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Rat cerebral cortex slices were incubated with 1muM [3H]4-aminobutyrate or [3H]L-glutamine. The subcellular distribution of the accumulated labelled substances were determined by fractionating the nuclei-free homogenates on a 5-step discontinuous sucrose density gradient in a B XIV zonal rotor. The gradient was designed to separate the synaptosomes into 3 subpopulations of increasing density. The patterns of distribution of [3H]4-aminobutyrate and [3H-a1L-glutamate in the three synaptosomal peaks were distinctly different. This indicates the presence of separate types of nerve ending accumulating these two potential neurotransmitters, which are known to be metabolically closely linked through the enzyme glutamate decarboxylase. Recentrifugation of the 3 synaptosomal peaks on flat step gradients in a swingout rotor did not result in any further enrichment in transmitter-specific synaptosomes.

The specificity of I125-MBP uptake and subcellular distribution on a discontinuous sucrose density gradient, compared to those of histone H 4 and cytochrome c, showed a high affinity of MBP for mitochondria and heavy synaptosomes, and of histone H 4 for lighter synaptosomes. One heavier synaptosomal subpopulation was almost equally labelled by both proteins. Cytochrome c showed only a low uptake into particular material. Receptor interaction studies of MBP with H3-labelled 5-hydroxytryptamine and naloxone gave negative results.