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

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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 neurotransmitter-specific 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 γ-aminobutyric acid and other amino acids. These data suggest that significant morphological variations may exist among different neurotransmitter-specific nerve terminals in the brain.