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“125-i-LSD: A High Sensitivity Ligand For Serotonin Receptors”. 
European Journal of Pharmacology. 1983;89:321-322.
125-I-labeled receptor ligands offer unique advantages over their 3H-labeled counterparts. Carrier-free 125-I-labeled ligands can he synthesized specific activities of up to 2170 Ci/mmol while (mono) tritium labeled ligands are limited to 29 Ci/mmol. Therefore, 125-I-labeled ligands can be approximately 70-fold more sensitive than 3H- labeled ligands in detecting receptor sites. In addition, 125-I-labeled ligands emit relatively energetic X-rays and y-rays which are readily detected by gamma counting equipment. We report here the serotonergic binding properties of 125-I-LSD the first reported 125-I-labeled ligand for serotonin receptors. For all experiments, frontal cortex membranes were prepared by a slight modification of the method of Peroutka and Snyder (1979). All receptor binding assays were performed with 10 mg wet weight of tissue per ml. Nonspecific binding was defined by 1 mcM (+)-butaclamol for [3H]spiro peridol assays and 1 mcM (+)-LSD for all other radioligands. I-LSD and 125-I-LSD were purified by thin-layer chromatography. The purity of I-LSD was greater than 95% as assayed by high performance liquid chromatography. 125-I-LSD (labeled at the 2 position) was first synthesized in a nonradioactive form by Troxler and Hofmann (1957). This ligand displays potent serotonin antagonist actions in peripheral (uterine) muscle tissue (Cerletti and Doepfner, 1958) but no reports of its CNS pharmacology or receptor binding properties have appeared. We synthesized 1- LSD by the method of Troxler and Hofmann; (1957) and characterized its serotonin receptor binding properties in rat frontal cortex membranes. 5-HT2 serotonin receptor sites labeled by [3H]spiroperidol are potently inhibited by both (+)-LSD and I-LSD whereas 5-HT1 receptor sites assayed by [3H]serotonin binding are much more readily inhibited by (+)-LSD than by I-LSD.
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