The effects of acetylcholine, carbamylcholine and gamma-aminobutyric acid on uropod motoneurons in the crayfish Procambarus clarkii and Cambaroides japonicus

The effects of acetylcholine (ACh), carbamylcholine and gamma-aminobutyric acid (GABA) on the spike activity of uropod motoneurons were investigated electrophysiologically in the crayfish Procambarus clarkii Girard and Cambaroides japonicus de Haan. High concentrations of ACh were required to bring about an increase in the spike discharge of uropod motoneurons while carbamylcholine, which is not destroyed by cholinesterase, caused a marked increase in the motoneuron spike discharge even in low concentrations. Application of GABA in concentrations of 10(-5)-10(-2) M caused the decrease in the spike discharge of uropod motoneurons. Under the condition that the synaptic transmission onto uropod motoneurons was blocked by perfusing EGTA containing Ca2+-free saline with high-Mg2+, ACh increased the spike discharge of uropod motoneurons whereas GABA decreased it. The results suggested that ACh and GABA function as excitatory and inhibitory transmitters, respectively, in the crayfish central nervous system.     

Kainate-induced stimulation of amino acid release from primary astroglial cultures of the rat hippocampus

The effects of the excitatory amino acid analogs kainate (KA) and N-methyl- -aspartate (NMDA) on release of amino acids from astrocytes in primary culture were investigated. Under basal conditions, glutamine was present in the medium at 15 μM. The levels of serine and taurine were 1.5 and 2.0 μM, respectively, while the concentration of other amino acids was below 1 μM. At 10 μM, KA did not affect amino acid release, whereas 100 μM KA enhanced glutamine release by 34% and taurine release by 85%. At 1 mM, KA stimulated the release of all amino acids measured. However, while most amino acids increased by 50–150%, glutamate and aspartate were elevated by more than 3000%. The effect of KA was greatly reduced by 1 mM kynurenate, an excitatory amino acid receptor antagonist. 1 mM NMDA did not stimulate amino acid release from the cultures. The results indicate that astrocytes are endowed with KA-receptive sites, but they do not seem to possess NMDA receptors.     

Excitatory Transmitter Amino Acid Release from the Ischemic Rat Cerebral Cortex: Effects of Adenosine Receptor Agonists and Antagonists

The effects of selective adenosine receptor agonists [N6-cyclopentyladenosine (CPA) and N-ethylcarboxamidoadenosine (NECA)] and antagonists [8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and 9-chloro-2-(2-furanyl)-5,6-dihydro-1,2,4-triazolo[1,5-c]quinazoline-5-im ine (CGS-15943A)] on aspartate and glutamate release from the ischemic rat cerebral cortex were studied with the cortical cup technique. Cerebral ischemia (for 20 min) was elicited by four-vessel occlusion. Excitatory amino acid releases were compared from control ischemic rats and drug-treated rats. Basal levels of aspartate and glutamate release were not greatly affected by pretreatment with the adenosine receptor agonists or antagonists. However, CPA (10(-10) M) and NECA (10(-9) M) significantly inhibited the ischemia-evoked release of aspartate and glutamate into cortical superfusates. The ability to block ischemia-evoked release of excitatory amino acids was not evident at higher concentrations of CPA (10(-6) M) or NECA (10(-5) M). The selective A1 receptor antagonist DPCPX also had no effect on release when administered at a low dosage (0.01 mg/kg, i.p.) but blocked the ischemia-evoked release of aspartate and glutamate at a higher dosage (0.1 mg/kg). Evoked release was inhibited by the selective A2 receptor antagonist CGS-15943A (0.1 mg/kg, i.p.). Thus, adenosine and its analogs may suppress ischemia-evoked release of excitatory neurotransmitter amino acids via high-affinity A1 receptors, whereas coactivation of lower-affinity A2 receptors may block (or reverse) the A1-mediated response.     

Gustatory responses of eel palatine receptors to amino acids and carboxylic acids

The gustatory receptors of the eel palate were found to be extremely sensitive to amino acids and carboxylic acids. The results obtained are as follows: (a) 11 amino acids which are among naturally occurring amino acids elicited responses in the palatine nerve, but 9 amino acids did not elicit a response even at a high concentration. The effect of D-amino acids was always much less than that of their corresponding L-isomers. There was no appreciable difference in the effectiveness of an alpha-amino acid (alpha-alanine) and beta-amino acid (beta-alanine). (b) The threshold concentrations of the most potent amino acids (arginine, glycine) were between 10(-8) and 10(-9) M. A linear relation between the magnitude of the response and log stimulus concentration held for a wide concentration range for all the amino acids examined. (c) The palatine receptors responded sensitively to various carboxylic acid solutions whose pH was adjusted to neutral. The threshold concentrations varied between 10(-4) and 10(-7) M. The magnitude of the response at 10(-2) M increased with an increase of carbon chain length. (d) The extent of cross-adaptation was examined with various combinations of amino acids. A variety of the response patterns showing complete cross-adaptation, no cross-adaptation, or synergetic interaction was observed. The synergetic interaction was also observed when one amino acid below its threshold concentration was added to the other amino acid below its threshold concentration was added to the other amino acid. No cross-adaptation was observed between amino acids and fatty acids. (e) The treatment of the palate with papain led to loss of the responses to arginine, glycine, and histidine without affecting those to proline and acetic acid. The treatment with pronase E eliminated selectively the response to proline. The possibility that the eel gustatory receptors are responsible for sensing food at a distance was discussed.     

Excitatory Amino Acid Receptors and Depolarization-Induced Ca2+ Influx into Hippocampal Slices

Hippocampal brain slices were incubated with depolarizing agents or excitatory amino acids either alone or in the presence of excitatory amino acid antagonists [omega-phosphonic alpha-aminocarboxylic acids--2-amino-4-phosphonobutyric acid (AP4), 2-amino-5-phosphonovaleric acid (AP5), or 2-amino-7-phosphonoheptanoic acid (AP7)--or gamma-D-glutamylaminomethylsulphonic acid (GAMS)] or a calcium-channel blocker, (S)-1-(3-methoxyphenyl)-3-methylaza-7-cyano-7-(3,4-dimethoxyphenyl )-8-methyl- nonane hydrochloride [(-)-D888]. The uptake of 45Ca2+ and the efflux of glutamate or aspartate induced by veratrine or high K+ was blocked (54-76%) by AP7 (IC50 46-250 microM). AP5 and AP4 were less effective. (-)-D888 (10 microM) caused 100% block of evoked 45Ca2+ uptake. Uptake of 45Ca2+ induced by exogenous glutamate, aspartate, and N-methyl-D-aspartate (NMDA) was also inhibited by AP7, whereas GAMS completely blocked the action of kainate and partially blocked that of glutamate. The action of NMDA in stimulating 45Ca2+ uptake was Mg2+-sensitive, low Mg2+ levels in the incubation medium selectively enhancing the response. It is concluded that Ca2+ uptake evoked by excitatory amino acids is receptor-mediated, and that released excitatory amino acids are responsible for a large part of the action of veratrine and high K+ in stimulating 45Ca2+ uptake.     

Comparison of effects of octopamine and insecticidal essential oils on activity in the nerve cord, foregut, and dorsal unpaired median neurons of cockroaches

Essential oil constituents were tested for their neurophysiological effects in Periplaneta americana and Blaberus discoidalis. Eugenol depressed spontaneous and stimulus-evoked impulses recorded extracellularly in the abdominal nerve cord, with an almost complete block of spikes at 2 x 10(-3) M. Geraniol and citral had similar depressive effects but increased spontaneous firing at lower doses (threshold 2.5 x 10(-4) M). Similar effects occurred in dorsal unpaired median (DUM) neurons, recorded intracellularly in the isolated terminal abdominal ganglion of P. americana. Spontaneous firing was progressively reduced by increasing concentrations of eugenol, whereas geraniol and citral produced biphasic effects (excitation at 10(-4) M, depression at 2 x 10(-3) M). All three oils decreased excitability of silent DUM neurons that were depolarised by applied current, but eugenol (at 10(-3) M) also changed the firing pattern from single spikes to bursts driven by plateau potentials. All oils reduced spike undershoot. Low doses of citral and geraniol (threshold ca. 10(-4) M) reversibly increased the frequency of spontaneous foregut contractions and abolished them at 2 x 10(-3) M (together with response to electrical stimulation). Eugenol reversibly reduced spontaneous activity at 10(-4) M and above. Eugenol has been reported to exert its insecticidal properties via a low-dose activation of octopamine receptors. In our studies, however, octopamine was found to have opposing effects to eugenol on DUM neurons and foregut activity (excitatory in both). Furthermore, eugenol did not affect the response to octopamine in DUM neurons. These results suggest that reported effects of eugenol were on a different sub-type of octopamine receptor.     

Conformational aspects of the actions of some piperidine dicarboxylic acids at excitatory amino acid receptors in the mammalian and amphibian spinal cord

A series of piperidine dicarboxylates (PDA) have been tested for excitatory amino acid agonist and antagonist activity and for synaptic depressant properties in the spinal cords of frogs and immature rats in vitro and of cats in vivo. The substances tested comprised (±)-cis-2,3-PDA, (±)-cis-2,4-PDA, (±)-cis-2,5-PDA, (±)-cis-2,6-PDA, (±)-trans-2,3-PDA, (±)-trans-2,3-PDA and both (+) and (–) forms ofcis-2,3-PDA. Peak excitatory amino acid agonist activity was observed with (±)-trans-2,3- and (±)-trans-2,4-PDA. Excitatory amino acid antagonism and synaptic depressant activity was observed only withcis-dicarboxylates, this activity being greatest in the 2,3-analogue. The agonist actions of piperidine dicarboxylates were effectively depressed by the specific NMDA receptor antagonist, (–)-2-amino-5-phosphonovalerate and, where tested, also byd--aminoadipate and low concentrations of Mg²⁺. It was concluded that the major part of these agonist actions were mediated by NMDA receptors. The main structural feature of the NMDA agonist actions of these substances was considered to be their close relationship to N-alkyl-aspartic and glutamic acid molecules, with thetrans arrangement of the respective 2,3- and 2,4-situated carboxyl groups promoting most effective interaction with the active sites of the NMDA receptor. (±)-Cis-2,3-PDA depressed excitatory responses induced by NMDA, kainate, quisqualate, (±)-trans-2,3-PDA and (±)-trans-2,4-PDA, or evoked by dorsal root stimulation. Both monosynaptic and polysynaptic excitation were susceptible to the depressant action of this substance. The (–) isomer ofcis-2,3-PDA carried both excitatory amino acid agonist and antagonist activity and also the synaptic depressant properties observed with the racemic form of this substance. The (+) isomer showed little pharmacological activity. It is proposed that the structure-activity features of these heterocyclic amino acids indicate some of the conformational requirements for interaction with physiological excitatory amino acid receptors.This paper is dedicated to Dr. Derek Richter on his seventy-fifth birthday.     

Mechanisms of excitatory actions of neurotensin on canine small intestinal circular muscle in vivo and in vitro

We have shown previously that close intra-arterial injections of neurotensin in vivo inhibited phasic activity induced by field stimulation of the canine small intestine during anaesthesia but had little effect during quiescence. In contrast, in vitro in the present study, full thickness strips of the muscularis externa cut in the circular axis responded to the lowest effective neurotensin concentrations (10(-12) to 10(-9) M) with an increase in frequency and amplitude of spontaneous contractions; as the concentration was increased from 10(-8) to 10(-7) M, neurotensin inhibited spontaneous activity. A small tonic contraction also occurred; it was maximal at 10(-7) M. Since sufficient tetrodotoxin to block field-stimulated nerve responses did not significantly reduce any of these responses in vitro, the neurotensin responses in vitro did not appear to involve actions on nerves. Indomethacin did not alter the excitatory response to 10(-11) M neurotensin but 5,8,11,14-eicosatetraynoic acid inhibited the excitatory response in a reversible fashion, without altering the response to acetylcholine. Thus excitation in vitro may require the release of excitatory metabolites of arachidonic acid via the lipoxygenase pathway. The neurotensin response in vivo was further studied by evaluating its actions against repetitive submaximal contractions induced by intra-arterial injections of acetylcholine given every minute. Doses that produced a short inhibition of the field-stimulated activity (10(-11) to 10(-10) mol intra-arterially) did not produce inhibition but 10(-10) mol significantly increased the response to acetylcholine. Higher doses (10(-9) mol) produced a significant inhibition of the first subsequent acetylcholine dose but no enhancement of later doses.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of dopamine on spontaneous activity generated by isolated spinal cord in 16- to 20-day-old chick embryos

The effects were investigated of applying L-DOPA, dopamine (DA), and noradrenaline (NA) on spontaneous activity (cyclic fluctuations in electrotonic dorsal and ventral root (DR and VR) potentials generated by a section of spinal cord isolated from 16 to 20-day-old chick embryos. A low concentration of L-DOPA (30–150 µm) intensified operation of the spinal generator, giving rise to above-threshold rhythm (i.e., spike activity in the DR and the VR). At a high concentration, L-DOPA produced inhibition of generator operation, although spontaneous activity did intensify during subsequent washout of the substance, with the onset of above-threshold rhythm. Both DA and NA failed to affect spontaneous activity in the VR and the DR at a concentration to 50 µM but a concentration of 100 µM produced inhibition. Application of 20 µM 2-amino-5-phosphonovaleric acid blocked the reinforced spontaneous activity produced by low L-DOPA concentrations. Activity generated by the neuronal network of the isolated dorsal horn rose under the effects of low L-DOPA concentrations; rhythmic activity was observed neither before nor after applying this substance in isolated ventral horn. Findings obtained would point to the occurrence of a direct (i.e., non-catecholamine dependent) excitatory influence of L-DOPA on the neuronal network of the chick embryo dorsal horn.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 23, No. 3, pp. 338–343, May–June, 1991.     

DNA conformation and thermostability of aqueous solutions of beta-alanine and gamma-aminobutyric acid

It has been shown that at low concentrations of rare amino acids (from 10(-3) M to 10(-1) M of the substance) stechiometric complexes amino acid -- DNA are formed, which bring about partial substitution of counterions screening phosphate groups and to a change of spatial structure of DNA water molecules. The DNA-solvent molecular interactions are changed, accompanied by an abrupt decrease of helix-coil enthalpy transition which leads to the unwinding of DNA double helix. In the region of amino acid high concentrations (greater than 1-1,5 M) a rise of thermostability and winding of DNA double helix is observed. It has been established that B----C-like conformational transition stimulated by the rise of DNA thermostability is a result of counterions dehydration and the increase of effective ionic strength of the solution which is due to the rise of amino acid-zwitterions content in it.     

Excitatory Amino Acid Agonist-Antagonist Interactions at 2-Amino-4-Phosphonobutyric Acid-Sensitive Quisqualate Receptors Coupled to Phosphoinositide Hydrolysis in Slices of Rat Hippocampus

Studies were carried out to define the relative affinities and intrinsic activities of excitatory amino acid agonists that activate receptor sites coupled to phosphoinositide hydrolysis in brain. Slices of rat hippocampus were prelabeled with myo-[3H]inositol, and agonist stimulation was indexed by measuring the accumulation of [3H]inositol monophosphate [( 3H]IP) in the presence of Li+. It was observed that ibotenic (IBO) and quisqualic (QUIS) acids both elicit highly significant, concentration-dependent stimulation of phosphoinositide hydrolysis. Whereas maximal stimulation by IBO (10(-3) M) was four- to fivefold over basal values, the maximal effect of QUIS (10(-4) M) was less (about twofold). Based on the relative concentrations required for 50% maximal stimulation, QUIS was 20 times more potent than IBO. Stimulation of phosphoinositide hydrolysis by either IBO or QUIS was additive to the effects of nonexcitatory amino acid agonists (carbachol and norepinephrine) in this tissue. However, the stimulatory effects of IBO plus QUIS were not additive. At greater than or equal to 10(-4) M, QUIS significantly inhibited phosphoinositide hydrolysis by a maximal stimulatory concentration of IBO (10(-3) M) to a level observed with QUIS alone. Other excitatory amino acid agonists, including kainate, N-methyl-D-aspartate, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), had no stimulatory effects at concentrations as high as 10(-3) M. The D,L or L forms of 2-amino-4-phosphonobutyric acid (AP4), but not D-AP4, significantly enhanced [3H]IP levels to approximately 135% of basal values.(ABSTRACT TRUNCATED AT 250 WORDS)     

Functional duality and structural uniqueness of depressant insect-selective neurotoxins

Depressant insect-selective neurotoxins derived from scorpion venoms (a) induce in blowfly larvae a short, transient phase of contraction similar to that induced by excitatory neurotoxins followed by a prolonged flaccid paralysis and (b) displace excitatory toxins from their binding sites on insect neuronal membranes. The present study was undertaken in order to examine the basis of these similarities by comparing the primary structures and neuromuscular effects of depressant and excitatory toxins. A new depressant toxin (LqhIT2) was purified from the venom of the Israeli yellow scorpion. The effects of this toxin on a prepupal housefly neuromuscular preparation mimic the effects on the intact animal; i.e., a brief period of repetitive bursts of junction potentials is followed by suppression of their amplitude and finally by a block of neuromuscular transmission. Loose patch clamp recordings indicate that the repetitive activity has a presynaptic origin in the motor nerve and closely resembles the effect of the excitatory toxin AaIT. The final synaptic block is attributed to neuronal membrane depolarization, which results in an increase in spontaneous transmitter release; this effect is not induced by excitatory toxin. The amino acid sequences of three depressant toxins were determined by automatic Edman degradation. The depressant toxins comprise a well-defined family of polypeptides with a high degree of sequence conservation. This group differs considerably in primary structure from the excitatory toxin, with which it shares identical or related binding sites, and from the two groups of scorpion toxins that affect sodium conductance in mammals. The two opposing pharmacological effects of depressant toxins are discussed in light of the above data.     

Effect of methionine-enkephalin on the spontaneous electrical and mechanical activity of the smooth muscle of the rat portal vein

In the longitudinal smooth muscle of the isolated rat portal vein, methionine-enkephalin (Met-enkephalin) increased the spontaneous contraction with a concentration as low as 10(-8)M. When the membrane activity was recorded using a microelectrode, Met-enkephalin enhanced the spike burst activity but without any effect on the resting membrane potential. Naloxone, phentolamine, atropine and reserpine pre-treatment did not inhibit the excitatory effect of Met-enkephalin on the spontaneous contraction. These results suggest that the excitatory effect of Met-enkephalin on the mechanisms involved in the automaticity may be a direct action on smooth muscle or relate to presynaptic action on a non-adrenergic non-cholinergic system.     

Autoreceptor Regulation of Glutamate and Aspartate Release from Slices of the Hippocampal CA1 Area

Slices of hippocampal area CA1 were employed to test the hypothesis that the release of glutamate and aspartate is regulated by the activation of excitatory amino acid autoreceptors. In the absence of added Mg2+, N-methyl-D-aspartate (NMDA)-receptor antagonists depressed the release of glutamate, aspartate, and gamma-aminobutyrate evoked by 50 mM K+. Conversely, the agonist NMDA selectively enhanced the release of aspartate. The latter action was observed, however, only when the K+ stimulus was reduced to 30 mM. Actions of the competitive antagonists 3-[(+/- )-2-carboxypiperazin-4-yl]-propyl-l-phosphonic acid (CPP) and D-2-amino-5-phosphonovalerate (D-AP5) differed, in that the addition of either 1.2 mM Mg2+ or 0.1 microM tetrodotoxin to the superfusion medium abolished the depressant effect of CPP without diminishing the effect of D-AP5. These results suggest that the activation of NMDA receptors by endogenous glutamate and aspartate enhances the subsequent release of these amino acids. The cellular mechanism may involve Ca2+ influx through presynaptic NMDA receptor channels or liberation of a diffusible neuromodulator linked to the activation of postsynaptic NMDA receptors. (RS)-alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, a selective quisqualate receptor agonist, and kainate, an agonist active at both kainate and quisqualate receptors, selectively depressed the K(+)-evoked release of aspartate. Conversely, 6-cyano-7-nitro-quinoxaline-2,3-dione, an antagonist active at both quisqualate and kainate receptors, selectively enhanced aspartate release. These results suggest that glutamate can negatively modulate the release of aspartate by activating autoreceptors of the quisqualate, and possibly also of the kainate, type. Thus, the activation of excitatory amino acid receptors has both presynaptic and postsynaptic effects.     

Interaction between morphine and putative excitatory neurotransmitters in cortical neurons in naive and tolerant rats.

Microelectrophoretically applied morphine depressed spontaneously discharging cortical neurones of rats and blocked excitation induced by electrophoretic administrations of either acetylcholine or l-glutamate. This depressant effect and both the anti-acetylcholine and the anti-glutamate effect were naloxone antagonizable and therefore regarded as specific morphine actions. The excitatory effects of morphine were not affected by naloxone application and were classified as non-specific.In chronically morphinized rats the depressant effect of morphine on spontaneous discharge activity and also its blocking action upon acetylcholine and l-glutamate-induced excitation were almost completely abolished. The predominant response in such pre-treated animals was non-specific excitation. Acetylcholine and l-glutamate were found to be more effective in tolerant rats (supersensitivity).     

Effect of nitric oxide on excitatory amino acid-evoked discharge of neurons in NTS

N-methyl-d-aspartate (NMDA) and non-NMDA excitatory amino acid (EAA) receptor subtypes are involved in the integration of visceral afferent inputs within the nucleus of the solitary tract (NTS). Microinjection studies indicate interactions between nitric oxide (NO) and EAA receptors within the NTS. To examine these interactions at the single cell level, this study characterized the effects of the NO synthase inhibitor N(G)-nitro-l-arginine methyl ester (l-NAME) and the NO donor 3-[2-hydroxy-2-nitroso-1-propylhydrazino]-1-propanamine (PAPA-NONOate) on the excitatory responses of vagus nerve (VN)-evoked NTS neurons to the activation of (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and NMDA receptors in rats. Iontophoresis of l-NAME did not alter spontaneous or VN-evoked discharges, but significantly decreased the number of action potentials (APs) evoked by iontophoretic application of AMPA. The effects of l-NAME on NMDA-evoked discharge were variable; for the population, l-NAME did not change the number of APs evoked by NMDA. PAPA-NONOate enhanced the spontaneous discharge and the number of APs elicited by AMPA but not NMDA. Iontophoresis of the inactive enantiomers N(G)-nitro-d-arginine methyl ester and hydroxydiazenesulfonic acid 1-oxide disodium salt had no effect on AMPA-evoked discharge. Our data suggest that NO facilitates AMPA-mediated neuronal transmission within the NTS.     

Effect of excitant amino acid antagonists on glutamate receptors in the locust and on convulsions induced by glutamate, aspartate, kynurenine and quinolinic acid in mice

All excitatory amino acid antagonists studied: diethyl esters of aspartic (DEEA) and glutamic (DEEG) acids, 2-amino-3-phosphono-propionic acid (APPA) and 2-amino-4-phosphono-butanoic acid (APBA), diminished the amplitude of excitatory postsynaptic potentials (EPP) of the locust (Locusta migratoria migratorioides) muscle fibers and arbitrary blocked glutamate (GLU) and aspartate (ASP) responses. Kynurenine (KYN) and quinolinic (QUI) acid had no effect on EPP even at a concentration of 2 X 10(-2) M. The antagonists were not strictly selective against intracerebroventricularly administered endogenous convulsants: GLU, ASP, KYN and QUI and in simulation of experimental seizures in mice. The antagonists structurally similar to ASP prevented ASP- and KYN-induced seizures in lower doses than GLU derivatives. Anti-KYN, but not anti-QUI DEEA, DEEG, APPA and APBA efficacy suggests that KYN and QUI act on different structures or binding sites.     

Characterization of the Quisqualate Receptor Linked to Phosphoinositide Hydrolysis in Neurocortical Cultures

Activation of phosphoinositide metabolism is an early event in signal transduction for a number of neurotransmitters and hormones. In primary cultures of rat neurocortical cells, various excitatory amino acids stimulate inositol phosphate production with a rank order of potency of quisqualate greater than ibotenate greater than glutamate greater than kainate, N-methyl-D-aspartate greater than alpha-amino-3-hydroxyl-5-methyl-4-isoxazole propionate. This response to excitatory amino acids was insensitive to a variety of excitatory amino acid antagonists including 6-cyano-7-nitroquinoxaline-2,3-dione, 3-3(2-carboxypiperazine-4-yl)propyl-1-phosphonate, and 2-amino-4-phosphonobutyrate. The individual responses of quisqualate-, ibotenate-, and kainate-stimulated inositol phosphate production were not additive. These results suggest that phosphoinositide metabolism activated by excitatory amino acids is mediated by a unique quisqualate-preferring receptor that is not antagonized by known N-methyl-D-aspartate and non-N-methyl-D-aspartate antagonists, and is relatively insensitive to alpha-amino-3-hydroxyl-5-methyl-4-isoxazole propionate.     

Extracellular Neuroactive Amino Acids in the Rat Striatum During Ischaemia: Comparison Between Penumbral Conditions and Ischaemia with Sustained Anoxic Depolarisation

Abstract: Changes in the extracellular levels of excitatory and inhibitory amino acid transmitters were studied in the rat striatum during penumbral ischaemia using intracerebral microdialysis. Effects of penumbral forebrain ischaemia were compared with those of ischaemia with sustained anoxic depolarisation and K⁺ (100 m M ). Comparisons were also made between different groups of animals at 2 and 24 h after dialysis probe implantation. The K⁺ stimulus did not provoke any release of excitatory amino acids in the 24-h group, probably reflecting a decrease of functional synapses adjacent to the probe. During 30 min of penumbral ischaemia, excitatory amino acids did not reach critical concentrations in the extracellular fluid, and increases in levels of inhibitory/modulatory amino acids were similar. On the other hand, severe transient ischaemia resulted in massive synchronous release of many neuroactive excitatory and inhibitory compounds, in both the 2- and 24-h groups. These and other data suggest that changes during severe ischaemia may arise from both neurotransmitter and metabolic pools. It is concluded that is- chaemic damage in the penumbra may not be related to extracellular neuroactive amino acid changes generated within this region.     

Kainate-enhanced release of D-[3H]aspartate from cerebral cortex and striatum: reversal by baclofen and pentobarbital

A study was made of the actions of the excitant neurotoxin, kainic acid, on the uptake and the release of D-[2,3-3H]aspartate (D-ASP) in slices of guinea pig cerebral neocortex and striatum. The slices took up D-ASP, reaching concentrations of the amino acid in the tissue which were 14-23 times that in the medium. Subsequently, electrical stimulation of the slices evoked a Ca2+-dependent release of a portion of the D-ASP. Kainic acid (10(-5)-10(-3) M) produced a dose-dependent inhibition of D-ASP uptake. The electrically evoked release of D-ASP was increased 1.6-2.0 fold by 10(-5) and 10(-4)M kainic acid. The kainate-enlarged release was Ca2+-dependent. Dihydrokainic acid, an analogue of kainic acid with little excitatory or toxic action, did not increase D-ASP release but depressed D-ASP uptake. Attempts were made to block the action of kainic acid with baclofen and pentobarbital, compounds which depress the electrically evoked release of L-glutamate (L-GLU) and L-aspartate (L-ASP). Baclofen (4 X 10(-6)M), an antispastic drug, and pentobarbital (10(-4)M), an anesthetic agent, each inhibited the electrically evoked release of D-ASP and prevented the enhancement of the release above control levels usually produced by 10(-4)M kainic acid. It is proposed that 10(-5) and 10(-4)M kainic acid may enhance the synaptic release of L-GLU and L-ASP from neurons which use these amino acids as transmitters. This action is prevented by baclofen and pentobarbital. In view of the possibility that cell death in Huntington's disease could involve excessive depolarization of striatal and other cells by glutamate, baclofen might be effective in delaying the loss of neurons associated with this condition.