Heterogeneous 3H-rauwolscine binding sites in rat cortex: two alpha 2-adrenoceptor subtypes or an additional non-adrenergic interaction?

Identification of Buctopamine and Mebuctopamine, a β2 Receptor Agonist and Its Metabolite, in Swine Hair and Feed Additives

4-[2-(t-Butylamino)-1-hydroxyethyl]phenol (buctopamine, 4), a new β₂ receptor agonist (β₂-agonist), was found to be an adulterant in feed additives for swine in Taiwan, where using β₂-agonists in food-production animals is prohibited. Buctopamine and its metabolite, 4-[2-(t-butylamino)-1-hydroxyethyl]-2-methoxyphenol (mebuctopamine, 2), were detected in swine hair specimens. Authentic compounds 2 and 4 were synthesized with 98.6% and 97.7% purity, respectively, as reference standards for analysis, and both compounds were more hydrophilic than ractopamine and clenbuterol. In a preliminary pharmacological evaluation, compounds 2 and 4 exhibited moderate human β₂ receptor binding affinity and did not show significant affinities for the human α₁, α₂, β₁, and β₃ receptors. After addition of compounds 2-4 into the β₂-agonist library, a multiresidue analysis of 26 β₂-agonists by using triple quadrupole LC/MS/MS for routine screening conducted by regulatory authorities was established, in which the common limits of quantification for the 26 β₂-agonists in swine feed and hair are 10 and 25 ng/g, respectively. In addition, the illegal use of buctopamine (4) has been effectively prevented. The results of this study are also useful for controlling the illegal use of new β₂-agonists in food-production animals.

Yokukansan inhibits morphine tolerance and physical dependence in mice: the role of α₂A-adrenoceptor

Yokukansan (YKS) is a traditional Japanese medicine consisting of seven medicinal herbs that is used for the treatment of neurosis, insomnia, and the behavioral/psychological symptoms of dementia. This study examined the effects of YKS on morphine tolerance and physical dependence in mice. Daily oral administration of YKS (0.5 or 1.0 g/kg) for 3 weeks significantly attenuated morphine tolerance and naloxone-precipitated morphine withdrawal signs (jumps and body weight loss) without affecting the analgesic effect of morphine. The inhibitory effect of YKS on withdrawal jumps in morphine-dependent mice was blocked by a single pretreatment with an α(2)-adrenoceptor antagonist, yohimbine, but not by an α(1)-adrenoceptor antagonist, prazosin. A similar inhibitory effect on withdrawal jumps was observed by repeated administration of yohimbine. The membrane expression of α(2A)-adrenoceptors in the pons/medulla was decreased in morphine withdrawn animals; this reduction was prevented by repeated administration of YKS or yohimbine. Competitive radioligand and [(35)S]guanosine-5'-O-(3-thiotriphosphate) binding assays revealed that YKS and its constituent herbs, Glycyrrhiza (GR) and Uncaria hook (UH), had specific binding affinity for and antagonist activity against the α(2A)-adrenoceptor. Certain chemical constituents, including GR -derived glycyrrhizin and its metabolite, 18β-glycyrrhetinic acid, and UH-derived geissoschizine methyl ether (GME), shared such activities. Repeated administration of GR, UH, glycyrrhizin or GME significantly inhibited morphine withdrawal signs. These results suggest that YKS and its active constituents inhibit morphine tolerance and physical dependence, and that the latter is due at least in part to the prevention of the decreased membrane expression of the α(2A)-adrenoceptor in the brainstem by its prolonged blockade.

Maturational changes in pontine and medullary alpha-adrenoceptor influences on respiratory rhythm generation in neonatal rats

We examined developmental changes in alpha-adrenoceptor influences and descending pontine inputs on the medullary respiratory network in the neonatal rat in vitro brainstem-spinal cord preparation. Using a split bath preparation to isolate the pons from the medulla, antagonists for alpha1 and alpha2 adrenoreceptors were applied to only the medulla at postnatal days 0, 2 and 4, before and after transection of the pons. Blocking alpha1 and alpha2 receptors in the medulla in the absence of a pons reduced burst frequency at all ages with a more pronounced effect in younger animals. At all ages the presence of a pons diminished the effect of blocking alpha2 receptors in the medulla and eliminated the effect of blocking alpha1 receptors. These results indicate that there is a tonic release of catecholamines within the medulla that is under influence from the pons. Additionally, transection experiments indicated that during development, the net influence of the pons changed from one of excitation to one of inhibition.

Pharmacology and metabolism of renzapride : a novel therapeutic agent for the potential treatment of irritable bowel syndrome

BACKGROUND AND OBJECTIVE

Renzapride (ATL-1251), a novel benzamide, is currently under clinical development for the treatment of irritable bowel syndrome (IBS). Previous in vitro and in vivo experimental studies have characterized renzapride as a full serotonin 5-HT(4) receptor agonist on the gut and a 5-HT(3) receptor antagonist. Clinical studies have confirmed the therapeutic efficacy, tolerability and safety of renzapride in patients with constipation-predominant IBS. This study set out to characterize the pharmacological profile of renzapride and its potential metabolic products at both 5-HT and other monoamine receptors in the gut.

METHODS

The affinity of renzapride, its (+) and (-) enantiomers, and its primary metabolite, renzapride N-oxide and its enantiomers, for serotonin receptors was assessed by means of in vitro radioligand binding inhibition studies. After membranes prepared from animal tissue or membranes of cell lines transfected with cloned human receptors had been incubated with radiolabelled ligand with high affinity for a specific receptor, renzapride was added to competitively inhibit this binding. Levels of bound radioligand were measured by filtration and counting of the bound radioactivity. In instances where >50% inhibition of radioligand binding had occurred, the inhibition constant (K(i)) was calculated. Metabolism of renzapride by liver microsomes was assessed by incubating 10 micromol/L renzapride with human liver microsome samples for 60 minutes at 37 degrees C. After the reaction was stopped, the samples were centrifuged and the supernatant analysed for metabolites by high-pressure liquid chromatography (HPLC). The potential inhibitory effects of renzapride on cytochrome P450 (CYP) enzymes were assessed by incubating renzapride at various concentrations over a 1-500 micromol/L concentration range with microsomes genetically engineered to express a single CYP.

RESULTS

Renzapride was selective for serotonergic receptors and, in particular, had high affinity for human 5-HT(3) and guinea-pig 5-HT(4) receptors (K(i) 17 and 477 nm, respectively). Inhibitory properties at 5-HT(2B) receptors were also identified for renzapride, as well as some affinity for 5-HT(2A) and 5-HT(2C) receptors. Renzapride N-oxide and its enantiomers demonstrated much lower affinity for all 5-HT receptors compared with renzapride. Renzapride was metabolized by liver microsomes to a limited extent and there was no significant non-microsomal metabolism of renzapride. Renzapride did not inhibit the major CYP drug-metabolizing enzymes CYP2C9, CYP2D6, CYP1A2, CYP2A6, CYP2C19, CYP2E1 or CYP3A4 at concentrations consistent with use in a clinical setting.

CONCLUSIONS

These results confirm and extend earlier studies in animal and human receptors that show renzapride is a potent and generally full 5-HT(4) receptor agonist and 5-HT(3) receptor antagonist. The results reported in the present study indicate that the metabolites of renzapride are minor and are unlikely to contribute to its therapeutic profile or lead to interaction of renzapride with other drugs that inhibit the major drug-metabolizing enzymes in the liver at therapeutic doses. These data contribute to the understanding of the pharmacological actions and metabolic fate of renzapride in vivo.

Regional distribution of alpha(2C)-adrenoceptors in brain and spinal cord of control mice and transgenic mice overexpressing the alpha(2C)-subtype: an autoradiographic study with [(3)H]RX821002 and [(3)H]rauwolscine

Behavioral studies on gene-manipulated mice have started to elucidate the neurobiological functions of the alpha(2C)-adrenoceptor (AR) subtype. In this study, we applied quantitative receptor autoradiography to investigate the potential anatomical correlates of the observed functional effects of altered alpha(2C)-AR expression. Labeling of brain and spinal cord sections with the subtype non-selective alpha(2)-AR radioligand [(3)H]RX821002 and the alpha(2C)-AR-preferring ligand [(3)H]rauwolscine revealed distinct binding-site distribution patterns. In control mice, [(3)H]rauwolscine binding was most abundant in the olfactory tubercle, accumbens and caudate putamen nuclei, and in the CA1 field of the hippocampus. A mouse strain with overexpression of alpha(2C)-AR regulated by a gene-specific promoter showed approximately two- to four-fold increased levels of [(3)H]rauwolscine binding in these regions. In addition, dramatic increases in [(3)H]rauwolscine binding were seen in the nerve layer of the olfactory bulb, the molecular layer of the cerebellum, and the ventricular system of alpha(2C)-AR-overexpressing mice, representing "ectopic" alpha(2C)-AR expression. Competition-binding experiments with several alpha(2)-AR ligands confirmed the alpha(2C)-AR identity of these sites. Our results provide quantitative evidence of the predominance of the alpha(2A)-AR subtype in most regions of the mouse CNS, but also disclose the wide distribution of alpha(2C)-AR in the normal mouse brain, although at relatively low density, except in the ventral and dorsal striatum and the hippocampal CA1 area. alpha(2C)-AR are thus present in brain regions involved in the processing of sensory information and in the control of motor and emotion-related activities such as the accumbens and caudate putamen nuclei, the olfactory tubercle, the lateral septum, the hippocampus, the amygdala, and the frontal and somatosensory cortices. The current results may help in specifying an anatomical framework for the functional roles of the alpha(2A)- and alpha(2C)-AR subtypes in the mouse CNS.

Distribution of alpha2-adrenoceptor mRNAs in the rat lumbar spinal cord in normal and axotomized rats

Using riboprobe in situ hybridization we have studied the distribution of alpha2A-, alpha2B- and alpha2c-adrenoceptor (AR) mRNAs in the lumbar spinal cord in normal rats and after peripheral axotomy. A strong alpha2A- and alpha2C AR mRNA labelling was found in motoneurons and other cells in the ventral horns. In the dorsal horns strong alpha2A-AR mRNA labelling was found in all layers and in the lateral spinal nucleus, whereas alpha2C-AR mRNA was found in lower numbers of cells in various layers. The alpha2B-AR mRNA signal was only detected in some small cells superficially in the dorsal horn. With regard to axotomy only a marginal effect was observed for alpha2C-AR mRNA in the ventral horn. The results suggest that alpha2-ARs are involved both in sensory and motor processing.

Aortic barodenervation up-regulates alpha2-adrenoceptors in the nucleus tractus solitarius and rostral ventrolateral medulla: an autoradiographic study

Earlier findings have shown that alpha2-adrenoceptors in the nucleus tractus solitarius and rostral ventrolateral medulla modulate baroreflexes. The present study investigated whether attenuation of baroreflexes induced by surgical interruption of aortic baroafferents is related to an alteration of alpha2-adrenoceptor binding in these regions of the brainstem. In vitro autoradiography was utilized to assess the density and binding dissociation constant (affinity) of alpha2-adrenoceptors in the rostral ventrolateral medulla and in the middle and rostral portions of the nucleus tractus solitarius of aortic-barodenervated and sham-operated rats. Compared to sham operation, aortic barodenervation caused an acute rise in mean arterial pressure and heart rate and a significant reduction in baroreflex sensitivity. Two days later, mean arterial pressure and heart rate of conscious aortic-barodenervated rats subsided to sham-operated levels, whereas the baroreflex sensitivity remained significantly (P<0.05) reduced when measured by phenylephrine (0.55+/-0.08 vs 1.26+/-0.07 ms/mmHg) or nitroprusside (0.43+/-0.06 vs 1.01+/-0.09ms/mmHg). Examination of brainstem coronal sections obtained from separate groups of rats 48 h after surgery and preincubated with [3H]rauwolscine (0.5-16 nM) revealed that labeling of alpha2 binding sites was saturable and of high affinity. Scatchard analysis of the saturation isotherms obtained from the three brain areas of sham-operated rats showed an uneven distribution of alpha2 binding sites; the rostral nucleus tractus solitarius exhibited the highest density and lowest affinity. Aortic barodenervation caused region-dependent changes in the binding activity of alpha2-adrenoceptors. These changes comprised significant (P<0.05) increases in the density of alpha2-adrenoceptors in the middle nucleus tractus solitarius (436+/-60 vs 240+/-50 fmol/mg protein) and rostral ventrolateral medulla (350+/-67 vs 194+/-35 fmol/mg protein) compared with sham-operated rats; no significant changes occurred in the rostral nucleus tractus solitarius. The affinity of alpha2 binding sites was not changed by aortic barodenervation in any of the three brain regions. These findings suggest that attenuation of baroreflexes produced by aortic barodenervation coincides with up-regulation of alpha2-adrenoceptors in brainstem areas that play critical roles in the control of cardiovascular functions.

Complex stimulus properties of LSD: a drug discrimination study with alpha 2-adrenoceptor agonists and antagonists

The influence of several alpha 2-adrenergic agents on the discriminative stimulus (DS) properties of lysergic acid diethylamide (LSD) was studied in rats trained to discriminate 0.08 mg/kg (186 nmol/kg) of LSD from saline in a two-lever operant paradigm. Only yohimbine fully mimicked LSD with an ED50 of 2.05 mg/kg (5.24 mumol/kg). Yohimbine's 5-HT1A agonist properties may be responsible for this substitution. Other alpha 2-adrenoceptor antagonists, idazoxan with an agonist/antagonist profile at 5-HT1A receptors and RS 26026-197, a highly selective alpha 2-adrenoceptor antagonist, failed to produce substitution. Clonidine, an alpha 2-adrenoceptor agonist, did not substitute for LSD but the response rate was dose-dependently reduced. None of the alpha 2-adrenergic agents used for pretreatment before LSD inhibited the response to the LSD training dose. Coadministration of clonidine with LSD produced a leftward shift of the dose-response relationship of LSD without a significant change in the slope of the dose-response line. Simultaneous administration of alpha 2-adrenergic agents with LSD shifted the dose-response curve to the left only when the adrenergic agent also possessed at least moderate affinity for the 5-HT1A receptor. In addition, radioligand competition experiments were performed that showed LSD to have relatively high affinity (Ki = 37 nM) for [3H]clonidine-labeled sites in rat cortex with lower affinity for [3H]yohimbine labeled sites. While previous studies have suggested that the nature of the LSD cue may be essentially expressed by 5-HT2 receptor activation, the present data show that this cue can be modulated by effects of LSD at 5-HT1A and at other monoamine neurotransmitter receptors.

Quantitative autoradiographic localization in rat brain of alpha 2-adrenergic and non-adrenergic I-receptor binding sites labelled by [3H]rilmenidine

alpha 2A-Adrenergic receptor (AR) and non-adrenergic imidazoline receptor (I-R) binding sites have been previously characterized in rat cerebral cortex membranes using the N-substituted oxazoline, [3H]rilmenidine ([3H]Ril) [King, P.R. et al., Eur. J. Pharmacol., 218 (1992) 101-108]. In the present study, in vitro autoradiography was used to quantify the regional distribution of these receptors throughout the rat neuroaxis. The distribution and relative density (fmol/mg tissue) of I-Rs was examined in the presence of 1 microM adrenaline to block the adrenergic component of 40 nM [3H]Ril binding and non-specific binding was measured in the presence of another oxazoline, Bay a6781 (10 microM). Both alpha 2A-ARs and I-Rs were broadly, but heterogeneously, distributed. In forebrain, high levels of [3H]Ril-labelled alpha 2A-AR sites were observed in the anterior olfactory nucleus, the piriform, entorhinal and perirhinal cortices, lateral septum, bed nucleus of the stria terminalis, several thalamic nuclei, the amygdala and the arcuate, dorsomedial and posterior hypothalamic nuclei. In hindbrain, alpha 2A-AR sites were concentrated in locus coeruleus, lateral parabrachial nucleus, nucleus of the solitary tract and area postrema. I-R sites accounted for 50% or more of specific [3H]Ril binding (40 nM) in most cortical and hypothalamic nuclei, nucleus of the solitary tract, cranial motor nuclei and most spinal cord layers. The highest densities of I-Rs were found in the arcuate, dorsomedial and posterior hypothalamic nuclei, the locus coeruleus, the area postrema, the cranial motor nuclei and associated with spinal motor neurones. A very high concentration of I-Rs was also detected in the pineal gland. The distribution of alpha 2-AR sites determined resembled that reported with [3H]p-aminoclonidine which appears to specifically label alpha 2-ARs and not I1-R sites in rat brain sections, and [3H]methoxyidazoxan which is a selective alpha 2-AR antagonist. The regional and cellular distribution of I-R binding sites was unlike the distribution of putative I1-R sites labelled by [3H]clonidine in human brain, although comparable autoradiographic mapping studies in rat brain have not been done using this ligand. The regional and cellular distribution of [3H]-labelled I-R binding sites had both similarities and differences to that reported using the imidazoline ligand, [3H]idazoxan, with common labelling of areas such as area postrema, arcuate and interpeduncular nuclei and pineal gland with the two ligands, and differential relative binding levels ([3H]Ril > [3H]idazoxan) associated with hippocampal pyramidal cells and brainstem and spinal motor neurones.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacological characterization of [3H]idazoxan, [3H]RX821002 and p-[125I]iodoclonidine binding to alpha 2-adrenoceptors in rat cerebral cortical membranes

Binding characteristics of alpha 2-adrenoceptors in rat cerebral cortical membranes were compared using the antagonist radioligands [3H]idazoxan, [3H]2-(2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline ([3H]RX821002), and the partial agonist radioligand [125I]2-[2,6-(dichloro-4-iodophenyl)imino]imidazoline ([125I]iodoclonidine). With [3H]RX821002 and alpha 2-adrenoceptor subtype-selective competitors, both alpha 2A/D- and alpha 2C-adrenoceptor subtypes were detected, suggesting rat cortical membranes contain approximately 90% alpha 2A/D-adrenoceptor subtype and 10% alpha 2C-adrenoceptor subtype. Only alpha 2A/D-adrenoceptors were detected with [3H]idazoxan and [125I]iodoclonidine. All three radioligands bound to a single high affinity site (Kd = 0.3-1.6 nM). However, the densities of sites labeled by [3H]idazoxan and [125I]iodoclonidine were 50% greater than the density labeled by [3H]RX821002, likely representing non-adrenoceptor binding sites. The density of [125I]iodoclonidine binding sites in glycylglycine buffer was similar to that labeled by [3H]RX821002. These results suggest that: (1) alpha 2A/D-adrenoceptors are the predominant subtype in rat cerebral cortex, (2) demonstrate that the small number of alpha 2C-adrenoceptors in this tissue can be detected using prazosin to displace [3H]RX821002 binding, and (3) non-adrenoceptor binding with [125I]iodoclonidine can be minimized with the use of glycylglycine buffer.

Mivazerol, a novel compound with high specificity for alpha 2 adrenergic receptors: binding studies on different human and rat membrane preparations

Mivazerol, 3-[1(H-imidazol-4-yl)methyl]-2-hydroxybenzamide hydrochloride, a new potential anti-ischemic drug designed by UCB S.A. Pharma Sector, has been studied in binding experiments on adrenergic, dopaminergic, serotoninergic, muscarinic and idazoxan binding sites. Our results indicate that this compound displays high affinity and marked specificity for alpha 2 adrenoceptors. Mivazerol displaced the binding of the alpha 2 adrenoceptor antagonist [3H]RX 821002 to the alpha 2A adrenoceptors in human frontal cortex membranes with an apparent Ki value of 37 nM. The competition curve was shallow (nH = 0.55), suggesting that this compound acts as an alpha 2 adrenergic agonist. Mivazerol was also a potent competitor for [3H]RX 821002 binding to human platelet membranes (containing alpha 2A adrenoceptors) and rat kidney membranes (75% of the alpha 2 adrenoceptors of the alpha 2B subtype), indicating that this compound is not alpha 2 adrenoceptor subtype selective. Equilibrium dissociation constants for alpha 1 adrenoceptors (displacement of [3H]prazosin) and 5-HT1A receptors (displacement of [3H]rauwolscine) were respectively about 120 times (Ki = 4.4 microM) and 14 times (Ki = 530 nM) higher than that for the alpha 2 adrenoceptors. Equilibrium dissociation constants were approximately 1000 times higher for all other receptors tested in this study; namely beta 1 and beta 2 adrenoceptors, D1- and D2-dopamine receptors, M1-, M2- and M3-muscarinic receptors, 5-HT2 receptors and non-adrenergic idazoxan binding sites.

WAY100135: a novel, selective antagonist at presynaptic and postsynaptic 5-HT1A receptors

The novel phenylpiperazine derivative, (+/-)-WAY100135 (N-tert-butyl-3-(4-(2-methoxyphenyl)piperazin-1-yl)-2-phenylpro pionamide dihydrochloride), is a selective antagonist at both somatodendritic and postsynaptic 5-HT1A receptors. The IC50 of (+/-)-WAY100135 at the rat hippocampal 5-HT1A receptor was 34 nM, whereas its IC50 at a range of other receptor sites was > 2 microM. Up to a dose of 2.5 mg/kg i.v. (+/-)-WAY100135 induced a maximum 30% inhibition of raphe neuronal firing and (at 0.5 mg/kg i.v.) antagonised the inhibition of firing induced by 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) in anaesthetised rats. (+/-)-WAY100135 antagonised the action of 5-carboxamidoiodotryptamine in the guinea-pig ileum, with a pA2 of 7.2. (+/-)-WAY100135 had no agonist-like behavioural effects but antagonised the behavioural syndrome and hypothermia induced by 8-OH-DPAT in the rat and mouse, respectively. The interaction of (+/-)-WAY100135 with the 5-HT1A receptor was stereoselective; the (+)-enantiomer being markedly more active in binding, functional and behavioural studies. These data indicate that (+/-)-WAY100135 is the first highly selective antagonist at both somatodendritic and postsynaptic 5-HT1A receptors.

Partial agonist properties of rauwolscine and yohimbine for the inhibition of adenylyl cyclase by recombinant human 5-HT1A receptors

Previous studies by another group have suggested that the alpha 2-adrenergic receptor antagonist rauwolscine may function as an agonist at the serotonin1A (5-HT1A) receptor expressed in human brain. To directly test that hypothesis, we transfected the human 5-HT1A receptor cDNA into CHO cells and examined the ability of rauwolscine and its isomer, yohimbine, to inhibit ligand binding of [3H]-(+/-)-8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT) and the activity of adenylyl cyclase in membranes derived from a single transformant that stably expresses approximately 225 fmol of 5-HT1A receptor/mg of membrane protein. Both ligands competitively antagonized the binding of [3H]8-OH-DPAT (Ki = 158 +/- 69 nM for rauwolscine and 690 +/- 223 nM for yohimbine), yielding shallow displacement curves consistent with agonist activity (Hill values = 0.69 +/- 0.2 for rauwolscine and 0.63 +/- 0.06 for yohimbine). Both ligands also inhibited forskolin-stimulated adenylyl cyclase activity in membranes derived from transfected (but not nontransfected) cells. For rauwolscine, the IC50 was 1.5 +/- 0.2 microM, and for yohimbine 4.6 +/- 1.0 microM, with activity ratios of 0.70 and 0.59, respectively, when compared to the full agonist serotonin. These studies demonstrated that rauwolscine and yohimbine are partial agonists for the human 5-HT1A receptor.

Alpha 2A adrenoceptors and non-adrenergic idazoxan binding sites in calf brain and retina are distinct from those in human brain

alpha 2 Adrenoceptors in membrane preparations of human and calf frontal cortex and of calf retina can be labelled by the antagonists [3H]idazoxan, [3H]rauwolscine and [3H]RX 821002. Present and previous data indicate that [3H]idazoxan possesses the highest affinity for the alpha 2 adrenoceptors in the calf tissues, whereas [3H]rauwolscine displays the highest affinity for those in the human frontal cortex. Competition binding experiments with adrenergic and serotonergic drugs further support the notion that the alpha 2 adrenoceptors in calf frontal cortex and retina are similar, but distinct from the receptors in human frontal cortex. The alpha 2 adrenoceptors in the three investigated tissues display low affinity for the antagonist prazosin, which suggests that they all belong to the alpha 2A subclass. The competition binding curves of the alpha 2A adrenoceptor subtype-selective agonist oxymetazoline are shallow, but undergo a rightward shift and steepening in the presence of GTP. The shallow curves can therefore be attributed to the coupling of the alpha 2 adrenoceptors to G proteins. The different binding characteristics of the alpha 2A adrenoceptors from the investigated human and bovine tissues are likely to reflect species-related differences in protein structure. [3H]Idazoxan binds also to non-adrenergic sites in membrane preparations from the three tissues. However, the affinity of [3H]idazoxan for these sites in calf cortex and retina is appreciably lower than for those in human cortex. The species-related differences of the non-adrenergic idazoxan binding sites may be due to differences in protein structure or even to differences in gene-product.

Effects of 5-HT-receptor and alpha 2-adrenoceptor ligands on the haemodynamic response to acute central hypovolaemia in conscious rabbits

1. We set out to elucidate the pharmacological mechanisms by which alpha 2-adrenoceptor and 5-HT-receptor ligands affect the haemodynamic response to acute central hypovolaemia in conscious rabbits. 2. Acute central hypovolaemia was produced by inflating an inferior vena caval cuff so that cardiac output fell at a constant rate of approximately 8.5% of its baseline level per min. 3. Drugs were administered into the fourth cerebral ventricle in either 154 mM NaCl (saline) or 20% w/v 2-hydroxypropyl-beta-cyclodextrin (beta-CDX). After vehicle treatments, the haemodynamic response to acute central hypovolaemia had the usual two phases. During Phase I, systemic vascular conductance fell in proportion to cardiac output so that mean arterial pressure fell by only 8 mmHg. Phase II commenced when cardiac output had fallen to approximately 60% of its baseline level, when vascular conductance rose abruptly and arterial pressure fell to < or = 40 mmHg. The haemodynamic response was not dependent on the vehicle used (saline or beta-CDX). 4. Methysergide delayed the occurrence of Phase II in a dose-dependent manner, and prevented it at a dose of 30- 600 nmol (geometric mean = 186 nmol). The effects and potency of methysergide were not dependent on the vehicle used, indicating that beta-CDX can be used as a vehicle for fourth ventricular administration of lipophilic drugs to conscious rabbits. Clonidine (10 nmol) reversed the effects of a critical dose of methysergide. 5. Phase II was also prevented by 8-hydroxy-2-(di-n-propylamino)tetralin (5-HT1A-selective agonist, geometric mean critical dose (range) = 13.1 (10-30) nmol), sumatriptan (5-HT1D-selective agonist, 72.1 (10-300) nmol), mesulergine (5-HT2/1C-selective antagonist, 173 (30-1000) nmol), idazoxan (alpha 2-adrenoceptor-selective antagonist, 548 (100-3000) nmol), and mianserin (5-HT2/1C-selective antagonist, 548 (100-3000) nmol). It was not affected by MDL 72222 (5-HT3-selective antagonist, 300 nmol) or ketanserin (5-HT2/1C-selective antagonist, 3000 nmol). 6. To characterize the nature of alpha 2-adrenoceptors in rabbit brainstem, we examined the binding of [3H]-rauwolscine to membrane homogenates of whole brainstem. [3H]-rauwolscine bound to a population of sites with the characteristics of alpha 2A-adrenoceptors. 7. From these results we suggest that activation of 5-HT1A receptors in the brainstem can prevent Phase II of the response to acute central hypovolaemia in conscious rabbits. Our results do not support the notion of an endogenous 5-hydroxytryptaminergic mechanism mediating Phase II. The mechanism by which the alpha 2-adrenoceptor antagonists yohimbine and idazoxan prevent Phase II remains to be elucidated. However, their potency relative to other 5-HT-receptor ligands indicates that an agonist action at 5-HT1A-receptors is more likely than an antagonist action at alpha 2-adrenoceptors.

Distributions of mRNAs for alpha-2 adrenergic receptor subtypes in rat brain: an in situ hybridization study

Selective 35S-labeled oligonucleotide probes were designed to sequences of the rat alpha-2A (RG20), alpha-2B (RNG), and alpha-2C (RG10) adrenoreceptor mRNAs for use in in situ hybridization experiments on sections of unfixed rat brain, spinal cord and kidney. After hybridized sections were exposed to film or dipped in autoradiographic emulsion, specific and selective labeling patterns characteristic for each probe and region of the central nervous system were observed. Alpha-2A mRNA labeling was most pronounced in neurons in layer six of the cerebral cortex, hypothalamic paraventricular nucleus, reticular thalamic nucleus, pontine nuclei, locus coeruleus, vestibular nuclei, trapezoid nuclei, deep cerebellar nuclei, nucleus tractus solitarii, ventrolateral medullary reticular formation, and the intermediolateral cell column of the thoracic spinal cord. In some of these locations, the receptor mRNA, in all probability, is present in noradrenaline and perhaps adrenaline neurons. The alpha-2B probe, which primarily labels the kidney, gave only a very light signal in the thalamus in the central nervous system after extended exposure times. Alpha-2C mRNA labeling was primarily observed in the olfactory bulb, cerebral cortex, islands of Calleja, striatum, hippocampal formation, cerebellar cortex, and dorsal root ganglia. Labeling patterns disappeared when excess unlabeled probes were added to their respective radiolabeled probes, or when sense probes were employed. When a hybrid antisense probe homologous to all three alpha-2 probes was used, labeling patterns also disappeared. The present study therefore justifies the pharmacological subclassification of alpha-2 receptors by providing anatomical evidence for specific and selective cell groups in the rat central nervous system containing mRNA for three alpha-2 receptor subtypes.

Galanin receptors inhibit the spontaneous firing of locus coeruleus neurones and interact with mu-opioid receptors

Electrophysiological experiments were performed in a pontine slice preparation of rat brain containing the locus coeruleus (LC). The extracellular part of this study showed that galanin (0.003-0.1 mumol/l), [Met5]enkephalin (0.01-10 mumol/l) and noradrenaline (0.1-100 mumol/l) concentration dependently inhibited the firing rate. Noradrenaline (1 and 3 mumol/l) had the same effect both before and during the application of galanin (0.001 or 0.01 mumol/l). Similarly, [Met5]enkephalin (0.03 and 0.1 mumol/l) produced identical inhibition, regardless of the presence or absence of 0.01 mumol/l galanin. Whereas rauwolscine (1 mumol/l) potentiated the effect of galanin (0.03 mumol/l), idazoxan (1 mumol/l) was inactive. In contrast, both naloxone (0.1 mumol/l) and beta-funaltrexamine (0.1 mumol/l) facilitated the galanin-induced inhibition. In the intracellular experiments, galanin (0.3 mumol/l) abolished the spontaneous discharge of action potentials, hyperpolarized the cells and decreased their input resistance. In conclusion, galanin may depress the firing rate by increasing a potassium permeability. Moreover, galanin receptors appear to interact with mu-opioid receptors but not with alpha 2-adrenoceptors.

[3H]-MK 912 binding delineates two alpha 2-adrenoceptor subtypes in rat CNS one of which is identical with the cloned pA2d alpha 2-adrenoceptor

1. Simultaneous computer modelling of control and guanfacine-masked [3H]-MK 912 saturation curves as well as guanfacine competition curves revealed that the drugs bound to two alpha 2-adrenoceptor subtypes in the rat cerebral cortex with very different selectivities. These alpha 2-adrenoceptor subtypes were designated alpha 2A and alpha 2C. The Kd value of [3H]-MK 912 for the alpha 2A-subtype was 1.77 nM and for the alpha 2C-subtype 0.075 nM; the receptor sites showing capacities 296 and 33 fmol mg-1 protein, respectively. The Kds of guanfacine were 19.9 and 344 nM, respectively. 2. Binding constants of 26 compounds for the two rat cerebral cortex alpha 2-adrenoceptor subtypes were determined by simultaneous computer modelling of control and guanfacine-masked drug competition curves as well as plain guanfacine competition curves using [3H]-MK912 as labelled ligand (i.e. a '3-curve assay'). Of the tested drugs WB4101, corynanthine, rauwolscine, yohimbine, ARC 239 and prazosin were found to be clearly alpha 2C-selective with selectivities ranging from 16 to 30 fold whereas guanfacine, oxymetazoline, BRL 44408 and BRL 41992 were found to be alpha 2A-selective with selectivities ranging from 9 to 22 fold. 3. The Kds of compounds obtained for the cerebral cortex alpha 2C-adrenoceptors showed an almost 1:1 correlation with the corresponding Kds for alpha 2-adrenoceptors expressed by the pA2d-gene (the rat 'alpha 2-C4' adrenoceptor) in CHO-cells. The cerebral cortex alpha 2A-adrenoceptors did not correlate well with the pA2d alpha 2-adrenoceptor Kds. 4. In the rat spinal cord [3H]-MK 912 bound to alpha 2A- and alpha 2C-adrenoceptor sites with similar affinities as in the cerebral cortex and with densities 172 and 7.4 fmol mg-1 protein, respectively. Drug affinities for some compounds showing major selectivity for alpha 2A- and alpha 2C-adrenoceptors were fully compatible with the notion that the spinal cord sites were alpha 2A- and alpha 2C-adrenoceptors.

[3H]-RS-15385-197, a selective and high affinity radioligand for alpha 2-adrenoceptors: implications for receptor classification

1. RS-15385-197 is the most potent and selective alpha 2-adrenoceptor antagonist available. We have used [3H]-RS-15385-197 to define alpha 2-adrenoceptor subtypes. The binding of [3H]-RS-15385-197 to membranes of rat cerebral cortex, rat neonatal lung and human platelets was reversible, saturable and of high affinity. Saturation experiments indicated that [3H]-RS-15385-197 bound to a single population of sites in all 3 tissues with high affinity (0.08-0.14 nM). The density of sites labelled by [3H]-RS-15385-197 was greater in the cortex (275 fmol mg-1 protein) than in the neonate lung (174 fmol mg-1 protein) and human platelet (170 fmol mg-1 protein). The density of sites labelled with [3H]-RS-15385-197 in the cortex was significantly greater than that labelled with [3H]-yohimbine (121 fmol mg-1 protein). 2. The selective alpha 2-adrenoceptor antagonists, idazoxan, yohimbine, rauwolscine and WY 26703 displaced [3H]-RS-15385-197 binding to rat cerebral cortex in a simple manner with Hill slopes close to unity. The affinities derived for these antagonists against [3H]-RS-15385-197 were similar to the values obtained for the displacement of [3H]-yohimbine indicating the alpha 2-adrenoceptor nature of the binding site. 3. alpha 2A-Adrenoceptor selective compounds, oxymetazoline and BRL 44409, showed high affinity for [3H]-RS-15385-197 binding in the human platelet and lower affinity in the neonate lung, while the alpha 2B-selective compounds, prazosin and imiloxan, showed high affinity for [3H]-RS-15385-197 binding in the neonate lung.This suggests that [3H]-RS-15385-197 labels both alpha2A- and alpha2B-adrenoceptor subtypes.4. Prazosin and methysergide inhibited the binding of [3H]-RS-15385-197 in the rat cerebral cortex in a simple manner consistent with an interaction at a single site. Although oxymetazoline inhibited [H]-RS- 15385-197 with a Hill slope significantly different from unity, the slope was increased to unity in the presence of Gpp(NH)p, suggesting an agonist-like interaction.5. The site labelled by [3H]-RS-15385-197 in the rat cortex shows high affinity for oxymetazoline and low affinity for prazosin which could be taken as evidence for classifying the site as an alpha2A-subtype.However, the affinities of yohimbine, rauwolscine and oxymetazoline at this site do not correspond to the population of sites in the human platelet. Yohimbine and rauwolscine were 20 fold selective for the platelet alph2A-subtype, whereas phentolamine was 2 fold and imiloxan was 10 fold selective for the cortex subtype. Indeed although the site showed some similarities with the alpha2A-subtype, the highest degree of homology was observed between this site and the rat submaxillary gland and the RG20 clone,tentatively called the alpha2D-adrenoceptor subtype. We propose that the alpha2-adrenoceptor in the rat cortex may therefore correspond to the putative alpha2D-subtype of the adrenoceptor.

Effects of α-adrenoceptor antagonists and clonidine on the haemodynamic response to acute hypovolaemia in conscious rabbits

In conscious rabbits an inferior vena caval cuff was progressively inflated so cardiac output fell at a constant approximately 8% of its baseline value. There was a biphasic haemodynamic response, consisting of an initial compensatory phase during which there was progressive systemic vasoconstriction and tachycardia, followed by a decompensatory phase in which systemic vasoconstriction failed abruptly, blood pressure plummeted and heart rate declined. We tested the effects on the haemodynamic response of prior 4th ventricular, and in some cases intravenous, infusions of saline, yohimbine, clonidine, yohimbine plus clonidine, and bunazosin. From the results we conclude that a yohimbine-sensitive mechanism in the brainstem, possibly alpha 2-adrenoceptor-mediated, may be an essential element of the cardiac receptor-mediated decompensatory phase of acute central hypovolaemia, but does not contribute to the arterial baroreflex-mediated compensatory phase.

Regional Distribution of ?2A-and ?2B-Adrenoceptor Subtypes in Postmortem Human Brain

The newly available and highly selective radiolabeled antagonist [3H]RX 821002 was used to examine the distribution of alpha 2 adrenoceptors in human brain. High densities of alpha 2 adrenoceptors were found in the hippocampus, frontal cortex, thalamus, amygdala, pons, and medulla oblongata. Intermediate densities were observed in the striatum (nucleus accumbens, nucleus caudatus, and putamen), globus pallidus, and substantia nigra. The KD values for [3H]RX 821002 were similar in all regions (ranging from 2.8 to 7.5 nM). On the basis of their different affinities for prazosin and oxymetazoline, the alpha 2 adrenoceptors have been divided into alpha 2A and alpha 2B subtypes. To examine the alpha 2A/alpha 2B-adrenoceptor ratio in the different brain regions, we performed oxymetazoline and prazosin/[3H]RX 821002 competition binding experiments. In frontal cortex membranes, the competition curves with prazosin were steep, indicating a single class of binding sites, whereas the competition curves with oxymetazoline were shallow and fitted by computer best to a two-site model. However, in the presence of GTP, the high-affinity sites for oxymetazoline were partially converted into low-affinity sites, indicating that this agonist interacts with high- and low-affinity states of the alpha 2 adrenoceptors. This implies that oxymetazoline is not very suitable for discriminating the alpha 2A- and alpha 2B-receptor subtypes in radioligand binding studies. Therefore, prazosin/[3H]RX 821002 competition binding experiments were used to investigate the distribution of the alpha 2-adrenoceptor subtypes in human brain. The alpha 2A-receptor subtype was detected in all brain regions examined. In contrast, alpha 2B receptors were only observed in striatum and globus pallidus.

Auditory-evoked response of the cortex after yohimbine administration: phase advance effect of central noradrenergic activation

The effects of central noradrenergic activation on an auditory-evoked cortical response were studied using systemic administration of yohimbine (2 mg/kg intravenously, IV), a noradrenergic stimulant, in 13 anesthetized rats. To analyze changes of the response, surface and intracortical evoked potentials (EP) as well as extracellular single-unit recordings with tungsten microelectrodes were employed. It was noted that the initial-positive wave of the surface EP corresponded to unit firing responses in a restricted area of the auditory cortex, where the surface EP was largest and a polarity inversion of the intracortical EP was observed. The following effects were produced by yohimbine: 1) The initial-positive surface potential (n = 10) and corresponding intracortical potential with inverted polarity (n = 6) both showed an increase in amplitude and a decrease in peak latency; 2) the unit firing response (n = 10) tended to show an increase in peak frequency and a decrease in peak firing latency; and 3) yohimbine produced an earlier ending of the firing period, and in paired stimulation experiments (n = 7) it prolonged the period during which the second response was suppressed, indicating an augmentation of postexcitation inhibition. Later histological examination suggested that most of the units recorded were pyramidal cells. These findings indicate that chemical stimulation of the central noradrenergic system by yohimbine enhances both the initial excitatory and following inhibitory processes in the auditory-evoked response of the cortical units (probably pyramidal cells), resulting not only in amplification of the response but also in advancement of the response phase.(ABSTRACT TRUNCATED AT 250 WORDS)

Autoradiographic distribution of alpha 2 adrenoceptors, NAIBS, and 5-HT1A receptors in human brain using [3H]idazoxan and [3H]rauwolscine

The regional distribution of [3H]idazoxan and [3H]rauwolscine was studied autoradiographically in human brain. [3H]Idazoxan binds with high affinity to alpha 2 adrenoceptors as well as to non-adrenergic sites (NAIBS). [3H]Rauwolscine, besides binding to alpha 2 adrenoceptors, also binds to 5-HT1A receptors. Both radioligands labelled the same population of alpha 2 adrenoceptors, defined as the epinephrine-displaceable binding component. The highest densities of alpha 2 adrenoceptors occur in the leptomeninges, cerebral cortex and claustrum; lower densities were visualised in the basal ganglia, thalamus, pons, substantia nigra, cerebellum and medulla oblongata; no alpha 2 adrenoceptors were detected in amygdala and nucleus ruber. NAIBS were present in all the examined brain areas, with the highest densities found in the basal ganglia and substantia nigra. The finding that certain brain regions, such as the amygdala, contained NAIBS but no detectable alpha 2 adrenoceptors, suggests that the binding sites are independent from each other. The regional distribution of 5-HT1A receptors labelled by [3H]rauwolscine is in agreement with previous studies using [3H]8-OH-DPAT.

Rat spinal cord alpha 2-adrenoceptors are of the alpha 2A-subtype: comparison with alpha 2A- and alpha 2B-adrenoceptors in rat spleen, cerebral cortex and kidney using 3H-RX821002 ligand binding

Binding of the alpha 2-adrenoceptor antagonist radioligand 3H-RX821002 was investigated in membranes from rat spinal cord, spleen, cerebral cortex and kidney. The ligand was found to bind to saturable binding sites with apparent uniform affinities within each tissue. Seven compounds, some of which have previously been reported to be selective for either alpha 2A- or alpha 2B-adrenoceptors, were used in competition with 3H-RX821002. By using computer modelling, competition curves generated for three of these compounds (ARC 239, prazosin and oxymetazoline) could be resolved into two site fits in the kidney, Kd's of the drugs being compatible with the notion that these sites corresponded to alpha 2A- and alpha 2B-adrenoceptors. Moreover, rauwolscine and yohimbine were found to be about 14 and 9-fold selective for alpha 2B-adrenoceptors in the kidney. In all other tissues studied drug competition curves were uniphasic and computer modelled into one site fits, drug Kd's being well correlated to those for the alpha 2A-adrenoceptor. In rat spinal cord 26 further drugs, which showed wide variation in structure, were evaluated in competition with 3H-RX821002. Of these compounds, competion curves of the agonists UK-14,304, (-) and (+) adrenaline were modelled into two site fits whereas those of the remaining compounds could be modelled only into one site fits. Since the high affinity site for UK-14,304, (-) and (+) adrenaline was eliminated when EDTA, Gpp(NH)p and 140 mM NaCl was present in the assay the heterogeniety observed in spinal cord was considered to be due to formation of high and low affinity conformations of the alpha 2-adrenoceptor for agonists. It is concluded that 3H-RX821002 is useful to label both alpha 2A- and alpha 2B-adrenoceptors in the rat. Moreover, the binding sites labelled by 3H-RX821002 in the spinal cord appear to consist of a single population of alpha 2-adrenoceptors of the alpha 2A-type.

[3H]rauwolscine behaves as an agonist for the 5-HT1A receptors in human frontal cortex membranes

The alpha 2 adrenergic antagonist [3H]rauwolscine binds with comparable nanomolar affinity to alpha 2 adrenoceptors and the nonadrenergic 5-HT1A receptors sites in human frontal cortex membranes. Addition of 0.5 mM GTP into the incubation medium produces a significant decrease in the amount of [3H]rauwolscine binding sites (Bmax = 230 +/- 16 and 115 +/- 11 fmol/mg protein in the absence and presence of GTP, respectively). The affinity for [3H]rauwolscine remains unchanged (i.e. KD = 40 +/- 0.9 nM and 4.1 +/- 1 nM). This effect of GTP can be attributed to decreased binding of the radioligand to the 5-HT1A receptors. GTP decreases binding of [3H]rauwolscine to nearly the same level as the one corresponding to the alpha 2 adrenoceptors in membranes from both the human frontal cortex and hippocampus. The venom of the marine cone snail, Conus tessulatus, preferentially inhibits [3H]rauwolscine binding to 5-HT1A receptors as compared with the alpha 2 adrenoceptors. Following complete masking of the 5-HT1A receptors by this venom. GTP no longer affects the saturation binding characteristics of [3H]rauwolscine for the remaining alpha 2 adrenoceptors. Nucleotides decrease the binding of [3H]rauwolscine to the 5-HT1A receptors with an order of potencies (i.e. GTP gamma S greater than GPP(NH)P much greater than GDP greater than GTP much greater than ATP) that is typical for nucleotide-mediated receptor-G protein dissociation. This suggests that [3H]rauwolscine is a 5-HT1A receptor agonist and this conclusion is compatible with earlier functional studies, indicating that rauwolscine (as well as yohimbine) has agonistic properties at the level of 5-HT autoreceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha 2-adrenergic binding sites in the medulla oblongata of tree shrews demonstrated by in vitro autoradiography: species related differences in comparison to the rat

Alpha 2-adrenergic binding sites in the medulla oblongata of tree shrews and rats were detected and quantified by in vitro-autoradiography with the alpha 2-antagonist 3H-rauwolscine (3H-RAUW). The autoradiographic pattern of the radioligand binding in the tree shrew medulla oblongata resembles that which has been described by others for the human myelencephalon. This pattern coincides well with the occurrence of catecholaminergic structures detected by immunocytochemistry with antibodies against phenylethanolamine-N-methyltransferase and tyrosine hydroxylase. In contrast to the rat, where only the nucleus tractus solitarii and the nucleus dorsalis nervi vagi were labeled, five discrete nuclei specifically bound 3H-RAUW in tree shrews. The highest number of binding sites was detected in the nucleus dorsalis nervi vagi (nX; Bmax: 333 fmoles/mg) and the nucleus tractus solitarii (NTS; 311 fmoles/mg), followed by the nucleus nervi hypoglossi (nXII; 297 fmoles/mg), the nucleus reticularis parvocellularis (FRS; 230 fmoles/mg), and the area of the catecholamine cell groups A1 and C1 (area C1; 202 fmoles/mg). Maximal binding in the two labeled nuclei of the rat was 158 fmoles/mg. The discrete nuclei of the two species also showed different affinities for 3H-RAUW with Kd ranging from 0.17 to 0.83 nM in tree shrews and 1.80 to 1.95 nM in rats. Competition experiments revealed that the radioligand bound specifically to alpha 2-binding sites. In the tree shrew, nX, nXII and the area C1, also have a relatively high affinity for the alpha 1-antagonist prazosin which is a quality of the adrenoceptor subtype alpha 2B. Furthermore, in the area C1, 3H-RAUW binding was inhibited by the dopamine antagonist haloperidol. There are thus species related as well as regional differences with respect to the number, the affinity, and the pharmacological properties of alpha 2-binding sites in the medulla oblongata. In tree shrews, alpha 2-adrenoceptors can be autoradiographically quantified in regions which are not labeled in the rat, although former data predicted the existence of such receptors, e.g., in the area of the adrenaline cell group C1.

Heterogeneity of alpha 2-adrenoceptors in rat cortex but not human platelets can be defined by 8-OH-DPAT, RU 24969 and methysergide

1. Saturation experiments indicated that [3H]-yohimbine binding was specific, saturable and labelled a single population of sites in rat cerebral cortex (Kd 5.3 +/- 0.9 nM, Bmax 121 +/- 10 fmol mg-1 protein) and human platelets (Kd 0.7 +/- 0.1 nM, Bmax 152 +/- 10 fmol mg-1 protein). 2. The alpha 2-adrenoceptor antagonists, yohimbine, rauwolscine, WY 26703, idazoxan and BDF 6143 displaced [3H]-yohimbine binding to each tissue in a simple manner, with high affinity and Hill slopes close to unity. 3. The alpha 1-adrenoceptor agonist, oxymetazoline and the antagonist prazosin inhibited the binding of [3H]-yohimbine to rat in a complex manner consistent with an interaction at more than one site. However, indoramin and WB 4101 only appeared to interact with one site. In contrast, in human platelets, all antagonists gave rise to monophasic displacement curves with Hill slopes close to unity suggesting a single site of interaction. 4. The 5-hydroxytryptamine (5-HT) receptor ligands, 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), RU 24969, and methysergide inhibited the binding of [3H]-yohimbine to rat cortex with high and low affinity, consistent with an interaction with two populations of binding sites. However, inhibition of [3H]-yohimbine binding to human platelets suggested a single site of interaction. The low affinity of 5-HT, 5-carboxyamidotryptamine (5-CT) and dipropyl-5-CT indicated that [3H]-yohimbine was not labelling a 5-HT1-like site in rat cortex. 5. The ability of 8-OH-DPAT, RU 24969 and methysergide in addition to prazosin and oxymetazoline to differentiate [3H]-yohimbine binding provides additional pharmacological evidence for heterogeneity within rat cortical alpha 2-adrenoceptors. However, if the two sites in rat cortex that are differentiated by the 5-HT ligands represent (alpha 2A- and alpha 2B-adrenoceptor subtypes as defined by prazosin and oxymetazoline, then they do not correspond to the population of sites in human platelets. As receptor classification should be linked to affinity of drugs rather than tissue distribution, the current classification of alpha 2-adrenoceptor subtypes does not appear to be satisfactory.

Assessment of imiloxan as a selective alpha 2B-adrenoceptor antagonist

1. The alpha 2-adrenoceptor binding sites of rabbit spleen and rat kidney, labelled with [3H]-rauwolscine, were characterized using a range of subtype selective ligands. 2. In rabbit spleen, the alpha-2-adrenoceptor binding sites displayed high affinity for oxymetazoline and WB 4101 and low affinity for prazosin and chlorpromazine suggesting the presence of an alpha 2A subtype. 3. There was evidence for heterogeneity of the alpha 2-adrenoceptor binding sites present in rabbit spleen. The results obtained with oxymetazoline and WB 4101 indicated that at least 75% of the [3H]-rauwolscine binding sites in this preparation displayed a pharmacology consistent with the presence of an alpha 2A subtype. 4. In rat kidney, the alpha 2-adrenoceptor binding sites displayed high affinity for prazosin and chlorpromazine and low affinity for oxymetazoline and WB 4101 suggesting the presence of an alpha 2B subtype. 5. The inclusion of guanylylimidodiphosphate (Gpp(NH)p, 0.1 mM) did not modify the pharmacology of the alpha 2-adrenoceptor binding sites present in the two preparations. Furthermore, when the two membrane preparations were combined, the resultant pharmacology was still consistent with the presence of two receptors that retained the characteristics of the alpha 2A and alpha 2B subtypes. 6. Imiloxan was identified as a selective alpha 2B ligand while benoxathian displayed a high degree of selectivity for the alpha 2A-adrenoceptor binding site. The selectivity of imiloxan for the alpha 2B-adrenoceptor binding site, coupled with its specificity for alpha 2-adrenoceptors, should make it a valuable tool in the classification of alpha 2-adrenoceptor subtypes.

Effects of sodium, lithium, and magnesium on in vitro binding of [3H]SCH23390 in rat neostriatum and cerebral cortex

The effects of sodium, lithium, and magnesium on the in vitro binding properties of the D1 antagonist [3H]SCH23390 were examined with membrane preparations from rat neostriatum (CPU; caudate-putamen) and cerebral cortex (CTX). The saturation binding isotherms for both tissues performed in the presence of 120 mM of either Na+ or Li+ revealed an increase in the affinity, as compared to that observed when the incubation buffer was composed of Tris-Cl 50 mM with MgCl2 1 mM alone. For the CPU there were no changes in the maximum binding capacity (Bmax) in the different buffers used. In the case of the CTX, there was a loss of [3H]SCH23390 binding sites when either Na+ or Li+ 120 mM were added to the incubations, suggesting a lack of selectivity of this ligand in the absence of group IA cations. The agonist state of the [3H]SCH23390 binding site was studied in competition experiments with dopamine. The highest agonist affinity was obtained in 50 mM Tris-Cl buffer with 1 mM MgCl2 while the addition of 120 mM of either Na+ or Li+ caused a 3- to 5-fold decrease in the potency of dopamine to compete with specific [3H]SCH23390 binding in both CPU and CTX. The presence of magnesium was essential for the competition experiments; i.e.: a concentration of 1 mM MgCl2 was optimum to obtain dopamine antagonism of ligand binding, while increasing Mg2+ to 2 or 5 mM did not appear to further improve the inhibitions. The results support both agonist and antagonist affinity shifts for the dopamine D1 receptor labeled with [3H]SCH23390.(ABSTRACT TRUNCATED AT 250 WORDS)

High affinity binding of 3H rauwolscine and 3H RX781094 to alpha 2 adrenergic receptors and non-stereoselective sites in human and rabbit brain cortex membranes

The radiolabeled antagonists 3H RX 781094 and 3H rauwolscine bind with high affinity to alpha 2 adrenergic receptors as well as to non-receptor sites in human and rabbit brain cortex membranes. These non-receptor sites form an important contaminant of the specific binding when non-specific binding is determined in the presence of 10 microM phentolamine or more. While phentolamine is no suitable ligand to discriminate both sites, (-)-epinephrine displays a sufficient affinity ratio to separate radioligand binding to these sites. When 1 microM (-)-epinephrine is used for the determination of the non-specific binding, both radioligands bind specifically to alpha 2 receptors. Under these conditions, 3H rauwolscine and 3H RX 781094 bind to the same amount of non-cooperative sites; binding isotherms for human brain are Bmax = 113 +/- 15 fmol/mg protein and Kd = 22.8 +/- 4.2 nM for 3H RX781094 and Bmax = 110 +/- 17 fmol/mg protein and Kd = 4.7 +/- 2.5 nM for 3H rauwolscine. Competition binding experiments show, for both radioligands and in both species, the typical pharmacological potency order of alpha 2 adrenergic receptors, i.e. phentolamine greater than yohimbine greater than prazosin for the antagonists and UK 14304 greater than p-aminoclonidine greater than or equal to (-)-epinephrine greater than (+)-epinephrine greater than isoproterenol for the agonists. Whereas the alpha 2 receptor sites display high affinity and stereoselectivity towards (-)-epinephrine and (+)-epinephrine, the non-receptor sites bind both epinephrine isomers with equal low affinity. Specific binding of both radioligands to these sites can be determined when total binding is performed in the presence of 1 microM (-)-epinephrine and non-specific binding the presence of 1 mM phentolamine. 3H rauwolscine binding to the non-stereoselective sites can be displaced with high affinity by 5-HT, suggesting binding to a 5-HT1-receptor. The 3H RX 781094 binding displays low affinity for most alpha adrenergic ligands and do not correspond to beta adrenergic, dopaminergic or serotonergic receptors.