NEURAL REGULATION OF ACETYLCHOLINESTERASE MESSENGER-RNAS AT MAMMALIANNEUROMUSCULAR SYNAPSES

We examined the role of innervation on acetylcholinesterase (AChE) gen e expression within mammalian skeletal muscle fibers. First, we showed the selective accumulation of AChE mRNAs within the junctional vs ext rajunctional sarcoplasm of adult muscle fibers using a quantitative re verse transcription PCR assay and demonstrated by in situ hybridizatio n experiments that AChE transcripts are concentrated immediately benea th the postsynaptic membrane of the neuromuscular junction. Next, we d etermined the influence of nerve-evoked activity vs putative trophic f actors on the synaptic accumulation of AChE mRNA levels in muscle fibe rs paralyzed by either surgical denervation or selective blockage of n erve action potentials with chronic superfusion of tetrodotoxin. Our r esults indicated that muscle paralysis leads to, a marked decrease in AChE transcripts from the postsynaptic sarcoplasm, yet the extent of t his decrease is less pronounced after tetrodotoxin inactivation than a fter denervation. These results suggest that although nerve-evoked act ivity per se appears a key regulator of AChE mRNA levels, the integrit y of the synaptic structure or the release of putative trophic factors contribute to maintaining the synaptic accumulation of AChE transcrip ts at adult neuromuscular synapses. Furthermore, the pronounced downre gulation of AChE transcripts in paralyzed muscles stands in sharp cont rast to the well-documented increase in nicotinic acetylcholine recept or mRNAs under these conditions, and indicates that expression of the genes encoding these two synaptic proteins are subjected to different regulatory mechanisms in adult muscle fibers in vivo.