Intracerebral microdialysis in the study of physiology and behaviour

Catecholamines: Knowledge and understanding in the 1960s, now, and in the future

The late 1960s was a heyday for catecholamine research. Technological developments made it feasible to study the regulation of sympathetic neuronal transmission and to map the distribution of noradrenaline and dopamine in the brain. At last, it was possible to explain the mechanism of action of some important drugs that had been used in the clinic for more than a decade (e.g. the first generation of antidepressants) and to contemplate the rational development of new treatments (e.g. l-dihydroxyphenylalanine therapy, to compensate for the dopaminergic neuropathy in Parkinson’s disease, and β₁-adrenoceptor antagonists as antihypertensives). The fact that drug targeting noradrenergic and/or dopaminergic transmission are still the first-line treatments for many psychiatric disorders (e.g. depression, schizophrenia, and attention deficit hyperactivity disorder) is a testament to the importance of these neurotransmitters and the research that has helped us to understand the regulation of their function. This article celebrates some of the highlights of research at that time, pays tribute to some of the subsequent landmark studies, and appraises the options for where it could go next.

A novel electrochemical approach for prolonged measurement of absolute levels of extracellular dopamine in brain slices

Tonic dopamine (DA) levels influence the activity of dopaminergic neurons and the dynamics of fast dopaminergic transmission. Although carbon fiber microelectrodes and fast-scan cyclic voltammetry (FSCV) have been extensively used to quantify stimulus-induced release and uptake of DA in vivo and in vitro, this technique relies on background subtraction and thus cannot provide information about absolute extracellular concentrations. It is also generally not suitable for prolonged (>90 s) recordings due to drift of the background current. A recently reported, modified FSCV approach called fast-scan controlled-adsorption voltammetry (FSCAV) has been used to assess tonic DA levels in solution and in the anesthetized mouse brain. Here we describe a novel extension of FSCAV to investigate pharmacologically induced, slowly occurring changes in tonic (background) extracellular DA concentration, and phasic (stimulated) DA release in brain slices. FSCAV was used to measure adsorption dynamics and changes in DA concentration (for up to 1.5 h, sampling interval 30 s, detection threshold < 10 nM) evoked by drugs affecting DA release and uptake (amphetamine, l-DOPA, pargyline, cocaine, Ro4-1284) in submerged striatal slices obtained from rats. We also show that combined FSCAV-FSCV recordings can be used for concurrent study of stimulated release and changes in tonic DA concentration. Our results demonstrate that FSCAV can be effectively used in brain slices to measure prolonged changes in extracellular level of endogenous DA expressed as absolute values, complementing studies conducted in vivo with microdialysis.

Upregulation of endogenous neurotrophin levels in the brain by intranasal administration of carnosic acid

The potential of intranasally administered carnosic acid to enhance the endogenous levels of neurotrophins [nerve growth factor and brain-derived neurotrophic factor] in the brain was investigated. Hydroxypropyl-β-cyclodextrin (HP-β-CD) was used to enhance the aqueous solubility of carnosic acid. The effect of different concentrations of chitosan on the permeation of carnosic acid was investigated across the bovine olfactory mucosa using Franz diffusion cell setup. The formulations were administered [intranasal (i.n.)/subcutaneous route] in Sprague-Dawley rats, and the neurotrophins were sampled from the brain by microdialysis after the treatment period and measured by enzyme-linked immunosorbent assay. Phase solubility studies revealed that the solubility of carnosic acid was enhanced significantly with increase in the concentration of HP-β-CD. The neurotrophin levels were enhanced significantly upon i.n. administration of carnosic acid with chitosan, which was approximately 1.5-2-fold more over the parenteral route. Nose-to-brain delivery of carnosic acid along with chitosan is a potential approach for treating disorders associated with depletion of neurotrophins.

Behavioral and neurochemical effects of chronic L-DOPA treatment on nonmotor sequelae in the hemiparkinsonian rat

Depression and anxiety are the prevalent nonmotor symptoms that worsen quality of life for Parkinson's disease (PD) patients. Although dopamine (DA) cell loss is a commonly proposed mechanism, the reported efficacy of DA replacement therapy with L-DOPA on affective symptoms is inconsistent. To delineate the effects of DA denervation and chronic L-DOPA treatment on affective behaviors, male Sprague-Dawley rats received unilateral 6-hydroxydopamine or sham lesions and were treated daily with L-DOPA (12 mg/kg+benserazide, 15 mg/kg, subcutaneously) or vehicle (0.9% NaCl, 0.1% ascorbic acid) for 28 days before commencing investigations into anxiety (locomotor chambers, social interaction) and depression-like behaviors (forced swim test) during the OFF phase of L-DOPA. One hour after the final treatments, rats were killed and striatum, prefrontal cortex, hippocampus, and amygdala were analyzed through high-performance liquid chromatography for monoamine levels. In locomotor chambers and social interaction, DA lesions exerted mild anxiogenic effects. Surprisingly, chronic L-DOPA treatment did not improve these effects. Although DA lesion reduced climbing behaviors on day 2 of exposure to the forced swim test, chronic L-DOPA treatment did not reverse these effects. Neurochemically, L-DOPA treatment in hemiparkinsonian rats reduced norepinephrine levels in the prefrontal cortex, striatum, and hippocampus. Collectively, these data suggest that chronic L-DOPA therapy in severely DA-lesioned rats does not improve nonmotor symptoms and may impair nondopaminergic processes, indicating that long-term L-DOPA therapy does not exert necessary neuroplastic changes for improving affect.

Delivery of nerve growth factor to brain via intranasal administration and enhancement of brain uptake

The main objective of the study was to investigate the efficacy of chitosan to facilitate brain bioavailability of intranasally administered nerve growth factor (NGF). In vitro permeability studies and electrical resistance studies were carried out across the bovine olfactory epithelium using Franz diffusion cells. The bioavailability of intranasally administered NGF in rat hippocampus was determined by carrying out brain microdialysis in Sprague-Dawley rats. The in vitro permeation flux across the olfactory epithelium of NGF solution without chitosan (control) was found to be 0.37 +/- 0.06 ng/cm(2)/h. In presence of increasing concentration of chitosan (0.1%, 0.25%, and 0.5%, w/v) the permeation flux of NGF was found to be 2.01 +/- 0.12, 3.88 +/- 0.19, and 4.12 +/- 0.21 ng/cm(2)/h respectively. Trans-olfactory epithelial electrical resistance decreased approximately 34.50 +/- 4.06% in presence of 0.25% (w/v) chitosan. The C(max) in rats administered with 0.25% (w/v) chitosan and NGF was 1008.62 +/- 130.02 pg/mL, which was significantly higher than that for rats administered with NGF only 97.38 +/- 10.66 pg/mL. There was approximately 14-fold increase in the bioavailability of intranasally administered NGF with chitosan than without chitosan. Chitosan can enhance the brain bioavailability of intranasally administered NGF.

Microdialysis: is it ready for prime time?

PURPOSE OF REVIEW

This review highlights recent advances in cerebral microdialysis for investigational and clinical neurochemical monitoring in patients with critical neurological conditions.

RECENT FINDINGS

Use of microdialysis with other methods, including PET, electrophysiological monitoring and brain tissue oximetry in traumatic brain injury, subarachnoid hemorrhage with vasospasm, and infarction with refractory increased intracranial pressure have been reported. Potentially adverse neurochemical effects of nonconvulsive status epilepticus and cortical slow depolarization waves, both of which are increasingly recognized in traumatic brain injury and stroke patients, have been reported. The explosive growth in the use of cerebral oximetry with targeted management of brain tissue oxygen levels is leading to greater understanding of derangements of cerebral bioenergetics in the critically ill brain, but there remain unresolved basic issues. Understanding of the analytes that are measurable at the bedside - glucose, lactate, pyruvate, glutamate and glycerol - continues to evolve with glucose, lactate, pyruvate and the lactate-pyruvate ratio taking center stage. Analytes including inflammatory biomarkers such as cytokines and metabolites of nitric oxide are presently investigational, but hold promise for future application in advancing our understanding of basic pathophysiology, therapeutic target selection and prognostication. Growing consensus on indications for use of clinical microdialysis and advances in commercially available equipment continue to make microdialysis increasingly 'ready for prime time.'

SUMMARY

Cerebral microdialysis is an established tool for neurochemical research in the ICU. This technique cannot be fruitfully used in isolation, but when combined with other monitoring methods provides unique insights into the biochemical and physiological derangements in the injured brain.

Microdialysis measures of functional increases in ACh release in the hippocampus with and without inclusion of acetylcholinesterase inhibitors in the perfusate

Because brain extracellular acetylcholine (ACh) levels are near detection limits in microdialysis samples, an acetylcholinesterase (AChE) inhibitor such as neostigmine is often added to microdialysis perfusates to increase ACh levels in the dialysate, a practice that raises concerns that the inhibitor might alter the results. Two experiments compared functional differences in ACh release with and without neostigmine. In the first experiment, 30-60% increases in extracellular ACh concentrations in the hippocampus were evident during food-rewarded T-maze training with 20-500 nm neostigmine in the perfusate but no increases were seen without neostigmine. In the second experiment, 78% increases in ACh release in the hippocampus were seen after injections of the GABA(A) receptor antagonist, bicuculline, into medial septum only if neostigmine (50 nm) was included in the perfusate. These findings suggest that, in the hippocampus, endogenous brain AChEs are very efficient at removing extracellular ACh, obscuring differences in ACh release in these experiments. Therefore, inclusion of AChE inhibitors in the microdialysis perfusate may be necessary under some conditions for observations of functional changes in release of ACh in the hippocampus.

Derivatization chemistries for determination of serotonin, norepinephrine and dopamine in brain microdialysis samples by liquid chromatography with fluorescence detection

The present paper provides an overview on currently developed derivatization chemistries and techniques for determination of monoamine neurotransmitters serotonin (5-HT), norepinephrine (NE) and dopamine (DA) in microdialysis samples by microbore liquid chromatography with fluorescence detection. In mild alkaline conditions, 5-hydroxyindoles and catecholamines react with benzylamine (BA), forming highly fluorescent 2-phenyl-4,5-pyrrolobenzoxazoles and 2-phenyl(4,5-dihydropyrrolo) [2,3-f]benzoxazoles, respectively. However, for derivatization of DA a higher fluorescence intensity was achieved for reaction with 1,2-diphenylethylenediamine (DPE) rather than with BA, therefore for simultaneous determination of 5-HT, NE and DA in brain microdialysates, a two-step derivatization with BA followed by DPE was developed. The detection limits for 5-HT, NE and DA were 0.2, 0.08 and 0.13 fmol, respectively, in an injection volume of 20 microL, which corresponds to concentrations of 30, 12 and 19.5 pm, respectively in standard solution prior to derivatization. The experimental data presented demonstrate the ability of the technique to simultaneously monitor neuronally releasable pools of monoamine neurotransmitters in the rat and mouse brains at basal conditions and following pharmacological treatments or physiological stimuli. These techniques play an important role in drug discovery and clinical investigation of psychiatric and neurological diseases such as depression, schizophrenia and Parkinson's disease.

The role of dopamine in conditioning and latent inhibition: what, when, where and how?

It is well established that dopamine is released in the nucleus accumbens (NAC) in animals in rewarding or reinforcing situations, and widely believed that this release is the substrate of, or at least closely related to, the experience of reward. The demonstration of conditioned release of dopamine by stimuli conditioned to primary rewards has reinforced this view. However, a number of observations do not sit comfortably with this interpretation, most notably that dopamine is released equally effectively in NAC by aversive stimuli, and stimuli conditioned to them. Furthermore, additional release of dopamine is seen during conditioning, even if motivational stimuli of either type are not involved. It is suggested here that one important action of NAC dopamine release is to restore the salience of potential conditioned stimuli, when this has been reduced by prior un-reinforced experience. The paradigm of latent inhibition (LI) demonstrates a behavioural effect of this type, and extensive studies on the role of dopamine in LI have been undertaken by us and others. Those studies are reviewed here, together with some previously unpublished data, to demonstrate that (1) amphetamine disruption of LI is indeed a function of calcium-dependant dopamine release in the NAC at the time of conditioning; (2) other drugs acting on LI via changes in dopamine transmission act at the same locus; (3) the disruptive effect of indirect dopamine agonists on LI can be prevented by either D-1 selective receptor antagonists, or D-2 selective receptor antagonists. It is concluded that dopamine release in these very varied behavioural contexts (reward, punishment, conditioning, modulation of salience) must be differentiated in some way, and that this should be investigated. An alternative explanation, if they are not differentiated, would be that the release in fact does have the same functional significance in each case. We suggest that this common significance might be the broadening of attention to take in potentially conditionable stimuli, which have previously been devalued.

Increased extracellular dopamine in nucleus accumbens in response to unconditioned and conditioned aversive stimuli: studies using 1 min microdialysis in rats

Previous microdialysis studies measuring extracellular dopamine levels in response to unconditioned and conditioned aversive stimuli have used relatively long (e.g. 10 min) sample durations, such that more than one stimulus event occurred within a single dialysis sample. The present study used 1 min dialysate sampling to measure changes in dopamine levels in response to individual stimulus presentations. The changes evoked by mild footshock showed an initial enhancement from the first to the second presentation, after which there was a steady decline in the response over subsequent presentations. Compared to the responses to footshock alone, when the footshock was paired with an unfamiliar tone, there was no change in the response to the first stimulus presentation, but a significant augmentation of responses during subsequent presentations, giving weight to the view that dopamine is not involved in the learning per se, but rather in the processing of learned information. Whilst an unfamiliar tone had no measurable effect on extracellular dopamine levels, the same tone which had previously been paired with footshock evoked a significant increase in dopamine during the tone presentation, indicating that it is the aversive nature of the stimulus onset rather than the 'rewarding' nature of its offset which increases extracellular dopamine.

Learning a new behavioral strategy in the shuttle-box increases prefrontal dopamine

Using microdialysis from medial prefrontal cortex of gerbils during aversive auditory conditioning in the shuttle-box we have previously shown a transient increase of dopamine efflux correlated with the establishment of avoidance behavior. We hypothesized that the acquisition of a new behavioral strategy is generally accompanied by this extra prefrontal dopamine release. The present experiment aimed at further testing this hypothesis. In a pre-training period in the shuttle-box the gerbils acquired an active avoidance response by generalizing two different tone signals to a GO-meaning (change of shuttle-box compartment). Thereafter, they were subjected in relearning sessions to differentially associate the known tone stimuli with GO- and NOGO- (no change of shuttle-box compartment) conditions, respectively. The following formation of discrimination behavior led to a similar extra dopamine increase as found during establishment of the avoidance strategy. This significant enhancement was limited to rapidly relearning individuals. Furthermore, the dopamine increase attenuated in these animals with increasing performance during the course of the discrimination training, similar to the retrieval stage of the avoidance strategy. Therefore, the dopamine system seems to be critically involved in the initial formation of associations for new behavioral strategies, i.e. learning. We assume that the prefrontal dopamine increase during initial learning of the complex discrimination behavior indicates an involvement of working memory principles and a goal-directed formation of a behavioral strategy.

Nitric oxide as modulator of neuronal function

The gas NO is a messenger that modulates neuronal function. The use of NO donors and NO synthase inhibitors as pharmacological tools revealed that this free radical is probably implicated in the regulation of excitability and firing, in long-term potentiation and long-term depression, as well as in memory processes. Moreover, NO modulates neurotransmitter release. In vivo and in vitro studies have shown that, in all brain structures investigated, endogenous NO modulates the release of several neurotransmitters, such as acetylcholine, catecholamines, excitatory and inhibitory amino acids, serotonin, histamine, and adenosine. In most cases, enhanced NO level in the tissue increases the release of neurotransmitters, although decreasing effects have also been observed. Cyclic 3'-5' guanosine monophosphate and glutamate mediate the modulation of transmitter release by NO. Recent observations suggest that the release of some transmitters is dually influenced by NO. Thus, besides modulation by presynaptically located auto- and heteroreceptors, NO released from nitrergic neurons seems to play a universal role in modulating the release of transmitters in the brain.

Activation of the dopaminergic system of medial prefrontal cortex of gerbils during formation of relevant associations for the avoidance strategy in the shuttle-box

1. A detailed analysis of behavior is a prerequisite for identification of components of information processing during learning. 2. Components of shuttle-box learning like the signal detection and signal evaluation can be differentiated using behavioral events such as the attention response and the orienting response. 3. Chiefly during evaluation of signal meaning in the acquisition phase of the avoidance strategy the extracellular DA is increased in mPFC. 4. The kinetics of prefrontal dopaminergic activation from trial to trial depends on the stage of avoidance learning. 5. The increase of DA in mPFC can be an indicator for the involvement of working memory principles in signal evaluation stages of conditioning.

Intracerebral microdialysis and its clinical application: a review

In vivo intracerebral microdialysis is an important neurochemical technique that has been used extensively in the experimental setting. Relatively recently, techniques have been developed to utilize this method in human subjects. The past decade has seen the advent of clinical investigations utilizing in vivo microdialysis in a number of neuropathological states. This review summarizes the principles of in vivo microdialysis techniques, as applied to humans, while discussing the significance of recent investigations for future clinical development.

In vivo microdialysis in an animal model of neurological disease: thiamine deficiency (Wernicke) encephalopathy

In vivo microdialysis allows for the constant monitoring of brain neurotransmitters in the extracellular fluid of awake and freely moving animals. Considerations including factors affecting probe recoveries, the blood-brain barrier, and tissue reactions to probe implantation are discussed in this paper. Details of the application of in vivo microdialysis to an animal model of encephalopathy are then presented. Thiamine deficiency encephalopathy is an animal model of Wernicke encephalopathy, a neurological disorder observed in alcoholics and in patients with severely compromised nutrition. Regionally selective neuronal cell death is observed in both patients and animals with thiamine deficiency (TD). Various thalamic nuclei suffer significant TD-induced cell death, and NMDA receptor-mediated glutamate excitotoxicity has been proposed as an underlying causative factor. A detailed methodology for the examination of the role of glutamate excitotoxicity using in vivo microdialysis in the neuronal cell death due to thiamine deficiency is presented.

Stages of avoidance strategy formation in gerbils are correlated with dopaminergic transmission activity

This detailed analysis of behavior is aimed at the differentiation of the components of information processing during associative conditioning. In gerbils, the influences of various acquired non-avoidance strategies as pre-experience were studied during the learning of a standard avoidance task in the same shuttle-box. Identical cue stimuli, frequency-modulated tones as conditioned stimuli and electric footshocks as unconditioned stimuli, were used in various behavioral tasks. In addition to common parameters such as avoidance performance and reaction times, behavioral events such as the attention response and the orienting response were quantified. Thereby, components of shuttle-box learning such as signal detection and signal evaluation were found to be affected by pre-experience-dependent dynamics. Using a microdialysis technique during avoidance learning in the shuttle-box, we found that only strategy formation was correlated with high dopamine levels in medial prefrontal cortex. The increase in dopamine in medial prefrontal cortex may be an indicator of the involvement of working memory principles in signal evaluation stages of conditioning.

Intracranial self-stimulation increases differentially in vivo hydroxylation of tyrosine but similarly in vivo hydroxylation of tryptophan in rat medial prefrontal cortex, nucleus accumbens and striatum

We have examined using microdialysis the effect of intracranial self-stimulation (ICSS) on the in vivo hydroxylation rate of tyrosine and tryptophan in the medial prefrontal cortex (mPFC), nucleus accumbens (NAC) and striatum (STR). A decarboxylase inhibitor NSD-1015 was included in the perfusate, which enabled the simultaneous measurement of 3,4-dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) as an index of the in vivo hydroxylation level of tyrosine and tryptophan. When rats were exposed to 1 h of ICSS at the medial forebrain bundle (MFB), their extracellular levels of DOPA significantly increased in the mPFC, NAC and STR, but with a different magnitude and time course. The same stimulation produced a delayed increase in extracellular 5-HTP, compared to DOPA, in these brain regions. The profile of 5-HTP response demonstrated no apparent difference among the regions. These findings indicate that ICSS of the MFB can increase differentially the in vivo hydroxylation of tyrosine but similarly the in vivo hydroxylation of tryptophan in the mPFC, NAC and STR.

Measurement of cholecystokinin release in vivo in the rat spinal dorsal horn

The microdialysis technique, used to monitor extracellular levels of transmitter substances in the central nervous system of laboratory animals as a reflection of transmitter release, is based on the ability of neurotransmitters to diffuse in the extracellular fluid from the site of release and to cross a semipermeable dialysis membrane. Even though the surgical procedure is not very complicated, the detection of released substances in the recovered dialysate may be difficult. Especially, the measurement of neuropeptide release is limited by the low extracellular concentration and of low recovery as compared to, for example, monoamines. Thus, for example, cholecystokinin (CCK), which is the most abundant neuropeptide in the central nervous system, is found at concentrations that are several orders of magnitude lower than those of classical transmitters. Therefore a highly sensitive detection method is of utmost importance. In the dorsal horn of the spinal cord CCK is found mainly in interneurons and in terminals of descending fibers. CCK seems to be involved in nociceptive transmission and CCK attenuates morphine-induced antinociception. We here describe in vivo microdialysis in the lumbar dorsal horn of the rat with subsequent quantification of the level of CCK-like immunoreactivity (-LI) by a highly sensitive radioimmunoassay.

Measurement of interstitial cetirizine concentrations in human skin: correlation of drug levels with inhibition of histamine-induced skin responses

BACKGROUND

The purpose of the present study was to measure the concentrations of cetirizine in the extracellular water compartment in intact human skin and assess simultaneously inhibition of histamine-induced wheal and flare reactions.

METHODS

Skin cetirizine levels were collected by the microdialysis technique and analyzed by high-pressure liquid chromatography with mass spectrometry detection. Skin levels in 20 subjects were compared to plasma levels for 4 h after a single oral dose of 10 or 20 mg of cetirizine. Skin prick tests were performed with histamine 100 mg/ml.

RESULTS

Plasma cetirizine levels increased within 30 min to reach peak values of 315+/-10 and 786+/-45 ng/ml 90-120 min after administration of 10 and 20 mg of cetirizine. This was followed by a slow decline. In the skin, dialysate cetirizine levels (non-protein-bound fraction only) peaked at 1.6+/-0.1 and 2.4+/-0.3 ng/ml at 120-180 min. In vivo recovery of cetirizine was 14.4+/-4.3%. It was estimated that the non-protein-bound concentration of cetirizine in the skin was 50-70% of corresponding plasma values. Both 10- and 20-mg doses of cetirizine inhibited wheal and flare reactions over 240 min. The time vs concentration profile of cetirizine in skin dialysate paralleled the inhibition of skin reactions, but no significant correlations were found between individual cetirizine levels in skin or plasma with wheal and flare reactions.

CONCLUSIONS

Cetirizine concentrations in the skin could be monitored by the microdialysis technique. The results indicate no simple linear correlation between cetirizine skin levels and inhibition of skin reactions.

Differential effect of immobilization stress on in vivo synthesis rate of monoamines in medial prefrontal cortex and nucleus accumbens of conscious rats

We have used microdialysis to measure the in vivo hydroxylation level of tyrosine and tryptophan in the medial prefrontal cortex and nucleus accumbens of conscious rats that were subjected to immobilization. The brain was perfused with an inhibitor of aromatic L-amino acid decarboxylase, 3-hydroxybenzylhydrazine, and the amount of 3,4-dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) accumulating in the dialysate was measured as an index of the in vivo hydroxylation rate of tyrosine and tryptophan. One hour of immobilization caused a significant increase in extracellular DOPAin the medial prefrontal cortex but not nucleus accumbens. The same manipulation produced a significant and more prolonged elevation in extracellular 5-HTP in the nucleus accumbens as well as medial prefrontal cortex. The observed profile of stress-induced 5-HTP response was comparable in two brain regions. The results suggest that in vivo catecholamine synthesis is heterogenous, whereas in vivo serotonin synthesis is homogenous, with respect to responsiveness to stress in the medial prefrontal cortex and nucleus accumbens.

Increase of extracellular dopamine in prefrontal cortex of gerbils during acquisition of the avoidance strategy in the shuttle-box

We examined whether extracellular dopamine (DA) increase in medial prefrontal cortex is correlated with the establishment or with the retrieval of an auditory avoidance strategy in a shuttle-box. Using microdialysis from right medial prefrontal cortex (mPFC) in combination with behavioral measures, gerbils were trained on a tone-footshock combination in two sessions on two successive days. Sessions in each animal involving avoidance strategy formation and avoidance strategy retrieval were differentiated post hoc by using the percentage of conditioned responses (CR) and other behavioral measures of training sessions. It was found that exclusively strategy formation correlated with high DA levels in mPFC.

Changes in blood-brain barrier permeability associated with insertion of brain cannulas and microdialysis probes

Blood-brain barrier (BBB) transcapillary transport was studied after insertion of cannulas and microdialysis probes into the brains of three groups of rats. Quantitative autoradiography was used to measure changes in BBB permeability around the insertion site. In the first group, BBB function was measured with 14C-sucrose at times from immediately, and up to 28 days, after insertion of a microdialysis probe. BBB function was disrupted biphasically: a 19-fold increase in the influx constant (K1) of sucrose occurred immediately after insertion with a second 17-fold increase at 2 days, followed by a slow decline to 5 times normal values at 28 days. In the second group, 14C-dextran (70 kDa) was used to measure BBB transcapillary transport; K1 was increased 90-fold after probe insertion. In the 3rd group, 14C-AIB (alpha-aminoisobutyric acid) was used to evaluate BBB transport after insertion of a 27 gauge cannula, which was used to infuse 1 microliter of saline over 5 min. The K1 of AIB was increased 25 times control values. We conclude that BBB transcapillary transport function is disturbed in response to insertion of brain cannulas and/or microdialysis probes, that BBB dysfunction is maximal at the cannula or probe tip, varies with time after insertion, may persist for at least 28 days after insertion, and occurs over a wide molecular range of solutes. These results suggest caution when using microdialysis as a method to study normal BBB function, and suggest that microdialysis may overestimate the rate of transfer into and out of the brain.

Increased extracellular dopamine in the nucleus accumbens of the rat during associative learning of neutral stimuli

Brain microdialysis was used to study changes in dopamine in the nucleus accumbens and the dorsal striatum during associative learning between two neutral stimuli, flashing light and tone, presented on a paired schedule during stage 1 of a sensory preconditioning paradigm. The tone was subsequently paired with mild footshock using standard aversive conditioning procedures and the formation of a conditioned association between the flashing light and the tone in stage 1 was assessed by measuring the ability of the flashing light to elicit the same conditioned response as the tone when presented at test. The first experiment used behavioural monitoring only, to establish stimulus parameters for subsequent microdialysis experiments. Animals receiving paired presentation of the light and tone in stage 1 showed a conditioned suppression of licking to the light as well as to the tone, indicating that associative learning between the flashing light and the tone had occurred during stage 1, whilst in a separate group of animals given the same stimuli over the same time period but on an explicitly non-paired schedule, the conditioned emotional response was seen to the tone, but not to the light, showing that no association had been formed between the two stimuli during stage 1. In dialysis experiments using the same procedure, we measured a two-fold rise in dopamine in the nucleus accumbens during paired presentation of flashing light and tone, but not during non-paired presentation of the two stimuli. On subsequent test presentation of the two stimuli, we saw increases in accumbal dopamine on presentation of the tone in both groups, reflecting the formation of an association with the footshock in both. However the flashing light elicited an increase in dopamine only in the group which had received paired presentation at stage 1. Thus accumbal dopamine release at test is correlated to the ability of the stimulus to evoke a conditioned response measured behaviourally. Hypotheses of the behavioural function of the mesolimbic dopamine system centre on its role in mediating the effects of biological reinforcers, both rewarding and aversive, conditioned and unconditioned. The present results, showing increases in extracellular dopamine in the nucleus accumbens when an association is formed between two stimuli of which neither is a biological reinforcer nor, prior to formation of the association, affects dopamine levels, suggest a role for accumbal dopamine in the modulation of associative learning in general, not only that involving reinforcement.

Opioid-induced release of neurotensin in the periaqueductal gray matter of freely moving rats

The midbrain periaqueductal gray matter (PAG) is an important region for endogenous pain suppression. Nerve terminals containing opioid peptides and neurotensin (NT), as well as high densities of opioid- and NT-receptors, have been demonstrated in the ventromedial PAG. Local administration of opioids or NT in this region induces antinociception in experimental animals. In the present microdialysis study, the effect of opioids on the release of NT in the ventromedial PAG was investigated. Perfusion of the microdialysis probe with 10 microM morphine induced a significant increase (P < 0.05; n = 5) of the extracellular level of NT-like immunoreactivity (NT-LI), while perfusion with a 10-fold higher concentration of morphine had no significant effect on the NT-LI release in the PAG. Also perfusion of the dialysis probe with the mu-opioid receptor-specific agonist [D-Ala2-N-Me-Phe4-Gly5-ol]-enkephaline (DAGO) (1 or 100 microM) induced a significant (P < 0.05; n = 7-9) increase of the NT-LI level. The increase in NT-LI release in response to 1 microM DAGO was both calcium-dependent and naloxone-reversible. Since opioid agonists generally inhibit neuronal activity, an indirect mechanism, involving inhibition of tonically active inhibitory neurons, e.g. gamma-aminobutyric acid (GABA) neurons, could be of importance for the opioid induced release of NT. However, local administration in the PAG of the GABA(A) antagonist bicuculline (0.1-10 microM) or the GABA(A) agonist muscimol (1-100 microM) had no significant effect on the extracellular NT-LI level in the PAG, suggesting that GABAergic mechanisms are not involved in the opioid-induced release of NT-LI. In conclusion, the present data provide in vivo evidence that mu-opioid receptors mediate stimulation of neurotensin release in the PAG.

Distinct activation of monoaminergic pathways in chick brain in relation to auditory imprinting and stressful situations: a microdialysis study

In the forebrain of the domestic chick (Gallus gallus domesticus), an area termed the mediorostral neostriatum/hyperstriatum ventrale is strongly involved in emotional learning paradigms such as acoustic filial imprinting. Furthermore, the involvement of the mediorostral neostriatum/hyperstriatum ventrale in stressful situations, such as social separation, has been demonstrated in 2-deoxyglucose studies. The aim of the present study was to examine whether quantitative changes of dopamine, serotonin and their metabolites occur during auditory filial imprinting and during social separation. Using in vivo microdialysis in tone-imprinted and in naive, control chicks, we compared the extracellular levels of homovanillic acid, a metabolite of dopamine, and 5-hydroxyindoleacetic acid, a metabolite of serotonin, during the presentation of the imprinting tone. A small, but statistically significant, decrease of extracellular homovanillic acid levels was found in the mediorostral neostriatum/hyperstriatum ventrale of imprinted chicks compared to control animals, whereas changes of 5-hydroxyindoleacetic acid were not detected. In a second experiment, we investigated the levels of homovanillic acid and 5-hydroxyindoleacetic acid in the mediorostral neostriatum/hyperstriatum ventrale of socially reared chicks during different stress situations, such as handling or separation from their cage mates. Handling induced a significant increase of homovanillic acid and 5-hydroxyindoleacetic acid, while social separation resulted in a significant increase of 5-hydroxyindoleacetic acid and only a slight increase of homovanillic acid. Despite considerable inter-individual variability, the increase of distress vocalizations (duration of distress calls) after social separation displayed a good correlation to the increased 5-hydroxyindoleacetic acid levels in all animals analysed. These results provide the first evidence that the physiological response of the mediorostral neostriatum/hyperstriatum ventrale related to different emotional conditions after acoustic imprinting and during stressful situations is, at least in part, mediated by dopaminergic and/or serotonergic pathways. Furthermore, the results from the present study indicate a distinct activation of dopaminergic and serotonergic pathways in relation to the behavioural situation and the associated changes of emotional status.

Rapid sampling of extracellular dopamine in the rat prefrontal cortex during food consumption, handling and exposure to novelty

We report the effects of physiological stimuli on extracellular dopamine (DA) in the medial prefrontal cortex (PFC) of the rat determined on-line in dialysates obtained every 5.5 min. The detection limit for DA was 0.03-0.1 pg/5 microl injection using a conventional HPLC set-up. Basal levels in PFC were at the detection limit, therefore 3 microM nomifensine was included in the Ringer perfusion fluid, producing readily detectable DA levels of 0.9 pg/injection. Perfusion with 3 microM TTX for 30 min decreased DA within 11 min to 10% of control. The routine use of rapid sampling of extracellular DA was applied to study cortical DA release in relation to behaviour. Exposure to a novel environment for 5.5 min led to an increase to 135%. Presentation of a food pellet to food-deprived rats resulted in a rapid increase to 150% within 5.5 min, which lasted 30-40 min, which is 10-20 min more than the time spent eating. Handling the rat for 5.5 min increased DA in PFC within 5.5 min to 160% and in 11 min to 190% of control followed by a 25-min period of a 50% increase, probably reflecting increased arousal. The results suggest that emotional arousal is a common denominator of increased cortical DA release and that responses are graded depending on the intensity of the stimulus.

Allergen-induced histamine release in intact human skin in vivo assessed by skin microdialysis technique: characterization of factors influencing histamine releasability

BACKGROUND

The purposes of the study were to characterize allergen-induced histamine release in intact human skin in vivo by using a novel microdialysis technique and to study covariates influencing histamine releasability.

METHODS

Hollow microdialysis fibers were inserted into the upper dermis in 15 timothy-sensitivity subjects. Up to 12 fibers were inserted in each subject. Each fiber was perfused with Krebs-Ringer's solution at a rate of 3.0 microliters/min. Three to four serial dilutions of allergen were applied to the skin by intracutaneous injections or skin prick test above individual fibers. Samples were collected in two 2-minute fractions before skin challenge and in 10 consecutive samples for 20 minutes after skin challenge. Histamine was assayed spectrofluorometrically.

RESULTS

A significant dose-response relationship for histamine release was demonstrated with intracutaneous tests and skin prick tests. The time to reach peak histamine release after an intracutaneous test was 4 to 8 minutes, compared with 12 to 14 minutes for a skin prick test. Histamine release correlated significantly with wheal size. Intrasubject coefficient of variation on histamine release was about 20%. A substantial intersubject variation in histamine releasability was observed. Seventy to seventy-five percent of the variation could be accounted for by a combination of gender, total and allergen-specific IgE, and an in vitro basophil histamine release test.

CONCLUSIONS

Using a skin microdialysis technique, we have described in detail histamine release in intact human skin by allergen. The microdialysis method proved to be a reproducible technique for monitoring histamine release in allergic skin reactions and for studying histamine releasability of skin mast cells in vivo.

Asymmetrical motor behavior in rats with unilateral striatal excitotoxic lesions as revealed by the elevated body swing test

Severe degeneration of basal ganglia neurons, particularly the intrinsic neurons of the striatum, is the major underlying neuropathology implicated in clinical attributes of Huntington's disease (HD). The excitotoxin-lesioned striatum provides a useful model for evaluating behavioral parameters of HD. Animals with unilateral excitotoxic lesions exhibit asymmetrical rotational behavior in response to dopamine agonists, such as apomorphine. However, the observed behavior is a pharmacological reaction, and subject to sensitization effects. A behavioral test using undrugged animals may demonstrate a more natural response of the animals to the lesion effects. Recently, we have developed the 'drug-free' elevated body swing test (EBST), and demonstrated that hemiparkinsonian rats exhibited significant biased swing activity. In the present study, we observed that animals with unilateral intrastriatal 3-nitropropionic acid or quinolinic acid lesions displayed a significant biased swing activity with the direction ipsilateral to the lesioned side of the brain. This ipsilateral swing corresponded to the ipsilateral rotational behavior exhibited by the lesioned animals when challenged with apomorphine. The present results demonstrated that the EBST is a sensitive measure for characterizing asymmetrical behavior in animals with striatal lesions.