AT-1001 Is a Partial Agonist with High Affinity and Selectivity at Human and Rat α3β4 Nicotinic Cholinergic Receptors

AT-1001 [N-(2-bromophenyl)-9-methyl-9-azabicyclo[3.3.1] nonan-3-amine] is a high-affinity and highly selective ligand at alpha 3 beta 4 nicotinic cholinergic receptors (nAChRs) that was reported to decrease nicotine self-administration in rats. It was initially reported to be an antagonist at rat alpha 3 beta 4 nAChRs heterologously expressed in HEK293 cells. Here we compared AT-1001 actions at rat and human alpha 3 beta 4 and alpha 4 beta 2 nAChRs similarly expressed in HEK 293 cells. We found that, as originally reported, AT-1001 is highly selective for alpha 3 beta 4 receptors over alpha 4 beta 2 receptors, but its binding selectivity ismuch greater at human than at rat receptors, because of a higher affinity at human than at rat alpha 3 beta 4 nAChRs. Binding studies in human and rat brain and pineal gland confirmed the selectivity of AT-1001 for alpha 3 beta 4 nAChRs and its higher affinity for human compared with rat receptors. In patch-clamp electrophysiology studies, AT-1001 was a potent partial agonist with 65-70% efficacy at both human and rat alpha 3 beta 4 nAChRs. It was also a less potent and weaker (18%) partial agonist at alpha 4 beta 2 nAChRs. Both alpha 3 beta 4 and alpha 4 beta 2 nAChRs are upregulated by exposure of cells to AT-1001 for 3 days. Similarly, AT-1001 desensitized both receptor subtypes in a concentration-dependent manner, but it was 10 and 30 times more potent to desensitize human alpha 3 beta 4 receptors than rat alpha 3 beta 4 and human alpha 4 beta 2 receptors, respectively. After exposure to AT-1001, the time to recovery from desensitization was longest for the human alpha 3 beta 4 nAChR and shortest for the human alpha 4 beta 2 receptor, suggesting that recovery from desensitization is primarily related to the dissociation of the ligand from the receptor.