Interaction of clonidine and beta-blockers

On the hypothesis that non-selective beta-blockers can antagonize or reverse the antihypertensive effect of clonidine (C), 12 hypertensive outpatients were treated with C alone and in combination with propranolol (P), atenolol (A) and prazosin (Pz). C alone (0.11 or 0.22 mg b.i.d.) or in combination with P (80 mg b.i.d.) did not provide normotension. Changing P to A (50 mg b.i.d.) reduced supine systolic and diastolic pressures, which now were significantly lower (p less than 0.01) than during C alone. Changing A to P again resulted in elevated pressures. Pz (1 mg t.i.d.) added to the C+P regimen lowered supine blood pressures to the levels otherwise recorded during C+A. C dose-dependently contracted rabbit aortic spiral in vitro, reaching about 50% of maximum responses to noradrenaline. Pz abolished this response. P (0.1--10 micrograms/ml) but not A somewhat enhanced responses to high doses of C. Sotalol rather antagonized C contractions. We conclude that A but not P enhances the antihypertensive action of C. No hypertensive interaction was observed.

Chloroethylclonidine reveals that alpha (1 A)-adrenoceptors mediate contraction in aorta of alpha (1 D)-adrenoceptor knockout mice

1 We have characterized the alpha(1)-adrenoceptor subtypes present in isolated aorta of the alpha(1D)-adrenoceptor knockout (KO) mice, by chloroethylclonidine (CEC)-induced alkylation and their protection by selective alpha(1)-adrenoceptor antagonists. 2 The alpha(1D)-adrenoceptor is involved in the contractile response to noradrenaline in wild type (WT) mouse aorta. 3 In WT mice 5-methylurapidil (5-MU, an alpha(1A)-adrenoceptor antagonist) or BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl] ethyl]-8-azaspiro[4.5] decane-7,9 dione, a selective alpha(1D)-adrenoceptor antagonist), protected the receptors from CEC-induced (alpha(1B/D)-adrenoceptor) alkylation, the combination of both antagonists resulted in complete protection, while AH11110A (1-[biphenyl-2-yloxy]-4-imino-4-piperidin-1-yl-butan-2-ol, an alpha(1B)-adrenoceptor antagonist) did not protect. 4 In aorta of KO mice there was a 19-fold rightward shift in noradrenaline effective concentration (EC(50)) compared with WT; while 5-MU alone or in combination with AH11110A protected alpha(1)-adrenoceptors to the same extent. 5 The data indicate that alpha(1A)-adrenoceptors mediate contraction and suggest their role in maintaining homeostasis in the alpha(1D)-adrenoceptors KO mice.

Clonidine in horses: identification, detection, and clinical pharmacology

Clonidine is classified as a class 3 performance-enhancing agent by the Association of Racing Commissioners International and thus has the potential to influence the outcome of a race. In this study, the authors developed and validated a sensitive gas chromatograph and mass spectrometer method to determine the pharmacokinetic parameters of clonidine in equine plasma samples after IV administration of a single dose (0.025 mg/kg) of clonidine in horses. At this dose, clonidine produced rapid and profound sedation, which cold be quickly reversed with yohimbine. Clonidine was able to produce an analgesic effect but failed to provide maximal analgesia in all horses; the limited analgesic effect persisted for about 60 minutes.

Effects of alpha- and beta-adrenergic agonists on Toxoplasma gondii infection in murine macrophages

We investigated the effects of alpha- and beta-adrenergic receptor agonists on the ability of Toxoplasma gondii to infect and proliferate in cultured murine macrophages. Macrophages pretreated in vitro with varying concentrations of alpha- and beta-adrenergic agonists and incubated with the RH strain of T. gondii did not result in a significant increase in the percentage of infected macrophages compared with negative controls. When parasites were pretreated with L-phenylephrine, an alpha-agonist, and L-isoproterenol, a beta-agonist, before infection, there was no significant change in the percentage of infected macrophages. Clonidine, an alpha2-adrenergic agonist, led to a significant decrease in the number of infected macrophages at all concentrations tested. The effects of clonidine were blocked by yohimbine, a specific alpha2-adrenergic antagonist, but not by phentolamine, an alpha1-adrenergic antagonist. These results suggest that the antiparasitic effects exhibited by clonidine (alpha2-adrenergic agonist) are mediated through an alpha2-adrenoreceptor found on the surface of T. gondii.

Intrathecal Neostigmine Prevents Intrathecal Clonidine from Attenuating Hypercapnic Cerebral Vasodilation in Rabbits

We previously demonstrated that lumbar intrathecal alpha(2) agonists attenuate hypercapnia-induced cerebral vasodilation. The combination of intrathecal clonidine and neostigmine is being investigated as pain therapy. The effects of their combination on cerebrovascular reactivity are unknown. We allocated rabbits anesthetized with pentobarbital to two groups: (a) clonidine (normal saline followed 30 min later by clonidine 2 microg/kg, both into the lumbar intrathecal space; n = 6), and (b) neostigmine-pretreatment (neostigmine 2 microg/kg followed 30 min later by clonidine 2 microg/kg, both into the lumbar intrathecal space; n = 6). We then evaluated the hypercapnia-induced changes in pial arteriolar diameter in these two groups using the closed cranial window preparation. The pial arteriolar dilator response to hypercapnia was significantly attenuated in the clonidine group (14% +/- 4%, 4% +/- 4%, 6% +/- 6%, and 5% +/- 7% for before and 30, 60, and 90 min, respectively). Neither normal saline nor neostigmine alone induced any change in the cerebral reactivity to hypercapnia. Pretreatment with neostigmine completely prevented the clonidine-induced attenuation of the hypercapnic cerebral vasodilation attenuated by intrathecal clonidine (16% +/- 7%, 15% +/- 6%, 12% +/- 6%, and 16% +/- 8%, respectively).

Mechanism of the Endothelium-Dependent Vasodilation and the Antihypertensive Effect of Brazilian Red wine

The mechanisms involved in the cardioprotector effect of red wine have not yet been completely elucidated but probably an endothelium-dependent vasodilator action may play a significant role in this effect. Experiments were undertaken to determine whether a Brazilian red wine (BRW) induces vasodilation in the mesenteric vascular bed (MVB) and an antihypertensive effect was also assessed in rats with NO-deficient hypertension. In MVB precontracted with norepinephrine, BRW (alcohol-free lyophilized) induces a long-lasting endothelium-dependent vasodilation that is not reduced by indomethacin. Inhibition of NO-synthase by NG-nitro-L-arginine methyl ester (L-NAME) and guanylyl cyclase by 1H-[1,2,3] oxadiazolo [4,4-a]quinoxalin-1-one (ODQ) reduces the vasodilator effect of BRW. In vessels precontracted with norepinephrine and depolarized with KCl (25 Mm) or treated with Ca-dependent K channel blockers charybdotoxin (ChTx) plus apamin, the effect of BRW was significantly reduced. However, this effect is not affected by ATP-dependent K (KATP) channel blocker (glibenclamide). The residual vasodilator effect of BRW observed in vessels pretreated with ChTx plus apamin is completely abolished by ChTx plus apamin plus L-NAME. Concentrations of atropine, pyrilamine, yohimbine, and HOE 140 that significantly reduced the vasodilator effect of acetylcholine, histamine, clonidine, and bradykinin, respectively did not change the vasodilator effect of BRW. Chronic oral administration of BRW induced a significant reduction in systolic, mean and diastolic arterial pressure in rats with L-NAME hypertension. The present results demonstrated that vasodilator effect of BRW is dependent on endothelium-derived hyperpolarizing factor (EDHF) in combination with nitric oxide (NO). The antihypertensive effect of red wine demonstrated in the present study may play a significant role on the cardioprotective action of chronic red wine consumption.

Clonidine induces rat aorta relaxation by nitric oxide-dependent and -independent mechanisms

The alpha1- and alpha2-adrenoceptors coexist in vascular smooth muscle cells producing vascular contraction and relaxation. This study was designed to investigate which is the mechanism activated by clonidine in the rat aorta, and the endothelial factors possibly involved in the relaxation induced by clonidine. The alpha2-adrenoceptors agonist clonidine relaxed rat aortas pre-contracted with phenylephrine, with or without endothelium. In non-contracted denuded arteries, clonidine produced contractions instead of relaxation. In intact endothelium aortic rings, clonidine induced greater relaxation than in denuded aortic rings. In aortas with intact endothelium, the NO-synthase inhibitor L-NAME (10 micromol/L) and the NO-scavenger hemoglobin (10 micromol/L) reduced the relaxation to clonidine. On the other hand, indomethacin (10 micromol/L) failed to alter the relaxation induced by clonidine. These results suggest the participation of NO, but not prostacyclin in clonidine-induced relaxation. In aortic rings pre-contracted with KCl (60 mmol/L) the relaxation induced by clonidine was abolished; however, the K+ channel blockers glibenclamide (K(ATP)), tetraethylamonium (K(Ca)), and the combination of apamin and charybdotoxin (K(Ca)) did not change the relaxation induced by clonidine. The relaxation induced by clonidine on PGF2alpha-contracted arteries was not affected by prazosin. However, in the absence of prazosin, clonidine had an additional contractile effect in PGF2alpha-contracted arteries. In conclusion, our results show that in rat aorta clonidine can activate alpha2-adrenoceptors in the smooth muscle cells and alpha2-adrenoceptors in the endothelial cells that activates NO production, but not prostacyclin and/or EDHF. In the absence of phenylephrine and prazosin, clonidine can also activate alpha1-adrenoceptors and rat aorta contraction.

Effect of ethanol on reductions in norepinephrine electrochemical signal in the rostral ventrolateral medulla and hypotension elicited by I1-receptor activation in spontaneously hypertensive rats

BACKGROUND

The mechanism of the antagonistic hemodynamic interaction between ethanol and centrally acting sympatholytics is not known. In this study, we tested the hypothesis that the imidazoline (I1)-receptor modulation of norepinephrine (NE) release within the rostral ventrolateral medulla (RVLM) plays a pivotal role in this clinically relevant hemodynamic interaction. METHOD In anesthetized spontaneously hypertensive rats, the effects of centrally acting sympatholytics on RVLM NE electrochemical signal were investigated by in vivo electrochemistry along with cardiovascular responses in the absence and presence of ethanol. In vivo microdialysis in conscious spontaneously hypertensive rats was used to confirm the electrochemical findings. RESULTS Clonidine (30 microg/kg, intravenously) or rilmenidine (400, 600, or 800 microg/kg) significantly reduced RVLM NE electrochemical signal (index of neuronal activity) and mean arterial pressure; rilmenidine effects were dose-related, and ethanol (1 g/kg) counteracted these responses. Ethanol (1 g/kg) pretreatment increased both RVLM NE electrochemical signal and blood pressure but did not influence the reductions in both variables elicited by subsequently administered clonidine. The alpha2-adrenergic antagonist 2-methoxyidazoxan (30 microg/kg) counteracted rilmenidine (800 microg/kg)-evoked responses. In vivo microdialysis in conscious spontaneously hypertensive rats confirmed the electrochemical findings since clonidine- (30 microg/kg, intravenously) evoked reductions in RVLM NE and the associated hypotension were counteracted by ethanol (1 g/kg). CONCLUSIONS (1) Ethanol counteracts centrally mediated hypotension, at least in part, by increasing RVLM NE; (2) the interaction involves the I1 receptor modulation of RVLM neuronal activity; (3) the alpha2-adrenergic receptor contributes to the electrochemical and cardiovascular effects of high doses of rilmenidine, and (4) the RVLM is a neuroanatomical target for systemically administered ethanol.

Clonidine-induced coronary vasodilatation in isolated guinea pig heart is not mediated by endothelial alpha2 adrenoceptors

Functional role of endothelial alpha(2)-adrenoceptor in coronary circulation remains unclear. Clonidine, an agonist of alpha(2)-adrenoceptors, was reported to induce coronary vasodilatation via stimulation of endothelial alpha(2)-adrenoceptors or coronary vasoconstriction involving vascular smooth muscle alpha(2)-adrenoceptors. Moreover, H(2) receptor-dependent responses to clonidine were described. Here, we reassess the contribution of endothelial alpha(2)-adrenoceptor and H(2) receptors to coronary flow and contractility responses induced by clonidine in the isolated guinea pig heart. We found that clonidine (10(-9) - 10(-6) M) produced concentration-dependent coronary vasoconstriction without a significant change in contractility. This response was inhibited by the alpha(1)/alpha(2)-adrenoceptor antagonist - phentolamine (10(-5) M) and the selective alpha(2)-adrenoceptor antagonist yohimbine (10(-6) M), but it was not changed by the selective alpha(1)-adrenoceptor antagonist prazosin (10(-6) M). In the presence of nitric oxide synthase inhibitor, L-NAME (10(-4) M) the clonidine-induced vasoconstriction was potentiated. Clonidine at high concentrations of 10(-5) - 3 x 10(-5) M produced coronary vasodilatation, and an increase in myocardial contractility. These responses were abolished by a selective H(2)-receptor antagonist, ranitidine (10(-5) M), but not by phentolamine (10(-5) M). We conclude that in the isolated guinea pig heart, clonidine-induced vasoconstriction is mediated by activation of smooth muscle alpha(2)-adrenoceptors whereas clonidine-induced coronary vasodilatation is mediated by activation of vascular H(2) histaminergic receptors. Accordingly, endothelial alpha(2)-adrenoceptors does not seem to play a major role in coronary flow response induced by clonidine.

Blockade of N-methyl-d-aspartate receptors within the rostral ventrolateral medulla antagonizes clonidine-induced cardiovascular effects

There is wide agreement that the rostral ventrolateral medulla (RVLM) plays a crucial role in the regulation of blood pressure (BP), and that there may be a close correlation between the actions of centrally acting antihypertensive agents and N-methyl-D-aspartate (NMDA) receptor functional states. The present study was done to test the hypothesis that NMDA receptors within the RVLM were involved in the cardiovascular effects of centrally acting antihypertensive drug clonidine in anesthetized and paralyzed rats. Prior unilateral microinjection of NMDA receptor antagonist dizocilpine (MK801, 500 pmol) into the RVLM significantly attenuated (p<0.01, n=9) the reductions of BP (-24+/-6 to -8+/-4 mm Hg) and heart rate (-49+/-9 to -14+/-7 bpm) induced by unilaterally injected clonidine (5 nmol) into the RVLM. Prior bilateral microinjection of MK801 (500 pmol for each side) into the RVLM effectively (p<0.01, n=7) antagonized the hypotension (-25+/-5 to -8+/-2 mm Hg) and bradycardia (-43+/-7 to -11+/-4 bpm) of intravenously administered clonidine (10 microg kg(-1)). Importantly, iontophoretic application of MK801 (60 nA) significantly (p<0.01, n=9) prevented the inhibitory effect of intravenously (10 microg kg(-1)) injected clonidine on the discharge of presympathetic neurons in the RVLM (neuronal inhibition: -39+/-6 to -10+/-2%). In conclusion, the present study shows that the RVLM administrated MK801 effectively antagonizes clonidine-induced cardiovascular effects, and suggests that NMDA receptors within the RVLM contribute to clonidine actions.

Rhythmic properties of neurons in the rostral ventrolateral medulla of the rat in vitro: effects of clonidine

The rostral ventrolateral medulla (RVLM) is thought to be the main central site for generation of tonic sympathetic activity. In the rat in vitro slice preparation, we used intracellular recordings to identify different populations of neurons in the RVLM: 43 spontaneously active neurons with regular (R) or irregular (I) patterns of spike firing and 10 silent neurons. The degree of regularity was quantified by the coefficient of variation (CV = SD/mean) of interspike interval durations, as well as by the rhythmic properties of the spike autospectrum and autocorrelation. The distribution of CVs was clustered: R and I neurons were defined as those with CVs </=12% (n = 21) or >12% (n = 22), respectively. The R-type and I-type neurons resemble the type II and type I neurons, respectively, which were previously characterized in the RVLM in vivo as barosensitive and bulbospinal. Both types may be important in generation of sympathetic tone. Clonidine (1-100 microM) was applied to 10 R-type neurons and 16 I-type neurons. The firing of 21/26 was depressed to the point of silence. However, 18/26 neurons were excited earlier in the perfusion. The later depression of firing occurred in both I and R neurons and in different cases was associated with either hyperpolarization or depolarization.

Perineural alpha(2A)-adrenoceptor activation inhibits spinal cord neuroplasticity and tactile allodynia after nerve injury

BACKGROUND

Nerve injury in animals increases alpha(2)-adrenoceptor expression in dorsal root ganglion cells and results in novel excitatory responses to their activation, perhaps leading to the phenomenon of sympathetically maintained pain. In contrast to this notion, peripheral alpha(2)-adrenoceptor stimulation fails to induce pain in patients with chronic pain. We hypothesized that alpha(2) adrenoceptors at the site of nerve injury play an inhibitory, not excitatory role.

METHODS

Partial sciatic nerve ligation was performed on rats, resulting in a reduction in withdrawal threshold to tactile stimulation. Animals received perineural injection at the injury site of clonidine, saline, or clonidine plus an alpha(2)-adrenergic antagonist, and withdrawal threshold was monitored. Immunohistochemistry was performed on the sciatic nerve ipsi- and contralateral to injury and on the spinal cord.

RESULTS

Clonidine reduced this hypersensitivity in a dose-dependent manner, and this was blocked by an alpha(2A)-preferring antagonist. Perineural clonidine injection had a slow onset (days) and prolonged duration (weeks). Systemic or intrathecal clonidine, or transient neural blockade with ropivacaine, had short lasting or no effect on hypersensitivity. alpha(2A)-adrenoceptor immunostaining was increased near the site of peripheral nerve injury, both in neurons and in immune cells (macrophages and T lymphocytes). Phosphorylated cAMP response element binding protein (pCREB) in lumbar spinal cord was increased ipsilateral to nerve injury, and this was reduced 1 week after perineural clonidine injection.

CONCLUSIONS

These data suggest that peripheral alpha(2) adrenoceptors are concentrated at the site of peripheral nerve injury, and their activation receptors produce long-lasting reductions in abnormal spinal cord gene activation and mechanical hypersensitivity.

Clonidine toxicity revisited

The incidence of clonidine overdose is increasing, yet there is a paucity of new information regarding treatment options for clonidine toxicity. Reported treatment approaches vary widely, demonstrating the lack of science on which current treatment is based. Available research needs to be reassessed. Neurotransmitters, receptors, endogenous opioids, and baseline sympathetic tone determine the clinical response to clonidine as well as the potential response to drug therapy following clonidine overdose. This article reviews aspects of clonidine toxicity that need to be further investigated. Multicenter research trials will be required to evaluate new treatment options.

Agmatine, an endogenous ligand at imidazoline binding sites, does not antagonize the clonidine-mediated blood pressure reaction

Since agmatine has been identified as a clonidine displacing substance (CDS), the aim of this study was to investigate whether agmatine can mimic CDS-induced cardiovascular reactions in organ bath experiments, pithed spontaneously hypertensive rats (SHR) and anaesthetized SHR. Intravenously-administered agmatine significantly reduced the blood pressure and heart rate of anaesthetized SHR at doses higher than 1 and 3 mg kg(-1), respectively. These effects are probably mediated via central mechanisms, since there was an approximate 8 fold rightward shift of the dose-response curve in the pithed SHR (indicating a weakened cardiovascular effect). Moreover, in organ bath experiments, agmatine failed to alter the contractility of intact or endothelium-denuded aortal rings. When agmatine was administered i.c.v. to anaesthetized SHR, blood pressure was increased without any alteration of heart rate, whereas blood pressure was unchanged and heart rate was increased after injection into the 4th brain ventricle. This suggests that haemodynamic reaction patterns after central application are related to distinct influences on central cardiovascular mechanisms. Agmatine reduces noradrenaline release in pithed SHR while alpha(2)-adrenoceptors are irreversibly blocked with phenoxybenzamine, but not while I(1)-binding sites are selectively blocked with AGN192403. This suggests that agmatine may modulate noradrenaline release in the same way that clonidine does, i.e. via imidazoline binding sites; this involves a reduction in sympathetic tone which in turn reduces blood pressure and heart rate. Finally, CDS-like cardiovascular activity appears not to be due to agmatine, since (i) blood pressure in anaesthetized SHR is decreased by agmatine and clonidine, and (ii) agmatine did not antagonize the blood pressure reaction to clonidine in pithed or anaesthetized SHR.

Ventilatory effects of alpha2-adrenoceptor blockade in awake goats

We sought to determine the extent to which alpha2-adrenoceptor (alpha2-AR) pathways exert a tonic influence on respiratory rhythm under normal physiological conditions. The ventilatory effects of alpha2-AR blockade with SKF-86466 were examined in awake adult goats. The antagonist effect of SKF-86466 at alpha(2)-ARs was evident both as a reversal of the ventilatory disturbances produced by the alpha2-AR agonist clonidine (5 microg/kg IV) and as a rightward shift in the potency of the agonist-evoked response after SKF-86466 pretreatment. Systemic administration of SKF-86466 in cumulative doses (25-250 microg/kg) or as a single bolus injection (500 microg/kg) produced dose-dependent increases in breathing in all animals. The excitatory effect was primarily mediated by increases in respiratory frequency that persisted for at least 120 min after injection (500 microg/kg). The time course and magnitude of the ventilatory response to SKF-86466 was similar in carotid body denervated animals. The ventilatory response to isocapnic hypoxia but not normoxic hypercapnia was significantly elevated after SKF-86466 administration (500 microg/kg). SKF-86466 (25-1000 microg/kg) produced dose-dependent increases in heart rate in all animals but did not significantly change mean arterial blood pressure at any dose. The results demonstrate that alpha2-AR pathways exert a tonic inhibitory influence on respiratory rhythm in the awake goat.

Clonidine-induced antinociception and locomotor hypoactivity are reduced by dexamethasone in mice

The effects of dexamethasone pretreatment on clonidine-induced antinociception and locomotor hypoactivity were investigated in mice. In the hot-plate and the tail-flick tests, dexamethasone administered intraperitoneally at a dose of 1 mg kg(-1), 30 or 60 min before clonidine, reduced clonidine antinociception in both tests and reduced clonidine-induced locomotor hypoactivity in the activity cage. When administered 15 min before clonidine, dexamethasone had no effect on clonidine antinociception. A higher dexamethasone dose (10 mg kg(-1)) induced the same effects observed at a dose of 1 mg kg(-1) in the hot-plate and the tail-flick tests, but the former dose had a stronger effect on locomotor hypoactivity. Dexamethasone (10 ng/mouse) administered intracerebroventricularly 30 min before clonidine was also able to reduce both clonidine-induced antinociception and locomotor hypoactivity. The protein synthesis inhibitor, cycloheximide, administered intraperitoneally at the dose of 10 mg kg(-1), 2 h before clonidine, was able to prevent dexamethasone effects on clonidine-induced antinociception. The glucocorticoid receptor antagonist RU-38486, administered intracerebroventricularly at the dose of 1 ng/mouse, was also able to block dexamethasone effects on clonidine-induced antinociception and locomotor hypoactivity, whereas both cycloheximide and RU-38486 per se did not influence pain sensitivity or locomotor activity. These results suggest that the dexamethasone effects on clonidine-induced antinociception and locomotor hypoactivity depend on the stimulating effects that dexamethasone exert, on the protein synthesis via the glucocorticoid receptor in the brain.

Norepinephrine release from spinal synaptosomes: auto-alpha2 -adrenergic receptor modulation

BACKGROUND

Clonidine produces analgesia after spinal injection by activating alpha2-adrenergic receptors. Recently, clonidine has been demonstrated to increase spinal release of norepinephrine (NE) in vivo, in contrast to that anticipated by classic presynaptic autoinhibition. The purpose of the current study was to determine if clonidine could inhibit release of NE in a preparation of spinal cord tissue lacking synaptic circuits.

METHODS

Crude synaptosomes were prepared from male Sprague-Dawley rat spinal cord, loaded with [3H]NE, and stimulated by potassium chloride to release [3H]NE. Samples were incubated with clonidine in the absence or presence of various inhibitors. To study the effect of alpha2a-adrenergic receptor subtypes, some animals were pretreated with an oligodeoxynucleotide (ODN) composed of a sense or antisense sequence to a portion of this receptor.

RESULTS

Potassium chloride produced a concentration-dependent increase in [3H]NE release, and this release was inhibited by clonidine with a concentration producing 50% maximal inhibition (IC50) of 1.3 microm. The effect of clonidine was inhibited by the alpha2-adrenergic antagonists, yohimbine and idazoxan, but not by alpha1-adrenergic, muscarinic, or opioid antagonists. Intrathecal pretreatment with antisense ODN to alpha2A-adrenergic receptors reduced alpha2A-adrenergic receptor protein expression compared with sense ODN control and also reduced clonidine-induced inhibition of [3H]NE release.

CONCLUSIONS

These data demonstrate the existence of classic autoinhibitory alpha2-adrenergic receptors in the spinal cord, probably of the alpha2Asubtype. They further suggest that clonidine-induced stimulation of spinal NE release must occur from indirect actions, presumably due to activation of a spinal circuit.

Effect of neurokinin-I receptor antagonists on the function of 5-HT and noradrenaline neurons

Substance P antagonists have been proposed to be a new class of antidepressants. The present study aimed to determine the effect of the selective non-peptide rat neurokinin-1 (NK1) receptor antagonists WIN 51,708 and CP-96,345 on the firing activity of rat dorsal raphe serotonin (5-HT) and locus coeruleus noradrenaline (NA) neurons. While WIN51,708 (2mg/kg, i.v.) and CP-96,345 (0.15 mg/kg, i.v.) did not modify the firing activity of 5-HT and NA neurons, both antagonists attenuated the suppressant effect of the alpha2-adrenoceptor agonist clonidine on the firing activity of both types of neurons. In contrast, the responsiveness of 5-HT neurons to the i.v. administration of the 5-HT autoreceptor agonist LSD and the 5-HT1A receptor agonist 8-OH-DPAT remained unchanged. These findings suggest that NK1 receptor antagonists affect markedly the NA system via an attenuation of the function of alpha2-adrenoceptors on the cell body of NA neurons and, consequently, may also modulate 5-HT neurotransmission.

Harmane produces hypotension following microinjection into the RVLM: possible role of I(1)-imidazoline receptors

The beta-carboline, harmane (0.1 - 1.0 nmol) produces dose dependent hypotension when microinjected unilaterally into the rostral ventrolateral medulla (RVLM) of the anaesthetized rat. The potency of harmane on blood pressure is similar to that of the imidazoline, clonidine. The hypotensive effects of both clonidine and harmane are reversed by microinjection of the relatively I(1)-receptor selective antagonist efaroxan (20 nmol). These results are consistent with harmane acting at an I(1)-receptor in the RVLM. This is the first report of an endogenous ligand for I(1)-receptors that has central effects on blood pressure.

Alpha-2 adrenergic and opioid receptor-mediated gastroprotection

Clonidine inhibited the development of gastric mucosal lesions induced by either acidified ethanol or indomethacin. The ED50 values were: 7.1 and 5.2 microg x kg(-1) orally, respectively. The gastroprotective effect was antagonised by the pre-synaptic alpha-2 antagonist yohimbine, the more selective alpha-2 antagonist Ch-38083 and the pre-synaptic alpha-2B antagonist prazosin. Moreover, the non-selective opioid receptor antagonist naloxone, the delta receptor selective naltrindole also reversed the clonidine-induced mucosal protective action. Clonidine was also effective following intracerebroventricular administration with the ED50 of 37 ng/rat against ethanol-induced mucosal damage. Our results suggest that: 1) the gastroprotective effect of clonidine is likely to be mediated by alpha-2B adrenoceptor subtype; 2) there is an interaction between pre-synaptic alpha-2 adrenoceptors and opioid system; and 3) clonidine can induce gastroprotection by central mechanism.

Ethanol counteraction of clonidine-evoked inhibition of norepinephrine release in rostral ventrolateral medulla of rats

Previous studies from our laboratory demonstrated an antagonistic hemodynamic interaction between ethanol and clonidine in conscious and in urethane-anesthetized rats. The present study tested the hypothesis that ethanol produces its effect by counteracting clonidine-evoked inhibition of norepinephrine (NE) release at its major site of action, the rostral ventrolateral medulla (RVLM). In vivo electrochemical measurement of real-time changes in NE level in the RVLM of urethane-anesthetized Sprague-Dawley rats was made along with blood pressure and heart rate. Clonidine (30 microg/kg, i.v.) produced significant decreases (p < 0.05) in NE electrochemical signal and blood pressure. Ethanol (1 g/kg, i.v.) administered 10 min after clonidine significantly (p < 0.05) increased NE signal and counteracted clonidine-evoked hypotension. Equal volume of saline had no effect on NE signal in the RVLM nor on the hypotensive response to clonidine. Pretreatment with the same dose of ethanol (1 g/kg) caused slight increases in RVLM NE level and in blood pressure, but did not influence the electrochemical and blood pressure responses to clonidine; clonidine (30 microg/kg) administration 10 min after ethanol resulted in significant (p < 0.05) decreases in NE signal and blood pressure. These findings suggest that: (i) ethanol counteraction of the hypotensive action of clonidine involves, at least in part, opposite effects on central pathways that use NE as a neurotransmitter; (ii) the RVLM represents a possible site for the adverse hemodynamic interaction between ethanol and clonidine; and (iii) ethanol-evoked increase in RVLM NE, which correlates with its pressor effect, is much enhanced when RVLM NE level is reduced by clonidine.

Clonidine and vasomotion physiology

It is known that clonidine exerts a hyperpolarizing action and alpha 2-adrenergic activity. The experimental work investigates the conditions under which each action of clonidine is developed on vascular smooth muscle. Two parameters were studied in vitro on bovine aortic media, Ca2+ uptake and vascular tone. The Ca2+ uptake measurement was performed by incubating in Krebs' solution small slices of the preparation in the presence of 45Ca. Studies on vascular tone were performed on deendothelialized bovine aortic rings suspended in Krebs' solution. Low concentrations of clonidine (1 nM-1 microM) decrease Ca2+ uptake and relax the preparation, indicating dominance of the inhibiting action of clonidine may be due to an hyperpolarization. Clonidine 10 microM results in equalization of the opposing actions of Ca2+ uptake and vascular tone. When the preparation is stimulated by alpha 1-adrenergic against phenylephrine 1 microM, clonidine 1 nM-10 microM potentates the Ca2+ uptake and vascular contraction in a monophasic way, indicating that the depolarizing mechanisms connected with the alpha 1-adrenergic stimulation totally inhibit the relaxing action of clonidine possibly due to hyperpolarization. This action is restricted in the presence of yohimbine (alpha 2-adrenergic blocker).

Clonidine inhibits gastric motility in the rat

We have studied the effect of clonidine on gastric motility, by examining the effect on gastric emptying of indigestible solids. In Wistar rats, either clonidine or saline was injected intraperitoneally, and ten steel balls (1.0 mm in diameter) were inserted into the stomach. Gastric emptying was examined at 3 h. Clonidine delayed gastric emptying of steel balls (ED50 = 0.0071 [95% confidence interval: 0.0033-0.013) mg kg-1). Yohimbine, but not naloxone, significantly antagonized the inhibitory effect of clonidine. We conclude that clonidine inhibits gastric motility through the alpha 2 adrenoceptor.

Effects of verapamil on electroconvulsive shock- and clonidine-induced amnesia

The effect of the Ca2+ blocker verapamil on amnesia induced by electroconvulsive shock (ECS) or by the alpha-adrenoceptor agonist clonidine was studied in male Wistar rats trained in passive avoidance task ("step down"). Clonidine (0.1 mg/kg, i.p.) and ECS induced a pronounced amnesia, significantly reducing the percentage of rats that had acquired the task upon retention tests, given 3 h, 24 h and 7 days after training. Verapamil (10 mg/kg) administered orally for 12 days (5 days before and 7 days after training) completely abolished the ECS- or clonidine-induced amnesia. These data suggest that calcium channel blocker verapamil has a protective effect against experimentally provoked memory deficit and might be useful for the treatment of cognitive disorders.

Contrasting effects of urethane, ketamine, and thiopental anesthesia on ethanol-clonidine hemodynamic interaction

In previous studies, we have shown that ethanol counteracts the hypotensive and sympathoinhibitory effects of clonidine in conscious rats but enhances these responses in alpha-chloralose anesthetized rats. This study investigated whether the latter finding pertains to other anesthetics that act by different mechanisms. The hemodynamic interaction between clonidine and ethanol was evaluated in conscious aortic barodenervated rats and in rats anesthetized with urethane, ketamine, or thiopental. The conscious aortic barodenervated rat was used because it exhibits a greater hypotensive response to clonidine compared with intact (sham-operated) rats. Starting from similar baseline values of blood pressure (BP) and heart rate, clonidine (30 micrograms/kg, iv) elicited similar decreases in BP in all groups of rats (conscious aortic barodenervated or anesthetized). Subsequent administration of ethanol (1 g/kg, iv) counteracted the hypotensive effect of clonidine in conscious aortic barodenervated rats and raised the BP to levels higher than the baseline (preclonidine) values. The pressor effect of ethanol persisted for at least 30 min. In contrast, preadministration of ethanol had no effect on the hypotensive effect of the subsequently administered clonidine. The antagonistic clonidine-ethanol hemodynamic interaction in conscious rats was not altered in urethane-anesthetized rats, both in terms of magnitude and duration. In ketamine-anesthetized rats, a short lasting (< 10 min) pressor effect of ethanol was observed. In contrast, ethanol augmented the hypotensive effect of clonidine in thiopental-anesthetized rats; the hypotensive effect of clonidine became slightly but significantly (p < 0.05) greater after ethanol compared with respective time control values. These findings suggest that the adverse hemodynamic effect of ethanol on centrally mediated hypotensive response is modified by different anesthetics. The antagonistic hemodynamic interaction, demonstrated in conscious rats, is replicated in urethane-anesthetized rats but weakened and reversed in ketamine- and thiopental-anesthetized rats, respectively. Therefore, urethane-anesthetized rats may be used in mechanistic studies on the antagonistic ethanol-clonidine hemodynamic interaction that cannot be undertaken, for technical reasons, in conscious rats.

Differential effects of two intraventricularly injected alpha 2 agonists, ST-91 and dexmedetomidine, on electroencephalogram, feeding, and electromyogram

Central alpha 2 agonists induce feeding in animals, electroencephalographic (EEG) synchronization, and sedation. Recent observations suggest that the investigational compound ST-91 and dexmedetomidine (DMET) may interact with distinct alpha 2 subclasses at spinal sites. We examined these drugs in brain. To accomplish these aims, ST-91, DMET, or methoxamine (METH), an alpha 1 agonist, were administered into the cerebral ventricles of unanesthetized rats prepared with chronic intracerebroventricular (ICVT) cannulae and cortical EEG electrodes. Behavior, EEG, electromyography (EMG), and feeding were assessed. We found that DMET resulted in dose-dependent (1-32 nmol) sedation, EEG synchronization, and a reduced gastrocnemeus EMG, while ST-91 (up to 153 nmol) had no effect. In contrast, ST-91 (1-153 nmol) resulted in a dose-dependent evocation of feeding, while the effects of DMET on feeding, if any, were obscured by the sedation. All effects were antagonized in a dose-dependent fashion by ICVT yohimbine (an alpha 2 antagonist). ICVT atipamezole (an alpha 2 antagonist), but not prazosin (an alpha 1 antagonist), reversed the effects of DMET. With ST-91, both atipamezole and prazosin had modest, but significant, antagonistic effects. ICVT METH had no effect. The differential physiological actions of these two intracerebroventricularly injected drugs, in concert with previous pharmacological studies, suggest two distinct subclasses of yohimbine-sensitive alpha 2 adrenergic receptors in the brain.

Involvement of supraspinal GABA receptors in majonoside-R2 suppression of clonidine-induced antinociception in mice

Majonoside-R2 (MR2) is a major constituent of Vietnamese ginseng (Panax vietnamensis, Ha et Grushv. Araliaceae) that is known to exhibit antagonistic activity against opioid-induced antinociception. In this study, we investigated the effect of MR2 on the antinociception caused in mice by the alpha2-adrenoceptor agonist clonidine, and elucidated the role of supraspinal GABAergic systems in this effect of MR2. The systemic administration of clonidine (0.5-2.5 mg/kg, s.c.) dose-dependently suppressed the nociceptive response of mice in the tail-pinch and hot-plate tests. The intraperitoneal (i.p.), intracerebroventricular (i.c.v.) or intrathecal (i.t.) administration of idazoxan (a selective alpha2-adrenoceptor antagonist) significantly antagonized the antinociceptive effect of clonidine in both tests. MR2 administered systemically (1.5-6.2 mg/kg, i.p.) or centrally (5-10 microg/mouse, i.c.v. or i.t.) dose-dependently antagonized the clonidine (1 mg/kg, s.c.)-induced antinociception in the tail-pinch test but not in the hot-plate test. The antagonistic effect of i.c.v. MR2 on the systemic clonidine-induced antinociception in the tail-pinch test was significantly reversed by i.c.v. administrations of the selective benzodiazepine receptor antagonist flumazenil (5 microg/mouse) and the GABA(A) antagonist picrotoxin (0.25 microg/mouse). These results suggest that the supraspinal GABA(A)/benzodiazepine receptors are involved in the antagonistic effect of MR2 on the clonidine-induced antinociception in the tail-pinch test in mice.

Antagonism of clonidine injected intracerebroventricularly in different models of salt intake

Clonidine, an alpha 2-adrenergic agonist, injected into the brain inhibits salt intake of animals treated by the diuretic model of sodium depletion. In th present study, we address the question of whether central injection of clonidine also inhibits salt intake in animals deprived of water or in the need-free state. Saline or clonidine (30 nmol) was injected into the anterior third ventricle of 24-h sodium-depleted (furosemide + removal of ambient sodium), of 24-h water-deprived and of normovolemic (need-free state) adult male rats. Clonidine injected intracerebroventricularly (i.c.v.) inhibited the 1.5% NaCl intake for 1209 min by 50 to 90% in every model tested. Therefore, different models of salt intake are inhibited by i.c.v. injection of clonidine. Idazoxan, an alpha 2-adrenergic antagonist, injected i.c.v. at a dose of 160 nmol, inhibited the effect of clonidine only in the furosemide + removal of ambient sodium model of salt intake. This indicates that the antagonism of this effect by idazoxan is dependent on the body fluid/sodium status of the animal.

Dexamethasone modifies the behavioral effects induced by clonidine in mice

The present study examines the influence of dexamethasone on behavioral effects induced by clonidine in mice. 2. The behavior elements considered were locomoter activity, rota rod, catalepsy and stereotyped behavior (rearing, grooming, social response test, crossing, smelling, washing face, scratching and bar holding). 3. Clonidine (0.1-0.5-1.0 mg/kg, IP) induced a significant reduction of all behavioral elements studied when compared to the saline treated group: the behavioral reduction was significant 10 min after administration and lasted for the entire recording period (120 min). 4. Dexamethasone (0.1-0.5-1.0 mg/kg, IP) per se did not induce significant changes in the behavior elements recorded. 5. Dexamethasone (0.1-0.5 mg/kg, IP) dod not affect behavioral effects induced by the 3 doses of clonidine, whereas the high dose (1 mg/kg) of the steroid significantly reduced its behavioral inhibition. 6. The results of the present study suggest that dexamethasone induces significant effects on clonidine-induced behavioral effects and that this may be related to an interference with the monoaminergic system.

[The role of serotonergic system in the mechanism of action of antidepressant agents]

Thirty eight years after the successful clinical introduction of antidepressant agents, there has been an important progress in the knowledge and changes in thinking about the role of central serotonergic system in depression and in the mechanism of their therapeutic efficacy. Although it is not clear whether an increase or decrease in serotoninergic function is more important in antidepressant action of agents, there is increasing evidence that almost all antidepressant drugs can induce changes in the sensitivity of somotodendritic 5-HT1A autoreceptors and postsynaptic 5-HT2 receptors in spite of very different pharmacological profiles after a single administration. The question arises as to the causal nature of the relationship between these effects and beneficial clinical action. Further studies are still required to dispel these doubts.

Omega-agatoxin IVA blocks spinal morphine/clonidine antinociceptive synergism

Involvement of P-type voltage-dependent Ca2+ channels in spinal morphine- or clonidine-induced antinociception and in the synergistic interaction between morphine and clonidine was examined in the present studies. Coadministration of the selective P-type antagonist, omega-agatoxin IVA (25 ng) intrathecally (i.t.) to mice along with morphine or clonidine enhanced the tail flick antinociception of each agonist 5-6-fold. The greater-than-additive (synergistic) interaction that occurred when morphine and clonidine were coadministered i.t. decreased to an additive interaction in the presence of omega-agatoxin IVA. In mice pretreated with pertussis toxin (10 ng) to inactivate G proteins, omega-agatoxin IVA did not alter the morphine/clonidine synergism. Surprisingly, omega-agatoxin IVA reversed the additive morphine/clonidine interaction that occurs in morphine-tolerant mice back to synergism. These results suggest that functional P-type Ca2+ channels play an essential role in the antinociceptive synergism between spinal morphine and clonidine.

Effects of clonidine on a C-fibre reflex in the rat

A C-fibre reflex elicited by electrical stimulation within the territory of the sural nerve, was recorded from the ipsilateral biceps femoris muscle in anaesthetized rats. The temporal evolution of the response was studied using a constant stimulus intensity (3 x threshold) and recruitment curves were built by varying stimulus intensity from 0 to 7 x threshold. The intravenous administration of 0.02-0.2 mg/kg clonidine resulted in a dose-dependent depression of the C-fibre reflex. The alpha 2-adrenoceptor antagonist idazoxan completely prevented this depressive effect of clonidine. The effects of clonidine on the C-fibre reflex elicited by a wide range of stimulus intensities were investigated using recruitment curves: following 0.16 mg/kg clonidine, a dramatic shift of the recruitment curve to the right was seen with both an increase in the threshold and a decrease in the slope. Clonidine also produced a dose-dependent increase in blood pressure, but this was not correlated with the depression of the nociceptive reflex.

alpha-Adrenoceptor and opioid receptor modulation of clonidine-induced antinociception

1. The antinociceptive action of clonidine (Clon) and the interactions with alpha 1, alpha 2 adrenoceptor and opioid receptor antagonists was evaluated in mice by use of chemical algesiometric test (acetic acid writhing test). 2. Clon produced a dose-dependent antinociceptive action and the ED50 for intracerebroventricular (i.c.v.) was lower than for intraperitoneal (i.p.) administration (1 ng kg-1 vs 300 ng kg-1). The parallelism of the dose-response curves indicates activation of a common receptor subtype. 3. Systemic administration of prazosin and terazosin displayed antinociceptive activity. Pretreatment with prazosin produced a dual action: i.c.v. Clon effect did not change, and i.p. Clon effect was enhanced. Yohimbine i.c.v. or i.p. did not induce antinonciception, but antagonized Clon-induced activity. These results suggest that alpha 1- and alpha 2-adrenoceptors, either located at the pre- and/or post-synaptic level, are involved in the control of spinal antinociception. 4. Naloxone (NX) and naltrexone (NTX) induced antinociceptive effects at low doses (microgram kg-1 range) and a lower antinociceptive effect at higher doses (mg kg-1 range). Low doses of NX or NTX antagonized Clon antinociception, possibly in relation to a preferential mu opioid receptor antagonism. In contrast, high doses of NX or NTX increased the antinociceptive activity of Clon, which could be due to an enhanced inhibition of the release of substance P. 5. The results obtained in the present work suggest the involvement of alpha 1-, alpha 2-adrenoceptor and opioid receptors in the modulation of the antinociceptive activity of clonidine, which seems to be exerted either at spinal and/or supraspinal level.

The role of prostaglandins and the hypothalamic and hippocampal histamine in the clonidine-induced pituitary-adrenocortical response

Involvement of prostaglandins (PGs) and histamine in the hypothalamus and hippocampus in the clonidine-induced pituitary-adrenocortical response was investigated in conscious rats. The hypothalamic-pituitary adrenocortical (HPA) activity was assessed indirectly by measuring corticosterone secretion. Clonidine, an alpha 2-adrenergic agonist, given intracerebroventricularly (10 micrograms icv), considerably increased the serum corticosterone and hypothalamic histamine levels and markedly elevated the hippocampal histamine levels. Systemic or icv pretreatment with indomethacin (2 mg/kg or 10 micrograms), an inhibitor of prostaglandin synthesis, significantly reduced the clonidine-induced corticosterone response and abolished the increase in the hypothalamic and hippocampal histamine levels elicited by clonidine. Indomethacin in the doses used did not substantially change the resting serum corticosterone or hypothalamic and hippocampal histamine levels. These results indicate that prostaglandins and hypothalamic histamine are considerably involved in the HPA response to alpha 2-adrenergic receptor stimulation. They also suggest involvement of prostaglandins and histamine of the hippocampus in the clonidine-induced HPA response.

Neurotransmitter-induced inhibition of exocytosis in insulin-secreting beta cells by activation of calcineurin

Neurotransmitters and hormones such as somatostatin, galanin, and adrenalin reduce insulin secretion. Their inhibitory action involves direct interference with the exocytotic machinery. We have examined the molecular processes underlying this effect using high resolution measurements of cell capacitance. Suppression of exocytosis was maximal at concentrations that did not cause complete inhibition of glucose-stimulated electrical activity. This action was dependent on activation of G proteins but was not associated with inhibition of the voltage-dependent Ca2+ currents or adenylate cyclase activity. The molecular processes initiated by the agonists culminate in the activation of the Ca(2+)-dependent protein phosphatase calcineurin, and suppression of the activity of this enzyme abolishes their action on exocytosis. We propose that mechanisms similar to those we report here may contribute to adrenergic and peptidergic inhibition of secretion in other neuroendocrine cells and in nerve terminals.

Effects of optical isomers of ephedrine and methylephedrine on the twitch response in the isolated rat vas deferens and the involvement of alpha 2-adrenoceptors

The effects of optical isomers of ephedrine (EPH) and methylephedrine (MEP) on the twitch response to electrical stimulation (1 msec, 1 Hz) in the isolated rat vas deferens were investigated to clarify the action on alpha 2-adrenoceptors. l-EPH (10(-7) 3 x 10(-5) M) and d-EPH (10(-6)-10(-4) M) markedly inhibited the twitch response in the presence of prazosin (10(-6) M). l-MEP also inhibited the twitch response at high concentrations (3 x 10(-5)-10(-3) M). The rank order of inhibitory potency was 1-EPH > d-EPH >> l-MEP. Yohimbine (3 x 10(-7) M), a selective alpha 2-adrenoceptor antagonist, attenuated the twitch-inhibitory effects of EPH isomers and l-MEP. Furthermore, the twitch-inhibitory effects of EPH isomers and l-MEP were attenuated by reserpine treatment (8 mg/kg, s.c.). On the other hand, d-MEP showed the potentiation of twitch response, and competitively antagonized the twitch-inhibitory effect of clonidine (10(-9)-10(-6) M) with the pA2 value of 4.3 in the presence of prazosin. The results suggest that EPH isomers and l-MEP have stimulating activity for presynaptic alpha 2-adrenoceptors. In addition, the twitch-inhibitory effect of EPH isomers and l-MEP may be at least partly mediated through the release of noradrenaline. It is also suggested that d-MEP has competitive alpha 2-adrenoceptor antagonist activity.

Antinociceptive effects of oral clonidine and S12813-4 in acute colon inflammation in rats

Acute colonic inflammation was induced by perendoscopic injection of 50 microleters of dilute formalin (5%) in the depth of the colonic wall (c.w.) in rats. Compared to saline injection, the procedure was followed by nociceptive behaviors from which visceral nociception was quantified. The alpha 2-adrenoceptor agonist, clonidine 2-[2,6-dichlorophenylamine]-2-imidazole hydrochloride (75, 150 and 300 mg/kg), administered orally 15 min after c.w. injection of formalin significantly reduced the nociceptive responses at the high dose only. However, when administered 30 min prior to nociceptive stimulation, the compound exhibited an antinociceptive effect at the three doses. A novel analgesic, the compound "S12813-4' 3-[2-(4-phenylpiperazine-1-yl)-ethyl]-2-oxo-2,3-dihydro-oxazolo[b] pyridine, chlorydrate (10, 30 and 90 mg/kg), given orally displayed antinociceptive effects whatever the administration schedule, before or after c.w. injection of formalin. The antinociceptive effect of S12813-4 (30 mg/kg given orally) was prevented by subcutaneous (s.c.) injection of yohimbine or idazoxan (1 mg/kg). We conclude that visceral nociception elicited by formalin-induced colonic inflammation is attenuated by clonidine and S12813-4. The pharmacological profiles of the two compounds and the inhibition of the antinociceptive effect of S12813-4 by yohimbine and idazoxan suggest that noradrenergic mechanisms are involved in the transmission and/or modulation of the nociceptive influx arising from the inflamed colon.

Dexamethasone reduced clonidine-induced hypoactivity in mice

Reduced clonidine anti-nociception in mice given low doses of dexamethasone has encouraged us to investigate the effects of dexamethasone pretreatment on locomotor hypoactivity, another example of clonidine-induced behaviour in mice. Dexamethasone administered intraperitoneally (0.1, 1.0, 10 mg kg-1) 30 min before clonidine reduced clonidine-induced locomotor hypoactivity in the activity cage to an extent which was dose-dependent. Dexamethasone administered centrally (10 ng/mouse) 30 min before clonidine was also able to reduce clonidine-induced locomotor hypoactivity. Cycloheximide administered at a dose of 10 mg kg-1 2 h before clonidine did not change the effects of clonidine but was able to prevent the effects of dexamethasone on clonidine-induced hypoactivity. The glucocorticoid receptor antagonist RU38486 administered centrally at the dose of 1 ng/mouse did not change the effects of clonidine, whereas it was able to block the effects of dexamethasone on clonidine-induced locomotor hypoactivity. These results suggest that the effects of dexamethasone on clonidine-induced locomotor hypoactivity depend on the stimulating effects that dexamethasone exerts on the protein synthesis via the glucocorticoid receptor in the brain.

The positive inotropic effect of alpha 1A-adrenoceptor stimulation is inhibited by 4-aminopyridine

This study was designed to determine if 4-aminopyridine, a reported inhibitor of the transient outward K+ current (Ito), alters the inotropic actions elicited via stimulation of WB4101- or chloroethylclonidine-sensitive receptors in rat myocardium. WB4101 (N-[2-(2, 6-dimethoxyphenoxy)ethyl]-2,3-dihydro-1,4-benzodioxin-2-m ethanamine) is a competitive antagonist that is selective for alpha 1A- and alpha 1C-adrenoceptors, while chloroethylclonidine is an irreversible blocker that is reported to antagonize alpha 1B-, alpha 1C-, and alpha 1D-adrenoceptor binding. Inotropic effects of the alpha 1-adrenoceptor agonist phenylephrine were examined in isolated left atrial and papillary muscle before and after addition of 4-aminopyridine, and before and after addition of 4-aminopyridine in preparations pretreated with chloroethylclonidine or WB4101. In addition, effects of phenylephrine were examined before and after treatment with staurosporine (an inhibitor of protein kinase C) in chloroethylclonidine-pretreated preparations. Phenylephrine (10 microM) elicited a sustained positive inotropic response in left atria and a triphasic inotropic action in papillary muscle (transient positive and negative inotropic components preceding a sustained positive inotropic response). 4-Aminopyridine (1.0, 1.7, 3.0 mM) reduced the sustained positive inotropic responses in the absence of antagonists and in chloroethylclonidine-pretreated preparations. However, in the presence of 10 nM WB4101, 4-aminopyridine had no effect on the remaining inotropic actions of phenylephrine. The sustained positive inotropic response to the alpha 1-agonist in chloroethylclonidine-pretreated preparations was not inhibited by 100 nM staurosporine. These data suggest that the sustained positive inotropic actions of alpha 1A-adrenoceptor stimulation in rat atrial and ventricular myocardium are mediated via non-protein kinase C-associated reductions in Ito.

A comparative study of the reversal by different alpha 2-adrenoceptor antagonists of the central sympatho-inhibitory effect of clonidine

1. The recovery of the clonidine-induced hypotension, bradycardia and sympatho-inhibition produced by several putative alpha 2-adrenoceptor antagonists was investigated in pentobarbitone anaesthetized rats. The activity of four substances containing an imidazoline structure: idazoxan, methoxy-idazoxan, BRL44408 and atipamezole was compared with the effect of fluparoxan, yohimbine and L-657,743; in addition the effect of the alpha 1-adrenoceptor antagonist, prazosin, was also studied. 2. Prazosin (0.03-1 mg kg-1, i.v.) failed to alter the sympatho-inhibitory and hypotensive effects of clonidine (10 micrograms kg-1, i.v.). L-657,743 (0.01-1 mg kg-1, i.v.) induced a recovery of blood pressure, heart rate and renal sympathetic nerve activity. Yohimbine (0.03-3 mg kg-1, i.v.) completely reversed the sympatho-inhibitory effect of clonidine but did not alter its hypotensive effect. 3. The four imidazoline drugs: idazoxan (10-300 micrograms kg-1, i.v.), methoxy-idazoxan (1-100 micrograms kg-1, i.v.), BRL44408 (0.1-3 mg kg-1, i.v.) and atipamezole (0.03-1 mg kg-1, i.v.) and fluparoxan (10-300 micrograms kg-1, i.v.) reversed the clonidine-induced hypotension but produced only a partial recovery of the renal sympathetic nerve activity and of the heart rate. After pretreatment with prazosin (0.1 mg kg-1, i.v.), the recovery of the sympathetic nerve activity elicited by these compounds was significantly higher. In hexamethonium (10 mg kg-1, i.v.) pretreated rats, these five drugs induced dose-related hypertension which was reduced by pretreatment with prazosin (0.1 mg kg-1, i.v.). 4. Our results indicate that the putative alpha 2-adrenoceptor antagonists idazoxan, methoxy-idazoxan, BRL44408, atipamezole and fluparoxan also have a peripheral hypertensive effect which is mediated through activation of vascular alpha 1-adrenoceptors; this property of the compounds may be partly responsible for the reversal of the hypotensive action of clonidine. Considering the structure and the affinities of the drugs tested, our data indirectly suggest that alpha 2A-adrenoceptors may be implicated in the central sympatho-inhibitory effects of clonidine.

Analysis of the mechanisms underlying the antinociceptive effect of the extracts of plants from the genus Phyllanthus

1. We examine some of the mechanisms underlying the analgesic effects of the hydroalcoholic extracts (HE) of Phyllanthus urinaria and P. niruri against formalin-induced nociception in mice. In addition, we also investigate the action of both HEs against capsaicin-mediated pain. 2. Both prazosin and yohimbine (0.15 mg/kg, i.p.) induced a marked inhibition of the analgesic effect caused by phenylephrine (10 mg/kg, i.p.) and clonidine (0.1 mg/kg, i.p.), respectively, but had no effect on the antinociceptive action caused by HE of P. urinaria (10 mg/kg, i.p.) or P. niruri (30 mg/kg, i.p.). 3. NG-nitro-L-arginine (L-NOARG, 75 mg/kg, i.p.) caused marked analgesic effect against the second phase of formalin-induced pain. Treatment of animals with L-arginine (600 mg/kg) completely antagonized the antinociceptive effect of L-NOARG but had no significant effect against the HE of P. urinaria (10 mg/kg, i.p.) or P. niruri (30 mg/kg. i.p.) analgesic properties. 4. The antinociceptive effects caused by the HEs of P. urinaria (10 mg/kg, i.p.) and P. niruri (30 mg/kg, i.p.) were unaffected by methysergide (5 mg/kg, i.p.), p-chloro-phenylalanine-methyl-ester (100 mg/kg, i.p., once a day for 4 consecutive days) or after previous adrenalectomy of animals. 5. The HE of P. urinaria and P. niruri given either intraperitoneally (1-30 mg/kg) or orally (25-200 mg/kg) caused marked and dose-related inhibition of capsaicin-induced pain with ID50 of 2.1 and 6.1 mg/kg given intraperitoneally and 39 and 35 mg/kg given orally, respectively.

Antagonizing effects of VA-045 on reduced activity of rat locus coeruleus neurons following head injury or intravenous injection of clonidine

Based on the finding that VA-045, a novel apovincaminic acid derivative, had improved disturbance in consciousness, we examined the effects of the drug on the electrical activity of locus coeruleus (LC) neurons in animal models of consciousness disturbance. The animal models of consciousness disturbance used in this experiment were closed head injury (CHI) and intravenous injection of clonidine. CHI as well as clonidine injection reduced the spontaneous activity of LC neurons. The reduction of the spontaneus activity of LC neurons following CHI or clonidine injection was restored by intravenous injection of VA-045. The change of LC neuronal activity induced VA-045 preceded desynchronization of EEG. These results suggest that VA-045 exerts its ameliorating effect on consciousness disturbances, at least in part, by augmenting the spontaneous activity of noradrenergic LC neurons.

Effect of I1-imidazoline receptor activation on responses of hypoglossal and phrenic nerve to chemical stimulation

Sedation elicited by some centrally acting antihypertensive agents may interfere with respiratory control, and by selectively inhibiting upper airway dilating muscle activity it may facilitate obstructive sleep apnea. Autoradiographic studies with [125I]p-iodoclonidine in the presence of 10 microM epinephrine to block alpha 2-adrenergic sites or 100 nM moxonidine to mask I1-imidazoline sites show that both I1- as well as alpha 2-sites are localized in putative chemosensory areas of the rostral ventrolateral medulla in the cat. We sought to determine the effect of activating I1 and alpha 2-receptors on central chemosensitivity by using moxonidine as a selective I1 agonist, clonidine as a mixed I1/alpha 2 agonist, SK&F-86466 as a specific alpha 2-antagonist, and efaroxan as a mixed I1/alpha 2 antagonist. We recorded responses of phrenic, hypoglossal, and cervical sympathetic nerve activities to progressive hypercapnia after hyperventilation to apnea. Moxonidine (3-100 micrograms/kg i.v.) caused dose-dependent decreases in tonic cervical sympathetic nerve activity and blood pressure, but had no effect on the CO2 threshold (after 30 or 100 micrograms/kg moxonidine, phrenic nerve activity reappeared at 5.8 +/- 0.2% CO2 versus 5.6 +/- 0.3% CO2 in control). Following moxonidine, the slope of the steep portion of the CO2 response tended to increase (10.3 +/- 1.8 versus 7.3 +/- 0.9). Peak phrenic nerve activity was comparable to control at 7.5% CO2 (20 +/- 2 U in control) and at 9.5% CO2 (30 +/- 3 versus 27. +/- 2 U). Similarly, the response of hypoglossal and inspiratory phasic cervical sympathetic nerve activity to a progressive CO2 rise was not affected by moxonidine. By contrast, clonidine in the same doses decreased CO2 sensitivity, because the CO2 threshold was elevated from 5.3 +/- 0.5% to 6.7 +/- 0.4% (p < 0.001). The slope of the CO2 response was decreased from 9.7 +/- 1.9 to 7.4 +/- 1.3 (p = 0.05). Peak phrenic nerve activity was reduced at 7.5% CO2 (11 +/- 5 versus 25 +/- 2 U; p < 0.05) and at 9.5% CO2 (21 +/- 4 versus 33 +/- 2 U; p = 0.06). Clonidine selectively inhibited the response of hypoglossal nerve activity to CO2. The depressive effects of clonidine were reversed by alpha 2-blockade with SK&F-86466 (0.5 or 1 mg/kg). Inspiratory phasic cervical sympathetic nerve activity increased after SK&F-86466 in parallel with phrenic and hypoglossal nerve activity, but the tonic component of cervical sympathetic nerve activity and blood pressure increased only transiently.(ABSTRACT TRUNCATED AT 400 WORDS)

Alpha-adrenoceptors and angiotensinogen gene expression in opossum kidney cells

To investigate whether alpha (alpha)-adrenoceptor agonists have a stimulatory effect on the expression of the angiotensinogen (Ang) gene in opossum kidney (OK) cells, we used OK 27 cells with a fusion gene containing the 5'-flanking regulatory sequence of the rat angiotensinogen gene fused with a human growth hormone (hGH) gene as a reporter, pOGH (Ang N-1498/+18), permanently integrated into their genomes. The level of expression of the pOGH (Ang N-1498/+18) was quantitated by the amount of immunoreactive-human growth hormone (IR-hGH) secreted into the medium. The addition of iodoclonidine (alpha 2-adrenoceptor agonist, 10(-13) to 10(-9) M) and phorbol 12-myristate 13-acetate (PMA, 10(-13) to 10(-5) M) stimulated the expression of pOGH (Ang N-1498/+18) in a dose-dependent manner, whereas the addition of phenylephrine (alpha 1-adrenoceptor agonist, 10(-13) to 10(-5) M) had no effect. The stimulatory effect of iodoclonidine was blocked by the presence of yohimbine (alpha 2-adrenoceptor antagonist) and staurosporine (an inhibitor of protein kinase C) but not blocked by the presence of prazosin (alpha 1-adrenoceptor antagonist) or Rp-cAMP (an inhibitor of cAMP-dependent protein kinase A). The addition of iodoclonidine, phenylephrine or PMA had no effect on the expression of pTKGH in OK 13 cells, an OK cell line, into which had been stably integrated a fusion gene, pTKGH containing the promoter/enhancer DNA sequence of the viral thymidine-kinase (TK) gene fused with a human growth hormone gene as a reporter.(ABSTRACT TRUNCATED AT 250 WORDS)

Antagonism of the effects of clonidine by the alpha 2-adrenoceptor antagonist, fluparoxan

1. The effects of fluparoxan, an alpha 2-adrenoceptor antagonist, on the pharmacodynamic changes induced by clonidine were investigated in this placebo-controlled, double-blind, two-period, cross-over study in 16 healthy male volunteers (aged 19 to 44 years). 2. Subjects received either fluparoxan or placebo, twice-daily for 5 1/2 days (11 doses). One hour after the first and last dose of each treatment period, clonidine (200 micrograms) was infused intravenously over 5 min. 3. Indices of clonidine-mediated pharmacodynamic responses (growth hormone secretion, bradycardia, hypotension, xerostomia and sedation) were taken before and after clonidine infusion. Growth hormone secretion was assessed by quantifying serum growth hormone concentrations; sedation was assessed by both visual analogue scales (VAS) and by a visual psychomotor response meter, measuring critical flicker fusion (CFF). 4. The majority of subjects reported minor adverse events such as lethargy, headache and dry mouth following clonidine infusion. All adverse events were likely to be related to clonidine, as they occurred consistently between treatment groups. Fluparoxan has, however, in previous studies been reported to cause headache and light-headedness. 5. Prior to the clonidine infusion, fluparoxan caused small but statistically significant increases in systolic blood pressure (4 mm Hg) and salivary flow (approximately 30%) after both single and repeated doses. A small increase in heart rate (2 beats min-1) was seen after a single dose which was also statistically significant.(ABSTRACT TRUNCATED AT 250 WORDS)

Systemic clonidine differentially modulates the abnormal reactions to mechanical and thermal stimuli in rats with peripheral mononeuropathy

The antinociceptive action of the systemically administered alpha 2-adrenoceptor agonist clonidine was evaluated in a rat model of peripheral unilateral mononeuropathy produced by loose ligatures around the common sciatic nerve, using nociceptive tests based on mechanical (vocalization threshold to paw pressure) or thermal (struggle latency to paw immersion in a cold (10 degrees C) or hot (44 degrees C) water bath) stimuli. Experiments were performed 2-3 weeks after surgery when pain-related behavior was fully developed. We demonstrated a dissociative action depending on the test used: clonidine (30-100 micrograms/kg i.v.) had a moderate effect on the abnormal reactions to the mechanical stimulus. By contrast it dramatically increased the struggle latency to hot or cold stimuli. These latter effects were completely prevented by prior administration of the alpha 2-adrenoceptor antagonist idazoxan (0.5 mg/kg i.v.).