Characterisation of an mGlu8 receptor-selective agonist and antagonist in the lateral and medial perforant path inputs to the dentate gyrus

Since its characterisation in 2001, the mGlu8-selective agonist DCPG has been widely used to explore the potential functional role of this group III mGlu receptor within the central nervous system. This research has implicated mGlu8 receptors in a number of disease states and conditions such as epilepsy and anxiety, suggesting that mGlu8-selective ligands may hold important therapeutic potential. However, there is evidence that DCPG exerts off-target effects at higher concentrations, limiting its use as an mGlu8-selective agonist. Here, we have used field recordings in rat hippocampal slices to investigate the effects of DCPG in the lateral perforant path (LPP), a pathway known to express high levels of mGlu8. We show that DCPG does inhibit excitatory transmission in this pathway, but produces a biphasic concentration-response curve suggesting activation of two distinct receptor types. The putative mGlu8-selective antagonist MDCPG antagonises the high, but not the low, potency component of this concentration-response curve. In addition, higher concentrations of DCPG also depress excitatory transmission in the medial perforant path (MPP), a pathway expressing very low levels of mGlu8 receptors. Experiments in slices from mice lacking mGlu8 receptors indicate that concentrations of DCPG >1 μM produce large non-selective effects in both the LPP and MPP. Further experiments in slices from mGlu2, 4 and 7 knock-out mice, as well as in an mGlu2-deficient substrain of Wistar rat, reveal that these non-selective effects are mediated primarily by mGlu2 receptors. Taken together, our results confirm the mGlu8-selectivity of DCPG at submicromolar concentrations, but suggest that care must be taken when employing higher concentrations of the agonist, which may additionally activate mGlu2 receptors, especially at synapses where their expression is high. MDCPG may be a useful tool in determining whether observable DCPG effects are attributable to mGlu8, versus mGlu2, receptor activation.

The Jak/STAT pathway is involved in synaptic plasticity

The Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway is involved in many cellular processes, including cell growth and differentiation, immune functions and cancer. It is activated by various cytokines, growth factors, and protein tyrosine kinases (PTKs) and regulates the transcription of many genes. Of the four JAK isoforms and seven STAT isoforms known, JAK2 and STAT3 are highly expressed in the brain where they are present in the postsynaptic density (PSD). Here, we demonstrate a new neuronal function for the JAK/STAT pathway. Using a variety of complementary approaches, we show that the JAK/STAT pathway plays an essential role in the induction of NMDA-receptor dependent long-term depression (NMDAR-LTD) in the hippocampus. Therefore, in addition to established roles in cytokine signaling, the JAK/STAT pathway is involved in synaptic plasticity in the brain.

The NMDA receptor as a target for cognitive enhancement

NMDA receptors (NMDARs) play an important role in neural plasticity including long-term potentiation and long-term depression, which are likely to explain their importance for learning and memory. Cognitive decline is a major problem facing an ageing human population, so much so that its reversal has become an important goal for scientific research and pharmaceutical development. Enhancement of NMDAR function is a core strategy toward this goal. In this review we indicate some of the major ways of potentiating NMDAR function by both direct and indirect modulation. There is good evidence that both positive and negative modulation can enhance function suggesting that a subtle approach correcting imbalances in particular clinical situations will be required. Excessive activation and the resultant deleterious effects will need to be carefully avoided. Finally we describe some novel positive allosteric modulators of NMDARs, with some subunit selectivity, and show initial evidence of their ability to affect NMDAR mediated events. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Inactivation of the constitutively active ghrelin receptor attenuates limbic seizure activity in rodents

Ghrelin is a pleiotropic neuropeptide that has been recently implicated in epilepsy. Animal studies performed to date indicate that ghrelin has anticonvulsant properties; however, its mechanism of anticonvulsant action is unknown. Here we show that the anticonvulsant effects of ghrelin are mediated via the growth hormone secretagogue receptor (GHSR). To our surprise, however, we found that the GHSR knockout mice had a higher seizure threshold than their wild-type littermates when treated with pilocarpine. Using both in vivo and in vitro models, we further discovered that inverse agonism and desensitization/internalization of the GHSR attenuate limbic seizures in rats and epileptiform activity in hippocampal slices. This constitutes a novel mechanism of anticonvulsant action, whereby an endogenous agonist reduces the activity of a constitutively active receptor.

Differences in kainate receptor involvement in hippocampal mossy fibre long-term potentiation depending on slice orientation

Long-term potentiation (LTP) is a well-established experimental model used to investigate the synaptic basis of learning and memory. LTP at mossy fibre - CA3 synapses in the hippocampus is unusual because it is normally N-methyl-d-aspartate (NMDA) receptor-independent. Instead it seems that the trigger for mossy fibre LTP involves kainate receptors (KARs). Although it is generally accepted that pre-synaptic KARs play an essential role in frequency facilitation and LTP, their subunit composition remains a matter of significant controversy. We have reported previously that both frequency facilitation and LTP can be blocked by selective antagonism of GluK1 (formerly GluR5/Glu(K5))-containing KARs, but other groups have failed to reproduce this effect. Moreover, data from receptor knockout and mRNA expression studies argue against a major role of GluK1, supporting a more central role for GluK2 (formerly GluR6/Glu(K6)). A potential reason underlying the controversy in the pharmacological experiments may reside in differences in the preparations used. Here we show differences in pharmacological sensitivity of synaptic plasticity at mossy fibre - CA3 synapses depend critically on slice orientation. In transverse slices, LTP of fEPSPs was invariably resistant to GluK1-selective antagonists whereas in parasagittal slices LTP was consistently blocked by GluK1-selective antagonists. In addition, there were pronounced differences in the magnitude of frequency facilitation and the sensitivity to the mGlu2/3 receptor agonist DCG-IV. Using anterograde labelling of granule cells we show that slices of both orientations possess intact mossy fibres and both large and small presynaptic boutons. Transverse slices have denser fibre tracts but a smaller proportion of giant mossy fibre boutons. These results further demonstrate a considerable heterogeneity in the functional properties of the mossy fibre projection.

Selective activation of either mGlu2 or mGlu3 receptors can induce LTD in the amygdala

Group II metabotropic glutamate (mGlu) receptors are known to induce a long-term depression (LTD) of synaptic transmission in many brain regions including the amygdala. However the roles of the individual receptor subtypes, mGlu2 and mGlu3, in LTD are not well understood. In particular, it is unclear whether activation of mGlu3 receptors is sufficient to induce LTD at synapses in the CNS. In the present study, advantage was taken of a Wistar rat strain not expressing mGlu2 receptors (Ceolin et al., 2011) to investigate the function of mGlu3 receptors in the amygdala. In this preparation, the group II agonist, DCG-IV induced an LTD of the cortical, but not the intra-nuclear, synaptic input to the lateral amygdala. This LTD was concentration dependent and was blocked by the group II mGlu receptor antagonist, LY341495. To investigate further the role of mGlu3 receptors, we used LY395756 (an mGlu2 agonist and mGlu3 antagonist), which acts as a pure mGlu3 receptor antagonist in this rat strain. This compound alone had no effect on basal synaptic transmission, but blocked the LTD induced by DCG-IV. Furthermore, we found that DCG-IV also induces LTD in mGlu2 receptor knock-out (KO) mice to a similar extent as in wild-type mice. This confirms that the activation of mGlu3 receptors alone is sufficient to induce LTD at this amygdala synapse. To address whether mGlu2 activation alone is also sufficient to induce LTD at this synapse we used LY541850 (the active enantiomer of LY395756) in wild-type mice. LY541850 induced a substantial LTD showing that either receptor alone is capable of inducing LTD in this pathway. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

A pivotal role of GSK-3 in synaptic plasticity

Glycogen synthase kinase-3 (GSK-3) has many cellular functions. Recent evidence suggests that it plays a key role in certain types of synaptic plasticity, in particular a form of long-term depression (LTD) that is induced by the synaptic activation of N-methyl-D-aspartate receptors (NMDARs). In the present article we summarize what is currently known concerning the roles of GSK-3 in synaptic plasticity at both glutamatergic and GABAergic synapses. We summarize its role in cognition and speculate on how alterations in the synaptic functioning of GSK-3 may be a major factor in certain neurodegenerative disorders.

Piperazine-2,3-dicarboxylic acid derivatives as dual antagonists of NMDA and GluK1-containing kainate receptors

Competitive N-methyl-d-aspartate receptor (NMDAR) antagonists bind to the GluN2 subunit, of which there are four types (GluN2A-D). We report that some N(1)-substituted derivatives of cis-piperazine-2,3-dicarboxylic acid display improved relative affinity for GluN2C and GluN2D versus GluN2A and GluN2B. These derivatives also display subtype selectivity among the more distantly related kainate receptor family. Compounds 18i and (-)-4 were the most potent kainate receptor antagonists, and 18i was selective for GluK1 versus GluK2, GluK3 and AMPA receptors. Modeling studies revealed structural features required for activity at GluK1 subunits and suggested that S674 was vital for antagonist activity. Consistent with this hypothesis, replacing the equivalent residue in GluK3 (alanine) with a serine imparts 18i antagonist activity. Antagonists with dual GluN2D and GluK1 antagonist activity may have beneficial effects in various neurological disorders. Consistent with this idea, antagonist 18i (30 mg/kg ip) showed antinociceptive effects in an animal model of mild nerve injury.

Synaptic plasticity in the hippocampal slice preparation

Synaptic plasticity is the process by which the brain alters the strength of its synaptic connections, a fundamental function of the brain that enables individuals to learn from experience. The study of synaptic plasticity often involves the application of standard in vitro electrophysiological techniques to hippocampal slice preparations. This unit discusses many of the special considerations that are applicable for the optimal study of synaptic plasticity in this system. Most of these principles also apply to the study of synaptic plasticity in other brain slice preparations.

Synergistic interactions between kainate and mGlu receptors regulate bouton Ca signalling and mossy fibre LTP

It is currently unknown why glutamatergic presynaptic terminals express multiple types of glutamate receptors. We have addressed this question by studying both acute and long-term regulation of mossy fibre function in the hippocampus. We find that inhibition of both mGlu₁ and mGlu₅ receptors together can block the induction of mossy fibre LTP. Furthermore, mossy fibre LTP can be induced by the pharmacological activation of either mGlu₁ or mGlu₅ receptors, provided that kainate receptors are also stimulated. Like conventional mossy fibre LTP, chemically-induced mossy fibre LTP (chem-LTPm) depends on Ca²⁺ release from intracellular stores and the activation of PKA. Similar synergistic interactions between mGlu receptors and kainate receptors were observed at the level of Ca²⁺ signalling in individual giant mossy fibre boutons. Thus three distinct glutamate receptors interact, in both an AND and OR gate fashion, to regulate both immediate and long-term presynaptic function in the brain.

ACET is a highly potent and specific kainate receptor antagonist: characterisation and effects on hippocampal mossy fibre function

Kainate receptors (KARs) are involved in both NMDA receptor-independent long-term potentiation (LTP) and synaptic facilitation at mossy fibre synapses in the CA3 region of the hippocampus. However, the identity of the KAR subtypes involved remains controversial. Here we used a highly potent and selective GluK1 (formerly GluR5) antagonist (ACET) to elucidate roles of GluK1-containing KARs in these synaptic processes. We confirmed that ACET is an extremely potent GluK1 antagonist, with a Kb value of 1.4+/-0.2 nM. In contrast, ACET was ineffective at GluK2 (formerly GluR6) receptors at all concentrations tested (up to 100 microM) and had no effect at GluK3 (formerly GluR7) when tested at 1 microM. The X-ray crystal structure of ACET bound to the ligand binding core of GluK1 was similar to the UBP310-GluK1 complex. In the CA1 region of hippocampal slices, ACET was effective at blocking the depression of both fEPSPs and monosynaptically evoked GABAergic transmission induced by ATPA, a GluK1 selective agonist. In the CA3 region of the hippocampus, ACET blocked the induction of NMDA receptor-independent mossy fibre LTP. To directly investigate the role of pre-synaptic GluK1-containing KARs we combined patch-clamp electrophysiology and 2-photon microscopy to image Ca2+ dynamics in individual giant mossy fibre boutons. ACET consistently reduced short-term facilitation of pre-synaptic calcium transients induced by 5 action potentials evoked at 20-25Hz. Taken together our data provide further evidence for a physiological role of GluK1-containing KARs in synaptic facilitation and LTP induction at mossy fibre-CA3 synapses.

An analysis of the stimulus requirements for setting the molecular switch reveals a lower threshold for metaplasticity than synaptic plasticity

The requirements for the synaptic activation of metabotropic glutamate (mGlu) receptors and for the induction of metaplasticity in the hippocampus are not known. In the present study, we have investigated the synaptic activation of mGlu5 receptors and the setting of the molecular switch, a form of metaplasticity, at CA1 synapses in the mouse hippocampus. We find that as few as eight stimuli (delivered at 100Hz) are sufficient to set the molecular switch, since a subsequent tetanus delivered to the same input is able to induce long-term potentiation (LTP) in the presence of the mGlu receptor antagonist MCPG ((S)-alpha-methyl-4-carboxyphenylglycine). In addition, we find that the molecular switch can be activated over a wide frequency range. When 10 shocks were delivered the threshold frequency was 4Hz. The ability of 10 shocks (delivered at 100Hz) to set the molecular switch was lost in the mGlu5 knockout. These data show that mGlu5 receptors can be activated synaptically and metaplasticity can be induced by relatively few stimuli. Indeed, metaplasticity was induced by stimuli that were subthreshold for the induction of LTP per se. Thus, metaplasticity has a lower threshold than the synaptic plasticity that it regulates.

The induction of long-term plasticity of non-synaptic, synchronized activity by the activation of group I mGluRs

It is well established that activation of group I metabotropic glutamate receptors (mGluRs) produces long-lasting alterations in synaptic efficacy. We now demonstrate that activation of mGluRs can also induce long-term alterations in synchronised network activity that are both induced and expressed in the absence of chemical synaptic transmission. Specifically, in hippocampal slices in which synaptic transmission was eliminated by perfusing with a Ca2+-free medium, the selective group I mGluR agonist 3,5-dihydroxyphenylglycine (DHPG) induced a persistent (>3h) enhancement (>2-fold) of the frequency of synchronised bursting activity. The underlying biochemical mechanism responsible for the induction of this form of plasticity was similar to that for DHPG-induced long-term depression (LTD) in that it required the activation of tyrosine phosphatases. Also, like DHPG-induced LTD, this form of neuronal plasticity could be reversed by application of the mGluR antagonist alpha-methyl-4-carboxyphenylglycine (MCPG). This unusual form of plasticity, which presumably also occurs when synaptic transmission is intact, could contribute to long-term alterations in synchronised activity in hippocampal neuronal networks.

The role of GSK-3 in synaptic plasticity

Glycogen synthase kinase-3 (GSK-3), an important component of the glycogen metabolism pathway, is highly expressed in the CNS. It has been implicated in major neurological disorders including Alzheimer's disease, schizophrenia and bipolar disorders. Despite its central role in these conditions it was not known until recently whether GSK-3 has neuronal-specific functions under normal conditions. However recent work has shown that GSK-3 is involved in the regulation of, and cross-talk between, two major forms of synaptic plasticity, N-methyl-D-aspartate receptor (NMDAR)-dependent long-term potentiation (LTP) and NMDAR-dependent long-term depression (LTD). The present article summarizes this recent work and discusses its potential relevance to the treatment of neurological disorders.

LTP inhibits LTD in the hippocampus via regulation of GSK3beta

Glycogen synthase kinase-3 (GSK3) has been implicated in major neurological disorders, but its role in normal neuronal function is largely unknown. Here we show that GSK3beta mediates an interaction between two major forms of synaptic plasticity in the brain, N-methyl-D-aspartate (NMDA) receptor-dependent long-term potentiation (LTP) and NMDA receptor-dependent long-term depression (LTD). In rat hippocampal slices, GSK3beta inhibitors block the induction of LTD. Furthermore, the activity of GSK3beta is enhanced during LTD via activation of PP1. Conversely, following the induction of LTP, there is inhibition of GSK3beta activity. This regulation of GSK3beta during LTP involves activation of NMDA receptors and the PI3K-Akt pathway and disrupts the ability of synapses to undergo LTD for up to 1 hr. We conclude that the regulation of GSK3beta activity provides a powerful mechanism to preserve information encoded during LTP from erasure by subsequent LTD, perhaps thereby permitting the initial consolidation of learnt information.

Differential roles of NR2A and NR2B-containing NMDA receptors in LTP and LTD in the CA1 region of two-week old rat hippocampus

The role of NMDA receptors in the induction of long-term potentiation (LTP) and long-term depression (LTD) is well established but which particular NR2 subunits are involved in these plasticity processes is still a matter of controversy. We have studied the effects of subtype selective NMDA receptor antagonists on LTP induced by high frequency stimulation (100 Hz for 1s) and LTD induced by low frequency stimulation (1 Hz for 15 min) in the CA1 region of hippocampal slices from 14 day old Wistar rats. Against recombinant receptors in HEK293 cells NVP-AAM077 (NVP) was approximately 14-fold selective for NR2A vs NR2B receptors, whilst Ro 25-6981 (Ro) was highly selective for NR2B receptors. On NMDA receptor-mediated EPSCs from Schaffer collaterals in CA1 neurones, NVP and Ro both reduced the amplitude but differentially affected the time constant of decay. The data are compatible with the selective effect of NVP (0.1 microM) and Ro (4 microM) on native NR2A and NBR2B receptors, respectively. NVP reduced both LTP and LTD whereas Ro reduced only LTP. Thus, LTP was reduced by 63% at 0.1 microM NVP and almost completely at 0.4 microM whereas 5 microM Ro reduced LTP by 45%. These data are consistent with a role for both NR2A and NR2B in the induction of LTP, under our experimental conditions. In comparison, LTD was unaffected by Ro (5 microM) even in the presence of a glutamate uptake inhibitor threo-beta-benzylaspartic acid (TBOA) to increase the concentration of glutamate at NR2B containing receptors. NVP (0.2-0.4 microM), however, produced a concentration dependent inhibition of LTD which was complete at 0.4 microM. The lack of effect of 0.1 microM NVP on LTD contrasts with its marked effect on LTP and raises the possibility that different NVP-sensitive NR2 subunit-containing NMDA receptors are required for LTP and LTD in this preparation.

Transient incorporation of native GluR2-lacking AMPA receptors during hippocampal long-term potentiation

Postnatal glutamatergic principal neuron synapses are typically presumed to express only calcium-impermeable (CI), GluR2-containing AMPARs under physiological conditions. Here, however, we demonstrate that long-term potentiation (LTP) in CA1 hippocampal pyramidal neurons causes rapid incorporation of GluR2-lacking calcium-permeable (CP)-AMPARs: CP-AMPARs are present transiently, being replaced by GluR2-containing AMPARs approximately 25 min after LTP induction. Thus, CP-AMPARs are physiologically expressed at CA1 pyramidal cell synapses during LTP, and may be required for LTP consolidation.

Rapid responses to thyroxine in the testis: active protein synthesis-independent pathway

We investigated the involvement of protein synthesis in the stimulatory action of thyroid hormones on amino acid accumulation and characterized K(+) currents involved in the hyperpolarizing effect of thyroxine (T(4)) on Sertoli cells. Immature rat testes were incubated in Krebs Ringer-bicarbonate buffer (KRb) in the presence of [(14)C]methylaminoisobutyric acid with and without T(4), 3,5,3'-l-triiodothyronine (T(3)) and/or cycloheximide. Sertoli cells were monitored by intracellular recording in a chamber perfused with KRb with and without T(4), T(3) and/or blockers, and the membrane potential was monitored. T(4) and T(3) stimulated amino acid accumulation and protein synthesis. Treatment with cycloheximide diminished T(3) stimulatory actions on amino acid accumulation but had no effect on T(4) action. Both hormones elicited a hyperpolarization of the Sertoli cell membrane potential which involved K(+) channels, since TEA and apamin abolished this effect. These findings on rapid membrane actions of thyroid hormone in the testis suggest that some effects of T(4) are modulated by non-genomic mechanisms.

The regulation of hippocampal LTP by the molecular switch, a form of metaplasticity, requires mGlu5 receptors

The role of metabotropic glutamate (mGlu) receptors in long-term potentiation (LTP) in the hippocampus is controversial. In the present study, we have used mice in which the mGlu1, mGlu5 or mGlu7 receptor has been deleted, by homologous recombination, to study the role of these receptor subtypes in LTP at CA1 synapses. We investigated the effects of the knockouts on both LTP and the molecular switch, a form of metaplasticity that renders LTP insensitive to the actions of the mGlu receptor antagonist MCPG ((S)-alpha-methyl-4-carboxyphenylglycine). We find that LTP is readily induced in the three knockouts and in an mGlu1 and mGlu5 double knockout. In addition, the molecular switch operates normally in either the mGlu1 or mGlu7 knockout. In contrast, the molecular switch is completely non-functional in the mGlu5 knockout, such that MCPG invariably blocks the induction of additional LTP in an input where LTP has already been induced. The effect of the mGlu5 receptor knockout was replicated in wildtype mouse slices perfused with the specific mGlu5 receptor antagonist MPEP (2-methyl-6-(phenylethynyl)-pyridine). In addition, the mGlu5 selective agonist CHPG ((RS)-2-chloro-5-hydroxyphenylglycine) sets the molecular switch. These data demonstrate that the operation of the molecular switch requires activation of mGlu5 receptors.

Synthesis and Pharmacology of Willardiine Derivatives Acting as Antagonists of Kainate Receptors

The natural product willardiine (8) is an AMPA receptor agonist while 5-iodowillardiine (10) is a selective kainate receptor agonist. In an attempt to produce antagonists of kainate and AMPA receptors analogues of willardiine with substituents at the N3 position of the uracil ring were synthesized. The N3-4-carboxybenzyl substituted analogue (38c) was found to be equipotent at AMPA and GLUK5-containing kainate receptors in the neonatal rat spinal cord. The N3-2-carboxybenzyl substituted analogue (38a) proved to be a potent and selective GLUK5 subunit containing kainate receptor antagonist when tested on native rat and human recombinant AMPA and kainate receptor subtypes. The GLUK5 kainate receptor antagonist activity was found to reside in the S enantiomer (44a) whereas the R enantiomer (44b) was almost inactive. 5-Iodo substitution of the uracil ring of 44a gave 45, which was found to have enhanced potency and selectivity for GLUK5.

Kainate receptors and mossy fiber LTP

There is considerable interest in understanding long-term potentiation (LTP) of glutamatergic synaptic transmission because the molecular mechanisms involved in its induction and expression are believed to be critical for learning and memory. There are two distinct forms of LTP. One type is triggered by synaptic activation of NMDA receptors and the other is NMDA receptor-independent. The latter type of LTP has been mostly studied at mossy fiber/CA3 synapses. Here we summarise some of our recent studies concerning the mechanisms of the induction of the NMDA receptor-independent form of LTP at these CA3 synapses. This form of LTP is triggered by the synaptic activation of kainate receptors. We also address the importance of Ca2+ availability in the extracellular environment and the release of Ca2+ from intracellular stores for this form of LTP.

Characterisation of UBP296: a novel, potent and selective kainate receptor antagonist

Willardiine derivatives with an N3-benzyl substituent bearing an acidic group have been synthesized with the aim of producing selective antagonists for GLUK5-containing kainate receptors. UBP296 was found to be a potent and selective antagonist of native GLUK5-containing kainate receptors in the spinal cord, with activity residing in the S enantiomer (UBP302). In cells expressing human kainate receptor subunits, UBP296 selectively depressed glutamate-induced calcium influx in cells containing GLUK5 in homomeric or heteromeric forms. In radioligand displacement binding studies, the willardiine analogues displaced [3H]kainate binding with IC50 values >100 microM at rat GLUK6, GLUK2 or GLUK6/GLUK2. An explanation of the GLUK5 selectivity of UBP296 was obtained using homology models of the antagonist bound forms of GLUK5 and GLUK6. In rat hippocampal slices, UBP296 reversibly blocked ATPA-induced depressions of synaptic transmission at concentrations subthreshold for affecting AMPA receptor-mediated synaptic transmission directly. UBP296 also completely blocked the induction of mossy fibre LTP, in medium containing 2 mM (but not 4 mM) Ca2+. These data provide further evidence for a role for GLUK5-containing kainate receptors in mossy fibre LTP. In conclusion, UBP296 is the most potent and selective antagonist of GLUK5-containing kainate receptors so far described.

Parallel kinase cascades are involved in the induction of LTP at hippocampal CA1 synapses

To identify the enzymes involved in the induction of N-methyl-D-aspartate (NMDA) receptor-dependent long-term potentiation (LTP) at CA1 synapses of two-week-old rats we have tested various kinase inhibitors. Surprisingly, given the large body of evidence supporting a role for calcium/calmodulin-dependent protein kinase II (CaMKII) in LTP, inhibition of this enzyme did not affect the induction of LTP at this age. Similarly inhibition of protein kinase A (PKA) or protein kinase C (PKC) was also without effect. However, inhibition of CaMKII together with inhibition of either PKA or PKC fully blocked the induction of LTP. These experiments reveal, unexpectedly, the existence of two parallel kinase pathways, one involving CaMKII and the other PKA and PKC, either of which can fully support the induction of LTP, at this stage of development.

A role for Ca2+ stores in kainate receptor-dependent synaptic facilitation and LTP at mossy fiber synapses in the hippocampus

Compared with NMDA receptor-dependent LTP, much less is known about the mechanism of induction of NMDA receptor-independent LTP; the most extensively studied form of which is mossy fiber LTP in the hippocampus. In the present study we show that Ca2+-induced Ca2+ release from intracellular stores is involved in the induction of mossy fiber LTP. This release also contributes to the kainate receptor-dependent component of the pronounced synaptic facilitation that occurs during high-frequency stimulation. We also present evidence that the trigger for this Ca2+ release is Ca2+ permeation through kainate receptors. However, these novel synaptic mechanisms can be bypassed when the Ca2+ concentration is raised (from 2 to 4 mM), via a compensatory involvement of L-type Ca2+ channels. These findings suggest that presynaptic kainate receptors at mossy fiber synapses can initiate a cascade involving Ca2+ release from intracellular stores that is important in both short-term and long-term plasticity.

Kainate receptors and the induction of mossy fibre long-term potentiation

There is intense interest in understanding the molecular mechanisms involved in long-term potentiation (LTP) in the hippocampus. Significant progress in our understanding of LTP has followed from studies of glutamate receptors, of which there are four main subtypes (alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA), N-methyl-D-aspartate (NMDA), mGlu and kainate). This article summarizes the evidence that the kainate subtype of glutamate receptor is an important trigger for the induction of LTP at mossy fibre synapses in the CA3 region of the hippocampus. The pharmacology of the first selective kainate receptor antagonists, in particular the GLU(K5) subunit selective antagonist LY382884, is described. LY382884 selectively blocks the induction of mossy fibre LTP, in response to a variety of different high-frequency stimulation protocols. This antagonist also inhibits the pronounced synaptic facilitation of mossy fibre transmission that occurs during high-frequency stimulation. These effects are attributed to the presence of presynaptic GLU(K5)-subunit-containing kainate receptors at mossy fibre synapses. Differences in kainate receptor-dependent synaptic facilitation of AMPA and NMDA receptor-mediated synaptic transmission are described. These data are discussed in the context of earlier reports that glutamate receptors are not involved in mossy fibre LTP and more recent experiments using kainate receptor knockout mice, that argue for the involvement of GLU(K6) but not GLU(K5) kainate receptor subunits. We conclude that activation of presynaptic GLU(K5)-containing kainate receptors is an important trigger for the induction of mossy fibre LTP in the hippocampus.