There is increasing evidence that neurotrophins (NTs) are involved in processes of neuronal plasticity besides their well-established actions in regulating the survival, differentiation, and maintenance of functions of specific populations of neurons. Nerve growth factor, brain-derived neurotrophic factor, NT-4/5, and corresponding antibodies dramatically modify the development of the visual cortex. Although the neuronal elements involved have not yet been identified, complementary studies of other systems have demonstrated that NT synthesis is rapidly regulated by neuronal activity and that NTs are released in an activity-dependent manner from neuronal dendrites. These data, together with the observation that NTs enhance transmitter release from neurons that express the corresponding signal-transducing Trk receptors, suggest a role for NTs as selective retrograde messengers that regulate synaptic efficacy.

Brain-derived neurotrophic factor (BDNF), a member of the nerve growth factor (NGF) gene family, has been shown to influence the survival and differentiation of specific classes of neurons in vitro and in vivo. The possibility that neurotrophins are also involved in processes of neuronal plasticity has only recently begun to receive attention. To determine whether BDNF has a function in processes such as long-term potentiation (LTP), we produced a strain of mice with a deletion in the coding sequence of the BDNF gene. We then used hippocampal slices from these mice to investigate whether LTP was affected by this mutation. Homo-and heterozygous mutant mice showed significantly reduced LTP in the CAl region of the hippocampus. The magnitude of the potentiation, as well as the percentage of cases in which LTP could be induced successfully, was clearly affected. According to the criteria tested, important pharmacological, anatomical, and morphological parameters in the hippocampus of these animals appear to be normal. These results suggest that BDNF might have a functional role in the expression of LTP in the hippocampus.Neurotrophic factors, in particular the members of the nerve growth factor (NGF) gene family, have so far been considered predominantly with regard to their function in regulating survival and differentiation of specific neuronal populations during embryonic development and the maintenance of characteristic neuronal function in adulthood (1-3). There is, however, evidence that neurotrophins might also be involved in neuronal plasticity (4-10). Long-term potentiation (LTP) is the most widely used paradigm to study cellular and molecular events underlying neuronal plasticity (11). We therefore used this paradigm in slices of the hippocampus from mice with targeted deletion of the brain-derived neurotrophic factor (BDNF) gene to test whether BDNF has a role in this important phenomenon of synaptic plasticity. MATERIALS AND METHODSIn the gene-targeting construct, a 560-bp fragment from the BDNF protein-coding exon was replaced by the selection marker-a neomycin-resistance gene flanked by a glycerate kinase gene promoter and a polyadenylylation signal-thus deleting most of the mature BDNF coding sequence (Fig. 1). Embryonic stem cells (D3, 129Sv) containing the disrupted BDNF gene were injected into BALB/c mouse blastocysts for subsequent generation of chimeric mice. Chimeric males were crossed with NMRI females to produce heterozygotes. In keeping with previously published reports (12, 13), homozygous BDNF (-/-) mutant mice were retarded in growth and had reduced weight (down to only 25% of the wild type) from postnatal day 3 (P3) on. They displayed aberrant limb coordination and balance, showed a loss of neurons in the dorsal root ganglia, and usually died between 2 and 4 weeks after birth. Such abnormalities were never observed in heterozygous BDNF (+/ -) mice.Transverse hippocampal slices (400 ,um thick) were prepared and maintained by standard procedures (medium, 124 mM NaCl/3 mM KCl/1.25 mM...

We report the purification from pig brain of a factor supporting the survival of, and fibre outgrowth from, cultured embryonic chick sensory neurons. The purified factor migrates as one single band, mol. wt. 12 300, on gel electrophoresis in the presence of sodium dodecylsulphate (SDS) and is a basic molecule (pI greater than or equal to 10.1). Approximately 1 microgram factor was isolated from 1.5 kg brain. The final degree of purification was estimated to be 1.4 X 10(6)‐fold, and the specific activity 0.4 ng/ml/unit, which is similar to that of nerve growth factor (NGF) using the same assay system. This factor is the first neurotrophic factor to be purified since NGF, from which it is clearly distinguished because it has different antigenic and functional properties.

During the development of the vertebrate nervous system, many neurons depend for survival on interactions with their target cells. Specific proteins are thought to be released by the target cells and to play an essential role in these interactions. So far, only one such protein, nerve growth factor, has been fully characterized. This has been possible because of the extraordinarily (and unexplained) large quantities of this protein in some adult tissues that are of no relevance to the developing nervous system. Whereas the dependency of many neurons on their target cells for normal development, and the restricted neuronal specificity of nerve growth factor have long suggested the existence of other such proteins, their low abundance has rendered their characterization difficult. Here we report the full primary structure of brain-derived neurotrophic factor. This very rare protein is known to promote the survival of neuronal populations that are all located either in the central nervous system or directly connected with it. The messenger RNA for brain-derived neurotrophic factor was found predominantly in the central nervous system, and the sequence of the protein indicates that it is structurally related to nerve growth factor. These results establish that these two neurotrophic factors are related both functionally and structurally.

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The mRNAs of nerve growth factor (NGF) and brain derived neurotrophic factor (BDNF) exhibit a similar, though not identical, regional and cellular distribution in the rodent brain. In situ hybridization experiments have shown that BDNF, like NGF, is predominantly expressed by neurons. The neuronal localization of the mRNAs of these two neurotrophic molecules raised the question as to whether neuronal activity might be involved in the regulation of their synthesis. After we had demonstrated that depolarization with high potassium (50 mM) resulted in an increase in the levels of both BDNF and NGF mRNAs in cultures of hippocampal neurons, we investigated the effect of a large number of transmitter substances. Kainic acid, a glutamate receptor agonist, was by far the most effective in increasing BDNF and NGF mRNA levels in the neurons, but neither N‐methyl‐D‐aspartic acid (NMDA) nor inhibitors of the NMDA glutamate receptors had any effect. However, the kainic acid mediated increase was blocked by antagonists of non‐NMDA receptors. Kainic acid also elevated levels of BDNF and NGF mRNAs in rat hippocampus and cortex in vivo. These results suggest that the synthesis of these two neurotrophic factors in the brain is regulated by neuronal activity via non‐NMDA glutamate receptors.

Abstract. Nerve growth factor (NGF) and brainderived neurotrophic factor (BDNF) are molecules which regulate the development and maintenance of specific functions in different populations of peripheral and central neurons, amongst them sensory neurons of neural crest and placode origin. Under physiological conditions NGF is synthesized by peripheral target tissues, whereas BDNF synthesis is highest in the CNS. This situation changes dramatically after lesion of peripheral nerves. As previously shown, there is a marked rapid increase in NGF mRNA in the nonneuronal cells of the damaged nerve. The prolonged elevation of NGF mRNA levels is related to the immigration of activated macrophages, interleukin-1 being the most essential mediator of this effect.Here we show that transsection of the rat sciatic nerve also leads to a very marked increase in BDNF mRNA, the final levels being even ten times higher than those of NGF mRNA. However, the time-course and spatial pattern of BDNF mRNA expression are distinctly different. There is a continuous slow increase of BDNF mRNA starting after day 3 post-lesion and reaching maximal levels 3-4 wk later. These distinct differences suggest different mechanisms of regulation of NGF and BDNF synthesis in non-neuronal cells of the nerve. This was substantiated by the demonstration of differential regulation of these mRNAs in organ culture of rat sciatic nerve and Schwann cell culture. Furthermore, using bioassays and specific antibodies we showed that cultured Schwann cells are a rich source of BDNF-and ciliary neurotrophic factor (CNTF)-like neurotrophic activity in addition to NGF. Antisera raised against a BDNF-peptide demonstrated BDNF-immunoreactivity in pure cultured Schwann cells, but not in fibroblasts derived from sciatic nerve.B RAIN-derived neurotrophic factor (BDNF) l and nerve growth factor (NGF) belong to a still growing family of neurotrophic molecules collectively called neurotrophins (6, 60). These proteins show a similar basic structure reflected by conserved domains arranged around the six cysteine residues which seem to be of great importance in determining the three-dimensional structure of these molecules, a prerequisite for their biological activity. However, BDNF and NGF also show distinctly different variable domains which determine the different spectra of their neuronal specificity. Both BDNF and NGF support neural crestderived sensory neurons in the periphery (placode-derived sensory neurons are supported only by BDNF and neurotrophin-3 [NT-3]) (27,28,37). The effects of different neurotrophins on DRG neurons are additive in vitro (BDNF and NGE see reference 37; BDNF and NT-3, see reference 28), suggesting that the various sub-populations of neurons are 1. Abbreviations used in this paper: BDNF, brain-derived neurotrophic factor; DRG, dorsal root ganglion; FK, forskolin; IL-1, intefleukin-1; NGF, nerve growth factor; supported by different members of the NGF gene family in a partly overlapping manner. Under physiological as well as culture conditions NGF ...

Abstract. The intact sciatic nerve contains levels of nerve growth factor (NGF) that are comparable to those of densely innervated peripheral target tissues of NGF-responsive (sympathetic and sensory) neurons. There, the high NGF levels are reflected by correspondingly high mRNA N~F levels. In the intact sciatic nerve, mRNA N~F levels were very low, thus indicating that the contribution of locally synthesized NGF by nonneuronal cells is small. However, after transection an increase of up to 15-fold in mRNA NC~ was measured in 4-mm segments collected both proximally and distally to the transection site. Distally to the transection site, augmented mRNA N~F levels occurred in all three 4-mm segments from 6 h to 2 wk after transection, the longest time period investigated. The augmented local NGF synthesis after transection was accompanied by a reexpression of NGF receptors by Schwann cells (NGF receptors normally disappear shortly after birth). Proximal to the transection site, the augmented NGF synthesis was restricted to the very end of the nerve stump that acts as a "substitute target organ" for the regenerating NGF-responsive nerve fibers. While the mRNA NCF levels in the nerve stump correspond to those of a densely innervated peripheral organ, the volume is too small to fully replace the lacking supply from the periphery. This is reflected by the fact that in the more proximal part of the transected sciatic nerve, where mRNA N~F remained unchanged, the NGF levels reached only 40% of control values. In situ hybridization experiments demonstrated that after transection all nonneuronal cells express mRNA NCF and not only those ensheathing the nerve fibers of NGF-responsive neurons. N'ERVE growth factor (NGF)', a well-characterized protein, is essential for the embryonic development and the maintenance of specialized properties of the sympathetic and neural crest-derived sensory neurons (9,23,38). The similarity of the effects resulting from the administration of anti-NGF antibodies and the interference with the retrograde axonal transport provided indirect evidence for NGF to act as a retrograde messenger, transferring information from the peripheral target tissues to the innervating neurons (12, 33). More recently this indirect evidence has been corroborated by the demonstration that while the density of sympathetic innervation of target tissues is correlated with the levels of NGF (18) and its mRNA (14, 34), the high levels of NGF in both sympathetic and sensory ganglia are not reflected by correspondingly high levels of mRNA N~F (5, 14), implying that NGF is accumulated in the ganglia by axonal transport from the periphery rather than by local synthesis. This interpretation was supported by the observation that the destruction of sympathetic nerve terminals by 6-hydroxydopamine or blockade of axonal transport S. Korsching's present address is California Institute of Technology, Division of Biology, 216-76, Pasadena, California 91125.