BDNF up-regulates evoked GABAergic transmission in developing hippocampus by potentiating presynaptic N- and P/Q-type Ca2+ channels signalling

Extracellular molecular signals shaping dendrite architecture during brain development

Proper growth and branching of dendrites are crucial for adequate central nervous system (CNS) functioning. The neuronal dendritic geometry determines the mode and quality of information processing. Any defects in dendrite development will disrupt neuronal circuit formation, affecting brain function. Besides cell-intrinsic programmes, extrinsic factors regulate various aspects of dendritic development. Among these extrinsic factors are extracellular molecular signals which can shape the dendrite architecture during early development. This review will focus on extrinsic factors regulating dendritic growth during early neuronal development, including neurotransmitters, neurotrophins, extracellular matrix proteins, contact-mediated ligands, and secreted and diffusible cues. How these extracellular molecular signals contribute to dendritic growth has been investigated in developing nervous systems using different species, different areas within the CNS, and different neuronal types. The response of the dendritic tree to these extracellular molecular signals can result in growth-promoting or growth-limiting effects, and it depends on the receptor subtype, receptor quantity, receptor efficiency, the animal model used, the developmental time windows, and finally, the targeted signal cascade. This article reviews our current understanding of the role of various extracellular signals in the establishment of the architecture of the dendrites.

The Role of Ryanodine Receptors in Regulating Neuronal Activity and Its Connection to the Development of Alzheimer's Disease

Research into the early impacts of Alzheimer's disease (AD) on synapse function is one of the most promising approaches to finding a treatment. In this context, we have recently demonstrated that the Abeta42 peptide, which builds up in the brain during the processing of the amyloid precursor protein (APP), targets the ryanodine receptors (RyRs) of mouse hippocampal neurons and potentiates calcium (Ca²⁺) release from the endoplasmic reticulum (ER). The uncontrolled increase in intracellular calcium concentration ([Ca²⁺]_i), leading to the development of Ca²⁺ dysregulation events and related excitable and synaptic dysfunctions, is a consolidated hallmark of AD onset and possibly other neurodegenerative diseases. Since RyRs contribute to increasing [Ca²⁺]_i and are thought to be a promising target for AD treatment, the goal of this review is to summarize the current level of knowledge regarding the involvement of RyRs in governing neuronal function both in physiological conditions and during the onset of AD.

The ryanodine receptor-calstabin interaction stabilizer S107 protects hippocampal neurons from GABAergic synaptic alterations induced by Abeta42 oligomers

The oligomeric form of the peptide amyloid beta 42 (Abeta42) contributes to the development of synaptic abnormalities and cognitive impairments associated with Alzheimer's disease (AD). To date, there is a gap in knowledge regarding how Abeta42 alters the elementary parameters of GABAergic synaptic function. Here we found that Abeta42 increased the frequency and amplitude of miniature GABAergic currents as well as the amplitude of evoked inhibitory postsynaptic currents. When we focused on paired pulse depression (PPD) to establish whether GABA release probability was affected by Abeta42, we did not observe any significant change. On the other hand, a more detailed investigation of the presynaptic effects induced by Abeta42 by means of multiple probability fluctuation analysis and cumulative amplitude analysis showed an increase in both the size of the readily releasable pool responsible for synchronous release and the number of release sites. We further explored whether ryanodine receptors (RyRs) contributed to exacerbating these changes by stabilizing the interaction between RyRs and the accessory protein calstabin. We observed that the RyR-calstabin interaction stabilizer S107 restored the synaptic parameters to values comparable to those measured in control conditions. In conclusion, our results clarify the mechanisms of potentiation of GABAergic synapses induced by Abeta42. We further suggest that RyRs are involved in the control of synaptic activity during the early stage of AD onset and that their stabilization could represent a new therapeutical approach for AD treatment. KEY POINTS: Accumulation of the peptide amyloid beta 42 (Abeta42) is a key characteristic of Alzheimer's disease (AD) and causes synaptic dysfunctions. To date, the effects of Abeta42 accumulation on GABAergic synapses are poorly understood. The findings reported here suggest that, similarly to what is observed on glutamatergic synapses, Abeta42 modifies GABAergic synapses by targeting ryanodine receptors and causing calcium dysregulation. The GABAergic impairments can be restored by the ryanodine receptor-calstabin interaction stabilizer S107. Based on this research, RyRs stabilization may represent a novel pharmaceutical strategy for preventing or delaying AD.

REST/NRSF drives homeostatic plasticity of inhibitory synapses in a target-dependent fashion

The repressor-element 1-silencing transcription/neuron-restrictive silencer factor (REST/NRSF) controls hundreds of neuron-specific genes. We showed that REST/NRSF downregulates glutamatergic transmission in response to hyperactivity, thus contributing to neuronal homeostasis. However, whether GABAergic transmission is also implicated in the homeostatic action of REST/NRSF is unknown. Here, we show that hyperactivity-induced REST/NRSF activation, triggers a homeostatic rearrangement of GABAergic inhibition, with increased frequency of miniature inhibitory postsynaptic currents (IPSCs) and amplitude of evoked IPSCs in mouse cultured hippocampal neurons. Notably, this effect is limited to inhibitory-onto-excitatory neuron synapses, whose density increases at somatic level and decreases in dendritic regions, demonstrating a complex target- and area-selectivity. The upscaling of perisomatic inhibition was occluded by TrkB receptor inhibition and resulted from a coordinated and sequential activation of the Npas4 and Bdnf gene programs. On the opposite, the downscaling of dendritic inhibition was REST-dependent, but BDNF-independent. The findings highlight the central role of REST/NRSF in the complex transcriptional responses aimed at rescuing physiological levels of network activity in front of the ever-changing environment.

Partial Activation of TrkB Receptors Corrects Interneuronal Calcium Channel Dysfunction and Reduces Epileptogenic Activity in Neocortex following Injury

Decreased GABAergic inhibition due to dysfunction of inhibitory interneurons plays an important role in post-traumatic epileptogenesis. Reduced N-current Ca2+ channel function in GABAergic terminals contributes to interneuronal abnormalities and neural circuit hyperexcitability in the partial neocortical isolation (undercut, UC) model of post-traumatic epileptogenesis. Because brain-derived neurotrophic factor (BDNF) supports the development and maintenance of interneurons, we hypothesized that the activation of BDNF tropomyosin kinase B (TrkB) receptors by a small molecule, TrkB partial agonist, PTX BD4-3 (BD), would correct N channel abnormalities and enhance inhibitory synaptic transmission in UC cortex. Immunocytochemistry (ICC) and western blots were used to quantify N- and P/Q-type channels. We recorded evoked (e)IPSCs and responses to N and P/Q channel blockers to determine the effects of BD on channel function. Field potential recordings were used to determine the effects of BD on circuit hyperexcitability. Chronic BD treatment 1) upregulated N and P/Q channel immunoreactivity in GABAergic terminals; 2) increased the effects of N or P/Q channel blockade on evoked inhibitory postsynaptic currents (eIPSCs); 3) increased GABA release probability and the frequency of sIPSCs; and 4) reduced the incidence of epileptiform discharges in UC cortex. The results suggest that chronic TrkB activation is a promising approach for rescuing injury-induced calcium channel abnormalities in inhibitory terminals, thereby improving interneuronal function and suppressing circuit hyperexcitability.

Amyloid Beta42 oligomers up-regulate the excitatory synapses by potentiating presynaptic release while impairing postsynaptic NMDA receptors

KEY POINTS

NMDA receptors (NMDARs) are key molecules for controlling neuronal plasticity, learning and memory processes. Their function is impaired during Alzheimer's disease (AD) but the exact consequence on synaptic function is not yet fully identified. An important hallmark of AD onset is represented by the neuronal accumulation of Amyloid Beta42 oligomers (Abeta42) that we have recently shown to be responsible for the increased intracellular Ca²⁺ concentration through ryanodine receptors (RyRs). Here we characterized the effects of Abeta42 on NMDA synapses showing specific pre- and post-synaptic functional changes that lead to a potentiation of basal and synchronous NMDA synaptic transmission. These overall effects can be abolished by decreasing Ca²⁺ release from RyRs with specific inhibitors that we propose as new pharmacological tools for AD treatment.

ABSTRACT

We have recently shown that Amyloid Beta42 oligomers (Abeta42) cause calcium dysregulation in hippocampal neurons by stimulating Ca²⁺ release from ryanodine receptors (RyRs) and inhibiting Ca²⁺ entry through NMDA receptors (NMDARs). Here, we found that Abeta42 decrease the average NMDA-activated inward current and that Ca²⁺ entry through NMDARs is accompanied by Ca²⁺ release from the stores. The overall amount of intraellular Ca²⁺ concentration([Ca²⁺ ]_i ) increase during NMDA application is 50% associated with RyR opening and 50% with NMDARs activation. Addition of Abeta42 does not change this proportion. We estimated the number of NMDARs expressed in hippocampal neurons and their unitary current. We found that Abeta42 decrease the number of NMDARs without altering their unitary current. Paradoxically, the oligomer increases the size of electrically evoked eEPSCs induced by NMDARs activation. We found that this is the consequence of the increased release probability (p) of glutamate and the number of release sites (N) of NMDA synapses, while the quantal size (q) is significantly decreased as expected from the decreased number of NMDARs. An increased number of release sites induced by Abeta42 is also supported by the increased size of the ready releasable pool (RRPsyn) and by the enhanced percentage of paired pulse depression (PPD). Interestingly, the RyRs inhibitor dantrolene prevents the increase of PPD induced by Abeta42 oligomers. In conclusion, Abeta42 up-regulates NMDA synaptic responses with a mechanism involving RyRs that occurs during the early stages of Alzheimer's disease (AD) onset. This suggests that new selective modulators of RyRs may be useful for designing effective therapies to treat AD patients.

Inhibitory Plasticity: From Molecules to Computation and Beyond

Synaptic plasticity is the cellular and molecular counterpart of learning and memory and, since its first discovery, the analysis of the mechanisms underlying long-term changes of synaptic strength has been almost exclusively focused on excitatory connections. Conversely, inhibition was considered as a fixed controller of circuit excitability. Only recently, inhibitory networks were shown to be finely regulated by a wide number of mechanisms residing in their synaptic connections. Here, we review recent findings on the forms of inhibitory plasticity (IP) that have been discovered and characterized in different brain areas. In particular, we focus our attention on the molecular pathways involved in the induction and expression mechanisms leading to changes in synaptic efficacy, and we discuss, from the computational perspective, how IP can contribute to the emergence of functional properties of brain circuits.

p140Cap Regulates GABAergic Synaptogenesis and Development of Hippocampal Inhibitory Circuits

The neuronal scaffold protein p140Cap was investigated during hippocampal network formation. p140Cap is present in presynaptic GABAergic terminals and its genetic depletion results in a marked alteration of inhibitory synaptic activity. p140Cap-/- cultured neurons display higher frequency of miniature inhibitory postsynaptic currents (mIPSCs) with no changes of their mean amplitude. Consistent with a potential presynaptic alteration of basal GABA release, p140Cap-/- neurons exhibit a larger synaptic vesicle readily releasable pool, without any variation of single GABAA receptor unitary currents and number of postsynaptic channels. Furthermore, p140Cap-/- neurons show a premature and enhanced network synchronization and appear more susceptible to 4-aminopyridine-induced seizures in vitro and to kainate-induced seizures in vivo. The hippocampus of p140Cap-/- mice showed a significant increase in the number of both inhibitory synapses and of parvalbumin- and somatostatin-expressing interneurons. Specific deletion of p140Cap in forebrain interneurons resulted in increased susceptibility to in vitro epileptic events and increased inhibitory synaptogenesis, comparable to those observed in p140Cap-/- mice. Altogether, our data demonstrate that p140Cap finely tunes inhibitory synaptogenesis and GABAergic neurotransmission, thus regulating the establishment and maintenance of the proper hippocampal excitatory/inhibitory balance.

Loss of TrkB Signaling in Parvalbumin-Expressing Basket Cells Results in Network Activity Disruption and Abnormal Behavior

The GABAergic system is regulated by the brain-derived neurotrophic factor (BDNF)/Tropomyosin-related kinase B (TrkB) pathway, but the cell-intrinsic role of TrkB signaling in parvalbumin cortical interneuron development and function is unclear. We performed conditional ablation of the TrkB receptor in parvalbumin-expressing (PV) interneurons to study whether postnatal loss of TrkB in parvalbumin cells affects their survival, connectivity, spontaneous and evoked neuronal activity and behavior. Using in vivo recordings of local field potentials, we found reduced gamma oscillations in the sensory cortex of PVcre+; TrkBF/F conditional knockout mice (TrkB cKO), along with increased firing of putative excitatory neurons. There was a significant downregulation in parvalbumin neuron number in cerebral and cerebellar cortices of TrkB cKO mice. In addition, inhibitory synaptic connections between basket cells and pyramidal neurons were profoundly reduced in the neocortex of TrkB cKO mice and there was a loss of cortical volume. TrkB cKO mice also showed profound hyperactivity, stereotypies, motor deficits and learning/memory defects. Our findings demonstrate that the targeting and/or synapse formation of PV-expressing basket cells with principal excitatory neurons require TrkB signaling in parvalbumin cells. Disruption of this signaling has major consequences for parvalbumin interneuron connectivity, network dynamics, cognitive and motor behavior.

Early Alterations of Hippocampal Neuronal Firing Induced by Abeta42

We studied the effect of Amyloid β 1-42 oligomers (Abeta42) on Ca2+ dependent excitability profile of hippocampal neurons. Abeta42 is one of the Amyloid beta peptides produced by the proteolytic processing of the amyloid precursor protein and participates in the initiating event triggering the progressive dismantling of synapses and neuronal circuits. Our experiments on cultured hippocampal network reveal that Abeta42 increases intracellular Ca2+ concentration by 46% and inhibits firing discharge by 19%. More precisely, Abeta42 differently regulates ryanodine (RyRs), NMDA receptors (NMDARs), and voltage gated calcium channels (VGCCs) by increasing Ca2+ release through RyRs and inhibiting Ca2+ influx through NMDARs and VGCCs. The overall increased intracellular Ca2+ concentration causes stimulation of K+ current carried by big conductance Ca2+ activated potassium (BK) channels and hippocampal network firing inhibition. We conclude that Abeta42 alters neuronal function by means of at least 4 main targets: RyRs, NMDARs, VGCCs, and BK channels. The development of selective modulators of these channels may in turn be useful for developing effective therapies that could enhance the quality of life of AD patients during the early onset of the pathology.

Related biomarkers of neurocognitive impairment in children with obstructive sleep apnea

OBJECTIVES

Different experiment approaches have demonstrated that children with obstructive sleep apnea (OSA) exhibit neurocognitive and behavioral deficits. This review summarized the potential biomarkers of OSA-associated neurocognitive impairment in children.

METHODS

A scoping review of studies on children with OSA that evaluated the potential value of different markers in identifying neurocognitive impairment was undertaken. Additionally, the biomarkers were categorized according to the different research methods, including brain imaging studies, serological indicators and urine markers.

RESULTS

Majority of the studies that evaluated blood biomarkers, plasma insulin growth factor-1 (IGF-1) and Alzheimer's disease (AD)-related biomarkers appeared to exhibit a favorable profile, and could discriminate between OSA children with or without neurocognitive impairments. Brain imaging studies and urinary neurotransmitters could also be helpful for screening OSA cognitive morbidity in children.

CONCLUSION

Due to limited research methods available in children, the cognitive susceptibility of children with OSA has been rarely studied. The main reason for this may be the limited research methods in children. Numerous study populations of children and complex psychological tests are required, which involve major labor and costs.Multi-center prospective studies are needed to identify suitable biomarkers for the timely prediction and effective intervention to prevent neurocognitive impairment in children with OSA and to explore further opportunities in this arena.

New Insights into Microglia-Neuron Interactions: A Neuron's Perspective

Microglia are the primary immune cells of the central nervous system. However, recent data indicate that microglia also contribute to diverse physiological and pathophysiological processes that extend beyond immune-related functions and there is a growing interest to understand the mechanisms through which microglia interact with other cells in the brain. In particular, the molecular processes that contribute to microglia-neuron communication in the healthy brain and their role in common brain diseases have been intensively studied during the last decade. In line with this, fate-mapping studies, genetic models and novel pharmacological approaches have revealed the origin of microglial progenitors, demonstrated the role of self-maintaining microglial populations during brain development or in adulthood, and identified the unexpectedly long lifespan of microglia that may profoundly change our view about senescence and age-related human diseases. Despite the exponentially increasing knowledge about microglia, the role of these cells in health and disease is still extremely controversial and the precise molecular targets for intervention are not well defined. This is in part due to the lack of microglia-specific manipulation approaches until very recently and to the high level of complexity of the interactions between microglia and other cells in the brain that occur at different temporal and spatial scales. In this review, we briefly summarize the known physiological roles of microglia-neuron interactions in brain homeostasis and attempt to outline some major directions and challenges of future microglia research.

Proliferative hippocampal activity in a group of patients with Rasmussen's encephalitis: Neuronal, glial, and BDNF tissue expression correlations

Rasmussen's encephalitis (RE) is a rare and devastating unilateral inflammatory brain disease that causes severe and intractable partial epilepsy. It has been shown that epilepsy and subsequent inflammation have deleterious influence on hippocampal cell survival and neurogenesis, but this still has not been systematically explored in human tissue. In this study, we investigated the correlation between inflammation and epilepsy as well as the rates of hippocampal gliogenesis and neurogenesis in a pediatric group of six patients with RE and six control cases. The dentate gyrus (DG) samples were obtained from patients who underwent surgery for intractable RE. Sections were processed for immunohistochemistry using antibodies against sex determining region Y-box 2 (Sox2), nestin, human protein encoded by MKI67 gen (Ki67), and brain-derived neurotrophic factor (BDNF). There was an increase in the number of Ki67-positive granule cells in the DG of patients with RE in comparison with the autopsy control group, but no statistical difference for Sox2-positive cells was observed between these groups. Nestin immunolabeling was less intense in the RE group while BDNF expression was increased. Neurons that were BDNF-positive were found in DG from patients with RE but not in the control group. In patients with RE, few nestin-positive cells in DG were also positive for BDNF, unlike in controls which showed no colocalization for these two markers. These results suggest a proliferation activity in the DG subfield of patients with RE, and also future studies are necessary to address the role of new cells in the hippocampus of patients with RE.

Nanodiamonds-induced effects on neuronal firing of mouse hippocampal microcircuits

Fluorescent nanodiamonds (FND) are carbon-based nanomaterials that can efficiently incorporate optically active photoluminescent centers such as the nitrogen-vacancy complex, thus making them promising candidates as optical biolabels and drug-delivery agents. FNDs exhibit bright fluorescence without photobleaching combined with high uptake rate and low cytotoxicity. Focusing on FNDs interference with neuronal function, here we examined their effect on cultured hippocampal neurons, monitoring the whole network development as well as the electrophysiological properties of single neurons. We observed that FNDs drastically decreased the frequency of inhibitory (from 1.81 Hz to 0.86 Hz) and excitatory (from 1.61 to 0.68 Hz) miniature postsynaptic currents, and consistently reduced action potential (AP) firing frequency (by 36%), as measured by microelectrode arrays. On the contrary, bursts synchronization was preserved, as well as the amplitude of spontaneous inhibitory and excitatory events. Current-clamp recordings revealed that the ratio of neurons responding with AP trains of high-frequency (fast-spiking) versus neurons responding with trains of low-frequency (slow-spiking) was unaltered, suggesting that FNDs exerted a comparable action on neuronal subpopulations. At the single cell level, rapid onset of the somatic AP (“kink”) was drastically reduced in FND-treated neurons, suggesting a reduced contribution of axonal and dendritic components while preserving neuronal excitability.

Partial TrkB receptor activation suppresses cortical epileptogenesis through actions on parvalbumin interneurons

Post-traumatic epilepsy is one of the most common and difficult to treat forms of acquired epilepsy worldwide. Currently, there is no effective way to prevent post-traumatic epileptogenesis. It is known that abnormalities of interneurons, particularly parvalbumin-containing interneurons, play a critical role in epileptogenesis following traumatic brain injury. Thus, enhancing the function of existing parvalbumin interneurons might provide a logical therapeutic approach to prevention of post-traumatic epilepsy. The known positive effects of brain-derived neurotrophic factor on interneuronal growth and function through activation of its receptor tropomyosin receptor kinase B, and its decrease after traumatic brain injury, led us to hypothesize that enhancing trophic support might improve parvalbumin interneuronal function and decrease epileptogenesis. To test this hypothesis, we used the partial neocortical isolation ('undercut', UC) model of posttraumatic epileptogenesis in mature rats that were treated for 2 weeks, beginning on the day of injury, with LM22A-4, a newly designed partial agonist at the tropomyosin receptor kinase B. Effects of treatment were assessed with Western blots to measure pAKT/AKT; immunocytochemistry and whole cell patch clamp recordings to examine functional and structural properties of GABAergic interneurons; field potential recordings of epileptiform discharges in vitro; and video-EEG recordings of PTZ-induced seizures in vivo. Results showed that LM22A-4 treatment 1) increased pyramidal cell perisomatic immunoreactivity for VGAT, GAD65 and parvalbumin; 2) increased the density of close appositions of VGAT/gephyrin immunoreactive puncta (putative inhibitory synapses) on pyramidal cell somata; 3) increased the frequency of mIPSCs in pyramidal cells; and 4) decreased the incidence of spontaneous and evoked epileptiform discharges in vitro. 5) Treatment of rats with PTX BD4-3, another partial TrkB receptor agonist, reduced the incidence of bicuculline-induced ictal episodes in vitro and PTZ induced electrographic and behavioral ictal episodes in vivo. 6) Inactivation of TrkB receptors in undercut TrkB^F616A mice with 1NMPP1 abolished both LM22A-4-induced effects on mIPSCs and on increased perisomatic VGAT-IR. Results indicate that chronic activation of the tropomyosin receptor kinase B by a partial agonist after cortical injury can enhance structural and functional measures of GABAergic inhibition and suppress posttraumatic epileptogenesis. Although the full agonist effects of brain-derived neurotrophic factor and tropomyosin receptor kinase B activation in epilepsy models have been controversial, the present results indicate that such trophic activation by a partial agonist may potentially serve as an effective therapeutic option for prophylactic treatment of posttraumatic epileptogenesis, and treatment of other neurological and psychiatric disorders whose pathogenesis involves impaired parvalbumin interneuronal function.

Activation of brain-derived neurotrophic factor-tropomyosin receptor kinase B signaling in the pedunculopontine tegmental nucleus: a novel mechanism for the homeostatic regulation of rapid eye movement sleep

Rapid eye movement (REM) sleep dysregulation is a symptom of many neuropsychiatric disorders, yet the mechanisms of REM sleep homeostatic regulation are not fully understood. We have shown that, after REM sleep deprivation, the pedunculopontine tegmental nucleus (PPT) plays a critical role in the generation of recovery REM sleep. In this study, we used multidisciplinary techniques to show a causal relationship between brain-derived neurotrophic factor (BDNF)-tropomyosin receptor kinase B (TrkB) signaling in the PPT and the development of REM sleep homeostatic drive. Rats were randomly assigned to conditions of unrestricted sleep or selective REM sleep deprivation (RSD) with PPT microinjections of vehicle control or a dose of a TrkB receptor inhibitor (2, 3, or 4 nmol K252a or 4 nmol ANA-12). On experimental days, rats received PPT microinjections and their sleep-wake physiological signals were recorded for 3 or 6 h, during which selective RSD was performed in the first 3 h. At the end of all 3 h recordings, rats were killed and the PPT was dissected out for BDNF quantification. Our results show that K252a and ANA-12 dose-dependently reduced the homeostatic responses to selective RSD. Specifically, TrkB receptor inhibition reduced REM sleep homeostatic drive and limited REM sleep rebound. There was also a dose-dependent suppression of PPT BDNF up-regulation, and regression analysis revealed a significant positive relationship between REM sleep homeostatic drive and the level of PPT BDNF expression. These data provide the first direct evidence that activation of BDNF-TrkB signaling in the PPT is a critical step for the development of REM sleep homeostatic drive.

Effects of fingolimod administration in a genetic model of cognitive deficits

Notwithstanding recent advances, cognitive impairments are among the most difficult-to-treat symptoms in neuropsychiatric disorders. Deficits in information processing contributing to memory and sociability impairments are found across neuropsychiatric-related disorders. Previously, we have shown that mutations in the DTNBP1 gene (encoding dystrobrevin-binding protein 1 [dysbindin-1]) lead to abnormalities in synaptic glutamate release in the prefrontal cortex (PFC) and hippocampus and to cognitive deficits; glutamatergic transmission is important for cortical recurrent excitation that allows information processing in the PFC. To investigate possible means of restoring glutamate release and improving cognitive impairments, we assess the effects of increasing endogenous levels of brain-derived neurotrophic factor (BDNF) in a dysbindin-1-deficient mouse model. Increasing endogenous levels of BDNF may aid in remediating cognitive deficits, given the roles of BDNF in synaptic transmission, plasticity, and neuroprotection. To increase BDNF, we use a novel strategy, repeated intraperitoneal injections of fingolimod (Gilenya). Sphingolipids have recently been shown to have therapeutic value in several neurology-related disorders. Both wild-type (WT) and mutant (MUT) genotypes were tested for sociability and recognition memory, followed by measuring endogenous BDNF levels and presynaptic [Ca²⁺ ]_i within the PFC. Both genotypes were treated for 1 week with either saline or fingolimod. Relative to WT mice, MUT mice demonstrated impairments in sociability and recognition memory and lower presynaptic calcium. After fingolimod treatment, MUT mice exhibited significant improvements in sociability and recognition memory and increases in presynaptic calcium and endogenous concentrations of BDNF. These results show promise for counteracting the cognitive impairments seen in neuropsychiatric disorders and may shed light on the role of dysbindin-1. © 2016 Wiley Periodicals, Inc.

Enhanced BDNF signalling following chronic hypoxia potentiates catecholamine release from cultured rat adrenal chromaffin cells

KEY POINTS

We investigated the role of the neurotrophin BDNF signalling via the TrkB receptor in rat adrenomedullary chromaffin cells (AMCs) exposed to normoxia (Nox; 21% O2) and chronic hypoxia (CHox; 2% O2) in vitro for ∼ 48 h. TrkB receptor expression was upregulated in primary AMCs and in immortalized chromaffin (MAH) cells exposed to CHox; this effect was absent in MAH cells deficient in the transcription factor, hypoxia inducible factor (HIF)-2α. Relative to normoxic controls, activation of the TrkB receptor in chronically hypoxic AMCs led to a marked increase in membrane excitability, intracellular [Ca(2+)], and catecholamine secretion. The BDNF-induced rise of intracellular [Ca(2+)] in CHox cells was sensitive to the selective T-type Ca(2+) channel blocker TTA-P2 and tetrodotoxin (TTX), suggesting key roles of low threshold T-type Ca(2+) and voltage-gated Na(+) channels in the signalling pathway. Environmental stressors, including chronic hypoxia, enhance the ability of adrenomedullary chromaffin cells (AMCs) to secrete catecholamines; however, the underlying molecular mechanisms remain unclear. Here, we investigated the role of brain-derived neurotrophic factor (BDNF) signalling in rat AMCs exposed to chronic hypoxia. In rat adrenal glands, BDNF and its tropomyosin-related kinase B (TrkB) receptor are highly expressed in the cortex and medulla, respectively. Exposure of AMCs to chronic hypoxia (2% O2; 48 h) in vitro caused a significant increase to TrkB mRNA expression. A similar increase was observed in an immortalized chromaffin cell line (MAH cells); however, it was absent in MAH cells deficient in the transcription factor HIF-2α. A specific TrkB agonist, 7,8-dihydroxyflavone (7,8-DHF), stimulated quantal catecholamine secretion from chronically hypoxic (CHox; 2% O2) AMCs to a greater extent than normoxic (Nox; 21% O2) controls. Activation of TrkB by BDNF or 7,8-DHF increased intracellular Ca(2+) ([Ca(2+)]i), an effect that was significantly larger in CHox cells. The 7,8-DHF-induced [Ca(2+)]i rise was sensitive to the tyrosine kinase inhibitor K252a and nickel (2 mm), but not the Ca(2+) store-depleting agent cyclopiazonic acid. Blockade of T-type calcium channels with TTA-P2 (1 μm) or voltage-gated Na(+) channels with TTX inhibited BDNF-induced [Ca(2+)]i increases. BDNF also induced a dose-dependent enhancement of action potential firing in CHox cells. These data demonstrate that during chronic hypoxia, enhancement of BDNF-TrkB signalling increases voltage-dependent Ca(2+) influx and catecholamine secretion in chromaffin cells, and that T-type Ca(2+) channels play a key role in the signalling pathway.

Stepping Out of the Shade: Control of Neuronal Activity by the Scaffold Protein Kidins220/ARMS

The correct functioning of the nervous system depends on the exquisitely fine control of neuronal excitability and synaptic plasticity, which relies on an intricate network of protein-protein interactions and signaling that shapes neuronal homeostasis during development and in adulthood. In this complex scenario, Kinase D interacting substrate of 220 kDa/ankyrin repeat-rich membrane spanning (Kidins220/ARMS) acts as a multi-functional scaffold protein with preferential expression in the nervous system. Engaged in a plethora of interactions with membrane receptors, cytosolic signaling components and cytoskeletal proteins, Kidins220/ARMS is implicated in numerous cellular functions including neuronal survival, neurite outgrowth and maturation and neuronal activity, often in the context of neurotrophin (NT) signaling pathways. Recent studies have highlighted a number of cell- and context-specific roles for this protein in the control of synaptic transmission and neuronal excitability, which are at present far from being completely understood. In addition, some evidence has began to emerge, linking alterations of Kidins220 expression to the onset of various neurodegenerative diseases and neuropsychiatric disorders. In this review, we present a concise summary of our fragmentary knowledge of Kidins220/ARMS biological functions, focusing on the mechanism(s) by which it controls various aspects of neuronal activity. We have tried, where possible, to discuss the available evidence in the wider context of NT-mediated regulation, and to outline emerging roles of Kidins220/ARMS in human pathologies.

Combination of early and delayed ischemic postconditioning enhances brain-derived neurotrophic factor production by upregulating the ERK-CREB pathway in rats with focal ischemia

Ischemic postconditioning, including early and delayed ischemic postconditioning, has been recognized as a simple and promising strategy in the treatment of stroke. However, the effects of the combination of early and delayed ischemic postconditioning, and the mechanisms underlying these effects, remain unclear. The aim of the present study was to determine whether the combination of early and delayed ischemic postconditioning offers greater protection against stroke, and enhances the production of brain‑derived neurotrophic factor (BDNF). A combination of early and delayed ischemic postconditioning was established by repeated, transient occlusion and reperfusion of the ipsilateral common carotid artery in a rat model of middle cerebral artery occlusion. Infarct size, motor function, cerebral blood flow and brain edema were then evaluated, in order to confirm the effects of combinative ischemic postconditioning. TUNEL staining was used to analyze the rate of apoptosis of cells in the penumbral area. BDNF, extracellular signal‑regulated kinases 1/2 (ERK1/2) and cAMP response element‑binding protein (CREB) expression was detected using immunofluorescence staining and western blot analysis. The results of the present study indicated that the combination of early and delayed ischemic postconditioning further reduced the infarct volume, stabilized cerebral blood disturbance and attenuated neuronal apoptosis, compared with either alone. However, combinative postconditioning exerted the same effect on neurological function and brain edema, compared with early or delayed ischemic postconditioning alone. Further investigation indicated that combinative ischemic postconditioning increased the expression of BDNF, and a significantly higher number of BDNF‑positive cells was observed in neurons and astrocytes from the combined group than in the early or delayed groups. Combinative ischemic postconditioning also induced the phosphorylation of ERK1/2 and CREB in the cortex, following focal ischemia. The results of the present study suggest that the combination of early and delayed ischemic postconditioning may further reduce brain ischemic reperfusion injury following focal ischemia, compared with either treatment alone. In addition, it induces the production of BDNF in neurons and astrocytes. Furthermore, the effects of combinative ischemic postconditioning may be mediated by the activation of ERK1/2 and CREB.

Bud extracts from Tilia tomentosa Moench inhibit hippocampal neuronal firing through GABAA and benzodiazepine receptors activation

ETHNOPHARMACOLOGICAL RELEVANCE

Tilia tomentosa Moench bud extracts (TTBEs) is used in traditional medicine for centuries as sedative compound. Different plants belonging to the Tilia genus have shown their efficacy in the treatment of anxiety but still little is known about the mechanism of action of their bud extracts.

AIM OF THE STUDY

To evaluate the action of TTBEs as anxiolytic and sedative compound on in vitro hippocampal neurons.

MATERIAL AND METHODS

The anxiolytic effect of TTBEs was assayed by testing the effects of these compounds on GABAA receptor-activated chloride current of hippocampal neurons by means of the patch-clamp technique and microelectrode-arrays (MEAs).

RESULTS

TTBEs acutely administered on mouse hippocampal neurons, activated a chloride current comparable to that measured in the presence of GABA (100 µM). Bicuculline (100 µM) and picrotoxin (100 µM) blocked about 90% of this current, while the remaining 10% was blocked by adding the benzodiazepine (BDZ) antagonist flumazenil (30 µM). Flumazenil alone blocked nearly 60% of the TTBEs activated current, suggesting that TTBEs binds to both GABAA and BDZ receptor sites. Application of high-doses of TTBEs on spontaneous active hippocampal neurons grown for 3 weeks on MEAs blocked the synchronous activity of these neurons. The effects were mimicked by GABA and prevented by picrotoxin (100µM) and flumazenil (30 µM). At minimal doses, TTBEs reduced the frequency of synchronized bursts and increased the cross-correlation index of synchronized neuronal firing.

CONCLUSIONS

Our data suggest that TTBEs mimics GABA and BDZ agonists by targeting hippocampal GABAergic synapses and inhibiting network excitability by increasing the strength of inhibitory synaptic outputs. Our results contribute toward the validation of TTBEs as effective sedative and anxiolytic compound.

Adolescent maturation of inhibitory inputs onto cingulate cortex neurons is cell-type specific and TrkB dependent

The maturation of inhibitory circuits during adolescence may be tied to the onset of mental health disorders such as schizophrenia. Neurotrophin signaling likely plays a critical role in supporting inhibitory circuit development and is also implicated in psychiatric disease. Within the neocortex, subcircuits may mature at different times and show differential sensitivity to neurotrophin signaling. We measured miniature inhibitory and excitatory postsynaptic currents (mIPSCs and mEPSCs) in Layer 5 cell-types in the mouse anterior cingulate (Cg) across the periadolescent period. We differentiated cell-types mainly by Thy1 YFP transgene expression and also retrobead injection labeling in the contralateral Cg and ipsilateral pons. We found that YFP− neurons and commissural projecting neurons had lower frequency of mIPSCs than neighboring YFP+ neurons or pons projecting neurons in juvenile mice (P21–25). YFP− neurons and to a lesser extent commissural projecting neurons also showed a significant increase in mIPSC amplitude during the periadolescent period (P21–25 vs. P40–50), which was not seen in YFP+ neurons or pons projecting neurons. Systemic disruption of tyrosine kinase receptor B (TrkB) signaling during P23–50 in TrkB_F616A mice blocked developmental changes in mIPSC amplitude, without affecting miniature excitatory post synaptic currents (mEPSCs). Our data suggest that the maturation of inhibitory inputs onto Layer 5 pyramidal neurons is cell-type specific. These data may inform our understanding of adolescent brain development across species and aid in identifying candidate subcircuits that may show greater vulnerability in mental illness.

New insight in expression, transport, and secretion of brain-derived neurotrophic factor: Implications in brain-related diseases

Brain-derived neurotrophic factor (BDNF) attracts increasing attention from both research and clinical fields because of its important functions in the central nervous system. An adequate amount of BDNF is critical to develop and maintain normal neuronal circuits in the brain. Given that loss of BDNF function has been reported in the brains of patients with neurodegenerative or psychiatric diseases, understanding basic properties of BDNF and associated intracellular processes is imperative. In this review, we revisit the gene structure, transcription, translation, transport and secretion mechanisms of BDNF. We also introduce implications of BDNF in several brain-related diseases including Alzheimer’s disease, Huntington’s disease, depression and schizophrenia.

BDNF enhances spontaneous and activity-dependent neurotransmitter release at excitatory terminals but not at inhibitory terminals in hippocampal neurons

Brain-derived neurotrophic factor (BDNF) is widely reported to enhance synaptic vesicle (SV) exocytosis and neurotransmitter release. But it is still unclear whether BDNF enhances SV recycling at excitatory terminals only, or at both excitatory and inhibitory terminals. In the present study, in a direct comparison using cultured rat hippocampal neurons, we demonstrate that BDNF enhances both spontaneous and activity-dependent neurotransmitter release from excitatory terminals, but not from inhibitory terminals. BDNF treatment for 5 min or 48 h increased both spontaneous and activity-induced anti-synaptotagmin1 (SYT1) antibody uptake at excitatory terminals marked with vGluT1. Conversely, BDNF treatment did not enhance spontaneous or activity-induced uptake of anti-SYT1 antibodies in inhibitory terminals marked with vGAT. Time-lapse imaging of FM1-43 dye destaining in excitatory and inhibitory terminals visualized by post-hoc immunostaining of vGluT1 and vGAT also showed the same result: The rate of spontaneous and activity-induced destaining was increased by BDNF at excitatory synapses, but not at inhibitory synapses. These data demonstrate that BDNF enhances SV exocytosis in excitatory but not inhibitory terminals. Moreover, BDNF enhanced evoked SV exocytosis, even if vesicles were loaded under spontaneous vesicle recycling conditions. Thus, BDNF enhances both spontaneous and activity-dependent neurotransmitter release on both short and long time-scales, by the same mechanism.

Defects in dendrite and spine maturation and synaptogenesis associated with an anxious-depressive-like phenotype of GABAA receptor-deficient mice

Mice that were rendered heterozygous for the γ2 subunit of GABAA receptors (γ2(+/-) mice) have been characterized extensively as a model for major depressive disorder. The phenotype of these mice includes behavior indicative of heightened anxiety, despair, and anhedonia, as well as defects in hippocampus-dependent pattern separation, HPA axis hyperactivity and increased responsiveness to antidepressant drugs. The γ2(+/-) model thereby provides strong support for the GABAergic deficit hypothesis of major depressive disorder. Here we show that γ2(+/-) mice additionally exhibit specific defects in late stage survival of adult-born hippocampal granule cells, including reduced complexity of dendritic arbors and impaired maturation of synaptic spines. Moreover, cortical γ2(+/-) neurons cultured in vitro show marked deficits in GABAergic innervation selectively when grown under competitive conditions that may mimic the environment of adult-born hippocampal granule cells. Finally, brain extracts of γ2(+/-) mice show a numerical but insignificant trend (p = 0.06) for transiently reduced expression of brain derived neurotrophic factor (BDNF) at three weeks of age, which might contribute to the previously reported developmental origin of the behavioral phenotype of γ2(+/-) mice. The data indicate increasing congruence of the GABAergic, glutamatergic, stress-based and neurotrophic deficit hypotheses of major depressive disorder.

Gephyrin plays a key role in BDNF-dependent regulation of amygdala surface GABAARs

Brain-derived neurotrophic factor (BDNF) is critically involved in synaptic plasticity and neurotransmission. Our lab has previously found that BDNF activation of neurotrophic tyrosine kinase, receptor, type 2 (TrkB) is required for fear memory formation and that GABAA receptor (GABAAR) subunits and the GABAA clustering protein gephyrin are dynamically regulated during fear memory consolidation. We hypothesize that TrkB-dependent internalization of GABAARs may partially underlie a transient period of amygdala hyperactivation during fear memory consolidation. We have previously reported that BDNF modulates GABAAR α1 subunit sequestration in cultured hippocampal and amygdala neurons by differential phosphorylation pathways. At present, no studies have investigated the regulation of gephyrin and GABAAR α1 subunits following BDNF activation in the amygdala. In this study, we confirm the association of GABAAR α1 and γ2 subunits with gephyrin on mouse amygdala neurons by coimmunoprecipitation and immunocytochemistry. We then demonstrate that rapid BDNF treatment, as well as suppression of gephyrin protein levels on amygdala neurons, induced sequestration of surface α1 subunits. Further, we find that rapid exposure of BDNF to primary amygdala cultures produced decreases in gephyrin levels, whereas longer exposure resulted in an eventual increase. While total α1 subunit levels remained unchanged, gephyrin was downregulated in whole cell homogenates, but enhanced in complexes with GABAARs. Our data with anisomycin suggest that BDNF may rapidly induce gephyrin protein degradation, with subsequent gephyrin synthesis occurring. Together, these findings suggest that gephyrin may be a key factor in BDNF-dependent GABAAR regulation in the amygdala. This work may inform future studies aimed at elucidating the pathways connecting BDNF, GABAA systems, gephyrin, and their role in underlying amygdala-dependent learning.

Microglia control neuronal network excitability via BDNF signalling

Microglia-neuron interactions play a crucial role in several neurological disorders characterized by altered neural network excitability, such as epilepsy and neuropathic pain. While a series of potential messengers have been postulated as substrates of the communication between microglia and neurons, including cytokines, purines, prostaglandins, and nitric oxide, the specific links between messengers, microglia, neuronal networks, and diseases have remained elusive. Brain-derived neurotrophic factor (BDNF) released by microglia emerges as an exception in this riddle. Here, we review the current knowledge on the role played by microglial BDNF in controlling neuronal excitability by causing disinhibition. The efforts made by different laboratories during the last decade have collectively provided a robust mechanistic paradigm which elucidates the mechanisms involved in the synthesis and release of BDNF from microglia, the downstream TrkB-mediated signals in neurons, and the biophysical mechanism by which disinhibition occurs, via the downregulation of the K⁺-Cl⁻ cotransporter KCC2, dysrupting Cl⁻ homeostasis, and hence the strength of GABA(A)- and glycine receptor-mediated inhibition. The resulting altered network activity appears to explain several features of the associated pathologies. Targeting the molecular players involved in this canonical signaling pathway may lead to novel therapeutic approach for ameliorating a wide array of neural dysfunctions.

Interneuronal calcium channel abnormalities in posttraumatic epileptogenic neocortex

Decreased release probability (Pr) and increased failure rate for monosynaptic inhibitory postsynaptic currents (IPSCs) indicate abnormalities in presynaptic inhibitory terminals on pyramidal (Pyr) neurons of the undercut (UC) model of posttraumatic epileptogenesis. These indices of inhibition are normalized in high [Ca++] ACSF, suggesting dysfunction of Ca2+ channels in GABAergic terminals. We tested this hypothesis using selective blockers of P/Q and N-type Ca2+ channels whose activation underlies transmitter release in cortical inhibitory terminals. Pharmacologically isolated monosynaptic IPSCs were evoked in layer V Pyr cells by extracellular stimuli in adult rat sensorimotor cortical slices. Local perfusion of 0.2/1 μM ω-agatoxin IVa and/or 1 μM ω-conotoxin GVIA was used to block P/Q and N-type calcium channels, respectively. In control layer V Pyr cells, peak amplitude of eIPSCs was decreased by ~50% after treatment with either 1 μM ω-conotoxin GVIA or 1 μM ω-agatoxin IVa. In contrast, there was a lack of sensitivity to 1 μM ω-conotoxin GVIA in UCs. Immunocytochemical results confirmed decreased perisomatic density of N-channels on Pyr cells in UCs. We suggest that decreased calcium influx via N-type channels in presynaptic GABAergic terminals is a mechanism contributing to decreased inhibitory input onto layer V Pyr cells in this model of cortical posttraumatic epileptogenesis.

The GABAergic deficit hypothesis of major depressive disorder

Increasing evidence points to an association between major depressive disorders (MDDs) and diverse types of GABAergic deficits. In this review, we summarize clinical and preclinical evidence supporting a central and causal role of GABAergic deficits in the etiology of depressive disorders. Studies of depressed patients indicate that MDDs are accompanied by reduced brain concentration of the inhibitory neurotransmitter γ-aminobutyric acid (GABA) and by alterations in the subunit composition of the principal receptors (GABA(A) receptors) mediating GABAergic inhibition. In addition, there is abundant evidence that suggests that GABA has a prominent role in the brain control of stress, the most important vulnerability factor in mood disorders. Furthermore, preclinical evidence suggests that currently used antidepressant drugs (ADs) designed to alter monoaminergic transmission and nonpharmacological therapies may ultimately act to counteract GABAergic deficits. In particular, GABAergic transmission has an important role in the control of hippocampal neurogenesis and neural maturation, which are now established as cellular substrates of most if not all antidepressant therapies. Finally, comparatively modest deficits in GABAergic transmission in GABA(A) receptor-deficient mice are sufficient to cause behavioral, cognitive, neuroanatomical and neuroendocrine phenotypes, as well as AD response characteristics expected of an animal model of MDD. The GABAergic hypothesis of MDD suggests that alterations in GABAergic transmission represent fundamentally important aspects of the etiological sequelae of MDDs that are reversed by monoaminergic AD action.

Rapid brain-derived neurotrophic factor-dependent sequestration of amygdala and hippocampal GABA(A) receptors via different tyrosine receptor kinase B-mediated phosphorylation pathways

During the consolidation of fear memory, it has been shown that GABA(A) receptors (GABA(A)R) are rapidly downregulated in amygdala. This rapid decrease in GABA(A)R functioning may permit transient hyperexcitablity, contributing to cellular mechanisms of memory consolidation. Memory consolidation also requires brain-derived neurotrophic factor (BDNF) activation of tyrosine receptor kinase B (TrkB) receptors in the amygdala and hippocampus. We hypothesized that rapid internalization of GABA(A)Rα1 is mediated via TrkB activation of PKA and PKC-dependent processes. Primary neuronal cell cultures, from postnatal day 14-21 mouse amygdala and hippocampus, were analyzed with immunofluorescence using cell-surface, whole-cell permeabilization, and antibody internalization techniques, as well as with (3)H-muscimol binding assays. In both hippocampal and amygdala cultures, we found a >60% reduction in surface GABA(A)Rα1 within 5 min of BDNF treatment. Notably, the rapid decrease in surface GABA(A)Rα1 was confirmed biochemically using surface biotinylation assays followed by western blotting. This rapid effect was accompanied by TrkB phosphorylation and increased internal GABA(A)Rα1 immunofluorescence, and was blocked by k252a, a broad-spectrum tyrosine kinase antagonist. To further demonstrate TrkB specificity, we used previously characterized TrkB(F616A) mice, in which the highly selective TrkB-mutant specific antagonist, 1NMPP1, prevented the BDNF-dependent GABA(A)Rα1 internalization. In hippocampus, we found both PKA and PKC inhibition, using Rp-8-Br-cAMP and Calphostin C, respectively, blocked GABA(A)Rα1 internalization, whereas inhibition of MAPK (U0126) and PI3K (LY294002) did not prevent rapid internalization. By contrast in amygdala cultures, Rp-8-Br-cAMP had no effect. Together, these data suggest that rapid GABA(A)R internalization during memory consolidation is BDNF-TrkB dependent. Further, it appears that hippocampal GABA(A)R internalization is PKA and PKC dependent, while it may be primarily PKC dependent in amygdala, implying differential roles for TrkB-dependent kinase activation in BDNF-dependent memory formation.

Correlation of cognitive performance and morphological changes in neocortical pyramidal neurons in aging

It is well established that the cerebral cortex undergoes extensive remodeling in aging. In this study, we used behaviorally characterized rats to correlate age-related morphological changes with cognitive impairment. For this, young and aged animals were tested in the Morris water maze to evaluate their cognitive performance. Following behavioral characterization, the animals were perfused and a combination of intracellular labeling and immunohistochemistry was applied. Using this approach, we characterized the dendritic morphology of cortical pyramidal neurons as well as the pattern of glutamatergic and GABAergic appositions on their cell bodies and dendrites. We focused on the association region of the parietal cortex (LtPA) and the medial prefrontal cortex (mPFC) for their involvement in the Morris water maze task. We found an age-related atrophy of distal basal dendrites that did not differ between aged cognitively unimpaired (AU) and aged cognitively impaired animals (AI). Dendritic spines and glutamatergic appositions generally decreased from young to AU and from AU to AI rats. On the other hand, GABAergic appositions only showed a trend towards a decrease in AU rats. Collectively, the data show that the ratio of excitatory/inhibitory inputs was only altered in AI animals. When cortical cholinergic varicosities were labeled on alternate sections, we found that AI animals also had a significant reduction of cortical cholinergic boutons compared with AU or young animals. In aged animals, the density of cortical cholinergic varicosities correlated with the excitatory/inhibitory ratio. Our data suggest that both cholinergic atrophy and an imbalance towards inhibition may contribute to the observed age-associated behavioral impairment.

Changes of Serum Brain-Derived Neurotrophic Factor in Children with Obstructive Sleep Apnoea–Hypopnoea Syndrome following Adenotonsillectomy

Children with obstructive sleep apnoea syndrome (OSAS) have substantial cognitive functional morbidity. Brain-derived neurotrophic factor (BDNF) is a key mediator of memory and cognition, but its regulation in OSAS is unknown. Circulating BDNF, transforming growth factor-β1 and 5-hydroxytryptamine levels, cognitive ability and overnight polysomnography were evaluated in 44 children with newly-diagnosed OSAS and in 26 healthy children. All parameters were monitored pre-operatively and at 3, 6 and 12 months after adenotonsillectomy. Pre-operative cognitive ability and sleep parameters were significantly poorer in children with OSAS compared with controls, but BDNF levels were similar. At 3 months post-operation, serum BDNF concentrations decreased, but cognitive ability and sleep parameters remained deficient. At 6 months post-operation, serum BDNF levels, sleep parameters and cognitive ability had improved and, by 12 months, there were no differences between the two groups. It is concluded that adenotonsillectomy can rapidly normalize sleep parameters and improve the cognitive ability of children with OSAS, by regulating the circulating level of BDNF.

Regulation of inhibitory neurotransmission by the scaffolding protein ankyrin repeat-rich membrane spanning/kinase D-interacting substrate of 220 kDa

Scaffolding proteins play a critical role in the proper development and function of neural circuits. In contrast to the case for excitatory circuits, in which the role of several scaffolding proteins has been characterized, less is known about the scaffolding proteins that regulate inhibitory neurotransmission. The ankyrin repeat-rich membrane spanning (ARMS)/kinase D-interacting substrate of 220 kDa (Kidins220) scaffolding protein is expressed during the establishment of γ-aminobutyric acid (GABA) neurotransmission and is highly regulated by activity. To evaluate whether ARMS/Kidins220 expression affects GABAergic neurotransmission, we modified the ARMS/Kidins220 levels during the period of its maximum expression in culture (DIV 1-10). Whereas a decrease in ARMS/Kidins220 levels suppressed GABAergic neurotransmission, overexpression of ARMS/Kidins220 produced an increase in GABAergic neurotransmission in hippocampal neurons. In addition, we found that ARMS/Kidins220 regulates GABAergic neurotransmission by a presynaptic mechanism. Our results suggest that the ARMS/Kidins220 scaffold protein plays a critical role in the regulation of inhibitory transmission in hippocampal neurons.

BDNF signaling in the formation, maturation and plasticity of glutamatergic and GABAergic synapses

In the past 15 years numerous reports provided strong evidence that brain-derived neurotrophic factor (BDNF) is one of the most important modulators of glutamatergic and GABAergic synapses. Remarkable progress regarding localization, kinetics, and molecular mechanisms of BDNF secretion has been achieved, and a large number of studies provided evidence that continuous extracellular supply of BDNF is important for the proper formation and functional maturation of glutamatergic and GABAergic synapses. BDNF can play a permissive role in shaping synaptic networks, making them more susceptible for the occurrence of plastic changes. In addition, BDNF appears to be also an instructive factor for activity-dependent long-term synaptic plasticity. BDNF release just in response to synaptic stimulation might be a molecular trigger to convert high-frequency synaptic activity into long-term synaptic memories. This review attempts to summarize the current knowledge in synaptic secretion and synaptic action of BDNF, including both permissive and instructive effects of BDNF in synaptic plasticity.

Impaired GABAergic inhibition in the visual cortex of brain-derived neurotrophic factor heterozygous knockout mice

Brain derived neurotrophic factor (BDNF) promotes the formation, maturation and stabilization of inhibitory synapses in the central nervous system. In addition, BDNF has been suggested to regulate the critical period for ocular dominance plasticity in the visual system. Here we further evaluated the role of BDNF in the visual cortex by studying the GABAergic synaptic transmission under conditions of chronically reduced levels of BDNF. Whole-cell patch-clamp recordings were performed from pyramidal neurons located in layers II/III of visual cortical slices in heterozygous BDNF knockout mice (BDNF (+/-)) and their wild-type littermates at the age of 21-25 days. The BDNF (+/-) mice showed a decreased frequency and amplitude of miniature inhibitory postsynaptic currents (mIPSCs) as well as a reduced amplitude and prolonged decay time constant of evoked IPSCs. Further analyses indicated an impaired presynaptic GABAergic function in BDNF (+/-) mice, as shown by the decreased release probability, steady-state release and synchronous release of GABA. However, the number of functional release sites remained unchanged. In line with these observations, an impaired glutamate-driven GABA release was observed in BDNF (+/-) mice. Furthermore, the overall balance in the strength of cortical excitation to inhibition shifted towards a decreased inhibition. Finally, the reversal potential for chloride-mediated evoked IPSCs was not affected. These findings suggested that chronically reduced levels of BDNF strongly impair the GABAergic inhibitory function in visual cortex by altering postsynaptic properties and by reducing presynaptic GABA release as well as the overall strength of inhibition onto pyramidal neurons within the cortical network. These impairments of inhibitory function are compatible with a rather immature status of the GABAergic system in BDNF (+/-) mice, which supports the hypothesis that the level of expression for BDNF critically affects maturation and function of the GABAergic inhibition.

Lack of synapsin I reduces the readily releasable pool of synaptic vesicles at central inhibitory synapses

Synapsins (Syns) are synaptic vesicle (SV) phosphoproteins that play a role in neurotransmitter release and synaptic plasticity by acting at multiple steps of exocytosis. Mutation of SYN genes results in an epileptic phenotype in mouse and man suggesting a role of Syns in the control of network excitability. We have studied the effects of the genetic ablation of the SYN1 gene on inhibitory synaptic transmission in primary hippocampal neurons. Inhibitory neurons lacking SynI showed reduced amplitude of IPSCs evoked by isolated action potentials. The impairment in inhibitory transmission was caused by a decrease in the size of the SV readily releasable pool, rather than by changes in release probability or quantal size. The reduction of the readily releasable pool was caused by a decrease in the number of SVs released by single synaptic boutons in response to the action potential, in the absence of variations in the number of synaptic contacts between couples of monosynaptically connected neurons. The deletion of SYN1 did not affect paired-pulse depression or post-tetanic potentiation, but was associated with a moderate increase of synaptic depression evoked by trains of action potentials, which became apparent at high stimulation frequencies and was accompanied by a slow down of recovery from depression. The decreased size of the SV readily releasable pool, coupled with a decreased SV recycling rate and refilling by the SV reserve pool, may contribute to the epileptic phenotype of SynI knock-out mice.

Postsynaptic secretion of BDNF and NT-3 from hippocampal neurons depends on calcium calmodulin kinase II signaling and proceeds via delayed fusion pore opening

The mammalian neurotrophins (NTs) NGF, BDNF, NT-3, and NT-4 constitute a family of secreted neuronal growth factors. In addition, NTs are implicated in several forms of activity-dependent synaptic plasticity. Although synaptic secretion of NTs has been described, the intracellular signaling cascades that regulate synaptic secretion of NTs are far from being understood. Analysis of NT secretion at the subcellular level is thus required to resolve the role of presynaptic and postsynaptic NT secretion for synaptic plasticity. Here, we transfected cultures of dissociated rat hippocampal neurons with green fluorescent protein-tagged versions of BDNF and NT-3, respectively, and identified NT vesicles at glutamatergic synapses by colocalization with the cotransfected postsynaptic marker PSD-95 (postsynaptic density-95)-DsRed. Depolarization-induced secretion of BDNF and NT-3 was monitored with live cell imaging. Direct postsynaptic depolarization with elevated K+ in the presence of blockers of synaptic transmission allowed us to investigate the signaling cascades that are involved in the postsynaptic NT vesicle secretion process. We show that depolarization-induced postsynaptic NT secretion is elicited by Ca2+ influx, either via L-type voltage-gated calcium channels or via NMDA receptors. Subsequent release of Ca2+ from internal stores via ryanodine receptors is required for the secretion process. Postsynaptic NT secretion is inhibited in the presence of KN-62 ([4(2S)-2-[(5-isoquinolinylsulfonyl)methylamino]-3-oxo-3-(4-phenyl-1-piperazinyl)propyl] phenyl isoquinolinesulfonic acid ester) and KN-93 (N-[2-[[[3-(4-chlorophenyl)-2-propenyl]methylamino]methyl]phenyl]-N-(2-hydroxyethyl)-4-methoxybenzenesulfonamide), indicating a critical dependence on the activation of alpha-calcium-calmodulin-dependent protein kinase II (CaMKII). The cAMP/protein kinase A (PKA) signaling inhibitor Rp-cAMP-S impaired NT secretion, whereas elevation of intracellular cAMP levels was without effect. Using the Trk inhibitor k252a, we show that NT-induced NT secretion does not contribute to the NT release process at synapses, and BDNF does not induce its own secretion at postsynaptic sites. Release experiments in the presence of the fluorescence quencher bromphenol blue provide evidence for asynchronous and prolonged fusion pore opening of NT vesicles during secretion. Because fusion pore opening is fast compared with compound release, the speed of NT release seems to be limited by diffusion of NTs out of the vesicle. Together, our results reveal a strong dependence of activity-dependent postsynaptic NT secretion on Ca2+ influx, Ca2+ release from internal stores, activation of CaMKII, and intact PKA signaling, whereas Trk signaling and activation of Na+ channels is not required.

A local reduction in cortical GABAergic synapses after a loss of endogenous brain-derived neurotrophic factor, as revealed by single-cell gene knock-out method

To address questions of whether brain-derived neurotrophic factor (BDNF) released from active excitatory neurons acts locally only on GABAergic presynaptic terminals contacting these neurons or generally also on GABAergic terminals contacting other inactive neurons, we developed a single-cell gene knock-out method in organotypic slice culture of visual cortex of floxed BDNF transgenic mice. A biolistic transfection of Cre recombinase with green fluorescence protein (GFP) plasmids to layer II/III of the cortex resulted in loss of BDNF in a single neuron or a small number of neurons, which expressed GFP at 13-14 d in vitro. Analysis with in situ hybridization and immunohistochemistry confirmed that neurons expressing GFP lacked BDNF mRNA and protein, respectively. Analysis with immunohistochemistry using antibody against GABA synthesizing enzyme showed that the number of GABAergic terminals on the soma of BDNF knock-out neurons was smaller than that of neighboring control neurons. Morphological analysis indicated that there was no significant difference in the soma size and branch points and length of dendrites between the BDNF knock-out and control neurons. Recordings of miniature IPSCs (mIPSCs) showed that the frequency of mIPSCs of BDNF knock-out neurons was lower than that of control neurons, although the amplitude was not significantly different, suggesting the smaller number of functional GABAergic synapses on whole the BDNF knock-out neuron. The present results suggest that BDNF released from postsynaptic target neurons promotes the formation or proliferation of GABAergic synapses through its local actions in layer II/III of visual cortex.

Reduced presynaptic efficiency of excitatory synaptic transmission impairs LTP in the visual cortex of BDNF-heterozygous mice

The neurotrophin brain-derived neurotrophic factor (BDNF) plays an important role in neuronal survival, axonal and dendritic growth and synapse formation. BDNF has also been reported to mediate visual cortex plasticity. Here we studied the cellular mechanisms of BDNF-mediated changes in synaptic plasticity, excitatory synaptic transmission and long-term potentiation (LTP) in the visual cortex of heterozygous BDNF-knockout mice (BDNF(+/-)). Patch-clamp recordings in slices showed an approximately 50% reduction in the frequency of miniature excitatory postsynaptic currents (mEPSCs) compared to wild-type animals, in the absence of changes in mEPSC amplitudes. A presynaptic impairment of excitatory synapses from BDNF(+/-) mice was further indicated by decreased paired-pulse ratio and faster synaptic fatigue upon prolonged repetitive stimulation at 40 Hz. In accordance, presynaptic theta-burst stimulation (TBS) failed to induce LTP at layer IV to layers II-III synapses during extracellular field-potential recordings in BDNF(+/-) animals. Changes in postsynaptic function could not be detected, as no changes were observed in either the amplitudes of evoked EPSCs, the ratios of AMPA : NMDA currents or the kinetics of evoked AMPA and NMDA EPSCs. In line with this observation, an LTP pairing paradigm that relies on direct postsynaptic depolarization under patch-clamp conditions could be induced successfully in BDNF(+/-) animals. These data suggest that a chronic reduction in the expression of BDNF to nearly 50% attenuates the efficiency of presynaptic glutamate release in response to repetitive stimulation, thereby impairing presynaptically evoked LTP in the visual cortex.

Regulation of glycinergic and GABAergic synaptogenesis by brain-derived neurotrophic factor in developing spinal neurons

Brain-derived neurotrophic factor (BDNF) effects on the establishment of glycinergic and GABAergic transmissions in mouse spinal neurons were examined using combined electrophysiological and calcium imaging techniques. BDNF (10 ng/ml) caused a significant acceleration in the onset of synaptogenesis without large effects on the survival of these neurons. Amplitude and frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) and miniature inhibitory postsynaptic currents (mIPSCs) associated to activation of glycine and GABA(A) receptors were augmented in neurons cultured with BDNF. The neurotrophin effect was blocked by long term tetrodotoxin (TTX) addition suggesting a dependence on neuronal activity. In addition, BDNF caused a significant increase in glycine- and GABA-evoked current densities that partly explains the increase in synaptic transmission. Presynaptic mechanisms were also involved in BDNF effects since triethylammonium(propyl)-4-(2-(4-dibutylamino-phenyl)vinyl)pyridinium (FM1-43) destaining with high K(+) was augmented in neurons incubated with the neurotrophin. The effects of BDNF were mediated by receptor tyrosine kinase B (TrkB) and mitogen-activated protein kinase kinase (MEK) activation since culturing neurons with either (9S,10R,12R)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'- kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid methyl ester (K252a) or 2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one (PD98059) blocked the augmentation in synaptic activity induced by the neurotrophin.

Transient receptor potential channels as novel effectors of brain-derived neurotrophic factor signaling: potential implications for Rett syndrome

In addition to their prominent role as survival signals for neurons in the developing nervous system, neurotrophins have established their significance in the adult brain as well, where their modulation of synaptic transmission and plasticity may participate in associative learning and memory. These crucial activities are primarily the result of neurotrophin regulation of intracellular Ca(2+) homeostasis and, ultimately, changes in gene expression. Outlined in the following review is a synopsis of neurotrophin signaling with a particular focus upon brain-derived neurotrophic factor (BDNF) and its role in hippocampal synaptic plasticity and neuronal Ca(2+) homeostasis. Neurotrophin signaling through tropomyosin-related kinase (Trk) and pan-neurotrophin receptor 75 kD (p75(NTR)) receptors are also discussed, reviewing recent results that indicate signaling through these two receptor modalities leads to opposing cellular outcomes. We also provide an intriguing look into the transient receptor potential channel (TRPC) family of ion channels as distinctive targets of BDNF signaling; these channels are critical for capacitative Ca(2+) entry, which, in due course, mediates changes in neuronal structure including dendritic spine density. Finally, we expand these topics into an exploration of mental retardation (MR), in particular Rett Syndrome (RTT), where dendritic spine abnormalities may underlie cognitive impairments. We propose that understanding the role of neurotrophins in synapse formation, plasticity, and maintenance will make fundamental contributions to the development of therapeutic strategies to improve cognitive function in developmental disorders associated with MR.

Anti-homeostatic synaptic plasticity of glycine receptor function after chronic strychnine in developing cultured mouse spinal neurons

In this study, we describe a novel form of anti-homeostatic plasticity produced after culturing spinal neurons with strychnine, but not bicuculline or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Strychnine caused a large increase in network excitability, detected as spontaneous synaptic currents and calcium transients. The calcium transients were associated with action potential firing and activation of gamma-aminobutyric acid (GABA(A)) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors as they were blocked by tetrodotoxin (TTX), bicuculline, and CNQX. After chronic blockade of glycine receptors (GlyRs), the frequency of synaptic transmission showed a significant enhancement demonstrating the phenomenon of anti-homeostatic plasticity. Spontaneous inhibitory glycinergic currents in treated cells showed a fourfold increase in frequency (from 0.55 to 2.4 Hz) and a 184% increase in average peak amplitude compared with control. Furthermore, the augmentation in excitability accelerated the decay time constant of miniature inhibitory post-synaptic currents. Strychnine caused an increase in GlyR current density, without changes in the apparent affinity. These findings support the idea of a post-synaptic action that partly explains the increase in synaptic transmission. This phenomenon of synaptic plasticity was blocked by TTX, an antibody against brain-derived neurotrophic factor (BDNF) and K252a suggesting the involvement of the neuronal activity-dependent BDNF-TrkB signaling pathway. These results show that the properties of GlyRs are regulated by the degree of neuronal activity in the developing network.

The molecular genetics of cognition: dopamine, COMT and BDNF

The important contribution of genetic factors to the development of cognition and intelligence is widely acknowledged, but identification of these genes has proven to be difficult. Given a variety of evidence implicating the prefrontal cortex and its dopaminergic circuits in cognition, most of the research conducted to date has focused on genes regulating dopaminergic function. Here we review the genetic association studies carried out on catechol-O-methyltransferase (COMT) and the dopamine receptor genes, D1, D2 and D4. In addition, the evidence implicating another promising candidate gene, brain-derived neurotrophic factor (BDNF) in neuropsychological function, is assessed. Both the COMT val158met polymorphism and the BDNF val66met variant appear to influence cognitive function, but the specific neurocognitive processes involved continue to be a matter of debate. Part of the difficulty is distinguishing between false positives, pleiotropy and the influence of a general intelligence factor, g. Also at issue is the complexity of the relevant neuromolecular pathways, which make the inference of simple causal relationships difficult. The implications of molecular genetic cognitive research for psychiatry are discussed in light of these data.

Specificity and timing of neocortical transcriptome changes in response to BDNF gene ablation during embryogenesis or adulthood

Brain-derived neurotrophic factor (BDNF) has been reported to be critical for the development of cortical inhibitory neurons. However, the effect of BDNF on the expression of transcripts whose protein products are involved in gamma amino butric acid (GABA) neurotransmission has not been assessed. In this study, gene expression profiling using oligonucleotide microarrays was performed in prefrontal cortical tissue from mice with inducible deletions of BDNF. Both embryonic and adulthood ablation of BDNF gave rise to many shared transcriptome changes. BDNF appeared to be required to maintain gene expression in the SST-NPY-TAC1 subclass of GABA neurons, although the absence of BDNF did not alter their general phenotype as inhibitory neurons. Furthermore, we observed expression alterations in genes encoding early-immediate genes (ARC, EGR1, EGR2, FOS, DUSP1, DUSP6) and critical cellular signaling systems (CDKN1c, CCND2, CAMK1g, RGS4). These BDNF-dependent gene expression changes may illuminate the biological basis for transcriptome changes observed in certain human brain disorders.

Brain-derived neurotrophic factor enhances GABA release probability and nonuniform distribution of N- and P/Q-type channels on release sites of hippocampal inhibitory synapses

Long-lasting exposures to brain-derived neurotrophic factor (BDNF) accelerate the functional maturation of GABAergic transmission in embryonic hippocampal neurons, but the molecular bases of this phenomenon are still debated. Evidence in favor of a postsynaptic site of action has been accumulated, but most of the data support a presynaptic site effect. A crucial issue is whether the enhancement of evoked IPSCs (eIPSCs) induced by BDNF is attributable to an increase in any of the elementary parameters controlling neurosecretion, namely the probability of release, the number of release sites, the readily releasable pool (RRP), and the quantal size. Here, using peak-scaled variance analysis of miniature IPSCs, multiple probability fluctuation analysis, and cumulative amplitude analysis of action potential-evoked postsynaptic currents, we show that BDNF increases release probability and vesicle replenishment with little or no effect on the quantal size, the number of release sites, the RRP, and the Ca2+ dependence of eIPSCs. BDNF treatment changes markedly the distribution of Ca2+ channels controlling neurotransmitter release. It enhances markedly the contribution of N- and P/Q-type channels, which summed to >100% ("supra-additivity"), and deletes the contribution of R-type channels. BDNF accelerates the switch of presynaptic Ca2+ channel distribution from "segregated" to "nonuniform" distribution. This maturation effect was accompanied by an uncovered increased control of N-type channels on paired-pulse depression, otherwise dominated by P/Q-type channels in untreated neurons. Nevertheless, BDNF preserved the fast recovery from depression associated with N-type channels. These novel presynaptic BDNF actions derive mostly from an enhanced overlapping and better colocalization of N- and P/Q-type channels to vesicle release sites.