Neurons in culture maintain acetylcholine receptor levels with far fewer transcripts than in vivo

Of the 10 neuronal nicotinic acetylcholine receptor (AChR) genes identified in chick, five are expressed by ciliary ganglion neurons in vivo (alpha 3, alpha 5, alpha 7, beta 2, and beta 4), and the mRNA levels produced increase during development approximately in parallel with the two major classes of AChRs present. Here we report that when chick ciliary ganglion neurons from 8-day embryos are transferred to dissociated cell culture, they express the same five genes but at much lower levels. The alpha 3 and alpha 7 transcripts, chosen for detailed analysis because they encode subunits segregated between the two AChR species, decrease rapidly in abundance on transfer to culture and, after 1 week, are at levels less than a 20th of those found in vivo for neurons of the same age. Co-culturing the neurons with skeletal myotubes did not increase the levels of AChR transcripts in the neurons. Despite low amounts of mRNA from all five genes, neither class of AChRs was much reduced in culture compared to in vivo. The numbers of AChRs on the cell surface actually increased with time in culture. Several culture conditions known to down-regulate the receptors in culture did not reduce the abundance of the alpha 3 and alpha 7 mRNAs. The results suggest that post-transcriptional controls can play an important role in determining AChR abundance on the neurons.