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

GSK-3 beta inhibits presynaptic vesicle exocytosis by phosphorylating P/Q-type calcium channel and interrupting SNARE complex formation

Glycogen synthase kinase-3 (GSK-3), a Ser/Thr protein kinase abundantly expressed in neurons, plays diverse functions in physiological and neurodegenerative conditions. Our recent study shows that upregulation of GSK-3 suppresses long-term potentiation and presynaptic release of glutamate; however, the underlying mechanism is elusive. Here, we show that activation of GSK-3beta retards the synaptic vesicle exocytosis in response to membrane depolarization. Using calcium imaging, whole-cell patch-clamp, as well as specific Ca(2+) channel inhibitors, we demonstrate that GSK-3beta phosphorylates the intracellular loop-connecting domains II and III (L(II-III)) of P/Q-type Ca(2+) channels, which leads to a decrease of intracellular Ca(2+) rise through the P/Q-type voltage-dependent calcium channel. To further illustrate the mechanisms of GSK-3beta's action, we show that activation of GSK-3beta interferes with the formation of the soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor (SNARE) complex through: (1) weakening the association of synaptobrevin with SNAP25 and syntaxin; (2) reducing the interactions among the phosphorylated L(II-III) and synaptotagmin, SNAP25, and syntaxin; and (3) inhibiting dissociation of synaptobrevin from synaptophysin I. These results indicate that GSK-3beta negatively regulates synaptic vesicle fusion events via interfering with Ca(2+)-dependent SNARE complex formation.

Cholinergic and glutamatergic alterations beginning at the early stages of Alzheimer disease: participation of the phospholipase A2 enzyme

RATIONALE

Alzheimer disease (AD), a progressive neurodegenerative disorder, is the leading cause of dementia in the elderly. A combination of cholinergic and glutamatergic dysfunction appears to underlie the symptomatology of AD, and thus, treatment strategies should address impairments in both systems. Evidence suggests the involvement of phospholipase A(2) (PLA(2)) enzyme in memory impairment and neurodegeneration in AD via actions on both cholinergic and glutamatergic systems.

OBJECTIVES

To review cholinergic and glutamatergic alterations underlying cognitive impairment and neuropathology in AD and attempt to link PLA(2) with such alterations.

METHODS

Medline databases were searched (no date restrictions) for published articles with links among the terms Alzheimer disease (mild, moderate, severe), mild cognitive impairment, choline acetyltransferase, acetylcholinesterase, NGF, NGF receptor, muscarinic receptor, nicotinic receptor, NMDA, AMPA, metabotropic glutamate receptor, atrophy, glucose metabolism, phospholipid metabolism, sphingolipid, membrane fluidity, phospholipase A(2), arachidonic acid, attention, memory, long-term potentiation, beta-amyloid, tau, inflammation, and reactive species. Reference lists of the identified articles were checked to identify additional studies of interest.

RESULTS

Overall, results suggest the hypothesis that persistent inhibition of cPLA(2) and iPLA(2) isoforms at early stages of AD may play a central role in memory deficits and beta-amyloid production through down-regulation of cholinergic and glutamate receptors. As the disease progresses, beta-amyloid induced up-regulation of cPLA(2) and sPLA(2) isoforms may play critical roles in inflammation and oxidative stress, thus participating in the neurodegenerative process.

CONCLUSION

Activation and inhibition of specific PLA(2) isoforms at different stages of AD could be of therapeutic importance and delay cognitive dysfunction and neurodegeneration.

Activation of glycogen synthase kinase-3 inhibits long-term potentiation with synapse-associated impairments

Activation of glycogen synthase kinase-3 (GSK-3) can cause memory deficits as seen in Alzheimer's disease, the most common age-associated dementia, but the mechanism is not understood. Here, we found that activation of GSK-3 by wortmannin or transient overexpression of wild-type GSK-3beta could suppress the induction of long-term potentiation (LTP) in rat hippocampus, whereas simultaneous inhibition of GSK-3 by lithium or SB216763 or transient expression of a dominant-negative GSK-3beta mutant (dnGSK-3beta) preserved the LTP. After high-frequency stimulation (HFS), the presynaptic release of glutamate and the expression/clustering of synapsin I, a synaptic vesicle protein playing an important role in neurotransmitter release, decreased markedly after upregulation of GSK-3. In vitro studies further demonstrated that GSK-3 inhibited the expression of SynI independent of HFS. In postsynaptic level, the expression of PSD93 and NR2A/B proteins decreased significantly when GSK-3 was activated. The LTP-associated synapse impairments including less presynaptic active zone, thinner postsynaptic density, and broader synaptic cleft were also prominent in the hippocampal slices after HFS with activation of GSK-3. These synaptic impairments were attenuated when GSK-3 was simultaneously inhibited by LiCl or SB216763 or transient expression of dnGSK-3. We conclude that upregulation of GSK-3 impairs the synaptic plasticity both functionally and structurally, which may underlie the GSK-3-involved memory deficits.

Essential fatty acids and mental illness

The treatment of psychiatric illness requires novel pharmacological strategies. There is a growing body of evidence examining the role of neuronal phospholipid abnormalities in the pathogenesis of psychiatric illness, particularly in schizophrenia. However, work in other conditions like mood disorders are also showing interesting outcomes with EPA supplementation. Diseases that are considered to have a genetic basis may be significantly influenced by environmental factors including dietary supplementation. The suggestion that EFA supplementation may prevent the onset of symptoms of a psychiatric disease or aberrant behaviour needs longitudinal randomized controlled research. In recent years the focus has shifted from omega-6 to omega-3. It is true that western diets have far more omega-6 than omega-3. In the 1980s, there were positive outcomes in research studies using GLA in schizophrenia (Vaddadi et al., 1989). Future research needs to incorporate studies using pure GLA. Research should not be restricted to parent fatty acid (omega-3) supplementation alone but be expanded to include bioactive down-the-chain metabolites. The recent identification of novel omega-3 derived mediators such as resolvins and neuroprotectins, which are a highly bioactive (1-10 nMol range), may well have some role to play in psychiatric disorders; however this remains highly speculative at this stage.

Activity-driven sharpening of the retinotectal projection: the search for retrograde synaptic signaling pathways

Patterned visual activity, acting via NMDA receptors, refines developing retinotectal maps by shaping individual retinal arbors. Because NMDA receptors are postsynaptic but the retinal arbors are presynaptic, there must be retrograde signals generated downstream of Ca(++) entry through NMDA receptors that direct the presynaptic retinal terminals to stabilize and grow or to withdraw. This review defines criteria for retrograde synaptic messengers, and then applies them to the leading candidates: nitric oxide (NO), brain-derived neurotrophic factor (BDNF), and arachidonic acid (AA). NO is not likely to be a general mechanism, as it operates only in selected projections of warm blooded vertebrates to speed up synaptic refinement, but is not essential. BDNF is a neurotrophin with strong growth promoting properties and complex interactions with activity both in its release and receptor signaling, but may modulate rather than mediate the retrograde signaling. AA promotes growth and stabilization of synaptic terminals by tapping into a pre-existing axonal growth-promoting pathway that is utilized by L1, NCAM, N-cadherin, and FGF and acts via PKC, GAP43, and F-actin stabilization, and it shares some overlap with BDNF pathways. The actions of both are consistent with recent demonstrations that activity-driven stabilization includes directed growth of new synaptic contacts. Certain nondiffusible factors (synapse-specific CAMs, ephrins, neurexin/neuroligin, and matrix molecules) may also play a role in activity-driven synapse stabilization. Interactions between these pathways are discussed.

Presynaptic protein kinase C controls maturation and branch dynamics of developing retinotectal arbors: possible role in activity-driven sharpening

Visual activity refines developing retinotectal maps and shapes individual retinal arbors via an NMDA receptor-dependent mechanism. As retinal axons grow into tectum, they slow markedly and emit many transient side branches behind the tip, assuming a "bottlebrush" morphology. Some branches are stabilized and branch further, giving rise to a compact arbor. The dynamic rate of branch addition and deletion is increased twofold when MK801 is used to block NMDA receptors, as if this prevents release of a stabilizing signal such as arachidonic acid (AA) from the postsynaptic neuron. In optic tract, AA mediates NCAM and L1 stimulation of axon growth by activating presynaptic protein kinase C (PKC) to phosphorylate GAP-43 and stabilize F-actin, and, if present in tectum, this growth control pathway could be modulated by postsynaptic activation. To test for the effects on arbor morphology of blocking PKC or AA release, we examined DiO-labeled retinal axons of larval zebrafish with time-lapse videomicroscopy. Bath application of the selective PKC inhibitor bisindolylmaleimide from 2 or 3 days onward doubled the rate at which side branches were added and deleted, as seen with MK801, and also prevented maturation of the arbor so that it retained a "bottlebrush" morphology. In order to selectively block the PKC being transported to retinal terminals, we injected the irreversible inhibitor calphostin C into the eye from which the ganglion cells were labeled, and this produced both effects seen with bath application. In contrast, there were no effects of control injections, which included Ringers into the same eye and the same dose into the opposite eye (actually much closer to the tectum of interest), to rule out the possibility that the inhibitor leaked from the eye to act on tectal cells. For comparison, we examined arbors treated with the NMDA blocker MK801 at half-hour time-lapse intervals, and detected the twofold rise in rates of branch addition and deletion previously reported in Xenopus larvae, but not the structural effect seen with the PKC inhibitors. In addition, we could produce both effects seen with PKC inhibitors by using RHC80267 to block AA release from DAG lipase, indicating that AA is the main drive for PKC activation. Thus, the results show a distinct role of AA and presynaptic PKC in both maturation of arbor structure and in the dynamic control of branching. The effects on branch dynamics were present regardless of the level of maturity of arbor structure. The fact that they mimicked those of MK801 suggests that presynaptic PKC may be involved in the NMDA receptor-driven stabilization of developing retinal arbors.

Long-term potentiation and memory

One of the most significant challenges in neuroscience is to identify the cellular and molecular processes that underlie learning and memory formation. The past decade has seen remarkable progress in understanding changes that accompany certain forms of acquisition and recall, particularly those forms which require activation of afferent pathways in the hippocampus. This progress can be attributed to a number of factors including well-characterized animal models, well-defined probes for analysis of cell signaling events and changes in gene transcription, and technology which has allowed gene knockout and overexpression in cells and animals. Of the several animal models used in identifying the changes which accompany plasticity in synaptic connections, long-term potentiation (LTP) has received most attention, and although it is not yet clear whether the changes that underlie maintenance of LTP also underlie memory consolidation, significant advances have been made in understanding cell signaling events that contribute to this form of synaptic plasticity. In this review, emphasis is focused on analysis of changes that occur after learning, especially spatial learning, and LTP and the value of assessing these changes in parallel is discussed. The effect of different stressors on spatial learning/memory and LTP is emphasized, and the review concludes with a brief analysis of the contribution of studies, in which transgenic animals were used, to the literature on memory/learning and LTP.

Lipopolysaccharide-induced increase in signalling in hippocampus is abrogated by IL-10--a role for IL-1 beta?

Parenterally administered lipopolysaccharide (LPS) increases the concentration of the pro-inflammatory cytokine interleukin-1beta (IL-1beta) in the rat hippocampus and evidence suggests that this effect plays a significant role in inhibiting long-term potentiation (LTP). The anti-inflammatory cytokine IL-10, antagonizes certain effects of IL-1beta, so if the effects of LPS are mediated through an increase in IL-1beta, it might be predicted that IL-10 would also abrogate the effect of LPS. Here, we report that IL-10 reversed the inhibitory effect of LPS on LTP and the data couple this with an inhibitory effect on the LPS-induced increase in IL-1beta. LPS treatment increased hippocampal expression of IL-1 receptor Type I protein. Consistent with the LPS-induced increases in IL-1beta concentration and receptor expression, were downstream changes which included enhanced phosphorylation of IRAK and the stress-activated kinases, JNK and p38; these LPS-induced changes were reversed by IL-10, which concurs with the idea that these events are triggered by increased activation of IL-1RI by IL-1beta. We provide evidence which indicates that LPS treatment leads to evidence of cell death and this was reversed in hippocampus prepared from LPS-treated rats which received IL-10. The evidence is therefore consistent with the idea that IL-10 acts to protect neuronal tissue from the detrimental effects induced by LPS.

Bioenergetics and transmitter release in the isolated nerve terminal

The isolated nerve terminal (or synaptosome) is the simplest preparation that allows mitochondrial bioenergetics to be studied in a physiological milieu, as well as facilitating investigation of the protein chemistry and regulation of synaptic vesicle exocytosis and recovery and providing a target for the study of the mechanism of action of numerous neurotoxins. This brief review discusses studies from our laboratory that may have provided some insight into these aspects of nerve terminal function.

Activation of the c-Jun N-terminal kinase signaling cascade mediates the effect of amyloid-beta on long term potentiation and cell death in hippocampus: a role for interleukin-1beta?

Amyloid-beta (Abeta) is a major constituent of the neuritic plaque found in the brain of Alzheimer's disease patients, and a great deal of evidence suggests that the neuronal loss that is associated with the disease is a consequence of the actions of Abeta. In the past few years, it has become apparent that activation of c-Jun N-terminal kinase (JNK) mediates some of the effects of Abeta on cultured cells; in particular, the evidence suggests that Abeta-triggered JNK activation leads to cell death. In this study, we investigated the effect of intracerebroventricular injection of Abeta(1-40) on signaling events in the hippocampus and on long term potentiation in Schaffer collateral CA1 pyramidal cell synapses in vivo. We report that Abeta(1-40) induced activation of JNK in CA1 and that this was coupled with expression of the proapoptotic protein, Bax, cytosolic cytochrome c, poly-(ADP-ribose) polymerase cleavage, and Fas ligand expression in the hippocampus. These data indicate that Abeta(1-40) inhibited expression of long term potentiation, and this effect was abrogated by administration of the JNK inhibitor peptide, D-JNKI1. In parallel with these findings, we observed that Abeta-induced changes in caspase-3 activation and TdT-mediated dUTP nick-end labeling staining in neuronal cultured cells were inhibited by D-JNKI1. We present evidence suggesting that interleukin (IL)-1beta plays a significant role in mediating the effects of Abeta(1-40) because Abeta(1-40) increased hippocampal IL-1beta and because several effects of Abeta(1-40) were inhibited by the caspase-1 inhibitor Ac-YVAD-CMK. On the basis of our findings, we propose that Abeta-induced changes in hippocampal plasticity are likely to be dependent upon IL-1beta-triggered activation of JNK.

Activation of p38 plays a pivotal role in the inhibitory effect of lipopolysaccharide and interleukin-1 beta on long term potentiation in rat dentate gyrus

Lipopolysaccharide (LPS), a component of the cell wall of Gram-negative bacteria, has been shown to induce profound changes both peripherally and centrally. It has recently been reported that intraperitoneal injection of LPS inhibited long term potentiation (LTP) in perforant path-granule cell synapses and that this effect was coupled with an increase in the concentration of the proinflammatory cytokine, interleukin-1 beta (IL-1 beta). The LPS-induced effects were abrogated by inhibition of caspase-1, suggesting that IL-1 beta may mediate the effects of LPS. Here we report that the inhibition of LTP induced by LPS and IL-1 beta was coupled with stimulation of the stress-activated protein kinase p38 in hippocampus and entorhinal cortex and that this effect was abrogated by the p38 inhibitor SB203580, while the effect of LPS was markedly attenuated in C57BL/6 IL-1RI-/- mice. The data also indicate that activation of the transcription factor, nuclear factor kappa B (NF kappa B), may play a role, since the inhibitory effect of LPS and IL-1 beta on LTP was attenuated by the NF kappa B inhibitor, SN50; consistently, LPS and IL-1 beta led to activation of NF kappa B in entorhinal cortex. We suggest that one consequence of these LPS and IL-1 beta-induced changes is a compromise in glutamate release in dentate gyrus, which was coupled with the inhibition of LTP. The evidence is consistent with the idea that the LPS-induced impairment in LTP is mediated by IL-1 beta and is a consequence of activation of p38.

Interleukin-1 receptor antagonist exerts agonist activity in the hippocampus independent of the interleukin-1 type I receptor

Interleukin-1 receptor antagonist (IL-1ra) selectively and competitively inhibits the actions of IL-1 at its receptors and has not been reported to have agonist activity. This study demonstrates that stimulation of synaptosomes with IL-1ra in vitro, mimicked the effects of IL-1 beta by decreasing glutamate release and increasing JNK phosphorylation. These effects of IL-1ra, but not IL-1 beta, were maintained in IL-1 type I receptor (IL-1RI) defective mice. IL-1 beta blocked these IL-1ra-induced effects suggesting that it may also act independently of IL-1RI in some circumstances. Furthermore, IL-1ra mimicked the inhibitory effect of IL-1 beta on long-term potentiation (LTP) in the hippocampus. These data, taken together with our findings that IL-1ra binds to hippocampal synaptosomes in the absence of IL-1RI, provide evidence that IL-1ra exerts agonist activity in the hippocampus independent of IL-1RI.

Chronic brain inflammation results in cell loss in the entorhinal cortex and impaired LTP in perforant path-granule cell synapses

Alzheimer's disease (AD) is characterized by chronic neuroinflammation, significant temporal lobe cell loss, and dementia. We investigated the influence of chronic neuroinflammation produced by chronic infusion of lipopolysaccharide (LPS) into the fourth ventricle for 4 weeks upon the induction and maintenance of long-term potentiation (LTP) in the dentate gyrus of the hippocampus, a well-characterized model of cellular synaptic plasticity. We also examined for pyramidal cell loss within the entorhinal cortex an area of the brain that contains the cell bodies of the perforant path. The results demonstrate that chronic neuroinflammation results in the loss of pyramidal cells within layers II and III of the entorhinal cortex and a significant attenuation of LTP within the dentate gyrus. Similar changes may underlie the temporal lobe pathology and dementia associated with AD.

Long-term potentiation in aged rats is restored when the age-related decrease in polyunsaturated fatty acid concentration is reversed

Several age-related changes have been identified in rat hippocampus; among these are deficits in glutamate release and long-term potentiation in dentate gyrus. These deficits correlate with a decrease in the concentration of arachidonic acid in hippocampus. In this study, the effects of dietary supplementation for 8 weeks with omega -6 or omega -3 fatty acids were assessed in groups of aged and young rats. The data presented indicate that dietary supplementation in aged rats restored the concentrations of arachidonic acid and docosahexanoic acid in hippocampal preparations to those observed in tissue prepared from young rats. In parallel, aged rats which received the experimental diets sustained long-term potentiation in a manner indistinguishable from young rats. The evidence presented supports the view that an age-related increase in reactive oxygen species production is linked with the decrease in polyunsaturated fatty acids and that a diet enriched in eicosapentanoic acid has antioxidant properties which may play a key role in reversal of the observed age-related deficits.

The anti-inflammatory cytokine, interleukin (IL)-10, blocks the inhibitory effect of IL-1 beta on long term potentiation. A role for JNK

Several effects of the proinflammatory cytokine, interleukin-1 beta (IL-1 beta), have been described in the central nervous system, and one area of the brain where marked changes have been reported is the hippocampus. Among these changes are an IL-1 beta-induced inhibition of long term potentiation (LTP) in perforant path-granule cell synapses and an attenuation of glutamate release in synaptosomes prepared from the hippocampus. Evidence suggests that, at least in circulating cells, the anti-inflammatory cytokine, IL-10, antagonizes certain effects of IL-1. We investigated the effect of IL-10 on IL-1 beta-induced inhibition of LTP and glutamate release. The evidence presented indicates that IL-1 beta stimulates the stress-activated protein kinase, c-Jun-activated protein kinase (JNK), and IL-1 receptor-associated kinase, which may explain its inhibitory effect on release and LTP, and that IL-10 reversed the IL-1 beta-induced stimulation of JNK activity and inhibition of release and LTP. We observed that IL-10 abrogated the stimulatory effect of IL-1 beta on superoxide dismutase activity and reactive oxygen species production, whereas the H(2)O(2)-induced inhibition of LTP was also blocked by IL-10. We present evidence that suggests that the action of IL-10 may be mediated by its ability to induce shedding of the IL-1 type I receptor.

Evidence that interleukin-1beta and reactive oxygen species production play a pivotal role in stress-induced impairment of LTP in the rat dentate gyrus

Long-term potentiation (LTP) in both area CA1 and the dentate gyrus is attenuated by stress and the evidence is consistent with the view that this is a consequence of increased activation of glucocorticoid receptors, in the hippocampus, following the stress-induced increase in circulating corticosterone. It has been shown that expression of the pro-inflammatory cytokine, interleukin-1beta (IL-1beta), is increased in hippocampus in response to stress; this finding together with the observation that IL-1beta exerts an inhibitory effect on LTP, suggests that IL-1beta may play a key role in mediating this inhibitory effect of stress on LTP. In this study, we explore this possibility and report that stress is also associated with increased reactive oxygen species production. The evidence presented supports the view that this is secondary to the stress-induced increase in IL-1beta concentration, as IL-1beta increased activity of superoxide dismutase and increased reactive oxygen species accumulation in hippocampus in vitro. We report that the inhibitory effect of stress on LTP is mimicked by H2O2, which increases reactive oxygen species accumulation, and by IL-1beta, the effect of which is overcome by the antioxidant, phenylarsine oxide. The hypothesis that the stress-induced increase in reactive oxygen species production may underlie the suppression of LTP is further supported by the finding that the effect of stress is abrogated by dietary manipulation with antioxidant vitamins E and C.

Protein synthesis in entorhinal cortex and long-term potentiation in dentate gyrus

Despite the concentration of effort in recent years, the mechanisms underlying the expression of long-term potentiation (LTP) in the hippocampus remain elusive, but amidst the uncertainty and sometimes controversy, one consistent finding is emerging; this is that late-phase LTP requires synthesis of proteins. This hypothesis was first proposed by a number of groups who reported that the more persistent components of LTP were blocked by protein synthesis inhibitors, and was supported by a significant literature which indicated that morphological changes accompanied LTP. Recent evidence indicated that the increase in protein synthesis may be cAMP-dependent and that subsequent activation of the transcription factor, CREB, represented one step in the cascade of events leading to protein synthesis. Whether protein synthesis occurs in presynaptic or postsynaptic neurons, or both, is still a subject of debate. Here we present evidence which suggests that LTP in perforant path-granule cell synapses is accompanied by protein synthesis, specifically synthesis of synaptic vesicle proteins, in the entorhinal cortex. We also show that protein synthesis is decreased in the entorhinal cortex of aged rats and a strain of rat which is genetically hypertensive, both of which exhibited impaired LTP. We propose that that the observed increase in protein synthesis in the entorhinal cortex, which accompanied LTP in the dentate gyrus, contributes to the reported changes in morphology in the presynaptic terminal.

Interleukin-1beta-dependent changes in the hippocampus following parenteral immunization with a whole cell pertussis vaccine

Neurological side effects are a major cause of concern following immunization with a number of vaccines, especially the whole cell pertussis vaccine (Pw). In this study we report that IL-1beta concentrations were significantly increased in the hippocampus following subcutaneous (s.c.) injection of Pw, and that this was accompanied by increased activity of the stress-activated kinase, c-Jun-N-terminal kinase (JNK) and a decrease in glutamate release. These effects were mimicked by s.c injection of active pertussis toxin (PT) or lipopolysaccharide (LPS). Incubation of hippocampal synaptosomes in the presence of Pw, PT or LPS also resulted in increased JNK activation and decreased glutamate release, effects which were mimicked by IL-1beta and blocked by the IL-1 receptor antagonist (IL-ra). Our observations are consistent with the hypothesis that IL-1beta induced by active bacterial toxins present in vaccine preparations, mediate the neurochemical and perhaps the neurological effects of Pw.

The inhibitory effect of interleukin-1beta on long-term potentiation is coupled with increased activity of stress-activated protein kinases

Long-term potentiation (LTP) in perforant path-granule cell synapses is decreased in aged rats, stressed rats, and rats injected intracerebroventricularly with the proinflammatory cytokine interleukin-1beta (IL-1beta). One factor that is common to these experimental conditions is an increase in the concentration of IL-1beta in the dentate gyrus, suggesting a causal relationship between the compromise in LTP and increased IL-1beta concentration. In this study, we have investigated the downstream consequences of an increase in IL-1beta concentration and report that the reduced LTP in rats injected intracerebroventricularly with IL-1beta was accompanied by a decrease in KCl-stimulated glutamate release in synaptosomes prepared from dentate gyrus, although unstimulated glutamate release was increased. These changes were paralleled by increased activity of the stress-activated kinases, c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase. Intracerebroventricular injection of IL-1beta increased reactive oxygen species production in hippocampal tissue, whereas IL-1beta and H(2)O(2) increased activities of both JNK and p38 in vitro. Dietary manipulation with antioxidant vitamins E and C blocked the increase in reactive oxygen species production, the stimulation of JNK and p38 activity, the attenuation of glutamate release, and the IL-1beta-induced inhibitory of LTP. We propose that IL-1beta stimulates activity of stress-activated kinases, which in turn may inhibit glutamate release and result in compromised LTP and that these actions are a consequence of increased production of reactive oxygen species.

Regulation of neurotransmitter release by metabotropic glutamate receptors

The G protein-coupled metabotropic glutamate (mGlu) receptors are differentially localized at various synapses throughout the brain. Depending on the receptor subtype, they appear to be localized at presynaptic and/or postsynaptic sites, including glial as well as neuronal elements. The heterogeneous distribution of these receptors on glutamate and nonglutamate neurons/cells thus allows modulation of synaptic transmission by a number of different mechanisms. Electrophysiological studies have demonstrated that the activation of mGlu receptors can modulate the activity of Ca(2+) or K(+) channels, or interfere with release processes downstream of Ca(2+) entry, and consequently regulate neuronal synaptic activity. Such changes evoked by mGlu receptors can ultimately regulate transmitter release at both glutamatergic and nonglutamatergic synapses. Increasing neurochemical evidence has emerged, obtained from in vitro and in vivo studies, showing modulation of the release of a variety of transmitters by mGlu receptors. This review addresses the neurochemical evidence for mGlu receptor-mediated regulation of neurotransmitters, such as excitatory and inhibitory amino acids, monoamines, and neuropeptides.

Lipopolysaccharide inhibits long term potentiation in the rat dentate gyrus by activating caspase-1

Lipopolysaccharide, a component of the cell wall of Gram-negative bacteria, may be responsible for at least some of the pathophysiological sequelae of bacterial infections, probably by inducing an increase in interleukin-1beta (IL-1beta) concentration. We report that intraperitoneal injection of lipopolysaccharide increased hippocampal caspase-1 activity and IL-1beta concentration; these changes were associated with increased activity of the stress-activated kinase c-Jun NH(2)-terminal kinase, decreased glutamate release, and impaired long term potentiation. The degenerative changes in hippocampus and entorhinal cortical neurones were consistent with apoptosis because translocation of cytochrome c and poly(ADP-ribose) polymerase cleavage were increased. Inhibition of caspase-1 blocked these changes, suggesting that IL-1beta mediated the lipopolysaccharide-induced changes.

Adenosine A2A receptor facilitation of synaptic transmission in the CA1 area of the rat hippocampus requires protein kinase C but not protein kinase A activation

Adenosine is a neuromodulator in the hippocampus acting mainly via inhibitory A(1) receptors but also via facilitatory A(2A) receptors. We now investigated the transducing system operated by hippocampal A(2A) receptors. The selective A(2A) receptor agonist, CGS 21680 (10 nM), facilitated synaptic transmission by 14%, an effect not modified by the phosphodiesterase IV inhibitor, rolipram (30 microM), or by the adenylate cyclase activator, forskolin (3 microM), or by the protein kinase A inhibitor, HA-1004 (10 microM), but nearly abolished by the protein kinase C inhibitors, chelerythrine (6 microM) or bisindolylmaleimide I (1 microM). Inhibition of protein kinase C also prevented the A(2A) receptor-induced attenuation of A(1) receptor-mediated inhibition of hippocampal synaptic transmission. These results indicate that adenosine A(2A) receptor facilitation of hippocampal synaptic transmission involves protein kinase C rather than protein kinase A activation.

Deficits in nerve growth factor release and tyrosine receptor kinase phosphorylation are associated with age-related impairment in long-term potentiation in the dentate gyrus

Previous findings have indicated that nerve growth factor may play a role in the expression of long-term potentiation in perforant path-granule cell synapses and that nerve growth factor treatment restores the ability of aged rats to sustain long-term potentiation. In this study, we have attempted to analyse the changes which occur in nerve growth factor release and tyrosine receptor kinase phosphorylation following tetanization in tissue prepared from dentate gyrus of young rats, as well as aged rats which did or did not sustain long-term potentiation. We report that KCl-stimulated nerve growth factor release was significantly increased in slices of the dentate gyrus or whole hippocampus, but not in synaptosomes prepared from the dentate gyrus. KCl-induced nerve growth factor release was also significantly enhanced in slices prepared from tetanized, compared with untetanized, tissue obtained from young rats and aged rats which sustained long-term potentiation; this response was absent in tissue prepared from aged rats which failed to sustain long-term potentiation, perhaps due to the enhanced basal nerve growth factor release observed in this tissue. Tetanization increased tyrosine receptor kinase phosphorylation in the dentate gyrus of young rats and aged rats which sustained long-term potentiation. In parallel with the changes in nerve growth factor release, tyrosine receptor kinase phosphorylation was markedly increased in untetanized tissue, which may contribute to the lack of effect in tetanized tissue prepared from aged rats which failed to sustain long-term potentiation. We observed that nerve growth factor concentration and tyrosine receptor kinase expression were decreased in aged, compared with young, rats. The data suggest that deficits in nerve growth factor release and subsequent signalling may contribute to age-related deficits in long-term potentiation.

Long-term potentiation in dentate gyrus of the rat is inhibited by the phosphoinositide 3-kinase inhibitor, wortmannin

The pivotal role of inositol phospholipids in cell signalling has been placed centre-stage again with the recognition that phosphoinositide (PI) 3-kinase is implicated in several cellular processes. Stimulation of PI-3 kinase requires activation of the 85 kD regulatory subunit which relies on tyrosine phosphorylation, one consequence of which is activation of the 110 kD catalytic subunit. In this study, we have investigated the role of PI 3-kinase in the expression of long-term potentiation (LTP) in perforant path-granule cell synapses of the rat. We report that intracerebroventricular injection of wortmannin inhibited expression of LTP, though it did not affect the early change in the synaptic response. Activation of PI 3-kinase was enhanced in tetanized tissue prepared from dentate gyrus, compared with untetanized tissue, but this effect was inhibited in tissue prepared from wortmannin-pretreated rats. LTP was associated with increased glutamate release, as previously described, but this effect was also inhibited in tissue prepared from wortmannin-pretreated rats. The results presented demonstrate that wortmannin also exerted an inhibitory effect on KCl-stimulated glutamate release and calcium influx in hippocampal synaptosomes in vitro. The evidence presented is consistent with the hypothesis that PI 3-kinase activation, possibly by NGF, plays a role in expression of LTP in dentate gyrus.

Age-related changes in synaptic function: analysis of the effect of dietary supplementation with omega-3 fatty acids

Depolarization-induced transmitter release in synaptosomes prepared from the hippocampus of aged rats is decreased compared with release from young animals. Although the underlying cause of this deficit is not known, some evidence suggests that increased membrane rigidity may contribute to these age-related synaptic changes. One possible consequence of the decreased transmitter release in the hippocampus of aged rats is a reduced ability to sustain long-term potentiation in perforant path-granule cell synapses, a pathway in which maintenance of long-term potentiation and increased glutamate release have been coupled. The observation that there is an age-dependent impairment in long-term potentiation is consistent with this view. If the age-related deficits in release and long-term potentiation are a consequence of increased membrane rigidity, it must be predicted that any manoeuvre which reverses membrane rigidity should reverse these functional deficits. In the present study, we investigated the effect of dietary manipulation of aged rats with omega-3 fatty acids on synaptic function. The data obtained indicate that an eight-week modified feeding schedule reversed the age-related impairments in long-term potentiation and depolarization-induced glutamate transmitter release. We also report that the concentrations of both docosahexanoic acid and arachidonic acid, two main polyunsaturated fatty acids in neuronal membranes, were decreased in the hippocampus of aged rats, and were restored by dietary manipulation. The data are consistent with the hypothesis that these deficits results from a change in membrane composition.

Activation of tyrosine receptor kinase plays a role in expression of long-term potentiation in the rat dentate gyrus

Long-term potentiation (LTP) in perforant path-granule cell synapses has been shown to be accompanied by an increase in glutamate release. The objective of this study was to examine the possibility that nerve growth factor (NGF), by activating tyrosine kinase, modulates glutamate release and, therefore, contributes to expression of LTP in dentate gyrus. The data indicate that NGF, in the presence of trans-1-aminocyclopentyl-1,3-dicarboxylate (ACPD), enhanced KCI-stimulated release and KCI-stimulated calcium influx in vitro and that these effects were blocked by the tyrosine receptor kinase (trk) inhibitor tyrphostin AG879. The data also indicate that NGF increased phosphorylation of trkA and the mitogen-activated protein kinase extracellular signal-regulated kinase (ERK) in dentate gyrus in vitro. In addition to its effects in vitro, tyrphostin AG879 inhibited the expression of LTP in perforant path-granule cell synapses and the accompanying increase in transmitter release. Analysis of phosphorylation of the two tyrosine kinase substrates trkA and ERK in synaptosomes prepared from untetanized and tetanized dentate gyrus revealed that LTP was associated with increased phosphorylation of both proteins; no evidence of such a change was observed in either tetanized or untetanized tissue prepared from tyrphostin-pretreated rats. These findings are consistent with the hypothesis that NGF, by interacting with trkA, triggers a sequence of tyrosine kinase-dependent phosphorylation steps that modulate glutamate release and calcium influx and impact on expression of LTP in dentate gyrus.

Age-related changes in LTP and antioxidant defenses are reversed by an alpha-lipoic acid-enriched diet

Among the age-related changes identified in rat hippocampus are impairments in LTP and glutamate release. These deficits have been coupled with decreased arachidonic acid concentration. In this study we compared LTP and glutamate release in groups of aged and young rats fed for 8 weeks on a control diet or on a diet enriched in alpha-lipoic acid. Dietary supplementation in aged rats restored hippocampal arachidonic acid concentration to levels observed in tissue prepared from young rats. We observed that aged rats that received the experimental diet sustained LTP in perforant path-granule cell synapses in a manner indistinguishable from young rats whereas the age-related impairment in glutamate release was reversed in synaptosomes prepared from dentate gyrus obtained from these rats. The evidence presented supports the hypothesis that the alpha-lipoic acid-enriched diet has antioxidant properties, because the age-related increase in superoxide dismutase activity and decrease in alpha-tocopherol concentration were reversed. The finding that the age-related increase in interleukin-1 (IL-1)beta concentration was also reversed suggests a possible role for this cytokine in ageing.

Age-related changes in oxidative mechanisms and LTP are reversed by dietary manipulation

Aged rats exhibit an impaired ability to sustain long-term potentiation in dentate gyrus which correlates with a decrease in arachidonic acid concentration. Here we confirm the previous finding that dietary supplementation with arachidonic acid and its precursor, gamma-linolenic acid, reversed the impairment in LTP in aged rats and report that there is a significant correlation between membrane arachidonic acid concentration and response to tetanic stimulation. We observed that age was associated with decreases in the concentration of vitamins C and E and increased activity of superoxide dismutase, indicative of a compromise in antioxidative defenses; these changes were paralleled by increases in interleukin-1beta (IL-1beta) concentration and lipid peroxidation. Dietary manipulation restored polyunsaturated fatty acid concentrations to values observed in tissue prepared from young rats and reversed the age-related changes in vitamins E and C, IL-1beta concentration and superoxide dismutase activity. We propose that these changes reverse the increase in lipid peroxidation and thereby the age-related change in polyunsaturated fatty acids.

Arachidonic acid reciprocally alters the availability of transient and sustained dendritic K(+) channels in hippocampal CA1 pyramidal neurons

The dendrites of hippocampal CA1 pyramidal cell dendrites express a high density of transient A-type K(+) channels, which play a critical role in the back-propagation of action potentials and in the determination of dendritic excitability. Recently, arachidonic acid and its nonmetabolizable analogue 5,8,11,14-eicosatetraynoic acid (ETYA) were shown to block transient K(+) channels in the somata of these cells (), but to have little effect on the somatic action potential. In the present study we have investigated the effects of arachidonic acid and ETYA on the gating of channels and the excitability of the apical dendrites of CA1 pyramidal neurons. We found not only a block of transient K(+) channels, but also an enhancement of sustained outward currents. The sustained currents consisted of at least two distinct channel types. The larger conductance channel (>50 pS) was identified as a K(+) channel. Arachidonic acid greatly enhanced the amplitude of back-propagating dendritic action potentials (>200 micrometer from the soma) but did not result in sustained depolarizations of the dendrites similar to those seen with 4-aminopyridine (4-AP) application. In fact, arachidonic acid reduced dendritic excitability when applied after 4-AP. Thus, arachidonic acid appears to cause a shift of available channels from the fast, transient type to the slower, sustained types. The net effect appears to be an enhancement of dendritic action potential amplitude that occurs without compromising the electrical stability of the dendrites.

Activation of p42 mitogen-activated protein kinase by arachidonic acid and trans-1-amino-cyclopentyl-1,3- dicarboxylate impacts on long-term potentiation in the dentate gyrus in the rat: analysis of age-related changes

Maintenance of long-term potentiation in perforant path-granule cell synapses is associated with an increase in glutamate release, which we have suggested relies on an interaction between arachidonic acid and the metabotropic glutamate receptor agonist, trans-1-amino-cyclopentyl-1,3-dicarboxylate (ACPD). Evidence suggests that this interaction is dependent on stimulation of tyrosine kinase, which phosphorylates and activates phospholipase Cgamma. In this study, we demonstrate that arachidonic acid and ACPD stimulate tyrosine phosphorylation of a protein of about 40,000 mol. wt and further analysis, using a specific antibody, suggested that this may be extracellular signal-regulated kinase, one member of the family of mitogen-activated protein kinases. Activity of extracellular signal-regulated kinase was increased by arachidonic acid and ACPD in vitro, but it was also increased by induction of long-term potentiation in perforant path-granule cell synapses. A role for extracellular signal-regulated kinase in long-term potentiation was supported by the observation that expression of long-term potentiation, as well as the associated increases in endogenous glutamate release and extracellular signal-regulated kinase activation, were inhibited by pretreatment with the mitogen-activated protein kinase inhibitor, PD98059, while PD98059 pretreatment inhibited the interaction between arachidonic acid and ACPD on glutamate release. An age-related decrease in extracellular signal-regulated kinase activity was observed in the dentate gyrus, and there was no evidence of increased extracellular signal-regulated kinase activity or endogenous glutamate release in tissue prepared from aged rats in which long-term potentiation was compromised. The evidence is consistent with the view that increased activation of extracellular signal-regulated kinase plays a role in long-term potentiation, and that activation of this kinase relies on the interaction between arachidonic acid and ACPD.

Neuropeptide Y inhibits glutamate release and long-term potentiation in rat dentate gyrus

The effect of intracerebroventricular injection of neuropeptide Y (NPY) was assessed on LTP in dentate gyrus. We report that NPY attenuated LTP and inhibited KCl-induced glutamate release in synaptosomes prepared from dentate gyrus. Activity of the stress-activated kinase, c-Jun NH2-terminal kinase (JNK) in synaptosomes was increased by incubation with NPY or following intracerebroventricular injection. Activation of JNK might underlie the inhibitory effect of NPY on LTP.

Arachidonic acid as a messenger for the expression of long-term potentiation

Arachidonic acid is suggested to play a role in the expression of long-term potentiation (LTP), a synaptic analog of memory and learning. However, it is unknown whether this free fatty acid is actually released during LTP or not. To address this question, we assayed arachidonic acid with an HPLC system using 9-anthryldiazomethane (ADAM) as a fluorescent probe. High frequency stimulation (tetanic stimulation) to a hippocampal slice from the guinea pig brain caused a huge increase in the release of glutamate from presynaptic terminals and in turn, a gradual increase in the release of arachidonic acid. A similar increase in the release of arachidonic acid was induced by application of glutamate and the increase was inhibited by either the selective AMPA/kainate receptor antagonist, DNQX, or to a lesser extent by the selective NMDA receptor antagonist, APV. These findings suggest that arachidonic acid is produced by activation of ionotropic glutamate receptors involving expression of LTP. Arachidonic acid exerted a long-lasting facilitatory action on synaptic transmission in the CA1 region of rat hippocampal slices and the facilitation occluded the tetanic LTP. Arachidonic acid, thus, appears to be a significant factor for the expression of LTP.

LTP occludes the interaction between arachidonic acid and ACPD and NGF and ACPD

We compared the interaction between the metabotropic glutamate receptor agonist ACPD and arachidonic acid with the interaction between ACPD and nerve growth factor (NGF) on presynaptic function in hippocampus. ACPD interacted with both NGF and arachidonic acid to increase KCl-stimulated endogenous glutamate release and calcium concentration in synaptosomes prepared from whole hippocampus and synaptosomes prepared from untetanized dentate gyrus. The data indicate that prior induction of long-term potentiation (LTP) in perforant path granule cells synapses occluded the interaction between ACPD and both NGF and arachidonic acid, suggesting that these agents may play a role in the generation of LTP in dentate gyrus.

Evidence for a role for synaptophysin in expression of long-term potentiation in rat dentate gyrus

Maintenance of long-term potentiation in perforant path-granule cell synapses is accompanied by increased glutamate release. Here we investigate the role of synaptophysin in release and in expression of long-term potentiation in dentate gyrus. We report that long-term potentiation was accompanied by increased endogenous glutamate release and increased tyrosine phosphorylation of synaptophysin, but these changes were attenuated when long-term potentiation was inhibited by the tyrosine kinase inhibitor tyrphostin AG879 or by the NMDA antagonist D-aminophosphonovalerate. In vitro analysis revealed that KCl-induced glutamate release was abolished in synaptosomes prepared in the presence of antisynaptophysin. The data suggest a role for synaptophysin in release and indicate that activation of tyrosine kinase and synaptophysin phosphorylation contribute to long-term potentiation perhaps by modulating glutamate release.

Modulation of ACh receptor currents by arachidonic acid

The present study investigated the effects of arachidonic acid on Torpedo (alpha beta gamma delta) and neuronal nicotinic acetylcholine (ACh) receptors (chick alpha7; rat alpha7, alpha3 beta2, alpha3 beta4, alpha4 beta2, and alpha4 beta4). Arachidonic acid (10 microM) depressed currents through normal Torpedo ACh receptors during treatment and afterward, persistently (>/=30 min) potentiated the currents. The potentiation was blocked by the selective protein kinase C (PKC) inhibitor, GF109203X or PKC inhibitor peptide (PKCI). The depression was not inhibited by any protein kinase inhibitor examined here, but greater in Ca2+-free extracellular solution. Arachidonic acid also potentiated currents through mutant Torpedo ACh receptors lacking PKC phosphorylation sites at Ser333 on the alpha subunit and Ser377 on the delta subunit without depression, but otherwise, it depressed currents through mutant receptors replacing of each Ser by negatively charged amino acid residue, possibly that mimics PKC phosphorylation of the receptors. These results suggest that the depression was due to the direct blocking effect on Ca2+-modulatory sites, which was accelerated under conditions of the receptors phosphorylated by PKC, and that the potentiation was caused by PKC activation, independently of PKC phosphorylation of the receptors. Arachidonic acid reduced currents through chick alpha7 receptors by a mechanism independent of protein kinase activation. In contrast, arachidonic acid potentiated currents through rat alpha7, alpha3 beta2, alpha4 beta2, and alpha4 beta4 receptors, perhaps by the same mechanism as the potentiation observed in Torpedo ACh receptors, although it had no effect on rat alpha3 beta4 receptors. The results of the present study thus demonstrate that arachidonic acid exerts diverse actions on nicotinic ACh receptors by different mechanisms.

Evidence that increased hippocampal expression of the cytokine interleukin-1 beta is a common trigger for age- and stress-induced impairments in long-term potentiation

Several cytokines and their receptors are identified in brain; one of these is the proinflammatory cytokine interleukin-1beta that is synthesized and released from neurons and glia in response to stress or insult. Among the actions of interleukin-1beta is its ability to inhibit long-term potentiation in the hippocampus in vitro, an action that mimics one of the consequences of stress and age. It has been shown that the concentration of interleukin-1beta in brain tissue is increased in neurodegenerative conditions, and recent evidence from our laboratory has indicated an increase in the concentration of interleukin-1beta in the hippocampus of aged rats. These observations led us to consider that the underlying common cause of impaired long-term potentiation in aged and stressed rats might be increased endogenous interleukin-1beta concentration in hippocampus. The data presented here indicate that there was an inverse relationship between concentration of interleukin-1beta in the dentate gyrus and long-term potentiation in perforant path-->granule cell synapses in aged rats, stressed rats, and rats pretreated with interleukin-1beta. The evidence suggested that the cytokine induces formation of reactive oxygen species that triggers lipid peroxidation in vivo, as well as in vitro, and that these changes lead to depletion of membrane arachidonic acid that correlates with impaired long-term potentiation. We propose that three theories of aging, the glucocorticoid theory, the membrane theory, and the free radical theory, constitute three facets of age with one underlying trigger: an increase in the endogenous concentration of interleukin-1beta in hippocampus.

Evidence that nerve growth factor plays a role in long-term potentiation in the rat dentate gyrus

An inbred strain of Wistar rat (GH), which is deficient in nerve growth factor (NGF), was used to assess the possible role of NGF in the generation of long-term potentiation in perforant path-granule cell synapses. The data show that NGF was significantly decreased in the dentate gyrus of GH rats, that this deficit was accompanied by an impairment in long-term potentiation (LTP) and that intraventricular injection of NGF substantially reversed this impairment. Analysis of depolarization-induced glutamate release in synaptosomes prepared from dentate gyrus of control rats revealed that NGF alone was without effect, but in combination with the metabotropic glutamate receptor agonist, aminocyclopentane-1,3-dicarboxylic acid (ACPD), NGF induced a significant increase in release. This effect was occluded by prior induction of LTP, suggesting that the interaction between these agents may be required to enhance transmitter release which accompanies LTP in dentate gyrus. In contrast to the effect of NGF and ACPD on glutamate release in control rats, the combination of these agents had no effect on release in synaptosomes prepared from GH rats, which might be explained by the marked decrease in trk receptors in dentate gyrus of GH rats. It was concluded that the impaired ability of GH rats to sustain LTP is associated with a reduction in NGF concentration, a reduction in stimulated release of NGF and a decrease in trk receptors in dentate gyrus. It is proposed that these data indicate a role for NGF in the generation of long-term potentiation in perforant path-granule cell synapses.

Analysis of the interaction between arachidonic acid and metabotropic glutamate receptor activation reveals that phospholipase C acts as a coincidence detector in the expression of long-term potentiation in the rat dentate gyrus

We have reported that arachidonic acid and the metabotropic glutamate receptor agonist, trans-1-amino-cyclopentyl-1,3-dicarboxylate (ACPD), act in synergy to increase release of glutamate from synaptosomes prepared from rat dentate gyrus. The observation that prior induction of LTP in perforant path-granule cell synapses occluded this synergism suggested that the interaction between arachidonic acid and ACPD might trigger the increase in glutamate release that accompanies LTP in dentate gyrus. Our objective was to identify the mechanism underlying the synergism between arachidonic acid and ACPD in LTP. The data indicate that both agents activate phospholipase C(PLC); the arachidonic acid-induced increase in phospholipase C activation was inhibited by the tyrosine kinase inhibitor, genistein, suggesting that PLCgamma, which is stimulated by tyrosine phosphorylation may be activated by arachidonic acid. The ACPD-induced increase was inhibited by neomycin, indicating the involvement of a G-protein and suggesting that PLCbeta may be activated by ACPD. We report that arachidonic acid stimulated phosphorylation of the specific tyrosine kinase substrate, poly(Glu80,Tyr20) and direct analysis indicated that arachidonic acid increased phosphorylation of PLCgamma. PLCgamma phosphorylation was assessed in control dentate gyrus and dentate gyrus in which LTP was induced in vivo. We report that the tyrosine kinase inhibitor, genistein, blocked expression of LTP and also blocked the associated increase in phosphorylation of PLCgamma. The data presented here indicate that tyrosine phosphorylation of PLCgamma was significantly enhanced following induction of LTP, but in separate experiments, in which LTP was inhibited by intraventricular injection of genistein, phosphorylation of PLCgamma was inhibited. The evidence presented is consistent with the hypothesis that PLC acts as a coincidence detector in LTP. The data indicate that PLCbeta is activated by ACPD, PLCgamma is activated by arachidonic acid, and coincident activation of both isoforms is necessary to stimulate an increase in glutamate release.

Arachidonic acid metabolites and the synaptic potentiation evoked by activation of metabotropic glutamate receptors

We have previously shown that coapplication of arachidonic acid (10 microM) and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD, 50 microM) evokes an enhancement of synaptic transmission in the CA1 region of the rat hippocampal slice. Here we have investigated whether the metabolites of arachidonic acid are implicated in this potentiation. Inclusion of the cyclo-oxygenase inhibitor indomethacin (10 microM) did not block the potentiation induced by coapplication of arachidonic acid and ACPD. However, the presence of either the cyclo-, lipo- and epoxygenase inhibitor 5,8,11,14-eicosatetraynoic acid (ETYA, 20 microM), or the lipoxygenase inhibitor nordihydroguaiaretic acid (10 microM), prevented the long-lasting enhancement. The results suggest that the lipoxygenase and epoxygenase metabolites of arachidonic acid may be involved in the induction of this form of synaptic potentiation.

The ability of aged rats to sustain long-term potentiation is restored when the age-related decrease in membrane arachidonic acid concentration is reversed

The ability of aged rats to sustain long-term potentiation in the dentate gyrus of the hippocampus is impaired and this impairment correlates with decreased release of glutamate and a decrease in membrane arachidonic acid concentration. Twenty-two-month-old rats receiving a diet supplemented with arachidonic acid and its precursor, gamma-linolenic acid, sustained long-term potentiation in a manner indistinguishable from four-month-old controls. Dietary supplementation also restored arachidonic acid concentrations in membranes prepared from hippocampus of these aged animals to levels observed in hippocampus of four-month-old rats. Glutamate release stimulated by depolarization was similar in dentate gyrus prepared from young rats and aged rats which received the experimental diet, but was markedly reduced in aged animals which received the control diet. In addition, the synergism between arachidonic acid and the metabotropic glutamate receptor agonist, trans-1-amino-cyclopentyl-1,3-dicarboxylate, on glutamate release, which was observed in hippocampal synaptosomes prepared from four-month-old rats, was also observed in hippocampal preparations obtained from aged rats which had been fed with the experimental diet, but was absent in hippocampal preparations obtained from aged animals which were fed with control diet. Thus, reversing the age-related decrease in membrane arachidonic acid concentration restored ability of aged animals to sustain long-term potentiation and reversed age-related changes in glutamate release.

Arachidonic acid inhibits uptake of amino acids and potentiates PKC effects on glutamate, but not GABA, exocytosis in isolated hippocampal nerve terminals

Arachidonic acid (AA), the putative retrograde messenger in long-term potentiation, enhanced extracellular aspartate, glutamate, and GABA levels in rat hippocampus synaptosomes. Whether this effect was determined by stimulating the release and/or inhibiting the uptake of amino acids was further investigated using different experimental conditions. To approach physiological conditions, a static incubation assay was used where both release and uptake occur. Under these conditions, AA dose-dependently (10-25 microM) enhanced basal extracellular amino acid levels in a completely Ca2+-independent way. AA still exerted this effect in the presence of inhibitors of PKC or of AA metabolism. When using the superfusion release assay, in which amino acid uptake cannot occur, no potentiating effect of AA on superfusate amino acid levels was observed. Therefore, AA possibly enhances the extracellular levels of aspartate, glutamate and GABA by inhibiting the uptake of these amino acids and not their efflux. Indeed, AA reduced the Na+-dependent uptake of endogenously released amino acids, which were labelled with traces of tritiated D-aspartate and GABA. When stimulating hippocampus synaptosomes with 4-aminopyridine, AA (2 microM) potentiated the Ca2+-dependent release of glutamate, but not of GABA, synergistically with PKC activation by 4beta-phorbol-12,13-dibutyric acid. In rat hippocampus, AA exerts different presynaptic effects to regulate extracellular amino acid levels, by inhibiting carrier-mediated uptake and, for glutamate, by stimulating exocytosis.

Changes in protein synthesis and synthesis of the synaptic vesicle protein, synaptophysin, in entorhinal cortex following induction of long-term potentiation in dentate gyrus: an age-related study in the rat

We have examined protein synthesis in entorhinal cortex following induction of long-term potentiation (LTP) in perforant path-granule cell synapses. The data presented here indicate that there was an increase in [35S]methionine labelling of TCA-precipitated proteins and [35S]methionine labelling of synaptophysin in the ipsilateral entorhinal cortex 40 min after induction of LTP in dentate gyrus. Intraventricular injection of both the NMDA antagonist, D-amino-phosphonovalerate, and the protein synthesis inhibitor, anisomycin reduced protein synthesis though the decrease caused by anisomycin was much more profound. Both agents blocked induction of LTP and the increase in protein synthesis and synaptophysin synthesis which accompanied LTP. These data indicate a close coupling of increased protein synthesis in the entorhinal cortex and expression of LTP in the dentate gyrus. This coupling was further suggested by the absence of an LTP-associated increase in protein synthesis in aged animals, in which LTP was markedly attenuated. The possibility that these changes impact on morphological changes which accompany LTP is discussed.

Training in the Morris water maze occludes the synergism between ACPD and arachidonic acid on glutamate release in synaptosomes prepared from rat hippocampus

We report here that release of glutamate, inositol phospholipid metabolism, and protein kinase C (PKC) activity are increased in synaptosomes prepared from hippocampi of rats that had been trained in a spatial learning task. In hippocampi obtained from animals that were untrained, activation of the metabotropic glutamate receptor by the specific agonist trans-1-amino-cyclopentyl-1,3-dicarboxylate (ACPD) increased release of glutamate but only in the presence of a low concentration of arachidonic acid. A similar interaction between arachidonic acid and ACPD was observed on inositol phospholipid turnover and on PKC activity. However, the synergistic effect of arachidonic acid and ACPD on glutamate release was occluded in hippocampal synaptosomes prepared from trained rats. Occlusion of the effect on inositol phospholipid turnover and PKC activation was also observed. These data suggest that the molecular changes that underlie spatial learning may include activation of metabotropic glutamate receptors in the presence of arachidonic acid and that the interaction between arachidonic acid and ACPD triggers the presynaptic changes that accompany learning.

The synergism between ACPD and arachidonic acid on glutamate release in hippocampus is age-dependent

Activation of the metabotropic glutamate receptor by the specific agonist trans-1-amino-cyclopentyl-1,3-dicarboxylate (ACPD) increases release of glutamate and activation of protein kinase C in the presence of a low concentration of arachidonic acid in hippocampal synaptosomes prepared from 4-month-old rats. The data presented indicate an age-related decrease in both [3H]glutamate release and protein kinase C activation and an age-related decrease in the release response to arachidonic acid and ACPD, with no corresponding change in protein kinase C activation. The finding that the interaction between arachidonic acid and ACPD on release was absent in synaptosomes prepared in the presence of heparin, an antagonist at inositol trisphosphate receptors, suggests that mobilization of intracellular calcium stores plays a role in the synergism between arachidonic acid and ACPD on [3H]glutamate release in hippocampal synaptosomes.