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To determine how frequently patients who present to an emergency department (ED) with a retinal artery occlusion (RAO) undergo brain imaging and cardiovascular testing and are hospitalized.Retrospective cross-sectional study.Patients who presented to an ED with an RAO in the National Emergency Department Sample (NEDS), a nationally representative United States database.The NEDS was queried to identify patients who presented to an ED with the primary diagnosis of RAO between 2006 and 2014. Patient and hospital characteristics were evaluated, and a multivariable regression was performed to determine predictors of hospitalization. Testing was categorized into 3 groups: (1) brain imaging performed using computed tomography or magnetic resonance; (2) carotid imaging performed using ultrasound, computed tomography, or magnetic resonance; and (3) cardiac testing performed using electrocardiogram or echocardiogram. The number of tests performed for each category was recorded.Proportions of patients undergoing brain imaging, carotid imaging, or cardiac testing. Rate and predictors of hospitalization.Among 259 343 582 ED visits, 2802 had a primary diagnosis of RAO. Patients were mostly aged ≥65 years (59%) and male (54%). Hypertension (59%), dyslipidemia (36%), and diabetes (20%) were the most common preexisting cardiovascular diseases. Brain imaging, carotid imaging, and cardiac testing were performed in 20.3%, 7.1%, and 23.8% of the patients, respectively; at least 1 test from each of these 3 categories was performed in 4.1% of the patients. Half of the patients were hospitalized. Factors that increased the chances of hospitalization included the following (P0.05): age of45 years; female sex; a history of smoking; presenting to a metropolitan hospital and having giant cell arteritis, carotid artery disease, atrial fibrillation, cardiac valve disease, obesity, dyslipidemia, hypertension, diabetes, and chronic ischemic heart disease.Most patients who presented to an ED with an RAO did not receive emergency brain imaging, carotid imaging, or basic cardiac testing. A multidisciplinary approach is needed to raise awareness that RAOs should be treated as a precursor of stroke or a stroke equivalent.

Reduced expression of brain-derived neurotrophic factor (BDNF) and impaired activation of the BDNF receptor, tropomyosin receptor kinase B (TrkB; also known as Ntrk2), are thought to contribute significantly to the pathophysiology of Rett syndrome (RTT), a severe neurodevelopmental disorder caused by loss-of-function mutations in the X-linked gene encoding methyl-CpG-binding protein 2 (MeCP2). Previous studies from this and other laboratories have shown that enhancing BDNF expression and/or TrkB activation in Mecp2-deficient mouse models of RTT can ameliorate or reverse abnormal neurological phenotypes that mimic human RTT symptoms. The present study reports on the preclinical efficacy of a novel, small-molecule, non-peptide TrkB partial agonist, PTX-BD4-3, in heterozygous female Mecp2 mutant mice, a well-established RTT model that recapitulates the genetic mosaicism of the human disease. PTX-BD4-3 exhibited specificity for TrkB in cell-based assays of neurotrophin receptor activation and neuronal cell survival and in in vitro receptor binding assays. PTX-BD4-3 also activated TrkB following systemic administration to wild-type and Mecp2 mutant mice and was rapidly cleared from the brain and plasma with a half-life of ∼2 h. Chronic intermittent treatment of Mecp2 mutants with a low dose of PTX-BD4-3 (5 mg/kg, intraperitoneally, once every 3 days for 8 weeks) reversed deficits in two core RTT symptom domains – respiration and motor control – and symptom rescue was maintained for at least 24 h after the last dose. Together, these data indicate that significant clinically relevant benefit can be achieved in a mouse model of RTT with a chronic intermittent, low-dose treatment paradigm targeting the neurotrophin receptor TrkB., Editor's choice: Long-term intermittent treatment with a newly developed partial agonist of the TrkB neurotrophin receptor reverses deficits in motor learning and respiration in a mouse model of Rett syndrome.

Lying at the intersection between neurobiology and epigenetics, Rett syndrome (RTT) has garnered intense interest in recent years, not only from a broad range of academic scientists, but also from the pharmaceutical and biotechnology industries. In addition to the critical need for treatments for this devastating disorder, optimism for developing RTT treatments derives from a unique convergence of factors, including a known monogenic cause, reversibility of symptoms in preclinical models, a strong clinical research infrastructure highlighted by an NIH-funded natural history study and well-established clinics with significant patient populations. Here, we review recent advances in understanding the biology of RTT, particularly promising preclinical findings, lessons from past clinical trials, and critical elements of trial design for rare disorders.

In 2007, Adrian Bird and his colleagues at the University of Edinburgh published a groundbreaking study demonstrating that Rett syndrome (RTT), a severe genetic neurodevelopmental disorder, is reversible in mouse models of the disease (1). Using Cre-Lox technology, the Bird group engineered a mouse in which the disease-causing gene, Mecp2, could be reversibly inactivated. Animals born with the gene switched off —a condition that mimics the loss of MECP2 function that underlies the human disease—developed full-blown symptoms of murine RTT. However, when the gene was reactivated in these severely ill mice, neurological dysfunction was reversed to a significant degree. This remarkable finding demonstrated that even prolonged loss of Mecp2 function does not lead to irreversible changes in brain structure or function, consistent with postmortem studies demonstrating the absence of neuronal cell loss or neurodegeneration in RTT patients. Against the backdrop of these encouraging results, there has been a flurry of activity in recent years to develop pharmacologic interventions that could achieve what Bird’s group had accomplished through genetic engineering in mice, i.e., symptom reversal in RTT. RTT is a complex and devastating disorder affecting approximately 1 in 10,000 female individuals worldwide. The complexity of RTT derives from the fact that MECP2 encodes a highly abundant DNA binding protein that influences the expression of many downstream genes and signaling pathways, particularly in the brain. Because MECP2 is an X-linked gene and most disease-causing mutations arise in the father’s germ line, the vast majority of RTT patients are female heterozygotes who are mosaic for normal and mutant MECP2. Once the disorder is fully manifest, which usually occurs within the first few years after birth, patients present with severe abnormalities in motor, respiratory, and autonomic control; loss of speech; increased risk of seizures; and musculoskeletal problems. During early stages of the disease, affected individuals can exhibit autistic-like behaviors, including social withdrawal, which often diminish within a few years. Currently, there are no treatments that ameliorate the pathological effects of MECP2 disruption in RTT, and the burden of care for affected families is enormous. One class of molecules that has shown promise in preclinical models of RTT is channel-blocking N-methyl-D-aspartate receptor (NMDAR) antagonists. Interest in the NMDAR as a therapeutic target in RTT arose from several lines of investigation, beginning with evidence that loss of MECP2 function may result in developmental dysregulation of NMDAR expression [reviewed in Katz et al. (2)]. In 2012, Kron et al. (3) demonstrated that treatment of heterozygous female Mecp2 mutant mice with a subanesthetic dose of ketamine (8 mg/kg) is highly effective at acutely reversing disease phenotypes, including abnormal patterns of neuronal activation in cortical and subcortical structures as well as sensorimotor dysfunction. Recently, another NMDAR channel blocker, of the low-trapping class, was also found effective at ameliorating symptoms in female RTT mice (4), strengthening the case for a therapeutic benefit of NMDAR antagonism. In this issue, Mierau et al. (5) and Patrizi et al. (6) provide further evidence for dysregulation of NMDAR expression (5) and the therapeutic potential of ketamine (6) in mouse models of RTT. Previously, the Fagiolini laboratory reported that loss of Mecp2 is associated with hyperinnervation of pyramidal neurons by gamma-aminobutyric acidergic inhibitory interneurons in visual cortex and postweaning regression in visual acuity. This was accompanied by a shift in the balance of expression in two NMDAR subunits, GluN2A and GluN2B, toward increased GluN2A (7). During development, GluN2B is expressed first during synapse and circuit formation, followed by expression of GluN2A as circuits and synapses stabilize (8). Significantly, rebalancing GluN2A/GluN2B by genetic reduction in GluN2A expression ameliorated the visual acuity regression in the Mecp2 knockout mice, strongly implicating NMDAR dysfunction in this RTT phenotype (7). The elegant study by Mierau et al. (5) builds on these observations to demonstrate that loss of Mecp2 disrupts NMDAR subunit expression in visual cortex in a cell-type selective manner. Specifically, the authors show that male mice that are null for Mecp2 exhibit an acceleration of the normal developmental shift from GluN2B to GluN2A subunits in visual cortical interneurons, whereas, in pyramidal neurons, this shift is retarded. This suggests that the beneficial effect of GluN2A knockdown noted previously (7) may be mediated at the level of cortical interneurons and selective pharmacologic targeting of GluN2A on these interneurons may be a therapeutic strategy. Whether it will be possible to develop cell-type selective GluN2A antagonists is an intriguing question. In addition, it is now clear that NMDAR subunit deployment differs markedly among different brain regions. Whereas visual cortex exhibits a robust developmental shift to a high GluN2A/GluN2B ratio, GluN2B subunits remain highly expressed in the adult prefrontal cortex in rodents and primates, putatively in support of high levels of synaptic plasticity that underlie cognitive functions such as working memory (9). Thus, therapeutic strategies aimed at shifting the balance between GluN2A and GluN2B activity in RTT are likely to impact NMDAR function in different brain regions in different ways, and efforts to develop NMDAR-targeted therapies for RTT will need to take this kind of regional heterogeneity into account. Finally, further studies will be required to understand how these findings in Mecp2 null male mice will translate to the heterozygous condition that characterizes RTT patients. In Patrizi et al. (6), the authors provide important new information on the therapeutic potential of ketamine in RTT by demonstrating that prolonged treatment of preweaning or postweaning Mecp2 nulls—with the same subanesthetic dose of ketamine used acutely in the Kron et al. study (3) (8 mg/kg) —significantly improves visual cortical function and extends lifespan. Possible improvements in other symptoms, including paw clasping and respiration, were also reported but are less clear, as statistical comparisons among saline- and drug-treated mutants are not described. Of particular importance is that positive treatment effects were measured the day after the last ketamine injection, long after the drug is undetectable in the brain. This indicates that transient NMDAR antagonism is sufficient to sustain improvement in this model. In addition, Patrizi et al. (6) provide evidence that prolonged ketamine treatment can ameliorate structural circuit defects that underlie or contribute to neurological dysfunction. Specifically, the authors show that gamma-aminobutyric acidergic hyperinnervation of visual cortical pyramidal neurons is reversed to wildtype levels, concomitant with increased spontaneous and evoked neuronal activity in ketamine-treated Mecp2 null mice. These findings argue strongly for durability of treatment effects at the level of circuit connectivity. Thus, it now appears that ketamine has beneficial effects in mouse RTT models during both drug-on (3) and drug-off (6) periods, and it will be important to determine which kind of activity may have the greatest therapeutic potential in patients. Toward this end, it will also be important to replicate these findings in Mecp2 heterozygotes that mimic the somatic mosaicism for normal and mutant MeCP2 that is characteristic of the human disorder. Potential parallels between the effectiveness of ketamine in mouse models of RTT and major depressive disorder in humans are inescapable, particularly with respect to the possibility of durability of action beyond the acute period of NMDAR antagonism. Trials have already been initiated to test the safety and efficacy of NMDAR antagonists in RTT patients, including dextromethorphan, a weak NMDAR antagonist, and low-dose ketamine [reviewed in Katz et al. (2)]. As the RTT field pursues the potential promise of NMDAR antagonists for the treatment of RTT, caution is also warranted, particularly in the case of ketamine. In addition to psychotomimetic risk, there is some evidence, albeit inconsistent, that ketamine may provoke epileptic discharges in some patients with a seizure disorder (10). Whether or not this will be a concern in RTT patients, who are prone to seizures, is currently unknown. Additionally, the Food and Drug Administration guidance for NMDAR-targeted drugs prescribes a series of specialized toxicology studies that need to be conducted in parallel with a drug’s clinical development program to establish and document the potential risk of neurotoxicity. Therefore, establishment of minimally effective dose and dosing interval will be critical for progressing NMDAR antagonists into juvenile populations. Regardless of whether ketamine itself turns out to be a RTT therapeutic, studies such as those highlighted here provide preclinical proof of concept for the ability of NMDAR antagonists to ameliorate neurological dysfunction and reverse at least some circuit-level defects caused by loss of MeCP2. A key step moving forward will be to identify translatable biomarkers of brain function that are improved by treatment with NMDAR antagonists and can be tested in the clinic. The RTT field seems well poised to respond to this challenge, given that ketamine and related molecules have already been shown to improve such end points in RTT mice (2,4,6) and some, such as respiratory function, are being evaluated in current trials (2). Though much work lies ahead, the progress made thus far in understanding the pathophysiology of RTT and in identifying potential therapeutic targets is truly exciting.

Rett syndrome (RTT) is a severe neurodevelopmental disorder caused by loss-of-function mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2; Amir et al., 1999), a transcriptional regulatory protein (Klose et al., 2005). Mouse models of RTT (Mecp2mutants) exhibit excitatory hypoconnectivity in the medial prefrontal cortex (mPFC; Sceniak et al., 2015), a region critical for functions that are abnormal in RTT patients, ranging from learning and memory to regulation of visceral homeostasis (Riga et al., 2014). The present study was designed to test the hypothesis that increasing the activity of mPFC pyramidal neurons in heterozygous femaleMecp2mutants (Hets) would ameliorate RTT-like symptoms, including deficits in respiratory control and long-term retrieval of auditory conditioned fear. Selective activation of mPFC pyramidal neurons in adult animals was achieved by bilateral infection with an AAV8 vector expressing excitatory hm3D(Gq) DREADD (Designer Receptors Exclusively Activated by Designer Drugs) (Armbruster et al., 2007) under the control of the CamKIIa promoter. DREADD activation inMecp2Hets completely restored long-term retrieval of auditory conditioned fear, eliminated respiratory apneas, and reduced respiratory frequency variability to wild-type (Wt) levels. Reversal of respiratory symptoms following mPFC activation was associated with normalization of Fos protein levels, a marker of neuronal activity, in a subset of brainstem respiratory neurons. Thus, despite reduced levels of MeCP2 and severe neurological deficits, mPFC circuits in Het mice are sufficiently intact to generate normal behavioral output when pyramidal cell activity is increased. These findings highlight the contribution of mPFC hypofunction to the pathophysiology of RTT and raise the possibility that selective activation of cortical regions such as the mPFC could provide therapeutic benefit to RTT patients.

Background The risk of optic disc haemorrhages on visual outcome in idiopathic intracranial hypertension (IIH) is unknown. We report the type and frequency of optic disc haemorrhages and other funduscopic abnormalities at baseline in the study eye of the 133 subjects enrolled in the Idiopathic Intracranial Hypertension Treatment Trial completing 6 months of follow-up. Methods We reviewed optic disc photographs to tabulate the frequency and type of optic disc haemorrhages, other funduscopic abnormalities and papilloedema grades of the study eye at baseline and analyse if their presence is associated with a poor visual outcome. Results 27.2% of subjects had nerve fibre layer haemorrhages in at least one eye. Five of seven, 71% of subjects that met criteria for treatment failure, had nerve fibre layer haemorrhages in at least one eye (Fisher9s exact test: p=0.02). There was a good correlation between presence of nerve fibre layer haemorrhages and Frisen grade (Spearman9s correlation, p=0.002; r=0.271). Subjects with nerve fibre layer haemorrhages had a higher cerebrospinal fluid pressure (40.0 mm water, p=0.04). There was poor correlation between nerve fibre layer haemorrhages at baseline and the perimetric mean deviation change at 6 months. Cotton wool spots were present in 4% of subjects, exudates in 3% and pseudodrusen in 4%. Conclusions Nerve fibre layer haemorrhages are common in patients with IIH with mild visual loss and correlate with the severity of the papilloedema. They occur more frequently in treatment failure subjects and therefore may be associated with poor visual outcomes. Trial registration number NCT01003639, Post-results.

Recent findings in the genetics of neurodevelopmental syndromes have ushered in an exciting era of discovery in which substrates of neurologic dysfunction are being identified at the synaptic and microcircuit levels in mouse models of these disorders. We review recent progress in this area, focusing on two examples of mouse models of autism spectrum disorders (ASDs): Mecp2 models of Rett syndrome, and a Met-knockout model of non-syndromic forms of autism. In both cases, a dominant theme is changes in synaptic strength, associated with hyper-connectivity or hypo-connectivity in specific microcircuits. Alterations in intrinsic neuronal excitability are also found, but do not appear to be as common. The microcircuit-specific nature of synaptic changes observed in these ASD models indicates that it will be necessary to define mechanisms of circuit dysfunction on a case-by-case basis, not only in neocortex but also in brainstem and other sub-cortical areas. Thus, functional microcircuit analysis is emerging as an important line of investigation, highly complementary to neurogenetic and molecular strategies, and holds promise for generating models of the underlying pathophysiology and for guiding development of novel therapeutic strategies.

Brain-derived neurotrophic factor (BDNF) and its receptor, TrkB, are highly expressed in the nucleus tractus solitarius (nTS), the principal target of cardiovascular primary afferent input to the brainstem. However, little is known about the role of BDNF signaling in nTS in cardiovascular homeostasis. We examined whether BDNF in nTS modulates cardiovascular functionin vivoand regulates synaptic and/or neuronal activity in isolated brainstem slices. Microinjection of BDNF into the rat medial nTS (mnTS), a region critical for baroreflex control of sympathetic outflow, produced dose-dependent increases in mean arterial pressure (MAP), heart rate (HR), and lumbar sympathetic nerve activity (LSNA) that were blocked by the tyrosine kinase inhibitor K252a. In contrast, immunoneutralization of endogenous BDNF (anti-BDNF), or microinjection of K252a alone, decreased MAP, HR, and LSNA. The effects of anti-BDNF were abolished by blockade of ionotropic glutamate receptors, indicating a role for glutamate signaling in the response to BDNF.In vitro, BDNF reduced the amplitude of miniature EPSCs as well as solitary tract (TS) evoked EPSC amplitude and action potential discharge (APD) in second-order nTS neurons. BDNF effects on EPSCs were independent of GABAergic signaling and abolished by AMPA receptor blockade. In contrast, K252a increased spontaneous EPSC frequency and TS evoked EPSC amplitude. BDNF also attenuated APD evoked by injection of depolarizing current into second-order neurons, indicating reduced intrinsic neuronal excitability. Our data demonstrate that BDNF signaling in mnTS plays a tonic role in regulating cardiovascular function, likely via modulation of primary afferent glutamatergic excitatory transmission and neural activity.

Postnatal deficits in brain-derived neurotrophic factor (BDNF) are thought to contribute to pathogenesis of Rett syndrome (RTT), a progressive neurodevelopmental disorder caused by mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2). InMecp2-null mice, a model of RTT, BDNF deficits are most pronounced in structures important for autonomic and respiratory control, functions that are severely affected in RTT patients. However, relatively little is known about how these deficits affect neuronal function or how they may be linked to specific RTT endophenotypes. To approach these issues, we analyzed synaptic function in the brainstem nucleus tractus solitarius (nTS), the principal site for integration of primary visceral afferent inputs to central autonomic pathways and a region in which we found markedly reduced levels of BDNF inMecp2mutants. Our results demonstrate that the amplitude of spontaneous miniature and evoked EPSCs in nTS neurons is significantly increased inMecp2-null mice and, accordingly, that mutant cells are more likely than wild- type cells to fire action potentials in response to primary afferent stimulation. These changes occur without any increase in intrinsic neuronal excitability and are unaffected by blockade of inhibitory GABA currents. However, this synaptopathy is associated with decreased BDNF availability in the primary afferent pathway and can be rescued by application of exogenous BDNF. On the basis of these findings, we hypothesize that altered sensory gating in nTS contributes to cardiorespiratory instability in RTT and that nTS is a site at which restoration of normal BDNF signaling could help reestablish normal homeostatic controls.

Rett syndrome (RTT) is a neurodevelopmental disorder caused by loss-of-function mutations in the Methyl-CpG-binding protein-2 (MECP2) gene and is characterized by derangements in cognition, behavior, motor control, respiration and autonomic homeostasis, as well as seizures. Deficits in norepinephrine (NE) are thought to contribute to RTT pathogenesis, but little is known about how MeCP2 regulates function of noradrenergic neurons. We therefore characterized morphological, electrical, and neurochemical properties of neurons in the locus ceruleus (LC), the major source of noradrenergic innervation to the central neuraxis, inMecp2mutant mice. We found that MeCP2 null LC neurons are electrically hyperexcitable, smaller in size, and express less of the NE-synthesizing enzyme tyrosine hydroxylase (TH) compared with wild-type neurons. Increased excitability of mutant neurons is associated with reductions in passive membrane conductance and the amplitude of the slow afterhyperpolarization. Studies inMecp2heterozygotes, which are mosaic for the null allele, demonstrated that electrical hyperexcitability and reduced neuronal size are cell-autonomous consequences of MeCP2 loss, whereas reduced TH expression appears to reflect both cell-autonomous and non-autonomous influences. Finally, we found reduced levels of TH and norepinephrine in cingulate cortex, a forebrain target of the LC. Thus, genetic loss of MeCP2 results in a somewhat paradoxical LC neuron phenotype, characterized by both electrical hyperexcitability and reduced indices of noradrenergic function. Given the importance of the LC in modulating activity in brainstem and forebrain networks, we hypothesize that dysregulation of LC function in the absence of MeCP2 plays a key role in the pathophysiology of RTT.

Rett syndrome (RTT) is a progressive developmental disorder resulting from loss-of-function mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2), a transcription regulatory protein. The RTT phenotype is complex and includes severe cardiorespiratory abnormalities, dysautonomia and behavioral symptoms of elevated stress. These findings have been attributed to an apparent hyperactivity of the sympathetic nervous system due to defects in brainstem development; however, the possibility that the peripheral sympathoadrenal axis itself is abnormal has not been explored. The present study demonstrates that the adrenal medulla and sympathetic ganglia of Mecp2 null mice exhibit markedly reduced catecholamine content compared with wild-type controls. Despite this, null animals exhibit significantly higher plasma epinephrine levels, suggesting enhanced secretory granule function in adrenal chromaffin cells. Indeed, we find that Mecp2 null chromaffin cells exhibit a cell autonomous hypersecretory phenotype characterized by significant increases in the speed and size of individual secretory granule fusion events in response to electrical stimulation. These findings appear to indicate accelerated formation and enhanced dilation of the secretory granule fusion pore, resulting in elevated catecholamine release. Our data therefore highlight abnormal catecholamine function in the sympathoadrenal axis as a potential source of autonomic dysfunction in RTT. These findings may help to explain the apparent 'overactivity' of the sympathetic nervous system reported in patients with RTT.

Severely arrhythmic breathing is a hallmark of Rett syndrome (RTT) and profoundly affects quality of life for patients and their families. The last decade has seen the identification of the disease-causing gene, methyl-CpG-binding protein 2 (Mecp2) and the development of mouse models that phenocopy many aspects of the human syndrome, including breathing dysfunction. Recent studies have begun to characterize the breathing phenotype of Mecp2 mutant mice and to define underlying electrophysiological and neurochemical deficits. The picture that is emerging is one of defects in synaptic transmission throughout the brainstem respiratory network associated with abnormal expression in several neurochemical signaling systems, including brain-derived neurotrophic factor (BDNF), biogenic amines and gamma-amino-butyric acid (GABA). Based on such findings, potential therapeutic strategies aimed at improving breathing by targeting deficits in neurochemical signaling are being explored. This review details our current understanding of respiratory dysfunction and underlying mechanisms in RTT with a particular focus on insights gained from mouse models.

Rett syndrome (RTT) is caused by loss-of-function mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2). Although MeCP2 is thought to act as a transcriptional repressor of brain-derived neurotrophic factor (BDNF),Mecp2null mice, which develop an RTT-like phenotype, exhibit progressive deficits in BDNF expression. These deficits are particularly significant in the brainstem and nodose cranial sensory ganglia (NGs), structures critical for cardiorespiratory homeostasis, and may be linked to the severe respiratory abnormalities characteristic of RTT. Therefore, the present study usedMecp2null mice to further define the role of MeCP2 in regulation of BDNF expression and neural function, focusing on NG neurons and respiratory control. We find that mutant neurons express significantly lower levels of BDNF than wild-type cellsin vitro, asin vivo, under both depolarizing and nondepolarizing conditions. However, BDNF levels in mutant NG cells can be increased by chronic depolarizationin vitroor by treatment ofMecp2null mice with CX546, an ampakine drug that facilitates activation of glutamatergic AMPA receptors. Ampakine-treatedMecp2null mice also exhibit marked functional improvement, characterized by restoration of normal breathing frequency and minute volume. These data demonstrate that BDNF expression remains plastic inMecp2null mice and raise the possibility that ampakine compounds could be of therapeutic value in the treatment of RTT.

Purpose of review Nonarteritic anterior ischemic optic neuropathy (NAION) is the most common cause of an acute optic neuropathy over the age 50 with an annual incidence of 2-10/100 000. Most patients are left with a permanent decrease in visual acuity and visual field loss. No approved treatment has conclusively reversed the process or prevented a second event that typically involves the previously unaffected eye. Many medical and surgical treatments have been proposed with conflicting results. The goal of this review is to present current data in order to permit clinicians and patients to make an educated decision about treatment. Recent findings Recently, there has been a flurry of case reports, small clinical trials and testing in animal models of NAION for various treatments for NAION and this review attempts to present the data concisely with the authors' opinions about the reliability of the data. Summary To date, there is no class I evidence of benefit for the treatment of NAION; however, the aphorism attributed to Carl Sagan, PhD aptly applies: 'Absence of evidence is not evidence of absence'.

Frontal cortical dysfunction is thought to contribute to cognitive and behavioral features of autism spectrum disorders; however, underlying mechanisms are poorly understood. The present study sought to define how loss of Mecp2, the gene mutated in Rett syndrome (RTT), disrupts function in the murine medial prefrontal cortex (mPFC) using acute brain slices and behavioral testing. Compared with wildtype, pyramidal neurons in the Mecp2 null mPFC exhibit significant reductions in excitatory postsynaptic currents, the duration of excitatory UP-states, evoked population activity, and the ratio of NMDA:AMPA currents, as well as an increase in the relative fraction of NR2B currents. These functional changes are associated with reductions in the density of excitatory dendritic spines, the ratio of vesicular glutamate to GABA transporters and GluN1 expression. In contrast to recent reports on circuit defects in other brain regions, we observed no effect of Mecp2 loss on inhibitory synaptic currents or expression of the inhibitory marker parvalbumin. Consistent with mPFC hypofunction, Mecp2 nulls exhibit respiratory dysregulation in response to behavioral arousal. Our data highlight functional hypoconnectivity in the mPFC as a potential substrate for behavioral disruption in RTT and other disorders associated with reduced expression of Mecp2 in frontal cortical regions.

The cerebral microvasculature has recently been identified as a source of factors that can influence the generation and survival of neurons, including brain-derived neurotrophic factor (BDNF). However, relatively little is known about signals that regulate secretion of endothelial cell derived BDNF. To approach this issue the present study examined BDNF secretion from brain endothelial cells in response to reduced oxygen availability (hypoxia), using the mouse brain microvascular endothelial cell line, bEnd.3. We found that exposure of bEnd.3 cells to either sustained or intermittent hypoxia (IH) stimulates BDNF expression and release and that IH is the more potent stimulus. IH-induced BDNF release can be partially inhibited by either N-acetyl-L-cysteine, a scavenger of reactive oxygen species, or by the stable superoxide dismutase mimetic manganese(III)tetrakis1-methyl-4-pyridylporphyrin, indicating that oxyradical formation contributes to enhanced secretion of BDNF. In addition, we found that IH-induced BDNF release requires Ca2+ mobilization from internal stores through ryanodine- and inositol (1,4,5-triphosphate) IP3 receptors and is completely blocked by SKF 96365, a nonselective inhibitor of transient receptor potential (TRP) channels. These data demonstrate that bEnd.3 cells respond to oxidative stress by increasing BDNF secretion and, in addition, highlight TRP channels as potential therapeutic targets for enhancing BDNF availability from the cerebral microvasculature.

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.

Cell-type-specific transcription factors may regulate phenotypic diversity by conferring selective responsiveness to relatively nonspecific environmental cues. To test this hypothesis, we examined whether the homeodomain transcription factors Phox2a/2b play a role in activity-dependent expression of the dopaminergic phenotype using petrosal ganglion (PG) sensory neurons as a model. The timing of Phox2a/2b expression is precisely correlated with the ability of PG neurons to express the dopamine-synthesizing enzyme, tyrosine hydroxylase (TH), in response to depolarizing stimuli. Phox2a/2b expression is highest at embryonic day 16.5, when virtually all PG neurons exhibit activity-dependent TH induction, and subsequently falls in parallel with the loss of activity-dependent TH induction. Expression is maintained, however, in all dopaminergic neurons. Physiologic stimulation of PG neurons in vivo induces TH expression exclusively in Phox2a/2b + cells. Our data suggest that constitutive expression of Phox2a/2b defines the potential of neurons to become dopaminergic in response to membrane depolarization during a critical window of phenotypic plasticity.

Importance To our knowledge, there are no large prospective cohorts of untreated patients with idiopathic intracranial hypertension (IIH) to characterize the disease. Objective To report the baseline clinical and laboratory features of patients enrolled in the Idiopathic Intracranial Hypertension Treatment Trial. Design, Setting, and Participants We collected data at baseline from questionnaires, examinations, automated perimetry, and fundus photography grading. Patients (n = 165) were enrolled from March 17, 2010, to November 27, 2012, at 38 academic and private practice sites in North America. All participants met the modified Dandy criteria for IIH and had a perimetric mean deviation between −2 dB and −7 dB. All but 4 participants were women. Main Outcomes and Measures Baseline and laboratory characteristics. Results The mean (SD) age of our patients was 29.0 (7.4) years and 4 (2.4%) were men. The average (SD) body mass index (calculated as weight in kilograms divided by height in meters squared) was 39.9 (8.3). Headache was the most common symptom (84%). Transient visual obscurations occurred in 68% of patients, back pain in 53%, and pulse synchronous tinnitus in 52%. Only 32% reported visual loss. The average (SD) perimetric mean deviation in the worst eye was −3.5 (1.1) dB, (range, −2.0 to −6.4 dB) and in the best eye was −2.3 (1.1) dB (range, −5.2 to 0.8 dB). A partial arcuate visual field defect with an enlarged blind spot was the most common perimetric finding. Visual acuity was 85 letters or better (20/20) in 71% of the worst eyes and 77% of the best eyes. Quality of life measures, including the National Eye Institute Visual Function Questionnaire–25 and the Short Form–36 physical and mental health summary scales, were lower compared with population norms. Conclusions and Relevance The Idiopathic Intracranial Hypertension Treatment Trial represents the largest prospectively analyzed cohort of untreated patients with IIH. Our data show that IIH is almost exclusively a disease of obese young women. Patients with IIH with mild visual loss have typical symptoms, may have mild acuity loss, and have visual field defects, with predominantly arcuate loss and enlarged blind spots that require formal perimetry for detection. Trial Registration clinicaltrials.gov Identifier:NCT01003639

Activity-dependent neuronal gene expression is thought to require activation of L-type calcium channels, a view based primarily on studies in which chronic potassium (K+) depolarization was used to mimic neuronal activity. However, N-type calcium channels are primarily inactivated during chronic depolarization, and their potential contribution to gene expression induced by physiological patterns of stimulation has not been defined. In the present study, electrical stimulation of dissociated primary sensory neurons at 5 Hz, or treatment with elevated K+, produced a large increase in the percentage of neurons that express tyrosine hydroxylase (TH) mRNA and protein. However, blockade of L-type channels, which completely inhibited K+-induced expression, had no effect on TH expression induced by patterned stimulation. Conversely, blockade of N-type channels completely inhibited TH induction by patterned stimulation, whereas K+-induced expression was unaffected. Similar results were obtained for depolarization-induced expression of the immediate early genesNurr1andNur77. In addition, TH induction by patterned stimulation was significantly reduced by inhibitors of PKA and PKC but was unaffected by inhibition of the mitogen-activated protein kinase (MAPK) pathway. On the other hand, K+-induced TH expression was significantly reduced by inhibition of the MAPK pathway but was unaffected by inhibitors of PKA or PKC. These results demonstrate that N-type calcium channels can directly link phasic membrane depolarization to gene expression, challenging the view that activation of L-type channels is required for nuclear responses to physiological patterns of activity. Moreover, our data show that phasic and chronic depolarizing stimuli act through distinct mechanisms to induce neuronal gene expression.

Null mutations affecting members of the transforming growth factor-β and neurotrophin families result in overlapping patterns of neuronal cell death. This is particularly striking in the cranial sensory nodose-petrosal ganglion complex (NPG), in which loss of either glial cell line-derived neurotrophic factor (GDNF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), or neurotrophin-4 (NT-4) results in a 30–50% reduction in neuronal survival. It is unknown, however, whether GDNF and any single neurotrophin support survival of the same cells, and if so, whether they are required simultaneously or sequentially during development. To approach these issues we defined survival requirements of nodose and petrosal neurons for GDNFin vitroand inbdnf,gdnf, andbdnf/gdnfnull mutant mice, as well as the distribution of GDNF in NPG target tissues. Our analyses focused on the total population of ganglion cells as well as the subset of NPG neurons that are dopaminergic. Neuron losses inbdnf/gdnfdouble mutants are not additive of the losses in singlebdnforgdnfnull mutants, indicating that many cells, including dopaminergic neurons, require both GDNF and BDNF for survivalin vivo. Moreover, both factors are required during the same period of development, between embryonic day (E) 15.5 and E17.5. In addition, GDNF, like BDNF is expressed in target tissues at the time of initial target innervation and coincident with GDNF dependence of the innervating neurons. Together, these findings demonstrate that both GDNF and BDNF can act as target-derived trophic factors and are required simultaneously for survival of some primary sensory neurons.

Brain-derived neurotrophic factor (BDNF) is expressed by many primary sensory neurons that no longer require neurotrophins for survival, indicating that BDNF may be used as a signaling molecule by the afferents themselves. Because many primary afferents also express glutamate, we investigated the possibility that BDNF modulates glutamatergic AMPA responses of newborn second-order sensory relay neurons. Perforated-patch, voltage-clamp recordings were made from dissociated neurons of the brainstem nucleus tractus solitarius (nTS), a region that receives massive primary afferent input from BDNF-containing neurons in the nodose and petrosal cranial sensory ganglia. Electrophysiological analysis was combined in some experiments with anterograde labeling of primary afferent terminals to specifically analyze responses of identified second-order neurons. Our data demonstrate that BDNF strongly inhibits AMPA-mediated currents in a large subset of nTS cells. Specifically, AMPA responses were either completely abolished or markedly inhibited by BDNF in 73% of postnatal day (P0) cells and in 82% of identified P5 second-order sensory relay neurons. This effect of BDNF is mimicked by NT-4, but not NGF, and blocked by the Trk tyrosine kinase inhibitor K252a, consistent with a requirement for TrkB receptor activation. Moreover, analysis of TrkB expression in culture revealed a close correlation between the percentage of nTS neurons in which BDNF inhibits AMPA currents and the percentage of neurons that exhibit TrkB immunoreactivity. These data document a previously undefined mechanism of acute modulation of AMPA responses by BDNF and indicate that BDNF may regulate glutamatergic transmission at primary afferent synapses.

Brain-derived neurotrophic factor (BDNF) supports survival of 50% of visceral afferent neurons in the nodose/petrosal sensory ganglion complex (NPG; Ernfors et al., 1994a; Jones et al., 1994; Conover et al., 1995; Liu et al., 1995; Erickson et al., 1996), including arterial chemoafferents that innervate the carotid body and are required for development of normal breathing (Erickson et al., 1996). However, the relationship between BDNF dependence of visceral afferents and the location and timing of BDNF expression in visceral tissues is unknown. The present study demonstrates that BDNF mRNA and protein are transiently expressed in NPG targets in the fetal cardiac outflow tract, including baroreceptor regions in the aortic arch, carotid sinus, and right subclavian artery, as well as in the carotid body. The period of BDNF expression corresponds to the onset of sensory innervation and to the time at which fetal NPG neurons are BDNF-dependentin vitro. Moreover, baroreceptor innervation is absent in newborn mice lacking BDNF. In addition to vascular targets, vascular afferents themselves express high levels of BDNF, both during and after the time they are BDNF-dependent. However, endogenous BDNF supports survival of fetal NPG neuronsin vitroonly under depolarizing conditions. Together, these data indicate two roles for BDNF during vascular afferent pathway development; initially, as a target-derived survival factor, and subsequently, as a signaling molecule produced by the afferents themselves. Furthermore, the fact that BDNF is required for survival of functionally distinct populations of vascular afferents demonstrates that trophic requirements of NPG neurons are not modality-specific but may instead be associated with innervation of particular organ systems.

Rett syndrome (RTT) is a devastating neurodevelopmental disorder with autistic features caused by loss-of-function mutations in the gene encoding methyl-CpG-binding protein 2 (MECP2), a transcriptional regulatory protein. RTT has attracted widespread attention not only because of the urgent need for treatments, but also because it has become a window into basic mechanisms underlying epigenetic regulation of neuronal genes, including BDNF. In addition, work in mouse models of the disease has demonstrated the possibility of symptom reversal upon restoration of normal gene function. This latter finding has resulted in a paradigm shift in RTT research and, indeed, in the field of neurodevelopmental disorders as a whole, and spurred the search for potential therapies for RTT and related syndromes. In this context, the discovery that expression of BDNF is dysregulated in RTT and mouse models of the disease has taken on particular importance. This chapter reviews the still evolving story of how MeCP2 might regulate expression of BDNF, the functional consequences of BDNF deficits in Mecp2 mutant mice, and progress in developing BDNF-targeted therapies for the treatment of RTT.

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.

Mutations in MECP2, encoding methyl CpG-binding protein 2, cause Rett syndrome, the most severe autism spectrum disorder. Re-expressing Mecp2 in symptomatic Mecp2-null mice markedly improves function and longevity, providing hope that therapeutic intervention is possible in humans. To identify pathways in disease pathology for therapeutic intervention, we carried out a dominant N-ethyl-N-nitrosourea (ENU) mutagenesis suppressor screen in Mecp2-null mice and isolated five suppressors that ameliorate the symptoms of Mecp2 loss. We show that a stop codon mutation in Sqle, encoding squalene epoxidase, a rate-limiting enzyme in cholesterol biosynthesis, underlies suppression in one line. Subsequently, we also show that lipid metabolism is perturbed in the brains and livers of Mecp2-null male mice. Consistently, statin drugs improve systemic perturbations of lipid metabolism, alleviate motor symptoms and confer increased longevity in Mecp2 mutant mice. Our genetic screen therefore points to cholesterol homeostasis as a potential target for the treatment of patients with Rett syndrome.

Rett syndrome, a severe X-linked neurodevelopmental disorder caused by mutations in the gene encoding methyl-CpG-binding protein 2 (Mecp2), is associated with a highly irregular respiratory pattern including severe upper-airway dysfunction. Recent work suggests that hyperexcitability of the Hering-Breuer reflex (HBR) pathway contributes to respiratory dysrhythmia in Mecp2 mutant mice. To assess how enhanced HBR input impacts respiratory entrainment by sensory afferents in closed-loop in vivo-like conditions, we investigated the input (vagal stimulus trains) - output (phrenic bursting) entrainment via the HBR in wild-type and MeCP2-deficient mice. Using the in situ perfused brainstem preparation, which maintains an intact pontomedullary axis capable of generating an in vivo-like respiratory rhythm in the absence of the HBR, we mimicked the HBR feedback input by stimulating the vagus nerve (at threshold current, 0.5 ms pulse duration, 75 Hz pulse frequency, 100 ms train duration) at an inter-burst frequency matching that of the intrinsic oscillation of the inspiratory motor output of each preparation. Using this approach, we observed significant input-output entrainment in wild-type mice as measured by the maximum of the cross-correlation function, the peak of the instantaneous relative phase distribution, and the mutual information of the instantaneous phases. This entrainment was associated with a reduction in inspiratory duration during feedback stimulation. In contrast, the strength of input-output entrainment was significantly weaker in Mecp2 (-/+) mice. However, Mecp2 (-/+) mice also had a reduced inspiratory duration during stimulation, indicating that reflex behavior in the HBR pathway was intact. Together, these observations suggest that the respiratory network compensates for enhanced sensitivity of HBR inputs by reducing HBR input-output entrainment.

To investigate the role of activity-dependent mechanisms in sensory transmitter development, we examined the effect of depolarizing stimuli on tyrosine hydroxylase expression and dopamine synthesis in cells of the fetal rat petrosal ganglion, a model of catecholaminergic sensory neurons. Although dopaminergic traits are normally detectable in only 10-20% of ganglion neurones, exposure to depolarizing concentrations of potassium chloride (40 mM) or veratridine (10 microM) in culture induced tyrosine hydroxylase expression in 100% of petrosal neurons and a 10-fold increase in dopamine content. Tyrosine hydroxylase expression remained elevated in a subset of neurons following return to control conditions, suggesting that chronic depolarization elicits a phenotypic switch in some cells. These data show for the first time that transmitter expression in developing sensory neurons can be regulated by activity-related cues.

The present study examined expression and plasticity of the neuropeptide, galanin, in carotid body afferent neurons in the petrosal ganglion of the adult rat. The pattern of galanin expression was compared with that of tyrosine hydroxylase, a selective marker of dopaminergic carotid body afferents in the petrosal ganglion. In normal animals, only 3% of tyrosine hydroxylase-containing petrosal ganglion neurons co-expressed galanin. Retrograde labeling studies, in which FluoroGold was injected into the vascularly isolated carotid body, demonstrated that all tyrosine hydroxylase-positive-galanin-positive cells in the petrosal ganglion project to this target. In addition, however, we unexpectedly found that galanin expression was markedly increased in the petrosal ganglion following FluoroGold injection into the carotid body. On the other hand, tyrosine hydroxylase expression was unchanged, indicating that monoaminergic and peptidergic traits can be differentially regulated in these cells. In summary, these data demonstrate that monoaminergic chemoafferent neurons can co-express a peptidergic trait, similar to catecholaminergic neurons within the central and autonomic nervous systems, and that these cells retain the potential for phenotypic plasticity in adulthood.

NERVE growth factor and other neurotrophins signal to neurons through the Trk family of receptor tyrosine kinases1–6. TrkB is relatively promiscuous in vitro, acting as a receptor for brain-derived neurotrophic factor (BDNF), neurotrophin-4 (NT4) and, to a lesser extent, NT3 (refs 3–5). Mice lacking TrkB7 show a more severe phenotype than mice lacking BDNF8,9, suggesting that TrkB may act as a receptor for additional ligands in vivo. To explore this possibility, we generated mice lacking NT4 or BDNF as well as mice lacking both neurotrophins. Unlike mice lacking other Trks7,10,11 or neurotrophins8,9,12–14, NT4-deficient mice are long-lived and show no obvious neurological defects. Analysis of mutant phenotypes revealed distinct neuronal populations with different neurotrophin requirements. Thus vestibular and trigemi-nal sensory neurons require BDNF but not NT4, whereas nodose-petrosal sensory neurons require both BDNF and NT4. Motor neurons, whose numbers are drastically reduced in mice lacking TrkB, are not affected even in mice lacking both BDNF and NT4. These results suggest that another ligand, perhaps NT3, does indeed act on TrkB in vivo.

Lectin cytochemistry was used to identify surface epitopes selectively expressed by chromaffin cell chemoreceptors (glomus cells) in the rat carotid body. Unexpectedly, these studies revealed that binding sites for peanut agglutinin (PNA; Arachis hypogea) were highly expressed by all neuroendocrine derivatives of the sympathoadrenal neural crest, including glomus cells, small, intensely fluorescent cells, and adrenal chromaffin cells in situ. In contrast, principal sympathetic neurons did not express PNA receptors. PNA binding was inhibited by 2% galactose. To determine whether expression of PNA receptors was selectively induced by neuroendocrine differentiation of sympathoadrenal precursors, we compared PNA labeling of embryonic sympathoblasts in the presence of either nerve growth factor (NGF) or the synthetic glucocorticoid dexamethasone (DEX). Dex-treated cells, which expressed several neuroendocrine traits, bound PNA, whereas NGF-treated neuronal derivatives did not. In addition, to examine whether expression of existing PNA receptors was down-regulated by neuronal differentiation of chromaffin cells, we compared labeling of PC12 cells, which normally bind PNA, in the presence and absence of NGF. Although PC12 cells acquired characteristic neuronal morphologies in the presence of NGF, they did not lose PNA labeling, even after 8 days of NGF treatment. These findings indicate that neuronal and neuroendocrine derivatives of the sympathoadrenal lineage can be distinguished by differential expression of carbohydrate epitopes and suggest that PNA receptors are induced by neuroendocrine differentiation. © 1995 John Wiley & Sons, Inc.

In September of 2011, the National Institute of Neurological Disorders and Stroke (NINDS), the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), the International Rett Syndrome Foundation (IRSF) and the Rett Syndrome Research Trust (RSRT) convened a workshop involving a broad cross-section of basic scientists, clinicians and representatives from the National Institutes of Health (NIH), the US Food and Drug Administration (FDA), the pharmaceutical industry and private foundations to assess the state of the art in animal studies of Rett syndrome (RTT). The aim of the workshop was to identify crucial knowledge gaps and to suggest scientific priorities and best practices for the use of animal models in preclinical evaluation of potential new RTT therapeutics. This review summarizes outcomes from the workshop and extensive follow-up discussions among participants, and includes: (1) a comprehensive summary of the physiological and behavioral phenotypes of RTT mouse models to date, and areas in which further phenotypic analyses are required to enhance the utility of these models for translational studies; (2) discussion of the impact of genetic differences among mouse models, and methodological differences among laboratories, on the expression and analysis, respectively, of phenotypic traits; and (3) definitions of the standards that the community of RTT researchers can implement for rigorous preclinical study design and transparent reporting to ensure that decisions to initiate costly clinical trials are grounded in reliable preclinical data.

Excitatory-inhibitory imbalance has been identified within specific brain microcircuits in models of Rett syndrome (RTT) and other autism spectrum disorders (ASDs). However, macrocircuit dysfunction across the RTT brain as a whole has not been defined. To approach this issue, we mapped expression of the activity-dependent, immediate-early gene product Fos in the brains of wild-type (Wt) and methyl-CpG-binding protein 2 (Mecp2)-null (Null) mice, a model of RTT, before and after the appearance of overt symptoms (3 and 6 weeks of age, respectively). At 6 weeks, Null mice exhibit significantly less Fos labeling than Wt in limbic cortices and subcortical structures, including key nodes in the default mode network. In contrast, Null mice exhibit significantly more Fos labeling than Wt in the hindbrain, most notably in cardiorespiratory regions of the nucleus tractus solitarius (nTS). Using nTS as a model, whole-cell recordings demonstrated that increased Fos expression in Nulls at 6 weeks of age is associated with synaptic hyperexcitability, including increased frequency of spontaneous and miniature EPSCs and increased amplitude of evoked EPSCs in Nulls. No such effect of genotype on Fos or synaptic function was seen at 3 weeks. In the mutant forebrain, reduced Fos expression, as well as abnormal sensorimotor function, were reversed by the NMDA receptor antagonist ketamine. In light of recent findings that the default mode network is hypoactive in autism, our data raise the possibility that hypofunction within this meta-circuit is a shared feature of RTT and other ASDs and is reversible.

Rett syndrome (RTT) results from loss-of-function mutations in the gene encoding the methyl-CpG-binding protein 2 (MeCP2) and is characterized by abnormal motor, respiratory and autonomic control, cognitive impairment, autistic-like behaviors and increased risk of seizures. RTT patients andMecp2-null mice exhibit reduced expression of brain-derived neurotrophic factor (BDNF), which has been linked in mice to increased respiratory frequency, a hallmark of RTT. The present study was undertaken to test the hypotheses that BDNF deficits inMecp2mutants are associated with reduced activation of the BDNF receptor, TrkB, and that pharmacologic activation of TrkB would improve respiratory function. We characterized BDNF protein expression, TrkB activation and respiration in heterozygous femaleMecp2mutant mice (Het), a model that recapitulates the somatic mosaicism for mutantMECP2found in typical RTT patients, and evaluated the ability of a small molecule TrkB agonist, LM22A-4, to ameliorate biochemical and functional abnormalities in these animals. We found that Het mice exhibit (1) reduced BDNF expression and TrkB activation in the medulla and pons and (2) breathing dysfunction, characterized by increased frequency due to periods of tachypnea, and increased apneas, as in RTT patients. Treatment of Het mice with LM22A-4 for 4 weeks rescued wild-type levels of TrkB phosphorylation in the medulla and pons and restored wild-type breathing frequency. These data provide new insight into the role of BDNF signaling deficits in the pathophysiology of RTT and highlight TrkB as a possible therapeutic target in this disease.

Recent studies in this and other laboratories demonstrated that leukemia inhibitory factor (LIF) can regulate differentiation and survival of nerve growth factor (NGF)-dependent primary sensory neurons. To determine whether development of NGF-independent sensory neurons could be similarly regulated, the present study examined the effects of LIF and the related cytokine-like growth factor, ciliary neurotrophic factor (CNTF), on survival of rat nodose ganglion (NG) cells in culture. In addition, survival in LIF-treated cultures was compared in the presence and absence of the neurotrophins brain-derived neurotrophic factor (BDNF), NT-3, and NT-4, factors previously shown to target NG sensory neurons, to determine whether the cytokine-like factors and neurotrophins act on the same population of ganglion cells. Treatment of dissociate cultures of Embryonic Day (E) 16.5 NG with LIF or CNTF (10 ng/ml) resulted in a four- to fivefold increase in neuronal survival; the number of neurons supported by either factor represented approximately 50% of the total NG population. In contrast, less than 10% of newborn NG neurons survived in the presence of LIF or CNTF, suggesting a loss in responsiveness to these factors during fetal development in vivo. In cultures of E16.5 ganglia, BDNF, NT-3, or NT-4 alone each increased cell survival to varying degrees (BDNF > NT-4 = LIF > NT-3); NGF, on the other hand, had no effect. Combining LIF with BDNF, NT-3, or NT-4 had only partially additive effects on NG neuron number (LIF + BDNF, 36% greater than BDNF alone; LIF + NT-3, 21% greater than LIF alone; LIF + NT-4, 49% greater than NT-4 or LIF alone; P < 0.05), indicating that LIF and these neurotrophins support survival of overlapping subsets of NG neurons in culture. To begin defining whether NG neurons were capable of responding to CNTF and LIF in vivo, Northern blots were used to examine expression of mRNAs coding for CNTF and LIF receptor components (CNTFR alpha, gp130, and LIFR beta) in the intact ganglion. mRNAs for all three receptor components were present in both E16.5 and newborn ganglia, suggesting that LIF and CNTF receptors are expressed in the NG in vivo. Taken together, these data demonstrate that LIF and CNTF can support survival of NGF-independent sensory neurons in culture and indicate that cytokine-like growth factors may play a role in regulating visceral sensory development in vivo.

Hypoxie stimulation of carotid body chemoreceptors is conveyed to the brainstem by primary sensory neurons whose peripheral axons run in the carotid sinus nerve. While considerable attention has focused on defining chemical neuroregulators released by glomus cells in the carotid body, our understanding of the morphology, distribution and transmitter phenotype of these carotid body afferent neurons remains limited. Carotid body afferent neurons were labeled by microinjection of the retrograde tracer, Fluorogold, into the vascularly isolated rat carotid body. In addition, immunoelectron microscopy was used to correlate transmitter phenotype with ultrastructural features of afferent terminals in the carotid body. Our results indicate that 41% of all carotid body afferent neurons express tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis, whereas 7% contain substance P. Tyrosine hydroxylase- and substance P-positive neurons constitute separate subpopulations of carotid body afferents, as these two phenotypes were not colocalized. Most of the tyrosine hydroxylase-containing carotid body afferent neurons were small- or medium-sized (mean cell diameter 15–20 μm) and located in the distal petrosal ganglion, whereas the majority of substance P-containing carotid body afferent neurons were medium- to large-sized (mean cell diameter 20–29 μm) and located in the proximal petrosal ganglion and jugular ganglion. These differences strengthen the notion that these catecholaminergic and peptidergic carotid body afferent neurons give rise to functionally distinct subsets of chemoafferent fibers. To further characterize the catecholaminergic phenotype expressed by tyrosine hydroxylase-positive cells in the petrosal ganglion, we examined the colocalization of tyrosine hydroxylase and DOPA decarboxylase, the dopamine-synthesizing enzyme. Eighty-six per cent of tyrosine hydroxylase-positive neurons in the distal petrosal ganglion also contained DOPA decarboxylase; as these cells do not express the norepinephrine-synthesizing enzyme, dopamine beta-hydroxylase, these data indicate that the catecholaminergic carotid body afferent neurons are dopaminergic. Finally, ultrastructural analysis of the peripheral processes of tyrosine hydroxylase-positive afferent terminals in the carotid body demonstrated endings in close opposition to Type I glomus cells, consistent with a role for dopaminergic afferent neurons in carotid body chemoreception. One possibility is that these cells, in addition to their role as afferents, constitute a morphologic substrate for dopaminergic “efferent” inhibition in the carotid body.

International audience; Disorders of respiratory control are a prominent feature of Rett syndrome (RTT), a severely debilitating condition caused by mutations in the gene encoding methyl-CpG-binding protein 2 (MECP2). RTT patients present with a complex respiratory phenotype that can include periods of hyperventilation, apnea, breath holds terminated by Valsalva maneuvers, forced and deep breathing and apneustic breathing, as well as abnormalities of heart rate control and cardiorespiratory integration. Recent studies of mouse models of RTT have begun to shed light on neurologic deficits that likely contribute to respiratory dysfunction including, in particular, defects in neurochemical signaling resulting from abnormal patterns of neurotransmitter and neuromodulator expression. The authors hypothesize that breathing dysregulation in RTT results from disturbances in mechanisms that modulate the respiratory rhythm, acting either alone or in combination with more subtle disturbances in rhythm and pattern generation. This article reviews the evidence underlying this hypothesis as well as recent efforts to translate our emerging understanding of neurochemical defects in mouse models of RTT into preclinical trials of potential treatments for respiratory dysfunction in this disease.

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