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F Darchen, D Scherman, PM Laduron and JP Henry
Institut de Biologie Physico-Chimique, Paris, France.
[3H]Ketanserin binding studies were performed on purified chromaffin granule membranes. Binding was found to occur on one class of sites and was temperature dependent. At 30 degrees the equilibrium dissociation constant KD was 45 nM. At 0 degrees, a KD value of 6 nM and a half-life of dissociation of 40 sec were measured. Methysergide, an antagonist of 5-hydroxytryptamine2 (5-HT2) receptors structurally unrelated to ketanserin, did not displace ketanserin binding. Tetrabenazine, an inhibitor of the monoamine transporter of chromaffin granules, displaced [3H]ketanserin binding. Conversely, ketanserin inhibited the binding of [3H] dihydrotetrabenazine, a ligand that specifically binds to the monoamine transporter. The inhibition was of the competitive type, indicating that both drugs bind to the same site. Ketanserin binding did not depend upon ATP-induced energization of chromaffin granules. ATP-dependent 5-HT uptake by chromaffin granule ghosts was inhibited by ketanserin with an IC50 value of 70 nM. A series of ketanserin derivatives were tested for their ability to displace [3H]dihydrotetrabenazine; EC50 values differed by more than 2 orders of magnitude and were not correlated to affinities on 5-HT2 receptors. In mouse brain, [3H]ketanserin was found to bind to methysergide-sensitive and to tetrabenazine-sensitive sites. In the striatum, tetrabenazine- sensitive sites represented a larger fraction than the methysergide- sensitive ones, whereas the reverse was true in the frontal cortex. It is concluded that nonspecific displaceable binding sites of [3H]ketanserin previously described in the striatum are tetrabenazine binding sites associated with the synaptic vesicle monoamine transporter.
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