Immobilization with Atrophy Induces De Novo Expression of Neuronal Nicotinic alpha 7 Acetylcholine Receptors in Muscle Contributing to Neurotransmission

Background: Mature acetylcholine receptor (AChR) isoform normally mediates muscle contraction. The hypothesis that alpha 7AChRs up-regulate during immobilization and contribute to neurotransmission was tested pharmacologically using specific blockers to mature (waglerin-1), immature (alpha A-OIVA), and alpha 7AChRs (methyllycaconitine), and nonspecific muscle AChR antagonist, alpha-bungarotoxin. Methods: Mice were immobilized; contralateral limbs were controls. Fourteen days later, anesthetized mice were mechanically ventilated. Nerve-stimulated tibialis muscle contractions on both sides were recorded, and blockers enumerated above sequentially administered via jugular vein. Data are mean +/- standard error. Results: Immobilization (N = 7) induced tibialis muscle atrophy (40.6 +/- 2.8 vs. 52.1 +/- 2.0 mg; P < 0.01) and decrease of twitch tension (34.8 +/- 1.1 vs. 42.9 +/- 1.5 g; P < 0.01). Waglerin-1 (0.3 +/- 0.05 mu g/g) significantly (P = 0.001; N = 9) depressed twitch tension on contralateral (>= 97%) versus immobilized side (approximately 45%). Additional waglerin-1 (total dose 1.06 +/- 0.12 mu g/g or approximately 15.0 x ED50 in normals) could not depress twitch of 80% or greater on immobilized side. Immature AChR blocker, alpha A-OIVA (17.0 +/- 0.25 mu g/g) did not change tension bilaterally. Administration of a-bungarotoxin (N = 4) or methyllycaconitine (N = 3) caused 96% or greater suppression of the remaining twitch tension on immobilized side. Methyllycaconitine, administered first (N = 3), caused equipotent inhibition by waglerin-1 on both sides. Protein expression of alpha 7AChRs was significantly (N = 3; P < 0.01) increased on the immobilized side. Conclusions: Ineffectiveness of waglerin-1 suggests that the twitch tension during immobilization is maintained by receptors other than mature AChRs. Because alpha A-OIVA caused no neuromuscular changes, it can be concluded that immature AChRs contribute minimally to neurotransmission. During immobilization approximately 20% of twitch tension is maintained by up-regulation of alpha-bungarotoxin- and methyllycaconitine-sensitive alpha 7AChRs.