Crucial role of TrkB ligands in the survival and phenotypic differentiation of developing locus coeruleus noradrenergic neurons

The dose-dependent neuroprotective effect of norepinephrine in improving memory retrieval in an experimental model of multiple sclerosis, experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is considered an immune-mediated inflammatory disorder that causes cognitive impairments by damaging the hippocampal tissue. Conversely, norepinephrine (NEP) has anti-inflammatory and re-myelinating properties, which improve cognitive impairments. The aim of this study was to assess the neuroprotective effects of NEP on learning and memory disorders in an experimental animal model of MS. Two guide cannulas were bilaterally implanted in the rat hippocampal CA1 regions. After recovery, the animals received 3 μl of 0.01% ethidium bromide (EtB) in each of both hippocampal regions. After three days, the rats were randomly divided into 6 groups (8 rats/group), including control, sham 1, sham 2, and three groups of NEP 0.25, 0.5, and 1 mg/kg by intrahippocampal injection. Behavioral tests (e.g. shuttle box test and open-field test) were then performed. Finally, ROS, MDA, GSH, TNF-α, IL-6, and IL-1β concentrations in the left CA1 area, as well as using western-blot analysis, p-p38, p-JNK, p-AKT, p-ERK1/2, p-NMDA, p-AMPA, p-CREB, and BDNF proteins in the right CA1 region evaluated. The EtB injection increased ROS, MDA, TNF-α, IL-6, and IL-1β levels, as well as p-JNK and p-P38, except all other proteins, while decreasing GSH content, as well as step-through latency and locomotor activity in sham groups compared to the control group. Conversely, NEP (0.5 and 1 mg/kg, particularly at the dose of 1 mg/kg) counterbalanced all the alterations mentioned above in comparison to the sham groups. The EtB induced learning and memory impairment; however, NEP dose-dependently restored these impairments to normal levels.

Widespread Reduced Density of Noradrenergic Locus Coeruleus Axons in the App Knock-In Mouse Model of Amyloid-β Amyloidosis

BACKGROUND

The locus coeruleus (LC), a brainstem nucleus comprising noradrenergic neurons, is one of the earliest regions affected by Alzheimer's disease (AD). Amyloid-β (Aβ) pathology in the cortex in AD is thought to exacerbate the age-related loss of LC neurons, which may lead to cortical tau pathology. However, mechanisms underlying LC neurodegeneration remain elusive.

OBJECTIVE

Here, we aimed to examine how noradrenergic neurons are affected by cortical Aβ pathology in AppNL-G-F/NL-G-F knock-in mice.

METHODS

The density of noradrenergic axons in LC-innervated regions and the LC neuron number were analyzed by an immunohistochemical method. To explore the potential mechanisms for LC degeneration, we also examined the occurrence of tau pathology in LC neurons, the association of reactive gliosis with LC neurons, and impaired trophic support in the brains of AppNL-G-F/NL-G-F mice.

RESULTS

We observed a significant reduction in the density of noradrenergic axons from the LC in aged AppNL-G-F/NL-G-F mice without neuron loss or tau pathology, which was not limited to areas near Aβ plaques. However, none of the factors known to be related to the maintenance of LC neurons (i.e., somatostatin/somatostatin receptor 2, brain-derived neurotrophic factor, nerve growth factor, and neurotrophin-3) were significantly reduced in AppNL-G-F/NL-G-F mice.

CONCLUSION

This study demonstrates that cortical Aβ pathology induces noradrenergic neurodegeneration, and further elucidation of the underlying mechanisms will reveal effective therapeutics to halt AD progression.

BDNF downregulates β-adrenergic receptor-mediated hypotensive mechanisms in the paraventricular nucleus of the hypothalamus

Brain-derived neurotrophic factor (BDNF) is upregulated in the paraventricular nucleus of the hypothalamus (PVN) in response to hypertensive stimuli such as stress and hyperosmolality, and BDNF acting in the PVN plays a key role in elevating sympathetic activity and blood pressure. However, downstream mechanisms mediating these effects remain unclear. We tested the hypothesis that BDNF increases blood pressure, in part by diminishing inhibitory hypotensive input from nucleus of the solitary tract (NTS) catecholaminergic neurons projecting to the PVN. Male Sprague-Dawley rats received bilateral PVN injections of viral vectors expressing either green fluorescent protein (GFP) or BDNF and bilateral NTS injections of vehicle or anti-dopamine-β-hydroxylase-conjugated saporin (DSAP), a neurotoxin that selectively lesions noradrenergic and adrenergic neurons. BDNF overexpression in the PVN without NTS lesioning significantly increased mean arterial pressure (MAP) in awake animals by 18.7 ± 1.8 mmHg. DSAP treatment also increased MAP in the GFP group, by 9.8 ± 3.2 mmHg, but failed to affect MAP in the BDNF group, indicating a BDNF-induced loss of NTS catecholaminergic hypotensive effects. In addition, in α-chloralose-urethane-anesthetized rats, hypotensive responses to PVN injections of the β-adrenergic agonist isoprenaline were significantly attenuated by BDNF overexpression, whereas PVN injections of phenylephrine had no effect on blood pressure. BDNF treatment was also found to significantly reduce β₁-adrenergic receptor mRNA expression in the PVN, whereas expression of other adrenergic receptors was unaffected. In summary, increased BDNF expression in the PVN elevates blood pressure, in part by downregulating β-receptor signaling and diminishing hypotensive catecholaminergic input from the NTS to the PVN.NEW & NOTEWORTHY We have shown that BDNF, a key hypothalamic regulator of blood pressure, disrupts catecholaminergic signaling between the NTS and the PVN by reducing the responsiveness of PVN neurons to inhibitory hypotensive β-adrenergic input from the NTS. This may be occurring partly via BDNF-mediated downregulation of β₁-adrenergic receptor expression in the PVN and results in an increase in blood pressure.

Role of catalpol in ameliorating the pathogenesis of experimental autoimmune encephalomyelitis by increasing the level of noradrenaline in the locus coeruleus

The endogenous neurotransmitter, noradrenaline, exerts anti-inflammatory and neuroprotective effects in vivo and in vitro. Reduced noradrenaline levels results in increased inflammation and neuronal damage. The primary source of noradrenaline in the central nervous system is tyrosine hydroxylase (TH)-positive neurons, located in the locus coeruleus (LC). TH is the rate-limiting enzyme for noradrenaline synthesis; therefore, regulation of TH protein expression and intrinsic enzyme activity represents the central means for controlling the synthesis of noradrenaline. Catalpol is an iridoid glycoside purified from Rehmannia glutinosa Libosch, which exerts a neuroprotective effect in multiple sclerosis (MS). The present study used an experimental mouse model of autoimmune encephalomyelitis to verify the neuroprotective effects of catalpol. Significant improvements in the clinical scores were observed in catalpol-treated mice. Furthermore, catalpol increased TH expression and increased noradrenaline levels in the spinal cord. In primary cultures, catalpol exerted a neuroprotective effect in rat LC neurons by increasing the noradrenaline output. These results suggested that drugs targeting LC survival and function, including catalpol, may be able to benefit patients with MS.

Noradrenergic fiber sprouting and altered transduction in neuropathic prefrontal cortex

Functional changes in hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels have been shown to contribute to medial prefrontal (mPFC) hyperexcitability after peripheral nerve injury. A reduction in the open probability of these neuronal channels might be relevant since this can enhance membrane input resistance and synaptic summation. However, the molecular mechanisms underlying neuropathy-associated alterations in HCN channel activity remain elusive. Using the spared nerve injury model of neuropathic pain in Long-Evans rats, we first discovered a significant increase in noradrenergic innervation within the mPFC of nerve-injured compared to control animals. Patch-clamp recordings in layer II/III pyramidal neurons of the mPFC revealed that adrenoceptors, primarily the α2 subtype, can modulate the voltage-dependent activation of HCN channels and the abnormal prefrontal excitability following peripheral neuropathy. Additionally, microinfusions of the α2 adrenoceptor agonist clonidine in the mPFC of neuropathic rats provided analgesic effects, indicating the behavioral significance for this noradrenergic pathway in manifestations of the chronic pain state. Taken together, our results provide insights into the role of cortical catecholaminergic neuromodulation in neuropathic pain and suggest that altered noradrenergic transduction may play a major role in the HCN channel dysfunction and pyramidal hyperactivity observed in several chronic pain conditions.

Conditional Depletion of Hippocampal Brain-Derived Neurotrophic Factor Exacerbates Neuropathology in a Mouse Model of Alzheimer's Disease

Damage occurring to noradrenergic neurons in the locus coeruleus (LC) contributes to the evolution of neuroinflammation and neurodegeneration in a variety of conditions and diseases. One cause of LC damage may be loss of neurotrophic support from LC target regions. We tested this hypothesis by conditional unilateral knockout of brain-derived neurotrophic factor (BDNF) in adult mice. To evaluate the consequences of BDNF loss in the context of neurodegeneration, the mice harbored familial mutations for human amyloid precursor protein and presenilin-1. In these mice, BDNF depletion reduced tyrosine hydroxylase staining, a marker of noradrenergic neurons, in the rostral LC. BDNF depletion also reduced noradrenergic innervation in the hippocampus, the frontal cortex, and molecular layer of the cerebellum, assessed by staining for dopamine beta hydroxylase. BDNF depletion led to an increase in cortical amyloid plaque numbers and size but was without effect on plaque numbers in the striatum, a site with minimal innervation from the LC. Interestingly, cortical Iba1 staining for microglia was reduced by BDNF depletion and was correlated with reduced dopamine beta hydroxylase staining. These data demonstrate that reduction of BDNF levels in an LC target region can cause retrograde damage to LC neurons, leading to exacerbation of neuropathology in distinct LC target areas. Methods to reduce BDNF loss or supplement BDNF levels may be of value to reduce neurodegenerative processes normally limited by LC noradrenergic activities.

Turning on the Light Within: Subcortical Nuclei of the Isodentritic Core and their Role in Alzheimer's Disease Pathogenesis

Pharmacological interventions in Alzheimer's disease (AD) are likely to be more efficacious if administered early in the course of the disease, foregoing the spread of irreversible changes in the brain. Research findings underline an early vulnerability of the isodendritic core (IC) network to AD neurofibrillary lesions. The IC constitutes a phylogenetically conserved subcortical system including the locus coeruleus in pons, dorsal raphe nucleus, and substantia nigra in the midbrain, and nucleus basalis of Meynert in basal forebrain. Through their ascending projections to the cortex, the IC neurons regulate homeostasis and behavior by synthesizing aminergic and cholinergic neurotransmitters. Here we reviewed the evidence demonstrating that neurons of the IC system show neurofibrillary tangles in the earliest stages of AD, prior to cortical pathology, and how this involvement may explain pre-amnestic symptoms, including depression, agitation, and sleep disturbances in AD patients. In fact, clinical and animal studies show a significant reduction of AD cognitive and behavioral symptoms following replenishment of neurotransmitters associated with the IC network. Therefore, the IC network represents a unique candidate for viable therapeutic intervention and should become a high priority for research in AD.

Central monoaminergic systems are a site of convergence of signals conveying the experience of exercise to brain circuits involved in cognition and emotional behavior

Physical activity can enhance cognitive function and increase resistance against deleterious effects of stress on mental health. Enhanced cognitive function and stress resistance produced by exercise are conserved among vertebrates, suggesting that ubiquitous mechanisms may underlie beneficial effects of exercise. In the current review, we summarize the beneficial effects of exercise on cognitive function and stress resistance and discuss central and peripheral signaling factors that may be critical for conferring the effects of physical activity to brain circuits involved in cognitive function and stress. Additionally, it is suggested that norepinephrine and serotonin, highly conserved monoamines that are sensitive to exercise and able to modulate behavior in multiple species, could represent a convergence between peripheral and central exercise signals that mediate the beneficial effects of exercise. Finally, we offer the novel hypothesis that thermoregulation during exercise could contribute to the emotional effects of exercise by activating a subset of temperature-sensitive serotonergic neurons in the dorsal raphe nucleus that convey anxiolytic and stress-protective signals to forebrain regions. Throughout the review, we discuss limitations to current approaches and offer strategies for future research in exercise neuroscience.

Causes, consequences, and cures for neuroinflammation mediated via the locus coeruleus: noradrenergic signaling system

Aside from its roles in as a classical neurotransmitter involved in regulation of behavior, noradrenaline (NA) has other functions in the CNS. This includes restricting the development of neuroinflammatory activation, providing neurotrophic support to neurons, and providing neuroprotection against oxidative stress. In recent years, it has become evident that disruption of physiological NA levels or signaling is a contributing factor to a variety of neurological diseases and conditions including Alzheimer's disease (AD) and Multiple Sclerosis. The basis for dysregulation in these diseases is, in many cases, due to damage occurring to noradrenergic neurons present in the locus coeruleus (LC), the major source of NA in the CNS. LC damage is present in AD, multiple sclerosis, and a large number of other diseases and conditions. Studies using animal models have shown that experimentally induced lesion of LC neurons exacerbates neuropathology while treatments to compensate for NA depletion, or to reduce LC neuronal damage, provide benefit. In this review, we will summarize the anti-inflammatory and neuroprotective actions of NA, summarize examples of how LC damage worsens disease, and discuss several approaches taken to treat or prevent reductions in NA levels and LC neuronal damage. Further understanding of these events will be of value for the development of treatments for AD, multiple sclerosis, and other diseases and conditions having a neuroinflammatory component. The classical neurotransmitter noradrenaline (NA) has critical roles in modulating behaviors including those involved in sleep, anxiety, and depression. However, NA can also elicit anti-inflammatory responses in glial cells, can increase neuronal viability by inducing neurotrophic factor expression, and can reduce neuronal damage due to oxidative stress by scavenging free radicals. NA is primarily produced by tyrosine hydroxylase (TH) expressing neurons in the locus coeruleus (LC), a relatively small brainstem nucleus near the IVth ventricle which sends projections throughout the brain and spinal cord. It has been known for close to 50 years that LC neurons are lost during normal aging, and that loss is exacerbated in neurological diseases including Parkinson's disease and Alzheimer's disease. LC neuronal damage and glial activation has now been documented in a variety of other neurological conditions and diseases, however, the causes of LC damage and cell loss remain largely unknown. A number of approaches have been developed to address the loss of NA and increased inflammation associated with LC damage, and several methods are being explored to directly minimize the extent of LC neuronal cell loss or function. In this review, we will summarize some of the consequences of LC loss, consider several factors that likely contribute to that loss, and discuss various ways that have been used to increase NA or to reduce LC damage. This article is part of the 60th Anniversary special issue.

Norepinephrine Protects against Amyloid-β Toxicity via TrkB

The locus coeruleus (LC), the brainstem noradrenergic nucleus that is the sole source of norepinephrine (NE) in the forebrain, is one of the first structures affected in Alzheimer's disease (AD). Experimental ablation of the LC exacerbates, while increasing NE abates, AD-like neuropathology and cognitive deficits in animal models of the disease. Some neuroprotective effects of NE appear to be mediated by tropomyosin-related kinase B (TrkB), the canonical receptor for brain-derived neurotrophic factor (BDNF). Here, we report that NE dose-dependently protected primary cortical and LC neurons from amyloid-β (Aβ) toxicity. The neuroprotective effects of NE were fully prevented by the Trk receptor antagonist K252a but only partially attenuated by adrenergic receptor antagonists and not mimicked by adrenergic agonists. Activation of TrkB by NE in cortical and LC neurons was confirmed by immunoblot and immunocytochemistry for phospho-TrkB. These results indicate that NE can activate TrkB and protect against Aβ toxicity, at least in part, via adrenergic receptor-independent mechanisms, and have implications for the consequences of LC degeneration in AD and potential therapies for the disease.

Noradrenergic regulation of glial activation: molecular mechanisms and therapeutic implications

It has been known for many years that the endogenous neurotransmitter noradrenaline (NA) exerts anti-inflammatory and neuroprotective effects both in vitro and in vivo. In many cases the site of action of NA are beta-adrenergic receptors (βARs), causing an increase in intracellular levels of cAMP which initiates a broad cascade of events including suppression of inflammatory transcription factor activities, alterations in nuclear localization of proteins, and induction of patterns of gene expression mediated through activity of the CREB transcription factor. These changes lead not only to reduced inflammatory events, but also contribute to neuroprotective actions of NA by increasing expression of neurotrophic substances including BDNF, GDNF, and NGF. These properties have prompted studies to determine if treatments with drugs to raise CNS NA levels could provide benefit in various neurological conditions and diseases having an inflammatory component. Moreover, increasing evidence shows that disruptions in endogenous NA levels occurs in several diseases and conditions including Alzheimer's disease (AD), Parkinson's disease (PD), Down's syndrome, posttraumatic stress disorder (PTSD), and multiple sclerosis (MS), suggesting that damage to NA producing neurons is a common factor that contributes to the initiation or progression of neuropathology. Methods to increase NA levels, or to reduce damage to noradrenergic neurons, therefore represent potential preventative as well as therapeutic approaches to disease.

Neurotrophic factors in development and regulation of respiratory control

Neurotrophic factors (NTFs) are a heterogeneous group of extracellular signaling molecules that play critical roles in the development, maintenance, modulation and plasticity of the central and peripheral nervous systems. A subset of these factors, including members of three multigene families-the neurotrophins, neuropoetic cytokines and the glial cell line-derived neurotrophic factor ligands-are particularly important for development and regulation of neurons involved in respiratory control. Here, we review the functional biology of these NTFs and their receptors, as well as their roles in regulating survival, maturation, synaptic strength and plasticity in respiratory control pathways. In addition, we highlight recent progress in identifying the role of abnormal NTF signaling in the molecular pathogenesis of respiratory dysfunction in Rett syndrome and in the development of potential new NTF-targeted therapeutic strategies.

Development of amyloid burden in African Green monkeys

The vervet is an old world monkey increasingly being used as a model for human diseases. In addition to plaques and tangles, an additional hallmark of Alzheimer's disease is damage to neurons that synthesize noradrenaline (NA). We characterized amyloid burden in the posterior temporal lobe of young and aged vervets, and compared that with changes in NA levels and astrocyte activation. Total amyloid beta (Aβ)40 and Aβ42 levels were increased in the aged group, as were numbers of amyloid plaques detected using antibody 6E10. Low levels of Aβ42 were detected in 1 of 5 younger animals, although diffusely stained plaques were observed in 4 of these. Increased glial fibrillary acidic protein staining and messenger RNA levels were significantly correlated with increased age, as were cortical NA levels. Levels of Aβ42 and Aβ40, and the number of 6E10-positive plaques, were correlated with NA levels. Interestingly messenger RNA levels of glial derived neurotrophic factor, important for noradrenergic neuronal survival, were reduced with age. These findings suggest that amyloid pathology in aged vervets is associated with astrocyte activation and higher NA levels.

Reduced tissue levels of noradrenaline are associated with behavioral phenotypes of the TgCRND8 mouse model of Alzheimer's disease

Noradrenergic cell loss is well documented in Alzheimer's disease (AD). We have measured the tissue levels of catecholamines in an amyloid precursor protein-transgenic 'TgCRND8' mouse model of AD and found reductions in noradrenaline (NA) within hippocampus, temporoparietal and frontal cortices, and cerebellum. An age-related increase in cortical NA levels was observed in non-Tg controls, but not in TgCRND8 mice. In contrast, NA levels declined with aging in the TgCRND8 hippocampus. Dopamine levels were unaffected. Reductions in the tissue content of NA were found to coincide with altered expression of brain-derived neurotrophic factor (BDNF) mRNA and to precede the onset of object memory impairment and behavioral despair. To test whether these phenotypes might be associated with diminished NA, we treated mice with dexefaroxan, an antagonist of presynaptic inhibitory α(2)-adrenoceptors on noradrenergic and cholinergic terminals. Mice 12 weeks of age were infused systemically for 28 days with dexefaroxan or rivastigmine, a cholinesterase inhibitor. Both dexefaroxan and rivastigmine improved TgCRND8 behavioral phenotypes and increased BDNF mRNA expression without affecting amyloid-β peptide levels. Our results highlight the importance of noradrenergic depletion in AD-like phenotypes of TgCRND8 mice.

Notch-Rbpj signaling is required for the development of noradrenergic neurons in the mouse locus coeruleus

The locus coeruleus (LC) is the main source of noradrenaline in the brain and is implicated in a broad spectrum of physiological and behavioral processes. However, genetic pathways controlling the development of noradrenergic neurons in the mammalian brain are largely unknown. We report here that Rbpj, a key nuclear effector in the Notch signaling pathway, plays an essential role in LC neuron development in the mouse. Conditional inactivation of Rbpj in the dorsal rhombomere (r) 1, where LC neurons are born, resulted in a dramatic increase in the number of Phox2a- and Phox2b-expressing early-differentiating LC neurons, and dopamine-β-hydroxylase- and tyrosine-hydroxylase-expressing late-differentiating LC neurons. In contrast, other neuronal populations derived from the dorsal r1 were either reduced or unchanged. In addition, a drastic upregulation of Ascl1, an essential factor for noradrenergic neurogenesis, was observed in dorsal r1 of conditional knockout mice. Through genomic sequence analysis and EMSA and ChIP assays, a conserved Rbpj-binding motif was identified within the Ascl1 promoter. A luciferase reporter assay revealed that Rbpj per se could induce Ascl1 transactivation but this effect was counteracted by its downstream-targeted gene Hes1. Moreover, our in vitro gene transfection and in ovo electroporation assays showed that Rbpj upregulated Ascl1 expression when Hes1 expression was knocked down, although it also exerted a repressive effect on Ascl1 expression in the presence of Hes1. Thus, our results provide the first evidence that Rbpj functions as a key modulator of LC neuron development by regulating Ascl1 expression directly, and indirectly through its target gene Hes1.

Onecut factors control development of the Locus Coeruleus and of the mesencephalic trigeminal nucleus

The Locus Coeruleus (LC), the main noradrenergic nucleus in the vertebrate CNS, contributes to the regulation of several processes including arousal, sleep, adaptative behaviors and stress. Regulators controlling the formation of the LC have been identified but factors involved in its maintenance remain unknown. Here, we show that members of the Onecut (OC) family of transcription factors, namely HNF-6, OC-2 and OC-3, are required for maintenance of the LC phenotype. Indeed, in embryos lacking any OC proteins, LC neurons properly differentiate but abnormally migrate and eventually lose their noradrenergic characteristics. Surprisingly, the expression of Oc genes in these neurons is restricted to the earliest differentiation stages, suggesting that OC factors may regulate maintenance of the LC in a non cell-autonomous manner. Accordingly, the OC factors are present throughout development in a population directly adjacent to the LC, the rhombencephalic portion of the mesencephalic trigeminal nucleus (MTN). In the absence of OC factors, rhombencephalic MTN neurons fail to be generated, suggesting that OC proteins cell-autonomously control their production. Hence, we propose that OC factors are required at early developmental stages for differentiation of the MTN neurons that are in turn necessary for maintenance of the LC.

The noradrenaline precursor L-DOPS reduces pathology in a mouse model of Alzheimer's disease

Damage to noradrenergic neurons in the locus coeruleus (LC) is a hallmark of Alzheimer's disease (AD) and may contribute to disease progression. In 5xFAD transgenic mice, which accumulate amyloid burden at early ages, the LC undergoes stress as evidenced by increased astrocyte activation, neuronal hypertrophy, reduced levels of LC-enriched messenger RNAs (mRNAs), and increased inflammatory gene expression. Central nervous system (CNS) noradrenaline (NA) levels in 5-month-old male 5xFAD mice were increased using the NA precursor L-threo-3,4-dihydroxyphenylserine (L-DOPS). After 1 month, L-DOPS treatment improved learning in the Morris water maze test compared with vehicle-treated mice. L-DOPS increased CNS NA levels, and average latency times in the water maze test were inversely correlated to NA levels. L-DOPS reduced astrocyte activation and Thioflavin-S staining; increased mRNA levels of neprilysin and insulin degrading enzyme, and of several neurotrophins; and increased brain-derived neurotrophic factor protein levels. These data demonstrate the presence of LC stress in a robust mouse model of AD, and suggest that raising CNS NA levels could provide benefit in AD.

Locus coeruleus damage and noradrenaline reductions in multiple sclerosis and experimental autoimmune encephalomyelitis

The endogenous neurotransmitter noradrenaline exerts anti-inflammatory and neuroprotective effects in vitro and in vivo. Several studies report that noradrenaline levels are altered in the central nervous system of patients with multiple sclerosis and rodents with experimental autoimmune encephalomyelitis, which could contribute to pathology. Since the major source of noradrenaline are neurons in the locus coeruleus, we hypothesized that alterations in noradrenaline levels are a consequence of stress or damage to locus coeruleus neurons. In C57BL/6 mice immunized with myelin oligodendrocyte glycoprotein peptide 35-55 to develop chronic disease, cortical and spinal cord levels of noradrenaline were significantly reduced versus control mice. Immunohistochemical staining revealed increased astrocyte activation in the ventral portion of the locus coeruleus in immunized mice. The immunized mice showed neuronal damage in the locus coeruleus detected by a reduction of average cell size of tyrosine hydroxylase stained neurons. Analysis of the locus coeruleus of multiple sclerosis and control brains showed a significant increase in astrocyte activation, a reduction in noradrenaline levels, and neuronal stress indicated by hypertrophy of tyrosine hydroxylase stained cell bodies. However, the magnitude of these changes was not correlated with extent of demyelination or of cellular infiltrates. Together these findings demonstrate the presence of inflammation and neuronal stress in multiple sclerosis as well as in experimental autoimmune encephalomyelitis. Since reduced noradrenaline levels could be permissive for increased inflammation and neuronal damage, these results suggest that methods to raise noradrenaline levels or increase locus coeruleus function may be of benefit in treating multiple sclerosis.

RanBPM contributes to TrkB signaling and regulates brain-derived neurotrophic factor-induced neuronal morphogenesis and survival

Tropomyosin-related kinase (Trk) B is a receptor tyrosine kinase for brain-derived neurotrophic factor (BDNF) which plays a critical role in neuronal survival, differentiation and morphogenesis. Ran-binding protein in the microtubule-organizing center (RanBPM) is a cytosolic scaffold protein that has been shown to interact with protein-tyrosine kinase receptor MET, Axl/Sky, and TrkA in addition to the pan-neurotrophin receptor pan-neurotrophin receptor 75 kDa. In this study, we report RanBPM is a novel TrkB-interacting protein that contributes to BDNF-induced MAPK and Akt activation together with neuronal morphogenesis and survival. Over-expression of RanBPM in PC1210 cells (PC12 cells stably over-expressing TrkB) can significantly enhance BDNF-induced MAPK and Akt activation. Moreover, RanBPM can promote BDNF-induced hippocampal neuronal morphogenesis and enhance BDNF-mediated trophic effects after serum deprivation, while siRNA knock down of RanBPM in cells has the opposite effects. Together, these results suggest that RanBPM may modulate TrkB-mediated downstream signaling and biological functions.

Candidate genes for panic disorder: insight from human and mouse genetic studies

Panic disorder is a major cause of medical attention with substantial social and health service cost. Based on pharmacological studies, research on its etiopathogenesis has been focused on the possible dysfunction of specific neurotransmitter systems. However, recent work has related the genes involved in development, synaptic plasticity and synaptic remodeling to anxiety disorders. This implies that learning processes and changes in perception, interpretation and behavioral responses to environmental stimuli are essential for development of complex anxiety responses secondary to the building of specific brain neural circuits and to adult plasticity. The focus of this review is on progress achieved in identifying genes that confer increased risk for panic disorder through genetic epidemiology and the use of genetically modified mouse models. The integration of human and animal studies targeting behavioral, systems-level, cellular and molecular levels will most probably help identify new molecules with potential impact on the pathogenetic aspects of the disease.

Constitutive Ret activity in knock-in multiple endocrine neoplasia type B mice induces profound elevation of brain dopamine concentration via enhanced synthesis and increases the number of TH-positive cells in the substantia nigra

Ret is the common signaling receptor for glial cell line-derived neurotrophic factor (GDNF) and other ligands of the GDNF family that have potent effects on brain dopaminergic neurons. The Met918Thr mutation leads to constitutive activity of Ret receptor tyrosine kinase, causing the cancer syndrome called multiple endocrine neoplasia type B (MEN2B). We used knock-in MEN2B mice with the Ret-MEN2B mutation to study the effects of constitutive Ret activity on the brain dopaminergic system and found robustly increased concentrations of dopamine (DA) and its metabolites in the striatum, cortex, and hypothalamus. The concentrations of brain serotonin were not affected and those of noradrenaline were slightly increased only in the lower brainstem. Tyrosine hydroxylase (TH) protein levels were increased in the striatum and substantia nigra/ventral tegmental area (SN/VTA), and TH mRNA levels were increased in SN/VTA of MEN2B mice, suggesting that constitutive Ret activity increases DA levels by increasing its synthesis. Also, the striatal DA transporter protein levels in the MEN2B mice were increased, which agrees with increased sensitivity of these mice to the stimulatory effects of cocaine. In the SN pars compacta of homozygous MEN2B mice, we found a 26% increase in the number of TH-positive cells, but no differences were found in the VTA. Thus, we show here that the constitutive Ret activity in mice is sufficient to increase the number of dopaminergic neurons and leads to profound elevation of brain DA concentration. These data clearly suggest that Ret activity per se can have a direct biological function that actively changes and shapes the brain dopaminergic system.

BMPs, FGF8 and Wnts regulate the differentiation of locus coeruleus noradrenergic neuronal precursors

In the present study, we investigated the involvement of rhombomere 1 patterning proteins in the regulation of the major noradrenergic centre of the brain, the locus coeruleus. Primary cultures of rat embryonic day 13.5 locus coeruleus were treated with fibroblast growth factor-8, noggin and members of the bone morphogenetic and Wnt protein families. We show that bone morphogenetic proteins 2, 5 and 7 increase and noggin decreases the number of tyrosine hydroxylase-positive locus coeruleus neurons. Interestingly, from all Wnts expressed in the first rhombomere by embryonic day 12.5 in the mice, we only found expression of wnt5a mRNA in the vicinity of the locus coeruleus. In agreement with this finding, from all Wnts studied in vitro, only Wnt5a increased the number of tyrosine hydroxylase-positive neurons in locus coeruleus cultures. Finally, we also found that fibroblast growth factor-8 increased the number of tyrosine hydroxylase-positive cells in locus coeruleus cultures. Neither of the identified factors affected the survival of tyrosine hydroxylase-positive locus coeruleus noradrenergic neurons or the proliferation of their progenitors or neurogenesis. Instead, our results suggest that these patterning signals of rhombomere 1 may work to promote the differentiation of noradrenergic progenitors at later stages of development.

Transgenic mice overexpressing the full-length neurotrophin receptor TrkC exhibit increased catecholaminergic neuron density in specific brain areas and increased anxiety-like behavior and panic reaction

Accumulating evidence has suggested that neurotrophins participate in the pathophysiology of mood disorders. We have developed transgenic mice overexpressing the full-length neurotrophin-3 receptor TrkC (TgNTRK3) in the central nervous system. TgNTRK3 mice show increased anxiety-like behavior and enhancement of panic reaction in the mouse defense test battery, along with an increase in the number and density of catecholaminergic (tyrosine hydroxylase positive) neurons in locus coeruleus and substantia nigra. Furthermore, treatment of TgNTRK3 mice with diazepam significantly attenuated the anxiety-like behaviors in the plus maze. These results provide evidence for the involvement of TrkC in the development of noradrenergic neurons in the central nervous system with consequences on anxiety-like behavior and panic reaction. Thus, changes in TrkC expression levels could contribute to the phenotypic expression of panic disorder through a trophic effect on noradrenergic neurons in the locus coeruleus. Our results demonstrate that the elevated NT3-TrkC tone via overexpression of TrkC in the brain may constitute a molecular mechanism for the expression of anxiety and anxiety.

Comparative analysis of neurotrophin receptors and ligands in vertebrate neurons: tools for evolutionary stability or changes in neural circuits?

To better understand the role of multiple neurotrophin ligands and their receptors in vertebrate brain evolution, we examined the distribution of trk neurotrophin receptors in representatives of several vertebrate classes. Trk receptors are largely expressed in homologous neuronal populations among different species/classes of vertebrates. In many neurons, trkB and trkC receptors are co-expressed. TrkB and trkC receptors are primarily found in neurons with more restricted, specialized dendritic and axonal fields that are thought to be involved in discriminative or 'analytical' functions. The neurotrophin receptor trkA is expressed predominantly in neurons with larger, overlapping dendritic fields with more heterogeneous connections ('integrative' or 'modulatory' systems) such as nociceptive and sympathetic autonomic nervous system, locus coeruleus and cholinergic basal forebrain. Surveys of trk receptor expression and function in the peripheral nervous system of different vertebrate classes reveal trends ranging from dependency on a single neurotrophin to a more complex dependency on increasing numbers of neurotrophins and their receptors, for example, in taste and inner ear innervation. Gene deletion studies in mice provide evidence for a complex regulation of neuronal survival of sensory ganglion cells by different neurotrophins. Although expression of neurotrophins and their receptors is predominantly conserved in most circuits, increasing diversity of neurotrophin ligands and their receptors and a more complex dependency of neurons on neurotrophins might have facilitated the formation of at least some new neuronal entities.

The phenotypic differentiation of locus ceruleus noradrenergic neurons mediated by brain-derived neurotrophic factor is enhanced by corticotropin releasing factor through the activation of a cAMP-dependent signaling pathway

We have developed a model system of locus ceruleus (LC) neurons in culture, in which brain-derived neurotrophic factor (BDNF) induces the emergence of noradrenergic neurons attested by the presence of tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase and the absence of phenylethanolamine N-methyl-transferase. Although inactive in itself, the neuropeptide corticotropin releasing factor (CRF) strongly amplified the effect of BDNF, increasing the number of cells expressing TH and the active accumulation of noradrenaline by a factor of 2 to 3 via a mechanism that was nonmitogenic. CRF also acted cooperatively with neurotrophin-4, which like BDNF is a selective ligand of the TrkB tyrosine kinase receptor. The effect of CRF but not that of BDNF was prevented by astressin, a nonselective CRF-1/CRF-2 receptor antagonist. However, only CRF-1 receptor transcripts were detectable in LC cultures, suggesting that this receptor subtype mediated the effect of CRF. Consistent with the positive coupling of CRF-1 receptors to adenylate cyclase, the trophic action of CRF was mimicked by cAMP elevating agents. Epac, a guanine nucleotide exchange factor directly activated by cAMP, contributed to the effect of CRF through the stimulation of extracellular signal-regulated kinases (ERKs) 1/2. However, downstream of ERK1/2 activation by CRF, the phenotypic induction of noradrenergic neurons relied upon the stimulation of the phosphatidylinositol-3-kinase/Akt transduction pathway by BDNF. Together, our results suggest that CRF participates to the phenotypic differentiation of LC noradrenergic neurons during development. Whether similar mechanisms account for the high degree of plasticity of these neurons in the adult brain remains to be established.

Neurotrophin receptor heterozygosity causes deficits in catecholaminergic innervation of amygdala and hippocampus in aged mice

We have recently shown that aged mice with haploinsufficiencies for the neurotrophin receptors trkB, trkC or both, trkB and trkC, display reduced cell numbers in the substantia nigra and in the dentate gyrus, but not in the amygdala. Moreover, both hippocampus and amygdala contain increased numbers of degenerated axonal fragments. Consistent with this observation and the expression of trkB and trkC by midbrain dopaminergic neurons, we show now that heterozygous deletion of the trkB or/and trkC receptor genes significantly reduces catecholaminergic, tyrosine hydroxylase (TH-) positive fiber densities in the hippocampus and amygdala mainly in aged (21-23 month old) mice. In the amygdala the phenotype was restricted to the lateral and basolateral nucleus of the amygdala. In adult (6 month old) mice, reductions in catecholaminergic fiber densities were only found in the hippocampal area CA3 and the dentate gyrus of heterozygous trkB and trkB/C mice. Our observations suggest that signaling through trkB and trkC neurotrophin receptors is important for the maintenance of the catecholaminergic innervation of two limbic key regions, the hippocampus and amygdala.

Endogenous noradrenaline affects the maturation and function of the respiratory network: Possible implication for SIDS

Breathing is a vital, rhythmic motor act that is required for blood oxygenation and oxygen delivery to the whole body. Therefore, the brainstem network responsible for the elaboration of the respiratory rhythm must function from the very first moments of extrauterine life. In this review, it is shown that the brainstem noradrenergic system plays a pivotal role in both the modulation and the maturation of the respiratory rhythm generator. Compelling evidence are reported demonstrating that genetically induced alterations of the noradrenergic system in mice affect the prenatal maturation and the perinatal function of the respiratory rhythm generator and have drastic consequences on postnatal survival. Sudden Infant Death Syndrome (SIDS), the leader cause of infant death in industrialised countries, may result from cardiorespiratory disorders during sleep. As several cases of SIDS have been observed in infants having noradrenergic deficits, a possible link between prenatal alteration of the noradrenergic system, altered maturation and function of the respiratory network and SIDS is suggested.

Ret deficiency in mice impairs the development of A5 and A6 neurons and the functional maturation of the respiratory rhythm

Although a normal respiratory rhythm is vital at birth, little is known about the genetic factors controlling the prenatal maturation of the respiratory network in mammals. In Phox2a mutant mice, which do not express A6 neurons, we previously hypothesized that the release of endogenous norepinephrine by A6 neurons is required for a normal respiratory rhythm to occur at birth. Here we investigated the role of the Ret gene, which encodes a transmembrane tyrosine kinase receptor, in the maturation of norepinephrine and respiratory systems. As Ret-null mutants (Ret-/-) did not survive after birth, our experiments were performed in wild-type (wt) and Ret-/- fetuses exteriorized from pregnant heterozygous mice at gestational day 18. First, in wt fetuses, quantitative in situ hybridization revealed high levels of Ret transcripts in the pontine A5 and A6 areas. Second, in Ret-/- fetuses, high-pressure liquid chromatography showed significantly reduced norepinephrine contents in the pons but not the medulla. Third, tyrosine hydroxylase immunocytochemistry revealed a significantly reduced number of pontine A5 and A6 neurons but not medullary norepinephrine neurons in Ret-/- fetuses. Finally, electrophysiological and pharmacological experiments performed on brainstem 'en bloc' preparations demonstrated impaired resting respiratory activity and abnormal responses to central hypoxia and norepinephrine application in Ret-/- fetuses. To conclude, our results show that Ret gene contributes to the prenatal maturation of A6 and A5 neurons and respiratory system. They support the hypothesis that the normal maturation of the respiratory network requires afferent activity corresponding to the A6 excitatory and A5 inhibitory input balance.

Development of pontine noradrenergic A5 neurons requires brain-derived neurotrophic factor

Pontine noradrenergic A5 neurons play a pivotal role in maturation and regulation of the brainstem respiratory rhythm-generating network. Analysis of newborn brain-derived neurotrophic factor (BDNF)-null mice revealed a marked loss of tyrosine hydroxylase-positive A5 neurons compared to wildtype controls that was rescued by null mutation of the proapoptotic gene Bax. In cultures of the A5 region from E12.5 rat embryos, BDNF significantly increased the number and branching of tyrosine hydroxylase-positive neurons. Immunoneutralization of endogenous glial cell line-derived neurotrophic factor partially inhibited the BDNF-dependent increase in the number of tyrosine hydroxylase-positive cells without affecting neurite number. The A5 nucleus is the first brainstem cell group identified at which BDNF is required in vivo for development of neurons critical for cardiorespiratory control.

The neurotransmitter noradrenaline rescues septal cholinergic neurons in culture from degeneration caused by low-level oxidative stress

We have developed a model system in which rat basal forebrain cholinergic neurons degenerate progressively when maintained in culture conditions that make them susceptible to low-level oxidative stress. In this study, we showed that cholinergic neurons identified by acetylcholinesterase cytochemistry or choline acetyl transferase immunocytochemistry are rescued efficiently by the neurotransmitter noradrenaline (NA). The effect of NA required neither adrenoceptor activation nor intracellular accumulation. NA operated via a mechanism that precluded activation of a cell death pathway in which reactive oxygen species (ROS) and proapoptotic caspases were crucially involved. It is noteworthy that NA remained protective even when applied late in the degenerative process but before intracellular ROS began to increase. The high efficacy of iron chelators and catalase in preventing the death of cholinergic neurons in this model suggested that NA neutralized the effects of hydroxyl radicals produced through a Fenton-type reaction. Pyrocatechol [the diphenolic moiety of NA] was sufficient in itself to prevent ROS production and cholinergic cell demise, indicating that the catechol structure was instrumental for the neuroprotective function of NA. Therefore, the noncatecholic neurotransmitter GABA failed to prevent neurodegeneration. Nerve growth factor and brain derived neurotrophic factor, two trophic peptides for septal cholinergic neurons, did not afford protection by themselves and did not improve neuroprotection provided by NA. However, in the presence of NA, they both retained their efficacy to stimulate cholinergic parameters. These data indicate that NA-based therapeutic strategies may be of interest in such neurodegenerative conditions as Alzheimer's disease, where progressive cholinergic deficits occur.

Glial cell line-derived neurotrophic factor (GDNF) is required for differentiation of pontine noradrenergic neurons and patterning of central respiratory output

Genetic mutations affecting signaling by glial cell line-derived neurotrophic factor (GDNF) perturb development of breathing in mice and are associated with congenital central hypoventilation syndrome in humans. However, the role of GDNF in development of brainstem neurons that control breathing is largely unknown. The present study demonstrates that genetic loss of GDNF decreases the number of tyrosine hydroxylase (TH) neurons in the pontine A5 noradrenergic cell group, a major source of inhibitory input to the medullary respiratory pattern generator. This phenotype is associated with a significant increase in the frequency of central respiratory output recorded from the fetal medulla-spinal cord in vitro. In dissociate cultures of the A5 region from rat embryos, GDNF increases TH cell number and neurite growth without affecting total neuronal survival or proliferation of TH neurons. These effects of GDNF are inhibited by function blocking antibodies against endogenous brain-derived neurotrophic factor (BDNF), indicating that GDNF requires BDNF as a cofactor to stimulate differentiation of A5 neurons. Our findings demonstrate that GDNF is required for development of pontine noradrenergic neurons in vivo and indicate that defects in the A5 cell group may contribute to the effects of genetic disruption of GDNF signaling on respiratory control.

Cranial sensory neuron development in the absence of brain-derived neurotrophic factor in BDNF/Bax double null mice

To investigate the role of brain-derived neurotrophic factor (BDNF) in differentiation of cranial sensory neurons in vivo, we analyzed development of nodose (NG), petrosal (PG), and vestibular (VG) ganglion cells in genetically engineered mice carrying null mutations in the genes encoding BDNF and the proapoptotic Bcl-2 homolog Bax. In bax(-/-) mutants, ganglion cell numbers were increased significantly compared to wild-type animals, indicating that naturally occurring cell death in these ganglia is regulated by Bax signaling. Analysis of bdnf(-/-)bax(-/-) mutants revealed that, although the Bax null mutation completely rescued cell loss in the absence of BDNF, it did not rescue the lethality of the BDNF null phenotype. Moreover, despite rescue of BDNF-dependent neurons by the bax null mutation, sensory target innervation was abnormal in double null mutants. Vagal sensory innervation to baroreceptor regions of the cardiac outflow tract was completely absent, and the density of vestibular sensory innervation to the cristae organs was markedly decreased, compared to wild-type controls. Moreover, vestibular afferents failed to selectively innervate their hair cell targets within the cristae organs in the double mutants. These innervation failures occurred despite successful navigation of sensory fibers to the peripheral field, demonstrating that BDNF is required locally for afferent ingrowth into target tissues. In addition, the bax null mutation failed to rescue expression of the dopaminergic phenotype in a subset of NG and PG neurons. These data demonstrate that BDNF signaling is required not only to support survival of cranial sensory neurons, but also to regulate local growth of afferent fibers into target tissues and, in some cells, transmitter phenotypic expression is required.

Ras/MEK pathway is required for NGF-induced expression of tyrosine hydroxylase gene

Neurotrophins are essential for the development and survival of catecholaminergic neurons. However, the critical pathway for expression of the tyrosine hydroxylase (TH) gene induced by neurotrophin is still unclear. Here we found that Ras/MEK pathway is required for NGF-induced expression of the TH gene in PC12D cells. Induction of TH mRNA by NGF was abolished by pretreatment of the cells with U0126, an inhibitor for MEK1/2, but not with inhibitors for p38 MAPK, PI3K, and PKA. U0126 inhibited TH promoter activity at the same concentration as it acted on ERK1/2 phosphorylation. A dominant-negative form of Ras suppressed the NGF-induced activation of the TH reporter gene, and transient transfection of cells with wild-type Ras and an active form of MEK1 increased the TH promoter activity. The reporter assay also demonstrated that the Ras/MEK pathway acted on both the AP-1-binding motif and the cAMP-responsive element in the TH promoter.