SUBCELLULAR-DISTRIBUTION OF RECEPTOR-SITES IN HUMAN BRAIN - DIFFERENTIATION BETWEEN HEAVY AND LIGHT STRUCTURES OF HIGH AND LOW-DENSITY

Studies of the subcellular localization of neuroreceptors in the rat b rain have shown that most of them are associated with light and low de nsity subcellular fractions. In two human brain areas, quite different subcellular distributions were observed. After fractionation by diffe rential centrifugation of frontal cortex homogenates, benzodiazepine a nd serotonin 5-HT2 receptors were mainly found in the heavy mitochondr ial (M) fraction, whereas mu-opiate and muscarinic cholinergic recepto rs were mainly concentrated in the microsomal (P) fraction. In human p utamen, the presynaptic markers of dopaminergic nerve terminals (neuro tensin receptors, dopamine uptake sites and amine vesicular transporte r-binding sites), benzodiazepine receptors and serotonin uptake sites were recovered both in the high and low density fractions, whereas the muscarinic, opiate and, to a lesser extent, dopamine D-2 receptors we re mostly concentrated in the microsomal fraction. In the cerebral cor tex, after isopycnic centrifugation in sucrose gradients, neurorecepto rs were found in the high density fractions where the peaks of cytochr ome oxidase and that of nerve endings, as identified by amine uptake a nd by means of electron microscopy were also found. A single peak of b enzodiazepine receptors was observed in high density (1.15-1.17 g/ml) fractions suggesting that these receptors are much more concentrated i n the nerve terminals or dendrites rather than in the dendritic spines or vesicles. The fact that muscarinic and opiate receptors were recov ered in the P fraction with plasma membrane constituents and also in M and L fractions, which is confirmed by a bimodal distribution in sucr ose gradient, suggests that they are localized in both the nerve termi nals or dendrites and in the small vesicles or dendritic spines. In th e putamen, much of the specific binding to uptake sites for dopamine a nd serotonin was recovered in the high density fractions, but the exis tence of another peak at a lower density indicates the presence of mic rosomal uptake sites. The results indicate that differential and isopy cnic fractionation methods performed on human brain samples, make it p ossible to separate tissue fractions enriched in nerve endings, dendri tes, dendritic spines, plasma membranes or vesicles.