Threshold For Long-term Potentiation Induction Research Articles

Quantitative analysis of the interaction between NMDA and AMPA receptors in glutamatergic synapses based on mathematical model

NMDA and AMPA receptors are co-localized at most glutamatergic synapses, where their numbers and distribution undergo dynamic changes. Glutamate binds to both the NMDA and AMPA receptors. Initially, I investigated whether there is competition between AMPA receptors and N-methyl-D-aspartic acid (NMDA) receptors for glutamate. Subsequently, I examined how these dynamic receptor changes affect synaptic response. To test the hypothesis, a synaptic model incorporating coexisting NMDA and AMPA receptors within the postsynaptic density (PSD) was developed. During long-term potentiation (LTP) induction, the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in the PSD increase. If there is competition for glutamate between AMPA receptors and NMDA receptors, the number of activated NMDA receptor channels will decrease. Since LTP induction relies on the activation of NMDA receptors, reducing their activation will raise the threshold for LTP induction. Consequently, the LTP of the synapse itself can establish negative feedback, preventing excessive dynamics and maintaining the stability of the neural network.

Individual NMDA receptor GluN2 subunit signaling domains differentially regulate the postnatal maturation of hippocampal excitatory synaptic transmission and plasticity but not dendritic morphology.

N-methyl-d-aspartate receptors (NMDARs) at hippocampal excitatory synapses undergo a late postnatal shift in subunit composition, from an initial prevalence of GluN2B subunit incorporation to a later predominance of GluN2A. This GluN2B to GluN2A shift alters NMDAR calcium conductance dynamics and intracellular molecular signaling that are individually regulated by distinct GluN2 signaling domains and temporally align with developmental alterations in dendritic and synaptic plasticity. However, the impacts of individual GluN2B to GluN2A signaling domains on neuronal development remain unknown. Ionotropic and intracellular signaling domains of GluN2 subunits were separated by creating chimeric GluN2 subunits that were expressed in two transgenic mouse lines. Western blot and immunoprecipitation revealed that roughly one third of native synaptic NMDARs were replaced by transformed NMDARs without altering total synaptic NMDAR content. Schaffer collateral synaptic strength was transiently increased in acutely prepared hippocampal slices at just over 3 weeks of age in animals overexpressing the GluN2B carboxy terminus. Long-term potentiation (LTP) induction following lower frequency stimulation was regulated by GluN2 ionotropic signaling domains in an age-dependent manner and LTP maintenance was enhanced by overexpression of the GluN2B CTD in mature animals. After higher frequency stimulation, the induction and maintenance of LTP were increased in young adult animals overexpressing the GluN2B ionotropic signaling domains but reduced in juveniles just over 3 weeks of age. Confocal imaging of green fluorescent protein (GFP)- labeled CA1 pyramidal neurons revealed no alterations in dendritic morphology or spine density in mice expressing chimeric GluN2 subunits. These results illustrate how individual GluN2 subunit signaling domains do or do not control physiological and morphological development of hippocampal excitatory neurons and better clarify the neurobiological factors that govern hippocampal maturation. SIGNIFICANCE STATEMENT: A developmental reduction in the magnitude of hippocampal long-term synaptic potentiation (LTP) and a concomitant improvement in spatial maze performance coincide with greater incorporation of GluN2A subunits into synaptic NMDARs. Corroborating our prior discovery that overexpression of GluN2A-type ionotropic signaling domains enables context-based navigation in immature mice, GluN2A-type ionotropic signaling domain overexpression reduces LTP induction threshold and magnitude in immature mice. Also, we previously found that GluN2B carboxy terminal domain (CTD) overexpression enhances long-term spatial memory in mature mice and now report that the GluN2B CTD is associated with greater amplitude of LTP after induction in mature mice. Thus, the late postnatal maturation of context encoding likely relies on a shift toward GluN2A-type ionotropic signaling and a reduction in the threshold to induce LTP while memory consolidation and LTP maintenance are regulated by GluN2B subunit CTD signaling.

Fear conditioning and extinction distinctively alter bidirectional synaptic plasticity within the amygdala of an animal model of post-traumatic stress disorder

Synaptic plasticity in the amygdala plays an essential role in the formation and inhibition of fear memory; however, this plasticity has mainly been studied in the lateral amygdala, making it largely uninvestigated in other subnuclei. Here, we investigated long-term potentiation (LTP) and long-term depression (LTD) in the basolateral amygdala (BLA) to the medial division of the central amygdala (CEm) synapses of juvenile C57BL/6N (B6) and 129S1/SvImJ (S1) mice. We found that in naïve B6 and S1 mice, LTP was not induced at the BLA to CEm synapses, whereas fear conditioning lowered the threshold for LTP induction in these synapses of both B6 and S1 mice. Interestingly, fear extinction disrupted the induction of LTP at the BLA to CEm synapses of B6 mice, whereas LTP was left intact in S1 mice. Both low-frequency stimulation (LFS) and modest LFS (mLFS) induced LTD in naïve B6 and S1 mice, suggesting that the BLA to CEm synapses express bidirectional plasticity. Fear conditioning disrupted both types of LTD induction selectively in S1 mice and LFS-LTD, presumably NMDAR-dependent LTD was partially recovered by fear extinction. However, mLFS-LTD which has been known to be endocannabinoid receptor 1 (CB1R)-dependent was not induced after fear extinction in both mouse strains. Our observations suggest that fear conditioning enhances LTP while fear extinction diminishes LTP at the BLA to the CEm synapses of B6 mice with successful extinction. Considering that S1 mice showed strong fear conditioning and impaired extinction, strong fear conditioning in the S1 strain may be related to disrupted LTD, and impaired extinction may be due to constant LTP and weak LFS-LTD at the BLA to CEm synapses. Our study contributes to the further understanding of the dynamics of synaptic potentiation and depression between the subnuclei of the amygdala in juvenile mice after fear conditioning and extinction.

Histamine and Corticosterone Modulate Acid Sensing Ion Channels (ASICs) Dependent Long-term Potentiation at the Mouse Anterior Cingulate Cortex

Increase in proton concentration [H+] or decrease in local and global extracellular pH occurs in both physiological and pathological conditions. Acid-sensing ion channels (ASICs), belonging to the ENaC/Deg superfamily, play an important role in signal transduction as proton sensor. ASICs and in particular ASIC1a (one of the six ASICs subunits) which is permeable to Ca2+, are involved in many physiological processes including synaptic plasticity and neurodegenerative diseases. Activity-dependent long-term potentiation (LTP) is a major type of long-lasting synaptic plasticity in the CNS, associated with learning, memory, development, fear and persistent pain. Neurons in the anterior cingulate cortex (ACC) play critical roles in pain perception and chronic pain and express ASIC1a channels. During synaptic transmission, acidification of the synaptic cleft presumably due to the co-release of neurotransmitter and H+ from synaptic vesicles activates postsynaptic ASIC1a channels in ACC of mice. This generates ASIC1a synaptic currents that add to the glutamatergic excitatory postsynaptic currents (EPSCs). Here we report that modulators like histamine and corticosterone, acting through ASIC1a regulate synaptic plasticity, reducing the threshold for LTP induction of glutamatergic EPSCs. Our findings suggest a new role for ASIC1a mediating the neuromodulator action of histamine and corticosterone regulating specific forms of synaptic plasticity in the mouse ACC.

The Effects of Exercise on Long-Term Potentiation: A Candidate Mechanism of the Exercise-Memory Relationship

Objective: The objective of this paper was to evaluate the extent to which exercise may influence long-term potentiation (LTP), a key cellular correlate of episodic memory function. Methods: Studies were identified using electronic databases, including PubMed, PsychInfo, Sports Discus and Google Scholar. Results: The computerized searches revealed 20 articles meeting the study criteria. Among these 20 evaluated articles, 17 were conducted in an animal model and 3 among humans. All 17 of these studies, with the exception of one, provided evidence that exercise enhances LTP. Each of the three human studies demonstrated evidence that exercise was favorably associated with LTP or LTP-like mechanisms. In animal models, exercise reliability increases LTP and lowers the threshold for LTP induction. Conclusion: Chronic exercise appears to robustly enhance LTP. The mechanisms of this effect are multifold and include, for example, exercise modulation of the structure and function of NMDA receptors.

Repeated shock stress facilitates basolateral amygdala synaptic plasticity through decreased cAMP-specific phosphodiesterase type IV (PDE4) expression.

Previous studies have shown that exposure to stressful events can enhance fear memory and anxiety-like behavior as well as increase synaptic plasticity in the rat basolateral amygdala (BLA). We have evidence that repeated unpredictable shock stress (USS) elicits a long-lasting increase in anxiety-like behavior in rats, but the cellular mechanisms mediating this response remain unclear. Evidence from recent morphological studies suggests that alterations in the dendritic arbor or spine density of BLA principal neurons may underlie stress-induced anxiety behavior. Recently, we have shown that the induction of long-term potentiation (LTP) in BLA principal neurons is dependent on activation of postsynaptic D1 dopamine receptors and the subsequent activation of the cyclic adenosine 5'-monophosphate (cAMP)-protein kinase A (PKA) signaling cascade. Here, we have used in vitro whole-cell patch-clamp recording from BLA principal neurons to investigate the long-term consequences of USS on their morphological properties and synaptic plasticity. We provided evidence that the enhanced anxiety-like behavior in response to USS was not associated with any significant change in the morphological properties of BLA principal neurons, but was associated with a changed frequency dependence of synaptic plasticity, lowered LTP induction threshold, and reduced expression of phosphodiesterase type 4 enzymes (PDE4s). Furthermore, pharmacological inhibition of PDE4 activity with rolipram mimics the effects of chronic stress on LTP induction threshold and baseline startle. Our results provide the first evidence that stress both enhances anxiety-like behavior and facilitates synaptic plasticity in the amygdala through a common mechanism of PDE4-mediated disinhibition of cAMP-PKA signaling.

Extrasynaptic NMDA receptor dependent long-term potentiation of hippocampal CA1 pyramidal neurons

In the adult mouse hippocampus, NMDA receptors (NMDARs) of CA1 neurons play an important role in the synaptic plasticity. The location of NMDARs can determine their roles in the induction of long-term potentiation (LTP). However, the extrasynaptic NMDARs (ES-NMDARs) dependent LTP haven’t been reported. Here, through the use of a 5-Hz stimulation and MK-801 (an irreversible antagonist of NMDARs) in the CA1 neurons of adult mice hippocampal slices, synaptic NMDARs were selectively inhibited and NMDAR-mediated excitatory postsynaptic currents were not recovered. We found that a robust LTP was induced by 3-train 100-Hz stimulation when the synaptic NMDARs and extrasynaptic NR2B containing NMDARs were blocked, but not in the any of the following conditions: blocking of all NMDARs (synaptic and extrasynaptic), blocking of the synaptic NMDARs, and blocking of the synaptic NMDARs and extrasynaptic NR2A-containing NMDARs. The results indicate that this LTP is ES-NMDARs dependent, and NR2B-containing ES-NMDARs modulates the threshold of LTP induction.

Early postnatal nicotine exposure causes hippocampus-dependent memory impairments in adolescent mice: Association with altered nicotinic cholinergic modulation of LTP, but not impaired LTP

Fetal nicotine exposure from smoking during pregnancy causes long-lasting cognitive impairments in offspring, yet little is known about the mechanisms that underlie this effect. Here we demonstrate that early postnatal exposure of mouse pups to nicotine via maternal milk impairs long-term, but not short-term, hippocampus-dependent memory during adolescence. At the Schaffer collateral (SC) pathway, the most widely studied synapses for a cellular correlate of hippocampus-dependent memory, the induction of N-methyl-D-aspartate receptor-dependent transient long-term potentiation (LTP) and protein synthesis-dependent long-lasting LTP are not diminished by nicotine exposure, but rather unexpectedly the threshold for LTP induction becomes lower after nicotine treatment. Using voltage sensitive dye to visualize hippocampal activity, we found that early postnatal nicotine exposure also results in enhanced CA1 depolarization and hyperpolarization after SC stimulation. Furthermore, we show that postnatal nicotine exposure induces pervasive changes to the nicotinic modulation of CA1 activity: activation of nicotinic receptors no longer increases CA1 network depolarization, acute nicotine inhibits rather than facilitates the induction of LTP at the SC pathway by recruiting an additional nicotinic receptor subtype, and acute nicotine no longer blocks LTP induction at the temporoammonic pathway. These findings reflect the pervasive impact of nicotine exposure during hippocampal development, and demonstrate an association of hippocampal memory impairments with altered nicotinic cholinergic modulation of LTP, but not impaired LTP. The implication of our results is that nicotinic cholinergic-dependent plasticity is required for long-term memory formation and that postnatal nicotine exposure disrupts this form of plasticity.

Regulation of hippocampal synaptic plasticity thresholds and changes in exploratory and learning behavior in dominant negative NPR-B mutant rats.

The second messenger cyclic GMP affects synaptic transmission and modulates synaptic plasticity and certain types of learning and memory processes. The impact of the natriuretic peptide receptor B (NPR-B) and its ligand C-type natriuretic peptide (CNP), one of several cGMP producing signaling systems, on hippocampal synaptic plasticity and learning is, however, less well understood. We have previously shown that the NPR-B ligand CNP increases the magnitude of long-term depression (LTD) in hippocampal area CA1, while reducing the induction of long-term potentiation (LTP). We have extended this line of research to show that bidirectional plasticity is affected in the opposite way in rats expressing a dominant-negative mutant of NPR-B (NSE-NPR-BΔKC) lacking the intracellular guanylyl cyclase domain under control of a promoter for neuron-specific enolase. The brain cells of these transgenic rats express functional dimers of the NPR-B receptor containing the dominant-negative NPR-BΔKC mutant, and therefore show decreased CNP-stimulated cGMP-production in brain membranes. The NPR-B transgenic rats display enhanced LTP but reduced LTD in hippocampal slices. When the frequency-dependence of synaptic modification to afferent stimulation in the range of 1–100 Hz was assessed in transgenic rats, the threshold for both, LTP and LTD induction, was shifted to lower frequencies. In parallel, NPR-BΔKC rats exhibited an enhancement in exploratory and learning behavior. These results indicate that bidirectional plasticity and learning and memory mechanism are affected in transgenic rats expressing a dominant-negative mutant of NPR-B. Our data substantiate the hypothesis that NPR-B-dependent cGMP signaling has a modulatory role for synaptic information storage and learning.

Early deprivation reduced anxiety and enhanced memory in adult male rats.

The effects of early deprivation (ED, which involves both dam and littermate deprivation) on anxiety and memory are less investigated in comparison with maternal separation (MS), and it is not yet clear how ED affects long-term potentiation (LTP) in the hippocampal Schaffer collateral pathway. By using a series of behavioral tests, enzyme-linked immunosorbent assay and field potential recording, we explored the effect of pre-weaning daily 3-h ED on anxiety, memory and potential mechanisms in adult male rats. Compared with control, ED rats spent longer time in open arms of elevated plus maze and in light compartment of light-dark transition box. Consistently, stress-induced blood plasma corticosterone level was also lower in ED rats. Moreover, ED rats showed better performance in social recognition and Morris water maze test. In accordance with results in memory tests, the threshold of LTP induction in hippocampal CA3-CA1 pathway of ED rats was also reduced. Our results indicate ED reduced anxiety, but enhanced social recognition and spatial reference memory. We suggest the diminished hypothalamic-pituitary-adrenal axis response and facilitated hippocampal LTP may contribute to the anxiety-reducing and memory-enhancing effects of ED, respectively.

Interleukin-1β Promotes Long-Term Potentiation in Patients with Multiple Sclerosis

The immune system shapes synaptic transmission and plasticity in experimental autoimmune encephalomyelitis (EAE), the mouse model of multiple sclerosis (MS). These synaptic adaptations are believed to drive recovery of function after brain lesions, and also learning and memory deficits and excitotoxic neurodegeneration; whether inflammation influences synaptic plasticity in MS patients is less clear. In a cohort of 59 patients with MS, we found that continuous theta-burst transcranial magnetic stimulation did not induce the expected long-term depression (LTD)-like synaptic phenomenon, but caused persisting enhancement of brain cortical excitability. The amplitude of this long-term potentiation (LTP)-like synaptic phenomenon correlated with the concentration of the pro-inflammatory cytokine interleukin-1β (IL-1β) in the cerebrospinal fluid. In MS and EAE, the brain and spinal cord are typically enriched of CD3(+) T lymphocyte infiltrates, which are, along with activated microglia and astroglia, a major cause of inflammation. Here, we found a correlation between the presence of infiltrating T lymphocytes in the hippocampus of EAE mice and synaptic plasticity alterations. We observed that T lymphocytes from EAE, but not from control mice, release IL-1β and promote LTP appearance over LTD, thereby mimicking the facilitated LTP induction observed in the cortex of MS patients. EAE-specific T lymphocytes were able to suppress GABAergic transmission in an IL-1β-dependent manner, providing a possible synaptic mechanism able to lower the threshold of LTP induction in MS brains. Moreover, in vivo blockade of IL-1β signaling resulted in inflammation and synaptopathy recovery in EAE hippocampus. These data provide novel insights into the pathophysiology of MS.

Reduced Threshold for Induction of LTP by Activation of Dopamine D1/D5 Receptors at Hippocampal CA1–Subiculum Synapses

The phasic release of dopamine in the hippocampal formation has been shown to facilitate the encoding of novel information. There is evidence that the subiculum operates as a detector and distributor of sensory information, which incorporates the novelty and relevance of signals received from CA1. The subiculum acts as the final hippocampal relay station for outgoing information. Subicular pyramidal cells have been classified as regular- and burst-spiking neurons. The goal of the present study was to study the effect of dopamine D1/D5 receptor activation on synaptic transmission and plasticity in the subicular regular-spiking neurons of 4–6 week old Wistar rats. We demonstrate that prior activation of D1/D5 receptors reduces the threshold for the induction of long-term potentiation (LTP) in subicular regular-spiking neurons. Our results indicate that D1/D5 receptor activation facilitates a postsynaptic form of LTP in subicular regular-spiking cells that is NMDA receptor-dependent, relies on postsynaptic Ca2+ signaling, and requires the activation of protein kinase A. The enhanced propensity of subicular regular-spiking cells to express postsynaptic LTP after activation of D1/D5 receptors provides an intriguing mechanism for the encoding of hippocampal output information.

The relationship between tetanus intensity and the magnitude of hippocampal long-term potentiation in vivo

In this study, we assessed the effects of varying tetanus and test-pulse intensity on the magnitude of long-term potentiation (LTP) in the perforant path–dentate gyrus projection of urethane-anaesthetized rats. We developed a novel within-subjects procedure in which test-pulse-stimulation intensity (60–1000μA) was varied quasi-randomly under computer control throughout the recording period. After a baseline period, we applied a high-frequency tetanus, the intensity of which was varied over the same range as test-pulse intensity, but between subjects. The time-course of LTP was thus monitored continuously across a range of test-pulse intensities in each rat. Intense high-frequency tetanization at 1000μA resulted in a paradoxical depression of the dentate field excitatory post-synaptic potential (fEPSP) slope at the lowest test intensity used (60μA), but caused a potentiation at higher test intensities in the same animal. Moreover, intense tetanization induced less LTP than a moderate tetanus over most of the test-intensity range. Explanations for this pattern of data include a potentiation of feed-forward inhibition in conjunction with LTP of excitatory neurotransmission, or local tissue damage at the stimulation site. To address this issue, we conducted an additional experiment in which a second stimulating electrode was placed in the perforant path at a site closer to the dentate, in order to activate a common population of afferents at a location ‘downstream’ of the original stimulation site. After 1000-μA tetanization of the original (‘upstream’) site, fEPSPs were again depressed in response to test stimulation of the upstream site, but only potentiation was observed in response to stimulation of the downstream site. This is consistent with the idea that the depression induced by intense tetanization results from local changes at the stimulation site. In conclusion, while tetanus intensity must exceed the LTP induction threshold, intensities above 500μA should be avoided; in the present study, tetanization at 250–500μA yielded maximal levels of LTP.

Neurogranin targets calmodulin and lowers the threshold for the induction of long-term potentiation.

Calcium entry and the subsequent activation of CaMKII trigger synaptic plasticity in many brain regions. The induction of long-term potentiation (LTP) in the CA1 region of the hippocampus requires a relatively high amount of calcium-calmodulin. This requirement is usually explained, based on in vitro and theoretical studies, by the low affinity of CaMKII for calmodulin. An untested hypothesis, however, is that calmodulin is not randomly distributed within the spine and its targeting within the spine regulates LTP. We have previously shown that overexpression of neurogranin enhances synaptic strength in a calmodulin-dependent manner. Here, using post-embedding immunogold labeling, we show that calmodulin is not randomly distributed, but spatially organized in the spine. Moreover, neurogranin regulates calmodulin distribution such that its overexpression concentrates calmodulin closer to the plasma membrane, where a high level of CaMKII immunogold labeling is also found. Interestingly, the targeting of calmodulin by neurogranin results in lowering the threshold for LTP induction. These findings highlight the significance of calmodulin targeting within the spine in synaptic plasticity.

Role of AMPA receptor regulation in stress-induced prolongation of incisional pain

Chronic pain after surgery is a serious issue in clinical practice that has received increased interest in recent years. Previous studies have shown that psychosocial and socio-environmental factors are involved in the development of chronic postsurgical pain. However, it is unclear how the transition from acute to chronic postsurgical pain occurs. In our studies, we found that forced swim stress markedly enhanced plantar incision-induced α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor phosphorylation in the spinal cord and greatly prolonged plantar incision-induced pain, but forced swim stress alone did not produce pain behaviors. We also found that targeted mutation of AMPA receptor GluA1 phosphorylation site Ser831 significantly inhibited stress-induced prolongation of incisional pain. It has been demonstrated that the stress hormone norepinephrine enhances AMPA receptor phosphorylation and in turn facilitates the synaptic delivery of GluA1-containing AMPA receptors, thereby lowering the threshold for long-term potentiation induction and increasing the probability of synaptic plasticity. Therefore, AMPA receptor regulation may play an important role in spinal neuronal plasticity by which chronic postsurgical pain develops after surgery. Chronic pain after surgery is a serious issue in clinical practice that has received increased interest in recent years. Previous studies have shown that psychosocial and socio-environmental factors are involved in the development of chronic postsurgical pain. However, it is unclear how the transition from acute to chronic postsurgical pain occurs. In our studies, we found that forced swim stress markedly enhanced plantar incision-induced α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor phosphorylation in the spinal cord and greatly prolonged plantar incision-induced pain, but forced swim stress alone did not produce pain behaviors. We also found that targeted mutation of AMPA receptor GluA1 phosphorylation site Ser831 significantly inhibited stress-induced prolongation of incisional pain. It has been demonstrated that the stress hormone norepinephrine enhances AMPA receptor phosphorylation and in turn facilitates the synaptic delivery of GluA1-containing AMPA receptors, thereby lowering the threshold for long-term potentiation induction and increasing the probability of synaptic plasticity. Therefore, AMPA receptor regulation may play an important role in spinal neuronal plasticity by which chronic postsurgical pain develops after surgery.

Metaplasticity gated through differential regulation of GluN2A versus GluN2B receptors by Src family kinases

Metaplasticity is a higher form of synaptic plasticity that is essential for learning and memory, but its molecular mechanisms remain poorly understood. Here, we report that metaplasticity of transmission at CA1 synapses in the hippocampus is mediated by Src family kinase regulation of NMDA receptors (NMDARs). We found that stimulation of G-protein-coupled receptors (GPCRs) regulated the absolute contribution of GluN2A-versus GluN2B-containing NMDARs in CA1 neurons: pituitary adenylate cyclase activating peptide 1 receptors (PAC1Rs) selectively recruited Src kinase, phosphorylated GluN2ARs, and enhanced their functional contribution; dopamine 1 receptors (D1Rs) selectively stimulated Fyn kinase, phosphorylated GluN2BRs, and enhanced these currents. Surprisingly, PAC1R lowered the threshold for long-term potentiation while long-term depression was enhanced by D1R. We conclude that metaplasticity is gated by the activity of GPCRs, which selectively target subtypes of NMDARs via Src kinases.

Synergistic Activation of Dopamine D1 and TrkB Receptors Mediate Gain Control of Synaptic Plasticity in the Basolateral Amygdala

Fear memory formation is thought to require dopamine, brain-derived neurotrophic factor (BDNF) and zinc release in the basolateral amygdala (BLA), as well as the induction of long term potentiation (LTP) in BLA principal neurons. However, no study to date has shown any relationship between these processes in the BLA. Here, we have used in vitro whole-cell patch clamp recording from BLA principal neurons to investigate how dopamine, BDNF, and zinc release may interact to modulate the LTP induction in the BLA. LTP was induced by either theta burst stimulation (TBS) protocol or spaced 5 times high frequency stimulation (5xHFS). Significantly, both TBS and 5xHFS induced LTP was fully blocked by the dopamine D1 receptor antagonist, SCH23390. LTP induction was also blocked by the BDNF scavenger, TrkB-FC, the zinc chelator, DETC, as well as by an inhibitor of matrix metalloproteinases (MMPs), gallardin. Conversely, prior application of the dopamine reuptake inhibitor, GBR12783, or the D1 receptor agonist, SKF39393, induced robust and stable LTP in response to a sub-threshold HFS protocol (2xHFS), which does not normally induce LTP. Similarly, prior activation of TrkB receptors with either a TrkB receptor agonist, or BDNF, also reduced the threshold for LTP-induction, an effect that was blocked by the MEK inhibitor, but not by zinc chelation. Intriguingly, the TrkB receptor agonist-induced reduction of LTP threshold was fully blocked by prior application of SCH23390, and the reduction of LTP threshold induced by GBR12783 was blocked by prior application of TrkB-FC. Together, our results suggest a cellular mechanism whereby the threshold for LTP induction in BLA principal neurons is critically dependent on the level of dopamine in the extracellular milieu and the synergistic activation of postsynaptic D1 and TrkB receptors. Moreover, activation of TrkB receptors appears to be dependent on concurrent release of zinc and activation of MMPs.

Deletion of neuronal gap junction protein connexin 36 impairs hippocampal LTP

In the mammalian CNS, deletion of neuronal gap junction protein, connexin 36 (Cx36), causes deficiencies in learning and memory. Here we tested whether Cx36 deletion affects the hippocampal long-term potentiation (LTP), which is considered as a cellular model of learning and memory mechanisms. We report that in acute slices of the hippocampal CA1 area, LTP is reduced in Cx36 knockout mice as compared to wild-type mice. Western blot analysis of NMDA receptor subunits indicates a higher NR2A/NR2B ratio in Cx36 knockout mice, indicating that there is shift in the threshold for LTP induction in knockout animals. Data suggest a possibility that learning and memory deficiencies in Cx36 knockout mice are due to deficiencies in LTP mechanisms.

Enhanced synaptic plasticity in mice with phosphomimetic mutation of the GluA1 AMPA receptor

Phosphorylation of the GluA1 subunit of AMPA receptors has been proposed to regulate receptor trafficking and synaptic transmission and plasticity. However, it remains unclear whether GluA1 phosphorylation is permissive or sufficient for enacting these functional changes. Here we investigate the role of GluA1 phosphorylation at S831 and S845 residues in the hippocampus through the analyses of GluA1 S831D/S845D phosphomimetic knock-in mice. S831D/S845D mice showed normal total and surface expression and subcellular localization of GluA1 as well as intact basal synaptic transmission. In addition, theta-burst stimulation, a protocol that was sufficient to induce robust long-term potentiation (LTP) in WT mice, resulted in LTP of similar magnitude in S831D/S845D mice. However, S831D/S845D mice showed LTP induced with 10-Hz stimulation, a protocol that is weaker than theta-burst stimulation and was not sufficient to induce LTP in WT mice. Moreover, S831D/S845D mice exhibited LTP induced with spike-timing-dependent plasticity (STDP) protocol at a long pre-post interval that was subthreshold for WT mice, although a suprathreshold STDP protocol at a short pre-post interval resulted in similarly robust LTP for WT and S831D/S845D mice. These results indicate that phosphorylation of GluA1 at S831 and S845 is sufficient to lower the threshold for LTP induction, increasing the probability of synaptic plasticity.

C-type natriuretic peptide modulates bidirectional plasticity in hippocampal area CA1 in vitro

C-type natriuretic peptide (CNP) and the natriuretic peptide receptor B (NPR-B) are expressed throughout the hippocampus. We tested whether CNP affected long-term potentiation (LTP) or long-term depression (LTD) in area CA1. Field potentials (FP) were simultaneously recorded in stratum pyramidale (SP) and stratum radiatum (SR) of area CA1 in rat hippocampal slices. To induce LTD and LTP stimulation was applied to SR in area CA1 at 1 and 5 Hz and 30–100 Hz, respectively. CNP (100 nM) increased LTD magnitude while LTP induction was impeded. Thus, in the presence of CNP the threshold for LTP induction was shifted to higher stimulus frequencies, a modulation that showed layer-specific differences in area CA1. Effects of CNP were prevented by the NPR-B antagonist HS-142-1. In the presence of the GABA A receptor blocker bicuculline (BMI, 5 μM), CNP-mediated effects were attenuated in SP and SR. Intracellular recordings under this condition revealed that CNP significantly reduced number of action potentials generated during depolarizing current steps. The input resistance of CA1 cells and amplitude of isolated excitatory postsynaptic potential (EPSPs) were significantly increased by CNP whereas these changes were not observed in the absence of BMI. 100 Hz stimulation induced stable potentiation of the EPSP amplitude in CA1 pyramidal cells while this effect was strongly attenuated by CNP. This effect was prevented by BMI. Immunohistochemistry indicated that the peptide binds to receptors expressed on pyramidal cells and GAD 65/67-immunopositive interneurons. 20 Hz stimulation, applied for 30 s, induced LTP in SR and SP. CNP attenuated LTP in SP and reversed LTP into LTD in SR. These effects were mimicked by low-dose dl -2-amino-5-phosphonopentanoic acid ( dl -APV) (10 μM) suggesting partial N-methyl d -aspartate (NMDA) receptor dependency of CNP-mediated effects. Together, our data suggest that CNP is involved in the regulation of bidirectional plasticity in area CA1 potentially by modulating GABA A -mediated inhibition and NMDA receptors.