Effect of direct and indirect dopamine agonists on brain extracellular ascorbate levels in the striatum and nucleus accumbens of awake rats

Systemic administration of direct and indirect dopamine agonists resulted in increased extracellular ascorbic acid levels in the striatum and, to a lesser degree, in the nucleus accumbens as measured by in vivo voltammetry. Intraperitoneal d-amphetamine sulfate (5mg/kg) increased ascorbate concentrations in striatal extracellular fluid. Amphetamine also increased extracellular ascorbate levels in the nucleus accumbens although more gradually and to a lesser extent. Intraperitoneal phenethylamine hydrochloride (20 mg/kg) following pargyline hydrochloride pretreatment (20 mg/kg) increased extracellular ascorbate levels in the striatum significantly above the small increase seen in the nucleus accumbens. The direct acting dopamine agonists Ly-141865 and Ly-163502 when given i.p. at 1 mg/kg, resulted in increased extracellular ascorbate concentrations in both brain areas, again with a significantly greater effect in the striatum. These results indicate that brain extracellular ascorbate levels can be modulated by dopaminergic neuro-transmission and that this modulation is quantitatively different in different dopamine-containing brain structures.     

Extracellular N-Acetylaspartate in the Rat Brain: In Vivo Determination of Basal Levels and Changes Evoked by High K+

Abstract: The purpose of this study was to determine the extracellular concentrations of N -acetylaspartate (NAA) in the rat cerebral cortex, striatum, and hippocampus of halo-thane-anaesthetised rats by intracerebral microdialysis, and to examine the effects of high K⁺-induced local depolarisation, which provokes synchronous neurotransmitter release, cell swelling, and acid-base changes. Basal levels of NAA in the extracellular fluid (EOF) were determined by the zero net flux method. Tissue levels of NAA in the cortex, striatum, and hippocampus were 8.4, 5.7, and 7.2 mmol/kg, respectively. The corresponding extracellular concentrations of NAA were much lower (35.1, 83.7, and 23.0 tiM). High tissue/ECF concentration ratios may suggest little release or leakage of NAA under basal conditions, and potent reuptake mechanisms for NAA in the cellular membrane of CNS cells. There was no change in ECF NAA during K⁺-induced local depolarising stimuli produced in the striatum, but NAA levels consistently increased after the K⁺ stimuli, irrespective of whether or not Ca²⁺ was present in the perfusion medium. These data confirm that NAA is not a neurotransmitter and suggest strongly that NAA is not directly involved in the release and reuptake or metabolism of neuroactive compounds. The increase of NAA in the ECF immediately after K⁺ stimulation may reflect an involvement in brain osmoregulation and/or acid-base homeostasis.     

Lactate levels in the brain are elevated upon exposure to volatile anesthetics: A microdialysis study

Anesthetic agents have well-defined pharmacological targets but their effects on energy metabolism in the brain are poorly understood. In this study, we examined the effects of different anesthetics on extracellular lactate and glucose levels in blood, CSF and brain of the mouse. In vivo-microdialysis was used to monitor extracellular energy metabolites in the brain of awake mice and during anesthesia with seven different anesthetic drugs. In separate groups, lactate and glucose concentrations in blood and CSF were measured for each anesthetic. We found that anesthesia with isoflurane caused a large increase of extracellular lactate levels in mouse striatum and hippocampus (300–400%). Pyruvate levels also increased while glucose and glutamate levels were unchanged. This effect was dose-dependent and was mimicked by other gaseous anesthetics such as halothane and sevoflurane but not by intravenous anesthetics. Ketamine/xylazine and chloral hydrate caused 2-fold increases of glucose levels in mouse blood and brain while lactate levels were only moderately increased. Propofol caused a minor increase of extracellular glucose levels while pentobarbital had no effect on either lactate or glucose. Volatile anesthetics also increased lactate levels in blood and CSF by 2–3-fold but had no effect on plasma glucose. Further experiments demonstrated that lactate formation by isoflurane in mouse brain was independent of neuronal impulse flow and did not involve ATP-dependent potassium channels. We conclude that volatile anesthetics, but not intravenous anesthetics, cause a specific, dose-dependent increase in extracellular lactate levels in mouse brain. This effect occurs in the absence of ischemia, is independent of peripheral actions and is reflected in strongly increased CSF lactate levels.     

Lactate levels in the brain are elevated upon exposure to volatile anesthetics: a microdialysis study

Anesthetic agents have well-defined pharmacological targets but their effects on energy metabolism in the brain are poorly understood. In this study, we examined the effects of different anesthetics on extracellular lactate and glucose levels in blood, CSF and brain of the mouse. In vivo-microdialysis was used to monitor extracellular energy metabolites in the brain of awake mice and during anesthesia with seven different anesthetic drugs. In separate groups, lactate and glucose concentrations in blood and CSF were measured for each anesthetic. We found that anesthesia with isoflurane caused a large increase of extracellular lactate levels in mouse striatum and hippocampus (300-400%). Pyruvate levels also increased while glucose and glutamate levels were unchanged. This effect was dose-dependent and was mimicked by other gaseous anesthetics such as halothane and sevoflurane but not by intravenous anesthetics. Ketamine/xylazine and chloral hydrate caused 2-fold increases of glucose levels in mouse blood and brain while lactate levels were only moderately increased. Propofol caused a minor increase of extracellular glucose levels while pentobarbital had no effect on either lactate or glucose. Volatile anesthetics also increased lactate levels in blood and CSF by 2-3-fold but had no effect on plasma glucose. Further experiments demonstrated that lactate formation by isoflurane in mouse brain was independent of neuronal impulse flow and did not involve ATP-dependent potassium channels. We conclude that volatile anesthetics, but not intravenous anesthetics, cause a specific, dose-dependent increase in extracellular lactate levels in mouse brain. This effect occurs in the absence of ischemia, is independent of peripheral actions and is reflected in strongly increased CSF lactate levels.     

An Electron Spin Resonance Study for Real-time Detection of Ascorbyl Free Radicals After Addition of Dimethyl Sulfoxide in Murine Hippocampus or Plasma During Kainic Acid-Induced Seizures

Electron spin resonance (ESR)-silent ascorbate solutions generate a detectable, likely concentration-dependent signal of ascorbyl free radicals (AFR) immediately upon addition of a molar excess of dimethyl sulfoxide (DMSO). We aimed to perform quantitative ESR analysis of AFR in real time after addition of DMSO (AFR/DMSO) to evaluate ascorbate concentrations in fresh hippocampus or plasma following systemic administration of kainate in mice. Use of a special tissue-type quartz cell allowed immediate detection of AFR/DMSO ESR spectra in fresh tissues from mice. AFR/DMSO content was increased significantly in fresh hippocampus or plasma obtained during kainate-induced seizures of mice, reaching maximum levels at 90 min after intraperitoneal administration of 50 mg/kg kainic acid. This suggests that oxidative injury of the hippocampus resulted from the accumulation of large amounts of ascorbic acid in the brain after kainic acid administration. AFR/DMSO content measured on an ESR spectrometer can be used for real-time evaluation of ascorbate content in fresh tissue. Due to the simplicity, good performance, low cost and real-time monitoring of ascorbate, this method may be applied to clinical research and treatment in the future.     

Changes in Antioxidant Defense Enzymes after d-amphetamine Exposure: Implications as an Animal Model of Mania

Studies have demonstrated that oxidative stress is associated with amphetamine-induced neurotoxicity, but little is known about the adaptations of antioxidant enzymes in the brain after amphetamine exposure. We studied the effects of acute and chronic amphetamine administration on superoxide dismutase (SOD) and catalase (CAT) activity, in a rodent model of mania. Male Wistar rats received either a single IP injection of d-amphetamine (1 mg/kg, 2 mg/kg, or 4 mg/kg) or vehicle (acute treatment). In the chronic treatment rats received a daily IP injection of either d-amphetamine (1 mg/kg, 2 mg/kg, or 4 mg/kg) or vehicle for 7 days. Locomotor behavior was assessed using the open field test. SOD and CAT activities were measured in the prefrontal cortex, hippocampus, and striatum. Acute and to a greater extent chronic amphetamine treatment increased locomotor behavior and affected SOD and CAT activities in the prefrontal cortex, hippocampus and striatum. Our findings suggest that amphetamine exposure is associated with an imbalance between SOD and CAT activity in the prefrontal cortex, hippocampus and striatum.     

Transport and distribution of homocarnosine after intracerebroventricular and intravenous injection in the rat

Rats were injected intracerebroventricularly (i.c.v.) or i.v. with [¹⁴C]homocarnosine (250 nmol). Distribution of the dipeptide in brain structures, transport from the brain to the blood, distribution in peripheral organs, and excretion in the urine were studied by measuring radioactivity in tissue, plasma, and urine samples by liquid scintillation counting 15–120 min after injection. After i.c.v. injection, [¹⁴C]homocarnosine was taken up into all parts of the brain investigated (highest uptake in structures close to the site of injection), it was transported to the blood, and radioactive substances were found in low concentration in muscle, spleen, and liver, in high concentration in the kidneys, and very high concentration in the urine. Investigations using high pressure liquid chromatography (HPLC) showed that no degradation took place in the brain, all radioactivity was found in the homocarnosine fraction. In the plasma 86% of the radioactivity was found in the GABA fraction presumed to be formed by cleavage of the peptide, while in the kidneys 35% and in the urine 40% was found in the GABA fraction. After i.v. injection of [¹⁴C]homocarnosine, no radioactivity was measured in hippocampus, striatum, cerebellum and cerebral cortex 15 min after injection, however, 60 min after injection a very low activity was detected in these structures (estimated intravascular radioactivity subtracted). A low activity was also measured in the spinal cord both 15 and 60 min after injection. When homocarnosine and GABA were separated on HPLC, all radioactivity in brain tissue was found in the GABA fraction, indicating either that [¹⁴C]homocarnosine did not cross the blood-brain barrier in amounts that could be measured with the method used, or that peptide entering the brain was rapidly transported back to the blood. [¹⁴C]Homocarnosine was not taken up either into crude synaptosomal preparations from hippocampus, striatum, cerebellum, cortex and spinal cord, or into slices prepared from the hippocampus and striatum. Transport from the brain to the kidneys and excretion in the urine seems to be a major route for disposal of this peptide in the rat.     

Increased levels of superoxide in brains from old female rats

Hypertension, aging and a range of neurodegenerative diseases are associated with increased oxidative damage. The present study examined whether superoxide (O₂^•-) levels in brain are increased during aging in female rats, and the role of superoxide dismutase (SOD) and oestrogen in regulating O₂^•- levels.

Young adult (3 month) and old (11 month) female spontaneously hypertensive stroke prone rats (SHRSP) and normotensive Wistar-Kyoto rats (WKY) were studied. O₂^•- levels were measured in brain homogenates by lucigenin chemiluminescence and SOD expression by Western blotting. Ageing significantly increased brain O₂^•- levels in WKY (cortex +216%, hippocampus +320%, striatum +225%) and to a greater extent in SHRSP (cortex +540%, hippocampus +580%, striatum +533%). Older SHRSP showed a decline in cortical Cu/Zn SOD expression compared to young adult SHRSP. Oestrogen did not attenuate O₂^•- levels.

The results show a significant age-dependent increase in brain O₂^•- levels which is exaggerated in SHRSP. The excess cortical O₂^•- levels in the SHRSP may be associated with a down-regulation of Cu/Zn SOD but are not related to a decrease in oestrogen.     

Increase in Exogenous Choline Fails to Elevate the Content or Turnover Rate of Cortical, Striatal, or Hippocampal Acetylcholine

Abstract: The present experiments were designed to test whether increasing the availability of choline to rat brain increases the rate of acetylcholine synthesis in that organ. The content of choline and acetylcholine and the turnover rate of acetylcholine in striatum, hippocampus, and cerebral cortex were measured following changes in dietary choline, intraperitoneal choline, or intravenous infusion of choline. Increasing plasma choline caused some increase in tissue choline but did not increase acetylcholine levels nor acetylcholine turn-over rate in any of the areas of brain studied. Indeed, in hippocampus, choline decreased the turnover rate of acetylcholine.     

Repeated immobilization stress alters rat hippocampal and prefrontal cortical morphology in parallel with endogenous agmatine and arginine decarboxylase levels

Agmatine, an endogenous amine derived from decarboxylation of l-arginine catalyzed by arginine decarboxylase, has been proposed as a neurotransmitter or neuromodulator in the brain. In the present study, we examined whether agmatine has neuroprotective effects against repeated immobilization-induced morphological changes in brain tissues and possible effects of immobilization stress on endogenous agmatine levels and arginine decarboxylase expression in rat brains. Sprague–Dawley rats were subjected to 2 h immobilization stress daily for 7 days. This paradigm significantly increased plasma corticosterone levels, and the glutamate efflux in the hippocampus as measured by in vivo microdialysis. Immunohistochemical staining with β-tubulin III showed that repeated immobilization caused marked morphological alterations in the hippocampus and medial prefrontal cortex that were prevented by simultaneous treatment with agmatine (50 mg/kg/day), i.p.). Likewise, endogenous agmatine levels measured by high-performance liquid chromatography in the prefrontal cortex, hippocampus, striatum and hypothalamus were significantly increased by immobilization, as compared to controls. The increased endogenous agmatine levels, ranging from 92 to 265% of controls, were accompanied by a significant increase of arginine decarboxylase protein levels in the same regions. These results demonstrate that the administration of exogenous agmatine protects the hippocampus and medial prefrontal cortex against neuronal insults caused by repeated immobilization. The parallel increase in endogenous brain agmatine and arginine decarboxylase protein levels triggered by repeated immobilization indicates that the endogenous agmatine system may play an important role in adaptation to stress as a potential neuronal self-protection mechanism.     

Increases in Striatal and Hippocampal Impedance and Extracellular Levels of Amino Acids by Cardiac Arrest in Freely Moving Rats

The time course of changes in the tissue impedance and the levels of extracellular transmitter and non-transmitter amino acids was studied in the striatum and hippocampus of the unanesthetized rat after cardiac arrest. Electrodes were implanted for the continuous measurement of tissue impedance so that a measure of the volume of extracellular space was provided. Alternatively, bilateral dialysis probes were used for monitoring levels of extracellular amino acids in subsequent 30-s samples using an automated precolumn derivatization technique for reversed-phase HPLC analysis and fluorimetric detection. The impedance started to rise approximately 1.2 min following cardiac arrest, increased rapidly during the first 5 min, and increased almost linearly thereafter. After 15 min, a decrease of approximately 50% in the extracellular space was calculated. The impedance rose more steeply in the striatum than in the hippocampus. The extracellular levels of taurine, which increased greater than 300% within 5 min after cardiac arrest, most closely resembled the time course of the change in impedance. Glutamate and aspartate levels did not increase until 5 min after circulatory arrest, and at 15 min they had risen to a level of 465 and 265% for the striatum and 298 and 140% for the hippocampus of the resting release, respectively. The release of gamma-aminobutyric acid (GABA) was multiphasic and did not resemble that of any of the other--putative--transmitter amino acids. Fifteen minutes after cardiac arrest, the levels of GABA were 617 and 774% of the resting release in the striatum and hippocampus, respectively. Glycine and alanine efflux substantially increased (232 and 151% in striatum and 141 and 154% in hippocampus, respectively) 15 min postmortem, whereas the glutamine level was slightly increased and levels of asparagine, histidine, threonine, ethanolamine, serine, arginine, and tyrosine were inconsistently higher in the two brain regions. At this time, the extracellular levels of glutamate, GABA, and aspartate were only slightly lower, as expected from the tissue levels and from levels of the other amino acids, an observation indicating that all the amino acids may diffuse through postmortem brain tissue to a nearly similar extent.(ABSTRACT TRUNCATED AT 400 WORDS)     

Brain and serum levels of choline and lecithin resulting from longterm administration

Choline and phosphatidylcholine tissue concentrations were examined in mice treated with long-term (18–22 month) dietary choline enrichment, choline deficiency, or phosphatidylcholine enrichment. There were no significant differences found in choline levels among the dietary groups in any of the tissues examined: plasma, erythrocytes, cortex, hippocampus, and striatum. In contrast, the concentration of phosphatidylcholine in both the choline enriched and phosphatidylcholine enriched groups were significantly increased in the cortex, compared to the choline deficient group, and in the striatum, compared to control. No differences in phosphatidylcholine concentration were found in the hippocampus or plasma between any of the dietary groups. These results are in contrast to the reported effects of acute or short-term choline and phosphatidylcholine treatment and indicate that there may be differences between the effects of short-term and long-term administration on the blood and brain levels of choline and phosphatidylcholine.     

Comparative Effect of Transient Global Ischemia on Extracellular Levels of Glutamate, Glycine, and γ-Aminobutyric Acid in Vulnerable and Nonvulnerable Brain Regions in the Rat

We evaluated whether regional differences in the magnitude of glutamate, gamma-aminobutyric acid (GABA), and glycine release could explain why some regions are vulnerable to ischemia whereas others are spared. By means of the microdialysis technique, the temporal profile of ischemia-induced changes in extracellular levels of glutamate, GABA, and glycine was compared in regions that demonstrate differing susceptibilities to a 10- and 20-min ischemic insult (dorsal hippocampus, anterior thalamus, somatosensory cortex, and dorsolateral striatum). The degree of ischemia (as established by local cerebral blood flow reduction) and the magnitude of histopathological neuronal damage were also evaluated in these regions. The blood flow reduction was severe and uniform in all regions; however, the histopathological outcome illustrated a different pattern. Whereas the CA1 sector of the hippocampus was severely damaged, the thalamus and cortex were relatively spared from both 10 and 20 min of ischemia. Striatal neurons were resistant to a 10-min insult but severely damaged after 20 min of ischemia. Ischemia-induced increase in glutamate and GABA content were of a similar magnitude and temporal profile in all four brain regions. A uniform increase in extracellular glycine levels was also observed in all four brain structures. The postischemic response, however, was different. Glycine levels remained twofold higher than baseline in the hippocampus but fell to baseline in the cortex and thalamus after both 10- and 20-min insults. In the striatum, glycine levels returned to baseline after 10 min of ischemia but remained relatively high after a 20-min insult.(ABSTRACT TRUNCATED AT 250 WORDS)     

The detection of age-, gender-, and region-specific changes in mouse brain tocopherol levels via the application of different validated HPLC methods

The aging process clearly increases the demand for antioxidant protection, especially in the brain, involving that provided by α-tocopherol (αT). However, little is known about the age-related changes in brain αT levels and the influencing effect of gender on it, in human or murine samples as well. Accordingly, the aim of the current study was to detect age-, gender- and region-specific changes in αT concentrations in mouse brain tissue and to assess the influencing effect of plasma αT levels on it. Female and male C57BL/6 mice at the ages of 6, 16 and 66 weeks (n?=?9 in each group) were applied. αT levels were determined with high performance liquid chromatography (HPLC) from the striatum, cortex, hippocampus, cerebellum, brainstem and from plasma samples. A detailed validation process was carried out for the applied HPLC method as well. The results demonstrated that brain αT levels significantly increased in the striatum, cortex, and hippocampus with aging in both genders, but in a more pronounced way in females with an increasing magnitude of this difference. In case of the cerebellum, a moderate elevation could be detected only in females, whereas in case of the brainstem there was no significant change in αT level. With regard to plasma samples, no clear trend could be identified. The current study is the first to present age-dependent gender-specific changes in αT level in certain brain regions of the C57Bl/6 mouse strain, and may provide meaningful information for future therapeutic studies targeting aging-related processes.     

Repeated immobilization stress alters rat hippocampal and prefrontal cortical morphology in parallel with endogenous agmatine and arginine decarboxylase levels

Agmatine, an endogenous amine derived from decarboxylation of L-arginine catalyzed by arginine decarboxylase, has been proposed as a neurotransmitter or neuromodulator in the brain. In the present study, we examined whether agmatine has neuroprotective effects against repeated immobilization-induced morphological changes in brain tissues and possible effects of immobilization stress on endogenous agmatine levels and arginine decarboxylase expression in rat brains. Sprague-Dawley rats were subjected to 2h immobilization stress daily for 7 days. This paradigm significantly increased plasma corticosterone levels, and the glutamate efflux in the hippocampus as measured by in vivo microdialysis. Immunohistochemical staining with beta-tubulin III showed that repeated immobilization caused marked morphological alterations in the hippocampus and medial prefrontal cortex that were prevented by simultaneous treatment with agmatine (50mg/kg/day), i.p.). Likewise, endogenous agmatine levels measured by high-performance liquid chromatography in the prefrontal cortex, hippocampus, striatum and hypothalamus were significantly increased by immobilization, as compared to controls. The increased endogenous agmatine levels, ranging from 92 to 265% of controls, were accompanied by a significant increase of arginine decarboxylase protein levels in the same regions. These results demonstrate that the administration of exogenous agmatine protects the hippocampus and medial prefrontal cortex against neuronal insults caused by repeated immobilization. The parallel increase in endogenous brain agmatine and arginine decarboxylase protein levels triggered by repeated immobilization indicates that the endogenous agmatine system may play an important role in adaptation to stress as a potential neuronal self-protection mechanism.     

Amphetamine Promotes Neostriatal Ascorbate Release via a Nigro-Thalamo-Cortico-Neostriatal Loop

Abstract: In the neostriatum, amphetamine and other dopamine agonists elevate the extracellular level of ascorbate, which is known to modulate neostriatal function. Although both D₁ and D₂ receptors have been linked to neostriatal ascorbate release, ample evidence suggests it is controlled by areas outside the neostriatum. The present series of experiments used selective lesions and intracerebral drug infusions to probe the involvement of the ventromedial thalamus and substantia nigra pars reticulata. Our results implicate both of these sites in amphetamine-induced increases in the release of neostriatal ascorbate. Thus, whereas unilateral electrolytic lesions of the substantia nigra pars reticulata completely abolished the ability of systemic amphetamine (2.5 mg/kg) to increase extracellular ascorbate in ipsilateral neostriatum, intranigral infusions of this drug (10 and 30 µg/µl) elevated neostriatal ascorbate release. This infusion effect, moreover, was blocked by electrolytic lesions of the ipsilateral ventromedial thalamus, which receives input from the substantia nigra pars reticulata and projects to the cerebral cortex. These results, combined with previous evidence implicating cortical projections to neostriatum as the source of extracellular ascorbate, suggest that neostriatal ascorbate release is regulated, at least in part, by a nigro-thalamo-cortico-neostriatal pathway.     

Metabolism Changes During Aging in the Hippocampus and Striatum of Glud1 (Glutamate Dehydrogenase 1) Transgenic Mice

The decline in neuronal function during aging may result from increases in extracellular glutamate (Glu), Glu-induced neurotoxicity, and altered mitochondrial metabolism. To study metabolic responses to persistently high levels of Glu at synapses during aging, we used transgenic (Tg) mice that over-express the enzyme Glu dehydrogenase (GDH) in brain neurons and release excess Glu in synapses. Mitochondrial GDH is important in amino acid and carbohydrate metabolism and in anaplerotic reactions. We monitored changes in nineteen neurochemicals in the hippocampus and striatum of adult, middle aged, and aged Tg and wild type (wt) mice, in vivo, using proton (¹H) magnetic resonance spectroscopy. Significant differences between adult Tg and wt were higher Glu, N-acetyl aspartate (NAA), and NAA + NAA–Glu (NAAG) levels, and lower lactate in the Tg hippocampus and striatum than those of wt. During aging, consistent changes in Tg and wt hippocampus and striatum included increases in myo-inositol and NAAG. The levels of glutamine (Gln), a key neurochemical in the Gln-Glu cycle between neurons and astroglia, increased during aging in both the striatum and hippocampus of Tg mice, but only in the striatum of the wt mice. Age-related increases of Glu were observed only in the striatum of the Tg mice.     

Physiological Stimulation Increases Nonoxidative Glucose Metabolism in the Brain of the Freely Moving Rat

Abstract: The effects of mild stress on nonoxidative glucose metabolism were studied in the brain of the freely moving rat. Extracellular lactate levels in the hippocampus and striatum were monitored at 2.5-min intervals with microdialysis coupled with an enzyme-based flow injection analysis system. Ten minutes of restraint stress led to a 235% increase in extracellular lactate levels in the striatum. A 5-min tail pinch caused an increase of 193% in the striatum and 170% in the hippocampus. Local application of tetrodotoxin in the striatum blocked the rise in lactate following tail pinch and inhibited the subsequent clearance of lactate from the extracellular fluid. Local application of the noncompetitive N -methyl- d -aspartate receptor antagonist MK-801 had no effect on the tail pinch-stimulated increase in lactate in the striatum. These results show that mild physiological stimulation can lead to a rapid increase in nonoxidative glucose metabolism in the brain.     

Regional Distribution of Extracellular 5-Hydroxytryptamine and 5-Hydroxyindoleacetic Acid in the Brain of Freely Moving Rats

The extracellular concentrations of 5-hydroxytryptamine (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) have been determined in six brain areas of awake rats (frontal cortex, striatum, hypothalamus, hippocampus, inferior colliculus, and raphe nuclei) using intracerebral microdialysis. The extracellular levels of 5-HT showed no significant differences among the brain regions studied. The tissue levels of 5-HT and 5-HIAA as well as the extracellular concentration of 5-HIAA were significantly higher in raphe nuclei. The regional distribution of tissue and extracellular 5-HIAA were very similar, suggesting that extracellular 5-HIAA depends mainly on the output from the intracellular compartment. On the other hand, extracellular 5-HT and tissue 5-HT showed a different distribution pattern. The tissue/extracellular ratio for 5-HT ranged from 739 in frontal cortex to 2,882 in raphe, whereas it only amounted to 1.8-3.6 for 5-HIAA. The relationship between the present results and the density of 5-HT uptake sites in these areas is discussed.