Release of neurotransmitter amino acids from synaptosomes: enhancement of calcium-independent efflux by oleic and arachidonic acids

Non-cell-autonomous OTX2 transcription factor regulates anxiety-related behavior in the mouse

The OTX2 homeoprotein transcription factor is expressed in the dopaminergic neurons of the ventral tegmental area, which projects to limbic structures controlling complex behaviors. OTX2 is also produced in choroid plexus epithelium, from which it is secreted into cerebrospinal fluid and transferred to limbic structure parvalbumin interneurons. Previously, adult male mice subjected to early-life stress were found susceptible to anxiety-like behaviors, with accompanying OTX2 expression changes in ventral tegmental area or choroid plexus. Here, we investigated the consequences of reduced OTX2 levels in Otx2 heterozygote mice, as well as in Otx2^+/AA and scFvOtx2^tg/0 mouse models for decreasing OTX2 transfer from choroid plexus to parvalbumin interneurons. Both male and female adult mice show anxiolysis-like phenotypes in all three models. In Otx2 heterozygote mice, we observed no changes in dopaminergic neuron numbers and morphology in ventral tegmental area, nor in their metabolic output and projections to target structures. However, we found reduced expression of parvalbumin in medial prefrontal cortex, which could be rescued in part by adult overexpression of Otx2 specifically in choroid plexus, resulting in increased anxiety-like behavior. Taken together, OTX2 synthesis by the choroid plexus followed by its secretion into the cerebrospinal fluid is an important regulator of anxiety-related phenotypes in the mouse.

Relationship between protein phosphatase type-2C activity and induction of apoptosis in cultured neuronal cells

The cellular composition and concentration of fatty acids are crucial for proliferation and survival. We recently showed stimulation of protein phosphatase type-2C (PP2C) by unsaturated fatty acids. Here, we describe that treatment of cultured chick neurons with 100 microM oleic acid for 24h increased the percentage of damaged neurons to 61+/-9% compared with 25+/-4% in controls. Oleic acid-induced cell death showed features of apoptosis such as chromatin condensation, shrinkage of the nucleus, DNA fragmentation and caspase-3 activation. Extensive studies with a variety of fatty acids revealed a striking correlation between activation of PP2C and induction of apoptosis. Lipophilicity, oxidizability, and an acidic group were required for both effects. In addition, activation of PP2C and induction of apoptosis could discriminate between cis- and trans-conformation of the fatty acids. The results are in favor of PP2C playing an important, yet unidentified role in apoptosis.

Role of excitatory aminoacids in neonatal hypoglycemia

BACKGROUND

In many neurological disorders, injury to neurons may be due in part to overstimulation of the receptors for the excitatory amino acids glutamate and aspartate. The same excitotoxic mechanism and high aspartate levels in experimental studies led to this study of the concentrations of glutamate and aspartate and zinc, copper, and magnesium levels in the cerebrospinal fluid (CSF) of hypoglycemic newborns.

METHODS

Aspartate and glutamate were determined by high-performance liquid chromatography, and magnesium, zinc and copper by atomic absorption spectrophotometer.

RESULTS

The CSF levels of aspartate (3.98 +/- 1.77 mumol/L) and glutamate (1.7 +/- 1.05 mumol/L) in 20 hypoglycemic newborns were significantly higher when compared with the values of aspartate (2.19 +/- 0.6 mumol/L) and glutamate (0.77 +/- 0.34 mumol/L) of 10 control newborns. In the hypoglycemic patients, the concentration of zinc (0.57 +/- 0.13 microgram/mL), but not copper (0.39 +/- 0.40 microgram/mL) was significantly lower when compared with the control values. There was no difference in the magnesium levels between the two groups.

CONCLUSIONS

The higher levels of excitatory amino acids found in the CSF of hypoglycemic infants than in controls were consistent with previous animal studies, which may indicate the role of excitatory amino acids in the late biochemical effects of hypoglycemia in newborn brain metabolism.

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.

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.

Secondary elevation of extracellular neurotransmitter amino acids in the reperfusion phase following focal cerebral ischemia

The purpose of this study was to evaluate amino acid neurotransmitter dynamics in the reperfusion phase after transient cerebral ischemia. In vivo microdialysis was used to measure extracellular amino acid levels in a rabbit model of focal ischemia. During 30 min of transient ischemia (n = 5), small but significant (p < 0.05) increases in glutamate, aspartate, gamma-aminobutyric acid (GABA), and taurine were noted. These elevations rapidly returned to baseline levels upon recirculation and remained constant for up to 5.5 h of reperfusion. In rabbits subjected to 2 h of transient ischemia (n = 5), two phases of amino acid release were seen. During ischemia, large (5- to 50-fold) elevations in glutamate, aspartate, GABA, and taurine occurred, as expected. These elevations rapidly normalized upon unocclusion. However, significant (p < 0.05) secondary elevations in glutamate, aspartate, and GABA occurred after 2-4 h of reperfusion. Regression analysis demonstrated significant correlations between primary (ischemic) and secondary (reperfusion) efflux. In permanent ischemia (n = 5), amino acid levels remained elevated throughout the entire experiment. Secondary elevations in excitatory amino acids may further contribute to the excitotoxic cascade during reperfusion.

Metabotropic glutamate receptors, transmitter output and fatty acids: studies in rat brain slices

1. The effects of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), a non-selective agonist of the metabotropic glutamate receptors (mGluRs), have been studied in rat cortical and striatal slices by measuring the depolarization-induced output of D-[3H]-aspartate (D-[3H]-Asp) and of [3H]-glutamate ([3H]-Glu), neosynthesized from [3H]-glutamine. 2. In cortical slices, 1S,3R-ACPD potentiated the depolarization-induced (KCl, 30 mM) output of both D-[3H]-Asp and [3H]-Glu. The potentiation, obtained at 300 microM 1S,3R-ACPD was 65 +/- 6% for D-[3H]-Asp and 56 +/- 10% for [3H]-Glu. Conversely, in striatal slices, 1S,3R-ACPD reduced the depolarization-induced transmitter output. The reduction, obtained at 300 microM of the agonist, was 60 +/- 8% for D-[3H]-Asp and 50 +/- 5% for neosynthesized [3H]-Glu. 3. Bovine serum albumin (BSA, 15 microM), which is able to bind locally produced fatty acids, completely eliminated the potentiating effect 1S,3R-ACPD had on D-[3H]-Asp output from cortical slices. Low concentrations of arachidonic acid (1-10 microM) or of oleic acid (1-10 microM) added to BSA-containing perfusion medium, restored this potentiating effect. BSA, however, had no effect on the inhibitory action of 1S,3R-ACPD in striatal slices. 4. Bromophenacyl bromide (100 microM), an inhibitor of phospholipase A2, and RG80267 (100 microM), an inhibitor of diacylglycerol lipase, have been shown to inhibit fatty acid production. These compounds prevented the potentiating effect of 1S,3R-ACPD on D-[3H]-Asp-output in cortical slices. 5. Indomethacin (100 microM), an inhibitor of cyclo-oxygenases, plus nordihydroguaiaretic acid (100 microM), an inhibitor of lipoxygenases, increased D-[3H]-Asp output in cortical slices perfused with BSA-containing medium. 6. These experiments suggest that the mGluR-mediated potentiation of transmitter output requires the availability of unsaturated fatty acids, such as arachidonic or oleic acids, in cortical slices. In contrast, the mGluR-induced inhibition of transmitter output is not dependent upon fatty acid availability in striatal slices. The requirement of both unsaturated fatty acids and 1S,3R-ACPD in the facilitation of transmitter exocytosis may play an important role in the regulation of synaptic plasticity.

Altered lipid metabolism in the presence and absence of extracellular Ca2+ during combined oxygen-glucose deprivation in primary astrocyte cultures

The effect of combined oxygen-glucose deprivation (COGD) on lipid metabolism in primary rat cortical astrocyte cultures was studied in both the presence and absence of extracellular Ca2+. In this study, increases in intracellular Ca2+ from internal Ca2+ stores were not inhibited nor were internal Ca2+ levels buffered. Combined oxygen-glucose deprivation resulted in a quantitative reduction in phospholipid levels and an increase in free fatty acid and lysophospholipid levels. Four hours after the onset of COGD, ethanolamine- and choline glycerophospholipid levels were decreased by 40 and 46% from control levels in the presence of Ca2+, respectively. A similar decrease was found 6 hr after onset of COGD in the absence of Ca2+. These changes were accompanied by elevated levels of the corresponding lysophospholipids. However, the increases in lysophospholipid content did not account for the entire loss of ethanolamine- or choline glycerophospholipid. Phosphatidylserine was reduced in both the presence and absence of extracellular Ca2+ but phosphatidylinositol was only decreased in the absence of Ca2+. Statistically significant increases in total fatty acid (FA) and polyunsaturated fatty acid (PUFA) levels occurred at 30 min and 3 hr after the onset of COGD in the absence and presence of Ca2+, respectively. Arachidonic acid levels were increased in both groups by 1 hr. These increases in FA, PUFA, and specifically arachidonic acid were time-dependent and increased over the 12 hr of COGD. Collectively, these results indicate the activation of an acylhydrolase mechanism in the possible presence of an inhibited reacylation pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Nordihydroguaiaretic acid and RHC 80267 potentiate astroglial injury during combined glucose-oxygen deprivation

Membrane phospholipid degradation has been proposed to play a key role in hypoxic-ischemic brain injury. We tested the hypotheses that both nordihydroguaiaretic acid, a phospholipase A2 and lipoxygenase inhibitor, and RHC 80267, a diacylglycerol lipase inhibitor, would decrease the release of [3H]arachidonic acid metabolites from prelabeled cultures of astroglia subjected to combined glucose-oxygen deprivation and that these inhibitors would also decrease astroglial injury during combined glucose-oxygen deprivation. Both nordihydroguaiaretic acid and RHC 80267 significantly inhibited the release of [3H]arachidonic acid metabolites during combined glucose-oxygen deprivation. This suggests that two separate enzymic pathways, the phospholipase A2 pathway and the phospholipase C/diacylglycerol lipase pathway, contribute to the release of astroglial [3H]arachidonic acid metabolites during combined glucose-oxygen deprivation. However, both of these lipase inhibitors increased astroglial cell death during combined glucose-oxygen deprivation, probably due to inhibition of arachidonic acid release. We speculate that arachidonic acid release may be a mechanism of astroglial self-preservation during combined glucose-oxygen deprivation.

Effect of arachidonic acid on [3H]D-aspartate outflow in the rat hippocampus

The aim of this study was to investigate the effect of arachidonic acid on [3H]d-aspartate outflow in rat hippocampus synaptosomes and slices. Arachidonic acid 1) increased basal outflow of [3H]d-aspartate in both synaptosomes and slices, and 2) increased K(+)-evoked overflow in slices but not in synaptosomes. The latter effect was dependent (at least in part) on arachidonic acid metabolism, most likely mediated by lipo-oxygenase metabolites and free radical production. It was prevented by nordihydroguairetic acid but not by indomethacin, and was significantly reduced by free radical scavengers (superoxide-dismutase and catalase). This effect was dependent upon stimulation since it could not be observed after a continuous perfusion of arachidonic acid in the absence of stimulation. Furthermore, it was long-lasting since a 30 min perfusion of arachidonic acid was sufficient to exert a significant effect on a stimulation following termination of the application.

The NMDA receptor complex

The synaptic responses elicited by glutamate and aspartate in the CNS are mediated by distinct groups of receptors which include the ionotropic NMDA receptor. The NMDA receptor is activated by high-strength synaptic input and produces relatively sustained depolarization which can lead to repetitive burst firing. These characteristics allow it to be involved in the maintainance of rhythmic neuronal activity and in the modulation of synaptic efficacy and plasticity. Overstimulation of the NMDA receptor appears to play a pivotal role in the physiopathology of ischemic brain injury. The NMDA receptor contains an integral cationic channel which is highly permeable to Ca2+ as well as to Na+ and K+. This receptor has several domains in addition to the NMDA recognition site: i) a divalent cation binding site within the channel pore, at which Mg2+ ions bind, ii) a binding site recognized by dissociative anesthetics and MK-801 within the channel; and iii) modulatory sites sensitive to glycine, Zn2+ and polyamines. The NMDA receptor is strictly controlled by Mg2+ ions in a voltage-dependent manner. Moreover, it is modulated by protons, by changes in the redox state and by endogenous physiological substances, eg NO and arachidonic acid. Selective antagonists now exist for the NMDA recognition site and glycine and polyamine modulatory sites. Molecular cloning of the NMDA receptor has identified a subunit termed NMDA-R1 and four additional subunits (NMDA-R2A through NMDA-R2D). Functionally distinct NMDA receptor subtypes are formed by heteromeric assembly of NMDA-R1 with NMDA-R2 subunits. NMDA receptor subunits contain consensus phosphorylation sites for protein kinases at the cytoplasmic domain. The high Ca2+ permeability and sensitivity to channel block by Mg2+ are imparted by asparagine residues in a putative channel-forming segment of the protein transmembrane 2. The knowledge of the molecular structure of the NMDA receptor will help to understand the molecular mechanisms responsible for its regulatory features and the molecular bases of neurotoxicity.

Membrane lipid peroxidation induces changes in gamma-[3H]aminobutyric acid transport and calcium uptake by synaptosomes

In the present study, we analyze the effect of Fe2+/ascorbate-induced lipid peroxidation on Ca(2+)-dependent and Ca(2+)-independent release and on the uptake of gamma-[3H]aminobutyric acid (GABA) by sheep brain synaptosomes. In addition, we study the effect of lipid peroxidation on the levels of cytosolic calcium and on the uptake of calcium (45Ca2+). After membrane lipid peroxidation, a decrease in the uptake of GABA is observed. After ascorbate/Fe(2+)-induced membrane lipid peroxidation, a significant decrease in [3H]GABA release in response to K(+)-depolarization occurs, in the absence and in the presence of Ca2+. The influx of 45Ca2+ induced by K(+)-depolarization is significantly depressed under peroxidative conditions, while basal calcium uptake is inhibited to a much lesser degree. The levels of free ionic calcium [Ca2+]i, as determined by the fluorescent dye Indo-1, are increased after synaptosomes were submitted to the ascorbate/Fe2+ oxidative stress. It is concluded that membrane lipid peroxidation induces a decrease in Ca(2+)-dependent and Ca(2+)-independent efflux of accumulated [3H]GABA in response to elevated K+ pulses (60 mM) and in the depolarization-induced calcium influx, while free ionic calcium levels increase. The Ca(2+)-dependent efflux is interpreted to reflect stimulus-secretion coupling process and the Ca(2+)-independent efflux may reflect membrane transport processes. Thus, the results suggest a possible relationship between a reduced calcium movement across the membrane, the decrease in neurotransmitters uptake and release and oxidative stress.

Prostaglandin F2 alpha synthesis in the hippocampal mossy fiber synaptosomal preparation: I. Dependence in arachidonic acid, phospholipase A2, calcium availability and membrane depolarization

Isolated hippocampal mossy fiber synaptosomes were used to characterize control mechanisms of prostaglandin F2 alpha (PGF2 alpha) synthesis at a central mammalian synapse. Exogenous arachidonic acid stimulated the dose-dependent synthesis of PGF2 alpha, as did the addition of phospholipase A2 or the activation of endogenous phospholipase A2. Phospholipase A2 inhibitors attenuated prostaglandin synthesis, but phospholipase C inhibitors had no effect. However, a diglyceride kinase inhibitor reduced PGF2 alpha accumulation. The cyclooxygenase inhibitor ibuprofen eliminated PGF2 alpha production, while the lipoxygenase inhibitors baicalein and NDGA reduced PGF2 alpha accumulation. The CA(2+)-ionophore-dependent stimulation of PGF2 alpha synthesis was abolished by Cd2+ or Ni2+. Further more, PGF2 alpha production appeared to be dependent on Ca2+ influx via L-type, but not N- or T-type, voltage-sensitive Ca2+ channels. Membrane depolarization with KC1, veratridine or 4-aminopyridine stimulated the synthesis of PGF2 alpha. This depolarization-dependent stimulation of PGF2 alpha synthesis was attenuated by L-type voltage-sensitive Ca2+ channel blockers, phospholipase A2 inhibitors, a K+ channel activator and a Na+ channel blocker. The activation of protein kinase C also led to a reduction of PGF2 alpha accumulation in depolarized nerve endings. These results may be used to suggest that PGF2 alpha production by hippocampal mossy fiber synaptosomes was controlled by the Ca(2+)- and phospholipase A2-dependent accumulation of unesterified arachidonic acid and was modulated by membrane depolarization and the activity of protein kinase C.

Modulation of Glutamate Release From Hippocampal Mossy Fiber Nerve Endings By Arachidonic Acid And Eicosanoids

Arachidonic acid has been implicated in normal synaptic transmission processes, including those related to the development of hippocampal long-term synaptic potentiation. Hippocampal mossy fiber (MF) synaptosomes were used to investigate the role of arachidonate in the evoked accumulation of presynaptic Ca2+ and the release of endogenous glutamate, since these nerve terminals express long-term potentiation and selectively release glutamate as the excitatory transmitter. It was demonstrated that membrane depolarization evoked the accumulation of Ca2+, the release of glutamate, and the production of unesterified arachidonic acid. These events may be functionally related, since exogenous arachidonate and phospholipase A2 activation mimicked the effects of depolarization on Ca2+ availability and glutamate release, while secretion processes were attenuated in the presence of phospholipase A2 inhibitors. In addition, pretreatment of the nerve terminals with arachidonate or melittin allowed for the facilitated release of glutamate in response to a subsequent depolarizing stimulus. Inhibition of cyclooxygenase or lipoxygenase activities also potentiated presynaptic responses to membrane depolarization. In contrast, 12-lipoxygenase products attenuated the depolarization-evoked accumulation of intraterminal free Ca2+ and glutamate release. It is suggested that arachidonic acid acts as a positive modulator of mossy fiber secretion processes, including those involved in the increased glutamate release required for the induction of long-term potentiation, while 12-lipoxygenase metabolites provide negative feedback signals designed to limit neurotransmitter secretion.

Inhibition of sodium-potassium-ATPase: a potentially ubiquitous mechanism contributing to central nervous system neuropathology

Direct and indirect evidence suggests that Na+/K(+)-ATPase activity is reduced or insufficient to maintain ionic balances during and immediately after episodes of ischemia, hypoglycemia, epilepsy, and after administration of excitotoxins (glutamate agonists). Recent results show that inhibition of this enzyme results in neuronal death, and thus a hypothesis is proposed that a reduction and/or inhibition of this enzyme contributes to producing the central neuropathy found in the above disorders, and identifies potential mechanisms involved. While the extent of inhibition of Na+/K(+)-ATPase during ischemia, hypoglycemia and epilepsy may be insufficient to cause neuronal death by itself, unless the inhibition is severe and prolonged, there are a number of interactions which can lead to a potentiation of the neurotoxic actions of glutamate, a prime candidate for causing part of the damage following trauma. Presynaptically, inhibition of the Na+/K(+)-ATPase destroys the sodium gradient which drives the uptake of acidic amino acids and a number of other neurotransmitters. This results in both a block of reuptake and a stimulation of the release not only of glutamate but also of other neurotransmitters which modulate the neurotoxicity of glutamate. An exocytotic release of glutamate can also occur as inhibition of the enzyme causes depolarization of the membrane, but exocytosis is only possible when ATP levels are sufficiently high. Postsynaptically, the depolarization could alleviate the magnesium block of NMDA receptors, a major mechanism for glutamate-induced neurotoxicity, while massive depolarization results in seizure activity. With less severe inhibition, the retention of sodium results in osmotic swelling and possible cellular lysis. A build-up of intracellular calcium also occurs via voltage-gated calcium channels following depolarization and as a consequence of a failure of the sodium-calcium exchange system, maintained by the sodium gradient.

The role of protein kinase C and its neuronal substrates dephosphin, B-50, and MARCKS in neurotransmitter release

This article focuses on the role of protein phosphorylation, especially that mediated by protein kinase C (PKC), in neurotransmitter release. In the first part of the article, the evidence linking PKC activation to neurotransmitter release is evaluated. Neurotransmitter release can be elicited in at least two manners that may involve distinct mechanisms: Evoked release is stimulated by calcium influx following chemical or electrical depolarization, whereas enhanced release is stimulated by direct application of phorbol ester or fatty acid activators of PKC. A markedly distinct sensitivity of the two pathways to PKC inhibitors or to PKC downregulation suggests that only enhanced release is directly PKC-mediated. In the second part of the article, a framework is provided for understanding the complex and apparently contrasting effects of PKC inhibitors. A model is proposed whereby the site of interaction of a PKC inhibitor with the enzyme dictates the apparent potency of the inhibitor, since the multiple activators also interact with these distinct sites on the enzyme. Appropriate PKC inhibitors can now be selected on the basis of both the PKC activator used and the site of inhibitor interaction with PKC. In the third part of the article, the known nerve terminal substrates of PKC are examined. Only four have been identified, tyrosine hydroxylase, MARCKS, B-50, and dephosphin, and the latter two may be associated with neurotransmitter release. Phosphorylation of the first three of these proteins by PKC accompanies release. B-50 may be associated with evoked release since antibodies delivered into permeabilized synaptosomes block evoked, but not enhanced release. Dephosphin and its PKC phosphorylation may also be associated with evoked release, but in a unique manner. Dephosphin is a phosphoprotein concentrated in nerve terminals, which, upon stimulation of release, is rapidly dephosphorylated by a calcium-stimulated phosphatase (possibly calcineurin [CN]). Upon termination of the rise in intracellular calcium, dephosphin is phosphorylated by PKC. A priming model of neurotransmitter release is proposed where PKC-mediated phosphorylation of such a protein is an obligatory step that primes the release apparatus, in preparation for a calcium influx signal. Protein dephosphorylation may therefore be as important as protein phosphorylation in neurotransmitter release.

Glutamate exocytosis evoked by 4-aminopyridine is inhibited by free fatty acids released from rat cerebrocortical synaptosomes

The Ca(2+)-dependent release of glutamate induced by 4-aminopyridine (4-AP) in rat cerebral cortical synaptosomes was reduced by removal of bovine serum albumin (BSA) from the incubation medium. The decrease in the glutamate release in the absence of BSA was consistent with a reduction in the rise in cytosolic free [Ca2+] after depolarization with 4-AP. Contrarily, neither the glutamate release nor the elevation in cytosolic free [Ca2+] after depolarization with 30 mM KCl was altered by the removal of BSA. The inhibitory action of the free fatty acids released during the incubation of synaptosomes was also observed when exogenous free fatty acids were added to the medium in the presence of BSA. The highest inhibition of 4-AP-induced release of glutamate was observed in the presence of arachidonic acid. The results strongly suggest an inhibitory action of free fatty acids by decreasing Ca2+ entry and glutamate release in rat cerebrocortical synaptosomes.

PGF2 alpha synthesis in isolated cerebellar glomeruli: effects of membrane depolarization, calcium availability and phospholipase activity

The controlling factors for PGF2 alpha production were assessed in isolated cerebellar glomeruli, since this prostaglandin has been shown to stimulate the release of neurotransmitters from the mossy fiber terminals associated with this synaptic preparation. The metabolism of PGE2 was also examined, in order to determine the specificity of any treatment effects. It was observed that K(+)-dependent membrane depolarization or the activation of voltage-sensitive Na+ channels with veratradine stimulated the production of PGF2 alpha. The syntheses of both prostanoids were dependent on available calcium and were blocked by cyclooxygenase inhibitors. The lipoxygenase inhibitor NDGA also reduced the accumulation of PGE2 and PGF2 alpha. In addition, PGF2 alpha synthesis was stimulated by the phospholipase A2 activator melittin and was reduced due to phospholipase inhibition with dibucaine. These results are consistent with a role for PGF2 alpha in the evoked release of neurotransmitter from cerebellar mossy fiber terminals.

Lithium ion does not protect brain against transient ischemia in gerbils

It has been proposed that lithium ion desensitizes neuronal receptors that function via the inositol phospholipid signaling mechanism. We examined the effects of lithium chloride on the morphologic outcome after 5 minutes of cerebral ischemia induced in gerbils by occluding both common carotid arteries under brief halothane anesthesia. In three treated groups of 10 gerbils each, 5 meq/kg i.p. lithium chloride was given 2 days, 1 day, and 2 hours before ischemia; 2 hours before ischemia; or immediately after the end of ischemia. Corresponding control groups of nine or 10 gerbils each received equivalent volumes of saline injected at comparable times. All gerbils were perfusion-fixed 1 week later, and neuronal density of the hippocampal CA1 pyramidal cells was determined. Lithium induced very mild intraischemic systemic hypothermia, but postischemic hyperthermia developed in both treated and control groups. Neuronal densities were equal in corresponding groups. The results indicate that our regimen of lithium administration provides no benefit in survival of hippocampal neurons, and intraischemic hypothermia of less than 0.8 degrees C is not protective. Other strategies to inactivate the signal transduction system that is specific for excitatory neurotransmission should be evaluated.

Arachidonic acid inhibits glycine transport in cultured glial cells

The effects of arachidonic acid on glycine uptake, exchange and efflux in C6 glioma cells were investigated. Arachidonic acid produced a dose-dependent inhibition of high-affinity glycine uptake. This effect was not due to a simple detergent-like action on membranes, as the inhibition of glycine transport was most pronounced with cis-unsaturated long-chain fatty acids, whereas saturated and trans-unsaturated fatty acids had relatively little or no effect. Endogenous unsaturated non-esterified fatty acids may exert a similar inhibitory effect on the transport of glycine. The mechanism for this inhibitory effect has been examined in a plasma membrane vesicle preparation derived from C6 cells, which avoids metabolic or compartmentation interferences. The results suggest that part of the selective inhibition of glycine transport by arachidonic acid could be due to the effects of the arachidonic acid on the lipid domain surrounding the carrier.

Arachidonic acid metabolism in gerbil cerebra: effects of ischemia and pentobarbital

Pentobarbital pretreatment may be used to predict biochemical events involved in ischemic brain damage following bilateral carotid artery ligations in the gerbil, since it reduces the subsequent edema and mortality. The effects of this anesthetic on the ischemia-induced modifications of cerebral arachidonic acid metabolism were investigated, in order to correlate observed alterations with tissue damage. Cerebral lipids were radiolabeled in vivo with [3H]arachidonic acid prior to 10 min of cerebral ischemia and 0-120 min of perfusion. Ischemia stimulated a 97.3% increase in unesterified [3H]arachidonate, which was due to the loss of label from choline, inositol, and ethanolamine glycerophospholipids. Tissue reperfusion stimulated further reductions in [3H]choline and [3H]inositol glycerophospholipids, while ethanolamine glycerophospholipid and triglyceride labeling increased. Inositol glycerophospholipid, but not choline glycerophospholipid, labeling returned to control level by 60 min of reperfusion. Pentobarbital pretreatment reduced the accumulation of [3H]arachidonate by 56.2% during ischemia. It increased the recovery of [3H]ethanolamine glycerophospholipids during the ischemic period and [3H]choline glycerophospholipids during the first 5 min of reperfusion. These effects accounted for the reduction of unesterified [3H]arachidonate observed during ischemia and reperfusion.

Arachidonic acid increases inositol phospholipid metabolism and glutamate release in synaptosomes prepared from hippocampal tissue

We have been interested in the possibility that arachidonic acid or one of its 12-lipoxygenase metabolites may function as a retrograde messenger in long-term potentiation (LTP) in the dentate gyrus of the hippocampus. One criterion required of a retrograde messenger is that it stimulates presynaptic changes. Here, two possible presynaptic actions of arachidonic acid and its 12-lipoxygenase metabolites, 12-hydroxyeicosatetraenoic acid (HETE) and 12-hydroperoxyeicosatetraenoic acid (HPETE), are examined. We report that arachidonic acid, HETE, and HPETE significantly increase both K(+)-stimulated release of [3H]glutamate and [3H]inositol labelling of inositol phosphates in synaptosomes, whereas other biologically important fatty acids (oleic, palmitic, and stearic) failed to induce a similar response. The findings of these experiments are consistent with the hypothesis that arachidonic acid, HETE, or HPETE may play the role of a retrograde messenger in LTP.

Presynaptic facilitation of glutamate release from isolated hippocampal mossy fiber nerve endings by arachidonic acid

Hippocampal mossy fiber synaptosomes were used to investigate the role of arachidonic acid in the release of endogenous glutamate and the long-lasting facilitation of glutamate release associated with long-term potentiation. Exogenous arachidonate induced a dose-dependent efflux of glutamate from the hippocampal mossy fiber synaptosomes and this effect was mimicked by melittin. Neither treatment induced the release of occluded lactate dehydrogenase at the concentrations used in these experiments. In each case, removal of the biochemical stimulus allowed for glutamate efflux to return to spontaneous levels. However, there was a persistent effect of exposure to either arachidonate or melittin, since these compounds facilitated the glutamate release induced by the subsequent addition of 35 mM KCl. This facilitation of glutamate release resulted from an enhancement of both the magnitude and duration of the response to depolarization. Although exogenous prostanoids were also able to stimulate the release of glutamate, they appeared to play no direct role in secretion processes, since inhibition of eicosanoid synthesis potentiated the glutamate efflux in response to membrane depolarization or exogenous arachidonic acid. We suggest that the calcium-dependent accumulation of arachidonic acid in presynaptic membranes plays a central role in the release of endogenous glutamate and that the persistent effects of arachidonic acid may be related to the maintenance of long-term potentiation in the hippocampal mossy fiber-CA3 synapse.

Calcium-dependent phospholipid catabolism and arachidonic acid mobilization in cerebral minces

Cerebral minces were used to investigate the role of calcium influx on trauma-induced alterations of brain lipid metabolism. Cerebral phospholipids, nonpolar lipids, and free fatty acids were radiolabeled in vivo with [3H]arachidonic acid. Tissue incubation stimulated the time-dependent catabolism of choline and inositol glycerophospholipids, and resulted in the accumulation of [3H]free fatty acids. These effects were attenuated in Ca2(+)-free incubations, and when EGTA or verapamil were present. The inhibition of calcium influx also reduced the labeling of diglycerides, whereas ethanolamine and serine glycerophospholipids were not affected by incubation or treatments. Replacing Ca2+ with other cations also attenuated the incubation-dependent alterations in lipid metabolism. However, only cadmium was able to compete with calcium and reduce the accumulation of [3H]free fatty acids. It appeared that about half of the observed phospholipid catabolism was dependent on Ca2+ influx and that at least 80% of the [3H]free fatty acid accumulation required calcium.

NMDA receptors activate the arachidonic acid cascade system in striatal neurons

Receptors for excitatory amino-acid transmitters on nerve cells fall into two main categories associated with non-selective cationic channels, the NMDA (N-methyl-D-aspartate) and non-NMDA (kainate and quisqualate) receptors. Special properties of NMDA receptors such as their voltage-dependent blockade by Mg2+ (refs 3, 4) and their permeability to Na+, K+ as well as to Ca2+ (refs 5, 6), have led to the suggestion that these receptors are important in plasticity during development and learning. They have been implicated in long-term potentiation (LTP), a model for the study of the cellular mechanisms of learning. We report here that glutamate and NMDA, acting at typical NMDA receptors, stimulate the release of arachidonic acid (as well as 11- and 12-hydroxyeicosatetraenoic acids from striatal neurons probably by stimulation of a Ca2+-dependent phospholipase A2. Kainate and quisqualate, as well as K+-induced depolarization were ineffective. Our results provide direct evidence in favour of the hypothesis, that arachidonic acid derivatives, produced by activation of the postsynaptic cell, could be messengers that cross the synaptic cleft to modify the presynaptic functions known to be altered during LTP. In addition, we suggest that NMDA receptors are the postsynaptic receptors which trigger the synthesis of these putative transynaptic messengers.

Depolarization-induced [3H]arachidonic acid accumulation: effects of external Ca2+ and phospholipase inhibitors

Isolated cerebellar glomeruli were prelabeled with [3H]arachidonate prior to assessment of the roles of external Ca2+ and phospholipases in the depolarization-induced accumulation of unesterified arachidonate. The glomerular particles have previously been shown to release neurotransmitters upon exposure to depolarizing conditions, calcium influx, exogenous arachidonate and added prostaglandins. It was observed that membrane depolarization caused an increased accumulation of [3H]arachidonate, which was inhibited by EGTA, verapamil or the lipase inhibitors mepacrine and dibucaine. The major effect of EGTA was expressed on the catabolism of [3H]triglycerides, while verapamil prevented the loss of radioactivity from inositol glycerophospholipids and the lipase inhibitors reduced by triglyceride and inositol glycerophospholipid catabolism.

[3H]arachidonic acid metabolism in rat brain minces: effects of nucleotide triphosphates, CDPcholine and CMP

Rat brain minces were used to investigate the effects of nucleotides on the metabolism of arachidonic acid in nerve tissue. Brain free fatty acids, neutral lipids and phospholipids, were radiolabeled in vivo following intracerebral injection of [3H]arachidonic acid. Minces were prepared from the radiolabeled cerebra and were incubated in a modified Krebs-Ringer buffer with and without various nucleotides. The incubation-induced accumulation of unesterified [3H]arachidonate was reduced in the presence of CDPcholine, ATP, CTP, GTP, and UTP. These nucleotides inhibited choline and inositol glycerophospholipid hydrolysis. They also reduced the amount of labeled diglycerides. However, CDPethanolamine had no effect on arachidonic acid metabolism in the mince preparation and CMP appeared to stimulate further hydrolysis of choline glycerophopholipids, resulting in increased accumulation of [3H]arachidonic acid and labeled diglycerides. We suggest that the production of unesterified [3H]arachidonate and labeled diglycerides is due to the involvement of more than one catabolic reaction, since the high energy nucleotides had similar effects on fatty acid accumulation, but different effects on phospholipid labeling.

Depolarization-induced release of glycine and beta-alanine from plasma membrane vesicles derived from rat brain synaptosomes

Glycine and beta-alanine actively loaded into brain synaptic plasma membrane vesicles were released into the external medium by using the classical depolarization agents high K+ and veratridine. This release occurs via a Ca2+-independent process. Measurements of membrane depolarization using tetraphenylphosphonium uptake show a close correlation between changes in the membrane potential and stimulation of the efflux process. Results shown herein and previously reported by our group (Aragón, M.C. and Giménez, C. (1986) Biochim. Biophys. Acta 855, 257-264; Agulló, L., Jiménez, B., Aragón, M.C. and Giménez, C. (1986) Eur. J. Biochem. 159, 611-617), suggest that the glycine and beta-alanine transport systems in synaptic plasma membranes are susceptible of modulation by changes in ionic fluxes and hence in the membrane potential, similar to those occurring during depolarization and repolarization.

Modulation of calcium-dependent neutral protease activity by fatty acids and lysophospholipids

The effect of fatty acids and lysophospholipids on calcium-activated neutral protease (CANP) was investigated. mu CANP, low calcium ion (microM concentration)-requiring CANP is more strongly inhibited by unsaturated fatty acids than is mCANP--the high calcium ion (mM concentration)-requiring form. Lysophospholipids in concentrations ranging from 10(-5) M to 10(-3) M inhibit mu CANP exclusively, whereas mCANP activity is unaffected or even slightly increased. Calpastatin decreases the activity of mCANP and, in the presence of polyunsaturated fatty acids such as docosahexaenoic acid, the inhibition is not increased. In the presence of lysophosphatidyl-ethanolamine, however, the inhibition of mCANP by calpastatin does not occur. The results indicate that fatty acids and lysocompounds liberated under different physiological and pathological conditions may modulate calcium-activated neutral protease.

Arachidonic acid inhibits choline uptake and depletes acetylcholine content in rat cerebral cortical synaptosomes

The effects of arachidonic acid on [3H]choline uptake, on [3H]acetylcholine accumulation, and on endogenous acetylcholine content and release in rat cerebral cortical synaptosomes were investigated. Arachidonic acid (10-150 microM) produced a dose-dependent inhibition of high-affinity [3H]choline uptake. Low-affinity [3H]choline uptake was also inhibited by arachidonic acid. Fatty acids inhibited high-affinity [3H]choline uptake with the following order of potency: arachidonic greater than palmitoleic greater than oleic greater than lauric; stearic acid (up to 150 microM) had no effect. Inhibition of [3H]choline uptake by arachidonic acid was reversed by bovine serum albumin. In the presence of arachidonic acid, there was an increased accumulation of choline in the medium, but this did not account for the inhibition of [3H]choline uptake produced by the fatty acid. Arachidonic acid inhibited the synthesis of [3H]acetylcholine from [3H]choline, and this inhibition was equal in magnitude to the inhibition of high-affinity [3H]choline uptake produced by the fatty acid. A K+-stimulated increase in [3H]acetylcholine synthesis was inhibited completely by arachidonic acid. Arachidonic acid also depleted endogenous acetylcholine stores. Concentrations of arachidonic acid and hemicholinium-3 that produced equivalent inhibition of [3H]choline uptake also produced equivalent depletion of acetylcholine content. In the presence of eserine, arachidonic acid had no effect on acetylcholine release. The results suggest that arachidonic acid may deplete acetylcholine content by inhibiting high-affinity choline uptake and subsequent acetylcholine synthesis. This raises the possibility that arachidonic acid may play a role in the impairment of cholinergic transmission seen in cerebral ischemia and other conditions in which large amounts of the free fatty acid are released in brain.

Ischemia-induced development of cerebral edema in awake and anesthetized gerbils

General anesthesia is often used to immobilize experimental animals prior to the induction of cerebral ischemia. However, anesthetics are known to alter many of the biochemical and physiological parameters used for the assessment of stroke-induced brain damage. We examined the effects of bilateral carotid artery ligations on mortality and the development of cerebral edema in unanesthetized gerbils. We found that increasing the length of the ischemic episode resulted in increased mortality, both during the ischemic period and during cerebral reperfusion. The duration of the ischemic episode was also correlated with the rate and degree of the development of cerebral edema. Both of these estimates of ischemia-induced brain damage were significantly reduced by the pretreatment of the animals with pentobarbital. Based on the variable effects of different anesthetics on CNS activities, and the observed effects of barbiturate anesthesia on ischemia-induced mortality and edema development in the present model, we suggest that it may be inappropriate to anesthetize experimental animals when investigating certain aspects of stroke-induced brain damage.

Evidence for an involvement of membrane lipids in the control of neuronal nicotinic receptor function using bungarotoxin II-S1

Previous work has shown that a toxin fraction, bungarotoxin (BGT) II-S1, isolated from Bungarus multicinctus venom could inhibit nicotinic receptor-mediated function. Experimental evidence suggested that this effect of the toxin might be due to a direct interaction of the toxin at the acetylcholine binding site and/or to its phospholipase activity. The toxin's enzymic activity has been further characterized; it has phospholipase activity of the A2 type with a Vmax of 12 pmol/min/ng protein and a Km of 300 microM. Phospholipases can produce their effects on a tissue through a variety of mechanisms including the disruption of important lipid protein bonds or the production of free fatty acids which interact with the tissue. To test for this latter possibility, various concentrations of fatty acid-free bovine serum albumin were added to the incubation medium. Fatty acid-free bovine serum albumin partially reversed the inhibition of carbachol-stimulated 1-[1,2-3H(N)]amino-4-guanidobutane ([3H]agmatine) uptake (used as a measure of ion flux) into the ganglion produced by BGT II-S1 (1.0 microM). In an attempt to determine which fatty acids might be responsible for this effect, various fatty acids were added to the incubation medium and their effect on nicotinic receptor-mediated [3H]agmatine uptake determined. Arachidonic acid decreased amine uptake by approximately 50% over the control carbachol-stimulated uptake; linoleic and oleic acid, on the other hand, did not significantly affect the response. This observation could imply that arachidonic acid is the fatty acid produced by the action of BGT II-S1 on the tissue to mediate the toxin's inhibitory effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization and localization of phospholipase A2 activity in sympathetic ganglia

Superior cervical ganglion phospholipase A2 activity was characterized using 1-palmitoyl-2-[1-14C]arachidonoyl-sn-glycero-3-phosphocholine as a substrate. The enzyme activity exhibited linearity with interval of incubation and tissue concentration; there appeared to be two pH optima of the enzyme, at pH 6.0 and 9.0. A Lineweaver-Burk plot of the reciprocal of activity versus substrate concentration yielded an apparent Km of 0.53 mM and a Vmax of 5.3 nmol/h/mg of protein. The enzyme exhibited a partial Ca2+ dependence; in the absence of Ca2+ and presence of EGTA, activity was reduced by 40%. The phospholipase A2 activity was heat sensitive and was completely inactivated after treatment at 100 degrees C for 30 min. For determination of whether the enzyme had a preference for hydrolysis of specific fatty acid substituents in the 2 position of phosphatidylcholine, several different substrates were tested. The order of preference for hydrolysis by the ganglionic enzyme was 1-palmitoyl-2-[1-14C]arachidonoyl-sn-glycero-3-phosphocholine = 1-palmitoyl-2-[1-14C]linoleoyl-sn-glycero-3-phosphocholine greater than 1-palmitoyl-2-[1-14C]palmitoyl-sn-glycero-3-phosphocholine. For determination of the localization of the phospholipase A2 enzyme in sympathetic ganglia, two approaches were used. Guanethidine, which results in destruction of adrenergic cell bodies in sympathetic ganglia, was administered to rats; an approximately 50% decline in phospholipase A2 activity was observed after this treatment. In other experiments, the preganglionic nerve to the ganglion was sectioned in rats; after 2 weeks of denervation, no significant change in ganglionic phospholipase A2 activity was seen.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of arachidonic acid on glutamate and gamma-aminobutyric acid uptake in primary cultures of rat cerebral cortical astrocytes and neurons

The effects of arachidonic acid on glutamate and gamma-aminobutyric acid (GABA) uptake were studied in primary cultures of astrocytes and neurons prepared from rat cerebral cortex. The uptake rates of glutamate and GABA in astrocytic cultures were 10.4 nmol/mg protein/min and 0.125 nmol/mg protein/min, respectively. The uptake rates of glutamate and GABA in neuronal cultures were 3.37 nmol/mg protein/min and 1.53 nmol/mg protein/min. Arachidonic acid inhibited glutamate uptake in both astrocytes and neurons. The inhibitory effect was observed within 10 min of incubation with arachidonic acid and reached approximately 80% within 120 min in both types of culture. The arachidonic acid effect was not only time-dependent, but also dose-related. Arachidonic acid, at concentrations of 0.015 and 0.03 mumol/mg protein, significantly inhibited glutamate uptake in neurons, whereas 20 times higher concentrations were required for astrocytes. The effects of arachidonic acid were not as deleterious on GABA uptake as on glutamate uptake in both astrocytes and neurons. In astrocytes, GABA uptake was not affected by any of the doses of arachidonic acid studied (0.015-0.6 mumol/mg protein). In neuronal cultures, GABA uptake was inhibited, but not to the same degree observed with glutamate uptake. Lower doses of arachidonic acid (0.03 and 0.015 mumol/mg protein) did not affect neuronal GABA uptake. Other polyunsaturated fatty acids, such as docosahexaenoic acid, affected amino acid uptake in a manner similar to arachidonic acid in both astrocytes and neurons. However, saturated fatty acids, such as palmitic acid, exerted no such effect. The significance of the arachidonic acid-induced inhibition of neurotransmitter uptake in cultured brain cells in various pathological states is discussed.

Beta-alanine transport in synaptic plasma membrane vesicles from rat brain. Efflux, exchange and stoichiometry

The efflux and exchange of beta-alanine were studied in synaptic plasma membrane vesicles from rat brain. The mechanism of beta-alanine translocation has been probed by comparing the ion dependence of net efflux to that of exchange. Dilution-induced efflux requires the simultaneous presence of internal sodium and chloride ions while influx is dependent on the presence of these two ions on the outside [Zafra, F., Aragón, M. C., Valdivieso, F. and Giménez, C. (1984) Neurochem Res. 9, 695-707]. These data show that the release of beta-alanine occurs via the carrier system and that it is cotransported with sodium and chloride ions. beta-Alanine efflux from the membrane vesicles is stimulated by external beta-alanine. This exchange does not require external sodium and chloride but it is dependent on the external concentration of beta-alanine. Half-maximal stimulation is obtained at a beta-alanine concentration similar to the Km for beta-alanine influx. Results of the direct measurements of the coupling of sodium and chloride to the transport of beta-alanine by using a kinetic approach allow us to propose a stoichiometry for the translocation cycle catalyzed by the beta-alanine transporter of three sodium ions and one chloride ion per beta-alanine zwitterion. To account for all the observed effects of external ions, beta-alanine concentrations and membrane potential on beta-alanine influx and efflux, a kinetic model of the Na+/Cl-/beta-alanine cotransport system is discussed.

Prostaglandin involvement in the evoked release of D-aspartate from cerebellar mossy fiber terminals

Isolated cerebellar glomeruli, containing mossy fiber terminals, were used to investigate the mechanisms involved in the evoked release of acid amino acids. The glomeruli contain a high affinity uptake system for D-aspartate, with a KT of 384 pmol/mg protein/min and the incorporated D-[3H]aspartate was released in response to various depolarizing agents, as well as exogenous arachidonic acid and prostaglandins. There was a marked inhibition of the release evoked by high K+ and exogenous arachidonate when the cyclooxygenase inhibitor ibuprofen was present. Also, exposure of the glomeruli to depolarizing conditions resulted in an increase in the amount of unesterified [3H]arachidonate. It appears that accumulation of unesterified arachidonate and subsequent production of prostaglandins are involved in the evoked release of the acidic amino acids from cerebellar mossy fiber terminals.

Extracellular overflow of neuroactive amino acids during severe insulin-induced hypoglycemia: in vivo dialysis of the rat hippocampus

Hypoglycemia-evoked changes in levels of extracellular excitatory and inhibitory amino acids were studied using the microdialysis technique. A newly designed dialysis probe was inserted stereotaxically into the rat hippocampus. Animals were then subjected to insulin-induced hypoglycemia; then blood glucose levels were restored by glucose injections after a 30-min period of isoelectric electroencephalography. Dialysates were collected before, during, and after the isoelectric period. Amino acids in the dialysates were analyzed by liquid chromatography and fluorescence detection following automatic precolumn derivatization with o-phthaldialdehyde. During the isoelectric phase, the concentration of aspartate increased 15-fold, whereas glutamate, gamma-amino-butyric acid, taurine, and phosphoethanolamine levels were elevated three- to sixfold. Smaller increases were observed for nonneuroactive amino acids such as asparagine, alanine, and phenylalanine. In contrast to all other amino acids, the glutamine content was reduced to less than 30% of preisoelectric values. The concentrations of the neuroactive amino acids were restored to normal in the post-isoelectric phase. These data demonstrate that there is an extracellular overflow of neuroactive amino acids, especially aspartate, during severe hypoglycemia.

Lithium mechanisms in bipolar illness and altered intracellular calcium functions

Calcium functions as an intracellular second messenger, transducing a variety of hormonal, electrical, and mechanical stimuli by activating a wide range of enzymes. There is evidence, ranging from definitive to strongly presumptive in quality, that lithium can alter many calcium-dependent processes. The list of enzyme systems dependent on calcium and altered by lithium includes adenylate cyclase, glycogen synthase, inositol-1-phosphatase, and calcium adenosine triphosphatase (ATPase). Lithium also interferes with calcium regulation of receptor sensitivity, parathyroid hormone release, microtubule structure, and other systems. All of the neural mechanisms that are hypothesized to explain various psychopharmacological treatments of bipolar illness involve functions that are critically controlled by calcium. Moreover, in every instance, a known action of lithium on calcium function could account for lithium's therapeutic or prophylactic results. From these considerations the dual hypotheses emerge that bipolar illnesses arise from disorders in calcium-regulated functions and that lithium acts by reversing or counterbalancing the effects of these calcium dysfunctions.

Interaction of enkephalins and des-tyrosyl-enkephalins with synaptosomal plasma membrane vesicles: enkephalin binding and inhibition of proline transport

Leucine- and methionine-enkephalins inhibit the Na+-dependent transport of proline into plasma membrane vesicles derived from synaptosomes. Glycine transport is weakly inhibited by enkephalins whereas there is no inhibition of transport of glutamic acid, aspartic acid, or gamma-aminobutyric acid. The inhibition of proline uptake is observed with des-tyrosyl-enkephalins but not with morphine, dynorphin(1-13), or beta-endorphins. Furthermore, enkephalin-induced inhibition of proline transport is not antagonized by naloxone. [Leu]enkephalinamide and modified [Leu]enkephalins with greater selectivity for the delta-subclass of enkephalin binding sites are less effective than [Leu]enkephalin in the inhibition of proline transport. Specific binding of [3H]Leu-enkephalin to the plasma membrane vesicles is demonstrated, and des-Tyr-[Leu]enkephalin competes with Leu-enkephalin for [Leu]enkephalin binding sites. The similarity in the concentrations of des-Tyr-[Leu]enkephalin required to compete for specific [Leu]enkephalin binding and to inhibit proline transport suggests that a specific subclass of enkephalin binding sites, distinguished by their recognition of both the enkephalins and their des-tyrosyl derivatives, may be associated with the synaptic proline transport system.

Enhanced synthesis of prostaglandins and hydroxyeicosatetraenoic acids in retina from a canine model of Batten's disease

The metabolism of [1-14C]arachidonic acid (20:4, n-6) was studied in intact retina and retinal pigment epithelial cells from normal English setters and English setters affected with hereditary canine ceroid lipofuscinosis. Acylation of arachidonic acid into membrane glycerolipids and oxygenation by lipoxygenase and cyclooxygenase to eicosanoids were measured by radiochromatographic techniques. In addition, the histopathology of accumulated ceroid particles in retinal ganglion cells and pigment epithelial cells was studied by electron microscopy. Synthesis of prostaglandins and hydroxyeicosatetraenoic acids was increased in canine ceroid lipofuscinosis retina, but not in retinal pigment epithelium. Prostaglandin D2, the putative neuronal eicosanoid, was increased nearly eightfold, whereas other eicosanoids increased two- to threefold. Ultrastructural studies revealed accumulation of ceroid and deterioration of neuronal and pigment epithelial cell architecture. These experiments demonstrate that, although lipopigment accumulates in both tissues, alterations of eicosanoid synthesis are specific for the retina, a neuronal tissue. The specific increase in prostaglandin D2 and the specificity of changes for the retina indicate that enhanced eicosanoid synthesis may be a result of an impairment of the control of oxygenation of arachidonic acid in neurons.

Biphasic liberation of arachidonic and stearic acids during cerebral ischemia

The mode of free fatty acid (FFA) liberation from the mouse brain during ischemia was investigated at various times after decapitation and under nizofenone treatment. Normal nonischemic brain FFAs consist mainly of palmitic acid (16:0), stearic acid (18:0), and oleic acid (18:1) with smaller amounts of arachidonic acid (20:4), docosahexaenoic acid (22:6), and others. Postdecapitative ischemia induced a rapid, biphasic release of 20:4 after a short lag of less than 30 s. The first phase showed a rapid 6.4-fold increase within 1 min of decapitation, followed by the second phase involving a slow release at less than one-fifth the rate of the first phase and lasting for at least 10 min. A similar, but not so marked, biphasic liberation was observed with 18:0. However, all of the other fatty acids (16:0, 18:1, 22:6, and others) were released only in a single phase at a slow rate. The time course for the rapid and specific liberation of 20:4 coincided with the time course for the decrease in brain ATP concentration during ischemia. Pretreatment of the animals with nizofenone resulted in a marked suppression of both FFA liberation and ATP depletion during ischemia. This suppression was particularly noteworthy with 20:4 and 18:0. The present study indicates that there is a specific and rapid liberation of 20:4 and 18:0 in a very early stage of ischemia and that this liberation seems to depend on availability of ATP in the brain. The physiological role of this transient 20:4 liberation during ischemia is discussed.