Nitric oxide and carbon monoxide as possible retrograde messengers in hgippocampal long-term potentiation

We have been investigating the hypothesis that the membrane-permeant molecules nitric oxide (NO) and carbon monoxide(CO) may act as retrograde messengers during long-term potentiation (LTP). Inhibitors of either NO synthase or heme oxygenase, the enzyme that produces CO, blocked induction of LTP in the CA1 region of hippocampal slices. Brief application of either NO or CO to slices produced a rapid and long-lasting increase in the size of synaptic potentials if, and only if, the application occurred at the same time as weak tetanic stimulation of the presynaptic fibers. The long-term enhancement by NO or CO was spatially restricted to synapses from active presynaptic fibers and appeared to involve mechanisms utilized by LTP, occluding the subsequent induction of LTP by strong tetanic stimulation. The enhancement by No or CO was not blocked by the NMDA receptor blocker APV, suggesting that NO and CO act downstream for the NMDA receptor. In other systems, both NO and CO produce many of their effects by activation of soluble guanylyl cyclase nd cGMP-dependent protein kinase. An inhibitor of soluble guabylyl cyclase blocked the induction of normal LTP. Conversely, membrane-permeabel analog 8-Br-cGMP produced a rapid onset and long-lasting synaptic enhancement if, and only if, it was applied at the same time as weak presynaptic stimulation. Similarly, two inhibitors of cGMP-dependent protein kinase blocked the induction of normal LTP, and a selective activator of cGMP-dependent protein kinase produced activity-dependent long-lasting synaptic enhancement. 8-Br-cGMP also produced and activity-dependent, long-lasting increase in the amplitude of evoked synaptic current between pairs of hippocampal neurons in dissociated cell culture. In addition, 8-Br-cGMP, like NO, produced a long-lasting increase in the frequency of spontaneous miniature synaptic currents. These results are consistent with the hypothesis that NO and CO, either alone or in combination, serve as retrograde messengers that produce activity-dependent presynaptic enhancement, perhaps by stimulating soluble guanbylyl cyclase and cGMP-dependent protein kinase, during LTP in hippocampus. 1994 John Wiley & Sons, Inc.     

Dopamine D4 receptor activation restores CA1 LTP in hippocampal slices from aged mice

Normal aging is characterized with a decline in hippocampal memory functions that is associated with changes in long‐term potentiation (LTP) of the CA3‐to‐CA1 synapse. Age‐related deficit of the dopaminergic system may contribute to impairment of CA1 LTP. Here we assessed how the modulation of CA1 LTP by dopamine is affected by aging and how it is dependent on the Ca²⁺ source. In slices from adult mice, the initial slope of the field potential showed strong LTP, but in slices from aged mice LTP was impaired. Dopamine did not affect LTP in adult slices, but enhanced LTP in aged slices. The dopamine D1/D5 receptor (D1R/D5R) agonist SKF‐81297 did not affect LTP in adult but caused a relative small increase in LTP in aged slices; however, although there was no difference in dopamine D4 receptor (D4R) expression, the D4R agonist PD168077 increased LTP in aged slices to a magnitude similar to that in adult slices. The N‐Methyl‐D‐aspartate receptor antagonist D‐AP5 reduced LTP in adult slices, but not in aged slices. However, in the presence of D‐AP5, PD168077 completely blocked LTP in aged slices. The voltage‐dependent calcium channel (VDCC) blocker nifedipine reduced LTP in adult slices, but surprisingly enhanced LTP in aged slices. Furthermore, in the presence of nifedipine, PD168077 caused a strong enhancement of LTP in aged slices to a magnitude exceeding LTP in adult slices. Our results indicate that the full rescue of impaired LTP in aging by the selective D4R activation and that a large potentiation role on LTP by co‐application of D4R agonist and VDCC blocker may provide novel strategies for the intervention of cognitive decline of aging and age‐related diseases.     

Time course of changes in long-term potentiation of synaptic transmission following subcortical deafferentation on the rat hippocampus.

Brief tetanic stimulation potentiates synaptic transmission both in the CA1 and dentate area of slices cut from normal rats. This long-term potentiation (LTP) was assayed in slices made at various times from rats subjected to complete bilateral sectioning of all subcortical afferents which enter the hippocampus. Over about one week survival time, LTP is present in the CA1 region of all and also in the fascia dentata of about 50% of slices. We found no signs of LTP in the dentate area of slices cut over 8 weeks after deafferentation, while the responses were clearly potentiated in the CA1 area of the same slices. Four week was the longest period when a somewhat modified version of LTP could be produced in the subcortically deafferented dentate area. The results confirm previous reports that subcortical afferents mediate some unknown factors essential for maintenance of long-term plasticity of intrinsic synapses in the fascia dentata. This unidentified, perhaps trophic influence diminishes in about 4 weeks after severing the subcortical fibers. In contrast, maintenance of subcortical inputs are apparently not required for the LTP in the intrinsic CA1 synapses.     

Effects of Chronic Stress and Phenytoin on the Long-term Potentiation （LTP） in Rat Hippocampal CA1 Region

Stress is the response to stimulation from inside andoutside with complicated effects on organisms. Appropri-ate stressful reactions are helpful in resisting diseases byactivating unspecific modulation system, while severe orprolonged stresses are harmful and even induce mentaland physical disorders such as recurrent depression, post-traumatic stress disorder (PTSD), Alzheimer’s disease andepilepsy [1]. Hippocampus, a main brain region of keyimportance for learning, memory and emotion, is t…     

Increased expression of phospho-cofilin in CA1 and subiculum areas after theta-burst stimulation of Schaffer collateral-commissural fibers in rat hippocampal slices

Activity-dependent structural plasticity of dendritic spines of pyramidal neurons in the central neuron system has been proposed to be a cellular basis of learning and memory. Long-term potentiation (LTP) is accompanied by changes in synaptic morphology and structural remodeling of dendritic spines. However, there is considerable uncertainty as to the nature of the adjustment. The present study tested whether immunoreactive phospho-cofilin, an index of altered actin filament assembly, could be increased by theta-burst stimulations (TBS), which is an effective stimulation pattern for inducing LTP in the hippocampus. The slope of fEPSPs evoked by TBS to Schaffer collateral-commissural fibers in hippocampal slices was measured, and p-cofilin expression was examined using immunofluorescence techniques. Results indicated that saturated L-LTP was produced by multiple TBS episodes to Schaffer collateral-commissural fibers in the hippocampal CA1 area, and TBSs also increased immunoreactive p-cofilin expression in the stratum radiatum of the hippocampal CA1 area and pyramidal layer of the subiculum. D-2-amino-5-phosphonovalerate (D-APV) prevented LTP and expression of p-cofilin immunoreactive induced by multiple TBS episodes in the stratum radiatum of the hippocampal CA1 area. Two paired-pulse low-frequency stimulation (PP-LFS) episodes to Schaffer collateral-commissural fibers induced long-term depression (LTD), and did not affect p-cofilin expression in the stratum radiatum of the hippocampal CA1 area. These results suggest that LTP induction is associated with altered actin filament assembly. Moreover, the CA1 and subiculum areas of the hippocampal formation possibly cooperate with each other in important physiological functions, such as learning and memory, or in pathological diseases, such as epilepsy.     

Antiserum Against S-100 Protein Prevents Long Term Potentiation Through a cAMP-Related Mechanism

Long term potentiation (LTP) was induced in the CA1 region of rat hippocampal slices by tetanization of the Schaffer collaterals. Local pretreatment of CA1 with serum of rabbits immunized against S-100 prevented the potentiation. However, treatment of the slices with a membrane permeant cAMP analogue, such as 8-Br-cAMP, could protect against the blocking effect of anti S-100 serum. We suggest that in the rat endogenous S-100_b is involved in transduction mechanisms during LTP induction, via its ability to stimulate adenylate cyclase. Possible mechanisms of this action are discussed.     

Effects of the mono- and tetrasialogangliosides GM1 and GQ1b on ATP-induced long-term potentiation in hippocampal CA1 neurons

The effects of the mono- and tetrasialogangliosides, GM1 and GQ1b, on ATP-induced long-term potentiation (LTP) were studied in CA1 neurons of guinea pig hippocampal slices. Application of 5 or 10 microM ATP for 10 min resulted in a transient depression followed by a slow augmentation of synaptic transmission, leading to LTP. LTP induced by treatment with 5 microM ATP was facilitated in hippocampal slices prepared from animals treated for 6 days with a ceramide analog, L-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propranol, which stimulates ganglioside biosynthesis. In addition, LTP induced by 5 microM ATP was significantly enhanced when naive slices were incubated with GQ1b but not with GM1. These results suggest that a cooperative effect between extracellular ATP and GQ1b enhances ATP-induced LTP in hippocampal CA1 neurons. In addition, the LTP induced by 10 microM ATP was blocked by coapplication of the NMDA antagonist AP5 (5 microM or 50 microM), and this effect was partially inhibited by GQ1b pretreatment of the slices, suggesting that in hippocampal CA1 neurons, the enhancing effect of GQ1b on ATP-induced LTP is mediated by modulation of NMDA receptors/Ca(2+) channels.     

Low intensity tetanic stimulation induces long-term potentiation in hippocampal neurons

A minimal intensity of the stimulation necessary for the induction of long-term potentiation of synaptic transmission (LTP) was investigated by intracellular recording in guinea pig in vitro hippocampal slices. High frequency stimulation of afferent fibres at intensities evoking in CA 1 neurons control excitatory postsynaptic potentials (EPSPs) of amplitudes 1-5 mV, resulted usually in a long-lasting increase in response amplitude. LTP was not observed at lower stimulus strength. The coactivation of a certain, though small number of synaptic contacts is thus necessary for the production of LTP.     

Tyrosine phosphatase STEP is a tonic brake on induction of long-term potentiation

The functional roles of protein tyrosine phosphatases (PTPs) in the developed CNS have been enigmatic. Here we show that striatal enriched tyrosine phosphatase (STEP) is a component of the N-methyl-D-aspartate receptor (NMDAR) complex. Functionally, exogenous STEP depressed NMDAR single-channel activity in excised membrane patches. STEP also depressed NMDAR-mediated synaptic currents whereas inhibiting endogenous STEP enhanced these currents. In hippocampal slices, administering STEP into CA1 neurons did not affect basal glutamatergic transmission evoked by Schaffer collateral stimulation but prevented tetanus-induced long-term potentiation (LTP). Conversely, inhibiting STEP in CA1 neurons enhanced transmission and occluded LTP induction through an NMDAR-, Src-, and Ca(2+)-dependent mechanism. Thus, STEP acts as a tonic brake on synaptic transmission by opposing Src-dependent upregulation of NMDARs.     

The effect of prior prolonged low frequency stimulation on the further synaptic plasticity at hippocampal CA1 synapses

The objective of this study is to determine the role of prior prolonged low frequency stimulation (900 pulses at 1 Hz) on the further induced long-term potentiation (LTP) and depression (LTD) of synaptic activity in the rat hippocampal CA1 area. Hippocampal slices and standard extracellular field potential recording techniques were employed. LTP and LTD were induced using stimulation at 5 Hz (900 pulses) paired with or without simultaneous application of 1 microM isoproterenol respectively, at either normal CA1 synapses or CA1 synapses that were pre-conditioned with prolonged low frequency stimulation at 1 Hz. LTD could be successfully induced upon 900 pulses of stimulation given at 5 Hz at normal synapses (82.1 +/- 2.9%; n = 5); it was, however, reduced to 96.5 +/- 4.7% (n = 6) at the preconditioned synapses. When paired with application of isoproterenol, 900 pulses of stimulation given at 5 Hz produced LTP (139.9 +/- 9.6%, n = 5) at normal synapses. The magnitude of LTP is decreased to (130 +/- 13.2%) (n = 6) at pre-conditioned synapses, though the difference is not significant. These results suggest that at a given CA1 synapses the expression of LTP and LTD is dependent on their history of use.     

Eugenol depresses synaptic transmission but does not prevent the induction of long-term potentiation in the CA1 region of rat hippocampal slices

Using field potential recording in the CA1 region of the rat hippocampal slices, the effects of eugenol on synaptic transmission and long-term potentiation (LTP) were investigated. Population spikes (PS) were recorded in the stratum pyramidal following stimulation of stratum fibers. To induce LTP, eight episodes of theta pattern primed-bursts (PBs) were delivered. Eugenol decreased the amplitude of PS in a concentration-dependent manner. The effect was fast and completely reversible. Eugenol had no effect on PBs-induced LTP of PS. It is concluded that while eugenol depresses synaptic transmission it does not affect the ability of CA1 synapses for tetanus-induced LTP and plasticity.     

The action of hydrogen peroxide on paired pulse and long-term potentiation in the hippocampus

The action of a reactive oxygen intermediate, that is, hydrogen peroxide (H2O2) on modulation of synaptic transmission was examined in the hippocampal brain slice preparation. Microinjection of H2O2 into the apical dendritic region of the CA1 pyramidal cells produced no change in either the pattern or amplitude of paired pulse facilitation compared to saline injection (control). Long term potentiation (LTP), induced by high frequency stimulation of homosynaptic inputs, however, was blocked by microinjection of H2O2 into the dendritic tree. LTP was seen in only 2 out of 10 slices investigated when treated with H2O2 while LTP was seen in 4 out of 5 slices when saline injected. The results suggest that a reactive oxygen intermediate can selectively modify synaptic mechanisms in the hippocampus.     

Time Course and Involvement of Protein Kinase C-Mediated Phosphorylation of F1/GAP-43 in Area CA3 After Mossy Fiber Stimulation

1. Protein kinase C (PKC) activity and phosphorylation of F1/growth associated protein (GAP)-43, a PKC substrate, have been proposed to play key roles in the maintenance of long-term potentiation (LTP) at the synapses of Schaffer collateral/commissural on pyramidal neurons in CA1 (Akers et al., 1986). We have studied in the involvement of PKC and PKC-dependent protein phosphorylation of F1/GAP-3 in in vitro LTP observed at the synapses of mossy fiber (MF) on CA3 pyramidal neurons of rat hippocampus by post hoc in vitro phosphorylation.2. After LTP was induced in CA3 in either the presence or absence of D-2-amino-5-phosphonovaleric acid (AP5), an NMDA receptor antagonist, the CA3 region was dissected for in vitro phosphorylation assay. In vivo phosphorylation of F1/GAP-43 was increased in membranes at 1 and 5 min after tetanic stimulation (TS) but not at 60 min after TS.3. The degree of phosphorylation of F1/GAP-43 in the cytosol was inversely related to that in membranes at each time point after LTP.4. The similar biochemical changes obtained from either control slices or AP5-treated slices indicate that LTP and the underlying biochemical changes are independent of the NMDA receptor. Immunoreactivity of the phophorylated F1/GAP-43 in LTP slices was not significantly different from control, indicating that results from western blotting and post hoc in vitro phosphorylation are consistent.5. Post hoc in vitro phosphorylation of F1/GAP-43 was PKC-mediated since phosphorylation of F1/GAP-43 was altered by the PKC activation cofactors, Ca²⁺, phosphatidylserine and phorbol ester.6. Calmodulin (CaM) at >5 M inhibited phosphorylation, consistent with the presence of CaM-binding activity at the site on F1/GAP-43 acted upon by PKC.7. We conclude that phosphorylation of F1/GAP-43 is associated with the induction but not the maintenance phase of MF-CA3 LTP.     

Enhancement of Endogenous Release of Glutamate and γ-Aminobutyric Acid from Hippocampus CA1 Slices After In Vivo Long-Term Potentiation

The effect of long-term potentiation (LTP) on endogenous amino acid release from rat hippocampus slices was studied. LTP was induced in vivo by application of a tetanus (200 Hz, 200 ms) to the Schaffer collateral fibers in unanesthetized rats. Endogenous release of glutamate and gamma-aminobutyric acid (GABA) was investigated 60 min after tetanization in CA1 subslices of potentiated and control rats. No significant effects of LTP were observed in basal and K(+)-induced Ca(2+)-independent release components of these amino acids. In contrast, K(+)-induced Ca(2+)-dependent release of both glutamate and GABA increased approximately 100% in slices from potentiated rats. No differences were observed in total content of glutamate and GABA between the subslices from control and LTP animals. These results suggest a persistent increase in the recruitment of the presynaptic vesicular pool of glutamate and GABA during LTP.     

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.     

Increased retention of calcium in the dendrites of long-term potentiated CA1 neurons of the hippocampal slice

Summary In view of the importance of calcium in the induction of long-term potentiation (LTP), experiments were carried out to localize calcium at the electron microscopic level in the CA1 region of guinea pig hippocampal slices, following high-frequency stimulation of the Schaffer collaterals. Apart from the ultrastructural localization, a semiquantitative method was used to count the calcium-containing deposits in electron micrographs. Significantly more calcium-containing deposits were seen in the dendrites of the stratum radiatum in slices with LTP than in those without it. A moderate increase of the extradendritic deposits was observed, too. The calcium content of the deposits was determined by means of EGTA incubation and X-ray analysis. The presented results, together with the relevant literature data, underline the importance of calcium-activated processes in postsynaptic structures probably involved in the generation of LTP.     

Increased retention of calcium in the dendrites of long-term potentiated CA1 neurons of the hippocampal slice. A combined electrophysiological and electron histochemical study

In view of the importance of calcium in the induction of long-term potentiation (LTP), experiments were carried out to localize calcium at the electron microscopic level in the CA1 region of guinea pig hippocampal slices, following high-frequency stimulation of the Schaffer collaterals. Apart from the ultrastructural localization, a semi-quantitative method was used to count the calcium-containing deposits in electron micrographs. Significantly more calcium-containing deposits were seen in the dendrites of the stratum radiatum in slices with LTP than in those without it. A moderate increase of the extradendritic deposits was observed, too. The calcium content of the deposits was determined by means of EGTA incubation and X-ray analysis. The presented results, together with the relevant literature data, underline the importance of calcium-activated processes in postsynaptic structures probably involved in the generation of LTP.     

Effects of A1 and A2 Adenosine Receptor Antagonists on the Induction and Reversal of Long-Term Potentiation in Guinea Pig Hippocampal Slices of CA1 Neurons

1. Using simultaneous recordings of the field EPSP and the population spike in the CA1 neurons of guinea pig hippocampal slices, we confirmed that delivery of a high-frequency stimulation (tetanus: 100 pulses at 100 Hz) produced robust long-term potentiation of synaptic efficacy (LTP) in two independent components, a synaptic component that increases field excitatory postsynaptic potentials (EPSPs) and a component that results in a larger population spike amplitude for a given EPSP size (E-S potentiation).2. In the same cells, reversal of LTP (depotentiation; DP) in the field EPSP and in the E-S component is achieved by delivering low-frequency afferent stimulation (LFS:1 Hz, 1000 pulses) 20 min after the tetanus.3. When the tetanus or LFS was applied to CA1 inputs in the presence of an adenosine A₁ receptor antagonist, 8-cyclopentyltheophylline (1 M), the field EPSP was enhances in LTP and attenuated in DP, while the E-S relationship was not significantly affected in either LTP or DP.4. When similar experiments were performed using an A₂ receptor antagonist, CP-66713 (10 M), the field EPSP was blocked in LTP but facilitated in DP, while E-S potentiation was enhanced during both LTP and DP.5. The results show that endogenous adenosine, acting via A₁ or A₂ receptors, modulates both the synaptic and the E-S components of the induction and reversal of LTP. Based on the results, we discuss the key issue of the contribution of these receptors to the dynamics of neuronal plasticity modification in hippocampal CA1 neurons.     

Enhanced Sensitivity of "Metabotropic" Glutamate Receptors After Induction of Long-Term Potentiation in Rat Hippocampus

Stimulation of [3H]inositol monophosphate ([3H]InsP) formation by ibotenate or trans-1-aminocyclopentyl-1,3-dicarboxylic acid (t-ACPD) in rat hippocampal slices was enhanced after tetanic stimulation of the Schaffer collaterals projecting to the CA1 region (in vitro) or the perforant pathway projecting to the dentate gyrus (in freely moving animals). This effect was observed 5 h (but not 2 h) after long-term potentiation (LTP) induction and was abolished if tetanic stimulation was performed in the presence of specific antagonists of N-methyl-D-aspartate receptors. The delayed increase in excitatory amino acid-induced polyphosphoinositide (PPI) hydrolysis was accompanied by an enhanced responsiveness to norepinephrine, whereas the basal and carbamylcholine-stimulated [3H]InsP formation were unchanged. These results suggest that an increased activity of "metabotropic" glutamate receptors may contribute to the synaptic mechanisms enabling the late expression and or maintenance of LTP. Accordingly, LTP decayed more rapidly (within 5 h) in rats repeatedly injected with LiCl (60-120 mg/kg, i.p., for 10 days), a treatment that led to a reduced efficacy of ibotenate and norepinephrine in stimulating PPI hydrolysis in hippocampal slices.     

Circadian regulation of hippocampal long-term potentiation

The goal of this study is to investigate the possible circadian regulation of hippocampal excitability and long-term potentiation (LTP) measured by stimulating the Schaffer collaterals (SC) and recording the field excitatory postsynaptic potential (fEPSP) from the CA1 dendritic layer or the population spike (PS) from the soma in brain slices of C3H and C57 mice. These 2 strains of mice were of interest because the C3H mice secrete melatonin rhythmically while the C57 mice do not. The authors found that the magnitude of the enhancement of the PS was significantly greater in LTP recorded from night slices compared to day slices of both C3H and C57 mice. They also found significant diurnal variation in the decay of LTP measured with fEPSPs, with the decay slower during the night in both strains of mice. There was evidence for a diurnal rhythm in the input/output function of pyramidal neurons measured at the soma in C57 but not C3H mice. Furthermore, LTP in the PS, measured in slices prepared during the day but recorded during the night, had a profile remarkably similar to the night group. Finally, PS recordings were carried out in slices from C3H mice maintained in constant darkness prior to experimentation. Again, the authors found that the magnitude of the enhancement of the PS was significantly greater in LTP recorded from subjective night slices compared to subjective day slices. These results provide the 1st evidence that an endogenous circadian oscillator modulates synaptic plasticity in the hippocampus.