Identification of α2 adrenergic receptors in human frontal cortex membranes by binding of [3H]RX 821002, the 2-methoxy analog of [3H]idazoxan

Androgen-induced norepinephrine release mediating guinea pig seminal vesicle smooth muscle proliferation: potential role of pre-synaptic alpha2-adrenoceptors

BACKGROUND

Recent studies from our laboratory have demonstrated that androgen-induced basal norepinephrine (NE) release is responsible for the onset of proliferation in seminal vesicle smooth muscle (SVM) cells during early puberty. With subsequent sexual maturation, SVM was irreversibly differentiated to an androgen-resistant-amitotic state in which basal NE release remained elevated and resistant to androgen withdrawal or repletion. Based on these findings, we hypothesized that this irreversible elevation of basal NE release during pubertal development is caused, at least in part, by the down-regulation of pre-synaptic NE feedback inhibition, secondary to irreversible reduction in the expression of neuronal (pre-synaptic) alpha(2)-adrenoceptors. Functional alpha(2)-adrenoceptors are selectively localized to pre-synaptic sites in SVM.

METHODS

To test this hypothesis, we employed ligand binding techniques with [(3)H]RX821002, an antagonist which labeled all alpha(2)-adrenoceptor sub-types. Initial experiments focused on analysis of competitor specificity to identify the predominant alpha(2)-adrenoceptor sub-type in SVM. Subsequently, we quantified the changes in the receptor concentration (B(max)) for [(3)H]RX821002 at the point of maximal dihydrotestosterone (DHT)-induced change in basal NE release.

RESULTS

Based on competitor specificity for [(3)H]RX821002, the alpha(2D)-adrenoceptor sub-type predominated in SVM. We treated pre-pubertal castrate animals with DHT for 7 days, which was previously demonstrated to maximally induce basal NE release. This treatment reduced the pre-synaptic alpha(2)-adrenoceptor B(max) 4-fold. In animals which had been castrated as adults, the B(max) for [(3)H]RX821002 remained irreversibly suppressed.

CONCLUSIONS

The DHT-dependent reduction in the alpha(2)-adrenoceptor concentration is consistent with the developmental pattern of increased basal NE release. These findings support the hypothesis that the down-regulation of pre-synaptic NE feedback is mechanistically involved in the irreversible elevation of basal NE release. NE mediates proliferation in SVM in early pubertal development. Thus, the androgen-dependent pubertal growth of smooth muscle cells may be indirectly controlled at the level of neurotransmission.

RX 821002 as a tool for physiological investigation of alpha(2)-adrenoceptors

RX 821002 is the 2-methoxy congener of idazoxan. In binding and tissue studies it behaves as a selective antagonist of alpha(2)-adrenoceptors, with at least 5 times greater affinity for these receptors than any other binding site. It does not select between the different types of alpha(2)-receptor. Although this drug probably has no future as a therapeutic agent, it remains a good probe for physiological activity at alpha(2)-adrenoceptors in animal experiments. A particularly useful feature of this compound is its lack of binding at I(1) and I(2) imidazoline receptors. However, it has relatively high affinity for 5-HT(1A) receptors (at which it acts as an antagonist) and a tendency to behave as an inverse agonist at alpha(2A)-adrenoceptors in some cell culture systems. These potential drawbacks may be overcome by careful design of experiments, and the greater selectivity of RX 821002 renders it much superior to yohimbine or idazoxan as a tool for probing physiological actions at alpha(2)-receptors. It can be compared favorably with other selective antagonists such as atipamezole. In physiological studies, RX 821002 augments norepinephrine release in the frontal cortex and increases drinking behavior in rat. In rabbit, intrathecal administration of this drug enhances somatic and autonomic motor outflows, showing that tonic adrenergic descending inhibition of withdrawal reflexes and sympathetic pre-ganglionic neurons is strong in this species. The potentiation of reflexes may be considered a pro-nociceptive action. In the same model, RX 821002 antagonizes the inhibitory effects of the mu opioid fentanyl, indicating that exogenous opioids synergize with endogenously released norepinephrine in the spinal cord. Thus, the careful use of RX 821002 has revealed several aspects of the physiological activity of alpha(2)-adrenoceptors in rabbit spinal cord and rat brain. We recommend that RX 821002 and/or compounds with similar selectivity for alpha(2)-adrenoceptors (atipamezole, MK-912, RS-79948) should be used in preference to yohimbine or idazoxan in all future studies of this type.

Brain alpha(2)-adrenoceptors in monoamine-depleted rats: increased receptor density, G coupling proteins, receptor turnover and receptor mRNA

1. This study was designed to assess the molecular and cellular events involved in the up-regulation (and receptor supersensitivity) of brain alpha(2)-adrenoceptors as a result of chronic depletion of noradrenaline (and other monoamines) by reserpine. 2. Chronic reserpine (0.25 mg kg(-1) s.c., every 48 h for 6 - 14 days) increased significantly the density (B(max) values) of cortical alpha(2)-adrenoceptor agonist sites (34 - 48% for [(3)H]-UK14304, 22 - 32% for [(3)H]-clonidine) but not that of antagonist sites (11 - 18% for [(3)H]-RX821002). Competition of [(3)H]-RX821002 binding by (-)-adrenaline further indicated that chronic reserpine was associated with up-regulation of the high-affinity state of alpha(2)-adrenoceptors. 3. In cortical membranes of reserpine-treated rats (0.25 mg kg(-1) s.c., every 48 h for 20 days), the immunoreactivities of various G proteins (Galphai(1/2), Galphai(3), Galphao and Galphas) were increased (25 - 34%). Because the high-affinity conformation of the alpha(2)-adrenoceptor is most probably related to the complex with Galphai(2) proteins, these results suggested an increase in signal transduction through alpha(2)-adrenoceptors (and other monoamine receptors) induced by chronic reserpine. 4. After alpha(2)-adrenoceptor alkylation, the analysis of receptor recovery (B(max) for [(3)H]-UK14304) indicated that the increased density of cortical alpha(2)-adrenoceptors in reserpine-treated rats was probably due to a higher appearance rate constant of the receptor ((Delta)r=57%) and not to a decreased disappearance rate constant ((Delta)k=7%). 5. Northern- and dot-blot analyses of RNA extracted from the cerebral cortex of saline- and reserpine-treated rats (0.25 mg kg(-1), s.c., every 48 h for 20 days) revealed that reserpine markedly increased the expression of alpha(2a)-adrenoceptor mRNA in the brain (125%). This transcriptional activation of the receptor gene expression appears to be the cellular mechanism by which reserpine induces up-regulation in the density of brain alpha(2)-adrenoceptors.

Modulation of sibutramine-induced increases in extracellular noradrenaline concentration in rat frontal cortex and hypothalamus by alpha2-adrenoceptors

1. The effects of sibutramine (0.25 - 10 mg kg-1 i.p.) on extracellular noradrenaline concentration in the frontal cortex and hypothalamus of freely-moving rats were investigated using microdialysis. The role of presynaptic alpha2-adrenoceptors in modulating the effects of sibutramine in these brain areas was also determined. 2. Sibutramine induced an increase in extracellular noradrenaline concentration, the magnitude of which paralleled dose, in both brain areas. In the cortex, this increase was gradual and sustained, whereas in the hypothalamus it was more rapid and of shorter duration. 3. In both the cortex and hypothalamus, pretreatment of rats with the alpha2-adrenoceptor antagonist RX821002 (3 mg kg-1 i.p.) potentiated increases in the accumulation of extracellular noradrenaline induced by sibutramine (10 mg kg-1 i. p.), by 7 and 10 fold respectively. RX821002 also reduced the latency of sibutramine to reach its maximum effect in the cortex, but not in the hypothalamus. 4. Infusion of RX821002 (1 microM) via the probe increased the accumulation of extracellular noradrenaline induced by sibutramine (10 mg kg-1 i.p.) in both brain areas. In the hypothalamus, the effects of RX821002 on the accumulation of noradrenaline induced by sibutramine were 2 fold greater than those in the cortex. 5. These findings support evidence that sibutramine inhibits the reuptake of noradrenaline in vivo, but that the accumulation of extracellular noradrenaline is limited by noradrenergic activation of presynaptic alpha2-adrenoceptors. Furthermore, the data suggest that terminal alpha2-adrenoceptors in the hypothalamus exert a greater inhibitory effect over the control of extracellular noradrenaline accumulation than do those in the cortex.

Differential modulation of alpha2-adrenoceptor subtypes in rat kidney by chronic desipramine treatment

The profile of [3H]RX821002 (2-methoxy idazoxan) binding to alpha2-adrenoceptor subtypes in rat kidney membranes was evaluated in controls and after chronic treatment with desipramine (10 mg/kg, i.p., every 12 h, 7 days) or clorgyline (2 mg/kg, i.p., every 24 h, 21 days). [3H]RX821002 recognized with high affinity (Kd=1.5+/-0.2 nM in controls) a single and saturable population of binding sites (Bmax=57+/-5 fmol/mg protein in controls). The competitions by (-)-adrenaline, the alpha2B-adrenoceptor selective drug ARC239 (2-[2-[4-(o-methoxyphenyl)-piperazin-1-yl]-ethyl]-4,4-dimethyl-1,3 (2H,4H)-isoquinolindione) and the alpha2A-adrenoceptor selective drug BRL44408 (2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidaz ole) suggested the existence of both alpha2A- and alpha2B-adrenoceptors together with a non-adrenoceptor binding site. After chronic desipramine but not after chronic clorgyline treatments, the density (Bmax) of alpha2-adrenoceptors was increased (46%). In the presence of ARC239 (50 nM), the density of alpha2A-adrenoceptors increased (44%) in the desipramine-treated group without changes in the clorgyline-treated group. Conversely, in the presence of BRL44408 (100 nM), the density of alpha2B-adrenoceptors was not affected by the treatments. The selective upregulation of the alpha2A-adrenoceptor subtype following chronic desipramine administration is compatible with a differential location and function of the alpha2-adrenoceptor subtypes in the rat kidney.

Identification of I1 and I2 imidazoline receptors in striatum membranes from different species

The alpha 2-adrenergic agonist [3H]clonidine and antagonist [3H]idazoxan also label I1 and I2 imidazoline receptors in striatum membranes. They are investigated here in striata from the dog, rat, mouse, rabbit, calf, monkey, and human. I1 receptors were barely detected in the dog, rat, and mouse and only further examined by competition binding experiments in calf, rabbit, and human. I2 receptors were further examined in all species. The centrally acting vasodilators clonidine and rilmenidine were more potent than moxonidine at the I1 receptors. They displayed low potency for the I2 receptors in all species except the rat. In all species examined, the nonsubstituted imidazoline derivatives idazoxan and RX801077 displayed high affinity for the I1 and I2 receptors. Conversely, both stereoisomers of the alkoxy-substituted imidazoline-derivative efaroxan displayed low affinity. The matching binding characteristics of these compounds further stress the structural similarity of the ligand binding sites of I1 and I2 receptors.

Tonic adrenergic and serotonergic inhibition of a withdrawal reflex in rabbits subjected to different levels of surgical preparation

The excitability of the heel-gastrocnemius withdrawal reflex pathway has been monitored in rabbits undergoing surgical preparation for electrophysiological experimentation under Saffan anaesthesia. Reflexes were evoked by percutaneous electrodes inserted at the heel and recorded as electromyograph signals from the ipsilateral medial gastrocnemius muscle. Two levels of surgery were carried out. The "full surgical" preparation was performed under deep Saffan anaesthesia. The trachea, carotid artery, jugular vein and intrathecal space (via a small laminectomy at L1) were cannulated, the animals were decerebrated by suction, and the left hindlimb was immobilized by screw clamps applied to the tibia and the femur. The sciatic nerve and its branches were exposed by bisection of the posterior biceps muscle and the anaesthetic was withdrawn. In the "reduced surgery" preparation, procedures were carried out with a lighter level of Saffan anaesthesia and operated tissues were infiltrated with local anaesthetic. Only the cannulations were performed in these animals. The excitability of the heel-gastrocnemius reflex declined throughout the full surgical preparation, with the median threshold increasing from 0.8 to 4.2 mA (n=19) and responses to suprathreshold stimuli reducing in size. Most of this effect was reversed after surgery was complete and anaesthesia withdrawn subsequent to decerebration. There were no significant changes in reflex excitability during the reduced surgery preparation (n = 15). Animals prepared by each of these protocols were given increasing intrathecal doses of either the selective alpha2-adrenoceptor antagonist RX 821002 (0.3 to 300 microg) or the serotonin/5-hydroxytryptamine (5-HT)1A-receptor antagonist WAY-100635 (0.01 to 30 microg). Both drugs caused significant, dose-dependent increases in reflex responses, to four to six times pre-drug control in both groups of animals. There were no differences in the effects on reflexes of either drug between the preparations. Thus, surgical preparation of decerebrated rabbits for electrophysiological recording results in depression of hindlimb withdrawal reflexes, although much of this effect did not persist beyond the completion of surgery. Tonic monoaminergic inhibition of reflexes was present to the same extent in both preparations investigated and is not therefore an epiphenomenon of the way in which the animals were prepared.

Effects of the alkylating agent EEDQ on regulatory G proteins and recovery of agonist and antagonist alpha2-adrenoceptor binding sites in rat brain

The aim of this study was to assess the effect of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ)-induced alpha2-adrenoceptor inactivation on regulatory G proteins and the recovery of agonist and antagonist binding sites. EEDQ induced a rapid increase in the abundance of rat brain cortical Galphai1/2 proteins (30% at 6 h) which reached a maximum at 4 days (45%) and which then slowly returned (7-30 days) to control values. EEDQ did not alter significantly the levels of Galphai3 and Galphao proteins. By using the standard monoexponential model, the analysis of the recovery of alpha2-adrenoceptor density (6 h-30 days) with [3H]UK 14304 (bromoxidine) and [3H]RX 821002 (2-metoxy idazoxan) in the cerebral cortex did not reveal differences in receptor turnover parameters. However, the recovery of [3H]UK 14304 binding fitted best to a new biphasic recovery model, suggesting the existence of two distinct phases of recovery of agonist sites (r1 and r2 = 15.7 and 7.4 fmol mg protein(-1) day(-1); k1 and k2 = 0.51 and 0.25 day(-1); (t1/2)1 and (t1/2)2 = 1.4 and 2.7 days). In contrast, the recovery of [3H]RX 821002 antagonist sites did not fit to the biphasic model (r = 8.1, k = 0.14, t1/2 = 4.9). Because agonist binding requires coupling to G proteins, the present results suggest that the rapid over-expression of Galphai1/2 proteins induced by EEDQ is related to the biphasic recovery of [3H]UK 14304 binding. The possible implication of the faster recovery of alpha2-adrenoceptor function after EEDQ inactivation is discussed.

Effect of RX 821002 at 5-HT1A-receptors in rabbit spinal cord in vivo

1. The activity of RX 821002 (2-methoxy idazoxan) at 5-HT1A-receptors in the spinal cord has been investigated in decerebrated, spinalized rabbits. Reflexes evoked in medial gastrocnemius motoneurones by electrical stimulation of the sural nerve were unaffected by intrathecal (i.th.) administration of RX 821002 (111 and 664 nmol cumulative, n = 7), although the highest dose of this drug did produce a significant increase in heart rate of 28 +/- 7 beats min(-1). Subsequent administration of the 5-HT1A-receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) at 300 nmol, i.th., facilitated reflexes to a median of 144% of pre-drug controls, an effect that was partially reversed (to a median value of 120% of pre-drug values) by subsequent administration of the 5-HT1A-receptor antagonist WAY-100635, at 185 nmol i.th. 2. In a separate set of experiments, 8-OH-DPAT was given at 30 nmol i.th. and potentiated reflexes to a median of 170% of pre-drug levels (n = 8). Subsequent administration of RX 821002 (at a cumulative dose of 1.11 micromol, i.th., n = 5) significantly reduced gastrocnemius responses to a median of 154% of control values. 3. After a 3 h recovery period, 8-OH-DPAT was re-administered at 30 nmol, i.th., and increased reflexes to a median value of 151% of pre-drug levels, an effect not significantly different from when it was given alone. WAY-100635 dose-dependently antagonized this effect, causing significant reductions in reflexes at a cumulative dose of 0.55 nmol, i.th., and complete reversal of the effects of 8-OH-DPAT at a cumulative dose of 5.5 nmol. 4. These data show that, at intrathecal doses up to 664 nmol, RX 821002 is devoid of agonist activity at 5-HT1A-receptors. It appears to be a very weak antagonist at these sites in vivo, being some 2000 times less potent than WAY-100635. The inability of WAY-100635 to block completely the effects of high doses of 8-OH-DPAT has been noted previously and can be explained by non-selective actions of the agonist. However, it would appear that a 30 nmol i.th. dose of 8-OH-DPAT is selective for 5-HT1A receptors in this preparation.

Modulation of catecholamine release by alpha 2-adrenoceptors and I1-imidazoline receptors in rat brain

The physiological and pharmacological effects of imidazoli(di)ne derivatives, such as clonidine, have been related not only to the interaction with alpha 2-adrenoceptors but also to their activity on non-adrenoceptor sites termed imidazoline receptors. The modulation of catecholamine release by imidazoline drugs was studied by monitoring extracellular levels of norepinephrine (NE), dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) with microdialysis in cingulate cortex of rats, with or without irreversible alpha 2-adrenoceptor blockade. NE and DA levels were in the 1 nM range whereas DOPAC and HIVA levels were approximately equal to 100 nM. NE and DA levels were increased when the uptake blocker desipramine (1 microM) or KCl (100 mM) were added to the perfusion medium. Clonidine induced a dose-dependent (0.3-1.2 mg/kg i.p.) decrease in NE (max 61%) and DA (max 40+) levels that was reversed by the alpha 2-adrenoceptor antagonist RX821002. After alpha 2-adrenoceptor irreversible blockade with the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), [3H]clonidine binding to alpha 2-adrenoceptors was reduced by 94 +/- 1%. Under such conditions, clonidine elicited a paradoxical dose-dependent (0.6-2.4 mg/kg i.p.) increase of NE (max 56%) without modifications in DA, DOPAC and HVA levels. The stimulatory effect of clonidine was prevented by the imidazoline receptor antagonist idazoxan (10 mg/kg i.p.) but not by RX821002 (5 mg/kg i.p.). In rats pretreated with EEDQ, cirazoline (I1/I2-imidazoline receptor agonist), moxonidine (I1-imidazoline receptor agonist), but not guanabenz (I2-imidazoline receptor agonist) (1.2-2.4 mg/kg i.p.) elicited an increase of NE levels in a similar manner to clonidine (11-82%). Idazoxan also abolished these responses to cirazoline or moxonidine. In contrast to systemic administration, local perfusion of clonidine (10-100 microM) through the microdialysis probe under alpha 2-adrenoceptor alkylating conditions, did not modify extracellular levels of NE and DA suggesting an indirect mechanism. The results demonstrate that clonidine and related imidazoli(di)ne drugs are able not only to inhibit NE release in rat cerebral cortex involving an alpha 2-adrenoceptor mechanism, but also to induce a paradoxical NE release through an indirect extracortical mechanism. The findings evidence that the indirect modulation of NE levels by imidazoline drugs is mainly due to a functional activity on I1-imidazoline receptors.

Pharmacological characterization of I1 and I2 imidazoline receptors in human striatum

[3H]RX821002, [3H]clonidine and [3H]idazoxan have previously been shown to selectively label alpha 2-adrenergic receptors, I1 and I2 imidazoline receptors in the human central nervous system, respectively. Idazoxan shows relatively high affinity for all three receptors. We investigated the possible selectivity of several compounds towards one of those receptors in human striatum. Addition of an alkoxy group at the 2-position of the benzodioxan moiety of idazoxan (ethoxy-idazoxan, methoxy-idazoxan) increases the alpha 2-selectivity in human brain. Efaroxan is also alpha 2-selective. On the contrary, BU224, BU239, cirazoline and RX801077 display imidazoline receptor selectivity. Our results indicate that for all molecules tested, idazoxan and 'flat' analogs possess I1/I2 receptor selectivity. A 'bulky' substituent at the 2-position of the benzodioxan ring gives rise to alpha 2-adrenergic receptor selectivity. Until now, we found no more than 3-fold difference in IC50 between both imidazoline receptors. Both receptors also display similar stereoselectivity, suggesting that they might be 'interconnected' in the human striatum.

Evaluation in rat of RS-79948-197 as a potential PET ligand for central alpha 2-adrenoceptors

Tritium-labelled RS-79948-197 {(8aR,12aS,13aS)-5, 8,8a,9,10,11,12,12a,13,13a-decahydro-3-methoxy-12-(ethylsulphon yl)-6H-iso- quino[2,1-g][1,6]naphthyridine} was evaluated in rat brain as an in vivo ligand for central alpha 2-adrenoceptors, as a preliminary step in the development of a radioligand for positron-emission tomography (PET) studies. The maximal receptor-specific signal was achieved within 90-120 min after i.v. injection of [ethyl-3H]RS-79948-197 and was selective for the alpha 2- compared with the alpha 1-adrenoceptor, with no detectable binding to the imidazoline-I2 site. Estimates for binding potential (approximating to Bmax/Kd) ranged between 3.4 in entorhinal cortex and 0.5 in medulla oblongata. The results, which indicate a similarly localised but 2-fold increase in specific binding compared with that previously demonstrated using [3H]RX 821002 (2-methoxy-idazoxan), are sufficiently encouraging as to support further investment in the development of 11C-labelled RS-79948-197, or a close structural analogue, as a ligand for clinical PET.

[3H]RX821002 (2-methoxyidazoxan) binds to alpha 2-adrenoceptor subtypes and a non-adrenoceptor imidazoline binding site in rat kidney

The binding of [3H]RX821002 (2-methoxyidazoxan) was evaluated in rat kidney membranes. [3H]RX821002 (0.13-16 nM) recognized a single, saturable binding site with high affinity. Different binding site densities were calculated depending on non-specific binding as defined by (-)-adrenaline or RX821002 (10 microM). Competition assays using (-)-adrenaline and the subtype-selective drugs ARC 239 (2-[2-[4-(o-methoxyphenyl)-piperazin-1-yl]-ethyl]-4,4-dimethyl-1,3 (2H,4H)-isoquinolindione), BRL 44408 (2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidaz ole), oxymetazoline or prazosin for [3H]RX821002 binding sites revealed the presence of alpha 2B-adrenoceptors (33-51%), alpha 2D-adrenoceptors (15-28%) and an adrenaline-insensitive population (34-40%), sensitive to imidazolines. After the addition of (-)-adrenaline (3 microM) to mask alpha 2-adrenoceptors, [3H]RX821002 specifically identified a saturable binding site with high affinity (Kd = 4.9 +/- 1.5 nM). The pharmacological profile of this non-adrenoceptor, [3H]RX821002 binding site (potencies: efaroxan > clonidine > guanabenz > BRL 44408 > ARC 239 > BU 224 (2-(4,5-dihydroimidaz-2-yl)quinoline) > moxonidine > (-)-nor-adrenaline > cimetidine) is different to that of imidazoline I1 or imidazoline I2 binding sites. Alternative incubation in the presence of ARC 239 (50 nM) to mask alpha 2B-adrenoceptors or BRL 44408 (100 nM) to mask alpha 2D-adrenoceptors confirmed the existence of both alpha 2-adrenoceptor subtypes and a non-adrenoceptor imidazoline binding site.

The subtype-selective alpha 2-adrenoceptor antagonists BRL 44408 and ARC 239 also recognize 5-HT1A receptors in the rat brain

Several alpha 2-adrenoceptor compounds have been reported to recognize 5-HT1A receptors. The interaction of the alpha 2A/D- and alpha 2B/C-adrenoceptor antagonists BRL 44408 (2-[2H-(1-methyl-1,3-dihydroisoindole) methyl]-4,5-dihydroimidazole) and ARC 239 (2-[2-[4-(o-methoxyphenyl)piperazin-1-yl] ethyl]-4,4-dimethyl-1,3-(2H,4H)-isoquinolinedione) with 5-HT1A receptors was evaluated in rat brain. Competition experiments in cortex with both compounds against the specific binding of the 5-HT1A receptor radioligand [3H]8-OH-DPAT (8-hydroxy-2-(n-dipropyl-amine)-tetralin) yielded Ki values in the nanomolar range, fairly close to their previously reported affinities for alpha 2-adrenoceptors. Similar Ki values were obtained under alpha 2-adrenoceptor masking conditions by competition assays of these compounds against the alpha 2-adrenoceptor and 5-HT1A receptor radioligand [3H]RX 821002 (2-methoxy idazoxan) specific binding in hippocampus. The results indicate that BRL 44408 and ARC 239 recognize 5-HT1A receptors in addition to alpha 2-adrenoceptors. The fact should be considered when using these compounds to study alpha 2-adrenoceptor subtypes.

Alpha 2-adrenoceptor subtypes in the human brain: a pharmacological delineation of [3H]RX-821002 binding to membranes and tissue sections

In order to study the characterization and localization of [3H]RX-821002 (2-methoxy-idazoxan) binding to alpha 2-adrenoceptor subtypes in several regions of the human brain, we have carried out competition studies using both autoradiography and membrane binding assays. The alpha 2A-adrenoceptor subtype was found to be predominant in the different layers of the frontal cortex, cerebellum and hippocampal formation, while in the neostriatum it was the non-alpha 2A- (alpha 2B- and alpha 2C-) adrenoceptor subtype. In the frontal cortex, in addition to binding to the alpha 2A-adrenoceptor subtype, [3H]RX-821002 bound also to a small portion of alpha 2B- and alpha 2C-adrenoceptors in layer III, and to an unidentified binding site in the external layers. In the hippocampus, both alpha 2A- and non-alpha 2A- (alpha 2B- and alpha 2C-) adrenoceptors were labelled in the dentate gyrus and the CA1 field, together with 5-HT1A receptors. 5-HT1A receptors were labelled predominantly in the stratum pyramidale layer. These results, in addition to delineate the relative presence of alpha 2-adrenoceptor subtypes, indicate that caution is needed when analyzing RX 821002 binding to human brain tissue.

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.

Imidazoline receptors, non-adrenergic idazoxan binding sites and alpha 2-adrenoceptors in the human central nervous system

Both [3H]clonidine and [3H]idazoxan bind to alpha 2 adrenoceptors. The former also labels imidazoline receptors, and the latter non-adrenergic idazoxan binding sites. In order to investigate whether the imidazoline receptors and non-adrenergic idazoxan binding sites are identical, we compared the binding characteristics of [3H]clonidine and [3H]idazoxan to these sites by radioligand binding experiments on ultra-thin slices and homogenates of human striatum. A good correlation was found between the effect of different ions on the binding characteristics of [3H]clonidine and [3H]idazoxan, and the affinities of most competing drugs. However, clonidine and rilmenidine displayed a 100- and 10-fold lower affinity, respectively, for the idazoxan binding sites than for the imidazoline receptors. Autoradiography with [3H]clonidine showed that high densities of imidazoline receptors were present in the striatum, pallidum, gyrus dentatus of the hippocampus, amygdala, and substantia nigra. Moderate densities were found throughout the cerebral cortex, thalamus and several brainstem nuclei including the nucleus olivarius inferior. Low densities were seen in the cerebellum, spinal cord and pituitary gland. As for the non-adrenergic sites labelled by [3H]idazoxan, the imidazoline receptors can be found in all major brain areas examined. However, there are some striking differences between the concentrations of imidazoline receptors and non-adrenergic idazoxan binding sites in certain brain regions. To reconcile distribution and pharmacologic data, we propose that imidazoline receptors and non-adrenergic idazoxan binding sites represent different proteins or protein complexes and that at least in the nucleus reticularis lateralis and the striatum, imidazoline receptors and non-adrenergic idazoxan binding sites may be physically associated. The regional distribution of alpha 2 adrenoceptors within the human CNS was determined by quantitative autoradiography with [3H]RX821002. The highest densities of alpha 2 adrenoceptors were found in the cerebral and cerebellar cortex, and certain regions in the medulla oblongata (floor of the IV ventricle, reticular formation, hypoglossal nucleus and nucleus olivarius inferior). No alpha 2 adrenoceptors were detected in the pituitary gland. There exists no relationship between the distribution pattern of imidazoline receptors and alpha 2 adrenoceptors, indicating that these binding sites are independent from each other.

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.

Relevance of the use of [3H]-clonidine to identify imidazoline receptors in the rabbit brainstem

1. [3H]-clonidine binding was investigated in membranes isolated from the ventral medulla oblongata of the rabbit, where clonidine produced a hypotensive effect which was not mediated by adrenoceptors. [3H]-clonidine specific binding, as defined by the difference between the binding of [3H]-clonidine in the presence and in the absence of 10 microM cirazoline, occurred at two sites: a high affinity site with a KD = 2.9 +/- 0.7 nM and a Bmax of 40 +/- 8 fmol mg-1 protein and a low affinity site with a KD = 18.2 +/- 0.4 nM and a Bmax of 66 +/- 14 fmol mg-1 protein. 2. The high affinity sites being catecholamine-sensitive were identified as alpha 2-adrenoceptors. The low affinity binding of [3H]-clonidine was insensitive to catecholamines, as well as to other alpha 2-adrenoceptor specific probes, and could be inhibited with high affinity only by compounds which lowered blood pressure when directly injected in the nucleus reticularis lateralis of the ventral brainstem, or by antagonists. 3. It was concluded that in the ventral medulla of the rabbit, [3H]-clonidine labelled alpha 2-adrenoceptors and imidazoline receptors (IRs). Only the latter were related to the hypotensive effects of clonidine and rilmenidine directly injected into the rostroventrolateral medulla oblongata (RVLM) of the rabbit. The methodological problems regarding the study of IRs with [3H]-clonidine are discussed.

Opposite age-dependent changes of alpha 2A-adrenoceptors and nonadrenoceptor [3H]idazoxan binding sites (I2-imidazoline sites) in the human brain: strong correlation of I2 with monoamine oxidase-B sites

In the postmortem human brain (27 specimens of frontal cortex, Brodmann area 9), the specific binding of the antagonists [3H]RX 821002 (2-methoxyidazoxan) to alpha 2A-adrenoceptors and that of [3H]idazoxan to I2-imidazoline sites (a nonadrenoceptor mitochondrial site) were determined in parallel to study the effect of aging (range, 4-89 years) on both brain proteins. The density of alpha 2A-adrenoceptors and age were negatively correlated (r = -0.71; p < 0.001). In contrast, the density of I2-imidazoline sites was positively correlated with aging (r = 0.59; p < 0.005). The ratio of receptor densities (alpha 2A/I2) also showed a marked negative correlation with age (r = -0.76; p < 0.001). In an age-selected group (range, 10-89 years), the density of monoamine oxidase (MAO)-B sites labeled by [3H]Ro 19-6327 (lazabemide) also showed a positive correlation with age (r = 0.80; p < 0.005). In these subjects, the density of I2-imidazoline sites correlated well with the density of MAO-B sites (r = 0.70; p < 0.005). The ratio of the density of these sites (MAO-B/I2) did not correlate with the age of the subject at death (r = -0.15). In the human frontal cortex, idazoxan displayed very low affinity (Ki = 89 microM) against the binding of [3H]Ro 19-6327 to MAO-B, which discounted a direct interaction of [3H]idazoxan with the active center of the enzyme and indicated that the I2-imidazoline site cannot be identified with MAO-B.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Quantification of in vivo binding of [3H]RX 821002 in rat brain: evaluation as a radioligand for central alpha 2-adrenoceptors

On the basis of its established in vitro characteristics, [3H]RX 821002 was evaluated in rats as an in vivo radioligand for central alpha 2-adrenoceptors. Estimates for in vivo binding potential, obtained by compartmental analyses of time-radioactivity data, ranged between 1.9 for hypothalamus and 0.2 for cerebellum, with a regional distribution in brain which was similar to that observed in vitro. Selectivity and specificity of the signal were checked by predosing with either the alpha 2-antagonists, idazoxan or yohimbine, the alpha 2-agonist, clonidine, or the alpha 1-antagonist, prazosin. Pretreatment of the rats with the selective neurotoxin, DSP-4, had no significant effect on [3H]RX 821002 binding, suggesting that the majority of labelled sites were situated post-junctionally. The studies indicate that [3H]RX 821002 can be used experimentally as an in vivo marker for central alpha 2-adrenoceptors. The size and rate of expression of the specific signal encourage the development and assessment of [11C]RX 821002 for clinical PET studies.

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.

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.

[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)