Transient appearance of Ca2+‐permeable AMPA receptors is crucial for the production of repetitive LTP‐induced synaptic enhancement (RISE) in cultured hippocampal slices

We have previously shown that repetitive induction of long‐term potentiation (LTP) by glutamate (100 μM, 3 min, three times at 24‐hr intervals) provoked long‐lasting synaptic enhancement accompanied by synaptogenesis in rat hippocampal slice cultures, a phenomenon termed RISE (repetitive LTP‐induced synaptic enhancement). Here, we examined the role of Ca²⁺‐permeable (CP) AMPA receptors (AMPARs) in the establishment of RISE. We first found a component sensitive to the Joro‐spider toxin (JSTX), a blocker of CP‐AMPARs, in a field EPSP recorded from CA3‐CA1 synapses at 2–3 days after stimulation, but this component was not found for 9–10 days. We also observed that rectification of AMPAR‐mediated current appeared only 2–3 days after stimulation, using a whole‐cell patch clamp recording from CA1 pyramidal neurons. These findings indicate that CP‐AMPAR is transiently expressed in the developing phase of RISE. The blockade of CP‐AMPARs by JSTX for 24 hr at this developing phase inhibited RISE establishment, accompanied by the loss of small synapses at the ultrastructural level. These results suggest that transiently induced CP‐AMPARs play a critical role in synaptogenesis in the developing phase of long‐lasting hippocampal synaptic plasticity, RISE.     

Activation of dopamine D1/D5 receptors facilitates the induction of presynaptic long‐term potentiation at hippocampal output synapses

Encoding of novel information has been proposed to rely on the time‐locked release of dopamine in the hippocampal formation during novelty detection. However, the site of novelty detection in the hippocampus remains a matter of debate. According to current models, the CA1 and the subiculum act as detectors and distributors of novel sensory information. Although most CA1 pyramidal neurons exhibit regular‐spiking behavior, the majority of subicular pyramidal neurons fire high‐frequency bursts of action potentials. The present study investigates the efficacy of dopamine D1/D5 receptor activation to facilitate the induction of activity‐dependent long‐term potentiation (LTP) in rat CA1 regular‐spiking and subicular burst‐spiking pyramidal cells. Using a weak stimulation protocol, set at a level subthreshold for the induction of LTP, we show that activation of D1/D5 receptors for 5–10 min facilitates LTP in subicular burst‐spiking neurons but not in CA1 neurons. The results demonstrate that D1/D5 receptor‐facilitated LTP is NMDA receptor‐dependent, and requires the activation of protein kinase A. In addition, the D1/D5 receptor‐facilitated LTP is shown to be presynaptically expressed and relies on presynaptic Ca²⁺ signaling. The phenomenon of dopamine‐induced facilitation of presynaptic NMDA receptor‐dependent LTP in subicular burst‐spiking pyramidal cells is in accordance with observations of the time‐locked release of dopamine during novelty detection in this brain region, and reveals an intriguing mechanism for the encoding of hippocampal output information.     

Changes in hippocampal synaptic functions and protein expression in monosodium glutamate‐treated obese mice during development of glucose intolerance

Glucose is the sole neural fuel for the brain and is essential for cognitive function. Abnormalities in glucose tolerance may be associated with impairments in cognitive function. Experimental obese model mice can be generated by an intraperitoneal injection of monosodium glutamate (MSG; 2 mg/g) once a day for 5 days from 1 day after birth. MSG‐treated mice have been shown to develop glucose intolerance and exhibit chronic neuroendocrine dysfunction associated with marked cognitive malfunctions at 28–29  weeks old. Although hippocampal synaptic plasticity is impaired in MSG‐treated mice, changes in synaptic transmission remain unknown. Here, we investigated whether glucose intolerance influenced cognitive function, synaptic properties and protein expression in the hippocampus. We demonstrated that MSG‐treated mice developed glucose intolerance due to an impairment in the effectiveness of insulin actions, and showed cognitive impairments in the Y‐maze test. Moreover, long‐term potentiation (LTP) at Schaffer collateral–CA1 pyramidal synapses in hippocampal slices was impaired, and the relationship between the slope of extracellular field excitatory postsynaptic potential and stimulus intensity of synaptic transmission was weaker in MSG‐treated mice. The protein levels of vesicular glutamate transporter 1 and GluA1 glutamate receptor subunits decreased in the CA1 region of MSG‐treated mice. These results suggest that deficits in glutamatergic presynapses as well as postsynapses lead to impaired synaptic plasticity in MSG‐treated mice during the development of glucose intolerance, though it remains unknown whether impaired LTP is due to altered inhibitory transmission. It may be important to examine changes in glucose tolerance in order to prevent cognitive malfunctions associated with diabetes.     

SK2 channels are neuroprotective for ischemia-induced neuronal cell death

In mouse hippocampal CA1 pyramidal neurons, the activity of synaptic small-conductance Ca²⁺-activated K⁺ channels type 2 (SK2 channels) provides a negative feedback on N-methyl--aspartate receptors (NMDARs), reestablishing Mg²⁺ block that reduces Ca²⁺ influx. The well-established role of NMDARs in ischemia-induced excitotoxicity led us to test the neuroprotective effect of modulating SK2 channel activity following cerebral ischemia induced by cardiac arrest and cardiopulmonary resuscitation (CA/CPR). Administration of the SK channel positive modulator, 1-ethyl-benzimidazolinone (1-EBIO), significantly reduced CA1 neuron cell death and improved CA/CPR-induced cognitive outcome. Electrophysiological recordings showed that CA/CPR-induced ischemia caused delayed and sustained reduction of synaptic SK channel activity, and immunoelectron microscopy showed that this is associated with internalization of synaptic SK2 channels, which was prevented by 1-EBIO treatment. These results suggest that increasing SK2 channel activity, or preventing ischemia-induced loss of synaptic SK2 channels, are promising and novel approaches to neuroprotection following cerebral ischemia.     

Atorvastatin enhances kainate‐induced gamma oscillations in rat hippocampal slices

Atorvastatin has been shown to affect cognitive functions in rodents and humans. However, the underlying mechanism is not fully understood. Because hippocampal gamma oscillations (γ, 20–80 Hz) are associated with cognitive functions, we studied the effect of atorvastatin on persistent kainate‐induced γ oscillation in the CA3 area of rat hippocampal slices. The involvement of NMDA receptors and multiple kinases was tested before and after administration of atorvastatin. Whole‐cell current‐clamp and voltage‐clamp recordings were made from CA3 pyramidal neurons and interneurons before and after atorvastatin application. Atorvastatin increased γ power by ~ 50% in a concentration‐dependent manner, without affecting dominant frequency. Whereas atorvastatin did not affect intrinsic properties of both pyramidal neurons and interneurons, it increased the firing frequency of interneurons but not that of pyramidal neurons. Furthermore, whereas atorvastatin did not affect synaptic current amplitude, it increased the frequency of spontaneous inhibitory post‐synaptic currents, but did not affect the frequency of spontaneous excitatory post‐synaptic currents. The atorvastatin‐induced enhancement of γ oscillations was prevented by pretreatment with the PKA inhibitor H89, the ERK inhibitor U0126, or the PI3K inhibitor wortmanin, but not by the NMDA receptor antagonist D‐AP5. Taken together, these results demonstrate that atorvastatin enhanced the kainate‐induced γ oscillation by increasing interneuron excitability, with an involvement of multiple intracellular kinase pathways. Our study suggests that the classical cholesterol‐lowering agent atorvastatin may improve cognitive functions compromised in disease, via the enhancement of hippocampal γ oscillations.     

17β estradiol recruits GluN2B‐containing NMDARs and ERK during induction of long‐term potentiation at temporoammonic‐CA1 synapses

When circulating 17β estradiol (E2) is elevated to proestrous levels, hippocampus‐dependent learning and memory is enhanced in female rodents, nonhuman primates, and women due to heightened synaptic function at hippocampal synapses. We previously reported that proestrous‐like levels of E2 administered to young adult ovariectomized (OVX) female rats increases the magnitude of LTP at CA3 Schaffer collateral (SC)‐CA1 synapses only when dendritic spine density, the NMDAR/AMPAR ratio, and current mediated by GluN2B‐containing NMDA receptors (NMDARs) are simultaneously increased. We also reported that this increase in GluN2B‐mediated NMDAR current in area CA1 is causally related to the E2‐induced increase in novel object recognition, tying together heightened synaptic function with improved learning and memory. In addition to SC inputs, innervation from the entorhinal cortex in the temporoammonic (TA) pathway onto CA1 distal dendrites in stratum lacunosum‐moleculare is critical for spatial memory formation and retrieval. It is not known whether E2 modulates TA‐CA1 synapses similarly to SC‐CA1 synapses. Here, we report that 24 hours post‐E2 injection, dendritic spine density on CA1 pyramidal cell distal dendrites and current mediated by GluN2B‐containing NMDARs at TA‐CA1 synapses is increased, similarly to our previous findings at SC‐CA1 synapses. However, in contrast to SC‐CA1 synapses, AMPAR transmission at TA‐CA1 synapses is significantly increased, and there is no effect on the LTP magnitude. Pharmacological blockade of GluN2B‐containing NMDARs or ERK activation, which occurs downstream of synaptic but not extrasynaptic GluN2B‐containing NMDARs, attenuates the LTP magnitude only in slices from E2‐treated rats. These data show that E2 recruits a causal role for GluN2B‐containing NMDARs and ERK signaling in the induction of LTP, cellular mechanisms not required for LTP induction at TA‐CA1 synapses in vehicle‐treated OVX female rats. © 2015 Wiley Periodicals, Inc.     

Prepubertal castration causes the age‐dependent changes in hippocampal long‐term potentiation

The effects of prepubertal castration on hippocampal CA3‐CA1 synaptic transmission and plasticity were studied at different ages in vitro. The field excitatory postsynaptic potentials (fEPSP) and population spikes (PS) were simultaneously recorded from stratum radiatum and stratum pylamidale of area CA1 following stimulation of Schaffer collaterals in slices taken from sham‐castrated and castrated rats at postnatal days (PND) 28, 35, 45, and 60. Castration had no effect on baseline responses at different ages except at PND 60 that a decrease in the fEPSP slope was seen. Prepubertal castration caused age‐specific changes in CA1‐long term potentiation (LTP) induction. The castration did decrease both fEPSP‐LTP and PS‐LTP at PND 35 but a decrease was seen only in PS‐LTP at PND 60. NMDA receptor antagonist AP5 (25 µM) completely blocked both fEPSP‐LTP and PS‐LTP at PND 60 and only PS‐LTP at PND 35 in both sham‐castrated and castrated groups. Although AP5 blocked fEPSP‐LTP at PND 35 in sham‐castrated group, it failed to inhibit fEPSP‐LTP at PND 35 in castrated one. These findings suggest that prepubertal castration causes the age‐dependent changes in CA1‐LTP induction, which might arise from alterations in the NMDA receptors. Synapse 67:235–244, 2013. © 2013 Wiley Periodicals, Inc.     

Long‐term high‐intensity sound stimulation inhibits h current (Ih) in CA1 pyramidal neurons

Afferent neurotransmission to hippocampal pyramidal cells can lead to long‐term changes to their intrinsic membrane properties and affect many ion currents. One of the most plastic neuronal currents is the hyperpolarization‐activated cationic current (I_h), which changes in CA1 pyramidal cells in response to many types of physiological and pathological processes, including auditory stimulation. Recently, we demonstrated that long‐term potentiation (LTP) in rat hippocampal Schaffer‐CA1 synapses is depressed by high‐intensity sound stimulation. Here, we investigated whether a long‐term high‐intensity sound stimulation could affect intrinsic membrane properties of rat CA1 pyramidal neurons. Our results showed that I_h is depressed by long‐term high‐intensity sound exposure (1 min of 110 dB sound, applied two times per day for 10 days). This resulted in a decreased resting membrane potential, increased membrane input resistance and time constant, and decreased action potential threshold. In addition, CA1 pyramidal neurons from sound‐exposed animals fired more action potentials than neurons from control animals; however, this effect was not caused by a decreased I_h. On the other hand, a single episode (1 min) of 110 dB sound stimulation which also inhibits hippocampal LTP did not affect I_h and firing in pyramidal neurons, suggesting that effects on I_h are long‐term responses to high‐intensity sound exposure. Our results show that prolonged exposure to high‐intensity sound affects intrinsic membrane properties of hippocampal pyramidal neurons, mainly by decreasing the amplitude of I_h.     

Heterosynaptic long‐term depression mediated by ATP released from astrocytes

Heterosynaptic long‐term depression (hLTD) at untetanized synapses accompanying the induction of long‐term potentiation (LTP) spatially sharpens the activity‐induced synaptic potentiation; however, the underlying mechanism remains unclear. We found that hLTD in the hippocampal CA1 region is caused by stimulation‐induced ATP release from astrocytes that suppresses transmitter release from untetanized synaptic terminals via activation of P2Y receptors. Selective stimulation of astrocytes expressing channelrhodopsin‐2, a light‐gated cation channel permeable to Ca²⁺, resulted in LTD of synapses on neighboring neurons. This synaptic modification required Ca²⁺ elevation in astrocytes and activation of P2Y receptors, but not N‐methyl‐D ‐aspartate receptors. Furthermore, blocking P2Y receptors or buffering astrocyte intracellular Ca²⁺ at a low level prevented hLTD without affecting LTP induced by SC stimulation. Thus, astrocyte activation is both necessary and sufficient for mediating hLTD accompanying LTP induction, strongly supporting the notion that astrocytes actively participate in activity‐dependent synaptic plasticity of neural circuits. © 2012 Wiley Periodicals, Inc.     

Involvement of intracellular Zn2+ signaling in LTP at perforant pathway–CA1 pyramidal cell synapse

Physiological significance of synaptic Zn²⁺ signaling was examined at perforant pathway–CA1 pyramidal cell synapses. In vivo long‐term potentiation (LTP) at perforant pathway–CA1 pyramidal cell synapses was induced using a recording electrode attached to a microdialysis probe and the recording region was locally perfused with artificial cerebrospinal fluid (ACSF) via the microdialysis probe. Perforant pathway LTP was not attenuated under perfusion with CaEDTA (10 mM), an extracellular Zn²⁺ chelator, but attenuated under perfusion with ZnAF‐2DA (50 μM), an intracellular Zn²⁺ chelator, suggesting that intracellular Zn²⁺ signaling is required for perforant pathway LTP. Even in rat brain slices bathed in CaEDTA in ACSF, intracellular Zn²⁺ level, which was measured with intracellular ZnAF‐2, was increased in the stratum lacunosum‐moleculare where perforant pathway–CA1 pyramidal cell synapses were contained after tetanic stimulation. These results suggest that intracellular Zn²⁺ signaling, which originates in internal stores/proteins, is involved in LTP at perforant pathway–CA1 pyramidal cell synapses. Because the influx of extracellular Zn²⁺, which originates in presynaptic Zn²⁺ release, is involved in LTP at Schaffer collateral‐CA1 pyramidal cell synapses, synapse‐dependent Zn²⁺ dynamics may be involved in plasticity of postsynaptic CA1 pyramidal cells.     

Theta burst stimulation‐induced LTP: Differences and similarities between the dorsal and ventral CA1 hippocampal synapses

The hippocampal synapses display a conspicuous ability for long‐term plasticity, which is thought to contribute to learning and memory. Previous research has shown that long‐term potentiation (LTP) greatly differs between the dorsal (DH) and ventral (VH) CA1 hippocampal synapses when induced by high‐frequency stimulation. In this study, using rat hippocampal slices and more physiologically relevant activity patterns based on the frequency of the theta rhythm (i.e., theta‐burst stimulation, TBS) we found that the DH compared with the VH displayed a higher ability for induction and stability of NMDA receptor‐dependent LTP of the field excitatory postsynaptic potential. Nevertheless, the maximal magnitude of LTP was similar in the two hippocampal segments. Blockade of GABA_B receptors prevented the LTP induction by the minimal effective TBS and reduced the magnitude of LTP induced by longer TBS. TBS produced a three‐fold higher facilitation of the synaptic burst responses in the DH compared with the VH, accompanied by a strong enhancement in the postsynaptic excitation in the DH but mostly depression in the VH. The DH displayed NMDA receptor‐dependent and NMDA receptor‐independent facilitation, but the facilitation in the VH was only NMDA receptor‐dependent. Also, the TBS‐associated activity of GABA_B receptors was higher in the DH than in the VH. The different response profiles during TBS could underlie the differences in LTP between the two hippocampal segments. L‐type voltage‐dependent calcium channels (L‐VDCC) and the metabotropic glutamate receptor‐5 (mGluR5) equally contributed in DH and VH to compound LTP induced by relatively long TBS. We propose that these dorsoventral differences in synaptic plasticity reflect specializations of the intrinsic circuitry of the hippocampus, that are involved in the distinct information processing performed by the two hippocampal segments and could effectively support the contribution of the dorsal and the ventral hippocampal segment to single event memory and to emotional memory respectively. © 2016 Wiley Periodicals, Inc.     

Presynaptic Ultrastructural Plasticity Along CA3→CA1 Axons During Long‐Term Potentiation in Mature Hippocampus

In area CA1 of the mature hippocampus, synaptogenesis occurs within 30 minutes after the induction of long‐term potentiation (LTP); however, by 2 hours many small dendritic spines are lost, and those remaining have larger synapses. Little is known, however, about associated changes in presynaptic vesicles and axonal boutons. Axons in CA1 stratum radiatum were evaluated with 3D reconstructions from serial section electron microscopy at 30 minutes and 2 hours after induction of LTP by theta‐burst stimulation (TBS). The frequency of axonal boutons with a single postsynaptic partner was decreased by 33% at 2 hours, corresponding perfectly to the 33% loss specifically of small dendritic spines (head diameters <0.45 μm). Docked vesicles were reduced at 30 minutes and then returned to control levels by 2 hours following induction of LTP. By 2 hours there were fewer small synaptic vesicles overall in the presynaptic vesicle pool. Clathrin‐mediated endocytosis was used as a marker of local activity, and axonal boutons containing clathrin‐coated pits showed a more pronounced decrease in presynaptic vesicles at both 30 minutes and 2 hours after induction of LTP relative to control values. Putative transport packets, identified as a cluster of less than 10 axonal vesicles occurring between synaptic boutons, were stable at 30 minutes but markedly reduced by 2 hours after the induction of LTP. APV blocked these effects, suggesting that the loss of axonal boutons and presynaptic vesicles was dependent on N‐methyl‐D‐aspartic acid (NMDA) receptor activation during LTP. These findings show that specific presynaptic ultrastructural changes complement postsynaptic ultrastructural plasticity during LTP. J. Comp. Neurol. 521:3898–3912, 2013. © 2013 Wiley Periodicals, Inc.     

Activation of SK channels inhibits epileptiform bursting in hippocampal CA3 neurons

The role of calcium-activated potassium channels in the regulation of neuronal hyperexcitability, as in epilepsy, is unclear. To examine this issue, we have used the acute hippocampal slice model of epileptiform activity to investigate the effects of an enhancer of SK channel activity, 1-ethyl-benzimidazolinone (EBIO). That EBIO is an SK channel modulator was confirmed by its potentiation of hSK1, hSK2, hSK3 and hIK currents (EC(50) values in the range of 130-870 microM) and its apamin (1 microM) sensitive reduction of the number of action potentials fired in CA3 pyramidal neurons in response to a depolarizing current step. In addition, while EBIO did not significantly affect electrically evoked glutamatergic synaptic transmission, it did inhibit epileptiform activity (IC(50) values in the range of 150-325 microM) induced by (1) modifying the extracellular ionic environment by removing extracellular Mg(2+) or elevating extracellular K(+) from 3.0 to 8.5 mM and (2) disinhibiting the slice using 3 mM pentylenetetrazol or combined application of 10 microM gabazine and 10 microM CGP55845. Furthermore, its inhibitory effect in the full disinhibition model of epileptiform activity (10 microM gabazine + 10 microM CGP55845) was occluded by the SK channel blocker apamin (300 nM-1 microM) which in its own right increased the duration and reduced the frequency of individual epileptiform bursts. In conclusion, compounds that enhance the activation of small conductance Ca(2+) -activated K(+) channels are effective inhibitors of epileptiform activity in vitro.     

NMDA receptor dependence of the input specific NMDA receptor-independent LTP in the hippocampal CA1 region

An important characteristic of long-term potentiation (LTP) in the hippocampal CA1 region is that it is specific for those synapses which are active during the induction event. This input specificity is commonly attributed to the location and properties of the N-methyl- -aspartate (NMDA) receptor channel. Experiments using strong high-frequency orthodromic activation have suggested that input-specific LTP can occur also in the absence of NMDA receptor activation. The present experiments have re-examined this question. They were performed in the CA1 region of hippocampal slices, and the synaptic strength was evaluated from the initial slope of the dendritically recorded field potential. In agreement with previous reports, 0.5 s. 200 Hz, orthodromic trains were found to lead to a substantial input-specific LTP (averaging 62%) in the presence of the competitive NMDA receptor antagonist -(−)-2-amino-5-phosphonopentanoic acid ( -AP5) (20 μM). Under conditions of higher NMDA receptor blockade considerably less LTP was evoked. In 50 μM -AP5 and 20 μM chloro-kynurenate LTP averaged 13.4%, and after addition of 20 μM (+)-dizicilpine malcate (MK-801) LTP averaged 5.9%. On the other hand, in 20 μM -AP5 and 20 μM of the calcium channel antagonist nifedipine LTP averaged 49.9%. The present results suggest that NMDA receptor activity remaining in high concentrations of AP5 is sufficient to underly LTP induction under strong induction conditions.     

Late phase of long‐term potentiation in the mossy fiber—CA3 hippocampal pathway is critically dependent on metalloproteinases activity

Matrix metalloproteinases (MMPs) are known to play a pivotal role in remodeling of the extracellular matrix and have been implicated in synaptic plasticity, learning and memory. In hippocampus, inhibition of MMPs impairs the maintenance of long term plasticity in Schaeffer collateral‐CA1 (Sch/CA1) synapses while its effect on short term plasticity remains a matter of debate. Surprisingly little is known on the role of MMPs in other hippocampal synapses. In this study we have investigated the impact of a broad spectrum MMPs inhibitor, FN‐439 on synaptic transmission in mossy fiber‐CA3 (MF/CA3) synapses exhibiting profoundly different mechanism of long term potentiation (LTP) as well as robust short‐term plasticity, features that clearly distinguish them from the Sch/CA1 synapses. We report, that MMPs blockade before and up to 30 minutes after LTP induction resulted in a severe disruption of the late phase of tetanically induced LTP. However, LTP time course was not changed when FN439 was administered 60 minutes post LTP induction indicating that MMPs activity is required for the consolidation of the synaptic plasticity within a specific time window. The paired‐pulse facilitation ratio or post‐tetanic potentiation or burst‐like pattern of mossy fiber stimulation were not changed in the presence of FN‐439 administered for 15 minutes suggesting that temporal pattern of presynaptic transmitter release and, in general, the MF‐CA3 fidelity is not significantly affected by MMPs inhibition. We conclude that although the mechanisms of long‐term plasticity in MF/CA3 and in Sch/CA1 are profoundly different, MMPs play a crucial role in both pathways in the maintenance of LTP, which is believed to play an important role in learning and memory in the hippocampus. © 2010 Wiley‐Liss, Inc.     

Novel nootropic drug sunifiram enhances hippocampal synaptic efficacy via glycine‐binding site of N‐methyl‐D‐aspartate receptor

Sunifiram is a novel pyrrolidone nootropic drug structurally related to piracetam, which was developed for neurodegenerative disorder like Alzheimer's disease. Sunifiram is known to enhance cognitive function in some behavioral experiments such as Morris water maze task. To address question whether sunifiram affects N‐methyl‐D ‐aspartate receptor (NMDAR)‐dependent synaptic function in the hippocampal CA1 region, we assessed the effects of sunifiram on NMDAR‐dependent long‐term potentiation (LTP) by electrophysiology and on phosphorylation of synaptic proteins by immunoblotting analysis. In mouse hippocampal slices, sunifiram at 10–100 nM significantly enhanced LTP in a bell‐shaped dose‐response relationship which peaked at 10 nM. The enhancement of LTP by sunifiram treatment was inhibited by 7‐chloro‐kynurenic acid (7‐ClKN), an antagonist for glycine‐binding site of NMDAR, but not by ifenprodil, an inhibitor for polyamine site of NMDAR. The enhancement of LTP by sunifilam was associated with an increase in phosphorylation of α‐amino‐3‐hydroxy‐5‐methylisozazole‐4‐propionate receptor (AMPAR) through activation of calcium/calmodulin‐dependent protein kinase II (CaMKII) and an increase in phosphorylation of NMDAR through activation of protein kinase Cα (PKCα). Sunifiram treatments at 1–1000 nM increased the slope of field excitatory postsynaptic potentials (fEPSPs) in a dose‐dependent manner. The enhancement was associated with an increase in phosphorylation of AMPAR receptor through activation of CaMKII. Interestingly, under the basal condition, sunifiram treatments increased PKCα (Ser‐657) and Src family (Tyr‐416) activities with the same bell‐shaped dose‐response curve as that of LTP peaking at 10 nM. The increase in phosphorylation of PKCα (Ser‐657) and Src (Tyr‐416) induced by sunifiram was inhibited by 7‐ClKN treatment. The LTP enhancement by sunifiram was significantly inhibited by PP2, a Src family inhibitor. Finally, when pretreated with a high concentration of glycine (300 μM), sunifiram treatments failed to potentiate LTP in the CA1 region. Taken together, sunifiram stimulates the glycine‐binding site of NMDAR with concomitant PKCα activation through Src kinase. Enhancement of PKCα activity triggers to potentiate hippocampal LTP through CaMKII activation. © 2013 Wiley Periodicals, Inc.     

Repetitive systemic morphine alters activity‐dependent plasticity of schaffer–collateral–CA1 pyramidal cell synapses: Involvement of adenosine A1 receptors and adenosine deaminase

The effectiveness of O‐pulse stimulation (TPS) for the reversal of O‐pattern primed bursts (PB)‐induced long‐term potentiation (LTP) were examined at the Schaffer–collateral–CA1 pyramidal cell synapses of hippocampal slices derived from rats chronically treated with morphine (M‐T). The results showed that slices derived from both control and M‐T rats had normal field excitatory postsynaptic potential (fEPSP)‐LTP, whereas PS‐LTP in slices from M‐T rats was significantly greater than that from control slices. When morphine was applied in vitro to slices derived from rats chronically treated with morphine, the augmentation of PS‐LTP was not seen. TPS given 30 min after LTP induction failed to reverse the fEPSP‐ or PS‐LTP in both groups of slices. However, TPS delivered in the presence of long‐term in vitro morphine caused the PS‐LTP reversal. This effect was blocked by the adenosine A1 receptor (A1R) antagonist CPX (200 nM) and furthermore was enhanced by the adenosine deaminase (ADA) inhibitor EHNA (10 μM). Interestingly, TPS given 30 min after LTP induction in the presence of EHNA (10 μM) can reverse LTP in morphine‐exposed control slices in vitro. These results suggest adaptive changes in the hippocampus area CA1 in particular in adenosine system following repetitive systemic morphine. Chronic in vivo morphine increases A1R and reduces ADA activity in the hippocampus. Consequently, adenosine can accumulate because of a stimulus train‐induced activity pattern in CA1 area and takes the opportunity to work as an inhibitory neuromodulator and also to enable CA1 to cope with chronic morphine. In addition, adaptive mechanisms are differentially working in the dendrite layer rather than the somatic layer of hippocampal CA1. © 2014 Wiley Periodicals, Inc.     

Galantamine enhancement of long‐term potentiation is mediated by calcium/calmodulin‐dependent protein kinase II and protein kinase C activation

Galantamine, a novel Alzheimer's drug, is known to inhibit acetylcholinesterase activity and potentiate nicotinic acetylcholine receptor (nAChR) in the brain. We previously reported that galantamine potentiates the NMDA‐induced currents in primary cultured rat cortical neurons. We now studied the effects of galantamine on long‐term potentiation (LTP) in the rat hippocampal CA1 regions. The field excitatory postsynaptic potentials (fEPSPs) were induced by stimulation of the Schaffer collateral/commissural pathways in the hippocampal CA1 region. Treatment with 0.01–10 μM galantamine did not affect the slope of fEPSPs in the CA1 region. Galantamine treatment increased calcium/calmodulin‐dependent protein kinase II (CaMKII) and protein kinase Cα (PKCα) activities with a bell‐shaped dose–response curve peaked at 1 μM, thereby increasing the phosphorylation of AMPA receptor, myristoylated alanine‐rich protein kinase C, and NMDA receptor as downstream substrates of CaMKII and/or PKCα. By contrast, galatamine treatment did not affect protein kinase A activity. Consistent with the bell‐shaped CaMKII and PKCα activation, galantamine treatment enhanced LTP in the hippocampal CA1 regions with the same bell‐shaped dose–response curve. Furthermore, LTP potentiation induced by galantamine treatment at 1 μM was closely associated with both CaMKII and PKC activation with concomitant increase in phosphorylation of their downstream substrates except for synapsin I. In addition, the enhancement of LTP by galantamine was accompanied with α7‐type nAChR activation. These results suggest that galantamine potentiates NMDA receptor‐dependent LTP through α7‐type nAChR activation, by which the postsynaptic CaMKII and PKC are activated. © 2009 Wiley‐Liss, Inc.     

Matrix metalloprotease activity shapes the magnitude of EPSPs and spike plasticity within the hippocampal CA3 network

Matrix metalloproteases (MMP) play a pivotal role in long‐term synaptic plasticity, learning, and memory. The roles of different MMP subtypes are emerging, but the proteolytic activity of certain MMPs was shown to support these processes through the structural and functional modification of hippocampal Schaeffer collateral and mossy fiber (MF) synapses. However, certain patterns of synaptic activity are additionally associated with non‐synaptic changes, such as the scaling of neuronal excitability. However, the extent to which MMPs affect this process remains unknown. We determined whether MMP activity interferes with excitatory post‐synaptic potential EPSP‐to‐spike (E–S) coupling under conditions of varying synaptic activity. We evoked short‐ and long‐term synaptic plasticity at associational/commissural (A/C) synapses of CA3 pyramidal neurons and simultaneously recorded population spikes (PSs) and EPSPs in acute rat (P30–60) brain slices in the presence of various MMP inhibitors. We found that MMP inhibition significantly reduced E–S coupling and shortened the PS latency associated with 4× 100 Hz stimulation or paired burst activity of MF–CA3 and A/C synapses. Moreover, MMP inhibition interfered with the scaling of amplitude of measured signals during high‐frequency trains, thus affecting the induction of long‐term potentiation (LTP). The inhibition of L‐type voltage‐gated calcium channels with 20 µM nifedipine or GABA‐A receptors with 1–30 µM picrotoxin did not occlude the effects of MMP inhibitors. However, MMP inhibition significantly reduced the LTP of NMDA receptor‐mediated EPSPs. Finally, the analysis of LTP saturation with multiple single (1× 100 Hz) or packed (4× 100 Hz) trains indicated that MMPs support E–S coupling evoked by selected synaptic activity patterns and set the ceiling for tetanically evoked E–S LTP. In conclusion, the activity of MMPs, particularly MMP‐3, regulated the magnitude of EPSPs and spike plasticity in the CA3 network and may affect information processing. Our data provide a novel link between MMP activity and neural excitability. Therefore, by limiting the number of firing neurons, MMP may functionally act beyond the synapse. © 2013 Wiley Periodicals, Inc.     

Ca2+/calmodulin-dependent protein kinase II and protein kinase C activities mediate extracellular glucose-regulated hippocampal synaptic efficacy

To define how extracellular glucose levels affect synaptic efficacy and long-term potentiation (LTP), we evaluated electrophysiological and neurochemical properties in hippocampal CA1 regions following alterations in glucose levels in the ACSF. In rat hippocampal slices prepared in ACSF with 3.5 mM glucose, fEPSPs generated by Schaffer collateral/commissural stimulation markedly increased when ACSF glucose levels were increased from 3.5 to 7.0 mM. The paired-pulse facilitation reflecting presynaptic transmitter release efficacy was significantly suppressed by elevation to 7.0 mM glucose because of potentiation of the input–output relationship (I/O relationship) of fEPSPs by single pulse stimulation. Prolonged potentiation of fEPSPs by elevation to 7.0 mM glucose coincided with increased autophosphorylation both of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and protein kinase Cα (PKCα). The increased I/O relationship of fEPSPs was also associated with markedly increased synapsin I phosphorylation by CaMKII. Transmitter-evoked postsynaptic currents were also measured in CA1 neurons by electrophoretical application of NMDA and AMPA to the apical dendrites of pyramidal neurons. NMDA- and AMPA-evoked currents were significantly augmented by elevation to 7.0 mM. Notably, high frequency stimulation of the Schaffer collateral/commissural pathway failed to induce LTP in the CA1 region at 3.5 mM glucose but LTP was restored dose-dependently by increasing glucose levels to 7.0 and 10.0 mM. LTP induction in the presence of 7.0 mM glucose was closely associated with further increases in CaMKII autophosphorylation without changes in PKCα autophosphorylation. Taken together, CaMKII and PKC activation likely mediate potentiation of fEPSPs by elevated glucose levels, and CaMKII activity is also associated with LTP induction in the hippocampal CA1 region.