Yukitoshi Izumi

Nitric oxide inhibitors attenuate N-methyl-d-aspartate excitotoxicity in rat hippocampal slices

To investigate whether nitric oxide (NO) plays a role in the neurotoxicity produced by N-methyl-D-aspartate (NMDA) we have examined the effects of NO inhibitors on NMDA-mediated neurodegeneration in the CA1 region of rat hippocampal slices. L-NG-Monomethylarginine, L-NG-nitroarginine and hemoglobin markedly diminished the toxicity produced by activation of NMDA receptors without interfering with NMDA receptor-mediated ion currents or synaptic responses. The neuroprotective effects are reversed by coapplication of L-arginine with the NO synthase inhibitors. These results suggest that activation of the NO system is an important component of the biochemical cascade leading to neurodegeneration produced by NMDA receptors.

Zinc Modulates Bidirectional Hippocampal Plasticity by Effects on NMDA Receptors

Zinc has complex effects on NMDA receptors (NMDARs) and may be an endogenous modulator of synaptic plasticity. In the CA1 region of rat hippocampal slices, we observed that low micromolar concentrations of zinc depress NMDAR synaptic responses by 40–50% and inhibit long-term depression (LTD) but not long-term potentiation (LTP). A combination of zinc plus ifenprodil, an inhibitor of NR1/NR2B receptors, produced no greater inhibition of synaptic NMDARs than either agent alone, suggesting overlapping effects on NMDARs. Similar to low micromolar zinc, ifenprodil inhibited LTD but not LTP. In contrast, low concentrations of 2-amino-5-phosphonovalerate (APV) did not block either LTP or LTD despite producing >50% inhibition of synaptic NMDARs. NVP-AAM077 ([(R)-[(S)-1-(4-bromo-phenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydro-quinoxalin-5-yl)-methyl]phosphonic acid), an antagonist with relative NR1/NR2A selectivity at low concentrations, also inhibited synaptic NMDARs by ∼50% at 0.05 μm but failed to completely block either LTP or LTD. These results suggest that LTD induction depends on specific NMDARs with sensitivity to low micromolar zinc and ifenprodil, but LTP is less dependent on specific NMDAR subtypes. Because high-affinity sites of NR2A are likely occupied by ambient zinc, we also examined effects of extracellular zinc chelators. Zinc chelation blocked LTP but had no effect on LTD. This LTP inhibition was overcome by APV and NVP-AAM077 but not ifenprodil, suggesting that zinc chelation unmasks tonic NR1/NR2A activation that negatively modulates LTP.

Effects of lactate and pyruvate on glucose deprivation in rat hippocampal slices

Rat hippocampal slices were used to evaluate the effects of glucose deprivation and the ability of lactate or pyruvate to preserve histological integrity and synaptic function. Dark cell changes were observed during 180 min incubations in glucose-free solutions. These changes were blocked by substituting 10 mM lactate or pyruvate for glucose during the incubation. Excitatory postsynaptic potentials disappeared during 60 min of glucose deprivation but were restored by subsequent introduction of glucose, lactate or pyruvate. Incubation of slices with iodoacetate revealed a distinct pattern of damage that was blocked completely by pyruvate and partially by lactate. These results indicate that exogenous pyruvate and lactate can serve as energy substrates in the hippocampus when glucose is unavailable or glycolytic metabolism is impaired.

Treatment with an Amyloid-β Antibody Ameliorates Plaque Load, Learning Deficits, and Hippocampal Long-Term Potentiation in a Mouse Model of Alzheimer's Disease

PDAPP transgenic mice overexpress a mutant form of human amyloid precursor protein under control of the platelet-derived growth factor promoter in CNS neurons that causes early onset, familial Alzheimers disease in humans. These mice, on a mixed genetic background, have been shown to have substantial learning impairments from early ages, as well as an age-dependent decline in learning ability that has been hypothesized to be caused by amyloid-β (Aβ) accumulation. The goals of this study were to determine: (1) whether PDAPP mice on a pure C57BL/6 background develop more severe age-dependent learning deficits than wild-type mice; (2) if so, whether Aβ accumulation accounts for the excessive decline in learning ability; and (3) whether the learning deficits are reversible, even after significant Aβ deposition. At 4-6, 10-12, or 17-19 months of age, PDAPP and littermate wild-type mice on a C57BL/6 background were tested on a 5 week water maze protocol in which the location of the escape platform changed weekly, requiring the mice to repeatedly learn new information. PDAPP mice exhibited impaired spatial learning as early as 4 months (pre-Aβ deposition), and the performance of both wild-type and PDAPP mice declined with age. However, PDAPP mice exhibited significantly greater deterioration with age. Direct evidence for the role of Aβ accumulation in the age-related worsening in PDAPP mice was provided by the observation that systemic treatment over several weeks with the anti-Aβ antibody 10D5 reduced plaque deposition, increased plasma Aβ, improved hippocampal long-term potentiation, and improved behavioral performance in aged PDAPP mice with substantial Aβ burden.

Nitric oxide and hippocampal synaptic plasticity

The dependence of NMDA receptor-dependent LTP on postsynaptic depolarization and increases in postsynaptic calcium, coupled with evidence supporting presynaptically mediated increases in transmitter release accompanying LTP, suggest that a retrograde transsynaptic messenger participates in the synaptic enhancement. Although many questions remain unanswered, the available evidence suggests a role for NO as such a messenger in certain LTP paradigms. It is unclear, however, whether NO contributes to LTP under differing experimental conditions and whether other messengers, acting in concert with or independent of NO, contribute to a retrograde signalling system. Furthermore, the conditions under which NMDA receptor activation, postsynaptic calcium increases and NO contribute to synaptic enhancement, synaptic depression and excitotoxic neuronal injury need to be clarified. Furthermore, efforts aimed at clarifying the molecular targets of NO must remain a priority of this line of research.

Norepinephrine promotes long‐term potentiation in the adult rat hippocampus in vitro

We previously found a reduction in the ability of a single 100 Hz × 1 sec tetanus to induce long‐term potentiation (LTP) in the CAl region of hippocampal slices prepared from adult animals. To determine whether this reduction in LTP generation results from changes in neuromodulator function, we examined the ability of several neuromodulators to promote LTP in slices prepared from mature rats. Although acetylcholine, N‐methyl‐D‐aspartate, and an agonist at metabotropic glutamate receptors failed to promote LTP, administration of norepinephrine allowed robust LTP. The effects of norepinephrine were mimicked by an α1‐adrenergic agonist and were blocked by an α1‐receptor antagonist. β‐adrenergic agonists and antagonists were ineffective. These results suggest that norepinephrine acting via α1‐adrenoceptors may be an important cofactor in promoting lasting synaptic plasticity in the adult central nervous system and that changes in adrenergic function may contribute to maturation‐ or aging‐associated changes in memory function. Synapse 31:196–202, 1999.

Neurosteroids, stress and depression: Potential therapeutic opportunities

Neurosteroids are potent and effective neuromodulators that are synthesized from cholesterol in the brain. These agents and their synthetic derivatives influence the function of multiple signaling pathways including receptors for γ-aminobutyric acid (GABA) and glutamate, the major inhibitory and excitatory neurotransmitters in the central nervous system (CNS). Increasing evidence indicates that dysregulation of neurosteroid production plays a role in the pathophysiology of stress and stress-related psychiatric disorders, including mood and anxiety disorders. In this paper, we review the mechanisms of neurosteroid action in brain with an emphasis on those neurosteroids that potently modulate the function of GABA(A) receptors. We then discuss evidence indicating a role for GABA and neurosteroids in stress and depression, and focus on potential strategies that can be used to manipulate CNS neurosteroid synthesis and function for therapeutic purposes.

2-Amino-3-phosphonopropionate blocks the induction and maintenance of long-term potentiation in rat hippocampal slices

Rat hippocampal slices were used to examine the effects of 2-amino-3-phosphonopropionate (AP3), an inhibitor of phosphatidylinositol (PI) turnover linked to metabotropic quisqualate receptors, on the development and maintenance of long-term potentiation (LTP) in area CA1. When perfused for 5 min prior to tetanization at concentrations of 100-1000 microM, D,L-AP3 had no effect on baseline synaptic transmission but blocked posttetanic potentiation (PTP) and the induction of LTP. Unlike the N-methyl-D-aspartate (NMDA) antagonists, 2-amino-5-phosphonovalerate (AP5) and MK-801, AP3 eliminated the late phase of LTP when applied immediately after tetanization. These data support the hypothesis that PI turnover is a factor in the expression and maintenance of LTP.

Long-term potentiation requires activation of calcium-independent phospholipase A2.

The predominant phospholipase activity present in rat hippocampus is a calcium‐independent phospholipase A2 (302.9 ± 19.8 pmol/mg·min for calcium‐independent phospholipase A2 activity vs. 14.6 ± 1.0 pmol/mg·min for calcium‐dependent phospholipase A2 activity). This calcium‐independent phospholipase A2 is exquisitely sensitive to inhibition by the mechanism‐based inhibitor, (E)‐6‐(bromomethylene)‐tetrahydro‐3‐(1‐naphthalenyl)‐2H‐pyran ‐2‐one (BEL). Moreover, treatment of hippocampal slices with BEL prior to tetanic stimulation prevents the induction of LTP (40.8 ± 5.6% increase in excitatory postsynaptic potential (EPSP) slope for control slices (n = 6) vs. 5.8 ± 8.5% increase in EPSP slope for BEL‐treated slices (n = 8)). Importantly, LTP can be induced following mechanism‐based inhibition of phospholipase A2 by providing the end product of the phospholipase A2 reaction, arachidonic acid, during the application of tetanic stimulation. Furthermore, the induction of LTP after treatment with BEL is dependent on the stereoelectronic configuration of the fatty acid provided since eicosa‐5,8,11‐trienoic acid, but not eicosa‐8,11,14‐trienoic acid, rescues LTP after BEL treatment (37.6 ± 16.1% increase in EPSP slope for eicosa‐5,8,11‐trienoic acid vs. −3.7 ± 5.2% increase in EPSP slope for eicosa‐8,11,14‐trienoic acid). Collectively, these results provide the first demonstration of the essential role of calcium‐independent phospholipase A2 in synaptic plasticity.

NEUROEXCITATORY ACTIONS OF TAMIFLU AND ITS CARBOXYLATE METABOLITE

Oseltamivir (Tamiflu) is now being stockpiled by several governments as a first line treatment for an anticipated outbreak of avian influenza caused by H5N1. However, abnormal behaviors and death associated with the use of Tamiflu have developed into a major issue in Japan where Tamiflu is often prescribed for seasonal influenza. Thus, it is critical to determine neuropsychiatric effects of oseltamivir and to establish methods for safe administration. Using juvenile rats and rat hippocampal slices, we investigated whether oseltamivir has adverse effects on the central nervous system. Systemic injection of oseltamivir (50mg/kg i.p.) produced no change in behavior within 2h. However, prior injection of oseltamivir significantly altered the duration of loss of lightning reflex following ethanol injection (3.3g/kg, i.p.). Ethanol injection in the presence of oseltamivir also resulted in enhanced hypothermia. In the CA1 region of hippocampal slices, oseltamivir (100 microM) induced paired-pulse facilitation in population spikes without changes in excitatory postsynaptic potentials. Similarly, 3 microM oseltamivir carboxylate, the active metabolite of oseltamivir, facilitated neuronal firing, though the facilitation did not involve GABAergic disinhibition. Moreover, oseltamivir carboxylate produced further facilitation following administration of 60mM ethanol. These findings indicate that oseltamivir has effects on the central nervous system, especially when combined with other agents.