AN IMPROVED METHOD DETECTS DIFFERENTIAL NGF AND BDNF GENE-EXPRESSION IN RESPONSE TO DEPOLARIZATION IN CULTURED HIPPOCAMPAL-NEURONS

Differential regulation of individual neurotrophins by impulse activit y potentially allows transformation of instantaneous signalling into d iverse, long-lasting neural alterations. To define the temporal profil es of trophin gene expression we examined nerve growth factor (NGF) an d brain-derived neurotrophic factor (BDNF) mRNAs in dissociated cell c ultures of rat hippocampus using an improved solution hybridization te chnique. Traditional methods lack the precision and sensitivity to det ect small changes during brief intervals and the facility to process l arge sample numbers simultaneously. This improved method has now allow ed us to better define the dynamics of depolarization-induced changes in expression of individual trophin genes. Using elevated K+ as a depo larizing stimulus, NGF mRNA increased 40% after 48 h. In contrast, BDN F message rose almost 4-fold within 3 h and attained a maximal 6-fold increase within 6 h. Similar increases in BDNF mRNA levels were exhibi ted following treatment of cultures with glutamate, and excitatory neu rotransmitter. To document the sensitivity of BDNF mRNA to depolarizin g conditions, we examined expression after K+ withdrawal. BDNF message began decreasing within one hour post-depolarization, and returned to basal levels after 6 h. Observations indicate that BDNF and NGF mRNAs are induced differentially in response to impulse activity; BDNF mess age is acutely responsive to ongoing changes, whereas NGF mRNA respond s more slowly and sluggishly. The physiological implications of this d ifferential regulation are discussed.