Different coupling of excitatory amino acid receptors with Ca2+ channels in primary cultures of cerebellar granule cells

Cerebellar granule cells in primary culture express receptors for excitatory amino acids. The activation of these receptors results in an increased uptake of Ca2+, however, the effects are different depending on the agonists used. Aspartate, NMDA and ibotenate are active only in depolarized conditions, whereas kainate and glutamate activate Ca2+ uptake independently from depolarization. The results indicate the presence of two receptor types: kainate recognition site coupled with voltage-independent Ca2+ channels and NMDA recognition site coupled with voltage-dependent Ca2+ channels.

Excitatory aminoacid antagonists provide a therapeutic approach to neurological disorders

Excessive excitation by neurotransmitters can cause the death of neurons. This excitotoxic action may be responsible for neuronal loss in stroke, cerebral palsy, epilepsy, ageing and Alzheimer's disease, Huntington's disease, and other chronic degenerative disorders. Compounds acting specifically to antagonise excitatory neurotransmission offer a novel therapeutic approach to these disorders.

The variation in action of excitatory amino acids in relation to distance of iontophoretic application to spinal motoneurones

Intracellular recordings were made from lumbosacral motoneurones of barbiturate-anaesthetized cats. DL-homocysteate (DLH) and L-glutamate were iontophoresed extracellularly over a range of distances from the impaled motoneurone. Movement of the iontophoretic electrode unit was controlled by a micromanipulator which was advanced independently of that moving the intracellular electrode. Depolarizations to DLH were first detected at a greater distance from the impaled motoneurones (mean, 383 micron) than depolarizations to L-glutamate (mean, 165 micron). As the point of application approached the soma of the motoneurone, depolarizations developed more rapidly, were larger and the latent period of the L-glutamate depolarization became shorter. Dendritic 'hot-spots' of the depolarizing action of L-glutamate were not detected.

Receptor types mediating the excitatory actions of exogenous L-aspartate and L-glutamate in rat olfactory cortex

Changes in potential between the pial and cut surfaces of rat olfactory cortex slices evoked by N-methyl-D-aspartate (NMDA), quisqualate, kainate, L-glutamate and L-aspartate and also by gamma-aminobutyric acid (GABA) have been monitored using extracellular electrodes. All agonists produced a pial-negative potential response when superfused onto the pial surface, GABA, L-aspartate and L-glutamate being less potent than the others. Repeated applications of NMDA, but not of the other agonists, led to a progressive reduction in response to approximately 30% of the initial depolarization. The responses to NMDA (100 microM) were selectively abolished by (+/-)2-amino-5-phosphonopentanoic acid (APP; 100 microM) while depolarizations evoked by L-glutamate and L-aspartate (both at 10 mM) were only antagonized by 21 +/- 2 (n = 12) and 36 +/- 3 (n = 12) percent respectively (means +/- S.E.M.). gamma-D-Glutamylglycine (gamma-DGG; 1 mM) and (+/-)cis-2,3-piperidine dicarboxylate (cis-PDA; 2 and 5 mM), in addition to antagonizing responses to NMDA, also partially blocked quisqualate- and kainate-evoked depolarizations. When a mixture of APP (100 microM), gamma-DGG (1 mM) and cis-PDA (5 mM) was applied to preparations, although NMDA receptors were completely blocked and responses to both quisqualate and kainate antagonized by approximately 80%, L-glutamate and L-aspartate evoked depolarizations were only reduced by 51 +/- 7 (n = 4) and 49 +/- 4 (n = 4) percent respectively (means +/- S.E.M.). The results are discussed in terms of the contributions made by NMDA, quisqualate and kainate receptors to the composite responses evoked by L-aspartate and L-glutamate.

Effects of zinc on markers of glutamate and aspartate neurotransmission in rat hippocampus

Receptor binding and synaptosomal uptake of L-[3H]glutamate and L-[3H]aspartate were measured in hippocampus derived from rats maintained on zinc restricted diets from weaning. Despite near lethal zinc deficiency, these markers of excitatory amino acid neurotransmission were unaffected compared to zinc-supplemented controls. However, addition of zinc in vitro markedly inhibited binding of glutamate and aspartate to hippocampal membranes. These data suggest that zinc can modulate the receptor affinities for glutamate and aspartate and may function as a tonic inhibitor of excitatory synapses in vivo.

Excitatory effect of amino acids on identified neuron R14 of Aplysia. II. Neutral amino acids and structure-activity relationships

The effects of 20 amino acids on identified neuron R14 of Aplysia kurodai were studied by conventional intracellular recording and voltage-clamp techniques. Neutral alpha-amino acids caused marked dose-dependent depolarizations of the neuron. The effects of several amino acids applied simultaneously were additive. L-isomers were much more effective than the corresponding D-isomers. R14 was at most slightly depolarized by perfusion with 2-aminoisobutyric acid and the non-alpha-amino acids beta-alanine and beta- and gamma-aminobutyric acid. Putative neurotransmitters ACh, 5-HT, dopamine, GABA, and taurine were much less effective in depolarizing R14 than neutral amino acids. Values of the Hill coefficient (nH) and the apparent dissociation constant (KA) were approximately 0.64 and 80 mM, respectively, for glycine. These results suggest that R14 has a general sensitivity to neutral amino acids and that amino acids in the hemolymph may be able to influence the electrical activity of R14.

An animal model for neuron-specific spinal cord lesions by the microinjection of N-methylaspartate, kainic acid, and quisqualic acid

It has been shown that N-methylaspartate (NMA), kainic acid (KA), and quisqualic acid (QA) can produce preferential neuronal damage in various parts of the striatum, hippocampus, and thalamus with relative sparing of axons in transit. Thus far, the evidence that axons in transit escape destruction has been based largely on histological observations. To test the functional integrity of axons in passage, we made unilateral lesions with these agents in the cervical spinal cord of rats and compared the subsequent functional deficits with those seen after spinal cord hemisections. Observations were made in 14 rats. In each case, a laminectomy at the C6-C7 level was performed under general anesthesia. Animals receiving microinjections of KA, QA, or NMA showed motor and sensory deficits only in the ipsilateral forepaw and remained able to use the hindpaws normally. By contrast, animals undergoing spinal cord hemisection developed obvious motor deficits in the ipsilateral hindpaw in addition to the deficits in the forepaw. Histological observations of the spinal cords confirmed an extensive gray matter destruction with relative preservation of the long tracts in animals injected with KA, QA, and NMA. In addition, it was noted that spinal cord neurons appear relatively less sensitive to KA and more sensitive to QA than neurons in the thalamus, striatum, or hippocampus. The possible application of these findings for the production of dorsal root entry zone lesions will be discussed.

An evaluation of selected brain constituents as putative excitatory neurotransmitters

Searching for the endogenous ligands of the 4 classes of excitatory amino acid receptors detected in the mammalian CNS, we have measured, using a 22Na+ efflux receptor assay, the excitatory activity of 42 brain constituents or analogs and established the receptor specificity of those substances which possess excitatory properties. Among the substances tested were methyltetrahydrofolate and N-acetylaspartylglutamate, two putative ligands of the kainate and glutamate receptors. These compounds were found to have very little or no excitatory activity, respectively. The 8 brain constituents possessing excitatory properties displayed a receptor specificity similar to either that of N-methyl-D-aspartate (e.g. quinolinate) or glutamate (e.g. cysteine sulfinate) but not of kainate or quisqualate. These results are discussed in relation with the problem of the identification of brain excitatory neurotransmitters.

[Satiation mechanism]

Food intake in man and animals occurs as meals. Because changes in daily food intake in laboratory animals are often associated with corresponding changes in meal size, while meal frequency is less affected, food intake appears to be regulated mainly by the satiety mechanism. Meal size is controlled by feedback or satiety signals originating from the stomach, the small intestine, the liver and the fat depots. Both neural and hormonal (Bombesin?, Cholecystokinin?) feedback signals are elicited in the stomach and intestine. The feedback signals of the liver apparently originate from hepatic chemoreceptors, whose activity is modulated by certain metabolites, in particular glucose. The satiety effect of pancreatic glucagon, whose plasma concentration in the portal vein increases during a meal, also appears to be mediated by the hepatic chemoreceptors. Unmyelated nerve endings are probably the morphological substrate of the hepatic chemoreceptors. There is some evidence that circulating fat metabolites (fatty acids, ketone bodies, glycerol), whose plasma levels are related to the size of the fat depots, function as lipostatic feedback signals. The signal transfer from the gastrointestinal chemo- and mechanoreceptors and from the hepatic chemoreceptors to the central nervous system occurs mainly through vagal afferents. The first relay for this afferents is the nucleus tractus solitarii in the medulla oblongata. The next relay is at the pons, from where neurons project to the hypothalamus and other forebrain areas. The hypothalamus has important integrative functions in the control of food intake.

Excitatory amino acid receptor interactions of a novel alpha-phosphinic acid analogue of alpha-methylaspartic acid

An alpha-phosphino analogue of alpha-methylaspartate has been synthesized. The compound may not interact with excitatory amino acid receptors directly, as assessed by direct in vitro radioreceptor binding methods; however, it possesses weak anticonvulsant activity and exhibits an excitant action in vitro that is apparently not mediated by a N-methyl-D-aspartate receptor.

Multiple receptor types for amino acids in the carp olfactory cells revealed by quantitative cross-adaptation method

A quantitative method of cross-adaptation was developed to explore the difference in the receptor sites for various amino acids in the carp olfactory receptors. The olfactory responses were measured by recording the stimulant-induced waves from the olfactory bulb. Cross-adaptation was carried out by varying concentrations of amino acids in a wide range. Typical examples of the results obtained are as follows. The response to Thr after Ser was decreased with increasing concentration of Ser applied first and reached the spontaneous level, while that to Thr after Glu was decreased to 77% of the level of the original response with increasing Glu concentration and stayed this constant level with a further increase in Glu concentration. Application of the amino acids in the reverse order gave essentially similar results. Such types of experiments were carried out between various pairs of amino acids. The results obtained suggested that amino acids of very close species (e.g. Thr and Ser, Asp and Glu, Tyr and Phe) stimulate the same respective receptor sites and that amino acids of most other pairs stimulate more or less different receptor sites, although there exist the receptor sites stimulated commonly by both amino acids of one pair. It was concluded that the carp olfactory cells have many different receptor sites stimulated only by one species of amino acid and its close analogues.

The release of (3H)-GABA from the rabbit retina

The neuronal and glial release of (3H)-GABA from rabbit retina has been studied. The results indicate, that neither are there any glutamate, aspartate or glycine receptors on the GABA accumulating neurons, nor any GABA autoreceptors. (3H)-GABA was found to be released by 40 mM K+ from retinal neurons, but not from glia, and the release was not dependent on extracellular Ca++. This indicates a release from a non vesicular transmitter pool. Ouabain has been proposed as a pharmacological tool for studying the release of (3H)-GABA located in neuronal cytoplasm. However, it induced an increased release of (3H)-GABA from both neurons and glia and it is therefore unlikely that it can be used for the specific purpose of studying release from neuronal cytoplasm.

Differential sensitivity of selected rat brain areas to excitatory neurotransmitters released by K+ depolarization

The depolarization of brain slices by high K+ concentrations in the presence of Ca2+ ions leads to the release from nerve terminals of endogenous excitatory neurotransmitters that interact with subsynaptic receptors controlling the neuronal membrane permeability to Na+ ions. This interaction has been studied by following the K+-evoked Ca2+-dependent 22Na+ efflux from 22Na+-preloaded slices from the cortex, striatum, hippocampus, cerebellum, substantia nigra and hypothalamus of the rat. The results indicate that the so-called N-methyl-D-aspartate receptor mediates the bulk of the excitatory transactions in the cortex and striatum whereas a subsynaptic glutamate receptor may mediate those in the hippocampus and cerebellum.

Identification and characterisation of L-[3H]aspartate binding sites on rat spinal cord synaptic membranes

The binding of L-[3H] aspartate to extensively-washed rat spinal cord synaptic membranes was investigated. Specific binding was enriched in synaptic membranes and was optimal under physiological conditions of temperature and pH. Equilibrium binding was established relatively slowly over a period of 30 min, and was totally reversible within 40 min. Saturation analysis revealed complex binding patterns. Two sites were clearly demonstrable, only one of which was shown to be saturable over the ligand concentration range employed in the study (0.1-10 microM). There was also some indication of the presence of a higher affinity site, although this was not investigated in any detail. Saturable binding demonstrated a KD = 1.4 microM and Bmax = 105 pmole/mg protein. Structure-activity studies with a range of amino acid analogues indicated that binding was stereospecific and was inhibited by a very restricted range of compounds. The most potent inhibitors of binding were L-glutamate and L-aspartate. There was no evidence for the involvement of NMDA receptors. Effects of possible endogenous modulators, including ions and guanosine nucleotides were investigated, and the chemical nature of the binding site probed with a number of protein-modifying agents.

Expression of functional GABA, glycine and glutamate receptors in Xenopus oocytes injected with rat brain mRNA

The use of Gurdon's Xenopus oocyte translation system has allowed the production of neurotransmitter receptors in a foreign cell membrane, following the translation of microinjected mRNA isolated from various sources. This very accessible and relatively simple preparation permits the study of the requirements for receptor-ionophore function, assembly and membrane integration. This analysis is presently feasible for the peripheral nicotinic acetylcholine receptor, the chick brain gamma-aminobutyrate (GABA) receptor and the rat brain serotonin receptor. We now report the novel and successful expression of the GABA, glycine, glutamate and related acidic amino acid receptors of mammalian brain, and show that they exhibit pharmacologically separate identities when their mRNAs are processed in the amphibian oocyte.

Sites of methyl esterification and deamination on the aspartate receptor involved in chemotaxis

The receptors involved in bacterial chemotaxis are post-translationally modified by specific enzymes which catalyze the deamination of glutaminyl residues and the methyl esterification and demethylation of glutamyl residues. In this work we identify the sites of these covalent modifications on the aspartate receptor from Salmonella typhimurium. These were identified using the properties of the Staphylococcus aureus V8 protease which cleaves peptide bonds following glutamyl but not glutaminyl residues. We show here that bonds following methyl-esterified glutamyl residues are also resistant to the protease. A comparison of the fragments obtained after V8 protease cleavage of methyl-esterified (or deaminated) peptides with the fragments from the corresponding unmodified peptides immediately yields the sites of modification. Three of the four methyl-esterified glutamyl residues are located near the middle of the receptor amino acid sequence; one of these is synthesized as a glutaminyl residue and is deaminated by the esterase to form a glutamyl residue. The fourth site of methyl esterification is located near the carboxyl terminus. All four sites occupy analogous positions in a well-conserved arrangement of residues which may form a binding site for the esterase and the methyltransferase.

Glutamate currents in mammalian spinal neurons: resolution of a paradox

Mouse spinal cord neurons grown in tissue culture were impaled with a pair of microelectrodes containing 1 M CsCl and voltage-clamped. Membrane currents evoked by excitatory amino acids were studied over the potential range -70 to +20 mV. Glutamate currents behaved as though generated by simultaneous activation of two conductance mechanisms, one voltage-sensitive, the other conventional. Block of NMDA receptors with the competitive antagonist 2-APV removed the voltage-sensitive component of the glutamate response. These results help to explain the paradoxical lack of conductance-change previously reported for glutamate responses recorded in the mammalian CNS.

Lateral hypothalamic responses to pre-absorptive and post-absorptive signals related to amino acid ingestion

This paper reports an investigation of the comparative effects of pre-absorptive and post-absorptive stimuli associated with amino acid ingestion on cat and rat lateral hypothalamic neuronal activity. Pre-absorptive signals, conveyed by the vagus nerve and originating from the small intestine while the amino acids are about to be absorbed, have been previously identified electrophysiologically in our laboratory [3]. They were found to selectively modulate the neuronal activity of some lateral hypothalamic (LHA) cells [4]. The effects of both pre-absorptive and post-absorptive signals associated with amino acid ingestion were analyzed with regard to their degree of convergence at the lateral hypothalamic level. The pre-absorptive signals consisted of either chemospecific information arising from intestinal chemoreceptors and related to the chemical nature of the ingested nutrient, or mechanical information arising from intestinal mechanoreceptors and related to distension. The post-absorptive signal consisted of a microiontophoretic application of amino acids known to selectively affect the neuronal activity of LHA amino acid-sensitive cells [15]. The convergence on these cells of the intestinal afferent amino acid-sensitive units was postulated in this study.

Localization of L-glutamate and L-aspartate synaptic receptors in chick retinal neurons

Kainic acid (KA) produces lesions in the chick retina when injected intravitreally. In order to ascribe L-glutamate and L-aspartate receptors to specific neuronal populations, we measured L-[3H]glutamate and L-[3H]aspartate specific binding to membranes from retinas treated with different doses of KA. A 20% reduction in glutamate and aspartate receptors was observed when amacrine cells were eliminated (50 nmol KA). When 120 nmol KA were injected, horizontal cells were highly decreased and so were glutamate (48%) and aspartate (22%) receptors. 200 nmol KA killed most of the bipolar cells in addition to the horizontal and amacrine cells; this lesion was associated with an additional 21% decrease in the specific binding of aspartate but not glutamate. Receptors for both compounds which remained after 200 nmol KA (30%) could be located in ganglion cells, which were spared by KA.

L-aspartic acid potentiates 'slow' inward current in cultured spinal cord neurons

Intracellular recordings (current- and two electrode voltage-clamp) were made from mouse spinal cord neurons grown dissociated in tissue culture. Neurons were bathed in elevated concentrations of calcium (Ca) and sometimes tetraethylammonium (TEA). Brief depolarizing current injections activated graded 'after-depolarizations' which summated to trigger prolonged all-or-none action potentials. Under voltage-clamp both of these active potentials were manifest as 'slow' inward current. Net inward current was observed in some neurons during 0.5-1.0 s depolarizing command steps. However, in the majority of cases the inward current was seen as large inward current tails (outward current relaxations) upon repolarization of the membrane potential to holding values. Cadmium (Cd) blocked this slow inward current, 'after-depolarizations' and prolonged action potentials. Applications of L-aspartic acid increased the magnitude of net inward current evoked by command steps and potentiated and prolonged inward current tails. This potentiation and prolongation of voltage-dependent inward current likely accounts for the prolonged action potentials or 'bursting' characteristic of responses to L-aspartic acid and related amino acids such as N-methyl-D-aspartic acid.

Amino acid neurotransmission between fimbria-fornix fibers and neurons in the lateral septum of the rat: a microiontophoretic study

We investigated the nature of the excitatory amino acid and the type of amino acid receptor involved in the projection of fimbria-fornix (fi-fx) fibers on neurons in the lateral septal complex (LSC) of the rat. It appeared that neurons which were strongly orthodromically activated (SOA) by stimulation of fi-fx fibers were excited by glutamate (GLU) and aspartate (ASP) at much lower ejecting currents than neurons which were only weakly orthodromically excited. In addition, GLU was a stronger agent than ASP, particularly in SOA septal cells. Two amino acid antagonists tested, glutamic acid diethylester (GDEE) and 2-amino-5-phosphonovaleric acid (2-APV), selectively antagonized responses to the amino acid agonists quisqualate (QUIS) and N-methyl-D-aspartate (NMDA), respectively. They also depressed GLU- and ASP-induced responses, although in that case the antagonists frequently had to be expelled with currents higher than those needed to block QUIS- and NMDA-evoked excitations. Furthermore, GDEE frequently antagonized GLU-induced responses better than ASP-evoked excitations, whereas 2-APV often blocked responses to ASP more effectively than those to GLU. It was observed that GDEE, ejected with currents that blocked responses to QUIS reversibly, decreased the number of synaptic responses induced in SOA cells by fi-fx stimuli. Synaptically induced excitation in these neurons was consistently unaffected by 2-APV, even when the antagonist was expelled with high currents. According to these results, LSC neurons, in particular the SOA neurons, are more readily activated by GLU than by ASP. Monosynaptic excitations elicited in SOA septal cells by fi-fx stimuli appear to be predominantly if not exclusively mediated by QUIS receptors. There are indications that GLU-induced responses in the LSC neurons are presumably mediated by the QUIS receptors. From these data it may be inferred that GLU rather than ASP is the transmitter involved in the projection of fi-fx fibers on LSC neurons.

Induction and decay of amino acid transport in the liver. Turnover of transport activity in isolated hepatocytes after stimulation by diabetes or glucagon

System A-mediated amino acid transport in liver tissue is stimulated by diabetes or by exogenous glucagon. The present report describes the decay process for stimulated System A activity in isolated rat hepatocytes. Transport induced by glucagon, insulin, or spontaneous diabetes (BB/G rats) decayed rapidly after initiation of primary cultures; the estimated half-life was about 1.5 h. In contrast, the stimulated activity in cultured hepatocytes from streptozotocin-diabetic rats had a half-life of about 2.5 h. It is not known if the loss of System A activity is the result of proteolysis or of another form of inactivation. The decay was blocked by either actinomycin or cycloheximide, but was unaffected by leupeptin, methylamine, chloroquine, dinitrophenol, rotenone, or tunicamycin. Studies with cycloheximide and actinomycin suggest the following: 1) within 30 min after initiation of cell cultures, synthesis of the corresponding mRNA for the transport-inactivating protein has begun; 2) the mRNA for transport-inactivating protein is relatively long-lived, but the inactivating protein itself has a half-life of less than 1 h; and 3) actinomycin blocks the decay through inhibition of transport-inactivating protein biosynthesis rather than by protection of the mRNA for the protein responsible for System A activity. A working model for the synthesis and decay of System A activity is presented. Cationic amino acid transport, System y+, was also stimulated severalfold after induction of diabetes or glucagon injection of rats. Systems ASC, X-, and N were enhanced to varying degrees in hepatocytes from diabetic or glucagon-injected rats, but the level of stimulation for each was not as great as that found for Systems A or y+.

Changes in the status of neurotransmitter receptors in a rabbit model of hepatic encephalopathy

It has previously been shown in an animal model of hepatic encephalopathy (HE) that the number of receptors for the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), increases and that the number of receptors for the excitatory neurotransmitter, glutamate, decreases. To determine the functional status of other neurotransmitter systems in HE, measurements were made of the specific binding of other neurotransmitters to synaptic membranes prepared from the brains of normal rabbits and rabbits in HE due to galactosamine-induced acute liver failure. The development of HE was associated with: (i) a decrease in the density (Bmax) of receptors for the two excitatory amino acid neurotransmitters, aspartate and kainic acid; (ii) an increase in the Bmax of both the low and high affinity binding site for strychnine, a marker for the inhibitory neurotransmitter glycine; (iii) a decrease in the affinity (Kd) of receptors for dopamine, and (iv) no appreciable change in either the specific binding of [3H]D-ala2-methionine enkephalinamide or [3H]naloxone, markers for opiate receptors, or in the Bmax or the Kd of receptors for acetylcholine. If it is assumed that the sensitivity of the brain to neurotransmitters varies directly with the density of neurotransmitter receptors, HE may be associated with increased sensitivity to inhibitory amino acid neurotransmitters and decreased sensitivity to excitatory amino acid neurotransmitters. Thus, the observed changes in neurotransmitter receptors in HE afford a feasible pathophysiological basis for the mediation of the neural inhibition of HE.

Partial purification and reconstitution of the aspartate transport system from Halobacterium halobium

Membrane vesicles of Halobacterium halobium R1Wrm bind to an aspartic acid-agarose affinity column. After disruption of the bound vesicles by low ionic strength, a protein fraction is eluted from the column with 2.5% cholate in 3 M NaCl. When this fraction is reconstituted with soybean lipids to form proteoliposomes, the proteoliposomes exhibit active aspartate accumulation. Aspartate transport in the reconstituted system is driven by a chemical sodium gradient (out greater than in), exhibits sensitivity to an electrical potential, and is specific for L-aspartate. These characteristics are consistent with observations on aspartate transport in intact membrane vesicles of H. halobium. Initial aspartate transport rates in the reconstituted system are about ninefold enhanced over the native system. The system developed should be useful in future purification schemes and studies of the molecular details of membrane transport.

Excitatory amino acid receptors in Xenopus embryo spinal cord and their role in the activation of swimming

Bath application of N-methyl-D-aspartate (NMDA), kainate or quisqualate to Xenopus embryos depolarized spinal cord motoneurones and reduced their input resistance in both normal salines and salines containing 20 mM-Mn2+ and 0.5 mM-Ca2+, or 2 X 10(-6) M-tetrodotoxin. This suggests that motoneurones possess all three types of excitatory amino acid receptor. These receptors have similar specificities to excitatory amino acid antagonists as those occurring in adult frog and cat spinal cords. Application of 30-40 microM-NMDA or 5-6.5 microM-kainate to the medium bathing spinalized embryos can cause a sustained patterned motor output similar to that of swimming evoked by natural stimulation of intact animals. At these concentrations NMDA and kainate depolarized motoneurones by 19.0 +/- 1.80 (mean +/- S.E. of mean) and 18.0 +/- 2.00 mV respectively and decreased their input resistance by 23.0 +/- 2.82% and 24.0 +/- 3.46%. These changes are similar to those associated with the tonic excitation which motoneurones receive during naturally evoked swimming. Bath application of 5-8 microM-quisqualate to spinal embryos can also cause a sustained motor output. However, this was different to that evoked by NMDA and kainate and was inappropriate for swimming. When applied to intact animals during swimming both 2-3 mM-cis-2,3-piperidine dicarboxylic acid (PDA) and 0.5 mM-gamma-D-glutamylglycine (DGG) selectively blocked the tonic excitation of motoneurones and in doing so abolished the motor output of the spinal cord. 50-200 microM-2-amino-5-phosphonovaleric acid reduced the tonic excitation but to a lesser extent than either PDA or DGG. The tonic excitation of motoneurones which occurs during swimming therefore appears to be mediated via an endogenous excitatory amino acid transmitter which acts on NMDA and kainate receptors.

Spinal seizures and excitatory amino acid-mediated synaptic transmission

In the isolated frog spinal cord penicillin or strychnine produced spinal seizures with spontaneous slow paroxysmal ventral root depolarizations (pVRDs) and superimposed motoneuron spikes. Mn2+, tetrodotoxin, mephenesin and low [Na+]o suppressed pVRDs, an indication that paroxysmal activity requires intact excitatory synaptic transmission involving interneurons. Compounds reducing the release of amino acids [-)baclofen) or interfering with the activation of N-methyl-D-aspartic acid (NMDA) receptors (D,L-alpha-aminoadipate, D-2-amino-5-phosphonovalerate, gamma-D-glutamylglycine) eliminated pVRDs. The results suggest that synaptic release of excitatory amino acids (e.g. L-glutamate, L-aspartate) and subsequent activation of specific receptors sensitive to the action of NMDA underlie spinal convulsions.

Baclofen reduces tone-evoked activity of cochlear nucleus neurons

Recent evidence suggests that an excitant amino acid may be a neurotransmitter at acoustic nerve synapses in cochlear nucleus (CN). Release of excitant amino acids is reportedly reduced by baclofen, a lipophilic GABA-mimetic used to treat the spasticity of multiple sclerosis and spinal injury. Microiontophoresis of (-)baclofen suppressed spontaneous and tone-evoked activity in CN neurons. GABA inhibited the responses of most neurons responsive to (-)baclofen. However, iontophoresis of these two substances onto the same CN neuron resulted in dramatic differences in time course to maximum effect and to recovery. Onset and offset of (-)baclofen-induced firing reduction were gradual at all doses (currents), but even the highest doses rarely caused total suppression of firing. Inhibition of firing by GABA was abrupt, and total suppression was frequently observed over the range of doses used. GABA desensitization (fading) commonly occurred while the (-)baclofen response never faded. The same CN neurons were also suppressed by D-alpha-aminoadipate, which blocks certain excitatory amino acid receptors, while the GABA antagonist bicuculline had no effect on the (-)baclofen response. These findings support the hypothesis that an excitant amino acid may be a transmitter at acoustic nerve synapses in CN.

Barbiturates, alcohols and the CNS excitatory neurotransmission: specific effects on the kainate and quisqualate receptors

The effects of barbiturates and straight-chain aliphatic alcohols on the responses of rat striatal neurons to excitatory amino acids have been investigated. The responses to N-methyl-D-aspartate, quisqualate, kainate, L-glutamate and L-aspartate were measured by the increase in 22Na+ efflux rate that they produce in brain slices. The responses to quisqualate and kainate, measured in the 22Na+ efflux assay, were found to be partially blocked by barbiturates whereas the responses to N-methyl-D-aspartate, glutamate and aspartate were not. The kainate and quisqualate-induced increases in 22Na+ efflux rate were much more readily blocked by the presence of aliphatic alcohols than were the responses to N-methyl-D-aspartate, glutamate and aspartate. These results strengthen the idea of the existence of 4 distinct receptors for excitatory amino acids in the rat striatum. They are consistent with the presence on the kainate and quisqualate receptors, but not on the N-methyl-D-aspartate and glutamate/aspartate receptors of a hydrophobic domain which would provide a site of interaction for barbiturates and alcohols. They suggest that receptors for excitatory amino acids can be targets for the actions of barbiturates and alcohols on the central nervous system, and may mediate some of the anesthetic and hypnotic effects of these drugs.

Neurochemical, pharmacological, and developmental studies on cerebellar receptors for dicarboxylic amino acids

Specific binding of L-[3H]glutamate ([3H]Glu) and L[3H]Asp) to cerebellar membranes represented a time-, temperature-, pH- and protein-dependent interaction which was both saturable and reversible. Binding sites for both radioligands appeared maximally enriched in synaptosomal fractions isolated by gradient centrifugation. Kinetically derived dissociation constant (K off/K on = Kd) for [3H]Glu binding to this fraction indicated high-affinity (433 nM). Competition experiments employing analogs of excitatory amino acids, including new antagonists, helped identify binding sites for [3H]Glu and [3H]Asp as receptors with differential pharmacological specificities. Membrane freezing reduced numbers of both receptor types, but binding activity could be recovered partially by incubation at 37 degrees C. Glu receptors exhibited a pronounced deleterious sensitivity to thiol modifying reagents and L-Glu (50-1000 microM) provided protection against these compounds during co-incubation with cerebellar membranes. It is suggested that cold storage may induce partially reversible receptor inactivation by promoting sulfhydryl group/bond modification. Rat cerebellar glutamatergic function (endogenous Glu content, Glu uptake and receptor sites) exhibited an apparent ontogenetic peak between days 8-12 postpartum with a plateauing profile from day 30 to adulthood. The accelerated development (days 8-12) coincides with the first demonstrable Glu release and kainic acid neurotoxicity, as described previously.

A comparative study of L[3H]-glutamate and L[3H]-cysteine sulfinate binding sites in subcellular fractions of rat brain

A comparative study of the binding of L-cysteine sulfinic acid (CSA) and L-glutamic acid (GLU) to various subcellular fractions of membranes from rat brain was made. Kinetic parameters were determined in all fractions for both types of binding. The effects of membrane preincubation, freezing, and thawing were also examined. The GLU and CSA specific binding levels increased in medium-density (C) and high-density (D) synaptic membranes as compared to the crude mitochondrial/synaptosomal membranes (wP2). Freezing and thawing reduced CSA binding in all tested subcellular fractions. GLU binding is reduced in wP2, C, and D. Binding to the "light" synaptic membranes (B) was not significantly affected, suggesting the presence of two GLU sites. Kinetics of the GLU binding indicated that the temperature-sensitive and -insensitive sites have Kd of 600 nM and 1,100/nM, respectively. Preincubation of fresh membranes conversely affected CSA and GLU binding to the various subcellular fractions, increasing CSA binding in wP2, B, C and decreasing it in D suggesting the existence of distinct sites for GLU and CSA. Preincubation of previously frozen membranes similarly modified CSA and GLU binding except in B fractions. CSA and GLU binding exhibited different pH sensitivities in both fresh and frozen membranes. These results indicate that multiple acid amino acid binding sites exist in membranes and that they can be differentiated according to their sensitivity to temperature. They also suggest the existence of distinct sites for CSA and GLU in fresh membranes, giving further support to the hypothesis that CSA may also serve a neurotransmitter role in the rat central nervous system.

Further structure activity studies on the excitatory amino acid receptors of the crustacean neuromuscular junction

Intracellular recordings of the membrane potential and evoked excitatory junction potentials were made from the opener muscle in the walking leg of the Hermit crab ( Eupagurus bernhardus ). A variety of amino acid analogues including cis and trans 1-amino-1,3- dicarboxycyclopentane were tested for agonist activity and their potencies were compared with L-glutamate. Quisqualic acid was the most powerful excitant whereas N-methyl-DL-aspartic acid and ibotenic acid were the least active. Threshold concentrations of L-glutamate and quisqualic acid potentiated the ionophoretic L-glutamate potential and the excitatory junction potentials without affecting membrane input resistance. The results are discussed in terms of the structure-activity relationship of the crustacean excitatory receptor compared to other vertebrate and invertebrate nervous systems.

The pharmacology of molluscan neurons

It is commonly accepted that the basic physiological properties of the neurons as well as the nature of transmitter substances have remained relatively unchanged through evolution, while brain size and neuron number have greatly increased. Among invertebrates the molluscs, due to the large size of their neurons and lesser complexity of the neural networks controlling specific behavior, have proved to be especially useful for studying elementary properties of single neurons, network organization as well as various forms of learning and memory. The study of putative neurotransmitters has indicated that molluscs use the same low molecular-weight substances and peptides or their metabolites and cyclic nucleotides as transmitters and second messengers as the other species of various phyla. At the same time the receptors of neurotransmitters were found to have certain characteristic properties in the molluscs. The large molluscan neurons have permitted the isolation of individual identifiable nerve cells, and the subsequent analysis of quantities of the transmitters and their metabolic enzymes. These studies have demonstrated that single neurons frequently can contain more than one putative neurotransmitter. It can be expected that this model will contribute to an understanding of the role of multiple transmitters within a single neuron assuring the plasticity of the nervous system. The cellular mechanisms of plasticity have been demonstrated first in molluscan nervous systems. It was proved in identified Aplysia neurons that the same transmitter (ACh) can be released from an interneuron onto two or more follower neurons and can excite one and inhibit another or evoke a biphasic response on a third type of cell. The biphasic response of the molluscan neurons to neurotransmitters was the first demonstration of the plastic synaptic changes. The discovery of individual neurons with their groups of follower cells acting as chemical units has provided an insight into the organization of various behavioral acts. Study of the gastropod molluscs has also shown that the giant serotonergic cells can act as peripheral modulator neurons, as well as interneurons, and in this way they can affect their target organs at more than one level. The molluscan studies have provided more information on transmitter receptors as it was shown that molluscan neurons have at least six different 5HT receptors, three Ach receptors which can be separated pharmacologically. This type of study has led to the discovery of numerous new antagonists and poisons.(ABSTRACT TRUNCATED AT 400 WORDS)

Recognition chemistry of anionic amino acids for hepatocyte transport and for neurotransmittory action compared

Comparison of neuronal and non-neuronal membrane transport of, and neuroexcitation by, the dicarboxylic amino acids bring out provocative similarities in structural selectivity, and hence in the strategies for studying them. Parallel anomalies in stereoselectivity show for both phenomena that the recognition sites are indeed chiral, as expected for biological functions, even though both fail in special instances to discriminate between DL pairs. High and low affinity, or Na+-dependence or Na+-independence, are not fully reliable bases for discriminating receptor sites serving one of these functions. Tolerance of N-methylation of the amino acid serves in discriminating families of recognition sites for both phenomena, as does substitution of the sulfonate or sulfinate for the distal carboxylate group, or other structural changes. Analogs in which the functional groups of aspartate or glutamate are presented in restrained arrays serve for both, although they have so far suggested identity neither of recognition sites for transport and excitation, nor of the events consequent to binding for these two phenomena.

Voltage clamp analysis of the effect of excitatory amino acids and derivatives on Purkinje cell dendrites in rat cerebellar slices maintained in vitro

A voltage clamp analysis of the effects of L-aspartate, L-glutamate and related derivatives on Purkinje cell dendrites was performed in rat cerebellar slices maintained in vitro. Short iontophoretic pulse applications of L-aspartate and L-glutamate in the dendritic field of Purkinje cells induced dose-dependent inward currents with fast onset and recovery. Quisqualate application also gave rise to well developed inward currents with fast onset and slow recovery, whereas N-methyl-D,L-aspartate had no or little effect on Purkinje cell membranes unless prolonged (several seconds) applications were used. Steady applications of low doses of N-methyl-D,L-aspartate much more severely depressed L-aspartate than L-glutamate mediated responses, whereas inward currents due to quisqualate were unaffected. Inward currents due to quisqualate were often more reduced than those due to L-aspartate by steady applications of 2-amino-5-phosphonovalerate, and the antagonistic action of this drug on responses due to L-glutamate was very weak. These results suggest that receptors of Purkinje cells for glutamate and aspartate are different, and are also different from N-methyl-D-aspartate and quisqualate receptors.

Human neutrophils contain an intracellular pool of putative receptors for the chemoattractant N-formyl-methionyl-leucyl-phenylalanine

Purified human peripheral blood neutrophils were disrupted by nitrogen cavitation or sonication and fractionated on sucrose density gradients in order to separate the plasma membranes and granule fractions. Quantitatively, the fractions containing the specific granules by marker enzyme/protein enrichment contained the most tritiated N-formyl-methionyl-leucyl-phenylalanine (fmet-leu-[3H]phe)-binding activity. Competitive binding experiments using unlabeled formyl peptide analogues indicated that the intracellular binding sites display the same structure-function specificity as formyl peptide receptors on intact polymorphonuclear leukocytes (PMN) or isolated plasma membranes. Analysis of the fractions for membrane, primary, and secondary granule markers, as well as the distribution of 125I-labeled plasma membranes in sucrose density gradients, indicated that the specific fmet-leu-[3H]phe binding to granule-containing fractions was not due to contamination by plasma membranes. In addition, membranes isolated from PMN previously stimulated with phorbol myristate acetate (PMA) demonstrated increased binding sites, while isolated membranes exposed to PMA under the same conditions failed to show such increases. The data lend direct support to the concept that there is an intracellular pool of fmet-leu-phe receptors that serves as a source of new surface membrane constituents and receptor material that may allow PMN to maintain functional responsiveness during chemotaxis.

Classification and properties of acidic amino acid receptors in hippocampus. II. Biochemical studies using a sodium efflux assay

The properties of excitatory amino acid receptors in hippocampal slices were analyzed using agonist-induced stimulation of 22Na efflux rate. Several amino acids (L- and D-glutamate, N-methylaspartate) produce progressively smaller responses upon successive applications, whereas D,L-homocysteate does not. Several lines of evidence suggest that depletion of an intracellular pool of 22Na is not responsible for the apparent desensitization. Addition of the amino acids in the presence of an antagonist does not affect the response of the slices to subsequent applications, indicating that desensitization is dependent upon the interaction of the agonist with its receptor. The antagonist D-alpha-aminoadipate discriminates between various excitatory amino acids, completely blocking the responses to N-methylaspartate, D-glutamate, and D,L-homocysteate; partially antagonizing those of quisqualate and kainate; and being without effect on L-glutamate. The order of potency of several excitatory amino acids on the stimulation of 22Na efflux rate in hippocampal slices is highly correlated with their relative effects measured with electrophysiological techniques, but does not correlate with their relative potencies to inhibit [3H]glutamate binding to hippocampal membranes. The similarities in the properties of excitatory amino acid receptors evidenced with the 22Na efflux assay or with the electrophysiological approach in the in vitro hippocampal slice preparation indicate that the same receptors are sampled by the two techniques. The results are discussed in terms of a classification of these receptors into four different groups: a synaptic receptor, activated by D,L-homocysteate (tentatively defined as a G1 receptor), an extrasynaptic glutamate receptor (defined as a G2 receptor), an N-methylaspartate receptor, and a kainate receptor.

Classification and properties of acidic amino acid receptors in hippocampus. III. Supersensitivity during the postnatal period and following denervation

The effects of excitatory amino acids on 22Na efflux rate in rat hippocampal slices were determined at various postnatal days and following removal of a major afferent system. Two weeks after a unilateral hippocampal aspiration, the 22Na efflux induced by potassium ions, D-glutamate, N-methylaspartate, and kainate is significantly decreased in the contralateral intact hippocampus whereas the effect of L-glutamate is substantially increased. Analysis of concentration-response curves suggests that the increased responsiveness to L-glutamate is due to an increase in the maximal effect rather than to changes in the half-maximal concentration for the amino acid. Partial denervation does not detectably change efflux elicited by D,L-homocysteic acid nor does it modify the properties of [3H]glutamate binding to hippocampal membranes. The effects of potassium ions, N-methylaspartate, and kainate but not of D,L-homocysteate are significantly decreased in slices incubated in the absence of calcium. All of the amino acids tested are considerably more potent in slices prepared from 11-day-old rats than in those from adult rats; the differences in responsiveness reflect an increase in maximal effect without changes in the half-maximal concentration. The responses to L-glutamate and D,L-homocysteate decline steadily between postnatal days 11 and 30, at which time adult values are reached. Together, the results from the denervation and development studies suggest a different localization and different modes of regulation for various classes of excitatory amino acid receptors.

Somatostatin release from cerebral cortical cells: influence of amino acid neurotransmitters

We examined the ability of several putative amino acid neurotransmitters to influence immunoreactive somatostatin (IRS) release from cultured rat cerebral cortical cells. The cells were exposed to either or sequential incubations in various concentrations of glutamate (Glu), aspartate (Asp), GABA, glycine, taurine and arginine. Glu and Asp were stimulatory to IRS release, whereas GABA was inhibitory. Glu-induced IRS release was calcium-dependent. Glycine and taurine were weak stimulants.

Sites of methyl esterification on the aspartate receptor involved in bacterial chemotaxis

The methyl esterification of the aspartate receptor involved in chemotaxis has been studied in order to clarify the role of receptor modification. Receptors were methyl esterified in an in vitro system using S-adenosyl-L-[methyl-3H]methionine as a methyl donor. Methyl esterified receptors were digested with trypsin and radioactive tryptic peptides were purified using high performance liquid chromatography. Comparing the amino acid composition of the modified peptides with the DNA sequence of the receptor gene, two regions of the polypeptide chain which contain methyl esterified residues were identified. The regions are homologous and contain a strongly conserved 13 amino acid sequence. One region, containing up to three modified residues, is near the middle of the protein; the other, containing one modified residue, is near the carboxyl terminus.

Pharmacology and regional variations of quinolinic acid-evoked excitations in the rat central nervous system

A physiological and pharmacological investigation of a novel endogenous excitant, quinolinic acid, was carried out in male rats using conventional iontophoretic techniques. It was established that quinolinic acid responses were preferentially reduced by antagonists acting at the N-methyl-D-aspartate (NMDA) preferring receptor, such as (+/-)-2-amino-7-phosphono-heptanoic acid and 1-hydroxy-3-amino-pyrrolidone-2. Glutamic acid diethyl ester reduced responses to quinolinic acid, quisqualic acid and NMDA with no clear specificity. Streptomycin, thought to act at the quisqualic acid receptor, largely spared quinolinic acid responses, being more effective against quisqualic acid evoked excitations. It is therefore suggested that quinolinic acid acts primarily at the NMDA receptor. In addition, the sensitivity of various components of the neuraxis to quinolinic acid was assessed and compared with glutamate and NMDA. Neurons in the spinal cord and cerebellum were largely unresponsive to quinolinate, whereas cells in the neocortex, striatum and hippocampus responded to this agonist to a similar degree as glutamate. In the cortex quinolinate was about one-fifth as active as NMDA, which together with quinolinic acid was much less active in the spinal cord and cerebellum. It is concluded that the possibility that quinolinic acid has a neurotransmitter type function at central "amino acid" receptors merits further investigation.

Separation of signal transduction and adaptation functions of the aspartate receptor in bacterial sensing

In order to investigate the functions of stimulus recognition, signal transduction, and adaptation, the aspartate receptor gene for bacterial chemotaxis in Salmonella typhimurium has been sequenced and modified. A carboxyl-terminal truncated receptor was shown to bind aspartate and to transmit a signal to change motility behavior. However, the truncated receptor showed greatly reduced methyl-accepting capacity, and did not allow adaptation to the sensory stimulation. The separation of receptor functions by alteration of primary structure emphasizes that the receptor is directly involved in adaptation and is not solely a device for transmitting a signal across a membrane.

Pharmacological differentiation between responses of rat medullary dorsal horn neurons to noxious mechanical and noxious thermal cutaneous stimuli

Response of lamina V medullary dorsal horn neurons to noxious thermal and noxious mechanical facial stimuli were challenged with iontophoretically applied cis-2,3-piperidine dicarboxylic acid, a broad spectrum excitatory amino acid antagonist. This antagonist reduced neuronal responses to noxious mechanical stimuli but not responses to noxious thermal stimuli. These results suggest that different neural mechanisms underlie the responses of lamina V neurons to different noxious stimuli, and that responses to noxious mechanical stimuli appear to involve excitatory amino acid receptors.

tRNA separation by high-performance liquid chromatography using an aggregate of ODS-Hypersil and trioctylmethylammonium chloride

High-performance liquid chromatography on a reversed-phase support treated with a tetraalkylammonium salt was used to separate tRNAs from baker's yeast. While resolution by this column appears to result from both anion-exchange and reversed-phase chromatography, it is the hydrophobic interactions which govern the separation of one tRNA from another. Chromatography of bulk tRNA resulted in a number of fractions with different amino acid acceptor activities and little cross-contamination. In some cases the column resolved several single nucleotide modifications of tRNAPhe. Using a 250 x 6.2 mm column it has been possible to chromatograph a minimum of 100 A260 units of tRNA without serious loss in resolution. tRNAs isolated from this column as the last step of a purification procedure have very high amino acid acceptor activities.

Effect of intestinal amino acid infusions on hypothalamic single unit activity in the anesthetized cat

Single unit discharges in the ventromedial hypothalamic nucleus (VMH) and lateral hypothalamic area (LH) were recorded extracellularly in anesthetized cats, while amino acid solutions were perfused through the small intestine via implanted cannulae. Test infusions consisted of 5 amino acid mixtures (arginine, leucine, phenylalanine, tryptophan, alanine: 50 mM each), 5.0% casein hydrolysate; arginine 125 mM (2.2%); leucine 125 mM (1.6%). Sixty-six units were recorded in 30 cats. Of 52 tested in the LH, 19 or 37% were affected: 14 increased and 5 decreased in firing rate in response to amino acid intestinal perfusions with a very short latency consistent with activation of intestinal amino acid receptors. Of 14 neurons tested in VMH, only 3 or 21% were activated by amino acid infusions but with a long latency of several minutes which cannot exclude the involvement of a postabsorptive signal. Results are discussed in terms of intestinal afferent control over hypothalamic neuronal activity related to amino acid induced satiety.

Excitatory amino acids and potassium release in the frog spinal cord

Synaptic release of excitatory amino acids such as L-glutamate and/or L-aspartate and subsequent activation of specific receptors by these putative transmitters appears necessary for the release of K+ by afferent stimulation in the isolated frog spinal cord. This conclusion is based on the findings that (-)baclofen, which is thought to reduce the presynaptic release of putative excitatory amino acid transmitters, and some amino dicarboxylic amino acids (D, L-alpha-aminoadipic acid, 2-amino-4-phosphonobutyric acid, and D, L-alpha, epsilon-diaminopimelic acid), which are believed to interfere with the activation of receptors by these same excitatory amino acids, significantly attenuate the increment in extracellular K+ evoked by tetanic dorsal root stimulation.