Different forms of LTD in the CA1 region of the hippocampus: role of age and stimulus protocol

In this study, we have investigated the developmental range over which different stimulus protocols induce long-term depression (LTD). Low-frequency stimulation (LFS; 900 stimuli, 1 Hz) produced LTD in hippocampal slices from rats younger than approximately 40 days old, but not in animals aged between approximately 40 days and 16 weeks. We demonstrate, however, that different stimulus protocols can result in LTD in the adult hippocampus. Whilst one paired-pulse low-frequency stimulus protocol [PP-LFS; 50 ms paired-pulse interval (PPI), 900 pairs of stimuli] produced N-methyl-D-aspartate (NMDA) receptor-independent LTD, another PP-LFS protocol (200 ms PPI; 900 pairs) produced NMDA receptor-dependent LTD. Furthermore, the saturation of NMDA receptor-dependent LTD did not prevent the induction of further NMDA receptor-independent LTD. This lack of occlusion suggests that different mechanisms of expression may underlie each of the above forms of LTD in the adult hippocampus. In contrast to the adult hippocampus, NMDA receptor-dependent LTD was induced by both LFS and PP-LFS (50 ms PPI) in slices from young animals (12-20 days). Although they share a common induction mechanism, LTD induced by PP-LFS may be expressed through other mechanisms in addition to those underlying LFS-induced LTD in the young hippocampus. In conclusion, the results in this study demonstrate that mechanisms of long-term synaptic depression within the hippocampus can alter radically with development of the central nervous system and with the use of different induction protocols.

Synaptic depression induced by pharmacological activation of metabotropic glutamate receptors in the perirhinal cortex in vitro

The perirhinal cortex is crucially involved in various forms of learning and memory. Decrements in neuronal responsiveness occur in the perirhinal cortex with stimulus repetition during visual recognition performance. However, very little is known concerning the underlying mechanisms of synaptic transmission and plasticity in this cortical region. In this study, we provide evidence demonstrating the presence of functional group I, II and III metabotropic glutamate receptors in the rat perirhinal cortex in vitro. Furthermore, the results demonstrate long-lasting synaptic depression in the perirhinal cortex. Extracellular synaptic responses were recorded from superficial layers of the perirhinal cortex directly below the rhinal sulcus, in response to electrical stimuli delivered in the superficial or intermediate layers to the entorhinal or temporal cortex sides of the rhinal sulcus. Evoked synaptic potentials were depressed during bath perfusion of each of the following: the broad-spectrum metabotropic glutamate receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid, the selective group I agonist (R,S)-3,5-dihydroxyphenylglycine, the group II agonist (2S,1'R,2'R,3'R)-(2',3'-dicarboxycyclopropyl)glycine and the group III agonist (S)-2-amino-4-phosphonobutanoate. Furthermore, there was a long-lasting depression of synaptic transmission following washout of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid, (R,S)-3,5-dihydroxyphenylglycine or (2S,1'R,2'R,3'R)-(2',3'-dicarboxy-cyclopropyl)glycine. Activation of group III metabotropic glutamate receptors by (S)-2-amino-4-phosphonobutanoate did not result in long-lasting changes in synaptic transmission. Thus, the pharmacological activation of metabotropic glutamate receptors can produce short- or long-term changes in synaptic transmission in the perirhinal cortex. It is possible therefore, that metabotropic glutamate receptors are involved in the decrement in neuronal responsiveness associated with visual recognition in the perirhinal cortex.

Input-and layer-dependent synaptic plasticity in the rat perirhinal cortex in vitro

The perirhinal cortex is crucially important in several forms of memory. Whilst it is important to understand the underlying mechanisms of this role in memory, little is known about the synaptic physiology or plasticity of this region of transitional cortex. In this study, we recorded evoked field potentials in superficial layers (approximately layer I) of the perirhinal cortex in vitro. One stimulating electrode was placed on the temporal side and the other on the entorhinal side of the rhinal sulcus in either the superficial or intermediate layers (approximately layers II/III). Paired stimuli resulted in depression of the second response. Paired-pulse depression was maximal at a 200-ms interpulse interval. Low-frequency stimulation resulted in synaptic depression, which returned to baseline within 60 min. The magnitude of both paired-pulse depression and low-frequency stimulation-induced depression was significantly greater at synapses activated from the temporal intermediate pathway than the other three pathways. Long-term potentiation, stable for at least 60 min, was induced by high-frequency stimulation of intermediate but not superficial pathways. Long-lasting depression (depotentiation) was induced by low-frequency stimulation following the induction of long-term potentiation. The induction of both long-term potentiation and depotentiation was N-methyl-D-aspartate receptor dependent. The group I/II metabotropic glutamate receptor antagonist (S)-alpha-methyl-4-carboxyphenylglycine was without effect on either of these forms of plasticity. Thus, both long- and short-lasting forms of synaptic plasticity exist at synapses in the perirhinal cortex, and these may mediate the changes in neuronal responses associated with visual recognition memory.

Induction of LTD in the adult hippocampus by the synaptic activation of AMPA/kainate and metabotropic glutamate receptors

It has been suggested that the induction of long-term depression (LTD) may be developmentally regulated since LTD can be readily induced by LFS in slices from young but not adult animals. However, we have recently reported that paired pulse low frequency stimulation (PP-LFS) can reliably induce LTD in the CA1 region of adult hippocampal slices. We now describe the role of glutamate receptors in the induction of LTD in adult hippocampus. The induction of LTD was prevented by the combined application of AMPA/kainate and metabotropic glutamate (mGlu) receptor antagonists (CNQX and LY341495). However, LTD was not blocked by the co-application of NMDA and mGlu receptor antagonists nor by the co-application of NMDA and AMPA/kainate receptor antagonists. Taken together, the above results suggest that activation of either AMPA/kainate or mGlu receptors is sufficient to induce LTD. Therefore, these results suggest that PP-LFS can efficiently activate AMPA/kainate and mGlu receptors in order to induce long-lasting synaptic depression in the CA1 region of the adult hippocampus in vitro.

A role for adenosine in the regulation of long-term depression in the adult rat hippocampus in vitro

We describe how endogenous adenosine can prevent the induction of homosynaptic long-term depression (LTD) in the CA1 region of slices of adult rat hippocampus. Neither of two consecutive periods of prolonged low frequency stimulation (LFS; 1 Hz, 900 stimuli) of the Schaffer collateral-commissural fibres resulted in the induction of LTD in the CA1 region of hippocampal slices from adult (8-30 week) animals. However, in the presence of adenosine deaminase or the selective adenosine A1 receptor antagonist, 1,3-dipropyl-8-cyclopentyl-xanthine (DPCPX), LTD was induced by each of the first and second of two periods of LFS. The first period of LFS did not, but the second period of LFS did, induce LTD in the presence of DPCPX and the NMDA receptor antagonist, D-2-amino-5-phosphonopentanoate (AP5). The present results show that A1 receptor activation by endogenous adenosine can prevent the induction of LTD in the adult hippocampus.

NMDA receptor-dependent and -independent long-term depression in the CA1 region of the adult rat hippocampus in vitro

We have investigated different stimulus parameters in an attempt to induce long-term depression (LTD) in the CA1 region of adult rat hippocampus in vitro. Whereas 900 stimuli delivered at 1 Hz failed to induce LTD, 900 stimuli when delivered as 450 pairs (50 msec inter-stimulus interval) at 1 Hz induced significant and stable N-methyl-D-aspartate (NMDA) receptor-dependent LTD. However, 900 paired stimuli at 1 Hz induced LTD which was only partly blocked by the NMDA receptor antagonist, D-2-amino-5-phosphonopentanoate (AP5).

Endogenous adenosine attenuates long-term depression and depotentiation in the CA1 region of the rat hippocampus

This study tested the hypothesis that endogenous adenosine, a neuromodulator which is known to modify long-term potentiation (LTP), might also affect other forms of long-lasting synaptic plasticity, namely long-term depression (LTD) and depotentiation, in the hippocampus. Long-term depression was induced by applying low-frequency stimulation (LFS; 1 Hz, 900 stimuli, test intensity) to the Schaffer collateral-commissural fibres in hippocampal slices taken from young (12-14-day old) animals. Depotentiation was induced by delivering LFS to a pathway in which LTP had previously been saturated. Under control conditions, LTD induced in two distinct pathways was similar. However, low-frequency stimulation, applied in either pathway in the presence of the selective adenosine A1 receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 10 nM), resulted in LTD which was larger than in control conditions. In a similar way, while under control conditions depotentiation induced in two distinct pathways was similar, when LFS was applied in the presence of DPCPX (10 nM) facilitation of depotentiation was observed. These results suggest that endogenous adenosine, acting through adenosine A1 receptors, is able to attenuate long-term depression and depotentiation in the hippocampus.

Effects of memantine and MK-801 on NMDA-induced currents in cultured neurones and on synaptic transmission and LTP in area CA1 of rat hippocampal slices

The effects of the uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists, memantine (1-amino-3,5-dimethyladamantane) and MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzocyclo-hepten-5,10-imin e maleate) were compared on synaptic transmission and long-term potentiation (LTP) in hippocampal slices and on NMDA-induced currents in cultured superior collicular neurones. 2. Memantine (10-100 microM) reversibly reduced, but did not abolish, NMDA receptor-mediated secondary population spikes recorded in area CA1 of hippocampal slices bathed in Mg(2+)-free artificial cerebrospinal fluid. 3. Memantine (100 microM) antagonized NMDA receptor-mediated excitatory postsynaptic currents recorded in area CA1 in a strongly voltage-dependent manner i.e. depressed to 11 +/- 4% of control at -35 mV and 95 +/- 5% of control at +40 mV (n = 9), with no apparent effect on response kinetics. 4. The effects of MK-801 and memantine on the induction of LTP were assessed after prolonged pre-incubations with these antagonists. When present for 6.6 +/- 0.4 h prior to tetanic stimulation, memantine blocked the induction of LTP with an IC50 of 11.6 +/- 0.53 microM. By comparison, similar long pre-incubations with MK-801 (6.4 +/- 0.4 h) blocked the induction of LTP with an IC50 of 0.13 +/- 0.02 microM. 5. Memantine and MK-801 reduced NMDA-induced currents in cultured superior colliculus neurones recorded at -70 mV with IC50s of 2.2 +/- 0.2 microM and 0.14 +/- 0.04 microM respectively. The effects of memantine were highly voltage-dependent and behaved as though the affinity decreased epsilon fold per 50 mV of depolarization (apparent delta = 0.71). In contrast, under the conditions used, MK-801 appeared to be much less voltage-dependent i.e. affinity decreased epsilon fold per 329 mV of depolarization (apparent delta = 0.15). 6. Depolarizing steps from -70 mV to +50 mV in the continuous presence of memantine (10 microM) caused a rapid relief of blockade of NMDA-induced currents from 83.7 +/- 1.9% to 21.8 +/- 1.8% (n = 5). This relief was best fitted by a double exponential function (17.2 +/- 11.7 and 698 +/- 204 ms), the faster component of which was most pronounced. 7. In conclusion, whereas MK-801 is equipotent in blocking NMDA-induced currents (at - 70 mV) and the induction of LTP, memantine is relatively less potent in blocking the induction of LTP. This is due to its rapid relief of blockade upon depolarization; a property which might explain its promising clinical profile in the treatment of chronic neurodegenerative diseases.

Studies on the role of metabotropic glutamate receptors in long-term potentiation: some methodological considerations

There has been considerable interest recently in trying to elucidate the roles of metabotropic glutamate receptors (mGluRs) in the induction of long-term potentiation (LTP) in area CA1 of rat hippocampal slices. This has come about principally because of the development of specific mGluR agonists and antagonists. Recently we reported that the competitive mGluR antagonist (+)-alpha-methyl-4-carboxyphenylglycine (MCPG) blocks the induction of LTP but not short-term potentiation (STP). We describe here the dose-dependency of the MCPG block; there is no effect at 100 microM while at 200 microM the block of LTP is normally complete but STP is spared. A higher concentration of MCPG (500 microM) has the same effect as 200 microM. We have also reported recently that high-frequency (tetanic) stimulation conditions a pathway such that MCPG fails to block the induction of subsequent LTP. We illustrate here that the conditioning effect of a tetanus lasts at least 6 h. We show how the pathway can be conditioned, without any persistent change in the synaptic response, by delivering tetanic stimulation in the presence of the specific NMDA receptor antagonist D-2-amino-5-phosphonopentanoate (AP5). The pathway can subsequently be deconditioned by delivering low-frequency stimulation (900 shocks at 2 Hz) so that MCPG blocks the induction of subsequent LTP. We also have reported that the specific mGluR agonist 1-aminocyclopentane-(1S,3R)-dicarboxylate (ACPD) can induce LTP without the need for STP.(ABSTRACT TRUNCATED AT 250 WORDS)

Motor deficit and impairment of synaptic plasticity in mice lacking mGluR1

Metabotropic glutamate receptor 1 (mGluR1) is a member of a large family of G-protein-coupled glutamate receptors, the physiological functions of which are largely unknown. Mice deficient in mGluR1 have severe motor coordination and spatial learning deficits. They have no gross anatomical or basic electrophysiological abnormalities in either the cerebellum or hippocampus, but they show impaired cerebellar long-term depression and hippocampal mossy fibre long-term potentiation. mGluR1-deficient mice should therefore be valuable models for studying synaptic plasticity.

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.

A molecular switch activated by metabotropic glutamate receptors regulates induction of long-term potentiation

Pharmacological studies of long-term potentiation (LTP) in the hippocampus are starting to provide a molecular understanding of synaptic plastic processes which are believed to be important for learning and memory in vertebrates. In the CA1 region of the hippocampus, the synaptic activation of glutamate receptors of the N-methyl-D-aspartate (NMDA) subtype is necessary for the induction of LTP under most experimental conditions. The synaptic activation of metabotropic glutamate receptors (mGluRs) is also needed for the induction of LTP. We now show that the role of mGluRs in the induction of LTP is fundamentally different from that of NMDA receptors. NMDA receptors initiate a molecular event that needs to be triggered each time a tetanus is delivered to induce LTP. In contrast, mGluRs activate a molecular switch which then negates the need for mGluR stimulation during the induction of LTP. This mGluR-activated switch is input-specific and can be turned off by a train of low-frequency stimulation. The molecular switch is a new feature of LTP which has fundamental consequences for our understanding of synaptic plastic mechanisms.

Phases in the development of a penicillin epileptiform focus in rat neocortex

Somatosensory evoked potentials and potentials evoked by direct cortical stimulation were recorded from layer IV of the somatosensory area of the cerebral cortex in urethane anaesthetised rats. Penicillin was expelled electrophoretically from the tip of a drug-filled micropipette at constant rates into layer IV. Small fluxes of penicillin (with electrophoretic currents of -50 to -90 nA) resulted in the appearance, after a delay of 1-2 min, of an enhancement of amplitude in the voltage of both types of evoked potential, unaccompanied by any prolongation of the waveform or spontaneous focal epileptiform discharges. The amplitude of the enhanced evoked potential exhibited a strength-response curve which was a scaled-up version of the curve before penicillin, the scaling factor reflecting the enhancement of amplitude. As the interval between a pair of stimuli was increased, the magnitude of the response to the second stimulus recovered, following a time course similar to that before penicillin. With larger fluxes of penicillin (with electrophoretic currents of -250 to -1000 nA) the amplitude of evoked potentials rose more rapidly and to a higher level; as the concentration of penicillin rose, this enhancement of amplitude led into a second phase, in which there was additionally an increase in the duration of the evoked potentials and the appearance of spontaneous epileptiform discharges. The evoked potentials in this situation showed physiological properties different from those before penicillin application. The strength-response curve exhibited a discontinuity, indicating the evoked potential to be the sum of a physiological response and an epileptiform discharge, the former being graded with stimulus strength and the latter being all or none.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabotropic glutamate receptors contribute to the induction of long-term depression in the CA1 region of the hippocampus

Long-term depression of synaptic transmission was induced following the prior induction of long-term potentiation in the CA1 region of rat hippocampal slices. We show that the induction of this form of synaptic depression can be prevented by (+)-alpha-methyl-4-carboxyphenylglycine, a selective antagonist of metabotropic glutamate receptors.

Induction of LTP in the hippocampus needs synaptic activation of glutamate metabotropic receptors

Understanding the mechanisms of long-term potentiation (LTP) should provide insights into the molecular basis of learning and memory in vertebrates. Ionotropic glutamate receptors play a central role in LTP; AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate) receptors and NMDA (N-methyl-D-aspartate) receptors mediate synaptic responses that are enhanced in LTP and, in addition, NMDA receptors are necessary for the induction of LTP in most pathways. There is also circumstantial evidence that metabotropic glutamate receptors (mGluRs) may be involved in LTP because the specific mGluR agonist aminocyclopentane dicarboxylate can augment tetanus-induced LTP2 and, under certain circumstances, can itself induce a slow-onset potentiation. But the absence of any effective mGluR antagonist has prevented the determination of whether mGluRs are involved in the induction of tetanus-induced LTP. We report here that (RS)-alpha-methyl-4-carboxyphenylglycine is a specific mGluR antagonist in the hippocampus and have used this compound to examine the nature of the involvement of mGluRs in LTP. We show that synaptic activation of mGluRs is necessary for the induction of both NMDA receptor-dependent and NMDA receptor-independent forms of LTP in the hippocampus.

Involvement of excitatory amino acid receptors in long-term potentiation in the Schaffer collateral-commissural pathway of rat hippocampal slices

The present article reviews studies from our laboratory, which have shown that excitatory amino acids receptors of the N-methyl-D-aspartate type are involved in the induction of long-term potentiation in the Schaffer collateral-commissural pathway of rat hippocampal slices. The nature of the excitatory amino acid receptors that mediate the response that is modified by the induction of long-term potentiation is also considered. The mechanism of induction of long-term potentiation is discussed, as are some possible stages that are required for the maintenance of this process. Some new data are presented concerning the ability of N-methyl-D-aspartate to potentiate synaptic transmission and to depress the amplitude of the presynaptic fibre volley. Concerning the potentiation, it is shown that brief (1-2 min) perfusion of slices with N-methyl-D-aspartate is sufficient to potentiate synaptic transmission for at least 3 h. The N-methyl-D-aspartate induced depression of the presynaptic fibre volley is shown to be transient and independent of synaptic transmission.

Long-term potentiation of NMDA receptor-mediated synaptic transmission in the hippocampus

Neurotransmission at most excitatory synapses in the brain operates through two types of glutamate receptor termed alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate (NMDA) receptors; these mediate the fast and slow components of excitatory postsynaptic potentials respectively. Activation of NMDA receptors can also lead to a long-lasting modification in synaptic efficiency at glutamatergic synapses; this is exemplified in the CA1 region of the hippocampus, where NMDA receptors mediate the induction of long-term potentiation (LTP). It is believed that in this region LTP is maintained by a specific increase in the AMPA receptor-mediated component of synaptic transmission. We now report, however, that a pharmacologically isolated NMDA receptor-mediated synaptic response can undergo robust, synapse-specific LTP. This finding has implications for neuropathologies such as epilepsy and neurodegeneration, in which excessive NMDA receptor activation has been implicated. It adds fundamentally to theories of synaptic plasticity because NMDA receptor activation may, in addition to causing increased synaptic efficiency, directly alter the plasticity of synapses.

Activation of the glycine site in the NMDA receptor is necessary for the induction of LTP

The effects of 7-chlorokynurenate (7-C1KY) were examined on N-methyl-D-aspartate (NMDA) receptor-mediated synaptic mechanisms in the CA1 region of rat hippocampal slices. 7-C1KY depressed both the NMDA receptor-mediated component of synaptic transmission recorded in Mg2(+)-free medium and the induction of long-term potentiation (LTP) in Mg2(+)-containing medium. Both of these effects were reversed by D-serine, suggesting that the action of 7-C1KY was at the allosteric glycine site on the NMDA receptor.