DEVELOPMENTALLY-REGULATED EXPRESSION OF MESSENGER-RNA FOR NEUROTROPHIN HIGH-AFFINITY (TRK) RECEPTORS WITHIN CHICK TRIGEMINAL SENSORY NEURONS

To investigate the distribution of neurons within the developing trige minal sensory system which express mRNA for each of the three known hi gh-affinity neurotrophin receptors (trk, trkB and trkC), we have perfo rmed in situ hybridization histochemistry on serial sections through t he trigeminal ganglion and trigeminal mesencephalic nucleus at various ages of development using specific antisense oligonucleotide probes. We show that trkC mRNA is first expressed in the chicken embryo at sta ge 13, in presumptive neurons prior to the formation of the ganglion, that trkB mRNA labelling is initially observed within peripheral neuro ns slightly later, at stage 19, and that trk mRNA expression is not de tectable until around embryonic day 3.5 (Istage 21/22). The neurons wh ich exhibit mRNA labelling for each of the high-affinity receptors occ upy discrete regions within the ganglion, indicating that the ganglion comprises distinct neuronal subpopulations, each of which has a diffe rent capacity to respond to the different neurotrophins. Neurons which express trk mRNA are confined to the proximal region of the ganglion, whereas those which express trkB mRNA and trkC mRNA are located in tw o distinct regions within the distal aspect and also within the trigem inal mesencephalic nucleus. From the estimation of the number of neuro ns which exhibit labelling between embryonic days 9 and 18, we determi ned that the expression of mRNA for the high-affinity receptors change s during embryonic development of the ganglion. This is consistent wit h the observed differences in the response to neurotrophins in vitro.