Nitric oxide regulates BDNF release from nodose ganglion neurons in a pattern-dependent and cGMP-independent manner

Mechanisms of vagal plasticity influencing feeding behavior

Sensory neurons of the vagus nerve receive many different peripheral signals that can change rapidly and frequently throughout the day. The ability of these neurons to convey the vast array of nuanced information to the brain requires neuronal adaptability. In this review we discuss evidence for neural plasticity in vagal afferent neurons as a mechanism for conveying nuanced information to the brain important for the control of feeding behavior. We provide evidence that synaptic plasticity, changes in membrane conductance, and neuropeptide specification are mechanisms that allow flexibility in response to metabolic cues that can be disrupted by chronic intake of energy dense diets.

Location and Number of Astrocytes Determine Dopaminergic Neuron Survival and Function Under 6-OHDA Stress Mediated Through Differential BDNF Release

While astrocytes throughout the CNS share many common traits, they exhibit significant differences in function and number among brain regions. The aim of the present study is to assess the effect of region-specificity and number of astrocytes on the survival of dopaminergic neurons under stress, and to understand the possible mechanism by which these astrocytes extend neuroprotection to dopaminergic neurons. Purified astrocytes obtained from forebrain, midbrain, and hindbrain region were characterized through FACS and immunofluorescence. Co-culture experiments (using trans-wells) were then performed to measure the effect of region-specificities and numbers of astrocytes on primary midbrain culture under 6-OHDA stress. Cell survival augmented with an increase in astrocyte seeding number and total cell survival was comparable among the different region-specific astrocytes for all numbers. However, striking differences were observed in dopaminergic neuronal (TH) cell survival in the presence of midbrain astrocytes in comparison to forebrain and hindbrain astrocytes at all seeding numbers. At 75 μM 6-OHDA insult, while cell survival was comparable in purified astrocytes from the different brain regions, a distinct increase in BDNF secretion (significantly higher than its constitutive release) was noted for midbrain astrocytes compared to forebrain and hindbrain astrocytes. The TH immunopositive population decreased when TrkB inhibitor was added to the co-culture under 6-OHDA toxicity, suggesting that BDNF released by co-cultured astrocytes plays a key role in the survival of dopaminergic neurons. This BDNF release decreased in presence of NO inhibitor and increased in the presence of NO donor (DETA/NO). We conclude that the BDNF released from astrocytes under 6-OHDA toxicity is mediated through NO release through both autocrine and paracrine signaling, and this BDNF release is primarily responsible for the differential effect of region-specific astrocytes on TH neuron survival under these conditions.

Nitric Oxide Synthase 1 Modulates Basal and β-Adrenergic-Stimulated Contractility by Rapid and Reversible Redox-Dependent S-Nitrosylation of the Heart

S-nitrosylation of several Ca²⁺ regulating proteins in response to β-adrenergic stimulation was recently described in the heart; however the specific nitric oxide synthase (NOS) isoform and signaling pathways responsible for this modification have not been elucidated. NOS-1 activity increases inotropism, therefore, we tested whether β-adrenergic stimulation induces NOS-1-dependent S-nitrosylation of total proteins, the ryanodine receptor (RyR2), SERCA2 and the L-Type Ca²⁺ channel (LTCC). In the isolated rat heart, isoproterenol (10 nM, 3-min) increased S-nitrosylation of total cardiac proteins (+46±14%) and RyR2 (+146±77%), without affecting S-nitrosylation of SERCA2 and LTCC. Selective NOS-1 blockade with S-methyl-L-thiocitrulline (SMTC) and N^ω-propyl-l-arginine decreased basal contractility and relaxation (−25–30%) and basal S-nitrosylation of total proteins (−25–60%), RyR2, SERCA2 and LTCC (−60–75%). NOS-1 inhibition reduced (−25–40%) the inotropic response and protein S-nitrosylation induced by isoproterenol, particularly that of RyR2 (−85±7%). Tempol, a superoxide scavenger, mimicked the effects of NOS-1 inhibition on inotropism and protein S-nitrosylation; whereas selective NOS-3 inhibitor L-N⁵-(1-Iminoethyl)ornithine had no effect. Inhibition of NOS-1 did not affect phospholamban phosphorylation, but reduced its oligomerization. Attenuation of contractility was abolished by PKA blockade and unaffected by guanylate cyclase inhibition. Additionally, in isolated mouse cardiomyocytes, NOS-1 inhibition or removal reduced the Ca²⁺-transient amplitude and sarcomere shortening induced by isoproterenol or by direct PKA activation. We conclude that 1) normal cardiac performance requires basal NOS-1 activity and S-nitrosylation of the calcium-cycling machinery; 2) β-adrenergic stimulation induces rapid and reversible NOS-1 dependent, PKA and ROS-dependent, S-nitrosylation of RyR2 and other proteins, accounting for about one third of its inotropic effect.

Diagnostic Accuracy of Brain-derived Neurotrophic Factor and Nitric Oxide in Patients with Schizophrenia: A pilot study

Background

Brain-derived neurotrophic factor (BDNF) and nitric oxide (NO) play multiple roles in the developing and adult CNS. Since BDNF and NO metabolisms are dysregulated in schizophrenia, we measured these markers simultaneously in the blood of schizophrenics and assessed their diagnostic accuracy.

Methods

Thirty-eight patients with schizophrenia classified according to demographic characteristics, symptomatologyand therapy and 39 age- and gender-matched healthy controls were enrolled. BDNF was determined by the ELISA technique while the concentration of nitrite/nitrate (NO2−/NO3−) was measured by the colorimetric method.

Results

Serum BDNF levels were significantly lower (20.38±3.73 ng/mL, P = 1.339E-05), whilst plasma NO2−/NO3− concentrations were significantly higher (84.3 (72–121) μmol/L, P=4.357E-08) in patients with schizophrenia than in healthy controls (25.65±4.32 ng/mL; 60.9 (50–76) μmol/L, respectively). The lowest value of BDNF (18.14±3.26 ng/mL) and the highest NO2−/NO3− concentration (115.3 (80–138) μmol/L) were found in patients treated with second-generation antipsychotics (SGA). The patients diseased before the age of 24 and the patients suffering for up to one year had significantly lower serum BDNF levels than those diseased after the age of 24 and the patients who were ill longer than one year. Both BDNF and NO2−/NO3− showed good diagnostic accuracy, but BDNF had better ROC curve characteristics, especially in patients with negative symptomatology.

Conclusions

BDNF and nitrite/nitrate showed inverse changes in schizophrenic patients. The most pronounced changes were found in patients treated with second-generation antipsychotics. Although BDNF is not specific of schizophrenia, it may be a clinically useful biomarker for the diagnosis of patients expressing predominantly negative symptoms.

Cellular mechanisms of activity-dependent BDNF expression in primary sensory neurons

Brain-derived neurotrophic factor (BDNF) is abundantly expressed by both developing and adult rat visceral sensory neurons from the nodose ganglion (NG) in vivo and in vitro. We have previously shown that BDNF is released from neonatal NG neurons by activity and regulates dendritic development in their postsynaptic targets in the brainstem. The current study was carried out to examine the cellular and molecular mechanisms of activity-dependent BDNF expression in neonatal rat NG neurons, using our established in vitro model of neuronal activation by electrical field stimulation with patterns that mimic neuronal activity in vivo. We show that BDNF mRNA (transcript 4) increases over threefold in response to a 4-h tonic or bursting pattern delivered at the frequency of 6 Hz, which corresponds to the normal heart rate of a newborn rat. No significant increase in BDNF expression was observed following stimulation at 1 Hz. The latter effect suggests a frequency-dependent mechanism of regulated BDNF expression. In addition to BDNF transcript 4, which is known to be regulated by activity, transcript 1 also showed significant upregulation. The increases in BDNF mRNA were followed by BDNF protein upregulation of a similar magnitude after 24h of stimulation at 6 Hz. Electrical stimulation-evoked BDNF expression was inhibited by pretreating neurons with the blocker of voltage-gated sodium channels tetrodotoxin and by removing extracellular calcium. Moreover, our data show that repetitive stimulation-evoked BDNF expression requires calcium influx through N-, but not L-type, channels. Together, our study reveals novel mechanisms through which electrical activity stimulates de novo synthesis of BDNF in sensory neurons, and points to the role of N-type calcium channels in regulating BDNF expression in sensory neurons in response to repetitive stimulation.

cAMP-mediated secretion of brain-derived neurotrophic factor in developing airway smooth muscle

Moderate hyperoxic exposure in preterm infants contributes to subsequent airway dysfunction and to risk of developing recurrent wheeze and asthma. The regulatory mechanisms that can contribute to hyperoxia-induced airway dysfunction are still under investigation. Recent studies in mice show that hyperoxia increases brain-derived neurotrophic factor (BDNF), a growth factor that increases airway smooth muscle (ASM) proliferation and contractility. We assessed the mechanisms underlying effects of moderate hyperoxia (50% O2) on BDNF expression and secretion in developing human ASM. Hyperoxia increased BDNF secretion, but did not alter endogenous BDNF mRNA or intracellular protein levels. Exposure to hyperoxia significantly increased [Ca2+]i responses to histamine, an effect blunted by the BDNF chelator TrkB-Fc. Hyperoxia also increased ASM cAMP levels, associated with reduced PDE4 activity, but did not alter protein kinase A (PKA) activity or adenylyl cyclase mRNA levels. However, 50% O2 increased expression of Epac2, which is activated by cAMP and can regulate protein secretion. Silencing RNA studies indicated that Epac2, but not Epac1, is important for hyperoxia-induced BDNF secretion, while PKA inhibition did not influence BDNF secretion. In turn, BDNF had autocrine effects of enhancing ASM cAMP levels, an effect inhibited by TrkB and BDNF siRNAs. Together, these novel studies suggest that hyperoxia can modulate BDNF secretion, via cAMP-mediated Epac2 activation in ASM, resulting in a positive feedback effect of BDNF-mediated elevation in cAMP levels. The potential functional role of this pathway is to sustain BDNF secretion following hyperoxic stimulus, leading to enhanced ASM contractility and proliferation.

The Physiology of BDNF and Its Relationship with ADHD

Brain-derived neurotrophic factor (BDNF) is a major neurotrophin in the central nervous system that plays a critical role in the physiological brain functions via its two independent receptors: tropomyosin-related kinase B (TrkB) and p75, especially in the neurodevelopment. Disrupting of BDNF and its downstream signals has been found in many neuropsychological diseases, including attention-deficit hyperactivity disorder (ADHD), a common mental disorder which is prevalent in childhood. Understanding the physiological functions of BDNF during neural development and its potential relationship with ADHD will help us to elucidate the possible mechanisms of ADHD and to develop therapeutic approaches for this disease. In this review, we summarized the important literatures for the physiological functions of BDNF in the neurodevelopment. We also performed an association study on the functional genetic variation of BDNF and ADHD by a case-control study in the Chinese mainland population and revealed the potential correlation between BDNF and ADHD which needs further research to confirm.

Putative roles of neuropeptides in vagal afferent signaling

The vagus nerve is a major pathway by which information is communicated between the brain and peripheral organs. Sensory neurons of the vagus are located in the nodose ganglia. These vagal afferent neurons innervate the heart, the lung and the gastrointestinal tract, and convey information about peripheral signals to the brain important in the control of cardiovascular tone, respiratory tone, and satiation, respectively. Glutamate is thought to be the primary neurotransmitter involved in conveying all of this information to the brain. It remains unclear how a single neurotransmitter can regulate such an extensive list of physiological functions from a wide range of visceral sites. Many neurotransmitters have been identified in vagal afferent neurons and have been suggested to modulate the physiological functions of glutamate. Specifically, the anorectic peptide transmitters, cocaine and amphetamine regulated transcript (CART) and the orexigenic peptide transmitters, melanin concentrating hormone (MCH) are differentially regulated in vagal afferent neurons and have opposing effects on food intake. Using these two peptides as a model, this review will discuss the potential role of peptide transmitters in providing a more precise and refined modulatory control of the broad physiological functions of glutamate, especially in relation to the control of feeding.

Upregulation of brain-derived neurotrophic factor expression in nodose ganglia and the lower brainstem of hypertensive rats

Hypertension leads to structural and functional changes at baroreceptor synapses in the medial nucleus tractus solitarius (NTS), but the underlying molecular mechanisms remain unknown. Our previous studies show that brain-derived neurotrophic factor (BDNF) is abundantly expressed by rat nodose ganglion (NG) neurons, including baroreceptor afferents and their central terminals in the medial NTS. We hypothesized that hypertension leads to upregulation of BDNF expression in NG neurons. To test this hypothesis, we used two mechanistically distinct models of hypertension, the spontaneously hypertensive rat (SHR) and the deoxycorticosterone acetate (DOCA)-salt rat. Young adult SHRs, whose blood pressure was significantly elevated compared with age-matched Wistar-Kyoto (WKY) control rats, exhibited dramatic upregulation of BDNF mRNA and protein in the NG. BDNF transcripts from exon 4, known to be regulated by activity, and exon 9 (protein-coding region) showed the largest increases. Electrical stimulation of dispersed NG neurons with patterns that mimic baroreceptor activity during blood pressure elevations led to increases in BDNF mRNA that were also mediated through promoter 4. The increase in BDNF content of the NG in vivo was associated with a significant increase in the percentage of BDNF-immunoreactive NG neurons. Moreover, upregulation of BDNF in cell bodies of NG neurons was accompanied by a significant increase in BDNF in the NTS region, the primary central target of NG afferents. A dramatic increase in BDNF in the NG was also detected in DOCA-salt hypertensive rats. Together, our study identifies BDNF as a candidate molecular mediator of activity-dependent changes at baroafferent synapses during hypertension.

Interactions between the NO-citrulline cycle and brain-derived neurotrophic factor in differentiation of neural stem cells

The diffusible messenger NO plays multiple roles in neuroprotection, neurodegeneration, and brain plasticity. Argininosuccinate synthase (AS) is a ubiquitous enzyme in mammals and the key enzyme of the NO-citrulline cycle, because it provides the substrate L-arginine for subsequent NO synthesis by inducible, endothelial, and neuronal NO synthase (NOS). Here, we provide evidence for the participation of AS and of the NO-citrulline cycle in the progress of differentiation of neural stem cells (NSC) into neurons, astrocytes, and oligodendrocytes. AS expression and activity and neuronal NOS expression, as well as l-arginine and NO(x) production, increased along neural differentiation, whereas endothelial NOS expression was augmented in conditions of chronic NOS inhibition during differentiation, indicating that this NOS isoform is amenable to modulation by extracellular cues. AS and NOS inhibition caused a delay in the progress of neural differentiation, as suggested by the decreased percentage of terminally differentiated cells. On the other hand, BDNF reversed the delay of neural differentiation of NSC caused by inhibition of NO(x) production. A likely cause is the lack of NO, which up-regulated p75 neurotrophin receptor expression, a receptor required for BDNF-induced differentiation of NSC. We conclude that the NO-citrulline cycle acts together with BDNF for maintaining the progress of neural differentiation.

Tumor necrosis factor-α increases brain-derived neurotrophic factor expression in trigeminal ganglion neurons in an activity-dependent manner

Many chronic trigeminal pain conditions, such as migraine or temporo-mandibular disorders, are associated with inflammation within peripheral endings of trigeminal ganglion (TG) sensory neurons. A critical role in mechanisms of neuroinflammation is attributed to proinflammatory cytokines, such as interleukin-1β and tumor necrosis factor-α (TNFα) that also contribute to mechanisms of persistent neuropathic pain resulting from nerve injury. However, the mechanisms of cytokine-mediated synaptic plasticity and nociceptor sensitization are not completely understood. In the present study, we examined the effects of TNFα on neuronal expression of brain-derived neurotrophic factor (BDNF), whose role in synaptic plasticity and sensitization of nociceptive pathways is well documented. We show that 4- and 24-h treatment with TNFα increases BDNF mRNA and protein, respectively, in neuron-enriched dissociated cultures of rat TG. TNFα increases the phosphorylated form of the cyclic AMP-responsive element binding protein (CREB), a transcription factor involved in regulation of BDNF expression in neurons, and activates transcription of BDNF exon IV (former exon III) and, to a lesser extent, exon VI (former exon IV), but not exon I. TNFα-mediated increase in BDNF expression is accompanied by increase in calcitonin gene-related peptide (CGRP), which is consistent with previously published studies, and indicates that both peptides are similarly regulated in TG neurons by inflammatory mediators. The effect of TNFα on BDNF expression is dependent on sodium influx through TTX-sensitive channels and on p38-mitogen-activated protein kinase. Moreover, electrical stimulation and forskolin, known to increase intracellular cAMP, potentiate the TNFα-mediated upregulation of BDNF expression. This study provides new evidence for a direct action of proinflammatory cytokines on TG primary sensory neurons, and reveals a mechanism through which TNFα stimulates de novo synthesis of BDNF in these neurons. Thus, TNFα should be considered in mechanisms of BDNF-dependent neuronal plasticity.