In vitro binding and autoradiographic studies with [ ³ H]8-OH-DPAT and [ ³ H]Lesopitron as radioligands confirm that Lesopitron binds to 5 -HT _1A receptors in the rat brain with a relatively high affinity (pK _i =7.35). As expected of a full agonist at postsynaptic 5-HT _{1A < /sub> receptors, Lesopitron (IC 50 =125 nM) inhibits forskolin-stimulated adenylate cyclase activity in rat hippocampal membranes to the same extent as 5-HT. Lesopitron inhibits the firing of serotoninergic neurons both in vitro (in brainstem slices, IC 50 =120 nM).}

Lesopitron inhibits the firing of serotoninergic neurons both in vivo (in chloral hydrate-anaesthetized rats, ID ₅₀ =35 μg/kg iv). It administered at a dose which induces anxiolytic behaviour in rats (30 μg/kg, ip) markedly reduces 5-HT levels (to 45% of the basal value) in cortical perfusates.