Regulation of sympathetic neuron differentiation by endogenous nerve growth factor and neurotrophin-3

Nerve growth factor (NGF) and neurotrophin-3 (NT3) play distinctive roles in sympathetic axon growth and target field innervation and are required for sympathetic neuron survival in vivo. To ascertain if these neurotrophins selectively regulate the expression of genes that determine the functional characteristics of differentiated sympathetic neurons, we measured the mRNA levels for several such genes in the superior cervical ganglion of NGF−/−, NT3−/− and wild type mouse embryos at a stage before excessive neuronal loss occurs in the absence of these neurotrophins. Despite the extensively documented ability of NGF to regulate the noradrenergic phenotype of sympathetic neurons, we found that tyrosine hydroxylase (TH) and dopamine β hydroxylase (DβH) mRNA levels were normal in NGF−/− embryos, but significantly reduced in NT3−/− embryos. In contrast, the β2 nicotinic acetylcholine receptor and PACAP receptor 1 mRNA levels were normal in NT3−/− embryos, but significantly reduced in NGF−/− embryos. Studies of mice lacking neurotrophin receptors suggested that the effects of NGF on gene expression require TrkA whereas those of NT3 require TrkA and p75NTR. These findings demonstrate that endogenous NGF and NT3 have distinctive and separate effects on gene expression in early sympathetic neurons and that these selective effects on gene expression require a different combination of neurotrophin receptors.