Changes in the expression of the nerve growth factor receptors TrkA and p75(LNGR) in the rat thymus with ageing and increased nerve growth factor plasma levels

The nerve growth factor (NGF) receptors p75(LNGR) and TrkA are expressed by thymic epithelial cells. Presumably, the NGF-TrkA system is involved in th e paracrine communication between thymic epithelial cells and thymocytes, w hereas the functional role of p75(LNGR) is Still unknown. The thymus of ver tebrates undergoes age-related changes that in part depend on hormonal fact ors. In order to find out whether thymic epithelial cells are responsive to NGF: during the whole lifespan of the rat, we studied NGF receptor express ion in the thymus from birth to 2 years of age, using immunohistochemistry. Furthermore, to evaluate whether increased plasma levels of NGF affected t he ageing process, either NGF or 4-methylcatechol (4MC), an inductor of NGF synthesis, was administered. Both TrkA and p75(LNGR) were expressed by a s ubpopulation of thymic epithelial cells during the whole age range studied and their expression peaked at around 3 months. TrkA was primarily found in subcortical and medullary epithelial cells, whereas p75(LNGR) was seen in a subpopulation of medullary cells. Cortical epithelial cells, neural crest -derived cells, other stromal cells and thymocytes were not immunoreactive for NGF receptors. Neither the administration of NGF nor the increased NGF plasma levels obtained after 4MC treatment seemed to affect the ageing of t he thymus as assessed by morphological and immunohistochemical criteria, bu t this increase in NGF levels did produce a shift in the expression of p75( LNGR) from epithelial cells to EDI-positive macrophages in animals of 6 mon ths and older. Present results indicate that the expression of p75(LMGR) an d TrkA in the rat thymus undergoes age-dependent changes that parallel thos e of epithelial cells. NGF could therefore be important for thymus homeosta sis, possibly acting on epithelial cells. Nevertheless, NGF did not seem to be able to prevent the involution of this organ, although it produced a sw itch in the expression of p75(LNGR), the significance of which remains to b e established.