THE PHARMACOLOGY OF CENTRALLY ACTING ANTIHYPERTENSIVE DRUGS

Exploring real-world evidence to uncover unknown drug benefits and support the discovery of new treatment targets for depressive and bipolar disorders

BACKGROUND

Major depressive and bipolar disorders are associated with impaired quality of life and high economic burden. Although progress has been made in our understanding of the underlying pathophysiology and the development of novel pharmacological treatments, a large unmet need remains for finding effective treatment options. The purpose of this study was to identify potential new mechanisms of actions or treatment targets that could inform future research and development opportunities for major depressive and bipolar disorders.

METHODS

A self-controlled cohort study was conducted to examine associations between 1933 medications and incidence of major depressive and bipolar disorders across four US insurance claims databases. Presence of incident depressive or bipolar disorders were captured for each patient prior to or after drug exposure and incident rate ratios were calculated. Medications that demonstrated ≥50% reduction in risk for both depressive and bipolar disorders within two or more databases were evaluated as potential treatment targets.

RESULTS

Eight medications met our inclusion criteria, which fell into three treatment groups: drugs used in substance use disorders; drugs that affect the cholinergic system; and drugs used for the management of cardiovascular-related conditions.

LIMITATIONS

This study was not designed to confirm a causal association nor inform current clinical practice. Instead, this research and the methods employed intended to be hypothesis generating and help uncover potential treatment pathways that could warrant further investigation.

CONCLUSIONS

Several potential drug targets that could aid further research and discovery into novel treatments for depressive and bipolar disorders were identified.

Impulsivity is associated with blood pressure and waist circumference among adolescents with bipolar disorder

OBJECTIVE

Cardiovascular risk factors (CVRFs) and impulsivity are common in bipolar disorder (BD), and CVRFs are also linked with impulsivity through a number of mechanisms, both behavioral and biological. This study examines the association between CVRFs and impulsivity in adolescents with BD.

METHODS

Subjects were 34 adolescents with BD and 35 healthy control (HC) adolescents. CVRFs were based on International Diabetes Federation metabolic syndrome criteria (triglycerides, high-density lipoprotein cholesterol, waist circumference, blood pressure (BP) and glucose). Impulsivity was measured using the computerized Cambridge Gambling Task (CGT). Analyses controlled for age, IQ, lifetime attention deficit hyperactivity disorder, and current antipsychotic use.

RESULTS

Adolescents with BD had higher diastolic BP (73.36 ± 9.57 mmHg vs. 67.91 ± 8.74 mmHg, U = 401.0, p = 0.03), higher triglycerides (1.13 ± 0.60 mmol/L vs. 0.78 ± 0.38 mmol/L, U = 373.5, p = 0.008), and were more likely to meet high-risk criteria for waist circumference (17.6% vs. 2.9%, p = 0.04) vs. HC. Within the BD group, CGT sub-scores were correlated with CVRFs. For example, overall proportion bet was positively correlated with systolic (r = 0.387, p = 0.026) and diastolic (ρ = 0.404, p = 0.020) BP. Quality of decision-making was negatively correlated with systolic BP (ρ = -0.401, p = 0.021) and waist circumference (ρ = -0.534, p = 0.003). Significant interactions were observed, such that BD diagnosis moderates the relationship between both waist circumference and BP with CGT sub-scores.

CONCLUSION

BP and waist circumference are associated with impulsivity in BD adolescents, but not in HC adolescents. Future studies are warranted to determine temporality and to evaluate whether optimizing CVRFs improves impulsivity among BD adolescents.

Topical combinations to treat microvascular dysfunction of chronic postischemia pain

BACKGROUND

Growing evidence indicates that patients with complex regional pain syndrome (CRPS) exhibit tissue abnormalities caused by microvascular dysfunction in the blood vessels of skin, muscle, and nerve. We tested whether topical combinations aimed at improving microvascular function would relieve allodynia in an animal model of CRPS. We hypothesized that topical administration of either α2-adrenergic (α2A) receptor agonists or nitric oxide (NO) donors given to increase arterial blood flow, combined with either phosphatidic acid (PA) or phosphodiesterase (PDE) inhibitors to increase capillary blood flow, would effectively reduce allodynia and signs of microvascular dysfunction in the animal model of chronic pain.

METHODS

Mechanical allodynia was induced in the hindpaws of rats with chronic postischemia pain (CPIP). Allodynia was assessed before and after topical application of vehicle, single drugs or combinations of an α2A receptor agonist (apraclonidine) or an NO donor (linsidomine), with PA or PDE inhibitors (lisofylline, pentoxifylline). A topical combination of apraclonidine + lisofylline was also evaluated for its effects on a measure of microvascular function (postocclusive reactive hyperemia) and tissue oxidative capacity (formazan production by tetrazolium reduction) in CPIP rats.

RESULTS

Each of the single topical drugs produced significant dose-dependent antiallodynic effects compared with vehicle in CPIP rats (N = 30), and the antiallodynic dose-response curves of either PA or PDE inhibitors were shifted 5- to 10-fold to the left when combined with nonanalgesic doses of α2A receptor agonists or NO donors (N = 28). The potent antiallodynic effects of ipsilateral treatment with combinations of α2A receptor agonists or NO donors with PA or PDE inhibitors were not reproduced by the same treatment of the contralateral hindpaw (N = 28). Topical combinations produced antiallodynic effects lasting up to 6 hours (N = 15) and were significantly enhanced by low-dose systemic pregabalin in early, but not late, CPIP rats (N = 18). An antiallodynic topical combination of apraclonidine + lisofylline was also found to effectively relieve depressed postocclusive reactive hyperemia in CPIP rats (N = 61) and to increase formazan production in postischemic tissues (skin and muscle) (N = 56).

CONCLUSIONS

The present results support the hypothesis that allodynia in an animal model of CRPS is effectively relieved by topical combinations of α2A receptor agonists or NO donors with PA or PDE inhibitors. This suggests that topical treatments aimed at improving microvascular function by increasing both arterial and capillary blood flow produce effective analgesia for CRPS.

Development and validation of a simple, sensitive and accurate LC-MS/MS method for the determination of guanfacine, a selective α2A -adrenergicreceptor agonist, in plasma and its application to a pharmacokinetic study

A simple, practical, accurate and sensitive liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed and fully validated for the quantitation of guanfacine in beagle dog plasma. After protein precipitation by acetonitrile, the analytes were separated on a C18 chromatographic column by methanol and water containing 0.1% (v/v) formic acid with a gradient elution. The subsequent detection utilized a mass spectrometry under positive ion mode with multiple reaction monitoring of guanfacine and enalaprilat (internal standard) at m/z 246.2 → 159.0 and m/z 349.2 → 205.9, respectively. Good linearity was obtained over the concentration range of 0.1-20 ng/mL for guanfacine in dog plasma and the lower limit of quantification of this method was 0.1 ng/mL. The intra- and inter-day precisions were <10.8% relative standard deviation with an accuracy of 92.9-108.4%. The matrix effects ranged from 89.4 to 100.7% and extraction recoveries were >90%. Stability studies showed that both analytes were stable during sample preparation and analysis. The established method was successfully applied to an in vivo pharmacokinetic study in beagle dogs after a single oral dose of 4 mg guanfacine extended-release tablets.

Topical combinations aimed at treating microvascular dysfunction reduce allodynia in rat models of CRPS-I and neuropathic pain

Growing evidence indicates that various chronic pain syndromes exhibit tissue abnormalities caused by microvasculature dysfunction in the blood vessels of skin, muscle, or nerve. We tested whether topical combinations aimed at improving microvascular function would relieve allodynia in animal models of complex regional pain syndrome type I (CRPS-I) and neuropathic pain. We hypothesized that topical administration of either α(2)-adrenergic (α(2)A) receptor agonists or nitric oxide (NO) donors combined with either phosphodiesterase (PDE) or phosphatidic acid (PA) inhibitors would effectively reduce allodynia in these animal models of chronic pain. Single topical agents produced significant dose-dependent antiallodynic effects in rats with chronic postischemia pain, and the antiallodynic dose-response curves of PDE and PA inhibitors were shifted 2.5- to 10-fold leftward when combined with nonanalgesic doses of α(2)A receptor agonists or NO donors. Topical combinations also produced significant antiallodynic effects in rats with sciatic nerve injury, painful diabetic neuropathy, and chemotherapy-induced painful neuropathy. These effects were shown to be produced by a local action, lasted up to 6 hours after acute treatment, and did not produce tolerance over 15 days of chronic daily dosing. The present results support the hypothesis that allodynia in animal models of CRPS-I and neuropathic pain is effectively relieved by topical combinations of α(2)A or NO donors with PDE or PA inhibitors. This suggests that topical treatments aimed at improving microvascular function may reduce allodynia in patients with CRPS-I and neuropathic pain.

PERSPECTIVE

This article presents the synergistic antiallodynic effects of combinations of α(2)A or NO donors with PDE or PA inhibitors in animal models of CRPS-I and neuropathic pain. The data suggest that effective clinical treatment of chronic neuropathic pain may be achieved by therapies that alleviate microvascular dysfunction in affected areas.

The effect of antihypertensive drugs on chronic kidney disease: a comprehensive review

Data from randomized clinical trials and epidemiological evidence identify systemic hypertension as the second most common modifiable risk factor for chronic kidney disease (CKD) progression after diabetes mellitus. CKD may progress silently over the years and early diagnosis and control of hypertension is of major importance in delaying renal function decline. Recent guidelines for the treatment of hypertension suggest the use of a variety of antihypertensive drugs in order to achieve the desired blood pressure levels. Renin-angiotensin system inhibitors have been undoubtedly studied the most and are suggested by guidelines and experts as first choice in patients with hypertension and renal injury, particularly in those with diabetes, as they have repeatedly shown to significantly reduce proteinuria. Other classes of antihypertensive drugs have been studied to a lesser extent and they have their own unique properties and effects. However, it is now common knowledge that adequate blood pressure control is the most important factor for the preservation of renal function, so every drug that effectively lowers hypertension is believed to be renoprotective. The present article will review the latest data on the role and properties of each class of antihypertensive drugs on CKD.

A selective alpha(2)-adrenergic agonist for cardiac resuscitation

The effects of selective alpha(2)-adrenergic agonist alpha-methylnorepinephrine on the initial success of resuscitation and postresuscitation myocardial function were compared with nonselective alpha- and beta-adrenergic epinephrine in a swine model of cardiac arrest. Epinephrine, the primary pharmacological intervention in the treatment of cardiac arrest, improves immediate outcome. However, epinephrine increases the severity of myocardial dysfunction after cardiac resuscitation. Both inotropic and chronotropic actions provoke disproportionate increases in myocardial oxygen consumption by the ischemic heart, prompting this study, in which we hypothesized that a selective alpha(2)-adrenergic agonist, alpha-methylnorepinephrine (alpha-MNE), would moderate these adverse effects of epinephrine and minimize postresuscitation myocardial dysfunction. After 7 minutes of untreated ventricular fibrillation (VF) in 14 anesthetized male domestic pigs, precordial compression at a fixed rate of 80 compressions/min was begun, along with mechanical ventilation. Either alpha-MNE (100 microg/kg) or epinephrine (20 microg/kg) was administered as a bolus after 2 minutes of precordial compression. After an additional 4 minutes of precordial compression, defibrillation was attempted. Left ventricular systolic and diastolic function was quantitated with the use of transesophageal echo-Doppler imaging. Comparable increases in coronary perfusion pressure to 15 mm Hg were observed after the administration of both drugs. All animals were successfully resuscitated; epinephrine and alpha-MNE were equally quick in restoring spontaneous circulation after 7 minutes of untreated VF. Ejection fraction was reduced by 35% and 14% by epinephrine and alpha-MNE, respectively, after resuscitation. Epinephrine and alpha-MNE increased postresuscitation heart rate by 38% and 15%, respectively. Accordingly, significantly less postresuscitation impairment followed the administration of alpha-MNE. alpha-MNE, a selective alpha-adrenergic agonist, was as effective as epinephrine in restoring spontaneous circulation after 7 minutes of untreated VF in a porcine model for CPR and demonstrated lesser postresuscitation myocardial injury.

In spontaneously hypertensive rats alterations in aortic wall properties precede development of hypertension

In hypertension arterial wall properties do not necessarily depend on increased blood pressure alone. The present study investigates the relationship between the development of hypertension and thoracic aortic wall properties in 1.5-, 3-, and 6-mo-old spontaneously hypertensive rats (SHR); Wistar-Kyoto rats (WKY) served as controls. During ketamine-xylazine anesthesia, compliance and distensibility were assessed by means of a noninvasive ultrasound technique combined with invasive blood pressure measurements. Morphometric measurements provided in vivo media cross-sectional area and thickness, allowing the calculation of the incremental elastic modulus. Extracellular matrix protein contents were determined as well. Blood pressure was not significantly different in 1.5-mo-old SHR and WKY, but compliance and distensibility were significantly lower in SHR. Incremental elastic modulus was not significantly different between SHR and WKY at this age. Media thickness and media cross-sectional area were significantly larger in SHR than in WKY, but there was no consistent difference in collagen density and content between the strains. Blood pressure was significantly higher in 3- and 6-mo-old SHR than in WKY, and compliance was significantly lower in SHR. The findings in this study show that in SHR, in which hypertension develops over weeks, alterations in functional aortic wall properties precede the development of hypertension. The decrease in compliance and distensibility at a young age most likely results from media hypertrophy rather than a change in intrinsic elastic properties.

Method for comparison of the hemodynamic effects of equi-antinociceptive oral doses of drugs in anesthetized rats

In a typical flowchart for discovery of novel analgesic (or other) agents, a critical path often involves maximization of the separation of the therapeutic endpoint from known adverse-effect (AE) endpoint(s). Although strategies can easily be designed for in vitro paradigms such as high-throughput screening, extension to in vivo testing can represent a major obstacle to the rapid progression to the next step in development. The problem can be particularly acute when the assessment is required for oral dosing, and when it is not known if the therapeutic and AE mechanism(s) of action are the same. As a case in point, alpha(2)-adrenoceptor (alpha(2)-AR) agonists have potential therapeutic use as analgesics, but they also produce cardiovascular (CV) effects. However, whether the two effects are inexorably linked has not been resolved, particularly for oral administration. The present study used a novel method for comparing the CV effects produced by alpha(2)-AR agonists given by intraduodenal administration to anesthetized rats at fixed ratios of the oral antinociceptive ED(50) dose of each agonist. The technique provided a useful screen of compounds. In addition,there was no correlation between CV endpoints and alpha(2A)-AR affinity, suggesting that oral alpha(2)-AR-mediated analgesia and CV effects might be separable or that other mechanisms might be involved.

Effects of sympathetic inhibition on receptive, proceptive, and rejection behaviors in the female rat

The present investigation was designed to examine the effects of sympathetic nervous system (SNS) inhibition on sexual behavior in ovariectomized, steroid-treated female rats. Clonidine, an alpha2-adrenergic agonist, guanethidine, a postganglionic noradrenergic blocker, and naphazoline, an alpha2-adrenoreceptor agonist were used to inhibit SNS activity. Intraperitoneal injections of either 33 micrograms/ml or 66 micrograms/ml clonidine significantly decreased receptive (lordosis) and proceptive (ear wiggles) behaviors and significantly increased rejection behaviors (vocalization, kicking, boxing). Either 25 mg/ml or 50 mg/ml guanethidine significantly decreased receptive and proceptive behavior and had no significant effect on rejection behaviors. Naphazoline significantly inhibited lordosis behavior at either 5 mg/ml or 10 mg/ml doses, significantly inhibited proceptive behavior at 5 mg/ml, and had no significant effect on rejection behaviors. These findings support the hypothesis that SNS inhibition decreases sexual activity in the female rat.

Aortic wall properties in normotensive and hypertensive rats of various ages in vivo

The distensibility of the arterial system, which is partly determined by arterial wall structure, smooth muscle tone, and actual pressure level, decreases with aging and hypertension. Our aim was to compare aortic wall properties in 3- and 6-month-old normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) at comparable blood pressures in vivo. During ketamine/xylazine anesthesia in rats we performed ultrasound arterial wall tracking and invasive pressure measurements to determine, at the level of the thoracic aorta, diastolic pressure, diastolic lumen area, changes in pressure and lumen area during the cardiac cycle, and indexes of compliance and distensibility. These observations were combined with histological measurements for determination of media cross-sectional area and thickness and the incremental elastic modulus under conditions as expected in situ. Anesthesia abolished the difference in diastolic pressure between SHR and WKY. Between 3 and 6 months of age in WKY, diastolic area and incremental elastic modulus increased significantly, distensibility decreased, and all other recorded variables were not modified. Between 3 and 6 months of age in SHR, diastolic area and incremental elastic modulus increased, distensibility of the aortic wall decreased, and all other mechanical and structural properties did not change significantly. At both ages, diastolic area and compliance were significantly smaller in SHR than WKY. The other mechanical and structural properties measured or calculated at comparable pressure did not differ between strains. Differences between the aorta of 3- and 6-month-old rats and between strains observed in vivo at comparable pressures can largely be attributed to differences in lumen caliber.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha-adrenoceptors

Major advances have been made in our understanding of the molecular structure and function of the alpha-adrenoceptors. Many new subtypes of the alpha-adrenoceptor have been identified recently through biochemical and pharmacological techniques and several of these receptors have been cloned and expressed in a variety of vector systems. Currently, at least seven subtypes of the alpha-adrenoceptor have been identified and the molecular structure and biochemical functions of these subtypes are beginning to be understood. The alpha-adrenoceptors belong to the super family of receptors that are coupled to guanine nucleotide regulatory proteins (G-proteins). A variety of G-proteins are involved in the coupling of the various alpha-adrenoceptor subtypes to intracellular second messenger systems, which ultimately produce the end-organ response. The mechanisms by which the alpha-adrenoceptor subtypes recognize different G-proteins, as well as the molecular interactions between receptors and G-proteins, are the topics of current research. Furthermore, the physiological and pathophysiological role that alpha-adrenoceptors play in homeostasis and in a variety of disease states is also being elucidated. These major advances made in alpha-adrenoceptor classification, molecular structure, physiologic function, second messenger systems and therapeutic relevance are the subject of this review.

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.

Influence of anaesthesia on the cardiovascular effects of rilmenidine and clonidine in spontaneously hypertensive rats

1. The acute cardiovascular effects of two alpha 2-adrenoceptor agonists, rilmenidine and clonidine, were studied in 15-week-old male spontaneously hypertensive rats (SHRs). The effects of these drugs were compared with intravenous (i.v.) and intracerebroventricular (i.c.v.) administration in conscious and pentobarbitone-anaesthetized SHRs, in which aortic blood pressure (BP) was continuously recorded. 2. In conscious SHRs, i.v. doses of either rilmenidine (30, 100, 300 micrograms kg-1) or clonidine (3, 10, 30 micrograms kg-1) induced dose-dependent short-lasting increases in BP followed by moderate decreases associated with bradycardia, while the same three doses of both drugs given i.c.v. were devoid of BP and heart rate (HR) effects. 3. Pentobarbitone-anaesthesia increased the sympathetic control of BP and suppressed the cardiac baroreflex sensitivity. 4. In anaesthetized SHRs, i.v. injections of the same 3 doses of rilmenidine and clonidine induced a slight increase in BP, rapidly followed by profound and long-lasting BP and HR decreases. Surprisingly, when given i.c.v., these 3 doses lowered BP and HR to the same extent but in a more progressive manner. 5. The lack of efficacy of both drugs in conscious SHRs after the i.c.v. administration of i.v. active doses and the lack of more marked and rapid effects in anaesthetized SHRs, after i.c.v. than after i.v. injections, question the involvement of a major central site of action for these antihypertensive alpha 2-adrenoceptor agonists. Moreover, these results show that the cardiovascular effects of these drugs are profoundly influenced by baseline sympathetic nervous system activity which is enhanced by pentobarbitone-anaesthesia.

Pharmacological evidence of alpha 1- and alpha 2-adrenergic supersensitivity in orthostatic hypotension due to spinal cord injury: a case report

Sympathetic efferent pathways and alpha-adrenergic receptivity were investigated in one patient with spinal cord transection (D1 level) and orthostatic hypotension. The lack of increase in catecholamine plasma levels during orthostasis and the paradoxical pressor effect of clonidine (2 micrograms/kg orally) suggested complete interruption of efferent sympathetic pathways. The pressor response to exogenous noradrenaline was significantly higher in the patient than in 6 normal controls (0.09 vs 0.72 micrograms.kg-1), indicating supersensitivity of vascular alpha-adrenoceptors. The platelet alpha 2-adrenergic receptor number, measured with [3H]yohimbine, was 507 in the patient vs 178 fmol.mg-1 protein in controls. The increase in systolic blood pressure induced by 10 mg midodrine, a specific alpha 1-agonist, was significantly higher in the patient (delta = 56 mm Hg) than in controls (delta = 15 mm Hg). The results indicate that in the patient there was alpha-adrenergic supersensitivity both of alpha 1- and alpha 2-adrenoceptors. This led to successfully oral treatment of the orthostatic hypotension with clonidine 150 micrograms bd and midodrine 10 mg bd.

Suppression of copulatory behavior in male rats following central administration of clonidine

Adrenergic transmitters have been implicated in the regulation of male sexual behavior. In the present study the contribution of alpha 2-adrenoceptors, located within the central nervous system, was evaluated. Sexually experienced male Long-Evans rats were implanted with a permanent cannula in either the third cerebral ventricle or, unilaterally, in the medial preoptic area. Baseline mating tests were administered 4, 7 and 10 days after surgery and only males ejaculating (at least) in the 7 and 10 day tests were used. Clonidine-evoked dose-dependent decrements in the number of males mounting, intromitting and ejaculating, with administration into the medial preoptic area more effective than into the 3rd ventricle. In those animals mating, administration of 20 nmol into the 3rd ventricle was associated with decreases in the latency to ejaculation and intercopulatory interval, whereas administration of 2 nmol into the medial preoptic area was associated with increases in these parameters and decreases in the ejaculatory threshold. Administration of yohimbine into the medial preoptic area attenuated the effects of systemically-administered clonidine and the systemic administration of yohimbine completely prevented the copulatory suppression induced by administration of clonidine into the medial preoptic area. It is suggested that central alpha 2-adrenoceptors are important in the control of male sexual behavior and that alterations in adrenergic mechanisms in the medial preoptic area may underlie sexual dysfunction of various etiologies.

Investigations into the effect of two sedatives on the stress response in cattle

The effects of the sedatives acepromazine (an alpha-adrenergic antagonist) and xylazine (an alpha 2-adrenergic agonist) on plasma indicators of stress in cows were assessed after intramuscular injection and transport. After blood samples had been taken for baseline values, nine cows were given an intramuscular injection of saline (2.5 ml), acepromazine (0.05 mg/kg in 2.5 ml) or xylazine (0.05 mg/kg in 2.5 ml) on different occasions at least 1 week apart. The animals were then transported for 5 min by truck to a different environment and blood sampled for a further 1-3 h. There was a significant increase in plasma cortisol concentration (3.29 +/- 1.59 x baseline) after the injection of saline and transport. The injection of acepromazine also resulted in a significant increase in cortisol concentration (2.84 +/- 0.84 x baseline). There was no similar increase after injection of xylazine. This suggests that alpha 2-adrenergic receptors are involved in the response of plasma cortisol concentrations to stressors. An hyperglycaemic response occurred after xylazine (1.66 +/- 0.49 x baseline) and saline (1.20 +/- 0.1 x baseline) but not after acepromazine. Both sedatives produced a metabolic alkalosis (1.13 +/- 0.01 x baseline pH after xylazine and 1.034 +/- 0.02 x baseline pH after acepromazine). A greater decrease in haematocrit was seen after both sedatives (0.88 +/- 0.04 x baseline after xylazine, 0.81 +/- 0.08 x baseline after acepromazine) than after the injection of saline (0.97 +/- 0.06 x baseline).

Cardiovascular effects of rilmenidine, a new alpha 2-adrenoceptor agonist, and clonidine in conscious spontaneously hypertensive rats

1. The acute and chronic effects of rilmenidine, a partial agonist of alpha 1- and alpha 2-adrenoceptors with antihypertensive properties, were compared to those of clonidine on blood pressure (BP), heart rate (HR) and the urinary excretion of catecholamines, which was used as an index of sympathetic activity. 2. As these drugs are known to interfere centrally and peripherally with the sympathetic nervous system, long-term arterial blood pressure recordings in freely moving unstressed adult spontaneously hypertensive rats (SHR) were used. 3. Acute i.v. administrations of rilmenidine (0.3 mg/kg at 1200 h, 1.2 mg/kg at 1700 and 2200 h) and clonidine (12 micrograms/kg at 1200 h, 50 micrograms/kg at 1700 and 2200 h) induced short-lasting increases in BP associated with a decrease in HR, which were followed by prolonged, dose-dependent decreases in BP without bradycardia. The pressor effect was less marked and the associated bradycardia was more marked in active SHR with physiologically high sympathetic activity than in resting SHR. 4. A 12-day oral treatment with rilmenidine (6.0 mg/kg daily) or clonidine (150 micrograms/kg daily) induced moderate decreases in BP without change in HR. Rilmenidine but not clonidine decreased normetanephrine (NMN) excretion in active but not in resting SHR. 5. Finally, during the 24 h following the cessation of the treatments, BP returned to normal, without significantly exceeding that of untreated controls. However, upswings in BP or HR were observed, more markedly and frequently after clonidine than after rilmenidine. 6. In conclusion the effects of alpha 2-adrenoceptor agonists appear to be influenced by the pre-existing sympathetic tone. The general agreement between these data and those observed in patients demonstrates that the use of conscious unstressed animals is of value to determine the cardiovascular effects of drugs which act on the sympathetic nervous system.

Dose-related effects of detomidine on autonomic responses in the horse

1. Detomidine is a novel veterinary sedative analgesic which is thought to act by stimulation of alpha 2 adrenoreceptors. The present study was undertaken to determine the direction, time course, and dose-response relationship of detomidine on specific autonomic responses in the unanaesthetized horse. 2. Detomidine was administered intravenously to eight adult thoroughbred racehorses at doses of 0.010-0.040 mg kg-1, according to a double-blind Latin square crossover design. Cardiac and respiratory rates, pupil diameter and rectal temperature were monitored for 180 min postinjection. 3. Detomidine produced prolonged dose-related bradycardia and bradypnoea. This was accompanied by a briefer period of dose-related mydriasis. Response duration, rather than peak was consistently increased as a function of dose. 4. Rectal temperature was not altered in a dose-dependent fashion. Low doses of detomidine produced late onset hypothermia, while high doses produced early and late onset hyperthermia. 5. These data indicate the uniqueness in autonomic response by the horse to alpha 2 adrenoreceptor stimulation.

Cardiovascular and central nervous system effects of rilmenidine (S 3341) in rats

Rilmenidine (S 3341) is a new alpha 2 agonist, with antihypertensive properties. Pharmacologic data concerning its hemodynamic and central nervous system effects in the rat are described in this report. In the anesthetized or conscious spontaneously hypertensive rat, rilmenidine was found effective and potent as an antihypertensive agent, lowering blood pressure in a dose-dependent manner after intravenous and oral administration. These effects are related to a reduction in sympathetic tone as seen by the decrease in plasma catecholamines induced by rilmenidine in the spontaneously hypertensive rat. Studies in the normotensive pithed rat (electrical stimulation and adrenalectomization) confirmed the presynaptic alpha 2-stimulating properties of rilmenidine and suggested that a component of the antihypertensive activity of rilmenidine could be exerted through these peripheral receptors. A study of the central effects of rilmenidine was performed using classic neuropharmacologic tests. No effect was observed on the pentobarbitone-induced sleeping time in the rat. Rilmenidine caused only a minimal and non-dose-dependent inhibition of the righting reflex in the chick. In the rat, rilmenidine did not decrease the motor activity at concentrations up to 50 times higher than the antihypertensive dose. These results confirmed the contrast between rilmenidine and clonidine and suggest that a dissociation between sedative and antihypertensive effects could occur with rilmenidine.

Pharmacological evidence for the involvement of alpha-2 adrenoceptors in the sedative effect of detomidine, a novel sedative-analgesic

The sedative effect and mechanism of action of a novel imidazole derivative, detomidine, were studied in laboratory animals. Three methods were used to quantify drug-induced sedation: (i) decrease in spontaneous activity of mice; (ii) increase in barbiturate induced anaesthesia time in mice; (iii) loss of righting reflex in chicks. Clonidine and xylazine were included in the studies for comparison. The sedative potency of detomidine was shown to be approximately equal to that of clonidine and much higher than that of xylazine. In all tests, the sedative effect of detomidine was inhibited by antagonists of alpha-2 adrenoceptors (yohimbine, rauwolscine and idazoxan) but not by alpha-1 antagonists (prazosin, corynanthine). Furthermore, an ex vivo receptor binding study in the rat showed that detomidine-induced decrease in spontaneous activity was significantly correlated to [3H]clonidine but not to [3H]prazosin displacement in brain membranes. These results show that detomidine has potent sedative effects in mice, rats and chicks, and suggest that this action is mediated through stimulation of alpha-2 adrenoceptors.

Enhancement of sexual motivation in male rats by yohimbine

Yohimbine hydrochloride, an alpha 2-adrenoceptor antagonist, increased sexual motivation in male rats as evidenced by increased mounting performance in mating tests conducted after genital anesthetization, increased percentage of male rats ejaculating in their first heterosexual encounter, and induction of copulatory behavior in sexually inactive male rats. These observations lead to the suggestion that alpha-adrenoceptors are important modulators of sexual arousal in intact male rats. These results indicate that pharmacological treatment of sexual (libido) dysfunction may be useful.

Antagonism of the antidiarrhoeal effect of clonidine and the lethal effect of noradrenaline in rats: a reliable procedure to evaluate the in-vivo alpha 1- and alpha 2-blocking activity of drugs?

Eight compounds with alpha-adrenergic blocking activity were tested for their ability to antagonize the antidiarrhoeal effect of clonidine (clonidine test) and the lethal effect of noradrenaline (noradrenaline test). Six of the compounds studied are alpha-adrenergic blocking agents with known alpha 2/alpha 1 selectivity. Two compounds, ketanserin (R 41 468) and butanserin (R 53 393), are 5-hydroxytryptamine S2-antagonists. The ED50-values (mg kg-1) obtained in the clonidine test were: phentolamine (0.34), RX781094 (0.34), yohimbine (0.51), piperoxan (9.36), butanserin (greater than 5.0), prazosin (greater than 10.0), phenoxybenzamine (greater than 40.0), and ketanserin (greater than 80.0). In the noradrenaline test the ED50's (mg kg-1) were: butanserin (0.014), prazosin (0.032), phentolamine (0.59), phenoxybenzamine (1.02), ketanserin (4.69), RX781094 (12.4), piperoxan (21.5), and yohimbine (25.0). The selectivity alpha 2/alpha 1-ratios (ED50 clonidine/ED50 noradrenaline were: yohimbine (0.020), RX781094 (0.027), piperoxan (0.44), phentolamine (0.58), ketanserin (greater than 39), prazosin (greater than 312), and butanserin (greater than 357). These results show that yohimbine and RX781094 are equipotent and relatively selective alpha 2-antagonists; piperoxan and phentolamine block both alpha 1- and alpha 2-receptors at closely related doses; ketanserin, prazosin and butanserin are selective blockers of alpha 1-receptors, ketanserin being very weak, prazosin and butanserin being very potent compounds in this respect. The potent and selective alpha 1-blocking activity of butanserin, combined to its 5-HT S2-antagonism makes butanserin a very interesting experimental drug in view of earlier reported data concerning the amplifying effects between 5-hydroxytryptaminergic and noradrenergic vascular mechanisms.

Identification of vascular postsynaptic alpha 1- and alpha 2-adrenoceptors in man

We studied postsynaptic alpha-adrenoceptors in human blood vessels by measuring the influence on forearm blood flow induced by intra-arterial infusions of selective alpha 1- and alpha 2-adrenoceptor agonists (methoxamine, B-HT 933, clonidine and guanfacine) and antagonists (doxazosin and yohimbine). The studies were done in healthy volunteers, and forearm blood flow was measured by plethysmography. All agonists produced a significant and dose-dependent vasoconstriction. The effect of B-HT 933 was completely abolished by the concomitant infusion of yohimbine, whereas it was hardly influenced by doxazosin. The effect of methoxamine was prevented by doxazosin and little influenced by yohimbine. The vasoconstriction by clonidine and guanfacine was partially prevented by both doxazosin and yohimbine. The single intra-arterial infusion of yohimbine, as well as doxazosin, resulted in vasodilation. These findings provide strong evidence for the existence of postsynaptic alpha 1- as well as alpha 2-adrenoceptors, both mediating vasoconstriction and contributing to basal vascular tone. The (patho-)physiological significance of this subdivision of alpha-adrenoceptors remains to be elucidated.