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

The relationship between adhesion molecules and neuronal plasticity

1. It is presently widely assumed that structural reorganization of synaptic architectures subserves the functional gains that define certain neuronal plasticities. 2. While target molecules thought to participate in such morphological dynamics are not well defined, growing evidence suggests a pivotal role for cell adhesion molecules. 3. Herein, brief discussions are presented on (i) the history of how adhesion molecules became implicated in plasticity and memory processes, (ii) the general biology of some of the major classes of such molecules, and (iii) the future of the adhesion molecule/plasticity relationship.

Epileptiform activity and changes in field potential responses induced by low [Mg2+]0 in a genetic rat model of absence epilepsy

The genetic absence epilepsy rats of Strasbourg (GAERS) display alterations in cortical synaptic transmission possibly facilitating the generation of ictaform activity and the late development into convulsive epilepsy. We studied low Mg2+-induced epileptiform activities and their long term effects on field potentials (fp) evoked by paired pulse stimulation in hippocampal area CA1 (CA1), medial entorhinal cortex (EC) and frontal cortex (FC) in in-vitro-slice preparations from GAERS and control (NE) adult rats (6 months). Omitting Mg2+-ions from artificial cerebrospinal fluid (ACSF) caused recurrent short discharges (in CA1) and seizure-like events (in EC) in both GAERS and NE rats. Latency to onset of activity as well as discharge pattern, frequency and amplitude of such events did not differ between the two strains, neither in CA1 nor in EC. In the FC, however, epileptiform events occurred in NE rats, but not in GAERS. Field potentials in normal ACSF were similar in both strains in CA1 and FC, while they were smaller in the EC of GAERS. Low [Mg2+]0 caused long-term changes of fp only in area CA1 where the population spikes were depressed in GAERS and increased in NE rats. We concluded that susceptibility to low [Mg2+]0-induced epileptic activity in EC and hippocampal area CA1 is not higher in GAERS than in NE adult rats. However, some properties like synaptic coupling in EC and long-term changes in synaptic efficacy induced by epileptiform activity in CA1 differ from that in NE rats. Whether the particularities in GAERS may be related to kindling by absence epileptic activities will be studied in further experiments.

Effects of GABA(A) inhibition on the expression of long-term potentiation in CA1 pyramidal cells are dependent on tetanization parameters

Long-term potentiation (LTP) of excitatory synaptic responses of principal neurons in the hippocampus is accompanied by changes in GABAergic inhibition mediated by interneurons. The impact of inhibition on LTP of excitatory postsynaptic responses in CA1 pyramidal cells was assessed by monitoring changes in field potentials evoked by Schaffer collateral stimulation in hippocampal slices in vitro. First, to determine the effect of inhibition on population EPSPs, slices were exposed to the GABA(A) receptor antagonist bicuculline (10 microM). Both the slope and amplitude of field EPSPs (fEPSPs) were significantly enhanced by bicuculline indicating that inhibition modulates excitatory postsynaptic responses of pyramidal cells. To assess if stimulation-dependent changes in inhibition influence LTP of excitatory responses of pyramidal cells, LTP was examined in the presence and absence of bicuculline (20 microM) following either 100 Hz tetanization, or theta-patterned stimulation (short bursts delivered at 5 Hz). In normal medium, 100 Hz stimulation produced marked short-term potentiation that decayed 5-10 min post-tetanus and both stimulation paradigms produced similar LTP at 30 min post-tetanus. In comparison, LTP of the fEPSP slope and amplitude was significantly enhanced after theta-patterned stimulation, but not after 100 Hz stimulation, in bicuculline. The greater potentiation of field responses following theta-patterned stimulation in the presence of bicuculline indicates that a larger potentiation of excitatory responses was unmasked during suppression of inhibitory inputs. These results suggest that a long-lasting enhancement of inhibition in pyramidal cells was also induced following theta-patterned stimulation in normal ACSF. Since suppression of inhibition did not uncover a significantly larger potentiation following 100 Hz tetanization, the influence of inhibition on LTP of excitatory responses appears to be stimulation-dependent. In conclusion, theta-patterned stimulation appears to be more effective at inducing plasticity within inhibitory circuits, and this plasticity may partially offset concurrent increases in the excitability of the CA1 network.

Memory and the brain: unexpected chemistries and a new pharmacology

Efforts to characterize long-term potentiation (LTP) and to identify its substrates have led to the discovery of novel synaptic chemistries, computational algorithms, and, most recently, pharmacologies. Progress has also been made in using LTP to develop a "standard model" of how unusual, but physiologically plausible, levels of afferent activity create lasting changes in the operating characteristics of synapses in the cortical telencephalon. Hypotheses of this type typically distinguish induction, expression, and consolidation stages in the formation of LTP. Induction involves a sequence consisting of theta-type rhythmic activity, suppression of inhibitory currents, intense synaptic depolarization, NMDA receptor activation, and calcium influx into dendritic spines. Calcium-dependent lipases, kinases, and proteases have been implicated in LTP induction. Regarding the last group, it has been recently reported that theta pattern stimulation activates calpain and that translational suppression of the protease blocks potentiation. It is thus likely that proteolysis is readily driven by synaptic activity and contributes to structural reorganization. LTP does not interact with treatments that affect transmitter release, has a markedly differential effect on the currents mediated by colocalized AMPA vs NMDA synaptic receptors, changes the waveform of the synaptic current, modifies the effects of drugs that modulate AMPA receptors, and is sensitive to the subunit composition of those receptors. These results indicate that LTP is expressed by changes in AMPA receptor operations. LTP is accompanied by modifications in the anatomy of synapses and spines, something which accounts for its extreme duration (weeks). As with various types of memory, LTP requires about 30 min to consolidate (become resistant to disruption). Consolidation involves adhesion chemistries and, in particular, activation of integrins, a class of transmembrane receptors that control morphology in numerous cell types. Platelet activating factor and adenosine may contribute to consolidation by regulating the engagement of latent integrins. How consolidation stabilizes LTP expression is a topic of intense investigation but probably involves modifications to one or more of the following: membrane environment of AMPA receptors; access of regulatory proteins (e.g., kinases, proteases) to the receptors; receptor clustering; and space available for receptor insertion. Attempts to enhance LTP have focused on the induction phase and resulted in a class of centrally active drugs ("ampakines") that positively modulate AMPA receptors. These compounds promote LTP in vivo and improve the encoding of variety of memory types in animals. Positive results have also been obtained in preliminary studies with humans.

Unilateral hippocampal ablation at birth causes a reduction in contralateral LTP

Subcortical damage in neonates often has more severe consequences than in adults. Unilateral electrolytic hippocampal lesions in adult rats typically result in transient memory deficits, whereas neonatal lesions cause lasting memory impairments. We hypothesized that unilateral lesions made at birth may affect synaptic physiology in the contralateral hippocampus. Consequently, the ability to sustain long-term potentiation (LTP), a form of synaptic plasticity believed to underlie certain forms of memory, was compared between slices from the remaining hippocampus of rats lesioned as newborns and as adults. Initial studies showed that a train of 10 stimulation bursts patterned after the hippocampal theta rhythm produced robust and stable LTP both in slices from controls and rats lesioned at birth. However, a theta burst pattern of stimulation closer to intrinsic physiology (five burst pairs separated by 30 s each), induced significantly less LTP in slices from rats lesioned at birth compared to those from controls and rats lesioned as adults. To investigate possible mechanisms underlying the deficit, the degree of paired-pulse facilitation (PPF) as well as the amount of depolarization occurring between two successive theta bursts were analyzed. The lesion did not detectably change PPF characteristics, suggesting that presynaptic mechanisms are normal. However, the extent to which a burst response was increased by a prior burst was significantly diminished in slices from rats lesioned at birth compared to those from controls and rats lesioned as adults, indicating that postsynaptic factors involved in the initial triggering events of LTP are affected by the lesion. Reduced ability to sustain LTP in the remaining hippocampus may contribute to impaired memory function after unilateral neonatal hippocampal lesion.

Spontaneous epileptiform bursts and long-term potentiation in rat CA3 hippocampal slices induced by chaotic stimulation of mossy fibers

The relation between long-term potentiation (LTP) and spontaneous rhythm in CA3 was investigated using rat hippocampal slices. Field potential response of CA3 to mossy fiber stimulation consisted of a mono-synaptic positive potential and subsequent poly-synaptic negative potentials. LTP of both field potentials was induced by chaotic mossy fiber stimulation. Although CA3 did not show any spontaneous rhythm before LTP induction in a normal perfusing medium, CA3 spontaneously caused epileptiform bursts after LTP induction by chaotic mossy fiber stimulation. The amplitude of those epileptiform bursts and the inter-burst interval were not uniform. After LTP induction, the cross-correlation function of spontaneous field potentials simultaneously recorded at two sites approximately 300 micron apart in CA3 showed a large central peak. This indicates that neuronal activity at two sites is synchronized. These results suggest that epileptiform bursts in CA3 are caused by synchronization of spontaneous CA3 pyramidal cell activity due to LTP induced by chaotic burst stimulation.

Induction and transient suppression of long-term potentiation in the peri- and postrhinal cortices following theta-related stimulation of hippocampal field CA1

During behavioral events associated with periods of likely mnemonic processing, CA1 pyramidal cells in rats typically discharge repetitively in either high-frequency bursts ('complex spikes') or single spikes, both of which are tightly phase-locked to the hippocampal theta rhythm. Interestingly, patterned stimulation which mimics the repetitive, learning-related complex spike discharges are optimal for inducing long-term potentiation (LTP) of excitatory field potentials in CA1, and patterned stimulation which mimics the theta-related single action potentials results in a robust and lasting depotentiation at these same synapses. The aim of the present study was to determine the extent to which these physiologically-relevant patterns of hippocampal stimulation have similar effects on synaptic efficacy in the monosynaptic projection from CA1 to the perirhinal and postrhinal cortices (PRh), areas thought to play a prominent role in many forms of learning and memory. Single-pulse stimulation of CA1 evoked a small amplitude, short latency population excitatory postsynaptic potential (EPSP) in the PRh. Theta-burst stimulation (TBS; n = 8) delivered to CA1 reliably potentiated the PRh EPSP slope for at least 30 min. Theta-pulse stimulation (TPS; 5 Hz; n = 4) delivered to CA1 5 min after TBS substantially but transiently suppressed EPSP slope relative to that of potentiated control preparations. Collectively these data suggest that theta-related patterns of hippocampal activation can reliably induce and transiently suppress LTP in PRh, and are consistent with the notion that behaviorally-relevant, theta-modulated patterns of CA1 unit activity may result in both long- and short-term alterations of synaptic strength within their rhinal cortical targets.

Estrogen facilitates induction of long term potentiation in the hippocampus of awake rats

In order to test the hypothesis that circulating levels of estrogen modulate synaptic plasticity in the hippocampus, we have studied the induction of long term potentiation (LTP) in awake rats. Ovariectomized animals, chronically implanted with a recording electrode in the cell body layer of CA1 and a stimulating electrode in stratum radiatum, were used to record evoked field potentials (population spike (PS) and summed EPSP) daily for at least 4 days before injection of sesame oil or 100 microg of estradiol benzoate per kg b.w. (E2). Basal levels of response to single square pulses (0.01 ms pulse width) delivered at 0.05 Hz through the stimulating electrode were recorded daily for 2 days after injection. To induce LTP a high-frequency 'theta pattern' stimulation was administered. Basal recordings at low-frequency stimulation did not change after injection. After high-frequency stimulation all (7/7) E2 injected animals showed LTP whereas only 1/6 oil injected controls did so; the mean increase in amplitude of the PS and slope of the EPSP after high-frequency stimulation were significantly greater in E2 treated rats. Input/output curves did not change significantly after E2 administration. These results show that at low-frequency stimulation, transynaptic responses of pyramidal neurones in CA1 are not affected by changes in levels of circulating estrogen, while synaptic plasticity -- which is at the basis of proposed hebbian associative memory -- is facilitated by estrogen treatment.

Frequency-dependent information flow from the entorhinal cortex to the hippocampus

Storage and retrieval of information in the hippocampus is dependent on information transfer from the entorhinal cortex (EC). We studied how the separate pathways from layer II and III of the EC to the hippocampus are selected for information transfer during repetitive synaptic stimulation. Intracellular recordings were made from EC layer II and III projection cells in horizontal combined EC-hippocampal slices. Synaptic responses to stimulation of deep layers or the lateral EC with stimulus intensities approximately 70% of that required to elicit an action potential were analyzed during short trains of repetitive stimulation. The threshold intensities for induction of action potentials were in layer II cells 8.2 +/- 3.8 (SE) V, significantly larger than 4.4 +/- 1.5 V in type 1, and 5.2 +/- 3.3 V in type 2 layer III cells, respectively. During repetitive subthreshold stimulation with frequencies below 5 Hz the pathway from the EC layer II remained quiet and was preferentially activated with stimulation frequencies above 5 Hz. In contrast the EC layer III cells responded preferentially to low stimulus frequencies (<10 Hz) and became strongly inhibited when synaptically stimulated with frequencies above 10 Hz. Interestingly during stimulus frequencies between 5 and 10 Hz the likelihood that both layer II and III cells fire was large. Thus a frequency switch operates in the entrohinal cortex regulating output of layer II and III cells to the hippocampus. We suggest that such frequency dependent regulation of information flow presents a new principle of neuronal information processing.

Block of theta-burst-induced long-term potentiation by (1S,3S)-1-aminocyclopentane-1,3-dicarboxylic acid: further evidence against long-term potentiation as a model for learning

It has been previously reported that block of high-frequency stimulation-induced long-term potentiation of synaptic transmission in the hippocampus does not necessarily lead to impairment of spatial learning. Here we show that (1S,3S)-1-aminocyclopentane-1,3-dicarboxylic acid, an agonist at group II metabotropic glutamate receptors, completely blocked long-term potentiation induced by a theta-burst type of stimulation protocol (five pulses at 75 Hz per train, 200 ms inter-train interval) in the CA1 region in vivo. The drug did not significantly affect synaptic responses during each train whereas inter-train facilitation of excitatory postsynaptic potentials was slightly reduced. It also produced a large reduction in paired-pulse facilitation (50 ms inter-stimulus interval), possibly indicating that an increase in inhibition might be involved in the block of long-term potentiation. The drug dose used (5 microliters of a 10 mM solution i.c.v.) was half the dose which inhibited high-frequency stimulation-induced long-term potentiation in earlier experiments but which did not prevent learning of spatial tasks. We conclude that long-term potentiation induced by a more physiological stimulation protocol which uses theta-like inter-train intervals does not appear to accurately model the synaptic changes which are believed to occur during learning either.

Time-dependent blockade of STP and LTP in hippocampal slices following acute stress in mice

The characteristics of short-term potentiation (STP) and long-term potentiation (LTP) in the CA1 region of hippocampal slices were determined at various times following exposure to acute stress produced by restraint and tail-shock in mice. In slices prepared from control animals, theta-burst stimulation resulted in a large increase in evoked field excitatory postsynaptic potentials (EPSPs) amplitude and slope that remained stable at least up to 30 min after stimulation. Slices prepared 1 h after stress exhibited a marked decrease in the extent of both STP and LTP. STP and LTP magnitude were still significantly decreased 24 h after stress exposure and were completely restored to control levels by 48 h. These results provide evidence for a reversible impairment of STP and LTP in CA1 following an acute episode of stress, and suggest that stress activates processes different from those activated by LTP-inducing stimuli.

Memory and behavior: a second generation of genetically modified mice

The use of standard genetic techniques, such as gene targeting and transgenesis, to study cognitive function in adult animals suffers from the limitations that the gene under study is often altered in many brain regions, and that this alteration is present during the entire developmental history of the animal. Furthermore, to relate cognitive defects to neuronal mechanisms of memory, studies have relied on examining long-term potentiation - an artificially induced form of synaptic plasticity. Recent technical advances allow the expression of a genetic alteration in mice to be restricted both anatomically and temporally, making possible a more precise examination of the role of various forms of synaptic plasticity, such as long-term potentiation and long-term depression, in memory formation. Recordings from so-called 'place cells' -hippocampal cells that encode spatial location -in freely moving, genetically modified mice have further advanced our understanding of how the actual cellular representation of space is influenced by genetic alterations that affect long-term potentiation.

Stimulation on the positive phase of hippocampal theta rhythm induces long-term potentiation that can Be depotentiated by stimulation on the negative phase in area CA1 in vivo

Long-term potentiation (LTP) of synaptic transmission induced by high-frequency stimulation (HFS) is considered to be a model for learning processes; however, standard HFS protocols consisting of long trains of HFS are very different from the patterns of spike firing in freely behaving animals. We have investigated the ability of brief bursts of HFS triggered at different phases of background theta rhythm to mimic more natural activity patterns. We show that a single burst of five pulses at 200 Hz given on the positive phase of tail pinch-triggered theta rhythm reliably induced LTP in the stratum radiatum of the hippocampus of urethane-anesthetized rats. Three of these bursts saturated LTP, and 10 bursts occluded the induction of LTP by long trains of HFS. Burst stimulation on the negative phase or at zero phase of theta did not induce LTP or long-term depression. In addition, stimulation with 10 bursts on the negative phase of theta reversed previously established LTP. The results show that the phase of sensory-evoked theta rhythm powerfully regulates the ability of brief HFS bursts to elicit either LTP or depotentiation of synaptic transmission. Furthermore, because complex spike activity of approximately five pulses on the positive phase of theta rhythm can be observed in freely moving rats, LTP induced by the present theta-triggered stimulation protocol might model putative synaptic plastic changes during learning more closely than standard HFS-induced LTP.

Transient and persistent consequences of acute stress on long-term potentiation (LTP), synaptic efficacy, theta rhythms and bursts in area CA1 of the hippocampus

Previous studies reported that exposure to an acute stressor of restraint and intermittent tailshock impairs long-term potentiation (LTP) in area CA1 of the rat hippocampus. In the first experiment, the longevity of the stress-induced impairment of LTP was determined. LTP of the excitatory postsynaptic potential (EPSP) was impaired 2 but not 4 days after stressor cessation. Exposure to the stressor also persistently enhanced the synaptic response to the tetanic stimulation patterned after theta rhythm activity (10, 100 Hz bursts delivered at 5 Hz). In a second experiment, we tested the hypothesis that exposure to the stressor enhanced synaptic efficacy itself. EPSPs were recorded from freely moving rats before, during and after stressor exposure. The synaptic response was not enhanced during stressor exposure. Instead, cessation of the stressor (and perhaps movement associated with release from restraint) induced a transient (< 2 min) decrease in synaptic efficacy. To determine whether exposure to the stressor enhances endogenous theta rhythms in area CA1, electroencephalographic (EEG) recordings were obtained from freely moving rats before, during and after exposure to the stressor. The power of theta (4-8 Hz) and low frequency (0.1-3.9 Hz) activity was enhanced in response to the tailshock aspect of the stressor. Together, the results indicate that exposure to an acute stressful event increases theta activity and its cessation transiently decreases synaptic efficacy. These transient effects are succeeded by a persistently sensitized response to theta burst stimulation and impaired LTP.

Brief theta-burst stimulation induces a transcription-dependent late phase of LTP requiring cAMP in area CA1 of the mouse hippocampus

Memory storage in the mammalian brain can be divided into a short-term phase that is independent of new protein synthesis and a long-term phase that requires synthesis of new RNA and proteins. A cellular model for these two phases has emerged from studies of long-term potentiation (LTP) in the three major excitatory synaptic pathways in the hippocampus. One especially effective protocol for inducing robust and persistent LTP is "theta-burst" stimulation, which is designed to mimic the firing patterns of hippocampal neurons recorded during exploratory behavior in intact awake animals. Unlike LTP induced by non-theta tetanization regimens, little is known about the biochemical mechanisms underlying theta-burst LTP in the hippocampus. In the present study, we examined theta-burst LTP in the Schaffer collateral pathway. We found that 3 sec of theta-burst stimulation induced a robust and persistent potentiation (theta L-LTP) in mouse hippocampal slices. This theta L-LTP was dependent on NMDA receptor activation. The initial or early phase of theta-LTP did not require either protein or RNA synthesis and was independent of cAMP-dependent protein kinase (PKA) activation. In contrast, the late phase of theta-LTP required synthesis of proteins and RNA and was blocked by inhibitors of PKA. Prior induction of theta-LTP also occluded the potentiation elicited by chemical activation of PKA. Our results show that, like non-theta LTP, theta-induced LTP in area CA1 of the mouse hippocampus also involves transcription, translation, and PKA and suggest that cAMP-mediated gene transcription may be a common mechanism responsible for the late phases of LTP induced by both theta and non-theta patterns of stimulation.

Studies on long-term depression in area CA1 of the anesthetized and freely moving rat

Homosynaptic long-term depression (LTD) is reported to occur in field CA1 of hippocampal slices collected from immature brains. Because the effect has been postulated to be a memory storage mechanism, it is of interest to test for its presence in adult, awake animals. Unfortunately, not only has hippocampal LTD proved difficult to obtain reliably in vivo, but the few successful studies vary with respect to protocols and evidence that the depression is input-specific. The present study tested for input-specific (homosynaptic) LTD in field CA1 after application of various stimulation protocols to the Schaffer collateral/commissural projections in freely moving, adult rats. The results indicate that although low-frequency trains do induce decrements in synaptic transmission lasting for hours to several days, the success rate of eliciting input-specific LTD in the awake rat is very modest compared with the ease with which stable potentiation is obtained in the same synapses. Moreover, it is questionable that the effective protocols represent patterns of activity likely to occur during behavior. The stronger the afferent activation during low-frequency stimulation, the greater was the probability of eliciting LTD accompanied by persistent heterosynaptic depression. Clear evidence for the occurrence of LTD, irrespective of stimulation protocol and current intensity, could not be obtained in rats under barbiturate anesthesia. In all, the results do not accord with the suggestion that LTD occurs routinely in the hippocampus in vivo as part of memory encoding.

Arg-Gly-Asp-Ser-selective adhesion and the stabilization of long-term potentiation: pharmacological studies and the characterization of a candidate matrix receptor

Peptides known to block the extracellular interactions of adhesion receptors belonging to a subclass of the integrin family were tested for their effects on the stabilization of long-term potentiation (LTP) in hippocampal slices. Theta burst stimulation delivered after infusions of Gly-Ala-Val-Ser-Thr-Ala (GAVSTA) resulted in a potentiation effect that decayed steadily over a period of 40 min; LTP elicited in the presence of inactive control peptides remained stable over this time period. GAVSTA had no detectible influence on baseline responses, induction processes, or the initial degree of potentiation. Infusions of integrin antagonists after application of theta bursts also resulted in the occurrence of a decremental form of LTP. Affinity chromatography was then used in an effort to identify targets of the structurally dissimilar integrin blockers that disrupt LTP stabilization. Both integrin antagonists Gly-Arg-Gly-Asp-Ser-Pro and GAVSTA eluted a major species of 55 kDa (synaptegrin-1) from GRGDSP-affinity columns that had been loaded with solubilized synaptic membranes; lesser concentrations of three polypeptides of approximately 20, 27, and 30 kDa were also collected. Synaptegrin-1 was labeled by antibodies to the RGDS-binding integrin alpha5beta1. In addition, the synaptegrin, as well as the 27 kDa, protein was found to copurify with pre- and postsynaptic markers during the isolation of forebrain synaptosomes. These results indicate that a matrix recognition event occurring several minutes after induction of LTP is a necessary step in the stabilization of potentiated synapses; they also identify an integrin-like matrix receptor of 55 kDa that may contribute to this event.

Microdialysis-coupled place cell detection in the hippocampus: a new strategy for the search for cognition enhancer drugs

1. The MPCD method in freely moving rats is a new neuroscience technique. It is able to detect the location-specific firing of hippocampal place cells, and to deliver, via microdialysis, various drug solutions into the extracellular environment of the detected neurons. Place cells are critical elements of the neural system in brain which governs cognitive processes. It is emphasized in this article that effective cognition enhancer drugs must selectively and significantly affect the firing of these cells. 2. By using MPCD, it is possible to recognize drug combinations which can increase the location-specific firing of place cells to an optimal level. This paper proposes that such pharmacological action facilitates engram-creation in extrahippocampal cortical areas, improving cognitive functions. Thus, an MPCD-based research strategy may lead to the rational development of a new generation of cognition enhancer drugs for the treatment of learning and memory disorders, including Alzheimer's disease (AD).

Piriform cortex efferents to the entorhinal cortex in vivo: kindling-induced potentiation and the enhancement of long-term potentiation by low-frequency piriform cortex or medial septal stimulation

The entorhinal cortex receives input from many cortical areas and mediates the flow of information between these sites and the hippocampal formation. Long-term synaptic plasticity in cortical efferents to the entorhinal cortex may contribute to the transmission of neural activity to the hippocampus, as well as the storage of information, but little is known about plasticity in these pathways. We describe here the use of evoked field potential recordings from chronically implanted electrodes in the rat entorhinal cortex to investigate synaptic plasticity in the large piriform (olfactory) cortex projection to the superficial layers of the entorhinal cortex. Both kindling-induced potentiation and long-term potentiation (LTP) were tested. In addition, we attempted to modulate LTP induction by the co-induction of frequency potentiation and by the co-activation of the medial septum. Epileptogenic kindling stimulations of the piriform cortex (1-s, 60-Hz trains 3 times/day for 5 days) were found to result in a reliable potentiation of field responses evoked by piriform cortex test pulses. Non-epileptogenic tetanization of the piriform cortex with 400-Hz 16-pulse trains reliably resulted in LTP effects. These effects could be augmented by embedding brief LTP induction stimuli within 11-pulse, 15-Hz trains that alone produce only frequency potentiation. Co-activating the medial septum with 10-Hz trains, just prior to tetanization of the piriform cortex, augmented LTP of piriform cortex inputs to the entorhinal cortex in an input-specific manner. All potentiation effects were found to last for periods of weeks. These findings demonstrate that both epileptogenic and non-epileptogenic piriform cortex stimulation induces lasting potentiation of population field responses in the entorhinal cortex of the awake rat. The LTP effects were inducible in a graded manner and were sensitive to the temporal context of stimulation. The finding that low-frequency activation of the septum can enhance plasticity in the entorhinal cortex adds to a body of data indicating a role for the medial septum in contributing to theta activity and plasticity in both the entorhinal cortex and hippocampal formation.

Stimulus-dependent induction of long-term potentiation in CA1 area of the hippocampus: experiment and model

In the CA1 area of the hippocampus, the magnitude of long-term potentiation (LTP) depends not only on the frequency of applied stimuli, but also on their number. With a slice preparation using extracellular recording in the hippocampus CA1 of a guinea pig, we investigate the magnitude of LTP induced by electrical stimuli with a range of frequencies and the number of applied stimuli. We find that the magnitude of the saturated potentiation obtained with periodic stimuli largely depends on the frequency and is insensitive to the number of stimuli, once the saturation level has been obtained. Furthermore, we investigated nonperiodic stimuli and found that the magnitude of the saturated potentiation is also sensitive to the statistical correlation between successive interstimulus intervals, even when their average frequency is held constant. In order to explain the LTP dependence on these various experimental parameters, we propose a simple mathematical model for the induction of LTP. In the model, an exponentially decaying element released as a result of previous stimuli is coupled with a new stimulus to act as the potentiation force, and the magnitude of potentiation is determined by this potentiation force. We can determine the decaying time constant of this hypothetical element as a model parameter by fitting the model to the experimental data. The time scale is found to be of the order of 200 msc. A molecular or cellular factor with this decaying time constant is likely to be induced in LTP induction.

Phase relations of rhythmic neuronal firing in the supramammillary nucleus and mammillary body to the hippocampal theta activity in urethane anesthetized rats

Structures in the caudal diencephalon including the posterior hypothalamic nucleus, the supramammillary nucleus (SUM) and the nuclei of the mammillary body (MB) occupy a strategic position in the crossroads of ascending and descending traffic between the brainstem and the limbic forebrain (septum/hippocampus). In this study we analyzed the phase relations of rhythmically discharging SUM/MB cells to hippocampal theta rhythm in urethane anesthetized rats with a dual aim of separating different functional types of SUM and MB neurons and characterizing their coupling to septohippocampal theta oscillators. We found that rhythmically firing neurons in the SUM/MB represent a functionally heterogenous population of cells that are coupled with forebrain theta oscillators at different preferred phases. Based on their phase relations to hippocampal theta four groups of rhythmic SUM/MB cells were identified. Neurons of the first and second groups fired out-of-phase relative to each other and synchronously with the positive (8 degrees +/- 7) or negative peaks (-177 degrees +/- 7) of theta field activity in the hippocampus, recorded above the CA1 pyramidal layer. Cells of the other two groups, also forming out-of-phase counter-parts, fired on the rising (97 degrees +/- 9) or falling segments (-97 degrees +/- 6) of CA1 theta waves. The peaks in the phase distribution histogram were well separated, and the empty zones between them were wider (40-70 degrees) than those comprising the phase data for different groups. The variations of phase values for individual neurons, when tested during several theta epochs, did not exceed the range of a single group. Theta field potentials were also recorded in the SUM/MB and were advanced by one quarter of the cycle (79 degrees +/- 9, range 56-99 degrees) relative to CA1 theta oscillations. The present results indicate that, similar to other theta-generating structures, rhythmically firing neurons can be classified on the basis of their phase relations in the SUM/MB as well. Different classes of SUM/MB neurons might play different roles in generating and/or transmitting theta rhythmic activity of the limbic system.

Long-term potentiation: a good model for learning and memory?

1. Long-term potentiation (LTP), long-term depression (LTD), and depotentiation of synaptic activity have been suggested to model synaptic plastic changes that occur during learning. Recent reports however show that neither LTP induced by high frequency stimulation (HFS) in the dentate, CA3, or CA1, nor depotentiation in area CA1 of the hippocampus, are reliable models of the learning abilities of rats or mice. LTD cannot reliably be obtained in the hippocampus in vivo and might be an artefact caused by altered inhibitory transmission. 2. Experiments with gene deletion ('knock out') mice strains show that mice that do not express HFS-induced LTP in the dentate are able to learn spatial tasks. 3. Studies of the effect of NMDA receptor blockers also showed that HFS-induced LTP in the dentate is not a model for processes that occur during learning. Studies using drugs that act on metabotropic glutamate receptors showed that HFS-induced LTP or depotentiation of LTP in area CA1 are not models for learning mechanisms either. 4. Neither in vivo recording of naturally-occurring LTP in the dentate nor synaptic saturation experiments in the hippocampus was able to support the theory that LTP occurs during learning. 5. While in vitro experiments are essential tools to investigate cellular and subcellular mechanisms that underlay synaptic transmission, measurements of LTP, LTD, or DP are not reliable models for learning processes and cannot replace experiments with intact animals that learn spatial tasks.

Epileptiform activity in the piriform cortex of the in vitro isolated guinea pig brain preparation

Brief intracerebral injections of bicuculline in the anterior portion of the piriform cortex of the isolated guinea pig brain preparation induce a transient, localized disinhibition confined to the site of drug application. The epileptiform activity generated at the bicuculline focus propagates and induces secondary excitability changes in remote cortical regions within the olfactory lobe. Long lasting changes in synaptic potentials were observed in the posterior piriform cortex, where long-projective cortico-cortical fibers originating from the site of bicuculline injection terminate. The activation of rhythmic, transient afterdischarges at the bicuculline focus was critical for the development of persistent 'epileptiform' associative synaptic potentials in remote cortical regions. When transient afterdischarges were prevented, epileptiform associative potentials in the posterior piriform cortex appeared only transiently and vanished within 20 min. The persistent secondary changes in synaptic excitability that occur in cortical regions distant from the acutely-induced primary epileptic focus may represent one of the transition mechanisms toward chronic epileptogenesis.

Effects of LTP on Response Selectivity of Simulated Cortical Neurons

We report here on specific ways in which synaptic long-term potentiation (LTP) affects the response selectivity of primary sensory cortical cells. LTP increases synaptic efficacy by incremental “steps,” up to a “ceiling” at which additional bursts of afferent stimulation cause no further potentiation. Endogenous and exogenous agents have been shown to modulate these two paramenters of LTP, raising the question of the functional implications associated with the sizes of steps and ceiling. We provide an analytical treatment of the effects of these two physiological LTP parameters on the behavior of simulated olfactory (piriform) cortex target cells in response to a range of inputs. A target cell's receptive field, i.e., the set of input patterns to which the cell responds, is broadened with potentiation of the cell's synapses, and is broadened more when the LTP step size is smaller, and when the LTP ceiling is higher. Moreover, the effects of step size and ceiling interact, and their joint relationship to receptive field breadth is nonlinear. Values of step size and ceiling are identified that balance the tradeoff between learning rate and receptive field breadth for particular sensory recognition tasks, and these model values are compared to corresponding known and inferred physiological values.

Synapse restructuring associated with the maintenance phase of hippocampal long-term potentiation

Synapses in the middle molecular layer of the rat dentate gyrus were analyzed by electron microscopy during the maintenance phase of long-term potentiation (LTP). LTP was induced by high-frequency stimulation of the medial perforant path carried out on each of 4 consecutive days. The dentate gyrus was examined electron microscopically 13 days following the fourth stimulation. At this time point, synaptic responses were still significantly enhanced relative to baseline, although the extent of their potentiation was lower than 1 hour after the last high-frequency stimulation. Stimulated, but not potentiated, rats served as controls. Using the stereological double disector method, estimates of the number of different morphological types of synapses per postsynaptic neuron were obtained. The number of asymmetrical axodendritic synapses increased (by 28%) during LTP maintenance, whereas the number of other synaptic types was not significantly altered. Our previous work demonstrated that the induction of LTP is followed by a selective increase in the number of axospinous perforated synapses with multiple, completely partitioned, transmission zones. Thus, the induction and maintenance phases of LTP are characterized by different structural synaptic alterations. These alterations may be related to each other as indicated by another finding of the present study regarding the existence of perforated synapses that appear to be transitional between axospinous and axodendritic junctions. This suggests a model of structural synaptic plasticity associated with LTP in which some axospinous perforated synapses increase in numbers shortly after the induction of LTP and are then converted into axodendritic ones during LTP maintenance.

Long-term potentiation and learning

Long-term potentiation (LTP), a relatively long-lived increase in synaptic strength, remains the mot popular model for the cellular process that may underlie information storage within neural systems. The strongest arguments for a role of LTP in memory are theoretical and involve Hebb's Postulate, Marr's theory of hippocampal function, and neural network theory. Considering LTP research as a whole, few studies have addressed the essential question: Is LTP a process involved in learning and memory? The present manuscript reviews research that attempts to link LTP with learning and memory, focusing on studies utilizing electrophysiological, pharmacological, and molecular biological methodologies. Most evidence firmly supports a role for LTP in learning memory. However, an unequivocal experimental demonstration of a contribution of LTP to memory is hampered by our lack of knowledge of the biological basis of memory and of the ways in which memories are represented in ensembles of neurons, the existence of a variety of cellular forms of LTP, and the likely resistance of distributed memory stores to degradation by treatments that incompletely disrupt LTP.

Distinct memory circuits composing the hippocampal region

The very different anatomical designs of the adjacent circuitries of the cortico-hippocampal pathway, along with their somewhat different synaptic plasticity mechanisms, suggest a nearly serial pathway of distinct memory circuits each contributing its own specialized processing operation to overall hippocampal function. Modeling and formal theoretical analysis of the prominent anatomical design features of particular circuits (piriform/entorhinal cortex; hippocampal field CA3; hippocampal field CA1) are found to identify potential emergent function not readily arrived at in the absence of these formal models, and yet which once derived can be seen potentially to confer unique capabilities to an integrated hippocampal mechanism for processing memories during behavior.

Bidirectional synaptic plasticity induced by a single burst during cholinergic theta oscillation in CA1 in vitro

In standard protocols, the frequency of synaptic stimulation determines whether CA1 hippocampal synapses undergo long-term potentiation or depression. Here we show that during cholinergically induced theta oscillation (theta) synaptic plasticity is greatly sensitized and can be induced by a single burst (4 pulses, 100 Hz). A burst given at the peak of theta induces homosynaptic LTP; the same burst at a trough induces homosynaptic LTD of previously potentiated synapses. Heterosynaptic LTD is produced at inactive synapses when others undergo LTP. The synaptic modifications during theta require NMDA receptors and muscarinic receptors. The enhancement is cooperative and occludes with standard LTP. These results suggest that the similar bursts observed during theta rhythm in vivo may be a natural stimulus for inducing LTP/LTD.

Difference in LTP at basal and apical dendrites of CA1 pyramidal neurons in urethane-anesthetized rats

In urethane-anesthetized rats, excitatory currents in hippocampal CA1 area following local stimulation were analyzed using the current source density technique. Systematic variation of stimulus depth revealed two dominant patterns of activation: basal versus apical dendritic excitation. The basal dendritic excitation (sink) was maximal after stimulation of stratum oriens; its onset latency was consistent with a monosynaptic excitation, and a late (presumed di- and polysynaptic) sink at the apical dendrites was observed. The apical dendritic excitation (sink) was maximal after stimulation of stratum radiatum; the early latency, presumed monosynaptic excitation was followed by a late, weak basal dendritic sink. Theta-frequency primed bursts of either high (400 microA) or low intensity (40-70 microA; 2 x threshold intensity) was delivered to either stratum oriens or radiatum, and long-term potentiation (LTP) was assessed. LTP was observed in both positive and negative components of the dipole field, as well as the (active) sink and (passive) source. Tetanus of stratum oriens resulted in a significant (P < 0.05) potentiation of the monosynaptic basal sink (at 2 ms from onset) following either high (135 + 11%, means +/- S.E.M., n = 7) or low intensity tetanus (178 +/- 21%, n = 7). Tetanus of stratum radiatum resulted in significant potentiation of the rise of the apical sink only for a high (173 +/- 51%, n = 6), but not for a low intensity tetanus (106 +/- 28%, n = 6). Significant LTP of the late apical sink was found following a high intensity tetanus of stratum oriens (155 +/- 15%, n = 7), but no significant change of the late basal sink was found following tetanization of stratum radiatum (118 +/- 20%, n = 6). UP of either the apical and basal sinks was blocked by intraventricular infusion of +/- 2-amino-5-phosphonovalerate, an N-methyl-D-aspartate antagonist. Thus, UP has a lower threshold at the basal than apical synapses of CA1 cells.

Effects of NMDA receptor modulation on hippocampal type 2 theta activity in rats

1. The present study was designed to investigate whether pharmacological modulation of N-methyl-D-aspartate (NMDA) receptor function could modify hippocampal type 2 theta activity in the dentate gyrus of rats. 2. The effects of pre-recording administration of d-cycloserine (DCS: 1.0, 3.0 and 9.0 mg/kg, i.p.), a partial agonist at the NMDA receptor associated glycine site, and MK-801 (0.1 mg/kg, i.p.), a noncompetitive NMDA receptor antagonist, were examined in freely moving rats. 3. Using adult Wistar rats, which had recording electrodes implanted unilaterally into the hilus of dentate gyrus, we recorded five 4 sec epochs of awake-immobility-related hippocampal EEG activity bands (1-20 Hz) 40 min after d-cycloserine and 2 hr after administration of MK-801. 4. In the off-line analysis, the spectral power and the frequency at the maximal theta power were calculated. 5. D-cycloserine (1.0-9.0 mg/kg) did not affect the frequency at the maximal theta power. However, the dose of 3.0 mg/kg, though not the 1.0 or 9.0 mg/kg doses, significantly increased the spectral power of the hippocampal immobility-related EEG activity. 6. In line with the previous findings, 0.1 mg/kg MK-801 decreased both the frequency at the maximal theta power as well as the spectral power of hippocampal type 2 EEG activity. 7. The present data show a clear relationship between NMDA receptors and hippocampal type 2 theta activity and suggest that the pharmacological modulation of the receptor function, using appropriate doses of glycine binding site agonist, d-cycloserine, may be a possible means to positively modulate the immobility-related hippocampal EEG activity.

Ascending projections of the posterior nucleus of the hypothalamus: PHA-L analysis in the rat

With the exception of a report by R.B. Veazey, D.G. Amaral, and W.M. Cowan (1982, J. Comp. Neurol. 207:135-156) that examined the projections of the posterior hypothalamic area in the monkey by using the autoradiographic technique, the ascending projections of the posterior nucleus (PH) of the hypothalamus have not been systematically examined in any species. The present report describes the ascending projections of PH in the rat by using the anterograde anatomical tracer, Phaseolus vulgaris-leucoagglutinin (PHA-L). The major ascending route for PH fibers is the medial forebrain bundle. PH fibers project densely to several subcortical and cortical sites. The subcortical sites are the subthalamus/hypothalamus (zona incerta, the supramammillary nucleus, lateral, perifornical, dorsal, and anterior nuclei/areas), the thalamus (lateroposterior, laterodorsal, parafascicular, reuniens, paraventricular, central medial, paracentral, central lateral and intermediodorsal nuclei), the amygdala (central, lateral, and medial nuclei), the septal area (bed nucleus of stria terminalis, medial and lateral septum), and the basal forebrain (horizontal/vertical limbs of diagonal band nuclei and lateral preoptic area). The cortical sites are the perirhinal, insular, frontal (lateral agranular), prelimbic, and infralimbic cortices. The diversity of PH projections to subcortical and cortical "limbic-related" sites and to several structures with direct input to the hippocampus (supramammillary nucleus, reuniens, paraventricular and laterodorsal nuclei of the thalamus, medial and lateral septum, and perirhinal cortex) suggest that the PH may serve a critical role in various components of emotional behavior, including mnemonic processes associated with significant emotional events.

Analysis of the decremental nature of LTP in the dentate gyrus

The persistence of long-term potentiation (LTP) in the dentate gyrus was compared for two tetanization protocols: 50 trains on one day, or 50 trains on 5 consecutive days. LTP induction was significantly greater in the 250 train condition, but the LTP decay rate over weeks was similar between conditions. The decay of LTP could not be accounted for by deterioration of the preparation. Successive days of stimulation caused repetitive induction of immediate early genes, but did not prolong LTP, suggesting that either the effects of gene expression on LTP stabilization had saturated, or that these genes play other roles in synaptic plasticity.

Expression, control, and probable functional significance of the neuronal theta-rhythm

The data on theta-modulation of neuronal activity in the hippocampus and related structures, obtained by the author and her colleagues have been reviewed. Analysis of extracellularly recorded neuronal activity in alert rabbits, intact and after various brain lesions, in slices and transplants of the hippocampus and septum allow one to make the following conclusions. Integrity of the medial septal area (MS-DB) and its efferent connections are indispensable for theta-modulation of neuronal activity and EEG of the hippocampus. The expression of hippocampal theta depends on the proportion of the MS-DB cells involved in the rhythmic process, and its frequency in the whole theta-range, is determined by the corresponding frequencies of theta-burst in the MS-DB. The neurons of the MS-DB have the properties of endogenous rhythmic burst and regular single spike oscillators. Input signals ascending to the MS-DB from the pontomesencephalic reticular formation increase both the frequency of the MS-DB theta-bursts and the proportion of neurons involved in theta-activity; serotonergic midbrain raphe nuclei have the opposite effect on the MS-DB rhythmic activity and hippocampal EEG theta. Increase of endogenous acetylcholine (by physostigmine) also increases the proportion of the MS-DB neurons discharging in theta-bursts (both in intact and basally-undercut septum), but does not influence the theta-frequency. The primary effect of the MS-DB on hippocampal neurons (pyramidal and non-pyramidal) consists in GABAergic reset inhibition. Reset inhibition, after which theta-modulation follows in constant phase relation, is triggered also by sensory stimuli. About two-thirds of the hippocampal pyramidal neurons are tonically inhibited by sensory stimuli which evoke EEG theta, while others are excited, or do not change their activity. Anticholinergic drugs restrict the population of rhythmic neurons but do not completely suppress theta-bursts in the MS-DB and hippocampus. Under their action, EEG theta can be evoked (presumably through GABAergic MS-DB influences) by strong reticular or sensory stimuli with corresponding high frequency. However information processing in this condition is defective: expression of reset is increased, responses to electrical stimulation of the perforant path and to sensory stimuli are often augmented, habituation to sensory stimuli is absent and tonic responses are curtailed. On a background of continuous theta induced by increase of endogenous acetylcholine, reset is absent or reduced, responsiveness of the hippocampal neurons to electrical and sensory stimulation is strongly reduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Extracellular phosphorylation of membrane protein modifies theta burst-induced long-term potentiation in CA1 neurons of guinea-pig hippocampal slices

The involvement of ecto-protein kinase activity in activity-dependent long-term potentiation (LTP) was studied in CA1 neurons of guinea-pig hippocampal slices. Application of 5 microM K-252b, an ecto-protein kinase inhibitor, blocked LTP induced by a theta-burst stimulation (3 bursts composed of 5 pulses at 100 Hz with inter-burst intervals of 200 ms). On the other hand, under 10 microM RK682, an ecto-phosphatase inhibitor, a robust LTP was induced by a weak theta-burst stimulation (3 bursts composed of 3 pulses) which was just at the threshold for the induction of LTP in the control perfusate. These findings suggest that ATP released from presynaptic terminals during the burst stimulation plays an important role in the induction of LTP through phosphorylation of extracellular domains of synaptic membrane proteins, as the substrate for ecto-protein kinase.

Alterations in the immunoreactivity for muscarinic acetylcholine receptors and colocalized PKC gamma in mouse hippocampus induced by spatial discrimination learning

This study describes changes in the immunoreactivity for muscarinic acetylcholine receptors (mAChRs) in the hippocampus of mice in relation to spatial discrimination behavior, employing the monoclonal antibody M35 raised against purified bovine mAChR protein. Performance in a hole board in which the animals learned the pattern of 4 baited holes out of 16 holes served as the measure of spatial discrimination learning and memory. Twenty-six adult male house mice were used, divided into four groups. Three groups served as various controls: group N (naive; blank controls); group H (habituated; animals were introduced to the hole board with all holes baited for 5 consecutive days), and group P (pseudo-trained; the animals were admitted to the hole board for 13 consecutive days with all holes baited). The T group (trained) was subjected to the hole board for 5 consecutive habituation days with all holes baited (similar to the H and P groups), followed by 8 successive training days with only four holes baited in a fixed pattern. During the 8 training days, the T group gradually acquired a pattern to visit the baited holes, whereas the P group continued visiting holes in a random fashion. The mice were killed 24 h after the last behavioral session. All principal cells in teh cornu ammonis (CA) and dentate gyrus (DG) of the habituated animals revealed increased levels of mAChR immunoreactivity (mAChR-ir) over the naive mice. A minor increase in mAChR-ir was found in the apical dendrites of the CA1 pyramidal cells. Pseudotraining resulted in a CA1-CA2 region with a low level of mAChR-ir, resembling naive animals, whereas the trained mice showed a further increase in mAChR-ir in the CA1-CA2 pyramidal cell bodies and apical dendrites. Optical density measures of the mAChR-ir in the CA1 region revealed a significant (P < 0.05) increase in the pyramidal cell bodies of the H and T group over the N and P group, and a significant (P < 0.05) increase in the apical dendrites of the T group over all other groups. In contrast to the CA1-CA2 region, both pseudotrained and trained mice revealed high mAChR staining in the CA3-CA4 region and the DG. These results indicate that prolonged exposure to the hole board is sufficient for an enhanced mAChR-ir in the CA3-CA4 and DG, whereas the increase in CA1-CA2 pyramidal cells is a training-specific feature related to spatial orientation. Nonpyramidal neurons within the CA1-CA2 region with enhanced mAChR-ir in the pyramidal cells, however, revealed a decreased level of mAChR-ir. The opposing effect of pyramidal and nonpyramidal cells suggests a shift in the excitability of the hippocampal microcircuitry. Previously we demonstrated an increase and redistribution of hippocampal protein kinase C gamma-immunoreactivity (PKC gamma-ir) induced by hole board learning in mice (Van der Zee et al., 1992, J Neurosci 12:4808-4815). Immunofluorescence double-labeling experiments conducted in the present study in naive and trained animals revealed that the principal cells and DG interneurons co-express mAChRs and PKC gamma, and that the immunoreactivity for both markers increased in relation to spatial orientation within these neurons. The mAChR-positive nonpyramidal cells of the CA1-CA2 region were devoid of PKC gamma and revealed an opposite training-induced effect. These results suggest that the postsynaptic changes in mAChR- and PKC gamma-ir reflect functional alterations of the hippocampal formation induced by spatial learning.

Further characteristics of long-term potentiation in piriform cortex

Mechanisms for the induction and expression of long-term potentiation (LTP) were studied in slices of piriform cortex. Cooperativity among afferent inputs as a controlling factor for induction of LTP was tested by pairing stimulation of one input that normally does not induce LTP with stimulation of another input. Combined stimulation, given either to two weak inputs with simultaneous bursts or by pairing single pulses with bursts, did effectively induce LTP. Tests for expression of LTP by NMDA vs. non-NMDA receptors indicated that non-NMDA receptor-mediated responses expressed much greater LTP than NMDA receptor-mediated responses. Ratios for paired-pulse facilitation and depression were not altered after induction of LTP. These characteristics are comparable to those exhibited by synapses in the CA1 field of hippocampus.

Centrally active modulators of glutamate receptors facilitate the induction of long-term potentiation in vivo

An experimental drug, 1-(1,3-benzodioxol-5-ylcarbonyl)piperidine, that facilitates glutamatergic transmission in brain after systemic administration was tested for its effects on the induction of long-term potentiation in the hippocampus of rats. Intraperitoneal injections of the drug markedly increased the degree and duration of long-term potentiation; similar results were obtained with an analogue of 1-(1,3-benzodioxol-5-ylcarbonyl)piperidine that was also found to improve retention of memory in a radial maze task and in an odor-matching problem. These results define tools for enhancing long-term potentiation in vivo and confirm an important prediction from the hypothesis that long-term potentiation is a substrate of memory.

Immediate early gene expression associated with the persistence of heterosynaptic long-term depression in the hippocampus

Long-term depression (LTD) of synaptic efficacy is likely to be as important in memory processing as the more well-known long-term potentiation (LTP). The case for LTD serving as a memory mechanism, however, requires that it be shown to persist across days or weeks at least. Here we examined the persistence of heterosynaptic LTD in the medial and lateral perforant path inputs to the dentate gyrus in awake rats and correlated this persistence with the degree of immediate early gene expression as assessed immunohistochemically. Rats were chronically implanted with separate stimulating electrodes in the medial and lateral perforant paths and an extracellular field potential recording electrode in the dentate hilus. After recovery from surgery, either the medial or the lateral perforant path was tetanized with 400-Hz trains, and homosynaptic LTP and heterosynaptic LTD were followed across time. Heterosynaptic LTD was shown to occur readily in awake animals and to persist across days or weeks, depending on the stimulation protocol. The persistence of LTD and LTP was highly correlated within animals. Additional animals, given the same tetanization protocols, showed that the greatest immediate early gene expression occurred following that protocol which consistently gave the longest-lasting LTP and LTD. These data support the proposed role of LTD in memory processing but question whether immediate early genes are important for the persistence of LTP, LTD, or both.

Non-Hebbian properties of long-term potentiation enable high-capacity encoding of temporal sequences

A hypothesis commonly found in biological and computational studies of synaptic plasticity embodies a version of the 1949 postulate of Hebb that coactivity of pre- and postsynaptic elements results in increased efficacy of their synaptic contacts. This general proposal presaged the identification of the first and still only known long-lasting synaptic plasticity mechanism, long-term potentiation (LTP). Yet the detailed physiology of LTP induction and expression differs in many specifics from Hebb's rule. Incorporation of these physiological LTP constraints into a simple non-Hebbian network model enabled development of "sequence detectors" that respond preferentially to the sequences on which they were trained. The network was found to have unexpected capacity (e.g., 50 x 10(6) random sequences in a network of 10(5) cells), which scales linearly with network size, thereby addressing the question of memory capacity in brain circuitry of realistic size.

Identification of a latent state arising in the hippocampus following the cessation of long-term potentiation

The possibility of the restoration of long-term potentiation in the CA1 region and the dentate gyrus of the hippocampus during stimulation respectively of the dorsal raphé nuclei and locus coeruleus, with stimulus parameters inducing behavioral reactions, was investigated in freely-behaving rats. It was demonstrated that stimulation of the locus coeruleus, which was ineffective prior to the tetanization of the perforant path, led to the restoration of extinguished long-term posttetanic potentiation in the dentate gyrus of the hippocampus induced by tetanization of the perforant path. Stimulation of the dorsal raphé nucleus, which was ineffective prior to the tetanization of the Schaffer collaterals, led to the restoration of long-term posttetanic potentiation in the CA1 region of the hippocampus induced by tetanization of the Schaffer collaterals. A mathematical model is proposed which has made it possible to describe the restoration of long-term posttetanic potentiation on the basis of the notion of the existence of several states of calcium/calmodulin-dependent protein kinase. The restoration of long-term potentiation during stimulation of emotiogenic zones was examined as a model of the phenomenon of emotional reminding.

Intrahippocampal administration of both the D- and the L-isomers of AP5 disrupt spontaneous alternation behavior and evoked potentials

We previously reported that systemically administered N-methyl-D-aspartate (NMDA) antagonists significantly impair spontaneous alternation behavior. Others have reported that the restricted blockade of hippocampal NMDA receptors disrupts performance on different tests of spatial learning and have suggested that the resulting impairments are attributable to a disruption of endogenous NMDA-dependent long-term potentiation (LTP). In the present study, we determined whether spontaneous alternation performance was disrupted by circumscribed blockade of hippocampal NMDA receptors as well as by a second class of compounds which disrupt LTP, protein kinase inhibitors. The effect of hippocampal NMDA blockade on inhibitory avoidance was also examined insofar as this behavior too is disrupted by systemically administered NMDA antagonists. When injected into the hippocampus 15 min prior to spontaneous alternation testing, the NMDA antagonists CPP and D,L-AP5 each decreased alternation rates. The specific protein kinase C (PKC) inhibitor, NPC 15437, also disrupted spontaneous alternation, whereas the more general kinase inhibitor, PMXB, did not. When injected 15 min prior to inhibitory avoidance training, CPP also impaired inhibitory avoidance learning as assessed during a subsequent test session, 48 h later. Interpretation of these data was complicated by the additional findings that intrahippocampal infusion of L-AP5 (which is inactive with respect to NMDA receptors) also disrupted alternation performance, and that both the D- and the L-isomers of AP5 as well as each kinase inhibitor dramatically disrupted evoked responses (i.e., population spike amplitude, spike latency, and EPSP slope), as recorded in the dentate gyrus and evoked by perforant path stimulation. These data indicate that behaviorally effective doses of AP5 may have effects which extend beyond NMDA blockade. Moreover, the effects of these compounds on hippocampal transmission, in general, suggest that attribution of the amnestic consequences of their administration to impaired LTP may be unwarranted.

Evaluation of the hypothesis that hippocampal interictal spikes are caused by long-term potentiation

Hippocampal spontaneous interictal spikes (SISs) occur in the EEG after repeated afterdischarges (ADs) induced by high-frequency (200 Hz) stimulation trains. Because SISs resemble population excitatory postsynaptic potentials (EPSPs), and SISs persist for several days like some types of hippocampal long-term potentiation (LTP), LTP has been suggested as a mechanism for SISs. We specifically examined the hypothesis that SISs are caused by basal-dendritic LTP in CA1. Rats were chronically implanted with bilateral electrodes in the hippocampal CA1 region. In the first experiment, 10-18 patterned primed burst (PB) stimulations were delivered hourly for 2-3 days to activate the commissural basal-dendritic EPSP in CA1. Robust LTP of the basal-dendritic CA1 synapse was detected, typically saturating at 100% enhancement after five stimulations. However, few SISs were detected if ADs were not elicited. In the second experiment, repeated commissurally evoked ADs induced a high rate of SISs, together with LTP of the basal-dendritic and apical-dendritic EPSP in CA1, but the SIS rate was not necessarily related to the level of LTP. In the third experiment, an intraventricular dose (20 micrograms) of an N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovalerate (APV) was used to block the basal-dendritic LTP in CA1. The increase in SISs induced by a single AD was not blocked, however, suggesting that NMDA receptors were not critical in generation of SISs. In the fourth experiment, PBs (that induced LTP but no ADs) were able to increase the rate of SISs marginally when SISs were already present. In all, the experiments suggest that LTP at the basal dendrites of CA1 is not critical in generation of hippocampal SISs, although an increase in LTP may increase the rate of SISs marginally when SISs are already present.

LTP in hippocampal CA1 of urethane-narcotized rats requires stronger tetanization parameters

Rats with chronically implanted electrodes in the hippocampal CA1 region were tested in their capacity to express and maintain long-term potentiation (LTP) of the population spike (PS) or of the field excitatory postsynaptic potential (fEPSP). Two different states were compared: a) freely moving animals; b) urethane-anesthetized animals (1 g/kg, IP). We found that a short, high-frequency tetanus (six bursts of 15 pulses; 200 Hz; double-pulse width; interburst interval 10 s) increased PS amplitudes and fEPSP slopes up to 300% in response to test stimuli in double-pulse width; interburst interval 10 s) increased PS amplitudes and fEPSP slopes up to 300% in response to test stimuli in the awake rat. The PS amplitude slowly decreased in time, returning to baseline levels 4 h post-tetanically, whereas the fEPSP slope remained at higher values for 24 h. Urethane injection reduced the fEPSP slope and abolished the PS to normal test pulses. We thus increased the strength of the test stimuli until we again recorded magnitudes of PSs and fEPSPs comparable to those in the awake animal. In conjunction with these stronger stimuli, tetanus-induced LTP was elicited that for the PS was increased in magnitude and prolonged in duration compared to the untreated control group. Although, stronger tetanic stimuli were applied to the narcotized fEPSP group too, no difference was found compared to controls. These results suggest that urethane narcosis influences the sensitivity of CA1 neurons to express LTP. Stronger stimulation was required to induce and maintain a long-lasting potentiation of the fEPSP slope and PS amplitude.

Psychological stress repeatedly blocks hippocampal primed burst potentiation in behaving rats

Primed burst (PB) potentiation is a long-term increase in CA1 population spike amplitude produced by brief physiologically patterned electrical stimulation of the hippocampal commissure. Exposure of rats to a novel environment resulted in a blockade of short-term (Post-tetanic potentiation, PTP) and long-term (PB potentiation) plasticity in all cases (n = 6). When the animals had extensive exposure to the environment (14 consecutive days), PTP and PB potentiation occurred. With placement of the rats in a second novel environment, once again, neither PTP nor PB potentiation was observed. Placement of rats in each of the two novel environments produced a significant increase in serum corticosterone levels, while placement in the familiar environment produced no increase in response. These findings support the hypothesis that hippocampal plasticity is repeatedly susceptible to modulation by the stress of forced exposure to a novel environment.

Pharmacological suppression of the median raphe nucleus with serotonin1A agonists, 8-OH-DPAT and buspirone, produces hippocampal theta rhythm in the rat

The effects on the hippocampal electroencephalogram of microinjections of procaine hydrochloride and the serotonin1A agonists, 8-OH-DPAT and buspirone, into the median raphe nucleus were examined in the urethane anesthetized rat. Injections of procaine, 8-OH-DPAT or buspirone into the median raphe nucleus produced a change in the hippocampal electroencephalogram from a spontaneous desynchronized pattern to a synchronized pattern (theta rhythm) within short latencies and for long durations post-injection. Procaine was shown to elicit theta at a mean latency of 52 s and for a mean duration of 21.75 min; buspirone at a mean latency of 2 min and for a mean duration of 34.5 min. A dose dependent relationship was observed between 8-OH-DPAT injections and latencies but not durations. Small doses (0.5 micrograms) of 8-OH-DPAT produced theta at a mean latency of 1.33 min and large doses (3.0 micrograms) at a mean latency of 1.17 min. 8-OH-DPAT injections generated theta for a mean duration of 62 min. Injections of each of these substances into structures dorsal, lateral or rostrocaudal to the median raphe (dorsal raphe nucleus, pontine reticular formation, caudal linear nucleus or raphe pontis, respectively) failed to generate theta or in a few cases produced theta at very long latencies (> 24 min). Saline injections in the median raphe nucleus or control structures were without effect. The demonstration that agents injected into the median raphe nucleus that inhibit its activity (procaine and serotonin1A agonists) produce theta indicate that serotonin-containing median raphe neurons normally suppress theta or are involved in the control of hippocampal desynchronization. The present findings are consistent with previous work showing that median raphe nucleus stimulation desynchronizes the hippocampal electroencephalogram and that median raphe nucleus lesions produce constant theta, but are at odds with the proposal that serotonergic mechanisms may play a role in the generation of the theta rhythm.

Serotonin controls the magnitude of LTP induced by theta bursts via an action on NMDA-receptor-mediated responses

The present studies examined the inhibitory effects of serotonin (5-HT) on LTP in the context of the theta burst stimulation paradigm and its known relationship to the induction chemistries of LTP. Comparisons were made between the effects of various dosages of 5-HT on: (i) the extent to which the second member of a pair of theta bursts was facilitated over the first member of the pair; and (ii) the degree of LTP produced by the paired bursts. Both LTP and burst facilitation were affected in a graded manner by the drug: at high concentrations LTP was completely blocked and burst enhancement was minimal, at lower dosages LTP stabilized at a reduced level while burst responses showed substantial but still impaired facilitation. The competitive antagonist AP-5 was then used to test if 5-HT blocked the NMDA receptor mediated synaptic currents which normally occur during the facilitated burst responses. AP-5 had no effect on the size of burst responses in slices pre-treated with 5-HT indicating that serotonin suppressed the activation of the NMDA receptors by theta stimulation. Serotonin depressed the facilitation of burst responses in slices pre-treated with AP-5 indicating that it also reduces the enhanced AMPA receptor mediated currents that occur during theta pattern stimulation. These results are discussed in terms of the known effects of serotonin on hippocampal physiology and how these might interact with the machinery whereby theta stimulation activates NMDA receptor mediated currents.

Pathway specificity of noradrenergic plasticity in the dentate gyrus

Previous experiments have described highly specific effects of noradrenergic agonists on synaptic transmission in the dentate gyrus (DG). For example, perfusion of hippocampal slices with the beta-noradrenergic agonist isoproterenol induces a long-lasting potentiation (LLP) of extracellularly recorded responses following stimulation of the medial perforant path (PP), and long-lasting depression (LLD) of responses evoked by stimulation of the lateral PP (Dahl D, Sarvey JM, 1989, Proc Natl Acad Sci USA 86:4776-4780). To examine the possible interactions of LLP, LLD, and long-term potentiation induced by tetanic stimulation (LTP), the authors recorded extracellular field potentials evoked in the DG by stimulation of the lateral or medial perforant path following LTP and LLP or LLD, invoked in different orders. After establishment of LLP or LLD by bath application of isoproterenol, subsequent tetanization of the respective afferents resulted in additional potentiation of the medial PP-evoked response and return of the lateral PP-evoked response to baseline levels. In other slices, application of isoproterenol after establishment of LTP resulted in further potentiation of medial PP-evoked responses but no change in the potentiated response evoked by lateral PP stimulation. Thus the pathway specificity was maintained irrespective of the history of previous potentiation or depression. Experiments using the specific beta 1 antagonist metoprolol further confirmed pathway specificity. Perfusion with 20 microM of metoprolol appeared to reduce LTP evoked by stimulation of the medial but not lateral PP. In a subsequent experiment, metoprolol in the absence of tetanization produced LLD of the medial PP-evoked response and LLP of the lateral PP-evoked response, opposite to the effects of ISO.(ABSTRACT TRUNCATED AT 250 WORDS)