Brain-derived neurotrophic factor increases survival and differentiated functions of rat septal cholinergic neurons in culture

Opposing effects of morphine and the neurotrophins, NT-3, NT-4, and BDNF, on locus coeruleus neurons in vitro

The development of noradrenergic locus coeruleus (LC) neurons is subject to regulation by multiple epigenetic signals. To examine the potential regulation of LC ontogeny by opiates and neurotrophins, we studied the effects of morphine and NT-3, NT-4, and BDNF on the survival and differentiation of LC neurons from prenatal rats in dissociated cell culture. Noradrenergic cells were identified and counted following tyrosine hydroxylase (TH) immunocytochemistry, and their state of differentiation was assessed by measuring norepinephrine (NE) uptake. Treating LC cultures with morphine starting on day 1 after plating resulted in a 20% decrease in NE uptake and a small (12%) but significant decrease in the number of TH-immunoreactive (TH +) cells. Application of morphine on day 4 after plating had the same effect on NE uptake without influencing TH + cell number. This effect of morphine was blocked by concomitant exposure to naloxone (an opioid receptor antagonist), and mimicked by exposure to opioid peptides. Treatment of cultures with the neurotrophins, NT-3 or NT-4, increased NE uptake and TH + cell number, as reported previously. Moreover, we show for the first time that brain-derived neurotrophic factor (BDNF) exerts similar effects, with a large (110%) increase in NE uptake and a modest (20%) increase in TH + cell number. Cotreatment of LC cultures with morphine and NT-3 resulted in an attenuation of the NT-3 effect on both NE uptake and the number of TH + cells. In contrast, cotreatment of LC cultures with morphine and NT-4 or BDNF attenuated the neurotrophin effect on TH + cell number but not on NE uptake. Our results raise the possibility that opioid peptides may modulate the influence of neurotrophins on LC neuronal survival and differentiation.

The neurotrophins BDNF, NT-3 and -4, but not NGF, TGF-beta 1 and GDNF, increase the number of NADPH-diaphorase-reactive neurons in rat spinal cord cultures

Neurotrophins have multiple functions for the development of the nervous system. They can promote survival and differentiation of select neuronal populations, but have also been shown to play instructive roles in the determination of the transmitter phenotype of neurons. We have investigated the influence of neurotrophins on the expression of nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d), a histochemical marker for nitric oxide synthase, in spinal cord cultures established from 16-day-old rat embryos. At this embryonic age we found NADPH-d reactivity becoming apparent in the spinal cord and predominantly expressed in preganglionic autonomic nuclei. Numbers of NADPH-d-positive neurons in spinal cord cultures were very low 24 h after plating. They did not change significantly until day 4 in vitro. However, treatment with the neurotrophins BDNF, NT-3 or NT-4 significantly increased their numbers. The effect became apparent after just 24 h, and was significant with concentrations as low as 1 ng/ml. Treatment with BDNF, NT-3 and NT-4 also augmented numbers of NADPH-d-positive neurons when initiated after three or five days in culture, and became consistently apparent within 24 h. This suggests that the neurotrophin-mediated increase in NADPH-d-positive neurons is unlikely to be due to promotion of neuron survival. NGF and two members of the transforming growth factor-beta superfamily, which have pronounced trophic effects on select neuron populations in vitro, TGF-beta 1 and GDNF, were not effective. Combined application of NT-4 and NT-3 had no additive effect. Our data therefore suggest that neurotrophins are involved in the developmental regulation of NADPH-d activity in neuron populations of the spinal cord. Neuron populations affected may include preganglionic autonomic neurons. NADPH-d activity may be induced in neurons expressing the enzyme constitutively, yet at undetectable levels, or may be induced de novo.

Monocular deprivation decreases the expression of messenger RNA for brain-derived neurotrophic factor in the rat visual cortex

We found that deprivation of pattern vision in one eye, that leaves luminance detection performance unaffected, is sufficient to reduce brain-derived neurotrophic factor (but not trkB) messenger RNA in the visual cortex of young and adult rats. Monocular deprivation by means of eyelids' suture was performed during or after the critical period and the cortical amount of brain-derived neurotrophic factor messenger RNA was analysed by in situ hybridization and RNAase protection after 15-30 days of deprivation. A reduction of brain-derived neurotrophic factor messenger RNA was observed in the visual cortex contralateral to the deprived eye in rats monocularly deprived during the critical period. The same reduction was also found in rats monocularly deprived after the end of the critical period, when anatomical or physiological signs of monocular deprivation are absent. The pharmacological blockade of retinal activity equally affected the expression of brain-derived neurotrophic factor messenger RNA in young and adults. Quantitative RNAase protection assays revealed that the cortical level of brain-derived neurotrophic factor messenger RNA was reduced to the same extent when intraocular injections of tetrodotoxin were performed within or after the critical period. A developmental study of brain-derived neurotrophic factor messenger RNA expression in rat visual cortex showed a marked increase around the time of natural eye-opening followed by a plateau from postnatal day 20 until adult age. Messenger RNA for the kinasic domain of brain-derived neurotrophic factor receptor (trkB) was found in the dorsal lateral geniculate nucleus and the visual cortex during development and in adults. Our results suggest that the reduction of brain-derived neurotrophic factor messenger RNA induced by monocular deprivation is related to the absence of pattern vision rather than to the competitive interactions that underlie the effects of monocular deprivation during the critical period.

Demonstration of deacetylated hippocampal cholinergic neurostimulating peptide and its precursor protein in rat tissues

This report concerns the demonstration of hippocampal cholinergic neurostimulating peptide (HCNP), its deacetylated analogue (free HCNP) and HCNP precursor protein in rat tissues. To avoid possible enzymatic degradation during sample manipulation, tissue extracts were prepared under acidic conditions using trifluoroacetic acid. The tissue contents of free HCNP and of precursor protein were determined by radioimmuno-assay (RIA) using two antibodies with different specificities, and by a combination of HPLC and RIA. Free HCNP was detected in neuronal and renal tissues, but not in liver. All tissues examined had measurable amounts of HCNP precursor protein. The concentrations of free HCNP and precursor in neuronal tissues were inversely related to the age. These results suggest that the deacetylated analogue of HCNP and its precursor protein may have significant physiological functions, especially in the central nervous system of young animals.

Influence of growth factors on neuronal differentiation

With so many neurotrophins and receptors now known, how is our picture of neurotrophism changing? Recent studies on knockout mice have confirmed our expectations of neurotrophin action in neuronal development. A notable exception is the activation of TrkB, on motor neurons, by an unknown ligand. It is also clear that some neurotrophins have diverse activities and influence early developmental stages. There are interesting new data concerning the role of p75, the low affinity neurotrophin receptor, as a modulator of neurotrophin activity. Even more exciting are new studies on glia-derived neurotrophic factor (GDNF) which demonstrate that this growth factor acts as a potential protector of motor neurons and striatal dopaminergic neurons.

Identification of a putative estrogen response element in the gene encoding brain-derived neurotrophic factor

We have been studying the role and mechanism of estrogen action in the survival and differentiation of neurons in the basal forebrain and its targets in the cerebral cortex, hippocampus, and olfactory bulb. Previous work has shown that estrogen-target neurons in these regions widely coexpress the mRNAs for the neurotrophin ligands and their receptors, suggesting a potential substrate for estrogen-neurotrophin interactions. Subsequent work indicated that estrogen regulates the expression of two neurotrophin receptor mRNAs in prototypic peripheral neural targets of nerve growth factor. We report herein that the gene encoding the neurotophin brain-derived neurotrophic factor (BDNF) contains a sequence similar to the canonical estrogen response element found in estrogen-target genes. Gel shift and DNA footprinting assays indicate that estrogen receptor-ligand complexes bind to this sequence in the BDNF gene. In vivo, BDNF mRNA was rapidly up-regulated in the cerebral cortex and the olfactory bulb of ovariectomized animals exposed to estrogen. These data suggest that estrogen may regulate BDNF transcription, supporting our hypothesis that estrogen may be in a position to influence neurotrophin-mediated cell functioning, by increasing the availability of specific neurotrophins in forebrain neurons.

Influence of neurotrophic factors on morphine- and cocaine-induced biochemical changes in the mesolimbic dopamine system

Previous research has shown an increase in tyrosine hydroxylase in the ventral tegmental area following chronic morphine and chronic cocaine treatments. Chronic morphine treatment also increases levels of glial fibrillary acidic protein in this brain region. In the present study, we investigated the effects of infusing neurotropic factors (nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, neurotrophin-4 or ciliary neurotrophic factor) via midline intra-ventral tegmental area cannulae on these biochemical changes. Our studies examined the effects of neurotrophic factor infusion alone, neurotrophic factor infusion followed by morphine treatment, morphine treatment followed by neurotrophic factor infusion, and concurrent neurotrophic factor infusion and cocaine treatment. Brain-derived neurotrophic factor, which by itself tended to decrease tyrosine hydroxylase levels in the ventral tegmental area, prevented the characteristic increase in tyrosine hydroxylase following morphine and cocaine exposure and reversed the increase in rats pretreated with morphine. Neurotrophin-4 and neurotrophin-3 exerted similar effects. In addition, neurotrophin-4 prevented the morphine-induced increase in glial fibrillary acidic protein. In contrast, ciliary neurotrophic factor infusions alone resulted in an increase in tyrosine hydroxylase levels, with no additional increase induced by morphine or cocaine coadministration. Nerve growth factor alone had no effect on tyrosine hydroxylase or glial fibrillary acidic protein levels and did not affect morphine's ability to induce these proteins. We also looked at the effects of intra-ventral tegmental area infusion of neurotrophic factor on cAMP-dependent protein kinase and adenylyl cyclase activity in the nucleus accumbens, both of which are increased by chronic morphine or cocaine exposure. In general, regulation of cAMP-dependent protein kinase and adenylyl cyclase morphine by neurotrophic factors paralleled effects seen in the ventral tegmental area. Intra-ventral tegmental area infusion of brain-derived neurotrophic factor (or neurotrophin-4) alone tended to decrease cAMP-dependent protein kinase and adenylyl cyclase activity in the nucleus accumbens and prevented the morphine-induced increases in these enzymes. These effects were not seen with ciliary neurotrophic factor or nerve growth factor. These studies demonstrate novel interactions within the ventral tegmental area, and its target the nucleus accumbens, between neurotrophic factors and drugs of abuse, which have potentially important implications for the pathophysiology and treatment of drug addiction.

Neurotrophic factors in central nervous system trauma

Although regeneration of injured neurons does not occur after trauma in the central nervous system (CNS), there is often significant recovery of functional capacity with time. Little is currently known about the molecular basis for such recovery, but the increased trophic activity in injured CNS tissue and the known properties of neurotrophic factors in neuronal growth and maintenance suggest that these polypeptides are probably involved in recovery of function. Members of the neurotrophin family, including nerve growth factor (NGF), brain-derived neurotrophic factors (BDNF), and neurotrophin 3 (NT-3), are capable of supporting survival of injured CNS neurons both in vitro and in vivo. They also stimulate neurite outgrowth, needed for reorganization of the injured CNS, and the expression of key enzymes for neurotransmitter synthesis that may need to be upregulated to compensate for reduced innervation. The effects of the neurotrophins are mediated through specific high affinity trk receptors (trk A, B, C) as well as a common low affinity receptor designated p75NGFR. Another class of neurotrophic polypeptides also provides candidate recovery-promoting molecules, the heparin-binding growth factors' acidic and basic fibroblast growth factor (aFGF, bFGF). FGFs not only sustain survival of injured neurons but also stimulate revascularization and certain glial responses to injury. Both the neurotrophins and the FGFs, as well as their respective receptors, have been shown to be upregulated after experimental CNS injury. Further, administration of neurotrophins or FGF has been shown to reduce the effects of experimental injury induced by axotomy, excitotoxins, and certain other neurotoxins. The cellular basis for the potential therapeutic use of neurotrophic molecules is discussed as well as new strategies to increase neurotrophic activity after CNS trauma based on the recently obtained information on pharmacological and molecular control of the expression of these genes.

Involvement of nerve growth factor in visual cortex plasticity

The physiological role of nerve growth factor (NGF), the prototype member of the neurotrophin family, has been widely studied. NGF has been shown to promote survival, sprouting and differentiation of sympathetic ganglion cells and sensory neurons in the peripheral nervous system; it has also been shown to support survival and regeneration of cholinergic neurons in the central nervous system. Recent evidence indicates that NGF is also involved in the neuronal plasticity of the visual cortex. Exogenous supplies of NGF have been shown to interfere with normal processes underlying activity- and age-dependent synaptic modifications in both developing and adult visual cortex. In parallel to these physiological effects, numerous neuronal markers in the visual cortex have been found to be influenced by NGF. Several proposals have been introduced to explain the physiological role of NGF in visual cortex plasticity. Although the mechanisms underlying NGF effects in the visual cortex are still under active investigation, current evidence implies that NGF, and perhaps other neurotrophins as well, may be useful for preventing or correcting inappropriate or anomalous connections in the visual cortex, and thus for treating visual dysfunctions such as amblyopia and strabismus.

NGF mRNA is expressed by GABAergic but not cholinergic neurons in rat basal forebrain

Nerve growth factor (NGF) supports the survival and biosynthetic activities of basal forebrain cholinergic neurons and is expressed by neurons within lateral aspects of this system including the horizontal limb of the diagonal bands and magnocellular preoptic areas. In the present study, colormetric and isotopic in situ hybridization techniques were combined to identify the neurotransmitter phenotype of the NGF-producing cells in these two areas. Adult rat forebrain tissue was processed for the colocalization of mRNA for NGF with mRNA for either choline acetyltransferase, a cholinergic cell marker, or glutamic acid decarboxylase, a GABAergic cell marker. In both regions, many neurons were single-labeled for choline acetyltransferase mRNA, but cells containing both choline acetyltransferase and NGF mRNA were not detected. In these fields, virtually all NGF mRNA-positive neurons contained glutamic acid decarboxylase mRNA. The double-labeled cells comprised a subpopulation of GABAergic neurons; numerous cells labeled with glutamic acid decarboxylase cRNA alone were codistributed with the double-labeled neurons. These data demonstrate that in basal forebrain GABAergic neurons are the principal source of locally produced NGF.

Astrocytes retrovirally transduced with BDNF elicit behavioral improvement in a rat model of Parkinson's disease

Neurotrophic factors that improve the survival of specific neuronal types during development and after exposure to various neuronal insults hold potential for treatment of neurodegenerative diseases. In particular, brain-derived neurotrophic factor (BDNF) has been shown to exert trophic and protective effects on dopaminergic neurons, the cell type known to degenerate in Parkinson's disease. To determine whether increased levels of biologically produced BDNF affect the function or regeneration of damaged dopaminergic neurons, the effects of grafting astrocytes transduced with the human BDNF gene into the striatum of the partially lesioned hemiparkinsonian rat were examined. Replication deficient retroviruses carrying either human prepro-BDNF or human alkaline phosphatase (AP) cDNA were used to transduce primary type 1 astrocytes purified from neonatal rat cortex. In vitro, BDNF mRNA was expressed by BDNF transduced astrocytes (BDNF astrocytes), but not control AP transduced astrocytes (AP astrocytes), as determined by reverse transcription polymerase chain reaction (RT-PCR). The modified astrocytes were injected into the right striatum 15 days after partial lesioning of the right substantia nigra with 6-hydroxydopamine. Transplantation of BDNF astrocytes, but not AP astrocytes, significantly attenuated amphetamine-induced rotation by 45% 32 days after grafting. Apomorphine-induced rotation increased over time in both groups, but was not significantly different in the BDNF-treated group. The modified BDNF astrocytes survived well with non-invasive growth in the brain for up to 42 days. Although BDNF mRNA positive cells were not detected within the graft site using in situ hybridization, alkaline phosphatase immunoreactive (IR) cells were present in control graft sites suggesting that the retroviral construct continued to be expressed at 42 days. Analysis of the density of tyrosine hydroxylase (TH)-IR fibers showed no effect of BDNF on TH-IR fiber density in the striatum on the lesioned side. These findings suggest that ex vivo gene therapy with BDNF ameliorates parkinsonian symptoms through a mechanism(s) other than one involving an effect of BDNF on regeneration or sprouting from dopaminergic neurons.

Chronic ethanol administration decreases brain-derived neurotrophic factor gene expression in the rat hippocampus

We have previously demonstrated that chronic ethanol consumption decreases neurotrophic activity in hippocampal extracts, as assessed by a chick dorsal root ganglia bioassay, but has no effect on hippocampal NGF mRNA or NGF protein levels. We presently report that hippocampal mRNAs encoding neurotrophin-3 and basic fibroblast growth factor are also unaffected. However, in contrast, brain-derived neurotrophic factor mRNA is reliably decreased, thereby suggesting that ethanol-induced damage of the septohippocampal system may at least partially result from an ethanol-induced decrease in hippocampal brain-derived neurotrophic factor expression.

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.

Functions of basic fibroblast growth factor and neurotrophins in the differentiation of hippocampal neurons

Restrictions in neuronal fate occur during the transition from a multipotential to a postmitotic cell. This and later steps in neuronal differentiation are determined by extracellular signals. We report that basic fibroblast growth factor is mitogenic for stem cells and is a differentiation factor for calbindin-expressing hippocampal neurons. The neurotrophin NT-3 is a differentiation factor for the same neurons but does not affect proliferation. NT-3 and brain-derived neurotrophic factor promote the maturation of neurons derived from stem cells that have been grown in vitro. These results define functions for basic fibroblast growth factor and neurotrophins in the differentiation processes that direct a multipotential stem cell to a specific neuronal fate.

Differential distribution of exogenous BDNF, NGF, and NT-3 in the brain corresponds to the relative abundance and distribution of high-affinity and low-affinity neurotrophin receptors

To evaluate effective means for delivering exogenous neurotrophins to neuron populations in the brain, we compared the distribution and transport of brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), and neurotrophin-3 (NT-3) following intracerebral delivery. Rats received an injection of radioiodinated or unlabeled neurotrophin into the lateral ventricle and were killed humanely after 1.5-24 hours. Other rats received continuous infusion of unlabeled neurotrophin into the lateral ventricle, the striatum, or the hippocampus for 3-14 days. The neurotrophins were detected by autoradiography or immunohistochemistry. There were striking differences between BDNF, NGF, and NT-3 in their penetration through brain tissue. These differences occurred regardless of the site or method of delivery, but were most pronounced following a bolus intracerebroventricular (ICV) injection. After ICV injection, NGF was widely distributed in tissues around the ventricles and at the surface of the brain, whereas the penetration of BDNF into brain tissue was distinctly less than that of NGF, and the penetration of NT-3 was intermediate. An ICV injection of NGF produced prominent but transient labeling of cells that contain the low-affinity NGF receptor, whereas an injection of BDNF prominently labeled the ventricular ependyma. During ICV infusion (12 micrograms/day), the distribution of BDNF was no greater than that observed after a 12-micrograms bolus injection. A sixfold increase in the amount of BDNF infused (72 micrograms/day) produced a more widespread distribution in the brain and doubled the depth of penetration into periventricular tissues near the cannula. Corresponding to their differences in penetration, NGF was retrogradely transported by basal forebrain cholinergic neurons after ICV or intrastriatal delivery, whereas NT-3 was transported by a few basal forebrain neurons after ICV delivery, and BDNF was rarely detected in neurons after ICV delivery. Delivery of BDNF directly to the striatum or the hippocampus labeled numerous neurons in nuclei afferent to these structures. In situ hybridization studies confirmed that the high-affinity BDNF receptor (TrkB) was much more widely expressed in neurons than was the high-affinity NGF receptor (TrkA). Moreover, mRNA for truncated forms of TrkB was expressed at high levels in the ependyma, the choroid epithelium, and the gray matter. It is likely that binding of BDNF to TrkB, which appears to be more abundant and ubiquitous than TrkA, restricts the diffusion of BDNF relative to that of NGF.

Cultured basal forebrain cholinergic neurons from postnatal rats show both overlapping and non-overlapping responses to the neurotrophins

Basal forebrain cholinergic neurons respond in vitro and in vivo to nerve growth factor (NGF) and to brain-derived neurotrophic factor (BDNF). It is not clear to what extent the neurons that respond to these two factors, or to neurotrophin-3 or -4/5 (NT-3; NT-4/5) are identical or only partially overlapping populations. We have addressed this issue in cultures of basal forebrain neurons derived from 2-week-old postnatal rats, using choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) as cholinergic markers. Cholinergic neuron survival was enhanced in the presence of NGF, BDNF and NT-4/5. NT-4/5 was as effective as BDNF. NT-3 was without effect at this age, although in cultures derived from embryonic forebrain, cholinergic differentiation was induced by NT-3. Cotreatment with NGF and BDNF resulted in small, but consistent increases in the number of ChAT-positive neurons, compared with either factor alone. NT-4/5 was also found to be additive with NGF, whereas cotreatment with BDNF and NT-4/5 showed no additivity. NT-3 had no additive effects with any other neurotrophin on any cholinergic parameters in postnatal cultures. Taken together, the results indicate the existence in postnatal rat brain of a large overlapping population of cholinergic neurons that are responsive to ligands for the neurotrophin receptors TrkA (NGF) and TrkB (BDNF and NT-4/5), but not TrkC (NT-3), and small distinct populations that show specificity for NGF or BDNF but not both. We hypothesize that cholinergic neurons projecting into different regions of the hippocampus may derive trophic support from distinct neurotrophins.

Regulatory region in choline acetyltransferase gene directs developmental and tissue-specific expression in transgenic mice

Acetylcholine, one of the main neurotransmitters in the nervous system, is synthesized by the enzyme choline acetyltransferase (ChAT; acetyl-CoA:choline O-acetyltransferase, EC 2.3.1.6). The molecular mechanisms controlling the establishment, maintenance, and plasticity of the cholinergic phenotype in vivo are largely unknown. A previous report showed that a 3800-bp, but not a 1450-bp, 5' flanking segment from the rat ChAT gene promoter directed cell type-specific expression of a reporter gene in cholinergic cells in vitro. Now we have characterized a distal regulatory region of the ChAT gene that confers cholinergic specificity on a heterologous downstream promoter in a cholinergic cell line and in transgenic mice. A 2342-bp segment from the 5' flanking region of the ChAT gene behaved as an enhancer in cholinergic cells but as a repressor in noncholinergic cells in an orientation-independent manner. Combined with a heterologous basal promoter, this fragment targeted transgene expression to several cholinergic regions of the central nervous system of transgenic mice, including basal forebrain, cortex, pons, and spinal cord. In eight independent transgenic lines, the pattern of transgene expression paralleled qualitatively and quantitatively that displayed by endogenous ChAT mRNA in various regions of the rat central nervous system. In the lumbar enlargement of the spinal cord, 85-90% of the transgene expression was targeted to the ventral part of the cord, where cholinergic alpha-motor neurons are located. Transgene expression in the spinal cord was developmentally regulated and responded to nerve injury in a similar way as the endogenous ChAT gene, indicating that the 2342-bp regulatory sequence contains elements controlling the plasticity of the cholinergic phenotype in developing and injured neurons.

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.

Spatiotemporal selective effects on brain-derived neurotrophic factor and trkB messenger RNA in rat hippocampus by electroconvulsive shock

Electroconvulsive therapy is used in the treatment of affective disorders and schizophrenia and experimental electroconvulsive shock may serve as an animal model for this treatment. The aim of this study was to investigate a possible role for neurotrophins in the mechanism of action of experimental electroconvulsive shock and thus in clinical electroconvulsive therapy. The effect of electroconvulsive shock on levels of messenger RNAs encoding the neurotrophin brain-derived neurotrophic factor and the receptor trkB in rat hippocampus was determined by in situ hybridization with RNA probes 1, 3, 9 and 27 h following the shock. Brain-derived neurotrophic factor messenger RNA levels were increased at 1, 3 and 9 h following the shock and normalized after 27 h. Granule cells of the dentate gyrus showed a more rapid response as compared to hilar cells and pyramidal cells of CA1. Total trkB messenger RNA levels, including the transcripts for both the truncated and full length trkB receptor protein (gp95trkB and gp145trkB, respectively), showed a pattern of increase very similar to that of the brain-derived neurotrophic factor messenger RNA. However, using a probe selective for the full length (gp145trkB) trkB messenger RNA, we determined a delayed pattern of activation with significant increase only at 3 and 9 h after the shock. In hippocampus total trkB messenger RNA was found to consist of approximately one-quarter of mRNA encoding gp145trkB and three-quarters encoding gp95trkB as revealed by RNAase protection. While brain-derived neurotrophic factor and the truncated trkB messenger RNAs appear to increase with a similar pattern, suggesting a similar mechanism of activation by electroconvulsive shock, full length receptor trkB messenger RNA appears to increase with a delayed pattern suggesting a separate mechanism of activation. Electroconvulsive shock-induced seizures seem to include activation of a brain neurotrophin known to be important for neuronal plasticity.

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.

BDNF produces analgesia in the formalin test and modifies neuropeptide levels in rat brain and spinal cord areas associated with nociception

Previous studies have demonstrated an antinociceptive effect of brain-derived neurotrophic factor (BDNF) following infusion into the midbrain, near the periaqueductal grey and dorsal raphe nuclei. BDNF administration attenuated the behavioural response in the tail-flick and hot-plate tests, two models employing a phasic, thermal high-intensity nociceptive stimulus; the present studies extend our previous findings to include a model of moderate, continuous pain resulting from a chemical stimulus, the formalin test. Midbrain infusion of BDNF decreased the behavioural paw flinch response to subcutaneous formalin injection in both the early and late phases of the test. As our previous studies showed that BDNF-induced analgesia was reversible by naloxone, we have examined the effects of BDNF administration on brain and spinal cord levels of neuropeptides involved in the modulation of nociceptive information, including the endogenous opioid peptides, met-enkephalin and beta-endorphin, as well as substance P and neuropeptide Y (NPY). At the site of infusion, within the PAG and dorsal raphe, BDNF increased the level of beta-endorphin by 63%, but had no effect on substance P, metenkephalin or NPY levels. In the dorsal spinal cord, substance P (113% increase), beta-endorphin (97% increase) and NPY (64% increase) were elevated, although ventral spinal cord levels of these peptides remained unchanged. These studies demonstrate a modulatory effect of BDNF on relevant neuropeptides within areas of the brain and spinal cord involved in the processing of nociceptive information.

The neurotrophins BDNF, NT-3 and NT-4/5 promote survival and morphological and biochemical differentiation of striatal neurons in vitro

The neurotrophins, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrophin 4/5 (NT-4/5) and nerve growth factor (NGF), were compared for their effects on the survival and differentiation of embryonic rat striatal neurons grown in low-density cultures. Treatment with BDNF for 8 days resulted in a 40% increase in overall neuronal survival, a 3- to 5-fold increase in the number of calbindin-immunoreactive neurons, and an 80% increase in GABA-positive neurons. Treatment with NT-3 or NT-4/5 produced a 2- to 3-fold increase in the number of calbindin-positive neurons and an increase in GABA-positive cell number similar to that induced by BDNF, BDNF treatment produced a striking morphological differentiation of striatal GABAergic neurons, which was characterized by a doubling of the number of neurite branch points, the total area of aborization and the perikaryal area compared to control cultures. All three of these factors increased high-affinity GABA uptake 2-fold. NGF had no effect on any of the parameters examined. Our results show that BDNF, NT-3 and NT-4/5 promote the survival and/or differentiation of calbindin-immunopositive and GABAergic striatal neurons.

Comparison of the effects of the neurotrophins on the morphological structure of dopaminergic neurons in cultures of rat substantia nigra

The effect of the various neurotrophin family members on the morphological structure of dopaminergic neurons was compared in dissociated cultures of embryonic rat ventral mesencephalon. Cultures were maintained in vitro in the presence of brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrohin-4/5 (NT-4/5), nerve growth factor (NGF) or no added growth factors. Three-dimensional reconstructions of 48 neurons were made in each of the experimental groups following immunocytochemical staining for tyrosine hydroxylase to detect dopaminergic neurons. In addition [3H]mazindol binding analyses were carried out in replicate cultures in order to quantify the effects of the neurotrophins on the number of dopamine uptake sites. Among the neurotrophins tested, NT-4/5 influenced the proximal morphological parameters most, as determined by a 36% increase in the soma profile area and 35% in the number of stem neurites. Analysis of neuritic size and complexity in these cultures revealed that combined neuritic length and number of segments/cell were increased by 45 and 40% respectively. A change in neurite complexity in the NT-4/5 treated cultures was further confirmed using Scholl's concentric sphere analysis. In addition, relative to the control, NT-4/5 increased the neuronal differentiation as evidenced by increases in varicosity density and [3H]mazindol binding by 114 and 101% respectively. BDNF and, to a lesser extent, NT-3 also increased both proximal parameters and parameters of differentiation, but were without effect on parameters of neuritic size and complexity. No effects on neuronal structure were observed in NGF treated cultures. These findings demonstrate that BDNF, NT-3 and NT-4/5 influence the morphological differentiation of dopaminergic neurons in vitro, suggesting they may play a role in the structural development and plasticity of these neurons in the mesencephalon.

Pan-neurotrophin 1: a genetically engineered neurotrophic factor displaying multiple specificities in peripheral neurons in vitro and in vivo

Pan-neurotrophin 1 (PNT-1) is a synthetic trophic factor engineered by combining active domains of the neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin 3 (NT-3) into an NT-3 backbone. This molecule was produced in transiently transfected COS cells or in baculovirus-infected insect cells transfected COS cells or in baculovirus-infected insect cells and subsequently purified to homogeneity. Saturation binding in embryonic spinal sensory neurons demonstrated a greater number of high-affinity binding sites for PNT-1 than for its parental molecule NT-3. PNT-1 was shown to efficiently block the chemical crosslinking of NGF, BDNF, and NT-3 to their cognate Trk receptors and to the low-affintiy NGF receptor expressed on neuronal and nonneuronal cells. PNT-1 stimulated survival and proliferation of MG87 fibroblasts expressing either TrkA, TrkB, or TrkC. PNT-1 also promoted survival of a greater number of embryonic dorsal root ganglion neurons than any of the other neurotrophins alone, and its effects were equivalent to a combination of NGF, BDNF, and NT-3. Analysis of receptor-specific neurotrophic activities demonstrated that PNT-1 efficiently rescued TrkA mRNA-containing sympathetic neurons and TrkB and TrkC mRNA-containing sensory neurons from the dorsal root and nodose ganglia. Finally, PNT-1 showed robust retrograde transport to DRG neurons in vivo after injection into the sciatic nerve. Radiolabeled PNT-1 accumulated in small-, medium-, and large-sized neurons. Coinjection with different unlabeled neurotrophins inhibited PNT-1 transport in distinct subpopulations of neurons of different sizes, suggesting that this molecule affects sensory neurons of different modalities. These results indicate that PNT-1 is a potent and multispecific neurotrophic factor that may be useful in the treatment of peripheral neurophathies and nerve damage.

Targeted disruption of the neurotrophin-3 gene with lacZ induces loss of trkC-positive neurons in sensory ganglia but not in spinal cords

We have replaced the NT-3 gene with Escherichia coli-derived lacZ gene by means of homologous recombination in embryonic stem cells and thus produced null mutant mice. Mice homozygous for this mutation developed to birth, but most of them could not suck well and died within 2 days after birth. The surviving homozygous mutant mice displayed movement disorder similar to ataxia. The expression of lacZ was widely distributed in the target tissues of peripheral nerves, spinal motor neurons, lumbar dorsal root ganglia and trigeminal ganglia during the prenatal periods. A neuroanatomical examination revealed that there was marked cell reduction present in trigeminal and lumbar dorsal root ganglia in the developing homozygous mutant mice. In these tissues, the expression of trkC, a high-affinity receptor for NT-3, was markedly reduced. In contrast, we did not find any morphological abnormalities, significant cell loss or decreased levels of trkC expression in the motor neurons present in the ventral horn of the spinal cord. These results indicate that the absence of the NT-3 gene leads to a defect in the sensory nervous system, but it may be complemented by other neurotrophins in the motor nervous system during the development.

Dendritic reorganisation in the basal forebrain under degenerative conditions and its defects in Alzheimer's disease. II. Ageing, Korsakoff's disease, Parkinson's disease, and Alzheimer's disease

Changes in the dendritic arborisation of Golgi-impregnated basal forebrain neurones with respect to size, shape, orientation, and topology of branching were quantitatively investigated in ageing, Alzheimer's disease (AD), Korsakoff's disease (KD), and Parkinson's disease (PD). A reorganisation of the whole dendritic tree characterized by an increase in both the total dendritic length and the degree of dendritic arborisation as well as by changes in the shape of the dendritic field was found during ageing, in KD, PD, and AD. Dendritic growth under these conditions was related to the extent of cell loss in basal forebrain nuclei. There appeared to be major differences, however, with respect to the overall pattern of dendritic reorganisation between AD on one side and ageing, KD, and PD on the other side. In both ageing and KD, dendritic growth was largely restricted to the terminal dendritic segments, resulting in an increase of the size of the dendritic field (pattern of "extensive growth") In AD, however, dendritic growth mainly resulted in an increase of the dendritic density within the dendritic field without being accompanied by an increase in the size of the volume occupied by the dendritic tree (pattern of "intensive growth"). In AD, aberrant growth processes were frequently observed in the perisomatic area or on distal dendritic segments of basal forebrain neurones of the reticular type. Neurones with aberrant growth profiles were typically located in the direct vicinity of deposits of beta/A4 amyloid. Perisomatic growth profiles were covered by the low-affinity receptor of nerve growth factor p75NGFR. Aberrant growth processes were not present in ageing, KD, and PD. On the basis of the present study, it is concluded that under certain degenerative conditions, reticular basal forebrain neurones undergo a compensatory reorganisation of their dendritic arborisation, a process that has become defective in AD, thereby converting a physiological signal into a cascade of events contributing to the pathology of the disease.

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.

Identification of brain-derived neurotrophic factor promoter regions mediating tissue-specific, axotomy-, and neuronal activity-induced expression in transgenic mice

The structure of rat brain-derived neurotrophic factor (BDNF) gene is complex; four 5' exons are linked to separate promoters and one 3' exon is encoding the BDNF protein. To analyze the relative importance of the regulatory regions in vivo, we have generated transgenic mice with six different promoter constructs of the BDNF gene fused to the chloramphenicol acetyl transferase reporter gene. High level and neuronal expression of the reporter gene, that in many respects recapitulated BDNF gene expression, was achieved by using 9 kb of genomic sequences covering the promoter regions that lie adjacent to each other in the genome (promoters I and II and promoters III and IV, respectively) and by including sequences of BDNF intron-exon splice junctions and 3' untranslated region in the constructs. The genomic regions responsible for the in vivo upregulation of BDNF expression in the axotomized sciatic nerve and in the brain after kainic acid-induced seizures and KCl-induced spreading depression were mapped. These data show that regulation of the different aspects of BDNF expression is controlled by different regions in vivo, and they suggest that these promoter constructs may be useful for targeted expression of heterologous genes to specific regions of the central and peripheral nervous systems in an inducible manner.

Expression of nerve growth factor, p75, and trk in the somatosensory and motor cortices of mature rats: evidence for local trophic support circuits

Neurotrophins such as nerve growth factor (NGF) are critical for the maintenance of CNS neurons. We determined the expression of NGF and the neurotrophin receptors p75 and trk in the somatosensory and motor cortices of mature rats with immunohistochemical techniques. Sections of mature rat cortex were processed immunohistochemically with primary antibodies directed against NGF, p75, or trk. The distribution of immunoreactive elements was examined, and stereological techniques were used to determine the density and size of immunoreactive cell bodies. Some sections processed for trk immunoreactivity were examined with an electron microscope. From the size and morphology of the labeled cells, it appeared that only neurons in the gray matter were NGF-positive. NGF was detected in one-third of the neurons in layers II-III, V, and VI of both somatosensory cortex and motor cortex; however, fewer than 1 in 12 of the layer IV neurons was NGF-positive. With the notable exception of layer V, few cell bodies (2-10% of the total population) were p75- or trk-immunoreactive. Layer Vb was replete with receptor-positive cell bodies; more than one-third of the layer Vb neurons were p75- or trk-positive. All labeled cells appeared to be pyramidal neurons. The distribution of p75 labeling with the two anti-p75 antibodies was indistinguishable. In addition, the neuropil in the supragranular laminae was p75- or trk-positive. Electron microscopy showed that trk immunoreactivity was also expressed by dendrites. Only rarely were immunoreactive axons detected. In summary, NGF is expressed by cortical neurons throughout cortex, and neurotrophin receptors are widely produced by postsynaptic targets. Thus, NGF appears to participate in an intracortical autoregulatory system. The strong expression of neurotrophin receptors by pyramidal neurons in layer Vb (the origin of brainstem and spinal cord projections) suggests that the neurotrophins are especially critical for the regulation of corticofugal projection systems.

Chronic alcohol ingestion: nerve growth factor gene expression and neurotrophic activity in rat hippocampus

Chronic ethanol treatment induces memory deficits accompanied by anatomical and biochemical changes in basal forebrain and hippocampus. Cholinergic neurons in the septohippocampal pathway are especially vulnerable to alcohol neurotoxicity. Several studies showed that an adequate supply of neurotrophins, such as Nerve Growth Factor and Brain-Derived Neurotrophic Factor, is required for the normal function and survival of cholinergic neurons in basal forebrain and medial septal nuclei. We tested the hypothesis that chronic alcohol ingestion may alter the gene expression level of Nerve Growth Factor in hippocampus, the major source of neurotrophins to the cholinergic neurons in the septohippocampal pathway. We measured Nerve Growth Factor protein and Nerve Growth Factor mRNA contents using sensitive two-site ELISA and Northern analysis. We also tested the endogenous neurotrophic activity, including and excluding Nerve Growth Factor, contained in 5%, 2%, 1%, 0.5% and 0.1% (w/v) hippocampal tissue extracts on sympathetic ganglia neurons. Twenty-eight weeks of chronic ethanol treatment did not reduce Nerve Growth Factor protein, Nerve Growth Factor mRNA, or total neurotrophic activity contained in the rat hippocampus when measured on sympathetic ganglia neurons.

Transient dendritic appendages on differentiating septohippocampal neurons are not the sites of synaptogenesis

The factors which determine the final shape and synaptic connections of a neuronal phenotype are largely unknown. In adult animals, a large number of projection neurons, e.g. cortical pyramidal neurons, bear spines which, in the case of pyramidal cells, are postsynaptic elements of mainly asymmetric synapses. In contrast, mature septohippocampal neurons do not bear spines. During maturation, however, septohippocampal projection neurons develop a variety of dendritic appendages. Because the appearance of these processes falls into the period of synaptogenesis, it has been hypothesized that these transient appendages may be the site of synaptogenesis. Here we have investigated whether these transient dendritic appendages are the site of initial synaptic contacts of septohippocampal neurons. Septohippocampal projection neurons in late embryonic and early postnatal rats were identified by retrograde tracing with the carbocyanine dye DiI or biocytin. Subsequently, selected cells were processed for electron microscopy. Serial thin sections through identified dendritic appendages did not reveal synaptic contacts with presynaptic boutons but immature to mature synapses were always found on dendritic shafts or somata. Often, synapses are located close to the appendages. These data indicate that the transient appendages are not the place where ingrowing afferent fibers make their synapses. The available information about transient dendritic appendages suggests, that they may be involved in short-term contacts with ingrowing axons, without being themselves the final site of the synaptic contact.

Role of neurotrophins in cholinergic-neurone function in the adult and aged CNS

Cholinergic neurones in the CNS undergo complex changes during normal aging. In recent years, considerable attention has focussed on the neurotrophins and, in particular, nerve growth factor, as potential maintenance factor for cholinergic-neurone function, and as therapeutic agents for use in a variety of neurodegenerative disorders including Alzheimer's disease. While brain cholinergic neurones from the neonate to the aged respond to nerve growth factor with enhanced expression of transmitter phenotype, there appears to be an age-related, region-specific decline in responsiveness. This age-related decrement in neurotrophin action might play a role in dysfunction of cholinergic neurones, and cognitive loss, and could limit the use of these factors as therapeutic agents.

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.

An improved method detects differential NGF and BDNF gene expression in response to depolarization in cultured hippocampal neurons

Differential regulation of individual neurotrophins by impulse activity potentially allows transformation of instantaneous signalling into diverse, long-lasting neural alterations. To define the temporal profiles of trophin gene expression we examined nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) mRNAs in dissociated cell cultures of rat hippocampus using an improved solution hybridization technique. Traditional methods lack the precision and sensitivity to detect small changes during brief intervals and the facility to process large sample numbers simultaneously. This improved method has now allowed us to better define the dynamics of depolarization-induced changes in expression of individual trophin genes. Using elevated K+ as a depolarizing stimulus, NGF mRNA increased 40% after 48 h. In contrast, BDNF message rose almost 4-fold within 3 h and attained a maximal 6-fold increase within 6 h. Similar increases in BDNF mRNA levels were exhibited following treatment of cultures with glutamate, an excitatory neurotransmitter. To document the sensitivity of BDNF mRNA to depolarizing conditions, we examined expression after K+ withdrawal. BDNF message began decreasing within one hour post-depolarization, and returned to basal levels after 6 h. Observations indicate that BDNF and NGF mRNAs are induced differentially in response to impulse activity; BDNF message is acutely responsive to ongoing changes, whereas NGF mRNA responds more slowly and sluggishly. The physiological implications of this differential regulation are discussed.

The stimulatory effect of brain-derived neurotrophic factor on dopaminergic phenotype expression of embryonic rat cortical neurons in vitro

Cells of embryonic (E12-16) rat cerebral cortex were cultured for 7 days in vitro (7DIV) in the presence of either brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF), with or without dopamine (DA). Chronic treatment of cells with DA or BDNF alone increased (300% and 600%, respectively) the number of the cells that expressed tyrosine hydroxylase (TH). However, the combination of BDNF and DA treatment greatly increased the expression of TH in E14 cortical cells in a dose-dependent manner, to a much greater extent than DA or BDNF alone. This marked response due to treatment with both BDNF and DA was greater in cortical tissue from E12 embryos than that from E14 embryos. The combination of CNTF and DA also increased expression of the dopaminergic phenotype whilst CNTF alone was ineffective, but this effect was largely due to DA. No effect of DA, or of neurotrophic factors, was observed on cortical cells from E16 embryos under any of the treatment conditions. The present study reveals how chemical environment plays an important role in determining the final phenotype of cortical neurons during early periods in brain development. BDNF, but not CNTF, may influence the differentiation of fetal cortical cells towards a dopaminergic phenotype via a unique mechanism, different from that due to DA. This combination of nerve growth factor and neurotransmitter may be of general importance in phenotype determination in the early developmental stages of the nervous system.

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)

Reversal of spatial memory impairments in aged rats by nerve growth factor and neurotrophins 3 and 4/5 but not by brain-derived neurotrophic factor

Aged rats, displaying impairments in spatial learning and memory associated with marked cellular atrophy of forebrain cholinergic neurons, received intracerebroventricular infusions of one of the four neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), or neurotrophin 4/5 (NT-4/5), or a combination of NGF and BDNF, or vehicle. During the 4-week infusion period rats receiving NGF, NT-3, or NT-4/5 showed improved acquisition and retention of spatial memory. With NGF and NT-3, but not NT-4/5, this was accompanied by a significant reduction in cholinergic neuron atrophy in septum, nucleus basalis, and striatum. BDNF, in contrast, was without effect either alone or in combination with NGF. These results show that memory deficits associated with aging can be reversed by several members of the neurotrophin family and that this effect may be mediated through activation of multiple neurotrophin receptors associated with cholinergic and possibly noncholinergic systems in the brain.

Promoting and directing axon outgrowth

Establishment of appropriate neuronal connections during development and regeneration requires the extension of processes that must then grow in the correct direction, find and recognize their targets, and make synapses with them. During development, embryonic neurons gradually establish central and peripheral connections in an evolving cellular environment in which neurotrophic factors are provided by supporting and target cells that promote neuronal survival, differentiation, and process outgrowth. Some cells also release neurotropic factors that direct the outgrowth of neuronal processes toward their targets. Following development the neurotrophic requirements of some adult neurons change so that, although they respond to neurotrophic factors, they no longer require exogenous neurotrophins to survive or to extend processes. Within the central nervous system (CNS), the ability of neurons to extend processes is eventually lost because of a change in their cellular environment from outgrowth permissive to inhibitory. Thus, neuronal connections that are lost in the adult CNS are rarely reestablished. In contrast, the environment of the adult peripheral nervous system fosters process outgrowth and synapse formation. This article discusses the neurotrophic requirements of embryonic and adult neurons, as well as the importance of neurotropic factors in directing the outgrowth of regenerating adult axons.

Monoamine-activated alpha 2-macroglobulin inhibits choline acetyltransferase of embryonic basal forebrain neurons and reversal of the inhibition by NGF and BDNF but not NT-3

Monoamine-activated alpha 2-macroglobulin (alpha 2M) has recently been shown to inhibit the growth and survival of cholinergic neurons of the basal forebrain (Liebl and Koo: J Neurosci Res 35:170-182, 1993). The mechanism of this inhibitory effect is believed to involve the regulation of growth factor activities by alpha 2M. The objectives of this study are to determine whether monoamine-activated alpha 2M can inhibit choline acetyltransferase (ChAT) activity of cholinergic basal forebrain neurons, and whether some common neurotrophins in the CNS can reverse the inhibition. This study demonstrates that both methylamine-activated alpha 2M (MA-alpha 2M) and serotonin-activated alpha 2M (5HT-alpha 2M) can dose-dependently suppress the expression of normal basal levels of ChAT activity in embryonic rat basal forebrain cells in vitro, while normal alpha 2M has little or no effect. As little as 0.35 microM monoamine-activated alpha 2M can suppress the ChAT activity, whereas either nerve growth factor (NGF) or brain-derived neurotrophic factor (BDNF), but not neurotrophin-3 (NT-3), stimulates ChAT expression of these cells. The addition of either NGF or BDNF to the alpha 2M-suppressed cells can increase ChAT activity back to its normal levels, while NT-3 can not. These results demonstrate that (1) monoamine-activated alpha 2M is a potent non-cytotoxic inhibitor of the ChAT activity in cholinergic basal forebrain neurons, and (2) NGF and BDNF are capable of not only stimulating the ChAT activity but can also specifically reverse the alpha 2M inhibition.(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.

Brain-derived neurotrophic factor protects against ischemic cell damage in rat hippocampus

The neuroprotective action of brain-derived neurotrophic factor (BDNF) was evaluated in a rat model of transient forebrain ischemia. A continuous intraventricular infusion of BDNF for 7 days starting immediately before the onset of ischemia significantly increased the number of pyramidal cells in the vulnerable CA1 sector of the hippocampus. In situ hybridization experiments suggest the neuroprotection to be mediated via trkB-receptors in the hippocampus. The data indicate a therapeutic potential for the treatment of cerebral ischemia.

Nerve growth factor and Alzheimer's disease

Nerve growth factor (NGF) is a well-characterized protein that exerts pharmacological effects on a group of cholinergic neurons known to atrophy in Alzheimer's disease (AD). Considerable evidence from animal studies suggests that NGF may be useful in reversing, halting, or at least slowing the progression of AD-related cholinergic basal forebrain atrophy, perhaps even attenuating the cognitive deficit associated with the disorder. However, many questions remain concerning the role of NGF in AD. Levels of the low-affinity receptor for NGF appear to be at least stable in AD basal forebrain, and the recent finding of AD-related increases in cortical NGF brings into question whether endogenous NGF levels are related to the observed cholinergic atrophy and whether additional NGF will be useful in treating this disorder. Evidence regarding the localization of NGF within the central nervous system and its presumed role in maintaining basal forebrain cholinergic neurons is summarized, followed by a synopsis of the relevant aspects of AD neuropathology. The available data regarding levels of NGF and its receptor in the AD brain, as well as potential roles for NGF in the pathogenesis and treatment of AD, are also reviewed. NGF and its low affinity receptor are abundantly present within the AD brain, although this does not rule out an NGF-related mechanism in the degeneration of basal forebrain neurons, nor does it eliminate the possibility that exogenous NGF may be successfully used to treat AD. Further studies of the degree and distribution of NGF within the human brain in normal aging and in AD, and of the possible relationship between target NGF levels and the status of basal forebrain neurons in vivo, are necessary before engaging in clinical trials.

Nerve growth factor reverses spatial memory impairments in aged rats

Aged rats, displaying impairments in spatial learning and memory associated with marked cellular atrophy of forebrain cholinergic neurons, received intracerebroventricular infusions of the neurotrophin nerve growth factor (NGF), or vehicle. During the 4-week infusion period rats receiving NGF showed improved acquisition and retention of spatial memory. With NGF this was accompanied by a significant reduction in cholinergic neuron atrophy in both septum, nucleus basalis and striatum. The cause of learning and memory deficits associated with ageing is not known. In the present paper we show that learning and memory deficits in aged rats can be reversed by NGF.

Changes in the response of cultured septal cholinergic neurons to nerve growth factor exposure and deprivation during the first postnatal month

The effects of nerve growth factor (NGF) and a blocking anti-NGF antibody were studied in cultures plated from postnatal day 1-28 (P1-P28) rat septum and maintained 3 weeks in vitro. 7S NGF (100 ng/ml = 0.75 nM) increased choline acetyltransferase (ChAT) activity in P7-P21 cultures. The largest increase was measured in P7-P14 cultures, where NGF addition produced ChAT activities 5-12 times higher than those measured in cultures grown in anti-NGF antibody. NGF also increased the number of acetylcholinesterase (AChE)-positive neurons in P7-P14 cultures. To determine whether this increase was due to enhanced survival of cholinergic neurons or simply to enhanced AChE expression, we examined cultures to which NGF was added only after an initial 1-2-week exposure to anti-NGF antibody. This delayed addition of NGF also increased ChAT activity and the number of AChE-positive neurons, indicating that cholinergic neurons survived the initial exposure to anti-NGF antibody. Thus even during a period when ChAT activity was most sensitive to NGF, postnatal septal cholinergic neurons did not require NGF for survival in vitro.

Cloning of a non-catalytic form of human trkB and distribution of messenger RNA for trkB in human brain

A truncated form of the human trkB gene has been cloned and sequenced. This gene is related to the trk family of tyrosine kinases, the products of which act as receptors for the neurotrophins. Of these, brain-derived neurotrophic factor and mammalian neurotrophin-4 are the known ligands for the TrkB receptor. Catalytic and non-catalytic (or truncated) forms of the trkB gene have been cloned for rat and mouse. In this study, using in situ hybridization, we describe the distribution of trkB messenger RNA in fetal and adult human brain.

Ciliary neurotrophic factor enhances the survival of Purkinje cells in vitro

We have examined the effects of ciliary neurotrophic factor (CNTF) on the development of rat Purkinje cells in vitro. Cerebellar cells, derived from embryonic day 16 rat fetuses, were found to respond rapidly to CNTF treatment by induction of c-Fos protein, such that 40% of the cells were immunopositive after 60 min. Treatment with low doses of CNTF (10-100 pg/ml) for 8 days resulted in an approximately 1.6-fold increase in the number of Purkinje cells, identified by immunohistochemical staining for calbindin. Immunohistochemical staining for other Purkinje cell markers--cyclic-GMP-dependent protein kinase and the low-affinity nerve growth factor receptor--verified increased Purkinje cell survival following CNTF treatment. In addition, CNTF increased specific high-affinity GABA uptake by 45%, and the number of GABAergic neurons by 70%. A maximal increase in the number of Purkinje cells and GABA-uptake was only achieved if CNTF was added within 48 h of plating the cells, further suggesting that CNTF enhances Purkinje cell survival in vitro. These results taken together strongly support a direct effect of CNTF in promoting the survival of Purkinje cells and possibly other GABAergic cerebellar neurons.