BDNF DOWN-REGULATES NEUROTROPHIN RESPONSIVENESS, TRKB PROTEIN AND TRKB MESSENGER-RNA LEVELS IN CULTURED RAT HIPPOCAMPAL-NEURONS

Regulation of Trk receptors by their ligands, the neurotrophins, was i nvestigated in dissociated cultures of embryonic day 18 rat hippocampa l neurons. Cultures were exposed to brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) or NT-4/5 for 24 h upon plating followed by factor washout. As determined by immunohistochemical staining and phosphotyrosine blotting, the functional responses to acute stimulatio n with BDNF, NT-3 and NT-4/5, including c-Fos induction and phosphoryl ation of Trk and extracellular signal-regulated kinase (ERK) proteins, were significantly decreased after 6 days in culture by prior exposur e to BDNF. As determined by Western and Northern blot analysis respect ively, there was a parallel down-regulation of TrkB protein as well as of trkB and trkC mRNA levels in BDNF-pretreated cultures. Exposure to NT-3 or NT-4/5 at the same concentrations as BDNF did not down-regula te any of the measured cellular responses or TrkB protein and/or trkB and trkC mRNA levels. Regulation of hippocampal neuronal TrkB protein does not appear to be just a developmental phenomenon, as infusion of BDNF into the hippocampus of adult rats for 6 days produced an 80% dec rease in levels of full-length TrkB protein. We thus show that exposur e of hippocampal neurons to BDNF, both in culture and in the adult bra in, results in down-regulation of TrkB. At least in vitro, this leads to long-term functional desensitization to BDNF, NT-3 and NT-4/5, as w ell as down-regulation of trkB and trkC mRNA.