RT Journal Article
SR Electronic
T1 Characterization of (<em>S</em>)-Des-4-amino-3-[<sup>125</sup>I]iodozacopride ([<sup>125</sup>I]DAIZAC), a Selective High-Affinity Radioligand for  5-Hydroxytryptamine<sub>3</sub> Receptors
JF Journal of Pharmacology and Experimental Therapeutics
JO J Pharmacol Exp Ther
FD American Society for Pharmacology and Experimental Therapeutics
SP 221
OP 231
VO 288
IS 1
A1 William A. Hewlett
A1 Bakula L. Trivedi
A1 Zhang-Jin Zhang
A1 Tomas de Paulis
A1 Dennis E. Schmidt
A1 David M. Lovinger
A1 M. Sib Ansari
A1 Michael H. Ebert
YR 1999
UL http://jpet.aspetjournals.org/content/288/1/221.abstract
AB The 5-hydroxytryptamine(HT)3 receptor subtype is present in the central nervous system (CNS) in low abundance, and few selective radiolabeled antagonists with high specific activity are available to study these sites. DAIZAC [desamino-3-iodo-(S)-zacopride; (S)-5-chloro-3-iodo-2-methoxy-N-(1-azobicyclo-[2.2.2]oct-3-yl)benzamide] is a compound with high affinity and selectivity for the 5-HT3 receptor. Scatchard analysis of specific binding to NCB-20 cell membranes gave a Bmax of 340 ± 58 fmol/mg protein and a KD of 0.14 ± 0.03 nM, which is in agreement with the value previously reported in rat brain (KD = 0.15 nM). Nonspecific binding of [125I]DAIZAC in NCB-20 cells was &lt;1% of total binding at the KD for DAIZAC compared with 17% in the rat brain preparation. Unlabeled DAIZAC (10 μM) showed minimal ability to displace binding of radiolabeled ligands selected for their affinities for other CNS receptor and uptake carrier binding sites. The discrimination ratio of DAIZAC for the 5-HT3 receptor over the M1 muscarinic binding site, the non-5-HT3 site at which it was most potent, was &gt;2800. Serotonergic antagonists at every other known CNS serotonergic binding sites (3–30 μM) were ineffective in displacing [125I]DAIZAC binding in rat brain membranes. Similarly, antagonists (3–30 μM) for other nonserotonergic receptors and uptake sites were ineffective in displacing [125I]DAIZAC binding. Autoradiographic studies showed highest specific binding in area postrema and nucleus solitarius, with intermediate levels of binding in entorhinal cortex and hippocampus. DAIZAC inhibited 5-HT3 receptor-mediated inward cation current in NCB-20 cells with an IC50 of 0.24 nM. [125I]DAIZAC is a potent and highly selective ligand for in vitro studies of the 5-HT3 receptor. The American Society for Pharmacology and Experimental Therapeutics