The 5HT 1A receptor ligand, S15535, antagonises G-protein activation: a [ 35S]GTPγS and [ 3H]S15535 autoradiography study

Abstract

4-(Benzodioxan-5-yl)1-(indan-2-yl)piperazine (S15535) is a highly selective ligand at 5-HT 1A receptors. The present study compared its autoradiographic labelling of rat brain sections with its functional actions, visualised by guanylyl-5′-[γ-thio]-triphosphate ([ 35S]GTPγS) autoradiography, which affords a measure of G-protein activation. [ 3H]S15535 binding was highest in hippocampus, frontal cortex, entorhinal cortex, lateral septum, interpeduncular nucleus and dorsal raphe, consistent with specific labelling of 5-HT 1A receptors. In functional studies, S15535 (10 μM) did not markedly stimulate G-protein activation in any brain region, but abolished the activation induced by the selective 5-HT 1A agonist, (+)-8-hydroxy-dipropyl-aminotetralin ((+)-8-OH-DPAT, 1 μM), in structures enriched in [ 3H]S15535 labelling. S15535 did not block 5-HT-stimulated activation in caudate nucleus or substantia nigra, regions where (+)-8-OH-DPAT was ineffective and [ 3H]S15535 binding was absent. Interestingly, S15535 attenuated (+)-8-OH-DPAT and 5-HT-stimulated G-protein activation in dorsal raphe, a region in which S15535 is known to exhibit agonist properties in vivo [Lejeune, F., Millan, M.J., 1998. Induction of burst firing in ventral tegmental area dopaminergic neurons by activation of serotonin (5-HT) 1A receptors: WAY100,635-reversible actions of the highly selective ligands, flesinoxan and S15535. Synapse 30, 172–180.]. The present data show that (i) [ 3H]S15535 labels pre- and post-synaptic populations of 5-HT 1A sites in rat brain sections, (ii) S15535 exhibits antagonist properties at post-synaptic 5-HT 1A receptors in corticolimbic regions, and (iii) S15535 also attenuates agonist-stimulated G-protein activation at raphe-localised 5-HT 1A receptors.