Microchemical synthesis of the serotonin receptor ligand, 125I-LSD.

The synthesis and properties of 2-[125I]-lysergic acid diethylamide, the first 125I-labeled serotonin receptor ligand, are described. A novel microsynthesis apparatus was developed for this synthesis. The apparatus employs a micromanipulator and glass micro tools to handle microliter to nanoliter volumes on a microscope stage. This apparatus should be generally useful for the synthesis of radioligands and other compounds when limited amounts of material must be handled in small volumes.

[(125)I]LSD binding to serotonin and dopamine receptors in bovine caudate membranes.

[(125)I]LSD (labeled at the 2 position) has been introduced as the first (125)I-labeled ligand for serotonin 5-HT(2) (S2) receptors. In the present study we examined the binding of [(125)I]LSD and its non-radioactive homologue, 2I-LSD, to bovine caudate homogenates. The binding of [(125)I]LSD is saturable, reversible, stereospecific and is destroyed by boiling the membranes. The specific to total binding ratio in this tissue is 75-80% and Scatchard plots of the binding data reveal K(d) = 1.1 nM, B(max) = 9.6 fmol/mg wet weight tissue. The association and dissociation rate constants are highly temperature dependent. At 0 degrees C the net dissociation is less than 5% after 1 h and the association rate is proportionately slow. IC(50) values for a variety of compounds show a clear 5-HT(2) (S2) serotonergic pattern at this [(125)I]LSD site. Blockage of this primary 5-HT(2) (S2) caudate binding site by 0.3 ?M mianserin reveals the presence of a weaker [(125)I]LSD binding site with a K(d) = 9.1 nM, B(max) = 7.6 fmol/mg tissue. This secondary site is a D3 dopaminergic receptor site, as shown by the relative abilities of various displacers to inhibit this binding. Binding studies with nonradioactive 2I-LSD reveal a clear preference for D2 over D3 dopamine receptor sites. [(125)I]LSD is a sensitive and selective label for 5-HT(2) (S2) serotonin receptor sites in both rat frontal cortex and bovine caudate membranes. Blockage of the primary bovine caudate [(125)I]LSD binding site with mianserin allows the high sensitivity of [(125)I]LSD to be applied to D2 dopamine receptor studies as well.

Characterization of 125I-lysergic acid diethylamide binding to serotonin receptors in rat frontal cortex.

125I-Lysergic acid diethylamide (125I-LSD) is the first 125I-labeled ligand for serotonin receptor studies. Its binding to rat frontal cortex membranes is saturable, reversible, and stereospecific. Specific binding is linearly dependent on tissue concentration and represents 70-80% of the total binding. Scatchard plots of the binding data are linear with a KD of 1.5 nM, a Bmax of 12.4 fmol/mg wet weight tissue, and a Hill slope of 1.02. The binding kinetics are highly temperature-dependent. At 37 degrees C the bimolecular association rate constant is 1.28 X 10(8) min-1 M-1 and the dissociation rate constant is 0.087 min-1 (t 1/2 = 8.0 min). At 0 degrees C less than 4% dissociation occurs over 40 min and the association rate is similarly depressed. Inhibition of 125I-LSD binding by a variety of serotonergic, dopaminergic, and adrenergic ligands reveals a 5-hydroxytryptamine2 (5-HT2) serotonergic profile for this binding site. Regional distribution studies of 125I-LSD binding in rat brain show that areas with the highest levels of binding include the cortex and striatum. Iodinated radioligands can be synthesized with specific activities exceeding 2,000 Ci/mmol, which makes them approximately 75-fold more sensitive than tritiated radioligands. This high specific activity, coupled with the selectivity of 125I-LSD for 5-HT2 sites, makes this ligand a sensitive new probe for 5-HT2 serotonin receptors.