Reversible inhibition of efflux transporters by hydrogel microdevices

Oral drug delivery is a preferred administration route due to its low cost, high patient compliance and fewer adverse events compared to intravenous administration. However, many pharmaceuticals suffer from poor solubility and low oral bioavailability. One major factor that contributes to low bioavailability are efflux transporters which prevent drug absorption through intestinal epithelial cells. P-glycoprotein (P-gp) and Breast Cancer Resistance Protein (BCRP) are two important efflux transporters in the intestine functioning to prevent toxic materials from entering systemic circulation. However, due to its broad substrate specificity, P-gp limits the absorption of many therapeutics, including chemotherapeutics and antibacterial agents. Methods to inhibit P-gp with competitive inhibitors have not been clinically successful. Here, we show that micron scale devices (microdevices) made from a commonly used biomaterial, polyethylene glycol (PEG), inhibit P-gp through a biosimilar mucus in Caco-2 cells and that transporter function is restored when the microdevices are removed. Microdevices were shown to inhibit P-gp mediated transport of calcein AM, doxorubicin, and rhodamine 123 (R123) and BCRP mediated transport of BODIPY-FL-prazosin. When in contact with Caco-2 cells, microdevices decrease the cell surface amount of P-gp without affecting the passive transport. Moreover, there was an increase in mucosal to serosal transport of R123 with microdevices in an ex-vivo mouse model and increased absorption in vivo. This biomaterial-based approach to inhibit efflux transporters can be applied to a range of drug delivery systems and allows for a nonpharmacologic method to increase intestinal drug absorption while limiting toxic effects.

The pharmacology of α1-adrenoceptor subtypes

This review examines the functions of α₁-adrenoceptor subtypes, particularly in terms of contraction of smooth muscle. There are 3 subtypes of α₁-adrenoceptor, α_1A- α_1B- and α_1D-adrenoceptors. Evidence is presented that the postulated α_1L-adrenoceptor is simply the native α_1A-adrenoceptor at which prazosin has low potency. In most isolated tissue studies, smooth muscle contractions to exogenous agonists are mediated particularly by α_1A-, with a lesser role for α_1D-adrenoceptors, but α_1B-adrenoceptors are clearly involved in contractions of some tissues, for example, the spleen. However, nerve-evoked responses are the most crucial physiologically, so that these studies of exogenous agonists may overestimate the importance of α_1A-adrenoceptors. The major α₁-adrenoceptors involved in blood pressure control by sympathetic nerves are the α_1D- and the α_1A-adrenoceptors, mediating peripheral vasoconstrictor actions. As noradrenaline has high potency at α_1D-adrenceptors, these receptors mediate the fastest response and seem to be targets for neurally released noradrenaline especially to low frequency stimulation, with α_1A-adrenoceptors being more important at high frequencies of stimulation. This is true in rodent vas deferens and may be true in vasopressor nerves controlling peripheral resistance and tissue blood flow. The α_lA-adrenoceptor may act mainly through Ca²⁺ entry through L-type channels, whereas the α_1D-adrenoceptor may act mainly through T-type channels and exhaustable Ca²⁺ stores. α₁-Adrenoceptors may also act through non-G-protein linked second messenger systems. In many tissues, multiple subtypes of α-adrenoceptor are present, and this may be regarded as the norm rather than exception, although one receptor subtype is usually predominant.

Virtual Screening against Phosphoglycerate Kinase 1 in Quest of Novel Apoptosis Inhibitors

Inhibition of apoptosis is a potential therapy to treat human diseases such as neurodegenerative disorders (e.g., Parkinson’s disease), stroke, and sepsis. Due to the lack of druggable targets, it remains a major challenge to discover apoptosis inhibitors. The recent repositioning of a marketed drug (i.e., terazosin) as an anti-apoptotic agent uncovered a novel target (i.e., human phosphoglycerate kinase 1 (hPgk1)). In this study, we developed a virtual screening (VS) pipeline based on the X-ray structure of Pgk1/terazosin complex and applied it to a screening campaign for potential anti-apoptotic agents. The hierarchical filters in the pipeline (i.e., similarity search, a pharmacophore model, a shape-based model, and molecular docking) rendered 13 potential hits from Specs chemical library. By using PC12 cells (exposed to rotenone) as a cell model for bioassay, we first identified that AK-918/42829299, AN-465/41520984, and AT-051/43421517 were able to protect PC12 cells from rotenone-induced cell death. Molecular docking suggested these hit compounds were likely to bind to hPgk1 in a similar mode to terazosin. In summary, we not only present a versatile VS pipeline for potential apoptosis inhibitors discovery, but also provide three novel-scaffold hit compounds that are worthy of further development and biological study.

The cytotoxicity of the α1-adrenoceptor antagonist prazosin is linked to an endocytotic mechanism equivalent to transport-P

Since the α1-adrenergic antagonist prazosin (PRZ) was introduced into medicine as a treatment for hypertension and benign prostate hyperplasia, several studies have shown that PRZ induces apoptosis in various cell types and interferes with endocytotic trafficking. Because PRZ is also able to induce apoptosis in malignant cells, its cytotoxicity is a focus of interest in cancer research. Besides inducing apoptosis, PRZ was shown to serve as a substrate for an amine uptake mechanism originally discovered in neurones called transport-P. In line with our hypothesis that transport-P is an endocytotic mechanism also present in non-neuronal tissue and linked to the cytotoxicity of PRZ, we tested the uptake of QAPB, a fluorescent derivative of PRZ, in cancer cell lines in the presence of inhibitors of transport-P and endocytosis. Early endosomes and lysosomes were visualised by expression of RAB5-RFP and LAMP1-RFP, respectively; growth and viability of cells in the presence of PRZ and uptake inhibitors were also tested. Cancer cells showed co-localisation of QAPB with RAB5 and LAMP1 positive vesicles as well as tubulation of lysosomes. The uptake of QAPB was sensitive to transport-P inhibitors bafilomycin A1 (inhibits v-ATPase) and the antidepressant desipramine. Endocytosis inhibitors pitstop(®) 2 (general inhibitor of endocytosis), dynasore (dynamin inhibitor) and methyl-β-cyclodextrin (cholesterol chelator) inhibited the uptake of QAPB. Bafilomycin A1 and methyl-β-cyclodextrin but not desipramine were able to preserve growth and viability of cells in the presence of PRZ. In summary, we confirmed the hypothesis that the cellular uptake of QAPB/PRZ represents an endocytotic mechanism equivalent to transport-P. Endocytosis of QAPB/PRZ depends on a proton gradient, dynamin and cholesterol, and results in reorganisation of the LAMP1 positive endolysosomal system. Finally, the link seen between the cellular uptake of PRZ and cell death implies a still unknown pro-apoptotic membrane protein with affinity towards PRZ.

Effect of amitriptyline on adrenergic receptor number and second messenger function in rat brain

Radioligand binding studies were done to investigate the effect of chronic administration of Amitriptyline on alpha1-adrenoceptor (alpha1-AR) receptor mediated response to inositol triphosphate (IP3) in rat brain. Our studies revealed a significant decrease in the densities of alpha1-ARs in cortex and cerebellum of rat brain after chronic administration of Amitriptyline (10 mg kg-1 b.wt.). However, there was no significant change in the affinity of [3H]prazosin to alpha1-ARs. Displacement studies showed that Amitriptyline has higher affinity for alpha1-AR with a Ki value of 182+/-16 nM. Significant change was observed in basal IP3 activity in cortex and cerebellum after Amitriptyline exposure. In cortex and cerebellum of experimental rats the NE (Norepinephrine) stimulated IP3 activity was significantly decreased (1460+/-102 DPM/g tissue; p<0.0001; 1188+/-112 DPM/g tissue; p<0.0001), when compared to NE stimulated IP3 activity (4152+/-286 and 3952+/-245 DPM/g tissue, respectively) in control rats. The decrease in NE stimulated IP3 activity in both regions may be due to the significant downregulation of alpha1-ARs in cortex after Amitriptyline exposure as these sites are positively coupled to IP3. The observed significant decrease in alphal-ARs with concomitant decrease in NE stimulated IP3 activity, after Amitriptyline treatment, suggests that Amitriptyline which has high affinity for these sites, acts by modulating the alpha1-AR receptor mediated response in brain.

α1-Adrenoceptor Subtypes in the Rat Ventricular Muscle

Scatchard analyses of [3H]prazosin binding in rat ventricular muscle membranes showed biphasic curves, which identified α1High- and α1Low-affinity sites. The α1High-affinity site was completely inhibited by 1 μm phenoxybenzamine. The displacement potencies of α1-adrenergic antagonists were characterized by [3H]prazosin binding to α1High. and α1Low-affinity sites in the absence and presence of 1 μm phenoxybenzamine. The affinities of most chemicals for α1Low-affinity sites were significantly lower than those for α1High-affinity sites, but WB-4101 (2-(2,6-dimethoxy-phenoxyethyl)aminomethyl-1,4-benzodioxane), arotinolol, cinanserin, nifedipine, and p-aminoclonidine had the same affinities for both α1Low- and α1High-affinity sites. These results show that two α1-adrenoceptor subtypes, α1High- and α1Low-affinity, are present in the rat heart, and that there are physical variations in α1-adrenoceptor binding sites, based on their selectivity to antagonists.

Quantification of [11C]GB67 binding to cardiac alpha1-adrenoceptors with positron emission tomography: validation in pigs

INTRODUCTION

An increase in human cardiac alpha(1)-adrenoceptor (alpha(1)-AR) density is associated with various diseases such as myocardial ischemia, congestive heart failure, hypertrophic cardiomyopathy and hypertension. Positron emission tomography (PET) with an appropriate radioligand offers the possibility of imaging receptor function in the normal and diseased heart. [(11)C]GB67, an analogue of prazosin, has been shown in rats to have potential as a PET ligand with high selectivity to alpha(1)-AR. However, alpha(1)-AR density is up to ten times higher in rat heart compared to that in man. The aim of the present preclinical study was to extend the previous evaluation to a large mammal heart, where the alpha(1)-AR density is comparable to man, and to validate a method for quantification before PET studies in man.

METHODS

Seven [(11)C]GB67 PET studies, with weight-adjusted target dose of either 5.29 MBq kg(-1) (pilot, test-retest and baseline-predose studies) or 8.22 MBq kg(-1) (baseline-displacement studies), were performed in four anaesthetised pigs (39.5 +/- 3.9 kg). Total myocardial volume of distribution (V (T)) was estimated under different pharmacological conditions using compartmental analysis with a radiolabelled metabolite-corrected arterial plasma input function. A maximum possible blocking dose of 0.12 mumol kg(-1) of unlabeled GB67 was given 20 min before [(11)C]GB67 administration in the predose study and 45 min after administration of [(11)C]GB67 in the displacement study. In addition, [(15)O]CO (3,000 MBq) and [(15)O]H(2)O, with weight adjusted target dose of 10.57 MBq kg(-1), were also administered for estimation of blood volume recovery (RC) of the left ventricular cavity and myocardial perfusion (MBF), respectively.

RESULTS

[(11)C]GB67 V (T) values (in ml cm(-3)) were estimated to be 24.2 +/- 5.5 (range, 17.3-31.3), 10.1 (predose) and 11.6 (displacement). MBF did not differ within each pig, including between baseline and predose conditions. Predose and displacement studies showed that specific binding of [(11)C]GB67 to myocardial alpha(1)-ARs accounts for approximately 50% of V (T).

CONCLUSION

The present study offers a methodology for using [(11)C]GB67 as a radioligand to quantify human myocardial alpha(1)-ARs in clinical PET studies.

Impaired expression and function of breast cancer resistance protein (Bcrp) in brain cortex of streptozocin-induced diabetic rats

The aim of this study was to investigate whether diabetes mellitus (DM) affected breast cancer resistance protein (Bcrp) function and expression in rat brain. 5-week and 8-week diabetic rats were induced by streptozocin (STZ). Bcrp expression and function in brain cortex were assessed by western blot and measuring the brain-to-plasma concentration ratios of two typical substrates prazosin and cimetidine, respectively. The diabetic rats were treated with three different agents insulin, aminoguanidine (AG) and metformin (MET). It was found that the brain-to-plasma ratios of prazosin and cimetidine in diabetic rats were significantly higher than those of control rats, which were dependent on duration of diabetes. Lower levels of Bcrp were found in brain cortex of diabetic rats, which were in parallel with increase of brain-to-plasma ratios. Insulin treatment may attenuate the impairment of Bcrp expression and function induced by diabetes. Aminoguanidine and metformin treatment did not prevent the impairment of Bcrp function and expression in brain cortex of diabetic rats. All results gave a conclusion that STZ-induced DM may induce the impairment of function and expression of Bcrp in brain cortex, and lower levels of insulin may mainly contribute to Bcrp dysfunction in brain.

Sipholenol A, a marine-derived sipholane triterpene, potently reverses P-glycoprotein (ABCB1)-mediated multidrug resistance in cancer cells

Through extensive screening of marine sponge compounds, the authors have found that sipholenol A, a sipholane triterpene isolated from the Red Sea sponge, Callyspongia siphonella, potently reversed multidrug resistance (MDR) in cancer cells that overexpressed P-glycoprotein (P-gp). In experiments, sipholenol A potentiated the cytotoxicity of several P-gp substrate anticancer drugs, including colchicine, vinblastine, and paclitaxel, but not the non-P-gp substrate cisplatin, and significantly reversed the MDR of cancer cells KB-C2 and KB-V1 in a concentration-dependent manner. Furthermore, sipholenol A had no effect on the response to cytotoxic agents in cells lacking P-gp expression or expressing MDR protein 1 or breast cancer resistance protein. Sipholenol A (IC(50) > 50 microM) is not toxic to all the cell lines that were used, regardless of their membrane transporter status. Accumulation and efflux studies with the P-gp substrate [(3)H]-paclitaxel demonstrated that sipholenol A time-dependently increased the intracellular accumulation of [(3)H]-paclitaxel by directly inhibiting P-gp-mediated drug efflux. In addition, sipholenol A did not alter the expression of P-gp after treating KB-C2 and KB-V1 cells for 36 h and 72 h. However, it efficaciously stimulated the activity of ATPase of P-gp and inhibited the photolabeling of this transporter with its transport substrate [(125)I]-iodoarylazidoprazosin. Overall, the present results indicate that sipholenol A efficiently inhibits the function of P-gp through direct interactions, and sipholane triterpenes are a new class of potential reversing agents for treatment of MDR in P-gp-overexpressing tumors.

Metabolism of prazosin in rat, dog, and human liver microsomes and cryopreserved rat and human hepatocytes and characterization of metabolites by liquid chromatography/tandem mass spectrometry

Prazosin (2-[4-(2-furanoyl)-piperazin-1-yl]-4-amino-6,7-dimethoxyquinazoline) is an antihypertensive agent that was introduced to the market in 1976. It has since established an excellent safety record. However, in vitro metabolism of prazosin has not been investigated. This study describes the in vitro biotransformation of prazosin in liver microsomes from rats, dogs, and humans, as well as rat and human cryopreserved hepatocytes and characterization of metabolites using liquid chromatography/tandem mass spectrometry. The major in vivo biotransformation pathways reported previously in rats and dogs include demethylation, amide hydrolysis, and O-glucuronidation. These metabolic pathways were also confirmed in our study. In addition, several new metabolites were characterized, including a stable carbinolamine, an iminium species, and an enamine-all formed via oxidation of the piperazine ring. Two ring-opened metabolites generated following oxidative cleavage of the furan ring were also identified. Using semicarbazide hydrochloride as a trapping agent, an intermediate arising from opening of the furan ring was captured as a pyridazine product. In the presence of glutathione, three glutathione conjugates were detected in microsomal incubations, although they were not detected in cryopreserved hepatocytes. These data support ring opening of the furan via a reactive gamma-keto-alpha,beta-unsaturated aldehyde intermediate. In the presence of UDP-glucuronic acid, prazosin underwent conjugation to form an N-glucuronide not reported previously. Our in vitro investigations have revealed additional metabolic transformations of prazosin and have shown the potential of prazosin to undergo bioactivation through metabolism of the furan ring to a reactive intermediate.

Cholesterol potentiates ABCG2 activity in a heterologous expression system: improved in vitro model to study function of human ABCG2

ABCG2, a transporter of the ATP-binding cassette family, is known to play a prominent role in the absorption, distribution, metabolism, and excretion of xenobiotics. Drug-transporter interactions are commonly screened by high-throughput systems using transfected insect and/or human cell lines. The determination of ABCG2-ATPase activity is one method to identify ABCG2 substrate and inhibitors. We demonstrate that the ATPase activities of the human ABCG2 transfected Sf9 cell membranes (MXR-Sf9) and ABCG2-overexpressing human cell membranes (MXR-M) differ. Variation due to disparity in the glycosylation level of the protein had no effect on the transporter. The influence of cholesterol on ABCG2-ATPase activity was investigated because the lipid compositions of insect and human cells are largely different from each other. Differences in cholesterol content, shown by cholesterol loading and depletion experiments, conferred the difference in stimulation of basal ABCG2-ATPase of the two cell membranes. Basal ABCG2-ATPase activity could be stimulated by sulfasalazine, prazosin, and topotecan, known substrates of ABCG2 in cholesterol-loaded MXR-Sf9 and MXR-M cell membranes. In contrast, ABCG2-ATPase could not be stimulated in MXR-Sf9 or in cholesterol-depleted MXR-M membranes. Moreover, cholesterol loading significantly improved the drug transport into inside-out membrane vesicles prepared from MXR-Sf9 cells. MXR-M and cholesterol-loaded MXR-Sf9 cell membranes displayed similar ABCG2-ATPase activity and vesicular transport. Our study indicates an essential role of membrane cholesterol for the function of ABCG2.

Synthesis and 5-HT2A, 5-HT1A and alpha1-binding affinities of 2-[2-Hydroxy-3-(pyridin-3-yl-methyl)amino]-, 2-[2-hydroxy-3-(2-pyridin-2-yl-ethyl)amino]- and 2-[2-hydroxy-3-(4-N-methyl-piperazin-1-yl)-amino]propoxybenzaldehyde-O-(substituted) benzyl oximes

Some oxime ether-substituted aryloxypropanolamines 3-5, structurally related to the active metabolite 2 of sarpogrelate 1, were synthesized and tested for their affinities at 5-HT2A and 5-HT1A serotoninergic receptors as well as at the alpha1-adrenoceptor. The results show that the compounds possess, at least partially, the ability of the model compounds 1 and 2 to interact with the 5-HT2A-receptors; they have the same selectivity towards 5-HT2A receptors vs alpha1-adrenoceptors.

Indorenate modifies a1-adrenergic and benzodiazepine receptor binding in the rat brain: an autoradiography study

Indorenate (5-methoxytryptamine-beta-methylcarboxylate) is a 5-HT1A receptor agonist that produces antihypertensive, anxiolytic, antidepressant and anticonvulsant effects. However, there is evidence suggesting that these effects could involve the activation of benzodiazepine (BZD) receptors but not the activation of a1-adrenergic receptors. The goal of this study was to analyse the effect of indorenate on a1-adrenergic and BZD receptor binding in specific rat brain areas by using in-vitro autoradiography. Coronal brain sections from male Wistar rats were used for labelling 5-HT1A (3H-8-OH-DPAT, 2 nM), a1-adrenergic (3H-prazosin, 2 nM) and BZD (3H-flunitrazepam, 2 nM) receptor binding in the presence or absence of indorenate (1 microM). Indorenate totally displaced 3H-8-OH-DPAT binding in all the brain areas evaluated. It decreased 3H-prazosin binding just in the frontal (30%) and sensorimotor (32%) cortices and in the thalamus (21%). Additionally, indorenate diminished 3H-flunitrazepam binding only in the cingulate (16%) and piriform (18%) cortices as well as in the dorsal raphe nucleus (18%). These results confirm that indorenate is a 5-HT1A ligand and suggest the possible participation of a1-adrenergic and BZD receptors in its pharmacological properties.

Modulation of the function of the multidrug resistance-linked ATP-binding cassette transporter ABCG2 by the cancer chemopreventive agent curcumin

Curcumin (curcumin I), demethoxycurcumin (curcumin II), and bisdemethoxycurcumin (curcumin III) are the major forms of curcuminoids found in the turmeric powder, which exhibit anticancer, antioxidant, and anti-inflammatory activities. In this study, we evaluated the ability of purified curcuminoids to modulate the function of either the wild-type 482R or the mutant 482T ABCG2 transporter stably expressed in HEK293 cells and drug-selected MCF-7 FLV1000 and MCF-7 AdVp3000 cells. Curcuminoids inhibited the transport of mitoxantrone and pheophorbide a from ABCG2-expressing cells. However, both cytotoxicity and [(3)H]curcumin I accumulation assays showed that curcuminoids are not transported by ABCG2. Nontoxic concentration of curcumin I, II, and III sensitized the ABCG2-expressing cells to mitoxantrone, topotecan, SN-38, and doxorubicin. This reversal was not due to reduced expression because ABCG2 protein levels were unaltered by treatment with 10 mumol/L curcuminoids for 72 hours. Curcumin I, II, and III stimulated (2.4- to 3.3-fold) ABCG2-mediated ATP hydrolysis and the IC(50)s were in the range of 7.5 to 18 nmol/L, suggesting a high affinity of curcuminoids for ABCG2. Curcuminoids also inhibited the photolabeling of ABCG2 with [(125)I]iodoarylazidoprazosin and [(3)H]azidopine as well as the transport of these two substrates in ABCG2-expressing cells. Curcuminoids did not inhibit the binding of [alpha-(32)P]8-azidoATP to ABCG2, suggesting that they do not interact with the ATP-binding site of the transporter. Collectively, these data show that, among curcuminoids, curcumin I is the most potent modulator of ABCG2 and thus should be considered as a treatment to increase the efficacy of conventional chemotherapeutic drugs.

Adrenergic receptor density in brown adipose tissue of active and hibernating hamsters and ground squirrels

The ligand-binding characteristics (B(max) and K(D)) of alpha(1)- and beta(1)/beta(2)-adrenoceptors were investigated in membranes prepared from brown adipose tissue (BAT) of warm-acclimated, cold-acclimated, hibernating and arousing ground squirrels (Spermophillus undulatus) and hamsters (Mesocricetus auratus) by specific binding of [(3)H]prazosin and [(3)H]CGP-12177, respectively. The physiological state did not change the affinity for the adrenoceptors in the BAT of ground squirrels and hamsters. There was a significant decrease in alpha(1)-receptor density in arousing ground squirrels and a significant decrease in beta(1)/beta(2) density in hibernating ground squirrels. The level of alpha(1)-receptors was in all conditions higher than that of beta(1)/beta(2) receptors. The results indicate a possible change in balance of adrenoceptor density in the processes of cold acclimation, hibernation and arousal. The balance between the various adrenoceptor subtypes may be important for the final effect of catecholamines in BAT in different physiological states.

Bivalent probes of the human multidrug transporter P-glycoprotein

A small library of bivalent agents was designed to probe the substrate binding sites of the human multidrug transporter P-glycoprotein (P-gp). The bivalent agents were composed of two copies of the P-gp substrate emetine, linked by tethers of varied composition. An optimum distance between the emetine molecules of approximately 10 A was found to be necessary for blocking transport of the known fluorescent substrate rhodamine 123. Additionally, it was determined that hydrophobic tethers were optimal for bridging the bivalent compounds; hydrophilic or cationic moieties within the tether had a detrimental effect on inhibition of transport. In addition to acting as probes of P-gp's drug binding sites, these agents were also potent inhibitors of P-gp. One agent, EmeC5, had IC50 values of 2.9 microM for inhibiting transport of rhodamine 123 and approximately 5 nM for inhibiting the binding of a known P-gp substrate, [125I]iodoarylazidoprazosin. Although EmeC5 is an inhibitor of P-gp and was shown to interact directly with P-gp in one or more of the substrate binding sites, our data suggest that it is either not a P-gp transport substrate itself or a poor one. Most significantly, EmeC5 was shown to reverse the MDR phenotype of MCF-7/DX1 cells when co-administered with a cytotoxic agent, such as doxorubicin.

Effect of cocaine sensitization on alpha1-adrenoceptors in brain regions of the rat: an autoradiographic analysis

We investigated the effects of repeated intermittent cocaine treatment, resulting in behavioral sensitization, on the density of alpha(1)-adrenoceptors in the rat brain measured by quantitative in vitro autoradiography of [(3)H]prazosin. Animals were decapitated following a short (2 h) and long (48 h) withdrawal period after an injection of cocaine (10 mg/kg) on day 10 given to either cocaine-naive (saline daily, days 1-5) or cocaine-sensitized (cocaine 10 mg/kg daily, days 1-5) rats. In cocaine-naive rats, significant decreases in alpha(1)-adrenoceptors 2 h after a single dose of cocaine were observed in the amygdaloid nuclei and hippocampus; the decreases in the centromedial nucleus of the amygdala persisted until 48th hour of withdrawal. On the contrary, increases in alpha(1)-adrenoceptors after 2-h withdrawal were seen in the nucleus accumbens core and retrosplenial cortex. In cocaine-sensitized rats, the density of alpha(1)-adrenoceptors 2 h after the challenge with cocaine increased in the centrolateral amygdala, while in the granular retrosplenial cortex and in the most of thalamic nuclei, the densities of alpha(1)-adrenoceptors decreased. After 48-h withdrawal, the density of alpha(1)-adrenoceptors increased in the nucleus accumbens core and shell (by 21% and 58%, respectively), and in the amygdaloid centromedial and basolateral nuclei (by ca. 24%), while the decline was still observed in some thalamic nuclei. Our study shows for the first time that cocaine sensitization produces significant (dependent on the withdrawal time) alterations in the alpha(1)-adrenoceptor density, and the changes in some parts of the thalamus seem to be related to processes of cocaine relapses.

Multiple drugbinding sites on the R482G isoform of the ABCG2 transporter

BACKGROUND & PURPOSE

Drug-resistant cancer cells frequently display efflux pumps such as P-glycoprotein (P-gp), the multidrug resistance associated protein (MRP1) or the transporter ABCG2. These transporters are each capable of mediating the active efflux of numerous anticancer drugs and display relatively distinct substrate preferences. The last, most recently discovered member, ABCG2, plays a major role in resistance in several types of cancer and the precise pharmacology of this multidrug transporter remain unresolved as does the nature of substrate binding.

EXPERIMENTAL APPROACH

Plasma membranes from insect cells expressing ABCG2 were used to characterise binding of [3H]daunomycin to the multidrug transporter. The kinetics of association and dissociation for this substrate and several other compounds were also determined in this experimental system.

KEY RESULTS

The dissociation constant for [3H]daunomycin binding was 564 +/- 57 nM and a Hill slope of 1.4 suggested cooperative binding. Doxorubicin, prazosin and daunomycin completely displaced the binding of radioligand, while mitoxantrone and Hoechst 33342 produced only a partial displacement. Analysis of the dissociation rates revealed that [3H]daunomycin and doxorubicin bind to multiple sites on the transporter.

CONCLUSIONS

Both kinetic and equilibrium data support the presence of at least two symmetric drug binding sites on ABCG2, which is distinct from the asymmetry observed for P-gp. The data provide the first molecular details underlying the mechanism by which this transporter is capable of interacting with multiple substrates.

The calcium channel blockers, 1,4-dihydropyridines, are substrates of the multidrug resistance-linked ABC drug transporter, ABCG2

The human ATP-binding cassette transporter, ABCG2, confers resistance to multiple chemotherapeutic agents and also affects the bioavailability of different drugs. [(125)I]Iodoarylazidoprazosin (IAAP) and [(3)H]azidopine were used for photoaffinity labeling of ABCG2 in this study. We show here for the first time that both of these photoaffinity analogues are transport substrates for ABCG2 and that [(3)H]azidopine can also be used to photolabel both wild-type R482-ABCG2 and mutant T482-ABCG2. We further used these assays to screen for potential substrates or modulators of ABCG2 and observed that 1,4-dihydropyridines such as nicardipine and nifedipine, which are clinically used as antihypertensive agents, inhibited the photolabeling of ABCG2 with [(125)I]IAAP and [(3)H]azidopine as well as the transport of these photoaffinity analogues by ABCG2. Furthermore, [(3)H]nitrendipine and bodipy-Fl-dihydropyridine accumulation assays showed that these compounds are transported by ABCG2. These dihydropyridines also inhibited the efflux of the known ABCG2 substrates, mitoxantrone and pheophorbide-a, from ABCG2-overexpressing cells, and nicardipine was more potent in inhibiting this transport. Both nicardipine and nifedipine stimulated the ATPase activity of ABCG2, and the nifedipine-stimulated activity was inhibited by fumitremorgin C, suggesting that these agents might interact at the same site on the transporter. In addition, nontoxic concentrations of dihydropyridines increased the sensitivity of ABCG2-expressing cells to mitoxantrone by 3-5-fold. In aggregate, results from the photoaffinity labeling and efflux assays using [(125)I]IAAP and [(3)H]azidopine demonstrate that 1,4-dihydropyridines are substrates of ABCG2 and that these photolabels can be used to screen new substrates and/or inhibitors of this transporter.

BML-241 fails to display selective antagonism at the sphingosine-1-phosphate receptor, S1P(3)

BACKGROUND AND PURPOSE

The thiazolidine carboxylic acid, BML-241, has been proposed as a lead compound in development of selective antagonists at the sphingosine-1-phosphate receptor (S1P3), based on its inhibition of the rise in intracellular calcium concentrations ([Ca2+]i) in HeLa cells overexpressing S1P receptors. We have studied the antagonistic properties of BML-241 for the S1P(3) receptor in more detail.

EXPERIMENTAL APPROACH

Chinese hamster ovary (CHO) cells stably transfected with the S1P3, S1P2 or alpha(1A)-adrenoceptors were used to investigate the effect of BML-241 on increases in [Ca2+]i mediated via different receptors. CHO-K1 cells were used to study ATP-induced [Ca2+]i elevations. Effects on S1P3 -mediated inhibition of forskolin-induced cAMP accumulation and on binding to alpha(1A)-adrenoceptors were also investigated. In addition, the effect of BML-241 on contractions of rat mesenteric artery induced by phenylephrine was studied in an organ bath.

KEY RESULTS

High concentrations of BML-241 (10 microM) inhibited the rise in [Ca2+]i induced by S1P3 and S1P2 receptor stimulation; lower concentrations were ineffective. This high concentration of BML-241 also inhibited [Ca2+]i increases via P2 (nucleotide) receptor or alpha(1A)-adrenoceptor stimulation. Moreover, BML-241 (10 microM) inhibited alpha(1)-adrenoceptor-mediated contraction of rat mesenteric artery but did not displace [3H]-prazosin from alpha(1A)-adrenoceptors in concentrations up to 100 microM. BML-241 (10 microM) did not affect the S1P3 -mediated decrease of forskolin-induced cAMP accumulation.

CONCLUSIONS AND IMPLICATIONS

We conclude that BML-241 is a low potency, non-selective inhibitor of increases in [Ca2+]i, rather than a specific antagonist at the S1P3 receptor.

Paraventricular-coerulear interactions: role in hypertension induced by prenatal undernutrition in the rat

Rats submitted to fetal growth retardation by in utero malnutrition develop hypertension when adult, showing increased hypothalamic mRNA expression for corticotropin-releasing hormone (CRH) and increased central noradrenergic activity. As hypothalamic CRH serves as an excitatory neurotransmitter within the locus coeruleus (LC) and coerulear norepinephrine plays a similar role within the paraventricular nucleus (PVN) of the hypothalamus, we studied, in both normal and prenatally undernourished 40-day-old anesthetized rats, the effects of intra-LC microinjection of CRH and intra-PVN microinjection of the alpha(1)-adrenoceptor antagonist prazosin on multiunit neuronal activity recorded simultaneously from the two nuclei, as well as the effects on systolic pressure. Undernutrition was induced during fetal life by restricting the diet of pregnant mothers to 10 g daily, whereas mothers of control rats received the same diet ad libitum. At day 40 of postnatal life: (i) undernourished rats showed increased neuronal activity in the PVN and LC, as well as increased systolic pressure; (ii) intra-LC CRH stimulated LC and PVN neurons and increased systolic pressure only in normal rats; (iii) intra-PVN prazosin decreased LC and PVN neuronal activity and systolic pressure only in undernourished rats; and (iv) in normal rats, prazosin prevented the stimulatory effect of CRH only in PVN activity; in undernourished rats, prazosin allowed CRH to regain its stimulatory effects. The results point to the existence of an excitatory PVN-LC closed loop, which seems to be hyperactive in prenatally undernourished rats as a consequence of fetal programming; this loop could be responsible, in part, for the hypertension developed by these animals.

Analysis of brain adrenergic receptors in dopamine-beta-hydroxylase knockout mice

The biosynthesis of norepinephrine occurs through a multi-enzymatic pathway that includes the enzyme dopamine-beta-hydroxylase (DBH). Mice with a homozygous deletion of DBH (Dbh-/-) have a selective and complete absence of norepinephrine. The purpose of this study was to assess the expression of alpha-1, alpha-2 and beta adrenergic receptors (alpha1-AR, alpha2-AR and beta-AR) in the postnatal absence of norepinephrine by comparing noradrenergic receptors in Dbh-/- mice with those in Dbh heterozygotes (Dbh+/-), which have normal levels of norepinephrine throughout life. The densities of alpha1-AR, alpha2-AR and beta-AR were assayed with [3H]prazosin, [3H]RX21002 and [125I]-iodo-pindolol autoradiography, respectively. The alpha2-AR agonist high affinity state was examined with [125I]-para-iodoclonidine autoradiography and alpha2-AR functionality by alpha2-AR agonist-stimulated [35S]GTPgammaS autoradiography. The density of alpha1-AR in Dbh-/- mice was similar to Dbh+/- mice in most brain regions, with an up-regulation in the hippocampus. Modest decreases in alpha2-AR were found in septum, hippocampus and amygdala, but these were not reflected in alpha2-AR functionality. The density of beta-AR was up-regulated to varying degrees in many brain regions of Dbh-/- mice compared to the heterozygotes. These findings indicate that regulation of noradrenergic receptors by endogenous norepinephrine depends on receptor type and neuroanatomical region.

Decreased alpha1-adrenergic receptor binding in the cerebral cortex and brain stem during pancreatic regeneration in rats

The purpose of this study was to investigate the role of brain alpha1-adrenergic receptor binding in the rat model of pancreatic regeneration using 60-70% pancreatectomy. The alpha1-adrenergic receptors kinetics was studied in the cerebral cortex and brain stem of sham operated, 72 h pancreatectomised and 7 days pancreatectomised rats. Scatchard analysis with [3H]prazosin in cerebral cortex and brain stem showed a significant decrease (P < 0.01), (P < 0.05) in maximal binding (Bmax) with a significant decrease (P < 0.001), (P < 0.01) in the Kd in 72 h pancreatectomised rats compared with sham respectively. Competition analysis in cerebral cortex and brain stem showed a shift in affinity during pancreatic regeneration. The sympathetic activity was decreased as indicated by the significantly decreased norepinephrine level in the plasma (P < 0.001), cerebral cortex (P < 0.01) and brain stem (P < 0.001) of 72 h pancreatectomised rats compared to sham. Thus, from our results it is suggested that the central alpha1-adrenergic receptors have a functional role in the pancreatic regeneration mediated through the sympathetic pathway.

Biochemical and pharmacological properties of an allosteric modulator site of the human P-glycoprotein (ABCB1)

The drug-transport function of the human P-glycoprotein (Pgp or ABCB1) is inhibited by a number of structurally unrelated compounds, known as modulators or reversing agents. Among them, the thioxanthene derivative flupentixol inhibits Pgp-mediated drug transport by an allosteric mechanism. Unlike most other Pgp modulators, the cis isomer of flupentixol [cis-(Z)-flupentixol] facilitates interaction of Pgp with its transport-substrate [125I]iodoarylazidoprazosin (or [125I]IAAP), yet inhibits transport. In this study, we show that the flupentixol site acts as a common site of interaction for the tricyclic ring-containing modulators thioxanthenes and phenothiazines. The allosteric stimulation of [125I]IAAP binding to Pgp occurs independent of the phosphorylation status of the transporter. Stimulation is retained in purified Pgp reconstituted into proteoliposomes, suggesting no involvement of any other cellular protein in the phenomenon. However, perturbation of the lipid environment of the reconstituted Pgp by nonionic detergent octylglucoside abolishes stimulation by cis-(Z)-flupentixol of [125I]IAAP binding. Extensive trypsin digestion of the [125I]IAAP-labeled Pgp generates a 5.5 kDa fragment with 80% of the stimulated level of labeling associated with it. Sensitivity to inhibition by transport-substrate vinblastine and competitive modulator cyclosporin A suggests that the elevated level of [125I]IAAP binding to the fragment represents a functionally relevant interaction with the substrate site of Pgp. In summary, we demonstrate that allosteric modulation by cis-(Z)-flupentixol is mediated through its interaction with Pgp at a site specific for tricyclic ring-containing Pgp modulators of thioxanthene and phenothiazine backbone, independent of other cellular components and the phosphorylation status of the protein.

Role of CNS α1-adrenoceptor activity in central fos responses to novelty

The present study investigated, by use of fos immunohistochemistry, whether the functional activity of alpha(1)-adrenoceptors is elevated during heightened behavioral activity in brain regions shown earlier to contain motoric alpha(1)-receptors. In confirmation, marked c-fos responses that were blocked by an alpha(1)-antagonist (prazosin) were found in four of these brain regions (secondary motor, cingulate, piriform cortices, and nucleus accumbens) of animals exposed to a mildly novel environment (clean cage), which elicits a high degree of sustained exploratory activity. Experimental restriction of exploratory activity in the novel cage by a small enclosure did not reduce the fos responses in these areas, and in fact, enhanced gene expression when carried out in home-caged animals suggesting that the fos response may be more closely associated with the motivation to be active rather than activity itself. Experiments with locally administered alpha(1)-agonists and antagonists in the cortex by reverse dialysis showed that the above mentioned alpha(1)-dependent-fos responses were the result of activation of local alpha(1)-receptors in these brain regions. Unlike the aforementioned brain regions, the fos response of the locus coeruleus was not blocked by prazosin, and this nucleus also showed a marked fos increase to prazosin itself possibly as a compensatory response to the blockade of forebrain alpha(1)-receptors.

Compensatory changes in the noradrenergic nervous system in the locus ceruleus and hippocampus of postmortem subjects with Alzheimer's disease and dementia with Lewy bodies

In Alzheimer's disease (AD), there is a significant loss of locus ceruleus (LC) noradrenergic neurons. However, functional and anatomical evidence indicates that the remaining noradrenergic neurons may be compensating for the loss. Because the noradrenergic system plays an important role in learning and memory, it is important to determine whether compensation occurs in noradrenergic neurons in the LC and hippocampus of subjects with AD or a related dementing disorder, dementia with Lewy bodies (DLB). We observed profound neuronal loss in the LC in AD and DLB subjects with three major changes in the noradrenergic system consistent with compensation: (1) an increase in tyrosine hydroxylase (TH) mRNA expression in the remaining neurons; (2) sprouting of dendrites into peri-LC dendritic zone, as determined by alpha2-adrenoreceptors (ARs) and norepinephrine transporter binding sites; and (3) sprouting of axonal projections to the hippocampus as determined by alpha2-ARs. In AD and DLB subjects, the postsynaptic alpha1-ARs were normal to elevated. Expression of alpha1A- and alpha2A-AR mRNA in the hippocampus of AD and DLB subjects were not altered, but expression of alpha1D- and alpha2C-AR mRNA was significantly reduced in the hippocampus of AD and DLB subjects. Therefore, in AD and DLB subjects, there is compensation occurring in the remaining noradrenergic neurons, but there does appear to be a loss of specific AR in the hippocampus. Because changes in these noradrenergic markers in AD versus DLB subjects were similar (except neuronal loss and the increase in TH mRNA were somewhat greater in DLB subjects), the presence of Lewy bodies in addition to plaques and tangles in DLB subjects does not appear to further affect the noradrenergic compensatory changes.

Pharmacological characterization of alpha1- and beta-adrenergic synergism of 5'DII activity in rat brown adipocytes

The role of adrenoceptor subtypes was studied in rat brown adipose tissue (BAT). The type II 5'-deiodinase (5'DII) was activated in response to simultaneous stimulation by beta3- and alpha1-adrenergic agonists, BRL 37344 or CGP 12177, and cirazoline, in brown adipocytes. Inhibition of the alpha1- and beta-adrenergic phenylephrine-stimulated 5'DII activity was obtained by the alpha1-adrenergic antagonists in the order of prazosin >/= wb 4101 > 5-methylurapidil. In comparison, the binding of [3H]prazosin to rat BAT plasma membranes was inhibited by alpha1-adrenergic antagonists in the order of prazosin > WB 4101 = benoxathian > 5-methylurapidil. Although the order of the alpha1-adrenergic competition seemed to be rather typical for the alpha1B-adrenergic receptors, a molecular analysis on adrenoceptor mRNAs should be made to confirm the exact alpha1-adrenergic subtypes at the level of brown adipocytes, since the possibility of a mixture of different receptor subtypes in brown fat cells and/or tissue may interact with the pharmacological characterization. Thus, specific alpha1- and beta-adrenoceptor subtypes participate in the regulation of 5'DII activity in the rat brown adipocytes, and therefore, an impaired alpha1- and beta-adrenergic co-work may be involved in a defective BAT function, e.g., in obese Zucker rats, too. An interesting possibility is that the decreased number of alpha1-adrenoceptors in the BAT of obese Zucker rats is due to the decrease in the alpha1B-adrenoceptor subtype which would further be involved especially in the regulation of BAT 5'DII activity.

The role of hydrogen bond acceptor groups in the interaction of substrates with Pdr5p, a major yeast drug transporter

The yeast ABC (ATP-binding cassette protein) multidrug transporter Pdr5p transports a broad spectrum of xenobiotic compounds, including antifungal and antitumor agents. Previously, we demonstrated that substrate size is an important factor in substrate-transporter interaction and that Pdr5p has at least three substrate-binding sites. In this study, we use a combination of whole cell transport assays and photoaffinity labeling of Pdr5p with [(125)I]iodoarylazidoprazosin in purified plasma membrane vesicles to study the behavior of two series of novel substrates: trityl (triphenylmethyl) and carbazole derivatives. The results indicate that site 2, defined initially by tritylimidazole efflux, requires at least a single hydrogen bond acceptor group (electron pair donor). In contrast, complete inhibition of rhodamine 6G efflux and [(125)I]iodoarylazidoprazosin binding at site 1 requires substrates with three electronegative groups. Carbazole and trityl substrates with two groups show saturating, incomplete inhibition at this site. This type of inhibition is frequently observed in bacterial multidrug-binding proteins that use a pocket with multiple binding sites. The presence of multiple sites with different requirements for substrate-Pdr5p interaction may explain the broad specificity of xenobiotic compounds transported by this protein.

Search for the pharmacophore in prazosin for Transport-P

Partial structures of prazosin have been synthesised and tested for inhibition of Transport-P in order to identify the structural features of prazosin, which appear to be involved in binding to the putative transporter. It is shown that the pyrimidinyl 4-amino group is critically important for binding but that the 6,7-dimethoxy and 2-furoyl groups are not essential.

Formalin hindpaw injection induces changes in the [3H]prazosin binding to alpha1-adrenoceptors in specific regions of the mouse brain and spinal cord

Involvement of the alpha1-adrenoceptor subtypes in early and late phases of formalin pain was investigated by quantitative in vitro autoradiography in the spinal cord and brain structures of CD-1 mice. Total alpha1-adrenoceptors binding (including all alpha1-adrenoceptor subtypes) was assessed with [3H]prazosin; alpha(1B)-adrenoceptor was assessed with [3H]prazosin in the presence of 10 nM WB4101 to mask remaining alpha1-adrenoceptor subtypes. Early after formalin injection the alpha1-adrenoceptors (mainly alpha1B receptor) binding was reduced in the contralateral hind limb area of the somatosensory cortex and in the secondary motor cortex. A reduction occurred also in the ipsilateral laminae I-III of the spinal cord (both alpha1B- and non-alpha1B-adrenoceptors). Lately an increase of alpha1-adrenoceptors binding (mostly subtypes other than alpha1B) appeared in discrete amygdaloid and thalamic nuclei. These results provide the first description of changes at the level of central alpha1-adrenoceptors' binding during the formalin-induced pain in mice. Their distribution suggests that they may have a functional meaning.

Biochemical mechanism of modulation of human P-glycoprotein (ABCB1) by curcumin I, II, and III purified from Turmeric powder

P-glycoprotein (Pgp, ABCB1) is an ATP-dependent drug efflux pump linked to development of multidrug resistance (MDR) in cancer cells. Previously [Biochem Pharmacol 2002;64:573-82], we reported that a curcumin mixture could modulate both function and expression of Pgp. This study focuses on the effect of three major curcuminoids--curcumin I, II and III purified from a curcumin mixture--on modulation of Pgp function in a multidrug resistant human cervical carcinoma cell line (KB-V1). The similar IC(50) values for cytotoxicity of curcuminoids of KB-V1, and KB-3-1 (parental drug sensitive cell line) suggest that these curcuminoids may not be substrates for Pgp. Treating the cells with non-toxic doses of curcuminoids increased their sensitivity to vinblastine only in the Pgp expressing drug resistant cell line, KB-V1, and curcumin I retained the drug in KB-V1 cells more effectively than curcumin II and III, respectively. Effects of each curcuminoid on rhodamine123, calcein-AM, and bodipy-FL-vinblastine accumulation confirmed these findings. Curcumin I, II and III increased the accumulation of fluorescent substrates in a dose-dependent manner, and at 15 microM, curcumin I was the most effective. The inhibitory effect in a concentration-dependent manner of curcuminoids on verapamil-stimulated ATPase activity and photoaffinity labeling of Pgp with the [(125)I]-iodoarylazidoprazosin offered additional support; curcumin I was the most potent modulator. Taken together, these results indicate that curcumin I is the most effective MDR modulator among curcuminoids, and may be used in combination with conventional chemotherapeutic drugs to reverse MDR in cancer cells.

3-D Image Analysis of Fluorescent Drug Binding

Fluorescent ligands provide the means of studying receptors in whole tissues using confocal laser scanning microscopy and have advantages over antibody- or non-fluorescence-based method. Confocal microscopy provides large volumes of images to be measured. Histogram analysis of 3-D image volumes is proposed as a method of graphically displaying large amounts of volumetric image data to be quickly analyzed and compared. The fluorescent ligand BODIFY FL-prazosin (QAPB) was used in mouse aorta. Histogram analysis reports the amount of ligand-receptor binding under different conditions and the technique is sensitive enough to detect changes in receptor availability after antagonist incubation or generic manipulations. QAPB binding was concentration dependent, causing concentration-related rightward shifts in histogram. In the presence of 10 microM phenoxybenzamine (blocking agent), the QAPB (50 nM) histogram overlaps the autofluorescence curve. The histogram obtained for the 1D knockout aorta lay to the left of that control and 1B knockout aorta, indicating a reduction in 1D receptors. We have shown, for the first time, that it is possible to graphically display binding of a fluorescent drug to a biological tissue. Although our application is specific to adrenergic receptors, the general method could be applied to any volumetric, fluorescence-image-based assay.

Beta-arrestin-dependent spontaneous alpha1a-adrenoceptor endocytosis causes intracellular transportation of alpha-blockers via recycling compartments

The antagonist ligand BODIPY-FL-prazosin (QAPB) fluoresces when bound to bovine alpha(1a)-adrenoceptors (ARs). Data indicate that the receptor-ligand complex is spontaneously internalized by beta-arrestin-dependent endocytosis. Internalization of the ligand did not occur in beta-arrestin-deficient cells; was blocked or reversed by another alpha(1) ligand, phentolamine, indicating it to reflect binding to the orthosteric recognition site; and was prevented by blocking clathrin-mediated endocytosis. The ligand showed rapid, diffuse, low-intensity, surface binding, superseded by punctate intracellular binding that developed to equilibrium in 50 to 60 min and was reversible on ligand removal, indicating a dynamic equilibrium. In cells expressing a human alpha(1a)-AR-enhanced green fluorescent protein (EGFP) 2 fusion protein, BODIPY-R-558/568-prazosin (RQAPB) colocalized with the fusion, indicating that the ligand gained access to all compartments containing the receptor, and, conversely, that the receptor has affinity for the ligand at all of these sites. The distribution of QAPB binding sites was similar for receptors with or without EGFP2, validating the fusion protein as an indicator of receptor location. The ligand partially colocalized with beta-arrestin in recycling and late endosomes, indicating receptor transit without destruction. Organelles containing receptors showed considerable movement consistent with a transportation function. This was absent in beta-arrestin-deficient cells, indicating that both constitutive receptor internalization and subsequent intracellular transportation are beta-arrestin-dependent. Calculations of relative receptor number indicate that at steady state, less than 30% of receptors reside on the cell surface and that recycling is rapid. We conclude that alpha(1a)-ARs recycle rapidly by an agonist-independent, constitutive, beta-arrestin-dependent process and that this can transport "alpha-blockers" into cells carrying these receptors.

Structural characterization of prazosin hydrochloride and prazosin free base

The three-dimensional solid-state structures of prazosin hydrochloride, C19H22N5O4+.Cl- (A), and prazosin free base, C19H21N5O4 (B), have been determined by synchrotron X-ray powder diffraction. A and B crystallize in triclinic P-1 and monoclinic Cc space groups, respectively, with one structural unit per asymmetric part. In A and B, the prazosin molecule adopts different conformations, which do not correspond to those obtained by DFT optimizations of protonated and free prazosin.

Multidrug transporter activity in lymphocytes

Multidrug transporters play a dual role in haematopoietic cells, mediating the efflux of xenobiotics and regulating cell migration. For several reasons including the lack of specific antibodies, reports of multidrug transporter distribution on lymphocytes conflict. Murine B cells have been reported to completely lack transporter activity. Through analysis of parental and 'knockout' mice we show that, contrary to previous studies, murine B and T lymphocytes possess at least three active multidrug transporters and also a hitherto unrecognised drug-specific import activity. Surprisingly, the drug specificity of P-glycoprotein appears cell type dependent. The data indicate that a range of developmentally regulated, multidrug transporters can impose a barrier to treatment of immune disorders.

Carvedilol blockade of rat myocardial alpha1-adrenoceptors

Carvedilol is a combined alpha(1)- and beta-adrenoceptor antagonist. The ability of carvedilol to antagonize functional effects mediated through myocardial alpha(1)-adrenoceptors has never been investigated. We tested the ability of carvedilol to antagonize the inotropic effect mediated by myocardial alpha(1)-adrenoceptors compared to the antagonism of beta-adrenoceptors. Papillary muscles from rat heart left ventricle were mounted in an organ bath and concentration-response experiments for the inotropic effects of separate alpha(1)- and beta-adrenoceptor stimulation were performed in the absence and presence of carvedilol. Carvedilol antagonized myocardial alpha(1)-adrenoceptors with an inhibition constant (K(i)) of 11.0+/-3.0 nmol/l and the functional experiments were supported by radioligand-binding studies. Corresponding functional studies on the response to beta-adrenoceptor stimulation revealed a K(i) of 1.2+/-0.35 nmol/l. Thus, carvedilol antagonizes the myocardial alpha(1)-adrenoceptors with a 9-fold lower potency than the beta-adrenoceptors. Antagonism of myocardial alpha(1)-adrenoceptor evoked effects may contribute to clinical effects of carvedilol.

Functional expression of rat ABCG2 on the luminal side of brain capillaries and its enhancement by astrocyte-derived soluble factor(s)

The purpose of the present study was to clarify the expression, transport properties and regulation of ATP-binding cassette G2 (ABCG2) transporter at the rat blood-brain barrier (BBB). The rat homologue of ABCG2 (rABCG2) was cloned from rat brain capillary fraction. In rABCG2-transfected HEK293 cells, rABCG2 was detected as a glycoprotein complex bridged by disulfide bonds, possibly a homodimer. The protein transported mitoxantrone and BODIPY-prazosin. In rat brain capillary fraction, rABCG2 protein was also detected as a glycosylated and disulfide-linked complex. Immunohistochemical analysis revealed that rABCG2 was localized mainly on the luminal side of rat brain capillaries, suggesting that rABCG2 is involved in brain-to-blood efflux transport. For the regulation study, conditionally immortalized rat brain capillary endothelial (TR-BBB13), astrocyte (TR-AST4) and pericyte (TR-PCT1) cell lines were used as an in vitro BBB model. Following treatment of TR-BBB13 cells with conditioned medium of TR-AST4 cells, the Ko143 (an ABCG2-specific inhibitor)-sensitive transport activity and rABCG2 mRNA level were significantly increased, whereas conditioned medium of TR-PCT1 cells had no effect. These results suggest that rat brain capillaries express functional rABCG2 protein and that the transport activity of the protein is up-regulated by astrocyte-derived soluble factor(s) concomitantly with the induction of rABCG2 mRNA.

Changes in c-fos expression in the rat heart during morphine withdrawal. Involvement of ?2-adrenoceptors

We previously demonstrated an increase in Fos expression in the heart during morphine withdrawal. In the present study we examined the role of beta- and alpha-adrenoceptors in naloxone-precipitated increases in Fos expression in the heart. Dependence on morphine was induced by 7-day chronic subcutaneous implantation of six morphine pellets (75 mg). Morphine withdrawal was precipitated by administration of naloxone (5 mg/kg subcutaneously) on day 8. Using immunohistochemical staining of Fos, the present results indicate that morphine withdrawal induced marked Fos immunoreactivity (Fos-IR) within the cardiomyocyte nuclei. Moreover, Western blot analysis revealed a peak expression of c-fos in the right and left ventricles after naloxone-precipitated withdrawal in parallel with an increase in noradrenaline (NA) turnover. In the second study, the effects of the administration of adrenoceptor antagonists on withdrawal-induced Fos expression in the heart were studied. Pretreatment with the beta antagonist, propranolol (3 mg/kg intraperitoneally) or alpha1-adrenoceptor antagonist, prazosin (1 mg/kg intraperitoneally) did not block the marked Fos-IR or the hyperactivity of catecholaminergic neurons observed in the heart during withdrawal. However, pre-treatment with alpha2-adrenoceptor antagonist, yohimbine (1 mg/kg intraperitoneally), 20 min before naloxone administration to morphine-dependent rats antagonized Fos expression and the enhancement of NA turnover in the heart. Collectively, these results suggest that noradrenergic neurons in the heart are active during morphine withdrawal, and that activation of transcriptional responses mediated by Fos are dependent upon cardiac alpha2-adrenoceptor.

Effects of xylazine on canine coronary artery vascular rings

OBJECTIVE

To determine the effects of xylazine on canine coronary artery smooth muscle tone.

SAMPLE POPULATION

Hearts of 26 healthy dogs.

PROCEDURE

Dogs were anesthetized with pentobarbital, and vascular rings of various diameters were prepared from the epicardial coronary arteries. Vascular rings were placed in tissue baths to which xylazine was added (cumulative concentrations ranging from 10(-10) to 10(-4) M), and changes in vascular ring tension were continuously recorded. Effects of the nitric oxide inhibitor NG-nitro-L-arginine methyl ester (L NAME; 5 mM), the alpha1-adrenoceptor antagonist prazosin (10 mM), and the alpha2-adrenoceptor antagonist atipamezole (10 mM) on xylazine-induced changes in vascular ring tension were determined. Results were expressed as percentage of maximal contraction for each vascular ring preparation.

RESULTS

Xylazine induced vasoconstriction of small (< 500-microm-diameter) and medium (500- to 1,000-microm-diameter) vascular rings but not of large (> 1,000-microm-diameter) rings. For large vascular rings, L-NAME, atipamezole, and prazosin did not significantly affect the contractile response to xylazine. For small vascular rings, the contractile response following addition of xylazine to rings treated with L-NAME was not significantly different from the contractile response following addition of xylazine to control rings, except at a xylazine concentration of 10(-6) M. Xylazine-induced vasoconstriction of small vascular rings was blocked by atipamezole, but the addition of prazosin had no effect on xylazine-induced vasoconstriction.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that xylazine increases smooth muscle tone of small canine coronary arteriesand that this effect is predominantly mediated by stimulation of alpha2adrenoceptors.

Biochemical basis of polyvalency as a strategy for enhancing the efficacy of P-glycoprotein (ABCB1) modulators: stipiamide homodimers separated with defined-length spacers reverse drug efflux with greater efficacy

Human P-glycoprotein (Pgp) is as an ATP-dependent efflux pump for a variety of chemotherapeutic drugs. The aim of this study is to evaluate whether Pgp modulators can be engineered to exhibit high-affinity binding using polyvalency. Five bivalent homodimeric polyenes based on stipiamide linked with polyethylene glycol ethers in the range of 3-50 A were synthesized and quantitatively characterized for their effect on Pgp function. The stipiamide homodimers displaced [(125)I]iodoarylazidoprazoin (IAAP), an analogue of the Pgp substrate prazosin. A minimal spacer of 11 A is necessary for inhibition of IAAP labeling, beyond which there is an inverse correlation between the length of the spacer and the IC(50) for the displacement of IAAP. ATP hydrolysis by Pgp on the other hand is stimulated by the dimers with spacers of up to 22 A, whereas dimers with longer spacers inhibit ATP hydrolysis. Finally, the homodimers reverse Pgp-mediated drug efflux in intact cells overexpressing Pgp, and 11 A is a threshold beyond which the effectiveness of the homodimers increases exponentially and levels off at 33 A. We demonstrate that dimerization and identification of an optimal spacer length increase by 11-fold the affinity of stipiamide, and this is reflected in the efficacy with which Pgp-mediated drug efflux is reversed. These results suggest that polyvalency could be a useful strategy for the development of more potent Pgp modulators.

Hepatocytes from alpha1B-adrenoceptor knockout mice reveal compensatory adrenoceptor subtype substitution

1 Alpha1-adrenoceptors (ARs) play an important functional role in the liver; yet little is known about their cellular location. We identified the subtypes present in wild-type (WT) and alpha1B-AR knockout (KO) mice livers at 3 and 4 months of age, and investigated their distribution in hepatocytes. 2 The fluorescent alpha1-AR antagonist quinazolinyl piperazine borate-dipyrromethene (QAPB) was used to visualise hepatic alpha1-ARs and radioligand binding with [3H]-prazosin was used to quantify the alpha1-AR population. 3 QAPB and [3H]-prazosin bound specifically to hepatic alpha1-ARs with nanomolar affinity. The cellular distribution of alpha1-ARs was similar in WT and alpha1B-AR KO hepatocytes; QAPB binding was distributed diffusely throughout the cell with no binding evident on the plasma membrane. Radioligand binding produced Bmax values as follows: 3-month WT - 76+/-3.3 fmol mg(-1); 4-month WT - 50+/-3.1 fmol mg(-1); 3-month alpha1B-AR KO - 7.4+/-0.73 fmol mg(-1); 4-month alpha1B-AR KO - 30+/-2.0 fmol mg(-1). 4 In 3- and 4-month WT liver, all antagonists acted competitively. RS100329 (alpha1A-selective) and BMY7378 (alpha1D-selective) bound with low affinities, indicating the presence of alpha1B-ARs. In 4-month alpha1B-AR KO liver prazosin produced a biphasic curve, whereas RS100329 and BMY7378 produced monophasic curves of high and low affinity, respectively, indicating the presence of alpha1A-ARs. 5 In conclusion, we have made the novel observation that alpha1-ARs can compensate for one another in the absence of the endogenously expressed receptor; yet there appears to be no subtype-specific subcellular location of alpha1-ARs; the WT livers express alpha1B-ARs, while alpha1B-AR KO livers express alpha1A-ARs. This study provides new insights into both hepatocyte and alpha1-AR biology.

Interaction of the breast cancer resistance protein with plant polyphenols

Multidrug transporters influence drug distribution in vivo and are often associated with tumour drug resistance. Here we show that plant-derived polyphenols that interact with P-glycoprotein can also modulate the activity of the recently discovered ABC transporter, breast cancer resistance protein (BCRP/ABCG2). In two separate BCRP-overexpressing cell lines, accumulation of the established BCRP substrates mitoxantrone and bodipy-FL-prazosin was significantly increased by the flavonoids silymarin, hesperetin, quercetin, and daidzein, and the stilbene resveratrol (each at 30 microM) as measured by flow cytometry, though there was no corresponding increase in the respective wild-type cell lines. These compounds also stimulated the vanadate-inhibitable ATPase activity in membranes prepared from bacteria (Lactococcus lactis) expressing BCRP. Given the high dietary intake of polyphenols, such interactions with BCRP, particularly in the intestines, may have important consequences in vivo for the distribution of these compounds as well as other BCRP substrates.

Identification of α-1L Adrenoceptor in Rabbit Ear Artery

The alpha-1L adrenoceptor (AR) was identified in rabbit ear artery by both functional and ligand binding studies. In functional studies using arterial rings, the contractile response to NS-49 [(R)-(-)-3'-(2-amino-1-hydroxyethyl)-4'-fluorometh-anesulfonanilide hydrochloride] (alpha-1A and alpha-1L AR-selective agonist) was competitively antagonized with low affinities by prazosin, RS-17053 [N-[2-(2-cyclopropylmethoxyphenoxy) ethyl]-5-chloro-alpha,alpha-dimethyl-1H-indole-3-ethamine hydrochloride], and 5-methylurapidil but with high affinities by tamsulosin and KMD-3213 [(-)-1-(3-hydroxypropyl)-5-[(2R)-2-([2-[(2,2,2-trifluoroethoxy)phenoxy]ethyl]amino)propyl]-2,3-dihydro-1H-indole-7-carboxamide]. In contrast, the response to noradrenaline (nonselective alpha-1 AR agonist) was inhibited noncompetitively by these antagonists (except 5-methylurapidil) with Schild slopes different from unity. These results suggest that the response to NS-49 was mediated predominantly via alpha-1L ARs, whereas the response to noradrenaline was produced through two distinct alpha-1 AR subtypes (presumably alpha-1B and alpha-1L ARs). In binding studies with intact segments of rabbit ear artery, [3H]KMD-3213 bound with high affinity (pKD=9.7) to alpha-1 ARs, which were subdivided by prazosin, RS-17053, and 5-methylurapidil into two subtypes (alpha-1A and alpha-1L ARs). In contrast, [3H]prazosin binding sites in ear artery segments (pKD = 9.8) were identified as alpha-1A and alpha-1B ARs. In conventional binding studies using isolated rabbit ear artery microsomal membranes, [3H]KMD-3213 binding sites were identified as alpha-1A ARs with high affinities for prazosin, RS-17053, and 5-methylurapidil. Our study indicates that an alpha-1L AR having a unique pharmacological profile coexists with alpha-1A and alpha-1B ARs in rabbit ear artery and can be identified either functionally or by binding studies using intact tissues but not microsomal membrane preparations.

Study of the behavioural responses related to the potential addictive properties of MDMA in mice

We investigated several behavioural responses induced by repeated administration of MDMA in mice that could be related to its potential abuse liability. Mice treated with MDMA at the dose of 10 mg/kg displayed a significant conditioned place preference with respect to saline treated controls, while lower doses (0.3, 1.0, 3.3 mg/kg) had no effect. The development of physical dependence was also investigated. Mice were treated with MDMA (10 mg/kg) twice daily for 5 days. On day 6, following a single administration of MDMA mice received the following monoaminergic antagonists: metergoline (0.1 and 1 mg/kg), ritanserin (0.25 and 1 mg/kg), timolol (2 and 10 mg/kg), prazosin (0.25 and 1 mg/kg), SCH 23390 (0.05 and 0.25 mg/kg), raclopride (0.1 and 0.5 mg/kg) or vehicle, and several somatic manifestations of withdrawal were evaluated for 45 min. Metergoline induced paw tremor, face rubbing, as well as an increase in locomotor activity in mice chronically treated with MDMA. Ritanserin, and timolol induced only paw tremor, while SCH 23390 and raclopride did not produce any somatic manifestation indicative of abstinence. The possible modification of the rewarding properties of MDMA (10 mg/kg) by the monoaminergic antagonists producing the most relevant somatic signs of withdrawal namely, metergoline (0.1 and 1 mg/kg) and timolol (2 and 10 mg/kg) were tested in the conditioned place preference paradigm. Results showed that metergoline did not significantly modify the rewarding properties of MDMA, whereas only the highest dose of timolol was able to decrease MDMA reward. No signs of dopaminergic neurotoxicity were observed following chronic treatment with MDMA as revealed by [(3)H] mazindol binding. The possible motivational and affective components of the withdrawal syndrome were assessed in the suppression of operant responding for food, the conditioned place aversion, and the lit/dark paradigms. Results showed that the somatic symptoms observed were not accompanied by any aversive/dysphoric or anxiogenic-like behaviours. These results reveal the rewarding properties of MDMA in mice, and suggest that chronic MDMA administration does not induce classical manifestations of physical dependence in mice.

Differential region-specific regulation of central alpha 1-adrenoceptor binding following chronic haloperidol and clozapine administration in the rat

RATIONALE

Many antipsychotics exhibit potent anti-alpha(1)-adrenergic receptor activity, which has been suggested to contribute to typical and atypical antipsychotic effects and to the production of centrally mediated side effects.

OBJECTIVES

To assess the relative contribution of alpha(1)-adrenoceptors to the mechanism of action of haloperidol and clozapine and to identify possible sites of action.

METHODS

We examined the effect of chronic haloperidol and clozapine treatment on alpha(1)-adrenoceptor characteristics in several rat brain regions. For comparison, D(2)-like dopamine receptor density in the striatum was also determined.

RESULTS

Clozapine administration (25 mg/kg/day i.p., 21 days) significantly increased alpha(1)-adrenoceptor density in the frontal cortex (44%), remaining cortex (49%) and thalamus (93%) but binding levels in the hippocampus and spinal cord were unchanged relative to vehicle. Haloperidol treatment (1.5 mg/kg/day i.p., 21 days) also significantly increased the density of alpha(1)-adrenoceptor binding in the thalamus (73%), but had no effect on alpha(1)-adrenoceptor levels in any other region examined. alpha(1)-Adrenoceptor affinity in the cortex was not significantly altered by either antipsychotic treatment. Haloperidol, in contrast to clozapine, significantly upregulated dopamine D(2)-like binding in the striatum.

CONCLUSIONS

Central alpha(1)-adrenoceptors are differentially regulated after chronic haloperidol and clozapine treatment. It is suggested that thalamic alpha(1)-adrenoceptors may represent a common anatomical locus contributing to the antipsychotic activity and/or alpha(1)-adrenoceptor centrally mediated side effects of both drugs, whereas the selective upregulation of cortical alpha(1)-adrenoceptor density by clozapine may contribute, in part, to its superior atypical properties.

Alpha-1 adrenoceptors: evaluation of receptor subtype-binding kinetics in intact arterial tissues and comparison with membrane binding

The binding kinetics of [3H]-prazosin were measured using intact segments of rat tail artery (RTA) and thoracic aorta (RAO), and the data were compared with those obtained using a conventional membrane ligand-binding method. In intact RTA and RAO segments, [3H]-prazosin bound reversibly in a time-dependent and receptor-specific manner at 4 degrees C to alpha-1 adrenoceptors (ARs) of the plasma membrane, with affinities (pKD): 9.5 in RTA; 9.9 in RAO) that were in agreement with values estimated by a conventional membrane ligand-binding method. However, nonspecific binding was considerably higher in RAO than RTA, failing to detect clearly the specific binding at high concentrations (>300 pm) of [3H]-prazosin in binding experiments with RAO segments and membranes. The abundance of receptor in the RTA and RAO (Bmax mg-1) of total tissue protein), estimated using the tissue segment-binding approach (527+/-14 fmol mg-1 for RTA; 138+/-4 fmol mg-1 for RAO), was about 25-fold higher than values estimated using a conventional membrane-binding method (22+/-5 fmol mg-1) for RTA; 5+/-1 fmol mg-1 for RAO). Binding competition experiments using intact tissue segments or membranes derived from RTA tissue yielded comparable data, indicating a coexistence of alpha-1A AR (high affinity for prazosin, KMD-3213 and WB4101 and low affinity for BMY 7378) and alpha-1B AR (high affinity for prazosin but low affinity for KMD-3213, WB4101 and BMY 7378). In RAO tissue, careful evaluation of the tissue segment-binding assay revealed the coexpression of alpha-1B AR (high affinity for prazosin, but low affinity for KMD-3213 and BMY 7378) and alpha-1D AR (high affinity for prazosin and BMY 7378, but low affinity for KMD-3213), whereas the membrane-binding approach failed to detect these receptor subtypes with certainty. The present study indicates that previous estimates of alpha-1 AR density and alpha-1 AR subtypes obtained by a conventional membrane-binding approach, as opposed to our improved tissue segment-binding assay, may have substantially underestimated the abundance of receptors present in arterial tissues, and may have failed to identify accurately the presence of receptor subtypes. Advantages and disadvantages of the tissue segment-binding approach are discussed.British Journal of Pharmacology (2004) 141, 468-476. doi:10.1038/sj.bjp.0705627