Effects of ethanol on extracellular amino acid levels in high-and low-alcohol sensitive rats: a microdialysis study.

Selectively bred high-alcohol sensitive (HAS) and low-alcohol sensitive (LAS) rats possess a number of behavioural and electrophysiological differences in their responses to alcohol. Using a microdialysis technique, we have evaluated whether the levels of the amino acids aspartate, glutamate, arginine, taurine, and alanine in HAS and LAS rats differ in their response to ethanol administration (2 g/kg, i.p.). The basal concentrations of each amino acid in these two groups were statistically similar. Following ethanol injection, alanine, arginine, and glutamate were significantly decreased in HAS rats, whereas, alanine, glutamate, and taurine were significantly increased in LAS rats by the end of the experiment. Interestingly, an increase in the sulphonated amino acid taurine was only evident 20 min after ethanol administration in the HAS rats, when compared to saline controls. No changes were observed in the other amino acids studied, aspartate and glycine, after ethanol administration. These data suggest that, in addition to differential behavioural responses to alcohol, HAS and LAS rats also differ in their neurochemical responses to ethanol.

Ethanol and amino acids in the central nervous system: assessment of the pharmacological actions of acamprosate.

Ethanol induces alterations in the central nervous system by differentially interfering with a number of neurotransmitter systems, although the mechanisms by which such effects are executed are not well understood. The present review therefore, is designed to ascertain the effect of ethanol on both excitatory and inhibitory amino acid neurotransmitters, as well as the sulphonated amino acid taurine, assayed by the microdialysis technique within specific brain regions of rat during different types of alcohol intoxication, acute and chronic, as well as during the withdrawal period. Such an understanding of these pharmacological actions of ethanol on neurotransmitters is essential in order to provide the impetus for the development of appropriate therapeutic intervention to ameliorate the multitude of neurochemical disorders induced by ethanol. In addition the possible mode of action of a new therapeutic drug for the treatment of alcoholism, acamprosate will be discussed. The first part of this review will be limited to studies of the effect of ethanol on both amino acid neurotransmitters and the sulphonated amino acid taurine, a possible neuromodulator. While, the second part will seek to establish the possible mechanism of action of a new therapeutic drug, acamprosate, which is used to combat the effects of ethanol, particularly during the craving period, as well as maintaining abstinence in weaned alcoholics.

Taurine blocks the glutamate increase in the nucleus accumbens microdialysate of ethanol-dependent rats.

During ethanol withdrawal, dramatic changes in the concentration of many neurotransmitters may be responsible for many of the adverse effects. In the present study, the technique of microdialysis was used to assay the changes in excitatory and inhibitory amino acids after withdrawal from chronic ethanol intoxication. Rats were made physically dependent on ethanol by vapor inhalation for 4 weeks. The basal concentrations of both arginine and GABA were significantly decreased in ethanol-dependent rats, although there were no significant changes in any of the other amino acid basal concentration assayed (i.e.. glutamate and taurine). During the first 12 h after withdrawal from ethanol, only glutamate increased significantly (p < 0.05) at 6 h, and for the duration of the study period of 12 h. To investigate whether either taurine and ethanol interact with amino acids during ethanol withdrawal, two other ethanol-dependent groups were injected with a single intraperitoneal injection of either taurine or ethanol 5 h after commencement of ethanol withdrawal. The IP injection of ethanol (2 g/kg) significantly increased taurine microdialysate content, and although this dose of ethanol was not able to block completely the increase of glutamate release after ethanol withdrawal, a delayed decrease in glutamate content was observed by the end of the period of the study (i.e., 11-12 h). However, IP injection of taurine (45 mg/kg) significantly blocked the increased glutamate release during ethanol withdrawal. This latter finding suggests that taurine may interact with glutamate, possibly by inducing a blockade of glutamate release during ethanol withdrawal.

Effect of repeated ethanol withdrawal on glutamate microdialysate in the hippocampus.

BACKGROUND: Our previous studies, which identified that ethanol withdrawal is associated with increases in glutamate microdialysate in the nucleus accumbens and reaches a maximum at 12 hr, have now been extended in order to assess whether repeated cycles of chronic ethanol intoxication followed by 12 hr withdrawal periods on three occasions alters glutamate release in the hippocampus of male rats. METHODS: In this study, the microdialysis technique has been used with the HPLC and electrochemical detection. RESULTS: During the first cycle of ethanol withdrawal, glutamate content increased significantly 8 hr after withdrawal (198.4% +/- 89.14%) by comparison with control rats. During the second period of ethanol withdrawal, 1 week after the initial withdrawal episode, glutamate microdialysate content increased significantly 10 hr after withdrawal, but to a much lower degree than in the first episode (179.08 +/- 25.68%), by comparison with control rats. During the third cycle of ethanol withdrawal, the concentration of glutamate in the hippocampus microdialysate did not significantly change at either of these time points except at 12 hr when glutamate was significantly decreased by comparison with control rats (52.09 +/- 14.38%). Apart from arginine, which was significantly decreased both at the cessation of alcoholization and during the 12 hr of the three withdrawal episodes, none of the other neurotransmitters assayed, aspartate, taurine, alanine, or GABA, showed any significant alteration. CONCLUSION: These results clearly indicate that elevated glutamate release during the first withdrawal episode is not paralleled in subsequent withdrawal episodes.

Acamprosate decreases the hypermotility during repeated ethanol withdrawal.

One of the known behavioral actions of acamprosate is to prevent relapse in weaned alcoholics. However, the mechanism underlying this effect remains unclear. In this study, the motility of Wistar male rats, which were either alcoholized by ethanol inhalation or simultaneously alcoholized and treated orally by acamprosate (400 mg/kg/ day) for 4 weeks, was measured during four episodes of the ethanol withdrawal. The concentrations of excitatory and inhibitory amino acids were also assayed by the microdialysis technique with OPA/BME precolumn derivatisation and electrochemical detection in the nucleus accumbens. Acamprosate reduces both the motility and the glutamate microdialysate content during the first 12 h of ethanol withdrawal in comparison to the alcoholized untreated group. The basal concentration of the sulfonated amino acid taurine increased significantly in alcoholized acamprosate-treated rats compared to alcoholized untreated rats. These results suggest that acamprosate is able to reduce the hypermotility during ethanol withdrawal syndrome directly by reducing the nucleus accumbens glutamate concentration or indirectly by increasing the taurine and GABA brain level.

Ethanol induces taurine release in the amygdala: an in vivo microdialysis study.

The effect of acute IP ethanol injections on the extracellular aspartate, glutamate, taurine and GABA content of the basolateral amygdala microdialysate was investigated in relationship with total brain ethanol. Each acute intraperitoneal injection of ethanol, 0.5, 1.0, 2.0 and 3.0 g/kg body weight, induced an immediate increase in microdialysate taurine; both 0.5 and 1.0 g/kg ethanol evoked an increase during the first 20 minutes following injection which returned to baseline value by 40 minutes, despite the fact that ethanol was detectable in the brain until 60 or 120 minutes, respectively. After either 2.0 or 3.0 g/kg ethanol there was an increase in taurine of gradual intensity which gradually declined to reach baseline values by 100 minutes. In contrast, the ethanol concentration for 2.0 g/kg remained elevated at the end of the 120 minutes; approximately 25 mg ethanol/mg protein. The stimulated release of taurine within the amygdala could participate in the regulation of ethanoli-nduced changes in osmolarity, since taurine is postulated to act as an osmoregulator in the brain. Taurine could also mediate or interact with ethanol-induced central nervous system effects, as it exerts a modulatory action on cell excitability and neurotransmitter processes.

Central effects of acamprosate: part 1. Acamprosate blocks the glutamate increase in the nucleus accumbens microdialysate in ethanol withdrawn rats.

One of the known behavioral actions of acamprosate is to decrease hypermotility during alcohol withdrawal. However, the mechanism of this effect remains unclear. In this study, the concentrations of excitatory and inhibitory amino acids were assayed by the microdialysis technique with OPA/BME precolumn derivatization and electrochemical detection in the nucleus accumbens of male Wistar rats which were either alcoholized by ethanol inhalation or simultaneously alcoholized and treated orally by acamprosate (400 mg/kg/day) for 4 weeks. Without treatment, extracellular glutamate increased during the withdrawal phase, while other amino acids tested (aspartate, arginine, taurine, alanine and GABA) remained stable. In contrast, the alcoholized rats treated with acamprosate failed to present the increase in glutamate during ethanol withdrawal, while other amino acids tested also remained stable. The observed glutamate increase could be responsible for the hyperexcitability observed during episodes of ethanol withdrawal. These results suggest that acamprosate is able to reduce the ethanol withdrawal syndrome by reducing the concentration of glutamate in the nucleus accumbens.

Central effects of acamprosate: part 2. Acamprosate modifies the brain in-vivo proton magnetic resonance spectrum in healthy young male volunteers.

Although acamprosate is a drug which is successfully used for therapy in maintaining alcohol abstinence following alcohol withdrawal in chronic alcoholism, little is understood about its mechanism of action in the central nervous system. Our objective was to assess the effects of acamprosate on the central nervous system in healthy subjects by dynamic proton magnetic resonance spectroscopy (MRS) measurements localized in brain tissue in vivo. Recordings were performed after intravenous administration of acamprosate or placebo to eight healthy male volunteers participating in a double-blind, randomized, cross-over, placebo-controlled study. The data were acquired using a spin-echo volume selective localized spectroscopy scheme on a 3-T whole body MRS system. Spectra obtained at baseline and at 20-min time intervals after the beginning of drug infusion were analyzed on the basis of five non-overlapping spectral integration regions. In the acamprosate-treated group, the median integral values in the regions for which N-acetylaspartate and glutamate are the main signal contributors showed decreases relative to placebo 20 min after the infusion began. Results suggest a central glutamatergic effect of acamprosate consistent with cerebral microdialysis glutamate measurements in vivo obtained from alcoholized rats treated with acamprosate (Part 1 of this study). This study is to our knowledge the first one describing a central effect of acamprosate in humans by MRS.

Acetaldehyde-induced changes in monoamine and amino acid extracellular microdialysate content of the nucleus accumbens.

The effect of an acute intraperitoneal (i.p.) injection of acetaldehyde, 20 mg/kg or 100 mg/kg, on the microdialysate content of both amino acids and monoamines was studies in the nucleus accumbens (NA) by a microdialysis technique. Acetaldehyde, ACH, which was detectable at levels of 50-130 mumol/g brain tissue 10 min after injection, evoked a significant decrease in the extracellular microdialysis dopamine content, which was sustained for the period of the study, i.e. 120 min. Homovanillic acid, HVA, decreased significantly when the lower dose of ACH was administered while dihydrophenylacetic acid, DOPAC, showed no significant change with either dose of ACH during the period of the study. Serotonin levels decreased significantly after both doses of acetaldehyde, with significant increases of its major metabolite, hydroxyindolacetic acid, 5-HIAA, with the higher acetaldehyde dose. Taurine increased significantly, only during the first twenty minutes, after both doses of acetaldehyde, although neither of the excitatory amino acids assayed, glutamate and aspartate, nor the inhibitory amino acid, GABA, showed any significant changes. Acetaldehyde clearly evokes significant perturbation in the monoamine content of the NA, such changes being the converse to those reported for monoamines after ethanol administration, which might indicate a negative reinforcement effect.

Taurine increases in the nucleus accumbens microdialysate after acute ethanol administration to naive and chronically alcoholised rats.

The extracellular changes of amino acids (glutamate, taurine and GABA) in the nucleus accumbens of freely moving rats were estimated using the microdialysis technique following acute and chronic ethanol injections (1, 2, and 3 g/kg body weight). Compared to baseline values, taurine increased by 154% +/- 73%, 142% +/- 40% and 162% +/- 75% 20 min after the acute injection of respectively, 1, 2, and 3 g/kg body weight ethanol, while 40 min after ethanol injection, taurine had increased by 124% +/- 36%, 146% +/- 54% and 168% +/- 98%. No changes in either glutamate or GABA were detected at any time points assayed. In the rats which had received chronic ethanol administration prior to a further acute ethanol injection (1, 2, and 3 g/kg body weight), taurine increased by 138% +/- 73%. 144% +/- 39% and 180% +/- 85% 20 min after the ethanol injection at 40 min post ethanol injection taurine had increased by 134% +/- 44%, 160% +/- 56% and 158% +/- 45%, compared to the basal baseline value. No significant changes were observed in either glutamate or GABA microdialysate content in these chronic studies. The biological role played by taurine after acute ethanol injection in the nucleus accumbens remains unclear but may be associated with a yet, undefined mechanism, in reducing the cytotoxicity of ethanol.

Acute ethanol increases taurine but neither glutamate nor GABA in the nucleus accumbens of male rats: a microdialysis study.

The effects of acute intraperitoneal administration of ethanol (1-3 g/kg body wt) on the extracellular concentrations of glutamate (GLU), taurine (TAU) and gamma-aminobutyric acid (GABA) in rat brain nucleus accumbens were studied by the microdialysis technique coupled to HPLC-EC detection. Levels of GLU and GABA were not affected by ethanol, whereas those of TAU were increased by doses of the drug of 2-3 g/kg.

Acute and chronic alcohol injections increase taurine in the nucleus accumbens.

Intracerebral microdialysis was used to investigate release of amino acid neurotransmitters in response to acute and chronic injection of alcohol in unanaesthetised animals. Increases in taurine levels but not of glutamate or GABA were found after acute injection of alcohol. Chronic injection of alcohol, in increasing doses over six weeks, resulted in substantially greater increases in taurine levels. Basal levels of all three amino acids were increased after chronic injection of alcohol, up to a dose of 2 g/kg body weight, but not at higher doses. Higher doses may overcome the compensatory capacity which induces increases concentrations of these neuromodulators after chronic exposure to alcohol.