Paying the brain's energy bill

How have animals managed to maintain metabolically expensive brains given the volatile and fleeting availability of calories in the natural world? Here we review studies in support of three strategies that involve: 1) a reallocation of energy from peripheral tissues and functions to cover the costs of the brain, 2) an implementation of energy-efficient neural coding, enabling the brain to operate at reduced energy costs, and 3) efficient use of costly neural resources during food scarcity. Collectively, these studies reveal a heterogeneous set of energy-saving mechanisms that make energy-costly brains fit for survival.

Neocortex saves energy by reducing coding precision during food scarcity

Information processing is energetically expensive. In the mammalian brain, it is unclear how information coding and energy use are regulated during food scarcity. Using whole-cell recordings and two-photon imaging in layer 2/3 mouse visual cortex, we found that food restriction reduced AMPA receptor conductance, reducing synaptic ATP use by 29%. Neuronal excitability was nonetheless preserved by a compensatory increase in input resistance and a depolarized resting potential. Consequently, neurons spiked at similar rates as controls but spent less ATP on underlying excitatory currents. This energy-saving strategy had a cost because it amplified the variability of visually-evoked subthreshold responses, leading to a 32% broadening of orientation tuning and impaired fine visual discrimination. This reduction in coding precision was associated with reduced levels of the fat mass-regulated hormone leptin and was restored by exogenous leptin supplementation. Our findings reveal that metabolic state dynamically regulates the energy spent on coding precision in neocortex.

Defying Expectations: How Neurons Compute Prediction Errors in Visual Cortex

In this issue of Neuron, Jordan and Keller (2020) explore subthreshold computations underlying predictive coding using whole-cell recordings in mouse visual cortex. Their findings suggest that layer 2/3, but not layer 5/6, neurons compute prediction errors by subtracting predicted and actual visual flow inputs generated by locomotion.

A two-compartment model of synaptic computation and plasticity

The synapse is typically viewed as a single compartment, which acts as a linear gain controller on incoming input. Traditional plasticity rules enable this gain control to be dynamically optimized by Hebbian activity. Whilst this view nicely captures postsynaptic function, it neglects the non-linear dynamics of presynaptic function. Here we present a two-compartment model of the synapse in which the presynaptic terminal first acts to filter presynaptic input before the postsynaptic terminal, acting as a gain controller, amplifies or depresses transmission. We argue that both compartments are equipped with distinct plasticity rules to enable them to optimally adapt synaptic transmission to the statistics of pre- and postsynaptic activity. Specifically, we focus on how presynaptic plasticity enables presynaptic filtering to be optimally tuned to only transmit information relevant for postsynaptic firing. We end by discussing the advantages of having a presynaptic filter and propose future work to explore presynaptic function and plasticity in vivo.

High and asymmetric somato-dendritic coupling of V1 layer 5 neurons independent of visual stimulation and locomotion

Active dendrites impact sensory processing and behaviour. However, it remains unclear how active dendritic integration relates to somatic output in vivo. We imaged semi-simultaneously GCaMP6s signals in the soma, trunk and distal tuft dendrites of layer 5 pyramidal neurons in the awake mouse primary visual cortex. We found that apical tuft signals were dominated by widespread, highly correlated calcium transients throughout the tuft. While these signals were highly coupled to trunk and somatic transients, the frequency of calcium transients was found to decrease in a distance-dependent manner from soma to tuft. Ex vivo recordings suggest that low-frequency back-propagating action potentials underlie the distance-dependent loss of signals, while coupled somato-dendritic signals can be triggered by high-frequency somatic bursts or strong apical tuft depolarization. Visual stimulation and locomotion increased neuronal activity without affecting somato-dendritic coupling. High, asymmetric somato-dendritic coupling is therefore a widespread feature of layer 5 neurons activity in vivo.

Intracellular Ca2+ Release and Synaptic Plasticity: A Tale of Many Stores

Ca2+ is an essential trigger for most forms of synaptic plasticity. Ca2+ signaling occurs not only by Ca2+ entry via plasma membrane channels but also via Ca2+ signals generated by intracellular organelles. These organelles, by dynamically regulating the spatial and temporal extent of Ca2+ elevations within neurons, play a pivotal role in determining the downstream consequences of neural signaling on synaptic function. Here, we review the role of three major intracellular stores: the endoplasmic reticulum, mitochondria, and acidic Ca2+ stores, such as lysosomes, in neuronal Ca2+ signaling and plasticity. We provide a comprehensive account of how Ca2+ release from these stores regulates short- and long-term plasticity at the pre- and postsynaptic terminals of central synapses.

Optical Quantal Analysis Using Ca2+ Indicators: A Robust Method for Assessing Transmitter Release Probability at Excitatory Synapses by Imaging Single Glutamate Release Events

Despite evidence that presynaptic efficacy and plasticity influence circuit function and behavior in vivo, studies of presynaptic function remain challenging owing to the difficulty of assessing transmitter release in intact tissue. Electrophysiological analyses of transmitter release are indirect and cannot readily resolve basic presynaptic parameters, most notably transmitter release probability (p r), at single synapses. These issues can be circumvented by optical quantal analysis, which uses the all-or-none optical detection of transmitter release in order to calculate p r. Over the past two decades, we and others have successfully demonstrated that Ca2+ indicators can be strategically implemented to perform optical quantal analysis at single glutamatergic synapses in ex vivo and in vitro preparations. We have found that high affinity Ca2+ indicators can reliably detect spine Ca2+ influx generated by single quanta of glutamate, thereby enabling precise calculation of pr at single synapses. Importantly, we have shown this method to be robust to changes in postsynaptic efficacy, and to be sensitive to activity-dependent presynaptic changes at central synapses following the induction of long-term potentiation (LTP) and long-term depression (LTD). In this report, we describe how to use Ca2+-sensitive dyes to perform optical quantal analysis at single synapses in hippocampal slice preparations. The general technique we describe here can be applied to other glutamatergic synapses and can be used with other reporters of glutamate release, including recently improved genetically encoded Ca2+ and glutamate sensors. With ongoing developments in imaging techniques and genetically encoded probes, optical quantal analysis is a promising strategy for assessing presynaptic function and plasticity in vivo.

Hippocampal mGluR1-dependent long-term potentiation requires NAADP-mediated acidic store Ca2+ signaling

Acidic organelles, such as endosomes and lysosomes, store Ca²⁺ that is released in response to intracellular increases in the second messenger nicotinic acid adenine dinucleotide phosphate (NAADP). In neurons, NAADP and Ca²⁺ signaling contribute to synaptic plasticity, a process of activity-dependent long-term potentiation (LTP) [or, alternatively, long-term depression (LTD)] of synaptic strength and neuronal transmission that is critical for neuronal function and memory formation. We explored the function of and mechanisms regulating acidic Ca²⁺ store signaling in murine hippocampal neurons. We found that metabotropic glutamate receptor 1 (mGluR1) was coupled to NAADP signaling that elicited Ca²⁺ release from acidic stores. In turn, this released Ca²⁺-mediated mGluR1-dependent LTP by transiently inhibiting SK-type K⁺ channels, possibly through the activation of protein phosphatase 2A. Genetically removing two-pore channels (TPCs), which are endolysosomal-specific ion channels, switched the polarity of plasticity from LTP to LTD, indicating the importance of specific receptor store coupling and providing mechanistic insight into how mGluR1 can produce both synaptic potentiation and synaptic depression.

Homeostatic Presynaptic Plasticity Is Specifically Regulated by P/Q-type Ca2+ Channels at Mammalian Hippocampal Synapses

Voltage-dependent Ca²⁺ channels (VGCC) represent the principal source of Ca²⁺ ions driving evoked neurotransmitter release at presynaptic boutons. In mammals, presynaptic Ca²⁺ influx is mediated mainly via P/Q-type and N-type VGCC, which differ in their properties. Changes in their relative contributions tune neurotransmission both during development and in Hebbian plasticity. However, whether this represents a functional motif also present in other forms of activity-dependent regulation is unknown. Here, we study the role of VGCC in homeostatic plasticity (HSP) in mammalian hippocampal neurons using optical techniques. We find that changes in evoked Ca²⁺ currents specifically through P/Q-type, but not N-type, VGCC mediate bidirectional homeostatic regulation of both neurotransmitter release efficacy and the size of the major synaptic vesicle pools. Selective dependence of HSP on P/Q-type VGCC in mammalian terminals has important implications for phenotypes associated with P/Q-type channelopathies, including migraine and epilepsy.

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P/Q-type VGCC regulate homeostatic synaptic plasticity (HSP) in mammals

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Changes in synaptic vesicle pool sizes during HSP are also mediated via P/Q-type VGCC

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Expression of HSP is independent of N-type VGCC regulation

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P/Q-type regulation of HSP may explain phenotypes of P/Q-type channelopathies

Subcellular spatial resolution achieved for deep-brain imaging in vivo using a minimally invasive multimode fiber

Achieving intravital optical imaging with diffraction-limited spatial resolution of deep-brain structures represents an important step toward the goal of understanding the mammalian central nervous system^1-4. Advances in wavefront-shaping methods and computational power have recently allowed for a novel approach to high-resolution imaging, utilizing deterministic light propagation through optically complex media and, of particular importance for this work, multimode optical fibers (MMFs)^5-7. We report a compact and highly optimized approach for minimally invasive in vivo brain imaging applications. The volume of tissue lesion was reduced by more than 100-fold, while preserving diffraction-limited imaging performance utilizing wavefront control of light propagation through a single 50-μm-core MMF. Here, we demonstrated high-resolution fluorescence imaging of subcellular neuronal structures, dendrites and synaptic specializations, in deep-brain regions of living mice, as well as monitored stimulus-driven functional Ca²⁺ responses. These results represent a major breakthrough in the compromise between high-resolution imaging and tissue damage, heralding new possibilities for deep-brain imaging in vivo.

Glutamate is required for depression but not potentiation of long-term presynaptic function

Hebbian plasticity is thought to require glutamate signalling. We show this is not the case for hippocampal presynaptic long-term potentiation (LTP_pre), which is expressed as an increase in transmitter release probability (P_r). We find that LTP_pre can be induced by pairing pre- and postsynaptic spiking in the absence of glutamate signalling. LTP_pre induction involves a non-canonical mechanism of retrograde nitric oxide signalling, which is triggered by Ca²⁺ influx from L-type voltage-gated Ca²⁺ channels, not postsynaptic NMDA receptors (NMDARs), and does not require glutamate release. When glutamate release occurs, it decreases P_r by activating presynaptic NMDARs, and promotes presynaptic long-term depression. Net changes in P_r, therefore, depend on two opposing factors: (1) Hebbian activity, which increases P_r, and (2) glutamate release, which decreases P_r. Accordingly, release failures during Hebbian activity promote LTP_pre induction. Our findings reveal a novel framework of presynaptic plasticity that radically differs from traditional models of postsynaptic plasticity.

Neurons communicate with one another at junctions called synapses. One neuron at the synapse releases a chemical substance called a neurotransmitter, which binds to and activates the other neuron. The release of neurotransmitter thus enables the electrical activity of one cell to influence the electrical activity of another. The efficiency of this communication can change over time, as is thought to occur during learning. If the neurons on both sides of a synapse are repeatedly active at the same time, the ability of the neurons to transmit electrical signals to each other increases.

One way that communication between neurons can become more efficient is if the first neuron becomes more likely to release neurotransmitter. Most synapses in the brain release a neurotransmitter called glutamate, and most types of learning involve changes in the efficiency of communication at glutamatergic synapses. But glutamate release is unreliable. Active glutamatergic neurons fail to release glutamate about 80% of the time. If glutamate has a key role in learning, how does the brain learn efficiently when glutamate release is so unlikely?

To find out, Padamsey et al. studied glutamatergic synapses in slices of tissue from mouse and rat brains. When both neurons at a synapse were repeatedly active at the same time, the first neuron would sometimes become more likely to release glutamate. But this only happened at synapses in which the first neuron usually failed to release glutamate in the first place. This suggests that communication failures help to drive change at synapses. When two neurons that are often active at the same time do not communicate efficiently, this failure triggers molecular changes that make future communication more reliable.

Previous results have shown that synapses can change when glutamate release occurs. The current results show that they can also change when it does not. This means that the brain can continue to learn despite frequent communication failures between neurons. Many neurological disorders, including Alzheimer’s disease, show altered glutamate signalling at synapses. Padamsey et al. hope that a better understanding of this process will lead to new therapies for these disorders.

Inhibition of lysosomal Ca2+ signalling disrupts dendritic spine structure and impairs wound healing in neurons

A growing body of evidence suggests that lysosomes, which have traditionally been regarded as degradative organelles, can function as Ca²⁺ stores, regulated by the second messenger nicotinic acid adenine dinucleotide phosphate (NAADP). We previously demonstrated that in hippocampal pyramidal neurons, activity-dependent Ca²⁺ release from these stores triggers fusion of the lysosome with the plasma membrane. We found that the physiological role of this Ca²⁺-dependent fusion was to maintain the long-term structural enlargement of dendritic spines induced by synaptic activity. Here, we examined the pathophysiological consequences of lysosomal dysfunction in hippocampal pyramidal neurons by chronically inhibiting lysosomal Ca²⁺ signalling using the NAADP antagonist, NED-19. We found that within just 20 hours, inhibition of lysosomal function led to a profound intracellular accumulation of lysosomal membrane. This was accompanied by a significant change in dendritic spine structure, which included a lengthening of dendritic spines, an increase in the number of filipodia, and an overall decrease in spine number. Inhibition of lysosomal function also inhibited wound healing in neurons by preventing lysosomal fusion with the plasma membrane. Neurons were therefore more susceptible to injury. Our findings suggest that dysfunction in lysosomal Ca²⁺ signalling and lysosomal fusion with the plasma membrane may contribute to the loss of dendritic spines and neurons seen in neurological disorders, such as Niemann-Pick disease type C1, in which lysosomal function is impaired.

Synaptic Transmission Optimization Predicts Expression Loci of Long-Term Plasticity

Long-term modifications of neuronal connections are critical for reliable memory storage in the brain. However, their locus of expression-pre- or postsynaptic-is highly variable. Here we introduce a theoretical framework in which long-term plasticity performs an optimization of the postsynaptic response statistics toward a given mean with minimal variance. Consequently, the state of the synapse at the time of plasticity induction determines the ratio of pre- and postsynaptic modifications. Our theory explains the experimentally observed expression loci of the hippocampal and neocortical synaptic potentiation studies we examined. Moreover, the theory predicts presynaptic expression of long-term depression, consistent with experimental observations. At inhibitory synapses, the theory suggests a statistically efficient excitatory-inhibitory balance in which changes in inhibitory postsynaptic response statistics specifically target the mean excitation. Our results provide a unifying theory for understanding the expression mechanisms and functions of long-term synaptic transmission plasticity.

Activity-Dependent Exocytosis of Lysosomes Regulates the Structural Plasticity of Dendritic Spines

Lysosomes have traditionally been viewed as degradative organelles, although a growing body of evidence suggests that they can function as Ca²⁺ stores. Here we examined the function of these stores in hippocampal pyramidal neurons. We found that back-propagating action potentials (bpAPs) could elicit Ca²⁺ release from lysosomes in the dendrites. This Ca²⁺ release triggered the fusion of lysosomes with the plasma membrane, resulting in the release of Cathepsin B. Cathepsin B increased the activity of matrix metalloproteinase 9 (MMP-9), an enzyme involved in extracellular matrix (ECM) remodelling and synaptic plasticity. Inhibition of either lysosomal Ca²⁺ signaling or Cathepsin B release prevented the maintenance of dendritic spine growth induced by Hebbian activity. This impairment could be rescued by exogenous application of active MMP-9. Our findings suggest that activity-dependent exocytosis of Cathepsin B from lysosomes regulates the long-term structural plasticity of dendritic spines by triggering MMP-9 activation and ECM remodelling.

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Back-propagating action potentials induce Ca²⁺ release from lysosomes in neurons

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Lysosomal Ca²⁺ release triggers exocytosis of the lysosomal protease Cathepsin B

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Cathepsin B maintains activity-dependent dendritic spine growth by activating MMP-9

Two sides to long-term potentiation: a view towards reconciliation

Almost since the discovery of long-term potentiation (LTP) in the hippocampus, its locus of expression has been debated. Throughout the years, convincing evidence has accumulated to suggest that LTP can be supported either presynaptically, by an increase in transmitter release, or postsynaptically, by an increase in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor number. However, whereas postsynaptic enhancement appears to be consistently obtained across studies following LTP induction, presynaptic enhancement is not as reliably observed. Such discrepancies, along with the failure to convincingly identify a retrograde messenger required for presynaptic change, have led to the general view that LTP is mainly supported postsynaptically, and certainly, research within the field for the past decade has been heavily focused on the postsynaptic locus. Here, we argue that LTP can be expressed at either synaptic locus, but that pre- and postsynaptic forms of LTP are dissociable phenomena mediated by distinct mechanistic processes, which are sensitive to different patterns of neuronal activity. This view of LTP helps to reconcile discrepancies across the literature and may put to rest a decades-long debate.

Imaging synaptic plasticity

Over the past decade, the use and development of optical imaging techniques has advanced our understanding of synaptic plasticity by offering the spatial and temporal resolution necessary to examine long-term changes at individual synapses. Here, we review the use of these techniques in recent studies of synaptic plasticity and, in particular, long-term potentiation in the hippocampus.

Increased expression of dysbindin-1A leads to a selective deficit in NMDA receptor signaling in the hippocampus

The effects of the major schizophrenia susceptibility gene disease DTNBP1 on disease risk are likely to be mediated through changes in expression level of the gene product, dysbindin-1. How such changes might influence pathogenesis is, however, unclear. One possible mechanism is suggested by recent work establishing a link between altered dysbindin-1 expression and changes in surface levels of N-methyl-d-aspartate receptors (NMDAR), although neither the precise nature of this relationship, nor the mechanism underlying it, are understood. Using organotypic slices of rat hippocampus, we show that increased expression of dysbindin-1A in pyramidal neurons causes a severe and selective hypofunction of NMDARs and blocks induction of LTP. Cell surface, but not cytoplasmic, expression of the NR1 subunit of the NMDAR is decreased, suggesting dysregulation of NMDAR trafficking and, consistent with this, pharmacological inhibition of clathrin-dependent endocytosis is sufficient to reverse the deficit in NMDAR signaling. These results support the idea that the level of the NMDAR at the plasma membrane is modulated by changes in dysbindin-1 expression and offer further insight into the role of dysbindin-1 at an important cellular pathway implicated in schizophrenia.