Review of microdialysis in brain tumors, from concept to application: First Annual Carolyn Frye-Halloran Symposium

In individuals with brain tumors, pharmacodynamic and pharmacokinetic studies of therapeutic agents have historically used analyses of drug concentrations in serum or cerebrospinal fluid, which unfortunately do not necessarily reflect concentrations within the tumor and adjacent brain. This review article introduces to neurological and medical oncologists, as well as pharmacologists, the application of microdialysis in monitoring drug metabolism and delivery within the fluid of the interstitial space of brain tumor and its surroundings. Microdialysis samples soluble molecules from the extracellular fluid via a semipermeable membrane at the tip of a probe. In the past decade, it has been used predominantly in neurointensive care in the setting of brain trauma, vasospasm, epilepsy, and intracerebral hemorrhage. At the first Carolyn Frye-Halloran Symposium held at Massachusetts General Hospital in March 2002, the concept of microdialysis was extended to specifically address its possible use in treating brain tumor patients. In doing so we provide a rationale for the use of this technology by a National Cancer Institute consortium, New Approaches to Brain Tumor Therapy, to measure levels of drugs in brain tissue as part of phase 1 trials. Originally published Neuro-oncology, Vol. 6, No. 1, Jan 200

Obesity-related changes in the vascular actions of insulin

Abstract Over the past 2 to 3 decades, research has focused on the changes in the vascular effects of insulin occurring in insulin resistant states like obesity. Consistent evidence indicates that obesity results in reduced endothelial release of nitric oxide in response to insulin, associated with concomitant enhancement in the production of endothelin-1. More recent work has pointed toward reduced vascular permeability and changes in the physical-chemical characteristics of the perivascular extracellular matrix as additional mechanisms of impaired insulin sensitivity in obesity. All these perturbations are important, because they contribute to impaired delivery of insulin itself and metabolic substrates to the target tissues and may play a role in the development of both diabetes and vascular damage. This review will describe the physiological vascular actions of insulin and their changes in obesity, focusing on some established pathophysiological determinants of the derangement of vascular insulin signaling, such as the lipid overflow from expanded fat depots and signals originating from inflamed obese adipose tissue (both distant and perivascular). Also, it will outline novel evidence underscoring the contribution of dysregulated adipokine secretion and changes in intestinal permeability and gut microbiome. Finally, it will touch upon some unresolved issues, such as the potential role of vascular insulin resistance in obesity-driven adipose tissue remodeling, and will discuss perspectives for future studies, regarding in particular possible therapeutic strategies with translational implications for the patients care

Extracellular Glutamate, Glutamine, and GABA in the Hippocampus and Cortex of Refractory Epilepsy Patients

Antiepileptic drug (AED) resistance affects one third of patients with epilepsy and is associated with significant disability. Brain microdialysis studies on the epileptogenic hippocampus of patients with medication refractory epilepsy have identified elevations in extracellular glutamate, the primary brain excitatory neurotransmitter, both acutely during seizures and chronically during the interictal periods. Whether extracellular glutamate, along with the metabolites glutamine and the inhibitory neurotransmitter GABA (gamma-aminobutyric acid), are elevated in other cortical regions is unknown. In addition, the effect of the administration of AEDs on the extracellular levels of these neurochemicals in patients with medication-refractory epilepsy is also unknown. Microdialysis samples were obtained from probes coupled to the EEG depth electrodes and implanted in the cortex and hippocampus of 81 awake patients with medication-refractory epilepsy undergoing intracranial electroencephalographic (EEG) monitoring. Probes were classified according to location and seizure activity into cortical or hippocampal non-epileptic, epileptogenic, propagated, non-localized or lesion sites. Samples were collected during the interictal period, in all subjects on their full AED dose during the first couple of days of their hospitalization, and then again (in a subset of 38 patients) after their AEDs were tapered. The samples were analyzed with high performance liquid chromatography (HPLC) for glutamate, glutamine and GABA levels. Data were log-transformed and compared by ANOVA or multiple t-tests with a Bonferroni correction, where appropriate. In the cortex, glutamate was significantly higher in epileptogenic (mean ± SEM, 17.3 ± 5.1 µM), propagated (25.8 ± 4.0 µM), non-localized (43.9 ± 9.9 µM), and lesion (46.9 ± 9.0 µM) sites compared to non-epileptic cortex (2.6 ± 0.3 µM). In the hippocampus, glutamate was significantly higher in the epileptogenic (10.3 ± 1.9 µM) and propagated sites (33.0 ± 13.8 µM) than non-epileptic sites (2.8 ± 0.5 µM). Glutamine was not significantly different between sites in both the cortex and hippocampus. In the cortex, GABA was significantly elevated in propagated (1503 ± 273 nM) and lesion (827 ± 183 nM) sites compared to non-epileptic sites (265 ± 62 nM). In the hippocampus, GABA was elevated in the propagated (1079 ± 395 nM) compared to non-epileptic sites (391 ± 169 nM). There were no significant differences in glutamate, glutamine, or GABA between the hippocampus and cortex within non-epileptic, epileptogenic, or propagated sites, which enabled cortical and hippocampal probes to be combined. Glutamate was now found to be significantly elevated in propagated compared to non-epileptic sites (p = 0.0001). GABA was significantly elevated in epileptogenic compared to non-epileptic sites (p = 0.011) and in propagated compared to epileptogenic sites (p = 0.028). After AED taper, there were no significant changes in glutamate in any site, although it decreased non-significantly in non-localized sites (p = 0.090). Glutamine decreased significantly in lesion sites (p = 0.0095). GABA declined significantly in non-localized sites (p = 0.014). When all sites were examined together, there were overall significant decreases in glutamine (p = 0.0011) and GABA (p \u3c 0.0001). In conclusion, elevations in glutamate and GABA extend beyond epileptogenic sites in patients with refractory epilepsy. Levels of glutamate, glutamine, and GABA were comparable between the hippocampus and cortex. AEDs may increase extracellular levels of glutamine and GABA but are inefficient in reducing glutamate to normal levels in these patients, which may relate to AED resistance

Identification and Modeling of Parameters that Influence Microdialysis Sampling in vitro and in vivo

This dissertation was digitized by the staff of the KU Libraries' Office of Scholarly Communication and Copyright.Microdialysis sampling has matured into a standard technique for sampling the rat brain in vivo. Microdialysis is beginning to be used for sampling from other tissues in laboratory animals. It has also been used in humans. The amount of material that enters or leaves a microdialysis probe is dependent upon many factors, some of which are presented in this dissertation. These factors include forced convection around the dialysis membrane, hindered diffusion through the membrane, and kinetic processes that occur in the tissue space. The aims of these studies were to identify factors that affect extraction efficiency in microdialysis systems and to develop develop methods to quantitate these factors. By setting up a specialized flow apparatus the effects of forced convection around a microdialysis membrane fiber were studied. From this same apparatus, values of the membrane permeability for hydroquinone, caffeine, theophylline, and theobromine were found for cuprophan, cellulose acetate and polyacrylonitrile membranes. A mathematical model was developed to determine the membrane permeability, and it fit the data well. The effects of inhibition of phenacetin and antipyrine metabolism in the liver and acetylcholine metabolism in the brain on the amount of each substance lost from the probe was studied. It was found through the development of a mechanistic model that micro vasculature exchange rates dominate metabolism rates in the liver. Inhibition of metabolism in the liver did not change the amount of material lost from the microdialysis probe during a local infusion. Acetylcholine is removed from the brain through only metabolic processes, thus inhibition affects the amount of material that is lost from the microdialysis probe after a local infusion of acetylcholine in the brain. A human study evaluated the usefulness of microdialysis for determining low levels of caffeine taken ad lib. This final study shows that microdialysis sampling although originally developed for brain studies in the rat, is finding more use in human studies

Mesure du flot sanguin dans le tissu adipeux : implantation et mise au point de la méthode à Sherbrooke

Le flot sanguin est un facteur déterminant de l'activité métabolique du tissu adipeux. En effet, tout échange métabolique ou hormonal dépend obligatoirement du produit du flot sanguin et de la différence artérioveineuse du métabolite étudié.Le stockage et la libération énergétique de même que la sécrétion d'hormones et d'adipocytokines nécessitent une régulation précise du flot sanguin dans le tissu adipeux (FSTA). Ce mémoire comportera essentiellement trois parties. Dans la première, nous ferons la recension des écrits qui concernent la régulation du FSTA. Nous avons écrit une revue sur le sujet. Nous décrirons le rôle de l'insuline, des hormones gastro-intestinales, du système nerveux sympathique et parasympathique, de l'endothéline et de diverses cytokines. Nous montrerons aussi que l'absence d'élévation du FSTA en condition postprandiale est une caractéristique de l'insulino-résistance et de l'obésité. Cette caractéristique justifie l'exploration de la physiologie vasculaire du tissu adipeux pour une meilleure compréhension des mécanismes patho-physiologiques qui sous-tendent ces deux pathologies. Dans la deuxième partie, nous discuterons des diverses techniques de mesure du FSTA chez l'humain, la mesure de clairance du 133 Xe, la microdialyse ou d'autres méthodes plus dispendieuses ou non validées. Nous décrirons ensuite une nouvelle méthode, la microinfusion, qui permet de mesurer le FSTA avec la technique de clairance du 133 Xe et de le faire varier simultanément en infusant dans le site radioactif, de façon continue, des agents vasoactifs. Un détecteur placé au-dessus de la source permet l'enregistrement de la clairance du 133 Xe du tissu adipeux vers la circulation sanguine en relation avec les changements environnementaux induits.Le tissu adipeux sous-cutané abdominal a été choisi comme site anatomique d'investigation. Cette méthode permet la comparaison directe entre des composés vasoactifs, agents pharmacologiques ou hormones, et un contrôle controlatéral situé à la même hauteur sur l'abdomen. La description précise de cette méthode fait l'objet d'un deuxième article. Nous y montrons aussi que la microinfusion améliore la précision et la faisabilité des études physiologiques et pharmacologiques in vivo chez l'humain et qu'elle a déjà permis l'évaluation de plusieurs acteurs potentiels impliqués dans la régulation du FSTA chez des sujets sains. Enfin, dans la troisième partie nous ferons le point de notre expérience de maîtrise, des difficultés et des déboires que nous avons eus dans la mise au point de la technique à Sherbrooke. Nous conclurons en montrant que la microinfusion va permettre d'améliorer notre compréhension des mécanismes sous-jacents au syndrome d'insulino-résistance, au développement du diabète de type 2 et des maladies cardiovasculaires. Son implantation et sa mise au point à Sherbrooke sont donc amplement justifiées

An investigation of the neurochemical mechanisms underlying the contrasting effects of d-amphetamine in two subregions of the rat anterior cingulate cortex.

d-Amphetamine inhibits neuronal uptake, and causes impulse-independent release of monoamines. There are several reports that d-amphetamine increases glutamate efflux in the rat cerebral cortex, but this has not been investigated systematically. It is unclear whether this is direct or secondary to its effects on dopamine transmission. These experiments aimed to compare regulation of extracellular glutamate in two adjacent subregions of the rat anterior cingulate cortex using in vivo microdialysis: the rostral anterior cingulate cortex (rACC) and caudal anterior cingulate cortex (cACC), which are innervated by dopaminergic projections from different brainstem nuclei. The first finding was that the glutamate response to d-amphetamine depended on subregion and route of administration. Glutamate in the cACC but not the rACC was increased by systemic d-amphetamine. Conversely, glutamate in the rACC but not the cACC was increased by local d-amphetamine. Local infusion of dopamine in the rACC mimicked the effect of d-amphetamine, suggesting the glutamate response is mediated by dopamine. This was confirmed by experiments where the glutamate response to local d-amphetamine in the rACC was blocked by the Di-like receptor antagonist SCH23390 but not the D2-like receptor antagonist haloperidol. Local infusion of dihydrokainate (DHK), which inhibits the glial GLT-1 glutamate transporter, did not affect spontaneous efflux of glutamate in either subregion. However, DHK increased glutamate efflux during local infusion of d-amphetamine in the cACC, indicating that GLT-1 normally contributes to clearance of glutamate released by d-amphetamine. In contrast, infusion of DHK reduced glutamate efflux in the rACC of rats given systemic d-amphetamine, suggesting that impairment of GLT-1 function leads to reduced glutamate release (possibly through activation of inhibitory autoreceptors). Such striking neurochemical asymmetries enable spatial focussing of the response to d-amphetamine in the ACC and could contribute to demarcation of the role of each of its subregions in regulation of mood and behaviour