Microdialysis--principles and applications for studies in animals and man

Microdialysis is a technique for sampling the chemistry of the individual tissues and organs of the body, and is applicable to both animal and human studies. The basic principle is to mimic the function of a capillary blood vessel by perfusing a thin dialysis tube implanted into the tissue with a physiological liquid. The perfusate is analysed chemically and reflects the composition of the extracellular fluid with time due to the diffusion of substances back and forth over the membrane. Microdialysis is thus a technique whereby substances may be both recovered from and supplied to a tissue. The most important features of microdialysis are as follows: it samples the extracellular fluid, which is the origin of all blood chemistry; it samples continuously for hours or days without withdrawing blood; and it purifies the sample and simplifies chemical analysis by excluding large molecules from the perfusate. However, the latter feature renders the technique unsuitable for sampling large molecules such as proteins. The technique has been extensively used in the neurosciences to monitor neurotransmitter release, and is now finding application in monitoring of the chemistry of peripheral tissues in both animal and human studies.

In vivo measurement of dopamine and its metabolites by intracerebral dialysis: changes after d-amphetamine

By using a new technique, intracerebral dialysis, in combination with high performance liquid chromatography and electrochemical detection, it was possible to recover and measure endogenous extracellular dopamine, together with its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) from the striatum and nucleus accumbens of anaesthetized or freely moving rats. In addition, measurements of extracellular 5-hydroxyindoleacetic acid, ascorbic acid, and uric acid were made. Basal extracellular concentrations of dopamine and DOPAC in the striatum were estimated to be 5 X 10(-8) M and 5 X 10(-6) M, respectively. d-Amphetamine (2 mg/kg s.c.) increased dopamine levels in the striatum perfusates by 14-fold, whereas levels of DOPAC and HVA decreased by 77% and 66%, respectively.

Changes in cortical extracellular levels of energy-related metabolites and amino acids following concussive brain injury in rats

The aim of this study was to measure extracellular chemical changes in the cerebral cortex in response to compression contusion trauma in rats. Energy-related metabolites (i.e., lactate, pyruvate, adenosine, inosine, and hypoxanthine) and amino acids were harvested from the extracellular fluid (ECF) using microdialysis and analyzed by high-performance liquid chromatography. The measurements were performed in cortical tissue, where neuronal injury occurs in this model. The severity of the trauma was varied by using different depths of impact: mild trauma, 1.5 mm; severe trauma, 2.5 mm. The trauma induced a dramatic increase in the ECF levels of energy-related metabolites that was conditioned by the severity of the insult. The ECF level of taurine, glutamate, aspartate, and gamma-aminobutyric acid (GABA) also rose markedly, while other amino acids did not change significantly. The results suggest that the trauma induced a transient, profound focal disturbance of energy metabolism in the cortical tissue, probably as a result of mechanically induced disruption of ion homeostasis and reduced blood flow in combination. The data support the potential role of glutamate and aspartate as mediators of traumatic brain injury. However, the concomitantly released adenosine, GABA, and taurine may be protective and ameliorate excitotoxicity. In analogy with the reported cumulative damaging effects of repeated ischemic insults, the observed ECF changes may help explain the vulnerability of traumatized brain tissue to secondary ischemia.

Purine levels in the intact rat brain. Studies with an implanted perfused hollow fibre

A thin dialysis tube was implanted stereotaxically under halothane anesthesia in the caudate nucleus of Sprague-Dawley rats and perfused with Ringer solution at a rate of 2 microliters/min. Initially there was a high rate of purine outflow but after 1-2 h of perfusion the rate was essentially constant (anesthetized - adenosine 0.4 +/- 0.04 microM, inosine 0.8 +/- 0.2 microM; non-anesthetized - adenosine 0.33 +/- 0.03 microM, inosine 0.21 +/- 0.07 microM). Hypoxia (9% O2) increased the levels more than 3-fold. The adenosine deaminase inhibitor erythro-2-(2-hydroxy-3-nonyl)adenine (EHNA) increased the adenosine level and decreased the inosine level. In vitro recovery of adenosine was about 30%. Therefore, we conclude that the free exchangable concentration of adenosine in the rat brain is likely to be 102 micro M. This level is high enough to potentially affect central nervous function.

In vivo measurement of extracellular dopamine and DOPAC in rat striatum after various dopamine-releasing drugs; implications for the origin of extracellular DOPAC

In order to further examine the likely origin of the dopamine (DA) metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), certain drugs known to release DA from different intraneuronal pools were tested for their effects on extracellular striatal DA and DOPAC levels by means of brain microdialysis in the halothane-anaesthetized rat. Amphetamine (10(-6) and 10(-5) M), nomifensine (10(-5) M), potassium chloride (30 and 60 mM), methylphenidate (10(-5) and 10(-4) M) and tyramine (10(-5) M), when added to the perfusion medium and administered locally into the striatum via the dialysis membrane, increased the level of DA in striatal perfusates during the 20 min of application. In comparison, the level of DOPAC in the perfusates was decreased by both amphetamine (10(-5) M) and potassium chloride (60 mM), but was not significantly changed by nomifensine, methylphenidate or tyramine. The effect of amphetamine (10(-6) M) and nomifensine (10(-5) M) on DA and DOPAC levels was further studied by administering the drugs over a longer period of time (3 X 20 min). Although both of these treatments produced a similar increase of DA, only amphetamine reduced the levels of DOPAC. DA (10(-4) but not (10(-5) M) increased the levels of DOPAC but this effect was also seen in DA-denervated animals. These data indicate that when the DA nerve terminal is exposed to drugs which release newly synthesized DA, DOPAC declines possibly because intraneuronal monoamine oxidase is deprived of its main substrate. We suggest that these findings support the hypothesis that a major portion of the DA metabolite, DOPAC, is derived from an intraneuronal pool of newly synthesized DA.

Dynamics of extracellular metabolites in the striatum after middle cerebral artery occlusion in the rat monitored by intracerebral microdialysis

The aim of this study was to measure changes in the extracellular fluid (ECF) concentration of lactate, pyruvate, purines, amino acids, dopamine, and dopamine metabolites in the striatum of rats subjected to focal cerebral ischemia, using intracerebral microdialysis as the sampling technique. Microdialysis probes were inserted into the lateral part of the caudate-putamen bilaterally 2 h before the experiment. Ischemia was induced by permanent middle cerebral artery occlusion (MCAO) on the left side. Microdialysis samples were analyzed by high performance liquid chromatography. Following MCAO, the concentration of lactate, adenosine, inosine, and hypoxanthine rose markedly in the ECF on the occluded side, while there was no significant change in pyruvate. These changes were accompanied by dramatically elevated levels of aspartate, glutamate, taurine, gamma-aminobutyric acid, and dopamine. There was also a marked increase in alanine/tyrosine, while minor or no changes occurred with other amino acids. Concomitantly, the ECF level of the dopamine metabolites 3,4-dihydroxyphenylacetate and homovanillic acid decreased. There was no significant increase in any of the metabolites measured on the right, nonoccluded side. In relation to the concept of excitotoxicity in brain ischemia, it is concluded that during the acute stage of focal cerebral ischemia, the ECF is flooded with both potentially harmful (e.g., aspartate, glutamate, and DA) and protective (e.g., taurine, GABA, and adenosine) agents. The relative importance of these events for the development of cell death in the ischemic penumbra needs to be elucidated. In addition, lactate, inosine, and hypoxanthine, measured in the ECF by intracerebral microdialysis, may prove to have diagnostic and/or prognostic value in neurometabolic monitoring of the ischemic brain.

A direct comparison of amphetamine-induced behaviours and regional brain dopamine release in the rat using intracerebral dialysis

An intracerebral microdialysis method was used in awake rats to directly compare the effect of amphetamine on dopamine (DA) release in the striatum and nucleus (n.) accumbens with alterations in behaviour. Amphetamine (0.5-5.0 mg/kg, s.c.) caused a dose-dependent release of DA in both brain regions; however the n. accumbens appeared for the most part more sensitive to amphetamine than the striatum. At each individual dose of the drug, 0.5, 2.0 and 5.0 mg/kg s.c., DA release was closely followed over the time course by the overall behavioural syndrome. Certain components of behaviour showed a regional-specific association with DA release. The intensity of stereotyped head and forepaw movements was closely correlated over the dose range with the amount of DA released in striatum but not n. accumbens. Over the time course, however, the occurrence of this behaviour was delayed compared to increased striatal DA release. In contrast, increased locomotor activity was correlated with the time course change in, and amount of, DA released in n. accumbens by low doses of amphetamine, but not at any dose with DA released in striatum. Repetitive sniffing was better correlated with DA released in n. accumbens than striatum. These in vivo measurements of DA release add further support to the hypothesis that amphetamine-induced stereotypy and locomotion are mediated via DA released in striatum and n. accumbens, respectively. Our data suggest that the occurrence of intense stereotypy rather than locomotor activity at high doses of amphetamine is not due to a selection action in striatum but probably competition between the two behaviours.(ABSTRACT TRUNCATED AT 250 WORDS)

Spinal cord stimulation attenuates augmented dorsal horn release of excitatory amino acids in mononeuropathy via a GABAergic mechanism

Neuropathic pain may be effectively relieved by electric stimulation of the spinal cord (SCS). However, the underlying mechanisms for the ensuing pain relief are poorly understood. In a rat model of neuropathy displaying hypersensitivity to innocuous tactile stimuli, (allodynia), we have earlier demonstrated that SCS may normalise withdrawal response thresholds. In the present study, using microdialysis, it is shown that SCS induces a decreased release of the dorsal horn excitatory amino acids (EAA), glutamate and aspartate, concomitant with an increase of the GABA release. Local perfusion with a GABA(B)-receptor antagonist in the dorsal horn transiently abolishes the SCS-induced suppression of the EAA release. Thus, the effect of SCS on neuropathic pain and allodynia may be due to an activation of local GABAergic mechanisms inhibiting the EAA release which is chronically elevated in such conditions.

Effect of neuroleptic drugs on striatal dopamine release and metabolism in the awake rat studied by intracerebral dialysis

This study investigated the effect of three neuroleptic drugs, (+/-)-sulpiride, haloperidol and cis-flupenthixol, on dopamine release and metabolism in the striatum of the awake rat. Endogenous extracellular dopamine and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), as well as the 5-hydroxytryptamine (5HT) metabolite 5-hydroxyindoleacetic acid (5HIAA), were determined in striatal perfusates in awake rats by using intracerebral dialysis together with high performance liquid chromatography with electrochemical detection. Sulpiride (10, 50 and 250 mg/kg), cis-flupenthixol (0.5 and 2 mg/kg) and haloperidol (2 mg/kg) all increased the levels of dopamine in striatal perfusates. However, the time course and magnitude of these effects differed markedly depending upon the neuroleptic used. Sulpiride (10, 50 and 250 mg/kg), cis-flupenthixol (0.05, 0.5 and 2 mg/kg) and haloperidol (0.05, 0.5 and 2 mg/kg) increased extracellular levels of DOPAC and HVA while having little effect on 5HIAA. In contrast to the effect on dopamine levels the changes in DOPAC and HVA followed similar time courses and were of similar magnitude independent of the neuroleptic used. The response of the dopamine metabolites seemed to occur at lower doses of the neuroleptics than the response of dopamine release itself. Furthermore, there was no close relationship between changes in dopamine as compared to changes in DOPAC and HVA. Finally, there was no correlation between any of the neurochemical changes measured and the occurrence of catalepsy. These data suggest that neuroleptic drugs have two separate actions on the dopamine neuron in vivo, one causing an increase in dopamine release and another producing an increase in dopamine metabolism, which is probably a consequence of increased dopamine synthesis. Furthermore neither of these effects are related to catalepsy.

An in vivo study of dopamine release and metabolism in rat brain regions using intracerebral dialysis

Intracerebral dialysis was used with a specifically designed HPLC with electrochemical detection assay to monitor extracellular levels of endogenous 3,4-dihydroxyphenylethylamine (dopamine, DA) and its major metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), in brain regions of the halothane-anesthetized rat. Significant amounts of DA, DOPAC, and HVA were detected in control perfusates collected from striatum and n. accumbens whereas the medial prefrontal cortex showed lower monoamine levels. The ratio of DA in perfusate to DA in whole tissue suggests that in f. cortex, compared to n. accumbens and striatum, there is a greater amount of DA in the extracellular space relative to the intraneuronal DA content. The DOPAC/HVA ratio in control perfusates varied between regions in accordance with whole tissue measurements. This ratio was highest in n. accumbens and lowest in f. cortex. The monoamine oxidase inhibitor pargyline (100 mg/kg i.p.) caused an exponential decline in DOPAC, but not of HVA, in regional perfusates, an effect that was associated with an increase in DA. The data indicated a higher turnover of extracellular DOPAC in n. accumbens than in striatum and the lowest DOPAC turnover in f. cortex. The rate of decline in extracellular DA metabolite levels was slow compared to whole tissue measurements. In the perfusates there was no statistical correlation between basal amounts of DA in the perfusates and DOPAC and HVA levels or DOPAC turnover for any of the areas, indicating that measurement of DA metabolism in the brain under basal conditions does not provide a good index of DA release. In summary, this study shows clear regional differences in basal DA release and metabolite levels, metabolite patterns, and DOPAC turnover rates in rat brain in vivo.

Chronic implants of chromaffin tissue into the dopamine-denervated striatum. Effects of NGF on graft survival, fiber growth and rotational behavior

Adult rat chromaffin tissue was transplanted into striatum of adult rat recipients whose nigrostriatal dopamine pathway had been lesioned on the grafted side by 6-hydroxydopamine. Long-term survival of the intrastriatal chromaffin grafts and the effects of treatment with nerve growth factor (NGF) was studied histochemically using Falck-Hillarp fluorescence histochemistry and functionally using rotational behavior induced by apomorphine. Small, cortex-free adrenal chromaffin tissue grafts survived permanently in striatum. The number of surviving cells was significantly increased by NGF. NGF treatment also caused transformation of many cells towards a more neuronal phenotype and greatly enhanced the adrenergic nerve fiber outgrowth into host brain tissue. NGF was either injected stereotaxically into the site of transplantation or infused continuously using implantable osmotic minipumps and a stereotaxically placed chronic indwelling dialysis fiber through striatum. The latter arrangement permitted continuous infusion of NGF for 14-28 days and caused a vigorous adrenergic nerve growth response by the grafts directed towards the source of NGF in the brain. There was a clearcut correlation between morphological signs of taking and rotational behavior. Grafts, and in particular grafts treated with NGF, were able to significantly and permanently counteract the rotational behavior induced by apomorphine. There seemed to be a dose relationship between NGF treatments and amount of reduction of asymmetric behavior. NGF treatment probably decreased the relative importance of diffuse release of catecholamines from chromaffin cells in the graft and increased the importance of adrenergic innervation of host striatum by cells in the graft. Immunofluorescence using antibodies against glial fibrillary acidic protein did not reveal any marked gliosis around the grafts nor were there any marked gliotic reactions around chronic indwelling dialysis fibers. We conclude that implantation of chromaffin tissue into striatum in conjunction with NGF treatments is an effective means of counteracting some of the symptoms of experimentally induced unilateral parkinsonism in rats.

Effects of apomorphine on the in vivo release of dopamine and its metabolites, studied by brain dialysis

The effect of apomorphine (0.05-0.5 mg/kg s.c.) on the release of endogenous dopamine and extracellular levels of the metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5HIAA) were examined in vivo by intracerebral dialysis. A dialysis tube was implanted stereotaxically through both caudate nuclei of rats and perfused with Ringer solution at a rate of 2 microliters/min. The amount of dopamine, DOPAC, HVA and 5HIAA in the perfusates was measured by HPLC with electrochemical detection. With the dialysis tube implanted into the striatum of anaesthetized rats it was possible to measure basal levels of dopamine and the metabolites in the perfusates; dopamine, 0.27 +/- 0.05 pmol/20 min (n = 15), DOPAC 43.3 +/- 2.57 pmol/20 min (n = 15), HVA 24.5 +/- 1.89 pmol/20 min (n = 15) and 5HIAA 13.9 +/- 1.77 pmol/20 min (n = 15). The % recoveries of the monoamines through the membrane were estimated to be 12% (dopamine), 21% (DOPAC), 23% (HVA) and 25% (5HIAA). Apomorphine 0.05-0.2 mg/kg decreased the spontaneous release of dopamine by a maximum of approximately 50%. When the dose of apomorphine was raised up to 0.5 mg/kg there was a 100% inhibition of dopamine release. Also, the extracellular levels of the metabolites DOPAC and HVA decreased following apomorphine administration; however there was no consistent change in 5HIAA. These findings indicate that the dopamine autoreceptors decrease dopamine release in vivo by 0-50% while larger decreases probably involve postsynaptic neurons engaging short- as well as long-loop reflexes.

Intracerebral microdialysis: I. Experimental studies of diffusion kinetics

Intracerebral microdialysis is a brain perfusion technique in which a tubular, semipermeable membrane perfused with a physiological solution is implanted into a selected brain region. Molecules in the extracellular space diffuse into the perfusate and may be recovered and their concentration determined. Hence, the level of substances such as neurotransmitters may be monitored, and the response to different treatments may be studied. The technique also allows for administration of substances locally to the region of the brain surrounding the perfused tubular membrane. Basic principles of the microdialysis technique are described, and the results from methodological experiments are examined. It is concluded that there is a direct linear relation between the concentration of a molecule in the medium surrounding the dialysis membrane and the concentration measured in the collected perfusate. Relative changes of molecular concentration in brain extracellular space may be calculated even when the molecular diffusion rate is unknown. In addition, a method is presented for calculating the real concentration of a substance in the extracellular space from its concentration in the perfusate. Applied in striatum of rat brain using microdialysis in vivo, the average extracellular concentration of the following substances is estimated to be: substance P, 0.9 nM; dopamine, 1 microM; and dihydroxyphenylacetic acid, 0.05 mM.

Neurometabolic monitoring of the ischaemic human brain using microdialysis

Recent animal research has provided evidence that brain ischaemia is associated with a shift of energy related metabolites (lactate, adenosine, inosine, and hypoxanthine) and several transmitters from the intracellular to the extracellular fluid (ECF). These chemical changes of the ECF reflect the energy crisis of the ischaemic tissue. We have proposed that measurement of these metabolites in the ECF using microdialysis may be a useful technique for detection of secondary ischaemia in neurosurgical intensive care patients. As a first step in the evaluation of such a possible clinical application of microdialysis the aim of this study was to measure energy related metabolites and amino acid transmitters during cerebral ischaemia in man. Microdialysis probes were inserted in tumour-free cortical tissue in the frontal lobe in patients undergoing frontal lobe resection as a treatment for brain tumours. Dialysis samples were collected in 10 minutes fractions before and during frontal lobe resection, thus serving as a simulated ischaemia model. The resection procedure was associated with markedly elevated levels of the energy related metabolites as well as transmitter amino acids. The tissue surrounding the probes was examined histologically, and the degree of oedema was estimated from CT scans. In two of the patients the tissue hosting the probes was oedematous. These patients had markedly higher basal levels of lactate. The main conclusions were (1) that the results support the clinical potential of microdialysis, (2) that lactate may be a sensitive indicator of the metabolic disturbances associated with brain oedema, and (3) that frontal lobe resection may be a useful human brain ischaemia model.