The technique of intracerebral microdialysis

Improved blood-brain barrier distribution: effect of borneol on the brain pharmacokinetics of kaempferol in rats by in vivo microdialysis sampling

ETHNOPHARMACOLOGICAL RELEVANCE

Kaempferol (KA) exists in a variety of herbal medicines. In vitro and in vivo studies have focused on the anti-Alzheimer effect of KA. However, little is known about its brain pharmacokinetic profile. The accumulated amount of KA in brain is very low because of the protection of blood-brain barrier (BBB). Borneol (BO) is a classical aromatic refreshing traditional Chinese medicine and commonly used as an adjuvant component of traditional Chinese medicines (e.g. compound Danshen dropping pills) in the treatment of cardiovascular and cerebrovascular diseases. According to the basic theories of traditional Chinese medicine, BO is called an "upper guiding drug", which can guide other components to the targeting tissues or organs in the upper part of the body, especially in the brain.

MATERIALS AND METHODS

The probes for blood and brain sampling were implanted within the jugular vein/right atrium and right hippocampus of SD rats, respectively. Rats were intravenous administered of KA (25 mg/kg) alone or combined with BO (15, 30 mg/kg) via caudal vein. The blood and brain microdialysates were collected every 15 min for 180 min and every 30 min for 180-300 min. A selective and sensitive high performance liquid chromatography-chemiluminescence method was developed for the determination of unbound KA in rat blood and brain microdialysates, which can be converted to their actual free-form concentrations based on the in vivo relative recoveries of KA across microdialysis probes.

RESULTS

KA quickly crossed the BBB to enter the extracellular fluid of hippocampus and reached the maximum concentration of 0.11 μg/mL within 30 min. The brain bioavailability and brain delivery of KA evidently increased with the co-administration of 15 and 30 mg/kg of BO. The AUC0-inf of KA in brain increased 1.84 and 2.19 times, and the Cmax of KA in brain increased 2.09 and 3.18 times than that without BO, respectively. In addition, the brain-to-blood distribution ratio of KA increased by 48.68% and 57.97% compared with that without BO. However, no significant difference in the T1/2 of unbound KA in blood aserved between three groups.

CONCLUSIONS

BO can enhance the BBB permeability and improve the transportation of KA to brain. The dose-dependent effect of BO on the brain pharmacokinetic parameters of KA was observed. This co-administration strategy can be designed to enhance the brain accumulation of other neuropsychiatric medications.

The importance of plasma protein binding in drug discovery

Plasma protein binding of drugs is a well-recognised phenomena, but it is only recently that the implications for drug action in vivo have been fully appreciated. Plasma proteins, by virtue of their high concentration, control the free drug concentration in plasma and in compartments in equilibrium with plasma, thereby, effectively attenuating drug potency in vivo. The historical background and thermodynamic basis for the 'Free Drug Principle' is presented, along with special considerations for intracellular targets, deep compartments and α1-acid glycoprotein binding. Real and apparent exceptions to the principle are discussed along with a survey of citations from the recent medicinal chemistry literature.

Fluorescent dopamine tracer resolves individual dopaminergic synapses and their activity in the brain

We recently introduced fluorescent false neurotransmitters (FFNs) as optical tracers that enable the visualization of neurotransmitter release at individual presynaptic terminals. Here, we describe a pH-responsive FFN probe, FFN102, which as a polar dopamine transporter substrate selectively labels dopamine cell bodies and dendrites in ventral midbrain and dopaminergic synaptic terminals in dorsal striatum. FFN102 exhibits greater fluorescence emission in neutral than acidic environments, and thus affords a means to optically measure evoked release of synaptic vesicle content into the extracellular space. Simultaneously, FFN102 allows the measurement of individual synaptic terminal activity by following fluorescence loss upon stimulation. Thus, FFN102 enables not only the identification of dopamine cells and their processes in brain tissue, but also the optical measurement of functional parameters including dopamine transporter activity and dopamine release at the level of individual synapses. As such, the development of FFN102 demonstrates that, by bringing together organic chemistry and neuroscience, molecular entities can be generated that match the endogenous transmitters in selectivity and distribution, allowing for the study of both the microanatomy and functional plasticity of the normal and diseased nervous system.

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.

Studying the central control of food intake and obesity in rats

The central nervous system regulates energy intake and expenditure through a complex network of neurotransmitters and neuromodulators. It is of great interest to understand the relevance of these systems to the physiological control of energy balance and to the disturbances of obesity. The present paper discusses some of the methods to address this field used at the laboratory of Endocrine Physiology of Universidade Federal de São Paulo. Initially, different experimental models of rat obesity are presented, namely the hypothalamic induced monosodium glutamate model, the Zucker genetic model, and the dietary model. The principles of brain microdialysis are also presented, the technique applied to obtain representative samples of the extracellular fluid of brain sites involved in feeding control. The microdialysate levels of serotonin, an important anorexigenic neurotransmitter, are determined by HPLC with electrochemical detection. The immunoblot technique (Western blot) is used to determine hypothalamic levels of proteins relevant to the anorexigenic effect of serotonin and to analyze the acute activation of the insulin signaling cascade in the hypothalamus. The final section addresses the potential applications of proteomics in the study of the central control of feeding.

Behavioral recovery in MPTP-treated monkeys: neurochemical mechanisms studied by intrastriatal microdialysis

Parkinson's disease (PD) patients express motor symptoms only after 60-80% striatal dopamine (DA) depletion. The presymptomatic phase of the disease may be sustained by biochemical modifications within the striatum. We used an appropriate specific 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) monkey model (Mounayar et al., 2007) to study the compensatory mechanisms operating in recovery from PD motor symptoms. We assessed the levels of DA and its metabolites (DOPAC, homovanillic acid), GABA, glutamate (Glu), serotonin (5-HT) and its metabolite (5HIAA) by repeated intracerebral microdialysis in awake animals before exposure to MPTP during full expression of the motor symptoms induced by MPTP and after recovery from these symptoms. Measurements were obtained from two functionally and anatomically different striatal areas: the associative-limbic territory and sensorimotor territory. Animals with motor symptoms displayed an extremely large decrease in levels of DA and its metabolites and an increase in Glu and GABA levels, as reported by other studies. However, we show here for the first time that serotonin levels increased in these animals. We found that increases in DA levels in the sensorimotor and/or associative-limbic territory and high levels of 5-HT and of its metabolite, 5HIAA, were associated with recovery from motor symptoms in this model. Determining whether similar changes in DA and 5-HT levels are involved in the compensatory mechanisms delaying the appearance of motor symptoms in the early stages of PD might make it possible to develop new treatment strategies for the disease.

A physiological model to evaluate drug kinetics in patients with hemorrhagic shock followed by fluid resuscitation. Application to amoxicillin-clavulanate

PURPOSE

To build a physiologically based pharmacokinetic model describing drug kinetics in interstitial fluid in case of hemorrhagic shock, and to propose a simple method to determine the subset of influential parameters that may be estimated with the data at hand.

METHODS

The model, which accounts for alterations of regional blood flows and body water distribution, was fitted to amoxicillin and clavulanate kinetic data, assessed in 12 trauma patients with hemorrhagic shock by comparison with 12 healthy volunteers. The predictions were the free concentrations of amoxicillin and clavulanate in 14 organs.

RESULTS

In all tissues of trauma patients, the rate of distribution was lower, but the steady-state level was higher than those in healthy participants. Blood volume was reduced by 25% and blood flow in organs other than lung, brain, and heart were reduced by 18%. Compared with healthy subjects, the time that free amoxicillin concentration remained above 8 mg/L in the interstitial fluid of trauma patients was higher in blood and muscles, and lower in the tendon compartment.

CONCLUSIONS

The results and predictions were consistent with the knowledge in this field. The model may be useful to optimize clinical trial designs and drug dosing regimens.

N1phenethyl-norcymserine, a selective butyrylcholinesterase inhibitor, increases acetylcholine release in rat cerebral cortex: a comparison with donepezil and rivastigmine

The effects of (-)-N(1)phenethyl-norcymserine (PEC, 5 mk/kg, i.p.) on acetylcholine release and cholinesterase activity in the rat cerebral cortex were compared with those of donepezil (1 mg/kg, i.p.), a selective acetylcholinesterase inhibitor, and rivastigmine (0.6 mg/kg, i.p.), an inhibitor of acetylcholinesterase and butyrylcholinesterase. Acetylcholine extracellular levels were measured by microdialysis coupled with HPLC; acetylcholinesterase and butyrylcholinesterase activity were measured with colorimetric and radiometric methods. It was found that comparable 2-3 fold increases in cortical extracellular acetylcholine level, calculated as areas under the curve, followed the administration of the three drugs at the doses used. At the peak of acetylcholine increase, a 27% acetylcholinesterase inhibition and no butyrylcholinesterase inhibition was found after donepezil (1 mg/kg, i.p) administration. At the same time point, rivastigmine (0.6 mg/kg, i.p.) inhibited acetylcholinesterase by 40% and butyrylcholinesterase by 25%. After PEC (5 mg/kg, i.p.) administration, there was a 39% butyrylcholinesterase inhibition and no effect on acetylcholinesterase. Since in the present study it was also confirmed that in the brain butyrylcholinesterase activity is only about 10% of acetylcholinesterase activity, it is surprising that its partial inhibition is sufficient to increase extracellular acetylcholine levels. The importance of butyrylcholinesterase as a "co-regulator" of synaptic acetylcholine levels should thus be reconsidered.

Microdialysis as a tool in local pharmacodynamics

In many cases the clinical outcome of therapy needs to be determined by the drug concentration in the tissue compartment in which the pharmacological effect occurs rather than in the plasma. Microdialysis is an in vivo technique that allows direct measurement of unbound tissue concentrations and permits monitoring of the biochemical and physiological effects of drugs throughout the body. Microdialysis was first used in pharmacodynamic research to study neurotransmission, and this remains its most common application in the field. In this review, we give an overview of the principles, techniques, and applications of microdialysis in pharmacodynamic studies of local physiological events, including measurement of endogenous substances such as acetylcholine, catecholamines, serotonin, amino acids, peptides, glucose, lactate, glycerol, and hormones. Microdialysis coupled with systemic drug administration also permits the more intensive examination of the pharmacotherapeutic effect of drugs on extracellular levels of endogenous substances in peripheral compartments and blood. Selected examples of the physiological effects and mechanisms of action of drugs are also discussed, as are the advantages and limitations of this method. It is concluded that microdialysis is a reliable technique for the measurement of local events, which makes it an attractive tool for local pharmacodynamic research.

Measurement of neurotransmitters from extracellular fluid in brain by in vivo microdialysis and chromatography–mass spectrometry

During the last three decades, a great deal of information has been discovered about chemical neurotransmission. However, the most important processes, namely the complex nature of neuronal circuitry, the "cross talk" between multiple neurotransmitter systems, and the varying effects neurochemicals have at different receptors, are still being explored. Techniques such as microdialysis are routinely employed to measure neurotransmitter levels in living tissue systems. Moreover, microdialysis studies have proven to be valuable in the investigation of neurodegenerative and psychiatric disease pathology, as well as in identifying novel drugs to treat such disorders. One particular challenge in performing these experiments is the requirement to couple microdialysis to sophisticated analytical equipment. Recently, considerable attention has been focused on the development of chromatographic-mass spectrometric techniques to provide more sensitive and accurate measurements of neurochemicals collected from in vivo microdialysis experiments. This review will provide a brief overview of the microdialysis technique, as well as how microdialysis and chromatography-mass spectrometry are being used to measure extracellular levels of neurotransmitters. The primary emphasis of this review will be on how these applications are used to measure levels of acetylcholine (ACh), dopamine, norepinephrine and gamma-aminobutyric acid (GABA).

Physiological Modeling for Indirect Evaluation of Drug Tissular Pharmacokinetics under Non-Steady-State Conditions: An Example of Antimicrobial Prophylaxis During Liver Surgery

Cefazolin, a time-dependent first-generation cephalosporin with non-linear binding to albumin, is widely recommended for antimicrobial prophylaxis during liver surgery to decrease the incidence of postoperative wound infections. The recommended protocol (2 g IV at anesthesia induction followed by 1 g 4 h later) is expected to maintain the free cefazolin concentration in exposed intratissular fluids above its minimal inhibitory concentration (MIC) for potentially encountered microorganisms, from skin incision to skin closure. Since this dosing protocol fails to take into account either of patients status (total body weight and renal function) or of surgical and anesthetic consequences (variations of cardiac output and regional blood flows, progressive decrease of plasma albumin concentration) on cefazolin tissular pharmacokinetics, a physiological modeling study was conducted to investigate protocol suitability for liver surgery in six populations: obese (body mass index >34), renal insufficiency (GFR = 10, 30 or 50 ml min(-1)) and high intraoperative blood loss (three times that usually observed during this surgery) and none of these features referred to as controls. A previously validated physiologically based pharmacokinetic (PB-PK) model for cefazolin in humans was used and then further adapted to simulate obese or renal insufficiency patients as well as the consequences of general anesthesia and liver surgery on cefazolin pharmacokinetics. Clinical data required for simulation (intraoperative kinetics of percent expired isoflurane and plasma albumin concentration, mean intraoperative blood loss) were obtained from 10 patients who underwent right hepatectomy in our institution. Using a fixed MIC of 2 microg ml(-1) against potentially encountered bacteria, it was concluded that the recommended dosing schedule was suitable in all tested populations, including obese patients, although prolongation of the interval between injections appeared advisable for renal insufficiency patients. Furthermore, when a MIC of 3 microg ml(-1) was considered, the recommended cefazolin-dosing regimen failed to maintain sufficient free cefazolin concentrations in the interstitial fluids during surgery in all tested populations except renal insufficiency patients (GFR < 50 ml min(-1)).

Clonazepam increases in vivo striatal extracellular glucose in diabetic rats after glucose overload

Hyperglycemia modulates brain function, including neuronal excitability, neurotransmitter release and behavioral changes. There may be connections between the GABAergic system, glucose sensing neurons and glucose in the neuronal environment that shed light on the mechanism by which GABA(A) agents influence depressive behavior in diabetic rats submitted to the forced swimming test. We aimed to investigate whether clonazepam (CNZ), a GABA(A) receptor positive modulator, modifies in vivo striatal extracellular glucose levels in diabetic rats under fasting condition or after oral glucose overload. Streptozotocin diabetic and nondiabetic rats were submitted to in vivo striatal microdialysis. Perfusate samples were collected at baseline, during fasting and following administration of CNZ (0.25 mg/kg) and oral glucose overload. Blood glucose and striatal extracellular glucose were measured simultaneously at several time points. Fasting striatal glucose levels were higher in diabetic than in nondiabetic rats and the differences between these animals were maintained after glucose overload. The increases in extracellular striatal glucose after glucose overload were around 40% and blood to brain transference was decreased in diabetics. CNZ treatment paradoxically increased striatal glucose after glucose overload in diabetic rats, which may mark the dysfunction in brain glucose homeostasis.

Haloperidol impairs auditory filial imprinting and modulates monoaminergic neurotransmission in an imprinting-relevant forebrain area of the domestic chick

In vivo microdialysis and behavioural studies in the domestic chick have shown that glutamatergic as well as monoaminergic neurotransmission in the medio-rostral neostriatum/hyperstriatum ventrale (MNH) is altered after auditory filial imprinting. In the present study, using pharmaco-behavioural and in vivo microdialysis approaches, the role of dopaminergic neurotransmission in this juvenile learning event was further evaluated. The results revealed that: (i) the systemic application of the potent dopamine receptor antagonist haloperidol (7.5 mg/kg) strongly impairs auditory filial imprinting; (ii) systemic haloperidol induces a tetrodotoxin-sensitive increase of extracellular levels of the dopamine metabolite, homovanillic acid, in the MNH, whereas the levels of glutamate, taurine and the serotonin metabolite, 5-hydroxyindole-3-acetic acid, remain unchanged; (iii) haloperidol (0.01, 0.1, 1 mm) infused locally into the MNH increases glutamate, taurine and 5- hydroxyindole-3-acetic acid levels in a dose-dependent manner, whereas homovanillic acid levels remain unchanged; (iv) systemic haloperidol infusion reinforces the N-methyl-d-aspartate receptor-mediated inhibitory modulation of the dopaminergic neurotransmission within the MNH. These results indicate that the modulation of dopaminergic function and its interaction with other neurotransmitter systems in a higher associative forebrain region of the juvenile avian brain displays similar neurochemical characteristics as the adult mammalian prefrontal cortex. Furthermore, we were able to show that the pharmacological manipulation of monoaminergic regulatory mechanisms interferes with learning and memory formation, events which in a similar fashion might occur in young or adult mammals.

Validity of the microdialysis technique for experimental in vivo studies of myocardial energy metabolism

OBJECTIVES

The validity of the microdialysis technique for experimental in vivo studies of myocardial energy metabolism is not known. To address this question interstitial levels of energy-related metabolites (lactate, adenosine, inosine and hypoxanthine) obtained by the microdialysis technique were compared with corresponding metabolites from myocardial biopsies at given intervals in a porcine heart model using different protocols of ischaemia and reperfusion.

METHODS

In an open chest porcine heart model, interstitial levels of energy-related metabolites were monitored using the microdialysis technique. All animals (n = 23) were subjected to 120-min pretreatment followed by 40 min of regional ischaemia and 120 min of reperfusion. Tissue biopsies were obtained in the beginning, middle and at the end of the 40-min ischaemic period and at the end of the reperfusion period. Pretreatment consisted of either rest (group 1, n = 7), or rest for 90 min and one ischaemia/reperfusion (10 + 20 min) cycle (group 2, n = 9), or four ischaemia/reperfusion cycles (10 + 20 min each) (group 3, n = 7).

RESULTS

Interstitial levels of energy-related metabolites monitored by the microdialysis technique correlated with tissue biopsy levels of lactate (r = 0.90, P < 0.001), adenosine (r = 0.89, P < 0.001), inosine (r = 0.88, P < 0.001) and hypoxanthine (r = 0.91, P < 0.001), respectively, which were obtained by tissue biopsies at given time intervals. These significant correlations were valid regardless of the functional state of the myocardium.

CONCLUSION

We observed significant correlations between microdialysis probe levels and tissue biopsy levels of energy-related metabolites in both ischaemic and non-ischaemic tissue. These data assess the validity of the microdialysis technique (in the current setting) for studying dynamic changes of myocardial energy metabolism.

Antiischemic effects of topiramate in a transient global forebrain ischemia model: a neurochemical, histological, and behavioral evaluation

The mechanisms of action of the anticonvulsant topiramate (TPM) are indicative of a potential benefit during cerebral ischemia. TPM was studied in a transient global forebrain ischemia (TGFI) model in gerbils in which 40 mg/kg was administered before or after TGFI. Control groups were administered 0.9% normal saline similarly. The evaluation consisted of neurochemical, histological, and functional analyses. The data obtained indicates that unlike the focal cerebral ischemia model, TPM is not neuroprotective in TGFI. The difference in effect, which may be due to the difference in species or the type of ischemia, points to the need for caution when extrapolating animal data from this drug to humans.

Protection of methylmercury effects on the in vivo dopamine release by NMDA receptor antagonists and nitric oxide synthase inhibitors

The possible protective effects of NMDA receptor antagonists dizocilpine (MK-801) and D(-)-2-amino-5-phosphonopentanoic acid (AP5), and nitric oxide synthase (NOS) inhibitors L-nitro-arginine methyl ester (L-NAME) and 7-nitro-indazol (7-NI) on the methylmercury (MeHg)-induced dopamine (DA) release from rat striatum were investigated using in vivo microdialysis. Intrastriatal infusion of 400 microM or 4 mM MeHg increased the extracellular DA levels to 1941+/-199 and 7971+/-534% with respect to basal levels. Infusion of 400 microM or 4 mM MeHg in 400 microM MK-801 pretreated animals, increased striatal DA levels to 677+/-126 and 2926+/-254%, with respect to basal levels, these increases being 65 and 63% smaller than those induced by MeHg in non-pretreated animals. Infusion of 400 microM or 4 mM MeHg in 400 microM AP5 pretreated animals, increased striatal DA levels to 950+/-234 and 2251+/-254% with respect to basal levels, these increases being 51 and 72% smaller than those induced by MeHg in non-pretreated animals. Infusion of 400 microM MeHg in 100 microM L-NAME or 7-NI pretreated animals, increased the extracellular DA levels to 1159+/-90 and 981+/-292%, with respect to basal levels, these increases being 40 and 50% smaller than those induced by MeHg in non-pretreated animals. In summary, MeHg acts, at last in part, through an overstimulation of NMDA receptors with possible NO production to induce DA release, and administration of NMDA receptor antagonists and NOS inhibitors protects against MeHg-induced DA release from rat striatum.