A rapid method for preparing synaptosomes: comparison, with alternative procedures

Protective effects of the antioxidant selenium on quinolinic acid-induced neurotoxicity in rats: in vitro and in vivo studies

Quinolinic acid (QUIN), a well known excitotoxin that produces a pharmacological model of Huntington's disease in rats and primates, has been shown to evoke degenerative events in nerve tissue via NMDA receptor (NMDAr) overactivation and oxidative stress. In this study, the antioxidant selenium (as sodium selenite) was tested against different markers of QUIN-induced neurotoxicity under both in vitro and in vivo conditions. In the in vitro experiments, a concentration-dependent effect of selenium was evaluated on the regional peroxidative action of QUIN as an index of oxidative toxicity in rat brain synaptosomes. In the in vivo experiments, selenium (0.625 mg per kg per day, i.p.) was administered to rats for 5 days, and 2 h later animals received a single unilateral striatal injection of QUIN (240 nmol/ micro L). Rats were killed 2 h after the induction of lesions with QUIN to measure lipid peroxidation and glutathione peroxidase (GPx) activity in striatal tissue. In other groups, the rotation behavior, GABA content, morphologic alterations, and the corresponding ratio of neuronal damage were all evaluated as additional markers of QUIN-induced striatal toxicity 7 days after the intrastriatal injection of QUIN. Selenium decreased the peroxidative action of QUIN in synaptosomes both from whole rat brain and from the striatum and hippocampus, but not in the cortex. A protective concentration-dependent effect of selenium was observed in QUIN-exposed synaptosomes from whole brain and hippocampus. Selenium pre-treatment decreased the in vivo lipid peroxidation and increased the GPx activity in QUIN-treated rats. Selenium also significantly attenuated the QUIN-induced circling behavior, the striatal GABA depletion, the ratio of neuronal damage, and partially prevented the morphologic alterations in rats. These data suggest that major features of QUIN-induced neurotoxicity are partially mediated by free radical formation and oxidative stress, and that selenium partially protects against QUIN toxicity.

Cytosolic choline acetyltransferase binds specifically to cholinergic plasma membrane of rat brain synaptosomes to generate membrane-bound enzyme

Uncovering the way membrane-bound choline acetyltransferase (ChAT) interacts with membranes and with which membrane in cholinergic neurons may help in understanding its role in acetylcholine metabolism. Subfractionation of rat hippocampal synaptosomes aiming to separate synaptic vesicles from plasma membranes shows that membrane-bound ChAT is bound to plasma membrane. Either detergents or urea and alkali can solubilize membrane-bound enzyme. Detergent-solubilized enzyme has a higher sedimentation rate than urea-alkali solubilized or cytosolic ChAT. Once dissociated, membrane-bound ChAT reassociates specifically with cholinergic plasma membranes, a process that was abolished by previous treatment of membranes with trypsin. Cytosolic ChAT behaves similarly. Thus, in cholinergic synaptosomes, ChAT exists as cytosolic and peripheral activity. Cytosolic ChAT generates peripheral enzyme most probably by interacting with a protein of plasma membrane of cholinergic nerve terminals. This "receptor" protein might regulate the amount of membrane-bound ChAT in cholinergic neurons.

Inhibition of brevetoxin binding to the voltage-gated sodium channel by gambierol and gambieric acid-A

Brevetoxins (BTXs) and ciguatoxins (CTXs) bind to site 5 of the voltage-gated sodium channel of excitable membranes. In the present study, we performed a competitive inhibition assay with other structurally distinct naturally occurring polyethers using isotope-labeled dihydro BTX-B ([3H]PbTx-3), which showed, for the first time, that gambierol and gambieric acid-A inhibit the binding of [3H]PbTx-3 while yessotoxins are inactive in this assay. The inhibition assay also suggested that there is a significant relationship between the size of the polycyclic region and inhibitory activity. Interestingly, the acute mouse toxicities of the compounds do not correspond directly to their inhibitory activities. These observations will serve as a guide for designing artificial polyethers with desired activity.

Selective depression by general anesthetics of glutamate versus GABA release from isolated cortical nerve terminals

The role of presynaptic mechanisms in general anesthetic depression of excitatory glutamatergic neurotransmission and facilitation of GABA-mediated inhibitory neurotransmission is unclear. A dual isotope method allowed simultaneous comparisons of the effects of a representative volatile (isoflurane) and intravenous (propofol) anesthetic on the release of glutamate and GABA from isolated rat cerebrocortical nerve terminals (synaptosomes). Synaptosomes were prelabeled with L-[(3)H]glutamate and [(14)C]GABA, and release was determined by superfusion with pulses of 30 mM K(+) or 1 mM 4-aminopyridine (4AP) in the absence or presence of 1.9 mM free Ca(2+). Isoflurane maximally inhibited Ca(2+)-dependent 4AP-evoked L-[(3)H]glutamate release (99 +/- 8% inhibition) to a greater extent than [(14)C]GABA release (74 +/- 6% inhibition; P = 0.023). Greater inhibition of L-[(3)H]glutamate versus [(14)C]GABA release was also observed for the Na(+) channel antagonists tetrodotoxin (99 +/- 4 versus 63 +/- 5% inhibition; P < 0.001) and riluzole (84 +/- 5 versus 52 +/- 12% inhibition; P = 0.041). Propofol did not differ in its maximum inhibition of Ca(2+)-dependent 4AP-evoked L-[(3)H]glutamate release (76 +/- 12% inhibition) compared with [(14)C]GABA (84 +/- 31% inhibition; P = 0.99) release. Neither isoflurane (1 mM) nor propofol (15 microM) affected K(+)-evoked release, consistent with a molecular target upstream of the synaptic vesicle exocytotic machinery or voltage-gated Ca(2+) channels coupled to transmitter release. These findings support selective presynaptic depression of excitatory versus inhibitory neurotransmission by clinical concentrations of isoflurane, probably as a result of Na(+) channel blockade.

Increased [3H]phorbol ester binding in rat cerebellar granule cells and inhibition of 45Ca(2+) buffering in rat cerebellum by hydroxylated polychlorinated biphenyls

Our previous structure-activity relationship (SAR) studies indicated that the effects of polychlorinated biphenyls (PCBs) on neuronal Ca(2+) homeostasis and protein kinase C (PKC) translocation were associated with the extent of coplanarity. Chlorine substitutions at ortho position on the biphenyl, which increase the non-coplanarity, are characteristic of the most active congeners in vitro. In the present study, we investigated the effects of selected hydroxylated PCBs, which are major PCB metabolites identified in mammals, on the same measures where PCBs had differential effects based on structural configuration. These measures include PKC translocation as determined by [3H]phorbol ester ([3H]PDBu) binding in cerebellar granule cells, and Ca(2+) sequestration as determined by 45Ca(2+) uptake by microsomes isolated from adult rat cerebellum. All the selected hydroxy-PCBs with ortho-chlorine substitutions increased [3H]PDBu binding in a concentration-dependent manner and the order of potency as determined by E(50) (concentration that increases control activity by 50%) is 2',4',6'-trichloro-4-biphenylol (32 +/- 4 microM), 2',5'-dichloro-4-biphenylol (70 +/- 9 microM), 2,2',4',5,5'-pentachloro-4-biphenylol (80 +/- 7 microM) and 2,2',5'-trichloro-4-biphenylol (93 +/- 14 microM). All the selected hydroxy-PCBs inhibited microsomal 45Ca(2+) uptake to a different extent. Among the hydroxy-PCBs selected, 2',4',6'-trichloro-4-biphenylol is the most active in increasing [3H]PDBu binding as well as inhibiting microsomal 45Ca(2+) uptake. 3,5-Dichloro-4-biphenylol and 3,4',5-trichloro-4-biphenylol did not increase [3H]PDBu binding, but inhibited microsomal 45Ca(2+) uptake. This effect was not related to ionization of these two hydroxy-PCBs. Hydroxylated PCBs seemed to be as active as parent PCBs in vitro. These studies indicate that PCB metabolites such as hydroxy-PCBs might contribute significantly to the neurotoxic responses of PCBs.

Enantiomeric resolution of a novel chiral cannabinoid receptor ligand

The enantiomeric resolution of a racemic novel cannabinoid receptor ligand conformationally restricted at the southern aliphatic chain was accomplished using a ChiralPak AD column. Both enantiomers were tested for their competitive binding to the rat brain CB1, mouse spleen CB2 and human CB2 receptors. The levorotatory isomer showed exceptionally high affinity for the CB1 receptor with a seven-fold selectivity over CB2.

The binding of BmK abT, a unique neurotoxin, to mammal brain and insect Na(+) channels using biosensor

The binding properties of BmK abT (a novel neurotoxic polypeptide abT from Chinese scorpion Buthus martensi Karsch), a unique neurotoxin from Chinese scorpion, on mammal brain and insect sodium channels were investigated using the BIAcore assay. Results showed that BmK abT could bind to rat brain synaptosomes with an association rate constant of about 2.49 x 10(6) M(-1) s(-1) and a dissociation rate constant of about 1.57 x 10(-4) s(-1), and to Heliothis nerve cord synaptosomes with an association rate constant of about 1.21 x 10(7) M(-1) s(-1) and a dissociation rate constant of about 0.99 x 10(-3) s(-1). The binding of BmK abT to rat brain synaptosomes could be partially inhibited by increasing the membrane potential, but not by BmK AS (a novel active polypeptide AS from B. martensi Karsch), BmK IT2 (a depressant insect-selective toxin IT2 from B. martensi Karsch), and BmK I (an alpha-like anti-mammal toxin I from B. martensi Karsch). Binding was not modulated by veratridine. In addition, the binding of BmK abT to Heliothis nerve cord synaptosomes was significantly enhanced by increasing the membrane potential and veratridine concentration and was inhibited by BmK IT2, but not by BmK AS or BmK I. The results suggest that BmK abT binds to a distinct receptor site on mammal brain Na(+) channels and associates with a related site for depressant insect-selective toxins on insect sodium channels.

Synaptosomal and vesicular accumulation of L-glutamate, L-aspartate and D-aspartate

We examined the vesicular accumulation of the excitatory amino-acid (EAA) neurotransmitters, L-glutamate and L-aspartate, together with the non-metabolisable EAA analogue D-aspartate. Synaptosomes derived from whole brain were incubated in various concentrations of [3H]-amino acids under conditions to facilitate vesicular turnover. Synaptosomes were then lysed in hypotonic medium and vesicles immunoprecipitated with monoclonal anti-synaptophysin antibodies coupled to sepharose beads. Using this method, saturable vesicular accumulation was observed for [3H]-L-glutamate, [3H]-L-aspartate, and [3H]-D-aspartate but not for the excitatory amino acid receptor ligands [3H]-AMPA or [3H]-kainate. Vesicular accumulation (t(1/2)=7.45 min) was markedly slower than synaptosomal accumulation (t(1/2)=1.03 min) and was substantially reduced at 4 degrees C. Maximal accumulation of [3H]-L-glutamate, [3H]-L-aspartate, and [3H]-D-aspartate was estimated to be 98, 68, and 112 pmol/mg of synaptosomal protein, respectively, and uptake affinities 1.6, 3.4, and 2.1 mM, respectively. Maximal accumulation of [3H]-L-glutamate was non-competitively inhibited by both 100 microM unlabeled L-aspartate and 100 microM D-aspartate, suggesting that all are accumulated into a common vesicular pool by different transporters.

Antioxidative activity of natural isorhapontigenin

Isorhapontigenin (ISOR), isolated from Belamcanda chinensis, is a derivative of stilbene. Its chemical structure is very similar to that of resveratrol, with a potent antioxidative effect. In the present study, we investigated the antioxidative activity of ISOR in vitro. Oxidative damage of rat liver microsomes, brain mitochondria and synaptosomes was induced by Fe2+-Cys, VitC-ADP-Fe2+ and H2O2, respectively. The formation of malondialdehyde (MDA), decrease of reduced glutathione (GSH) and increase of ultra-weak chemiluminescence during the lipid peroxidation process were determined. In addition, the characteristic ultra-weak chemiluminescence of oxidative DNA damage induced by CuSO4-Phen-VitC-H2O2 system was studied. The results showed that ISOR significantly inhibited MDA formation in liver microsomes, brain mitochondria and synaptosomes induced by Fe2+-Cys. Also, ISOR markedly prevented the decrease of GSH in mitochondria and synaptosomes induced by H2O2 and the increase of ultra-weak chemiluminescence during lipid peroxidation induced by VitC-ADP-Fe2+ as well as oxidative DNA damage induced by CuSO4-Phen-VitC-H2O2. The effects of ISOR at 10(-5) and 10(-6) mol/L on the MDA formation and decrease of GSH were similar to that of the classical antioxidant vitamin E (10(-4) mol/L). It may be concluded that ISOR possessed potent antioxidative activity and was much more potent than vitamin E.

Stereochemical selectivity of methanandamides for the CB1 and CB2 cannabinoid receptors and their metabolic stability

Several chiral, analogues of the endogenous cannabinoid receptor ligand, arachidonylethanolamide (anandamide), methylated at the 2,1' and 2' positions using asymmetric synthesis were evaluated in order to study (a) stereoselectivity of binding to CB1 and CB2 cannabinoid receptors; and (b) metabolic stability with regard to anandamide amidase. Enantiomerically pure 2-methyl arachidonic acids were synthesized through diastereoselective methylation of the respective chiral 2-oxazolidinone enolate derivatives and CB1 and CB2 receptor affinities of the resulting chiral anandamides were evaluated using a standard receptor binding assay. Introduction of a single 2-methyl group increased affinity for CB1, led to limited enantioselectivity and only modestly improved metabolic stability. However, a high degree of enantio- and diastereoselectivity was observed for the 2,1'-dimethyl analogues. (R)-N-(1-methyl-2-hydroxyethyl)-2-(R)-methyl-arachidonamide (4) exhibited the highest CB1 receptor affinity in this series with a K(i) of 7.42 nM, an at least 10-fold improvement on anandamide (K(i)=78.2 nM). The introduction of two methyl groups at the 2-position of anandamide led to no change in affinity for CB1 but somewhat enhanced metabolic stability. Conversely, chiral headgroup methylation in the 2-gem-dimethyl series led to chiral analogues possessing a wide range of CB1 affinities. Of these the (S)-2,2,2'-trimethyl analogue (12) had the highest affinity for CB1 almost equal to that of anandamide. In agreement with our previous anandamide structure-activity relationship work, the analogues in this study showed high selectivity for the CB1 receptor over CB2. The results are evaluated in terms of stereochemical factors affecting the ligand's affinity for CB1 using receptor-essential volume mapping as an aid. Based on the results, a partial CB1 receptor site model is proposed, that bears two hydrophobic pockets capable of accommodating 1'- and 2-methyl groups

Regulation of phospholipase Cbeta activity by muscarinic acetylcholine and 5-HT(2) receptors in crude and synaptosomal membranes from human cerebral cortex

Stimulation of phospholipase Cbeta by receptor agonists and G proteins has been characterized in crude cerebral membrane preparations, but little is known about their presynaptic localizations and little information is currently available for human brain tissue. The characteristics of phosphoplipase C transmembrane signaling were studied in crude and synaptosomal plasma membranes from postmortem human prefrontal cortex by measuring the hydrolysis of exogenous [(3)H]phosphatidylinositol4,5bisphosphate(PIP(2)) and the immunoreactive levels of phospholipase C (PLC) and G(alphaq/11) proteins. Regulation of PLC activity by Ca(2+) and the 5-HT(2) receptor agonist 5-methyltryptamine, but not by guanosine 5'-O-[3-thiotriphosphate] and the muscarinic acetylcholine receptor agonist carbachol were different between crude and synaptosomal membranes. KCl (20 mM) stimulation was absent in both preparations. Levels of G(alphaq/11)-protein subunits differed between preparations. The functional inhibition carried out with pirenzepine in crude membranes in order to reverse the carbachol-induced PLC stimulation indicates the existence of a component (53%) of the response that is activated by the M(1) muscarinic acetylcholine receptor subtype, and another component (47%) probably mediated by the M(3) muscarinic acetylcholine receptor subtype. In synaptosomal plasma membranes an increased inhibition of carbachol-induced PLC activation through M(1) was found. The PLC activation by 5-methyltryptamine (ketanserin-sensitive in crude membranes) was absent in synaptosomal plasma membranes suggesting the lack of activity mediated by 5-HT(2)-serotonin receptors.

Neurochemical effects of environmental chemicals: in vitro and in vivo correlations on second messenger pathways

Polychlorinated biphenyls (PCBs) are persistent, bioaccumulative, toxic, and widely distributed environmental chemicals. There is now both epidemiological and experimental evidence that PCBs cause cognitive deficits; however, the underlying cellular or molecular mechanism(s) is not known. We have hypothesized that altered signal transduction/second messenger homeostasis by PCBs may be associated with these effects since second messengers in signal transduction pathways, such as calcium, inositol phosphates (IP), and protein kinase C (PKC), play key roles in neuronal development and their function. In vitro studies using cerebellar granule neurons and isolated organelle preparations indicate that ortho-PCBs increase intracellular free Ca2+ levels by inhibiting microsomal and mitochondrial Ca2+ buffering and the Ca2+ extrusion process. Ortho-PCBs also increase agonist-stimulated IP accumulation and cause PKC translocation at low micromolar concentrations where no cytotoxicity is observed. On the other hand, non-ortho-PCBs are not effective in altering these events. Further SAR studies indicate that congeners with chlorine substitutions favoring non-coplanarity are active in vitro, while congeners favoring coplanarity are relatively inactive. Subsequent in vivo studies have shown that repeated exposure to a PCB mixture, Aroclor 1254, increases PKC translocation and decreases Ca2+ buffering in the brain, similar to in vitro studies. These changes in vivo are associated with elevated levels of non-coplanar ortho-PCB congeners at levels equivalent to 40-50 microM in brain, the concentrations that significantly inhibited second messenger systems in neuronal cultures in vitro. Current research is focusing on PCB-induced alterations in second messenger systems following developmental exposure.

Tamulustoxin: A Novel Potassium Channel Blocker from the Venom of the Indian Red Scorpion Mesobuthus tamulus

We have characterized tamulustoxin, a novel 35-amino-acid peptide found in the venom of the Indian red scorpion (Mesobuthus tamulus). Tamulustoxin was identified through a [125I]toxin I screen, designed to identify toxins that block voltage-activated potassium channels. Tamulustoxin has also been cloned by RT-PCR, using RNA extracted from scorpion venom glands. Tamulustoxin shares no homology with other scorpion venom toxins, although the positions of its six cysteine residues would suggest that it shares the same structural scaffold. Tamulustoxin rapidly inhibited both peak and steady-state currents (18.9 +/- 1.0 and 37 +/- 1.1%, respectively) produced by injecting CHO cells with mRNA encoding the hKv1.6 channel.

The binding of BmK IT2, a depressant insect-selective scorpion toxin on mammal and insect sodium channels

Binding assay of (125)I-BmK IT2, a depressant insect-selective scorpion toxin showed two non-interacting binding sites on insect neuronal membranes: a high affinity (K(d(1))=0.65+/-0.20 nM) and low capacity (B(max(1))=0.46+/-0.13 pmol/mg protein) binding site, as well as a low-affinity (K(d(2))=78.7+/-16.4 nM) and high capacity (B(max(2))=33.1+/-8.5 pmol/mg protein) binding site. BmK IT2 could associate with and dissociate from its binding sites on insect neuronal membranes in quick manner (k(1)=5.4 x 10(5) S(-1) M(-1) and k(2)=3.2 x 10(4) S(-1) M(-1); k(-1)=7.4 x 10(-4) S(-1) and k(-2)=5.3 x 10(-3) S(-1)). The binding of (125)I-BmK IT2 to insect synaptosomes could be significantly inhibited by native BmK IT2, BmK AS and BmK AS-1 in a dose-dependent manner, and partially by BmK I, but not modified by depolarization of membrane potential and veratridine, In addition, specific binding of (125)I-BmK IT2 seem to be undetectable on rat brain synaptosomes even at high concentration. Whole cell patch-clamping recording found that BmK IT2 could partially inhibit total sodium currents of rat DRG neurons, the inhibitory effects were reversible. The results suggest that the receptor binding site of BmK IT2 on insect sodium channels might be similar to that on sodium channels of mammal peripheral nervous system, but different from that of mammal central nervous system.

Binding characteristics of BmK I, an alpha-like scorpion neurotoxic polypeptide, on cockroach nerve cord synaptosomes

In this study, the binding characteristics of BmK I, an alpha-like neurotoxic polypeptide purified from the venom of the Chinese scorpion Buthus martensi Karsch, were investigated on rat brain and cockroach nerve cord synaptosomes. The results showed that BmK I can bind to a single class of noninteracting binding sites on cockroach nerve cord synaptosomes with medium affinity (Kd = 16.5 +/ - 4.4 nM) and low binding capacity (Bmax = 1.05 +/- 0.23 pmol/mg protein), but lacks specific binding on rat brain synaptosomes. BmK AS, BmK AS-1 (two novel sodium channel-blocking ligands), BmK IT (an excitatory insect-selective toxin) and BmK IT2 (a depressant insect-selective toxin) from the same venom were found to be capable of depressing BmK I binding in cockroach nerve cord synaptosomes, which might be attributed to either allosteric modulation of voltage-gated Na+ channels by these toxic polypeptides or partial overlapping between the receptor binding sites of BmK I and these toxins. This thus supported the notion that alpha-like scorpion neurotoxic polypeptides bind to a distinct receptor site on sodium channels, which might be similar to the binding receptor site of alpha-type insect toxins, and also related to those of BmK AS type and insect-selective scorpion toxins on insect sodium channels.

Sibutramine, a serotonin uptake inhibitor, increases dopamine concentrations in rat striatal and hypothalamic extracellular fluid

We measured, using microdialysis, the effects of sibutramine, given intraperitoneally, on brain dopamine and serotonin flux into striatal and hypothalamic dialysates of freely moving rats, and on the uptake of [(3)H]-DA into striatal synaptosomes. For microdialysis experiments, samples collected every 30 min were assayed by high-pressure liquid chromatography, in a single run. Administration of a low dose of sibutramine (2.0 mg/kg, i.p) had no effect on dopamine or serotonin concentrations in striatal dialysates but higher doses increased both: 5 mg/kg increased these concentrations to 196+/-24% (p<0.01) and 221+/-28% (p<0.01) of baseline, respectively; 10 mg/kg increased dopamine to 260+/-66% (p<0.01) and serotonin to 160+/-20% (p<0.05) of baseline. In hypothalamus, the 5 mg/kg sibutramine dose increased the dopamine concentration to 186+/-40% (p<0.05) and that of serotonin to 312+/-86% (p<0.01) of baseline, while the 10 mg/kg (i.p.) dose increased dopamine to 392+/-115% (p<0.01), and serotonin to 329+/-104% (p<0.01) of baseline. In vitro, sibutramine blocked [(3)H]-dopamine uptake into striatal synaptosomes, with an IC(50) value of 3.8 microM. These findings indicate that sibutramine has at least as great an effect on brain extracellular dopamine levels as on brain serotonin, and suggest that the drug's antiobesity action may result from the changes it produces in brain dopamine as well as serotonin metabolism.

BmK AS: new scorpion neurotoxin binds to distinct receptor sites of mammal and insect voltage-gated sodium channels

This study was undertaken to assess the binding properties of BmK AS on both mammal and insect excitable cell membranes. It was found that BmK AS bound specifically to a single class of non-interacting binding sites on both rat brain and cockroach nerve cord synaptosomes with high affinity (K(d) = 1.49 +/- 0.14 and 0.79 +/- 0.29 nM) and low capacity (B(max) = 1.39 +/- 0.09 and 6.60 +/- 1.25 pmol/mg protein), respectively. Binding kinetics showed that BmK AS could bind and reach equilibrium quickly, and dissociate partially from its binding sites on both kinds of synaptosomes. The binding of BmK AS was independent of membrane potential. Veratridine could not modify the binding of BmK AS. The competitive binding assay showed that specific binding of (125)I-BmK AS could be significantly inhibited by native BmK AS, BmK AS-1, BmK IT2 and BmK IT on both synaptosomes. Unexpectedly, only about 20-30% binding of BmK AS on mammal synaptosomes was inhibited by BmK I at 10(-5)-10(-9) M, but not on insect synaptosomes. It thus suggests that BmK AS type neurotoxins might bind to a distinct receptor site of sodium channels on mammal and insect excitable cell membranes with a manner similar to beta-scorpion toxins.

Insulin-like growth factor 1 regulates developing brain glucose metabolism

The brain has enormous anabolic needs during early postnatal development. This study presents multiple lines of evidence showing that endogenous brain insulin-like growth factor 1 (Igf1) serves an essential, insulin-like role in promoting neuronal glucose utilization and growth during this period. Brain 2-deoxy-d- [1-(14)C]glucose uptake parallels Igf1 expression in wild-type mice and is profoundly reduced in Igf1-/- mice, particularly in those structures where Igf1 is normally most highly expressed. 2-Deoxy-d- [1-(14)C]glucose is significantly reduced in synaptosomes prepared from Igf1-/- brains, and the deficit is corrected by inclusion of Igf1 in the incubation medium. The serine/threonine kinase Akt/PKB is a major target of insulin-signaling in the regulation of glucose transport via the facilitative glucose transporter (GLUT4) and glycogen synthesis in peripheral tissues. Phosphorylation of Akt and GLUT4 expression are reduced in Igf1-/- neurons. Phosphorylation of glycogen synthase kinase 3beta and glycogen accumulation also are reduced in Igf1-/- neurons. These data support the hypothesis that endogenous brain Igf1 serves an anabolic, insulin-like role in developing brain metabolism.

Quantitative characterization of crude synaptosomal fraction (P-2) components by flow cytometry

Flow cytometry, which definitively identifies each particle as positive or negative with respect to fluorescent markers, is used to characterize the P-2 fraction (crude synaptosomal fraction) with respect to primary components, size, and intactness. Particle size ranged from a few tenths of a microm to greater than 4.5 microm. The viable dye calcein AM labeled 90% of the preparation, indicating that the majority of particles were intact and esterase-positive. 66% of the P-2 fraction is neuronal in origin, as demonstrated by labeling with an antibody directed against SNAP-2. An antibody directed against glial fibrillary acidic protein (GFAP) labeled 35% of the particles in this preparation. The mitochondrial dye nonyl acridine orange (NAO) stained 74% of particles, indicating intra- and extrasynaptosomal mitochondria. Gating analysis reveals that SNAP-25 is enriched in the larger particles. These results suggest that flow cytometry may be used to take advantage of the increased viability, yield, and convenience of the P-2 fraction for studies of nerve terminal function.

Altered regulation of dopaminergic activity and impairment in motor function in rats after subchronic exposure to styrene

Animal and human studies suggest a dopamine-mediated effect of styrene neurotoxicity. However, the results reported to date are incomplete and not consistent. As such, the mechanism of its neurotoxicity is still unclear. The present study has, therefore, reexamined the central dopaminergic system in relation to some neurobehavioral effects in rats following subchronic exposure to styrene. Groups of adult male Sprague-Dawley rats received 0, 0.25, or 0.5 g styrene per kg b.wt. by gavage for 13 consecutive weeks. Twenty-four hours after cessation of such treatment with the higher dose (0.5 g/kg), the contents of dopamine (DA) and its metabolites were significantly reduced in the corpus striatum, hypothalamus, and lateral olfactory tract regions. In vitro styrene showed a significant increase in DA release from rat striatal synaptosomes similar to that of tyramine. Significant loss of motor function was observed on days 56, 70, and 84 during the styrene treatment with the higher dose, and lasted over a month after such treatment. However, the treated animals recovered their motor function within 45-60 days after cessation of such treatment, along with the recovery of normal levels of dopamine and its metabolites. Furthermore, styrene-induced initial impairments in measures of dopaminergic activity cannot be attributed to altered regulation of tyrosine hydroxylase activity. Specific [3H]-spiroperidol binding was also unaltered 7 or 15 days after subchronic treatment with styrene. These data imply that despite the dopaminergic neuronal loss due to styrene, dopaminergic transmission was not reduced to a level that would result in an overall development of dopamine receptor supersensitivity in the striatum. Collectively, these studies indicate that the subchronic neurotoxic action of styrene may be primarily presynaptic in nature and may involve impaired regulation of DA content and stimulation of DA release.

Brevetoxin derivatives that inhibit toxin activity

BACKGROUND

The brevetoxins are marine neurotoxins that interfere with the normal functions of the voltage-gated Na(+) channel. We have identified two brevetoxin derivatives that do not exhibit pharmacological properties typical of the brevetoxins and that function as brevetoxin antagonists.

RESULTS

PbTx-3 and benzoyl-PbTx-3 elicited Na(+) channel openings during steady-state depolarizations; however, two PbTx-3 derivatives retained their ability to bind to the receptor, but did not elicit Na(+) channel openings. alpha-Naphthoyl-PbTx-3 acted as a PbTx-3 antagonist but did not affect Na(+) channels that were not exposed to PbTx-3. beta-Naphthoyl-PbTx-3 reduced openings of Na(+) channels that were not exposed to PbTx-3.

CONCLUSIONS

Some modifications to the brevetoxin molecule do not alter either the binding properties or the activity of these toxins. Larger modifications to the K-ring sidechain do not interfere with binding but have profound effects on their pharmacological properties. This implies a critical function for the K-ring sidechain of the native toxin.

A late phase of exocytosis from synaptosomes induced by elevated [Ca2+]i is not blocked by Clostridial neurotoxins

Treatment of rat cerebrocortical synaptosomes with botulinum toxin types E and C1 or tetanus toxin removed the majority of intact SNAP-25, syntaxin 1A/1B, and synaptobrevin and diminished Ca(2+)-dependent K+ depolarization-induced noradrenaline secretion. With botulinum toxin type E, <10% of intact SNAP-25 remained and K(+)-evoked release of glutamate and GABA was inhibited. The large component of noradrenaline release evoked within 120 s by inclusion of the Ca2+ ionophore A23187 with the K+ stimulus was also attenuated by these toxins; additionally, botulinium neurotoxin type E blocked the first 60 s of ionophore-induced GABA and glutamate exocytosis. However, exposure to A23187 for longer periods induced a phase of secretion nonsusceptible to any of these toxins (>120 s for noradrenaline; >60 s for glutamate or GABA). Most of this late phase of release represented exocytosis because of its Ca2+ dependency, ATP requirement, and sensitivity to a phosphatidylinositol 4-kinase inhibitor. Based on these collective findings, we suggest that the ionophore-induced elevation of [Ca2+]i culminates in the disassembly of complexes containing nonproteolyzed SNAP receptors protected from the toxins that can then contribute to neuroexocytosis.

Introduction of macromolecules into synaptosomes using electroporation

Synaptic terminals are sites of high metabolic activity and thus are particularly vulnerable to oxidative stress. Oxidative damage to proteins can be toxic to neurons and may cause irreversible cell damage and neurodegeneration. A neuroprotective mechanism used by cells to combat oxidative damage is to selectively degrade damaged proteins. Therefore, it is of interest to study the mechanism of degradation of oxidatively damaged proteins in synaptosomes. One way of oxidizing synaptosomal proteins in vitro is by incubating intact synaptosomes in the presence of an oxidizing agent. A problem with this approach is that it may also cause oxidative damage to the machinery required to recognize and degrade oxidized proteins. We have, therefore, introduced a fluorescent macromolecule into synaptosomes to assess the feasibility of using this technique to study how oxidized proteins are degraded and removed from synaptic terminals. Synaptosomes were subjected to electroporation in the presence of FITC labelled-dextran with an average molecular weight of 70000 (FD-70) and non-specific binding was determined by running parallel experiments in lysed synaptosomes. Following extensive washing, synaptosomes were assayed for the presence of intra-synaptosomal FD-70 by measuring fluorescence in a microplate fluorescence reader. Significant differences in fluorescence were found between intact and lysed synaptosomes with maximal uptake at 100 V/ 1500 microF (approx. 36 pmol/mg protein). To determine if membrane transport was compromised by electroporation, uptake of 3H-arginine was compared in control and electroporated synaptosomes. While untreated electroporated synaptosomes showed a loss of 22% in the ability to transport arginine, preincubation in the presence of 1 mM ATP resulted in a complete restoration of arginine transport. These results show that electroporation is a potentially useful technique for introducing a specific oxidized protein, into synaptic terminals so its metabolic fate can be examined.

Effects of tranylcypromine on thyroid hormone metabolism and concentrations in rat brain

The effect of 14 days administration of the anti-depressant tranylcypromine (TCP) on iodothyronine deiodinase activities and the concentrations of thyroxine (T4) and triiodothyronine (T3) were investigated in homogenates of up to nine regions of the rat brain. The activity of the 5III deiodinase isoenzyme, which catalyses the inactivation of T3 to 3,3'-diiodothyronine (3,3'-T2), was enhanced in eight brain regions. However, the brain levels of T4 were completely unchanged and the T3 concentrations were significantly reduced in the frontal cortex only. Therefore, we also measured the T3 concentrations of three subcellular fractions (nuclei, synaptosomes and mitochondria) of six brain regions. TCP induced a significant reduction in T3 levels in the synaptosomes of the frontal cortex and significant increases in the mitochondrial T3 concentrations in the amygdala. The latter effect was replicated after 14 days administration of 5 mg/kg desipramine. No effects of either drug on nuclear concentrations of T3 were seen in any brain region. As the amygdala is critically involved in the affective coloring of sensory stimuli, the increase in T3 concentrations in the mitochondria of this brain region may be of relevance for the mechanism of action of anti-depressant drugs.

Calcium-dependent dissociation of synaptotagmin from synaptic SNARE complexes

The formation of the synaptic core (SNARE) complex constitutes a crucial step in synaptic vesicle fusion at the nerve terminal. The interaction of synaptotagmin I with this complex potentially provides a means of conferring Ca2+-dependent regulation of exocytosis. However, the subcellular compartments in which interactions occur and their modulation by Ca2+ influx remain obscure. Sodium dodecyl sulfate (SDS)-resistant core complexes, associated with synaptotagmin I, were enriched in rat brain fractions containing plasma membranes and docked synaptic vesicles. Depolarization of synaptosomes triggered [3H]GABA release and Ca2+-dependent dissociation of synaptotagmin from the core complex. In perforated synaptosomes, synaptotagmin dissociation was induced by Ca2+ (30-300 microM) but not Sr2+ (1 mM); it apparently required intact membrane bilayers but did not result in disassembly of trimeric SNARE complexes. Synaptotagmin was not associated with unstable v-SNARE/t-SNARE complexes, present in fractions containing synaptic vesicles and cytoplasm. These complexes acquired SDS resistance when N-ethylmaleimide-sensitive fusion protein (NSF) was inhibited with N-ethylmaleimide or adenosine 5'-O-(3-thiotriphosphate), suggesting that constitutive SNARE complex disassembly occurs in undocked synaptic vesicles. Our findings are consistent with models in which the Ca2+ triggered release of synaptotagmin precedes vesicle fusion. NSF may then dissociate ternary core complexes captured by endocytosis and recycle/prime individual SNARE proteins.

1-Methyl-4-phenyl-2,3-dihydropyridinium is transformed by ubiquinone to the selective nigrostriatal toxin 1-methyl-4-phenylpyridinium

We have studied the interaction of coenzyme Q with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its metabolites, 1-methyl-4-phenyl-2,3-dihydropyridinium (MPDP(+)) and 1-methyl-4-phenylpyridinium (MPP(+)), the real neurotoxin to cause Parkinson's disease. Incubation of MPTP or MPDP(+) with rat brain synaptosomes induced complete reduction of endogenous ubiquinone-9 and ubiquinone-10 to corresponding ubiquinols. The reduction occurred in a time- and MPTP/MPDP(+) concentration-dependent manner. The reduction of ubiquinone induced by MPDP(+) went much faster than that by MPTP. MPTP did not reduce liposome-trapped ubiquinone-10, but MPDP(+) did. The real toxin MPP(+) did not reduce ubiquinone in either of the systems. The reduction by MPTP but not MPDP(+) was completely prevented by pargyline, a type B monoamine oxidase (MAO-B) inhibitor, in the synaptosomes. The results indicate that involvement of MAO-B is critical for the reduction of ubiquinone by MPTP but that MPDP(+) is a reductant of ubiquinone per se. It is suggested that ubiquinone could be an electron acceptor from MPDP(+) and promote the conversion from MPDP(+) to MPP(+) in vivo, thus accelerating the neurotoxicity of MPTP.

Extracellular calcium is required for the polychlorinated biphenyl-induced increase of intracellular free calcium levels in cerebellar granule cell culture

Recent studies from the laboratory indicate that polychlorinated biphenyl (PCB) congeners can alter signal transduction and calcium homeostasis in neuronal preparations. These effects were more pronounced for the ortho-substituted, non-coplanar congeners, although the mechanisms underlying these effects are not clear. In the present study the time-course and concentration-dependent effects of coplanar and non-coplanar PCBs on intracellular free calcium concentration ([Ca2+]i) in cerebellar granule cell cultures were compared using the fluorescent probe fura-2. The ortho-substituted congeners 2,2'-dichlorobiphenyl (DCB) and 2,2',4,6,6'-pentachlorobiphenyl (PeCB) caused a gradual increase of [Ca2+]i while the non-ortho-substituted congeners 4,4'-DCB and 3,3',4,4',5-PeCB had no effect. The increase of [Ca2+]i produced by 2,2'-DCB was time- and concentration-dependent. Further studies examined possible mechanisms for this rise in [Ca2+]i. In contrast to the muscarinic agonist carbachol, the effects of 2,2'-DCB on [Ca2+]i were not blocked by thapsigargin and required the presence of extracellular calcium. The effects of ortho-substituted PCBs may depend on their ability to inhibit calcium sequestration as 2,2'-DCB significantly inhibited 45Ca2+-uptake by microsomes and mitochondria while 3,3',4,4',5-PeCB had no effect. In addition, 2,2'-DCB significantly increased the binding of [3H]inositol 1,4,5-trisphosphate to receptors on cerebellar microsomes, suggesting another possible mechanism by which ortho-substituted PCBs can mobilize [Ca2+]i. These results show that PCBs increase [Ca2+]i in vitro via a mechanism that requires extracelluar calcium, and support previous structure-activity studies indicating that ortho-substituted PCBs are more potent than non-ortho-substituted PCBs.

Inhibition of glutamine transport in rat brain mitochondria by some amino acids and tricarboxylic acid cycle intermediates

Glutamine transport into rat brain synaptic and non-synaptic mitochondria has been monitored by the uptake of [3H]glutamine and by mitochondrial swelling. The concentration of glutamate in brain mitochondria is calculated to be high, 5-10 mM, indicating that phosphate activated glutaminase localized inside the mitochondria is likely to be dormant and the glutamine taken up not hydrolyzed. The uptake of [3H]glutamine is largely stereospecific. It is inhibited by glutamate, asparagine, aspartate, 2-oxoglutarate and succinate. Glutamate inhibits this uptake into synaptic and non-synaptic mitochondria by 95 and 85%, respectively. The inhibition by glutamate, asparagine, aspartate and succinate can be explained by binding to an inhibitory site whereas the inhibition by 2-oxoglutarate is counteracted by aminooxyacetic acid, which indicates that it is dependent on transamination. The glutamine-induced swelling, a measure of a very low affinity uptake, is inhibited by glutamate at a glutamine concentration of 100 mM, but this inhibition is abolished when the glutamine concentration is raised to 200 mM. This suggests that the very low affinity glutamine uptake is competitively inhibited by glutamate. Furthermore, glutamine-induced swelling is inhibited by 2-oxoglutarate, succinate and malate, similarly to that of the [3H]glutamine uptake. The properties of the mitochondrial glutamine transport are not identical with those of a recently purified renal glutamine carrier.

Design and synthesis of the CB1 selective cannabinoid antagonist AM281: a potential human SPECT ligand

In the search for a radioligand capable of imaging cannabinoid CB1 receptors in the living human brain by SPECT (single photon emission computed tomography),N-(morpholin-4-yl)-1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM281) was synthesized. This compound is an analog of the potent, CB1 receptor selective antagonist SR141716A [N-(piperidin-1-yl)-1-(2,4-dichlorophenyl)-5-(4-chlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide]. AM281 bound to brain and spleen membrane preparations (CB1 and CB2 receptors, respectively) with K(i) values of 12 nM and 4200 nM, respectively. AM281 also inhibited the response of guinea-pig small intestine preparation to a cannabinoid receptor agonist. Thus, AM281 behaves as a CB1 receptor selective antagonist. Methods for the rapid, high-yield synthesis and purification of [123I]AM281 were developed, and transaxially reconstructed brain SPECT images obtained after continuous infusion of [123I]AM281 in baboons. Thus [123I]AM281 may be suitable for imaging CB1 receptors in humans.

Relevance of benzyloxy group in 2-indolyl methylamines in the selective MAO-B inhibition

1. Previous studies with indolyl derivatives as monoamine oxidase (MAO) inhibitors have shown the relevance of the indole structure for recognition by the active site of this enzyme. We now report a new series of molecules with structural features which determine the selectivity of MAO inhibition. 2. A benzyloxy group attached at position 5 of the indole ring is critical for this selective behaviour. Amongst all of these benzyloxy-indolyl methylamines, N-(2-propynyl)-2-(5-benzyloxyindol)methylamine FA-73 was the most potent MAO-B 'suicide' inhibitor studied. 3. The Ki values for MAO-A and MAO-B were 800+/-60 and 0.75+/-0.15 nM, respectively. These data represent a selectivity value of 1066 for MAO-B, being 48 times more selective than L-deprenyl (Ki values of 376+/-0.032 and 16.8+/-0.1 nM for MAO A and MAO-B, respectively). The IC50 values for dopamine uptake in striatal synaptosomal fractions from rats were 150+/-8 microM for FA-73 and 68 +/- 10 microM for L-deprenyl whereas in human caudate tissue the IC50 values were 0.36+/-0.015 microM for FA-73 and 0.10+/-0.007 microM for L-deprenyl. Moreover, mouse brain MAO-B activity was 90% ex vivo inhibited by both compounds 1 h after 4 mg kg(-1) administration, MAO-A activity was not affected. 4. These novel molecules should provide a better understanding of the active site of monoamine oxidase and could be the starting point for the design of further selective, non-amphetamine-like MAO-B inhibitors with therapeutic potential for the treatment of neurological disorders.

Formation of phospholipid hydroperoxides and its inhibition by alpha-tocopherol in rat brain synaptosomes induced by peroxynitrite

Peroxynitrite resulted from the reaction of nitric oxide and superoxide anion has been implicated in the genesis of neurotoxicity. In this study, the oxidation of phospholipids in rat brain synaptosomes induced by peroxynitrite generated from 3-morpholinosydnonimine (SIN-1) was studied in vitro. The formation and accumulation of phospholipid hydroperoxides, including phosphatidylcholine hydroperoxide (PCOOH) and phosphatidyl-ethanolamine hydroperoxide (PEOOH) in rat brain synaptosomes induced by peroxynitrite, were observed. PEOOH and PCOOH were formed rapidly and SIN-1 concentration-dependently. The hydroperoxides formed in synaptosomes were unstable and it was suggested that phospholipase A2 played a role in degradation of the hydroperoxides. The endogenous alpha-tocopherol acted as a potent antioxidant. It was oxidized very rapidly and concentration-dependently by SIN-1 to alpha-tocopheryl quinone. Furthermore, uric acid was found to be an effective antioxidant in inhibiting oxidative damage to synaptosomal lipids induced by SIN-1. The results provide direct evidence to show that peroxynitrite can not only deplete alpha-tocopherol, but also cause production of phospholipid hydroperoxides resulting in disrupted brain tissue.

Extraction and quantification of thyroid hormones in selected regions and subcellular fractions of the rat brain

There is increasing evidence of an involvement of thyroid hormones in numerous physiological processes of the adult vertebrate brain. However, the only valid method available for measuring triiodothyronine (T3) in brain tissue is time-consuming and not sufficiently sensitive to determine hormone concentrations in small, but physiologically important areas such as the amygdala and septum. We therefore developed a protocol for reliable measurement of the concentrations of thyroxine (T4) and T3 in brain tissue. This was achieved by combining a new method of extracting iodothyronines with highly sensitive, accurate and reproducible radioimmunoassays (RIAs) in order to be able to detect T4 and T3 in homogenates and even subcellular fractions (nuclear, synaptosomal and mitochondrial) in up to 11 regions of the rat brain. The iodothyronines were extracted from tissue samples by adding 100% methanol containing 1 mM PTU. Recoveries of 72.8 +/- 5.5% and 83.2 +/- 3.3% for T4 and T3, respectively, were obtained. The RIA detection thresholds were 10 fmol/g for T4 and 18 fmol/g for T3. Only 0.2% of the antibody for T4 cross-reacted with T3 and 0.95% reverse T3. T3 antibody (0.05%) reacted with T4 and 0.01% with 3,5-T2. The T4 concentrations in the homogenates of selected areas of the brain ranged between 1 and 4 pmol/g, whereas those of T3 ranged between 0.5 and 4 pmol/g. The T3 levels ranged between 190 and 470 fmol/mg protein, 38 and 110 fmol/g protein and 25 and 180 fmol/mg protein in the nuclei, synaptosomes and mitochondriae, respectively. In conclusion, the newly developed method enabled us to determine both T4 and T3 concentrations in homogenates and T3 in subcellular fractions of regions of the brain as small as the septum and amygdala.

Shifted cytosolic NADP+-dependent isocitrate dehydrogenase on 2-D gel in the brain of genetically epileptic E1 mice

We observed a spot on two-dimensional (2-D) gel in the epileptic mutant strain E1 mice with a similar molecular weight but with a different isoelectric point of approximately 0.2, compared with its mother strain ddY mice. The collected protein from the E1 mice was identified as cytosolic NADP+-dependent isocitrate dehydrogenase by internal amino acid sequencing. The enzyme is known to be maximally active during the development of the brain and to play an important role in NADPH production for fatty acids and cholesterol synthesis. In addition, alterations in cholesterol synthesis early in the development of the mammalian brain have been reported to lead to chronic epilepsy. The results in the present study therefore suggest that cytosolic NADP+-dependent isocitrate dehydrogenase might be involved in the epileptogenesis of the E1 mouse.

Glutamine transport in rat brain synaptic and non-synaptic mitochondria

Glutamine transport into rat brain mitochondria (synaptic and non-synaptic) was monitored by the uptake of [3H]glutamine as well as by mitochondrial swelling. The uptake is inversely correlated to medium osmolarity, temperature-dependent, saturable and inhibited by mersalyl, and glutamine is upconcentrated in the mitochondria. These results indicate that glutamine is transported into an osmotically active space by a protein catalyzed mechanism. The uptake is slightly higher in synaptic mitochondria than in non-synaptic ones. It is inhibited both by rotenone and the protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone, the latter at pH 6.5, showing that the transport is activated by an electrochemical proton gradient. The K+/H+ ionophore nigericin also inhibits the uptake at pH 6.5 in the presence of external K+, which indicates that glutamine, at least in part, is taken up by a proton symport transporter. In addition, glutamine uptake as measured by the swelling technique revealed an additional glutamine transport activity with at least 10 times higher Km value. This uptake is inhibited by valinomycin in the presence of K+ and is thus also activated by the membrane potential. Otherwise, the two methods show similar results. These data indicate that glutamine transport in brain mitochondria cannot be described by merely a simple electroneutral uniport mechanism, but are consistent with the uptake of both the anionic and the zwitterionic glutamine.

Ciguatoxins and brevetoxins, neurotoxic polyether compounds active on sodium channels

Ciguatoxins (CTXs) and brevetoxins (PbTxs) modify the activation and inactivation processes of voltage-sensitive sodium channels (VSSC). In this study, the specific binding to rat brain synaptosomes of two commercial PbTxs, five purified CTXs and their derivatives was evaluated in competition with various concentrations of radiolabelled brevetoxin ([3H]PbTx-3). The results indicate that all CTXs bind specifically and with high affinity to sodium channels. Statistical analysis of the calculated inhibition constants identified two classes of toxins: the PbTxs and the less polar CTXs, and a group of CTXs of very high affinity. Relatively small chemical differences between the CTXs gave rise to significant differences in their affinity to the rat brain sodium channels. Cytotoxic effects associated with sodium channel activation were evaluated for the two classes of toxins on murine neuroblastoma cells, and their acute toxicity was determined in mice. CTXs have shown high affinities to VSSC of rat brain membranes and strong cytotoxic effects on neuroblastoma cells which correlate with their very low LD50 in mice. For PbTxs, it is different. Although binding with high affinity to VSSC and giving rise to significant cytotoxic effects, they are known to be poorly toxic intraperitoneally to mice. Furthermore, within the CTXs family, even though the most toxic compound (CTX-1B) has the highest affinity and the less toxic one (CTX-4B) the lowest affinity, a detailed analysis of the data pointed out a complex situation: (i) high affinity and toxicity seem to be related to the hydroxylation of the molecule on the A-ring rather than to the backbone type, (ii) acute toxicity in mice does not follow exactly the sodium-dependent cytotoxicity on neuroblastoma cells. These data suggest that the high toxicity of CTXs is related to sodium-dependent disturbances of the excitable membranes but might also involve other cellular mechanisms.

Repeated exposure of adult rats to Aroclor 1254 causes brain region-specific changes in intracellular Ca2+ buffering and protein kinase C activity in the absence of changes in tyrosine hydroxylase

Polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants, some of which may be neurotoxic. In vitro studies from this laboratory indicated that noncoplanar PCBs perturbed intracellular signal transduction mechanisms including Ca2+ homeostasis, receptor-mediated inositol phosphate production, and translocation of protein kinase C (PKC). In the present study, we examined the effects of PCBs in vivo by dosing adult male Long-Evans rats orally with Aroclor 1254 (0, 10, or 30 mg/kg/day; 5 days/week for 4 weeks) in corn oil. At 24 h after the last dose, rats were tested for motor activity in a photocell device for 30 min. Immediately, the rats were euthanized, blood was collected for thyroid hormone analysis, and brains were removed, dissected into regions (cerebellum, frontal cortex, and striatum), and subcellular fractions were obtained for neurochemical analysis. Following Aroclor 1254 treatment, body weight gain in the high-dose group was significantly lower than the control and low-dose groups. Horizontal motor activity was significantly lower in rats dosed with 30 mg/kg Aroclor 1254. Ca2+ buffering by microsomes was significantly lower in all three brain regions from the 30 mg/kg group. In the same dose group, mitochondrial Ca2+ buffering was affected in cerebellum but not in cortex or striatum. Similarly, total cerebellar PKC activity was decreased significantly while membrane-bound PKC activity was significantly elevated at 10 and 30 mg/kg. PKC activity was not altered either in cortex or the striatum. Neurotransmitter levels in striatum or cortex were slightly altered in PCB-exposed rats compared to controls. Furthermore, repeated oral administration of Aroclor 1254 to rats did not significantly alter forebrain tyrosine hydroxylase immunoreactivity or enzymatic activity. Circulating T4 (total and free) concentrations were severely depressed at both doses in Aroclor 1254-exposed rats compared to control rats, suggesting a severe hypothyroid state. These results indicate that (1) in vivo exposure to a PCB mixture can produce changes in second messenger systems that are similar to those observed after in vitro exposure of neuronal cell cultures; (2) second messenger systems seem to be more sensitive than alterations in neurotransmitter levels or tyrosine hydroxylase involved in dopamine synthesis during repeated exposure to PCBs; and (3) the observed motor activity changes were independent of changes in striatal dopamine levels.

New toxins acting on sodium channels from the scorpion Leiurus quinquestriatus hebraeus suggest a clue to mammalian vs insect selectivity

Two new toxins were purified from Leiurus quinquestriatus hebraeus (Lqh) scorpion venom, Lqh II and Lqh III. Lqh II sequence reveals only two substitutions, as compared to AaH II, the most active scorpion alpha-toxin on mammals from Androctounus australis Hector. Lqh III shares 80% sequence identity with the alpha-like toxin Bom III from Buthus occitanus mardochei. Using bioassays on mice and cockroach coupled with competitive binding studies with 125I-labeled scorpion alpha-toxins on rat brain and cockroach synaptosomes, the animal selectivity was examined. Lqh II has comparable activity to mammals as AaH II, but reveals significantly higher activity to insects attributed to its C-terminal substitution, and competes at low concentration for binding on both mammalian and cockroach sodium channels. Lqh II thus binds to receptor site 3 on sodium channels. Lqh III is active on both insects and mammals but competes for binding only on cockroach. The latter indicates that Lqh III binds to a distinct receptor site. Thus, Lqh II and Lqh III represent two different scorpion toxin groups, the alpha- and alpha-like toxins, respectively, according to the structural and pharmacological criteria. These new toxins may serve as a lead for clarification of the structural basis for insect vs mammal selectivity of scorpion toxins.

Preferential utilization of acetate by astrocytes is attributable to transport

Exogenous acetate is preferentially metabolized by astrocytes in the CNS, but the biochemical basis for this selectivity is unknown. We observed that rat cortical astrocytes produce 14CO2 from 0.2 mM [14C]acetate at a rate of 0.43 nmol/min per milligram of protein, 18 times faster than cortical synaptosomes. Subsequent studies examined whether this was attributable to cellular differences in the transport or metabolism of acetate. The activity of acetyl-CoA synthetase, the first enzymatic step in acetate utilization, was greater in synaptosomes than in astrocytes (5.0 and 2.9 nmol/min per milligram of protein), indicating that slower metabolism in synaptosomes cannot be attributed to lack of enzymatic activity. [14C]Acetate uptake in astrocytes is rapid and time-dependent and follows saturation kinetics (Vmax, 498 nmol/min per milligram of protein; Km, 9.3 mM). Uptake is inhibited stereospecifically by L-lactate as well as by pyruvate, fluoroacetate, propionate, and alpha-cyano-4-hydroxycinnamate (CHC). Preloading astrocytes with L-lactate or acetate, but not D-lactate, pyruvate, or glyoxylate, transaccelerates [14C]acetate uptake. Acetate uptake by astrocytes appears to be mediated by a carrier with properties similar to that of monocarboxylate transport. In contrast, studies with synaptosomes provided no evidence for time-dependent, saturable, transaccelerated, or CHC-inhibitable uptake of [14C]acetate. The high rate of transport in astrocytes compared with synaptosomes explains the rapid incorporation of [14C]acetate into brain glutamine over glutamate. These findings provide support for the use of acetate as a marker for glial metabolism and suggest that extracellular acetate in the brain generated from acetylcholine and ethanol metabolism is accumulated first by astrocytes.

"In vitro" effect of some 5-hydroxy-indolalkylamine derivatives on monoamine uptake system

Three different indolalkylamine derivatives (FA 102, FA 69, FA 70) having in common an -OH group at 5 position of the indole ring and differing in the presence of a methyl group at the N or the acetylenic group of the side chain, have been synthesized and assayed as monoamine oxidase-A (MAO-A) [E.C.1.4.3.4] inhibitors. They were effective inhibitors with, in some cases, similar potencies to clorgyline. "In vitro" experiments were performed on rat brain synaptosomes to investigate whether these MAO-A inhibitors had any effect on noradrenaline (NA), dopamine (DA) and 5-hydroxytryptamine (5-HT) transport systems in different rat brain regions. The effect of these drugs were compared with those of clorgyline and 1-deprenyl. FA 102, FA 69, FA 70 behaved as inhibitors of 3H-monoamine uptake with similar rank of order of potency for amine uptake inhibition: 5-HT > DA > NA. The IC50 values for FA 102, FA 69, FA 70, respectively, were: 17 microM, 60 microM, 18 microM for 5HT uptake in cortex and 37 microM, 55 microM and 20 microM in hippocampus; 70 microM, 385 microM for NA uptake in cortex and 315 microM, 255 microM and 600 microM in hypothalamus; 270 microM, 160 microM, 40 microM for DA uptake in striatum. 1-Deprenyl was a very poor inhibitor of monoamine uptake, whereas clorgyline behaved similarly to these indolalkylamine derivatives. Comparing these results with the IC50 values of citalopram, nisoxetine and GBR12909, specific and selective inhibitors of 5-HT, NA and DA transport systems respectively, indicated that these indolalkylamine derivatives interact more strongly with the 5HT uptake system.

Evidence that glypican is a receptor mediating beta-amyloid neurotoxicity in PC12 cells

Docking of beta-amyloid fibrils to neuronal or glial cell membranes may be an early, necessary and intervenable step during the progression of Alzheimer's disease. Formation of neurofibrillary tangles and amyloid plaques as well as neurotoxicity and inflammation may be direct or indirect consequences. In an attempt to find a receptor that mediates those effects, we assessed rat pheochromocytoma PC12 cell 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) reduction after addition of beta-amyloid to the culture medium. Presence of competitive substances in the medium, cell-surface treatment and specific block of cellular synthesis pathways helped to identify the heparan sulphate moiety of a glycosylphosphatidylinositol-anchored protein likely to represent glypican as a possible receptor mediating beta-amyloid neurotoxicity.

Modulation of monoamine oxidase activity in different brain regions and platelets following exposure of rats to methylmercury

Monoamine oxidase (MAO; EC 1.4.3.4) is known to have an important role in the regulation of biogenic amines in the brain and peripheral tissues. It is also known that circulating platelets represent an excellent model for an easy assessment of the effect of MAO-B inhibitors in extracerebral tissue. The present study was carried out to determine the effects of methylmercury (MeHg) on the activity of MAO in synaptosomes of different brain regions of male Sprague-Dawley rats as well as in rat blood platelets both in vitro and in vivo. MeHg pretreatment inhibited the activity of MAO in the synaptosomes of the cortex, hypothalamus, hippocampus, striatum, cerebellum, and brain stem in a concentration-dependent (0-10 microM) manner. The threshold concentration of MeHg for such inhibition in different brain synaptosomes was found to be the same (i.e., 1 microM) except for in the rat striatum it was 2.5 microM, and the IC50 value for MeHg was found to be around 2.1 microM. Significant inhibition of the MAO activity was also observed in synaptosomes of the cortex, cerebellum, hypothalamus, and hippocampus as well as in platelets of rats 24 h after treatment by gavage with a total cumulative dose of 35 mg/kg (5 mg/kg/day for 7 days). The decrease of such activity was found to be at maximum in different brain synaptosomes and platelets 24 h following treatment with a cumulative total dose of 75 mg/kg (7.5 mg/kg/day for 10 days); the treated animals showed signs of ataxia under these conditions. The data have further shown that methylmercury is capable of inhibiting the MAO activity in different brain synaptosomes to different degrees but without showing any specificity towards any specific brain region. The present in vivo results suggest that the platelet MAO activity may be used as a potential biomarker of early neurotoxicity due to repeated exposure to MeHg in rats.

Extracellular acidification modifies Ca2+ fluxes in rat brain synaptosomes

We examined the influence of external acidification on Ca2+ fluxes (45Ca2+ influx and 45Ca2+ efflux) in rat brain synaptosomes. A change on external pH (pHe) from 7.5 to 6.5 linearly decreased the 45Ca2+ uptake (5nmoles/mg protein/pHunit) and increased the 45Ca2+ efflux (1.5 fold/pH unit). These changes were both Na+ dependent and amiloride sensitive suggesting that the Na+/Ca2+ exchanger could be involved. The addition of the Ca2+ channel blockers (diltiazem, verapamil, nifedipine) did not prevent the decrease of the 45Ca2+ uptake evoked by acid pHe and so the involvement of the voltage-sensitive Ca2+ channels could be discarded. In order to determine whether the Na+/Ca2+ exchanger was directly activated by H+ or was indirectly activated by an internal mobilization of Ca2+ from intrasynaptosomal stores we examined the effect of pHe variation on phophoinositide hydrolisis. An increase on phosphoinositide hydrolisis was observed at acid pHe values (7 and 6.5). The hydrolisis was amiloride insensitive. On the other hand 1mM neomycin did inhibit the effect of acidic pHe on Ca2+ fluxes. Taken together, the results of our study provide evidence that external acidification stimulates phospholipase C leading to an increase in phosphoinositide hydrolisis and Ca2+ mobilization. The increase in intracellular Ca2+ would stimulate the Na+/Ca2+ exchanger, increasing Ca2+ efflux and reducing the global Ca2+ influx.

Simultaneous isolation of glial and neuronal fractions from rat brain homogenates: comparison of high-affinity L-glutamate transport properties

An improved three-step Percoll density gradient centrifugation technique is described for simultaneous isolation of glial plasmalemmal vesicles (GPV) and synaptosomal vesicles (SYN) from a rat brain homogenate. While electron microscopy revealed that fractions contained intact vesicles with markedly distinct morphological features, measures of high-affinity [3H]choline uptake, glutamine synthetase and carbonic anhydrase activities, as well as Western blot analyses for glial fibrillary acidic protein and neuron specific enolase, served to confirm the low level of neuronal contamination in GPV fractions as well as the low level of glial contamination in SYN fractions. In addition, GPV and SYN fractions were used to characterize the kinetic and pharmacological properties of sodium-dependent [3H]L-glutamate transport. In conclusion, these results demonstrate the usefulness of this method for obtaining highly-enriched, functionally viable populations of glial and neuronal elements which are suitable for studies of their respective cell functions in vitro.

Effect of piribedil and its metabolite, S584, on brain lipid peroxidation in vitro and in vivo

We studied the effect of piribedil (1-3,4-methylendioxybenzyl-4-(2-pyrimidyl) piperazine) and its catechol metabolite, S584 (1-(3,4-dihydroxybenzyl-4-(2-pyrimidinyl)-piperazine), on rat brain lipid peroxidation (a) in vitro in rat synaptosomes and cortical slices after induction of an oxidative stress and (b) in vivo in mouse brain after short-term exposure (two and three 4-h cycles) to O2/CO2 (95%:5%). The metabolite (10[-4]-10[-5] M), but not piribedil, prevented Fe3+-stimulated lipid peroxidation in rat synaptosomes and in rat cortical slices incubated with high oxygen concentrations. Piribedil (7.5 and 30 mg/kg, orally), counteracted the increase in thiobarbituric reactive substances in the brain of mice only when these were exposed to two or three cycles of a high oxygen concentration. S584 (30 mg/kg, orally) reduced thiobarbituric acid reactive substances in brain in mice exposed either to air (control) or to three cycles of a high oxygen concentration. These results suggest that piribedil has an antiperoxidative effect in brain, which may be partly related to the in vivo formation of the catechol metabolite, S584.

Use of zonal centrifugation method for the preparation of mu-opioid receptor enriched membranes from bovine corpus striatum

Earlier attempts to purify the opioid receptors met with limited success because of the use of brain crude membranes. Subcellular fractionation procedures adopted now to get enriched membranes resulted in 2-3 fold enrichment. However, a procedure has been developed in our laboratory in which a crude membrane fraction obtained at 17,500 x g was lysed with 1 mM sodium bicarbonate and later subjected to zonal fractionation, employing a density gradient of 0.6-1.2 M sucrose. This could yield a membrane fraction highly enriched with mu-opioid receptors which is 9.3 fold higher than the crude membranes.

Development and preliminary validation of a microtiter plate-based receptor binding assay for paralytic shellfish poisoning toxins

More than 20 countries have either established or proposed regulatory limits for one or more of the paralytic shellfish poisoning (PSP) toxins as they occur in seafood products. PSP toxin levels are generally estimated using the standard AOAC mouse bioassay, yet because of various limitations of this method [e.g. high variability (+/-20%), low sensitivity, limited sample throughput and use of live animals], there remains a need for alternative testing protocols. A sensitive and selective, high capacity assay was developed for the PSP toxins which exploits the highly specific interaction of these toxins with their biological receptor (i.e. voltage-dependent sodium channel) and is thus based on functional activity. This receptor binding assay provides a radioactive endpoint, and is performed in a microtiter filter plate format with results determined by standard liquid scintillation counting within 24 hr. The Ki for the assay is 3.66 +/- 0.86 nM saxitoxin, with a limit of detection of c. 5 ng saxitoxin/ml in a sample extract. Good quantitative agreement of the assay with both mouse bioassay and high-performance liquid chromatographic analysis of crude extracts of contaminated shellfish, as well as PSP toxin-producing algae, was observed. Our findings indicate that the receptor binding assay has a strong predictive value for toxicity determined by mouse bioassay, and that this approach warrants consideration as a rapid, reliable and cost-effective alternative to live animal testing for detection and estimation of PSP-related toxicity in seafood and toxic algae.

High affinity binding of pyrethroids to the alpha subunit of brain sodium channels

Na+ channels are the primary molecular targets of the pyrethroid insecticides. Na+ channels consisting of only a type IIA alpha subunit expressed in Chinese hamster ovary cells responded to pyrethroid treatment in a normal manner: a sustained Na+ current was induced progressively after each depolarizing pulse in a train of stimuli, and this Na+ current decayed slowly on repolarization. These modified Na+ channels could be reactivated at much more negative membrane potentials (V0.5 = -139 mV) than unmodified Na+ channels (V0.5 = -28 mV). These results indicate that pyrethroids can modify the functional properties of the Na+ channel alpha subunit expressed alone by blocking their inactivation, shifting their voltage dependence of activation, and slowing their deactivation. To demonstrate directly the specific interaction of pyrethroids with the alpha subunit of voltage-gated Na+ channels, a radioactive photosensitive derivative, [3H]RU58487, was used in binding and photolabeling studies. In the presence of a low concentration of the nonionic detergent Triton X-100, specific pyrethroid binding to Na+ channels in rat brain membrane preparations could be measured and reached 75% of total binding under optimal conditions. Binding approached equilibrium within 1 hr at 4 degrees, dissociated with a half-time of approximately 10 min, and had K(D) values of approximately 58-300 nM for three representative pyrethroids. Specific pyrethroid binding was enhanced by approximately 40% in the presence of 100 nM alpha-scorpion toxin, but no allosteric enhancement was observed in the presence of toxins acting at other Na+ channel receptor sites. Extensive membrane washing increased specific binding to 89%. Photolabeling with [3H]RU58487 under these optimal binding conditions revealed a radiolabeled band with an apparent molecular mass of 240 kDa corresponding to the Na+ channel alpha subunit. Anti-peptide antibodies recognizing sequences within the alpha subunit were able to specifically immunoprecipitate the covalently modified channel. Together, these results demonstrate that the pyrethroids can modify the properties of cells expressing only the alpha subunit of Na+ channels and can bind specifically to a receptor site on the alpha subunit.

The depolarisation-induced release of [125I]BDNF from brain tissue

The pattern of release of radioactive brain-derived neurotrophic factor ([125I]BDNF) from brain tissue was studied. Rat brain slices from cerebral cortex and synaptosomes from cerebral cortex and hippocampus were preloaded with [125I]BDNF. Depolarising stimulation by veratridine (final conc. 50 microM) and high KCl (final conc. 45 mM) caused a short-term, greatly enhanced depolarisation-induced release of [125I]BDNF during superfusion and batch protocol experiments. The results suggested that the evoked release was independent of the presence of extracellular calcium ions, but dependent on intracellular calcium ion stores, since the intracellular calcium ion chelator BAPTA-AM, but not the extracellular chelator EGTA abolished the high-potassium-induced [125I]BDNF release from synaptosomes. The release was blocked by tetrodotoxin (1 microM) when synaptosomes were stimulated by veratridine or potassium chloride. Short time-fraction (30 s) superfusion experiments showed that the [125I]BDNF release from synaptosomes appeared in two temporal phases.

Studies on protective mechanisms of four components of green tea polyphenols against lipid peroxidation in synaptosomes

The comparison of the protective effects of four components of "green tea polyphenols' (GTP) - (-)-epigallocatechin gallate, EGCG; (-)-epicatechin gallate, ECG; (-)epigallocatechin, EGC; and (-)epicatechin, EC - against iron-induced lipid peroxidation in synaptosomes showed that: (1) the inhibitory effects of those compounds on TBA reactive materials from lipid peroxidation decreased in the order of EGCG > ECG > EGC > EC; (2) the scavenging effects of those compounds on lipid free radicals produced by lipid peroxidation could be classified as follows: ECG > EGCG > EC > EGC. Furthermore, we investigated the iron-chelating activity and the free radical scavenging activity of those compounds as their protective mechanisms against lipid peroxidation in synaptosomes. As for the iron-chelating activity, the ratio of EGC, EGCG, ECG or EC to iron(III) was 3:2, 2:1, 2:1 and 3:1, respectively. The hydroxyl radical (HO) scavenging activity of those compounds was investigated in a photolysis of the H2O2 system. It was found that their ability to scavenge hydroxyl radicals decreased in the order of ECG > EC > EGCG >> EGC. It was also found that they could scavenge lipid free radicals in the lecithin/lipoxidase system and their scavenging activity was classified as follows: ECG > EGCG >> EGC > EC. Moreover, we found that their antioxidant active positions were different from each other and the stability of the semiquinone free radicals produced by those compounds in NaOH solution decreased in the order of EGCG > ECG >> EC. The results indicated that the ability of those compounds to protect synaptosomes from the damage of lipid peroxidation initiated by Fe2+/Fe3+ was dependent not only on their iron-chelating activity and free-radical scavenging activity, but also on the stability of their semiquinone free radicals.

The characteristics of arginine transport by rat cerebellar and cortical synaptosomes

The uptake of L-[3H]arginine into synaptosomes prepared from rat cerebellum and cortex occurred by a high-affinity carrier-mediated process. The uptake of arginine appeared to be potentiated by removal of extracellular Na+, inhibited by high levels of extracellular K+, but not by depolarization with veratridine or 4-amino pyridine. The effect of Na+ removal or K+ elevation did not seem to be due to changes in intracellular Ca2+ or pH. In both brain regions, uptake was significantly inhibited by L-arginine, L-lysine, L-ornithine, and L-homoarginine, but not by D-arginine nor L-citrulline. Uptake was also inhibited by NG-monomethyl-L-arginine acetate, but not by NG-nitro-L-arginine methyl ester nor NG-nitro-L-arginine except in the cortex at a concentration of 1 mM. The results indicate that the carrier system operating in synaptosomes showed many of the characteristics of the ubiquitous y+ system seen in many other tissues, although its apparent sensitivity to variations in extracellular Na+ was unusual.