||Acetylcholinesterase (AChE) is highIy concentrated at vertebrate neuromuscular junctions. Several lines of evidence indicate that the pre-synaptic motor neuron is able to synthesize and secrete AChE at the junctions. In order to determine the retrograde role of muscle in regulating the expression of AChE in motor neuron, a chimeric co-culture of neuroblastoma X glioma hybrid cell NG108-15, a cholinergic cell line that resembles motor neuron, with chick myotube was established to mimic the neuromuscular contact in vitro. A DNA construct of human AChE promoter tagged with luciferase as a reporter, namely pAChE-Luc, was stably transfected into co-cultured NG108-15 cells. The co-culture with myotubes robustly stimulated the promoter activity, as well as the endogenous expression of AChE, in pAChE-Luc stably transfected NG108-15 cells; however, this inductive effect was blocked by the mutation on the CRE site of human AChE promoter. Furthermore, the activity of the cAMP-responsive element (CRE) derived from human AChE promoter in the transfected NG108-15 cells was induced by the co-culture. In parallel, the extract derived from embryonic chick muscles induced the promoter activity and endogenous AChE, when applied onto the pAChE-Luc transfected NG108-15 cells. In addition, the accumulation of intracellular cAMP and the phosphorylation of cAMP-responsive element-binding protein (CREB) in the cultured NG108-15 cells were stimulated by the applied muscle extract. In pAChE-Luc expressing NG108-15 cells the application of calcitonin gene-related peptide (CGRP) was able to mimic part of the muscle-induced neuronal AChE expression in activating the accumulation of intracellular cAMP and in inducing the CREB phosphorylation in NG108-15 cells. In addition, the inductive effect of muscle extract was partially blocked by the antagonist of CGRP receptor, CGRP8-37, suggesting the possibility of CGRP as a muscle-derived factor to retrogradely regulate the neuronal AChE expression. These results suggest that the muscle-induced neuronal AChE expression in the neuron-muscle co-culture is mediated by a cAMP-dependent signaling pathway, and the muscle-derived CGRP could explain part of the muscle effect.