NEUROTROPHINS AND THEIR RECEPTORS IN RAT PERIPHERAL TRIGEMINAL SYSTEMDURING MAXILLARY NERVE GROWTH

We examined the expression of the neurotrophins (NTFs) and their recep tor mRNAs in the rat trigeminal ganglion and the first branchial arch before and at the time of maxillary nerve growth. The maxillary nerve appears first at embryonic day (E)10 and reaches the epithelium of the first branchial arch at E12, as revealed by anti-L1 immunohistochemis try. In situ hybridization demonstrates, that at E10-E11, neurotrophin -3 (NT-3) mRNA is expressed mainly in the mesenchyme, but neurotrophin -4 (NT-4) mRNA in the epithelium of the first branchial arch. NGF and brain-derived neurotrophic factor (BDNF) mRNAs start to be expressed i n the distal part of the first branchial arch shortly before its inner vation by the maxillary nerve. Trigeminal ganglia strongly express the mRNA of trkA at E10 and thereafter. The expression of mRNAs for low-a ffinity neurotrophin receptor (LANR), trkB, and trkC in trigeminal gan glia is weak at E10, but increases by E11-E12. NT-3, NT-4, and more pr ominently BDNF, induce neurite outgrowth from explant cultures of the E10 trigeminal ganglia but no neurites are induced by NGF, despite the expression of trkA. By E12, the neuritogenic potency of NGF also appe ars. The expression of NT-3 and NT-4 and their receptors in the trigem inal system prior to target field innervation suggests that these NTFs have also other functions than being the target-derived trophic facto rs.