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

Relationship between the ventromedullary clonidine-sensitive area and the posterior hypothalamus

The connections between the areas 'S' which have been previously described as the ventromedullary sites of the action of clonidine and the posterior hypothalamus have been investigated. Superficial electrocoagulation of the left area 'S' suppresses the pressor response to electrical stimulation of the homolateral part of the posterior hypothalamus. Although such medullary lesions cause a significant reduction of the mean arterial pressure, the contralateral hypothalamic stimulation can still increase blood pressure. Clonidine it self applied topically (8 micrograms/kg) to the ventral face of the brain stem decreases the blood pressure response to liminal hypothalamic stimulation. It is concluded that efferent pathways, which are involved in vasomotor regulation, originate in the posterior hypothalamus and run through the ventrolateral part of the brain stem. The mechanism of the blocking effect of clonidine on these pathways is discussed.

Susceptibility of spontaneous sympathetic outflow and sympathetic reflexes to depression by clonidine

The hypotensive activity of clonidine has been attributed to inhibition of sympathetic outflow. The present series of experiments examined the relative influence of clonidine on spontaneous outflow and various sympathetic reflexes in anesthetized cats. Clonidine resulted in dose-related decreases in the former. Increases in efferent sympathetic activity mediated by baroreceptors and supraspinally integrated potentials evoked in the splanchnic nerve by somatic nerve stimulation were inhibited to a lesser extent. Splanchnic potentials evoked by spinal cord stimulation in spinal animals were also diminished. In contrast, baroreceptor induced reflex decreases in sympathetic outflow were only minimally affected. These experiments indicate that clonidine can inhibit central sympathetic mechanisms at various levels including the spinal cord but that considerable differences exist in the susceptibility of sympathetic outflow and various reflexes to depression by the compound.

Cardiovascular changes induced by large doses of clonidine in mice

Clonidine at doses of 0.5-1.0 mg/kg i.p. produced an initial rise followed by a sustained fall in blood pressure. The initial pressor response became more marked and the onset of hypotensive effect was delayed as the dose was increased to large doses such as 10-50 mg/kg given intraperitoneally. The heart rate was markedly reduced soon after clonidine administration and the bradycardia lasted for more than 2 hours. Both the initial pressor and subsequent hypotensive effects of clonidine were reduced by pretreatment with phentolamine, the initial pressor effects were suppressed by propranolol which did not affect the hypotensive effects. This initial pressor effect was potentiated while the hypotensive effect was reduced after bilateral vagotomy and pretreatment with either 6-hydroxydopamine or atropine. The bradycardia was significantly reduced by propranolol, atropine and bilateral vagotomy. Central sympathetic as well as parasympathetic mechanisms may be involved in cardiovascular changes after large doses of clonidine in urethanized mice.

Central sites and mechanisms of the hypotensive and bradycardic effects of the narcotic analgesic agent fentanyl

In dogs, anaesthetized with chloralose, fentanyl (5 mug/kg i.v.) augmented the bradycardia produced by electrical stimulation of the carotid sinus nerves. In contrast, the same dose of the drug did not change the bradycardic response to stimulation of the nucleus of the solitary tract (NTS) indicating that a central facilitation of baroreceptor impulses occurs within the NTS, probably at the first synapse of baroreceptor reflex fibres. Bilateral destruction of the NTS caused a fulminating hypertension and tachycardia similar to that after cutting the baroreceptor afferent fibres. After both procedures, fentanyl (20 mug/kg i.v.) produced marked hypotension and bradycardia. The bradycardic effect was abolished by cutting both vagal nerves when the dogs were pretreated with a beta-adrenoceptor blocking agent (S 2395, 50 mug/kg i.v.). The results provide evidence that the NTS is not the main site of action either for the hypotensive effect or for the vagally mediated bradycardia of fentanyl. Since the dorsal nucleus of the vagal nerve was destroyed together with the NTS, this nucleus does also not appear to be a major site of the action of fentanyl. Blockade of dopamine receptors by haloperidol or pimozide or of serotonin receptors by methysergide did not change the hypotensive, bradycardic and sympathoinhibitory effects of fentanyl.

Effect of clonidine and chlorpromazine on centrally evoked electrodermal responses and their interaction with yohimbine

Both clonidine and chlorpromazine reduced the amplitude of electrodermal responses (EDR) evoked by stimulation of the hypothalamus at a constant submaximal frequency (10-16 Hz). The ED50 for clonidine was approximately 5 mug/kg and that for chlorpromazine was about 1 mg/kg. Yohimbine pretreatment (0.5 mg/kg, i.v.) antagonized the effects of clonidine but did not alter the effectiveness of clorpromazine in inhibiting these responses. Yohimbine alone was without effect on these sympathetic-cholinergic responses. These results suggest that clonidine and chlorpromazine deress central sympathetic reactivity by different mechanisms.

Action of clonidine on the baroreceptor pathway and medullary sites mediating vagal bradycardia

In anaesthetized dogs, clonidine (10 mug/kg i.v.) increased the spontaneous firing of the carotid sinus nerve and decreased blood pressure and heart rate. After transection of the spinal cord, clonidine decreased heart rate and this bradycardia was abolished by selective baroreceptor denervation. Clonidine (1 mug/kg) injected into the vertebral artery of anaesthetized dogs, pretreated with a beta-adrenoceptor blocking agent (S 2395: 50 mug/kg i.v.) potentiated the bradycardia induced by stimulation of the carotid sinus nerve but did not change the hypotension and bradycardia produced by stimulation of the nucleus tractus solitarius or of the nucleus ambiguus. In anaesthetized cats with bilateral destruction of nuclei tractus solitarii, clonidine (10 mug/kg i.v.) decreased blood pressure and heart rate. Clonidine (2 mug/kg), injected into the vertebral artery of anaesthetized dogs pretreated with a beta-adrenergic blocking agent (S 2395: 50 mug/kg i.v.) or guanethidine, induced a bradycardia but the discharges of the carotid sinus nerve were not increased. Selective baroreceptor denervation abolished this bradycardia. In conclusion, these experiments provide direct evidence that the central facilitory effect of clonidine on baroreceptor impulses play a role in the bradycardic effect of the drug. This facilitation is likely localized in the nucleus tractus solitarius at the first synapse of baroreceptor fibres. The vagally mediated bradycardia can be explained by an increase in baroreceptor discharges and by the central facilitation of baroreceptor impulses. The site of the hypotensive effect of clonidine did not seem to be localized in the nucleus tractus solitarius.

Studies on the site of action of clonidine utilizing a sympathetic-cholinergic system

Clonidine (30 mug/kg, i.v.) reduced centrally evoked electrodermal responses (EDR) following stimulation of reactive loci in the hypothalamus and medulla. The responses were most depressed following low frequency stimulation. Similar results were observed on the EDR evoked by stimulation of the cervical cord in the spinal cat. Little effect was seen following peripheral nerve stimulation. These results demonstrate that clonidine depresses the reactivity of this sympathetic-cholinergic system at all central levels including the cervical cord.

The essential action of propranolol in hypertension

The unique action of propranolol and other beta blockers in lowering raised arterial pressure is discussed. Although the onset of the antihypertensive effect is not immediate, many trials have confirmed the efficacy of these drugs. Animal experiments have thrown little light on the mechanism of action of beta blockers in hypertension: this may be because in animals, especially the rat, peripheral beta adrenoceptor vasodilatation is relatively more important than in man. Five principal theories have been advanced to explain the antihypertensive effect. None of these, the renin, central nervous system, cardiac, baroceptor or metabolite theory, is totally satisfactory. A new theory is proposed suggesting that the essential action is to diminish sympathetic nerve output by damping sensory input to the central nervous system from a heart whose capacity to respond to exercise and stress is blunted by beta adrenoceptor blockade.

The electrodermal response as a model for central sympathetic reactivity: the action of clonidine

Electrodermal responses (EDR) were evoked centrally by stimulation of reactive loci in the posterior hypothalamus and peripherally by stimulation of the distal portion of the sectioned median or ulnar nerve. Moderate doses of clonidine (3-30 mug/kg, i.v.) reduced the amplitude of the centrally evoked EDR while having no effect on the peripherally evoked responses. This central action of clonidine occurred concomitantly with the clonidine-induced bradycardia and hypotension. Administration of clonidine shifted the centrally evoked EDR frequency-response curve to the right in a dose related manner at 3, 10 and 30 mu/kg, i.v. 1 mug/kg was without effect on these responses. This central depressant action of clonidine was partially reversed following administration of yohimbine (0.5-1.0 mug/kg, i.v.). These results suggest that clonidine inhibits central reactivity in this sympathetic-cholinergic system in a manner analogous to its action on other sympathetic systems, and that a central adrenergic inhibitory mechanism may be involved.

Studies on the centrally mediated hypotensive activity of guanabenz

Prior studies demonstrated that guanabenz reduces systemic blood pressure by inhibiting central sympathetic outflow as well as by adrenergic neuron blockade. Potential mechanisms responsible for the reduction of efferent sympathetic activity were examined in the present series. Guanabenz failed to modify carotid sinus nerve activity in a perfused sinus preparation. It reduced sympathetic outflow, heart rate and blood pressure in debuffered cats indicating that its actions are not mediated primarily by baroreceptor mechanisms. alpha-Adrenergic blockade greatly attenuated the response suggesting that the central sympathoinhibitory effect of guanabenz results from alpha-adrenergic receptor activation. Only a high dose of the compound attenuated the increase in sympathetic nerve activity produced by stimulation of the posterior hypothalamus. These experiments lead to the overall conclusion that guanabenz acts primarily at sites which regulate the basal level of sympathetic outflow.

Differential effect of baroreceptor reflexes and clonidine on frequency components of sympathetic discharge

The effects of the baroreceptor reflexes and clonidine on the 3 and 10 Hz components of spontaneously occurring renal sympathetic nervous discharge (SND) were studied in the cat. The proportion of 3 : 10 Hz SND was assessed by selective filtering and integration of activity in the 2-4 Hz and 9-11 Hz frequency bands. Baroreceptor denervation increased activity in both frequency bands while significantly lowering the 3 : 10 Hz ratio. Baroreceptor reflex activation (i.v. phenylephrine) decreased activity in both frequency bands while significantly increasing the 3 : 10 ratio. Clonidine (4-20 mug/kg, i.v.) affected the 3 : 10 Hz ratio of SND in a manner similar to baroreceptor reflex activation. These results indicate that the 3 and 10 Hz components of SND, both of which have been previously shown to be inherent to the central vasomotor system, are differentially affected by the baroreceptor reflexes and clonidine. It is concluded that it is possible to characterize certain cardiovascular states with respect to the 3 : 10 Hz ratio of SND.

Effect of the activation of alpha-adrenoreceptors on the synthesis and release of noradrenaline by peripheral adrenergic nerves in vivo

The synthesis and release of noradrenaline (NA) in the heart and submaxillary glands were studied in the rat following s.c. injections of oxymetazoline (50 mug/kg) or noradrenaline (500 mug/kg). NA release was evaluated from the decline in tissular specific radioactivity after administration of 3H-NA and NA synthesis by the estimation of the amounts of 3H-NA synthesized from 3H-tyrosine (TY) or 3H-Dopa, 30 min after the injection. Oxymetazoline treatment delayed the release of NA, the NA biological half-lives rising from 12 up to 36 hours in the heart and from 5.9 up to 21 hours in sub-maxillary glands. This inhibitory effect on NA release was interpreted as the consequence of the stimulation of alpha-adrenoreceptors. Thirty minutes after its injection, oxymetazoline increased both NA endogenous levels and 3H-NA amounts formed from 3H-TY: 3H-NA specific activities were not significantly altered. NA treatment led to an acceleration of NA release in the heart (NA biological half-life decreasing from 12 to 2.2 hours) but not in sub-maxillary glands. After injection of 3H-TY, the amounts of 3H-NA found in the heart and sub-maxillary glands were strongly reduced. Similar results were observed in the heart using 3H-Dopa as a precursor. These data are interpreted as the consequence of the removal of the newly synthesized 3H-NA by exogenous NA. The results obtained with oxymetazoline point out a dissociation between the NA release which is reduced and the NA synthesis which is unaltered. This indicates that NA synthesis rate by sympathetic nerve terminals is not immediately regulated by its release intensity. These data do not support the end-product feedback inhibition hypothesis according to which tyrosine hydroxylase is regulated by the intraneuronal NA concentration.

Effect of the norepinephrine receptor stimulating agent "clonidine" on the turnover of 5-hydroxytryptamine in some areas of the rat brain

The effect of clonidine (0.05 mg/kg i.p.) on 5-hydroxytryptamine (5-HT) turnover has been studied in the whole brain and in various cerebral areas of the rat (brain stem, hypothalamus, striatum and "rest" of the brain). In the whole brain, clonidine produced a significant decrease (-23%) in 5-hydroxyindole acetic acid level and a slight increase (+7%) in 5-HT level. These variations were not observed in all the cerebral structures. The 5-hydroxyindole acetic acid levels were reduced in the hypothalamus and in the rest of the brain; they were not significantly altered in the brain stem and increased in the striatum. The effect of clonidine on the 5-HT synthesis has been studied by evaluation of the rises in 5-HT levels induced by pargyline: these rises were found significantly increased in the brain stem and the hypothalamus and not significantly altered in the other cerebral structures. These findings give support to the hypothesis according to which the norepinephrine receptor stimulation inhibits the activtity in the 5-HT neurons: clonidine reduces primarily the relase of 5-HT without an immediate effect on its synthesis. The differences observed among the various cerebral areas might be explained by the fact that relations between nor-epinephrine and 5-HT neurons do not exist identically in all cerebral structures.

Localization of the central sympatho-inhibitory effect of a narcotic analgesic agent, fentanyl, in cats

Fentanyl (10 and 30 mug/kg), a narcotic analgesic, induced in cats a decrease in blood pressure and heart rate and reduced spontaneous splanchnic nerve activity. Fentanyl reduced the pressor response to medullary stimulation, but did not change the pressor response to hypothalamic or cervical spinal cord stimulation. Fentanyl reduced the potential evoked in the splanchnic nerve by stimulation at low frequency of a pressor area of the medulla oblongata. The potentials evoked in the splanchnic nerve by hypothalamic or cervical spinal cord stimulation were only slightly changed. Nalorphine (0.5 mg/kg) or naloxone (30 mug/kg) induced a recovery in blood pressure, heart rate and spontaneous splanchnic discharges which had been reduced by fentanyl, but nalorphine or naloxone did not restore pressor response to medullary stimulation or potentials evoked in the splanchnic nerve by medullary stimulation, which had been decreased by fentanyl.

Central and peripheral contribution to the antihypertensive action of indoramin

Indoramin has been reported to reduce blood pressure in experimental animals and humans. The complex pharmacological profile of the compound suggests that it may also exert central actions. Effects on spontaneous sympathetic outflow and on sympathetic nerve and blood pressure responses to hypothalamic stimulation were examined in anesthetized cats in the present study. At a low dose (1 mg/kg) indoramin reduced blood pressure but did not significantly influence the level of efferent sympathetic nerve activity. A higher dose (5 mg/kg) lowered pressure further and inhibited activity in the splanchnic, cardiac and renal nerves indicating that reduced sympathetic outflow is a contributing factor to the hypotensive response at this dose. However, in contrast to other centrally acting antihypertensive agents, e.g., clonidine, indoramin did not inhibit the increase in sympathetic nerve activity produced by hypothalamic stimulation.

Central alpha- and beta-adrenoceptors modifying arterial blood pressure and heart rate in conscious cats

1 In conscious unrestrained cats noradrenaline, alpha-methylnoradrenaline and clonidine, infused into the lateral cerebral ventricles (i.c.v.) caused dose-related falls in blood pressure and heart rate; both effects were abolished after i.c.v. phentolamine.2 In 12 out of 20 cats, i.c.v. isoprenaline and salbutamol when given caused dose-related pressor responses and tachycardias. These effects were abolished after i.c.v. beta-adrenoceptor blocking drugs but were unaffected by alpha-adrenoceptor blocking agents.3 In 5 out of 20 cats, i.c.v. isoprenaline regularly produced dose-related falls in blood pressure with associated tachycardias; both effects were abolished after i.c.v. beta-adrenoceptor blocking agents.4 Intracerebroventricular dopamine produced cardiovascular responses which were qualitatively similar to those produced by i.c.v. isoprenaline.5 Intracerebroventricular adrenaline produced complex responses in untreated animals but typical alpha-effects were obtained after prior i.c.v. treatment with a beta-adrenoceptor blocking agent and typical beta-effects after i.c.v. pretreatment with an alpha-adrenoceptor blocking agent.6 The cardiovascular changes produced by i.c.v. beta-adrenoceptor agonists were abolished after systemic administration of hexamethonium or bethanidine.7 The results are discussed in the light of the mode of action of beta-adrenoceptor stimulants and beta-adrenoceptor blocking agents in the treatment of hypertension.

On the mechanism of action of clonidine: effects on single central neurones

1 Noradrenaline and clonidine were applied by microiontophoresis to single neurones in the cerebral cortex and medullary reticular formation of anaesthetized rats.2 Of a total of 247 neurones studied, 79% of medullary units and 60% of cortical units responded in the same manner to both noradrenaline and clonidine. The usual response was a depression of neuronal firing rate.3 It proved possible to antagonize some responses to both substances by the microiontophoresis of bulbocapnine, whilst leaving unaffected similar responses to 5-hydroxytryptamine.4 On 13% of the cells, clonidine produced an increase of firing rate. This effect could not be attributed to a post-synaptic antagonism of tonically released endogenous noradrenaline, but may indicate a presynaptic action of clonidine, reducing noradrenaline release.5 These observations are thought to support the idea that clonidine may have an agonist action on noradrenaline receptors in the brain.

Localization of the central cardiovascular action of clonidine

1. The fall in arterial blood pressure with bradycardia that occurs on injection of clonidine into the cerebral ventricles and into the cisterna magna is attributed to an action on ;chemosensitive zones' situated at the ventral surface of the brain stem. This conclusion is based on the following results obtained in cats anaesthetized with pentobarbitone sodium.2. The fall in blood pressure no longer occurs on injection of clonidine (10 to 100 mug) into the cerebral ventricles when the passage of clonidine into the subarachnoid space is prevented by cannulation of the aqueduct. In this condition, the injections produce instead a rise in blood pressure.3. Applied bilaterally, by means of perspex rings, to the ventral surface of the brain stem, clonidine (10 mul of a 50 to 1,000 mug/ml solution placed in each ring) produces a fall in blood pressure with bradycardia, but only when the perspex rings cover the ;chemosensitive zones' from which changes in blood pressure and heart rate are obtained with various drugs.

Interaction between clonidine and desipramine in man

Interaction between the tricyclic antidepressant desipramine and the antihypertensive agent clonidine has been investigated in five hypertensive patients in a double-blind placebo controlled study. Introduction of the tricyclic antidepressant led to loss of blood pressure control in four of the patients. The average blood pressure rise in the desipramine period compared with the placebo period was 22/15 mm Hg in the lying position and 12/11 mm Hg standing. Thus addition of a tricyclic antidepressant may lead to loss of blood pressure control in a hypertensive patient treated with clonidine.

Central noradrenergic neurones and the cardiovascular actions of clonidine in the rabbit

1. Clonidine (1 mug/kg), given by intracisternal injection to anaesthetized rabbits, lowered mean arterial blood pressure by 33 mmHg and heart rate by 32 beats/min.2. In animals pre-treated with 6-hydroxydopamine (6-OHD 500 mug/kg intracisternally) 7-10 days before, intracisternal clonidine (1 mug/kg) reduced mean arterial blood pressure by only 2.5 mmHg and heart rate by 4 beats/minute.3. The hypotensive action of intravenous clonidine was reduced to 49% of control by pre-treatment with intracisternal 6-OHD. In unanaesthetized normal animals intravenous clonidine (30 mug/kg) lowered mean arterial blood pressure by 19.3 mmHg, while after 6-OHD it fell only 9.4 mmHg.4. These studies suggest that the central hypotensive effect of clonidine is dependent on the integrity of central monoaminergic neurones.

Assessment of autonomic function in patients with a Parkinsonian syndrome

The integrity of the autonomic nervous system was assessed in 11 Parkinsonian patients with symptoms suggestive of autonomic dysfunction. Three had the additional clinical features of the Shy-Drager variant of idiopathic orthostatic hypotension and were found to have a gross disturbance of vasomotor, sudomotor, pilomotor, and bladder function; assessment indicated that a lesion was present at sympathetic ganglionic level or beyond in two cases, though a more centrally placed lesion may well have been present also, as in the third case. In the remaining eight patients with paralysis agitans no unequivocal functional disturbance was found except in the bladder; nevertheless, the low resting blood pressure and the supersensitivity to intravenously infused L-noradrenaline in the three patients in whom it was tested is taken to imply defective regulation from higher centres, with a consequent reduction in impulse traffic at sympathetic nerve terminals. Such a concept is supported by experimental studies in animals and would account for the low renin and aldosterone secretion rates and reduced noradrenaline formation reported by others in patients with paralysis agitans.