Characterization and localization of [125I]RTI-55-labeled cocaine binding sites in fetal and adult rat brain

The present studies examined the characteristics of fetal (gestational day 20) and adult rat brain cocaine recognition sites labeled with the potent cocaine congener [125I]RTI-55 [3 beta-(4-iodophenyl)-tropane-2 beta-carboxylic acid methyl ester]. Saturation analyses of [125I]RTI-55 binding to membrane fractions from both fetal and adult whole-brain yielded curvilinear Scatchard plots that were resolved by nonlinear curve-fitting into high- and low-affinity components. Mean Kd values were 0.13 nM and 12 nM for fetal brain high- and low-affinity sites, respectively, compared to 0.26 and 18 nM for adult brain. The Kd for high-affinity binding was significantly different between the groups, suggesting a possible developmental change in the properties of [125I]RTI-55 binding sites. Drug displacement studies with various monoamine uptake inhibitors indicated that at a 10 pM concentration of [125I]RTI-55, almost all binding to fetal brain membranes could be accounted for by interaction with the serotonin (5-HT) and (to a lesser extent) dopamine (DA) transporters. This conclusion was supported by autoradiographic studies of both adult and fetal brain, demonstrating a predominance of [125I]RTI-55 binding in areas with high densities of 5-HT and/or DA uptake sites. The present results are in accordance with our previous demonstration of [3H]cocaine binding sites in fetal brain (Meyer et al., 1993) and further suggest that [125I]RTI-55 should be a useful ligand for assessing the effects of prenatal cocaine exposure on the subsequent development of cocaine recognition sites on the DA and 5-HT transporters.