Observation of rovibrational dephasing of molecules in parahydrogen crystals by frequency domain spectroscopy

Rotation-vibration transitions of methane molecules embedded in parahydrogen crystals were investigated through Fourier transform infrared spectroscopy. Each transition shows extremely sharp peaks with a Lorentzian line shape profile, which indicates the spectra are free from inhomogeneous broadening. The steep temperature dependence of the linewidths observed in the range between 3.7 and 8. 5 K is interpreted to be a result of the pure dephasing relaxation mechanism. A remarkable difference in population relaxation widths between stretching and bending vibrational excited states was also found.

Comparisons of the anesthetic potency and intracellular concentrations of S(-) and R() bupivacaine and ropivacaine in crayfish giant axon in vitro

Levobupivacaine and ropivacaine are both single S(-) enantiomers that have less severe cardiotoxic and convulsant effects than racemic bupivacaine. We compared the anesthetic actions of S(-) bupivacaine, R(+) bupivacaine, and ropivacaine in vitro by studying their effects on action potential amplitude and the maximal rate of rise of action potential in crayfish giant axon. To clarify the difference of intracellular anesthetic concentration, the intracellular ionized anesthetic concentration was measured. Desheathed crayfish axons were stimulated at a frequency of either 0. 1 or 5 Hz and perfused with 1 mM of each anesthetic at pH 7.0. Intracellular anesthetic concentration was measured by us- ing local anesthetic-sensitive glass microelectrodes. At 0.1-Hz stimulation, no differences were observed in their potency. At 5-Hz stimulation, the order of magnitude of the mean percentage decrease in maximal rate of rise of action potential was S(-) bupivacaine > R(+) bupivacaine > ropivacaine. Intracellular local anesthetic concentration did not differ among the three anesthetics at 0.1 Hz and 5 Hz. We conclude that, compared with ropivacaine, S(-) bupivacaine has a more potent phasic blocking effect in crayfish giant axon. The intracellular local anesthetic concentrations of S(-), R(+) bupivacaine and ropivacaine were not significantly different, regardless of differences in blocking effect and stimulation frequency.

IMPLICATIONS

S(-) bupivacaine has a more potent phasic blocking effect than ropivacaine or R(+) bupivacaine in crayfish giant axons in vitro. An equivalent intracellular local anesthetic concentration for the three anesthetics was found, suggesting that the intracellular cationic local anesthetic concentration is not directly correlated with the intensity of block.

sigma receptor ligands attenuate N-methyl-D-aspartate cytotoxicity in dopaminergic neurons of mesencephalic slice cultures

We investigated the potential neuroprotective effects of several sigma receptor ligands in organotypic midbrain slice cultures as an excitotoxicity model system. When challenged with 100-microM N-methyl-D-aspartate (NMDA) for 24 h, dopaminergic neurons in midbrain slice cultures degenerated, and this was prevented by (5R, 10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,b]-cyclohepten-5, 10-imine (MK-801; 1-10 microM). Concomitant application of ifenprodil (1-10 microM) or haloperidol (1-10 microM), both of which are high-affinity sigma receptor ligands, significantly attenuated the neurotoxicity of 100 microM NMDA. The sigma(1) receptor-selective ligand (+)-N-allylnormetazocine ((+)-SKF 10047; 1-10 microM) was also effective in attenuating the toxicity of NMDA. The effect of R(-)-N-(3-phenyl-1-propyl)-1-phenyl-2-aminopropane hydrochloride ((-)-PPAP), a sigma receptor ligand with negligible affinity for the phencyclidine site of NMDA receptors, was also examined. (-)-PPAP (3-100 microM) caused a concentration-dependent reduction of NMDA cytotoxicity, with significant protection at concentrations of 30 and 100 microM. In contrast, (+)-SKF 10047 (10 microM) and (-)-PPAP (100 microM) showed no protective effects against cell death induced by the Ca(2+) ionophore ionomycin (1-3 microM). These results indicate that sigma receptor ligands attenuate the cytotoxic effects of NMDA on midbrain dopaminergic neurons, possibly via inhibition of NMDA receptor functions.

p75-mediated neuroprotection by NGF against glutamate cytotoxicity in cortical cultures

Accumulating evidence suggests that the neurotrophin receptors, Trks and p75, play distinct roles in regulating cells survival and death, with Trks important for cell survival, and p75 acting to induce cell death. Here, we provide evidence that, in neuronal cultures from rat cerebral cortex, nerve growth factor (NGF) exerts neuroprotective actions via p75. Incubating cultures with NGF for 1-24 h protected cortical neurons from delayed cytotoxicity induced by brief exposure to glutamate. Delayed neurotoxicity induced by a calcium ionophore, ionomycin, or nitric oxide (NO) donors such as S-nitrosocysteine (SNOC) and 3-morpholinosydnonimine (SIN-1), was also attenuated by pretreatment with NGF. RT-PCR analysis revealed the presence of p75 and trkB transcripts in cortical cultures, but did not detect transcripts of trkA, a high-affinity receptor for NGF. Brain-derived neurotrophic factor (BDNF), but not NGF, induced tyrosine phosphorylation of Trks, indicating that NGF does not activate Trks in cortical neurons. Concurrent application of anti-p75 neutralizing antibody markedly reduced the neuroprotective effect of NGF, but resulted in only a modest reduction of that of BDNF. BDNF-induced neuroprotection, but not NGF-induced neuroprotection, was inhibited by a protein synthesis inhibitor cycloheximide. Distinct signaling pathways mobilized by NGF and BDNF were also revealed in that NGF but not BDNF stimulated significant production of ceramides, whereas BDNF but not NGF caused persistent activation of mitogen-activated protein kinases. These results indicate that, although NGF and BDNF both protect cortical neurons from excitotoxicity, the mechanisms involved in their effects are totally different. The present results are, to our knowledge, the first to demonstrate the principal involvement of p75 in cytoprotective actions of neurotrophins.

Depletion of intracellular glutathione increases susceptibility to nitric oxide in mesencephalic dopaminergic neurons

Using primary neuronal cultures, we investigated the effects of GSH depletion on the cytotoxic effects of glutamate and NO in dopaminergic neurons. Intracellular GSH was depleted by 24-h exposure to L-buthionine-[S,R]-sulfoximine (BSO), an irreversible inhibitor of GSH synthase. BSO exposure caused concentration-dependent reduction of the viability of both dopaminergic and nondopaminergic neurons. In contrast, 24-h exposure of cultures to glutamate or NOC18, an NO-releasing agent, significantly reduced the viability of nondopaminergic neurons without affecting that of dopaminergic neurons. Pretreatment with N-acetyl-L-cysteine for 24 h ameliorated the NOC18-induced toxicity in nondopaminergic neurons. In dopaminergic neurons, sublethal concentrations of BSO reduced intracellular GSH content and markedly potentiated glutamate- and NOC18-induced toxicity. These results suggested that glutamate toxicity was enhanced in dopaminergic neurons by suppression of defense mechanisms against NO toxicity under conditions of GSH depletion. Under such conditions, free iron plays an important role because BSO-enhanced NO toxicity was ameliorated by the iron-chelating agent, deferoxamine. These results suggest that GSH plays an important role in the expression of NO-mediated glutamate cytotoxicity in dopaminergic neurons. Free iron may be related to enhanced NO cytotoxicity under GSH depletion.

[Factors regulating survival and death of midbrain dopamine neurons]

We utilized organotypic midbrain slice cultures for the assessment of survival and degeneration of dopaminergic neurons in the substantia nigra. Application of N-methyl-D-aspartate (NMDA) to midbrain slice cultures for 24 h caused a concentration-dependent decrease in the number of surviving dopaminergic neurons visualized by tyrosine hydroxylase immunohistochemistry. Simultaneous application of (-)-deprenyl significantly attenuated the cytotoxic effect of NMDA. Because pretreatment with (-)-deprenyl failed to reduce NMDA toxicity, it is suggested that the neuroprotective effect of (-)-deprenyl is not mediated by its irreversible inhibitory action on monoamine oxidase B. We also prepared co-cultures of midbrain and striatal slices to investigate whether the presence of target tissue influences toxic actions of several drugs on dopaminergic neurons. Co-cultured dopaminergic neurons formed dense innervation to the striatal tissue. Dopaminergic neurons in midbrain--striatum co-cultures were more resistant to the cytotoxic actions of NMDA and a nitric oxide donor NOC-18, than the same neuronal population in single midbrain cultures. On the other hand, the toxicity of 1-methyl-4 phenylpyridinium ion or buthionine-[S,R]-sulfoximine was more prominent in midbrain--striatum co-cultures than that in single midbrain cultures. Organotypic slice cultures appeared to be a useful system for evaluation of dopaminergic neuronal death under experimental conditions relevant to physiological/pathophysiological situations.

Deprenyl rescues dopaminergic neurons in organotypic slice cultures of neonatal rat mesencephalon from N-methyl-D-aspartate toxicity

The potential neuroprotective effect of (-)-deprenyl (R-N,alpha-dimethyl-N-2-propynylbenzeneethanamine) against N-methyl-D-aspartate (NMDA) excitotoxicity was investigated on rat mesencephalic dopaminergic neurons in organotypic slice cultures. While 24 h application of NMDA (100 microM) caused a marked decrease in the number of surviving dopaminergic neurons, simultaneous application of (-)-deprenyl significantly attenuated the cytotoxic effect of NMDA. (+)-Deprenyl showed a less potent but still significant protective effect against NMDA insult. Pre-treatment of cultures with (-)-deprenyl conferred no protection against subsequent NMDA insult, suggesting that the protective effect of (-)-deprenyl may be independent of its irreversible inhibitory action on monoamine oxidase B. (-)-Deprenyl was also ineffective in preventing cell death induced by H2O2. These results indicated that (-)-deprenyl protects dopaminergic neurons from NMDA excitotoxicity through a mechanism distinct from monoamine oxidase inhibition or detoxification of reaction oxygen species.

A potential role of Ras-mediated signal transduction for the enhancement of depolarization-induced Ca2+ responses in hippocampal neurons by basic fibroblast growth factor

Chronic treatment with basic fibroblast growth factor (bFGF) increases the expression of functional L-type voltage-dependent Ca2+ channels (VDCCs) in fetal rat hippocampal neurons. We investigated the intracellular signaling mechanisms involved in this effect, using high K+ depolarization-induced elevation of intracellular Ca2+ concentrations as a measure. Genistein, a protein tyrosine kinase inhibitor, significantly attenuated the effect of bFGF. The effect of bFGF was also diminished by concurrent application of a Ras inactivator, N-acetyl-S-farnesyl-l-cysteine. In contrast, a phospholipase C inhibitor U73122, a phosphatidylinositol-3 kinase inhibitor wortmannin, Li+ which inhibits inositol phospholipid turnover, or a protein kinase inhibitor calphostin C did not inhibit the effect of bFGF. Phorbol 12-myristate 13-acetate, a protein kinase C activator, did not mimic the effect of bFGF. On the other hand, an adenylyl cyclase activator forskolin and a cyclic AMP analog 8-Br-cyclic AMP markedly attenuated the effect of bFGF, which indicates the presence of a cyclic AMP-mediated negative regulatory mechanism, possibly the interference of Ras-Raf interaction. These results suggest that Ras-mediated signal transduction is required for the enhancement by bFGF of VDCC responses in hippocampal neurons.

Pharmacokinetic differences between lansoprazole enantiomers in rats

Because limited information is available about potential differences between the pharmacokinetics and pharmacodynamics of the enantiomers of lansoprazole, the enantioselective pharmacokinetics of the compound have been investigated in rats. There was a noticeable difference between the serum levels of the enantiomers of lansoprazole and of their metabolites, 5-hydroxylansoprazole enantiomers, after oral administration of the racemate (50 mg kg(-1)) to rats. Cmax (maximum serum concentration) and AUC (area under the serum concentration-time curve) for (+)-lansoprazole were 5-6 times greater than those for (-)-lansoprazole, whereas for (+)-5-hydroxylansoprazole both values were significantly smaller than those for the (-) enantiomer. CLtot/F values (where CLtot is total clearance and F is the fraction of the dose absorbed) for (+)-lansoprazole were significantly smaller than those for the (-) enantiomer. There was no significant difference between the absorption rate constants of the lansoprazole enantiomers in the in-situ absorption study. The in-vitro protein-binding study showed that binding of (+)-lansoprazole to rat serum proteins was significantly greater than for the (-) enantiomer. The in-vitro metabolic study showed that the mean metabolic ratio (45.9%) for (-)-lansoprazole was significantly greater than that (19.8%) for the (+) enantiomer in rat liver microsomes at 5.6 microM lansoprazole. These results show that the enantioselective disposition of lansoprazole could be a consequence of the enantioselectivity of plasma-protein binding and the hepatic metabolism of the enantiomers.

[Protective effect of neurotrophin against glutamate neurotoxicity in cortical cultures]

This study was performed to investigate the effects of neurotrophins on glutamate cytotoxicity by using cultured cortical neurons. Primary cultures obtained from the cerebral cortex of fetal rats (17-19 days gestation) were used for experiments. NGF did not elicit tyrosine phosphorylation of Trks whereas BDNF induced Trk tyrosine phosphorylation within 10 min, followed by time-dependent decrease. Brief glutamate exposure to the cell induced delayed cytotoxicity. Similar cytotoxicity was observed with the brief application of a calcium ionophore, ionomycin, and nitric oxide (NO) generating agents, S-nitrosocysteine (SNOC) and SIN-1. Exposure of the cultures to NGF and BDNF for 1 or 24 hr prior to glutamate exposure reduced glutamate-induced cytotoxicity. In contrast, simultaneous addition of NGF and BDNF with glutamate did not affect glutamate-induced cytotoxicity. Ionomycin-induced cytotoxicity was prevented by exposing cultures to NGF and BDNF for 24-hr. Moreover, NGF and BDNF ameliorated cytotoxicity induced by SNOC and SIN-1. These results suggest that neurotrophins prevent NO mediated glutamate cytotoxicity.

The effects of extracellular pH with and without bicarbonate on intracellular procaine concentrations and anesthetic effects in crayfish giant axons

BACKGROUND

The potentiating effect of sodium bicarbonate on local anesthetic action is attributed to two mechanisms: (1) an increase in the un-ionized local anesthetic due to extracellular alkalinization, and (2) an accelerated conversion of local anesthetic from un-ionized to ionized form with intracellular acidification caused by bicarbonate. To evaluate these hypotheses, the intracellular pH, intracellular ionized procaine concentration, and evoked action potentials were measured in crayfish giant axons.

METHODS

In all measurements, axon preparations from crayfish were perfused extracellularly for 15 min with either bicarbonate-containing solution at pH 7.6 (bicarb/7.6) or bicarbonate-free solution at pH 7.6 (nonbicarb/7.6) or pH 8.0 (nonbicarb/8.0). Intracellular pH was measured using a pH-sensitive microelectrode. Intracellular anesthetic concentration was measured using a specially designed procaine-sensitive microelectrode with each of three solutions containing 1 mM procaine hydrochloride. Membrane potential was measured and, as an index of anesthetic action, the dV/dt of evoked action potential was calculated during perfusion with procaine.

RESULTS

Mean intracellular pH was significantly lower in the bicarb/7.6 (7.16+/-0.07) group than in the nonbicarb/7.6 (7.33+/-0.09) and nonbicarb/8.0 (7.33+/-0.12) groups (P < 0.01). The mean intracellular ionized procaine concentration was significantly higher in the bicarb/7.6 (0.53+/-0.08 mM; P < 0.05) and nonbicarb/8.0 (0.58+/-0.13 mM; P < 0.01) than in nonbicarb/7.6 (0.32+/-0.14 mM) group but did not differ between the bicarb/7.6 and nonbicarb/8.0 groups. The mean percentage decrease in dV/dtmax was approximately coincident with the mean intracellular procaine concentration in each solution.

CONCLUSION

The presence of bicarbonate or extracellular alkalinization increased the intracellular concentration of ionized procaine and the anesthetic effect.

beta-Hydroxybutyrate fuels synaptic function during development. Histological and physiological evidence in rat hippocampal slices

To determine whether ketone bodies sustain neuronal function as energy substrates, we examined the effects of beta-hydroxybutyrate (betaHB) on synaptic transmission and morphological integrity during glucose deprivation in rat hippocampal slices. After the depression of excitatory postsynaptic potentials (EPSPs) by 60 min of glucose deprivation, administration of 0.5-10 mM D-betaHB restored EPSPs in slices from postnatal day (PND) 15 rats but not in slices from PND 30 or 120 rats. At PND 15, adding D-betaHB to the media allowed robust long-term potentiation of EPSPs triggered by high frequency stimulation, and prevented the EPSP-spike facilitation that suggests hyperexcitability of neurons. Even after PND 15,D-betaHB blocked morphological changes produced by either glucose deprivation or glycolytic inhibition. These results indicate that D-betaHB is not only able to substitute for glucose as an energy substrate but is also able to preserve neuronal integrity and stability, particularly during early development.

Graded, irreversible changes in crayfish giant axon as manifestations of lidocaine neurotoxicity in vitro

High concentrations of lidocaine induce irreversible conduction block with little effect on resting membrane potential (Em). We assumed the mechanism of persistent neurologic deficit caused by local anesthetics may result from neural death, as represented by the loss of Em. We investigated the effects of lidocaine on Em and action potential (AP) in single crayfish giant axons in vitro. Axons were perfused with two doses of lidocaine for either 15 or 30 min, and they were continuously washed. No axons exposed to 80 mM lidocaine for 30 min showed recovery of AP and Em. Those exposed to 40 mM for 30 min and 80 mM for 15 min showed a return to baseline for Em, but no recovery of AP. Those exposed to 40 mM lidocaine for 15 min showed full recovery of Em and AP immediately after washing. The membrane depolarization was significantly greater during exposure to 80 mM lidocaine for 30 min than in other groups. We conclude that lidocaine has a direct neurotoxic effect on crayfish giant axons and that the generation of AP is more vulnerable than the maintenance of Em. The irreversibility of AP and Em is dose- and time-dependent.

IMPLICATIONS

Highly concentrated lidocaine induced an irreversible conduction block and a complete loss of resting membrane potential in crayfish giant axons in vitro. Our results may represent a possible explanation for various grades of local anesthetic-induced neurotoxicity in clinical cases if the same toxicity occurs in mammalian nerves in vivo.

3-Hydroxykynurenine, an endogenous oxidative stress generator, causes neuronal cell death with apoptotic features and region selectivity

3-Hydroxykynurenine (3-HK) is a potential endogenous neurotoxin whose increased levels have been described in several neurodegenerative disorders. Here, we characterized in vitro neurotoxicity of 3-HK. Of the tested kynurenine pathway metabolites, only 3-HK, and to a lesser extent 3-hydroxyanthranilic acid, were toxic to primary cultured striatal neurons. 3-HK toxicity was inhibited by various antioxidants, indicating that the generation of reactive oxygen species is essential to the toxicity. 3-HK-induced neuronal cell death showed several features of apoptosis, as determined by the blockade by macromolecule synthesis inhibitors, and by the observation of cell body shrinkage with nuclear chromatin condensation and fragmentation. In addition, 3-HK toxicity was dependent on its cellular uptake via transporters for large neutral amino acids, because uptake inhibition blocked the toxicity. Cortical and striatal neurons were much more vulnerable to 3-HK toxicity than cerebellar neurons, which may be attributable to the differences in transporter activities of these neurons. These results indicate that 3-HK, depending on transporter-mediated cellular uptake and on intracellular generation of oxidative stress, induces neuronal cell death with brain region selectivity and with apoptotic features, which may be relevant to pathology of neurodegenerative disorders.

Oxygen deprivation produces delayed inhibition of long-term potentiation by activation of NMDA receptors and nitric oxide synthase

The acute and delayed effects of anoxia on synaptic transmission and long-term potentiation (LTP) were examined in the CA1 region of rat hippocampal slices. Oxygen deprivation for 20 minutes completely but reversibly depressed excitatory postsynaptic potentials mediated by both N-methyl-D-aspartate receptors (NMDAR) and non-NMDAR. Although LTP was reliably produced by a single tetanus delivered 30 minutes after reoxygenation, LTP could not be induced when a tetanus was delivered 70 to 100 minutes after reoxygenation. A tetanus delivered 100 minutes after reoxygenation produced lasting synaptic enhancement when 100 mumol/L D,L-amino-phosphonovaleric acid (APV), a competitive NMDAR antagonist, was administered during the period of oxygen deprivation. The delayed effects of oxygen deprivation were not blocked when APV was administered after oxygen deprivation. Similarly, the delayed effects on LTP induction were overcome by inhibitors of nitric oxide synthase when the nitric oxide synthase inhibitors were administered during anoxia, but not when administered after oxygen deprivation. These results suggest that untimely activation of NMDAR and nitric oxide release during anoxia produce delayed inhibition of LTP induction and may be involved in the memory defects that occur subsequent to cerebral hypoxia.

Endogenous monocarboxylates sustain hippocampal synaptic function and morphological integrity during energy deprivation

The ability to fuel neurons via glycogenolysis is believed to be an important function of glia. Indeed, the slow, rather than immediate, depression of synaptic transmission in hippocampal slices during exogenous glucose deprivation suggests that intrinsic energy reservoirs help to sustain neurotransmission. It is believed that glia fuel neighboring neurons via diffusible monocarboxylates such as pyruvate and lactate, although a role for glucose has been proposed also. Using alpha-cyano-4-hydroxycinnamate (4-CIN) to inhibit monocarboxylate transport and cytochalasin B (CCB) to inhibit glucose transport, we examined the role of glucose and monocarboxylates in supporting the functional and morphological integrity of hippocampal neurons during glucose deprivation. Although 200 microM 4-CIN failed to depress EPSPs supported by 10 mM glucose, pretreatment with 4-CIN accelerated the depression of EPSPs during glucose deprivation. 4-CIN also accelerated the decline in glucose-supported EPSPs after administration of 50 microM CCB, whereas CCB failed to alter the slow decay of pyruvate-supported EPSPs during pyruvate deprivation. 4-CIN did not alter the morphology of pyramidal neurons in the presence of 10 mM glucose but produced significant damage during glucose deprivation or CCB administration. These results suggest that endogenous monocarboxylates rather than glucose maintain neuronal integrity during energy deprivation. Furthermore, EPSPs supported by 2-3.3 mM glucose were sensitive to 4-CIN, suggesting that endogenous monocarboxylates are involved in maintaining neuronal function even under conditions of mild glucose deprivation.

Neurotrophic activity of organosulfur compounds having a thioallyl group on cultured rat hippocampal neurons

Several organosulfur compounds found in garlic extract promoted the survival of rat hippocampal neurons in vitro. From the analysis of structure-activity relationship, thioallyl group in these compounds is essential for the manifestation of neurotrophic activity. S-Allyl-L-cysteine (SAC), one of the organosulfur compounds having thioallyl group in garlic extract, also promoted the axonal branching of cultured neurons. These results suggest that thioallyl compounds make a unique group of neurotrophic factors.

Allixin, a phytoalexin produced by garlic, and its analogues as novel exogenous substances with neurotrophic activity

Effects of allixin, a phytoalexin of garlic, and its analogues were studied on the survival and morphology of primary cultured neurons from fetal rat brain. Addition of allixin (1-100 ng/ml) to medium significantly promoted the survival of neurons derived from various regions of brain and increased the number of branching points per axon in hippocampal neurons. Allixin, however, was cytotoxic at higher concentrations (>1 microg/ml). Among the analogues of allixin, 2,6-dimethyl-3-hydroxy-4H-pyran-4-one (DHP) possessed potent neurotrophic activity at concentrations over 10 ng/ml without any obvious cytotoxicity up to 10 microg/ml. DHP also retained the activity to promote axonal branching. These results indicate that DHP is a novel exogenous low molecular weight neurotrophic substance without apparent cytotoxicity. This compound may be a useful prototype leading chemical for developing therapeutic and/or prophylactic drugs for neurodegenerative disorders.

Biphasic effect of hydrogen peroxide on field potentials in rat hippocampal slices

In the CA1 region of rat hippocampal slices, H2O2 (0.294-2.94 mM) caused initial augmentation, and subsequent long-lasting depression, of population spikes and excitatory postsynaptic potentials. The effect of H2O2 may not be mediated by its degradation product, hydroxyl radicals, because an iron chelator deferoxamine did not block the effect. A catalase inhibitor 3-amino-1,2,4-triazole only modestly attenuated the initial augmentation, suggesting that the effect of H2O2 is not attributable to catalase-dependent O2 generation, either. An N-methyl-D-aspartate receptor antagonist DL-2-amino-5-phosphonovaleric acid had no influence on the effect of H2O2, whereas a gamma-aminobutyric acid type A receptor channel blocker picrotoxin attenuated long-lasting depression, indicating that gamma-aminobutyric acid-mediated inhibition is altered during the depression phase. The initial augmentation but not subsequent depression was attenuated by a phospholipase A2/C inhibitor 4-bromophenacyl bromide, suggesting the involvement of lipid signaling molecule(s) in the enhancement of excitatory synaptic transmission. These results suggest that H2O2 regulates hippocampal synaptic transmission via multiple mechanisms.

Monocarboxylates (pyruvate and lactate) as alternative energy substrates for the induction of long-term potentiation in rat hippocampal slices

We previously reported that mild hypoglycemic conditions hamper the induction of long-term potentiation (LTP) in the CA1 region of rat hippocampal slices. In the present study, we examined whether monocarboxylates (pyruvate and lactate) permit LTP induction when glucose is unavailable. During glucose deprivation, pyruvate and lactate not only maintained basal synaptic transmission but also allowed LTP induction, with pyruvate being more effective than lactate. Additionally, LTP inhibition by 5 microM iodoacetate, a glycolytic inhibitor, was prevented by addition of pyruvate but not lactate. These results suggest that monocarboxylates can be affected alternative energy substrates for LTP induction when glucose is not available. However, lactate and pyruvate are not identical as energy sources for LTP induction.

Furosemide-sensitive calcium rise induced by GABAA-receptor stimulation in cultures of embryonic rat striatal neurons

Changes in [Ca2+]i induced by gamma-aminobutyric acid (GABA) were investigated in primary cultured neurons obtained from fetal rat striatum. GABA and muscimol induced [Ca2+]i rise, and bicuculline blocked the effect of GABA. The [Ca2+]i elevating effect of GABA was also abolished by removal of extracellular Ca2+ or by application of nicardipine. Furthermore, furosemide, an inhibitor of Na+/K+/ 2Cl- co-transport, reversibly inhibited the GABA-induced [Ca2+]i rise. These results suggest that due to the elevated level of intracellular Cl- maintained by Na+/K+/2Cl- transport activity in these neurons, opening of GABAA-receptor-associated Cl- channels results in Cl- efflux, leading to membrane depolarization and activation of L-type voltage-dependent Ca2+ channels.

Noradrenergic regulation of synaptic plasticity in the hippocampal CA1 region

The effects of norepinephrine (NE) and related agents on long-lasting changes in synaptic efficacy induced by several patterns of afferent stimuli were investigated in the CA1 region of rat hippocampal slices. NE (10 microM) showed little effect on the induction of long-term potentiation (LTP) triggered by theta-burst-patterned stimulation, whereas it inhibited the induction of long-term depression (LTD) triggered by 900 pulses of 1-Hz stimulation. In nontreated slices, 900 pulses of stimuli induced LTD when applied at lower frequencies (1-3 Hz), and induced LTP when applied at a higher frequency (30 Hz). NE (10 microM) caused a shift of the frequency-response relationship in the direction preferring potentiation. The effect of NE was most prominent at a stimulus frequency of 10 Hz, which induced no changes in control slices but clearly induced LTP in the presence of NE. The facilitating effect of NE on the induction of LTP by 10-Hz stimulation was blocked by the beta-adrenergic receptor antagonist timolol (50 microM), but not by the alpha receptor antagonist phentolamine (50 microM), and was mimicked by the beta-agonist isoproterenol (0.3 microM), but not by the alpha1 agonist phenylephrine (10 microM). The induction of LTD by 1-Hz stimulation was prevented by isoproterenol but not by phenylephrine, indicating that the activation of beta-receptors is responsible for these effects of NE. NE (10 microM) also prevented the reversal of LTP (depotentiation) by 900 pulses of 1-Hz stimulation delivered 30 min after LTP induction. In contrast to effects on naive (nonpotentiated) synapses, the effect of NE on previously potentiated synapses was only partially mimicked by isoproterenol, but fully mimicked by coapplication of phenylephrine and isoproterenol. In addition, the effect of NE was attenuated either by phentolamine or by timolol, indicating that activation of both alpha1 and beta-receptors is required. These results show that NE plays a modulatory role in the induction of hippocampal synaptic plasticity. Although beta-receptor activation is essential, alpha1 receptor activation is also necessary in determining effects on previously potentiated synapses.

Monocarboxylic acids enhance the anesthetic action of procaine by decreasing intracellular pH

Sodium monocarboxylates are known to enhance the anesthetic action of procaine, and also decrease intracellular pH (pHi). We studied the effect of 30 mM Na monocarboxylates (formate, acetate, propionate, butyrate, and salicylate) on the pHi and on the anesthetic action of procaine HCl using giant axons of crayfish (Procambarus clarkii). The pHi was measured using pH sensitive microelectrode method and the anesthetic action was evaluated by the change in the action potential (AP) amplitude. The tested acids except for formate showed apparent decrease in pHi and enhancement of the action of 2 mM procaine. Other organic acids (maleate and benzensulfonate) did not affect pHi and anesthetic action of procaine. In the bicarbonate free solution, pHi increased and the anesthetic action was weakened. The EC25 values (the concentration of procaine which depresses the AP amplitude by 25%) of acetate, propionate, and bicarbonate free solution were coincided with the predicted EC25 values from the simple simulation on intracellular procaine increase according to the pHi change. But the EC25 value of salicylate group was less than half of the predicted. These results suggested that the enhancing action of straight chain monocarboxylic acids is due to pHi decrease, and salicylate has other additional mechanisms.

GABAA receptor stimulation promotes survival of embryonic rat striatal neurons in culture

In order to clarify the functional role of gamma-aminobutyric acid (GABA) in developing brain, we investigated the effect of GABA on the survival of embryonic rat striatal neurons in dissociated cell culture. Chronic exposure of striatal cultures to GABA resulted in a significant increase in the number of surviving neurons. The effect of GABA was concentration-dependent (1-1000 microM) and was blocked by a GABAA receptor antagonist, bicuculline (100 microM), or a GABAA chloride channel blocker, picrotoxin (100 microM), but not by a GABAB receptor antagonist, 2-hydroxysaclofen (100 microM). In addition, the GABAA receptor agonist muscimol mimicked the effect of GABA, promoting cell survival in a concentration-dependent manner (0.01-100 microM), while the GABAB receptor agonist baclofen (up to 100 microM) had no significant effect. The GABA-induced enhancement of neuronal survival was suppressed by the L-type voltage-dependent Ca2+ channel blockers nifedipine (1-3 microM) and nicardipine (1-5 microM). Protein kinase inhibitors, H-7 (10-30 microM) or genistein (3 microM), also suppressed GABA-induced enhancement of neuronal survival. These results suggest that stimulation of GABAA receptors enhances survival of embryonic striatal neurons, and that the effect is mediated by Ca2+ influx through L-type voltage-dependent Ca2+ channels, initiating intracellular signaling cascades that involve activation of H-7- and genistein-sensitive protein kinases.