Synaptosomal distribution

SNAP-25 (synaptosomal-associated protein of 25kDa) is a 206 residue protein that lacks a classical transmembrane segment, but is bound to the cytosolic surface of the neuronal plasmalemma via pal-mitoylation of four cysteine residues located at the center of the molecule (Fig. 3) (Bennett and Scheller, 1994 Sudhof, 1995). SNAP-25 is required for axonal growth during development, and for nerve terminal plasticity in the mature nervous system (Osen-Sand et al., 1993). The tissue distribution of SNAP-25 is less well characterized than that of VAMP however, its presence in pancreatic cells (Jacobson et al., 1994 Sadoul et al., 1995) may indicate that it is also expressed outside the nervous system. A SNAP-25 related protein required for post-Golgi transport has been cloned from yeast (Brenwald efal., 1994). 

An asymmetric lipid distribution between the exoplasmic and cytoplasmic leaflets of plasma membranes is typical (Devaux, 1991). Por example, it is well documented that highly unsaturated species of PE and PS are found primarily on the inner leaflet of many membranes. This has been reported for human erythrocytes (Knapp et al., 1994), murine synaptosomal plasma membranes (Pontaine et al., 1980), and human lymphocytes (Bougnoux et al., 1985), among others. It has also been shown that polyunsaturated fatty acids, including DHA, are found in higher concentrations in the aminophospholipids in the inner, cytoplasmic leaflet (Crinier et al., 1990 Hullin et al., 1991). As a result, the cytoplasmic leaflet of erythrocytes is more fluid than the exoplasmic leaflet (Morrot et al., 1986). The addition of polyunsaturated fatty acids to membranes have been shown to translocate cholesterol to the outer leaflet, where its efflux from membranes is enhanced (Dusserre et al., 1995). 

More recently, after allowing synaptosomes to reach a steady state Ca uptake in high Na+ medium (61), naloxone-reversible inhibition of basal Ca uptake by morphine and JB-endorphin could be consistently demonstrated (62,63) This inhibitory effect was noted only when ATP was present in milli-molar concentrations in the incubation medium. Efflux of Ca from preloaded synaptosomes was not significantly affected by morphine. The, inhibitory effect on Ca- "" " uptake was seen with synaptosomes prepared from frontal cortex, thalamus and hypothalamus, but not from the cerebellum, thus corresponding well with data on opiate receptor distribution (20,64). 

Kuhar M. J., Shaskan, E. G. and Snyder, S. H. (1971) The subcellular distribution of endogenous and exogenous serotonin in brain tissue comparison of synaptosomes storing serotonin, norepinephrine and y-aminobutyric acid. J. Neurochem., 18 333-343. 

Fig. 3. Distribution of non-occluded ( ) and occluded ill ID ChAc in fractions separated by discontinuous density gradient centrifuging. The blocks correspond to the fractions 0-1 described by Whittaker et at. (1964). A, Suspension of hypo-osmotically treated synaptosomes and B, After binding of soluble ChAc to synaptosome membranes. (Reproduced from Biochem. J. (1968), F.

Choline acetyltransferase can be obtained in 4 different forms on subcellular fractionation of brain tissue. If the tissue is homogenized in sucrose the enzyme is obtained either in an occluded particulate form (the synaptosomes) or in the soluble form (disrupted cell bodies, axons and dendrites). The ratio of soluble to particulate ChAc in sucrose homogenate varies for the different regions of the brain and reflects regional differences in distribution of cholinergic structures. If the synaptosomes are hypo-osmotically treated, the enzyme is obtained either in a soluble (cytoplasma)... 

Since P-endorphin is located within the hypothalamus and the pituitary, and has a relatively longer duration of action, it tends to be viewed as a neurohormone. Enkephalins, on the other hand, are more extensively distributed, are very rapidly degraded, and are primarily located in synaptosomal areas. The additional observation that enkephalin release following depolarization of brain (and intestinal) tissues is calcium dependent makes it more realistic to categorize them as neurotransmitters or modulators of synaptic function. Binding sites (receptors) for opioids are found, particularly in synaptosomal brain fractions. The enkephalins are located in neurons whose distribution correlates well with that of the receptors. In fact, regional distribution of peptides and their receptors are closely parallel, as would be predicted for a neurotransmitter system. 

MAO is a more fascinating enzyme. It is also widely distributed throughout the body tissues. However, unlike COMT, it is also associated with neuronal mitochondria. This is significant, because catecholamines in the cytosol of the neuronal cell are not immune from its oxidative ravages unless protected within the synaptosomal vesicles. In addition, unlike COMT, inhibitors of MAO have significant effects on amine levels with physiological consequences. Several such MAO inhibitors have pharmacological applications in the treatment of depression and hypertension, as will be seen. 

K. Takahashi, and H. Imura, Synaptosomal T3 binding sites in rat brain their localization on synaptic membrane and regional distribution, Acta Endocrinol. 104 134 (1983). 

It is well known that the CNS has both muscarinic and nicotinic receptor sites (for a review, see De Robertis, 1975) thus the PI effect stimulated by ACh could be mediated by one or the other type of receptors. Scattered observations favor the view that the muscarinic receptors are related to the metabolism of PI. In particulate fractions Hokin Hokin (1958) found that the PI effect was blocked by atropine, and Schacht Agranoff (1973), In a synaptosomal fraction, observed that the ACh stimulation was insensitive to d-tubocurarine. To investigate this problem further, in addition to using appropriate cholinergic agonists and antagonists, an approach would be to take Into consideration the regional distribution of both types of receptors in the CNS. For example,... 

The results on the distribution of hypotaurme oxidase activity In subcellular fractions of ox retina Is given In Table V, About 78% of enzyme activity was found In the soluble fraction while about 22% was associated with particulate components. In the synaptosomal fraction was present about 55% of enzyme activity, recovered from crude mitochondria. 

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