Zinc selectively blocks the action of N-methyl-D-aspartate on cortical neurons

Large amounts of zinc are present in synaptic vesicles of mammalian central excitatory boutons and may be released during synaptic activity, but the functional significance of the metal for excitatory neurotransmission is currently unknown. Zinc (10 to 1000 micromolar) was found to have little intrinsic membrane effect on cortical neurons, but invariably produced a zinc concentration-dependent, rapid-onset, reversible, and selective attenuation of the membrane responses to N-methyl-D-aspartate, homocysteate, or quinolinate. In contrast, zinc generally potentiated the membrane responses to quisqualate or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate and often did not affect the response to kainate. Zinc also attenuated N-methyl-D-aspartate receptor-mediated neurotoxicity but not quisqualate or kainate neurotoxicity. The ability of zinc to specifically modulate postsynaptic neuronal responses to excitatory amino acid transmitters, reducing N-methyl-to-aspartate receptor-mediated excitation while often increasing quisqualate receptor-mediated excitation, is proposed to underlie its normal function at central excitatory synapses and furthermore could be relevant to neuronal cell loss in certain disease states.     

Molecular cloning and functional expression of glutamate receptor subunit genes

Three closely related genes, GluR1, GluR2, and GluR3, encode receptor subunits for the excitatory neurotransmitter glutamate. The proteins encoded by the individual genes form homomeric ion channels in Xenopus oocytes that are sensitive to glutamatergic agonists such as kainate and quisqualate but not to N-methyl-D-aspartate, indicating that binding sites for kainate and quisqualate exist on single receptor polypeptides. In addition, kainate-evoked conductances are potentiated in oocytes expressing two or more of the cloned receptor subunits. Electrophysiological responses obtained with certain subunit combinations show agonist profiles and current-voltage relations that are similar to those obtained in vivo. Finally, in situ hybridization histochemistry reveals that these genes are transcribed in shared neuroanatomical loci. Thus, as with gamma-aminobutyric acid, glycine, and nicotinic acetylcholine receptors, native kainate-quisqualate-sensitive glutamate receptors form a family of heteromeric proteins.     

Glutamate, the dominant excitatory transmitter in neuroendocrine regulation

Glutamate has been found to play an unexpectedly important role in neuroendocrine regulation in the hypothalamus, as revealed in converging experiments with ultrastructural immunocytochemistry, optical physiology with a calcium-sensitive dye, and intracellular electrical recording. There were large amounts of glutamate in boutons making synaptic contact with neuroendocrine neurons in the arcuate, paraventricular, and supraoptic nuclei. Almost all medial hypothalamic neurons responded to glutamate and to the glutamate agonists quisqualate and kainate with a consistent increase in intracellular calcium. In all magnocellular and parvocellular neurons of the paraventricular and arcuate nuclei tested, the non-NMDA (non-N-methyl-D-aspartate) glutamate antagonist CNQX (cyano-2,3-dihydroxy-7-nitroquinoxaline) reduced electrically stimulated and spontaneous excitatory postsynaptic potentials, suggesting that the endogenous neurotransmitter is an excitatory amino acid acting primarily on non-NMDA receptors. These results indicate that glutamate plays a major, widespread role in the control of neuroendocrine neurons.     

Contributions of quisqualate and NMDA receptors to the induction and expression of LTP

The contributions of two subclasses of excitatory amino acid transmitter receptors to the induction and expression of long-term potentiation (LTP) were analyzed in hippocampal slices. The quisqualate/kainate receptor antagonist DNQX (6,7-dinitro-quinoxaline-2,3-dione) blocked 85% of the evoked field potential, leaving a small response that was sensitive to D-AP5 (D-2-amino-5-phosphonopentanoate), an N-methyl-D-aspartate (NMDA) receptor blocker. This residual D-AP5-sensitive response was of comparable size in control and previously potentiated inputs. High-frequency stimulation in the presence of DNQX did not result in the development of robust LTP. Washout of the drug, however, revealed the potentiation effect. Thus NMDA-mediated responses can induce, but are not greatly affected by, LTP; non-NMDA receptors, conversely, mediate responses that are not needed to elicit LTP but that are required for its expression.     

Ethanol inhibits NMDA-activated ion current in hippocampal neurons

The ion current induced by the glutamate receptor agonist N-methyl-D-aspartate (NMDA) in voltage-clamped hippocampal neurons was inhibited by ethanol (EtOH). Inhibition increased in a concentration-dependent manner over the range 5 to 50 mM, a range that also produces intoxication. The amplitude of the NMDA-activated current was reduced 61 percent by 50 mM EtOH; in contrast, this concentration of EtOH reduced the amplitude of current activated by the glutamate receptor agonists kainate and quisqualate by only 18 and 15 percent, respectively. The potency for inhibition of the NMDA-activated current by several alcohols is linearly related to their intoxicating potency, suggesting that alcohol-induced inhibition of responses to NMDA receptor activation may contribute to the neural and cognitive impairments associated with intoxication.     

Quinoxalinediones: potent competitive non-NMDA glutamate receptor antagonists

The N-methyl-D-aspartate (NMDA)-subtype of glutamate receptors has been well described as a result of the early appearance of NMDA antagonists, but no potent antagonist for the "non-NMDA" glutamate receptors has been available. Quinoxalinediones have now been found to be potent and competitive antagonists at non-NMDA glutamate receptors. These compounds will be useful in the determination of the structure-activity relations of quisqualate and kainate receptors and the role of such receptors in synaptic transmission in the mammalian brain.     

Sustained dendritic gradients of Ca2+ induced by excitatory amino acids in CA1 hippocampal neurons

Spatially resolved measurements of intracellular free calcium and of the changes produced by excitatory amino acids were made in neurons isolated from adult mammalian brain. Extremely long-lasting (minutes) Ca2+ gradients were induced in the apical dendrites of hippocampal CA1 neurons after brief (1 to 3 seconds), local application of either glutamate or N-methyl-D-aspartate (NMDA). These gradients reflect the continuous flux of Ca2+ into the dendrite. The sustained gradients, but not the immediate transient response to the agonists, were prevented by prior treatment with the protein kinase C inhibitor sphingosine. Expression of the long-lasting Ca2+ gradients generally required a priming or conditioning stimulus with the excitatory agonist. The findings demonstrate a coupling between NMDA receptor activation and long-lasting intracellular Ca2+ elevation that could contribute to certain use-dependent modifications of synaptic responses in hippocampal CA1 neurons.     

Quisqualate activates a rapidly inactivating high conductance ionic channel in hippocampal neurons

Glutamate activates a number of different receptor-channel complexes, each of which may contribute to generation of excitatory postsynaptic potentials in the mammalian central nervous system. The rapid application of the selective glutamate agonist, quisqualate, activates a large rapidly inactivating current (3 to 8 milliseconds), which is mediated by a neuronal ionic channel with high unitary conductance (35 picosiemens). The current through this channel shows pharmacologic characteristics similar to those observed for the fast excitatory postsynaptic current (EPSC); it reverses near 0 millivolts and shows no significant voltage dependence. The amplitude of the current through this channel is many times larger than that through the other non-NMDA (N-methyl-D-aspartate) channels. These results suggest that this high-conductance quisqualate-activated channel may mediate the fast EPSC in the mammalian central nervous system.     

Rat brain N-methyl-D-aspartate receptors expressed in Xenopus oocytes

N-methyl-D-aspartate (NMDA) activates a class of excitatory amino acid receptor involved in a variety of plastic and pathological processes in the brain. Quantitative study of the NMDA receptor has been difficult in mammalian neurons, because it usually exists with other excitatory amino acid receptors of overlapping pharmacological specificities. Xenopus oocytes injected with messenger RNA isolated from primary cultures of rat brain have now been used to study NMDA receptors. The distinguishing properties of neuronal NMDA receptors have been reproduced in this amphibian cell, including voltage-dependent block by magnesium, block by the NMDA receptor antagonist D-2-amino-5-phosphonovaleric acid, and potentiation by glycine. This preparation should facilitate the quantitative study of the regulation of NMDA receptor activation and serve as a tool for purification of the encoding messenger RNA.     

Role for excitatory amino acids in methamphetamine-induced nigrostriatal dopaminergic toxicity

The systemic administration of either methamphetamine or 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to experimental animals produces degenerative changes in nigrostriatal dopaminergic neurons or their axon terminals. This study was conducted to determine if excitatory amino acids, which appear to be involved in various neurodegenerative disorders, might also contribute to the dopaminergic neurotoxicity produced in mice by either methamphetamine or MPTP. MK-801, phencyclidine, and ketamine, noncompetitive antagonists of one subtype of excitatory amino acid receptor, the N-methyl-D-aspartate receptor, provided substantial protection against neurotoxicity produced by methamphetamine but not that produced by MPTP. These findings indicate that excitatory amino acids play an important role in the nigrostriatal dopaminergic damage induced by methamphetamine.     

Treatment of ischemic brain damage by perturbing NMDA receptor- PSD-95 protein interactions

N-methyl-D-aspartate receptors (NMDARs) mediate ischemic brain damage but also mediate essential neuronal excitation. To treat stroke without blocking NMDARs, we transduced neurons with peptides that disrupted the interaction of NMDARs with the postsynaptic density protein PSD-95. This procedure dissociated NMDARs from downstream neurotoxic signaling without blocking synaptic activity or calcium influx. The peptides, when applied either before or 1 hour after an insult, protected cultured neurons from excitotoxicity, reduced focal ischemic brain damage in rats, and improved their neurological function. This approach circumvents the negative consequences associated with blocking NMDARs and may constitute a practical stroke therapy.     

A family of AMPA-selective glutamate receptors

Four cloned cDNAs encoding 900-amino acid putative glutamate receptors with approximately 70 percent sequence identity were isolated from a rat brain cDNA library. In situ hybridization revealed differential expression patterns of the cognate mRNAs throughout the brain. Functional expression of the cDNAs in cultured mammalian cells generated receptors displaying alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-selective binding pharmacology (AMPA = quisqualate greater than glutamate greater than kainate) as well as cation channels gated by glutamate, AMPA, and kainate and blocked by 6,7-dinitroquinoxaline-2,3-dione (CNQX).     

恒河猴纹状体一氧化氮合酶阳性神经元的形态学观察

为研究恒河猴纹状体一氧化氮合酶阳性神经元的形态特征和分布特点,利用尼克酰胺腺嘌呤二核苷酸磷酸黄递酶(NADPH-d)组织化学和神经元型一氧化氮合酶(nNOS)免疫组织化学法。结果发现尾壳核内分布着大量形态多样的NADPH-d和nNOS阳性神经元,其胞体呈锥体形、多角形、纺锤形、梭形和卵圆形,直径9.6~27.2μm。在内囊中有少量的阳性细胞,形态多样。外髓板中有淡染的NADPH-d、nNOS弱阳性大细胞,直径27.2~54.4μm。恒河猴纹状体内有大量的形态大小不一的一氧化氮合酶阳性神经元。     

Both mu and delta opiate receptors exist on the same neuron

Low concentrations of the relatively selective opiate receptor agonists dihydromorphine and normorphine (mu receptor agonists) and D-Ala 2-D-Leu 5-enkephalin (a delta receptor agonist) were applied to single enteric neurons while the frequency of action potential firing was recorded. Most neurons that were inhibited by the mu agonists were also inhibited by the delta agonist, but the two receptors could be distinguished by the higher concentration of naloxone required to antagonize the delta agonist. The results indicate that enteric neurons bear both mu and delta receptors and that cell firing is inhibited if either receptor type is activated.     

D1 dopamine receptor activation required for postsynaptic expression of D2 agonist effects

D1 and D2 dopamine receptors exert synergistic effects on the firing rates of basal ganglia neurons and on the expression of stereotyped behavior in rats. Moreover, the ability of D2 agonists to induce changes in basal ganglia single unit activity and spontaneous motor activity is dependent upon the presence of endogenous dopamine to stimulate D1 receptors; in rats treated with alpha-methyl-rho-tyrosine to reduce endogenous dopamine levels, the neurophysiological and behavioral effects of the D2 agonist quinpirole are significantly attenuated, while the effects of nonselective agonists like apomorphine, which stimulate both D1 and D2 receptors, or combinations of a D2 agonist and a D1 agonist are not attenuated. Thus, the previously held view that D2 receptors alone are responsible for evoking the changes in behavior and basal ganglia output induced by nonselective dopamine agonists and endogenous dopamine is not supported by these results, which indicate that these phenomena require concurrent stimulation of both dopamine receptor subtypes.     

Long-term modulation of electrical synapses in the mammalian thalamus

Electrical synapses are common between inhibitory neurons in the mammalian thalamus and neocortex. Synaptic modulation, which allows flexibility of communication between neurons, has been studied extensively at chemical synapses, but modulation of electrical synapses in the mammalian brain has barely been examined. We found that the activation of metabotropic glutamate receptors, via endogenous neurotransmitter or by agonist, causes long-term reduction of electrical synapse strength between the inhibitory neurons of the rat thalamic reticular nucleus.     

Indole-2-carboxylic acid: a competitive antagonist of potentiation by glycine at the NMDA receptor

The N-methyl-D-aspartate (NMDA) class of excitatory amino acid receptors regulates the strength and stability of excitatory synapses and appears to play a major role in excitotoxic neuronal death associated with stroke and epilepsy. The conductance increase gated by NMDA is potentiated by the amino acid glycine, which acts at an allosteric site tightly coupled to the NMDA receptor. Indole-2-carboxylic acid (I2CA) specifically and competitively inhibits the potentiation by glycine of NMDA-gated current. In solutions containing low levels of glycine, I2CA completely blocks the response to NMDA, suggesting that NMDA alone is not sufficient for channel activation. I2CA will be useful for defining the interaction of glycine with NMDA receptors and for determining the in vivo role of glycine in excitotoxicity and synapse stabilization.     

L-cysteine, a bicarbonate-sensitive endogenous excitotoxin

After systemic administration to immature rodents, L-cysteine destroys neurons in the cerebral cortex, hippocampus, thalamus, and striatum, but the underlying mechanism has never been clarified. This neurotoxicity of L-cysteine, in vitro or in vivo, has now been shown to be mediated primarily through the N-methyl-D-aspartate subtype of glutamate receptor (with quisqualate receptor participation at higher concentrations). In addition, the excitotoxic potency of L-cysteine was substantially increased in the presence of physiological concentrations of bicarbonate ion. L-Cysteine is naturally present in the human brain and in the environment, and is much more powerful than beta-N-methylamino-L-alanine, a bicarbonate-dependent excitotoxin, which has been implicated in an adult neurodegenerative disorder endemic to Guam. Thus, the potential involvement of this common sulfur-containing amino acid in neurodegenerative processes affecting the central nervous system warrants consideration.     

Glutamate induces calcium waves in cultured astrocytes: long-range glial signaling

The finding that astrocytes possess glutamate-sensitive ion channels hinted at a previously unrecognized signaling role for these cells. Now it is reported that cultured hippocampal astrocytes can respond to glutamate with a prompt and oscillatory elevation of cytoplasmic free calcium, visible through use of the fluorescent calcium indicator fluo-3. Two types of glutamate receptor--one preferring quisqualate and releasing calcium from intracellular stores and the other preferring kainate and promoting surface-membrane calcium influx--appear to be involved. Moreover, glutamate-induced increases in cytoplasmic free calcium frequently propagate as waves within the cytoplasm of individual astrocytes and between adjacent astrocytes in confluent cultures. These propagating waves of calcium suggest that networks of astrocytes may constitute a long-range signaling system within the brain.     

Identification and functional characterization of brainstem cannabinoid CB2 receptors

The presence and function of CB2 receptors in central nervous system (CNS) neurons are controversial. We report the expression of CB2 receptor messenger RNA and protein localization on brainstem neurons. These functional CB2 receptors in the brainstem were activated by a CB2 receptor agonist, 2-arachidonoylglycerol, and by elevated endogenous levels of endocannabinoids, which also act at CB1 receptors. CB2 receptors represent an alternative site of action of endocannabinoids that opens the possibility of nonpsychotropic therapeutic interventions using enhanced endocannabinoid levels in localized brain areas.