Physiological Concentrations of Choline Activate Native α7-Containing Nicotinic Acetylcholine Receptors in the Presence of PNU-120596 [1-(5-Chloro-2,4-dimethoxyphenyl)-3-(5-methylisoxazol-3-yl)-urea]

The use of PNU-120596 [1-(5-chloro-2,4-dimethoxyphenyl)-3-( 5-methylisoxazol-3-yl)-urea], a positive allosteric modulator of alpha 7 nicotinic acetylcholine receptor (nAChR), may be beneficial for enhancing cholinergic therapies. However, the effects of PNU-120596 on activation of native alpha 7-containing nAChRs by physiological concentrations of choline are not known and were investigated in this study using patch-clamp electrophysiology and histaminergic tuberomammillary neurons in hypothalamic slices. In the presence of PNU-120596, subthreshold (i.e., inactive) physiological concentrations of choline (similar to 10 mu M) elicited repetitive step-like whole-cell responses reminiscent of single ion channel openings that were reversibly blocked by 20 nM methyllycaconitine, a selective alpha 7 nAChR antagonist. The effects of choline and PNU-120596 were synergistic as administration of 10 to 40 mu M choline or 1 to 4 mu M PNU-120596 alone did not elicit responses. In voltage clamp at -60 mV, the persistent activation of alpha 7-containing nAChRs by 10 mu M choline plus 1 mu M PNU-120596 was estimated to produce a sustained influx of Ca2+ ions at a rate of 8.4 pC/min (similar to 0.14 pA). In the presence of PNU-120596 in current clamp, transient steplike depolarizations (similar to 5 mV) enhanced neuronal excitability and triggered voltage-gated conductances; a single opening of an alpha 7-containing nAChR channel appeared to transiently depolarize the entire neuron and facilitate spontaneous firing. Therefore, this study tested and confirmed the hypothesis that PNU120596 enhances the effects of subthreshold concentrations of choline on native alpha 7-containing nAChRs, allowing physiological levels of choline to activate these receptors and produce whole-cell responses in the absence of exogenous nicotinic agents. In certain neurological disorders, this activation may be therapeutically beneficial, more efficacious, and safer than treatments with nAChR agonists.