Calcium-induced long-term potentiation in the hippocampus

Interplay between hippocampal TACR3 and systemic testosterone in regulating anxiety-associated synaptic plasticity

Tachykinin receptor 3 (TACR3) is a member of the tachykinin receptor family and falls within the rhodopsin subfamily. As a G protein-coupled receptor, it responds to neurokinin B (NKB), its high-affinity ligand. Dysfunctional TACR3 has been associated with pubertal failure and anxiety, yet the mechanisms underlying this remain unclear. Hence, we have investigated the relationship between TACR3 expression, anxiety, sex hormones, and synaptic plasticity in a rat model, which indicated that severe anxiety is linked to dampened TACR3 expression in the ventral hippocampus. TACR3 expression in female rats fluctuates during the estrous cycle, reflecting sensitivity to sex hormones. Indeed, in males, sexual development is associated with a substantial increase in hippocampal TACR3 expression, coinciding with elevated serum testosterone and a significant reduction in anxiety. TACR3 is predominantly expressed in the cell membrane, including the presynaptic compartment, and its modulation significantly influences synaptic activity. Inhibition of TACR3 activity provokes hyperactivation of CaMKII and enhanced AMPA receptor phosphorylation, associated with an increase in spine density. Using a multielectrode array, stronger cross-correlation of firing was evident among neurons following TACR3 inhibition, indicating enhanced connectivity. Deficient TACR3 activity in rats led to lower serum testosterone levels, as well as increased spine density and impaired long-term potentiation (LTP) in the dentate gyrus. Remarkably, aberrant expression of functional TACR3 in spines results in spine shrinkage and pruning, while expression of defective TACR3 increases spine density, size, and the magnitude of cross-correlation. The firing pattern in response to LTP induction was inadequate in neurons expressing defective TACR3, which could be rectified by treatment with testosterone. In conclusion, our study provides valuable insights into the intricate interplay between TACR3, sex hormones, anxiety, and synaptic plasticity. These findings highlight potential targets for therapeutic interventions to alleviate anxiety in individuals with TACR3 dysfunction and the implications of TACR3 in anxiety-related neural changes provide an avenue for future research in the field.

Drug-Responsive Inhomogeneous Cortical Modulation by Direct Current Stimulation

OBJECTIVE

Cathodal direct current stimulation (cDCS) induces long-term depression (LTD)-like reduction of cortical excitability (DCS-LTD), which has been tested in the treatment of epilepsy with modest effects. In part, this may be due to variable cortical neuron orientation relative to the electric field. We tested, in vivo and in vitro, whether DCS-LTD occurs throughout the cortical thickness, and if not, then whether drug-DCS pairing can enhance the uniformity of the cortical response and the cDCS antiepileptic effect.

METHODS

cDCS-mediated changes in cortical excitability were measured in vitro in mouse motor cortex (M1) and in human postoperative neocortex, in vivo in mouse somatosensory cortex (S1), and in a mouse kainic acid (KA)-seizure model. Contributions of N-methyl-D-aspartate-type glutamate receptors (NMDARs) to cDCS-mediated plasticity were tested with application of NMDAR blockers (memantine/D-AP5).

RESULTS

cDCS reliably induced DCS-LTD in superficial cortical layers, and a long-term potentiation (LTP)-like enhancement (DCS-LTP) was recorded in deep cortical layers. Immunostaining confirmed layer-specific increase of phospho-S6 ribosomal protein in mouse M1. Similar nonuniform cDCS aftereffects on cortical excitability were also found in human neocortex in vitro and in S1 of alert mice in vivo. Application of memantine/D-AP5 either produced a more uniform DCS-LTD throughout the cortical thickness or at least abolished DCS-LTP. Moreover, a combination of memantine and cDCS suppressed KA-induced seizures.

INTERPRETATION

cDCS aftereffects are not uniform throughout cortical layers, which may explain the incomplete cDCS clinical efficacy. NMDAR antagonists may augment cDCS efficacy in epilepsy and other disorders where regional depression of cortical excitability is desirable. ANN NEUROL 2020;88:489-502.

Methodological standards for in vitro models of epilepsy and epileptic seizures. A TASK1-WG4 report of the AES/ILAE Translational Task Force of the ILAE

In vitro preparations are a powerful tool to explore the mechanisms and processes underlying epileptogenesis and ictogenesis. In this review, we critically review the numerous in vitro methodologies utilized in epilepsy research. We provide support for the inclusion of detailed descriptions of techniques, including often ignored parameters with unpredictable yet significant effects on study reproducibility and outcomes. In addition, we explore how recent developments in brain slice preparation relate to their use as models of epileptic activity.

REVIEW ■ : GABAergic Inhibitory Processes and Hippocampal Long-term Potentiation

Long-term potentiation (LTP) is a widely studied form of activity-dependent synaptic plasticity. Hippocampal LTP evoked in the dentate and CA1 areas requires calcium influx through N-methyl-D-aspartate (NMDA) receptor-channel complexes, a process triggered during high-frequency stimulation by conjunctive presy naptic glutamate release and postsynaptic depolarization. It has been suggested that alterations in GABAergic recurrent and/or feedforward inhibitory synaptic transmission may accompany LTP induction in these hippocampal areas. To this end, possible LTP-related modifications in functional inhibition are ad dressed in the context of both paired-pulse depression and the excitatory postsynaptic potential-spike (E-S) relationship. Consideration is also given as to how GABAergic processes may contribute mechanistically to the induction of NMDA receptor-dependent LTP. It is concluded that although GABAergic disinhibition may contribute to the induction of LTP, it is not yet clear whether or not the induction of LTP has a lasting impact on inhibitory processes. NEUROSCIENTIST 3:226-236, 1997

Calcium, Reactive Oxygen Species, and Synaptic Plasticity

In this review article, we address how activity-dependent Ca2+ signaling is crucial for hippocampal synaptic/structural plasticity and discuss how changes in neuronal oxidative state affect Ca2+ signaling and synaptic plasticity. We also analyze current evidence indicating that oxidative stress and abnormal Ca2+ signaling contribute to age-related synaptic plasticity deterioration.

NMDA-dependent phase synchronization between septal and temporal CA3 hippocampal networks

Increasing evidence suggests that synchronization between brain regions is essential for information exchange and memory processes. However, it remains incompletely known which synaptic mechanisms contribute to the process of synchronization. Here, we investigated whether NMDA receptor-mediated synaptic plasticity was an important player in synchronization between septal and temporal CA3 areas of the rat hippocampus. We found that both the septal and temporal CA3 regions intrinsically generate weakly synchronized δ frequency oscillations in the complete hippocampus in vitro. Septal and temporal oscillators differed in frequency, power, and rhythmicity, but both required GABAA and AMPA receptors. NMDA receptor activation, and most particularly the NR2B subunit, contributed considerably more to rhythm generation at the temporal than the septal region. Brief activation of NMDA receptors by application of extracellular calcium dramatically potentiated the septal-temporal coherence for long durations (>40 min), an effect blocked by the NMDA antagonist AP-5. This long-lasting NMDA-receptor-dependent increase in coherence was also associated with an elevated phase locking of spikes locally and across regions. Changes in coherence between oscillators were associated with increases in phase locking between oscillators independent of oscillator amplitude. Finally, although the septal CA3 rhythm preceded the oscillations in temporal regions in control conditions, this was reversed during the NMDA-dependent enhancement in coherence, suggesting that NMDA receptor activation can change the direction of information flow along the septotemporal CA3 axis. These data demonstrate that plastic changes in communication between septal and temporal hippocampal regions can arise from the NMDA-dependent phase locking of neural oscillators.

Long-term potentiation in cultured hippocampal neurons

Studies performed on low-density primary neuronal cultures have enabled dissection of molecular and cellular changes during N-methyl-D-aspartate (NMDA) receptor-dependent long-term potentiation (LTP). Various electrophysiological and chemical induction protocols were developed for the persistent enhancement of excitatory synaptic transmission in hippocampal neuronal cultures. The characterisation of these plasticity models confirmed that they share many key properties with the LTP of CA1 neurons, extensively studied in hippocampal slices using electrophysiological techniques. For example, LTP in dissociated hippocampal neuronal cultures is also dependent on Ca(2+) influx through post-synaptic NMDA receptors, subsequent activation and autophosphorylation of the Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and an increase in alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor insertion at the post-synaptic membrane. The availability of models of LTP in cultured hippocampal neurons significantly facilitated the monitoring of changes in endogenous postsynaptic receptor proteins and the investigation of the associated signalling mechanisms that underlie LTP. A central feature of LTP of excitatory synapses is the recruitment of AMPA receptors at the postsynaptic site. Results from the use of cell culture-based models started to establish the mechanism by which synaptic input controls a neuron's ability to modify its synapses in LTP. This review focuses on key features of various LTP induction protocols in dissociated hippocampal neuronal cultures and the applications of these plasticity models for the investigation of activity-induced changes in native AMPA receptors.

Reversal of Hippocampal LTP by Spontaneous Seizure-Like Activity: Role of Group I mGluR and Cell Depolarization

Memory impairment is a common consequence of epileptic seizures. The hippocampal formation is particularly prone to seizure-induced amnesia due to its prominent role in mnemonic processes. We used the isolated CA1 slice preparation to examine effects of seizure-like activity on hippocampal plasticity, long-term potentiation (LTP), and long-term depression (LTD). Repeated spontaneous ictal events, generated in the presence of antagonists of GABAA receptor function, led to a stepwise erasure of LTP (termed spontaneous depotentiation, SDP). SDP could be initiated at various stages of LTP consolidation (tested ≤120 min after the induction of LTP). Renewed tetanic stimulation re-established LTP. SDP was remarkably specific: baseline transmission and other forms of hippocampal plasticity, i.e., Ca2+-induced LTP and two forms of LTD [(RS)-3,5-dihydroxyphenyglycine (DHPG) mediated and low-frequency stimulation mediated] were not affected by the same type of seizure activity. SDP was blocked in the presence of the group I mGluR antagonist ( S)-4-carboxyphenylglycine. The mGluR1 antagonist ( S)-(+)-α-amino-methylbenzeneacetic acid blocked ∼80%, the mGluR5-specific antagonist 2-methyl-6-(phenylethynyl)-pyridine ∼30% of SDP. Most efficient implementation of SDP was observed during seizures in the combined presence of the group I mGluR agonist DHPG and the GABAA antagonist bicuculline. However, similar ictal activity generated in the presence of DHPG alone did not lead to SDP in the vast majority of recordings. Complete disinhibition and at least partial activation of group I mGluR were necessary conditions for the induction of SDP. The depotentiating pharmacological conditions were accompanied by tonic membrane depolarization of CA1 pyramidal cells. Since hyperpolarization (by negative current injection) prevented intracellular SDP under depotentiating pharmacological conditions and depolarization (by positive current injection) led to selective intracellular SDP in the non-depotentiating seizure protocol of DHPG, it is concluded that cell depolarization was a sufficient condition for seizure-like activity to reverse hippocampal LTP.

Long-term potentiation in the optic tectum of rainbow trout

We examined synaptic plasticity in the optic tectum of rainbow trout by extracellular recordings. We found that the field-excitatory postsynaptic potential in the retinotectal synapses was potentiated by repetitive stimuli of 1.0 Hz for 20 s to the retinotectal afferents. The long-term potentiation (LTP) developed slowly, and was maintained for at least 2 h. Applications of an antagonist for N-methyl-D-aspartic acid (NMDA) receptors or Mg2+ -free saline showed that activation of NMDA receptors was required to form the LTP beyond the induction period. The present findings indicate that presynaptic stimulation in the retinotectal synapses causes LTP mediated by NMDA receptors in the optic tectum of rainbow trout.

Chemically-induced long-term potentiation in rat motor cortex involves activation of extracellular signal-regulated kinase cascade

The involvement of the extracellular signal-regulated kinase 1/2 (ERK1/2) cascade in long-lasting potentiation of synaptic transmission, induced by tetraethylammonium (TEA) or by elevated extracellular calcium concentration, was investigated in layer V horizontal connections within motor cortex in rat brain slices. Brief application of TEA (25 mM) resulted in a long-lasting potentiation of field potentials by 54+/-12%. A transient exposure of slices to elevated extracellular calcium (5 mM) induced long-lasting potentiation of responses reaching 30+/-8%. The induction of both forms of potentiation was prevented by the exposure of slices to inhibitors of the upstream activator of ERK 1/2, MEK (ERK kinase), U0126 (20 microM) and PD 98059 (50 microM). PhosphoERK2 immunoreactivity was transiently increased above baseline levels 15 min after termination of the exposure of slices to either TEA or elevated calcium concentration. Both forms of potentiation were partially occluded by Sp-adenosine 3',5'-cyclic monophosphorothioate triethylammonium salt (Sp-cAMPS; 100 microM), an activator of cAMP-dependent protein kinase (PKA), and they were blocked after preincubation with Rp-adenosine 3',5'-cyclic monophosphorothioate triethylammonium salt (Rp-cAMPS; 100 microM), a specific inhibitor of PKA activation by cAMP. It has previously been shown that TEA-induced potentiation represents a N-methyl-d-aspartate (NMDA) receptor-independent form of persistent synaptic enhancement, and, on the contrary, calcium-induced potentiation depends on NMDA receptors. Thus, the activation of PKA and the ERK1/2 cascade are required for two forms of chemically induced long-lasting increases of synaptic efficacy in slices of rat motor cortex.

Calcium-induced long-term potentiation in horizontal connections of rat motor cortex

A transient (10 min) exposure of brain slices of young adult rats to elevated extracellular calcium (5 mM) resulted in a long-lasting potentiation of field potentials evoked in layer II/III and layer V horizontal connections of the primary motor cortex. This form of synaptic plasticity was blocked by D,L-2-amino-5-phosphonovalerate (APV, 100 micro M), an antagonist of NMDA receptors.

Neural plasticity and the brain renin-angiotensin system

The brain renin-angiotensin system mediates several classic physiologies including body water balance, maintenance of blood pressure, cyclicity of reproductive hormones and sexual behaviors, and regulation of pituitary gland hormones. In addition, angiotensin peptides have been implicated in neural plasticity and memory. The present review initially describes the extracellular matrix (ECM) and the roles of cell adhesion molecules (CAMs), matrix metalloproteinases, and tissue inhibitors of metalloproteinases in the maintenance and degradation of the ECM. It is the ECM that appears to permit synaptic remodeling and thus is critical to the plasticity that is presumed to underlie mechanisms of memory consolidation and retrieval. The interrelationship among long-term potentiation (LTP), CAMs, and synaptic strengthening is described, followed by the influence of angiotensins on LTP. There is strong support for an inhibitory influence by angiotensin II (AngII) and a facilitory role by angiotensin IV (AngIV), on LTP. Next, the influences of AngII and IV on associative and spatial memories are summarized. Finally, the impact of sleep deprivation on matrix metalloproteinases and memory function is described. Recent findings indicate that sleep deprivation-induced memory impairment is accompanied by a lack of appropriate changes in matrix metalloproteinases within the hippocampus and neocortex as compared with non-sleep deprived animals. These findings generally support an important contribution by angiotensin peptides to neural plasticity and memory consolidation.

Loss of Calbindin-28K Immunoreactivity in Hippocampal Slices from Aged Rats: a Role for Calcium?

Calbindin-28K (CaBP) is a calcium-binding protein widely distributed in the brain. This protein appears to be involved in the sequestration and the translocation of intracellular free calcium. In this study, we have examined the distribution pattern of the structures immunoreactive for CaBP in the hippocampal formation from slices of young (4 months) and aged (24 - 27 months) rats previously submitted to electrophysiological measurements. We demonstrated a marked loss in the number of pyramidal cells immunoreactive for CaBP in aged rats as compared to young rats. A consistent decrease in the staining intensity was also revealed by optical density measurements. Some experiments have suggested that calcium homeostasis is modified in hippocampal neurons of aged rats. The loss of CaBP-like immunoreactivity (CaBP-LI) labelling could result from an increase in intracellular calcium concentrations. To support this hypothesis, we showed that in young rats (i) the CaBP-LI was enhanced in pyramidal neurons when the slice was preincubated in a calcium-free medium, and (ii) CaBP-LI was strongly decreased when the slice was preincubated in a high-calcium medium (5 mM) and when the entry of calcium into the cell was increased by a short application of an excitatory amino acid in the medium. Our results suggest that the loss of CaBP-LI in the hippocampus of aged rats could be due to an increase in intracellular calcium concentration. Preliminary observations of hippocampal slices at different times after induction of long-term potentiation (LTP) failed to show significant changes in CaBP immunoreactivity, suggesting that this calcium-binding protein is not directly involved in LTP processes.

Extracellular matrix molecules, long-term potentiation, memory consolidation and the brain angiotensin system

Considerable evidence now suggests an interrelationship among long-term potentiation (LTP), extracellular matrix (ECM) reconfiguration, synaptogenesis, and memory consolidation within the mammalian central nervous system. Extracellular matrix molecules provide the scaffolding necessary to permit synaptic remodeling and contribute to the regulation of ionic and nutritional homeostasis of surrounding cells. These molecules also facilitate cellular proliferation, movement, differentiation, and apoptosis. The present review initially focuses on characterizing the ECM and the roles of cell adhesion molecules (CAMs), matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs), in the maintenance and degradation of the ECM. The induction and maintenance of LTP is described. Debate continues over whether LTP results in some form of synaptic strengthening and in turn promotes memory consolidation. Next, the contribution of CAMs and TIMPs to the facilitation of LTP and memory consolidation is discussed. Finally, possible roles for angiotensins, MMPs, and tissue plasminogen activators in the facilitation of LTP and memory consolidation are described. These enzymatic pathways appear to be very important to an understanding of dysfunctional memory diseases such as Alzheimer's disease, multiple sclerosis, brain tumors, and infections.

Long term depletion of serotonin leads to selective changes in glutamate receptor subunits

The present study was carried out to clarify possible modulation mechanism of serotonin (5-HT) on glutamatergic neurotransmission in the rat cerebral cortex. 5-HT was depleted by a 5-HT metabolite blocker (para-chlorophenylalanine; pCPA) for a week. Receptor binding experiments using (S)-[(3)H]alpha-amino-3-hydroxy-5-methylisoxazol-4-propionic acid (AMPA) showed a considerable increase in B(max) value of the membrane samples prepared from the cerebral cortex of rats compared with that of control animals received saline. In contrast, B(max) value of the [(3)H]MK-801 binding experiments for NMDA receptor was not changed by pCPA-treatment. Changes in the density of each AMPA receptor subtype were examined in the cerebral cortex by immunoblot analyses using antibodies against AMPA receptor subunits. The density of immunoreactive bands with receptor subtype specific antibodies against GluR2/3 and GluR2 receptors was increased, whereas that of GluR1 receptors was decreased. Considering GluR2 receptor subtype inhibits Ca(2+) influx into neurons, the present study suggests that 5-HT appears to modulate synaptic plasticity by regulating the density of each AMPA receptor subtype.

Adenylyl cyclase activation modulates activity-dependent changes in synaptic strength and Ca2+/calmodulin-dependent kinase II autophosphorylation

Activation of the Ca2+- and calmodulin-dependent protein kinase II (CaMKII) and its conversion into a persistently activated form by autophosphorylation are thought to be crucial events underlying the induction of long-term potentiation (LTP) by increases in postsynaptic Ca2+. Because increases in Ca2+ can also activate protein phosphatases that oppose persistent CaMKII activation, LTP induction may also require activation of signaling pathways that suppress protein phosphatase activation. Because the adenylyl cyclase (AC)-protein kinase A signaling pathway may provide a mechanism for suppressing protein phosphatase activation, we investigated the effects of AC activators on activity-dependent changes in synaptic strength and on levels of autophosphorylated alphaCaMKII (Thr286). In the CA1 region of hippocampal slices, briefly elevating extracellular Ca2+ induced an activity-dependent, transient potentiation of synaptic transmission that could be converted into a persistent potentiation by the addition of phosphatase inhibitors or AC activators. To examine activity-dependent changes in alphaCaMKII autophosphorylation, we replaced electrical presynaptic fiber stimulation with an increase in extracellular K+ to achieve a more global synaptic activation during perfusion of high Ca2+ solutions. In the presence of the AC activator forskolin or the protein phosphatase inhibitor calyculin A, this treatment induced a LTP-like synaptic potentiation and a persistent increase in autophosphorylated alphaCaMKII levels. In the absence of forskolin or calyculin A, it had no lasting effect on synaptic strength and induced a persistent decrease in autophosphorylated alphaCaMKII levels. Our results suggest that AC activation facilitates LTP induction by suppressing protein phosphatases and enabling a persistent increase in the levels of autophosphorylated CaMKII.

Induction of LTD by increasing extracellular Ca2+ from a low level in the dentate gyrus in vitro

Long-term depression (LTD) of field excitatory postsynaptic potentials was induced in the medial path of the dentate gyrus in juvenile rats in vitro by a protocol involving returning the Ca2+ concentration of the external media to the control level (2 mM) following a period (30 min) in low Ca2+ (1.2 mM), with stimulation at the test frequency throughout. Directly increasing Ca2+ from the control level did not induce LTD (or long-term potentiation). Such LTD showed mutual occlusion with low frequency stimulation (LFS, 1 Hz, 900 stimuli)-induced LTD. The Ca2+-induced LTD was shown to be induced by the procedure of elevating the Ca2+ from the low to the control concentration, and not by the procedure of lowering of Ca2+, as full amplitude LFS induced LTD could be induced in the low Ca2+ media. Ca2+-induced LTD was inhibited by Ni2+ (25 microM) and 2-amino-5-phosphonopentanoate (50 microM), demonstrating the necessity for activation of T/R-type voltage-gated Ca2+ channels and N-methyl-D-aspartate receptors, respectively.

Metabotropic glutamate receptors are involved in calcium-induced LTP of AMPA and NMDA receptor-mediated responses in the rat hippocampus

Effects of metabotropic glutamate (mGlu) receptors on calcium-induced long-term potentiation (LTP) of alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA) and N-methyl-D-aspartate (NMDA) receptor-mediated components were investigated in rat hippocampal slices using whole-cell patch-clamp recordings of excitatory postsynaptic currents (EPSCs). Calcium-induced LTP comprises a parallel, long-lasting increase of AMPA and NMDA receptor-mediated components. The calcium-induced LTP of the AMPA receptor-mediated component can be significantly attenuated by the use of a selective NMDA antagonist. (R.S)-alpha-methyl-4-carboxyphenylglycine (MCPG), a selective antagonist of mGlu receptors, abolished the long-lasting increase of both AMPA and NMDA receptor-mediated components observed in calcium-induced LTP. In current clamp mode, the application of a high calcium alone or Schaffer fiber stimulation alone (20 Hz) only generated a short-term increase in the firing rate of evoked action potentials. Conversely, a long-term increase in the firing rate was observed if Schaffer fiber stimulation (20 Hz) accompanied the perfusion of high calcium. These results suggest that calcium-induced LTP involves a parallel, long-lasting enhancement in ionotropic AMPA and NMDA receptor-mediated components. More importantly, the mGlu receptor plays a critical role in the establishment of both AMPA and NMDA receptor-mediated components underlying calcium-induced LTP. In addition, the present study also described an experimental condition in which the coapplication of the high calcium pulse and Schaffer fiber stimulation (20 Hz) can synergistically elicit a long-term increase of neuronal excitability.

Activation of NMDA receptors is necessary for the induction of associative long-term potentiation in area CA1 of the rat hippocampal slice

1. It is commonly assumed that the role of the strongly activated heterosynaptic input during the induction of associative long-term potentiation (LTP) is to relieve the magnesium blockade of NMDA receptors located at the weakly stimulated synapses and thereby allow the weak input to undergo potentiation. We tested this assumption by using a caged form of the NMDA receptor antagonist, D-(-)-2-amino-5-phosphonopentanoic acid (D-AP5) to block the activation of NMDA receptors at the weak input in a conditioning protocol for the induction of associative LTP in area CA1 of the rat hippocampal slice. 2. The effect of releasing D-AP5 by flash photolysis of 100 microM caged D-AP5 (N-[1-(2-nitrophenyl)ethoxycarbonyl]-D-AP5) on pharmacologically isolated NMDA receptor-mediated field EPSPs was examined in area CA1. The slope of the EPSP was reduced by 71% within 50 ms of the initiation of the photolytic reaction when the concentration of released D-AP5 had reached 2.0-2.5 microM and was reduced by 95% within 1 min (10 microM D-AP5 released). 3. Associative LTP was induced by pairing a strong tetanus to one input with a weak tetanus (subthreshold for homosynaptic LTP) to a second input. The strong tetanus preceded the weak by 50 ms. Rapid application of D-AP5, by flash photolysis of caged D-AP5, coincident with the last shock of the strong tetanus, resulted in the blockade of NMDA receptor activation during the period of the weak tetanus. Associative LTP was blocked by photolysis of caged D-AP5 but was normally expressed in experiments using caged L-AP5. 4. We conclude that activation of NMDA receptors at the weakly activated input is an essential requirement for synaptically induced associative LTP.

Chronic cerebral hypoperfusion inhibits calcium-induced long-term potentiation in rats

BACKGROUND AND PURPOSE

Long-term potentiation (LTP) in the rat hippocampus induced by tetanic stimulation is impaired by chronic cerebral hypoperfusion. The effects of chronic cerebral hypoperfusion on other forms of LTP are unknown. Such data could help delineate the pathways of cellular alteration caused by chronic cerebral hypoperfusion. The in vitro phenomenon of calcium-induced LTP was thus examined in rat hippocampal CA1 cells that had undergone chronic hypoperfusion with a reduction in cerebral blood flow of between 25% and 50% maintained for 26 weeks.

METHODS

Ten Sprague-Dawley rats had a cervical arteriovenous fistula surgically constructed, and an additional 10 animals were used as age-matched controls. Hippocampal slices were prepared after 26 weeks of hypoperfusion, and in vitro extracellular field potential recordings were taken from the Schäffer collateral CA1 region. Properties of LTP induced through transient exposure to a hypercalcemic solution were analyzed.

RESULTS

LTP was impaired in animals with an arteriovenous fistula (P < .05). Control animals demonstrated potentiation lasting for the entire 2 hours of recording, whereas fistula animals showed only transient potentiation (< 60 minutes) before returning to baseline values.

CONCLUSIONS

Calcium-induced LTP is impaired by chronic cerebral hypoperfusion. This form of LTP is different from that induced by tetanic stimulation. It is the most sensitive test available for in vitro detection of the changes induced in neuronal function by chronic noninfarctional reductions in cerebral blood flow of 25% to 50% and may indicate that the most basic cellular parameters involving calcium homeostasis and metabolism are being altered. The precise mechanisms remain to be elucidated, and several postulates are discussed.

Enhanced LTP in mice deficient in the AMPA receptor GluR2

AMPA receptors (AMPARs) are not thought to be involved in the induction of long-term potentiation (LTP), but may be involved in its expression via second messenger pathways. However, one subunit of the AMPARs, GluR2, is also known to control Ca2+ influx. To test whether GluR2 plays any role in the induction of LTP, we generated mice that lacked this subunit. In GluR2 mutants, LTP in the CA1 region of hippocampal slices was markedly enhanced (2-fold) and nonsaturating, whereas neuronal excitability and paired-pulse facilitation were normal. The 9-fold increase in Ca2+ permeability, in response to kainate application, suggests one possible mechanism for enhanced LTP. Mutant mice exhibited increased mortality, and those surviving showed reduced exploration and impaired motor coordination. These results suggest an important role for GluR2 in regulating synaptic plasticity and behavior.

LTD, LTP, and the sliding threshold for long-term synaptic plasticity

LTD of synaptic transmission is a form of long-term synaptic plasticity with the potential to be as significant as LTP to both the activity-dependent development of neural circuitry and adult memory storage. In addition, interactions between LTP and LTD and the dynamic regulation of the gain of synaptic plasticity mechanisms are also very important. In particular, the computational ability of LTD to properly counterbalance LTP may be essential to maintaining synaptic strengths in the linear range, and to maximally sharpen the ability of synapses to compute and store frequency-based information about the phase relation between synapses. Experimental data confirm the presence of an activity-dependent "sliding threshold" with the expected properties. That is, when levels of neuronal activity are high, indicating circumstances increasing the likelihood of inducing LTP, compensatory changes cause the suppression of LTP and an enhanced likelihood of LTD. Conversely, we would predict that low levels of synaptic activity would shift the threshold in favor of greater LTP and less LTD, a hypothesis which has yet to be tested. The sliding threshold for LTP and LTD also has implications for underlying cellular mechanisms of both forms of long-term synaptic plasticity. If the thresholds for LTP and LTD are tightly and reciprocally co-regulated, that could imply that at least one component of LTD is a true depotentiation caused by reversal of a change mediating LTP. If so, the intuitively simplest hypothesis is that phosphorylation of AMPA glutamate receptors causes LTP of synaptic e.p.s.p.s, while dephosphorylation of the same site or sites causes depotentiation LTD. Of course, this hypothesis would refer only to a postsynaptic component of both LTP and LTD. There has been a recent report that, in neonatal rat hippocampus, a form of LTD that is expressed developmentally earlier than LTP appears to have a postsynaptic induction site, but is expressed as decreased presynaptic transmitter release (Bolshakov and Siegelbaum, 1994). Whether these properties will be retained as LTD matures is unknown, as is the likelihood that, if a component of LTP is expressed presynaptically, depotentiation of that presynaptic component can also occur. Equally unclear is the persistence of LTD relative to LTP. The few rigorous long-term anatomical studies available suggest that the latest phases of LTP may be expressed as changes in dendritic spine shapes and/or synaptic morphology. While heterosynaptic LTD has been reported to have a duration of weeks in vivo (Abraham et al., 1994), we do not know whether LTP-induced morphological changes that take many days to appear can be reversed in an activity-dependent manner. An important feature of the consolidation of memories may turn out to be the slow development of LTP that is resistant to reversal by LTD. While we still at an earlier stage in our understanding of the mechanisms underlying LTD compared to LTP, some things are becoming clear. LTD is induced by afferent neuronal activity that is relatively ineffective in exciting the postsynaptic cell--an "anti-hebbian" condition. This property, coupled with the hebbian properties of LTP and the dynamic nature of membrane conductances, necessarily confers upon synapses the ability to compute and store the results of a covariance function. However, the role of such a computation in processing and/or memory is unclear. In addition, LTD appears to require the activation of NMDA and metabotropic subtypes of glutamate receptors, release of Ca2+ from intracellular stores, and an increase in intracellular [Ca2+] that is lower than that necessary to induce LTP. The early evidence is consistent with some overlap of targets for modification by LTP and LTD, with some forms of LTD likely to be a reversal, or "depotentiation," of previous LTP, perhaps through dephosphorylation of AMPA receptors.

NMDA/R1-antisense oligonucleotide influences the early stage of long-term potentiation in the CA1-region of rat hippocampus

We have studied the role of the N-methyl-D-aspartate (NMDA)/R1 receptor subunit in the mechanism of long-term potentiation (LTP) using an antisense-oligodeoxynucleotide strategy. Antisense-oligodeoxynucleotide (aDON; 10 nmol) or sense-oligodeoxynucleotide (sDON) were applied into the right ventricle of 7 week old male Wistar rats every 12 h for 3 days. Thereafter, in hippocampal slices extracellular field potential recordings were made from the CA1 region. LTP was induced by tetanization of the Schaffer collaterals. In slices of rats pretreated with aDON the initial potentiation of the population spike (POP-spike) was significantly smaller than in those from saline controls and naive animals. The impairment of potentiation lasted for about 50 min posttetanus. However, the potentiation of the field excitatory postsynaptic potentials (fEPSPs) was significantly influenced for 15 min. In slices of rats pretreated with sDON only a small, insignificant difference in POP-spike and fEPSP potentiation was seen compared to saline controls. In the group pretreated with aDON the specific binding of [3H]glutamate to the NMDA-receptor subtype of hippocampal membranes was reduced to about 63% in comparison with the group treated with sDON. These results indicate that DONs reached the target region when applied intraventricularly and were able to suppress the translation of mRNA of the NMDA/R1 receptor subunit. They further support the assumption of the essential role of the NMDA/R1 receptor subunit in the induction of LTP.

Long-term potentiation and N-methyl-D-aspartate receptors: foundations of memory and neurologic disease?

Understanding the physiology of learning and memory is one of the great challenges of neuroscience. The discovery in recent years of long-term potentiation (LTP) of synaptic transmission and the elaboration of the mechanisms involved, in particular the NMDA receptor, offers the prospect not only of improving our understanding of normal memory storage and retrieval, but may also yield insights about various neurological and psychiatric clinical disorders. In this review, we begin by examining the different forms, properties, and methods of inducing LTP, followed by a description of molecular mechanisms thought to underlie the phenomenon. Molecular structure of the receptor is discussed, along with the roles of Ca2+ second messenger systems, synaptic morphology changes, and retrograde messengers in LTP. Finally, implications of the NMDA receptor and LTP in learning, memory, and certain clinical conditions such as epilepsy, Alzheimer's disease, and schizophrenia are discussed.

Synaptic transmission and paired-pulse behaviour of CA1 pyramidal cells in hippocampal slices from a hibernator at low temperature: importance of ionic environment

To investigate the effects of ionic changes possibly associated with hibernation, hippocampal slices prepared from golden hamsters were studied in artificial cerebrospinal fluid (ACSF) of variable composition (K+ 3-5 mM, Ca2+ 2-4 mM, Mg2+ 2-4 mM, pH 7.0-7.7) at temperatures of 15-20 degrees C, just above the temperature below which synaptic transmission is blocked. Population action potentials (population spikes, PSs) of CA1 pyramidal cells were evoked by stimulation of the Schaffer collaterals/commissural fibers with paired pulses (interpulse interval 50 ms, interval between pairs 30 s). The responses evoked at given temperatures were investigated as a function of extracellular ion concentrations. In ACSF containing 3 mM K+, 2 mM Ca2+ and 2 mM Mg2+, PSs could be evoked at temperatures of > approximately 16 degrees C whereas at lower temperatures synaptic transmission was blocked. The threshold temperature was slightly higher for the first (PS1) than for the second PS (PS2) evoked by paired-pulse stimulation. The slices displayed paired-pulse facilitation (PPF) at all temperatures. Elevation of [K+]o from 3 to 5 mM depressed the amplitudes of both PS1 and PS2, with a stronger effect on PS2. PPF was reduced and, at near-threshold temperatures, turned into paired-pulse depression (PPD). Elevation of [Ca2+]o from 2 to 4 mM increased the amplitude of PS1. The amplitude of PS2, in contrast, was reduced at near-threshold temperatures. PPF turned into PPD. Elevation of [Mg2+]o from 2 to 4 mM reduced the amplitudes of both PS1 and PS2, with a stronger effect on PS1. Accordingly, PPF was increased. Acidification by 0.3 pH units strongly depressed the amplitudes of PS1 as well as PS2 and increased PPF. Alkalization by 0.4 pH units had only weak effects in the opposite direction. Changes in the ionic composition comparable to those investigated in the present study presumably occur in the brain interstitium of hamsters during entrance into hibernation. According to our results, such changes depress synaptic transmission at low temperatures in the hamster hippocampus in vitro. This modulation may be important for the regulation of neuronal activity during entrance into hibernation.

Reduced hippocampal CA1 Ca(2+)-induced long-term potentiation is associated with age-dependent impairment of spatial learning

Expression of Ca(2+)-induced CA1 long-term potentiation (LTP) was analysed in hippocampal slices obtained from (1) 3-month-old and (2) 18-20-month-old Sprague-Dawley rats selected for their performances in the Morris water maze task. In all slices, a transient (10 min) increase of extracellular Ca2+ concentration (4 mM) caused a long-lasting enhancement of potentials evoked by electrical stimulation of radiatum fibers. However, a significant difference was found in the degree of potentiation among groups. In particular, increases of the CA1 response amplitudes were significantly lower in old rats impaired in spatial learning than in young at 30 (P < 0.05), 60, 90 and 120 min (P < 0.01) after restoring the normal Ca2+ concentration. On the contrary, no differences were observed between young animals and the old ones with good performances in spatial learning. The data suggest that amplitude of CA1 Ca(2+)-induced LTP in old rats is related to spatial learning abilities.

The S-100: a protein family in search of a function

The S-100 is a group of low molecular weight (10-12 kD) calcium-binding proteins highly conserved among vertebrates. It is present in different tissues as dimers of homologous or different subunits (alpha, beta). In the nervous system, the S-100 exists as a mixture composed of beta beta and alpha beta dimers with the monomer beta represented more often. Its intracellular localisation is mainly restricted to the glial cytoplasmic compartment with a small fraction bound to membranes. In this compartment the S-100 acts as a potent inhibitor of phosphorylation on several substrates including the synaptosomal C-Kinase and Tau, a microtubule-associated protein. The S-100 in particular conditions, after binding with specific membrane sites (Kd = 0.2 microM; Bmax = 4.5 nM), is able to modify the activity of adenylate cyclase, probably via G-proteins. In addition, the Ca2+ homeostasis is also modulated by S-100 via an increase of specific membrane conductance and/or Ca2+ release from intracellular stores. "In vitro" and "in vivo" experiments showed that lower (nM) concentrations of extracellular S-100 beta act on glial and neuronal cells as a growth-differentiating factor. On the other hand, higher concentrations of the protein induce apoptosis of some cells such as the sympathetic-like PC12 line. Finally, data obtained from physiological (development, ageing) or pathological (dementia associated with Down's syndrome, Alzheimer's disease) conditions showed that a relationship could be established between the S-100 levels and some aspects of the statii.

Associative EPSP--spike potentiation induced by pairing orthodromic and antidromic stimulation in rat hippocampal slices

1. Pairing low-frequency orthodromic stimulation with high-frequency antidromic conditioning of pyramidal cells in area CA1 of the rat hippocampus resulted in long-lasting potentiation of the extracellular population spike of the cells, without an accompanying increase in the extracellular excitatory postsynaptic potential (EPSP), indicating an increase in EPSP-spike (E-S) coupling, also called E-S potentiation. 2. The amplitude of the antidromically conditioned E-S potentiation took up to 60 min to reach its peak, much longer than synaptic long-term potentiation (LTP) induced by orthodromic tetanic stimulation. 3. The population spike amplitude of a control orthodromic input, which stimulated a separate set of fibres and which was inactive during the pairing, was also increased in over half the slices tested. That it can affect a silent pathway suggests that antidromically conditioned E-S potentiation is not generated locally at tetanized synapses. 4. Bath application of 50 microM D,L-2-amino-5-phosphonovaleric acid (AP5) blocked induction of antidromically conditioned E-S potentiation. After washing out the AP5, the same stimulation resulted in population spike increases. This suggests that activation of the NMDA subtype of glutamate receptor is necessary for the induction of this form of E-S potentiation. 5. Application of 10 microM picrotoxin and/or 10 microM bicuculline, which block inhibition mediated by gamma-aminobutyric acid A (GABAA) receptors, did not reduce antidromically conditioned E-S potentiation. Thus, plasticity in GABAA-mediated inhibition cannot account for the increased population spike amplitude. 6. E-S potentiation did not increase the amplitude of either extracellular or intracellular EPSPs recorded at the cell body.

Different mechanisms may be required for maintenance of NMDA receptor-dependent and independent forms of long-term potentiation

In hippocampal area CA1, long-term potentiation (LTP) is induced by tetanic stimulation protocols that activate N-methyl-D-aspartate (NMDA) receptors. In addition, some stimulation protocols can induce LTP during NMDA receptor blockade. An initial signal in both NMDA receptor-dependent and independent LTPs is increased intracellular Ca2+ concentration in postsynaptic neurons. It therefore seems possible that subsequent steps leading to expression and maintenance of potentiation are shared whether or not LTP is induced through NMDA receptor activation. We tested this hypothesis by applying a broad spectrum protein kinase inhibitor, previously shown to inhibit NMDA receptor-dependent LTP. In agreement with earlier reports, we found that H-7 inhibited NMDA receptor-dependent LTP when applied either during tetanic stimulation, or beginning 30 min following tetanic stimulation. In contrast, NMDA receptor-independent LTP was not inhibited by H-7 applied during or following tetanic stimulation. We also tested for mutual occlusion between NMDA receptor-dependent and independent LTPs. Although induction of NMDA receptor-independent LTP did not occlude later induction of NMDA receptor-dependent LTP, induction of NMDA receptor-dependent LTP did occlude NMDA receptor-independent LTP. While the kinase inhibitor experiment showed a clear difference between NMDA receptor-dependent and independent LTPs, the occlusion experiments suggest an interaction between the signalling pathways for the two LTPs.

The origin of synaptic neuroplasticity: crucial molecules or a dynamical cascade?

What is neuroplasticity and what are its origins? These questions have been the subject of intense theoretical and experimental research in the neurosciences for decades. Basically, the term neuroplasticity refers to the ability of neurons to alter some functional property in response to alterations in input. Traditional definitions, however, are often imprecise and restricted to particular 'model' neural systems. In the present article we will consider several of the most widely studied models of synaptic-level neuroplasticity including alterations in response properties of two types of invertebrate sensory neurons, long-term potentiation (LTP) in mammalian hippocampus and cortex, and ocular dominance shifts in cat visual cortex. While many other forms of neuroplasticity have been studied, these examples typify the diversity of the subject, as well as illustrate our contention that no unitary model of the phenomena is possible for all conditions. This last point is of particular importance for the mammalian literature, since many hypotheses concerning the mechanism(s) underlying the initiation of neuroplasticity have proposed a single crucial molecular element as the primary causal agent. A closer examination of these various hypotheses, in concert to several examples from the invertebrate literature, leads, however, to the conclusion that synaptic neuroplasticity must arise from a series of inter-related molecular events of a particular form, a cascade, in which individual elements may differ radically from system to system. We next provide an overview of our studies of age-dependent regulation of excitatory and inhibitory ionotropic neurotransmitter receptor populations in cortex in response to agonist and depolarizing stimulation. We provide evidence that such regulation for ionotropic receptors is under the control of ionically driven receptor kinase and phosphatase activity which is also age-dependent in function. These data provide the basis for a cascade model of receptor regulation. Based on this qualitative model, we describe a quantitative computer simulation of certain age-dependent stages in the receptor regulatory cascade which may interact to produce LTP-like effects. While such a model is not exclusive, it nevertheless provides a demonstration that elements in the proposed cascade may comprise the necessary and sufficient conditions for some forms of neuroplasticity. We also propose some of the principles underlying our model as a means of unifying much of the diverse phenomenology reported in the literature. Finally, we make a series of explicit predictions which are testable with current experimental techniques.(ABSTRACT TRUNCATED AT 250 WORDS)

Block of induction and maintenance of calcium-induced LTP by inhibition of protein kinase C in postsynaptic neuron in hippocampal CA1 region

Calcium-induced LTP (Ca-LTP) refers to the long-lasting potentiation of synaptic transmission in hippocampal synapses that can be induced by a transient (7-10 min) and small increase (from 2 mM to 4 mM) in extracellular calcium concentration. In this paper the effects of protein kinase C (PKC) inhibitors, polymyxin B (PMB) and PKC(19-31), given intracellularly to the postsynaptic neuron in the CA1 region, on the induction and maintenance of Ca-LTP were studied and compared with those found in a similar study earlier made in this laboratory on tetanic stimulation-induced LTP (TS-LTP) [18]. When the intracellular delivery of the inhibitor(s) was made to begin 30 min before the exposure to increased [Ca2+]0, the development of Ca-LTP was completely blocked, leaving only a brief enhancement of EPSPs directly attributable to the brief increase in [Ca2+]0. When the intracellular delivery of either PKC(19-31) alone or PMB+PKC(19-31) was made to begin after the full establishment of Ca-LTP, it soon made the maintained potentiation begin to decline, the EPSP amplitude gradually returning to the control value before the exposure to increased calcium. Thus postsynaptic PKC inhibition blocks both the induction and the maintenance of Ca-LTP, just as it has been shown to do to TS-LTP. But quantitative differences exist: both the induction and the maintenance of Ca-LTP appeared to be more susceptible to block by postsynaptic PKC inhibition than those of TS-LTP.

High concentrations of glycine induce long-lasting changes in synaptic efficacy in rat hippocampal slices

Brief perfusion of adult rat hippocampal slices with high concentrations of glycine results in a slowly developing, long-lasting increase in synaptic responses in field CA1. Two observations indicated that the effect requires the activation of NMDA receptors by glycine. First, the glycine-induced potentiation is reduced by ketamine, an NMDA receptor channel blocker. Second, glycine potentiates the NMDA receptor-mediated epileptiform activity recorded in the presence of low magnesium concentration and picrotoxin. In slices prepared from rat pups (5-8 postnatal day), perfusion with glycine results in a slowly developing, long-lasting depression of EPSP amplitude. These results provide a new way of producing potentiation of synaptic efficacy and suggest new properties of NMDA receptors.

A synaptic model of memory: long-term potentiation in the hippocampus

Long-term potentiation of synaptic transmission in the hippocampus is the primary experimental model for investigating the synaptic basis of learning and memory in vertebrates. The best understood form of long-term potentiation is induced by the activation of the N-methyl-D-aspartate receptor complex. This subtype of glutamate receptor endows long-term potentiation with Hebbian characteristics, and allows electrical events at the postsynaptic membrane to be transduced into chemical signals which, in turn, are thought to activate both pre- and postsynaptic mechanisms to generate a persistent increase in synaptic strength.

Biochemical correlates of long-term potentiation in hippocampal synapses

Figure 2 summarizes biochemical events which are currently known or hypothesized to participate in LTP induction/maintenance. Current evidence strongly suggests that postsynaptic Ca2+, both entered from the outside of cells and released from intracellular stores, is the initial key substance for the induction of LTP. A rise of [Ca2+]i triggers a variety of enzymatic reactions and initiates the enhancement of synaptic transmission. This first step may be achieved by direct/indirect phosphorylations of protein molecules in postsynaptic receptors/ion channels. This would result in an increase in receptor sensitivity. An immediate increase in the number of available postsynaptic receptors by modifications of spine morphology is another candidate. Such modifications may be accomplished by cytoskeleton rearrangements or changes in extracellular environments. A change in spine structure may also cause an increase in spine neck conductance. Although it is unknown to what extent the increase in [Ca2+]i affects cellular chemistry, Ca2+ probably also directly/indirectly stimulates cascades which exert effects more slowly. A delayed increase in metabotropic receptor sensitivity may occur. New synthesis of protein molecules may be involved in late periods of LTP by replacing turnovered molecules and/or by supplying new materials. Some of these chains of biochemical events may also apply to presynaptic terminals, although the existence of retrograde messenger substances must still be confirmed. In addition, interactions between different protein kinases and second messengers appear to occur to bring about final effects.

Absence of calcium-induced LTP-like response in the dentate area of seizure-prone gerbils and its relation to parvalbumin in the entorhinal perforant path synapse of this species

Mongolian gerbils (Meriones unguiculatus) are genetically predisposed to seizures, for which an involvement of hippocampal hyperexcitability and disinhibition has been suggested. The response in vitro of the hippocampal synaptic circuit upon exposure to an elevated extracellular calcium concentration is well known in the rat, and its dependence on inhibitory and excitatory transmission has been thoroughly studied. The purpose of the present investigation was to compare the influence of elevated extracellular calcium on inhibitory and excitatory transmission in the dentate area and the CA1 field of gerbil and rat hippocampal slices. Elevated calcium induced in the CA1 area of both animal species a long-term potentiation (LTP)-like response. Upon calcium exposure in the dentate area a decrease in population spike amplitude occurred in both gerbil and rat slices, indicating a similar degree of synaptic inhibition in the two species. However, in contrast to the effects known in the rat, elevated extracellular calcium failed to enhance the excitatory postsynaptic potential in the gerbil dentate area. This difference may depend on the species-specific, selective presence of the calcium-binding protein parvalbumin in perforant path terminals of the gerbil, which may be relevant to the susceptibility to seizures of this animal species.

Adenosine-induced suppression of synaptic responses and the initiation and expression of long-term potentiation in the CA1 region of the hippocampus

The results of several previous studies have suggested that pretreatment with adenosine can block the induction of long-term potentiation (LTP), although other studies have found no effect of adenosine on the induction of LTP. The interaction of adenosine with the induction of LTP in the rat hippocampal slice was investigated. Inhibition of synaptic responses by adenosine either prior to or immediately after high-frequency or theta-burst stimulation did not affect LTP measured after washout of the adenosine. The only conditions under which adenosine blocked the development of LTP was when it was given 3-5 minutes prior to the stimulation train. To understand how it was possible to induce LTP, during the period 1-3 minutes following adenosine when synaptic responses were virtually eliminated, evoked responses during the 100 Hz stimulation train were recorded. Although synaptic responses to low-frequency stimulation were virtually eliminated by adenosine, they reappeared during high-frequency stimulation. These results suggest that although adenosine can depress synaptic responses, an increase in neurotransmission during a high-frequency train can partially overcome this effect of adenosine, and the hypothesis that adenosine can selectively block LTP is not supported.

Long-term potentiation as synaptic dialogue

We have proposed a testable model of the physiological and biochemical events underlying LTP that offers the following novel features. (1) The focus is not on a single mechanism or synaptic site, but rather on the integration and interaction of mechanisms occurring on both sides of the synapse, (2) beta PKC plays a critical presynaptic role in LTP, while gamma PKC functions postsynaptically. (3) These stages can be ordered in a time-delimited sequence of post- then presynaptic molecular events based on the period of effectiveness of inhibitor compounds. (4) The distinction is made between the time when kinase activation occurs and the time when the potentiated response requiring this kinase activation is observed.

A model of NMDA receptor-mediated activity in dendrites of hippocampal CA1 pyramidal neurons

1. The role of synaptic activation of NMDA (N-methyl-D-aspartate) receptor-mediated conductances on CA1 hippocampal pyramidal cells in short-term excitability changes was studied with the use of a computational model. Model parameters were based on experimental recordings from dendrites and somata and previous hippocampal simulations. Representation of CA1 neurons included NMDA and non-NMDA excitatory dendritic synapses, dendritic and somatic inhibition, five intrinsic membrane conductances, and provision for activity-dependent intracellular and extracellular ion concentration changes. 2. The model simulated somatic and dendritic potentials recorded experimentally. The characteristic CA1 spike afterdepolarization was a consequence of the longitudinal spread of dendritic charge, reactivation of slow Ca(2+)-dependent K+ conductances, slow synaptic processes (NMDA-dependent depolarizing and gamma-aminobutyric acid-mediated hyperpolarizing currents) and was sensitive to extracellular potassium accumulation. Calcium currents were found to be less important in generating the spike afterdepolarization. 3. Repetitive activity was influenced by the cumulative activation of the NMDA-mediated synaptic conductances, the frequency-dependent depression of inhibitory synaptic responses, and a shift in the potassium reversal potential. NMDA receptor activation produced a transient potentiation of the excitatory postsynaptic potential (EPSP). The frequency dependence of EPSP potentiation was similar to the experimental data, reaching a maximal value near 10 Hz. 4. Although the present model did not have compartments for dendritic spines, Ca2+ accumulation was simulated in a restricted space near the intracellular surface of the dendritic membrane. The simulations demonstrated that the Ca2+ component of the NMDA-operated synaptic current can be a significant factor in increasing the Ca2+ concentration at submembrane regions, even in the absence of Ca2+ spikes. 5. Elevation of the extracellular K+ concentration enhanced the dendritic synaptic response during repetitive activity and led to an increase in intracellular Ca2+ levels. This increase in dendritic excitability was partly mediated by NMDA receptor-mediated conductances. 6. Blockade of Ca(2+)-sensitive K+ conductances in the dendrites increased the size of EPSPs leading to a facilitation of dendritic and somatic spike activity and increased [Ca2+]i. NMDA receptor-mediated conductances appeared as an amplifying component in this mechanism, activated by the relatively depolarized membrane potential. 7. The results suggest that dendritic NMDA receptors, by virtue of their voltage-dependency, can interact with a number of voltage-sensitive conductances to increase the dendritic excitatory response during periods of repetitive synaptic activation. These findings support experimental results that implicate NMDA receptor-mediated conductances in the short-term response plasticity of the CA1 hippocampal pyramidal neuron.

Potassium-induced long-term potentiation in rat hippocampal slices

We observed that a transient increase in extracellular potassium concentration (50 mM for 40 s) was sufficient to induce long-term potentiation (LTP) of synaptic transmission in area CA1 of the hippocampal slice. Potassium-induced potentiation of the Schaffer collateral/commissural synapses demonstrated several features characteristic of tetanus-induced LTP: (1) population excitatory post-synaptic potential (EPSP) amplitudes were enhanced to a similar magnitude (on average 70% above baseline) which (2) lasted for more than 20 min; (3) induction was blocked by bath application of the specific N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonovalerate (D-APV), and (4) was attenuated by reduction of the concentration of calcium in the extracellular medium. Induction of either potassium-induced LTP or tetanus-induced LTP occluded the subsequent expression of the other. Finally, exposure to high potassium in the absence of electrical stimulation was sufficient to induce LTP. Taken together, these data indicate that brief depolarizing stimuli other than tetanus can induce LTP. Because potassium-induced LTP is not restricted to the subset of afferents examined electrophysiologically, such a method could facilitate analyses of the biochemical events underlying both the induction and expression of LTP.

Thapsigargin blocks the induction of long-term potentiation in rat hippocampal slices

Experiments were performed to investigate whether intact intracellular Ca2+ pools are necessary for long-term potentiation (LTP) in the CA1 region of rat hippocampal slices. Thapsigargin (1 microM), which depletes most intracellular Ca2+ pools by blocking ATP-dependent Ca2+ uptake into intracellular compartments, blocked the induction but not the expression of LTP. Thapsigargin had no effect on synaptic transmission or on responses mediated by N-methyl-D-aspartate (NMDA) receptor activation. These data suggest that Ca2+ release from intracellular stores is required for the induction of LTP.

The effects of external calcium on the N-methyl-D-aspartate induced short-term potentiation in the rat hippocampal slice

The effect of altered extracellular Ca concentration was studied on the N-methyl-D-aspartate (NMDA) induced short-term potentiation (STP) of the population excitatory post-synaptic potential recorded from the stratum radiatum of CA1 of the rat hippocampal slice. Perfusion of 130 microM of NMDA for 10 s in control media containing 2.0 mM extracellular Ca evoked an STP with a maximum amplitude of 46% and a duration of 16 min. Perfusion of media containing a reduced Ca concentration of 0.8 mM or 1.0 mM did not alter the amplitude or time course of the STP. However, raising the Ca concentration to 3.0 mM or 4.0 mM caused a significant reduction in the amplitude of the STP to 23% and 2% respectively. The abolition of the NMDA induced STP in 4 mM Ca could not have been produced by response saturation since an identical long-term potentiation (LTP) was produced in this high Ca media as in the control media. These studies show that the NMDA induced STP has a very different Ca dependency to LTP.

Activation of multifunctional Ca2+/calmodulin-dependent kinase in intact hippocampal slices

In vitro phosphorylation of multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase) converts it to a form that is independent of Ca2+. We demonstrate that significant Ca(2+)-independent CaM kinase activity is present in untreated hippocampal slices. Two manipulations that produce a long-lasting enhancement of neuronal activity in hippocampal slices, elevated extracellular Ca2+ or depolarization with high K+, generate additional Ca(2+)-independent activity. This increase is dependent on extracellular Ca2+ and is correlated with an increased phosphorylation of CaM kinase. In contrast, CaM kinase in posterior pituitary, a brain structure that is not thought to be involved in memory-related processes, is not modulated by depolarization. These results suggest that the Ca(2+)-independent form of CaM kinase may modulate neuronal activity in the hippocampus.

Developmental changes of calcium currents in the visual cortex of the cat

During a critical period of postnatal development the visual cortex of kittens is susceptible to experience-dependent modifications of neuronal response properties. Evidence is accumulating that these modifications are triggered by a transient neuronal calcium (Ca) influx. To further investigate this issue we measured extracellular Ca concentrations with ion-sensitive microelectrodes and compared the magnitude and the distribution of stimulus-evoked Ca fluxes in slices of the visual cortex of 4- to 5-week-old kittens and of 6-month-old adult cats. Stimulation of the white matter at 15 Hz for 8 s caused transient decreases of the extracellular Ca concentration (delta Cao) in slices of both age groups and in all cortical layers. However, there were developmental changes in the laminar distribution of the delta Cao: in kittens, they were maximal in layer IV whereas in adult cats they were most pronounced in the supragranular layers. The ratios between the amplitudes of delta Cao in layer IV and the supragranular layers were 1.65 +/- 0.26 in kittens and 0.43 +/- 0.2 in adult cats. These changes in laminar distribution resemble the laminar specific decay of neuronal malleability and parallel the developmental redistribution of 1,4-Dihydropyridine-sensitive Ca channels. Because of these correlations we interpret our findings as support for the hypothesis that experience-dependent modifications are triggered by Ca influx.

Calcimycin potentiates responses of rat hippocampal neurons to N-methyl-D-aspartate

We examined the effect of elevating intracellular calcium ([Ca2+]i) on responses to iontophoretically applied N-methyl-D-aspartate (NMDA), and quisqualate in CA1 neurons of the hippocampal slice. Topical application of calcimycin (A23187), a calcium ionophore, potentiated responses to NMDA but not to quisqualate. This potentiation was prevented by loading cells with the calcium chelator, BAPTA, suggesting that the action of calcimycin on NMDA receptors was mediated by an elevation of [Ca2+]i in the recorded cell. The potentiation was also recorded in voltage-clamped and in cesium-loaded cells, suggesting that it was not mediated by non-specific changes in voltage or input resistance of the cell that may have resulted from the rise in [Ca2+]i. We propose that intracellular calcium plays a crucial role in regulating the activity of the NMDA subtype of L-glutamate receptor.

Aminoglycoside antibiotics affect hippocampal LTP: a comparative study with the N-type calcium antagonist omega-conotoxin-GVIA

The in vitro activity of N-type calcium antagonists such as omega-conotoxin-GVIA and the aminoglycoside antibiotics neomycin and streptomycin was studied in rat hippocampal slices. The effects of the drugs were tested on basal CA1 synaptic transmission and on the hippocampal long-term potentiation (LTP) induced by tetanic electrical stimulation and by increasing (4mM) the calcium concentration. Omega-conotoxin-GVIA, neomycin and streptomycin were able to significantly reduce the amplitude of the CA1 population spike at 1 microM, 0.5 mM and 1 mM, respectively. In addition, the drugs affected the induction and maintenance of the CA1 tetanic and calcium-induced LTP at concentrations which did not modify the magnitude of the control CA1 population spike. Omega-conotoxin-GVIA (0.5 microM), neomycin (0.3 mM) and streptomycin (0.7 mM) perfused for 60 min, before inducing LTP, prevented the subsequent increase of the CA1 population spike in all the experiments. The same concentrations of these drugs perfused for 60-min after a previously established LTP significantly reduced the amplitude of the CA1 population spike. The results promote a role for the N-type calcium channels and for the release of neurotransmitters in both the induction and the maintenance of hippocampal LTP.