THE DEVELOPMENT OF A HIGH SPECIFIC ACTIVITY RADIOLIGAND, IODINE-125-LSD, AND ITS APPLICATION TO THE STUDY OF SEROTONIN RECEPTORS

{dollar}sp{lcub}125{rcub}{dollar}I-Labeled receptor ligands can be synthesized with specific activities exceeding 2000 Ci/mmol, making them nearly 70-fold more sensitive in receptor site assays than (mono) tritiated ligands. We have synthesized and characterized {dollar}sp{lcub}125{rcub}{dollar}I-lysergic acid diethylamide ({dollar}sp{lcub}125{rcub}{dollar}I-LSD), the first radioiodinated ligand for serotonin receptor studies. The introduction of {dollar}sp{lcub}125{rcub}{dollar}I at the 2 position of LSD increased both the affinity and selectivity of this compound for serotonin 5-HT{dollar}sb{lcub}2{rcub}{dollar} receptors in rat cortex. The high specific activity of {dollar}sp{lcub}125{rcub}{dollar}I-LSD and its high ratio of specific to nonspecific binding make this ligand especially useful for autoradiographic studies of serotonin receptor distribution.; In addition to its utility in the vertebrate nervous system, {dollar}sp{lcub}125{rcub}{dollar}I-LSD is also useful for the study of invertebrate serotonin receptors. We have found that {dollar}sp{lcub}125{rcub}{dollar}I-LSD binds with high affinity to a class of serotonin receptors in the CNS of the marine mollusc Aplysia californica. These receptors are located primarily in the neuropil, but are also observed on a small subset of neuronal cell bodies including neurons L1 and R1 of the abdominal ganglion. The pharmacological properties of this molluscan serotonin receptor distinguish it from any of the vertebrate serotonin receptor subtypes. Guanine nucleotides modulate agonist but not antagonist binding affinities for this receptor, suggesting that this site is coupled to a G-protein.; Serotonin-evoked currents were recorded from the abdominal ganglion left upper quadrant neurons and from R1 under voltage clamp. Among the six neurons studied, four distinct ionic currents were activated by serotonin. One of these currents, resulting from an increase in Cl{dollar}sp{lcub}-{rcub}{dollar} conductance, was observed only in neurons L1 and R1, and was inhibited by the serotonergic antagonists cyproheptadine and mianserin. Thus, the distribution and pharmacology of this serotonin-evoked Cl{dollar}sp{lcub}-{rcub}{dollar} current match that of the {dollar}sp{lcub}125{rcub}{dollar}I-LSD-labeled receptor on these neurons. In addition, the kinetic properties of this Cl{dollar}sp{lcub}-{rcub}{dollar} current suggest that it is mediated via a second messenger system, consistent with the in vitro observation that the {dollar}sp{lcub}125{rcub}{dollar}I-LSD-labeled receptor is coupled to a G-protein. In these molluscan neurons, it appears that {dollar}sp{lcub}125{rcub}{dollar}I-LSD labels a subset of serotonin receptors which regulate a Cl{dollar}sp{lcub}-{rcub}{dollar} conducting ion channel.