Mouse muscle denervation increases expression of an alpha7 nicotinic receptor with unusual pharmacology.
Neuronal nicotinic alpha7 subunits have been found in chick and rat skeletal muscle during development and denervation. In the present study, reverse transcriptase-polymerase chain reaction was used to detect alpha7 subunit mRNA in denervated mouse muscle. To determine whether the alpha7 subunit forms functional nicotinic acetylcholine receptors (nAChRs) in muscle, choline was used to induce a membrane depolarization because choline has been considered a specific agonist of alpha7-containing (alpha7*) nAChRs. We found, however, that choline (3-10 mM) also weakly activates muscle nAChRs. After inhibiting muscle nAChRs with a specific muscle nAChR inhibitor, alpha-conotoxin GI (alphaCTxGI), choline was used to activate the alpha7* nAChRs on muscle selectively. Four weeks after denervation, rapid application of choline (10 mM) elicited a substantial depolarization in the presence of alphaCTxGI (0.1 microM). This component of the depolarization was never present in denervated muscles obtained from mutant mice lacking the alpha7 subunit (i.e. alpha7-null mice). The depolarization component that is resistant to alphaCTxGI was antagonized by pancuronium (3-10 microM) and by a 4-oxystilbene derivative (F3, 0.1-0.5 microM) at concentrations considered highly specific for alpha7* nAChRs. Another selective alpha7 antagonist, methyllycaconitine (0.05-5 microM), did not strongly inhibit this choline-induced depolarization. Furthermore, the choline-sensitive nAChRs showed little desensitization over 10 s of application with choline (10-30 mM). These results indicate that functional alpha7* nAChRs are significantly present on denervated muscle, and that these receptors display unusual functional and pharmacological characteristics.
Tsuneki
Hiroshi
H
Division of Neuroscience, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030-3498, USA.
Salas
Ramiro
R
Dani
John A
JA
D000818:Animals; D002794:Choline / Q000494:pharmacology; D020916:Conotoxins / Q000494:pharmacology; D005260:Female; D015870:Gene Expression; D008082:Lipotropic Agents / Q000494:pharmacology; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D051379:Mice; D008810:Mice, Inbred C57BL; D008817:Mice, Mutant Strains; D009121:Muscle Denervation; D018482:Muscle, Skeletal / Q000294:innervation* / Q000502:physiology*; D011978:Receptors, Nicotinic / Q000235:genetics* / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D064569:alpha7 Nicotinic Acetylcholine Receptor
C579166:Chrna7 protein, mouse; C579165:Chrna7 protein, rat; D020916:Conotoxins; D008082:Lipotropic Agents; D011978:Receptors, Nicotinic; D064569:alpha7 Nicotinic Acetylcholine Receptor; C029960:conotoxin GI; D002794:Choline