Electrophysiological, biochemical, neurohormonal and behavioural studies with WAY-100635, a potent, selective and silent 5-HT1A receptor antagonist.

Abstract Although considerable progress has been made in characterising the 5-HT 1 A receptor using agonists, partial agonists or non-selective antagonists, further studies of 5-HT 1 A receptor function have been hindered by the lack of highly selective antagonists. The term ‘silent’ antagonist has been used for such compounds in order to distinguish them unequivocally from several 5-HT 1 A receptor partial agonists which were initially designated ‘antagonists’. In this report we provide a comprehensive review of the biochemical, pharmacological and behavioural properties of the first potent, selective and silent 5-HT 1 A receptor antagonist, WAY-100635 ( N -{2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}- N -(2-pyridinyl)cyclohexanecarboxamide trihydrochloride). WAY-100635 had an IC 50 (displacement of specific [ 3 H]8-OH-DPAT binding to 5-HT 1 A receptors in the rat hippocampus) of 1.35 nM and was >100-fold selective for the 5-HT 1 A site relative to a range of other CNS receptors. [ 3 H]WAY-100635 was also characterised as the first 5-HT 1 A antagonist radioligand, displaying the same regional distribution of binding sites as [ 3 H]8-OH-DPAT in rat brain. As would be expected for the binding of an antagonist to a G-protein-coupled receptor, the B max of [ 3 H]WAY-100635 specific binding was consistently 50–60% greater than that of the agonist radioligand, [ 3 H]8-OH-DPAT. Mn 2+ , but not guanine nucleotides, inhibited [ 3 H]WAY-100635-specific binding. [ 3 H]WAY-100635 was also shown to bind selectively to brain 5-HT 1 A receptors in vivo, following intravenous administration to mice. In vitro electrophysiological studies demonstrated that WAY-100635 had no 5-HT 1 A receptor agonist actions, but dose-dependently blocked the effects of agonists at both the postsynaptic 5-HT 1 A receptor in the CA1 region of the hippocampus, and the somatodendritic 5-HT 1 A receptor located on dorsal raphe 5-HT neurones. In vivo, WAY-100635 also dose-dependently blocked the ability of 8-OH-DPAT to inhibit the firing of dorsal raphe 5-HT neurones, and to induce the ‘5-HT syndrome’, hypothermia, hyperphagia and to elevate plasma ACTH levels. In the mouse light/dark box anxiety model, WAY-100635 induced anxiolytic-like effects. WAY-100635 had no intrinsic effect on cognition in the delayed-matching-to-position model of short-term memory in the rat, but reversed the disruptive effects of 8-OH-DPAT on motor/motivational performance. These data clearly demonstrate that WAY-100635 is the first potent, selective and silent 5-HT 1 A receptor antagonist. Furthermore, [ 3 H]WAY-100635 is the first antagonist radioligand to become available for 5-HT 1 A receptor binding studies both in vitro and in vivo. The positive effects of WAY-100635 in an anxiety model also indicate that a postsynaptic 5-HT 1 A receptor antagonist action may contribute to the anxiolytic properties of 5-HT 1 A receptor partial agonists.