Inactivity produces increases in neurotransmitter release and synapse size

Timing Matters: Preconditioning Effects of Cathodal Transcranial Direct Current Stimulation on Intermittent Theta-Burst Stimulation-Induced Neuroplasticity in the Primary Motor Cortex

BACKGROUND

Recent advances have highlighted the interplay between intermittent theta-burst stimulation (iTBS) and transcranial direct current stimulation (tDCS) in neuroplasticity modulation. However, the synergistic potential of these modalities in optimizing plasticity, particularly with cathodal tDCS preconditioning before iTBS, remains poorly understood.

OBJECTIVE

This study examined the effects of cathodal high-definition tDCS (HD-tDCS) preconditioning on iTBS-induced neuroplasticity in the primary motor cortex at different timing intervals.

MATERIALS AND METHODS

Twenty healthy participants underwent four stimulation sessions in a randomized cross-over design, receiving iTBS either immediately or at 10-minute and 30-minute intervals after cathodal HD-tDCS preconditioning, in addition to a control session with iTBS immediately after sham HD-tDCS. Motor evoked potentials (MEPs) were measured at baseline and 5, 10, 15, and 30 minutes after iTBS to assess changes in neuroplasticity. Each session was separated by ≥one week to prevent carry-over effects.

RESULTS

Compared with sham sessions, immediate cathodal HD-tDCS preconditioning significantly enhanced MEPs across all measured intervals after iTBS, with sustained neuroplasticity persisting for up to 30 minutes. Immediate preconditioning produced significant MEP enhancements at 5 and 10 minutes when compared with the 30-minute delayed condition.

CONCLUSIONS

The effectiveness of cathodal tDCS preconditioning in enhancing iTBS-induced neuroplasticity decreased with increasing intervals between tDCS and iTBS application. These findings highlight the essential role of precise timing in tDCS preconditioning for maximizing the neuroplastic effects of iTBS and offer valuable insights for optimizing neurorehabilitation protocols.

Neuronal network inactivity potentiates neuropeptide release from mouse cortical neurons

Neurons adapt to chronic activity changes by modifying synaptic properties, including neurotransmitter release. However, whether neuropeptide release via dense core vesicles (DCVs)-a distinct regulated secretory pathway-undergoes similar adaptation remains unclear. Here, we demonstrate that 24-hour action potential blockade leads to significant DCV accumulation in primary mouse cortical neurons of both sexes. Reactivation with action potential trains induced enhanced Ca2+-influx and 700% more DCV exocytosis compared to control neurons. Notably, total DCV cargo protein levels were unchanged, while mRNA levels of corresponding genes were reduced. Blocking neurotransmitter release with Tetanus toxin induced DCV accumulation, similar to that induced by network silencing with TTX. Hence, chronic network silencing triggers increased DCV accumulation due to reduced exocytosis during silencing. These accumulated DCVs can be released upon reactivation resulting in a massive potentiation of DCV exocytosis, possibly contributing to homeostatic mechanisms.Significance Statement This study addresses an unexplored area - how dense core vesicles (DCVs) exocytosis adapts to chronic changes in activity - and demonstrates accumulation of DCVs and a massive upregulation of DCV exocytosis in response to 24h inactivity. The potentiation of neuropeptide release might contribute to homeostatic regulation of neuronal networks in the brain.

Tuning synapse strength by nanocolumn plasticity

The precise organization of the complex set of synaptic proteins at the nanometer scale is crucial for synaptic transmission. At the heart of this nanoscale architecture lies the nanocolumn. This aligns presynaptic neurotransmitter release with a high local density of postsynaptic receptor channels, thereby optimizing synaptic strength. Although synapses exhibit diverse protein compositions and nanoscale organizations, the role of structural diversity in the notable differences observed in synaptic physiology remains poorly understood. In this review we examine the current literature on the molecular mechanisms underlying the formation and maintenance of nanocolumns, as well as their role in modulating various aspects of synaptic transmission. We also discuss how the reorganization of nanocolumns contributes to functional dynamics in both synaptic plasticity and pathology.

Network effects of traumatic brain injury: from infra slow to high frequency oscillations and seizures

Traumatic brain injury (TBI) can have a multitude of effects on neural functioning. In extreme cases, TBI can lead to seizures both immediately following the injury as well as persistent epilepsy over years to a lifetime. However, mechanisms of neural dysfunctioning after TBI remain poorly understood. To address these questions, we analyzed human and animal data and we developed a biophysical network model implementing effects of ion concentration dynamics and homeostatic synaptic plasticity to test effects of TBI on the brain network dynamics. We focus on three primary phenomena that have been reported in vivo after TBI: an increase in infra slow oscillations (<0.1 Hz), increase in Delta power (1 - 4 Hz), and the emergence of broadband Gamma bursts (30 - 100 Hz). Using computational network model, we show that the infra slow oscillations can be directly attributed to extracellular potassium dynamics, while the increase in Delta power and occurrence of Gamma bursts are related to the increase in strength of synaptic weights from homeostatic synaptic scaling triggered by trauma. We also show that the buildup of Gamma bursts in the injured region can lead to seizure-like events that propagate across the entire network; seizures can then be initiated in previously healthy regions. This study brings greater understanding of the network effects of TBI and how they can lead to epileptic activity. This lays the foundation to begin investigating how injured networks can be healed and seizures prevented.

Data-driven synapse classification reveals a logic of glutamate receptor diversity

The rich diversity of synapses facilitates the capacity of neural circuits to transmit, process and store information. We used multiplex super-resolution proteometric imaging through array tomography to define features of single synapses in mouse neocortex. We find that glutamatergic synapses cluster into subclasses that parallel the distinct biochemical and functional categories of receptor subunits: GluA1/4, GluA2/3 and GluN1/GluN2B. Two of these subclasses align with physiological expectations based on synaptic plasticity: large AMPAR-rich synapses may represent potentiated synapses, whereas small NMDAR-rich synapses suggest "silent" synapses. The NMDA receptor content of large synapses correlates with spine neck diameter, and thus the potential for coupling to the parent dendrite. Overall, ultrastructural features predict receptor content of synapses better than parent neuron identity does, suggesting synapse subclasses act as fundamental elements of neuronal circuits. No barriers prevent future generalization of this approach to other species, or to study of human disorders and therapeutics.

Adult visual deprivation engages associative, presynaptic plasticity of thalamic input to cortex

Associative plasticity at thalamocortical synapses is thought to be constrained by age in the mammalian cortex. However, here we show for the first time that prolonged visual deprivation induces robust and reversible plasticity at synapses between first order visual thalamus and cortical layer 4 pyramidal neurons. The plasticity is associative and expressed by changes in presynaptic function, thereby amplifying and relaying the change in efferent drive to the visual cortex.

Rasopathy-Associated Mutation Ptpn11D61Y has Age-Dependent Effect on Synaptic Vesicle Recycling

Rasopathies are genetic disorders often associated with developmental delay and intellectual disability. Noonan syndrome (NS) is one of the most common Rasopathies, caused by mutations in PTPN11 in more than 50% of cases. In mammalian neurons, PTPN11 controls the trafficking of postsynaptic glutamate receptors. This process is disrupted in neurons expressing PTPN11 variants associated with Rasopathies and is thought to contribute to the cognitive impairments in Noonan syndrome. Recent work revealed presynaptic impairments upon expression of RASopathy-linked PTPN11 variants in Drosophila. However, the presynaptic role of PTPN11 has not yet been addressed in mammals. Here, we investigated membrane trafficking of synaptic vesicles in cultured mouse cortical neurons expressing Rasopathy-associated PTPN11D61Y variant. We observed a significantly smaller readily releasable and total recycling pool of synaptic vesicles. The drop in synaptic vesicle release competence was accompanied by a decreased rate of SV retrieval. Interestingly, the presynaptic phenotype was evident in mature (DIV21) but not in immature (DIV12) neurons. Thus, our data reveal importance of balanced PTPN11 activity for normal trafficking of neurotransmitter-filled synaptic vesicles in the presynaptic ending of mature neurons.

Morphological correlates of synaptic protein turnover in the mouse brain

Synaptic proteins need to be replaced regularly, to maintain function and to prevent damage. It is unclear whether this process, known as protein turnover, relates to synaptic morphology. To test this, we relied on nanoscale secondary ion mass spectrometry, to detect newly synthesized synaptic components in the brains of young adult (6 mo old) and aged mice (24 mo old), and on transmission electron microscopy, to reveal synapse morphology. Several parameters correlated to turnover, including pre- and postsynaptic size, the number of synaptic vesicles and the presence of a postsynaptic nascent zone. In aged mice, the turnover of all brain compartments was reduced by ∼20%. The turnover rates of the pre- and postsynapses correlated well in aged mice, suggesting that they are subject to common regulatory mechanisms. This correlation was poorer in young adult mice, in line with their higher synaptic dynamics. We conclude that synapse turnover is reflected by synaptic morphology.

Differences in motor learning-related structural plasticity of layer 2/3 parvalbumin-positive interneurons of the young and aged motor cortex

Changes to neuronal connectivity are believed to be a key factor in cognitive impairments associated with normal aging. Because of its effect on activities of daily living, deficient motor control is a critical type of cognitive decline to understand. Diminished inhibitory networks in the cortex are implicated in such motor control deficits, pointing to the connectivity of inhibitory cortical interneurons as an important area for study. Here, we used chronic two-photon microscopy to track the structural plasticity of en passant boutons (EPBs) of parvalbumin-positive interneurons in the mouse motor cortex in the first longitudinal, in vivo study of inhibitory interneuron synapses in the context of aging. Young (3-5 months) and aged (23-28 months) mice underwent training on the accelerating rotarod to evoke motor learning-induced structural plasticity. Our analysis reveals that, in comparison with axons from young mice, those from aged mice have fewer EPBs at baseline that also tend to be larger in size. Aged axons also express learning-related structural plasticity-like new bouton stabilization and bouton enlargement-that is less persistent than that of young axons. This study reveals striking baseline differences in young and aged axon morphology as well as differences in the deployment of learning-related structural plasticity across axons.

Prolonged Activity Deprivation Causes Pre- and Postsynaptic Compensatory Plasticity at Neocortical Excitatory Synapses

Homeostatic plasticity stabilizes firing rates of neurons, but the pressure to restore low activity rates can significantly alter synaptic and cellular properties. Most previous studies of homeostatic readjustment to complete activity silencing in rodent forebrain have examined changes after 2 d of deprivation, but it is known that longer periods of deprivation can produce adverse effects. To better understand the mechanisms underlying these effects and to address how presynaptic as well as postsynaptic compartments change during homeostatic plasticity, we subjected mouse cortical slice cultures to a more severe 5 d deprivation paradigm. We developed and validated a computational framework to measure the number and morphology of presynaptic and postsynaptic compartments from super-resolution light microscopy images of dense cortical tissue. Using these tools, combined with electrophysiological miniature excitatory postsynaptic current measurements, and synaptic imaging at the electron microscopy level, we assessed the functional and morphological results of prolonged deprivation. Excitatory synapses were strengthened both presynaptically and postsynaptically. Surprisingly, we also observed a decrement in the density of excitatory synapses, both as measured from colocalized staining of pre- and postsynaptic proteins in tissue and from the number of dendritic spines. Overall, our results suggest that cortical networks deprived of activity progressively move toward a smaller population of stronger synapses.

Connectivity of the olfactory tubercle: inputs, outputs, and their plasticity

The olfactory tubercle (OT) is a unique part of the olfactory cortex of the mammal brain in that it is also a component of the ventral striatum. It is crucially involved in motivational behaviors, particularly in adaptive olfactory learning. This review introduces the basic properties of the OT, its synaptic connectivity with other brain areas, and the plasticity of the connectivity associated with learning behavior. The adaptive properties of olfactory behavior are discussed further based on the characteristics of OT neuronal circuits.

Parallel processing of quickly and slowly mobilized reserve vesicles in hippocampal synapses

Vesicles within presynaptic terminals are thought to be segregated into a variety of readily releasable and reserve pools. The nature of the pools and trafficking between them is not well understood, but pools that are slow to mobilize when synapses are active are often assumed to feed pools that are mobilized more quickly, in a series. However, electrophysiological studies of synaptic transmission have suggested instead a parallel organization where vesicles within slowly and quickly mobilized reserve pools would separately feed independent reluctant- and fast-releasing subdivisions of the readily releasable pool. Here, we use FM-dyes to confirm the existence of multiple reserve pools at hippocampal synapses and a parallel organization that prevents intermixing between the pools, even when stimulation is intense enough to drive exocytosis at the maximum rate. The experiments additionally demonstrate extensive heterogeneity among synapses in the relative sizes of the slowly and quickly mobilized reserve pools, which suggests equivalent heterogeneity in the numbers of reluctant and fast-releasing readily releasable vesicles that may be relevant for understanding information processing and storage.

Glial Cx43 hemichannels and neuronal Panx1 hemichannels and P2X7 receptors orchestrate presynaptic homeostatic plasticity

The emerging role of glial cells in modulating neuronal excitability and synaptic strength is a growing field in neuroscience. In recent years, a pivotal role of gliotransmission in homeostatic presynaptic plasticity has been highlighted and glial-derived ATP arises as a key contributor. However, very little is known about the glial non-vesicular ATP-release pathway and how ATP participates in the modulation of synaptic strength. Here, we investigated the functional changes occurring in neurons upon chronic inactivity and the role of the purinergic signaling, connexin43 and pannexin1 hemichannels in this process. By using hippocampal dissociated cultures, we showed that blocking connexin43 and pannexin1 hemichannels decreases the amount of extracellular ATP. Moreover, Ca2+ imaging assays using Fluo-4/AM revealed that blocking connexin43, neuronal P2X7Rs and pannexin1 hemichannels decreases the amount of basal Ca2+ in neurons. A significant impairment in synaptic vesicle pool size was also evidenced under these conditions. Interestingly, rescue experiments where Panx1HCs are blocked showed that the compensatory adjustment of cytosolic Ca2+ was recovered after P2X7Rs activation, suggesting that Panx1 acts downstream P2X7Rs. These changes were accompanied by a modulation of neuronal permeability, as revealed by ethidium bromide uptake experiments. In particular, the permeability of neuronal P2X7Rs and pannexin1 hemichannels is increased upon 24 h of inactivity. Taken together, we have uncovered a role for connexin43-dependent ATP release and neuronal P2X7Rs and pannexin1 hemichannels in the adjustment of presynaptic strength by modulating neuronal permeability, the entrance of Ca2+ into neurons and the size of the recycling pool of synaptic vesicles.

Progressive Circuit Hyperexcitability in Mouse Neocortical Slice Cultures with Increasing Duration of Activity Silencing

Forebrain neurons deprived of activity become hyperactive when activity is restored. Rebound activity has been linked to spontaneous seizures in vivo following prolonged activity blockade. Here, we measured the time course of rebound activity and the contributing circuit mechanisms using calcium imaging, synaptic staining, and whole-cell patch clamp in organotypic slice cultures of mouse neocortex. Calcium imaging revealed hypersynchronous activity increasing in intensity with longer periods of deprivation. While activity partially recovered 3 d after slices were released from 5 d of deprivation, they were less able to recover after 10 d of deprivation. However, even after the longer period of deprivation, activity patterns eventually returned to baseline levels. The degree of deprivation-induced rebound was age-dependent, with the greatest effects occurring when silencing began in the second week. Pharmacological blockade of NMDA receptors indicated that hypersynchronous rebound activity did not require activation of Hebbian plasticity. In single-neuron recordings, input resistance roughly doubled with a concomitant increase in intrinsic excitability. Synaptic imaging of pre- and postsynaptic proteins revealed dramatic reductions in the number of presumptive synapses with a larger effect on inhibitory than excitatory synapses. Putative excitatory synapses colocalizing PSD-95 and Bassoon declined by 39 and 56% following 5 and 10 d of deprivation, but presumptive inhibitory synapses colocalizing gephyrin and VGAT declined by 55 and 73%, respectively. The results suggest that with prolonged deprivation, a progressive reduction in synapse number is accompanied by a shift in the balance between excitation and inhibition and increased cellular excitability.

Synaptic homeostasis transiently leverages Hebbian mechanisms for a multiphasic response to inactivity

Homeostatic regulation of synapses is vital for nervous system function and key to understanding a range of neurological conditions. Synaptic homeostasis is proposed to operate over hours to counteract the destabilizing influence of long-term potentiation (LTP) and long-term depression (LTD). The prevailing view holds that synaptic scaling is a slow first-order process that regulates postsynaptic glutamate receptors and fundamentally differs from LTP or LTD. Surprisingly, we find that the dynamics of scaling induced by neuronal inactivity are not exponential or monotonic, and the mechanism requires calcineurin and CaMKII, molecules dominant in LTD and LTP. Our quantitative model of these enzymes reconstructs the unexpected dynamics of homeostatic scaling and reveals how synapses can efficiently safeguard future capacity for synaptic plasticity. This mechanism of synaptic adaptation supports a broader set of homeostatic changes, including action potential autoregulation, and invites further inquiry into how such a mechanism varies in health and disease.

Intracellular magnesium optimizes transmission efficiency and plasticity of hippocampal synapses by reconfiguring their connectivity

Synapses at dendritic branches exhibit specific properties for information processing. However, how the synapses are orchestrated to dynamically modify their properties, thus optimizing information processing, remains elusive. Here, we observed at hippocampal dendritic branches diverse configurations of synaptic connectivity, two extremes of which are characterized by low transmission efficiency, high plasticity and coding capacity, or inversely. The former favors information encoding, pertinent to learning, while the latter prefers information storage, relevant to memory. Presynaptic intracellular Mg2+ crucially mediates the dynamic transition continuously between the two extreme configurations. Consequently, varying intracellular Mg2+ levels endow individual branches with diverse synaptic computations, thus modulating their ability to process information. Notably, elevating brain Mg2+ levels in aging animals restores synaptic configuration resembling that of young animals, coincident with improved learning and memory. These findings establish intracellular Mg2+ as a crucial factor reconfiguring synaptic connectivity at dendrites, thus optimizing their branch-specific properties in information processing.

SV2B defines a subpopulation of synaptic vesicles

Synaptic vesicles can undergo several modes of exocytosis, endocytosis, and trafficking within individual synapses, and their fates may be linked to different vesicular protein compositions. Here, we mapped the intrasynaptic distribution of the synaptic vesicle proteins SV2B and SV2A in glutamatergic synapses of the hippocampus using three-dimensional electron microscopy. SV2B was almost completely absent from docked vesicles and a distinct cluster of vesicles found near the active zone. In contrast, SV2A was found in all domains of the synapse and was slightly enriched near the active zone. SV2B and SV2A were found on the membrane in the peri-active zone, suggesting the recycling from both clusters of vesicles. SV2B knockout mice displayed an increased seizure induction threshold only in a model employing high-frequency stimulation. Our data show that glutamatergic synapses generate molecularly distinct populations of synaptic vesicles and are able to maintain them at steep spatial gradients. The almost complete absence of SV2B from vesicles at the active zone of wildtype mice may explain why SV2A has been found more important for vesicle release.

Increased presynaptic excitability in a migraine with aura mutation

Migraine is a common and disabling neurological disorder. The headache and sensory amplifications of migraine are attributed to hyperexcitable sensory circuits, but a detailed understanding remains elusive. A mutation in casein kinase 1 delta (CK1δ) was identified in non-hemiplegic familial migraine with aura and advanced sleep phase syndrome. Mice carrying the CK1δT44A mutation were more susceptible to spreading depolarization (the phenomenon that underlies migraine aura), but mechanisms underlying this migraine-relevant phenotype were not known. We used a combination of whole-cell electrophysiology and multiphoton imaging, in vivo and in brain slices, to compare CK1δT44A mice (adult males) to their wild-type littermates. We found that despite comparable synaptic activity at rest, CK1δT44A neurons were more excitable upon repetitive stimulation than wild-type, with a reduction in presynaptic adaptation at excitatory but not inhibitory synapses. The mechanism of this adaptation deficit was a calcium-dependent enhancement of the size of the readily releasable pool of synaptic vesicles, and a resultant increase in glutamate release, in CK1δT44A compared to wild-type synapses. Consistent with this mechanism, CK1δT44A neurons showed an increase in the cumulative amplitude of excitatory post-synaptic currents, and a higher excitation-to-inhibition ratio during sustained activity compared to wild-type. At a local circuit level, action potential bursts elicited in CK1δT44A neurons triggered an increase in recurrent excitation compared to wild-type, and at a network level, CK1δT44A mice showed a longer duration of 'up state' activity, which is dependent on recurrent excitation. Finally, we demonstrated that the spreading depolarization susceptibility of CK1δT44A mice could be returned to wild-type levels with the same intervention (reduced extracellular calcium) that normalized presynaptic adaptation. Taken together, these findings show a stimulus-dependent presynaptic gain of function at glutamatergic synapses in a genetic model of migraine, that accounts for the increased spreading depolarization susceptibility and may also explain the sensory amplifications that are associated with the disease.

Postsynaptic cell type and synaptic distance do not determine efficiency of monosynaptic rabies virus spread measured at synaptic resolution

Retrograde monosynaptic tracing using glycoprotein-deleted rabies virus is an important component of the toolkit for investigation of neural circuit structure and connectivity. It allows for the identification of first-order presynaptic connections to cell populations of interest across both the central and peripheral nervous system, helping to decipher the complex connectivity patterns of neural networks that give rise to brain function. Despite its utility, the factors that influence the probability of transsynaptic rabies spread are not well understood. While it is well established that expression levels of rabies glycoprotein used to trans-complement G-deleted rabies can result in large changes in numbers of inputs labeled per starter cell (convergence index [CI]), it is not known how typical values of CI relate to the proportions of synaptic contacts or input neurons labeled. And it is not known whether inputs to different cell types, or synaptic contacts that are more proximal or distal to the cell body, are labeled with different probabilities. Here, we use a new rabies virus construct that allows for the simultaneous labeling of pre- and postsynaptic specializations to quantify the proportion of synaptic contacts labeled in mouse primary visual cortex. We demonstrate that with typical conditions about 40% of first-order presynaptic excitatory synapses to cortical excitatory and inhibitory neurons are labeled. We show that using matched tracing conditions there are similar proportions of labeled contacts onto L4 excitatory pyramidal, somatostatin (Sst) inhibitory, and vasoactive intestinal peptide (Vip) starter cell types. Furthermore, we find no difference in the proportions of labeled excitatory contacts onto postsynaptic sites at different subcellular locations.

Distributed dopaminergic signaling in the basal ganglia and its relationship to motor disability in Parkinson's disease

The degeneration of mesencephalic dopaminergic neurons that innervate the basal ganglia is responsible for the cardinal motor symptoms of Parkinson's disease (PD). It has been thought that loss of dopaminergic signaling in one basal ganglia region - the striatum - was solely responsible for the network pathophysiology causing PD motor symptoms. While our understanding of dopamine (DA)'s role in modulating striatal circuitry has deepened in recent years, it also has become clear that it acts in other regions of the basal ganglia to influence movement. Underscoring this point, examination of a new progressive mouse model of PD shows that striatal dopamine DA depletion alone is not sufficient to induce parkinsonism and that restoration of extra-striatal DA signaling attenuates parkinsonian motor deficits once they appear. This review summarizes recent advances in the effort to understand basal ganglia circuitry, its modulation by DA, and how its dysfunction drives PD motor symptoms.

Fully-primed slowly-recovering vesicles mediate presynaptic LTP at neocortical neurons

Pre- and postsynaptic forms of long-term potentiation (LTP) are candidate synaptic mechanisms underlying learning and memory. At layer 5 pyramidal neurons, LTP increases the initial synaptic strength but also short-term depression during high-frequency transmission. This classical form of presynaptic LTP has been referred to as redistribution of synaptic efficacy. However, the underlying mechanisms remain unclear. We therefore performed whole-cell recordings from layer 5 pyramidal neurons in acute cortical slices of rats and analyzed presynaptic function before and after LTP induction by paired pre- and postsynaptic neuronal activity. LTP was successfully induced in about half of the synaptic connections tested and resulted in increased synaptic short-term depression during high-frequency transmission and a decelerated recovery from short-term depression due to an increased fraction of a slow recovery component. Analysis with a recently established sequential two-step vesicle priming model indicates an increase in the abundance of fully-primed and slowly-recovering vesicles. A systematic analysis of short-term plasticity and synapse-to-synapse variability of synaptic strength at various types of synapses revealed that stronger synapses generally recover more slowly from synaptic short-term depression. Finally, pharmacological stimulation of the cyclic adenosine monophosphate and diacylglycerol signaling pathways, which are both known to promote synaptic vesicle priming, mimicked LTP and slowed the recovery from short-term depression. Our data thus demonstrate that LTP at layer 5 pyramidal neurons increases synaptic strength primarily by enlarging a subpool of fully-primed slowly-recovering vesicles.

Electrophysiological properties of layer 2/3 pyramidal neurons in the primary visual cortex of a retinitis pigmentosa mouse model (rd10)

Retinal degeneration is one of the main causes of visual impairment and blindness. One group of retinal degenerative diseases, leading to the loss of photoreceptors, is collectively termed retinitis pigmentosa. In this group of diseases, the remaining retina is largely spared from initial cell death making retinal ganglion cells an interesting target for vision restoration methods. However, it is unknown how downstream brain areas, in particular the visual cortex, are affected by the progression of blindness. Visual deprivation studies have shown dramatic changes in the electrophysiological properties of visual cortex neurons, but changes on a cellular level in retinitis pigmentosa have not been investigated yet. Therefore, we used the rd10 mouse model to perform patch-clamp recordings of pyramidal neurons in layer 2/3 of the primary visual cortex to screen for potential changes in electrophysiological properties resulting from retinal degeneration. Compared to wild-type C57BL/6 mice, we only found an increase in intrinsic excitability around the time point of maximal retinal degeneration. In addition, we saw an increase in the current amplitude of spontaneous putative inhibitory events after a longer progression of retinal degeneration. However, we did not observe a long-lasting shift in excitability after prolonged retinal degeneration. Together, our results provide evidence of an intact visual cortex with promising potential for future therapeutic strategies to restore vision.

Learning-dependent structural plasticity of intracortical and sensory connections to functional domains of the olfactory tubercle

The olfactory tubercle (OT), which is a component of the olfactory cortex and ventral striatum, has functional domains that play a role in odor-guided motivated behaviors. Learning odor-guided attractive and aversive behavior activates the anteromedial (am) and lateral (l) domains of the OT, respectively. However, the mechanism driving learning-dependent activation of specific OT domains remains unknown. We hypothesized that the neuronal connectivity of OT domains is plastically altered through olfactory experience. To examine the plastic potential of synaptic connections to OT domains, we optogenetically stimulated intracortical inputs from the piriform cortex or sensory inputs from the olfactory bulb to the OT in mice in association with a food reward for attractive learning and electrical foot shock for aversive learning. For both intracortical and sensory connections, axon boutons that terminated in the OT domains were larger in the amOT than in the lOT for mice exhibiting attractive learning and larger in the lOT than in the amOT for mice exhibiting aversive learning. These results indicate that both intracortical and sensory connections to the OT domains have learning-dependent plastic potential, suggesting that this plasticity underlies learning-dependent activation of specific OT domains and the acquisition of appropriate motivated behaviors.

Glial Draper signaling triggers cross-neuron plasticity in bystander neurons after neuronal cell death in Drosophila

Neuronal cell death and subsequent brain dysfunction are hallmarks of aging and neurodegeneration, but how the nearby healthy neurons (bystanders) respond to the death of their neighbors is not fully understood. In the Drosophila larval neuromuscular system, bystander motor neurons can structurally and functionally compensate for the loss of their neighbors by increasing their terminal bouton number and activity. We term this compensation as cross-neuron plasticity, and in this study, we demonstrate that the Drosophila engulfment receptor, Draper, and the associated kinase, Shark, are required for cross-neuron plasticity. Overexpression of the Draper-I isoform boosts cross-neuron plasticity, implying that the strength of plasticity correlates with Draper signaling. In addition, we find that functional cross-neuron plasticity can be induced at different developmental stages. Our work uncovers a role for Draper signaling in cross-neuron plasticity and provides insights into how healthy bystander neurons respond to the loss of their neighboring neurons.

Rethinking the network determinants of motor disability in Parkinson's disease

For roughly the last 30 years, the notion that striatal dopamine (DA) depletion was the critical determinant of network pathophysiology underlying the motor symptoms of Parkinson's disease (PD) has dominated the field. While the basal ganglia circuit model underpinning this hypothesis has been of great heuristic value, the hypothesis itself has never been directly tested. Moreover, studies in the last couple of decades have made it clear that the network model underlying this hypothesis fails to incorporate key features of the basal ganglia, including the fact that DA acts throughout the basal ganglia, not just in the striatum. Underscoring this point, recent work using a progressive mouse model of PD has shown that striatal DA depletion alone is not sufficient to induce parkinsonism and that restoration of extra-striatal DA signaling attenuates parkinsonian motor deficits once they appear. Given the broad array of discoveries in the field, it is time for a new model of the network determinants of motor disability in PD.

Increased pyramidal and VIP neuronal excitability in rat primary auditory cortex directly correlates with tinnitus behaviour

Tinnitus affects roughly 15%-20% of the population while severely impacting 10% of those afflicted. Tinnitus pathology is multifactorial, generally initiated by damage to the auditory periphery, resulting in a cascade of maladaptive plastic changes at multiple levels of the central auditory neuraxis as well as limbic and non-auditory cortical centres. Using a well-established condition-suppression model of tinnitus, we measured tinnitus-related changes in the microcircuits of excitatory/inhibitory neurons onto layer 5 pyramidal neurons (PNs), as well as changes in the excitability of vasoactive intestinal peptide (VIP) neurons in primary auditory cortex (A1). Patch-clamp recordings from PNs in A1 slices showed tinnitus-related increases in spontaneous excitatory postsynaptic currents (sEPSCs) and decreases in spontaneous inhibitory postsynaptic currents (sIPSCs). Both measures could be correlated to the rat's behavioural evidence of tinnitus. Tinnitus-related changes in PN excitability were independent of changes in A1 excitatory or inhibitory cell numbers. VIP neurons, part of an A1 local circuit that can control the excitation of layer 5 PNs via disinhibitory mechanisms, showed significant tinnitus-related increases in excitability that directly correlated with the rat's behavioural tinnitus score. That PN and VIP changes directly correlated to tinnitus behaviour suggests an important role in A1 tinnitus pathology. Tinnitus-related A1 changes were similar to findings in studies of neuropathic pain in somatosensory cortex suggesting a common pathology of these troublesome perceptual impairments. Improved understanding between excitatory, inhibitory and disinhibitory sensory cortical circuits can serve as a model for testing therapeutic approaches to the treatment of tinnitus and chronic pain. KEY POINTS: We identified tinnitus-related changes in synaptic function of specific neuronal subtypes in a reliable animal model of tinnitus. The findings show direct and indirect tinnitus-related losses of normal inhibitory function at A1 layer 5 pyramidal cells, and increased VIP excitability. The findings are similar to what has been shown for neuropathic pain suggesting that restoring normal inhibitory function at synaptic inputs onto A1 pyramidal neurons (PNs) could conceptually reduce tinnitus discomfort.

Glial Draper signaling triggers cross-neuron plasticity in bystander neurons after neuronal cell death

Neuronal cell death and subsequent brain dysfunction are hallmarks of aging and neurodegeneration, but how the nearby healthy neurons (bystanders) respond to the cell death of their neighbors is not fully understood. In the Drosophila larval neuromuscular system, bystander motor neurons can structurally and functionally compensate for the loss of their neighbors by increasing their axon terminal size and activity. We termed this compensation as cross-neuron plasticity, and in this study, we demonstrated that the Drosophila engulfment receptor, Draper, and the associated kinase, Shark, are required in glial cells. Surprisingly, overexpression of the Draper-I isoform boosts cross-neuron plasticity, implying that the strength of plasticity correlates with Draper signaling. Synaptic plasticity normally declines as animals age, but in our system, functional cross-neuron plasticity can be induced at different time points, whereas structural cross-neuron plasticity can only be induced at early stages. Our work uncovers a novel role for glial Draper signaling in cross-neuron plasticity that may enhance nervous system function during neurodegeneration and provides insights into how healthy bystander neurons respond to the loss of their neighboring neurons.

Organizing principles of astrocytic nanoarchitecture in the mouse cerebral cortex

Astrocytes are increasingly understood to be important regulators of central nervous system (CNS) function in health and disease; yet, we have little quantitative understanding of their complex architecture. While broad categories of astrocytic structures are known, the discrete building blocks that compose them, along with their geometry and organizing principles, are poorly understood. Quantitative investigation of astrocytic complexity is impeded by the absence of high-resolution datasets and robust computational approaches to analyze these intricate cells. To address this, we produced four ultra-high-resolution datasets of mouse cerebral cortex using serial electron microscopy and developed astrocyte-tailored computer vision methods for accurate structural analysis. We unearthed specific anatomical building blocks, structural motifs, connectivity hubs, and hierarchical organizations of astrocytes. Furthermore, we found that astrocytes interact with discrete clusters of synapses and that astrocytic mitochondria are distributed to lie closer to larger clusters of synapses. Our findings provide a geometrically principled, quantitative understanding of astrocytic nanoarchitecture and point to an unexpected level of complexity in how astrocytes interact with CNS microanatomy.

Elevated amyloid beta disrupts the nanoscale organization and function of synaptic vesicle pools in hippocampal neurons

Alzheimer's disease is linked to increased levels of amyloid beta (Aβ) in the brain, but the mechanisms underlying neuronal dysfunction and neurodegeneration remain enigmatic. Here, we investigate whether organizational characteristics of functional presynaptic vesicle pools, key determinants of information transmission in the central nervous system, are targets for elevated Aβ. Using an optical readout method in cultured hippocampal neurons, we show that acute Aβ42 treatment significantly enlarges the fraction of functional vesicles at individual terminals. We observe the same effect in a chronically elevated Aβ transgenic model (APPSw,Ind) using an ultrastructure-function approach that provides detailed information on nanoscale vesicle pool positioning. Strikingly, elevated Aβ is correlated with excessive accumulation of recycled vesicles near putative endocytic sites, which is consistent with deficits in vesicle retrieval pathways. Using the glutamate reporter, iGluSnFR, we show that there are parallel functional consequences, where ongoing information signaling capacity is constrained. Treatment with levetiracetam, an antiepileptic that dampens synaptic hyperactivity, partially rescues these transmission defects. Our findings implicate organizational and dynamic features of functional vesicle pools as targets in Aβ-driven synaptic impairment, suggesting that interventions to relieve the overloading of vesicle retrieval pathways might have promising therapeutic value.

Circadian time- and sleep-dependent modulation of cortical parvalbumin-positive inhibitory neurons

Parvalbumin-positive neurons (PVs) are the main class of inhibitory neurons in the mammalian central nervous system. By examining diurnal changes in synaptic and neuronal activity of PVs in the supragranular layer of the mouse primary visual cortex (V1), we found that both PV input and output are modulated in a time- and sleep-dependent manner throughout the 24-h day. We first show that PV-evoked inhibition is stronger by the end of the light cycle (ZT12) relative to the end of the dark cycle (ZT0), which is in line with the lower inhibitory input of PV neurons at ZT12 than at ZT0. Interestingly, PV inhibitory and excitatory synaptic transmission slowly oscillate in opposite directions during the light/dark cycle. Although excitatory synapses are predominantly regulated by experience, inhibitory synapses are regulated by sleep, via acetylcholine activating M1 receptors. Consistent with synaptic regulation of PVs, we further show in vivo that spontaneous PV activity displays daily rhythm mainly determined by visual experience, which negatively correlates with the activity cycle of surrounding pyramidal neurons and the dorsal lateral geniculate nucleus-evoked responses in V1. These findings underscore the physiological significance of PV's daily modulation.

Rapid homeostatic plasticity and neuropsychiatric therapeutics

Neuronal and synaptic plasticity are widely used terms in the field of psychiatry. However, cellular neurophysiologists have identified two broad classes of plasticity. Hebbian forms of plasticity alter synaptic strength in a synapse specific manner in the same direction of the initial conditioning stimulation. In contrast, homeostatic plasticities act globally over longer time frames in a negative feedback manner to counter network level changes in activity or synaptic strength. Recent evidence suggests that homeostatic plasticity mechanisms can be rapidly engaged, particularly by fast-acting antidepressants such as ketamine to trigger behavioral effects. There is increasing evidence that several neuropsychoactive compounds either directly elicit changes in synaptic activity or indirectly tap into downstream signaling pathways to trigger homeostatic plasticity and subsequent behavioral effects. In this review, we discuss this recent work in the context of a wider paradigm where homeostatic synaptic plasticity mechanisms may provide novel targets for neuropsychiatric treatment advance.

Retinoic acid-gated BDNF synthesis in neuronal dendrites drives presynaptic homeostatic plasticity

Homeostatic synaptic plasticity is a non-Hebbian synaptic mechanism that adjusts synaptic strength to maintain network stability while achieving optimal information processing. Among the molecular mediators shown to regulate this form of plasticity, synaptic signaling through retinoic acid (RA) and its receptor, RARα, has been shown to be critically involved in the homeostatic adjustment of synaptic transmission in both hippocampus and sensory cortices. In this study, we explore the molecular mechanism through which postsynaptic RA and RARα regulates presynaptic neurotransmitter release during prolonged synaptic inactivity at mouse glutamatertic synapses. We show that RARα binds to a subset of dendritically sorted brain-derived neurotrophic factor (Bdnf) mRNA splice isoforms and represses their translation. The RA-mediated translational de-repression of postsynaptic BDNF results in the retrograde activation of presynaptic tropomyosin receptor kinase B (TrkB) receptors, facilitating presynaptic homeostatic compensation through enhanced presynaptic release. Together, our study illustrates an RA-mediated retrograde synaptic signaling pathway through which postsynaptic protein synthesis during synaptic inactivity drives compensatory changes at the presynaptic site.

NMDAR-dependent presynaptic homeostasis in adult hippocampus: Synapse growth and cross-modal inhibitory plasticity

Homeostatic plasticity (HP) encompasses a suite of compensatory physiological processes that counteract neuronal perturbations, enabling brain resilience. Currently, we lack a complete description of the homeostatic processes that operate within the mammalian brain. Here, we demonstrate that acute, partial AMPAR-specific antagonism induces potentiation of presynaptic neurotransmitter release in adult hippocampus, a form of compensatory plasticity that is consistent with the expression of presynaptic homeostatic plasticity (PHP) documented at peripheral synapses. We show that this compensatory plasticity can be induced within minutes, requires postsynaptic NMDARs, and is expressed via correlated increases in dendritic spine volume, active zone area, and docked vesicle number. Further, simultaneous postsynaptic genetic reduction of GluA1, GluA2, and GluA3 in triple heterozygous knockouts induces potentiation of presynaptic release. Finally, induction of compensatory plasticity at excitatory synapses induces a parallel, NMDAR-dependent potentiation of inhibitory transmission, a cross-modal effect consistent with the anti-epileptic activity of AMPAR-specific antagonists used in humans.

Brain circuit pathology in Down syndrome: from neurons to neural networks

Down syndrome (DS), a genetic pathology caused by triplication of chromosome 21, is characterized by brain hypotrophy and impairment of cognition starting from infancy. While studies in mouse models of DS have elucidated the major neuroanatomical and neurochemical defects of DS, comparatively fewer investigations have focused on the electrophysiology of the DS brain. Electrical activity is at the basis of brain functioning. Therefore, knowledge of the way in which brain circuits operate in DS is fundamental to understand the causes of behavioral impairment and devise targeted interventions. This review summarizes the state of the art regarding the electrical properties of the DS brain, starting from individual neurons and culminating in signal processing in whole neuronal networks. The reported evidence derives from mouse models of DS and from brain tissues and neurons derived from individuals with DS. EEG data recorded in individuals with DS are also provided as a key tool to understand the impact of brain circuit alterations on global brain activity.

Enhanced GIRK2 channel signaling in Down syndrome: A feasible role in the development of abnormal nascent neural circuits

The most distinctive feature of Down syndrome (DS) is moderate to severe cognitive impairment. Genetic, molecular, and neuronal mechanisms of this complex DS phenotype are currently under intensive investigation. It is becoming increasingly clear that the abnormalities arise from a combination of initial changes caused by triplication of genes on human chromosome 21 (HSA21) and later compensatory adaptations affecting multiple brain systems. Consequently, relatively mild initial cognitive deficits become pronounced with age. This pattern of changes suggests that one approach to improving cognitive function in DS is to target the earliest critical changes, the prevention of which can change the ‘trajectory’ of the brain development and reduce the destructive effects of the secondary alterations. Here, we review the experimental data on the role of KCNJ6 in DS-specific brain abnormalities, focusing on a putative role of this gene in the development of abnormal neural circuits in the hippocampus of genetic mouse models of DS. It is suggested that the prevention of these early abnormalities with pharmacological or genetic means can ameliorate cognitive impairment in DS.

Synaptic spinules are reliable indicators of excitatory presynaptic bouton size and strength and are ubiquitous components of excitatory synapses in CA1 hippocampus

Synaptic spinules are thin, finger-like projections from one neuron that become embedded within the presynaptic or postsynaptic compartments of another neuron. While spinules are conserved features of synapses across the animal kingdom, their specific function(s) remain unknown. Recent focused ion beam scanning electron microscopy (FIB-SEM) image volume analyses have demonstrated that spinules are embedded within ∼25% of excitatory boutons in primary visual cortex, yet the diversity of spinule sizes, origins, and ultrastructural relationships to their boutons remained unclear. To begin to uncover the function of synaptic spinules, we sought to determine the abundance, origins, and 3D ultrastructure of spinules within excitatory presynaptic spinule-bearing boutons (SBBs) in mammalian CA1 hippocampus and compare them with presynaptic boutons bereft of spinules (non-SBBs). Accordingly, we performed a comprehensive 3D analysis of every excitatory presynaptic bouton, their embedded spinules, and postsynaptic densities, within a 5 nm isotropic FIB-SEM image volume from CA1 hippocampus of an adult male rat. Surprisingly, we found that ∼74% of excitatory presynaptic boutons in this volume contained at least one spinule, suggesting they are fundamental components of excitatory synapses in CA1. In addition, we found that SBBs are 2.5-times larger and have 60% larger postsynaptic densities (PSDs) than non-SBBs. Moreover, synaptic spinules within SBBs are clearly differentiated into two groups: small clathrin-coated spinules, and 29-times larger spinules without clathrin. Together, these findings suggest that the presence of a spinule is a marker for stronger and more stable presynaptic boutons in CA1, and that synaptic spinules serve at least two separable and distinct functions.

Randomly fluctuating neural connections may implement a consolidation mechanism that explains classic memory laws

How can we reconcile the massive fluctuations in neural connections with a stable long-term memory? Two-photon microscopy studies have revealed that large portions of neural connections (spines, synapses) are unexpectedly active, changing unpredictably over time. This appears to invalidate the main assumption underlying the majority of memory models in cognitive neuroscience, which rely on stable connections that retain information over time. Here, we show that such random fluctuations may in fact implement a type of memory consolidation mechanism with a stable very long-term memory that offers novel explanations for several classic memory 'laws', namely Jost's Law (1897: superiority of spaced learning) and Ribot's Law (1881: loss of recent memories in retrograde amnesia), for which a common neural basis has been postulated but not established, as well as other general 'laws' of learning and forgetting. We show how these phenomena emerge naturally from massively fluctuating neural connections.

Methamphetamine Exposure in Adolescent Impairs Memory of Mice in Adulthood Accompanied by Changes in Neuroplasticity in the Dorsal Hippocampus

Methamphetamine (METH) has been shown to alter learning and memory by affecting the neuroplasticity of the dorsal hippocampus, a key structure that undergoes extensive remodeling during adolescence. In this study, we investigated whether mid-to-late adolescent exposure to METH leads to long-lasting memory impairment. To do this, adolescents (35–48 postnatal days) were exposed to different doses of METH for 14 days and then evaluated by the Morris water maze (MWM), new object recognition test (NORT), and the Y-maze, to investigate the learning and memory abilities of mice in their adolescence and adulthood, respectively. We also detected the mRNA levels of genes associated with neuroplasticity in the dorsal hippocampus. The synaptic ultrastructure and the number of neurons and astrocytes in the dorsal hippocampus were also determined by transmission electron microscopy (TEM) and immunofluorescence (IF). Exposure to METH in mid-to-late adolescence impaired spatial memory retrieval ability and the long-term recognition memory of mice in their adulthood, but not in their adolescence. Of note, the impairment of memory capacity in adulthood was accompanied by molecular and structural changes in synapses in the dorsal hippocampus. Our results indicate that mice exposed to METH in mid-to-late adolescence have impaired memory ability in their adulthood; this may be the result of abnormal changes in the structural plasticity of the dorsal hippocampus; the causal relationship between changes in synaptic structural plasticity and memory impairment needs to be further confirmed. In summary, our study provides evidence for the detrimental consequences of adolescent addiction and the prevention of adolescent drug abuse.

A presynaptic phosphosignaling hub for lasting homeostatic plasticity

Stable function of networks requires that synapses adapt their strength to levels of neuronal activity, and failure to do so results in cognitive disorders. How such homeostatic regulation may be implemented in mammalian synapses remains poorly understood. Here we show that the phosphorylation status of several positions of the active-zone (AZ) protein RIM1 are relevant for synaptic glutamate release. Position RIMS1045 is necessary and sufficient for expression of silencing-induced homeostatic plasticity and is kept phosphorylated by serine arginine protein kinase 2 (SRPK2). SRPK2-induced upscaling of synaptic release leads to additional RIM1 nanoclusters and docked vesicles at the AZ and is not observed in the absence of RIM1 and occluded by RIM^S1045E. Our data suggest that SRPK2 and RIM1 represent a presynaptic phosphosignaling hub that is involved in the homeostatic balance of synaptic coupling of neuronal networks.

Modeling and Evaluation of Vesicle Release Mechanisms in Neuro-Spike Communication

Neuro-spike communication (NSC) is a communication method that includes electrical communication process and molecular communication process, which has been investigated extensively in recent years. The vesicle release process has a great influence on the accuracy of NSC systems. So the modeling of the vesicle release process has become a hot spot. There exist different vesicle release mechanisms, including univesicular release (UVR) case, multivesicular release (MVR) case and hybrid vesicle release (HVR) case. When a spike arrives, the UVR case refers to that at most one vesicle can be released. The MVR case refers to that more than one vesicle can be released. The HVR case is a mixed case of the UVR and MVR cases. This paper proposes the modeling of these three vesicle release cases. The theoretical analysis is conducted to compare them in terms of efficiency. Simulations are performed to evaluate the impacts of main vesicle releasing parameters on signal transmission accuracy. The simulation results show that the HVR model can effectively improve the transmission accuracy compared with UVR and MVR models under some conditions.

Synaptic counts approximate synaptic contact area in Drosophila

The pattern of synaptic connections among neurons defines the circuit structure, which constrains the computations that a circuit can perform. The strength of synaptic connections is costly to measure yet important for accurate circuit modeling. Synaptic surface area has been shown to correlate with synaptic strength, yet in the emerging field of connectomics, most studies rely instead on the counts of synaptic contacts between two neurons. Here we quantified the relationship between synaptic count and synaptic area as measured from volume electron microscopy of the larval Drosophila central nervous system. We found that the total synaptic surface area, summed across all synaptic contacts from one presynaptic neuron to a postsynaptic one, can be accurately predicted solely from the number of synaptic contacts, for a variety of neurotransmitters. Our findings support the use of synaptic counts for approximating synaptic strength when modeling neural circuits.

Heterogeneous CaMKII-Dependent Synaptic Compensations in CA1 Pyramidal Neurons From Acute Hippocampal Slices

Prolonged changes in neural activity trigger homeostatic synaptic plasticity (HSP) allowing neuronal networks to operate within functional ranges. Cell-wide or input-specific adaptations can be induced by pharmacological or genetic manipulations of activity, and by sensory deprivation. Reactive functional changes caused by deafferentation may partially share mechanisms with HSP. Acute hippocampal slices are a suitable model to investigate relatively rapid (hours) modifications occurring after denervation and explore the underlying mechanisms. As during slicing many afferents are cut, we conducted whole-cell recordings of miniature excitatory postsynaptic currents (mEPSCs) in CA1 pyramidal neurons to evaluate changes over the following 12 h. As Schaffer collaterals constitute a major glutamatergic input to these neurons, we also dissected CA3. We observed an average increment in mEPSCs amplitude and a decrease in decay time, suggesting synaptic AMPA receptor upregulation and subunit content modifications. Sorting mEPSC by rise time, a correlate of synapse location along dendrites, revealed amplitude raises at two separate domains. A specific frequency increase was observed in the same domains and was accompanied by a global, unspecific raise. Amplitude and frequency increments were lower at sites initially more active, consistent with local compensatory processes. Transient preincubation with a specific Ca2+/calmodulin-dependent kinase II (CaMKII) inhibitor either blocked or occluded amplitude and frequency upregulation in different synapse populations. Results are consistent with the concurrent development of different known CaMKII-dependent HSP processes. Our observations support that deafferentation causes rapid and diverse compensations resembling classical slow forms of adaptation to inactivity. These results may contribute to understand fast-developing homeostatic or pathological events after brain injury.

Correlative Live-Cell and Super-Resolution Imaging to Link Presynaptic Molecular Organisation With Function

Information transfer at synapses occurs when vesicles fuse with the plasma membrane to release neurotransmitters, which then bind to receptors at the postsynaptic membrane. The process of neurotransmitter release varies dramatically between different synapses, but little is known about how this heterogeneity emerges. The development of super-resolution microscopy has revealed that synaptic proteins are precisely organised within and between the two parts of the synapse and that this precise spatiotemporal organisation fine-tunes neurotransmission. However, it remains unclear if variability in release probability could be attributed to the nanoscale organisation of one or several proteins of the release machinery. To begin to address this question, we have developed a pipeline for correlative functional and super-resolution microscopy, taking advantage of recent technological advancements enabling multicolour imaging. Here we demonstrate the combination of live imaging of SypHy-RGECO, a unique dual reporter that simultaneously measures presynaptic calcium influx and neurotransmitter release, with post hoc immunolabelling and multicolour single molecule localisation microscopy, to investigate the structure-function relationship at individual presynaptic boutons.

Determinants of synapse diversity revealed by super-resolution quantal transmission and active zone imaging

Neural circuit function depends on the pattern of synaptic connections between neurons and the strength of those connections. Synaptic strength is determined by both postsynaptic sensitivity to neurotransmitter and the presynaptic probability of action potential evoked transmitter release (Pr). Whereas morphology and neurotransmitter receptor number indicate postsynaptic sensitivity, presynaptic indicators and the mechanism that sets Pr remain to be defined. To address this, we developed QuaSOR, a super-resolution method for determining Pr from quantal synaptic transmission imaging at hundreds of glutamatergic synapses at a time. We mapped the Pr onto super-resolution 3D molecular reconstructions of the presynaptic active zones (AZs) of the same synapses at the Drosophila larval neuromuscular junction (NMJ). We find that Pr varies greatly between synapses made by a single axon, quantify the contribution of key AZ proteins to Pr diversity and find that one of these, Complexin, suppresses spontaneous and evoked transmission differentially, thereby generating a spatial and quantitative mismatch between release modes. Transmission is thus regulated by the balance and nanoscale distribution of release-enhancing and suppressing presynaptic proteins to generate high signal-to-noise evoked transmission.

Heterogeneity of glutamatergic synapses: cellular mechanisms and network consequences

Chemical synapses are commonly known as a structurally and functionally highly diverse class of cell-cell contacts specialized to mediate communication between neurons. They represent the smallest "computational" unit of the brain and are typically divided into excitatory and inhibitory as well as modulatory categories. These categories are subdivided into diverse types, each representing a different structure-function repertoire that in turn are thought to endow neuronal networks with distinct computational properties. The diversity of structure and function found among a given category of synapses is referred to as heterogeneity. The main building blocks for this heterogeneity are synaptic vesicles, the active zone, the synaptic cleft, the postsynaptic density, and glial processes associated with the synapse. Each of these five structural modules entails a distinct repertoire of functions, and their combination specifies the range of functional heterogeneity at mammalian excitatory synapses, which are the focus of this review. We describe synapse heterogeneity that is manifested on different levels of complexity ranging from the cellular morphology of the pre- and postsynaptic cells toward the expression of different protein isoforms at individual release sites. We attempt to define the range of structural building blocks that are used to vary the basic functional repertoire of excitatory synaptic contacts and discuss sources and general mechanisms of synapse heterogeneity. Finally, we explore the possible impact of synapse heterogeneity on neuronal network function.

Brain is modulated by neuronal plasticity during postnatal development

Neuroplasticity is referred to the ability of the nervous system to change its structure or functions as a result of former stimuli. It is a plausible mechanism underlying a dynamic brain through adaptation processes of neural structure and activity patterns. Nevertheless, it is still unclear how the plastic neural systems achieve and maintain their equilibrium. Additionally, the alterations of balanced brain dynamics under different plasticity rules have not been explored either. Therefore, the present article primarily aims to review recent research studies regarding homosynaptic and heterosynaptic neuroplasticity characterized by the manipulation of excitatory and inhibitory synaptic inputs. Moreover, it attempts to understand different mechanisms related to the main forms of synaptic plasticity at the excitatory and inhibitory synapses during the brain development processes. Hence, this study comprised surveying those articles published since 1988 and available through PubMed, Google Scholar and science direct databases on a keyword-based search paradigm. All in all, the study results presented extensive and corroborative pieces of evidence for the main types of plasticity, including the long-term potentiation (LTP) and long-term depression (LTD) of the excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs).

A Correspondence Between Normalization Strategies in Artificial and Biological Neural Networks

A fundamental challenge at the interface of machine learning and neuroscience is to uncover computational principles that are shared between artificial and biological neural networks. In deep learning, normalization methods such as batch normalization, weight normalization, and their many variants help to stabilize hidden unit activity and accelerate network training, and these methods have been called one of the most important recent innovations for optimizing deep networks. In the brain, homeostatic plasticity represents a set of mechanisms that also stabilize and normalize network activity to lie within certain ranges, and these mechanisms are critical for maintaining normal brain function. In this article, we discuss parallels between artificial and biological normalization methods at four spatial scales: normalization of a single neuron's activity, normalization of synaptic weights of a neuron, normalization of a layer of neurons, and normalization of a network of neurons. We argue that both types of methods are functionally equivalent-that is, both push activation patterns of hidden units toward a homeostatic state, where all neurons are equally used-and we argue that such representations can improve coding capacity, discrimination, and regularization. As a proof of concept, we develop an algorithm, inspired by a neural normalization technique called synaptic scaling, and show that this algorithm performs competitively against existing normalization methods on several data sets. Overall, we hope this bidirectional connection will inspire neuroscientists and machine learners in three ways: to uncover new normalization algorithms based on established neurobiological principles; to help quantify the trade-offs of different homeostatic plasticity mechanisms used in the brain; and to offer insights about how stability may not hinder, but may actually promote, plasticity.

The glutamatergic synapse: a complex machinery for information processing

Being the most abundant synaptic type, the glutamatergic synapse is responsible for the larger part of the brain's information processing. Despite the conceptual simplicity of the basic mechanism of synaptic transmission, the glutamatergic synapse shows a large variation in the response to the presynaptic release of the neurotransmitter. This variability is observed not only among different synapses but also in the same single synapse. The synaptic response variability is due to several mechanisms of control of the information transferred among the neurons and suggests that the glutamatergic synapse is not a simple bridge for the transfer of information but plays an important role in its elaboration and management. The control of the synaptic information is operated at pre, post, and extrasynaptic sites in a sort of cooperation between the pre and postsynaptic neurons which also involves the activity of other neurons. The interaction between the different mechanisms of control is extremely complicated and its complete functionality is far from being fully understood. The present review, although not exhaustively, is intended to outline the most important of these mechanisms and their complexity, the understanding of which will be among the most intriguing challenges of future neuroscience.

Quantum propensities in the brain cortex and free will

Capacity of conscious agents to perform genuine choices among future alternatives is a prerequisite for moral responsibility. Determinism that pervades classical physics, however, forbids free will, undermines the foundations of ethics, and precludes meaningful quantification of personal biases. To resolve that impasse, we utilize the characteristic indeterminism of quantum physics and derive a quantitative measure for the amount of free will manifested by the brain cortical network. The interaction between the central nervous system and the surrounding environment is shown to perform a quantum measurement upon the neural constituents, which actualize a single measurement outcome selected from the resulting quantum probability distribution. Inherent biases in the quantum propensities for alternative physical outcomes provide varying amounts of free will, which can be quantified with the expected information gain from learning the actual course of action chosen by the nervous system. For example, neuronal electric spikes evoke deterministic synaptic vesicle release in the synapses of sensory or somatomotor pathways, with no free will manifested. In cortical synapses, however, vesicle release is triggered indeterministically with probability of 0.35 per spike. This grants the brain cortex, with its over 100 trillion synapses, an amount of free will exceeding 96 terabytes per second. Although reliable deterministic transmission of sensory or somatomotor information ensures robust adaptation of animals to their physical environment, unpredictability of behavioral responses initiated by decisions made by the brain cortex is evolutionary advantageous for avoiding predators. Thus, free will may have a survival value and could be optimized through natural selection.