Long-term potentiation phenomena in the rat limbic forebrain

Frequency dependence of long-term potentiation and depression in the dentate gyrus of the freely moving rat

Long-term potentiation (LTP) or long-term depression (LTD) can be produced in the dentate gyrus (DG) of the hippocampus with high-or low-frequency stimulation trains, respectively. Although LTP can be elicited in a variety of preparations, we know of no reports of LTD unaccompanied by seizure activity in the awake rat. In this experiment, test pulses at alternating high (95% of maximum response) and moderate (50-75% of maximum) intensities were presented at 0.05 Hz to the perforant path of freely moving rats in order to assess changes in DG population spike amplitude. Trains were delivered at 10-min intervals, and intratrain frequency was adjusted either upward from 3 Hz or downward from 400 Hz until all subjects had received three consecutive tetani at each of 3, 6, 12.5, 25, 50, 100, 200, and 400 Hz. Potentiation was observed at high frequencies regardless of whether ascending (ASC) or descending (DES) test order was used. Depression occurred at low frequencies, but only in ASC rats. The LTD observed in this preparation was not very robust and was clearly seen only when moderate-intensity test pulses were used. The threshold frequency (at which depression gives way to potentiation) was approximately 6-9 Hz for DES rats but was 100-120 Hz for ASC animals. Prior stimulation therefore affected the response to subsequent trains. These results are generally consistent with the hypothesis of a variable threshold for LTP induction. Our findings can also be explained by postulating a wide "labile range" at moderate frequencies within which no plastic changes occur.

Angiotensin II blockade of long-term potentiation at the perforant path--granule cell synapse in vitro

Field recordings of evoked excitatory postsynaptic potentials (pEPSPs) were carried out in the granule cell stratum moleculare following stimulation of the perforant path in rat hippocampal slices. Under control conditions tetanic stimulation produced long-term potentiation (LTP) as measured by an increase in the initial slope of the pEPSPs that lasted for at least 1 h. LTP experiments were repeated with 0.5, 5.0, 50, or 500 nM angiotensin II (AII) present in the bath at the time of tetanization. Induction of LTP was blocked by 50 nM AII; however, normal baseline responses were not affected. At the highest dose tested, 500 nM, a decrease in the amplitude and slope of baseline pEPSPs was observed. When the AII AT1 receptor antagonist losartan was present in the bath AII inhibition of LTP was blocked. The application of losartan alone had no effect on LTP expression. These findings support previous results from in vivo studies demonstrating that activation of AT1 receptors in the dentate gyrus blocks the induction of LTP at the perforant path-granule cell synapse.

Differences in the refractory properties of two distinct inhibitory circuitries in field CA1 of the hippocampus

Extracellular reflections of IPSPs were examined in two distinct circuitries in field CA1 of the hippocampus. Stimulation in the stratum radiatum in the presence of AMPA receptor antagonists elicited positive potentials in the same stratum that were eliminated by picrotoxin, a blocker of GABAA receptors. Laminar profile analysis revealed that the response was maximal in the stratum radiatum at a point well distal to the pyramidal cell body layer and had a negative reflection in the stratum oriens. These field IPSPs presumably mediate the feedforward inhibition normally activated by the Schaffer-commissural projections to field CA1. Stimulation of the alveus produced an antidromic response followed by a much slower positive potential in recordings collected in the pyramidal cell layer. The latter response was suppressed by AMPA receptor antagonists or picrotoxin, as expected for disynaptic, recurrent (feedback) inhibition. The laminar profile for the feedback field IPSPs had its maximum near the pyramidal cell layer and its negative dipole in the stratum radiatum. Feedforward IPSPs were inhibited by about 50% if they were preceded within 200 ms by a priming pulse while feedback IPSPs were reduced by less than 20% under comparable conditions. The refractory effect was minimally dependent on stimulation intensity but was strongly affected by an antagonist of GABAB receptors. Attempts to modify IPSPs in the s. radiatum with long trains of low frequency stimulation or with theta-burst stimulation were not successful, suggesting that GABAergic synapses do not have the plasticities found in their glutamatergic counterparts. These results indicate that interneurons contacted by the extrinsic afferents of hippocampus form GABAergic synapses that differ in terms of spatial location and functional properties from the synapses generated by interneurons innervated by the recurrent collaterals of the pyramidal cells. The findings also suggest that repetitive afferent activity, while reducing the influence of dendritic IPSPs on excitatory input, will leave feedback suppression of cell spiking largely intact.

Post-activation potentiation in the neocortex. III. Kindling-induced potentiation in the chronic preparation

Previous experiments have shown the neocortex to be very resistant to the induction of long-term potentiation in chronic preparations. We show here that kindling-induced potentiation effects can be reliably produced in the neocortex of awake, freely moving rats. These effects develop rather slowly. In sites contralateral to the stimulation electrode, potentiation effects did not become clear until the animals had received about 5 days or more of stimulation. Ipsilateral sites required even longer (approximately 10 days), and both sites required more than 13 days to reach asymptotic levels of potentiation. Both monosynaptic and polysynaptic components were present in the neocortical field potentials. When population spikes were absent, the surface negative monosynaptic EPSP component tended to show a potentiation effect. If population spikes were present, they were generally enhanced while the monosynaptic population EPSP tended to be depressed. Consequently, the apparent depression may have been due to competing field currents. The later polysynaptic components (15-28 ms latency to peak) always showed a potentiation effect with 5 or more kindling stimulations and is presumed to result from activation of cortico-cortical associational fibers. All of these effects were long-lasting, showing little decay over a period of several weeks.

Post-activation potentiation in the neocortex. IV. Multiple sessions required for induction of long-term potentiation in the chronic preparation

The neocortex in chronically prepared rats is very resistant to the induction of long-term potentiation (LTP). In the first of two experiments described in this paper, we tried unsuccessfully to induce neocortical LTP within one session by coactivating basal forebrain cholinergic and cortical inputs to our neocortical recording site. In the second experiment, we tested a new procedure which involved the application of repeated conditioning sessions over several days. This procedure was suggested by our finding that kindling-induced potentiation (KIP) of cortical field potentials could be reliably triggered but was slow to develop. We administered 30 high frequency trains per day to the corpus callosum for 25 days. LTP in callosal-neocortical field potentials became clear after about 5 days of stimulation and reached asymptotic levels by about 15 days. After the termination of treatment, LTP persisted for at least 4 weeks, the duration of our post-stimulation test period. As in previous experiments on kindling-induced potentiation, the potentiation effects were clear in both early population spike components and in a late (probably disynaptic) component. The monosynaptic EPSP component was often depressed, but this may have been due to competing field currents generated by the enhanced population spike activity. We discuss these results in the context of theories emphasizing slower but more permanent memory storage in neocortex compared to the hippocampus.

Long-term potentiation and N-methyl-D-aspartate receptors: foundations of memory and neurologic disease?

Understanding the physiology of learning and memory is one of the great challenges of neuroscience. The discovery in recent years of long-term potentiation (LTP) of synaptic transmission and the elaboration of the mechanisms involved, in particular the NMDA receptor, offers the prospect not only of improving our understanding of normal memory storage and retrieval, but may also yield insights about various neurological and psychiatric clinical disorders. In this review, we begin by examining the different forms, properties, and methods of inducing LTP, followed by a description of molecular mechanisms thought to underlie the phenomenon. Molecular structure of the receptor is discussed, along with the roles of Ca2+ second messenger systems, synaptic morphology changes, and retrograde messengers in LTP. Finally, implications of the NMDA receptor and LTP in learning, memory, and certain clinical conditions such as epilepsy, Alzheimer's disease, and schizophrenia are discussed.

Glutamate, GABA and epilepsy

The nature and value of various animal models of epilepsy for the study and understanding of the human epilepsies are reviewed, with special reference to the ILAE classification of seizures. Kindling as a model of complex-partial seizures with secondary generalisation is treated in detail, dwelling principally on the evidence that the neurotransmitters glutamate and GABA are centrally involved in the kindling process. Kindling in the entorhinal cortex-hippocampus system and its relationship to LTP are analysed in detail. Changes in amino acid content in animal and human brain tissue following onset of the epileptic state are reviewed with special reference to glutamate and GABA. Studies of changes in the extent of basal and stimulus-evoked release of glutamate and GABA both in vivo (microdialysis) and in vitro (brain slices) are evaluated. This includes both kindling and other models of epilepsy, and microdialysis of human patients with epilepsy. Experiments which study the influence of pre-synaptic metabotropic glutamate receptors on glutamate release, and consequently on the extent of electrical kindling, are described. This pre-synaptic control of glutamate release can be studied using synaptosomes. The significance of the ability of focal intracerebrally injected glutamate and NMDA to cause (chemical) kindling and the strong sensitivity of this process to pre-treatment with NMDA receptor antagonists is analysed. Electrical and chemical kindling effects are additive, indicating the existence of mechanisms in common. They are both sensitive to NMDA antagonists and the common mechanism is probably NMDA receptor activation due to the presence of exogenous (chemical) or endogenous (electrically-released) extracellular glutamate. The participation of the NMDA receptor in the generation of the spontaneous hyperactivity which characterises the chronic epileptic state is reviewed. This includes the entry of Ca2+ to stimulate various post-synaptic phosphorylation processes, and possible modulation of NMDA receptor population size and sensitivity. The question of whether neurotransmitter glutamate is involved in initiation and/or spread of seizures is discussed.

Long-term changes in entorhinal-dentate evoked potentials induced by electroconvulsive shock seizures in rats

Entorhinal-dentate evoked potentials were measured in rats before and after: (1) eight electroconvulsive shock (ECS) seizures, or (2) matched handling. In animals that received ECS, evoked potentials were significantly enhanced, as evidenced by a long-lasting increase in the amplitude of the population spike. This increase in population-spike amplitude lasted for at least 3 months after the last ECS trial. No evoked-potential changes were observed in the subjects that received matched handling. These data suggest that ECS seizures produce long-lasting, perhaps permanent, changes in the brain.

Relation of the enhancement of entorhinal tetanic responses by 50-Hz amygdala stimulation to the progression of kindling in the rat

We recorded entorhinal tetanic responses to 50-Hz kindling stimulations applied at the amygdala in conscious rats, which produced facilitation and depression during the train pulses, in order to analyze the relationship of the changes in the tetanic responses to the development of both after-discharges (ADs) and behavioral seizures. Facilitation was always produced in the earlier tetanic responses and was followed by depression which reached a quasi-steady level in the later tetanic responses during each kindling stimulation. To estimate the changes in magnitude of the excitatory synaptic activation in the tetanic responses, with reference to the development of seizure stages, tetanic responses which produced the same behavioral seizure stage in each rat were averaged and the area between the negative (excitatory) potentials and the baseline of the averaged tetanic response was measured in terms of mV x ms. Magnitudes of the averaged negative components were significantly enhanced with an increase in the order of seizure stages in eight rats (P < 0.01). In addition, the mean magnitude of the averaged negative components had a linear correlation (r = 0.95, P < 0.05) with the mean AD duration with reference to the order of seizure stages in the eight rats. The magnitude of the positive (inhibitory) component in the averaged tetanic responses was also measured and found to decrease with an increase in the seizure stages (P < 0.01). The magnitude of the negative component in the test responses to single (0.3 Hz) stimuli just before kindling stimulations also increased with an increase in the order of seizure stages, indicating long term potentiation of the responses by kindling stimulations. We concluded from the results that the enhancement of facilitation of the excitatory synaptic activation and the reduction of the inhibitory synaptic activation in entorhinal tetanic responses by 50-Hz amygdala kindling stimulation is involved in the electrophysiological source of the progression of kindling epilepsy.

An integrative analysis to sleep functions

Various studies suggest that some sleep functions, especially some slow wave sleep functions, are indispensable in mammals and related to brain regulation. It has been proposed that two of these functions are the adjustment of emotional balance and the processing of acquired emotional memories. During waking, the gradual accumulation of various randomly learned emotional memories in the limbic structures would inevitably imbalance and disorganize emotional behaviors. Although the emotional balance can be restored during waking by the ascending NA, DA, ACh and 5-HT systems, their roles in memory retention and emotional regulation may sometimes be dissociated and their adjustment of the emotional balance can only be a transient effect. On the other hand, the function of slow wave sleep for emotional adjustment can be long-lasting and is in agreement with its function on the processing of emotional memories. As a result, these sleep functions become indispensable in preventing the emotional imbalance inevitably caused by the accumulation of emotional memories. The effects of rapid eye movement sleep on memory and emotional regulation are just opposite to those of slow wave sleep. Low vigilance is required as premise for sleep to accomplish these indispensable functions.

The effect of amygdaloid kindling on heart period and heart period variability

Two studies were conducted on rats to assess the effects of amygdaloid kindling on baseline measures of heart period and heart period variance. The results indicate that seizure activity was associated with increased vagal influence on heart period marked by sinus bradycardia and decreased beat-to-beat variability. The resultant bradycardia was enhanced following each seizure and persisted for at least a one-week period of time. The results are discussed in terms of the role of vagal tone in influencing abnormal cardiac patterns which could result in sudden unexplained death in some epileptic patients.

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.

Repeated exposure to lindane leads to behavioral sensitization and facilitates electrical kindling

Repeated intermittent exposure to some chemicals produces behavioral sensitization and seizure induction through a kindling mechanism. Although many pesticides are convulsant at high dosages, the persistent neurological effects of chronic low level exposure are unclear. The impact of intermittent exposure to lindane on behavioral seizure development and subsequent electrical kindling was assessed in the present study. Rats were administered lindane (0 or 10 mg/kg, po) for 30 days, or 3 times/week for 10 weeks. Enhanced behavioral responsiveness to lindane (myoclonic jerks, clonic seizures) emerged over the course of dosing and persisted 2 to 4 weeks after the last dose. The incidence of generalized convulsions was increased from 0% to 15% between the first and final day of dosing. In addition, electrographic recordings from the amygdala revealed brief rhythmic bursts and isolated interictal spike and wave discharge in the absence of overt behavioral seizures. Electrical kindling of the amygdala, beginning 4 to 6 weeks after the final dose, was facilitated. In contrast, prior administration of a single convulsive dose of lindane (20 mg/kg) was without effect on kindling development. These data indicate that repeated exposure to subconvulsant doses of lindane produces a persistent alteration in the central nervous system as evidenced by an enhanced susceptibility to kindled seizures. The pattern of behavioral development whereby the sensitivity is built up gradually over time is suggestive of a chemical kindling mechanism. Savings in the number of stimulation sessions required to induce electrical kindling following a history of lindane treatment provides further evidence that prior lindane exposure may lead to a state of partial kindling. Thus, intermittent subconvulsive lindane treatment induces alterations in limbic excitability that persist for at least 1 month.

Evolution of afterdischarge and seizure characteristics during electrical kindling of the guinea-pig

Interspecies comparisons may help us understand the mechanisms which underlie brain plasticity. In this study, we examined the electrical kindling phenomenon in the amygdala, piriform and perirhinal regions of the guinea-pig. The changes in afterdischarge (AD) characteristics and behavioural seizures were assessed under different stimulation intervals and parameters as well as under reduced inhibitory neurotransmitter systems. We report that the guinea-pigs displayed a number of similarities with other species, such as the progressive increases in AD characteristics and seizure behaviours, but also a number of differences, such as the behavioural manifestations of the seizures, failing to reach a fully generalized tonic-clonic seizure and an apparent insensitivity to both low-frequency stimulation and reduced GABA and catecholamine levels.

Neurophysiological analysis of long-term potentiation in mammalian brain

Long-term potentiation (LTP) is a persistent increase in postsynaptic response following a high-frequency presynaptic activation. Characteristic LTP features, including input specificity and associativity, make it a popular model to study memory mechanisms. Mechanisms of LTP induction and maintenance are briefly reviewed. Increased intracellular Ca2+ concentration is shown to be critical for LTP induction. This increase is believed to be based on Ca2+ influx secondary to activation of N-methyl-D-aspartate (NMDA) subtype of glutamate receptors. Existence of other sources of Ca2+ increase and other critical factors is now becoming evident. They include voltage-dependent Ca2+ channels, Ca2+ intracellular stores, metabotropic glutamate receptors, 'modulatory' transmitters. An example of an involvement of voltage-dependent Ca2+ channels is potentiation induced by intracellular depolarizing pulses. LTP can be divided into decremental earlier (E-LTP) and non-decremental late (L-LTP) phases which explains some inconsistencies in studies of LTP mechanisms. E-LTP is suggested to be based on a transient increase in presynaptic release probabilities. A hypothesis is considered which explains L-LTP by suggesting that Ca2+ activates structural changes leading to an increase in the synaptic gap resistance. This enhances positive synaptic electrical feedback and augments release probability. The hypothesis predicts specific morphological changes, synchronous transmitter release of two or several quanta in some central synapses and the amplification of such synchronization following LTP induction. Data are discussed which maintain these predictions.

Unscheduled brain DNA synthesis, long-term potentiation, and depression at the perforant path-granule cell synapse in the rat

We investigated the effect of long-term potentiation (LTP) of the perforant path-granule cell synapse, on the synthesis of DNA in the target area and in polysynaptically stimulated hippocampal (CA3/CA1) and cortical areas (entorhinal, temporal, and occipital cortices) in the rat. The contralateral nonstimulated side was used as a control. The degree of LTP was indexed by the field EPSP and population spike amplitude recorded in the dentate area of the stimulated side before and after high frequency stimulation (250 Hz, 250 ms) every 30 min. DNA synthesis was evaluated in tissue homogenates after a 3-h period of incorporation of 3H-thymidine. DNA synthesis was significantly lower in the stimulated side in the hippocampal cortex CA3/CA1 (-25%), and in the entorhinal cortex (-50%), but not in the dentate area. In addition, the occurrence of preparations without expression of LTP allowed the analysis of unscheduled brain DNA synthesis (UBDS) in a supposedly long-term depression (LTD) subgroup. UBDS was higher in the group without LTP (no-LTP group) than in that with a significant LTP expression (LTP-group) on both sides of the brain. Furthermore, correlative analyses revealed that UBDS covaried with LTP of the EPSP (but not of population spike) in the dentate area and in extratarget hippocampal subregions on both sides and in dorsal cortex on the stimulated side. Further, regional crosscorrelation analyses revealed a high degree of coupling among brain sites following LTP.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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

Spontaneous potentiation of focal potentials of the CA1 field in long-surviving hippocampal slices of the rat in the absence of electrical stimulation

The changes in the efficiency of the synaptic transmission in the Schaffer collaterals-field CA1 neurons pathways following prolonged deprivation of afferentation, the cessation (up to four hours) of stimulation of the tested input with brief series of tests every hour (the control afferent inputs, were stimulated throughout the entire experiment at a frequency of 0.05/sec), were investigated in surviving slices of the hippocampus of rats. The evoked focal potentials, population spikes (PS) and population excitatory postsynaptic potentials (pEPSP) were recorded. The prolonged cessation of stimulation led to a significant (p < 0.001) increase in the amplitude of the PS (up to 208% in relation to the baseline level). This phenomenon of potentiation was specific in relation to input and exhibited the properties of "E-S potentiation". A hypothesis is advanced regarding the association of mechanisms responsible for the development of "deprivational" potentiation and of the late phase of long-term posttetanic potentiation.

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.

Effects of intraseptally injected glutamatergic drugs on hippocampal sodium-dependent high-affinity choline uptake in "naive" and "trained" mice

We have previously reported that spatial reference memory (RM) training-induced alterations in hippocampal cholinergic activity as measured by sodium-dependent high-affinity choline uptake (SDHACU). Each training session was found to induce an immediate (30 s) increase in SDHACU followed (30 s to 15 min posttest) by a deactivation and long-lasting inhibition (15 min to 24 h) of this cholinergic marker. The present experiments were designed to assess the role of septal glutamatergic receptors in this posttraining cholinergic deactivation. In the first experiment, the effects of intraseptal injections of different doses of glutamic acid and glutamatergic antagonists (kynurenic acid, KYN, and AP5) on hippocampal SDHACU were studied in awake but otherwise resting (i.e., naive) mice. The results showed that glutamic acid at the lowest dose used (5 ng) produced a decrease in SDHACU, whereas both glutamatergic antagonists produced a dose-related increase in this cholinergic marker. It was concluded that septal glutamatergic receptors mediate a tonic inhibitory input on the cholinergic cells. Hence, in a second experiment the effect of intraseptal injections of KYN (5 ng) on the training-induced changes in hippocampal cholinergic activity were assessed following variable amounts of radial maze RM training. Trained mice were injected 20 min before the first or the ninth training session and killed 30 s or 15 min posttraining for determination of SDHACU. KYN slowed the posttesting cholinergic deactivation (disinhibition), this effect being more marked in good learners than in bad learners. The present findings suggest that septal glutamatergic receptors mediate an inhibitory input on the cholinergic cells, and that this input could play a role in memory consolidation.

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.

Attenuated hippocampal long-term potentiation in basolateral amygdala-lesioned rats

Possible involvement of the amygdaloid input in long-term potentiation (LTP) in the medial perforant path-dentate gyrus granule cell synapses in vivo was investigated by evaluating the effects of lesions of the amygdaloid nucleus. The dentate gyrus synaptic potential evoked by low-frequency test stimulation did not change following lesions of the basolateral and central amygdala. However, when tetanic stimulation (30 pulses at 60 Hz) was applied 60 min after lesioning of the ipsilateral basolateral amygdala, the magnitude of LTP was significantly attenuated. Since lesions of the ipsilateral central amygdala and the contralateral basolateral amygdala did not affect the dentate gyrus LTP, the attenuation of the dentate gyrus LTP is a specific effect of acute lesions of the ipsilateral basolateral amygdala. The basolateral amygdaloid lesions significantly attenuated both LTP induced by weak (20 pulses at 60 Hz) and strong (100 pulses at 100 Hz) tetanus, indicating that the effect of the lesions does not depend on the strength of tetanus applied to induce LTP. When the ipsilateral basolateral amygdala was destroyed after application of tetanus, it did not affect the established LTP. The attenuation of LTP was also observed after the basolateral amygdala-lesioned rats were allowed a recovery period of 2 weeks. This is the first report providing evidence that the ipsilateral basolateral amygdala modulates hippocampal synaptic plasticity in vivo.

Chronic epileptogenicity following focal status epilepticus

We examined chronic epileptogenicity in the perforant path stimulation model of focal status epilepticus. After 24 h of perforant path stimulation, every stimulation elicited multiple population spike discharges, and this phenomenon persisted more than 2-3 months after stimulation. Short (10-100 ms) interstimulus interval-dependent paired-pulse inhibition was almost completely lost right after stimulation, but recovered progressively over the following month. Long (200-1000 ms) interstimulus interval-dependent paired-pulse inhibition decreased, and in spite of a partial recovery, remained significantly reduced 4 weeks after stimulation. Frequency-dependent paired-pulse inhibition was lost immediately after stimulation. One month later, inhibition at 2 Hz remained significantly reduced, although in individual rats recovery ranged from poor to complete. Input/output response curves showed increased population spike amplitude but no change of the slope of excitatory postsynaptic potentials. 3-4 weeks after stimulation, spontaneous generalized motor convulsions were observed in half of the stimulated rats. In all of the stimulated rats, the kindling phenomenon was significantly accelerated compared with non-stimulated controls, and class 5 convulsions were elicited in 3.3 +/- 1.0 trials in stimulated rats, against 11.0 +/- 2.5 trials in controls.

Ipsilateral associational pathway in the dentate gyrus: an excitatory feedback system that supports N-methyl-D-aspartate-dependent long-term potentiation

Axons from granule cells in the dentate gyrus of the rat hippocampus project to cells in the hilar region, including mossy cells, which project along the longitudinal axis of the hippocampus and synapse in the inner (proximal) one-third of the molecular layer of the dentate gyrus. To study this feedback system, multiple recording electrodes were located along the longitudinal (septo-temporal) axis in the dorsal leaf of the dentate gyrus in urethane-anesthetized rats. Single pulse electrical stimuli delivered to the hilar region evoked negative-going, monosynaptic field potentials that were largest in the inner one-third of the molecular layer (commissural zone). These evoked field potentials (EFPs) were recorded simultaneously at three to five locations. The latency to onset and peak amplitude of the EFP varied linearly with distance from point of stimulation, and EFPs were elicited in both directions along the longitudinal axis. The transmission speed was estimated to be 1.4 m/s. Tetanic stimulation of the hilar region potentiated the EFP slopes (mean = 26%). Potentiation lasted at least 2 hours and was specific to responses from the tetanized stimulating electrode; the responses to other stimulating electrodes in the hilus and the angular bundle of the perforant path changed less than 4%. Combined stimulation of the hilus and the medial perforant path increased the magnitude of recorded field potentials and population spikes, demonstrating that both pathways are excitatory. NMDA antagonist NPC-17742 blocked potentiation of EFP slopes in both the medial perforant path and hilus pathways. The results suggest that the ipsilateral associational system of the dentate gyrus is excitatory and capable of supporting long-lasting NMDA-dependent, synapse-specific plasticity.

Maturation of long-term potentiation in the hippocampal dentate gyrus of the freely moving rat

The ability of the perforant path/dentate granule cell synapse of the hippocampal formation to establish and maintain enhanced levels of synaptic transmission in response to tetanization (long-term potentiation, LTP) was investigated in freely moving rats at 15, 30, and 90 days of age. Measures of 1) the slope of the population excitatory postsynaptic potential (EPSP), and 2) the population spike amplitude (PSA) obtained before, and at several times following tetanization, were used to evaluate the magnitude and duration of LTP as a function of age. Significant enhancement of both EPSP slope and PSA measures was obtained from animals of all three ages in response to perforant path tetanization. The initial degree of enhancement was essentially the same across the age groups, ranging from +27% to +38% of pretetanization levels for EPSP slope measures and +60% to +75% of pretetanization levels for PSA measures, obtained 15 min after tetanization. The duration of this enhancement obtained from animals of the preweaning group was significantly longer than that obtained from either 30- or 90-day-old animals. Enhanced measures of both EPSP slope and PSA decayed to baseline levels in these older animals 18 to 24 h after tetanization, while animals tetanized at 15 days of age maintained potentiated levels of both measures for a period of 5 days following tetanization. Tetanization of 15-day-old animals resulted in a significant reduction in the latency to EPSP onset without affecting the time-based relationships among the other measured parameters, which included latency of the population spike onset, population spike minimum, and population spike offset.(ABSTRACT TRUNCATED AT 250 WORDS)

Potassium-induced long-term potentiation in area CA1 of the hippocampus involves phospholipase activation

Previous studies have shown that potassium-induced long-term potentiation (LTP) of the Schaffer collateral/commissural synapses in area CA1 of the hippocampus shares common properties with tetanus-induced LTP. In the present investigation, we performed electrophysiological and binding experiments on CA1 hippocampal slices to evaluate the location and nature of the changes underlying potassium-induced LTP. Paired-pulse facilitation, which represents an index of transmitter release, was markedly reduced by potassium-induced LTP. In addition, KCl-induced LTP was associated with an increase in 3H-AMPA ([3H]-amino-3-hydroxy-5-methylisoxazole-4-propionate) binding to CA1 synaptic membranes when measured 40 min after high-potassium exposure; however, no changes were detected in binding of an antagonist ([3H]-6-cyano-7-nitroquinoxaline-2,3-dione; 3H-CNQX) to AMPA receptors in slices expressing KCl-induced LTP. Administration of the phospholipase A2 (PLA2) inhibitor bromophenacyl bromide (BPB) prior to potassium application prevented LTP formation as well as the changes in paired-pulse facilitation and 3H-AMPA binding that characterized this type of potentiation. Taken together, these data indicate that potassium-induced LTP may be related to modifications in both pre- and postsynaptic properties and confirm the hypothesis that PLA2 activation is an important mechanism in long-term changes of synaptic operation.

Dimensionality alterations of the hippocampal electroencephalographic activity following the induction of long-term potentiation in rats

In the present in vivo experimental study the complexity alterations of the hippocampal electroencephalographic (EEG) activity were investigated prior and during the maintenance phase of long-term potentiation (LTP), using analytical methods based on the recent concepts of deterministic chaos. LTP was induced in the hippocampal dentate hilus after stimulation of the medial perforant path of the rat's brain. During the experimental procedure hippocampal EEG epochs were recorded prior and after the induction of LTP. Dimensionality computations performed on these epochs showed a maintained relative reduction in the correlation dimension during the maintenance phase of LTP. This result might suggest that different functional states of the brain are governed by different degrees of complexity and that the altered efficacy in the information process, as it is achieved by the induction of LTP, modifies the spontaneous EEG activity of the potentiated hippocampal area in a plastic manner.

Brain angiotensin receptor subtypes in the control of physiological and behavioral responses

This review summarizes emerging evidence that supports the notion of a separate brain renin-angiotensin system (RAS) complete with the necessary precursors and enzymes for the formation and degradation of biologically active forms of angiotensins, and several binding subtypes that may mediate their diverse functions. Of these subtypes the most is known about the AT1 site which preferentially binds angiotensin II (AII) and angiotensin III (AIII). The AT1 site appears to mediate the classic angiotensin responses concerned with body water balance and the maintenance of blood pressure. Less is known about the AT2 site which also binds AII and AIII and may play a role in vascular growth. Recently, an AT3 site was discovered in cultured neoblastoma cells, and an AT4 site which preferentially binds AII(3-8), a fragment of AII now referred to as angiotensin IV (AIV). The AT4 site has been implicated in memory acquisition and retrieval, and the regulation of blood flow. In addition to the more well-studied functions of the brain RAS, we review additional less well investigated responses including regulation of cellular function, the modulation of sensory and motor systems, long term potentiation, and stress related mechanisms. Although the receptor subtypes responsible for mediating these physiologies and behaviors have not been definitively identified research efforts are ongoing. We also suggest potential contributions by the RAS to clinically relevant syndromes such as dysfunctions in the regulation of blood flow and ischemia, changes in cognitive affect and memory in clinical depressed and Alzheimer's patients, and angiotensin's contribution to alcohol consumption.

Post-activation potentiation and depression in the neocortex of the rat: II. Chronic preparations

Although long-term potentiation (LTP) has been demonstrated in a number of subcortical sites in chronic preparations, there have been no demonstrations of LTP in the neocortex of chronic preparations. Even neocortical slice and acute preparations often require a drug-induced suppression of inhibition before LTP effects can be reliably induced. We have attempted to induce LTP in neocortical sites in 7 different experiments using chronically prepared adult rats. We were unable to obtain any evidence, even a trend, for the induction of LTP. The following manipulations were tested: (1) standard stimulation train parameters that have been shown to be highly effective in subcortical and hippocampal sites; (2) a 10-fold increase in the intra-train pulse durations; (3) variations in train pulse frequency (1 Hz to 300 Hz) and train duration (100 ms to 15 min); (4) co-activation of multiple inputs by stimulation of combinations of cortical sites or cortical and thalamic sites; (5) reduction of inhibition by administration of picrotoxin; 5) Housing of animals in an enriched environment; (6) utilization of the neocortical stimulation trains as a cue in a learning task; (7) application of pilocarpine to co-activate cholinergic systems. Although none of these manipulations produced LTP, the application of pilocarpine did facilitate the induction of a long-lasting depression effect. These findings contrast with the results obtained from anesthetized rats and from studies using brain slices, where LTP can be reliably induced. These results are discussed in light of other recent findings with respect to LTP and LTD effects.

Post-activation potentiation in the neocortex: I. Acute preparations

Long-term potentiation is widely studied as a memory model, and has been demonstrated in a number of subcortical sites in both acute and chronic preparations. In the neocortex, however, most of the demonstrations of LTP have been in neocortical slice or acute preparations, and even these have often required a drug-induced attenuation of inhibition before the LTP could be reliably expressed. In this paper we show that LTP can be reliably expressed in adult rats in a number of neocortical sites, both ipsilateral and contralateral to the site of callosal stimulation. We also show that, when recording field potentials, LTP is expressed roughly equally at all cortical depths. In a third experiment, we monitored input/output (I/O), paired-pulse inhibition and short-term potentiation effects over the course of LTP induction. The ipsilateral responses were, as expected, of shorter latency and larger amplitude than contralateral responses. They also showed small spike-like components that correlated with cell discharge. Nevertheless, the contralateral responses tended to show the largest LTP effects. The paired pulse effect was mainly depression, lasting for up to 3000 ms, at both ipsilateral and control sites. The short-term potentiation components were best fit by two summed exponentials with time constants of about 70 s and 12 min. The LTP effect lasted at least two h which was the longest period monitored in these experiments.

The effect of kindling beyond the 'stage 5' criterion on paired-pulse depression and hilar cell counts in the dentate gyrus

Recent experiments have indicated that recurrent inhibition in the dentate gyrus, as measured with paired-pulse tests, is reduced following the induction of status epilepticus. Also, a loss of cells in the hilus has been reported, and it has been suggested that the two effect might be related. In this experiment, we have monitored paired-pulse depression and counted cells in the hilus in animals that have been kindled well beyond the typical stage 5 criterion. Responses evoked in the dentate gyrus by paired-pulse stimulation of the perforant path were monitored before and after kindling of the perforant path. One group of animals served as controls and received no kindling stimulations. Another group was kindled to 4 stage 5 seizures and then allowed to recover for 2 months. A third group was kindled to 44 stage 5 seizures and then allowed to recover for at least 5 weeks. Paired-pulse tests were taken at 1 week intervals during the kindling and recovery phases. Paired-pulse inhibition increased during kindling, peaked after 4 stage 5 seizures, remained enhanced throughout the additional 40 stage 5 seizures, and recovered towards baseline over a period of about 5 weeks. Upon completion of this phase of the experiment, cell counts were taken in the hilar regions of the Nissl stained brain sections. There was a significant reduction in number of cells in the tissue from kindled animals, compared to controls, but there was no significant difference between the 2 kindled groups.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Opioid receptor-dependent long-term potentiation at the lateral perforant path-CA3 synapse in rat hippocampus

The involvement of opioid receptors in the induction of long-term potentiation (LTP) was investigated in the lateral and medial perforant path projections to area CA3 of the hippocampus in anesthetized rats. The opioid receptor antagonist naloxone (10 nmol), applied to the hippocampal CA3 region 10 min prior to tetanization, blocked the induction lateral perforant path-CA3 LTP induced by high-frequency stimulation. By contrast, LTP induction in medial perforant path-CA3 was not attenuated by a 10 nmol quantity of naloxone. (+)-Naloxone (10 nmol), the inactive stereoisomer of naloxone, was without effect on the induction of lateral perforant path-CA3 LTP. Naloxone applied 1 h following LTP induction did not reverse established lateral perforant path-CA3 LTP, indicating that opioid receptors are involved in the induction but not the maintenance of LTP in this pathway. LTP of medial perforant path responses developed immediately, while LTP of lateral perforant path responses was slow to develop. The latter pattern is similar to the time course of the development of LTP observed at the mossy fiber-CA3 synapse and suggests that lateral and medial perforant path synapses may use distinct mechanisms of both induction and expression of LTP. These data extend previous findings demonstrating opioid receptor-dependent mechanisms of LTP induction at both the mossy fiber-CA3 synapse and the lateral perforant path-dentate gyrus synapse. We suggest that lateral perforant path and mossy fiber synapses may utilize similar, opioid receptor-dependent, mechanisms of LTP induction and expression.

Long-term potentiation as a function of test pulse intensity: a study using input/output profiles

Input/output profiles of the population responses in CA1 were recorded in the rat hippocampal slice in vitro following stimulation of the Schaffer collaterals before and after theta-frequency patterned primed-bursts (PBs). The most robust potentiation was found for the population spike amplitude, which reached > 200% at low intensity test pulses, but decreased at high intensity. The latency of the population spike was more consistently decreased at high than low stimulus intensities. The enhancement of the population dendritic EPSPs was larger at low than high intensity. Intracellular recordings from CA1 neurons indicated that the intracellular and population EPSPs showed a similar saturation with stimulus intensity, while all single neurons fired at 60 microA intensity when the population spike only reached a mean 70% of its maximal amplitude, suggesting that population spike increase at > 60 microA intensity or following potentiation was caused by increased firing synchrony among neurons. It is suggested that input/output profiles are necessary for the standardization of the degree of long-term potentiation among different laboratories.

The effect of kindling of different nuclei in the left and right amygdala on anxiety in the rat

The effects on rodent anxiety of kindling in the medial or basolateral amygdaloid nuclei in each hemisphere were examined. Anxiety was measured using the hole board and elevated plus maze tests. The animals were kindled in medial or basolateral amygdalas, of either the left or right hemisphere. Controls had electrodes implanted in comparable areas, but were not kindled. Analysis of electrode location showed that some animals were kindled in amygdaloid nuclei other than medial or basolateral amygdala. These animals were labelled outliers. Kindling of the medial/basolateral amygdala in the left hemisphere decreased anxiety for at least 1 week after the last kindled seizure. Right hemisphere medial/basolateral kindling tended to increase anxiety. Outlier-kindled rats were less anxious than their controls regardless of hemisphere 1 week after their last kindled seizure. Clear anxiogenic effects were not likely seen in the right hemisphere in this study because of the electrode locations. The degree of anxiety following kindling was correlated with electrode location in the anterior-posterior plane. More anterior foci in the amygdala were associated with more anxiety. More posterior amygdala foci were associated with less anxiety. These findings point to the importance of kindled focus in the amygdala for behavioral effect. Future research should carefully control the location of kindled foci when investigating effects of amygdala kindling on anxiety and other behaviors.

Potassium currents operated by thyrotrophin-releasing hormone in dissociated CA1 pyramidal neurones of rat hippocampus

1. Membrane currents activated by thyrotrophin-releasing hormone (TRH) were investigated in the dissociated rat hippocampal CA1 pyramidal neurone using the nystatin perforated patch recording configuration. 2. Under current-clamp condition, TRH caused a transient hyperpolarization accompanied by a decrease of firing activity and a successive long-lasting depolarization. The latter induced a blockade of firing. 3. When neurones were held at a holding potential (VH) of -40 mV under voltage clamp, TRH elicited a transient outward current with an increase in the membrane conductance, which was followed by a sustained inward current with a decrease in membrane conductance. The inactive TRH metabolite, TRH free acid, did not induce any currents. 4. The reversal potential of TRH-induced outward current (ETRH) was close to the K+ equilibrium potential (EK). The change in ETRH for a 10-fold change in extracellular K+ concentration was 56.4 mV, indicating that the membrane behaves like a K+ electrode in the presence of TRH. On the other hand, the TRH-induced inward current was due to suppression of a slow inward current relaxation during hyperpolarizing voltage commands to -50 mV from a VH of -40 mV, indicating the suppression of the voltage- and time-dependent component of the K+ current (M-current). 5. The TRH-induced outward current (ITRH) increased in a concentration-dependent manner over the concentration range 10(-8)-10(-6) M. The half-maximum concentration was 7.4 x 10(-8) M and the Hill coefficient was 1.5. 6. The TRH-induced outward current (ITRH) was antagonized by K+ channel blockers such as tetraethylammonium (TEA), 4-aminopyridine (4-AP) and Ba2+ in a concentration-dependent manner. ITRH was insensitive to both apamin and iberiotoxin. 7. The first application of TRH to neurones perfused with Ca(2+)-free external solution containing 2 mM EGTA could induce ITRH but the TRH response diminished dramatically with successive applications. Intracellular perfusion with a Ca2+ chelator, 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), also diminished the TRH response. 8. The depletion of Ca2+ from the intracellular Ca2+ store by thapsigargin blocked the TRH response without affecting the caffeine response. Pretreatment with Li+ significantly enhanced ITRH, suggesting that ITRH is involved in the elevation of intracellular free Ca2+ released from the inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ store site but not from the caffeine-sensitive one. 9. Staurosporine, a protein kinase C (PKC) inhibitor, suppressed ITRH in a concentration-dependent manner (the half-maximum inhibitory concentration (IC50), was 2.45 x 10(-8) M).(ABSTRACT TRUNCATED AT 400 WORDS)

Ipsilateral corticotectal pathway inhibits the formation of long-term potentiation (LTP) in the rat superior colliculus through GABAergic mechanism

The purpose of the present experiments was to clarify the possible mechanism for the depression of long term potentiation (LTP) induction in the superficial gray layer of the rat superior colliculus after optic nerve stimulation. A postsynaptic field potential was recorded in vitro in the superficial gray layer of superior colliculus slices after stimulation of the optic layer. Tetanic optic layer stimulation (50 Hz, 20 s) induced LTP of the postsynaptic field potential elicited in the superficial gray layer. The postsynaptic field potential, with unitary discharges, produced in the superficial gray layer by optic nerve stimulation in vivo was depressed by a conditioning stimulus to the visual cortex. Identical inhibition of the cortical response of the superficial gray layer was produced by optic nerve stimulation. The application of picrotoxin (2.5 mg/kg, i.p.), a GABAA antagonist or methoxypyridoxine (100 mg/kg, i.v.), an anti-glutamate decarboxylase agent which reduces GABA levels, blocked the inhibitory interaction between the optic nerve-superficial gray layer and visual cortex-superficial gray layer. Tetanic optic nerve stimulation (50 Hz, 20 s) failed to induce LTP in the superficial gray layer of the intact rat. LTP was only elicited by tetanic optic nerve stimulation when picrotoxin or methoxypyridoxine was administered prior to the tetanic stimulation and when the ipsilateral visual cortex was removed. These results indicate that GABAergic interneurons in the superficial gray layer activated by corticotectal input, may stop the formation of LTP in the superficial gray layer.

Increased spontaneous unit discharge rates following electrical kindling in the rat

Changes in neuronal excitability that persist after seizures may play a key role in epilepsy. In this study, extracellular single unit recording techniques were used to investigate the changes in cell discharge patterns that result from kindling in vivo. The spontaneous activity of piriform and perirhinal cortical units in chronically implanted adult rats was recorded before, during and after kindling the amygdala. We observed that kindling-induced seizures resulted in a prolonged and dramatic increase in the rate of spontaneous background neuronal discharge in both the piriform and perirhinal regions.

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.

Determinants of non-spatial working memory deficits in rats given intraventricular infusions of the NMDA antagonist AP5

Two series of experiments using rats assessed the effects of intraventricular administration of the NMDA antagonist AP5 on performance of non-spatial working memory tasks. The first series used a continuous delayed non-matching to sample (DNMS) design; the second series used a discrete trial delayed matching to sample (DMS) design. Performance was assessed at retention intervals ranging from approximately 5 to 90 sec. The subjects had acquired the behavioural tasks before drug testing commenced. In the DNMS series, minipumps containing vehicle, 5, 10 or 15 nM D-AP5 were implanted. Every 10 days, each rat's minipump was removed and replaced with a fresh pump containing a new drug dose in a counterbalanced design, so that all rats were tested under all four conditions. There were no drug effects on performance at any retention interval. In the DMS series, there were three different basic task variants. Minipumps filled either with 15 mM D-AP5 or vehicle solution were implanted. Vehicle rats performed at approximately pre-operative levels; AP5 rats were impaired only on task variants using repeated stimulus presentations within session. There was no interaction between retention interval and drug treatment. This pattern of results closely resembles that seen following hippocampectomy or fornicotomy, as would be expected if this drug, administered intraventricularly, selectively affected hippocampal function.

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.

A comparative study of age-related changes in inhibitory processes and long-term potentiation in the lateral septum of mice

Anaesthetized C57 BL/6 mice of different ages (young: 5 months; middle-aged: 15 months; and old: 21 months) were used to determine whether aging alters the efficiency of synaptic inhibition and long-term potentiation (LTP) in the lateral septum (LS). Electrical stimulation of the fimbria induced field potentials in the ipsilateral LS comprising two initial negative components (N2 and N3) followed by a positive wave of low amplitude. Paired-pulse experiments showed a facilitation of the N2 component and a concomitant depression of the N3 components. Facilitation of the N2 component was stronger in both middle-aged and old mice as compared to young mice, whereas an inverse pattern of changes was observed for inhibition of the N3 component. High-frequency stimulation of the fimbria produced a persistent increase in the N3 amplitude. This LTP was of significantly higher amplitude in both young and middle-aged mice as compared to old mice. These results suggest that aging impairs both inhibitory processes and synaptic plasticity in the mouse LS, but that inhibitory processes appear to be affected earlier.

Effects of adrenocortical steroids on long-term potentiation in the limbic system: basic mechanisms and behavioral consequences

Hippocampal structures are a major target for adrenal steroid hormones, and hence these neural regions are some of the most likely mediators of the effects of adrenocortical steroids on behavior. Memory disturbance, in particular biasing toward negative contents, are part of the symptomatology presented by depressive patients. In turn, a sizeable subset of depression also presents with hypercortisolemia. Adrenocortical hormones are also known to affect memory processes. Hippocampal formation is essential for declarative memory. We thought it appropriate then to study the effects of adrenal steroids on long-term potentiation, a putative memory mechanism in the hippocampus. Two clearly distinguished components of the evoked response to perforant path stimulation can be studied in the hippocampus: the excitatory postsynaptic potential (EPSP) which denotes the graded depolarization of the somatodendritic region of the neuron and the population spike (PS), a manifestation of the all-or-none-discharge of the cell action potential. Corticosterone had a significant depressant effect on the EPSP component of the evoked response immediately and 15 min after injection. Thereafter EPSP amplitudes were within normal values. Corticosterone significantly decreased the PS immediately after the train, the component remaining low 30 min after the train. 5 alpha-Dihydrocorticosterone (a ring A-reduced metabolite of corticosterone) significantly reduced the PS component of the response at all times after injection. 18-Hydroxydeoxycorticosterone and deoxycorticosterone significantly decreased both EPSP and PS components of the evoked response from the time of infusion. Contrary to expectation, tetrahydrodeoxycorticosterone was ineffective in decreasing and if anything, enhanced the development of long-term potentiation. 18-Hydroxydeoxycorticosterone 21-acetate behaved like vehicle, except for the first 30 min after injection when the EPSP was decreased. Allotetrahydroprogesterone decreased all EPSP's values and had no effect in the PS development in comparison with vehicle. The suggestion is made that the study of steroidal effects on hippocampal LTP can serve as a preclinical model of some aspects of depression in a specific subset of the disease.

Comparison of long-term enhancement and short-term exploratory modulation of perforant path synaptic transmission

Long-term enhancement (LTE/LTP) is an artificially induced form of synaptic change that may underlie memory storage in the hippocampus; however, there is as yet no evidence that this process occurs naturally as a result of normal neural activity. In the dentate gyrus, synaptic change does occur in conjunction with an animal's recent history of exploratory behavior. This change, which persists for a short time (ca. 30 min) following cessation of exploration, has been called short-term exploratory modulation (STEM). This experiment examined the relationship between LTE and STEM by comparing the magnitude of STEM before and after induction of LTE in rats with chronically implanted stimulating electrodes in the perforant path and recording electrodes in the fascia dentata. The absolute magnitude of STEM was the same before and after LTE saturation, suggesting that the processes are independent of each other. Furthermore, quantitative and qualitative analyses of the types of changes seen in the evoked-potential waveforms reveal different types of alteration. LTE includes an increase in EPSP slope, whereas STEM reflects an increase in EPSP onset. These data suggest that it is unlikely that STEM and LTE reflect the same synaptic process, and are at least partly consistent with recent reports suggesting that STEM may be mediated by activity-dependent changes in brain temperature.