Radioiodinated D-(+)-N1-ethyl-2-iodolysergic acid diethylamide: a ligand for in vitro and in vivo studies of serotonin receptors

Molecular and Functional Imaging Studies of Psychedelic Drug Action in Animals and Humans

Hallucinogens are a loosely defined group of compounds including LSD, N,N-dimethyltryptamines, mescaline, psilocybin/psilocin, and 2,5-dimethoxy-4-methamphetamine (DOM), which can evoke intense visual and emotional experiences. We are witnessing a renaissance of research interest in hallucinogens, driven by increasing awareness of their psychotherapeutic potential. As such, we now present a narrative review of the literature on hallucinogen binding in vitro and ex vivo, and the various molecular imaging studies with positron emission tomography (PET) or single photon emission computer tomography (SPECT). In general, molecular imaging can depict the uptake and binding distribution of labelled hallucinogenic compounds or their congeners in the brain, as was shown in an early PET study with N1-([11C]-methyl)-2-bromo-LSD ([11C]-MBL); displacement with the non-radioactive competitor ketanserin confirmed that the majority of [11C]-MBL specific binding was to serotonin 5-HT2A receptors. However, interactions at serotonin 5HT1A and other classes of receptors and pleotropic effects on second messenger pathways may contribute to the particular experiential phenomenologies of LSD and other hallucinogenic compounds. Other salient aspects of hallucinogen action include permeability to the blood-brain barrier, the rates of metabolism and elimination, and the formation of active metabolites. Despite the maturation of radiochemistry and molecular imaging in recent years, there has been only a handful of PET or SPECT studies of radiolabeled hallucinogens, most recently using the 5-HT2A/2C agonist N-(2[11CH3O]-methoxybenzyl)-2,5-dimethoxy- 4-bromophenethylamine ([11C]Cimbi-36). In addition to PET studies of target engagement at neuroreceptors and transporters, there is a small number of studies on the effects of hallucinogenic compounds on cerebral perfusion ([15O]-water) or metabolism ([18F]-fluorodeoxyglucose/FDG). There remains considerable scope for basic imaging research on the sites of interaction of hallucinogens and their cerebrometabolic effects; we expect that hybrid imaging with PET in conjunction with functional magnetic resonance imaging (fMRI) should provide especially useful for the next phase of this research.

Synthesis and in vivo studies of a selective ligand for the dopamine transporter: 3 beta-(4-[125I]iodophenyl) tropan-2 beta-carboxylic acid isopropyl ester ([125I]RTI-121)

A selective ligand for the dopamine transporter 3 beta-(4-iodophenyl)tropan-2 beta-carboxylic acid isopropyl ester (RTI-121) has been labeled with iodine-125 by electrophilic radioiododestannylation. The [125I]RTI-121 was obtained in good yield (86 +/- 7%, n = 3) with high radiochemical purity (> 99%) and specific radioactivity (1210-1950 mCi/mumol). After i.v. administration of [125I]RTI-121 to mice, the rank order of regional brain tissue radioactivity (striatum > olfactory tubercles > > cortex, hippocampus, thalamus, hypothalamus, cerebellum) was consistent with dopamine transporter labeling. Specific in vivo binding in striatum and olfactory tubercles was saturable, and was blocked by the dopamine transporter ligands GBR 12,909 and (+/-)-nomifensine. By contrast, binding was not reduced by paroxetine, a serotonin transporter inhibitor, or desipramine, a norepinephrine transporter inhibitor. A variety of additional drugs having high affinities for recognition sites other than the neuronal dopamine transporter also had no effect. The [125I]RTI-121 binding in striatum and olfactory tubercles was inhibited by d-amphetamine in dose-dependent fashion. Nonmetabolized radioligand represents 85% of the signal observed in extracts of whole mouse brain. Thus, [125I]RTI-121 is readily prepared, and is a useful tracer for dopamine transporter studies in vivo.

Selective in vivo labelling of brain 5-HT1A receptors by [3H]WAY 100635 in the mouse

The novel selective 5-HT1A receptor antagonist radioligand [3H]WAY 100635 ([O-methyl-3H]N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2- pyridyl)cyclohexane-carboxamide) was injected i.v. to mice in an attempt to label in vivo central 5-HT1A receptors. Although 5 min after the i.v. injection of [3H]WAY 100635 (4-7.6 muCi per mouse) the amount of tritium found in the whole brain only accounted for 1.5-1.8% of the injected radioactivity, regional differences in 3H accumulation already corresponded to those of 5-HT1A receptor density. Optimal data were obtained 1 h after [3H]WAY 100635 injection as the distribution of 3H in brain was exactly that of 5-HT1A receptor binding sites in mouse brain sections labelled in vitro with [3H]WAY 100635. In particular, high level of labelling was found in the lateral septum, gyrus dentatus and CA1 area of Ammon's horn in the hippocampus, dorsal raphe nucleus and entorhinal cortex. No labelling was found in he substantia nigra, and 3H accumulated in the cerebellum represented only 12-14% of that found in the hippocampus. Pretreatment with various drugs indicated that only 5-HT1A receptor ligands were able to decrease the accumulation of 3H in all the brain areas examined except in the cerebellum. Assuming that only non-specific binding took place in the latter structure, it was possible to calculate the ID50 values of 5-HT1A receptor agonists (8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin), S 14506 (1-[2-(4-fluorobenzoylamino)ethyl]-4-(7-methoxynaphthyl+ ++)piperazine) and S 20499 ((+)-4-[N-(5-methoxy-chroman-3-yl)-N-propylamino]butyl-8- azaspiro-(4,5)-decane-7,9-dione)) and antagonists (spiperone, (-)-tertatolol, (+)-WAY 100135 (N-tert-butyl-3,4-(2-methoxyphenyl)piperazin-1-yl-2-phenyl- propanamide)) as inhibitors of 3H accumulation in the hippocampus of [3H]WAY 100635-injected mice. Comparison of these values with the in vitro affinity of the same ligands for hippocampal 5-HT1A receptors revealed marked variations in the capacity of 5-HT1A receptor agonists and antagonists to reach the brain when injected via the subcutaneous route in mice.

Repeated administration of MDMA causes transient down-regulation of serotonin 5-HT2 receptors

The present study examined short- and long-term effects of MDMA (3,4-methylene-dioxymethamphetamine) on serotonin (5-HT2 and 5-HT1c) receptors in the brain of the rat. N1-Methyl-2-[125I]lysergic acid diethylamide ([125I]MIL) was used to label these receptors in vitro and in vivo. The usefulness of [125I]MIL for in vivo detection of changes in 5-HT2 receptors was confirmed in preliminary experiments in which rats were treated chronically with mianserin (5 mg/kg, once daily for 10 days). Decreases in specific in vivo binding of [125I]MIL, after treatment with mianserin were found to be of the same magnitude as those determined by others, using in vitro methods. The MDMA (8 doses; 5-20 mg/kg each) was administered to rats over a period of 4 days. At various times after administration of the last dose of MDMA, the binding of [125I]MIL was measured. Acutely, treatment with MDMA (20 mg/kg) reduced specific in vivo binding of [125I]MIL in all regions of brain studied. For example, in the frontal cortex, specific binding of [125I]MIL was decreased by 80% at 6 hr and by 62% at 24 hr, after cessation of treatment with MDMA. Twenty-one days after administration of MDMA however, the number of binding sites for [125I]MIL was back to control levels. Reductions in in vivo binding of [125I]MIL in frontal cortex were dependent on the dose of MDMA injected and were associated with decreases in the number of binding sites for [125I]MIL (Bmax values) in tissue homogenates of the same area. Autoradiographic studies of MDMA-treated rats confirmed the decreased density of 5-HT2 receptors and also suggested that the 5-HT1c receptor of the choroid plexus was not affected. These results indicate that repeated administration of MDMA caused transient down-regulation of 5-HT2 receptors in the brain of the rat. Further, they demonstrated that [125I]MIL is a suitable radioligand for labeling 5-HT2 receptors, both in vitro and in vivo. Once labeled with an appropriate radionuclide for SPECT (single photon emission computed tomography) or PET (positron emission tomography), MIL should prove useful for monitoring changes in the density of serotonin receptors in the living mammalian brain.