Nerve-ending synaptosomal

Wood, J. D., Schousboe, A., and Krogsgaard-Larsen, P. (1980) In vivo changes in the GABA content of nerve endings (synaptosomes) induced by inhibitors of GABA uptake. Neuropharmacology 19,1149-1152. 

Table 1. Location of H3 heteroreceptors inhibiting the release of monoamines, acetylcholine and glutamate in the brain. To prove or disprove the presynaptic location of H3 receptors, transmitter release was studied in isolated nerve endings (synaptosomes) or in brain slices superfused with K+-rich Ca2+-free medium containing tetrodotoxin (TTX) (in the latter case, transmitter release was evoked by introduction of Ca2+ ions into the medium). The experimental approaches used in the electrophysiological study to show the presynaptic location of H3 receptors on glutamatergic neurones are described in the text.

Gangliosides. sialidase and sialvltransferase in the membranes surrounding nerve endings ( synaptosomal membranes )... 

Isolated nerve endings (synaptosomes) in superfusion represent an ideal preparation for the study of neurotoxicant action. Indeed the use of such an approach to examine the interaction of pharmacological agents with ion channels in vertebrate synaptosomal preparations is well established ( 3), (40. It is only relatively recently that fractionation procedures for invertebrate synaptosomes have been optimized and superfusion techniques applied to the study of drug and toxin action in insects (J ), ( 5). In the present communication we have attempted to clarify the mode of action of certain insecticidal neurotoxicants by determining their effects on transmitter release in the presence and absence of channel specific modulators. 

Geddes J W and Wood J D. (1984) Changes in the amino acid content of nerve endings (synaptosomes) induced by drugs that alter the metabolism of glutamate and y-aminobutyric acid / Neurochem 42, 16-24... 

Wood J D, Russell M P, and Kurylo E (1980) The y-aminobutyrate content of nerve endings (synaptosomes) in mice after the instramuscular injection of y-aminobutyrate-elevatmg agents. A possible role in anticonvulsant activity. / Neurochem 35, 125-130... 

COMT activity has been demonstrated in many tissues of the body in almost all cases it appears to be a cytoplasmic enzyme. However, in adipose tissue and erythro< es some enzyme activity is associate with the cell-membranes. In sympathetically innervated tissues most of the COMT activity is located in cells other than the adrenergic neurons (extraneuronal). Recent evidence sug ts. however, that small amounts of COMT are also present in adrenergic nerves. In the brain considerable amounts of COMT are recovered in fractions containing nerve endings (synaptosomes), suggesting a partially neuronal location of the en me in this organ. 

A NT might be expected to be concentrated in nerve terminals and this can be ascertained since when nervous tissue is appropriately homogenised the nerve endings break off from their axons and surrounding elements and then reseal. Such elements are known as synaptosomes. They have been widely used to study NT release in vitro (Chapter 4) and some NT should always be found in them, at least if it is released from vesicles. 

Regulation of transmitter release does not rest solely on the frequency at which nerve impulses reach the terminals. Early experiments using stimulated sympathetic nerve/end-organ preparations in situ, or synaptosomes, indicated that release of [ HJnoradrenaline was attenuated by exposure to unlabelled, exogenous transmitter. This action was attributed to presynaptic adrenoceptors, designated a2-adrenoceptors, which were functionally distinct from either aj- or )S-adrenoceptors. Later experiments have confirmed that ai-adrenoceptors comprise a family of pharmacologically and structurally distinct adrenoceptor subtypes. 

Schematic illustration of a generalized cholinergic junction (not to scale). Choline is transported into the presynaptic nerve terminal by a sodium-dependent choline transporter (CHT). This transporter can be inhibited by hemicholinium drugs. In the cytoplasm, acetylcholine is synthesized from choline and acetyl -A (AcCoA) by the enzyme choline acetyltransferase (ChAT). Acetylcholine is then transported into the storage vesicle by a second carrier, the vesicle-associated transporter (VAT), which can be inhibited by vesamicol. Peptides (P), adenosine triphosphate (ATP), and proteoglycan are also stored in the vesicle. Release of transmitter occurs when voltage-sensitive calcium channels in the terminal membrane are opened, allowing an influx of calcium. The resulting increase in intracellular calcium causes fusion of vesicles with the surface membrane and exocytotic expulsion of acetylcholine and cotransmitters into the junctional cleft (see text). This step can he blocked by botulinum toxin. Acetylcholine s action is terminated by metabolism by the enzyme acetylcholinesterase. Receptors on the presynaptic nerve ending modulate transmitter release. SNAPs, synaptosome-associated proteins VAMPs, vesicle-associated membrane proteins.

Figure 5. Electron microscopic examination of the "nerve ending fraction (X13,000) and of the synaptosomal membrane fraction (X.7150)...

Biochemical characteristics of the "Nerve ending fraction" and of the "Synaptosomal fraction", obtained from calf brain. Ganglio-sides are expressed as nmoles bound N-acetylneuraminic acid enzyme activities in milli International Units ( 1 nmole transformed substrate min at 37°C - 30° for NADH-, and NADPH-Cyt C reductase and LDH). The data shown, referred to 1 g starting fresh tissue, are the mean values of 6 experiments the S. E. was in all cases lower than + 10 % of the mean values. 

Parameter Nerve ending fraction Activity (or concentration) total specific Synaptosomal membrane fraction Activity (or concentration) total specific Enrichment ... 

Whittaker, V.P. and Michaelson, I.A. et al. (1964) The separation of synaptic vesicles from nerve-ending particles ( synaptosomes ). Biochem. J. 90, 293-303. 

By directly testing the effects of NO in a reductionist system such as cardiac synaptosomes, where no local reflex mechanisms come into play, we have uncovered a potent concentration-dependent biphasic effect of NO on adrenergic nerve endings. Thus, our findings on cardiac synaptosomes may help to explain the discrepancies in the literature on the modulatory effects of NO on adrenergic nerve function. 

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