DOWN-REGULATION OF PHOSPHATIDYLINOSITOL RESPONSE TO BDNF AND NT-3 IN CULTURES OF CORTICAL-NEURONS

The hydrolysis of phosphatidyl 4,5-bisphosphate (PI), which is involve d in the transduction mechanism of neurotransmitters and growth factor s, is stimulated by brain-derived neurotrophic factor (BDNF) and neuro trophic-3 (NT-3) in primary cultures of fetal brain neurons. In the Pr esent study we sought to examine the effect of pretreatment with these factors on their acute stimulation capabilities and, furthermore, to substantiate that the effects of BDNF and NT-3 reflect actions on neur ons rather than glial cells. Pretreatment with BNDF and NT-3 for 4 day s followed by 1 day without growth factor abolished the effect of an a cute stimulation with these factors. The growth factors were mutually effective so that BDNF pretreatment abolished the acute response to NT -3 and vice versa. In contrast, the effects of bFGF (basic fibroblast growth factor, a non-neurotrophic growth factor) also stimulating PI h ydrolysis in these culture systems, were not reduced by neurotrophic p retreatment. Pretreatment with K-252b, at concentrations known to inhi bit trk receptors, did not alter the acute stimulation of PI hydrolysi s induced by the neurotrophins. PI hydrolysis stimulated by BDNF and N T-3 in cultures grown in presence of cytosine arabinoside C, containin g > 95% neurons, was higher than in cultures containing non-neuronal c ells, indicating that the neurotrophic stimulation occurs in neuronal cells. No stimulatory effect was detected in bFGF treated pure neurona l cultures. The findings suggest that prolonged exposure of responsive neurons to BDNF and NT-3 down-regulates their stimulatory effects on PI hydrolysis.