Established Long-term Potentiation Research Articles

Enhanced Activities of δ Subunit-containing GABAA Receptors Blocked Spinal Long-term Potentiation and Attenuated Formalin-induced Spontaneous Pain

The δ subunit-containing γ-Aminobutyric acid type A receptors (δ-GABAARs) are located at extrasynaptic sites and persistently active in the control of neuronal excitability. Here we recorded primary afferent C fiber-evoked field potentials in the superficial dorsal horn of rat spinal cords in vivo and investigated the possible influence of δ-GABAARs activities on nociceptive synaptic transmission. We found that δ-GABAARs-preferring agonist 4,5,6,7-tetrahydroisoxazolol [4,5-c] pyridine-3-ol (THIP), when topically applied onto spinal cord dorsum, inhibited the basal synaptic responses in a dose-dependent manner. Low-frequency stimulation (LFS) of sciatic nerves elicited long-term potentiation (LTP) of C fiber transmission, a synaptic correlate of central sensitization. Pretreatment with THIP before LFS delivery blocked the induction of LTP. When applied at 30 min and 180 min post-LFS, THIP reduced the magnitudes of established LTP. Intraplantar injection of formalin naturally evoked LTP in anesthetized rats. Spinal administration of THIP not only reversed formalin-induced LTP, but alleviated the spontaneous painful behaviors and mechanical hyperalgesia. Biochemical analysis demonstrated that δ-GABAARs activation by THIP decreased the synaptic expression and phosphorylation of AMPA receptor GluA1 subunit in formalin-injected rats, and meanwhile, increased synaptic GluA2 content, allowing the switch of GluA2-lacking AMPA receptors to GluA2-containing ones at synapses. THIP also suppressed the synaptic accumulation and phosphorylation of NMDA receptor GluN1 subunit in formalin-injected rats. Our data suggested that enhanced δ-GABAARs activities blunted the initiation and maintenance of spinal LTP, which correlated with the amelioration of central sensitization of nociceptive behaviors.

Status Epilepticus Enhances Depotentiation after Fully Established LTP in an NMDAR-Dependent but GluN2B-Independent Manner.

N-Methyl-D-aspartate (NMDA) receptor-dependent long-term potentiation (LTP) can be reversed by low-frequency stimulation (LFS) referred to as depotentiation (DP). We previously found GluN2B upregulated in CA1 neurons from post-status epilepticus (post-SE) tissue associated with an enhanced LTP. Here, we tested whether LFS-induced DP is also altered in pathological GluN2B upregulation. Although LTP was enhanced in post-SE tissue, LTP was significantly reversed in this tissue, but not in controls. We next tested the effect of the GluN2B subunit-specific blocker Ro 25-6981 (1 μM) on LFS-DP. As expected, LFS had no effect on synaptic strength in the presence of the GluN2B blocker in control tissue. In marked contrast, LFS-DP was also attained in post-SE tissue indicating that GluN2B was obviously not involved in depotentiation. To test for NMDA receptor-dependence, we applied the NMDA receptor antagonist D-AP5 (50 μM) prior to LFS and observed that DP was abolished in both control and post-SE tissue confirming NMDA receptor involvement. These results indicate that control Schaffer collateral-CA1 synapses cannot be depotentiated after fully established LTP, but LFS was able to reverse LTP significantly in post-SE tissue. However, while LFS-DP clearly required NMDA receptor activation, GluN2B-containing NMDA receptors were not involved in this form of depotentiation.

PKM-ζ is not required for hippocampal synaptic plasticity, learning and memory.

Long-term potentiation (LTP), a well-characterized form of synaptic plasticity, has long been postulated as a cellular correlate of learning and memory. Although LTP can persist for long periods of time, the mechanisms underlying LTP maintenance, in the midst of ongoing protein turnover and synaptic activity, remain elusive. Sustained activation of the brain-specific protein kinase C (PKC) isoform protein kinase M-ζ (PKM-ζ) has been reported to be necessary for both LTP maintenance and long-term memory. Inhibiting PKM-ζ activity using a synthetic zeta inhibitory peptide (ZIP) based on the PKC-ζ pseudosubstrate sequence reverses established LTP in vitro and in vivo. More notably, infusion of ZIP eliminates memories for a growing list of experience-dependent behaviours, including active place avoidance, conditioned taste aversion, fear conditioning and spatial learning. However, most of the evidence supporting a role for PKM-ζ in LTP and memory relies heavily on pharmacological inhibition of PKM-ζ by ZIP. To further investigate the involvement of PKM-ζ in the maintenance of LTP and memory, we generated transgenic mice lacking PKC-ζ and PKM-ζ. We find that both conventional and conditional PKC-ζ/PKM-ζ knockout mice show normal synaptic transmission and LTP at Schaffer collateral-CA1 synapses, and have no deficits in several hippocampal-dependent learning and memory tasks. Notably, ZIP still reverses LTP in PKC-ζ/PKM-ζ knockout mice, indicating that the effects of ZIP are independent of PKM-ζ.

Long-Term Potentiation in Spinal Nociceptive Pathways as a Novel Target for Pain Therapy

Long-term potentiation (LTP) in nociceptive spinal pathways shares several features with hyperalgesia and has been proposed to be a cellular mechanism of pain amplification in acute and chronic pain states. Spinal LTP is typically induced by noxious input and has therefore been hypothesized to contribute to acute postoperative pain and to forms of chronic pain that develop from an initial painful event, peripheral inflammation or neuropathy. Under this assumption, preventing LTP induction may help to prevent the development of exaggerated postoperative pain and reversing established LTP may help to treat patients who have an LTP component to their chronic pain. Spinal LTP is also induced by abrupt opioid withdrawal, making it a possible mechanism of some forms of opioid-induced hyperalgesia. Here, we give an overview of targets for preventing LTP induction and modifying established LTP as identified in animal studies. We discuss which of the various symptoms of human experimental and clinical pain may be manifestations of spinal LTP, review the pharmacology of these possible human LTP manifestations and compare it to the pharmacology of spinal LTP in rodents.

Different roles of two nitric oxide activated pathways in spinal long-term potentiation of C-fiber-evoked field potentials

There is accumulating evidence implicating the involvement of nitric oxide (NO) in spinal central sensitization. The long-term potentiation (LTP) of spinal C-fiber-evoked field potentials is considered as a fundamental mechanism of sensitization of nociceptive neurons in the spinal cord. The present study examined the roles of soluble guanylate cyclase (sGC) or ADP-ribosyltransferase (ADPRT), two potential NO targets, in spinal LTP. The results showed that (1) administration of sGC inhibitors, methyl blue (MB, 4 mM, 20 μl) or 1 H-[1,2,4]oxadiazolo[4,3- a]-quiloxalin-1-one (ODQ, 10 μM, 20 μl) before tetanic stimulation, significantly inhibited the induction of spinal LTP, and this was reversed by 8-Br-cGMP, a membrane-permeable cGMP analog. However, the maintenance of spinal LTP was not changed when application of ODQ 2 h after tetanic stimulation. (2) Although our previous experiments have identified a key role for NO in the induction of spinal LTP, NO synthase (NOS) inhibitor, L-NAME (1 mM, 20 μl) or hemoglobin (2 mg/ml, 20 μl), a scavenger of NO, had no effect on established spinal LTP when applied 2 h after the induction of spinal LTP. (3) The mono-ADPRT inhibitor, nicotinamide (10 mM, 20 μl), had no effect on the induction and maintenance of spinal LTP. However, the poly-ADPRT inhibitor, benzamide (100 μM, 20 μl), inhibited its maintenance, but not its induction. The results suggest that NO-stimulated guanylyl cyclase activity plays a critical role in the induction of LTP of C-fiber-evoked field potentials in the spinal cord, whereas NO-related poly-ADPRT activity contributes to the maintenance of spinal LTP.

Measurement of presynaptic zinc changes in hippocampal mossy fibers

The hippocampal mossy fiber terminals of CA3 area contain high levels of vesicular zinc that is released in a calcium-dependent way, following high-frequency stimulation. However the properties of zinc release during normal synaptic transmission, paired-pulse facilitation and mossy fiber long-term potentiation are still unknown. Using the fluorescent zinc probe N-(6-methoxy-8-quinolyl)- para-toluenesulfonamide, we measured fast mossy fiber zinc changes indicating that zinc is released following single and low levels of electrical stimulation. The observed presynaptic zinc signals are maintained during the expression of mossy fiber long-term potentiation, assumed to be mediated by an increase in transmitter release, and are enhanced during paired-pulse facilitation. This zinc enhancement is, like paired-pulse facilitation, reduced during established long-term potentiation. The correlation between the paired-pulse evoked zinc and field potential responses supports the idea that zinc is co-released with glutamate.

The inhibition of long-term potentiation in the rat dentate gyrus by pro-inflammatory cytokines is attenuated in the presence of nicotine

Nicotine has previously been shown to affect both long-term potentiation (LTP) and long-term depression and to reverse age-related impairments of LTP in the hippocampus. Levels of proinflammatory cytokines are known to be elevated with age and to inhibit LTP. In the present study we have investigated the effects of three pro-inflammatory cytokines on nicotine-enhanced LTP in the rat hippocampus in vitro. In the presence of nicotine the inhibitory effect of interleukin-1β, interleukin-18 and tumour necrosis factor-α on LTP was eliminated. Furthermore, significant depotentiation of established LTP could not be obtained in slices treated with nicotine. These experiments demonstrate that nicotine can reverse the inhibitory effects of pro-inflammatory cytokines on LTP.

Phosphatidylinositol 3-kinase is required for the expression but not for the induction or the maintenance of long-term potentiation in the hippocampal CA1 region.

Several signal transduction pathways have been implicated in the induction of long-term potentiation (LTP), yet the signal transduction mechanisms behind the maintenance-expression phase of LTP are still poorly understood. We investigated the role of phosphatidylinositol 3-kinase (PI3-kinase) in LTP at Schaffer collateral/commissural fiber-CA1 synapses in rat hippocampal slices using biochemical approaches and extracellular electrophysiological recordings. We observed that PI3-kinase activity was induced in the CA1 region during LTP of field EPSPs (fEPSPs) and that two structurally unrelated PI3-kinase inhibitors, LY294002 and wortmannin, abated established LTP, suggesting that PI3-kinase is involved in the maintenance-expression phase of LTP. However, LTP recovered after washout of the reversible PI3-kinase inhibitor LY294002, confirming that LTP maintenance and expression are distinct events and indicating that PI3-kinase activity is required for LTP expression rather than for its maintenance. Interestingly, preincubation with LY294002 did not prevent LTP induction. In fact, if LY294002 was withdrawn 5 min after high-frequency stimulation, an LTP of fEPSP was seen. Last, a voltage-dependent calcium channel-dependent form of LTP in the CA1 could also be reversibly abated by LY294002, raising the possibility that PI3-kinase could be required for the expression of multiple forms of synaptic potentiation.

Phosphatidylinositol 3-Kinase Is Required for the Expression But Not for the Induction or the Maintenance of Long-Term Potentiation in the Hippocampal CA1 Region

Several signal transduction pathways have been implicated in the induction of long-term potentiation (LTP), yet the signal transduction mechanisms behind the maintenance-expression phase of LTP are still poorly understood. We investigated the role of phosphatidylinositol 3-kinase (PI3-kinase) in LTP at Schaffer collateral/commissural fiber-CA1 synapses in rat hippocampal slices using biochemical approaches and extracellular electrophysiological recordings. We observed that PI3-kinase activity was induced in the CA1 region during LTP of field EPSPs (fEPSPs) and that two structurally unrelated PI3-kinase inhibitors, LY294002 and wortmannin, abated established LTP, suggesting that PI3-kinase is involved in the maintenance-expression phase of LTP. However, LTP recovered after washout of the reversible PI3-kinase inhibitor LY294002, confirming that LTP maintenance and expression are distinct events and indicating that PI3-kinase activity is required for LTP expression rather than for its maintenance. Interestingly, preincubation with LY294002 did not prevent LTP induction. In fact, if LY294002 was withdrawn 5 min after high-frequency stimulation, an LTP of fEPSP was seen. Last, a voltage-dependent calcium channel-dependent form of LTP in the CA1 could also be reversibly abated by LY294002, raising the possibility that PI3-kinase could be required for the expression of multiple forms of synaptic potentiation.

Mediation of hippocampal mossy fiber long-term potentiation by presynaptic Ih channels.

Hippocampal mossy fiber long-term potentiation (LTP) is expressed presynaptically, but the exact mechanisms remain unknown. Here, we demonstrate the involvement of the hyperpolarization-activated cation channel (Ih) in the expression of mossy fiber LTP. Established LTP was blocked and reversed by Ih channel antagonists. Whole-cell recording from granule cells revealed that repetitive stimulation causes a calcium- and Ih-dependent long-lasting depolarization mediated by protein kinase A. Depolarization at the terminals would be expected to enhance transmitter release, whereas somatic depolarization would enhance the responsiveness of granule cells to afferent input. Thus, Ih channels play an important role in the long-lasting control of transmitter release and neuronal excitability.

Time-dependent plasticity of synaptic transmission produced by long-term potentiation of C-fiber evoked field potentials in rat spinal dorsal horn

Our previous work has shown that repetitive stimulation of Aδ-fibers depresses long-term potentiation (LTP) of C-fiber evoked field potentials in spinal dorsal horn. Here, we tested the effects of the Aδ stimulation on the spinal LTP at different time points following LTP induction. Fifteen minutes after LTP induction Aδ stimulation depressed LTP by 44.1±4.2% (mean±SEM, n=7) for 69.3±18.5 min and 1 h after LTP the same Aδ stimulation depressed spinal LTP by only 16.9±3.1% for 21.9±2.0 min ( n=7). Three hours after LTP, however, the Aδ stimulation produced a further potentiation (31.9±6.3%, n=7) lasting for all the recording periods (1–3 h). These data indicate that the effects of repetitive stimulation of Aδ-fibers on established spinal LTP of C-fiber evoked field potentials is time-dependent.

Long-term depression of C-fibre-evoked spinal field potentials by stimulation of primary afferent A delta-fibres in the adult rat.

Long-term potentiation (LTP) of spinal C-fibre-evoked field potentials can be induced by brief electrical stimulation of afferent C-fibres, by natural noxious stimulation of skin or by acute nerve injury. Here, we report that in urethane anaesthetized, adult rats prolonged high frequency burst stimulation of the sciatic nerve at Adelta-fibre strength produced long-term depression (LTD) of C-fibre-evoked field potentials, and also depressed the increased amplitudes of C-fibre-evoked field potentials recorded after LTP had been established (depotentiation). Electrical stimulation of Abeta-fibres failed to induce LTD or depotentiation. In spinalized rats, prolonged Adelta-fibre conditioning stimulation induced LTP rather than LTD of C-fibre-evoked field potentials. Thus, tonic descending inhibition may determine the direction of plastic changes in C-fibre-mediated synaptic transmission. Spinal application of the N-methyl-D-aspartic acid receptor antagonist D-APV blocked induction of LTD in intact rats and LTP in spinalized rats. The presently described LTD and the depotentiation of established LTP of C-fibre-evoked field potentials in spinal dorsal horn may underlie some forms of prolonged analgesia induced by peripheral nerve stimulation procedures.

A Requirement for the Mitogen-activated Protein Kinase Cascade in Hippocampal Long Term Potentiation

The mitogen-activated protein kinase (MAPK) cascade has been intensely studied as a primary biochemical pathway through which a variety of extracellular stimuli initiate and regulate processes of cellular transformation. That MAPKs are abundantly expressed in postmitotic neurons, however, suggests different yet currently unknown functions for this cascade in the mature nervous system. Here we report that the MAPK cascade is required for hippocampal long term potentiation (LTP), a robust and widely studied form of synaptic plasticity. We observed that PD 098059, a selective inhibitor of the MAPK cascade, blocked MAPK activation in response to direct stimulation of the NMDA receptor as well as to LTP-inducing stimuli. Furthermore, inhibition of the MAPK cascade markedly attenuated the induction of LTP. PD 098059, however, had no effect on the expression of established LTP, and the MAPK cascade was not persistently activated during LTP expression. Our observations provide the first demonstration of a role for the MAPK cascade in the activity-dependent modification of synaptic connections between neurons in the adult mammalian nervous system.

Requirement of basolateral amygdala neuron activity for the induction of long-term potentiation in the dentate gyrus in vivo

We investigated the effect of injection of a local anesthetic, tetracaine, into the ipsilateral basolateral amygdala (BLA) on long-term potentiation (LTP) in the medial perforant path-dentate gyrus granule cell synapses of anesthetized rats. The dentate gyrus synaptic potential evoked by low frequency test stimulation did not change following tetracaine injection into BLA. However, when tetanic stimulation (30 pulses at 60 Hz) was applied 10 min after tetracaine injection, the magnitude of LTP was significantly attenuated in a dose-dependent manner. Injection of tetracaine after tetanic stimulation did not affect the established LTP. The small LTP observed in the BLA-lesioned rats was regarded as the BLA-independent component of LTP, which was not affected by tetracaine injection. These results suggest that neuron activities in the ipsilateral BLA are partly required for the induction of LTP in the dentate gyrus in vivo.

Chapter 5 The involvement of PKC and multifunctional CaM kinase II of the postsynaptic neuron in induction and maintenance of long-term potentiation

Publisher Summary This chapter discusses the role of postsynaptic PKC and CaMKII in long-term potentiation (LTP) in the rat hippocampal CA1 region. The chapter presents a research in which the effects of various inhibitors of PKC and CaMKII were studied, given intracellularly to the postsynaptic neuron, either singly or jointly, on the induction and maintenance of tetanus induced LTP. The study shows that the peptide pseudosubstrate inhibitors—PKC and CaMKII—can each block the induction of LTP and the maintenance of established LTP; and when given together, there is a strong synergism between the two, greatly accelerating the development of blocking effects. The induction of the usual type of LTP produced by brief high-frequency stimulation (HFS) in the CA1 region of the hippocampus takes place in the postsynaptic neuron triggered by calcium ion influx through the NMDA receptor channel. The biochemical processes in the postsynaptic neuron following calcium entry that underlie the development and maintenance of LTP have now logically become the focus of research on the cellular and molecular mechanisms of LTP.

Attenuated hippocampal long-term potentiation in basolateral amygdala-lesioned rats

Possible involvement of the amygdaloid input in long-term potentiation (LTP) in the medial perforant path-dentate gyrus granule cell synapses in vivo was investigated by evaluating the effects of lesions of the amygdaloid nucleus. The dentate gyrus synaptic potential evoked by low-frequency test stimulation did not change following lesions of the basolateral and central amygdala. However, when tetanic stimulation (30 pulses at 60 Hz) was applied 60 min after lesioning of the ipsilateral basolateral amygdala, the magnitude of LTP was significantly attenuated. Since lesions of the ipsilateral central amygdala and the contralateral basolateral amygdala did not affect the dentate gyrus LTP, the attenuation of the dentate gyrus LTP is a specific effect of acute lesions of the ipsilateral basolateral amygdala. The basolateral amygdaloid lesions significantly attenuated both LTP induced by weak (20 pulses at 60 Hz) and strong (100 pulses at 100 Hz) tetanus, indicating that the effect of the lesions does not depend on the strength of tetanus applied to induce LTP. When the ipsilateral basolateral amygdala was destroyed after application of tetanus, it did not affect the established LTP. The attenuation of LTP was also observed after the basolateral amygdala-lesioned rats were allowed a recovery period of 2 weeks. This is the first report providing evidence that the ipsilateral basolateral amygdala modulates hippocampal synaptic plasticity in vivo.

Involvement of Carbon Monoxide in Long-Term Potentiation in the Dentate Gyrus of Anesthetized Rats

To elucidate the role of carbon monoxide (CO) in hippocampal long-term potentiation (LTP), the effect of an inhibitor of heme oxygenase, which produces carbon monoxide, was investigated in the dentate gyrus of anesthetized rats. Administration of zinc protoporphyrin IX (100 nmol, i.c.v.) 30 min before the tetanic stimulation of 30 pulses at 60 Hz attenuated the intensity of LTP. However, this drug did not affect the established LTP. These results suggest that CO participates in the generation of LTP in vivo.

Modulation by metabotropic glutamate receptor of long-term potentiation of population spikes in the dentate gyrus in vivo

The effect of 2-amino-3-phosphonopropionate (AP3), a metabotropic glutamate receptor antagonist, on long-term potentiation (LTP) of evoked potentials in the dentate gyrus was investigated using anesthetized rats, Intracerebroventricular injection of AP3 (50 nmol) prior to application of tetanic stimulation did not significantly affect LTP induced by 100-pulse, 100-Hz tetanus, but inhibited the formation of LTP by weaker tetanus (30 pulses at 60 Hz). When AP3 was injected after tetanus, it did not significantly affect the established LTP. These results suggest that metabotropic glutamate receptors play a role in facilitation of the formation of LTP in the dentate gyrus in vivo.

The characteristics and pharmacology of olfactory cortical LTP induced by theta-burst high frequency stimulation and 1S,3R-ACPD

Long-term potentiation (LTP) of monosynaptic excitations in the olfactory cortex slice by theta burst high frequency stimulation (theta-HFS) and application of the metabotropic glutamate receptor-selective agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) has been studied. Theta-HFS optimally induced LTP when given 4 times at intervals of 20–30 min. The degree of LTP was significantly potentiated by the inclusion of picrotoxin in the perfusion medium but induction was prevented by d-2-amino-5-phosphonopentanoate (25 μM), l-2-amino-3-phosphonopropionate (125 μM), 5-(isoquinolinylsulphonyl)-2-methylpiperazine (50 μM), sangivamycin (25 μM) and thapsigargin (1 μM). Of the drugs tested, only d-2-amino-5-phosphonopentanoate failed to depotentiate established LTP. Application of 1S,3R-ACPD (100 μM) repeated 4 times at intervals of 20–30 min also optimally induced an LTP which was significantly less in unstimulated preparations and showed the same pattern of sensitivity to the drugs tested as LTP induced by theta-HFS. It is concluded that the induction of LTP by theta-HFS and 1S,3R-ACPD requires activation of both N- methyl- d-aspartate and metabotropic glutamate receptors and that a protein kinase is essential for the induction and maintenance of LTP. The likely mechanisms of induction and maintenance of olfactory cortical and hippocampal LTP are contrasted.

Tyrosine kinase inhibitors, herbimycin A and lavendustin A, block formation of long-term potentiation in the dentate gyrus in vivo

The effects of two specific tyrosine kinase inhibitors on long-term potentiation (LTP) in the dentate gyrus were investigated using anesthetized rats. I.c.v. injection of herbimycin A (1.74 nmol) or lavendustin A (1.31 nmol) before application of tetanic stimulation significantly inhibited the formation of LTP. However, these inhibitors, when administered after tetanic stimulation, did not affect the established LTP. These results suggest that protein tyrosine phosphorylation is involved in formation of LTP at the dentate gyrus in vivo.