L-Tryptophan-induced depression of the pressor response to clinidine in anaesthetized rats

1. The interaction of serotonin precursor L-tryptophan with the pressor responses of the anaesthetized rat to the intravenous injection of clonidine, adrenaline and angiotensin has been studied. 2. Pretreatment of rats with L-tryptophan (100 mg/kg) depressed the pressor response to clonidine but had no effect on the responses elicited by adrenaline or angiotensin. 3. The L-tryptophan-induced depression of the clonidine response was prevented by pretreating rats with either Rö 4-4602, carbidopa, BW 172C58, methysergide or by pithing. 4. Intravenous infusions of serotonin depressed the pressor responses to clonidine, adrenaline and angiotensin in both intact anaesthetized and pithed rats. 5. It is concluded that the depressant action of L-tryptophan is dependent on its conversion within the periphery to serotonin. This action is also dependent on or mediated by the sympathetic nervous system.

Antagonism by mianserin and classical alpha-adrenoceptor blocking drugs of some cardiovascular and behavioral effects of clonidine

Antagonism of pressor responses to sympathetic outflow stimulation and alpha-adrenoceptor agonists in pithed spontaneously hypertensive rats was used to estimate postsynaptic alpha-adrenoceptor blocking activity of mianserin, phentolamine, phenoxybenzamine, piperoxan and yohimbine. Estimation of presynaptic alpha-adrenoceptor blocking activity of these drugs was obtained by studying their ability to antagonize clonidine-induced suppression of positive chronotropic responses to sympathetic outflow stimulation. In this manner, evidence was obtained that mianserin causes selective presynaptic alpha-adrenoceptor blockade. Mianserin, piperoxan and yohimbine antagonized clonidine-induced avoidance blockade or hypotension in spontaneously hypertensive rats, but methysergide, phenoxybenzamine and phentolamine were ineffective. These results suggest that mianserin may antagonize the central effects of clonidine by blockade of noradrenergic presynaptic or autoreceptors and possibly explain the antidepressant effect of mianserin as due to indirect activation of central noradrenergic neurons.

Effect of clonidine on sleep patterns in man

Clonidine (300 microgram orally) increased in man the total duration of sleep and strikingly reduced the duration of REM sleep. Yohimbine (10 mg per os) did not alter the sleep patterns in man but antagonized the effects of clonidine. These results provide evidence that an alpha sympathomimetic mechanism could suppress REM sleep and increased the total duration of sleep.

Inhibitory effects of clonidine on responses to sympathetic nerve stimulation in the pithed rat

1. The spinal sympathetic outflow to the eyelid, heart, splanchnic blood vessels, vas deferens and anococcygeus muscle was stimulated in pithed rats. 2. Clonidine inhibited sympathetic outflow to all of the tissues studied. The inhibitory effects of clonidine on cardiac nerves and hypogastric nerves were antagonized by phentolamine. 3. Clonidine produced a postsynaptic alpha-adrenoceptor agonist action on the eyelid, splanchnic blood vessels and the anococcygeus muscle. These effects were also antagonized by phentolamine. 4. The effects of clonidine, naphazoline and oxymetazoline on pre- and postsynaptic alpha-adrenoceptors were determined. 5. The presynaptic alpha-adrenoceptors employed were situated in either the sympathetic cardiac or hypogastric nerve terminals. Increases in diastolic blood pressure were used to assess concurrent postsynaptic alpha-adrenoceptor agonist activity. 6. The presynaptic alpha-adrenoceptor agonist potencies of clonidine, naphazoline and oxymetazoline were very similar on cardiac nerve terminals whereas on the hypogastric nerve terminals oxymetazoline was about 6 times more potent than either naphazoline or clonidine. 7. The results support the view that presynaptic alpha-adrenoceptors regulate transmitter release in sympathetic nerves. There appear to be subtle differences between the presynaptic alpha-adrenoceptors of different sympathetic nerve endings.

On the mechanism of central hypotensive action of clonidine

The hypotensive effect of clonidine in anaesthetised (pentobarbitone) cat has been analysed with the help of pharmacological tools. Application of clonidine (0.1%) to the exposed ventral surface of medulla oblongata produced hypotension (28.6%) and bradycardia (18%). Similar application of glycine (5%) and GABA (10%) also lowered the blood pressure of cat by 20.3% and 29.3%, respectively. The hypotension as well as the bradycardia owing to clonidine were significantly (p less than 0.01) blocked by similar prior application of atropine methylnitrate (1%) and hemicholinium-3 (HC3, 1%), whereas HC3 pretreatment only insignificantly blocked the hypotension produced by glycine (p greater than 0.80) and GABA (p less than 0.70). Topical application of atropine (1%) also blocked (p less than 0.05) the hypotensive effect of clonidine. Intravenous administration of clonidine (50 microgram/kg) produced hypotension (34.6%) after an initial hypertensive response and bradycardia (38.8%). The hypotension was significantly (p less than 0.01) blocked by pretreatment of the cat with intracerebroventricular atropine (4 mg) or HC3 (0.5 mg). Topical application of atropine (1%) to the ventral surface of medulla also significantly (p less than 0.05) reduced the hypotension and bradycardia resulting from intravenous administration of clonidine. It is concluded that an intact cholinergic link in the brainstem is essential for the hypotensive effect of clonidine.

Some cardiovascular effects of ST-91 and clonidine

St-91, 2(2,6-diethylphenylamino)-2-imidazoline, is a clonidine derivative which does not penetrate the blood-brain barrier. In spontaneously hypertensive (SH) rats is acutely increased arterial pressure and reduced heart rate while at 8 to 12 h after oral administration, it slightly lowered arterial pressure. In contrast, clonidine had acute antihypertensive activity at all doses used. By intracerebroventricular administration to SH rats, both drugs (St-91 and clonidine) reduced arterial pressure and heart rate; in this respect, clonidine was more potent then St-91. Cardiac acceleration induced by low frequency electrical stimulation of right cardiac sympathetic nerves in anesthetized and vagotomized dogs was reduced by St-91 at the same doses by clonidine. Phenoxybenzamine, phentolamine and desipramine antagonized the inhibitory effects of St-91 on electrically induced cardiac acceleration. It was concluded that St-91, like clonidine, stimulates inhibitory alpha-adrenergic receptors at the sympathetic nerve endings but, unlike clonidine, is substantially devoid of acute antihypertensive activity. This suggests that stimulation of peripheral presynaptic inhibitory alpha-adrenergic receptors is not likely to represent the sole mechanism of antihypertensive action of clonidine.

A survey of drug interaction

A survey is presented of the interaction phenomena between two or more simultaneously used drugs. The well known mechanisms of drug interaction, like chemical incompatibility, displacement of protein binding, enzyme induction, etc., are summarized and illustrated by means of practically relevant examples. Attention is paid to the fact that drug interaction can sometimes be advantageous from the clinical point of view, although in many cases it is an unwanted phenomenon in practical pharmacotherapy. Furthermore, the predictability and detection of interaction phenomena are mentioned briefly, together with the possibilities of distributing appropriate information to physicians and pharmacists.

Influence of clonidine on cardiac vagal component of baroreceptor reflex in normotensive and spontaneously hypertensive rats

In normotensive (NR) and spontaneously hypertensive rats (SHR) the cardiac vagal component of baroreceptor reflex was checked before and after intravenous infusion of clonidine (25-30 microgram/kg). 1) The decrease of heart rate due to the arterial baroreceptor reflex was the same in NR and in SHR. 2) Intravenous injection of clonidine produced much greater decrease of the heart rate in SHR than in NR. 3) After administration of clonidine the stimulation of arterial baroreceptors decreased the heart rate to the same low level in NR and SHR. In these conditions the maximal neurogenic slowing of the heart rate was probably achieved. 4) Atropine abolished almost completly heart rate change following baroreceptor stimulation or/and clonidine administration.

Further evidence for central histamine H2-receptor involvement in the hypotensive effect of clonidine in the rat

In urethane-anaesthetised rats, the administration of the specific histamine H2-receptor antagonist metiamide intracerebroventricularly (i.c.v.) raised the blood pressure and increased the heart rate. Metiamide (i.c.v.) antagonised the hypotensive effect of clonidine (i.c.v.) in an apparently competitive manner. 4-Methylhistamine i.c.v. did not significantly change the blood pressure. The results are consistent with the concept that the hypotensive effect of clonidine is at least partly due to a stimulation of cerebral H2-receptors. The existence of cerebral H2-receptors mediating hypotensive effects is supported by the hypertensive effect of metiamide but not by the lack of hypotensive effects of 4-methylhistamine.

Possible involvement of a central noradrenergic system in automutilation induced by clonidine in mice

Automutilation induced by a single large dose of clonidine was potentiated by pretreatment with methamphetamine, caffeine and theophyline, while it was inhibited by acute administration of reserpine, alpha-methyl-p-tyrosine, phenoxybenzamine, phentolamine and chlorpromazine. L-Dopa, 5-hydroxytryptophan and p-chlorophenylalanine had no effect on this abnormal behavior. Biochemical studies on brain monoamines revealed that noradrenaline was markedly increased and dopamine slightly so, but 5-hydroxytryptamine was never changed by clonidine. These results suggest that a central noradrenergic system may be involved in automutilation induced by clonidine in mice.

Effect of clonidine on blood pressure, heart rate and body temperature in conscious rats

Effects of clonidine on blood pressure, heart rate and rectal temperature in conscious rats were examined. Clonidine (0.1-1 mg/kg s.c.) caused a prevailing pressor response and dose-dependently a fall in heart rate and body temperature. The pressor response to clonidine (0.3 mg/kg s.c.) was completely reduced by phentolamine (10 mg/kg s.c.), chlorpromazine (10 mg/kg s.c.) but not by hexamethonium (30 mg/kg i.p.), guanethidine (30 mg/kg s.c.) or reserpine (5 mg/kg s.c. 18 hr + mg/kg i.p. 4 hr prior to clonidine). Conversely, a remarkable potentiation of the pressor response to clonidine was observed after treatment with reserpine. The bradycardia with clonidine (0.3 mg/kg s.c.) was significanlty reduced by phentolamine, chlorpromazine or atropine (5 mg/kg s.c.) but was potentiated by reserpine. The hypothermia with clonidine (0.3 mg/kg s.c.) was not influenced by phentolamine or atropine but was significanlty potentiated by chlorpromazine. From the above results it is suggested that the prevailing pressor response to clonidine in conscious rats is due to a stimulation of peripheral alpha-adrenoceptors, the bradycardia with clonidine is exerted through the sympathetic pathway and the baroceptor-vagal reflex, and that the hypothermia with clonidine is mainly due to the central mechanism.

Inhibition of the central hypotensive effect of clonidine by trazodone, a novel antidepressant

Trazodone, a non-tricyclic drug with antidepressant properties, diminished the central hypotensive response to clonidine in a dose-dependent manner. Both drugs were infused into the left vertebral artery of chloralose-anaesthetized cats. Intravenously injected trazodone did not reduce the central hypotensive effect of clonidine. The antagonism probably occurs at the level of central alpha-adrenoreceptors in the brain stem, where clonidine is the alpha-agonist and trazodone the alpha-antagonist. Such an antagonism, already known for clonidine and the tricyclic drugs also holds for non-tricyclic antidepressants, provided that they possess alpha-adrenoreceptor-blocking activity.

Reduction of the hypotensive effect of clonidine and alpha-methyldopa by various psychotropic drugs

1. In chloralose-anaesthetized cats the centrally induced hypotensive effect of clonidine, and in some cases alpha-methyldopa, was diminished by pretreatment with a variety of tricyclic antidepressants and neuroleptic agents. 2. The interaction is assumed to occur at the central alpha-adrenoreceptors. Clonidine or alpha-methyl-noradrenaline (from alpha-methyldopa) are the agonists, and the aforementioned psychotropic drugs are the antagonists. 3. Psychotropic drugs which are not alpha-receptor blockers, like butyrophenone neuroleptics (pimozide and haloperidol) or benzodiazepine tranquillizers, do not significantly diminsh the centrally induced hypotensive effect of clonidine.

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.

Evidence for a renal alpha-adrenergic receptor inhibiting renin release

The mechanism by which clonidine suppresses renin release was investigated in conscious rats. This suppression was studied by means of selected autonomic interventions in conjunction with changes in sodium balance. Serum renin activity and direct arterial pressure were monitored. Clonidine administration suppressed basal (by 68-85%), diuretic-induced (by 89%), and sympathetic nervous system-mediated (by 75-100%) renin release. Cholinergic, ganglionic, and peripheral sympathetic neuronal blockade did not prevent this inhibitory effect of clonidine. These results indicate a peripheral site of action for suppression of renin release by clonidine. The alpha-adrenergic blocking drug phentolamine prevented clonidine suppression of renin release in sodium-depleted rats and was partially effective in normal rats. Phentolamine blocked the decrease in renin caused by clonidine in ganglion-blocked rats. Clozapine, a new neuroleptic agent with alpha-adrenergic blocking activity, or phenoxybenzamine blocked the effect of clonidine on renin release in both sodium-depleted and normal rats. After ganglionic blockade in sodium-depleted rats, clonidine caused a significantly greater suppression of renin release than did an equipressor dose of methoxamine. These data, combined with hemodynamic correlates, suggest that clonidine inhibits renin release by activation of an intrarenal alpha-adrenergic receptor.

Effects of alpha-adrenoceptor agonists and antagonists on pre- and postsynaptically located alpha-adrenoceptors

The effects of alpha adrenoceptor agonists and antagonists have been examined at pre- and post-synaptically located alpha-adrenoceptors in the pithed rat. The presynaptic receptors were those located at the cardiac sympathetic nerve terminals and the postsynaptic receptors were those present in vascular smooth muscle. Clonidine was approximately equipotent at pre- and post-synaptic alpha-adrenoceptors, whilst LSD and BAY-1470 were more active at the pre- than at post-synaptic sites. Oxymetazoline, naphazoline, methoxamine and phenylephrine were all much more active at the postsynaptic alpha-adrenoceptors. Phentolamine was the most potent antagonist at both pre- and post-synaptic alpha-adrenoceptors. Piperoxan, yohimbine and tolazoline were about 3-7X less potent than phentolamine at both sites. Thymoxamine was about 10X less potent than phentolamine at postsynaptic alpha-adrenoceptors but about 1000X less active at the presynaptic receptors. The differential actions of both agonsists and antagonists at pre- and post-synaptic alpha-adrenoceptors suggest that the receptors may be of different types.

The reversal of clonidine-induced hypotension by protriptyline and desipramine

The present paper deals with further studies on the interaction between clonidine and tricyclic antidepressants. The pronounced central hypotensive action of 1 mug clonidine/kg, administered into the left vertebral artery of chloralose-anaesthetized cats was readily reversed by protriptyline (300 mug/kg), infused via the same route shortly after the development of the maximum hypotensive effect of clonidine. In earlier studies it has been demonstrated that pretreatment with tricyclic antidepressants significantly diminishes the central hypotensive action of clonidine. This interaction has been presumed to occur at the level of central alpha-adrenoreceptors, where clonidine would be the agonist and tricyclic antidepressants the antagonist. The present findings suggest that a competitive antagonism at the central level, which can occur in either sense, may be involved.

Different alpha-adrenoreceptors in the central nervous system mediating biochemical and functional effects of clonidine and receptor blocking agents

The influence of clonidine on alpha-adrenoreceptors in the central nervous system of rats and mice has been investigated. Both functional events due to postsynaptic receptor stimulation (flexor reflex activity, motor activity) and biochemical changes have been considered. 1. Clonidine was less potent in stimulating the hindlimb flexor reflex activity of spinal rats than in inhibiting the alpha-methyltyrosine-induced disappearance of noradrenaline in the spinal cord and in the whole brain of rats. 2. The increase in flexor reflex activity due to clonidine (0.4 mg/kg) was virtually completely inhibited by phenoxybenzamine (20 mg/kg) and haloperidol (10 mg/kg), was partially inhibited by yohimbine (10 mg/kg) and piperoxan (60 mg/kg) and was not significantly inhibited by yohimbine (3 mg/kg) and tolazoline (50 mg/kg). 3. The potentiation by clonidine of the apomorphine-induced locomotor stimulation of reserpine-treated mice was almost completely inhibited by phenoxybenzamine (20 mg/kg) but was not significantly affected by yohimbine (10 or 3 mg/kg) and only slightly inhibited by tolazoline (50 mg/kg). 4. Clonidine (0.1 mg/kg) caused a considerable inhibition of the alpha-methyltyrosine-induced disappearance of noradrenaline in the spinal cord and brain or rats and in the brain of mice. This effect of clonidine was completely antagonized by yohimbine (10 mg/kg). It was markedly antagonized by yohimbine (3 mg/kg), piperoxan (60 mg/kg) or tolazoline (50 mg/kg) but not by phenoxybenzamine (20 mg/kg) or haloperidol (10 mg/kg). 5. Clonidine (0.1 mg/kg) caused an inhibition of the accumulation of Dopa after decarboxylase inhibition in the noradrenaline-rich regions of the rat central nervous system. This effect was counteracted by yohimbine (10 mg/kg), piperoxan (60 mg/kg) or tolazoline (50 mg/kg) but not by phenoxybenzamine (20 mg/kg). 6. The postsynaptic functional effects and the biochemical effects of clonidine may be due to stimulation of different alpha-adrenoreceptors since the two effects were inhibited differently by various alpha-adrenoreceptor blocking agents and since the two effects were produced by different doses of clonidine. The alpha-adrenoreceptors mediating the biochemical changes might be located on the noradrenergic neurones.

Central effects of clonidine 2-(2,6-dichlorophenylamino)-2-imidazoline hydrochloride in fowls

1 The effects of clonidine infused into the IIIrd cerebral ventricle, the hypothalamus or intravenously were studied on behaviour, electrocortical activity, body, comb and leg temperatures, respiration and carbon dioxide elimination in adult and young fowls (Gallus domesticus). 2 Behavioural and electrocortical slow wave sleep were induced by clonidine infused into IIIrd cerebral ventricle, the hypothalamus or intravenously. Suprisingly, sleep elicited by intravenous clonidine was much longer-lasting than that induced by an identical dose given intraventricularly. 3 Body temperature was lowered by clonidine given intraventricularly or infused into the hypothalamus. Depending on initial comb temperature and ambient temperature, comb temperature was elevated, unaffected or lowered as body temperature fell; temperature of the unfeathered legs also rose as body temperature declined after clonidine. 4 Following clonidine, but before any considerable decline of body temperature, tachypnoea and wing abduction developed; during recovery of body temperature, the wings were lowered and applied closely to the trunk and the feathers partly erected. 5 CO2 elimination fell more swiftly than body temperature following intrahypothalamic clonidine in young chicks; initial recovery developed sooner than that of body temperature, but eventual recovery was delayed compared to that for body temperature. The effects of clonidine were much more marked in young chicks studied at an ambient temperature below thermoneutrality as compared to thermoneutrality. 6 The soporific effects of clonidine were attenuated by intraventricular phentolamine; its hypothermic effects were prevented by phenoxybenzamine and prevented or attenuated by phentolamine. Intraventricular atropine, haloperidol, methysergide and propranolol were ineffective. 7 Larger doses of intraventricular phentolamine elicited shivering, tachypnoea and wing abduction; body temperature was elevated, to the extent even of lethal hyperthermia. Intraventricular atropine also elevated body temperature. 8 Clonidine infused intravenously, intraventricularly or into the hypothalamus, replaced the behavioural and electrocortical arousal evoked with dexamphetamine, by sleep associated with slow wave electrocortical activity.

Inhibition of both noradrenergic and serotonergic neurons in brain by the alpha-adrenergic agonist clonidine

By means of single unit recording techniques it was found that a small systemically administered (intravenous) dose of the alpha-adrenergic agonist clonidine inhibited the spontaneous firing of brain norepinephrine (NE)-containing neurons in the locus coeruleus. In addition, the NE neurons were consistently inhibited by the direct (microiontophoretic) application of minute amounts of NE or clonidine. Intravenous clonidine also inhibited the firing of the great majority of (5-HT) neurons in the midbrain dorsal raphe nucleus. However, this action does not appearto be a direct one since clonidine (and NE) had relatively weak or variable effects when applied microiontophoretically to raphe neurons. The clonidine-induced depression of raphe firing may be secondary to an impairment in adrenergic transmission since (1) the depression could be reversed by the NE-releasing agents D- and L-amphetamine, (2) high doses of clonidine itself (which have been reported to have postsynaptic alpha-agonistic activity) reversed the depression produced by a low dose of clonidine and (3) prior destruction of NE neurons by 6-hydroxydopamine (7-12 days) rendered raphe neurons insensitive to the depressant effect of i.v. clonidine. Dopaminergic (substantia nigra, zona compacta) neurons did not respond to either low or high doses of clonidine. These results are consistent with previous data showing that clonidine decreases NE and 5-HT but not dopamine turnover. We conclude that systemically administered clonidine inhibits the firing of brain NE neurons by acting directly upon adrenergic receptors located on or near the soma of these neurons but that the concomitant inhibition of 5-HT neurons is an indirect effect (possibly secondary to an impairment in noracrenergic transmission).

Central catecholamine receptor blocking actions of BE-2254 ('heat'): comparison with chlorpromazine and haloperidol

BE-2254, 2-[beta-(4-hydroxyphenyl)-ethylaminomethyl]-tetralone, (ED50 = 3.4 mg/kg i.p.) was about equal to chlorpromazine (ED50 = 4.4) as an antagonist of central noradrenergic receptor stimulation produced by clonidine (enhancement of the flexor reflex in spinalized rats). Haloperidol and phentolamine had essentially no effect at 9 mg/kg i.p...

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.

The central action of clonidine and its antagonism

1. We have examined the central actions of clonidine (2-(2-6-dichlorphenylamine)-2-imidazoline hydrochloride). It has been confirmed that when infused into the vertebral artery at 2 mug/min, it caused a decrease in blood pressure and a slight increase in heart rate. The same dose given intravenously or into the carotid artery had no effect.2. Intravertebral clonidine also greatly reduced the reflex response to carotid occlusion and the effects of an intravertebral infusion of angiotensin (1 ng/kg)/min.3. This central action of clonidine was antagonized by the adrenergic neurone blocking drug bethanidine (4-5 mg/kg intravenously) even after the cervical cord had been transected at C(4)-C(6) suggesting that bethanidine also has central actions.4. Other drugs which also antagonized the central effects of clonidine were guanethidine (4-5 mg/kg intravenously), bretylium (10 mg/kg intravenously) and phentolamine (0.2 mg/kg intravenously).5. It is suggested that there are central adrenergic neurones which inhibit cardiovascular autonomic reflexes and that the central autonomic effects of clonidine are due to stimulation of inhibitory adrenoceptors. The antagonism by adrenergic neurone blocking drugs of the effect of clonidine could therefore be due to blockade of these inhibitory pathways.6. The central action of clonidine could only be demonstrated when a high concentration was infused into the vertebral artery and could not be shown with oral doses of (20 mug/kg)/day for seven days. It is concluded that the hypotensive action of therapeutic doses is unlikely to be due to the central action of clonidine.

Sleep produced by clonidine (2-(2,6-dichlorophenylamino)-2-imidazoline hydrochloride)

1. The dose of clonidine (given intravenously) required to elicit sleep in the young chick is 1/25th to 1/50th of an equiactive dose of noradrenaline. The approximate ED50 is 0.01 mumol/kg. Phentolamine (10-15 mg/kg, but not 5 mg/kg) antagonizes the action of both clonidine and noradrenaline.2. Intensive treatment with p-chlorophenylalnine (700 mg/kg for 3-4 days) does not prevent the hypnotic effect of clonidine in the chick, although brain 5-HT is reduced to 15% of normal. Neither is natural sleep modified.3. Sleep after clonidine is not affected by methysergide (0.1-1 mumol/kg, i.m.), but prevented by LSD (0.1-0.3 mumol/kg). The effect of LSD is interpreted as a physiological antagonism.4. Clonidine (50 mg/kg) injected intravenously into adult rats causes sleep which is not abolished by phentolamine (5 mg/kg) or by p-chlorophenylalanine in doses which interfere with natural sleep.5. When, per kg body weight, the same dose of clonidine is injected into the lateral cerebral ventricle of rats, sleep ensues in more than half the animals, and persistent eating in about a third; only one of seventeen rats showed no change in behaviour. Eating and sleeping remained unaltered after p-chlorophenylalanine. The actual dose of clonidine injected into the lateral ventricle was 0.037 mumol, amounting to about 0.15 mumol/kg or 15 times the dose required intravenously in the chick. Noradrenaline 0.15 mumol per (intraventricular) injection caused eating but no sleep, whereas higher doses produced ataxia and paresis.6. The work suggests that clonidine does not elicit sleep by an action requiring the integrity of the 5-HT-containing neurones arising in the raphé nuclei, and that its action is not on tryptamine receptors. In the chick, sleep appears to be produced by a central sympathomimetic effect; it is possible, but not certain, that this also holds for the rat.7. The intravenous hypnotic dose of clonidine for the cat is about the same as that for the rat, but injection is not accompanied by signs of peripheral sympathetic stimulation.