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

A unified model of the biology of peripartum depression

Peripartum depression (PPD) is a prevalent and debilitating disorder that adversely affects the development of mothers and infants. Recently, there has been a plea for increased mental health screening during the peripartum period; however, currently, there is no accurate screening tool to identify women at risk of PPD. In addition, some women do not respond to current treatment schemes and develop treatment-resistant depression. The current perspective aims to propose a unified understanding of the biological underpinnings of PPD (UmPPD) that considers the heterogeneity in the onset, symptoms cluster, and severity of PPD. Such a model could promote basic and applied research on PPD and suggest new treatment avenues. The central hub of the model is the kynurenine pathway (KP) and the KP-serotonin ratio. The forces and specific processes at play that cause an imbalance within the KP and between KP and serotonin are inflammation, stress, reproductive hormones (especially estradiol and progesterone), and oxytocin. UmPPD predicts that the most severe PPD would comprise prolonged inflammation, ongoing or multiple stressors, excessive estrogen, progesterone resistance, and avoidance of breastfeeding, skin-to-skin contact, and social proximity. These factors would be associated with a higher likelihood of developing PPD, early onset, and more significant symptom severity. In addition, subtypes of PPD would consist of different compositions and expressions of these components, with one central common factor. UmPPD could aid in directing future research and possibly detecting critical processes that could help discover, develop, and utilize novel treatments for PPD.

Selective Depletion of CREB in Serotonergic Neurons Affects the Upregulation of Brain-Derived Neurotrophic Factor Evoked by Chronic Fluoxetine Treatment

Neurotrophic factors are regarded as crucial regulatory components in neuronal plasticity and are postulated to play an important role in depression pathology. The abundant expression of brain-derived neurotrophic factor (BDNF) in various brain structures seems to be of particular interest in this context, as downregulation of BDNF is postulated to be correlated with depression and its upregulation is often observed after chronic treatment with common antidepressants. It is well-known that BDNF expression is regulated by cyclic AMP response element-binding protein (CREB). In our previous study using mice lacking CREB in serotonergic neurons (Creb1^TPH2CreERT2 mice), we showed that selective CREB ablation in these particular neuronal populations is crucial for drug-resistant phenotypes in the tail suspension test observed after fluoxetine administration in Creb1^TPH2CreERT2 mice. The aim of this study was to investigate the molecular changes in the expression of neurotrophins in Creb1^TPH2CreERT2 mice after chronic fluoxetine treatment, restricted to the brain structures implicated in depression pathology with profound serotonergic innervation including the prefrontal cortex (PFC) and hippocampus. Here, we show for the first time that BDNF upregulation observed after fluoxetine in the hippocampus or PFC might be dependent on the transcription factor CREB residing, not within these particular structures targeted by serotonergic projections, but exclusively in serotonergic neurons. This observation may shed new light on the neurotrophic hypothesis of depression, where the effects of BDNF observed after antidepressants in the hippocampus and other brain structures were rather thought to be regulated by CREB residing within the same brain structures. Overall, these results provide further evidence for the pivotal role of CREB in serotonergic neurons in maintaining mechanisms of antidepressant drug action by regulation of BDNF levels.

Effects of postnatal dietary choline manipulation against MK-801 neurotoxicity in pre- and postadolescent rats

Prenatal supplementation of rat dams with dietary choline has been shown to provide their offspring with neuroprotection against N-methyl-d-aspartate (NMDA) antagonist-mediated neurotoxicity. This study investigated whether postnatal dietary choline supplementation exposure for 30 and 60 days of rats starting in a pre-puberty age would also induce neuroprotection (without prenatal exposure). Male and female Sprague-Dawley rats (postnatal day 30 of age) were reared for 30 or 60 concurrent days on one of the four dietary levels of choline: 1) fully deficient choline, 2) 1/3 the normal level, 3) the normal level, or 4) seven times the normal level. After diet treatment, the rats received one injection of MK-801 (dizocilpine 3mg/kg) or saline control. Seventy-two hours later, the rats were anesthetized and transcardially perfused. Their brains were then postfixed for histology with Fluorojade-C (FJ-C) staining. Serial coronal sections were prepared from a rostrocaudal direction from 1.80 to 4.2mm posterior to the bregma to examine cell degeneration in the retrosplenial and piriform regions. MK-801, but not control saline, produced significant numbers of FJ-C positive neurons, indicating considerable neuronal degeneration. Dietary choline supplementation or deprivation in young animals reared for 30-60days did not alter NMDA antagonist-induced neurodegeneration in the retrosplenial region. An interesting finding is the absence of the piriform cortex involvement in young male rats and the complete absence of neurotoxicity in both hippocampus regions and DG. However, neurotoxicity in the piriform cortex of immature females treated for 60days appeared to be suppressed by low levels of dietary choline.

Water maze experience and prenatal choline supplementation differentially promote long-term hippocampal recovery from seizures in adulthood

Status epilepticus (SE) in adulthood dramatically alters the hippocampus and produces spatial learning and memory deficits. Some factors, like environmental enrichment and exercise, may promote functional recovery from SE. Prenatal choline supplementation (SUP) also protects against spatial memory deficits observed shortly after SE in adulthood, and we have previously reported that SUP attenuates the neuropathological response to SE in the adult hippocampus just 16 days after SE. It is unknown whether SUP can ameliorate longer-term cognitive and neuropathological consequences of SE, whether repeatedly engaging the injured hippocampus in a cognitive task might facilitate recovery from SE, and whether our prophylactic prenatal dietary treatment would enable the injured hippocampus to more effectively benefit from cognitive rehabilitation. To address these issues, adult offspring from rat dams that received either a control (CON) or SUP diet on embryonic days 12-17 first received training on a place learning water maze task (WM) and were then administered saline or kainic acid (KA) to induce SE. Rats then either remained in their home cage, or received three additional WM sessions at 3, 6.5, and 10 weeks after SE to test spatial learning and memory retention. Eleven weeks after SE, the brains were analyzed for several hippocampal markers known to be altered by SE. SUP attenuated SE-induced spatial learning deficits and completely rescued spatial memory retention by 10 weeks post-SE. Repeated WM experience prevented SE-induced declines in glutamic acid decarboxylase (GAD) and dentate gyrus neurogenesis, and attenuated increased glial fibrilary acidic protein (GFAP) levels. Remarkably, SUP alone was similarly protective to an even greater extent, and SUP rats that were water maze trained after SE showed reduced hilar migration of newborn neurons. These findings suggest that prophylactic SUP is protective against the long-term cognitive and neuropathological effects of KA-induced SE, and that rehabilitative cognitive enrichment may be partially beneficial.

Decreased neurotrophic response to birth hypoxia in the etiology of schizophrenia

BACKGROUND

Obstetric complications, particularly fetal hypoxia, are associated with increased risk for schizophrenia later in life. Such factors are also related to increased severity of certain neuropathological features of schizophrenia, including hippocampal and cortical gray matter reduction, among individuals with a genetic susceptibility to the disorder. However, the molecular mechanisms underlying these associations are unknown. Here, we sought to determine whether neurotrophic factors, which are stimulated as part of a neuroprotective response to fetal distress, are differentially expressed in cord blood samples at the time of birth following fetal hypoxia, maternal hypertension/small for gestational age status, and/or prematurity among individuals who developed schizophrenia as adults, as compared with control subjects.

METHODS

One hundred eleven cases with psychotic disorders (70 with schizophrenia) and 333 control subjects matched for gender, race, and date of birth were drawn from the Philadelphia cohort of the National Collaborative Perinatal Project in a nested case-control study. Brain-derived neurotrophic factor (BDNF) was assayed from cord and maternal blood samples taken at delivery and stored at -20 degrees C for 45 to 50 years.

RESULTS

Among control subjects, birth hypoxia was associated with a significant (10%) increase in BDNF in cord samples, while among cases, hypoxia was associated with a significant (20%) decrease in BDNF. This differential response to fetal hypoxia was specific to schizophrenia and was not explained by other obstetric complications or by the BDNF valine (val) to methionine (met) polymorphism at codon 66 (val66met).

CONCLUSIONS

These findings provide serologically based prospective evidence of disrupted neurotrophic signaling in response to birth hypoxia in the molecular pathogenesis of schizophrenia.

Prenatal choline supplementation attenuates neuropathological response to status epilepticus in the adult rat hippocampus

Prenatal choline supplementation (SUP) protects adult rats against spatial memory deficits observed after excitotoxin-induced status epilepticus (SE). To examine the mechanism underlying this neuroprotection, we determined the effects of SUP on a variety of hippocampal markers known to change in response to SE and thought to underlie ensuing cognitive deficits. Adult offspring from rat dams that received either a control or SUP diet on embryonic days 12-17 were administered saline or kainic acid (i.p.) to induce SE and were euthanized 16 days later. SUP markedly attenuated seizure-induced hippocampal neurodegeneration, dentate cell proliferation, and hippocampal GFAP mRNA expression levels, prevented the loss of hippocampal GAD65 protein and mRNA expression, and altered growth factor expression patterns. SUP also enhanced pre-seizure hippocampal levels of BDNF, NGF, and IGF-1, which may confer a neuroprotective hippocampal microenvironment that dampens the neuropathological response to and/or helps facilitate recovery from SE to protect cognitive function.

Alterations in IGF-I, BDNF and NT-3 levels following experimental brain trauma and the effect of IGF-I administration

The effects of a unilateral, penetrating brain trauma on IGF-I, BDNF and NT-3 were studied immunocytochemically in the rat. BDNF and NT-3 were decreased in the peritraumatic area, but increased in the adjacent region, 4 and 12 h post-injury. One week following the trauma, BDNF remained low in the peritraumatic area, but was restored to normal levels in the adjacent, while no effect of injury on NT-3 levels was detected in either area. Injury resulted in an increase in IGF-I levels in the peritraumatic area, which was most pronounced 1 week following the trauma, indicating that IGF-I could participate in endogenous repair processes. We thus administered IGF-I immediately following the trauma and investigated its effects on injury-induced changes in neurotrophin levels. Administration of IGF-I partially reversed the injury-induced decrease in BDNF and NT-3 in the peritraumatic area observed 4 and 12 h post-injury, while at the same time-points, it completely cancelled the effects of injury in the adjacent region. One week after the trauma, BDNF levels were dramatically increased in both the peritraumatic and adjacent area, reaching levels even higher than those of the sham-operated animals, following IGF-I administration. Our results showing that IGF-I not only counteracts injury-induced changes in neurotrophins, but can also further increase their levels, indicate that this growth factor could mediate repair and/or protective processes, following brain trauma.

Persistent regional increases in brain-derived neurotrophic factor in the flurothyl model of epileptogenesis are dependent upon the kindling status of the animal

Brain-derived neurotrophic factor (BDNF) appears to be both regulated by and a regulator of epileptogenesis. In the flurothyl (HFE) model of kindling mice exposed to successive flurothyl trials over 8 days express a rapid, long-lasting reduction in generalized seizure threshold and a more slowly evolving change in seizure phenotype in response to subsequent flurothyl exposure. The BDNF genotype of particular mouse strains appears to influence the epileptogenic progression in this model. Thus, we hypothesized that BDNF signaling pathways are altered by flurothyl-induced seizures. Following HFE kindling, fully kindled (eight seizures) adult male C57BI/6J mice had significantly elevated whole brain BDNF levels through at least 28 days after their final seizure. Mice that received only four HFE seizures (not kindled) had elevated BDNF levels, but only at 1 day post-seizure (DPSz), while BDNF levels were not significantly altered in mice receiving just one HFE seizure at any time point studied. Regional expression patterns of BDNF in the hippocampus, hypothalamus, and frontal cortex were also elevated by one DPSz and returned to control values by 14 DPSz in mice that received four HFE seizures. No changes were seen in the cerebellum, striatum, or piriform cortex. In contrast, fully kindled mice had significantly elevated BDNF levels within the hippocampus, hypothalamus, neocortex, and striatum that remained elevated through at least 14 DPSz, while levels were unchanged in the cerebellum and piriform cortex. Regional results were confirmed using anti-BDNF immunohistochemistry (IHC). Despite changes in BDNF levels following HFE kindling, we were unable to demonstrate alterations either in full-length tyrosine kinase receptor B (TrkB) expression (Western blot and IHC) or in truncated TrkB (IHC) expression levels. Together, these data suggest a model of a positive feedback loop involving seizure activity and seizure number and persistently modified BDNF signaling pathways that influences seizure phenotypes within the HFE kindling paradigm. Thus, long-term elevations in BDNF may be responsible in part for epileptogenic processes and the development of human refractory epilepsies.

Neurotrophic factors and CNS disorders: findings in rodent models of depression and schizophrenia

Nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are proteins involved in neuronal survival and plasticity of dopaminergic, cholinergic and serotonergic neurons in the central nervous system (CNS). Loss of neurons in specific brain regions has been found in depression and schizophrenia, and this chapter summarizes the findings of altered neurotrophins in animal models of those two disorders under baseline condition and following antidepressive and antipsychotic treatments. In a model of depression (Flinders sensitive line/Flinders resistant line; FSL/FRL rats), increased NGF and BDNF concentrations were found in frontal cortex of female, and in occipital cortex of male 'depressed' FSL compared to FRL control rats. Using the same model, the effects of electroconvulsive stimuli (ECS) and chronic lithium treatment on brain NGF, BDNF and glial cell line-derived neurotrophic factors were investigated. ECS and lithium altered the brain concentrations of neurotrophic factors in the hippocampus, frontal cortex, occipital cortex and striatum. ECS mimic the effects of electroconvulsive therapy (ECT) that is an effective treatment for depression and also schizophrenia. Since NGF and BDNF may also be changed in the CNS of animal models of schizophrenia, we investigated whether treatment with antipsychotic drugs (haloperidol, risperidone, and olanzapine) affects the constitutive levels of NGF and BDNF in the CNS. Both typical and atypical antipsychotic drugs altered the regional brain levels of NGF and BDNF. Other studies also demonstrated that these drugs differentially altered neurotrophin mRNAs. Overall, these studies indicate that alteration of brain level of NGF and BDNF could constitute part of the biochemical alterations induced by antipsychotic drugs.

Absence of hippocampal mossy fiber sprouting in transgenic mice overexpressing brain-derived neurotrophic factor

Excess neuronal activity upregulates the expression of two neurotrophins, nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) in adult hippocampus. Nerve growth factor has been shown to contribute the induction of aberrant hippocampal mossy fiber sprouting in the inner molecular layer of the dentate gyrus, however the role of prolonged brain-derived neurotrophic factor exposure is uncertain. We examined the distribution and plasticity of mossy fibers in transgenic mice with developmental overexpression of brain-derived neurotrophic factor. Despite 2--3-fold elevated BDNF levels in the hippocampus sufficient to increase the intensity of neuropeptide Y immunoreactivity in interneurons, no visible changes in mossy fiber Timm staining patterns were observed in the inner molecular layer of adult mutant hippocampus compared to wild-type mice. In addition, no changes of the mRNA expression of two growth-associated proteins, GAP-43 and SCG-10 were found. These data suggest that early and persistent elevations of brain-derived neurotrophic factor in granule cells are not sufficient to elicit this pattern of axonal plasticity in the hippocampus.

Neuronal death enhanced by N-methyl-D-aspartate antagonists

Glutamate promotes neuronal survival during brain development and destroys neurons after injuries in the mature brain. Glutamate antagonists are in human clinical trials aiming to demonstrate limitation of neuronal injury after head trauma, which consists of both rapid and slowly progressing neurodegeneration. Furthermore, glutamate antagonists are considered for neuroprotection in chronic neurodegenerative disorders with slowly progressing cell death only. Therefore, humans suffering from Huntington's disease, characterized by slowly progressing neurodegeneration of the basal ganglia, are subjected to trials with glutamate antagonists. Here we demonstrate that progressive neurodegeneration in the basal ganglia induced by the mitochondrial toxin 3-nitropropionate or in the hippocampus by traumatic brain injury is enhanced by N-methyl-d-aspartate antagonists but ameliorated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate antagonists. These observations reveal that N-methyl-d-aspartate antagonists may increase neurodestruction in mature brain undergoing slowly progressing neurodegeneration, whereas blockade of the action of glutamate at alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptors may be neuroprotective.

Spatiotemporal expression of BDNF in the hippocampus induced by the continuous intracerebroventricular infusion of beta-amyloid in rats

The beta-amyloid protein (Abeta) is the major component of senile plaques found in the brain in Alzheimer's disease (AD). Its neurotoxic properties in vivo, however, are not well defined. Since the expression of neurotrophin genes is considered an important component of the intrinsic neuroprotective response to insults, we analyzed the gene expression of neurotrophins in the brains of rats which received a continuous infusion of Abeta-(1-42) into the cerebroventricle. Northern blot analysis revealed a significant increase in brain-derived neurotrophic factor (BDNF) expression in the hippocampus but no change in the cerebral cortices. The alteration peaked on days 3-7 and returned to the basal level on day 14 after the start of Abeta-(1-42) infusion. No significant changes in nerve growth factor or neurotrophin-3 mRNA expression were observed. The infusion of Abeta-(1-40) and (25-35) also triggered the expression of BDNF mRNA, whereas neither Abeta-(40-1) nor (1-16) had any effect. In situ hybridization histochemistry revealed that the expression mainly occurred in the hilus and granular layer of the dentate gyrus and to a lesser extent in the pyramidal neurons of the CA1 region. These results demonstrate that the continuous intracerebroventricular infusion of Abeta induces selective and spatiotemporal expression of BDNF mRNA in the hippocampus.

Developmental regulation of BDNF and NT-3 expression by quinolinic acid in the striatum and its main connections

Interactions between neurotrophic factors and neurotransmitters participate in the formation and maintenance of appropriate connections, as well as in neurodegenerative processes. Here we have measured changes in the developmental expression pattern of BDNF and NT-3 in the striatum, cortex, and substantia nigra induced by intrastriatal injection of the N-methyl-d-aspartate glutamate receptor agonist quinolinic acid (QUIN). Animals were injected at different postnatal ages, and BDNF and NT-3 mRNA levels were determined 6 h after lesion using a ribonuclease protection assay. Our results show a biphasic increase in BDNF mRNA levels in striatum and in the ipsilateral cortex at postnatal day (P)5 and P21. In contrast, although NT-3 expression did not change in the striatum, it was down-regulated in the ipsilateral cortex at P5 and P30. Intrastriatal QUIN injection did not induce changes in either BDNF or NT-3 expression in the ipsilateral substantia nigra. These findings show that neurotrophin expression is developmentally regulated after excitotoxic injury, which suggests that this endogenous response may be involved in different neuronal maturation and vulnerability during development.

Data and hypotheses on the role of nerve growth factor and other neurotrophins in psychiatric disorders

Nerve growth factor (NGF) was discovered and characterized for its role on the growth, differentiation and maintenance of specific neurons of the peripheral nervous system. Subsequent studies revealed that NGF is synthesized and released within the central nervous system and exerts a trophic and functional role on basal forebrain cholinergic neurons; it is involved in a protective role following brain insults induced by an epileptic status, seizure, as well as surgical and chemical lesions.More recently our collaborative studies provided evidence that NGF is implicated in neurobehavioral response including cerebral alterations associated with psychiatric disorders. In this brief review, ongoing and emerging data are presented and discussed.

Studies in animal models and humans suggesting a role of nerve growth factor in schizophrenia-like disorders

Neurotrophic factors, such as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), are known to play a crucial role in growth, differentiation and function in a variety of brain neurons during development and in adult life. We have recently shown that environmental changes, aggressive behavior and anxiety-like responses alter both circulating and brain basal NGF levels. In the present review, we present data obtained using animal models which suggest that neurotrophic factors, particularly NGF and BDNF, might be implicated in mechanism(s) leading to a condition associated with schizophrenic-like behaviors. The hypothesis that neurotrophins of the NGF family can be implicated in some maldevelopmental aspects of schizophrenia is supported by findings indicating that the constitutive levels of NGF and BDNF are affected in schizophrenic patients.

Concentration- and cell type-specific effects of calbindin D28k on vulnerability of hippocampal neurons to seizure-induced injury

The calcium-binding protein calbindin D28k (CB) is expressed in limited subpopulations of neurons in the brain. In the hippocampus, CB is expressed in all dentate granule cells and a subpopulation of CA1 pyramidal neurons, but is absent from CA3 neurons. This pattern of CB expression is inversely correlated with neuronal vulnerability to seizure-induced damage suggesting the possibility that expression of CB confers resistance to excitotoxicity. While data from cell culture studies support an excitoprotective role for calbindin, it is not known whether CB is a key determinant of neuronal vulnerability in vivo. We therefore examined the pattern of damage to hippocampal neurons following intrahippocampal injection of the seizure-inducing excitotoxin kainate in CB homozygous (CB-/-) and CB heterozygous (CB+/-) knockout mice in comparison with wild-type mice (CB+/+). Whereas the extent of damage to CA1 neurons was similar in CB-/- and CB+/+ mice, damage to CA1 neurons was significantly reduced in CB+/- mice. Dentate granule neurons were not damaged following kainate-induced seizures in CB+/+, CB+/- or CB-/- mice. These findings suggest that CB can modify vulnerability of hippocampal CA1 neurons to seizure-induced injury, and that either CB is not a critical determinant of resistance of dentate granule neurons, or compensatory changes occur and lack of CB is not the only difference between CB-/- and CB+/+ mice.

Neurotrophin-3 levels in cerebrospinal fluid from children with bacterial meningitis, viral meningitis, or encephalitis

Neurotrophin-3 levels were measured in the cerebrospinal fluid of 35 patients with bacterial meningitis, viral meningitis, or encephalitis by two-site enzyme immunoassay. Elevated cerebrospinal fluid levels of neurotrophin-3 were demonstrated in 8 of 18 patients with bacterial meningitis. Follow-up examination of the eight patients at the convalescent stage showed diminished cerebrospinal fluid levels of neurotrophin-3. In contrast, none of the 17 patients with viral meningitis or encephalitis showed an elevation of neurotrophin-3 levels in cerebrospinal fluid. No relationships were observed between neurotrophin-3 levels and cerebrospinal fluid cell numbers, cerebrospinal fluid protein levels, serum C-reactive protein concentrations, or outcome in bacterial meningitis. Since neurotrophin-3 is involved in the survival of neurons and the modulation of the immune system, neurotrophin-3 could play a neuroprotective or immunomodulatory role in bacterial meningitis.

The effect of hypothermia on the expression of neurotrophin mRNA in the hippocampus following transient cerebral ischemia in the rat

The expression of the mRNAs of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT3) and the neurotrophin receptor, TrkB, was studied in the rat hippocampus by in situ hybridization following normothermic (37 degreesC) and protective hypothermic (33 degreesC) transient cerebral ischemia of 15 min duration. In the resistant dentate gyrus, normothermic ischemia transiently induced NGF mRNA at around 8 h of recovery, while the NT3 mRNA levels were depressed over at least a 24-h recovery period. The levels of BDNF and TrkB were transiently and markedly elevated with a maximal expression at 24 h of recovery. Intraischemic hypothermia reduced the induction of NGF mRNA, while the increase of BDNF mRNA expression occurred earlier during recovery, and the post-ischemic NT3 mRNA depression was not affected. Also, the expression of TrkB mRNA was enhanced, and occurred concomitantly with the elevation of BDNF mRNA. In contrast, there were no changes in neurotrophin and TrkB mRNA in the CA3 and CA1 regions. The expression of BDNF mRNA at 24 h after normothermic ischemia, was attenuated by intraischemic hypothermia. We conclude that, the expressions of NGF, BDNF, NT3 or TrkB mRNA in ischemia-sensitive hippocampal subregions are not increased by protective hypothermia. In contrast, hypothermia induces neurotrophin mRNA alterations in the ischemia-resistant dentate gyrus that may convey protection to sensitive regions.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Differential regulation of the expression of nerve growth factor, brain-derived neurotrophic factor, and neurotrophin-3 after excitotoxicity in a rat model of Huntington's disease

In the present study we have evaluated changes in nerve growth factor (NGF), brain-derived neurotrophic factor, and neurotrophin 3 (NT-3) mRNA expression induced by different glutamate receptor agonists injected into the neostriatum. Up-regulation of NGF expression was observed at 24 h after intrastriatal quinolinate injection, an N-methyl-D-aspartate receptor agonist, and this increase was maintained up to 7 days after lesion. NGF up-regulation was also apparent in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) treatment from 6 to 16 h postinjection. Instead, BDNF was up-regulated only at 6 h after kainate or AMPA excitotoxicity. Interestingly, NT-3 mRNA was down-regulated from 10 to 16 h following AMPA lesion, while 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid injection enhanced NT-3 mRNA levels at 10 h. Our results show a specific neurotrophin response induced by stimulation of each glutamate receptor. These activity-dependent changes might be involved in neuronal plasticity processes and may underlie the differential vulnerability of striatal neurons observed in neurodegenerative disorders.

Brain-derived neurotrophic factor antisense oligonucleotide impairs memory retention and inhibits long-term potentiation in rats

We have examined the relationship between brain-derived neurotrophic factor gene expression in the hippocampus and memory retention as well as long-term potentiation of rats. One-way inhibitory avoidance learning was adopted as the behavioural paradigm. Results revealed that brain-derived neurotrophic factor messenger RNA levels in the dentate gyrus of the hippocampus were markedly increased at 1 h, 3 h and 6 h post-training in rats showing good retention performance when compared with the poor retention controls. Direct injection of brain-derived neurotrophic factor antisense oligonucleotide into the dentate gyrus of the hippocampus before memory consolidation takes place markedly impaired retention performance in rats. It also significantly decreased brain-derived neurotrophic factor messenger RNA level in the dentate gyrus. The same antisense treatment also markedly reduced the amplitude and slope of excitatory postsynaptic potential as well as the brain-derived neurotrophic factor messenger RNA level in the dentate gyrus. These results suggest that hippocampal brain-derived neurotrophic factor gene expression plays an important role in the memory consolidation process and in the expression of long-term potentiation in rats. These results provide the first evidence to relate brain-derived neurotrophic factor gene expression and memory function in vertebrates. It further suggests that brain-derived neurotrophic factor gene expression is involved in behavioural plasticity.

Inhibition of GABAA synaptic responses by brain-derived neurotrophic factor (BDNF) in rat hippocampus

Brain-derived neurotrophic factor (BDNF) is one of neurotrophins involved in the development and maintenance of both the peripheral nervous system and CNS. Although the expression of BDNF and its receptor TrkB still occurs in the adult stage, their physiological role in the mature CNS is not fully understood. In the present study we examined in detail the possibility that BDNF modulates synaptic neurotransmissions by using patch-clamp technique in rat hippocampal CA1 region. BDNF (20-100 ng/ml) did not show any appreciable effect on evoked EPSCs, but it markedly reduced both evoked and spontaneous IPSCs within 5 min, and the reduction persisted while BDNF was present. BDNF also attenuated GABAA receptor-mediated response to applied GABA. However, BDNF failed to attenuate IPSCs when the postsynaptic pyramidal neuron was loaded intracellularly with 200 nM K252a, an alkaloid that inhibits the kinase activity of Trk receptor family, through the patch pipette. Intracellular application of 200 nM K252b, a weaker inhibitor of Trk-type kinase, did not affect the inhibition. The attenuating effect also was prevented by postsynaptic injection of U73122 (5 microM), a broad-spectrum PLC inhibitor, and by strong chelation of intracellular Ca2+ with 10 mM BAPTA. These data suggest that BDNF modulates GABAA synaptic responses by postsynaptic activation of Trk-type receptor and subsequent Ca2+ mobilization in the CNS.

Suppressed kindling epileptogenesis and perturbed BDNF and TrkB gene regulation in NT-3 mutant mice

In the kindling model of epilepsy, repeated electrical stimulations lead to progressive and permanent intensification of seizure activity. We find that the development of amygdala kindling is markedly retarded in mice heterozygous for a deletion of the neurotrophin-3 (NT-3) gene (NT-3+/- mice). These mice did not reach the fully kindled state (3rd grade 5 seizure) until after 28 +/- 4 days of stimulation compared to 17 +/- 2 days in the wild-type animals. The deficit in the NT-3+/- mice reflected dampening of the progression from focal to generalized seizures. The number of stimulations required to evoke focal (grade 1 and 2) seizures did not differ between the groups, but the NT-3 mutants spent a considerably longer period of time (13 +/- 3 days) than wild-type mice (2 +/- 1 days) in grade 2 seizures. As assessed by test stimulation 4-12 weeks after the 10th grade 5 seizure, kindling was maintained in the NT-3 mutants. In situ hybridization showed 30% reduction of basal NT-3 mRNA levels and lack of upregulation of TrkC mRNA expression at 2 h after a generalized seizure in dentate granule cells of the NT-3+/- mice, whereas the seizure-evoked increase in brain-derived neurotrophic factor (BDNF) and TrkB mRNA levels was enhanced. These results indicate that endogenous NT-3 levels can influence the rate of epileptogenesis, and suggest a link between NT-3 and BDNF gene regulation in dentate granule cells.

Effects of X-irradiation on nerve growth factor in the developing mouse brain

The involvement of neurotrophins after radiation injury during brain development were studied in pregnant mice (C 57/B1) exposed on gestation day 15 to X-ray doses of 0.02-2 Gy. Nerve growth factor protein (NGF) and different cholinergic markers were investigated on postnatal day 1 (P1) and day 21 (P21); in situ hybridization with brain-derived neurotrophic factor (BDNF) and trkC (receptor serving to bind neurotrophin-3) probes was investigated on P21 in cortex, hippocampus, septum and cerebellum. The level of NGF protein was increased in irradiated forebrain on P1 in a dose-related manner. However, on P21 the NGF protein dropped down below the control levels in irradiated hippocampus and cerebellum. The response of acetylcholine esterase (AChE) activity in cerebellum at P21 was correlated with the changes in the amount of NGF. The intensity of cell labelling with trkC probe decreased after irradiation in the region of the hippocampus at P21, especially in dentate gyrus. The expression of BDNF mRNA was increased at P21 by low doses of irradiation (0.02-1 Gy) but was decreased by a high dose (2 Gy) in the same area. Thus, the radiation induced an alteration of neurotrophins, and the changes varied depending on the dose or time after irradiation. Such alterations in the pattern of growth factor production may modulate the response of cells to radiation. Furthermore, NGF protein levels and the expression of BDNF and trkC mRNA were affected by radiation doses as low as 0.02 Gy, indicating that during development the neurotrophins and their receptors are very sensitive to radiation.

Upregulation of BDNF mRNA expression in the barrel cortex of adult mice after sensory stimulation

Upregulation of brain-derived neurotrophic factor (BDNF) mRNA expression by neuronal activity has been reported in cultured hippocampal cells and in different in vivo excitotoxic paradigms. The aim of the present study was to determine whether sensory stimulation of the whisker-to-barrel pathway alters BDNF mRNA expression in the cortex and, if so, to evaluate the specificity of this effect. To this end, a set of mystacial whiskers was unilaterally stimulated by mechanical deflection, and the expression of BDNF mRNA was analyzed in the barrel cortex by in situ hybridization (ISH) using a 35S-labeled antisense BDNF riboprobe and emulsion autoradiography. A clear-cut and specific upregulation of the BDNF mRNA expression was found at the level of the somatosensory cortex after the increased peripheral stimulation. In the barrel cortex of control mice, BDNF mRNA was present in a few cells in layers II/III and VI, whereas it was almost undetectable in layer IV. After 6 hr of whisker stimulation, increased levels of BDNF mRNA were found in layers II to VI of the contralateral barrel cortex. In layer IV, BDNF upregulation was confined to the barrels corresponding to the stimulated follicles. ISH combined with immunocytochemistry against the three calcium-binding proteins parvalbumin, calretinin, and calbindin-D28K revealed that BDNF mRNA-expressing cells do not belong to the GABAergic cell population of the barrel cortex. The present results support a role for BDNF in activity-dependent modifications of the adult cerebral cortex.

Expression of NGF and NT3 mRNAs in hippocampal interneurons innervated by the GABAergic septohippocampal pathway

We used in situ hybridization for the detection of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin 3 (NT3) mRNAs combined with immunocytochemistry against the calcium-binding proteins parvalbumin (PARV), calbindin 28k (CALB), and calretinin (CALR) to determine the expression of neurotrophins in functionally distinct subsets of hippocampal interneurons. Most PARV-immunoreactive neurons in the hippocampus were NGF mRNA-positive (82%), which corresponds to 71% of NGF-positive neurons in the hippocampus proper and in the dentate gyrus (excluding granule cells). In contrast, only a subset of CALB- and CALR-immunoreactive interneurons (24% and 23%, respectively) displayed hybridization signals for NGF. Small subsets of PARV- and CALR-positive cells expressed NT3 mRNA, but we did not find hippocampal interneurons expressing BDNF mRNA. These results show that NGF and NT3 genes are differentially regulated in distinct subsets of GABAergic cells, and these interneurons are a major source of NGF production in the hippocampus. We also addressed whether hippocampal interneurons expressing neurotrophins were targets of the GABAergic septohippocampal pathway. We developed a triple-labeling method that combines anterograde tracing of this pathway by means of Phaseolus vulgaris leucoagglutinin injections, with in situ hybridization for the detection of neurotrophins, and immunocytochemistry for calcium-binding proteins. Virtually every PARV-positive neuron innervated by GABAergic septohippocampal baskets expressed NGF mRNA (86%), whereas 39-59% of CALR- and CALB-positive interneurons that were contacted by GABAergic septohippocampal axons showed NGF gene expression. A small subset of NT3 mRNA-expressing interneurons was also innervated by septohippocampal baskets. These findings show that the GABAergic septohippocampal pathway preferentially terminates on interneurons expressing NGF mRNA, suggesting that this neurotrophic factor might be involved in the specification of this connection and in its maintenance and normal function in the adult brain.

Neurotrophin-3 as an essential signal for the developing nervous system

Rapid advances in characterization of the biological actions mediated by the third member of the neurotrophin family, neurotrophin-3 (NT-3), have been made recently in vitro as well as in situ. These have been made possible by the cloning of the genes for NT-3 and for its transducing receptor tyrosine kinase TrkC. This article will focus on the roles of NT-3 in the nervous system. In situ localization of NT-3 consistent with that of its receptor is manifested at all developmental stages studied and into adulthood. Through TrkC, NT-3 signals a number of trophic effects, ranging from mitogenesis, promotion of survival, or differentiation, depending on the developmental stage of the target cells. The sites of action of NT-3 reside primarily in the peripheral nervous system (PNS), various areas of the central nervous system (CNS), and in the enteric system (ENS). Analyses of the phenotypes of transgenic mice lacking NT-3 or injection of embryos with a blocking antibody have so far revealed the essential role of NT-3 in development of specific populations of the PNS, and in particular of proprioceptive, nodose, and auditory sensory neurons and of sympathetic neurons. The actions of NT-3 also extend to modulation of transmitter release at several types of synapses in the periphery as well as in the adult CNS. In addition, NT-3 may play a role in the development of tissues other than the nervous system, such as the cardiovascular system. Future investigations will widen the understanding of the many roles of NT-3 on both neuronal and nonneuronal cells.

Trophic factor response to neuronal stimuli or injury

Neurotrophic factors are produced by CNS neurons, and have both paracrine and autocrine activities. In nerve cells, expression of neurotrophic factors is regulated by physiological afferent activity, which implies that these factors play a role in activity-dependent plasticity and survival. Neurotrophic factor levels are also altered following injury, which suggests that they play a part in the neurodegenerative response and synaptic reorganization as well. Recent studies have examined extensively the regulation and functional roles of the neurotrophin family, and have also identified other neurotrophic factors present in brain that are regulated by different, as well as similar mechanisms.