Alpha 2-adrenoceptor antagonism and other pharmacological antagonist properties of some substituted benzoquinolizines and yohimbine in vitro

Synthesis and biological evaluation of [11C]MK-912 as an alpha2-adrenergic receptor radioligand for PET studies

In vitro studies showed that MK-912 ((2S, 12bS)1',3'-dimethylspiro(1,3,4,5',6,6',7,12b-octahydro -2H-benzo[b]furo[2,3-a]quinolizine)-2,4'-pyrimidin-2'-one) is a potent alpha2-adrenergic receptor antagonist with high affinity (Ki = 0.42, 0.26 and 0.03 nM to alpha2A, alpha2B and alpha2C, respectively) and high selectivity (alpha2A/alpha1A = 240; alpha2A/D-1 = 3600; alpha2A/D-2 = 3500; alpha2A/5-HT1 = 700; alpha2A/5-HT2 = 4100). The compound was labeled with 11C and evaluated in rodents and monkey as a specific radioligand for studying alpha2-adrenergic receptors using PET. [11C]MK-912 was synthesized by methylation of its desmethyl precursor, L-668,929, with [11C]CH3I in (Bu3O)P=O at 85 degrees C for 8 min followed by purification with HPLC in 18% yield in a synthesis time of 45 min from end of bombardment (EOB). The specific activity was 0.83-0.93 Ci/micromol and the radiochemical purity was 97%. The initial uptake of [11C]MK-912 in mouse brain, heart, lung, liver and kidney was high (5%, 4%, 5%, 17% and 8% per gram of organ, respectively, at 5 min postinjection) and the activities were then slowly cleared from these organs. The uptake of [11C]MK-912 in rat olfactory tubercle, a brain region with high density of alpha2-adrenergic receptors, was reduced by 30%, and the ratio of radioactivity in olfactory tubercle/cerebellum was reduced from 2:1 to 1:1 by coinjection of [11C]MK-912 with a potent alpha2-adrenergic receptor antagonist, atipamezole (3 mg/kg), indicating that compound 2 binds to alpha2-adrenergic receptors. However, a PET study in a rhesus monkey revealed that the initial influx of [11C]MK-912 into various brain regions (cerebellum, cortex, olfactory tubercle and striatum) was high (0.02%/cc), and the radioactivity was then washed out slowly and without significantly differential retention in these brain regions. This, coupled with the fact that none of the high-density alpha2-adrenergic receptor brain regions exceeds a few millimeters in diameter, suggests that [11C]MK-912 is probably not an ideal radioligand for studying alpha2-adrenergic receptors in humans using commercially available PET.

Further characterization of 5-hydroxytryptamine receptors (putative 5-HT2B) in rat stomach fundus longitudinal muscle

1. The present study was undertaken to isolate and characterize pharmacologically homogeneous populations of 5-hydroxytryptamine (5-HT) receptors from a possible mixed receptor population mediating concentration of the longitudinal muscle of rat stomach fundus. Our aim was to extend the pharmacological characterization of the 5-HT2B receptor which is reported to be expressed in this preparation. 2. To minimize spontaneous activity and any influence of circular muscle on the contractile response, narrow (1-1.5 x 20 mm) segments of mucosa-denuded longitudinal muscle were used. Under these conditions, blockade of monoamine oxidase with pargyline (100 microM for 15 min) caused a leftward displacement of concentration-effect curves for both 5-methoxytryptamine (5-MeO-T) and tryptamine. Neither pargyline nor a number of uptake inhibitors affected responses to 5-HT. 3. In pargyline pretreated preparations, the order of potency of a number of tryptamine analogues was as follows: 5-MeO-T > or = alpha-Me-5-HT > or = 5-HT > 5-carboxamidotryptamine (5-CT) > tryptamine > 2-Me-5-HT. In addition several ligands known to act as agonists at either 5-HT2A or 5-HT2C receptors including 1-m-chlorophenylpiperazine (m-CPP), Ru 24969, MK 212 and SCH 23390 were also agonists in rat fundus whilst sumatriptan, renzapride and 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) were very weak or inactive. With the exception of 2-Me-5-HT and m-CPP, most agonists produced monophasic concentration-effect curves consistent with an interaction at a single site. High concentrations of 2-Me-5-HT evoked relaxations which were blocked by phentolamine (1 MicroM) suggesting an interaction with alpha-adrenoceptors. m-CPP often evoked biphasic concentration-effect curves with a second contractile phase which was insensitive to yohimbine at concentrations higher than required for antagonism of responses to 5-HT.4. LY 53857, methiothepin, methysergide, ritanserin and ICI 170809 were potent but non-surmountable antagonists of 5-HT in rat fundus. In contrast, several ligands behaved as surmountable antagonists with the following order of potency: rauwolscine >yohimbine = mesulergine > mianserin = SB 204070 >WY 26703 > SB 200646> pirenpirone> renzapride. DAU 6285, granisetron, spiperone, ketanserin,phentolamine and GR 127935 did not affect responses to 5-HT at concentrations up to 1 pM. The agonist and concentration independent profile of antagonism supported a single site interaction for both agonists and antagonists.5. We conclude that despite small differences concerning the enantiomeric selectivity and affinity of rauwolscine and yohimbine, the close pharmacological identity of 5-HT receptors in rat stomach fundus and the recently cloned 5-HT2B receptor is maintained. SB 200646, which demonstrates some selectivity for 5-HT receptors in rat stomach fundus, should provide a useful ligand for confirmation of this view and allow discrimination of 5-HT2B function both in vitro and in vivo.

In vivo pharmacological profile of SL 84.0418, a new selective, peripherally active alpha 2-adrenoceptor antagonist

SL 84.0418 is a novel pyrrolo-indole derivative with potent and selective alpha 2-adrenoceptor antagonist activity in vitro and anti-hyperglycemic properties in vivo. In the present study we demonstrated that SL 84.0418 and its active (+) enantiomer, SL 86.0715, show a high degree of selectivity for alpha 2-adrenoceptors in vivo, preferentially blocking alpha 2-adrenoceptors in the periphery. While having no effects on basal blood pressure and heart rate, p.o. (3 and 10 mg/kg) or i.v. (0.3 mg/kg) administered SL 84.0418 or SL 86.0715 antagonized the hypertensive responses mediated by postsynaptic vascular alpha 2-adrenoceptors after the administration of UK 14304 or norepinephrine to pithed rats. The (-) enantiomer of SL 84.0418, SL 86.0714, was devoid of alpha 2-adrenoceptor antagonist properties in vivo, while the (+) enantiomer of SL 84.0418, SL 86.0715, showed alpha 2-adrenoceptor antagonist activity similar to that of the racemate. SL 84.0418 (3 mg/kg p.o.) did not modify the centrally mediated hypotensive and bradycardic effects of i.c.v. administered clonidine. Furthermore, in the mouse, SL 84.0418 potently antagonized the hyperglycemic responses to epinephrine or UK 14304, which are mediated by peripheral alpha 2-adrenoceptors, but failed (in doses up to 30 mg/kg i.p.) to antagonize the clonidine-induced hypolocomotion, which is mediated by central alpha 2-adrenoceptors. These results suggest that SL 84.0418 preferentially antagonizes peripheral alpha 2-adrenoceptors. SL 84.0418 administered to pithed rats (3 mg/kg p.o. or 0.3 mg/kg i.v.) did not modify the vasoconstriction induced by cirazoline (effect mediated by alpha 1-adrenoceptors) nor the tachycardia induced by norepinephrine (effect mediated by beta-adrenoceptors).(ABSTRACT TRUNCATED AT 250 WORDS)

[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.

Stimulation of α2-adrenergic receptors in nucleus tractus solitarius is required for the baroreceptor reflex

Bilateral injection into the nucleus tractus solitarius (NTS) of the alpha 2-adrenergic receptor antagonist yohimbine produced a dose-related (10-500 pmol) increase in arterial pressure, with a maximal response of approximately 60 mm Hg. Idazoxan, also an alpha 2-adrenergic receptor antagonist, produced a similar response although idazoxan was less potent than yohimbine. The pressor response elicited by these drugs was attenuated by stimulation of adrenergic receptors in the NTS by local administration of either clonidine or tyramine. Doses of yohimbine (200 pmol) or idazoxan (5 nmol) that maximally increased arterial pressure also completely inhibited the depressor and bradycardic responses to electrical stimulation of the aortic depressor nerve. These results indicate that tonic stimulation of alpha 2-adrenergic receptors in the NTS is required for baroreceptor reflex function.

Yohimbine can induce ethanol tolerance in an in vitro preparation of rat locus coeruleus

Noradrenergic neurons have been implicated in the development of ethanol dependence and tolerance. Moreover, the development of an hyposensitivity of alpha 2 adrenoceptors has been postulated during long-term exposition to ethanol. In order to test the putative role of alpha 2 receptors in ethanol intoxication, we have studied the interaction between ethanol and yohimbine, an alpha 2 antagonist, on the spontaneous firing rate of rat locus coeruleus (LC) in an in vitro slice model. The spikes from single neurons were recorded by glass microelectrodes. Ethanol at 100 mM, a concentration that parallels the behavioral effects in the human and in the animals, inhibits the firing activity of some LC cells. This inhibition was quickly reversed after stopping the ethanol perfusion and was observed for each further administration. However, if yohimbine (20 microM) was simultaneously perfused, the ethanol-induced inhibition was rapidly antagonized. This effect is reversible after long time washout of yohimbine. This suggests that alpha 2 adrenoceptors could be implicated in the inhibitory effect of ethanol on LC noradrenergic neurons and perhaps in the development of tolerance. However, other hypotheses are discussed, because yohimbine can also antagonize other types of receptors.

Yohimbine and naloxone: effects on male rat sexual behavior

We evaluated the effects of yohimbine (2 mg/kg) and naloxone (5 mg/kg), separately and in combination, on copulatory behavior in male rats. In Experiment 1, yohimbine evinced decrements in intromission frequency, ejaculation latency, and copulatory efficiency, whereas naloxone administration was followed by an increased ejaculation latency, and the combination of yohimbine plus naloxone was without effect. In Experiment 2, yohimbine evinced decreases in intromission frequency, ejaculation latency, copulatory efficiency in the first, but not subsequent, copulatory series, as well as a decreased latency to sexual exhaustion. Further, treatment with yohimbine alone, naloxone alone, or yohimbine plus naloxone was followed by a reduction in the number of ejaculation prior to sexual exhaustion. Thus, at the doses tested, no synergistic effects were observed for the combination of yohimbine plus naloxone.

The paradox of alpha 2 adrenergic regulation of prolactin (PRL) secretion. II. PRL-releasing action of the alpha 2 receptor antagonists

It has been suggested that the stimulation of the secretion of PRL by the alpha 2 adrenergic receptor antagonists (yohimbine, piperoxane) results from blockade of an inhibitory influence imposed on PRL release by the central alpha 2 receptors (7, 15). Our present results do not support these conclusions for the following reasons: 1) The effectiveness of the alpha 2 receptor antagonists yohimbine (YOH), rauwolscine (RAU), Wy 26392 and idazoxan (IDAZ) respectively to activate secretion of PRL was not related to their alpha 2 antagonist potencies. RAU was more effective in activation of PRL secretion than either YOH or Wy 26392 although it had a similar alpha 2 antagonist activity, while IDAZ, the most potent alpha 2 blocker among the four compounds, did not stimulate PRL secretion. 2) The PRL-releasing effect of YOH or Wy 26392 was reversed by the alpha 2 agonist clonidine but the same effect of RAU was not, speaking against a common alpha 2-mediated mechanism of action of the three antagonists. 3) The PRL-stimulating effect of YOH, RAU or Wy 26392 persisted following inhibition of NE synthesis and presumably release with FLA 63, DDC or combination of reserpine and DDC. 4) Conversely, we found no indication for an inhibiting influence of activation of the alpha 2 receptors on the secretion of PRL. We conclude that the stimulation of PRL secretion by the alpha 2 receptor antagonists is not derived from blockade of the central alpha 2 receptors but from other, not yet defined properties of the drugs.

An insulin-releasing property of imidazoline derivatives is not limited to compounds that block alpha-adrenoceptors

As we have demonstrated previously phentolamine stimulates the release of additional insulin from isolated mouse islets and raises plasma insulin levels in the whole rat. This effect was independent of the well known property of phentolamine to block alpha-adrenoceptors. In experiments on isolated pancreatic islets from mice we now demonstrate that tolazoline and antazoline which are chemically closely related to phentolamine, share its ability to potentiate insulin release. The following results were taken as evidence that this effect does not result from an alpha-adrenoceptor blocking action of imidazoline compounds. More than 10 times higher concentrations of phentolamine were required to liberate additional insulin from isolated islets than were effective in counteracting the inhibitory effect of clonidine on insulin release. The newly introduced alpha 2-adrenoceptor antagonist BDF 8933, which is an imidazoline derivative, stimulates insulin release as well, while the irreversible alpha-adrenoceptor blocking agent benextramine of different structure failed to do so, even when being present in concentrations blocking the alpha 2-adrenoceptor-mediated effects of clonidine. Antazoline shared the ability of phentolamine to stimulate insulin release despite having no or only very little alpha-adrenoceptor blocking activity. When used under our conditions, it almost entirely failed to alleviate the inhibition of insulin release induced by clonidine. We conclude that the response of the islet cells to imidazoline derivatives is not limited to those capable of blocking alpha-adrenoceptors. On the other hand, alpha-adrenoceptor blocking agents of different chemical structure fail to induce the release of additional insulin.(ABSTRACT TRUNCATED AT 250 WORDS)

5-HT1-like receptors requiring functional cyclo-oxygenase and 5-HT2 receptors independent of cyclo-oxygenase mediate contraction of the human umbilical artery

1. The interactions between 5-hydroxytryptamine (5-HT) and the antagonists ketanserin, methysergide and phentolamine were studied in isolated preparations of human umbilical artery (HUA) at physiological oxygen tension (Po2 approximately 15 mmHg) and at high PO2 (approximately 120 mmHg). 2. At physiological Po2 ketanserin, methysergide and phentolamine behaved as silent competitive antagonists of the 5-HT-induced contraction of HUA. pA2 values calculated by Schild analysis were 8.92, 8.52 and 6.37, respectively. 3. At high Po2, 5-HT-induced contractions were antagonised in a biphasic manner by ketanserin (0.1 microM); the response to low but not to high concentrations of 5-HT was resistant to blockade by ketanserin. The ketanserin-resistant component was abolished following cyclo-oxygenase inhibition by indomethacin (1 microM). 4. At high Po2, methysergide behaved as a partial agonist. Methysergide-induced contractions were inhibited but not abolished by indomethacin, and resistant to 5-HT2 receptor and alpha 1-adrenoceptor blockade. 5. At high Po2 the component of the response to 5-HT mediated by the ketanserin-resistant receptor was mimicked by the selective 5-HT1-like receptor agonist 5-carboxamidotryptamine (5-CT): 5-CT was 7 fold more potent than 5-HT. 6. At high Po2 the component of the response to 5-HT mediated by the ketanserin-resistant receptor was antagonised by phentolamine and the selective alpha 2-adrenoceptor antagonist Wy 26703. 7. These results suggest that (i) at physiological Po2 5-HT2 receptors almost exclusively mediate contractions induced by 5-HT, and (ii) at high Po2 the agonist potency order of 5-CT greater than 5-HT greater than methysergide suggests that ketanserin-resistant responses are mediated by 5-HT1-like receptors which require functional cyclo-oxygenase.

Failure of selective antagonists (CH-38083 and idazoxan) to distinguish between prejunctional and postjunctional alpha 2-adrenoreceptors

1. The prejunctional alpha 2-adrenoreceptor antagonist activity of CH-38083 (7,8-(methylenedioxi)-14-alpha-hydroxyalloberbane HCl), idazoxan and prazosin was determined against B-HT 920 on the tachycardia induced by cardiac nerve stimulation in pithed rats. Antagonism of cirazoline and B-HT 920 pressor responses was used to assess postjunctional alpha 1- and alpha 2-adrenoreceptor antagonist activities. 2. CH-38083 was more potent than idazoxan in blocking pre- and postjunctional alpha 2-adrenoreceptor sites. There was no difference between the activities of the two selective alpha 2-adrenoreceptor blocking agents on pre- and postjunctional alpha 2-adrenoreceptors. 3. These data classify CH-38083 as a potent and highly selective alpha 2-adrenoreceptor antagonist in vivo and further support evidence of the homogeneity of pre- and postjunctional alpha 2-adrenoreceptors.

Effect of CH-38083, a selective antagonist of alpha 2-adrenoceptors, on renal sympathetic function

The effects of 7,8-(methylenedioxi)-14-alpha-hydroxyalloberbane HCl (CH-38083), a structurally new, selective and potent alpha 2-adrenoceptor antagonist, and of idazoxan, were studied on both the spontaneous activity of the postganglionic sympathetic renal nerve and on the clonidine- or xylazine-induced inhibitory action in anaesthetized cats. The drug CH-38083 (30-200 micrograms/kg, i.v.) caused a sustained increase of the sympathetic activity and blood pressure. Larger doses of idazoxan (200-500 micrograms/kg, i.v.) were needed to induce similar effects. Both antagonists were effective in antagonizing the sympatho-inhibitory effect of clonidine or xylazine. The excitatory response to selective alpha 2-adrenoceptor antagonists suggests that the sympathetic output undergoes tonic inhibition due to a permanent alpha 2-adrenoceptor stimulation.

Yohimbine's anxiogenic action: evidence for noradrenergic and dopaminergic sites

Yohimbine (2.5 or 4 mg/kg) reduced the percentage of open arm entries and the percentage of time spent on the open arms displayed by rats on an elevated plus-maze indicating anxiogenic activity. These effects were reversed by the alpha 2-adrenoceptor agonist clonidine (0.01 mg/kg) and by the dopamine receptor agonist apomorphine (0.57 mg/kg). The following failed to reverse the effects of yohimbine: the selective alpha 2-adrenoceptor agonists, guanfacine (0.25 and 1 mg/kg), B-HT920 (0.025 and 0.1 mg/kg), B-HT933 (1 and 10 mg/kg); the beta-blocker propranolol (2.5 and 10 mg/kg); the alpha 1-adrenoceptor agonist phenylephrine; the D1 agonist SK&F 38393 (5 and 10 mg/kg) and the D2 agonist LY 171555 (0.5 and 1 mg/kg). Therefore, it is unlikely that activity at only the alpha 1, alpha 2, beta, D1 or D2 sites can entirely account for the anxiogenic actions of yohimbine in the elevated plus-maze. Evidence that clonidine affects the dopaminergic system and that apomorphine affects the noradrenergic system suggest that yohimbine may produce its anxiogenic response by activity on both the noradrenergic and dopaminergic systems.

Pharmacological analysis of postjunctional alpha-adrenoceptors mediating contractions to (-)-noradrenaline in the rabbit isolated lateral saphenous vein can be explained by interacting responses to simultaneous activation of alpha 1- and alpha 2-adrenoceptors

1. The pharmacological characteristics of the alpha-adrenoceptor population in the rabbit isolated saphenous vein has been examined with (-)-noradrenaline (NA), as principal agonist, and a number of antagonists with selectivity for either alpha 1- or alpha 2-adrenoceptors. 2. The rank order of potency of various agonists is consistent with a population of alpha 2-adrenoceptors; UK-14304 greater than (-)-noradrenaline = (-)-adrenaline greater than B-HT 920 = cirazoline greater than phenylephrine greater than amidephrine, but the rank order of pA2 values for the antagonists against (-)-noradrenaline: BDF-6143 greater than rauwolscine = prazosin greater than CH-38083 = YM-12617 greater than Wy-26703 = phentolamine greater than corynanthine, is indicative of a mixed population of alpha 1- and alpha 2-adrenoceptors or, alternatively, a new subtype with characteristics of both the alpha 1- and alpha 2-subtypes. 3. Further evidence for two discrete populations of alpha-adrenoceptors is provided by, (a) the potent but non-competitive effect of prazosin against (-)-noradrenaline, (b) the presence of a component of the contractions elicited by NA and phenylephrine which is resistant to the selective alpha 2-adrenoceptor antagonists rauwolscine and CH-38083: these responses were inhibited by the selective alpha 1-adrenoceptor antagonists prazosin and YM-12617, but not by the selective alpha 2-adrenoceptor antagonist BDF-6143 and, (c) the relative potency of the yohimbine diastereoisomers rauwolscine and corynanthine against NA, phenylephrine and UK-14304. 4. In spite of the overwhelming evidence for a population of postjunctional alpha 2-adrenoceptors, prazosin was similarly effective against all agonists and failed to discriminate between those with putative selectivity for alpha 1- and alpha 2-adrenoceptors. This suggests an interaction of the effects of agonists at the two alpha-adrenoceptor subtypes. 5. An attempt has been made to reconcile a number of paradoxical observations with regard to the identification of postjunctional alpha 2-adrenoceptors in vitro, and it is suggested that in many of the isolated blood vessels presently available for examination both subtypes reside on the same smooth muscle cell. The pharmacological consequences of multiple subtypes of receptors mediating the same response is considered.

Evidence that the population of postjunctional-adrenoceptors mediating contraction of smooth muscle in the rabbit isolated ear vein is predominantly alpha 2

1. Noradrenaline (NA), phenylephrine and UK-14304 elicited concentration-dependent contractions of the rabbit isolated ear vein of similar maximal magnitude. The rank order of potency, UK-14304 greater than noradrenaline greater than phenylephrine, is consistent with that of an effect mediated through an alpha 2-subtype. 2. The potent and highly selective alpha 1-adrenoceptor antagonists prazosin and YM-12617, at concentrations as high as 1 microM, produced less than a 4 fold rightward displacement of the NA concentration-response curve. 3. The selective alpha 2-adrenoceptor antagonists rauwolscine, Wy-26703 and CH-38083 antagonized responses to noradrenaline in a competitive manner. For all three antagonists, the pA2 values were consistent with an effect at alpha 2-adrenoceptors. However, 0.1 microM YM-12617 increased the potency of rauwolscine 2 fold indicating the presence of a small population of postjunctional alpha 1-adrenoceptors. 4. The relative antagonist potency of the yohimbine diastereoisomers rauwolscine and corynanthine against noradrenaline (rauwolscine 30 fold greater than corynanthine) is also consistent with an effect at alpha 2-adrenoceptors. 5. Contractions elicited by noradrenaline in the rabbit isolated ear vein appear to be mediated predominantly by postjunctional alpha 2-adrenoceptors.

Excitatory and inhibitory cholinergic effects of yohimbine on isolated guinea-pig small intestine

The interaction of yohimbine with the cholinergic intestinal system was investigated in the isolated guinea-pig ileum using a wide range of drug concentrations from 3 x 10(-13) to 2 x 10(-4) g/ml. Low concentrations of yohimbine (3 x 10(-13) to 3 x 10(-11) g/ml) caused dose-dependent contractions of the ileal longitudinal muscle, which were potentiated by eserine 1 x 10(-8) g/ml and prevented by tetrodotoxin 1 x 10(-6) g/ml or by atropine 1 x 10(-12) g/ml; methysergide and diphenydramine were ineffective up to 3 x 10(-7) g/ml dose. Submaximal stimulatory responses evoked by twitch stimulation or by acetylcholine were significantly potentiated by the same concentrations of yohimbine (3 x 10(-13) to 3 x 10(-11) g/ml) and blocked by atropine 1 x 10(-12) g/ml. By contrast, high concentrations of yohimbine (1 x 10(-6) to 2 x 10(-4) g/ml) displayed dose-dependent inhibitory effects on cholinergic responses. The stimulant effect of yohimbine seems to be indirect and mediated by the increase in the release of acetylcholine, while the inhibitory action may be due to a molecular interaction with the muscarinic receptors allowing non-specific receptor blockade.

A proconvulsant action of selective alpha 2-adrenoceptor antagonists

The effects of alpha 2-adrenoceptor agonists and antagonists were examined on seizure thresholds determined by intravenous infusions of convulsants in rodents. alpha 2-Adrenoceptor antagonists were proconvulsant; dose dependently reducing the threshold for pentylenetetrazol- or bicuculline-induced tonic seizures. Strychnine-induced tonic seizures were unaffected. The alpha 2-adrenoceptor agonists clonidine, BHT-933 and UK 14,304 did not modify pentylenetetazol-induced seizures at low or moderate doses but at high doses clonidine and BHT-933 were proconvulsant. The facilitatory effect of alpha 2-adrenoceptor antagonists on pentylenetetrazol-induced tonic seizures was blocked by clonidine or UK 14,304. The proconvulsant action of alpha 2-adrenoceptor antagonists was contrasted with that of ethyl-beta-carboline-3-carboxylate, a benzodiazepine receptor contragonist, which markedly reduced the threshold for seizure initiation rather than the tonic seizure threshold. The selective facilitatory action of alpha 2-adrenoceptor blockade on tonic seizures suggests that a noradrenergic mechanism is involved in the control of seizure propagation rather than seizure initiation.

Selective agonists and antagonists for 5-hydroxytryptamine receptor subtypes, and interactions with yohimbine and FG 7142 using the elevated plus-maze test in the rat

The effects of some 5-HT receptor ligands were investigated on measures of anxiety in an elevated plus-maze test in the rat. Quipazine (2 and 4 mg kg-1), a non-specific 5-HT agonist and ritanserin (0.25-10 mg kg-1), a 5-HT2 receptor antagonist displayed anxiogenic profiles by reducing both of the measures of anxiety used in this test. Two 5-HT1A receptor ligands, buspirone (4 and 8 mg kg-1) and ipsapirone (2.5-10 mg kg-1) and the 5-HT1 agonist, RU 24969 (0.1875-1.5 mg kg-1) significantly reduced only the percentage of time spent on the open arms. (-)-Propranolol (5 and 10 mg kg-1), a 5-HT1 receptor antagonist significantly reduced only the percentage of entries made onto the open arms. Metergoline (4 mg kg-1), a non-specific 5-HT antagonist displayed anxiolytic effects in this test by increasing both measures of anxiety. The 5-HT1A receptor agonist, 8-OH-DPAT (0.0625-0.25 mg kg-1) had no effect on either of the measures of anxiety. The results from the non-specific ligands (quipazine and metergoline) are consistent with the theory that a reduction in 5-HT function reduces anxiety. However, in spite of their more selective effects on 5-HT receptors the results in this test from the more specific ligands are not consistent with a strong involvement of any single receptor subtype. The interaction studies with yohimbine and FG 7142 (beta-carboline-3-carboxylate methylamide) provided no clear evidence for a major role of 5-HT pathways in the mediation of their anxiogenic effects.

Pharmacological profile of a new potent and specific alpha 2-adrenoceptor antagonist, L-657,743

L-657,743,(2S,12bS)1',3'-dimethylspiro(1,3,4,5',6,6',7,12 b-octahydro-2H- benzo[b]furo[2,3-a]quinolizine)-2,4'-pyrimidin-2'-one, was tested in several in vitro and in vivo models for alpha 2-adrenoceptor antagonism. L-657,743 exhibited a high affinity (less than or equal to 1 nM) for alpha 2-adrenoceptors labelled by [3H] rauwolscine or [3H]clonidine with a 240-fold selectivity versus alpha 1-adrenoceptors labelled by [3H]prazosin. L-657,743 was a potent, selective, and competitive alpha 2-adrenoceptor antagonist in the rat isolated vas deferens (pA2 = 9.3 vs. clonidine; pA2 = 7.1 vs methoxamine). In vivo, L-657,743 potently blocked clonidine-induced mydriasis in the rat and stimulated cerebrocortical norepinephrine synthesis, two indices of central alpha 2-adrenoceptor antagonism. L-657,743 exhibited a comparatively low affinity for several monoamine receptor subtypes (D1, D2, 5-HT1, 5-HT2) in radioligand binding assays in vitro and a comparatively low potency to alter the synthesis of brain DA and 5-HT in vivo indicating a marked alpha 2-specificity versus other monoamine receptor mechanisms. Compared to yohimbine, L-657,743 had considerably higher alpha 2-antagonist potency and alpha 2/alpha 1 selectivity and was significantly more alpha 2-specific (i.e., vs. DA, 5-HT receptors).

Some pharmacological properties of Wy 27127 a more selective alpha 2:alpha 1-adrenoceptor antagonist than idazoxan in vitro

Wy 27127 and idazoxan were approximately equipotent as antagonists at alpha 2-adrenoceptors as estimated by their ability to block clonidine-induced inhibition of electrically-evoked contractions of the rat isolated vas deferens. Idazoxan was seven times as potent as Wy 27127, as an antagonist at alpha 1-adrenoceptors as indicated by blockade of methoxamine-induced contractions of the rat isolated anococcygeus muscle. Thus, the alpha 2:alpha 1 selectivity ratio, as calculated from these tests was 407 for Wy 27127 and 76 for idazoxan. Wy 27127 and idazoxan were equipotent in enhancing stimulation-evoked overflow of tritium from rabbit isolated pulmonary arteries preloaded with [3H]-noradrenaline as expected for alpha 2-adrenoceptor antagonists. At higher concentrations both compounds reduced the stimulation-evoked contraction of the pulmonary artery but idazoxan was 15 times as potent as Wy 27127 in this respect. Neither compound had marked antagonist actions at 5-hydroxytryptamine (D), muscarinic, presynaptic dopamine or histamine (H1) receptors or at beta 1-adrenoceptors. Thus, idazoxan and Wy 27127 were equipotent alpha 2-adrenoceptor antagonists in vitro, however, the alpha 2:alpha 1 selectivity of Wy 27127 was considerably greater than that of idazoxan by virtue of weaker alpha 1-adrenoceptor antagonism.

The effect of age on the sensitivity of pre- and postsynaptic alpha-adrenoceptors to agonists and antagonists in the rat

Age-related changes in presynaptic alpha-2 and postsynaptic alpha-1 adrenoceptors have been determined using the rat isolated vas deferens and the thoracic aorta, respectively. The IC50 values of clonidine, B-HT 933 and UK 14,304 for inhibition of the electrically evoked contractions of the vas deferens were significantly higher in 50 week old rats when compared with rats of 5 weeks. Similarly, EC50 values for the contraction of the thoracic aorta by noradrenaline, methoxamine and phenylephrine were significantly increased in 50 week old rats compared with 5 week old rats. No age-related changes in the potency of the selective alpha-2 adrenoceptor antagonists yohimbine and Wy 26392 were detected in the vas deferens. Similarly, there were no age-related changes in the alpha-1 adrenoceptor antagonist potency of indoramin or prazosin on the aorta. The results of the present study suggest that the potency of both alpha-1 and alpha-2 adrenoceptor agonists, as measured by their respective EC and IC50 values decreases with increasing age.

Heterogeneity between soluble human and rabbit splenic alpha 2-adrenoceptors

The pharmacological and biochemical characteristics of soluble alpha 2-adrenoceptors were investigated to determine whether differences observed in membranes were maintained in solution and to probe the nature of any such differences. alpha 2-Adrenoceptors were solubilized from purified plasma membrane preparations of human and rabbit spleen using digitonin. [3H]yohimbine bound to one population of alpha 2-adrenoceptors in the preparations with dissociation constants of 2.4 nM and 7.8 nM respectively. The pharmacological profile of the alpha 2-adrenoceptors has been examined. Upon solubilization the affinity of the alpha 2-adrenoceptors for yohimbine was unchanged. In contrast, the potency of idazoxan and RX 811066 were increased, whereas the potency for prazosin (human only), phentolamine and WY 26392 was decreased 2-3-fold. The potency of the agonists oxymetazoline, UK 14304 and adrenaline were all reduced upon solubilization of alpha 2-adrenoceptors. The selectivity of yohimbine, idazoxan, RX 811066 and WY 26392 for human rather than rabbit alpha 2-adrenoceptors was maintained in solution. Possible sources of heterogeneity between human and rabbit alpha 2-adrenoceptors were investigated. The protein structure was probed by comparing the susceptibility of the receptors to inactivation by sulphydryl modifying agents. No differences were observed in the potency of N-ethylmaleimide or p-chloromercuribenzoate to inactivate the receptor. The carbohydrate component of the receptors was investigated using agarose-linked lectins. Rabbit splenic alpha 2-adrenoceptors had a lower affinity for the lectins wheatgerm agglutinin (Triticum vulgaris) and soybean (Glycine max) which bind the sugars N-acetyl d-glucosamine and N-acetyl d-galactosamine respectively. These findings suggest that heterogeneity of the alpha 2-adrenoceptor derives from its structural characteristics rather than its environment in the membrane.

Evidence that endogenous catecholamines can regulate acetylcholine release in a sympathetic ganglion

The objective of this study was to test whether activation of alpha-adrenoceptors by endogenously released catecholamines alters the release of acetylcholine (ACh) from the cat superior cervical ganglion. The alpha-adrenoceptor agonists noradrenaline and clonidine depressed evoked ACh release; this effect was concentration-dependent; it was apparent during preganglionic stimulation at 20 Hz, but not so at lower frequencies of stimulation. The inhibitory effect of noradrenaline on evoked ACh release was reversed by yohimbine, by phentolamine and, to a lesser extent, by prazosin. Thus, exogenous amines can depress evoked ACh release by an action on presynaptic alpha-adrenoceptors. To determine if activation of these receptors by endogenous amines inhibits ACh release, we tested whether the alpha-adrenoreceptor antagonists enhance ACh release. Yohimbine and phentolamine increased evoked ACh release during preganglionic stimulation at 20 Hz, but not during stimulation at 5 Hz, suggesting that endogenous, like exogenous, amine can depress evoked ACh release from preganglionic nerve terminals.

L-654,284 a new potent and selective alpha 2-adrenoceptor antagonist

L-654,284 [(2R, 12bS)-N-(1,3,4,6,7,12b-hexahydro-2H-benzo[b]-furo[2,3-a] quinolizine-2-yl)-N-methyl-2-hydroxyethanesulfonamide) was tested in several in vitro and in vivo models for alpha 2-adrenoceptor antagonist activity and compared to several reference agents. In vitro L-654,284 completed for the binding of 3H-clonidine or 3H-rauwolscine (Ki's 0.8 nM, 1.1 nM) and blocked the presynaptic effects of clonidine in the rat isolated vas deferens (pA2, 9.1). L-654,284 exhibited marked alpha 2-vs. alpha 1-adrenoceptor selectivity in vitro, inhibiting 3H-prazosin binding with a Ki of 110 nM and blocking the effects of methoxamine on the vas deferens with a pA2 of 7.5. In vivo L-654,284 at 22 nmoles/kg i.v. doubled the ED50 of clonidine to produce mydriasis in rats. Given orally, the potency of L-654,284 in this test was reduced by a factor of 5.5. L-654,284 also potently increased cerebrocortical NE turnover in the rat, another in vivo index of alpha 2-adrenoceptor blockade in the central nervous system. In the periphery, L-654,284 demonstrated alpha 2-adrenoceptor selectivity by preferentially blocking the pressor effects of UK 14304 versus those of methoxamine in the pithed rat. Overall, L-654,284 was generally a more potent alpha 2-adrenoceptor antagonist than RX 781094 with comparable alpha 2/alpha 1 selectivity and was several times more potent and alpha 2-selective than WY 26703 or yohimbine. In addition, L-654,284 had better (5-6 times) oral bioavailability than RX 781094 or WY 26703.

Characterization of flufylline, fluprofylline, ritanserin, butanserin and R 56413 with respect to in-vivo alpha 1-,alpha 2- and 5-HT2-receptor antagonism and in-vitro affinity for alpha 1-,alpha 2- and 5-HT2-receptors: comparison with ketanserin

The experimental drugs butanserin (R 53393), ritanserin (R 55667), R 56413, flufylline (Sgd 195/78) and fluprofylline (Sgd 144/80) were evaluated with respect to their antagonism at postjunctional alpha 1- and alpha 2-adrenoceptors and 5-HT2-receptors in pithed rats. Moreover, affinity for [3H]mianserin, [3H]prazosin and [3H]yohimbine binding sites was assessed using rat brain preparations. In all experiments ketanserin was taken as a reference compound. It is concluded that of the compounds investigated butanserin is the most potent and selective alpha 1-adrenoceptor antagonist, whereas ritanserin was found to be a potent and selective 5-HT2-antagonist. Of the other compounds, fluprofylline was a very selective though not very potent alpha 1-adrenoceptor antagonist. The other compounds were less active and less selective in this respect.

Heterogeneity of mammalian alpha 2-adrenoceptors delineated by [3H]yohimbine binding

[3H]Yohimbine binding to membrane preparations of human colon, cerebral cortex, kidney, spleen and platelets was compared with binding to preparations of animal tissues (rabbit spleen, kidney and cerebral cortex; rat cerebral cortex; guinea-pig and cat spleen). Specific binding to all preparations was saturable and indicative of binding to a uniform population of sites. The equilibrium dissociation constants (KD) of [3H]yohimbine ranged from 1.6 to 2.6 nM for human tissue and from 5.1 to 9.4 nM for the animal tissues. Binding to all tissues was displaced by drugs with an order of potency yohimbine greater than phentolamine greater than prazosin, indicating an alpha 2-adrenoceptor classification of the labelled sites. Whilst certain drugs (phentolamine, corynanthine) possessed similar affinities for all alpha 2-adrenoceptors, others (prazosin, idazoxan, WY 26392) exhibited differential potencies for alpha 2-adrenoceptors in certain species. The pharmacological characteristics of human alpha 2-adrenoceptors were conserved within the tissues examined. These results suggest that human alpha 2-adrenoceptors differ in a number of ways from those present in tissues from the other mammalian species examined. The possible existence of a spectrum of alpha 2-adrenoceptors is discussed in the light of these findings.

Neuronal dopamine receptors of the rabbit ear artery: pharmacological characterization of the receptor

Dopamine and apomorphine were examined in the rabbit isolated perfused ear artery for both direct effects on vascular smooth muscle and effects on the response to field stimulation of sympathetic nerve terminals. The neuroinhibitory effect of both dopamine (EC50 = 37 nM) and apomorphine (EC50 = 44 nM) occurred at concentrations which did not produce vasoconstriction. The neuroinhibitory effect of dopamine was shown to be due to inhibition of noradrenaline release by measurement of 3H-overflow from prelabelled tissues. At relatively high concentrations dopamine produced vasoconstriction. In a superfused segment of ear artery, dopamine was found to be a full agonist at the alpha 1-adrenoreceptor, with an EC50 (15 microM) about 75 fold higher than the EC50 for noradrenaline. At concentrations up to 3 microM, apomorphine had no vasoconstrictor activity in the perfused ear artery. Representative examples of several classes of dopamine antagonists, including the phenothiazines, butyrophenones, diphenylbutylpiperidines and benzamides produced competitive antagonism of dopamine or apomorphine-induced inhibition, with nearly identical Kb values against these two agonists. The pharmacological characteristics of the neuronal dopamine receptor on the rabbit ear artery would indicate this receptor to be typical of the D2 subclass, and this tissue to be a useful model for quantitative studies on dopamine receptor agonists and antagonists.

A difference in the affinity of some selective alpha 2-adrenoceptor antagonists when compared on isolated vasa deferentia of rat and rabbit

The alpha 2-adrenoceptor antagonist potencies of the benzoquinolizines (Wy 26 703, Wy 25 309, Wy 26 392), the benzodioxans (RX 781 094, RS 21 361), yohimbine and rauwolscine have been compared at presynaptic alpha 2-adrenoceptors in the isolated vasa deferentia of the rat and rabbit. Yohimbine and rauwolscine are of equal potency as antagonists in both the rat and rabbit against the agonists clonidine or UK 14304. The benzoquinolizines and benzodioxans are very weak antagonists of clonidine or UK 14304 at the presynaptic alpha 2-adrenoceptors of the rabbit vas deferens when compared to their potency at the presynaptic alpha 2-adrenoceptors of the rat vas deferens. This suggests that the presynaptic alpha 2-adrenoceptors present in the rat vasa deferentia may be different from those present in the rabbit vasa deferentia.