Low-frequency stimulation induces a pathway-specific late phase of LTP in the amygdala that is mediated by PKA and dependent on protein synthesis

Activity-dependent changes in synaptic efficacy are thought to be the key cellular mechanism for the formation and storage of both explicit and implicit memory. Different patterns of stimulation can elicit different changes in the efficiency on excitatory synaptic transmission. Here, we examined the synaptic changes in the amygdala of adult mice produced by low-frequency stimulation (1 Hz, 15 min, LFS). We first compared the synaptic changes induced by LFS in three different synaptic pathways of amygdala: cortical–lateral amygdala, thalamic–lateral amygdala, and lateral–basolateral amygdala pathways. We find that the plastic changes induced by LFS are different between synaptic pathways. Low-frequency stimulation selectively elicits a slow onset and protein synthesis-dependent late-phase LTP in the cortical–lateral amygdala pathway, but not in the thalamic–lateral or lateral–basolateral pathways. We next analyzed LTP induced by LFS in the cortical–lateral amygdala pathway and found that three PKA-coupling neurotransmitter receptors are involved: 5-HT4, Dopamine D1, and β-adrenergic receptors. Antagonists of these receptors block the LFS L-LTP, but the effects of agonists of these receptors are clearly different. These results indicate that the threshold for the induction of LFS L-LTP is different among these pathways and that the maintenance of LFS L-LTP requires a cross-talk among multiple neurotransmitters.     

Alzheimer amyloid beta-peptide inhibits the late phase of long-term potentiation through calcineurin-dependent mechanisms in the hippocampal dentate gyrus

The perforant path projecting from the entorhinal cortex to the hippocampal dentate gyrus is a particularly vulnerable target to the early deposition of amyloid beta (Abeta) peptides in Alzheimer's brain. The authors previously showed that brief applications of Abeta at subneurotoxic concentrations suppressed the early-phase long-term potentiation (E-LTP) in rat dentate gyrus. The current study further examines the effect of Abeta on the late-phase LTP (L-LTP) in this area. Using multiple high-frequency stimulus trains, a stable L-LTP lasting for at least 3 h was induced in the medial perforant path of rat hippocampal slices. Bath application of Abeta(1-42) (0.2-1.0 microM) during the induction trains attenuated both the initial and late stages of L-LTP. On the other hand, Abeta(1-42) perfusion within the first hour following the induction primarily impaired the late stage of L-LTP, which resembled the action of the protein synthesis inhibitor emetine. Blockade of calcineurin activity with FK506 or cyclosporin A completely prevented Abeta-induced L-LTP deficits. These results suggest that Abeta(1-42) impaired both the induction and maintenance phase of dentate L-LTP through calcineurin-dependent mechanisms. In the concentration range effective for inhibiting L-LTP, Abeta(1-42) also reduced the amplitude of NMDA receptor-mediated synaptic currents in dentate granule cells via a postsynaptic mechanism. In addition, concurrent applications of Abeta(1-42) with the protein synthesis inhibitor caused no additive reduction of L-LTP, indicating a common mechanism underlying the action of both. Thus, inhibition of NMDA receptor channels and disruption of protein synthesis were two possible mechanisms contributing to Abeta-induced L-LTP impairment.     

Protein kinase C mediates amyloid β-protein fragment 31–35-induced suppression of hippocampal late-phase long-term potentiation in vivo

Amyloid β-protein (Aβ) in the brain of Alzheimer’s disease (AD) plays a detrimental role in synaptic plasticity and cognitive function. The effects of Aβ on the early-phase long-term potentiation (E-LTP) have been reported widely. However, whether the late-phase long-term potentiation (L-LTP), which differs from E-LTP mechanistically, is also affected by Aβ is still an open question. The present study examined the effects of intracerebraventricular injection of Aβ fragments 25–35 and 31–35 on the L-LTP in the CA1 area of rat hippocampus in vivo, and further investigated its possible underlying mechanism. Our results showed that: (1) Aβ25–35 (6.25–25 nmol) did not affect the baseline field excitatory postsynaptic potentials, but dose-dependently suppressed multiple high-frequency stimuli-induced L-LTP; (2) Aβ31–35, a shorter Aβ fragment than Aβ25–35, also significantly suppressed L-LTP, with the same suppressive effects as Aβ25–35; (3) pretreatment with PMA (6 nmol/5 μl), a membrane permeable PKC agonist, effectively prevented Aβ31–35-induced deficits in the early and the late components of L-LTP; (4) co-application of Aβ31–35 and chelerythrine (12 nmol/5 μl), a PKC antagonist, caused no additive suppression of L-LTP. These results indicate that both Aβ25–35 and Aβ31–35 can impair hippocampal synaptic plasticity in vivo by suppressing the maintenance of L-LTP, and PKC probably mediates the Aβ-induced suppression of hippocampal L-LTP. In addition, the similar efficacy of Aβ31–35 and Aβ25–35 in L-LTP suppression supports the hypothesis we suggested previously that the sequence 31–35 in Aβ might be the shortest active sequence responsible for the neuronal toxicity induced by full length of Aβ molecules.     

Protein kinase Mzeta is essential for the induction and maintenance of dopamine-induced long-term potentiation in apical CA1 dendrites

Dopaminergic D1/D5-receptor-mediated processes are important for certain forms of memory as well as for a cellular model of memory, hippocampal long-term potentiation (LTP) in the CA1 region of the hippocampus. D1/D5-receptor function is required for the induction of the protein synthesis-dependent maintenance of CA1-LTP (L-LTP) through activation of the cAMP/PKA-pathway. In earlier studies we had reported a synergistic interaction of D1/D5-receptor function and N-methyl-D-aspartate (NMDA)-receptors for L-LTP. Furthermore, we have found the requirement of the atypical protein kinase C isoform, protein kinase Mζ (PKMζ) for conventional electrically induced L-LTP, in which PKMζ has been identified as a LTP-specific plasticity-related protein (PRP) in apical CA1-dendrites. Here, we investigated whether the dopaminergic pathway activates PKMζ. We found that application of dopamine (DA) evokes a protein synthesis-dependent LTP that requires synergistic NMDA-receptor activation and protein synthesis in apical CA1-dendrites. We identified PKMζ as a DA-induced PRP, which exerted its action at activated synaptic inputs by processes of synaptic tagging.     

Cholinergic afferents to the locus coeruleus and noradrenergic afferents to the medial septum mediate LTP-reinforcement in the dentate gyrus by stimulation of the amygdala

Transient long-term potentiation (E-LTP) can be transformed into a long-lasting LTP (L-LTP) in the dentate gyrus (DG) by behavioral stimuli with high motivational content. Previous research from our group has identified several brain structures, such as the basolateral amygdala (BLA), the locus coeruleus (LC), the medial septum (MS) and transmitters as noradrenaline (NA) and acetylcholine (ACh) that are involved in these processes. Here we have investigated the functional interplay among brain structures and systems which result in the conversion of a E-LTP into a L-LTP (reinforcement) by stimulation of the BLA (BLA-R). We used topical application of specific drugs into DG, and other targets, while following the time course of LTP induced by stimulation of the perforant pathway (PP) to study their specific contribution to BLA-R. One injection cannula, a recording electrode in the DG and stimulating electrodes in the PP and the BLA were stereotactically implanted one week before electrophysiological experiments. Topical application of atropine or propranolol into the DG blocked BLA-R in both cases, but the effect of propranolol occurred earlier, suggesting a role of NA within the DG during an intermediate stage of LTP maintenance. The injection of lidocaine into the LC abolished BLA-R indicating that the LC is part of the functional neural reinforcing system. The effect on the LC is mediated by cholinergic afferents because application of atropine into the LC produced the same effect. Injection of lidocaine inactivating the MS also abolished BLA-R. This effect was mediated by noradrenergic afferents (probably from the LC) because the application of propranolol into the MS prevented BLA-R. These findings suggest a functional loop for BLA-R involving cholinergic afferents to the LC, a noradrenergic projection from the LC to the DG and the MS, and finally, the cholinergic projection from the MS to the DG.     

Concentration-dependent effects of protein phosphatase (PP) inhibitors implicate PP1 and PP2A in different stages of memory formation

Numerous studies have demonstrated roles for protein phosphorylation and for specific kinases in memory formation; however, a role for specific protein phosphatases has not been established. Previous studies using pharmacobehavioral methods to implicate protein phosphatase activity in memory formation have been unable to discriminate between protein phosphatases 1 (PP1) and 2A (PP2A), as available cell-permeable agents generally inhibit both enzyme classes. To address this difficulty the present study exploited differences in the potency of the selective phosphatase inhibitor, okadaic acid, toward PP1 and PP2A. Within the context of a temporally precise animal model of memory, developed using the day-old chick (Gallus domesticus), acute administration of various concentrations of okadaic acid was found to disrupt two temporally distinct stages of memory formation. When administered bilaterally into an area of the chick brain implicated in memory formation, concentrations of okadaic acid known to selectively inhibit PP2A in vitro disrupted memory from 50 min posttraining. Higher concentrations, reported to inhibit both PP2A and PP1 in vitro, produced significant retention deficits from 20 min posttraining. Identical temporally specific effects were also obtained by varying the concentration and time of administration of calyculin A, a phosphatase inhibitor with equal potency toward both enzyme classes. Hence, different phosphatase enzymes may contribute to different stages of the enzymatic cascade believed to underlie memory formation.     

Removal of S6K1 and S6K2 leads to divergent alterations in learning, memory, and synaptic plasticity

Protein synthesis is required for the expression of enduring memories and long-lasting synaptic plasticity. During cellular proliferation and growth, S6 kinases (S6Ks) are activated and coordinate the synthesis of de novo proteins. We hypothesized that protein synthesis mediated by S6Ks is critical for the manifestation of learning, memory, and synaptic plasticity. We have tested this hypothesis with genetically engineered mice deficient for either S6K1 or S6K2. We have found that S6K1-deficient mice express an early-onset contextual fear memory deficit within one hour of training, a deficit in conditioned taste aversion (CTA), impaired Morris water maze acquisition, and hypoactive exploratory behavior. In contrast, S6K2-deficient mice exhibit decreased contextual fear memory seven days after training, a reduction in latent inhibition of CTA, and normal spatial learning in the Morris water maze. Surprisingly, neither S6K1- nor S6K2-deficient mice exhibited alterations in protein synthesis-dependent late-phase long-term potentiation (L-LTP). However, removal of S6K1, but not S6K2, compromised early-phase LTP expression. Furthermore, we observed that S6K1-deficient mice have elevated basal levels of Akt phosphorylation, which is further elevated following induction of L-LTP. Taken together, our findings demonstrate that removal of S6K1 leads to a distinct array of behavioral and synaptic plasticity phenotypes that are not mirrored by the removal of S6K2. Our observations suggest that neither gene by itself is required for L-LTP but instead may be required for other types of synaptic plasticity required for cognitive processing.     

'Silent' priming of translation-dependent LTP by ß-adrenergic receptors involves phosphorylation and recruitment of AMPA receptors

The capacity for long-term changes in synaptic efficacy can be altered by prior synaptic activity, a process known as "metaplasticity." Activation of receptors for modulatory neurotransmitters can trigger downstream signaling cascades that persist beyond initial receptor activation and may thus have metaplastic effects. Because activation of β-adrenergic receptors (β-ARs) strongly enhances the induction of long-term potentiation (LTP) in the hippocampal CA1 region, we examined whether activation of these receptors also had metaplastic effects on LTP induction. Our results show that activation of β-ARs induces a protein synthesis-dependent form of metaplasticity that primes the future induction of late-phase LTP by a subthreshold stimulus. β-AR activation also induced a long-lasting increase in phosphorylation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) GluA1 subunits at a protein kinase A (PKA) site (S845) and transiently activated extracellular signal-regulated kinase (ERK). Consistent with this, inhibitors of PKA and ERK blocked the metaplastic effects of β-AR activation. β-AR activation also induced a prolonged, translation-dependent increase in cell surface levels of GluA1 subunit-containing AMPA receptors. Our results indicate that β-ARs can modulate hippocampal synaptic plasticity by priming synapses for the future induction of late-phase LTP through up-regulation of translational processes, one consequence of which is the trafficking of AMPARs to the cell surface.     

Neural functions of calcineurin in synaptic plasticity and memory

Major brain functions depend on neuronal processes that favor the plasticity of neuronal circuits while at the same time maintaining their stability. The mechanisms that regulate brain plasticity are complex and engage multiple cascades of molecular components that modulate synaptic efficacy. Protein kinases (PKs) and phosphatases (PPs) are among the most important of these components that act as positive and negative regulators of neuronal signaling and plasticity, respectively. In these cascades, the PP protein phosphatase 2B or calcineurin (CaN) is of particular interest because it is the only Ca(2+)-activated PP in the brain and a major regulator of key proteins essential for synaptic transmission and neuronal excitability. This review describes the primary properties of CaN and illustrates its functions and modes of action by focusing on several representative targets, in particular glutamate receptors, striatal enriched protein phosphatase (STEP), and neuromodulin (GAP43), and their functional significance for synaptic plasticity and memory.     

Hippocampal low-frequency stimulation and chronic mild stress similarly disrupt fear extinction memory in rats

Disruptions of fear extinction-related potentiation of synaptic efficacy in the connection between the hippocampus (HPC) and the medial prefrontal cortex (mPFC) have been shown to impair the recall of extinction memory. This study was undertaken to examine if chronic mild stress (CMS), which is known to alter induction of HPC–mPFC long-term potentiation, would also interfere with both extinction-related HPC–mPFC potentiation and extinction memory. Following fear conditioning (5 tone-shock pairings), rats were submitted to fear extinction (20 tone-alone presentations), which produced an increase in the amplitude of HPC–mPFC field potentials. HPC low-frequency stimulation (LFS), applied immediately after training, suppressed these changes and induced fear return during the retention test (5 tone-alone presentations). CMS, delivered before fear conditioning, did not interfere with fear extinction but blocked the development of extinction-related potentiation in the HPC–mPFC pathway and impaired the recall of extinction. These findings suggest that HPC LFS may provoke metaplastic changes in HPC outputs that may mimic alterations associated with a history of chronic stress.     

A nitric oxide-independent and beta-adrenergic receptor-sensitive form of metaplasticity limits theta-frequency stimulation-induced LTP in the hippocampal CA1 region

The induction of long-term potentiation (LTP) and long-term depression (LTD) at excitatory synapses in the hippocampus can be strongly modulated by patterns of synaptic stimulation that otherwise have no direct effect on synaptic strength. Likewise, patterns of synaptic stimulation that induce LTP or LTD not only modify synaptic strength but can also induce lasting changes that regulate how synapses will respond to subsequent trains of stimulation. Collectively known as metaplasticity, these activity-dependent processes that regulate LTP and LTD induction allow the recent history of synaptic activity to influence the induction of activity-dependent changes in synaptic strength and may thus have an important role in information storage during memory formation. To explore the cellular and molecular mechanisms underlying metaplasticity, we investigated the role of metaplasticity in the induction of LTP by υ-frequency (5-Hz) synaptic stimulation in the hippocampal CA1 region. Our results show that brief trains of υ-frequency stimulation not only induce LTP but also activate a process that inhibits the induction of additional LTP at potentiated synapses. Unlike other forms of metaplasticity, the inhibition of LTP induction at potentiated synapses does not appear to arise from activity-dependent changes in NMDA receptor function, does not require nitric oxide signaling, and is strongly modulated by β-adrenergic receptor activation. Together with previous findings, our results indicate that mechanistically distinct forms of metaplasticity regulate LTP induction and suggest that one way modulatory transmitters may act to regulate synaptic plasticity is by modulating metaplasticity.     

Synaptic plasticity in hippocampal CA1 neurons of mice lacking type 1 inositol-1,4,5-trisphosphate receptors

In hippocampal CA1 neurons of wild-type mice, delivery of a standard tetanus (100 pulses at 100 Hz) or a train of low-frequency stimuli (LFS; 1000 pulses at 1 Hz) to a naive input pathway induces, respectively, long-term potentiation (LTP) or long-term depression (LTD) of responses, and delivery of LFS 60 min after tetanus results in reversal of LTP (depotentiation, DP), while LFS applied 60 min before tetanus suppresses LTP induction (LTP suppression). To evaluate the role of the type 1 inositol-1,4,5-trisphosphate receptor (IP3R1) in hippocampal synaptic plasticity, we studied LTP, LTD, DP, and LTP suppression of the field excitatory postsynaptic potentials (EPSPs) in the CA1 neurons of mice lacking the IP3R1. No differences were seen between mutant and wild-type mice in terms of the mean magnitude of the LTP or LTD induced by a standard tetanus or LFS. However, the mean magnitude of the LTP induced by a short tetanus (10 pulses at 100 Hz) was significantly greater in mutant mice than in wild-type mice. In addition, DP or LTP suppression was attenuated in the mutant mice, the mean magnitude of the responses after delivery of LFS or tetanus being significantly greater than in wild-type mice. These results suggest that, in hippocampal CA1 neurons, the IP3R1 is involved in LTP, DP, and LTP suppression but is not essential for LTD. The facilitation of LTP induction and attenuation of DP and LTP suppression seen in mice lacking the IP3R1 indicates that this receptor plays an important role in blocking synaptic potentiation in hippocampal CA1 neurons.     

Long-term potentiation in the dentate gyrus in freely moving rats is reinforced by intraventricular application of norepinephrine, but not oxotremorine

Growing evidence suggests that processes of synaptic plasticity, such as long-term potentiation (LTP) occurring in one synaptic population, can be modulated by consolidating afferents from other brain structures. We have previously shown that an early-LTP lasting less than 4 h (E-LTP) in the dentate gyrus can be prolonged by stimulating the basolateral amygdala, the septum or the locus coeruleus within a specific time window. Pharmacological experiments have suggested that noradregeneric (NE) and/or cholinergic systems might be involved in these effects. We have therefore investigated whether the direct intraventricular application of agonists for NE- or muscarinic receptors is able to modulate synaptic plasticity. E-LTP was induced at the dentate gyrus of freely moving rats using a mild tetanization protocol that induces only an E-LTP. NE or oxotremorine (OXO) were applied icv 10 min after the tetanus. Results show that low doses of NE (1.5 and 5 nM) effectively prolong LTP. A higher dose (50 nM) was not effective. None of the OXO doses employed (5, 25, and 50 nM) showed similar effects. These results stress the importance of transmitter-specific modulatory influences on the time course of synaptic plasticity, in particular NE whose application mimics the reinforcing effect of directly stimulating limbic structures on LTP.     

BDNF: a key regulator for protein synthesis-dependent LTP and long-term memory?

It is generally believed that late-phase long-term potentiation (L-LTP) and long-term memory (LTM) require new protein synthesis. Although the full complement of proteins mediating the long-lasting changes in synaptic efficacy have yet to be identified, several lines of evidence point to a crucial role for activity-induced brain-derived neurotrophic factor (BDNF) expression in generating sustained structural and functional changes at hippocampal synapses thought to underlie some forms of LTM. In particular, BDNF is sufficient to induce the transformation of early to late-phase LTP in the presence of protein synthesis inhibitors, and inhibition of BDNF signaling impairs LTM. Despite solid evidence for a critical role of BDNF in L-LTP and LTM, many issues are not resolved. Given that BDNF needs to be processed in Golgi outposts localized at the branch point of one or few dendrites, a conceptually challenging problem is how locally synthesized BDNF in dendrites could ensure synapse-specific modulation of L-LTP. An interesting alternative is that BDNF-TrkB signaling is involved in synaptic tagging, a prominent hypothesis that explains how soma-derived protein could selectively modulate the tetanized (tagged) synapse. Finally, specific roles of BDNF in the acquisition, retention or extinction of LTM remain to be established.     

Age-related enhancement of a protein synthesis-dependent late phase of LTP induced by low frequency paired-pulse stimulation in hippocampus

Protein synthesis-dependent late phase of LTP (L-LTP) is typically induced by repeated high-frequency stimulation (HFS). This form of L-LTP is reduced in the aged animal and is positively correlated with age-related memory loss. Here we report a novel form of protein synthesis-dependent late phase of LTP in the CA1 region of hippocampus induced by a brief 1-Hz paired-pulse stimulation (PP-1 Hz, 1 min). In contrast to L-LTP induced by HFS, the late phase of PP-1 Hz LTP does not exist in young adult animals. Rather, it emerges and becomes enhanced in an age-related way. Thus, in 1.5- to 2-mo-old mice, a brief PP-1 Hz stimulation induces only a short lasting LTP, decaying to baseline in about 90 min. By contrast, PP-1 Hz stimulation induces an enduring and protein synthesis dependent LTP in 12- to 18-mo-old mice. The PP-1 Hz-induced L-LTP is dependent on NMDA receptor activation, requires voltage-dependent calcium channels, and is modulated by dopamine D1/D5 receptors. Because memory ability declines with aging, the age-related enhancement of L-LTP induced by PP-1 Hz stimulation indicates that this form of L-LTP appears to be inversely correlated with memory ability.     

Activation of CB1 specifically located on GABAergic interneurons inhibits LTD in the lateral amygdala

Previously, we found that in the lateral amygdala (LA) of the mouse, WIN55,212-2 decreases both glutamatergic and GABAergic synaptic transmission via activation of the cannabinoid receptor type 1 (CB1), yet produces an overall reduction of neuronal excitability. This suggests that the effects on excitatory transmission override those on inhibitory transmission. Here we show that CB1 activation by WIN55,212-2 and Delta(9)-THC inhibits long-term depression (LTD) of basal synaptic transmission in the LA, induced by low-frequency stimulation (LFS; 900 pulses/1 Hz). The CB1 agonist WIN55,212-2 blocked LTD via G(i/o) proteins, activation of inwardly rectifying K+ channels (K(ir)s), inhibition of the adenylate cyclase-protein kinase A (PKA) pathway, and PKA-dependent inhibition of voltage-gated N-type Ca2+ channels (N-type VGCCs). Interestingly, WIN55,212-2 effects on LTD were abolished in CB1 knock-out mice (CB1-KO), and in conditional mutants lacking CB1 expression only in GABAergic interneurons, but were still present in mutants lacking CB1 in principal forebrain neurons. LTD induction per se was unaffected by the CB1 antagonist SR141716A and was normally expressed in CB1-KO as well as in both conditional CB1 mutants. Our data demonstrate that activation of CB1 specifically located on GABAergic interneurons inhibits LTD in the LA. These findings suggest that CB1 expressed on either glutamatergic or GABAergic neurons play a differential role in the control of synaptic transmission and plasticity.     

A role for ERK MAP kinase in physiologic temporal integration in hippocampal area CA1

Recent studies demonstrate a requirement for the Extracellular signal Regulated Kinase (ERK) mitogen-activated protein kinase (MAPK) cascade in both the induction of long-lasting forms of hippocampal synaptic plasticity and in hippocampus-dependent associative and spatial learning. In the present studies, we investigated mechanisms by which ERK might contribute to synaptic plasticity at Schaffer collateral synapses in hippocampal slices. We found that long-term potentiation (LTP) induced with a pair of 100-Hz tetani does not require ERK activation in mice whereas it does in rats. However, in mice, inhibition of ERK activation blocked LTP induced by two LTP induction paradigms that mimicked the endogenous θ rhythm. In an additional series of studies, we found that mice specifically deficient in the ERK1 isoform of MAPK showed no impairments in tests of hippocampal physiology. To investigate ERK-dependent mechanisms operating during LTP-inducing stimulation paradigms, we monitored spike production in the cell body layer of the hippocampus during the period of θ-like LTP-inducing stimulation. θ-burst stimulation (TBS) produced a significant amount of postsynaptic spiking, and the likelihood of spike production increased progressively over the course of the three trains of TBS independent of any apparent increase in Excitatory Post-Synaptic Potential (EPSP) magnitude. Inhibition of ERK activation dampened this TBS-associated increase in spiking. These data indicate that, for specific patterns of stimulation, ERK may function in the regulation of neuronal excitability in hippocampal area CA1. Overall, our data indicate that the progressive increase in spiking observed during TBS represents a form of physiologic temporal integration that is dependent on ERK MAPK activity.