Concentration of acetylcholine receptor mRNA in synaptic regions of adult muscle fibres

Abstract

Acetylcholine receptors (AChRs) are highly concentrated in the small fraction (∼0.1%) of the skeletal muscle fibre surface that comprises the postsynaptic membrane of the neuromuscular junction (Fig. 1a). In adult murine muscle, for example, AChRs are packed at a density of over 15,000 per µm² in postsynaptic membrane, whereas their density is <30 per µm² in extrasynaptic membrane^1,2. Because synaptic AChRs turn over^2,3 they must be replaced, and it is interesting to consider where the new AChRs that maintain synaptic aggregates are synthesized. One possibility is that AChRs are synthesized uniformly along the length of the multinucleated muscle fibre; in this case, AChRs might be redistributed to or selectively stabilized at the synapse, as probably occurs during synapse formation^4–7. Alternatively, AChRs might be eferentially ynthesized near synapses, a possibility that would suggest that innervation can influence not only where AChRs are inserted⁸ or accumulate but also where they are synthesized. In support of this second possibility, we report here that AChR messenger RNA is more abundant near to than far from synapses in adult muscle fibres.