BDNF increases the early expression of TRH rnRNA in fetal TrkB(+) hypothalamic neurons in primary culture

Known effects of neurotrophins in the developing central nervous system inc lude induction or regulation of peptide expression. Hypothalamic postmitoti c thyrotropin-releasing hormone (TRH)-producing neurons may require neurotr ophins for survival and/or differentiation. This issue was investigated usi ng primary cell cultures derived from 17-day-old fetal rat hypothalamus see ded in serum-free medium and analysed up to 4 days in vitro culture. Neurot rophin receptor (TrkB and TrkC) mRNA expression was detected by RT-PCR in f etal hypothalamus and throughout the culture period. Western blots confirme d the expression of the full-length proteins in vitro. Semi-quantitative RT -PCR showed that the addition of brain-derived neurotrophic factor (BDNF) i ncreases TRH mRNA levels while the addition of neurotrophin-3 does not. TRH cell content was not modified. Studies on the effect of cell density or ho mologous conditioned medium demonstrated that endogenous factors probably c ontribute to determine TRH mRNA levels. One of these factors was BDNF becau se basal TRH mRNA levels were reduced by the addition of a Trk inhibitor or anti-BDNF. TrkB mRNA was expressed in 27% of cells and TRH mRNA in 2% of c ells. The number of TRH+ cells was not affected by BDNF treatment. Forty-ei ght per cent of TRH neurons contained TrkB mRNA; these neurons had higher a mounts of TRH mRNA than TrkB(-) neurons. Only TrkB(+) cells responded to BD NF by increasing their TRH mRNA levels suggesting that BDNF may directly af fect TRH biosynthesis. In conclusion, fetal hypothalamic TRH neurons are pr obably heterogeneous in regard to the neurotrophic factors enhancing peptid e and mRNA levels. BDNF enhances TRH mRNA levels in a population of TrkB(+) fetal hypothalamic TRHergic neurons in primary culture. However, additiona l influences may be necessary for the establishment of peptide phenotype in the TrkB(+) neurons.