Nicotinic Receptor: Target for Therapeutic Drugs

BOUZAT C

Cordoba

Conferencia; Congreso Anual de la Sociedad Argentina de Farmacología Experimental; 2006

Sociedad Argentina de Farmacología Experimental

Nicotinic receptors (AChRs) are members of the Cys-loop receptor superfamily. They mediate rapid synaptic transmission throughout the nervous system in both vertebrates and invertebrates. AChRs are involved in muscle contraction and they contribute to a wide range of brain activities. Their essential function is to couple the binding of agonist at the extracellular domain to the opening of an intrinsic ion pore. This mechanism, which is known as gating, has not been elucidated. However, it is known that mutations that change gating kinetics lead to neurological disorders, such as slow-channel myasthenic syndromes. To investigate the structural basis for gating, we generated a chimeric receptor composed of the acetylcholine binding protein (AChBP) and the pore domain from the 5-HT3A receptor. The chimeric AChBP-5HT3A receptor shows high surface expression on mammalian cells but it does not function, suggesting that although binding and pore domains are corrected folded, the interface between them is not compatible, preventing inter-domain coupling. Only when amino-acid sequences of three extracellular loops in AChBP are changed to their 5HT3A counterparts does ACh bind with low affinity characteristic of the activatable receptor and trigger opening of the ion pore. Thus functional coupling requires structural compatibility at the interface of the binding and pore domains of Cys-loop receptors. AChR function is modified by drugs, which can act through different mechanisms and at different conformational states. Nematode muscle AChRs are targets for anthelmintic drugs, which produce spastic paralysis and ultimately death. We have shown that levamisole and pyrantel are very low-efficacy agonists of mammalian muscle AChRs. By combining site-directed mutagenesis with single- and macroscopic-current recordings we identified residues located at different faces of the binding site that govern the selectivity of these agents. Mutations at these sites in the parasite subunits may lead to anthelmintic resistance. We also have characterized AChR activity in muscle cells from C. elegans, which is a model for the study of parasitic nematodes. We determined that the main channel activity is mediated by levamisole-sensitive AChRs, for which levamisole is a more potent agonist than ACh. AChRs are also targets for many neuroactive drugs, and consequently, these receptors may be involved in their therapeutic or adverse actions. We have elucidated the action of antidepressants at the molecular level. Our results reveal that these drugs inhibit muscle and neuronal a7 AChRs by different mechanisms. In muscle AChRs, they increase the desensitization rate from the open state, and in a7 AChRs, they interact with the closed state and allosterically inhibit channel opening.  For designing new drugs targeted to a specific AChR subtype, mutated receptor or conformational state, we require an intimate knowledge of the structure-function relationship. New understanding of the molecular pharmacology of AChRs will lead to a new generation of more selective drugs with improved safety profiles. Supported by grants from CONICET, FONCYT, UNS.