The alpha7 neuronal nicotinic acetylcholine receptor (AchR) subtype may be an important therapeutic target, with potential significance for the treatment of Alzheimer's disease, stroke, and schizophrenia. Activation of the alpha7 subtype has been shown to have cytoprotective effects and enhance synaptic transmission, which may underlie reported positive cognitive effects obtained with nicotinic agents. Our data indicate that alpha7 receptors have two contrasting modes of activation. Typically, studies have focused on the relatively large transient currents that can be stimulated by rapid application of high concentrations of agonist. However, in the continued presence of such high agonist concentrations, virtually all steady-state current is suppressed by the desensitization process, while at lower agonist concentrations a small amount of steady-state activation persists. Our preliminary data regarding the biophysics of the receptor and the cytoprotective effects of alpha7-selective agonists indicate that steady-state activation of this calcium permeable receptor subtype by low agonist concentrations may represent the functional modality of greatest therapeutic significance. We will therefore study the activation and desensitization properties of alpha7 receptors in detail, expanding our analysis of concentration/response function to include an analysis of the steady-state currents. We will conduct single-channel and whole-cell patch-clamp analysis, using the endogenous activators, Ach and choline, as well as the alpha7-selective partial agonist HMBA. We will test models of the biophysical properties of alpha7 receptors, towards the goal of improving therapeutic targeting of alpha7 receptors. HMBA is the active metabolite of DMXB (GTS-21), a drug just entering phase 2 clinical trials for Alzheimer's disease. DMXB itself is the prototype for a family of alpha7-selective anabaseine derivatives, many of which we have shown, like nicotine, have two phases of action, initially stimulating the receptor, then subsequently causing a long lasting inhibition. We will investigate the nature of this inhibitory activity and the molecular interactions which underlie it. We will characterize the desensitized states of the alpha7 receptor induced by Ach and choline, and determine whether the residual inhibition observed after the application of DMXB and similar agents represents accelerated desensitization or alternative forms of inhibition. In order to improve our ability to target alpha7 receptors for therapeutics, chimeras of the human and rat alpha7 receptors will be made. Then, through the use of agents which show selectivity for the activation or inhibition of the human and rat alpha7 wild-type receptors, it will be possible to identify structural elements of the proteins that regulate activation and desensitization.
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