Cells that Express Brain-Derived Neurotrophic Factor mRNA in the Developing Postnatal Rat Brain

Comparison of neurotrophin regulation of human and rat neuropeptide Y (NPY) neurons: induction of NPY production in aggregate cultures derived from rat but not from human fetal brains

Previous studies established that brain-derived neurotrophic factor (BDNF) induces neuropeptide Y (NPY) production and accumulation of NPY-mRNA in cultures of rat fetal brain tissues. In this study, we addressed the question: Are cultured human NPY neurons regulated by BDNF and/or by another member of the neurotrophin (NT) family of growth factors? Using aggregate cultures derived from human fetal cortical hemispheres, we assessed the effect of BDNF on NPY production varying the following experimental conditions: fetal and culture age; medium composition (with and without serum), dose and duration of exposure to BDNF, and neurotrophin species tested (BDNF, NT-4/5, NT-3 or NGF). Under none of these conditions did BDNF, NT-4/5, NT-3 or NGF induce an increase in NPY production. This was in contrast to forskolin + phorbol 12 myristate 13-acetate (PMA) which were highly effective in inducing NPY production, verifying that expression of NPY is a regulated process in these cultures. None of these neurotrophins enhanced the response to forskolin + PMA. By comparison, using aggregate cultures derived from rat fetal cortices, BDNF and NT-4/5 were equipotent in inducing NPY production but NT-3 and NGF were essentially ineffective. Moreover, the effects of BDNF or NT-4/5 and forskolin + PMA on NPY production were additive, indicating the involvement of distinct intracellular signalling pathways. Western blot analyses of human- and rat-derived aggregates indicated the presence of full-length Trk receptors which are tyrosine-phosphorylated in response to either BDNF, NT-4/5 or NT-3. Primary cultures of astrocytes (rat as well as human) were devoid of a functional TrkB receptor, strongly suggesting a neuronal expression of TrkB in the aggregates. Thus, a functional TrkB receptor is expressed by both the human and rat aggregates, but only the rat aggregates responded to BDNF or NT-4/5. These results are consistent with a difference in a post TrkB-receptor event(s) mediating BDNF action in the cultured human and rat fetal NPY neurons.

Comparative study of brain-derived neurotrophic factor messenger RNA and protein at the cellular level suggests multiple roles in hippocampus, striatum and cortex

Brain-derived neurotrophic factor (BDNF) is important for the development and trophic support of several neuronal groups in the rat. In the present study, the distribution of BDNF messenger RNA was studied by in situ hybridization, and the cellular localization of BDNF protein was investigated with anti-peptide antibodies. Anatomical investigations were also made in animals with prolonged epileptic seizures which show an enhanced expression of BDNF messenger RNA. Major forebrain areas studied were the hippocampus, striatum and cortex. The messenger RNA coding for the putative high-affinity receptor, tyrosine kinase B, was also visualized using in situ hybridization with a probe specific for the full-length form. In the hippocampus, granule cells and pyramidal neurons expressed BDNF messenger RNA and BDNF-like immunoreactivity. Interneurons in dendritic layers did not show labelling with either method. Tyrosine kinase B messenger RNA was found within neurons in all these regions. In the medial septum-diagonal band, nucleus basalis and lateral hypothalamus, neurons with punctate cytoplasmic immunofluorescence were found, and neurons in the lateral septum were diffusely positive for BDNF. In striatum, positive labelling of medium-sized neurons was found with the antibody, whereas BDNF messenger RNA was only detectable during seizures. A laminar pattern of neuronal labelling for BDNF messenger RNA and protein was found in the neocortex. The analysis of the anatomical distribution of BDNF-producing cells suggests a number of possible cellular interactions. In the hippocampus, BDNF might act in an autocrine or paracrine manner for granule cells and pyramidal neurons, and, in addition, may serve as a signal from these principal cells to interneurons. BDNF could be a target-derived and a locally produced trophic factor for cholinergic neurons in the medial septum. The expression of BDNF in the striatum suggests that this factor could be a target-derived factor for dopaminergic neurons of substantia nigra and/or work as an autocrine/ paracrine factor within the striatum itself.

Brain-derived neurotrophic factor, but not neurotrophin-3, prevents ischaemia-induced neuronal cell death in organotypic rat hippocampal slice cultures

We have investigated the neuroprotective actions of neurotrophins in a model of ischaemia using slice cultures. Ischaemia was induced in organotypic hippocampal cultures by simultaneous oxygen and glucose deprivation. Cell death was assessed 24 h later by propidium iodide fluorescence. Pre- but not post-ischaemic addition of brain-derived neurotrophic factor (BDNF) produced a concentration-dependent reduction in neuronal damage. Neurotrophin-3 was not neuroprotective. These data suggest that BDNF may form part of an endogenous neuroprotective mechanism.

Regulation of synaptic responses to high-frequency stimulation and LTP by neurotrophins in the hippocampus

Neurotrophins promote neuronal survival and differentiation, but the fact that their expression is modified by neuronal activity, suggests a role in regulating synapse development and plasticity. In developing hippocampus, the expression of brain derived neurotrophic factor (BDNF) and its receptor TrkB increases in parallel with the ability to undergo long-term potentiation (LTP). Here we report a mechanism by which BDNF modulates hippocampal LTP. Exogenous BDNF promoted the induction of LTP by tetanic stimulation in young (postnatal day 12-13) hippocampal slices, which in the absence of BDNF show only short-term potentiation (STP). This effect was due to an enhanced ability of hippocampal synapses to respond to tetanic stimulation, rather than to a direct modulation of the LTP-triggering mechanism. A TrkB-IgG fusion protein, which scavenges endogenous BDNF, reduced the synaptic responses to tetanus as well as the magnitude of LTP in adult hippocampus. Our results suggest that BDNF may regulate LTP in developing and adult hippocampus by enhancing synaptic responses to tetanic stimulation.

Intrastriatal and intraventricular infusion of brain-derived neurotrophic factor in the cynomologous monkey: distribution, retrograde transport and co-localization with substantia nigra dopamine-containing neurons

The distribution and retrograde transport of brain-derived neurotrophic factor was examined using magnetic resonance imaging guided stereotaxic intracerebroventricular and intrastriatal infusion in the cynomologous monkey. Two intracerebroventricular animals were infused with brain-derived neurotrophic factor at a dose of 3 micrograms/h for 21 and 28 days. A third intracerebroventricular animal received sequential infusions of 15, 30 and 60 micrograms/h brain-derived neurotrophic factor each for seven days using an Alzet 2002 minipump. For the multiple intrastriatal animals (n = 5) a dose of 3 micrograms/h was infused into each site. One intrastriatal monkey was infused with vehicle solution of 10 mM phosphate-buffered saline pH 7.4 for 14 days resulting in no brain-derived neurotrophic factor immunoreactivity. Following the lower dose intracerebroventricular infusion, brain-derived neurotrophic factor immunoreactivity was confined to the ventricular ependymal layer. In the sequential higher dose intracerebroventricular case, the cannula was located mainly within the lateral ventricle, although there was damage to the ependymal wall and adjacent caudate nucleus. Brain-derived neurotrophic factor immunoreactivity revealed spread of injectate within the ipsilateral and to a lesser extent the contralateral caudate nucleus, septum, orbital cortex and ventricular ependymal wall. In this case, retrogradely labelled brain-derived neurotrophic factor neurons were found within the parafascicular thalamus and substantia nigra, pars compacta, as well as within cortex, vertical limb of the diagonal band and nucleus basalis. Brain-derived neurotrophic factor intrastriatal infusion retrogradely labelled perikarya within sensory motor cortex, parafascicular thelamus and substantia nigra, pars compacta. Sections from these cases dual-immunoreacted for brain-derived neurotrophic factor and tyrosine hydroxylase, the synthesizing enzyme for dopamine, revealed a subpopulation of pars compacta dopaminergic neurons which contained retrogradely transported brain-derived neurotrophic factor. These findings indicate that a select subgroup of nigral dopamine neurons retrogradely transport brain-derived neurotrophic factor in the primate. Furthermore it remains to be determined whether select nigral cells are responsive to the trophic influences of brain-derived neurotrophic factor in the normal and neuropathologic condition.

NGF prevents the changes induced by monocular deprivation during the critical period in rats

Photic evoked responses were recorded from the striate cortex of Long-Evans hooded intact, monocular visual deprivation (MD) and MD treated with NGF rats. The averaged visual evoked responses (AVER) were obtained from both hemispheres and provided comparison after binocular photic stimuli between the contralateral and the ipsilateral striate cortex with relation to the MD eye. One month of monocular visual deprivation at the critical period of development resulted in marked reduction of the amplitudes of AVER components as compared to the control recordings (P < 0.001). These changes of the AVER could be prevented by NGF infusion to lateral ventricle at the dosage of 2.0-2.4 micrograms/day for four weeks during the monocular deprivation. In conclusion, the change of AVER amplitudes induced by monocular visual deprivation during the critical period of development can be prevented by NGF infusion to lateral ventricle.

BDNF protein measured by a novel enzyme immunoassay in normal brain and after seizure: partial disagreement with mRNA levels

Messenger RNA for brain-derived neurotrophic factor (BDNF) is distributed in many brain regions and regulated by excitatory neuronal activity. Despite numerous studies of BDNF mRNA, the distribution and regulation of BDNF protein are poorly understood because of the difficulty of its quantitative measurement. We have established a two-site enzyme immunoassay that detects trace amounts of BDNF protein (> 1 pg/assay) but not other neurotrophins or growth factors. The highest levels of BDNF in adult rat brain were found in the hippocampus, followed by the hypothalamus, neocortex, cerebellum, thalamus and striatum. This pattern is similar, but not identical, to the distribution of BDNF mRNA. A similar disparity between BDNF protein and mRNA levels was observed in their changes after hilus lesion-induced limbic seizures. In limbic structures, BDNF concentrations remained elevated 4 days after seizure onset, whereas BDNF mRNA has been reported previously to return to basal levels within 46 h. The temporal and spatial differences between the dynamics of protein and mRNA levels suggest the importance of post-translational and/or subcellular processes for BDNF production. The persistence of the increases in BDNF content was also reflected in its biological activity, e.g. peptidergic differentiation activity. After limbic seizures, neuropeptide Y content was most markedly and persistently elevated in the entorhinal/amygdaloid region, where the most sustained up-regulation of BDNF protein was observed. These results suggest that the sustained increase of BDNF protein in these limbic structures is involved in prolonged post-seizure phenomena, including peptidergic alterations.

NGF and BDNF are differentially modulated by visual experience in the developing geniculocortical pathway

Neuronal activity and trophic factors have been implicated in shaping the connectivity of functional synaptic circuits. We studied the development and regulation by sensory input of the neurotrophins NGF, BDNF and NT-3 in the developing rat visual system. In the occipital cortex, NT-3 mRNA was transiently expressed in the neonate. In contrast, BDNF and NGF mRNA's increased during postnatal development, and reached mature levels around 3 weeks of age. BDNF mRNA was ten times more abundant than NGF mRNA. In the lateral geniculate nucleus (LGN), NT-3 mRNA was also transiently expressed, whereas NGF and BDNF mRNA's did not vary significantly during development. The high-affinity neurotrophin receptors trkB and trkC were expressed both in the developing LGN and occipital cortex. These receptors for BDNF and NT-3, respectively, were expressed at birth, with little change during development. In contrast, trkA mRNA, which encodes the high-affinity NGF receptor, was undetectable in either region. Visual experience differentially modulated expression of NGF and BDNF mRNA's. NGF mRNA was slightly increased after 3 weeks of light-deprivation. In contrast, BDNF mRNA expression in visual cortex was significantly lower than normal in rats dark-reared from birth. Decreased BDNF expression after sensory deprivation was reversible by exposure to light. Thus, all three neurotrophins were detected in visual cortex and LGN. Differences in abundance developmental profiles, and regulation imply distinct functions for each factor in the visual system.

Brain-derived neurotrophic factor-transduced fibroblasts: production of BDNF and effects of grafting to the adult rat brain

Local delivery of brain-derived neurotrophic factor (BDNF) by genetically modified cells provides the unique opportunity to examine the effects of BDNF on adult dopaminergic and cholinergic neurons in vivo. Primary rat fibroblasts were genetically engineered to produce BDNF. Conditioned media from BDNF-transduced fibroblasts supported embryonic chick dorsal root ganglion neurons as well as rat fetal mesencephalic neurons. BDNF-transduced fibroblasts grafted to the rat brain survived and showed continued mRNA production for at least 2 weeks. The effects of BDNF-transduced fibroblast grafts on the dopaminergic and cholinergic systems were then assessed. BDNF-transduced fibroblasts grafted into the normal intact substantia nigra induced sprouting of tyrosine hydroxylase- and neurofilament-immunoreactive fibers into the graft. Fibroblast grafts implanted into the normal intact striatum and midbrain as well as the 6-hydroxydopamine-lesioned brain did not induce sprouting of dopaminergic fibers; neither did they affect drug-induced rotational behavior. BDNF-transduced fibroblasts did, however, significantly increase the homovanillic acid/dopamine ratio when grafted into the normal midbrain. Following transection of the fimbriafornix, BDNF-transduced fibroblasts grafted into the septum were unable to rescue the septal cholinergic population, as did nerve growth factor-producing fibroblast grafts. Genetically modified fibroblast grafts may provide an effective, localized method of BDNF delivery in vivo to test biological effects of this factor on the central nervous system.

Regulation of neuropeptide expression in the brain by neurotrophins. Potential role in vivo

Neurotrophins, which are structurally related to nerve growth factor, have been shown to promote survival of various neurons. Recently, we found a novel activity of a neurotrophin in the brain: Brain-derived neurotrophic factor (BDNF) enhances expression of various neuropeptides. The neuropeptide differentiation activity was then compared among neurotrophins both in vivo and in vitro. In cultured neocortical neurons, BDNF and neurotrophin-5 (NT-5) remarkably increased levels of neuropeptide Y and somatostatin, and neurotrophin-3 (NT-3) also increased these peptides but required higher concentrations. At elevating substance P, however, NT-3 was as potent as BDNF. In contrast, NGF had negligible or no effect. Neurotrophins administered into neonatal brain exhibited slightly different potencies for increasing these neuropeptides: The most marked increase in neuropeptide Y levels was obtained in the neocortex by NT-5, whereas in the striatum and hippocampus by BDNF, although all three neurotrophins increased somatostatin similarly in all the brain regions examined. Overall spatial patterns of the neuropeptide induction were similar among the neurotrophins. Neurons in adult rat brain can also react with the neurotrophins and alter neuropeptide expression in a slightly different fashion. Excitatory neuronal activity and hormones are known to change expression of neurotrophins. Therefore, neurotrophins, neuronal activity, and hormones influence each other and all regulate neurotransmitter/peptide expression in developing and mature brain. Physiological implication of the neurotransmitter/peptide differentiation activities is also discussed.

Synergistic trophic actions on rat basal forebrain neurons revealed by a synthetic NGF/BDNF chimaeric molecule

Basal forebrain cholinergic neurons, which degenerate in Alzheimer's disease, respond to multiple trophic factors, including the neurotrophins, nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF). This dual responsiveness prompted us to investigate the effects of a synthetic chimaeric molecule, containing the active domains of both NGF and BDNF. The NGF/BDNF chimaeric factor exhibited synergistic actions, and was 100-fold more potent than wild-type BDNF in enhancing survival of cultured dissociated basal forebrain cholinergic neurons. This effect was apparently due to true BDNF/NGF synergy, since addition of the two wild-type trophins simultaneously reproduced the effect of the chimaera. Synergy was selective for neurons which respond to both factors; substantia nigra dopaminergic neurons, which respond to BDNF but not NGF, exhibited no potentiation. The chimaeric factor thus revealed a synergy that may normally occur in the brain, and constitutes a potentially novel therapeutic agent with greater potency than naturally occurring individual trophins.

Expression of BDNF and trkB mRNAs in comparison to dopamine D1 and D2 receptor mRNAs and tyrosine hydroxylase-immunoreactivity in nigrostriatal in oculo co-grafts

Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, expresses potential effects on survival and outgrowth from dopaminergic neurons in ventral mesencephalon. In this study we have examined the expression of BDNF mRNA and its high affinity trkB receptor mRNA in the nigrostriatal system after grafting to the anterior chamber of the eye. The BDNF mRNA expression has been compared to the dopaminergic innervation of striatum as revealed by tyrosine hydroxylase (TH) immunohistochemistry and the development of D1 and D2 subtypes of the dopamine receptor mRNAs. Ventral mesencephalon and striatum anlage were either co-grafted or grafted alone and evaluated 2 weeks (immature grafts) or 6 weeks (mature grafts) after transplantation. In situ hybridization for BDNF revealed a positive signal over large neurons in the ventral mesencephalic grafts with an increased silver grain density in the mature grafts. The striatal grafts were negative for BDNF mRNA at both time points evaluated, but in situ hybridization for trkB truncated mRNA revealed increased silver grain density in both the ventral mesencephalic grafts and striatum, with a patchy appearance. The D1 and D2 mRNAs were expressed in a patchy pattern in the striatum both in single grafts and when co-implanted with ventral mesencephalon at both time points evaluated. Often the patches of D1 mRNA did not overlap with the D2 mRNA patches. TH-immunohistochemistry revealed positive neurons in all ventral mesencephalic grafts and a dense patchy innervation of the striatal co-grafts. In conclusion, the trkB truncated mRNA and the dopamine receptor mRNAs were expressed in the striatal graft independent of the contact to a ventral mesencephalic transplant and the dopaminergic input, and BDNF mRNA expression in the ventral mesencephalic transplants was independent of the contact to its striatal target.

Expression of brain-derived neurotrophic factor and of its functional receptor in neonatal and adult rat retina

The expression of mRNA coding for brain-derived neurotrophic factor (BDNF) and for its functional receptor, the full-length tyrosine kinase receptor trkB (trkB mRNA), was examined in early postnatal and adult rat retina by in situ hybridization using digoxygenin and radioactively-labeled oligonucleotide probes. BDNF and trkB mRNAs are expressed in the ganglion cell layer at postnatal-days (PN) 1, 4, 7, 14, 60, in proximal neuroblastic layer (PN 1, 4, 7), and proximal inner nuclear layer (PN 14, 60). Subpopulations of developing and mature retinal cells are thus capable of synthesizing BDNF.

Brain-derived neurotrophic factor messenger RNA is expressed in the septum, hypothalamus and in adrenergic brain stem nuclei of adult rat brain and is increased by osmotic stimulation in the paraventricular nucleus

We have detected scattered brain-derived neurotrophic factor mRNA-producing neurons in the medial septal nucleus, which contains cholinergic neurons that are responsive to brain-derived neurotrophic factor and nerve growth factor. In the brainstem, many adrenergic neurons showed a positive signal for brain-derived neurotrophic factor messenger RNA. Several hypothalamic nuclei contain brain-derived neurotrophic factor messenger RNA-positive neurons, among them paraventricular, median preoptic, vetromedial and dorsomedial nuclei. Osmotic stimulus, which activates vasopressin-producing neurons increased brain-derived neurotrophic factor messenger RNA levels in the paraventricular nucleus demonstrating that this factor is regulated by neuronal activity not only in the hippocampus and cortex but also in the hypothalamus.

Brain-derived neurotrophic factor and neurotrophin-4 increase neurotrophin-3 expression in the rat hippocampus

Hippocampal levels of mRNA encoding nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are rapidly induced by enhanced neuronal activity following seizures and glutamate or muscarinic receptor activation. However, the levels of neurotrophin-3 (NT-3) mRNA acutely decrease after limbic seizures suggesting that a different mode of regulation may exist for these neurotrophins. Here we show that BDNF and neutrotrophin-4 (NT-4), but not NT-3 itself, up-regulate NT-3 mRNA in cultured hippocampal neurons. In the rat hippocampus, the muscarinic receptor agonist, pilocarpine increased BDNF mRNA levels rapidly and those of NT-3 with a delay of several hours. Injection of BDNF into neonatal rats elevated NT-3 mRNA in the hippocampus which demonstrates that BDNF is able to enhance NT-3 expression in vivo. The regulation of NT-3 by BDNF and NT-4 enlargens the neurotrophic spectrum of these neurotrophins to include neuron populations responsive primarily to NT-3.

Neurotrophin-4/5 promotes survival and differentiation of rat striatal neurons developing in culture

Cultures of dissociated striatal neurons from fetal rats were prepared, and were grown in the presence of neurotrophin-4/5 (NT-4/5) as well as the other known neurotrophins, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3). We found that acute administration of NT-4/5 to 7-day-old cultures stimulates the hydrolysis of phosphatidylinositol, an event involved in neurotrophin signal transduction. Growth of striatal cultures in the presence of NT-4/5 resulted in increased cell survival, as indicated by elevations in cell number, protein content, and a measure of mitochondrial enzyme activity (MTT assay). NT-4/5 increased GABA uptake and staining intensity in these cultures, as indicated by GABA immunocytochemistry, indicating a trophic action on GABAergic neurons, the predominant neuron type in the striatum. To further identify responsive cell populations we analysed for calretinin, a calcium-binding protein known to colocalize with GABA in a number of neuronal cells. In cultures prepared from rats of embryonic day 15, NT-4/5 strongly increased the number of calretinin-positive cells as well as calretinin levels, as determined by Western blot analysis. When the cultures were prepared from embryonic day 18 rats, NT-4/5 very strongly increased the morphological differentiation of calretinin-positive cells, whereas the increase in cell number was less prominent. All effects produced by NT-4/5 were mimicked by BDNF with similar potency. NT-3 was less effective than NT-4/5 and BDNF, and its effects were limited to cultures prepared from embryonic day 15 rats, suggesting a role in the regulation of cell survival at early developmental stages. NGF did not affect any of the measured parameters. Our findings identify NT-4/5 as potent neurotrophic factor for striatal neurons, able to promote their survival and differentiation.

Activity-dependent and hormonal regulation of neurotrophin mRNA levels in the brain--implications for neuronal plasticity

The neurotrophins exhibit neurotrophic effects on specific, partially overlapping populations of neurons both in the peripheral and the central nervous system (CNS). In the periphery, they are synthesized by a variety of nonneuronal cells, and their synthesis seems to be independent of the neuronal input. In contrast, in the CNS all neurotrophins are expressed under physiological conditions primarily by neurons. The production of NGF and BDNF is controlled by neuronal activity: up-regulation by glutamate and acetylcholine, down-regulation by gamma-aminobutyric acid. In contrast, NT-3 regulation is independent of neuronal activity, but it is up-regulated by thyroid hormones and BDNF. The latter observation suggests that NT-3 might be controlled indirectly by neuronal activity via BDNF. In peripheral nonneuronal tissues, glucocorticoid hormones down-regulate NGF mRNA levels both in vitro and in vivo. In contrast, in the CNS, neuronal production of NGF is enhanced by glucocorticoids. The rapid regulation of NGF and BDNF by subtle physiological stimuli together with the recent demonstration that the neurotrophins release neurotransmitters such as acetylcholine opens up interesting perspectives for the function of neurotrophins as mediators of neuronal plasticity.

Differential regulation of the expression of nerve growth factor, brain-derived neurotrophic factor and neurotrophin-3 mRNAs in adult rat brain after intrahippocampal injection of quinolinic acid

Intrahippocampal injection of the endogenous excitotoxin quinolinic acid (QUIN) induces seizures together with local, delayed neurodegeneration in specific cell layers. In situ hybridization histochemistry was used to study the spatio-temporal pattern of expression of neurotrophins (NTFs) after this treatment. As in other excitatory paradigms, nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) mRNA levels increased dramatically and transiently in dentate gyrus after the administration of 120 nmol of QUIN to the left hippocampus. BDNF, but not NGF, mRNA also increased in the hippocampal pyramidal cell layer, mainly in the CA1 field. Neurotrophin-3 (NT3) mRNA levels decreased in dentate gyrus, practically disappeared around 12 h after the insult and returned to basal levels four days later. A very different pattern of expression of NTFs was found locally: (a) upregulation of NGF and BDNF mRNAs expression was prevented in a spherical region of 1-2 mm diameter around the injection site, (b) a delayed increase in NT3 mRNA levels, beginning at 12 h and lasting for at least 4 days after the administration of QUIN, was found in the same region, in cell layers showing neurodegeneration. Pretreatment with the non-competitive NMDA antagonist MK-801 (2 mg/kg, 30 min before the insult), partially blocked the increase in both BDNF and NGF mRNAs, as well as the decrease in NT3, in the contralateral hippocampus. However, this treatment did not prevent the QUIN-induced local downregulation of NGF and BDNF. Treatment with the AMPA/kainate antagonist NBQX (30 mg/kg, 15 and 5 min before, and 10 min after the insult) did not influence the effect of QUIN upon NGF or BDNF mRNA levels, although it partially prevented the hippocampal contralateral decrease in NT3 mRNA. In conclusion, the present study strongly supports previous work concerning different regulation of BDNF/NGF respect to NT3 in seizure inducing paradigms. Moreover, the different and to some extent opposite regulation of NTFs in the hippocampal region contiguous to the injection site, respect to the remaining hippocampus, suggests a differential regulation of NTFs in QUIN-induced neurodegenerative and seizural processes. Finally, our pharmacological data, (i) show that the upregulation of NGF and BDNF mRNAs, indirectly induced by QUIN, is not mediated by AMPA receptors, and (ii) suggest other effects for QUIN, apart from the stimulation of NMDA receptors.

Glucocorticoids and the expression of mRNAs for neurotrophins, their receptors and GAP-43 in the rat hippocampus

The genes encoding brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and basic fibroblast growth factor (bFGF) are all expressed in the adult rat hippocampus. The colocalization of the these factors with the receptors to which they bind, namely trkB, trkC and the bFGF receptor, respectively, suggests that in the hippocampus they may exert their putative protective and trophic effects through an autocrine mechanism. The morphology and survival of hippocampal neurons are also affected by glucocorticoids, which can act as transcriptional activators of gene expression. In this study we have used in situ hybridization to investigate the adrenal steroid regulation of the mRNAs encoding the neurotrophic factors BDNF, NT-3, and bFGF, their respective receptors, and the growth-associated protein GAP-43. After 7 days of adrenalectomy (ADX), there was an increase in the level of GAP-43 mRNA expression in the CA1 and CA3 pyramidal cell layers of the hippocampus, that was prevented by corticosterone replacement to the ADX animals. In the CA2 subregion, adrenalectomy resulted in a decrease in bFGF mRNA expression, that was reversed by steroid treatment. There was evidence for glucocorticoid modulation of the BDNF and NT-3 mRNAs in pyramidal cell layers and in the dentate gyrus, but not of the mRNAs encoding the trkB, trk C or bFGF receptors.

The response of human and rat fetal ventral mesencephalon in culture to the brain-derived neurotrophic factor treatment

Brain-derived neurotrophic factor (BDNF) has been shown to increase the survival of dopaminergic neurons in rodent mesencephalic cultures. The mRNAs of BDNF and trkB receptor have been found to be expressed in the substantia nigra of rat. In this study, the action of BDNF was studied on the survival and transmitter-specific differentiation of dopaminergic neurons of fetal human CNS aged 9-10-week in vitro. Dopaminergic neuron viability and phenotypic expression were monitored by tyrosine hydroxylase (TH) immunohistochemistry and measurement of dopamine (DA) content with HPLC, respectively. After seven days of treatment with BDNF there were 2.2-fold greater number of TH+ neurons surviving than in untreated cultures. Although very low levels of DA were detectable in human tissue, considerable amounts of DA was found in the culture medium from around 13 days in vitro (DIV), indicating that DA in human fetal tissue tended to be synthesised and released into the incubation medium more readily than from cultured rat fetal tissue during the same period. The content of DA in the BDNF-treated cultures was approximately double that of untreated cultures after 7 days. In rat fetal tissue, the capacity of each TH+ neuron to produce DA was not changed in the BDNF-treated cultures (7 DIV) compared with control cultures, suggesting that BDNF does not up-regulate the production of DA but rather acts to reduce cell death rates. Ciliary neurotrophic factor (CNTF) treatment of rat mesencephalic culture failed to improve the period of survival of fetal dopaminergic neurons and had no effect on the production of DA in cultures.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of BDNF promoters in the rat hippocampus

Brain-derived neurotrophic factor belongs to the neurotrophin family of trophic factors. Recently we have described four promoters in the BDNF gene. We have analyzed the hippocampal expression pattern of BDNF mRNA's containing different 5' exons during postnatal development. This analysis has revealed distinct regulation of promoters I and II compared to promoters III and IV, which suggests the presence of common regulatory elements for these clusters of promoters. Induction of different BDNF promoters after treatment with kainic acid combined with glutamate antagonists MK801 and NBQX discloses the differential participation of different glutamate receptor subtypes in regulation of the BDNF gene in the hippocampus.

bcl-2 messenger RNA is localized in neurons of the developing and adult rat brain

Overexpression of the proto-oncogene bcl-2 blocks programmed cell death in sympathetic and sensory neurons that normally die after the withdrawal of neurotrophic factors. The role of endogenous bcl-2 in the development and function of the peripheral and central nervous system is, however, not known. We have found that low levels of bcl-2 messenger RNA are widely distributed in the adult and developing rat brain. In situ hybridization localized bcl-2 messenger RNA in mitral cells of the olfactory bulb, granule and pyramidal neurons of hippocampus, pontine nuclei, cerebellar granule neurons, and in ependymal cells in adult rat brain. bcl-2 messenger RNA levels were higher in late prenatal development than in postnatal and adult brain. High levels of bcl-2 messenger RNA were expressed in the neuroepithelium and in the cortical plate in prenatal cortex. During postnatal development the distribution of the message resembled that found in adult brain. We have also tested the hypothesis that induction of bcl-2 messenger RNA expression might be part of the survival-promoting action of neurotrophic factors. Brain-derived neurotrophic factor, which supports survival of cultured cerebellar granule neurons, failed to influence the levels of bcl-2 messenger RNA in these cultures.

Neurotrophic factors: from molecule to man

Recent advances in the understanding of the physiological role of nerve growth factor (NGF) have raised the question of whether neurotrophic factors might have clinical potential in the treatment of neurodegenerative disease or nerve trauma. Although NGF was first characterized as a target-derived survival factor for developing sympathetic and sensory neurons, it is now clear that it plays an important role in the maintenance and regeneration of mature peripheral neurons. However, the highly restricted specificity of NGF for sympathetic neurons, subpopulations of neural-crest-derived sensory neurons, and striatal and basal forebrain cholinergic neurons has, for almost two decades, stimulated the search for other neurotrophic factors that might act on the many classes of neurons that do not respond to NGF. In this article, the biology of the recently discovered NGF-related family of neurotrophic factors and ciliary neurotrophic factor and their receptors are reviewed, especially in the context of the therapeutic potential of these factors in the treatment of neurological disorders of the CNS.

Developmental regulation of brain-derived neurotrophic factor messenger RNAs transcribed from different promoters in the rat brain

During the development of the nervous system many types of neurons are produced in excess and die because they fail to obtain sufficient amounts of target-derived neurotrophic factors (for review, see Refs 1, 16). Brain-derived neurotrophic factor is a protein that belongs to the nerve growth factor family of neurotrophins (for review, see Ref. 17) which promotes the survival and differentiation of distinct neuronal populations that partially overlap with those affected by the other members of the family (for review, see Ref. 11). We have recently shown that four promoters direct tissue specific expression of the rat brain-derived neurotrophic factor gene. In the present study we have quantified the levels of brain-derived neurotrophic factor messenger RNAs transcribed from different promoters during the rat brain development. Our data show that different promoters have specific regulation patterns during embryonic development but are regulated coordinately in several regions of rat brain during postnatal development, suggesting that the four transcription units have both distinct and common regulatory elements. Quantification of absolute levels of brain-derived neurotrophic factor messenger RNA revealed that it is expressed significantly higher in the rat brain than previously estimated.

Dopaminergic neurons in rat ventral midbrain express brain-derived neurotrophic factor and neurotrophin-3 mRNAs

Studies of the trophic activities of brain-derived neurotrophic factor and neurotrophin-3 indicate that both molecules support the survival of a number of different embryonic cell types in culture. We have shown that mRNAs for brain-derived neurotrophic factor and neurotrophin-3 are localized to specific ventral mesencephalic regions containing dopaminergic cell bodies, including the substantia nigra and ventral tegmental area. In the present study, in situ hybridization with 35S-labeled cRNA probes for the neurotrophin mRNAs was combined with neurotoxin lesions or with immunocytochemistry for the catecholamine-synthesizing enzyme tyrosine hydroxylase to determine whether the dopaminergic neurons, themselves, synthesize the neurotrophins in adult rat midbrain. Following unilateral destruction of the midbrain dopamine cells with 6-hydroxydopamine, a substantial, but incomplete, depletion of brain-derived neurotrophic factor and neurotrophin-3 mRNA-containing cells was observed in the ipsilateral substantia nigra pars compacta and ventral tegmental area. In other rats, combined in situ hybridization and tyrosine hydroxylase immunocytochemistry demonstrated that the vast majority of the neurotrophin mRNA-containing neurons in the substantia nigra and ventral tegmental area were tyrosine hydroxylase immunoreactive. Of the total population of tyrosine hydroxylase-positive cells, double-labeled neurons constituted 25-50% in the ventral tegmental area and 10-30% in the substantia nigra pars compacta, with the proportion being greater in medial pars compacta. In addition, tyrosine hydroxylase/neurotrophin mRNA coexistence was observed in neurons in other mesencephalic regions including the retrorubral field, interfascicular nucleus, rostral and central linear nuclei, dorsal raphe nucleus, and supramammillary region. The present results demonstrate brain-derived neurotrophic factor and neurotrophin-3 expression by adult midbrain dopamine neurons and support the suggestion that these neurotrophins influence dopamine neurons via autocrine or paracrine mechanisms. These data raise the additional possibility that inappropriate expression of the neurotrophins by dopaminergic neurons could contribute to the neuropathology of disease states such as Parkinson's disease and schizophrenia.

BDNF in the development of the visual system of Xenopus

To explore the role of BDNF during Xenopus visual system development, the expression of BDNF and TrkB, as well as the effect of BDNF during retinal ganglion cell (RGC) development in culture, was examined. BDNF mRNA was found to be expressed in both the developing eye and tectum, peaking at stage 45/46. The expression of BDNF coincided with RGC expression of full-length trkB transcripts, suggesting a functional role for BDNF. In culture, BDNF significantly increased the number of RGCs. The ability of BDNF to rescue differentiated RGCs that had projected to the tectum, the time course of the effect, and the lack of mitogenic response indicate that this neurotrophin promotes survival. The expression of BDNF and TrkB and the responsiveness of RGCs to BDNF coincide with retinal axon terminal arborization and patterning. Our results indicate that BDNF is a relevant neurotrophin for Xenopus RGC development and suggest that it plays a role during visual system patterning.

NT-3 and BDNF protect CNS neurons against metabolic/excitotoxic insults

Neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF) were recently shown to have biological activity in central neurons. In the present study, NT-3 and BDNF attenuated glucose deprivation-induced neuronal damage dose-dependently in rat hippocampal, septal and cortical cultures. Direct measurements of intraneuronal free calcium levels ([Ca2+]i) and manipulations of calcium influx demonstrated that NT-3 and BDNF each prevented the elevation of [Ca2+]i that mediated glucose deprivation-induced injury. Studies in cultures depleted of glia indicated a direct action of NT-3 and BDNF on neurons. Neurons pretreated with NT-3 or BDNF for 24 hr were more resistant to glutamate neurotoxicity, and showed attenuated [Ca2+]i responses to glutamate. TrkB (BDNF receptor) and trkC (NT-3 receptor) proteins were present in hippocampal, cortical and septal cultures where they were localized to neuronal cell bodies and neurites. The data demonstrate that NT-3 and BDNF can protect neurons against metabolic and excitotoxic insults, and suggest that these neurotrophins may serve [Ca2+]i-stabilizing and neuroprotective functions in the brain.

Dopamine neoinnervation in the substantia nigra and hyperinnervation in the interpeduncular nucleus of adult rat following neonatal cerebroventricular administration of 6-hydroxydopamine

An aberrant network of dopamine axons was found to pervade the rat substantia nigra following neonatal destruction of its dopamine nerve cell bodies and dendrites by cerebroventricular administration of 6-hydroxydopamine. Light-microscopic immunocytochemistry with a primary monoclonal antibody directed against dopamine-glutaraldehyde-protein was used to investigate the time-course of development and the critical period of induction of this ectopic dopamine innervation (neoinnervation). In rats 6-hydroxydopamine-lesioned at postnatal day 3 (P3) and examined at P7, P10, P15, P30 or later, some dopamine fibers were already present in the substantia nigra at P7; their number increased sharply until P15 and only slightly thereafter, assuming a topographic distribution reminiscent of the missing dopamine nerve cell bodies and dendrites. A similar growth of dopamine fibers took place in the substantia nigra after lesions made at P6, P9 and P12, but was less pronounced after lesion at P15 and absent after lesion at P21 or later. Excessive innervation by dopamine axons (hyperinnervation) was concomitantly observed in the nearby interpeduncular nucleus. The sprouting of dopamine axons in both regions was therefore rapid and coincided in time and space with the developmental redistribution of mesencephalic dopamine neurons in normal rat. It is conceivable that these aberrant dopamine innervations play a role in the peculiar behavior and responsiveness to dopaminergic agents manifested by neonatally 6-hydroxydopamine-lesioned rats. It will be of particular interest to investigate the functional consequences of the dopamine neoinnervation in the substantia nigra, where an eventual axonal release might thus be replacing the normal somatodendritic release of this amine.

Neurotrophin-3 enhances sprouting of corticospinal tract during development and after adult spinal cord lesion

The number of neurotrophic factors found in the central nervous system is rapidly growing, but their functions in vivo are largely unknown. In the peripheral nervous system they promote the survival of developing and lesioned neurons and enhance nerve fibre growth and regeneration. Here we study the effects of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) on the largest tract system leading from the brain to the spinal cord, the corticospinal tract (CST). The developing CST grows down the spinal cord during the first postnatal days and innervates its targets after a waiting period by collateral sprouting. We find that NT-3 injected locally specifically enhances this sprouting, whereas BDNF has no effect. In adult rats, injection of NT-3 (but not BDNF) into the lesioned spinal cord increases the regenerative sprouting of the transected CST. The distance of growth of the sprouts is very restricted, but application of an antibody that neutralizes myelin-associated neurite growth inhibitory proteins results in long-distance regeneration of CST fibres.

Growth factors in Parkinson's disease

The etiology of Parkinson's disease, one of the most frequent neurodegenerative disorders in human, is unknown. New hopes concerning satisfactory therapies include transplants of autologous adrenal medullary chromaffin tissue, fetal mesencephalic dopaminergic neurons, and local application of growth factors with a neurotrophic capacity. A large body of evidence supports the notion that neurons require trophic support not only during a limited period of ontogenesis, but during their whole lifespan. Relevant molecules promote survival, transmitter synthesis and other differentiated properties, and become crucially important when a neuron is metabolically or toxically impaired. Several molecules, most of which occur in the striatum and the substantia nigra, have been identified that protect lesioned dopaminergic nigrostriatal neurons in culture or in animal models of Parkinson's disease. These include members of the neurotrophin, fibroblast growth factor, and insulin-like growth factor families as well as epidermal growth factor/transforming growth factor alpha, interleukins and ciliary neurotrophic factor. Whether their effects are merely pharmacological, or reflect a physiological role in the nigrostriatal system, is unclear as yet. This article reviews experiments that document the trophic effects of these factors on dopaminergic neurons and discusses their possible physiological and therapeutic relevance.

NT-3 stimulates sympathetic neuroblast proliferation by promoting precursor survival

Although proliferation is fundamental to the generation of neuronal populations, little is known about the function of trophic mechanisms during neurogenesis. We now describe a novel role for neurotrophin-3 (NT-3): the neurotrophin stimulates proliferation of sympathetic neuroblasts through trophic mechanisms. NT-3 promotes survival of the dividing precursors, but does not directly stimulate mitosis. NT-3 trophic effects differ markedly from those of the sympathetic mitogen, insulin. Furthermore, whereas NT-3 exhibits trophic activity for dividing neuroblasts, nerve growth factor characteristically promotes survival of postnatal sympathetic neurons. The stage-specific activity of NT-3 and nerve growth factor in culture parallels the sequence of trkC and trkA receptor gene expression detected in vivo. Thus, neurotrophins apparently serve as trophic factors during ontogeny, acting sequentially during establishment of individual populations.

Ganglioside GM1 cooperates with brain-derived neurotrophic factor to protect dopaminergic neurons from 6-hydroxydopamine-induced degeneration

Brain-derived neurotrophic factor (BDNF) has recently been shown to enhance the survival of dopaminergic neurons in cultures derived from the embryonic rat mesencephalon. In the present study BDNF was found to protect cultured dopaminergic neurons from injury induced by acute exposure to the dopaminergic-selective neurotoxin 6-hydroxydopamine. The BDNF effect was concentration (ED50 approximately 10 ng/ml) and time-dependent, as determined by tyrosine hydroxylase immunocytochemistry. More importantly, subthreshold amounts of BDNF were rendered efficacious in the presence of ganglioside GM1: loss of tyrosine hydroxylase positive cells was reduced from 80% to only 20%. Thus GM1 may provide a fruitful treatment strategy for disorders of dopamine function such as Parkinson's disease.

Trophic actions of transforming growth factor alpha on mesencephalic dopaminergic neurons developing in culture

Transforming growth factor alpha messenger RNA and protein levels are highest in the striatum, the target area of mesencephalic dopaminergic neurons of the substantia nigra, suggesting a role as a target-derived neurotrophic factor for these cells. To test this hypothesis, we characterized the actions of transforming growth factor alpha on fetal rat dopaminergic neurons in culture. Transforming growth factor alpha promoted dopamine uptake in a dose- and time-dependent manner. Administration of transforming growth factor alpha at the time of plating for 2 h produced a significant increase in dopamine uptake after five days of growth in vitro. As cultures aged they became less responsive to transforming growth factor alpha, such that longer times of exposure were required to elicit a similar, but weaker, response. Dopaminergic cell survival was selectively promoted by transforming growth factor alpha, since there was an increase in the number of tyrosine hydroxylase-immunostained cells without a parallel increase in the total number of neuron-specific enolase-immunopositive cells. Neurite length, branch number and soma area of tyrosine hydroxylase-immunopositive cells also were enhanced by transforming growth factor alpha treatment. Increases in each of the dopaminergic parameters due to transforming growth factor alpha were accompanied by a rise in glial cell number, making it possible that these effects were mediated by this cell population. The neurotrophin antagonist, K252b, failed to inhibit the transforming growth factor alpha-induced increase in dopamine uptake, indicating that transforming growth factor alpha's effects were not mediated by neurotrophin mechanisms. The actions of transforming growth factor alpha on the differentiation of dopaminergic neurons only partially overlapped with those of epidermal growth factor. Thus, while transforming growth factor alpha and epidermal growth factor are believed to share the same receptor they differentially affect dopaminergic cell development in vitro. These results indicate that transforming growth factor alpha is a trophic factor for mesencephalic cells in culture and suggests that transforming growth factor alpha plays a physiological role in the development of these cells in vivo.

Localization of trkB mRNA in postnatal brain development

We investigated the localization of trkB mRNA, which encodes a putative component of high-affinity brain-derived neurotrophic factor (BDNF) or the neurotrophin-3 (NT-3) receptor, in the postnatal rat brain by in situ hybridization histochemistry. At birth, trkB mRNA was strongly expressed in various regions with the thalamus and cerebral cortex showing the strongest expression. As the rat grows, expression generally persisted or declined in most regions with the exception of the hippocampus where trkB mRNA expression increased during postnatal development. In the adult brain, trkB mRNA was detected in the olfactory system, cerebral cortex, hippocampal formation, amygdala, and cerebellar cortex. These findings, together with the developmental profiles of BDNF and NT-3 mRNA expressions, suggest that trkB product (gp145trkB) mainly transduces NT-3 signals early in the postnatal period, and BDNF signals later in the period.

Time course, localization and pharmacological modulation of immediate early inducible genes, brain-derived neurotrophic factor and trkB messenger RNAs in the rat brain following photochemical stroke

A focal, unilateral thrombotic stroke was produced in the rat sensorimotor cortex. The time course of expression and localization of the immediate early inducible genes: c-fos, c-jun, zif268; nerve growth factor, brain-derived neurotrophic factor and the related tyrosine kinase high-affinity receptor (trkB) messenger RNAs were studied by in situ hybridization. The levels of messenger RNAs for c-fos, zif268, brain-derived neurotrophic factor (but not nerve growth factor) and trkB were consistently increased in cortex ipsilaterally to the lesion, while c-jun messenger RNA content was only slightly increased. The brain-derived neurotrophic factor messenger RNA was increased from 2 to 18 h following the stroke, mainly in cells having a normal morphological appearance. The trkB messenger RNA displayed temporal and spatial increases similar to brain-derived neurotrophic factor messenger RNA. The time course and pattern of expression of immediate early inducible gene and trophic factor messenger RNAs did not clearly support a causal relationship between these two families of factors. The observed messenger RNA increases were greatly attenuated by the non-competitive N-methyl-D-aspartate-sensitive glutamate receptor antagonist (+)-5-methyl-10,11-dihydroxy-5H-dibenzo(a,d)cyclohepten-5,10-imine , but substantially unaffected by the non-N-methyl-D-aspartate receptor antagonist 2,3-dihydroxy-6-nitrosulphanoylbenzoquinoxaline. The results suggest a major contribution of N-methyl-D-aspartate-sensitive glutamate receptor activation to the transcriptionally directed events subsequent to stroke. Future studies should clarify the contribution of these processes to either the progression of neuronal degeneration or the establishment of protective compensatory responses.

Survival and neurite formation of mesencephalic trigeminal neurones of the rat in vitro

In order to study the development and functional properties of single, isolated, rat mesencephalic trigeminal neurones, a cell-culture procedure was developed for these specific primary sensory neurones. Mesencephalic trigeminal neurones were isolated from the brainstem of 16-day-old rat embryos. Various factors thought to promote the survival and growth of these neurones in vitro were examined. Outgrowth and maintenance of mesencephalic trigeminal neurones in vitro appeared to be stimulated by a muscle-derived factor, present in muscle-conditioned medium or in muscle extract. Of the neurotrophic factors examined, brain-derived neurotrophic factor and neurotrophin-3, but not nerve-growth factor, promoted the survival of rat mesencephalic trigeminal neurones. Optimal survival of these neurones was found to occur on a monolayer of astrocytes, an effect mediated through direct cell-to-cell interactions.

Widespread and developmentally regulated expression of neurotrophin-4 mRNA in rat brain and peripheral tissues

The neurotrophin gene family includes four structurally related proteins with neurotrophic activities. Two of them, nerve growth factor and brain-derived neurotrophic factor (BDNF), have been studied in detail and information has recently emerged on the expression and function of the third member, neurotrophin-3. In contrast, little information is available on neurotrophin-4 (NT-4), the most recently isolated member of this family. In this report we have used a sensitive RNAase protection assay to analyse the developmental expression of NT-4 mRNA in the rat brain and in 12 different rat peripheral organs. In heart, liver and muscle plus skin NT-4 mRNA levels were maximal at embryonic day (E) E13 (the earliest time point tested), with reduced levels at later times of development. In lung, kidney and thymus similar levels were seen from E13 to postnatal day (P) 1, with reduced levels in the adult. In testis, ovary and salivary gland NT-4 mRNA was detected at E16 with a peak shortly after birth. During brain development, NT-4 mRNA was maximal at E13 followed by a decrease around birth, after which the level increased. The postnatal increase of NT-4 mRNA was also seen in cerebral cortex and brain stem analysed separately, while in the hippocampus similar levels were found from P1 to adulthood. NT-4 mRNA was detected in all ten adult rat brain regions analysed with only small regional variations, being highest in pons-medulla, hypothalamus, thalamus and cerebellum.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotrophin-4 is a target-derived neurotrophic factor for neurons of the trigeminal ganglion

The cellular localization of mRNA for neurotrophin-4 (NT-4), a novel neurotrophic factor, in the developing whisker follicles and skin of the embryonic rat is demonstrated by in situ hybridization. Levels of NT-4 mRNA in the whisker pad decrease between embryonic day 13 (E13) and E20, correlating in time with the onset of naturally occurring neuronal death in the innervating trigeminal ganglion. In addition to NT-4, brain-derived neuotrophic factor (BDNF) mRNA is also shown to be expressed in the rat embryonic whisker follicles although in a different cellular localization, which combined with previous data on the expression of NGF and NT-3 mRNAs, shows that all four neurotrophins are expressed during development of this structure. NT-4 protein is shown to elicit neurite outgrowth from explanted embryonic trigeminal ganglia and to promote neuronal survival of dissociated trigeminal ganglion neurons when cultured during the phase of cell death. NT-4 and NT-3 mainly support different neuronal subpopulations, whereas some NT-4-responsive cells appear to respond also to NGF and BDNF. Analysis of mRNAs for members of the Trk family of neurotrophin receptors in neurons rescued by different neurotrophins demonstrates the presence of distinct neuronal subpopulations that respond to specific combinations of these factors. Based on these results we propose that NT-4, together with the other three neurotrophins, orchestrate the innervation of the different structures of the developing whisker pad by the trigeminal ganglion, acting as target-derived neurotrophic factors for different subpopulations of trigeminal ganglion neurons.

Expression of members of the trk family in the developing postnatal rat brain

Tyrosine protein kinases trk, trkB and trkC are essential components of the high affinity receptors necessary to mediate biological effects of the neurotrophins NGF, BDNF, NT-3 and NT-4. Here we report on the expression of these receptors during postnatal development in the rat brain. Cells expressing mRNAs encoding different members of the trk family were identified by in situ hybridization using oligonucleotides complementary to their respective mRNA. In septum, striatum and brainstem, higher levels of trk mRNA were detected at 2 and 4 weeks than at 1 weeks of age. In thalamic nuclei associated with the limbic system, trkB and trkC mRNA were highly expressed at P1 to P7, but the expression declined gradually in 2 and 4 week old animals. Other structures where a developmentally regulated expression was seen included the tenia tecta and piriform cortex where trkB mRNA was not detected until 2 weeks of age. A high labeling was found for trkC mRNA in the deeper parts of neocortex in P1 and P4 animals, while in 2 and 4 weeks old animals the highest labeling was seen over the outer neocortical layers. Several brainstem nuclei showed a higher labeling for trkC mRNA at P1 to P7 than in animals of older age. These data show that expression of members of the trk family is developmentally regulated during postnatal brain development and suggest that high affinity neurotrophin receptors mediate a transient response to neurotrophins in many regions during brain ontogeny.

Brain-derived neurotrophic factor selectively rescues mesencephalic dopaminergic neurons from 2,4,5-trihydroxyphenylalanine-induced injury

Brain-derived neurotrophic factor (BDNF) supports the survival of sensory neurons as well as retinal ganglion cells, basal forebrain cholinergic neurons, and mesencephalic dopaminergic neurons in vitro. Here we examined the ability of BDNF to confer protection on cultured dopaminergic neurons against the neurotoxic effects of 6-hydroxyDOPA (TOPA or 2,4,5-trihydroxyphenylalanine), a metabolite of the dopamine pathway suggested to participate in the pathology of Parkinson's disease. Cells prepared from embryonic day 14-15 rat mesencephalon were maintained with 10-50 ng/ml BDNF for 7 days prior to addition of TOPA (10-30 microM) for 24 hr. In BDNF-treated cultures, the extensive loss (> 90%) of tyrosine hydroxylase immunopositive cells was virtually (< 10%) eliminated, while the equally drastic loss (> 90%) of the overall cell population was limited to only a 25-30% recovery. Furthermore, the monosialoganglioside GM1 (1-10 microM), although inactive alone, acted synergistically with subthreshold amounts of BDNF to rescue tyrosine hydroxylase-positive cells against TOPA neurotoxicity. These results add impetus to exploring the therapeutic potential of gangliosides and BDNF in Parkinson's disease.

Multiple promoters direct tissue-specific expression of the rat BDNF gene

Brain-derived neurotrophic factor (BDNF) supports the survival of a specific set of neurons in the vertebrate nervous system. Here we show that the rat BDNF gene consists of four short 5' exons and one 3' exon encoding the mature BDNF protein. Eight different BDNF mRNAs with four different 5' ends and two alternative polyadenylation sites are transcribed from this gene. BDNF mRNAs containing exons I, II, and III are expressed predominantly in the brain, whereas exon IV transcripts predominate in the lung and heart. mRNAs containing exons I, II, and III increase markedly in the brain after kainic acid-induced seizures, whereas exon IV mRNA increases only slightly. Several transcription initiation sites were mapped upstream of the four 5' exons, and transfection of promoter-reporter gene constructs confirmed that these sequences act as promoters. Combined, the data demonstrate that alternative usage of four promoters within the BDNF gene and differential splicing control tissue-specific and seizure-induced expression of BDNF mRNA.

The nature of the trophic action of brain-derived neurotrophic factor, des(1-3)-insulin-like growth factor-1, and basic fibroblast growth factor on mesencephalic dopaminergic neurons developing in culture

Brain-derived neurotrophic factor, basic fibroblast growth factor and des(1-3)-insulin-like growth factor-1, a brain specific form of insulin-like growth factor-1, were analysed, in the rat, for their influence on survival, morphological growth, and transmitter-specific differentiation of dopaminergic neurons in vitro. Brain-derived neurotrophic factor, des-insulin-like growth factor-1, and basic fibroblast growth factor were found to differentially regulate development of dopaminergic cells. Brain-derived neurotrophic factor stimulated survival, the formation of primary neurites and dopamine uptake activity. des-Insulin-like growth factor-1 was most effective in promoting survival, stimulated dopamine uptake less effectively than brain-derived neurotrophic factor and did not alter the morphology of dopaminergic cells. Basic fibroblast growth factor produced comparatively mild increases in survival and dopamine uptake, and slightly reduced neurite growth of the cells. None of the factors stimulated the expression of the tyrosine hydroxylase gene. These findings suggest that (i) effective growth factors may stimulate different, but partially overlapping, molecular pathways during developmental differentiation, (ii) none of the factors stimulates dopaminergic cell differentiation comparable to the pronounced trophic action of nerve growth factor on peripheral sympathetic or basal forebrain cholinergic neurons, and (iii) localization and effects of none of the factors are compatible with a role as target-derived survival-regulating neurotrophic factor.