Acetylcholine potentiates glutamate-induced neurodegeneration in cultured hippocampal neurons

Enhancement of nicotinic receptors alleviates cytotoxicity in neurological disease models

The common pathological mechanisms among the spectrum of neurodegenerative diseases are supposed to be shared. Multiple lines of evidence, from molecular and cellular to epidemiological, have implicated nicotinic transmission in the pathology of the two most common neurodegenerative disorders, namely Alzheimer's disease (AD) and Parkinson's disease (PD). In this review article we present evidence of nicotinic acetylcholine receptor (nAChR)-mediated protection against neurotoxicity induced by β amyloid (Aβ), glutamate, rotenone, and 6-hydroxydopamine (6-OHDA) and the signal transduction involved in this mechanism. Our studies have clarified that survival signal transduction, the α7 nAChR/Src family/PI3K/AKT pathway and subsequent upregulation of Bcl-2 and Bcl-x, would lead to neuroprotection. In addition to the PI3K/AKT pathway, two other survival pathways, JAK2/STAT3 and MEK/ERK, are proposed by other groups. In rotenone- and 6-OHDA-induced PD models, nAChR-mediated neuroprotection was also observed, and the effect was blocked not only by α7 but also by α4β2 nAChR antagonists. We also document that nAChR stimulation blocks glutamate neurotoxicity in spinal cord motor neurons. These findings suggest that nAChR-mediated neuroprotection is achieved through subtypes of nAChRs and common signal cascades. An early diagnosis and protective therapy with nAChR stimulation could be effective in delaying the progression of neurodegenerative diseases such as AD, PD and amyotrophic lateral sclerosis.

Effect of acute soman exposure on GABA(A) receptors in rat hippocampal slices and cultured hippocampal neurons

Exposure of the central nervous system to organophosphorus (OP) nerve agents causes seizures and neuronal cell death. Benzodiazepines are commonly used to treat seizures induced by OPs. However, it is known that soman-induced seizures are particularly resistant to benzodiazepine treatment, as compared with other OPs. This study investigated the effect of soman on γ-aminobutyric acid (GABA) neurotransmission in acute rat hippocampal slices and the surface expression of GABA(A) receptors in cultured rat hippocampal neurons. Results showed that GABA-mediated inhibitory post synaptic currents (IPSCs) are significantly reduced by soman in a concentration-dependent manner in acute rat hippocampal slices. Furthermore, confocal microscopic and cell-based ELISA assays revealed that soman caused rapid internalization of GABA(A) receptors in cultured rat hippocampal neurons. The effect of soman on GABA(A)R endocytosis was not due to inhibition of acetylcholinesterase (AChE) because (1) the acetylcholine muscarinic receptor antagonist atropine did not block soman-induced GABA(A)R endocytosis; and (2) physostigmine, at concentrations that completely inhibit AChE activity, did not cause GABA(A)R endocytosis. Moreover, blocking of the N-methyl-D-aspartate (NMDA) receptors by 2-amino-5-phosphonovalerate (APV) had no effect on soman-induced GABA(A)R endocytosis, suggesting that the soman effect was not secondary to glutamate receptor over activation. Regardless of the exact mechanism, the observation that soman induces rapid GABA(A)R endocytosis may have significant implications in the development of effective countermeasures against soman-induced seizures.

Regulated hypoxia/reperfusion-dependent modulation of ERK1/2, cPLA2, and Bcl-2/Bax: a potential mechanism of neuroprotective effect of penehyclidine hydrochloride

The activation of event-related kinase 1/2 (ERK1/2) and cytosolic phospholipaseA2 (cPLA2), which can aggravate hypoxia/reoxygenation (H/R) damage related to their downstream Bcl-2/Bax and Caspase-3 pathway, plays a key role in H/R. The M1 receptors could be responsible for activation of ERK1/2. Thus, it seems that the regulation of M1 receptors mediated the ERK1/2; cPLA2-mediated Bcl-2/Bax pathway may be a significant responsive signal in H/R. Penehyclidine hydrochloride (PHC) is an anticholinergic agent with high degree of selectivity for M1 and M3 receptor subtypes, it is reported that PHC has a protective effect against H/R damage. Here we hypothesize and demonstrate that PHC could downregulate the expression of pERK1/2, cPLA2, and Caspase-3, increased the ratio of Bcl-2/Bax. This study may widen the application of PHC and therapeutic agents of stroke.

Nicotinic receptor-mediated neuroprotection in neurodegenerative disease models

Multiple lines of evidence, from molecular and cellular to epidemiological, have implicated nicotinic transmission in the pathology of Alzheimer's disease (AD) and Parkinson's disease (PD). This review article presents evidence for nicotinic acetylcholine receptor (nAChR)-mediated protection and the signal transduction involved in this mechanism. The data is based mainly on our studies using rat-cultured primary neurons. Nicotine-induced protection was blocked by an alpha7 nAChR antagonist, a phosphatidylinositol 3-kinase (PI3K) inhibitor, and an Src inhibitor. Levels of phosphorylated Akt, an effector of PI3K, Bcl-2 and Bcl-x were increased by nicotine administration. From these experimental data, our hypothesis for the mechanism of nAChR-mediated survival signal transduction is that the alpha7 nAChR stimulates the Src family, which activates PI3K to phosphorylate Akt, which subsequently transmits the signal to up-regulate Bcl-2 and Bcl-x. Up-regulation of Bcl-2 and Bcl-x could prevent cells from neuronal death induced by beta-amyloid (Abeta), glutamate and rotenone. These findings suggest that protective therapy with nAChR stimulation could delay the progress of neurodegenerative diseases such as AD and PD.

Protective effects of N-methyl-D-aspartate receptor antagonism on VX-induced neuronal cell death in cultured rat cortical neurons

Exposure of the central nervous system to organophosphorus (OP) nerve agents induces seizures and neuronal cell death. Here we report that the OP nerve agent, VX, induces apoptotic-like cell death in cultured rat cortical neurons. The VX effects on neurons were concentration-dependent, with an IC(50) of approximately 30 microM. Blockade of N-methyl-D-aspartate receptors (NMDAR) with 50 microM. D-2-amino-5-phosphonovalerate (APV) diminished 30 microM VX-induced total cell death, as assessed by alamarBlue assay and Hoechst staining. In contrast, neither antagonists of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors (AMPARs) nor metabotropic glutamate receptors (mGluRs) had any effect on VX-induced neurotoxicity. VX-induced neuronal cell death could not be solely attributed to acetylcholinesterase (AChE) inhibition, since neither the reversible pharmacological cholinesterase inhibitor, physostigmine, nor the muscarinic receptor antagonist, atropine, affected VX-induced cell death. Importantly, APV was found to be therapeutically effective against VX-induced cell death up to 2 h post VX exposure. These results suggest that NMDARs, but not AMPARs or mGluRs, play important roles in VX-induced cell death in cultured rat cortical neurons. Based on their therapeutic effects, NMDAR antagonists may be beneficial in the treatment of VX-induced neurotoxicities.

Na+-dependent sources of intra-axonal Ca2+ release in rat optic nerve during in vitro chemical ischemia

The contribution of intracellular stores to axonal Ca2+ overload during chemical ischemia in vitro was examined by confocal microscopy. Ca2+ accumulation was measured by fluo-4 dextran (low-affinity dye, KD approximately 4 microM) or by Oregon Green 488 BAPTA-1 dextran (highaffinity dye, KD approximately 450 nM). Axonal Na+ was measured using CoroNa Green. Ischemia in CSF containing 2 mM Ca2+ caused an approximately 3.5-fold increase in fluo-4 emission after 30 min, indicating a large axonal Ca2+ rise well into the micromolar range. Axonal Na+ accumulation was enhanced by veratridine and reduced, but not abolished, by TTX. Ischemia in Ca2+-free (plus BAPTA) perfusate resulted in a smaller but consistent Ca2+ increase monitored by Oregon Green 488 BAPTA-1, indicating release from intracellular sources. This release was eliminated in large part when Na+ influx was reduced by replacement with N-methyl-D-glucamine (NMDG+; even in depolarizing high K+ perfusate), Li+, or by the application of TTX and significantly increased by veratridine. Intracellular release also was reduced significantly by neomycin or 1-(6-[(17beta-methoxyestra-1,3,5 [10]-trien-17-yl) amino] hexyl)-1H-pyrrole-2,5-dione (U73122 [GenBank]) (phospholipase C inhibitors), heparin [inositol trisphosphate (IP3) receptor blocker], or 7-chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one (CGP37157; mitochondrial Na+/Ca2+ exchange inhibitor) as well as ryanodine. Combining CGP37157 with U73122 [GenBank] or heparin decreased the response more than either agent alone and significantly improved electrophysiological recovery. Our conclusion is that intra-axonal Ca2+ release during ischemia in rat optic nerve is mainly dependent on Na+ influx. This Na+ accumulation stimulates three distinct intra-axonal sources of Ca2+: (1) the mitochondrial Na+/Ca2+ exchanger driven in the Na+ import/Ca2+ export mode, (2) positive modulation of ryanodine receptors, and (3) promotion of IP3 generation by phospholipase C.

Protective effect of rhynchophylline and isorhynchophylline on in vitro ischemia-induced neuronal damage in the hippocampus: putative neurotransmitter receptors involved in their action

Rhynchophylline and isorhynchophylline are major tetracyclic oxindole alkaloid components of Uncaira species, which have been long used as medicinal plants. In this study we examined the protective effects of rhynchophylline and isorhynchophylline on in vitro ischemia-induced neuronal damage in the hippocampus and interaction of these alkaloids with neurotransmitter receptors in a receptor expression model of Xenopus oocytes. In vitro ischemia was induced by exposing the hippocampal slices to oxygen- and D-glucose-deprived medium over 8 min. The resultant neuronal damage was elucidated as deterioration of population spike (PS) amplitudes evoked trans-synaptically by electrical stimulation of Schaffer collaterals and recorded in the CA1 area. Rhynchophylline and isorhynchophylline, as well as the N-methyl-D-aspartate (NMDA) antagonist (+/-)-2-amino-5-phosphono-valeric acid (APV), the muscarinic M1 receptor antagonist pirenzepine, and the 5-HT2 receptor antagonist ketanserin, attenuated the in vitro ischemia-induced neuronal damage in a concentration-dependent manner. There was no difference in the extent of protection against the neuronal damage between rhynchophylline and isorhynchophylline treatment. In Xenopus oocytes expressing the rat brain receptors encoded by total RNA, both rhynchophylline and isorhynchophylline reduced muscarinic receptor- and 5-HT2 receptor-mediated current responses in a competitive manner. Together with our previous findings that rhynchophylline and isorhynchophylline have a non-competitive antagonistic effect on the NMDA-type ionotropic glutamate receptors, the present results suggest that these alkaloids exert their protective action against ischemia-induced neuronal damage by preventing NMDA, muscarinic M1, and 5-HT2 receptors-mediated neurotoxicity during ischemia.

Modification of brain aging and neurodegenerative disorders by genes, diet, and behavior

Multiple molecular, cellular, structural, and functional changes occur in the brain during aging. Neural cells may respond to these changes adaptively, or they may succumb to neurodegenerative cascades that result in disorders such as Alzheimer's and Parkinson's diseases. Multiple mechanisms are employed to maintain the integrity of nerve cell circuits and to facilitate responses to environmental demands and promote recovery of function after injury. The mechanisms include production of neurotrophic factors and cytokines, expression of various cell survival-promoting proteins (e.g., protein chaperones, antioxidant enzymes, Bcl-2 and inhibitor of apoptosis proteins), preservation of genomic integrity by telomerase and DNA repair proteins, and mobilization of neural stem cells to replace damaged neurons and glia. The aging process challenges such neuroprotective and neurorestorative mechanisms. Genetic and environmental factors superimposed upon the aging process can determine whether brain aging is successful or unsuccessful. Mutations in genes that cause inherited forms of Alzheimer's disease (amyloid precursor protein and presenilins), Parkinson's disease (alpha-synuclein and Parkin), and trinucleotide repeat disorders (huntingtin, androgen receptor, ataxin, and others) overwhelm endogenous neuroprotective mechanisms; other genes, such as those encoding apolipoprotein E(4), have more subtle effects on brain aging. On the other hand, neuroprotective mechanisms can be bolstered by dietary (caloric restriction and folate and antioxidant supplementation) and behavioral (intellectual and physical activities) modifications. At the cellular and molecular levels, successful brain aging can be facilitated by activating a hormesis response in which neurons increase production of neurotrophic factors and stress proteins. Neural stem cells that reside in the adult brain are also responsive to environmental demands and appear capable of replacing lost or dysfunctional neurons and glial cells, perhaps even in the aging brain. The recent application of modern methods of molecular and cellular biology to the problem of brain aging is revealing a remarkable capacity within brain cells for adaptation to aging and resistance to disease.

The effects of citicoline and/or MK-801 on survival, neurological and behavioral outcome of mice exposed to transient hyperglycemia and oligemic hypoxia

The effects of citicoline and/or low dose of MK-801 (sufficient to prevent the development of seizures) on survival, neurological and behavioral recovery following transient hyperglycemic-oligemic-hypoxic insult have been evaluated in mice. Neurological recovery was assessed semi-quantitatively on the third and the 10th day after the insult, and behavioral tests evaluating spontaneous locomotor activity, motor coordination and spontaneous alternation performance were performed on day 10. Neither drug given alone did influence survival rate, but the combination of MK-801 and higher citicoline dose decreased mortality on day 10. Behavioral performance was markedly compromised by the insult. Citicoline, but not MK-801, slightly but significantly improved behavioral outcome in all three tests.

CONCLUSION

when brain ischemic insult is complicated with acute hyperglycemia, post-treatment with citicoline combined with MK-801 in low anti-convulsive dose improves survival and neurological recovery, and citicoline but not MK-801 enhances behavioral recovery.

Nicotinic receptor-mediated protection against beta-amyloid neurotoxicity

Multiple lines of evidence, from molecular and cellular to epidemiologic, have implicated nicotinic transmission in the pathology of Alzheimer's disease. In this review we present evidence for nicotinic receptor-mediated protection against beta-amyloid and glutamate neurotoxicity, and the signal transduction involved in this mechanism. The data are based mainly on our studies using rat-cultured primary neurons. Nicotine-induced protection was blocked by an alpha7 nicotinic receptor antagonist, a phosphatidylinositol 3-kinase inhibitor, and an Src inhibitor. Levels of phosphorylated Akt, an effector of phosphatidylinositol 3-kinase; Bcl-2; and Bcl-x were increased by nicotine administration. From these experimental data, our hypothesis for the mechanism of nicotinic receptor-mediated survival signal transduction is that the alpha7 nicotinic receptor stimulates the Src family, which activates phosphatidylinositol 3-kinase to phosphorylate Akt, which subsequently transmits the signal to upregulate Bcl-2 and Bcl-x. Upregulation of Bcl-2 and Bcl-x could prevent cells from neuronal death induced by beta-amyloid and glutamate. These findings suggest that an early diagnosis of Alzheimer's disease and protective therapy with nicotinic receptor stimulation could delay the progress of Alzheimer's disease.

Traumatic brain injury reduces hippocampal alpha7 nicotinic cholinergic receptor binding

Changes in the expression of central nervous system (CNS) neurotransmitter receptors may contribute to behavioral and physiological deficits that occur following traumatic brain injury (TBI). Studies investigating the neurochemical basis for the protracted cognitive dysfunction that follows TBI have focused in part on cholinergic mechanisms. The present study compared the effects of mild and moderate cortical contusion injury (CCI) on the density of cholinergic receptor subtypes, NMDA-type glutamate receptors, and calcium channel expression. Quantitative autoradiography was used to determine the effects of CCI on receptor expression, 48 h following injury. The most robust and consistent change in receptor binding was in the density of alpha7 nicotinic receptors as determined by alpha-[125I]-bungarotoxin (BTX) binding. Bilateral deficits in BTX binding were present following both mild and moderate levels of injury. In contrast, changes in the density of alpha3/alpha4 nAChr's, muscarinic AChr's, NMDA-type glutamate receptors, and L-type calcium channel expression were more regionally restricted and lower in magnitude, as compared to changes in BTX binding. The high calcium permeability of the alpha7 nAChr may be related to the extensive decrease in BTX binding that occurs following TBI.

Calcium signaling in the ER: its role in neuronal plasticity and neurodegenerative disorders

Endoplasmic reticulum (ER) is a multifaceted organelle that regulates protein synthesis and trafficking, cellular responses to stress, and intracellular Ca2+ levels. In neurons, it is distributed between the cellular compartments that regulate plasticity and survival, which include axons, dendrites, growth cones and synaptic terminals. Intriguing communication networks between ER, mitochondria and plasma membrane are being revealed that provide mechanisms for the precise regulation of temporal and spatial aspects of Ca2+ signaling. Alterations in Ca2+ homeostasis in ER contribute to neuronal apoptosis and excitotoxicity, and are being linked to the pathogenesis of several different neurodegenerative disorders, including Alzheimer's disease and stroke.

Muscarinic-receptor antagonist scopolamine rescues hippocampal neurons from death induced by glutamate

Cultured hippocampal neurons were used to test the hypothesis that modulation of muscarine receptors can modify glutamate-induced neurodegeneration. Treatment of hippocampal cultures with scopolamine (1 nM to 1 mM) under glutamate incubation had beneficial effect on neuronal viability. Thus, blockade of muscarinic-receptor sites increased the threshold for glutamate neurotoxicity. These data show that interactions between the NMDA, muscarinic receptors and their corresponding neurotransmitter inputs to hippocampal neurons may play a crucial role in neurodegeneration.

Differential effects of scopolamine on neuronal survival in ischemia and glutamate neurotoxicity: relationships to the excessive vulnerability of the dorsoseptal hippocampus

The neurodegeneration in the CA1 subfield of hippocampus exhibited a dorsal-ventral gradient of susceptibility in global ischemia (82% dorsoseptally and only 16% ventrotemporally). Scopolamine (SCOP) did not improve the neuronal damage caused by the global ischemic challenge in rats and did not reduce the infarct area after the focal MCA-occlusion in mice. No differences were observed between saline and SCOP-treated animals in the physiologic parameters, except for a slight increase in rectal temperature. In contrast, treatment of hippocampal cultures with increasing concentrations of SCOP (1 nM to 1 mM) under glutamate incubation had a beneficial effect on neuronal viability. These data show that (1) there is substantial gradient of vulnerability of the hippocampus from dorsal to ventral in global ischemia and (2) that interactions between the NMDA, muscarinic receptors and their corresponding neurotransmitter inputs to hippocampal neurons are evident in vitro and may play a crucial role in neuronal neurodegeneration. However, the mechanisms underlying the high vulnerability of dorsal hippocampus still remain enigmatic.

A role of nitric oxide in organophosphate-induced convulsions

The effects of nitric oxide-regulating compounds on convulsions and mortality of rats administered i.p. with diisopropylfluorophosphate was investigated. l-N(G)-nitroarginine methyl ester, a nitric oxide synthase inhibitor possessing an anticholinergic action, markedly attenuated the intensity of convulsions and significantly reduced the mortality rate. A similar result was obtained with anticholinergic procyclidine, an N-methyl-d-aspartate receptor antagonist. Noteworthy, l-N(G)-nitroarginine, another inhibitor of nitric oxide synthase, significantly attenuated the seizure intensity when administered in combination with atropine sulfate (5 mg/kg), though either l-N(G)-nitroarginine or atropine sulfate was inactive alone. It is suggested that nitric oxide may be a proconvulsant or a convulsion-promoting factor in anticholinesterase poisoning, and both the reduction of nitric oxide level and blockade of cholinergic systems may be required for more effective protection of seizures.

Nicotine protects against the dexamethasone potentiation of kainic acid-induced neurotoxicity in cultured hippocampal neurons

To elucidate the neuroprotective effect of nicotine, we investigated whether nicotine may attenuate dexamethasone potentiation of kainic acid-induced neurotoxicity. Primary hippocampal culture was pre-treated with nicotine for 24 h followed by dexamethasone (10(-4) M) for 24 h. Then, cultures were exposed with kainic acid (10(-4) M) and cellular viability was determined by LDH effluxmetry. Nicotine pre-treatment (10(-9)-10(-7) M) dose-dependently attenuated dexamethasone potentiation of kainic acid-induced neurotoxicity. These results may support the epidemiological data suggesting a neuroprotective effect of cigarette smoking on Alzheimer's disease or Parkinson's disease.

Nicotine-induced protection against glutamate cytotoxicity. Nicotinic cholinergic receptor-mediated inhibition of nitric oxide formation

Cortical neurodegeneration in Alzheimer's disease (AD) is suggested to be attributable not only to beta-protein but also to glutamate. Although degeneration of cholinergic projection to the cerebral cortex is recognized to be one of the most prominent pathological changes in AD, there is only limited information concerning the cholinergic interaction with the cortical neurodegeneration. This study was performed to examine the protective effect of nicotine against glutamate-induced cytotoxicity using rat cultured cortical neurons. The cell viability was significantly reduced when cultures were briefly exposed to glutamate or N-methyl-D-aspartate (NMDA). The simultaneous addition of nicotine did not reduce glutamate cytotoxicity. In contrast, the simultaneous application of NMDA receptor antagonists such as MK-801 reduced glutamate cytotoxicity. Incubating the cultures with nicotine (10 microM) for 0.5-24 h prior to glutamate exposure reduced its cytotoxicity. Neuroprotection by nicotine was dependent on both the concentration and incubation period. In contrast to nicotine, muscarine (10 microM) weakly potentiated glutamate cytotoxicity. The neuroprotective effect of nicotine against glutamate cytotoxicity was antagonized by hexamethonium but not by artopine. Nicotine prevented NMDA cytotoxicity but did not affect cytotoxicity induced by either kainate or alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA). Cell viability was significantly reduced by a brief exposure of cultures to ionomycin, a calcium ionophore. Ionomycin-induced cytotoxicity was abolished by removing Ca2+ from incubating medium. S-nitrosocysteine (SNOC), which spontaneously releases nitric oxide (NO), also induced delayed cell death. Nicotine prevented ionomycin-induced cytotoxicity without affecting SNOC-induced cytotoxicity. These results suggest that nicotinic cholinergic receptor stimulation induces neuroprotection against glutamate cytotoxicity by its inhibitory action on NO-formation. Therefore, we propose that acetylcholine, acting through nicotinic cholinergic receptors, can function as a putative neuroprotective factor against neurodegeneration caused by the excessive release of glutamate and/or NMDA receptor activation.

Neuroprotection and selective vulnerability of neurons within the nucleus basalis magnocellularis

Neurons within the nucleus basalis may die due to their selective vulnerability to endogenous excitatory amino acid neurotransmitters, nitric oxide and free radicals. The factors influencing the selective vulnerability of neurons within the nucleus basalis depend upon many different factors related to the presence of these agents and the neuron's ability to defend itself against the consequences of exposure. Many different mechanisms have been investigated to provide neuroprotection for neurons within the nucleus basalis and throughout the central nervous system. This review summarizes the results of studies that have investigated our current capability to either attenuate the neurotoxicity of endogenous excitatory amino acids, such as glutamate, or to provide effective neuroprotection during circumstances of neurotoxin exposure.

Assessment of primary neuronal culture as a model for soman-induced neurotoxicity and effectiveness of memantine as a neuroprotective drug

An in vitro mammalian model neuronal system to evaluate the intrinsic toxicity of soman and other neurotoxicants as well as the efficacy of potential countermeasures was investigated. The link between soman toxicity, glutamate hyperactivity and neuronal death in the central nervous system was investigated in primary dissociated cell cultures from rat hippocampus and cerebral neocortex. Exposure of cortical or hippocampal neurons to glutamate for 30 min produced neuronal death in almost 80% of the cells examined at 24 h. Hippocampal neurons exposed to soman for 15-120 min at 0.1 microM concentration caused almost complete inhibition (> or = 90%) of acetylcholinesterase but failed to show any evidence of effects on cell viability, indicating a lack of direct cytotoxicity by this agent. Acetylcholine (ACh, 0.1 mM), alone or in combination with soman, did not potentiate glutamate toxicity in hippocampal neurons. Memantine, a drug used for the therapy of Parkinson's disease, spasticity and other brain disorders, significantly protected hippocampal and cortical neurons in culture against glutamate and N-methyl-D-aspartate (NMDA) excitotoxicity. In rats a single dose of memantine (18 mg/kg) administered 1 h prior to a s.c. injection of a 0.9 LD50 dose of soman reduced the severity of convulsions and increased survival. Survival, however, was accompanied by neuronal loss in the frontal cortex, piriform cortex and hippocampus.

YM796, a novel muscarinic agonist, improves the impairment of learning behavior in a rat model of chronic focal cerebral ischemia

We studied effects of YM796, a novel muscarinic agonist, on behavioral, histological and regional cerebral blood flow changes in the chronic phase after focal cerebral ischemia in rats. YM796 (0.03, 0.1, 0.3 and 1 mg/kg) was administered orally once a day from the 7th to the 13th day after the permanent occlusion of left middle cerebral artery. On the 7th day, rats were trained in one-trial step-through passive avoidance task 45 min after drug administration. Test trials were carried out on the 8th and 14th days. Neurological deficits, including hemiplegia and abnormal posture, were observed on the 7th and 14th days. After the completion of behavioral studies, the rats were decapitated and cerebral infarction was measured. Regional cerebral blood flow was also measured by the hydrogen clearance technique 7 days after MCA occlusion. YM796 (0.1-1 mg/kg) significantly (P < 0.05) attenuated the impairment of learning behavior in a dose-dependent manner without affecting spontaneous locomotor activity. The ameliorating effect of YM796 (0.3 mg/kg) on the impaired learning behavior was significantly (P < 0.05) suppressed by intracerebroventricular injection of pirenzepine (10 micrograms/rat), an M1 antagonist. No significant difference in either neurological deficits or cerebral infarction was found between the vehicle- and YM796-treated groups. Further, YM796 (0.3 mg/kg) had little effect on the reduced blood flow in the ipsilateral frontal cortex 7 days after occlusion. These results suggest that YM796 improves the impaired learning behavior probably by activating central M1 receptors in a rat model of chronic focal cerebral ischemia.

Atropine and cerebral ischemic injury in the Mongolian gerbil

1. Cerebral ischemia of 5 min duration was induced in unanesthetized gerbils by bilateral occlusion of the carotid arteries. 2. The extent of cerebral damage was assessed by the elevation of motor activity in comparison with control animals and by a histological assessment of the extent of neuronal degeneration in the CA1 area of the hippocampus. 3. Atropine, an antagonist of ACh, at either a low (1 mg/kg) or a high (10 mg/kg) dose administered 15 min prior to the ischemic episode, did not confer protection against cerebral ischemic damage. 4. This finding suggests that ACh does not play a critical role in the generation of ischemia reperfusion injury.

Nicotine-induced protection of cultured cortical neurons against N-methyl-D-aspartate receptor-mediated glutamate cytotoxicity

The effects of nicotine on glutamate-induced cytotoxicity were examined using primary cultures of rat cortical neurons. The cell viability was significantly reduced when cultures were briefly exposed to glutamate or N-methyl-D-aspartate (NMDA) then incubated with normal medium for 1 h. A 1-h exposure of the cultures to kainate or alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) reduced cell viability. Incubating cultures with nicotine for 1-24 h protected cortical neurons against glutamate cytotoxicity. Maximum protection against glutamate cytotoxicity was induced with a 2-h nicotine incubation. Exposure to nicotine for up to 2 h did not affect cell viability by itself although cell viability was reduced in a time-dependent manner when the exposure exceeded 4 h. Neuroprotection by nicotine was dependent on both the concentration and incubation period. Nicotine reduced the NMDA cytotoxicity but did not attenuate that of kainate and AMPA. The neuroprotective effects of nicotine against glutamate cytotoxicity were antagonized by mecamylamine and hexamethonium but not by atropine. These results indicate that nicotinic receptor stimulation induces neuroprotection against glutamate cytotoxicity mediated by NMDA receptors.

Endogenous neuroprotection factors and traumatic brain injury: mechanisms of action and implications for therapy

Throughout evolution the brain has acquired elegant strategies to protect itself against a variety of environmental insults. Prominent among these are signals released from injured cells that are capable of initiating a cascade of events in neurons and glia designed to prevent further damage. Recent research has identified a remarkably large number of neuroprotection factors (NPFs), whose expression is increased in response to brain injury. Examples include the neurotrophins (NGF, NT-3, NT-5, and BDNF), bFGF, IGFs, TGFs, TNFs and secreted forms of the beta-amyloid precursor protein. Animal and cell culture studies have shown that NPFs can attenuate neuronal injury initiated by insults believed to be relevant to the pathophysiology of traumatic brain injury (TBI) including excitotoxins, ischemia, and free radicals. Studies of the mechanism of action of these NPFs indicate that they enhance cellular systems involved in maintenance of Ca2+ homeostasis and free radical metabolism. Recent work has identified several low-molecular-weight lipophilic compounds that appear to mimic the action of NPFs by activating signal transduction cascades involving tyrosine phosphorylation. Such compounds, alone or in combination with antioxidants and calcium-stabilizing agents, have proved beneficial in animal studies of ischemic brain injury and provide opportunities for development of preventative/therapeutic approaches for TBI.

Acetylcholine output from the ischemic rat cerebral cortex: effects of adenosine agonists

The efflux of acetylcholine (ACh) from the ischemic rat cerebral cortex was examined using the cortical cup technique and an HPLC with electrochemical detection assay. Four vessel occlusion of the cerebral circulation caused a rapid increase in ACh efflux into the cortical superfusates, which was then sustained during the 20 min period of occlusion. Reperfusion was associated with a rapid return of ACh efflux to basal levels. The A1 and A2 selective adenosine receptor agonists, N6-cyclopentyladenosine (10(-8) and 10(-10) M) and CGS 21680 (10(-8)), failed to significantly alter ischemia-evoked release of ACh. Because ACh is known to enhance NMDA receptor mediated neuronal depolarization and intracellular Ca2+ levels, and to potentiate L-glutamate-induced neural degeneration, the present findings suggest that ACh could contribute to ischemic brain injury.

Extracellular amino acid levels in hippocampus during pilocarpine-induced seizures

Extracellular levels of aspartate, glutamate and glutamine were monitored by microdialysis in the dorsal hippocampus of freely moving rats following the administration of a convulsant dose of pilocarpine (400 mg/kg, i.p.). Rats were either pretreated with the glutamate uptake inhibitor, 1-trans-pyrrolidine-2,4-dicarboxylic acid (PDC, 1 mM in the perfusion medium, -25 min), or received pilocarpine directly. All rats injected with pilocarpine (with or without PDC pretreatment) developed limbic seizures (latency 15.4 +/- 2.4 min). Without PDC pretreatment there were no significant changes in extracellular levels of aspartate, glutamate and glutamine following pilocarpine administration until the onset of limbic seizures when glutamine levels fell by 35%. Following PDC pretreatment there were large and sustained increases in extracellular hippocampal aspartate (250%) and glutamate (55%) levels, but no significant change in the glutamine level. When pilocarpine was administered to this group of rats, there were further selective, significant, transient increases in the extracellular levels of aspartate (31%) and glutamate (18%) which preceded the onset of seizures. Aspartate and glutamate levels were not significantly increased (relative to PDC controls) during seizures. The conditions for pilocarpine-induced increases in aspartate and glutamate release were established in parallel groups of anaesthetised rats where pilocarpine was administered via a microdialysis probe in the dorsal hippocampus. Following the infusion of 10 mM pilocarpine there were large and rapid increases in the levels of aspartate (143%) and glutamate (179%), which were completely abolished by the absence of calcium in the perfusion medium, or by the presence of atropine (20 mM) or tetrodotoxin (1 microM).

Antiparkinsonian drugs and in vitro excitotoxicity

N-Methyl-D-aspartate (NMDA) receptor activation has been implicated in the pathogenesis and clinical expression of Parkinson's disease. Because some antiparkinsonian drugs have NMDA antagonist properties, we examined their effects on NMDA toxicity, measured by lactate dehydrogenase (LDH) release, in neuron-enriched cerebrocortical cultures. Amantadine reduced NMDA toxicity with half-maximal reduction at approximately 30 microM, while trihexphenidyl, L-3,4-dihydroxyphenylalanine (L-DOPA), bromocriptine and selegiline were ineffective, and benztropine was itself toxic. Amantadine and related drugs could not only reduce parkinsonian symptoms, but also modify underlying neurodegenerative processes.

Inhibitory effect of CCK-8 and ceruletide on glutamate-induced rises in intracellular free calcium concentrations in rat neuron cultures

To study the mechanism by which cholecystokinin octapeptide (CCK-8) and its potent analogue, ceruletide, prevent glutamate-induced neuronal cell death in rat neuron cultures, we examined the effect of both peptides on glutamate-induced increases in the intracellular free calcium concentrations ([Ca2+]i), which are known to be a crucial trigger of the neurodegeneration induced by glutamate. CCK-8 itself did not alter [Ca2+]i in rat neuron cultures. Glutamate increased [Ca2+]i in neuron cultures rapidly and markedly. CCK-8 and ceruletide significantly suppressed the increases in [Ca2+]i induced by glutamate. The maximum inhibitory effects of CCK-8 and ceruletide at 10(-6) M reached 43 and 46% of the response to glutamate, respectively. Gastrin-I and CCK-4 also significantly attenuated the increases in [Ca2+]i induced by glutamate. The inhibitory effect of CCK-8 was completely blocked by the selective antagonist for CCK-B receptors, (+)L-365,260, but not by (-)L-364,718, which is a selective antagonist for CCK-A receptors. CCK-8 significantly suppressed [Ca2+]i response to kainate and high concentrations of extracellular K+, but not to N-methyl-D-aspartate. With cultured astrocytes, CCK-8 did not inhibit the increment of [Ca2+]i induced by glutamate. These findings clearly demonstrated that CCK-8 and ceruletide inhibit glutamate-induced increases in [Ca2+]i in neuron cultures through CCK-B receptors, suggesting that CCK-8 may participate in the central actions of glutamate.

Neurotransmitters as Neurotrophic Factors: a New Set of Functions

At the start of this review, factors were deemed trophic if they stimulated mitosis, permitted neural cell survival, promoted neurite sprouting and growth cone motility, or turned on a specific neuronal phenotype. The in vitro evidence from cell cultures is overwhelming that both neurotransmitters and neuropeptides can have such actions. Furthermore, the same chemical can exert several of these effects, either on the same or on different cell populations. Perhaps the most striking example is that of VIP, which can stimulate not only mitosis, but also survival and neurite sprouting of sympathetic ganglion neuroblasts (Pincus et al., 1990a,b). The in vivo data to support the in vitro experiments are starting to appear. A role for VIP in neurodevelopment is supported by in vivo studies that show behavioral deficits produced in neonatal rats by treatment with a VIP antagonist (Hill et al., 1991). The work of Shatz' laboratory (Chun et al., 1987; Ghosh et al., 1990) suggests that neuropeptide-containing neurons, transiently present, serve as guideposts for thalamocortical axons coming in to innervate specific cortical areas. Along similar lines, Wolff et al. (1979) demonstrated gamma-aminobutyric acid-accumulating glia in embryonic cortex that appeared to form axoglial synapses and suggested the possibility that gamma-aminobutyric acid released from the glia might play a role in synaptogenesis by increasing the number of postsynaptic thickenings. Meshul et al. (1987) have provided evidence that astrocytes can regulate synaptic density in the developing cerebellum. The work of Zagon and McLaughlin (1986a,b, 1987) has shown that naltrexone, an antagonist of the endogenous opioid peptides, affects both cell number and neuronal sprouting. Lauder's laboratory (Lauder et al., 1982) has shown a role for 5-HT in regulation of the proliferation of numerous cell types. These studies illustrate another important point, that neurotransmitters and neuropeptides function in communication not only between neurons, but also between neurons and glial cells, and between glial cells. Given that astrocytes can express virtually all of the neural receptors and can produce at least some of the neurotransmitters and neuropeptides, they must now be considered equal partners in the processes of intercellular communication in the nervous system, including the trophic responses. The actions of neurotransmitters and neuropeptides have to be considered in terms of a broad spectrum of actions that range from the trophic actions described in this review, to the classic transmitter actions, to potential roles in neurotoxicity.(ABSTRACT TRUNCATED AT 400 WORDS)

Activation of the metabotropic glutamate receptor attenuates N-methyl-D-aspartate neurotoxicity in cortical cultures

Excitatory amino acid receptor-mediated neurotoxicity (excitotoxicity) has been proposed to contribute to neuronal loss in a wide variety of neurodegenerative conditions. Although considerable evidence has accumulated implicating N-methyl-D-aspartate (NMDA), kainate, and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors in the processes of excitotoxicity, relatively little research has focused on the ability of other neurotransmitter systems to influence excitotoxic neuronal injury. In the present study, we examined the effects of trans-1-aminocyclopentyl-1,3-dicarboylic acid (ACPD), a selective agonist for the metabotropic glutamate, or ACPD, receptor, and carbachol, an agonist at the acetylcholine receptor, on neuronal degeneration produced by brief exposure to NMDA in murine cortical cultures. Since excitotoxic neuronal injury is probably caused by increases in intracellular Ca2+ concentrations, the two transmitter agonists were of particular interest as both have been shown to mobilize intracellular calcium stores. Contrary to what might be expected, ACPD and, to a lesser degree, carbachol attenuated NMDA neurotoxicity. The neuroprotective effect of ACPD, but not of carbachol, was dependent upon the developmental state of cultures; in older cultures (greater than or equal to 18 days in vitro), the protective effect decreased. The neuroprotection by ACPD may be, in part, mediated by protein kinases, since protection is partially reversed by the protein kinase antagonists H-7 and HA-1004. These data suggest that concomitant activation of the ACPD receptor may serve as a protective mechanism against neurotoxicity that could be produced by brief intense NMDA receptor activation during normal or abnormal brain function.

A metabotropic glutamate receptor agonist does not mediate neuronal degeneration in cortical culture

In light of the evidence that calcium plays a critical role in excitotoxic neuronal death, it has been speculated that the metabotropic glutamate receptor may also contribute to excitotoxic damage through the mobilization of Ca2+ from intracellular stores. In the present study we examined this possibility by studying the neurotoxicity of trans-1-amino-cyclopentyl-1,3-dicarboxylate (trans-ACPD), a selective agonist of the metabotropic glutamate receptor. Exposure of cortical neurons to 100 microM trans-ACPD substantially increased phosphoinositide hydrolysis and intraneuronal free calcium in the presence of CPP and CNQX. Despite the presence of functional metabotropic receptors on cultured neurons, however, exposure of cultures to as high as 1 mM trans-ACPD for 24 h failed to produce any morphological or chemical signs of neuronal damage. Furthermore, trans-ACPD did not potentiate submaximal neurotoxicity produced by other non-N-methyl-D-aspartate (NMDA) agonists, kainate and D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionic acid (AMPA).

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

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

Analysis of the convulsant-potentiating effects of lithium in rats

Lithium pretreatment of rats has previously been shown to potentiate the convulsant effects of cholinomimetic drugs, such as pilocarpine. The first objective of this project was to determine if lithium also potentiates seizures induced by other classes of drugs. Lithium pretreatment of rats did not affect seizure activity induced by administration of N-methyl-D-aspartate, kainic acid, bicuculline, or pentylenetetrazole. This suggests that the proconvulsant effect of lithium is largely selective for cholinomimetics. A second series of experiments investigated possible mechanisms of the lithium potentiation of pilocarpine-induced seizures. The alpha 2-adrenergic receptor agonist clonidine suppressed seizure development, and the antagonist idazoxan enhanced the onset of seizures, suggesting that endogenous norepinephrine provides anticonvulsant properties. Administration of the norepinephrine depleter DSP-4 potentiated pilocarpine-induced seizures. These results suggest that the previously reported impairment of noradrenergic function by lithium may play a role in its potentiation of cholinomimetic-induced seizures.

Somato-, branchio- and viscero-motor neurons contain glutaminase-like immunoreactivity

Immunocytochemistry combined with a fluorescent dye tracer method revealed that somatic, branchial and visceral motoneurons in the brainstem and spinal cord of the rat contain phosphate-activated glutaminase (PAG). An excitatory neurotransmitter glutamate is synthesized mainly through this enzyme. Among these motoneurons, neurons in the dorsal motor nucleus of the vagus nerve (dmnX), autonomic preganglionic neurons in the spinal cord and urethral sphincter motoneurons (DL) were most intensely immunostained. PAG is co-expressed with choline acetyltransferase, calcitonin gene-related peptide or galanin in these neurons. These findings, together with the findings that motor endplates in urethral sphincter muscle contain PAG and PAG-like immunostaining in dmnX motoneurons was decreased after axotomy, suggest that glutamate is a co-transmitter of acetylcholine in motoneurons. Brainstem motoneurons were moderately stained, while somatic motoneurons in the spinal cord other than DL, showed very weak staining for PAG. However, they showed intense PAG-like immunoreactivity at their premature stage, suggesting that glutamate has some effects on the maturation of these neurons. A variety of functional roles of glutamate in motoneurons is discussed.

Modulation of NMDA receptor responsiveness by neurotransmitters, drugs and chemical modification

The NMDA receptor is intimately involved an a wide range of pathophysiological processes in the mammalian brain, including epilepsy and ischemia-induced neurodegeneration. The widespread distribution of NMDA receptors places almost every area of the brain at risk from NMDA receptor over-activity. However, it is clear that the central nervous system can function effectively without imminent danger of self-destruction. The focus of this review is the processes that control NMDA receptor responsiveness in vivo. The review will cover the modulation of the receptor by Mg2+, glycine, Zn2+ and polyamines that is believed to occur by virtue of interaction with distinct ligand binding sites on the NMDA receptor complex. Studies suggesting a role for receptor phosphorylation and for redox modulation will be discussed. Finally, some evidence for indirect regulation of cellular responses to NMDA receptor activation by other neurotransmitters will be presented.