Cellular localization of NGF and NT-3 mRNAs in postnatal rat forebrain

The presence of transiently elevated levels of mRNA for nerve growth factor (NGF) and neurotrophin-3 (NT-3) in postnatal development of several brain areas suggests that these factors may be expressed by a greater number of cell types in the immature than in the adult brain. To evaluate this possibility, in situ hybridization was used to determine the cellular localization of NGF mRNA and NT-3 mRNA in hippocampus, cingulate cortex, posterolateral neocortex, thalamus, and cerebellum of postnatal rat. In areas expressing both neurotrophins (i.e., hippocampus, cingulate cortex, and anteroventral thalamus), NT-3 mRNA was detected at earlier ages than NGF mRNA. Patterns of hybridization in hippocampus and cerebellum indicate that NT-3 is expressed by neurons soon after leaving the mitotic cycle whereas NGF expression is a feature of more mature neurons. The exception to this pattern was NGF expression in the lateral geniculate nuclei which was present by Postnatal Day 1 and retained in the adult. Both neurotrophins were transiently expressed in several brain areas. The loss of expression with age was most striking in thalamus with transient expression of NT-3 mRNA by the majority of dorsal thalamic relay nuclei and of NGF mRNA by fewer nuclei including the posterior, anteroventral, ventrolateral, and ventromedial nuclei. NT-3 expression also was transient in caudal cingulate/retrosplenial cortex, hippocampal CA3 stratum pyramidale, and the granule cells of archicerebellum. In early postnatal cingulate and retrosplenial cortices there were reciprocal rostrocaudal gradients of NGF and NT-3 expression. These results suggest both distinct and overlapping functions for NT-3 and NGF in early developmental processes including involvement of NT-3 in cerebellar development and of NGF in the development and maintenance of visual afferents to thalamus. Patterns of neurotrophin expression in medial limbic cortex may establish trophic gradients which influence the topography of thalamic innervation.