New 5-Hydroxytryptamine<INF>1A</INF> Receptor Ligands Containing a Norbornene Nucleus:  Synthesis and in Vitro Pharmacological Evaluation

New arylpiperazine derivatives were prepared to identify highly selective and potent ligands for the 5-hydroxytryptamine 1A (5-HT<INF>1A</INF>) receptor as potential pharmacological tools in studies of central nervous system (CNS) disorders. The combination of structural elements (heterocyclic nucleus, oxyalkyl chain, and arylpiperazine) known to introduce 5-HT<INF>1A</INF> receptor affinity and the proper selection of substituents led to compounds with higher receptor specificity and affinity. In binding studies, several molecules showed affinity in the nanomolar and subnanomolar ranges at 5-HT<INF>1A</INF> and moderate to no affinity for other relevant receptors (5-HT<INF>2A</INF>, 5-HT<INF>2C</INF>, D<INF>1</INF>, D<INF>2</INF>, α<INF>1</INF>, and α<INF>2</INF>). The 4-[3-[4-(o-methoxyphenyl)piperazin-1-yl]propoxy]-4-aza-tricyclo[5.2.1.02,6]dec-8-ene-3,5-dione (<BO>3b</BO>), with K<INF>i</INF> = 0.021 nM, was the most active and selective derivative for the 5-HT<INF>1A</INF> receptor with respect to other serotonin receptors, whereas the most selective derivative for dopaminergic and adrenergic receptors was a CF<INF>3</INF>-substituted arylpiperazine (<BO>2e</BO>). As a general trend, compounds with a piperazinylpropoxy chain (<BO>3b</BO>−<BO>g</BO>) showed a preferential affinity for the 5-HT<INF>1A</INF> receptor, suggesting that the alkyl chain length represents a critical structural feature in determining 5-HT<INF>1A</INF> receptor affinity and selectivity, as confirmed by the molecular modeling invoked for explaining the differential binding affinities of the new arylpiperazines.