Autoradiographic characterization of (+-)-1-(2,5-dimethoxy-4-[125I] iodophenyl)-2-aminopropane ([125I]DOI) binding to 5-HT2 and 5-HT1c receptors in rat brain

The 5-HT2 (serotonin) receptor has traditionally been labeled with antagonist radioligands such as [3H]ketanserin and [3H]spiperone, which label both agonist high-affinity (guanyl nucleotide-sensitive) and agonist low-affinity (guanyl nucleotide-insensitive) states of this receptor. The hallucinogen 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) is an agonist which labels the high-affinity guanyl nucleotide-sensitive state of brain 5-HT2 receptors selectively. In the present study, conditions for autoradiographic visualization of (+/-)-[125I]DOI-labeled 5-HT2 receptors were optimized and binding to slide-mounted sections was characterized with respect to pharmacology, guanyl nucleotide sensitivity and anatomical distribution. In slide-mounted rat brain sections (+/-)-[125I]DOI binding was saturable, of high affinity (KD approximately 4 nM) and displayed a pharmacologic profile typical of 5-HT2 receptors. Consistent with coupling of 5-HT2 receptors in the high-affinity state to a guanyl nucleotide regulatory protein, [125I]DOI binding was inhibited by guanyl nucleotides but not by adenosine triphosphate. Patterns of autoradiographic distribution of [125I]DOI binding to 5-HT2 receptors were similar to those seen with [3H]ketanserin- and [125I]-lysergic acid diethylamide-labeled 5-HT2 receptors. However, the density of 5-HT2 receptors labeled by the agonist [125I]DOI was markedly lower (30-50%) than that labeled by the antagonist [3H]ketanserin. High densities of [125I]DOI labeling were present in olfactory bulb, anterior regions of cerebral cortex (layer IV), claustrum, caudate putamen, globus pallidus, ventral pallidum, islands of Calleja, mammillary nuclei and inferior olive. Binding in hippocampus, thalamus and hypothalamus was generally sparse