Effect of varying the ionic concentration of a microdialysis perfusate on basal striatal dopamine levels in awake rats

HIV-Associated Apathy/Depression and Neurocognitive Impairments Reflect Persistent Dopamine Deficits

Individuals living with human immunodeficiency virus type 1 (HIV-1) are often plagued by debilitating neurocognitive impairments and affective alterations;the pathophysiology underlying these deficits likely includes dopaminergic system dysfunction. The present review utilized four interrelated aims to critically examine the evidence for dopaminergic alterations following HIV-1 viral protein exposure. First, basal dopamine (DA) values are dependent upon both brain region andexperimental approach (i.e., high-performance liquid chromatography, microdialysis or fast-scan cyclic voltammetry). Second, neurochemical measurements overwhelmingly support decreased DA concentrations following chronic HIV-1 viral protein exposure. Neurocognitive impairments, including alterations in pre-attentive processes and attention, as well as apathetic behaviors, provide an additional line of evidence for dopaminergic deficits in HIV-1. Third, to date, there is no compelling evidence that combination antiretroviral therapy (cART), the primary treatment regimen for HIV-1 seropositive individuals, has any direct pharmacological action on the dopaminergic system. Fourth, the infection of microglia by HIV-1 viral proteins may mechanistically underlie the dopamine deficit observed following chronic HIV-1 viral protein exposure. An inclusive and critical evaluation of the literature, therefore, supports the fundamental conclusion that long-term HIV-1 viral protein exposure leads to a decreased dopaminergic state, which continues to persist despite the advent of cART. Thus, effective treatment of HIV-1-associated apathy/depression and neurocognitive impairments must focus on strategies for rectifying decreases in dopamine function.

Overview of brain microdialysis

The technique of microdialysis enables sampling and collecting of small-molecular-weight substances from the interstitial space. It is a widely used method in neuroscience and is one of the few techniques available that permits quantification of neurotransmitters, peptides, and hormones in the behaving animal. More recently, it has been used in tissue preparations for quantification of neurotransmitter release. This unit provides a brief review of the history of microdialysis and its general application in the neurosciences. The authors review the theoretical principles underlying the microdialysis process, methods available for estimating extracellular concentration from dialysis samples (i.e., relative recovery), the various factors that affect the estimate of in vivo relative recovery, and the importance of determining in vivo relative recovery to data interpretation. Several areas of special note, including impact of tissue trauma on the interpretation of microdialysis results, are discussed. Step-by-step instructions for the planning and execution of conventional and quantitative microdialysis experiments are provided.

Translational neurochemical research in acute human brain injury: the current status and potential future for cerebral microdialysis

Microdialysis (MD) was introduced as an intracerebral sampling method for clinical neurosurgery by Hillered et al. and Meyerson et al. in 1990. Since then MD has been embraced as a research tool to measure the neurochemistry of acute human brain injury and epilepsy. In general investigators have focused their attention to relative chemical changes during neurointensive care, operative procedures, and epileptic seizure activity. This initial excitement surrounding this technology has subsided over the years due to concerns about the amount of tissue sampled and the complicated issues related to quantification. The interpretation of mild to moderate MD fluctuations in general remains an issue relating to dynamic changes of the architecture and size of the interstitial space, blood-brain barrier (BBB) function, and analytical imprecision, calling for additional validation studies and new methods to control for in vivo recovery variations. Consequently, the use of this methodology to influence clinical decisions regarding the care of patients has been restricted to a few institutions. Clinical studies have provided ample evidence that intracerebral MD monitoring is useful for the detection of overt adverse neurochemical conditions involving hypoxia/ischemia and seizure activity in subarachnoid hemorrhage (SAH), traumatic brain injury (TBI), thromboembolic stroke, and epilepsy. There is some data strongly suggesting that MD changes precede the onset of secondary neurological deterioration following SAH, hemispheric stroke, and surges of increased ICP in fulminant hepatic failure. These promising investigations have relied on MD-markers for disturbed glucose metabolism (glucose, lactate, and pyruvate) and amino acids. Others have focused on trying to capture other important neurochemical events, such as excitotoxicity, cell membrane degradation, reactive oxygen species (ROS) and nitric oxide (NO) formation, cellular edema, and BBB dysfunction. However, these other applications need additional validation. Although these cerebral events and their corresponding changes in neurochemistry are important, other promising MD applications, as yet less explored, comprise local neurochemical provocations, drug penetration to the human brain, MD as a tool in clinical drug trials, and for studying the proteomics of acute human brain injury. Nevertheless, MD has provided new important insights into the neurochemistry of acute human brain injury. It remains one of very few methods for neurochemical measurements in the interstitial compartment of the human brain and will continue to be a valuable translational research tool for the future. Therefore, this technology has the potential of becoming an established part of multimodality neuro-ICU monitoring, contributing unique information about the acute brain injury process. However, in order to reach this stage, several issues related to quantification and bedside presentation of MD data, implantation strategies, and quality assurance need to be resolved. The future success of MD as a diagnostic tool in clinical neurosurgery depends heavily on the choice of biomarkers, their sensitivity, specificity, and predictive value for secondary neurochemical events, and the availability of practical bedside methods for chemical analysis of the individual markers. The purpose of this review was to summarize the results of clinical studies using cerebral MD in neurosurgical patients and to discuss the current status of MD as a potential method for use in clinical decision-making. The approach was to focus on adverse neurochemical conditions in the injured human brain and the MD biomarkers used to study those events. Methodological issues that appeared critical for the future success of MD as a routine intracerebral sampling method were addressed.

Differential effects of post-implantation time on potassium- versus D-amphetamine-evoked dopamine overflow in the striatum of F344 rats

Effects of post-implantation time on potassium (K+)- versus D-amphetamine (D-AMPH)-evoked striatal dopamine (DA) overflow were measured using microdialysis in freely moving young and aged Fischer 344 rats. In one group, samples were collected on the day of probe insertion (Day 1 group). In a second group, samples were collected 24 h after probe insertion (Day 2 group). While analyses revealed no significant differences between the two age groups, the 100 mM K+ stimulus evoked a significantly greater amount of DA overflow in the Day 1 group compared to the Day 2 group. The decrease in 3,4-dihydroxyphenylacetic acid (DOPAC) produced by K+ stimulation was not influenced by post-implantation time. The effect of the 250 microM D-AMPH stimulus on DA overflow did not differ between the Day 1 and Day 2 groups, nor did the decrease in DOPAC that accompanied D-AMPH stimulation. These results support the hypothesis that under some stimulus conditions, post-implantation time is an important variable in microdialysis studies.

Utilization of in vivo ultrafiltration in biomedical research and clinical applications

Ultrafiltration (UF) is a filtrate selection method with a wide range of biomedical and clinical applications, including detoxification of blood in hemodialysis and peritoneal dialysis. New is, however, the use of UF as a convenient in vivo sampling method that, for example, has been used in diabetics. Ultrafiltration avoids complicated and time-consuming recovery calculations that are necessary when using in vivo microdialysis, as recoveries of low molecular weight molecules are near 100%. The subcutaneously or intravenously placed UF probes have been studied for off-line sample analysis and for continuous on-line monitoring, in a wide variety of species, including dogs, rats, pigs and humans. This review discusses the potential of in vivo UF as a continuous tissue sampling technique in clinical research areas, and in several major biomedical applications including glucose and lactate monitoring and drug kinetic studies.

Perfusate oxygen and carbon dioxide concentration influence basal microdialysate levels of striatal glucose and lactate in conscious rats

O(2) concentration ([O(2)]) in air equilibrated solutions at room temperature is three fold higher than that in brain extracellular fluid (ECF), and CO(2) concentration ([CO(2)]) is 100 times lower. Using microdialysis the ECF is routinely dialyzed against glucose free isotonic perfusates containing 200 microM O(2) and 10 microM CO(2). In conscious rats, 2 days after probe implantation, decreasing perfusate [O(2)] from 200 to 68 microM (physiologic level) for 60 min, while maintaining a low [CO(2)] (10 microM), increased striatal dialysate glucose and lactate by 12% and 33%, respectively. The same protocol on the third day essentially had no effect on monoamine metabolites. Decreasing [O(2)] concurrent with increasing [CO(2)] to 1.3 mM (physiologic level) for 60 min increased glucose and lactate by 17% and 37%, respectively. This study demonstrates for dialysis studies of glucose and lactate, perfusates that mimic physiologic ECF [O(2)] and [CO(2)] are more appropriate.

Acute and long-term changes in the mesolimbic dopamine pathway after systemic or local single nicotine injections

We have examined several neurochemical and behavioural parameters related to the function of the mesolimbic dopamine (DA) pathway in animals treated with nicotine following three modes of drug administration, i.e. systemic intraperitoneal injection, intra-accumbens (Acb) infusion or intraventral tegmental area (intra-VTA) microinjection. The present modes of systemic, intra-Acb and intra-VTA nicotine administration elicited comparable acute increases in dialysate DA levels from the Acb. The increase in extracellular DA levels was paralleled by a significant enhancement of locomotion in a habituated environment in the case of systemic or intra-VTA nicotine administration, whereas unilateral or bilateral intra-Acb nicotine infusion was ineffective, showing that accumbal DA increase is not sufficient to elicit locomotion in this experimental paradigm. Intra-VTA, but not systemic or intra-Acb, nicotine administration caused a long-term (at least 24-h) increase in basal dialysate DA levels from the Acb. In addition, significant increases in tyrosine hydroxylase (TH) and GluR1 (but not dopamine transporter or NR1) mRNA levels in the VTA were detected 24 h after intra-VTA nicotine administration. Systemic nicotine injection caused only an increase in TH mRNA levels while intra-Acb infusion did not modify any of the mRNAs tested. The long-term increase in basal DA levels in the Acb and TH, and GluR1 mRNA levels in the VTA upon intra-VTA nicotine microinjection indicates that even a single nicotine injection can induce plastic changes of the mesolimbic DA pathway.

Functional striatal hypodopaminergic activity in mice lacking adenosine A(2A) receptors

Adenosine and caffeine modulate locomotor activity and striatal gene expression, partially through the activation and blockade of striatal A(2A) receptors, respectively. The elucidation of the roles of these receptors benefits from the construction of A(2A) receptor-deficient mice (A(2A)-R(-/-)). These mice presented alterations in locomotor behaviour and striatal expression of genes studied so far, which are unexpected regarding the specific expression of A(2A) receptor by striatopallidal neurones. To clarify the functions of A(2A) receptors in the striatum and to identify the mechanisms leading to these unexpected modifications, we studied the basal expression of immediate early and constitutive genes as well as dopamine and glutamate neurotransmission in the striatum. Basal zif268 and arc mRNAs expression was reduced in mutant mice by 60-80%, not only in the striatum but also widespread in the cerebral cortex and hippocampus. Striatal expression of substance P and enkephalin mRNAs was reduced by about 50% and 30%, respectively, whereas the expression of GAD67 and GAD65 mRNAs was slightly increased and unaltered, respectively. In vivo microdialysis in the striatum revealed a 45% decrease in the extracellular dopamine concentration and three-fold increase in extracellular glutamate concentration. This was associated with an up-regulation of D(1) and D(2) dopamine receptors expression but not with changes in ionotropic glutamate receptors. The levels of tyrosine hydroxylase and of striatal and cortical glial glutamate transporters as well as adenosine A(1) receptors expression were indistinguishable between A(2A)-R(-/-) and wild-type mice. Altogether these results pointed out that the lack of A(2A) receptors leads to a functional hypodopaminergic state and demonstrated that A(2A) receptors are necessary to maintain a basal level in immediate early and constitutive genes expression in the striatum and cerebral cortex, possibly via their control of dopamine pathways.

Halothane potentiates the effect of methamphetamine and nomifensine on extracellular dopamine levels in rat striatum: a microdialysis study

Brain microdialysis was used to study the in vivo release and metabolism of dopamine (DA) in the rat striatum during halothane anaesthesia. Concentrations were measured in microdialysates collected every 20 min and applied directly to an on-line high-performance liquid chromatograph. Halothane was administered at concentrations of 0.5, 1.0, 1.5 and 2.0%. In another series of experiments, rats were treated intraperitoneally or locally with methamphetamine, a drug of abuse, or with nomifensine, a dopamine uptake blocker and antidepressant, in combination with 0.5 or 1.5% halothane. Halothane anaesthesia did not affect the dialysate (extracellular) concentration of DA at 2.0%. By contrast, the concentrations of DA metabolites [3-methoxytyramine (3-MT), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA)] increased during inhaled halothane anaesthesia in a dose-dependent manner and recovered after anaesthesia. Halothane potentiated the ability of methamphetamine to increase the extracellular concentration of DA when administered systemically, whereas only a small increase in DA accumulation was seen when methamphetamine was administered locally via the perfusate. Similarly, the increase in extracellular DA was accentuated by systemic nomifensine during halothane anaesthesia, but no obvious enhancement was observed when it was applied locally. It has been shown that the neurotoxic effect of methamphetamine is mediated by the suboxidation of DA released from the cytoplasm into the extracellular space and transformed into highly reactive free radicals. On the basis of our results, it is suggested that care should be exercised when halothane anaesthesia is used in patients abusing phenylethylamines (amphetamines) or being treated with DA uptake blockers (nomifensine).

Overview of microdialysis

The technique of microdialysis enables the monitoring of neurotransmitters and other molecules in the extracellular environment. This method has undergone several modifications and is now widely used for sampling and quantitating neurotransmitters, neuropeptides, and hormones in the brain and periphery. This unit describes the principles of conventional and quantitative microdialysis as well as strategies in designing a dialysis experiment. It establishes the groundwork for the basic techniques of preparation, conduct, and analysis of dialysis experiments in rodents and subhuman primates. Although the methods described are those used for monitoring CNS function, they can be easily applied with minor modification to other organ systems.

Methodological issues in microdialysis sampling for pharmacokinetic studies

Microdialysis is an in vivo technique that permits monitoring of local concentrations of drugs and metabolites at specific sites in the body. Microdialysis has several characteristics, which makes it an attractive tool for pharmacokinetic research. About a decade ago the microdialysis technique entered the field of pharmacokinetic research, in the brain, and later also in peripheral tissues and blood. Within this period much has been learned on the proper use of this technique. Today, it has outgrown its child diseases and its potentials and limitations have become more or less well defined. As microdialysis is a delicate technique for which experimental factors appear to be critical with respect to the validity of the experimental outcomes, several factors should be considered. These include the probe; the perfusion solution; post-surgery interval in relation to surgical trauma, tissue integrity and repeated experiments; the analysis of microdialysate samples; and the quantification of microdialysate data. Provided that experimental conditions are optimized to give valid and quantitative results, microdialysis can provide numerous data points from a relatively small number of individual animals to determine detailed pharmacokinetic information. An example of one of the added values of this technique compared with other in vivo pharmacokinetic techniques, is that microdialysis reflects free concentrations in tissues and plasma. This gives the opportunity to assess information on drug transport equilibration across membranes such as the blood-brain barrier, which already has provided new insights. With the progress of analytical methodology, especially with respect to low volume/low concentration measurements and simultaneous measurement of multiple compounds, the applications and importance of the microdialysis technique in pharmacokinetic research will continue to increase.

Dopamine release in the rat globus pallidus characterised by in vivo microdialysis

Brain microdialysis has been used to examine the in vivo effects of potassium and calcium on dopamine release in the dorsal globus pallidus (GP) of rats. Furthermore, the effects of food presentation and consumption on dopamine release in the GP were investigated. Basal dopamine levels in the GP were below the detection limit, therefore nomifensine (30 microM) was added to the perfused artificial cerebrospinal fluid (aCSF). A prominent increase of dopamine release to 370% was observed after perfusion with elevated potassium (100 mM), while perfusion with calcium-free aCSF produced a significant decrease of dopamine efflux to 36% of control levels. Furthermore, presentation and consumption of food resulted in a rapid increase of extracellular dopamine to 130%. The present experiments demonstrate that in the GP extracellular dopamine can be measured by in vivo brain microdialysis. The data suggest that the dopamine release in the GP can be stimulated by a depolarising agent and involves a partially calcium-dependent release mechanism. The data further suggest that dopamine in basal ganglia structures downstream the striatum as the GP is involved in signalling of important stimuli in the environment, e.g. food.

Striatal extracellular dopamine levels in rats with haloperidol-induced depolarization block of substantia nigra dopamine neurons

Correlations between substantia nigra (SN) dopamine (DA) cell activity and striatal extracellular DA were examined using simultaneous extracellular single-unit recordings and in vivo microdialysis performed in drug-naive rats and in rats treated repeatedly with haloperidol (HAL). Intact rats treated with HAL for 21-28 d exhibited significantly fewer active DA cells, indicating the presence of depolarization block (DB) in these cells. However, in rats that received surgical implantation of the microdialysis probe followed by a 24 hr recovery period, HAL-induced DA cell DB was reversed, as evidenced by a number of active DA neurons that was significantly higher than that in HAL-treated intact rats and similar to that of drug-naive rats. In contrast, using a modified probe implantation procedure that did not reverse SN DA neuron DB, we found striatal DA efflux to be significantly lower than in controls and significantly correlated with the reduction in DA neuron spike activity. Furthermore, although basal striatal DA efflux was independent of SN DA cell burst-firing activity in control rats, these variables were significantly correlated in rats with HAL-induced DA cell DB. Therefore, HAL-induced DB of SN DA neurons is disrupted by implantation of a microdialysis probe into the striatum using standard procedures. However, a modified microdialysis method that allowed reinstatement of DA neuron DB revealed that the HAL-induced inactivation of SN DA neurons was associated with significantly lower extracellular DA levels in the striatum. Moreover, the residual extracellular DA maintained in the presence of DB may, in part, depend on the burst-firing pattern of the noninactivated DA neurons in the SN.

Effects of different perfusion medium on the extracellular basal concentration of dopamine in striatum and medial prefrontal cortex: a zero-net flux microdialysis study

The zero-net flux microdialysis method was used to determine (1) the basal concentration of dopamine (DA) in the extracellular space, and (2) the in vivo recovery of Da in the striatum and the medial prefrontal cortex by using three different kinds of perfusion medium. They were, (a) commercial Ringer's solution, (b) low Ca2+ Ringer's solution, and (c) artificial cerebrospinal fluid (aCSF). Our results not only support previous findings that the high Ca2+ concentration in the perfusion medium could increase the baseline concentration of DA in the dialysate, which was collected from extracellular space through dialysis probe; but also provides evidence that this baseline increase was primarily due to an increase of basal DA concentration, and not from the increase of the in vivo recovery. Additionally, there was no significant difference in the basal DA concentration by using either commercial Ringer's solution or aCSF. This indicates that both commercial Ringer's solution and aCSF are suitable as good perfusion medium to determine the basal DA in the rat's brain.

Magnesium ion augmentation of inhibitory effects of adenosine on dopamine release in the rat striatum

The effects of adenosine and magnesium ion (Mg2+) on striatal dopamine release were studied in awake rats by in vivo microdialysis. The mean striatal basal levels of dopamine release at Mg2+ free perfusate were 56.95 +/- 5.30 fmol/sample (for 20 min). By varying the Mg2+ levels in perfusate from 0 mmol/L to 1, 10 or 40 mmol/L, the dopamine release was inhibited by Mg2+ in a level-dependent manner. Perfusion with modified Ringer's solution containing zero Mg2+ and from 5 to 50 mumol/L adenosine, non-selective adenosine agonist, as well as 0.1 mumol/L 2-chloro-N6-cyclopentyladenosine (CCPA), selective adenosine A1 agonist, showed no effect on dopamine release. However, from 5 to 50 mumol/L adenosine and from 0.1 to 1 mumol/L CCPA plus Mg2+ (1 and 40 mumol/L) perfusion decreased the dopamine release. This inhibitory effect of adenosine and CCPA on striatal dopamine release was enhanced by an increase in extracellular Mg2+ levels. Levels of 50 mumol/L of 8-cyclopentyl-1,3-dimethylxanthine (CPT), a selective adenosine A1 receptor antagonist, in perfusate increased the dopamine release under conditions both with and without Mg2+. This stimulatory effect of CPT on striatal dopamine release was reduced by an increase in extracellular Mg2+ levels. As a result, CPT antagonized the inhibitory effects of adenosine and CCPA on dopamine release under conditions of the presence and absence of Mg2+. These results suggest that the inhibition of striatal dopamine release by adenosine was mediated by adenosine A1 receptor. This inhibition was intensified by Mg2+. This study also revealed that the concentrations of Mg2+, which ranged from physiological to supraphysiological, reduced the striatal dopamine release; furthermore it was found that the physiological concentration of Mg2+ potentiated the effects of adenosine agonists, but inhibited adenosine antagonist. Thus, the present study, using in vivo microdialysis preparations, suggests Mg2+ inhibits the calcium ion channels and enhances the adenosinergic function in the central nervous system.

Evaluation of the function of microdialysis probes permanently implanted into the rat CNS and coupled to an on-line HPLC system of analysis

The aim of the microdialysis technique is to reflect as closely as possible the status and fluctuations of substances contained in the extracellular space. Most often, microdialysis is performed with repetitively implanted probes. We have recently devised an experimental set-up which allows microdialysis to be performed in the spinal cord of unrestrained rats through chronically permanently implanted probes. In the present study, we have compared the in vitro recovery of a non-biogenic amine (DHBA) and of 5-HT, and the in vivo recovery of the former. Thus, we could extrapolate the in vivo recovery of endogenous 5-HT released. Moreover, we have found that the recovery does not vary irrespective of whether the animal is at rest or performing sustained physical exercise, and that it also remains stable with time, from 8 to 36 days after permanent implantation of the probe. We conclude that this simple method can be applied to standard experiments of microdialysis, and thus allow one to measure the actual rate of recovery for a given probe. Moreover, it permits control of the stability of dialysis parameters with time for long-term permanently implanted probes.

Functional neuroanatomy of the nigrostriatal and striatonigral pathways as studied with dual probe microdialysis in the awake rat--I. Effects of perfusion with tetrodotoxin and low-calcium medium

In the present study we employed the dual probe approach to investigate functional interactions between the nigrostriatal dopaminergic and striatonigral GABAergic pathways in the awake, freely moving rat and their role in motor function. One microdialysis probe of concentric design was implanted in the substantia nigra pars reticulata and another in the ipsilateral dorsolateral striatum. Perfusion with a low-Ca2+ (0.1 mM) medium and with the voltage-dependent Na(+)-channel blocker tetrodotoxin (10 microM) was alternatively performed in both brain regions and the dialysate dopamine, glutamate and GABA levels were simultaneously measured in the dorsolateral striatum, whereas GABA levels alone were monitored in the substantia nigra. Perfusion with a low-Ca2+ medium in the substantia nigra pars reticulata did not affect local GABA levels, but transiently increased striatal dopamine release (+40%) without modifying striatal glutamate and GABA levels. Conversely, intranigral perfusion with tetrodotoxin transiently increased local GABA levels (+40%), while it decreased striatal dopamine (-60%) and increased glutamate (+70%) and GABA (+50%) levels. Perfusion with a low-Ca2+ medium in the dorsolateral striatum reversibly decreased local dopamine (-70%), glutamate (-20%) and GABA (-20%) levels, while local perfusion with tetrodotoxin decreased dopamine (-70%), increased glutamate (+30%) but did not affect dialysate GABA levels in this brain area. Neither of these intrastriatal treatments significantly affected GABA levels in the substantia nigra. Intranigral but not intrastriatal perfusion with tetrodotoxin was also associated with an increase in spontaneous locomotor activity as expressed by contralateral turning. Intranigral and intrastriatal perfusion with low-Ca2+ medium did not influence locomotor activity. On the basis of these neurochemical and behavioural findings, we propose a new dynamic model for the study of motor behaviour as mediated by basal ganglia circuitry.

Changes in cortical acetylcholine release in the rat during day and night: differences between motor and sensory areas

By sampling simultaneously from two microdialysis probes placed in the left and right hindlimb somatosensory cortex, or in the somatosensory and visual or in the somatosensory and motor cortices, we compared the release of acetylcholine in functionally different regions. Samples were taken hourly from freely moving, adult male Sprague-Dawley rats for periods of 10-24h. A generalized increase in acetylcholine release occurred in all cortical regions with the transition to the night-time period of wakefulness and activity; however, the change was significantly greater in the two sensory regions (56%) than in the motor cortex (20%). Decrements in release during the active period seldom decreased the amount released below the values observed during sleep. During the active period, the amount of acetylcholine released in the somatosensory cortex was strongly correlated with the amount released in the contralateral somatosensory region and was only slightly less well correlated with the amount released in either the visual or motor cortex. The correlation between release in the somatosensory and motor cortex was not present during the day, when rats habitually sleep. These data confirm that a global change in the level of acetylcholine release occurs with a transition in behavioural state; however, because the change is not equal in all areas and, because the correlation between motor and sensory cortex can be uncoupled, it seems likely that there are additional mechanisms available for independent control of acetylcholine release within specific cortical regions.

Codeine-induced histamine release in intact human skin monitored by skin microdialysis technique: comparison of intradermal injections with an atraumatic intraprobe drug delivery system

BACKGROUND

The skin microdialysis technique makes it possible to measure histamine release in intact human skin in vivo directly. In this study we have used the microdialysis technique to characterize histamine release by codeine after intracutaneous injections and following skin challenge by a novel atraumatic delivery technique.

OBJECTIVE

The purpose of the study was to compare histamine release in human skin by codeine, delivered by an intraprobe drug delivery system (IPD) and intracutaneous injections (ICT), with respect to dose-response relations, kinetics of histamine appearance and decay, correlations between histamine release and skin responses, and reproducibility.

METHODS

Hollow dialysis fibres were inserted intradermally in 12 healthy subjects. Twelve fibres were inserted in each subject, six fibres in each arm. Each fibre was perfused at a rate of 3 microM/min, and samples were collected in 2 min fractions. By the IPD technique, codeine was administered to the skin by adding codeine to the perfusion medium. Sequential IPD challenges were performed in one arm, and ICTs were done on the other arm.

RESULTS

Sixfold serial dilutions of codeine (0.01-3 mg/mL) caused a significant dose-related histamine release by ICT and IPD. Peak histamine release was found within the first 4 min after skin challenge by ICT and IPD, followed by a fast decline with a dialysate histamine half life of approximately 2-3 min. Peak histamine release was linearly correlated with cumulative release of the 20 min sampling period, and histamine release correlated with weal size. The coefficient of variation on peak histamine release was 18.9% and 4.8% for codeine ICT and IPD, respectively.

CONCLUSION

We have described in detail codeine-induced histamine release in intact human skin in vivo by the microdialysis technique. It was possible to administer codeine atraumatically to the skin by intraprobe delivery. The skin microdialysis technique opens up possibilities for measurement of inflammatory mediators release in normal and diseased skin, and it will be possible to deliver immunopharmacologically active drugs to the skin by intraprobe delivery.

Acute versus chronic haloperidol: relationship between tolerance to catalepsy and striatal and accumbens dopamine, GABA and acetylcholine release

Using in vivo microdialysis, changes in extracellular dorsolateral striatum and nucleus accumbens dopamine, GABA and acetylcholine following acute and chronic haloperidol (0.25 mg/kg, s.c.) were evaluated in rats concurrent with the measurement of catalepsy. When administered to drug-naive and chronically treated rats, haloperidol was associated with a consistent and prolonged (> 150 min) increase in dorsolateral striatum and nucleus accumbens DA release and a transient (60 min) increase in dorsolateral striatum GABA release. Haloperidol was also associated with a transient (30 min) increase in dorsolateral striatum acetylcholine release in the chronically treated rats. Basal dopamine and acetylcholine levels were similar in both brain regions; however, basal dorsolateral striatum GABA levels were two-fold higher in the chronically treated rats. Administration of haloperidol was associated with a prolonged (> 150 min) catalepsy in the drug-naive rats which was greatly diminished or absent in chronically treated rats. Additionally, serum haloperidol levels were shown to be similar 120 min following administration of haloperidol in both groups. These results indicate a marked behavioral difference in the effects of haloperidol in drug-naive and chronically treated rats which is not related to an altered bioavailability of the drug and which is dissociated from both basal and haloperidol induced effects on dopamine and acetylcholine release in both brain regions. However, the selective elevation of basal dorsolateral striatum GABA release following chronic administration of haloperidol may contribute to the development of tolerance to catalepsy as well as providing an in vivo neurochemical marker of the long-term effects of haloperidol.

Improved method for the routine analysis of acetylcholine release in vivo: quantitation in the presence and absence of esterase inhibitor

An improved high-performance liquid chromatographic (HPLC) method using electrochemical detection (ED) is described capable of routinely measuring the low levels of acetylcholine (ACh) typically found in rat brain microdialysis samples. Microdialysis was performed in the striatum of the urethane anesthetized rat using a 4-mm membrane length, high recovery (40% at 1.0 microliters/min; ambient conditions), loop-design probe perfused with an artificial cerebrospinal fluid (aCSF) solution containing physiologically normal calcium levels (1.2 mM). The HPLC method utilizes a polymeric stationary phase to resolve choline (Ch) from ACh. These analytes are then converted to hydrogen peroxide (H2O2) by a solid-phase reactor (containing immobilized choline oxidase and acetylcholinesterase enzymes). The H2O2 is detected amperometrically and quantitated on a platinum (Pt) working electrode (+300 mV; with a unique analytical cell featuring a solid-state palladium reference electrode). Two designs of the Pt working electrode were examined, differing only in the support material used (Kel-F or PEEK). The Kel-F/Pt electrode had a limit of detection (LOD) for both analytes of < 30 fmol per 10 microliters with a signal-to-noise ratio of 3:1. Striatal microdialysis perfusates were monitored for ACh and Ch over a 0-1000 nM range of neostigmine (NEO) in the CSF perfusion medium. Using the 4-mm probe, basal ACh and Ch levels were detected with a NEO level as low as 10 nM and were found to be 37 +/- 3 fmol and 22 +/- 1 pmol per 10 microliters (mean +/- S.E.M., n = 6 replicates) respectively. In similar experiments using 3-mm concentric probes comparable (lower) levels of ACh were found with the 50 and 1000 nM NEO doses (n = 4-21 animals). ACh could not be reliably quantitated when animals were perfused with the 10 nM dose of NEO (n = 4). The PEEK/Pt electrode had an improved LOD of < 20 fmol per 10 microliters due to a two- to three-fold decrease in the background noise component. Basal striatal levels of ACh in the absence of NEO approached the LOD and were found to be 15 +/- 2 fmol per 10 microliters; Ch was 5 +/- 1 pmol per 10 microliters (n = 2, mean of five basal samples). The analytical system requires very little maintenance; a simple electrochemical electrode cleaning step eliminates the need for routine polishing of the Pt electrode and the mobile phase is stable for up to one week.(ABSTRACT TRUNCATED AT 400 WORDS)

Determination of endogenous ions in intercellular fluid using capillary ultrafiltration and microdialysis probes

Capillary ultrafiltration probes are novel sampling tools for continuously monitoring small molecules in the extracellular fluid of awake animals. Capillary ultrafiltration uses a vacuum applied to hydrophilic membrane fibres and extracts intercellular fluid and quantitatively recover many small hydrophilic molecules. The effects of continuously removing a small amount of fluid from the interstitial space are not known. The concentration of sodium, potassium, calcium and inorganic phosphorus were determined in the collected ultrafiltrates from subcutaneous tissue. These values were compared to literature values and to concentrations determined for the same animals using microdialysis. The concentrations of sodium, potassium, calcium and inorganic phosphorous were found to be 140 +/- 4, 3.7 +/- 0.1, 1.1 +/- 0.1 and 1.7 +/- 0.1 mM, respectively, in the subcutaneous ultrafiltrates obtained from rats. These corresponded very well with literature values and microdialysates, obtained, using pure water as the perfusate, in subcutaneous tissue. The concentration of sodium and potassium were determined to be 142 +/- 2 mM and 3.6 +/- 0.2 mM, respectively, for the dialysates. Hyperinsulinemic-induced decrease in intercellular potassium levels under a euglycemic clamp were monitored using capillary ultrafiltration probes in rats to further validate this technique for monitoring small molecule dynamics in the intercellular space. The intercellular level of potassium in rats decreased from 3.6 +/- 0.5 to 2.6 +/- 0.3 mM after an acute dose of pork insulin.

Influence of potassium concentration in microdialysis perfusate on basal and stimulated striatal dopamine release: effect of ceruletide, a cholecystokinin-related peptide

The in vivo microdialysis method was used to study the effect of the cholecystokinin-related peptide, ceruletide, on extracellular levels of dopamine (DA) in the striatum following perfusion with various K+ concentrations. Increasing the K+ concentration in the perfusate from 4 to 15 or 17.5 mM did not change basal DA release or release evoked by electrical stimulation of the medial forebrain bundle (MFB). However, when the perfusing solution contained 20 or 30 mM K+, dose-dependent reductions of both basal and MFB-stimulated DA release occurred. Subcutaneous administration of ceruletide at 160 micrograms/kg had no influence on the basal or MFB-stimulated DA release with 4 or 15 mM K+ in the perfusate. However, after perfusion with 17.5 mM K+, ceruletide significantly attenuated the basal and MFB-stimulated DA release. Carbachol (10 microM) locally applied via the dialysis probe also attenuated MFB-stimulated DA release after perfusion with 17.5 mM K+. From these results, we conclude that under appropriate depolarization of striatal DA terminals, ceruletide induces further depolarization and inactivation of nigrostriatal DA terminals. The present data suggest that this effect may be mediated via intrinsic cholinergic neurons in the striatum.

Daily changes in the release of acetylcholine from rat primary somatosensory cortex

Using microdialysis, acetylcholine (ACh) release was measured in the somatosensory cortex of 14 rats over a 24-h period. The release of ACh was 0.195 pmol/min during the day and 0.344 pmol/min at night. The length of exposed dialysis membrane within the cortex was an important source of variability in the absolute amounts of ACh collected. Even after rejecting some cases where the membrane contacted only the superficial cortical layers, this factor accounted for 25% of the variation of absolute amounts collected in different animals. After correcting for the length of exposed membrane, the release of ACh was shown to increase 52% at night during the time when the animals were awake, feeding and grooming. Variability in the measures of ACh release obtained during periods of activity was greater than its variability during periods of inactivity. These data were interpreted in the context of several hypothesized roles for ACh in sensory cortex.

Overview of chemical sampling techniques

Although many of the ideas for sampling the chemical microenvironment of the brain were present, at least in nascent form, three decades ago or more, the last 10 years have witnessed a particularly spectacular surge of development, refinement, and use. We are now able to measure virtually any endogenous brain chemical in vivo at commendable levels of sensitivity, selectivity, and speed. The long-dreamt-of goal of being able to correlate neurochemical events with ongoing behavior and/or presentation of salient environmental cues and stimuli has already been largely achieved. Further refinements of existing techniques may well lead to levels of analysis inconceivable even a few years ago. The implications for theory-building and hypothesis-testing are enormous, particularly within such essentially virgin domains as behavioral neuroscience and biological psychiatry. These are truly exciting times.

Effect of the neurotoxin AF64A on intrinsic and extrinsic neuronal systems of rat neostriatum measured by in vivo microdialysis

In the present in vivo microdialysis study the aziridinium ion of ethylcholine mustard, AF64A and the excitotoxin ibotenic acid were compared for their effects on extracellular striatal acetylcholine, choline, gamma-aminobutyric acid (GABA), dopamine and its metabolites, glutamate and aspartate, measured in the same perfusate sample, under basal and high KCL conditions. Ten days following unilateral striatal injections of AF64A (2 x 0.08 to 2 x 8 mM) there was a dose-dependent decrease in the extracellular striatal levels of acetylcholine and GABA, the two major intrinsic striatal neurotransmitter systems. No significant effects were observed on any of the monitored neurotransmitter systems following the lowest (2 x 0.08 mM) dose of AF64A, while at the intermediate (2 x 0.8 mM) dose, AF64A produced a unilateral > 50% and > 70% decrease in basal extracellular striatal acetylcholine and GABA levels respectively. The effects of K(+)-depolarization on extracellular acetylcholine and GABA levels were diminished by approximately 50%. At the highest dose (2 x 8 mM), extracellular striatal acetylcholine levels were non-detectable under basal conditions, while the GABA levels were decreased by > 50%, when compared with the contralateral intact side. However, at this dose, GABA levels were bilaterally decreased compared to levels observed in control animals. Basal extracellular striatal dopamine and glutamate levels, representing the two major extrinsic neurotransmitter systems innervating the neostriatum were only affected by the highest dose of AF64A. The excitotoxin ibotenic acid (2 x 28.4 mM) produced a strong unilateral decrease in extracellular striatal acetylcholine (> 80%) and GABA (> 90%) levels, without significantly affecting basal dopamine and glutamate levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Striatal extracellular dopamine in conscious vs. anesthetized rats: effects of chloral hydrate anesthetic on responses to drugs of different classes

Many investigations using the microdialysis technique have been performed in anesthetized animals, both in this laboratory and elsewhere. Concern arises with this preparation that the anesthetic may compromise neuronal function, or that it may interact with test drugs affecting neurotransmitter overflow. In addition, in these studies the microdialysis probe typically is introduced into the brain on the day of testing, and data collection commences within an hour or two following probe insertion. It has been suggested that transmitter recovered in the perfusate probably represents leakage due to tissue damage as well as exocytotic release, and may not accurately reflect neuronal responses to the manipulations of interest. Such potential confounds present important implications for the interpretation of data from these studies. The present investigation examined the effects of chloral hydrate anesthetic on (1) basal dopamine (DA) overflow in the anterior striatum, and (2) DA responses to systemically delivered drugs of two different classes known to influence DA activity. Three putative indices of impulse-dependent release were measured: (a) the time course and stability of basal DA overflow over several hours; (b) sodium channel involvement by adding tetrodotoxin (TTX) to the artificial CSF; and (c) calcium channel involvement using magnesium (Mg) in a calcium-free perfusate. Basal DA levels became stable in both conscious and anesthetized preparations by the second hour after probe insertion. Levels of recovered DA overflow in the anterior striata of conscious rats were approximately double those in chloral hydrate-anesthetized rats. Consistent with other findings, this suggests a general depression of CNS function by chloral hydrate.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in the in vitro and in vivo 5-hydroxytryptamine extraction performance among three common microdialysis membranes

The present study summarizes the results of an in vitro and in vivo comparison of the apparent 5-hydroxytryptamine (5-HT), 5-hydroxyindoleacetic acid, and 3,4-dihydroxyphenylacetic acid dialysis performance of three types of membrane frequently used in intracerebral microdialysis experiments. The dialysis fiber types examined were a regenerated cellulose Cuprophan (GF), a proprietary polycarbonate ether (CMA), and a polyacrylonitrile/sodium methallylsulfonate copolymer (HOSPAL). The experiments unexpectedly revealed that the HOSPAL membrane-equipped probes displayed clearly aberrant 5-HT diffusion dynamics compared with GF and CMA probes, demonstrable not only in vitro, but also in in vivo experiments. In vitro, the GF and CMA membrane-equipped probes exhibited maximum relative recovery for 5-HT already in the first 20-min sample, whereas the 5-HT recovery of HOSPAL probes increased in a very slow and protracted manner over a period of a little less than 2 h. The GF and CMA probes further displayed an immediate washout of 5-HT when the probes were subsequently transferred to artificial CSF only-containing medium (no 5-HT), whereas approximately 2 h was required to yield near-total extinction of dialysate 5-HT with the standard HOSPAL probes. In vivo, the rat ventral hippocampal dialysate 5-HT output responses to K+ (100 mM) infusion, to Ca2+ omission, and to systemic 8-hydroxy-2-(di-n-propylamino)tetralin injection were all markedly retarded and blunted when HOSPAL instead of GF membrane-equipped probes were used. However, the 5-hydroxyindoleacetic acid and 3,4-dihydroxyphenylacetic acid extraction in vitro and in vivo were comparable using either of the membrane types.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of voltage-sensitive calcium channel antagonists on the release of 5-hydroxytryptamine from rat hippocampus in vivo

The effect of calcium channel antagonists on the release of 5-hydroxytryptamine from the hippocampus of the chloral hydrate-anaesthetised rat was studied using the technique of intracerebral microdialysis. As the basal concentration of 5-hydroxytryptamine was close to the limit of detection of the HPLC method (8 fmol), the 5-hydroxytryptamine reuptake inhibitor, fluoxetine (10 microM), was included in the perfusion fluid. The L-type voltage-sensitive calcium channel antagonists, PN200-110, diltiazem, and verapamil, all passed through the dialysis membrane, giving a recovery of 20-30%. The N-type voltage-sensitive calcium channel antagonist, omega-conotoxin, penetrated less readily (12% recovery). The dihydropyridine, PN200-110, adhered to the probe, resulting in an effective concentration at the membrane 30% of that in the perfusion fluid. The concentration of 5-hydroxytryptamine in the dialysate samples was reduced by 60% in the absence of calcium. The L channel antagonists had little effect on the release of 5-hydroxytryptamine, which was inhibited, in a dose-dependent manner, to a maximum of 40% by omega-conotoxin. It is concluded that, under physiological conditions, the release of 5-hydroxytryptamine from the rat hippocampus is dependent on the entry of calcium through N-type voltage-sensitive calcium channels, although another calcium channel may also be involved.

In vivo sampling using loop microdialysis probes coupled to a liquid chromatograph

Microdialysis probes with longer membranes (20-100 mm) provide increased relative recovery over traditional shorter probes (1-4 mm) developed for neuroscience applications. The characterization and optimization of "straight through" or "loop type" probes for use in subcutaneous tissue are considered. Membrane area, probe size, inlet and outlet tubing dimensions, and flow-rate are examined for their effects on relative recovery, the total collection rate, and bulk flow through the membrane wall. Polyacrylonitrile and regenerated cellulose membrane fibers with different geometries were compared. Sampling probes used fibers 3-10 cm long. Inlet and outlet tubing was varied from 25 to 110 microns I.D. with lengths of 10 to 50 cm. Probe configurations optimized for relative recovery, flow-rate, and utility for in vivo use are presented. Utilizing microdialysis probes with large membrane surface areas results in relative concentration recovery of greater than 50% at flow-rates of greater than 5 microliters/min. Therapeutic drug monitoring in subcutaneous tissue of awake animals is explored.

Evidence for a substrate of neuronal plasticity based on pre- and postsynaptic neurotensin-dopamine receptor interactions in the neostriatum

The major mechanism underlying the neuroleptic action of the tridecapeptide neurotensin (NT) appears to be an interaction with dopamine receptor mechanisms based on biochemical binding and behavioral experiments. In vivo microdialysis was used in conscious rats to investigate the effects of local perfusion with NT on the sensitivity of striatal dopamine D1 and D2 receptors for their selective agonists by monitoring extracellular dopamine, 3,4-dihydroxyphenylacetic acid, homovanilic acid, and gamma-aminobutyric acid levels in the awake unrestrained male rat. Perfusion with NT (10 nM) counteracted the inhibitory effects of the dopamine D2 agonist pergolide (500 nM) on extracellular levels of dopamine and gamma-aminobutyric acid. In contrast, NT (10 mM) significantly enhanced the reduction of extracellular striatal levels of dopamine after perfusion with the D1 agonist SKF 38393 (5 microM), and this combined treatment also resulted in a significant increase in the extracellular striatal levels of gamma-aminobutyric acid. These results provide in vivo evidence that NT regulates central dopamine transmission by reducing pre-and postsynaptic dopamine D2 and enhancing D1 receptor sensitivity possibly through an antagonistic NT receptor-D2 receptor interaction. This heteroregulation has the potential to substantially increase the plasticity within the dopamine synapse.

Microdialysis perfusion increases the sensitivity of rat striatal neurons to ischemic insult

Brief periods of global forebrain ischemia which never produced striatal cell loss in control, non-dialyzed striatum resulted in significant cell loss in the contralateral, dialyzed striatum. No striatal cell loss was seen following these short periods of ischemia in animals implanted with dialysis probes that were not perfused. Dialysis perfusion alters striatal extracellular fluid composition in such a way as to render this tissue more sensitive to brief ischemic insult.

Opposing tonically active endogenous opioid systems modulate the mesolimbic dopaminergic pathway

The mesolimbic dopaminergic system has been implicated in mediating the motivational effects of opioids and other drugs of abuse. The site of action of opioids within this system and the role of endogenous opioid peptides in modulating dopamine activity therein remain unknown. Employing the technique of in vivo microdialysis and the administration of highly selective opioid ligands, the present study demonstrates the existence of tonically active and functionally opposing mu and kappa opioid systems that regulate dopamine release in the nucleus accumbens, the major terminal area of A10 dopaminergic neurons. Thus, stimulation of mu-type receptors in the ventral tegmental area, the site of origin of A10 dopaminergic neurons, increases dopamine release whereas the selective blockade of this opioid receptor type results in a significant decrease in basal dopamine release. In contrast, stimulation of kappa-type receptors within the nucleus accumbens decreases dopamine release whereas their selective blockade markedly increases basal dopamine release. These data show that tonic activation of mu and kappa receptors is required for the maintenance of basal dopamine release in the nucleus accumbens. In view of the postulated role of the mesolimbic system in the mediation of drug-induced alterations in mood and affect, such findings may have implications for the treatment of opiate dependence and affective disorders.

High-performance liquid chromatography with electrochemical detection for the determination of levodopa, catecholamines and their metabolites in rat brain dialysates

A high-performance liquid chromatographic assay with electrochemical detection is described for the simultaneous determination of levodopa, 3-O-methyldopa, dopamine, dihydroxyphenylacetic acid, homovanillic acid, 3-methoxytyramine, noradrenaline, adrenaline, 3-methoxy-4-hydroxyphenylethylene glycol and 5-hydroxyindoleacetic acid in rat brain dialysates. Samples are obtained in vivo using the microdialysis technique. Microdialysis probes are placed in the brain area to be studied and neurochemicals are collected by perfusion of the probe with modified Ringer's solution. Direct injection of the dialysates allows rapid and reliable results to be obtained.

Effects of acute and chronic clozapine on dopaminergic function in medial prefrontal cortex of awake, freely moving rats

We previously showed that chronic administration of the clinically atypical and clinically superior antipsychotic drug clozapine selectively reduces dopamine (DA) release in the nucleus accumbens but not neostriatum, and that this effect appears mediated by anatomically selective mesolimbic DA depolarization blockade. The present study extends that research to another mesocorticolimbic DA locus, the medial prefrontal cortex. Acute clozapine challenge (5-40 mg/kg i.p.) produced dose-dependent increased extracellular levels of DA and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), in the medial prefrontal cortex of awake, free-moving rats as measured by in vivo brain microdialysis. Chronic clozapine treatment (20 mg/kg/day for 21 days) did not significantly change basal extracellular levels of DA, DOPAC or HVA. Acute clozapine challenge on day 22 in the chronic clozapine-treated animals produced no significant differences in medial prefrontal cortex DA, DOPAC or HVA as compared to chronic vehicle-treated animals, indicating that tolerance to clozapine does not develop in the mesocortical DA system, in contrast to the mesolimbic system. The DA agonist apomorphine (100 micrograms/kg) produced decreased basal extracellular levels of DA, DOPAC and HVA in medial prefrontal cortex of both chronic clozapine-treated and chronic vehicle-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo characterisation of extracellular dopamine, GABA and acetylcholine from the dorsolateral striatum of awake freely moving rats by chronic microdialysis

Basal extracellular (EC) DA, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), gamma aminobutyric acid (GABA) and acetylcholine (ACh) were measured in dialysates from the dorsolateral striatum (DLS) of awake rats, every 30 min for 4.5 h each day over a 4-day period. The responsiveness of basal EC DA, DOPAC, HVA and GABA to local perfusion with tetrodotoxin (1 micron) was measured 1 and 4 days after implantation. In addition EC ACh was also measured 4 days after probe implantation. The results of this study indicate that EC levels of DA, DOPAC, HVA, GABA and ACh can be reliably monitored for up to 4 days after probe implantation. In addition, we show that striatal EC levels of DA, GABA and ACh may be regarded as a reflection of ongoing neuronal activity for up to 4 days after implantation of a microdialysis probe.