Guanabenz mitigates the neuropathological alterations and cell death in Alzheimer's disease.

Alzheimer's disease (AD) pathology is characterized by cognitive impairment, increased acetylcholinesterase (AChE) activity, and impaired neuronal communication. Clinically, AChE inhibitors are being used to treat AD patients; however, these remain unable to prevent the disease progression. Therefore, further development of new therapeutic molecules is required having broad spectrum effects on AD-related various neurodegenerative events. Since repurposing is a quick mode to search the therapeutic molecules; henceforth, this study was conducted to evaluate the anti-Alzheimer activity of drug guanabenz which is already in use for the management of high blood pressure in clinics. The study was performed employing both cellular and rat models of AD along with donepezil as reference drug. Guanabenz treatment in both the experimental models showed significant protection against AD-specific behavioral and pathological indicators like AChE activity, tau phosphorylation, amyloid precursor protein, and memory retention. In conjunction, guanabenz also attenuated the AD-related oxidative stress, impaired mitochondrial functionality (MMP, cytochrome-c translocation, ATP level, and mitochondrial complex I activity), endoplasmic reticulum stress (GRP78, GADD153, cleaved caspase-12), neuronal apoptosis (Bcl-2, Bax, cleaved caspase-3), and DNA fragmentation. In conclusion, findings suggested the panoptic protective effect of guanabenz on disease-related multiple degenerative markers and signaling. Furthermore, clinical trial may shed light and expedite the availability of new therapeutic anti-Alzheimer's molecule for the wellbeing of AD patients.

In silico screening of GMQ-like compounds reveals guanabenz and sephin1 as new allosteric modulators of acid-sensing ion channel 3.

Acid-sensing ion channels (ASICs) are voltage-independent cation channels that detect decreases in extracellular pH. Dysregulation of ASICs underpins a number of pathologies. Of particular interest is ASIC3, which is recognised as a key sensor of acid-induced pain and is important in the establishment of pain arising from inflammatory conditions, such as rheumatoid arthritis. Thus, the identification of new ASIC3 modulators and the mechanistic understanding of how these compounds modulate ASIC3 could be important for the development of new strategies to counteract the detrimental effects of dysregulated ASIC3 activity in inflammation. Here, we report the identification of novel ASIC3 modulators based on the ASIC3 agonist, 2-guanidine-4-methylquinazoline (GMQ). Through a GMQ-guided in silico screening of Food and Drug administration (FDA)-approved drugs, 5 compounds were selected and tested for their modulation of rat ASIC3 (rASIC3) using whole-cell patch-clamp electrophysiology. Of the chosen drugs, guanabenz (GBZ), an α2-adrenoceptor agonist, produced similar effects to GMQ on rASIC3, activating the channel at physiological pH (pH 7.4) and potentiating its response to mild acidic (pH 7) stimuli. Sephin1, a GBZ derivative that lacks α2-adrenoceptor activity, has been proposed to act as a selective inhibitor of a regulatory subunit of the stress-induced protein phosphatase 1 (PPP1R15A) with promising therapeutic potential for the treatment of multiple sclerosis. However, we found that like GBZ, sephin1 activates rASIC3 at pH 7.4 and potentiates its response to acidic stimulation (pH 7), i.e. sephin1 is a novel modulator of rASIC3. Furthermore, docking experiments showed that, like GMQ, GBZ and sephin1 likely interact with the nonproton ligand sensor domain of rASIC3. Overall, these data demonstrate the utility of computational analysis for identifying novel ASIC3 modulators, which can be validated with electrophysiological analysis and may lead to the development of better compounds for targeting ASIC3 in the treatment of inflammatory conditions.

Decoding the selectivity of eIF2α holophosphatases and PPP1R15A inhibitors.

The reversible phosphorylation of proteins controls most cellular functions. Protein kinases have been popular drug targets, unlike phosphatases, which remain a drug discovery challenge. Guanabenz and Sephin1 are selective inhibitors of the phosphatase regulatory subunit PPP1R15A (R15A) that prolong the benefit of eIF2α phosphorylation, thereby protecting cells from proteostatic defects. In mice, Sephin1 prevents two neurodegenerative diseases, Charcot-Marie-Tooth 1B (CMT-1B) and SOD1-mediated amyotrophic lateral sclerosis (ALS). However, the molecular basis for R15A inhibition is unknown. Here we reconstituted human recombinant eIF2α holophosphatases, R15A-PP1 and R15B-PP1, whose activity depends on both the catalytic subunit PP1 (protein phosphatase 1) and either R15A or R15B. This system enabled the functional characterization of these holophosphatases and revealed that Guanabenz and Sephin1 induced a selective conformational change in R15A, detected by resistance to limited proteolysis. This altered the recruitment of eIF2α, preventing its dephosphorylation. This work demonstrates that regulatory subunits of phosphatases are valid drug targets and provides the molecular rationale to expand this concept to other phosphatases.

PPP1R15A-mediated dephosphorylation of eIF2α is unaffected by Sephin1 or Guanabenz.

Dephosphorylation of translation initiation factor 2 (eIF2α) terminates signalling in the mammalian integrated stress response (ISR) and has emerged as a promising target for modifying the course of protein misfolding diseases. The [(o-chlorobenzylidene)amino]guanidines (Guanabenz and Sephin1) have been proposed to exert protective effects against misfolding by interfering with eIF2α-P dephosphorylation through selective disruption of a PP1-PPP1R15A holophosphatase complex. Surprisingly, they proved inert in vitro affecting neither stability of the PP1-PPP1R15A complex nor substrate-specific dephosphorylation. Furthermore, eIF2α-P dephosphorylation, assessed by a kinase shut-off experiment, progressed normally in Sephin1-treated cells. Consistent with its role in defending proteostasis, Sephin1 attenuated the IRE1 branch of the endoplasmic reticulum unfolded protein response. However, repression was noted in both wildtype and Ppp1r15a deleted cells and in cells rendered ISR-deficient by CRISPR editing of the Eif2s1 locus to encode a non-phosphorylatable eIF2α (eIF2αS51A). These findings challenge the view that [(o-chlorobenzylidene)amino]guanidines restore proteostasis by interfering with eIF2α-P dephosphorylation.

Preventing proteostasis diseases by selective inhibition of a phosphatase regulatory subunit.

Protein phosphorylation regulates virtually all biological processes. Although protein kinases are popular drug targets, targeting protein phosphatases remains a challenge. Here, we describe Sephin1 (selective inhibitor of a holophosphatase), a small molecule that safely and selectively inhibited a regulatory subunit of protein phosphatase 1 in vivo. Sephin1 selectively bound and inhibited the stress-induced PPP1R15A, but not the related and constitutive PPP1R15B, to prolong the benefit of an adaptive phospho-signaling pathway, protecting cells from otherwise lethal protein misfolding stress. In vivo, Sephin1 safely prevented the motor, morphological, and molecular defects of two otherwise unrelated protein-misfolding diseases in mice, Charcot-Marie-Tooth 1B, and amyotrophic lateral sclerosis. Thus, regulatory subunits of phosphatases are drug targets, a property exploited here to safely prevent two protein misfolding diseases.

Synthesis of conjugates of 6-aminophenanthridine and guanabenz, two structurally unrelated prion inhibitors, for the determination of their cellular targets by affinity chromatography.

The synthesis of affinity matrices for 6-aminophenanthridine (6AP) and 2,6-dichlorobenzylidenaminoguanidine (Guanabenz, GA), two unrelated prion inhibitors, is described. In both cases, the same simple spacer, epsilon-aminocaproylaminopentanol, was introduced by a Mitsunobu reaction and the choice of the anchoring position of the linker was determined by the study of the residual antiprion activity of the corresponding 6AP or GA conjugates. Very recently, these two affinity matrices were used for chromatography assays leading to the identification of ribosome (via the rRNA) as a common target of these two antiprion drugs. Here, we show, using competition experiments with Quinacrine (QC) and Chlorpromazine (CPZ), two other antiprion drugs, that QC, but not CPZ, may also directly target the rRNA.

Hepatic, extrahepatic, microsomal, and mitochondrial activation of the N-hydroxylated prodrugs benzamidoxime, guanoxabenz, and Ro 48-3656 ([[1-[(2s)-2-[[4-[(hydroxyamino)iminomethyl]benzoyl]amino]-1-oxopropyl]-4-piperidinyl]oxy]-acetic acid).

In previous studies, it was shown that liver microsomes from rabbit, rat, pig, and human are involved in the reduction of N-hydroxylated amidines, guanidines, and amidinohydrazones of various drugs and model compounds (Drug Metab Rev 34: 565-579). One responsible enzyme system, the microsomal benzamidoxime reductase, consisting of cytochrome b5, its reductase, and a cytochrome P450 isoenzyme, was isolated from pig liver microsomes (J Biol Chem 272:19615-19620). Further investigations followed to establish whether such enzyme systems are also present in microsomes of other organs such as brain, lung, and intestine. In addition, the mitochondrial reduction in human and porcine liver and kidney preparations was studied. The reductase activities were measured by following the reduction of benzamidoxime to benzamidine, guanoxabenz to guanabenz, and Ro 48-3656 ([[1-[(2S)-2-[[4-[(hydroxyamino)iminomethyl]benzoyl]amino]-1-oxopropyl]-4-piperidinyl]oxy]-acetic acid) to Ro 44-3888 ([[1-[(2S)-2-[[4-(aminoiminomethyl)benzoyl]amino]-1-oxopropyl]-4-piperidinyl]oxy]-acetic acid). Interestingly, preparations of all tested organs were capable of reducing the three compounds. The highest specific rates were found in kidney followed by liver, brain, lung, and intestine, and usually the mitochondrial reduction rates were superior. From the determined characteristics, similarities between the enzyme systems in the different organs and organelles were detected. Furthermore, properties of the benzamidoxime reductase located in the outer membrane of pig liver mitochondria were studied. In summary, these results demonstrate that in addition to the microsomal reduction, mitochondria are involved to a great extent in the activation of amidoxime prodrugs. The importance of extrahepatic metabolism in the reduction of N-hydroxylated prodrugs is demonstrated.

Identification of an N-hydroxyguanidine reducing activity of xanthine oxidase.

A guanoxabenz [1-(2,6-dichlorobenzylideneamino)-3-hydroxyguanidine; an N-hydroxyguanidine] reducing enzymatic activity of rat spleen cytosol was investigated. By means of protein purification and N-terminal amino acid sequencing, the reducing activity was shown to reside in xanthine oxidase. The action of the enzyme on guanoxabenz resulted in the formation of guanabenz [1-(2,6-dichlorobenzylidene-amino)-3-guanidine]; the product formation could be monitored by HPLC and its identity was confirmed by NMR analysis. The reduction of guanoxabenz required xanthine or NADH as reducing substrates, while the process could be blocked by allopurinol, a selective inhibitor of xanthine oxidase. By using bovine milk xanthine oxidase, the guanoxabenz reducing activity of the enzyme was also verified. We conclude that guanoxabenz is a novel electron acceptor structure for xanthine oxidase.

Characterization of guanoxabenz reducing activity in rat brain.

Guanoxabenz (1-(2,6-dichlorobenzylidene-amino)-3-hydroxyguanidine) and guanabenz (1-(2,6-dichlorobenzylidene-amino)-3-guanidine) are both known as centrally active antihypertensive drugs. We have previously shown that enzymatic activity in the rat spleen can induce N-reduction of guanoxabenz, leading to high affinity alpha 2-adrenoceptor binding, due to the formation of the alpha 2-adrenoceptor active drug, guanabenz. The spleen activity appears to reside in xanthine oxidase as it is activated by xanthine and blocked by allopurinol. We report that high affinity guanoxabenz binding is also induced in rat brain membranes after addition of NADH or NADPH cofactors. However, the brain process was clearly different from that of the spleen, as the formation of high affinity binding in the brain was not blocked by allopurinol. Moreover the NADH/NADPH activated mechanism of the brain membranes was not blocked by carbon monoxide and SKF525A, thus the activity appears not to reside in cytochrome P450 enzymes. Instead the activity was blocked by menadione and dicumarol. We conclude that the rat cerebral cortex contains an enzymatic activity that may activate guanoxabenz leading to formation of a metabolite showing high affinity for alpha 2-adrenoceptors. We also conclude that the rat brain activity is clearly distinct from that of the rat spleen.

Characterization of the enzymatic activity for biphasic competition by guanoxabenz (1-(2,6-dichlorobenzylidene-amino)-3-hydroxyguanidine) at alpha2-adrenoceptors. II. Description of a xanthine-dependent enzymatic activity in spleen cytosol.

The mechanism for formation of high affinity binding of guanoxabenz (1-(2,6-dichlorobenzylidene-amino)-3-hydroxyguanidine) to alpha2-adrenoceptors by the rat spleen cytosol was studied. We report here that the spleen cytosolic fraction mediated the reduction of guanoxabenz to guanabenz (1-(2,6-dichlorobenzylidene-amino)-3-guanidine), the latter having an almost 100-fold higher affinity for rat alpha2A-adrenoceptors than guanoxabenz itself. The reaction product could be separated by high-performance liquid chromatography and its identity as guanabenz confirmed by nuclear magnetic resonance. The spleen cytosolic activity could be separated into high and low molecular weight components, the high molecular weight component requiring low molecular weight factors for maximal activity. Xanthine oxidase seems to be the most likely candidate responsible for the activity, as the guanoxabenz-reducing activity of the high molecular weight component could be sustained by exogenously applied xanthine, while it was potently blocked by allopurinol. The conversion of guanoxabenz by the cytosolic activity was also quite potently blocked by DWO1, 1-(3,4-dimethoxybenzylideneamino)3-hydroxyguanidine, a hydroxyguanidine analogue to guanoxabenz.

Characterization of the enzymatic activity for biphasic competition by guanoxabenz (1-(2,6-dichlorobenzylidene-amino)-3-hydroxyguanidine) at alpha2-adrenoceptors. I. Description of an enzymatic activity in spleen membranes.

The mechanism for formation of high-affinity binding of 1-(2,6-dichlorobenzylidene-amino)-3-hydroxyguanidine (guanoxabenz) to alpha2-adrenoceptors was studied in particulate fractions from the rat spleen. The proportion of apparent high versus low-affinity alpha2-adrenoceptor binding sites increased with increasing incubation time and was also augmented by Mg2+ ions. The formation of high-affinity guanoxabenz binding seemed to be inhibited by a series of N-hydroxyguanidine analogs to guanoxabenz, as well as by a series of metabolic inhibitors that included allopurinol, 1-chloro-2,4-dinitrobenzene, 5,5'-dithiobis-(2-nitrobenzoic acid), cibacron blue, phenyl-p-benzoquinone, didox, and trimidox. The formation of guanoxabenz high-affinity binding was also inhibited in a time- and concentration-dependent fashion by preincubating the membranes with the LW03 N-hydroxyguanidine analogue of guanoxabenz. Moreover, when the spleen membranes were extensively washed for 30 min with buffers at 25 degrees, the guanoxabenz high-affinity binding disappeared. However, when these washed membranes were supplemented with xanthine, the apparent affinity of guanoxabenz increased four to five-fold. Taken together, all data were compatible with the theory that the formation of high-affinity binding was dependent on the generation of a guanoxabenz metabolite that showed an approximate 100-fold greater affinity for the alpha2-adrenoceptors than guanoxabenz itself. Because the most potent blocker of the formation of high-affinity binding was allopurinol (apart from some N-hydroxyguanidine analogs to guanoxabenz) and since the activity could be restored with xanthine, a likely candidate responsible for the metabolic activation is xanthine oxidase.

Formation of guanoxabenz from guanabenz in human liver. A new metabolic marker for CYP1A2.

The in vitro N-hydroxylation of guanabenz as well as the corresponding N-dehydroxylation of guanoxabenz has been previously detected in biotransformation studies with microsomal fractions of different species including human hepatic microsomes. Furthermore, the N-hydroxylation of guanabenz was found to be catalyzed by enriched cytochrome P450 (P450) fractions in reconstituted systems. Strong correlations between 7-ethoxyresorufin O-deethylation (r = 0. 96; p < 0.001), caffeine N-demethylation (r = 0.92; p < 0.001), respectively, and guanabenz N-hydroxylation activities were demonstrated in 10 human liver microsomal preparations. Studies with microsomes from human B-lymphoblastoid cell lines expressing human cytochrome P450 enzymes proved that CYP1A2 is the major isozyme responsible for this metabolic pathway. Further, P450 isozymes did not show any detectable conversion rates. The reaction was inhibited in presence of the potent CYP1A2 inhibitors alpha-naphthoflavone (7, 8-benzoflavone) and furafylline. The N-reduction of guanoxabenz to guanabenz exhibits a significant correlation to the benzamidoxime N-reduction after incubation with 10 human liver microsomal preparations (r = 0.97; p < 0.001). The formation of benzamidine from benzamidoxime was described previously to be catalyzed by the benzamidoxime reductase. These results suggest that the guanabenz N-hydroxylation is mediated via CYP1A2, whereas the corresponding guanoxabenz N-reduction is catalyzed by an enzyme system composed of cytochrome b5, NADH cytochrome b5-reductase, and benzamidoxime reductase. The high affinity of guanabenz to CYP1A2 and the distinct selectivity of this P450 isozyme toward guanabenz confirms the in vitro guanabenz N-hydroxylation to be a suitable metabolic marker for CYP1A2 in biotransformation studies.

Microsomal catalyzed N-hydroxylation of guanabenz and reduction of the N-hydroxylated metabolite: characterization of the two reactions and genotoxic potential of guanoxabenz.

The N-reduction of the centrally acting alpha 2-adrenoreceptor agonist guanoxabenz (Benzérial), an N-hydroxyamidinohydrazone, to the amidinohydrazone guanabenz (Wytensin, Hipten, Rexitene) by microsomal fractions from rabbits, pigs and humans has been detected in vitro. The conversion rates with rabbit microsomal fractions were markedly slower than those in the cases of fractions from humans and pigs. It was also possible to demonstrate the N-oxidation of guanabenz to guanoxabenz by the use of microsomal fractions from rabbits, pigs, and humans. Furthermore, the oxidation was also observed in reconstituted systems in the presence of enriched cytochrome P450 fractions, purified isoenzyme P450 2C3, and heterologously expressed P450 2C3 of the subforms 6 beta H and 6 beta L. The analyses were performed with a newly developed HPLC technique and were confirmed by LC-MS methods. The kinetic parameters determined for the metabolic cycle (bioreversible reactions) are indicative of a predominance of the reduction of guanoxabenz to guanabenz in vivo. Accordingly, guanoxabenz in part constitutes a prodrug of guanabenz. Examination of guanabenz and guanoxabenz for mutagenicity by means of the Ames test revealed that guanoxabenz has pronounced mutagenic effects in the strains TA 98 and TA 1537. Guanabenz did not exhibit mutagenicity so that the N-reduction of guanoxabenz has significance in terms of detoxification.

Renal effects of infusion of rilmenidine and guanabenz in conscious dogs: contribution of peripheral and central nervous system alpha 2-adrenoceptors.

1. We tested the renal effects of the alpha 2-adrenoceptor agonists, rilmenidine and guanabenz and the antagonists, 2-methoxyidazoxan and idazoxan, in conscious dogs. Our aim was to test the hypothesis that putative imidazoline (I) receptors influence renal function. We reasoned that since rilmenidine and guanabenz are selective for I1- and I2-binding sites respectively, an influence of one of these receptive sites on renal function would be reflected in qualitative differences between the effects of these agents. Moreover, effects mediated by putative I-receptors should be relatively resistant to antagonism by the selective alpha 2-adrenoceptor antagonist, 2-methoxyidazoxan. Since the effects of these drugs on renal function could be mediated in the central nervous system or periphery, the dogs were studied under both normal and ganglion-blocked conditions. 2. In dogs with intact autonomic reflexes, 2-methoxyidazoxan (15 micrograms kg-1 plus 0.6 micrograms kg-1 min-1) produced effects consistent with a generalized increase in sympathetic drive, including increases in mean arterial pressure and plasma renin activity, and a reduction in sodium excretion. In ganglion-blocked dogs, 2-methoxyidazoxan reduced sodium excretion but had no discernible effect on systemic or renal haemodynamics. We conclude that an alpha 2-adrenoceptor-mediated mechanism in the central nervous system tonically inhibits sympathetic drive in the conscious dog. 3. In ganglion-blocked dogs idazoxan (3-300 micrograms kg-1) dose-dependently increased arterial pressure. This was not abolished by concomitant administration of 2-methoxyidazoxan (0.3-30 micrograms kg-1). The pressor effect of idazoxan is therefore probably mediated by an agonist action at alpha 1-adrenoceptors. 4. The effects of infusions of rilmenidine (0.1-1.0 mg kg-1) and guanabenz (10-100 micrograms kg-1) were indistinguishable. They comprised dose-dependent increases in mean arterial pressure, urine excretion, and glomerular filtration rate (the latter in ganglion blocked dogs only), and dose-dependent reductions in heart rate, renal blood flow and sodium excretion (only in dogs with intact autonomic reflexes). All of these effects were antagonized by 2-methoxyidazoxan. 5. We conclude that the renal effects of rilmenidine and guanabenz infusions in conscious dogs are predominantly, if not completely, attributable to activation of alpha 2-adrenoceptors. Our results do not support the hypothesis that putative I-receptors contribute towards the renal effects of these agents.

[Evidence for two alpha 2B-adrenoreceptor isoforms in the renal cortex of salt-sensitive and salt resistant Sabra rats. Effect of salt loading]. / Distinction de deux isofromes de récepteurs alpha 2B-adrénergiques dans le cortex rénal des rats Sabra sensibles et résistants au sel. Effet d'une surcharge en sel.

alpha 2-adrenoceptors are involved in various renal functions regulating blood pressure. They were classified in subtypes whom genes were identified in both humans and rats. In rat renal cortex it was evidenced that the alpha 2B isoform is predominant. This result was confirmed in Sabra rats. However, the renal cortex alpha 2B density is higher in salt-sensitive (SBH) than in salt-resistant (SBN) Sabra rats. alpha 2B-adrenoceptors were recently subclassified in two pharmacologically distinct subtypes exhibiting high and low affinity for guanoxabenz and respectively called alpha 2B1 and alpha 2B2. We studied sodium loading effect on alpha 2B1 and alpha 2B2 distribution in Sabra rat renal cortex using competition experiments between [3H]-yohimbine and guanoxabenz. The rats were submitted to normal (0.2%) or high sodium diet (8%) for six weeks. Under normal diet, proportion alpha 2B1 and alpha 2B2 was similar in SBH and SBN. Nevertheless, their respective densities were significantly higher in SBH as compared to SBN (alpha 2B1: 90.6 +/- 4.1 vs 57.4 +/- 2.5 fmoles/mg prot, p < 0.0001; n = 5; alpha 2B2: 102.7 +/- 4.0 vs 66.4 +/- 4.6 fmoles/mg prot; p < 0.0001; n = 5). Under high sodium diet the distribution of these two isoforms was altered. The densities of alpha 2B1 were decreased by 27.0 +/- 5.9% in SBH (68.0 +/- 4.0 fmoles/mg prot; p < 0.0001, n = 5) and by 47.3 +/- 7.4% for SBN (29.2 +/- 3.1 fmoles/mg prot; p < 0.0001; n = 5). Conversely, the densities of alpha 2B2 were increased by 28.3 +/- 5.4% in SBH (131.1 +/- 9.5 fmoles/mg prot; p < 0.001; n = 5) and by 75.0 +/- 17% in SBN (123.2 +/- 9.1 fmoles/mg prot; p < 0.0001; n = 5). In conclusion, alpha 2B1- and alpha 2B2-adrenoceptor subtypes are found in renal cortex of both SBH and SBN. Our data demonstrated an equal distribution of these two isoforms between SBH and SBN under normal salt diet. This distribution is largely altered, especially in SBN, by the high sodium diet. From these modifications might result differential renal responses to activation of alpha 2B-adrenoceptors between SBH and SBN, and consequently responsible for normal or high blood pressure after high sodium diet.

Alpha 2-adrenoceptor subtypes identified by [3H]RX821002 binding in the human brain: the agonist guanoxabenz does not discriminate different forms of the predominant alpha 2A subtype.

Competition [3H]RX821002 ([3H]2-methoxyidazoxan) binding experiments with alpha 2-adrenoceptor subtype-specific antagonists--BRL 44408 (alpha 2A selectively), ARC 239 (alpha 2B selective), and others--were performed to delineate through rigorous computer modeling receptor subtypes in the postmortem human brain. In the hippocampus, hypothalamus, cerebellum, and brainstem the whole population of alpha 2-adrenoceptors appears to belong to the alpha 2A subtype (100%; Bmax = 34-90 fmol/mg of protein). In the frontal cortex, the predominant receptor was the alpha 2A subtype (87%; Bmax = 53 fmol/mg of protein), although a small population of the alpha 2B/C subtype (13%; Bmax = 8 fmol/mg of protein) was also detected. In the caudate nucleus, a mixed population of alpha 2A (64%; Bmax = 9 fmol/mg of protein) and alpha 2B/C (36%; Bmax = 5 fmol/mg of protein) subtypes was detected. In the cortex and caudate and in the presence of ARC 239 (to mask the alpha 2B/C-adrenoceptors), competition experiments with the agonist guanoxabenz clearly modeled the high- and low-affinity states of the alpha 2A subtype. In the presence of ARC 239 and the GTP analogue guanylyl-5'-imidodiphosphate together with NaCl and EDTA (to eliminate the high-affinity alpha 2A-adrenoceptor) guanoxabenz only recognized the low-affinity alpha 2A-adrenoceptor. The results indicate that in the human brain the predominant alpha 2-adrenoceptor is of the alpha 2A subtype and that this functionally relevant receptor subtypes is not heterogeneous in nature.

Differential interaction of guanabenz with receptor binding sites in rat brain and kidney.

The alpha 2-adrenoceptor selective agonist, [3H]guanabenz ([ 3H]GBZ), labels a unique population of binding sites in whole kidney which are not labeled by [3H]p-aminoclonidine ([3H]PAC). These binding sites are saturable and of high affinity (Kd = 10-12 nM). [3H]GBZ was not displaced from these sites by other alpha 1- or alpha 2-ligands, suggesting that they are non-adrenergic. This hypothesis is further supported by the insensitivity of renal guanabenz binding to regulation by guanyl nucleotides or to destruction by trypsin. Also, there appears to be no effect of guanabenz on the potency of isoproterenol in competing for beta-adrenoceptors in the kidney, which has been previously reported to be sensitive to clonidine. The absence of any effect of guanabenz on isoproterenol displacement of [3H]dihydroalprenolol in kidney suggests there are subtle differences in activation of alpha-receptors by clonidine and guanabenz in the kidney. In the brain, [3H]GBZ labels two binding sites. Part of the binding of [3H]GBZ in the brain is to sites essentially identical to the alpha 2-adrenoceptors labeled by [3H]PAC. The remainder of the binding resembles the non-adrenergic binding in kidney. The relationship of this unique binding site to the pharmacologic actions of guanabenz is currently not known.

Guanabenz, guanochlor, guanoxan and idazoxan bind with high affinity to non-adrenergic sites in pig kidney membranes.

[3H]Idazoxan is a labelled ligand that is frequently used to study alpha 2-adrenoceptors in the central nervous system. In pig kidney membranes, [3H]idazoxan labelled high-affinity binding sites (Kd = 1.5 nM) that were not alpha 2-adrenoceptors and which recognized clonidine with low affinity. This new class of binding sites was recognized by amiloride derivatives; however, it is not likely that these sites are the well-known targets of amiloride in the kidney: the Na+/H+ exchanger and the epithelium Na+ channel. These binding sites may be the normal target of a series of imidazolidines derivatives (guanabenz, guanochlor, guanoxan), which are known for their antihypertensive properties.

Effect of polyethylene glycol 400 on the penetration of drugs through human cadaver skin in vitro.

The effect of polyethylene glycol 400 on the penetration of drugs through human cadaver skin is reported. Polyethylene glycol 400 was used in various concentrations in the donor and the receptor compartments. It was observed that polyethylene glycol 400 had significant effects on the penetration rates of compounds, both when used in the donor as well as in the receptor solutions. These effects were barrier specific and are related to the alteration of the skin structure and the mass flow of water.

Beta-adrenergic activity and conformation of the antihypertensive specific alpha 2-agonist drug, guanabenz.

In recent research a new series of specific drugs, one of which is guanabenz (GBZ, 2,6(dichlorobenzyliden)-aminoguanidine) has been introduced into the clinical treatment of centrally mediated hypertension. Guanabenz (GBZ) is considered to be among the most specific alpha 2-adrenergic agonists, acting similarly to clonidine by decreasing the sympathetic outflow from the brain to the peripheral circulatory system. In the present report we show that GBZ displays a significant affinity for beta-adrenoceptors. In displacement studies of the iodinated beta-antagonist [125I]cyanopindolol (CYP) from turkey erythrocyte membranes, the dissociation constant of GBZ was 3.8 microM. Inhibition of the (-) epinephrine induced adenylate cyclase activity by GBZ is competitive, with an apparent dissociation constant of 30 microM. A similar value was obtained by studies of GBZ's effect on the (-) epinephrine-induced [3H]cAMP accumulation in intact turkey erythrocytes. In view of its unexpected affinity for beta-adrenoceptors, we examined the three-dimensional structure of crystalline GBZ. In these studies substantial differences between clonidine and GBZ were observed, despite their strong structural resemblance. These dissimilarities (angle of rotation phi = 39.7 degrees as compared to 76 degrees in clonidine, and the rotational restriction of clonidine as compared to the greater mobility in rotation of GBZ) could explain the difference of specificity between these two compounds.