Does rilmenidine act in vivo on central alpha 2-adrenoceptors modulating noradrenaline release?

Noradrenaline release evoked by electrical stimulation was recorded in the rat hypothalamus by in vivo electrochemistry. Rilmenidine (0.3-10 mg kg-1 i.v.) did not diminish this evoked release while clonidine induced a dose-dependent decrease. These results further suggest that the antihypertensive action of rilmenidine is not mediated by central alpha 2-adrenoceptors and might explain why, unlike clonidine, rilmenidine does not have sedative effects.     

Clonidine produces mydriasis in conscious mice by activating central alpha 2-adrenoceptors

Intraperitoneal (i.p.) injection of the alpha 2-adrenoceptor agonist clonidine (1-3000 micrograms/kg) produced dose-dependent pupil dilatation in conscious C57/Bl/6 mice with an ED50 of 54 micrograms/kg (95% confidence limits 40-74 micrograms/kg). This response was rapid in onset and of approximately 30 min duration. The alpha 2-adrenoceptor antagonists idazoxan (1 or 3 mg/kg i.p.) and yohimbine (1 or 3 mg/kg i.p.) both produced dose-related miosis, but the alpha 1- and beta-adrenoceptor antagonists prazosin (1 or 3 mg/kg i.p.) and pindolol (1 or 3 mg/kg i.p.) were without effect. These doses of idazoxan and yohimbine potently reversed the mydriasis induced by clonidine (100 micrograms/kg i.p.), while prazosin and pindolol were again ineffective. Clonidine-induced mydriasis was also unaltered by the 5-HT antagonists, methysergide (2.5 mg/kg i.p.) and ketanserin (0.1 mg/kg i.p.) or 0.1 mg/kg i.p. of the dopamine antagonists, haloperidol, SCH 23390 and BRL 34778. A dose of 0.25 microgram clonidine, which was ineffective when administered i.p., produced marked mydriasis after intracerebroventricular (i.c.v.) injection. In addition, the mydriasis produced by i.p. injection of clonidine (100 micrograms/kg) was abolished by i.c.v. dosing of 2.5 micrograms idazoxan or yohimbine, but again not by prazosin or pindolol. Together, these data provide strong evidence to indicate that clonidine-induced mydriasis is exclusively mediated via central alpha 2-adrenoceptors and that this response provides a useful model for studying the function of these receptors.     

Comparison of central and peripheral alpha1-adrenoceptors

Summary A series of alpha-adrenergic agonists and antagonists having diverse chemical structure was examined for both central and peripheral alpha₁-adrenoceptor activity. The agonists tested included several novel aminotetralin derivatives which were potent and selective alpha₁-agonists. Peripheral alpha₁-activity was determined in the isolated rabbit ear artery; central alpha₁-receptor affinity was measured as the ability to inhibit ³H-WB 4101 binding to rat brain homogenates. In the agonist series, an excellent correlation between peripheral alpha₁-activity and central alpha₁-affinity was obtained, providing that partial agonists were excluded. Likewise, the receptor dissociation constant for blockade of the peripheral alpha₁-adrenoceptor correlated well with affinity for the central receptor for all of the alphaantagonists. These data support the conclusion that central and peripheral alpha₁-adrenoceptors are similar or identical.     

Acute and chronic effects of rilmenidine on baroreflex function in conscious dogs

The effects of acute and chronic administration of rilmenidine on blood pressure, heart rate and baroreflex function were assessed in two groups of six conscious dogs. Baroreflex function was evaluated using increasing doses of glyceryl trinitrate and phenylephrine to decrease and increase blood pressure respectfully. Acute administration of rilmenidine (1 mg/kg p.o.) reduced (P less than 0.05) blood pressure and heart rate, and enhanced (P less than 0.05) baroreflex function to increases in blood pressure with phenylephrine, but not to decreases in blood pressure by glyceryl trinitrate. Following chronic administration of rilmenidine, blood pressure remained lowered while heart rate and baroreflex function returned to control values.     

The vascular relaxant effects of guanabenz are not mediated by alpha 2-adrenoceptors in rat aortic rings

The present experiments show that the alpha 2-adrenoceptor agonist guanabenz produced a concentration-dependent relaxation in 5-hydroxytryptamine- and noradrenaline-preconstricted rat aortic rings, but not when tone was raised by high potassium in the medium. Similar findings were made for pinacidil. In 5-hydroxytryptamine-preconstricted preparations the relaxant effect of guanabenz was not affected by yohimbine, propranolol or by removal of the endothelium. In noradrenaline-preconstricted rings the relaxant effects of guanabenz and pinacidil were significantly antagonized by tetraethylammonium. The results suggest that guanabenz might act in the muscle tissue by opening potassium channels or antagonism of calcium mobilization from intracellular stores.     

Blockade of central alpha 2-adrenoceptors enhances the carotid occlusion response in dogs

Intravertebral artery injection of yohimbine at total doses of 10, 50, 100 and 200 micrograms caused dose-related enhancement of the systemic pressor response to bilateral carotid artery occlusion (BCO) in dogs; the response was depressed by 5000 micrograms yohimbine. Basal blood pressure was increased by 100 and 200 micrograms yohimbine, and decreased by 5000 micrograms. Intravertebral artery injection of rauwolscine, 5 micrograms enhanced the BCO response but had no effect on basal blood pressure. Corynanthine, 200 or 500 micrograms was ineffective. The pressor response to intravenously administered noradrenaline was not affected by any of the drugs. It is concluded that enhancement of the BCO response by yohimbine and rauwolscine is caused by blockade of central alpha 2-adrenoceptors.     

Evidence for pharmacological similarity between alpha 2-adrenoceptors in the vas deferens and central nervous system of the rat.

Seven alpha 2-adrenoceptor antagonists with diverse chemical structures have been examined for their effects at alpha 2-adrenoceptors in the vas deferens and central nervous system of the rat. Antagonist potency assessed against the presynaptic alpha 2-adrenoceptor agonist action of clonidine in the isolated vas deferens (RX 781094 greater than Wy 26703 greater than yohimbine greater than rauwolscine greater than piperoxan greater than mianserin greater than RS 21361) was highly correlated with the ability of these drugs to displace saturable [3H]-RX 781094 binding from cerebral cortex membranes. Similarly, antagonist potency in the vas deferens was highly correlated with antagonist activity in reversing the centrally-mediated mydriasis induced by the selective alpha 2-adrenoceptor agonist, guanoxabenz, in pentobarbitone-anaesthetized rats. The results indicate that the presynaptic alpha 2-adrenoceptors in the vas deferens are pharmacologically similar to characterized these alpha 2-adrenoceptors in the central nervous system of the rat.     

Anticonvulsant effects of clonidine mediated through central alpha2-adrenoceptors

The effects of centrally (0.3-30 ng/kg) and peripherally (0.01 microgram/kg-1.0 mg/kg) administered clonidine on PTZ-induced seizures was studied in rats. At low doses, dose-dependent decreases in the duration of seizures was observed but at higher doses the duration returned to control levels. Anticonvulsant activity was antagonized by yohimbine (100 microgram/kg i.p.) indicating alpha2-adrenoceptor involvement, whereas the second phase of the response was antagonized by prazosin (10 ng/kg i.c.v.) indicating that it involved alpha1-adrenoceptors.     

Alpha 2-adrenoceptors in the enteric nervous system: a study in alpha 2A-adrenoceptor-deficient mice

Mammals possess three types of alpha(2)-adrenoceptor, alpha(2A), alpha(2B) and alpha(2C). Our aim was to determine the type of alpha(2)-adrenoceptor involved in the control of gastrointestinal motility. In transmitter overflow experiments, myenteric plexus longitudinal muscle (MPLM) preparations of the ileum were preincubated with [(3)H]-choline and then superfused. The alpha(2)-adrenoceptor agonist medetomidine reduced the electrically evoked overflow of tritium from preparations taken from wild type but not alpha(2A)-adrenoceptor-knockout mice. In a second series of overflow experiments, MPLM preparations were preincubated with [(3)H]-noradrenaline and then superfused. Again medetomidine reduced the electrically evoked overflow of tritium from wild type but not alpha(2A)-knockout preparations. In organ bath experiments, medetomidine reduced electrically evoked contractions of segments of the ileum from wild type but not alpha(2A)-knockout mice. In each of these three series, phentolamine antagonized the effect of medetomidine in wild-type preparations with greater potency than rauwolscine. In conscious mice, gastrointestinal transit was assessed by means of an intragastric charcoal bolus. In alpha(2A)-knockout mice, the speed of gastrointestinal transit was doubled compared to wild-type. Medetomidine, injected intraperitoneally, slowed gastrointestinal transit in wild type but not alpha(2A)-knockout mice. We conclude that the cholinergic motor neurons of the enteric nervous system of mice possess alpha(2)-heteroreceptors which mediate inhibition of acetylcholine release, of neurogenic contractions and of gastrointestinal transit. The noradrenergic axons innervating the intestine possess alpha(2)-autoreceptors. Both hetero- and autoreceptors are exclusively alpha(2A). It is the alpha(2A)-adrenoceptor which in vivo mediates the inhibition of intestinal motility by the sympathetic nervous system.     

Effects of peripheral alpha 1- and alpha 2-adrenoceptor agonists on baroreceptor responsiveness in conscious dogs

Baroreceptor responsiveness was investigated in conscious dogs following increasing doses (i.v.) of the selective alpha-adrenoceptor agonists methoxamine (alpha 1) and oxymetazoline (alpha 2), in the presence and absence of beta-adrenoceptor blockade. The study was repeated in another group of dogs with background afferent baroreceptor nerve activity reduced by continuous infusion of sodium nitroprusside. Both agonists dose dependently increased mean arterial pressure and reflexly decreased heart rate. In dogs pretreated with a beta-adrenoceptor antagonist a correlation between increase in mean arterial pressure (increase up to 70 mmHg) and increase in heart period (baroreceptor responsiveness) indicated no difference in the regression lines between methoxamine and oxymetazoline for both the normotensive and the sodium nitroprusside groups. However, in the dogs not pretreated with a beta-adrenoceptor antagonist the slope of the regression line for oxymetazoline was steeper than that for methoxamine (P less than 0.01) in the normotensive group. In the sodium nitroprusside group the regression line for oxymetazoline was situated significantly to the left of the methoxamine line (P less than 0.05). It is suggested that this greater bradycardic response to the alpha 2-adrenoceptor agonist oxymetazoline was caused by suppression of the cardiac sympathetic component (presynaptic modulation of noradrenaline release) in addition to the vagal activation and the sympathetic withdrawal component of the reflex. This indicates that drugs with alpha 2-adrenoceptor agonistic activity can influence a reflex physiological situation under conditions of low sympathetic nerve activity.     

Comparison of agonistic and antagonistic actions of guanabenz and guanfacin on alpha 1 and alpha 2-adrenoceptors in isolated smooth muscles

The effects of guanabenz, guanfacine and clonidine on alpha-adrenoceptors were investigated in isolated rat vas deferens and rabbit aortic strip. All 3 drugs at low concentrations (10(-9)-10(-8) M) caused inhibition of twitch responses of the rat vas deferens induced by nerve stimulation. These effects were competitively antagonized by yohimbine. Guanfacine and clonidine at relatively high concentrations (10(-6)-10(-4) M) produced contractions of the rat vas deferens which were antagonized by prazosin. These contractile responses were not much affected by denervation. Prazosin-sensitive contractions by guanfacine and clonidine were also observed in the rabbit aortic strip, which were not affected by reserpine pretreatment. In both tissues, the intrinsic activity of guanfacine was almost identical to that of norepinephrine, whereas that of clonidine was less than one half. Guanabenz and clonidine showed a competitive antagonistic effect against norepinephrine and phenylephrine in both the rat vas deferens and rabbit aorta, the antagonisms being similar in potency. The results indicate that all 3 drugs are potent agonists on the presynaptic alpha 2-adrenoceptor. In contrast, on the postsynaptic alpha 1-adrenoceptor, guanfacine and guanabenz showed only agonistic and antagonistic actions, respectively, whereas clonidine exhibited partial agonistic characteristics.     

Functional evidence for heterogeneity of peripheral prejunctional alpha 2-adrenoceptors.

1. We have examined the potencies of a series of alpha 2-adrenoceptor antagonists in functional studies of prejunctional alpha 2-adrenoceptors in rat atrium and vas deferens, and compared potencies with affinities for the alpha 2A-ligand binding site of human platelet and the alpha 2B-site of rat kidney. 2. Antagonist potency in rat atrium was expressed as an EC30 (concentration producing 30% increase in the stimulation-evoked overflow of tritium in tissues pre-incubated with [3H]-noradrenaline). Antagonist potency in rat vas deferens was expressed as a pA2 or KB at antagonizing the inhibition by the alpha 2-adrenoceptor agonist xylazine of the isometric twitch to a single stimulus, or as an EC30. 3. In ligand binding studies, Ki values were obtained for the displacement by alpha-adrenoceptor antagonists of [3H]-yohimbine binding to human platelet or rat kidney membranes. 4. In functional studies, three antagonists (ARC 239, prazosin and chlorpromazine) distinguished between prejunctional alpha 2-adrenoceptors of rat atrium (EC30) and rat vas deferens (pA2) and showed 49, 12 and 7 times higher potency in rat atrium, respectively. ARC 239 was also 17 times more potent in rat atrium than rat vas deferens when EC30 values were compared. 5. The correlation of affinity for the alpha 2A-site of human platelet was better with prejunctional potency in rat vas deferens than rat atrium. 6. The correlation of affinity for the alpha 2B-site of rat kidney was better with prejunctional potency in rat atrium than rat vas deferens.(ABSTRACT TRUNCATED AT 250 WORDS)     

Species-selective binding of [3H]-idazoxan to alpha 2-adrenoceptors and non-adrenoceptor, imidazoline binding sites in the central nervous system.

1. We have used the imidazoline derivative [3H]-idazoxan to define alpha 2-adrenoceptors and non-adrenoceptor, imidazoline binding sites in cerebral cortex membranes of calf, mouse, rat, guinea-pig and man. 2. Competition experiments using the selective alpha-adrenoceptor drugs, rauwolscine and corynanthine, indicated that [3H]-idazoxan bound to a single population of sites in the calf and mouse membranes. However, [3H]-idazoxan also labelled non-adrenoceptor, imidazoline binding sites in the rat (15%), guinea-pig (30%) and human (40%) cerebral cortex membranes. 3. Competition experiments with adrenaline and cirazoline in the guinea-pig cortex, verified [3H]-idazoxan binding to both alpha 2-adrenoceptors and to non-adrenoceptor, imidazoline binding sites. 4. It has been postulated by several groups that [3H]-idazoxan may possess partial agonist activity. To investigate this further, saturation experiments were performed in the cerebral cortex membranes of all five species in the absence and presence of 300 microM guanosine triphosphate (GTP). GTP had no effect on [3H]-idazoxan binding in guinea-pig cerebral cortex; in both rat and mouse membranes 300 microM GTP increased the dissociation constant for [3H]-idazoxan by 2-3 fold without significantly affecting the Bmax. GTP reduced the Bmax by approximately 30% and 60% in calf and human cerebral cortex membranes, respectively, without significantly altering the Kd. 5. Saturation experiments were performed in the calf cerebral cortex membranes in the absence and presence of 300 microM GTP with the selective alpha 2-adrenoceptor agonist [3H]-clonidine and the selective muscarinic antagonist [3H]-quinuclidinyl benzilate (QNB).(ABSTRACT TRUNCATED AT 250 WORDS)     

Cardiovascular effects of rilmenidine, moxonidine and clonidine in conscious wild-type and D79N alpha2A-adrenoceptor transgenic mice

1. We investigated the cardiovascular effects of rilmenidine, moxonidine and clonidine in conscious wild-type and D79N alpha2A-adrenoceptor mice. The in vitro pharmacology of these agonists was determined at recombinant (human) alpha2-adrenoceptors and at endogenous (dog) alpha2A-adrenoceptors. 2. In wild-type mice, rilmenidine, moxonidine (100, 300 and 1000 microg kg(-1), i.v.) and clonidine (30, 100 and 300 microg kg(-1), i.v.) dose-dependently decreased blood pressure and heart rate. 3. In D79N alpha2A-adrenoceptor mice, responses to rilmenidine and moxonidine did not differ from vehicle control. Clonidine-induced hypotension was absent, but dose-dependent hypertension and bradycardia were observed. 4. In wild-type mice, responses to moxonidine (1 mg kg(-1), i.v.) were antagonized by the non-selective, non-imidazoline alpha2-adrenoceptor antagonist, RS-79948-197 (1 mg kg(-1), i.v.). 5. Affinity estimates (pKi) at human alpha2A-, alpha2B- and alpha2C-adrenoceptors, respectively, were: rilmenidine (5.80, 5.76 and 5.33), moxonidine (5.37, <5 and <5) and clonidine (7.21, 7.16 and 6.87). In a [35S]-GTPgammaS incorporation assay, moxonidine and clonidine were alpha2A-adrenoceptor agonists (pEC50/intrinsic activity relative to noradrenaline): moxonidine (5.74/0.85) and clonidine (7.57/0.32). 6. In dog saphenous vein, concentration-dependent contractions were observed (pEC50/intrinsic activity relative to noradrenaline): rilmenidine (5.83/0.70), moxonidine (6.48/0.98) and clonidine (7.22/0.83). Agonist-independent affinities were obtained with RS-79948-197. 7. Thus, expression of alpha2A-adrenoceptors is a prerequisite for the cardiovascular effects of moxonidine and rilmenidine in conscious mice. There was no evidence of I1-imidazoline receptor-mediated effects. The ability of these compounds to act as alpha2A-adrenoceptor agonists in vitro supports this conclusion.     

Adrenergic-induced hyperglycemia in anaesthetized rats: involvement of peripheral alpha 2-adrenoceptors

The selective alpha 2-adrenoceptor agonist, UK 14.304, induces a time- and dose-dependent hyperglycemic response in the anaesthetized rat. This hyperglycemia seems to be mediated through the pancreas because lesions of the pancreatic beta-cells induced with streptozotocin completely abolished this hyperglycemic response to UK 14.304 while ganglionic blockade by chlorisondamine did not affect the response. The non-selective agonist, adrenaline, induces a similar hyperglycemic response which is antagonized selectivity by the alpha 2-adrenoceptor antagonist, idazoxan. Combined alpha 1- and beta-adrenoceptor blockade resulted in an increase in the alpha 2-adrenoceptor-mediated hyperglycemic response to adrenaline. Since adrenaline is known not to cross the blood-brain barrier it is concluded that alpha 2-adrenoceptor stimulation induces hyperglycemic responses through a peripheral effect that involves postsynaptic alpha 2-adrenoceptors in the pancreatic beta-cell which are linked to the inhibition of insulin release.     

Contribution of alpha 2-adrenoceptors to the central cardiovascular effects of clonidine and S 8350 in anaesthetized rats.

1. The alpha 2-adrenoceptor agonist clonidine elicits centrally mediated effects through an interaction with both alpha 2-adrenoceptors and imidazoline binding sites. 2. We selected a new oxazoline derivative, S 8350, which competes with [3H]-yohimbine for binding to cerebral alpha 2-adrenoceptors (IC50, 67 +/= 17 nmol/L) and displays a higher affinity (35-fold) for alpha 2- than for alpha 1-adrenoceptors. 3. As observed for clonidine, intravenous (i.v.) administration of S 8350 resulted in a brief pressor effect followed by a prolonged hypotension. When S 8350 was administered i.v. to spinally pithed rats, only a rise in blood pressure was observed. 4. In order to discriminate the cardiovascular effects related to the central imidazoline receptor or alpha 2-adrenoceptor activation, the effects of intracisternal (i.c.) administration of clonidine and S 8350 were investigated in the rat. 5. In the anaesthetized rat, both clonidine and S 8350 displayed a profound central (i.c. route) hypotensive effect associated with a bradycardia. 6. The cardiovascular effects of S 8350 were abolished by the central administration of the selective alpha 2-adrenoceptor antagonist rauwolscine. Conversely, rauwolscine completely prevented bradycardia but it induced only a partial reversion of the hypotension elicited by clonidine. 7. These results suggest that central alpha 2-adrenoceptors are responsible for hypotension and bradycardia while imidazoline binding sites do not apparently contribute to heart rate control.     

Neuroprotective effects of TEMPOL in central and peripheral nervous system models of Parkinson's disease

TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl) is a stable nitroxyl antioxidant. Previous studies have suggested that TEMPOL is protective in acute disorders thought to involve reactive oxygen species (ROS), such as ischemic stroke and cardiac reperfusion injury. Oxidized TEMPOL can be recycled to its redox-active reducing form by co-administration with polynitroxylated albumin, making it a candidate as a pharmacological "reservoir" for reducing potential of use in chronic disorders involving ROS. The present studies examine the efficacy of TEMPOL in cell culture and animal models of the central and peripheral dysfunction associated with Parkinson's disease, a disorder in the pathogenesis of which ROS generated from dopamine have been implicated. Antioxidants have been proposed as both preventive and symptomatic therapy for Parkinson's disease. TEMPOL protects MN9D dopaminergic mesencephalic cells in culture from 6-hydroxydopamine (6-OHDA)-induced apoptosis. Translocation of the p65 component of NF-kappaB to the nucleus accompanies protection by TEMPOL. In vivo, intraperitoneal TEMPOL protects mice from intrastriatal 6-OHDA-induced cell and dopamine metabolite loss in the striatum. TEMPOL also protects mice against the 6-OHDA-induced rotational behavior elicited by intrastriatal administration of d-amphetamine. In addition, TEMPOL protects mice from the ptosis, activity level decrement, and mortality induced by intraperitoneal administration of 6-OHDA, a model of autonomic dysfunction in Parkinson's disease. Adjunctive use of polynitroxylated albumin enhances the in vitro and in vivo effects of TEMPOL.     

Discontinued drugs in 2006: central and peripheral nervous system drugs

This perspective describes compounds that target the central and peripheral nervous system, whose development was discontinued in 2006 and which were being developed for the treatment of a range of neurological disorders, including chronic pain, Alzheimer's disease, Parkinson's disease, multiple sclerosis, epilepsy and anxiety. These discontinued candidates are described by disease area, based on information available from the Pharmaprojects database.