RESUMO
Human cellular models of neurodegeneration require reproducibility and longevity, which is necessary for simulating age-dependent diseases. Such systems are particularly needed for TDP-43 proteinopathies1, which involve human-specific mechanisms2-5 that cannot be directly studied in animal models. Here, to explore the emergence and consequences of TDP-43 pathologies, we generated induced pluripotent stem cell-derived, colony morphology neural stem cells (iCoMoNSCs) via manual selection of neural precursors6. Single-cell transcriptomics and comparison to independent neural stem cells7 showed that iCoMoNSCs are uniquely homogenous and self-renewing. Differentiated iCoMoNSCs formed a self-organized multicellular system consisting of synaptically connected and electrophysiologically active neurons, which matured into long-lived functional networks (which we designate iNets). Neuronal and glial maturation in iNets was similar to that of cortical organoids8. Overexpression of wild-type TDP-43 in a minority of neurons within iNets led to progressive fragmentation and aggregation of the protein, resulting in a partial loss of function and neurotoxicity. Single-cell transcriptomics revealed a novel set of misregulated RNA targets in TDP-43-overexpressing neurons and in patients with TDP-43 proteinopathies exhibiting a loss of nuclear TDP-43. The strongest misregulated target encoded the synaptic protein NPTX2, the levels of which are controlled by TDP-43 binding on its 3' untranslated region. When NPTX2 was overexpressed in iNets, it exhibited neurotoxicity, whereas correcting NPTX2 misregulation partially rescued neurons from TDP-43-induced neurodegeneration. Notably, NPTX2 was consistently misaccumulated in neurons from patients with amyotrophic lateral sclerosis and frontotemporal lobar degeneration with TDP-43 pathology. Our work directly links TDP-43 misregulation and NPTX2 accumulation, thereby revealing a TDP-43-dependent pathway of neurotoxicity.
Assuntos
Esclerose Lateral Amiotrófica , Proteína C-Reativa , Proteínas de Ligação a DNA , Degeneração Lobar Frontotemporal , Rede Nervosa , Proteínas do Tecido Nervoso , Neurônios , Humanos , Esclerose Lateral Amiotrófica/metabolismo , Esclerose Lateral Amiotrófica/patologia , Proteína C-Reativa/metabolismo , Proteínas de Ligação a DNA/deficiência , Proteínas de Ligação a DNA/metabolismo , Degeneração Lobar Frontotemporal/metabolismo , Degeneração Lobar Frontotemporal/patologia , Rede Nervosa/metabolismo , Rede Nervosa/patologia , Proteínas do Tecido Nervoso/metabolismo , Células-Tronco Neurais/citologia , Neuroglia/citologia , Neurônios/citologia , Neurônios/metabolismo , Reprodutibilidade dos TestesRESUMO
Bayesian Active Learning (BAL) is an efficient framework for learning the parameters of a model, in which input stimuli are selected to maximize the mutual information between the observations and the unknown parameters. However, the applicability of BAL to experiments is limited as it requires performing high-dimensional integrations and optimizations in real time. Current methods are either too time consuming, or only applicable to specific models. Here, we propose an Efficient Sampling-Based Bayesian Active Learning (ESB-BAL) framework, which is efficient enough to be used in real-time biological experiments. We apply our method to the problem of estimating the parameters of a chemical synapse from the postsynaptic responses to evoked presynaptic action potentials. Using synthetic data and synaptic whole-cell patch-clamp recordings, we show that our method can improve the precision of model-based inferences, thereby paving the way towards more systematic and efficient experimental designs in physiology.
Assuntos
Aprendizagem Baseada em Problemas , Projetos de Pesquisa , Teorema de Bayes , Potenciais de Ação , Técnicas de Patch-ClampRESUMO
Animal behavior is remarkably robust despite constant changes in neural activity. Homeostatic plasticity stabilizes central nervous system (CNS) function on time scales of hours to days. If and how CNS function is stabilized on more rapid time scales remains unknown. Here, we discovered that mossy fiber synapses in the mouse cerebellum homeostatically control synaptic efficacy within minutes after pharmacological glutamate receptor impairment. This rapid form of homeostatic plasticity is expressed presynaptically. We show that modulations of readily releasable vesicle pool size and release probability normalize synaptic strength in a hierarchical fashion upon acute pharmacological and prolonged genetic receptor perturbation. Presynaptic membrane capacitance measurements directly demonstrate regulation of vesicle pool size upon receptor impairment. Moreover, presynaptic voltage-clamp analysis revealed increased Ca2+-current density under specific experimental conditions. Thus, homeostatic modulation of presynaptic exocytosis through specific mechanisms stabilizes synaptic transmission in a CNS circuit on time scales ranging from minutes to months. Rapid presynaptic homeostatic plasticity may ensure stable neural circuit function in light of rapid activity-dependent plasticity.
Assuntos
Exocitose , Homeostase/fisiologia , Terminações Pré-Sinápticas/fisiologia , Sinapses/fisiologia , Animais , Cálcio/metabolismo , Transporte de Íons , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Knockout , Terminações Pré-Sinápticas/metabolismo , Receptores de AMPA/fisiologia , Sinapses/metabolismoRESUMO
Fast synchronous neurotransmitter release at the presynaptic active zone is triggered by local Ca(2+) signals, which are confined in their spatiotemporal extent by endogenous Ca(2+) buffers. However, it remains elusive how rapid and reliable Ca(2+) signaling can be sustained during repetitive release. Here, we established quantitative two-photon Ca(2+) imaging in cerebellar mossy fiber boutons, which fire at exceptionally high rates. We show that endogenous fixed buffers have a surprisingly low Ca(2+)-binding ratio (â¼ 15) and low affinity, whereas mobile buffers have high affinity. Experimentally constrained modeling revealed that the low endogenous buffering promotes fast clearance of Ca(2+) from the active zone during repetitive firing. Measuring Ca(2+) signals at different distances from active zones with ultra-high-resolution confirmed our model predictions. Our results lead to the concept that reduced Ca(2+) buffering enables fast active zone Ca(2+) signaling, suggesting that the strength of endogenous Ca(2+) buffering limits the rate of synchronous synaptic transmission.
Assuntos
Sinalização do Cálcio , Cálcio/metabolismo , Animais , Feminino , Técnicas In Vitro , Cinética , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Neurotransmissores/metabolismoRESUMO
Over a century ago, the Spanish anatomist Ramón y Cajal described 'mossy fibres' in the hippocampus and the cerebellum, which contain several presynaptic boutons. Technical improvements in recent decades have allowed direct patch-clamp recordings from both hippocampal and cerebellar mossy fibre boutons (hMFBs and cMFBs, respectively), making them ideal models to study fundamental properties of synaptic transmission. hMFBs and cMFBs have similar size and shape, but each hMFB contacts one postsynaptic hippocampal CA3 pyramidal neuron, while each cMFB contacts â¼50 cerebellar granule cells. Furthermore, hMFBs and cMFBs differ in terms of their functional specialization. At hMFBs, a large number of release-ready vesicles and low release probability (<0.1) contribute to marked synaptic facilitation. At cMFBs, a small number of release-ready vesicles, high release probability (â¼0.5) and rapid vesicle reloading result in moderate frequency-dependent synaptic depression. These presynaptic mechanisms, in combination with faster postsynaptic currents of cerebellar granule cells compared with hippocampal CA3 pyramidal neurons, enable much higher transmission frequencies at cMFB compared with hMFB synapses. Analysing the underling mechanisms of synaptic transmission and information processing represents a fascinating challenge and may reveal insights into the structure-function relationship of the human brain.
Assuntos
Fibras Musgosas Hipocampais/fisiologia , Fibras Nervosas/fisiologia , Animais , Plasticidade Neuronal , Transmissão SinápticaRESUMO
AIM: We aimed to investigate the induction of long-term potentiation (LTP)-like plasticity by paired associative stimulation (PAS) in patients with high-functioning autism and Asperger syndrome (HFA/AS). METHOD: PAS with an interstimulus interval between electrical and transcranial magnetic stimulation of 25 ms (PAS(25)) was performed in patients with HFA/AS (n=9; eight males, one female; mean age 17 y 11 mo, SD 4 y 5 mo) and in typically developing age-matched volunteers (n=9; five males, four females; mean age 22 y 4 mo, SD 5 y 2 mo). The amplitude of motor-evoked potentials was measured before PAS(25), immediately after stimulation, and 30 minutes and 60 minutes later. A PAS protocol adapted to individual N20 latency (PAS(N20+2)) was performed in six additional patients with HFA/AS. Short-interval intracortical inhibition was measured using paired-pulse stimulation. RESULTS: In contrast to the typically developing participants, the patients with HFA/AS did not show a significant increase in motor-evoked potentials after PAS(25). This finding could also be demonstrated after adaptation for N20 latency. Short-interval intracortical inhibition of patients with HFA/AS was normal compared with the comparison group and did not correlate with PAS effect. INTERPRETATION: Our results show a significant impairment of LTP-like plasticity induced by PAS in individuals with HFA/AS compared with typically developing participants. This finding is in accordance with results from animal studies as well as human studies. Impaired LTP-like plasticity in patients with HFA/AS points towards reduced excitatory synaptic connectivity and deficits in sensory-motor integration in these patients.
Assuntos
Síndrome de Asperger/patologia , Transtorno Autístico/patologia , Potencial Evocado Motor/fisiologia , Potenciação de Longa Duração/fisiologia , Córtex Motor/fisiopatologia , Adolescente , Adulto , Análise de Variância , Síndrome de Asperger/fisiopatologia , Transtorno Autístico/fisiopatologia , Estimulação Elétrica , Eletromiografia , Feminino , Humanos , Masculino , Inibição Neural/fisiologia , Tempo de Reação/fisiologia , Fatores de Tempo , Estimulação Magnética Transcraniana , Adulto JovemRESUMO
AIM: Early unilateral brain lesions can lead to a persistence of ipsilateral corticospinal projections from the contralesional hemisphere, which can enable the contralesional hemisphere to exert motor control over the paretic hand. In contrast to the primary motor representation (M1), the primary somatosensory representation (S1) of the paretic hand always remains in the lesioned hemisphere. Here, we report on differences in exercise-induced neuroplasticity between individuals with such ipsilateral motor projections (ipsi) and individuals with early unilateral lesions but 'healthy' contralateral motor projections (contra). METHOD: Sixteen children and young adults with congenital hemiparesis participated in the study (contralateral [Contra] group: n=7, four females, three males; age range 10-30y, median age 16y; ipsilateral [Ipsi] group: n=9, four females, five males; age range 11-31y, median age 12y; Manual Ability Classification System levels I to II in all individuals in both groups). The participants underwent a 12-day intervention of constraint-induced movement therapy (CIMT), consisting of individual training (2h/d) and group training (8h/d). Before and after CIMT, hand function was tested using the Wolf Motor Function Test (WMFT) and diverging neuroplastic effects were observed by transcranial magnetic stimulation (TMS), functional magnetic resonance imaging (fMRI), and magnetoencephalography (MEG). Statistical analysis of TMS data was performed using the non-parametric Wilcoxon signed-rank test for pair-wise comparison; for fMRI standard statistical parametric and non-parametric mapping (SPM5, SnPM3) procedures (first level/second level) were carried out. Statistical analyses of MEG data involved analyses of variance (ANOVA) and t-tests. RESULTS: While MEG demonstrated a significant increase in S1 activation in both groups (p=0.012), TMS showed a decrease in M1 excitability in the Ipsi group (p=0.036), but an increase in M1 excitability in the Contra group (p=0.043). Similarly, fMRI showed a decrease in M1 activation in the Ipsi group, but an increase in activation in the M1-S1 region in the Contra group (for both groups p<0.001 [SnPM3] within the search volume). INTERPRETATION: Different patterns of sensorimotor (re)organization in individuals with early unilateral lesions show, on a cortical level, different patterns of exercise-induced neuroplasticity. The findings help to improve the understanding of the general principles of sensorimotor learning and will help to develop more specific therapies for different pathologies in congenital hemiparesis.
Assuntos
Encéfalo/patologia , Técnicas de Exercício e de Movimento/métodos , Plasticidade Neuronal/fisiologia , Paresia/etiologia , Paresia/terapia , Adolescente , Adulto , Análise de Variância , Encéfalo/irrigação sanguínea , Encéfalo/fisiopatologia , Criança , Feminino , Lateralidade Funcional/fisiologia , Mãos/fisiopatologia , Humanos , Processamento de Imagem Assistida por Computador , Imageamento por Ressonância Magnética , Magnetoencefalografia , Masculino , Paresia/classificação , Paresia/congênito , Tempo de Reação , Estimulação Magnética Transcraniana , Adulto JovemRESUMO
BACKGROUND: Transcranial magnetic stimulation (TMS) commonly uses so-called monophasic pulses where the initial rapidly changing current flow is followed by a critically dampened return current. It has been shown that a monophasic TMS pulse preferentially excites different cortical circuits in the human motor hand area (M1-HAND), if the induced tissue current has a posterior-to-anterior (PA) or anterior-to-posterior (AP) direction. Here we tested whether similar direction-specific effects could be elicited in M1-HAND using TMS pulses with a half-sine wave configuration. RESULTS: In 10 young participants, we applied half-sine pulses to the right M1-HAND which elicited PA or AP currents with respect to the orientation of the central sulcus.Measurements of the motor evoked potential (MEP) revealed that PA half-sine stimulation resulted in lower resting motor threshold (RMT) than AP stimulation. When stimulus intensity (SI) was gradually increased as percentage of maximal stimulator output, the stimulus-response curve (SRC) of MEP amplitude showed a leftward shift for PA as opposed to AP half-sine stimulation. Further, MEP latencies were approximately 1 ms shorter for PA relative to AP half-sine stimulation across the entire SI range tested. When adjusting SI to the respective RMT of PA and AP stimulation, the direction-specific differences in MEP latencies persisted, while the gain function of MEP amplitudes was comparable for PA and AP stimulation. CONCLUSIONS: Using half-sine pulse configuration, single-pulse TMS elicits consistent direction-specific effects in M1-HAND that are similar to TMS with monophasic pulses. The longer MEP latency for AP half-sine stimulation suggests that PA and AP half-sine stimulation preferentially activates different sets of cortical neurons that are involved in the generation of different corticospinal descending volleys.
Assuntos
Potencial Evocado Motor/fisiologia , Córtex Motor/fisiologia , Estimulação Magnética Transcraniana , Biofísica , Estimulação Elétrica , Eletromiografia , Feminino , Mãos/inervação , Humanos , Masculino , Músculo Esquelético/inervação , Tempo de Reação/fisiologiaRESUMO
AMPA receptors (AMPARs) mediate excitatory neurotransmission in the central nervous system (CNS) and their subunit composition determines synaptic efficacy. Whereas AMPAR subunits GluA1-GluA3 have been linked to particular forms of synaptic plasticity and learning, the functional role of GluA4 remains elusive. Here, we demonstrate a crucial function of GluA4 for synaptic excitation and associative memory formation in the cerebellum. Notably, GluA4-knockout mice had ~80% reduced mossy fiber to granule cell synaptic transmission. The fidelity of granule cell spike output was markedly decreased despite attenuated tonic inhibition and increased NMDA receptor-mediated transmission. Computational network modeling incorporating these changes revealed that deletion of GluA4 impairs granule cell expansion coding, which is important for pattern separation and associative learning. On a behavioral level, while locomotor coordination was generally spared, GluA4-knockout mice failed to form associative memories during delay eyeblink conditioning. These results demonstrate an essential role for GluA4-containing AMPARs in cerebellar information processing and associative learning.
Assuntos
Piscadela/fisiologia , Cerebelo/fisiologia , Condicionamento Clássico/fisiologia , Memória/fisiologia , Receptores de AMPA/metabolismo , Animais , Feminino , Masculino , Camundongos , Camundongos Knockout , Receptores de AMPA/genéticaRESUMO
Cerebellar granule cells (GCs) make up the majority of all neurons in the vertebrate brain, but heterogeneities among GCs and potential functional consequences are poorly understood. Here, we identified unexpected gradients in the biophysical properties of GCs in mice. GCs closer to the white matter (inner-zone GCs) had higher firing thresholds and could sustain firing with larger current inputs than GCs closer to the Purkinje cell layer (outer-zone GCs). Dynamic Clamp experiments showed that inner- and outer-zone GCs preferentially respond to high- and low-frequency mossy fiber inputs, respectively, enabling dispersion of the mossy fiber input into its frequency components as performed by a Fourier transformation. Furthermore, inner-zone GCs have faster axonal conduction velocity and elicit faster synaptic potentials in Purkinje cells. Neuronal network modeling revealed that these gradients improve spike-timing precision of Purkinje cells and decrease the number of GCs required to learn spike-sequences. Thus, our study uncovers biophysical gradients in the cerebellar cortex enabling a Fourier-like transformation of mossy fiber inputs.
The timing of movements such as posture, balance and speech are coordinated by a region of the brain called the cerebellum. Although this part of the brain is small, it contains a huge number of tiny nerve cells known as granule cells. These cells make up more than half the nerve cells in the human brain. But why there are so many is not well understood.The cerebellum receives signals from sensory organs, such as the ears and eyes, which are passed on as electrical pulses from nerve to nerve until they reach the granule cells. These electrical pulses can have very different repetition rates, ranging from one pulse to a thousand pulses per second. Previous studies have suggested that granule cells are a uniform population that can detect specific patterns within these electrical pulses. However, this would require granule cells to identify patterns in signals that have a range of different repetition rates, which is difficult for individual nerve cells to do.To investigate if granule cells are indeed a uniform population, Straub, Witter, Eshra, Hoidis et al. measured the electrical properties of granule cells from the cerebellum of mice. This revealed that granule cells have different electrical properties depending on how deep they are within the cerebellum. These differences enabled the granule cells to detect sensory signals that had specific repetition rates: signals that contained lots of repeats per second were relayed by granule cells in the lower layers of the cerebellum, while signals that contained fewer repeats were relayed by granule cells in the outer layers.This ability to separate signals based on their rate of repetition is similar to how digital audio files are compressed into an MP3. Computer simulations suggested that having granule cells that can detect specific rates of repetition improves the storage capacity of the brain.These findings further our understanding of how the cerebellum works and the cellular mechanisms that underlie how humans learn and memorize the timing of movement. This mechanism of separating signals to improve storage capacity may apply to other regions of the brain, such as the hippocampus, where differences between nerve cells have also recently been reported.
Assuntos
Córtex Cerebelar , Neurônios , Animais , Fenômenos Biofísicos/fisiologia , Córtex Cerebelar/citologia , Córtex Cerebelar/metabolismo , Córtex Cerebelar/fisiologia , Análise de Fourier , Camundongos , Modelos Neurológicos , Fibras Nervosas/metabolismo , Fibras Nervosas/fisiologia , Neurônios/citologia , Neurônios/metabolismo , Neurônios/fisiologia , Células de Purkinje/citologia , Células de Purkinje/metabolismo , Células de Purkinje/fisiologia , Potenciais Sinápticos/fisiologia , Substância Branca/citologia , Substância Branca/metabolismo , Substância Branca/fisiologiaRESUMO
Plastic changes in synaptic transmission are thought to underlie learning and memory formation. However, changes in synaptic function are only meaningful in the context of stable baseline function. Accumulating evidence suggests that homeostatic signaling systems actively stabilize synaptic transmission in response to neural activity perturbation. Homeostatic mechanisms control both presynaptic and postsynaptic function. Here, we review recent advances in the field of presynaptic homeostatic plasticity (PHP). We discuss PHP in the context of basic mechanisms controlling neurotransmitter release, highlight emerging similarities between different synapses in different species, and summarize new insights into the molecular mechanisms underlying this evolutionary conserved form of synaptic plasticity.
Assuntos
Homeostase/fisiologia , Plasticidade Neuronal/fisiologia , Terminações Pré-Sinápticas/fisiologia , Transdução de Sinais/fisiologia , AnimaisRESUMO
OBJECTIVE: To assess effects of deafferentation of the arm representation of primary motor cortex (M1) on practice-dependent plasticity in healthy adults. METHODS: Twelve healthy, right-handed adults (18-48 years, median 20.2 years) performed two consecutive experiments (exp. 1 and exp. 2). Exp. 1 consisted of a motor practice (MP) of repeated ballistic flexion movements of the left thumb. This was followed by exp. 2 consisting of selective anaesthesia of the upper brachial plexus (SPA) to disinhibit the training M1 and a second period of the same MP. Peak acceleration of the trained thumb movement and the motor evoked potential (MEP) amplitude in the flexor pollicis brevis muscle elicited by single-pulse transcranial magnetic stimulation of the training M1 were studied before and after exp. 1 and after exp. 2. RESULTS: After exp. 1 all subjects demonstrated an increase of peak acceleration (baseline: 19.23+/-3.81ms(-2); after exp. 1: 43.28+/-17.63ms(-2), p=0.008) and MEP amplitude (from 0.46+/-0.23mV to 1.26+/-0.77mV, p=0.03). There was no additional increase of these measures after exp. 2 (44.37+/-19.56ms(-2), p=0.78, 1.69+/-1.21mV (p=0.07)). CONCLUSIONS: Training of ballistic thumb movements leads to behavioural improvement as well as to an increased excitability of the corresponding M1 representation. These effects do not increase further during deafferentation of the training M1. In contrast to stroke patients [Muellbacher W, Richards C, Ziemann U, Wittenberg G, Weltz D, Boroojerdi B, et al. Improving hand function in chronic stroke. Arch Neurol 2002;59:1278-82], practice-dependent plasticity in healthy subjects cannot be enhanced by deafferentation of neighbouring motor cortex areas. SIGNIFICANCE: Healthy subjects, in contrast to patients with central motor lesions, are capable of saturating practice-dependent plasticity to a level that cannot be further enhanced by experimental manipulation.
Assuntos
Potencial Evocado Motor/fisiologia , Atividade Motora/fisiologia , Córtex Motor/fisiologia , Plasticidade Neuronal/fisiologia , Adolescente , Adulto , Anestésicos Locais/farmacologia , Plexo Braquial/efeitos dos fármacos , Plexo Braquial/fisiologia , Feminino , Humanos , Lidocaína/farmacologia , Masculino , Pessoa de Meia-Idade , Polegar/fisiologia , Estimulação Magnética TranscranianaRESUMO
In the central nervous system, the frequency at which reliable synaptic transmission can be maintained varies strongly between different types of synapses. Several pre- and postsynaptic processes must interact to enable high-frequency synaptic transmission. One of the mechanistically most challenging issues arises during repetitive neurotransmitter release, when synaptic vesicles fuse in rapid sequence with the presynaptic plasma membrane within the active zone (AZ), potentially interfering with the structural integrity of the AZ itself. Here we summarize potential mechanisms that help to maintain AZ integrity, including arrangement and mobility of release sites, calcium channel mobility, as well as release site clearance via lateral diffusion of vesicular proteins and via endocytotic membrane retrieval. We discuss how different types of synapses use these strategies to maintain high-frequency synaptic transmission.
Assuntos
Terminações Pré-Sinápticas/metabolismo , Sinapses/fisiologia , Transmissão Sináptica/fisiologia , Vesículas Sinápticas/fisiologia , Animais , Canais de Cálcio , Endocitose , Neurônios/citologia , Neurônios/fisiologia , Sinapses/classificaçãoRESUMO
Here we explore the relationship between presynaptic homeostatic plasticity and proteasome function at the Drosophila neuromuscular junction. First, we demonstrate that the induction of homeostatic plasticity is blocked after presynaptic proteasome perturbation. Proteasome inhibition potentiates release under baseline conditions but not during homeostatic plasticity, suggesting that proteasomal degradation and homeostatic plasticity modulate a common pool of vesicles. The vesicles that are regulated by proteasome function and recruited during homeostatic plasticity are highly EGTA sensitive, implying looser Ca2+ influx-release coupling. Similar to homeostatic plasticity, proteasome perturbation enhances presynaptic Ca2+ influx, readily-releasable vesicle pool size, and does not potentiate release after loss of specific homeostatic plasticity genes, including the schizophrenia-susceptibility gene dysbindin. Finally, we provide genetic evidence that Dysbindin levels regulate the access to EGTA-sensitive vesicles. Together, our data suggest that presynaptic protein degradation opposes the release of low-release probability vesicles that are potentiated during homeostatic plasticity and whose access is controlled by dysbindin.
Assuntos
Disbindina/metabolismo , Junção Neuromuscular/metabolismo , Plasticidade Neuronal , Complexo de Endopeptidases do Proteassoma/metabolismo , Vesículas Sinápticas/fisiologia , Animais , Animais Geneticamente Modificados , Cálcio/metabolismo , Drosophila , Proteínas de Drosophila/metabolismo , Ácido Egtázico , Homeostase , Proteínas rab3 de Ligação ao GTP/metabolismoRESUMO
The speed of neuronal information processing depends on neuronal firing frequency. Here, we describe the evolutionary advantages and ubiquitous occurrence of high-frequency firing within the mammalian nervous system in general. The highest firing frequencies so far have been observed at the cerebellar mossy fiber to granule cell synapse. The mechanisms enabling high-frequency transmission at this synapse are reviewed and compared with other synapses. Finally, information coding of high-frequency signals at the mossy fiber synapse is discussed. The exceptionally high firing frequencies and amenability to high-resolution technical approaches both in vitro and in vivo establish the cerebellar mossy fiber synapse as an attractive model to investigate high-frequency signaling from the molecular up to the network level.
Assuntos
Cerebelo/fisiologia , Fibras Nervosas/fisiologia , Plasticidade Neuronal/fisiologia , Neurônios/fisiologia , Sinapses/fisiologia , Animais , Potenciais Pós-Sinápticos Excitadores/fisiologia , HumanosRESUMO
The fusion of neurotransmitter-filled vesicles during synaptic transmission is balanced by endocytotic membrane retrieval. Despite extensive research, the speed and mechanisms of synaptic vesicle endocytosis have remained controversial. Here, we establish low-noise time-resolved membrane capacitance measurements that allow monitoring changes in surface membrane area elicited by single action potentials and stronger stimuli with high-temporal resolution at physiological temperature in individual bona-fide mature central synapses. We show that single action potentials trigger very rapid endocytosis, retrieving presynaptic membrane with a time constant of 470 ms. This fast endocytosis is independent of clathrin but mediated by dynamin and actin. In contrast, stronger stimuli evoke a slower mode of endocytosis that is clathrin, dynamin, and actin dependent. Furthermore, the speed of endocytosis is highly temperature dependent with a Q10 of â¼3.5. These results demonstrate that distinct molecular modes of endocytosis with markedly different kinetics operate at central synapses.
Assuntos
Potenciais de Ação/fisiologia , Clatrina/metabolismo , Endocitose/fisiologia , Estriol/análogos & derivados , Animais , Estriol/fisiologia , Hipocampo/metabolismo , Camundongos Endogâmicos C57BL , Ratos , Transmissão Sináptica/fisiologia , Temperatura , Fatores de TempoRESUMO
Cerebellar granule cells (GCs), the smallest neurons in the brain, have on average four short dendrites that receive high-frequency mossy fiber inputs conveying sensory information. The short length of the dendrites suggests that GCs are electrotonically compact allowing unfiltered integration of dendritic inputs. The small average diameter of the dendrites (~0.7 µm), however, argues for dendritic filtering. Previous studies based on somatic recordings and modeling indicated that GCs are electrotonically extremely compact. Here, we performed patch-clamp recordings from GC dendrites in acute brain slices of mice to directly analyze the electrotonic properties of GCs. Strikingly, the input resistance did not differ significantly between dendrites and somata of GCs. Furthermore, spontaneous excitatory postsynaptic potentials (EPSP) were similar in amplitude at dendritic and somatic recording sites. From the dendritic and somatic input resistances we determined parameters characterizing the electrotonic compactness of GCs. These data directly demonstrate that cerebellar GCs are electrotonically compact and thus ideally suited for efficient high-frequency information transfer.
RESUMO
BACKGROUND: Transcranial magnetic stimulation (TMS) of the human primary motor hand area (M1-HAND) can produce multiple descending volleys in fast-conducting corticospinal neurons, especially so-called indirect waves (I-waves) resulting from trans-synaptic excitation. Facilitatory interaction between these I-waves can be studied non-invasively using a paired-pulse paradigm referred to as short-interval intracortical facilitation (SICF). OBJECTIVE/HYPOTHESIS: We examined whether SICF depends on waveform and current direction of the TMS pulses. METHODS: In young healthy volunteers, we applied single- and paired-pulse TMS to M1-HAND. We probed SICF by pairs of monophasic or half-sine pulses at suprathreshold stimulation intensity and inter-stimulus intervals (ISIs) between 1.0 and 5.0 ms. For monophasic paired-pulse stimulation, both pulses had either a posterior-anterior (PA) or anterior-posterior (AP) current direction (AP-AP or PA-PA), whereas current direction was reversed between first and second pulse for half-sine paired-pulse stimulation (PA-AP and AP-PA). RESULTS: Monophasic AP-AP stimulation resulted in stronger early SICF at 1.4 ms relative to late SICF at 2.8 and 4.4 ms, whereas monophasic PA-PA stimulation produced SICF of comparable size at all three peaks. With half-sine stimulation the third SICF peak was reduced for PA-AP current orientation compared with AP-PA. CONCLUSION: SICF elicited using monophasic as well as half-sine pulses is affected by current direction at clearly suprathreshold intensities. The impact of current orientation is stronger for monophasic compared with half-sine pulses. The direction-specific effect of paired-pulse TMS on the strength of early versus late SICF shows that different cortical circuits mediate early and late SICF.
Assuntos
Córtex Motor/fisiologia , Estimulação Magnética Transcraniana/métodos , Adulto , Potencial Evocado Motor/fisiologia , Feminino , Humanos , Masculino , Adulto JovemRESUMO
A full-sine (biphasic) pulse waveform is most commonly used for repetitive transcranial magnetic stimulation (TMS), but little is known about how variations in duration or amplitude of distinct pulse segments influence the effectiveness of a single TMS pulse to elicit a corticomotor response. Using a novel TMS device, we systematically varied the configuration of full-sine pulses to assess the impact of configuration changes on resting motor threshold (RMT) as measure of stimulation effectiveness with single-pulse TMS of the non-dominant motor hand area (M1). In young healthy volunteers, we (i) compared monophasic, half-sine, and full-sine pulses, (ii) applied two-segment pulses consisting of two identical half-sines, and (iii) manipulated amplitude, duration, and current direction of the first or second full-sine pulse half-segments. RMT was significantly higher using half-sine or monophasic pulses compared with full-sine. Pulses combining two half-sines of identical polarity and duration were also characterized by higher RMT than full-sine stimuli resulting. For full-sine stimuli, decreasing the amplitude of the half-segment inducing posterior-anterior oriented current in M1 resulted in considerably higher RMT, whereas varying the amplitude of the half-segment inducing anterior-posterior current had a smaller effect. These findings provide direct experimental evidence that the pulse segment inducing a posterior-anterior directed current in M1 contributes most to corticospinal pathway excitation. Preferential excitation of neuronal target cells in the posterior-anterior segment or targeting of different neuronal structures by the two half-segments can explain this result. Thus, our findings help understanding the mechanisms of neural stimulation by full-sine TMS.