Recent progress in α1-adrenergic receptor research

Alpha1-Adrenergic receptors (AR) play an important role in the regulation of physiological responses mediated by norepinephrine and epinephrine, particularly in the cardiovascular system. The three cloned alpha1-AR subtypes (alpha1A, alpha1B, and alpha1D) are G protein-coupled receptors that signal through the Gq/11 signaling pathway, each showing distinct pharmacological properties and tissue distributions. However, due to the lack of highly subtype-selective drugs, the functional roles of individual subtypes are still not clear. Development of new subtype-specific drugs will greatly facilitate the identification of the functions of each subtype. Conopeptide rho-TIA has been found to be a new alpha1B-AR selective antagonist with different modes of inhibition at alpha1-AR subtypes. In addition, recent studies using genetically engineered mice have shed some light on alpha1-AR functions in vivo, especially in the cardiovascular system and brain. Several proteins have been shown to interact directly with particular alpha1-AR, and may be important in regulating receptor function. Receptor heterodimerization has been shown to be important for cell surface expression, signaling and internalization. These new observations are likely to help elucidate the functional roles of individual alpha1-AR subtypes.     

Antagonism of the hypothermic effect of clozapine in mice by centrally-active α2-adrenergic antagonists and α1-adrenergic agonists

Hypothermia induced by either clozapine or clonidine in mice was blocked by the ₂-adrenergic antagonists yohimbine, idazoxan, CH-38083, SKF 86466, and L-657,743. These effects were dose related, and the ID₅₀ values for inhibition of clozapine- or clonidine-induced hypothermia were fairly comparable. The order of potency for blocking clonidine-induced hypothermia was: L-657,743>CH-38083>yohimbine>idazoxan>SKF 86466. A very similar blockade hierarchy for clozapine-induced hypothermia was observed, with the order of the two most effective compounds being reversed. Hypothermia induced by either compound was not blocked by the peripherally-acting, selective ₂-adrenergic antagonist, L-659,066, indicating that blockade by the other compounds occurred centrally. The centrally-acting, ₁-adrenergic agonists St 587, cirazoline, and SKF 89748 were very effective in blocking the response to clozapine, but ineffective in antagonizing clonidine-induced hypothermia. The ED₅₀ values for the blockade of this response to clozapine, however, did not correlate with their reported potencies in stimulating either peripheral or central ₁-adrenergic receptors. This indicates that clozapine-induced hypothermia in mice is not a suitable model for evaluating the properties of central ₁-adrenergic compounds. Moreover, since the clonidine-induced hypothermia is not influenced by ₁-adrenergic agonists, this paradigm is preferable to clozapine-induced hypothermia in the assessment of ₂-adrenergic antagonism. The ability of ₂-adrenergic antagonists to block clozapine-induced hypothermia may result from the central overflow of norepinephrine, which is known to be brought about by this group of compounds. The neurochemical mechanism responsible for the anti-clozapine effect of the ₁-adrenergic agonists in mice is not clear. An as yet unknown property of these compounds, unrelated to ₁-agonism, may have to be considered. An interaction of these compounds at the high-affinity clozapine binding site is a possibility. Discovery of antagonists to clozapine may help to elucidate the mechanism of action of this atypical neuroleptic.     

Effects of noradrenergic denervation on L-DOPA-induced dyskinesia and its treatment by α- and β-adrenergic receptor antagonists in hemiparkinsonian rats

While L-3,4-dihydroxyphenylalanine (L-DOPA) remains the standard treatment for Parkinson's disease (PD), long-term efficacy is often compromised by L-DOPA-induced dyskinesia (LID). Recent research suggests that targeting the noradrenergic (NE) system may provide relief from both PD and LID, however, most PD patients exhibit NE loss which may modify response to such strategies. Therefore this investigation aimed to characterize the development and expression of LID and the anti-dyskinetic potential of the α2- and β-adrenergic receptor antagonists idazoxan and propranolol, respectively, in rats receiving 6-OHDA lesions with (DA lesion) or without desipramaine protection (DA + NE lesion). Male Sprague-Dawley rats (N = 110) received unilateral 6-hydroxydopamine lesions. Fifty-three rats received desipramine to protect NE neurons (DA lesion) and 57 received no desipramine reducing striatal and hippocampal NE content 64% and 86% respectively. In experiment 1, the development and expression of L-DOPA-induced abnormal involuntary movements (AIMs) and rotations were examined. L-DOPA efficacy using the forepaw adjusting steps (FAS) test was also assessed in DA- and DA + NE-lesioned rats. In experiment 2, DA- and DA + NE-lesioned rats received pre-treatments of idazoxan or propranolol followed by L-DOPA after which the effects of these adrenergic compounds were observed. Results demonstrated that moderate NE loss reduced the development and expression of AIMs and rotations but not L-DOPA efficacy while anti-dyskinetic efficacy of α2- and β-adrenergic receptor blockade was maintained. These findings suggest that the NE system modulates LID and support the continued investigation of adrenergic compounds for the improved treatment of PD.     

Functional α1-adrenergic receptor subtypes in human right gastroepiploic artery

AIM: To study the functional alpha1-adrenergic receptor (alpha1-AR) subtypes in human right gastroepiploic artery (RGA). METHODS: The effects of alpha2-AR, alpha1-AR, and alpha1-AR subtype selective antagonists on norepinephrine (NE)-induced vasoconstriction in isolated human RGA were observed by contractile function experiment. RESULTS: Cumulative concentration-response curves for NE were competitively antagonized in RGA by alpha2-AR selective antagonist yohimbine (pA2 6.82+/-0.28, slope 1.12+/-0.40),alpha1-AR selective antagonist prazosin (pA2 9.77+/-0.22, slope 0.90+/-0.22),alpha1A-AR selective antagonists RS17053 (pA2 8.42+/-0.20, slope 0.93+/-0.20) and 5-MU (pA2 8.42+/-0.22, slope 0.88+/-0.18),alpha1D-AR selective antagonist BMY7378 (pA2 6.84+/-0.32, slope 1.05+/-0.17), and alpha1A-,alpha1B-AR selective antagonist WB4101 (pA2 8.88+/-0.20, slope 1.15+/-0.16). The correlation coefficients between these pA2 values of alpha1-AR selective antagonists with pKi values of which obtained from alpha1A-, alpha1B- and alpha1D-AR cloned cells are 0.95, 0.82, and 0.42. After the vessels were pretreated by chlorethylclonidine (CEC), an alpha1B- and alpha1D-AR irreversible alkylating agent, the pD2 values were changed from 5.9+/-0.5 to 5.6+/-0.6 and the maximal contraction was changed from (8.9+/-3.2) g to (8.0+/-3.2) g, respectively. The difference was not significant. CONCLUSION: In human RGA, the contraction response is mainly mediated by alpha1-AR, of which alpha1A-AR plays an important role, whereas alpha1B- and alpha1D-AR are not involved in the contraction response.     

Muscarinic M1receptor agonists and M2 receptor antagonists as therapeutic targets in Alzheimer’s disease

Alzheimer’s disease is the most common form of senile dementia and is of increasing medical and economic importance. One of the earliest findings in the systematic study of Alzheimer’s disease was the progressive loss of acetylcholinergic neurons. This lead to the so-called cholinergic hypothesis of Alzheimer’s disease and provoked a search for cholinergic replacement therapies. This has resulted in compounds capable of inhibiting the catabolism of acetylcholine (acetylcholinesterase inhibitors) being licensed for the treatment of Alzheimer’s disease. However, the search for directly acting, selective, muscarinic M₁ receptor agonists and M₂ receptor antagonists, that are capable of increasing acetylcholine levels in the neocortex, has not yet proved successful. For the muscarinic agonists this failure has been largely due to the difficulty in finding genuinely selective compounds. As a result, there has been a reduction in the number published patents in this area. This review discusses the rationale, progress and the possible future for these two potential approaches to the therapy of Alzheimer’s disease especially in light of some recent patents containing selective compounds.     

Actions of adenosine A1 and A2 receptor antagonists on CFTR antibody-inhibited β-adrenergic mucin secretion response

1. The cystic fibrosis gene protein, the cystic fibrosis transmembrane conductance regulator (CFTR) acts as a chloride channel and is a key regulator of mucin secretion. The mechanism by which 3-isobutyl-1-methylxanthine (IBMX) corrects the defect in CFTR mediated β-adrenergic stimulation of mucin secretion has not been determined. The present study has investigated the actions of adenosine A₁ and A₂ receptor antagonists to determine whether ability to stimulate mucin secretion correlates with correction of CFTR antibody inhibited β-adrenergic response and whether excessive cyclic AMP rise is required.
2. CFTR antibodies were introduced into living rat submandibular acini by hypotonic swelling. Following recovery, mucin secretion in response to isoproterenol was measured.
3. The adenosine A₁ receptor antagonist, 8 cyclopentyltheophylline (CPT) was a less potent stimulator of mucin secretion than was the A₂ receptor antagonist dimethylpropargylxanthine (DMPX). A concentration of CPT close to the K_i for A₁ receptor antagonism (10 nM) did not stimulate mucin secretion.
4. DMPX, although a potent stimulator of mucin secretion, did not correct CFTR antibody inhibited mucin secretion.
5. CPT corrected defective CFTR antibody inhibited mucin secretion at a high (1 mM) concentration, suggesting a mechanism other than adenosine receptor antagonism.
6. DMPX potentiated the isoproterenol induced cyclic AMP rise, whereas CPT did not.
7. Correction of the defective CFTR mucin secretion response did not correlate with ability to stimulate mucin secretion and did not require potentiation of β-adrenergic induced increases in cyclic AMP. This affords real promise for the development of a selective drug treatment for cystic fibrosis.

     

Non-peptidic δ opioid receptor agonists and antagonists (2000 – 2012)

Introduction: The δ opioid receptor mediates various pharmacological effects such as antinociceptive and antidepressant effects, whereas it does not appear to induce μ opioid-like side effects such as dependence, respiratory depression and constipation. Therefore, the δ opioid receptor is a promising drug target.

Areas covered: This review covers literature and patents concerning non-peptidic δ opioid receptor agonists, antagonists, modulators and ligands from 2000 to 2012. Pharmacological effects induced by δ receptor agonists or antagonists are also discussed.

Expert opinion: Potential therapeutic effects by δ receptor agonists are antinociceptive, antidepressant, anxiolytic, cardioprotective and neuroprotective effects. Among them, anxiolytic effects are of particular interest because the anxiolytic effects by a δ receptor agonist have been observed in humans. Although non-peptidic δ receptor agonists were reported to show convulsive effects via the δ opioid receptor, some δ receptor agonists are known to produce no convulsive behaviors. Therefore, it may be possible to eliminate convulsion induced by a δ receptor agonist. Many δ receptor antagonists were also reported but there is little new information about pharmacological effects by a δ receptor antagonist. Although detailed results were not revealed, two δ receptor antagonists with μ receptor agonistic or antagonistic properties are in the late stages of the clinical trial.     

The pharmacogenetics of β-adrenergic receptor antagonists in the treatment of hypertension and heart failure

INTRODUCTION: β-Blockers have an important therapeutic role throughout the cardiovascular continuum. However, there is considerable variation in response to these drugs, which may be related to genetic influences on their pharmacokinetics and pharmacodynamic effects. AREAS COVERED: This review focuses on genetic variations in the drug metabolizing enzymes which influence the pharmacokinetics and potentially the pharmacodynamics of some β-blockers. It also reviews the polymorphisms in the adrenergic receptors (ARs) and their related pathways which are likely to influence the responses to β-blockers. EXPERT OPINION: The CYP2D6 genotypes influence the pharmacokinetics of some β-blockers but the effects on β-blocker responses have been inconsistent and there is currently no general role for CYP2D6 genotyping prior to choosing a particular β-blocker or dose. The common polymorphisms producing changes in the β(1)-ARs, and their signaling pathways, have been associated with clinical outcomes in several studies in hypertension and heart failure. Treatment with β-blockers, especially with higher doses, appears to have greater benefits in patients with the genetic forms of the β(1)-ARs which are more responsive to both agonists and antagonists. However, current data are not sufficiently consistent to support genotyping for these polymorphisms before selecting or initiating β-blocker treatment and further study results are needed to clarify the situation.     

Effects of intrathecal 6-hydroxydopamine, α1 and α2 adrenergic receptor antagonists on antinociception of propofol in mice

AIM: To investigate the relationship between spinal cord norepinephrine, alpha1 and alpha2 adrenergic receptors and antinociception of propofol in mice. METHODS: Kunming mice were used. Antinociceptive tests were investigated with the tail-immersion test and the acetic acid-induced writhing test. The effects of subcutaneous (sc), intrathecal (ith) and intracerebroventricular (icv) injection propofol on pain threshold were observed. The influences of pretreatment with ith 6-hydroxydopamine, alpha1R antagonist prazosin, or alpha2R antagonist yohimbine on the antinociception of propofol were studied. RESULTS: Significant antinociception was produced by propofol (25, 50 mg/kg, sc) and propofol (20, 40 microg, ith) in tail-immersion test and acetic the acid-induced writhing test (P<0.05 or P<0.01). Icv propofol (10, 20, and 40 microg) did not produce any effect on pain threshold in mice (P>0.05). The 6-hydroxydopamine (5 and 10 microg), prazosin (5 and 10 microg), or yohimbine (5 and 10 microg) ith alone did not affect basal tail-flick latency (TFL) in conscious mice, but significantly reduced the TFL as measured by tail-immersion test in propofol (50 mg/kg, sc)-treated mice, compared with basal TFL and vehicle groups (P<0.05 or P<0.01). CONCLUSION: The spinal cord is a target of propofol antinociception. In mice propofol antinociception is partly mediated by spinal norepinephrine, alpha1R and alpha2R.     

Development of β-amino-carbonyl compounds as androgen receptor antagonists

Aim： Androgen receptor （AR） antagonists have proven to be useful in the early control of prostate cancer. The aim of this study was to identify and characterize a novel β-amino-carbonyl-based androgen receptor antagonist. Methods Different isomers of the β-amino-carbonyl compounds were obtained by chiral separation. The bioactivities of the isomers were evaluated by AR nuclear translocation, mammalian two-hybrid, competitive receptor binding and cell proliferation assays. The expression of genes downstream of AR was analyzed with real-time PCR. The therapeutic effects on tumor growth in vivo were observed in male SClD mice bearing LNCaP xenografts. Results： Compound 21 was previously identified as an AR modulator by the high-throughput screening of a diverse compound library. In the present study, the two isomers of compound 21, termed compounds 21-1 and 2.1.-2, were characterized as partial AR agonists in terms of androgen-induced AR nuclear translocation, prostate-specific antigen expression and cell proliferation. Further structural modifications led to the discovery of a androgen receptor antagonist （compound 6012）, which blocked androgen receptor nuclear translocation, androgen-responsive gene expression and androgen-dependent LNCaP cell proliferation. Four stereoisomers of compound 6012 were isolated, and their bioactivities were assessed. The pharmacological effects of 6012, including AR binding, androgen-induced AR translocation, NH2- and COOH-terminal interaction, growth inhibition of LNCaP cells in vitro and LNCaP xenograft growth in nude mice, were mainly restricted to isomer 6012-4 （1R, 3S）. Conclusion： Compound 6012-4 was determined to be a novel androgen receptor antagonist with prostate cancer inhibitory activities comparable to bicalutamide both in vitro and in vivo.     

Effects of clonidine and some α-adrenergic antagonists alone and in combination on schedule-controlled behavior in pigeons and mice

Schedule-controlled responding was maintained under multiple fixed-interval, fixed-ratio schedules in pigeons and single fixed-ratio schedules in mice. In pigeons, clonidine, an ₂-receptor agonist, produced dose-related decreases in responding under both fixed-interval and fixedratio schedules; fixed-interval responding was decreased at a lower dose than fixed-ratio responding. Low to intermediate doses of yohimbine, an ₂-receptor antagonist, increased responding under the fixed-interval schedule without appreciably affecting responding under the fixed-ratio schedule; higher doses decreased responding under both schedules. In mice, both clonidine and yohimbine produced dose-related decreases in responding under fixed-ratio schedules. Decreases in response rates produced by clonidine were antagonized by low to intermediate doses of yohimbine. Decreases in response rates under fixed-ratio schedules produced by yohimbine were antagonized only slightly, if at all, by clonidine. Under the fixed-interval schedule, clonidine potentiated the response-rate increasing effects of intermediate doses of yohimbine and slightly antagonized the rate-decreasing effects. Although some effects of clonidine were antagonized by yohimbine, at no dose combination did performances completely resemble control performances. Prazosin, an ₁-receptor antagonist, was ineffective both when administered alone and as an antagonist of the effects of clonidine. The behavioral effects of clonidine appeared to be mediated by ₂ rather than ₁ receptors. Additionally, yohimbine appears to have significant behavioral effects other than ₂-antagonist actions.     

6-Hydroxydopamine pretreatment effects on α- and β-adrenergic receptor adaptation to clorgyline

Summary The effects of 6-hydroxydopamine (6-OHDA) lesions on brain adrenergic receptor adaptation to 21 days of treatment with the selective monoamine oxidase type A (MAO-A) inhibitor clorgyline were studied in rats. 6-OHDA pretreatment effectively blocked the decrease in ₁-, ₂- and -adrenergic receptor densities observed in response to clorgyline treatment. In saline-treated rats, 6-OHDA reduced norepinephrine (NE) to 8% of control levels, modestly reduced dopamine (DA) to 67% of controls, but did not affect serotonin (5HT) levels in the cortex, Clorgyline administration to shams increased NE and 5HT to 239% and 160% of their respective control levels, but did not effect DA levels. 6-OHDA lesions attenuated clorgyline's effect on cortical NE levels but not 5HT. The results suggest that -adrenergic receptor adaptation to MAOI's as with tricyclic antidepressants is a response to an increase in catecholamine receptor occupancy, and that a similar molecular mechanism is responsible for the observed clorgyline induced changes in -adrenergic receptors.     

Anxiolytic-like profile of mirtazapine in rat conditioned fear stress model: Functional significance of 5-hydroxytryptamine 1A receptor and α1-adrenergic receptor

Mirtazapine is an antidepressant with a unique mechanism of action and has been categorized as a Noradrenergic and Specific Serotonergic Antidepressant (NaSSA). Although numerous clinical trials suggested the usefulness of mirtazapine for not only major depressive disorders but also a variety of anxiety disorders, efficacy studies in animal anxiety models have been rarely reported. The present study investigated a potential anxiolytic-like profile of mirtazapine in rat conditioned fear stress model. A 5-hydroxytryptamine (5-HT) 1A receptor partial agonist, buspirone (1-5 mg/kg) exhibited a significant reduction in freezing time, and its maximal effect was reversed by a selective 5-HT_1A antagonist, WAY-100635 (1 mg/kg). Mirtazapine (1-10 mg/kg) also reduced the freezing time in a dose-related fashion, a substantial proportion (approx. 50%) of which was likewise antagonized by WAY-100635 (1 mg/kg). Mianserin (1-30 mg/kg), a structural analogue for mirtazapine, was ineffective. Furthermore, co-administration of α1 adrenoceptor antagonist, prazosin (0.03 mg/kg) completely reversed mirtazapine (10 mg/kg)-induced reduction of freezing time. These findings represent the first demonstration that the anxiolytic-like action of mirtazapine involves activation of 5-HT_1A receptor and α1 adrenoceptor to different extents, and are compatible with one aspect of mirtazapine's pharmacological profile as NaSSA.     

Crystal structures,absolute configurations and molecular docking studies of naftopidil enantiomers as α1D-adrenoceptor antagonists

Chiral drug naftopidil (NAF), a specific α_1D-adrenoceptor (AR) antagonist for the treatment of benign prostatic hyperplasia, was used in racemic form for several decades. Our recent work declared that NAF enantiomers showed the same antagonistic effects on the α_1D-AR, but the binding mechanism of these two stereochemical NAF isomers to the α_1D receptor remained unclear. Herein, we reported the crystallographic structures of optically pure NAF stereoisomers for the first time and unambiguously determined their absolute configurations. The crystal data of R and S enantiomers matched satisfactorily the pharmacophore model for α_1D-selective antagonists. Based on the constructed α_1D homology model, molecular docking studies shed light on the molecular mechanism of NAF enantiomers binding to α_1D-AR. The results indicated that NAF enantiomers exhibited the very similar binding poses and occupied the same binding pocket.     

Analogues of γ-aminobutyric acid (GABA) and trans-4-aminocrotonic acid (TACA) substituted in the 2 position as GABAC receptor antagonists

1. γ-Aminobutyric acid (GABA) and trans-4-aminocrotonic acid (TACA) have been shown to activate GABA_C receptors. In this study, a range of C2, C3, C4 and N-substituted GABA and TACA analogues were examined for activity at GABA_C receptors.
2. The effects of these compounds were examined by use of electrophysiological recording from Xenopus oocytes expressing the human ρ₁ subunit of GABA_C receptors with the two-electrode voltage-clamp method.
3. trans-4-Amino-2-fluorobut-2-enoic acid was found to be a potent agonist (K_D=2.43 μM). In contrast, trans-4-amino-2-methylbut-2-enoic acid was found to be a moderately potent antagonist (IC₅₀=31.0 μM and K_B=45.5 μM). These observations highlight the possibility that subtle structural substitutions may change an agonist into an antagonist.
4. 4-Amino-2-methylbutanoic acid (K_D=189 μM), 4-amino-2-methylenebutanoic acid (K_D=182 μM) and 4-amino-2-chlorobutanoic acid (K_D=285 μM) were weak partial agonists. The intrinsic activities of these compounds were 12.1%, 4.4% and 5.2% of the maximal response of GABA, respectively. These compounds more effectively blocked the effects of the agonist, GABA, giving rise to K_B values of 53 μM and 101 μM, respectively.
5. The sulphinic acid analogue of GABA, homohypotaurine, was found to be a potent partial agonist (K_D=4.59 μM, intrinsic activity 69%).
6. It was concluded that substitution of a methyl or a halo group in the C2 position of GABA or TACA is tolerated at GABA_C receptors. However, there was dramatic loss of activity when these groups were substituted at the C3, C4 and nitrogen positions of GABA and TACA.
7. Molecular modelling studies on a range of active and inactive compounds indicated that the agonist/competitive antagonist binding site of the GABA_C receptor may be smaller than that of the GABA_A and GABA_B receptors. It is suggested that only compounds that can attain relatively flat conformations may bind to the GABA_C receptor agonist/competitive antagonist binding site.

     

Novel antagonists for α1-adrenoceptor subtypes

α₁-Adrenergic receptors (α₁-ARs) belong to the seven-transmembrane-domain receptor superfamily and play a primary role in the regulation of several physiological processes, particularly in the cardiovascular system. Three different α₁-AR subtypes, namely α_1A-AR, α_1B-AR and α_1D-AR, have been cloned and characterised so far. In the past, several non-subtype-selective α₁-AR antagonists, such as prazosin (the prototype of these substances), doxazosin and terazosin, have been used as effective antihypertensive drugs and, more recently, in the symptomatic treatment of benign prostatic hypertrophy (BPH). Intensive research in academia and pharmaceutical industries has led to the discovery of a number of subtype-selective antagonists in the last decade particularly for the α_1A-AR, which seems to be the main subtype involved in bladder outlet obstruction in patients with BPH. This review, which is mainly based on the patent literature (2000 – 2003), is focused on the recent advances in the development of new subtype-selective α₁-AR antagonists and their potential therapeutic applications.     

Anti-emetic potential of tachykinin NK_1 receptor antagonists

AIM: Evidence has accumulated to indicate the importance ofsubstance P in the emetic reflex. The present investigations weredesigned to charaterize tachykinin NK_1 receptors in the emeticreflex of Suncus murinus. METHODS: Several antagonistswere assessed for a potential to inhibit nicotine (5 mg·kg~(-1),sc)-, copper sulphate pentahydrate (120 mg·kg~(-1) ig)-, andmotion (4 cm lateral displacement at 1 Hz)-induced emesis.RESULTS: CP 122 721, CP-99 994,RP 67 580, and FK 888