Brain-Derived Neurotrophic Factor and Neurotrophin-3 Enhance Somatostatin Gene Expression through a Likely Direct Effect on Hypothalamic Somatostatin Neurons

Abstract

Although neurotrophins (NTs) have been extensively studied as neuronal survival factors in some areas of the central nervous system, little is known about their function or cellular targets in the hypothalamus. To understand their functional significance and sites of action on hypothalamic neurons, we examined the effects of their cognate ligands on neuropeptide content and messenger RNA (mRNA) expression in somatostatin neurons present in fetal rat hypothalamic cultures. Treatments were performed in defined insulinfree medium between days 6 and 8 of culture, since the maximal effects of NTs on somatostatin content and mRNA expression were observed after 48-h incubations. Brain-derived neurotrophic factor and NT-3, but not nerve growth factor, induced a dose-dependent increase in somatostatin content, which was influenced by plating density. The same treatment increased somatostatin mRNA and immunostaining intensity of somatostatin neurons, but had no effect on the number of these labeled neurons. The increased levels of somatostatin (peptide and mRNA) induced by NTs were not blocked by tetrodotoxin or by glutamate receptor antagonists, suggesting that endogenous neurotransmitters (e.g. glutamate) were not involved in these effects. In contrast, the stimulatory effects were completely blocked by K-252a, an inhibitor of tyrosine kinase (Trk) receptors, whereas the less active analog K-252b was ineffective. Double-labeling studies demonstrated that both TrkB or TrkC receptors were located on somatostatin neurons. Our results show that, in rat hypothalamic cultures, brain-derived neurotrophic factor, and NT-3 have a potent stimulatory effect on peptide synthesis in somatostatinergic neurons, likely through direct activation of TrkB and TrkC receptors. (Endocrinology 140: 909 -916, 1999) T HE NEUROTROPHIN (NT) family, a group of structurally related neurotrophic factors, all expressed in neurons of the central nervous system, includes nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), NT-3, NT-4/5, and NT-6 (1-6). Their biological effects are mediated by high-affinity tyrosine kinase (Trk) receptors, although all bind to a low-affinity receptor (p75) (7). NGF binds to TrkA receptors, BDNF preferentially activates Trk B receptors, whereas NT-3 mainly interacts with TrkC receptors but also stimulates TrkB receptors. Trk receptor activation initiates differentiation and survival processes in selected neuronal populations (8).Within the hypothalamus, moderate levels of BDNF, low levels of NGF, and no NT-3 messenger RNA (mRNA) have been localized in adult or newborn rats (9 -15). It is noteworthy that, besides the hippocampus, the hypothalamus is the region where the highest levels of BDNF protein are found in adult rat brain (16,17). In situ hybridization studies have shown that, in adult rats, TrkB and TrkC mRNA are expressed in most cells of all hypothalamic nuclei, whereas TrkA mRNA is rarely detected in the hypothalamus (13, 18). In RT-PCR studies, we detected the expression of the three...

Involvement of brain-derived neurotrophic factor in the regulation of hypothalamic somatostatin in vivo

“…In contrast, 48 h later BDNF exerted a stimulatory effect on all of these parameters. These late effects are in line with our previous data showing a BDNF stimulatory effect on SRIH synthesis in cultured hypothalamic neurons after a 48 h treatment (Rage et al 1999).…”

“…In BDNFknockout mice, the density of SRIH cells is drastically reduced in hippocampus and cortex (Gro e et al 2005). In hypothalamic cultures, BDNF has been reported to have a stimulatory effect on SRIH content and mRNA expression (Rage et al 1999) and on differentiation (Loudes et al 1999). …”

<b>Neurotrophins in Neuroendocrine Control: Brain Derived-neurotrophic Factor (BDNF) and Somatostatin Involvement in the Stress Response and Reproductive Physiology <b/>

“…In addition, we have shown, with double immunocytochemical staining, that somatostatin neurons possess BDNF receptors, i.e. TrkB (Rage et al, 1999). These data provide morphological and functional evidence indicating a direct effect of BDNF on somatostatin neurons in vivo.…”

“…This effect clearly mimicks the stimulatory effect of stress on somatostatin release (Benyassi et al, 1993). In contrast, since stress induces a BDNF mRNA decrease in the PeV nucleus (Arancibia et al, 2000 b ) and given that BDNF seems to stimulate somatostatin synthesis (Rage et al, 1999), a decrease in BDNF due to a stress stimulus could result in a decrease in somatostatin mRNA, as observed in ISH experiments following stress (Arancibia et al, 2000 b ). If this decrease in the messenger results in diminished release, this effect might arise from activation of ultra-short feedback, which might be involved in the somatostatin regulation (Epelbaum et al, 1986).…”

Protective Role for Protease-Activated Receptor-2 against Influenza Virus Pathogenesis via an IFN-γ-Dependent Pathway

“…Total RNA were extracted and RT-PCR analysis was done as previously described (25,26). The following primers were used for amplification: actin, 5Ј-ATTCCTATGTGGGCGACGAGGC CCA-3Ј (forward) and 5Ј-TTGGCGTACAGGTCTTTGCGGATGTC-3Ј (reverse); PAR 2 , 5Ј-AGAAGCCTTATTGGTAAGGTT-3Ј (forward) and 5Ј-AACATCATGACAGGTCGTGAT-3Ј (reverse); and M2, 5Ј-AAGAC CAATCCTGTCACCTCTGA-3Ј (forward) and 5Ј-CAAAGCGTCTA CGCTGCAGTCC-3Ј (reverse).…”