Asynaptic expression of the adult nicotinic acetylcholine receptor in long-term cultures of mammalian myotubes.

To determine whether synaptic contact is required to express adult-type nicotinic acetylcholine receptors (A-AChR) in developing mammalian muscle, we have examined single-channel AChR activity in primary muscle cultures maintained for up to 29 days. A-AChRs were first expressed after day 12 in culture (CD12), during a period characterized by the accumulation of embryonic acetylcholine receptors (E-AChR). The highest rate of A-AChR expression was observed between CD15 and 19, during a period of maximal E-AChR accumulation. Although the level of A-AChR expression between individual patches was quite variable during this period, A-AChRs accounted for up to 40% of the events produced by receptors expressed over a 3-day interval. Between CD19 and 29, the density of E-AChRs diminished while the expression of A-AChRs per patch continued to increase but at a lower rate than that observed between CD15 and 19. In 25-29-day cultures, 70.6% of patches exhibited both E-AChR and A-AChR activity, and the percentage of A-AChR events per patch ranged between 0 and 47% with a mean of 11.7 +/- 3.2%. These results demonstrate that endogenous muscle mechanisms promote developmental increases in the expression of A-AChRs in myotubes that have no history of synaptic contact. This conclusion suggests that synaptic imprinting at developing junctions is mediated in part by endogenous muscle mechanisms, and does not require direct neurotrophic activation of epsilon mRNA transcription.