The effects of repetitive low frequency stimulation on control and "potentiated" synaptic responses in the hippocampus

Involvement of nerve growth factor in visual cortex plasticity

The physiological role of nerve growth factor (NGF), the prototype member of the neurotrophin family, has been widely studied. NGF has been shown to promote survival, sprouting and differentiation of sympathetic ganglion cells and sensory neurons in the peripheral nervous system; it has also been shown to support survival and regeneration of cholinergic neurons in the central nervous system. Recent evidence indicates that NGF is also involved in the neuronal plasticity of the visual cortex. Exogenous supplies of NGF have been shown to interfere with normal processes underlying activity- and age-dependent synaptic modifications in both developing and adult visual cortex. In parallel to these physiological effects, numerous neuronal markers in the visual cortex have been found to be influenced by NGF. Several proposals have been introduced to explain the physiological role of NGF in visual cortex plasticity. Although the mechanisms underlying NGF effects in the visual cortex are still under active investigation, current evidence implies that NGF, and perhaps other neurotrophins as well, may be useful for preventing or correcting inappropriate or anomalous connections in the visual cortex, and thus for treating visual dysfunctions such as amblyopia and strabismus.

Hippocampal long-term depression and depotentiation are defective in mice carrying a targeted disruption of the gene encoding the RI beta subunit of cAMP-dependent protein kinase

The cAMP-dependent protein kinase (PKA) has been shown to play an important role in long-term potentiation (LTP) in the hippocampus, but little is known about the function of PKA in long-term depression (LTD). We have combined pharmacologic and genetic approaches to demonstrate that PKA activity is required for both homosynaptic LTD and depotentiation and that a specific neuronal isoform of type I regulatory subunit (RI beta) is essential. Mice carrying a null mutation in the gene encoding RI beta were established by use of gene targeting in embryonic stem cells. Hippocampal slices from mutant mice show a severe deficit in LTD and depotentiation at the Schaffer collateral-CA1 synapse. This defect is also evident at the lateral perforant path-dentate granule cell synapse in RI beta mutant mice. Despite a compensatory increase in the related RI alpha protein and a lack of detectable changes in total PKA activity, the hippocampal function in these mice is not rescued, suggesting a unique role for RI beta. Since the late phase of CA1 LTP also requires PKA but is normal in RI beta mutant mice, our data further suggest that different forms of synaptic plasticity are likely to employ different combinations of regulatory and catalytic subunits.

Hippocampal homosynaptic long-term depression/depotentiation induced by adrenal steroids

The effects of adrenal steroids on synaptic plasticity were investigated in the dentate gyrus of the hippocampus. Experiments were performed in either adrenalectomized or intact (non-adrenalectomized), anesthetized rats. High-frequency stimulation was applied to the medial perforant pathway at three different frequencies; 100, 200 or 400 Hz, either post- or pre- and post-administration of the specific Type-II adrenal steroid receptor agonist RU 28362. High-frequency stimulation prior to RU 28362 administration produced a frequency-dependent long-term potentiation of the population spike, with 100 Hz showing no long-term potentiation and 400 Hz the highest degree of potentiation. In contrast, following administration of RU 28362, high-frequency stimulation produced a long-term depression (in comparison to baseline). In the experiments in which high-frequency stimulation was applied both pre- and post-RU 28362 administration, the size of the population spike was initially potentiated and then depotentiated after the RU 28362 injection. This effect was also frequency dependent, although opposite to the long-term potentiation effect. That is, 400 Hz was the least effective frequency for inducing long-term depression/depotentiation, while 100 Hz was the most effective. Long-term depression/depotentiation was immediate following high-frequency stimulation and lasted for the extent of the recording session, in some cases longer than 1 h. Similar to the finding reported in the accompanying paper, induction of long-term potentiation was substantially suppressed by RU 28362. However, in a number of experiments long-term potentiation could still be induced after RU 28362 administration, even after long-term depression/depotentiation had been established. In these cases, stimulation at the higher frequencies was necessary.(ABSTRACT TRUNCATED AT 250 WORDS)

On the mechanism of long-term potentiation induced by (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) in rat hippocampal slices

We have reported previously that transient application of a specific metabotropic glutamate receptor (mGluR) agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD) can induce a slow-onset form of long-term potentiation (LTP) of synaptic transmission in the CA1 region of rat hippocampal slices [Bortolotto Z. A. and Collingridge G. L. (1993) Neuropharmacology 32, 1-9]. Here we have investigated further the mechanisms involved in the induction and expression of ACPD-induced LTP. Unless otherwise stated, field excitatory postsynaptic potentials (EPSPs) were recorded in stratum radiatum in response to low frequency (0.033 Hz stimulation) of the Schaffer collateral-commissural pathway and 10 microM ACPD was added for 20 min to the perfusate. ACPD-induced LTP was still observed following blockade of GABAA receptor-mediated synaptic inhibition using picrotoxin (50 microM) and was not the result of a change in the presynaptic fibre volley. Intracellular recording from area CA1 revealed an increase in the size of the EPSP but no associated change in membrane potential or input resistance. However, ACPD-induced potentiation was never seen when intracellular electrodes contained the Ca(2+)-chelating agent 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA; 0.5 M). In area CA3, ACPD elicited a slow-onset LTP of the intracellularly recorded EPSP, evoked by stimulation of associational fibres. In contrast to area CA1, 10 microM ACPD depolarized CA3 neurones. Unlike certain other forms of tetanus- and chemically-induced potentiation, ACPD-induced LTP was not affected by the L-type Ca2+ channel antagonist nimodipine (50 microM). It was, however, prevented by delivering low frequency stimulation (900 shocks at 1 Hz) immediately following termination of the application of ACPD; an effect which was inhibited by the specific N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonopentanoate (AP5; 50 microM). ACPD failed to induce LTP of pharmacologically-isolated NMDA receptor-mediated EPSPs. The induction of ACPD-induced LTP was blocked by the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), in a reversible manner. In slices in which area CA3 had been removed ACPD failed to induce LTP when applied alone or together with AMPA. However, a slow-onset form of LTP was induced, in slices lacking area CA3, when a tetanus (100 Hz, 1 sec) was delivered in the presence of ACPD and 50 microM AP5 (the latter applied to prevent conventional tetanus-induced LTP). ACPD-induced LTP was associated with a parallel increase in the sensitivity of CA1 neurones to AMPA. Considered together, these data suggest that ACPD-induced LTP is due to a direct increase in the AMPA receptor-mediated synaptic conductance and involves postsynaptic induction and expression mechanisms.

Metabotropic glutamate receptor-induced homosynaptic long-term depression and depotentiation in the dentate gyrus of the rat hippocampus in vitro

We have investigated the role of metabotropic glutamate receptors (mGluR) in the induction of homosynaptic long-term depression (LTD) and depotentiation (DP) in the dentate gyrus of the adult rat. Perfusion of the mGluR agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) for a prolonged period (20 min) induced long-term depression (LTD) of field excitatory postsynaptic field potentials (epsps) from the baseline level and also depotentiation (DP) from the long-term potentiated level. Both the ACPD-and the low frequency stimulation (LFS)-induced LTD and DP were inhibited in the presence of the mGluR antagonist (+)-alpha-methyl-4-carboxyphenylglycine (MCPG), demonstrating the necessity for the activation of metabotropic glutamate receptors in the induction of LTD/DP. The LFS and ACPD-induced LTD were independent of the activation of N-methyl-D-aspartate (NMDA) receptors, as they were not blocked by the NMDA receptor antagonist D-2-amino-5-phophonopentanoate (AP5).

Brain RNA synthesis, long-term potentiation and depression at the perforant path-granule cell synapse in the guinea pig

The effects of long-term changes in synaptic efficacy at the perforant path-granule cell synapse on the de-novo synthesis of ribonucleic acid (RNA) were investigated in hippocampal and cortical areas in anaesthetized Guinea pig preparations. Two experiments were run with stimulating and recording microelectrodes aimed at the perforant bundle and dentate gyrus hilus on both sides. In Experiment 1, a low-frequency (LFS; 0.02 Hz, 3 h) or high-frequency stimulation (HFS; 400 Hz, 250 ms) was delivered to the left perforant bundle with the contralateral side as control. In Experiment 2, animals received LFS or HFS trains with implanted nonstimulated animals used as controls. The latency and amplitude of the field postsynaptic potentials (FPSP) and population spike (POPS) were monitored under baseline conditions and following stimulation over a 3 h period. In addition, two HFS groups were tested with few (HFS-F: every 15 min) or several test stimuli (HFS-S: every 3 min). In both experiments RNA synthesis was determined by measuring the amount of 3H-5,6-uridine incorporated into the RNA 3 h after bilateral intraventricular injection. In Exp. 1 the LFS group showed a higher synthesis of RNA than both HFS groups. The rate of RNA synthesis did not differ between the stimulated and nonstimulated side. In Exp. 2 the HFS groups showed a decreased RNA synthesis. In the HFS-F group, it pertained to the dorsal dentate area, CA1, subiculum, cingulate and dorsal cortices bilaterally, and to the ventral dentate area and CA3 on the nonstimulated side. In contrast, the HFS-S group showed decreased RNA synthesis at the dorsal dentate area and dorsal cortex on the stimulated side, and at CA1, subiculum, and cingulate cortex bilaterally. The decrease was stronger in the HFS-F than in the HFS-S group. Moreover, the subgroup with a low (0-60%) and that with a high (61-240%) level of long-term potentiation of FPSP revealed lower and higher RNA synthesis, respectively, both in homosynaptic target areas, and in heterosynaptic sites. Further, correlative analyses between FPSP, POPS and RNA synthesis revealed a complex pattern, depending upon the type of stimulation and on the brain side. Finally, cross-correlation analyses revealed a high degree of coupling among brain sites in the stimulated groups, indicating distributed covariant changes in RNA synthesis across different brain sites. Thus, changes in synaptic efficacy covary with changes in RNA synthesis, and presumably exert a modulatory role on gene expression.

Synaptic plasticity. A molecular switch for memory

The metabotropic glutamate receptor controls a molecular switch that, when activated, generates a conditioned state essential for the induction of long-term potentiation of synaptic transmission in the hippocampus.

Nerve growth factor-induced ocular dominance plasticity in adult cat visual cortex

Activity-dependent modifiability of cortical ocular dominance occurs only during early postnatal life, within the so-called "critical period," but not thereafter in adult visual cortex. To examine the role of neurotrophins in the activity- and age-dependent stimulation-induced modifiability of visual cortex, we tested whether intracortical infusion of nerve growth factor could induce ocular dominance plasticity in adult visual cortex. Nerve growth factor was continuously infused, by means of osmotic minipumps, into striate cortex of adult cats for 2 weeks. At the time of minipump implantation, one eyelid of the experimental animals was sutured closed. After 3 weeks of monocular deprivation, the ocular dominance distribution of neurons in the striate cortex was assessed using single unit recording. We found that monocular deprivation imposed on adult animals in conjunction with nerve growth factor infusion causes an ocular dominance shift toward the deprived eye. Although the underlying mechanisms remain uncertain, the results indicate that nerve growth factor can enhance activity-dependent synaptic modification and remodeling in adult visual cortex.

An investigation of depotentiation of long-term potentiation in the CA1 region of the hippocampus

We have investigated long-term synaptic depression in the CA1 region of rat hippocampal slices. Prolonged low-frequency stimulation (LFS; 900 stimuli delivered at 2 Hz) of the Schaffer collateral-commissural pathway in naïve slices did not induce long-term depression (LTD) of synaptic transmission. However, if long-term potentiation (LTP) was firstly induced in the pathway then LFS generated an LTD-like effect (i.e. depotentiation of LTP). Depotentiation could be induced 2 h (the longest time studied) after the induction of LTP and was stable for the duration of the experiment (followed for up to 40 min). The induction of depotentiation was not blocked by the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonopentanoate, the L-type voltage-gated Ca2+ channel blocker nimodipine or the nitric oxide synthase inhibitor N omega-nitro-L-arginine. However, the magnitude of depotentiation was reversibly reduced, in a stereoselective manner, by the specific metabotropic glutamate receptor (mGluR) antagonist (+)-alpha-methyl-4-carboxyphenylglycine. These results show that prolonged low frequency stimulation can result in an mGluR-dependent depotentiation of LTP.

Induction in the rat hippocampus of long-term potentiation (LTP) and long-term depression (LTD) in the presence of a nitric oxide synthase inhibitor

Several recent studies have suggested a critical role for nitric oxide (NO) production in hippocampal LTP and LTD. In this study we show that normal LTP and LTD can be induced in rat hippocampal slices incubated in the NO synthase inhibitor L-NG-nitroarginine (NOArg) (100 microM). A test of NMDA-stimulated cGMP production demonstrated that incubation of slices in 100 microM NOArg effectively inhibited NO synthase. Our results suggest that NO synthase activity may not be required for the generation of LTP or LTD in CA1 of rat hippocampus.

Synaptic plasticity: LTP and LTD

Long-term potentiation (LTP) is a synaptic enhancement that follows brief, high-frequency electrical stimulation in the hippocampus and neocortex. Recent evidence suggests that induction of LTP may require, in addition to postsynaptic Ca2+ entry, activation of metabotropic glutamate receptors and the generation of diffusible intercellular messengers. A new form of synaptic plasticity, homosynaptic long-term depression (LTD) has also recently been documented, which, like LTP, requires Ca2+ entry through the NMDA receptor. Current work suggests that this LTD is a reversal of LTP, and vice versa, and that the mechanisms of LTP and LTD may converge at the level of specific phosphoproteins.

A molecular switch activated by metabotropic glutamate receptors regulates induction of long-term potentiation

Pharmacological studies of long-term potentiation (LTP) in the hippocampus are starting to provide a molecular understanding of synaptic plastic processes which are believed to be important for learning and memory in vertebrates. In the CA1 region of the hippocampus, the synaptic activation of glutamate receptors of the N-methyl-D-aspartate (NMDA) subtype is necessary for the induction of LTP under most experimental conditions. The synaptic activation of metabotropic glutamate receptors (mGluRs) is also needed for the induction of LTP. We now show that the role of mGluRs in the induction of LTP is fundamentally different from that of NMDA receptors. NMDA receptors initiate a molecular event that needs to be triggered each time a tetanus is delivered to induce LTP. In contrast, mGluRs activate a molecular switch which then negates the need for mGluR stimulation during the induction of LTP. This mGluR-activated switch is input-specific and can be turned off by a train of low-frequency stimulation. The molecular switch is a new feature of LTP which has fundamental consequences for our understanding of synaptic plastic mechanisms.

Flip side of synaptic plasticity: long-term depression mechanisms in the hippocampus

There is growing interest in the phenomenon of long-term depression (LTD) of synaptic efficacy that, together with long-term potentiation (LTP), is a putative information storage mechanism in mammalian brain. In neural network models, multiple learning rules have been used for LTD induction. Similarly, in neurophysiological studies of hippocampal synaptic plasticity, a variety of activity patterns have been effective at inducing LTD, although experimental paradigms are still being optimized. In this review the authors summarize the major experimental paradigms and compare what is known about the mechanisms of LTD induction. Although all paradigms appear to initiate a cascade of events leading to an elevated level of Ca2+ postsynaptically, the extent to which these paradigms involve common expression mechanisms has not yet been tested. The authors discuss several critical experiments that would address this latter issue. Numerous questions about the properties and mechanisms of LTD(s) in the hippocampus remain to be answered, but it is clear that LTD has finally arrived, and will soon be attracting attention equal to its flip side, LTP.

Resonant activation of calcium signal transduction in neurons

The relevant parameters of calcium fluxes mediating activation of immediate-early genes and the collapse of growth cones in mouse DRG neurons in response to action potentials delivered in different temporal patterns were measured in a multicompartment cell culture preparation using digital fluorescence videomicroscopy. Growth cone collapse was produced by trains of action potentials causing a large rise in [Ca2+]i, but after chronic exposure to patterned stimulation growth cones regenerated and became insensitive to the stimulus-induced increase in [Ca2+]i. Calcium reached similar peak concentrations, but the [Ca2+]i increased more slowly than in naive growth cones (time constant of 6.0 s versus 1.4 s in naive growth cones). Semiquantitative PCR measurements of gene expression showed that pulsed stimulation delivered at 1-min intervals for 30 min induced expression of c-fos, but the same total number of action potentials delivered at 2-min intervals failed to induce c-fos expression, even though this stimulus induces a larger peak [Ca2+]i than the effective stimulus pattern. The experiments suggest that the kinetics of calcium fluxes produced by different patterns of stimulation, and changes in the kinetics of calcium flux in neurons under different states of activation, are critical in determining the effects of action potentials on growth cone motility or expression of IE genes during development of neuronal circuits. We propose that differences in kinetics of individual reactions in the stimulus-response pathway may lead to resonance of activation in the neuron, such that certain processes will be selectively activated by particular temporal patterns of stimulation.

Low-frequency stimulation erases LTP through an NMDA receptor-mediated activation of protein phosphatases

In the CA1 region of adult guinea pig hippocampal slices, long trains of theta frequency (5 Hz) stimulation produced a small enhancement of basal synaptic transmission but depressed the strength of synaptic transmission at synapses that had recently undergone long-term potentiation (LTP). Five hertz stimulation delivered immediately prior to high-frequency stimulation also inhibited the subsequent induction of LTP. The depression of potentiated synapses by 5 Hz stimulation (depotentiation) was blocked by 2-amino-5-phosphonovalerate and was observed only during the early phases of LTP. Furthermore, the protein phosphatase inhibitors okadaic acid and calyculin A blocked both depotentiation and the ability of 5 Hz stimulation to inhibit subsequent LTP, suggesting that protein phosphatases are involved in the ability of 5 Hz stimulation to modulate synaptic plasticity in the CA1 region of the hippocampus.

The sensitivity of hippocampal long-term potentiation to nitric oxide synthase inhibitors is dependent upon the pattern of conditioning stimulation

Inhibition of nitric oxide synthase prior to conditioning has been previously found to reduce levels of hippocampal long-term potentiation. In the present experiments in the rat, the reduction of long-term potentiation by nitric oxide synthase inhibitors was highly conditioning-dependent. We have characterized the relative importance of the number of conditioning stimulus trains, pulse number, intensity, and pattern in the reduction of long-term potentiation by nitric oxide synthase inhibitors. Long-term potentiation was reduced relative to control values only when multiple conditioning stimulus trains were presented at maximal stimulus intensity; potentiation produced by an equivalent number and intensity of stimuli presented in a single conditioning train was not reduced by nitric oxide synthase inhibitors, and multiple-train-induced potentiation was sensitive to nitric oxide synthase inhibitors only when maximal stimulation intensity was employed. Another form of synaptic potentiation, primed-burst potentiation, was reduced by nitric oxide synthase inhibition, while homosynaptic and heterosynaptic depression were unaffected. Our results support the hypothesis that conditioning-dependent release of nitric oxide can contribute to long-term potentiation, but also show that its blockade by nitric oxide synthase inhibitors is dependent on the nature of the conditioning stimulus, and that long-term potentiation can be generated that is apparently resistant to the effects of these drugs.

Failure to induce long-term depression by an anti-correlation procedure in area CA1 of the rat hippocampal slice

We have independently in our two laboratories re-examined the report by Stanton and Sejnowski (Nature, 339, 215-218, 1989) that single stimuli to a test pathway in area CA1 of the hippocampal slice, when delivered between short bursts of stimuli to a second, convergent pathway, produce an associative long-term depression (LTD) in the test pathway. While robust associative LTP was observed when stimuli to the two inputs were correlated in time, the anti-correlation procedure failed to induce LTD; rather, a trend towards potentiation was observed. This result was obtained using both submerged and interface chambers, and in two different strains of rat. A transient depression lasting for a few minutes could usually be elicited by strong tetanic stimulation; this depression was not restricted to activated pathways.

An essential role for protein phosphatases in hippocampal long-term depression

The effectiveness of long-term potentiation (LTP) as a mechanism for information storage would be severely limited if processes that decrease synaptic strength did not also exist. In area CA1 of the rat hippocampus, prolonged periods of low-frequency afferent stimulation elicit a long-term depression (LTD) that is specific to the stimulated input. The induction of LTD was blocked by the extracellular application of okadaic acid or calyculin A, two inhibitors of protein phosphatases 1 and 2A. The loading of CA1 cells with microcystin LR, a membrane-impermeable protein phosphatase inhibitor, or calmodulin antagonists also blocked or attenuated LTD. The application of calyculin A after the induction of LTD reversed the synaptic depression, suggesting that phosphatase activity is required for the maintenance of LTD. These findings indicate that the synaptic activation of protein phosphatases plays an important role in the regulation of synaptic transmission.

Metabotropic glutamate receptors contribute to the induction of long-term depression in the CA1 region of the hippocampus

Long-term depression of synaptic transmission was induced following the prior induction of long-term potentiation in the CA1 region of rat hippocampal slices. We show that the induction of this form of synaptic depression can be prevented by (+)-alpha-methyl-4-carboxyphenylglycine, a selective antagonist of metabotropic glutamate receptors.

Reversal of LTP by theta frequency stimulation

Reversal of long-term potentiation (LTP) by physiological stimulation was tested in the CA1 field of hippocampal slices. In control medium, a one minute episode of 5 Hz (theta frequency) stimulation beginning 1-3 min after LTP had no effect on the degree of potentiation measured 30 min later. However, in the presence of norepinephrine (200 microM), 5 Hz stimulation reduced LTP by about 30%. Theta frequency stimulation was only effective when administered within 10 min of LTP induction and had no lasting effects on non-potentiated synapses. Stimulation at 1 Hz did not reverse LTP and stimulation at 10 Hz was no more effective than 5 Hz stimulation. LTP could be nearly completely reversed by theta frequency stimulation when potentiation was induced by milder and more naturalistic stimulation patterns. Under these conditions, LTP reversal was blocked by an antagonist of adenosine A1 receptors. These results suggest that the hippocampal theta rhythm promotes both the induction of LTP and its subsequent reversal with the latter process involving activation of adenosine receptors. Reversal of LTP may function to refine or sharpen recently encoded representations.

Mechanisms underlying induction of homosynaptic long-term depression in area CA1 of the hippocampus

The mechanisms responsible for long-lasting, activity-dependent decreases in synaptic efficacy are not well understood. We have examined the initial steps required for the induction of long-term depression (LTD) in CA1 pyramidal cells by repetitive low frequency (1 Hz) synaptic stimulation. This form of LTD was synapse specific, was saturable, and required activation of post-synaptic NMDA receptors. Loading CA1 cells with the Ca2+ chelator BAPTA prevented LTD, whereas lowering extracellular Ca2+ resulted in the induction of LTD by stimulation that previously elicited long-term potentiation. Following LTD, synaptic strength could be increased to its original maximal level, indicating that LTD is reversible and not due to deterioration of individual synapses. Induction of homosynaptic LTD therefore requires an NMDA receptor-dependent change in postsynaptic Ca2+ which may be distinct from that required for long-term potentiation.

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.

Quantal analysis of long-term potentiation of "minimal" excitatory postsynaptic potentials in guinea pig hippocampal slices: binomial approach

"Minimal" excitatory postsynaptic potentials (EPSPs) were recorded from 13 neurones in area CA1 of guinea pig hippocampal slices after double-pulse stimulation of stratum radiatum (str. rad.) and stratum oriens (str. or.). Amplitudes of EPSPs significantly increased in 8 neurones 5 to 55 min after 9 tetanizations in str. rad.. The increase was considered to represent long-term potentiation (LTP). Altogether 26 EPSPs (42 post-tetanic regions) were statistically analysed by four methods of the quantum hypothesis assuming the binomial model of transmitter release: the deconvolution (histogram), the variance, the failures, and the combined (variance-failures) methods. The mean quantal content (m) significantly increased after LTP induction according to all methods used. Quantal size (v) also tended to increase but according to some methods, the increase was not statistically significant and it did not correlate with LTP magnitude. However, for an EPSP subset with a LTP magnitude of less than 1.55, the increase in v correlated with LTP magnitude, whereas the increase in m did not. The relative contribution of the increase in v to LTP magnitude was larger for cases with small LTP than for the whole EPSP set. In general, the increase in m corresponds to previous studies and favours the presynaptic location of major mechanisms of LTP maintenance, i.e. an increase in the average number of transmitter quanta released by each presynaptic volley. The post-tetanic increase in v might reflect some additional mechanisms which presumably include an increase in the amount of transmitter in one quantum.

Quantal parameters of "minimal" excitatory postsynaptic potentials in guinea pig hippocampal slices: binomial approach

Binomial distributions of amplitudes of excitatory postsynaptic potentials (EPSPs) mixed with Gaussian noise were simulated. The objective of Monte Carlo simulations was, firstly, to study influences of sampling size (N) and noise standard deviation (Sn) on estimates of mean quantal content (m), quantal size (v) and binomial parameters (n and p) by four methods of quantal analysis (histogram, variance, failures and combined method) based on the binomial model and, secondly, to modify these methods on the basis of comparison of estimated with simulated parameters. Reliable estimates (within +/- 10% of the simulated values) were obtained for large sample sizes (N = 500-1000) with Sn less than or equal to v by the histogram (deconvolution) method and with Sn less than or equal to 2v by the other three methods. Similar results were obtained by averages from about 10 simulations if smaller samples were used (N = 50-200). In electrophysiological experiments on slices, "minimal" EPSPs were recorded from CA1 pyramidal cells after low-intensity stimuli to stratum radiatum or stratum oriens. Amplitudes of minimal EPSPs fluctuated in a manner predicted by the quantum hypothesis. Amplitude distributions of EPSPs in the non-facilitated state were adequately described either by binomial statistics with an average p equal to about 0.4 (a range of 0.3-0.7) and an average n of about 3 (range 2-6) or by Poisson statistics with m of about 1. The quantal analysis suggests that typical values of m and v for a single activated fibre in stratum radiatum might be about 0.5-1 and 300-400 microV, respectively, with low p (0.1-0.3) and n (2-4). However, the estimates of binomial parameters should be considered as coarse approximations in view of the simulation results and a possible nonuniformity of parameter p. The comparison of results of various methods based on the binomial model, in both simulation and physiological experiments, indicates the reliability of estimates of basic quantal parameters (m and v) under realistic conditions of physiological experiments. The methods are considered to be sufficiently sensitive to make use of them for studies on mechanisms of long-term synaptic plasticity.

Distributed changes in rat brain DNA synthesis with long-term habituation and potentiation of the perforant path-granule cell synapse

The involvement of brain deoxyribonucleic acid (DNA) synthesis in adaptive neural events was studied in the adult rat during long-term habituation (LTH) or potentiation (LTP) of the perforant path-granule cell synapse. Male Long-Evans rats were given 50 muCi [3H]thymidine intraventricularly under urethane anesthesia. Soon thereafter, field excitatory postsynaptic potential (EPSP) slope and population spike were monitored from the right dentate gyrus before and at various times (5, 10, 15, 60 min) following the delivery to the ipsilateral perforant bundle of a low frequency (LFS: 1.0 Hz, 160 s) or a high-frequency train (HFS: 400 Hz, 200 ms), repeated once after 5 min. Unstimulated implanted rats served as controls. DNA synthesis was evaluated by the incorporation of the radioactive precursor into DNA of several brain areas at the end of a 1 h incorporation period. In CA1, LTH and LTP increased DNA synthesis by 30% on the stimulated side. In the entorhinal cortex, LTH but not LTP increased DNA synthesis (by 30%) on the stimulated side. Conversely, in the frontal cortex, LTP but not LTH increased DNA synthesis (by 100%) on both sides. Long-lasting changes in synaptic efficacy covaried non-linearly with DNA synthesis in mono- and polysynaptically stimulated hippocampal regions, and in functionally associated neocortical areas. The co-variations of population spike amplitude were positive for LTH and negative for LTP in the dentate gyrus and frontal cortex of both sides, and in CA3/CA1 of the stimulated side, indicating higher DNA synthesis at lower values of LTH and LTP, and viceversa. Further, regional cross-correlation analyses revealed a high degree of synchronization among brain sites, following low- or high-frequency train pulses, indicating that (i) extra-target sites participate on the stimulated and on the contralateral side, and (ii) small distributed changes take place across the sampled neural networks. A modulatory role of information flow on brain DNA synthesis is inferred to take place in a diffuse, distributed manner.

Reversal of long-term potentiation (depotentiation) induced by tetanus stimulation of the input to CA1 neurons of guinea pig hippocampal slices

The reduction of the long-term potentiated response induced by tetanus (depotentiation (DP) of LTP) was investigated by the delivery of a train of low-frequency afferent stimuli (depotentiating stimulation: DPS) after the tetanus (100 Hz, 100 pulses) in CA1 neurons of the guinea pig's hippocampal slice. The parameters of DPS (frequencies of 1, 2, 5 and 10 Hz; number of pulses of 200 and 1000; and the time-lag after tetanus of 20 and 100 min) were altered systematically and their effects on LTP were evaluated through the analysis of the slope of field EPSP (S-EPSP) and amplitude and peak latency of population spike (A- and L-PS). DPS of 1 Hz, 1000 pulses, given 20 min after tetanus, reduced the potentiated component of S-EPSP, A-PS and L-PS by 68.5%, 80.1% and 56.1%, respectively (mean, n = 6), whereas it reduced the control response by 4.3%, 7.1%, and 1.9%, respectively (n = 6). Significantly less effectiveness was observed for DPS at higher frequencies (2-10 Hz), with smaller numbers of pulses, featuring a longer time-lag after tetanus and under APV administration. When DPS was applied before tetanus, significantly less robust LTP was observed. However, these effects were blocked by the administration of APV during DPS.

Stable depression of potentiated synaptic responses in the hippocampus with 1-5 Hz stimulation

Adult rats with two chronic stimulating electrodes in the Schaffer collateral/commissural system of the hippocampus and one recording electrode in the stratum radiatum (apical dendrites) of field CA1 were administered high-frequency stimulation (10 brief bursts at theta frequency) to produce long-term potentiation (LTP). 'Low frequency' stimulation (100 pulses at 1 Hz alone or followed by 250 pulses at 5 Hz) delivered 5-15 min later had no effect on LTP in 18% of the rats, caused a transient reversal in 18% of the group, but produced an apparent reversal of LTP for the remainder of a 1 h test session in 64% of the animals. LTP did not recover in animals tested 24 h later, at which point a second episode of high-frequency stimulation but without subsequent low-frequency stimulation was administered. This produced an LTP effect that persisted for a 1 h test session in 94% of the cases and that was still present in 86% of the animals tested 24 h later. Low-frequency stimulation applied prior to induction of LTP had no lasting effects on evoked responses not did it affect responses to a control stimulating electrode in those cases in which it reversed LTP. Possible implications of these results for hypotheses concerning the substrates of LTP and mechanisms of forgetting are discussed.

Eye movement desensitization: a new treatment for post-traumatic stress disorder

The use of saccadic eye movements for treating post-traumatic stress disorder is described. The procedure involves eliciting from clients sequences of large-magnitude, rhythmic saccadic eye movements while holding in mind the most salient aspect of a traumatic memory. This results in (1) a lasting reduction of anxiety, (2) changes in the cognitive assessment of the memory, and (3) cessation of flashbacks, intrusive thoughts, and sleep disturbances. The procedure can be extremely effective in only one session, as indicated by a previous controlled study and a case history presented here. It does not require a hierarchical approach, as in desensitization, or the elicitation of disturbingly high levels of anxiety over a prolonged period of time, as in flooding. Some speculations are offered concerning the basis for the effectiveness of the procedure.

Habituation of monosynaptic field potentials in the dentate gyrus of freely moving rats interferes with LTP

In 30 male Wistar rats, monosynaptically evoked field potentials in the dentate gyrus were registered under different conditions of repetitive stimulation and the long-term changes in synaptic transmission studied. Low-frequency stimulation induces a habituation-like decrement of the field-EPSP, but not of the population spike which interferes with the maintenance of LTP when administered after tetanic stimulation. The habituation-like response decrement, however, does not influence paired pulse-depression seen with an interstimulus interval of 20.0 ms. The paired pulse-plasticity, on the other hand, can be influenced by the tetanic stimulation (diminution of facilitation or depression), being dependent on the intensity of test stimuli. The results can be interpreted in terms of a complex influence of low-frequency- and tetanic stimulation on the elements of the dentate local circuitry and point to the necessity of considering long-term plastic changes in the investigation of substance- or stimulation-induced deviations of neuronal responses.

Induction of long-term depression and potentiation by low- and high-frequency stimulation in the dentate area of the anesthetized rat: magnitude, time course and EEG

We investigated the possible importance of stimulus train frequency for the induction and magnitude of long-term synaptic plasticity in the perforant path-granule cell pathway. Under the same experimental conditions, low- (15 Hz) or high-frequency (400 Hz) stimulation could elicit a profound long-term depression (LTD), or typical long-term potentiation (LTP), of the population spike amplitude, excitatory postsynaptic potential (EPSP) amplitude and spike onset latency. In addition, changes in the relationship between the EPSP and population spike amplitude indicated that granule cell excitability was enhanced during LTP and reduced during LTD. LTD occurred primarily after low-frequency stimulation (5 of 6 cases), and was always accompanied by striking changes in the EEG, most notably a biphasic slow potential. While the EEG changes were confined to the first 5 min after the tetanus, LTD lasted from 1 to 4 h. The nature of the EEG events is still unclear, it is suggested that they may represent a spreading depression-like episode. Finally, we found that LTP evoked by high-frequency stimulation was larger and generally reached peak magnitude faster than when it followed low-frequency stimulation. A possible mechanism and role for hippocampal LTD is proposed.

Tetanic stimulation-induced changes in [3H]glutamate binding and uptake in rat hippocampus

Tetanus-induced (400 Hz, 200 pulses) long-lasting potentiation of the stratum radiatum-evoked CA1 population spike in hippocampal slices is not accompanied by any change in Na+-independent [3H]glutamate binding sites. Homosynaptic depression that occurs subsequent to either a low frequency tetanus (20 Hz, 600 pulses) or a transient exposure to Cl(-)-free (containing NO3-) medium is associated with an elevation in the amino acid binding. [3H]Glutamate uptake into slices was decreased following a high frequency (400 Hz, 200 pulses) tetanus but in the majority of cases was increased following a low frequency (20 Hz, 600 pulses) tetanus to stratum radiatum. When the high frequency tetanus was given in the absence of extracellular Ca2+, there was a further reduction in [3H]glutamate uptake.

The duration of long-term potentiation in the CA1 region of the hippocampal slice preparation

The duration of long-term potentiation (LTP) of the monosynaptic excitatory Schaffer collateral-commissural input to hippocampal neurons of the CA1 region was examined in the in vitro slice. Relatively stable evoked potentials were obtained under conventional perfusion conditions at least for 10 hours. Tetanic stimulation (100 Hz, 1 sec) increased the population spike (pop-spike) amplitude by about 150% and the slope of the field-EPSP by about 30% over the pre-LTP baseline, whereas the latency and peak latency of the pop-spike decreased. In comparison to control experiments (same number of stimuli at 0.2 Hz) the differences were statistically significant for 2 hr (field-EPSP) and for greater than or equal to 10 hr (pop-spike), respectively. Repeated tetanization (3 X 100 Hz/1 sec), however, substantially prolongs EPSP-LTP (greater than or equal to 10 hr) and doubles the approximated half-life of pop-spike LTP. The threshold current intensity to elicit pop-spike responses decreased after the induction of LTP. Furthermore, the smaller field-EPSP values necessary to evoke near-threshold pop-spikes demonstrate an E-S potentiation (left-shift) at least in the low-intensity range. While the total duration of potentiation of the different parameters has not been determined, all the above mentioned effects could be observed at least 10 hr following the repeated tetanization. It is proposed that the slice preparation is suitable for the investigation of mechanisms of a postulated late phase of LTP if appropriate conditions are used.

An analysis of the increase in granule cell excitability accompanying habituation in the dentate gyrus of the anesthetized rat

Repeated low-frequency stimulation of the perforant path results in a decrement in the population EPSP and population spike recorded in the hilus of the dentate gyrus. The EPSP decrement is accompanied, however, by an increase in the population spike height/population EPSP slope relation, suggesting that an increase in granule cell excitability also occurs. The present experiments explored the mechanisms of this apparent increase in excitability using standard field potential recording techniques to assess perforant path input/output curves in rats anesthetized with sodium pentobarbital. Low-frequency homosynaptic stimulation (512 pulses, 1 Hz) of the perforant path resulted in a decreased spike threshold and overall shift to the left of the function relating population spike height to EPSP slope. These changes were consistently produced, even when granule cell discharge was inhibited by conditioning stimulation of the contralateral hilus. On the other hand, low-frequency heterosynaptic (lateral perforant path) or antidromic (mossy fiber) driving of the granule cells only slightly increased the medial path spike/EPSP relation, and did not alter the spike threshold. The excitability shift accompanying habituation was qualitatively different from that associated with long-term potentiation, but these shifts did not summate. The interpretation which best explains these various results is that granule cell excitability is increased during low-frequency perforant path stimulation by a process of disinhibition, caused by habituation of perforant path excitatory synaptic drive onto feed-forward inhibitory interneurons.

Long-lasting potentiation in hippocampus is not due to an increase in glutamate receptors

In transversely sectioned rat hippocampal slices, the effects of low (20 Hz, 600 pulses) and high (400 Hz, 200 pulses) frequency tetani of Schaffer collaterals were examined on the CA1 population spike as well as on the binding of 3H-glutamate. The population spike was suppressed while 3H-glutamate binding greatly enhanced following a low frequency tetanus. Verapamil (1 micron), which does not block long-lasting potentiation (LLP), counteracted the depression of the population spike as well as the associated increase in 3H-glutamate binding. The high frequency tetanus induced LLP of the population spike but caused no change in the amino acid binding. These results indicate that the increase in the number of glutamate receptors is not a requirement for LLP.

Long-term potentiation as a candidate mnemonic device

One approach to studying the neurophysiological correlates of long-term memory is to search for, and study properties of the nervous system that impart plasticity of synaptic efficacy. Within this context, we argue that long-term potentiation is, currently, the most plausible device for subserving or initiating long-term information storage in the mammalian brain. This argument is derived from examining features of LTP with respect to the constraints posed by our current concepts of learning and memory. In addition, we examine evidence for a role by LTP in behavioral learning. We conclude that studying LTP within the context of behavioral learning and memory may provide new insights into the neurophysiological bases of learning and memory.

Long-term potentiation in the hippocampus

Long-term potentiation of field and single neuronal responses recorded in various hippocampal fields is described on the basis of author's and literary data. Most of intrahippocampal and extrinsic connections in both in vivo and in vitro hippocampal preparations show this phenomenon after one or several conditioning trains of comparatively short duration (20 s or less) at various frequencies (from 10 to 400 Hz). Properties of hippocampal potentiation are described. The properties include long term persistence (hours and days) of the potentiated response, its low frequency depression, self-restoration after the depression, specificity of the potentiation for the tetanized pathway, necessity of activation of a sufficient number of neuronal elements ('cooperativity') to produce the potentiation, possible involvement of 'reinforcing' brain structures during conditioning tetanization. These properties are distinct from those of 'usual' short-term post-tetanic potentiation and lead to the suggestion that the neuronal mechanisms underlying long-term post-tetanic are similar to those underlying memory and behavioral-conditioned reflex. Neurophysiological mechanisms of long-term potentiation are discussed. The main mechanism consists in an increase in efficacy of excitatory synapses as shown by various methods including intracellular recording and quantal analysis. The latter favours presynaptic localization of changes of synaptic efficacy showing increase in the number of transmitter quanta released per presynaptic impulse. However, changes in the number of subsynaptic receptors or localized changes in dendritic postsynaptic membrane are not excluded. Biochemical studies indicate the increase in transmitter release and calcium-dependent phosphorylation of pyruvate dehydrogenase after tetanization. Instances of persistent response facilitations at other levels of the vertebrate central nervous system (especially at neocortical level) are considered and compared with hippocampal long-term potentiation. It is suggested that modifiable excitatory synapses necessary for learning have been identified in studies of long-term potentiation. These synapses are presumably modified as a result of close sequential activation of the following three structures: excitatory presynaptic fibers, the postsynaptic neuron and a 'reinforcing' brain system.