Efflux of potassium from neurones excited by glutamate and aspartate causes a depolarization of cultured glial cells

The time course of the depolarization of cultured astrocytes and neurones by glutamate and aspartate corresponds well with the increase of the extracellular K+ concentration ([K +]0) measured with an ion-sensitive microelectrode placed in the close vicinity of the cells tested. It is concluded that the depolarization of glial cells is caused by an efflux of K+ from neighbouring neurones during their excitation by the amino acids.

Effects of tetrodotoxin on changes in extracellular free calcium induced by repetitive electrical stimulation and iontophoretic application of excitatory amino acids in the sensorimotor cortex of cats

Extracellular free calcium ([Ca2+]o) was measured with double barreled ion-sensitive reference electrodes in the sensorimotor cortex of cats before and after application of tetrodotoxin (TTX). Electrical stimulation of the cortical surface or of the thalamic ventrobasal complex resulted in reductions of [Ca2+]o (delta Ca) by up to 0.45 mM (baseline 1.2--1.3 mM). Iontophoretic applications of the excitatory amino acids glutamate, aspartate and DL-homocysteate evoked delta Ca by up to 1.2 mM. delta Ca were largest at a depth of 100-300 micron below cortical surface. After application of 10(-5) M TTX to the cortical surface, the delta Ca evoked by electrical stimulation disappeared and the accompanying slow negative potentials were reduced in amplitude. In contrast, delta Ca evoked by excitatory amino acids were only slightly affected. It is suggested that excitatory amino acids activate voltage-dependent postsynaptic Ca2+ conductances in neocortical neurones.

Pharmacological dissection of pyloric network of the lobster stomatogastric ganglion using picrotoxin

1. Picrotoxin (PTX) (10(-7)-10(-6) M) completely blocked most inhibitory synapses in the pyloric pattern generator of the lobster (Panulirus interruptus) stomatogastric ganglion. The sensitivity of synapses from most classes of identified neurons was examined. Blockade was at least partly reversible with prolonged washing. 2. The synapses from pyloric dilator (PD) neurons were the only inhibitory synapses that picrotoxin failed to block completely. 3. A correlation is derived that brief, fast-rise inhibitory postsynaptic potentials (IPSPs) are picrotoxin sensitive, whereas a slow rounded component of IPSPs from PD neurons is not picrotoxin sensitive. 4. Picrotoxin caused specific changes in the pattern of the motor rhythm produced by the 16-cell pyloric network. This sheds some light on the functional role of particular synapses in the pyloric generator. 5. The endogenously bursting neurons (PD and anterior burster (AB)), which drive the pyloric rhythm, kept a similar burst rate. 6. Under picrotoxin, the pyloric "follower" neurons all moved to later phase relative to the "driver" group. Some normally antagonistic cells, related by reciprocal inhibitor connections, became in-phase. These and other pattern changes could be related to blockade of particular synapses. 7. The pyloric rhythm was still quite recognizable under picrotoxin despite the drastically altered circuitry of the synaptic network. This supports the idea that periodic inhibition from the PD driver neurons plays a primary role in creating the pyloric pattern.

L-glutamate as possible neurotransmitter in the ampullae of Lorenzini of the skate

Effects of Mg2+, L-glutamate (L-Glu) and L-glutamic acid diethylester (GDEE) on resting and evoked activities of the ampullae of Lorenzini were studied in marine skates (Raja clavata). Perfusion of the basal membrane with 10(-4)-10(-5) M L-Glu produced significant and reversible changes in resting firing rate depending on initial resting rate. When synaptic transmission was blocked by perfusion with elevated Mg2+, the resting activity was restored if L-Glu (10(-4)-10(-5) M) was added. GDEE (10(-3)-10(-4) M) blocked the synaptic transmission. The results are consistent with the view that L-Glu is the synaptic transmitter in the ampullae of Lorenzini of the skate.

Aspartate and glutamate as possible neurotransmitters of cells in layer 6 of the visual cortex

Earlier work has suggested that aspartate, glutamate and gamma-aminobutyric acid (GABA) act as transmitters in the cerebral cortex. There is reasonable evidence for the identity of the cell population responsible for GABA release but until now there has been little evidence concerning the sources for release of aspartate and glutamate. Here we have used two approaches to identify possible neurotransmitters used by cells in the visual cortex: measurement of the efflux of endogenous compounds in conditions of synaptic release and localization of these compounds to particular cell classes using neurotransmitter-specific histochemical techniques. Our results suggest that the acidic amino acids aspartate and glutamate may be cortical neurotransmitters, as shown by calcium-dependent release from endogenous stores and by uptake specific to pyramidal cells in layer 6 of the cortex. These substances may therefore have a role in the function of layer 6 cells, which are responsible for the recurrent projection from the cortex to the lateral geniculate nucleus and for the projection within the cortex from layer 6 to layer 4.

Modulation of lateral geniculate neurone excitability by noradrenaline microiontophoresis or locus coeruleus stimulation

The dorsal lateral geniculate nucleus (LGNd) receives afferents from the brainstem which regulate its capacity to transmit visual information from the retina to the striate cortex. One such pathway consists of noradrenaline (NA)-containing fibres originating in the locus coeruleus (LC). These provide a dense, uniform, noradrenergic innervation of the LGNd. Electrical stimulation in the LC region has been reported to enhance the responsiveness of LGNd neurones to afferent excitation. Although this effect was abolished when brain NA stores were pharmacologically depleted, it was not established as a direct action of NA on LGNd neurones because of the widespread distribution of LC fibres to many parts of the brain and the long latency of the response. Recently, we observed that NA, applied locally by microiontophoresis with low ejection currents, produced a delayed increase in the firing rate of most spontaneously active LGNd neurones, an effect selectively blocked by alpha-adrenoceptor antagonists. We show here that microiontophoretic NA can mimic the ability of LC stimulation to enhance the synaptic excitation of LGNd neurones. As neither NA nor LC stimulation activated LGNd neurones in the absence of synaptic or glutamate-induced excitation, both appear to act through a neuromodulatory mechanism. The postsynaptic alpha-adrenoceptor antagonist WB-4101 blocks the facilitation produced by locally applied NA and by coeruleo-geniculate pathway stimulation, providing evidence for pharmacological identity of the two effects.

[Role of acetylglutamate in the stimulation of citrullinogenesis by glucagon]

Acute glucagon treatment of Rats has been found to increase in liver the intramitochondrial concentration of acetylglutamate which is an activator of carbamylphosphate synthetase l. A part of the stimulation of citrulline formation by glucagon is certainly related to this increase of acetylglutamate concentrations.

Evidence against an exclusive role of glutamate in kainic acid neurotoxicity

Transection of the non-glutamatergic septohippocampal fibers 3-5 days prior to intrahippocampal microinjection of 1 microgram kainic acid (KA) or 3 micrograms ibotenic acid (IBO) protect dentate granule cells against KA but not IBO. In the transected animals, a significant reduction of KA-induced behavioral seizures can be observed. Neither transection of the hippocampal commissural system nor the fornix contralateral to the intrahippocampal injections protected against KA or IBO neurotoxicity. These findings are discussed in the light of the current hypothesis of KA- and IBO-induced neuronal degeneration.

Evidence for L-glutamate as the neurotransmitter of baroreceptor afferent nerve fibers

Microinjection of L-glutamate into the intermediate nucleus tractus solitarii in anesthetized rats elicits hypotension, bradycardia, and apnea, simulating baroreceptor reflexes. Ablation of the nodose ganglion results in selective reduction of high-affinity uptake of L-glutanate in the nucleus tractus solitarii. L-Glutamate may be the neurotransmitter of afferent nerve fibers from arterial baroreceptors.

Kainate-induced lesion in the optic tectum: dependency upon optic nerve afferents or glutamate

Kainic acid is known to induce characteristic lesions in neurons receiving an intact input with presumed glutamate-mediated neurotransmission. There are indications for glutamate as a transmitter of retinal afferent terminals in the pigeon optic tectum. After tectal injection of kainic acid (0.5-2.0 microgram in 0.5 microliter) the optic tectum was studied by light and electron microscopy and the following changes were observed: (a) within 1-48 h important neuropil vacuolization predominantly in lower part of layer 5. Such vacuoles were sometimes postsynaptic to identified retinal afferent terminals: (b) within 1 h to 21 days progressive neuronal cell loss throughout the tectal layers. These toxic effects were not observed 2-12 weeks after contralateral retinal ablation but could partially be restored by combined glutamate (0.2 mg) and kainate injection. Thus in the pigeon tectum, kainic acid neurotoxicity is dependent upon an intact retinal input, a finding consistent with a special role for glutamate - possibly as a transmitter - in retinal terminals.

Structure-activity studies on an excitatory receptor for glutamate on leech Retzius neurones

1 Intracellular recordings were made from Retzius cells from the segmental ganglia of Hirudo medicinalis and Haemopis sanguisuga. Glutamate had a direct excitatory effect on the leech Retzius cells. 2 L-Glutamate was 25 times more potent than D-glutamate. 3 L-Glutamate was approximately equipotent with ibotenic acid and 11.2 times more potent than L-aspartic acid. 4 Quisqualic acid and kainic acid were both approximately 100 times more potent than L-glutamate. DL-1-Amino-cis-1-3-dicarboxyclyclopentane was approximately 5 times more potent than L-glutamate, while the trans isomer was 105 times less potent. 5 alpha-NH2-pimelic acid and beta-CH3-glutamic acid reduced the response to L-glutamate. 6 It is suggested that glutamic acid may interact with the Retzius cell glutamate receptor in an extended conformation.

Aspartate, glutamate and GABA levels in pallidum, substantia nigra, center median and dorsal raphe nuclei after cylindric lesion of caudate nucleus in cat

Aspartate, glutamate and GABA levels were determined in afferent and efferent projection nuclei of the striatum after unilateral cylindric lesion in the head of the caudate nucleus in cats. Two and four weeks after operation, GABA content was significantly reduced in substantia nigra and pallidum ipsilateral to the lesion. Glutamate (GLU) level was decreased in substantia nigra and pallidum only 4 weeks after lesion, whereas aspartate content in substantia nigra decreased significantly already after 2 weeks. No changes in the contents of these amino acids were detected in the dorsal raphe nucleus, which receives a projection from the caudate, as well as in the center median nucleus, which projects to the striatum. These experiments using longer survival times substantiate the role of GABA in caudato-pallidal and caudatonigral projections. The possibility is discussed that aspartate (ASP) could function as transmitter of cortico-nigral fibers.

Brain cholinergic dysfunction and memory in aged rats

Age related alterations in mnemonic ability and in the functional status of muscarinic receptors were evaluated and compared to biochemical measures of pre and post-synaptic cholinergic functioning. Retention of a single trial passive avoidance task was considerably disturbed as a function of aging. The functional status of muscarinic receptors, as measured by the ability of microiontophoretically applied acetylcholine to stimulate the firing of hippocampal pyramidal cells, was similarly disturbed in aged rats. A small, but significant decrease in muscarinic receptors was detected in the dorsal hippocampi of these same aged rats, while choline acetyltransferase activity did not change. When considered with prior psychopharmacological studies, these data suggest that specific muscarinic receptor impairments may play a critical role in the memory disturbances associated with old age.

Differential effects of diltiazem on glutamate potentials and excitatory junctional potentials at the crayfish neuromuscular junction

1. The effects of diltiazem on glutamate potentials and excitatory junctional potentials (e.j.p.s) were investigated in the crayfish neuromuscular junction. 2. When diltiazem (0.3 mM) was added to the perfusion fluid, the ionophoretic glutamate potential was reduced to about half, whereas the peak amplitude of successive e.j.p.s elicited by a train of pulses of 100/sec increased by about 2 times. 3. It was suggested that diltiazem was a non-competitive inhibitor of L-glutamate. The reduction of the response to applied glutamate was not due to the acceleration of desensitization of the glutamate receptor. The rate of recovery from desensitization was delayzed by diltiazem. 4. The increase in amplitude of e.j.p.s caused by diltiazem was due to the increase in membrane resistance. The quantum content and size of extracellular e.j.p.s were not affected by diltiazem. 5. It was substantiated using the micro-electrode technique that the glutamate sensitive area coincided with the neuromuscular junctional area. 6. The pharmacological difference between glutamate potentials and e.j.p.s revealed in the present study is difficult to explain on the glutamate transmitter hypothesis. One explanation worthy to be considered is that there are two pharmacologically different kinds of receptors sensitive to L-glutamate.

Amino acids as neurotransmitters of corticofugal neurones in the rat: a comparison of glutamate and aspartate

1 The relative sensitivities to aspartate and glutamate of neurones receiving a corticofugal innervation were examined by microiontophoresis, and compared with the relative sensitivities of neurones not appearing to receive such an input.2 On all the cells tested, glutamate appeared to be a more potent excitant than aspartate in terms of neuronal response size or effective dose.3 DL-alpha-Aminoadipate (alphaAA) reduced the excitatory amino acid responses on all the neurones tested. On many of these cells a control excitation could be produced, by acetylcholine or hydrogen ions, which was in most cases unaffected by doses of alphaAA producing antagonism of amino acid excitation.4 On 70% of the cells, aminoadipate showed no selectivity for aspartate compared with glutamate but a differential action, involving blockade of aspartate but not glutamate, was apparent on the other 30%.5 Doses of alphaAA which selectively reduced responses to aspartate had no effect on short latency evoked spikes, but doses which also reduced responses to glutamate reduced the short-latency synaptic excitation induced by electrical stimulation of either the surface of the cerebral cortex, or of the pyramidal tracts in the medulla.6 These findings suggest that corticofugal neurones having an excitatory action on cells in various parts of the brain may use an amino acid, probably glutamate, as a common neurotransmitter.7 As no significant difference could be demonstrated in the potency ratios of glutamate:aspartate on monosynaptically activated cells compared with other cells, doubt is cast on the validity of drawing conclusions about transmitter identity from potency ratios alone, without the support of antagonist studies.

Opposite turning effects of dainic and ibotenic acid injected in the rat substantia nigra

Unilateral intranigral administration of kainic and ibotenic acid, two putative stimulants of glutamatergic mechanisms, elicited turning behaviour starting from doses of 10 ng. While the turning produced by kainic acid was ipsilateral, that produced by ibotenic acid was contralateral to the injected side. Previous destruction of dopaminergic neurons on the side of the intranigral injection failed to reduce the turning behaviour. Peripheral treatment with picrotoxin did not reduce the turning in response to ibotenic acid. The results might suggest the existence of excitatory and inhibitory glutamate receptors which control nigral non-dopaminergic neurons mediating turning-behaviour.

Stretching and yawning: a role of glutamate

Systemic injection of GDEE (glutamate diethyl ester), an antagonist of glutamate and aspartate receptors, induces stretching and yawning in rats. This was not accompanied by excessive grooming. Coupled with previous work these findings give evidence that a glutamatergic mechanism is involved in stretching and yawning.

Fine structural analysis of the cortico-striatal pathway

Considerable evidence has accumulated indicating that one neurotransmitter in the excitatory cortico-striatal tract is glutamate. Lesions of the tract result in reductions in the striatum of glutamate levels as well as high affinity uptake of glutamate into synaptosomes. Furthermore, such lesions eliminate the neurotoxicity of the glutamate analog kainic acid when injected into the striatum. The fine structure of the cortico-striatal pathway was studied to provide evidence regarding the morphology of glutamate nerve endings. Seven days after injection of 3H-proline (20-25 mu Ci) into the rat frontal cortex, axonally transported label appeared in the striatum with uniform distribution in a single type of nerve ending. The labeled boutons had common round vesicles and made asymmetrical contacts, mostly with dendritic spines. This morphology is typical of excitatory synapses, and similar to that previously shown for cholinergic boutons in the striatum. In four animals similarly injected with 3H-proline, kainic acid was administered directly into the striatum to induce degeneration of postsynaptic elements eight to ten hours before sacrifice. In areas affected by these injections, grains appear in patches, possibly resulting from glial swelling. Labeled boutons were seen almost four times as often in synaptic contact with degenerating dendritic elements as with normal ones. The data provide morphological evidence as to the nature of the probable glutamatergic boutons in the striatum, and show the close relationship of such boutons with the neurotoxic effects of kainic acid. This would be anticipated in view of the dependency of kainic acid neurotoxicity on the integrity of the cortico-striatal pathway.

Desensitization of glutamate receptors on innervated and denervated locust muscle fibres

Depolarizations to L-glutamate, applied locally by microinophoresis to the extrajunctional membrane of locust extensor tibiae muscle fibres and measured either in current clamp or voltage clamp, increased in amplitude for equivalent doses of glutamate following chronic denervation of the muscle. 2. A two-pulse method was used to examine recovery from desentization of junctional and extrajunctional receptors. A 'response ratio', i.e. the amplitude of response to the second (test) of a pair of glutamate pulses over to the pulses. The 'response ratio' for extrajunctional depolarizations of innervated fibres increased exponentially with pulse interval, with a time constant of 15.6 +/- 4.7 sec (n = 11). Recovery of extrajunctional receptor populations from desensitization was accelerated after denervation. The recovery kinetics for responses from fibres 6-22 days after denervation were generally described by two exponential terms, with time constants in the range 0.5-10 sec which were inversely related to the glutamate sensitivity of the extrajunctional membrane. For junctional receptors on both innervated and denervated fibres the recovery kinetics were described by a single exponential with a time constant of 0.2-1 sec. 3. The results suggest that the increased extrajunctional glutamate sensitivity which occurs after denervation results from the 'appearance' of glutamate receptors with properties similar to those found at the post-junctional membrane on locust muscle fibres.

The effect of acidic amino acid antagonists on synaptic transmission in the hippocampal formation in vitro

The effects on synaptic efficacy of the putative acidic amino acid antagonists, 2-amino-4-phosphonobutyric acid (APB), 2-amino-3-phosphonopropionic acid (APP), 1-hydroxy-3-amino-pyrrolidone-2 (HA-966) and glutamic acid diethyl ester (GDEE), were tested by bath application to the hippocampal slice preparation. On the basis of previous work, we hypothesized that APB, HA-966 and GDEE might antagonize synaptic responses to either glutamate or aspartate, but APP should antagonize only synaptic responses to aspartate. APB and HA-966 reduced the amplitude of the extracellular EPSP recorded during stimulation of the perforant path fibers, but APP and GDEE were without effect. APB, APP and HA-966, but not GDEE, consistently inhibited transmission at Schaffer collateral and commissural synapses. The mossy fiber evoked extracellular EPSP was unaffected by these agents. At the concentrations used in this study (usually 2.5 mM) none of these drugs affected the amplitude of presynaptic fiber potentials or antidromic responses, indicating that they probably acted at synapses. The spontaneous activity of hippocampal pyramidal cells, but not of dentate granule cells, increased in the presence of 2.5 mM APB. The amplitude of the population spike generated by Schaffer commissural stimulation initially increased following introduction of APB into the medium and then declined in parallel with the extracellular EPSP. In addition, APB reduced the duration of recurrent inhibition during the period when pyramidal cell firing was enhanced. These results can be explained by an antagonism at the synapse between pyramidal cell and inhibitory interneuron.

Effect of hippocampus extirpation in the rat on glutamate levels in target structures of hippocampal efferents

Twenty days after complete uni- or bilateral hippocampus extirpation in rats, a 25% decrease in glutamate concentration was observed in the septum. Glutamate content also decreased in other terminal structures of the hippocampo-subicular system, i.e. entorhinal cortex, nucleus accumbens septi, mammillary bodies and contralateral hippocampus. It is concluded that the fall in glutamate content which is absent in caudate nucleus is specific for target regions of the hippocampal efferents, adding further support to the suggested transmitter role of glutamate in the limbic system.

Pharmacological distinction between the excitatory junctional potential and the glutamate potential revealed by concanavalin A at the crayfish neuromuscular junction

The effect of concanavalin A(Con A) on desensitization of the glutamate receptor was investigated in the crayfish opener muscle. The depolarization of the crayfish muscle fiber caused by bath-applied L-glutamate was greatly augmented by Con A. The time course of the appearance of the augmentation was slow. Con A completely prevented the development of desensitization of the glutamate receptor. When L-glutamate was applied iontophoretically with a constant current pulse, a decline of the depolarization was seen during the course of the drug application which was presumably due to desensitization of the glutamate receptor. The glutamate potential was slightly increased by Con A, though the increase was transient. On the other hand, the amplitude of excitatory junctional potentials (EJPs) was not increased but decreased by addition of Con A. In normal saline, the amplitudes of both glutamate potentials and EJPs remarkably decreased because of desensitization of the glutamate receptor, but the decrease in amplitude of the glutamate potential was completely prevented by previous application of Con A. On the other hand, Con A had no influence upon the decrease in amplitude of EJPs. These results show that there is a pharmacological difference between the glutamate potential and EJPs.

The excitation of mammalian central neurones by amino acids

1. The relative potencies of a number of analogues of L-glutamate as excitants of thalamic neurones in the rat have been compared. The most powerful compounds were kainate, ibotenate and (+/-)cis-1-amino-1,3-dicarboxycyclopentane. The D- and L-isomers of glutamate and aspartate were also compared. Whereas D-glutamate is approximately one-half as active as the L-form, D-aspartate is more potent than L-aspartate. 2. Computer analysis has indicated that ibotenate and cis-1-amino-1,3-dicarboxy-cyclopentane have relatively fixed and similar C alpha-N, Comega-N and C alpha-Comega interatomic distances which can also be achieved by glutamate in certain conformations of the molecule, but not by aspartate. 3. Parallel examination of the antagonists glutamate diethylester and D-alpha-aminoadipate has shown that the former preferentially reduces L-glutamate effects while the latter blocks the actions of other amino acid excitants more readily than those of L-glutamate. 4. The evidence is consistent with the hypothesis that at least two populations of neuronal receptors for the excitatory amino acids exist.

Kainic acid injections result in degeneration of cochlear nucleus cells innervated by the auditory nerve

When kainic acid, a putative neurotoxin for neurons with glutamatergic input, is injected into the brainstem, it produces a selective pattern of degeneration in the cochlear nucleus. The rate and extent of degeneration is correlated with the distribution of the primary auditory fibers. This evidence supports the hypothesis that glutamate is the neurotransmitter for primary auditory fibers.

Glutamate current noise: post-synaptic channel kinetics investigated under voltage clamp

1. Analysis of voltage-clamped noise has been used to investigate the operation of glutamate receptors and associated channels at the locust nerve-muscle junction. Channels opened by glutamate and an agonist have been compared. 2. Glutamate-induced current fluctuations have a power spectrum with a single (1/frequency2) component which fits a simple model for the operation of channels. The form of the spectra for glutamate voltage noise and for 'background' noise has been determined. 3. The single channel conductance was estimated from the spectra, gamma glutamate = 122 +/- 0.4 (S.E.) pS. This estimate is independent of membrane potential and of the amplitude of membrane current change produced by glutamate. 4. The rate constant, alpha, for the closing of glutamate-operated channels depends exponentially on membrane potential, conforming to the equation alpha = approximately alphaeetaVm (approximately alpha = 0.26 +/- 0.014 msec-1, eta = 0.0054 +/- 0.001 msec-1); the duration of the channel lifetime (tau) decreases with hyperpolarization. Membrane potential dependence of alpha reduces as temperature is lowered. 5. For glutamate-operated channels, the temperature dependence of alpha and gamma fits the Arrhenius equation; alpha and gamma decrease exponentially as a function of T-1 (degrees K) with a descrete change in slope at about 6 degrees C, indicating a change in the activaiton energies of the respective rate processes. 6. Spectra of quisqualate-induced current fluctuations have the same form as spectra for glutamate noise. The single channel conductance was estimated from the spectra, gamma quisqualate = 120 +/- 3.9 (S.E.) pS. 7. The rate constnt, alpha, for the closing of quisqualate-induced channels depends exponentially on membrane potential. The duration of the open state for quisqualate channels was 2.2 times longer than for glutamate channels. 8. For glutamate receptors the voltage-sensitivity of the channel life-time is in the opposite direction to that of ACh receptors in vertebrate muscle. Possible explanations for the sharp change in the activation energy of the rate processes associated with the channel are discussed.

Evidence for glutamate as a neurotransmitter in the corticofugal fibres to the dorsal lateral geniculate body and the superior colliculus in rats

The high-affinity uptake of L-glutamate, D-aspartate and GABA were examined in homogenates from the dorsal lateral geniculate body and the superior colliculus after removal of the right visual cortex of adult rats. The high-affinity uptake of D-aspartate and L-glutamate were reduced by 53 to 75% respectively in the dorsal lateral geniculate body and by 46 and 53% respectively in the superior colliculus ipsilateral to the lesion. The uptake on the contralateral side was unaffected. The reductions were detected 3 days after the lesion and were maximally developed after 7 days. Subcellular fractionation showed that the main part of the uptake was confined to the synaptosomal fraction of both regions and that the reductions were most prominent in this fraction. The lesion was not accompanied by significant changes in high-affinity uptake of GABA nor in changes of choline acetyltransferase and glutamate decarboxylase activities. The high-affinity L-glutamate uptake on the contralateral visual cortex was unchanged from control values. After ablation of the visual cortex the level of L-glutamate was reduced by 32 and 17% in the ipsilateral dorsal lateral geniculate body and superior colliculus, respectively. The levels of the other amino acids examined, including L-aspartate, were unchanged. Enucleation had no effect on the uptake of L-glutamate and of GABA in the dorsal lateral geniculate body or in the superior colliculus.

Glutamic acid and ethanol dependence

Glutamate diethyl ester, a specific glutamate antagonist, attenuated the seizures and decreases in behavioral activity that were observed in mice during withdrawal. Prior to withdrawal, ethanol-dependent animals were supersensitive to kainic acid, a potent glutamate agonist, but they were not supersensitive to the convulsant drug pentylenetetrazol. These findings suggest that supersensitivity to glutamate develops during ethanol dependence, and that this phenomenon contributes to the signs of ethanol withdrawl.

Glutamate sensitivity and distribution of receptors along normal and denervated locust muscle fibres

1. Factors influencing the glutamate sensitivity of extrajunctional regions of innervated and denervated locust muscle fibres have been investigated. Properties of the two types of extrajunctional glutamate receptors, D- and H-receptors, have been studied in regions of high and low sensitivity. 2. The low level of extrajunctional sensitivity which is normally present in innervated fibres was 20-30 times higher at the muscle-tendon junction than at other sites; increased sensitivity extended about 20-40 micron from the tendon. After denervation or localized damage the entire extrajunctional sensitivity was increased approximately 100 times above control levels. 3. Applying L-glutamate (which activates D- and H-receptors) and DL-ibotenate (which activates H-receptors) from multibarrelled micropipettes showed that increased extrajunctional sensitivity resulted from an increase in D-receptors while H-receptors were apparently unchanged. 4. Coulomb dose vs. response relationships for the action of glutamate on D-receptors were similar when obtained at the muscle-tendon junction and nerve-muscle junction of innervated fibres or at extrajunctional regions in denervated fibres. 5. Time course of onset and percentage desensitization of D- and H-receptors in innervated fibres were similar. The percentage desensitization of D-receptors in extrajunctional regions of high sensitivity was greatly reduced. 6. It is suggested that D- and H-receptors are independent and that the trigger for increased receptor sensitivity acts specifically on D-receptors. In all respects so far studied, the D-receptors resemble extrajunctional ACh-receptors in vertebrate muscle.