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Dopamine Transporter (DAT) binding sites in medial temporal lobe structures are much less dense than in striatum and have been difficult to image and quantitate. The recently synthesized compound [125I]RTI-121 ([125I]2 beta-carboxylic acid isopropyl ester-3 beta-(4-iodophenyl)tropane) has demonstrated high specificity and affinity for the DAT in preliminary animal studies. The present experiments were designed to delineate the pharmacological and anatomical characteristics of [125I]RTI-121 binding sites in the striatum and medial temporal lobe of normal humans. A series of saturation experiments performed with striatal membrane preparations generated a one-site model with a KD averaging 1.49 +/- 0.06 nM, and a Bmax that was comparable to those of earlier reports. Competition experiments confirmed the selectivity of [125I]RTI-121 for DAT binding sites. After the assay conditions for autoradiography were optimized, [125I]RTI-121 binding was visualized, pharmacologically characterized and quantitated in human temporal lobe structures. In the hippocampus, specific binding was distributed in the CA4 and CA3 pyramidal cell layers, the outer stratum radiatum and the dentate molecular layer, but not in the dentate granule cells or outer pyramidal cells. In the amygdala specific binding was limited to the basolateral nuclei. Dopamine nerve terminals, as identified with [125I]RTI-121 binding, also displayed a discrete and homologous innervation pattern in the amygdala and hippocampus of several other mammalian species. In summary, the kinetic, saturation, competition and autoradiographic experiments demonstrated that [125I]RTI-121 can be used to identify DAT binding sites and to assess their functional state in post mortem human brain samples.

The time course of beta adrenergic receptor adaptation in response to chronic imipramine treatment (10 or 20 mg/kg) was assessed by quantitative autoradiographic analysis of 125I-pindolol binding in rat brain. Binding of the radioligand was assessed in 18 brain areas, including subregions of the hippocampus, amygdala, septum, hypothalamus and specific cerebral cortical regions. After only 2 days treatment with imipramine at a dose of 20 mg/kg, select cortical regions exhibited a reduction in 125I-pindolol binding. These rapidly adapting cortical regions included the medial prefrontal, lateral frontal, ventrolateral orbital and piriform cortices. After 7 or 21 days treatment with imipramine at 20 mg/kg, 16 of 18 brain regions examined exhibited significant reduction in 125I-pindolol binding. The only regions examined that did not show reduced 125I-pindolol binding for these treatment conditions were the caudate-putamen and anterior hypothalamic area. After 2 days treatment with 10 mg/kg of imipramine, down-regulation of beta adrenergic receptors was not observed in any region. After 7 days treatment with 10 mg/kg, down-regulation of beta adrenergic receptor binding was found only in certain cortical regions: medial prefrontal, lateral frontal, ventrolateral orbital and piriform cortices. Thus, the cortical regions that were most rapidly affected with the 20 mg/kg dose of imipramine (i.e., after 2 days) were also the first to respond with the 10 mg/kg dose of the drug. After 21 days treatment with imipramine at 10 mg/kg, 125I-pindolol binding was reduced in 13 of the 18 regions examined.(ABSTRACT TRUNCATED AT 250 WORDS)