Facilitation of the induction of long-term potentiation by GABAB receptors

Unraveling the role of cholecystokinin in epilepsy: Mechanistic insight into neuroplasticity

Epilepsy is a disorder characterized by an imbalance between excitability and inhibition, leading to uncontrolled hyperexcitability of neurons in the central nervous system. Despite the prevalence of epileptic seizures, the underlying mechanisms driving this hyperexcitability remain poorly understood. This review article aims to enhance our understanding of the mechanisms of epilepsy, with a specific focus on the role of cholecystokinin (CCK) in this debilitating disease. We will begin with an introduction to the topic, followed by an examination of the role of GABAergic neurons and the synaptic plasticity mechanisms associated with seizures. As we delve deeper, we will elucidate how CCK and its receptors contribute to seizure behavior. Finally, we will discuss the CCK-dependent synaptic plasticity mechanisms and highlight their potential implications in seizure activity. Through a comprehensive examination of these aspects, this review provides valuable insights into the involvement of CCK and its receptors in epilepsy. By improving our understanding of the mechanisms underlying this condition, particularly the role of CCK, we aim to contribute to the development of more effective treatment strategies.

Postsynaptic GABAB-receptor mediated currents in diverse dentate gyrus interneuron types

The processing of rich synaptic information in the dentate gyrus (DG) relies on a diverse population of inhibitory GABAergic interneurons to regulate cellular and circuit activity, in a layer-specific manner. Metabotropic GABAB-receptors (GABABRs) provide powerful inhibition to the DG circuit, on timescales consistent with behavior and learning, but their role in controlling the activity of interneurons is poorly understood with respect to identified cell types. We hypothesize that GABABRs display cell type-specific heterogeneity in signaling strength, which will have direct ramifications for signal processing in DG networks. To test this, we perform in vitro whole-cell patch-clamp recordings from identified DG principal cells and interneurons, followed by GABABR pharmacology, photolysis of caged GABA, and extracellular stimulation of endogenous GABA release to classify the cell type-specific inhibitory potential. Based on our previous classification of DG interneurons, we show that postsynaptic GABABR-mediated currents are present on all interneuron types albeit at different amplitudes, dependent largely on soma location and synaptic targets. GABABRs were coupled to inwardly-rectifying K+ channels that strongly reduced the excitability of those interneurons where large currents were observed. These data provide a systematic characterization of GABABR signaling in the rat DG to provide greater insight into circuit dynamics.

N-acetyltransferase 10 mediates cognitive dysfunction through the acetylation of GABABR1 mRNA in sepsis-associated encephalopathy

Sepsis-associated encephalopathy (SAE) is a critical neurological complication of sepsis and represents a crucial factor contributing to high mortality and adverse prognosis in septic patients. This study explored the contribution of NAT10-mediated messenger RNA (mRNA) acetylation in cognitive dysfunction associated with SAE, utilizing a cecal ligation and puncture (CLP)-induced SAE mouse model. Our findings demonstrate that CLP significantly upregulates NAT10 expression and mRNA acetylation in the excitatory neurons of the hippocampal dentate gyrus (DG). Notably, neuronal-specific Nat10 knockdown improved cognitive function in septic mice, highlighting its critical role in SAE. Proteomic analysis, RNA immunoprecipitation, and real-time qPCR identified GABABR1 as a key downstream target of NAT10. Nat10 deletion reduced GABABR1 expression, and subsequently weakened inhibitory postsynaptic currents in hippocampal DG neurons. Further analysis revealed that microglia activation and the release of inflammatory mediators lead to the increased NAT10 expression in neurons. Microglia depletion with PLX3397 effectively reduced NAT10 and GABABR1 expression in neurons, and ameliorated cognitive dysfunction induced by SAE. In summary, our findings revealed that after CLP, NAT10 in hippocampal DG neurons promotes GABABR1 expression through mRNA acetylation, leading to cognitive dysfunction.

Somatostatin Interneurons Recruit Pre- and Postsynaptic GABAB Receptors in the Adult Mouse Dentate Gyrus

The integration of spatial information in the mammalian dentate gyrus (DG) is critical to navigation. Indeed, DG granule cells (DGCs) rely upon finely balanced inhibitory neurotransmission in order to respond appropriately to specific spatial inputs. This inhibition arises from a heterogeneous population of local GABAergic interneurons (INs) that activate both fast, ionotropic GABAA receptors (GABAAR) and slow, metabotropic GABAB receptors (GABABR), respectively. GABABRs in turn inhibit pre- and postsynaptic neuronal compartments via temporally long-lasting G-protein-dependent mechanisms. The relative contribution of each IN subtype to network level GABABR signal setting remains unknown. However, within the DG, the somatostatin (SSt) expressing IN subtype is considered crucial in coordinating appropriate feedback inhibition on to DGCs. Therefore, we virally delivered channelrhodopsin 2 to the DG in order to obtain control of this specific SSt IN subpopulation in male and female adult mice. Using a combination of optogenetic activation and pharmacology, we show that SSt INs strongly recruit postsynaptic GABABRs to drive greater inhibition in DGCs than GABAARs at physiological membrane potentials. Furthermore, we show that in the adult mouse DG, postsynaptic GABABR signaling is predominantly regulated by neuronal GABA uptake and less so by astrocytic mechanisms. Finally, we confirm that activation of SSt INs can also recruit presynaptic GABABRs, as has been shown in neocortical circuits. Together, these data reveal that GABABR signaling allows SSt INs to control DG activity and may constitute a key mechanism for gating spatial information flow within hippocampal circuits.

Effects of (S)-3,4-DCPG, an mGlu8 receptor agonist, on hippocampal long-term potentiation at perforant pathway-dentate gyrus synapses in prenatal valproic acid-induced rat model of autism

Autism spectrum disorder (ASD) is a pervasive neurodevelopmental condition characterized by social interaction deficits, communication impairments, repetitive behaviors, and sensory sensitivities. While the etiology of ASD is multifaceted, abnormalities in glutamatergic neurotransmission and synaptic plasticity have been implicated. This study investigated the role of metabotropic glutamate receptor 8 (mGlu8) in modulating long-term potentiation (LTP) in a rat model of ASD induced by prenatal valproic acid (VPA) exposure. To induce an animal model with autism-like characteristics, pregnant rats received an intraperitoneal injection of 500 mg/kg of sodium valproate (NaVPA) on embryonic day 12.5. High-frequency stimulation was applied to the perforant path-dentate gyrus (PP-DG) synapse to induce LTP, while the mGlu8 receptor agonist (S)-3,4-dicarboxyphenylglycine (DCPG) was administered into the DG. The results revealed that VPA-exposed rats exhibited reduced LTP compared to controls. DCPG had contrasting effects, inhibiting LTP in controls and enhancing it in VPA-exposed rats. Moreover, reduced social novelty preference index (SNPI) in VPA-exposed rats was reversed by intra-DG administration of S-3,4-DCPG. In conclusion, our study advances our understanding of the complex relationship between glutamatergic neurotransmission, synaptic plasticity, and VPA-induced autism model. The findings suggest that mGlu8 receptor dysfunction plays a role in the impaired synaptic plasticity seen in ASD.

Distinct GABAergic modulation of timing-dependent LTP in CA1 pyramidal neurons along the longitudinal axis of the mouse hippocampus

Synaptic plasticity in the hippocampus underlies episodic memory formation, with dorsal hippocampus being instrumental for spatial memory whereas ventral hippocampus is crucial for emotional learning. Here, we studied how GABAergic inhibition regulates physiologically relevant low repeat spike timing-dependent LTP (t-LTP) at Schaffer collateral-CA1 synapses along the dorsoventral hippocampal axis. We used two t-LTP protocols relying on only 6 repeats of paired spike-firing in pre- and postsynaptic cells within 10 s that differ in postsynaptic firing patterns. GABAA receptor mechanisms played a greater role in blocking 6× 1:1 t-LTP that recruits single postsynaptic action potentials. 6× 1:4 t-LTP that depends on postsynaptic burst-firing unexpectedly required intact GABAB receptor signaling. The magnitude of both t-LTP-forms decreased along the dorsoventral axis, despite increasing excitability and basal synaptic strength in this direction. This suggests that GABAergic inhibition contributes to the distinct roles of dorsal and ventral hippocampus in memory formation.

Synaptic configuration and reconfiguration in the neocortex are spatiotemporally selective

Brain computation relies on the neural networks. Neurons extend the neurites such as dendrites and axons, and the contacts of these neurites that form chemical synapses are the biological basis of signal transmissions in the central nervous system. Individual neuronal outputs can influence the other neurons within the range of the axonal spread, while the activities of single neurons can be affected by the afferents in their somatodendritic fields. The morphological profile, therefore, binds the functional role each neuron can play. In addition, synaptic connectivity among neurons displays preference based on the characteristics of presynaptic and postsynaptic neurons. Here, the author reviews the "spatial" and "temporal" connection selectivity in the neocortex. The histological description of the neocortical circuitry depends primarily on the classification of cell types, and the development of gene engineering techniques allows the cell type-specific visualization of dendrites and axons as well as somata. Using genetic labeling of particular cell populations combined with immunohistochemistry and imaging at a subcellular spatial resolution, we revealed the "spatial selectivity" of cortical wirings in which synapses are non-uniformly distributed on the subcellular somatodendritic domains in a presynaptic cell type-specific manner. In addition, cortical synaptic dynamics in learning exhibit presynaptic cell type-dependent "temporal selectivity": corticocortical synapses appear only transiently during the learning phase, while learning-induced new thalamocortical synapses persist, indicating that distinct circuits may supervise learning-specific ephemeral synapse and memory-specific immortal synapse formation. The selectivity of spatial configuration and temporal reconfiguration in the neural circuitry may govern diverse functions in the neocortex.

Somatostatin-expressing interneurons modulate neocortical network through GABAb receptors in a synapse-specific manner

The firing activity of somatostatin-expressing inhibitory neurons (SST-INs) can suppress network activity via both GABAa and GABAb receptors (Rs). Although SST-INs do not receive GABAaR input from other SST-INs, it is possible that SST-IN-released GABA could suppress the activity of SST-INs themselves via GABAbRs, providing a negative feedback loop. Here we characterized the influence of GABAbR modulation on SST-IN activity in layer 2/3 of the somatosensory cortex in mice. We compared this to the effects of GABAbR activation on parvalbumin-expressing interneurons (PV-INs). Using in vitro whole-cell patch clamp recordings, pharmacological and optogenetic manipulations, we found that the firing activity of SST-INs suppresses excitatory drive to themselves via presynaptic GABAbRs. Postsynaptic GABAbRs did not influence SST-IN spontaneous activity or intrinsic excitability. Although GABAbRs at pre- and postsynaptic inputs to PV-INs are modestly activated during cortical network activity in vitro, the spontaneous firing of SST-INs was not the source of GABA driving this GABAbR activation. Thus, SST-IN firing regulates excitatory synaptic strength through presynaptic GABAbRs at connections between pyramidal neurons (Pyr-Pyr) and synapses between pyramidal neurons and SST-INs (Pyr-SST), but not Pyr-PV and PV-Pyr synapses. Our study indicates that two main types of neocortical inhibitory interneurons are differentially modulated by SST-IN-mediated GABA release.

Advances in the Electrophysiological Recordings of Long-Term Potentiation

Understanding neuronal firing patterns and long-term potentiation (LTP) induction in studying learning, memory, and neurological diseases is critical. However, recently, despite the rapid advancement in neuroscience, we are still constrained by the experimental design, detection tools for exploring the mechanisms and pathways involved in LTP induction, and detection ability of neuronal action potentiation signals. This review will reiterate LTP-related electrophysiological recordings in the mammalian brain for nearly 50 years and explain how excitatory and inhibitory neural LTP results have been detected and described by field- and single-cell potentials, respectively. Furthermore, we focus on describing the classic model of LTP of inhibition and discuss the inhibitory neuron activity when excitatory neurons are activated to induce LTP. Finally, we propose recording excitatory and inhibitory neurons under the same experimental conditions by combining various electrophysiological technologies and novel design suggestions for future research. We discussed different types of synaptic plasticity, and the potential of astrocytes to induce LTP also deserves to be explored in the future.

Mechanisms of Hebbian-like plasticity in the ventral premotor - primary motor network

The functional connection between ventral premotor cortex (PMv) and primary motor cortex (M1) is critical for the organization of goal-directed actions. Repeated activation of this connection by means of cortico-cortical paired associative stimulation (cc-PAS), a transcranial magnetic stimulation (TMS) protocol, may induce Hebbian-like plasticity. However, the physiological modifications produced by Hebbian-like plasticity in the PMv-M1 network are poorly understood. To fill this gap, we investigated the effects of cc-PAS on PMv-M1 circuits. We hypothesized that specific interactions would occur with I₂ -wave interneurons as measured by the short intracortical facilitation protocol (SICF). We used different paired-pulse TMS protocols to examine the effects of PMv-M1 cc-PAS on SICF, on GABAergic circuits as measured by short (SICI) and long (LICI) intracortical inhibition protocols, and varied the current direction in M1 to target different M1 neuronal populations. Finally, we examined the effects of cc-PAS on PMv-M1 connectivity using a dual coil approach. We found that PMv-M1 cc-PAS induces both a long-term potentiation (LTP)- or long-term depression (LTD)-like after-effect in M1 neuronal activity that is strongly associated with a bidirectional-specific change in I₂ -wave activity (SICF = 2.5 ms ISI). Moreover, cc-PAS induces a specific modulation of the LICI circuit and separately modulates PMv-M1 connectivity. We suggest that plasticity within the PMv-M1 circuit is mediated by a selective mechanism exerted by PMv on M1 by targeting I₂ -wave interneurons. These results provide new mechanistic insights into how PMv modulates M1 activity that are relevant for the design of brain stimulation protocols in health and disease. KEY POINTS: The I₂ -wave is specifically modulated by the induction of ventral premotor cortex - primary motor cortex (PMv-M1) plasticity. After PMv-M1 cortico-cortical paired associative stimulation (cc-PAS), corticospinal excitability correlates negatively with I₂ -wave amplitude. Different cc-PAS coil orientations can lead to a long-term potentiation- or long-term depression-like after-effect in M1.

Impairment in social interaction and hippocampal long-term potentiation at perforant pathway-dentate gyrus synapses in a prenatal valproic acid-induced rat model of autism

It is well established that prenatal valproic acid exposure in rats leads to autism-like behaviours and social deficits. Long-term potentiation changes in the brain have been proposed as a potential mechanism in the development of autistic behaviour. However, there are controversies regarding the effect of in utero valproic acid exposure on long-term potentiation. This study examined the social interaction and long-term potentiation induction in perforant pathway-dentate gyrus synapses in male offspring of a rat model of autism induced by prenatal exposure to valproic acid. On Embryonic Day 12.5, the pregnant dams received an injection of 500 mg/kg valproic acid (intraperitoneal) to produce the autism model. The sociability test was performed between Postnatal Days 37 and 40. The offsprings were urethane-anaesthetized and placed into a stereotaxic apparatus for surgery, electrode implantation and field potential recording on Postnatal Days 45–55. In the dentate gyrus region, excitatory postsynaptic potential slope and population spike amplitude were measured. Valproic acid-exposed offspring showed significantly impaired social interaction. The birth weight in valproic acid-exposed rats was significantly lower than in control rats. The ability of dentate gyrus synapses to induce long-term potentiation was hampered by valproic acid exposure. The decreasing excitatory postsynaptic potential slope and population spike amplitude of long-term potentiation provide evidence in favour of this notion. It is widely supposed that the hippocampus plays a central role in the process of learning and memory as well as social interaction and social memory. Therefore, deficiencies in hippocampal synaptic plasticity may be responsible, at least in part, for the social interaction deficits in valproic acid-exposed rats.

Novel types of frequency filtering in the lateral perforant path projections to dentate gyrus

Despite its evident importance to learning theory and models, the manner in which the lateral perforant path (LPP) transforms signals from entorhinal cortex to hippocampus is not well understood. The present studies measured synaptic responses in the dentate gyrus (DG) of adult mouse hippocampal slices during different patterns of LPP stimulation. Theta (5 Hz) stimulation produced a modest within-train facilitation that was markedly enhanced at the level of DG output. Gamma (50 Hz) activation resulted in a singular pattern with initial synaptic facilitation being followed by a progressively greater depression. DG output was absent after only two pulses. Reducing release probability with low extracellular calcium instated frequency facilitation to gamma stimulation while long-term potentiation, which increases release by LPP terminals, enhanced within-train depression. Relatedly, per terminal concentrations of VGLUT2, a vesicular glutamate transporter associated with high release probability, were much greater in the LPP than in CA3-CA1 connections. Attempts to circumvent the potent gamma filter using a series of short (three-pulse) 50 Hz trains spaced by 200 ms were only partially successful: composite responses were substantially reduced after the first burst, an effect opposite to that recorded in field CA1. The interaction between bursts was surprisingly persistent (>1.0 s). Low calcium improved throughput during theta/gamma activation but buffering of postsynaptic calcium did not. In all, presynaptic specializations relating to release probability produce an unusual but potent type of frequency filtering in the LPP. Patterned burst input engages a different type of filter with substrates that are also likely to be located presynaptically. KEY POINTS: The lateral perforant path (LPP)-dentate gyrus (DG) synapse operates as a low-pass filter, where responses to a train of 50 Hz, γ frequency activation are greatly suppressed. Activation with brief bursts of γ frequency information engages a secondary filter that persists for prolonged periods (lasting seconds). Both forms of LPP frequency filtering are influenced by presynaptic, as opposed to postsynaptic, processes; this contrasts with other hippocampal synapses. LPP frequency filtering is modified by the unique presynaptic long-term potentiation at this synapse. Computational simulations indicate that presynaptic factors associated with release probability and vesicle recycling may underlie the potent LPP-DG frequency filtering.

Cortical Inhibition State-Dependent iTBS Induced Neural Plasticity

Background

Intermittent theta burst stimulation (iTBS) is an effective stimulus for long-term potentiation (LTP)-like plasticity. However, iTBS-induced effects varied greatly between individuals. Ample evidence suggested that an initial decrease in local γ-aminobutyric acid (GABA) or enhancement in N-methyl-D-aspartate (NMDA) facilitation neurotransmission is of vital importance for allowing LTP-like plasticity to occur. Therefore, we aimed to investigate whether the individual level of GABA or NMDA receptor-mediated activity before stimulation is correlated with the after-effect in cortical excitability induced by iTBS.

Methods

Fifteen healthy volunteers were recruited for the present study. We measured short-interval intracortical inhibitory (SICI), long-interval intracortical inhibitory (LICI), and intracortical facilitation (ICF), which index GABA_A receptor-, GABA_B receptor-, and glutamate receptor-mediated activity, respectively, in the cortex before conducting iTBS. After iTBS intervention, the changes of motor-evoked potential (MEP) amplitude were taken as a measure for cortical excitability in response to iTBS protocol.

Results

There was a significant negative correlation between the amount of SICI measured before iTBS and the after-effect of iTBS-induced LTP-like plasticity at the time points of 5, 10, and 15 min after inducing iTBS. A multiple linear regression model indicated that SICI was a good predictor of the after-effect in cortical excitability induced by iTBS at 5, 10, and 15 min following stimulation.

Conclusion

The present study found that GABA_A receptor-mediated activity measured before stimulation is negatively correlated with the after-effect of cortical excitability induced by iTBS. SICI, as the index of GABA_A receptor-mediated activity measured before stimulation, might be a good predictor of iTBS-induced LTP-like plasticity for a period lasting 15 min following stimulation.

Prepubescent female rodents have enhanced hippocampal LTP and learning relative to males, reversing in adulthood as inhibition increases

Multiple studies indicate that adult male rodents perform better than females on spatial problems and have a lower threshold for long-term potentiation (LTP) of hippocampal CA3-to-CA1 synapses. We report here that, in rodents, prepubescent females rapidly encode spatial information and express low-threshold LTP, whereas age-matched males do not. The loss of low-threshold LTP across female puberty was associated with three inter-related changes: increased densities of α5 subunit-containing GABAARs at inhibitory synapses, greater shunting of burst responses used to induce LTP and a reduction of NMDAR-mediated synaptic responses. A negative allosteric modulator of α5-GABAARs increased burst responses to a greater degree in adult than in juvenile females and markedly enhanced both LTP and spatial memory in adults. The reasons for the gain of functions with male puberty do not involve these mechanisms. In all, puberty has opposite consequences for plasticity in the two sexes, albeit through different routes.

Hippocampal Somatostatin Interneurons, Long-Term Synaptic Plasticity and Memory

A distinctive feature of the hippocampal structure is the diversity of inhibitory interneurons. These complex inhibitory interconnections largely contribute to the tight modulation of hippocampal circuitry, as well as to the formation and coordination of neuronal assemblies underlying learning and memory. Inhibitory interneurons provide more than a simple transitory inhibition of hippocampal principal cells (PCs). The synaptic plasticity of inhibitory neurons provides long-lasting changes in the hippocampal network and is a key component of memory formation. The dendrite targeting interneurons expressing the peptide somatostatin (SOM) are particularly interesting in this regard because they display unique long-lasting synaptic changes leading to metaplastic regulation of hippocampal networks. In this article, we examine the actions of the neuropeptide SOM on hippocampal cells, synaptic plasticity, learning, and memory. We address the different subtypes of hippocampal SOM interneurons. We describe the long-term synaptic plasticity that takes place at the excitatory synapses of SOM interneurons, its singular induction and expression mechanisms, as well as the consequences of these changes on the hippocampal network, learning, and memory. We also review evidence that astrocytes provide cell-specific dynamic regulation of inhibition of PC dendrites by SOM interneurons. Finally, we cover how, in mouse models of Alzheimer’s disease (AD), dysfunction of plasticity of SOM interneuron excitatory synapses may also contribute to cognitive impairments in brain disorders.

Early Sensory Deprivation Leads to Differential Inhibitory Changes in the Striatum During Learning

The corticostriatal circuit has been identified as a vital pathway for associative learning. However, how learning is implemented when the sensory striatum is permanently impaired remains unclear. Using chemogenetic techniques to suppress layer five auditory cortex (AC) input to the auditory striatum, learning of a sound discrimination task was significantly impacted in freely moving Mongolian gerbils, in particular when this suppression occurs early on during learning. Whole-cell recordings sampled throughout learning revealed a transient reduction in postsynaptic (GABAA) inhibition in both striatal D1 and D2 cells in normal-hearing gerbils during task acquisition. In contrast, when the baseline striatal inhibitory strengths and firing rates were permanently reduced by a transient period of developmental sensory deprivation, learning was accompanied by augmented inhibition and increased firing rates. Direct manipulation of striatal inhibition in vivo and in vitro revealed a key role of the transient inhibitory changes in task acquisition. Together, these results reveal a flexible corticostriatal inhibitory synaptic plasticity mechanism that accompanies associative auditory learning.

Impaired plasticity of intrinsic excitability in the dentate gyrus alters spike transfer in a mouse model of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by cognitive decline related to deficits in synaptic transmission and plasticity. We report in APP/PS1 mice, a double transgenic mouse model of AD, that females displayed an early burden of Aβ plaques load in the stratum moleculare of the dentate gyrus (DG) together with prominent neuroinflammatory activation of astrocytes and microglia. Robust deficits in hippocampus-dependent memory tasks were observed in APP/PS1 female mice as early as 3 months of age. We then studied the functional properties of the lateral perforant path (LPP) to DG granule cells. Remarkably DG granule cells displayed higher intrinsic excitability in APP/PS1 female mice. We showed that the long term potentiation of population spike amplitude induced by high frequency stimulation (HFS) at LPP-DG granule cells synapse is impaired in APP/PS1 female mice. HFS induced plasticity of intrinsic excitability in DG granule cells without inducing noticeable modification of synaptic strength. Furthermore, the enhanced intrinsic excitability was potentiated to a greater extent in APP/PS1 as compared to control mice following HFS. Our study shows that changes in the intrinsic excitability of DG granule cells in AD contribute to the dysfunctional transfer of information from the entorhinal cortex to the hippocampus.

Reelin signaling modulates GABAB receptor function in the neocortex

Reelin is a protein that is best known for its role in controlling neuronal layer formation in the developing cortex. Here, we studied its role for post-natal cortical network function, which is poorly explored. To preclude early cortical migration defects caused by Reelin deficiency, we used a conditional Reelin knock-out (RelncKO ) mouse, and induced Reelin deficiency post-natally. Induced Reelin deficiency caused hyperexcitability of the neocortical network in vitro and ex vivo. Blocking Reelin binding to its receptors ApoER2 and VLDLR resulted in a similar effect. Hyperexcitability in RelncKO organotypic slice cultures could be rescued by co-culture with wild-type organotypic slice cultures. Moreover, the GABAB receptor (GABAB R) agonist baclofen failed to activate and the antagonist CGP35348 failed to block GABAB Rs in RelncKO mice. Immunolabeling of RelncKO cortical slices revealed a reduction in GABAB R1 and GABAB R2 surface expression at the plasma membrane and western blot of RelncKO cortical tissue revealed decreased phosphorylation of the GABAB R2 subunit at serine 892 and increased phosphorylation at serine 783, reflecting receptor deactivation and proteolysis. These data show a role of Reelin in controlling early network activity, by modulating GABAB R function. Cover Image for this issue: https://doi.org/10.1111/jnc.15054.

Probiotic treatment differentially affects the behavioral and electrophysiological aspects in ethanol exposed animals

OBJECTIVES

Harmful effects of alcohol on brain function including cognitive phenomena are well known. Damage to gut microbiota is linked to neurological disorders. Evidence indicates that intestinal flora can be strengthened by probiotic bacteria. In this study, we evaluated the effect of probiotics administration on LTP induction in rats receiving ethanol.

MATERIALS AND METHODS

To assess if probiotic treatment influences toxic effect of ethanol, vehicle (CON) and probiotic treated (CON+PRO) control rats, and chronic ethanol (CE) exposed and CE probiotic treated (CE+PRO) animals were entered into the experiments. Shuttle box test and in vivo electrophysiological recordings were accomplished to evaluate memory and hippocampal baseline filed excitatory postsynaptic potentials (fEPSPs) and long term potentiation (LTP), respectively.

RESULTS

Ethanol impaired memory in the CE rats. It also diminished the slope size of fEPSPs and prevented LTP induction. While the probiotic supplementation improved memory in the CE+PRO rats, it did not influence synaptic transmission in these animals.

CONCLUSION

Conclusively, behavioral but not electrophysiological aspect of cognition is sensitive to probiotic treatment in the ethanol exposed animals.

GABAB receptors: modulation of thalamocortical dynamics and synaptic plasticity

GABA_B-receptors (GABA_B-Rs) are metabotropic, G protein-coupled receptors for the neurotransmitter GABA. Their activation induces slow inhibitory control of the neuronal excitability mediated by pre- and postsynaptic inhibition. Presynaptically GABA_B-Rs reduce GABA and glutamate release inhibiting presynaptic Ca²⁺ channels in both inhibitory and excitatory synapses while postsynaptic GABA_B-Rs induce robust slow hyperpolarization by the activation of K⁺ channels. GABA_B-Rs are activated by non-synaptic or volume transmission, which requires high levels of GABA release, either by the simultaneous discharge of GABAergic interneurons or very intense discharges in the thalamus or by means of the activation of a neurogliaform interneurons in the cortex. The main receptor subunits GABA_B1a, GABA_B1b and GABA_B2 are strongly expressed in neurons and glial cells throughout the central nervous system and GABA_B-R activation is related to many neuronal processes such as the modulation of rhythmic activity in several brain regions. In the thalamus, GABA_B-Rs modulate the generation of the main thalamic rhythm, spindle waves. In the cerebral cortex, GABA_B-Rs also modulate the most prominent emergent oscillatory activity-slow oscillations-as well as faster oscillations like gamma frequency. Further, recent studies evaluating the complexity expressed by the cortical network, a parameter associated with consciousness levels, have found that GABA_B-Rs enhance this complexity, while their blockade decreases it. This review summarizes the current results on how the activation of GABA_B-Rs affects the interchange of information between brain areas by controlling rhythmicity as well as synaptic plasticity.

Neural activity related to volitional regulation of cortical excitability

To date there exists no reliable method to non-invasively upregulate or downregulate the state of the resting human motor system over a large dynamic range. Here we show that an operant conditioning paradigm which provides neurofeedback of the size of motor evoked potentials (MEPs) in response to transcranial magnetic stimulation (TMS), enables participants to self-modulate their own brain state. Following training, participants were able to robustly increase (by 83.8%) and decrease (by 30.6%) their MEP amplitudes. This volitional up-versus down-regulation of corticomotor excitability caused an increase of late-cortical disinhibition (LCD), a TMS derived read-out of presynaptic GABAB disinhibition, which was accompanied by an increase of gamma and a decrease of alpha oscillations in the trained hemisphere. This approach paves the way for future investigations into how altered brain state influences motor neurophysiology and recovery of function in a neurorehabilitation context.

β-adrenergic receptors reduce the threshold for induction and stabilization of LTP and enhance its magnitude via multiple mechanisms in the ventral but not the dorsal hippocampus

The hippocampus is a functionally heterogeneous structure with the cognitive and emotional signal processing ascribed to the dorsal (DH) and the ventral hippocampus (VH) respectively. However, the underlying mechanisms are poorly understood. Noradrenaline is released in hippocampus during emotional arousal modulating synaptic plasticity and memory consolidation through activation of β adrenergic receptors (β-ARs). Using recordings of field excitatory postsynaptic potentials from the CA1 field of adult rat hippocampal slices we demonstrate that long-term potentiation (LTP) induced either by theta-burst stimulation (TBS) that mimics a physiological firing pattern of hippocampal neurons or by high-frequency stimulation is remarkably more sensitive to β-AR activation in VH than in DH. Thus, pairing of subthreshold primed burst stimulation with activation of β-ARs by their agonist isoproterenol (1 μM) resulted in a reliable induction of NMDA receptor-dependent LTP in the VH without affecting LTP in the DH. Activation of β-ARs by isoproterenol during application of intense TBS increased the magnitude of LTP in both hippocampal segments but facilitated voltage-gated calcium channel-dependent LTP in VH only. Endogenous β-AR activation contributed to the stabilization and the magnitude of LTP in VH but not DH as demonstrated by the effects of the β-ARs antagonist propranolol (10 μM). Exogenous (but not endogenous) β-AR activation strongly increased TBS-induced facilitation of postsynaptic excitability in VH. In DH, isoproterenol only produced a moderate and GABAergic inhibition-dependent enhancement in the facilitation of synaptic burst responses. Paired-pulse facilitation did not change with LTP at any experimental condition suggesting that expression of LTP does not involve presynaptic mechanisms. These findings suggest that β-AR may act as a switch that selectively promotes synaptic plasticity in VH through multiple ways and provide thus a first clue to mechanisms that underlie VH involvement in emotionality.

Neurophysiological and cognitive impairment following repeated sports concussion injuries in retired professional rugby league players

BACKGROUND

Concussion is regarded as a common injury in rugby league, however no studies have explored the long-term neurophysiological and cognitive effects of repeated concussion injuries in this sport.

METHODS

Former professional rugby athletes (n = 25) were compared to 25 age-matched participants with no history of a concussion. All participants completed standardised motor dexterity, reaction time, and cognitive tasks for working memory, associative learning and rule acquisition and reversal. Single-pulse transcranial magnetic stimulation (TMS) acquired motor evoked potentials and cortical silent period (cSP), as well as paired-pulse TMS for short latency intracortical inhibition and long intracortical inhibition (LICI).

RESULTS

Compared to controls, dexterity and visuomotor reaction time was slower in the rugby group compared to controls (p = 0.02, p < 0.01, respectively). The rugby group also demonstrated poorer cognitive performance than controls (p range 0.02 to < 0.01). TMS revealed significantly reduced cSP at suprathreshold stimulation intensities (p range 0.02 to <0.01), and increased LICI (p = 0.03) in the rugby group.

DISCUSSION

These findings of motor and cognitive changes, along with neurophysiological alterations, particularly with intracortical inhibition, nearly two decades post-concussion provides evidence for long-term sequelae for athletes with a history of repeated head trauma in contact sports.

Impact of different intensities of intermittent theta burst stimulation on the cortical properties during TMS-EEG and working memory performance

Intermittent theta burst stimulation (iTBS) is a noninvasive brain stimulation technique capable of increasing cortical excitability beyond the stimulation period. Due to the rapid induction of modulatory effects, prefrontal application of iTBS is gaining popularity as a therapeutic tool for psychiatric disorders such as depression. In an attempt to increase efficacy, higher than conventional intensities are currently being applied. The assumption that this increases neuromodulatory may be mechanistically false for iTBS. This study examined the influence of intensity on the neurophysiological and behavioural effects of iTBS in the prefrontal cortex. Sixteen healthy participants received iTBS over prefrontal cortex at either 50, 75 or 100% resting motor threshold in separate sessions. Single-pulse TMS and concurrent electroencephalography (EEG) was used to assess changes in cortical reactivity measured as TMS-evoked potentials and oscillations. The n-back task was used to assess changes in working memory performance. The data can be summarised as an inverse U-shape relationship between intensity and iTBS plastic effects, where 75% iTBS yielded the largest neurophysiological changes. Improvement in reaction time in the 3-back task was supported by the change in alpha power, however, comparison between conditions revealed no significant differences. The assumption that higher intensity results in greater neuromodulatory effects may be false, at least in healthy individuals, and should be carefully considered for clinical populations. Neurophysiological changes associated with working memory following iTBS suggest functional relevance. However, the effects of different intensities on behavioural performance remain elusive in the present healthy sample.

Hippocampal GABAergic Inhibitory Interneurons

In the hippocampus GABAergic local circuit inhibitory interneurons represent only ~10-15% of the total neuronal population; however, their remarkable anatomical and physiological diversity allows them to regulate virtually all aspects of cellular and circuit function. Here we provide an overview of the current state of the field of interneuron research, focusing largely on the hippocampus. We discuss recent advances related to the various cell types, including their development and maturation, expression of subtype-specific voltage- and ligand-gated channels, and their roles in network oscillations. We also discuss recent technological advances and approaches that have permitted high-resolution, subtype-specific examination of their roles in numerous neural circuit disorders and the emerging therapeutic strategies to ameliorate such pathophysiological conditions. The ultimate goal of this review is not only to provide a touchstone for the current state of the field, but to help pave the way for future research by highlighting where gaps in our knowledge exist and how a complete appreciation of their roles will aid in future therapeutic strategies.

Inhibition shapes the organization of hippocampal representations

Hippocampal neurons become tuned to stimuli that predict behaviorally salient outcomes. This plasticity suggests that memory formation depends upon shifts in how different anatomical inputs can drive hippocampal activity. Here, I present evidence that inhibitory neurons can provide such a mechanism for learning-related changes in the tuning of pyramidal cells. Inhibitory currents arriving on the dendrites of pyramidal cells determine whether an excitatory input can drive action potential output. Specificity and plasticity of this dendritic modulation allows for precise, modifiable changes in how afferent inputs are integrated, a process that defines a neuron's receptive field. In addition, feedback inhibition plays a fundamental role in biasing which excitatory neurons may be co-active. By defining the rules of synchrony and the rules of input integration, interneurons likely play an important role in the organization of memory representation within the hippocampus.

GABAergic inhibition gates excitatory LTP in perirhinal cortex

The perirhinal cortex (PRh) is a key region downstream of auditory cortex (ACx) that processes familiarity linked mnemonic signaling. In gerbils, ACx-driven EPSPs recorded in PRh neurons are largely shunted by GABAergic inhibition (Kotak et al., 2015, Frontiers in Neural Circuits, 9). To determine whether inhibitory shunting prevents the induction of excitatory long-term potentiation (e-LTP), we stimulated ACx-recipient PRh in a brain slice preparation using theta burst stimulation (TBS). Under control conditions, without GABA blockers, the majority of PRh neurons exhibited long-term depression. A very low concentration of bicuculline increased EPSP amplitude, but under this condition TBS did not significantly increase e-LTP induction. Since PRh synaptic inhibition included a GABA_B receptor-mediated component, we added a GABA_B receptor antagonist. When both GABA_A and GABA_B receptors were blocked, TBS reliably induced e-LTP in a majority of PRh neurons. We conclude that GABAergic transmission is a vital mechanism regulating e-LTP induction in the PRh, and may be associated with auditory learning.

The Small GTPase Rac1 Contributes to Extinction of Aversive Memories of Drug Withdrawal by Facilitating GABAA Receptor Endocytosis in the vmPFC

Extinction of aversive memories has been a major concern in neuropsychiatric disorders, such as anxiety disorders and drug addiction. However, the mechanisms underlying extinction of aversive memories are not fully understood. Here, we report that extinction of conditioned place aversion (CPA) to naloxone-precipitated opiate withdrawal in male rats activates Rho GTPase Rac1 in the ventromedial prefrontal cortex (vmPFC) in a BDNF-dependent manner, which determines GABA_A receptor (GABA_AR) endocytosis via triggering synaptic translocation of activity-regulated cytoskeleton-associated protein (Arc) through facilitating actin polymerization. Active Rac1 is essential and sufficient for GABA_AR endocytosis and CPA extinction. Knockdown of Rac1 expression within the vmPFC of rats using Rac1-shRNA suppressed GABA_AR endocytosis and CPA extinction, whereas expression of a constitutively active form of Rac1 accelerated GABA_AR endocytosis and CPA extinction. The crucial role of GABA_AR endocytosis in the LTP induction and CPA extinction is evinced by the findings that blockade of GABA_AR endocytosis by a dynamin function-blocking peptide (Myr-P4) abolishes LTP induction and CPA extinction. Thus, the present study provides first evidence that Rac1-dependent GABA_AR endocytosis plays a crucial role in extinction of aversive memories and reveals the sequence of molecular events that contribute to learning experience modulation of synaptic GABA_AR endocytosis.SIGNIFICANCE STATEMENT This study reveals that Rac1-dependent GABA_AR endocytosis plays a crucial role in extinction of aversive memories associated with drug withdrawal and identifies Arc as a downstream effector of Rac1 regulations of synaptic plasticity as well as learning and memory, thereby suggesting therapeutic targets to promote extinction of the unwanted memories.

Differential Regulation of NMDA Receptor-Mediated Transmission by SK Channels Underlies Dorsal-Ventral Differences in Dynamics of Schaffer Collateral Synaptic Function

Behavioral, physiological, and anatomical evidence indicates that the dorsal and ventral zones of the hippocampus have distinct roles in cognition. How the unique functions of these zones might depend on differences in synaptic and neuronal function arising from the strikingly different gene expression profiles exhibited by dorsal and ventral CA1 pyramidal cells is unclear. To begin to address this question, we investigated the mechanisms underlying differences in synaptic transmission and plasticity at dorsal and ventral Schaffer collateral (SC) synapses in the mouse hippocampus. We find that, although basal synaptic transmission is similar, SC synapses in the dorsal and ventral hippocampus exhibit markedly different responses to θ frequency patterns of stimulation. In contrast to dorsal hippocampus, θ frequency stimulation fails to elicit postsynaptic complex-spike bursting and does not induce LTP at ventral SC synapses. Moreover, EPSP-spike coupling, a process that strongly influences information transfer at synapses, is weaker in ventral pyramidal cells. Our results indicate that all these differences in postsynaptic function are due to an enhanced activation of SK-type K⁺ channels that suppresses NMDAR-dependent EPSP amplification at ventral SC synapses. Consistent with this, mRNA levels for the SK3 subunit of SK channels are significantly higher in ventral CA1 pyramidal cells. Together, our findings indicate that a dorsal-ventral difference in SK channel regulation of NMDAR activation has a profound effect on the transmission, processing, and storage of information at SC synapses and thus likely contributes to the distinct roles of the dorsal and ventral hippocampus in different behaviors.SIGNIFICANCE STATEMENT Differences in short- and long-term plasticity at Schaffer collateral (SC) synapses in the dorsal and ventral hippocampus likely contribute importantly to the distinct roles of these regions in cognition and behavior. Although dorsal and ventral CA1 pyramidal cells exhibit markedly different gene expression profiles, how these differences influence plasticity at SC synapses is unclear. Here we report that increased mRNA levels for the SK3 subunit of SK-type K⁺ channels in ventral pyramidal cells is associated with an enhanced activation of SK channels that strongly suppresses NMDAR activation at ventral SC synapses. This leads to striking differences in multiple aspects of synaptic transmission at dorsal and ventral SC synapses and underlies the reduced ability of ventral SC synapses to undergo LTP.

Synaptic plasticity and spatial working memory are impaired in the CD mouse model of Williams-Beuren syndrome

Mice heterozygous for a complete deletion (CD) equivalent to the most common deletion found in individuals with Williams-Beuren syndrome (WBS) recapitulate relevant features of the neurocognitive phenotype, such as hypersociability, along with some neuroanatomical alterations in specific brain areas. However, the pathophysiological mechanisms underlying these phenotypes still remain largely unknown. We have studied the synaptic function and cognition in CD mice using hippocampal slices and a behavioral test sensitive to hippocampal function. We have found that long-term potentiation (LTP) elicited by theta burst stimulation (TBS) was significantly impaired in hippocampal field CA1 of CD animals. This deficit might be associated with the observed alterations in spatial working memory. However, we did not detect changes in presynaptic function, LTP induction mechanisms or AMPA and NMDA receptor function. Reduced levels of Brain-derived neurotrophic factor (BDNF) were present in the CA1-CA3 hippocampal region of CD mice, which could account for LTP deficits in these mice. Taken together, these results suggest a defect of CA1 synapses in CD mice to sustain synaptic strength after stimulation. These data represent the first description of synaptic functional deficits in CD mice and further highlights the utility of the CD model to study the mechanisms underlying the WBS neurocognitive profile.

REVIEW ■ : GABAergic Inhibitory Processes and Hippocampal Long-term Potentiation

Long-term potentiation (LTP) is a widely studied form of activity-dependent synaptic plasticity. Hippocampal LTP evoked in the dentate and CA1 areas requires calcium influx through N-methyl-D-aspartate (NMDA) receptor-channel complexes, a process triggered during high-frequency stimulation by conjunctive presy naptic glutamate release and postsynaptic depolarization. It has been suggested that alterations in GABAergic recurrent and/or feedforward inhibitory synaptic transmission may accompany LTP induction in these hippocampal areas. To this end, possible LTP-related modifications in functional inhibition are ad dressed in the context of both paired-pulse depression and the excitatory postsynaptic potential-spike (E-S) relationship. Consideration is also given as to how GABAergic processes may contribute mechanistically to the induction of NMDA receptor-dependent LTP. It is concluded that although GABAergic disinhibition may contribute to the induction of LTP, it is not yet clear whether or not the induction of LTP has a lasting impact on inhibitory processes. NEUROSCIENTIST 3:226-236, 1997

GABAB receptor-mediated, layer-specific synaptic plasticity reorganizes gamma-frequency neocortical response to stimulation

Repeated presentations of sensory stimuli generate transient gamma-frequency (30-80 Hz) responses in neocortex that show plasticity in a task-dependent manner. Complex relationships between individual neuronal outputs and the mean, local field potential (population activity) accompany these changes, but little is known about the underlying mechanisms responsible. Here we show that transient stimulation of input layer 4 sufficient to generate gamma oscillations induced two different, lamina-specific plastic processes that correlated with lamina-specific changes in responses to further, repeated stimulation: Unit rates and recruitment showed overall enhancement in supragranular layers and suppression in infragranular layers associated with excitatory or inhibitory synaptic potentiation onto principal cells, respectively. Both synaptic processes were critically dependent on activation of GABAB receptors and, together, appeared to temporally segregate the cortical representation. These data suggest that adaptation to repetitive sensory input dramatically alters the spatiotemporal properties of the neocortical response in a manner that may both refine and minimize cortical output simultaneously.

Associative spike timing-dependent potentiation of the basal dendritic excitatory synapses in the hippocampus in vivo

Spike timing-dependent plasticity in the hippocampus has rarely been studied in vivo. Using extracellular potential and current source density analysis in urethane-anesthetized adult rats, we studied synaptic plasticity at the basal dendritic excitatory synapse in CA1 after excitation-spike (ES) pairing; E was a weak basal dendritic excitation evoked by stratum oriens stimulation, and S was a population spike evoked by stratum radiatum apical dendritic excitation. We hypothesize that positive ES pairing-generating synaptic excitation before a spike-results in long-term potentiation (LTP) while negative ES pairing results in long-term depression (LTD). Pairing (50 pairs at 5 Hz) at ES intervals of -10 to 0 ms resulted in significant input-specific LTP of the basal dendritic excitatory sink, lasting 60-120 min. Pairing at +10- to +20-ms ES intervals, or unpaired 5-Hz stimulation, did not induce significant basal dendritic or apical dendritic LTP or LTD. No basal dendritic LTD was found after stimulation of stratum oriens with 200 pairs of high-intensity pulses at 25-ms interval. Pairing-induced LTP was abolished by pretreatment with an N-methyl-d-aspartate receptor antagonist, 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), which also reduced spike bursting during 5-Hz pairing. Pairing at 0.5 Hz did not induce spike bursts or basal dendritic LTP. In conclusion, ES pairing at 5 Hz resulted in input-specific basal dendritic LTP at ES intervals of -10 ms to 0 ms but no LTD at ES intervals of -20 to +20 ms. Associative LTP likely occurred because of theta-rhythmic coincidence of subthreshold excitation with a backpropagated spike burst, which are conditions that can occur naturally in the hippocampus.

Neural plasticity and behavior - sixty years of conceptual advances

This brief review summarizes 60 years of conceptual advances that have demonstrated a role for active changes in neuronal connectivity as a controller of behavior and behavioral change. Seminal studies in the first phase of the six-decade span of this review firmly established the cellular basis of behavior - a concept that we take for granted now, but which was an open question at the time. Hebbian plasticity, including long-term potentiation and long-term depression, was then discovered as being important for local circuit refinement in the context of memory formation and behavioral change and stabilization in the mammalian central nervous system. Direct demonstration of plasticity of neuronal circuit function in vivo, for example, hippocampal neurons forming place cell firing patterns, extended this concept. However, additional neurophysiologic and computational studies demonstrated that circuit development and stabilization additionally relies on non-Hebbian, homoeostatic, forms of plasticity, such as synaptic scaling and control of membrane intrinsic properties. Activity-dependent neurodevelopment was found to be associated with cell-wide adjustments in post-synaptic receptor density, and found to occur in conjunction with synaptic pruning. Pioneering cellular neurophysiologic studies demonstrated the critical roles of transmembrane signal transduction, NMDA receptor regulation, regulation of neural membrane biophysical properties, and back-propagating action potential in critical time-dependent coincidence detection in behavior-modifying circuits. Concerning the molecular mechanisms underlying these processes, regulation of gene transcription was found to serve as a bridge between experience and behavioral change, closing the 'nature versus nurture' divide. Both active DNA (de)methylation and regulation of chromatin structure have been validated as crucial regulators of gene transcription during learning. The discovery of protein synthesis dependence on the acquisition of behavioral change was an influential discovery in the neurochemistry of behavioral modification. Higher order cognitive functions such as decision making and spatial and language learning were also discovered to hinge on neural plasticity mechanisms. The role of disruption of these processes in intellectual disabilities, memory disorders, and drug addiction has recently been clarified based on modern genetic techniques, including in the human. The area of neural plasticity and behavior has seen tremendous advances over the last six decades, with many of those advances being specifically in the neurochemistry domain. This review provides an overview of the progress in the area of neuroplasticity and behavior over the life-span of the Journal of Neurochemistry. To organize the broad literature base, the review collates progress into fifteen broad categories identified as 'conceptual advances', as viewed by the author. The fifteen areas are delineated in the figure above. This article is part of the 60th Anniversary special issue.

Role of GABA(B) receptors in learning and memory and neurological disorders

Although it is evident from the literature that altered GABAB receptor function does affect behavior, these results often do not correspond well. These differences could be due to the task protocol, animal strain, ligand concentration, or timing of administration utilized. Because several clinical populations exhibit learning and memory deficits in addition to altered markers of GABA and the GABAB receptor, it is important to determine whether altered GABAB receptor function is capable of contributing to the deficits. The aim of this review is to examine the effect of altered GABAB receptor function on synaptic plasticity as demonstrated by in vitro data, as well as the effects on performance in learning and memory tasks. Finally, data regarding altered GABA and GABAB receptor markers within clinical populations will be reviewed. Together, the data agree that proper functioning of GABAB receptors is crucial for numerous learning and memory tasks and that targeting this system via pharmaceuticals may benefit several clinical populations.

Theta-burst LTP

This review covers the spatial and temporal rules governing induction of hippocampal long-term potentiation (LTP) by theta-burst stimulation. Induction of LTP in field CA1 by high frequency stimulation bursts that resemble the burst discharges (complex-spikes) of hippocampal pyramidal neurons involves a multiple-step mechanism. A single burst is insufficient for LTP induction because it evokes both excitatory and inhibitory currents that partially cancel and limit postsynaptic depolarization. Bursts repeated at the frequency (~5 Hz) of the endogenous theta rhythm induce maximal LTP, primarily because this frequency disables feed-forward inhibition and allows sufficient postsynaptic depolarization to activate voltage-sensitive NMDA receptors. The disinhibitory process, referred to as "priming", involves presynaptic GABA autoreceptors that inhibit GABA release. Activation of NMDA receptors allows a calcium flux into dendritic spines that serves as the proximal trigger for LTP. We include new data showing that theta-burst stimulation is more efficient than other forms of stimulation for LTP induction. In addition, we demonstrate that associative interactions between synapses activated during theta-bursts are limited to major dendritic domains since such interactions occur within apical or basal dendritic trees but not between them. We review evidence that recordings of electrophysiological responses during theta burst stimulation can help to determine if experimental manipulations that affect LTP do so by affecting events antecedent to the induction process, such as NMDA receptor activation, or downstream signaling cascades that result from postsynaptic calcium fluxes. Finally, we argue that theta-burst LTP represents a minimal model for stable, non-decremental LTP that is more sensitive to a variety of experimental manipulations than is LTP induced by other stimulation paradigms. This article is part of a Special Issue entitled SI: Brain and Memory.

Cortical Synaptic Inhibition Declines during Auditory Learning

Auditory learning is associated with an enhanced representation of acoustic cues in primary auditory cortex, and modulation of inhibitory strength is causally involved in learning. If this inhibitory plasticity is associated with task learning and improvement, its expression should emerge and persist until task proficiency is achieved. We tested this idea by measuring changes to cortical inhibitory synaptic transmission as adult gerbils progressed through the process of associative learning and perceptual improvement. Using either of two procedures, aversive or appetitive conditioning, animals were trained to detect amplitude-modulated noise and then tested daily. Following each training session, a thalamocortical brain slice was generated, and inhibitory synaptic properties were recorded from layer 2/3 pyramidal neurons. Initial associative learning was accompanied by a profound reduction in the amplitude of spontaneous IPSCs (sIPSCs). However, sIPSC amplitude returned to control levels when animals reached asymptotic behavioral performance. In contrast, paired-pulse ratios decreased in trained animals as well as in control animals that experienced unpaired conditioned and unconditioned stimuli. This latter observation suggests that inhibitory release properties are modified during behavioral conditioning, even when an association between the sound and reinforcement cannot occur. These results suggest that associative learning is accompanied by a reduction of postsynaptic inhibitory strength that persists for several days during learning and perceptual improvement.

GABAB receptor subtypes differentially modulate synaptic inhibition in the dentate gyrus to enhance granule cell output

BACKGROUND AND PURPOSE

Activation of GABAB receptors in the dentate gyrus (DG) enhances granule cell (GC) activity by reducing synaptic inhibition imposed by hilar interneurons. This disinhibitory action facilitates signal transfer from the perforant path to the hippocampus. However, as the two main molecular subtypes, GABA(B(1a,2)) and GABA(B(1b,2)) receptors, prefer axonal terminal and dendritic compartments, respectively, they may modulate the hilar pathways at different synaptic localizations. We examined their relative expression and functions in the DG.

EXPERIMENTAL APPROACH

The localization of GABAB subtypes was revealed immunohistochemically using subunit-selective antibodies in GABA(B1a)(-/-) and GABA(B1b)(-/-) mice. Effects of subtype activation by the GABAB receptor agonist, baclofen, were examined on the perforant path-stimulated GC population activities in brain slices.

KEY RESULTS

GABA(B(1a,2)) receptors were concentrated in the inner molecular layer, the neuropil of the hilus and hilar neurons at the border zone; while GABA(B(1b,2)) receptors dominated the outer molecular layer and hilar neurons in the deep layer, showing their differential localization on GC dendrite and in the hilus. Baclofen enhanced the GC population spike to a larger extent in the GABA(B1b)(-/-) mice, demonstrating exclusively disinhibitory roles of the GABA(B(1a,2)) receptors. Conversely, in the GABA(B1a)(-/-) mice baclofen not only enhanced but also inhibited the population spike during GABAA blockade, revealing both disinhibitory and inhibitory effects of GABA(B(1b,2)) receptors.

CONCLUSIONS AND IMPLICATIONS

The GABA(B(1a,2)) and GABA(B(1b,2)) receptor subtypes differentially modulate GC outputs via selective axonal terminal and dendritic locations in the hilar pathways. The GABA(B(1a,2)) receptors exclusively mediate disinhibition, thereby playing a greater role in gating signal transfer for hippocampal spatial and pattern learning.

Altered neuronal excitability underlies impaired hippocampal function in an animal model of psychosis

Psychosis is accompanied by severe attentional deficits, and impairments in associational-memory processing and sensory information processing that are ascribed to dysfunctions in prefrontal and hippocampal function. Disruptions of glutamatergic signaling may underlie these alterations: Antagonism of the N-methyl-D-aspartate receptor (NMDAR) results in similar molecular, cellular, cognitive and behavioral changes in rodents and/or humans as those that occur in psychosis, raising the question as to whether changes in glutamatergic transmission may be intrinsic to the pathophysiology of the disease. In an animal model of psychosis that comprises treatment with the irreversible NMDAR-antagonist, MK801, we explored the cellular mechanisms that may underlie hippocampal dysfunction in psychosis. MK801-treatment resulted in a profound loss of hippocampal LTP that was evident 4 weeks after treatment. Whereas neuronal expression of the immediate early gene, Arc, was enhanced in the hippocampus by spatial learning in controls, MK801-treated animals failed to show activity-dependent increases in Arc expression. By contrast, a significant increase in basal Arc expression in the absence of learning was evident compared to controls. Paired-pulse (PP) facilitation was increased at the 40 ms interval indicating that NMDAR and/or fast GABAergic-mediated neurotransmission was disrupted. In line with this, MK801-treatment resulted in a significant decrease in GABA(A), and increase in GABA(B)-receptor-expression in PFC, along with a significant increase of GABA(B)- and NMDAR-GluN2B expression in the dentate gyrus. NMDAR-GluN1 or GluN2A subunit expression was unchanged. These data suggest that in psychosis, deficits in hippocampus-dependent memory may be caused by a loss of hippocampal LTP that arises through enhanced hippocampal neuronal excitability, altered GluN2B and GABA receptor expression and an uncoupling of the hippocampus-prefrontal cortex circuitry.

Frequency-Dependent Gating of Hippocampal-Neocortical Interactions

How and where hippocampal-neocortical interactions required for memory formation take place is a major issue of current research. Using a combined in vivo functional magnetic resonance imaging/electrophysiology approach, we have investigated whether specific frequencies of CA3 neuronal activation, inducing different forms of short-term plasticity at CA1 synapses, contribute to differential activity propagation in brain-wide networks connected to the hippocampus. We report that localized activation of CA3 neurons in dorsal hippocampus produced activity propagation within the hippocampal formation, including the subiculum and entorhinal cortex, which increased monotonically with frequency to a maximum at 20-40 Hz. However, robust extrahippocampal propagation was seen specifically at theta-beta frequencies (10-20 Hz), reaching a network of midline neocortical and mesolimbic structures. Activation in those regions correlated with a frequency-dependent facilitation of spiking activity recorded in CA1. These results provide a mechanistic link between the dynamic properties of short-term plasticity in the efferent synapses of CA3 neurons in CA1 and activity propagation in brain-wide networks, and identify polysynaptic information channels segregated in the frequency domain.

Augmenting Plasticity Induction in Human Motor Cortex by Disinhibition Stimulation

Cellular studies showed that disinhibition, evoked pharmacologically or by a suitably timed priming stimulus, can augment long-term plasticity (LTP) induction. We demonstrated previously that transcranial magnetic stimulation evokes a period of presumably GABA(B)ergic late cortical disinhibition (LCD) in human primary motor cortex (M1). Here, we hypothesized that, in keeping with cellular studies, LCD can augment LTP-like plasticity in humans. In Experiment 1, patterned repetitive TMS was applied to left M1, consisting of 6 trains (intertrain interval, 8 s) of 4 doublets (interpulse interval equal to individual peak I-wave facilitation, 1.3-1.5 ms) spaced by the individual peak LCD (interdoublet interval (IDI), 200-250 ms). This intervention (total of 48 pulses applied over ∼45 s) increased motor-evoked potential amplitude, a marker of corticospinal excitability, in a right hand muscle by 147% ± 4%. Control experiments showed that IDIs shorter or longer than LCD did not result in LTP-like plasticity. Experiment 2 indicated topographic specificity to the M1 hand region stimulated by TMS and duration of the LTP-like plasticity of 60 min. In conclusion, GABA(B)ergic LCD offers a powerful new approach for augmenting LTP-like plasticity induction in human cortex. We refer to this protocol as disinhibition stimulation (DIS).

The long-term effects of sports concussion on retired Australian football players: a study using transcranial magnetic stimulation

This study investigated corticomotor excitability and inhibition, cognitive functioning, and fine motor dexterity in retired elite and amateur Australian football (AF) players who had sustained concussions during their playing careers. Forty male AF players who played at the elite level (n=20; mean age 49.7±5.7 years) or amateur level (n=20; mean age 48.4±6.9 years), and had sustained on average 3.2 concussions 21.9 years previously, were compared with 20 healthy age-matched male controls (mean age 47.56±6.85 years). All participants completed assessments of fine dexterity, visuomotor reaction time, spatial working memory (SWM), and associative learning (AL). Transcranial magnetic stimulation (TMS) was used to measure corticospinal excitability: stimulus-response (SR) curves and motor evoked potential (MEP) 125% of active motor threshold (aMT); and intracortical inhibition: cortical silent period (cSP), short-interval intracortical inhibition (SICI), and long-interval intracortical inhibition (LICI). Healthy participants performed better in dexterity (p=0.003), reaction (p=0.003), and movement time (p=0.037) than did both AF groups. Differences between AF groups were found in AL (p=0.027) and SWM (p=0.024). TMS measures revealed that both AF groups showed reduced cSP duration at 125% aMT (p>0.001) and differences in SR curves (p>0.001) than did healthy controls. Similarly, SICI (p=0.012) and LICI (p=0.009) were reduced in both AF groups compared with controls. Regression analyses revealed a significant contribution to differences in motor outcomes with the three measures of intracortical inhibition. The measures of inhibition differed, however, in terms of which performance measure they had a significant and unique predictive relationship with, reflecting the variety of participant concussion injuries. This study is the first to demonstrate differences in motor control and intracortical inhibition in AF players who had sustained concussions during their playing career two decades previously.

Dysbindin-1 loss compromises NMDAR-dependent synaptic plasticity and contextual fear conditioning

Genetic variants in DTNBP1 encoding the protein dysbindin-1 have often been associated with schizophrenia and with the cognitive deficits prominent in that disorder. Because impaired function of the hippocampus is thought to play a role in these memory deficits and because NMDAR-dependent synaptic plasticity in this region is a proposed biological substrate for some hippocampal-dependent memory functions in schizophrenia, we hypothesized that reduced dysbindin-1 expression would lead to impairments in NMDAR-dependent synaptic plasticity and in contextual fear conditioning. Acute slices from male mice carrying 0, 1, or 2 null mutant alleles of the Dtnbp1 gene were prepared, and field recordings from the CA1 striatum radiatum were obtained before and after tetanization of Schaffer collaterals of CA3 pyramidal cells. Mice homozygous for the null mutation in Dtnbp1 exhibited significantly reduced NMDAR-dependent synaptic potentiation compared to wild type mice, an effect that could be rescued by bath application of the NMDA receptor coagonist glycine (10 μM). Behavioral testing in adult mice revealed deficits in hippocampal memory processes. Homozygous null mice exhibited lower conditional freezing, without a change in the response to shock itself, indicative of a learning and memory deficit. Taken together, these results indicate that a loss of dysbindin-1 impairs hippocampal plasticity which may, in part, explain the role dysbindin-1 plays in the cognitive impairments of schizophrenia.