Selective in vivo labelling of brain 5-HT 1A receptors by [ 3H]WAY 100635 in the mouse

Abstract

The novel selective 5-HT 1A 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-HT 1A receptors. Although 5 min after the i.v. injection of [ 3H]WAY 100635 (4–7.6 μCi 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-HT 1A 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-HT 1A 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 the 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-HT 1A 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 ID 50 values of 5-HT 1A receptor agonists (8-OH-DPAT (8-hydroxy-2-(di- n-propylamino)tetralin), S 14506 (1-[2-(4-fluorobenzoylamino) ethyl]-4-(7-methoxynapthyl)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-HT 1A receptors revealed marked variations in the capacity of 5-HT 1A receptor agonists and antagonists to reach the brain when injected via the subcutaneous route in mice.