Long-term Potentiation Response Research Articles

Heat-shock pretreatment prevents suppression of long-term potentiation induced by scopolamine in rat hippocampal CA1 synapses

We examined the effect of heat-shock pretreatment on long-term potentiation (LTP) in the CA1 hippocampal slices of the rat using the muscarinic blocker scopolamine as the LTP (memory) suppressor. Time course study using immunohistochemical techniques indicated peak expression of HSP70 16 h after heat-shock treatment. Focusing on that time point we found tetanic stimulation (at 100 Hz) induced LTP of 191.1±12.2% in control slices ( n=7), which was suppressed by scopolamine to 114.5±2.8 %. Heat-shock pretreatment successfully prevented such suppression (216.6±38.2% and 190.2±10.6% with and without scopolamine, respectively, n=7). Both HSP expression and LTP responses were relatively small taken either 2 or 48 h after heat-shock or sham pretreatment. These results suggest that the induction of HSPs is time-dependent and can prevent scopolamine-mediated LTP suppression.

Effects of high power microwave pulses on synaptic transmission and long term potentiation in hippocampus.

Effects of short, extremely high power microwave pulses (EHPP) on neuronal network function were explored by electrophysiological techniques in the isolated rat hippocampal slice model. Population spikes (PS) in the CA1 area were evoked by repeated stimulation (1 per 30 s) of the Schaffer collateral pathway. A brief tetanus (2 s at 50 Hz) was used to induce long term potentiation (LTP) of synaptic transmission. In three different series of experiments with a total of 160 brain slices, the EHPP irradiation was performed before, during, or after the tetanus. The EHPP carrier frequency was 9.3 GHz, the pulse width and repetition rate were from 0.5 to 2 micros and from 0.5 to 10 Hz, respectively, and the peak specific absorption rate (SAR) in brain slices reached up to 500 MW/kg. Microwave heating of the preparation ranged from 0.5 degrees C (at 0.3 kW/kg time average SAR) to 6 degrees C (at 3.6 kW/kg). The experiments established that the only effect caused by EHPP exposure within the studied range of parameters was a transient and fully reversible decrease in the PS amplitude. Recovery took no more than a few minutes after the cessation of exposure and return to the initial temperature. This effect's features were characteristic of an ordinary thermal response: it was proportional to the temperature rise but not to any specific parameter of EHPP, and it could also be induced by a continuous wave (CW) irradiation or conventional heating. Irradiation did not affect the ability of neurons to develop LTP in response to tetanus or to retain the potentiated state that was induced before irradiation. No lasting or delayed effects of EHPP were observed. The results are consistent with the thermal mechanism of EHPP action and thus far provided no indication of EHPP-specific effects on neuronal function.

Fimbrial control of bidirectional synaptic plasticity of medial perforant path-dentate transmission.

Lesions of the fimbria-fornix (FF) tract cause profound impairments of cognitive ability in animals. Our previous study showed that spatial performance correlates with long-term potentiation (LTP) of the dentate gyrus (DG), but not of the CA1 region, in rats with bilateral FF lesions, suggesting that FF lesions selectively inhibited LTP in the DG. The cortical input to the DG is anatomically and physiologically divided into two types of afferents, i.e., the medial perforant path (MPP) and the lateral perforant path (LPP), which show distinct synaptic properties. To elucidate the difference in the FF modulation of these two inputs, field responses were recorded from MPP- or LPP-DG synapses in anesthetized rats. MPP-DG synapses of rats with FF lesions displayed neither LTP in response to theta-burst stimulation nor long-term depression (LTD) in response to low-frequency burst stimulation. In contrast to the MPP, LPP-DG synapses showed normal LTP in rats with FF lesions. The low-frequency burst stimulation could not induce LTD at LPP-DG synapses in either intact or FF-lesioned rats. These results suggest that the FF pathway selectively supports the mechanisms of bidirectional synaptic plasticity at MPP-DG synapses. This study provides new insights into external control of information processing in the hippocampus.

Estimating a hidden Bernoulli parameter by sequential Bayesian analysis

A tetanically-stimulated (TS) neuron is said to have failed to fire if its voltage-clamped excitatory postsynaptic current (EPSC) measurement is devoid of a long-term potentiation (LTP) response. This paper provides a method for evaluating the posterior probability of “failure” for TS neurons. A sequential Bayes algorithm is employed on an imperfect Bernoulli trial model in order to refine the posterior density of the failure parameter with each EPSC data record processed. The algorithm is applied to simulated EPSC data with TS elicited LTP responses and is shown to coincide very well with the expected presynaptically-induced LTP failure rate observed in vitro.

Gαq-Deficient Mice Lack Metabotropic Glutamate Receptor-Dependent Long-Term Depression But Show Normal Long-Term Potentiation in the Hippocampal CA1 Region

Long-term potentiation (LTP) and depression (LTD) are potential cellular mechanisms involved in learning and memory. Group I metabotropic glutamate receptors (mGluR), which are linked to heterotrimeric G-proteins of the G(q) family (G(q) and G(11)), have been reported to facilitate both hippocampal LTP and LTD. To evaluate their functional role in synaptic plasticity, we studied LTD and LTP in the CA1 region of the hippocampus from wild-type, Galpha(q)(-/-), and Galpha(11)(-/-) mice. Basic parameters of the synaptic transmission were not altered in Galpha(q)(-/-) and Galpha(11)(-/-) mice. Moreover, these mice showed normal LTP in response to a strong tetanus and to a weak tetanus. However, LTD induced either by a group I mGluRs agonist or by paired-pulse low-frequency stimulation (PP-LFS) was absent in Galpha(q)(-/-) mice. Moreover, PP-LFS caused potentiation of the synaptic transmission in these mice that was not affected by the NMDAR antagonist AP-5. These results show that G(q) plays a crucial role in the mGluR-dependent LTD, whereas hippocampal LTP is not affected by the lack of a single member of the G(q) family.

Synaptic plasticity in morphologically identified CA1 stratum radiatum interneurons and giant projection cells.

Long-term potentiation (LTP) of synaptic efficacy was examined in interneurons and giant cells in the stratum radiatum region of the hippocampal CA1 subfield. Cells were visually selected using differential interference contrast (DIC) optics and filled with biocytin while being recorded using whole-cell patch-clamp techniques. Electrophysiological criteria, including spike height, width, and degree of spike adaptation shown to sustained depolarization, proved inadequate for differentiating interneurons from giant cells. We found that cells in the stratum radiatum, however, could be reliably differentiated using DIC optics or following intracellular staining. The response of the two cell types to tetanic stimulation further dissociated them. Long-term potentiation, dependent on the activation of NMDAr (N-methyl-D-aspartate receptors), could reliably be induced in interneurons with stimuli administered at 200 Hz, but not 100 Hz. Giant cells, in contrast, exhibited NMDA receptor-dependent LTP in response to 100-Hz stimuli, but not the 200-Hz stimuli. LTP induction in interneurons also appeared temperature-dependent, being much more robust at 34 degrees C than at room temperature. The LTP in both cell types required postsynaptic calcium influx, and was not due to the passive propagation of LTP induction in neighboring pyramidal cells. These results suggest that different cell types within the hippocampal formation may preferentially alter synaptic connectivity in a frequency-specific manner.

Reg1ulatory Role and Molecular Interactions of a Cell-Surface Heparan Sulfate Proteoglycan (N-syndecan) in Hippocampal Long-Term Potentiation

The cellular mechanisms responsible for synaptic plasticity involve interactions between neurons and the extracellular matrix. Heparan sulfates (HSs) constitute a group of glycosaminoglycans that accumulate in the beta-amyloid deposits in Alzheimer's disease and influence the development of neuron-target contacts by interacting with other cell surface and matrix molecules. However, the contribution of HSs to brain function is unknown. We found that HSs play a crucial role in long-term potentiation (LTP), a finding that is consistent with the idea that converging molecular mechanisms are used in the development of neuron-target contacts and in activity-induced synaptic plasticity in adults. Enzymatic cleavage of HS by heparitinase as well as addition of soluble heparin-type carbohydrates prevented expression of LTP in response to 100 Hz/1 sec stimulation of Schaffer collaterals in rat hippocampal slices. A prominent carrier protein for the type of glycans implicated in LTP regulation in the adult hippocampus was identified as N-syndecan (syndecan-3), a transmembrane proteoglycan that was expressed at the processes of the CA1 pyramidal neurons in an activity-dependent manner. Addition of soluble N-syndecan into the CA1 dendritic area prevented tetanus-induced LTP. A major substrate of src-type kinases, cortactin (p80/85), and the tyrosine kinase fyn copurified with N-syndecan from hippocampus. Moreover, association of both cortactin and fyn to N-syndecan was rapidly increased after induction of LTP. N-syndecan may thus act as an important regulator in the activity-dependent modulation of neuronal connectivity by transmitting signals between extracellular heparin-binding factors and the fyn signaling pathway.

Soluble HIV-1 infected macrophage secretory products mediate blockade of long-term potentiation: a mechanism for cognitive dysfunction in HIV-1-associated dementia.

It is generally accepted that viral and cellular products from immune competent mononuclear phagocytes (MP) (brain macrophages and microglia) underlie the neuropathogenesis of HIV-1-associated dementia (HAD). What remains unanswered, however, is the composition of and mechanisms for such MP-induced neurological dysfunctions. In attempts to address these issues culture fluids from HIV-1ADA-infected monocyte-derived macrophages (MDMs) (depleted or enriched with progeny virus) were placed onto the CA1 area of rat hippocampal brain slices (the site of mammalian learning and memory) and neuronal long-term potentiation (LTP) assayed. LTP was induced by high frequency stimulation (HFS). Lipopolysaccharide (LPS) served as a surrogate macrophage activator. Synaptic strength was assayed by the initial slope of evoked field excitatory postsynaptic potentials (EPSPs). Synaptic potentiation following HFS was observed in slices incubated with uninfected (control) MDM culture fluids. The magnitude of the LTP response was 150.2 +/- 21.10% compared to basal levels (n=6). Synaptic strength was enhanced in virus-infected (135.7+/-28.9%, n=8) and LPS-activated MDM (123.3+/-5.1%, n=7) but at lower levels than controls. The lowest levels of LTP were in brain slices incubated with virus-infected and LPS-activated MDM fluids at (109.5+/-9.9% n=12). Interestingly, bath application of progeny HIV-1 virions showed minimal LTP effects. Virus-infected, LPS-activated MDM fluids, with progenyvirus, reduced synaptic strength but were not statistically different than replicate culture fluids depleted of virus. In contrast, IL-1beta and quinolinic acid, significantly diminished synaptic strength. These results, taken together, suggest that soluble HIV-1-infected MDM secretory products, but not virus per se, significantly affect LTP. This electrophysiological system, which monitors neuronal function following cell exposure to HIV-1 infected materials could provide a novel testing ground for therapeutics designed to protect brain function in HAD.

Control of memory formation through regulated expression of a CaMKII transgene.

One of the major limitations in the use of genetically modified mice for studying cognitive functions is the lack of regional and temporal control of gene function. To overcome these limitations, a forebrain-specific promoter was combined with the tetracycline transactivator system to achieve both regional and temporal control of transgene expression. Expression of an activated calcium-independent form of calcium-calmodulin-dependent kinase II (CaMKII) resulted in a loss of hippocampal long-term potentiation in response to 10-hertz stimulation and a deficit in spatial memory, a form of explicit memory. Suppression of transgene expression reversed both the physiological and the memory deficit. When the transgene was expressed at high levels in the lateral amygdala and the striatum but not other forebrain structures, there was a deficit in fear conditioning, an implicit memory task, that also was reversible. Thus, the CaMKII signaling pathway is critical for both explicit and implicit memory storage, in a manner that is independent of its potential role in development.

Mice Expressing Activated CaMKII Lack Low Frequency LTP and Do Not Form Stable Place Cells in the CA1 Region of the Hippocampus

To relate different forms of synaptic plasticity to the formation and maintenance of place cells in the hippocampus, we have recorded place cells in freely behaving, transgenic mice that express a mutated Ca2+-independent form of CaM Kinase II. These mice have normal long-term potentiation (LTP) at 100 Hz, but they lack LTP in response to stimulation at 5–10 Hz and are impaired on spatial memory tasks. In these transgenic mice, the place cells in the CA1 region have three important differences from those of wild types: they are less common, less precise, and less stable. These findings suggest that LTP in the 5–10 Hz range may be important for the maintenance of place-field stability and that this stability may be essential for the storage of spatial memory.

Effects of chronic ethanol ingestion on long-term potentiation remain even after a prolonged recovery from ethanol exposure.

Previous work in our laboratory has demonstrated that memory formation, a behavioral process thought to be at least in part attributed to hippocampal functioning, is severely attenuated following 5 months of chronic ethanol treatment (CET) and 2 months of recovery from CET. Additionally, 48 h following CET, a well-recognized physiological correlate of memory formation, long-term potentiation (LTP), is reduced in the hippocampus. We hypothesized that the reduction in LTP may in part contribute to the behavioral deficit in memory formation, which is a permanent consequence of CET. In order for the reduction of LTP to be involved with the permanent deficit in memory acquisition, it too must be present following a prolonged period of ethanol abstinence. The present study examined the permanent effect of CET on LTP. Animals were fed a nutritionally complete, ethanol containing diet for 28 weeks and then allowed a 5-7 month abstinence period. A control group was fed the same diet except sucrose was isocalorically substituted for ethanol. Neurophysiological methods measured the capacity of synaptic connections onto CA1 pyramidal cells to support LTP in response to a variety of conditioning trains. The magnitude of LTP was reduced in CET animals as compared with pair-fed controls. LTP induction is mediated by activation of the N-methyl-d-aspartate (NMDA) receptor complex and is modulated by activation of gamma-aminobutyric acid (GABA)ergic synaptic transmission. The effect of CET on LTP magnitude may be due to effects on the NMDA-induced induction itself or on the GABAergic modulation of induction.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of high frequency activity in the somatosensory thalamus of rats in vivo results in long-term potentiation of responses in SI cortex.

Extracellular single-unit techniques were employed to record unitary activity simultaneously from the thalamic ventral posterior medial (VPM) nucleus and the ipsilateral primary somatosensory cortex of adult rats. Cross-correlation analysis triggered by the spontaneous firing of thalamocortical relay neurons in VPM and the discharge of layer IV neurons in the corresponding ipsilateral cortical barrel indicated that the paired-units included in this study were strongly correlated in their activity. The baseline responses of highly correlated cortical/thalamic pairs to a 10 ms deflection of a vibrissa on the contralateral side were measured using poststimulus time histograms. After establishing the baseline response, high frequency activity in VPM was induced in one of two ways: i) direct electrical stimulation of thalamic neurons or ii) whisker stimulation in the presence of bicuculline methiodide (BIC) released near the thalamic neurons. Both methods resulted in a conditioning stimulus (CS) paradigm consisting of "burst" of high-frequency activity (50-100 Hz) with an inter-burst interval of 150 ms (approximately 7 Hz). Almost immediately following the presentation of the CS, the response of layer IV cortical neurons to vibrissa stimulation increased by 37-62% over baseline values, which was maintained after the effects of BIC had worn off in VPM. This enhancement in the response of the cortical neurons was not accompanied by a concomitant increase in the thalamic responses. Thus, these results strongly suggest that the potentiation first occurred at the thalamocortical synapse.

Homosynaptic long-term depression in area CA1 of hippocampus and effects of N-methyl-D-aspartate receptor blockade.

We tested a theoretical prediction that patterns of excitatory input activity that consistently fail to activate target neurons sufficiently to induce synaptic potentiation will instead cause a specific synaptic depression. To realize this situation experimentally, the Schaffer collateral projection to area CA1 in rat hippocampal slices was stimulated electrically at frequencies ranging from 0.5 to 50 Hz. Nine hundred pulses at 1-3 Hz consistently yielded a depression of the CA1 population excitatory postsynaptic potential that persisted without signs of recovery for greater than 1 hr after cessation of the conditioning stimulation. This long-term depression was specific to the conditioned input, ruling out generalized changes in postsynaptic responsiveness or excitability. Three lines of evidence suggest that this effect is accounted for by a modification of synaptic effectiveness rather than damage to or fatigue of the stimulated inputs. First, the effect was dependent on the stimulation frequency; 900 pulses at 10 Hz caused no lasting change, and at 50 Hz a synaptic potentiation was usually observed. Second, the depressed synapses continued to support long-term potentiation in response to a high-frequency tetanus. Third, the effects of conditioning stimulation could be prevented by application of NMDA receptor antagonists. Thus, our data suggest that synaptic depression can be triggered by prolonged NMDA receptor activation that is below the threshold for inducing synaptic potentiation. We propose that this mechanism is important for the modifications of hippocampal response properties that underlie some forms of learning and memory.

NMDA receptor blockade prevents the increase in protein kinase C substrate (protein F1) phosphorylation produced by long-term potentiation

Recent evidence has implicated activation of the N-methyl- d-aspartate (NMDA) class of glutamate receptor in the initiation of hippocampal long-term potentiation (LTP), an electrophysiological model of information storage in the brain. A separate line of evidence has suggested that activation of protein kinase C (PKC) and the consequent phosphorylation of it substrates is necessary for the maintenance of the LTP response. To determine if PKC activation is a consequence of NMDA receptor activation during LTP, we applied the NMDA receptor antagonist drug, dl-aminophosphonovalerate (APV) both immediately prior to and following high frequency stimulation, resulting in successful and unsuccessful blockade of LTP initiation, respectively. We then measured the phosphorylation of a PKC substrate (protein F1) in hippocampal tissue dissected from these animals. Only successful blockade of LTP initiation by prior application of APV was seen to block the LTP-associated increase in protein F1 phosphorylation measured in vitro ( P < 0.0001 by ANOVA). This suggests that NMDA receptor-mediated initiation triggers maintenance processes that are, at least in part, mediated by protein F1 phosphorylation. These data provide the first evidence linking two mechanisms associated with LTP, NMDA receptor activation and PKC substrate phosphorylation.

Enhancement of long-term potentiation by cis-unsaturated fatty acid: relation to protein kinase C and phospholipase A2

Previous correlative and interventive work from this laboratory has suggested that activation of protein kinase C (PKC) is important for the maintenance of the hippocampal long-term potentiation (LTP) response. One such study demonstrated that application of the cis-unsaturated fatty acid, oleate, a newly discovered PKC activator, could prolong the time course of LTP. The present study explored the mechanism of cis-unsaturated fatty acid action on LTP produced by perforant path stimulation. First, neither oleate application nor high-frequency stimulation alone produced a persistent change in synaptic transmission, while the 2 in conjunction did so. This suggests that oleate acts synergistically with the consequences of this stimulation to produce an enhancement of the LTP response. Second, oleate enhancement of LTP was more potent when applied in the perforant path synaptic terminal zone than in the dentate hilus, implying that the site of oleate action is at the synapse (where PKC is reported to be enriched). Third, translocation of PKC activity to the membrane was significantly increased after oleate-enhanced LTP relative to vehicle controls. PKC translocation was found to be unaltered by oleate application alone. Fourth, mepacrine blockade of the Ca2+-dependent enzyme phospholipase A2, which releases endogenous oleate from membrane phospholipids, inhibited the time-course of a persistent LTP response. This inhibition was shown to be reversible with oleate application. We propose that high-frequency stimulation produces an elevation of intracellular Ca2+, which then triggers phospholipase A2-mediated oleate release. This free oleate then could act in synergy with processes that render PKC oleate-sensitive to produce a persistent activation of PKC, which is critical for and leads to the persistence of the LTP response.

Selective increase in phosphorylation of a 47-kDa protein (F1) directly related to long-term potentiation

Five minutes after the induction of long-term potentiation (LTP) in the intact hippocampal formation of chloralose/urethane-anesthetized rats there was a selective increase in the in vitro phosphorylation state of a 47-kDa band (designated Protein F1). When low frequency, nonpotentiating stimulation was used, no change in Protein F1 was observed. LTP had no significant effect on other phosphoproteins measured at the 5-min time point. In vitro phosphorylation of Protein F1 5 min after LTP was directly related to the change in synaptic efficiency (r = +0.86, p less than .01). The calcium-dependent, synaptically localized Protein F1 may be the same as the brain-specific Protein B-50. The regulation of the plastic LTP response may involve the novel multifunctional phospholipid-dependent enzyme, Protein Kinase C, which has been shown to phosphorylate B50.