Dopamine-induced protection of striatal neurons against kainate receptor-mediated glutamate cytotoxicity in vitro

REVIEW ■ : ATP-sensitive Potassium Channels: Diverse Functions in the Central Nervous System

ATP-sensitive potassium channels open when cytoplasmic levels of ATP drop, thus linking membrane potential to the metabolic state of the cell. Cloning studies have suggested that these channels are related structurally to the inward rectifier family of potassium channels, with two putative membrane-spanning regions. Sulfonylurea drugs, which are used in the treatment of diabetes, inhibit these channels by binding to an associated membrane protein. Other drugs, including some vasodilators, activate ATP-sensitive potassium channels. Diverse neurotransmitter and hormone receptors can modulate these channels, in some cases through interactions with guanyl nucleotide binding proteins. There appear to be multiple subtypes of these channels, differing in electrical properties as well as in drug sensitivities. In the brain, these channels appear to play a role in mediating satiety after feeding. They also function in neurons to protect against excitotoxicity, by counteracting the membrane depolarization associated with metabolic stress. Brain dopamine receptors appear to modulate a novel subtype of ATP-sensitive potassium channel. The association of dopamine receptors with a mechanism involved in protection against neurodegeneration may have implications for the causes of diseases in which dopaminergic regions of brain undergo structural changes, possibly including schizophrenia. NEUROSCIENTIST 2:145-152, 1996

The effect of dopamine on adult hippocampal neurogenesis

Cumulative studies indicated that adult hippocampal neurogenesis might be involved in the action mechanism of antidepressant drugs and/or the pathophysiology of depression. Dopamine (DA) is involved in the regulation of motivation, volition, interest/pleasure, and attention/concentration, all of which are likely to be impaired in depressed patients. Several previous reports suggest that depression may often be accompanied by a relative hypo-dopaminergic state, and some DA receptor agonists are beneficial effects in the treatment for refractory and bipolar depression. In the present study, to clarify the direct effect of DA on neural progenitor cells, we examined the effect of DA on the proliferation of adult rat dentate gyrus-derived neural precursor cells (ADPs). In addition, we examined the effect of DA receptor agonists on adult rat hippocampal neurogenesis in vivo. Results showed that DA promoted the increase of ADPs via D1-like receptor and D1-like receptor agonist promoted the survival of newborn cells in the adult hippocampus. On the contrary, D2-like receptor agonist did not affect both proliferation and survival. These results suggested that DA might play, at least in part, a role in adult hippocampal neurogenesis via D1-like receptor and the activation of D1-like receptor has a therapeutic potential for depression.

MPP(+) and glutamate in the degeneration of nigral dopaminergic neurons

Glutamate neurotoxicity can be an experimental oxidative stress, and we investigated glutamate toxicity against cultured rat mesencephalic neurons. Although glutamate showed similar toxicity against dopaminergic and nondopaminergic neurons, nitric oxide (NO) showed neurotoxicity restricted exclusively in nondopaminergic neurons. An inhibitor of NO synthase had no significant effect on the glutamate toxicity against dopaminergic neurons, however, it had a significant antagonistic effect on that against nondopaminergic neurons. These findings indicate the presence of two mechanisms of glutamate neurotoxicity, one being not mediated by NO, found in dopaminergic neurons, and the other being mediated via NO, found in nondopaminergic neurons. In contrast to NO, peroxynitrite (ONOO(-)), an active metabolite of NO, caused significant cytotoxicity against dopaminergic and nondopaminergic neurons, suggesting that conversion of NO to ONOO(-) is suppressed in dopaminergic neurons. After pretreatment with small doses of methyl-4-phenylpyridium ion (MPP(+)), NO caused significant cytotoxicity against dopaminergic neurons, and glutamate toxicity was enhanced only against dopaminergic neurons. Therefore, sublethal dose of MPP(+) enhances glutamate toxicity against dopaminergic neurons, probably by the facilitation of suppressed NO conversion to ONOO(-) in dopaminergic neurons. Finally, to provide basic data for neuroprotective therapy in Parkinson's disease, we investigated neuroprotection against glutamate toxicity by dopamine agonists. Preincubation with the D2 type dopamine agonists provides neuroprotection against glutamate neurotoxicity and the protective effects blocked by a D2 antagonist, indicating that D2 agonists provide protection mediated not only by the inhibition of dopamine turnover, but also via D2 type dopamine receptor.

Dopamine promotes striatal neuronal apoptotic death via ERK signaling cascades

Although the mechanisms underlying striatal neurodegeneration are poorly understood, we have shown that striatal pathogenesis may be initiated by high synaptic levels of extracellular dopamine (DA). Here we investigated in rat striatal primary neurons the mobilization of the mitogen-activated protein kinase (MAPK) signaling pathways after treatment with DA. Instead of observing an elevation of the archetypical pro-cytotoxic MAPKs, p-JNK and p-p38 MAPK, we found that DA, acting through D1 DA receptors, induced a sustained stimulation of the phosphorylated form of extracellular signal-regulated kinase (p-ERK) via a cAMP/protein kinase A (PKA)/Rap1/B-Raf / MAPK/ERK kinase (MEK) pathway. Blockade of D2 DA receptors, beta-adrenergic receptors or N-methyl-D-aspartate receptors with receptor-specific antagonists had no significant effect on this process. Activation of D1 DA receptors and PKA by DA caused phosphorylation and inactivation of the striatal-enriched tyrosine phosphatase, an important phosphatase for the dephosphorylation and subsequent inactivation of p-ERK in the striatum. Interestingly, p-ERK was primarily retained in the cytoplasm, with only low amounts translocated to the nucleus. The scaffold protein beta-arrestin2 interacted with both p-ERK and D1 DA receptor, triggering the cytosolic retention of p-ERK and inducing striatal neuronal apoptotic death. These data provide unique insight into a novel role of p-ERK in striatal neurodegeneration.

Common key-signals in learning and neurodegeneration: focus on excito-amino acids, beta-amyloid peptides and alpha-synuclein

In this paper a hypothesis that some special signals ("key-signals" excito-amino acids, beta-amyloid peptides and alpha-synuclein) are not only involved in information handling by the neuronal circuits, but also trigger out substantial structural and/or functional changes in the Central Nervous System (CNS) is introduced. This forces the neuronal circuits to move from one stable state towards a new state, but in doing so these signals became potentially dangerous. Several mechanisms are put in action to protect neurons and glial cells from these potentially harmful signals. However, in agreement with the Red Queen Theory of Ageing (Agnati et al. in Acta Physiol Scand 145:301-309, 1992), it is proposed that during ageing these neuroprotective processes become less effective while, in the meantime, a shortage of brain plasticity occurs together with an increased need of plasticity for repairing the wear and tear of the CNS. The paper presents findings supporting the concept that such key-signals in instances such as ageing may favour neurodegenerative processes in an attempt of maximizing neuronal plasticity.

Dopamine D1-Like Receptors Depress Excitatory Synaptic Transmissions in Striatal Neurons After Transient Forebrain Ischemia

Background and Purpose— Spiny neurons in the neostriatum are highly vulnerable to ischemia. Despite an enormous body of research suggesting that dopamine is involved in ischemia-induced neuronal loss in the striatum, it remains unclear how dopamine interacts with the glutamatergic excitotoxicity that is widely accepted as a major cause of ischemic cell death. Our study was designed to investigate the effects of dopamine D1 receptor (D1R) activation on excitatory neurotransmission in postischemic striatal neurons.

Methods— We used the 4-vessel occlusion ischemia model and brain slice preparations. Whole-cell voltage-clamp recording was performed on striatal neurons to measure excitatory postsynaptic currents (EPSCs). Systemic administration of a D1R agonist after ischemia and hematoxylin/eosin staining were performed to evaluate the effects of D1R activation on ischemia-induced neuronal degeneration in the striatum.

Results— D1R activation depressed EPSCs in postischemic striatal neurons. The depression was attributable to inhibition of presynaptic release. An activator of cAMP-dependent protein kinase A (PKA) mimicked the depressive effects of D1R activation. Bath application of a PKA inhibitor blocked the depression of EPSCs, whereas intracellular postsynaptic application of the PKA inhibitor had no effect. The D1R agonist failed to reduce EPSC amplitude in the presence of an adenosine A1 receptor antagonist. Systemic administration of a D1R agonist after ischemia significantly attenuated ischemia-induced cell death in the striatum.

Conclusions— These results indicate that D1R activation presynaptically depresses excitatory synaptic transmission in striatal neurons after ischemia through activation of PKA and adenosine A1 receptors and thus demonstrate a novel mechanism of D1R-mediated protection against ischemia.

Dopamine in neurotoxicity and neuroprotection: what do D2 receptors have to do with it?

Accurate control of dopamine levels and/or the resulting dopamine-receptor interaction is essential for brain function. Indeed, several human neurological and psychiatric disorders are characterized by dysfunctions of the dopaminergic system. Dopamine has been reported to exert either protective or toxic effects on neurons, yet it is unclear whether these effects are receptor-dependent and, if so, which dopamine receptor could be involved. The D(2) dopamine receptor occupies a privileged position because its signalling might be neuroprotective in human diseases, such as Parkinson's disease, ischaemia and epilepsy. Unravelling the role of D(2) receptors in neuronal death and survival might be central to understanding the mechanisms that underlie several neuropathologies.

Clozapine, ziprasidone and aripiprazole but not haloperidol protect against kainic acid-induced lesion of the striatum in mice, in vivo: Role of 5-HT1A receptor activation

Excessive activation of non-NMDA receptors, AMPA and kainate, contributes to neuronal degeneration in acute and progressive pathologies, possibly including schizophrenia. Because 5-HT(1A) receptor agonists have neuroprotective properties (e.g., against NMDA-induced neurotoxicity), we compared the effects of the antipsychotics, clozapine, ziprasidone and aripiprazole, that are partial agonists at 5-HT(1A) receptor, with those of haloperidol, which is devoid of 5-HT(1A) agonist properties, on kainic acid (KA)-induced striatal lesion volumes, in C57Bl/6N mice. The involvement of 5-HT(1A) receptors was determined by antagonist studies with WAY100635, and data were compared with those obtained using the potent and high efficacy 5-HT(1A) receptor agonist, F13714. Intra-striatal KA lesioning and measurement of lesion volumes using cresyl violet staining were carried out at 48 h after surgery. F13714, antipsychotics or vehicle were administered ip twice, 30 min before and 3 1/2 h after KA injection. WAY100635 (0.63 mg/kg) or vehicle were given sc 30 min before each drug injection. Clozapine (2 x 10 mg/kg), ziprasidone (2 x 20 mg/kg) and aripiprazole (2 x 10 mg/kg) decreased lesion volume by 61%, 59% and 73%, respectively. WAY100635 antagonized the effect of ziprasidone and of aripiprazole but only slightly attenuated that of clozapine. In contrast, haloperidol (2 x 0.16 mg/kg) did not affect KA-induced lesion volume. F13714 dose-dependently decreased lesion volume. The 61% decrease of lesion volume obtained with F13714 (2 x 0.63 mg/kg) was antagonized by WAY100635. WAY100635 alone did not affect lesion volume. These results show that 5-HT(1A) receptor activation protects against KA-induced striatal lesions and indicate that some atypical antipsychotic agents with 5-HT(1A) agonist properties may protect against excitotoxic injury, in vivo.

Dopamine D2 and D3 receptor agonists limit oligodendrocyte injury caused by glutamate oxidative stress and oxygen/glucose deprivation

Dopamine receptor activation is thought to contribute adversely to several neuropathological disorders, including Parkinson's disease and schizophrenia. In addition, dopamine may have a neuroprotective role: dopamine receptor agonists are reported to protect nerve cells by virtue of their antioxidant properties as well as by receptor-mediated mechanisms. White matter injury can also be a significant factor in neurological disorders. Using real-time RT-PCR, we show that differentiated rat cortical oligodendrocytes express dopamine D2 receptor and D3 receptor mRNA. Oligodendrocytes were vulnerable to oxidative glutamate toxicity and to oxygen/glucose deprivation injury. Agonists for dopamine D2 and D3 receptors provided significant protection of oligodendrocytes against these two forms of injury, and the protective effect was diminished by D2 and D3 antagonists. Levels of oligodendrocyte D2 receptor and D3 receptor protein, as measured by Western blotting, appeared to increase following combined oxygen and glucose deprivation. Our results suggest that dopamine D2 and D3 receptor activation may play an important role in oligodendrocyte protection against oxidative glutamate toxicity and oxygen-glucose deprivation injury.

Dopaminergic modulation of oxidative stress and apoptosis in human peripheral blood lymphocytes: evidence for a D1-like receptor-dependent protective effect

Dopamine (DA) is a neurotransmitter in the central and peripheral nervous system, which can be either cytotoxic or cytoprotective under selected conditions. Such effects involve oxidative mechanisms and are likely to play a role in neurodegenerative disorders. Because increasing evidence points to peripheral blood lymphocytes (PBL) as a feasible model for studying DA-related mechanisms of cell death and survival, we have explored in these cells the effects of DA on oxidative metabolism and apoptosis. Our results show that, whereas DA 100-500 microM resulted in increased intracellular reactive oxygen species (ROS) levels and apoptotic cell death through oxidative stress, DA 0.1-5 microM decreased ROS levels and apoptosis. DA (both 1 and 500 microM) partially counteracted the decrease in Cu/Zn superoxide dismutase levels observed in untreated PBL. However, whereas the effect of the low dose lasted for the whole incubation period (24 h), the effect of DA 500 microM was transient. DA-dependent reduction of ROS levels and apoptosis was prevented by D1-like (but not D2-like) receptor antagonism. The present findings add knowledge about the sensitivity of PBL to DA and strengthen the rationale for exploiting these cells as an easily accessible peripheral model for the ex vivo investigation of oxidative stress-related dopaminergic mechanisms underlying human neurodegenerative diseases.

Neuroprotective effects of alpha-tocopherol on oxidative stress in rat striatal cultures

Oxidative stress caused by an increase in free radicals plays an important role in neuronal death. We investigated the effects of alpha-tocopherol on oxidative stress-induced cytotoxicity using primary cultures of rat striatal neurons. alpha-Tocopherol at concentrations of 1-10 microM significantly prevented cytotoxicity induced by superoxide radical (O(2(-)) donor, 1,1'-dimethyl-4,4'-bipyridium dichloride (paraquat). In contrast, alpha-tocopherol did not affect the cytotoxicity of hydrogen peroxide (H(2)O(2)), which enhances hydroxyl radical (.OH) formation by metal-catalyzed Fenton reactions. alpha-Tocopherol significantly inhibited the cytotoxicity of nitric oxide (NO) donors, S-nitrosocysteine and 3-morpholinosydnonimine (SIN-1). alpha-Tocopherol showed potent protection against cytotoxicity induced by L-buthionine-[S,R]-sulfoximine (BSO), which causes depletion of intracellular glutathione. Moreover, alpha-tocopherol afforded a moderate but significant inhibition of cytotoxicity induced by a non-specific protein kinase inhibitor, staurosporine, which is known to induce apoptosis in many types of cells including neurons. These results suggest that alpha-tocopherol protects striatal neurons by the reduction of oxidative stress, presumably by decreasing intracellular O(2)(-) levels, and at least partly by the inhibition of apoptosis.

Neuroprotection of striatal neurons against kainate excitotoxicity by neurotrophins and GDNF family members

Neurotrophic factors are regarded as potential therapeutic tools in neurodegenerative disorders. Here, we analysed the protective effects of brain-derived neurotrophic factor, neurotrophin-3, glial cell line-derived neurotrophic factor and neurturin against the excitotoxic damage induced by kainate in striatal neurons in vitro and in vivo. Our results show that the decrease in the number of cultured striatal calbindin-positive neurons induced by kainate was prevented by treatment with any of these factors. To characterize their protective effects in vivo, cell lines overexpressing brain-derived neurotrophic factor, neurotrophin-3, glial cell line-derived neurotrophic factor or neurturin were grafted into the striatum. We found that the numbers of striatal projection neurons (calbindin-positive) and striatal interneurons (parvalbumin- or choline acetyltransferase-positive) were differentially decreased after kainate lesion. These neurotrophic factors prevented the loss of striatal projection neurons and interneurons with differing efficiency: brain-derived neurotrophic factor was the most efficient, whereas neurturin was the least. Our findings show that brain-derived neurotrophic factor, neurotrophin-3, glial cell line-derived neurotrophic factor and neurturin have specific neuroprotective profiles in striatal neurons and indicate that they are specific modulators of the survival of distinct subsets of striatal neurons in pathophysiological conditions.

Neuroprotective role of dopamine against hippocampal cell death

Glutamate excitotoxicity plays a key role in the induction of neuronal cell death occurring in many neuropathologies, including epilepsy. Systemic administration of the glutamatergic agonist kainic acid (KA) is a well characterized model to study epilepsy-induced brain damage. KA-evoked seizures in mice result in hippocampal cell death, with the exception of some strains that are resistant to KA excitotoxicity. Little is known about the factors that prevent epilepsy-related neurodegeneration. Here we show that dopamine has such a function through the activation of the D2 receptor (D2R). D2R gene inactivation confers susceptibility to KA excitotoxicity in two mouse strains known to be resistant to KA-induced neurodegeneration. D2R-/- mice develop seizures when administered KA doses that are not epileptogenic for wild-type (WT) littermates. The spatiotemporal pattern of c-fos and c-jun mRNA induction well correlates with the occurrence of seizures in D2R-/- mice. Moreover, KA-induced seizures result in extensive hippocampal cell death in D2R-/- but not WT mice. In KA-treated D2R-/- mice, hippocampal neurons die by apoptosis, as indicated by the presence of fragmented DNA and the induction of the proapoptotic protein BAX. These results reveal a central role of D2Rs in the inhibitory control of glutamate neurotransmission and excitotoxicity.

Neuronal cell death in Huntington's disease: a potential role for dopamine

Huntington's disease is an inherited neurodegenerative disorder, the cause of which is unknown. Excitotoxicity, mitochondrial dysfunction and oxidative stress are all likely to contribute to the striatal cell death that occurs in this disorder. There are accumulating data indicating that under specific circumstances, dopamine, which occurs in high concentrations in the basal ganglia, might be neurotoxic. In this article, the current models used to study Huntington's disease are reviewed and the recent findings that implicate dopamine in the pathophysiology of this progressive disorder are discussed. Although many questions remain unanswered, the dopaminergic system could contribute to striatal vulnerability in Huntington's disease and provide a novel avenue for the development of new therapies.

Stress, aging, and brain oxidative damage

Stress may contribute to aging acceleration and age-related degenerative diseases. Stress and adaptation to stress require numerous homeostatic adjustments including hormones, neurotransmitters, oxidants, and other mediators. The stress-induced hormones, neurotransmitters, and oxidants all have beneficial, but also harmful effects if out of balance. Therefore, the homeostasis of stress and adaptation should be governed by the hormone balance, neurotransmitter balance, and oxidant balance, as well as the interactions among these substances. The imbalance and the over-interaction of these balances may ultimately cause increased oxidant generation and oxidative damage to biomolecules. This increased oxidative damage may add to the oxidant burden associated with normal aerobic metabolism, which in itself, generates oxidants, causes accumulation of oxidative damage in mitochondria, and contributes to normal aging. Therefore, the stress-associated increase of oxidative damage may, in part, contribute to stress-associated aging acceleration and age-related neurodegenerative diseases.

Dopaminergic modulation of early signs of excitotoxicity in visualized rat neostriatal neurons

Cell swelling induced by activation of excitatory amino acid receptors is presumably the first step in a toxic cascade that may ultimately lead to cell death. Previously we showed that bath application of N-methyl-D-aspartate (NMDA) or kainate (KA) produces swelling of neostriatal cells. The present experiments examined modulation of NMDA and KA-induced cell swelling by dopamine (DA) and its receptor agonists. Nomarski optics and infra-red videomicroscopy were utilized to visualize neostriatal medium-sized neurons in thick slices from rat pups (12-18 postnatal days). Increase in somatic cross-sectional area served as the indicator of swelling induced by bath application of glutamate receptor agonists. NMDA induced cell swelling in a dose-dependent manner. Activation of DA receptors in the absence of NMDA did not produce swelling. DA and the D1 receptor agonist SKF 38393, increased the magnitude of swelling produced by NMDA. This effect was reduced in the presence of the D1 receptor antagonist, SCH 23390. In contrast, activation of D2 receptors by quinpirole decreased the magnitude of NMDA-induced cell swelling. DA slightly attenuated cell swelling induced by activation of KA receptors. Quinpirole produced a significant concentration-dependent reduction in KA-induced swelling while SKF38393 increased KA-induced swelling, but only at a low concentration of KA. Together, these results provide additional support for the hypothesis that the direction of DA modulation depends on the glutamate receptor subtype, as well as the DA receptor subtype activated. One possible consequence of these observations is that endogenous DA may be an important contributing factor in the mechanisms of cell death in Huntington's disease.

Protective effects of vasoactive intestinal peptide against delayed glutamate neurotoxicity in cultured retina

The effects of vasoactive intestinal peptide (VIP) on glutamate-induced delayed death were examined using the primary cultures of rat retinal neurons. Effects of VIP on glutamate-induced neurotoxicity were evaluated by double staining with fluorescein diacetate and propidium iodide. Glutamate (1 mM) was applied to the culture for 10 min in the presence and absence of VIP, and visible cells enumerated 24 h after culture in normal medium. Effects of VIP on increase in the intracellular Ca2+ concentration and currents induced by glutamate in retinal neurons were investigated using the Ca2+ image analyzing system with fura-2 and whole-cell patch-clamp recording, respectively. The cAMP contents in retinal cultures were measured by radioimmunoassay. VIP (10 nM-1 microM) dose-dependently protected against glutamate-induced neurotoxicity in cultured retinal neurons. Protection by VIP (100 nM) against glutamate (1 mM)-induced neurotoxicity was antagonized by VIP6-28 (1 microM), a VIP antagonist, and H-89 (100 nM and 1 microM), a protein kinase A inhibitor. However, VIP had no effect on glutamate-induced inward currents nor glutamate-induced increase in the intracellular Ca2+ concentration. A 10-min exposure of VIP (100 nM) with glutamate (1 mM) resulted in an increase in the cAMP level to 446+/-58 from 22+/-1 pmol/mg protein. These findings suggest that VIP protects against the glutamate-induced neurotoxicity in retinal cultures by elevating the cAMP level via VIP receptors and thereby activating protein kinase A.

Dopamine D2-type agonists protect mesencephalic neurons from glutamate neurotoxicity: mechanisms of neuroprotective treatment against oxidative stress

Oxidative stress, a process in which neurotoxic oxygen free radicals cause dopaminergic neuronal degeneration, has been implicated in the degenerative process in Parkinson's disease. Glutamate-induced neurotoxicity is a model of oxidative stress. We demonstrated that preincubation with D2-type dopamine agonists bromocriptine and quinpirole provides neuroprotection against glutamate-induced neurotoxicity in cultured rat mesencephalic neurons. Simultaneous administration of D2 agonists, however, did not provide neuroprotection. The protective effects were dependent on the duration of preincubation and were blocked by a D2 antagonist and a protein synthesis inhibitor. Furthermore, preincubation with D2 agonists provided neuroprotection against toxicity induced by calcium overload and exposure to superoxide anions. Confocal microscopic analysis, using 2,7-dichlorofluorescin diacetate, revealed that bromocriptine preincubation suppressed the action of radicals on neurons. These findings indicate that dopamine D2 agonists provide protection mediated not only by the inhibition of dopamine turnover but also via D2-type dopamine receptor stimulation and the subsequent synthesis of proteins that scavenge free radicals.

Prenatal effects of drugs of abuse on brain development

Drugs of abuse modify signaling of neurotransmitter systems and intracellular messengers. Recent studies of central nervous system development show that these same neurotransmitters may serve as molecules that regulate specific aspects of cell proliferation, survival, migration, circuit formation and establishment of topography. Moreover, the convergence of neurotransmitter, growth factor and hormone activity on similar intracellular signaling systems suggests the potential for significant interactions among molecular components that regulate development. The application of modern strategies used by developmental and cell biologists to the question of whether prenatal drug exposure alters brain structure and function has led to discoveries of specific, targeted changes. Studies of the mechanisms of drug action that lead to altered neural development are now reality.

Death by a dopaminergic neurotoxin, 1-methyl-4-phenylpyridinium ion (MPP+) and protection by EGF in GH3 cells

In this study we investigated the uptake and effect of a dopaminergic neurotoxin, 1-methyl-4-phenylpyridinium ion (MPP+) on a clonal strain, GH3 cells, established from rat anterior pituitary. Although the level was very low compared with that in PC12 cells, a clonal rat pheochromocytoma cell line, there was a detectable amount of tyrosine hydroxylase protein in GH3 cells. The levels of monoamines including dopamine in GH3 cells were also very low compared with those in PC12 cells. [3H]MPP+ was incorporated to GH3 cells in a concentration-dependent manner and the uptake was inhibited by nomifensine, an inhibitor of dopamine transporter. Addition of 200 microM MPP+ stimulated the leakage of lactate dehydrogenase (LDH) after a lag of 24 h. Pretreatment with 50 ng/ml of epidermal growth factor (EGF), but not nerve growth factor (NGF) or brain-derived neurotrophic factor (BDNF), protected against MPP+-induced cell death. These findings show that: (1) MPP+ uptake to GH3 cells was via an effective dopamine transport system and causes delayed cell death, and (2) EGF protects against MPP+-induced cell death. A possible role for GH3 cells as dopaminergic neurons is discussed.

Culture medium components modulate retina cell damage induced by glutamate, kainate or "chemical ischemia"

The aim of this study was to determine whether culture-conditioned medium (CCM) can prevent neuronal damage caused by excitotoxicity or by "chemical ischemia" in cultured chick retina cells. Excitotoxic conditions were obtained by incubating retina cells with glutamate or kainate and "chemical ischemia" was induced by metabolic inhibition. In this case, cultures were briefly exposed to sodium cyanide, to block oxidative phosphorylation and iodoacetic acid, to block glycolysis. The assessment of neuronal injury was made spectrophotometrically by quantification of cellularly reduced MTT. Stimulation of retina cells with glutamate or kainate in serum deprived culture medium (BME-FCS), lead to a decrease in the MTT metabolism that was dependent on the time of exposure to the toxic agents. CCM prevented cell damage, either when present during the stimulation period or during the recovery period. This protection was more prominent in the case of kainate-induced neuronal death. "Chemical ischemia" also lead to a decrease of the MTT metabolism in a time-dependent manner and CCM protected retina cells from "ischemia"-induced lesions when present during the stimulation period and during the recovery period. The protective effect of CCM was partially decreased by the tyrosine kinase inhibitor, genistein, when the cells were stimulated with kainate, but not with glutamate, or when the cells were subjected to "chemical ischemia". CCM protected retina cells against both the acute and the delayed toxicity induced by either glutamate or kainate, or by "chemical ischemia", when present during both the insult and the recovery period. The presence of survival factors in the media may effectively inhibit the cell death signals generated by glutamate receptor activation or by "chemical ischemia".

An increase in intracellular levels of cyclic AMP produces trophic effects on striatal neurons developing in culture

Cyclic AMP-dependent kinases have been suggested to constitute signal transduction pathways involved in the regulation of neuronal development and survival. The present study examined whether elevated levels of cyclic AMP exhibit trophic activities on rat striatal neurons grown under serum-free culture conditions. Treatment with dibutyryl cyclic AMP, a permeable cyclic AMP, increased GABA uptake and immunocytochemically detectable levels of proteins such as c-Fos and calbindin-D28k. Neuronal survival was promoted by dibutyryl cyclic AMP only in lower density cultures. Chronic exposure of neurons to dibutyryl cyclic AMP enhanced the morphological development of calbindin-D28k-positive neurons. Furthermore, pretreatment with dibutyryl cyclic AMP afforded neuroprotection against N-methyl-D-aspartate-induced excitotoxicity. The dibutyryl cyclic AMP-induced trophic effects above were blocked by adenosine 3',5'-cyclic monophosphothioate, a specific inhibitor of cyclic AMP-dependent kinases. We also examined whether cyclic AMP is involved in trophic effects provided by membrane depolarization induced by high K+ and growth factors such as basic fibroblast growth factor and insulin-like growth factor-1. Depolarization, but not the growth factors, increased intracellular levels of cyclic AMP. Adenosine 3',5'-cyclic monophosphothioate diminished depolarization increases in GABA uptake, whereas it did not affect the trophic effect of the growth factors. Co-treatment with the growth factors and dibutyryl cyclic AMP produced additive effects on both increases in GABA uptake and neuroprotection against excitotoxicity. The present results indicate that cyclic AMP-dependent kinases play roles in mediating differentiation and survival of developing striatal neurons. Signalling pathways activated by either basic fibroblast growth factor or insulin-like growth factor-1 are independent of those involving cyclic AMP. In contrast, depolarization-induced trophic effects are mediated, at least in part, by cyclic AMP-dependent pathways. Protective actions of dibutyryl cyclic AMP against excitotoxic injury as well as the additive effects with the growth factors are of potential interest in the experimental therapy of acute or chronic neurodegenerative diseases.

L-DOPA neurotoxicity is mediated by glutamate release in cultured rat striatal neurons

The exposure of cultured rat striatal neurons to L-DOPA caused marked cell death. The L-DOPA cytotoxicity was inhibited by the addition of Mg2+ to and by the removal of Ca2+ from the culture medium, and also by the application of tetrodotoxin. Moreover, prolonged application of L-DOPA increased the glutamate content in the culture medium. These results indicate that L-DOPA produces neurotoxicity by facilitating glutamate release.

The biochemical basis of synaptic plasticity and neurocomputation: a new theory

The recent finding that dendritic spines (on which 90% of all excitatory synapses on pyramidal cells are formed) are not permanent structures but are continually being formed and adsorbed has implications for the present theoretical basis of neurocomputation, which is largely based on the concept of fixed nerve nets. This evidence would tend to support the recent theories of Edelman, Freeman, Globus, Pribram and others that neuronal networks in the brain operate mainly as nonlinear dynamic, chaotic systems. This paper presents a hypothesis of a possible neurochemical mechanism underlying this synaptic plasticity based on reactive oxygen species and toxic 0-semiquinones derived from catecholamines (i) by the enzyme prostaglandin H synthetase induced by glutamatergic NMDA receptor activation and (ii) by reactive nitrogen species derived from nitric oxide in a low ascorbate environment. A key factor in this neuromodulation may be the fact that catecholamines are potent antioxidants and free radical scavengers and are thus able to affect the redox mediated balance at the glutamate receptors between synapse formation and synapse removal that may be a key factor in neurocomputational plasticity. But catecholamines are also easily oxidized to neurotoxic 0-semiquinones and this may be relevant to the pathology of several diseases including schizophrenia. The relationship between dopamine release and positive reinforcement is relevant to this hypothesis.

Mechanisms of dopamine-induced cell death in cultured rat forebrain neurons: interactions with and differences from glutamate-induced cell death

Injury to the brain, whether by ischemia or trauma, results in the uncontrolled release of many neurotransmitters, including glutamate and dopamine. Both of these neurotransmitters are neurotoxic in high concentrations, and the oxidative stress caused by reactive oxygen species generation has been implicated in the mechanism of neurotoxicity. In this study, we used cultured rat forebrain neurons to characterize cell death caused by exposure to dopamine and/or glutamate and to investigate potential acute mechanisms of toxicity. Dopamine exposure (250 microM for 2 h) reduced cell viability to 34. 3 +/- 5.5% of untreated control 20 h later and increased the number of neurons with apoptotic morphology. The antioxidant N-acetylcysteine (100 microM) inhibited dopamine-induced toxicity and prevented the covalent binding of dopamine quinones to protein. In contrast, glutamate toxicity lacked the hallmark characteristics of apoptosis. When neurons were exposed successively to sublethal concentrations of dopamine and glutamate, cell viability at 20 h was reduced to 62.3 +/- 5.2% of untreated control. Apoptosis was not evident, and N-acetylcysteine blocked the potentiating effect of dopamine on glutamate-induced toxicity. We used single-cell fluorescence assays to measure changes in intraneuronal glutathione, intraneuronal Ca2+, mitochondrial membrane potential, and DNA integrity as potential acute inducers of neuronal injury. While changes in these parameters could be demonstrated, none were identified as the sole acute inducer of cell death caused by dopamine. In summary, we have characterized a number of neuronal responses to lethal dopamine injury. Also, we have demonstrated that dopamine and glutamate can interact in vitro to potentiate cell death and that the potentiation appears to be induced by oxidative stress.

Differential neurotoxicity induced by L-DOPA and dopamine in cultured striatal neurons

The neurotoxicity of L-DOPA and dopamine (DA) on striatal neurons was examined by using primary cultures of rat striatum. Exposure to L-DOPA and DA at concentrations of 30-300 microM induced dose-dependent cell death in both younger cultures (3 days in culture, 3 DIC) and elder cultures (10 days in culture, 10 DIC). The cytotoxicity of L-DOPA and DA was also dependent on the exposure time (6-24 h). Ascorbic acid (200 microM) inhibited both L-DOPA- and DA-induced cytotoxicity in 3 DIC cultures, whereas it provided significant protection against DA- but not L-DOPA-induced cytotoxicity in 10 DIC cultures. The L-DOPA cytotoxicity in 10 DIC cultures was prevented by a non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and by an NMDA receptor antagonist, MK-801. Neither antagonist prevented DA cytotoxicity. D-DOPA did not affect the viability of 10 DIC cultures, though it elicited marked toxicity in 3 DIC cultures. These results suggest that there are two components in the mechanisms that mediate the L-DOPA neurotoxicity on striatal neurons: one is autoxidation-relevant and the other is autoxidation-irrelevant. With respect to the latter, glutamate receptor stimulation may be involved. In contrast, autoxidation plays an important role in DA neurotoxicity.

PCP and ketamine inhibit non-NMDA glutamate receptor mediated hsp70 induction

The physiological model for glutamate receptor mediated excitotoxicity entails elevation of intraneuronal calcium levels. Excessive activation of the NMDA receptor leads to excitotoxicity by prolonged calcium influx via its calcium channel. The purpose of this research was to examine the mechanism of non-NMDA glutamate receptor mediated excitotoxicity. Mammalian AMPA receptors do not show significant calcium conductance. However, some kainate receptors show significant calcium conductance. The hypothesis of this research states that non-NMDA glutamate agonists (quisqualate (5 microliters of 2 mg/ml i.c.v.), AMPA (4 microliters of 1 mg/ml i.c.v.), and kainate (15 mg/kg i.p.)) produce significant heat shock gene, hsp70, induction via glutamate release with subsequent opening of the NMDA receptor calcium channel. PCP (phencyclidine) and ketamine are noncompetitive blockers of the NMDA calcium channel. They act to prevent significant NMDA receptor excitotoxicity. PCP (20 mg/kg i.p.) and ketamine (60 mg/kg i.p.) both diminished quisqualate and AMPA hsp70 induction in the CA1, CA2, CA3 areas of the hippocampus, in the polymorph area of the dentate gyrus, and in the parietal neocortex. PCP significantly (P < 0.05) diminished kainate hsp70 induction only in the CA1 area and the neocortex. Ketamine failed to reduce kainate hsp70 induction. AMPA receptors appear to result in excitotoxic damage via glutamate release. Glutamate opens NMDA receptor calcium channels which increases intraneuronal calcium levels. Kainate receptors probably mediate excitotoxicity via direct calcium conductance with glutamate release being important in the CA1 area and neocortex.

Ibudilast protects against neuronal damage induced by glutamate in cultured hippocampal neurons

1. The effect of ibudilast, a drug that has been clinically used for asthma and the improvement of cerebrovascular disorders, was examined on glutamate neurotoxicity in cultured neurons from rat hippocampus. 2. The extent of neuronal damage induced by exposure of the neurons to glutamate for 5 min was estimated by the activity of lactate dehydrogenase (LDH) released from degenerated neurons into the medium during a 24 h postexposure period. When ibudilast was added into all pre-incubation, exposure and postexposure media, the extent of neuronal damage decreased to approximately half that of control at an ibudilast concentration of 43 mumol/L. 3. The neuroprotective effects of ibudilast were dose-dependent. Sufficient protection was detected even when ibudilast was added only into the postexposure medium. 4. The extent of 45Ca2+ influx during glutamate exposure was slightly reduced by the addition of ibudilast. Intracellular cAMP, as measured by radioimmunoassay, was increased by neuronal exposure to glutamate and then decreased after the removal of glutamate; however in the presence of ibudilast, AMP was maintained at the high level. 5. These results suggest that protection against glutamate neurotoxicity by ibudilast is not only attributable to the inhibition of phenomena that occur during glutamate exposure, such as Ca2+ influx, but also to some beneficial metabolic changes that are induced by a sustained high level of intracellular cAMP.

Intrauterine cocaine exposure of rabbits: persistent elevation of GABA-immunoreactive neurons in anterior cingulate cortex but not visual cortex

The effects of prenatal cocaine exposure on the development of the rabbit cerebral cortex were studied. Two cortical areas were compared: primary visual cortex (VC) and anterior cingulate cortex (ACC). ACC was selected because behavioral deficits observed in cocaine-exposed infants suggest the involvement of ACC. In addition, ACC receives dense dopaminergic innervation and cocaine's action in inhibiting the re-uptake of dopamine is believed to underly the rewarding properties of cocaine. VC was selected as a control area because there is no evidence of behavioral deficits associated with visual perception in cocaine-exposed infants, and because VC receives minimal dopaminergic innervation. Two aspects of cortical development were studied: (i) cortical morphology, growth and cytoarchitectonic organization; and (ii) the development of the GABAergic neurotransmitter system. Measures of postnatal cortical growth, including cortical lamination, cell number and soma size, were compared in cocaine-exposed or control (saline) rabbits aged P5-P60. There was no difference between cocaine and saline animals in any of these parameters, and cortical cytoarchitecture appeared normal. However, despite the absence of major abnormalities in cortical development, we found that the number of GABA-immunoreactive neurons in cocaine-exposed animals was significantly higher than normal in ACC. This effect was highly consistent, was present in all laminae and at all ages studied, and persisted into maturity (P60). In contrast, in VC, the number of GABA-immunoreactive neurons in cocaine-exposed animals did not differ from normal. We suggest that increased GABA immunoreactivity may reflect a compensatory response to excessive excitatory input to ACC. A change in the balance of excitation and inhibition in ACC, reflecting 'noisy' or dysfunctional intracortical circuitry, may underly the emotional lability and attentional deficits characteristically described in infants exposed in utero to cocaine.