A quantum of neurotransmitter causes minis in multiple postsynaptic cells at the Caenorhabditis elegans neuromuscular junction

The polyadic synapse, where a single presynaptic active zone associates with two or more postsynaptic cells, exists in both mammals and invertebrates. An important but unresolved question is whether synaptic transmission occurs between the presynaptic site and its various postsynaptic partners. Using the dual whole-cell voltage clamp technique, we analyzed miniature postsynaptic currents (mPSCs or minis) at the C. elegans neuromuscular junction (NMJ), which is a polyadic synapse. We found that neighboring muscle cells at the same position along the body axis had high frequencies of concurrent mPSCs, which could not be explained by pure chance. Although body-wall muscle cells are electrically coupled, the high frequency of concurrent mPSCs was not due to electrical coupling because there was no correlation between the frequency of concurrent mPSCs and the degree of electrical coupling; the rise time of concurrent mPSCs was identical to that of nonconcurrent mPSCs but distinct from that of junctional currents (I(j)); and a mutant defective in electrical coupling showed normal frequency of concurrent mPSCs. Our analyses suggest that a single quantum of neurotransmitter may cause mPSCs in multiple postsynaptic cells at polyadic synapses, and that high-fidelity synaptic transmission occurs between the presynaptic site and its various postsynaptic partners. Thus, polyadic synapses could be a distinct mechanism for synaptic divergence and for synchronizing activities of postsynaptic cells.     

Modelling Feedback Excitation,Pacemaker Properties and Sensory Switching of Electrically Coupled Brainstem Neurons Controlling Rhythmic Activity

What cellular and network properties allow reliable neuronal rhythm generation or firing that can be started and stopped by brief synaptic inputs? We investigate rhythmic activity in an electrically-coupled population of brainstem neurons driving swimming locomotion in young frog tadpoles, and how activity is switched on and off by brief sensory stimulation. We build a computational model of 30 electrically-coupled conditional pacemaker neurons on one side of the tadpole hindbrain and spinal cord. Based on experimental estimates for neuron properties, population sizes, synapse strengths and connections, we show that: long-lasting, mutual, glutamatergic excitation between the neurons allows the network to sustain rhythmic pacemaker firing at swimming frequencies following brief synaptic excitation; activity persists but rhythm breaks down without electrical coupling; NMDA voltage-dependency doubles the range of synaptic feedback strengths generating sustained rhythm. The network can be switched on and off at short latency by brief synaptic excitation and inhibition. We demonstrate that a population of generic Hodgkin-Huxley type neurons coupled by glutamatergic excitatory feedback can generate sustained asynchronous firing switched on and off synaptically. We conclude that networks of neurons with NMDAR mediated feedback excitation can generate self-sustained activity following brief synaptic excitation. The frequency of activity is limited by the kinetics of the neuron membrane channels and can be stopped by brief inhibitory input. Network activity can be rhythmic at lower frequencies if the neurons are electrically coupled. Our key finding is that excitatory synaptic feedback within a population of neurons can produce switchable, stable, sustained firing without synaptic inhibition.     

Effect of early malnutrition on dynamic properties of axodendritic synapses in the rat prefrontal cortex

The influence of early protein-calorie malnutrition on synaptic transmission in the rat prefrontal cortex was studied by evoking direct cortical responses with different trains of threshold electrical pulses. Malnutrition resulted in a requirement for increased pulse train stimulating current, reflecting a diminished release of neurotransmitter from the preterminal endings. In addition there was an increase in the time constant of pyramidal cells, indicating that axodendritic synapses have a disability for integrating high frequency inputs at the post synaptic level. It is concluded that early malnutrition alters dynamic properties of axodentritic synapses at both the pre- and the postsynaptic levels.     

The Frequency Response, Coherence, and Information Capacity of Two Neuronal Models

Two neuronal models are analyzed in which subthreshold inputs are integrated either without loss (perfect integrator) or with a decay which follows an exponential time course (leaky integrator). Linear frequency response functions for these models are compared using sinusoids, Poisson-distributed impulses, or gaussian white noise as inputs. The responses of both models show the nonlinear behavior characteristic of a rectifier for sinusoidal inputs of sufficient amplitude. The leaky integrator shows another nonlinearity in which responses become phase locked to cyclic stimuli. Addition of white noise reduces the distortions due to phase locking. Both models also show selective attenuation of high-frequency components with white noise inputs. Input, output, and cross-spectra are computed using inputs having a broad frequency spectrum. Measures of the coherence and information transmission between the input and output of the models are also derived. Steady inputs, which produce a constant “carrier” rate, and intrinsic sources, which produce variability in the discharge of neurons, may either increase or decrease coherence; however, information transmission using inputs with a broad spectrum is generally increased by steady inputs and reduced by intrinsic variability.     

Frequency Characteristics of Retinal Neurons in the Carp

Frequency characteristics of various retinal neurons in the carp were studied using sinusoidally modulated light as an input. They were affected by both intensity and pattern of illumination. In the horizontal cells, in which the effect of light intensity was studied most extensively, an increase in the light intensity brought about a decrease of the gain, which was more marked at lower frequencies, resulting in a shift of cutoff frequency towards higher frequencies and in a slight low frequency attenuation. A decrease in the area illuminated had an effect similar to a decrease in the light intensity. In the receptor, the low frequency attenuation was not apparent even at high light intensities. The adaptation process in receptors was not sufficient to explain the low frequency attenuation in the horizontal cells, and a possible contribution of negative feedback from horizontal cells to receptors was suggested. In the bipolar cell, the lateral interaction played an important role. An increase in an area resulted in the suppression of the response at low frequencies where the phases of the center and the surround responses were opposed, but in the augmentation near 5 Hz where the two responses were in phase. In amacrine cells, a low frequency attenuation and a phase advance at low frequencies were very prominent, and were considered to be due mainly to a process designated here as the neural adaptation.     

Changes in signal transmission by neurons of the cat lateral geniculate nucleus by acute deafferentation and early degeneration.

1. The responses of single principal cells of the cat lateral geniculate nucleus (LGN) were recorded extracellulary from the optic radiation (OR) axons or intracellularly from the LGN. Electrical stimuli at different frequencies were applied to the optic tract (OT) to test the transneuronal and the synaptic signal transmission in the LGN. 2. The effect of acute deafferentation (by photocoagulation of the retinal receptive field) or of synaptic degeneration induced 2-4 days prior to the recording time on the LGN neuron signal transfer was studied. Immediately after deafferentation, the synaptic signal transfer by LGN neurons exhibits signs of hyperexcitability leading to multiple neuronal discharges. This acute deafferentation hyperexicitability is probably caused mainly by the disapperance of lateral inhibition mediated by LGN interneurons. The deafferentation hyperexcitability disappeared during electrical stimulation of the OT at frequencies greater than 10/sec. 3. With progressing degeneration of the synaptic terminals during the 2nd to 4th day after interruption of the optic nerve axoplasmic flow, the synaptic signal transfer by LGN neurons gradually deteriorates and ceases at the end of the fourth day. The signs of this deterioration (larger temporal scatter, increased exhaustability and reduced upper frequency limit of the transneuronal signal transmission and gradual reduction of the EPSP amplitude in D-neurons) were quantitatively investigated. 4. The neurophysiological data obtained at different levels of synaptic terminal degeneration are well correlated with morphological changes found within the degenerating synaptic terminals.     

NMDA受体参与小鼠的前额扣带回的神经突触传递

谷氨酸性突触是哺乳动物神经系统的主要兴奋性突触。在正常条件下,大多数的突触反应是由谷氨酸的AMPA受体传递的。NMDA受体在静息电位下为镁离子抑制。在被激活时,NMDA受体主要参与突触的可塑性变化。但是,许多NMDA受体拮抗剂在全身或局部注射时能产生行为效应,提示NMDA受体可能参与静息状态的生理功能。此文中,我们在离体的前额扣带回脑片上进行电生理记录,发现NMDA受体参与前额扣带回的突触传递。在重复刺激或近于生理性温度时,NMDA受体传递的反应更为明显。本文直接显示了NMDA受体参与前额扣带回的突触传递,并提示NMDA受体在前额扣带回中起着调节神经元兴奋的重要作用。     

The effect of photoreceptor coupling and synapse nonlinearity on signal:noise ratio in early visual processing

Electrical coupling of vertebrate photoreceptors is well known to improve the signal: noise ratio in the photoreceptor layer for large-area stimuli. For example, if N photoreceptors are perfectly coupled to each other, the signal: noise ratio is improved for stimuli illuminating more than a number M = square root of N of the receptors but is made worse for small-area stimuli illuminating less than M of the N receptors. Using the model of Lamb & Simon (J. Physiol., Lond. 263, 257 (1976], which treats the photoreceptor layer as a square array of cells, each coupled through a resistive gap junction to the four cells around it, we show that the signal:noise ratio for small-area stimuli is much greater than would be expected from a model in which receptors are assumed to be perfectly coupled. Contrary to predictions made assuming perfect coupling, receptor coupling should not prevent rods from detecting single photons, but whether the single photon signal can be detected at the bipolar cell level depends on how signals are read out of the receptor layer. The signal:noise ratio in bipolar cells postsynaptic to the photo-receptor layer is determined partly by synaptic convergence and nonlinearity in synaptic transmission from receptors. If the synaptic gain decreases with light-induced receptor hyperpolarization, as is found experimentally, then receptor coupling can improve the postsynaptic signal:noise ratio for stimuli illuminating only one receptor, even though coupling decreases the presynaptic signal:noise ratio for such stimuli. Moreover, increasing the number of coupled receptors projecting to a bipolar cell can improve the signal:noise ratio for localized stimuli if the synapse is sufficiently nonlinear (although, for the degree of nonlinearity seen in lower vertebrates, synaptic convergence makes the ratio worse for the single photon event). The fact that receptor coupling and synaptic convergence can, under some circumstances, improve the signal:noise ratio in bipolar cells suggests a principle of retinal design that may compete with the requirements of high spatial resolution.     

The mechanism of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor desensitization after removal of glutamate.

We have examined responses of AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate) receptors in the chick nucleus magnocellularis to pairs of pulses of glutamate and determined the extent of desensitization and the rate of recovery. Receptors recovered from desensitization with a time constant of 16 ms, regardless of the concentration or duration of the conditioning pulse. Even with very brief conditioning pulses, evoking submaximal currents, desensitization occurred at a consistent rate after the removal of free ligand. A quantitative kinetic model based on these data shows that receptors must desensitize from a closed state. The results provide evidence that very brief exposure to glutamate, on the time scale of uniquantal synaptic transmission, will result in a significant reduction in sensitivity of postsynaptic receptors.     

Synaptic Integration in Electrically Coupled Neurons

Interactions among chemical and electrical synapses regulate the patterns of electrical activity of vertebrate and invertebrate neurons. In this investigation we studied how electrical coupling influences the integration of excitatory postsynaptic potentials (EPSPs). Pairs of Retzius neurons of the leech are coupled by a nonrectifying electrical synapse by which chemically induced synaptic currents flow from one neuron to the other. Results from electrophysiology and modeling suggest that chemical synaptic inputs are located on the coupled neurites, at 7.5 μm from the electrical synapses. We also showed that the space constant of the coupled neurites was 100 μm, approximately twice their length, allowing the efficient spread of synaptic currents all along both coupled neurites. Based on this cytoarchitecture, our main finding was that the degree of electrical coupling modulates the amplitude of EPSPs in the driving neurite by regulating the leak of synaptic current to the coupled neurite, so that the amplitude of EPSPs in the driving neurite was proportional to the value of the coupling resistance. In contrast, synaptic currents arriving at the coupled neurite through the electrical synapse produced EPSPs of constant amplitude. This was because the coupling resistance value had inverse effects on the amount of current arriving and on the impedance of the neurite. We propose that by modulating the amplitude of EPSPs, electrical synapses could regulate the firing frequency of neurons.     

Intensity and frequency dependence of laryngeal afferent inputs to respiratory hypoglossal motoneurons

Mifflin, Steven W. Intensity and frequency dependenceof laryngeal afferent inputs to respiratory hypoglossal motoneurons. J. Appl. Physiol. 83(6):1890-1899, 1997.Inspiratory hypoglossal motoneurons (IHMs)mediate contraction of the genioglossus muscle and contribute to theregulation of upper airway patency. Intracellular recordings wereobtained from antidromically identified IHMs in anesthetized,vagotomized cats, and IHM responses to electrical activation ofsuperior laryngeal nerve (SLN) afferent fibers at various frequenciesand intensities were examined. SLN stimulus frequencies <2 Hz evokedan excitatory-inhibitory postsynaptic potential (EPSP-IPSP) sequence oronly an IPSP in most IHMs that did not change in amplitude as thestimulus was maintained. During sustained stimulus frequencies of5-10 Hz, there was a reduction in the amplitude of SLN-evokedIPSPs with time with variable changes in the EPSP. At stimulusfrequencies >25 Hz, the amplitude of EPSPs and IPSPs was reduced overtime. At a given stimulus frequency, increasing stimulus intensityenhanced the decay of the SLN-evoked postsynaptic potentials (PSPs).Frequency-dependent attenuation of SLN inputs to IHMs also occurred innewborn kittens. These results suggest that activation of SLN afferentsevokes different PSP responses in IHMs depending on the stimulusfrequency. At intermediate frequencies, inhibitory inputs areselectively filtered so that excitatory inputs predominate. At higherfrequencies there was no discernible SLN-evoked PSP temporally lockedto the SLN stimuli. Alterations in SLN-evoked PSPs could play a role inthe coordination of genioglossal contraction during respiration,swallowing, and other complex motor acts where laryngeal afferents areactivated.

     

Analytical description of the activation of multi-state receptors by continuous neurotransmitter signals at brain synapses.

Chemical synaptic transmission is a fundamental component of interneuronal communications in the central nervous system (CNS). Discharge of a presynaptic vesicle containing a few thousand molecules (a quantum) of neurotransmitter into the synaptic cleft generates a transmitter concentration signal that drives postsynaptic ion-channel receptors. These receptors exhibit multiple states, with state transition kinetics dependent on neurotransmitter concentration. Here, a novel and simple analytical approach for describing gating of multi-state receptors by signals with complex continuous time courses is used to describe the generation of glutamate-mediated quantal postsynaptic responses at brain synapses. The neurotransmitter signal, experienced by multi-state N-methyl-D-aspartate (NMDA)- and L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-type glutamate receptors at specific points in a synaptic cleft, is approximated by a series of step functions of different intensity and duration and used to drive a Markovian, multi-state kinetic scheme that describes receptor gating. Occupancy vectors at any point in time can be computed interatively from the occupancy vectors at the times of steps in transmitter concentration. Multi-state kinetic schemes for both the low-affinity AMPA subtype of glutamate receptor and for the high-affinity NMDA subtype are considered, and expected NMDA and AMPA components of synaptic currents are calculated. The amplitude of quantal responses mediated by postsynaptic receptor clusters having specific spatial distributions relative to foci of quantal neurotransmitter release is then calculated and related to the displacement between the center of the postsynaptic receptor cluster and the focus of synaptic vesicle discharge. Using this approach we show that the spatial relation between the focus of release and the center of the postsynaptic receptor cluster affects synaptic efficacy. We also show how variation in this relation contributes to variation in synaptic current amplitudes.     

Intracellular recording from the giant synapse of the squid

1. Recording with glass micropipette electrodes inserted close to the synaptic region, in the presynaptic and in the postsynaptic fibers of the giant synapse in the stellate ganglion of the squid, has been accomplished. 2. The forms of the spike and of the synaptic potential are very much like those reported earlier (Bullock, 1948) from macroelectrodes. The crest time and the rate of fall are labile and depend on the state of fatigue, though the time of initiation of the postsynaptic potential does not. 3. It is concluded after examination of both intra- and extracellular recordings that there is a real synaptic delay of the order of 1 or 2 milliseconds at 15–20°C. 4. There is sometimes a very small and sometimes no visible deflection in the intracellular postsynaptic record attributable to the presynaptic spike. It is concluded that transmission cannot be electrical. 5. The amplitude of the postsynaptic potential can be controlled over some range by the amplitude of the presynaptic potential. 6. Hyperpolarization of the postsynaptic membrane results in increase in amplitude of spikes up to 200 millivolts, in increase in the membrane potential level at which the spike flares up, but in no considerable change in the amplitude in postsynaptic potential. 7. The postsynaptic potential can add to the late falling phase and the undershoot of an antidromic spike in the postfiber but cannot add to the crest or early part of the falling phase. The earliest part of the antidromic spike during which the postsynaptic potential can add is probably a period of refractoriness to electrical shock by analogy with the properties of the axon.     

Neural Organization of the Median Ocellus of the Dragonfly : I. Intracellular electrical activity

Intracellular responses from receptors and postsynaptic units have been recorded in the median ocellus of the dragonfly. The receptors respond to light with a graded, depolarizing potential and a single, tetrodotoxin-sensitive impulse at "on." The postsynaptic units (ocellar nerve dendrites) hyperpolarize during illumination and show a transient, depolarizing response at "off." The light-evoked slow potential responses of the postsynaptic units are not altered by the application of tetrodotoxin to the ocellus. It appears, therefore, that the graded receptor potential, which survives the application of tetrodotoxin, is responsible for mediating synaptic transmission in the ocellus. Comparison of pre- and postsynaptic slow potential activity shows (a) longer latencies in postsynaptic units by 5–20 msec, (b) enhanced photosensitivity in postsynaptic units by 1–2 log units, and (c) more transient responses in postsynaptic units. It is suggested that enhanced photosensitivity of postsynaptic activity is a result of summation of many receptors onto the postsynaptic elements, and that transients in the postsynaptic responses are related to the complex synaptic arrangements in the ocellar plexus to be described in the following paper.     

Responses of the neurons of the cortical secondary auditory area to acoustic and non-acoustic stimuli in cats

The responses of the cortical secondary auditory area (AII) to the non-acoustic stimuli (electrical stimulation of the skin in the vibrissae area and light flash) and their combination with acoustic stimulation (sound click or tone) were studied in experiments on cats anesthetized by kalipsol using extra- and intracellular recording. Of the total number of neurons, 69% of the units generating spike responses to the acoustic stimulation responded to the non-acoustic stimulation too. The responses to the modal-nonspecific stimulation, as a rule, were weak and variable; they were mostly represented by a tonic change in the neuronal discharge frequency. The nonspecific stimulation evoked primary excitatory and inhibitory postsynaptic potentials in 77% and 20% of the examined neurons, respectively. We found that synaptic effects of the nonspecific and specific stimulations interact with each other, ensuring considerable modulation of the latter (mostly a significant facilitation resulting from the EPSP summation and suppression of an inhibitory component of the response to acoustic stimulation). Possible participation of the midbrain reticular formation in the transmission of the modal-nonspecific influences to the cortical neurons is considered; stimulation of this structure evoked responses similar to those evoked by the modal-nonspecific sensory stimuli.Neirofiziologiya/Neurophhysiology, Vol. 26, No. 5, pp. 356–364, September–October, 1994.     

Resistance of frog olfactory bulb slow potential and afferent input inhibition to hypoxia and synaptic transmission blockade by manganese,cobalt, and magnesium ions

The effect of hypoxia and application of manganese, cobalt, and magnesium ions on electrical responses of the frog olfactory bulb to adequate stimulation and to direct electrical stimulation of the olfactory nerve were studied. The slow potential evoked by adequate stimulation and the associated inhibition of the afferent input of the olfactory bulb were found to be much more resistant to inhibition of synaptic transmission by all methods used than the postsynaptic components of the orthodromic response and associated postsynaptic inhibition. A slow potential was recorded even when synaptic transmission in the olfactory bulb was completely blocked by magnesium ions. It is concluded that the slow potential of the olfactory bulb and inhibition of its afferent input are nonsynaptic in nature. It is postulated that the slow potential reflects mainly depolarization of glial cells in the glomerular layer of the bulb evoked by accumulation of potassium ions. The possible mechanisms of inhibition of the afferent input are discussed.     

Thematic Minireview Series: Molecular Mechanisms of Synaptic Plasticity

The human brain contains ∼86 billion neurons, which are precisely organized in specific brain regions and nuclei. High fidelity synaptic communication between subsets of neurons in specific circuits is required for most human behaviors, and is often disrupted in neuropsychiatric disorders. The presynaptic axon terminals of one neuron release neurotransmitters that activate receptors on multiple postsynaptic neuron targets to induce electrical and chemical responses. Typically, postsynaptic neurons integrate signals from multiple presynaptic neurons at thousands of synaptic inputs to control downstream communication to the next neuron in the circuit. Importantly, the strength (or efficiency) of signal transmission at each synapse can be modulated on time scales ranging up to the lifetime of the organism. This “synaptic plasticity” leads to changes in overall neuronal circuit activity, resulting in behavioral modifications. This series of minireviews will focus on recent advances in our understanding of the molecular and cellular mechanisms that control synaptic plasticity.     

Adaptation to synaptic inactivity in hippocampal neurons

In response to activity deprivation, CNS neurons undergo slow adaptive modification of unitary synaptic transmission. The changes are comparable in degree to those induced by brief intense stimulation, but their molecular basis is largely unknown. Our data indicate that prolonged AMPAR blockade acts through loss of Ca2+ entry through L-type Ca2+ channels to bring about an increase in both vesicle pool size and turnover rate, as well as a postsynaptic enhancement of the contribution of GluR1 homomers, concentrated at the largest synapses. The changes were consistent with a morphological scaling of overall synapse size, but also featured a dramatic shift toward synaptic drive contributed by the Ca2+-permeable homomeric GluR1 receptors. These results extend beyond "synaptic homeostasis" to involve more profound changes that can be better described as "metaplasticity".     

Potentiation of electrical and chemical synaptic transmission mediated by endocannabinoids

Endocannabinoids are well established as inhibitors of chemical synaptic transmission via presynaptic activation of the cannabinoid type 1 receptor (CB1R). Contrasting this notion, we show that dendritic release of endocannabinoids mediates potentiation of synaptic transmission at mixed (electrical and chemical) synaptic contacts on the goldfish Mauthner cell. Remarkably, the observed enhancement was not restricted to the glutamatergic component of the synaptic response but also included a parallel increase in electrical transmission. This effect involved the activation of CB1 receptors and was indirectly mediated via the release of dopamine from nearby varicosities, which in turn led to potentiation of the synaptic response via a cAMP-dependent protein kinase-mediated postsynaptic mechanism. Thus, endocannabinoid release can potentiate synaptic transmission, and its functional roles include the regulation of gap junction-mediated electrical synapses. Similar interactions between endocannabinoid and dopaminergic systems may be widespread and potentially relevant for the motor and rewarding effects of cannabis derivatives.     

The suprachiasmatic nucleus exhibits diurnal variations in spontaneous excitatory postsynaptic activity

A most prominent feature of neurons in the suprachiasmatic nucleus (SCN) is the circadian rhythm in spontaneous firing frequency. To disclose synaptic mechanisms associated with the rhythmic activity, the spontaneous postsynaptic activity was studied using whole-cell, patch clamp recordings in the ventral region of the SCN in slice preparations from rats. The synaptic events were compared between two time intervals corresponding to the highest and lowest electrical activity within the SCN during subjective daytime and nighttime, respectively. The gamma-aminobutyric acid (GABA)-mediated spontaneous inhibitory activity showed no diurnal variations, but the excitatory activity was markedly higher in frequency, without differences in amplitude, during the subjective day compared to the subjective night. Spontaneous and evoked inhibitory synaptic events were blocked by the GABA(A) receptor antagonist bicuculline. The alpha-amino-hydroxy-5-methylisoxazole-4-propionic acid (AMPA/kainate) receptor antagonist 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX) blocked most of the excitatory activity. In addition, CNQX reduced the spontaneous inhibitory activity. The N-methyl-D-aspartate antagonist D-2-amino-5-phosphonopentanoic acid reduced the inhibitory activity to a lesser degree, and there was no significant difference in amplitude or frequency of synaptic events in control and Mg2+-free solutions, indicating that the AMPA receptor plays an important role in regulating the inhibitory release of GABA within the SCN. Ipsi- and contralateral stimulation of the SCN consistently evoked excitatory synaptic responses. Inhibitory synaptic responses occurred in some neurons upon increasing stimulus strength. In conclusion, this study shows that there is a substantial influence from spontaneous glutamatergic synapses on the ventral part of the SCN and that these exhibit daily variations in activity. Diurnal fluctuations in spontaneous excitatory postsynaptic activity within this network may contribute to the mechanisms for synchronization of rhythms between individual SCN neurons and may underlie the daily variations in the spontaneous firing frequency of SCN neurons.