Synergistic potentiation of glutamate release by arachidonic acid and oleoyl-acetyl-glycerol

Physiological activation of presynaptic metabotropic glutamate receptors increases intracellular calcium and glutamate release

Activation of metabotropic glutamate receptors (mGluRs) has diverse effects on the functioning of vertebrate synapses. The cellular mechanisms that underlie these changes, however, are largely unknown. The role of presynaptic mGluRs in modulating Ca(2+) dynamics and regulating neurotransmitter release was investigated at the vestibulospinal-reticulospinal (VS-RS) synapse in the lamprey brain stem. Application of the specific Group I mGluRs antagonist 7-(hydroxyimino) cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) reduced the amplitude of consecutive high-frequency evoked excitatory postsynaptic currents (EPSCs). A series of experiments using techniques of electrophysiology and calcium imaging were carried out to determine the cellular mechanisms by which this phenomenon occurs. Concentration-dependent increases in the pre- and postsynaptic [Ca(2+)](i) were seen with the application of mGluR agonists. Similarly, high-frequency stimulation of axons caused a Group I mGluR-dependent enhancement in presynaptic Ca(2+) transients. Application of mGluR agonist caused a depolarization of the presynaptic elements, while thapsigargin decreased the high-frequency stimulus- and agonist-induced rises in [Ca(2+)](i). These data suggest that both membrane depolarization and the release of Ca(2+) from intracellular stores potentially play a role in mGluR-induced Ca(2+) signaling. To determine the effect of this modulation of Ca(2+) dynamics on spontaneous glutamate release, miniature EPSCs were recorded from postsynaptic reticulospinal neurons. A potent Group I mGluR agonist, (S)-homoquisqualic acid, caused a large increase in the frequency of events. These results demonstrate the presence of presynaptic Group I mGluRs at the VS-RS synapse. Activation of these receptors leads to a rise in [Ca(2+)](i) and enhances the spontaneous and evoked release of glutamate. Taken together, these studies highlight the importance of synaptic activation of these facilitatory autoreceptors in both short-term plasticity and synaptic transmission.

Molecules, Motion, and Man

The application of cell and molecular biology techniques to vestibular research is resulting in rapid changes in our understanding of the fundamental mechanisms of vestibular function. The clinical problems encountered in space travel together with the acute and chronic vestibular dysfunction affecting many of the patients otolaryngologists care for have driven this research at a rapid pace. A review of these methods and highlights of the major advances are discussed. (Otolaryngol Head Neck Surg 1998;118:S16-S24.)

Regulation of phosphatidylethanolamine degradation by enzyme(s) of subcellular fractions from cerebral cortex

Hydrolysis of 1-acyl-2-[14C]arachidonoyl-sn-glycero-3-phosphoethanolamine was studied in cerebral cortex homogenate and subcellular fractions. The enzyme(s) confined to the synaptic plasma membrane (SPM) hydrolyze(s) [14C-arachidonoyl]phosphatidylethanolamine (PE) in the presence of EGTA to [14C-arachidonoyl]diacylglycerol (DAG) and a small amount of [14C]arachidonic acid (AA). Degradation of PE is time-, protein- and substrate-dependent with a pH optimum of 7.8. The highest activity of PE degradation was observed in the presence of 10 mM EGTA. Under this condition GTP gamma S has no effect on PE hydrolysis. In the presence of Ca2+ ions degradation of PE was significantly lower as compared to the conditions with EGTA. However, the percentage distribution of free AA in the sum of both products of PE hydrolysis (AA + DAG) increases from 16 and 20% observed in the presence of EGTA 2 mM and 10 mM to 34% and 43% in the presence of 0.5 mM CaCl2 alone and together with GTP gamma S, respectively. Cytosolic enzymes also degrade PE in the presence of 2 mM EGTA with the formation of DAG and AA. Radioactivity in the AA represents about 80% of the total radioactivity of the products of PE degradation. The hydrolysis of PE by cytosolic enzymes is almost completely inhibited by neomycin but the hydrolysis by the SPM-bound enzyme(s) is inhibited only 70%. Other studies with quinacrine indicated that only a small pool of PE is degraded by SPM-bound Ca(2+)-independent phospholipase A2 (PLA2). All of these data suggest that PE in cerebral cortex is mainly degraded by cytosolic and SPM-bound Ca(2+)-independent phospholipase C. Further studies towards a better understanding of the mechanisms of cerebral degradation and the physiological significance of Ca(2+)-independent pathways of PE hydrolysis are necessary.

Lipid-dependent modulation of Ca2+ availability in isolated mossy fiber nerve endings

An enhancement of glutamate release from hippocampal neurons has been implicated in long-term potentiation, which is thought to be a cellular correlate of learning and memory. This phenomenom appears to be involved the activation of protein kinase C and lipid second messengers have been implicated in this process. The purpose of this study was to examine how lipid-derived second messengers, which are known to potentiate glutamate release, influence the accumulation of intraterminal free Ca2+, since exocytosis requires Ca2+ and a potentiation of Ca2+ accumulation may provide a molecular mechanism for enhancing glutamate release. The activation of protein kinase C with phorbol esters potentiates the depolarization-evoked release of glutamate from mossy fiber and other hippocampal nerve terminals. Here we show that the activation of protein kinase C also enhances evoked presynaptic Ca2+ accumulation and this effect is attenuated by the protein kinase C inhibitor staurosporine. In addition, the protein kinase C-dependent increase in evoked Ca2+ accumulation was reduced by inhibitors of phospholipase A2 and voltage-sensitive Ca2+ channels, as well as by a lipoxygenase product of arachidonic acid metabolism. That some of the effects of protein kinase C activation were mediated through phospholipase A2 was also indicated by the ability of staurosporine to reduce the Ca2+ accumulation induced by arachidonic acid or the phospholipase A2 activator melittin. Similarly, the synergistic facilitation of evoked Ca2+ accumulation induced by a combination of arachidonic acid and diacylglycerol analogs was attenuated by staurosporine. We suggest, therefore, that the protein kinase C-dependent potentiation of evoked glutamate release is reflected by increases in presynaptic Ca2+ and that the lipid second messengers play a central role in this enhancement of chemical transmission processes.

Stimulatory effect of arachidonic acid on the release of GABA in matrix-enriched areas from the rat striatum

Arachidonic acid was shown to stimulate the release of preloaded [3H]GABA from microdiscs of tissue punched out in matrix-enriched areas of the rat striatum. This effect, which was calcium- and dose-dependent, persisted in the presence of inhibitors of arachidonic acid catabolism. Other fatty acids were less or not effective. Arachidonic acid also inhibited [3H]GABA uptake into purified striatal synaptosomes, however the arachidonic acid-evoked release of [3H]GABA persisted following inhibition of the GABA neuronal uptake process. The stimulatory effect of arachidonic acid on GABA release may largely result from the activation of a protein kinase C since the arachidonic acid response was reduced by several protein kinase C inhibitors. Arachidonic acid also dose-dependently stimulated the release of preloaded [3H]GABA from purified striatal synaptosomes. Similar results were obtained when synaptosomes were previously incubated with [3H]glutamine to study the release of endogenously synthesized [3H]GABA. Further indicating a direct action of the fatty acid on GABAergic neurons, the arachidonic acid-induced release of [3H]GABA from microdiscs was not modified in the presence of the D1 dopaminergic antagonist SCH23390 or of glutamatergic antagonists. Finally, the release of [3H]GABA evoked by the combined application of NMDA and carbachol (a treatment known to markedly stimulate arachidonic acid formation) was reduced by inhibitors of phospholipase A2 further indicating that endogenously formed arachidonic acid significantly facilitates the release of GABA in the striatum.

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.

Identification and localization of an actin-binding motif that is unique to the epsilon isoform of protein kinase C and participates in the regulation of synaptic function

Individual isoforms of the protein kinase C (PKC) family of kinases may have assumed distinct responsibilities for the control of complex and diverse cellular functions. In this study, we show that an isoform specific interaction between PKC epsilon and filamentous actin may serve as a necessary prelude to the enhancement of glutamate exocytosis from nerve terminals. Using a combination of cosedimentation, overlay, and direct binding assays, we demonstrate that filamentous actin is a principal anchoring protein for PKC epsilon within intact nerve endings. The unusual stability and direct nature of this physical interaction indicate that actin filaments represent a new class of PKC-binding protein. The binding of PKC epsilon to actin required that the kinase be activated, presumably to expose a cryptic binding site that we have identified and shown to be located between the first and second cysteine-rich regions within the regulatory domain of only this individual isoform of PKC. Arachidonic acid (AA) synergistically interacted with diacylglycerol to stimulate actin binding to PKC epsilon. Once established, this protein-protein interaction securely anchored PKC epsilon to the cytoskeletal matrix while also serving as a chaperone that maintained the kinase in a catalytically active conformation. Thus, actin appears to be a bifunctional anchoring protein that is specific for the PKC epsilon isoform. The assembly of this isoform-specific signaling complex appears to play a primary role in the PKC-dependent facilitation of glutamate exocytosis.

7-Nitro indazole, a selective neuronal nitric oxide synthase inhibitor in vivo, impairs spatial learning in the rat

Nitric oxide (NO) is an intercellular messenger that has been suggested to have a role in learning and memory formation. Previous studies with nonselective NO synthase inhibitors have produced contradictory results in learning experiments. However, these drugs also produced blood pressure changes, as NO is an endothelial-derived relaxing factor. A novel NO synthase inhibitor, 7-nitro indazole (7-NI), as a dose (30 mg/kg i.p.) shown previously to inhibit neuronal NO synthase by 85% without affecting blood pressure, produced amnesic effects both in a water maze and in an 8-arm radial maze. Latency as well as distance was greater in the 7-NI group in the water maze while swim speed was not affected. Latency, working memory (WM), and reference memory (RF) errors were also higher in the 7-NI group in the 8-arm maze. At the end of the second training day, these differences were no longer apparent. However, on the fourth training day, a transfer test in the water maze showed that 7-NI had produced a spatial memory deficit, reducing quadrant bias and the number of annulus crossings. Learning of a visual cue task was not affected. No difference between groups was visible in an open field test. We conclude that neuronal NO synthase activity plays a role in learning and memory formation in the rat.

Mediators of injury in neurotrauma: intracellular signal transduction and gene expression

Membrane lipid-derived second messengers are generated by phospholipase A2 (PLA2) during synaptic activity. Overstimulation of this enzyme during neurotrauma results in the accumulation of bioactive metabolites such as arachidonic acid, oxygenated derivatives of arachidonic acid, and platelet-activating factor (PAF). Several of these bioactive lipids participate in cell damage, cell death, or repair-regenerative neural plasticity. Neurotransmitters may activate PLA2 directly when linked to receptors coupled to G proteins and/or indirectly as calcium influx or mobilization from intracellular stores is stimulated. The release of arachidonic acid and its subsequent metabolism to prostaglandins are early responses linked to neuronal signal transduction. Free arachidonic acid may interact with membrane proteins, i.e., receptors, ion channels, and enzymes, modifying their activity. It can also be acted upon by prostaglandin synthase isoenzymes (the constitutive prostaglandin synthase PGS-1 or the inducible PGS-2) and by lipoxygenases, with the resulting formation of different prostaglandins and leukotrienes. Glutamatergic synaptic activity and activation of postsynaptic NMDA receptors are examples of neuronal activity, linked to memory and learning processes, which activate PLA2 with the consequent release of arachidonic acid and platelet-activating factor (PAF), another lipid mediator. Both mediators may exert presynaptic and postsynaptic effects contributing to long-lasting changes in glutamate synaptic efficacy or long-term potentiation (LTP), PAF, a potential retrograde messenger in LTP, stimulates glutamate release. The PAF antagonist BN 52021 competes for receptors in presynaptic membranes and blocks this effect. PAF may also be involved in plasticity responses because PAF leads to the expression of early response genes and subsequent gene cascades. The PAF antagonist BN 50730, selective for PAF intracellular binding, blocks PAF-mediated induction of gene expression. A consequence of neural injury induced by ischemia, trauma, or seizures is an increased release of neurotransmitters, that in turn generates an overproduction of second messengers. Glutamate, a key player in excitotoxic neuronal damage, triggers increased permeation of calcium mediated by NMDA receptors and activation of PLA2 in postsynaptic neurons. NMDA receptor antagonists reduce the accumulation of free fatty acids and elicit neuroprotection in ischemic damage. Increased production of free arachidonic acid and PAF converges to exacerbate glutamate-mediated neurotransmission. These neurotoxic actions may be brought about by arachidonic acid-induced potentiation of NMDA receptor activity and decreased glutamate reuptake. On the other hand, PAF stimulates the further release of glutamate at presynaptic endings. The neuroprotective effects of the PAF antagonist BN 52021 in ischemia-reperfusion are due, at least in part, to an inhibition of presynaptic glutamate release. PAF also induces expression of the inducible prostaglandin synthase gene, and PAF antagonists selective for the intracellular sites inhibit this effect. The PAF antagonist also inhibits the enhanced abundance, due to vasogenic cerebral edema and ischemia-reperfusion damage, of inducible prostaglandin synthase mRNA in vivo. Therefore, PAF, an injury-generated mediator, may favor the formation of other cell injury and inflammation mediators by turning on the expression of the gene that encodes prostaglandin synthase.

Arachidonic acid as a neurotoxic and neurotrophic substance

In this article we summarize a wide variety of properties of arachidonic acid (AA) in the mammalian nervous system especially in the brain. AA serves as a biologically-active signaling molecule as well as an important component of membrane lipids. Esterified AA is liberated from the membrane by phospholipase activity which is stimulated by various signals such as neurotransmitter-mediated rise in intracellular Ca2+. AA exerts many biological actions which include modulation of the activities of protein kinases and ion channels, inhibition of neurotransmitter uptake, and enhancement of synaptic transmission. AA serves also as a precursor of a variety of eicosanoids, which are formed by oxidative metabolism of AA. AA cascade is activated under several pathological conditions in the brain such as ischemia and seizures, and may be involved in irreversible tissue damage. On the other hand, AA can show beneficial influences on brain tissues and cells in several situations. In a recent study using cultured brain neurons, we have found that AA shows quite distinct actions at a narrow concentration range, such as induction of cell death, promotion of cell survival and enhancement of neurite extension. The neurotoxic action is mediated by free radicals generated by AA metabolism, whereas the neurotrophic actions are exerted by AA itself. The observed in vitro actions of AA might be related to important roles of AA in brain pathogenesis and neural development.

Arachidonic acid and diacylglycerol ACT synergistically through protein kinase C to persistently enhance synaptic transmission in the hippocampus

In model membranes, arachidonic acid and diacylglycerol have been proposed to synergistically induce a membrane-inserted, constitutively active form of protein kinase C. We have investigated the effects of these lipid protein kinase C activators on synaptic efficacy in the Schaffer collateral input to CA1 hippocampal pyramidal cells. Arachidonic acid (5 microM) perfusion combined with repetitive afferent stimulation had no consistent effect on field excitatory postsynaptic potentials recorded in stratum radiatum, while treatment with a cell-permeable diglyceride, oleoyl-acetylglycerol (5 micrograms/ml), followed by stimulation, led to a short-term potentiation. By contrast, the combination of oleoyl-acetylglycerol and arachidonic acid gave rise to a long-lasting non-decremental potentiation of field excitatory postsynaptic potentials. The induction of potentiation was "activity dependent", as there was either no significant effect or there was a measurable depression when repetitive synaptic stimulation was omitted. Furthermore, consistent with a protein kinase C-dependent process, the potentiation was blocked by the kinase inhibitors H-7 and staurosporine. The results suggest that relatively low concentrations of arachidonic acid and diacylglycerol work synergistically through protein kinase C to persistently enhance synaptic transmission. This synergy has the makings of an associative (Hebbian) device for long-term potentiation induction operating at the second messenger level.