The effects of repetitive low frequency stimulation on control and "potentiated" synaptic responses in the hippocampus

Integrins, synaptic plasticity and epileptogenesis

A number of processes are thought to contribute to the development of epilepsy including enduring increases in excitatory synaptic transmission, changes in GABAergic inhibition, neuronal cell death and the development of aberrant innervation patterns in part arising from reactive axonal growth. Recent findings indicate that adhesion chemistries and, most particularly, activities of integrin class adhesion receptors play roles in each of these processes and thereby are likely to contribute significantly to the cell biology underlying epileptogenesis. As reviewed in this chapter, studies of long-term potentiation have shown that integrins are important for stabilizing activity-induced increases in synaptic strength and excitability. Other work has demonstrated that seizures, and in some instances subseizure neuronal activity, modulate the expression of integrins and their matrix ligands and the activities of proteases which regulate them both. These same adhesion proteins and proteases play critical roles in axonal growth and synaptogenesis including processes induced by seizure in adult brain. Together, these findings indicate that seizures activate integrin signaling and induce a turnover in adhesive contacts and that both processes contribute to lasting changes in circuit and synaptic function underlying epileptogenesis.

Regulation of hippocampal synaptic plasticity by cyclic AMP-dependent protein kinases

Protein kinases critically regulate synaptic plasticity in the mammalian hippocampus. Cyclic-AMP dependent protein kinase (PKA) is a serine-threonine kinase that has been strongly implicated in the expression of specific forms of long-term potentiation (LTP), long-term depression (LTD), and hippocampal long-term memory. We review the roles of PKA in activity-dependent forms of hippocampal synaptic plasticity by highlighting particular themes that have emerged in ongoing research. These include the participation of distinct isoforms of PKA in specific types of synaptic plasticity, modification of the PKA-dependence of LTP by multiple factors such as distinct patterns of imposed activity, environmental enrichment, and genetic manipulation of signalling molecules, and presynaptic versus postsynaptic mechanisms for PKA-dependent LTP. We also discuss many of the substrates that have been implicated as targets for PKA's actions in hippocampal synaptic plasticity, including CREB, protein phosphatases, and glutamatergic receptors. Future prospects for shedding light on the roles of PKA are also described from the perspective of specific aspects of synaptic physiology and brain function that are ripe for investigation using incisive genetic, cell biological, and electrophysiological approaches.

Reversal of pentylenetetrazol-induced potentiation phenomenon by theta pulse stimulation in the CA1 region of rat hippocampal slices

The effect of theta pulse stimulation (TPS) on pentylenetetrazol (PTZ)-induced long-term potentiation of population spikes was studied in the CA1 region of rat hippocampal slices. The field excitatory postsynaptic potential (fEPSP) and population spikes (PS) were recorded from strata radiatum and pyramidale, respectively, following stimulation of Schaffer collaterals. A transient PTZ application produced a long-lasting enhancement of PS amplitude. A 3-min episode of TPS delivered at test-pulse intensity failed to reverse the PTZ potentiation. However, the same stimulation at a higher intensity produced complete reversal of the PTZ potentiation when delivered during the last minutes of PTZ application. Prior application of high-intensity TPS also decreased the amount of PTZ potentiation, whereas it had no long-lasting effect on baseline synaptic responses. High-intensity TPS induced reversal was blocked by adenosine A1 receptor antagonist and, furthermore, was reduced by protein phosphatase 1 inhibitor. The results suggest that mechanism of PTZ-induced LTP reversal involves activation of adenosine receptors and protein phosphatases.

Neural circuit of tail-elicited siphon withdrawal in Aplysia. II. Role of gated inhibition in differential lateralization of sensitization and dishabituation

In the preceding report, we observed that tail-shock-induced sensitization of tail-elicited siphon withdrawal reflex (TSW) of Aplysia was expressed ipsilaterally but that dishabituation induced by an identical tail shock was expressed bilaterally. Here we examined the mechanisms of this differential lateralization. We first isolated the modulatory pathway responsible for the induction of contralateral dishabituation by making selective nerve cuts. We found that an intact pleural-abdominal connective, the descending pathway connecting the ring ganglia with the abdominal ganglion, ipsilateral to the shock was required for contralateral dishabituation. We examined whether network inhibition suppresses the contralateral effects of tail shock in nonhabituated preparations. We found that blockade of inhibitory transmission in the CNS by the nicotinic ACh inhibitor d-tubocurarine (d-TC) rendered tail shock capable of inducing bilateral sensitization. We next asked whether serotonin (5-HT), a neuromodulator released in the CNS in response to tail shock, was affected by d-TC. We found that d-TC does not alter 5-HT processes in the ring ganglia: it had no effect on the lateralized pattern of tail nerve shock-induced changes in tail sensory neuron excitability, a 5-HT-dependent process, and it did not alter tail nerve shock-evoked release of 5-HT. By contrast, d-TC enhanced 5-HT release in the abdominal ganglion. Consistent with this observation, restricting d-TC to the abdominal ganglion rendered tail nerve shock capable of producing bilateral sensitization. Together with the results of the preceding paper, our results suggest a model in which TSW sensitization and dishabituation can be dissociated both anatomically and mechanistically.

Different phosphatase-dependent mechanisms mediate long-term depression and depotentiation of long-term potentiation in mouse hippocampal CA1 area

Two types of synaptic depression have been described in the hippocampus, long-term depression and depotentiation of long-term potentiation known to recruit the serine/threonine protein phosphatases PP1, PP2A and PP2B (calcineurin). The contribution of each of these protein phosphatases is controversial. To examine the role of the Ca2+/calmodulin-dependent protein phosphatase calcineurin in long-term depression and depotentiation, we analysed the effect of genetically inhibiting calcineurin reversibly in the hippocampus, using the doxycycline-dependent rtTA system in transgenic mice. We show that reducing calcineurin activity has no effect on long-term depression but reversibly affects depotentiation. Consistently, the calcineurin inhibitor FK-506 reproduces the depotentiation impairment observed in the mutant mice but does not affect long-term depression in control animals. In contrast, the PP1/PP2A inhibitor okadaic acid fully blocks both long-term depression and depotentiation. These data demonstrate that the nature of signalling cascades induced by synaptic activity depends on the initial synaptic state. While depression of potentiated synaptic responses requires activation of PP1/PP2A and/or calcineurin, depression of basal synaptic responses depends only on PP1/PP2A activation.

Transient and sustained types of long-term potentiation in the CA1 area of the rat hippocampus

Synaptic potentiation induced by high frequency stimulation was investigated by recording field excitatory postsynaptic potentials (f-EPSPs) in rat hippocampal slices. Potentiation consisted of a transient period of decaying f-EPSPs (short-term potentiation, STP) that led to a plateau of continuously potentiated f-EPSPs (long-term potentiation, LTP). Here we show that a previously unknown type of transient, use-dependent, long-lasting potentiation (t-LTP) can account for STP. t-LTP could be stored for more than 6 h and its decay was caused by synaptic activation. Both the expression and the decay of t-LTP were input specific. t-LTP was induced differently from conventional LTP in that the amplitude of t-LTP was dependent upon the stimulation frequency, whereas the magnitude of LTP was dependent on the number of stimuli in the induction train. The decay of t-LTP could not be prevented by the blockage of glutamate receptors, but was prevented by the blockage of stimulus-evoked neurotransmitter release, suggesting that t-LTP is expressed presynaptically. Paired-pulse stimulation experiments showed that the decay of t-LTP was mediated by a decrease in the probability of neurotransmitter release. The decline of t-LTP could be prolonged by the activation of NMDA receptors. Hence, both single and paired-pulse stimuli prolonged the decline of the t-LTP. This decline could be prevented by high frequency burst stimulation (200 Hz). We conclude that t-LTP allows dynamic modulation of synaptic transmission by providing not only spatial association but also temporal convergence between synaptic inputs. Therefore, t-LTP might be a substrate for the encoding of synaptic memory.

Compatibility of bidirectional synaptic plasticity on hippocampo-prefrontal cortex pathway in rats

The hippocampo-prefrontal cortex pathway reportedly expresses long-term potentiation (LTP) and depression (LTD) in anesthetized rats. We examined whether there were any effects governing the induction of LTD after prior induction of LTP, or vice versa. Induction in sequence of LTP and LTD resulted in significantly stable changes of about 140 and 70% of a common control for 1 h each. The reversed sequence, LTD and LTP, showed a mirror image of about 65 and 135% of control, which were not different from the respective changes in the first sequence (P>0.3 for each). The correlation coefficient between changes was significantly positive in the first sequence and weakly negative in the reverse. These results indicate that this pathway can express compatibility of bidirectional synaptic plasticity while historical changes remain covert.

Developmental and regional differences in the consolidation of long-term potentiation

The alpha5beta1 integrin is present in high concentrations in the apical dendrites of pyramidal neurons in adult rats but is virtually absent in the basal dendrites. Moreover, alpha5beta1 does not appear in apical dendritic branches until the third post-natal week. Given that integrins contribute to the consolidation of synaptic plasticity, these results raise the possibility of developmental and regional differences in the stability of long-term potentiation (LTP). The present study tested this point using a LTP reversal paradigm in field CA1 of hippocampal slices. In accord with earlier reports, low-frequency afferent stimulation (5 Hz) introduced 30 s after theta burst stimulation (TBS) completely reversed LTP but was ineffective 30 min and 60 min later in slices from adult rats. The same low-frequency trains caused a partial reversal of LTP when applied 30 and 60 min post-TBS in slices from 21-day-old rats and a complete reversal at all time points in slices from 10-day-old rats. LTP in the basal dendrites of adult rats did not fully consolidate; i.e. potentiation was partially reversed by low-frequency stimulation even after delays of 30 or 60 min. Moreover, spaced (10 min) applications of 5- Hz pulses beginning at 30 min post-TBS completely erased LTP. The reversal effect in both apical and basal dendrites was blocked by N-methyl-D-aspartic acid receptor antagonists but an integrin antagonist had differential effects across the two dendritic domains. These results constitute evidence that the stability of LTP increases with age in the apical dendrites but remains incomplete even in adulthood in the basal dendrites. The possibilities that the developmental and regional variations in LTP consolidation are correlated with integrin expression and linked to different types of memory processing are discussed.

Long-term potentiation in the Eocene

The first ten years of long-term potentiation (LTP) research are reviewed. Surprisingly, given the intensity of current interest, the discovery paper did not trigger a wave of follow-on experiments. Despite this, the initial work laid out what ultimately became standard questions and paradigms. The application of the then still novel hippocampal slice technique oriented LTP towards basic neuroscience, perhaps somewhat at the cost of lesser attention to its functional significance. The use of slices led to the discovery of the events that trigger the formation of LTP and provided some first clues about its extraordinary persistence. Signs of the intense controversy over the nature of LTP expression (release vs receptors) emerged towards the end of the first decade of work. What appears to be lacking in the literature of that time is a widespread concern about LTP and memory. This may reflect a somewhat different attitude that neurobiologists then had towards memory research and a perceived need to integrate the new potentiation phenomenon into the web of established science before advancing extended arguments about its contributions to behaviour.

Bidirectional synaptic plasticity: from theory to reality

Theories of receptive field plasticity and information storage make specific assumptions for how synapses are modified. I give a personal account of how testing the validity of these assumptions eventually led to a detailed understanding of long-term depression and metaplasticity in hippocampal area CA1 and the visual cortex. The knowledge of these molecular mechanisms now promises to reveal when and how sensory experience modifies synapses in the cerebral cortex.

Protein synthesis is required for synaptic immunity to depotentiation

De novo protein synthesis and transcription are necessary for the expression of long-lasting synaptic potentiation [long-term potentiation (LTP)] in hippocampal area CA1 and for the consolidation of long-term memory. The stability of LTP and its longevity require macromolecular synthesis at later stages, but a specific role for early protein synthesis has not been identified. Using electrophysiological recording methods in mouse hippocampal slices, we show that multiple trains of high-frequency stimulation provide immediate synaptic immunity to depotentiation. This immunity to depotentiation is dependent on the amount of synaptic stimulation used to induce LTP, it is input specific, and it is prevented by inhibitors of protein synthesis. We propose that local translation mediates input-specific synaptic immunity against depotentiation. We also present evidence suggesting that, in addition to translation, products of transcription can provide cell-wide immunity to depotentiation via heterosynaptic transfer of synaptic immunity between distinct pathways in area CA1. Protein synthesis and transcription may importantly regulate long-term storage of information by conferring synaptic immunity to depotentiation at previously potentiated synapses.

Time-dependent depotentiation in the dentate gyrus of freely moving rats by repeated brief 7 Hz stimulation

Long-term potentiation (LTP) in the hippocampus of awake animals is known to be reversed by long trains of low-frequency stimulation (LFS). Here we show that a persistent LTP reversal, i.e. depotentiation (DP), can be induced in the dentate gyrus of freely moving rats by brief 7 Hz stimulations (three trains of 100 pulses, 1 min intertrain-interval) if LFS was applied 2 min after tetanization. In contrast, the same LFS protocol given 15 min after tetanization failed to induce long-lasting DP. Likewise, reduction of LFS (given 2 min post-tetanus) to two trains resulted only in short-term DP. Because naturally occurring hippocampal 7 Hz oscillatory events are correlated to new sensory input, our results might be relevant for the understanding of the mechanisms of LTP reversal induced by processing new information.

Impact of aging on hippocampal function: plasticity, network dynamics, and cognition

Aging is associated with specific impairments of learning and memory, some of which are similar to those caused by hippocampal damage. Studies of the effects of aging on hippocampal anatomy, physiology, plasticity, and network dynamics may lead to a better understanding of age-related cognitive deficits. Anatomical and electrophysiological studies indicate that the hippocampus of the aged rat sustains a loss of synapses in the dentate gyrus, a loss of functional synapses in area CA1, a decrease in the NMDA-receptor-mediated response at perforant path synapses onto dentate gyrus granule cells, and an alteration of Ca(2+) regulation in area CA1. These changes may contribute to the observed age-related impairments of synaptic plasticity, which include deficits in the induction and maintenance of long-term potentiation (LTP) and lower thresholds for depotentiation and long-term depression (LTD). This shift in the balance of LTP and LTD could, in turn, impair the encoding of memories and enhance the erasure of memories, and therefore contribute to cognitive deficits experienced by many aged mammals. Altered synaptic plasticity may also change the dynamic interactions among cells in hippocampal networks, causing deficits in the storage and retrieval of information about the spatial organization of the environment. Further studies of the aged hippocampus will not only lead to treatments for age-related cognitive impairments, but may also clarify the mechanisms of learning in adult mammals.

The group I metabotropic glutamate receptor agonist (S)-3,5-dihydroxyphenylglycine induces a novel form of depotentiation in the CA1 region of the hippocampus

The ability of activation of group I metabotropic glutamate receptor (mGluR) to induce depotentiation was investigated at Schaffer collateral-CA1 synapses of rat hippocampal slices. Brief bath application (5 min) of group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) (10 microm) induced a long-term depression of synaptic transmission or depotentiation (DEP) of previously established long-term potentiation (LTP), which was independent of NMDA or A(1) adenosine receptor activation. This DHPG-DEP was observed when DHPG was delivered 3 min after LTP induction. However, when DHPG was applied at 10 or 30 min after LTP induction, significantly less depotentiation was found. DHPG-DEP (1) is reversible and has the ability to unsaturate LTP, (2) is synapse specific, (3) does not require concurrent synaptic stimulation, (4) is mechanistically distinct from NMDA receptor-dependent depotentiation, (5) requires mGluR5 activation, (6) requires rapamycin-sensitive mRNA translation signaling, (7) does not require phospholipase C or protein phosphatase activation, and (8) is not associated with a change in paired-pulse (PP) facilitation. In addition, the ability of DHPG to reverse LTP was mimicked by a long train of low-frequency (1 Hz/15 min) PP stimulation. Moreover, the expression of DHPG-DEP is associated with a reduction in the increase of the surface expression of AMPA receptors seen with LTP. These results suggest that the activation of mGluR5 and in turn the triggering of a protein synthesis-dependent internalization of synaptic AMPA receptors may contribute to the DHPG-DEP in the CA1 region of the hippocampus.

Memory enhancement: the search for mechanism-based drugs

Rapid progress has been made in understanding the synaptic changes required for memory encoding. Several companies are now attempting to use information about the induction and consolidation phases of this process to build memory-enhancing drugs. These efforts have produced novel compounds that improve retention scores across a broad range of tests and species. Initial clinical results are encouraging. Issues now arise about appropriate applications of candidate drugs and optimal cellular targets for future development.

Role for cAMP and protein phosphatase in the presynaptic expression of mouse hippocampal mossy fibre depotentiation

Long-term potentiation (LTP) at the hippocampal mossy fibre-CA3 synapses can be reversed (depotentiated) by long trains of low-frequency stimulation (LFS). In the present study, we showed that this depotentiation is triggered by a presynaptic group II metabotropic glutamate receptor (mGluR), which reduces cytosolic cAMP level, leading to a reversal of cellular processes responsible for mossy fibre LTP expression. Furthermore, we found that both the presynaptic activity-induced elevation of Ca(2+) and the activation of protein phosphatase (PP) activity are required for the induction of depotentiation. Thus, we conclude that mossy fibre depotentiation is expressed presynaptically through the activation of both presynaptic mGluR- and PP-coupled signalling cascades, and that the bidirectional long-term plasticity at the mossy fibre-CA3 synapses is likely to be regulated by presynaptic Ca(2+)-dependent processes.

Group I metabotropic glutamate receptor (mGluR)-dependent long-term depression mediated via p38 mitogen-activated protein kinase is inhibited by previous high-frequency stimulation and activation of mGluRs and protein kinase C in the rat dentate gyrus in vitro

The induction of synaptic plasticity is known to be influenced by the previous history of the synapse, a process termed metaplasticity. Here we demonstrate a novel metaplasticity in which group I metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD) of synaptic transmission is regulated by previous mGluR activation. In these studies, the group I mGluR-dependent LTD induced by the selective agonist (RS)-3,5-dihydroxyphenylglycine (DHPG-LTD) was inhibited by previous preconditioning brief high-frequency stimulation (HFS), regardless of whether the preconditioning HFS induced long-term potentiation. Blockade of NMDA receptors during the preconditioning HFS did not alter the inhibition of DHPG-LTD by the HFS. However, antagonism of mGluRs during the preconditioning HFS did prevent the inhibition of DHPG-LTD by the HFS. In addition, blocking PKC stimulation during the preconditioning HFS also prevented the inhibitory effect of HFS on DHPG-LTD. The DHPG-LTD itself was not inhibited by blocking PKC stimulation but was inhibited by blocking the p38 mitogen-activated protein kinase (MAPK) pathway. Thus, whereas the DHPG-LTD is mediated via activation of the p38 MAPK pathway, the inhibitory effects of preconditioning HFS on DHPG-LTD are mediated via stimulation of group I/II mGluRs, activation of PKC, and subsequent blocking of the functioning of group I mGluR.

NMDA Receptor-dependent Transient Homo- and Heterosynaptic Depression in Picrotoxin-treated Hippocampal Slices

Extracellular recording was used to study the effects of high-frequency (tetanic) stimulation on excitatory synaptic transmission in the CA1 region of rat hippocampal slices in the presence of the gamma-aminobutyric acid (GABA) type A antagonist, picrotoxin (50 microM). Under these conditions tetanic stimulation (100 Hz, 1 s) at the test intensity resulted in homosynaptic long-term potentiation (LTP). In contrast, tetanic stimulation of higher intensity (100 Hz, 1 s, double test intensity) resulted in homo- and heterosynaptic depression which recovered within 45 min. A transient (1 - 3 min) negative shift in DC potential and a transient (5 - 10 min) depression of the homosynaptic fibre volley occurred immediately following the higher intensity tetanus. The DC shift, induction of homo- and heterosynaptic depression and depression of the fibre volley were reversibly prevented by the N-methyl-d-aspartate (NMDA) receptor antagonist, d-2-amino-5-phosphonopentanoate (AP5; 20 microM) but were not prevented by a variety of L-type calcium channel antagonists. Transient (30 - 45 min) synaptic depression of pharmacologically isolated NMDA receptor-mediated field excitatory postsynaptic potentials also occurred following tetanic stimulation (100 Hz, 1 s) at double test intensity. These results demonstrate an NMDA receptor-dependent form of reversible synaptic depression in the CA1 region of the hippocampus.

"Silent" metaplasticity of the late phase of long-term potentiation requires protein phosphatases

The late phase of long-term potentiation (L-LTP) is correlated with some types of long-term memory, but the mechanisms by which L-LTP is modulated by prior synaptic activity are undefined. Activation of protein phosphatases by low-frequency stimulation (LFS) given before induction of L-LTP may significantly modify L-LTP. Using cellular electrophysiological recording methods in mouse hippocampal slices, we show that LFS given before induction of L-LTP inhibited L-LTP in an activity-dependent manner without affecting either basal synaptic strength or the early phase of LTP (E-LTP). This anterograde inhibitory effect of LFS was persistent, required N-methyl-D-aspartate (NMDA) receptor activation, and was blocked by inhibitors of protein phosphatase 1 (PP1) and protein phosphatase 2A (PP2A). These data indicate that certain patterns of LFS can activate PP1 and/or PP2A, and that long-lasting activation of these phosphatases by prior LFS can suppress the subsequent expression of L-LTP without affecting E-LTP. Because this inhibition of L-LTP is caused by prior synaptic activity that, alone, produced no net effect on synaptic efficacy, we suggest that this is a "silent" form of metaplasticity that may influence long-term information storage by modulating the capacity of synapses to express L-LTP after repeated bouts of activity.

Alpha3 integrin receptors contribute to the consolidation of long-term potentiation

Several lines of evidence suggest that integrin receptors play a pivotal role in consolidation of long-term potentiation (LTP), but which of the many integrin dimers are involved remains to be discovered. The present study used an LTP reversal paradigm to test if alpha3 integrins make an important contribution. Function blocking alpha3 monoclonal antibodies or vehicle were locally infused into recording sites in field CA1 of rat hippocampal slices and LTP induced with theta burst stimulation. Low frequency trains of pulses were applied 30 min after the theta bursts. Previous work indicates that low frequency stimulation reverses LTP when applied immediately after induction but is largely ineffective after 30-45-min delays. If the antibodies were to block consolidation, then they should extend the period over which potentiation is vulnerable to disruption. There was no detectable difference between the two groups in the initial degree of LTP or within slice decay of potentiation 1-10 min after induction; a small but reliable decay occurred from 10 to 30 min with antibody treatment (P<0.01) but not in control slices. Percent potentiation was not statistically different for vehicle (55 +/- 19%, mean +/- S.D.) and anti-alpha3 (43 +/- 21%) slices at 30 min post-theta bursts. Five-Hz stimulation ("theta pulse" stimulation) 30 min after induction caused a reduction of LTP. The percent loss of potentiation after the 1-min trains was greater in the antibody-treated slices than in controls (98 +/- 4% vs. 62 +/- 28%, P<0.01, U-test) and correlated (r=0.84, alpha3 slices) with the percent LTP present prior to low frequency stimulation, as expected if the stimulation reversed potentiation. Recovery occurred in both groups but percent LTP was significantly smaller in experimental slices at 10 min post-theta pulses (5 +/- 11% vs. 36 +/- 15%, P<0.01). Recovery continued for 20 min after theta pulses and, in accordance with earlier work, was nearly complete for the control slices (50 +/- 19% vs 55 +/- 15%, 40 min post- vs. immediately pre-theta pulses). LTP remained depressed after 40 min of recovery in the anti-alpha3 slices (23 +/- 19% vs. 43 +/- 21%) at which point it was substantially less than that found in controls (P<0.01). Western blots with anti-alpha3 antibodies identified a polypeptide with the molecular mass (155 kDa) expected for the alpha3 subunit and further showed that it is broadly distributed in brain. Subcellular fractionation experiments demonstrated that alpha3 is concentrated in synaptic membranes over homogenates to about the same degree as the GluR1 subunit of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate-type glutamate receptor. From these results we suggest that alpha3-containing integrins are localized to synapses and are needed to stabilize a slowly decaying form of LTP. The findings also show that vulnerability to reversal can be used in place of extended recording sessions in studying consolidation.

Modulation by serotonin 5-HT(4) receptors of long-term potentiation and depotentiation in the dentate gyrus of freely moving rats

Tetanization-induced long-term potentiation (LTP) in the hippocampus can be depotentiated by low-frequency stimulation. 5-HT(4) receptors are expressed in the hippocampus and are suggested to be involved in hippocampus-dependent cognitive processes. Since the role of these receptors in the dentate gyrus has yet not been characterized, this study investigated the effects of 5-HT(4) receptors on basal synaptic transmission, LTP and depotentiation in the dentate gyrus of freely moving rats. Male Wistar rats were chronically implanted with a recording electrode in the dentate gyrus granule cell layer, a stimulation electrode in the medial perforant path and a cannula for drug administration in the ipsilateral ventricle. The 5-HT(4) agonist methoxytryptamine dose-dependently inhibited basal synaptic transmission and LTP. Priming of receptors by a dose of this agonist which elicited no significant change of basal synaptic transmission inhibited depotentiation. These effects could be prevented by the 5-HT(4) antagonist RS 39604, which did not produce independent effects on synaptic transmission, LTP or depotentiation. The effects of methoxytryptamine were confirmed with the highly selective 5-HT(4) agonist, RS 67333. These results strongly support a role for 5-HT(4) receptors in hippocampal synaptic plasticity and provide an important link to findings with regard to the involvement of 5-HT in processes related to learning and memory.

Evidence that integrins contribute to multiple stages in the consolidation of long term potentiation in rat hippocampus

Three structurally distinct groups of antagonists were used to test the hypothesis that integrin adhesion receptors play an essential role in consolidating (stabilizing) long term potentiation of the Schaffer collaterals in rat hippocampus. Comparisons were made of percent potentiation at antagonist-treated versus control sites within CA1 stratum radiatum of the same hippocampal slice. Function blocking antibodies against the alpha5 subunit of the fibronectin receptor had no effect on baseline responses or initial potentiation but resulted in a >30% reduction, relative to within-slice control long term potentiation, 45 min later. Larger reductions were recorded in separate experiments continued for 4 h after the induction of potentiation. Alpha(v) and alpha2 subunit antibodies did not reliably affect the stabilization of potentiation. An antagonist peptide with preference for beta1 integrins produced a slowly developing decline of the type seen with alpha5 antibodies. A cyclic peptide antagonist reduced potentiation within 10 min of induction and caused an almost 40% decrease over 45 min. Two disintegrins (snake toxins that potently block integrins) were very effective in preventing the consolidation of long term potentiation: echistatin reduced potentiation by >70%, while triflavin caused approximately 50% decrease. The suppressing effects of echistatin were concentration-dependent, obtained with treatment after induction, and much more rapid than the effects of antibodies. Rapid declines in potentiation were particularly evident when the two disintegrins were applied together. These results indicate that hippocampal fibronectin receptors (alpha5/beta1 integrin) contribute importantly to a slowly developing phase of long term potentiation consolidation. They also suggest that other integrins are critical to aspects of consolidation occurring in the first few minutes after induction.

Characterization of the mechanism underlying the reversal of long term potentiation by low frequency stimulation at hippocampal CA1 synapses

Reversal of long term potentiation (LTP) may function to increase the flexibility and storage capacity of neuronal circuits; however, the underlying mechanisms remain incompletely understood. We show that depotentiation induced by low frequency stimulation (LFS) (2 Hz, 10 min, 1200 pulses) was input-specific and dependent on N-methyl-d-aspartate (NMDA) receptor activation. The ability of LFS to reverse LTP was mimicked by a brief application of NMDA. This NMDA-induced depotentiation was blocked by adenosine A(1) receptor antagonist. However, the reversal of LTP by LFS was unaffected by metabotropic glutamate receptor antagonism. This LFS-induced depotentiation was specifically prevented by protein phosphatase (PP)1 inhibitors, okadaic acid, and calyculin A but not by the PP2A or PP2B inhibitors. Furthermore, by using phosphorylation site-specific antibodies, we found that LFS-induced depotentiation is associated with a persistent dephosphorylation of the GluR1 subunit of amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor at serine 831, a protein kinase C and calcium/calmodulin-dependent protein kinase II (CaMKII) substrate, but not at serine 845, a substrate of cAMP-dependent protein kinase. This effect was mimicked by bath-applied adenosine or NMDA and was specifically prevented by okadaic acid. Also, the increased phosphorylation of CaMKII at threonine 286 and the decreased PP activity seen with LTP were overcome by LFS, adenosine, or NMDA application. These results suggest that LFS erases LTP through an NMDA receptor-mediated activation of PP1 to dephosphorylate amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and CaMKII in the CA1 region of the hippocampus.

Time-dependent plasticity of synaptic transmission produced by long-term potentiation of C-fiber evoked field potentials in rat spinal dorsal horn

Our previous work has shown that repetitive stimulation of Adelta-fibers depresses long-term potentiation (LTP) of C-fiber evoked field potentials in spinal dorsal horn. Here, we tested the effects of the Adelta stimulation on the spinal LTP at different time points following LTP induction. Fifteen minutes after LTP induction Adelta stimulation depressed LTP by 44.1+/-4.2% (mean+/-SEM, n=7) for 69.3+/-18.5 min and 1 h after LTP the same Adelta stimulation depressed spinal LTP by only 16.9+/-3.1% for 21.9+/-2.0 min (n=7). Three hours after LTP, however, the Adelta stimulation produced a further potentiation (31.9+/-6.3%, n=7) lasting for all the recording periods (1-3 h). These data indicate that the effects of repetitive stimulation of Adelta-fibers on established spinal LTP of C-fiber evoked field potentials is time-dependent.

Long-term depression: a cascade of induction and expression mechanisms

The aims of this paper are to provide a comprehensive and up to date review of the mechanisms of induction and expression of long-term depression (LTD) of synaptic transmission. The review will focus largely on homosynaptic LTD and other forms of LTD will be considered only where appropriate for a fuller understanding of LTD mechanisms. We shall concentrate on what are felt to be some of the most interesting recent findings concerning LTD in the central nervous system. Wherever possible we shall try to consider some of the disparities in results and possible reasons for these. Finally, we shall briefly consider some of the possible functional consequences of LTD for normal physiological function.

Long-term depression: a cellular basis for learning?

Long-term depression (LTD) comprises a persistent activity-dependent reduction in synaptic efficacy which typically occurs following repeated low frequency afferent stimulation. Hippocampal LTD has been a subject of particular interest due to the established role of the hippocampus in certain forms of information storage and retrieval. Recently, it was reported that LTD in the CA1 region may be associated with novelty acquisition in rats. CA1 LTD expression may also be increased in stressful conditions. This suggests a more complex role for this form of plasticity than the oft-cited hypothesis that it simply serves to prevent synapse saturation, by means, for example, of enabling reversal of long-term potentiation (LTP). One possibility is that LTD may be directly involved in the creation of a memory trace. Alternatively, LTD may prime a synapse in readiness for the expression of LTP, thereby contributing indirectly to information storage. There is increasing evidence that LTD is not mechanistically the reverse of LTP. Although some common processes exist, molecular, biochemical, electrophysiological and pharmacological studies all point to several quite distinct induction and maintenance mechanisms for this form of synaptic plasticity. Taken together these findings suggest that hippocampal LTD must be considered in a new light. This review focuses on the interpretation of novel and established information with regard to LTD in the hippocampal CA1 region in terms of its possible role as a cellular basis for learning and memory.

Simultaneous activation and opioid modulation of long-term potentiation in the dentate gyrus and the hippocampal CA3 region after stimulation of the perforant pathway in freely moving rats

Recent investigations indicate monosynaptic activation by the perforant pathway (pp) of the dentate gyrus and the CA3 region. While short-term potentiation and long-term potentiation (LTP) and its opioid modulation are frequently described for the dentate gyrus, data for the CA3 region are rare. Therefore, evoked potentials and opioid modulation of LTP were directly compared in both target regions of the pp. Male Wistar rats were chronically implanted with a bipolar stimulation electrode in the pp (angular bundle) and two recording electrodes in the dorsal dentate gyrus and the CA3 region. Stimulation of the pp in the freely behaving animals induced short-latency evoked potentials in both target structures which were compared with respect to waveform, latency, amplitude and signs of short- and long-term neuronal plasticity. The short-latency potential in the CA3 region seemed to be a monosynaptic potential which displayed LTP sensitive to the N-methyl-D-aspartate receptor antagonist, MK 801, and depotentiating stimulation. After application of specific opioid antagonists at the mu-, delta- and kappa-opioid receptor subtypes, naloxone, funaltrexamine, naltrindole and binaltorphimine, different effects on induction and maintenance of LTP of the population spike were found both within the dentate gyrus and between the dentate gyrus and the CA3 region. The results show marked diminution of LTP in the dentate gyrus only for naloxone and naltrindole and only small, if any, effects of naloxone on LTP in the CA3 region. Thus, neuronal plasticity in the direct perforant pathway input to the CA3 region seems not to be under such substantial opioidergic control. LTP would be inducible in that region even when LTP in the input formation, the dentate gyrus, and transsynaptic LTP via the mossy fibres are blocked.

Effects of lead exposure on long-term potentiation induced by 2-deoxy-D-glucose in area CA1 of rat hippocampus in vitro

Chronic developmental lead exposure is known to be associated with cognitive dysfunction in children. Previous studies have demonstrated that chronic lead exposure could impair the induction and maintenance of long-term potentiation induced by high-frequency stimulation (HFS-LTP). In area CA1 of rat hippocampus, long-term potentiation could also be induced following temporary replacement of 10 mM 2-deoxy-D-glucose (2-DG) for 10 mM glucose in the normal perfusate (artificial cerebrospinal fluid). The present study was carried out to investigate whether chronic lead exposure affected long-term potentiation induced by 2-DG (2-DG-LTP). Neonatal Wistar rats were exposed to lead from parturition to weaning via milk of dams whose drinking water contained 0.2% lead acetate. Field excitatory postsynaptic potentials (EPSPs) in area CA1 of hippocampus were recorded on postnatal days 25-30. 2-DG application was followed by an increase in EPSP slopes in a time-course-dependent manner in both control and lead-exposed rats, while the amplitude of 2-DG-LTP in the lead-exposed rats (225.9+/-19.0%, n=12) was significantly greater than that in controls (155.2+/-9.8%, n=12). In contrast to the effects of lead exposure on 2-DG-LTP, the amplitude of HFS-LTP in the lead-exposed rats (121.5+/-13.7%, n=12) was significantly less than that in controls (183.9+/-18.6%, n=12). These results indicate that chronic lead exposure had opposite effects on the two types of LTP induced by HFS and 2-DG. This would suggest that the effects of lead on HFS-LTP and 2-DG-LTP are the result of different sites of lead toxicity.

Synaptic plasticity in the medial vestibular nuclei: role of glutamate receptors and retrograde messengers in rat brainstem slices

The analysis of cellular-molecular events mediating synaptic plasticity within vestibular nuclei is an attempt to explain the mechanisms underlying vestibular plasticity phenomena. The present review is meant to illustrate the main results, obtained in vitro, on the mechanisms underlying long-term changes in synaptic strength within the medial vestibular nuclei. The synaptic plasticity phenomena taking place at the level of vestibular nuclei could be useful for adapting and consolidating the efficacy of vestibular neuron responsiveness to environmental requirements, as during visuo-vestibular recalibration and vestibular compensation. Following a general introduction on the most salient features of vestibular compensation and visuo-vestibular adaptation, which are two plastic events involving neuronal circuitry within the medial vestibular nuclei, the second and third sections describe the results from rat brainstem slice studies, demonstrating the possibility to induce long-term potentiation and depression in the medial vestibular nuclei, following high frequency stimulation of the primary vestibular afferents. In particular the mechanisms sustaining the induction and expression of vestibular long-term potentiation and depression, such as the role of various glutamate receptors and retrograde messengers have been described. The relevant role of the interaction between the platelet-activating factor, acting as a retrograde messenger, and the presynaptic metabotropic glutamate receptors, in determining the full expression of vestibular long-term potentiation is also underlined. In addition, the mechanisms involved in vestibular long-term potentiation have been compared with those leading to long-term potentiation in the hippocampus to emphasize the most significant differences emerging from vestibular studies. The fourth part, describes recent results demonstrating the essential role of nitric oxide, another retrograde messenger, in the induction of vestibular potentiation. Finally the fifth part suggests the possible functional significance of different action times of the two retrograde messengers and metabotropic glutamate receptors, which are involved in mediating the presynaptic mechanism sustaining vestibular long-term potentiation.

The LTP Program: a data acquisition program for on-line analysis of long-term potentiation and other synaptic events

The LTP Program is a stimulation, acquisition and on-line analysis program for studying long-term potentiation (LTP), long-term depression (LTD), and stimulus-evoked synaptic responses in general. The program is freely available from the website: www.ltp-program.com. It is a 32-bit DOS program that runs on Windows 3/95/98 computers having a Pico Technologies ADC-42, Axon Instruments' Digidata 1200, or Scientific Solution's Labmaster acquisition board. The program records two channels of activity in extracellular, current- or voltage clamp modes. It acquires < or =1,000,000 samples per sweep, and has extracellular dual pathway stimulation and epoch-like intracellular stimulation. Basic protocols include slow alternating dual pathway stimulation. LTP is induced by single train, theta burst, or primed burst stimulation. LTD is induced using fast repetitive 1 pulse sweeps (< or =2 Hz). The program analyzes all stimulus-evoked synaptic responses in both acquisition channels. Analyzes include: slope, peak amplitude/latency, population spike amplitude/latency, average amplitude, duration, area, rise time, decay time, coastline, cell resistance and patch electrode series resistance. Sweeps can be averaged and digitally filtered. Trains can be analyzed by measuring the responses of all pulses relative to the baseline of the first pulse. Stimulus artifacts can be automatically removed for accurate determination of synaptic areas and peaks during a train.

Time-dependent reversal of long-term potentiation by low-frequency stimulation at the hippocampal mossy fiber-CA3 synapses

Using mouse hippocampal slices, we studied the induction of depotentiation of long-term potentiation (LTP) at the mossy fiber synapses onto CA3 pyramidal neurons. A long train of low-frequency (1 Hz/900 pulses) stimulation (LFS) induced a long-term depression of baseline synaptic transmission or depotentiation of previously established LTP, which was reversible and was independent of NMDA receptor activation. This LFS-induced depotentiation was observed when the stimulus was delivered 1 or 10 min after LTP induction. However, when LFS was applied at 30 min after induction, significantly less depotentiation was found. The induction of depotentiation on one input was associated with a heterosynaptic reverse of the LTP induced previously on a separate pathway. In addition, this LFS-induced depotentiation appeared to be mediated by the activation of group 2 metabotropic glutamate receptors (mGluRs), because it was mimicked by the bath-applied group 2 agonist (2S,2'R,3'R)-2-(2', 3'-dicarboxycyclopropyl) glycine and was specifically inhibited by the group 2 antagonists (S)-alpha-methyl-4-carboxyphenylglycine and (alphaS)-alpha-amino-alpha-(1S,2S)-2-carboxycyclopropyl-9H-xanthine-9-propanic acid. Moreover, the induction of depotentiation was entirely normal when synaptic transmission is blocked by glutamate receptor antagonist kynurenic acid and was associated with a reversal of paired-pulse facilitation attenuation during LTP expression. Pretreatment of the hippocampal slices with G(i/o)-protein inhibitor pertussis toxin (PTX) prevented the LFS-induced depotentiation. These results suggest that the activation of presynaptic group 2 mGluRs and in turn triggering a PTX-sensitive G(i/o)-protein-coupled signaling cascade may contribute to the LFS-induced depotentiation at the mossy fiber-CA3 synapses.

Neuronal plasticity in hippocampal mossy fiber–CA3 synapses of mice lacking the inositol-1,4,5-trisphosphate type 1 receptor

In the present study, we used inositol-1,4,5-trisphosphate (IP3) type 1 receptor (IP3R1) knockout mice to examine the role of this receptor in the induction of LTP, LTD, and DP at mossy fiber-CA3 synapses. No difference in synaptically induced field-EPSPs was seen between the wild-type (IP3R1(+/+)) and IP3R1 knockout mice (IP3R1(-/-)), showing that basic synaptic transmission does not involve IP3R1 activation. Tetanus induced LTP in both wild-type and IP(3)R1(-/-) mice, but the magnitude of LTP was significantly greater in IP3R1(-/-) mice (149.8+/-3.5%, mean+/-S.E.M., n=15) than in wild-type mice (132.4+/-1.5%, n=17), suggesting that the IP3R1 has a suppressive effect on LTP induction. To determine whether this effect involved N-methyl-D-aspartate receptor (NMDAR)-dependent LTP, the effect of tetanus was tested in the present of the NMDAR antagonist, D,L-AP5 (50 microM); under these conditions, the LTP in both IP3R1(-/-) and IP3R1(+/+) mice was not significantly reduced. In addition, group I mGluR activation was shown to be necessary for LTP induction, as the LTP was almost blocked by the group I mGluR antagonist, RS-4CPG (500 microM) in both IP3R1(-/-) (117.6+/-1.7%, n=8) and IP3R1(+/+) (116.9+/-1.8%, n=5) mice. The IP3R1 also plays an essential role in LTD induction, as low-frequency stimulation (LFS) failed to induce LTD in the mutant mice (104.5+/-2.1%, n=10). DP was induced in both IP3R1(-/-) and wild-type mice.

Nicotine accelerates reversal of long-term potentiation and enhances long-term depression in the rat hippocampal CA1 region

In the hippocampal CA1 region, low-frequency stimulation (LFS; 200 pulses at 1 Hz) causes reversal of long-term potentiation (depotentiation, DP) and long-term depression (LTD), both of which are thought to be the cellular substrate of learning and memory. Because nicotine enhances learning and memory, we examined if nicotine modulates DP and LTD in the hippocampal CA1 region. Bath application of nicotine during LFS accelerated DP, that is, potentiated synaptic responses in hippocampal CA1 neurons returned to pre-tetanic control levels more rapidly in the presence of nicotine. Because a similar acceleration of DP was observed using the alpha7 nicotinic acetylcholine receptor (nAChR)-selective antagonist methyllcaconitine (MLA), the nicotine effect appeared to be at least partly mediated by nicotine-induced desensitization of alpha7 nAChRs. Delivery of LFS in the presence of nicotine or MLA also depressed synaptic responses in a naive pathway and facilitated LTD, that is, the magnitude of LTD was larger when the drug was present during LFS. Thus, these results demonstrate that nicotine facilitates DP and LTD, which may represent, at least in part, the cellular mechanism underlying nicotine-induced cognitive enhancement.

Long-term potentiation/depotentiation are accompanied by complex changes in spontaneous unit activity in the hippocampus

Typically, long-term potentiation (LTP) has been assessed as long-lasting changes in field potentials or intracellularly recorded postsynaptic potentials evoked by activation of a set of afferents. In the present experiment, we determined changes in spontaneous unit activity in the dentate gyrus (DG) following high-frequency (HFS) or low-frequency stimulation (LFS) of the medial perforant pathway. Experiments were performed in anesthetized rats. Field potentials and unit recordings were obtained alternatively from the same recording electrode. Of 39 single units isolated (from 25 independent sessions), the spontaneous discharges of 13 units (33%) increased, while 7 units (18%) decreased their discharges following HFS that induced significant LTP of the field potentials. Such opposing modulations of unit discharges following HFS were observed on simultaneously recorded units. LFS applied following HFS also induced bi-directional effects on unit discharges. Of 20 single units isolated from a subset of recordings (12 experiments) to which LFS was applied, 6 units increased and 4 units decreased their discharges. LFS produced a long-lasting (>20 min) depotentiation, to the baseline level, on field potentials in four recording cases. The autocorrelation functions indicated that the isolated unit discharges were comparable to those of the putative DG granule cells and interneurons, shown in previous studies. The results suggest that changes in synaptic efficacy following HFS or LFS produce rather dynamic changes in cell activity in the DG.

Synaptic plasticity and memory: an evaluation of the hypothesis

Changing the strength of connections between neurons is widely assumed to be the mechanism by which memory traces are encoded and stored in the central nervous system. In its most general form, the synaptic plasticity and memory hypothesis states that "activity-dependent synaptic plasticity is induced at appropriate synapses during memory formation and is both necessary and sufficient for the information storage underlying the type of memory mediated by the brain area in which that plasticity is observed." We outline a set of criteria by which this hypothesis can be judged and describe a range of experimental strategies used to investigate it. We review both classical and newly discovered properties of synaptic plasticity and stress the importance of the neural architecture and synaptic learning rules of the network in which it is embedded. The greater part of the article focuses on types of memory mediated by the hippocampus, amygdala, and cortex. We conclude that a wealth of data supports the notion that synaptic plasticity is necessary for learning and memory, but that little data currently supports the notion of sufficiency.

Effects of chronic lead exposure on short-term and long-term depression in area CA1 of the rat hippocampus in vivo

Chronic developmental lead (Pb) exposure to the rat has been reported to impair the long-term potentiation (LTP) in area CA1 and DG of the hippocampus. The present study was performed to investigate the effects of chronic Pb exposure on homosynaptic short-term depression (STD) and long-term depression (LTD) of population spikes (PS) in area CA1 of the rat hippocampus in vivo. Neonatal Wistar rats were exposed to Pb from parturition to weaning via the milk of dams fed with 0.2% lead acetate solution. The input/output (I/O) function, paired-pulse reaction (PPR), the PS were measured in the area CA1 in response to low frequency stimulation (LFS). The results showed that the homo-STD amplitude of PS depotentiation in Pb-exposed rats (87.48 +/- 7.44%, n = 14) was less significant than that in control rats (72.34 +/- 6.05%, n = 18, P<0.05), and the homo-LTD amplitude of PS depotentiation in Pb-exposed rats (72.80 +/- 5.86%, n = 14) was even less significant than that in control rats (47.80 +/- 5.03%, n = 18, P<0.01). The results suggest that chronic Pb exposure in neonatal rats caused impairments in the STD and LTD of area CA1 of hippocampus.

Effects of A1 and A2 adenosine receptor antagonists on the induction and reversal of long-term potentiation in guinea pig hippocampal slices of CA1 neurons

1. Using simultaneous recordings of the field EPSP and the population spike in the CA1 neurons of guinea pig hippocampal slices, we confirmed that delivery of a high-frequency stimulation (tetanus: 100 pulses at 100 Hz) produced robust long-term potentiation of synaptic efficacy (LTP) in two independent components, a synaptic component that increases field excitatory postsynaptic potentials (EPSPs) and a component that results in a larger population spike amplitude for a given EPSP size (E-S potentiation). 2. In the same cells, reversal of LTP (depotentiation; DP) in the field EPSP and in the E-S component is achieved by delivering low-frequency afferent stimulation (LFS: 1 Hz, 1000 pulses) 20 min after the tetanus. 3. When the tetanus or LFS was applied to CA1 inputs in the presence of an adenosine A1 receptor antagonist, 8-cyclopentyltheophylline (1 microM), the field EPSP was enhances in LTP and attenuated in DP, while the E-S relationship was not significantly affected in either LTP or DP. 4. When similar experiments were performed using an A2 receptor antagonist, CP-66713 (10 microM), the field EPSP was blocked in LTP but facilitated in DP, while E-S potentiation was enhanced during both LTP and DP. 5. The results show that endogenous adenosine, acting via A1 or A2 receptors, modulates both the synaptic and the E-S components of the induction and reversal of LTP. Based on the results, we discuss the key issue of the contribution of these receptors to the dynamics of neuronal plasticity modification in hippocampal CA1 neurons.

Synaptic activity modulates the induction of bidirectional synaptic changes in adult mouse hippocampus

Activity-dependent synaptic plasticity is critical for learning and memory. Considerable attention has been paid to mechanisms that increase or decrease synaptic efficacy, referred to as long-term potentiation (LTP) and long-term depression (LTD), respectively. It is becoming apparent that synaptic activity also modulates the ability to elicit subsequent synaptic changes. We provide direct experimental evidence that this modulation is attributable, at least in part, to variations in the level of postsynaptic depolarization required for inducing plasticity. In slices from adult hippocampal CA1, a brief pairing protocol known to produce LTP can also induce LTD. The voltage-response function for the induction of LTD and LTP in naive synapses exhibits three parts: at a postsynaptic membrane potential during pairing (V(m)) </= -40 mV, no synaptic modification is obtained; at V(m) between -40 and -20 mV, LTD is induced; and, finally, at V(m) > -20 mV, LTP is generated. This function varies with initial synaptic efficacy. In depressed synapses, Theta(-), the V(m) above which LTD is generated, is shifted toward more depolarized V(ms) and Theta(+), the LTD-LTP crossover point or, equivalently, the V(m) above which LTP is induced, toward more polarized V(ms). Conversely in potentiated synapses, Theta(-) is shifted toward more polarized V(ms). Therefore synaptic activity changes synaptic efficacy and accordingly adjusts the voltages for eliciting subsequent synaptic modifications. The concomitant shifts in the voltages for inducing LTD and LTP in opposite directions promote synaptic potentiation and inhibit synaptic depression in depressed synapses and vice versa in potentiated synapses.

Long-term depression and depotentiation in the sensorimotor cortex of the freely moving rat

Activity-dependent reductions in synaptic efficacy are central components of recent models of cortical learning and memory. Here, we have examined long-term synaptic depression (LTD) and the reversal of long-term potentiation (depotentiation) of field potentials evoked in sensorimotor cortex by stimulation of the white matter in the adult, freely moving rat. Prolonged, low-frequency stimulation (1 Hz for 15 min) was used to induce either depotentiation or LTD. LTD was expressed as a reduction in the amplitude of both monosynaptic and polysynaptic field potential components. Both LTD and depotentiation were reliably induced by stimulation of the ipsilateral white matter. Stimulation of the contralateral neocortex induced only a depotentiation effect, which decayed more rapidly than that induced by ipsilateral stimulation (hours vs days). Although ipsilateral LTD was effectively induced by a single session of low-frequency stimulation, multiple sessions of stimulation, either massed or spaced, induced LTD effects that were larger in magnitude and longer lasting. Previously, we showed that the induction of long-term potentiation in the neocortex of chronic preparations required multiple, spaced stimulation sessions to reach asymptotic levels. Here, we report that LTD also required multiple stimulation sessions to reach asymptotic levels, but massed and spaced patterns of low-frequency stimulation were equally effective.

Heterogeneity of synaptic plasticity at unitary CA3-CA1 and CA3-CA3 connections in rat hippocampal slice cultures

Long-term potentiation (LTP) of unitary EPSPs, generated by pairs of monosynaptically connected CA3 and CA1 pyramidal cells, was compared with LTP of extracellularly evoked, multi-unitary EPSPs in rat hippocampal slice cultures. LTP was induced by repeated, synchronous pairing of low-frequency presynaptic and postsynaptic activity. Three differences were observed. First, LTP of multi-unitary EPSPs displayed two phases: transient (<5 min) and sustained. Potentiation of unitary EPSPs displayed both phases in 42% of experiments; the remainder showed sustained potentiation only. Unitary EPSPs displaying transient-sustained and only sustained potentiation could be recorded from single postsynaptic cells, indicating that excitatory synapses on a given cell are heterogeneous with respect to short-term plasticity. Second, whereas LTP of multi-unitary EPSPs never resulted in greater than twofold increases in amplitude (mean potentiation of 175% of control), maximal LTP of unitary EPSPs was as great as 13-fold (mean potentiation of 250%). Third, LTP could not be induced in 24% of unitary EPSPs. We provide here the first evidence for the coexistence of potentiatable and nonpotentiatable synapses on individual postsynaptic neurons. Thirty-seven percent of connections not displaying LTP exhibited long-term depression (LTD), suggesting that the connections were already maximally potentiated. In the remaining 63% of these pairs, neither LTP nor LTD could be induced, despite the existence of a pharmacologically identified, NMDA receptor-mediated EPSP component. In conclusion, there is considerable heterogeneity in the amplitude and time course of LTP expression at different synaptic connections. A substantial proportion of apparently nonplastic synapses probably accounts for the weaker potentiation displayed by compound EPSPs.

A role for extracellular adenosine in time-dependent reversal of long-term potentiation by low-frequency stimulation at hippocampal CA1 synapses

The involvement of adenosine on the development of time-dependent reversal of long-term potentiation (LTP) by low-frequency stimulation (LFS) was investigated at Schaffer collateral-CA1 synapses of rat hippocampal slices. A train of LFS (2 Hz, 10 min, 1200 pulses) had no long-term effects on synaptic transmission but produced lasting depression of previously potentiated responses. This reversal of LTP (depotentiation) was observed when the stimulus was delivered </=3 min after induction of LTP. However, application at 10 min after induction had no detectable effect on potentiation. This time-dependent reversal of LTP by LFS appeared to be mediated by extracellular adenosine, because it was mimicked by bath-applied adenosine and was specifically inhibited by the selective A(1) adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (100 nM). The effect of adenosine could be mimicked by 5-HT(1A) receptor agonist buspirone, but the LFS-induced depotentiation could not be antagonized by 5-HT(1A) receptor antagonist NAN-190. The source of extracellular adenosine in response to LFS appeared to be attributable to the efflux of cAMP. In addition, this LFS-induced depotentiation was blocked by bath application of adenylyl cyclase activator forskolin or injection of a cAMP analog Sp-adenosine cAMP (10 mM) into postsynaptic neurons. Moreover, the selective protein phosphatase 1 and 2A inhibitors okadaic acid and calyculin A prevented the LFS-induced depotentiation. These results thus suggest that increasing extracellular adenosine appears to underlie the LFS-induced depotentiation via acting on the A(1) receptor subtype to interrupt the cAMP-dependent biochemical processes leading to the LTP expression.

A nitric oxide-independent and beta-adrenergic receptor-sensitive form of metaplasticity limits theta-frequency stimulation-induced LTP in the hippocampal CA1 region

The induction of long-term potentiation (LTP) and long-term depression (LTD) at excitatory synapses in the hippocampus can be strongly modulated by patterns of synaptic stimulation that otherwise have no direct effect on synaptic strength. Likewise, patterns of synaptic stimulation that induce LTP or LTD not only modify synaptic strength but can also induce lasting changes that regulate how synapses will respond to subsequent trains of stimulation. Collectively known as metaplasticity, these activity-dependent processes that regulate LTP and LTD induction allow the recent history of synaptic activity to influence the induction of activity-dependent changes in synaptic strength and may thus have an important role in information storage during memory formation. To explore the cellular and molecular mechanisms underlying metaplasticity, we investigated the role of metaplasticity in the induction of LTP by theta-frequency (5-Hz) synaptic stimulation in the hippocampal CA1 region. Our results show that brief trains of theta-frequency stimulation not only induce LTP but also activate a process that inhibits the induction of additional LTP at potentiated synapses. Unlike other forms of metaplasticity, the inhibition of LTP induction at potentiated synapses does not appear to arise from activity-dependent changes in NMDA receptor function, does not require nitric oxide signaling, and is strongly modulated by beta-adrenergic receptor activation. Together with previous findings, our results indicate that mechanistically distinct forms of metaplasticity regulate LTP induction and suggest that one way modulatory transmitters may act to regulate synaptic plasticity is by modulating metaplasticity.

Time-dependent induction of depotentiation in the dentate gyrus of freely moving rats: involvement of group 2 metabotropic glutamate receptors

Depotentiation comprises a reversal of tetanization-induced long-term potentiation (LTP) which occurs following low-frequency stimulation (LFS) in the hippocampus in vivo. Although depotentiation has been consistently demonstrated in the CA1 region, no positive reports of the existence of depotentiation in the dentate gyrus in vivo have occurred. This study therefore investigated whether depotentiation is possible in the dentate gyrus in vivo. We found that depotentiation can be induced, but it is very tightly dependent on the interval between tetanization and LFS. Thus, LFS given 2 or 5 min following tetanization produced significant depotentiation, whereas LFS given 10-30 min following tetanization had no significant effect on the expression of LTP. Depotentiation occurred in two phases: a transient depression of evoked responses to below pre-tetanization values, which occurred in the first 60 min following LFS, and a recovery of this response to a stable level of synaptic transmission which comprised a significant reduction in the magnitude of LTP. Group 2 metabotropic glutamate receptors (mGluRs) play an important role in the expression of long-term depression in vivo. We therefore investigated whether group 2 mGluRs contribute to depotentiation. The group 2 antagonist (2S)-alpha-ethylglutamic acid (EGLU) inhibited the early transient depression at a concentration which inhibits LTD in vivo, but did not block the expression of depotentiation. EGLU also inhibited the transient depression induced by 5 Hz given alone. Increasing the concentration of EGLU prevented depotentiation, however. The group 2 agonist (S)-4-carboxy-3-hydroxyphenyl- glycine (4C3HPG) inhibited LTP and enhanced depotentiation. These data suggest a role for group 2 mGluRs in depotentiation.

An investigation into signal transduction mechanisms involved in DHPG-induced LTD in the CA1 region of the hippocampus

Previously, we have found that activation of mGlu receptors using a group I-specific mGlu receptor agonist, (RS)-3,5-DHPG, can induce long-term depression (LTD) in the CA1 region of the hippocampus and that, once established, this synaptic depression can be reversed by application of the mGlu receptor antagonist, (S)-MCPG [Palmer et al., 1997. Neuropharmacology 36, 1517-1532]. We have started to investigate the signal transduction mechanisms involved in these effects. Group I mGlu receptors couple to phospholipase C and therefore can activate protein kinase C and mobilise Ca2+ from intracellular stores. However, neither protein kinase C inhibitors (chelerythrine or Ro 31-8220) nor agents which deplete intracellular Ca2+ stores (thapsigargin or cyclopiazonic acid) were able to prevent DHPG-induced LTD. Furthermore, the ability of MCPG to reverse DHPG-induced LTD was not prevented by these compounds. These results suggest that it is unlikely that DHPG-induced LTD, or its reversal by MCPG, is produced via activation of either protein kinase C or by release of Ca2+ from intracellular stores.

DHPG-induced LTD in area CA1 of juvenile rat hippocampus; characterisation and sensitivity to novel mGlu receptor antagonists

We have used extracellular microelectrode recording to characterise a form of long-term depression (LTD) of synaptic transmission that can be induced by metabotropic glutamate (mGlu) receptor activation in the CA1 region of the young (12-18 day old) rat hippocampus. Activation of group I mGlu receptors by the specific agonist 3,5-dihydroxyphenylglyine (DHPG) induced LTD of field excitatory postsynaptic potentials (fEPSPs). The mGlu5 selective agonist 2-chloro-5-hydroxyphenylglycine was also capable of inducing LTD. In contrast, the group II specific agonist DCG-IV had no effect on synaptic transmission, whilst the group III receptor agonist (S)-2-amino-4-phosphonobutyrate elicited a depression that reversed fully upon agonist washout. DHPG-induced LTD could still be generated after prior saturation of electrically-induced NMDA receptor-dependent LTD. DHPG-induced LTD was reversed by tetanic stimulation comprising 100 shocks delivered at 100 Hz. A novel mGlu receptor antagonist, (RS)-2-amino-2-(3-cis and trans-carboxycyclobutyl-3-(9-thioxanthyl)propionic acid) (LY393053) that potently inhibits mGlu1 and mGlu5 receptors, prevented the induction of DHPG-induced LTD. Like other mGlu receptor antagonists, LY393053 also reversed pre-established DHPG-induced LTD. In contrast, a potent mGlu1 selective antagonist (S)-2-methyl-4-carboxyphenylglycine (LY367385) did not prevent the induction of DHPG-induced LTD. In conclusion, DHPG, probably via activation of mGlu5 receptors, is able to induce a robust form of LTD in the CA1 region of the young rat hippocampus that is mechanistically distinct from NMDA receptor-dependent homosynaptic LTD.

Chronic treatment with repetitive transcranial magnetic stimulation inhibits seizure induction by electroconvulsive shock in rats

BACKGROUND

Studies in laboratory animals suggest that repetitive transcranial magnetic stimulation (rTMS) and electroconvulsive shock (ECS) increase seizure inhibition acutely. This study was designed to explore whether chronic rTMS would also have seizure inhibition properties.

METHODS

To this purpose we administered rTMS (Magstim Rapid) and sham rTMS twice daily (2.5 T, 4-sec train duration, 20 Hz) to two groups of 10 rats for 16 days. The rTMS coil was a 50-mm figure-8 coil held directly over the rat's head. Raters were blind to experimental groups. On days 11, 17, and 21 (5 days after the last rTMS) ECS was administered with a Siemens convulsator using three electrical charge levels. Variables examined were the presence or absence of seizures and seizure length (measured from the initiation of the tonic contraction until the end of the limb movement).

RESULTS

At day 11 rTMS had no effect on seizures, and both rTMS and sham rTMS animals convulsed equally. At day 17, however, rTMS-treated animals convulsed significantly less (both at presence/absence of seizures, and at seizure length) than sham rTMS animals. At day 21 the effects of rTMS had disappeared.

CONCLUSIONS

These findings suggest that rTMS administered chronically leads to changes in seizure threshold similar to those reported for ECS and ECT; however, these effects were short-lived.

Hippocampal neurochemical and electrophysiological measures from freely moving rats

This paper describes surgical and recording procedures that have been developed which permit the simultaneous monitoring of levels of select neurochemicals (via microdialysis) and measures of dentate-evoked field potentials within the hippocampal formation of freely moving adult rats. To test and evaluate these procedures, they were employed to examine changes in hippocampal neurochemistry and neuronal excitability associated with the establishment and maintenance of hippocampal long-term potentiation (LTP). Measures of hippocampal norepinephrine (NE) and glutamate levels along with measures of the dentate granule cell population spike amplitude (PSA) were obtained before, during, and after tetanization of the medial perforant path using two separate tetanization paradigms. Results obtained using these new procedures in several animals indicated that changes in NE and glutamate levels were strongly correlated with increases in the dentate granule cell PSA measure obtained following tetanization. The results indicate that this newly developed procedure can be effectively used to directly examine the relationship between neurochemical and neurophysiological changes associated with hippocampal neuroplasticity.

Time-dependent reversal of dentate LTP by 5 Hz stimulation

Field potential recordings were made from the dentate gyrus of urethane-anaesthetized rats in order to investigate the ability of 5 Hz stimulation to reverse long-term potentiation (LTP) induced by a high frequency tetanus. A 10 min train of 5 Hz was found to reverse LTP in a time-dependent fashion: as the interval between tetanus and 5 Hz was increased, LTP became progressively less susceptible to reversal. If 10 min or 30 min intervened between tetanization and 5 Hz stimulation, LTP was unaffected. These results indicate that dentate LTP in vivo exhibits a similar limited time window of vulnerability to reversal by low frequency stimulation to that previously reported in area CA1 in vitro.

Seizure induced synthesis of fibronectin is rapid and age dependent: implications for long-term potentiation and sprouting

Extracellular matrix proteins are induced by activity in adult brain but the time course of these responses, and hence the possibility of their involvement in use-dependent synaptic plasticity, is not known. To evaluate this issue, the influence of seizures on fibronectin expression was evaluated in the adult and developing hippocampus. In adult rats, kainic acid-induced seizures increased fibronectin mRNA and immunoreactivity (ir) by about 1 h after the first behavioral seizure. In situ hybridization analysis indicated that fibronectin mRNA was increased in broadly distributed glial cells as well as within discrete neuronal populations that normally express this transcript. Western blots demonstrated that increased fibronectin-ir was evident in both soluble and non-soluble fractions at the same time point. Immunocytochemical colocalization confirmed that fibronectin-ir was indeed elevated in broadly distributed glial fibrillary acidic protein-ir astroglia. Seizures had no detectable effect on fibronectin-ir in the hippocampus of nine day old rats. Together with previous results, the above findings suggest that intense physiological activity triggers a 'matrix response' (i.e., release proteases, activate integrins, secrete matrix proteins) that is sufficiently rapid to participate in the consolidation of long-term potentiation (LTP). The absence of such reactions in the immature hippocampus is in accord with the hypothesis that matrix proteins generated by mature astroglia impose temporal and spatial limitations on axonal remodeling.