Rapid stimulatory effects of brain-derived neurotrophic factor and neurotrophin-3 on somatostatin release and intracellular calcium rise in primary hypothalamic cell cultures

Although the long-lasting effects of neurotrophins have been extensively st udied, less data are available on their rapid effects, especially on peptid e release. In the present report, we investigated rapid effects of neurotro phins on somatostatin release and on intracellular calcium concentration (I Ca2+](i)) in primary cultures of hypothalamic neurons. RT-PCR experiments r evealed mRNA expression of the three high-affinity neurotrophin receptors t yrosine kinase (Trk) TrkA, TrkB and TrkC, indicating potential responses to their preferential ligands: nerve growth factor (NGF), brain-derived neuro trophic factor (BDNF) and neurotrophin 3 (NT-3), respectively. We demonstra ted that BDNF, and to a lesser extent NT-3, induced significant time- and c oncentration-dependent somatostatin release, while NGF was devoid of any ef fect. BDNF or NT-3 induction of somatostatin release was inhibited by the T rk inhibitors K-252a and genistein, whereas K-252b, a less effective inhibi tor, had no effect. BDNF- and NT-3-induced somatostatin release depended up on extra- and intracellular Ca2+ since it was completely abolished in the p resence of the Ca2+ chelators BAPTA (bis-(alpha -aminophenoxy)-ethane-N,N,N ' ,N ' -tetraacetic acid) or BAPTA-AM (bis-(alpha -aminophenoxy)ethane-N,N ,N ' ,N ' -tetraacetoxymethylester respectively. In addition, BDNF and NT-3 induced a sustained and rapid increase in [Ca2+](i) which depended on the extracellular Ca2+ concentration. MK-801 (dizocilpine) and tetrodotoxin (TT X) entirely blocked neurotrophin-evoked somatostatin release and [Ca2+](i) rise in response to BDNF and NT-3 application in most neurons. Neurotrophin -induced [Ca2+](i) rise was completely blocked by K-252a. The present resul ts are consistent with: (1) an indirect effect of neurotrophins on somatost atin release via endogenous glutamate release and subsequent NMDA receptor activation, (2) a major indirect effect of neurotrophins on Ca2+ rise in hy pothalamic neurons which very likely occurs through NMDA receptor activatio n. Taken altogether, these results indicate that BDNF and NT-3 can rapidly affect the activity of hypothalamic neurons. Copyright (C) 2001 S. Karger A G, Basel.