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

Induction of a novel form of hippocampal long-term depression by muscimol: involvement of GABAA but not glutamate receptors

1. Unlike long-term potentiation, long-term depression (LTD) in the central nervous system remains poorly understood. The present study was undertaken to investigate the role of GABAA receptors in LTD and synaptic plasticity. 2. Extracellular recordings were made in the CA1 pyramidal cell layer of rat hippocampal slices following orthodromic stimulation of Schaffer collateral fibres in stratum radiatum (0.01 Hz). 3. Muscimol induced a time- and concentration-dependent LTD of the amplitude of orthodromic potentials. Increasing the stimulation frequency from 0.01 Hz to 1 Hz for 10 s reversed the LTD induced by muscimol. Muscimol also induced LTD in the absence of electrical stimulation. 4. Adenosine decreased the spike size in a concentration-dependent manner, but failed to induce LTD. 5. Alphaxalone and 5 alpha-pregnan-3 alpha-ol-20-one at concentrations that did not have any effect themselves on the population spike (0.5 and 1 microM), potentiated the inhibitory effect of muscimol on the population spike size, including concentrations which were not effective by themselves. Both steroids were able to potentiate the ability of muscimol to induce LTD. 6. Bicuculline, 5 microM, reversed the LTD induced by muscimol, 10 microM. 7. The NMDA receptor antagonist (+/-)-2-amino-5-phosphonopentanoic acid (2-AP5), the NMDA/metabotropic antagonist 2-AP3 and selective metabotropic antagonist L-(+)-2-amino-3-phosphonopropionic acid (L(+)-AP3) failed to modify the LTD. Similarly, quisqualic acid and (1S, 3R)-aminocyclopentane dicarboxylic acid (ACPD) a selective agonist at metabotropic receptors did not induce LTD or short-term depression, whereas kynurenic acid prevented the reversal of the LTD obtained at 1 Hz. 8. It is concluded that LTD can be induced by the selective activation of GABAA receptors. The lack of involvement of glutamate receptors in our protocol confirms the unique nature of the LTD described here. The phenomenon of GABA-induced LTD and its reversal by 1 Hz stimulation may represent a novel type of long-lasting depression by which inhibitory interneurones can modulate pyramidal cell excitability in a frequency-dependent manner.

A long-lasting increase and decrease in synaptic excitability in the rat lateral septum are associated with high and low shuttle box performance, respectively

In a series of experiments with rats, using evoked field potentials, the influence of massed trial training in 2-way shuttle box avoidance and step-through passive avoidance tasks was studied on the synaptic excitability of the lateral septum (LS) neurons and on the induction of long-term potentiation in the lateral septum in vivo. The majority of rats that attained a high performance level in the shuttle box task exhibited, after the shuttle box training, a long-lasting enhancement of synaptic excitability of lateral septum neurons, whereas most of the rats with low performance in the shuttle box showed a long-lasting depression in the LS synaptic excitability. Both types of excitability changes disappeared within 24 h. Neither the first habituation session in the passive avoidance apparatus nor the subsequent one-trial learning in passive avoidance task had a marked influence on lateral septum synaptic excitability. Both high-performance and low-performance rats exhibited a long-term potentiation (LTP)-like potentiation of synaptic excitability of the lateral septum neurons after high frequency stimulation of the fimbria fibers although the amount of LTP in high performance rats was slightly higher than that in low performance animals.

Transient protein kinase C activation primes long-term depression and suppresses long-term potentiation of synaptic transmission in hippocampus

Activity-dependent long-lasting plasticity in hippocampus and neocortex includes long-term potentiation (LTP) and long-term depression (LTD) of synaptic strength. Recent studies have confirmed theoretical predictions that the sensitivity of LTP- and LTD-inducing mechanisms is dynamically regulated by previous synaptic history. In particular, prior induction of either repeated short-term potentiations or LTP lowers the threshold for induction of LTD and raises the threshold for LTP. In the current study, transient activation of protein kinase C with phorbol 12,13-diacetate was able to substitute for synaptic activity in priming synapses to exhibit enhanced homosynaptic LTD and to suppress the induction of LTP at Schaffer collateral synapses in area CA1 of hippocampal slices. This priming lasted 30 min, but not 3 hr, following phorbol 12,13-diacetate bath application. These data suggest that a protein kinase C-sensitive phosphorylation site may be an activity-sensitive target mediating the rapid expression of LTP and LTD.

Long-term potentiation and depression in the dentate gyrus, and effects of nicotine

Effects of a brief high-frequency stimulation or a prolonged low-frequency stimulation in the presence and absence of nicotine were studied in thin transverse slices of the dentate gyrus prepared from the guinea pig. Test and conditioning stimulations were delivered to the middle one-third of the molecular layer, and the slope of the population excitatory postsynaptic potentials (EPSPs) elicited by the test stimulation was taken as an indicator of potentiation or depression. Nicotine was without effects on the N-methyl-D-aspartate (NMDA) component of EPSPs at 50 or 100 microM. A brief high-frequency stimulation induced long-term potentiation (LTP) in the presence of bicuculline. Nicotine (50 microM) almost doubled the magnitude of LTP. In the absence of bicuculline, an identical high-frequency stimulation induced a brief depression, the duration and magnitude of which were increased by nicotine. The increase was not statistically significant, however. In contrast to observations in the region CA1, 720 pulses at 1 Hz delivered after induction of LTP failed to induce long-term depression. Nicotine did not modify the after-effect of the low-frequency stimulation. It was discussed that the facilitation of LTP by nicotine probably resulted from suppression of inhibitory processes.

Synaptic plasticity. A molecular switch for memory

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

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

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

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

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

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

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

Synaptic plasticity: LTP and LTD

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

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.

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

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

Resonant activation of calcium signal transduction in neurons

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

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

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

NMDA-receptor-dependent synaptic plasticity: multiple forms and mechanisms

Long-term potentiation in the CA1 region of the hippocampus is the most extensively studied model of activity-dependent synaptic plasticity in the mammalian brain. Its induction normally involves activation of postsynaptic N-methyl-D-aspartate (NMDA) receptors, which are thought to control the occurrence of long-term potentiation at individual synapses. Recent work in the hippocampus indicates that NMDA receptor activation does not necessarily lead to induction of long-term potentiation but instead may elicit a repertoire of distinct forms of synaptic plasticity including short-term potentiation or long-term depression. Furthermore, mechanisms exist such that the induction of long-term potentiation can be inhibited by modest activation of NMDA receptors. Experimental results are beginning to clarify the mechanistic relationships between these different phenomena, although much remains unknown. Whatever their underlying mechanisms, these additional forms of NMDA-receptor-dependent synaptic plasticity confer increased flexibility to neural circuits involved in information processing and storage.

Partial limbic kindling--brain, behavior, and the benzodiazepine receptor

Partial kindling (PK) of the left perforant path (PP) lastingly increased feline defensiveness. Perforant path PK produced long-term potentiation (LTP) in the amygdalo-ventromedial hypothalamic (AM-VMH) pathways in both hemispheres, and in the ventroamygdalofugal (VAF)-VMH efferents of the amygdala of the left hemisphere. Long-term potentiation paralleled behavioral changes. Perforant path PK did not affect recurrent inhibition in area CA3 of the ventral hippocampus. Long-term potentiation of CA3 EPSP and population spikes appeared, but before behavioral changes. Changes in excitability of the periaqueductal grey also accompanied behavioral changes. After kindling, the benzodiazepine receptor antagonist, flumazenil, reduced defensive response to rats in a drug-dependent manner. Flumazenil also reduced LTP in the AM-VMH pathway, but did not affect LTP in the VAF-VMH pathway. Therefore, flumazenil was acting in the amygdala, and not at the VAF-VMH synapse. Kindling caused flumazenil to act like an agonist on behavior, and in the amygdala, and as an agonist or an inverse agonist in area CA3 depending upon the physiological measure taken.

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

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

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

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

Quantal analysis and synaptic anatomy--integrating two views of hippocampal plasticity

The excitatory synapses onto CA1 pyramidal cells have become a model system for understanding the activity-dependent changes in synapses that underlie learning and memory. Here we examine physiological and anatomical results that are relevant to understanding the mechanisms of synaptic transmission and plasticity at these synapses. Three main points are discussed. First, quantal analysis indicates a large heterogeneity of postsynaptic efficacies for different synapses on the same cell. Reconstructions from electron microscopy show that synapse size is also highly heterogeneous. Reasons for suspecting a relationship between synaptic size and efficacy are discussed. Second, physiological evidence indicates that the changes during long-term potentiation are both pre- and postsynaptic. Similarly, several lines of anatomical evidence suggest that plasticity affects the structure of both the pre- and postsynaptic elements. The detailed registration of structures across the synapse and the physical linkage between pre- and postsynaptic elements suggest a 'structural unit hypothesis' for coordinating pre- and postsynaptic modifications. Third, quantal analysis indicates that stimulation of a single axon can release multiple quanta. Anatomical evidence shows that cell pairs can be connected by multiple synapses, suggesting that multiple quanta may be released at independent sites. These results raise the possibility that one component of synaptic plasticity is mediated by changes in the number of functional synaptic sites.

Reversal of LTP by theta frequency stimulation

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

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

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

Development of long-term potentiation in the somatosensory cortex of rats of different ages

The age dependence of possible long-term potentiation (LTP) induction in rat somatosensory cortex was studied in in vitro slice experiments. Coronal slices were prepared from the somatosensory cortex of rats of different ages, and excitatory postsynaptic potentials evoked by stimulation of the white matter (0.1 Hz, subthreshold for spike) were recorded intracellularly. In 70% of the slices taken from 2-week-old rats, a moderate potentiation (20-30%) could be induced by either 5 or 100 Hz stimulation. No LTP was observed in younger (1 week) or older (3 weeks) cortex. On the basis of our experiments an important ontogenetic role of increased synaptic efficacy is suggested in a critical developmental period of rats after birth.

Homosynaptic long-term depression in area CA1 of hippocampus and effects of N-methyl-D-aspartate receptor blockade

We tested a theoretical prediction that patterns of excitatory input activity that consistently fail to activate target neurons sufficiently to induce synaptic potentiation will instead cause a specific synaptic depression. To realize this situation experimentally, the Schaffer collateral projection to area CA1 in rat hippocampal slices was stimulated electrically at frequencies ranging from 0.5 to 50 Hz. Nine hundred pulses at 1-3 Hz consistently yielded a depression of the CA1 population excitatory postsynaptic potential that persisted without signs of recovery for greater than 1 hr after cessation of the conditioning stimulation. This long-term depression was specific to the conditioned input, ruling out generalized changes in postsynaptic responsiveness or excitability. Three lines of evidence suggest that this effect is accounted for by a modification of synaptic effectiveness rather than damage to or fatigue of the stimulated inputs. First, the effect was dependent on the stimulation frequency; 900 pulses at 10 Hz caused no lasting change, and at 50 Hz a synaptic potentiation was usually observed. Second, the depressed synapses continued to support long-term potentiation in response to a high-frequency tetanus. Third, the effects of conditioning stimulation could be prevented by application of NMDA receptor antagonists. Thus, our data suggest that synaptic depression can be triggered by prolonged NMDA receptor activation that is below the threshold for inducing synaptic potentiation. We propose that this mechanism is important for the modifications of hippocampal response properties that underlie some forms of learning and memory.

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

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

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

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

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

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

NMDA receptor antagonists block the induction of long-term depression in the hippocampal dentate gyrus of the anesthetized rat

The present study tested the effect of two non-competitive NMDA receptor antagonists, ketamine and phencyclidine, on the induction of long-term depression (LTD) in the dentate gyrus of urethane-anesthetized rats. Both drugs blocked the induction of LTD as well as long-term potentiation (LTP). NMDA receptor activation thus seems to be required for the induction of both LTD and LTP in the dentate gyrus. High-intensity conditioning stimulation did not overcome the phencyclidine block of LTD. Strong, but brief, postsynaptic depolarization is apparently not the only event needed to trigger LTD.