Neurotransmitter-Specific Synaptosomes in Rat Corpus Striatum: Morphological Variations ^*

Abstract

This communication describes ultrastructural variations among synaptosomal fractions isolated from the corpus striatum of the rat by incomplete equilibrium sedimentation in sucrose density gradients, and attempts to relate the variations to neurotransmitterspecific synaptosomes. The concentration of synaptosomes in each fraction of the density gradient was found to be correlated with the concentration of potassium, a marker for cytoplasm occluded within synaptosomes. Monoamine oxidase activity was found to be correlated with the incidence of free mitochondria in the gradients.Synaptosomes from denser gradient fractions showed a markedly increased frequency of adherent postsynaptic elements and intraterminal mitochondria. These denser gradient fractions were rich in synaptosomes containing norepinephrine, dopamine, serotonin, and acetylcholine, while synaptosomes in lighter portions of the gradients were rich in -y-aminobutyric acid and other amino acids. These data suggest that significant morphological variations may exist among different neurotransmitterspecific nerve terminals in the brain. mossy fiber endings have been separated from smaller synap tosomes in the cerebellum; however, both populations are rich in acetylcholine (7). Cotman et al. (8) also reported morphological differences in synaptosomal populations separated by differential centrifugation. Recently (9), various neurotransmitters in brain tissue were labeled with different radioactive isotopes, and subtle variations in the sedimentation properties of synaptosomes that store different neurotransmitters were thereby discriminated. Using this approach and centrifuging sucrose density gradients for brief intervals (a procedure designated "incomplete equilibrium sedimentation"), we have been able to enhance the resolution of these synaptosomes (10, 11).We describe here the ultrastructural variations among subcellular fractions isolated from the corpus striatum of the rat by incomplete equilibrium sedimentation, and attempt to correlate them with the neurotransmitter-specific synaptosomes contained in different subcellular fractions. Ascertaining the function of morphologically identifiable synapses in the brain is a fundamental problem of neurobiology. A knowledge of which synapses utilize which neurotransmitters would constitute a great advance toward an understanding of synaptic organization in the brain. Subcellular fractionation of brain has been a useful approach to elucidating chemical features of ultrastructural elements. When brain tissue is homogenized in isotonic sucrose, large numbers of nerve terminals "pinch off" to form intact, membrane-bound particles called "synaptosomes," which can be isolated by differential and density gradient centrifugation (1, 2). Such particles contain various putative transmitter substances, such as acetylcholine, norepinephrine, dopamine, serotonin, and 'y-aminobutyric acid (GABA) (1-3). Isolation of the "synaptosomal" fraction of brain tissue, however, does not provide information about the morphol...

A Unique Synaptosomal Fraction, Which Accumulates Glutamic and Aspartic Acids, in Brain Tissue

“…However, Whittaker (4) failed to find any significant contamination of synaptosomal preparations by glia. Our own electron microscopic examination of fractions from gradients prepared in exactly the same vR ay as those in the present study also showed only minimal contamination (17,26). Although synaptosomal fractions prepared by incomplete equilibrium sedimentation contain free mitochondria, as well as synaptosomes, these occur in the denser area of the synaptosomal distribution (17,26) and, hence, could not account for the profiles of the acidic amino acids that preponderate in the less dense areas.…”

Somatostatin receptors: identification and characterization in rat brain membranes.

“…Synaptosomes have been shown to sediment in a broad band in continuous gradients of sucrose and Ficoll and appear to be heterogenous (28,29). They also have been subfractionated in discontinuous gradients (30). The two subcellular fractions P5 and P6 identified in the present study appear to be enriched in synaptosomes as shown morphologically for P5 ( Fig.…”

Neuronal vs glial glutamate uptake: Resolving the conundrum

“…Nevertheless, these experiments showed that the synaptosome assay mostly detects neuronal components as originally assumed (Gfeller et al, 1971;Logan and Snyder, 1972;Levi and Raiteri, 1973a;Levi and Raiteri, 1973b;Fonnum, 1984;Erecinska et al, 1996;Robinson, 1998;Raiteri and Raiteri, 2000) despite contamination from astrocytes (e.g. Delaunoy et al, 1979;Henn et al, 1976;Nakamura et al, 1993).…”