Studies of the peripheral nervous system have led to the concept of target-derived neurotrophic support. Neurotrophic factors such as nerve growth factor are now known to act as retrograde trophic factors – retrophins – that are produced in the target cells and released to presynaptic neurons. However, using brain-derived neurotrophic factor (BDNF) tagged with green- fluorescent protein to monitor the subcellular dynamics of BDNF in neurites, Tsumoto and colleagues have provided persuasive visual evidence that BDNF can be released at the synapses of brain neurons in an activity-dependent manner to act on postsynaptic neurons. Accordingly, BDNF serves as an anterophin to regulate postsynaptic development and plasticity in the central nervous system. Remarkable progress in the field of cell technology has enabled the cloning of various genes for fluorescent proteins, and these are becoming commercially available in the form of plasmids carrying eukaryotic expression promoters. Tagging a target protein with such a fluorescent protein at the gene level makes it possible to visualize the location of the target protein and to monitor its subcellular dynamics using fluorescence microscopy. Using such a molecular genetic approach, brain-derived neurotrophic factor (BDNF) protein has been fused to green fluorescent protein (GFP) in a plasmid vector, and gene transfer of the plasmid construct into cells then expresses this GFP-tagged BDNF (Fig. 1). The GFP–BDNF was shown to be released from neurons and mimic the conventional activities of BDNF through activation of TrkB receptors 1 . When Kohara, Tsumoto and colleagues microinjected the gene encoding GFP–BDNF and the gene for another fluorescent protein (DsRed) into the nuclei of cultured neurons, puncta of GFP–BDNF were found in axon-like processes and migrated away from the soma with a velocity of 0.3 ± 0.1 μm s -1 (Ref. 2). Thus, the green-fluorescent spots in the axons presumably represent GFP–BDNF-containing vesicles that are moving anterogradely. Surprisingly, the somatic region of the neurons on which the GFP–BDNF- carrying axons terminated and formed synapses also showed green fluorescence 2 . Because the postsynaptic neurons did not exhibit the red fluorescence of the DsRed that had been microinjected simultaneously, the green fluorescence in the soma cannot have been ascribed to their expression of the GFP–BDNF gene, and instead must have been supplied from the presynaptic neurons. The effects of neuronal activity on the synaptic release of GFP–BDNF were also evaluated. In neocortical cultures prepared from rat embryos, spontaneous synaptic activities were generated, and were manipulated with TTX or picrotoxin. Treatment with TTX for 48 hrs blocked the synaptic currents completely, and markedly reduced the fluorescence of GFP–BDNF in the postsynaptic neurons. Conversely, treatment with picrotoxin enhanced the firing rates of postsynaptic neurons and significantly increased the fluorescence. The activity-dependent synaptic transfer of GFP–BDNF indicates that BDNF acts as an anterograde trophic factor in the CNS. Several pieces of indirect evidence show that BDNF can be released from presynaptic terminals and act on postsynaptic neurons (Fig. 2). BDNF has been found in the dense core vesicles of nerve terminals or recovered in the synaptic vesicular fraction 3–5 , and its TrkB receptors are localized in dendrites and axons 6 . In addition, axotomy of BDNF- containing neurons results in depletion of BDNF in their target area, suggesting that BDNF must be transported anterogradely to the nerve terminals 7,8 . The present results clearly show that this is indeed the case: BDNF is stored in the presynaptic terminals and released to postsynaptic neurons. In spite of this conclusion, significant amounts of GFP–BDNF fluorescence have also been found in dendritic processes 2,9 , raising the question of how BDNF is released from dendrites. Non-neuronal cells are known to liberate BDNF, which is presumably packed in constitutive vesicles, whose dynamics and turnover are very rapid 10 . In agreement with the present data, previous neurochemical studies have shown that BDNF is, in part, released from CNS neurons in an activity-dependent manner and in constitutive fashion 11,12 . Whether a proportion of BDNF can be released toward presynaptic CNS neurons, serving as a ‘traditional’ retrograde trophic factor, remains to be clarified. Postsynaptic influences of neurotrophins have received less attention than their presynaptic effects. Target molecules for BDNF include NMDA-type and AMPA-type glutamate receptors, nitric oxide synthase (nNOS), and calcium binding proteins 13–15 . Phosphorylation of NMDAR1 subunits becomes apparent, leading to enhancement of their channel activity in the acute phase 16 . Exposure to BDNF for a period of days elevates the expression of TRENDS in Neurosciences Vol.24 No.12 December 2001 http://tins.trends.com 0166-2236/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S0166-2236(00)01955-X 683 Research Update BDNF as an anterophin;a novel neurotrophic relationship between brain neurons Hiroyuki Nawa and Nobuyuki Takei TRENDS in Neurosciences Presynaptic Postsynaptic Axon Picrotoxin TTX None Dendrite (b) (a) (c) Fig. 1 . Axonal transport and synaptic transfer of green fluorescent protein (GFP)-tagged brain-derived neurotrophic factor (BDNF) between cultured brain neurons. (a) GFP–BDNF can be found in dendritic processes and axonal vesicles of neurons transformed with its gene. In addition, the somatic region of postsynaptic neuron also exhibits the fluorescence of GFP–BDNF, which is presumably derived from the presynaptic neuron. (b) The fluorescence in the postsynaptic neuron disappears when the neurons are treated with tetrodotoxin (TTX) or TrkB-Ig. (c) By contrast, it is enhanced in the presence of picrotoxin.