A reassessment of the binding of [3H]rauwolscine to membranes from the rat cortex

Pharmacological profile of (-)HT-90B, a novel 5-HT1A receptor agonist/5-HT2 receptor antagonist

1. HT-90B ((-)-N-([2-(8-methyl-l, 4-benzodioxane-2-ylmethyl)amino]ethyl) tricyclo[3,3,1,1(3.7)] decane-1-carboxamide) had high affinities for the 5-HT1A (Ki = 0.18 nM) and 5-HT2 (Ki = 9.2 nM) receptors. 2. HT-90B inhibited forskolin activated adenylate cyclase in rat hippocampal membranes as a 5-HT1A full agonist (IC50 = 2 nM), and the potency of the drug was higher than that of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a standard 5-HT1A agonist. 3. In the serotonin syndrome test, HT-90B behaved as a weak partial 5-HT1A agonist in reserpinized rats. 4. 5-HT2 receptor-mediated potentiation of rabbit platelet aggregation by serotonin (5-HT) was reduced by HT-90B (IC50 = 1.73 microM). 5. Head twitch response induced by 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a 5-HT2 agonist, was inhibited by HT-90B in mice. 6. It is concluded that HT-90B has potent 5-HT1A receptor agonist as well as 5-HT2 receptor antagonist properties in vitro and in vivo.

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.

Central nervous system receptor binding profiles of some 2-amino-4-phenyl quinolines: a novel class of alpha 2-adrenoceptor selective ligands

The 2-amino-4-phenyl quinoline moiety is a structural motif common to a number of central nervous system active agents. Extensive radio-ligand receptor binding profiles for several derivatives of this common structural feature have been determined. A high base level of central nervous system receptor affinity was observed with a distinct preference for the alpha-, and in particular the alpha 2-, adrenoceptors.

Characterization and autoradiographical localization of non-adrenoceptor idazoxan binding sites in the rat brain

1. In rat whole brain homogenates, saturation analysis revealed that both [3H]-idazoxan and [3H]-RX821002, a selective alpha 2-adrenoceptor ligand, bound with high affinity to an apparent single population of sites. However, the Bmax for [3H]-idazoxan was significantly (P less than 0.01) greater than that for [3H]-RX821002. 2. In competition studies, (-)-adrenaline displaced 3 nM [3H]-idazoxan binding with an affinity consistent with [3H]-idazoxan labelling alpha 2-adrenoceptors. However, this displacement was incomplete since 23.68 +/- 1.11% of specific [3H]-idazoxan binding remained in the presence of an excess concentration (100 microM) of (-)-adrenaline. In contrast, unlabelled idazoxan promoted a complete displacement of [3H]-idazoxan binding with a Hill slope close to unity and an affinity comparable with its KD determined in saturation studies. 3. Displacement of [3H]-idazoxan binding by the alpha 2-adrenoceptor antagonists yohimbine, RX821002 (2-(2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline) and RX811059 (2-(2-ethoxy-1,4-benzodioxan-2-yl)-2-imidazoline) was more complex, with Hill slopes considerably less than unity, and best described by a two-site model of interaction comprising a high and low affinity component. The proportion of sites with high affinity for each antagonist was similar (60-80%). 4. The rank order of antagonist potency for the high affinity component in each displacement curve (RX821002 greater than RX811059 greater than yohimbine) is similar to that determined against the binding of [3H]-RX821002 to rat brain, suggesting that these components reflect the inhibition of [3H]-idazoxan binding to alpha 2-adrenoceptors.The remaining component in each displacement curve exhibiting low affinity towards these antagonists is attributable to the displacement of [3H]-idazoxin from a non-adrenoceptor idazoxan binding site (NAIBS) since a comparable amount of [3H]-idazoxan binding was not displaced by an excess concentration of (-)-adrenaline.5. The displacement of [3H]-idazoxan binding by RX801023 (6-fluoro-(2-(1,4-benzodioxan-2-yl)-2-imidazoline) was also best described by a model assuming a two site interaction with 20.07 +/- 3.11% of the sites labelled displaying high affinity for RX801023. The Ki of RX801023 for the remainder of the sites labelled was similar to its Ki versus [3H]-RX821002, indicating that this drug displays improved affinity and NAIBS/z2-adrenoceptor selectivity compared with idazoxan.6. In autoradiographical studies, the distribution of 5 nM [3H]-idazoxan binding to sections of rat whole brain was consistent with that reported from previous studies and resembled the distribution ofM2-adrenoceptors. However, when sections of brain were coincubated with concentrations of alpha2-adrenoceptor agonists or antagonists predicted to saturate alpha2-adrenoceptors, there remained distinct areas of binding corresponding to discrete brain nuclei. This remaining binding was however displaced by unlabelled idazoxan (3 microM) or RX801023 (3 microM) indicative of the labelling of NAIBS.7. Quantitative autoradiography of NAIBS revealed several brain nuclei which contained higher densities of these sites than alpha2-adrenoceptors, notably the area postrema, interpeduncular nucleus,arcuate nucleus, ependyma and pineal gland.

[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.

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.

Novel alpha 2-adrenoceptor antagonists show selectivity for alpha 2A- and alpha 2B-adrenoceptor subtypes

Pharmacological characterization of mammalian alpha 2-adrenoceptors in various tissues and species has provided evidence for the existence of two alpha 2-adrenoceptor subtypes. These subtypes can be defined in rat and human tissues by prazosin which is alpha 2B selective and oxymetazoline which is alpha 2A selective. In addition to these agents, two types of alpha 2-adrenoceptor antagonists are described which show high affinity and selectivity for the alpha 2A-adrenoceptor and the alpha 2B-adrenoceptor respectively.

[3H]L-657,743 (MK-912): a new, high affinity, selective radioligand for brain alpha 2-adrenoceptors

L-657,743 (MK-912), a highly potent and selective alpha 2-adrenoceptor antagonist was tritiated to a high specific activity and its binding characteristics to brain tissue were determined. The specific binding of [3H]L-657,743 to rat cerebrocortex was saturable, reversible, and dependent on tissue concentration. In saturation studies, [3H]L-657,743 binding was resolved into two high affinity components exhibiting Kd values of 86 pM and 830 pM with densities of 82 fmol/mg protein and 660 fmol/mg protein, respectively. Based on the binding potencies of a variety of compounds with differing receptor selectivities, the sites labeled by [3H]L-657,743 were characteristic of alpha 2-adrenoceptors. In contrast to alpha 2-antagonists, alpha 2-agonists displayed shallow competition curves. In the presence of 100 microM GTP, Gpp(NH)p or 150 mM NaCl, the competition curve for epinephrine was shifted to the right, whereas that for yohimbine was unaffected. In studies utilizing human cerebrocortical tissue, [3H]L-657,743 also bound with high affinity to sites characteristic of alpha 2-adrenoceptors.

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

Ligand binding and isolated tissue data have provided evidence for the existence of two, tissue-specific, alpha 2-adrenoceptor subtypes in various rodent and non-rodent species. Thus it has been proposed that the complex binding of alpha 2-antagonists to rat cortical membranes is due to the presence of both subtypes in this tissue. We have previously shown that the alpha 2-antagonist 3H-rauwolscine binds to two sites on rat cortical membranes: a high affinity component characterised pharmacologically as an alpha 2-binding site, and a low affinity, spiperone-sensitive, serotonergic-like component. By the use of computerised non-linear curve-fitting, and the inclusion of (in the incubation buffer of displacement experiments) a concentration of spiperone previously shown to selectively occlude the low affinity component of the 3H-rauwolscine saturation isotherm, we have determined the rank order of affinity at each of the two sites. Whereas the rank order of affinity at the high affinity site retains the pharmacological profile of a single, monophasic alpha 2-binding site, that at the low affinity component is markedly different and is similar to that at the putative 5HT1A subtype. These data, together with the additional, functional serotonergic interactions of rauwolscine and yohimbine, indicate that there is no evidence to support the existence of heterogeneous alpha 2-binding sites, as measured by 3H-rauwolscine binding, on rat cortical membranes. Furthermore, we present evidence that the specific, low affinity serotonergic interaction of 3H-rauwolscine could be avoided by a more judicial estimation of specific binding.

Agonist-induced regulation of adrenoceptor subtypes in cerebral cortical slices

Cerebral cortical slices from rat brain were incubated at 37 degrees C for 2 h in the presence of isoproterenol, noradrenaline, or adrenaline, and binding affinities and densities of adrenoceptor subtypes were subsequently examined in homogenized tissue. The density of alpha 2- and total beta-adrenoceptors was estimated using the radioligands [3H]rauwolscine and [3H]dihydroalprenolol (DHA), respectively. The percentages of beta 1- and beta 2-adrenoceptors were defined by inhibiting the binding of [3H]DHA with the beta 1-selective antagonist metoprolol. Exposure of slices to noradrenaline and adrenaline significantly decreased the maximal number of binding sites (Bmax) of alpha 2-adrenoceptors (48 and 37% respectively) without significantly affecting affinity; isoproterenol had no effect. Exposure to isoproterenol, noradrenaline, and adrenaline significantly decreased the Bmax of beta-adrenoceptors (by 60, 34, and 24%, respectively) but did not affect the affinity. Isoproterenol and adrenaline significantly decreased the density of beta 1-adrenoceptors by 75 and 24% and beta 2-adrenoceptors by 23 and 28%, respectively. Noradrenaline significantly decreased the density of beta 1-adrenoceptors by 42% without affecting the number of beta 2-adrenoceptors. These findings indicate that subtypes of adrenoceptors in rat cerebral cortex are differentially regulated by adrenergic agonists.

Acute cold-restraint stress affects alpha 2-adrenoceptors in specific brain regions of the rat

The effect of acute cold-restraint stress on binding of [3H]rauwolscine to alpha 2-adrenoceptors was investigated in 10 regions of rat brain. Acute stress: significantly decreased the density but had no significant effect on the affinity of binding sites in the hippocampus; decreased density and increased affinity in the amygdala; and increased density and decreased the affinity in the midbrain. Seven other brain regions showed no significant changes in binding parameters in response to stress. These results, together with previous findings in this laboratory showed that alteration by restraint stress of noradrenaline levels in amygdala and hippocampus, but not other regions, indicate an association between neurotransmitter turnover and receptor function.