Synaptic pruning is a critical neurodevelopmental process that eliminates redundant or weak synaptic connections to optimize brain circuitry. In schizophrenia, converging evidence from imaging, genetic, and postmortem studies suggests that this process is pathologically accelerated, particularly in the prefrontal cortex during adolescence. The resulting reduction in synaptic density has been implicated in disrupted neural connectivity observed in psychosis, with the onset of cognitive impairment and negative symptoms. This review explores whether modulating aberrant synaptic pruning could serve as a preventive or early intervention strategy for schizophrenia. We analyze domains with emerging therapeutic relevance: tetracycline antibiotics, the complement system and C4 gene, kynurenine pathway modulation, epigenetic therapies, neuroprotective strategies (e.g., BDNF, NF-κB, progranulin), genetic and transcriptional regulators of pruning, and other new, mostly hypothetical, options. We also discuss the limitations of the impact on pruning. We conducted a structured review of the mechanisms involved in pruning, as well as clinical trials, preclinical studies, and mechanistic models that investigate molecular targets influencing synaptic pruning in schizophrenia. Several molecular pathways have been implicated in abnormal synaptic pruning in schizophrenia, including complement C4A overexpression, kynurenine pathway imbalance (KYNA/QUIN), and dysregulation of microglial and transcriptional modulators such as MEF2C and TCF4. While retrospective studies suggest minocycline or doxycycline may reduce psychosis risk, randomized trials remain inconclusive. Emerging interventions, including LSD1 inhibitors, BDNF/progranulin enhancers, and lifestyle-based epigenetic modulation, show promise but require further validation in clinical settings. We also discuss the limitations of these methods, including safety considerations. Targeted modulation of synaptic pruning represents a promising but complex therapeutic strategy. The timing, specificity, and reversibility of interventions are crucial to avoid disrupting essential neurodevelopment. Future efforts should focus on identifying biomarkers for patient stratification and validating preventive strategies in high-risk populations.
Mitochondria regulate intracellular calcium ion (Ca2+) signaling by a fine-tuned process of mitochondrial matrix (m) Ca2+ influx, mCa2+ buffering (sequestration) and mCa2+ release (Ca2+ efflux). This process is critically important in the neurosynaptic terminal, where there is a simultaneous high demand for ATP utilization, cytosolic (c) Ca2+ regulation, and maintenance of ionic gradients across the cell membrane. Brain synaptic and non-synaptic mitochondria display marked differences in Ca2+ retention capacity. We hypothesized that mitochondrial Ca2+ handling in these two mitochondrial populations is determined by the net effects of Ca2+ uptake, buffering or efflux with increasing CaCl2 boluses. We found first that synaptic mitochondria have a more coupled respiration than non-synaptic mitochondria; this may correlate with the higher local energy demand in synapses to support neurotransmission. When both mitochondrial fractions were exposed to increasing mCa2+ loads we observed decreased mCa2+ sequestration in synaptic mitochondria as assessed by a significant increase in the steady-state free extra matrix Ca2+ (ss[Ca2+]e) compared to non-synaptic mitochondria. Since, non-synaptic mitochondria displayed a significantly reduced ss[Ca2+]e, this suggested a larger mCa2+ buffering capacity to maintain [Ca2+]m with increasing mCa2+ loads. There were no differences in the magnitude of the transient depolarizations and repolarizations of the membrane potential (ΔΨm) and both fractions exhibited similar gradual depolarization of the baseline ΔΨm during additional CaCl2 boluses. Adding the mitochondrial Na+/Ca2+ exchanger (mNCE) inhibitor CGP37157 to the mitochondrial suspensions unmasked the mCa2+ sequestration and concomitantly lowered ss[Ca2+]e in synaptic vs. non-synaptic mitochondria. Adding complex V inhibitor oligomycin plus ADP (OMN + ADP) bolstered the matrix Ca2+ buffering capacity in synaptic mitochondria, as did Cyclosporin A (CsA), in non-synaptic. Our results display distinct differences in regulation of the free [Ca2+]m to prevent collapse of ΔΨm during mCa2+ overload in the two populations of mitochondria. Synaptic mitochondria appear to rely mainly on mCa2+ efflux via mNCE, while non-synaptic mitochondria rely mainly on Pi-dependent mCa2+ sequestration. The functional implications of differential mCa2+ handling at neuronal synapses may be adaptations to cope with the higher metabolic activity and larger mCa2+ transients at synaptosomes, reflecting a distinct role they play in brain function.
Anorexia nervosa (AN) is increasingly understood as a metabo-psychiatric disorder in which metabolic biology and neural circuit function are intrinsically intertwined. Genetic studies reveal that AN is associated with heritable metabolic traits suggesting that metabolic vulnerability contributes to the disorder. The metabolic profile of AN further shapes brain responses; endocrine signals such as insulin, leptin, ghrelin, and adiponectin elicit atypical neural responses in circuits regulating appetite, reward, interoception, and cognitive control. This altered signaling is accompanied by circuit-specific remodeling, suggesting that the chronic metabolic dysregulation seen in AN affects synaptic plasticity across distributed brain regions. Neural systems that integrate metabolic, emotional, and cognitive information-including hypothalamic, striatal, prefrontal, and limbic circuits-show altered plasticity under starvation. Glucose and insulin modulate excitatory-inhibitory balance and synaptic efficacy, while ketone bodies act as starvation-associated neuromodulators influencing transmitter release and structural plasticity. These and other body-to-brain signals recalibrate network dynamics central to food intake, motivation, and learning. At the molecular level, intracellular metabolic sensors such as AMPK, mTOR, and O-GlcNAc function as transducers that convert nutrient availability into changes in protein synthesis, receptor trafficking, and dendritic spine architecture, providing mechanistic links between metabolic state and synaptic remodeling. Overall, converging evidence supports a model in which AN arises from interactions between metabolic traits and the plastic neural circuits mediating food intake, emotion, and cognition. By clarifying how metabolic signals reshape synaptic ensembles in AN, we present a framework for understanding mechanisms of vulnerability and identify targets capable of restoring adaptive plasticity. This review suggests a trajectory in which treatments jointly address metabolic physiology and brain-based processes of learning, motivation, and affect.
Synaptic failure is one of the earliest and most significant contributors to the cognitive decline in Alzheimer's disease (AD), preceding extensive neuronal loss. Although amyloid beta (Aβ) plaques and neurofibrillary tangles (NFTs) of tau protein characterize the disease, memory impairment primarily results from the gradual deterioration of synaptic communications. This decline is caused by a complex interaction among mitochondrial energy deficits, cytoskeletal instability, disrupted exosomal signaling, and immune-mediated synaptic pruning. Mitochondrial dysfunction, particularly affecting complexes I and IV, leads to reduced ATP production, faulty mitophagy and disrupted calcium (Ca2+) homeostasis, placing the synapse under constant metabolic stress. Elevated reactive oxygen species (ROS) further activate stress pathways, including p38 MAPK and JNK, contributing to synaptic protein damage and impaired long-term potentiation (LTP). Furthermore, tau hyperphosphorylation destabilizes the neuronal cytoskeleton, weakening dendritic spine integrity and synaptic connectivity. At the same time, Aβ alters the cargo carried by exosomes, facilitating the spread of pathogenic Aβ and tau species between the neurons and modulating microglial activation and complement-mediated synaptic pruning. Additionally, emerging studies highlight the role of NETosis in exacerbating neuroinflammation and compromising the blood-brain barrier (BBB) integrity, thereby increasing synaptic damage. In contrast, phytochemicals such as resveratrol, ginkgolide B, curcumin, ferulic acid, epigallocatechin gallate (EGCG), and quercetin exert neuroprotection by restoring redox balance, altering exosomal communications, stabilizing cytoskeletal signaling, and reducing neuroinflammation. Moreover, delivery techniques such as nanoparticles and engineered exosomes enhance BBB permeability and enable targeted synaptic intervention. Overall, this review summarizes current mechanistic findings and highlights the potential of phytochemicals as multitarget therapeutic agents for synaptic repair and functional recovery in AD.
Epilepsy is a prevalent neurological condition, with underlying neuronal mechanisms involving hyperexcitability and hypersynchrony. Imbalance between excitatory and inhibitory circuits, as well as histological reorganization are relatively well-documented in animal models or even in the human hippocampus, but less is known about human neocortical epileptic activity. Our knowledge about changes in the excitatory signaling is especially scarce, compared to that about the inhibitory cell population. This study investigated the firing properties of single neurons in the human neocortex in vitro, during pharmacological blockade of glutamate receptors, and additionally evaluated anatomical changes in the excitatory circuit in tissue samples from epileptic and non-epileptic patients. Both epileptic and non-epileptic tissues exhibited spontaneous population activity (SPA), NMDA receptor antagonization reduced SPA recurrence only in epileptic tissue, whereas further blockade of AMPA/kainate receptors reversibly abolished SPA emergence regardless of epilepsy. Firing rates did not significantly change in excitatory principal cells and inhibitory interneurons during pharmacological experiments. Granular layer (L4) neurons showed an increased firing rate in epileptic compared to non-epileptic tissue. The burstiness of neurons remained unchanged, except for that of inhibitory cells in epileptic recordings, which decreased during blockade of glutamate receptors. Crosscorrelograms computed from single neuron discharge revealed both mono- and polysynaptic connections, particularly involving intrinsically bursting principal cells. Histological investigations found similar densities of SMI-32-immunopositive long-range projecting pyramidal cells in both groups, and shorter excitatory synaptic active zones with a higher proportion of perforated synapses in the epileptic group. These findings provide insights into epileptic modifications from the perspective of the excitatory system and highlight discrete alterations in firing patterns and synaptic structure. Our data suggest that NMDA-dependent glutamatergic signaling, as well as the excitatory synaptic machinery are perturbed in epilepsy, which might contribute to epileptic activity in the human neocortex.
Programmed axon degeneration significantly affects neural connectivity, however, the underlying mechanisms remain poorly understood, particularly in cortical regions. Sterile Alpha and TIR motif-containing protein 1 (SARM1) is a known regulator of axon degeneration in the peripheral nervous system, but its role in cortical axon plasticity, particularly during injury conditions, remains unclear. This study examined the role of SARM1 in synaptic connectivity and remodelling in the adult sensory-motor cortex under normal physiological conditions and following acute axonal injury. Adult male Thy1-GFP-M mice (3-12 months) expressing EGFP in excitatory neurons were also either wild-type (WT-GFP) or null for SARM1 (SARM1KO-GFP). Using in vivo multiphoton microscopy, long cortical axon segments (~335 μm ± 140 μm), with terminaux and en passant synaptic boutons in the upper layers of the cortical neuropil, were repeatedly imaged at 48-h intervals to assess axon morphology, synaptic density, and synaptic turnover in the presence and absence of SARM1. Without injury, axon morphology, synaptic density, and turnover were similar between WT and SARM1KO groups, suggesting that SARM1 is not necessary for maintaining baseline cortical synaptic connectivity. Following axotomy by laser lesion, the non-degenerating proximal axon (still connected to the soma) showed significant changes in synaptic plasticity, with an increased rate of loss of synapses. Our findings suggest that SARM1 plays no role in the remodelling of synapses in the proximal axon after an acute axonal injury.
Traumatic Brain Injury (TBI) is a leading cause of mortality and morbidity in adults and can lead to long-term disability, including cognitive and motor deficits. Despite advances in research, there are currently no pharmacological interventions to improve outcomes after TBI. Studies suggest that non-selective transient receptor potential melastatin 2 (TRPM2) channels contribute to brain injury in models of ischemia, however TRPM2 remains understudied following TBI. Thus, we utilized TRPM2 KO mice and a novel TRPM2 inhibiting peptide, tatM2NX, to assess the role of TRPM2 in TBI-induced injury and functional recovery. This study used histological analysis of injury, neurobehavior and electrophysiology to assess the role of TRPM2 on injury and cognitive recovery (memory) impairments using the controlled cortical impact (CCI) model to induce TBI in mice. Histological analysis used to investigate brain injury volume at 7 days after TBI showed sex differences in response to injury in TRPM2 KO mice but no pharmacological effects in our WT mice. A contextual fear-conditioning task was used to study memory function 7 or 30 days after TBI and demonstrates that sham-operated mice exhibited significant freezing behavior compared to TBI-operated mice, indicating impaired memory function. Mice administered tat-M2NX 2 h after TBI exhibited a significant reduction of freezing behavior compared to control tat-scrambled (tat-SCR)-treated mice, suggesting improvement in memory function after TBI. To test the effect of TBI on hippocampal long-term potentiation (LTP), a well-established cellular model of synaptic plasticity associated with changes in learning and memory, extracellular field recordings of CA1 neurons were performed in hippocampal slices prepared 7 days after TBI. Consistent with our behavioral testing, we observed impaired hippocampal LTP in mice following TBI (tat-SCR), compared to sham control mice. However, mice treated with tat-M2NX after TBI exhibited preserved LTP, consistent with the improved memory function observed in our behavioral studies. While this data implicates TRPM2 in brain pathology following TBI, the improvement in memory function without providing histological protection suggests that administration of tatM2NX at an acute time point differentially affects hippocampal regions compared to cortical regions.
Short-term plasticity is an important feature in the brain for shaping neural dynamics and for information processing. Short-term plasticity is known to depend on many factors including brain region, cortical layer, and cell type. Here we focus on vasoactive-intestinal peptide (VIP) interneurons (INs). VIP INs play a key disinhibitory role in cortical circuits by inhibiting other IN types, including Martinotti cells (MCs) and basket cells (BCs). Despite this prominent role, short-term plasticity at synapses to and from VIP INs is not well described. In this study, we therefore characterized the short-term plasticity at inputs and outputs of genetically targeted VIP INs in mouse motor cortex. To explore inhibitory to inhibitory (I → I) short-term plasticity at layer 2/3 (L2/3) VIP IN outputs onto L5 MCs and BCs, we relied on a combination of whole-cell recording, 2-photon microscopy, and optogenetics, which revealed that VIP IN→MC/BC synapses were consistently short-term depressing. To explore excitatory (E) → I short-term plasticity at inputs to VIP INs, we used extracellular stimulation. Surprisingly, unlike VIP IN outputs, E → VIP IN synapses exhibited heterogeneous short-term dynamics, which we attributed to the target VIP IN cell rather than the input. Computational modeling furthermore linked the diversity in short-term dynamics at VIP IN inputs to a wide variability in probability of release. Taken together, our findings highlight how short-term plasticity at VIP IN inputs and outputs is specific to synapse type. We propose that the broad diversity in short-term plasticity of VIP IN inputs forms a basis to code for a broad range of contrasting signal dynamics.
Long-term potentiation (LTP) is a biochemical process that underlies learning in excitatory glutamatergic synapses in the Central Nervous System (CNS). A critical early driver of LTP is autophosphorylation of the abundant postsynaptic enzyme, Ca2+/calmodulin-dependent protein kinase II (CaMKII). Autophosphorylation is initiated by Ca2+ flowing through NMDA receptors activated by strong synaptic activity. Its lifetime is ultimately determined by the balance of the rates of autophosphorylation and of dephosphorylation by protein phosphatase 1 (PP1). Here we have modeled the autophosphorylation and dephosphorylation of CaMKII during synaptic activity in a spine synapse using MCell4, an open source computer program for creating particle-based stochastic, and spatially realistic models of cellular microchemistry. The model integrates four earlier detailed models of separate aspects of regulation of spine Ca2+ and CaMKII activity, each of which incorporate experimentally measured biochemical parameters and have been validated against experimental data. We validate the composite model by showing that it accurately predicts previous experimental measurements of effects of NMDA receptor activation, including high sensitivity of induction of LTP to phosphatase activity in vivo, and persistence of autophosphorylation for a period of minutes after the end of synaptic stimulation. We then use the model to probe aspects of the mechanism of regulation of autophosphorylation of CaMKII that are difficult to measure in vivo. We examine the effects of "CaM-trapping," a process in which the affinity for Ca2+/CaM increases several hundred-fold after autophosphorylation. We find that CaM-trapping does not increase the proportion of autophosphorylated subunits in holoenzymes after a complex stimulus, as previously hypothesized. Instead, CaM-trapping may dramatically prolong the lifetime of autophosphorylated CaMKII through steric hindrance of dephosphorylation by protein phosphatase 1. The results provide motivation for experimental measurement of the extent of suppression of dephosphorylation of CaMKII by bound Ca2+/CaM. The composite MCell4 model of biochemical effects of complex stimuli in synaptic spines is a powerful new tool for realistic, detailed dissection of mechanisms of synaptic plasticity.
Group I metabotropic glutamate receptors (mGluRs) play a critical role in regulating neuronal excitability, synaptic strength, and cortical network activity. Although their physiological functions and involvement in neurological disorders are well established, direct experimental evidence for their role in human cortical neurons remains limited. We investigated the effects of group I mGluR activation on excitatory synaptic transmission in the human supragranular cortex using paired whole-cell patch-clamp recordings from synaptically connected pyramidal cells and interneurons in acute slices of human neocortex resected during neurosurgery. Activation of mGluRs with the agonist (S)-3,5-dihydroxyphenylglycine (DHPG) altered excitatory synaptic efficacy in an interneuron subtype-dependent manner. Specifically, we observed acute enhancement of excitatory postsynaptic current (EPSC) amplitudes in 54% of fast-spiking interneurons and in 15% of non-fast-spiking interneuron types. Applying the same experimental protocol in slices from Wistar rats resulted in a similar increase in synaptic strength in fast-spiking interneurons. However, paired-pulse ratio analysis showed species-dependent differences, which may reflect distinct contributions of pre- and postsynaptic factors to the observed modulation. Together, these results demonstrate that acute modulation of pyramidal cell-fast-spiking interneuron synapses via group I mGluRs is conserved between human and rodent neocortex, while pointing to species-specific underlying mechanisms. Moreover, mGluR-mediated modulation exhibits cell-type specificity in human cortical circuits. Collectively, these findings provide direct functional evidence for group I mGluR-dependent synaptic regulation in the human cortex and highlight important species- and cell-type-specific differences that should be considered when extrapolating rodent data to human cortical physiology and disease mechanisms.
Homo sapiens has evolved a large and complex neocortex that underlies advanced cognitive capabilities. Neural computation, however, is inherently energy-intensive, and evolutionary pressures have shaped mechanisms that optimize both computational performance and energy efficiency in the human brain. Fast-spiking interneurons, particularly basket cells, are among the most active neuron types in the neocortex, where they play a key role in coordinating time and space in the activity of neuronal networks, but their high activity levels require high metabolic resources. Because the human neocortex is significantly larger than that of rodents-and contains a higher proportion of inhibitory interneurons relative to pyramidal cells-this expansion may have created evolutionary pressure to reduce the energetic cost of fast-spiking neurons. Compared with rodents, human fast-spiking neurons exhibit adaptations that appear to lower energy expenditure while preserving rapid and precise inhibition. One such adaptation is increased input resistance, which allows both excitation and inhibition to occur with reduced transmembrane ion currents, thereby decreasing the energy required to maintain ionic gradients across the plasma membrane. Since higher input resistance also slows down membrane potential changes, these cells show secondary adaptations that maintain rapid electrical signaling. Additional modifications-such as optimized ion channel composition in soma and axon initial segment, enhanced axon myelination, simplified structure of dendritic tree, and multivesicular synapses-further improve electrical signaling and are likely to reduce metabolic demand, collectively reducing ATP consumption in the neuronal network. By integrating cellular and synaptic perspectives, this review highlights how fast-spiking neurons in the human neocortex have evolved differently from those in rodents to balance energy efficiency while maintaining computational power, providing insight into the metabolic constraints of the human brain.
Palmitoylation and depalmitoylation represent dichotomic processes by which a labile posttranslational lipid modification regulates protein trafficking and degradation. The depalmitoylating enzyme, palmitoyl-protein thioesterase 1 (PPT1), is associated with the devastating pediatric neurodegenerative condition, infantile neuronal ceroid lipofuscinosis (CLN1). CLN1 is characterized by the accumulation of autofluorescent lysosomal storage material (AFSM) in neurons and robust neuroinflammation. Converging lines of evidence suggest that in addition to cellular waste accumulation, the symptomology of CLN1 corresponds with disruption of synaptic processes. Indeed, loss of Ppt1 function in cortical neurons dysregulates the synaptic incorporation of the GluA1 AMPA receptor (AMPAR) subunit during a type of synaptic plasticity called synaptic scaling. However, the mechanisms causing this aberration are unknown. Here, we used the Ppt1-/- mouse model (both sexes) to further investigate how Ppt1 regulates synaptic plasticity and how its disruption affects downstream signaling pathways. To this end, we performed a palmitoyl-proteomic screen, which provoked the discovery that Akap5 is excessively palmitoylated at Ppt1-/- synapses. Extending our previous data, in vivo induction of synaptic scaling, which is regulated by Akap5, caused an excessive upregulation of GluA1 in Ppt1-/- mice. This synaptic change was associated with exacerbated disease pathology. Furthermore, the Akap5- and inflammation-associated transcriptional regulator, nuclear factor of activated T cells (NFAT), was sensitized in Ppt1-/- cortical neurons. Suppressing the upstream regulator of NFAT activation, calcineurin, with the FDA-approved therapeutic FK506 (Tacrolimus) modestly improved neuroinflammation in Ppt1-/- mice. These findings indicate that the absence of depalmitoylation stifles synaptic protein trafficking and contributes to neuroinflammation via an Akap5-associated mechanism.
The amygdala is involved in processing and memory of emotional stimuli. The cortical regions of the amygdala are situated medially to the piriform cortex (PC) and are also considered part of the olfactory system, which may explain why scents can evoke strong emotional responses and trigger vivid memories. Similarly to the PC, the anterior cortical nucleus of the amygdala (ACo) is a three-layered structure receiving direct input from the main olfactory bulb (MOB) through the lateral olfactory tract (LOT), without a thalamic relay. While activity processing, plasticity and behavioral relevance of the PC have been extensively studied, evidence on the functional properties of the olfactory amygdala is scarce. ACo participates in the innate response to aversive stimuli and could be involved in olfactory emotional learning. Primary cortices of different sensory modalities display early periods in which afferent connections are particularly plastic, and this is crucial for network maturation. Here, we studied the plastic properties of LOT-ACo synapses during olfactory system maturation, by assessing long-term potentiation (LTP) induction during the first weeks of life. We performed field recordings of LOT-ACo synaptic potentials in brain slices from rats of 6 to 35 postnatal days (P6-P35) to assess LTP induction by theta-burst stimulation. To investigate if the developmental regulation of plasticity can be altered by early sensory deprivation, we performed unilateral naris occlusion. We found that LTP was significantly higher between P16-P25 (reaching ~30%) compared to earlier or later stages. Interestingly, LTP was missing around the end of the first postnatal month (P26-35). Furthermore, we demonstrated that LTP induction at one month of age was recovered in both hemispheres in rats that were subjected to unilateral sensory deprivation. This result suggests the existence of a critical period for LTP induction in olfactory connections to the amygdala. Therefore, ACo may not only be involved in innate responses to biologically relevant odorants, but could also participate in early experience-dependent associative learning. Moreover, olfactory deprivation could extend the critical period for plasticity, making the system prone to undergo plastic changes at later developmental stages.
Age-related hearing difficulties have a complex etiology that includes degenerative processes in the sensory cochlea. The cochlea comprises the start of the afferent, ascending auditory pathway, but also receives efferent feedback innervation by two separate populations of brainstem neurons: the medial olivocochlear and lateral olivocochlear pathways, innervating the outer hair cells and auditory-nerve fibers synapsing on inner hair cells, respectively. Efferents are believed to improve hearing under difficult conditions, such as high background noise. Here, we compare olivocochlear efferent innervation density along the tonotopic axis in young-adult and aged gerbils (at ~50% of their maximum lifespan potential), a classic animal model for age-related hearing loss. Efferent synaptic terminals and sensory hair cells were labeled immunohistochemically with anti-synaptotagmin and anti-myosin VIIa, respectively. Numbers of hair cells, numbers of efferent terminals, and the efferent innervation area were quantified at seven tonotopic locations along the organ of Corti. The tonotopic distribution of olivocochlear innervation in the gerbil was similar to that previously shown for other species, with a slight apical cochlear bias in presumed lateral olivocochlear innervation (inner-hair-cell region), and a broad mid-cochlear peak for presumed medial olivocochlear innervation (outer-hair-cell region). We found significant, age-related declines in overall efferent innervation to both the inner-hair-cell and the outer-hair-cell region. However, when accounting for the age-related losses in efferent target structures, the innervation density of surviving elements proved unchanged in the inner-hair-cell region. For outer hair cells, a pronounced increase of orphaned outer hair cells, i.e., lacking efferent innervation, was observed. Surviving outer hair cells that were still efferently innervated retained a nearly normal innervation. A comparison across species suggests a basic aging scenario where outer hair cells, type-I afferents, and the efferents associated with them, steadily die away with advancing age, but leave the surviving cochlear circuitry largely intact until an advanced age, beyond 50% of a species' maximum lifespan potential. In the outer-hair-cell region, MOC degeneration may precede outer-hair-cell death, leaving a putatively transient population of orphaned outer hair cells that are no longer under efferent control.
Epileptiform activity is the most striking result of hyperexcitation of a group of neurons that can occur in different brain regions and then spread to other sites. Later it was shown that these rhythms have a cellular correlate in vitro called paroxysmal depolarization shift (PDS). In 13-15 DIV neuron-glial cell culture, inhibition of the GABA(A) receptors induces bursts of action potential in the form of clasters PDS and oscillations of intracellular Ca2+ concentration ([Ca2+]i). We demonstrate that GABAergic neurons expressing calcium-permeable AMPA receptors (CP-AMPARs) as well as Kv7-type potassium channels regulate hippocampal glutamatergic neurons' excitability during epileptiform activity in culture. A combination of whole-cell patch-clamp in current clamp mode and calcium imaging microscopy was used to simultaneously register membrane potential and [Ca2+]i level. To identify GABAergic cell cultures were fixed and stained with antibodies against glutamate decarboxylase GAD 65/67 and neuron-specific enolase (NSE) after vital [Ca2+]i imaging. It was shown that CP-AMPARs are involved in the regulation of the PDS clusters and [Ca2+]i pulses accompanied them. Activation of CP-AMPARs of GABAergic neurons is thought to cause the release of GABA, which activates the GABA(B) receptors of other GABAergic interneurons. It is assumed that activation of these GABA(B) receptors leads to the release of beta-gamma subunits of Gi protein, which activate potassium channels, resulting in hyperpolarization and inhibition of these interneurons. The latter causes disinhibition of glutamatergic neurons, the targets of these interneurons. In turn, the CP-AMPAR antagonist, NASPM, has the opposite effect. Measurement of membrane potential in GABAergic neurons by the patch-clamp method in whole-cell configuration demonstrated that NASPM suppresses hyperpolarization in clusters and individual PDSs. It is believed that Kv7-type potassium channels are involved in the control of hyperpolarization during epileptiform activity. The blocker of Kv7 channels, XE 991, mimicked the effect of the CP-AMPARs antagonist on PDS clusters. Both drugs increased the duration of the PDS cluster. In turn, the Kv7 activator, retigabine, decreased the duration of the PDS cluster and Ca2+ pulse. In addition, retigabine led to deep posthyperpolarization at the end of the PDS cluster. The Kv7 channel is believed to be involved in the formation of PDS, as the channel blocker reduced the rate of hyperpolarization in the PDS almost three times. Thus, GABAergic neurons expressing CP-AMPARs, regulate the membrane potential of innervated glutamatergic neurons by modulating the activity of postsynaptic potassium channels of other GABAergic neurons.
Rapid, synapse-specific neurotransmission requires the precise alignment of presynaptic neurotransmitter release and postsynaptic receptors. How postsynaptic glutamate receptor accumulation is induced during maturation is not well understood. We find that in cultures of dissociated hippocampal neurons at 11 days in vitro (DIV) numerous synaptic contacts already exhibit pronounced accumulations of the pre- and postsynaptic markers synaptotagmin, synaptophysin, synapsin, bassoon, VGluT1, PSD-95, and Shank. The presence of an initial set of AMPARs and NMDARs is indicated by miniature excitatory postsynaptic currents (mEPSCs). However, AMPAR and NMDAR immunostainings reveal rather smooth distributions throughout dendrites and synaptic enrichment is not obvious. We found that brief periods of Ca2+ influx through NMDARs induced a surprisingly rapid accumulation of NMDARs within 1 min, followed by accumulation of CaMKII and then AMPARs within 2-5 min. Postsynaptic clustering of NMDARs and AMPARs was paralleled by an increase in their mEPSC amplitudes. A peptide that blocked the interaction of NMDAR subunits with PSD-95 prevented the NMDAR clustering. NMDAR clustering persisted for 3 days indicating that brief periods of elevated glutamate fosters permanent accumulation of NMDARs at postsynaptic sites in maturing synapses. These data support the model that strong glutamatergic stimulation of immature glutamatergic synapses results in a fast and substantial increase in postsynaptic NMDAR content that required NMDAR binding to PSD-95 or its homologues and is followed by recruitment of CaMKII and subsequently AMPARs.
The unitary postsynaptic response to presynaptic quantal glutamate release is the fundamental basis of excitatory information transfer between neurons. The view, however, of individual glutamatergic synaptic connections in a population as homogenous, fixed-strength units of neural communication is becoming increasingly scrutinized. Here, we used minimal stimulation of individual glutamatergic afferent axons to evoke single synapse resolution postsynaptic responses from central sensory lamina I neurons in an ex vivo adult rat spinal slice preparation. We detected unitary events exhibiting a NMDA receptor component with distinct kinetic properties across synapses conferred by specific GluN2 subunit composition, indicative of GluN2 subtype-based postsynaptic heterogeneity. GluN2A, 2A and 2B, or 2B and 2D synaptic predominance functioned on distinct lamina I neuron types to narrowly, intermediately, or widely tune, respectively, the duration of evoked unitary depolarization events from resting membrane potential, which enabled individual synapses to grade differentially depolarizing steps during temporally patterned afferent input. Our results lead to a model wherein a core locus of proteomic complexity prevails at this central glutamatergic sensory synapse that involves distinct GluN2 subtype configurations. These findings have major implications for subthreshold integrative capacity and transmission strength in spinal lamina I and other CNS regions.
Electrical brain stimulation has been used to treat epilepsy with therapeutic success, but without complete understanding of its cellular mechanisms of seizure suppression. Using acute human brain slices obtained from pharmaco-resistant epilepsy patients, we delivered high-frequency, high-intensity extracellular electrical stimulation while recording from neocortical layer 2/3 pyramidal neurons (PNs), fast-spiking interneurons (FSINs), and non-fast-spiking interneurons (nFSINs). Electrical stimulation led to an increasingly selective activation of FSINs at higher stimulation frequencies, which consistently fired with high fidelity throughout prolonged stimulation periods while both PNs and nFSINs were suppressed rapidly. In addition, stimulation caused a long-term rebalancing of synaptic weights through selective and differential depression of synaptic inputs, resulting in an approximately twofold increase in the normalized excitatory-to-inhibitory postsynaptic conductance at FSINs while not affecting PNs, thereby providing support for FSIN activation toward increased inhibitory tone and network stabilization. These stimulation-induced effects were accompanied by only minimal changes in neuronal intrinsic excitability.
In vitro maintained human brain slices provide a unique experimental platform for investigating rhythmic neuronal network activity, bridging the gap between animal models and clinical studies. A wide range of spontaneous and induced oscillatory activities has been described in human brain slices. However, their occurrence and characteristics are strongly shaped by methodological determinants spanning tissue origin, slice preparation, recording conditions, and induction strategies. This has been shown to have a profound impact on the reproducibility and interpretation of oscillatory dynamics. This review synthesizes current evidence on rhythmic network activity in acute human brain slices, with a particular emphasis on how methodological determinants interact with intrinsic circuit properties to generate oscillatory dynamics. We discuss how different experimental manipulations influence oscillation frequency, stability, and spatial organization. We further examine the cellular and circuit mechanisms underlying rhythmic activity, highlighting the roles of excitatory-inhibitory balance, synaptic dynamics, neuromodulatory influences, and distinct interneuron populations. Finally, we consider how oscillatory patterns differ across disease contexts, particularly epilepsy and tumor-associated cortex, and discuss the translational value and limitations of human brain slices for linking microcircuit mechanisms to pathological and functional brain states.
Long-term plasticity at pyramidal cell to basket cell (PC → BC) synapses is important for the functioning of cortical microcircuits. It is well known that at neocortical PC → PC synapses, dendritic calcium (Ca2+) dynamics signal coincident pre-and postsynaptic spiking which in turn triggers long-term potentiation (LTP). However, the link between dendritic Ca2+ dynamics and long-term plasticity at PC → BC synapses of primary visual cortex (V1) is not as well known. Here, we explored if PC → BC synaptic plasticity in developing V1 is sensitive to postsynaptic spiking. Two-photon (2P) Ca2+ imaging revealed that action potentials (APs) in dendrites of V1 layer-5 (L5) BCs back-propagated decrementally but actively to the location of PC → BC putative synaptic contacts. Pairing excitatory inputs with postsynaptic APs elicited dendritic Ca2+ supralinearities for pre-before-postsynaptic but not post-before-presynaptic temporal ordering, suggesting that APs could impact synaptic plasticity. In agreement, extracellular stimulation as well as high-throughput 2P optogenetic mapping of plasticity both revealed that pre-before-postsynaptic but not post-before-presynaptic pairing resulted in anti-Hebbian long-term depression (LTD). Our results demonstrate that V1 BC dendritic Ca2+ nonlinearities and synaptic plasticity at PC → BC connections are both sensitive to somatic spiking.