Functional evidence that BDNF is an anterograde neuronal trophic factor in the CNS

Lumbar transplant of neurons genetically modified to secrete brain-derived neurotrophic factor attenuates allodynia and hyperalgesia after sciatic nerve constriction

Chronic delivery of anti-nociceptive molecules by means of cell grafts near the pain processing centers of the spinal cord is a newly developing technique for the treatment of neuropathic pain. The rat neuronal cell line, RN33B, derived from E13 rat brainstem raphe and immortalized with the SV40 temperature-sensitive allele of large T antigen (tsTag), was transfected with rat brain-derived neurotrophic factor cDNA (BDNF), and the BDNF-synthesizing cell line, 33BDNF.4, was isolated. The 33BDNF.4 cells synthesized mature BDNF protein at permissive temperature (33 degrees C), when the cells were proliferating, and during differentiation at non-permissive temperature (39 degrees C) in vitro. The bio-active BDNF protein was also secreted by the cells during both growth conditions, as measured by ELISA analysis of BDNF content and secretion. The bio-activity of the BDNF in 33BDNF.4 cell conditioned media was assessed by neurite outgrowth from E15 dorsal root ganglion (DRG) cultures. A control cell line, 33V1, transfected with the vector alone, did not synthesize or secrete any significant BDNF at either growth condition. Both cell lines were used as grafts in a model of chronic neuropathic pain induced by unilateral chronic constriction injury (CCI) of the sciatic nerve. Pain-related behaviors, including cold and tactile allodynia and thermal and tactile hyperalgesia, were evaluated after CCI in the affected hindpaw. When 33BDNF.4 and 33V1 cells were transplanted in the lumbar subarachnoid space of the spinal cord 1 week after CCI, they survived greater than 7 weeks on the pia mater around the spinal cord and the 33BDNF.4 cells continued to synthesize BDNF in vivo. Furthermore, the tactile and cold allodynia and tactile and thermal hyperalgesia induced by CCI was significantly reduced during the 2-7 week period after grafts of 33BDNF.4 cells. The maximal effect on chronic pain behaviors with the BDNF grafts occurred 2-3 weeks after transplant and the anti-nociceptive effects of the BDNF cell grafts was permanent. Transplants of the control 33V1 cells had no effect on the allodynia and hyperalgesia induced by CCI and these cells did not synthesize BDNF in vivo. These data suggest that a chronically applied, low local dose of BDNF supplied by transplanted cells near the spinal dorsal horn was able to reverse the development of chronic neuropathic pain following CCI. The use of neural cell lines that are able to deliver anti-nociceptive molecules, such as BDNF, in a model of chronic pain offers a novel approach to pain management and such 'biologic minipumps' can be developed for safe use in humans.

Brain-derived neurotrophic factor differentially regulates excitatory and inhibitory synaptic transmission in hippocampal cultures

Brain-derived neurotrophic factor (BDNF) has been postulated to be a key signaling molecule in regulating synaptic strength and overall circuit activity. In this context, we have found that BDNF dramatically increases the frequency of spontaneously initiated action potentials in hippocampal neurons in dissociated culture. Using analysis of unitary synaptic transmission and immunocytochemical methods, we determined that chronic treatment with BDNF potentiates both excitatory and inhibitory transmission, but that it does so via different mechanisms. BDNF strengthens excitation primarily by augmenting the amplitude of AMPA receptor-mediated miniature EPSCs (mEPSCs) but enhances inhibition by increasing the frequency of mIPSC and increasing the size of GABAergic synaptic terminals. In contrast to observations in other systems, BDNF-mediated increases in AMPA-receptor mediated mEPSC amplitudes did not require activity, because blocking action potentials with tetrodotoxin for the entire duration of BDNF treatment had no effect on the magnitude of this enhancement. These forms of synaptic regulations appear to be a selective action of BDNF because intrinsic excitability, synapse number, and neuronal survival are not affected in these cultures. Thus, although BDNF induces a net increase in overall circuit activity, this results from potentiation of both excitatory and inhibitory synaptic drive through distinct and selective physiological mechanisms.

Age-dependent changes in projections from locus coeruleus to hippocampus dentate gyrus and frontal cortex

Age-dependent changes in noradrenergic innervations of the hippocampal dentate gyrus (DG) and the frontal cortex (FC) have been studied in male F344 rats. The projections from the nucleus locus coeruleus (LC) to DG or FC with advancing age (from 7 to 27 months) in rats have been quantified by electrophysiological and immunohistochemical methods. In the electrophysiological study, we observed that the percentage of LC neurons activated antidromically by electrical stimulation (P-index) of DG or FC decreased with age. We found that the percentage of LC neurons showing multiple antidromic latencies (M-index), which suggests axonal branching of individual LC neurons, increased markedly between 15 and 17 months in DG or FC. In DG, the M-index increased steadily between 15 and 24 months. In contrast, the increased M-index in FC was maintained until 24 months. The increased M-index in both targets declined at 27 months. These results suggest that LC neurons give rise to axonal branching following the loss of projections to DG or FC with age. In the immunohistochemical study, the density of dopamine-beta-hydroxylase-positive axonal varicosities was measured in molecular, granule cell and polymorphic layers of DG. The density in the polymorphic layer significantly decreased in the earlier stage of ageing (7-19 months), whilst the density in the molecular and granule cell layers decreased in the later stage (27 months). These findings suggested that a layer-specific decline occurred with age in the noradrenergic axon terminals in DG.

Brain-derived neurotrophic factor induces excitotoxic sensitivity in cultured embryonic rat spinal motor neurons through activation of the phosphatidylinositol 3-kinase pathway

Neurotrophic factors (NTFs) can protect against or sensitize neurons to excitotoxicity. We studied the role played by various NTFs in the excitotoxic death of purified embryonic rat motor neurons. Motor neurons cultured in brain-derived neurotrophic factor, but not neurotrophin 3, glial-derived neurotrophic factor, or cardiotrophin 1, were sensitive to excitotoxic insult. BDNF also induces excitotoxic sensitivity (ES) in motor neurons when BDNF is combined with these other NTFs. The effect of BDNF depends on de novo protein and mRNA synthesis. Reagents that either activate or inhibit the 75-kDa NTF receptor p75NTR do not affect BDNF-induced ES. The low EC50 for BDNF-induced survival and ES suggests that TrkB mediates both of these biological activities. BDNF does not alter glutamate-evoked rises of intracellular Ca2+, suggesting BDNF acts downstream. Both wortmannin and LY294002, which specifically block the phosphatidylinositol 3-kinase (PI3K) intracellular signaling pathway in motor neurons, inhibit BDNF-induced ES. We confirm this finding using a herpes simplex virus (HSV) that expresses the dominant negative p85 subunit of PI3K. Infecting motor neurons with this HSV, but not a control HSV, blocks activation of the PI3K pathway and BDNF-induced ES. Through the activation of TrkB and the PI3K signaling pathway, BDNF renders developing motor neurons susceptible to glutamate receptor-mediated cell death.

Upregulation of BDNF mRNA and trkB mRNA in the nigrostriatal system and in the lesion site following unilateral transection of the medial forebrain bundle

We have performed unilateral transection of the medial forebrain bundle (MFB) and studied BDNF mRNA and trkB mRNA levels at different postlesion times in the nigrostriatal system by means of in situ hybridization. BDNF mRNA levels were transiently induced in the substantia nigra pars compacta at 1 day postaxotomy. The disposition of BDNF mRNA expressing cells at this postlesion time in substantia nigra mimicked that of the dopaminergic neurons expressing the mRNA for the dopamine transporter. TrkB mRNA levels remained unaltered in the ventral mesencephalon at the different postlesion times examined-1 to 14 days. In contrast, trkB mRNA levels were significantly induced in the striatum at the longer postlesion time examined-14 days-when all neurodegenerative events are completed. It is becoming apparent that nigral BDNF mRNA levels are anterogradely transported to its target tissue in striatum. However, following axotomy, the lesion site represents a second potential target for BDNF action. Consequently, we also analyzed the pattern of mRNA expression for BDNF and trkB at the lesion site where dopaminergic axons are disconnected. There, we found notable inductions of both BDNF mRNA and trkB mRNA levels at 4 days postaxotomy. BDNF mRNA expressing cells were confined at the site of axotomy, which coincided precisely to that showing induction of trkB mRNA. Altogether, our results anticipate promising trophic roles of BNDF in the injured nigrostriatal system.

Evidence that brain-derived neurotrophic factor from presynaptic nerve terminals regulates the phenotype of calbindin-containing neurons in the lateral septum

Brain-derived neurotrophic factor (BDNF) is transported anterogradely in neurons of the CNS and can be released by activity-dependent mechanisms to regulate synaptic plasticity. However, few neural networks have been identified in which the production, transport, and effects of BDNF on postsynaptic neurons can be analyzed in detail. In this study, we have identified such a network. BDNF has been colocalized by immunocytochemistry with tyrosine hydroxylase (TH) in nerve fibers and nerve terminals within the lateral septum of rats. BDNF-containing nerve fibers terminate on a population of calbindin-containing neurons in lateral septum that contain TrkB, the high-affinity receptor for BDNF. Overexpression of BDNF in noradrenergic neurons increased levels of calbindin in septum, as well as in whole-brain lysates. Septal levels of calbindin and BDNF partially decreased after unilateral lesions of the medial forebrain bundle (MFB), induced with 6-hydroxydopamine, a treatment that abolished TH staining. These data suggest that BDNF is anterogradely transported within the MFB in catecholaminergic neurons arising from brainstem nuclei. To determine whether BDNF affects the production of calbindin in lateral septal neurons directly, we tested the effects of BDNF on cultures of septal neurons from embryonic day 16-17 rats. BDNF promoted the expression of calbindin, as well as the arborization of calbindin-containing neurons, but BDNF had no effect on cell division or survival. Together, these results suggest that BDNF, anterogradely transported in catecholaminergic neurons, regulates calbindin expression within the lateral septum.

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.

Ultrastructural localization of full-length trkB immunoreactivity in rat hippocampus suggests multiple roles in modulating activity-dependent synaptic plasticity

Neurotrophins acting at the trkB receptor have been shown to be important modulators of activity-dependent plasticity in the hippocampus, but the mechanisms underlying these effects are not yet well understood. To identify the cellular and subcellular targets of trkB ligands in the adult rat hippocampal formation, full-length trkB receptor immunoreactivity (trkB-IR) was localized using electron microscopy. trkB-IR was present in the glutamatergic pyramidal and granule cells. Labeling in these neurons appeared as discrete clusters and was primarily in axons, excitatory-type axon terminals, and dendritic spines and to a lesser extent in somata and dendritic shafts. trkB-IR was commonly found on the plasma membrane of dendritic spines, whereas in other subcellular regions trkB-IR was often intracellular. Labeling was strikingly dense within axon initial segments, suggesting extensive receptor trafficking. trkB-IR was not confined to pyramidal and granule cells. Dense trkB-IR was found in occasional interneuron axon initial segments, some axon terminals forming inhibitory-type synapses onto somata and dendritic shafts, and excitatory-type terminals likely to originate extrahippocampally. This suggests that trkB is contained in some GABAergic interneurons, neuromodulatory (e.g., cholinergic, dopaminergic, and noradrenergic) afferents, and/or glutamatergic afferents. These data indicate that full-length trkB receptor activation may modulate glutamatergic pathways of the trisynaptic circuit both presynaptically at axon terminals and initial segments and postsynaptically at dendritic spines and shafts. Signaling via catalytic trkB may also presynaptically affect inhibitory and modulatory neurons. A pan-trkB antibody labeled the same neuronal populations as the full-length-specific trkB antiserum, but the labels differed in density at various subcellular sites. These findings provide an ultrastructural foundation for further examining the mechanisms through which neurotrophins acting at trkB receptors contribute to synaptic plasticity.

Effects of mild protein prenatal malnutrition and subsequent postnatal nutritional rehabilitation on noradrenaline release and neuronal density in the rat occipital cortex

There is evidence that severe malnutrition started during gestation and continued through lactation affects adversely the morphologic development of the neocortex leading to increased neuronal cell packing density and decreased dendritic branching. Nevertheless, the effect of purely mild protein prenatal malnutrition on neocortical development remains rather unexplored. This study evaluates the effects of mild protein prenatal malnutrition (8% casein diet, calorically compensated by carbohydrates) and subsequent postnatal nutritional rehabilitation (25% casein diet) on: (i) the potassium-induced release of [(3)H]-noradrenaline (NA) in occipital cortex slices obtained from 1- and 22-day-old pups; and (ii) the packing density of neurons in lateral, dorso-lateral and dorsal regions of the occipital cortex of 22-day-old pups by using the optical dissector method. The experiments were performed in rats normally fed during gestation and lactation (G(+)L(+)), malnourished during gestation but rehabilitated during lactation (G(-)L(+)) and malnourished during gestation and lactation (G(-)L(-)). At day 1 of age, no significant differences in body and brain weights were observed between prenatally well-nourished and malnourished pups. Nevertheless, at this early age, pups born from mothers submitted to the 8% casein diet had significantly higher cortical net percent NA release than pups born from mothers receiving the 25% casein diet. At weaning (22 days of age) G(-)L(+) rats had, compared to the G(+)L(+) group, similar body weight, brain weight and [(3)H]-NA release values, but significantly higher neuron density scores in the lateral region of the occipital cortex. In contrast, at 22 days of age, G(-)L(-) rats exhibited, compared to G(+)L(+) animals, significant deficits in body and brain weights as well as significant increases in cortical net percent NA release together with enhanced packing density of neurons in the lateral, dorso-lateral and dorsal regions of the occipital cortex. Moreover, in G(-)L(-) animals was not found the laterodorsal histogenetic gradient of neuronal cell packing density observed in G(+)L(+)rats. Results suggest that mild prenatal malnutrition per se is able to induce deleterious effects on cortical neuronal density, in spite of nutritional rehabilitation during lactation, through a mechanism involving central NA hyperactivity during gestation. Prosecution of malnutrition after birth magnifies both neurochemical and morphometric disorders.

Expression of brain-derived neurotrophic factor in catecholaminergic neurons of the rat lower brainstem after colchicine treatment or hemorrhage

Widespread brain-derived neurotrophic factor messenger RNA expression has been detected in the region of catecholamine groups of the rat lower brainstem, while few brain-derived neurotrophic factor-immunoreactive cells were found in this area. In the present study, a double-color immunofluorescence technique for brain-derived neurotrophic factor and tyrosine hydroxylase after colchicine treatment was employed to evaluate the possible presence of brain-derived neurotrophic factor immunoreactivity in the catecholaminergic cells of the rat lower brainstem. We detected many new brain-derived neurotrophic factor-immunoreactive cells in the A1, A2, A4, A6-A10 and C1-C3 cell groups and in the other lower brainstem nuclei where, without colchicine treatment, brain-derived neurotrophic factor messenger RNA was expressed, but not brain-derived neurotrophic factor immunoreactivity. In addition, the catecholaminergic neurons were found to express brain-derived neurotrophic factor immunoreactivity with the co-existence being greatest, in percentage terms, in medullary catecholaminergic cell groups. Hypotensive hemorrhage, which activates medullary catecholaminergic neurons, induced the expression of brain-derived neurotrophic factor immunoreactivity in catecholaminergic neurons (A1/C1 and C2). The results demonstrate that brain-derived neurotrophic factor is regulated by neuronal activity in medullary catecholaminergic cell groups involved in central cardiovascular regulation.

Deprivation of endogenous brain-derived neurotrophic factor results in impairment of spatial learning and memory in adult rats

Brain-derived neurotrophic factor (BDNF) is abundantly expressed in the hippocampus and cerebral cortex and is involved in synaptic plasticity and long-term potentiation (LTP). The present study was under taken to investigate whether endogenous BDNF was required for spatial learning and memory in a rat model. Antibodies to BDNF (anti-BDNF, n=7) or control immunoglobulin G (control, n=6) were delivered into the rat brain continuously for 7 days with an osmotic pump. The rats were then subjected to a battery of behavioral tests. The results show that the average escape latencies in the BDNF antibody treated group were dramatically longer than those of the control (F=13.3, p<0.001). The rats treated with control IgG swam for a significantly longer distance in the P quadrant (where the escape plane had been placed) compared with the other three quadrants (p<0.05). In contrast, anti-BDNF-treated rats swam an equivalent distance in all four quadrants. The average percentage of swimming distance in the P quadrant by anti-BDNF-treated rats was much less than that by control IgG treated rats (p<0.001). These results suggest that endogenous BDNF is required for spatial learning and memory in adult rats.

Functionally antagonistic interactions between the TrkA and p75 neurotrophin receptors regulate sympathetic neuron growth and target innervation

In this report, we provide evidence that NGF and BDNF have functionally antagonistic actions on sympathetic neuron growth and target innervation, with NGF acting via TrkA to promote growth and BDNF via p75NTR to inhibit growth. Specifically, in cultured sympathetic neurons that themselves synthesize BDNF, exogenous BDNF inhibits and function-blocking BDNF antibodies enhance process outgrowth. Both exogenous and autocrine BDNF mediate this effect via p75NTR because (1) BDNF does not inhibit growth of neurons lacking p75NTR, (2) function-blocking p75NTR antibodies enhance NGF-mediated growth, and (3) p75NTR-/- sympathetic neurons grow more robustly in response to NGF than do their wild-type counterparts. To determine the physiological relevance of this functional antagonism, we examined the pineal gland, a well defined sympathetic target organ. BDNF is present in the pineal gland during target innervation, and incoming sympathetic axons are p75NTR positive. Moreover, the pineal glands of BDNF+/- and BDNF-/- mice are hyperinnervated with sympathetic fibers, and tyrosine hydroxylase (TH) levels are elevated. Increased tyrosine hydroxylase is also observed in the BDNF+/- carotid artery, another sympathetic neuron target. Thus, BDNF, made by sympathetic neurons and/or their target organs, acts via p75NTR to antagonize NGF-mediated growth and target innervation, suggesting that sympathetic target innervation is determined by the balance of positively and negatively acting neurotrophins present in developing and potentially mature targets.

Distribution of BDNF, NT-3 and NT-4 in the developing auditory brainstem

This study describes the developmental expression of three neurotrophins, brain derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3) and neurotrophin (NT-4) in the rat auditory brain-stem using immunohistochemistry. At postnatal day 0 (PND 0), neurotrophins expression was virtually absent from all auditory nuclei in the brainstem, even though some positive neurons were observed in the mesencephalic trigeminal nucleus at this age. However, BDNF, NT-3 and NT-4 positive neurons were observed in most brainstem auditory nuclei by PND 6. At the following stages, there was a general increase in the intensity of the neurotrophins immunoreactivity and BDNF labeling was particularly prominent in most cochlear nucleus neurons. A differential pattern of staining emerged in cochlear nucleus subdivisions, with more intense staining present in the ventral part. The superior olivary complex nuclei followed a similar pattern of BDNF staining compared to the cochlear nucleus. In the adult, BDNF heavily labeled most neurons of the superior olivary nuclei and moderately labeled neurons of the inferior colliculus (IC). NT-3 and NT-4 showed a similar pattern of staining in most auditory brainstem nuclei. The first staining was observed by PND 6 in some neuronal cell bodies. NT-3 and NT-4 immunoreactivity increased in the following stages and in the adult moderate labelings were observed in most neurons of the cochlear nucleus, the superior olivary nuclei and the IC. These results show that neurotrophins are expressed 1 week before the onset of hearing and the increase of their expressions correlate with the appearance of sound-evoked activity in the system. The temporal distribution of neurotrophins does not correlate with neuronal birth, axonal outgrowth or the formation of connection in the auditory structures, suggesting a role primarily in the maintenance and/ or modulation of postnatal and adult functions.

Brain-derived neurotrophic factor modulates nociceptive sensory inputs and NMDA-evoked responses in the rat spinal cord

Central sensitization, the hyperexcitability of spinal processing that often accompanies peripheral injury, is a major component of many persistent pain states. Here we report that the neurotrophin, brain-derived neurotrophic factor (BDNF), is a modulator of excitability within the spinal cord and contributes to the mechanism of central sensitization. BDNF, localized in primary sensory neuron cell bodies and central terminals, potentiates nociceptive spinal reflex responses in an in vitro spinal cord preparation and induces c-fos expression in dorsal horn neurons. NMDA receptor-mediated responses, known as a major contributor to central sensitization, were significantly enhanced by exogenous BDNF. Systemic NGF treatment, a procedure that mimics peripheral inflammatory states, raises BDNF levels in sensory neurons and increases nociceptive spinal reflex excitability. This increased central excitability is reduced by trkB-IgG, a BDNF "antagonist." We also show directly that inflammatory pain-related behavior depends on BDNF release in vivo. Thus behavioral nociceptive responses induced by intraplantar formalin and by intraplantar carageenan are significantly attenuated by trkB-IgG. Hence BDNF is appropriately localized and regulated in inflammatory states and is sufficient and necessary for the expression of central sensitization in the spinal cord. We propose that BDNF may function as a modulator of central sensitization in pathological states, and our results suggest that pharmacological antagonism of BDNF may prove an effective and novel analgesic strategy for the treatment of persistent inflammatory pain states.

Activity-dependent activation of TrkB neurotrophin receptors in the adult CNS

In this paper we have investigated the hypothesis that neural activity causes rapid activation of TrkB neurotrophin receptors in the adult mammalian CNS. These studies demonstrate that kainic acid-induced seizures led to a rapid and transient activation of TrkB receptors in the cortex. Subcellular fractionation demonstrated that these activated Trk receptors were preferentially enriched in the synaptosomal membrane fraction that also contained postsynaptic glutamate receptors. The fast activation of synaptic TrkB receptors could be duplicated in isolated cortical synaptosomes with KCl, presumably as a consequence of depolarization-induced BDNF release. Importantly, TrkB activation was also observed following pharmacological activation of brain-stem noradrenergic neurons, which synthesize and anterogradely transport BDNF; treatment with yohimbine led to activation of cortical TrkB receptors within 30 min. Pharmacological blockade of the postsynaptic alpha1-adrenergic receptors with prazosin only partially inhibited this effect, suggesting that the TrkB activation was partially due to a direct effect on postsynaptic cortical neurons. Together, these data support the hypothesis that activity causes release of BDNF from presynaptic terminals, resulting in a rapid activation of postsynaptic TrkB receptors. This activity-dependent TrkB activation could play a major role in morphological growth and remodelling in both the developing and mature nervous systems.

Activity-Dependent Activation of TrkB Neurotrophin Receptors in the Adult CNS

In this paper we have investigated the hypothesis that neural activity causes rapid activation of TrkB neurotrophin receptors in the adult mammalian CNS. These studies demonstrate that kainic acid-induced seizures led to a rapid and transient activation of TrkB receptors in the cortex. Subcellular fractionation demonstrated that these activated Trk receptors were preferentially enriched in the synaptosomal membrane fraction that also contained postsynaptic glutamate receptors. The fast activation of synaptic TrkB receptors could be duplicated in isolated cortical synaptosomes with KCl, presumably as a consequence of depolarization-induced BDNF release. Importantly, TrkB activation was also observed following pharmacological activation of brain-stem noradrenergic neurons, which synthesize and anterogradely transport BDNF; treatment with yohimbine led to activation of cortical TrkB receptors within 30 min. Pharmacological blockade of the postsynaptic α1-adrenergic receptors with prazosin only partially inhibited this effect, suggesting that the TrkB activation was partially due to a direct effect on postsynaptic cortical neurons. Together, these data support the hypothesis that activity causes release of BDNF from presynaptic terminals, resulting in a rapid activation of postsynaptic TrkB receptors. This activity-dependent TrkB activation could play a major role in morphological growth and remodelling in both the developing and mature nervous systems.

Ponto-geniculo-occipital-wave suppression amplifies lateral geniculate nucleus cell-size changes in monocularly deprived kittens

We have previously shown that during the post-natal critical period of development of the cat visual system, 1 week of instrumental rapid eye movement (REM) sleep deprivation (IRSD) during 2 weeks of monocular deprivation (MD) results in significant amplification of the effects of solely the 2-week MD on cell-size in the binocular segment of the lateral geniculate nucleus (LGN) [36,40]. In this study, we examined whether elimination of ponto-geniculo-occipital (PGO)-wave phasic activity in the LGN during REM sleep (REMS), rather than suppression of all REMS state-related activity, would similarly yield enhanced plasticity effects on cell-size in LGN. PGO-activity was eliminated in LGN by bilateral pontomesencephalic lesions [8,32]. This method of removing phasic activation at the level of the LGN preserved sleep and wake proportions as well as the tonic activities (low voltage, fast frequency ECoG and low amplitude EMG) that characterize REM sleep. The lesions were performed in kittens on post-natal day 42, at the end of the first week of the 2-week period of MD, the same age when IRSD was started in the earlier study. LGN interlaminar cell-size disparity increased in the PGO-wave-suppressed animals as it had in behaviorally REM sleep-deprived animals. Smaller A1/A-interlaminar ratios reflect the increased disparity effect in both the REM sleep- and PGO-suppressed groups compared to animals subjected to MD-alone. With IRSD, the effect was achieved because the occluded eye-related, LGN A1-lamina cells tended to be smaller relative to their size after MD-alone, whereas after PGO-suppressing lesions, the A1-lamina cells retained their size and the non-occluded eye-related, A-lamina cells tended to be larger than after MD-alone. Despite this difference, for which several possible explanations are offered, these A1/A-interlaminar ratio data indicate that in conjunction either with suppression of the whole of the REMS state or selective removal of REM sleep phasic activity at the LGN, altered visual input evokes more LGN cell plasticity during the developmental period than it would otherwise. These data further support involvement of the REM sleep state in reducing susceptibility to plasticity changes and undesirable variability in the course of normative CNS growth and maturation.

Differential sorting of nerve growth factor and brain-derived neurotrophic factor in hippocampal neurons

Nerve growth factor (NGF) is released through the constitutive secretory pathway from cells in peripheral tissues and nerves where it can act as a target-derived survival factor. In contrast, brain-derived neurotrophic factor (BDNF) appears to be processed in the regulated secretory pathway of brain neurons and secreted in an activity-dependent manner to play a role in synaptic plasticity. To determine whether sorting differences are intrinsic to the neurotrophins or reflect differences between cell types, we compared NGF and BDNF processing in cultured hippocampal neurons using a Vaccinia virus expression system. Three independent criteria (retention or release from cells after pulse-chase labeling, depolarization-dependent release, and immunocytochemical localization) suggest that the bulk of newly synthesized NGF is sorted into the constitutive pathway, whereas BDNF is primarily sorted into the regulated secretory pathway. Similar results occurred with AtT 20 cells, including those transfected with cDNAs encoding neurotrophin precursor-green fluorescent protein fusions. The NGF precursor, but not the BDNF precursor, is efficiently cleaved by the endoprotease furin in the trans-Golgi network (TGN). Blocking furin activity in AtT 20 cells with alpha1-PDX as well as increasing the expression of NGF precursor partially directed NGF into the regulated secretory pathway. Therefore, neurotrophins can be sorted into either the constitutive or regulated secretory pathways, and sorting may be regulated by the efficiency of furin cleavage in the TGN. This mechanism may explain how neuron-generated neurotrophins can act both as survival factors and as neuropeptides.

BDNF is a target-derived survival factor for arterial baroreceptor and chemoafferent primary sensory neurons

Brain-derived neurotrophic factor (BDNF) supports survival of 50% of visceral afferent neurons in the nodose/petrosal sensory ganglion complex (NPG; Ernfors et al., 1994a; Jones et al., 1994; Conover et al., 1995; Liu et al., 1995; Erickson et al., 1996), including arterial chemoafferents that innervate the carotid body and are required for development of normal breathing (Erickson et al., 1996). However, the relationship between BDNF dependence of visceral afferents and the location and timing of BDNF expression in visceral tissues is unknown. The present study demonstrates that BDNF mRNA and protein are transiently expressed in NPG targets in the fetal cardiac outflow tract, including baroreceptor regions in the aortic arch, carotid sinus, and right subclavian artery, as well as in the carotid body. The period of BDNF expression corresponds to the onset of sensory innervation and to the time at which fetal NPG neurons are BDNF-dependent in vitro. Moreover, baroreceptor innervation is absent in newborn mice lacking BDNF. In addition to vascular targets, vascular afferents themselves express high levels of BDNF, both during and after the time they are BDNF-dependent. However, endogenous BDNF supports survival of fetal NPG neurons in vitro only under depolarizing conditions. Together, these data indicate two roles for BDNF during vascular afferent pathway development; initially, as a target-derived survival factor, and subsequently, as a signaling molecule produced by the afferents themselves. Furthermore, the fact that BDNF is required for survival of functionally distinct populations of vascular afferents demonstrates that trophic requirements of NPG neurons are not modality-specific but may instead be associated with innervation of particular organ systems.

Differential regulation of neurotrophin and trk receptor mRNAs in catecholaminergic nuclei during chronic opiate treatment and withdrawal

The neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) and their receptors trkB and trkC, respectively, are expressed in the locus coeruleus (LC) and ventral tegmental area (VTA), brain regions known to be involved in opiate addiction. Previously, administration of exogenous neurotrophins has been shown to oppose effects of chronic morphine treatment on LC and VTA neurons. However, the response of endogenous neurotrophins in LC and VTA to opiate treatment is unknown. In this study, BDNF, NT-3, trkB, and trkC mRNAs were analyzed in these regions after chronic morphine treatment and during antagonist precipitated withdrawal. Although chronic morphine exposure resulted in only modest increases in BDNF and NT-3 mRNA expression in LC, precipitated withdrawal led to a marked, rapid, and prolonged increase in BDNF mRNA and a delayed decrease in NT-3 mRNA. Levels of trkB and trkC mRNAs, which were unchanged by chronic morphine treatment, were elevated in LC at 2 and 6 hr of withdrawal. By 20 hr, trkB mRNA levels in LC had returned to control, whereas trkC mRNA levels fell below control values. In contrast to the substantial alterations observed in LC, there was no regulation of the neurotrophins or trk mRNAs within the VTA during chronic opiate treatment or withdrawal, with the exception of an increase in trkB mRNA at 6 hr of withdrawal. These results suggest that neurotrophins and their receptors per se may be involved in opiate-induced plasticity of the LC, whereas other mechanisms would appear to be involved in the VTA.

Highest trkB mRNA expression in the entorhinal cortex among hippocampal subregions in the adult rat: contrasting pattern with BDNF mRNA expression

Brain-derived neurotrophic factor (BDNF) and its receptor, TrkB, regulate synaptic functions in the hippocampus of the adult rodent. In previous studies, in situ hybridization methods have been used to evaluate regional differences in BDNF and trkB mRNA expression levels in hippocampal subregions. However, these studies have failed to reach consensus regarding the regional differences in the mRNA expression levels. In the present study, we quantitated mRNA expression levels using two different methods, ribonuclease protection assays and a quantitative reverse-transcription polymerase chain reaction technique, in four hippocampal subregions: the entorhinal cortex, dentate gyrus (DG), CA3 and CA1. These two methods yielded the same results. We found that BDNF and trkB mRNA expression levels did not covary in the four subregions. BDNF and full length trkB (trkB FL) mRNA in the entorhinal cortex and the DG show contrasting expression patterns. The expression level of BDNF mRNA was highest in the DG among the hippocampal subregions and low in the entorhinal cortex and the CA1, whereas the trkB FL mRNA expression level was highest in the entorhinal cortex, low in the DG and lowest in the CA3. These results suggest regional differences in BDNF/TrkB signaling for maintenance and modifiability of neuronal connections in the hippocampal formation.

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.

[Diffuse primary leptomeningeal gliomatosis]

A woman died at the age of 22 years. Lifetime diagnosis had been inflammatory arachnopathy. The course of the disease had taken at least 6 years. Autopsy revealed primary diffuse leptomeningeal astrocytoma of the brain and spinal cord without neoplastic foci in the parenchyma of the central nervous system. Patchy dystrophic calcifications were recorded from the cerebral and cerebellar cortex.