already contained within. 

The nerve fibers in the cochlea can be classified as afferent fibers (toward the brain) or efferent fibers (toward the periphery). The afferent fibers that terminate on IHCs constitute approximately 90-95% of all afferent fibers in the cochlea. These afferent fibers originate from type I ganglion cells and are coated with a thick myelin sheath. Each fiber is connected to only one IHC, but each IHC is innervated by 2-10 individual afferent fibers. The principal neurotransmitter released by IHCs, glutamate, activates the afferent fibers in a quantal manner. The remaining 5-10% of afferent fibers are connected to OHCs and are unmyelinated. They originate from type II ganglion cells. Each afferent fiber is highly branched and is connected to 6-100 OHCs. OHCs are innervated mainly by efferent fibers, which are derived from the medial olivocochlear bundle. Efferent fibers release acetylcholine (ACh) as their principal neurotransmitter. Each OHC is in contact with 2-5 efferent synaptic boutons at its base. IHCs receive axodendritic efferent innervation onto afferent fibers from the lateral olivocochlear bundle (reviewed by Raphael and Altschuler, 2003). 

Ingham CA, Bolam JP, Smith AD (1988) GABA-immunoreactive synaptic boutons in the rat basal forebrain comparison of neurons that project to the neocortex with pallidosubthalamic neurons. J. Comp. Neurol, 273, 263-282. 

Fournier, J.G., Escaig, H.F, Billette, d.V.T., and Robain, O. (1995). Ultrastructural localization of cellular prion protein (PrPc) in synaptic boutons of normal hamster hippocampus. C. R. Acad. Sci 7//318, 339-344. 

Kosaka T., Hama K. and Wu J -Y (1984) GABAergic synaptic boutons in the rat dentate gyrus Brain Res 293, 353-359... 

Budnik V. and Gorczyca M. 1992. SSB, an antigen that selectively labels morphologically distinct synaptic boutons at the Drosophila larval neuromuscular junction. /. Neurobiol. 23 1054-1065. 

Estes P.S., Roos J., van der Bliek A., Kelly R.B., Krishnan K.S., and Ramaswami M. 1996. Traffic of dynamin within individual Drosophila synaptic boutons relative to compartment-specific markers. J. Neurosci. 16 5443-5456. 

Lahey X, Gorczyca, M., Jia X., and Budnik V. 1994. The Drosophila tumor suppressor gene, dig, is required for normal synaptic bouton structure. Neuron 13 823-835. 

Iontophoresis. Iontophoresis of charged molecules (e.g., L-glutamate) is performed essentially as described for the embryo (see p. 287). A stock solution of 0.1 m L-glutamate (pH 8.0) is iontophoresed from a glass pipette (10-20 M 2) positioned directly over the NMJ synaptic boutons. 

Seals are made by applying gentle suction to identified synaptic boutons visualized with Nomarski optics. Seal resistances of up to 30 G 2 form, and currents can be recorded in cell-attached, inside-out or whole-cell recording configurations (Martinez-Padron and Ferrus 1997). Records from enlarged, mutant type III boutons on muscle 12 (>2 im in diameter) showed an average input resistance of 8-10 G 2 and mean bouton capaci-... 

DaryaK,GangulyA,LeeD(2009)Quantitative analysis of synaptic boutons in Drosophila primary neuronal cultures. Brain Res 1280 1-12... 

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