A biochemist's view of long-term potentiation

Excitation-transcription coupling, neuronal gene expression and synaptic plasticity

Excitation-transcription coupling (E-TC) links synaptic and cellular activity to nuclear gene transcription. It is generally accepted that E-TC makes a crucial contribution to learning and memory through its role in underpinning long-lasting synaptic enhancement in late-phase long-term potentiation and has more recently been linked to late-phase long-term depression: both processes require de novo gene transcription, mRNA translation and protein synthesis. E-TC begins with the activation of glutamate-gated N-methyl-D-aspartate-type receptors and voltage-gated L-type Ca2+ channels at the membrane and culminates in the activation of transcription factors in the nucleus. These receptors and ion channels mediate E-TC through mechanisms that include long-range signalling from the synapse to the nucleus and local interactions within dendritic spines, among other possibilities. Growing experimental evidence links these E-TC mechanisms to late-phase long-term potentiation and learning and memory. These advances in our understanding of the molecular mechanisms of E-TC mean that future efforts can focus on understanding its mesoscale functions and how it regulates neuronal network activity and behaviour in physiological and pathological conditions.

Karim Nader and the unification of memory erasure: PKMζ inhibition and reconsolidation blockade

Karim Nader is rightly celebrated for his seminal studies on memory reconsolidation. This commentary celebrates another related contribution - his work on memory maintenance by the autonomously active PKC isoform, PKMζ. There are two methods for "erasing" previously established long-term memory maintenance: 1) inhibiting PKMζ, and 2) blocking reconsolidation. Prior to Nader's research on PKMζ, these two forms of memory erasure were thought to be fundamentally different. Inhibiting PKMζ in a brain region disrupts memory held in storage. But if the inhibitor is injected into the same region immediately after memory retrieval, the drug has no effect. Conversely, inhibiting protein synthesis immediately after memory retrieval blocks reconsolidation. But protein synthesis inhibitors have no effect on memory held in storage without retrieval. The work of Paolo Virginia Migues, Nader, and colleagues, however, revealed an unexpected link between the mechanisms of memory maintenance by PKMζ and the kinase's regulation of postsynaptic AMPAR trafficking that potentiates synaptic transmission and expresses memory during retrieval. This insight led Matteo Bernabo, Nader, and colleagues to observe that memory retrieval first rapidly degrades PKMζ, and then induces the resynthesis of the kinase to restore maintenance of the retrieved memory. This finding explains why a PKMζ inhibitor such as ZIP, if injected in a brain region storing a memory, does not erase the memory immediately after retrieval - the kinase maintaining the retrieved memory has been degraded but not yet resynthesized. Moreover, Bernabo et al. showed that suppressing the resynthesis of PKMζ after its degradation prevents memory reconsolidation, reproducing the effect of general protein synthesis inhibition. Thus, Nader and colleagues demonstrated PKMζ inhibition and reconsolidation blockade disrupt in different ways the same molecular mechanism of memory maintenance - PKMζ inhibition erases all memories maintained in storage by the kinase; reconsolidation blockade disrupts specific recalled memories maintained by PKMζ by preventing resynthesis of the kinase after its degradation.

Glucocorticoid modulation of synaptic plasticity in the human temporal cortex of epilepsy patients: Does chronic stress contribute to memory impairment?

OBJECTIVE

Memory impairment is common in patients with temporal lobe epilepsy and seriously affects life quality. Chronic stress is a recognized cofactor in epilepsy and can also impair memory function. Furthermore, increased cortisol levels have been reported in epilepsy patients. Animal models have suggested that aggravating effects of stress on memory and synaptic plasticity were mediated via glucocorticoids. The aim of this study was, therefore, to investigate the effect of glucocorticoid receptor (GR) modulation on synaptic plasticity in the human cortex of epilepsy patients.

METHODS

We performed field potential recordings in acute slices from the temporal neocortex of patients who underwent surgery for drug-resistant temporal lobe epilepsy. Synaptic plasticity was investigated by a theta-burst stimulation (TBS) protocol for induction of long-term potentiation (LTP) in the presence of GR modulators.

RESULTS

LTP was impaired in temporal cortex from epilepsy patients. Pretreatment of the slices with the GR antagonist mifepristone (RU486) improved LTP induction, suggesting that LTP impairment was due to baseline GR activation in the human cortex. The highly potent GR agonist dexamethasone additionally weakened synaptic strength in an activity-dependent manner when applied after TBS.

SIGNIFICANCE

Our results show a direct negative glucocorticoid effect on synaptic potentiation in the human cortex and imply chronic activation of GRs. Chronic stress may therefore contribute to memory impairment in patients with temporal lobe epilepsy. Furthermore, the activity-dependent acute inhibitory effect of dexamethasone suggests a mechanism of synaptic downscaling by which postictally increased cortisol levels may prevent pathologic plasticity upon seizures.

Dynamic Integrative Synaptic Plasticity Explains the Spacing Effect in the Transition from Short- to Long-Term Memory

Repeated stimuli that are spaced apart in time promote the transition from short- to long-term memory, while massing repetitions together does not. Previously, we showed that a model of integrative synaptic plasticity, in which plasticity induction signals are integrated by a low-pass filter before plasticity is expressed, gives rise to a natural timescale at which to repeat stimuli, hinting at a partial account of this spacing effect. The account was only partial because the important role of neuromodulation was not considered. We now show that by extending the model to allow dynamic integrative synaptic plasticity, the model permits synapses to robustly discriminate between spaced and massed repetition protocols, suppressing the response to massed stimuli while maintaining that to spaced stimuli. This is achieved by dynamically coupling the filter decay rate to neuromodulatory signaling in a very simple model of the signaling cascades downstream from cAMP production. In particular, the model's parameters may be interpreted as corresponding to the duration and amplitude of the waves of activity in the MAPK pathway. We identify choices of parameters and repetition times for stimuli in this model that optimize the ability of synapses to discriminate between spaced and massed repetition protocols. The model is very robust to reasonable changes around these optimal parameters and times, but for large changes in parameters, the model predicts that massed and spaced stimuli cannot be distinguished or that the responses to both patterns are suppressed. A model of dynamic integrative synaptic plasticity therefore explains the spacing effect under normal conditions and also predicts its breakdown under abnormal conditions.

The modulation of actin dynamics via atypical Protein Kinase-C activated Cofilin regulates metastasis of colorectal cancer cells

Colorectal cancer (CRC) is the third most common cancer in the United States. The exact mechanism of CRC cells metastasis is poorly understood. Actin polymerization is thought to be an initial step in the cancer cell motility cycle which drives the formation of cell protrusions and defines the direction of migration. Cofilin, a significant actin-regulating molecule, regulates the migration of cancer cells by the formation of lamellipodia and filopodia, however, little is known about the upstream regulation of cofilin. In this study, the effect of atypical Protein Kinase C (atypical PKC) on Cofilin activity in CRC was studied. This study demonstrates that the atypical PKC inhibition impedes the metastasis of CRC cells by increasing phospho-Cofilin (S3) and changing actin organization.

Variations on the Theme of Synaptic Filtering: A Comparison of Integrate-and-Express Models of Synaptic Plasticity for Memory Lifetimes

Integrate-and-express models of synaptic plasticity propose that synapses integrate plasticity induction signals before expressing synaptic plasticity. By discerning trends in their induction signals, synapses can control destabilizing fluctuations in synaptic strength. In a feedforward perceptron framework with binary-strength synapses for associative memory storage, we have previously shown that such a filter-based model outperforms other, nonintegrative, "cascade"-type models of memory storage in most regions of biologically relevant parameter space. Here, we consider some natural extensions of our earlier filter model, including one specifically tailored to binary-strength synapses and one that demands a fixed, consecutive number of same-type induction signals rather than merely an excess before expressing synaptic plasticity. With these extensions, we show that filter-based models outperform nonintegrative models in all regions of biologically relevant parameter space except for a small sliver in which all models encode memories only weakly. In this sliver, which model is superior depends on the metric used to gauge memory lifetimes (whether a signal-to-noise ratio or a mean first passage time). After comparing and contrasting these various filter models, we discuss the multiple mechanisms and timescales that underlie both synaptic plasticity and memory phenomena and suggest that multiple, different filtering mechanisms may operate at single synapses.

Detection of single unit spikes during orthodromic-high frequency stimulation in rat hippocampus

Deep brain stimulation (DBS) has been used as a treatment of brain diseases such as Parkinson's disease and is a promising therapy for epilepsy. But the mechanisms of high frequency stimulation (HFS) used by DBS are still uncertain. In order to investigate the changes of action penitential firings of individual neurons (single unit activity, SUA) during the period of HFS, a new algorithm based on window detection was designed to detect spikes in broadband-frequency recording signals. The results show that orthodromic-HFS (O-HFS) could excite the neurons in CA1 regions, and the firing rate of interneurons and pyramidal neurons increased significantly. In particular, a decrease in spike amplitude for both interneurons and pyramidal neurons was observed during the period of O-HFS. The amplitude decrease of unit spikes was most remarkable with the presence of HFS-induced population spike (PS). These results suggest that the stimulation pulses of O-HFS could activate the downstream neurons continuously, leading to the downstream neurons being unable to repolarize completely. The results are important for tracking individual neuron activity during HFS and for further understanding of DBS mechanisms.

Neural plasticity and behavior - sixty years of conceptual advances

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

Age-related synaptic dysfunction in Tg2576 mice starts as a failure in early long-term potentiation which develops into a full abolishment of late long-term potentiation

Tg2576 mice are widely used to study amyloid-dependent synaptic dysfunction related to Alzheimer's disease. However, conflicting data have been reported for these mice with regard to basal transmission as well as the in vitro correlate of memory, long-term potentiation (LTP). Some studies show clear impairments, whereas others report no deficiency. The present study uses hippocampal slices from 3-, 10-, and 15-month-old wild-type (WT) and Tg2576 mice to evaluate synaptic function in each group, including experiments to investigate basal synaptic transmission, short- and long-term plasticity by inducing paired-pulse facilitation, and both early and late LTP. We show that synaptic function remains intact in hippocampal slices from Tg2576 mice at 3 months of age. However, both early and late LTP decline progressively during aging in these mice. This deterioration of synaptic plasticity starts affecting early LTP, ultimately leading to the abolishment of both forms of LTP in 15-month-old animals. In comparison, WT littermates display normal synaptic parameters during aging. Additional pharmacological investigation into the involvement of NMDA receptors and L-type voltage-gated calcium channels in LTP suggests a distinct mechanism of induction among age groups, demonstrating that both early and late LTP are differentially affected by these channels in Tg2576 mice during aging.

A CMOS IC-based multisite measuring system for stimulation and recording in neural preparations in vitro

In this report, we describe the system integration of a complementary metal oxide semiconductor (CMOS) integrated circuit (IC) chip, capable of both stimulation and recording of neurons or neural tissues, to investigate electrical signal propagation within cellular networks in vitro. The overall system consisted of three major subunits: a 5.0 × 5.0 mm CMOS IC chip, a reconfigurable logic device (field-programmable gate array, FPGA), and a PC. To test the system, microelectrode arrays (MEAs) were used to extracellularly measure the activity of cultured rat cortical neurons and mouse cortical slices. The MEA had 64 bidirectional (stimulation and recording) electrodes. In addition, the CMOS IC chip was equipped with dedicated analog filters, amplification stages, and a stimulation buffer. Signals from the electrodes were sampled at 15.6 kHz with 16-bit resolution. The measured input-referred circuitry noise was 10.1 μ V root mean square (10 Hz to 100 kHz), which allowed reliable detection of neural signals ranging from several millivolts down to approximately 33 μ Vpp. Experiments were performed involving the stimulation of neurons with several spatiotemporal patterns and the recording of the triggered activity. An advantage over current MEAs, as demonstrated by our experiments, includes the ability to stimulate (voltage stimulation, 5-bit resolution) spatiotemporal patterns in arbitrary subsets of electrodes. Furthermore, the fast stimulation reset mechanism allowed us to record neuronal signals from a stimulating electrode around 3 ms after stimulation. We demonstrate that the system can be directly applied to, for example, auditory neural prostheses in conjunction with an acoustic sensor and a sound processing system.

Contribution of Ca2+ release channels to hippocampal synaptic plasticity and spatial memory: potential redox modulation

SIGNIFICANCE

Memory is an essential human cognitive function. Consequently, to unravel the cellular and molecular mechanisms responsible for the synaptic plasticity events underlying memory formation, storage and loss represents a major challenge of present-day neuroscience.

RECENT ADVANCES

This review article first describes the wide-ranging functions played by intracellular Ca2+ signals in the activity-dependent synaptic plasticity processes underlying hippocampal spatial memory, and next, it focuses on how the endoplasmic reticulum Ca2+ release channels, the ryanodine receptors, and the inositol 1,4,5-trisphosphate receptors contribute to these processes. We present a detailed examination of recent evidence supporting the key role played by Ca2+ release channels in synaptic plasticity, including structural plasticity, and the formation/consolidation of spatial memory in the hippocampus.

CRITICAL ISSUES

Changes in cellular oxidative state particularly affect the function of Ca2+ release channels and alter hippocampal synaptic plasticity and the associated memory processes. Emphasis is placed in this review on how defective Ca2+ release, presumably due to increased levels of reactive oxygen species, may cause the hippocampal functional defects that are associated to aging and Alzheimer's disease (AD).

FUTURE DIRECTIONS

Additional studies should examine the precise molecular mechanisms by which Ca2+ release channels contribute to hippocampal synaptic plasticity and spatial memory formation/consolidation. Future studies should test whether redox-modified Ca2+ release channels contribute toward generating the intracellular Ca2+ signals required for sustained synaptic plasticity and hippocampal spatial memory, and whether loss of redox balance and oxidative stress, by altering Ca2+ release channel function, presumably contribute to the abnormal memory processes that occur during aging and AD.

Protein phosphatases 1 and 2A are both required for long-term depression and associated dephosphorylation of cAMP response element binding protein in hippocampal area CA1 in vivo

Evidence shows that the serine/threonine protein phosphatase 1 (PP1) plays a critical role in synaptic plasticity and memory. Little is known about the contribution of the serine/threonine phosphatase 1 (PP2A) to synaptic plasticity. Both protein phosphatases can target the transcription factor cAMP response element binding protein (CREB), whose phosphorylation at Ser133, we previously found, was downregulated during long-term depression (LTD) of glutamatergic transmission in area CA1 of the adult hippocampus in vivo. Other work from our group showed that the activity of PP2A, as well as that of PP1, is increased after LTD induction in area CA1 in vivo. We therefore investigated here whether both protein phosphatases are necessary for LTD in area CA1, and whether they both are involved in the LTD-associated modification of CREB. We found that inhibition of either PP1 or PP2A interferes with the establishment of LTD. Furthermore, inhibition of either enzyme alone abrogated the LTD-associated dephosphorylation of CREB. Interestingly, inhibition of PP1 disrupted CREB dephosphosphorylation rapidly after LTD-inducing stimulation, whereas inhibition of PP2A did not blunt the CREB modification until a later time point. Thus, both PP1 and PP2A regulate CREB during LTD in area CA1, although possibly through different signaling pathways. Our results demonstrate that PP2A, similar to PP1, plays an essential role in the molecular events that underlie LTD at glutamatergic synapses in hippocampal area CA1 in vivo. We propose that one of the mechanisms through which these protein phosphatases may contribute to the prolonged maintenance of LTD is through the regulation of CREB.

Traces of Drosophila memory

Studies using functional cellular imaging of living flies have identified six memory traces that form in the olfactory nervous system after conditioning with odors. These traces occur in distinct nodes of the olfactory nervous system, form and disappear across different windows of time, and are detected in the imaged neurons as increased calcium influx or synaptic release in response to the conditioned odor. Three traces form at or near acquisition and coexist with short-term behavioral memory. One trace forms with a delay after learning and coexists with intermediate-term behavioral memory. Two traces form many hours after acquisition and coexist with long-term behavioral memory. The transient memory traces may support behavior across the time windows of their existence. The experimental approaches for dissecting memory formation in the fly, ranging from the molecular to the systems, make it an ideal system for elucidating the logic by which the nervous system organizes and stores different temporal forms of memory.

Early growth response-1 induction by fibroblast growth factor-1 via increase of mitogen-activated protein kinase and inhibition of protein kinase B in hippocampal neurons

BACKGROUND AND PURPOSE

The transcription factor early growth response-1 (Egr-1) and the acidic fibroblast growth factor (FGF-1) are involved in many regulatory processes, including hippocampus-associated learning and memory. However, the intracellular signalling mechanisms regulating Egr-1 in hippocampal cells are not entirely understood.

EXPERIMENTAL APPROACH

We used primary mouse hippocampal neurons and the mouse hippocampal neuronal cell line HT22 to investigate how FGF-1 transiently induces Egr-1 protein. This was accomplished by a range of techniques including Western blotting, immunofluorescence, specific protein kinase inhibitors and transfectable constitutively active protein kinase constructs.

KEY RESULTS

Protein kinase B (PKB) and mitogen-activated protein kinase (MAPK) were both initially phosphorylated and activated by FGF-1 treatment, but when phosphorylated MAPK reached maximal activation, phosphorylated PKB was at its lowest levels, suggesting an interaction between MAPK kinase (MEK-1/2) and phosphatidyl inositol-3-kinase (PI3K) during Egr-1 induction. Interestingly, pharmacological inhibition of MEK-1/2 resulted in a robust increase in the phosphorylation of PKB, which was repressed in the presence of increasing doses of a PI3K inhibitor. FGF-1-mediated Egr-1 induction was impaired by inhibition of MEK-1/2, but not of PI3K. However, elevated levels of PKB, induced by transfection of constitutively active PKB (myrAkt) into hippocampal neuronal HT22 cells, led to reduced levels of Egr-1 after FGF-1 application.

CONCLUSIONS AND IMPLICATIONS

Our data indicate a contribution of inactive (dephosphorylated) PKB to FGF-1-mediated induction of Egr-1, and strongly suggest a functionally and pharmacologically interesting cross-talk between MEK-1/2 and PI3K signalling in hippocampal neurons after FGF-1 stimulation that may play a role in hippocampal synaptic plasticity.

Maternal immune activation by polyriboinosinic-polyribocytidilic acid injection produces synaptic dysfunction but not neuronal loss in the hippocampus of juvenile rat offspring

It has been suggested that maternal immune activation increases the risk of psychiatric disorders such as schizophrenia in offspring. There are many reports about hippocampal structural pathology in schizophrenia. Antipsychotic drug administration in adolescence prevented postpubertal hippocampal structural pathology in the maternal immune activation animal model. These findings suggest the possibility that maternal immune activation induces hippocampal dysfunction in juvenile offspring. To test this hypothesis, we investigated hippocampal function in juvenile offspring of maternal immune activation model rat. A synthetic double-stranded RNA polyriboinosinic-polyribocytidilic acid (Poly I:C; 4 mg/kg/day, I.P.) was injected to pregnant rats on gestation days 15 and 17, in order to cause immune activation by stimulating Toll-like receptor 3. Hippocampal synaptic function and morphology in their juvenile offspring (postnatal days 28-31) were compared to those in vehicle-injected control offspring. Field responses were recorded in the hippocampal CA1 region by stimulating commissural/Schaffer collaterals. Pre-synaptic fiber volley amplitudes (mV) and field excitatory post-synaptic potential slopes (mV/ms) were significantly lower in treated offspring. In addition, short-term synaptic plasticity, namely, the paired-pulse facilitation ratio, was significantly higher and long-term synaptic plasticity (long-term potentiation) was significantly impaired in treated offspring. Furthermore, major pre-synaptic protein (synaptophysin) expressions were decreased, but not major post-synaptic proteins (GluR1, GluR2/3, and NR1), in hippocampal CA1 of treated offspring, whereas neuronal loss was not detected in the hippocampal CA1-CA3 regions. These results indicate that maternal immune activation leads to synaptic dysfunction without neuronal loss in the hippocampus of juvenile offspring, and this may be one of the early stages of schizophrenia pathologies.

Expression of P-aPKC-iota, E-cadherin, and beta-catenin related to invasion and metastasis in hepatocellular carcinoma

OBJECTIVES

Atypical protein kinase C iota (aPKC-iota) and its associated intracellular molecules, E-cadherin and beta-catenin, are important for cell polarization in tumorigenesis and progression. Expression of aPKC-iota, P-aPKC-iota (activated aPKC-iota), E-cadherin, and beta-catenin in hepatocellular carcinoma (HCC) was measured, and correlation with clinicopathological characteristics of HCC was analyzed.

METHODS

Paraffin-embedded tumor tissue was obtained from patients with HCC after resection without preoperative radiotherapy or chemotherapy. Gene expression was detected by polymerase chain reaction (PCR), and protein expression was detected by immunohistochemistry and Western blot analysis. Expressions of aPKC-iota, P-aPKC-iota, E-cadherin, and beta-catenin were analyzed with relation to the clinicopathological data.

RESULTS

The gene and protein expression of aPKC-iota are obviously higher in HCC tissues than that in peritumoral tissues and normal tissues by semiquantitative PCR and immunohistochemistry methods. Accumulation of aPKC-iota in HCC cytoplasm and nucleolus inhibited the later formation of belt-like adherens junctions (AJs) and/or tight junctions (TJs) in cell-cell contact. E-cadherin was reduced and accumulation of cytoplasm beta-catenin was increased in HCC. The expression of aPKC-iota was closely related to pathological differentiation, tumor size, invasion, and metastasis of HCC.

CONCLUSION

Accumulation of cytoplasm aPKC-iota may reflect pathological differentiation, invasion, and metastasis potential of HCC. In this regard, our study on HCC revealed the potential usefulness of aPKC-iota, E-cadherin, and beta-catenin as a prognostic marker, closely related to pathological differentiation, invasion, metastasis, and prognosis of HCC.

Atypical protein kinase C phosphorylates IKKalphabeta in transformed non-malignant and malignant prostate cell survival

Mechanistic pathways involving atypical protein kinase C-iota (aPKC-iota) have been targeted in various cancer cells such as lung cancer, brain and prostate due to PKCiota's antiapoptotic function, and role in cell proliferation and cell survival. In the current study, we examined the involvement of PKC-iota in the NF-kappaB pathway following treatment of prostate cells with the pro-inflammatory cytokine tumor necrosis factor alpha (TNFalpha). Results demonstrated that androgen-independent DU-145 prostate carcinoma is insensitive to TNFalpha while transformed non-tumorigenic prostate RWPE-1 cells showed a slight sensitivity to TNFalpha. However, androgen-dependent LNCaP prostate cells are more sensitive to TNFalpha treatment and undergo apoptosis. Results demonstrated that in DU-145 cells, TNFalpha-induced PKC-iota in phosphorylation of IKKalphabeta. In RWPE-1 cells, PKC-zeta phosphorylates IKKalphabeta. Degradation of IkappaBalpha was observed in all three cell lines, allowing NF-kappaB/p65 translocation to the nucleus. Although, IKKalpha is weakly activated in LNCaP cells, the upstream kinase phosphorylation of IKKalphabeta via aPKCs was not observed. Hence, aPKCs may play a role in activation of NFkappaB pathway in prostate cancer cells.

Neuropsin (KLK8)-dependent and -independent synaptic tagging in the Schaffer-collateral pathway of mouse hippocampus

Hippocampal early long-term potentiation (LTP) elicited by a weak (one or two) tetanic stimulus normally fades away within 90 min. Late LTP elicited by strong (four) stimuli lasts >180 min and requires new protein synthesis to persist. If a strong tetanus is injected once into a synapse, even a weak tetanus injected into another synapse can evoke persistent LTP. It was hypothesized that a synaptic tag enables capture of newly synthesized synaptic molecules. Here, we found two synaptic capture mechanisms for a weakly stimulated synapse to acquire persistency (i.e., neuropsin dependent and independent). The single tetanus evokes a neuropsin-dependent form that follows downstream signaling into integrin/actin signal and L-type voltage-dependent Ca2+ channel (LVDCC) pathway. Additionally, a neuropsin-independent form of synaptic capture is evoked by a stronger (two) tetanus than the former. Both forms converging on LVDCC might serve different associative memories depending on their input strength. Our study strongly supports the hypothesis of synaptic tagging and demonstrates that neuropsin-dependent late associativity is particularly important in nonstressful associative memory.

Learning-induced glutamate receptor phosphorylation resembles that induced by long term potentiation

Long term potentiation and long term depression of synaptic responses in the hippocampus are thought to be critical for certain forms of learning and memory, although until recently it has been difficult to demonstrate that long term potentiation or long term depression occurs during hippocampus-dependent learning. Induction of long term potentiation or long term depression in hippocampal slices in vitro modulates phosphorylation of the alpha-amino-3-hydrozy-5-methylisoxazole-4-propionic acid subtype of glutamate receptor subunit GluR1 at distinct phosphorylation sites. In long term potentiation, GluR1 phosphorylation is increased at the Ca2+/calmodulin-dependent protein kinase and protein kinase C site serine 831, whereas in long term depression, phosphorylation of the protein kinase A site serine 845 is decreased. Indeed, phosphorylation of one or both of these sites is required for long term synaptic plasticity and for certain forms of learning and memory. Here we demonstrate that training in a hippocampus-dependent learning task, contextual fear conditioning is associated with increased phosphorylation of GluR1 at serine 831 in the hippocampal formation. This increased phosphorylation is specific to learning, has a similar time course to that in long term potentiation, and like memory and long term potentiation, is dependent on N-methyl-D-aspartate receptor activation during training. Furthermore, the learning-induced increase in serine 831 phosphorylation is present at synapses and is in heteromeric complexes with the glutamate receptor subunit GluR2. These data indicate that a biochemical correlate of long term potentiation occurs at synapses in receptor complexes in a final, downstream, postsynaptic effector of long term potentiation during learning in vivo, further strengthening the link between long term potentiation and memory.

Superoxide dismutase and hippocampal function: age and isozyme matter

Superoxide dismutases (SODs) are the major antioxidant enzymes that inactivate superoxide and thereby control oxidative stress as well as redox signaling. Transgenic mice overexpressing different isozymes of SOD have been used to study the effect of SOD overexpression on hippocampal synaptic plasticity and hippocampus-dependent learning and memory. Studies with transgenic and wild-type animals of different ages show that the function of SOD overexpression changes across the life span of an animal, and comparisons between animals that overexpress different SOD isozymes suggest that the functional value of overexpression as well as the mechanisms through which the respective functional values are effected vary depending on isozyme. The work discussed in this review has important implications for the use of antioxidant treatments and for our understanding of the role of superoxide in physiological and pathological processes.

Hippocampal long-term potentiation, memory, and longevity in mice that overexpress mitochondrial superoxide dismutase

Superoxide has been shown to be critically involved in several pathological manifestations of aging animals. In contrast, superoxide also can act as a signaling molecule to modulate signal transduction cascades required for hippocampal synaptic plasticity. Mitochondrial superoxide dismutase (SOD-2 or Mn-SOD) is a key antioxidant enzyme that scavenges superoxide. Thus, SOD-2 may not only prevent aging-related oxidative stress, but may also regulate redox signaling in young animals. We used transgenic mice overexpressing SOD-2 to study the role of mitochondrial superoxide in aging, synaptic plasticity, and memory-associated behavior. We found that overexpression of SOD-2 had no obvious effect on synaptic plasticity and memory formation in young mice, and could not rescue the age-related impairments in either synaptic plasticity or memory in old mice. However, SOD-2 overexpression did decrease mitochondrial superoxide in hippocampal neurons, and extended the lifespan of the mice. These findings increase our knowledge of the role of mitochondrial superoxide in physiological and pathological processes in the brain.

Divergent regulation of Pyk2/CAKbeta phosphorylation by Ca2+ and cAMP in the hippocampus

Proline-rich tyrosine kinase 2 (Pyk2) is activated in neurones following NMDA receptor stimulation via PKC. Pyk2 is involved in hippocampal LTP and acts to potentiate NMDA receptor function. Elevations of intracellular Ca2+ and cAMP levels are key NMDA receptor-dependent triggering events leading to induction of hippocampal LTP. In this study, we compared the ability of A23187 (Ca2+ ionophore) or forskolin (adenylate cyclase activator) to modulate the phosphorylation of Pyk2 in rat hippocampal slices. Using an immunoprecipitation assay, phosphorylated Pyk2 levels were increased following treatment with A23187, levels peaking at around 10 min. Staurosporine, at concentrations inhibiting conventional and novel isoforms of PKC, and chelerythrine, at concentrations inhibiting the atypical PKC isoform PKMxi, were compared for their ability to attenuate the effect of A23187. Exposure of acute hippocampal slices to either chelerythrine or staurosporine completely blocked enhanced phosphorylation of Pyk2 by A23187, suggesting a possible involvement of PKMxi and typical PKCs in Pyk2 activation by Ca2+. In contrast, application of forskolin reduced phosphorylated Pyk2 below basal levels, suggesting that cAMP inhibits Pyk2. These results implicate Ca2+ and multiple forms of PKC in the activation of Pyk2 downstream of NMDA receptors and suggest that cAMP-dependent processes exert a suppressive action on Pyk2.

Activity-dependent decrease of excitability in rat hippocampal neurons through increases in Ih

Hippocampal long-term potentiation (LTP) induced by theta-burst pairing of Schaffer collateral inputs and postsynaptic firing is associated with localized increases in synaptic strength and dendritic excitability. Using the same protocol, we now demonstrate a decrease in cellular excitability that was blocked by the h-channel blocker ZD7288. This decrease was also induced by postsynaptic theta-burst firing alone, yet it was blocked by NMDA receptor antagonists, postsynaptic Ca2+ chelation, low concentrations of tetrodotoxin, omega-conotoxin MVIIC, calcium/calmodulin-dependent protein kinase II (CaMKII) inhibitors and a protein synthesis inhibitor. Increasing network activity with high extracellular K+ caused a similar reduction of cellular excitability and an increase in h-channel HCN1 protein. We propose that backpropagating action potentials open glutamate-bound NMDA receptors, resulting in an increase in I(h) and a decrease in overall excitability. The occurrence of such a reduction in cellular excitability in parallel with synaptic potentiation would be a negative feedback mechanism to normalize neuronal output firing and thus promote network stability.

The superoxide anion is involved in the induction of long-term potentiation in the rat somatosensory cortex in vitro

The involvement of the superoxide anion (O2-) in the induction of neocortical long-term potentiation (LTP) was examined in rat brain slices containing the primary somatosensory cortex. Field potentials evoked by stimulation in cortical layer IV were recorded from layer II/III. In control experiments, tetanic high-frequency stimulation (HFS) resulted in essentially input-specific, NMDA receptor-dependent LTP (20.2+/-3.0% increase in field potential amplitude). When the availability of intracellular O2- was reduced by application of the cell membrane-permeable O2- scavengers MnTBAP or CP-H (spin trap), HFS-induced LTP was attenuated to 12.0+/-1.7% and 8.7+/-3.1% increase, respectively. In contrast, HFS-induced LTP was not significantly affected by the cell membrane-impermeable O2- scavenger superoxide dismutase (SOD). Induction of the generation of O2- by the cell membrane-permeable redox-cycling quinone DMNQ resulted in a HFS-independent slow-onset LTP (21.8+/-6.0%) in three of eight brain slices. Together, these results suggest the contribution of O2- to the induction of LTP in the primary somatosensory cortex by an action on intracellular induction mechanisms.

Heterosynaptic co-activation of glutamatergic and dopaminergic afferents is required to induce persistent long-term potentiation

The persistence of protein synthesis-dependent long-term potentiation (late-LTP) is thought to require heterosynaptic activation of both glutamate and neuromodulatory receptors in the hippocampus. The present series of experiments contrasts two alternative accounts of heterosynaptic activation. The original version of the synaptic-tag hypothesis of the variable persistence of LTP implied that neuromodulatory and glutamatergic activation could occur independently, albeit within a critical time-window; an alternative view is that there needs to be simultaneous co-activation of both receptors to trigger the up-regulation of relevant protein synthesis (Neuron 34 (2002) 235). Our findings include a replication, over 6 h post-LTP-induction, of earlier findings showing heterosynaptic influences on LTP persistence. Specifically, 'strong' tetanisation with multiple trains of stimulation of one input pathway in a conventional hippocampal slice preparation induces a D1/D5 receptor-dependent form of late-LTP that enables 'weak' tetanic stimulation to induce late-LTP on an independent pathway. However, we also observed that when the first pathway was tetanised in the presence of AP5, not only was no LTP observed on that pathway, but there was also no rescue of late-LTP on the second pathway. Thus, it appears that DA receptors must be co-activated with NMDA receptors in a common pool of neurons to enable LTP persistence, although late-LTP can still be induced by selective activation of glutamatergic synapses if this occurs at time periods shortly before or shortly after this essential coactivation.

Active dendrites, potassium channels and synaptic plasticity

The dendrites of CA1 pyramidal neurons in the hippocampus express numerous types of voltage-gated ion channel, but the distributions or densities of many of these channels are very non-uniform. Sodium channels in the dendrites are responsible for action potential (AP) propagation from the axon into the dendrites (back-propagation); calcium channels are responsible for local changes in dendritic calcium concentrations following back-propagating APs and synaptic potentials; and potassium channels help regulate overall dendritic excitability. Several lines of evidence are presented here to suggest that back-propagating APs, when coincident with excitatory synaptic input, can lead to the induction of either long-term depression (LTD) or long-term potentiation (LTP). The induction of LTD or LTP is correlated with the magnitude of the rise in intracellular calcium. When brief bursts of synaptic potentials are paired with postsynaptic APs in a theta-burst pairing paradigm, the induction of LTP is dependent on the invasion of the AP into the dendritic tree. The amplitude of the AP in the dendrites is dependent, in part, on the activity of a transient, A-type potassium channel that is expressed at high density in the dendrites and correlates with the induction of the LTP. Furthermore, during the expression phase of the LTP, there are local changes in dendritic excitability that may result from modulation of the functioning of this transient potassium channel. The results support the view that the active properties of dendrites play important roles in synaptic integration and synaptic plasticity of these neurons.

Mood stabilizer psychopharmacology

Mood stabilizers represent a class of drugs that are efficacious in the treatment of bipolar disorder. The most established medications in this class are lithium, valproic acid, and carbamazepine. In addition to their therapeutic effects for treatment of acute manic episodes, these medications often are useful as prophylaxis against future episodes and as adjunctive antidepressant medications. While important extracellular effects have not been excluded, most available evidence suggests that the therapeutically relevant targets of this class of medications are in the interior of cells. Herein we give a prospective of a rapidly evolving field, discussing common effects of mood stabilizers as well as effects that are unique to individual medications. Mood stabilizers have been shown to modulate the activity of enzymes, ion channels, arachidonic acid turnover, G protein coupled receptors and intracellular pathways involved in synaptic plasticity and neuroprotection. Understanding the therapeutic targets of mood stabilizers will undoubtedly lead to a better understanding of the pathophysiology of bipolar disorder and to the development of improved therapeutics for the treatment of this disease. Furthermore, the involvement of mood stabilizers in pathways operative in neuroprotection suggests that they may have utility in the treatment of classical neurodegenerative disorders.

Role of reactive oxygen species in hippocampal long-term potentiation: contributory or inhibitory?

Reactive oxygen species (ROS) typically are characterized as molecules involved in neurotoxicity and neurodegeneration. However, recent evidence from both neuronal and nonneuronal cells suggests that ROS also function as small messenger molecules that are normal components of signal transduction cascades during physiological processes. Consistent with this idea, ROS have been shown to be critical for hippocampal long-term potentiation (LTP), a form of synaptic plasticity widely studied as a cellular substrate for learning and memory. On the other hand, ROS also have been shown to be involved in aging-related impairment of LTP. This review discusses the evidence supporting the notion that ROS both contribute to normal LTP and are involved in age-related impairment of LTP. We also discuss possible sources that might be responsible for the production of ROS after the induction of LTP. Finally, we propose a functional ROS continuum to help explain this dichotomy of ROS function in hippocampal LTP.

Potentiation of hippocampal synaptic transmission by superoxide requires the oxidative activation of protein kinase C

Recent evidence suggests that reactive oxygen species (ROS), including superoxide, are not only neurotoxic but function as small messenger molecules in normal neuronal processes such as synaptic plasticity. Consistent with this idea, we show that brief incubation of hippocampal slices with the superoxide-generating system xanthine/xanthine oxidase (X/XO) produces a long-lasting potentiation of synaptic transmission in area CA1. We found that X/XO-induced potentiation was associated with a persistent superoxide-dependent increase in autonomous PKC activity that could be isolated via DEAE column chromatography. The X/XO-induced potentiation was blocked by the inhibition of PKC, indicating that the superoxide-dependent increase in autonomous PKC activity was necessary for the potentiation. We also found that X/XO-induced potentiation and long-term potentiation (LTP) occluded one another, suggesting that these forms of plasticity share similar cellular mechanisms. In further support of this idea, we found that a persistent, superoxide-dependent increase in autonomous PKC activity isolated via DEAE column chromatography also was associated with LTP. Taken together, our findings indicate that X/XO-induced potentiation and LTP share similar cellular mechanisms, including superoxide-dependent increases in autonomous PKC activity. Finally, our findings suggest that superoxide, in addition to its well known role as a neurotoxin, also can be considered a small messenger molecule critical for normal neuronal signaling.

Learning in honeybees: from molecules to behaviour

Studies in a variety of organisms as diverse as molluscs, insects, birds and mammals have shown that memories can exist in a variety of temporal domains ranging from short-term memories in the range of minutes to long-term memories lasting a lifetime. While transient covalent modifications of proteins underlie short-term memory, the formation of long-term memory requires gene expression and protein synthesis. Different intracellular signalling cascades have been implicated in distinct aspects of learning and memory formation. Little is known however, about how learning in intact animals is related to the modulation of these signalling cascades and how this contributes to distinct neuronal and behavioural changes in vivo. Associative learning in the honeybee provides the opportunity to study processes of memory formation by analysing its progression through different phases, across levels of behaviour, neural circuits, and cellular signalling pathways. The findings reveal evidence that various cellular signalling pathways in the neuronal circuit of distinct brain areas play a role in different processes during learning and memory formation.

Peroxynitrite-induced tyrosine nitration and inhibition of protein kinase C

Protein kinase C (PKC) is an important intracellular signaling molecule whose activity is essential for a number of aspects of neuronal function including synaptic plasticity. We investigated the regulation of PKC activity by reactive nitrogen species in order to examine whether such species regulate PKC in neurons. Neither autonomous nor cofactor-dependent PKC activity was altered when either hippocampal homogenates or rat brain purified PKC were incubated briefly with three different nitric oxide donor compounds. However, brief incubation of either hippocampal homogenates or purified PKC with peroxynitrite (ONOO(-)) inhibited cofactor-dependent PKC activity in a manner that correlated with the nitration of tyrosine residues on PKC, suggesting that this modification was responsible for the inhibition of PKC. Consistent with this idea, reducing agents had no effect on the inhibition of PKC activity caused by ONOO(-). Because there are numerous PKC isoforms that differ in the composition of the regulatory domain, we studied the effect of ONOO(-) on various PKC isoforms. ONOO(-) inhibited the cofactor-dependent activity of the alpha, betaII, epsilon, and zeta isoforms, indicating that inhibition of enzymatic activity by ONOO(-) was not PKC isoform-specific. We also were able to isolate nitrated PKCalpha and PKCbetaII from ONOO(-)-treated hippocampal homogenates via immunoprecipitation. Collectively, our findings support the hypothesis that ONOO(-) inhibits PKC activity via tyrosine nitration in neurons.

Impairment of long-term potentiation and associative memory in mice that overexpress extracellular superoxide dismutase

Reactive oxygen species, including superoxide, generally are considered neurotoxic molecules whose effects can be alleviated by antioxidants. Different from this view, we show that scavenging of superoxide with an antioxidant enzyme is associated with deficits in hippocampal long-term potentiation (LTP), a putative neural substrate of memory, and hippocampal-mediated memory function. Using transgenic mice that overexpress extracellular superoxide dismutase (EC-SOD), a superoxide scavenger, we found that LTP was impaired in hippocampal area CA1 despite normal LTP in area CA3. The LTP impairment in area CA1 could be reversed by inhibition of EC-SOD. In addition, we found that EC-SOD transgenic mice exhibited impaired long-term memory of fear conditioning to contextual cues despite exhibiting normal short-term memory of the conditioning experience. These findings strongly suggest that superoxide, rather than being considered exclusively a neurotoxic molecule, should also be considered a signaling molecule necessary for normal neuronal function.

Does the kindling model of epilepsy contribute to our understanding of multiple chemical sensitivity?

Multiple chemical sensitivity (MCS) is a phenomenon whereby individuals report an increased sensitivity to low levels of chemicals in the environment. Kindling is a model of synaptic plasticity whereby repeated low-level electrical stimulation to a number of brain sites leads to permanent increases in seizure susceptibility. Stimulation that is initially subthreshold for subclinical seizure provocation comes, over time, to elicit full-blown motor seizures. Kindling can also be induced by chemical stimulation, and repeated exposures to some pesticides have been shown to induce signs of behavioral seizure, facilitate subsequent electrical kindling, and induce subclinical electrographic signs of hyperexcitability in the amygdala. Many of the symptoms of MCS suggest that CNS limbic pathways involved in anxiety are altered in individuals reporting MCS. Limbic structures are among the most susceptible to kindling-induced seizures, and persistent cognitive and emotional sequelae have been associated with temporal lobe epilepsy (TLE) in humans and kindling in animals. Thus, a number of parallels exist between kindling and MCS phenomena, leading to initial speculations that MCS may occur via a kindling-like mechanism. However, kindling requires the activation of electrographic seizure discharge and has thus been primarily examined as a model for TLE. Events leading to the initial evocation of a subclinical electrographic seizure have been much less well studied. It is perhaps these events that may serve as a more appropriate model for the enhanced chemical responsiveness characteristic of MCS. Alternatively, kindling may be useful as a tool to selectively increase sensitivity in subcomponents of the neural fear circuit to address questions relating the role of anxiety in the development and expression of MCS.

Plasticity at hippocampal to prefrontal cortex synapses: dual roles in working memory and consolidation

The involvement of the hippocampus and the prefrontal cortex in cognitive processes and particularly in learning and memory has been known for a long time. However, the specific role of the projection which connects these two structures has remained elusive. The existence of a direct monosynaptic pathway from the ventral CA1 region of the hippocampus and subiculum to specific areas of the prefrontal cortex provides a useful model for conceptualizing the functional operations of hippocampal-prefrontal cortex communication in learning and memory. It is known now that hippocampal to prefrontal cortex synapses are modifiable synapses and can express different forms of plasticity, including long-term potentiation, long-term depression, and depotentiation. Here we review these findings and focus on recent studies that start to relate synaptic plasticity in the hippocampo-prefrontal cortex pathway to two specific aspects of learning and memory, i.e., the consolidation of information and working memory. The available evidence suggests that functional interactions between the hippocampus and prefrontal cortex in cognition and memory are more complex than previously anticipated, with the possibility for bidirectional regulation of synaptic strength as a function of the specific demands of tasks.

Protein phosphatase-mediated regulation of protein kinase C during long-term depression in the adult hippocampus in vivo

The neural substrates of learning and memory are thought to involve use-dependent long-term changes in synaptic function, including long-term depression (LTD) of synaptic strength. One biochemical event hypothesized to contribute to the maintenance and expression of LTD is decreased protein phosphorylation, caused by a decrease in protein kinase activity and/or an increase in protein phosphatase activity. We tested whether the activity of protein kinase C (PKC) decreases after the induction of LTD in area CA1 of the adult hippocampus in vivo, and then investigated the mechanism responsible for the LTD-associated alteration in PKC activity. We found that LTD was associated with a significant decrease in both autonomous and cofactor-dependent PKC activity. The decrease in PKC activity was prevented by NMDA receptor blockade and was not accompanied by a decrease in the level of either PKCalpha, beta, gamma, or zeta. Western blot analysis with phosphospecific antibodies revealed that phosphorylation of Ser-657 on the catalytic domain of PKCalpha (Ser-660 on PKCbetaII) was decreased significantly after the induction of LTD, and that this dephosphorylation was prevented by the protein phosphatase inhibitor okadaic acid. The decrease in autonomous and cofactor-dependent PKC activity likewise was prevented by okadaic acid. These findings suggest that LTD in the adult hippocampus in vivo involves a decrease in PKC activity that is mediated, at least in part, by dephosphorylation of the catalytic domain of PKC by protein phosphatases activated after LTD-inducing stimulation. Our findings are consistent with the idea that protein dephosphorylation contributes to the expression of LTD.

Experience-dependent changes in extracellular spike amplitude may reflect regulation of dendritic action potential back-propagation in rat hippocampal pyramidal cells

Activity-dependent attenuations in extracellular spike amplitude have been shown to correlate with a decrease in the effectiveness with which somatic action potentials back-propagate into the dendritic arbor of hippocampal pyramidal cells. In this paper we demonstrate that activity-dependent attenuations in amplitude occur during behavior and that the amount of attenuation is reduced with an animal's experience in an environment. The observed reductions are caused by an animal's experience within a specific environmental context, are dependent on functional NMDA receptors, and are accompanied by an increase in the effective coupling of pyramidal cells and interneurons. These results provide an important step in linking together in vivo studies with in vitro data and suggest that mechanisms of plasticity engaged during behavior may be sufficient to alter the biophysical and integrative properties of hippocampal pyramidal cells.

PDZ proteins interacting with C-terminal GluR2/3 are involved in a PKC-dependent regulation of AMPA receptors at hippocampal synapses

We investigated the role of PDZ proteins (GRIP, ABP, and PICK1) interacting with the C-terminal GluR2 by infusing a ct-GluR2 peptide ("pep2-SVKI") into CA1 pyramidal neurons in hippocampal slices using whole-cell recordings. Pep2-SVKI, but not a control or PICK1 selective peptide, caused AMPAR-mediated EPSC amplitude to increase in approximately one-third of control neurons and in most neurons following the prior induction of LTD. Pep2-SVKI also blocked LTD; however, this occurred in all neurons. A PKC inhibitor prevented these effects of pep2-SVKI on synaptic transmission and LTD. We propose a model in which the maintenance of LTD involves the binding of AMPARs to PDZ proteins to prevent their reinsertion. We also present evidence that PKC regulates AMPAR reinsertion during dedepression.

A role for protein kinase C in a form of metaplasticity that regulates the induction of long-term potentiation at CA1 synapses of the adult rat hippocampus

The possibility that protein kinase C (PKC) is involved in the induction of N-methyl-D-aspartate (NMDA) receptor-dependent long-term potentiation (LTP) at CA1 synapses in the hippocampus has been the subject of considerable investigation. However, many of the conclusions have been drawn from the use of relatively nonspecific PKC inhibitors. In the present study we have examined the role of PKC in tetanus-induced LTP of AMPA receptor-mediated synaptic transmission in hippocampal slices obtained from adult rats. In particular, we have investigated the possible role of PKC in a molecular switch process that is triggered by the synaptic activation of metabotropic glutamate receptors and regulates the induction of LTP. We find that the three PKC inhibitors examined, chelerythrine, Ro-31-8220 and Gö 6983, all block the setting of the molecular switch at concentrations consistent with inhibition of PKC. In contrast, these inhibitors are without affect on the induction of LTP, even when applied in very much higher concentrations. A PKA inhibitor, Rp-cAMPS, had no effect on either process. We suggest that neither PKC nor PKA is required to induce LTP at this synapse. However, PKC is involved in the regulation of LTP induction, via the molecular switch process.

Long term depression in the CA1 field is associated with a transient decrease in pre- and postsynaptic PKC substrate phosphorylation

Induction of homosynaptic long term depression (LTD) in the CA1 field of the hippocampus is thought to require activation of N-methyl-d-aspartate receptors, an elevation of postsynaptic Ca(2+) levels, and a subsequent increase in phosphatase activity. To investigate the spatial and temporal changes in protein phosphatase activity following LTD induction, we determined the in situ phosphorylation state of a pre- (GAP-43/B-50) and postsynaptic (RC3) protein kinase C substrate during N-methyl-d-aspartate receptor-dependent LTD in the CA1 field of rat hippocampal slices. We show that LTD is associated with a transient (<30 min) and D-AP5-sensitive reduction in GAP-43/B-50 and RC3 phosphorylation and that LTD is prevented by the phosphatase inhibitors okadaic acid and cyclosporin A. Our data provide strong evidence for a transient increase in pre- and postsynaptic phosphatase activity during LTD. Since the in situ phosphorylation of the calmodulin-binding proteins GAP-43/B-50 and RC3 changes during both LTD and long term potentiation, these proteins may form part of the link between the Ca(2+) signal and Ca(2+)/calmodulin-dependent processes implicated in long term potentiation and LTD.

Superoxide-induced stimulation of protein kinase C via thiol modification and modulation of zinc content

We investigated the effects of mild oxidation on protein kinase C (PKC) using the xanthine/xanthine oxidase system of generating superoxide. Exposure of various PKC preparations to superoxide stimulated the autonomous activity of PKC. Similarly, thiol oxidation increased autonomous PKC activity, consistent with the notion that superoxide stimulates PKC via thiol oxidation. The superoxide-induced stimulation of PKC activity was partially reversed by reducing agents, suggesting that disulfide bond formation contributed to the oxidative stimulation of PKC. In addition, superoxide increased the autonomous activity of the alpha, beta(II), epsilon, and zeta PKC isoforms, all of which contain at least one cysteine-rich region. Taken together, our observations suggested that superoxide interacts with PKC at the cysteine-rich region, zinc finger motif of the enzyme. Therefore, we examined the effects of superoxide on this region by testing the hypothesis that superoxide stimulates PKC by promoting the release of zinc from PKC. We found that a zinc chelator stimulated the autonomous activity of PKC and that superoxide induced zinc release from an PKC-enriched enzyme preparation. In addition, oxidized PKC contained significantly less zinc than reduced PKC. Finally, we have isolated a persistent, autonomously active PKC by DEAE-cellulose column chromatography from hippocampal slices incubated with superoxide. Taken together, these data suggest that superoxide stimulates autonomous PKC activity via thiol oxidation and release of zinc from cysteine-rich region of PKC.

The extracellular signal-regulated kinase cascade is required for NMDA receptor-independent LTP in area CA1 but not area CA3 of the hippocampus

Activation of extracellular signal-regulated kinase (ERK) has been shown to be necessary for NMDA receptor-dependent long-term potentiation (LTP). We studied the role of ERK in three forms of NMDA receptor-independent LTP: LTP induced by very high-frequency stimulation (200 Hz-LTP), LTP induced by the K(+) channel blocker tetraethylammonium (TEA) (TEA-LTP), and mossy fiber (MF) LTP (MF-LTP). We found that ERK was activated in area CA1 after the induction of both 200 Hz-LTP and TEA-LTP and that this activation required the influx of Ca(2+) through voltage-gated Ca(2+) channels. Inhibition of the ERK signaling cascade with either PD 098059 or U0126 prevented the induction of both 200 Hz-LTP and TEA-LTP in area CA1. In contrast, neither PD 098059 nor U0126 prevented MF-LTP in area CA3 induced by either brief or long trains of high-frequency stimulation. U0126 also did not prevent forskolin-induced potentiation in area CA3. However, incubation of slices with forskolin, an activator of the cAMP-dependent protein kinase (PKA) cascade, did result in increases in active ERK and cAMP response element-binding protein (CREB) phosphorylation in area CA3. The forskolin-induced increase in active ERK was inhibited by U0126, whereas the increase in CREB phosphorylation was not, which suggests that in area CA3 the PKA cascade is not coupled to CREB phosphorylation via ERK. Overall, our observations indicate that activation of the ERK signaling cascade is necessary for NMDA receptor-independent LTP in area CA1 but not in area CA3 and suggest a divergence in the signaling cascades underlying NMDA receptor-independent LTP in these hippocampal subregions.

Activation of pre- and postsynaptic protein kinase C during tetraethylammonium-induced long-term potentiation in the CA1 field of the hippocampus

Tetraethylammonium (TEA) induces a form of long-term potentiation (LTP) that is independent on N-methyl-D-aspartate (NMDA) receptor activation (LTP(K)). LTP(K) may be a suitable chemical model to study molecular mechanisms underlying LTP. We monitored the phosphorylation state of two identified neural-specific protein kinase C (PKC) substrates (the presynaptic protein GAP-43/B-50 and postsynaptic protein RC3) after different chemical depolarisations. TEA induced a long-lasting increase in synaptic efficacy in the CA1 field of the hippocampus and increased the phosphorylation of both GAP-43/B-50 and RC3 (51 and 56.1%, respectively). These effects were blocked by the voltage-dependent calcium channel antagonist nifedipine, but not by the NMDA receptor antagonist AP5. These data show that in LTP(K) the in situ phosphorylation of pre-and postsynaptic PKC substrates is increased, indicating that NMDA receptor-dependent and NMDA receptor-independent LTP share common Ca(2+)-dependent expression mechanisms, including activation of pre- and postsynaptic PKC.

Dendritic potassium channels in hippocampal pyramidal neurons

Potassium channels located in the dendrites of hippocampal CA1 pyramidal neurons control the shape and amplitude of back-propagating action potentials, the amplitude of excitatory postsynaptic potentials and dendritic excitability. Non-uniform gradients in the distribution of potassium channels in the dendrites make the dendritic electrical properties markedly different from those found in the soma. For example, the influence of a fast, calcium-dependent potassium current on action potential repolarization is progressively reduced in the first 150 micrometer of the apical dendrites, so that action potentials recorded farther than 200 micrometer from the soma have no fast after-hyperpolarization and are wider than those in the soma. The peak amplitude of back-propagating action potentials is also progressively reduced in the dendrites because of the increasing density of a transient potassium channel with distance from the soma. The activation of this channel can be reduced by the activity of a number of protein kinases as well as by prior depolarization. The depolarization from excitatory postsynaptic potentials (EPSPs) can inactivate these A-type K+ channels and thus lead to an increase in the amplitude of dendritic action potentials, provided the EPSP and the action potentials occur within the appropriate time window. This time window could be in the order of 15 ms and may play a role in long-term potentiation induced by pairing EPSPs and back-propagating action potentials.

Modulation of presynaptic action potential kinetics underlies synaptic facilitation of type B photoreceptors after associative conditioning in Hermissenda

Descriptions of conditioned response generation in Hermissenda stipulate that the synaptic interaction between type B and A photoreceptors should be enhanced after associative pairings of light and rotation. Although evidence from several laboratories has confirmed this assumption, the mechanism underlying this synaptic facilitation has not been elucidated. Here we report that in vitro conditioning (i.e., light paired with stimulation of vestibular hair cells) modifies the kinetics of presynaptic action potentials in the B photoreceptor in a manner sufficient to account for this synaptic facilitation. After paired training, we observed an increase in the duration of evoked action potentials and a decrease in the amplitude of the spike afterhyperpolarization in the B-cell. As previously reported, paired training also enhanced the excitability (i.e., input resistance and evoked spike rate) of the B photoreceptor. In a second experiment, simultaneous recordings were made in type B and A photoreceptors, and paired training was found to produce an increase in the amplitude of the IPSP in the A photoreceptor in response to an evoked spike in the B-cell. Importantly, there was no change in the initial slope of the postsynaptic IPSP in the A photoreceptor, suggesting that spike duration-independent mechanisms of neurotransmitter exocytosis or postsynaptic receptor sensitivity did not contribute to the observed synaptic facilitation. Perfusion of 4-aminopyridine (4-AP) mimicked a known effect of behavioral conditioning in that it specifically reduced the amplitude of the transient voltage-dependent K(+) current (I(A)) in the B-cell, but in addition, produced action potential broadening and synaptic facilitation that was analogous to that observed after in vitro conditioning. Finally, the effect of 4-AP on B-cell action potentials and on the postsynaptic IPSP in the A-cell was occluded by previous paired (but not unpaired) training, suggesting that the prolongation of the B-cell action potential by a reduction of I(A) was sufficient to account for the observed synaptic facilitation. The occlusion of the effects of 4-AP by paired training was not attributable to a saturation of the capacity of the B-cell for transmitter exocytosis, because it was observed that tetraethylammonium (TEA)-induced inhibition of the delayed voltage-dependent K(+) current induced both spike broadening and synaptic facilitation regardless of training history. Collectively, these results demonstrate that training-induced facilitation at B-cell synapses is attributable to the effects of a reduction of a presynaptic K(+) conductance on action potential kinetics and suggest another critical similarity between the cellular basis for learning in Hermissenda and other invertebrate systems.

Long-term depression in the hippocampus in vivo is associated with protein phosphatase-dependent alterations in extracellular signal-regulated kinase

There is growing evidence that activation of either protein kinases or protein phosphatases determines the type of plasticity observed after different patterns of hippocampal stimulation. Because activation of the extracellular signal-regulated kinase (ERK) has been shown to be necessary for long-term potentiation, we investigated the regulation of ERK in long-term depression (LTD) in the adult hippocampus in vivo. We found that ERK immunoreactivity was decreased following the induction of LTD and that this decrease required NMDA receptor activation. The LTD-associated decrease in ERK immunoreactivity could be simulated in vitro via incubation of either purified ERK2 or hippocampal homogenates with either protein phosphatase 1 or protein phosphatase 2A. The protein phosphatase-dependent decrease in ERK immunoreactivity was inhibited by microcystin. Intrahippocampal administration of the protein phosphatase inhibitor okadaic acid blocked the LTD-associated decrease in ERK2, but not ERK1, immunoreactivity. Collectively, these data demonstrate that protein phosphatases can decrease ERK immunoreactivity and that such a decrease occurs with ERK2 during LTD. These observations provide the first demonstration of a biochemical alteration of ERK in LTD.

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 theta-frequency (5-Hz) synaptic stimulation in the hippocampal CA1 region. Our results show that brief trains of theta-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 beta-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.