[I-125] 5-IODO-6-NITROQUIPAZINE - A POTENT AND SELECTIVE LIGAND FOR THE 5-HYDROXYTRYPTAMINE UPTAKE COMPLEX .1. IN-VITRO STUDIES

In search of a potent and selective radioiodinated ligand for the 5-hydroxytryptamine (serotonin or 5-HT) uptake complex, we synthesized and evaluated the in vitro properties of [I-125]5-iodo-6-nitroquipazine. The binding properties and pharmacological profile of this radioligand were studied in rat brain homogenates, and it was found to display high affinity and selectivity for the serotonin uptake complex. Scatchard analysis of the binding data indicated a single population of sites with a K(d) of 23 +/- 6 pM and a B(max) of 430 +/- 50 fmol/mg protein (mean +/- S.E.M., n = 7). Inhibitors of serotonin uptake were the most efficient competitors for [I-125]5-iodo-6-nitroquipazine binding with K(i) values similar in rank order and magnitude to those obtained in studies of other established serotonin uptake blockers. Inhibitors of dopamine and norepinephrine uptake as well as a wide variety of postsynaptic receptor agents were relatively ineffective in inhibiting [I-125]5-iodo-6-nitroquipazine binding to rat brain membranes. Serotonin was the only monoaminergic neurotransmitter capable of effectively competing for [1251]5-iodo-6-nitroquipazine binding sites and gave a K(i) value of 2.8 +/- 0.6 mu M. Lesions of the serotonergic system with p-chloroamphetamine resulted in a dramatic loss ( > 90%) of [I-125]5-iodo-6-nitroquipazine binding to rat cortical membranes. Non-radiolabeled 5-iodo-6-nitroquipazine potently inhibited the binding of [H-3]paroxetine to serotonin reuptake sites in rat cortical membranes with a K(i) of 0.17 +/- 0.06 nM. These results demonstrate that 5-iodo-6-nitroquipazine is a potent and selective ligand for the serotonin uptake complex, and the [I-125]-labeled radioligand has an advantage of detecting low levels of binding sites.