Elevated levels of neurotrophins in human biceps brachii tissue of amyotrophic lateral sclerosis

Previous studies suggest that neurotrophins support regeneration and survival of injured motoneurons. Based on these findings, brain-derived neurotrophic factor (BDNF) has been clinically investigated for its therapeutic potential in amyotrophic lateral sclerosis (ALS), a rapidly progressing and fatal motoneuronal disease. We questioned whether imbalances of neurotrophic levels are indeed involved in the pathology of ALS. Therefore the expression of nerve growth factor (NGF), BDNF, neurotrophin-3 (NT-3), and neurotrophin-4/5 (NT-4/5) was investigated in postmortem muscle tissue of the biceps from 15 patients with neuropathologically confirmed sporadic ALS and 15 age-matched controls. Using mRNA analysis techniques and quantitative protein measurements, we have demonstrated that both mRNA and protein levels of all four neurotrophins are increased in muscle tissue of ALS patients. The production levels displayed a disease duration dependency and different expression patterns emerged for the four neurotrophins. Whereas the early phase of the disease was characterized by a strong upregulation of BDNF, levels of NGF, NT-3, and NT-4/5 gradually increased in the course of the disorder, peaking at later stages. We conclude that decreased neurotrophic support from muscle tissue is most likely not the cause of motoneuron degeneration in ALS. On the contrary, our results suggest that degenerating motoneurons in ALS are exposed to elevated levels of muscle-derived neurotrophins.     

Regulation of Neurotrophin mRNA Expression in the Rat Brain by Glucocorticoids

Northern blot analysis was used to examine the effects of glucocorticoids on neurotrophin mRNA expression in the rat cerebral cortex and hippocampus. The results show that 3 days after adrenalectomy the mRNA levels for nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) decreased significantly in both these regions. In adrenalectomized animals given dexamethasone replacement the mRNA levels for the three neurotrophins were restored to control levels. The effect of a single dose of dexamethasone (5 mg/kg) administered i p. to intact animals on the expression of neurotrophins was also examined. NGF and NT-3 mRNAs showed a 2.5-fold and a 1.4-fold increase, respectively, during the first 4 h after the injection. The increase was followed by a decrease, with levels -50% of control 24 and 48 h after the injection. In contrast, the level of BDNF mRNA did not change during the first 10 h after the injection, but decreased to 70% of control 48 h after the injection. These data indicate that glucocorticoids regulate neurotrophin mRNA expression both in the cortex and in the hippocampus, and suggest further that the known effects of glucocorticoids on neuronal survival in the brain could be due to changes in the levels of neurotrophins in the brain.     

Different expressions of BDNF, NT3, and NT4 in muscle and nerve after various types of peripheral nerve injuries

The changes in the expression of brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4) in the rat neuromuscular system as a result of three different types of sciatic nerve injuries have been evaluated. The changes in mRNA and protein levels for BDNF, NT-3, and NT-4 in the soleus muscle and sciatic nerve were assessed 4-28 days after sciatic nerve transection (neurotmesis), sciatic nerve crush (axonotmesis), and mild acute compression (neurapraxia). BDNF mRNA levels increased dramatically with nerve transection in the soleus muscle and the sciatic nerve 7-14 days after injury, whereas the changes were low in other types of injury. The changes of protein levels for BDNF were also similar. The mRNA and the protein levels of NT-3 in the soleus muscle did not show any significant difference. The mRNA for NT-4 in the soleus muscle decreased from 4 to 14 days after sciatic nerve transection, and the protein level was also minimum 14 days after sciatic nerve transection. Our results indicate that the neurotrophic factors in the neuromuscular system could play a role in differentiating peripheral nerve injury.     

Highly selective effects of nerve growth factor,brain-derived neurotrophic factor,and neurotrophin-3 on intact and injured basal forebrain magnocellular neurons

Cholinergic neurons of the basal nucleus complex (BNC) respond to nerve growth factor (NCF), the first member of a polypeptide gene family that also includes brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4/5 (NT-4/5), NGF, BDNF, and NT-3 are enriched in hippocampus. In addition, NGF and, more recently, BDNF have been shown to stimulate the cholinergic differentiation and enhance the survival of BNC cells in vitro. The present investigation was designed to test, in a comparative fashion, the in vivo effects of human recombinant NGF, BDNF, and NT-3 with confirmed activities in vitro on cholinergic and γ-aminobutyric acid (GABA)-ergic BNC neurons. The specific questions asked were whether and, to what extent, biologically active recombinant neurotrophins stimulate the transmitter phenotypes of intact cholinergic and GABAergic neurons of the BNC, and whether, and to what extent, recombinant neurotrophins protect the transmitter phenotypes of axotomized cholinergic and GABAergic neurons of the BNC following complete transections of the fimbria-fornix (measured by ChAT mRNA hybridization). Our results confirm the profound stimulatory and p^75NGFR expression in both intact and axotomized cholinergic neurons and to exert minor effects on some cholinergic markers (e.g., ChAT immunoreactivity). NT-3 had no influence on GABAergic neurons. Taken together, these results indicate that, despite their significant sequence homologies and their shared abundance in target fields of BNC neurons, NGF, BDNF, and NT-3 show striking differences in their efficacies as cholinergic trophic factors. GABAergic neurons of the BNC are resistant to neurotrophins. The result of the present investigation establish that NGF excels among neurotrophins as a trophic factor for intact and injured basal forebrain cholinergic neurons. © 1994 Wiley-Liss, Inc.     

Neuronal signals regulate neurotrophin expression in oligodendrocytes of the basal forebrain

Previous studies suggest that oligodendrocytes express trophic molecules, including neurotrophins. These molecules have been shown to influence nearby neurons. To determine whether neuronal signals may, in turn, affect oligodendrocyte-derived trophins, we examined regulation of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) mRNA expression in cultured oligodendrocytes of the basal forebrain. Neuronal signals had distinct effects on individual neurotrophins. KCl elicited increases in BDNF mRNA, but did not affect expression of NGF or NT-3. The cholinergic agonist, carbachol, increased expression of NGF, but did not affect expression of BDNF or NT-3. Glutamate elicited a decrease in BDNF, but did not affect expression of NGF or NT-3. This glutamate effect is not due to toxicity, since the number of total cells was unchanged, while the number of mature myelin basic protein positive (MBP+) cells increased. Our observations suggest that individual neuronal signals distinctly influence the trophic function of oligodendrocytes.     

Distinct usages of phospholipase C gamma and Shc in intracellular signaling stimulated by neurotrophins

Nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), members of the neurotrophin family, bind to and activate TrkA, TrkB and TrkC, respectively, members of the Trk receptor tyrosine kinase family, to exert various effects including promotion of differentiation and survival, and regulation of synaptic plasticity in neuronal cells. Many reports have suggested that different neurotrophins show distinct biological functions, although molecular mechanisms by which neurotrophins exert their different functions remain unclear. In the present study, we found distinct usages of phospholipase Cgamma (PLCgamma) and Shc in intracellular signaling stimulated by neurotrophins. BDNF stimulated much stronger interactions of PLCgamma with Trk than NGF and NT-3 in PC12 cells stably expressing TrkB and cultured cerebral cortical neurons, respectively, although BDNF, NGF and NT-3 induced similar levels of tyrosine phosphorylation of Trk. Furthermore, the cultured cortical neurons showed large PLCgamma-dependent increases in intracellular Ca(2+) levels in response to BDNF compared with NT-3. In Shc signaling, NGF, but not BDNF, displayed interactions between Trk and Shc in a phenylarsine oxide (PAO; an inhibitor of tyrosine phosphatase)-dependent manner in TrkB-expressing PC12 cells. These results indicated that neurotrophins stimulate distinct kinds of interactions between Trk and PLCgamma and between Trk and Shc. These differences may lead to the distinct biological functions of neurotrophins.     

Neurotrophins regulate proliferation and survival of two microglial cell lines in vitro

Microglia are thought to play a key role in the development and regeneration of the central nervous system although the mechanisms regulating their presence and activity are not fully understood. Substantial evidence suggests that members of the neurotrophin family such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 and -4 (NT-3/4) have a dramatic effect on both neurons and perineuronal cells. This study employed two murine microglial lines, BV-2 and N9, to examine the action of these neurotrophins on the mitotic activity and survival of microglia in vitro. Neurotrophins were incorporated into the media at the time of plating and cell number and levels of mitochondrial dehydrogenase activity (MTT) were determined at various time points in vitro. NGF increased cell number and MTT levels of both cell lines in a dose-dependent manner. BV-2 was more sensitive to NGF than N9. Similar responses were elicited by BDNF, although the sensitivity of each cell line was different than that found for NGF. NT-3 and NT-4 had no effect on cell proliferation. However, NT-4 had an effect on the survival of BV-2 and N9 cells. The response of these cells to neurotrophins was blocked by K252a, a tyrosine kinase inhibitor, suggesting that actions of neurotrophins were mediated by high-affinity tyrosine kinase receptors (Trk). Immunolocalization studies revealed positive Trk (pan) reactivity in the above cell lines and in primary microglia, but an absence of the low-affinity p75 neurotrophin receptor. Western blot analysis supported the above observations. These studies suggest that in addition to their neurotrophic actions, NGF and BDNF may also regulate microglial dynamics, thereby influencing the surrounding milieu during neuronal regeneration.     

Preparation of brain-derived neurotrophic factor-and neurotrophin-3-secreting schwann cells by infection with a retroviral vector

One reason that the central nervous system of adult mammals does not regenerate after injury is that neurotrophic factors are present only in low concentrations in these tissues. Recent studies have shown that the application of brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) acts to encourage the regrowth of motor and sensory fibers after spinal cord injury. Other studies have reported that the regrowth of axons after injury was enhanced by the implantation of Schwann cells, which normally secrete BDNF and NT-3. The purpose of the present study was to genetically modify Schwann cells to secrete increased amounts of BDNF or NT-3 by infection with a retroviral vector. Retroviral vectors were constructed by the ligation of BDNF or NT-3 cDNA to the LXSN vector. Viruses were generated from the plasmid forms of the vectors by transient transfection of PA317 amphotrophic retroviral packaging cells. Viruses were harvested and used to infect the human Schwann cell line designated NF-1T. Northern blot analysis of poly (A⁺) RNA prepared from Schwann cells that were infected with BDNF- or NT-3-containing virus showed the presence of BDNF or NT-3 mRNA. An enzyme-linked immunosorbent assay (ELISA) for BDNF and NT-3 was performed on media the cells were grown in, and on cellular extracts prepared from the BDNF- and NT-3-infected Schwann cells. The ELISA results demonstrated that the Schwann cells were secreting increased levels of immunologically active BDNF or NT-3. Immunocytochemical staining of these cells revealed the presence of these two neurotrophic factors located in perinuclear granules. These neurotrophic factor-secreting Schwann cells are currently being evaluated for their efficacy in the treatment of spinal cord injury.     

Age-dependent time course of cerebral brain-derived neurotrophic factor, nerve growth factor, and neurotrophin-3 in APP23 transgenic mice

Neurotrophins, including brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), and neurotrophin-3 (NT-3), have repeatedly been shown to be involved in the pathophysiology of Alzheimer's disease (AD). Recent studies have claimed that these neurotrophic factors are important tools for therapeutic intervention in neurodegenerative diseases. So far, little is known about the age- and disease-modulated time course of cerebral neurotrophins. Therefore, we have studied protein concentrations of BDNF, NGF, and NT-3 in different brain areas and sciatic nerve, a neurotrophin-transporting peripheral nerve, in a well-characterized AD model of amyloid precursor protein-overexpressing rodents (APP23 mice) at the ages of 5.0, 10.5, and 20.0 months. In APP23 mice, there was a significant increase of BDNF and NGF in the frontal and occipital cortices (for BDNF also in the striatum) of old 20.0-month-old mice (with respect to median values up to 8.2-fold), which was highly correlated with amyloid concentrations of these brain areas. Median values of NGF and NT-3 showed up to a 6.0-fold age-dependent increase in the septum that was not detectable in APP23 mice. Hippocampus, olfactory bulb, and cerebellum (except NT-3) did not show substantial age- or genotype-related regulation of neurotrophins. In the sciatic nerve, BDNF and NGF levels are increased in5-month-old APP23 mice and decrease with age to control levels. In conclusion, APP23 mice show a genotype-dependent increase of cortical BDNF and NGF that is highly correlated with amyloid concentrations and may reflect an amyloid-related glia-derived neurotrophin secretion or an altered axonal transport of these neurotrophic factors.     

Expression of trkB and trkC receptors and their ligands brain-derived neurotrophic factor and neurotrophin-3 in the murine amygdala

The neurotrophin brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) and their cognate receptors, trkB and trkC, have a variety of physiological brain functions, ranging from cell survival to mechanisms involved in learning and memory and long-term potentiation (LTP). LTP can be induced in the cortex and hippocampus, as well as within the amygdala. However, the role of neurotrophins in amygdalar LTP is largely unknown. Expression patterns of BDNF and NT-3 and their cognate receptors in the adult mouse amygdala have not been analyzed in detail. We have therefore examined the expression of trkB, trkC, BDNF, and NT-3 mRNA and protein in different amygdalar nuclei as well as in the hippocampal areas CA1-CA3 and the dentate gyrus. The distribution pattern of trkB, trkC, BDNF, and NT-3 mRNA in the murine hippocampus is comparable to that seen in rats. Within most amygdalar nuclei, a moderate BDNF mRNA expression was found; however, BDNF mRNA was virtually absent from the central nucleus. No expression of NT-3 mRNA was found within the amygdala, but trkC mRNA-expressing cells were widely distributed within this brain region. trkB mRNA was strongly expressed in the amygdala. Because trkB is expressed in a full-length and a truncated form (the latter form is also expressed by nonneuronal cells), we also investigated the distribution of full-length trkB mRNA-expressing cells and could demonstrate that this version of trkB receptors is also widely expressed in the amygdala. These results can serve as a basis for studies elucidating the physiological roles of these receptors in the amygdala.     

Expression of neurotrophins BDNF and NT-3, and their receptors in rat brain after administration of antipsychotic and psychotrophic agents

We have investigated the potential role of neurotrophic factors in antipsychotic drug action by examining the effects of antipsychotic and psychotropic treatments on the mRNA expression of brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and their receptors, trkB and trkC, respectively, in rat brain. Neither acute nor chronic clozapine treatment significantly affected the expression of these mRNAs in any brain area investigated, except for a decrease in trkB expression in the granule cells of the olfactory bulb. We then examined the effects of the psychotropic agent MK-801. MK-801 (5 mg/kg; 4h) significantly increased BDNF mRNA in the entorhinal cortex, but did not influence NT-3, trkB, or trkC expression in any brain area except for the olfactory bulb. The induction of BDNF mRNA by MK-801 was attenuated by pre-treatment (1 h prior to MK-801 administration) with the antipsychotics, clozapine (25 mg/kg) and haloperidol (2 mg/kg), but not with the antidepressant desipramine (15 mg/kg). Finally, we confirmed that the effects of MK-801 on BDNF mRNA were reflected in the respective changes in BDNF protein levels: MK-801 significantly increased anti-BDNF reactivity in the entorhinal cortex (126 ± 7% of control) while concomitantly decreasing in the hippocampus (71 ± 2% of control). These data do not support the hypothesis that neurotrophins play an important role in antipsychotic drug action, but rather suggest that induction of BDNF in the entorhinal cortex may play a significant role in the psychotropic action of MK-801.     

Short-term response of postnatal rat vestibular neurons following brain-derived neurotrophic factor or neurotrophin-3 application

The effects of the application of brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) neurotrophins on the intracellular calcium level ([Ca²⁺]_i) were studied in vestibular ganglion neurons (VGNs) from postnatal day 3 (P3) rats cultured for 50 hr. We first assessed the expression of trkB and trkC mRNA receptors in cultured VGNs. Immunobloting and immunocytochemistry confirmed the presence of the neurotrophin receptors on neurons. Both neurotrophins induced transient [Ca²⁺]_i elevations in VGNs: BDNF-treated neurons responded in 65% and NT-3-treated neurons in 56%. The responses could be inhibited by anti-BDNF or anti-NT-3 antibodies. The [Ca²⁺]_i elevation was dependent on extracellular calcium since it was abolished in calcium-free medium but also implicates the release of calcium from intracellular stores as tested by prior depletion with thapsigargin. Our results suggest the implication of a short-term calcium regulation in VGNs, which could reflect specific fast effects of neurotrophins in the early postnatal rat vestibular system. J. Neurosci. Res. 50:443–449, 1997. © 1997 Wiley-Liss, Inc.     

Function and evolution in the NGF family and its receptors.

The gene family of neurotrophins includes nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4). Recently, neurotrophin-5 (NT-5), a possible mammalian homologue to NT-4 described in the frog Xenopus, has been cloned in man and rat. The neurotrophins stimulate survival and differentiation of a range of target neurons by binding to cell surface receptors. The structure of NGF has recently been clarified from crystallographic data. The similarities between the different neurotrophins are substantial with the variable regions, giving specificity to each of the family members, being localized to some exposed loop regions. Low-affinity binding (Kd of 10(-9) M) of all tested neurotrophins is mediated via a 75 K glycoprotein (LNGFR) that has been cloned and characterized. A 140 K tyrosine protein kinase encoded by the proto-oncogene trk has been found to bind NGF with high affinity (Kd of 10(-11) M) and to evoke the cellular neurotrophic responses. In addition, a protein encoded by the trk-related gene trkB has been shown to bind BDNF. Recently, a third member of the trk family, trkC, has been cloned and demonstrated to function as a high-affinity receptor for NT-3. The expression of trk and LNGFR mRNA are co-localized in the rat brain to the medial septal nucleus and the nucleus of Broca's diagonal band containing the NGF-responsive magnocellular cholinergic neurons projecting to hippocampus and cerebral cortex. In sharp contrast, the pattern of expression of trkB is widely spread in many areas of the cortex as well as lateral septum. The trkB protein might serve general functions in large areas of the cortex. Site-directed mutagenesis and expression of recombinant chimaeric neurotrophin proteins have made it possible to localize a likely region for the interaction between NGF and the LNGFR. This region could be altered, resulting in the total loss of LNGFR binding by the mutant NGF protein without affecting the binding to the trk receptor which was sufficient for the full biological activity. Cladistic analysis of likely phylogenies within the neurotrophins shows BDNF and NT-4 to be most closely related whereas NGF may be the sister group to NT-3, BDNF, and NT-4. Neurotrophins offer obvious clinical possibilities for treatment of neurodegenerative diseases.     

Brain-derived Neurotrophic Factor is a Survival Factor for Cultured Rat Cerebellar Granule Neurons and Protects them Against Glutamate-induced Neurotoxicity

We have studied the effects of different neurotrophins on the survival and proliferation of rat cerebellar granule cells in culture. These neurons express trkB and trkC, the putative neuronal receptors for brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) respectively. Binding studies using iodinated BDNF and NT-3 demonstrated that both BDNF and NT-3 bind to the cerebellar granule neurons with a similar affinity of ˜ 2x10^-9 M. The number of receptors per granule cell was surprisingly high, ∼30x10^-4 and 2x 10⁵ for BDNF and NT-3, respectively. Both NT-3 and BDNF elevated c-fos mRNA in the granule neurons, but only BDNF up-regulated the mRNA encoding the low-affinity neurotrophin receptor (p75). In contrast to NT-3, BDNF acted as a survival factor for the granule neurons. BDNF also induced sprouting of the granule neurons and significantly protected them against neurotoxicity induced by high (1 mM) glutamate concentrations. Cultured granule neurons also expressed low levels of BDNF mRNA which were increased by kainic acid, a glutamate receptor agonist. Thus, BDNF, but not NT-3, is a survival factor for cultured cerebellar granule neurons and activation of glutamate receptor(s) up-regulates BDNF expression in these cells.     

Brain derived neurotrophic factor and neurotrophin-4 employ different intracellular pathways to modulate norepinephrine uptake and release in rat hypothalamus

Classical actions of the neurotrophin family are related to cellular survival and differentiation. Moreover, acute effects of neurotrophins have been reported. Although neurotrophins effects on synaptic transmission at central nervous system level have been largely studied, acute effects of neurotrophins on hypothalamic noradrenergic transmission are still poorly understood. Thus, we have studied the effects of the neurotrophin family members nerve growth factor (NGF), brain derived neurotrophic factor (BDNF) and neurotrophin-4 (NT-4) on norepinephrine (NE) neuronal uptake and its evoked release, as well as the receptor and the intracellular pathways involved in these processes in rat hypothalamus.Present results indicate that BDNF increased NE uptake and decreased its evoked release through a mechanism that involve Trk B receptor and phospholipase C. Moreover, NT-4, also through the Trk B receptor, decreased NE uptake and its evoked release by activating phosphatidylinositol 3-OH-kinase. These effects were observed in whole hypothalamus as well as in the anterior hypothalamic zone. On the other hand, NGF did not modify noradrenergic transmission.In conclusion, we showed for the first time that BDNF and NT-4 activate two different intracellular signalling pathways through a Trk B receptor dependent mechanism. Furthermore, present findings support the hypothesis that BDNF and NT-4 acutely applied, could be considered as modulators of noradrenergic transmission and thus may regulate hypothalamic physiological as well as pathophysiological responses.     

Neurotrophin and neuropeptide expression in mouse brain is regulated by knockout of the norepinephrine transporter

Neurotrophins [e.g. nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3)] and neuropeptides such as corticotropin-releasing factor (CRF) are reported to contribute to the action of antidepressants (ADs). Norepinephrine transporter (NET) knockout (NETKO) mice represent a model of chronic AD treatment. In the present study, we examined brain-region-specific regulations of NT-3, NGF, BDNF and CRF at the mRNA and protein level in NET wild-type (NETWT) and NETKO mice by means of quantitative real-time PCR (qPCR) and two-site enzyme-linked immunosorbent assays (ELISAs), respectively. NETKO-induced changes were detected for NT-3 in olfactory bulb, brainstem and whole brain at the mRNA and for olfactory bulb at the protein level, for NGF mRNA and protein in olfactory bulb, cerebellum and brainstem and for CRF mRNA and protein in the hippocampus. In contrast, BDNF levels remained unaltered. Our results suggest that NETKO mice represent a useful model to examine gene regulation of downstream targets potentially involved in the action of ADs. We could delineate NT-3, NGF and CRF as being regulated in distinct brain regions by KO of the NET.     

Neurotrophins modulate neuron-glia interactions at a vertebrate synapse

Neurotrophins are important modulators of synaptic function at both developing and mature synapses in the CNS and PNS. At the neuromuscular junction (NMJ), neurotrophins, as well as perisynaptic Schwann cells (PSCs) are critical for the long-term maintenance and stability of the synapse. Considering this correlation and the acute interactions that occur at the synapse between PSCs and the nerve terminal, we wondered if neurotrophins could also be involved in neuron-glia signalling. To test if neurotrophins were able to signal to PSCs we used brief applications of neurotrophin-3 (NT-3), brain-derived neurotophic factor (BDNF) or nerve growth factor (NGF; 100 ng/mL). Soleus muscles of mice were incubated with the Ca(2+) indicator Fluo-4AM and Ca(2+) responses in PSCs were elicited through nerve stimulation (50 Hz, 30 s). Our results indicate that acute application of both NT-3 and BDNF, but not NGF, increased PSC Ca(2+) responses. Investigation of the mechanisms involved in these increases revealed distinct pathways for BDNF and NT-3. BDNF increased PSC responsiveness through potentiation of ATP responses while NT-3 modulated muscarinic acetylcholine receptor signalling. Using local applications of the neurotrophins, we found that both neurotrophins were able to elicit Ca(2+) responses in PSCs where BDNF used a phospholipase C-inositol 1,4,5-triphosphate (PLC-IP(3)) mechanism, while NT-3 required extracellular Ca(2+). Our results demonstrate a neurotrophin-dependent modulation of neuron-glia signalling through differential mechanisms employed by NT-3 and BDNF. Hence, neurotrophins precisely and differentially regulate PSC functions through modulation of either purinergic or cholinergic signalling pathways.     

Positive Feedback between Acetylcholine and the Neurotrophins Nerve Growth Factor and Brain-derived Neurotrophic Factor in the Rat Hippocampus

In the rat hippocampus, nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are synthesized by neurons in an activity-dependent manner. Glutamate receptor activation increases whereas GABAergic stimulation decreases NGF and BDNF mRNA levels. Here we demonstrate that NGF and BDNF mRNA and NGF protein are up-regulated in the rat hippocampus by the activation of muscarinic receptors. Conversely, NGF and BDNF enhance the release of acetylcholine (ACh) from rat hippocampal synaptosomes containing the nerve endings of the septal cholinergic neurons. NGF also rapidly increases the high-affinity choline transport into synaptosomes. The reciprocal regulation of ACh, NGF and BDNF in the hippocampus suggests a novel molecular framework by which the neurotrophins might influence synaptic plasticity.