Neurotrophins suppress apoptosis induced by deafferentation of an avian motor-cortical region

Auditory feedback in learning and maintenance of vocal behaviour

Songbirds are one of the best-studied examples of vocal learners. Learning of both human speech and birdsong depends on hearing. Once learned, adult song in many species remains unchanging, suggesting a reduced influence of sensory experience. Recent studies have revealed, however, that adult song is not always stable, extending our understanding of the mechanisms involved in song maintenance, and their similarity to those active during song learning. Here we review some of the processes that contribute to song learning and production, with an emphasis on the role of auditory feedback. We then consider some of the possible neural substrates involved in these processes, particularly basal ganglia circuitry. Although a thorough treatment of human speech is beyond the scope of this article, we point out similarities between speech and song learning, and ways in which studies of these disparate behaviours complement each other in developing an understanding of general principles that contribute to learning and maintenance of vocal behaviour.

An associational model of birdsong sensorimotor learning II. Temporal hierarchies and the learning of song sequence

Understanding the neural mechanisms underlying serially ordered behavior is a fundamental problem in motor learning. We present a computational model of sensorimotor learning in songbirds that is constrained by the known functional anatomy of the song circuit. The model subsumes our companion model for learning individual song "syllables" and relies on the same underlying assumptions. The extended model addresses the problem of learning to produce syllables in the correct sequence. Central to our approach is the hypothesis that the Anterior Forebrain Pathway (AFP) produces signals related to the comparison of the bird's own vocalizations and a previously memorized "template." This "AFP comparison hypothesis" is challenged by the lack of a direct projection from the AFP to the song nucleus HVc, a candidate site for the generator of song sequence. We propose that sequence generation in HVc results from an associative chain of motor and sensory representations (motor --> sensory --> next motor. ) encoded within the two known populations of HVc projection neurons. The sensory link in the chain is provided, not by auditory feedback, but by a centrally generated efference copy that serves as an internal prediction of this feedback. The use of efference copy as a substitute for the sensory signal explains the ability of adult birds to produce normal song immediately after deafening. We also predict that the AFP guides sequence learning by biasing motor activity in nucleus RA, the premotor nucleus downstream of HVc. Associative learning then remaps the output of the HVc sequence generator. By altering the motor pathway in RA, the AFP alters the correspondence between HVc motor commands and the resulting sensory feedback and triggers renewed efference copy learning in HVc. Thus, auditory feedback-mediated efference copy learning provides an indirect pathway by which the AFP can influence sequence generation in HVc. The model makes predictions concerning the role played by specific neural populations during the sensorimotor phase of song learning and demonstrates how simple rules of associational plasticity can contribute to the learning of a complex behavior on multiple time scales.

Nerve growth factor but not neurotrophin-3 is synthesized by hippocampal GABAergic neurons that project to the medial septum

Conventional uptake of neurotrophins takes place at axon terminals via specific receptors, and is followed by retrograde transport. Recent studies demonstrated that, with the exception of nerve growth factor, other neurotrophins may be delivered anterogradely to the region containing the receptor expressing neurons. In this study we used a triple labeling method that combines retrograde tract tracing, in situ hybridization and immunocytochemistry to examine whether non-principal cells projecting from the hippocampus to the septum synthesize nerve growth factor. Our results show that, on average, 59% of the horseradish peroxidase-labeled hippocamposeptal nonpyramidal neurons also display nerve growth factor messenger RNA hybridization signal. The ratio was slightly higher in the CA1 stratum oriens and the hilus of the dentate gyrus (64 and 62%, respectively) compared to stratum oriens of the CA3 region (58%). In addition, we demonstrated that many nerve growth factor-positive septally projecting neurons also contain the calcium-binding protein calbindin D-28K, whereas nerve growth factor-negative projecting cells mostly lack this neurochemical marker. In contrast to nerve growth factor, neurotrophin-3 has never been found in hippocamposeptal cells. Hippocamposeptal GABAergic cells are reciprocally connected with the medial septum, thus they are in a key position to regulate nerve growth factor release as a function of the activity level in the septohippocampal system. Furthermore, our results raise the intriguing possibility that nerve growth factor may be transported also in an anterograde manner. Regardless of the direction of transport, the presence of nerve growth factor in hippocamposeptal cells suggests that long distance fast synaptic mechanisms and slow neurotrophin action are coupled in these neurons.

A relationship between behavior, neurotrophin expression, and new neuron survival

The high vocal center (HVC) controls song production in songbirds and sends a projection to the robust nucleus of the archistriatum (RA) of the descending vocal pathway. HVC receives new neurons in adulthood. Most of the new neurons project to RA and replace other neurons of the same kind. We show here that singing enhances mRNA and protein expression of brain-derived neurotrophic factor (BDNF) in the HVC of adult male canaries, Serinus canaria. The increased BDNF expression is proportional to the number of songs produced per unit time. Singing-induced BDNF expression in HVC occurs mainly in the RA-projecting neurons. Neuronal survival was compared among birds that did or did not sing during days 31-38 after BrdUrd injection. Survival of new HVC neurons is greater in the singing birds than in the nonsinging birds. A positive causal link between pathway use, neurotrophin expression, and new neuron survival may be common among systems that recruit new neurons in adulthood.

Deafferentation-induced apoptosis of neurons in thalamic somatosensory nuclei of the newborn rat: critical period and rescue from cell death by peripherally applied neurotrophins

This study shows that unilateral transection of the infraorbital nerve (ION) in newborn (P0) rats induces apoptosis in the contralateral ventrobasal thalamic (VB) complex, as evidenced by terminal transferase-mediated deoxyuridine triphosphate-biotin nick end labelling (TUNEL) and electron miscroscopy. Double-labelling experiments using retrograde transport of labelled microspheres injected into the barrel cortex, followed by TUNEL staining, show that TUNEL-positive cells are thalamocortical neurons. The number of TUNEL-positive cells had begun to increase by 24 h postlesion, increased further 48 h after nerve section, and decreased to control levels after 120 h. Lesion-induced apoptosis in the VB complex is less pronounced if ION section is performed at P4, and disappears if the lesion is performed at P7. This time course closely matches the critical period of lesion-induced plasticity in the barrel cortex. Nerve growth factor (NGF) or brain-derived neurotrophic factor (BDNF), applied on the ION stump alone or in combination, are able to partially rescue thalamic neurons from apoptosis. Total cell counts in the VB complex of P7 animals that underwent ION section at P0 confirm the rescuing effect of BDNF and NGF. Blockade of axonal transport in the ION mimics the effect of ION section. These data suggest that survival-promoting signals from the periphery, maybe neurotrophins, are required for the survival of higher-order neurons in the somatosensory system during the period of fine-tuning of neuronal connections. We also propose that anterograde transneuronal degeneration in the neonatal rat trigeminal system may represent a new animal model for studying the pathways of programmed cell death in vivo.

Seasonal plasticity in the adult brain

Seasonal plasticity of structure and function is a fundamental feature of nervous systems in a wide variety of animals that occupy seasonal environments. Excellent examples of seasonal brain changes are found in the avian song control system, which has become a leading model of morphological and functional plasticity in the adult CNS. The volumes of entire brain regions that control song increase dramatically in anticipation of the breeding season. These volumetric changes are induced primarily by vernal increases in circulating sex steroids and are accompanied by increases in neuronal size, number and spacing. In several species, these structural changes in the song control circuitry are associated with seasonal changes in song production and learning. Songbirds provide important insights into the mechanisms and behavioral consequences of plasticity in the adult brain.

TrkB-like immunoreactivity in the song system of developing zebra finches

The neural song system in zebra finches develops for approximately the first 2 months after hatching. During that time, male-biased sexual dimorphisms emerge in the volume of song control nuclei as well as in the number and size of neurons within them. Brain derived neurotrophic factor (BDNF) has been documented in song control nuclei at various stages of development. Its high affinity receptor (tyrosine kinase B; trkB) is also in the song system, at least at around I month of age. The present study was designed to more completely describe the timing and potential location of BDNF action by investigating trkB expression during sexual differentiation of the song control nuclei. The pattern of immunoreactivity to a trkB antibody was examined in male and female zebra finches at post-hatching days 3-60. Labeling in somata and neuropil appeared to define the telencephalic components of the motor pathway (high vocal center and robust nucleus of the archistriatum) for song production in males from days 30 to 60, and in females on days 45 and 60 (high vocal center). These results are consistent with the hypothesis that the receptor, and its ligand BDNF, play a role in processes related to song learning in both sexes, including perhaps the motor component exhibited by developing males.

Dynamic regulation of BDNF and NT-3 expression during visual system development

Recent studies have proposed roles for neurotrophins in the formation and plasticity of ocular dominance columns as well as in the regulation of dendritic arborization in visual cortex of higher mammals. To assess potential roles for neurotrophins in these processes, we have examined the developmental expression of BDNF and NT-3 mRNA in the cat's visual system using in situ hybridization. BDNF and NT-3 mRNAs are dynamically regulated in many CNS structures during embryonic and postnatal development, and both mRNAs undergo striking developmental changes in laminar specificity and levels of expression within primary visual cortex during the critical period for ocular dominance column formation. Within visual cortex, BDNF mRNA is found in neurons in deep cortical layers (5 and 6) prior to eye opening, and in both deep and superficial layers (2 and 3) shortly afterwards. Within layer 4, the target of thalamocortical axons, BDNF mRNA is low initially and rises to high levels by the end of the critical period for ocular dominance column formation. NT-3 mRNA is first detectable in small stellate neurons at the base of layer 4 (4c) after eye opening, and levels decrease near the end of the critical period. BDNF and NT-3 mRNAs can be detected in the lateral geniculate nucleus at birth, and levels peak during the critical period. In both structures, BDNF mRNA expression is maintained into adulthood, while NT-3 is undetectable in the adult. The presence and dynamic regulation of these neurotrophins in visual structures is consistent with suggested roles for both of these neurotrophins in axonal and dendritic remodeling known to accompany the formation of ocular dominance columns.

Increased expression of endogenous biotin, but not BDNF, in telencephalic song regions during zebra finch vocal learning

Brain-derived neurotrophic factor (BDNF) is thought to regulate multiple aspects of brain development and neural plasticity in vertebrates. We have examined BDNF expression in two telencephalic nuclei (RA and HVC) in the zebra finch brain that control song learning by juvenile males and the production of already-learned song by adults. Using two different antibody-labeling techniques (avidin-biotin complex and horseradish peroxidase), we were unable to detect BDNF-like immunoreactivity in RA of juvenile or adult birds. BDNF-like immunoreactive labeling of somata was detected in HVC, but the density of labeled cells was not different between juvenile and adult birds. Immunocytochemical findings were confirmed by RT-PCR for BDNF mRNA. Thus, in contrast to a previous report (Akutagawa and Konishi, Proc. Natl. Acad. Sci. USA 95 (1998) 11429-11434), we did not observe elevated levels of BDNF immunoreactivity in RA and HVC of juvenile birds that were learning to sing. However, RA and HVC of juvenile birds were found to express elevated levels of endogenous biotin (as detected by avidin peroxidase), suggesting a possible role for biotin-regulated mechanisms in songbird vocal learning.

Expression of neurotrophin-3 (NT-3) and anterograde axonal transport of endogenous NT-3 by retinal ganglion cells in chick embryos

Anterograde axonal transport of neurotrophins has been demonstrated recently, but to date such transport has only been shown for brain-derived neurotrophic factor and no other endogenous neurotrophin. Endogenous neurotrophin-3 (NT-3) protein is present in the ganglion cell layer of the chicken retina, as well as the superficial layers of the optic tectum. NT-3 immunolabel in these tectal layers is largely reduced or abolished after treatment of the eye with colchicine or monensin, demonstrating that endogenous NT-3 is transported to the optic tectum by retinal ganglion cells (RGCs). Reverse transcription-PCR analysis of RGCs purified to 100% shows that RGCs, but not tectal cells, express NT-3 mRNA. Blockade of the intercellular transfer of NT-3 within the retina does not reduce the anterograde transport of endogenous NT-3 to the tectum, indicating that a major fraction of the anterogradely transported NT-3 is produced by RGCs rather than taken up from other retinal cells. Immunolabel for the neurotrophin receptor p75, but not trkB or trkC, in the superficial tectum coincides with the NT-3 label. The p75 label in the neuropil of superficial tectal layers is largely reduced or eliminated by injection of monensin in the eye, indicating that p75 protein is exported along RGC axons to the retinotectal terminals and may act as a neurotrophin carrier. These results show that NT-3 is produced by RGCs and that some of this NT-3 is transported anterogradely along the axons to the superficial layers of the tectum, possibly to regulate the survival, synapse formation, or dendritic growth of tectal neurons.

Cerebral cortical neuron apoptosis after mild excitotoxic injury in vitro: different roles of mesencephalic and cortical astrocytes

OBJECTIVE

Increasing evidence supports the presence of neuronal apoptosis after ischemic or excitotoxic brain injury. Astrocytes, which exhibit significant regional differences in function, may exert a protective effect on neurons exposed to ischemic injury. We examined the effects of astrocytes derived from different regions of the central nervous system on neuronal apoptosis after mild excitotoxic injury in tissue culture.

METHODS

Purified astrocyte cultures derived from P4 rat cerebral cortex or mesencephalon showed transient cell swelling but no cell death when exposed to 50 micromol/L glutamate for 5 minutes. When mixed neuronal/glial cocultures were exposed to the same glutamate dose, neuron death was observed. Necrotic and apoptotic cell death during 24 hours was examined using morphological criteria, nuclear staining, triphosphate nick end labeling, and trypan blue exclusion.

RESULTS

We found that cortical neurons that elaborate a more extensive dendritic arbor when grown on homotypic astrocytes are more likely to undergo apoptosis than neurons with a limited dendritic arbor grown on heterotypic astrocytes. By contrast, a similar number of neurons undergo necrotic cell death.

CONCLUSION

This finding may be associated with 1) increased vulnerability of neurons with a more elaborate dendrite structure to mild excitotoxic injury, or 2) regional differences in the ability of astrocytes to attenuate apoptosis.

Lesions of an avian forebrain nucleus that disrupt song development alter synaptic connectivity and transmission in the vocal premotor pathway

The avian forebrain nucleus, the lateral magnocellular nucleus of the anterior neostriatum (LMAN), is necessary for normal song development because LMAN lesions made in juvenile birds disrupt song production but do not disrupt song when made in adults. Although these age-limited behavioral effects implicate LMAN in song learning, a potential confound is that LMAN lesions could disrupt normal vocal motor function independent of any learning role by altering LMAN's premotor target, the song nucleus, the robust nucleus of the archistriatum (RA). To date, however, no studies have examined directly the effects of LMAN lesions on the circuitry of the RA. We report here that juvenile LMAN lesions rapidly and profoundly affect RA, altering synaptic connectivity within this nucleus, including descending inputs from the song nucleus HVc. Specifically, morphological assays of the dendritic spines of RA projection neurons and axon terminal boutons arising from HVc show a numerical decline in the density of connections in RA in LMAN-lesioned juveniles compared with controls. Concurrently, LMAN lesions alter excitatory transmission within the juvenile RA: after LMAN lesions, the stimulus-response relationship between HVc fibers and RA neurons steepens, and the amplitude of spontaneous monophasic EPSCs increases. Rather than arresting RA in a juvenile state, LMAN lesions transform the structure and function of RA and its connections, such that it is distinct from that of the normal juvenile. In many ways, RA circuitry in LMAN-lesioned juveniles resembles that of normal adults, suggesting that LMAN lesions induce a premature maturation of the vocal motor pathway, which may lead to a loss of behavioral plasticity and abnormal song development.

Characterization of dopaminergic midbrain neurons in a DBH:BDNF transgenic mouse

The neurotrophin brain-derived neurotrophic factor (BDNF) has been implicated in the survival and differentiation of central nervous system neurons, including dopaminergic cells in culture. To determine whether BDNF might play a role in the development of dopaminergic neurons in vivo, we used a previously characterized transgenic mouse (DBH:BDNF) that overexpresses BDNF in adrenergic and noradrenergic neurons as a result of fusion of the BDNF gene to the dopamine beta-hydroxylase (DBH) gene promoter. We quantified dopaminergic neuronal profiles at four midbrain coronal levels and compared DBH:BDNF transgenic animals with wild-type mice of the same genetic background. Analysis of sections immunostained with tyrosine hydroxylase (TH) showed that the mean number of dopaminergic neurons in the four selected midbrain sections was 52% greater (one-way analysis of variance, P < 0.0005) in transgenic mice (2,165 +/- 55 S. E.M., n = 4) than in control mice (1,428 +/- 71 S.E.M., n = 4). The increase in dopaminergic neuron profile count in DBH:BDNF transgenic animals was confirmed by analysis of the pars compacta of the substantia nigra on Nissl-stained sections. Surface area of the reference region of interest containing TH-immunoreactive neurons was similar in transgenic and control mice. Regional analysis of different midbrain areas containing dopaminergic neurons suggested that the increase in cell profile count occurs in a relatively homogeneous manner. Comparison of TH-immunoreactive cell size showed a tendency for smaller neurons in transgenic animals, but the difference was not statistically significant. We conclude that DBH:BDNF transgenic mice show increased number of TH-immunoreactive cells in the midbrain. We propose that BDNF rescues dopaminergic neurons from the perinatal period of developmental cell death as a consequence of increased anterograde transport of the neurotrophin via the coeruleonigral projection.

Roles of thapsigargin-sensitive Ca2+ stores in the survival of developing cultured neurons

The roles of the intracellular calcium pool involved in regulating the Ca2+ profile and the neuronal survival rate during development were studied by using thapsigargin (TG), a specific inhibitor of endoplasmic reticulum (ER) Ca2+-ATPase in cultured cerebellar granule neurons. Measuring the neuronal [Ca2+]i directly in the culture medium, we found a bell-shaped curve for [Ca2+]i versus cultured days in cerebellar granule neurons maintained in medium containing serum and 25 mM K+. The progressive increase in [Ca2+]i of the immature granule neurons (1-4 days in vitro) was abolished by TG, which resulted in massive neuronal apoptosis. When the [K+] was lowered from 25 to 5 mM, neither the progressively increasing [Ca2+]i nor the survival of immature granule neurons was significantly changed over 24-h incubation. Similarly, TG caused a dramatic decrease in the [Ca2+]i and survival rate of these immature neurons when switched to 5 mM K+ medium. Following maturation, the granule neurons became less sensitive to TG for both [Ca2+]i and neuronal survival. However, TG can protect mature granule neurons from the detrimental effect of switching to a 5 mM K+ serum-free medium by decreasing [Ca2+]i to an even lower level than in the respective TG-free group. Based on these findings, we propose that during the immature stage, TG-sensitive ER Ca2+-ATPase plays a pivotal role in the progressive increase of [Ca2+]i, which is essential for the growth and maturation of cultured granule neurons.

Interactions between nerve growth factor binding and estradiol in early development of the zebra finch telencephalon

The zebra finch telencephalon exhibits rapid and substantial development in the first few weeks after hatching. In parallel, the rate of estradiol synthesis is very high in the zebra finch forebrain, and estradiol can have potent neurotrophic effects in specific telencephalic regions, including those that control the learning and production of song. In an attempt to elucidate mechanisms regulating telencephalic development, potentially including a role for the large capacity for estrogen production, (125)I-nerve growth factor (NGF) binding was measured in homogenates of telencephalon from zebra finches age 3, 15, 30, 60, and 120 days. The highest density of low- and high-affinity (125)I-NGF binding sites was observed in 3-day-old finches. Using an aromatase inhibitor, Fadrozole, to reduce estradiol levels in 1 to 4-day-old zebra finches significantly decreased both high- and low-affinity (125)I-NGF binding sites. Conversely, treating adult or 8 to 14-day-old hatchlings with estradiol increased high-affinity (125)I-NGF binding sites. These results are consistent with the hypothesis that estradiol influences the level of NGF receptors, and suggest one mechanism through which the steroid could affect brain development. The data also indicate that estradiol and NGF activity may be important for very early development of the telencephalon.

Estrogen-inducible, sex-specific expression of brain-derived neurotrophic factor mRNA in a forebrain song control nucleus of the juvenile zebra finch

The expression of brain-derived neurotrophic factor (BDNF) mRNA is increased significantly within the high vocal center (HVc) of male but not female zebra finches from posthatching day 30-35 on. The population of HVc cells expressing BDNF mRNA included 35% of the neurons projecting to the nucleus robustus of the archistriatum (RA). In the RA and in RA-projecting neurons of the lateral portion of the magnocellular nucleus of the anterior neostriatum, BDNF mRNA was expressed at very low levels in both sexes. The BDNF-receptor trkB mRNA was expressed in the RA, in RA-projecting neurons of lateral portion of the magnocellular nucleus of the anterior neostriatum, and in the HVc, except in most of its RA-projecting neurons. Premature stimulation and an inhibitory effect on the normal increase of the BDNF mRNA expression in juvenile males occurred after treatments with 17beta-estradiol and the aromatase inhibitor fadrozole, respectively. The up-regulation of the BDNF expression in the HVc could be a mechanism by which estrogen triggers the differentiation of cells within and connected to the HVc of male zebra finches.

Song behavior, NGF level and NPY distribution in the brain of adult male zebra finches

The aim of the present study was to investigate the role of nerve growth factor (NGF) and neuropeptide Y (NPY) in the higher vocal center (HVC) on singing behavior of adult male zebra finches. The results of our studies show: (a) that NGF is present in the brain of these birds and it is higher in the HVC than in the other neostriatal tissues; (b) that exogenous administration of NGF or NGF-antibody had no discernible effect on singing behavior; and (c) that NGF enhances the NPY immunoreactivity in neurons and fibers localized in HVC and other areas of the neostriatum and hippocampus whereas anti-NGF decreased NPY stained cells in the hippocampus. These studies indicate that NGF is produced in the brain of zebra finch and that it plays a role in the regulation of NPY.

Sensitive periods and circuits for learned birdsong

Experience influences the development of certain behaviors and their associated neural circuits during a discrete period after birth. Songbirds, with their highly quantifiable vocal output and well-delineated vocal control circuitry, provide an excellent context in which to examine the neural mechanisms regulating sensitive periods for learning. Recent discoveries indicate that auditory input to the vocal control circuitry in songbirds is dynamically modulated and show that neural circuitry previously thought to be used only in plastic juvenile song may also actively maintain stable adult song. These findings provide important clues to how sensitive periods for auditory feedback and vocal plasticity are regulated during song development.

Breed differences in deafferentation-induced neuronal cell death and shrinkage in chick cochlear nucleus

Removal of functional presynaptic input can result in a variety of changes in postsynaptic neurons in the central nervous system, including altered metabolism, changes in neuronal cell size, and even death of the postsynaptic cell. Age-dependent neuronal cell death and shrinkage has been documented in second order auditory neurons in the chick brainstem (nucleus magnocellularis, NM) following cochlea removal (Born and Rubel, 1985. J. Comp. Neurol. 231, 435-445). Here we examined whether the extent of neuronal cell death and shrinkage is also breed-dependent. We performed unilateral cochlea removal on both hatchling and adult birds of either a broiler breed (Arbor Acres Cross) or egg layer breed (Hy-Line, H and N) and killed birds one week later. Changes in neuronal cell number and cross sectional area were determined from Nissl-stained sections. We observed 25% neuronal cell loss and a 15-20% decrease in neuronal cross sectional area after cochlea removal in either broiler or egg layer hatchling birds. In adult birds, however, neuronal cell loss is breed-dependent. Adult egg layer birds lose an average of 37% of NM neurons after cochlea removal, while adult broiler birds show no cell loss. In both breeds of adult birds, cochlea removal results in a 20% decrease in neuronal cross sectional area. These results suggest that analysis of differences between breeds as well as ages of birds will prove fruitful in determining how afferent input controls neuronal survival and metabolism.

Evidence for loss and recovery of chick brainstem auditory neurons during gentamicin-induced cochlear damage and regeneration

It is well documented that damage to the chick cochlea caused by acoustic overstimulation or ototoxic drugs is reversible. Second-order auditory neurons in nucleus magnocellularis (NM) are sensitive to changes in input from the cochlea. However, few experiments studying changes in NM during cochlear hair cell loss and regeneration have been reported. Chicks were given a single systemic dose of gentamicin, which results in maximal hair cell loss in the base of the cochlea after 5 days. Many new hair cells are present by 9 days. These new hair cells are mature but not completely recovered in organization by 70 days. We counted neurons in Nissl-stained sections of the brainstem within specific tonotopic regions of NM, comparing absolute cell number between gentamicin- and saline-treated animals at both short and long survival times. Our data suggest that neuronal number in rostral NM parallels hair cell number in the base of the cochlea. That is, after a single dose of gentamicin, we see a loss of both cochlear hair cells and NM neurons early, followed by a recovery of both cochlear hair cells and NM neurons later. These results suggest that neurons, like cochlear hair cells, can recover following gentamicin-induced damage.

Neurotrophin trafficking by anterograde transport

The ever-unfolding biology of NGF is consistent with a target-derived retrograde mode of action in peripheral and central neurons. However, another member of the neurotrophin family, brain-derived neurotrophic factor (BDNF), is present within nerve terminals in certain regions of the brain and PNS that do not contain the corresponding mRNA. Recent studies have shown that the endogenous neurotrophins, BDNF and neurotrophin-3 (NT-3), are transported anterogradely by central and peripheral neurons. The supply of BDNF by afferents is consistent with their presynaptic synthesis, vesicular storage, release and postsynaptic actions. Anterograde axonal transport provides an 'afferent supply' of BDNF and NT-3 to neurons and target tissues, where they function as trophic factors and as neurotransmitters.

Transient expression and transport of brain-derived neurotrophic factor in the male zebra finch's song system during vocal development

The distribution of brain-derived neurotrophic factor (BDNF) in the song system of male zebra finches changes with posthatching age. At day 20, the hyperstriatum ventrale, pars caudale is the only song nucleus in which neurons showed BDNF immunoreactivity. At day 45, the staining in hyperstriatum ventrale, pars caudale was denser than at day 20 and the robust nucleus of the archistriatum, another song nucleus, showed BDNF labeling. By day 65, two additional song nuclei, area X and the lateral magnocellular nucleus of the anterior neostriatum, have become immunoreactive. In the adult, however, the amount of BDNF labeling in all of these brain nuclei is sharply reduced. These sequential events, the anatomical connections between these song nuclei, and the labeling of relevant axons and terminals suggest anterograde transport of BDNF. Furthermore, the timing of BDNF expression coincident with the development of singing behavior suggests that this neurotrophin may be directly involved with the differentiation of the song system.

Anterograde transport of neurotrophin proteins in the CNS--a reassessment of the neurotrophic hypothesis

The basic tenets of the neurotrophic hypothesis are that i) limiting quantities of a given factor are produced in specific target tissues; ii) responsive neurons projecting to these targets compete for the limiting amounts of the factor; iii) the factor is bound within the target by selective receptors on afferent terminals, internalized, and retrogradely transported to the neuronal cell body where it provides signals affecting neuronal survival and differentiation. Although originally formulated on the basis of evidence for NGF's actions on peripheral sensory and sympathetic neurons, the neurotrophic hypothesis appeared to be upheld for CNS neuronal systems as well, where NGF was found to function primarily as a target-derived trophic factor for basal forebrain cholinergic neurons. With the discovery of additional neurotrophins sharing considerable structural homology with NGF, the question arose of whether the neurotrophic hypothesis held true for all members of this protein family. Recent investigations into the localization and function of neurotrophins other than NGF, particularly BDNF and NT-3, have provided evidence indicating that these molecules may not act in a manner consistent with the neurotrophic hypothesis, as originally postulated. Numerous studies in the peripheral and central nervous systems have now demonstrated that BDNF (and NT-3) may be preferentially trafficked anterogradely along axonal processes and stored within pre-synaptic terminals. Other studies have suggested that these factors may be released in an activity-dependent, rather than constitutive, manner and can act in autocrine or paracrine fashions to subserve an assortment of biological functions including anterograde effects on cell survival and differentiation, as well as more novel roles in synaptic transmission. These recent findings strongly suggest that, while the various neurotrophin proteins may be grouped into a single family based upon their structural homology, they should be considered as a heterogeneous group of trophic factors based upon function and mode of action.

Role of afferents in the development and cell survival of the vertebrate nervous system

1. During normal development of the vertebrate central nervous system, a considerable number of neurons die. The factors controlling which neurons die and which survive are not fully understood. 2. Target populations are known to maintain their innervating neurons. However, the role of afferents in maintaining their targets is still under review. 3. In the developing nervous system, deafferentation of a neuron population is difficult to achieve because plasticity (structural re-organization) can cause re-innervation of the area. Re-innervation alters, rather than removes, the afferent supply. 4. Afferent input is important for neuronal survival during development because deafferentation increases neuronal death by 20-30% and increasing input diminishes neuronal death. 5. Deafferented neurons die at the normal time for cell death for any given population. This occurs after the arrival of afferent axons but before the completion of connectivity and the onset of function. 6. Neuronal survival is maintained by any input, such as reinnervation by inappropriate fibres, but for optimal survival, morphological maturation and the acquisition of normal physiology, the correct input is required. 7. Afferents maintain their target neurons via a combination of electrical activity and delivery of trophic agents, which adjust intracellular calcium, thereby facilitating protein synthesis, mitochondrial function and suppressing apoptosis. 8. Evidence from animal and in vitro experiments indicates that afferents play an extremely important role in the survival of developing neurons in the immature vertebrate nervous system.

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.

Contributions of the optic tectum and the retina as sources of brain-derived neurotrophic factor for retinal ganglion cells in the chick embryo

Retinal ganglion cells (RGC) are supported by brain-derived neurotrophic factor (BDNF), but it is not known if BDNF acts as a target-derived factor or as an afferent or autocrine trophic factor. Here we demonstrate that BDNF mRNA is expressed in the retinorecipient layer of the chick optic tectum as well as in the inner nuclear layer and ganglion cell layer of the retina. Amacrine cells rather than RGC were the main source of BDNF mRNA in the ganglion cell layer, as determined by in situ hybridization that was combined with retrograde labeling of RGC and destruction of RGC by optic stalk transection, followed by quantitative RT-PCR. Cells in the ganglion cell layer as well as the retinorecipient layers of the optic tectum were BDNF-immunolabeled. After injections into the tectum, radio-iodinated BDNF was transported to the retina where autoradiographic label accumulated in the inner plexiform and ganglion cell layers. After intraocular injection, iodinated BDNF accumulated in these same retinal layers and correlated with the distribution of p75 neurotrophin receptor protein. The majority of cross-linked receptor-bound BDNF in the retina immunoprecipitated with p75 antibodies. No difference in the intensity of BDNF immunolabel was observed in the experimental retina or tectum after optic stalk transection, indicating that most of the BDNF in the RGC was not derived from the optic tectum. These data indicate that a substantial fraction of the BDNF in the ganglion cell layer is derived from local sources, afferents within the retina, rather than from the optic tectum via retrograde transport.

BDNF injected into the superior colliculus reduces developmental retinal ganglion cell death

The role of neurotrophins as survival factors for developing CNS neurons, including retinal ganglion cells (RGCs), is uncertain. Null mutations for brain-derived neurotrophic factor (BDNF) or neurotrophin 4 (NT4), individually or together, are without apparent effect on the number of RGCs that survive beyond the period of normal, developmental RGC death. This contrasts with the BDNF dependence of RGCs in vitro and the effectiveness of BDNF in reducing RGC loss after axotomy. To investigate the effect of target-derived neurotrophins on the survival of developing RGCs, we injected BDNF into the superior colliculus (SC) of neonatal hamsters. At the age when the rate of developmental RGC death is greatest, BDNF produces, 20 hr after injection, a 13-15-fold reduction in the rate of RGC pyknosis compared with the rates in vehicle-injected and untreated hamsters. There is no effect 8 hr after injection. Electrochemiluminescence immunoassay measurements of BDNF protein in the retinae and SC of normal and BDNF-treated hamsters demonstrate that the time course of BDNF transport to RGCs supports a role for target-derived BDNF in promoting RGC survival. The effectiveness of pharmacological doses of BDNF in reducing developmental RGC death may be useful in further studies of the mechanisms of stabilization and elimination of immature central neurons.

Individual variation in neuron number predicts differences in the propensity for avian vocal imitation

Avian song learning involves memorizing and reproducing song material produced by conspecifics. In several species song repertoire size correlates with the overall volume of two song-related brain regions, the HVc (acronym used as the proper name) and the robust nucleus of the archistriatum (RA). We raised male zebra finches with two adult tutors and found that individual differences in HVc volume and neuron number correlated positively with differences in the number of tutor syllables accurately copied. These results were replicated in a second study. The relationship between RA volume and song learning was similar, but less robust. Importantly, total repertoire size (number of song syllables) did not correlate significantly with anatomical measures of either the HVc or RA. Because previous work suggests that the volume and neuron number of these regions are not regulated by song learning, it is possible that naturally occurring variation in neuron number constrains how much song material can be copied or reproduced.

Anatomical and synaptic substrates for avian song learning

In songbirds, vocal learning occurs during periods of major cellular and synaptic change. This neural reorganization includes massive synaptogenesis associated with the addition of new neurons into the vocal motor pathway, as well as pruning of connections between song regions. These observations, coupled with behavioral evidence that song development requires NMDA receptor activation in specific song nuclei, suggest that experiences associated with vocal learning are encoded by activity driven, Hebbianlike processes of synaptic change akin to those implicated in many other forms of developmental plasticity and learning. In this review we discuss the hypothesis that develpmental and/or seasonal changes in NMDA receptor function and the availability of new synapses may modulate thresholds for plasticity and thereby define sensitive periods for vocal learning.

Selective expression of insulin-like growth factor II in the songbird brain

Neuronal replacement occurs in the forebrain of juvenile and adult songbirds. To address the molecular processes that govern this replacement, we cloned the zebra finch insulin-like growth factor II (IGF-II) cDNA, a factor known to regulate neuronal development and survival in other systems, and examined its expression pattern by in situ hybridization and immunocytochemistry in juvenile and adult songbird brains. The highest levels of IGF-II mRNA expression occurred in three nuclei of the song system: in the high vocal center (HVC), in the medial magnocellular nucleus of the neostriatum (mMAN), which projects to HVC, and to a lesser extent in the robust nucleus of the archistriatum (RA), which receives projections from HVC. IGF-II mRNA expression was developmentally regulated in zebra finches. In canary HVC, monthly changes in IGF-II mRNA expression covaried with previously reported monthly differences in neuron incorporation. Combining retrograde tracers with in situ hybridization and immunocytochemistry, we determined that the HVC neurons that project to area X synthesize the IGF-II mRNA, whereas the adjacent RA-projecting neurons accumulate the IGF-II peptide. Our findings raise the possibility that within HVC IGF-II acts as a paracrine signal between nonreplaceable area X-projecting neurons and replaceable RA-projecting neurons, a mode of action that is compatible with the involvement of IGF-II with the replacement of neurons. Additional roles for IGF-II expression in songbird brain are likely, because expression also occurs in some brain areas outside the song system, among them the cerebellar Purkinje cells in which neurogenesis is not known to occur.

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.