Abnormalities in the levels of extracellular and tissue amino acids in the brain of the seizure-susceptible rat

Metabolism of brain amino acids following pentylenetetrazole treatment

We studied the effects of pentylenetetrazole (PTZ) on brain amino acid metabolism in mice. Administration of this convulsant did not change forebrain concentrations of amino acids, but when treated animals also received an injection of [15N]leucine, which served as a tracer of brain nitrogen metabolism, total (14N+15N) forebrain [leucine] exceeded control and [glutamate] and [aspartate] were less than control, as were forebrain concentrations of [15N]glutamate and [2-15N]glutamine. These data suggest greater uptake of [15N]leucine but diminished transamination of leucine to glutamate in experimental mice. In contrast to the [15N]leucine studies, which were associated with increased brain [leucine], the administration of [15N]alanine did not alter levels of alanine, glutamate or glutamine. However, label appeared in [2-15N]glutamine much more readily with [15N]alanine than with [15N]leucine as precursor and the ratio of enrichment in [2-15N]glutamine/[15N]alanine was much higher than that in [2-15N]glutamine/[15N]leucine, a finding that is compatible with preferential metabolism of alanine in astrocytes, which are the primary site of brain glutamine synthetase. We conclude that PTZ treatment favors the uptake of selected amino acids such as leucine but also diminishes transamination of leucine to yield glutamate via branched-chain amino acid transaminase. PTZ treatment may favor the "reverse" transamination of 2-keto-isocaproate (KIC), the ketoacid of leucine, to form leucine and to consume glutamate. A net result of these processes may be to enable the brain more readily to dispose of the glutamate that is released from neurons during convulsive activity.

Glutamate receptors and transporters in genetic and acquired models of epilepsy

Glutamate, the principal excitatory neurotransmitter in the brain, acts on three families of ionotropic receptor--AMPA (alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid), kainate and NMDA (N-methyl-D-aspartate) receptors and three families of metabotropic receptor (Group I: mGlu1 and mGlu5; Group II: mGlu2 and mGlu3; Group III: mGlu4, mGlu6, mGlu7 and mGlu8). Glutamate is removed from the synaptic cleft and the extracellular space by Na+-dependent transporters (GLAST/EAAT1, GLT/EAAT2, EAAC/EAAT3, EAAT4, EAAT5). In rodents, genetic manipulations relating to the expression or function of glutamate receptor proteins can induce epilepsy syndromes or raise seizure threshold. Decreased expression of glutamate transporters (EAAC knockdown, GLT knockout) can lead to seizures. In acquired epilepsy syndromes, a wide variety of changes in receptors and transporters have been described. Electrically-induced kindling in the rat is associated with functional potentiation of NMDA receptor-mediated responses at various limbic sites. Group I metabotropic responses are enhanced in the amygdala. To date, no genetic epilepsy in man has been identified in which the primary genetic defect involves glutamate receptors or transporters. Changes are found in some acquired syndromes, including enhanced NMDA receptor responses in dentate granule cells in patients with hippocampal sclerosis.

Glutamate receptors in epilepsy

Glutamatergic synapses play a critical role in all epileptic phenomena. Broadly enhanced activation of post-synaptic glutamate receptors (ionotropic and metabotropic) is proconvulsant. Antagonists of NMDA receptors and AMPA receptors are powerful anticonvulsants in many animal models of epilepsy. A clinical application of pure specific glutamate antagonists has not yet been established. Many different alterations in glutamate receptors or transporters can potentially contribute to epileptogenesis. Several genetic alterations have been shown to be epileptogenic in animal models but no specific mutation relating to glutamatergic function has yet been linked to a human epilepsy syndrome. There is clear evidence for altered NMDA receptor function in acquired epilepsy in animal models and in man. Changes in metabotropic receptor function may also play a key role in epileptogenesis.

Ethanol and neurotransmitter interactions--from molecular to integrative effects

There is extensive evidence that ethanol interacts with a variety of neurotransmitters. Considerable research indicates that the major actions of ethanol involve enhancement of the effects of gamma-aminobutyric acid (GABA) at GABAA receptors and blockade of the NMDA subtype of excitatory amino acid (EAA) receptor. Ethanol increases GABAA receptor-mediated inhibition, but this does not occur in all brain regions, all cell types in the same region, nor at all GABAA receptor sites on the same neuron, nor across species in the same brain region. The molecular basis for the selectivity of the action of ethanol on GaBAA receptors has been proposed to involve a combination of benzodiazepine subtype, beta 2 subunit, and a splice variant of the gamma 2 subunit, but substantial controversy on this issue currently remains. Chronic ethanol administration results in tolerance, dependence, and an ethanol withdrawal (ETX) syndrome, which are mediated, in part, by desensitization and/or down-regulation of GABAA receptors. This decrease in ethanol action may involve changes in subunit expression in selected brain areas, but these data are complex and somewhat contradictory at present. The sensitivity of NMDA receptors to ethanol block is proposed to involve the NMDAR2B subunit in certain brain regions, but this subunit does not appear to be the sole determinant of this interaction. Tolerance to ethanol results in enhanced EAA neurotransmission and NMDA receptor upregulation, which appears to involve selective increases in NMDAR2B subunit levels and other molecular changes in specific brain loci. During ETX a variety of symptoms are seen, including susceptibility to seizures. In rodents these seizures are readily triggered by sound (audiogenic seizures). The neuronal network required for these seizures is contained primarily in certain brain stem structures. Specific nuclei appear to play a hierarchical role in generating each stereotypical behavioral phases of the convulsion. Thus, the inferior colliculus acts to initiate these seizures, and a decrease in effectiveness of GABA-mediated inhibition in these neurons is a major initiation mechanism. The deep layers of superior colliculus are implicated in generation of the wild running behavior. The pontine reticular formation, substantia nigra and periaqueductal gray are implicated in generation of the tonic-clonic seizure behavior. The mechanisms involved in the recruitment of neurons within each network nucleus into the seizure circuit have been proposed to require activation of a critical mass of neurons. Achievement of critical mass may involve excess EAA-mediated synaptic neurotransmission due, in part, to upregulation as well as other phenomena, including volume (non-synaptic diffusion) neurotransmission. Effects of ETX on receptors observed in vitro may undergo amplification in vivo to allow the excess EAA action to be magnified sufficiently to produce synchronization of neuronal firing, allowing participation of the nucleus in seizure generation. GABA-mediated inhibition, which normally acts to limit excitation, is diminished in effectiveness during ETX, and further intensifies this excitation.

Aberrant neuronal responsiveness in the genetically epilepsy-prone rat: acoustic responses and influences of the central nucleus upon the external nucleus of inferior colliculus

The inferior colliculus (IC) central nucleus (ICc), is critical for audiogenic seizure (AGS) initiation in the genetically epilepsy-prone rat (GEPR). The ICc lacks direct motor outputs but sends a major projection to the external nucleus of IC (ICx), which does project to the sensorimotor integration nuclei within the AGS neuronal network. The present study compared acoustic responses of ICx neurons in the GEPR and normal anesthetized rat and evaluated whether the GEPR exhibits functional abnormalities in the pathway from ICc to ICx. There is a significantly greater incidence of sustained repetitive response patterns to the acoustic stimulus in GEPR ICx neurons (75%) than in normal ICx neurons (24%). Following unilateral microinjection of N-methyl-D-aspartate (NMDA) into the contralateral ICc, acoustically-evoked ICx excitation and inhibition were each increased in normal animals, which is consistent with the mixed projections previously reported in this pathway and observed with electrical stimulation in the present study. The NMDA-induced ICx firing increase may be relevant to AGS, since, in previous studies, bilateral focal microinjection of NMDA into the ICc induced AGS susceptibility in normal rats [23]. However, the incidence and degree of the ICx neuronal response changes after NMDA microinjection was not abnormal in the GEPR. These data suggest that the hyperresponsiveness of ICx neurons may not involve abnormal transmission between the ICc and ICx, despite the elevated ICx neuronal responses to acoustic stimuli. However, the ICx hyperresponsivess of the GEPR, which is likely due to the known decrease in effectiveness of GABA-mediated inhibition in GEPR neurons, may be a major mechanism subserving the critical role that this structure plays in the AGS network.

Effect of alpha 2-adrenergic drugs dexmedetomidine and atipamezole on extracellular amino acid levels in vivo

alpha 2-Adrenoceptors are known to be involved in a variety of physiological functions and pathological conditions, including epilepsy and the extent of excitotoxin-induced cell death. In this study we evaluated whether selective alpha 2-adrenergic drugs can modulate the release of neurotransmitter amino acids. The effect of the alpha 2-adrenoceptor agonist dexmedetomidine (5 micrograms/kg, s.c.) and the alpha 2-adrenoceptor antagonist atipamezole (0.1 mg/kg and 1 mg/kg, s.c.) on the release of extracellular glutamate, aspartate and gamma-aminobutyric acid (GABA) was studied with microdialysis in the hippocampus of freely moving rats under basal and K(+)-evoked conditions. Atipamezole (1 mg/kg) decreased K(+)-evoked glutamate efflux by 30% compared to the control group (P < 0.05) but did not affect significantly the effluxes of aspartate and GABA. Dexmedetomidine and the lower dose of atipamezole (0.1 mg/kg) did not significantly alter the evoked overflow of amino acids. The results suggest that alpha 2-adrenergic drugs have only modest effects on the K(+)-stimulated overflow of extracellular neurotransmitter amino acids in rat hippocampus.

Decreased GABA release following tonic-clonic seizures is associated with an increase in extracellular glutamate in rat hippocampus in vivo

The effects of maximal electroshock, used as a model of generalized seizures, were studied on extracellular GABA and glutamate levels in the ventral hippocampus of the freely-moving rat, using in vivo microdialysis. Following a maximal electroshock there was a rapid decline in GABA levels (46 +/- 5%) in the 20 min immediately after the seizure and levels remained depressed for a further 60 min. However, although there was a transient small decrease (11 +/- 2%) in glutamate levels in the first 20 min post-ictally, there followed a more prolonged, larger increase in the next 40 min. Maximal electroshock, administered in the absence of extracellular calcium, did not change GABA levels, while glutamate levels were again increased (42 +/- 8%) in the 40-80 min after the shock. Local perfusion with nickel (1 mM) to block T-type calcium channels had no effect on basal GABA or glutamate levels but prevented maximal electroshock-induced changes in both amino acids. Experiments were carried out to test the hypothesis that the post-ictal increased glutamate release was due to the decrease in GABA release. Perfusion with the potent GABA re-uptake inhibitor NNC-711, for 60 min prior to administration of maximal electroshock, increased GABA levels (436 +/- 58%) and abolished the seizure-induced decrease. Basal glutamate levels were not affected by perfusion with NNC-711 but subsequent maximal electroshock also failed to affect levels. Local perfusion with the GABAA receptor antagonist bicuculline (1, 10 and 100 microM) had no effect on basal GABA levels but glutamate levels were increased (46 +/- 5%) after perfusion with 100 microM bicuculline.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of in vivo amino acid neurotransmitters by high-performance liquid chromatography with o-phthalaldehyde-sulphite derivatisation

The measurement of amino acid neurotransmitters by high-performance liquid chromatography (HPLC) has emerged as a reliable and sensitive method. This paper describes a method which employs electrochemical (EC) detection of amino acid derivatives formed by a reaction with o-phthalaldehyde (OPA) in the presence of sulphite ions. This is discussed in relation to the problems of previously reported methods based on OPA derivatisation. Precise separation of the following 7 amino acid standards is achieved using isocratic elution: serine, glycine, taurine, glutamate, arginine, alanine and GABA, in order of increasing retention time. Total elution time is 25 min. Derivatisation proceeds at room temperature and the derivatives are stable for up to 5 h. This technique has the sensitivity to determine the concentrations of amino acid neurotransmitters in cerebrospinal fluid (CSF) and an in vivo microdialysis method is discussed for the detection of basal and potassium-stimulated levels of gamma-aminobutyric acid and glutamate from rat hippocampus.

Diminished potassium-stimulated GABA release in vivo in genetically epilepsy-prone rats

This experiment was conducted to assess the physiological relevance of observed changes in transmitter amino acid content in severe seizure genetically epilepsy-prone rats (GEPR-9s) by use of microdialysis. Adult male GEPR-9s and non-epileptic control rats were implanted with guide cannulae, and 6 mm (loop) dialysis probes were inserted unilaterally into rostral caudate and perfused with artificial cerebrospinal fluid. Each subject was perfused in the awake state with 100 or 150 mM K+ for 80 min in separate counterbalanced sessions, and 20-min fractions collected. High K+ perfusion increased extracellular fluid GABA and glutamate (GLU) in a concentration-dependent manner in both GEPR-9s and non-epileptic control rats. However, in the presence of 150 mM K+ GABA release was decreased in GEPR-9s relative to controls throughout the stimulation interval. In contrast, the increase in extracellular fluid GLU after high K+ was not different in the two groups. These results suggest an important role for mechanisms underlying GABA release in the seizure susceptibility observed in GEPR-9s.

Quantitative analysis of neuroactive amino acids in brain tissue by liquid chromatography using fluorescent pre-column labelling with o-phthalaldehyde and N-acetyl-L-cysteine

A high-performance liquid chromatographic method for the determination of aspartic acid, glutamic acid, glutamine, glycine, taurine, and gamma-aminobutyric acid in brain tissue is described. Amino acids were derivatized with o-phthalaldehyde/N-acetyl-L-cysteine, a fluorescent labelling mixture, in the presence of 0.1 M borate buffer pH 9.5. The derivatization reaction was sensitive to the pH and concentration of borate buffer. A drift in the fluorescent response less than 4% was obtained with the reported conditions after 4 h of reaction. The resolution of the amino acid derivatives was accomplished in a reversed-phase column with a methanol gradient in 50 mM acetate buffer pH 5.5. These conditions also allowed the separation of the major tissue free physiological amino acids. L-Norvaline was used as an internal standard for both peak identification and quantification. Within-day and between-day precision were less than 6.2%, and the accuracy ranged from 99.1 to 104%. The applicability of the method was demonstrated in a study in rats in which the levels of the assayed amino acids in discrete areas of brain were examined.

A microdialysis study of amino acid concentrations in the extracellular fluid of the substantia nigra of freely behaving GEPR-9s: relationship to seizure predisposition

Substantia nigra (SN) is known to play an important role in seizure generalization. Both excitatory and inhibitory neurotransmitters can modulate this role of SN. Previous studies have shown that GABA as well as aspartate and glutamate participate in seizure regulation through this site. Evidence for such a role comes from studies on the genetically epilepsy-prone rat (GEPR) and other seizure models. In the GEPR, bilateral microinjections of NMDA receptor antagonists in SN block or reduce seizure severity. In order to further evaluate which neurotransmitters are specifically involved at the SN level of seizure regulation in the GEPR, we undertook a microdialysis study of K+ stimulated release of amino acids in the SN of GEPR-9s- and non-epileptic controls. A 1 mm loop-type microdialysis probe was inserted through pre-implanted guides into the SN of awake and freely moving rats (seven GEPR-9s and four non-epileptic controls), and used to perfuse a 100 mM K+ (high K+) solution for 2 h. Four 30 microliters samples were collected prior to high K+ stimulation (basal release), during high K+ perfusion, and after high K+ infusion. After precolumn derivatization with phenylisothiocyanate, levels of aspartic (ASP) and glutamic (GLU) acids, glycine (GLY), taurine (TAU) and GABA were measured by reversed phase high performance liquid chromatography. Two hours after the initiation of high K+ infusion, the increases relative to basal were, for non-epileptic controls, 35%, 74%, 68%, 847% and 283% respectively for ASP, GLU, GLY, TAU and GABA. Corresponding increases for GEPR-9s were 14%, 10%, 41%, 505% and 123% respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Contrasting effects of D- and L-(E)-4-(3-phosphono-2-propenyl)piperazine-2-carboxylic acid as anticonvulsants and as inhibitors of potassium-evoked increases in hippocampal extracellular glutamate and aspartate levels in freely moving rats

Microdialysis experiments performed in the dorsal hippocampus of freely moving rats showed that L-(E)-4-(3-phosphono-2-propenyl)piperazine-2-carboxylic acid (L-CPPene) is 10 times as potent as D-CPPene in inhibiting potassium-induced increases in extracellular levels of aspartate and glutamate. In control experiments, two 100 mM KCl stimuli (S1 and S2) applied for 10 min each (separated by a 40-min recovery period) produced substantial (300-500%) increases in the extracellular levels of aspartate, glutamate, taurine, and GABA and a 50% decrease in the glutamine level. S2/S1 ratios in the control groups were 0.67 (aspartate), 0.78 (glutamate), 0.83 (GABA), and 0.85 (taurine). In the experimental groups, D- or L-CPPene was applied via the probe during the second potassium stimulus (S2). L-CPPene (25 or 250 microM) produced selective suppression of potassium-induced increases of extracellular glutamate (S2/S1 ratio: 0.25) and aspartate (S2/S1 ratio: 0.20) levels, whereas 250 microM D-CPPene was required to inhibit the extracellular aspartate and glutamate increases. Neither enantiomer of CPPene affected the potassium-induced increases of GABA and taurine or the decrease in extracellular glutamine concentration. An additional study comparing the anticonvulsant potencies of D- and L-CPPene was performed using audiogenic DBA/2 mice. The anticonvulsant potency of D-CPPene, as assessed against sound-induced seizures in DBA/2 mice, was an order of magnitude higher than that of L-CPPene [ED50 clonic phase (intraperitoneal, 45 min): 1.64 mumol/kg and 16.8 mumol/kg, respectively]. We attribute the anticonvulsant action of D-CPPene to its antagonist action at the NMDA receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Heterogeneous distribution of functionally important amino acids in brain areas of adult and aging humans

The regional distribution of seven amino acids thought to have inhibitory neurotransmitter or neurotransmitter precursor function--GABA, glycine, taurine, serine, threonine, phenylalanine, and tyrosine--was determined in 52 discrete areas from brain of adult and old humans. Significant heterogeneity was found, with 3- to 16-fold differences in levels in the various regions analyzed. The patterns of distribution were somewhat different from those in the adult or old rat brain. Relatively few changes were seen in old brain. Heterogeneity in distribution has to be taken into account in assessing physiological changes in amino acid levels and metabolism.

Effects of soman-induced seizures on different extracellular amino acid levels and on glutamate uptake in rat hippocampus

Extracellular amino acid levels in CA3 and CA1 fields of rat hippocampus, an area highly sensitive to seizures, were determined by intracranial microdialysis during seizures induced by systemic administration of soman (o-1,2,2-trimethylpropyl methylphosphonofluoridate), a potent inhibitor of acetylcholinesterase. The glutamate uptake level was determined on another series of animals in hippocampus homogenates. An early and transient increase in the extracellular glutamate level occurred in CA3 within 30 min of seizures, with correlated brief elevations of taurine, glycine and glutamine levels. The glutamate level increased early in CA1, declined and then became more sustained (after 50 min of seizures). Apparent elevations of taurine, glycine and glutamine levels in CA1 accompanied changes in glutamate concentrations. Changes of glutamate level correlated with an increase in the glutamate uptake which rapidly declined after 40 min of seizures. The role of the transient release of glutamate in CA3 and of the sustained release in CA1 in prolonged soman-induced seizures is considered. The correlation between glutamate and other amino acid release is studied.

Roles of neurotransmitter amino acids in seizure severity and experience in the genetically epilepsy-prone rat

This investigation was designed to compare seizure-naive and seizure-experienced genetically epilepsy-prone rats (GEPRs) in order to distinguish transmitter amino acid changes related to seizure severity from those associated with seizure experience. Moderate (GEPR-3) and severe (GEPR-9) seizure male GEPRs were divided into seizure-naive and seizure-experienced groups based on whether seizure-inducing acoustical stimuli had been presented between 45 and 60 days of age, and then were sacrificed at 76 +/- 3 days. gamma-Aminobutyric acid (GABA) concentrations were lower in both GEPR-3s and GEPR-9s compared to non-epileptic controls in each brain region examined. Aspartate content was elevated in 5 of 6 brain areas in GEPR-9s compared to non-epileptic controls, and in 3 regions was higher in GEPR-9s than in GEPR-3s. In contrast, taurine concentrations were higher in GEPR-3s than in non-epileptic controls in each region, and in 4 areas were higher in GEPR-3s than in GEPR-9s. Changes resulting from seizure experience consisted of increases in aspartate, glutamate and glycine in seizure-experienced compared to seizure-naive groups in inferior colliculus and in motor-sensory and frontal cortices. These findings suggest that the high levels of taurine in GEPR-3s and the elevated content of aspartate in GEPR-9s have roles as determinants of seizure severity. The low concentrations of GABA in both types of GEPRs are consistent with a role for this amino acid in determination of seizure susceptibility. Furthermore, the seizure-induced changes in aspartate and glutamate in both types of GEPRs support the concept that these excitatory amino acids mediate changes in seizure predisposition.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-lasting effects of audiogenic seizures on neurotransmitter amino acids in Rb mice

The existence of long-lasting (15-18 h) alterations of neurotransmitter amino acid levels following a single or repeated acoustic stimulations in audiogenic seizure-prone Rb1 and Rb2 mice and seizure-resistant Rb3 mice were investigated. The levels of glutamate, aspartate, glycine, taurine, and of some of their precursors: glutamine and serine were determined. Fourteen brain areas were examined. Alterations were found only in 6 brain areas (pons, olfactory bulbs, superior colliculus, inferior colliculus, olfactory tubercles and raphe). Most frequent occurring changes were observed in pons and olfactory tubercles. These changes concerned mainly the excitatory amino acids, glutamate, and aspartate. Alterations of taurine, glycine and serine were also recorded.

The high pressure neurological syndrome in genetically epilepsy prone rats: protective effect of 2-amino-7-phosphono heptanoate

Genetically epilepsy prone rats (GEPR) are hypersensitive to various epileptogenic treatments and undergo characteristic generalized seizures when exposed to potent acoustic stimulation. We have studied the sensitivity of GEPR to high atmospheric pressure. Threshold pressures for behavioral symptoms of the high pressure neurological syndrome (HPNS) were recorded in normal Sprague-Dawley (SD) and GEPR (which originate from the SD strain) of both sexes. The threshold pressure (TP) for tremor and for convulsion was significantly lower in GEPR than in SD rats. The protective action of the NMDA receptor antagonist D-2-amino-7-phosphono-heptanoate (D-APH) was tested on both strains of rats. D-APH, 90 mg/kg ip was more protective against tremor in SD than in GEPR. Female GEPR were not protected against tremor. Protection against clonic seizures was similar in both sexes of GEPR and female SD rats while SD males were not significantly protected. None of the animals treated with D-APH developed the tonic phase of seizures. Blockade of the NMDA receptor with D-APH brought the threshold for convulsions in GEPR to a similar pressure to that obtained in SD vehicle-injected controls. This findings suggests the involvement of the excitatory amino acid system in the hypersensitivity of GEPR to high atmospheric pressure.

Amino acid neurotransmitter alterations in three sublines of Rb mice differing by their susceptibility to audiogenic seizures

The levels of inhibitory amino acids (Tau, Gly), or excitatory amino acids (Glu, Asp) and Gln, precursor of GABA, have been determined, under resting conditions, in 17 brain areas of 3 sublines of inbred Rb mice displaying different responses to an acoustic stimulus. Rb1 mice were clonictonic seizure-prone, Rb2 mice were clonic seizure-prone and Rb3 mice were seizure resistant. Profile of distribution in the brain of each one of these amino acids differed. Maximum to minimum level ratio was higher for Tau (3.8) than for Glu or Asp or Gln (2). The level of Gly was similar in 13 out of the 17 areas examined. Multiple inter-subline differences were recorded for each amino acid. These differences have been analyzed considering the seizure susceptibility or severity of the three Rb sublines. Common lower levels (approximately -20%: Rb1/Rb3, Rb2/Rb3) of Gln in Temporal Cortex may be implicated in seizure susceptibility. Seizure severity (Rb1/Rb2) seems to correlate, in some areas, with additional lower amounts of GABA already reported and, to a lower extent, of Asp (-19% in striatum, inferior colliculus and cerebellum), of Tau and Gly; a tendency for a rise in Gln content was observed in certain others (10-20% in olfactory bulb, thalamus, hypothalamus, substantia nigra, and frontal, temporal and occipital cortex). The data and correlations recorded provide guidelines for further investigations for synaptosomal and metabolic alterations in the three sublines of the same strain of Rb mice.

Effects of microdialysis-perfusion with anisoosmotic media on extracellular amino acids in the rat hippocampus and skeletal muscle

Changes in the levels of amino acids have been implicated as being important in osmoregulation both within and outside the CNS. The present study addressed the question of whether changes in osmolarity affect the extracellular concentration of amino acids in the rat hippocampus and femoral biceps muscle (FBM). Microdialysis probes were implanted in these tissues and perfused with standard physiological saline. Amino acid concentrations in the dialysate were determined with HPLC separation of o-phthaldialdehyde derivatives and fluorescence detection. The osmolarity of the perfusion buffer was gradually decreased by reduction of the concentration of NaCl from 122 to 61 to 0 mM. In other experiments, the osmolarity was increased by elevation of the NaCl level from 122 to 183 to 244 mM or by addition of mannitol. Glutamate, aspartate, gamma-aminobutyrate, and alanine levels in dialysate from the hippocampus increased when the concentration of NaCl was decreased by 61 mM, and they were further elevated when NaCl was omitted. Taurine and phosphoethanolamine (PEA) levels were maximally elevated at the intermediary decrease of NaCl concentration, and glutamine in particular but also methionine and leucine were suppressed by perfusion with hypoosmolar medium. The amino acid response of the FBM differed substantially from that of the hippocampus. The aspartate content increased slightly, and there was a marginal transient increase in PEA level. Perfusion with media containing high concentrations of NaCl induced diminished dialysate levels of taurine, PEA, and glutamate, whereas levels of other amino acids were either unaffected or increased. Mannitol administration via the perfusion fluid led to reduced levels of taurine, PEA, glutamate, and aspartate. In contrast to the effects of high NaCl levels, hyperosmotic mannitol did not induce increases in level of any of the amino acids detected. The results suggest that taurine and PEA are involved in osmoregulation in the mammalian brain. From a quantitative viewpoint, taurine seems to be most important. Transmitter amino acids may also be involved in the maintenance of the volume of neural cells subjected to severe disturbances in osmotic equilibrium.