Synaptopodin is required for long-term depression at Schaffer collateral-CA1 synapses.

Synaptopodin (SP), an actin-associated protein found in telencephalic neurons, affects activity-dependant synaptic plasticity and dynamic changes of dendritic spines. While being required for long-term depression (LTD) mediated by metabotropic glutamate receptor (mGluR-LTD), little is known about its role in other forms of LTD induced by low frequency stimulation (LFS-LTD) or spike-timing dependent plasticity (STDP). Using electrophysiology in ex vivo hippocampal slices from SP-deficient mice (SPKO), we show that absence of SP is associated with a deficit of LTD at Sc-CA1 synapses induced by LFS-LTD and STDP. As LTD is known to require AMPA- receptors internalization and IP3-receptors calcium signaling, we tested by western blotting and immunochemistry if there were changes in their expression which we found to be reduced. While we were not able to induce LTD, long-term potentiation (LTP), albeit diminished in SPKO, can be recovered by using a stronger stimulation protocol. In SPKO we found no differences in NMDAR, which are the primary site of calcium signalling to induce LTP. Our study shows, for the first time, the key role of the requirement of SP to allow induction of activity-dependant LTD at Sc-CA1 synapses.

The 5HT2b Receptor in Alzheimer's Disease: Increased Levels in Patient Brains and Antagonist Attenuation of Amyloid and Tau Induced Dysfunction.

BACKGROUND: Background: Neurodegenerative diseases manifest behavioral dysfunction with disease progression. Intervention with neuropsychiatric drugs is part of most multi-drug treatment paradigms. However, only a fraction of patients responds to the treatments and those responding must deal with drug-drug interactions and tolerance issues generally attributed to off-target activities. Recent efforts have focused on the identification of underexplored targets and exploration of improved outcomes by treatment with selective molecular probes. Objective: As part of ongoing efforts to identify and validate additional targets amenable to therapeutic intervention, we examined levels of the serotonin 5-HT2b receptor (5-HT2bR) in Alzheimer's disease (AD) brains and the potential of a selective 5-HT2bR antagonist to counteract synaptic plasticity and memory damage induced by AD-related proteins, amyloid-ß, and tau. Methods: This work used a combination of biochemical, chemical biology, electrophysiological, and behavioral techniques. Biochemical methods included analysis of protein levels. Chemical biology methods included the use of an in vivo molecular probe MW071, a selective antagonist for the 5HT2bR. Electrophysiological methods included assessment of long-term potentiation (LTP), a type of synaptic plasticity thought to underlie memory formation. Behavioral studies investigated spatial memory and associative memory. Results: 5HT2bR levels are increased in brain specimens of AD patients compared to controls. 5HT2bR antagonist treatment rescued amyloid-ß and tau oligomer-induced impairment of synaptic plasticity and memory. Conclusions: The increased levels of 5HT-2bR in AD patient brains and the attenuation of disease-related synaptic and behavioral dysfunctions by MW071 treatment suggest that the 5HT-2bR is a molecular target worth pursuing as a potential therapeutic target.

Mismatch novelty exploration training shifts VPAC1 receptor-mediated modulation of hippocampal synaptic plasticity by endogenous VIP in male rats.

Novelty influences hippocampal-dependent memory through metaplasticity. Mismatch novelty detection activates the human hippocampal CA1 area and enhances rat hippocampal-dependent learning and exploration. Remarkably, mismatch novelty training (NT) also enhances rodent hippocampal synaptic plasticity while inhibition of VIP interneurons promotes rodent exploration. Since VIP, acting on VPAC1 receptors (Rs), restrains hippocampal LTP and depotentiation by modulating disinhibition, we now investigated the impact of NT on VPAC1 modulation of hippocampal synaptic plasticity in male Wistar rats. NT enhanced both CA1 hippocampal LTP and depotentiation unlike exploring an empty holeboard (HT) or a fixed configuration of objects (FT). Blocking VIP VPAC1Rs with PG 97269 (100 nM) enhanced both LTP and depotentiation in naïve animals, but this effect was less effective in NT rats. Altered endogenous VIP modulation of LTP was absent in animals exposed to the empty environment (HT). HT and FT animals showed mildly enhanced synaptic VPAC1R levels, but neither VIP nor VPAC1R levels were altered in NT animals. Conversely, NT enhanced the GluA1/GluA2 AMPAR ratio and gephyrin synaptic content but not PSD-95 excitatory synaptic marker. In conclusion, NT influences hippocampal synaptic plasticity by reshaping brain circuits modulating disinhibition and its control by VIP-expressing hippocampal interneurons while upregulation of VIP VPAC1Rs is associated with the maintenance of VIP control of LTP in FT and HT animals. This suggests VIP receptor ligands may be relevant to co-adjuvate cognitive recovery therapies in aging or epilepsy, where LTP/LTD imbalance occurs.

Postweaning Development Influences Endogenous VPAC1 Modulation of LTP Induced by Theta-Burst Stimulation: A Link to Maturation of the Hippocampal GABAergic System.

Long-term potentiation (LTP) induced by theta-burst stimulation (TBS) undergoes postweaning developmental changes partially linked to GABAergic circuit maturation. Endogenous vasoactive intestinal peptide (VIP) acting on its VPAC1 receptor strongly influences LTP induced by theta-burst stimulation (TBS), an effect dependent on GABAergic transmission. Although VPAC1 receptor levels are developmentally regulated during embryogenesis, their variation along postweaning development is unknown, as is the VPAC1 modulation of LTP or its relation to hippocampal GABAergic circuit maturation. As such, we investigated how VPAC1 modulation of LTP adjusts from weaning to adulthood along with GABAergic circuit maturation. As described, LTP induced by mild TBS (5 bursts, 4 pulses delivered at 100 Hz) was increasingly greater from weaning to adulthood. The influence of the VPAC1 receptor antagonist PG 97-269 (100 nM) on TBS-induced LTP was much larger in juvenile (3-week-old) than in young adult (6-7-week-old) or adult (12-week-old) rats. This effect was not associated with a developmental decrease in synaptic VPAC1 receptor levels. However, an increase in pre and post-synaptic GABAergic synaptic markers suggests an increase in the number of GABAergic synaptic contacts that is more prominent than the one observed in glutamatergic connections during this period. Conversely, endogenous VPAC2 receptor activation did not significantly influence TBS-induced LTP. VPAC2 receptor levels enhance pronouncedly during postweaning development, but not at synaptic sites. Given the involvement of VIP interneurons in several aspects of hippocampal-dependent learning, neurodevelopmental disorders, and epilepsy, this could provide important insights into the role of VIP modulation of hippocampal synaptic plasticity during normal and altered brain development potentially contributing to epileptogenesis.

Estrogen and testosterone secretion from the mouse brain.

Estrogen and testosterone are typically thought of as gonadal or adrenal derived steroids that cross the blood brain barrier to signal via both rapid nongenomic and slower genomic signalling pathways. Estrogen and testosterone signalling has been shown to drive interlinked behaviours such as social behaviours and cognition by binding to their cognate receptors in hypothalamic and forebrain nuclei. So far, acute brain slices have been used to study short-term actions of 17ß-estradiol, typically using electrophysiological measures. For example, these techniques have been used to investigate, nongenomic signalling by estrogen such as the estrogen modulation of long-term potentiation (LTP) in the hippocampus. Using a modified method that preserves the slice architecture, we show, for the first time, that acute coronal slices from the prefrontal cortex and from the hypothalamus maintained in aCSF over longer periods i.e. 24 h can be steroidogenic, increasing their secretion of testosterone and estrogen. We also show that the hypothalamic nuclei produce more estrogen and testosterone than the prefrontal cortex. Therefore, this extended acute slice system can be used to study the regulation of steroid production and secretion by discrete nuclei in the brain.

Heterosynaptic plasticity of the visuo-auditory projection requires cholecystokinin released from entorhinal cortex afferents.

The entorhinal cortex is involved in establishing enduring visuo-auditory associative memory in the neocortex. Here we explored the mechanisms underlying this synaptic plasticity related to projections from the visual and entorhinal cortices to the auditory cortex in mice using optogenetics of dual pathways. High-frequency laser stimulation (HFS laser) of the visuo-auditory projection did not induce long-term potentiation. However, after pairing with sound stimulus, the visuo-auditory inputs were potentiated following either infusion of cholecystokinin (CCK) or HFS laser of the entorhino-auditory CCK-expressing projection. Combining retrograde tracing and RNAscope in situ hybridization, we show that Cck expression is higher in entorhinal cortex neurons projecting to the auditory cortex than in those originating from the visual cortex. In the presence of CCK, potentiation in the neocortex occurred when the presynaptic input arrived 200 ms before postsynaptic firing, even after just five trials of pairing. Behaviorally, inactivation of the CCK+ projection from the entorhinal cortex to the auditory cortex blocked the formation of visuo-auditory associative memory. Our results indicate that neocortical visuo-auditory association is formed through heterosynaptic plasticity, which depends on release of CCK in the neocortex mostly from entorhinal afferents.

A voltage-based Event-Timing-Dependent Plasticity rule accounts for LTP subthreshold and suprathreshold for dendritic spikes in CA1 pyramidal neurons.

Long-term potentiation (LTP) is a synaptic mechanism involved in learning and memory. Experiments have shown that dendritic sodium spikes (Na-dSpikes) are required for LTP in the distal apical dendrites of CA1 pyramidal cells. On the other hand, LTP in perisomatic dendrites can be induced by synaptic input patterns that can be both subthreshold and suprathreshold for Na-dSpikes. It is unclear whether these results can be explained by one unifying plasticity mechanism. Here, we show in biophysically and morphologically realistic compartmental models of the CA1 pyramidal cell that these forms of LTP can be fully accounted for by a simple plasticity rule. We call it the voltage-based Event-Timing-Dependent Plasticity (ETDP) rule. The presynaptic event is the presynaptic spike or release of glutamate. The postsynaptic event is the local depolarization that exceeds a certain plasticity threshold. Our model reproduced the experimentally observed LTP in a variety of protocols, including local pharmacological inhibition of dendritic spikes by tetrodotoxin (TTX). In summary, we have provided a validation of the voltage-based ETDP, suggesting that this simple plasticity rule can be used to model even complex spatiotemporal patterns of long-term synaptic plasticity in neuronal dendrites.

Synaptic Plasticity and Cognitive Ability in Experimental Adult-Onset Hypothyroidism.

Adult-onset hypothyroidism impairs normal brain function. Research on animal models of hypothyroidism has revealed critical information on how deficiency of thyroid hormones impacts the electrophysiological and molecular functions of the brain, which leads to the well known cognitive impairment in untreated hypothyroid patients. Currently, such information can only be obtained from experiments on animal models of hypothyroidism. This review summarizes important research findings that pertain to understanding the clinical cognitive consequences of hypothyroidism, which will provide a better guiding path for therapy of hypothyroidism. SIGNIFICANCE STATEMENT: Cognitive impairment occurs during adult-onset hypothyroidism in both humans and animal models. Findings from animal studies validate clinical findings showing impaired long-term potentiation, decreased CaMKII, and increased calcineurin. Such findings can only be gleaned from animal experiments to show how hypothyroidism produces clinical symptoms.

Long-lasting forms of plasticity through patterned ultrasound-induced brainwave entrainment.

Achieving long-lasting neuronal modulation with low-intensity, low-frequency ultrasound is challenging. Here, we devised theta burst ultrasound stimulation (TBUS) with gamma bursts for brain entrainment and modulation of neuronal plasticity in the mouse motor cortex. We demonstrate that two types of TBUS, intermittent and continuous TBUS, induce bidirectional long-term potentiation or depression-like plasticity, respectively, as evidenced by changes in motor-evoked potentials. These effects depended on molecular pathways associated with long-term plasticity, including N-methyl-d-aspartate receptor and brain-derived neurotrophic factor/tropomyosin receptor kinase B activation, as well as de novo protein synthesis. Notably, bestrophin-1 and transient receptor potential ankyrin 1 play important roles in these enduring effects. Moreover, pretraining TBUS enhances the acquisition of previously unidentified motor skills. Our study unveils a promising protocol for ultrasound neuromodulation, enabling noninvasive and sustained modulation of brain function.

Pre- versus Post-synaptic Forms of LTP in Two Branches of the Same Hippocampal Afferent.

There has been considerable controversy about pre- versus postsynaptic expression of memory-related long-term potentiation (LTP), with corresponding disputes about underlying mechanisms. We report here an instance in male mice, in which both types of potentiation are expressed but in separate branches of the same hippocampal afferent. Induction of LTP in the dentate gyrus (DG) branch of the lateral perforant path (LPP) reduces paired-pulse facilitation, is blocked by antagonism of cannabinoid receptor type 1, and is not affected by suppression of postsynaptic actin polymerization. These observations are consistent with presynaptic expression. The opposite pattern of results was obtained in the LPP branch that innervates the distal dendrites of CA3: LTP did not reduce paired-pulse facilitation, was unaffected by the cannabinoid receptor blocker, and required postsynaptic actin filament assembly. Differences in the two LPP termination sites were also noted for frequency facilitation of synaptic responses, an effect that was reproduced in a two-step simulation by small adjustments to vesicle release dynamics. These results indicate that different types of glutamatergic neurons impose different forms of filtering and synaptic plasticity on their afferents. They also suggest that inputs are routed to, and encoded by, different sites within the hippocampus depending upon the pattern of activity arriving over the parent axon.

Protocol for induction of heterosynaptic long-term potentiation in the mouse hippocampus via dual-opsin stimulation technique.

Cholecystokinin (CCK) is the most abundant neuropeptide that broadly regulates the physiological status of animals. Here, we present a two-color laser theta burst stimulation (L-TBS) protocol for simultaneous activation of Schaffer collateral and perforant pathway in the hippocampus of CCK Cre mice. We describe steps for heterosynaptic long-term potentiation induction by L-TBS. This technique allows for the examination of the neurotransmitter roles in synaptic modulation and facilitates the exploration of pathological mechanisms in genetic models of brain disorders in mice. For complete details on the use and execution of this protocol, please refer to Su et al.1.

A biochemical description of postsynaptic plasticity-with timescales ranging from milliseconds to seconds.

Synaptic plasticity [long-term potentiation/depression (LTP/D)], is a cellular mechanism underlying learning. Two distinct types of early LTP/D (E-LTP/D), acting on very different time scales, have been observed experimentally-spike timing dependent plasticity (STDP), on time scales of tens of ms; and behavioral time scale synaptic plasticity (BTSP), on time scales of seconds. BTSP is a candidate for a mechanism underlying rapid learning of spatial location by place cells. Here, a computational model of the induction of E-LTP/D at a spine head of a synapse of a hippocampal pyramidal neuron is developed. The single-compartment model represents two interacting biochemical pathways for the activation (phosphorylation) of the kinase (CaMKII) with a phosphatase, with ion inflow through channels (NMDAR, CaV1,Na). The biochemical reactions are represented by a deterministic system of differential equations, with a detailed description of the activation of CaMKII that includes the opening of the compact state of CaMKII. This single model captures realistic responses (temporal profiles with the differing timescales) of STDP and BTSP and their asymmetries. The simulations distinguish several mechanisms underlying STDP vs. BTSP, including i) the flow of [Formula: see text] through NMDAR vs. CaV1 channels, and ii) the origin of several time scales in the activation of CaMKII. The model also realizes a priming mechanism for E-LTP that is induced by [Formula: see text] flow through CaV1.3 channels. Once in the spine head, this small additional [Formula: see text] opens the compact state of CaMKII, placing CaMKII ready for subsequent induction of LTP.

T-Type Ca2+ Channels Mediate a Critical Period of Plasticity in Adult-Born Granule Cells.

Adult-born granule cells (abGCs) exhibit a transient period of elevated synaptic plasticity that plays an important role in hippocampal function. Various mechanisms have been implicated in this critical period for enhanced plasticity, including minimal GABAergic inhibition and high intrinsic excitability conferred by T-type Ca2+ channels. Here we assess the contribution of synaptic inhibition and intrinsic excitability to long-term potentiation (LTP) in abGCs of adult male and female mice using perforated patch recordings. We show that the timing of critical period plasticity is unaffected by intact GABAergic inhibition such that 4-6-week-old abGCs exhibit LTP that is absent by 8 weeks. Blocking GABAA receptors, or partial blockade of GABA release from PV and nNos-expressing interneurons by a µ-opioid receptor agonist, strongly enhances LTP in 4-week-old GCs, suggesting that minimal inhibition does not underlie critical period plasticity. Instead, the closure of the critical period coincides with a reduction in the contribution of T-type Ca2+ channels to intrinsic excitability, and a selective T-type Ca2+ channel antagonist prevents LTP in 4-week-old but not mature GCs. Interestingly, whole-cell recordings that facilitate T-type Ca2+ channel activity in mature GCs unmasks LTP (with inhibition intact) that is also sensitive to a T-type Ca2+ channel antagonist, suggesting T-type channel activity in mature GCs is suppressed by native intracellular signaling. Together these results show that abGCs use T-type Ca2+ channels to overcome inhibition, providing new insight into how high intrinsic excitability provides young abGCs a competitive advantage for experience-dependent synaptic plasticity.

Personalized depth-specific neuromodulation of the human primary motor cortex via ultrasound.

Non-invasive brain stimulation has the potential to boost neuronal plasticity in the primary motor cortex (M1), but it remains unclear whether the stimulation of both superficial and deep layers of the human motor cortex can effectively promote M1 plasticity. Here, we leveraged transcranial ultrasound stimulation (TUS) to precisely target M1 circuits at depths of approximately 5 mm and 16 mm from the cortical surface. Initially, we generated computed tomography images from each participant's individual anatomical magnetic resonance images (MRI), which allowed for the generation of accurate acoustic simulations. This process ensured that personalized TUS was administered exactly to the targeted depths within M1 for each participant. Using long-term depression and long-term potentiation (LTD/LTP) theta-burst stimulation paradigms, we examined whether TUS over distinct depths of M1 could induce LTD/LTP plasticity. Our findings indicated that continuous theta-burst TUS-induced LTD-like plasticity with both superficial and deep M1 stimulation, persisting for at least 30 min. In comparison, sham TUS did not significantly alter M1 excitability. Moreover, intermittent theta-burst TUS did not result in the induction of LTP- or LTD-like plasticity with either superficial or deep M1 stimulation. These findings suggest that the induction of M1 plasticity can be achieved with ultrasound stimulation targeting distinct depths of M1, which is contingent on the characteristics of TUS. KEY POINTS: The study integrated personalized transcranial ultrasound stimulation (TUS) with electrophysiology to determine whether TUS targeting superficial and deep layers of the human motor cortex (M1) could elicit long-term depression (LTD) or long-term potentiation (LTP) plastic changes. Utilizing acoustic simulations derived from individualized pseudo-computed tomography scans, we ensured the precision of TUS delivery to the intended M1 depths for each participant. Continuous theta-burst TUS targeting both the superficial and deep layers of M1 resulted in the emergence of LTD-like plasticity, lasting for at least 30 min. Administering intermittent theta-burst TUS to both the superficial and deep layers of M1 did not lead to the induction of LTP- or LTD-like plastic changes. We suggest that theta-burst TUS targeting distinct depths of M1 can induce plasticity, but this effect is dependent on specific TUS parameters.

Abnormal motor cortical plasticity as a useful neurophysiological biomarker for Alzheimer's disease pathology.

OBJECTIVE: Amyloid-beta (Aß) and tau accumulations impair long-term potentiation (LTP) induction in animal hippocampi. We investigated relationships between motor-cortical plasticity and biomarkers for Alzheimer's disease (AD) diagnosis in subjects with cognitive decline. METHODS: Twenty-six consecutive subjects who complained of memory problems participated in this study. We applied transcranial quadripuse stimulation with an interstimulus interval of 5 ms (QPS5) to induce LTP-like plasticity. Motor-evoked potentials were recorded from the right first-dorsal interosseous muscle before and after QPS5. Cognitive functions, Aß42 and tau levels in the cerebrospinal fluid (CSF) were measured. Amyloid positron-emission tomography (PET) with11C-Pittsburg compound-B was also conducted. We studied correlations of QPS5-induced plasticity with cognitive functions or AD-related biomarkers. RESULTS: QPS5-induced LTP-like plasticity positively correlated with cognitive scores. The degree of LTP-like plasticity negatively correlated with levels of CSF-tau, and the amount of amyloid-PET accumulation at the precuneus, and correlated with the CSF-Aß42 level positively. In the amyloid-PET positive subjects, non-responder rate of QPS5 was higher than the CSF-tau positive rate. CONCLUSIONS: Findings suggest that QPS5-induced LTP-like plasticity is a functional biomarker of AD. QPS5 could detect abnormality at earlier stages than CSF-tau in the amyloid-PET positive subjects. SIGNIFICANCE: Assessing motor-cortical plasticity could be a useful neurophysiological biomarker for AD pathology.

Gamma oscillation plasticity is mediated via parvalbumin interneurons.

Understanding the plasticity of neuronal networks is an emerging field of (patho-) physiological research, yet the underlying cellular mechanisms remain poorly understood. Gamma oscillations (30 to 80 hertz), a biomarker of cognitive performance, require and potentiate glutamatergic transmission onto parvalbumin-positive interneurons (PVIs), suggesting an interface for cell-to-network plasticity. In ex vivo local field potential recordings, we demonstrate long-term potentiation of hippocampal gamma power. Gamma potentiation obeys established rules of PVI plasticity, requiring calcium-permeable AMPA receptors (CP-AMPARs) and metabotropic glutamate receptors (mGluRs). A microcircuit computational model of CA3 gamma oscillations predicts CP-AMPAR plasticity onto PVIs critically outperforms pyramidal cell plasticity in increasing gamma power and completely accounts for gamma potentiation. We reaffirm this ex vivo in three PVI-targeting animal models, demonstrating that gamma potentiation requires PVI-specific signaling via a Gq/PKC pathway comprising mGluR5 and a Gi-sensitive, PKA-dependent pathway. Gamma activity-dependent, metabotropically mediated CP-AMPAR plasticity on PVIs may serve as a guiding principle in understanding network plasticity in health and disease.

Long-term plasticity of NMDA GluN2B (NR2B) receptor in anterior cingulate cortical synapses.

The anterior cingulate cortex (ACC) is a key cortical area for pain perception, emotional fear and anxiety. Cortical excitation is thought to be the major mechanism for chronic pain and its related emotional disorders such as anxiety and depression. GluN2B (or called NR2B) containing NMDA receptors play critical roles for such excitation. Not only does the activation of GluN2B contributes to the induction of the postsynaptic form of LTP (post-LTP), long-term upregulation of GluN2B subunits through tyrosine phosphorylation were also detected after peripheral injury. In addition, it has been reported that presynaptic NMDA receptors may contribute to the modulation of the release of glutamate from presynaptic terminals in the ACC. It is believed that inhibiting subtypes of NMDA receptors and/or downstream signaling proteins may serve as a novel therapeutic mechanism for future treatment of chronic pain, anxiety, and depression.

Activity-Dependent Stabilization of Nascent Dendritic Spines Requires Nonenzymatic CaMKIIα Function.

The outgrowth and stabilization of nascent dendritic spines are crucial processes underlying learning and memory. Most new spines retract shortly after growth; only a small subset is stabilized and integrated into the new circuit connections that support learning. New spine stabilization has been shown to rely upon activity-dependent molecular mechanisms that also contribute to long-term potentiation (LTP) of synaptic strength. Indeed, disruption of the activity-dependent targeting of the kinase CaMKIIα to the GluN2B subunit of the NMDA-type glutamate receptor disrupts both LTP and activity-dependent stabilization of new spines. Yet it is not known which of CaMKIIα's many enzymatic and structural functions are important for new spine stabilization. Here, we used two-photon imaging and photolysis of caged glutamate to monitor the activity-dependent stabilization of new dendritic spines on hippocampal CA1 neurons from mice of both sexes in conditions where CaMKIIα functional and structural interactions were altered. Surprisingly, we found that inhibiting CaMKIIα kinase activity either genetically or pharmacologically did not impair activity-dependent new spine stabilization. In contrast, shRNA knockdown of CaMKIIα abolished activity-dependent new spine stabilization, which was rescued by co-expressing shRNA-resistant full-length CaMKIIα, but not by a truncated monomeric CaMKIIα. Notably, overexpression of phospho-mimetic CaMKIIα-T286D, which exhibits activity-independent targeting to GluN2B, enhanced basal new spine survivorship in the absence of additional glutamatergic stimulation, even when kinase activity was disrupted. Together, our results support a model in which nascent dendritic spine stabilization requires structural and scaffolding interactions mediated by dodecameric CaMKIIα that are independent of its enzymatic activities.

Phosphorylation of 4.1N by CaMKII Regulates the Trafficking of GluA1-containing AMPA Receptors During Long-term Potentiation in Acute Rat Hippocampal Brain Slices.

OBJECTIVE: GluA1-containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (AMPARs) inserted into postsynaptic membranes are key to the process of long-term potentiation (LTP). Some evidence has shown that 4.1N plays a critical role in the membrane trafficking of AMPARs. However, the underlying mechanism behind this is still unclear. We investigated the role of 4.1N-mediated membrane trafficking of AMPARs during theta-burst stimulation long-term potentiation (TBS-LTP), to illustrate the molecular mechanism behind LTP. METHODS: LTP was induced by TBS in rat hippocampal CA1 neuron. Tat-GluA1 (MPR), which disrupts the association of 4.1N-GluA1, and autocamtide-2-inhibitory peptide, myristoylated (Myr-AIP), a CaMKII antagonist, were used to explore the role of 4.1N in the AMPARs trafficking during TBS-induced LTP. Immunoprecipitation (IP) and immunoblotting (IB)were used to detect protein expression, phosphorylation, and the interaction of p-CaMKII-4.1N-GluA1. RESULTS: We found that Myr-AIP attenuated increases of p-CaMKII (T286), p-GluA1 (ser831), and 4.1N phosphorylation after TBS-LTP, and decreased the association of p-CaMKII-4.1N-GluA1, along with the expression of GluA1, at postsynaptic densities during TBS-LTP. We also designed interfering peptides to disrupt the interaction between 4.1N and GluA1, which showed that Tat-GluA1 (MPR) or Myr-AIP inhibited TBS-LTP and attenuated increases of GluA1 at postsynaptic sites, while Tat-GluA1 (MPR) or Myr-AIP had no effects on miniature excitatory postsynaptic currents (mEPSCs) in non-stimulated hippocampal CA1 neurons. CONCLUSION: Active CaMKII enhanced the phosphorylation of 4.1N and facilitated the association of p-CaMKII with 4.1N-GluA1, which in turn resulted in GluA1 trafficking during TBS-LTP. The association of 4.1N-GluA1 is required for LTP, but not for basal synaptic transmission.

Temporal and quantitative analysis of the functional expression of Ca2+-permeable AMPA receptors during LTP.

In the present study, we attempted to temporally and quantitatively analyze the functional contributions of Ca2+-permeable (CP) α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) during long-term potentiation (LTP) expression using electrophysiological and pharmacological approaches. In hippocampal CA1 neurons, using 1-naphthyl acetyl spermine (NASPM), a CP-AMPAR antagonist, we began by demonstrating that NASPM-sensitive components, probably including the GluA1 homomer, functionally contributed to about 15% of AMPAR-mediated EPSC amplitude in basal conditions. Then, when NASPM was treated at different time points (3-30 min) after LTP induction, it was found that LTP was almost completely impaired at 3 or 10 min but maintained at 20 or 30 min, although its potentiation was reduced. Further temporal and quantitative analysis revealed that the functional expression of CP-AMPARs began increasing approximately 20 min after LTP induction and reached more than twice the basal level at 30 min. These results suggest that CP-AMPARs in the first 3-10 min of LTP might play an important role in LTP maintenance. Moreover, their decay time was also significantly increased at 30 min, suggesting that CP-AMPARs changed not only quantitatively in LTP but also qualitatively.