Cultured hippocampal neurons show responses to BDNF, NT-3, and NT-4, but not NGF

To investigate the possibility of neurotrophins acting directly on hippocampal neurons, we first examined expression of the trk receptors in sections of adult rat brain and in cultures of embryonic rat hippocampus, and then investigated general and specific responses of cultured hippocampal neurons to each of the neurotrophins. In situ hybridization studies indicated high levels of expression of trkB and trkC but not trkA in pyramidal cells, dentate granule neurons, and scattered interneurons. Cultures of embryonic day 18 (E18) hippocampal neurons were also found by Northern analysis to express trkB and trkC but not trkA, indicating potential responsiveness to brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and NT-4, but not NGF. Phosphorylation experiments indeed showed that BDNF, NT-3, and NT-4 produced rapid tyrosine phosphorylation of Trk proteins, as detected by immunoprecipitation using a pan-Trk-specific antibody, whereas NGF produced no detectable tyrosine phosphorylation in hippocampal cultures. Similarly, all of the neurotrophins, except NGF, induced expression of c-fos mRNA and c-fos protein in these cultures. c-Fos protein induction was detectable in approximately 40-50% of the cells. While we observed no major effect of any of the neurotrophins upon the survival of E18 hippocampal neurons, BDNF, NT-3, and NT-4, but not NGF, produced marked increases in the number of neurons expressing detectable levels of either calbindin or AChE. NT-3 produced the greatest increase in the number of calbindin-positive neurons, whereas BDNF and NT-4 produced the greater increase in the number of AChE-positive neurons. Our results suggest that several of the neurotrophins have important effects in the differentiation and maintenance of function of subpopulations of hippocampal neurons.