Synaptosomes, neurotransmitter

Turner TJ, Adams ME, Dunlap K (1993) Multiple Ca2+ channel types coexist to regulate synaptosomal neurotransmitter release. Proc Natl Acad Sci USA 90 9518-9522... 

There is some evidence that receptors for other neurotransmitters on 5-HT nerve terminals also modify release of 5-HT. These include nicotinic receptors (increase release from striatal synaptosomes), a2A-adrenoceptors (depress cortical release) and H3-receptors (cortical depression). Because changes in 5-HT release on activation of these receptors is evident in synaptosomal preparations, it is likely that these are true heteroceptors . 

Some intracellular signal transduction molecules are reduced in schizophrenia. The release of neurotransmitters is regulated by a family of proteins that coordinate vesicular trafficking (see Ch. 9). Of these, the expression of complexin I and II appears to be decreased in prefrontal cortex and subfields of the hippocampal formation, and the ratio of complexin I to complexin II is elevated in the hippocampus [35], SNAP-25 (Synaptosomal Associated Protein, kDa 25) has inconsistently been found to be down-regulated in both these regions. Synapsin expression is also reduced, but more robust decrements have been observed in bipolar disorder (Ch. 55). 

The metabolism of norepinephrine is reported to be altered by other drugs used in the treatment of the affective disorders and a number of studies have shown a change in the metabolism of norepinephrine as a result of Li+ treatment. In rat brain, acute Li+ treatment enhances the uptake of norepinephrine in synaptosomes [151] and the enhanced turnover of this neurotransmitter may be due to an increase in its deamination in the brain, although Li+ also causes a slight increase in the levels of the amino acid precursor, tyrosine, in the brain and plasma of rats [152]. Also, acute Li+ treatment induces a decrease in the release of norepinephrine after electrical stimulation of rat brain [153]. Interest-... 

Adding the second alpha-methyl to MDA gives a compound that apparently is only weakly active in man (199). Again, the 3,4-methylenedioxy substitution seems anomalous. This may also be due to an indirect effect, such as release of endogenous neurotransmitter from nerve terminals. However, neither this a,a-dimethylated MDA analog nor a,a-dimethyl-4-methoxy-/3-phenethylamine had any ability to induce the release of 3H-serotonin from rat whole brain synaptosomes (160). 

Neurotransmitter release induced by potassium-dependent depolarization is a physiologically relevant way to investigate pyrethroid effects on calcium-dependent neurotransmitter release since this process is independent of voltage-sensitive sodium channels [71]. Furthermore, potassium-stimulated calcium influx and subsequent neurotransmitter release by synaptosomes is blocked by a variety of voltage-sensitive calcium channel antagonists but not by TTX [4, 71, 72]. 

At the cellular level, chlordecone causes spontaneous neurotransmitter release (End et al. 1981) and increases in free intracellular calcium in synaptosomes (Bondy and Halsall 1988 Bondy and McKee 1990 Bondy et al. 1989 Komulainen and Bondy 1987). This appears to be due at least in part to increased permeability of the plasma membrane (Bondy and Halsall 1988 Bondy and McKee 1990 Bondy et al. 1989 Komulainen and Bondy 1987), activation of voltage-dependent calcium channels (Komulainen and Bondy 1987), and inhibition of brain mitochondrial calcium uptake (End et al. 1979, 1981). 

Amino acid neurotransmitter Constituents of hypericum also appear to have effects on amino acid neurotransmission, particularly GABA. Hypericin and a crude extract bind to GABAA and GABAB receptors (Cott 1997). Hyperforin also inhibits synaptosomal GABA reuptake in the low micromolar range (IC50 values of 0.05-0.10 ug/ml). Activity at GABAA benzodiazepine receptors was noted in extracts of four hy-... 

All botulin neurotoxins act in a similar way. They only differ in the amino-acid sequence of some protein parts (Prabakaran et al., 2001). Botulism symptoms are provoked both by oral ingestion and parenteral injection. Botulin toxin is not inactivated by enzymes present in the gastrointestinal tracts. Foodborne BoNT penetrates the intestinal barrier, presumably due to transcytosis. It is then transported to neuromuscular junctions within the bloodstream and blocks the secretion of the neurotransmitter acetylcholine. This results in muscle limpness and palsy caused by selective hydrolysis of soluble A-ethylmalemide-sensitive factor activating (SNARE) proteins which participate in fusion of synaptic vesicles with presynaptic plasma membrane. SNARE proteins include vesicle-associated membrane protein (VAMP), synaptobrevin, syntaxin, and synaptosomal associated protein of 25 kDa (SNAP-25). Their degradation is responsible for neuromuscular palsy due to blocks in acetylcholine transmission from synaptic terminals. In humans, palsy caused by BoNT/A lasts four to six months. 

Mariussen E, Fonnum F (2003) The effect of brominated flame retardants on neurotransmitter uptake into rat brain synaptosomes and vesicles. Neurochem Int. 43 533-542 Marshall KA (2003) Chlorocarbons and chlorohydrocarbons, survey. In Kirk-Othmer Encyclopedia of Chemical Technology. Wiley, 6 226-253. Available at http //www.mrw.interscience.wiley. com/emrw/9780471238966/search/firstpage... 

The toxin produced by the bacterium C. botulinum is a mixture of six large molecules and is one of the two most potent toxins known to humans. Each consists of two components, a heavy (100 kDa) and light (50 kDa) polypeptide chain. The toxin molecule is transported into nerve cells and destroys a synaptosomal protein, which prevents the release of the neurotransmitter acetylcholine. This blocks muscle contraction, causing paralysis. This can be fatal. 

In aged animal models, chronic administration of ginkgo for 3-4 weeks led to modifications in central nervous system receptors and neurotransmitters. Receptor densities increased for muscarinic, 2, and 5-HTla receptors and decreased for B-adrenoceptors. Increased serum levels of acetylcholine and norepinephrine and enhanced synaptosomal reuptake of serotonin have also been reported. Additional mechanisms that may be involved include reversible inhibition of MAO-A and MAO-B, reduced corticosterone synthesis, inhibition of amyloid-beta fibril formation, and enhanced GABA levels. 

Fig. 4 Superfusion neurotransmitter release assay in synaptosomes. (a) Schematic drawing of a superfusion setup. Synaptosomes are preloaded with radioactive neurotransmitter and captured on fiberglass filters in superfusion chambers under continuous superfusion with gassed physiological salt solution (e.g., Krebs bicarbonate buffer) using a peristaltic pump. After a 10- to 15-minute wash, neurotransmitter release is triggered by rapid switching of superfusion lines to a stimulating buffer (e.g., high-potassium solution). Superfusate is collected on a fraction collector, and radioactivity is measured by liquid scintillation, (b) Typical trace recording of tritium-labeled norepinephrine fractional release in rat cortical synaptosomes stimulated by high potassium and a-latrotoxin in the presence or absence of external calcium.

Mariussen E, Fonnum F (2003) The effect of brominated flame retardants on neurotransmitter uptake into rat brain synaptosomes and vesicles. Neurochem Int 43 533-542. 

These observations are particularly relevant to the experimental use of a-LTX in neurotransmission studies, since a-LTX has been shown in several systems to cause nonvesicular release by allowing leakage of cytoplasmic neurotransmitters (McMahon et al. 1990 Deri et al. 1993 Davletov et al. 1998). This flux could be mediated by the a-LTX channel itself, by local disruptions of cellular membranes, or by reversal of transmitter uptake pumps driven by Na+ gradient (see Section 3.4.3). Synaptosomes seem to be particularly sensitive to an increase in hydrostatic pressure, which may occur when influx of Na+ or Ca2+ leads to a concomitant influx of water. 

Mee CJ, Tomlinson SR, Perestenko PV et al (2004) Latrophilin is required for toxicity of black widow spider venom in Caenorhabditis elegans. Biochem J 378 185-91 Meiri H, Erulkar SD, Lerman T et al (1981) The action of the sodium ionophore, monensin, or transmitter release at the frog neuromuscular junction. Brain Res 204 204-8 Meldolesi J, Huttner WB, Tsien RY et al (1984) Free cytoplasmic Ca2+ and neurotransmitter release studies on PCI 2 cells and synaptosomes exposed to a-latrotoxin. Proc Natl Acad Sci US A 81 6204... 

Shimazaki Y, Nishiki T, Omori A et al (1996) Phosphorylation of 25-kDa synaptosome-associated protein. Possible involvement in protein kinase C-mediated regulation of neurotransmitter release. J Biol Chem 271 14548-53... 

In cell culture preparations, diphenylhydantoin, carbamazepine and valproate have been shown to reduce membrane excitability at therapeutically relevant concentrations. This membrane-stabilizing effect is probably due to a block in the sodium channels. High concentrations of diazepam also have similar effects, and the membrane-stabilizing action correlates with the action of these anticonvulsants in inhibiting maximal electroshock seizures. Intracellular studies have shown that, in synaptosomes, most anticonvulsants inhibit calcium-dependent calmodulin protein kinase, an effect which would contribute to a reduction in neurotransmitter release. This action of anticonvulsants would appear to correlate with the potency of the drugs in inhibiting electroshock seizures. The result of all these disparate actions of anticonvulsants would be to diminish synaptic efficacy and thereby reduce seizure spread from an epileptic focus. 

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