Bidirectional plasticity gated by hyperpolarization controls the gain of postsynaptic firing responses at central vestibular nerve synapses

Linking synaptic plasticity with behavioral learning requires understanding how synaptic efficacy influences postsynaptic firing in neurons whose role in behavior is understood. Here, we examine plasticity at a candidate site of motor learning: vestibular nerve synapses onto neurons that mediate reflexive movements. Pairing nerve activity with changes in postsynaptic voltage induced bidirectional synaptic plasticity in vestibular nucleus projection neurons: long-term potentiation relied on calcium-permeable AMPA receptors and postsynaptic hyperpolarization, whereas long-term depression relied on NMDA receptors and postsynaptic depolarization. Remarkably, both forms of plasticity uniformly scaled synaptic currents evoked by pulse trains, and these changes in synaptic efficacy were translated into linear increases or decreases in postsynaptic firing responses. Synapses onto local inhibitory neurons were also plastic but expressed only long-term depression. Bidirectional, linear gain control of vestibular nerve synapses onto projection neurons provides a plausible mechanism for motor learning underlying adaptation of vestibular reflexes.     

Synaptic secretion of BDNF after high-frequency stimulation of glutamatergic synapses.

The protein brain-derived neurotrophic factor (BDNF) has been postulated to be a retrograde or paracrine synaptic messenger in long-term potentiation and other forms of activity-dependent synaptic plasticity. Although crucial for this concept, direct evidence for the activity-dependent synaptic release of BDNF is lacking. Here we investigate secretion of BDNF labelled with green fluorescent protein (BDNF-GFP) by monitoring the changes in fluorescence intensity of dendritic BDNF-GFP vesicles at glutamatergic synaptic junctions of living hippocampal neurons. We show that high-frequency activation of glutamatergic synapses triggers the release of BDNF-GFP from synaptically localized secretory granules. This release depends on activation of postsynaptic ionotropic glutamate receptors and on postsynaptic Ca(2+) influx. Release of BDNF-GFP is also observed from extrasynaptic dendritic vesicle clusters, suggesting that a possible spatial restriction of BDNF release to specific synaptic sites can only occur if the postsynaptic depolarization remains local. These results support the concept of BDNF being a synaptic messenger of activity-dependent synaptic plasticity, which is released from postsynaptic neurons.     

Ultrastructure of synapses and cellular relationships in the oculomotor nucleus of the rhesus monkey

Summary The fine structure of synapses and of cellular relations was examined in the somatic efferent portion of the oculomotor nucleus of the adult rhesus monkey. Axosomatic and axodendritic synapses are characterized by distinct synaptic clefts which usually measure 20–30 nm between pre- and postsynaptic membranes. Cytoplasmic thickenings of pre- and postsynaptic membranes are often observed. Subjunctional bodies are present at both axosomatic and axodendritic synapses. Somatic and dendritic spine synapses are present. Serial synapses are also found, suggesting the operation of presynaptic inhibition in this nucleus. At some synapses the extracellular gap between pre- and postsynaptic membranes is reduced to 5–9 nm. However, junctions similar to the latter are also present between neurons and glia, and at the junctions between adjacent glial elements. The present results provide no evidence for a clear morphological substrate for electrotonic transmission in the somatic efferent portion of the primate oculomotor nucleus.Supported in part by grants from the National Institutes of Health (NS-15320), the Kroc Foundation, and by the Medical Research Service of the Veterans Administration (to S.G.W.), and from the National Science Foundation (BNS 77-28493) and the Muscular Dystrophy Association of America (to G.D.P.)     

Engineered synaptic tools reveal localized cAMP signaling in synapse assembly

The physiological mechanisms driving synapse formation are elusive. Although numerous signals are known to regulate synapses, it remains unclear which signaling mechanisms organize initial synapse assembly. Here, we describe new tools, referred to as “SynTAMs” for synaptic targeting molecules, that enable localized perturbations of cAMP signaling in developing postsynaptic specializations. We show that locally restricted suppression of postsynaptic cAMP levels or of cAMP-dependent protein-kinase activity severely impairs excitatory synapse formation without affecting neuronal maturation, dendritic arborization, or inhibitory synapse formation. In vivo, suppression of postsynaptic cAMP signaling in CA1 neurons prevented formation of both Schaffer-collateral and entorhinal-CA1/temporoammonic-path synapses, suggesting a general principle. Retrograde trans-synaptic rabies virus tracing revealed that postsynaptic cAMP signaling is required for continuous replacement of synapses throughout life. Given that postsynaptic latrophilin adhesion-GPCRs drive synapse formation and produce cAMP, we suggest that spatially restricted postsynaptic cAMP signals organize assembly of postsynaptic specializations during synapse formation.     

Suppression of inhibitory synaptic potentiation by presynaptic activity through postsynaptic GABA(B) receptors in a Purkinje neuron

At inhibitory synapses on a cerebellar Purkinje neuron, the depolarization caused by heterosynaptic climbing fiber activation induces long-lasting potentiation accompanied by an increase in GABA(A) receptor responsiveness. Here we show that activation of a presynaptic inhibitory interneuron during the conditioning postsynaptic depolarization suppresses the potentiation. The suppression is due to postsynaptic GABA(B) receptor activation by GABA released from presynaptic terminals. The results suggest that GABA(B) receptor activation decreases the activity of cAMP-dependent protein kinase through the G(i)/G(o) proteins. The presynaptic activity-dependent suppression of synaptic plasticity is a novel regulatory mechanism of synaptic efficacy at individual synapses and may contribute to the learning and computational ability of the cerebellar cortex.     

Dissecting neurotransmission with artificial synapses

JGP study demonstrates how recordings from neuron–HEK cell cocultures provide a clearer picture of the factors involved in synaptic transmission.

Resolving the rapid series of steps involved in synaptic transmission and assessing the contributions of different molecules to each of them is an enormous challenge. In this issue of JGP, Chiang et al. show that the process can be studied with greater resolution at the artificial synapses formed between neurons and cocultured human embryonic kidney (HEK) cells (1).Chung-Wei Chiang (left), Meyer Jackson (right), and colleagues studied synaptic transmission between hippocampal neurons and cocultured HEK cells. Mutations in the SNARE protein synaptobrevin 2 alter the shape of mEPSCs generated in HEK cells, an effect made clearer by the absence of dendritic filtering in this artificial system.Meyer Jackson and colleagues at the University of Wisconsin School of Medicine and Public Health are particularly interested in exocytosis. Though this process can be measured directly in endocrine cells, its role in controlling the dynamics of synaptic transmission can be difficult to separate from all the downstream steps required to elicit a response in the postsynaptic neuron. “We wanted to study a surrogate synapse with a simplified response to neurotransmitter that would allow us to focus on vesicle release with greater resolution,” Jackson explains.For years, researchers have studied synaptogenesis by transfecting HEK cells with a handful of postsynaptic factors that enable them to assemble functional synapses when cocultured with neurons (2, 3). Jackson realized that these artificial synapses lack two key sources of variability that can obscure the contribution of vesicle release to synaptic transmission. First, the postsynaptic apparatus of neuron–HEK synapses is consistent and can be precisely controlled (in contrast to regular synapses, where the molecular composition may vary from synapse to synapse). Second, the compact shape of HEK cells greatly reduces the influence of dendritic filtering, the phenomenon by which synaptic inputs take varying lengths of time to reach the cell body, depending on the location of the synapse within the dendritic arbor.Jackson and colleagues, including first author Chung-Wei Chiang, transfected HEK cells with four postsynaptic proteins—the AMPA receptor subunit GluA2, the adhesion molecule neuroligin 1, the scaffold protein PSD-95, and the accessory factor stargazin—and cocultured them with hippocampal neurons (1). The researchers then measured the miniature excitatory postsynaptic currents (mEPSCs) evoked in the HEK cells by the spontaneous release of single synaptic vesicles from neighboring neurons.The mEPSCs generated at these artificial synapses were larger and faster than mEPSCs produced by regular, neuron–neuron synapses (though they involved comparable amounts of charge, suggesting that the vesicle populations are similar at both types of synapse).Notably, the rise rate of mEPSCs in HEK cells was faster and less variable, in keeping with the absence of dendritic filtering and the consistent, shorter distance between the artificial synapses and the HEK cell body. This allowed Chiang et al. (1) to resolve the contribution of vesicle release to mEPSC dynamics using mutant versions of the SNARE protein synaptobrevin 2 that impede the flux of neurotransmitters through synaptic fusion pores (4). These mutant proteins decreased the rise rate and decay rate of mEPSCs at artificial synapses. “However, the effect was much clearer in HEK cells than we’d previously seen in neurons,” Jackson says.Chiang et al. (1) were also able to resolve the contribution of postsynaptic receptors to mEPSC shape, but Jackson is most interested in using the neuron–HEK coculture system to investigate synaptic vesicle release in more detail. “It opens up a new approach that will allow us to study synaptic exocytosis more precisely and look for much subtler effects,” Jackson says. For example, Jackson hopes to explain why mutations in some exocytotic proteins have major effects on endocrine cells but little to no effect at synapses.     

Neural Organization of the Median Ocellus of the Dragonfly : II. Synaptic structure

Two types of presumed synaptic contacts have been recognized by electron microscopy in the synaptic plexus of the median ocellus of the dragonfly. The first type is characterized by an electron-opaque, button-like organelle in the presynaptic cytoplasm, surrounded by a cluster of synaptic vesicles. Two postsynaptic elements are associated with these junctions, which we have termed button synapses. The second synaptic type is characterized by a dense cluster of synaptic vesicles adjacent to the presumed presynaptic membrane. One postsynaptic element is observed at these junctions. The overwhelming majority of synapses seen in the plexus are button synapses. They are found most commonly in the receptor cell axons where they synaptically contact ocellar nerve dendrites and adjacent receptor cell axons. Button synapses are also seen in the ocellar nerve dendrites where they appear to make synapses back onto receptor axon terminals as well as onto adjacent ocellar nerve dendrites. Reciprocal and serial synaptic arrangements between receptor cell axon terminals, and between receptor cell axon terminals and ocellar nerve dendrites are occasionally seen. It is suggested that the lateral and feedback synapses in the median ocellus of the dragonfly play a role in enhancing transients in the postsynaptic responses.     

Differential Modulation of GABAA Receptors Underlies Postsynaptic Depolarization- and Purinoceptor-Mediated Enhancement of Cerebellar Inhibitory Transmission: A Non-Stationary Fluctuation Analysis Study

Cerebellar GABAergic inhibitory transmission between interneurons and Purkinje cells (PCs) undergoes a long-lasting enhancement following different stimulations, such as brief depolarization or activation of purinergic receptors of postsynaptic PCs. The underlying mechanisms, however, are not completely understood. Using a peak-scaled non-stationary fluctuation analysis, we therefore aimed at characterizing changes in the electrophysiological properties of GABA_A receptors in PCs of rat cerebellar cortex during depolarization-induced “rebound potentiation (RP)” and purinoceptor-mediated long-term potentiation (PM-LTP), because both RP and PM-LTP likely depend on postsynaptic mechanisms. Stimulation-evoked inhibitory postsynaptic currents (eIPSCs) were recorded from PCs in neonatal rat cerebellar slices. Our analysis showed that postsynaptic membrane depolarization induced RP of eIPSCs in association with significant increase in the number of synaptic GABA_A receptors without changing the channel conductance. By contrast, bath application of ATP induced PM-LTP of eIPSCs with a significant increase of the channel conductance of GABA_A receptors without affecting the receptor number. Pretreatment with protein kinase A (PKA) inhibitors, H-89 and cAMPS-Rp, completely abolished the PM-LTP. The CaMKII inhibitor KN-62 reported to abolish RP did not alter PM-LTP. These results suggest that the signaling mechanism underlying PM-LTP could involve ATP-induced phosphorylation of synaptic GABA_A receptors, thereby resulting in upregulation of the channel conductance by stimulating adenylyl cyclase-PKA signaling cascade, possibly via activation of P2Y₁₁ purinoceptor. Thus, our findings reveal that postsynaptic GABA_A receptors at the interneuron-PC inhibitory synapses are under the control of two distinct forms of long-term potentiation linked with different second messenger cascades.     

Maintained depolarization of synaptic terminals facilitates nerve-evoked transmitter release at a crayfish neuromuscular junction

Presynaptic and postsynaptic potentials were examined by intracellular recording at a crayfish neuromuscular junction. During normal synaptic transmission, the action potentials were recorded in the terminal region of the excitatory axon and postsynaptic responses were obtained in the muscle fibers. We found that it was possible to modify the synaptic transmission by applying depolarizing or hyperpolarizing currents through the presynaptic intracellular electrode. Typically, a 7-15 mV depolarization lasting longer than 50 msec leads to a large (500%) enhancement of transmitter release, even though the preterminal action potential is reduced in amplitude. Hyperpolarization increases the amplitude of the action potential, but slightly reduces the transmitter release. These results are different from those reported for other neuromuscular synapses and the squid giant synapse, but are similar in many respects to the results reported for several invertebrate central synapses. We conclude, first, that different synapses may have markedly different responses to conditioning by membrane polarization and, secondly, that maintained low-level depolarization may induce a potentiated state in the nerve terminal, perhaps brought about by slow entry of calcium.     

Communicating synapses: types and functional interpretation. Exceptions to Cajal's neuron theory.

The neurons of the dorsal periaqueductal nucleus of the mesencephalon and their synaptic contacts were observed under a transmission electron microscope. We found various types of synapses which constituted an exception to Cajal's neuron theory (law of neuron independence). Some of these synapses had an open communicating or continuity 'passage' between the presynaptic bouton of a neuron (first neuron) and the postsynaptic portion of another neuron (second neuron). The 'communicating' passage (located in the synaptosome) is formed by the continuity of the presynaptic and postsynaptic membrane, and its limits or rims are the reflexion points of the membranes. When only two neurons intervene they could be termed 'simple communicating synapses'. We found three types: I = communicating axosomatic synapses; II = communicating axodendritic synapses, and III = communicating axoaxonic synapses'. When three neurons intervene in the synaptic contact, they could be termed 'complex communicating synapses'. In these, the first and second neurons form a normal synapse, but the lateral portion of the presynaptic bouton of the first neuron also enters into contact with a third neuron, with which it establishes an open communicating or continuity passage. The points of these passages are collateral to the synapse, and may be in the presynaptic or pre-postsynaptic portions simultaneously, communicating collaterally with the third neuron. We found a further three types: IV = complex communicating axosomatic and dendritic synapses; V = complex communicating axoaxonic and somatic synapses, and VI = complex communicating axodendritic and double-somatic synapses. It is suggested that communicating synapses may constitute an exception to Cajal's neuron theory, representing functional states for the acceleration, retardation or modulation of the synaptic function. The neurotransmitters would pass en masse through the communicating passage and the depolarization wave would pass through the rims without being retarded. In the simple communicating synapses, their action would be intensifying. In the complex communicating synapses, their action would be modulating or retarding, since the collateral communicating passage would function as an 'escape valve' through which part of the impulse reaching the presynaptic bouton would escape.     

Spatial Distribution of Excitatory Synapses on the Dendrites of Ganglion Cells in the Mouse Retina

Excitatory glutamatergic inputs from bipolar cells affect the physiological properties of ganglion cells in the mammalian retina. The spatial distribution of these excitatory synapses on the dendrites of retinal ganglion cells thus may shape their distinct functions. To visualize the spatial pattern of excitatory glutamatergic input into the ganglion cells in the mouse retina, particle-mediated gene transfer of plasmids expressing postsynaptic density 95-green fluorescent fusion protein (PSD95-GFP) was used to label the excitatory synapses. Despite wide variation in the size and morphology of the retinal ganglion cells, the expression of PSD95 puncta was found to follow two general rules. Firstly, the PSD95 puncta are regularly spaced, at 1–2 µm intervals, along the dendrites, whereby the presence of an excitatory synapse creates an exclusion zone that rules out the presence of other glutamatergic synaptic inputs. Secondly, the spatial distribution of PSD95 puncta on the dendrites of diverse retinal ganglion cells are similar in that the number of excitatory synapses appears to be less on primary dendrites and to increase to a plateau on higher branch order dendrites. These observations suggest that synaptogenesis is spatially regulated along the dendritic segments and that the number of synaptic contacts is relatively constant beyond the primary dendrites. Interestingly, we also found that the linear puncta density is slightly higher in large cells than in small cells. This may suggest that retinal ganglion cells with a large dendritic field tend to show an increased connectivity of excitatory synapses that makes up for their reduced dendrite density. Mapping the spatial distribution pattern of the excitatory synapses on retinal ganglion cells thus provides explicit structural information that is essential for our understanding of how excitatory glutamatergic inputs shape neuronal responses.     

Remote Modulation of Current Transfer from Dendritic Glutamatergic Synapses by GABA-ergic Synapses of the Somatic Zone of Motoneurons: a Simulation Study

Until now, information concerning spatial interaction of postsynaptic excitation and inhibition in neuronal dendrites remains rather limited. In model experiments, we studied spatial effects of tonic co-activation of GABA-ergic synapses situated on the soma and axon hillock of a motoneuron and dendritic glutamatergic synapses with receptors sensitive or insensitive to N-methyl-D-aspartate. We analyzed distribution maps of the transmembrane potentials and excitatory currents transferred toward the soma over the reconstructed dendritic arborization of a rat abducens motoneuron (three-dimensional reconstruction). In the motoneuron, isolated tonic excitation of glutamatergic synapses induced two stable states of low (downstate) or high (upstate) spatially heterogeneous dendritic depolarization, which decayed with unequal rates along different dendritic paths. In this case, the local steady-state current-voltage relation of the dendritic membrane became N-shaped due to a limb of the negative slope within a certain voltage range. The upstate corresponding to plateau potentials associated with stereotyped motor activity patterns was analyzed in detail. In this state, most proximal dendritic sites were the main sources of the excitatory current reaching the soma, while the contribution from distal sites was negligible. Co-activation of GABA-synapses located at the soma and axon hillock reduced this depolarization and shifted the main excitatory current source from a perisomatic location to the middle, structurally more complex, region of the dendritic arborization. The more remote dendritic region having a greater membrane area and receiving a greater number of synaptic contacts became directly involved in the supply of the trigger zone by the excitatory current. We suggest that a special, not described earlier, operational mechanism of postsynaptic inhibition is manifested in the above spatial effects of activation of strategically located inhibitory synapses, and that the list of known crucial inhibitory mechanisms (namely hyperpolarization and shunting of the postsynaptic membrane) must be expanded.     

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 gradient clusteron: A model neuron that learns to solve classification tasks via dendritic nonlinearities,structural plasticity,and gradient descent

Synaptic clustering on neuronal dendrites has been hypothesized to play an important role in implementing pattern recognition. Neighboring synapses on a dendritic branch can interact in a synergistic, cooperative manner via nonlinear voltage-dependent mechanisms, such as NMDA receptors. Inspired by the NMDA receptor, the single-branch clusteron learning algorithm takes advantage of location-dependent multiplicative nonlinearities to solve classification tasks by randomly shuffling the locations of “under-performing” synapses on a model dendrite during learning (“structural plasticity”), eventually resulting in synapses with correlated activity being placed next to each other on the dendrite. We propose an alternative model, the gradient clusteron, or G-clusteron, which uses an analytically-derived gradient descent rule where synapses are "attracted to" or "repelled from" each other in an input- and location-dependent manner. We demonstrate the classification ability of this algorithm by testing it on the MNIST handwritten digit dataset and show that, when using a softmax activation function, the accuracy of the G-clusteron on the all-versus-all MNIST task (~85%) approaches that of logistic regression (~93%). In addition to the location update rule, we also derive a learning rule for the synaptic weights of the G-clusteron (“functional plasticity”) and show that a G-clusteron that utilizes the weight update rule can achieve ~89% accuracy on the MNIST task. We also show that a G-clusteron with both the weight and location update rules can learn to solve the XOR problem from arbitrary initial conditions.     

Connecting the dots: from actin polymerization to synapse formation

Protrusive behavior of dendritic spines on developing neurons has been previously suggested to mediate the formation of new axodendritic synaptic contacts. A study by Zito et al. in this issue of Neuron links actin polymerization in dendritic spines with the motility that the spines exhibit and the synapses that they form.     

Generalized Stein's model for anatomically complex neurons

A neuron with a large dendritic structure is considered. The number of synapses located on the dendrites is substantially higher than on the soma. The synaptic input effect on the neuronal excitability decreases with distance between a synapse ending and the trigger zone. Two areas are distinguished in accordance with the effect of synaptic input--dendritic and somatic. The dendritic area, when compared to the soma, is characterized by much higher intensity of its activation but the amplitudes of synaptically evoked changes of the membrane potential at the trigger zone are in general small. This situation is suitable for a diffusion approximation. However, on the soma, especially in the proximity of the trigger zone, the membrane potential changes are a large fraction of the threshold depolarization. The membrane potential at the trigger zone is modelled by a one-dimensional stochastic process. The diffusion Ornstein-Uhlenbeck process serves as a basis of the model; however, at the moments of somatic synapses activation its voltage changes in jumps. Their sizes represent the amplitudes of the evoked postsynaptic potentials. The unimodal histograms of interspike intervals can be explained by the model. The values of the coefficient of variation greater than one are connected with substantial inhibition.