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Fillion G, Fillion MP, Spirakis C, Bahers JM, Jacob J. 
“5-Hydroxytryptamine Binding to Synaptic Membranes from Rat Brain”. 
Life Sci.. 1976;18(1):65-74.
The binding of 5-hydroxytryptamine (5HT) to synaptic membranes from rat brain was studied. Methods Brains from rats (200-250g) were divided into regions and synaptosomal and crude mitochondrial fractions isolated by density gradient centrifugation. The fractions, and some kidney samples, were assayed for 5HT binding in the presence and absence of antagonists. Samples were incubated with 3H-5HT alone or with antagonist and bound and free 3H-5HT separated by rapid ultrafiltration. The identity of free and bound radioactivity was checked against 5HT, 5-hydroxy IAA (5HIAA), and bufatenine. Purity of the synaptosomal fraction was assessed by electron microscopy. Results Total radioactivity bound to the synaptosomal membrane increased with increasing concentrations of 3H-5HT. Total binding had 2 components, a specific, high affinity (KD=l.l x l0-9 M), saturable, binding displaceable by 5HT, and a non-saturable, nondisplaceable binding. Crude mitochondrial fractions showed a similar high affinity, saturable binding. Free and bound radioactivity had the same chromatographic Rf as authentic 5HT. Specific binding of 5HT was thermosensitive, temperature dependent, and pH dependent. Binding capacity of 5HT (/mg protein) was lower for the crude mitochondrial fraction than for synaptic membranes and the 0f specific binding to total binding was higher in synaptic membranes than in homogenates. Specific binding was greatest in the striatum and decreased in the order hippocampus, diencephalon, cortex, raphe nucleus, cerebellum, and kidney. 5HT, methoxytryptamine, tryptamine, dimethyltryptamine, lysergic acid diethylamide (Roche) and dihydroergotamine displaced bound 3H-5HT (DIso%, 10- -10-6M). Cinanserine, 5HIAA, imipramine, and chlorpromazine (10-6M) and noradrenaline, dopamine, octopamine, and tetrahydroisoquinoline (salsolinol) (10-5M) were unable to displace bound 5HT. Conclusion The binding sites of 5HT probably represent receptor sites.
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