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 involved in the transduction mechanism of neurotransmitters and growth factors, is stimulated by brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) in primary cultures of fetal brain neurons. In the present 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 neurons rather than glial cells. Pretreatment with BNDF and NT-3 for 4 days followed by 1 day without growth factor abolished the effect of an acute 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-neurotrophin growth factor) also stimulating PI hydrolysis in these culture systems, were not reduced by neurotrophin pretreatment. Pretreatment with K-252b, at concentrations known to inhibit trk receptors, did not alter the acute stimulation of PI hydrolysis induced by the neutrophins. PI hydrolysis stimulated by BDNF and NT-3 in cultures grown in presence of cytosine arabinoside C, containing > 95% neurons, was higher than in cultures containing non-neuronal cells, indicating that the neurotrophin stimulation occurs in neuronal cells. No stimulatory effect was detected in bFGF treated pure neuronal cultures. The findings suggest that prolonged exposure of responsive neurons to BDNF and NT-3 down-regulates their stimulatory effects on PI hydrolysis.