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Robaut C, Fillion MP, Dufois S, Fayole-Bauguen C, Rousselle J-C, Gillet G, Benkirane S, Fillion G. 
“Multiple High Affinity Binding Sites for 5-Hydroxytryptamine: a New Class of Sites Distinct from 5-HT1 and S2”. 
Brain Res. 1985;346(2):250-262.
Two different classes of binding sites probably related to serotonergic receptors have already been reported: 5-HT1 binding sites recognize [3H]5-hydroxytryptamine with a high affinity (Kd = 3 nM) and S2 binding sites recognize [3H]spiropcridol and [3H]ketanserine. An additional population of sites-has been observed in crude membrane preparations or fractions enriched with synaptosomal membranes obtained from rat brain cortex. This population was observed as a single class of sites in a synaptosomal fraction according to Laduron (1977). It corresponded to a dissociation constant Kd= 13-15 nM, and Bmax = 0.80 +/- 0.15 pmol/mg protein. Displacement experiments showed that it recognized preferentially the 5-HT structure (bufotenin, 5-MeO-tryptomine). Tryptamine was a weak displacer and 5,7-dihydroxytryptamine totally inefficient. Neither 8-OH-DPAT, nor quipazine had any effect. Methiothepin, cinanserin and cyproheptadine displaced 5-HT from these sites where ergot derivatives did not. Contrary to 5-HT1 binding, this recently observed binding was not altered by GTP; alpha-MSH reduced the corresponding Bmax whereas Leu-enkephalin did not. The degenerative lesion of the serotonergic fibers led to a slight increase in the Bmax of the binding without altering the Kd which means that corresponding sites are not located on serotonergic fibers and might he postsynaptically located.
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