CHARACTERIZATION OF DESAMINO-5-[I-125]IODO-3-METHOXYZACOPRIDE ([I-125]MIZAC) BINDING TO 5-HT3 RECEPTORS IN THE RAT-BRAIN

1. Antagonists at 5-HT3 receptors have shown activity in animal models of mental illness, however, few radiolabeled 5-HT3 ligands are availa ble for preclinical studies. MIZAC, an analogue of the selective 5-HT3 antagonist, zacopride, binds with high affinity (1.3-1.5 nM) to CNS 5 -HT3 sites. The authors report here the selectivity of MIZAC for these sites in rat brain homogenates. 2. Ninety-seven percent of total spec ific binding of [I-125]MIZAC (0.1 nM) of was displaced by bemesetron ( 3 mu M), a selective 5-HT3 antagonist. Competition studies using ligan ds with known affinities for 5-HT3 sites give a high correlation with reported pK(i) values (r(2) 0.98). Bemesetron displaceable binding has a regional distribution consistent with that of the 5-HT3 receptor, i .e. highest in cortex and hippocampus, and lowest in striatum and cere bellum. 3. Potent antagonists present at concentrations sufficient to occupy 95% of other 5-HT receptor populations (1A, 1B, 1D, 2A, 2B, 2C, 5A, 5B, 6, and 7) showed minimal ability to displace [I-125]MIZAC bin ding (3 nM). Specificity studies using radioligand binding assays sele ctive for 5-HT4, 5-HT6, and 5-HT7 receptors, and for binding sites of other neurotransmitters indicate a high degree of selectivity of [I-12 5]MIZAC for the 5-HT3 receptor. 4. [I-125]MIZAC binds to an apparent l ow affinity (benzac) site having a unique pharmacology. Low affinity b inding was displaceable by benztropine, but not by other muscarinic ag ents nor inhibitors of dopamine uptake. The regional distribution of t he low affinity site differed markedly from that of the high affinity site. The apparent affinity of [I-125]MIZAC for the benzac site is two orders of magnitude lower than for the 5-HT3 receptor. Given its high selectivity for 5-HT3 binding sites, [I-125]MIZAC appears to be a pro mising ligand for labeling 5-HT3 receptors in vitro and in vivo.