Activity-dependent scaling of GABAergic synapse strength is regulated by brain-derived neurotrophic factor

The homeostatic plasticity hypothesis suggests that neuronal activity scales synaptic strength. This study analyzed effects of activity deprivation on GABAergic synapses in cultured hippocampal neurons using patch clamp electrophysiology to record mIPSCs and immunocytochemistry to visualize presynaptic GAD-65 and the gamma2 subunit of the GABA(A) receptor. When neural activity was blocked for 48 h with tetrodotoxin (TTX, 1 microM), the amplitude of mIPSCs was reduced, corresponding with diminished sizes of GAD-65 puncta and gamma2 clusters. Treatment with the NMDA receptor antagonist APV (50 microM) or the AMPA receptor antagonist DNQX (20 microM) mimicked these effects, and co-application of brain-derived neurotrophic factor (BDNF, 100 ng/mL) overcame them. Moreover, when neurons were treated with BDNF alone for 48 h, these effects were reversed via the TrkB receptor. Overall, these results suggest that activity-dependent scaling of inhibitory synaptic strength can be modulated by BDNF/TrkB-mediated signaling.     

Prevention of muscimol-induced long-term depression by brain-derived neurotrophic factor

1. The authors have recently reported a new protocol for inducing long-term depression through activation of GABAA receptors in the hippocampal slices. This long-term depression is reversed by bicuculline and potentiated by neurosteroids such as alphaxalone. It was also shown that glutamate receptor activity is not involved in the induction of this novel type of long-term depression. Brain derived neurotrophic factor is a member of the neurotrophins family widely expressed in the central nervous system. There is increasing evidence that indicate an important role for brain-derived neurotrophic factor in synaptic plasticity. It has been reported that brain-derived neurotrophic factor level is downregulated by GABA system. The present study investigated a possible relation between muscimol-induced long-term depression and brain-derived neurotrophic factor level. 2. Extracellular recordings were made in the CA1 pyramidal cell layer of rat hippocampal slices following orthodromic stimulation of Schaffer collateral fibers in stratum radiatum. 3. It was observed that brain-derived neurotrophic factor at concentration that did not have any effect itself on the population spike, prevents the induction of long-term depression by muscimol. In addition to this, K-252a an inhibitor of Trk type kinase blocked the prevention of muscimol-induced LTD by brain-derived neurotrophic factor. 4. The results suggest that there is an interaction between muscimol-induced long-term depression and brain-derived neurotrophic factor and may explain the post receptor mechanism of muscimol-induced long-term depression through a bilateral relation between GABAA activity and brain-derived neurotrophic factor.     

Stabilization of growing retinal axons by the combined signaling of nitric oxide and brain-derived neurotrophic factor.

The pattern of axonal projections early in the development of the nervous system lacks the precision present in the adult. During a developmental process of refinement, mistargeted projections are eliminated while correct projections are retained. Previous studies suggest that during development nitric oxide (NO) is involved in the elimination of mistargeted retinal axons, whereas brain-derived neurotrophic factor (BDNF) may stabilize retinal axon arbors. It is unclear whether these neuromodulators interact. This study showed that NO induced growth cone collapse and retraction of developing retinal axons. This effect was not attributable to NO-induced neurotoxicity. BDNF protected growth cones and axons from the effects of NO. This effect was specific to BDNF, because neither nerve growth factor (NGF) nor neurotrophin-3 (NT-3) prevented NO-induced growth cone collapse and axon retraction. Exposure to both BDNF and NO, but not either factor alone, stabilized growth cones and axons. Stabilized axons exhibited minimal retraction or extension. This response appears to be a new axon "state" and not simply a partial amelioration of the effect of NO, because lower doses of BDNF or NO allowed axon extension. Furthermore, BDNF/NO-induced growth cone stabilization correlated with the appearance of a cytochalasin D-resistant population of actin filaments. BDNF protection from NO likely was mediated locally at the level of the growth cone, because growth cones or individual filopodia in contact with BDNF-coated beads were protected from NO-induced collapse. These findings suggest a cellular mechanism by which some axonal connections are stabilized and some are eliminated during development.     

Upregulation of brain-derived neurotrophic factor in the sensory pathway by selective motor nerve injury in adult rats

Selective motor nerve injury by lumbar 5 ventral root transection (L5 VRT) induces neuropathic pain, but the underlying mechanisms remain unknown. Previously, increased expression and secretion of brain-derived neurotrophic factor (BDNF) had been implicated in injury-induced neuropathic pain in the sensory system. In this study, as a step to examine potential roles of BDNF in L5 VRT-induced neuropathic pain, we investigated BDNF gene and protein expression in adult rats with L5 VRT. L5 VRT induced a dramatic upregulation of BDNF mRNA in intact sensory neurons in the ipsilateral L5 dorsal root ganglia (DRG), in non-neuronal cells in the ipsilateral sciatic nerve, and in motoneurons in the ipsilateral spinal cord. L5 VRT also induced de novo synthesis of BDNF mRNA in spinal dorsal horn neurons and in glial cells in the white matter of the ipsilateral spinal cord. Consistent with the mRNA expression pattern, BDNF protein was also mainly upregulated in all populations of sensory neurons in the ipsilateral L5 DRG and in spinal neurons and glia. Quantitative analysis by ELISA showed that the BDNF content in the DRG and sciatic nerve peaked on day 1 and remained elevated 14 days after L5 VRT. These results suggest that increased BDNF expression in intact primary sensory neurons and spinal cord may be an important factor in the induction of neuropathic pain without axotomy of sensory neurons.     

侧脑室内注入脑源性神经营养因子对AD小鼠酪氨酸激酶B及内源性BDNF表达的影响

目的 探讨侧脑室内注入脑源性神经营养因子(BDNF)对APP/PS1双转基因阿尔茨海默病(AD)小鼠酪氨酸激酶B (TrkB)及内源性BDNF表达的影响. 方法 10只10月龄APP/PS1雄性小鼠按随机数字表法分为2组,实验组5只,双侧侧脑室内注入BDNF;磷酸盐缓冲液(PBS)组5只,双侧侧脑室内注入PBS,为阳性对照组;干预时间均为6周.同时选择5只同窝生10月龄野生型小鼠,不予任何处理,为阴性对照组.采用荧光免疫组化法观察小鼠皮层区β-淀粉样蛋白(Aβ)斑块形态学改变,硫磺素S法检测致密斑的数量,同时检测小鼠皮层区TrkB、BDNF蛋白表达的情况. 结果 (1)治疗前、后BDNF组Aβ斑块总数分别为(101.58±7.86)个、(102.83±8.22)个,与PBS组(97.23±1 1.62)个、(103.6±6.46)个比较差异均无统计学意义(t=0.695、-0.171,P=-0.509、0.869);治疗6周后BDNF组Aβ斑块直径缩小至(34.65±9.33)μm,TS+斑块数量减少至(51.70±4.18)个,与PBS组(46.17±10.16)μm、(58.85±7.55)个比较,差异均具有统计学意义(t=-2.401、-2.536,P=0.047、0.039);(2)治疗后BDNF组TrkB、BDNF蛋白表达明显增强. 结论 侧脑室内注入BDNF减少了Aβ致密斑的形成,使Aβ蛋白沉积导致的神经毒性作用减弱,从而促进皮层区TrkB表达增强,导致内源性BDNF表达增强,可在一定程度上延缓AD小鼠的病程.     

Expression disparity of brain-derived neurotrophic factor immunoreactivity and mRNA in ischemic hippocampal neurons

We studied the spatial and temporal expression of BDNF immunoreactivity and mRNA in the hippocampal formation after transient forebrain ischemia in gerbils. Our study demonstrated that in the vulnerable CA1 neurons, there was a prolonged expression disparity between BDNF immunoreactivity and mRNA and the BDNF level was reduced, in contrast to the ischemia-resistant dentate gyrus neurons that showed transient expression disparity and maintained the BDNF level. This expression disparity of the neurotrophic factor may be related to delayed neuronal death. Double immunostaining showed that reactive astroglia and microglia could express BDNF, suggesting a possible involvement of these cells in the mechanism of neuronal survival after ischemia.     

Expression of brain-derived neurotrophic factor in rat hippocampus following focal cerebral ischemic injury

BACKGROUND： The functional role of brain-derived neurotrophic factor （BDNF） is enhanced following cerebral ischemic injury providing neurons with an important self-protection mechanism in early stage ischemia/hypoxia. OBJECTIVE： To investigate the expression pattern of BDNF in different rat hippocampal regions following focal cerebral ischemic injury. DESIGN, TIME AND SETTING： We performed a comparative and neurobiological study of animals in the Department of Histology and Embryology and the Central Laboratory, Hebei Medical University from March to December 2003. MATERIALS： Forty healthy Sprague Dawley rats were randomly divided into a cerebral ischemia group and a sham operation group, with 20 rats per group. METHODS： In the cerebral ischemia group, we occluded the right middle cerebral artery with a suture, threading it to a depth of 17-19 mm. In the sham operation group, the threading depth was approximately 10mm. MAIN OUTCOME MEASURES： We analyzed the expression of BDNF in different hippocampal regions by immunohistochemical staining of brain sections taken on post-operative days 7, 14, 21 and 30. RESULTS： Sham operation group： We observed a number of a few BDNF-positive cells with light staining in the hippocampal CA1 CA4 regions and dentate gyms. Cerebral ischemia group： compared with the sham operation group, BDNF increased on day 7, significantly increased on day 14, and reached a peak on day 21 （P 〈 0.05）. Furthermore, irnmunologically reactive products were darkly stained, and neurons had long axons. BDNF was particularly highly expressed in the hippocampal CA3 and CA4 regions and dentate gyms. CONCLUSION： Cerebral ischemic injury can damage hippocampal neurons. Neurons can increase their anti-ischemic capacity by increasing BDNF expression in the hippocampal CA3 and CA4 regions and dentate gyms.     

Expression of hippocampal brain-derived neurotrophic factor and its receptors in Stanley consortium brains

Several lines of evidence implicate BDNF in the pathophysiology of psychiatric illness. BDNF polymorphisms have also been associated with the risk of schizophrenia and mood disorders. We therefore investigated whether levels of (pro)BDNF and receptor proteins, TrkB and p75, are altered in hippocampus in schizophrenia and mood disorder and whether polymorphisms in each gene influenced protein expression. Formalin-fixed paraffin-embedded hippocampal sections from subjects with schizophrenia, major depressive disorder (MDD), bipolar disorder (BPD) and non-psychiatric controls were obtained from the Stanley Foundation Neuropathology Consortium. (pro)BDNF, TrkB(T1) and p75 protein densities were quantified by immunoautoradiography and DNA extracted from each subject was used to determine the effect of genotype on protein expression. In MDD, reductions in (pro)BDNF were seen in all layers of the right but not the left hippocampus with no changes in the dentate gyrus. The pattern was similar but less marked for BPD. In addition, BPD but not MDD patients, had bilateral reductions in p75 in hippocampal layers but not in dentate gyrus. No changes in TrkB(T1) density were seen in any diagnosis. These findings suggest MDD and BPD may share impairment in (pro)BDNF expression. However, BPD may involve impairments of both (pro)BDNF and p75 receptor, whereas MDD may involve impaired (pro)BDNF alone. Moreover, the lateralisation of changes may indicate a role of asymmetry in vulnerability to MDD. Hippocampal (pro)BDNF and receptor levels were also affected by genotype, suggesting that allelic variations are important in the hippocampal abnormalities seen in these psychiatric disorders.     

In vivo insular cortex LTP induced by brain-derived neurotrophic factor

Recent studies suggest that brain-derived neurotrophic factor (BDNF) plays a critical role in long-term synaptic plasticity in the adult brain. Previous studies on the insular cortex (IC), a region of the temporal cortex implicated in the acquisition and storage of different aversive learning tasks, have demonstrated that tetanic stimulation of the basolateral nucleus of the amygdala (Bla) induces an N-methyl-D-aspartate (NMDA)-dependent form of long-term potentiation (LTP) in the IC of adult rats in vivo. Here, we show that acute intracortical microinfusion of BDNF induces a lasting potentiation of synaptic efficacy in the Bla-IC projection of anesthetized adult rats. This constitutes an in vivo demonstration of neurotrophin-induced potentiation of synaptic transmission in the neocortex. These findings support the concept that BDNF could be a synaptic messenger involved in activity-dependent synaptic plasticity.     

Nerve growth factor, brain-derived neurotrophic factor, and neurotrophin-3 are sorted to dense-core vesicles and released via the regulated pathway in primary rat cortical neurons

Neurotrophins (NTs) play an important role in the modulation of synaptic transmission and in morphological changes in synaptic structures. Although there is agreement that brain-derived neurotrophic factor (BDNF) is sorted to large dense-core vesicles (LDCVs) and released via the regulated secretory pathway, there has been some dispute regarding the mode of secretion of nerve growth factor (NGF) and neurotrophin-3 (NT-3), two structurally related members of the NT family. In this study, we examined the subcellular localization and release characteristics of NGF, BDNF, and NT-3 in adenovirus-infected primary cortical neurons. We found that all members of the NT family colocalized with markers for the endoplasmic reticulum and Golgi within cell bodies and in a punctate manner with a marker for LDCVs within processes. Moreover, their release was triggered by depolarization, indicating that NGF, BDNF, and NT-3 are released via the regulated secretory pathway. When neurons were coinfected with two separate adenoviruses coding for NGF or BDNF, both NTs showed almost complete vesicular colocalization within single cells, suggesting that different NTs might be packaged into shared vesicles. We also examined whether the two splice variants of NGF, the short and long precursors, differ in their release characteristics. We found that neurons infected with viruses coding for either splice variant released NGF in a regulated way. Overall, our study supports the notion that all members of the NT family undergo activity-dependent regulated release from neurons, enabling them to act as "synaptotrophins" on electrically active neurons.     

Expression of mRNA for brain-derived neurotrophic factor in the dorsal root ganglion following peripheral inflammation

It is well known that the nerve growth factor (NGF) may serve as a link between inflammation and hyperalgesia. Recent experiments showed that systemic injection of NGF dramatically stimulated the expression of brain-derived neurotrophic factor (BDNF) mRNA in the dorsal root ganglion (DRG). In the present study, we evaluated the change of BDNF mRNA in the DRG following peripheral inflammation and also observed colocalization of BDNF and trkA mRNAs by means of in situ hybridization histochemistry in rats. Peripheral tissue inflammation produced by an intraplantar injection of Freund's adjuvant into the paws significantly increased BDNF mRNA levels in the DRG and many neurons expressing trkA mRNA showed increased expession of BDNF mRNA. Intraplantar injection of antibody to NGF together with Freund's adjuvant prevented the increase in BDNF mRNA. These findings suggest that peripheral inflammation induces an increased expression of BDNF mRNA which is mediated by NGF in DRG.     

Infarct tolerance induced by intra-cerebral infusion of recombinant brain-derived neurotrophic factor

Neuronal expression of brain-derived neurotrophic factor (BDNF) has been implicated in the mechanism of infarct tolerance (resistance to stroke) (H. Yanamoto et al., Infarct tolerance accompanied enhanced BDNF-like immunoreactivity in neuronal nuclei, submitted to Brain Res.), a process that takes more than 7 days following a preconditioning of repetitive cortical spreading depression (CSD). To investigate whether an elevated level of BDNF protein in the brain solely protects neurons against temporary focal ischemia, recombinant (r)BDNF was infused into the rat neocortex. Recombinant BDNF (or vehicle: saline) was administered into the left neocortex via an implanted osmotic minipump for 2.5, 7, 10 or 14 days pre-ischemia, during ischemia and for 2 days post-ischemia (8 microgram in total) in male Sprague-Dawley rats (n=6 each). Temporary focal ischemia was induced in the left middle cerebral artery (MCA) territory by three-vessel occlusion of bilateral common carotid arteries (CCAs) and MCA for 2 h, and the cerebral infarct volume was analyzed 2 days after ischemia using TTC staining. Regional cerebral blood flow (rCBF) of the left neocortex was monitored after 14 days of intracerebral administration of BDNF or vehicle (n=10 each). The distribution of BDNF following different periods of rBDNF or vehicle-infusion was analyzed using immunohistochemical techniques (n=5 each). In the groups treated with 8 microgram of rhBDNF for 7, 10, or 14 days pre-ischemia, there were significant reductions of neocortical infarct volume compared to in the control or vehicle-treated groups (p<0.05). In the rCBF study, there was no significant change after the infusion of 8 microgram rhBDNF for 14 days. In the histological study, a wide distribution of BDNF-like immunoreactivity in the neuronal nuclei in the ipsilateral neocortex was demonstrated after the infusion of 8 microgram rhBDNF for 14 days. The BDNF-like immunoreactivity in the neuronal nuclei was enhanced at the time that the resistance to stroke was achieved by direct intra-cerebral infusion of exogenous rBDNF. Elucidating the function of the BDNF-like protein located in the neuronal nuclei should reveal a new strategy for neuroprotection against ischemic brain attack in humans.     

Hippocampal brain-derived neurotrophic factor gene regulation by exercise and the medial septum

Brain-derived neurotrophic factor (BDNF) enhances synaptic plasticity and neuron function. We have reported that voluntary exercise increases BDNF mRNA levels in the hippocampus; however, mechanisms underlying this regulation have not been defined. We hypothesized that medial septal cholinergic and/or gamma amino butyric acid (GABA)ergic neurons, which provide a major input to the hippocampus, may regulate the baseline gene expression and exercise-dependent gene upregulation of this neurotrophin. Focal lesions were produced by medial septal infusion of the saporin-linked immunotoxins 192-IgG-saporin or OX7-saporin. 192-IgG-saporin produced a selective and complete loss of medial septal cholinergic neurons with no accompanying GABA loss. Baseline BDNF mRNA was reduced in the hippocampus of sedentary animals, but exercise-induced gene upregulation was not impaired, despite complete loss of septo-hippocampal cholinergic afferents. OX7-saporin produced a graded lesion of the medial septum characterized by predominant GABA neuron loss with less reduction in the number of cholinergic cells. OX7-saporin lesion reduced baseline hippocampal BDNF mRNA and attenuated exercise-induced gene upregulation, in a dose-dependent manner. These results suggest that combined loss of septal GABAergic and cholinergic input to the hippocampus may be important for exercise-dependent BDNF gene regulation, while cholinergic activity on its own is not sufficient. These results are discussed in relation to their implications for aging and Alzheimer's disease.