Actions of L-NAME and methylene blue on the hypotensive effects of clonidine and rilmenidine in the anesthetized rat

Blood pressure reduction induced by chronic intracerebroventricular or peroral clonidine administration in rats with salt-dependent or angiotensin II-dependent hypertension

The agonists of α2-adrenergic receptors such as clonidine, rilmenidine or monoxidine are known to lower blood pressure (BP) through a reduction of brain sympathetic outflow but their chronic antihypertensive effects in rats with low-renin or high-renin forms of experimental hypertension were not studied yet. Moreover, there is no comparison of mechanisms underlying BP reduction elicited by chronic peroral (po) or intracerebroventricular (icv) clonidine treatment. Male salt-sensitive Dahl rats fed 4% NaCl diet and Ren-2 transgenic rats were treated with clonidine administered either in the drinking fluid (0.5 mg/kg/day po) or as the infusion into lateral brain ventricle (0.1 mg/kg/day icv) for 4 weeks. Basal BP and the contributions of renin-angiotensin system (captopril 10 mg/kg iv) or sympathetic nervous system (pentolinium 5 mg/kg iv) to BP maintenance were determined in conscious cannulated rats at the end of the study. Both peroral and intracerebroventricular clonidine treatment lowered BP to the same extent in either rat model. However, in both models chronic clonidine treatment reduced sympathetic BP component only in rats treated intracerebroventricularly but not in perorally treated animals. In contrast, peroral clonidine treatment reduced angiotensin II-dependent vasoconstriction in Ren-2 transgenic rats, whereas it lowered residual blood pressure in Dahl rats. In conclusions, our results indicate different mechanisms of antihypertensive action of clonidine when administered centrally or systemically.

The role of α2-adrenergic receptors in hypertensive preeclampsia: A hypothesis

Preeclampsia, a major disorder of human pregnancy, manifests as persistent hypertension and proteinuria presenting after 20 weeks of pregnancy. Multiple systemic symptoms might be associated with preeclampsia including thrombocytopenia, liver impairment, pulmonary edema, and cerebral disturbances. However, vascular dysfunction remains the core pathological driver of preeclampsia. Defective placental implantation followed by dysfunctional placental spiral artery development promotes a hypoxic environment. Massive endothelial dysfunction characterized by reduced vasodilation, augmented vasoconstriction, and increased vascular permeability and inflammation ensues. Interestingly, the same signaling and inflammatory pathways implicated in preeclampsia appear to be shared with other vascular disorders involving alteration of α₂ -AR function. The role of α₂ -ARs in the regulation of microcirculatory function has long been recognized, thus raising the question of whether they are involved in the pathogenesis of vascular dysfunction in preeclampsia. Here, we review possible interplay between signaling and inflammatory pathways common to preeclampsia and α₂ -AR function/regulation. We speculate on the potential contribution of these receptors to the observed phenotype and the potential role for their pharmacological modulators as therapeutic interventions with preeclampsia.

Pharmacological mechanisms involved in the antinociceptive effects of dexmedetomidine in mice

Dexmedetomidine (DEX) is a α₂ -adrenoceptor (α₂ -AR) agonist used as an anesthetic adjuvant and as sedative in critical care settings. Typically, α₂ -AR agonists release nitric oxide (NO) and subsequently activate NO-GMPc pathway and have been implicated with antinociception. In this study, we investigate the pharmacological mechanisms involved in the antinociceptive effects of DEX, using an acetic acid-induced writhing assay in mice. Saline or DEX (1, 2, 5, or 10 μg/kg) was intravenously injected 5 min before ip administration of acetic acid and the resulting abdominal constrictions were then counted for 10 min. To investigate the possible mechanisms related to antinociceptive effect of DEX (10 μg/kg), the animals were also pretreated with one of the following drugs: 7-nitroindazole (7-NI; 30 mg/kg ip); 1H-[1,2,4] oxadiazole [4,3-a] quinoxaline-1-one (ODQ; 2.5 mg/kg, ip); yohimbine (YOH; 1 mg/kg, ip); atropine (ATRO; 2 mg/kg, ip); glibenclamide (GLIB; 1 mg/kg, i.p.) and naloxone (NAL; 0.2 mg/kg, ip). A rotarod and open-field performance test were performed with DEX at 10 μg/kg dose. DEX demonstrated its potent antinociceptive effect in a dose-dependent manner. The pretreatment with 7-NI, ODQ, GLIB, ATRO, and YOH significantly reduced the antinociceptive affects of DEX. However, NAL showed no effecting DEX-induced antinociception. The rotarod and open-field tests confirmed there is no detectable sedation or even significant motor impairment with DEX at 10 μg/kg dose. Our results suggest that the α₂ -AR and NO-GMPc pathways play important roles in the systemic antinociceptive effect of DEX in a murine model of inflammatory pain. Furthermore, the antinociceptive effect exerted by DEX appears to be dependent on KATP channels, independent of opioid receptor activity.

The role of NO-cGMP pathway and potassium channels on the relaxation induced by clonidine in the rat mesenteric arterial bed

The mechanisms involved in the vasodilation action of clonidine have not yet been completely elucidated. We investigated the potential mechanisms that seem to be involved in the clonidine vasodilator effect using rat isolated mesenteric arterial bed (MAB). In precontracted MAB, clonidine (10-300 pmol) induced a dose-dependent relaxation, that was inhibited by endothelium removal (deoxycholic acid - 2.5 mM) and reduced by the alpha(2) adrenoceptor inhibitors yohimbine (1-3 microM) and rauwolscine (1 microM). The endothelium-dependent vasodilation induced by clonidine was reduced by the nitric oxide (NO) synthase inhibitor L-NAME (0.3 mM) and guanylyl cyclase inhibitor ODQ (10 microM) but was not affected by indomethacin (3-10 microM) alone. High K+ (25 mM) solution reduced the vasodilator effect of clonidine that was further attenuated by L-NAME. In the presence of high K+ plus L-NAME, the residual vasodilator effect of clonidine was further reduced by indomethacin (3 microM). The Ca(2+)-dependent K+ channel (K+(Ca2+)) inhibitors, charybdotoxin (ChTx; 0.1 microM) plus apamin (0.1 microM), also reduced the vasodilation induced by clonidine, however this response was not further reduced in the presence of L-NAME as observed with acetylcholine (10 pmol). In the presence of ATP-dependent K+ channel (K+(ATP)) blocker, glibenclamide (10 microM), the inhibitory effect of ChTx plus apamin plus L-NAME was increased. In contrast, the vasodilation induced by clonidine was not affected by voltage-dependent K+ channels (K(V)) blocker, 4-aminopyridine (4-AP, 1 mM). In conclusion, our results demonstrate that clonidine activates alpha(2)-adrenoceptors in rat MAB and that the endothelium-dependent vasodilation is mediated by activation of NO-cGMP pathway, hyperpolarization due to activation of K+(Ca) and K+(ATP) channels. Prostaglandins might participate in the vasodilator effect of clonidine when NO and EDHF mechanisms are blunted.

Modulated hemodynamic response to clonidine in bile duct-ligated rats: the role of nitric oxide

Despite the well-known involvement of the peripheral sympathetic abnormalities in the development of cardiovascular complications of cholestasis, the role of the central sympathetic system is still elusive. The goal of this study was to evaluate the effects of central sympathetic tone reduction, through clonidine administration, on hemodynamic parameters of 7-day bile duct-ligated rats. The contributions of nitric oxide and endogenous opioids were also examined by acute intravenous (10 min before clonidine) or chronic daily subcutaneous administrations of N(omega)-nitro-L-arginine methyl ester (L-NAME, 3 mg/kg) or naltrexone (20 mg/kg). Seven days after bile duct ligation or sham operation, animals were anesthetized with sodium pentobarbital. After hemodynamic stabilization, clonidine (10 microg/kg) was injected intravenously, which elicited an initial hypertension (the peripheral effect) followed by persistent hypotension and bradycardia (the central effects). Cholestatic rats demonstrated significant basal bradycardia (P<0.001) and hypotension (P<0.05), which were corrected by chronic naltrexone but not L-NAME treatment. While the peripheral effect of clonidine was blunted, the central effects were exaggerated in cholestatic rats (P<0.01). Acute L-NAME treatment accentuated the hypertensive phase in sham-operated and cholestatic rats (P<0.05). However, the difference between the two groups was preserved (P<0.01). This treatment attenuated the central effects in both sham-operated and cholestatic rats to the same level (P<0.001). Chronic L-NAME treatment resulted in exaggeration of the peripheral response in cholestatic and central responses in sham-operated rats (P<0.05), and abolished the difference between the groups. Naltrexone treatment had no significant effect on either the central or the peripheral responses to clonidine. This study shows that both central and peripheral hemodynamic responses to clonidine are altered in cholestasis. It also provides evidence that nitric oxide contributes to the development of these abnormalities.

Central Adenosine Signaling Plays a Key Role in Centrally Mediated Hypotension in Conscious Aortic Barodenervated Rats

We tested the hypothesis that clonidine-evoked hypotension is dependent on central adenosinergic pathways. Five groups of male, conscious, aortic baroreceptor-denervated (ABD) rats received clonidine (10 microg/kg i.v.) 30 min after i.v. 1) saline, 2) theophylline (10 mg/kg), or 3) 8-(p-sulfophenyl)theophylline (8-SPT) (2.5 mg/kg) or 1 h after i.p. 4) dipyridamole (5 mg/kg) or 5) an equal volume of sesame oil. Blockade of central (theophylline) but not peripheral (8-SPT) adenosine receptors abolished clonidine hypotension. In contrast, dipyridamole substantially enhanced the bradycardic response to clonidine. In additional groups, intracisternal (i.c.) dipyridamole (150 microg) and 8-SPT (10 microg) enhanced and abolished, respectively, clonidine (0.6 microg i.c.)-evoked hypotension. Because clonidine is a mixed I1/alpha2 agonist, we also investigated whether adenosine signaling is linked to the I1 or the alpha2A receptor by administering the selective I1 (rilmenidine, 25 microg) or alpha2A [alpha-methylnorepinephrine (alpha-MNE), 4 microg] agonist 30 min after central adenosine receptor blockade (8-SPT; 10 microg i.c.) or artificial cerebrospinal fluid. The hypotensive response elicited by rilmenidine or alpha-MNE was abolished in 8-SPT-pretreated rats. To delineate the role of the adenosine A2A receptor in clonidine-evoked hypotension, i.c. clonidine (0.6 microg) was administered 30 min after central adenosine receptor A2A blockade [5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo[4,3-epsilon]-1,2,4-triazolo[1,5-c]-pyrimidine (SCH58261); 150 microg i.c.]. The latter virtually abolished the hypotensive and bradycardic responses elicited by clonidine. In conclusion, central adenosine A2A signaling plays a key role in clonidine-evoked hypotension in conscious aortic barodenervated rats.

Clonidine-evoked respiratory effects in anaesthetized rats

The respiratory effects of stimulation of alpha2-adrenergic receptors were studied in spontaneously breathing anaesthetized rats that were neurally intact, or bilaterally vagotomized, or subjected to bilateral combined midcervical vagotomy and section of the carotid sinus nerves. An intravenous clonidine bolus (15 microg kg(-1)) evoked a prolonged slowing of the respiratory rate in all the neural states explored. Vagotomy reduced the early clonidine-evoked decline, but not the augmentation of tidal volume that followed the decline. After section of the carotid sinus nerves, clonidine challenge continued to decrease the respiratory rate, but not the tidal volume. Blockade of alpha2-adrenergic receptors with intravenous doses of SKF 86466 (200 microg kg(-1)) abolished all respiratory effects of the clonidine challenge. In all the neural states studied, clonidine evoked a significant short-lived rise in mean arterial blood pressure followed by a decrease below the respective prechallenge value. The SKF 86466 pretreatment lowered mean arterial blood pressure control values and reduced the magnitude of postclonidine changes. These results indicate that: (i) clonidine-evoked activation of alpha2-adrenergic receptors affects the two components of the breathing pattern differently, and this occurs beyond the lung vagi; and (ii) changes in tidal volume result from excitation of the carotid bodies and are coupled with centrally mediated slowing of the respiratory rhythm.

Role of the NO-cGMP pathway in the systemic antinociceptive effect of clonidine in rats and mice

The mechanism underlying the analgesic effect of clonidine, an alpha(2)-adrenoceptor agonist, remains uncertain. Activation of alpha(2)-adrenoceptor induces the release of nitric oxide (NO) from endothelial cells, which has led us to test the hypothesis that the observed antinociceptive effect induced by the systemic administration of clonidine depends on the NO-cGMP pathway. The possible involvement of an opioid link in the antinociceptive effect of clonidine was also evaluated. The antinociceptive effect induced by systemic administration (intravenous or intraperitoneal) of clonidine was evaluated using the rat paw formalin, mice tail-flick and writhing tests. Clonidine (3-120 microg/kg) induces a dose-dependent antinociceptive effect in the formalin, tail-flick and writhing tests. The antinociceptive effect of clonidine in a dose that had no sedative effect assessed by rota rod test, was significantly reduced by NO-synthase and guanylyl cyclase inhibition. The antinociceptive effect of morphine, but not clonidine, was inhibited by naloxone. Our current results suggest that the antinociceptive effect of systemic clonidine does not involve the opioid receptor and is modulated by the NO-cGMP pathway.

Central blockade of nitric oxide synthesis reduces moxonidine-induced hypotension

1. Nitric oxide (NO) and alpha(2)-adrenoceptor and imidazoline agonists such as moxonidine may act centrally to inhibit sympathetic activity and decrease arterial pressure. 2. In the present study, we investigated the effects of pretreatment with l-NAME (NO synthesis inhibitor), injected into the 4th ventricle (4th V) or intravenously (i.v.), on the hypotension, bradycardia and vasodilatation induced by moxonidine injected into the 4th V in normotensive rats. 3. Male Wistar rats with a stainless steel cannula implanted into the 4th V and anaesthetized with urethane were used. Blood flows were recorded by use of miniature pulsed Doppler flow probes implanted around the renal, superior mesenteric and low abdominal aorta. 4. Moxonidine (20 nmol), injected into the 4th V, reduced the mean arterial pressure (-42+/-3 mmHg), heart rate (-22+/-7 bpm) and renal (-62+/-15%), mesenteric (-41+/-8%) and hindquarter (-50+/-8%) vascular resistances. 5. Pretreatment with l-NAME (10 nmol into the 4th V) almost abolished central moxonidine-induced hypotension (-10+/-3 mmHg) and renal (-10+/-4%), mesenteric (-11+/-4%) and hindquarter (-13+/-6%) vascular resistance reduction, but did not affect the bradycardia (-18+/-8 bpm). 6. The results indicate that central NO mechanisms are involved in the vasodilatation and hypotension, but not in the bradycardia, induced by central moxonidine in normotensive rats.

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.

The effects of nitric oxide synthase inhibitors on the sedative effect of clonidine

The mechanism underlying the Niteroi, Rio de Janeiro sedative effect of clonidine, an alpha2-adrenoceptor agonist, remains uncertain. Because activation of alpha2-adrenoceptors induces release of nitric oxide (NO), we tested the hypothesis that the sedative effect of clonidine depends on NO-related mechanisms. The effect of 7-nitro indazole on the sleeping time induced by clonidine was studied in Wistar rats. In addition, we examined the effect of clonidine, alpha-methyldopa, and midazolam on the thiopental-induced sleeping time in rats pretreated with N(G)-nitro-L-arginine-methyl-ester (L-NAME). The sleeping time induced by clonidine was significantly decreased by 7-nitro indazole. Thiopental sleeping time was increased by clonidine, alpha-methyldopa, and midazolam. L-NAME reduced the prolongation effect of clonidine and alpha-methyldopa, but did not alter the effect of midazolam on the thiopental-induced sleeping time. The inhibitory effect of L-NAME on clonidine-dependent prolongation of thiopental-induced sleeping time was reversed by L-arginine. These results suggest that NO-dependent mechanisms are involved in the sedative effect of clonidine. In addition, this effect seems to be specific for the sedative action of alpha2-adrenoceptors agonists.

IMPLICATIONS

Clonidine, an antihypertensive drug, is also a sedative. This sedative effect, although an adverse event in the treatment of hypertensive patients, can be helpful for sedation of surgical patients. The mechanism of this effect, however, is unknown. In this study, we show that the sedative effect of clonidine is mediated by nitric oxide, because it could be prevented by pretreatment with nitric oxide synthase inhibitors.

Prolonged general anesthesia in MR studies of rats

RATIONALE AND OBJECTIVES

Magnetic resonance (MR) imaging of laboratory animals may require general anesthesia to minimize body movements over many hours. The anesthetization technique should allow physiologic parameters to remain as close to normal as possible, permit fast recovery, allow safe, repeated use, and avoid attachment of ferrous metal components to the animal. The purpose of this study was to evaluate an anesthetization technique that was developed to meet each of these qualifications.

MATERIALS AND METHODS

In 15 rats (280-483-g body weight), general anesthesia was induced (with intramuscular ketamine hydrochloride, xylazine hydrochloride, acepromazine maleate, and atropine), a tail vein catheter was inserted, and preimaging surgical procedures were performed. A face mask was applied, the animal was positioned in a dorsal recumbent position on an acrylic board, and an isothermal heating pad was placed on the ventral aspect of the abdominal wall. The rat, on the board, was then inserted into a trough that contained a custom-built, linearly polarized birdcage head coil and placed in the bore of a 4.7-T horizontal-bore magnet. The face mask was connected to a non-rebreathing gaseous anesthetic system, and anesthesia was maintained with 1.5-2.0 L/min oxygen and 0.25%-1.50% isoflurane. Oxygen saturation, heart rate, and rectal temperature were continuously monitored.

RESULTS

The duration of intramuscular anesthesia was 110 minutes +/- 12, and the duration of gaseous anesthesia was 106 minutes +/- 43. The monitoring equipment permitted display of vital signs.

CONCLUSION

The method appeared safe, was easy to perform, maintained a stable physiologic state for the parameters monitored, and could be used for repeated anesthesia in the same animal.

Clonidine-induced nitric oxide-dependent vasorelaxation mediated by endothelial alpha(2)-adrenoceptor activation

1. To assess the involvement of endothelial alpha(2)-adrenoceptors in the clonidine-induced vasodilatation, the mesenteric artery of Sprague Dawley rats was cannulated and perfused with Tyrode solution (2 ml min(-1)). We measured perfusion pressure, nitric oxide (NO) in the perfusate using chemiluminescence, and tissue cyclic GMP by RIA. 2. In phenylephrine-precontracted mesenteries, clonidine elicited concentration-dependent vasodilatations associated to a rise in luminal NO. One hundred nM rauwolscine or 100 microM L(omega)-nitro-L-arginine antagonized the clonidine-induced vasodilatation. Guanabenz, guanfacine, and oxymetazoline mimicked the clonidine-induced vasorelaxation. 3. In non-contracted mesenteries, 100 nM clonidine elicited a maximal rise of NO (123+/-13 pmol); associated to a peak in tissue cyclic GMP. Endothelium removal, L(omega)-nitro-L-arginine, or rauwolscine ablated the rise in NO. One hundred nM aminoclonidine, guanfacine, guanabenz, UK14,304 and oxymetazoline mimicked the clonidine-induced surge of NO. Ten microM ODQ obliterated the clonidine-induced vasorelaxation and the associated tissue cyclic GMP accumulation; 10 - 100 nM sildenafil increased tissue cyclic GMP accumulation without altering the clonidine-induced NO release. 4. alpha(2)-Adrenergic blockers antagonized the clonidine-induced rise in NO. Consistent with a preferential alpha(2D)-adrenoceptor activation, the K(B)s for yohimbine, rauwolscine, phentolamine, WB-4101, and prazosin were: 6.8, 24, 19, 165, and 1489 nM, respectively. 5. Rat pretreatment with 100 mg kg(-1) 6-hydroxydopamine reduced 95% tissue noradrenaline and 60% neuropeptide Y. In these preparations, 100 nM clonidine elicited a rise of 91.9+/-15.5 pmol NO. Perfusion with 1 microM guanethidine or 1 microM guanethidine plus 1 microM atropine did not modify the NO surge evoked by 100 nM clonidine. 6. Clonidine and congeners activate endothelial alpha(2D)-adrenoceptors coupled to the L-arginine pathway, suggesting that the antihypertensive action of clonidine involves an endothelial vasorelaxation mediated by NO release, in addition to presynaptic mechanisms.