Effects of blood flow on the in vivo recovery of a small diffusible molecule by microdialysis in human skin

Systematic Review on Transdermal/Topical Cannabidiol Trials: A Reconsidered Way Forward

Introduction: This systematic review aimed to assess efficacy and safety for skin-applied formulations containing CBD. Methods: Bibliographic and clinical trial registries were searched for interventional human trials using cutaneously administered CBD or reported plasma CBD concentrations (any species). Results: Eight of 544 articles fitted the selection criteria: 3 placebo-controlled randomized and 5 single-arm trials. Eleven more studies were found in clinical trial databases but not accessible. Symptoms targeted were dermatopathologies or safety (two studies), pain (two), and behavior (one). Doses were 50-250 mg or 0.075-1.0% CBD, but coformulated with other ingredients. Risk of bias was high and reporting deficiencies further compromised data reliability. Diverse methodologies and formulations hampered syntheses for CBD dose, efficacy, and safety. Plasma CBD levels in dogs and rodents were 0.01-5 μM translating to <100 nM free, unbound CBD in humans. Adverse events were uncommon and mild, but meaningless without CBD's contribution to efficacy data. Achievable free CBD plasma concentrations ∼100 nM can interact predominantly with high-affinity CBD targets, for example, TRPA1 and TRPM8 membrane channels that are abundantly expressed in pathological conditions. Even if reached, higher CBD concentrations on less susceptible targets risk complex and unsafe CBD therapy. A conceptual framework is proposed where dermal capillary loops create sinking for topical CBD demonstrating parallels between topical and transdermal CBD administration. Conclusions: Users risk generalizing inadequately designed trials to all CBD preparations. New clinical trials are urgently needed: they must demonstrate that outcomes are solely from CBD pharmacology, are reliable, unbiased, safe, and comparable. Measurements of sustained plasma CBD levels are mandatory, irrespective of administration route for successful translation from in vitro systems that express human molecular targets. Placebos must be appropriate. Transcutaneous and topical formulations need preliminary in vitro studies to optimize CBD skin penetration. Then, users can rationally balance efficacy against potential harms and cost-effectiveness of CBD formulations.

Microdialysis techniques and microdialysis-based patient-near diagnostics

This article will debate the usefulness of POCT measurements and the contribution microdialysis can make to generating valuable information. A particular theme will be the rarely considered difference between ex vivo sampling, which typically generates only a static measure of concentration, and in vivo measurements that are subject to dynamic changes due to mass transfer. Those dynamic changes provide information about the patients' physiological state.

Neuro-inflammatory Sequelae of Minimal Trauma in the Non-traumatized Human Brain: A Microdialysis Study

Cytokine measurement directly from the brain parenchyma by means of microdialysis has documented the activation of certain procedures in vivo, after brain trauma in humans. However, the intercalation of the micro-catheter insertion with the phenomena triggered by the head trauma renders the assessment of the findings problematic. The present study attempts to elucidate the pure effect of minimal trauma, represented by the insertion of the micro-catheter, on the non-traumatized human brain. Microdialysis catheters were implanted in 12 patients with drug-resistant epilepsy, and subjected to invasive electroencephalography with intracranial electrodes. Samples were collected during the first 5 days of monitoring. The dialysate was analyzed using bead flow cytometry, and the concentrations of interleukin (IL)-1, IL-6, IL-8, IL-10, IL-12, and tumor necrosis factor-α (TNF-α) were measured. The levels of IL-1 and IL-8 were found to be raised until 48 h post-implantation, and thereafter they reached a plateau of presumably baseline values. The temporal profile of the IL-6 variation was different, with the increase being much more prolonged, as its concentration had not returned to baseline levels at the fifth day post-insertion. TNF-α was found to be significantly raised only 2 h after implantation. IL-10 and IL-12 did not have any significant response to micro-trauma. These findings imply that the reaction of the neuro-inflammatory mechanisms of the brain exist even after minimal trauma, and is unexpectedly intense for IL-6. Questions may arise regarding the objectivity of findings attributed by some studies to inflammatory perturbation after head injury.

Sampling insulin in different tissue compartments using microdialysis: methodological aspects

Sampling the concentration of insulin in human skin using microdialysis is challenging because of low intracutaneous concentrations and low recovery, presumably due to adsorption of insulin to the microdialysis system. In this study, we aimed to (1) measure how the concentration of insulin varies in three different tissue compartments (intracutaneous, subcutaneous and intravenous) and (2) to study how much insulin is adsorbed to the microdialysis catheter membranes and tubing during a typical microdialysis experiment, both in vivo and in vitro. We hypothesized that (1) the concentration of insulin decreases from the intravenous compartment to the intracutaneous and subcutaneous tissue, and that (2) adsorption of insulin to the microdialysis membrane and tubing impairs the recovery of insulin from the tissue. In this experimental study, microdialysis catheters were inserted intracutaneously, subcutaneously and intravenously in 11 healthy subjects. Systemic endogenous hyperinsulinemia was induced by intake of an oral glucose load. Insulin concentration was measured in the dialysate and in the extracted samples from the catheter membrane and tubings. In vitro microdialysis was performed to investigate the temporal resolution of the adsorption. After an oral glucose load insulin concentration increased intravenously, but not in the intracutaneous or subcutaneous compartments, while glucose, lactate and pyruvate concentrations increased in all compartments. The adsorption of insulin to the microdialysis membrane in vivo was highest in the intravenous compartment (p = 0.01), compared to the intracutaneous and subcutaneous compartments. In vitro, the adsorption to the microdialysis membrane was highest one hour after sampling, then the concentration gradually decreased after three and five hours of sampling. The concentration of insulin in peripheral tissues is low, probably due to decreasing tissue vascularity. Adsorption of insulin to the microdialysis membrane is modest but time-dependent. This finding highlights the importance of a stabilization time for the microdialysis system before sampling tissue analytes.

Modeling Temperature-Dependent Dermal Absorption and Clearance for Transdermal and Topical Drug Applications

A computational model was developed to better understand the impact of elevated skin temperatures on transdermal drug delivery and dermal clearance. A simultaneous heat and mass transport model with emphasis on transdermal delivery system (TDS) applications was developed to address transient and steady-state temperature effects on dermal absorption. The model was tested using representative data from nicotine TDS applied to human skin either in vitro or in vivo. The approximately 2-fold increase of nicotine absorption with a 10°C increase in skin surface temperature was consistent with a 50-65 kJ/mol activation energy for diffusion in the stratum corneum, with this layer serving as the primary barrier for nicotine absorption. Incorporation of a dermal clearance component into the model revealed efficient removal of nicotine via the dermal capillaries at both normal and elevated temperatures. Two-compartment pharmacokinetic simulations yielded systemic drug concentrations consistent with the human pharmacokinetic data. Both in vitro skin permeation and in vivo pharmacokinetics of nicotine delivered from a marketed TDS under normal and elevated temperatures can be satisfactorily described by a simultaneous heat and mass transfer computational model incorporating realistic skin barrier properties and dermal clearance components.

Skin microdialysis: methods, applications and future opportunities-an EAACI position paper

Skin microdialysis (SMD) is a versatile sampling technique that can be used to recover soluble endogenous and exogenous molecules from the extracellular compartment of human skin. Due to its minimally invasive character, SMD can be applied in both clinical and preclinical settings. Despite being available since the 1990s, the technique has still not reached its full potential use as a tool to explore pathophysiological mechanisms of allergic and inflammatory reactions in the skin. Therefore, an EAACI Task Force on SMD was formed to disseminate knowledge about the technique and its many applications. This position paper from the task force provides an overview of the current use of SMD in the investigation of the pathogenesis of chronic inflammatory skin diseases, such as atopic dermatitis, chronic urticaria, psoriasis, and in studies of cutaneous events during type 1 hypersensitivity reactions. Furthermore, this paper covers drug hypersensitivity, UVB-induced- and neurogenic inflammation, and drug penetration investigated by SMD. The aim of this paper is to encourage the use of SMD and to make the technique easily accessible by providing an overview of methodology and applications, supported by standardized operating procedures for SMD in vivo and ex vivo.

Intrabronchial Microdialysis: Effects of Probe Localization on Tissue Trauma and Drug Penetration into the Pulmonary Epithelial Lining Fluid

Recent intrabronchial microdialysis data indicate that the respiratory epithelium is highly permeable to drugs. Of concern is whether intrabronchial microdialysis disrupts the integrity of the respiratory epithelium and thereby alters drug penetration into the pulmonary epithelial lining fluid (PELF). The objective of this study was to investigate the effect of intrabronchial microdialysis on the integrity of the bronchial epithelium. Microdialysis sampling in PELF in proximal (n = 4) and distal bronchi (n = 4) was performed after intravenous inulin and florfenicol administration in anaesthetized pigs. Inulin was used as a marker molecule of permeability of the epithelium, and florfenicol was used as test drug. Bronchial tissue was examined by histopathology (distal and proximal bronchi) and gene expression analysis (RT-qPCR, proximal bronchi) at the termination of the experiment (6.5 hr). The microdialysis probe caused overt tissue trauma in distal bronchi, whereas no histopathological lesions were observed in proximal bronchi. A moderate up-regulation of the pro-inflammatory cytokines IL1B, IL6 and acute-phase reactant serum amyloid A was seen in proximal bronchi surrounding the microdialysis probes suggesting initiation of an inflammatory response. The observed up-regulation is considered to have limited impact on drug penetration during short-term studies. Inulin penetrated the respiratory epithelium in both proximal and distal bronchi without any correlation to histopathological lesions. Likewise, florfenicol penetration into PELF was unaffected by bronchial histopathology. However, this independency of pathology on drug penetration may not be valid for other antibiotics. We conclude that short-term microdialysis drug quantification can be performed in proximal bronchi without disruption of tissue integrity.

Retrodialysis: a review of experimental and clinical applications of reverse microdialysis in the skin

Microdialysis is a method that has been used for decades to recover endogenous mediators, metabolites and drugs from the interstitial space in several tissues of both animals and humans. The principle of microdialysis is the flux of compounds across a semipermeable membrane. The application of microdialysis as a method of drug delivery is a process referred to as retrodialysis, i.e. the introduction of a substance into the extracellular space via a microdialysis probe. Thus, microdialysis also offers opportunities to deliver mediators and drugs to target tissues by adding solutes to the perfusion medium. In this context, retrodialysis combines a method for minimally invasive delivery with a sampling method to study biological processes in health and disease. The aim of this review is to give insight into the use of retrodialysis by outlining examples of retrodialysis studies focusing on applications in skin in animal studies, human experimental investigations and clinical settings.

Continuous online microdialysis using microfluidic sensors: dynamic neurometabolic changes during spreading depolarization

Microfluidic glucose biosensors and potassium ion selective electrodes were used in an in vivo study to measure the neurochemical effects of spreading depolarizations (SD), which have been shown to be detrimental to the injured human brain. A microdialysis probe implanted in the cortex of rats was connected to a microfluidic PDMS chip containing the sensors. The dialysate was also analyzed using our gold standard, rapid sampling microdialysis (rsMD). The glucose biosensor performance was validated against rsMD with excellent results. The glucose biosensors successfully monitored concentration changes, in response to SD wave induction, in the range of 10-400 μM with a second time-resolution. The data show that during a SD wave, there is a time delay of 62 ± 24.8 s (n = 4) between the onset of the increase in potassium and the decrease in glucose. This delay can be for the first time demonstrated, thanks to the high-temporal resolution of the microfluidic sensors sampling from a single tissue site (the microdialysis probe), and it indicates that the decrease in glucose is due to the high demand of energy required for repolarization.

Modeling the human skin barrier--towards a better understanding of dermal absorption

Many drugs are presently delivered through the skin from products developed for topical and transdermal applications. Underpinning these technologies are the interactions between the drug, product and skin that define drug penetration, distribution, and elimination in and through the skin. Most work has been focused on modeling transport of drugs through the stratum corneum, the outermost skin layer widely recognized as presenting the rate-determining step for the penetration of most compounds. However, a growing body of literature is dedicated to considering the influence of the rest of the skin on drug penetration and distribution. In this article we review how our understanding of skin physiology and the experimentally observed mechanisms of transdermal drug transport inform the current models of drug penetration and distribution in the skin. Our focus is on models that have been developed to describe particular phenomena observed at particular sites of the skin, reflecting the most recent directions of investigation.

A human vascular model based on microdialysis for the assessment of the vasoconstrictive dose-response effects of norepinephrine and vasopressin in skin

OBJECTIVE

Microdialysis enables drug delivery in the skin and simultaneous measurement of their effects. The present study aimed to evaluate dose-dependent changes in blood flow and metabolism during microdialysis of norepinephrine and vasopressin.

METHODS

We investigated whether increasing concentrations of norepinephrine (NE, 1.8-59 μmol/L) and vasopressin (VP, 1-100 nmol/L), delivered sequentially in one catheter or simultaneously through four catheters, yield dose-dependent changes in blood flow (as measured using urea clearance) and metabolism (glucose and lactate).

RESULTS

We found a significant dose-dependent vasoconstriction with both drugs. Responses were characterized by a sigmoid dose response model. Urea in the dialysate increased from a baseline of 7.9 ± 1.7 to 10.9 ± 0.9 mmol/L for the highest concentration of NE (p < 0.001) and from 8.1 ± 1.4 to 10.0 ± 1.7 mmol/L for the highest concentration of VP (p = 0.037). Glucose decreased from 2.3 ± 0.7 to 0.41 ± 0.18 mmol/L for NE (p = 0.001) and from 2.7 ± 0.6 to 1.3 ± 0.5 mmol/L for VP (p < 0.001). Lactate increased from 1.1 ± 0.4 to 2.6 ± 0.5 mmol/L for NE (p = 0.005) and from 1.1 ± 0.4 to 2.6 ± 0.5 mmol/L for VP (p = 0.008). There were no significant differences between responses from a single catheter and from those obtained simultaneously using multiple catheters.

CONCLUSIONS

Microdialysis in the skin, either with a single catheter or using multiple catheters, offers a useful tool for studying dose response effects of vasoactive drugs on local blood flow and metabolism without inducing any systemic effects.

Implications for burn shock resuscitation of a new in vivo human vascular microdosing technique (microdialysis) for dermal administration of noradrenaline

INTRODUCTION

Skin has a large dynamic capacity for alterations in blood flow, and is therefore often used for recruitment of blood during states of hypoperfusion such as during burn shock resuscitation. However, little is known about the blood flow and metabolic consequences seen in the dermis secondary to the use vasoactive drugs (i.e. noradrenaline) for circulatory support. The aims of this study were therefore: to develop an in vivo, human microdosing model based on dermal microdialysis; and in this model to investigate effects on blood flow and metabolism by local application of noradrenaline and nitroglycerin by the microdialysis system simulating drug induced circulatory support.

METHOD

Nine healthy volunteers had microdialysis catheters placed intradermally in the volar surface of the lower arm. The catheters were perfused with noradrenaline 3 or 30 mmol/L and after an equilibrium period all catheters were perfused with nitroglycerine (2.2 mmol/L). Dermal blood flow was measured by the urea clearance technique and by laser Doppler imaging. Simultaneously changes in dermal glucose, lactate, and pyruvate concentrations were recorded.

RESULTS

Noradrenaline and nitroglycerine delivered to the dermis by the microdialysis probes induced large time- and dose-dependent changes in all variables. We particularly noted that tissue glucose concentrations responded rapidly to hypoperfusion but remained higher than zero. Furthermore, vasoconstriction remained after the noradrenaline administration implicating vasospasm and an attenuated dermal autoregulatory capacity. The changes in glucose and lactate by vasoconstriction (noradrenaline) remained until vasodilatation was actively induced by nitroglycerine.

CONCLUSION

These findings, i.e., compromised dermal blood flow and metabolism are particularly interesting from the burn shock resuscitation perspective where noradrenaline is commonly used for circulatory support. The importance and clinical value of the results obtained in this in vivo dermal model in healthy volunteers needs to be further explored in burn-injured patients.

Application of dermal microdialysis for the determination of bioavailability of clobetasol propionate applied to the skin of human subjects

Dermal microdialysis was used to assess the bioavailability of a topical corticosteroid, clobetasol propionate, following application onto the skin of human subjects. The penetration of clobetasol propionate from a 4% m/v ethanolic solution applied onto 4 sites on one forearm of healthy human volunteers was studied. A lipid emulsion, Intralipid®, was used as the perfusate and linear microdialysis probes with a 2-kDa cutoff were inserted intradermally at the designated sites. The results indicated that Intralipid could be used as a suitable perfusate for in vivo microdialysis of this lipophilic drug of interest. Furthermore, the study clearly demonstrated the application of dermal microdialysis as a valuable tool to assess the bioavailability/bioequivalence of clobetasol propionate penetration into the skin following topical application.

Probe depth matters in dermal microdialysis sampling of benzoic acid after topical application: an ex vivo study in human skin

Microdialysis (MD) in the skin - dermal microdialysis (DMD) - is a unique technique for sampling of topically as well as systemically administered drugs at the site of action, e.g. sampling of dermatological drug concentrations in the dermis. Debate has concerned the existence of a correlation between the depth of the sampling device - the probe - in the dermis and the amount of drug sampled following topical drug administration. This study evaluates the relation between probe depth and drug sampling using dermal DMD sampling ex vivo in human skin. We used superficial (<1 mm), intermediate (1-2 mm) and deep (>2 mm) positioning of the linear MD probe in the dermis of human abdominal skin, followed by topical application of 4 mg/ml of benzoic acid (BA) in skin chambers overlying the probes. Dialysate was sampled every hour for 12 h and analysed for BA content by high-performance liquid chromatography. Probe depth was measured by 20-MHz ultrasound scanning. The area under the time-versus-concentration curve (AUC) describes the drug exposure in the tissue during the experiment and is a relevant parameter to compare for the 3 dermal probe depths investigated. The AUC(0-12) were: superficial probes: 3,335 ± 1,094 μg·h/ml (mean ± SD); intermediate probes: 2,178 ± 1,068 μg·h/ml, and deep probes: 1,159 ± 306 μg·h/ml. AUC(0-12) sampled by the superficial probes was significantly higher than that of samples from the intermediate and deeply positioned probes (p value <0.05). There was a significant inverse correlation between probe depth and AUC(0-12) sampled by the same probe (p value <0.001, r(2) value = 0.5). The mean extrapolated lag-times (±SD) for the superficial probes were 0.8 ± 0.1 h, for the intermediate probes 1.7 ± 0.5 h, and for the deep probes 2.7 ± 0.5 h, which were all significantly different from each other (p value <0.05). In conclusion, this paper demonstrates that there is an inverse relationship between the depth of the probe in the dermis and the amount of drug sampled following topical penetration ex vivo. The result is of relevance to the in vivo situation, and it can be predicted that the differences in sampling at different probe depths will have a more significant impact in the beginning of a study or in studies of short duration. Based on this study it can be recommended that studies of topical drug penetration using DMD sampling should include measurements of probe depth and that efforts should be made to minimize probe depth variability.

Gastric submucosal microdialysis in the detection of rat stomach ischemia--a comparison of the 3H2O efflux technique with metabolic monitoring

Microdialysis has been utilized for nutritive blood flow measurements, but both the advantages and disadvantages of various approaches have not been evaluated in parallel in the stomach yet. Our aim was to compare the (3)H(2)O efflux technique with biochemical monitoring during temporary celiac artery occlusion in anesthetized rats. Microdialysis probes were implanted in the gastric submucosa and perfused with (3)H(2)O; samples were analyzed for β-activity, glucose, lactate, pyruvate and glycerol. Gastric mucosa and plasma were subjected to morphometry and analysis of myeloperoxidase, total thiols and lactatdehydrogenase. The most dramatic responses to ischemia were observed in lactate/pyruvate and lactate/glucose (%) ratios (6.1-9.3×, p < 0.0001); the changes in (3)H(2)O efflux and glycerol were less pronounced (1.1-1.7×, p < 0.0001 and < 0.01, respectively). (3)H(2)O efflux correlated best with the lactate/glucose ratio and glucose alone (r = 0.693 and -0.681, respectively, p < 0.0001). A correlation was also found between plasma lactatdehydrogenase and relative glycerol release (r = 0.600, p < 0.05). Myeloperoxidase, lactatdehydrogenase and histology score were increased by ischemia/reperfusion (0.06-0.12 nkat g(-1), p < 0.05, 0.26-0.44 nkat g(-1), p < 0.05 and 1.79-2.33, p < 0.05, respectively), macroscopy and plasma thiols remained unchanged. Microdialysis is useful in monitoring gastric ischemia, metabolic monitoring being superior to the (3)H(2)O efflux technique. The results question the efficacy of the utilized model to produce standardized major gastric damage.

How minimally invasive is microdialysis sampling? A cautionary note for cytokine collection in human skin and other clinical studies

It is common to refer to microdialysis as a minimally invasive procedure, likening it to insertion of an artificial capillary. While a comparison of this type allows the process to be easily visualized by those outside the field, it tends to provide a false impression of the localized perturbation of the tissue space that is caused by catheter insertion. With the increased acceptance of microdialysis sampling as a viable in vivo sampling method, many researchers have begun to use the technique to explore inflammatory and immune-mediated diseases in the skin and other organs. Unfortunately, many of the molecules of interest, particularly chemokines and cytokines, are known to be generated during the inflammatory response to wounding and the subsequent cellular events leading to wound repair. With more than 11,000 reports citing the use of microdialysis sampling, only a few researchers have sought to assess the tissue damage that is incurred by probe insertion. For this reason, caution is warranted when collecting these molecules and inferring a role for them in clinical disease states. This commentary seeks to remind the research community of the confounding effects that signaling molecules related to the wounding response will have on clinical studies. Proper controls must be incorporated into all studies in order to assess whether or not particular molecules are truly related to the disease state under investigation or have been generated as part of the tissue response to the wound incurred by microdialysis catheter implantation.

Urea clearance: a new method to register local changes in blood flow in rat skeletal muscle based on microdialysis

SUMMARY

Increasing evidence suggests that local blood flow should be monitored during microdialysis (MD) as the recovery of analytes is affected by local blood flow. At present ethanol clearance is the standard technique for this purpose, but it is not functional at very low perfusion velocities. Here, we introduce a technique for MD whereby local tissue blood flow is recorded by the use of urea clearance (changes inflow/outflow concentration), in conjunction with measurements of tissue metabolism (glucose, lactate and puruvate). MD probes were inserted into the gracilis muscle of 15 rats and perfused with a medium containing urea (20 mmol l(-1)). Changes in muscle blood flow were made by addition of noradrenaline (5 microg ml(-1)) to the perfusion medium at two perfusion velocities (0.6 and 0.4 microl min(-1)). The clearance of urea from the perfusion medium was then calculated and examined in relation to the dose of noradrenaline and to the coexisting changes in extracellular metabolites. The results showed reproducible and dose-dependent changes in blood flow that were induced by noradrenaline. These were characterized by dose-dependent changes in the urea clearance as well as blood-flow-specific changes in the MD metabolic markers (reduction in glucose and increase in lactate). The sensitivity for blood flow changes as assessed by urea clearance (MD) was increased at 0.4 compared with the 0.6 microl min(-1) perfusion speed. The results indicate that inclusion of urea to the perfusion medium may be used to monitor changes in skeletal muscle blood flow at low perfusion velocities and in parallel assess metabolic variables with a high recovery (>90%).

Moxifloxacin distribution in the interstitial space of infected decubitus ulcer tissue of patients with spinal cord injury measured by in vivo microdialysis

We investigated the distribution of moxifloxacin in the interstitial space of normal and infected subcutaneous tissue in patients with spinal cord injury and decubitus ulcers using in vivo microdialysis. Drug concentrations achieved in serum, saliva, normal and infected tissues showed approximately parallel time profiles. The interstitial tissue fluids reached bactericidal levels for common bacteria found in infected skin lesions. Our findings suggest that moxifloxacin exhibits good and similar penetration into the interstitial space fluid in normal subcutaneous tissue and infected decubitus ulcers in patients with spinal cord injury.

Nitric oxide and prostaglandin inhibition during acetylcholine-mediated cutaneous vasodilation in humans

Acetylcholine-induced endothelium-dependent vasodilation in conduit arteries primarily depends on nitric oxide (NO). However, the biochemical mediators in the microvasculature remain less well defined. We tested whether prostaglandins and NO are responsible for cutaneous acetylcholine-mediated vasodilation and if they interact to modulate vasodilation. We measured skin blood flow (SBF) using laser Doppler flow (LDF) with intradermal microdialysis in the calves of 23 healthy volunteers. We examined the response of SBF to different doses of acetylcholine (0.01-100 mM), the nonisoform-specific NO synthase inhibitor, nitro-L-arginine (NLA, 10 mM), the nonspecific cyclo-oxygenase (COX) inhibitor, ketorolac (Keto, 10 mM), and combined NLA + Keto. NLA had no effect on baseline SBF, while Keto increased baseline SBF by approximately 150%. The increase was blunted with combined NLA + Keto. SBF increased by approximately 700% with the highest acetylcholine concentration and reduced by approximately 60% by NLA. Ketorolac alone also reduced the response to acetylcholine, although the reduction varied between 10 and 20% at differing acetylcholine doses. NLA plus ketorolac reduced the responses to different doses of acetylcholine by some 30%, which was intermediate to NOS or COX inhibition alone. These data suggest that cutaneous acetylcholine-mediated endothelium-dependent vasodilation is highly NO-dependent and is also strongly related to the interactions of NO with prostaglandins.

Insulin and contraction increase nutritive blood flow in rat muscle in vivo determined by microdialysis of L-[14C]glucose

In the present study, a mathematical model using the microdialysis outflow: inflow (O/I) ratio of the novel analogue L-[14C]glucose has been developed which allows the calculation of the nutritive (and non-nutritive) flow in muscle as a proportion of total blood flow. Anaesthetized rats had microdialysis probes carrying L-[14C]glucose inserted through a calf muscle group (tibialis/plantaris/gastrocnemius). The nutritive fraction of total blood flow was determined under basal conditions and in response to contraction (electrical field stimulation), insulin (hyperinsulinaemic euglycaemic clamp with 10 mU min(-1) kg(-1) insulin) or saline control from limb blood flow and the microdialysis O/I ratio of L-[14C]glucose. Both contraction and insulin infusion decreased the O/I ratio of L-[14C]glucose and increased total limb blood flow. Calculations based on mathematical models using L-[14C]glucose O/I and limb blood flow revealed that during basal conditions, the nutritive fraction of total flow was 0.38 +/- 0.06, indicating that basal flow was predominantly non-nutritive. Contraction and insulin increased the nutritive fraction to 0.82 +/- 0.24 (P < 0.05) and 0.52 +/- 0.12 (P < 0.05). Thus the increase in limb blood flow from insulin was fully accommodated by nutritive flow, while contraction increased nutritive flow at the expense of non-nutritive flow. This novel method using microdialysis and the O/I ratio of L-[14C]glucose allows the determination of the nutritive fraction of total flow in muscle as well as the proportion of total flow that may be redistributed in response to contraction and insulin.

Estimation of proinflammatory biomarkers of skin irritation by dermal microdialysis following exposure with irritant chemicals

The aim of the present study was to quantify the release of proinflammatory biomarkers by dermal microdialysis after topical exposure with irritant chemicals, Jet fuel (JP-8) and xylene in rat skin. Occlusive dermal exposure (2h) was carried out with 230microl of JP-8 or xylene using Hill top chambers((R)). Linear microdialysis probes (10mm) were inserted in the dermis under urethane anesthesia. The dialysis fluid was pumped at a flow rate of 2microl/min and the dialysate was collected for 7h following probe insertion. The expression of substance P (SP), calcitonin-gene related peptide (CGRP) and prostaglandin E(2) (PGE(2)) in the dialysate following microdialysis was measured by enzyme immunoassay (EIA). The effect of pretreatment with an SP antagonist (SR-140333) and a PGE(2) inhibitor (celecoxib), 6 and 18h before the application of JP-8 was also assessed to further establish the sensitivity of the microdialysis set up. On similar lines, untreated and capsaicin treated control experiments were performed to compare with the SP release following JP-8 treatment. Further, we also investigated the SP release following topical application of xylene. The mean concentrations of SP after the application of JP-8 (90.01+/-3.31) and 3h after its removal (58.66+/-9.36) indicated that JP-8 induced significantly higher release of SP as compared to the baseline value (P<0.05). The release of SP following JP-8 treatment (58.66+/-9.36pg/ml) was comparable to capsaicin (58.18+/-11.29pg/ml). JP-8 exposure resulted in a significant increase (P<0.001) in PGE(2) levels over the baseline control at the end of 1 and 2h of exposure. JP-8 treatment also produced significant increase (P<0.001) in PGE(2) levels as compared to the untreated control during occlusion and 1h following its removal. There was a significant drop (P<0.05) in the PGE(2) levels by the end of 3h following exposure. Pretreatment with SR-140333 and celecoxib significantly reduced (P<0.05) SP and PGE(2) release induced by JP-8. The mean concentrations of SP following xylene exposure (25.50+/-8.80pg/ml) and 3h after its removal (34.37+/-5.61pg/ml) indicated its skin irritation potential. Unlike JP-8, xylene produced a significant increase in SP release only after the removal of occlusion. Pretreatment with SR-140333 significantly blocked the xylene induced SP release. CGRP was not detected in any of the samples. This study demonstrates that dermal microdialysis can be used to quantify skin irritation potential of JP-8 and related irritant chemicals.

What can microdialysis tell us about the temporal and spatial generation of cytokines in allergen-induced responses in human skin in vivo?

This study examined the suitability of microdialysis to assess the time course of cytokine generation from discrete sites within the skin following intradermal injection of allergen. Cytokines were recovered using two microdialysis probes, one close to the point of allergen injection and the other 1 cm away but within the area of the late-phase induration. Skin biopsies taken at both sites were stained immunocytochemically to investigate possible relationships between cytokine generation, expression of adhesion molecules, and recruitment of neutrophils and eosinophils during the late-phase allergic response. The cytokine response to probe insertion was assessed using a single probe in the opposite arm (control). At baseline, microdialysate contained low levels of IL-1alpha, IL-5, IL-8, IL-12, GM-CSF, and TNFalpha (n=27-33). At control sites, this was followed by increases in IL-6 and IL-8 at 3 and 6 hours. Allergen increased TNFalpha levels in 3/11 individuals within 30 minutes at the injection site. Levels of IL-6 and IL-8 rose rapidly and were significantly greater (P<0.05) than that of controls at 3 and 6 hours at both injection and distant sites. Adhesion molecule expression and leukocyte infiltration were elevated only at the allergen injection site, suggesting a complex relationship between cytokine generation and cellular events in allergic inflammation. In conclusion, microdialysis can be used to distinguish temporal and spatial changes in protein profiles in the skin. Furthermore, when used in conjunction with skin biopsies, it provides novel information about the mechanisms of dermal inflammation.

Spatial distribution of cutaneous microvasculature and local drug clearance after drug application on the skin

We analysed quantitatively blood microvessels distribution in normal skin. We conclude that the segmental area of blood vessels peaks approximately 0.1 mm below the skin surface, where the upper cutaneous blood plexus resides. Total blood vessels area then decreases quasi-exponentially to a depth of approx. -0.75 mm, with a decay length of approximately 0.1 mm, which is site and skin condition dependent, but at greater depths the decrease is approx. 6-times less steep. The corresponding permeability sink exhibits a similar, but superficially steeper, depth-profile. The lateral localisation of superficial blood vessels is such that ensures maximum diffusion from and into the capillaries, which affects transdermal drug delivery: each hairpin-like loop is in the centre of a papilla that corresponds to a cluster of corneocytes surrounded by main diffusion pathways. The aggregate area of blood vessels in the skin is >or=2.5-fold greater than total organ surface area under normal physiological conditions. The molecules diffusing through the skin barrier are thus largely cleared in outermost 20% of the organ, which may create a drug concentration maximum in the dermis, if clearance increases significantly with time. Skin microdialysis data are therefore extremely sensitive to cutaneous blood flow (distribution) and sampling. Skin microvasculature and its distribution must consequently be considered in all topical or transdermal drug transport studies, for example, by including suitably formulated clearance term into generalised diffusion equation.

Peripheral amplification of sweating--a role for calcitonin gene-related peptide

Neuropeptides are the mediators of neurogenic inflammation. Some pain disorders, e.g. complex regional pain syndromes, are characterized by increased neurogenic inflammation and by exaggerated sudomotor function. The aim of this study was to explore whether neuropeptides have a peripheral effect on human sweating. We investigated the effects of different concentrations of calcitonin gene-related peptide (CGRP), vasoactive intestinal peptide (VIP) and substance P (SP) on acetylcholine-induced axon reflex sweating in healthy subjects (total n = 18). All substances were applied via dermal microdialysis. The experiments were done in a parallel setting: ACh alone and ACh combined with CGRP, VIP or SP in various concentrations were applied. Acetylcholine (10(-2) m) always elicited a sweating response, neuropeptides alone did not. However, CGRP significantly enhanced ACh-induced sweating (P < 0.01). Post hoc tests revealed that CGRP in physiological concentrations of 10(-7)-10(-9) m was most effective. VIP at any concentration had no significant effect on axon reflex sweating. The duration of the sweating response (P < 0.01), but not the amount of sweat, was reduced by SP. ACh-induced skin blood flow was significantly increased by CGRP (P < 0.01), but unaltered by VIP and SP. The results indicate that CGRP amplifies axon reflex sweating in human skin.

Changes in plasma protein extravasation in rat skin during inflammatory challenges evaluated by microdialysis

Docetaxel and prostaglandin E1 (PGE1) increase transcapillary albumin extravasation and reduce interstitial fluid pressure in the skin. In this study the microdialysate concentration (Cm) of 125I-labeled human serum albumin (125I-HSA) and different-sized endogenous plasma proteins (EPP) was compared to evaluate changes in transcapillary extravasation of plasma proteins. 125I-HSA was also used to estimate changes in the specific activity of albumin. Extravasation of 125I-HSA and EPP from plasma to interstitium in the rat skin was compared during continuous administration of docetaxel and PGE1 by using microdialysis in anesthetized rats. Also, 20 ml of Ringer solution (RS) were injected intravenously during 10 min in a separate group. Two hollow plasmapheresis fibers (3 cm, cut off 3,000 kDa), one acting as control, were placed subcutaneously on the back skin and perfused with RS (5 μl/min, 140 min, collected every 10 min). The size of the different EPP was estimated to be 73, 65, 56, 47, and 39 Å, separated by a size-exclusion high-performance liquid chromatography column and quantified by UV detection (280 nm). Docetaxel (0.5 mg/ml, n = 5) increased Cm of 125I-HSA and EPP of sizes 73, 65, 56, and 39 Å significantly ( P < 0.05) compared with control. PGE1 (20 μg/ml, n = 6) increased Cm of 125I-HSA significantly ( P < 0.05) but none of the different-sized EPP was increased compared with control. Intravenous RS (20 ml, n = 6) increased Cm of 125I-HSA and increased all the different-sized EPP significantly ( P < 0.05) compared with control. Although the microdialysis method is able to monitor qualitative changes in capillary permeability, a quantitative determination of the capillary reflection coefficient or permeability-surface area product was not possible, because steady state between plasma and dialysate was not achieved during the measurement period. The different pattern of extravasation of EPP and 125I-HSA after docetaxel, PGE1, and RS indicates increased interstitial transport rate and/or increased capillary permeability after docetaxel and RS, whereas PGE1 seems to increase transcapillary fluid flux without altering the permeability.

Dermal capillary clearance: physiology and modeling

Substances applied to the skin surface may permeate deeper tissue layers and pass into the body's systemic circulation by entering blood or lymphatic vessels in the dermis. The purpose of this review is an in-depth analysis of the dermal clearance/exchange process and its constituents: transport through the interstitium, permeability of the microvascular barrier and removal via the circulation. We adapt an 'engineering' viewpoint with emphasis on quantifying the dermal microcirculatory physiology, providing the theoretical framework for the physics of key transport processes and reviewing the available computational clearance models in a comparative manner. Selected experimental data which may serve as valuable input to modeling attempts are also reported.

Microdialysis of large molecules

Microdialysis has been used in many tissues, including skin, brain, adipose tissue, muscle, kidney, and gastrointestinal tract, to recover low-molecular mass endogenous mediators, metabolites, and xenobiotics from the interstitial space. Recently, molecules of larger molecular mass, such as plasma proteins, cytokines, growth factors, and neuropeptides, have also been recovered successfully using larger-pore membranes. Microdialysis recovery of large molecules offers the opportunity to identify patterns of protein expression in a variety of tissue spaces and to evaluate clinically useful biomarkers of disease. From this may develop a better understanding of the disease process and its diagnosis and more targeted approaches to therapy.

Vasoactive intestinal peptide fragment VIP10-28 and active vasodilation in human skin

A recent study reported the vasoactive intestinal peptide (VIP) fragment VIP(10-28) inhibited the rise in skin blood flow during heat stress. Our laboratory has reported that the nitric oxide (NO) pathway and histamine receptor-1 (H1)-receptor activation is common to both exogenous VIP-mediated dilation and active vasodilation (AVD). The present study aimed to further examine the specific role for VIP in AVD by using VIP(10-28) to antagonize VIP-mediated dilation in the presence of NO synthase (NOS) inhibition and an H1 antagonist. Study 1 (n = 12) examined whether VIP(10-28) antagonizes vasodilation to exogenous VIP via inhibition of NO-dependent mechanisms. Study 2 (n = 6) investigated AVD in skin sites receiving VIP(10-28) alone and in combination with NOS inhibition. Study 3 (n = 6) examined AVD in sites receiving VIP(10-28) alone and combined VIP(10-28) and H1 antagonism. Due to differences in our findings and those previously published, study 4 (n = 6) investigated whether an increase in baseline skin blood flow could result in a diminished rise in AVD. Red blood cell flux was measured using laser Doppler flowmetry, and cutaneous vascular conductance (flux/mean arterial pressure) was normalized to maximal vasodilation (28 mM sodium nitroprusside). VIP(10-28) augmented vasodilation to exogenous VIP (P < 0.05 vs. control) and hyperthermia (P < 0.05 vs. control). NOS inhibition had no effect on the augmented dilation during exogenous VIP or hyperthermia (P > 0.05). Similarly, H1-receptor antagonists had no effect on the augmented dilation during hyperthermia (P > 0.05 vs. VIP(10-28)). In study 4, percentage of maximal cutaneous vascular conductance was attenuated when baseline skin blood flow was elevated before whole body heating. Our results suggest that VIP(10-28) may be an unsuitable antagonist for examining a role for VIP-mediated dilation in human skin.

Transcutaneous penetration of toluene in rat skin a microdialysis study

Percutaneous absorption of lipophilic substances has major implications for therapeutical use or toxicological effects. We, therefore, using dermal microdialysis, measured local toluene concentrations and assessed the effects of duration of exposure, skin barrier disruption and the use of skin-care products. Three microdialysis membranes (3000 kDa) were inserted intradermally at a length of 2 cm in the abdominal skin of 82 anaesthetized male Wistar rats. They were perfused with albumin solution (5%) at 10 microl/min. A skin area of 1.5 x 0.6 cm above the membranes was exposed to toluene (100%, 200 microl) for 15 or 240 min. Dialysate was sampled at 20-min intervals. Using GC-FPD (gas charomotography flame photometric detector), it was analysed for toluene. In addition, the effects of tape stripping and pretreatment with topical products were assessed. In each of the 12 permutations of exposure time, pretreatments and tape stripping, five to eight animals were investigated. Maximum toluene concentrations were reached at 60 min after exposure (3.07 +/- 0.40 microg/ml, 15 min; 5.38 +/- 0.92 microg/ml, 240 min). In 15-min exposure experiments, dermal toluene concentrations decreased slowly to reach baseline values after 240 min. After 240-min exposure, a plateau of approximately 6 microg/ml was reached after 60 min. Neither tape stripping nor the pretreatment with barrier cream induced a significant change on dermal toluene concentrations. The slow kinetics of toluene penetration results in a steep concentration gradient in the skin with very-high local toluene concentrations and a delayed wash out, which might be relevant not only toxicologically, but also therapeutically.

Microdialysis--a model for studying chronic wounds

Microdialysis has been used for more than 20 years as a method of sampling the interstitial fluid space. It has been used in both animals and human tissues, in vivo. The principle of microdialysis is based on the passive diffusion of a compound along its concentration gradient. One major advantage of this sampling technique is that it is simple, relatively cheap, and minimally invasive. Consequently, microdialysis has been employed in a variety of research and clinical settings to recover endogenous molecules and metabolites from the tissue space. It has also been used to measure the tissue penetration of xenobiotics and to follow their temporal and spatial distribution. Most recently, microdialysis has begun to be used as a diagnostic tool and its application to clinical investigation at the bedside explored. This review describes the principles of the technique of microdialysis and its current uses in both an experimental and clinical setting. It goes on to consider current methods of wound fluid sampling and the range of bioactive molecules that have been detected in wound fluid recovered using these techniques. Finally, the use of microdialysis as a novel method for sampling wound fluid in vivo and its ability to provide a fluid that is unaffected by the sampling method and that is representative of the wound environment is discussed.

Microdialysis

Microdialysis is a probe-based sampling method, which, if linked to analytical devices, allows for the measurement of drug concentration profiles in selected tissues. During the last two decades, microdialysis has become increasingly popular for preclinical and clinical pharmacokinetic studies. The advantage of in vivo microdialysis over traditional methods relates to its ability to continuously sample the unbound drug fraction in the interstitial space fluid (ISF). This is of particular importance because the ISF may be regarded as the actual target compartment for many drugs, e.g. antimicrobial agents or other drugs mediating their action through surface receptors. In contrast, plasma concentrations are increasingly recognised as inadequately predicting tissue drug concentrations and therapeutic success in many patient populations. Thus, the minimally invasive microdialysis technique has evolved into an important tool for the direct assessment of drug concentrations at the site of drug delivery in virtually all tissues. In particular, concentrations of transdermally applied drugs, neurotransmitters, antibacterials, cytotoxic agents, hormones, large molecules such as cytokines and proteins, and many other compounds were described by means of microdialysis. The combined use of microdialysis with non-invasive imaging methods such as positron emission tomography and single photon emission tomography opened the window to exactly explore and describe the fate and pharmacokinetics of a drug in the body. Linking pharmacokinetic data from the ISF to pharmacodynamic information appears to be a straightforward approach to predicting drug action and therapeutic success, and may be used for decision making for adequate drug administration and dosing regimens. Hence, microdialysis is nowadays used in clinical studies to test new drug candidates that are in the pharmaceutical industry drug development pipeline.

Excised porcine skin experimental systems to validate quantitative microdialysis methods for determination of drugs in skin after topical application

Microdialysis is useful as a method to evaluate the disposition of drugs in the skin to design improved transdermal delivery systems (TDDSs). In this study, quantitative microdialysis methods were validated in excised porcine skin experimental systems in vitro. Flurbiprofen (FP), used as a model drug, showed high affinity for the skin tissues in equilibrium states between the medium and skin. The membrane clearances of FP for permeation through the membrane of a dialysis fiber placed in the skin (CL(m in S)) were lower than that in the medium. The adsorption of components in the skin to the membrane surface of the dialysis fiber and accumulation of FP near the dialysis fiber are the most likely reasons for this. When CL(m in S) was used to predict the extracellular FP concentration in skin (C(T)), the value obtained was lower than that expected from the FP concentration in the medium on the dermis side, which should be equal to C(T) at equilibrium. In the zero net flux (ZNF) method, in which the concentration difference of perfusate (DeltaC) between the inflow and outflow were used to obtain C(T), the predicted C(T) was similar to the expected value. In an in vitro skin permeation experiment, the ZNF method was used for the prediction of C(T) near the dialysis fiber. The predicted C(T) was over 10 times higher than the FP concentration in the medium on the dermis side, suggesting a concentration gradient in the dermis. Although the ZNF method is good for predicting the C(T) in skin, the mass balance has to be considered for the quantitative evaluation of the skin permeation of drugs. In this study, the effect of the mass transfer of FP from the perfusate to the skin on the cumulative amount of FP passing through the skin was relatively low because of the use of suitable solutions as perfusate. The perfusion conditions and schedules should be designed carefully for quantitative evaluations using the ZNF method. These results provide useful information for the in vivo application of quantitative microdialysis to evaluate TDDS.

Modulation of microvascular function following low-dose exposure to the organophosphorous compound malathion in human skin in vivo

This study investigates whether malathion, a widely used organophosphate insecticide, has its effects on cutaneous vasculature in healthy human volunteers through its anticholinergic activity or through the modulation of other, noncholinergic pathways. Acute, low-dose exposure to malathion (10 mg/ml for 5 h under occlusive dressing) caused a significant increase in cutaneous blood flux, monitored by using laser-Doppler flowmetry and imaging. It had little effect on tissue levels of ACh, nitric oxide, and histamine assayed in dermal dialysate collected from malathion-exposed and control-treated skin. The duration of the cutaneous vascular response to exogenous ACh (2%) delivered by iontophoresis was significantly enhanced by preexposure to malathion, both <1 h after its removal and 24 h later ( P < 0.001). At <1 h, the time to 50% decay of the response was 24 ± 4 and 50 ± 8 min in control and malathion-treated skin, respectively. Malathion also enhanced the size and duration of the axon reflex-mediated vasoresponse to ACh. The increase in blood flux to malathion and the endothelium-mediated response to exogenous ACh, both in the presence and absence of malathion, were attenuated by pretreatment of the skin with atropine and local anesthesia ( P < 0.01). We conclude that short-term exposure to a single low dose of malathion causes prolonged modulation of the physiological function of the cutaneous vasculature and that this is, in part, through its action on acetylcholinesterase at both neuronal and nonneuronal sites.

A rabbit ear flap perfusion experiment to evaluate the percutaneous absorption of drugs

A rabbit ear flap single-pass perfusion system was examined as an experimental method for studying the relationship between the physiological conditions of tissues and drug disposition after topical applications. Tyrode solutions containing bovine serum albumin (BSA) and sucrose or flurbiprofen (FP), used as a model drug, were perfused through the vessel in the ear flap to evaluate the physiological conditions prior to the application of FP to the skin surface. The extracellular volume and distribution properties of FP in the perfused ear were similar to those in an in vivo experimental system. In addition, the perfused ear flap exhibited a pharmacological response to bradykinin (BK). The amount of FP in the outflow Tyrode solution containing BSA after application to the skin surface of the perfused ear decreased with the addition of BK, while that in the tissues under the application site increased. FP binds to BSA, which leaked from the intravascular space, and could be retained in the tissues under the application site. The protein binding also affected the redistribution of FP to other tissues in the ear flap after application to the skin. The rabbit ear perfusion system is a useful method for studying the percutaneous absorption of drugs especially variations in the disposition of drugs in oedematous tissues.

Penetration of moxifloxacin into healthy and inflamed subcutaneous adipose tissues in humans

The present study addressed the ability of moxifloxacin to penetrate into healthy and inflamed subcutaneous adipose tissues in 12 patients with soft tissue infections (STIs). Penetration of moxifloxacin into the interstitial space fluid of healthy and inflamed subcutaneous adipose tissues was measured by use of in vivo microdialysis following administration of a single intravenous dosage of 400 mg in six diabetic and six nondiabetic patients with STIs. For the entire study population, the mean time-concentration profile of free moxifloxacin in plasma was identical to the time-concentration profile of free moxifloxacin in tissue (P was not significant). For healthy and inflamed adipose tissues for the diabetic subgroup, the mean moxifloxacin areas under the concentration-time curves (AUCs) from 0 to 8 h (AUC(0-8)s) were 8.1 +/- 7.1 and 3.7 +/- 1.9 mg.h/liter, respectively (P was not significant). The ratios of the mean AUC(0-8) for inflamed tissue/AUC(0-8) for free moxifloxacin in plasma were 0.5 +/- 0.4 for diabetic patients and 1.2 +/- 0.8 for nondiabetic patients (P was not significant). The ratios of the AUCs from 0 to 24 h for free moxifloxacin in plasma/MIC at which 90% of isolates are inhibited were >58 and 121 h for Streptococcus species and methicillin-sensitive Staphylococcus aureus, respectively. Concentrations of moxifloxacin effective against clinically relevant bacterial strains are reached in plasma and in inflamed and healthy adipose tissues. Thus, the pharmacokinetics of moxifloxacin in tissue and plasma support its use for the treatment of STIs in diabetic and nondiabetic patients.

Opioid-induced mast cell activation and vascular responses is not mediated by mu-opioid receptors: an in vivo microdialysis study in human skin

Activation of mast cells and the systemic release of histamine is a common side effect of opioids. Nevertheless, fentanyl and its derivatives show only a slight activation of mast cells with a subsequent liberation of histamine and tryptase. In this study, we used intradermal microdialysis to assess whether this stimulatory effect of opioids on mast cells depends on the activation of opioid receptors. This new approach allowed us to measure the dose-dependent release of histamine and tryptase from mast cells and the subsequent vascular and sensory effect without systemic side effects in volunteers. The opiate codeine and the synthetic opioids meperidine, fentanyl, alfentanil, sufentanil, remifentanil, buprenorphine, and the opioid antagonist naloxone were tested. Only codeine and meperidine induced mast cell activation with the release of tryptase and histamine, leading to protein extravasation, flare reactions, and itch sensations. Because naloxone did not attenuate these effects, it is unlikely that mu-opioid receptors are involved in the activation of mast cells.

IMPLICATIONS

Opioid effects on mast cells were assessed using intradermal microdialysis. Mast cell activation was seen with codeine and meperidine; no other opioid induced degranulation. Therefore, histamine release seen at large concentrations of potent micro agonists is caused by an unspecific effect rather than an activation of opioid receptors.