Novel Nicotinic Acetylcholine Receptor Ligands based on Cytisine, Ferruginine, Anatoxin-a and Choline

Abstract

Nicotinic acetylcholine receptors (nAChRs), excitatory ligand-gated cation channels, are transmembranic allosteric proteins composed of five polypeptide chains. The subtype at the neuromuscular junction consists of α1, β1, γ and δ subunits whereas the neuronal subtypes are formed by a combination of different α (α2 to α10) and β(β2 to β4) subunits (heteromeric type) or identical α subunits (homomeric type). NAChRs are widely distributed in the central nervous system and in the periphery. In recent years, there has been increasing interest in nAChR ligands as potential analgesics and therapeutics for the treatment of various neurological and mental disorders related to a decrease in cholinergic function such as Alzheimer’s and Parkinson’s diseases, attention deficit/hyperactivity disorder, schizophrenia, and depression.<br /> Natural toxic alkaloids like (-)-cytisine, (+)-ferruginine, and (+)-anatoxin-a possess high affinity for α4β2 nAChR, representing the major population of nACh receptors in mammalian brain, but no appropriate selectivity. Structure-activity relationship studies for these ligands are missing for other subtypes, like α7, α3β4 and (α1)₂β1γδ. In order to evaluate the affinities of novel analogues of (-)-cytisine, (+)-ferruginine, and (+)-anatoxin-a, <i>in vitro</i> radioligand binding studies were performed for four different nAChR subtypes: α4β2*, α7*, α3β4* and (α1)₂β1γδ nAChRs, using membranes of native tissues (rat brains, calf/pig adrenals and Torpedo calif. electroplax). Important information could be obtained concerning the structural requirements that enhance selectivity of these novel analogues for α4β2* nAChR over other nAChRs investigated. In general, since the molecular recognition between ligands and nAChRs might be based on cation-pi interactions and a hydrogen bond formation between the receptor site and the ligand (HBD-HBA interaction), all high affinity compounds bear the cationic and HBA motifs.<br /> Beside the search and development of novel nAChR ligands exhibiting selectivity for the α4β2* nAChR, interest is growing to develop novel compounds selective for the α7 and α3-containing subtypes. Choline has proved to be a selective ligand for the α7, showing also neuroprotective actions. In this study, choline derivatives are evaluated in <i>in vitro</i> radioligand binding studies to gain more information about the structural requirements to enhance the selectivity for α7* versus the α4β2* nicotinic receptor.