Activation of Protein Kinase C by Phorbol Esters and Arachidonic Acid Required for the Optimal Potentiation of Glutamate Exocytosis

Bidirectional modulation of glutamatergic synaptic transmission by metabotropic glutamate type 7 receptors at Schaffer collateral-CA1 hippocampal synapses

KEY POINTS

Neurotransmitter release is inhibited by metabotropic glutamate type 7 (mGlu₇ ) receptors that reduce Ca²⁺ influx, yet synapses lacking this receptor also produce weaker release, suggesting that mGlu₇ receptors may also prime synaptic vesicles for release. Prolonged activation of mGlu₇ receptors with the agonist l-AP4 first reduces and then enhances the amplitude of EPSCs through a presynaptic effect. The inhibitory response is blocked by pertussis toxin, while the potentiating response is prevented by a phospholipase C inhibitor (U73122) and an inhibitor of diacylglycerol (DAG) binding (calphostin C), suggesting that this receptor also couples to pathways that generate DAG. Release potentiation is associated with an increase in the number of synaptic vesicles close to the plasma membrane, which was dependent on the Munc13-2 and RIM1α proteins. The Glu7 receptors activated by the glutamate released following high frequency stimulation provoke a bidirectional modulation of synaptic transmission.

ABSTRACT

Neurotransmitter release is driven by Ca²⁺ influx at synaptic boutons that acts on synaptic vesicles ready to undergo exocytosis. Neurotransmitter release is inhibited when metabotropic glutamate type 7 (mGlu₇ ) receptors provoke a reduction in Ca²⁺ influx, although the reduced release from synapses lacking this receptor suggests that they may also prime synaptic vesicles for release. These mGlu₇ receptors activate phospholipase C (PLC) and generate inositol trisphosphate, which in turn releases Ca²⁺ from intracellular stores and produces diacylglycerol (DAG), an activator of proteins containing DAG-binding domains such as Munc13 and protein kinase C (PKC). However, the full effects of mGlu₇ receptor signalling on synaptic transmission are unclear. We found that prolonged activation of mGlu₇ receptors with the agonist l-AP4 first reduces and then enhances the amplitude of EPSCs, a presynaptic effect that changes the frequency but not the amplitude of the mEPSCs and the paired pulse ratio. Pertussis toxin blocks the inhibitory response, while the PLC inhibitor U73122, and the inhibitor of DAG binding calphostin C, prevent receptor mediated potentiation. Moreover, this DAG-dependent potentiation of the release machinery brings more synaptic vesicles closer to the active zone plasma membrane in a Munc13-2- and RIM1α-dependent manner. Electrically evoked release of glutamate that activates mGlu₇ receptors also bidirectionally modulates synaptic transmission. In these conditions, potentiation now occurs rapidly and it overcomes any inhibition, such that potentiation prevails unless it is suppressed with the PLC inhibitor U73122.

The metabotropic glutamate receptor mGlu7 activates phospholipase C, translocates munc-13-1 protein, and potentiates glutamate release at cerebrocortical nerve terminals

At synaptic boutons, metabotropic glutamate receptor 7 (mGlu7 receptor) serves as an autoreceptor, inhibiting glutamate release. In this response, mGlu7 receptor triggers pertussis toxin-sensitive G protein activation, reducing presynaptic Ca(2+) influx and the subsequent depolarization evoked release. Here we report that receptor coupling to signaling pathways that potentiate release can be seen following prolonged exposure of nerve terminals to the agonist l-(+)-phosphonobutyrate, l-AP4. This novel mGlu7 receptor response involves an increase in the release induced by the Ca(2+) ionophore ionomycin, suggesting a mechanism that is independent of Ca(2+) channel activity, but dependent on the downstream exocytotic release machinery. The mGlu7 receptor-mediated potentiation resists exposure to pertussis toxin, but is dependent on phospholipase C, and increased phosphatidylinositol (4,5)-bisphosphate hydrolysis. Furthermore, the potentiation of release does not depend on protein kinase C, although it is blocked by the diacylglycerol-binding site antagonist calphostin C. We also found that activation of mGlu7 receptors translocate the active zone protein essential for synaptic vesicle priming, munc13-1, from soluble to particulate fractions. We propose that the mGlu7 receptor can facilitate or inhibit glutamate release through multiple pathways, thereby exerting homeostatic control of presynaptic function.

Stochastic aspects of transmitter release and bioenergetic dysfunction in isolated nerve terminals

Synaptosomes (isolated nerve terminals) have been studied for more than 40 years. The preparation allows aspects of transmitter metabolism and release to be studied ex vivo from specific brain regions of animals of any age. Conditions can be devised to enable the terminals to fire spontaneous action potentials, allowing the presynaptic control of glutamate exocytosis to be studied. Recent developments have greatly increased the sensitivity with which the bioenergetics of the intra-synaptosomal mitochondria can be investigated.

Partial compensation for N-type Ca(2+) channel loss by P/Q-type Ca(2+) channels underlines the differential release properties supported by these channels at cerebrocortical nerve terminals

N-type and P/Q-type Ca(2+) channels support glutamate release at central synapses. To determine whether the glutamate release mediated by these channels exhibits distinct properties, we have isolated each release component in cerebrocortical nerve terminals from wild-type mice by specifically blocking N-type Ca(2+) channels with omega-conotoxin-GVIA and P/Q-type Ca(2+) channels with omega-agatoxin-IVA. In addition, we have determined the release properties at terminals from mice lacking the alpha(1B) subunit of N-type channels (Ca(v) 2.2) to test the possibility that P/Q-type channels can compensate for the loss of N-type Ca(2+) channels. We recently demonstrated that, while evoked glutamate release depends on P/Q- and N-type channels in wild-type nerve terminals, only P/Q-type channels participate in these knockout mice. Moreover, in nerve terminals expressing solely P/Q-type channels, metabotropic glutamate receptor 7 (mGluR7) fails to inhibit the evoked Ca(2+) influx and glutamate release. Here, we show that the failure of mGluR7 to modulate evoked glutamate release is not due to a lack of receptors, as nerve terminals from mice lacking N-type Ca(2+) channels express mGluR7. Indeed, we show that other receptor responses, such as the inhibition of forskolin-induced release, are preserved in these knockout mice. N-type channels are more loosely coupled to release than P/Q-type channels in nerve terminals from wild-type mice, as reflected by the tighter coupling of release in knockout nerve terminals. We conclude that the glutamate release supported by N- and P/Q-type channels exhibits distinct properties, and that P/Q-type channels cannot fully compensate for the loss of N-type channels.

Protein kinase C activity blocks neuropeptide Y-mediated inhibition of glutamate release and contributes to excitability of the hippocampus in status epilepticus

The unbalanced excitatory/inhibitory neurotransmitter function in the neuronal network afflicted by seizures is the main biochemical and biophysical hallmark of epilepsy. The aim of this work was to identify changes in the signaling mechanisms associated with neuropeptide Y (NPY)-mediated inhibition of glutamate release that may contribute to hyperexcitability. Using isolated rat hippocampal nerve terminals, we showed that the KCl-evoked glutamate release is inhibited by NPY Y2 receptor activation and is potentiated by the stimulation of protein kinase C (PKC). Moreover, we observed that immediately after status epilepticus (6 h postinjection with kainate, 10 mg/kg), the functional inhibition of glutamate release by NPY Y2 receptors was transiently blocked concomitantly with PKC hyperactivation. The pharmacological blockade of seizure-activated PKC revealed again the Y2 receptor-mediated inhibition of glutamate release. The functional activity of PKC immediately after status epilepticus was assessed by evaluating phosphorylation of the AMPA receptor subunit GluR1 (Ser-831), a substrate for PKC. Moreover, NPY-stimulated [35S]GTPgammaS autoradiographic binding studies indicated that the common target for Y2 receptor and PKC on the inhibition/potentiation of glutamate release was located downstream of the Y2 receptor, or its interacting G-protein, and involves voltage-gated calcium channels.

Presynaptic group I metabotropic glutamate receptors modulate synaptic transmission in the rat superior colliculus via 4-AP sensitive K(+) channels

1. Group I metabotropic glutamate receptors (mGluRs) are thought to be important modulators of neuronal function in the superior colliculus (SC). Here, we investigated the pharmacology and signalling mechanisms underlying group I mGluR-mediated inhibition of neuronal excitability and synaptic transmission in the rat SC slice. 2. The group I agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) potently depressed synaptically evoked excitatory postsynaptic potentials (EPSPs), currents (EPSCs), and action potentials in a dose-dependent manner (IC50: 6.3 microm). This was strongly reduced by the broad-spectrum antagonist (+)-alpha-methyl-4-carboxyphenylglycine (MCPG, 1 mm, approximately 95% reduction), by the mGluR1 antagonist LY367385 (100 microm, approximately 80% reduction) but not by the mGluR5 antagonist 6-methyl-2-(phenylethynyl)-pyridine (MPEP, 1-100 microm). 3. The putative mGluR5-specific agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG, 500 microm) also inhibited EPSPs. Interestingly, CHPG's actions were not blocked by MPEP, but LY367385 (100 microm) reduced the effect of CHPG by 50%. 4. Inhibition induced by DHPG was independent of phospholipase C (PLC)/protein kinase C pathways, and did not require intact intracellular Ca2+ stores. It was not abolished but enhanced by the GABAA antagonist bicuculline (5 microm), suggesting that DHPG's action was not due to facilitated inhibition or changes in neuronal network activity. 5. The K+ channel antagonist 4-aminopyridine (4-AP, 50-100 microm) converted the inhibitory effect of DHPG into facilitation. Paired-pulse depression was strongly reduced by DHPG, an effect that was also prevented by 4-AP. 6. Our data indicate that group I agonists regulate transmitter release, presumably via an autoreceptor in the SC. This receptor may be involved in adaptation to repetitive stimulation via a non-PLC mediated pathway.

Effect of ionotropic glutamate receptors antagonists on the modifications in extracellular glutamate and aspartate levels during picrotoxin seizures: a microdialysis study in freely moving rats

Our previous studies have shown a local decrease in glutamate and aspartate levels during seizures, induced by picrotoxin microdialysis in the hippocampus of chronic freely moving rats. In this paper, we study the effect of continuous hippocampal microperfusion of the NMDA, AMPA and kainate glutamate receptor inhibitors 5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine (MK-801); 6,7-dinitroquinoxaline-2,3-dione (DNQX), and 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI 52466). We also examine the action of L(-)-threo-3-hydroxyaspartic acid (THA), a glutamate and aspartate reuptake blocker, on the modification of extracellular glutamate and aspartate levels induced by picrotoxin, using the microdialysis method in freely moving rats. We found that changes in extracellular hippocampal concentrations in both amino acids are prevented by NMDA, AMPA and kainate receptor inhibitors. Seizures elicited under DNQX also induce a transient increase in aspartate extracellular levels coincident with seizure time. L(-)-threo-3-hydroxyaspartic acid increased the basal extracellular concentrations of both amino acids, but did not prevent the seizure-related decrease. Our results suggest that glutamate, the major neurotransmitter at the synaptic level, may also play an important role in non-synaptic transmission during seizures.

Protein kinase C: a physiological mediator of enhanced transmitter output

Protein kinase C (PKC), activated by either diacylglycerol and/or arachidonic acid, through the activation of presynaptic receptors or nerve or nerve depolarization is involved is involved in the enhancement of transmitter release from many neural types. This facilities is most likely mediated by the phosphorylation of proteins involved in vesicle dynamics although a role for ion channels cannot be ruled out. PKC is not fundamental to the release process but rather has a modulatory role of PKC is to help maintain transmitter output during prolonged or elevated levels of activation and this seems to parallel suggestions that PKC is involved in the movement of reserve pools of vesicles into release-study sites. presynaptic facilitatory actions mediated by PKC are also involved in integrated modulatory functions such as long term potentiation, again where it elevates or maintains transmitter output. Although studies have tried to identify specific roles for various PKC isoforms, the actions of phorbol esters in elevators transmitter release do not fit with known potencies on individual isoforms and lit suggests that PKC may be located at an intraneuronal location which is difficult to access for lipophilic phorbol esters and further work is required in this area.

Hyperalgesia and allodynia induced by intrathecal (RS)-dihydroxyphenylglycine in rats

To investigate the role of Group I mGluRs in allodynia and hyperalgesia, we examined the behavioural responses of rats to noxious and non-noxious mechanical and thermal stimuli following intrathecal (i.t.) treatment (25 nmol) with the selective mGluR1/5 agonist, (RS)-dihydroxyphenylglycine ((RS)-DHPG). (RS)-DHPG administration produced a persistent decrease in response latency on a 48 degrees C hotplate, a reduction in the 50% response threshold to von Frey hairs, and an increase in responses to a tail pinch. These data suggest that activation of spinal mGluR1/5 receptors plays a role in the development of persistent allodynia and hyperalgesia associated with tissue or nerve injury.

The effect of arachidonic acid and free fatty acids on vesicular uptake of glutamate and gamma-aminobutyric acid

The manner in which arachidonic acid and other free fatty acids influence the vesicular uptake of glutamate and gamma-aminobutyric acid (GABA) has been investigated. The cis-polyunsaturated fatty acid arachidonic acid (20:4), eicosapentanoic acid (20:5) and linolenic acid (18:3) at 150 nmol/mg protein (50 microM) inhibited the vesicular uptake of glutamate and GABA more than 70%. Reduced inhibition of vesicular uptake was seen with the cis-monounsaturated fatty acid oleic acid (18:1) and the trans-mono-unsaturated fatty acid elaidic acid (18:1). The saturated fatty acids stearic acid (16:0) and arachidic acid (20:0) had no significant effect on the uptake. The inhibition of vesicular uptake by arachidonic acid was prevented by the addition of fatty acid free bovine serum albumin. Arachidonic acid inhibited in a dose-dependent manner the generation of the transmembrane pH gradient of the synaptic vesicles. This inhibition was proportional to the inhibition of the vesicular uptake of glutamate and GABA. The saturated fatty acid arachidic acid showed no inhibition of delta pH generation. Arachidonic acid at 200 nmol/mg of protein did not increase the uptake-independent leakage of glutamate and GABA from the vesicles, showing that the effect of arachidonic acid is not caused by an unspecific detergent effect. These results suggest that arachidonic acid and other polyunsaturated fatty acids are acting like proton-ionophores on the vesicular uptake of these neurotransmitters. This finding may have implications for the increased fatty acid concentration during pathological conditions like ischemia and in long term potentiation.

Increased presynaptic protein kinase C activity and glutamate release in rats with a prenatally induced hippocampal lesion

We have previously shown that protein kinase C (PKC) activity is up-regulated in nerve terminals of animals that have been subjected to targeted cellular ablation of cortical and hippocampal neurons by treatment with methylazoxymethanol (MAM), which results in impaired long-term potentiation (LTP) and cognitive deficit. In this study we investigated the consequences of increased membrane-bound PKC in the regulation of release of glutamate, the major excitatory transmitter involved in LTP. We show that nerve terminals of MAM-treated rats show higher PKC activity, as monitored by the in situ phosphorylation of B-50/GAP-43, in both basal and phorbol ester-stimulated conditions. In these animals, hippocampal nerve endings release a greater amount of glutamate than those of controls, both in basal conditions and when synaptosomes are stimulated with KCl or 3,4-diaminopyridine. The potentiation observed in MAM-treated rats was counteracted by the PKC blocker H-7 and the clostridial tetanus toxin. On the contrary, GABA release was not significantly up-regulated, either in basal or in depolarization-evoked conditions. Therefore our data show that the increase in synaptosomal PKC activity is paralleled by increased glutamate but not GABA release in this animal model. Whether this reflects specific up-regulation of membrane PKC activity in glutamatergic terminals or an alteration in the regulation of glutamate release remains to be determined.

Characterization of melittin effects in synaptosomes

The effects of melittin at increasing concentrations on: [3H]GABA release from mouse brain synaptosomes; on the radioactivity released from [3H]arachidonic acid labeled synaptosomal membranes; on synaptosomes ultrastructure and on the leakage of the cytoplasmic marker, lactate-dehydrogenase (LDH) was investigated. Melittin 0.3, 1, 3, 7, and 10 microM progressively increases [3H]GABA release, but the efficacy of melittin is decreased when the amount of tissue exposed to a constant concentration of the toxin increases. The release of [3H]GABA induced by melittin below 3 microM is Ca2+ dependent, but not that induced by the higher concentrations. The Ca2+ dependent fraction of the [3H]GABA released by 0.3 microM melittin is selectively inhibited by 10 microM quinacrine and 1 microM nordihydroguaiaretic acid (NDGA) and facilitated by 3 microM indomethacin, whereas the Ca2+ independent fraction of the [3H]GABA released by melittin is not. In the presence of Ca2+, melittin 0.3, 1 and 10 microM progressively increases [3H]arachidonic acid release over control release, but the effectiveness of melittin is also decreased as the amount of tissue increases. No apparent changes in synaptosomes ultrastructure are observed in 0.3 microM treated synaptosomes, but a noticeable disorganization is produced in 10 microM melittin-treated synaptosomes, independently on the presence of external Ca2+. LDH activity only increases over control activity in the supernatant solutions of 10 microM melittin treated synaptosomes, also in a Ca2+ independent manner. Our interpretation of these results is that the Ca2+-dependent, pharmacologic sensitive component of melittin-induced release of [3H]GABA, unmasked when 0.3 microM melittin was used, involves the activation of a Ca2+-dependent type of membrane PLA2. The Ca2+-independent release of [3H]GABA is in contrast, highly probable to be due to the membrane perturbation produced by complex melittin/lipid interactions.

L-AP4 (L-(+)-2-amino-4-phosphonobutyric acid) induced impairment of spatial learning in the rat is antagonized by MAP4 ((S)-2-amino-2-methyl-4-phosphonobutanoic acid)

L-AP4, an agonist at the metabotrophic glutamate receptors 4, 6, 7, 8 and 9 produced a selective spatial learning impairment in a water maze as well as in an 8-arm maze task when injected i.c.v. (5 microliters of a 80 mM solution), a dose previously reported to block consolidation of long-term potentiation in vivo. Acquisition and recall of the spatial water-maze task, as measured by escape latency and quadrant bias, respectively, were impaired, whereas swim speed was not affected. In contrast, ability to perform a non-spatial control task was not impaired; latency to reach a visible escape platform was not delayed in L-AP4-treated animals. No behavioral difference was visible in the open field. MAP4, an antagonist of mGluRs mediating L-AP4 induced reduction of transmitter release, when administered pretraining i.c.v. (5 microliters of an 80 mM solution) did not affect motor activity in the open field test but did impair learning of both spatial tasks. In addition, swim speed was increased. However, injecting L-AP4 and MAP4 in combination at equimolar concentrations had no effect on learning in both spatial tasks or on swim speed in the water maze. Neither latency in the visible-platform test nor behavior in the open field was affected. We conclude that L-AP4 sensitive metabotropic glutamate receptors play a selective role in learning and memory formation of the rat.

Synergistic activation of protein kinase C by arachidonic acid and diacylglycerols in vitro: generation of a stable membrane-bound, cofactor-independent state of protein kinase C activity

The present study examines the synergistic activation of PKC by arachidonic acid and diacylglycerols in phospholipid vesicles and demonstrates that this combination of activators leads to the formation of a constitutively active, phospholipid-bound form of the enzyme. Activation of PKC was almost entirely calcium-dependent with vesicles containing dioleoylglycerol alone. In contrast, considerable calcium-independent activity was observed when vesicles contained both a diacylglycerol and free arachidonic acid. High-affinity association of enzyme activity with diacylglycerol-containing vesicles was calcium dependent and reversible. However, addition of arachidonic acid to diacylglycerol-containing vesicles resulted in irreversible PKC binding in the absence of calcium. Immunoblot analysis indicated that the calcium-independent binding was not isozyme-specific. The activity of the vesicle-associated PKC, bound to vesicles in the absence of calcium, was predominantly calcium-dependent. On the other hand, when the binding and isolation of vesicle-bound enzyme was conducted in the presence of calcium, the subsequent activity was almost entirely resistant to calcium chelation. This vesicle-associated form of the enzyme, when detergent extracted and recombined with phospholipid vesicles, maintained significant 'constitutive' activity (activity in the absence of both diacylglycerol and calcium). The data from this in vitro system provide the basis for a model of the physiological regulation of PKC in which the combined actions of arachidonate and diacylglycerol facilitate the stable formation of a tightly membrane-associated, intrinsically active form of PKC.

Presynaptic receptors and the control of glutamate exocytosis

When a typical glutamate-containing neurone fires, an action potential is propagated down the branching axon through more than a thousand varicosities. At each of these release sites the probability that a synaptic vesicle will be exocytosed into the synaptic cleft is individually controlled by means of presynaptic receptors: autoreceptors responding by positive or negative feedback to previously released transmitter, or heteroreceptors under the influence of other neurotransmitters or modulators. The simplest system in which to investigate presynaptic modulation is the isolated nerve terminal or synaptosome; studies with this preparation have revealed a complex interplay of signal-transduction pathways.

Characterization of protein kinase and phosphatase systems in chick ciliary ganglion

The aim of the present study was to characterize the second messenger activated protein kinase and phosphatase systems in chick ciliary ganglion using biochemical and immunochemical techniques. Using synthetic peptide substrates cyclic-AMP-, cyclic-GMP-, Ca2+/calmodulin- and Ca2+/phospholipid-dependent protein kinase activities were detected in homogenates of ciliary ganglion dissected from 15-16-day-old embryos. Autophosphorylation of the alpha and beta subunits of Ca2+/calmodulin-dependent protein kinase II in the presence of Ca2+/calmodulin or 5 mM ZnSO4 was detected by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and autoradiography. Protein kinase C was shown to be present using a monoclonal antibody. Two cyclic-AMP binding proteins whose molecular weights corresponded to the regulatory subunits of cyclic AMP-dependent protein kinase (RI and RII) were detected in ciliary ganglia using 8-azido-cyclic-AMP. The most heavily labelled band following incubation with [gamma-32P]ATP under most conditions had an apparent molecular weight of 65,000 which corresponds to the chicken form of myristoylated alanine-rich C kinase substrate, a known substrate of protein kinase C. Another substrate for protein kinase C was a 45,000 molecular weight protein which was tentatively identified as neuromodulin (B-50/GAP-43). Although no endogenous substrate proteins for cyclic-GMP-dependent protein kinase were detected, protein kinase A strongly labelled a 40,000 molecular weight protein. Using 32P(i)-labelled glycogen phosphorylase, protein phosphatases 1 and 2A were identified in ciliary ganglia homogenates at levels which were indistinguishable from forebrain at the same age. The major endogenous protein substrates in ciliary ganglion homogenates from 15-16-day-old embryos were also labelled to a similar extent in homogenates of ciliary ganglia from newly hatched chickens. Intact ciliary ganglia remained viable for several hours after dissection and, after incubation with 32P(i), responded to phorbol ester stimulation by an increased endogenous phosphorylation of several proteins, but especially myristoylated alanine-rich C kinase substrate. These results represent the first systematic characterization of the protein phosphorylation systems in chicken ciliary ganglion and provide a basis for future studies on the biochemical mechanisms responsible for regulating synaptic transmission in this tissue.

Pre-incubation of synaptosomes with arachidonic acid potentiates inhibition of [3H]D-aspartate transport

The ability of low micromolar concentrations of the polyunsaturated fatty acid, arachidonic acid (cis-5,8,11,14-eicosatetraenoic acid) to inhibit the high-affinity, sodium-dependent transport of [3H]D-aspartate into purified synaptosomes of rat brain has been examined. Pre-incubation of the synaptosomes with arachidonic acid for 10-60 min produced a marked potentiation of the response to 10 microM arachidonic acid compared to co-incubation, and the threshold for inhibition of [3H]D-aspartate transport occurred at a concentration of 1 microM. Minimal inhibition of transport was seen with the unsaturated fatty acids, cis-oleic (cis-9-octadecenoic acid) and cis-linolenic (cis-9,12,15-octadecatrienoic acid), nor with the 20-carbon saturated fatty acid, arachidic acid (n-eicosanoic acid). Inclusion of the cyclo-oxygenase inhibitor, nor-dihydroguaretic acid (NDGA), in the presence of 5 microM arachidonic acid did not alter the inhibition of [3H]D-aspartate transport between 0-10 min, but did enhance the response at longer pre-incubation times. Inhibition of [3H]D-aspartate transport by arachidonic acid persisted during addition of the calcium ionophore, A23187, whereas removal of calcium ions from the incubation medium potentiated the response to arachidonic acid. The results are discussed in terms of the physiological relevance of the inhibition of glutamate transport by arachidonic acid, and suggest that regulation of inhibition of the glutamate transporter by arachidonic acid may be achieved by changes in the extracellular, as well as the intracellular, concentration of calcium ions.

Synergistic activation by cis-fatty acid and diacylglycerol of protein kinase C and protein phosphorylation in hippocampal slices

cis-Unsaturated fatty acid, which activates protein kinase C in vitro, stimulates protein phosphorylation in intact hippocampal slices. Two protein bands (44,000 and 47,000 mol. wt) are particularly sensitive to cis-fatty acid and are phosphorylated in a dose- and time-dependent manner. The cis-fatty acid-stimulated protein phosphorylation can be further potentiated with diacylglycerol or 12-O-tetradecanoylphorbol 13-acetate. Several lines of evidence indicate that the cis-fatty acid-stimulated phosphorylation of these proteins is mediated by protein kinase C. First, the cis-fatty acid effect is mimicked by other protein kinase C activators such as diacylglycerol. Second, the stimulation of the phosphorylation by these activators can be blocked by staurosporine, which potently inhibits protein kinase C. Third, a concomitant application of cis-fatty acid and diacylglycerol or 12-O-tetradecanoylphorbol 13-acetate enhances the 44,000 and 47,000 mol. wt phosphorylation in a synergistic manner, which is a novel activation mode for protein kinase C. Fourth, they can be phosphorylated by purified protein kinase C (type III: alpha). Moreover, the synergistic activation of purified protein kinase C by cis-fatty acid and diacylglycerol leads to a drastic increase in the phosphorylation of these two protein bands. Two-dimensional gel electrophoresis and immunoblot analysis revealed that they are both acidic proteins. The 47,000 mol. wt band consists of two protein components; one is found to be F1/growth-associated protein-43 (pI = 4.5), and the other 47,000 mol, wt protein has broad pI ranging from 4.6 to 4.9. The 44,000 mol. wt component is a major phosphoprotein with pI of 4.8-5.1. Our results strongly indicate that cis-fatty acid can act as a regulator of endogenous protein kinase C in concert with diacylglycerol, and stimulate protein phosphorylation of its substrates such as F1/growth-associated protein-43 in the hippocampus.

NMDA and carbachol but not AMPA affect differently the release of [3H]GABA in striosome- and matrix-enriched areas of the rat striatum

The effects of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA; 10(-3) M), N-methyl-D-aspartate (10(-3) M, in the absence of magnesium or presence of AMPA) and carbachol (10(-3) M) on the release of preloaded [3H]gamma-aminobutyric acid ([3H]GABA) from microdiscs of tissue punched out from sagittal brain slices in striosome- or matrix-enriched areas of the rat striatum have been compared. Although AMPA stimulated similarly the release of [3H]GABA in both striatal compartments, the release of [3H]GABA evoked by either N-methyl-D-aspartate (in the presence of AMPA) or carbachol was more pronounced in matrix- than in striosome-enriched areas. AMPA- and N-methyl-D-aspartate- (in the absence of magnesium) evoked responses were reduced but not abolished in the presence of tetrodotoxin (10(-6) M) in both compartments while the carbachol-evoked release of [3H]GABA was decreased by tetrodotoxin only in the matrix. The interruption of cholinergic transmission by the combined application of atropine (10(-5) M) and pempidine (10(-4) M) was without effect on the AMPA-evoked release of [3H]GABA, but it reduced the N-methyl-D-aspartate- (in the absence of magnesium or presence of AMPA) evoked release of [3H]GABA in both compartments, these reductions being of similar amplitude than those observed with tetrodotoxin.

Effects of temperature and dietary n-3 and n-6 fatty acids on endocytic processes in isolated rainbow trout (Oncorhynchus mykiss, Walbaum) hepatocytes

Effects of different incubation temperatures (2, 8, 14 and 20°C) and hepatocyte membrane fatty acid composition on the rate of internalization and lysosomal degradation of the ligand, mannosylated albumin, that is taken up by receptor-mediated endocytosis, were investigated in rainbow trout (Oncorhynchus mykiss, Walbaum). The fish were kept at a water temperature ranging from 9 to 14°C and fed pelleted diets coated with either capelin oil (control), EPA/DHA-concentrate (rich in n-3 polyunsaturated fatty acids) or soybean oil (rich in n-6 unsaturated fatty acids) for at least 3 months prior to sampling. The endocytic uptake mediated by the mannose receptor was very efficient at all temperatures studied. Lysosomal degradation, on the other hand, came to a halt below 8°C. The activation energies for uptake and degradation were 54.6 and 164.2 kJ/mol respectively. No negative effects of increased amounts of either n-3 or n-6 fatty acids were observed on the endocytic parameters studied. On the contrary, multivariate analysis indicated a positive relationship between high levels of n-6 fatty acids and low unsaturation index in the phosphatidylcholine (PC) fraction of the hepatocytes and the internalization rate of 2°C, meaning that the rate of receptor-mediated endocytosis may be affected by membrane fatty acid composition.

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

Both the activation of protein kinase C (PKC) and the facilitation of depolarization-evoked glutamate release have been implicated in the induction of hippocampal long-term synaptic potentiation. These observations may be functionally related, since stimulation of PKC activity enhances evoked glutamate release. Recently, it was shown that arachidonic acid and the diacylglycerol analog oleoyl-acetyl-glycerol activate brain PKC in a synergistic fashion. We report the facilitation of depolarization-induced glutamate efflux from hippocampal mossy fiber synaptosomes due to a combination of arachidonic acid and oleoyl-acetyl-glycerol. The potentiating effects appeared to depend on the activation of PKC since they were attenuated by staurosporine. In addition, the effects of arachidonic acid and oleoyl-acetyl-glycerol appeared to be limited to calcium-dependent processes.