Abstract
α7 nicotinic acetylcholine receptors (nAChRs) have been a puzzle since their discovery in brain and non-neuronal tissues. Maximal transient probability of an α7 nAChR being open with rapid agonist applications is only 0.002. The concentration dependence of α7 responses measured from transfected cells and Xenopus laevis oocytes shows the same disparity in potency estimations for peak currents and net charge, despite being studied at 1000-fold different time scales. In both cases the EC50 was approximately 10-fold lower for net charge than for peak currents. The equivalence of the data obtained at such disparate time scales indicates that desensitization of α7 is nearly instantaneous. At high levels of agonist occupancy, the receptor is preferentially converted to a ligand-bound nonconducting state, which can be destabilized by the positive allosteric modulator N-(5-chloro-2,4-dimethoxyphenyl)-N′-(5-methyl-3-isoxazolyl)-urea (PNU-120596). Such currents can be sufficiently large to be cytotoxic to the α7-expressing cells. Both the potentiating effect of PNU-120596 and the associated cytotoxicity have a high temperature dependence that can be compensated for by serum factors. Therefore, despite reduced potentiation at body temperatures, use of type II positive allosteric modulators may put cells that naturally express high levels of α7 nAChRs, such as neurons in the hippocampus and hypothalamus, at risk. With a low intrinsic open probability and high propensity toward the induction of nonconducting ligand-bound states, it is likely that the well documented regulation of signal transduction pathways by α7 nAChRs in cells such as those that regulate inflammation may be independent of ion channel activation and associated with the nonconducting conformational states.
Footnotes
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The online version of this article (available at http://molpharm.aspetjournals.org) contains supplemental material.
This work was supported by the National Institutes of Health National Institute of General Medical Science [Grant R01-GM57481]; National Institutes of Health National Institute on Aging [Grant T32-AG000196]; and Florida Biomedical James and Esther King Research [Grant KG12].
Article, publication date, and citation information can be found at http://molpharm.aspetjournals.org.
ABBREVIATIONS:
- α-btx
- α-bungarotoxin
- nAChR
- nicotinic acetylcholine receptor
- ACh
- acetylcholine
- PAM
- positive allosteric modulator
- PNU-120596
- N-(5-chloro-2,4-dimethoxyphenyl)-N'-(5-methyl-3-isoxazolyl)-urea
- HEK
- human embryonic kidney
- BSA
- bovine serum albumin
- HBSS
- Hanks' balanced saline solution
- DMSO
- dimethyl sulfoxide
- 5-HI
- 5-hydroxyindole
- NS-1738
- N-(5-chloro-2-hydroxyphenyl)-N′-[2-chloro-5-(trifluoromethyl)phenyl]urea
- TQS
- 3a,4,5,9b-tetrahydro-4-(1-naphthalenyl)-3H-cyclopentan[c]quinoline-8-sulfonamide
- DMEM
- Dulbecco's modified Eagle's medium
- FBS
- fetal bovine serum
- MLA
- methyllycaconitine
- GTS-21
- 3-(2,4-dimethoxybenzylidene)anabaseine
- NS-6740
- (1,4-diazabicyclo[3.2.2]nonan-4-yl(5-(3-(trifluoromethyl)phenyl)furan-2-yl)methanone.
- Received May 30, 2012.
- Accepted July 24, 2012.
- Copyright © 2012 The American Society for Pharmacology and Experimental Therapeutics
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