Block of HFS-induced LTP in the dentate gyrus by 1S,3S-ACPD: further evidence against LTP as a model for learning

Variation of CS salience reveals group II mGluR-dependent and -independent forms of conditioning in the rat

There is good evidence that metabotropic glutamate receptors (mGluRs) are involved in some types of learning, and we have previously suggested that this involvement may reflect the modulation by mGluRs of the signal-to-noise ratio in neural networks. This hypothesis supposes that unspecific activation of mGluRs increases background noise level, so reducing the effectiveness of behaviourally relevant stimuli as signals in the network. We report here that intraperitoneal (i.p.) injection of 4-aminopyrrolidine-2,4-dicarboxylic acid (APDC), a specific agonist of group II mGluRs, disrupts conditioning to context (but not to cue) using conventional procedures. The hypothesis predicts, however, that the effect of the drug should be counteracted by the use of more salient stimuli, which would provide stronger signals to the network. In accordance with this prediction, we find that increases in the salience of either the CS (context) or the UCS (shock) abolish the drug-induced disruption of conditioning. These results suggest that group II mGluRs modulate neural networks involved in association formation.

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.

Metabotropic glutamate receptor activation and blockade: their role in long-term potentiation, learning and neurotoxicity

Metabotropic glutamate receptors represent a fairly recent addition to the family of glutamate receptors. These receptors have the distinguishing feature of being coupled to G-proteins rather than ion channels and they appear to have a variety of functional characteristics. These receptors play a vital role, for example, in the induction and maintenance of long-term potentiation, the most popular current model of the biological correlates of learning and memory. Blockade of metabotropic glutamate receptors prevents long-term potentiation induction and learning in a variety of tasks in different species. Chronic metabotropic glutamate receptor activation is also associated with neurodegeneration and selective neuronal loss when agonists of these receptors are injected in high concentrations directly into the brain. Metabotropic glutamate receptors also play a role in the normal development of the nervous system and these sites within the central nervous system offer possible routes for drug therapies; selective receptor antagonists, for example, may prove to have the very desirable feature of endowing neuroprotection during ischaemic episodes whilst allowing normal excitatory neurotransmission to occur.

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

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

Group 1 metabotropic glutamate receptors contribute to slow-onset potentiation in the rat CA1 region in vivo

It has been demonstrated in the CA1 region of the hippocampus in vitro, and in the dentate gyrus and CA1 region in vivo, that application of the metabotropic glutamate receptor (mGluR) agonist, 1S, 3R-amino cyclopentane 2,3-dicarboxylic acid triggers a slow-onset potentiation of synaptic transmission in the hippocampus. This study examined the involvement of group 1 and 2 mGluRs in this phenomenon in the CA1 region of freely moving rats. Drugs were applied via the lateral cerebral ventricle, and measurements were obtained from the CA1 region via permanently implanted electrodes. The group 1 mGluR agonists, 3,5-dihydroxyphenylglycine (DHPG, 20-100 nmol/5 microl) and trans-azetidine-2,4-dicarboxylic acid (ADA, 100 nmol-1 micromol/5 microl) induced a dose-dependent potentiation of basal synaptic transmission. The mGluR antagonist R,S-alpha-methyl-carboxyphenylglycine (MCPG, 1 micromol), and the group 1 mGluR antagonist, S-4-carboxyphenylglycine (4CPG, 100 nmol) completely inhibited the effects of both DHPG and ADA. The group 2 mGluR agonist, (S)-4-carboxy-3-hydroxy phenylglycine (4C3H-PG, 50-200 nmol/5 microl) induced a dose-dependent decrease of basal synaptic transmission. These results suggest that in the CA1 region in vivo, slow-onset potentiation may be mediated by group 1 mGluRs.

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

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

Nitric oxide, the enigmatic neuronal messenger: its role in synaptic plasticity

The discovery of the intercellular messenger nitric oxide (NO) stimulated new concepts of how synaptic plasticity could be induced in the nervous system. While initial reports found evidence that NO is of importance for the formation of long-term potentiation of synaptic transmission (LTP) and spatial learning in rats, later reports failed to confirm these results. Novel approaches such as deletion of the gene that encodes NO synthase in mice showed that the neuronal and the endothelial isoforms are expressed in neurones. Deletion of both isoforms reduced the inducibility of LTP. Furthermore, novel selective inhibitors of NO synthase impaired spatial learning. These results support the hypothesis that NO plays an important role in synaptic transmission and explain some but not all previously contradictory results.