Amygdalofugal axon terminals immunoreactive for L-aspartate or L-glutamate in the nucleus accumbens of rats and domestic chickens: a comparative electron microscopic immunocytochemical study combined with anterograde pathway tracing

Several studies have shown that L-aspartate (Asp) is present in synaptic vesicles and released exocytotically from presynaptic terminals, possibly by Ca²⁺-dependent corelease of Asp and L-glutamate (Glu). It has been demonstrated that both excitatory amino acids (EAAs) are released from the rat striatum as part of corticostriatal neurotransmission. The single or colocalized occurrence of Asp and Glu in specific synaptic boutons of the chicken medial striatum/nucl. accumbens has been demonstrated by our group using ultrastructural immunocytochemistry. However, evidence for the presence of EAAs in any specific striatal pathway was only circumstantial. Here, we report on the distribution of Asp and Glu in specific synaptic terminals of the amygdalostriatal pathway, both in rat and chicken brains, combining anterograde tracing with postembedding immunogold labeling of Asp or Glu. Immunoreactivity for Asp and Glu was observed in amygdalofugal terminals with asymmetrical synaptic junctions (morphologically representing excitatory synapses) in both species. The postsynaptic targets were either dendritic spines or small dendrites, whereas axosomatic or axo-axonic connections were not observed. Ultrastructurally, the synaptic terminals immunoreactive for Asp were indistinguishable from those immunoreactive for Glu. The findigs are consistent with an Asp?CGlu corelease mechanism, with a distinct synaptic contingent, evolutionarily conserved in the amygdalostriatal pathway.     

Physiological requirement for the glutamate transporter dEAAT1 at the adult Drosophila neuromuscular junction

L-glutamate is the major excitatory neurotransmitter in the mammalian brain. Specific proteins, the Na+/K+-dependent high affinity excitatory amino acid transporters (EAATs), are involved in the extracellular clearance and recycling of this amino acid. Type I synapses of the Drosophila neuromuscular junction (NMJ) similarly use L-glutamate as an excitatory transmitter. However, the localization and function of the only high-affinity glutamate reuptake transporter in Drosophila, dEAAT1, at the NMJ was unknown. Using a specific antibody and transgenic strains, we observed that dEAAT1 is present at the adult, but surprisingly not at embryonic and larval NMJ, suggesting a physiological maturation of the junction during metamorphosis. We found that dEAAT1 is not localized in motor neurons but in glial extensions that closely follow motor axons to the adult NMJ. Inactivation of the dEAAT1 gene by RNA interference generated viable adult flies that were able to walk but were flight-defective. Electrophysiological recordings of the thoracic dorso-lateral NMJ were performed in adult dEAAT1-deficient flies. The lack of dEAAT1 prolonged the duration of the individual responses to motor nerve stimulation and this effect was progressively increased during physiological trains of stimulations. Therefore, glutamate reuptake by glial cells is required to ensure normal activity of the Drosophila NMJ, but only in adult flies.     

Pharmacologically distinct glutamate receptors on cerebellar granule cells

Cultured cerebellar granule cells were found to exhibit calcium-dependent release of 3H-D-aspartate when stimulated with excitatory amino acids. L-glutamate and L-aspartate were found to be potent stimulators of 3H-D-aspartate release, D-aspartate was weaker and only minor effects were seen with D-glutamate, quisqualate, kainate, N-methyl-D-aspartate (NMDA) and L-alpha-aminoadipate (L-alpha AA). It was also found that only L-glutamate and L-aspartate showed high affinity for the 3H-L-glutamate binding sites on granule cell membranes. Stimulation by L-glutamate of 3H-D-aspartate release could be blocked by various excitatory amino acid antagonists. From the relative potencies of agonists and antagonists on D-aspartate release it is suggested that cerebellar granule cells express functionally active glutamate receptors with pharmacological characteristics different from all known excitatory amino acid receptors.     

Effects of N-Acetyl Aspartate, Aspartate, and Glutamate on cAMP and cGMP Levels in Developing Rat Cerebral Cortex

N-Acetyl-aspartate (N-Ac-Asp) incubated with minced cerebral cortex caused a dose-dependent increase in the levels of cAMP and cGMP. This effect was followed during postnatal development. N-Ac-Asp elicits the greatest increase in cAMP in 5-day-old and in cGMP in 40-day-old rats. The levels of cyclic AMP were always higher than those of cGMP. We also studied the effects of L-aspartate (Asp) and L-glutamate (Glu) on the levels of cyclic nucleotides in the cerebral cortex minces of rats different ages, and observed that both amino acids produced the maximum increase in cAMP at 10 days, whereas in the case of cGMP the maximal effect of Asp occurs earlier than 20 days and of Glu after 40 days. In the adult rat, the N-Ac-Asp effect on cAMP was greater than that produced by either Asp or Glu, whereas the levels of cGMP were similarly affected by all three. The data show a peak response of cAMP and cGMP to N-Ac-Asp, Asp, and Glu during cortical maturation. Because this response varies with postnatal time, N-Ac-Asp, and Glu may act upon different receptor sites.     

Excitatory amino acids inhibit stimulated phosphoinositide hydrolysis in the rat prefrontal cortex

In rat prefrontal cortical slices, the excitatory amino acids N-methyl-D-aspartate (NMDA), ibotenate, L-aspartate, quisqualate, kainate and L-glutamate inhibit carbachol-induced phosphoinositide hydrolysis as measured by the accumulation of [3H]inositol-1-phosphate ([3H]IP1). NMDA dose-dependently inhibited the carbachol response (IC50 = 14.4 microM), and this inhibition was blocked by the NMDA receptor antagonist D,L-aminophosphonovaleric acid. Lowering medium Na+ concentration to 10 mM or exposing slices to pertussis toxin alleviated the inhibitory effect of NMDA on carbachol-induced [3H]IP1 formation. Serotonin-induced stimulation of [3H]IP1 was also inhibited by NMDA; in contrast, stimulation by norepinephrine, epinephrine or dopamine was unaffected. The results suggest that excitatory amino acids, besides their traditional role as stimulatory substances, can also act to inhibit the production of 2nd messengers activated by certain neurotransmitters in the brain.     

Glutamate Increases Glycogen Content and Reduces Glucose Utilization in Primary Astrocyte Culture

The glycogen content of primary cultured astrocytes was approximately doubled by incubation with 1 mM L-glutamate or L-aspartate. Other amino acids and excitatory neurotransmitters were without effect. The increase in glycogen level was not blocked by the glutamate receptor antagonist kynurenic acid but was completely blocked by the glutamate uptake inhibitor threo-3-hydroxy-D,L-aspartate and by removal of Na+ from the medium. Incubation with radiolabeled glucose and glutamate revealed that the increased glycogen content was derived almost entirely from glucose. Glutamate at 1 mM was also found to cause a 53 +/- 12% decrease in glucose utilization and a 112 +/- 69% increase in glucose-6-phosphate levels. These results suggest that the glycogen content of astrocytes is linked to the rate of glucose utilization and that glucose utilization can, in turn, be affected by the availability of alternative metabolic substrates. These relationships suggest a mechanism by which brain glycogen accumulation occurs during decreased neuronal activity.     

Ethanol-induced inhibition of testosterone biosynthesis in rat Leydig cells: role of culture medium composition

The biochemical mechanisms responsible for the ethanol-induced inhibition of testicular testosterone synthesis were studied in isolated rat Leydig cells in vitro. This inhibition was removed when HAM-F12 nutrient mixture was added to the DME culture medium. The components of HAM-F12, i.e. vitamins, amino acids and other supplements, were tested individually and the amino acids L-glutamate (Glu) and L-aspartate (Asp) were found to potentiate strongly the hCG stimulated testosterone synthesis. None of the other components of HAM-F12 had any effect upon testosterone synthesis or its ethanol-induced inhibition. Moreover, Glu, but not Asp, effectively reversed the acute inhibition of steroidogenesis by ethanol. These results demonstrate the importance of the composition of the culture media and provide the first piece of evidence that the metabolic stress in rat Leydig cells in vitro induced by the metabolism of ethanol can be overcome by proper culture medium supplementation.     

The pharmacology of Limulus central neurons

1. Intracellular recordings have been made from neurons in the central nervous system of the horse-shoe crab, Limulus polyphemus. Neurons possess resting potentials between -40 and -60 mV, with action potentials ranging from 2-3 mV up to 60 mV in amplitude. Neurons also have excitatory and inhibitory postsynaptic potentials. 2. All the neurons studied are inhibited by GABA and excited by cholinomimetics. The GABA response is chloride mediated and reversibly antagonised by picrotoxinin but not by bicuculline or bicuculline methochloride or methoiodide. The cholinergic response is nicotinic and blocked by pentolinium, hexamethonium, chlorisondamine and dihydro-beta-erythroidine. 3. L-Glutamate can excite some cells, inhibit others and have a biphasic action, inhibition followed by excitation, on other cells. The inhibitory effect is chloride mediated and blocked by picrotoxinin. Ibotenate mimics the action of glutamate both in terms of inhibition and excitation but kainate and quisqualate only mimic the excitatory action of L-glutamate. 4. Dopamine, octopamine, 5-hydroxytryptamine and histamine excite some neurons while inhibiting others or have a biphasic action. Dopamine and octopamine normally have different effects on the same cell, suggesting they act via different receptors. Octopamine shows stereospecificity for the (-) isomer which is more than 100 times more active than the (+) isomer and octopamine is reversibly antagonised by phentolamine and cyproheptadine. 5. Proctolin has an excitatory action on these neurons and this effect is long lasting and can be potentiated by dibutyl cyclic AMP. 6. The pharmacology of Limulus central neurons is compared to the pharmacology of insect and crustacean central neurons. It is concluded that GABA and acetylcholine are central transmitters throughout the arthropods. It is also probable that L-glutamate and octopamine have a physiological role in the arthropod central nervous system. Proctolin appears to modify neuronal and muscle activity in the arthropods and has a modulatory or transmitter function.     

Heterogeneity of high affinity uptake of L-glutamate and L-aspartate in the mammalian central nervous system.

Characteristics of high affinity uptake of L-glutamate are examined in order to evaluate the possible use of the uptake of [3H]L-glutamate, [3H]L-aspartate or any other suitable [3H]-labelled substrate as a marker for glutamatergic and aspartergic synapses in autoradiographic studies in the mammalian brain. Review of data on substrate specificity indicates the presence of at least two high affinity uptake systems specific for acidic amino acids in the central nervous tissue; one which takes up L-glutamate and L-aspartate and the other which is selective for L-glutamate only. Studies on ionic requirements, too, point to the existence of at least two distinct uptake systems with high affinity for L-glutamate. The Na(+)-dependent uptake system(s) handle(s) both L-glutamate and L-aspartate whereas the Na(+)-independent uptake system(s) show(s) selectivity for L-glutamate only. Available data do not favour the Na(+)-dependent binding of [3H]D-aspartate to thaw-mounted sections of frozen brain tissue as a suitable marker for glutamatergic/aspartergic synaptic nerve endings. However, there are reasons--such as the results of lesion studies and the existence of uptake sites which have a higher affinity for L-aspartate than for D-aspartate--to suggest that Na(+)-dependent binding of [3H]L-aspartate, rather than that of [3H]D-aspartate, should be further investigated as a possible marker for the glutamatergic/aspartergic synapses in the autoradiographic studies using sections of frozen brain.     

Na+ entry via glutamate transporter activates the reverse Na+/Ca2+ exchange and triggers Ca(i)2+-induced Ca2+ release in rat cerebellar Type-1 astrocytes

We have previously demonstrated that rat cerebellar Type-1 astrocytes express a very active genistein sensitive Na(+)/Ca(2+) exchanger, which accounts for most of the total plasma membrane Ca(2+) fluxes and for the clearance of loads induced by physiological agonists. In this work, we have explored the mechanism by which the reverse Na(+)/Ca(2+) exchange is involved in agonist-induced Ca(2+) signaling in rat cerebellar astrocytes. Microspectrofluorometric measurements of Cai(2+) with Fluo-3 demonstrate that the Cai(2+) signals associated long (> 20 s) periods of reverse operation of the Na(+)/Ca(2+) exchange are amplified by a mechanism compatible with calcium-calcium release, while those associated with short (< 20 s) pulses are not amplified. This was confirmed by pharmacological experiments using ryanodine receptors agonist (4-chloro-m-cresol) and the endoplasmic reticulum ATPase inhibitor (thapsigargin). Confocal microscopy demonstrates a high co-localization of immunofluorescent labeled Na(+)/Ca(2+) exchanger and RyRs. Low (< 50 micromol/L) or high (> 500 micromol/L) concentrations of L-glutamate (L-Glu) or L-aspartate causes a rise in which is completely blocked by the Na(+)/Ca(2+) exchange inhibitors KB-R7943 and SEA0400. The most important novel finding presented in this work is that L-Glu activates the reverse mode of the Na(+)/Ca(2+) exchange by inducing Na(+) entry through the electrogenic Na(+)-Glu-co-transporter and not through the ionophoric L-Glu receptors, as confirmed by pharmacological experiments with specific blockers of the ionophoric L-Glu receptors and the electrogenic Glu transporter.     

L-Glutamate effects on electrical potentials of synaptic plasma membrane vesicles

The electrogenic nature of the L-glutamate-stimulated Na+ flux was examined by measuring the distribution of the lipophilic anion [35S]thiocyanate (SCN-) into synaptic membrane vesicles that were incubated in a NaCl medium. Concentrations of L-glutamate from 10(-7) to 10(-4) M added to the incubation medium caused an enhanced intravesicular accumulation of SCN-. Based on the SCN- distribution in synaptic membrane vesicles it was calculated that 10 microM L-glutamate induced an average change in the membrane potential of + 13 mV. L-Glutamate enhanced both the Na+ and K+ conductance of these membranes as determined by increases in SCN- influx. Other neuroexcitatory amino acids and amino acid analogs (D-glutamate, L-aspartate, L-cysteine sulfinate, kainate, ibotenate, quisqualate, N-methyl-D-aspartate, and DL-homocysteate) also increased SCN- accumulation in synaptic membrane vesicles. These observations are indicative of the activation by L-glutamate and some of its analogs of excitatory amino acid receptor ion channel complexes in synaptic membranes.     

Induction of locomotion in spinal tadpoles by excitatory amino acids and their agonists

Bath application of the excitatory amino acids L-aspartate and/or L-glutamate or their agonists N-methyl-D,L-aspartate and/or kainate elicited swimming movements in spinal tadpoles. Swimming cycles induced by the amino acids were in the frequency range of natural movements, and could be evoked after sectioning all dorsal roots in the exposed spinal segments. Locomotion was only elicited by L-aspartate or L-glutamate at low concentrations when the bath medium was rapidly circulated over the exposed surface of the spinal cord, and was of much shorter duration than the agonist-induced movements. These results indicate some differences between the actions of L-aspartate and L-glutamate and their agonists on the tadpole spinal cord.     

Differential Changes of Extracellular Aspartate and Glutamate in the Striatum of Domestic Chicken Evoked by High Potassium or Distress: An In Vivo Microdialysis Study

It has long been proposed that L: -aspartate (Asp) is an excitatory neurotransmitter similar to L: -glutamate (Glu) but with distinct signaling properties. The presence of Asp in excitatory synapses of the medial striatum/nucleus accumbens of domestic chicks suggests that Asp plays a role of neurotransmitter also in the avian brain. Neurotransmitters are released from the presynaptic bouton mostly by Ca(2+) dependent exocytosis. We used in vivo microdialysis to monitor the simultaneous changes of the extracellular levels of Asp and Glu in the medial striatum of young post-hatch domestic chicks. Microdialysis samples were collected from freely moving birds at 5 min intervals and analysed off-line using capillary electrophoresis. Event-related elevations of extracellular Glu and Asp concentrations in response to handling stress and to high KCl (50 mM) were observed. Increase of Glu and Asp on handling stress was 200 and 250 %, whereas on KCl stimulation the values were 300 and 1,000 %, respectively, if stress was applied before high KCl, and 150 and 200 %, respectively, in the absence of stress. In most cases, the amino acids showed correlated changes, Asp concentrations being consistently smaller at resting but exceeding Glu during stimulation. Using Ca(2+) free medium, the KCl triggered elevation of Glu was reduced. When KCl stimulation was combined with tetrodotoxin infusion, there was no significant elevation in Asp or in Glu suggesting that most of the extracellular excitatory amino acids were released by synaptic mechanisms. The results support the suggestion that Asp is co-released with Glu and may play a signaling role (as distinct from that of glutamate) in the striatum of birds.     

Differential action of adenosine triphosphate on high affinity transport of L-glutamate and L-aspartate in rat brain synaptosomes

Uptake of labeled L-glutamate into rat cortical synaptosomes was inhibited by ATP in a dose-dependent fashion; maximum inhibition was obtained at an ATP concentration of 500μM and was approximately 50%. Transport of L-aspartate was not affected by ATP. Thus, although L-glutamate and L-aspartate are believed to share a common transport system, it appears that transport of these two excitatory amino acids may be differentially regulated.     

Adriamycin-liposome interactions. A magnetic resonance study of the differential effects of cardiolipin on drug-induced fusion and permeability

L-Glutamate and L-aspartate transport into osmotically active intestinal brush border membrane vesicles is specifically increased by Na+ gradient (extravesicular greater than intravesicular) which in addition energizes the transient accumulation (overshoot) of the two amino acids against their concentration gradients. The "overshoot" is observed at minimal external Na+ concentration of 100 mM for L-glutamate and 60 mM for L-aspartate; saturation with respect to [Na+] was observed at a concentration near 100 mM for both amino acids. Increasing amino acid concentration, saturation of the uptake rate was observed for L-glutamate and L-aspartate in the concentration range between 1 and 2 mM. Experiments showing mutual inhibition and transtimulation of the two amino acids indicate that the same Na+ -dependent transport system is shared by the two acidic amino acids. The imposition of diffusion potentials across the membrane vesicles artificially induced by addition of valinomycin in the presence of a K+ gradient supports the conclusion that the cotransport Na+/dicarboxylic amino acid in rat brush border membrane vesicles is electroneutral.     

Evidence for Neuronal Origin and Metabotropic Receptor-Mediated Regulation of Extracellular Glutamate and Aspartate in Rat Striatum In Vivo Following Electrical Stimulation of the Prefrontal Cortex

Abstract: Extracellular levels of glutamate (Glu) and aspartate (Asp) were measured at 5-s intervals in the striatum of chloral hydrate-anesthetized rats by using microdialysis coupled to an automated assay system based on capillary electrophoresis with laser-induced fluorescence. Application of a single 10-s train of depolarizing pulses to the prefrontal cortex caused a rapid increase in Glu and Asp concentrations (200–300% of basal value), which returned to basal level within 60 s. The stimulated rise in Glu and Asp concentrations was blocked completely by 2 µ M tetrodotoxin or depletion of extracellular Ca²⁺, suggesting a neuronal origin of the Glu and Asp. Infusion of the Glu transport inhibitor l - trans -pyrrolidine-2,4-dicarboxylic acid (200 µ M ) increased resting Glu and Asp levels by 300–500% without altering electrically stimulated changes in Glu and Asp concentration. Stimulated Glu and Asp concentration changes were suppressed by 91 and 73%, respectively, by the metabotropic Glu receptor agonist (1 S ,3 R )-1-aminocyclopentane- trans -1,3-dicarboxylate (200 µ M ). This effect was blocked by the metabotropic Glu receptor antagonist ( RS )-α-methylcarboxyphenylglycine (MCPG; 200 µ M ). MCPG alone produced no effect on electrically stimulated changes in Glu and Asp levels; however, in the presence of l - trans -pyrrolidine-2,4-dicarboxylic acid, MCPG produced a five- to sixfold increase in stimulated overflow. Based on these results, it is concluded that release of Glu and Asp from corticostriatal neurons can be inhibited by activation of metabotropic Glu autoreceptors, which may be an important determinant of excitatory transmission at striatal synapses.     

Differential activation and blockade of excitatory amino acid receptors in the mammalian and amphibian central nervous systems

1. Experiments were conducted in vitro on isolated spinal cords of frogs and immature rats and in vivo on cat spinal neurones. 2. The concept of two major types of excitatory amino acid receptors present in these preparations is summarized, one type (NMDA receptors) being activated specifically by N-methyl-D-aspartate (NMDA) and blocked by specific antagonists such as D(-)-2-amino-5-phosphonovalerate (APV), and a second type (non-NMDA receptors) characterized by insensitivity to specific NMDA antagonists. This second type may be comprised of two sub-types activated selectively by the agonists quisqualate and kainate. The putative transmitters L-glutamate and L-aspartate have mixed action on both NMDA and non-NMDA receptors. The major action of both transmitter candidates is considered to be on non-NMDA receptors, but the proportion of the composite responses mediated by NMDA receptors (at least for spinal neurones) appears to be greater for L-aspartate than for L-glutamate. 3. The preference of NMDA and non-NMDA receptors for a range of agonists is discussed. Some newer agonists are considered, in addition to several known agonists not previously discussed in terms of NMDA- and non-NMDA-receptor preference. Structure-activity relations of agonists are discussed. 4. The actions of some new amino acid antagonists are reported. Some of these have useful kainate and quisqualate blocking activity, in addition to their ability to block NMDA induced responses. 5. Evidence is presented suggesting that excitatory amino acid receptors are involved in both polysynaptic and monosynaptic excitation in the spinal cord, NMDA receptors mediating polysynaptic excitation and non-NMDA receptors monosynaptic excitation. 6. The unusual effect is reported of L-2-amino-4-phosphonobutyrate, which potently blocks spinal synaptic excitation in the absence of depressant action on excitatory amino acid-induced responses.     

Salt bridge induced changes in the secondary structure of ionic polypeptides.

The carboxylate-containing homopolypeptides poly(L-glutamate) [poly(Glu)] and poly(L-aspartate) [poly(Asp)] were found to form different types of ordered structures in the presence of poly(L-lysine) [poly(Lys)]. Mixing poly(Glu) with poly(Lys) in aqueous solution at neutral pH results in the instantaneous formation of a gel-like precipitate. The secondary structure of the gel precipitate can be best described as intermolecular antiparallel beta-strands, involving the backbone amide groups, as evidenced by the presence of characteristic amide I bands in the ir spectrum at 1684 and 1612 cm-1. Mixing poly(Asp) with poly(Lys) under identical conditions results in the formation of a fine precipitate with a different morphology. Examination of the ir spectrum of the precipitate revealed that unlike poly(Glu), poly(Asp) did not yield any discrete secondary structure upon precipitation with poly(Lys). Addition of solutions containing Ca2+ or Mg2+ to the poly(Glu)/poly(Lys) aggregates resulted in complete dissolution of the gel, with the disappearance of the ir bands characteristic of the intermolecular hydrogen-bonded network. The results demonstrate the importance of salt bridges in establishing strong hydrogen bonds between the backbone amide groups. Reaggregation occurred upon heating the poly(Glu)/poly(Lys) mixture in the presence of Ca2+, but not in the presence of Mg2+ ions. In the presence of Ca2+ ions, aggregation and formation of an extended hydrogen-bonded network occurred upon heating. The aggregates formed upon heating poly(Glu)/poly(Lys) in the presence of Ca2+ were attributed solely to complexation of Ca2+ to the carboxylate groups of poly(Glu) with poly(Lys) remaining free in solution. Dissolution of the aggregate could be accomplished through addition of Mg2+ at room temperature.     

Extracellular Concentrations of Taurine,Glutamate, and Aspartate in the Cerebral Cortex of Rats at the Asymptomatic Stage of Thioacetamide-Induced Hepatic Failure: Modulation by Ketamine Anesthesia

Subclinical hepatic encephalopathy (SHE) was produced in rats by two intraperitoneal injections of TAA at 24 h intervals and the animals were examined 21 days later. Concentrations of the neuroactive amino acids taurine (Tau), glutamate (Glu) and aspartate (Asp), were measured in the cerebral cortical microdialysates of thioacetamide (TAA)-treated and untreated control rats. During microdialysis some animals were awake while others were anesthetized with ketamine plus xylazine. There was no difference in the water content of cerebral cortical slices isolated from control and SHE rats, indicating a recovery from cerebral cortical edema that accompanies the acute, clinical phase of hepatic encephalopathy in this model. When microdialysis was carried out in awake rats, dialysate concentrations of all the three amino acids were 30% to 50% higher in SHE rats than in control rats. Ketamine anesthesia caused a 2.2% increase of water content of cerebral cortical slices and increased Asp, Glu, and Tau concentration in microdialysates of control rats. In SHE rats, ketamine anesthesia produced a similar degree of cerebral edema, however, it did not alter Asp and Glu concentrations in the microdialysates. These data may reflect on one hand a neuropathological process of excitotoxic neuronal damage related to increased Glu and Asp, on the other hand neuroprotection from neuronal swelling indicated by Tau redistribution in the cerebral cortex. The reduction of the effects of SHE on Glu and Asp content in ketamine-anesthesized rats is likely to be due to interference of ketamine with the NMDA receptor-mediated component of the SHE-evoked excitatory neurotransmitter efflux and/or reuptake of the two amino acids. By contrast, the SHE-related increase of Tau content was not affected by ketamine anesthesia, indicating that the mechanism(s) underlying SHE-evoked accumulation of Tau must be different from the mechanism causing release of excitatory amino acids. The results with ketamine advocate caution when using this anesthetic in studies employing the cerebral microdialysis technique for measurement of extracellular amino acids.     

Somatostatin Potently Stimulates In Vivo Striatal Dopamine and γ-Aminobutyric Acid Release by a Glutamate-Dependent Action

Abstract: We have used in vivo microdialysis in anaesthetised rats to investigate whether somatostatin (SRIF) can play a neuromodulatory role in the striatum. When 100 n M SRIF was retrodialysed for 15 min, it increased concentrations of dopamine (DA) by 28-fold, γ-aminobutyric acid (GABA) by eightfold, and glutamate (Glu) by sixfold as well as those of aspartate (Asp) and taurine (Tau). These effects were both calcium- and tetrodotoxin-sensitive. Lower (10 or 50 n M ) and higher (1 µ M ) SRIF concentrations were less effective. Rapid sampling showed that whereas Asp and Glu concentrations were raised for 3 min at the start of 15-min SRIF infusions, those of DA were increased for 12 min. A second 15-min application of 100 n M SRIF given 135 min after the first application failed to increase transmitter release. An NMDA receptor antagonist, 2-amino-5-phosphonopentanoic acid (200 µ M ), blocked SRIF (100 n M )-evoked Asp, Glu, Tau, and GABA release and reduced that of DA. An α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate antagonist, 6,7-dinitroquinoxaline-2,3-dione (100 µ M ), blocked SRIF-induced DA and Tau release and reduced that of Asp, Glu, and GABA. These results show that SRIF increases DA, Glu, Asp, GABA, and Tau release in the rat striatum and suggest that its actions on DA and GABA release are mainly mediated through increased excitatory amino acid release.