Release-independent short-term synaptic depression in cultured hippocampal neurons

The computational power of the human brain

At the end of the 20th century, analog systems in computer science have been widely replaced by digital systems due to their higher computing power. Nevertheless, the question keeps being intriguing until now: is the brain analog or digital? Initially, the latter has been favored, considering it as a Turing machine that works like a digital computer. However, more recently, digital and analog processes have been combined to implant human behavior in robots, endowing them with artificial intelligence (AI). Therefore, we think it is timely to compare mathematical models with the biology of computation in the brain. To this end, digital and analog processes clearly identified in cellular and molecular interactions in the Central Nervous System are highlighted. But above that, we try to pinpoint reasons distinguishing in silico computation from salient features of biological computation. First, genuinely analog information processing has been observed in electrical synapses and through gap junctions, the latter both in neurons and astrocytes. Apparently opposed to that, neuronal action potentials (APs) or spikes represent clearly digital events, like the yes/no or 1/0 of a Turing machine. However, spikes are rarely uniform, but can vary in amplitude and widths, which has significant, differential effects on transmitter release at the presynaptic terminal, where notwithstanding the quantal (vesicular) release itself is digital. Conversely, at the dendritic site of the postsynaptic neuron, there are numerous analog events of computation. Moreover, synaptic transmission of information is not only neuronal, but heavily influenced by astrocytes tightly ensheathing the majority of synapses in brain (tripartite synapse). At least at this point, LTP and LTD modifying synaptic plasticity and believed to induce short and long-term memory processes including consolidation (equivalent to RAM and ROM in electronic devices) have to be discussed. The present knowledge of how the brain stores and retrieves memories includes a variety of options (e.g., neuronal network oscillations, engram cells, astrocytic syncytium). Also epigenetic features play crucial roles in memory formation and its consolidation, which necessarily guides to molecular events like gene transcription and translation. In conclusion, brain computation is not only digital or analog, or a combination of both, but encompasses features in parallel, and of higher orders of complexity.

Presynaptic Plasticity Is Associated with Actin Polymerization

Modulation of presynaptic short-term plasticity induced by actin polymerization was studied in rat hippocampal slices using the paired-pulse paradigm. Schaffer collaterals were stimulated with paired pulses with a 70-ms interstimulus interval every 30 s before and during perfusion with jasplakinolide, an activator of actin polymerization. Jasplakinolide application resulted in the increase in the amplitudes of CA3-CA1 responses (potentiation) accompanied by a decrease in the paired-pulse facilitation, suggesting induction of presynaptic modifications. Jasplakinolide-induced potentiation depended on the initial paired-pulse rate. These data indicate that the jasplakinolide-mediated changes in actin polymerization increased the probability of neurotransmitter release. Less typical for CA3-CA1 synapses responses, such as a very low paired-pulse ratio (close to 1 or even lower) or even paired-pulse depression, were affected differently. Thus, jasplakinolide caused potentiation of the second, but not the first response to the paired stimulus, which increased the paired-pulse ratio from 0.8 to 1.0 on average, suggesting a negative impact of jasplakinolide on the mechanisms promoting paired-pulse depression. In general, actin polymerization facilitated potentiation, although the patterns of potentiation differed depending on the initial synapse characteristics. We conclude that in addition to the increase in the neurotransmitter release probability, jasplakinolide induced other actin polymerization-dependent mechanisms, including those involved in the paired-pulse depression.

Physiological effects of cathodal electrode configuration for transspinal stimulation in humans

Transspinal stimulation modulates neuronal excitability and promotes recovery in upper motoneuron lesions. The recruitment input-output curves of transspinal evoked potentials (TEPs) recorded from knee and ankle muscles, and their susceptibility to spinal inhibition, were recorded when the position, size, and number of the cathode electrode were arranged in four settings or protocols (Ps). The four Ps were the following: 1) one rectangular electrode placed at midline (KNIKOU-LAB4Recovery or K-LAB4Recovery; P-KLAB), 2) one square electrode placed at midline (P-2), 3) two square electrodes 1 cm apart placed at midline (P-3), and 4) one square electrode placed on each paravertebral side (P-4). P-KLAB and P-3 required less current to reach TEP threshold or maximal amplitudes. A rightward shift in TEP recruitment curves was evident for P-4, whereas the slope was increased for P-2 and P-4 compared with P-KLAB and P-3. TEP depression upon single and paired transspinal stimuli was pronounced in ankle TEPs but was less prominent in knee TEPs. TEP depression induced by single transspinal stimuli at 1.0 Hz was similar for most TEPs across protocols, but TEP depression induced by paired transspinal stimuli was different between protocols and was replaced by facilitation at 100-ms interstimulus interval for P-4. Our results suggest that P-KLAB and P-3 are preferred based on excitability threshold of motoneurons. P-KLAB produced more TEP depression, thereby maximizing the engagement of spinal neuronal pathways. We recommend P-KLAB to study neurophysiological mechanisms underlying transspinal stimulation or when used as a neuromodulation method for recovery in neurological disorders.NEW & NOTEWORTHY Transspinal stimulation with a rectangular cathode electrode (P-KLAB) requires less current to produce transspinal evoked potentials and maximizes spinal inhibition. We recommend P-KLAB for neurophysiological studies or when used as a neuromodulation method to enhance motor output and normalize muscle tone in neurological disorders.

Analog Transmission of Action Potential Fine Structure in Spiral Ganglion Axons

Action potential waveforms generated at the axon initial segment (AIS) are specialized between and within neuronal classes. But is the fine structure of each electrical event retained when transmitted along myelinated axons or is it rapidly and uniformly transmitted to be modified again at the axon terminal? To address this issue action potential axonal transmission was evaluated in a class of primary sensory afferents that possess numerous types of voltage-gated ion channels underlying a complex repertoire of endogenous firing patterns. In addition to their signature intrinsic electrophysiological heterogeneity, spiral ganglion neurons are uniquely designed. The bipolar, myelinated somata of type I neurons are located within the conduction pathway, requiring that action potentials generated at the first heminode must be conducted through their electrically excitable membrane. We utilized this unusual axonal-like morphology to serve as a window into action potential transmission to compare locally-evoked action potential profiles to those generated peripherally at their glutamatergic synaptic connections with hair cell receptors. These comparisons showed that the distinctively-shaped somatic action potentials were highly correlated with the nodally-generated, invading ones for each neuron. This result indicates that the fine structure of the action potential waveform is maintained axonally, thus supporting the concept that analog signaling is incorporated into each digitally-transmitted action potential in the specialized primary auditory afferents.

Dynamic Factors for Transmitter Release at Small Presynaptic Boutons Revealed by Direct Patch-Clamp Recordings

Small size of an axon and presynaptic structures have hindered direct functional analysis of axonal signaling and transmitter release at presynaptic boutons in the central nervous system. However, recent technical advances in subcellular patch-clamp recordings and in fluorescent imagings are shedding light on the dynamic nature of axonal and presynaptic mechanisms. Here I summarize the functional design of an axon and presynaptic boutons, such as diversity and activity-dependent changes of action potential (AP) waveforms, Ca²⁺ influx, and kinetics of transmitter release, revealed by the technical tour de force of direct patch-clamp recordings and the leading-edge fluorescent imagings. I highlight the critical factors for dynamic modulation of transmitter release and presynaptic short-term plasticity.

Past and Future of Analog-Digital Modulation of Synaptic Transmission

Action potentials (APs) are generally produced in response to complex summation of excitatory and inhibitory synaptic inputs. While it is usually considered as a digital event, both the amplitude and width of the AP are significantly impacted by the context of its emission. In particular, the analog variations in subthreshold membrane potential determine the spike waveform and subsequently affect synaptic strength, leading to the so-called analog-digital modulation of synaptic transmission. We review here the numerous evidence suggesting context-dependent modulation of spike waveform, the discovery analog-digital modulation of synaptic transmission in invertebrates and its recent validation in mammals. We discuss the potential roles of analog-digital transmission in the physiology of neural networks.

Kinetics of transmitter release at the calyx of Held synapse

Synaptic contacts mediate information transfer between neurons. The calyx of Held, a large synapse in the mammalian auditory brainstem, has been used as a model system for the mechanism of transmitter release from the presynaptic terminal for the last 20 years. By applying simultaneous recordings from pre- and postsynaptic compartments, the calcium-dependence of the kinetics of transmitter release has been quantified. A single pool of readily releasable vesicles cannot explain the time course of release during repetitive activity. Rather, multiple pools of vesicles have to be postulated that are replenished with distinct kinetics after depletion. The physical identity of vesicle replenishment has been unknown. Recently, it has become possible to apply total internal reflection fluorescent microscopy to the calyx terminal. This technique allowed the visualization of the dynamics of individual synaptic vesicles. Rather than recruitment of vesicles to the transmitter release sites, priming of tethered vesicles in the total internal reflection fluorescent field limited the number of readily releasable vesicles during sustained activity.

Diverse Synaptic Plasticity Induced by the Interplay of Ionic Polarization and Doping at Salt-Doped Electrolyte/Semiconducting Polymer Interface

Pt/Ca²⁺-polyethylene oxide/polymer poly[3-hexylthiophene-2,5-diyl]/Pt devices were fabricated, and their pulse responses were studied. The discharging peak, represented by the postsynaptic current (PSC), first increases and then decreases with increasing input number in a pulse train. The weight of the PSC decreased for low-frequency stimulations but increased for high-frequency stimulations. However, the peak of the negative differential resistance during the charging process varied following the opposite trend. These behaviors suggested the ability for transferring the signal bidirectionally, confirming the equivalence between the ionic kinetics of our device and the transmitter kinetics of one kind of synapse. A facilitation (F)-depression (D) interplay model corresponding to the ionic polarization and doping interplay at the electrolyte/semiconducting polymer interface was adopted to successfully mimic the weight modification of the PSC. The simulation results showed that the observed synaptic plasticity was caused by the great disparity between the recovery time constants of F and D (τ _F and τ _D ). Moreover, such an interplay could inspire the features of responses to post-tetanic stimulations. Our study suggested a means to realize synaptic computation.

Synaptic Vesicles: Turning Reluctance Into Action

Vesicle availability partly determines the efficacy of synaptic communication in the CNS. The authors recently found that some hippocampal glutamate vesicles exhibit reluctance to exocytose during short, high-frequency action potential trains. These same vesicles can be “coaxed” into exocytosis by increased Ca2+entry, by direct depolarization of synaptic terminals, or by challenge with hypertonic sucrose, a tool used to cause fusion of the population of release-ready synaptic vesicles. Interestingly, the authors did not find evidence of reluctance at hippocampal GABA synapses, suggesting that vesicle reluctance might be a negative feedback mechanism to prevent runaway excitation. It is also possible that synapses exhibit reluctance to retain a dormant population of readily accessible vesicles, ready to respond to triggers such as enhanced Ca2+ influx or neuromodulators. Recent work from the calyx of Held synapse suggests that reluctance might arise from inactivation of Ca2+ channels. The authors review this work, along with several other potential mechanisms of reluctance.

A Review on Locomotor Training after Spinal Cord Injury: Reorganization of Spinal Neuronal Circuits and Recovery of Motor Function

Locomotor training is a classic rehabilitation approach utilized with the aim of improving sensorimotor function and walking ability in people with spinal cord injury (SCI). Recent studies have provided strong evidence that locomotor training of persons with clinically complete, motor complete, or motor incomplete SCI induces functional reorganization of spinal neuronal networks at multisegmental levels at rest and during assisted stepping. This neuronal reorganization coincides with improvements in motor function and decreased muscle cocontractions. In this review, we will discuss the manner in which spinal neuronal circuits are impaired and the evidence surrounding plasticity of neuronal activity after locomotor training in people with SCI. We conclude that we need to better understand the physiological changes underlying locomotor training, use physiological signals to probe recovery over the course of training, and utilize established and contemporary interventions simultaneously in larger scale research studies. Furthermore, the focus of our research questions needs to change from feasibility and efficacy to the following: what are the physiological mechanisms that make it work and for whom? The aforementioned will enable the scientific and clinical community to develop more effective rehabilitation protocols maximizing sensorimotor function recovery in people with SCI.

Modulation of Synaptic Plasticity by Glutamatergic Gliotransmission: A Modeling Study

Glutamatergic gliotransmission, that is, the release of glutamate from perisynaptic astrocyte processes in an activity-dependent manner, has emerged as a potentially crucial signaling pathway for regulation of synaptic plasticity, yet its modes of expression and function in vivo remain unclear. Here, we focus on two experimentally well-identified gliotransmitter pathways, (i) modulations of synaptic release and (ii) postsynaptic slow inward currents mediated by glutamate released from astrocytes, and investigate their possible functional relevance on synaptic plasticity in a biophysical model of an astrocyte-regulated synapse. Our model predicts that both pathways could profoundly affect both short- and long-term plasticity. In particular, activity-dependent glutamate release from astrocytes could dramatically change spike-timing-dependent plasticity, turning potentiation into depression (and vice versa) for the same induction protocol.

Frequency-dependent learning achieved using semiconducting polymer/electrolyte composite cells

Frequency-dependent learning has been achieved using semiconducting polymer/electrolyte composite cells. The cells composed of polymer/electrolyte double layers realized the conventional spike-rate-dependent plasticity (SRDP) learning model. These cells responded to depression upon low-frequency stimulation and to potentiation upon high-frequency stimulation and presented long-term memory. The transition threshold θm from depression to potentiation varied depending on the previous stimulations. A nanostructure resembling a bio-synapse in its transport passages was demonstrated and a random channel model was proposed to describe the ionic kinetics at the polymer/electrolyte interface during and after stimulations with various frequencies, accounting for the observed SRDP.

The active-zone protein Munc13 controls the use-dependence of presynaptic voltage-gated calcium channels

Presynaptic calcium channel function is critical for converting electrical information into chemical communication but the molecules in the active zone that sculpt this function are poorly understood. We show that Munc13, an active-zone protein essential for exocytosis, also controls presynaptic voltage-gated calcium channel (VGCC) function dictating their behavior during various forms of activity. We demonstrate that in vitro Munc13 interacts with voltage-VGCCs via a pair of basic residues in Munc13's C2B domain. We show that elimination of this interaction by either removal of Munc13 or replacement of Munc13 with a Munc13 C2B mutant alters synaptic VGCC's response to and recovery from high-frequency action potential bursts and alters calcium influx from single action potential stimuli. These studies illustrate a novel form of synaptic modulation and show that Munc13 is poised to profoundly impact information transfer at nerve terminals by controlling both vesicle priming and the trigger for exocytosis.

Fidelity of frequency and phase entrainment of circuit-level spike activity during DBS

High-frequency stimulation is known to entrain spike activity downstream and upstream of several clinical deep brain stimulation (DBS) targets, including the cerebellar-receiving area of thalamus (VPLo), subthalamic nucleus (STN), and globus pallidus (GP). Less understood are the fidelity of entrainment to each stimulus pulse, whether entrainment patterns are stationary over time, and how responses differ among DBS targets. In this study, three rhesus macaques were implanted with a single DBS lead in VPLo, STN, or GP. Single-unit spike activity was recorded in the resting state in motor cortex during VPLo DBS, in GP during STN DBS, and in STN and pallidal-receiving area of motor thalamus (VLo) during GP DBS. VPLo DBS induced time-locked spike activity in 25% (n = 15/61) of motor cortex cells, with entrained cells following 7.5 ± 7.4% of delivered pulses. STN DBS entrained spike activity in 26% (n = 8/27) of GP cells, which yielded time-locked spike activity for 8.7 ± 8.4% of stimulus pulses. GP DBS entrained 67% (n = 14/21) of STN cells and 32% (n = 19/59) of VLo cells, which showed a higher fraction of pulses effectively inhibiting spike activity (82.0 ± 9.6% and 86.1 ± 16.6%, respectively). Latency of phase-locked spike activity increased over time in motor cortex (58%, VPLo DBS) and to a lesser extent in GP (25%, STN DBS). In contrast, the initial inhibitory phase observed in VLo and STN during GP DBS remained stable following stimulation onset. Together, these data suggest that circuit-level entrainment is low-pass filtered during high-frequency stimulation, most notably for glutamatergic pathways. Moreover, phase entrainment is not stationary or consistent at the circuit level for all DBS targets.

Abnormal cortical synaptic transmission in CaV2.1 knockin mice with the S218L missense mutation which causes a severe familial hemiplegic migraine syndrome in humans

Familial hemiplegic migraine type 1 (FHM1) is caused by gain-of-function mutations in Ca_V2.1 (P/Q-type) Ca²⁺ channels. Knockin (KI) mice carrying the FHM1 R192Q missense mutation show enhanced cortical excitatory synaptic transmission at pyramidal cell synapses but unaltered cortical inhibitory neurotransmission at fast-spiking interneuron synapses. Enhanced cortical glutamate release was shown to cause the facilitation of cortical spreading depression (CSD) in R192Q KI mice. It, however, remains unknown how other FHM1 mutations affect cortical synaptic transmission. Here, we studied neurotransmission in cortical neurons in microculture from KI mice carrying the S218L mutation, which causes a severe FHM syndrome in humans and an allele-dosage dependent facilitation of experimental CSD in KI mice, which is larger than that caused by the R192Q mutation. We show gain-of-function of excitatory neurotransmission, due to increased action-potential evoked Ca²⁺ influx and increased probability of glutamate release at pyramidal cell synapses, but unaltered inhibitory neurotransmission at multipolar interneuron synapses in S218L KI mice. In contrast with the larger gain-of-function of neuronal Ca_V2.1 current in homozygous than heterozygous S218L KI mice, the gain-of-function of evoked glutamate release, the paired-pulse ratio and the Ca²⁺ dependence of the excitatory postsynaptic current were similar in homozygous and heterozygous S218L KI mice, suggesting compensatory changes in the homozygous mice. Furthermore, we reveal a unique feature of S218L KI cortical synapses which is the presence of a fraction of mutant Ca_V2.1 channels being open at resting potential. Our data suggest that, while the gain-of-function of evoked glutamate release may explain the facilitation of CSD in heterozygous S218L KI mice, the further facilitation of CSD in homozygous S218L KI mice is due to other Ca_V2.1-dependent mechanisms, that likely include Ca²⁺ influx at voltages sub-threshold for action potential generation.

Controlling Ion Conductance and Channels to Achieve Synaptic-like Frequency Selectivity

Enhancing ion conductance and controlling transport pathway in organic electrolyte could be used to modulate ionic kinetics to handle signals. In a Pt/Poly(3-hexylthiophene-2,5-diyl)/Polyethylene+LiCF₃SO₃/Pt hetero-junction, the electrolyte layer handled at high temperature showed nano-fiber microstructures accompanied with greatly improved salt solubility. Ions with high mobility were confined in the nano-fibrous channels leading to the semiconducting polymer layer, which is favorable for modulating dynamic doping at the semiconducting polymer/electrolyte interface by pulse frequency. Such a device realized synaptic-like frequency selectivity, i.e., depression at low frequency stimulation but potentiation at high-frequency stimulation.

Frequency selectivity in pulse responses of Pt/poly(3-hexylthiophene-2,5-diyl)/polyethylene oxide +Li+/Pt hetero-junction

Pt/poly(3-hexylthiophene-2,5-diyl)/polyethylene oxide + Li+/Pt hetero junctions were fabricated, and their pulse responses were studied. The direct current characteristics were not symmetric in the sweeping range of ±2 V. Negative differential resistance appeared in the input range of 0 to 2 V because of de-doping (or reduction) in the side with the semiconductor layer. The device responded stably to a train of pulses with a fixed frequency. The inverse current after a pulse was related to the back-migrated ions. Importantly, the weight calculated based on the inverse current strength, was depressed during low-frequency stimulations but was potentiated during high-frequency stimulations when pulses were positive. Therefore, frequency selectivity was first observed in a semiconducting polymer/electrolyte hetero junction. Detailed analysis of the pulse response showed that the input frequency could modulate the timing of ion doping, de-doping, and re-doping at the semiconducting polymer/electrolyte interface, which then resulted in the frequency selectivity. Our study suggests that the simple redox process in semiconducting polymers can be modulated and used in signal handling or the simulation of bio-learning.

Mechanism underlying unaltered cortical inhibitory synaptic transmission in contrast with enhanced excitatory transmission in CaV2.1 knockin migraine mice

Familial hemiplegic migraine type 1 (FHM1), a monogenic subtype of migraine with aura, is caused by gain-of-function mutations in Ca_V2.1 (P/Q-type) calcium channels. In FHM1 knockin mice, excitatory neurotransmission at cortical pyramidal cell synapses is enhanced, but inhibitory neurotransmission at connected pairs of fast-spiking (FS) interneurons and pyramidal cells is unaltered, despite being initiated by Ca_V2.1 channels. The mechanism underlying the unaltered GABA release at cortical FS interneuron synapses remains unknown. Here, we show that the FHM1 R192Q mutation does not affect inhibitory transmission at autapses of cortical FS and other types of multipolar interneurons in microculture from R192Q knockin mice, and investigate the underlying mechanism. Lowering the extracellular [Ca²⁺] did not reveal gain-of-function of evoked transmission neither in control nor after prolongation of the action potential (AP) with tetraethylammonium, indicating unaltered AP-evoked presynaptic calcium influx at inhibitory autapses in FHM1 KI mice. Neither saturation of the presynaptic calcium sensor nor short duration of the AP can explain the unaltered inhibitory transmission in the mutant mice. Recordings of the P/Q-type calcium current in multipolar interneurons in microculture revealed that the current density and the gating properties of the Ca_V2.1 channels expressed in these interneurons are barely affected by the FHM1 mutation, in contrast with the enhanced current density and left-shifted activation gating of mutant Ca_V2.1 channels in cortical pyramidal cells. Our findings suggest that expression of specific Ca_V2.1 channels differentially sensitive to modulation by FHM1 mutations in inhibitory and excitatory cortical neurons underlies the gain-of-function of excitatory but unaltered inhibitory synaptic transmission and the likely consequent dysregulation of the cortical excitatory–inhibitory balance in FHM1.

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Multipolar interneuron autaptic transmission is unaltered in FHM1 knockin mice.

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This is due to unaltered action potential (AP)-evoked presynaptic Ca influx.

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Unaltered AP-evoked Ca influx is not due to short duration of interneuron APs.

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Ca_V2.1 channels of multipolar interneurons are barely affected by the FHM1 mutation.

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This may explain unaltered inhibitory neurotransmission in FHM1.

Locomotor training improves premotoneuronal control after chronic spinal cord injury

Spinal inhibition is significantly reduced after spinal cord injury (SCI) in humans. In this work, we examined if locomotor training can improve spinal inhibition exerted at a presynaptic level. Sixteen people with chronic SCI received an average of 45 training sessions, 5 days/wk, 1 h/day. The soleus H-reflex depression in response to low-frequency stimulation, presynaptic inhibition of soleus Ia afferent terminals following stimulation of the common peroneal nerve, and bilateral EMG recovery patterns were assessed before and after locomotor training. The soleus H reflexes evoked at 1.0, 0.33, 0.20, 0.14, and 0.11 Hz were normalized to the H reflex evoked at 0.09 Hz. Conditioned H reflexes were normalized to the associated unconditioned H reflex evoked with subjects seated, while during stepping both H reflexes were normalized to the maximal M wave evoked after the test H reflex at each bin of the step cycle. Locomotor training potentiated homosynaptic depression in all participants regardless the type of the SCI. Presynaptic facilitation of soleus Ia afferents remained unaltered in motor complete SCI patients. In motor incomplete SCIs, locomotor training either reduced presynaptic facilitation or replaced presynaptic facilitation with presynaptic inhibition at rest. During stepping, presynaptic inhibition was modulated in a phase-dependent manner. Locomotor training changed the amplitude of locomotor EMG excitability, promoted intralimb and interlimb coordination, and altered cocontraction between knee and ankle antagonistic muscles differently in the more impaired leg compared with the less impaired leg. The results provide strong evidence that locomotor training improves premotoneuronal control after SCI in humans at rest and during walking.

Continuously tunable Ca(2+) regulation of RNA-edited CaV1.3 channels

CaV1.3 ion channels are dominant Ca(2+) portals into pacemaking neurons, residing at the epicenter of brain rhythmicity and neurodegeneration. Negative Ca(2+) feedback regulation of CaV1.3 channels (CDI) is therefore critical for Ca(2+) homeostasis. Intriguingly, nearly half the CaV1.3 transcripts in the brain are RNA edited to reduce CDI and influence oscillatory activity. It is then mechanistically remarkable that this editing occurs precisely within an IQ domain, whose interaction with Ca(2+)-bound calmodulin (Ca(2+)/CaM) is believed to induce CDI. Here, we sought the mechanism underlying the altered CDI of edited channels. Unexpectedly, editing failed to attenuate Ca(2+)/CaM binding. Instead, editing weakened the prebinding of Ca(2+)-free CaM (apoCaM) to channels, which proves essential for CDI. Thus, editing might render CDI continuously tunable by fluctuations in ambient CaM, a prominent effect we substantiate in substantia nigral neurons. This adjustability of Ca(2+) regulation by CaM now looms as a key element of CNS Ca(2+) homeostasis.

Neurophysiological characterization of transpinal evoked potentials in human leg muscles

The objectives of this study were to characterize the neurophysiological properties of the compound muscle action potentials (CMAPs) evoked by transcutaneous electric stimulation of the spine (tsESS), and the effects of tsESS on the soleus H-reflex in seated and standing healthy human subjects. In seated semi-prone subjects with the trunk semi-flexed, two re-usable self-adhering electrodes (cathode), connected to act as one electrode, were placed bilaterally on the iliac crests. A re-usable pregelled electrode (anode) was placed on the thoracolumbar region at thoracic 10-lumbar 1 and held under constant pressure throughout the experiment. CMAPs were recorded bilaterally from ankle muscles with subjects seated semi-prone at 1.0, 0.3, 0.2, 0.125, and 0.1 Hz following tsESS. The soleus H-reflex, evoked by posterior tibial nerve stimulation via conventional methods, was measured following tsESS at inter-stimulus intervals (ISIs) that ranged from -100 to 100 ms with the subjects seated semi-prone and during standing. The tsESS-induced CMAPs were not decreased at low stimulation frequencies, and the soleus H-reflex excitability was profoundly decreased by tsESS at ISIs that ranged from -5 to 20 ms with the subjects seated semi-prone and during standing. CMAPs induced by tsESS may be utilized to assess spinal-to-muscle conduction time while bypassing spinal motoneuron excitability and tsESS can be used as a modality to decrease spinal reflex hyper-excitability in neurological disorders.

Fast spiking interneuron control of seizure propagation in a cortical slice model of focal epilepsy

In different animal models of focal epilepsy, seizure-like ictal discharge propagation is transiently opposed by feedforward inhibition. The specific cellular source of this signal and the mechanism by which inhibition ultimately becomes ineffective are, however, undefined. We used a brain slice model to study how focal ictal discharges that were repetitively evoked from the same site, and at precise times, propagate across the cortex. We used Ca(2+) imaging and simultaneous single/dual cell recordings from pyramidal neurons (PyNs) and different classes of interneurons in rodents, including G42 and GIN transgenic mice expressing the green fluorescence protein in parvalbumin (Pv)-fast spiking (FS) and somatostatin (Som) interneurons, respectively. We found that these two classes of interneurons fired intensively shortly after ictal discharge generation at the focus. The inhibitory barrages that were recorded in PyNs occurred in coincidence with Pv-FS, but not with Som interneuron burst discharges. Furthermore, the strength of inhibitory barrages increased or decreased in parallel with increased or decreased firing in Pv-FS interneurons but not in Som interneurons. A firing impairment of Pv-FS interneurons caused by a membrane depolarization was found to precede ictal discharge onset in neighbouring pyramidal neurons. This event may account for the collapse of local inhibition that allows spatially defined clusters of PyNs to be recruited into propagating ictal discharges. Our study demonstrates that Pv-FS interneurons are a major source of the inhibitory barrages that oppose ictal discharge propagation and raises the possibility that targeting Pv-FS interneurons represents a new therapeutic strategy to prevent the generalization of human focal seizures.

Quantifying impacts of short-term plasticity on neuronal information transfer

Short-term changes in efficacy have been postulated to enhance the ability of synapses to transmit information between neurons, and within neuronal networks. Even at the level of connections between single neurons, direct confirmation of this simple conjecture has proven elusive. By combining paired-cell recordings, realistic synaptic modeling, and information theory, we provide evidence that short-term plasticity can not only improve, but also reduce information transfer between neurons. We focus on a concrete example in rat neocortex, but our results may generalize to other systems. When information is contained in the timings of individual spikes, we find that facilitation, depression, and recovery affect information transmission in proportion to their impacts upon the probability of neurotransmitter release. When information is instead conveyed by mean spike rate only, the influences of short-term plasticity critically depend on the range of spike frequencies that the target network can distinguish (its effective dynamic range). Our results suggest that to efficiently transmit information, the brain must match synaptic type, coding strategy, and network connectivity during development and behavior.

Beyond faithful conduction: short-term dynamics, neuromodulation, and long-term regulation of spike propagation in the axon

Most spiking neurons are divided into functional compartments: a dendritic input region, a soma, a site of action potential initiation, an axon trunk and its collaterals for propagation of action potentials, and distal arborizations and terminals carrying the output synapses. The axon trunk and lower order branches are probably the most neglected and are often assumed to do nothing more than faithfully conducting action potentials. Nevertheless, there are numerous reports of complex membrane properties in non-synaptic axonal regions, owing to the presence of a multitude of different ion channels. Many different types of sodium and potassium channels have been described in axons, as well as calcium transients and hyperpolarization-activated inward currents. The complex time- and voltage-dependence resulting from the properties of ion channels can lead to activity-dependent changes in spike shape and resting potential, affecting the temporal fidelity of spike conduction. Neural coding can be altered by activity-dependent changes in conduction velocity, spike failures, and ectopic spike initiation. This is true under normal physiological conditions, and relevant for a number of neuropathies that lead to abnormal excitability. In addition, a growing number of studies show that the axon trunk can express receptors to glutamate, GABA, acetylcholine or biogenic amines, changing the relative contribution of some channels to axonal excitability and therefore rendering the contribution of this compartment to neural coding conditional on the presence of neuromodulators. Long-term regulatory processes, both during development and in the context of activity-dependent plasticity may also affect axonal properties to an underappreciated extent.

Axon Physiology

Axons are generally considered as reliable transmission cables in which stable propagation occurs once an action potential is generated. Axon dysfunction occupies a central position in many inherited and acquired neurological disorders that affect both peripheral and central neurons. Recent findings suggest that the functional and computational repertoire of the axon is much richer than traditionally thought. Beyond classical axonal propagation, intrinsic voltage-gated ionic currents together with the geometrical properties of the axon determine several complex operations that not only control signal processing in brain circuits but also neuronal timing and synaptic efficacy. Recent evidence for the implication of these forms of axonal computation in the short-term dynamics of neuronal communication is discussed. Finally, we review how neuronal activity regulates both axon morphology and axonal function on a long-term time scale during development and adulthood.

Synaptically activated burst-generating conductances may underlie a group-pacemaker mechanism for respiratory rhythm generation in mammals

Breathing, chewing, and walking are critical life-sustaining behaviors in mammals that consist essentially of simple rhythmic movements. Breathing movements in particular involve the diaphragm, thorax, and airways but emanate from a network in the lower brain stem. This network can be studied in reduced preparations in vitro and using simplified mathematical models that make testable predictions. An iterative approach that employs both in vitro and in silico models argues against canonical mechanisms for respiratory rhythm in neonatal rodents that involve reciprocal inhibition and pacemaker properties. We present an alternative model in which emergent network properties play a rhythmogenic role. Specifically, we show evidence that synaptically activated burst-generating conductances-which are only available in the context of network activity-engender robust periodic bursts in respiratory neurons. Because the cellular burst-generating mechanism is linked to network synaptic drive we dub this type of system a group pacemaker.

Dynamic modulation of phasic and asynchronous glutamate release in hippocampal synapses

Although frequency-dependent short-term presynaptic plasticity has been of long-standing interest, most studies have emphasized modulation of the synchronous, phasic component of transmitter release, most evident with a single or a few presynaptic stimuli. Asynchronous transmitter release, vesicle fusion not closely time locked to presynaptic action potentials, can also be prominent under certain conditions, including repetitive stimulation. Asynchrony has often been attributed to residual Ca(2+) buildup in the presynaptic terminal. We verified that a number of manipulations of Ca(2+) handling and influx selectively alter asynchronous release relative to phasic transmitter release during action potential trains in cultured excitatory autaptic hippocampal neurons. To determine whether other manipulations of vesicle release probability also selectively modulate asynchrony, we probed the actions of one thoroughly studied modulator class whose actions on phasic versus asynchronous release have not been investigated. We examined the effects of the phorbol ester PDBu, which has protein kinase C (PKC) dependent and independent actions on presynaptic transmitter release. PDBu increased phasic and asynchronous release in parallel. However, while PKC inhibition had relatively minor inhibitory effects on PDBu potentiation of phasic and total release during action potential trains, PKC inhibition strongly reduced phorbol-potentiated asynchrony, through actions most evident late during stimulus trains. These results lend new insight into PKC-dependent and -independent effects on transmitter release and suggest the possibility of differential control of synchronous versus asynchronous vesicle release.

Calcium-activated nonspecific cation current and synaptic depression promote network-dependent burst oscillations

Central pattern generators (CPGs) produce neural-motor rhythms that often depend on specialized cellular or synaptic properties such as pacemaker neurons or alternating phases of synaptic inhibition. Motivated by experimental evidence suggesting that activity in the mammalian respiratory CPG, the preBötzinger complex, does not require either of these components, we present and analyze a mathematical model demonstrating an unconventional mechanism of rhythm generation in which glutamatergic synapses and the short-term depression of excitatory transmission play key rhythmogenic roles. Recurrent synaptic excitation triggers postsynaptic Ca(2+)-activated nonspecific cation current (I(CAN)) to initiate a network-wide burst. Robust depolarization due to I(CAN) also causes voltage-dependent spike inactivation, which diminishes recurrent excitation and thus attenuates postsynaptic Ca(2+) accumulation. Consequently, activity-dependent outward currents-produced by Na/K ATPase pumps or other ionic mechanisms-can terminate the burst and cause a transient quiescent state in the network. The recovery of sporadic spiking activity rekindles excitatory interactions and initiates a new cycle. Because synaptic inputs gate postsynaptic burst-generating conductances, this rhythm-generating mechanism represents a new paradigm that can be dubbed a 'group pacemaker' in which the basic rhythmogenic unit encompasses a fully interdependent ensemble of synaptic and intrinsic components. This conceptual framework should be considered as an alternative to traditional models when analyzing CPGs for which mechanistic details have not yet been elucidated.

Heterogeneous Ca2+ influx along the adult calyx of Held: a structural and computational study

The calyx of Held is a morphologically complex nerve terminal containing hundreds to thousands of active zones. The calyx must support high rates of transient, sound-evoked vesicular release superimposed on a background of sustained release, due to the high spontaneous rates of some afferent fibers. One means of distributing vesicle release in space and time is to have heterogeneous release probabilities (Pr) at distinct active zones, which has been observed at several CNS synapses including the calyx of Held. Pr may be modulated by vesicle proximity to Ca2+ channels, by Ca2+ buffers, by changes in phosphorylation state of proteins involved in the release process, or by local variations in Ca2+ influx. In this study, we explore the idea that the complex geometry of the calyx also contributes to heterogeneous Pr by impeding equal propagation of action potentials through all calyx compartments. Given the difficulty of probing ion channel distribution and recording from adult calyces, we undertook a structural and modeling approach based on computerized reconstructions of calyces labeled in adult cats. We were thus able to manipulate placement of conductances and test their effects on Ca2+ concentration in all regions of the calyx following an evoked action potential in the calyceal axon. Our results indicate that with a non-uniform distribution of Na+ and K+ channels, action potentials do not propagate uniformly into the calyx, Ca2+ influx varies across different release sites, and latency for these events varies among calyx compartments. We suggest that the electrotonic structure of the calyx of Held, which our modeling efforts indicate is very sensitive to the axial resistivity of cytoplasm, may contribute to variations in release probability within the calyx.

Facilitation versus depression in cultured hippocampal neurons determined by targeting of Ca2+ channel Cavbeta4 versus Cavbeta2 subunits to synaptic terminals

Ca²⁺ channel β subunits determine the transport and physiological properties of high voltage–activated Ca²⁺ channel complexes. Our analysis of the distribution of the Ca_vβ subunit family members in hippocampal neurons correlates their synaptic distribution with their involvement in transmitter release. We find that exogenously expressed Ca_vβ_4b and Ca_vβ_2a subunits distribute in clusters and localize to synapses, whereas Ca_vβ_1b and Ca_vβ₃ are homogenously distributed. According to their localization, Ca_vβ_2a and Ca_vβ_4b subunits modulate the synaptic plasticity of autaptic hippocampal neurons (i.e., Ca_vβ_2a induces depression, whereas Ca_vβ_4b induces paired-pulse facilitation [PPF] followed by synaptic depression during longer stimuli trains). The induction of PPF by Ca_vβ_4b correlates with a reduction in the release probability and cooperativity of the transmitter release. These results suggest that Ca_vβ subunits determine the gating properties of the presynaptic Ca²⁺ channels within the presynaptic terminal in a subunit-specific manner and may be involved in organization of the Ca²⁺ channel relative to the release machinery.

Preparation and maintenance of single-cell micro-island cultures of basal forebrain neurons

Micro-island cultures provide a simplified system for studying the expression of cellular phenotype, excitability, synapse formation and pre- and postsynaptic regulatory mechanisms without the usual problems that arise from complex interactions between large numbers of other cells. The technique relies on the ability to constrain the attachment and growth of either single or small groups of neurons to discrete (20-500 microm) 'islands' of cell-permissive substrate applied over a nonadherent background layer. Constrained in this way, neurons form large numbers of conventional synaptic and/or autaptic contacts that can be easily visualized, making them ideally suited for studying synaptic physiology using electrophysiological and/or high-resolution optical imaging techniques. The protocol described here requires approximately 2 h for preparation of the culture dishes and a further 3-4 h for isolation and plating out the cells. Once established, the cultures can be maintained for prolonged periods (>6 weeks) permitting manipulations to be made to their local environment and the effects on individually identified cells to be repeatedly monitored.

Discharge of the readily releasable pool with action potentials at hippocampal synapses

A readily releasable pool (RRP) of synaptic vesicles has been identified at hippocampal synapses with application of hypertonic solution. RRP size correlates with important properties of synaptic function such as release probability. However, a discrepancy in RRP size has been reported depending on the method used to evoke synaptic release. This study was undertaken to determine quantitative relationships between the RRP defined with hypertonic solution and that released with trains of action potentials. We find that asynchronous release at cell culture synapses contributes significantly to the discharge of the RRP with trains of action potentials and that RRP size is the same when elicited by either nerve stimuli or hypertonic challenge.

Paired-pulse ratio of synaptically induced transporter currents at hippocampal CA1 synapses is not related to release probability

When a synapse is stimulated in rapid succession, the second post-synaptic response can be larger than the first and termed paired-pulse facilitation. It has been reported that the paired-pulse ratio (PPR), which is the ratio of the amplitude of the second response to that of the first, depends on the probability of vesicular release at the synapse, and PPR has been used as an easy measure of the release probability. To re-examine the relation of PPR with transmitter release probability, we made whole-cell recordings from astrocytes and pyramidal neurons in the CA1 area of rat hippocampal slices, and studied responses evoked by paired-pulse stimulus of the Schaffer collaterals. In a control condition in which blockers for ionotropic glutamate receptors were added to the artificial cerebrospinal fluid, synaptically induced transporter currents (STCs) recorded from astrocytes showed PPF with similar dependency on stimulus interval as the AMPA-receptor-mediated excitatory post-synaptic currents (AMPA-EPSCs) recorded from pyramidal neurons. When the transmitter release was enhanced by raising Ca2+ concentration in the bathing medium or by applying 8-CPT, an adenosine A1 receptor antagonist, the PPR of the neuronal AMPA-EPSCs decreased significantly. In the same condition, although the amplitude of STCs was significantly increased, the PPR of STCs did not show significant change. The PPR of AMPA-EPSCs, however, recovered by lowering the stimulus intensity or by applying low concentration of NBQX, a competitive antagonist for AMPA-receptor. These results imply that the PPR of transmitter release at Schaffer collateral synapses stays constant as the release probability was altered.

Effective release rates at single rat Schaffer collateral-CA1 synapses during sustained theta-burst activity revealed by optical imaging

To understand how information is coded at single hippocampal synapses during high-frequency activity, we imaged NMDA receptor-mediated Ca(2+) responses in spines of CA1 neurons using two-photon microscopy. Although discrete quantal events were not readily apparent during continuous theta-burst stimulation (TBS), we found that the steady-state dendritic Ca(2+) response was spatially restricted (half-width < 1 microm), voltage dependent and sensitive to MK-801, indicating that that it was mediated by activation of NMDA receptors at single synapses. Partial antagonism of NMDA receptors caused a similar reduction of NMDA EPSCs (measured at the soma) and local dendritic Ca(2+) signals, suggesting that, like EPSCs, the steady-state Ca(2+) signal was made up of a linear addition of quantal events. Statistical analyses of the steady-response suggested that the quantal size did not change dramatically during TBS. Deconvolution of TBS-evoked Ca(2+) responses revealed a heterogeneous population of synapses differing in their capacity to signal high-frequency information, with an average effective steady-state release rate of approximately 2.6 vesicles synapse(-1)s(-1). To assess how the optically determined release rates compare with population measures we analysed the rate of decay of peak EPSCs during train stimulation. From these studies, we estimated a unitary vesicular replenishment rate of 0.02 s(-1), which corresponds to an average release rate of approximately 0.8-2 vesicles s(-1) at individual synapses. Additionally, extracellular recordings from single Schaffer collaterals revealed that spikes propagate reliably during TBS. Hence, during high-frequency activity, Schaffer collaterals conduct spikes with high fidelity, but release quanta with relatively lower efficiency, leaving NMDA receptor function largely intact and synapse specific. Heterogeneity in release rates between synapses suggests that similar patterns of presynaptic action potentials could trigger different forms of plasticity at individual synapses.

Transforming growth factor-beta2 modulates synaptic efficacy and plasticity and induces phosphorylation of CREB in hippocampal neurons

Transforming growth factor-betas (TGF-betas) are widely expressed and play roles as multifunctional growth factors and regulators of key events in development, disease, and repair. However, it is not known whether TGF-betas affect the plasticity of hippocampal neurons. As a first step to address this issue, we examined whether TGF-beta2 modulated the electrophysiological and biochemical properties of cultured hippocampal neurons. We found that prolonged 24 h treatment with TGF-beta2 induced facilitation of evoked postsynaptic currents (ePSCs). This facilitation was associated with a decrease in short-term synaptic depression of ePSCs and increases in both the amplitude and frequency of spontaneous miniature postsynaptic currents (mPSCs). The long-term changes of ePSCs and mPSCs may be associated with cAMP response element-binding protein (CREB), which has been previously implicated in long-term potentiation. Immunofluorescence techniques and Western blot analysis both revealed that TGF-beta2 enhanced the phosphorylation of CREB. Together, these results suggest that TGF-beta2 may play a role in the cascade of events underlying long-term synaptic facilitation in hippocampus, and that CREB may be an important mediator of these effects.

Spinal use-dependent plasticity of synaptic transmission in humans after a single cycling session

The spinal cord is able to express use-dependent plasticity, as demonstrated in spinalized cats following treadmill training. In humans, spinal use-dependent plasticity is inferred from modifications in the size of H reflex, which are often more prominent after skilled motor training. Plasticity can develop at synaptic connections between afferent fibres and/or descending tracts and motoneurones or interneurones interposed in the spinal pathways. Here we explore whether skilled training induces a change in synaptic efficacy at the synapse between Ia afferents and soleus (Sol) motoneurones. Synaptic efficacy can be modulated presynaptically through changes of the probability of transmitter release (homosynaptic depression, HD). The frequency-related depression of the Sol H reflex, thought to reflect HD, was tested at rest, before and after one single skilled (14 subjects) or non-skilled (9 subjects) cycling training session. Performance improved in both groups but to a larger extent with skilled training, while HD increased immediately after and the day following skilled training in the absence of changes with non-skilled training. These results support the view that spinal cord function is able to encode a local motor memory.

Presynaptic fluctuations and release-independent depression

Although vesicle depletion contributes to short-term depression, several studies have reported that the release probability can be transiently depressed even if an action potential fails to evoke release. Here we argue that stochastic fluctuation in the release machinery can give rise to apparent release-independent depression as a result of sampling bias. The relationship between this apparent depression and the interstimulus interval provides a window on the kinetics of state transitions of the release apparatus.

Visual stimuli–induced LTD of GABAergic synapses mediated by presynaptic NMDA receptors

Local GABA (gamma-aminobutyric acid) circuits contribute to sensory experience-dependent refinement of neuronal connections in the developing nervous system, but whether GABAergic synapses themselves can be rapidly modified by sensory stimuli is largely unknown. Here we report that repetitive light stimuli or theta burst stimulation (TBS) of the optic nerve in the developing Xenopus retinotectal system induces long-term potentiation (LTP) of glutamatergic inputs but long-term depression (LTD) of GABAergic inputs to the same tectal neuron. The LTD is due to a reduction in presynaptic GABA release and requires activation of presynaptic NMDA (N-methyl-D-aspartate) receptors (NMDARs) and coincident high-level GABAergic activity. Thus, the presynaptic NMDAR may function as a coincidence detector for adjacent glutamatergic and GABAergic activities, leading to coordinated synaptic modification by sensory experience.

Mechanisms of target-cell specific short-term plasticity at Schaffer collateral synapses onto interneurones versus pyramidal cells in juvenile rats

Although it is presynaptic, short-term plasticity has been shown at some synapses to depend upon the postsynaptic cell type. Previous studies have reported conflicting results as to whether Schaffer collateral axons have target-cell specific short-term plasticity. Here we investigate in detail the short-term dynamics of Schaffer collateral excitatory synapses onto CA1 stratum radiatum interneurones versus pyramidal cells in acute hippocampal slices from juvenile rats. In response to three stimulus protocols that invoke different forms of short-term plasticity, we find differences in some but not all forms of presynaptic short-term plasticity, and heterogeneity in the short term plasticity of synapses onto interneurones. Excitatory synapses onto the majority of interneurones had less paired-pulse facilitation than synapses onto pyramidal cells across a range of interpulse intervals (20-200 ms). Unlike synapses onto pyramidal cells, synapses onto most interneurones had very little facilitation in response to short high-frequency trains of five pulses at 5, 10 and 20 Hz, and depressed during trains at 50 Hz. However, the amount of high-frequency depression was not different between synapses onto pyramidal cells versus the majority of interneurones at steady state during 2-10 Hz trains. In addition, a small subset of interneurones (approximately 15%) had paired-pulse depression rather than paired-pulse facilitation, showed only depression in response to the high-frequency five pulse trains, and had more steady-state high-frequency depression than synapses onto pyramidal cells or the majority of interneurones. To investigate possible mechanisms for these differences in short-term plasticity, we developed a mechanistic mathematical model of neurotransmitter release that explicitly explores the contributions to different forms of short-term plasticity of the readily releasable vesicle pool size, release probability per vesicle, calcium-dependent facilitation, synapse inactivation following release, and calcium-dependent recovery from inactivation. Our model fits the responses of each of the three cell groups to the three different stimulus protocols with only two parameters that differ with cell group. The model predicts that the differences in short-term plasticity between synapses onto CA1 pyramidal cells and stratum radiatum interneurones are due to a higher initial release probability per vesicle and larger readily releasable vesicle pool size at synapses onto interneurones, resulting in a higher initial release probability. By measuring the rate of block of NMDA receptors by the open channel blocker MK-801, we confirmed that the initial release probability is greater at synapses onto interneurones versus pyramidal cells. This provides a mechanism by which both the initial strength and the short-term dynamics of Schaffer collateral excitatory synapses are regulated by their postsynaptic target cell.

Reluctant vesicles coaxed into the limelight

Synapses respond to brief, repetitive stimulation with synaptic depression when initial transmitter release probability is high. Vesicle depletion has been a long-standing hypothesis for depression, but results unexplained by the depletion hypothesis have been nagging. In this issue of Neuron, Xu and Wu show that, under some conditions, calcium current inactivation explains stimulus-dependent depression at the calyx of Held.

The decrease in the presynaptic calcium current is a major cause of short-term depression at a calyx-type synapse

Repetitive nerve firings cause short-term depression (STD) of release at many synapses. Its underlying mechanism is largely attributed to depletion of a readily releasable vesicle pool (RRP) and a decreased probability of releasing a readily releasable vesicle during an action potential. Which of these two mechanisms is dominant and the mechanism that decreases the release probability remain debated. Here, we report that a decreased release probability is caused by a calcium-induced inhibition of presynaptic calcium channels, particularly P/Q-type channels at the calyx of Held in rat brainstem. This mechanism was the dominant cause of STD in a wide range of stimulation conditions, such as during 2 to 20 action potential-equivalent stimuli (AP-e) at 0.2-30 Hz and after 2 to 20 AP-e at 0.2-100 Hz. Only during > or = 100 Hz AP-e was depletion the dominant mechanism.

Properties of short-term synaptic depression at larval neuromuscular synapses in wild-type and temperature-sensitive paralytic mutants of Drosophila

The larval neuromuscular synapse of Drosophila serves as an important model for genetic and molecular analysis of synaptic development and function. Further functional characterization of this synapse, as well as adult neuromuscular synapses, will greatly enhance the impact of this model system on our understanding of synaptic transmission. Here we describe a form of short-term synaptic depression observed at larval, but not adult, neuromuscular synapses and explore the underlying mechanisms. Larval neuromuscular synapses exhibited a form of short-term depression that was strongly dependent on stimulation frequency over a narrow range of low frequencies (0.1-1 Hz). This form of synaptic depression, referred to here as low-frequency short-term depression (LF-STD), results from an activity-dependent reduction in neurotransmitter release. However, in contrast to the predictions of depletion models, the degree of depression was independent of the initial level of neurotransmitter release over a range of extracellular calcium concentrations. This conclusion was confirmed in two temperature-sensitive (TS) paralytic mutants, cacophony and shibire, which exhibit reduced neurotransmitter release resulting from conditional disruption of presynaptic calcium channels and dynamin, respectively. Higher stimulation frequencies (40 or 60 Hz) produced two components of depression that appeared to include LF-STD as well as a more conventional component of short-term depression. These findings reveal novel properties of short-term synaptic depression and suggest that complementary genetic analysis of larval and adult neuromuscular synapses will further define the in vivo mechanisms of neurotransmitter release and short-term synaptic plasticity.

Brain plasticity and ion channels

It is generally believed that spatio-temporal configurations of distributed activity in the brain contribute to the coding of neuronal information and that synaptic contacts between nerve cells could play a central role in the formation of privileged pathways of activity. Synaptic plasticity is not the only mode of regulation of information processing in the brain and persistent regulations of ionic conductances in some specialized neuronal areas such as the dendrites, the cell body and the axon could also modulate, in the short- and the long-term, the propagation of information in the brain. Persistent changes in intrinsic excitability have been reported in several brain areas in which activity is modified during a classical conditioning. The role of synaptic activity seems to be determinant in the induction but the learning rules and the underlying mechanisms remain to be defined. This review discusses the role of neuronal activity in the induction of intrinsic plasticity in cortical, hippocampal and cerebellar neurons. Activation and inactivation properties of ionic channels in the axon determine the short-term dynamics of axonal propagation and synaptic transmission. Activation of glutamate receptors initiates a long-term modification in neuronal excitability that may represent the substrate for the mnesic engram and for the stabilization of the epileptic state. Similarly to synaptic plasticity, long-lasting intrinsic plasticity appears to be reversible and to express a certain level of input or cellular specificity. These non-synaptic forms of plasticity affect the signal propagation in the axon, the dendrites and the soma. They not only share common learning rules and induction pathways with the better known synaptic plasticity such as NMDA receptor-dependent LTP and LTD but also contribute in synergy with these synaptic changes to the formation of a coherent mnesic engram.

Activity-dependent excitability changes in hippocampal CA3 cell Schaffer axons

The membrane potential changes following action potentials in thin unmyelinated cortical axons with en passant boutons may be important for synaptic release and conduction abilities of such axons. In the lack of intra-axonal recording techniques we have used extracellular excitability testing as an indirect measure of the after-potentials. We recorded from individual CA3 soma in hippocampal slices and activated the axon with a range of stimulus intensities. When conditioning and test stimuli were given to the same site the excitability changes were partly masked by local effects of the stimulating electrode at intervals < 5 ms. Therefore, we elicited the conditioning action potential from one axonal branch and tested the excitability of another branch. We found that a single action potential reduced the axonal excitability for 15 ms followed by an increased excitability for approximately 200 ms at 24 degrees C. Using field recordings of axonal action potentials we show that raising the temperature to 34 degrees C reduced the magnitude and duration of the initial depression. However, the duration of the increased excitability was very similar (time constant 135 +/- 20 ms) at 24 and 34 degrees C, and with 2.0 and 0.5 mM Ca2+ in the bath. At stimulus rates > 1 Hz, a condition that activates a hyperpolarization-activated current (Ih) in these axons, the decay was faster than at lower stimulation rates. This effect was reduced by the Ih blocker ZD7288. These data suggest that the decay time course of the action potential-induced hyperexcitability is determined by the membrane time constant.

Differential presynaptic modulation of excitatory and inhibitory autaptic currents in cultured hippocampal neurons

Short-term synaptic plasticity has an important role in higher cortical function. Hyperpolarization may effect a form of short-term plasticity by promoting recovery from sodium channel inactivation or by activating axonal A-type potassium channels. To determine whether one or both processes occur, we examined the effect of hyperpolarizing prepulses on autaptic currents in cultured postnatal rat hippocampal neurons. As expected of enhanced recovery from sodium channel inactivation, hyperpolarizing prepulses reversibly increased fast excitatory autaptic currents (eacs) mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), slow eacs mediated by N-methyl-D-aspartate receptors (NMDARs), and inhibitory autaptic currents (iacs) mediated by gamma-aminobutyric acidA receptors (GABAARs). Hyperpolarizing prepulses augmented nearly all fast and slow eacs but only half of the iacs. This change occurred without a change in autaptic current kinetics. Of note, hyperpolarizing prepulses did not significantly reduce autaptic currents in any neuron studied. The rapidly dissociating competitive antagonists kynurenate and L-2-amino-5-phosphonovaleric acid (LAPV) inhibited fast and slow eacs, respectively, to the same extent with and without a hyperpolarizing prepulse. In addition, hyperpolarizing prepulses revealed a slow eac even after the slow eac evoked without a prepulse was completely blocked by the open channel blocker, (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801). Finally, hyperpolarizing prepulses did not alter currents evoked by exogenous applications of glutamate and GABA. These findings suggest that hyperpolarizing prepulses preferentially enhance eacs over iacs, and that they do so, in part, by overcoming conduction block or by activating silent synapses.

Variance-Mean Analysis in the Presence of a Rapid Antagonist Indicates Vesicle Depletion Underlies Depression at the Climbing Fiber Synapse

Many types of synapses throughout the nervous system are transiently depressed during high-frequency stimulation. Several mechanisms have been proposed to account for this depression, including depletion of release-ready vesicles. However, numerous studies have raised doubts about the importance of depletion in depression of central synapses and have implicated alternative mechanisms, such as decreased release probability. We use variance-mean analysis to determine the mechanism of depression at the climbing fiber to Purkinje cell synapse. We find that postsynaptic receptor saturation makes it difficult to distinguish between a decrease in available vesicles and a reduction in release probability. When AMPA receptor saturation is relieved with a low-affinity antagonist, variance-mean analysis reveals that depression arises from a decrease in the number of release-ready vesicles. Vesicle depletion is prominent, despite numerous docked vesicles at each release site, due to multivesicular release. We conclude that vesicle depletion can contribute significantly to depression of central synapses.