STUDIES ON NICOTINIC ACETYLCHOLINE RECEPTORS IN MAMMALIAN BRAIN. CHARACTERIZATION OF A MICROSOMAL SUBFRACTION ENRICHED IN RECEPTOR FUNCTION FOR DIFFERENT NEUROTRANSMITTERS

Abstract— A subfraction, derived from the microsomal fraction of rat cerebral cortex, with a buoyant density of 1.112 g μ ml⁻¹ appears to be enriched in receptor sites for a number of potential neurotrans-mitters. These include the cholinergic (nicotinic and muscarinic) and ß-adrenergic receptors. This microsomal subfraction (P₃B₂) has been isolated on a preparative scale by two sequential isopycnic sedimentations in discontinuous sucrose gradients.
We have studied the morphology, enzymatic markers and protein composition of this fraction and have compared them with the properties of other subcellular fractions from the same source. Synaptic plasma membranes resembled P₃B₂ by exhibiting the same high extent of enrichment in receptors. However, the synaptic membranes appear to contain more mitochondrial and presynaptic (axonal and cell surface) membranes than does P₃B₂, and the postsynaptic membranes in the two fractions appear morphologically distinct since P₃B₂ does not contain the characteristic postsynaptic densities. Thus these membranes may be derived from Gray's type II synapses.