Regulation of neurotransmitter metabolic enzymes and tyrosine hydroxylase mRNA level by nerve growth factor in cultured sympathetic neurones.

The survival of new-born rat sympathetic neurones in culture was increased in a dose-dependent manner by 7S nerve growth factor (NGF). NGF also increased, in a parallel manner, the specific activities of tyrosine hydroxylase (TOH) and choline acetyltransferase (CAT). Total acetylcholinesterase (AcChE) activity increased with NGF concentration, although less distinctly than TOH and CAT. However, NGF caused a large induction of the asymmetric A12 form of AcChE, and to a lesser extent of the globular G1 and G2 forms, whereas the globular G4 form was little affected. This suggests that NGF differentially regulates the synthesis and/or assembly of the various AcChE molecular forms. The levels of TOH mRNA in neurone cultures grown with increasing NGF concentrations were measured by Northern blot analysis with a rat cDNA probe. To correct for variations in the total mass of RNA per neurone, the filters were rehybridized with an 18S rRNA probe. The level of TOH mRNA, measured by the ratio (TOH:18S) of the hybridization signals increased 3.4-fold between 92 and 740 ng ml-1 7S NGF. Increases of TOH specific activity of the same order of magnitude were observed in sister cultures. The deficit in the level of mature TOH mRNA at low NGF concentration was not accompanied by a compensatory accumulation in unprocessed TOH transcripts. As TOH induction is insensitive to RNA polymerase inhibitors, we suggest that NGF regulates the maturation of TOH pre-mRNAs, and that the unprocessed transcripts are rapidly degraded. The long-term regulation of TOH by NGF may thus constitute a case of process-versus-discard control, as defined by J.E. Darnell.