Neurotrophins regulate dendritic growth in developing visual cortex

Growth factors sculpt the synapse

The brain's ability to learn and remember is at least partly due to the changing strength of its cell-to-cell connections, the synapses. Growth factors, previously thought to confine their action on synapses to the developing organism, can also change synaptic strength in the adult; an example in the invertebrate Aplysia is reported by Zhang et al . on p. 1318 of this issue. In her Perspective, Schuman discusses how these new results add to what is known about the actions of growth factors in the adult and developing brain.

Somatostatin and brain-derived neurotrophic factor mRNA expression in the primate brain: decreased levels of mRNAs during aging

The expression of the genes for somatostatin (SRIF) and brain-derived neurotrophic factor (BDNF) was investigated in the central nervous system (CNS) of the macaque monkey (Macaca fuscata fuscata). Using Northern blot analysis, one SRIF mRNA transcript, 0.65 kb, and two BDNF mRNA transcripts, 1.6 and 4.0 kb in length, were detected in the monkey brain tissues. During the aging process (2 years, 10 years, and > 30 years), the ratio of SRIF mRNA/glyceraldehyde-3 phosphate dehydrogenase (G3PDH) mRNA significantly decreased (60-70%) in the hippocampus and in several cerebral subdivisions such as frontal cortex, temporal cortex, motor cortex, somatosensory cortex and visual cortex. BDNF mRNA was expressed in the various cerebral subdivisions and in the hippocampus. During the aging process, the gene expression of BDNF declined (20-50% for the 4.0 kb transcript, and 40-70% for the 1.6 kb transcript) in the various cerebral subdivisions. In the hippocampus, the level of the 1.6 kb mRNA at > 30 years old declined to 60% of the level at 2 years old, while the 4.0 kb mRNA did not change significantly during the aging process. Recent studies have shown that BDNF enhances the expression of SRIF mRNA in the rodent cerebral cortex (Nawa, H. et al., J. Neurochem., 60 (1993) 772-775; Nawa, H. et al., J. Neurosci., 14 (1994) 3751-3765). These studies and our present results suggest that the decrease in gene expression for a neurotrophic molecule, such as BDNF, might cause the levels of SRIF mRNA to decline in the primate brain during the aging process.

Changing patterns of expression and subcellular localization of TrkB in the developing visual system

Neurotrophins play important roles in the survival, differentiation, and maintenance of CNS neurons. To begin to investigate specific roles for these factors in the mammalian visual system, we have examined the cellular localization of the neurotrophin receptor trkB within the developing cerebral cortex and thalamus of the ferret using extracellular domain-specific antibodies. At prenatal ages (gestation is 41 d), trkB-immunostained fibers were observed in the internal capsule and as two distinct fascicles within the intermediate zone of the cerebral cortex. The staining of these fiber tracts declined with increasing age, whereas soma and dendrite staining of cortical neurons was first evident in early postnatal life and increased during subsequent development. Staining of subplate neurons [by prenatal day 5 (P5)] was followed by staining of cortical layer 5 neurons (at P10). By P31, trkB immunoreactivity was particularly prominent in layers 3 and 5 but was absent from subplate neurons. Staining included cells, especially pyramidal neurons, in all cortical layers by P45, and this pattern was maintained into adulthood. The optic tract and fibers within the lateral geniculate nucleus (LGN) were also strongly trkB immunoreactive at prenatal ages. Cellular staining of a subset of LGN neurons, those within the C-layers and perigeniculate nucleus, was apparent by P10 and maintained until P45, when the adult pattern of highly trkB-immunoreactive neurons in all layers of the LGN first appeared. The pattern of trkB immunoreactivity suggests that specific subsets of cortical and thalamic neurons may respond to neurotrophins such as brain-derived neurotrophic factor and/or NT-4/5 at discrete developmental times and locations. The appearance of trkB on axon fibers early in development and then on cell bodies and dendritic processes later is consistent with roles for both long-range and local, including autocrine and/or paracrine, delivery of neurotrophins in cell survival and maturation.

Structural and functional characterization of the rat neurotrophin-4 gene

Neurotrophin-4 (NT-4) is a member of the neurotrophin family of growth factors. To study the molecular mechanisms that govern NT-4 expression, we have cloned and characterized the rat genome region encoding NT-4. The rat NT-4 gene consists of three exons: two 5'-flanking noncoding exons and a coding exon. NT-4 mRNA transcription is controlled by two promoters flanking the noncoding exons. Alternative splicing of the second intron results in a NT-4 mRNA with a different open reading frame, encoding a shorter protein lacking pre-NT-4 sequence. A rat NT-4 gene fragment, containing all exons and introns in addition to 1.4 kb of the upstream genomic sequence, has been introduced into mice. This transgene enables partial recapitulation of the expression pattern of NT-4 mRNA and confers activity-dependent expression of the NT-4 mRNA in muscle.

Developmental plasticity in neural circuits for a learned behavior

The neural substrate underlying learned vocal behavior in songbirds provides a textbook illustration of anatomical localization of function for a complex learned behavior in vertebrates. The song-control system has become an important model for studying neural systems related to learning, behavior, and development. The song system of zebra finches is characterized by a heightened capacity for both neural and behavioral change during development and has taught us valuable information regarding sensitive periods, rearrangement of synaptic connections, topographic specificity, cell death and neurogenesis, experience-dependent neural plasticity, and sexual differentiation. The song system differs in some interesting ways from some well-studied mammalian model systems and thus offers fresh perspectives on specific theoretical issues. In this highly selective review, we concentrate on two major questions: What are the developmental changes in the song system responsible for song learning and the restriction of learning to a sensitive period, and what factors explain the highly sexually dimorphic development of this system? We discuss the important role of sex steroid hormones and of neurotrophins in creating a male-typical neural song circuit (which can learn to produce complex vocalizations) instead of a reduced, female-typical song circuit that does not produce learned song.

Transplant of Schwann cells allows normal development of the visual cortex of dark-reared rats

Visual experience is necessary for the correct development of the visual cortex. Dark-rearing from birth affects normal maturation of the functional properties of mammalian visual cortex: cortical cells show rapid habituation to repeated stimulation, decreased orientation selectivity, and enlarged receptive fields. Spatial resolution and response latency are also impaired. Recent experiments have demonstrated that visual deprivation reduces the expression of neurotrophins in the visual cortex. We formulated the hypothesis that visual experience drives the maturation of functional properties of the visual cortex by regulating cortical levels of neurotrophins. If this hypothesis is correct, exogenous supply of neurotrophins during dark-rearing from birth should prevent, or at least ameliorate, the effects of a lack of visual experience. Since Schwann cells are efficient biological minipumps of neurotrophic factors, we transplanted 1.0 or 1.5 x 10(6) Schwann cells or infused vehicle solution as a control into the lateral ventricles of 13 day old rats reared in total darkness from birth until the end of the critical period (postnatal day 45). Single-cell responses and visual-evoked potentials were recorded from the binocular zone of the primary visual cortex of each group. We found that in Schwann cell-transplanted animals all parameters tested were significantly improved upon those of dark-reared control rats and were in the range of normal adult values. Thus, Schwann cell transplant contributed to the normal development of visual response properties in the visual cortex, compensating for a complete absence of visual experience.

Brain-derived neurotrophic factor and nerve growth factor potentiate excitatory synaptic transmission in the rat visual cortex

1. The effect of brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) on excitatory synaptic transmission in the developing visual cortex was studied by whole-cell patch-clamp recordings from rat brain slices. 2. Both neurotrophins induced a rapid increase in the amplitude of impulse-evoked excitatory postsynaptic currents (EPSCs). BDNF also increased the frequency of spontaneous EPSCs. 3. Analysis of the currents revealed that alpha-amino-3-hydroxy-5-methyl-isoxazole propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor-mediated components contributing to the EPSC peak amplitude were equally potentiated by the neurotrophins. 4. When synaptic transmission was studied by minimal stimulation of intracortical afferents, neurotrophins induced a decrease in the occurrence of release failures. 5. A number of neurones were insensitive to the effects of the neurotrophins, possibly related to the considerable heterogeneity of neuronal types and to the uneven distribution of neurotrophin receptors in the visual cortex. 6. The probability of neurotransmitter release represents a rapidly modifiable synaptic feature by which neurotrophins can potentiate the efficacy of excitatory synaptic transmission in the visual cortex.

Role of neurotrophins in synapse development and plasticity

Neurotrophic factors are traditionally viewed as secretory proteins that regulate long-term survival and differentiation of neurons. The role of neurotrophic factors in the structural integrity of the nervous system makes them attractive candidates as therapeutic agents for neurodegenerative diseases. However, the fact that expression of many neurotrophic factors in the central nervous system is rapidly enhanced by neuronal activity suggests a new role for these factors in activity-dependent processes, such as synaptic development and plasticity. A series of recent studies has provided strong evidence for this novel function of neurotrophic factors. The neurotrophin family of proteins has been shown to acutely potentiate synaptic transmission at the neuromuscular junction and in the brain. These factors are also involved in the maturation of the neuromuscular synapses and in the development of synapses in the visual system. Gene targeting and physiological experiments demonstrate that brain-derived neurotrophic factor (BDNF) plays an important role in long-term potentiation (LTP), a cellular model for learning and memory. These findings have brought together two hotly pursued areas of neuroscience, namely, the function of neurotrophic factors and the mechanisms for synaptic plasticity. Continuous studies in this new field will help understand how synapses develop and function in the brain, and may have significant implications in treating learning disorders in both children and adults.

mRNAS for one, two or three members of trk receptor family are expressed in single rat trigeminal ganglion neurons

We studied the expression of mRNAs of neurotrophin (NTF) receptors trkA, trkB and trkC in single rat trigeminal ganglion neurons at embryonic days 12 and 16 to determine, whether single trigeminal ganglion neurons express one trk family member or coexpress several of them. For that purpose we elaborated a sensitive technique of reverse transcriptase-polymerase chain reaction to detect all neurotrophin receptors in a single neuron. Expression of neurofilament light chain mRNA was used as a positive marker to confirm the recovery of mRNAs from single neurons. Neurofilament-positive samples were subsequently analyzed for the expression of mRNAs for catalytic trkA, trkB, and trkC, and in some cases, low-affinity neurotrophin receptor (p75). We found neurons expressing one, coexpressing two, or even all three trk receptors. In many neurons analyzed, p75 mRNA was coexpressed with trks, but we also found neurons expressing only trks without p75, and a neuron expressing p75 alone. There were also neurons containing neither trk receptors nor p75. We provide here first direct evidence that single sensory neurons can simultaneously express three or even four neurotrophin receptors.

Neurotrophin regulation of cortical dendritic growth requires activity

Neurotrophins have been proposed to mediate several forms of activity-dependent competition in the central nervous system. A key element of such hypotheses is that neurotrophins act preferentially on active neurons; however, little direct evidence supports this postulate. We therefore examined, in ferret cortical brain slices, the interactions between activity and neurotrophins in regulating dendritic growth of layer 4 pyramidal neurons. Inhibition of spontaneous electrical activity, synaptic transmission, or L-type calcium channels each prevented the otherwise dramatic increase in dendritic arborizations elicited by brain-derived neurotrophic factor. In developing cortex, this requirement for conjoint neurotrophin signaling and activity provides a mechanism for selectively enhancing the growth and connectivity of active neurons.

Molecular characterization of the dendritic growth cone: regulated mRNA transport and local protein synthesis

The molecular mechanisms that regulate growth cone guidance of dendrite outgrowth remain to be elucidated. We hypothesized that mRNA localization in dendritic growth cones and their local protein synthesis may be important for growth cone functioning. The appearance of 23 of 31 growth cone mRNAs was developmentally regulated. Also, alteration of growth cone morphology affected the relative levels of three mRNAs. Finally, using single dendrite transfection, it was shown that local protein synthesis occurs in dendrites and growth cones. A heterogeneous population of mRNAs exists in dendritic growth cones of cultured hippocampal neurons whose relative abundances are developmentally regulated and can vary with changes in growth cone physiology. The demonstration of protein synthesis in growth cones suggests that translation of the localized mRNAs may contribute to regulation of growth cone motility and dendrite outgrowth.

Mice lacking brain-derived neurotrophic factor exhibit visceral sensory neuron losses distinct from mice lacking NT4 and display a severe developmental deficit in control of breathing

The neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-4/5 (NT4) act via the TrkB receptor and support survival of primary somatic and visceral sensory neurons. The major visceral sensory population, the nodose-petrosal ganglion complex (NPG), requires BDNF and NT4 for survival of a full complement of neurons, providing a unique opportunity to compare gene dosage effects between the two TrkB ligands and to explore the possibility that one ligand can compensate for loss of the other. Analysis of newborn transgenic mice lacking BDNF or NT4, or BDNF and NT4, revealed that survival of many NPG afferents is proportional to the number of functional BDNF alleles, whereas only one functional NT4 allele is required to support survival of all NT4-dependent neurons. In addition, subpopulation analysis revealed that BDNF and NT4 can compensate for the loss of the other to support a subset of dopaminergic ganglion cells. Together, these data demonstrate that the pattern of neuronal dependencies on BDNF and NT4 in vivo is far more heterogeneous than predicted from previous studies in culture. Moreover, BDNF knockout animals lack a subset of afferents involved in ventilatory control and exhibit severe respiratory abnormalities characterized by depressed and irregular breathing and reduced chemosensory drive. BDNF is therefore required for expression of normal respiratory behavior in newborn animals.

Synaptic activity and the construction of cortical circuits

Vision is critical for the functional and structural maturation of connections in the mammalian visual system. Visual experience, however, is a subset of a more general requirement for neural activity in transforming immature circuits into the organized connections that subserve adult brain function. Early in development, internally generated spontaneous activity sculpts circuits on the basis of the brain's "best guess" at the initial configuration of connections necessary for function and survival. With maturation of the sense organs, the developing brain relies less on spontaneous activity and increasingly on sensory experience. The sequential combination of spontaneously generated and experience-dependent neural activity endows the brain with an ongoing ability to accommodate to dynamically changing inputs during development and throughout life.

Differential distribution of AMPA receptors and glutamate during pre- and postnatal development in the visual cortex of ferrets

Immunohistochemical methods were used to study the distribution and time-course of appearance of cells expressing glutamate and alpha-amino-3-hydroxy-5-methyl-4-isoaxazole propionic acid (AMPA)-type glutamate receptors (GluR1 and GluR2/3) during development of the ferret visual cortex. Glutamate is present in many neurons in the ventricular zone, intermediate zone, developing cortical plate, and marginal zone as early as embryonic day (E) 34 (birth is at E41 in ferrets). Glutamate attains its adult distribution coincident with the completion of cellular migration. By contrast, GluR1 immunoreactivity emerges more slowly. By birth, GluR1 immunoreactivity is present only in a few neurons in the marginal zone and ventricular zone but is abundant in the marginal zone and subplate, where synaptogenesis commences. The number and staining intensity of GluR1-positive cells increases dramatically during the first two postnatal weeks and is maximal between the second and third week, before slowly declining to adult levels. Cortical cells immunopositive for GluR2/3 follow a similar pattern, although their distribution differs: GluR2/3-positive cells are mainly pyramidal cells. During the first postnatal week, GluR2/3 is also transiently present in fibers in the intermediate zone, which at this stage contains many thalamocortical and callosal and corticofugal axons. The abundance of glutamate at fetal stages, especially in the ventricular zone, is consistent with the previously proposed role of glutamate in mediating trophic effects in vivo, as previously demonstrated in vitro. The expression of AMPA receptors, as well as their transient overexpression, confirms the results of in situ hybridization studies and may imply a developmental role in neuronal differentiation for these receptors, in addition to their mature role in mediating cortical transmission.

Suppression of trkB expression by antisense oligonucleotides alters a neuronal phenotype in the rod pathway of the developing rat retina

trkB is the high-affinity receptor for brain-derived neurotrophic factor (BDNF), a trophic molecule with demonstrated effects on the survival and differentiation of a wide variety of neuronal populations. In the mammalian retina, trkB is localized to both ganglion cells and numerous cells in the inner nuclear layer. Much information on the role of BDNF in neuronal development has been derived from the study of trkB- and BDNF-deficient mutant mice. This includes an attenuation of the numbers of cortical neurons immunopositive for the calcium-binding proteins, parvalbumin, and calbindin. Unfortunately, these mutant animals typically fail to survive for > 24-48 hr after birth. Since most retinal neuronal differentiation occurs postnatally, we have devised an alternative scheme to suppress the expression of trkB in the retina to examine the role of BDNF on the postnatal development of neurons of the inner retina. Neonatal rats were treated with intraocular injection of an antisense oligonucleotide (1-2 microliters of 10-100 microM solution) targeted to the trkB mRNA. Immunohistochemistry with a polyclonal antibody to trkB showed that the expression of trkB in retinal neurons was suppressed 48-72 hr following a single injection. Northern blot analysis demonstrated that antisense treatment had no effect on the level of trkB mRNA, even after multiple injections. This suggests an effect of trkB antisense treatment on protein translation, but not on RNA transcription. No alterations were observed in the thickness of retinal cellular or plexiform layers, suggesting that BDNF is not the sole survival factor for these neurons. There were, however, alterations in the patterns of immunostaining for parvalbumin, a marker for the narrow-field, bistratified AII amacrine cell-a central element of the rod (scotopic) pathway. This was evidenced by a decrease in both the number of immunostained somata (> 50%) and in the intensity of immunolabeling. However, the immunostaining pattern of calbindin was not affected. These studies suggest that the ligands for trkB have specific effects on the neurochemical phenotypic expression of inner retinal neurons and in the development of a well-defined retinal circuit.

Developmental and mature expression of full-length and truncated TrkB receptors in the rat forebrain

The neurotrophins brain-derived neurotrophic factor (BDNF) and NT-4/5 exert their trophic effects on the nervous system via signaling through trkB receptors. These receptors occur as splice variants of the trkB gene that encodes a full-length receptor containing the signal transducing tyrosine kinase domain as well as truncated forms lacking this domain. Because the importance of the trkB isoforms for development and maturation of the nervous system is unknown, we have examined the expression of trkB receptor isoforms during development of the rat forebrain using 1) a sensitive ribonuclease protection assay to distinguish full-length and truncated trkB transcripts, 2) western blot analysis to characterize developmental changes in trkB proteins, and 3) immunohistochemistry to determine the cellular localization of trkB receptors. In the rat forebrain, adult mRNA levels for full-length trkB are reached by birth, whereas truncated trkB message does not peak until postnatal days 10-15. Western blot analysis indicates that full-length trkB protein is the major form during early development, whereas truncated trkB protein predominates in all forebrain regions of late postnatal and adult rats. These data also suggest that the glycosylation state of these receptors changes during postnatal maturation. TrkB immunoreactivity is present predominately within neurons, where it is localized to axons, cell soma, and dendrites. Strong dendritic immunostaining is particularly evident in certain neuronal populations, such as pyramidal neurons in the hippocampus and in layer V of the neocortex. The dendritic localization of trkB receptors supports the hypothesis that dendrites, as well as axons, are important sites for neurotrophin actions in the central nervous system.

Effects of BDNF and NT-4/5 on striatonigral neuropeptides or nigral GABA neurons in vivo

Supranigral infusions of the TrkB-receptor-preferring neurotrophins BDNF or NT-4/5 augment locomotor behaviours, pars compacta firing rates and striatal dopamine metabolism. However these actions of BDNF or NT-4/5 may involve other neurotransmitter systems in addition to dopamine neurons in the substantia nigra. We thus investigated the effects of 2-week supranigral infusions of BDNF or NT-4/5 on rat peptidergic striatonigral neurons and nigral GABAergic neurons. Radioimmunoassay revealed that BDNF and NT-4/5 elevated substantia nigra levels of substance P (by 46 and 57% respectively) and substance K (by 64 and 81%). In addition, BDNF elevated substance K by 59% in a nigral projection area, the superior colliculus. NT-4/5 elevated dynorphin A in the substantia nigra (by 52%) and met-enkephalin in substantia nigra and globus pallidus (by 89%). None of these neuropeptides were altered in the striatum. Consistent with these findings, supranigral infusions of BDNF elevated the mRNA for preprotachykinin A in striatal neurons. In the same animals, glutamic acid decarboxylase (GAD)67 mRNA was increased by 48% in the substantia nigra. The cross-sectional area of GAD67-positive neuronal somata in the BDNF-infused nigra was increased by 59%, and 70% of nigral GABAergic neurons had a cross-sectional area > 550 microns2, whereas 95% of the neurons in vehicle-infused animals had cross-sectional areas < 550 microns2. Thus, supranigral infusions of BDNF or NT-4/5 increase tachykinin mRNA and protein levels within striatonigral neurons and increase the size and GAD67 mRNA expression levels of nigral GABAergic neurons. These results suggest that BDNF or NT-4/5 may modify the output of the basal ganglia not only through effects on dopamine neurons but also by increasing neurotransmission in striatonigral peptidergic and nigral GABAergic pathways.

Fast actions of neurotrophic factors

A diversity of neurotrophic factors are required for the differentiation and survival of neurons and for maintaining their phenotype. By virtue of the rapid time scale of signal transduction in the cytosol, many of these factors also acutely regulate neuronal functions as diverse as synaptic transmission and nerve growth. These fast actions greatly expand the regulatory role of neurotrophic factors, particularly in the synaptic plasticity of developing nervous systems.

Recombinant BDNF rescues deficits in basal synaptic transmission and hippocampal LTP in BDNF knockout mice

Brain-derived neurotrophic factor (BDNF) is expressed at high levels in hippocampal neurons, and its expression is modulated by neural activity. Knockout mice can be used to study the roles of molecules like BDNF in synaptic plasticity with more molecular specificity than is possible using pharmacological approaches. Because in conventional knockouts the disrupted gene product is absent in all tissues throughout the life of the animal, developmental effects may complicate the interpretation of deficits in the adult. Rescue experiments can help to distinguish between developmental and acute requirements for the missing gene product. We here demonstrate that treatment of hippocampal slices from BDNF knockout mice with recombinant BDNF completely reverses deficits in long-term potentiation and significantly improves deficits in basal synaptic transmission at the Schaffer collateral-CA1 synapse. Thus, BDNF has an acute role in hippocampal synaptic function.

The action of neurotrophins in the development and plasticity of the visual cortex

Nerve growth factor (NGF) and the other members of the NGF gene family have been extensively characterized as neurotrophic factors. Recently a modulatory action of these neurotrophic factors on synapse efficacy has emerged. The developing visual system has provided a convenient model to test the role of neurotrophins on neural plasticity in vivo.

Cortical development: with an eye on neurotrophins

Recent observations suggest that neurotrophins are involved in activity-dependent plasticity of the developing cerebral cortex. What molecular mechanisms underlie activity-dependent competition between axons for trophic factors?

Neurotrophins and activity-dependent development of the neocortex

A number of recent results suggest that neurotrophins play an important role in early development as well as in the later, activity-dependent processes important for the final shaping of cortical connections. Many neurotrophins and their receptors are regulated in parallel with the 'critical period' in development, and their application to the neocortex can dramatically alter the functional organization of the cortex, as well as the morphological properties of neocortical neurons. In addition, recent data show that a different phenomenon of synaptic plasticity, hippocampal long-term potentiation, also critically depends on neurotrophins. Thus, neurotrophins may play a role in linking functional modifications of synapses to the morphological effects of synaptic stabilization and rearrangement, as observed in the neocortex.

Sorting signals and targeting of infectious agents through axons: an annotation to the 100 years' birth of the name "axon"

A brief review is given on mechanisms by which axons may be initiated during development and by which the polarity of neurons is maintained by selective sorting and delivery of molecules to axons and dendrites. The use of viruses as tools to study targeting of newly synthesized proteins to axons is described. Emphasis is then given to the hazards that are presented to the individual by the retrograde transport of infectious agents in axons to the brain. Borna disease virus, prions, and Listeria monocytogenes are examined briefly as examples of these mechanisms. These agents have attracted interest previously in veterinary medicine for the most part, but they may present potential and substantial threats to human health. Such infectious agents also represent a new type of virus, a new principle for disease transmission, and a new mechanism for intracellular transport, respectively.

Effects of brain-derived neurotrophic factor on optic axon branching and remodelling in vivo

Neurotrophins are thought to be important for the survival and differentiation of vertebrate neurons. Roles have been suggested for target-derived neurotrophins, based both on their expression in target tissues at the time of neuron innervation, and on their effects on axonal sprouting. However, direct in vivo evidence of their involvement in axon arborization has remained elusive. We have used in vivo microscopy to follow individual optic axons over time, and have examined the role of the neurotrophin brain-derived neurotrophic factor (BDNF) in their development. Here we show that injection of BDNF into the optic tectum of live Xenopus laevis tadpoles increased the branching and complexity of optic axon terminal arbors. In contrast, injection of specific neutralizing antibodies to BDNF reduced axon arborization and complexity. The onset of these effects was rapid (within 2 hours) and persisted throughout the 24-hour observation period. Other neurotrophins had little or no significant effects. These results demonstrate the involvement of neurotrophins in the dynamic elaboration of axon terminals, and suggest a direct role for target-derived BDNF during synaptic patterning in the developing central nervous system.

NT-4-mediated rescue of lateral geniculate neurons from effects of monocular deprivation

Altering the balance of activity between the two eyes during the critical period for visual-system development profoundly affects competitive interactions among neurons in the lateral geniculate nucleus and primary visual cortex. Neurons in the lateral geniculate nucleus that are deprived of activity by closing or silencing one eye atrophy as a result of competition with non-deprived neurons for some critical factor(s) presumed to be present in the cortex. Based on their actions in the developing visual system, neurotrophins are attractive candidates for such factors. We tested whether neurotrophins mediate intracortical competition of afferents from the lateral geniculate nucleus by using monocular deprivation and a new method for highly localized, in vivo delivery of neurotrophins. This method allowed unambiguous identification of neurons that were exposed to neurotrophin. Here we report that only one neurotrophin, the TrkB ligand NT-4, rescued neurons in the lateral geniculate nucleus from the dystrophic effects of monocular deprivation.