Trigeminal inputs to reticulospinal neurones in lampreys are mediated by excitatory and inhibitory amino acids

Reticulospinal (RS) neurones integrate sensory inputs from several modalities to generate appropriate motor commands for maintaining body orientation and initiation of locomotion in lampreys. As in other vertebrates, trigeminal afferents convey sensory inputs from the head region. The in vitro brainstem/spinal cord preparation of the lamprey was used for characterizing trigeminal inputs to RS neurones as well as the transmitter systems involved. The trigeminal nerve on each side was electrically stimulated and synaptic responses, which consisted of mixed excitation and inhibition, were recorded intracellularly in the middle and posterior rhombencephalic reticular nuclei. The EPSPs were mediated by activation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors. An increase in the late phase of the excitatory response occurred when Mg²⁺ ions were removed from the Ringer's solution. This effect was antagonized by 2-amino-5-phosphonopentanoate (2-AP5) or reversed by restoring Mg²⁺ ions to the perfusate suggesting the activation of N-methyl-d-aspartate (NMDA) receptors. IPSPs were mediated by glycine. These findings are similar to those reported for other types of sensory inputs conveyed to RS neurones, where excitatory and inhibitory amino acid transmission is also involved.     

Rods and cones activate different excitatory amino acid receptors on the mudpuppy retinal horizontal cell

We have studied rod and cone neurotransmission onto horizontal cells (HCs) of the mudpuppy retina by: (1) dual whole-cell recordings from synaptically connected pairs in a superfused retinal slice preparation, and (2) intracellular recordings from HCs in a superfused retina-eyecup preparation. In the presence of relatively weak concentrations of broad spectrum excitatory amino acid antagonists, HC recordings show that cone-mediated responses were significantly more attenuated than those mediated by rods. These observations suggest that the HC postsynaptic receptors for rods and cones are pharmacologically different. One advantage of having distinct rod- and cone-activated receptors on second-order neurons may be to preserve different rod and cone functions under varying levels of light stimulation and provide separate means of regulating rod and cone neurotransmission onto the second-order cells.     

Glutamate metabotropic receptor mediated depression of synaptic inputs to lamprey reticulospinal neurones

The transmission of vestibular inputs to reticulospinal (RS) neurones of the posterior rhombencephalic nucleus (PRRN) has been shown to be depressed by the bath application of N-methyl-D-aspartate (NMDA). The aim of this study was to investigate the pharmacological mechanism involved using patch clamp recordings of reticulospinal neurones. It is demonstrated that the chemical component of vestibular inputs to the PRRN is mediated by glutamatergic synapses utilizing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors on the PRRN neurones. Monosynaptic excitatory postsynaptic currents (EPSCs) from octavomotorius relay cells to RS neurones are markedly depressed by the application of NMDA, a depression which was insensitive to competitive and non-competitive NMDA receptor antagonists. The effect of NMDA was eliminated by inactivation of G proteins. A similar depressive effect was observed following application of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) to the superfusate. It is concluded that NMDA acts at a metabotropic receptor located most likely presynaptically to reticulospinal neurones on terminals of octavomotorius relay cells.     

Activation of 5-HT1C/2 receptors depresses polysynaptic reflexes and excitatory amino acid-induced motoneuron responses in frog spinal cord

Sucrose gap recordings from the ventral roots of isolated, hemisected frog spinal cords were used to evaluate the effects of high concentrations of serotonin (5-HT) and alpha-methyl-5-HT (alpha-Me-5-HT) on the changes in motoneuron potential produced by dorsal root stimulation and by excitatory amino acids and agonists. Bath application of 5-HT in concentrations of 10 microM or greater produced a concentration-dependent motoneuron depolarization. Polysynaptic ventral root potentials evoked by dorsal root stimuli were reduced in both amplitude and area by 5-HT or alpha-Me-5-HT (both 100 microM). This may result from a reduction of the postsynaptic sensitivity of motoneurons to excitatory amino acid transmitters because 5-HT significantly depressed motoneuron depolarizations produced by addition of L-glutamate and L-aspartate to the superfusate. Similarly, 5-HT reduced depolarizations produced by the excitatory amino acid agonists N-methyl-D-aspartate (NMDA), quisqualate, alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA), and kainate. alpha-Me-5-HT reduced NMDA depolarizations. Tetrodotoxin (TTX) did not affect the ability of 5-HT to attenuate NMDA or kainate depolarizations, but did eliminate the 5-HT-induced attenuation of quisqualate and AMPA depolarizations. The glycine receptor site associated with the NMDA receptor did not appear to be affected by 5-HT because saturation of the site by excess glycine did not alter the 5-HT-induced depression of NMDA responses. The 5-HT1C/2 antagonist ketanserin and the 5-HT1A/2 antagonist spiperone significantly attenuated the 5-HT-induced depression of NMDA-depolarizations.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of neonatal cholinergic basal forebrainlesions on excitatory amino acid receptors in neocortex

The role of cholinergic basal forebrain projections in the modulation of corticalplasticity and associated functional changes is currently the subject of renewed attention.Excitatory amino acid receptors have been identified as mediators of cortical topographic efferentand afferent information. In addition some of these receptors, notably the NMDA andmetabotropic [mGluR] type, participate in cortical plasticity. Growing evidence suggests thatinteractions between cholinergic and glutamatergic systems contribute to cognitive corticalfunctions and their anatomical and physiological substrates. Though cholinergic and glutamatergicmechanisms have both been shown to be involved in cortical morphogenesis, few studies haveattempted to study their interactions in development. The present study investigates the effect ofneonatal lesions to the cholinergic basal forebrain on NMDA, AMPA and mGluR receptors inBALB/CByJ mice, at two different developmental ages. We demonstrated previously that nBMlesions at birth result in transient cholinergic depletion for the first two postnatal weeks,substantial morphogenetic alterations in neocortex and cognitive deficits by adulthood. We showhere that unilateral neonatal lesions result in decreases in NMDA and AMPA receptors butincreases in mGluRs during the second postnatal week (PND 14). At 30 days postnatal, lesionmediated changes were attenuated, compared with PND 14, but significant sex differences incontrol and nBM lesioned mice were apparent. These data support the notion thatcholinergic/glutamatergic interactions are important during early cortical morphogenesis.Moreover, our results highlight the fact that cholinergic as well glutamatergic developmentalmechanisms are sexually dimorphic.     

A1 neurons and excitatory amino acid receptors in rat caudal medulla mediate vagal excitation of supraoptic vasopressin cells

Extracellular recordings from the supraoptic nucleus of the rat established that vasopressinergic neurosecretory cells were excited by stimulation of cervical but not abdominal vagal afferents. This response was absent or significantly attenuated after microinjection of gamma-aminobutyric acid into a region of the caudal medulla known to contain the A1 noradrenaline cell group. Consistent with the possible involvement of the A1 group, vagal stimulation approximately doubled the frequency of proto-oncogene expression in A1 noradrenaline neurons, as indicated by the occurrence of nuclear Fos-like immunoreactivity in tyrosine hydroxylase-positive neurons of the caudal ventrolateral medulla. Finally, A1 region microinjection of either the N-methyl-D-aspartic acid (NMDA) receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV), or the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), significantly reduced vasopressin cell responses to vagal stimulation. These findings suggest that: (i) the A1 group is an essential component in a pathway which relays facilitatory vagal input of cardiopulmonary origin to neurosecretory vasopressin cells, and (ii) the activation of A1 neurons in this pathway involves both NMDA and non-NMDA excitatory amino acid receptors, an observation consistent with an input to A1 cells which generates 'mixed' excitatory postsynaptic potentials.     

Localization of striatal excitatory amino acid binding site subtypes to striatonigral projection neurons

Quantitative autoradiography was used to examine the cellular localization of excitatory amino acid binding sites in the striatum following selective lesion of striatonigral projection neurons. Degeneration of striatonigral neurons was induced unilaterally by injection of the suicide transport toxin, volkensin, into the left substantia nigra. Twelve days following nigral volkensin injection there was a reduction of all excitatory amino acid binding site subtypes in the striatum ipsilateral to the injected nigra. The reduction in (NMDA) binding sites was significantly greater than the loss of -α-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA), kainate and metabotropic binding. These results indicate that there are NMDA, AMPA, metabotropic and kainate binding sites on striatonigral projection neurons and suggest that the NMDA subtype may be selectively enriched on striatonigral neurons.     

兴奋性氨基酸、氧自由基与缺血再灌注脑损伤关系的实验观察

目的：观察脑缺血再灌注过程中兴奋性氨基酸（Ｅｘｃｉｔａｔｏｒｙ ａｍｉｎｏ ａｃｉｄ,ＥＡＡ）、氧自由基的变化,研究探索脑缺血再灌注损伤的机制。方法：测定假手术组、缺血３０ｍｉｎ再灌注６０ｍｉｎ生理盐水（ＮＳ）处理组和单唾液酸四已糖神经节苷脂（ＧＭ１,１０ｍｇ／ｋｇ,ＩＰ）处理组,鼠脑海马组织ＥＡＡ、丙二醛（Ｍａｌｏｎｄｉａｄｅｈｙｄｅ,ＭＤＡ）的含量。实验应用全脑缺血（４ＶＯ）模型,ＥＡＡ采用Ｈ     

板党对模型小鼠兴奋性氨基酸的影响

目的观察富硒板党对精神分裂症模型小鼠兴奋性氨基酸(EAA)的影响。方法采用地卓西平马来酸盐(MK-801)建立谷氨酸功能低下精神分裂症模型小鼠,观察富硒板党对小鼠脑组织中谷氨酸(Glu)、天冬氨酸(Asp)、r-氨基丁酸(GABA)、甘氨酸(Gly)及超氧化物歧化酶(SOD)、过氧化氢酶(CAT)的影响。结果富硒板党对正常小鼠脑组织EAA含量无影响,可使MK-801所致谷氨酸功能低下精神分裂症模型小鼠脑组织的Glu、Asp、CAT含量降低,SOD活性增强。结论富硒板党具有调节精神分裂症模型小鼠脑组织中EAA的代谢功能,从而发挥对脑组织的保护作用。     

Actions of excitatory amino acid agonists and antagonists on the primary afferents of the vestibular system of the axolotl (Ambystoma mexicanum)

In order to determine the nature of the transmitter in the synapse between hair cells and primary afferent fibers, both resting and evoked spike activity of vestibular system afferents were recorded. Excitatory amino acid agonists and antagonists were applied by micro perfusion. Excitatory amino acid agonists consistently increased the firing rate of these afferents. The rank order in potencies of the agonists tested was:kainatequisqualate>d-aspartatel-glutamatel-aspartate> N-methyld-aspartate. Blockade of synaptic transmission with high-Mg²⁺ and low-Ca²⁺ solutions did not seem to affect the responses to the excitatory amino acid agonists indicating their postsynaptic action. Excitatory amino acid antagonists inhibit both resting and physiologically evoked activity. The rank order of inhibitory potency was:kynurenate>l-glutamate diethyl ester>d,l-2-amino-4-phosphono-butyrate>d-α-amino adipate>d,l-2-amino-5-phosphonovalerate. This findings suggest that an amino acid-related compound may be transmitter at this synapse. The relative potencies of agonists and antagonists tested provide evidence that the transmitter released from the hair cell's basal pole in the axolotl vestibular system interacts with postsynaptic kainic/quisqualic type receptors.     

Propentofylline administered by microdialysis attenuates ischemia-induced hippocampal damage but not excitatory amino acid release in gerbils

Systemic administration of propentofylline (PPF), and adenosine uptake inhibitor, has been demonstrated to protect CA1 pyramidal cells from death following transient cerebral ischemia in gerbils. In order to examine the direct effects of this inhibitor, we tested whether or not PPF administered into the hippocampus in situ through a microdialysis probe could attenuate ischemia-induced excitatory amino acid (EAA) release and prevent subsequent death of CA1 pyramidal cells in the gerbil. The EAA release and death of CA1 pyramidal cells observed in the hippocampus were compared with those in the contralateral hippocampus of the same animal into which vehicle alone was administered. The results indicated that pre- as well as post-treatments with PPF inhibited the death of CA1 pyramidal cells after 5-min ischemia in a dose-dependent manner, but did not significantly alter the EAA release during ischemia and reperfusion in the same animals. While the neuroprotective effect of PPF against ischemic damage has commonly been ascribed to attenuation of EAA release during ischemia, other actions of adenosine such as those influencing the synaptic responses, neuronal excitation, and local cerebral circulation, or as yet unidentified actions may be involved in the observed neuroprotective effects of PPF.     

Participation of AMPA- and NMDA-type excitatory amino acid receptors in the spinal reflex transmission,in rat

Classical in vitro and in vivo models and electrophysiological techniques were used to investigate the role of AMPA- and NMDA-type glutamate receptors in various components of spinal segmental reflex potentials. In the rat hemisected spinal cord preparation, the AMPA antagonists NBQX and GYKI 52466 abolished the monosynaptic reflex (MSR) potential but caused only partial inhibition of the motoneuronal population EPSP. NMDA antagonists had no noticeable effect on the MSR in normal medium, but markedly depressed the late part of EPSP. However, an NMDA receptor antagonist sensitive monosynaptic response was recorded in magnesium-free medium at complete blockade of the AMPA receptors. In spinalized rats, the AMPA antagonists completely blocked all components of the dorsal root stimulation evoked potential. MK-801 (2mg/kg, i.v.) reduced monosynaptic responses in a frequency dependent way, with no effect at 0.03 Hz and 22% inhibition at 0.25 Hz. The reduction of the di- and polysynaptic reflex components was about 30% and did not depend on stimulation frequency. Long-latency reflex discharge responses, especially when evoked by train stimulation, were more sensitive to MK-801 than the polysynaptic reflex.These results suggest that glutamate activates MSR pathways through AMPA receptors. However, under certain conditions, NMDA receptors can modulate this transmission through plastic changes in the underlying neuronal circuits. AMPA and NMDA receptors play comparable roles in the mediation of longer latency reflex components.     

Voltage-dependency of the responses of cerebellar Purkinje cells to excitatory amino acids

The voltage-dependency of the responses of Purkinje cells to excitatory amino acids was examined in rat cerebellar slices, using intrasomatic recordings with the single electrode voltage-clamp. In standard perfusion medium, the depolarizations evoked in these neurones by ionophoretic pulse applications (less than 300 ms) of L-glutamate, L-aspartate and quisqualate in their dendritic fields had underlying inward currents which did not increase or even decreased, as the holding potential was shifted to values more negative than -65 mV. This 'abnormal' voltage-dependency was still present in Mg2+ -free solution but was abolished in the presence of CsCl2 (10 mM) in the perfusion medium. When TTX (5 microM) and CdCl2 (0.1 mM) were further added to the bath in order to block regenerative conductances, thus broadening the range of the clamp voltages to more positive values than -50 mV, the current-voltage relation between -80 and 0 mV for responses to L-glutamate and L-asparate was almost linear. Our results support the view that low doses of both amino acids act on Purkinje cells essentially via the activation of receptors which are not of the N-methyl-D-aspartate type.     

Inhibition of the early phase of free fatty liberation during cerebral ischemia by excitatory amino acid antagonist administered

In order to ditermine the role of excitatory amino acids (EAAs) in free fatty acid (FFA) liberation during cerebral ischemia, we examined the effect on in situ adminstration of kynurenic acid, a broad-spectrum antagonist of EAA receptors, by microdialysis on the increase in FFA levels during ischemia in the rat hippocampus. A transient rapid increase in FFA levels, superimposed on a continued slow increase, was observed beginning at 1–2 min after ischemia induction. The early phase of FFA liberation. Development of massive ionic shifts during cerebral ischemia can be delayed for several minutes by kynurenic acid administered by the same procedure, suggesting a vital role for EAAs in the early appearance of anaxonic depolarization. The observed inhibition of early FFA liberation may thus be attributable to thedelay in development of massive ionic shifts and resulatant neurotransmitter release which may activate phospholipase A₂and C.     

Polyamine amides are neuroprotective in cerebellar granule cell cultures challenged with excitatory amino acids

Primary cultures of rat cerebellar granule cells have been used to assess the potential neuroprotective effects of philanthotoxins and argiotoxin-636 (ArgTX-636). These polyamine amides are potent antagonists of ionotropic -glutamate ( -Glu) receptors. In granule cells loaded with fluo-3, ArgTX-636 and philanthotoxin-343 (PhTX-343) antagonised increases of intracellular free calcium concentration ([Ca²⁺];) that were stimulated by N-methyl- -aspartate (NMDA). The antagonism was use-dependent. Antagonism by PhTX-343 was fully reversible, but recovery following antagonism by ArgTX-636 was slow and only partial during the time-course of an experiment. Neither compound inhibited K+-induced increases in [Ca 2+];. In excitotoxicity studies with cerebellar granule cells, the release of lactate dehydrogenase (LDH) and morphological observations were used to assess cell death. A 20–30 min exposure to 500 μM NMDA, 100 μM t,-Glu or 500 μM kainate was sufficient to kill > 90% of the cells after 18–20 h. When added 5 min prior to, and during agonist exposure, PhTX-343 and ArgTX-636 provided total neuroprotection. ArgTX-636 was about 20–30 fold more potent than PhTX-343 against NMDA, but was approximately equipotent with PhTX-343 against a kainate challenge. Neither of the toxins showed any inherent toxicity even at 400 μM and 100 μM respectively. Some analogues of PhTX-343 are more potent, both in terms of antagonism of NMDA-stimulated increases of [Ca ²⁺]_i and neuroprotection, than PhTX-343 and ArgTX-636.     

Evidence that excitatory amino acids not only activate the receptor channel complex but also lead to use-dependent block

The effects of fast application of excitatory amino acids N-methyl-D-aspartate (NMDA), L-aspartate (ASP), L-glutamate (GLU), quisqualate (QU) and kainate (KAIN) were studied in neurons from the embryonic spinal cord of the chick in monolayer cultures by employing the 'patch clamp' technique in the 'whole cell' mode. It was found that NMDA, ASP, GLU and QU, but not KAIN, induced responses that exhibited several components. The early component decayed with a time constant of 2 s to a lower level of membrane current and discontinuation of the application was followed by an after-current which returned to the base-line with a time constant of about 7 s. It is suggested that NMDA, ASP, GLU and QU, but not KAIN, not only activate the receptor channel complex but also induce use-dependent block.     

咯利普兰对大鼠脊髓损伤后保护作用及其机制

目的探讨咯利普兰对大鼠脊髓损伤(SCI)后的保护作用及其可能机制。方法将84只SD大鼠随机分为三组:假手术组(n=4)、损伤组(n=40)和治疗组(n=40)。大鼠SCI模型采用纽约大学脊髓重物坠落伤模型。治疗组伤后即刻腹腔内注射咯利普兰,剂量为0.5 mg(/kg.d),2次/d,连续3d。损伤前后对损伤组和治疗组大鼠进行开放场地试验(BBB)评估大鼠脊髓功能。免疫组化染色分析三组大鼠损伤前后脊髓兴奋性氨基酸转运蛋白4(EAAT4)表达情况。结果损伤组和治疗组伤后即刻大鼠BBB评分均为0,处于完全瘫痪状态;然后BBB评分逐渐增高;伤后42、56和64d,治疗组BBB评分明显高于损伤组(P<0.01)。假手术组脊髓组织形态正常,EAAT4表达较少;脊髓损伤1周后,损伤中央出现较大空洞,EAAT4表达明显升高;而治疗组,组织空洞缩小,EAAT4表达较损伤组明显增加。结论咯利普兰有助于大鼠脊髓损伤后功能恢复,其机制可能与增加EAAT4表达有关。     

Evidence for an excitatory amino acid pathway in the brainstem and for its involvement in cardiovascular control

The source and possible role of excitatory amino acid projections to areas of the ventrolateral medulla (VLM) involved in cardiovascular control were studied. Following the injection of [3H]D-aspartate ([3H]D-Asp), a selective tracer for excitatory amino acid pathways, into vasopressor or vasodepressor areas of the VLM in rats, more than 90% of retrogradely labelled neurones were found in the nucleus of the solitary tract (NTS). Very few of the [3H]D-Asp-labelled cells were immunoreactive for tyrosine hydroxylase, none for phenylethanolamine-N-methyltransferase or gamma-aminobutyric acid. The density of labelled cells in the NTS was similar to that obtained with the non-selective tracers wheat germ agglutinin-horseradish peroxidase (WGA-HRP) and WGA-colloidal gold, but these tracers also labelled other cell groups in the medulla. Furthermore, the decrease in blood pressure, caused by pharmacological activation of neurones in the NTS of rats, or by electrical stimulation of the aortic depressor nerve in rabbits could be blocked by the selective N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovalerate injected into the caudal vasodepressor area of the VLM. This area corresponds to the termination of [3H]D-Asp transporting NTS neurones. These results provide evidence that a population of NTS neurones projecting to the VLM use excitatory amino acids as transmitters. Among other possible functions, this pathway may mediate tonic and reflex control of blood pressure via NMDA receptors in the VLM.     

Role of excitatory amino acids in rat vagal and sympathetic baroreflexes

Vagal baroreflexes were studied by measuring the atropine-sensitive cardioinhibition produced by raising arterial pressure with phenylephrine in anesthetized rats pretreated with the beta-adrenergic receptor antagonist nadolol. Sympathetic baroreflexes were determined in halothane-anesthetized rats by measuring the inhibition of lumbar sympathetic discharge produced by elevating arterial pressure with gradual aortic constriction. Both reflexes were drastically reduced by bilateral injections of 2.2 nmol of the glutamate receptor antagonist kynurenic acid (KYN) into either the nucleus of the solitary tract (NTS) or the ventrolateral medulla between 0 and 1 mm posterior to the level of the obex. Injections of KYN elsewhere in the medulla were generally ineffective and injections of 8-OH kynurenate (an inactive analog) into the ventrolateral medulla or NTS were also without effect. KYN injections (2.2 nmol) into the intermediate portion of the NTS produced small increases in mean arterial pressure (0-15 mm Hg) and no change in heart rate while injections of similar amounts into the ventrolateral medulla at obex level were followed by large (35-116 mm Hg) increases in pressure and bradycardia. Both types of injections produced a similar degree of blockade of vagal and sympathetic baroreflexes. These results support previous evidence that baroreceptor primary afferents may release a glutamate-like transmitter in the NTS and indicate that a similar type of excitatory transmitter is involved at the level of the ventrolateral medulla in mediating or modulating both vagal and sympathetic baroreflexes. Finally the bradycardia and hypertension produced by blocking amino acid receptors in the ventrolateral medulla appear largely unrelated to the disruption of peripheral baroreceptor inputs.