NGF RECEPTOR EXPRESSION IN SENSORY NEURONS DEVELOPS NORMALLY IN EMBRYOS LACKING NGF

Numerous in vivo and in vitro studies have shown that NGF increases th e expression of its receptors, p75 and TrkA, in NGF-responsive cell li nes and in NGF-responsive neurons of the developing and mature nervous system, To determine if endogenous NGF is required for the normal dev elopmental increase in p75 and TrkA expression that occurs in sensory neurons shortly after they innervate their targets, we used quantitati ve RT/PCR to compare the levels of p75 and trkA mRNAs in the trigemina l ganglia of normal mouse embryos and embryos that are homozygous for a null mutation in the NGF gene. We show that the marked increase in p 75 and trkA mRNA expression that occurs between Ell and E13 in normal embryos takes place on time and to the same extent in NGF(-/-) embryos . We also show that trigeminal neurons from E13 NGF(+/+) and NGF(-/-) embryos have very similar dose responses for survival induced by NGF. These findings clearly show that the expression of both p75 and TrkA a nd the sensitivity of developing sensory neurons to NGF do not require and are not modulated by target-derived NGF during the early stages o f target held innervation. (C) 1995 Academic Press, Inc.