Dendritic excitability and synaptic plasticity

ER-mitochondria tethering by PDZD8 regulates Ca2+ dynamics in mammalian neurons

Interfaces between organelles are emerging as critical platforms for many biological responses in eukaryotic cells. In yeast, the ERMES complex is an endoplasmic reticulum (ER)-mitochondria tether composed of four proteins, three of which contain a SMP (synaptotagmin-like mitochondrial-lipid binding protein) domain. No functional ortholog for any ERMES protein has been identified in metazoans. Here, we identified PDZD8 as an ER protein present at ER-mitochondria contacts. The SMP domain of PDZD8 is functionally orthologous to the SMP domain found in yeast Mmm1. PDZD8 was necessary for the formation of ER-mitochondria contacts in mammalian cells. In neurons, PDZD8 was required for calcium ion (Ca²⁺) uptake by mitochondria after synaptically induced Ca²⁺-release from ER and thereby regulated cytoplasmic Ca²⁺ dynamics. Thus, PDZD8 represents a critical ER-mitochondria tethering protein in metazoans. We suggest that ER-mitochondria coupling is involved in the regulation of dendritic Ca²⁺ dynamics in mammalian neurons.

Optogenetic Control of Endoplasmic Reticulum-Mitochondria Tethering

The organelle interface emerges as a dynamic platform for a variety of biological responses. However, their study has been limited by the lack of tools to manipulate their occurrence in live cells spatiotemporally. Here, we report the development of a genetically encoded light-inducible tethering (LIT) system allowing the induction of contacts between endoplasmic reticulum (ER) and mitochondria, taking advantage of a pair of light-dependent heterodimerization called an iLID system. We demonstrate that the iLID-based LIT approach enables control of ER-mitochondria tethering with high spatiotemporal precision in various cell types including primary neurons, which will facilitate the functional study of ER-mitochondrial contacts.

Indistinguishable Synapses Lead to Sparse Networks

Neurons integrate information from many neighbors when they process information. Inputs to a given neuron are thus indistinguishable from one another. Under the assumption that neurons maximize their information storage, indistinguishability is shown to place a strong constraint on the distribution of strengths between neurons. The distribution of individual synapse strengths is found to follow a modified Boltzmann distribution with strength proportional to [Formula: see text]. The model is shown to be consistent with experimental data from Caenorhabditis elegans connectivity and in vivo synaptic strength measurements. The [Formula: see text] dependence helps account for the observation of many zero or weak connections between neurons or sparsity of the neural network.

Dynamic Hebbian Cross-Correlation Learning Resolves the Spike Timing Dependent Plasticity Conundrum

Spike Timing-Dependent Plasticity has been found to assume many different forms. The classic STDP curve, with one potentiating and one depressing window, is only one of many possible curves that describe synaptic learning using the STDP mechanism. It has been shown experimentally that STDP curves may contain multiple LTP and LTD windows of variable width, and even inverted windows. The underlying STDP mechanism that is capable of producing such an extensive, and apparently incompatible, range of learning curves is still under investigation. In this paper, it is shown that STDP originates from a combination of two dynamic Hebbian cross-correlations of local activity at the synapse. The correlation of the presynaptic activity with the local postsynaptic activity is a robust and reliable indicator of the discrepancy between the presynaptic neuron and the postsynaptic neuron's activity. The second correlation is between the local postsynaptic activity with dendritic activity which is a good indicator of matching local synaptic and dendritic activity. We show that this simple time-independent learning rule can give rise to many forms of the STDP learning curve. The rule regulates synaptic strength without the need for spike matching or other supervisory learning mechanisms. Local differences in dendritic activity at the synapse greatly affect the cross-correlation difference which determines the relative contributions of different neural activity sources. Dendritic activity due to nearby synapses, action potentials, both forward and back-propagating, as well as inhibitory synapses will dynamically modify the local activity at the synapse, and the resulting STDP learning rule. The dynamic Hebbian learning rule ensures furthermore, that the resulting synaptic strength is dynamically stable, and that interactions between synapses do not result in local instabilities. The rule clearly demonstrates that synapses function as independent localized computational entities, each contributing to the global activity, not in a simply linear fashion, but in a manner that is appropriate to achieve local and global stability of the neuron and the entire dendritic structure.

Rapid State-Dependent Alteration in Kv3 Channel Availability Drives Flexible Synaptic Signaling Dependent on Somatic Subthreshold Depolarization

In many neurons, subthreshold depolarization in the soma can transiently increase action-potential (AP)-evoked neurotransmission via analog-to-digital facilitation. The mechanisms underlying this form of short-term synaptic plasticity are unclear, in part, due to the relative inaccessibility of the axon to direct physiological interrogation. Using voltage imaging and patch-clamp recording from presynaptic boutons of cerebellar stellate interneurons, we observed that depolarizing somatic potentials readily spread into the axon, resulting in AP broadening, increased spike-evoked Ca²⁺ entry, and enhanced neurotransmission strength. K_v3 channels, which drive AP repolarization, rapidly inactivated upon incorporation of K_v3.4 subunits. This leads to fast susceptibility to depolarization-induced spike broadening and analog facilitation independent of Ca²⁺-dependent protein kinase C signaling. The spread of depolarization into the axon was attenuated by hyperpolarization-activated currents (I_h currents) in the maturing cerebellum, precluding analog facilitation. These results suggest that analog-to-digital facilitation is tempered by development or experience in stellate cells.

FGF-FGFR Mediates the Activity-Dependent Dendritogenesis of Layer IV Neurons during Barrel Formation

Fibroblast growth factors (FGFs) and FGF receptors (FGFRs) are known for their potent effects on cell proliferation/differentiation and cortical patterning in the developing brain. However, little is known regarding the roles of FGFs/FGFRs in cortical circuit formation. Here we show that Fgfr1/2/3 and Fgf7/9/10/22 mRNAs are expressed in the developing primary somatosensory (S1) barrel cortex. Barrel cortex layer IV spiny stellate cells (bSCs) are the primary recipients of ascending sensory information via thalamocortical axons (TCAs). Detail quantification revealed distinctive phases for bSC dendritogenesis: orienting dendrites toward TCAs, adding de novo dendritic segments, and elongating dendritic length, while maintaining dendritic patterns. Deleting Fgfr1/2/3 in bSCs had minimal impact on dendritic polarity but transiently increased the number of dendritic segments. However, 6 d later, FGFR1/2/3 loss of function reduced dendritic branch numbers. These data suggest that FGFs/FGFRs have a role in stabilizing dendritic patterning. Depolarization of cultured mouse cortical neurons upregulated the levels of several Fgf/Fgfr mRNAs within 2 h. In vivo, within 6 h of systemic kainic acid administration at postnatal day 6, mRNA levels of Fgf9, Fgf10, Fgfr2c, and Fgfr3b in S1 cortices were enhanced, and this was accompanied by exuberant dendritogenesis of bSCs by 24 h. Deleting Fgfr1/2/3 abolished kainic acid-induced bSC dendritic overgrowth. Finally, FGF9/10 gain of function also resulted in extensive dendritogenesis. Together, our data suggest that FGFs/FGFRs can be regulated by glutamate transmission to modulate/stabilize bSC dendritic complexity. Both male and female mice were used for our study.SIGNIFICANCE STATEMENT Glutamatergic transmission plays critical roles in cortical circuit formation. Its dysregulation has been proposed as a core factor in the etiology of many neurological diseases. We found that excessive glutamate transmission upregulated mRNA expression of Fgfrs and their ligands Fgfs Deleting Fgfr1/2/3 not only impaired bSC dendritogenesis but also abolished glutamate transmission-induced dendritic overgrowth. Overexpressing FGF9 or FGF10 in cortical glutamatergic neurons results in excessive dendritic outgrowth within 24 h, resembling the changes induced by excessive glutamate transmission. Our findings provide strong evidence for the physiological role of fibroblast growth factors (FGFs) and FGF receptors (FGFRs) in establishing and maintaining cortical circuits. Perturbing the expression levels of FGFs/FGFRs by excessive glutamatergic neurotransmission could lead to abnormal neuronal circuits, which may contribute to neurological and psychiatric disease.

Theta-frequency selectivity in the somatic spike-triggered average of rat hippocampal pyramidal neurons is dependent on HCN channels

The ability to distill specific frequencies from complex spatiotemporal patterns of afferent inputs is a pivotal functional requirement for neurons residing in networks receiving frequency-multiplexed inputs. Although the expression of theta-frequency subthreshold resonance is established in hippocampal pyramidal neurons, it is not known if their spike initiation dynamics manifest spectral selectivity, or if their intrinsic properties are tuned to process gamma-frequency inputs. Here, we measured the spike-triggered average (STA) of rat hippocampal pyramidal neurons through electrophysiological recordings and quantified spectral selectivity in their spike initiation dynamics and their coincidence detection window (CDW). Our results revealed strong theta-frequency selectivity in the STA, which was also endowed with gamma-range CDW, with prominent neuron-to-neuron variability that manifested distinct pairwise dissociations and correlations with different intrinsic measurements. Furthermore, we demonstrate that the STA and its measurements substantially adapted to the state of the neuron defined by its membrane potential and to the statistics of its afferent inputs. Finally, we tested the effect of pharmacologically blocking the hyperpolarization-activated cyclic-nucleotide-gated (HCN) channels on the STA and found that the STA characteristic frequency reduced significantly to the delta-frequency band after HCN channel blockade. This delta-frequency selectivity in the STA emerged in the absence of subthreshold resonance, which was abolished by HCN channel blockade, thereby confirming computational predictions on the dissociation between these two forms of spectral selectivity. Our results expand the roles of HCN channels to theta-frequency selectivity in the spike initiation dynamics, apart from underscoring the critical role of interactions among different ion channels in regulating neuronal physiology.NEW & NOTEWORTHY We had previously predicted, using computational analyses, that the spike-triggered average (STA) of hippocampal neurons would exhibit theta-frequency (4-10 Hz) spectral selectivity and would manifest coincidence detection capabilities for inputs in the gamma-frequency band (25-150 Hz). Here, we confirmed these predictions through direct electrophysiological recordings of STA from rat CA1 pyramidal neurons and demonstrate that blocking HCN channels reduces the frequency of STA spectral selectivity to the delta-frequency range (0.5-4 Hz).

Cocaine Promotes Coincidence Detection and Lowers Induction Threshold during Hebbian Associative Synaptic Potentiation in Prefrontal Cortex

Addictive drugs usurp neural plasticity mechanisms that normally serve reward-related learning and memory, primarily by evoking changes in glutamatergic synaptic strength in the mesocorticolimbic dopamine circuitry. Here, we show that repeated cocaine exposure in vivo does not alter synaptic strength in the mouse prefrontal cortex during an early period of withdrawal, but instead modifies a Hebbian quantitative synaptic learning rule by broadening the temporal window and lowers the induction threshold for spike-timing-dependent LTP (t-LTP). After repeated, but not single, daily cocaine injections, t-LTP in layer V pyramidal neurons is induced at +30 ms, a normally ineffective timing interval for t-LTP induction in saline-exposed mice. This cocaine-induced, extended-timing t-LTP lasts for ∼1 week after terminating cocaine and is accompanied by an increased susceptibility to potentiation by fewer pre-post spike pairs, indicating a reduced t-LTP induction threshold. Basal synaptic strength and the maximal attainable t-LTP magnitude remain unchanged after cocaine exposure. We further show that the cocaine facilitation of t-LTP induction is caused by sensitized D1-cAMP/protein kinase A dopamine signaling in pyramidal neurons, which then pathologically recruits voltage-gated l-type Ca²⁺ channels that synergize with GluN2A-containing NMDA receptors to drive t-LTP at extended timing. Our results illustrate a mechanism by which cocaine, acting on a key neuromodulation pathway, modifies the coincidence detection window during Hebbian plasticity to facilitate associative synaptic potentiation in prefrontal excitatory circuits. By modifying rules that govern activity-dependent synaptic plasticity, addictive drugs can derail the experience-driven neural circuit remodeling process important for executive control of reward and addiction.

SIGNIFICANCE STATEMENT

It is believed that addictive drugs often render an addict's brain reward system hypersensitive, leaving the individual more susceptible to relapse. We found that repeated cocaine exposure alters a Hebbian associative synaptic learning rule that governs activity-dependent synaptic plasticity in the mouse prefrontal cortex, characterized by a broader temporal window and a lower threshold for spike-timing-dependent LTP (t-LTP), a cellular form of learning and memory. This rule change is caused by cocaine-exacerbated D1-cAMP/protein kinase A dopamine signaling in pyramidal neurons that in turn pathologically recruits l-type Ca²⁺ channels to facilitate coincidence detection during t-LTP induction. Our study provides novel insights on how cocaine, even with only brief exposure, may prime neural circuits for subsequent experience-dependent remodeling that may underlie certain addictive behavior.

Emulating Bilingual Synaptic Response Using a Junction-Based Artificial Synaptic Device

Excitatory and inhibitory postsynaptic potentials are the two fundamental categories of synaptic responses underlying the diverse functionalities of the mammalian nervous system. Recent advances in neuroscience have revealed the co-release of both glutamate and GABA neurotransmitters from a single axon terminal in neurons at the ventral tegmental area that can result in the reconfiguration of the postsynaptic potentials between excitatory and inhibitory effects. The ability to mimic such features of the biological synapses in semiconductor devices, which is lacking in the conventional field effect transistor-type and memristor-type artificial synaptic devices, can enhance the functionalities and versatility of neuromorphic electronic systems in performing tasks such as image recognition, learning, and cognition. Here, we demonstrate an artificial synaptic device concept, an ambipolar junction synaptic devices, which utilizes the tunable electronic properties of the heterojunction between two layered semiconductor materials black phosphorus and tin selenide to mimic the different states of the synaptic connection and, hence, realize the dynamic reconfigurability between excitatory and inhibitory postsynaptic effects. The resulting device relies only on the electrical biases at either the presynaptic or the postsynaptic terminal to facilitate such dynamic reconfigurability. It is distinctively different from the conventional heterosynaptic device in terms of both its operational characteristics and biological equivalence. Key properties of the synapses such as potentiation and depression and spike-timing-dependent plasticity are mimicked in the device for both the excitatory and inhibitory response modes. The device offers reconfiguration properties with the potential to enable useful functionalities in hardware-based artificial neural network.

Calcium Dynamics in Basal Dendrites of Layer 5A and 5B Pyramidal Neurons Is Tuned to the Cell-Type Specific Physiological Action Potential Discharge

Layer 5 (L5) is a major neocortical output layer containing L5A slender-tufted (L5A-st) and L5B thick-tufted (L5B-tt) pyramidal neurons. These neuron types differ in their in vivo firing patterns, connectivity and dendritic morphology amongst other features, reflecting their specific functional role within the neocortical circuits. Here, we asked whether the active properties of the basal dendrites that receive the great majority of synaptic inputs within L5 differ between these two pyramidal neuron classes. To quantify their active properties, we measured the efficacy with which action potential (AP) firing patterns backpropagate along the basal dendrites by measuring the accompanying calcium transients using two-photon laser scanning microscopy in rat somatosensory cortex slices. For these measurements we used both "artificial" three-AP patterns and more complex physiological AP patterns that were previously recorded in anesthetized rats in L5A-st and L5B-tt neurons in response to whisker stimulation. We show that AP patterns with relatively few APs (3APs) evoke a calcium response in L5B-tt, but not L5A-st, that is dependent on the temporal pattern of the three APs. With more complex in vivo recorded AP patterns, the average calcium response was similar in the proximal dendrites but with a decay along dendrites (measured up to 100 μm) of L5B-tt but not L5A-st neurons. Interestingly however, the whisker evoked AP patterns-although very different for the two cell types-evoke similar calcium responses. In conclusion, although the effectiveness with which different AP patterns evoke calcium transients vary between L5A-st and L5B-tt cell, the calcium influx appears to be tuned such that whisker-evoked calcium transients are within the same dynamic range for both cell types.

Serotonergic modulation of the activity of GLP-1 producing neurons in the nucleus of the solitary tract in mouse

Objective

Glucagon-like peptide-1 (GLP-1) and 5-HT are potent regulators of food intake within the brain. GLP-1 is expressed by preproglucagon (PPG) neurons in the nucleus tractus solitarius (NTS). We have previously shown that PPG neurons innervate 5-HT neurons in the ventral brainstem. Here, we investigate whether PPG neurons receive serotonergic input and respond to 5-HT.

Methods

We employed immunohistochemistry to reveal serotonergic innervation of PPG neurons. We investigated the responsiveness of PPG neurons to 5-HT using in vitro Ca²⁺ imaging in brainstem slices from transgenic mice expressing the Ca²⁺ indicator, GCaMP3, in PPG neurons, and cell-attached patch-clamp recordings.

Results

Close appositions from 5-HT-immunoreactive axons occurred on many PPG neurons. Application of 20 μM 5-HT produced robust Ca²⁺ responses in NTS PPG dendrites but little change in somata. Dendritic Ca²⁺ spikes were concentration-dependent (2, 20, and 200 μM) and unaffected by blockade of glutamatergic transmission, suggesting 5-HT receptors on PPG neurons. Neither activation nor blockade of 5-HT₃ receptors affected [Ca²⁺]_i. In contrast, inhibition of 5-HT₂ receptors attenuated increases in intracellular Ca²⁺ and 5-HT_2C receptor activation produced Ca²⁺ spikes. Patch-clamp recordings revealed that 44% of cells decreased their firing rate under 5-HT, an effect blocked by 5-HT_1A receptor antagonism.

Conclusions

PPG neurons respond directly to 5-HT with a 5-HT_2C receptor-dependent increase in dendritic [Ca²⁺]_i. Electrical responses to 5-HT revealed additional inhibitory effects due to somatic 5-HT_1A receptors. Reciprocal innervation between 5-HT and PPG neurons suggests that the coordinated activity of these brainstem neurons may play a role in the regulation of food intake.

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Brainstem PPG neurons receive close appositions from 5-HT-containing axons.

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5-HT activates NTS PPG dendrites directly via 5-HT₂ receptors.

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5-HT inhibits a subset of somata via 5-HT_1A receptors.

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Activation of 5-HT₃ receptors does not affect PPG cell [Ca²⁺]_i.

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5-HT_2C receptor activation induces spatially confined Ca²⁺ spikes in PPG neurons.

A Model of Fast Hebbian Spike Latency Normalization

Hebbian changes of excitatory synapses are driven by and enhance correlations between pre- and postsynaptic neuronal activations, forming a positive feedback loop that can lead to instability in simulated neural networks. Because Hebbian learning may occur on time scales of seconds to minutes, it is conjectured that some form of fast stabilization of neural firing is necessary to avoid runaway of excitation, but both the theoretical underpinning and the biological implementation for such homeostatic mechanism are to be fully investigated. Supported by analytical and computational arguments, we show that a Hebbian spike-timing-dependent metaplasticity rule, accounts for inherently-stable, quick tuning of the total input weight of a single neuron in the general scenario of asynchronous neural firing characterized by UP and DOWN states of activity.

Neurophysiology and Regulation of the Balance Between Excitation and Inhibition in Neocortical Circuits

Brain function relies on the ability of neural networks to maintain stable levels of activity, while experiences sculpt them. In the neocortex, the balance between activity and stability relies on the coregulation of excitatory and inhibitory inputs onto principal neurons. Shifts of excitation or inhibition result in altered excitability impaired processing of incoming information. In many neurodevelopmental and neuropsychiatric disorders, the excitability of local circuits is altered, suggesting that their pathophysiology may involve shifts in synaptic excitation, inhibition, or both. Most studies focused on identifying the cellular and molecular mechanisms controlling network excitability to assess whether they may be altered in animal models of disease. The impact of changes in excitation/inhibition balance on local circuit and network computations is not clear. Here we report findings on the integration of excitatory and inhibitory inputs in healthy cortical circuits and discuss how shifts in excitation/inhibition balance may relate to pathological phenotypes.

Functional consequences of inhibitory plasticity: homeostasis, the excitation-inhibition balance and beyond

Computational neuroscience has a long-standing tradition of investigating the consequences of excitatory synaptic plasticity. In contrast, the functions of inhibitory plasticity are still largely nebulous, particularly given the bewildering diversity of interneurons in the brain. Here, we review recent computational advances that provide first suggestions for the functional roles of inhibitory plasticity, such as a maintenance of the excitation-inhibition balance, a stabilization of recurrent network dynamics and a decorrelation of sensory responses. The field is still in its infancy, but given the existing body of theory for excitatory plasticity, it is likely to mature quickly and deliver important insights into the self-organization of inhibitory circuits in the brain.

Synaptic plasticity in dendrites: complications and coping strategies

The elaborate morphology, nonlinear membrane mechanisms and spatiotemporally varying synaptic activation patterns of dendrites complicate the expression, compartmentalization and modulation of synaptic plasticity. To grapple with this complexity, we start with the observation that neurons in different brain areas face markedly different learning problems, and dendrites of different neuron types contribute to the cell's input-output function in markedly different ways. By committing to specific assumptions regarding a neuron's learning problem and its input-output function, specific inferences can be drawn regarding the synaptic plasticity mechanisms and outcomes that we 'ought' to expect for that neuron. Exploiting this assumption-driven approach can help both in interpreting existing experimental data and designing future experiments aimed at understanding the brain's myriad learning processes.

Developmental control of spike-timing-dependent plasticity by tonic GABAergic signaling in striatum

Activity-dependent long-term potentiation (LTP) and depression (LTD) of synaptic strength underlie multiple forms of learning and memory. Spike-timing-dependent plasticity (STDP) has been described as a Hebbian synaptic learning rule that could account for experience-dependent changes in neural networks, but little is known about whether and how STDP evolves during development. We previously showed that GABAergic signaling governs STDP polarity and thus operates as a Hebbian/anti-Hebbian switch in the striatum. Although GABAergic networks are subject to important developmental maturation, it remains unclear whether STDP is developmentally shaped by GABAergic signaling. Here, we investigated whether STDP rules are developmentally regulated at corticostriatal synapses in the dorsolateral striatum. We found that striatal STDP displays unidirectional plasticity (Hebbian tLTD) in young rats (P_7-10) whereas STDP is bidirectional and anti-Hebbian in juvenile (P_20-25) and adult (P_60-90) rats. We also provide evidence that the appearance of tonic (extrasynaptic) GABAergic signaling from the juvenile stage is a crucial factor in shaping STDP rules during development, establishing bidirectional anti-Hebbian STDP in the adult striatum. Thus, developmental maturation of GABAergic signaling tightly drives the polarity of striatal plasticity.

Comparative ultrastructural features of excitatory synapses in the visual and frontal cortices of the adult mouse and monkey

The excitatory glutamatergic synapse is the principal site of communication between cortical pyramidal neurons and their targets, a key locus of action of many drugs, and highly vulnerable to dysfunction and loss in neurodegenerative disease. A detailed knowledge of the structure of these synapses in distinct cortical areas and across species is a prerequisite for understanding the anatomical underpinnings of cortical specialization and, potentially, selective vulnerability in neurological disorders. We used serial electron microscopy to assess the ultrastructural features of excitatory (asymmetric) synapses in the layers 2-3 (L2-3) neuropil of visual (V1) and frontal (FC) cortices of the adult mouse and compared findings to those in the rhesus monkey (V1 and lateral prefrontal cortex [LPFC]). Analyses of multiple ultrastructural variables revealed four organizational features. First, the density of asymmetric synapses does not differ between frontal and visual cortices in either species, but is significantly higher in mouse than in monkey. Second, the structural properties of asymmetric synapses in mouse V1 and FC are nearly identical, by stark contrast to the significant differences seen between monkey V1 and LPFC. Third, while the structural features of postsynaptic entities in mouse and monkey V1 do not differ, the size of presynaptic boutons are significantly larger in monkey V1. Fourth, both presynaptic and postsynaptic entities are significantly smaller in the mouse FC than in the monkey LPFC. The diversity of synaptic ultrastructural features demonstrated here have broad implications for the nature and efficacy of glutamatergic signaling in distinct cortical areas within and across species.

Strings on a Violin: Location Dependence of Frequency Tuning in Active Dendrites

Strings on a violin are tuned to generate distinct sound frequencies in a manner that is firmly dependent on finger location along the fingerboard. Sound frequencies emerging from different violins could be very different based on their architecture, the nature of strings and their tuning. Analogously, active neuronal dendrites, dendrites endowed with active channel conductances, are tuned to distinct input frequencies in a manner that is dependent on the dendritic location of the synaptic inputs. Further, disparate channel expression profiles and differences in morphological characteristics could result in dendrites on different neurons of the same subtype tuned to distinct frequency ranges. Alternately, similar location-dependence along dendritic structures could be achieved through disparate combinations of channel profiles and morphological characteristics, leading to degeneracy in active dendritic spectral tuning. Akin to strings on a violin being tuned to different frequencies than those on a viola or a cello, different neuronal subtypes exhibit distinct channel profiles and disparate morphological characteristics endowing each neuronal subtype with unique location-dependent frequency selectivity. Finally, similar to the tunability of musical instruments to elicit distinct location-dependent sounds, neuronal frequency selectivity and its location-dependence are tunable through activity-dependent plasticity of ion channels and morphology. In this morceau, we explore the origins of neuronal frequency selectivity, and survey the literature on the mechanisms behind the emergence of location-dependence in distinct forms of frequency tuning. As a coda to this composition, we present some future directions for this exciting convergence of biophysical mechanisms that endow a neuron with frequency multiplexing capabilities.

Pyramidal Neurons Are Not Generalizable Building Blocks of Cortical Networks

A key challenge in cortical neuroscience is to gain a comprehensive understanding of how pyramidal neuron heterogeneity across different areas and species underlies the functional specialization of individual neurons, networks, and areas. Comparative studies have been important in this endeavor, providing data relevant to the question of which of the many inherent properties of individual pyramidal neurons are necessary and sufficient for species-specific network and areal function. In this mini review, the importance of pyramidal neuron structural properties for signaling are outlined, followed by a summary of our recent work comparing the structural features of mouse (C57/BL6 strain) and rhesus monkey layer 3 (L3) pyramidal neurons in primary visual and frontal association cortices and their implications for neuronal and areal function. Based on these and other published data, L3 pyramidal neurons plausibly might be considered broadly “generalizable” from one area to another in the mouse neocortex due to their many similarities, but major differences in the properties of these neurons in diverse areas in the rhesus monkey neocortex rules this out in the primate. Further, fundamental differences in the dendritic topology of mouse and rhesus monkey pyramidal neurons highlight the implausibility of straightforward scaling and/or extrapolation from mouse to primate neurons and cortical networks.

The Sodium Channel β4 Auxiliary Subunit Selectively Controls Long-Term Depression in Core Nucleus Accumbens Medium Spiny Neurons

Voltage-gated sodium channels are essential for generating the initial rapid depolarization of neuronal membrane potential during action potentials (APs) that enable cell-to-cell communication, the propagation of signals throughout the brain, and the induction of synaptic plasticity. Although all brain neurons express one or several variants coding for the core pore-forming sodium channel α subunit, the expression of the β (β1–4) auxiliary subunits varies greatly. Of particular interest is the β4 subunit, encoded by the Scn4b gene, that is highly expressed in dorsal and ventral (i.e., nucleus accumbens – NAc) striata compared to other brain regions, and that endows sodium channels with unique gating properties. However, its role on neuronal activity, synaptic plasticity, and behaviors related to drugs of abuse remains poorly understood. Combining whole-cell patch-clamp recordings with two-photon calcium imaging in Scn4b knockout (KO) and knockdown mice, we found that Scn4b altered the properties of APs in core accumbens medium spiny neurons (MSNs). These alterations are associated with a reduction of the probability of MSNs to evoke spike-timing-dependent long-term depression (tLTD) and a reduced ability of backpropagating APs to evoke dendritic calcium transients. In contrast, long-term potentiation (tLTP) remained unaffected. Interestingly, we also showed that amphetamine-induced locomotor activity was significantly reduced in male Scn4b KO mice compared to wild-type controls. Taken together, these data indicate that the Scn4b subunit selectively controls tLTD by modulating dendritic calcium transients evoked by backpropagating APs.

Bidirectional Hebbian Plasticity Induced by Low-Frequency Stimulation in Basal Dendrites of Rat Barrel Cortex Layer 5 Pyramidal Neurons

According to Hebb's original hypothesis (Hebb, 1949), synapses are reinforced when presynaptic activity triggers postsynaptic firing, resulting in long-term potentiation (LTP) of synaptic efficacy. Long-term depression (LTD) is a use-dependent decrease in synaptic strength that is thought to be due to synaptic input causing a weak postsynaptic effect. Although the mechanisms that mediate long-term synaptic plasticity have been investigated for at least three decades not all question have as yet been answered. Therefore, we aimed at determining the mechanisms that generate LTP or LTD with the simplest possible protocol. Low-frequency stimulation of basal dendrite inputs in Layer 5 pyramidal neurons of the rat barrel cortex induces LTP. This stimulation triggered an EPSP, an action potential (AP) burst, and a Ca²⁺ spike. The same stimulation induced LTD following manipulations that reduced the Ca²⁺ spike and Ca²⁺ signal or the AP burst. Low-frequency whisker deflections induced similar bidirectional plasticity of action potential evoked responses in anesthetized rats. These results suggest that both in vitro and in vivo similar mechanisms regulate the balance between LTP and LTD. This simple induction form of bidirectional hebbian plasticity could be present in the natural conditions to regulate the detection, flow, and storage of sensorimotor information.

Dendritic Spikes in Sensory Perception

What is the function of dendritic spikes? One might argue that they provide conditions for neuronal plasticity or that they are essential for neural computation. However, despite a long history of dendritic research, the physiological relevance of dendritic spikes in brain function remains unknown. This could stem from the fact that most studies on dendrites have been performed in vitro. Fortunately, the emergence of novel techniques such as improved two-photon microscopy, genetically encoded calcium indicators (GECIs), and optogenetic tools has provided the means for vital breakthroughs in in vivo dendritic research. These technologies enable the investigation of the functions of dendritic spikes in behaving animals, and thus, help uncover the causal relationship between dendritic spikes, and sensory information processing and synaptic plasticity. Understanding the roles of dendritic spikes in brain function would provide mechanistic insight into the relationship between the brain and the mind. In this review article, we summarize the results of studies on dendritic spikes from a historical perspective and discuss the recent advances in our understanding of the role of dendritic spikes in sensory perception.

Degeneracy in the regulation of short-term plasticity and synaptic filtering by presynaptic mechanisms

KEY POINTS

We develop a new biophysically rooted, physiologically constrained conductance-based synaptic model to mechanistically account for short-term facilitation and depression, respectively through residual calcium and transmitter depletion kinetics. We address the specific question of how presynaptic components (including voltage-gated ion channels, pumps, buffers and release-handling mechanisms) and interactions among them define synaptic filtering and short-term plasticity profiles. Employing global sensitivity analyses (GSAs), we show that near-identical synaptic filters and short-term plasticity profiles could emerge from disparate presynaptic parametric combinations with weak pairwise correlations. Using virtual knockout models, a technique to address the question of channel-specific contributions within the GSA framework, we unveil the differential and variable impact of each ion channel on synaptic physiology. Our conclusions strengthen the argument that parametric and interactional complexity in biological systems should not be viewed from the limited curse-of-dimensionality standpoint, but from the evolutionarily advantageous perspective of providing functional robustness through degeneracy.

ABSTRACT

Information processing in neurons is known to emerge as a gestalt of pre- and post-synaptic filtering. However, the impact of presynaptic mechanisms on synaptic filters has not been quantitatively assessed. Here, we developed a biophysically rooted, conductance-based model synapse that was endowed with six different voltage-gated ion channels, calcium pumps, calcium buffer and neurotransmitter-replenishment mechanisms in the presynaptic terminal. We tuned our model to match the short-term plasticity profile and band-pass structure of Schaffer collateral synapses, and performed sensitivity analyses to demonstrate that presynaptic voltage-gated ion channels regulated synaptic filters through changes in excitability and associated calcium influx. These sensitivity analyses also revealed that calcium- and release-control mechanisms were effective regulators of synaptic filters, but accomplished this without changes in terminal excitability or calcium influx. Next, to perform global sensitivity analysis, we generated 7000 randomized models spanning 15 presynaptic parameters, and computed eight different physiological measurements in each of these models. We validated these models by applying experimentally obtained bounds on their measurements, and found 104 (∼1.5%) models to match the validation criteria for all eight measurements. Analysing these valid models, we demonstrate that analogous synaptic filters emerge from disparate combinations of presynaptic parameters exhibiting weak pairwise correlations. Finally, using virtual knockout models, we establish the variable and differential impact of different presynaptic channels on synaptic filters, underlining the critical importance of interactions among different presynaptic components in defining synaptic physiology. Our results have significant implications for protein-localization strategies required for physiological robustness and for degeneracy in long-term synaptic plasticity profiles.

Astrocytes gate Hebbian synaptic plasticity in the striatum

Astrocytes, via excitatory amino-acid transporter type-2 (EAAT2), are the major sink for released glutamate and contribute to set the strength and timing of synaptic inputs. The conditions required for the emergence of Hebbian plasticity from distributed neural activity remain elusive. Here, we investigate the role of EAAT2 in the expression of a major physiologically relevant form of Hebbian learning, spike timing-dependent plasticity (STDP). We find that a transient blockade of EAAT2 disrupts the temporal contingency required for Hebbian synaptic plasticity. Indeed, STDP is replaced by aberrant non-timing-dependent plasticity occurring for uncorrelated events. Conversely, EAAT2 overexpression impairs the detection of correlated activity and precludes STDP expression. Our findings demonstrate that EAAT2 sets the appropriate glutamate dynamics for the optimal temporal contingency between pre- and postsynaptic activity required for STDP emergence, and highlight the role of astrocytes as gatekeepers for Hebbian synaptic plasticity.

An NMDA receptor-dependent mechanism for subcellular segregation of sensory inputs in the tadpole optic tectum

In the vertebrate CNS, afferent sensory inputs are targeted to specific depths or layers of their target neuropil. This patterning exists ab initio, from the very beginning, and therefore has been considered an activity-independent process. However, here we report that, during circuit development, the subcellular segregation of the visual and mechanosensory inputs to specific regions of tectal neuron dendrites in the tadpole optic tectum requires NMDA receptor activity. Blocking NMDARs during the formation of these sensory circuits, or removing the visual set of inputs, leads to less defined segregation, and suggests a correlation-based mechanism in which correlated inputs wire to common regions of dendrites. This can account for how two sets of inputs form synapses onto different regions of the same dendrite. Blocking NMDA receptors during later stages of circuit development did not disrupt segregation, indicating a critical period for activity-dependent shaping of patterns of innervation.

DOI:http://dx.doi.org/10.7554/eLife.20502.001

Subcellular Patch-clamp Recordings from the Somatodendritic Domain of Nigral Dopamine Neurons

Dendrites of dopaminergic neurons receive and convey synaptic input, support action potential back-propagation and neurotransmitter release. Understanding these fundamental functions will shed light on the information transfer in these neurons. Dendritic patch-clamp recordings provide the possibility to directly examine the electrical properties of dendrites and underlying voltage-gated ion channels. However, these fine structures are not easily accessible to patch pipettes because of their small diameter. This report describes a step-by-step procedure to collect stable and reliable recordings from the dendrites of dopaminergic neurons in acute slices. Electrophysiological measurements are combined with post hoc recovery of cell morphology. Successful experiments rely on improved preparation of slices, solutions and pipettes, adequate adjustment of the optics and stability of the pipette in contact with the recorded structure. Standard principles of somatic patch-clamp recording are applied to dendrites but with a gentler approach of the pipette. These versatile techniques can be implemented to address various questions concerning the excitable properties of dendrites.

Analogue modulation of back-propagating action potentials enables dendritic hybrid signalling

We report that back-propagating action potentials (bAPs) are not simply digital feedback signals in dendrites but also carry analogue information about the overall state of neurons. Analogue information about the somatic membrane potential within a physiological range (from -78 to -64 mV) is retained by bAPs of dentate gyrus granule cells as different repolarization speeds in proximal dendrites and as different peak amplitudes in distal regions. These location-dependent waveform changes are reflected by local calcium influx, leading to proximal enhancement and distal attenuation during somatic hyperpolarization. The functional link between these retention and readout mechanisms of the analogue content of bAPs critically depends on high-voltage-activated, inactivating calcium channels. The hybrid bAP and calcium mechanisms report the phase of physiological somatic voltage fluctuations and modulate long-term synaptic plasticity in distal dendrites. Thus, bAPs are hybrid signals that relay somatic analogue information, which is detected by the dendrites in a location-dependent manner.

Nonlinear Hebbian Learning as a Unifying Principle in Receptive Field Formation

The development of sensory receptive fields has been modeled in the past by a variety of models including normative models such as sparse coding or independent component analysis and bottom-up models such as spike-timing dependent plasticity or the Bienenstock-Cooper-Munro model of synaptic plasticity. Here we show that the above variety of approaches can all be unified into a single common principle, namely nonlinear Hebbian learning. When nonlinear Hebbian learning is applied to natural images, receptive field shapes were strongly constrained by the input statistics and preprocessing, but exhibited only modest variation across different choices of nonlinearities in neuron models or synaptic plasticity rules. Neither overcompleteness nor sparse network activity are necessary for the development of localized receptive fields. The analysis of alternative sensory modalities such as auditory models or V2 development lead to the same conclusions. In all examples, receptive fields can be predicted a priori by reformulating an abstract model as nonlinear Hebbian learning. Thus nonlinear Hebbian learning and natural statistics can account for many aspects of receptive field formation across models and sensory modalities.

The question of how the brain self-organizes to develop precisely tuned neurons has puzzled neuroscientists at least since the discoveries of Hubel and Wiesel. In the past decades, a variety of theories and models have been proposed to describe receptive field formation, notably V1 simple cells, from natural inputs. We cut through the jungle of candidate explanations by demonstrating that in fact a single principle is sufficient to explain receptive field development. Our results follow from two major insights. First, we show that many representative models of sensory development are in fact implementing variations of a common principle: nonlinear Hebbian learning. Second, we reveal that nonlinear Hebbian learning is sufficient for receptive field formation through sensory inputs. The surprising result is that our findings are robust of specific details of a model, and allows for robust predictions on the learned receptive fields. Nonlinear Hebbian learning is therefore general in two senses: it applies to many models developed by theoreticians, and to many sensory modalities studied by experimental neuroscientists.

The effects of arousal on apical amplification and conscious state

Neocortical pyramidal cells can integrate two classes of input separately and use one to modulate response to the other. Their tuft dendrites are electrotonically separated from basal dendrites and soma by the apical dendrite, and apical hyperpolarization-activated currents (Ih) further isolate subthreshold integration of tuft inputs. When apical depolarization exceeds a threshold, however, it can enhance response to the basal inputs that specify the cell's selective sensitivity. This process is referred to as apical amplification (AA). We review evidence suggesting that, by regulating Ih in the apical compartments, adrenergic arousal controls the coupling between apical and somatic integration zones thus modifying cognitive capabilities closely associated with consciousness. Evidence relating AA to schizophrenia, sleep, and anesthesia is reviewed, and we assess theories that emphasize the relevance of AA to consciousness. Implications for theories of neocortical computation that emphasize context-sensitive modulation are summarized. We conclude that the findings concerning AA and its regulation by arousal offer a new perspective on states of consciousness, the function and evolution of neocortex, and psychopathology. Many issues worthy of closer examination arise.

Cellular and System Biology of Memory: Timing, Molecules, and Beyond

The storage of information in the mammalian nervous systems is dependent on a delicate balance between change and stability of neuronal networks. The induction and maintenance of processes that lead to changes in synaptic strength to a multistep process which can lead to long-lasting changes, which starts and ends with a highly choreographed and perfectly timed dance of molecules in different cell types of the central nervous system. This is accompanied by synchronization of specific networks, resulting in the generation of characteristic "macroscopic" rhythmic electrical fields, whose characteristic frequencies correspond to certain activity and information-processing states of the brain. Molecular events and macroscopic fields influence each other reciprocally. We review here cellular processes of synaptic plasticity, particularly functional and structural changes, and focus on timing events that are important for the initial memory acquisition, as well as mechanisms of short- and long-term memory storage. Then, we cover the importance of epigenetic events on the long-time range. Furthermore, we consider how brain rhythms at the network level participate in processes of information storage and by what means they participating in it. Finally, we examine memory consolidation at the system level during processes of sleep.

Consensus Paper: Probing Homeostatic Plasticity of Human Cortex With Non-invasive Transcranial Brain Stimulation

Homeostatic plasticity is thought to stabilize neural activity around a set point within a physiologically reasonable dynamic range. Over the last ten years, a wide range of non-invasive transcranial brain stimulation (NTBS) techniques have been used to probe homeostatic control of cortical plasticity in the intact human brain. Here, we review different NTBS approaches to study homeostatic plasticity on a systems level and relate the findings to both, physiological evidence from in vitro studies and to a theoretical framework of homeostatic function. We highlight differences between homeostatic and other non-homeostatic forms of plasticity and we examine the contribution of sleep in restoring synaptic homeostasis. Finally, we discuss the growing number of studies showing that abnormal homeostatic plasticity may be associated to a range of neuropsychiatric diseases.

Paired associative transspinal and transcortical stimulation produces plasticity in human cortical and spinal neuronal circuits

Anatomical, physiological, and functional connectivity exists between the neurons of the primary motor cortex (M1) and spinal cord. Paired associative stimulation (PAS) produces enduring changes in M1, based on the Hebbian principle of associative plasticity. The present study aimed to establish neurophysiological changes in human cortical and spinal neuronal circuits by pairing noninvasive transspinal stimulation with transcortical stimulation via transcranial magnetic stimulation (TMS). We delivered paired transspinal and transcortical stimulation for 40 min at precise interstimulus intervals, with TMS being delivered after (transspinal-transcortical PAS) or before (transcortical-transspinal PAS) transspinal stimulation. Transspinal-transcortical PAS markedly decreased intracortical inhibition, increased intracortical facilitation and M1 excitability with concomitant decreases of motor threshold, and reduced the soleus Hoffmann's reflex (H-reflex) low frequency-mediated homosynaptic depression. Transcortical-transspinal PAS did not affect intracortical circuits, decreased M1 excitability, and reduced the soleus H-reflex-paired stimulation pulses' mediated postactivation depression. Both protocols affected the excitation threshold of group Ia afferents and motor axons. These findings clearly indicate that the pairing of transspinal with transcortical stimulation produces cortical and spinal excitability changes based on the timing interval and functional network interactions between the two associated inputs. This new PAS paradigm may constitute a significant neuromodulation method with physiological impact, because it can be used to alter concomitantly excitability of intracortical circuits, corticospinal neurons, and spinal inhibition in humans.

Long-Term Potentiation by Theta-Burst Stimulation Using Extracellular Field Potential Recordings in Acute Hippocampal Slices

This protocol describes how to carry out theta-burst long-term potentiation (LTP) with extracellular field recordings in acute rodent hippocampal slices. This method is relatively simple and noninvasive and provides a way to sample many neurons simultaneously, making it suitable for applications requiring higher throughput than whole-cell recording.

Active dendrites regulate the impact of gliotransmission on rat hippocampal pyramidal neurons

An important consequence of gliotransmission, a signaling mechanism that involves glial release of active transmitter molecules, is its manifestation as N-methyl-d-aspartate receptor (NMDAR)-dependent slow inward currents in neurons. However, the intraneuronal spatial dynamics of these events or the role of active dendrites in regulating their amplitude and spatial spread have remained unexplored. Here, we used somatic and/or dendritic recordings from rat hippocampal pyramidal neurons and demonstrate that a majority of NMDAR-dependent spontaneous slow excitatory potentials (SEP) originate at dendritic locations and are significantly attenuated through their propagation across the neuronal arbor. We substantiated the astrocytic origin of SEPs through paired neuron-astrocyte recordings, where we found that specific infusion of inositol trisphosphate (InsP3) into either distal or proximal astrocytes enhanced the amplitude and frequency of neuronal SEPs. Importantly, SEPs recorded after InsP3 infusion into distal astrocytes exhibited significantly slower kinetics compared with those recorded after proximal infusion. Furthermore, using neuron-specific infusion of pharmacological agents and morphologically realistic conductance-based computational models, we demonstrate that dendritically expressed hyperpolarization-activated cyclic-nucleotide-gated (HCN) and transient potassium channels play critical roles in regulating the strength, kinetics, and compartmentalization of neuronal SEPs. Finally, through the application of subtype-specific receptor blockers during paired neuron-astrocyte recordings, we provide evidence that GluN2B- and GluN2D-containing NMDARs predominantly mediate perisomatic and dendritic SEPs, respectively. Our results unveil an important role for active dendrites in regulating the impact of gliotransmission on neurons and suggest astrocytes as a source of dendritic plateau potentials that have been implicated in localized plasticity and place cell formation.

Symmetric spike timing-dependent plasticity at CA3-CA3 synapses optimizes storage and recall in autoassociative networks

CA3–CA3 recurrent excitatory synapses are thought to play a key role in memory storage and pattern completion. Whether the plasticity properties of these synapses are consistent with their proposed network functions remains unclear. Here, we examine the properties of spike timing-dependent plasticity (STDP) at CA3–CA3 synapses. Low-frequency pairing of excitatory postsynaptic potentials (EPSPs) and action potentials (APs) induces long-term potentiation (LTP), independent of temporal order. The STDP curve is symmetric and broad (half-width ∼150 ms). Consistent with these STDP induction properties, AP–EPSP sequences lead to supralinear summation of spine [Ca²⁺] transients. Furthermore, afterdepolarizations (ADPs) following APs efficiently propagate into dendrites of CA3 pyramidal neurons, and EPSPs summate with dendritic ADPs. In autoassociative network models, storage and recall are more robust with symmetric than with asymmetric STDP rules. Thus, a specialized STDP induction rule allows reliable storage and recall of information in the hippocampal CA3 network.

STDP is dependent on the timing of pre- and post-synaptic activity. Here, the authors describe a symmetric STDP induction rule at CA3-CA3 synapses, which induces LTP over a broad range of paring intervals. Modelling suggests that this STDP rule may enhance storage capacity and pattern completion in the CA3 cell network.

Location-dependent synaptic plasticity rules by dendritic spine cooperativity

Nonlinear interactions between coactive synapses enable neurons to discriminate between spatiotemporal patterns of inputs. Using patterned postsynaptic stimulation by two-photon glutamate uncaging, here we investigate the sensitivity of synaptic Ca(2+) signalling and long-term plasticity in individual spines to coincident activity of nearby synapses. We find a proximodistally increasing gradient of nonlinear NMDA receptor (NMDAR)-mediated amplification of spine Ca(2+) signals by a few neighbouring coactive synapses along individual perisomatic dendrites. This synaptic cooperativity does not require dendritic spikes, but is correlated with dendritic Na(+) spike propagation strength. Furthermore, we show that repetitive synchronous subthreshold activation of small spine clusters produces input specific, NMDAR-dependent cooperative long-term potentiation at distal but not proximal dendritic locations. The sensitive synaptic cooperativity at distal dendritic compartments shown here may promote the formation of functional synaptic clusters, which in turn can facilitate active dendritic processing and storage of information encoded in spatiotemporal synaptic activity patterns.

Transient potassium channels augment degeneracy in hippocampal active dendritic spectral tuning

Hippocampal pyramidal neurons express an intraneuronal map of spectral tuning mediated by hyperpolarization-activated cyclic-nucleotide-gated nonspecific-cation channels. Modeling studies have predicted a critical regulatory role for A-type potassium (KA) channels towards augmenting functional robustness of this map. To test this, we performed patch-clamp recordings from soma and dendrites of rat hippocampal pyramidal neurons, and measured spectral tuning before and after blocking KA channels using two structurally distinct pharmacological agents. Consistent with computational predictions, we found that blocking KA channels resulted in a significant reduction in resonance frequency and significant increases in input resistance, impedance amplitude and action-potential firing frequency across the somato-apical trunk. Furthermore, across all measured locations, blocking KA channels enhanced temporal summation of postsynaptic potentials and critically altered the impedance phase profile, resulting in a significant reduction in total inductive phase. Finally, pair-wise correlations between intraneuronal percentage changes (after blocking KA channels) in different measurements were mostly weak, suggesting differential regulation of different physiological properties by KA channels. Our results unveil a pivotal role for fast transient channels in regulating theta-frequency spectral tuning and intrinsic phase response, and suggest that degeneracy with reference to several coexisting functional maps is mediated by cross-channel interactions across the active dendritic arbor.

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.

Differential dopaminergic regulation of inwardly rectifying potassium channel mediated subthreshold dynamics in striatal medium spiny neurons

The dorsal striatum plays a key role in motor control and cognitive processes. Proper functioning of the striatum relies on the fine dynamic balance between the direct pathway projection medium spiny neurons (MSNs) that express D1 dopamine receptor (D1 MSNs) and indirect pathway projection MSNs that express D2 dopamine receptor (D2 MSNs). The inwardly rectifying K(+) channels (Kir), which express on both D1 and D2 MSNs, participate in the subthreshold dynamics including the membrane resonance and dendritic integration. However, it remains unclear whether dopamine differentially regulates Kir mediated subthreshold dynamics in two subtypes MSNs. Using transgenic mice that express either tdTomato in D1 MSNs or eGFP in D2 MSNs, we explored the Kir mediated subthreshold dynamics in D1 or D2 MSNs with whole cell patch clamp recording in acute brain slices. We found that D1 receptor agonist increased the Kir current while D2 receptor activation decreased the Kir conductance. The dopamine regulation of the Kir enhanced the resonant frequency and reduced the resonant impedance of D1 MSNs. The converse is ture for D2 MSNs. It also caused an opposing effect on dendritic integration between D1 and D2 MSNs, which can promote stability of the two pathways. The D1 receptor activation modulated Kir through cAMP-PKA signaling, whereas the D2 receptor modulated Kir through PLC-PKC signaling. Our findings demonstrated the differential dopaminergic regulation role of Kir, which mediates distinct subthreshold dynamics, and thus, contributes to the role of dopamine in fine tuning the balance of the striatal direct and indirect pathway activities.

Inhibition as a Binary Switch for Excitatory Plasticity in Pyramidal Neurons

Synaptic plasticity is thought to induce memory traces in the brain that are the foundation of learning. To ensure the stability of these traces in the presence of further learning, however, a regulation of plasticity appears beneficial. Here, we take up the recent suggestion that dendritic inhibition can switch plasticity of excitatory synapses on and off by gating backpropagating action potentials (bAPs) and calcium spikes, i.e., by gating the coincidence signals required for Hebbian forms of plasticity. We analyze temporal and spatial constraints of such a gating and investigate whether it is possible to suppress bAPs without a simultaneous annihilation of the forward-directed information flow via excitatory postsynaptic potentials (EPSPs). In a computational analysis of conductance-based multi-compartmental models, we demonstrate that a robust control of bAPs and calcium spikes is possible in an all-or-none manner, enabling a binary switch of coincidence signals and plasticity. The position of inhibitory synapses on the dendritic tree determines the spatial extent of the effect and allows a pathway-specific regulation of plasticity. With appropriate timing, EPSPs can still trigger somatic action potentials, although backpropagating signals are abolished. An annihilation of bAPs requires precisely timed inhibition, while the timing constraints are less stringent for distal calcium spikes. We further show that a wide-spread motif of local circuits—feedforward inhibition—is well suited to provide the temporal precision needed for the control of bAPs. Altogether, our model provides experimentally testable predictions and demonstrates that the inhibitory switch of plasticity can be a robust and attractive mechanism, hence assigning an additional function to the inhibitory elements of neuronal microcircuits beyond modulation of excitability.

We must constantly learn in order to meet the demands of a dynamically changing environment. The basis of learning is believed to be synaptic plasticity, i.e. the potential of neuronal connections to change. Depending on context, however, it may be either useful to learn and modify connections or, alternatively, to keep an established network structure stable to maintain what has been already learned (also referred to as the plasticity-stability dilemma). The ability to switch synaptic plasticity on and off in a flexible way hence constitutes an attractive feature of neuronal processing. Here, we analyze a cellular mechanism based on the inhibition-mediated gating of coincidence signals required for Hebbian forms of excitatory synaptic plasticity. While experimental evidence in support of individual steps involved in this mechanism is accumulating, it is as of now unclear whether this mechanism can indeed operate robustly under physiologically realistic parameters of pyramidal cells, in particular, without impairing information flow in these cells altogether. Computational modeling allows us to demonstrate that this is indeed possible if inhibition is well timed (on the order of 1 ms). Moreover, we show that a specific design of the local circuit can ensure the necessary timing.

Endocannabinoid dynamics gate spike-timing dependent depression and potentiation

Synaptic plasticity is a cardinal cellular mechanism for learning and memory. The endocannabinoid (eCB) system has emerged as a pivotal pathway for synaptic plasticity because of its widely characterized ability to depress synaptic transmission on short- and long-term scales. Recent reports indicate that eCBs also mediate potentiation of the synapse. However, it is not known how eCB signaling may support bidirectionality. Here, we combined electrophysiology experiments with mathematical modeling to question the mechanisms of eCB bidirectionality in spike-timing dependent plasticity (STDP) at corticostriatal synapses. We demonstrate that STDP outcome is controlled by eCB levels and dynamics: prolonged and moderate levels of eCB lead to eCB-mediated long-term depression (eCB-tLTD) while short and large eCB transients produce eCB-mediated long-term potentiation (eCB-tLTP). Moreover, we show that eCB-tLTD requires active calcineurin whereas eCB-tLTP necessitates the activity of presynaptic PKA. Therefore, just like glutamate or GABA, eCB form a bidirectional system to encode learning and memory.

A general homeostatic principle following lesion induced dendritic remodeling

INTRODUCTION

Neuronal death and subsequent denervation of target areas are hallmarks of many neurological disorders. Denervated neurons lose part of their dendritic tree, and are considered "atrophic", i.e. pathologically altered and damaged. The functional consequences of this phenomenon are poorly understood.

RESULTS

Using computational modelling of 3D-reconstructed granule cells we show that denervation-induced dendritic atrophy also subserves homeostatic functions: By shortening their dendritic tree, granule cells compensate for the loss of inputs by a precise adjustment of excitability. As a consequence, surviving afferents are able to activate the cells, thereby allowing information to flow again through the denervated area. In addition, action potentials backpropagating from the soma to the synapses are enhanced specifically in reorganized portions of the dendritic arbor, resulting in their increased synaptic plasticity. These two observations generalize to any given dendritic tree undergoing structural changes.

CONCLUSIONS

Structural homeostatic plasticity, i.e. homeostatic dendritic remodeling, is operating in long-term denervated neurons to achieve functional homeostasis.

Emergence of functional subnetworks in layer 2/3 cortex induced by sequential spikes in vivo

During cortical circuit development in the mammalian brain, groups of excitatory neurons that receive similar sensory information form microcircuits. However, cellular mechanisms underlying cortical microcircuit development remain poorly understood. Here we implemented combined two-photon imaging and photolysis in vivo to monitor and manipulate neuronal activities to study the processes underlying activity-dependent circuit changes. We found that repeated triggering of spike trains in a randomly chosen group of layer 2/3 pyramidal neurons in the somatosensory cortex triggered long-term plasticity of circuits (LTPc), resulting in the increased probability that the selected neurons would fire when action potentials of individual neurons in the group were evoked. Significant firing pattern changes were observed more frequently in the selected group of neurons than in neighboring control neurons, and the induction was dependent on the time interval between spikes, N-methyl-D-aspartate (NMDA) receptor activation, and Calcium/calmodulin-dependent protein kinase II (CaMKII) activation. In addition, LTPc was associated with an increase of activity from a portion of neighboring neurons with different probabilities. Thus, our results demonstrate that the formation of functional microcircuits requires broad network changes and that its directionality is nonrandom, which may be a general feature of cortical circuit assembly in the mammalian cortex.

Neuromodulated Spike-Timing-Dependent Plasticity, and Theory of Three-Factor Learning Rules

Classical Hebbian learning puts the emphasis on joint pre- and postsynaptic activity, but neglects the potential role of neuromodulators. Since neuromodulators convey information about novelty or reward, the influence of neuromodulators on synaptic plasticity is useful not just for action learning in classical conditioning, but also to decide "when" to create new memories in response to a flow of sensory stimuli. In this review, we focus on timing requirements for pre- and postsynaptic activity in conjunction with one or several phasic neuromodulatory signals. While the emphasis of the text is on conceptual models and mathematical theories, we also discuss some experimental evidence for neuromodulation of Spike-Timing-Dependent Plasticity. We highlight the importance of synaptic mechanisms in bridging the temporal gap between sensory stimulation and neuromodulatory signals, and develop a framework for a class of neo-Hebbian three-factor learning rules that depend on presynaptic activity, postsynaptic variables as well as the influence of neuromodulators.

Effects of testosterone on synaptic plasticity mediated by androgen receptors in male SAMP8 mice

Synaptic changes are closely associated with cognitive deficits. In addition, testosterone (T) is known to exert regulative effects on synaptic plasticity. T may improve cognitive deficits in Alzheimer's disease (AD) patients, but the underlying mechanisms of androgenic action on cognitive performance remain unclear. The aim of this study was thus to examine the protective mechanism attributed to T on cognitive performance in an AD senescence, accelerated mouse prone 8 (SAMP8) animal model. Using Golgi staining to quantify the dendritic spine density in hippocampal CA1 region, molecular biomarkers of synapse function were analyzed using immunohistochemistry and western blot. T significantly increased the dendritic spine density in hippocampal CA1 region, while flutamide (F) inhibited these T-mediated effects. Immunohistochemistry and western blot analysis showed that the expression levels of brain derived neurotrophic factor (BDNF), postsynaptic density 95 (PSD-95), and p-cyclic-AMP response element binding protein (CREB)/CREB levels were significantly elevated in the T group, but F reduced the T-induced effects in these biomarkers to control levels. There were no significant differences in the expression levels of PSD-95, BDNF, and p-CREB/CREB between C and F. These findings indicate that the effects of T on improvement in synaptic plasticity were mediated via androgen receptor (AR). It is conceivable that new treatments targeted toward preventing synaptic pathology in AD may involve the use of androgen-acting drugs.

Synapse-specific expression of calcium-permeable AMPA receptors in neocortical layer 5

Key points

In the hippocampus, calcium‐permeable AMPA receptors have been found in a restricted subset of neuronal types that inhibit other neurons, although their localization in the neocortex is less well understood.

In the present study, we looked for calcium‐permeable AMPA receptors in two distinct populations of neocortical inhibitory neurons: basket cells and Martinotti cells. We found them in the former but not in the latter. Furthermore, in basket cells, these receptors were associated with particularly fast responses.

Computer modelling predicted (and experiments verified) that fast calcium‐permeable AMPA receptors enable basket cells to respond rapidly, such that they promptly inhibit neighbouring cells and shut down activity.

The results obtained in the present study help our understanding of pathologies such as stroke and epilepsy that have been associated with disordered regulation of calcium‐permeable AMPA receptors.

Abstract

AMPA‐type glutamate receptors (AMPARs) lacking an edited GluA2 subunit are calcium‐permeable (CP) and contribute to synaptic plasticity in several hippocampal interneuron types, although their precise role in the neocortex is not well described. We explored the presence of CP‐AMPARs at pyramidal cell (PC) inputs to Martinotti cells (MCs) and basket cells (BCs) in layer 5 of the developing mouse visual cortex (postnatal days 12–21). GluA2 immunolabelling was stronger in MCs than in BCs. A differential presence of CP‐AMPARs at PC‐BC and PC‐MC synapses was confirmed electrophysiologically, based on measures of spermine‐dependent rectification and CP‐AMPAR blockade by 1‐naphtyl acetyl spermine using recordings from synaptically connected cell pairs, NPEC‐AMPA uncaging and miniature current recordings. In addition, CP‐AMPAR expression in BCs was correlated with rapidly decaying synaptic currents. Computer modelling predicted that this reduces spike latencies and sharpens suprathreshold responses in BCs, which we verified experimentally using the dynamic clamp technique. Thus, the synapse‐specific expression of CP‐AMPARs may critically influence both plasticity and information processing in neocortical microcircuits.

In the hippocampus, calcium‐permeable AMPA receptors have been found in a restricted subset of neuronal types that inhibit other neurons, although their localization in the neocortex is less well understood.

In the present study, we looked for calcium‐permeable AMPA receptors in two distinct populations of neocortical inhibitory neurons: basket cells and Martinotti cells. We found them in the former but not in the latter. Furthermore, in basket cells, these receptors were associated with particularly fast responses.

Computer modelling predicted (and experiments verified) that fast calcium‐permeable AMPA receptors enable basket cells to respond rapidly, such that they promptly inhibit neighbouring cells and shut down activity.

The results obtained in the present study help our understanding of pathologies such as stroke and epilepsy that have been associated with disordered regulation of calcium‐permeable AMPA receptors.

Synapse-type-specific plasticity in local circuits

Neuroscientists spent decades debating whether synaptic plasticity was presynaptically or postsynaptically expressed. It was eventually concluded that plasticity depends on many factors, including cell type. More recently, it has become increasingly clear that plasticity is regulated at an even finer grained level; it is specific to the synapse type, a concept we denote synapse-type-specific plasticity (STSP). Here, we review recent developments in the field of STSP, discussing both long-term and short-term variants and with particular emphasis on neocortical function. As there are dozens of neocortical cell types, there is a multiplicity of forms of STSP, the vast majority of which have never been explored. We argue that to understand the brain and synaptic diseases, we have to grapple with STSP.

Induction of Long-term Depression-like Plasticity by Pairings of Motor Imagination and Peripheral Electrical Stimulation

Long-term depression (LTD) and long-term potentiation (LTP)-like plasticity are models of synaptic plasticity which have been associated with memory and learning. The induction of LTD and LTP-like plasticity, using different stimulation protocols, has been proposed as a means of addressing abnormalities in cortical excitability associated with conditions such as focal hand dystonia and stroke. The aim of this study was to investigate whether the excitability of the cortical projections to the tibialis anterior (TA) muscle could be decreased when dorsiflexion of the ankle joint was imagined and paired with peripheral electrical stimulation (ES) of the nerve supplying the antagonist soleus muscle. The effect of stimulus timing was evaluated by comparing paired stimulation timed to reach the cortex before, at and after the onset of imagined movement. Fourteen healthy subjects participated in six experimental sessions held on non-consecutive days. The timing of stimulation delivery was determined offline based on the contingent negative variation (CNV) of electroencephalography brain data obtained during imagined dorsiflexion. Afferent stimulation was provided via a single pulse ES to the peripheral nerve paired, based on the CNV, with motor imagination of ankle dorsiflexion. A significant decrease (P = 0.001) in the excitability of the cortical projection of TA was observed when the afferent volley from the ES of the tibial nerve (TN) reached the cortex at the onset of motor imagination based on the CNV. When TN stimulation was delivered before (P = 0.62), or after (P = 0.23) imagined movement onset there was no significant effect. Nor was a significant effect found when ES of the TN was applied independent of imagined movement (P = 0.45). Therefore, the excitability of the cortical projection to a muscle can be inhibited when ES of the nerve supplying the antagonist muscle is precisely paired with the onset of imagined movement.