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

Increases in neuronal bursting recorded from the chick lobus parolfactorius after training are both time-dependent and memory-specific

Day-old-chicks can be trained in one trial to avoid a methylanthranilate-coated bead (methyl-chicks). The lobus parolfactorius of the chick forebrain is an important structure for memory of this avoidance response. To examine training-induced electrophysiological changes in this structure, spontaneous neuronal bursting activity was measured from the lobus parolfactorius of anaesthetized, day-old methyl- and water-chicks (the latter chicks trained to peck at a water-coated bead) over the period 1-10 h post-test. Bursting was significantly higher in methyl-chicks over this period. This post-test increase was time-dependent: bursting in methyl-chicks was significantly higher only during the period 4-7 h post-test. In a second experiment, methyl-chicks were subjected to brief, subconvulsive electroshock 5 min post-training. When tested 1 h later about half of these chicks showed recall (avoided the bead) and half were amnesic (pecked the bead). These chicks were anaesthetized and bursting was recorded from the lobus parolfactorius. Chicks that showed recall exhibited a significantly higher level of bursting over the period 1-10 h post-test when compared to chicks that were amnesic. The time course of bursting was similar to that seen in non-electroshocked methyl-chicks. These results suggest that passive avoidance training induces a memory-specific, time-dependent increase in neuronal activity within the lobus parolfactorius of day-old chicks. This increase may be directly associated with long-term consolidation of memory for the task.

(R,S)-alpha-methyl-4-carboxyphenylglycine (MCPG) blocks spatial learning in rats and long-term potentiation in the dentate gyrus in vivo

Recently, it was demonstrated by the use of the competitive and selective antagonist (R,S)-alpha-methyl-4-carboxyphenylglycine (MCPG) that metabotropic glutamate receptor (mGluR) activation is required to induce long-term potentiation (LTP) in the hippocampus. Accordingly, we investigated whether MCPG also inhibits spatial learning. Rats were trained on a spatial alternation task in a Y-maze with footshock reinforcement, and MCPG (0.0208 mg) was injected intracerebroventricularly prior to training and/or retention test. Animals injected pre-training are clearly impaired in retention, whereas preretention application was without effect. A state dependency could be excluded. Additionally, MCPG at the same concentration completely blocks a potentiation at perforant path/dentate gyrus synapses in vivo. These results strongly implicate a role of mGluRs in spatial learning and LTP.

Facilitation of glutamate receptors enhances memory

A benzamide drug that crosses the blood-brain barrier and facilitates DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated synaptic responses was tested for its effects on memory in three behavioral tasks. The compound reversibly increased the amplitude and prolonged the duration of field excitatory postsynaptic potentials in hippocampal slices and produced comparable effects in the dentgate gyrus in situ after intraperitoneal injections. Rats injected with the drug 30 min prior to being given a suboptimal number of training trials in a two-odor discrimination task were more likely than controls to select the correct odor in a retention test carried out 96 hr later. Evidence for improved memory was also obtained in a water maze task in which rats were given only four trials to find a submerged platform in the presence of spatial cues; animals injected with the drug 30 min before the training session were significantly faster than vehicle-injected controls in returning to the platform location when tested 24 hr after training. Finally, the drug produced positive effects in a radial maze test of short-term memory. Well trained rats were allowed to retrieve rewards from four arms of an eight-arm maze and then tested for reentry errors 8 hr later. The number of such errors was substantially reduced on days in which the animals were injected with the drug before initial learning. These results indicate that a drug that facilitates glutamatergic transmission enhances the encoding of memory across tasks involving different sensory cues and performance requirements. This may reflect an action on the cellular mechanisms responsible for producing synaptic changes since facilitation of AMPA receptors promotes the induction of the long-term potentiation effect.

The pharmacology and function of central GABAB receptors

In conclusion, GABAB receptors enable GABA to modulate neuronal function in a manner not possible through GABAA receptors alone. These receptors are present at both pre- and postsynaptic sites and can exert both inhibitory and disinhibitory effects. In particular, GABAB receptors are important in regulating NMDA receptor-mediated responses, including the induction of LTP. They also can regulate the filtering properties of neural networks, allowing peak transmission in the frequency range of theta rhythm. Finally, GABAB receptors are G protein-coupled to a variety of intracellular effector systems, and thereby have the potential to produce long-term changes in the state of neuronal activity, through actions such as protein phosphorylation. Although the majority of the effects of GABAB receptors have been reported in vitro, recent studies have also demonstrated that GABAB receptors exert electrophysiological actions in vivo. For example, GABAB receptor antagonists reduce the late IPSP in vivo and consequently can decrease inhibition of spontaneous neuronal firing following a stimulus (Lingenhöhl and Olpe, 1993). In addition, blockade of GABAB receptors can increase spontaneous activity of central neurons, suggesting the presence of GABAB receptor-mediated tonic inhibition (Andre et al., 1992; Lingenhöhl and Olpe, 1993). Despite these electrophysiological effects, antagonism of GABAB receptors has generally been reported to produce few behavioral actions. This lack of overt behavioral effects most likely reflects the modulatory nature of the receptor action. Nevertheless, two separate behavioral studies have recently reported an enhancement of cognitive performance in several different animal species following blockade of GABAB receptors (Mondadori et al., 1992; Carletti et al., 1993). Because of their small number of side effects, GABAB receptor antagonists may represent effective therapeutic tools for modulation of cognition. Alternatively, the lack of overt behavioral effects of GABAB receptors may indicate that these receptors are more important in pathologic rather than normal physiological states (Wojcik et al., 1989). For example, a change in receptor affinity or receptor number brought on by the pathology could enhance the effectiveness of GABAB receptors. Of significance, CGP 35348 has been shown to block absence seizures in genetically seizure prone animals, while inducing no seizures in control animals (Hosford et al., 1992; Liu et al., 1992). Thus, GABAB receptors may represent effective sites for pharmacological regulation of absence seizures. Perhaps further behavioral effects of these receptors will become apparent only after additional studies have been performed using the highly potent antagonists that have been recently introduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuronal transmission of hippocampal CA1 neurones is modulated by corticotropin-like intermediate lobe peptide [CLIP; ACTH(18-39)]

The study was conducted to test whether CLIP [ACTH(18-39)] influences the neuronal transmission and the induction of long-term potentiation (LTP) in the hippocampus. The population spike was recorded in the hippocampal CA1 region of freely moving rats before and after intracerebroventricular (ICV) administration of CLIP in comparison to ACTH and saline (controls). After infusion of CLIP, the population spike amplitude (PSA) rose to about 200% of baseline values. After reaching this level, it was impossible to induce a further increase of PSA by tetanization. However, if the stimulus intensity was reduced to a new baseline level, electrically induced LTP could be observed. There were no significant changes after infusion of ACTH. Our results indicate that the ICV administration of CLIP leads to an enhancement of excitability in the hippocampal CA1 region, which might be independent of LTP.

Structural synaptic correlate of long-term potentiation: formation of axospinous synapses with multiple, completely partitioned transmission zones

Synapses were analyzed in the middle molecular layer (MML) and inner molecular layer (IML) of the rat dentate gyrus following the induction of long-term potentiation (LTP) by high-frequency stimulation of the medial perforant path carried out on each of 4 consecutive days. Potentiated animals were sacrificed 1 hour after the fourth high frequency stimulation. Stimulated but not potentiated and implanted but not stimulated animals served as controls. Using the stereological disector technique, unbiased estimates of the number of synapses per postsynaptic neuron were differentially obtained for various subtypes of axospinous junctions: For atypical (giant) nonperforated synapses with a continuous postsynaptic density (PSD), and for perforated ones distinguished by (1) a fenestrated PSD and focal spine partition, (2) a horseshoe-shaped PSD and sectional spine partition, (3) a segmented PSD and complete spine partition(s), and (4) a fenestrated, (5) horseshoe-shaped, or (6) segmented PSD without a spine partition. The major finding of this study is that the induction of LTP in the rat dentate gyrus is followed by a significant and marked increase in the number of only those perforated axospinous synapses that have multiple, completely partitioned transmission zones. No other synaptic subtype exhibits such a change as a result of LTP induction. Moreover, this structural alteration is limited to the terminal synaptic field of activated axons (MML) and does not involve an immediately adjacent one (IML) that was not directly activated by potentiating stimulation. The observed highly selective modification of synaptic connectivity involving only one particular synaptic subtype in the potentiated synaptic field may represent a structural substrate of the long-lasting enhancement of synaptic responses that characterizes LTP.

Correlations between immediate early gene induction and the persistence of long-term potentiation

The duration of long-term potentiation in the dentate gyrus of awake rats was examined following systematic manipulation of the number of stimulus trains delivered. This was correlated with the induction of immediate early genes in separate groups of animals given identical stimulus regimes. Following 10 trains of stimulation, long-term potentiation decayed with a time constant of up to several days (long-term potentiation 2), and this correlated with the appearance of an increase in the messenger RNA and protein levels of zif/268. Increasing the number of stimulus trains resulted in a greater probability of eliciting long-term potentiation with a time constant of several weeks (long-term potentiation 3), as well as increasing the induction of zif/268, c-Jun, Jun-B, Jun-D and Fos-related proteins. When 10 trains were delivered repeatedly on up to five consecutive days, only the zif/268 protein levels showed associated changes. These data provide support for the hypothesis that long-term potentiation 3 involves mechanisms additional to those for long-term potentiation 2. One possible mechanism is altered gene expression, initiated by immediate early gene transcription factors such as zif/268 and possibly homo- or heterodimers of Fos and Jun family members, that then contributes to the stabilization or maintenance of long-term potentiation 3.

Long-term potentiation of hippocampal afferents and efferents to prefrontal cortex: implications for associative learning

It has been proposed that the physical substrate of memory resides in alterations of the strengths or weights of modifiable synaptic connections. In recent years, the hypothesis that the mechanisms underlying a particular form of synaptic plasticity, known as long-term potentiation, or LTP, are activated during learning and may actually subserve the formation of associative memories, has gained much empirical support. This paper reviews experimental studies suggesting that changes in synapse physiology and chemistry are involved in the formation of neural associative representation in hippocampal networks during classical conditioning. Recent experiments investigating LTP and learning-induced synaptic changes at hippocampal outputs to the prefrontal cortex are reported. The results provide a working framework within which the dynamics of information storage in hippocampal and prefrontal cortical networks is profiled.

Heterosynaptic short-term depression of population spike amplitude in the pyramidal layer of the CA1 hippocampal region evoked by a theta-like tetanization

Heterosynaptic short-term depression (STD) of the stratum radiatum and stratum oriens inputs to the CA1 region was studied in rat hippocampal slices. STD was evoked by trains of 1050 impulses with interstimulus interval (ISI) variable from 10 to 700 ms. The STD was found to be very pronounced for tetanizations with ISI around 200 ms, and almost absent for ISI less than 50 ms or more than 500 ms. These data show that theta-like tetanization is an effective pattern not only for induction of the long-term potentiation (LTP), as has been shown previously, but for production of the heterosynaptic STD as well. This implies that heterosynaptic STD can effectively modulate induction of LTP by theta-like tetanization, and plays an important role in differentiation of potentiated pathways. It is discussed that the theta-like tetanization-induced release of ACh is a possible mechanism of the STD.

Entorhinal cortex long-term potentiation evoked by theta-patterned stimulation of associative fibers in the isolated in vitro guinea pig brain

Long-term potentiation (LTP) induced in the lateral entorhinal cortex by theta-patterned tetanic stimulation of the piriform cortex was analyzed in the isolated guinea pig brain maintained in vitro. Monosynaptic excitatory postsynaptic potentials (EPSPs) evoked by stimulation of the piriform cortex are composed of an early and late component selectively blocked by non-N-methyl-D-aspartate (non-NMDA) and NMDA receptor antagonists, respectively. LTP induction was dependent on NMDA receptor activation, being blocked by perfusing the preparation with 2-amino-5-phosphonovalerate (AP-5). LTP was expressed through synaptic enhancement of both early non-NMDA and late, possibly NMDA receptor-mediated responses.

Factors regulating the magnitude of long-term potentiation induced by theta pattern stimulation

Electrical stimulation patterned after the hippocampal theta rhythm produces a robust and stable long-term potentiation (LTP) effect. Pharmacological manipulations were used in the present studies in an effort to relate characteristics of the responses occurring during theta stimulation to the magnitude of potentiation which follows it. Comparisons were made using five or ten bursts of stimulation which respectively induce sub-maximal or near maximal degrees of LTP. DPCPX, a drug that increases release by blocking adenosine A1 receptors, was used to enhance the depolarization produced by individual theta bursts. This resulted in a marked increase in the amount of stable LTP induced by five theta bursts but did not affect that resulting from ten bursts. This finding suggested that depolarization occurring during a burst response influences per burst potentiation but not the ceiling on maximum LTP. Aniracetam, a nootropic drug that enhances responses via an action on glutamate (AMPA) receptors, was used to test this conclusion. Like DPCPX, aniracetam increased the size of the burst response and enhanced the degree of LTP caused by five but not ten theta bursts. Forskolin, an activator of adenylate cyclase, was used to test the effects of blocking the hyperpolarization normally present between theta bursts on the induction of LTP. The drug augmented the degree of LTP resulting from five theta bursts and, in contrast to DPCPX and aniracetam, nearly doubled that obtained with ten bursts. Thus the drug affected both per burst potentiation and the ceiling on LTP. These results are discussed in terms of an hypothesis in which the magnitude of NMDA receptor mediated currents affects the degree of potentiation produced by individual theta bursts while the duration of the currents is related to the limit on the maximum LTP induced by a series of bursts. The possible implications of the findings for learning are also considered.

Structural synaptic plasticity associated with the induction of long-term potentiation is preserved in the dentate gyrus of aged rats

Changes in synaptic numbers were examined in the hippocampal dentate gyrus of aged (28 months old) rats following the induction of long-term potentiation (LTP) by high-frequency stimulation of the medial perforant path carried out on each of 4 consecutive days. Potentiated animals were sacrificed 1 hour after the fourth stimulation. Stimulated but not potentiated and implanted but not stimulated rats of the same chronological age served as controls. Synapses were analyzed in the middle (MML) and inner (IML) molecular layer of the dentate gyrus. Using the stereological dissector technique, unbiased estimates of the number per neuron were obtained for the following morphological varieties of synapses: axodendritic synaptic junctions involving dendritic shafts, nonperforated axospinous synapses having a continuous postsynaptic density (PSD), and perforated ones distinguished by a fenestrated, horseshoe-shaped, or segmented PSD. The induction of LTP resulted in a selective increase in the number of synapses with segmented PSDs. This change was detected only in the potentiated synaptic field (MML), but not in an immediately adjacent one (IML), which was not directly stimulated during the induction of LTP. Comparison of these data with the results of our previous LTP study in young adult rats (Geinisman, Y. et al., 1991, Brain Res. 566:77-88) showed that the only significant difference in the absolute number of synaptic contacts per neuron between potentiated animals of the two chronological ages was an age-related reduction in segmented synapses of the MML. Relative increases in the number of segmented synapses per neuron were, however, virtually of the same magnitude in potentiated rats of both ages as compared with their respective controls. This finding may explain why senescent rats can be potentiated to the same extent as young ones.

Muscimol infused into the medial septal area impairs long-term memory but not short-term memory in inhibitory avoidance, water maze place learning and rewarded alternation tasks

These experiments investigated the effects of injections of muscimol (1 or 5 nmol), administered into the medial septal area prior to training, on memory tested at different retention delays after training in 3 tasks: an inhibitory avoidance task, a one-trial place learning task, and a rewarded alternation task. In all 3 tasks, intraseptal injections of muscimol did not impair memory performance at short retention delays, but impaired memory at the longer retention delays. These findings are consistent with the view that GABAergic regulation of the septohippocampal cholinergic system plays a selective role in the establishment of long-term memory.

Variations in Synaptic Plasticity and Types of Memory in Corticohippocampal Networks

If Synaptic long-term potentiation (LTP) represents a memory storage mechanism, its induction and expression characteristics may constitute rules governing encoding and read-out of memory in cortical circuitry, The presence of variants of the LTP effect in different anatomical networks provides grounds for predictions about the types of memory operations to which potentiation contributes. Computer modeling studies incorporating the complex rules for LTP induction and the characteristics of expressed potentiation can be used to make such predictions specific. We review ttie types of synaptic plasticity found in the successive stages of the corticohippocampal pathway, and present results indicating that LTP does participate in definably different forms of memory, suggesting a classification of memory types differing somewhat from categories deduced from behavioral studies. Specifically, the results suggest that subtypes of memory operate serially, in an “assembly line” of specialized functions, each of which adds a unique aspect to the processing of memories. The effects of lesions on the encoding versus expression of memory can be interpreted from the perspective of this hypothesis.

Benzodiazepines block long-term potentiation in slices of hippocampus and piriform cortex

The effects of two benzodiazepines, diazepam and triazolam, on long-term potentiation were tested in slices of hippocampus and piriform cortex. The drugs had little influence on baseline synaptic responses but both were very effective in blocking LTP elicited by theta pattern stimulation. The effects were fully reversible upon washout. Diazepam reduced the increase in burst responses that occurs during theta stimulation and thus appears to interfere with the initial triggering events for long-term potentiation. This may reflect the enhancing action of the drug on GABA-mediated inhibitory potentials. Triazolam did not detectably change the burst responses elicited by theta pattern stimulation. Experiments with slices of piriform cortex indicated that triazolam also failed to disrupt the development of long-term potentiation but instead caused the potentiation to decay back to baseline in 15-30 min. Triazolam thus seems to act on the mechanisms that stabilize long-term potentiation. These results provide a possible explanation for the amnestic effects of benzodiazepines in humans and animals and support the hypothesis that long-term potentiation contributes to memory encoding.

Long-term potentiation induced by patterned stimulation of the commissural pathway to hippocampal CA1 region in freely moving rats

In urethane-anesthetized rats, stimulation of the contralateral hippocampal CA1 region resulted in activation of the homotopic CA1 region. Current-source-density analysis revealed that both basal and apical dendrites were activated. However, alveolar and stratum oriens stimulation in CA1 gave about equal peak excitation of the basal and apical dendrites while CA1 stratum radiatum/moleculare and CA3c stimulation gave stronger apical than basal dendritic excitation. In chronically implanted and freely moving rats, tetanic patterned stimulation of the contralateral CA1, irrespective of depth, resulted in a robust long-term potentiation of the ipsilateral CA1 basal dendritic synapse. The population basal dendritic excitatory postsynaptic potential was initially potentiated to greater than 200% of the baseline and decayed with a 3 h time constant; it lasted at least two days. Patterned stimulation of the commissural inputs at 2 x threshold stimulus intensity seldom potentiated the apical dendritic synapse in CA1; rather, long-term depression was sometimes observed. After tetanic stimulations at 3 x threshold, a small potentiation of the apical dendritic excitation was seen in about half of the experiments. The average apical dendritic potentiation peaked at about 25% and persisted to at least one day. This study provides original evidence that the properties of long-term potentiation are different at the commissural basal dendritic and apical dendritic synapses in CA1 of the behaving rat. Basal dendritic potentiation is low-threshold, high-amplitude and decayed rapidly in the first 3 h. Apical dendritic potentiation is high-threshold, low-amplitude and not rapidly decaying. A long-lasting enhancement of synaptic transmission has been postulated as a physiological correlate of memory. This paper reports properties of this synaptic enhancement for two different types of synapses on the same cells in the behaving animal. The basal dendritic synapse on hippocampal pyramidal cells readily increased their efficacy, up to at least two days, after a brief, patterned stimulation. In the same preparation, it was difficult to obtain a long-lasting increase in the apical dendritic excitation, in contrast to studies on isolated hippocampal slices in vitro.

Serotonin blocks the long-term potentiation induced by primed burst stimulation in the CA1 region of rat hippocampal slices

The effect of 5-hydroxytryptamine on the induction of long-term potentiation by a train of high frequency pulses (100 Hz; 1 s) or by a stimulation consisting of one burst of five pulses at 100 Hz delivered 170 ms after a single pulse (primed burst) was investigated in the CA1 region of the rat hippocampal slice in vitro with extracellular recordings. Superfusion with 5-hydroxytryptamine (3-30 microM) produced a concentration-dependent decrease in amplitude of the population spikes evoked by test stimuli. The presence of 5-hydroxytryptamine (30 microM) did not affect the magnitude of long-term potentiation produced by the high-frequency stimulation but it prevented the long-term potentiation induced by a primed burst. The action of 5-hydroxytryptamine was mimicked by the 5-hydroxytryptamine1A agonist 5-carboxamidotryptamine (0.3 microM) and blocked by the 5-hydroxytryptamine2/5-hydroxytryptamine1A antagonist spiperone (3 microM) or by the 5-hydroxytryptamine1/5-hydroxytryptamine2 antagonist methiothepin (1-10 microM). The selective 5-hydroxytryptamine2 antagonist ritanserin (1 microM) did not antagonize the block of long-term potentiation produced by 5-hydroxytryptamine. The selective 5-hydroxytryptamine3 antagonists (3-tropanyl)-1H-indole-3-carboxylic acid ester (ICS 205-930; 1 nM) and ondansetron (GR-38032; 30 nM) did not affect the reduction in the population spike produced by application of 5-hydroxytryptamine. In contrast, a primed burst delivered at the fifth minute of 5-hydroxytryptamine application in the presence of a 5-hydroxytryptamine3 antagonist induced a long-term potentiation.(ABSTRACT TRUNCATED AT 250 WORDS)

Linear relationship between the maintenance of hippocampal long-term potentiation and retention of an associative memory

The hypothesis that the maintenance or decay of an associative memory trace after an extended retention interval is a function of the residual strength of the synapses originally strengthened during learning was examined in a classical conditioning paradigm in which high-frequency stimulation of a hippocampal input--the medial perforant path--served as a conditioned stimulus. Rats received perforant path stimulus-foot shock pairings while engaged in a previously acquired food-motivated lever-pressing task. Conditioned suppression of lever pressing was the behavioral measure of learning and retention of the association. Stimulus trains to the perforant path at an intensity above the threshold for eliciting a population spike induced long-term potentiation of synaptic transmission in the dentate gyrus. Synaptic potentials recorded extracellularly in the dentate gyrus were subsequently monitored for 31 days to examine quantitatively the decay of synaptic potentiation, a period after which retention of the learned association was assessed. All rats learned the association to a similar extent and displayed equivalent amounts of long-term potentiation by the end of conditioning. A slowly decaying function of synaptic potentiation was observed in remembering rats, i.e., rats with high retention performance after the 31-day learning-to-retention interval, while forgetting was associated with a rapid decay of long-term potentiation. Behavioral performance at the long-term memory test was linearly correlated with the amplitude of long-term potentiation maintained just prior to the retention test. The results favor the hypothesis that long-term associative memory depends, at least in part, on the maintenance of elevated synaptic strengths in the pathway activated during learning and suggest a role for the lasting component of long-term potentiation in the maintenance of memory.

Induction of long-term potentiation is associated with an increase in the number of axospinous synapses with segmented postsynaptic densities

Long-term potentiation (LTP) is characterized by a long-lasting enhancement of synaptic efficacy which may be due to an increase in synaptic numbers. The present study was designed to verify the validity of this suggestion using recently developed unbiased methods for synapse quantitation. LTP was elicited in young adult rats by high-frequency stimulation of the medial perforant path carried out on each of 4 consecutive days. Potentiated animals were sacrificed 1 h after the fourth stimulation. Stimulated but not potentiated and implanted but not stimulated rats served as controls. Synapses were examined in the middle (MML) and inner (IML) molecular layer of the hippocampal dentate gyrus. Using the stereological disector technique, unbiased estimates of the number of synapses per neuron were differentially obtained for the following morphological synaptic types: axodendritic synapses involving dendritic shafts, non-perforated axospinous synapses exhibiting a continuous postsynaptic density (PSD) and perforated axospinous synapses distinguished by a fenestrated, horseshoe-shaped or segmented PSD. A major finding of this study is that the induction of LTP is accompanied by a selective increase in the number of synapses with segmented PSDs. This change was detected only in the potentiated synaptic field (MML), but not in an immediately adjacent one (IML) which was not directly stimulated during the induction of LTP. It is strongly suggested by the latter finding that the increase in the number of axospinous synapses exhibiting segmented PSDs is associated with LTP. Such a highly selective modification of connectivity, which involves only one particular subtype of synapses in the potentiated synaptic field, is likely to represent a structural substrate of the enduring augmentation of synaptic efficacy typical of LTP.

Morphological differentiation of rat pheochromocytoma cells (PC12 cells) by electric stimulation

The effects of electric stimulation on the morphological differentiation of PC12 cells are described. PC12 cells were stimulated with the 'theta' (4-7 Hz electroencephalogram (EEG) rhythm) pattern-electric stimulation, which was known to elicit stable long-term potentiation (LTP) in the CA1 region of the hippocampus. The stimulation induced the neurite outgrowth of PC12 cells, as well as nerve growth factor (NGF). This result suggests that the electric signal has a differentiating potential equivalent to the receptor-ligand interaction.

Activation of the NMDA receptor: a correlate in the dentate gyrus field potential and its relationship to long-term potentiation and kindling

Stimulation trains, but not stimulation pulses, are capable of inducing long-term potentiation (LTP). In this paper we report experiments designed to examine, in chronic preparations, the characteristics of a component unique to the train-evoked response. Stimulation trains applied to the perforant path evoked population EPSP's and population spikes in the dentate gyrus that were nearly identical to those evoked by single pulses of comparable intensity. The trains also triggered a prolonged potential, negative at the dendritic pole of our electrodes, which far outlasted the pulse-evoked response. We substracted pulse-evoked responses from these train-evoked responses which left us with a waveform that peaked at about 15 ms and lasted for about 50-70 ms. The GABA agonists, diazepam and sodium pentobarbital, had no significant effect on this component, but the NMDA antagonists, ketamine and MK-801, both depressed it by over 30%. The late component had a very low threshold, which might account for the frequent observation of LTP induction at very low thresholds. Also, the late component is reliably seen in all animals showing LTP, even in the occasional animals that show no population spikes. The late component did not appear to be affected by the induction of LTP, and was either not affected or was depressed following the completion of kindling. When the 'NMDA-component' of the train-evoked response was monitored, along with LTP, in an ascending intensity train series, it was found that both the NMDA-component and the LTP increased smoothly. There was no sudden appearance of the NMDA-component at the LTP threshold. The presence of an NMDA component in the field potential of the chronic preparation allows the monitoring of the levels of NMDA activation over prolonged periods.

Hippocampal CA1 evoked response and radial 8-arm maze performance after hippocampal kindling

Rats with chronically implanted electrodes in the hippocampal CA1 region were trained in the open radial 8-arm maze and then subjected to kindling (afterdischarges, ADs) or 0.17 Hz low-frequency stimulations (LFSs) as controls. Partial kindling (21 ADs) induced a general increase of AD threshold but no motor convulsions. The commissurally evoked average evoked potentials (AEPs) in CA1 were enhanced above the pre-AD baseline or the AEPs in LFS control rats at 1 day after the 1st, 6th, 11th and 16th AD and for at least 25 days after 21 ADs. Similarly, maze performance was significantly worse in kindled than LFS rats for about 4 weeks after 21 ADs/LFSs. The study confirms the long-lasting behavioral effect of partial kindling and suggests that synaptic enhancement may underlie the behavioral disruption.

Long-term potentiation, protein kinase C, and glutamate receptors

Among the various molecular events that have been proposed to contribute to the mechanisms of long-term potentiation (LTP), one of the most cited possibilities has been the activation of protein kinase C (PKC). Here we review various aspects of the cellular actions of PKC activation and inhibition, with special emphasis on the effects of the kinase on synaptic transmission and the N-methyl-D-aspartate (NMDA) and non-NMDA receptor-mediated components of synaptic responses. We discuss the implications of these effects for interpretations of the role of PKC in the mechanisms of LTP induction and maintenance.

Learning-related patterns of CA1 spike trains parallel stimulation parameters optimal for inducing hippocampal long-term potentiation

Recent studies have revealed 3 stimulation parameters that together comprise the temporal pattern of neuronal activation optimal for the induction of hippocampal LTP: high-frequency bursts, activity 100-200 ms prior to a burst, and burst delivery in phase with the ongoing hippocampal theta rhythm. The present paper reports that these 3 aspects of patterned neural activity, collectively referred to as "theta-bursting," are characteristic of the spike trains of CA1 pyramidal cells in rats during the sampling and analysis of learning cues in an odor discrimination task and during performances of a spatial memory task. In contrast, theta-bursting occurs relatively infrequently during behavioral events less directly related to task-relevant mnemonic processing. These findings suggest that the optimal conditions for the induction of LTP occur naturally in behaving animals, time-locked to behavioral events critical to learning.

Evidence that changes in spine neck resistance are not responsible for expression of LTP

From modeling studies it is known that changes in spine neck resistance can influence the shape of the non-linear curve relating synaptic current to synaptic conductance if the resistance of the neck approaches the synaptic input resistance. Such work also indicates that the effects of resistance will be much more pronounced for fast rather than slow synaptic currents. Accordingly, a reduction in neck resistance could produce an increase in the rapid responses generated by the quisqualate/AMPA class of glutamate receptors while only minimally affecting the slower NMDA receptor-mediated responses and thus account for the pattern of changes known to be associated with long-term potentiation (LTP). This hypothesis predicts that large reductions in synaptic conductance should have disproportionate effects on potentiated versus control responses. This was tested by using field potential recordings of synaptic currents in CA1 pyramidal cells in hippocampal slices in response to stimulation of Schaffer/commissural inputs that either received LTP-inducing stimulation or did not. Two manipulations were used to systematically reduce synaptic conductances: reductions of extracellular Ca++ and partial blockade of postsynaptic receptors. Reductions of synaptic field potentials by 40-75% by either method at control synapses were accompanied by equivalent reductions at previously potentiated synapses. These results suggest that LTP expression is not due to a change in the curves relating synaptic current to synaptic conductance as would be predicted by the spine resistance hypothesis.

Development of hippocampal long-term potentiation is reduced by recently introduced calpain inhibitors

The effects of two recently synthesized inhibitors of calpains, calpain inhibitor I (CiI) and calpain inhibitor II (CiII) were tested on the development of long-term potentiation (LTP) in region CA1 of rat hippocampus. Slices maintained in 100 microM of CiI or CiII showed an initial degree of potentiation after theta burst stimulation that, in contrast to controls, slowly decayed across time. The effects of CiI and CiII appeared to be independent of possible actions on the physiological mechanisms that take place during the induction stage of LTP. Since these inhibitors are more potent and specific than leupeptin in blocking calpain activity, their effects on LTP can be more convincingly ascribed to a selective blockade of the calcium-sensitive protease. Accordingly, the results favor the idea that a proteolytic event of the kind found after N-methyl-D-aspartate receptor activation is an intermediary step in the development of LTP.

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.

Kindling with stimulation of the dentate gyrus. II. Effects on evoked field potentials

Once daily for 60 days, male hooded rats received unilateral high-frequency stimulation in the hilus of the dentate gyrus (DG), at an intensity sufficient to evoke afterdischarge (AD). Every 2nd day, evoked potentials were recorded from the hilus following stimulation of the PP with single 0.1 ms pulses at 6 current intensities. Changes in synaptic excitability of the dentate granule cells were monitored by measuring the amplitudes of the population spikes; changes in the strength of excitatory synaptic transmission were monitored by measuring the slopes of the excitatory postsynaptic potentials (EPSPs). Control rats, which were not given kindling stimulation, were tested for changes in synaptic transmission and excitability in the same way, at comparable times. In general, hilar stimulation resulted in a large decrease in population spike amplitudes to below baseline and control levels, accompanied by a paradoxical potentiation of EPSPs. Population spike amplitudes decreased more in rats that developed generalized stage-5 seizures (Generalized group) than in rats that did not progress beyond partial seizures despite 60 days of stimulation (Partial group). Conversely, EPSP slopes increased more in the Partial group than in the Generalized group. These results suggest that kindling stimulation may potentiate responsiveness of the directly activated dentate granule cells to inputs from the PP, but at the same time suppress the output of the granule cells resulting from this input. Furthermore, the results indicate that kindling is more closely allied to the suppression of output than to the potentiation of responsiveness to input.

The nature and causes of hippocampal long-term potentiation

One of the most fascinating features of the hippocampus is its capacity for plasticity. Long-term potentiation (LTP), a stable facilitation of synaptic potentials after high-frequency synaptic activity, is very prominent in hippocampus and is a leading candidate memory storage mechanism. Here, we discuss the nature and causes of LTP and relate them to endogenous rhythmic neuronal activity patterns and their potential roles in memory. Anatomical studies indicate that LTP is accompanied by postsynaptic structural modifications while pharmacological studies strongly suggest that LTP is not due to an increase in presynaptic transmitter release. In field CA1, LTP induction appears to be triggered by a postsynaptic influx of calcium through NMDA receptor-linked channels. Possible roles of several calcium-sensitive enzyme systems in LTP are discussed and it is argued that activation of a calcium-dependent protease (calpain) could produce the structural changes linked to LTP. Rhythmic bursting activity is highly effective in inducing LTP and it is argued that the endogenous hippocampal theta rhythm plays a role in LTP induction in vivo. Finally, studies indicate that LTP and certain types of memory share a common pharmacology and the use of electrical brain stimulation as a sensory cue suggests that LTP develops when the significance of that cue is learned.

Long-term potentiation of monosynaptic EPSPs in rat piriform cortex in vitro

Induction of long-term potentiation (LTP) by burst stimulation patterned after the limbic system theta rhythm was studied in slices of piriform cortex. Monosynaptic responses were evoked by stimulation of afferent fibers of the lateral olfactory tract (LOT) or the intrinsic associational (ASSN) feedback system. LTP was difficult to elicit at LOT synapses in the presence of 2.5 mM extracellular Mg2+, and when it was induced potentiation increased for 20-30 min after burst stimulation before stabilizing. The probability of inducing LTP was increased when the extracellular Mg2+ concentration was reduced to 50 microM. In ASSN synapses LTP developed in about 1 min after burst stimulation and then remained stable. ASSN system LTP was more readily induced in slices from caudal than rostral piriform. Induction of LTP at both LOT and ASSN synapses was blocked by D-2-amino-5-phosphonopentanoate, indicating that NMDA receptor activation was required. Neither system exhibited the decremental short-term potentiation effect observed after burst stimulation of inputs to the CA1 field of hippocampus.

Role of excitatory amino acid neurotransmission in synaptic plasticity and pathology. An integrative hypothesis concerning the pathogenesis and evolutionary advantages of schizophrenia-related genes

N-methyl-D-aspartate (NMDA) receptors are involved in long-term potentiation, burst firing and the generation of patterned activity in neuronal networks; in use-dependent stabilization of synaptic connectivity in developing animals; in some forms of learning in mature animals; and in pathologies as found in brain aging. A number of these characteristics are reminiscent of several manifestations of schizophrenia and therefore we present the hypothesis that one of the genes modified in schizophrenia is directly or indirectly linked to the control of excitatory neurotransmission; possibly the normal switching on of the expression of the adult form of the NMDA receptor is delayed. Alternatively the adult form of the NMDA receptor is altered, resulting in inappropriate functioning of this receptor. The delayed or faulty expression of the adult form of the NMDA receptor, in turn, should confer a series of evolutionary advantages including protection against aging-associated brain pathologies.

Long-term potentiation in the hippocampal CA1 region: its induction and early temporal development

Long-term potentiation (LTP) is a process that due to its prolonged time course and associative nature of induction is believed to be involved in learning and memory in the mammalian brain. In this chapter the experimental evidence for the view that LTP is initiated by an influx of calcium ions through synaptically controlled N-methyl-D-aspartate (NMDA) receptor channels is discussed. It will also be described how LTP develops following its induction. It will be shown that there is a considerable delay, about 2-3 s, between a tetanus and the initiation of LTP, and that additional 20-30 s are needed for the potentiation to reach peak levels. The potentiation subsequently decays to a degree which depends primarily on tetanus length. It will be argued that this early phase of tetanus-induced LTP is of the same nature as that present a few hours later.

Effects of aging on the dynamics of information processing and synaptic weight changes in the mammalian hippocampus

It is clear that the properties of LTE make it a plausible mechanism for associative information storage at some synapses in the central nervous system. While many of the factors that regulate LTE's induction and expression have been discovered and a strong case is being developed for its role in learning and memory processes, until we understand more clearly the mechanisms underlying both the expression and maintenance of LTE, an understanding of its change with age will be difficult. Judging by the progress that has been made over the past several years in uncovering some of the molecular events that are critical for LTE's expression, one may be optimistic that answers will be forthcoming reasonably soon. Of particular importance to aging mammals, such answers may provide insights into why older organisms show faster forgetting. This may have a profound impact on therapeutic strategies for memory disorders in both normal and pathological conditions of aging.

Morphological correlates of long-term potentiation imply the modification of existing synapses, not synaptogenesis, in the hippocampal dentate gyrus

This report evaluates two morphological markers of synaptogenesis following the induction of long-term potentiation (LTP) in the dentate gyrus of the anesthetized rat. These two morphological features, polyribosomes and multiple synaptic contacts, are known to increase in number with synaptogenesis in the mature hippocampus. The analysis focused on the middle third of the dentate molecular layer. As shown previously, this is the region of primary synaptic activation in our electrophysiological protocol and the region of localized morphological changes with LTP. Here the incidence of a polyribosome at the base of a dendritic spine declined 57% with LTP. In addition, the number of multiple synaptic contacts decreased 18% there with LTP. Both decreases were more pronounced immediately following conditioning stimulation than at later intervals. Because both morphological features decrease with LTP but increase with synaptogenesis, the data do not support the hypothesis that new synapses form with LTP. Instead, the data add further support to the view that the strengthening of existing excitatory synapses underlies LTP.

Recovery of spatial learning deficits after decay of electrically induced synaptic enhancement in the hippocampus

A widespread interest in a long-lasting form of synaptic enhancement in hippocampal circuits has arisen largely because it might reflect the activation of physiological mechanisms that underlie rapid associative learning. As its induction normally requires the 'Hebbian' association of activity on a number of input fibres, we refer to the process as long-term enhancement (LTE) rather than long-term potentiation (LTP), to emphasize its distinction from the ubiquitous, non-associative 'potentiation' phenomena that occur at most synapses, including those exhibiting LTE. Among other evidence that LTE might actually have a role in associative memory is the demonstration that repeated high-frequency stimulation, which saturated the inducible LTE, caused a severe deficit in spatial learning, although it had no effect on well established spatial memory. These results were consistent with a widespread view that information need only temporarily be stored in the hippocampal formation in order for long-term memories to be established in neocortical circuits. In this context, it is important to understand whether the possible underlying synaptic changes are of a permanent character, or are relatively transient. A second question is whether the actual cause of the observed learning deficit is the distruption of the synaptic weight distribution, and/or the limitation of further synaptic change, which presumably results from experimental saturation of the LTE mechanism. Alternatively, the deficit could be a consequence of some unobserved secondary effect of the high-frequency electrical stimulation. Here we demonstrate that learning capacity recovers in about the same time that it takes LTE to decay, which strongly favours the first possibility and supports the idea that LTE-like processes actually underlie associative memory.

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.

Absence of long-term potentiation in the subcortically deafferented dentate gyrus

All subcortical afferents to the dorsal hippocampus, running in the fimbria-fornix and supracallosal path, were removed by aspiration. Three to 5 months later the rats were implanted with chronic recording electrodes in the dentate gyrus and CA1 region, and stimulating electrodes in the angular bundle. In non-lesioned rats, high-frequency trains delivered to the angular bundle gave rise to a sustained increase of the evoked population spike in the dentate gyrus. In lesioned animals, high-frequency stimulation resulted in only short-lasting changes, and by 15 min after the conditioning trains the amplitude of both the population spike and field postsynaptic potentials returned to baseline. In lesioned rats large amplitude interictal spikes (less than 40 ms, 3-8 mV) occurred spontaneously. These findings suggest that either (1) coactivation of entorhinal and subcortical inputs is essential for the induction of long-lasting plastic changes in the dentate gyrus, or (2) the long-term potentiation mechanism is saturated by the chronically occurring interictal discharges in the subcortically denervated dentate gyrus.

Evidence that changes in presynaptic calcium currents are not responsible for long-term potentiation in hippocampus

We used two approaches to test the possibility that changes in presynaptic calcium currents might be responsible for the long-term potentiation (LTP) effect induced by high-frequency stimulation in area CA1 of hippocampal slices. In a first series of experiments, we compared the effect of LTP induction on paired-pulse facilitation with that produced by changes in extracellular calcium concentration, a procedure that modifies presynaptic calcium currents during depolarization by changing the ionic gradient for calcium. In hippocampus, as in peripheral synapses, increasing concentrations of extracellular calcium caused a marked reduction in the degree of facilitation obtained with paired-pulse stimulation; LTP, conversely, did not affect the facilitation ratio. The differential effect of changing calcium concentrations versus LTP induction on paired-pulse facilitation was observed with different interpulse intervals as well as in conditions in which the changes in response size produced by the two manipulations were comparable. In the second approach, we measured calcium dependency curves of synaptic responses before and after LTP induction or application of 4-aminopyridine, a blocker of potassium channels that increases presynaptic calcium currents by slowing spike repolarization. Procedures that increase calcium entry into terminals during transmission should shift to the left the sigmoidal function relating extracellular calcium to the slope of the extracellular response. This in turn should result in disproportionate effects of the procedure as a function of the calcium concentration. This prediction was realized with 4-aminopyridine but did not occur following LTP induction: control and potentiated responses were similarly affected by changes in calcium concentration. Although indirectly, these data strongly suggest that LTP is not accompanied by alterations in the presynaptic calcium dynamics associated with transmitter release.

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.

Long-term potentiation and kindling: how similar are the mechanisms?

Long-term potentiation (LTP) and kindling are strikingly similar in many respects. Both are believed to model CNS plasticity, both are induced by the localized application of brief, high-frequency trains of electrical pulses through implanted electrodes, and both result in a lasting increase in the response to a constant stimulus. In addition to these formal similarities, recent findings have indicated that the two models may share aspects of an underlying neural mechanism, and this has led to the suggestion that LTP may constitute the cellular mechanism of kindling. However, other findings have indicated two models. This article discusses the differences in mechanisms and the relations between LTP and kindling.

Two-stage model of memory trace formation: a role for "noisy" brain states

Review of the normally occurring neuronal patterns of the hippocampus suggests that the two principal cell types of the hippocampus, the pyramidal neurons and granule cells, are maximally active during different behaviors. Granule cells reach their highest discharge rates during theta-concurrent exploratory activities, while population synchrony of pyramidal cells is maximum during immobility, consummatory behaviors, and slow wave sleep associated with field sharp waves. Sharp waves reflect the summed postsynaptic depolarization of large numbers of pyramidal cells in the CA1 and subiculum as a consequence of synchronous discharge of bursting CA3 pyramidal neurons. The trigger for the population burst in the CA3 region is the temporary release from subcortical tonic inhibition. An overview of the experimentally explored criteria of synaptic enhancement (intensity, frequency, and pattern of postsynaptic depolarization, calcium influx, cooperativity, threshold) suggests that these requirements may be present during sharp wave-concurrent population bursts of pyramidal cells. Experimental evidence is cited showing that (a) population bursts in CA3 may lead to long-term potentiation in their postsynaptic CA1 targets, (b) tetanizing stimuli are capable of increasing the synchrony of the sharp wave-burst, and (c) activity patterns of the neocortical input to the hippocampus determine which subgroup of CA3 neurons will trigger subsequently occurring population bursts (initiator cells). Based on the experimental evidence reviewed a formal model of memory trace formation is outlined. During exploratory (theta) behaviors the neocortical information is transmitted to the hippocampus via the fast-firing granule cells which may induce a weak and transient heterosynaptic potentiation in a subgroup of CA3 pyramidal cells. The weakly potentiated CA3 neurons will then initiate population bursts upon the termination of exploratory activity (sharp wave state). It is assumed that recurrent excitation during the population burst is strongest on those cells which initiated the population event. It is suggested that the strong excitatory drive brought about by the sharp wave-concurrent population bursts during consummatory behaviors, immobility, and slow wave sleep may be sufficient for the induction of long-term synaptic modification in the initiator neurons of the CA3 region and in their targets in CA1. In this two-stage model both exploratory (theta) and sharp wave states of the hippocampus are essential and any interference that might modify the structure of the population bursts (e.g. epileptic spikes) is detrimental to memory trace formation.