Brain-derived neurotrophic factor rapidly enhances synaptic transmission in hippocampal neurons via postsynaptic tyrosine kinase receptors

Modulation of unitary glutamatergic synapses by neurotrophin-4/5 or brain-derived neurotrophic factor in hippocampal microcultures: presynaptic enhancement depends on pre-established paired-pulse facilitation

The neurotrophins, nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3 and neurotrophin-4/5, have--in addition to their known effects as neuronal survival factors--recently been found to modulate synaptic transmission in the rat hippocampus and neocortex. Using standard whole-cell patch-clamp recordings, we have now investigated the acute effects of brain-derived neurotrophic factor and neurotrophin-4/5 on unitary (i.e. single cell activated) glutamatergic synaptic connections in microcultures of postnatal rat hippocampal neurons. We show that, in approximately 30% of the cells, glutamatergic synaptic transmission is enhanced to 170 +/- 52% (neurotrophin-4/5, 100 ng/ml) and 143 +/- 35% (brain-derived neurotrophic factor, 100 ng/ml) of control values, respectively. The enhancement is abolished in the presence of the specific Trk tyrosine kinase inhibitor k252a (200 nM). Depending on the particular cell investigated, the enhancement consisted of transient and sustained components in varying quantities. A minority of neurons (10%) showed a depression of glutamatergic synaptic transmission to 64 +/- 14% (brain-derived neurotrophic factor) and 61 +/- 11% of control (neurotrophin-4/5). The enhancement of unitary glutamatergic synaptic transmission is mediated predominantly by presynaptic modifications, as is evident from (i) the concomitant decrease in paired-pulse facilitation, (ii) the concomitant increase in the variance of the evoked unitary synaptic currents and (iii) the enhanced miniature excitatory postsynaptic/autaptic current frequencies that could be observed in the absence of an effect on miniature excitatory postsynaptic/autaptic current amplitudes. Finally, we show that the successful enhancement of synaptic transmission by neurotrophin-4/5 critically depends on the degree of paired-pulse facilitation prior to the start of neurotrophin application, with autapses/synapses initially showing a higher degree of paired-pulse facilitation being enhanced more effectively. Taken together, these results suggest that the brain-derived neurotrophic factor- and neurotrophin-4/5-mediated enhancement of unitary glutamatergic synaptic transmission in hippocampal cultures results predominantly from a presynaptic modulation of transmitter release, and this modulation could participate in the neurotrophin-dependent modification of glutamatergic synaptic transmission in the hippocampus in situ.

Cerebellar brain-derived neurotrophic factor-TrkB defect associated with impairment of eyeblink conditioning in Stargazer mutant mice

In the spontaneous ataxic mutant mouse stargazer, there is a selective reduction of brain-derived neurotrophic factor (BDNF) mRNA expression in the cerebellum. BDNF protein levels in the cerebellum are reduced by 70%. Despite normal levels of full-length and truncated TrkB receptor, constitutive and neurotrophin-4/5-induced tyrosine phosphorylation was significantly reduced in several signal transduction molecules, including phospholipase-Cgamma1, erk1, and erk2. Morphological examination revealed an increased number of external granule cells at postnatal day 15 and the presence of abnormal neurons resembling immature granule cells in the adult. These abnormalities are associated with a severe impairment in the acquisition of classical eyeblink conditioning, indicating cerebellar malfunction. Our data suggest that normal BDNF expression and TrkB signal transduction in the cerebellum are necessary for learning and plasticity in this model.

Impaired eye-blink conditioning in waggler, a mutant mouse with cerebellar BDNF deficiency

In addition to their trophic functions, neurotrophins are also implicated in synaptic modulation and learning and memory. Although gene knockout techniques have been used widely in studying the roles of neurotrophins at molecular and cellular levels, behavioral studies using neurotrophin knockouts are limited by the early-onset lethality and various sensory deficits associated with the gene knockout mice. In the present study, we found that in a spontaneous mutant mouse, waggler, the expression of brain-derived neurotrophic factor (BDNF) was selectively absent in the cerebellar granule cells. The cytoarchitecture of the waggler cerebellum appeared to be normal at the light microscope level. The mutant mice exhibited no sensory deficits to auditory stimuli or heat-induced pain. However, they were massively impaired in classic eye-blink conditioning. These results suggest that BDNF may have a role in normal cerebellar neuronal function, which, in turn, is essential for classic eye-blink conditioning.

Impaired Eye-Blink Conditioning in waggler, a Mutant Mouse With Cerebellar BDNF Deficiency

In addition to their trophic functions, neurotrophins are also implicated in synaptic modulation and learning and memory. Although gene knockout techniques have been used widely in studying the roles of neurotrophins at molecular and cellular levels, behavioral studies using neurotrophin knockouts are limited by the early-onset lethality and various sensory deficits associated with the gene knockout mice. In the present study, we found that in a spontaneous mutant mouse, waggler, the expression of brain-derived neurotrophic factor (BDNF) was selectively absent in the cerebellar granule cells. The cytoarchitecture of the wagglercerebellum appeared to be normal at the light microscope level. The mutant mice exhibited no sensory deficits to auditory stimuli or heat-induced pain. However, they were massively impaired in classic eye-blink conditioning. These results suggest that BDNF may have a role in normal cerebellar neuronal function, which, in turn, is essential for classic eye-blink conditioning.

BDNF, and full length and truncated TrkB expression in the hippocampus of the rat following kainic acid excitotoxic damage. Evidence of complex time-dependent and cell-specific responses

Systemic administration of kainic acid (KA) at convulsant doses results in irreversible cell damage and neuron loss in the hilus of the dentate gyrus and in the CA1 area of the hippocampus. This is followed by reactive astrocytosis in these regions, and sprouting of mossy fibers into the molecular layer of the dentate gyrus. Since trophic factors are probably implicated in the cellular responses to the excitotoxic insult, and early induction of BDNF and TrkB mRNAs has been observed following KA injection, the present study examines BDNF, full-length and truncated TrkB protein expression in the hippocampus, as revealed by immunohistochemistry, up to 30 days following KA administration to adult rats. Reduction in BDNF and full-length TrkB immunoreactivity preceding neuron loss is observed in the damaged areas. However, transient increase in BDNF immunoreactivity is observed in surviving CA1 neurons and in granule cells of the dentate gyrus. In contrast, full-length TrkB immunoreactivity progressively increases in the molecular layer of the dentate gyrus up to day 30 following KA administration. A second peak in BDNF immunoreactivity is observed in reactive astrocytes, as revealed with double-labeling immunohistochemistry to BDNF and GFAP, in the plexiform layers of CA1 and, to a lesser degree, in the molecular layer of the dentate gyrus. In addition, strong truncated TrkB immunoreactivity is found in reactive astrocytes, as revealed with double-labeling immunohistochemistry to truncated TrkB and GFAP, in the same regions. These results, in concert with previous observations in the same model of hippocampal damage, suggest that BDNF participates in the early response to excitotoxic damage, and that expression of full-length TrkB at strategic sites in the molecular layer of the dentate gyrus has a role in the regenerative response linked to mossy fiber sprouting. Interestingly, delayed expression of BDNF and truncated TrkB in reactive astrocytes may act as negative regulators of neurite growth in devastated regions, such as the CA1 area, which are impoverished of putative postsynaptic sites.

Brain-derived neurotrophic factor modulates hippocampal synaptic transmission by increasing N-methyl-D-aspartic acid receptor activity

Neurotrophins (NTs) have recently been found to regulate synaptic transmission in the hippocampus. Whole-cell and single-channel recordings from cultured hippocampal neurons revealed a mechanism responsible for enhanced synaptic strength. Specifically, brain-derived neurotrophic factor augmented glutamate-evoked, but not acetylcholine-evoked, currents 3-fold and increased N-methyl-D-aspartic acid (NMDA) receptor open probability. Activation of trkB NT receptors was critical, as glutamate currents were not affected by nerve growth factor or NT-3, and increased open probability was prevented by the tyrosine kinase inhibitor K-252a. In addition, the NMDA receptor antagonist MK-801 blocked brain-derived neurotrophic factor enhancement of synaptic transmission, further suggesting that NTs modulate synaptic efficacy via changes in NMDA receptor function.

Brain-derived neurotrophic factor rapidly potentiates synaptic transmission through NMDA, but suppresses it through non-NMDA receptors in rat hippocampal neuron

Brain-derived neurotrophic factor (BDNF) rapidly enhances synaptic transmission among the hippocampal neurons. In order to examine which component of glutamate receptors participates in synaptic potentiation by BDNF, we have studied the effect of glutamate antagonists on excitatory postsynaptic currents (EPSCs) enhanced by BDNF, using cultured embryonic hippocampal neurons. In the presence of AP5, a N-methyl-D-aspartate (NMDA) antagonist, BDNF depressed the EPSCs. In contrast, BDNF enhanced the EPSCs in the presence of a non-NMDA antagonist CNQX. Our results suggest that BDNF acutely activates synaptic transmission via NMDA, but suppresses it via non-NMDA receptors in the hippocampus.

cAMP-mediated regulation of neurotrophin-induced collapse of nerve growth cones

Neurotrophins are known to promote the survival, differentiation, and neurite outgrowth of developing neurons. Here we report that acutely applied brain-derived neurotrophic factor (BDNF) induces rapid growth cone collapse and neurite retraction of embryonic Xenopus spinal neurons in culture. The collapsing effect of BDNF depends on the activation of Trk receptor tyrosine kinase, requires an influx of extracellular Ca2+, and is regulated by cAMP-dependent activity. Elevation of intracellular cAMP levels ([cAMP]i) by forskolin or (Sp)-cAMP completely blocked the collapsing effect, whereas inhibition of protein kinase A (PKA) by (Rp)-cAMP potentiated the collapsing action. BDNF-induced growth cone collapse was only observed in 6 hr cultures but not in 24 hr cultures. However, inhibition of PKA by (Rp)-cAMP restored the collapsing response of these "old" neurons in 24 hr cultures, suggesting that embryonic Xenopus spinal neurons may upregulate their endogenous cAMP-dependent activity during development in culture, leading to the blockade of their collapsing response to BDNF. Taken together, our results suggest the presence of cross-talk between Ca2+- and cAMP-signaling pathways involved in the collapsing action of neurotrophins, in which the cAMP-pathway regulates the Ca2+-mediated signal transduction required for BDNF-induced collapse. By modulating the cAMP-dependent activity through the intrinsic programming or interaction with other factors present in the environment, a neuron thus could respond to the same extracellular factors with different morphological and cellular changes at different stages during development.

Localized synaptic actions of neurotrophin-4

Neurotrophins secreted by the postsynaptic target cell may participate in activity-dependent synaptic modification during development and in the mature brain. A fundamental question of how neurotrophins can sculpt synaptic connections is whether neurotrophin-induced synaptic changes are spatially restricted to the site of neurotrophin secretion or whether they can spread to neighboring synapses. Using a model system of nerve-muscle coculture in which neurotrophin-4 (NT-4) is overexpressed in a subpopulation of postsynaptic myocytes, we demonstrated that presynaptic potentiation is restricted to synapses on myocytes overexpressing NT-4 without affecting nearby synapses formed by the same neuron on control myocytes. Likewise, postsynaptic modulation of acetylcholine channels by secreted NT-4 is spatially restricted to <60 micron from the site of NT-4 secretion. Therefore, activity-dependent secretion of neurotrophins can result in highly localized modification of neuronal connections.

cAMP-mediated regulation of neurotrophin-induced collapse of nerve growth cones

Neurotrophins are known to promote the survival, differentiation, and neurite outgrowth of developing neurons. Here we report that acutely applied brain-derived neurotrophic factor (BDNF) induces rapid growth cone collapse and neurite retraction of embryonic Xenopus spinal neurons in culture. The collapsing effect of BDNF depends on the activation of Trk receptor tyrosine kinase, requires an influx of extracellular Ca2+, and is regulated by cAMP-dependent activity. Elevation of intracellular cAMP levels ([cAMP]i) by forskolin or (Sp)-cAMP completely blocked the collapsing effect, whereas inhibition of protein kinase A (PKA) by (Rp)-cAMP potentiated the collapsing action. BDNF-induced growth cone collapse was only observed in 6 hr cultures but not in 24 hr cultures. However, inhibition of PKA by (Rp)-cAMP restored the collapsing response of these "old" neurons in 24 hr cultures, suggesting that embryonic Xenopus spinal neurons may upregulate their endogenous cAMP-dependent activity during development in culture, leading to the blockade of their collapsing response to BDNF. Taken together, our results suggest the presence of cross-talk between Ca2+- and cAMP-signaling pathways involved in the collapsing action of neurotrophins, in which the cAMP-pathway regulates the Ca2+-mediated signal transduction required for BDNF-induced collapse. By modulating the cAMP-dependent activity through the intrinsic programming or interaction with other factors present in the environment, a neuron thus could respond to the same extracellular factors with different morphological and cellular changes at different stages during development.

Localized synaptic actions of neurotrophin-4

Neurotrophins secreted by the postsynaptic target cell may participate in activity-dependent synaptic modification during development and in the mature brain. A fundamental question of how neurotrophins can sculpt synaptic connections is whether neurotrophin-induced synaptic changes are spatially restricted to the site of neurotrophin secretion or whether they can spread to neighboring synapses. Using a model system of nerve-muscle coculture in which neurotrophin-4 (NT-4) is overexpressed in a subpopulation of postsynaptic myocytes, we demonstrated that presynaptic potentiation is restricted to synapses on myocytes overexpressing NT-4 without affecting nearby synapses formed by the same neuron on control myocytes. Likewise, postsynaptic modulation of acetylcholine channels by secreted NT-4 is spatially restricted to <60 micron from the site of NT-4 secretion. Therefore, activity-dependent secretion of neurotrophins can result in highly localized modification of neuronal connections.

BDNF and trkB mRNAs oscillate in rat brain during the light-dark cycle

In this study, we investigated whether in basal conditions the different functional states occurring during a 24-h cycle are reflected by the expression of brain-derived neurotrophic factor (BDNF) and its receptor, trkB, in rat cerebral cortex and hippocampus. Using semiquantitative RT-PCR assay, the levels of both BDNF and trkB mRNAs were found to undergo significant variation in a 24-h period. The strongest variation was detected in the hippocampus, where the ratio between maximum and minimum levels was about 3.5 and 17.5 for BDNF and trkB, respectively. These findings provide the first evidence that, in the absence of any experimental manipulation, the expression of a neurotrophin and its receptor undergoes diurnal oscillation, possibly related to the physiological variations of the activity level.

Axotomy upregulates the anterograde transport and expression of brain-derived neurotrophic factor by sensory neurons

In addition to the known retrograde transport of neurotrophins, it is now evident that endogenous brain-derived neurotrophic factor (BDNF) is transported in the anterograde direction in peripheral and central neurons. We used a double-ligation procedure that distinguishes between anterograde and retrograde flow to quantify the anterograde transport of endogenous neurotrophins and neuropeptides in the peripheral nervous system before and after axotomy. BDNF accumulation proximal to the ligation (anterograde transport) was twice that distal to the ligation (retrograde direction). Anterograde transport of nerve growth factor and neurotrophin-3 was not evident. Furthermore, BDNF anterograde transport increased 3.5-fold within 24 hr after sciatic nerve injury or dorsal rhizotomy. Anterograde transport of substance P and calcitonin gene-related peptide decreased after peripheral nerve lesion, demonstrating that there was no generalized increase in anterograde transport. To determine the source of the anterogradely transported BDNF, we performed in situ hybridization in a variety of tissues before and after axotomy. Expression of BDNF mRNA in proximal nerve segments did not change with treatment, showing that the increased accumulation of BDNF was not a result of increased local synthesis. BDNF mRNA and protein were expressed by dorsal root ganglion sensory neurons but not by motor neurons. BDNF mRNA expression was increased 1 d after nerve injury, and BDNF protein was also increased twofold to threefold, suggesting that sensory neurons are the major contributing source of the increased BDNF traffic in the sciatic nerve. Our results suggest that increased anterogradely transported BDNF plays a role in the early neuronal response to peripheral nerve injury at sites distal to the cell body.

Granule neuron regulation of Purkinje cell development: striking a balance between neurotrophin and glutamate signaling

Granule neurons, presynaptic afferents of Purkinje cells, are potent regulators of Purkinje cell development. Purified Purkinje cells survive and differentiate poorly, whereas coculture with granule neurons enhances their survival and dendritic development. Here we investigate the role of neurotrophins in granule-Purkinje cell interactions. BDNF or NT-4 improves, but NT-3 or CNTF reduces, survival of isolated Purkinje cells. When granule neurons are present, however, BDNF or NT-4 treatment leads to Purkinje cell loss. This decrease is overcome by anti-BDNF or TrkB-IgG-blocking reagents or by CNQX, a non-NMDA glutamate receptor antagonist. Furthermore, BDNF increases the spine density on the surviving Purkinje cells. These results suggest that Purkinje cell survival and differentiation are context-dependent and require a balance between neurotrophin- and activity-dependent signaling.

Expression of brain-derived neurotrophic factor and its cognate receptor, TrkB, in the rat suprachiasmatic nucleus

Photic entrainment of mammalian circadian rhythms occurs because the pacemaker in the suprachiasmatic nuclei (SCN) of the hypothalamus is endowed with a rhythmic sensitivity to photic signals conveyed by the retinohypothalamic tract. Since brain-derived neurotrophic factor (BDNF) has been implicated in the functional modulation of other retinal targets, the rat SCN was examined for expression and cellular distribution of this neurotrophin and TrkB, the tyrosine kinase receptor that preferentially binds BDNF. The rat SCN was found to express the mature BDNF peptide and mRNA by Western blotting, enzyme-linked immunosorbent assay (ELISA), and reverse transcription-polymerase chain reaction (RT-PCR) analyses. BDNF-immunoreactivity and hybridization signal for its mRNA were coextensively localized within a number of SCN cells throughout the rostrocaudal axis of each nucleus. In addition, some cells intercalated within the optic chiasm were distinguished by expression of BDNF immunoreactivity or mRNA. Immunostaining for the TrkB receptor was also evident in the SCN within terminals or fibers predominantly located along the SCN/optic chiasm interface and within scattered perikarya near the medial border of each nucleus. Combined in situ hybridization and immunocytochemical analysis revealed that BDNF mRNA-expressing cells within the ventrolateral SCN were often closely apposed to TrkB-positive fibers extending from the optic chiasm. These findings raise the possibility that target-derived interactions between BDNF and TrkB receptors could play a role in the circadian modulation of SCN pacemaker sensitivity to photic input transmitted by the retinohypothalamic tract.

Nerve growth factor and brain-derived neurotrophic factor increase neurotransmitter release in the rat visual cortex

A number of experiments have shown that neurotrophins are involved in the development and plasticity of the visual cortex (Bonhoeffer, T., Curr. Op. Neurobiol., 6, 119, 1996). A possible mechanism underlying these effects is the neurotrophin modulation of synaptic transmission. We investigated whether nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) can modulate the release of neurotransmitter in the rat visual cortex at the peak of the critical period for plasticity (P23). The release of glutamate, acetylcholine and gamma-aminobutyric acid (GABA) from visual cortical synaptosomes was analysed in continuous perfusion conditions. We found that NGF enhances the depolarization-evoked release of glutamate (approximately 90%) and acetylcholine (approximately 35%) but not that of GABA. By contrast, BDNF enhances the depolarization-evoked release of all three neurotransmitters investigated (approximately 30%). BDNF and NGF were ineffective on basal release of neurotransmitters. The effect of NGF was not blocked by cholinergic antagonists atropine and mecamylamine. NGF and BDNF potentiation of transmitter release was strongly but not completely blocked by K252a, a tyrosine kinase inhibitor. The role of TrkA and p75NTR receptors was investigated in NGF-induced potentiation of glutamate release. Block of NGF binding to p75NTR using specific blocking antibodies (REX-IgG) slightly but significantly reduced the effect of NGF. Activation of TrkA in isolation by RTA-IgG, an antibody that specifically activates TrkA, was less effective than activation of both receptors by NGF. These results show that neurotrophin action on neurotransmitter release was mostly mediated by Trk receptors with p75NTR having a little but significant positive role. Antigen blot analysis showed the presence of TrkA, TrkB and p75NTR receptors in the visual cortex.

Axotomy upregulates the anterograde transport and expression of brain-derived neurotrophic factor by sensory neurons

In addition to the known retrograde transport of neurotrophins, it is now evident that endogenous brain-derived neurotrophic factor (BDNF) is transported in the anterograde direction in peripheral and central neurons. We used a double-ligation procedure that distinguishes between anterograde and retrograde flow to quantify the anterograde transport of endogenous neurotrophins and neuropeptides in the peripheral nervous system before and after axotomy. BDNF accumulation proximal to the ligation (anterograde transport) was twice that distal to the ligation (retrograde direction). Anterograde transport of nerve growth factor and neurotrophin-3 was not evident. Furthermore, BDNF anterograde transport increased 3.5-fold within 24 hr after sciatic nerve injury or dorsal rhizotomy. Anterograde transport of substance P and calcitonin gene-related peptide decreased after peripheral nerve lesion, demonstrating that there was no generalized increase in anterograde transport. To determine the source of the anterogradely transported BDNF, we performed in situ hybridization in a variety of tissues before and after axotomy. Expression of BDNF mRNA in proximal nerve segments did not change with treatment, showing that the increased accumulation of BDNF was not a result of increased local synthesis. BDNF mRNA and protein were expressed by dorsal root ganglion sensory neurons but not by motor neurons. BDNF mRNA expression was increased 1 d after nerve injury, and BDNF protein was also increased twofold to threefold, suggesting that sensory neurons are the major contributing source of the increased BDNF traffic in the sciatic nerve. Our results suggest that increased anterogradely transported BDNF plays a role in the early neuronal response to peripheral nerve injury at sites distal to the cell body.

Granule neuron regulation of Purkinje cell development: striking a balance between neurotrophin and glutamate signaling

Granule neurons, presynaptic afferents of Purkinje cells, are potent regulators of Purkinje cell development. Purified Purkinje cells survive and differentiate poorly, whereas coculture with granule neurons enhances their survival and dendritic development. Here we investigate the role of neurotrophins in granule-Purkinje cell interactions. BDNF or NT-4 improves, but NT-3 or CNTF reduces, survival of isolated Purkinje cells. When granule neurons are present, however, BDNF or NT-4 treatment leads to Purkinje cell loss. This decrease is overcome by anti-BDNF or TrkB-IgG-blocking reagents or by CNQX, a non-NMDA glutamate receptor antagonist. Furthermore, BDNF increases the spine density on the surviving Purkinje cells. These results suggest that Purkinje cell survival and differentiation are context-dependent and require a balance between neurotrophin- and activity-dependent signaling.

Brain-derived neurotrophic factor modulates the development of the dopaminergic network in the rodent retina

Dopaminergic cells in the retina express the receptor for brain-derived neurotrophic factor (BDNF) (). To investigate whether BDNF can influence the development of the retinal dopaminergic pathway, we performed intraocular injections of BDNF during the second or third postnatal week and visualized the dopaminergic system with tyrosine hydroxylase (TH) immunohistochemistry. Both regimens of BDNF treatment caused an increase in TH immunoreactivity in stratum 1 and stratum 3 of the inner plexiform layer (IPL). D2 dopamine receptor immunoreactivity, a presynaptic marker of dopaminergic cells (), was also increased in stratum 1 and stratum 3 of the inner plexiform layer. These data suggest that BDNF causes sprouting of dopaminergic fibers in the inner plexiform layer. Other neurochemical systems, for example, the cholinergic amacrine cells, remained unaffected. Similar effects were observed after injections of neurotrophin-3 and neurotrophin-4, but not nerve growth factor. Analysis of whole-mounted TH-immunolabeled retinae revealed hypertrophy of dopaminergic cells (+41% in soma areas; p < 0.01) and an increase of labeled dopaminergic varicosities in stratum 1 of the IPL (+51%; p < 0.01) after BDNF treatment. The opposite was observed in mice homozygous for a null mutation of the bdnf gene: dopaminergic cells were atrophic (-22.5% in soma areas; p < 0.05), and the density of TH-positive varicosities in stratum 1 was reduced (57%; p < 0.01). We conclude that BDNF controls the development of the retinal dopaminergic network and may be particularly important in determining the density of dopaminergic innervation in the retina.

Brain-derived neurotrophic factor modulates the development of the dopaminergic network in the rodent retina

Dopaminergic cells in the retina express the receptor for brain-derived neurotrophic factor (BDNF) (). To investigate whether BDNF can influence the development of the retinal dopaminergic pathway, we performed intraocular injections of BDNF during the second or third postnatal week and visualized the dopaminergic system with tyrosine hydroxylase (TH) immunohistochemistry. Both regimens of BDNF treatment caused an increase in TH immunoreactivity in stratum 1 and stratum 3 of the inner plexiform layer (IPL). D2 dopamine receptor immunoreactivity, a presynaptic marker of dopaminergic cells (), was also increased in stratum 1 and stratum 3 of the inner plexiform layer. These data suggest that BDNF causes sprouting of dopaminergic fibers in the inner plexiform layer. Other neurochemical systems, for example, the cholinergic amacrine cells, remained unaffected. Similar effects were observed after injections of neurotrophin-3 and neurotrophin-4, but not nerve growth factor. Analysis of whole-mounted TH-immunolabeled retinae revealed hypertrophy of dopaminergic cells (+41% in soma areas; p < 0.01) and an increase of labeled dopaminergic varicosities in stratum 1 of the IPL (+51%; p < 0.01) after BDNF treatment. The opposite was observed in mice homozygous for a null mutation of the bdnf gene: dopaminergic cells were atrophic (-22.5% in soma areas; p < 0.05), and the density of TH-positive varicosities in stratum 1 was reduced (57%; p < 0.01). We conclude that BDNF controls the development of the retinal dopaminergic network and may be particularly important in determining the density of dopaminergic innervation in the retina.

Brain-derived neurotrophic factor alters the synaptic modification threshold in visual cortex

The effects of brain-derived neurotrophic factor (BDNF) were investigated on synaptic transmission and two forms of activity-dependent synaptic plasticity, long-term potentiation (LTP) and long-term depression (LTD), in visual cortex slices prepared from young (P21 -28) rats. The slices treated for 2-5 h in BDNF showed no difference from control slices when a 'strong' tetanus was used (theta-burst stimulation) to elicit a maximal level of LTP but displayed significantly greater synaptic potentiation in response to a 'weak' (20 Hz) tetanus. The BDNF-treated slices also showed significantly less LTD in response to a 1 Hz tetanus. Thus, BDNF treatment alters the relationship between stimulation frequency and synaptic plasticity in the visual cortex, shifting the modification threshold to the left. The effects of BDNF on LTP and LTD induction may be attributed to the significant enhancement of synaptic responses that was observed during conditioning stimulation. These data suggest that one role of BDNF during development of the visual cortex may be to modulate the properties of synaptic plasticity, enhancing synaptic strengthening and reducing synaptic weakening processes which contribute to the formation of specific synaptic connections.

Ca2+ influx regulates BDNF transcription by a CREB family transcription factor-dependent mechanism

CREB is a transcription factor implicated in the control of adaptive neuronal responses. Although one function of CREB in neurons is believed to be the regulation of genes whose products control synaptic function, the targets of CREB that mediate synaptic function have not yet been identified. This report describes experiments demonstrating that CREB or a closely related protein mediates Ca2+-dependent regulation of BDNF, a neurotrophin that modulates synaptic activity. In cortical neurons, Ca2+ influx triggers phosphorylation of CREB, which by binding to a critical Ca2+ response element (CRE) within the BDNF gene activates BDNF transcription. Mutation of the BDNF CRE or an adjacent novel regulatory element as well as a blockade of CREB function resulted in a dramatic loss of BDNF transcription. These findings suggest that a CREB family member acts cooperatively with an additional transcription factor(s) to regulate BDNF transcription. We conclude that the BDNF gene is a CREB family target whose protein product functions at synapses to control adaptive neuronal responses.

Unilateral hippocampal lesions in newborn and adult rats: effects on spatial memory and BDNF gene expression

Subcortical damage at birth often produces more severe deficits than similar lesions in an adult. In the present study, effects of unilateral electrolytic hippocampal ablations made on postnatal day 1 or in 3-month-old adult rats, were compared. Exploratory behavior and spatial navigation in the Morris water maze (MWM) were assessed 8 and 20 weeks after hippocampal damage. Rats with neonatal damage did not respond to novelty in the environment and did not learn to find the hidden platform in the MWM. Rats lesioned as adults did learn the water maze task, but slower than controls. We hypothesized that behavioral deficits observed in rats lesioned at birth, may be due, in part, to neurochemical dysfunction of the contralateral hippocampus. Specifically, cholinergic and GABAergic neurotransmission were assessed by measuring choline-acetyltransferase (ChAT) and GABAdecarboxylase (GAD) activity. In addition, nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) mRNA levels were assayed in the remaining (contralateral) hippocampus. Of these molecules, only BDNF gene expression was significantly reduced (by 30%) at 8 and 20 weeks after neonatal and adult unilateral ablation. The similar reduction in BDNF mRNA in both treatment groups does not correspond with the lesion's differential effect on memory function. However, the more severe learning impairment after neonatal lesion may reflect increased dependence on trophins during development.

A role for BDNF in the late-phase of hippocampal long-term potentiation

The neurotrophin family of growth factors has received enormous attention recently for its role in modulating synaptic strength in the developing and adult nervous system. Several recent studies have indicated a role for brain-derived neurotrophic factor (BDNF) in long-term potentiation (LTP), a form of long-lasting plasticity observed at synapses in the hippocampus and other brain areas. The late-phase (L-LTP; e.g. > 2 h) of LTP has been shown to require the synthesis of new proteins. We have examined whether BDNF or other TrkB ligands participate in L-LTP in two ways: by examining transgenic mice which lack BDNF or by acutely blocking TrkB function using function-blocking antibodies. Slices from BDNF knock-out animals or slices treated with TrkB antibodies failed to exhibit L-LTP, indicating that TrkB ligands participate in extending synaptic enhancement from a short-lasting to a long-lasting form.

BDNF acutely increases tyrosine phosphorylation of the NMDA receptor subunit 2B in cortical and hippocampal postsynaptic densities

While neurotrophins are critical for neuronal survival and differentiation, recent work suggests that they acutely regulate synaptic transmission as well. Brain-derived neurotrophic factor (BDNF) enhances excitatory postsynaptic currents in cultured dissociated hippocampal neurons within 2-3 min through postsynaptic, phosphorylation-dependent mechanisms. Moreover, BDNF modulates hippocampal long-term potentiation, in which postsynaptic NMDA (N-methyl-D-aspartate) receptors (NRs) play a key role. We now report that BDNF acutely increases tyrosine phosphorylation of the specific NMDA receptor subunit NR2B, which has recently been shown to play a role in long-term potentiation. Incubation of BDNF with cortical or hippocampal postsynaptic densities for 5 min increased tyrosine phosphorylation of the NR2B subunits in a dose-dependent manner. A maximal increase to 165% of control phosphorylation occurred at a BDNF concentration of 2 ng/ml. The BDNF action appeared to be specific, since nerve growth factor, another member of the neurotrophin gene family, had no effect on NR2B phosphorylation. Further, BDNF action was selective, since it did not alter tyrosine phosphorylation of NR2A subunits. Our results suggest that tyrosine phosphorylation of NR2B subunits of the NMDA receptor may contribute to neurotrophin modulation of postsynaptic responsiveness and long-term potentiation.

Transcript-specific effects of adrenalectomy on seizure-induced BDNF expression in rat hippocampus

Activity-induced brain-derived neurotrophic factor (BDNF) expression is negatively modulated by circulating adrenal steroids. The rat BDNF gene gives rise to four major transcript forms that each contain a unique 5' exon (I-IV) and a common 3' exon (V) that codes for BDNF protein. Exon-specific in situ hybridization was used to determine if adrenalectomy has differential effects on basal and activity-induced BDNF transcript expression in hippocampus. Adrenalectomy alone had only modest effects on BDNF mRNA levels with slight increases in exon III-containing mRNA with 7-10-day survival and in exon II-containing mRNA with 30-days survival. In the dentate gyrus granule cells, adrenalectomy markedly potentiated increases in exon I and II cRNA labeling, but not increases in exon III and IV cRNA labeling, elicited by one hippocampal afterdischarge. Similarly, for the granule cells and CA1 pyramidal cells, hilus lesion (HL)-induced recurrent limbic seizures elicited greater increases in exon I and II cRNA hybridization in adrenalectomized (ADX) as compared to adrenal-intact rats. In this paradigm, adrenalectomy modestly potentiated the increase in exon III-containing mRNA in CA1 but had no effect on exon IV-containing mRNA content. These results demonstrate that the negative effects of adrenal hormones on activity-induced BDNF expression are by far the greatest for transcripts containing exons I and II. Together with evidence for region-specific transcript expression, these results suggest that the effects of stress on adaptive changes in BDNF signalling will be greatest for neurons that predominantly express transcripts I and II.

Dynamic changes of brain-derived neurotrophic factor protein levels in the rat forebrain after single and recurring kindling-induced seizures

Regional levels of brain-derived neurotrophic factor protein were measured in the rat brain using enzyme immunoassay following seizures evoked by hippocampal kindling stimulations. One stimulation, which induced a brief, single episode of epileptiform activity in hippocampus and piriform cortex but not in parietal cortex or striatum, gave rise to a transient increase of brain-derived neurotrophic factor levels in dentate gyrus and CA3 region and a decrease in piriform cortex. After 40 rapidly recurring seizures, with epileptiform activity also involving parietal cortex and striatum, increases were observed in dentate gyrus, CA3 and CA1 regions, piriform cortex and striatum. Maximum levels were reached at 2-24 h and brain-derived neurotrophic factor then returned to baseline except in dentate gyrus, where elevated protein content was sustained for four days. The differential regulation of brain-derived neurotrophic factor protein levels in various forebrain structures, which only partly correlates to messenger RNA changes, could indicate regional differences in protein release, antero- or retrograde transport, or brain-derived neurotrophic factor promotor activation. The dynamic changes of brain-derived neurotrophic factor levels in regions involved in the generation and spread of seizure activity may regulate excitability and trigger plastic responses in the post-seizure period.

Secreted amyloid precursor protein alpha selectively suppresses N-methyl-D-aspartate currents in hippocampal neurons: involvement of cyclic GMP

The secreted form of beta-amyloid precursor protein (sAPP alpha) is released from neurons in an activity-dependent manner; data suggest sAPP alpha may play roles in regulating neuronal excitability, plasticity, and survival. In cultured hippocampal neurons sAPP alpha can suppress elevation of [Ca2+]i induced by glutamate and can protect neurons against excitotoxicity. We now report whole-cell patch-clamp data from studies of cultured embryonic rat hippocampal neurons which demonstrate that sAPP alpha selectively suppresses N-methyl-D-aspartate currents without affecting currents induced by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate or kainate. sAPP alpha suppressed N-methyl-D-aspartate current rapidly and reversibly at concentrations of 0.011 nM. Suppression of N-methyl-D-aspartate current by sAPP alpha is apparently mediated by cyclic guanosine monophosphate because 8-bromo-cyclic guanosine monophosphate suppressed N-methyl-D-aspartate current in a manner similar to sAPP alpha, and two different inhibitors of cyclic guanosine monophosphate-dependent protein kinase prevented sAPP alpha-induced suppression of N-methyl-D-aspartate current. In addition, okadaic acid prevented suppression of N-methyl-D-aspartate-induced current suggesting the involvement of a protein phosphatase in modulation of N-methyl-D-aspartate current by sAPP alpha. These data identify a mechanism whereby sAPP alpha can modulate cellular responses to glutamate, and suggest important roles for sAPP alpha in the various physiological and pathophysiological processes in which N-methyl-D-aspartate receptors participate.

Role of MAP kinase in neurons

Extracellular stimuli such as neurotransmitters, neurotrophins, and growth factors in the brain regulate critical cellular events, including synaptic transmission, neuronal plasticity, morphological differentiation and survival. Although many such stimuli trigger Ser/Thr-kinase and tyrosine-kinase cascades, the extracellular signal-regulated kinases, ERK1 and ERK2, prototypic members of the mitogen-activated protein (MAP) kinase family, are most attractive candidates among protein kinases that mediate morphological differentiation and promote survival in neurons. ERK1 and ERK2 are abundant in the central nervous system (CNS) and are activated during various physiological and pathological events such as brain ischemia and epilepsy. In cultured hippocampal neurons, simulation of glutamate receptors can activate ERK signaling, for which elevation of intracellular Ca2+ is required. In addition, brain-derived neurotrophic factor and growth factors also induce the ERK signaling and here, receptor-coupled tyrosine kinase activation has an association. We describe herein intracellular cascades of ERK signaling through neurotransmitters and neurotrophic factors. Putative functional implications of ERK and other MAP-kinase family members in the central nervous system are give attention.

LTP and activity-dependent synaptogenesis: the more alike they are, the more different they become

Recent data suggest that long-term potentiation and activity-dependent synaptogenesis share the same mechanism at the initiation stage during which NMDA receptor activity is necessary to increase the postsynaptic response via AMPA receptor currents. However, several fundamental differences between the environments of young and mature synapses and the neurons that support them suggest that the same cellular mechanism is facilitated by very different parameters in the young versus the mature brain.

Nerve growth factor modulates synaptic transmission between sympathetic neurons and cardiac myocytes

Regulation of heart rate by the sympathetic nervous system involves the release of norepinephrine (NE) from nerve terminals onto heart tissue, resulting in an elevation in beat rate. Nerve growth factor (NGF) is a neurotrophin produced by the heart that supports the survival and differentiation of sympathetic neurons. Here we report that NGF also functions as a modulator of sympathetic synaptic transmission. We determined the effect of NGF on the strength of synaptic transmission in co-cultures of neonatal rat cardiac myocytes and sympathetic neurons from the superior cervical ganglion (SCG). Synaptic transmission was assayed functionally, as an increase in the beat rate of a cardiac myocyte during stimulation of a connected neuron. Application of NGF produced a pronounced, reversible enhancement of synaptic strength. We found that TrkA, the receptor tyrosine kinase that mediates many NGF responses, is expressed primarily by neurons in these cultures, suggesting a presynaptic mechanism for the effects of NGF. A presynaptic model is further supported by the finding that NGF did not alter the response of myocytes to application of NE. In addition to the acute modulatory effects of NGF, we found that the concentration of NGF in the growth medium affects the level of synaptic transmission in cultures of sympathetic neurons and cardiac myocytes. These results indicate that in addition to its role as a survival factor, NGF plays both acute and long-term roles in the regulation of developing sympathetic synapses in the cardiac system.

Activity-dependent dendritic targeting of BDNF and TrkB mRNAs in hippocampal neurons

The mechanisms underlying the subcellular localization of neurotrophins and their receptors are poorly understood. We show that in cultured hippocampal neurons, the mRNAs for BDNF and TrkB have a somatodendritic localization, and we quantify the extent of their dendritic mRNA localization. In the dendrites the labeling covers on average the proximal 30% of the total dendritic length. On high potassium depolarization, the labeling of BDNF and TrkB mRNA extends on average to 68% of the dendritic length. This increase does not depend on new RNA synthesis, is inhibited by the Na+ channel blocker tetrodotoxin, and involves the activation of glutamate receptors. Extracellular Ca2+, partly flowing through L-type Ca2+ channels, is absolutely required for this process to occur. At the protein level, a brief stimulation of hippocampal neurons with 10 mM KCl leads to a marked increase of BDNF and TrkB immunofluorescence density in the distal portion of dendrites, which also occurs, even if at lower levels, when transport is inhibited by nocodazole. The protein synthesis inhibitor cycloheximide abolishes this increase. The activity-dependent modulation of mRNA targeting and protein accumulation in the dendrites may provide a mechanism for achieving a selective local regulation of the activity of neurotrophins and their receptors, close to their sites of action.

Distribution of brain-derived neurotrophic factor in cranial and spinal ganglia

In a previous study we have shown that a subpopulation of primary sensory neurons contain brain-derived neurotrophic factor immunoreactivity. In the present study we investigated the distribution of brain-derived neurotrophic factor and its mRNA in cranial and spinal ganglia at different segmental levels, using immunohistochemical and quantitative reverse transcriptase-polymerase chain reaction techniques. Our results show that there is no significant difference in the percentage of brain-derived neurotrophic factor-immunoreactive neurons in spinal ganglia of different segmental levels. In contrast, more brain-derived neurotrophic factor-immunoreactive neurons were found in placode-derived than neural crest-derived ganglia. The percentage of brain-derived neurotrophic factor-immunoreactive neurons is consistent with the percentage of neurons lost after deletion of brain-derived neurotrophic factor or trkB genes. However, there is no correlation between brain-derived neurotrophic factor mRNA levels and the number of brain-derived neurotrophic factor immunoreactive neurons in these ganglia, suggesting that some neurons synthesize brain-derived neurotrophic factor while others accumulate the factor following its retrograde transport within nerve fibers. In particular, the proportion of brain-derived neurotrophic factor that is derived from extraganglionic sources in the placode-derived ganglia appears greater than that in the neural crest-derived ganglia.

Acute intrahippocampal infusion of BDNF induces lasting potentiation of synaptic transmission in the rat dentate gyrus

The effect of acute intrahippocampal infusion of brain-derived neurotrophic factor (BDNF) on synaptic transmission in the dentate gyrus was investigated in urethan-anesthetized rats. Medial perforant path-evoked field potentials were recorded in the dentate hilus and BDNF-containing buffer was infused (4 microl, 25 min) immediately above the dentate molecular layer. BDNF led to a slowly developing increase of the field excitatory postsynaptic potential (fEPSP) slope and population spike amplitude. The potentiation either reached a plateau level at approximately 2 h after BDNF infusion or continued to increase for the duration of experiment; the longest time point recorded was 10 h. Mean increases at 4 h after BDNF infusion were 62.2 and 224% for the fEPSP slope and population spike, respectively. No changes in responses were observed in controls receiving buffer medium only or buffer containing cytochrome C. BDNF-induced potentiation developed in the absence of epileptiform activity in the hippocampal electroencephalogram or changes in recurrent inhibition on granule cells as assessed by paired-pulse inhibition of the population spike. We conclude that exogenous BDNF induces a lasting potentiation of synaptic efficacy in the dentate gyrus of anesthetized adult rats.

The messenger RNA encoding VGF, a neuronal peptide precursor, is rapidly regulated in the rat central nervous system by neuronal activity, seizure and lesion

The VGF gene encodes a neuronal secretory-peptide precursor that is rapidly induced by neurotrophic growth factors and by depolarization in vitro. VGF expression in the animal peaks during critical periods in the developing peripheral and central nervous systems. To gain insight into the possible functions and regulation of VGF in vivo, we have used in situ hybridization to examine the regulation of VGF messenger RNA by experimental manipulations, and have found it to be regulated in the CNS by paradigms that affect electrical activity and by lesion. Inhibition of retinal electrical activity during the critical period of visual development rapidly repressed VGF messenger RNA in the dorsal lateral geniculate nucleus of the thalamus. In the adult, kainate-induced seizures transiently induced VGF messenger RNA in neurons of the dentate gyrus, hippocampus, and cerebral cortex within hours. Cortical lesion strongly induced VGF messenger RNA in ipsilateral cortex within hours, and strongly repressed expression in ipsilateral striatum. Ten days postlesion there was a delayed induction of VGF messenger RNA in a portion of deafferented striatum where compensatory cortical sprouting has been detected. Expression of the neuronal secretory-peptide precursor VGF is therefore modulated in vivo by monocular deprivation, seizure, and cortical lesion, paradigms which lead to neurotrophin induction, synaptic remodeling and axonal sprouting.

BDNF restores the expression of Jun and Fos inducible transcription factors in the rat brain following repetitive electroconvulsive seizures

The expression of inducible transcription factors was studied following repetitive electroconvulsive seizures (ECS), c-Fos, c-Jun, JunB, and JunD immunoreactivities were investigated following a single (1 x ECS) or repetitive ECS evoked once per day for 4, 5, or 10 days (4 x ECS, 5 x ECS, or 10 x ECS). Animals were killed 3 or 12 h following the last ECS. Three hours after 1 x ECS, c-Fos was expressed throughout the cortex and hippocampus. After 5 x ECS and 10 x ECS, c-Fos was reexpressed in the CA4 area, but was completely absent in the other hippocampal areas and cortex. In these areas, c-Fos became only reinducible when the time lag between two ECS stimuli was 5 days. In contrast to c-Fos, intense JunB expression was inducible in the cortex and hippocampus, but not CA4 subfield, after 1 x ECS, 5 x ECS, and 10 x ECS. Repetitive ECS did not effect c-Jun and JunD expression. In a second model of systemic excitation of the brain, repetitive daily injection of kainic acid for 4 days completely failed to express c-Fos, c-Jun, and JunB after the last application whereas injection of kainic acid once per week did not alter the strong expressions compared to a single application of kainic acid. In order to study the maintenance of c-Fos expression during repetitive seizures, brain-derived neurotrophic factor (BDNF) was applied in parallel for 5 or 10 days via miniosmotic pumps and permanent cannula targeted at the hippocampus or the parietal cortex. Infusion of BDNF completely reinduced c-Fos expression during 5 x ECS or 10 x ECS in the cortex ipsilaterally to the cannula and, to a less extent, also increased the expression of c-Jun and JunB when compared to saline-treated controls. BDNF had no effect on the expression patterns in the hippocampus. ECS with or without BDNF infusion did not change the expression patterns of the constitutive transcription factors ATF-2, CREB, and SRF. These data demonstrate that various transcription factors substantially differ in their response to acute and chronic neural stimulation. Repetitive pathophysiological excitation decreases the transcriptional actions of neurons over days in the adult brain, and this decrement can be prevented by BDNF restoring the neuroplasticity at the level of gene transcription.

BDNF potentiates spontaneous Ca2+ oscillations in cultured hippocampal neurons

Brain-derived neurotrophic factor (BDNF) is thought to regulate neuronal plasticity in developing and matured neurons, although the molecular mechanisms are less well characterized. We monitored changes in the intracellular calcium (Ca2+) levels induced by BDNF using a fluorescence Ca2+ indicator (Fluo-3) by means of confocal laser microscopy in rat cultured hippocampal neurons. BDNF acutely potentiated spontaneous Ca2+ oscillations in dendrites and also in the soma of several neurons, although it increased intracellular Ca2+ in only selective proportion of resting neurons without Ca2+ oscillations. The potentiation was observed both in the frequency and the amplitude of Ca2+ oscillations, completely blocked by K-252a, and significantly reduced by 2-aminophosphonovaleric acid. These findings suggest that BDNF increases glutamate release and N-methyl-D-aspartate (NMDA) channel-gated Ca2+ influx via TrkB and regulates the frequency and the amplitude of Ca2+ oscillations. BDNF may have the potential to modulate spontaneous Ca2+ oscillations to regulate neuronal plasticity in developing hippocampal neurons.

Activity-dependent dendritic targeting of BDNF and TrkB mRNAs in hippocampal neurons

The mechanisms underlying the subcellular localization of neurotrophins and their receptors are poorly understood. We show that in cultured hippocampal neurons, the mRNAs for BDNF and TrkB have a somatodendritic localization, and we quantify the extent of their dendritic mRNA localization. In the dendrites the labeling covers on average the proximal 30% of the total dendritic length. On high potassium depolarization, the labeling of BDNF and TrkB mRNA extends on average to 68% of the dendritic length. This increase does not depend on new RNA synthesis, is inhibited by the Na+ channel blocker tetrodotoxin, and involves the activation of glutamate receptors. Extracellular Ca2+, partly flowing through L-type Ca2+ channels, is absolutely required for this process to occur. At the protein level, a brief stimulation of hippocampal neurons with 10 mM KCl leads to a marked increase of BDNF and TrkB immunofluorescence density in the distal portion of dendrites, which also occurs, even if at lower levels, when transport is inhibited by nocodazole. The protein synthesis inhibitor cycloheximide abolishes this increase. The activity-dependent modulation of mRNA targeting and protein accumulation in the dendrites may provide a mechanism for achieving a selective local regulation of the activity of neurotrophins and their receptors, close to their sites of action.

Nerve growth factor modulates synaptic transmission between sympathetic neurons and cardiac myocytes

Regulation of heart rate by the sympathetic nervous system involves the release of norepinephrine (NE) from nerve terminals onto heart tissue, resulting in an elevation in beat rate. Nerve growth factor (NGF) is a neurotrophin produced by the heart that supports the survival and differentiation of sympathetic neurons. Here we report that NGF also functions as a modulator of sympathetic synaptic transmission. We determined the effect of NGF on the strength of synaptic transmission in co-cultures of neonatal rat cardiac myocytes and sympathetic neurons from the superior cervical ganglion (SCG). Synaptic transmission was assayed functionally, as an increase in the beat rate of a cardiac myocyte during stimulation of a connected neuron. Application of NGF produced a pronounced, reversible enhancement of synaptic strength. We found that TrkA, the receptor tyrosine kinase that mediates many NGF responses, is expressed primarily by neurons in these cultures, suggesting a presynaptic mechanism for the effects of NGF. A presynaptic model is further supported by the finding that NGF did not alter the response of myocytes to application of NE. In addition to the acute modulatory effects of NGF, we found that the concentration of NGF in the growth medium affects the level of synaptic transmission in cultures of sympathetic neurons and cardiac myocytes. These results indicate that in addition to its role as a survival factor, NGF plays both acute and long-term roles in the regulation of developing sympathetic synapses in the cardiac system.

Learning deficit in BDNF mutant mice

Brain-derived neurotrophic factor (BDNF) has been implicated in the regulation of high-frequency synaptic transmission and long-term potentiation in the hippocampus, processes that are also thought to be involved in the learning of spatial tasks such as the Morris water maze. In order to determine whether BDNF is required for normal spatial learning, mice carrying a deletion in one copy of the BDNF gene were subjected to the Morris water maze task. Young adult BDNF mutant mice were significantly impaired compared with wild-type mice, requiring twice the number of days to reach full performance. Aged wild-type mice performed significantly worse than young wild-type mice and the effect was even more pronounced in the BDNF mutant mice, which did not learn at all. Although there was no difference in mean swimming speed between BDNF mutant and wild-type mice, we cannot exclude the possibility that developmental or peripheral deficits also contribute to the learning deficits in these mice. In situ hybridization and RNase protection analysis revealed that BDNF mRNA expression was indeed decreased in BDNF mutant mice. Furthermore, a pronounced effect of age on BDNF mRNA expression was seen, displayed as both a reduced level of mRNA expression and a reduced or entirely absent layer-specific expression pattern in the cerebral cortex of aged animals. Thus, our data suggest that BDNF expression may be linked to learning.

Neurotrophin release by neurotrophins: implications for activity-dependent neuronal plasticity

Neurotrophins, secreted in an activity-dependent manner, are thought to be involved in the activity-dependent refinement of synaptic connections. Here we demonstrate that in hippocampal neurons and the rat pheochromocytoma cell line PC12 application of exogenous neurotrophins induces secretion of neurotrophins, an effect that is mediated by the activation of tyrosine kinase neurotrophin receptors (Trks). Like activity-dependent secretion of neurotrophins, neurotrophin-induced neurotrophin secretion requires mobilization of calcium from intracellular stores. Because neurotrophins are likely to be released from both dendrites and axons, neurotrophin-induced neurotrophin release represents a potential positive feedback mechanism, contributing to the reinforcement and stabilization of synaptic connections.

Neurotrophin modulation of NMDA receptors in cultured murine and isolated rat neurons

Neurotrophin modulation of NMDA receptors in cultured murine and isolated rat neurons. J. Neurophysiol. 78: 2363-2371, 1997. Patch-clamp and calcium imaging techniques were used to assess the acute effects of the neurotrophins, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and nerve growth factor (NGF), on the responses of cultured and acutely isolated hippocampal and cultured striatal neurons to the glutamate receptor agonist N-methyl--aspartic acid (NMDA). The effects of BDNF on NMDA-activated currents were examined in greater detail. Currents evoked by NMDA, and the accompanying changes in intracellular calcium, were enhanced by low concentrations of the neurotrophins (1-20 ng/ml). The potentiation by the neurotrophins was rapid in onset and offset (<1 s). The neurotrophins also reduced desensitization of these currents in most cells. The enhancement of NMDA-activated currents by BDNF was observed using both perforated and whole cell patch recording techniques and could be demonstrated in outside-out patches. Furthermore, its effects were not attenuated by pretreatment with the protein kinase inhibitors genistein or 1-(5-isoquinolynesulfony)2-methylpiperazine (H7). Therefore, the actions of BDNF do not appear to be mediated by phosphorylation. Similar enhancements were observed with NT-3 and NT-4 and with NGF despite the fact that hippocampal neurons lack TrkA receptors. All together this evidence suggests that the enhancement of NMDA-evoked currents is unlikely to be mediated through the activation of growth factor receptors. Modulation of NMDA responses by BDNF was dependent on the concentration of extracellular glycine. The most pronounced potentiation by BDNF was observed at low concentrations, whereas no potentiation was observed in saturating concentrations of glycine, suggesting that BDNF may have increased the affinity of the NMDA receptor for glycine. However, the competitive glycine-site antagonist 7-chloro-kynurenic acid blocked the enhancement by BDNF without shifting the dose-inhibition relationship for this antagonist, and Mg2+ consistently depressed the potentiation of NMDA-evoked currents by BDNF, indicating that BDNF does not alter glycine affinity. BDNF also reversibly increased the probability of opening of NMDA channels recorded from outside-out patches taken from cultured hippocampal neurons. Other unrelated peptides including dynorphin and somatostatin also caused a glycine-dependent enhancement of NMDA currents and depressed the currents in saturating concentrations of glycine. In contrast, a shortened analogue dynorphin (6-17), which lacks N-terminus glycine residues, and another peptide met-enkephalin were without effects on NMDA currents recorded in low concentrations of glycine. Our results suggest that neurotrophins and other peptides can serve as glycine-like ligands for the NMDA receptor.

CREB: a major mediator of neuronal neurotrophin responses

Neurotrophins regulate neuronal survival, differentiation, and synaptic function. To understand how neurotrophins elicit such diverse responses, we elucidated signaling pathways by which brain-derived neurotrophic factor (BDNF) activates gene expression in cultured neurons and hippocampal slices. We found, unexpectedly, that the transcription factor cyclic AMP response element-binding protein (CREB) is an important regulator of BDNF-induced gene expression. Exposure of neurons to BDNF stimulates CREB phosphorylation and activation via at least two signaling pathways: by a calcium/calmodulin-dependent kinase IV (CaMKIV)-regulated pathway that is activated by the release of intracellular calcium and by a Ras-dependent pathway. These findings reveal a previously unrecognized, CaMK-dependent mechanism by which neurotrophins activate CREB and suggest that CREB plays a central role in mediating neurotrophin responses in neurons.

Potentiation of developing synapses by postsynaptic release of neurotrophin-4

The hypothesis that synaptic functions can be regulated by neurotrophins secreted from the postsynaptic cell was examined in Xenopus nerve-muscle cultures. Neuromuscular synapses formed on myocytes overexpressing neurotrophin-4 (M+ synapses) exhibited a higher level of spontaneous synaptic activity and enhanced evoked synaptic transmission as compared to those formed on normal control myocytes (M- synapses). The NT-4 effects involve a potentiation of presynaptic transmitter secretion as well as a lengthening of the mean burst duration of postsynaptic low conductance acetylcholine channels. Repetitive stimulation of either the presynaptic neuron or the postsynaptic myocyte led to a potentiation of synaptic transmission at M+ synapses. All potentiation effects of NT-4 overexpression were abolished by the extracellular presence of TrkB-IgG but not by the presence of TrkA-IgG, indicating that postsynaptic secretion of NT-4 was responsible for the synaptic modification.

Brain-derived neurotrophic factor enhances long-term potentiation in rat visual cortex

Brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), members of the nerve growth factor (NGF) gene family, have been suggested to play a role in experience-dependent modification of neural networks in the developing nervous system. In this study we addressed the question of whether these neurotrophins are involved in long-term potentiation (LTP) in developing visual cortex. We recorded layer II/III field potentials and whole-cell currents evoked by test stimulation of layer IV at 0.1 Hz in visual cortical slices prepared from young rats (postnatal day 15-25) and observed effects of BDNF, NT-3, and NGF on these responses. Then we analyzed the effects of these neurotrophins on LTP induced by tetanic (Theta-burst type) stimulation of layer IV. We found that BDNF at 200 ng/ml potentiated field potentials and EPSCs in most cases and that this potentiation lasted after cessation of the BDNF application. At the concentration of 20 ng/ml, BDNF did not show such an effect, but it enhanced the magnitude of expressed LTP. On the other hand, NT-3 and NGF had none of these effects. Immunohistochemical staining of slices with antibody against BDNF showed that exogenous BDNF penetrated into the whole slice within approximately 5 min of its application. The actions of BDNF were blocked by preincubation of slices with TrkB-IgG fusion protein, a BDNF scavenger, or coapplication of K252a, an inhibitor for receptor tyrosine kinases. TrkB-IgG or K252a itself completely blocked LTP, suggesting that endogenous BDNF or another TrkB ligand plays a role in LTP in the developing visual cortex.