Patterned stimulation at the theta frequency is optimal for the induction of hippocampal long-term potentiation

The role of N-methyl-D-aspartate receptors in the generation of short-term potentiation in the rat hippocampus

The effects of stimulus intensity and the N-methyl-D-aspartate (NMDA) antagonist 2-amino-5-phosphonovalerate (AP5) were studied on the induction of short-term potentiation (STP) and long-term potentiation (LTP) in CA1 of the rat hippocampal slice. A tetanus of very weak intensity stimuli produced STP, and also LTP providing the stimuli were applied in the form of a series of high frequency trains rather than one continuous train. Increasing the intensity of the stimuli to just threshold for spike initiation produced larger amplitude STP and LTP. AP5 strongly inhibited the STP as well as the LTP produced by a series of high frequency trains, indicating a large component of this STP was generated by activation of NMDA receptors. A further residual component of STP in AP5, which was associated with a decrease in paired pulse facilitation, is probably generated by a presynaptic increase in the probability of transmitter release.

Intraseptal administration of muscimol produces dose-dependent memory impairments in the rat

The present study examined the effects of intraseptal administration of the GABAergic agonist muscimol on performance of a radial-arm maze (RAM) task. Male Long-Evans rats were trained to perform a RAM task in which a 1-h delay was imposed between the sample and the test session. In this task rats have access to four out of eight maze arms during a predelay session. Following a 1-h delay, rats are returned to the maze and allowed to freely choose among all eight arms. Arms not blocked during the predelay session are baited, and entry into an arm chosen during the predelay session or a repeated entry into a postdelay chosen arm constitutes an error. Following acquisition, animals were implanted with a single cannula aimed at the medial septum. A within-subjects design was utilized to examine the effects of intraseptal administration of muscimol (0.0, 0.75, 1.5 or 3.0 nmol) on performance in this task. All drugs or artificial cerebrospinal fluid were administered immediately following the predelay session. Muscimol, a GABA-A agonist, produced a dose-dependent impairment in maze performance as evidenced by fewer correct choices in the first four postdelay choices and an increase in the number of errors. Intraseptal administration of muscimol did not significantly alter latency per choice on the RAM task nor did it affect locomotor activity levels. Muscimol-induced impairments were also observed when a 4-h delay was imposed between the fourth and the fifth maze selection, suggesting that the behavioral deficit represents an inability to store or retain spatial working memories rather than a general performance deficit. These data indicated that pharmacological manipulation of GABA-A receptors within the medial septum modifies working memory processes. The potential interaction of GABAergic and cholinergic mechanisms in the modulation of working memory processes is discussed.

The neurobiology of learning and memory

The study of memory is a great challenge, perhaps the greatest in biological sciences. Memory involves changes in a tiny fraction of an extremely large pool of elements, a conclusion that makes the task of finding those changes using current technologies formidable. What can be done about this roadblock to neurological investigations of learning? One response that has become particularly productive in recent years is to study learning or learning-like phenomena in relatively simple "model" systems. The idea is to extract basic principles from these models in which molecular and anatomical details can be studied and then to use these in analyzing learning in higher regions of the brain. In this article we discuss current progress and emerging concepts derived from the simple system approach using animal models.

Developmental changes in synaptic properties in hippocampus of neonatal rats

The properties of synaptic responses in area CA1 of hippocampus were analyzed in slices prepared from 7-9 and 12-15 day old neonate rats. As expected from earlier work, only slices of two-week-old animals showed a consistent degree of long-term potentiation (LTP) in response to patterned high frequency stimulation. Several other synaptic properties were found to change during this developmental period. Inhibitory responses were absent in 7-9 day old but not in 12-15 day old neonates. Paired-pulse facilitation and the calcium sensitivity of postsynaptic responses were considerably reduced in 7-9 as compared to 12-15 day old rats. However, phorbol esters and 4-aminopyridine treatment still produced a strong facilitation of field potentials. The N-methyl-D-aspartate (NMDA) component of responses to single pulse stimulation in low magnesium medium was found to be larger in slices of 7-9 than 12-15 day old or adult animals. At the two time periods examined, trains of high frequency stimulation applied in the presence of regular magnesium elicited an NMDA dependent response. It is concluded that the differences in synaptic properties observed between 7-9 and 12-15 day old neonates may not account for the absence of LTP in the younger animals.

Onset Characteristics of Long-Term Potentiation in the Guinea-Pig Hippocampal CA1 Region in Vitro

The temporal development of long-term potentiation (LTP) was examined in the CA1 region of the hippocampal slice preparation (bath temperature 30 degrees C). LTP was evoked by a single brief afferent tetanus (3 - 40 impulses at 50 Hz) given in the presence of picrotoxin (to facilitate LTP induction). Short-lasting potentiation processes unrelated to LTP were excluded by comparing the potentiation obtained in picrotoxin solution with that obtained in normal solution or in the presence of the N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonovalerate. LTP was also evoked by pairing single test volleys with brief (2 - 3 impulses) heterosynaptic tetani in picrotoxin solution. Both methods showed no significant rise of LTP until about 3 s after the induction event. LTP thereafter developed almost linearly towards a peak within 20 - 25 s after the tetanus, the time course being practially independent of the induction method and of the relative amount of LTP evoked. The latency and rise time of LTP depended on bath temperature, being about twice as long at 25 degrees C as at 30 degrees C. Following the peak, LTP rapidly decayed to less than half its peak value in 8 min, the decay tending to be less with longer trains. The LTP component reaching its peak 20 - 25 s after a tetanus was practically occluded after a saturating homosynaptic tetanization, and was only partially recovered 1 h afterwards. The latency to the onset of LTP suggests an indirect coupling between the calcium influx, presumed to trigger the potentiation, and the expression of LTP. The independence of the early time course with respect to the induction strength indicates that the intervening system(s) operates in a linear manner.

Theta pattern stimulation and the induction of LTP: the sequence in which synapses are stimulated determines the degree to which they potentiate

Induction of long-term potentiation (LTP) by asynchronous stimulation of converging afferents was studied in hippocampal slices. Three stimulation electrodes were positioned to activate separate groups of Schaffer-commissural inputs to a population of CA1 pyramidal cells. Patterned stimulation consisted of a single coincident priming pulse to all 3 electrodes followed by a burst of 4 pulses (100 Hz) to the first input (S1) at a delay of 180 ms, to the second (S2) at a delay of 200 ms, and to the third (S3) at a delay of 220 ms. This pattern was repeated 10 times at 5-s intervals. The magnitude of LTP induced (measured 20 min after stimulation) was greatest for the first stimulated input, intermediate for the second, and least for the third. Intracellular recordings indicated that the greatest postsynaptic depolarization occurred during the period of S2 stimulation; thus the magnitude of LTP induced was not simply dependent on the degree of depolarization during afferent activation. Rather, sustained depolarization after synaptic activation could contribute to LTP induction by prolonging the activity of N-methyl-D-aspartate receptor-gated channels. Earlier-arriving bursts may also trigger an inhibitory process that reduces the effectiveness of later bursts for inducing LTP.

Activity-induced depression of synaptic inhibition during LTP-inducing patterned stimulation

In the hippocampus, patterns of electrical stimulation that approximate bursting neuronal activity during theta rhythm have been shown to induce a long-term potentiation (LTP) of excitatory synapses. In this study, a single subthreshold stimulus applied to one set of Schaffer/commissural fibers affected the response to a second stimulation delivered 200 ms later to a separate set of Schaffer/commissural fibers in the CA1 field of rat hippocampal slices. The first (priming) stimulus caused a prolongation of the synaptic response elicited by the second (primed) stimulus. In addition, the priming stimulation facilitated the induction of LTP by bursts of stimulation (4 pulses at 100 Hz) of the second afferent pathway. Analysis of the shape of the synaptic responses indicates that the prolongation is due to the removal of an inhibitory component rather than the addition of a novel excitatory component. Blockade of GABAA-ergic transmission with picrotoxin mimicked the priming effect in that it also widened synaptic responses and facilitated burst-induced LTP. We suggest that these patterns of stimulation result in a transient loss of inhibition during the primed stimulation. This, in turn, brings about a prolongation of the synaptic response that allows short bursts of excitatory synaptic activity to depolarize postsynaptic cells sufficiently to trigger LTP.

Long-term potentiation: studies in the hippocampal slice

Long-term potentiation (LTP) is an example of activity-dependent plasticity that was discovered in the hippocampal formation. There is growing evidence that LTP not only is a useful model for mnemonic processes, but also may represent the cellular substrate for at least some kinds of learning and memory. The hippocampal slice preparation has proven exceptionally useful in pharmacological studies of possible mechanisms of LTP. A slice remains viable and stable for several hours, and known concentrations of drugs in the bathing medium can be added and then washed out. Drugs can also be applied under visual guidance from micropipettes to discrete neuronal regions, an accomplishment that is aided by the lamellar organization of the hippocampus. Electrical stimulation of the perforant path (PP) in the molecular layer of the dentate gyrus produces a monosynaptic excitatory postsynaptic potential (EPSP) and action potential, which can be recorded extracellularly as a population EPSP and population spike, respectively. Presentation of a high-frequency train (HFT; 100 Hz X 1 s) to the PP results in a long-lasting (greater than 30 min) potentiation of the maximal EPSP slope and of the population spike amplitude. Similarly, exposure of the slice to norepinephrine (e.g. 20 microM for 30 min) results in a long-lasting potentiation (LLP) of both EPSP and population spike (Stanton and Sarvey (1987) Brain Res. Bull., 18: 115). No such LLP was seen in field CA1 following NE application (Stanton and Sarvey (1985) Brain Res., 361: 276). beta-Adrenergic antagonists, such as propranolol, inhibit both LTP and NE-induced LLP in dentate (Stanton and Sarvey, J. Neurosci., 5: 2169 (1985); Stanton and Sarvey (1985) Brain Res., 361: 276). Cyclic AMP levels are increased by either an HFT or NE (Stanton and Sarvey (1985) Brain Res., 358: 343). Thus, NE, acting through a beta-receptor, appears to be both necessary and sufficient to produce long-lasting enhancement of synaptic responses. Finally, inhibitors of protein synthesis, such as emetine, also block both LTP and NE-induced LLP (Stanton and Sarvey, J. Neurosci., (1984) 4: 3080; Stanton and Sarvey (1985) Brain Res., 361: 276). The N-methyl-D-aspartate (NMDA) excitatory amino acid receptor subtype appears to play a role in a number of forms of neuronal plasticity. Bath-application of a 1 microM concentration of the NMDA antagonists D-2-amino-5-phosphonavaleric acid (AVP) or 3-((+/-)2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) blocked both LTP and NE-induced LLP in the dentate gyrus.(ABSTRACT TRUNCATED AT 400 WORDS)

The NMDA receptor-mediated components of responses evoked by patterned stimulation are not increased by long-term potentiation

The participation of N-methyl-D-aspartate (NMDA) receptors in synaptic transmission before and after induction of long-term potentiation (LTP) was studied in field CA1 of hippocampal slices. NMDA receptor-mediated postsynaptic responses were determined by comparing responses recorded in the presence and absence of the selective antagonist, D-2-amino-5-phosphonopentanoate (D-AP5, 50 microM). In the presence of physiological magnesium concentrations (1 mM), robust D-AP5-sensitive responses could be evoked by high frequency bursts (4 pulses, 100 Hz) when burst stimulation was preceded 200 ms earlier by 'priming' stimulation (2 pulses, 15 ms apart) of a separate input. Induction of LTP resulted in a substantial potentiation (35%) of non-NMDA-mediated responses to primed bursts but not of NMDA-mediated responses. These results suggest that long-term postsynaptic modifications are at least partly responsible for the expression of LTP.

Long-lasting physiological effects of bath applied N-methyl-D-aspartate

The present experiments describe a long-lasting form of potentiation induced in field CA1 of rat hippocampal slices by bath application of N-methyl-D-aspartate (NMDA), in association with low magnesium concentrations, glycine and spermine. The potentiation effect consisted of a 50% increase in slope of field potentials and was stable for at least 80 min post treatment. It was not accompanied by detectable changes in antidromic responses and was completely blocked by an antagonist of NMDA receptor. The possible relationship of pharmacologically induced potentiation to long-term potentiation (LTP) is discussed.

The development of long-term potentiation in hippocampus and neocortex

The development of long-term potentiation (LTP), an enduring alteration in synaptic efficacy following afferent activation, was examined in CA1 hippocampus and primary visual cortex of rat. Both regions show little LTP prior to postnatal day 5, demonstrate a maximal potentiated response around postnatal day 15, and a subsequent decline to adult levels. These results are discussed with respect to the underlying mechanism of action and behavioral significance of these critical-period phenomena.

Linear relation between the magnitude of long-term potentiation in the dentate gyrus and associative learning in the rat. A demonstration using commissural inhibition and local infusion of an N-methyl-D-aspartate receptor antagonist

Field potentials were recorded in the dentate gyrus of freely-moving rats in a classical conditioning paradigm in which high-frequency stimulation of the perforant path served as a conditioned stimulus. Paired or unpaired perforant path stimulus-footshock presentations were given to animals engaged in a previously acquired food-motivated lever-pressing task. Conditioned suppression of lever-pressing was the behavioural measure of conditioning. Perforant path stimulus trains at an intensity above spike threshold induced long-term potentiation of synaptic transmission in the dentate gyrus. In this condition, animals learned the perforant path stimulus-shock association. Three strategies were employed to block the induction or reduce the magnitude of long-term potentiation induced by the conditioned stimulus: (1) reduction of the intensity of the stimulus below the spike threshold resulted in no long-term potentiation and a failure by the animals to learn the perforant path stimulus-shock association; (2) inhibitory modulation of long-term potentiation by high-frequency activation of commissural input to the dentate gyrus resulted in learning deficits; (3) chronic infusion of DL-2-amino-5-phosphonovalerate, a selective antagonist of the N-methyl-D-aspartate subtype of glutamate receptor, blocked the induction of long-term potentiation and prevented associative learning. A highly significant linear relation emerged from a correlational analysis between the magnitude of the change in synaptic efficacy at the activated synapses and the amount the animals learned about the perforant path stimulus-shock association. The results presented in this paper are consistent with the hypothesis that associative learning depends on the development of lasting changes in synaptic function. We propose that the activation of N-methyl-D-aspartate receptors in the dentate gyrus is involved in this process and that the more change in synaptic efficacy is produced in the activated network, the more the animals learn.

GABA-containing neurons in the septum control inhibitory interneurons in the hippocampus

The hippocampus, in particular the neocortex-hippocampus-neocortex circuit, is widely believed to be crucial in memory. Information flow in this circuit is strongly influenced by relatively sparse afferents derived from subcortical centres, such as the septum, involved in arousal, emotions and autonomic control. A powerful mechanism, by which numerically small inputs can produce profound effects, is feed-forward inhibition, that is, the activation of local inhibitory interneurons, which, in turn, control the activity of large populations of principal cells in the hippocampus. An example is the cholinergic input to the hippocampus from the septum, which is likely to be involved in feed-forward operations. Here, we demonstrate the existence of a circuit underlying another powerful mechanism of subcortical control of hippocampal information processing. We show that GABA-containing afferents originating in the septum innervate most of the GABA-containing interneurons in the hippocampus, making many synaptic contacts with each of them. Activation of the GABA-containing neurons in the septum is likely to lead to disinhibition of the principal neurons in the hippocampal formation and so this pathway is probably crucial in the induction of hippocampal electrical activity patterns, and may be involved in NMDA (N-methyl-D-aspartate) receptor-mediated functions, such as memory, in a permissive manner.

Long-term potentiation: persisting problems and recent results

In this paper we discuss recent experimental results pertinent to three unresolved issues regarding the long-term potentiation (LTP) effect: the nature of its enduring substrates, the biochemical mechanisms that produce it, and its potential role in memory. LTP appears to be triggered by a postsynaptic influx of calcium and is associated with alterations in the shape of dendritic spines and probably the formation of new synapses. We discuss the possibility that morphological reorganization also modifies membrane surface chemistry of synaptic elements. Evidence is presented that LTP is not associated with changes in presynaptic calcium currents. Activation of protein kinase C is shown to be insufficient for the induction of LTP, although it may play a modulatory role. The hypothesis that activation of a calcium-sensitive protease (calpain) is pivotal to the establishment of LTP is supported by experiments showing that a calpain inhibitor, leupeptin, blocks LTP. Furthermore, activation of NMDA receptors, an event implicated in LTP induction, is accompanied by calcium-sensitive proteolysis of spectrin, a major dendritic cytoskeletal protein. The finding that stimulation patterns designed to mimic naturally-occurring cell discharge patterns are highly effective for LTP induction greatly strengthens the hypothesis that LTP actually occurs during the encoding of information in cortical systems. Potential contributions of LTP to learning are explored using computer simulations of a simple cortical network.

Chronic administration of a thiol-proteinase inhibitor blocks long-term potentiation of synaptic responses

It has been proposed that activation of a calcium-sensitive protease (calpain) is a crucial step in the induction of long-term potentiation (LTP). To test this hypothesis, we used chronic recording techniques to measure the effects of intraventricular infusion of leupeptin, a calpain inhibitor, on LTP in the hippocampus. Rats implanted bilaterally with stimulating electrodes in the Schaffer-commissural system and one recording electrode in the apical dendrites of field CA1 were fitted with osmotic mini-pumps delivering either leupeptin (20 mg/ml) or saline at a rate of 0.5 microliter/h into the lateral ventricle. Short bursts of high-frequency stimulation with the bursts delivered at 5/s were used to induce LTP in those animals which had stable responses for several days. Rats in the saline group (n = 11) exhibited an immediate LTP effect that remained in place over successive days of testing, while only 3 of 13 leupeptin treated animals showed evidence of LTP 24 h after high-frequency stimulation, and in only one of those was a sizeable effect recorded over several days. The average change in responses at the 24-h test point was +33% for the controls and +4% for the leupeptin group (P less than 0.01). The block of LTP induction was reversible, since high-frequency stimulation applied after disconnecting the pumps led to a robust LTP effect that lasted for several days in 6 of 7 animals tested. There were no detectable differences in baseline responses in the presence and absence of leupeptin.

Role of N-methyl-D-aspartate receptors in the induction of synaptic potentiation by burst stimulation patterned after the hippocampal theta-rhythm

Short bursts of high frequency stimulation produce maximal long-term potentiation (LTP) at Schaffer-commissural synapses on CA1 neurons in hippocampal slices when the bursts are spaced 200 ms apart. A burst to one input (S1) does not induce LTP but 'primes' the postsynaptic neurons such that 200 ms later the postsynaptic response to a burst to a second input (S2) is greatly enhanced and LTP is induced. The role of N-methyl-D-aspartate (NMDA) receptors in this response enhancement and LTP induction was studied by perfusing slices with the NMDA antagonist, 2-amino-5-phosphonovalerate (AP5). AP5 (100 microM) had no effect on the field excitatory postsynaptic potential evoked by single pulse stimulation, but completely eliminated both the decremental short-term potentiation (lasting less than 10 min) and stable LTP effects elicited by burst stimulation. AP5 reduced the response to a non-primed burst by about 10% and reduced the relative enhancement of a primed burst response by about 35%. These results indicate that part of the postsynaptic response to a primed burst is mediated by NMDA receptors and that this component is necessary for all forms of synaptic potentiation (including LTP) resulting from burst stimulation. The similarity of the short bursts with the complex-spike discharges of hippocampal neurons as well as the 200 ms optimal interval with the period of the hippocampal theta-rhythm suggest links between theta and the NMDA receptor in the induction of hippocampal synaptic plasticity.

Long-term potentiation in the dentate gyrus is induced preferentially on the positive phase of theta-rhythm

Long-term potentiation (LTP), a long lasting enhancement of synaptic efficacy is considered a model for learning and memory. In anesthetized rats, theta-rhythm was induced in the dentate gyrus by midbrain stimulation. Short trains of pulses were applied to the perforant pathway either at the peak of theta-rhythm or its trough. Trains applied at the peak of theta-rhythm induced LTP while trains applied at the trough produced a decrease of synaptic efficacy or had no effect. Thus, theta-rhythm may play a modulating role in the induction of LTP, suggesting a possible mnemonic function for the rhythm during the behaviors in which it occurs.

Modification of primary afferent depolarization in cat group Ia afferents following high frequency intra-axonal tetanization of individual afferents

Intra-axonal tetanization of a single, functionally-identified group Ia afferent from the triceps surae muscle in the anesthetized cat produces marked enhancement and slowing of the primary afferent depolarization (PAD) generated in the Ia afferent by volleys in flexor muscle group Ia afferents, plus a pronounced transmembrane hyperpolarizing undershoot (HPU) which disappears more rapidly than the enhanced PAD. These alterations are qualitatively and quantitatively similar to those found after conditioning tetani are applied to the whole muscle nerve. The occurrence of these PAD changes after intra-axonal tetanization of a single group Ia afferent appears to rule out the participation of non-specific alterations in extracellular ionic concentrations or activation of polysynaptic pathways in their genesis.

Long-term potentiation in the dentate gyrus is preferentially induced at theta rhythm periodicity

In urethane-anesthetized rats, high frequency stimulation was applied to the medial perforant pathway at various time intervals (50, 100, 200, 350 and 500 ms) following stimulation of the same pathway by a single pulse of equal intensity. Recordings of dentate gyrus granule cell evoked responses were made to investigate the range of stimuli that are effective in inducing long-term potentiation (LTP). LTP was induced almost exclusively at the 200 ms interval, corresponding to the periodicity of the theta rhythm. Taken in conjunction with similar findings reported in the CA1 field of the hippocampal slice, these results suggest that the correlation between theta rhythm periodicity and LTP is a general phenomenon within the hippocampal formation and lends further support to the hypothesis that the naturally occurring theta rhythm may play a modulatory role in the induction of LTP.

Stable hippocampal long-term potentiation elicited by 'theta' pattern stimulation

The stability of long-term potentiation (LTP) elicited by a stimulation paradigm in which short high-frequency bursts of pulses were given in a 'theta' pattern (i.e. 5 bursts/s) was tested in a chronic recording study. Stimulation electrodes were implanted bilaterally in the Schaffer-commissural system while the recording electrode was placed in the apical dendritic field of the Ca1 zone of the hippocampus. Following 4 days of baseline testing, 'theta' stimulation was applied to one electrode for a total of 2 s (ten 30 ms bursts), after which testing was continued for 3 weeks or until the responses fell to below baseline levels. Data were collected from 25 animals and 3 types of results were obtained: (1) no LTP (n = 4), (2) LTP that decreased steadily from 24 h after high frequency stimulation onward (n = 4), and (3) LTP that was stable until recording was terminated or until the responses began a precipitous decrease to below baseline values (n = 17). The mean of the slopes of the curves relating degree of potentiation to days after 'theta' stimulation was less than 1%/day with a mean correlation coefficient of only 0.1 prior to the point at which the responses began their rapid decline. Control responses were unaffected by the induction of LTP in neighboring CA1 afferents and did not exhibit a reliable relationship with time. These results suggest that, for most rats, LTP elicited by theta pattern stimulation is stable until such time that stimulation-recording arrangements begin to deteriorate.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain spectrin, calpain and long-term changes in synaptic efficacy

This chapter discusses the possibility that proteolytic digestion of cytoskeletal proteins, in particular spectrin, is part of the mechanisms through which physiological activity elicits structural and chemical changes in brain synapses. Recent work from several laboratories has produced a description of the initial events that trigger the long-term potentiation (LTP) of synaptic responses that appears in hippocampus after brief episodes of high frequency electrical stimulation. A likely sequence is as follows: suppression of IPSPs, prolongation of EPSPs, activation of N-methyl-D-aspartate (NMDA) receptors, influx of calcium into target cells. After briefly describing the evidence for this triggering sequence, the review takes up the question of what types of calcium sensitive chemistries are available to synaptic region that could produce functional changes lasting for weeks (i.e., for LTP). It is argued that the partial degradation of spectrin by a calcium-activated protease (calpain) provides a mechanism of this type. Spectrin is a substrate for calpain and both it and a breakdown product comparable to that produced by calpain are found in postsynaptic densities. Moreover, there is substantial evidence that spectrin regulates the surface chemistry and morphology of cells and thus its partial degradation would be expected to produce pronounced and persistent modifications in synapses. To reinforce this point, the review discusses recent findings suggesting that calpain mediated proteolysis of spectrin and other cytoskeletal proteins produces substantial changes in the shape of blood-borne cells and the distribution of their surface receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Induction of synaptic potentiation in hippocampus by patterned stimulation involves two events

Electrical stimulation of axons in the hippocampus with short high-frequency bursts that resemble in vivo activity patterns produces stable potentiation of postsynaptic responses when the bursts occur at intervals of 200 milliseconds but not 2 seconds. When a burst was applied to one input and a second burst applied to a different input to the same target neuron 200 milliseconds later, only the synapses activated by the second burst showed stable potentiation. This effect was observed even when the two inputs innervated completely different regions of the postsynaptic cells; but did not occur when the inputs were stimulated simultaneously or when the second burst was delayed by 2 seconds. Intracellular recordings indicated that the first burst extended the decay phase of excitatory postsynaptic potentials evoked 200 milliseconds later. These results suggest that a single burst of axonal stimulation produces a transient, spatially diffuse "priming" effect that prolongs responses to subsequent bursts, and that these altered responses trigger spatially restricted synaptic modifications. The similarity of the temporal parameters of the priming effect and the theta rhythm that dominates the hippocampal electroencephalogram (EEG) during learning episodes suggests that this priming may be involved in behaviorally induced synaptic plasticity.