Cardiovascular effects of 2-(2,6-dichlorophenylamino)-2-imidazoline hydrochloride (ST 155). II. Central sympathetic structures

Effect of the imidazoline-derivative ST-91 on heart rate and blood pressure in healthy man

1 ST-91, 2-(2,6-diethylphenylamino)-2-imidazoline hydrochloride produces in animal experiments bradycardia, and in larger doses, also hypertension, due to α-sympathomimetic activity and probable central sympathetic inhibition. 2 The effects of ST-91, after oral administration, were studied in 10 healthy volunteers. A dose-dependent, significant decrease of the heart rate was observed. On average the resting heart rate was reduced by 11-19 beats/min (15-29%). During exercise the decrease of the heart rate was less pronounced. After the largest dose there was a fall of the systolic blood pressure during exercise averaging at 14-15 mmHg (11-12%). Otherwise the blood pressure remained unchanged. 3 Eight out of the 10 test-subjects complained of hair-raising and goose-flesh after all doses, due to the α-sympathomimetic effect of the drug. No toxic effects were observed.

Site and mode of action of clonidine in the central nervous system

In urethane-anaesthetized rats clonidine was administered intravenously (i.v.), intracerebroventricularly (i.c.v.) or onto the surface of the area postrema which protrudes into the fourth cerebral ventricle. In each instance clonidine induced a dose-dependent lowering of the blood pressure. The region of the area postrema appears to be the most sensitive site for the action of clonidine so far studied. In order to obtain similar blood pressure effects, approximately 8 times higher amounts were needed i.c.v., and about 80 times higher amounts i.v., than onto the surface of the area postrema. A pretreatment of the rats with the specific histamine H2-receptor blocking drug, metiamide (4.5 micronmoles/rat i.c.v.) shifted the dose-response curve of clonidine (i.c.v.) to the right. The results suggest that clonidine exerts its hypotensive effect in the rat via a stimulation of histamine H2-receptors in, or in the vicinity of, the area postrema.

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.

Functional and autoradiographic studies to locate the sites at which clonidine acts to cause hyperglycaemia and inhibition of opiate-induced sympathetic outflow

The effect of intracisternal administration of clonidine (1 microgram/kg) on the response to intravenous injection of morphine (4 mg/kg) was examined in conscious rabbits. Morphine acts on central opiate receptors to increase sympathetic outflow and cause hypertension. Clonidine, given intracisternally, prevented the morphine-induced rise in mean arterial pressure, fall in heart rate and increase in catecholamines in plasma. Using in vitro autoradiography, alpha 2-adrenoceptors were localised in the nucleus of the tractus solitarius and the dorsal motor nucleus of the vagus and these may be two of the sites at which clonidine acts. Clonidine also causes hyperglycaemia after intravenous administration and the site of action was investigated by comparing the effects of intravenous and intracerebroventricular administration of clonidine (1, 2 and 2 micrograms/kg), given at intervals of 30 min. Similar increases in glucose occurred after intraventricular and intravenous administration of clonidine, indicating that it has both central and peripheral actions, which increase glucose by different mechanisms. Clonidine, given intraventricularly also reduced mean arterial pressure and heart rate but there were no effects after intravenous administration. These studies demonstrate that the inhibitory effect of clonidine on opiate-induced stimulation of sympatho-adrenal outflow is central, whereas the hyperglycaemic effect of clonidine depends on both central and peripheral mechanisms.

Interaction between clonidine and physostigmine in normal rats and in rats after sinoaortic denervation

Clonidine (3-30 micrograms.kg-1, i.v.) induced a fall in mean arterial pressure in rats after sinoaortic denervation but not in sham-operated animals. Moreover, sinoaortic denervation reduced the bradycardic action of this antihypertensive drug. Pressor and tachycardic response to physostigmine (60 micrograms.kg-1, i.v.) were greater in denervated than in sham-operated rats. The increase of mean arterial pressure was 26.2 +/- 2.2 mm Hg in sham-operated rats (n = 12) and 53.8 +/- 2.0 mm Hg in denervated rats (n = 12, P less than 0.005). Pretreatment with 3 micrograms.kg-1 (i.v.) of clonidine did not alter the pressor response to physostigmine (60 micrograms.kg-1) in either of the two groups; 10 and 30 micrograms.kg-1 of clonidine reduced the physostigmine-induced increase of mean arterial pressure in sham-operated rats but enhanced the pressor response in denervated animals. Furthermore, an ineffective dose of physostigmine (30 micrograms.kg-1, i.v.) induced a pressor response after pretreatment with clonidine (10 micrograms.kg-1) in denervated rats. Clonidine (10 micrograms.kg-1) did not affect the pressor effect of 1,1 dimethyl-4-phenylpiperazinium iodide (DMPP: 50 micrograms.kg-1, i.v.) or phenylephrine (4 micrograms.kg-1, i.v.) in either group. The anticholinergic effect of clonidine in sham-operated rats may be explained by an inhibitory action on the release of acetylcholine in several brain structures but the facilitatory effect of clonidine observed in denervated animals is not clear. The results did not suggest a peripheral involvement in this facilitatory effect.

Acute and short-term effects of clonidine in heart failure

The authors performed a randomized, double-blind, placebo-controlled trial to assess the tolerance and effects of chronic oral clonidine administration on the clinical status, exercise tolerance, and ventricular function of 10 male patients with chronic heart failure. Patients were given either oral clonidine (400 micrograms/day) or matching placebo for twelve weeks and then tapered off medication over a two-week period. Results are summarized as follows: Profiles over time between groups were significantly different for resting heart rate (p = 0.0005), were different for arterial pressure (p = 0.04), were different for left ventricular ejection fraction (p less than 0.006), and were different for mean accumulated workload (p = 0.076). Exercise double product at 25 watts changed little in three patients and showed a decrease in 2 patients following six weeks of oral clonidine; after twelve weeks, it decreased in three patients, increased in 1, and changed little in another. After washout, double product returned toward baseline values. In the placebo groups, double product showed little change. Resting of the heart by decreasing heart rate, systemic arterial blood pressure, and venous tone are thought by the authors to be major contributing factors to the observed beneficial effect of chronic oral clonidine in chronic heart failure.

Interaction between clonidine and histamine on the guinea-pig isolated trachea

In experiments on guinea-pig isolated tracheal spirals, clonidine, in concentrations of 10(-6) to 3 X 10(-4) M, had a contracting effect which was strongly inhibited by prazosin but not significantly modified by yohimbine. Moreover, clonidine (3 X 10(-5) to 3 X 10(-4) M) potentiated histamine-induced contractions; this latter effect was inhibited specifically by alpha 1-adrenoceptor antagonists (e.g. prazosin, AR-C 239) but unmodified by yohimbine, nicardipine or agents acting on the arachidonic acid cascade. It would therefore appear that clonidine in high concentrations contracts the guinea-pig trachea by stimulating alpha 1-adrenoceptors and that, contrary to what has been reported with other animal species, notably the dog, the guinea-pig trachea is devoid of alpha 2-adrenoceptors that mediate contractions.

Increase in insulin response to glucose in the rat chronically treated with clonidine

Effect of chronic clonidine treatment on the response to glucose of rat pancreatic B-cells was investigated. Clonidine treatment was carried out for 10 days by dissolving the drug into drinking water at a concentration of 10 micrograms/ml. Control rats were given drug-free tap water. Serum insulin responses to glucose (750 mg/kg, i.v.) of clonidine-treated rats were much smaller than those of control rats. However, after 1 day's withdrawal of clonidine, the rise in the serum insulin level induced by glucose was approximately 2-fold larger in clonidine-treated rats as compared to that in control rats. Since clonidine treatment decreased body weight of the rat by 10%-20% in 10 days, the same experiments were carried out with rats whose body weight loss was made comparable to that of clonidine-treated rats by restricting food for 10 days. Then, some animals of the group thus treated had food-restriction discontinued for 1 day. In both of the above two groups, no increment in glucose-induced rise in serum insulin level was observed. Islets of Langerhans isolated from clonidine-treated rats showed pronounced insulin releasing capacity in response to glucose. Insulin content per islet of the clonidine-treated rat was slightly larger than that of control rat. These results indicate that the enhancement of serum insulin response to glucose following clonidine treatment is mainly attributable to the hyper-responsiveness developed in the pancreatic B-cells.

Central and peripheral cardiovascular actions of urapidil in normotensive and Goldblatt hypertensive animals

The central effect of urapidil on mean arterial blood pressure, heart rate, and reflex tachycardia was studied in anesthetized normotensive and Goldblatt hypertensive dogs and anesthetized cats. The administration of 1 mg intracisternally of urapidil decreased blood pressure and heart rate in normotensive dogs, whereas 2 and 4 mg decreased heart rate and returned blood pressure to the control level. Reflex tachycardia evoked by the intravenous administration of bradykinin was suppressed in a dose-dependent manner by urapidil. In the anesthetized cat, urapidil was administered intracisternally (50 to 400 micrograms) and had no effect on heart rate. The reflex bradycardia elicited by central vagal nerve stimulation was accentuated by urapidil given intracisternally, but this only occurred in the presence of beta adrenoceptor blockade with propranolol.

Distribution of alpha 2 agonist binding sites in the rat and human central nervous system: analysis of some functional, anatomic correlates of the pharmacologic effects of clonidine and related adrenergic agents

Using [3H]para-aminoclonidine, alpha 2 adrenergic binding sites have been mapped in the rat and human CNS using in vitro labeling autoradiographic techniques. In both the rat and human thoracic spinal cord, high densities of alpha 2 binding sites were associated with the substantia gelatinosa and the intermediolateral cell column. In the rat medulla, high binding site density was observed in the medial nucleus of the solitary tract, dorsal motor nucleus of the vagus, raphe pallidus and the substantia gelatinosa of the trigeminal nucleus, while lower levels of specific binding were found in the lateral and ventrolateral medulla. In the human, a similar distribution was observed. However, significantly lower levels of specific binding were seen in the medial nts as opposed to the dmv. In the rat, high levels of specific binding were seen at pontine and midbrain levels in the locus coeruleus, parabrachial nucleus and periaqueductal gray. In the forebrain, several hypothalmic and limbic regions, including the paraventricular and arcuate nuclei of the hypothalamus, the central, medial and basal nuclei of the amygdala, lateral septum and bed nucleus of the stria terminalis and pyriform, entorhinal and insular cortex were labeled. Each of these regions are involved in either modulating autonomic functions directly or integrating somatosensory and/or affective function with autonomic mechanisms. Further, these regions are interrelated by reciprocal connections, and neurons that utilize noradrenaline or adrenaline as their neurotransmitter form a vital part of these connections. Thus, these functional, anatomical and neurochemical correlates of the alpha 2 binding site distribution establish a neurological basis for the complex pharmacological effects of centrally acting alpha 2 agonists.

Analysis of CNS sympatho-inhibition produced by guanabenz

The effects of guanabenz on several autonomic systems were observed in vagotomized, anesthetized cats with the aim of determining, in a quantitative sense, the degree to which guanabenz produces a clonidine-like central nervous system action. Guanabenz given as a single dose (50 micrograms/kg i.v.) produced a transient hypertension associated with a more sustained bradycardia and depression of centrally (hypothalamic) evoked electrodermal responses (EDR). Increasing cumulative doses of guanabenz (3-1000 micrograms/kg i.v.) also resulted in a dose-related depression of EDR amplitude, transient hypertension followed by hypotension, sustained bradycardia, and mydriasis. All responses were antagonized by pretreatment with yohimbine (0.5 mg/kg i.v.). The ED50 for depression of the centrally evoked EDR was in the range of 50-100 micrograms/kg i.v. in the non-pretreated preparations. Guanabenz (100 micrograms/kg i.v.) was shown to be devoid of significant ganglionic blocking properties. These experiments suggest that guanabenz acts like clonidine in the CNS and that an alpha 2-adrenergic inhibitory mechanism is involved in its myriad of central autonomic effects.

Bronchopulmonary effects of clonidine on the bronchomotor responses of the guinea-pig

In conscious guinea pigs, clonidine (10 and 100 micrograms/kg i.v.) lowered diastolic (-7.9 +/- 3.5 and -12.4 +/- 5.2%) and systolic (-8.6 +/- 3.0 and -11.9 +/- 4.2%) arterial pressure and reduced heart rate (-14.5 +/- 3.9 and -27.7 +/- 3.8%), but did not significantly modify pulmonary airway resistance. Hypotension was suppressed by yohimbine and bradycardia was partially suppressed by atropine and yohimbine, which demonstrates in this animal an alpha 2-adrenergic effect for hypotension and a mixed cholinergic and alpha 2-adrenergic effect for bradycardia. Clonidine (10 and 100 micrograms/kg i.v.) enhanced the bronchoconstrictor effects of histamine 20 micrograms/kg (+80.0 +/- 22.5 and 89.1 +/- 26.5%), acetylcholine 25 micrograms/kg (+66.4 +/- 19.8 and +95.4 +/- 25.4%) and serotonin 15 micrograms/kg (+68.5 +/- 23.2 and +81.4 +/- 34.1%). The duration of this effect was comparable to that of the hypotensive and cardiac effects of clonidine. The effects of clonidine were suppressed after pretreatment with propranolol, reserpine or pentobarbitone, all drugs which enhance the bronchoconstrictor effect of ACh. Yohimbine (1 mg/kg), piperoxan (0.3 mg/kg) or prazosin in high dosage (0.3 mg/kg) inhibited the potentiation by clonidine of ACh-induced bronchoconstriction, whereas prazosin in lower doses (0.03 mg/kg) or AR-C 239 (0.05 mg/kg) had no action. A specific involvement of alpha 2-adrenoceptors stimulated by clonidine with subsequent reduction of the adrenergic activity associated with bronchospasm could therefore be demonstrated in the conscious guinea-pig during bronchomotor reactions.

Absence of an effect of mianserin on the actions of clonidine or methyldopa in hypertensive patients

The concurrent administration of tricyclic antidepressants has been shown in man to result in a clinically significant impairment of the antihypertensive effect of clonidine. This interaction is thought to be related to competition for central alpha 2 receptors where clonidine acts as an agonist and the tricyclics act as antagonists. Although it seems to cause less cardiovascular effects than tricyclic antidepressants, the tetracyclic antidepressant, mianserin also has been reported to be an alpha receptor antagonist and may, therefore, also interfere with the antihypertensive activity of centrally-acting drugs. This study investigates the effects of acute and chronic mianserin administration in patients with essential hypertension established on long term treatment with either clonidine or methyldopa. The first dose of mianserin was not associated with an increase in blood pressure and during a further two weeks of mianserin therapy there were no significant alterations in blood pressure, supine or erect. Similarly, mianserin did not alter heart rate either after acute or after chronic administration. Mianserin itself had a sedative effect but there was no interference with the sedation attributable to clonidine or methyldopa. Mianserin caused no reduction salivary flow and did not influence the reduced saliva production caused by clonidine. Both clonidine and methyldopa are associated with a reduction in sympathetic outflow but there was no evidence in this study of any further change in plasma noradrenaline or 24 h urinary catecholamine excretion. This study demonstrates that if mianserin is given acutely or chronically, it does not interfere with the effects of the centrally acting anti-hypertensive drugs, clonidine and methyldopa. Mianserin may therefore be a suitable antidepressant for patients receiving these antihypertensive agents if drug treatment for depression is indicated.

Assessment of the interaction between mianserin and centrally-acting antihypertensive drugs

1 The interaction between mianserin and centrally-acting antihypertensive drugs was evaluated in normal volunteers and in patients with essential hypertension receiving either clonidine or methyldopa. 2 The administration of the first dose of 20 mg mianserin to the normal volunteers was associated with a significant sedative effect and transient postural hypotension. 3 In the normal volunteers, the blood pressure responses to a single oral dose of 300 micrograms clonidine were not modified by pretreatment with mianserin. The bradycardia associated with clonidine alone, however, was significantly attenuated. 4 In the patient study, no significant changes in blood pressure control were observed, either after the first dose of 30 mg mianserin or after one and two weeks' continued treatment with mianserin. 5 There is no evidence from these studies that the addition of mianserin therapy results in a clinically significant impairment of the antihypertensive effects of clonidine or methyldopa.

Inhibition of efferent sympathetic nerve activity by centrally administered paraoxon in the cat

We recently found that central administration of the cholinesterase inhibitor paraoxon lowered blood pressure substantially. It was postulated that the decrease in pressure was mediated by a reduction of sympathetic outflow. In the present study, efferent splanchnic nerve activity in anaesthetized and paralysed cats was recorded, and quantified by measuring the variance of signal amplitude. After administration of 8 micrograms paraoxon into the vertebral arteries, blood pressure and splanchnic nerve activity decreased simultaneously. A mean fall of 46 +/- 6% and 45 +/- 13% (mean +/- S.E.M.) respectively was reached within 12 min and was maintained during the period studied (30 min). When the effect of paraoxon was antagonized by dexetimide, both blood pressure and splanchnic nerve activity returned to control values. Since previous work has shown that the depressor action could not be prevented by efferent vagal blockade it seems likely that the fall in blood pressure after paraoxon was mainly caused by a decreased sympathetic outflow. In addition, we varied the amplifier band width in recording splanchnic nerve activity. The measurement of frequencies between 10 and 225 Hz appeared to be sufficient for studying the change in activity after paraoxon.

Influence of alpha-receptor blocking agents upon metabolic activity in nuclei involved in central control of blood pressure

The energy metabolism of the central nervous structures involved in the regulation of blood pressure was examined by means of the autoradiographic 2-deoxy-D-[14C]glucose method in normal, awake rats during hypotension induced by the alpha-adrenergic blockers, phentolamine, phenoxybenzamine, and yohimbine. Administration of these alpha-blockers produced a dose-dependent increase of glucose consumption in the medial nucleus of the solitary tract (NTSm), the dorsal motor nucleus of the vagus (DMX), the nucleus ambiguus (NA), and the paraventricular (PAVH) and supraoptic (SO) hypothalamic nuclei. However, in the external cuneate nucleus and in the paramedian reticular nucleus there was a decrease in glucose utilization. These changes appear to be at least partly due to the hypotension produced by the drugs in addition to their central effects; when the hypotension was prevented by administration of the plasma expander, dextran, in phenoxybenzamine-treated rats, the effects were less than those observed in animals with hypotension not limited by dextran infusion.

The antihypertensive mechanism of clonidine in man. Evidence against a generalized reduction of sympathetic activity

Recording of multi-unit sympathetic activity were made from muscle branches of the peroneal nerve during i.v. bolus injection of 100 to 275 micrograms clonidine in seven hypertensive patients. Blood pressure was reduced in all patients, but sympathetic activity and heart rate could either increase or decrease. When plasma levels of clonidine were low, sympathetic activity tended to increase, and when plasma levels were high, activity tended to decrease. Irrespective of whether mean level of sympathetic activity increased or decreased with the fall in blood pressure level, transient fluctuations of blood pressure continued to cause dynamic baroreflex modulation of the sympathetic outflow. It is suggested that the drug influences sympathetic outflow by a combination of central and peripheral effects.

Pharmacodynamic studies on mianserin and its interaction with clonidine

There is evidence that clonidine's hypotensive effect is reduced by the concurrent administration of tricyclic antidepressants. It has been proposed that this results from an interaction at alpha 2-receptors in the brain stem where clonidine acts as a relatively selective agonist and the tricyclic antidepressants as antagonists. Mianserin is an antidepressant with a tetracyclic structure and, although it has been reported to cause less cardiovascular disturbance, there is evidence that it also has alpha-adrenoceptor blocking effects. This study in 6 normotensive healthy male volunteers was designed to investigate a possible interaction between clonidine and the antidepressant mianserin. Administration of the first dose of 20 mg mianserin was associated with acute cardiovascular effects, notably transient postural hypotension, but no significant disturbance of heart rate or blood pressure was detected after 3 days continuous treatment with mianserin 20 mg tid. Following pre-treatment with mianserin or placebo the responses to a single oral dose of 300 micrograms clonidine were then assessed. The combination of mianserin and clonidine was not associated with any attenuation of clonidine's hypotensive effect, erect or supine, but there was significant attenuation of clonidine's supine bradycardic effect. There was no evidence that mianserin interfered with the ability of clonidine to diminish salivary flow, cause sedation, and reduce catecholamine output, but it was noted that mianserin itself had a very pronounced sedative effect. Mianserin alone had no significant effect on salivary flow. This short term study demonstrates that mianserin does not significantly interfere with the responses to a single oral dose of clonidine.

Comparative study of the effect of azepexole and clonidine on blood pressure and pulse rate in normotensive subjects

In a double-blind cross-over study in 8 normotensive subjects azepexole was found to have a pharmacological profile identical to that of clonidine, although chemically there is no structural similarity between the two compounds. Compared to placebo, oral azepexole 5 and 10 mg and clonidine 0.15 mg produced a significant reduction in systolic and diastolic blood pressures both supine and standing (tilted). The reduction was greater in the standing than in the supine position. There was a significant slowing of the pulse rate only in the supine position, whereas the reflex increase in pulse rate on tilting was enhanced after both substances. The rise in diastolic blood pressure on tilting, however, was partically blocked by azepexole 10 mg. No dose dependent change in the pulse rate was observed. The maximum fall in blood pressure and pulse rate occurred between 1.5 and 4 h after ingestion of either substance, and had not returned to placebo level after 6 h, except for the supine systolic pressure after azepexole 5 mg. Dose-dependent sedation and dryness of the mouth were observed both after azepexole and clonidine. They followed approximately the same time-course as the haemodynamic changes. Although the hypotensive effect of azepexole 10 mg was larger than that of clonidine 0.15 mg, the orthostatic side-effects were also greater. As the hypotensive effect and side-effects of azepexole 5 mg and 0.15 mg clonidine were almost the same, it seems justifiable to investigate the properties of azepexole in hypertensive patients.

Effects of low doses of clonidine in the monkey cranial circulation

Migraine therapy using low doses of clonidine has been based on the proposal that clonidine directly inhibits vascular smooth muscle reactivity. In anaesthetized monkeys in which internal and external carotid vascular resistances were measured, the only significant effects of clonidine administered acutely (0.5 and 2 microgram x kg-1 i.v.) or chronically (2 microgram x kg-1 i.m. daily for 7 days) on cranial vascular responses to the constrictors noradrenaline and 5-hydroxytryptamine, and the dilators histamine, prostaglandin E1 and bradykinin, were small potentiations of some of the responses. Acute clonidine initially increased blood pressure and constricted the cranial vasculature, then induced hypotension without involvement of the cranial circulation. It also decreased the external carotid vasoconstrictor response to low frequency cervical sympathetic nerve stimulation. The low chronic dose of clonidine had no hypotensive effect. The pressor response to common carotid occlusion was inhibited by both acute and chronic clonidine. These experiments thus provide no evidence that clonidine inhibits cranial vascular reactivity at doses equivalent to those used in migraine.

Furter investigations on the interaction between alpha-adrenoceptor antagonists and the central hypotensive effect of clonidine in rats, rabbits and dogs

1. The interactions between five alpha-adrenoceptor blocking agents and clonidine have been studied in rats, rabbits and dogs after intracisternal injections. 2. Dibozane, ethomoxane, azapetine, dibenamine and thymoxamine reduced blood pressure in rats and an antagonism of the hypotensive effects of clonidine was detected for the first four drugs. 3. In rabbits, azapetine, dibenamine, dibozane and ethomoxane were hypotensive while thymoxamine had no effect on blood pressure. Dibozane and dibenamine reduced the hypotensive effect of clonidine. 4. In dogs, azapetine, dibozane and ethomoxane reduced blood pressure, while dibenamine induced an increase in blood pressure and thymoxamine was without effect. Only dibenamine antagonized the blood pressure lowering effect of clonidine. A definite conclusion could not be drawn with azapetine due to its long duration of action of action in both rabbits and dogs. 5. These results suggest that the central receptors involved in the hypotensive effect of clonidine differ from many other central receptors and vary according to the animal species. In addition the alpha-adrenoceptor blocking agents appear not to have a unique site and mechanism of action on central cardiovascular control.

Effects of long-term administration and withdrawal of clonidine on activity of sympathetic efferent nerve unit in spontaneously hypertensive rats

Daily administration of clonidine for 5 weeks (approx. 300 micrograms/kg/day, p.o.) produced significant decreases in blood pressure and renal sympathetic efferent activity in spontaneously hypertensive rats (SHR). After cessation of clonidine administration, blood pressure and sympathetic nerve activity recovered rapidly from the depressed levels to the untreated control levels. These findings could indicate that clonidine-induced hypotension and its rapid recovery to control after withdrawal of clonidine are caused by corresponding changes in sympathetic efferent nerve activity.

Effects of clonidine and chlorpromazine on a sympathetic-cholinergic reflex

Intravenous administration of clonidine and chlorpromazine resulted in a dose-dependent inhibition of the amplitude of reflexly evoked electrodermal responses in intact and spinal cats. Yohimbine pretreatment (0.5 mg/kg, i.v.) antagonized the effects of clonidine but not chlorpromazine in both preparations. These findings confirm and expand previous observations that both clonidine and chlorpromazine inhibit the amplitude of centrally evoked responses in this sympathetic-cholinergic system. In addition, both drugs appeared to have a spinal site of action. The antagonism of the effects of clonidine by yohimbine suggests that the mechanism of the action of clonidine may be a result of activation of central inhibitory alpha-adrenergic receptors. The failure of yohimbine to antagonize the effects of chlorpromazine suggests that clonidine and chlorpromazine may depress these sympathetic reflexes by different mechanisms.

Vagal bradycardia produced by microinjections of morphine-like drugs into the nucleus ambiguus in anaesthetized dogs

The site in the dog medulla oblongata where fentanyl or met-enkephalinamide produced vagal bradycardia was determined using microinjections. The nucleus ambiguus was found to be a selective and highly sensitive area for the vagal bradycardia.

Multiple receptor responses: a new concept to describe the relationship between pharmacological effects and pharmacokinetics of a drug: studies on clonidine in the rat and cat

The time course of an observed pharmacological effect is affected not only by the kinetics of the drug levels at the site of action but also by parameters such as the slope and maximum effect of the functional relationship between drug level and response. Using clonidine as a test drug, it was found that the kinetics of its effects on blood pressure and pain responses cannot be described by the time course of clonidine levels in the blood, brain, or the hypothetical tissue compartment of the two-compartment characteristics of this drug. However, the results can be explained assuming that the observed pharmacological effects of a drug are composed of the sum of responses from at least two receptor sites with different slopes and maximal effects. The effect of intravenously administered clonidine on blood pressure in the rat was found to be related to the blood concentrations at least at two receptor sites with opposite effects, one leading to a hypertensive and the other to a hypotensive response. Predictions indicate that a maximum decrease of arterial blood pressure is obtained when the steady-state blood concentration of clonidine is about 1 ng/ml and that no effect is seen at 10 ng/ml. Higher levels will produce an increase of the pressure. The kinetics of the analgesic effect of clonidine in the rat could best be related to the brain levels if the observed effect was considered to be derived from the sum of activity at two receptor sites each producing analgesia. The kinetics of the effects of clonidine on the nictitating membrane of the cat was found to be determined by the kinetics of the drug in the peripheral compartment of the two-compartment open model. Consideration of multiple receptor responses is suggested for future studies on the relationship between the kinetics of drug levels and pharmacological responses.

Pharmacokinetics of clonidine in the rat and cat

To investigate the pharmacokinetic behavior of clonidine, rats were given clonidine intravenously at 125, 250, and 500 micrograms/kg and blood clonidine concentrations were followed for 6 hr. The disposition of clonidine in two brain regions was studied in rats after an i.v. dose of 500 micrograms/kg. The liver clearance in rats was investigated by liver perfusion techniques. The results obtained indicate that the disposition characteristics of clonidine can be described by a two-compartment open model in both rats and cats. The penetration of clonidine into tissues is rapid, and brain levels in rats were about 1.7 times higher than blood levels. Brain tissues were found to be an indistinguisible part of the central (blood) compartment. Dose-dependent pharmacokinetic behavior was found for clonidine in rats at the doses used. This was demonstrated by a decrease of both the rate constant of distribution to the peripheral compartment and the overall elimination rate constant from the body, with increase in dose. As a consequence, the volume of distribution and the clearance both decreased with increasing dose. Possible explanations for the dose-dependent behavior of clonidine are discussed.

Pharmacological studies of lycorenine, an alkaloid of Lycoris radiata Herb.: Vasodepressor mechanism in rats

Vasodepressor mechanism of lycorenine (an alkaloid of Lycoris radiata Herb.) was investigated in anesthetized rats. Lycorenine (1--10 mg/kg i.v.) produced dose-related decreases in blood pressure and heart rate and tachyphylaxis developed with repeated injections. In the blood-perfused rat hindquarters, lycorenine (62.5--500 micrograms i.a.) produced dose-related decreases both in mean blood pressure and in perfusion pressure, and the lycorenine-induced decrease in perfusion pressure was abolished by phenoxybenzamine or hexamethonium. Lycorenine (more than 1 mg/kg i.v.) blocked the pressor response to sympathetic nerve stimulation, but failed to block the tachycardia induced by sympathetic nerve stimulation. Lycorenine (7.5 or 15 mg/kg i.v.) reduced the spontaneous splanchnic nerve activity. Lycorenine when given intracerebroventricularly produced decreases in blood pressure and heart rate only in large doses (over 500 micrograms). The maximal bradycardia induced by lycorenine was abolished by bilateral vagotomy. It is suggested that lycorenine may produce a decrease in blood pressure as the result of alpha-adrenergic blockade in conjunction with the reduction of the spontaneous sympathetic nerve activity, and produce bradycardia by modifying vagal activity.

Topical clonidine produces mydriasis by a central nervous system action

One drop of clonidine solution (0.125, 0.25 or 0.5%) was administered topically to one eye in cats anesthetized with pentobarbital in which the vagosympathetic nerves had been sectioned. Clonidine caused a simultaneous dose-related mydriasis in both eyes along with a decrease in heart rate. The peak effects were observed in about 20--30 min. Topical administration of clonidine (0.5%) produced no effect on the parasympathectomized, eserinized iris but did dilate the opposite pupil. Epinephrine (0.1--30 microgram, i.a.) produced equal pupillary dilation in both eyes. In addition, topical clonidine caused a dramatic decrease in postganglionic ciliary nerve activity. All of the effects of clonidine were antagonized by yohimbine (0.5 mg/kg, i.v.). These results demonstrate that topical administration of clonidine causes my driasis in the cat and that this effect is mediated totally by means of CNS inhibition of parasympathetic tone to the iris.

Evidence for a central postsynaptic action of clonidine

Intravenous administration of clonidine (1--100 micrograms/kg) produces a dose-dependent mydriasis in cats by inhibition of parasympathetic tone to the iris. The magnitude of CNS-induced pupillary dilation was similar in both normal anaesthetized cats and in anaesthetized preparations pretreated with reserpine (5 mg/kg i.p.) and alpha-methyl-p-tyrosine (2 x 300 mg/kg i.p.). Pretreatment reduced the concentrations of noradrenaline, dopamine and serotonin to less than 3% of that control levels in most parts of the CNS in which these amines were measured. Clonidine produced bradycardia in control animals but not in pretreated cats. In amine depleted animals in which only one eye was innervated by the ciliary nerves (parasympathetic), clonidine produced mydriasis only on the innervated side. These experiments confirm our previous observations that clonidine produces mydriasis in the cat by means of inhibition of parasympathetic tone to the iris. It is concluded that if clonidine produces this effect by stimulating noradrenergic, dopaminergic or serotonergic receptors, then clonidine exerts its centrally-induced mydriatic effect by acting on post-synaptic mechanisms.

Pharmacological study of a cholinergic mechanism within the rat posterior hypothalamic nucleus which mediates a hypertensive response

In unanesthetized freely moving rats, microinjection of a variety of cholinergic agonists into the posterior hypothalamic nucleus (PHN) consistently produced an elevation in mean arterial pressure (MAP). Experiments were undertaken to pharmacologically characterize this cholinergic mechanism. Microinjection of carbachol (0.1--100 nmol) into the PHN elicited reproducible and dose-related increase in MAP (17--47 mm Hg) and variable changes in heart rate. Similar responses, although longer in onset and duration, were produced by microinjection of the cholinesterase inhibitors, neostigmine, physostigmine and echothiophate. The pressor responses produced by neostigmine and physostigmine, but not by carbachol, were shown to be dependent upon intact stores of acetylcholine in the PHN. Blockade of postsynaptic muscarinic receptors by prior microinjection of atropine abolished the rise in MAP to subsequent injection of cholinergic agonists; however, similar pretreatment with the antinicotinic agent, mecamylamine, was without effect. The peripheral mechanism through which a rise in MAP was produced by cholinergic stimulation of PHN was the sympathetic nervous system since i.v. injection of an alpha-adrenergic blocking agent, phentolamine, attenuated the pressor response to intrahypothalamic injection of carbachol or neostigmine. Adrenal catecholamine release or vasopressin release were not important mechanisms in this regard. From this study we conclude that within the rat PHN there exists a muscarinic, cholinergic mechanism which, upon activation, mediates a rise in MAP through an increase in sympathetic tone.

Evidence for a central monoaminergic influence on urinary bladder control mechanism

The aim of the present study was to investigate a possible central monoaminergic influence on the control mechanism of the urinary bladder in rats. A selective central nervous stimulation was accomplished by injection of monoaminergic precursor (L-dopa and 5-HTP) after enzymatic blockade of its peripheral metabolisation. The bladder response was recorded with a cytometric procedure. The central adrenergic stimulation with L-dopa resulted in a hyperactive bladder response, with higher intravesical pressure and more prominent detrusor contractions than in control rats. Injection of 5-HTP had no such effect. The hyperactive bladder response to L-dopa was abolished by prior administration of a central dopamine receptor blocking agent (spiroperiodol). This indicated that the central effect on the bladder was elicited by dopaminergic structures. Peripheral adrenergic stimulation with metabolites of L-dopa--dopamine and noradrenaline--gave no measurable bladder response. The study showed that activation of central adrenergic mechanisms influenced urinary bladder control, i.e. evoked a hyperactive bladder response.

Effect of hypothalamic stimulation in spontaneously hypertensive and Wistar-Kyoto rats

This study was undertaken to determine if central nervous system differences in blood pressure regulation exist between spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) controls. Central control mechanisms were examined by observing the effects of posterior hypothalamic (PH) stimulation upon preganglionic sympathetic activity in 14--18 week old SHR and WKY rats. A bipolar, concentric electrode was stereotaxically placed in the PH. Stimulation was delivered at 20, 60 and 100 Hz (3-sec duration, 0.1 msec pulse width) at a voltage twice that producing an increase in blood pressure (less than 5 mm Hg) at 60 Hz. Sympathetic activity was recorded from a portion of the splanchnic nerve just distal to the diaphragm. Blood pressure was measured from a femoral artery catheter. SHR responded with greater increases in sympathetic activity than WKY; the differences were statistically significant at 60 and 100 Hz. SHR also responded with significantly greater increases in blood pressure at all frequencies of stimulation. To determine if the enhanced sympathetic response to PH stimulation seen in adult SHR is an intrinsic difference rather than secondary to sustained hypertension, we maintained SHR normotensive from four weeks of age with antihypertensive drug therapy (clonidine or hydralazine). Chronically treated animals were then tested at 14--18 weeks of age while on antihypertensives or four days after drug discontinuance. Sympathetic and blood pressure responses to PH stimulation were significantly greater in SHR maintained normotensive than untreated or chronically treated WKY. These data support the concept that a central factor is involved in the etiology of hypertension in the SHR.

Central adrenoceptors and cholinoceptors in cardiovascular control

1. In cats anaesthetized with chloralose, adrenoceptor and cholinoceptor agonists and antagonists were localized to the posterior hypothalamus (PH), lateral medullary pressor area (LMPA) and spinal autonomic loci to delineate the role of central cholinoceptors and adrenoceptors in cardiovascular control. 2 All along the neuroaxis, the alpha-adrenoceptors seem to subserve an inhibitory and the beta-adrenoceptors a facilitatory role in cardiovascular control. There appear to be a predominance of alpha-adrenoceptors at the medullary level and beta-adrenoceptors at the hypothalamic level. 3 The nicotinic cholinoceptors at the hypothalamic, medullary and spinal levels were facilitatory, whereas muscarinic cholinoceptors were inhibitory for cardiovascular control. However, muscarinic receptors were undetectable at the posterior hypothalamus. 4 The central cardiovascular effects of nicotine are attributed to nicotinic receptor activation and release of central catecholamines. 5 There appears to be a relationship between central cholinergic and adrenergic mechanisms in cardiovascular control.

Effects of clonidine on vulnerability to fibrillation in the normal and ischemic canine ventricle

Clonidine infusion (10 microgram/kg, i.v.) elicited a 30% increase in repetitive extrasystole (RE) threshold in 6 chloralose-anesthetized dogs. A reduction in heart rate and arterial blood pressure accompanied the increased threshold. Intracisternal injection of clonidine (2 microgram/kg) in 6 dogs caused similar alterations in these parameters. Bilateral vagotomy, performed in 6 dogs prior to intravenous clonidine, prevented the increase in RE threshold but did not prevent the drug-induced bradycardia. Atropine (0.2 and 0.6 mg/kg), however, did not attenuate the effect of clonidine on RE threshold. Clonidine administration did not prevent the reduction in ventricular fibrillation threshold associated with a 10 min occlusion of the left anterior descending coronary artery or following reperfusion. We conclude that: (1) clonidine reduces ventricular vulnerability in the normal but not the ischemic heart, and (2) its protective effect is mediated by enhanced afferent vagal input to midbrain cardiovascular regulatory centers. This central nervous system action causes a reduction in sympathetic tone to the heart.

Central cardiovascular effects of morphinomimetic peptides in dogs

Morphinomimetic peptides were injected into the cisterna magna of chloralosed dogs. Methionine enkepalin (500 microgram/kg i.c.) was found ineffective but beta-endorphin (50 microgram/kg i.c.) induced an initial and transient increase in blood pressure and heart rate followed by a delayed hypotension and bradycardia. The synthetic pentapeptides, [d-ala2]met-enkephalin (500 microgram/kg i.c.), [d-ala2]met-enkephalinamide 500 microgram/kg i.c.) also induced a marked hypotensive, bradycardic and sympatho-inhibitory effect. High doses of naloxone (100 microgram/kg i.v.) were required to antagonize these effects transiently.