Effects of intracisternal and intravenous alpha-methyldopa and clonidine on haemodynamics and baroreceptor--heart rate reflex properties in conscious rabbits

Cardiopulmonary effects of dexmedetomidine, with and without vatinoxan, in isoflurane-anesthetized cats

OBJECTIVE

To characterize the cardiopulmonary effects of dexmedetomidine, with or without vatinoxan, in isoflurane-anesthetized cats.

STUDY DESIGN

Randomized, crossover experimental study.

ANIMALS

A group of six adult healthy male neutered cats.

METHODS

Cats were instrumented during anesthesia with isoflurane in oxygen. Isoflurane end-tidal concentration was set to 1.25 minimum alveolar concentration (MAC). Dexmedetomidine was administered using a target-controlled infusion system to achieve and maintain 10 target plasma concentrations ranging from 0 to 40 ng mL^-1. Furthermore, vatinoxan or an equivalent volume of saline was administered using a target-controlled infusion system to achieve and maintain a target plasma concentration of 4 μg mL^-1. Isoflurane concentration was adjusted after each change in dexmedetomidine concentration to maintain a concentration equivalent to 1.25 MAC. Heart rate (HR), arterial blood pressure, central venous pressure (CVP), pulmonary artery pressure (PAP), pulmonary artery occlusion pressure (PAOP), body temperature, cardiac output, arterial and mixed-venous blood gas and pH and drug concentrations were measured. Additional variables were calculated from the measurements.

RESULTS

Dexmedetomidine alone resulted in decreased HR, cardiac index, stroke index and oxygen delivery, and increased systolic, mean (MAP) and diastolic arterial pressure, CVP, PAP, PAOP, systemic vascular resistance index, rate-pressure product, left ventricular stroke work index and oxygen extraction ratio. Vatinoxan resulted in severe hypotension at target plasma dexmedetomidine concentrations <10 ng mL^-1. Vatinoxan attenuated the cardiovascular effects of dexmedetomidine at the 10 and 20 ng mL^-1 targets, but MAP could be maintained above 60 mmHg only when isoflurane concentration was <1.25 MAC. Less improvement in cardiovascular function was seen with vatinoxan at the 40 ng mL^-1 target plasma dexmedetomidine concentration.

CONCLUSIONS AND CLINICAL RELEVANCE

Vatinoxan, at the plasma concentration maintained in this study, attenuated the cardiovascular effects of dexmedetomidine in isoflurane-anesthetized cats. However, its administration resulted in hypotension, which may limit its clinical usefulness.

Comparison in Conscious Rabbits of the Baroreceptor-Heart Rate Reflex Effects of Chronic Treatment with Rilmenidine, Moxonidine, and Clonidine

We investigated the effects of chronic subcutaneous treatment with centrally-acting antihypertensive agents moxonidine, rilmenidine, and clonidine on the baroreflex control of heart rate (HR) in conscious normotensive rabbits over 3 weeks. Infusions of phenylephrine and nitroprusside were performed at week 0 and at weeks 1 and 3 of treatment to determine mean arterial pressure (MAP)-HR baroreflex relationships. A second curve was performed after intravenous methscopolamine to determine the sympathetic baroreflex relationship. The vagal component of the reflex was determined by subtracting the sympathetic curve from the intact curve. Clonidine and moxonidine (both 1 mg/kg/day), and rilmenidine (5 mg/kg/day), reduced MAP by 13 ± 3, 15 ± 2, and 13 ± 2 mmHg, respectively, but had no effect on HR over the 3-week treatment period. Whilst all three antihypertensive agents shifted baroreflex curves to the left, parallel to the degree of hypotension, moxonidine and rilmenidine decreased the vagal contribution to the baroreflex by decreasing the HR range of the reflex but moxonidine also increased sympathetic baroreflex range and sensitivity. By contrast clonidine had little chronic effect on the cardiac baroreflex. The present study shows that second generation agents moxonidine and rilmenidine but not first generation agent clonidine chronically shift the balance of baroreflex control of HR toward greater sympathetic and lesser vagal influences. These changes if translated to hypertensive subjects, may not be particularly helpful in view of the already reduced vagal contribution in hypertension.

Effect of Endothelin-1 on Baroreflexes and the Cardiovascular Action of Clonidine in Conscious Rabbits

We studied the influence of pretreatment with endothelin–1 on cardiac baroreflexes and on the effect of clonidine on blood pressure and heart rate. In order to avoid the complication of the direct vasoconstrictor effects of endothelin-1, initial dose-response studies in animals treated with a ganglion blocker were performed. Intravenous administration of 50, 200, and 1200 ng/kg of endothelin-1 produced biphasic changes in blood pressure, consisting of an immediate depressor response, followed by a long lasting and dose-dependent pressor effect (peak response 3 ± 1, 9 ± 3, and 33 ± 5 mmHg, respectively). Thus, the 50 ng/kg dose of endothelin-1 was used in subsequent studies. Conscious rabbits were pretreated on separate days with endothelin-1, either intravenously (50 ng/kg) or intracisternally (10 and 50 ng/kg), or with vehicle. The animals then received an intravenous dose (20 μg/kg) or an intracisternal dose (1 μg/kg) of clonidine and the effects on blood pressure and heart rate were measured. In vehicle-treated rabbits, the intravenous administration of clonidine induced a significant decrease in blood pressure and heart rate (15 min after injection: −15.7 ± 4.7 mmHg and −33 ± 4 b/min, respectively). Similarly, the intracisternal injection of clonidine lowered blood pressure (−16.0 ± 2.5 mmHg), but produced a less pronounced bradycardia (−18 ± 4 b/min). Endothelin pretreatment, either 50 ng/kg centrally or peripherally, had no significant effect on the hypotension or bradycardia produced either by central or peripheral injection of clonidine. At this dose, endothelin by itself did not produce significant changes in blood pressure or heart rate. There was a reduction of the gain of the baroreceptor-heart rate reflex with intracisternal endothelin-1. These results suggest that central ₂–adrenoceptor mechanisms involved in clonidine-induced hypotension and bradycardia do not appear to be influenced by activation of endothelin receptors.

Lidocaine, dexmedetomidine and their combination reduce isoflurane minimum alveolar concentration in dogs

The effects of intravenous (IV) lidocaine, dexmedetomidine and their combination delivered as a bolus followed by a constant rate infusion (CRI) on the minimum alveolar concentration of isoflurane (MAC_ISO) in dogs were evaluated. Seven healthy adult dogs were included. Anaesthesia was induced with propofol and maintained with isoflurane. For each dog, baseline MAC (MAC_ISO/BASAL) was determined after a 90-minute equilibration period. Thereafter, each dog received one of the following treatments (loading dose, CRI): lidocaine 2 mg kg⁻¹, 100 µg kg⁻¹ minute⁻¹; dexmedetomidine 2 µg kg⁻¹, 2 µg kg⁻¹ hour⁻¹; or their combination. MAC was then determined again after 45- minutes of treatment by CRI. At the doses administered, lidocaine, dexmedetomidine and their combination significantly reduced MAC_ISO by 27.3% (range: 12.5–39.2%), 43.4% (33.3–53.3%) and 60.9% (46.1–78.1%), respectively, when compared to MAC_ISO/_BASAL. The combination resulted in a greater MAC_ISO reduction than the two drugs alone. Their use, at the doses studied, provides a clinically important reduction in the concentration of ISO during anaesthesia in dogs.

Chronic ethanol attenuates centrally-mediated hypotension elicited via alpha(2)-adrenergic, but not I(1)-imidazoline, receptor activation in female rats

AIMS

This study dealt with the effect of chronic ethanol administration on hemodynamic responses elicited by alpha(2)-adrenergic (alpha-methyldopa) or I(1)-imidazoline (rilmenidine) receptor activation in telemetered female rats.

MAIN METHODS

The effects of alpha-methyldopa or rilmenidine on blood pressure (BP), heart rate (HR) and their variability were investigated in rats that received liquid diet without or with ethanol (5% w/v) for 12 weeks. To evaluate the effect of each drug on cardiovascular autonomic control (BP and HR variability) in the absence or presence of ethanol, three time-domain indices of hemodynamic variability were measured: (i) standard deviation of mean arterial pressure (SDMAP), (ii) standard deviation of beat-to-beat intervals, and (iii) root mean square of successive differences in R-R intervals.

KEY FINDINGS

In liquid diet-fed control rats, i.p. rilmenidine (600 microg/kg) or alpha-methyldopa (100 mg/kg) reduced BP along with decreases and increases, respectively, in HR. Both drugs had no effect on HR variability but reduced BP variability (SDMAP), suggesting a reduced vasomotor sympathetic tone. Ethanol feeding attenuated reductions in BP and SDMAP evoked by alpha-methyldopa but not by rilmenidine.

SIGNIFICANCE

We conclude that chronic ethanol preferentially compromises alpha(2)- but not I(1)-receptor-mediated hypotension in female rats probably via modulation of vasomotor sympathetic activity. These findings highlight the adequacy of rilmenidine use to lower BP in hypertensive alcoholic females.

Involvement of central α1- and α2-adrenoceptors on cardiovascular responses to moxonidine

In the present study we compared the effects produced by moxonidine (alpha2-adrenoceptor/imidazoline agonist) injected into the 4th cerebral ventricle and into the lateral cerebral ventricle on mean arterial pressure, heart rate and on renal, mesenteric and hindquarter vascular resistances, as well as the possible action of moxonidine on central alpha1- or alpha2-adrenoceptors to produce cardiovascular responses. Male Holtzman rats (n=7-8) anesthetized with urethane (0.5 g/kg, intravenously-i.v.) and alpha-chloralose (60 mg/kg, i.v.) were used. Moxonidine (5, 10 and 20 nmol) injected into the 4th ventricle reduced arterial pressure (-19+/-5, -30+/-7 and -43+/-8 mmHg vs. vehicle: 2+/-4 mmHg), heart rate (-10+/-6, -16+/-7 and -27+/-9 beats per minute-bpm, vs. vehicle: 4+/-5 bpm), and renal, mesenteric and hindquarter vascular resistances. Moxonidine (5, 10 and 20 nmol) into the lateral ventricle only reduced renal vascular resistance (-77+/-17%, -85+/-13%, -89+/-10% vs. vehicle: 3+/-4%), without changes on arterial pressure, heart rate and mesenteric and hindquarter vascular resistances. Pre-treatment with the selective alpha2-adrenoceptor antagonist yohimbine (80, 160 and 320 nmol) injected into the 4th ventricle attenuated the hypotension (-32+/-5, -25+/-4 and -12+/-6 mmHg), bradycardia (-26+/-11, -23+/-5 and -11+/-6 bpm) and the reduction in renal, mesenteric and hindquarter vascular resistances produced by moxonidine (20 nmol) into the 4th ventricle. Pre-treatment with yohimbine (320 nmol) into the lateral ventricle did not change the renal vasodilation produced by moxonidine (20 nmol) into the lateral ventricle. The alpha1-adrenoceptor antagonist prazosin (320 nmol) injected into the 4th ventricle did not affect the cardiovascular effects of moxonidine. However, prazosin (80, 160 and 320 nmol) into the lateral ventricle abolished the renal vasodilation (-17+/-4, -6+/-9 and 2+/-11%) produced by moxonidine. The results indicate that the decrease in renal vascular resistance due to moxonidine action in the forebrain is mediated by alpha1-adrenoceptors, while the cardiovascular effects produced by moxonidine acting in the brainstem depend at least partially on the activation of alpha2-adrenoceptors.

Medetomidine and dexmedetomidine: a review of cardiovascular effects and antinociceptive properties in the dog

Alpha(2)-adrenoreceptor agonists (alpha(2)-agonists) are commonly used in small animal anaesthesia for their potent sedative and analgesic properties, although concerns about their cardiovascular effects have prevented their full adoption into veterinary practice. Research into alpha(2) adrenoreceptor agonists and their clinical use is extensive, therefore this review focuses on the use of dexmedetomidine and medetomidine in dogs. Emphasis is given to the cardiovascular effects and antinociceptive action of these agents.

Chronic ethanol administration attenuates imidazoline I1 receptor- or alpha 2-adrenoceptor-mediated reductions in blood pressure and hemodynamic variability in hypertensive rats

Our previous studies have demonstrated that acute ethanol administration counteracts imidazoline I(1) receptor but not alpha(2)-adrenoceptor-mediated hypotension in spontaneously hypertensive rats (SHR). In the present study, we investigated the effect of chronic ethanol administration on hypotensive responses elicited by acute administration of selective imidazoline I(1) receptor (rilmenidine) or alpha(2)-adrenoceptor (alpha-methyldopa) agonist along with ethanol effects on: (i) locomotor activity and (ii) time-domain indices of variability in blood pressure (standard deviation of mean arterial pressure) and heart rate (standard deviation of beat-to-beat intervals and root mean square of successive differences in R-R intervals). Hemodynamic and locomotor responses elicited by rilmenidine or alpha-methyldopa were assessed in radiotelemetered ethanol-fed (2.5% or 5% w/v, 12 week) and control SHR. In control SHR, i.p. rilmenidine (600 microg/kg) or alpha-methyldopa (100 mg/kg) significantly reduced blood pressure. Rilmenidine had no effect on heart rate whereas alpha-methyldopa elicited a biphasic response (tachycardia followed by bradycardia). Blood pressure and heart rate oscillations were also reduced by both drugs, which may conform to sympathoinhibition. The hypotensive effect of rilmenidine or alpha-methyldopa was significantly attenuated by ethanol feeding (2.5% or 5%) in a concentration-dependent manner. In addition, ethanol attenuated alpha-methyldopa-evoked reduction in heart rate, but not blood pressure, variability in marked contrast to attenuating rilmenidine-evoked reductions in blood pressure, but not heart rate, variability. These findings demonstrate that, unlike its acute effects, chronic ethanol attenuates both imidazoline I(1) receptor and alpha(2)-adrenoceptor-mediated hypotension whereas its effect on hemodynamic variability depended on the nature of the hypotensive stimulus.

Effects of Long-Term Ovariectomy and Estrogen Replacement on Clonidine-Evoked Reductions in Blood Pressure and Hemodynamic Variability

The present study investigated the influence of 12-week ovariectomy (OVX) and estrogen supplementation (OVXE2) on the acute effects of the centrally acting antihypertensive agent clonidine on blood pressure, hemodynamic variability, and locomotor activity in Sprague-Dawley rats. The effects of estrogen depletion and repletion on the peripherally mediated hypotensive response to hydralazine were also evaluated to determine the selectivity of estrogen-clonidine interaction. The radiotelemetry technique was used for blood pressure and locomotor activity measurements. Three time-domain indices of hemodynamic variability were employed: (1). the standard deviation of mean arterial pressure (SDMAP) as a measure of blood pressure variability, and (2). the standard deviation of R-R intervals (SDRR) and the root mean square of successive differences in R-R intervals (rMSSD) as measures of heart rate variability. In control (sham-operated) rats, clonidine (30 microg/kg, i.p.) elicited significant decreases in MAP, blood pressure variability (SDMAP), and overall heart rate variability (SDRR). The reductions in MAP and its variability index (SDMAP) were significantly augmented in OVX rats and restored to sham-operated levels after estrogen replacement (17beta-estradiol subcutaneous pellet, 14.2 microg/d, 12 weeks). The locomotor activity was reduced by clonidine only in OVX rats. In contrast to clonidine, hydralazine (0.5 mg/kg, i.p.) hypotension was not altered by OVX or estrogen replacement. These findings suggest that estrogen negatively modulates centrally evoked hypotension versus no effect on hypotension of peripheral origin. Further, the results implicate the cardiovascular autonomic control in the enhanced hypotensive response to clonidine in OVX rats.

α-2 Adrenergic Transmission and Human Baroreflex Regulation

We observed earlier that central α-2 adrenoceptor stimulation in mice greatly augments parasympathetic tone. To test the effects in humans, we assessed autonomic vasomotor tone and baroreflex regulation in 9 normal young adults on 2 occasions, once with and once without clonidine. We determined heart rate (HR), beat-by-beat blood pressure (BP), and muscle sympathetic nerve activity. HR variability was analyzed in the time and frequency domain. Pharmacological baroreflex slopes were determined using incremental phenylephrine and nitroprusside infusions. Clonidine lowered resting BP (122±4/73±3 versus 100±7/55±3 mm Hg, P <0.01), muscle sympathetic nerve activity (18±3 versus 4±2 bursts/min, P <0.01), and HR (62±3 versus 56±3 bpm, P <0.05). The baroreflex heart rate curve was reset to much lower HR values and showed no saturation at low HR. HR variability profoundly increased during clonidine plus phenylephrine (total power: 3224±843 versus 8943±2329 ms 2 , P <0.05). High-frequency power was 1451±520 at baseline and 6720±2475 ms 2 during baroreceptor loading ( P <0.05). The low-frequency/high-frequency ratio decreased (1.94±0.41 versus 0.69±0.10, P <0.05). In contrast, clonidine reduced resting sympathetic vasomotor tone and shifted the operating point of the sympathetic baroreflex to a flat part of the sympathetic baroreflex curve. The shift decreased the ability of the baroreflex to withdraw sympathetic vasomotor tone during baroreflex loading. These baroreflex changes were associated with a moderate increase in phenylephrine responsiveness. We conclude that α-2 adrenoceptor stimulation has a differential effect on baroreflex HR and vasomotor regulation. α-2 Adrenoceptor stimulation greatly augments baroreflex-mediated bradycardia, most likely by parasympathetic activation.

Modulation of the baroreceptor reflex by alpha 2A-adrenoceptors: a study in alpha 2A knockout mice

1. Our objective was to determine whether alpha(2A)-adrenoceptors modulate the baroreceptor reflex. The efficacy of the reflex was evaluated by measuring the spontaneous blood pressure and heart rate variability at rest and the heart rate responses to evoked changes in blood pressure. Experiments were carried out in conscious, unrestrained, and anaesthetized alpha(2A)-adrenoceptor-deficient (alpha(2A)-KO) mice and WT mice. 2. In conscious alpha(2A)-KO mice, the spontaneous blood pressure variability was greater, and the spontaneous heart rate variability was lower than in conscious WT mice. This was also observed in anaesthetized animals. 3. The reflex bradycardia after intravenous injection of phenylephrine was greatly attenuated in conscious alpha(2A)-KO compared to conscious WT mice; the baroreceptor reflex gain (ratio maximal change in heart rate/maximal change in mean arterial pressure) was decreased by 40%. 4. Similar results were obtained when reflex bradycardia was elicited by intra-arterial volume loading of conscious WT and alpha(2A)-KO mice. The baroreceptor reflex gain upon volume loading was also low in anaesthetized alpha(2A)-KO mice. 5. The reflex tachycardia evoked by intravenous sodium nitroprusside injection was also significantly less in alpha(2A)-KO mice as compared to WT, conscious as well as anaesthetized; the baroreceptor reflex gains were decreased by 50 and 65%, respectively. 6. Direct stimulation of cardiac beta-adrenoceptors by the agonist isoprenaline produced similar cardioacceleration in alpha(2A)-KO and WT animals. 7. Our results show that the baroreceptor reflex function is impaired in mice lacking alpha(2A)-adrenoceptors. We conclude that central alpha(2A)-adrenoceptors facilitate the reflex response to both loading and unloading of the arterial baroreceptors.

A genetic polymorphism of the alpha2-adrenergic receptor increases autonomic responses to stress

We hypothesized that individual differences in autonomic responses to psychological, physiological, or environmental stresses are inherited, and exaggerated autonomic responsiveness may represent an intermediate phenotype that can contribute to the development of essential hypertension in humans over time. alpha(2)-Adrenergic receptors (alpha(2)-ARs), encoded by a gene on chromosome 10, are found in the central nervous system and also mediate release of norepinephrine from the presynaptic nerve terminals of the peripheral sympathetic nervous system and the exocytosis of epinephrine from the adrenal medulla. We postulated that, because this receptor mediates central and peripheral autonomic responsiveness to stress, genetic mutations in the gene encoding this receptor may explain contrasting activity of the autonomic nervous system among individuals. The restriction enzyme Dra I identifies a polymorphic site in the 3'-transcribed, but not translated, portion of the gene encoding the chromosome 10 alpha(2)-AR. Southern blotting of genomic DNA with a cDNA probe after restriction enzyme digestion results in fragments that are either 6.7 kb or 6.3 kb in size. Transfection studies of these two genotypes resulted in contrasting expression of a reporter gene, and it is suggested from these findings that this is a functional polymorphism. In a study of 194 healthy subjects, we measured autonomic responses to provocative motion, a fall in blood pressure induced by decreasing venous return and cardiac output, or exercise. Specifically, we measured reactions to 1) Coriolis stress, a strong stimulus that induces motion sickness in man; 2) heart rate responses to the fall in blood pressure induced by the application of graded lower body negative pressure; and 3) exercise-induced sweat secretion. In all of these paradigms of stress, subjective and objective evidence of increased autonomic responsiveness was found in those individuals harboring the 6.3-kb allele. Specifically, volunteers with the 6.3-kb allele had greater signs and symptoms of motion sickness mediated by the autonomic nervous system after off-axis rotation at increasing velocity (number of head movements a subject could complete during rotation before emesis +/- SE: 295 +/- 18 vs. 365 +/- 11; P = 0.001). They also had greater increases in heart rate in responses to the lower body negative pressure-induced fall in blood pressure (increase in heart rate +/- SE: 3.0 +/- 0.4 vs. 1.8 +/- 0.3; P = 0.012), and the 6.3-kb group had higher sweat sodium concentrations during exercise (mean sweat sodium concentration in meq/l over 30 min of exercise +/- SE: 43.2 +/- 7.1 vs. 27.6 +/- 3.4; P < 0.05). This single-nucleotide polymorphism may contribute to contrasting individual differences in autonomic responsiveness among healthy individuals.

Evidence for the involvement of central I1 imidazoline receptor in ethanol counteraction of clonidine hypotension in spontaneously hypertensive rats

Our previous studies have shown that ethanol counteracts centrally mediated hypotensive responses to clonidine. In this study, we investigated the relative roles of central alpha2-adrenergic and I1 imidazoline receptors in the antagonistic ethanol-clonidine hemodynamic interaction. The effects of selective blockade of alpha2- or I1 receptor by 2-methoxyidazoxan and efaroxan, respectively, on the blood pressure and heart rate responses to clonidine and subsequent ethanol administration were evaluated in conscious spontaneously hypertensive rats. Intracisternal administration of clonidine (1.5 microg/kg) produced significant (30 mm Hg; p < 0.05) and sustained (at least 60 min) decreases in blood pressure and heart rate. Systemic ethanol (1 g/kg), administered 10 min after clonidine, counteracted the hypotensive response and restored blood pressure to the preclonidine levels. Treatment with 2-methoxyidazoxan (0.16 microg/kg, intracisternal) or efaroxan (0.45 microg/kg, intracisternal) produced similar attenuation of the hypotensive and bradycardic responses to clonidine. The ability of ethanol to counteract the hypotensive action of clonidine was significantly (p < 0.05) attenuated in rats pretreated with efaroxan. The pressor response to ethanol lasted only 10 min compared with at least 60 min in the absence of efaroxan. In contrast, ethanol counteraction of clonidine-evoked hypotension was not altered when alpha2-adrenoceptors were blocked by 2-methoxyidazoxan. These findings suggest that centrally mediated hypotensive and bradycardic effects of clonidine in conscious spontaneously hypertensive rats involve activation of both alpha2-adrenergic and I1 imidazoline receptors. Furthermore, the findings suggest the dependence of a fully expressed ethanol counteraction of the hypotensive action of clonidine on functional I1 receptor within the central nervous system.

Dexmedetomidine and haemodynamic responses to acute central hypovolaemia in conscious rabbits

1. Effects of the alpha2-adrenoceptor agonist dexmedetomidine on vasoconstrictor and heart rate (HR) responses to acute central hypovolaemia were studied in eight chronically instrumented rabbits. We compared intravenous (i.v.) and fourth ventricular (V4) dexmedetomidine (0.1-10 microg/kg) and the reversal of effects by the alpha2-adrenoceptor antagonist idazoxan and the opioid agonist alfentanil. 2. Gradual inflation of an inferior vena cava (IVC) cuff reduced cardiac index (CI) by 8%/min, with progressive vasoconstriction and increased HR. In control rabbits, at approximately 40% baseline CI, there was sudden decompensation with failure of vasoconstriction and a fall in mean arterial pressure (MAP). 3. Dexmedetomidine (i.v. and V4) reduced resting MAP and HR and caused an earlier decompensation during central hypovolaemia. Intravenous dexmedetomidine (3 and 10 microg/kg) also reduced the slope of the initial vasoconstrictor response and the maximum HR. 4. The effects of dexmedetomidine were reversed by the antagonist idazoxan, which prevented the decompensation phase. Intravenous alfentanil was also effective in restoring the vasoconstrictor response and delaying decompensation with hypovolaemia after dexmedetomidine. Combining dexmedetomidine with an opioid, such as alfentanil, may provide the benefit of reduced sympathetic tone without increased risk of cardiovascular collapse.

Effects of moxonidine on corticocerebral blood flow under normal and ischemic conditions in conscious rabbits

Hypertension associated with excessive liberation of circulating and tissue catecholamines is an independent risk factor for further cardiovascular complications and an important predictor of stroke. Moxonidine is a centrally acting anti-hypertensive drug with potent action on I1-imidazoline receptors. It inhibits catecholamine release and is therefore expected to exert an antiadrenergic effect at various levels in the regulation of the cardiovascular system. The aim of this study was to investigate the effect of moxonidine (0.025-0.1 mg/kg, i.v.) on the normal and unilateral carotid occlusion-induced impaired corticocerebral blood flow (cCBF) determined by hydrogen polarography, on mean arterial blood pressure (MABP) and heart rate (HR) in conscious rabbits. Moxonidine produced a reduction of MABP and HR. On the other hand, after administration of the drug, a significant increase in the normal and impaired cCBF was observed. Because the improvement in cCBF was conspicuous in both normal and ischemic conditions, moxonidine might be beneficial not only in the treatment of hypertension but also in the management of cerebral ischemia.

Ethanol abolishes clonidine-induced impairment of baroreflex control of heart rate in conscious rats

Our previous studies showed that the ability of ethanol or clonidine to alter the baroreflex control of heart rate (baroreflex sensitivity, BRS) depends on the functional activity of aortic baroreflexes. In this study, we investigated the interaction between the two drugs on BRS in conscious rats with intact baroreflexes (shamoperated, SO) and after aortic baroreceptor denervation (ABD). The slope of the curve relating increments in mean arterial pressure induced by phenylephrine to corresponding reflex bradycardic responses was taken as an index of BRS. Ethanol (1 g/kg i.v.) significantly (p < 0.05) attenuated BRS in SO rats (-1.7 +/- 0.13 versus -1.04 +/- 0.15 beats/min/mm Hg) but not in ABD rats. Clonidine (30 microg/kg, i.v.) elicited significantly (p < 0.05) greater hypotensive responses in conscious ABD compared with SO rats. The BRS was not affected by clonidine administration in SO rats but showed significant (p < 0.05) reductions in ABD rats. Ethanol (1 g/kg, i.v.) had no effect on the hypotensive response to subsequently administered clonidine in ABD and SO rats; however, the effect of the two drugs on BRS was variable. In ABD rats, the BRS values before and after administration of ethanol and clonidine were similar, suggesting that pretreatment with ethanol counteracted clonidine-evoked attenuation of BRS in this rat preparation. In SO rats, the ethanol-clonidine combination produced a significant (p < 0.05) decrease in BRS, similar to the effect of ethanol when administered alone. These data confirm earlier findings that the aortic baroreflex arc modulates the interaction of ethanol and clonidine with baroreflex function. Further, the ability of ethanol to abolish clonidine-induced attenuation of BRS in ABD rats may relate to the compound effects of the two drugs on neuronal pathways participating in the central processing of baroreflexes in these rats.

alpha-2 and imidazoline receptor agonists. Their pharmacology and therapeutic role

Clonidine has proved to be a clinically useful adjunct in clinical anaesthetic practice as well as in chronic pain therapy because it has both anaesthetic and analgesic-sparing activity. The more selective alpha-2 adrenoceptor agonists, dexmedetomidine and mivazerol, may also have a role in providing haemodynamic stability in patients who are at risk of peri-operative ischaemia. The side-effects of hypotension and bradycardia have limited the routine use of alpha-2 adrenoceptor agonists. Investigations into the molecular pharmacology of alpha-2 adrenoceptors have elucidated their role in the control of wakefulness, blood pressure and antinociception. We discuss the pharmacology of alpha-2 adrenoceptors and their therapeutic role in this review. The alpha-2 adrenoceptor agonists are agonists at imidazoline receptors which are involved in central blood pressure control. Selective imidazoline agonists are now available for clinical use as antihypertensive agents and their pharmacology is discussed.

Effect of moxonidine on blood pressure and sympathetic tone in conscious spontaneously hypertensive rats

The effects of moxonidine on blood pressure, heart rate and sympathetic tone were studied in conscious spontaneously hypertensive rats. Intravenous moxonidine (80 nmol) transiently increased blood pressure without affecting heart rate or splanchnic nerve activity. Moxonidine (20-80 nmol) given into the fourth cerebral ventricle dose-dependently lowered mean arterial pressure, heart rate and sympathetic outflow (maximally by 60 +/- 3 mm Hg, 148 +/- 10 beats min(-1) and 15 +/- 3 microV). Moxonidine was more effective by this route than after the injection into the lateral ventricle. Clonidine (20-80 nmol) produced an initial pressor response after both intracerebroventricular routes of administration. A decrease in blood pressure was observed only when clonidine was given into the fourth ventricle. Clonidine decreased heart rate and splanchnic nerve activity similarly like moxonidine when the substances were given into the fourth ventricle. The data imply that the hypotensive effect of moxonidine is related to central sympathoinhibition. The main site of this action appears to be in the brainstem region.

Comparison of the baroreceptor-heart rate reflex effects of moxonidine, rilmenidine and clonidine in conscious rabbits

In 10 conscious rabbits, the baroreceptor-heart rate (HR) reflex effects of centrally acting antihypertensive agents with high affinity for imidazoline receptors (IRs), moxonidine and rilmenidine, were compared with clonidine which acts predominantly via central alpha2-adrenoceptors. Dose regimens were chosen to give similar hypotension (-17+/-1 mm Hg) and bradycardia (-27+/-2 b/min) for all three agents given into the fourth ventricle. Baroreceptor-HR reflex curves were assessed by i.v. drug induced changes in blood pressure. With all treatments, the baroreflex curves with both vagal and sympathetic effectors intact were shifted to the left, corresponding to the hypotension, and the bradycardia plateau was reduced. Rilmenidine and moxonidine also reduced the upper plateau such that the curves were shifted parallel down the HR scale with no change in the HR range. By contrast, clonidine only decreased the lower plateau, and thus increased HR range (+19+/-6%). Moxonidine, but not rilmenidine, reduced the baroreflex gain by reducing the curvature. Clonidine also decreased curvature but this did not result in a reduction in gain as it was offset by the increase in HR range. The gain and range of the cardiac sympathetic component, as assessed after vagal blockade, was reduced by rilmenidine by 53 and 40% respectively, but was not affected by the other agents. The calculated vagal component of the curves showed that all agents produced a greater vagal bradycardia in response to a rise in pressure and that both rilmenidine and clonidine increased vagal HR range. The present study results show that many of the baroreflex effects of clonidine, such as facilitating cardiac vagal responses, are shared by the second generation agent rilmenidine, suggesting that they are primarily due to alpha2-adrenoceptor activation. In addition, the inhibition of the sympathetic component of the baroreflex, observed with rilmenidine, and not clonidine suggests that this effect may involve IRs. By contrast moxonidine, the most specific agent for I1 receptors, produces mainly a baroreflex independent inhibition of cardiac sympathetic activity with little effect on vagal activity.

Relationship between imidazoline and alpha2-adrenoceptors involved in the sympatho-inhibitory actions of centrally acting antihypertensive agents

Since the first suggestion of the existence of imidazoline receptors, there has been a continuing and yet unresolved debate as to their contribution to the antihypertensive actions of clonidine-like agents. In this review we bring together a number of studies from our laboratory which have examined the importance and interdependence of imidazoline receptors and alpha2-adrenoceptors in the mechanism of action of centrally acting antihypertensive drugs. Using conscious rabbits and a range of imidazoline and specific alpha2-adrenoceptor antagonists we have consistently found that second generation agents rilmenidine and moxonidine preferentially act via imidazoline receptors but that alpha2-adrenoceptors are important for the hypotension produced by clonidine and alpha-methyldopa. Despite this difference in receptor mechanism, the hypotension produced by all these drugs is dependent on central noradrenergic pathways. In other studies using anaesthetised rabbits and direct measures of sympathetic nerve activity we confirmed the generally held view that the major site of sympatho-inhibitory actions and sympathetic baroreflex effects of centrally acting antihypertensive agents is the rostral ventrolateral medulla (RVLM). We also found, using microinjection of specific antagonists, that alpha2-adrenoceptors in this nucleus appear to be activated as a consequence of imidazoline receptor activation. Thus, there appears to be a close relationship between imidazoline receptors and alpha2-adrenoceptors located in the RVLM in mediating the hypotension and inhibition of renal sympathetic nerve activity. Furthermore in recent studies using a noradrenergic neurotoxin microinjected into the RVLM we found that this treatment selectively blocked the actions of moxonidine but not clonidine, suggesting that I1-imidazoline receptors may be located on adrenergic terminals in situ. By contrast, clonidine acts predominantly via alpha2-adrenoceptors, perhaps located on cell bodies in the nucleus. We conclude that there is indeed a close nexus between 'presynaptic' imidazoline receptors on noradrenergic terminals and 'downstream' alpha2-adrenoceptors within the RVLM. Our hypothesis brings together opposing points of view that the mechanism for hypotension must involve either the imidazoline receptor or the alpha2-adrenoceptor. Clearly both are important.

Ethanol selectively counteracts hypotension evoked by central I(1)-imidazoline but not alpha2-adrenergic receptor activation in spontaneously hypertensive rats

Previous studies from our laboratory showed that ethanol counteracts hypotensive responses to clonidine in spontaneously hypertensive rats. This study investigated whether this effect of ethanol involves interaction with central alpha2-adrenoceptors or I(1)-imidazoline receptors or both. The effects of ethanol (0.5 or 1 g/kg, i.v.) or an equal volume of saline on hypotensive and bradycardic responses to clonidine (mixed alpha2-adrenoceptor/I(1)-imidazoline receptor agonist), rilmenidine (selective I(1)-imidazoline receptor agonist), or alpha-methylnorepinephrine (selective alpha2-adrenoceptor agonist) were studied in conscious spontaneously hypertensive rats. Intracisternal administration of clonidine (0.5 microg), rilmenidine (25 microg), or alpha-methylnorepinephrine (4 microg) elicited similar decreases in mean arterial pressure (MAP; 25-30 mm Hg) that lasted > or =60 min. Subsequent administration of ethanol (0.5 and 1 g/kg, i.v.) counteracted the hypotensive effect of clonidine in a dose-related manner. Ethanol (1 g/kg) increased the blood pressure to levels similar to baseline (preclonidine) levels, and blood pressure remained significantly (p < 0.05) higher compared with the corresponding values in saline-treated rats. Similarly, ethanol (0.5 and 1 g/kg, i.v.) dose-dependently counteracted the hypotensive effect of rilmenidine. The antagonizing effects of ethanol on hypotension evoked by clonidine and rilmenidine were comparable in terms of both magnitude and duration. In contrast, ethanol (0.5 or 1 g/kg) had no effect on hypotension evoked by alpha-methylnorepinephrine. Except for a brief increase in blood pressure by ethanol (1 g/kg) at 5 min, blood pressure values obtained in alpha-methylnorepinephrine-treated rats receiving any of the two doses of ethanol were similar to postsaline values. Ethanol had no effect on bradycardic responses to any of the three hypotensive agents. Blood ethanol concentrations were similar regardless of the antihypertensive drug used. We concluded that the adverse hemodynamic effect of ethanol on centrally mediated hypotensive responses depends on the types of receptors involved in the elicitation of this response. That ethanol counteracts decreases in blood pressure evoked by clonidine and rilmenidine but not by alpha-methylnorepinephrine suggests an interaction between ethanol and central pathways involved in I(1)-imidazoline receptor-mediated hypotension.

Facilitation of reflex bradycardia does not contribute to the enhanced hypotensive effect of clonidine in aortic barodenervated rats

Our previous studies showed that the hypotensive effect of clonidine is enhanced in rats with surgically eliminated aortic baroafferents. In this study, we investigated whether this effect of clonidine is related to facilitation of baroreceptor control of reflex bradycardia. The effects of clonidine on blood pressure (BP), heart rate (HR), and baroreflex-mediated decreases in HR (baroreflex sensitivity, BRS) were studied in conscious aortic barodenervated (ABD) and sham-operated (SO) rats. The slope of the baroreflex curve relating increments in mean arterial pressure (MAP) induced by phenylephrine to corresponding baroreflex-mediated bradycardic responses was taken as an index of BRS. ABD but not the sham operation caused immediate and significant (p < 0.05) increases in BP and HR and an impairment of BRS. Two days after ABD, these parameters, except the BRS, subsided to near control levels. Starting from similar baseline values of BP and HR, clonidine (30 microg/kg, i.v.) elicited significantly (p < 0.05) greater decreases in MAP in conscious ABD rats compared with SO rats (-23 +/- 2 mm Hg vs. -7 +/- 2 mm Hg). The enhanced hypotensive effect of clonidine in ABD rats was associated with a significant reduction in baroreceptor-mediated reflex bradycardic responses to increments in BP evoked by phenylephrine. The slope of the baroreflex curves that represented the BRS showed approximately 40% reduction after treatment with clonidine (baseline BRS, -1.2 +/- 0.06 beats/min/mm Hg; clonidine, -0.73 +/- 0.07 beats/min/mm Hg). On the other hand, a threefold lower decrease in BP by clonidine in SO rats was not associated with any alteration in BRS. These findings support the hypothesis that aortic baroreceptors exert a tonically active restraining influence on centrally mediated hypotension. More important, the results do not favor a role for facilitation of baroreflexes in the enhanced hypotensive effect of clonidine in denervated rats.

Relative importance of medullary brain nuclei for the sympatho-inhibitory actions of rilmenidine in the anaesthetized rabbit

OBJECTIVE

To determine the contribution of the rostral ventrolateral medulla and the nucleus of the solitary tract in mediating the attenuation of the renal sympathetic baroreflex produced by administration of rilmenidine to anaesthetized rabbits and to examine the relative contribution of alpha2-adrenoceptors and imidazoline receptors at these sites to the cardiovascular effects of rilmenidine.

METHODS AND RESULTS

Rilmenidine micro-injected into the rostral ventrolateral medulla produced hypotension and inhibition of renal sympathetic nerve activity with doses an order of magnitude lower than those required in the nucleus tractus solitarius. Alpha-methylnoradrenaline, however, was similarly potent at producing hypotension when it was injected into the rostral ventrolateral medulla or nucleus tractus solitarius but, unlike rilmenidine, did not lower renal sympathetic nerve activity when it was injected into the nucleus tractus solitarius. The alpha2-adrenoceptor antagonist 2-methoxyidazoxan partially reversed the hypotension and renal sympathetic nerve activity inhibition due to alpha-methylnoradrenaline when it was administered into the rostral ventrolateral medulla, whereas the mixed alpha2-adrenoceptor/imidazoline receptor antagonists, idazoxan and efaroxan, did not. 2-Methoxyidazoxan, but not idazoxan, also reversed the hypotension when alpha-methylnoradrenaline was administered into the nucleus tractus solitarius. The hypotension induced by rilmenidine in the rostral ventrolateral medulla was completely reversed both by 2-methoxyidazoxan and by idazoxan, as was the sympathetic inhibition. To assess any interaction between the nucleus tractus solitarius and the rostral ventrolateral medulla in mediating the baroreflex effects of rilmenidine, we injected rilmenidine into the rostral ventrolateral medulla, the nucleus tractus solitarius or both nuclei and determined renal baroreflex responses of sympathetic nerve activity using drug-induced changes in blood pressure. Injection of 0.5 nmol rilmenidine into the rostral ventrolateral medulla reduced mean arterial pressure and basal renal sympathetic nerve activity as well as renal sympathetic baroreflex range (by 27%) and gain (by 35%). In contrast, injection of rilmenidine into the nucleus tractus solitarius had no effect on basal renal sympathetic nerve activity and renal sympathetic baroreflex parameters. The effect of combined injection was similar to that of administration into the rostral ventrolateral medulla alone.

CONCLUSION

Our results show that the rostral ventrolateral medulla, rather than the nucleus tractus solitarius, is the major site involved in the hypotension and inhibition of the renal sympathetic baroreflex by rilmenidine. Comparison of the actions of alpha2-adrenoceptor and imidazoline receptor antagonists on the effects of rilmenidine and alpha-methylnoradrenaline suggests that these agents are acting at different receptors, presumably imidazoline and alpha2-adrenoceptors receptors, respectively, and that both are important in lowering sympathetic tone and blood pressure in the rostral ventrolateral medulla.

Pretreatment with the dopamine agonist quinpirole inhibits central antihypertensive mechanisms in rats

1. Intravenous or central treatment of spontaneously hypertensive rats (SHR) with the dopamine D2 receptor agonist quinpirole caused a short-lasting pressor response with little effect on heart rate. 2. At 30 min after intravenous administration of quinpirole, the antihypertensive effect of rilmenidine was significantly inhibited. This interaction of quinpirole and rilmenidine was similarly observed when quinpirole was administered either intravenously (0.3 or 0.1 mg/kg), in the lateral cerebral ventricles (0.1 mg/kg) or intracisternally (0.1 mg/kg) or when rilmenidine was administered intravenously (1 mg/kg) or intracisternally (0.1 mg/kg). 3. The apparent desensitization to the antihypertensive effect of rilmenidine 30 min after pretreatment with quinpirole was not observed after a 4 or 24 h interval. 4. These data suggest that quinpirole has prolonged effects on central sympathetic vasomotor mechanisms that are the target of centrally acting antihypertensive drugs. These and previous results show a functional interaction between central dopamine D2 receptor activation and sympathetic responses mediated by a wide range of different receptors, including imidazoline and 5-hydroxytryptamine 5-HT1A-receptors and alpha 2-adrenoceptors.

Effect of rilmenidine on arterial pressure and urinary output in the spontaneously hypertensive rat

Rilmenidine is an antihypertensive agent acting at the imidazoline receptor that may have both central effects in the ventral lateral medulla and direct effects on the kidney to alter Na+ excretion. The present experiments examined whether rilmenidine induces a leftward shift or change in the slope of the pressure-natriuresis curve in the spontaneously hypertensive rat (SHR). A single oral gavage dose indicated that 3 and 10 mg/kg rilmenidine significantly lowers arterial pressure at 4-12 h after administration by oral gavage. The effect of rilmenidine on pressure-natriuresis was studied using twice daily doses of 1 and 3 mg/kg for control and treated SHR drinking tap water or 1% NaCl for 3 days. Na+ excretion was measured over 24 h, and mean arterial pressure was measured 6-8 h after the morning dose of rilmenidine. The results indicate that 1 mg/kg had no effect, while the pressure-natriuresis relationship for the rats receiving the 3 mg/kg dose was shifted to the left and was not significantly different from the vertical slope of the untreated SHR. This experiment also suggested that rilmenidine may attenuate the salt preference of the rats. This was confirmed in an additional series of experiments in which the rats had access to both tap water and 1% NaCl. Thus, rilmenidine shifts the pressure-natriuresis relationship to the left and reduces salt preference in SHR.

Importance of imidazoline receptors in the cardiovascular actions of centrally acting antihypertensive agents

Increasing evidence indicates that the hypotensive effect of centrally acting antihypertensive drugs is not due to stimulation of alpha 2-adrenoceptors but to action on imidazoline receptors (IR). This has led to the development and recent clinical use of second generation agents such as rilmenidine and moxonidine that possess a much greater selectivity toward these nonadrenergic receptors. However, relatively few studies have examined the role of these receptors in conscious animals or have adequately accounted for the alpha 2-adrenoceptor antagonist properties of IR antagonists such as idazoxan. We have taken the approach of initially calibrating the alpha 2-adrenoceptor antagonist potency of intracisternally (ic) administered idazoxan and the IR-1 receptor antagonist efaroxan against 2-methoxyidazoxan, a highly selective alpha 2-adrenoceptor antagonist with little or no imidazoline antagonist effect. This was done using alpha-methyldopa, a hypotensive agent affecting only alpha 2-adrenoceptors. Thus, we chose doses of the antagonists with equal alpha 2-adrenoceptor blocking action such that differences in the ability of idazoxan or efaroxan compared to 2-methoxy-idazoxan to reverse the hypotension produced by rilmenidine, moxonidine, or clonidine indicate an interaction with IR. By this method we found that the hypotensive effects of rilmenidine and moxonidine at moderate intracisternal doses were more readily reversed by the imidazoline antagonists than by 2-methoxy-idazoxan, indicating that IR were largely responsible for their hypotensive actions. By contrast, clonidine's effects were equally reversed by all antagonists, suggesting interaction mainly with alpha 2-adrenoceptors. In conscious rabbits with chronic renal sympathetic nerve electrodes we examined the effect of rilmenidine and alpha-methyldopa on the renal sympathetic baroreflex. Both drugs reduced renal sympathetic nerve activity and sympathetic baroreflex responses, but only the effect of rilmenidine was preferentially reversed by idazoxan. Thus, both IR and central alpha 2-adrenoceptor receptors can influence the renal baroreflex, but the former are relatively more important for the actions of rilmenidine. We recently examined the possible sites of action of rilmenidine in anesthetized rabbits and showed that sixfold lower doses were required to reduce blood pressure when the drug was injected into the rostral ventrolateral medulla compared to intracisternal administration. At this site rilmenidine also reduced renal sympathetic tone and inhibited renal sympathetic baroreflex responses. By contrast, rilmenidine was relatively ineffective when injected into the nucleus of the solitary tract. These experiments support the view that rilmenidine acts primarily at IR in the rostral ventrolateral medulla to reduce sympathetic tone and modulate sympathetic baroreflexes.

Clonidine and rilmenidine suppress hypotension-induced Fos expression in the lower brainstem of the conscious rabbit

Our current knowledge of the sites of action of the centrally-acting antihypertensive drug clonidine is based almost entirely on experiments in anesthetized animals. The aim of this study was to determine, in conscious rabbits, the sites of action in the brainstem of systemically administered clonidine, as well as its oxazoline analog rilmenidine. Three groups of experiments were carried out. In the first group, hypotension was produced by continuous intravenous infusion of sodium nitroprusside, at a rate sufficient to decrease arterial pressure by 20-30 mmHg, maintained for a period of 60 min. In the second and third groups of experiments, sustained hypotension was also produced by nitroprusside infusion as in the first group, but this was preceded by intravenous injection of clonidine (7-30 micrograms/kg i.v.) or rilmenidine (150-300 micrograms/kg i.v.), respectively. In confirmation of our previous study [Li Y.-W. and Dampney R. A. L. (1994) Neuroscience 61, 613-634], hypotension produced by nitroprusside alone induced a large increase (compared to sham control experiments) in the neuronal expression of Fos (a marker of neuronal activation) in the nucleus of the solitary tract, area postrema, the rostral, intermediate and caudal parts of the ventrolateral medulla, A5 area, locus coeruleus and subcoeruleus, and parabrachial nucleus. In comparison with this group, in rabbits pretreated with clonidine the numbers of Fos-positive cells were greatly reduced (by 76-94%) in the rostral, intermediate and caudal parts of the ventrolateral medulla, area postrema, A5 area, locus coeruleus and subcoeruleus. Clonidine pretreatment also caused a more moderate reduction (by 45%) in the number of Fos-positive cells in the nucleus of the solitary tract, but had no effect on Fos expression in the parabrachial nucleus. Double-labeling for tyrosine hydroxylase and Fos immunoreactivity showed that clonidine pretreatment greatly reduced the numbers of both catecholamine and non-catecholamine Fos-positive neurons. Rilmenidine pretreatment also greatly reduced Fos expression in the lower brainstem, with a very similar pattern to that observed after clonidine pretreatment. The results indicate that in conscious animals both clonidine and rilmenidine cause a widespread but selective inhibition of neurons in the pons and medulla that are normally activated by a hypotensive stimulus. In contrast to previous observations in anesthetized animals, the results suggest that (i) systemic administration of both drugs inhibits non-catecholamine as well as catecholamine neurons in the ventrolateral medulla, and (ii) the regional pattern of neuronal inhibition following administration of equipotent hypotensive doses of both drugs is very similar.

Central monoamine systems and new antihypertensive agents

This review describes the relationship between central monoamine pathways and centrally acting antihypertensive agents. By using antagonists with affinity for alpha 2-adrenoceptors and also imidazoline receptors we have found that the first generation agents clonidine and alpha-methyldopa activate alpha 2-adrenoceptors while the newer second generation antihypertensive agents rilmenidine and moxonidine activate imidazoline receptors. Despite the difference in receptors activated, the hypotension produced by central administration of all agents was attenuated after chemical lesioning of the brainstem noradrenergic or serotonergic pathways suggesting a similar dependence on central monoamine pathways. Since the acute 6-hydroxydopamine-induced release of noradrenaline in the brainstem produces hypotension it suggests that these agents normally mimic brainstem noradrenergic function. By contrast the pressor response shortly following 5,6-dihydroxytryptamine suggests serotonergic neurones in the brainstem are pressor and that part of the anti-hypertensive action of centrally acting antihypertensive agents is mediated by inhibition of bulbar serotonergic pathways. We suggest that the similar haemodynamic and baroreflex effects of the two generations of agents can be explained by the alpha 2-adrenoceptors and the imidazoline receptors being in series along the noradrenergic and serotonergic pathways.

Involvement of imidazoline-preferring receptors in regulation of sympathetic tone

We examined the contribution of imidazoline-preferring receptors (IPR) and alpha 2-adrenoceptors at different levels of the central nervous system in the antihypertensive and sympathoinhibitory actions of rilmenidine in 2 conscious animal models, the spontaneously hypertensive rat (SHR) and the normotensive rabbit. In conscious SHRs, we compared the potency of rilmenidine and clonidine administered intravenously into the lateral cerebral ventricle, the cisterna magna, and into the subarachnoidal space of the thoracolumbar spinal cord. In SHRs, we found that rilmenidine was more potent and more effective by the intrathecal than the intracisternal route. By contrast, clonidine was most effective after administration into the cisterna magna. Intravenous administration of rilmenidine or clonidine induced dose-dependent and prolonged decreases in blood pressure and heart rate. Neither rilmenidine nor clonidine altered mean arterial pressure or heart rate when given into the lateral cerebral ventricle. These data suggest that in SHRs the spinal cord may be an important site for the antihypertensive action of rilmenidine. We therefore characterized the receptor type involved. We observed in conscious SHRs that intrathecal post-treatment with idazoxan, a mixed alpha 2-adrenoceptor and IPR antagonist, abolished the antihypertensive effect of rilmenidine, whereas 2-methoxyidazoxan, a selective alpha 2-adrenoceptor antagonist, caused only a partial reversal of the blood pressure effects of rilmenidine. These results suggest that rilmenidine acts mainly through IPR rather than alpha 2-adrenoceptors in the spinal cord. In view of these findings, we compared the hypotensive actions of rilmenidine and clonidine, administered into the lateral cerebral ventricle, the cisterna magna, and the subarachnoid space of the thoracolumbar spinal cord in conscious normotensive rabbits. Both drugs were less potent and effective when administered intrathecally than intracisternally. These experiments suggest that the hypotensive action of rilmenidine and clonidine in the rabbit is mediated through receptors mainly located in the brainstem. Further, we found that idazoxan reversed the hypotensive action of rilmenidine more readily than 2-methoxyidazoxan. Surprisingly, both idazoxan and 2-methoxyidazoxan completely reversed the depressor effects of clonidine. Therefore, in the rabbit, rilmenidine acts through IPR located in the brainstem and clonidine acts predominantly through alpha 2-adrenoceptors. In conclusion, our studies demonstrate that IPR are involved in the vasodepressor action of rilmenidine in both conscious SHRs and rabbits. However, although the main site of action of rilmenidine in SHRs may be located in the thoracolumbar spinal cord, in the rabbit it appears to be in the brainstem.(ABSTRACT TRUNCATED AT 250 WORDS)

Behavioral and autonomic responses to intermittent social stress: differential protection by clonidine and metoprolol

The present study investigated physiological and pharmacological characteristics of socially "stressed" animals. Specifically, we examined (1) to what degree autonomic and behavioral "stress" reactions during intermittent confrontations between an intruder male adult Long-Evans rat with an aggressive resident undergo habituation, and (2) to what extent the defeat-experienced animal can be protected against these "stress" reactions with clonidine or metoprolol, two adrenergic agents with clinical anxiolytic effects. We developed an acute social stress situation that consisted of initially placing an experimental rat as an intruder into the homecage of a resident while the resident was not present, thereafter permitting brief physical agonistic interactions with the reintroduced resident until the intruder was forced into a submissive supine posture and emitted ultrasonic vocalizations (USV), and eventually exposing the intruder to the resident's threats for one hour, while being shielded from potentially injurious attacks ("threat encounter"). Over the course of the initial 4-weekly threat encounters the acute tachycardia but not the hyperthermic stress responses decreased in magnitude. Following the first three threat encounters core temperature (Tc) was significantly elevated for at least 3 h. The Tc was already elevated when the repeatedly defeated intruder was confronted with the olfactory cues of the resident's cage. This conditioned "anticipatory" hyperthermia developed in the course of the first three confrontations and was paralleled by a decrease in exploratory and motor behavior and by an increase in defensive behaviors and in both types of USV emitted in the "low" (20-30 kHz) and the "high" (31-70 kHz) frequency range. Clonidine (0.01-0.1 mg/kg, IP), an alpha 2-adrenergic agonist and metoprolol, a beta-adrenergic blocker (1.0-10.0 mg/kg, IP), dose-dependently prevented the tachycardic response to stress. Only clonidine, but not metoprolol, also attenuated the rise in Tc during the 1-h agonistic interaction. Clonidine decreased those aspects of motor behavior (e.g. rearing, walking) that are of lesser "cost" for the individual but maintained high levels of defensive reactions and increased the duration of "low" USV. The high doses of clonidine (0.06, 0.1 mg/kg) attenuated the homeostatic regulation and sedated the intruder while exposed to threats during a social confrontation. The absence of attenuation of the high level of defensive behavior and the prolonged "low" USV suggest a stress intensification by the higher doses of clonidine. In conclusion, after the fourth encounter, the autonomic, behavioral and vocal response pattern prior to and during repeated weekly confrontations show no evidence for habituation for the following 6 weeks.(ABSTRACT TRUNCATED AT 400 WORDS)

Cardiac baroreflexes and hypertension

1. The role of cardiac reflexes in baroreflex control mechanisms and the changes that occur in chronic hypertension is reviewed. The rapid resetting properties of the arterial baroreceptors ensures its role in short-term rather than long-term control of blood pressure. 2. In hypertensive humans and animals, the baroreceptor-heart rate reflex has diminished sensitivity due mainly to reduced maximum capacity of the cardiac vagal component rather than a change to the sympathetic. 3. The development of this vagal deficit in spontaneously hypertensive rats (SHR) coincides with the onset of cardiac hypertrophy rather than vascular hypertrophy. A combination of chronic perindopril treatment regimens in young and older SHR showed that the vagal deficit was better correlated with the degree of cardiac hypertrophy than with other variables such as blood pressure, hypertension or indices of vascular hypertrophy. Similar results have been shown for renovascular hypertension in rats. 4. The bradycardia produced by electrical stimulation of the vagus in urethane anaesthetized young SHR was enhanced compared to Wistar-Kyoto rats (WKY) while responses observed in adult SHR and WKY were similar, suggesting that the vagal deficit in hypertensive rats is not due to a defect in the efferent arm of the baroreflex. 5. The association of the vagal deficit with cardiac hypertrophy, but not with the vagal efferent pathways, suggests an important role for cardiac afferents in hypertension in mediating the baroreflex deficit. 6. A diminished baroreflex and also a reduced heart rate variability is an independent risk factor for sudden death following myocardial infarction. Thus, antihypertensive therapy, which more effectively reduces cardiac hypertrophy, should have a desirable effect on baroreceptor reflexes and therefore in reducing blood pressure variability.

Spectral analysis of blood pressure and heart rate variability in response to stress from air-jet in the Lyon rat

1. Power spectral analysis of the frequency of wave-forms of blood pressure and heart rate was used to characterize short-term fluctuations of these parameters in three strains of conscious Lyon rat, normotensive (LL and LN) and hypertensive LH. 2. A mild stress produced by means of a jet of air elicited blood pressure rises, associated with tachycardia. This response was of similar magnitude in the three strains. The stressor amplified the medium frequency (195-605 mHz) Mayer waves of blood pressure and heart rate which are under autonomic control. 3. Clonidine (10 micrograms kg-1 i.v.) lowered blood pressure and heart rate and dramatically reduced the amplitude of blood pressure and heart rate oscillations in the frequency region of 195-605 mHz. 4. A jet of air applied after clonidine administration led to blood pressure rise associated with tachycardia and enhanced oscillations in the 195-605 mHz region. 5. These results indicate that in Lyon normotensive and hypertensive rats, a mild emotional stressor elicits blood pressure and heart rate rises associated with spectral modifications reflecting sympathetic hyperactivity. Clonidine lowers blood pressure and heart rate and reduces their related variabilities. This effect is more pronounced in LH rats than in their normotensive controls. Clonidine appears to reduce the autonomic response to stress as indicated by the medium frequency oscillations.

L 4 - IMPORTANCE OF IMIDAZOLINE RECEPTORS IN THE CARDIOVASCULAR RESPONSES TO CLONIDINE AND RILMENIDINE IN CONSCIOUS RABBITS

The present paper summarizes our studies concerning the involvement of imidazoline and alpha 2-adrenoceptors in the cardiovascular actions of centrally acting drugs rilmenidine, clonidine and methyldopa. We have found that they produce very similar cardiovascular autonomic effects which relate directly to the function of central monoamine neurotransmitters. They mimic certain elements of the noradrenergic neuron system in the central nervous system, in particular the brainstem actions which involve hypotension, bradycardia and resetting of the baroreceptor heart rate reflex. By contrast they turn off serotonergic pathways that are pressor, produce tachycardia and inhibit the baroreceptor heart rate reflex. Recent studies using specific receptors antagonist drugs idazoxan and 2-methoxy-idazoxan indicate that in conscious rabbits the imidazoline receptor actions of rilmenidine is of primary importance at doses which would be considered clinically relevant. We further conclude that the alpha 2-adrenoceptors and the imidazoline receptors are likely to be located in series i.e. along the same cardiovascular autonomic pathways in the brainstem but presumably at different sites.

Effects of α-adrenoceptor antagonists and clonidine on the haemodynamic response to acute hypovolaemia in conscious rabbits

In conscious rabbits an inferior vena caval cuff was progressively inflated so cardiac output fell at a constant approximately 8% of its baseline value. There was a biphasic haemodynamic response, consisting of an initial compensatory phase during which there was progressive systemic vasoconstriction and tachycardia, followed by a decompensatory phase in which systemic vasoconstriction failed abruptly, blood pressure plummeted and heart rate declined. We tested the effects on the haemodynamic response of prior 4th ventricular, and in some cases intravenous, infusions of saline, yohimbine, clonidine, yohimbine plus clonidine, and bunazosin. From the results we conclude that a yohimbine-sensitive mechanism in the brainstem, possibly alpha 2-adrenoceptor-mediated, may be an essential element of the cardiac receptor-mediated decompensatory phase of acute central hypovolaemia, but does not contribute to the arterial baroreflex-mediated compensatory phase.

Cardiovascular responses of sinoaortic-denervated rats to intracerebroventricular injection of α1- and α2-adrenoceptor agonists

The aim of the present work was to analyse the cardiovascular responses induced by i.c.v. administration of the alpha 1- and alpha 2-adrenoceptor agonists, phenylephrine and clonidine, respectively, in conscious normal and sinoaortic-denervated rats. Sinoaortic denervation involves changes in central and peripheral catecholaminergic pathways. Clonidine (1-10 micrograms) produced a dose-dependent rise in blood pressure and a bradycardiac response in sham-operated animals, whereas in sinoaortic-denervated rats it provoked a brief rise in blood pressure followed by a marked fall as well as bradycardia. The responses involved mostly activation of central alpha 2-adrenoceptors, but the blood pressure responses induced by clonidine in sinoaortic-denervated rats may also have involved alpha 1-adrenoceptors. The bradycardia induced by the alpha 2-agonist in both groups of rats involved preferentially central alpha 2-adrenoceptors but also partially stimulated alpha 1-adrenoceptors. Phenylephrine, at a dose of 10-60 micrograms, induced a rise in blood pressure and a bradycardiac response while 90 micrograms produced a biphasic pressure response (early transient rise followed by a fall) as well as bradycardia in both sham-operated and sinoaortic-denervated animals. Phenylephrine activated alpha 1-adrenoceptors in every case, but the fall in blood pressure and the bradycardia also involved alpha 2-adrenoceptors. The responses were significantly higher in the sinoaortic-denervated rats than in the sham-operated. Our findings suggest that arterial baroreceptor reflexes can modify the effects of alpha-agonists initiated in the central nervous system. Sinoaortic denervation preparations enable one to unmask the depressor response to clonidine and also demonstrate the true magnitude of the phenylephrine response.

Contribution of forebrain noradrenaline innervation to the central circulatory effects of alpha-methyldopa and 6-hydroxydopamine

We studied the circulatory effects of chronic lesions of the ascending noradrenergic (NA) projections to the forebrain on the acute effects of intracisternal (i.c.) alpha-methyldopa (alpha-MD) and 6-hydroxydopamine (6-OHDA) on mean arterial pressure (MAP) and heart rate (HR) in conscious rabbits with arterial baroreceptors either intact or denervated (sinoaortic denervation, SAD). Both drugs acutely release neurotransmitter from central NA neurons. I.c. 6-OHDA produced acute hypertension and bradycardia while i.c. alpha-MD produced acute hypotension and bradycardia. The responses are qualitatively similar in SAD rabbits except that after 6-OHDA, HR increased. In another group we studied the effects of the drugs 3-4 weeks after localised injections of 6-OHDA in the midbrain dorsal and ventral NA bundles. Local 6-OHDA depleted forebrain regions of NA by 44-76%, and had no effects on basal values of MAP or HR. The pressor and depressor effects, of 6-OHDA and alpha-MD respectively, were little affected by the lesions in either intact or SAD rabbits. By contrast, in rabbits with intact baroreceptors, the lesion abolished the bradycardia produced by i.c. alpha-MD and 6-OHDA. The latter drug now produced a late tachycardia. In SAD rabbits, however, there was no effect on the alpha-MD-induced bradycardia, but the 6-OHDA tachycardia was enhanced. Since the major effects of the lesions were confined to the rabbits with intact baroreceptor afferents, it suggests that the ascending NA pathways are important for the cardiac responses dependent on baroreceptor input. In intact animals, both drugs produce bradycardia through facilitation of the vagal component of the baroreceptor-heart rate reflex. In SAD rabbits, almost all the changes to HR are mediated through the cardiac sympathetic and the lesions have little effect on HR.

Cardiovascular functions of central noradrenergic neurons in rabbits

1. The cardiovascular actions of central noradrenergic (NA) neurons was examined using the acute neurotransmitter releasing actions of intracisternal 6-hydroxydopamine in conscious rabbits. 2. The predominant actions of NA pathways in the brain-stem are to inhibit vasomotor and cardiac sympathetic activity and to facilitate cardiac vagal responses. 3. NA projections to the spinal cord tonically inhibit vasoconstrictor tone and form part of a high gain control system for responding to changes in specific afferent information. 4. The forebrain NA projections influence heart rate through modulation of the baroreflex gain of the cardiac vagus. 5. Central antihypertensive drugs such as clonidine and alpha-methyldopa mimic most of the brainstem actions of the central NA neurons. They also utilize the ascending NA pathways influencing the baroreflex vagal gain to minimize the baroreflex effects of reducing mean arterial pressure. 6. Thus, NA neurons influence the circulatory system through actions at all levels of the central nervous system and provide a comprehensive framework for integrating cardiovascular information from multiple inputs. This provides a number of targets for future development of useful pharmacological agents.

Reversal by ethanol of the hypotensive effect of clonidine in conscious spontaneously hypertensive rats

We studied the acute effect of ethanol on the hypotensive response to clonidine in conscious spontaneously hypertensive rats. When administered during the hypotensive response to clonidine, ethanol not only reversed the response but also caused a slight but significant short-lived pressor effect. The maximal hypotensive effect of graded doses of clonidine was significantly (p less than 0.05) attenuated by a dose of 1 g/kg ethanol, which resulted in a peak blood ethanol concentration of 54.2 +/- 6.3 mg/dl. The data strongly suggest the adverse effect of ethanol on the hypotensive response to clonidine is ethanol mediated and that their antagonistic interaction is both reversible and reproducible because: 1) an equal volume of saline had no effect on the hemodynamic responses to clonidine and 2) crossing over ethanol and saline treatments on days 2 and 3, which allowed longitudinal comparisons, showed that the effect of ethanol was similar both in naive rats (day 1) and in rats that were pre-exposed to ethanol (day 3). Whether this negative effect of ethanol also involves other antihypertensive agents that do not act primarily by a central nervous system mechanism was investigated. The same dose of ethanol had little or no effect on the hypotensive response to hydralazine, suggesting the negative effect of ethanol is selective to centrally acting antihypertensive agents. Although the mechanism by which ethanol reverses the hypotensive effect of clonidine is not known, it is possible that it involves an ethanol-evoked increase in plasma catecholamine levels, which are known to be decreased by clonidine. That ethanol did not reverse the hypotensive effect of hydralazine, which is also known to be associated with increased plasma catecholamine levels, supports this notion. The findings of the present study may explain, at least in part, why regular use of alcohol is associated with an inadequate control of blood pressure in treated hypertensive patients who are regular consumers of alcohol.

Comparative pharmacokinetics of D- and L-alphamethyldopa in plasma, aqueous humor, and cerebrospinal fluid in rabbits

The 2 stereoisomers of alphamethyldopa (alpha MD) were separately injected IV at 3 different doses (3, 10, 30 mg/kg) in anesthetized rabbits. Samples of plasma, aqueous humor (AH), and cerebrospinal fluid (CSF) were collected over a 300-min period. The concentration of the aminoacid (AA) was determined by liquid chromatography and electrochemical detection. Parameters obtained from kinetic analyses of the plasma concentrations were close to the values reported in other species. Linear elimination kinetics were observed in the dose range studied. A marked dose-dependent entry of alpha MD was observed in AH. A stereospecific active transport of alpha MD was evidenced in the AH since the concentration of the L-isomer reached values above the plasma levels. CSF entry of the AA was small when compared to AH kinetics. A limited passive diffusion of the AA in the brain could account for this phenomenon. However, greater availability of the L-stereoisomer was still observed in CSF. These alpha MD kinetic analyses illustrate the adaptation of AH and CSF removal procedures to the pharmacokinetic studies of the brain and ocular entry of AA isomers.

Cardiovascular role of A1 catecholaminergic neurons in the rabbit. Effect of chronic lesions on responses to methyldopa, clonidine and 6-OHDA induced transmitter release

We confirmed the findings of previous investigators that bilateral anodal lesions of the A1 region were associated with hypertension, bradycardia, pulmonary edema and a high mortality. All these sequelae (except the bradycardia) no longer occurred after cathodal lesions and these were therefore used to investigate the role of the catecholaminergic (CA) neurons of the A1 region in circulatory regulation. Conscious rabbits were studied 2-4 weeks after A1 lesions or sham-operation, when resting mean arterial pressure (MAP) and heart rate (HR) were closely similar in both groups. We tested for differences in MAP and HR responses between lesioned and sham-operated groups: to intracisternal (i.c.) alpha-methyldopa (MD) and to clonidine; and to the acute effects of i.c. 6-hydroxydopamine (6-OHDA) which elicits central CA release. Since these tests depend on the integrity of the central CA neurons, response differences between lesioned and sham-operated groups denote participation by the CA neurons of the A1 region in the central circulatory pathways. The bradycardia responses in the above tests were all smaller in lesioned than sham-operated rabbits, but there were no differences in MAP responses. Electrical stimulation of the region under alfathesin anaesthesia produced depressor responses at low frequencies and pressor responses at high frequencies. From the results in conscious rabbits CA neurons of the A1 region mainly influence the pathways regulating HR, rather than blood pressure. The changes in MAP during electrical stimulation are thus probably mediated through non-CA neurons.

Effect of urapidil, clonidine, and prazosin on sympathetic tone in conscious rats

To test the hypothesis that the hypotensive action of urapidil is in part related to a direct action on the brain, the central (intracerebroventricular) and peripheral (intravenous) effects of urapidil were studied and compared with those obtained with clonidine and prazosin. All studies were conducted in conscious, chronically instrumented stroke-prone spontaneously hypertensive rats (SHRSP). Efferent sympathetic nervous system activity was estimated by means of a bipolar electrode implanted on the splanchnic nerve. Only clonidine, administered intracerebroventricularly and intravenously, decreased sympathetic nerve activity. Urapidil and prazosin either did not affect sympathetic nerve activity after central administration or increased it after peripheral administration at low and high doses, respectively. Centrally administered urapidil and prazosin lowered blood pressure but also blocked the response to intravenously administered phenylephrine; this result suggests a peripheral effect. Centrally administered urapidil decreased heart rate. Urapidil given either intracerebroventricularly or into the cisterna magna had no influence on baroreceptor responses. Intravenous infusions of urapidil and prazosin in sufficient doses to lower blood pressure in spontaneously hypertensive rats by 50 mm Hg completely blocked the actions of phenylephrine. These data suggest that in conscious SHRSP urapidil lowers blood pressure through peripheral blockade of alpha 1-adrenergic receptors rather than by means of central sympathetic suppression. In this regard urapidil resembles prazosin rather than clonidine; however, the effect of urapidil on heart rate is consistent with a central mode of action.

Centrally mediated cardiovascular effects of nicergoline in the dog compared to those of clonidine

The intracisternal administration of nicergoline (5 micrograms/kg) or clonidine (4 micrograms/kg) in chloralose-anesthetized dogs induced significant decreases in blood pressure and heart rate. The same dose of nicergoline induced similar effects on atropine-pretreated dogs. Guanethidine pretreatment (30 mg/kg i.v. the day before) prevented the hypotension but not the bradycardia induced by clonidine. Guanethidine prevented both the hypotension and the bradycardia induced by nicergoline. Thus, nicergoline, unlike clonidine, does not increase cardiac parasympathetic activity. When administered by the same route at the same doses, nicergoline did not change the slope and reduced the amplitude whereas clonidine increased both the slope and the amplitude of the heart period vs. blood pressure curve obtained by intravenous administration of phenylephrine. Taken together, these results suggest that nicergoline and clonidine probably act on different structures within the central nervous system.

The plasma noradrenaline and growth hormone response to alpha-methyldopa and clonidine in hypertensive subjects

The mechanism of the antihypertensive effect of alpha-methyldopa was compared with clonidine by administering equipotent single doses of clonidine (0.2 mg) and alpha-methyldopa (750 mg) to nine hypertensive patients. Plasma noradrenaline was followed for 8 h thereafter as an index of peripheral sympathetic activity. alpha-Methyldopa and clonidine produced the same hypotensive response at 6 and 8 h after dosing with a similar fall in plasma noradrenaline levels at these times. Linear regression analysis between the systolic blood pressure fall and the corresponding plasma noradrenaline fall, showed that the slopes of the two regression lines were similar for alpha-methyldopa as for clonidine. Equipotent doses of alpha-methyldopa and clonidine produce the same fall in plasma noradrenaline. This supports the current hypothesis that an alpha-methyldopa metabolite acts centrally, like clonidine, to reduce peripheral sympathetic activity.

Central and peripheral autonomic mechanisms involved in the circulatory actions of methyldopa

Intracisternal (i.c.) and intravenous (i.v.) administration of methyldopa in conscious rabbits produced closely similar changes in hemodynamics, heart rate, and falls in plasma norepinephrine levels. Two weeks after giving i.c. 6-hydroxydopamine (6-OHDA), when there is widespread destruction of central noradrenergic neurons, the effects of i.c. methyldopa virtually were abolished. This suggests that noradrenergic neurons are the major central site of biotransformation into active metabolites. The circulatory and norepinephrine effects of i.v. methyldopa were attenuated but not completely abolished after giving i.c. 6-OHDA. Hence, in the rabbit about 70% of the action of methyldopa was central and about 30% was peripheral in the human therapeutic range of methyldopa concentrations. Preliminary lesion experiments suggest that the A5 nucleus plays an important role in the bradycardia. Two weeks after giving 5,6-dihydroxytryptamine (5,6-DHT) to destroy serotonergic (5HT) neurons the effects of i.c. methyldopa on mean arterial pressure (MAP) and heart rate were attenuated to approximately 50% of control effects. Therefore, some of the central effects of methyldopa apparently are mediated through 5HT pathways. We also compared the effects of i.c. methyldopa with those of i.c. clonidine (an alpha 2-adrenergic receptor agonist) and with the effects of transmitter release from the endings of noradrenergic and 5HT neurons during the first few hours after either 6-OHDA or 5,6-DHT administration. Our findings suggest that after biotransformation of methyldopa its active metabolites increase the activity of the bulbospinal noradrenergic neurons that control MAP and heart rate and reduce the activity of bulbospinal 5HT neurons.