Metabotropic receptor stimulation coupled to weak tetanus leads to long-term potentiation and a rapid elevation of cytosolic protein kinase C activity

mGluR1α expression in the hippocampus, subiculum, entorhinal cortex and superior temporal gyrus in Alzheimer's disease

Glutamate is the main excitatory neurotransmitter in the central nervous system, responsible for a plethora of cellular processes including memory formation and higher cerebral function and has been implicated in various neurological disease states. Alzheimer's disease (AD) is the leading neurodegenerative disorder worldwide and is characterized by significant cell loss and glutamatergic dysfunction. While there has been a focus on ionotropic glutamatergic receptors few studies have attempted to elucidate the pathological changes of metabotropic glutamate receptors (mGluRs) in AD. mGluRs are G-protein coupled receptors which have a wide-ranging functionality, including the regulation of neuronal injury and survival. In particular, the group I mGluRs (mGluR1 and mGluR5) are associated with ionotropic receptor activation and upregulation with resultant glutamate release in normal neuronal functioning. The mGluR subtype 1 splice variant a (mGluR1α) is the longest variant of the mGluR1 receptor, is localized to dendritic processes and is mainly plasma membrane-bound. Activation of mGluR1a has been shown to result in increased constitutive activity of ionotropic receptors, although its role in neurodegenerative and other neurological diseases is controversial, with some animal studies demonstrating potential neuroprotective properties in excito- and neurotoxic environments. In this study, the expression of mGluR1a within normal and AD human hippocampal tissue was quantified using immunohistochemistry. We found a significantly reduced expression of mGluR1α within the stratum pyramidale and radiatum of the CA1subregion, subiculum, and entorhinal cortex. This downregulation could result in potential dysregulation of the glutamatergic system with consequences on AD progression by promoting excitotoxicity, but alternatively may also be a neuroprotective mechanism to prevent mGluR1α associated excitotoxic effects. In summary, more research is required to understand the role and possible consequences of mGluR1α downregulation in the human AD hippocampus, subiculum and entorhinal cortex and its potential as a therapeutic target.

Loss of long-term depression in the insular cortex after tail amputation in adult mice

The insular cortex (IC) is an important forebrain structure involved in pain perception and taste memory formation. Using a 64-channel multi-electrode array system, we recently identified and characterized two major forms of synaptic plasticity in the adult mouse IC: long-term potentiation (LTP) and long-term depression (LTD). In this study, we investigate injury-related metaplastic changes in insular synaptic plasticity after distal tail amputation. We found that tail amputation in adult mice produced a selective loss of low frequency stimulation-induced LTD in the IC, without affecting (RS)-3,5-dihydroxyphenylglycine (DHPG)-evoked LTD. The impaired insular LTD could be pharmacologically rescued by priming the IC slices with a lower dose of DHPG application, a form of metaplasticity which involves activation of protein kinase C but not protein kinase A or calcium/calmodulin-dependent protein kinase II. These findings provide important insights into the synaptic mechanisms of cortical changes after peripheral amputation and suggest that restoration of insular LTD may represent a novel therapeutic strategy against the synaptic dysfunctions underlying the pathophysiology of phantom pain.

Platelet-activating factor-induced synaptic facilitation is associated with increased calcium/calmodulin-dependent protein kinase II, protein kinase C and extracellular signal-regulated kinase activities in the rat hippocampal CA1 region

Platelet-activating factor (PAF) is an important inflammatory lipid mediator affecting neural plasticity. In the present study, we demonstrated how PAF affects synaptic efficacy through activation of protein kinases in the rat hippocampal CA1 region. In cultured hippocampal neurons, 10 to 1000 nM PAF stimulated autophosphorylation of calcium/calmodulin-dependent protein kinase II (CaMKII) and phosphorylation of synapsin I and myristoylated alanine-rich protein kinase C substrate (MARCKS). In hippocampal CA1 slices, field excitatory postsynaptic potentials (fEPSPs) induced by stimulation of the Schaffer collateral/commissural pathways were significantly increased 10-50 min after exposure to 100 to 1000 nM PAF. Immunoblotting analysis showed that 100 nM PAF treatment for 10 or 50 min significantly and persistently increased CaMKII autophosphorylation in the hippocampal CA1 region. Increased protein kinase Calpha (PKCalpha) autophosphorylation was also seen at the same time point after PAF exposure. By contrast, extracellular signal-regulated kinase (ERK) phosphorylation was slightly but significantly increased at 10 min after PAF exposure. Consistent with increased CaMKII autophosphorylation, AMPA-type glutamate receptor subunit 1 (GluR1) (Ser-831) phosphorylation as a CaMKII postsynaptic substrate significantly increased after 10 or 50 min of treatment, whereas synapsin I (Ser-603) phosphorylation as a presynaptic substrate increased at 10 min in the hippocampal CA1 region. Phosphorylation of MARCKS (Ser-152/156) and NMDA receptor subunit 1 (NR1) (Ser-896) as PKCalpha substrates also significantly increased after 10 min but had not further increased by 50 min in the CA1 region. Increased of fEPSPs induced by PAF treatment completely and/or partly inhibited by KN93 and/or U0126 treatment. These results suggest that PAF induces synaptic facilitation through activation of CaMKII, PKC and ERK in the hippocampal CA1 region.

Reactive oxygen species and respiratory plasticity following intermittent hypoxia

The neural network controlling breathing exhibits plasticity in response to environmental or physiological challenges. For example, while hypoxia initiates rapid and robust increases in respiratory motor output to defend against hypoxemia, it also triggers persistent changes, or plasticity, in chemosensory neurons and integrative pathways that transmit brainstem respiratory activity to respiratory motor neurons. Frequently studied models of hypoxia-induced respiratory plasticity include: (1) carotid chemosensory plasticity and metaplasticity induced by chronic intermittent hypoxia (CIH), and (2) acute intermittent hypoxia (AIH) induced phrenic long-term facilitation (pLTF) in naïve and CIH preconditioned rats. These forms of plasticity share some mechanistic elements, although they differ in anatomical location and the requirement for CIH preconditioning. Both forms of plasticity require serotonin receptor activation and formation of reactive oxygen species (ROS). While the cellular sources and targets of ROS are not well known, recent evidence suggests that ROS modify the balance of protein phosphatase and kinase activities, shifting the balance towards net phosphorylation and favoring cellular reactions that induce and/or maintain plasticity. Here, we review possible sources of ROS, and the impact of ROS on phosphorylation events relevant to respiratory plasticity.

Respiratory long-term facilitation following intermittent hypoxia requires reactive oxygen species formation

Acute intermittent hypoxia (AIH) elicits a form of respiratory plasticity known as long-term facilitation (LTF). LTF is a progressive and sustained increase in respiratory motor output as expressed in phrenic and hypoglossal (XII) nerve activity. Since reactive oxygen species (ROS) play important roles in several forms of neuroplasticity, and ROS production is increased by intermittent hypoxia, we tested the hypothesis that ROS are necessary for phrenic and XII LTF following AIH. Urethane-anesthetized, paralyzed, vagotomized and pump-ventilated Sprague-Dawley rats were exposed to AIH (11% O2, 3, 5 min episodes, 5 min intervals), and both phrenic and XII nerve activity were monitored for 60 min post-AIH. Although phrenic and XII LTF were observed in control rats, i.v. manganese (III) tetrakis (1-methyl-4-pyridyl) porphyrin pentachloride (MnTMPyP), a superoxide anion scavenger, attenuated both phrenic and XII LTF in a dose dependent manner. Localized application of MnTMPyP (5.5 mM; 10 microl) to the intrathecal space of the cervical spinal cord (C4) abolished phrenic, but not XII LTF. Thus, ROS are necessary for AIH-induced respiratory LTF, and the relevant ROS appear to be localized near respiratory motor nuclei since cervical MnTMPyP injections impaired phrenic (and not XII) LTF. Phrenic LTF is a novel form of ROS-dependent neuroplasticity since its ROS-dependence resides in the spinal cord.

Tamoxifen effect on L-DOPA induced response complications in parkinsonian rats and primates

The contribution of striatal protein kinase C (PKC) isoform changes in levodopa (L-DOPA) induced motor response complications in parkinsonian rats was investigated and the ability of tamoxifen, an antiestrogen with a partial PKC antagonist property, to prevent these response alterations in 6-hydroxydopamine (6-OHDA) lesioned rats as well as in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treated cynomologous monkeys was studied. Following treatment of adult male rats with L-DOPA twice daily for 3 weeks, protein levels of left (lesioned) and right (intact) striatal PKC isoforms were measured. Western blot analysis showed increased protein expression of both the novel PKC epsilon isoform and the atypical PKC lambda isoform ipsilateral to the lesion (174+/-17% for epsilon, 140+/-9% for lambda, of intact striatum in 6-OHDA lesioned plus chronic L-DOPA treated animals) in acute L-DOPA treated rats. No enhancement was observed in PKC immunoreactivity for other isoforms. Tamoxifen (5.0 mg/kg p.o.) significantly attenuated the L-DOPA induced augmentation of protein expression of PKC epsilon and PKC lambda, but had no effect on immunoreactivity for other PKC isoforms. In chronic L-DOPA treated parkinsonian rats, tamoxifen prevented (5.0 mg/kg p.o.) as well as ameliorated (5.0 mg/kg p.o.) the characteristic shortening in duration of motor response to L-DOPA challenge. In MPTP lesioned primates, similar to the ameliorative effect seen in rats, tamoxifen (1 and 3 mg/kg p.o) reduced the appearance of L-DOPA induced dyskinesia by 61% and 55% respectively (p<0.05). These results suggest that changes in specific striatal PKC isoforms contribute to the pathogenesis of L-DOPA induced motor complications and further that drugs able to selectively inhibit these signaling kinases might provide adjunctive benefit in the treatment of Parkinson's disease.

Hemispheric differences in protein kinase C betaII levels in the rat amygdala: baseline asymmetry and lateralized changes associated with cue and context in a classical fear conditioning paradigm

The amygdala is critically important for fear learning, and specific kinases have been implicated as contributors to the mechanisms that underlie learning. We examined levels of protein kinase C betaII (PKC betaII) in the left and right lateral and basolateral nuclei (LA/BLA) of the amygdala from animals that were classically fear conditioned with tones as cues and footshocks. Groups consisted of animals that received neither tones nor shocks, paired tones and shocks, or unpaired tones and shocks. At 1 h after conditioning, some animals from each group were used for biochemical measurements of PKC betaII levels and other animals were given probe trials to assess freezing behavior to cue and context. The levels of PKC betaII were greater in the left hemisphere in animals receiving neither tones nor shocks and animals receiving paired tones and shocks. PKC betaII levels were greater in the right hemisphere of animals receiving randomly presented tones and shocks. Freezing times to cue were long (>80% of probe trial time) in both the paired tone/shock and randomly unpaired tone/shock groups. Freezing times to context were long in the unpaired tone/shock group, but not the paired tone/shock group. Correlational analyses showed that freezing times to context, but not cue, precisely predicted the right/left relation of PKC betaII levels in the LA/BLA: the greater the time spent freezing to context, the greater the increase in right hemisphere PKC betaII levels. We conclude that fear conditioning causes hemisphere and input specific increases in PKC betaII in the rat LA/BLA.

The metabotropic glutamate receptor 5 is necessary for late-phase long-term potentiation in the hippocampal CA1 region

Selective antagonists of the metabotropic receptors 1 (mGluR1), +/-2-methyl-4-carboxyphenylglycine (LY367385), and mGluR5, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), were used to investigate the role of group I metabotropic receptors in late-phase long-term potentiation (L-LTP) at Schaffer collateral/commissural fiber-CA1 synapses in rat hippocampal slices. L-LTP was induced with three trains of tetanization of 1 s duration at 100 Hz separated by 10-min intervals. Neither LY367385 nor MPEP affected basal synaptic responses at the doses used (200 and 10 microM, respectively) and only the mGluR5 inhibitor MPEP blocked L-LTP. However, in agreement with previous mouse mutant studies, we found that both LY367385 and MPEP inhibited the induction of an LTP obtained with a single train of tetanization of 1 s duration at 100 Hz. MPEP's ability to disrupt L-LTP was not due to an effect on NMDA responses since it did not affect pharmacologically isolated N-methyl-D-aspartate (NMDA) excitatory postsynaptic potentials (EPSPs). However, MPEP prevented the increased phosphorylation in dendrites of p70 S6 kinase (p70(S6K)) at Thr3889, a major regulator of translation required for the induction of protein synthesis-dependent forms of LTP.

Depression of GABAergic input to identified hippocampal neurons by group III metabotropic glutamate receptors in the rat

The release of GABA in synapses is modulated by presynaptic metabotropic glutamate receptors (mGluRs). We tested whether GABA release to identified hippocampal neurons is influenced by group III mGluR activation using the agonist L-(+)-2-amino-4-phosphonobutyric acid (L-AP4) on inhibitory postsynaptic currents (IPSCs) evoked in CA1 interneurons and pyramidal cells. In interneurons, characterized with biocytin and immunolabelling for somatostatin, evoked IPSCs were depressed by 50 micro m L-AP4 (activating mGluR4 and 8) to 68 +/- 6% of control, but they were rarely depressed in pyramidal cells (96 +/- 4% of control). At 300-500 micro m concentration (activating mGluR4, 7 and 8), L-AP4 depressed IPSCs in both interneurons (to 70 +/- 6%) and pyramidal cells (to 67 +/- 4%). The change in trial-to-trial variability and in paired-pulse depression indicated a presynaptic action. In interneurons, the degree of IPSC depression was variable (to 9-87%), and a third of IPSCs were not affected by L-AP4. The L-AP4-evoked IPSC depression was blocked by LY341495. The depression of IPSCs was similar in O-LM cells and other interneurons. The lack of cell-type selectivity and the similar efficacy of different concentrations of L-AP4 suggest that several group III mGluRs are involved in the depression of IPSCs. Electron microscopic immunocytochemistry confirmed that mGluR4, mGluR7a and mGluR8a occur in the presynaptic active zone of GABAergic terminals on interneurons, but not on those innervating pyramidal cells. The high variability of L-AP4-evoked IPSC suppression is in line with the selective expression of presynaptic mGluRs by several distinct types of GABAergic neuron innervating each interneuron type.

Group I metabotropic glutamate receptors regulate the frequency-response function of hippocampal CA1 synapses for the induction of LTP and LTD

Synaptically released glutamate binds to ionotropic or metabotropic glutamate receptors. Metabotropic glutamate receptors (mGluRs) are G-protein-coupled receptors and can be divided into three subclasses (Group I-III) depending on their pharmacology and coupling to signal transduction cascades. Group I mGluRs are coupled to phospholipase C and are implicated in several important physiological processes, including activity-dependent synaptic plasticity, but their exact role in synaptic plasticity remains unclear. Synaptic plasticity can manifest itself as an increase or decrease of synaptic efficacy, referred to as long-term potentiation (LTP) and long-term depression (LTD). The likelihood, degree and direction of the change in synaptic efficacy depends on the history of the synapse and is referred to as 'metaplasticity'. We provide direct experimental evidence for an involvement of group I mGluRs in metaplasticity in CA1 hippocampal synapses. Bath application of a low concentration of the specific group I agonist 3,5-dihydroxyphenylglycine (DHPG), which does not affect basal synaptic transmission, resulted in a leftward shift of the frequency-response function for the induction of LTD and LTP in naïve synapses. DHPG resulted in the induction of LTP at frequencies which induced LTD in control slices. These alterations in the induction of LTD and LTP resemble the metaplastic changes observed in previously depressed synapses. In addition, in the presence of DHPG additional potentiation could be induced after LTP had apparently been saturated. These findings provide strong evidence for an involvement of group I mGluRs in the regulation of metaplasticity in the CA1 field of the hippocampus.

Chromatin remodeling and neuronal response: multiple signaling pathways induce specific histone H3 modifications and early gene expression in hippocampal neurons

Plasticity in gene expression is achieved by a complex array of molecular mechanisms by which intracellular signaling pathways directly govern transcriptional regulation. In addition to the remarkable variety of transcription factors and co-regulators, and their combinatorial interaction at specific promoter loci, the role of chromatin remodeling has been increasingly appreciated. The N-terminal tails of histones, the building blocks of nucleosomes, contain conserved residues that can be post-translationally modified by phosphorylation, acetylation, methylation and other modifications. Depending on their nature, these modifications have been linked to activation or silencing of gene expression. We wanted to investigate whether neuronal stimulation by various signaling pathways elicits chromatin modifications that would allow transcriptional activation of immediate early response genes. We have analysed the capacity of three drugs - SKF82958 (a dopaminergic receptor agonist), pilocarpine (a muscarinic acetylcholine receptor agonist) and kainic acid (a kainate glutamate receptor agonist) - to induce chromatin remodeling in hippocampal neurons. We show that all stimulations induce rapid, transient phosphorylation of histone H3 at serine 10. Importantly, the same agonists induce rapid activation of the mitogen-activated protein kinase pathway with similar kinetics to extracellular-regulated-kinase phosphorylation. In the same neurons where this dynamic signaling cascade is activated, there is induction of c-fos transcription. Histone H3 Ser10 phosphorylation is coupled to acetylation at the nearby Lys14 residue, an event that has been linked to local opening of chromatin structure. Our results underscore the importance of dynamic chromatin remodeling in the transcriptional response to various stimuli in neuronal cells.

Time-restricted role for dendritic activation of the mTOR-p70S6K pathway in the induction of late-phase long-term potentiation in the CA1

Mammalian target of rapamycin (mTOR) is a key regulator of translational capacity. The mTOR inhibitor rapamycin can prevent forms of protein synthesis-dependent synaptic plasticity such as long-term facilitation in Aplysia and late-phase long-term potentiation (L-LTP) in the hippocampal CA1 region of rodents. In the latter model, two issues remain to be addressed: defining the L-LTP phase sensitive to rapamycin and identifying the site of rapamycin-sensitive protein synthesis. Here, we show that L-LTP is sensitive to application of rapamycin only during the induction paradigm, whereas rapamycin application after the establishment of L-LTP was ineffective. Second, we observed that Thr-389-phosphorylated p70 S6 kinase (p70S6K), the main active phosphoform of the mTOR effector p70S6K, was induced in an N-methyl-D-aspartate and phosphatidylinositol 3-kinase-dependent manner throughout the dendrites but not in the cell bodies of CA1 neurons in hippocampal slices after L-LTP induction. A similar dendrite-wide activation of p70S6K was induced in primary hippocampal neurons by depolarization with KCL or glutamate. In primary hippocampal neurons, the sites of dendritic activation of p70S6K appeared as discrete compartments along dendritic shafts like the hotspots for fast dendritic translation. Conversely, only a subset of dendritic spines also displayed activated p70S6K. Taken together, the present data suggest that the N-methyl-d-aspartate-, phosphatidylinositol 3-kinase-dependent dendritic activation of the mTOR-p70S6K pathway is necessary for the induction phase of protein synthesis-dependent synaptic plasticity. Newly synthesized proteins in dendritic shafts could be targeted selectively to activity-tagged synapses. Thus, coordinated activation of dendrite-wide translation and synaptic-specific activation is likely to be necessary for long-term synaptic plasticity.

High frequency stimulation-induced dendritic calcium waves in rat hippocampal neurons

An activity-dependent intracellular Ca(2+) increase represents a key signal for the activation of mechanisms involved in synaptic long-term plasticity. Here we present data from confocal microscopic imaging in conjunction with electrophysiological studies, that local 100 Hz stimulations of the hippocampal Schaffer collateral fibers, usually used for the induction of synaptic long-term potentiation (LTP), elicits an intradendritic Ca(2+) rise, that propagates as a short-distance wave within dendrites of CA1-neurons, which could be involved in triggering the dendritic (local signal cascade) machinery of LTP generation. Pharmacological investigations elucidated the coincidental involvement of N-methyl-D-aspartate- and of group I metabotropic glutamate receptors in the generation of the Ca(2+) wave.

Molecular psychology: roles for the ERK MAP kinase cascade in memory

In this review we describe an emerging understanding of the roles of the Extracellular-signal regulated kinase/mitogen-activated protein kinase (ERK/MAPK) cascade in learning and memory. We begin by describing several behavioral memory paradigms and review data implicating ERK as an essential component of the signal transduction mechanisms subserving these processes. We then describe evidence implicating ERK as a critical player in synaptic and neuronal plasticity-a cellular role likely to underlie ERK's behavioral role in the animal. We then proceed to parsing the complexities of biochemical regulation of ERK in neurons and to a description of a few likely cellular targets of ERK. This is in order to begin discussing the possible molecular basis of ERK-mediated behavioral change. We close our review with speculations concerning how the complexities and idiosyncrasies of ERK regulation may allow for sophisticated information processing at the biochemical level in neurons-attributes that may make the ERK cascade well-suited for triggering complex and long-lasting behavioral change. Our goal in this review is not so much to portray ERK as unique regarding its role as a signal transducter in memory, but rather to use ERK as one specific example of recent studies beginning to address the molecules and signal transduction pathways subserving cognition.

G(alpha)q-deficient mice lack metabotropic glutamate receptor-dependent long-term depression but show normal long-term potentiation in the hippocampal CA1 region

Long-term potentiation (LTP) and depression (LTD) are potential cellular mechanisms involved in learning and memory. Group I metabotropic glutamate receptors (mGluR), which are linked to heterotrimeric G-proteins of the G(q) family (G(q) and G(11)), have been reported to facilitate both hippocampal LTP and LTD. To evaluate their functional role in synaptic plasticity, we studied LTD and LTP in the CA1 region of the hippocampus from wild-type, Galpha(q)(-/-), and Galpha(11)(-/-) mice. Basic parameters of the synaptic transmission were not altered in Galpha(q)(-/-) and Galpha(11)(-/-) mice. Moreover, these mice showed normal LTP in response to a strong tetanus and to a weak tetanus. However, LTD induced either by a group I mGluRs agonist or by paired-pulse low-frequency stimulation (PP-LFS) was absent in Galpha(q)(-/-) mice. Moreover, PP-LFS caused potentiation of the synaptic transmission in these mice that was not affected by the NMDAR antagonist AP-5. These results show that G(q) plays a crucial role in the mGluR-dependent LTD, whereas hippocampal LTP is not affected by the lack of a single member of the G(q) family.

Tetanic stimulation and metabotropic glutamate receptor agonists modify synaptic responses and protein kinase activity in rat auditory cortex

We investigated whether tetanic-stimulation and activation of metabotropic glutamate receptors (mGluRs) can modify field-synaptic-potentials and protein kinase activity in rat auditory cortex, specifically protein kinase A (PKA) and protein kinase C (PKC). Tetanic stimulation (50 Hz, 1 s) increases PKA and PKC activity only if the CNQX-sensitive field-EPSP (f-EPSP) is also potentiated. If the f-EPSP is unchanged, then PKA and PKC activity remains unchanged. Tetanic stimulation decreases a bicuculline-sensitive field-IPSP (f-IPSP), and this occurs whether the f-EPSP is potentiated or not. Potentiation of the f-EPSP is blocked by antagonists of mGluRs (MCPG) and PKC (calphostin-C, tamoxifen), suggesting that the potentiation of the f-EPSP is dependent on mGluRs and PKC. PKC antagonists block the rise in PKC and PKA activity, which suggests that these may be coupled. In contrast, ACPD (agonist at mGluRs) decreases both the f-EPSP and the f-IPSP, but increases PKC and PKA activity. Quisqualate (group I mGluR agonist), decreases the f-IPSP, and increases PKA activity, suggesting that the increase in PKA activity is a result of activation of group I mGluRs. Additionally, the increase in PKC and PKA activity appears to be independent of the decrease of the f-EPSP and f-IPSP, because PKC antagonists block the increase in PKC and PKA activity levels but do not block ACPD's effect on the f-EPSP or f-IPSP. These data suggest that group I mGluRs are involved in potentiating the f-EPSP by a PKC and possibly PKA dependent mechanism which is separate from the mechanism that decreases the f-EPSP and f-IPSP.

Choline availability to the developing rat fetus alters adult hippocampal long-term potentiation

Supplementation with choline during pregnancy in rats causes a long-lasting improvement of visuospatial memory of the offspring. To determine if the behavioral effects of choline are related to physiological changes in hippocampus, the effect of perinatal choline supplementation or deficiency on long-term potentiation (LTP) was examined in hippocampal slices of 6-8 and 12-14 month old rats born to dams consuming a control, choline-supplemented, or a choline-free diet during pregnancy. Stimulating and recording electrodes were placed in stratum radiatum of area CA1 to record extracellular population excitatory postsynaptic potentials (pEPSPs). To induce LTP, a theta-like stimulus train was generated. The amplitude of the stimulus pulses was set at either 10% or 50% of the stimulus intensity which had induced the maximal pEPSP slope on the input/output curve. We found that at both ages, a significantly smaller percentage of slices from perinatally choline-deficient rats displayed LTP after 10% stimulus intensity (compared with control and choline-supplemented rats), and a significantly larger percentage of slices from choline-supplemented rats displayed LTP at 50% stimulus intensity (compared with control and choline-deficient rats). Results reveal that alterations in the availability of dietary choline during discrete periods of development lead to changes in hippocampal electrophysiology that last well into adulthood. These changes in LTP threshold may underlie the observed enhancement of visuospatial memory seen after prenatal choline supplementation and point to the importance of choline intake during pregnancy for development of brain and memory function.

Transient translocation of protein kinase Cgamma in hippocampal long-term potentiation depends on activation of metabotropic glutamate receptors

Protein kinase C has been implicated in long-term regulation of cellular functions including induction and maintenance of hippocampal long-term potentiation. In the present study the time-course of long-term potentiation-induced translocation of Ca(2+)-dependent protein kinase C isoenzymes (PKCalpha/beta and PKCgamma) was investigated. Quantitative immunoblot analysis was used to measure translocation of these isoenzymes between cytosolic, membrane-associated and membrane-inserted fraction at 5, 15 and 60 min after induction of long-term potentiation in the dentate gyrus in vivo. To investigate the involvement of metabotropic glutamate receptors in protein kinase C regulation during long-term potentiation induction, additional animals were treated before tetanization with (R,S)-alpha-methyl-4-carboxyphenylglycine, an antagonist of metabotropic glutamate receptors. Brief tetanic stimulation of the perforant path resulted in a 100-150% increase in the population spike amplitude in response to test stimuli 5, 15 or 60 min after stimulation in both untreated and (R,S)-alpha-methyl-4-carboxyphenylglycine-treated animals. Only those rats showing clear potentiation were selected for further biochemical analysis of the potentiated dentate gyrus. Five minutes after high-frequency stimulation the subcellular distribution of all studied protein kinase C isoenzymes was unchanged compared with controls. PKC-gamma translocated into the cytosol 15 min after tetanization and this redistribution was blocked by (R,S)-alpha-methyl-4-carboxyphenylgly-cine pretreatment. By contrast, PKC alpha/beta levels increased in the cytosolic fraction only 60 min after tetanization, but in a (R,S)-alpha-methyl-4-carboxyphenylglycine-independent manner. In an additional set of experiments it was shown that (R,S)-alpha-methyl-4-carboxyphenylglycine alone applied intraventricularly had no effect on the subcellular distribution of the studied isoenzymes. The data suggest that PKCalpha/beta and PKCgamma are activated during different post-tetanic phases and metabotropic glutamate receptor activation might be essential for tetanus-induced translocation of postsynaptic PKCgamma only.

Group I metabotropic glutamate receptors: implications for brain diseases

Glutamate is the major excitatory neurotransmitter in the brain and plays a unique role in a variety of central nervous system (CNS) functions. The discovery of the metabotropic receptors (mGluRs), a family of G-protein coupled receptors than can be activated by glutamate, has led to an impressive number of studies in recent years aimed at understanding their biochemical, physiological and pharmacological characteristics. The eight mGluRs now known are divided into three groups according to their sequence homology, signal transduction mechanisms, and agonist selectivity. Group I mGluRs include mGluR1 and mGluR5, which are linked to the activation of phospholipase C; Groups II and III include all others and are negatively coupled to adenylyl cyclases. The availability in recent years of agents selective for Group I mGluRs has made possible the study of the physiological roles of these receptors in the CNS. In addition to mediating glutamatergic neurotransmission, Group I mGluRs can modulate other neurotransmitter receptors, including GABA and the ionotropic glutamate receptors. Group I mGluRs are involved in many CNS functions and may participate in a variety of disorders such as pain, epilepsy, ischemia, and chronic neurodegenerative diseases. This class of receptor may provide important pharmacological therapeutic targets and elucidating its functions will be relevant to develop new treatments for neurological and psychiatric disorders in which glutamatergic neurotransmission is abnormally regulated. In this review anatomical, physiological and pharmacological results are presented with a special emphasis on the role of Group I mGluRs in functional and pathological processes.

The mitogen-activated protein kinase cascade couples PKA and PKC to cAMP response element binding protein phosphorylation in area CA1 of hippocampus

Activation of the mitogen-activated protein kinase (MAPK) cascade recently was discovered to play an important role in synaptic plasticity in area CA1 of rat hippocampus. However, the upstream mechanisms regulating MAPK activity and the downstream effectors of MAPK in the hippocampus are uncharacterized. In the present studies we observed that hippocampal MAPK activation is regulated by both the PKA and PKC systems; moreover, we found that a wide variety of neuromodulatory neurotransmitter receptors (metabotropic glutamate receptors, muscarinic acetylcholine receptors, dopamine receptors, and beta-adrenergic receptors) couple to MAPK activation via these two cascades. In additional studies we observed that PKC is a powerful regulator of CREB phosphorylation in area CA1. MAPK plays a critical role in transcriptional regulation by PKC, because MAPK activation is a necessary component for increased CREB phosphorylation in response to the activation of this kinase. Surprisingly, we also observed that MAPK activation is necessary for PKA coupling to CREB phosphorylation in area CA1. Overall, these studies indicate an unexpected richness of diversity in the regulation of MAPK in the hippocampus and suggest the possibility of a broad role for the MAPK cascade in regulating gene expression in long-term forms of hippocampal synaptic plasticity.

Roles of metabotropic glutamate receptors in LTP and LTD in the hippocampus

Metabotropic L-glutamate receptors are involved in various forms of synaptic plasticity in the hippocampus. The use of a new antagonist (LY341495) that blocks all known metabotropic L-glutamate receptors in the brain, together with subtype-selective antagonists, has identified multiple roles both for cloned and novel metabotropic L-glutamate receptors in hippocampal long-term potentiation and long-term depression.

Evidence for NMDA and mGlu receptor-dependent long-term potentiation of mossy fiber-granule cell transmission in rat cerebellum

Long-term potentiation (LTP) is a form of synaptic plasticity that can be revealed at numerous hippocampal and neocortical synapses following high-frequency activation of N-methyl--aspartate (NMDA) receptors. However, it was not known whether LTP could be induced at the mossy fiber-granule cell relay of cerebellum. This is a particularly interesting issue because theories of the cerebellum do not consider or even explicitly negate the existence of mossy fiber-granule cell synaptic plasticity. Here we show that high-frequency mossy fiber stimulation paired with granule cell membrane depolarization (-40 mV) leads to LTP of granule cell excitatory postsynaptic currents (EPSCs). Pairing with a relatively hyperpolarized potential (-60 mV) or in the presence of NMDA receptor blockers [5-amino--phosphonovaleric acid (APV) and 7-chloro-kynurenic acid (7-Cl-Kyn)] prevented LTP, suggesting that the induction process involves a voltage-dependent NMDA receptor activation. Metabotropic glutamate receptors were also involved because blocking them with (+)-alpha-methyl-4-carboxyphenyl-glycine (MCPG) prevented potentiation. At the cytoplasmic level, EPSC potentiation required a Ca2+ increase and protein kinase C (PKC) activation. Potentiation was expressed through an increase in both the NMDA and non-NMDA receptor-mediated current and by an NMDA current slowdown, suggesting that complex mechanisms control synaptic efficacy during LTP. LTP at the mossy fiber-granule cell synapse provides the cerebellar network with a large reservoir for memory storage, which may be needed to optimize pattern recognition and, ultimately, cerebellar learning and computation.

Priming of long-term potentiation induced by activation of metabotropic glutamate receptors coupled to phospholipase C

Activation of metabotropic glutamate receptors (mGluRs) with 1-aminocyclopentane-1S,3R-dicarboxylic acid 20 min prior to tetanus facilitates, or "primes," subsequent induction of long-term potentiation (LTP; Cohen and Abraham, J Neurophysiol 1996;76:953-962). In the present study, we investigated the receptor specificity and associated second messenger pathways involved in the mGluR priming effect by using field potentials recorded from area CA1 of rat hippocampal slices. In controls, mild theta-burst or high-frequency (100 Hz) stimulation induced 16% and 21% LTP, respectively. A 10-min application of the group I mGluR agonist 3,5-dihydroxyphenylglycine (DHPG) caused a transient depression of synaptic responses but a significant enhancement of subsequent LTP for both tetanus protocols (45% and 41% LTP, respectively). Maximal LTP, induced by stronger tetanization protocols, was not enhanced by DHPG, nor was mild LTP facilitated by post-tetanic application of DHPG. Priming with agonists selective for group II or III mGluRs had no effect on LTP. The mGluR antagonists L-2-amino-3-phosphonopropionic acid and 1-aminoindan-1,5-dicarboxylic acid inhibited the LTP facilitatory effect of DHPG but not the transient response depression, whereas alpha-methyl-4-carboxyphenylglycine produced the opposite effects. Priming with N-methyl-D-aspartate or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid did not facilitate LTP induction. Prior activation of muscarinic acetylcholine receptors produced at best a weak priming effect. Inhibition of phospholipase C by U-73122 completely abolished the priming of LTP by DHPG. We conclude that mGluR priming of LTP results from biochemical cascades triggered by activation of phospholipase C coupled to group I mGluRs.

A role for superoxide in protein kinase C activation and induction of long-term potentiation

The induction of several forms of long-term potentiation (LTP) of synaptic transmission in the CA1 region of the mammalian hippocampus is dependent on N-methyl-D-aspartate receptor activation and the subsequent activation of protein kinase C (PKC), but the mechanisms that underlie the regulation of PKC in this context are largely unknown. It is known that reactive oxygen species, including superoxide, are produced by N-methyl-D-aspartate receptor activation in neurons, and recent studies have suggested that some reactive oxygen species can modulate PKC in vitro. Thus, we have investigated the role of superoxide in both the induction of LTP and the activation of PKC during LTP. We found that incubation of hippocampal slices with superoxide scavengers inhibited the induction of LTP. The effects of superoxide on LTP induction may involve PKC, as we observed that superoxide was required for appropriate modulation of PKC activation during the induction of LTP. In this respect, superoxide appears to work in conjunction with nitric oxide, which was required for a portion of the LTP-associated changes in PKC activity as well. Our observations indicate that superoxide and nitric oxide together regulate PKC in a physiologic context and that this type of regulation occurs during the induction of LTP in the hippocampus.

Protein kinase C in the parabrachial nucleus of rats during conditioned taste aversion induced by amphetamine

D-Amphetamine (AM) is a potent inducer of conditioned taste aversion (CTA) the mechanism of which differs from that induced by lithium. The aim of the present communication is to see whether AM-induced CTA will produce shift in the protein kinase (PKC) activity in the parabrachial nucleus (PBN). Activity of PKC was measured in PBN of rats during AM-induced CTA. In the control experiments a single intraperitoneal (i.p.) injection of AM (3 mg/kg) alone (not paired with saccharin drinking) resulted in rise of particulate bound PKC by 77% and a tendency to decrease its activity in cytosol 60 min but not 24 and 48 h after AM administration. The results suggest translocation of the enzyme from cytosol to membrane. Cytosolic PKC increased by 17 and 50%, 24 and 48 h, respectively, after acquisition of CTA (15 min after the retrieval test), when the direct effect of AM on PKC had already disappeared. Particulate PKC did not change at either of the two time intervals. Thus the total PKC activity was increased. Since we have previously observed the same PKC shifts using LiCl or CuSO4 as CTA unconditioned stimuli, we assume that any CTA inducer will elicit the same alteration of PKC in PBN.

Receptor-mediated activation of protein kinase C in hippocampal long-term potentiation: facts, problems and implications

During the last decade hippocampal long-term potentiation has become one of the most frequently used models to study cellular mechanisms of learning and memory. Receptor-mediated activation of protein kinase C is thought to be involved in LTP stabilisation. In the present review, 1. the molecular structure and activation mechanisms of PKC isoenzymes, 2. the biochemical evidences for PKC activation after induction of LTP using different stimulation paradigms as well as 3. the involvement of metabotropic glutamate receptors in PKC activation after induction of LTP are critically discussed.

Implication of protein kinase C in mechanisms of potassium-induced long-term potentiation in rat hippocampal slices

The involvement of Ca2+/phospholipid-dependent (alpha, beta, gamma, PKCs) and Ca(2+)-independent PKC (epsilon and zeta isoforms) in mechanisms of long-term potentiation was investigated in CA1 hippocampal slices, using a brief high potassium pulse (50 mM, 40 s) to induce long-term potentiation (K+/LTP). The K+ pulse induced first, in 15 s a translocation of PKC activity to the membrane. This was rapidly followed, from 1 to 60 min after the pulse, by a selective activation of PKC in the cytosol. This activation, which could be blocked by the NMDA (N-methyl-D-aspartate) receptor antagonist 2-amino-5-phosphonovalerate (APV), was associated with a significant increase n immunoreactivity for gamma PKC in he cytosol, and also to a less degree for beta PKC. In contrast, application of the phorbol ester PMA (phorbol 12-mirystate 13 acetate) to other slices induced a rapid and persistent translocation to the membrane of alpha, beta, epsilon and zeta PKCs. A major role for the activation role for the activation of cytosolic gamma PKC in the maintenance of LTP is discussed.

Activation of metabotropic glutamate receptors increases endogenous protein kinase C substrate phosphorylation in adult hippocampal slices

We previously reported (Staak, S., Behnisch, T. and Angenstein, F., Hippocampal long-term potentiation: transient increase but no persistent translocation of protein kinase C (PKC) isoenzymes alpha and beta, Brain Res., 682 (1995) 55-62) that Ca(2+)-dependent PKC isoenzymes alpha/beta and gamma are not translocated between subcellular compartments after stimulation of glutamate receptor subtypes in hippocampal slices. Extending our previous work in this study in situ phosphorylation of endogenous PKC substrates and the translocation of novel PKC isoenzymes delta and epsilon was analysed to detect PKC activation. Two proteins of approximately 94 kDa and 18 kDa were first characterised to be specific PKC substrates. As control of the technique carbachol was shown to increase in situ phosphorylation of the two substrates without any measurable translocation of PKC protein. Activation of metabotropic glutamate receptors by 50 microM DHPG also increased the situ-phosphorylation by 43.9% (94 kDa) and 32.8% (18 kDa) compared to controls but did not induce a measurable subcellular redistribution of conventional and novel PKC isoenzymes. Stimulation by 50 microM trans-ACPD or 0.1 mM quisqualate enhanced the situ phosphorylation in the same range, whereas 0.1 mM NMDA was ineffective. To our knowledge this is the first report showing a direct link between metabotropic glutamate receptor activation and increased endogenous PKC substrate phosphorylation in adult hippocampal slices. This PKC activation was not detectable by a redistribution of enzyme protein between subcellular compartments. We, therefore, conclude, that the failure to detect PKC translocation in physiological experiments is not an indicator for unchanged enzyme activity.

Comparing the role of metabotropic glutamate receptors in long-term potentiation and in learning and memory

1. Neuronal plasticity has been suggested to be the physical substrate for changes underlying the expression of memory. One model which has attracted wide attention as a possible candidate of such neuronal plasticity is long-term potentiation (LTP), mainly investigated in the hippocampus of rodents. Moreover, various processes with different time constants may underlie LTP, and these phases show striking correspondence to different phases of memory. 2. Pharmacological evidence strongly implicates that the neurotransmitter glutamate plays a major role in LTP. Although the involvement of ionotropic glutamate receptors has been proven, the role of the newly discovered metabotropic glutamate receptors is still uncertain. 3. Metabotropic glutamate receptors (mGluRs) comprise a whole family with currently eight members grouped into three classes according to their amino acid sequence identity and pharmacological profile. They are G-protein coupled, either positively linked to phospholipase C (class I) or negatively linked to adenylate cyclase (class II and III), and among other effects are known to induce phosphorylation of ionotropic glutamate receptors as well as modulate the excitability of neurons. Finally, they are heterogeneously distributed throughout the brain. 4. In hippocampal slice preparations, mGluRs have been shown to be involved in the induction of LTP in CA1 and dentate gyrus by some investigators, but others have failed to reproduce such experiments, leaving the question: what are the appropriate conditions for mGluR-mediated LTP? 5. In vivo, metabotropic receptor antagonists have been shown to block, and agonists to facilitate, induction and maintenance of LTP, mainly at perforant path/dentate granule cell synapses. As demonstrated in behavioral investigations, mGluRs apparently play an important part in hippocampus-dependent learning paradigms. As in LTP, antagonists block memory formation; in contrast to LTP, agonists also prevent memory formation. In memory recall metabotropic receptors seem to play no role. 6. Based on current information the authors develop models for a role of mGluRs in both LTP and memory formation. Activation of metabotropic receptors plays a particular modulatory role when high frequency stimulation is weak. Strong tetanization may bypass mGluRs by stimulating other systems leading to, at least phenomenologically, similar LTP, Behaviorally, mGluRs possibly set the signal to noise ratio of the hippocampal circuit.

The synergism between metabotropic glutamate receptor activation and arachidonic acid on glutamate release is occluded by induction of long-term potentiation in the dentate gyrus

In synaptosomes prepared from dentate gyrus, activation of the metabotropic glutamate receptor by the specific agonist, trans-1-amino-cyclopentyl-1,3-dicarboxylate, increases release of glutamate in the presence of a low concentration of arachidonic acid. A similar interaction between trans-1-amino-cyclopentyl-1,3-dicarboxylate and arachidonic acid is observed on inositol phospholipid turnover and on protein kinase C activity. We report here that when long-term potentiation is induced in the dentate gyrus by high frequency tetanic stimulation to the perforant path, the synergism between arachidonic acid and trans-1-amino-cyclopentyl-1,3-dicarboxylate is occluded. The occlusion of the synergistic action between arachidonic acid and trans-1-amino-cyclopentyl-1,3-dicarboxylate on glutamate release extended to occlusion of the effect in inositol phospholipid turnover and protein kinase C activation in synaptosomes prepared from dentate gyrus in which long-term potentiation was induced in vivo. One interpretation of the results presented here is that tetanic stimulation is followed by stimulation of metabotropic glutamate receptors at a time when arachidonic acid concentration in the synaptic region is elevated, and that this interaction triggers the presynaptic changes required for expression of long-term potentiation.

Differential long-lasting potentiation of the NMDA and non-NMDA synaptic currents induced by metabotropic and NMDA receptor coactivation in cerebellar granule cells

Whole-cell patch-clamp recordings in rat cerebellar slices were used to investigate the effect of metabotropic glutamate receptor activation on mossy fibre-granule cell synaptic transmission. Transient application of 20 microM 1S, 3R-aminocyclopentane-1, 3-dicarboxylic acid simultaneously with low-frequency NMDA receptor activation induced long-lasting non-decremental potentiation of both NMDA and non-NMDA receptor-mediated synaptic transmission. Potentiation could be prevented by application of the metabotropic glutamate receptor antagonist (+)-O-methyl-4-carboxyphenyl-glycine at 500 microM. Characteristically, NMDA potentiation was two to three times as large as non-NMDA current potentiation, occurred only in a slow subcomponent, and was voltage-independent. This result demonstrates a pivotal role of NMDA receptors in the metabotropic potentiation of transmission, which may be important in regulating cerebellar information processing.

KEYWORDS

cerebellum, LTP, metabotropic receptors, NMDA receptors, patch-clamp, rat

Protein kinase C in the rat cerebral cortex during spreading depression

Activity and distribution of protein kinase C (PKC) in the rat cerebral cortex was correlated with the development of spreading depression. When the 'waves' of the slow potential shift, induced by topical application of concentrated KCl solutions, were allowed to spread over the cerebral cortex for 10 min, both cytosolic and particulate fractions of the enzyme were increased to 169% and 143%, respectively, of the control values obtained from the contralateral, relatively intact hemicortex. When the enzyme activities were correlated with development of a single slow potential shift, it appeared that in the cortical area fully depolarized (under the maximum of negativity), the respective values were 175% and 157%. One min after recovery of the single wave of spreading depression both cytosolic and particulate fractions continued to rise up to 218% and 239%, respectively. During 5 min of recovery both the cytosolic and particulate fractions fell to 71% and 57%, respectively, of control levels. Even at 10 min the cytosolic enzyme was still decreased to 80%. At 20 min no difference between control and experimental values was found (soluble, 111%; particulate, 96%). The results are discussed in the context of data obtained in a few studies dealing with depolarization-induced changes of PKC in vitro.

Glutamate receptors and development of the visual cortex: effect of metabotropic agonists on cAMP

Glutamate receptors are more active in several respects in young animals than in adults. Here we examine the effect of metabotropic glutamate agonists on rat cortical cAMP during and after the critical period for visual cortex plasticity. Quisqualate produced a substantial increase in cAMP, which was larger during the critical period than in the adult. The increase was not affected by CNQX or APV, showing that it was not due to the action of quisqualate on ionotropic glutamate receptors. Both Type I mGluRs (mGluRs 1 and/or 5) and Type II mGluRs (mGluRs 2 and/or 3) probably contributed to the cAMP increase because (i) ACPD and L-CCG-I, which are more active on Type II mGluRs, were more effective than DHPG, which is more active on Type I mGluRs; and (ii) there was a significant difference in the effect of ACPD on the increase in cAMP, comparing mGluR1 knockout mice with control mice. Agonists which produce large stimulation of cAMP production (ACPD, L-CCG-I), as well as L-AP4, also produced small attenuations of forskolin-stimulated cAMP, but only at high concentrations. Thus, we conclude that it is the stimulation and/or potentiation of cAMP production that is significant, rather than the attenuation of forskolin-stimulated cAMP. Since this stimulation and/or potentiation is higher during the critical period than in the adult, and the cAMP second messenger system has been implicated in hippocampal plasticity, it may also play a role in visual cortex plasticity.

Mechanisms of 1S,3R-ACPD-induced neuroprotection in rat hippocampal slices subjected to oxygen and glucose deprivation

The efficacy and mechanisms of 1-amino-cyclopentyl-1S,3R-dicarboxylate (1S,3R-ACPD)-induced neuroprotection were investigated in rat hippocampal slices subjected to 10 min of oxygen and glucose deprivation. Neuronal viability was assessed by measuring both the amplitude of evoked population spike in the CA1 pyramidale and by imaging CA1 neurons using a live/dead fluorescence assay with confocal microscopy. CA1 pyramidal neurons in oxygen-glucose deprived slices remained viable for up to 120 min following the insult but were dead by 240 min. Pretreatment with 1S,3R-ACPD significantly protected the oxygen-glucose deprived slices in a concentration-dependent fashion. Oxygen-glucose deprived slices pretreated for the same period with the protein kinase C (PKC) activation phorbol 12-myristate 13-acetate (PMA; 1 microM) were significantly protected whereas oxygen-glucose deprived slices treated with the adenylyl cyclase activator, forskolin (30 microM) were not. Oxygen-glucose deprivation induced a rapid and persistent decrease (approximately 50%) in PKC activity and a > 6 fold increase in cyclic adenosine monophosphate (cAMP) levels in whole hippocampal slices. While 1S,3R-ACPD did not stimulate PKC activity and had no effect on basal cAMP in whole slices, it significantly enhanced the rate of return of cAMP to basal levels following reperfusion. Consistent with this observation, the 1S,3R-ACPD-induced neuroprotection was inhibited by forskolin (30 microM). These results suggest that in vitro neuroprotection of CA1 neurons by 1S,3R-ACPD involves metabotropic glutamate receptors negatively linked to cAMP and possibly those which increase PKC activity.

Concurrent blockade of hippocampal metabotropic glutamate and N-methyl-D-aspartate receptors disrupts working memory in the rat

In order to clarify the roles of hippocampal metabotropic glutamate and N-methyl-D-aspartate receptors in working and reference memory performance of rats, the effects of intrahippocampal administration of selective antagonists for both receptors on these behaviours were examined with a three-panel runway task. In the working memory task, the potent and competitive N-methyl-D-aspartate receptor antagonist, (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), significantly increased the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points), when injected bilaterally at 10 and 32 ng/side into the dorsal hippocampus. Intrahippocampal injection of CPP at a dose of 3.2 ng/side had no effect on the number of working memory errors. The metabotropic glutamate receptor antagonist, (+)-alpha-methyl-4-carboxyphenylglycine (+)-(MCPG), injected into the hippocampus at doses up to 3.2 micrograms/side, did not significantly affect the number of working memory errors. Combined administration of (+)-MCPG (3.2 micrograms/side) and CPP (3.2 ng/side) into the hippocampus, neither of which had an individual effect on errors, significantly increased the number of working memory errors. However, intrahippocampal administration of the relatively inactive isomer, (-)-MCPG, at 3.2 micrograms/side did not affect working memory errors, whether given independently or concurrently with the behaviourally ineffective dose of CPP (3.2 ng/side). In the reference memory task, intrahippocampal injection of CPP at doses up to 32 ng/side had no effect on the number of errors. Intrahippocampal (+)-MCPG at doses up to 3.2 micrograms/side did not affect the number of reference memory errors, whether administered alone or together with 3.2 ng/side of CPP. These results indicate that blockade of hippocampal metabotropic glutamate receptors aggravates impairment of working memory resulting from deficiency of N-methyl-D-aspartate receptor-mediated glutamatergic neurotransmission, suggesting that mechanisms regulated by co-activation of hippocampal metabotropic glutamate and N-methyl-D-aspartate receptors are involved in working memory performance of rats.

Class I metabotropic glutamate receptor agonists do not facilitate the induction of long-term potentiation in the dentate gyrus of the rat in vitro

The possibility that activation of class I (phospholipase C-coupled) metabotropic glutamate receptors (mGluRs) can facilitate the induction of long-term potentiation (LTP) was investigated in the dentate gyrus of rat hippocampal slices. In the presence of picrotoxin, a weak tetanus led to a short-term potentiation (STP) lasting 10-15 min. Application of the class I mGluR agonists trans-azetidine-2,4-dicarboxylic acid (tADA, 100 microM) or 3,5-dihydroxyphenylglycine (DHPG, 100 microM) for 15 or 30 min before the weak tetanus did not affect baseline synaptic transmission or the magnitude of the subsequent potentiation. DHPG (70 microM) did, however, reduce accommodation of neuronal firing in response to depolarizing current injection. These results suggest that at the medial perforant path-granule cell synapse, class I mGluR activation by exogenous agonist application does not facilitate the induction of LTP.

Identification and cellular localization of protein kinase C isoforms in cultures of rat type-1 astrocytes

We have examined which isoforms of protein kinase C were present in rat brain astrocytes and found that: (1) the total of calcium-independent isoforms was greater than the total of calcium-dependent isoforms; (2) there were differences in the intracellular distribution of different isoforms; and (3) the abundance of total protein kinase C was greater in astrocytes from cortex than astrocytes from diencephalon.

Possible involvement of protein kinases in physical dependence on opioids: studies using protein kinase inhibitors, H-7 and H-8

Effects of a cAMP-dependent protein kinase and protein kinase C inhibitor, H-7 (1-(5-isoquinolinesulfonyl)-2-methylpiperazine) and a cAMP- and cGMP-dependent protein kinase inhibitor, H-8 (N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide), on the behavioral signs of naloxone (an opioid receptor antagonist)-precipitated withdrawal syndrome and effects of H-7 on the change of protein kinase C activity in the pons/medulla region induced by morphine (a mu-opioid receptor agonist) or butorphanol (a mu/delta/kappa mixed opioid receptor agonist) were investigated in this study. Rats were intracerebroventricularly (i.c.v.) infused with morphine (26 nmol/microliters/h) or butorphanol (26 nmol/microliters/h) through osmotic minipumps for 3 days. In some groups, either saline or drug-treated groups were concomitantly infused with H-7 (1 and 10 nmol/microliters/h) or H-8 (10 nmol/microliters/h). The expression of physical dependence produced by morphine or butorphanol, as evaluated by naloxone (5 mg/kg i.p.)-precipitated withdrawal signs, was reduced by concomitant infusion of H-7 or H-8. In the same condition, morphine and butorphanol chronic treatment enhanced (28.1% and 26.3% enhancement over the saline-treated group, respectively) cytosolic protein kinase C activity in the pons/medulla, but not in the membrane fraction. Furthermore, concomitant infusion of H-7 inhibited the enhancement of protein kinase C activity. These results indicate that various types of protein kinases may play an important role in the development and/or expression of physical dependence on opioids. Among them, the enhancement of cytosolic protein kinase C activity in the pons/medulla region seems to be one of the major underlying mechanisms in opioid physical dependence.

Neuronal Ca2+ stores: activation and function

The intracellular concentration of free Ca2+ ([Ca2+]i) displays complex fluctuations in response to a variety of stimuli, and acts as a pluripotent signal for many neuronal functions. It is well established that various 'metabotropic' neurotransmitter receptors can mediate the mobilization of Ca2+ stores via actions of inositol-polyphosphate second messengers, and more recent evidence suggests that 'ionotropic' receptor-mediated Ca2+ signals in neurones might also involve release of Ca2+ from intracellular stores. These two mechanisms of release of Ca2+ enable considerable temporal and spatial complexity of increases in the [Ca2+]i via multiple interactions at the level of intracellular-receptor activation. The complexity of Ca2+ signalling that is elicited via these interconnecting pathways might underlie mechanisms that are central to information transfer and integration within neuronal compartments.

Ca2+ channel blocker, diltiazem, prevents physical dependence and the enhancement of protein kinase C activity by opioid infusion in rats

The influence of an L-type Ca2+ channel blocker, diltiazem [(2S-cis)-3-(acetyloxy)-5-[2-(dimethylamino)-ethyl]-2,3-dihydro-2- (4- methoxyphenyl)-1,5-benzothiazepin-4(5H)-one], on the behavioral signs of naloxone (opioid receptor antagonist)-precipitated withdrawal syndrome and the enhancement of protein kinase C activity in the pons/medulla regions of rats rendered dependent on morphine (mu-opioid receptor agonist) or butorphanol (mu/delta/kappa mixed opioid receptor agonist) was investigated. The expression of physical dependence produced by continuous intracerebroventricular (i.c.v.) infusion of morphine (26 nmol/microliters per h) or butorphanol (26 nmol/microliters per h) for 3 days, as evaluated by naloxone (5 mg/kg, i.p.)-precipitated withdrawal signs, was dose dependently attenuated by concomitant infusion of diltiazem (10 and 100 nmol/microliters per h). Furthermore, diltiazem (100 nmol/microliters per h) completely inhibited the enhancement of cytosolic protein kinase C activity in the pons/medulla regions in rats rendered dependent by continuous infusion with morphine or butorphanol. These results suggest that the augmentation of intracellular Ca2+ concentration mediated through L-type Ca2+ channels during continuous opioid infusion leads to the enhancement of cytosolic protein kinase C activity in the pons/medulla region which is intimately involved in the development and/or expression of physical dependence on opioids.

Hippocampal long-term potentiation: transient increase but no persistent translocation of protein kinase C isoenzymes alpha and beta

Using a monoclonal antibody the translocation of the Ca(2+)-dependent protein kinase C (PKC) isoenzymes alpha/beta was studied in hippocampal slices after stimulation of glutamate receptors or induction of long-term potentiation. In submerged slices preincubated for 60 min in a medium usually used in electrophysiological studies, cytosolic PKC was not detectable and the amount of membrane-associated enzyme was increased. The treatment of these slices with 10(-6) M phorbol-12,13-dibutyrate induced a time-dependent translocation of alpha/beta PKC from the membrane-associated into the membrane-inserted state. The glutamatergic agonists N-methyl-D-aspartate, quisqualate and trans-ACPD did not cause a membrane insertion of alpha/beta PKC as observed for the phorbol ester when applied alone or in combination. Furthermore, 2 min and 15 min after induction of LTP in the Schaffer collateral-CA1 pathway the distribution of alpha/beta PKC between the two membrane fractions remained unchanged. An increase in the total amount of PKC immunoreactivity was measured immediately after tetanization (142.6% of controls). The data suggest that a membrane insertion of alpha/beta PKC is not a prerequisite for the LTP-induced increased phosphorylation of PKC substrates and that the enzyme might be recruited from a previously inactive pool.

Activation of Metabotropic Glutamate Receptors in Conjunction with Postsynaptic Depolarization Triggers a Long-Term Depression of the N-Methyl-D-Aspartate Receptor-Mediated Synaptic Potential in the Rat Hippocampus

The mechanism responsible for long-term depression (LTD) of pharmacologically isolated N-methyl-D-aspartate (NMDA) receptor-mediated excitatory postsynaptic potential (EPSP(NMDA)) was studied. Intracellular recordings were made from CA1 cells of rat hippocampal slices in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione (10 &mgr;M) and picrotoxin (50 &mgr;M), which block non-NMDA and GABA(A) receptors, respectively. Intracellular injections of depolarizing pulses (500 ms, 0.3-0.7 nA) at 1 Hz for 5 min in the absence of synaptic stimulation caused a persistent increase in the amplitude of EPSP(NMDA). However, coupling postsynaptic depolarization with synaptic activity induced LTD. The EPSP(NMDA) LTD could be blocked by L-2-amino-3-phosphonopropionic acid (50 &mgr;M) or (RS)-alpha-methyl-4-carboxyphenylglycine (200 &mgr;M), specific antagonists for metabotropic glutamate receptors (mGluR). Furthermore, application of trans-1-aminocyclopentane-1,3-dicarboxylic acid (t-ACPD, 50 &mgr;M), a specific mGluR agonist, in conjunction with postsynaptic depolarizing elicited LTD. In contrast, the mGluR agonists quisqualate or t-ACPD when given alone produced a sustained enhancement of EPSP(NMDA). Finally, coupled depolarization did not evoke LTD in slices pretreated with the protein kinase C (PKC) inhibitor calphostin c (60 nM). The present results demonstrate that activation of mGluR is necessary for the induction of LTD of EPSP(NMDA) and suggest that NMDA receptors are subject to bidirectional regulation by mGluR. Furthermore, the induction of LTD is likely to involve the stimulation of PKC. Copyright 1995 S. Karger AG, Basel

Multiple forms of long-term potentiation and multiple regulatory sites of N-methyl-D-aspartate receptors: role of the redox site

Long-term potentiation (LTP) is a form of synaptic plasticity thought to be involved in learning and memory. Although extensively studied, mainly in the CA1 region of the hippocampus, the mechanisms underlying the induction and expression of LTP are poorly elucidated. This is probably due to the fact that LTP is not a unique process and indeed recent studies have shown that several forms of LTP could be generated depending on the experimental conditions. Furthermore, LTP is generally associated with a long-lasting increase of the synaptic efficacy of AMPA receptors but an increasing number of data also suggested that NMDA receptors could be potentiated as well. NMDA receptor responses are modulated by a large number of extracellular and intracellular events, providing additional possibilities for the generation of LTP. The role of these different modulatory sites of the NMDA receptor and their relation with LTP are reviewed with a particular attention to the redox site which seems to be a selective target to distinguish between AMPA and NMDA-LTP.

Glutamate receptor update

The past year has seen significant advances in matching the actions of recombinant glutamate receptors with the actions of native receptors, and in mapping their distribution and regulation. The discovery of a novel RNA editing mechanism for AMPA receptors and a revised view of the transmembrane topology of the NMDA receptor subunit, NR1, are particularly noteworthy. Seven metabotropic glutamate receptor subtypes have been identified with several interesting expression patterns and transduction mechanisms; results from work on these subtypes has led to a provocative model of the ligand-binding site. Functional studies of metabotropic receptors have been enhanced by the development of the first subtype-specific antagonist.

(RS)-alpha-methyl-4-carboxyphenylglycine (MCPG) does not block theta burst-induced long-term potentiation in area CA1 of rat hippocampal slices

We have used the selective metabotropic glutamate receptor antagonist (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG) to investigate in the CA1 hippocampal subregion in vitro whether coactivation of N-methyl-D-aspartate (NMDA) and metabotropic glutamate receptors is necessary for the induction of long-term potentiation (LTP) when LTP is induced by theta burst stimulation (TBS). When MCPG (500 microM) was bath applied 14-30 min prior to a triple high-frequency tetanization (100 Hz, 1 s) and washed out immediately afterwards the potentiation of the extracellularly recorded field potentials decayed gradually to baseline (P < 0.05) over 2-3 h. However, when MCPG was applied in the same manner before a triple TBS (10 bursts at 5 Hz, 100 Hz within the bursts) the resulting potentiation remained stable for at least 4 h. MCPG had no effect on baseline synaptic transmission or post-tetanic potentiation. These results demonstrate a clear difference in the mechanisms underlying these two different forms of LTP.