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 μg/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 depress central sympathetic reactivity by different mechanisms.     

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

Clonidine (30 μg/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 act. 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.     

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.     

Effects of clonidine and yohimbine on the social play of juvenile rats

The social play of juvenile rats was observed following administration of either the alpha-2-adrenergic agonist clonidine (0.1, 0.5, 1, 5, 10, and 50 micrograms/kg) or antagonist yohimbine (0.5, 1, 2, and 5 mg/kg). Using pins (one animal on its back with the other on top) as the dependent measure, we found that clonidine reliably reduced the amount of play at all but the lowest dose tested, while yohimbine had no effect at any but the highest dose. In addition, we tested a clonidine-yohimbine interaction to assess neurospecificity of the results. While yohimbine alone (0.5 mg/kg) had no effect on play, it partially reversed the clonidine-induced suppression, indicating that the effect may be mediated to some degree through an alpha-2-adrenergic mechanism.     

Amphetamine,chlorpromazine and clonidine effects on self-stimulation in caudate or hypothalamus of the squirrel monkey

In 2 separate groups of squirrel monkeys and within 3 animals low rates of intracranial self-stimulation (ICSS) elicited from caudate or lateral hypothalamic brain sites were increased by as much as 200% above control levels by amphetamine (0.5 mg/kg). Thresholds for responding were decreased by 50%. Increasing the drug dose from 2 to 10mg/kg produced response inhibition at both brain sites. The duration of inhibitory action of amphetamine (2.0 mg/kg) on ICSS from the medial forebrain bundle (MFB) area of the lateral hypothalamus was 6 hr. At caudate sites ICSS did not occur until 48 hr had elapsed. A 10 mg/kg dose of amphetamine produced a duration of action of 36 hr in the MFB and 84 hr in the caudate. Chlorpromazine (CPZ) doses of 0.5 and 1.0 mg/kg decreased caudate ICSS significantly more than lateral hypothalamic ICSS. At 1.0 mg/kg the duration of action of CPZ was 6 hr at lateral hypothalamic brain sites and 24 hr at caudate sites. At a 2.0 mg/kg CPZ dose the duration of action was 12 hr in the MFB and 36 hr in the caudate. A dose of 0.10 mg/kg of clonidine blocked high rates of MFB ICSS while within the same animal caudate ICSS was much less affected. Higher doses (0.25 mg/kg) sedated the animal and ICSS was equally inhibited at both sites. These findings, using ICSS as a behavioral measure, suggest that the effects of amphetamine and CPZ involve not only hypothalamic structures but more anterior telencephalic sites as well. The prolonged actions of amphetamine and CPZ on caudate ICSS suggest that drugs acting, in part, on dopamine containing neurons will interfere with certain caudate mediated behavior. Further, since hypothalamic but not caudate ICSS sites are more dose sensitive to drugs that selectively act on NE containing neurons, other amines in addition to NE may play a role in the support of ICSS.     

Effect of clonidine on sleep patterns in man

Summary Clonidine (300 µg 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 sympathomimetic mechanism could suppress REM sleep and increased the total duration of sleep.     

Analysis of pupillary dilation produced by analogs of clonidine

Pupillary responses to intravenous administration of clonidine and five congeneric derivatives of clonidine were observed in anesthetized cats. All of the agents tested produced a dose-dependent mydriasis of long duration. The order of potency for these compound was clonidine (St-155) > St-375 > St-606 > St-600 > St-608 > St-91. Pretreatment with yohimbine hydrochloride (0.5 mg/kg i.v.) shifted the dose-response curve for all of the compounds tested to the right. A CNS inhibition of parasympathetic nerve activity was demonstrated for clonidine and St-375 by means of direct recordings from the postganglionic ciliary nerves. An effect of high doses of St-91 on nerve activity was also observed. These results suggest that all the analogs of clonidine tested (with the possible exception of St-91) act like clonidine to produce mydriasis by a central α-adrenergic inhibition of parasympathetic outflow to the eye.     

Effects of clonidine on baroreceptor function in anesthetized dogs

The effects of clonidine (15–30 μg/kg i.v.) on carotid sinus and other baroreceptors were investigated in anesthetized dogs. In 14 control dogs, right carotid sinus pressure was controlled by retrograde perfusion through the common carotid artery at constant flow with femoral arterial blood. Graded reductions in heart rate and blood pressure induced by graded increases in carotid sinus pressure were prevented, whereas reflex bradycardias associated with norepinephrine pressor activity were potentiated by clonidine. Norepinephrine-induced bradycardia, although reduced, still persisted after chronic bilateral sinusectomy and these responses were also potentiated by clonidine. In contrast, clonidine did not potentiate reflex bradycardia in dogs 20 days after aortic stripping. In intact dogs, clonidine inhibited the response to bilateral carotid artery occlusion and to carotid sinus nerve stimulation. These studies suggest that clonidine can inhibit carotid sinus baroreceptor function and simultaneously potentiate other, presumably aortic, baroreceptor activity.     

Inhibition of somato-sympathetic reflex via peripheral presynaptic alha-adrenoceptors.

In cats anaesthetized with chloralose--urethane (70/100 mg/kg) a somato-sympathetic reflex increase of blood pressure and heart rate was evoked by electrical stimulation of sensory fibres running within the tibial nerve. I.V. injection of 30 microgram/kg of the alpha-adrenoceptor agonist clonidine caused a reflex inhibition in intact and vagotomized animals. In contrast, injection of 2 microgram/kg clonidine into the vertebral artery elicited hypotension but no reflux inhibition. In small doses, phenylephrine which is known from in vitro experiments to prefer the postsynaptic alpha-adrenoceptor, potentiated the reflex increase in blood pressure by an additive effect on the vascular alpha-adrenoceptor, while oxymetazoline and tramazoline, which preferentially activate the presynaptic receptor, were inhibitory. The inhibitory effect of tramazoline on the reflex increase in blood pressure and heart rate was antagonized by the presynaptic alpha-adrenolytic drug yohimbine. The results suggest that peripheral presynaptic alpha-adrenoceptors are responsible for somato-sympathetic reflex inhibition. The importance of peripheral presynaptic alpha-adrenoceptors in the in vivo regulation of blood pressure is discussed.     

Effects of the training dose on generalization of morphine stimulus to clonidine

The relationship between the stimulus properties of morphine and clonidine was tested in rats trained to discriminate morphine sulfate (4, 2 or 1 mg/kg) from saline in a two-lever food-rewarded task. The response trained in the low dose group generalized only to higher doses of clonidine (0.125 to 0.5 mg/kg) whereas the response trained with the high dose of morphine generalized only to higher doses of clonidine (0.625 to 1.0 mg/kg). Naloxone blocked the generalization in the low dose group but only partially blocked it in the high dose group. Yohimbine blocked the generalization to clonidine in the high morphine dose group and reversed the response rate suppressant effect of clonidine in all groups.     

The central modulatory effect of clonidine on the cardiodepressor reflex after suppression of synthesis and storage of noradrenaline.

Rats were decerebrated and treated with a β-adrenoceptor blocker (toliprolol, Kö 592, 5 mg/kg). Increase of blood pressure were repeatedly elicited by i.v. injection of angiotensin (0.03–0.125 μg/kg) and the accompanying reflex bradycardia was measured. The vagally mediated reflex bradycardia was significantly increased by clonidine, 30 μg/kg i.v., in non-pretreated animals as well as after pretreatment with reserpine (7.5 mg/kg s.c., 20 hr) and α-methyl-p-tyrosine-methylester (250 mg/kg i.p., 5 hr), separately or in combination. The facilitatory effect of clonidine was antagonized in all groups by i.v. injection of the α-adrenoceptor blocking drugs, phentolamine (5 mg/kg) and piperoxan (1 mg/kg). In the control periods before clonidine, reflex bradycardia was similar in pretreated animals to that in animals without pretreatment. It was concluded therefore that catecholamines have no essential transmitter functions in the cardiodepressor reflex loop. Therefore, the action of clonidine on cardiovascular centers in the medulla is independent of endogenous noradrenaline storage and synthesis. A direct effect is assumed on central α-adrenoceptors, which have a modulatory ‘effector’ function on the cardiodepressor reflex.     

Effect of clonidine on the human acoustic startle reflex

The present study investigated in healthy human volunteers whether clonidine reduced the amplitude of the acoustic startle reflex and whether this effect, if found, was due to an accelerated rate of habituation. Subjects were presented with startleeliciting noise-bursts after intravenous (iv) infusion of clonidine (1.5 µg/kg) and saline on separate days. Clonidine significantly reduced the amplitude of the acoustic startle reflex (as indexed by the eyeblink component) relative to the saline treated condition. This effect was neither due to an accelerated rate of habituation of the startle reflex nor to the sedative effect of clonidine. These findings complement an earlier report (Morgan et al. 1993) that yohimbine augments the amplitude of the startle reflex in man. Taken together, the two reports indicate a new model for the clinical investigation of central alpha₂ adrenoceptor function in humans.     

Clonidine inhibits nicotinic effects in ganglia of the cholinergic-sympathetic system

The electrodermal potential (EDP) recorded with surface electrodes between the palm and the shaven back of the forepaws of anaesthetized cats was taken as a measure of sudomotor activity in response to submaximal activation of cholinergic-sympathetic efferents with the nicotinic ganglionic stimulant DMPP. The intravenous injection of 30 micrograms/kg clonidine inhibited EDPs in despinalized cats and in vagotomized cats with chronic denervation of the stellate ganglion. In vagotomized cats clonidine (30 micrograms/kg i.v.) shifted the dose-response curve of DMPP (4-256 micrograms/kg i.v.) to the right. An inhibitory effect was also observed in despinalized cats upon topical application of 0.1 and 1 microgram clonidine to the stellate ganglion. In all preparations the inhibitory effect of clonidine could be antagonized by intravenous injection of 200 micrograms/kg yohimbine. As all substances used were also active when injected during arrest of blood flow to the foreleg by means of a tourniquet, actions at the level of the sweat gland could be excluded. The results lead to the conclusion that the activation of postsynaptic somadendritic alpha 2-adrenoceptors in sudomotor ganglia of the cholinergic-sympathetic nervous system inhibits the effects of nicotinic ganglionic stimulation. It is assumed that this action is due to the known ganglionic hyperpolarization induced by alpha 2-adrenoceptor stimulants.     

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.     

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.     

The potentiating effects of 4-aminopyridine on adrenergic transmission in the rabbit vas deferens.

The effects of 4-aminipyridine on adrenergic transmission in rabbit vas deferens were investigated. 10(-5)-10(-3) M 4-aminopyridine markedly potentiated the sustained, secondary contractile response to transmural stimulation and significantly increased the amount of (+/-)-|3H]-metaraminol released at 5 Hz, but had little effect on responses to exogenous (-)-noradrenaline. 10(-5)-10(-4) M 4-aminopyridine did not significantly alter the accumulation or efflux of (-)-[3H]-noradrenaline in rabbit heart. It is suggested that 4-aminopyridine potentiated responses of rabbit vas deferens to transmural stimulation mainly by increasing transmitter release, possibly as a result of prolongation of the action potential. Responses to transmural stimulation were also potentiated by by 2- and 3-aminopyridine and by 3,4-diaminopyridine but not by pyridine, aniline, 4-dimethylaminopyridine, 4-aminoethylpyridine or 2-aminopyrimidine.     

Spinal 5-HT pathways and the antinociception induced by intramedullary clonidine in rats

Summary The possible involvement of spinal 5-hydroxytryptamine (5-HT) pathways in antinociception induced by microinjection of clonidine into the ventrolateral surface of the medulla oblongata was investigated in rats. Microinjection of clonidine (10-20 µg), but not yohimbine (1 µg) or 0.9% saline, into the lateral medulla prolonged the hot plate latency in rats. This clonidine-induced antinociception was abolished by intramedullary injection of the alpha₂-adrenoceptor antagonist, yohimbine. Selective destruction of spinal 5-HT neurons produced by intraspinal injection of 5,7-dihydroxytryptamine (5,7-DHT; 10 µg) or postsynaptic blockade of spinal 5-HT receptors produced by intrathecal injection of cyproheptadine (1 µg; a mixed 5-HT₁/5-HT₂ antagonist) also abolished clonidine-induced antinociception. Rats given 5,7-DHT intraspinally or cyproheptadine intrathecally showed a decrease in hot plate latency as compared with the controls. In anesthetized rats, the 5-HT release from the thoracic spinal cord was enhanced by microinjection of clonidine into the lateral medulla. This enhanced spinal 5-HT release evoked by intramedullary injection of clonidine was abolished by pretreatment of rats with intraspinal injection of 5,7-DHT. These results indicate that 5-HT pathways to the spinal cord mediate the antinociceptive effect induced by microinjection of clonidine into the ventrolateral surface of the medulla oblongata in rats.This study was supported by grants from the National Science Council (Taipei, Taiwan, Republic of China) Send offprint requests to M.T. Lin at the above address     

A physiological and pharmacological analysis of the electrodermal response in the rat

Electrodermal responses (EDR) of the sympathetic-cholinergic sudomotor system were elicited in the footpads of the hindpaws of anesthetized rats. The most reactive CNS loci were caudal to the region of the posterior hypothalamus. Peripherally evoked responses were elicited by electrical stimulation of the decentralized tibial nerve. The amplitude of these evoked EDR was stable over time and both the centrally and peripherally elicited responses were frequency-dependent. Atropine (200 μg/kg) depressed the EDR elicited by both peripheral and central stimulation whereas hexamethonium (20 mg/kg) only inhibited the central EDR. Clonidine had no effect on the EDR evoked at the periphery but produced a significant dose-dependent depressant effect on the centrally evoked EDR; this effect was partially antagonized by yohimbine (0.75 mg/kg). It is suggested that the rat is a suitable species for the use of the sudomotor system in the investigation of adrenergic agents which are thought to have a central sympatho-inhibitory action.     

The influence of chronic treatment with clonidine, yohimbine and idazoxan on morphine withdrawal

Numerous previous attempts have been made to study the involvement of α₂-adrenoceptors in the expression of morphine withdrawal by studying the effects of selective α₂-agonists and antagonists administered immediately before precipitation of withdrawal by an opioid antagonist such as naloxone. In the present investigation, we examined the effects of chronic treatment with clonidine (α₂-agonist), idazoxan and yohimbine (α₂-antagonists), concomitantly administered with morphine, on the expression of the withdrawal signs. In contrast to their acute effects, clonidine potentiated, while yohimbine and idazoxan attenuated the withdrawal signs precipitated by naloxone in morphine-dependent mice. In addition, mice chronically treated only with yohimbine displayed withdrawal signs similar to those reported with morphine withdrawal and these signs were not influenced by naloxone administration. Mice chronically treated with clonidine displayed withdrawal signs similar to those reported with morphine withdrawal and these signs were further potentiated by naloxone administration. The results suggest that down-regulation of α₂-adrenoceptors by morphine is a major adaptation contributing to development of dependence on opioids and also point the way to more effective treatment of narcotic dependence. This suggestion was based on the hypothesis that the suppression of noradrenergic system during chronic morphine treatment by α₂-antagonists might diminish noradrenergic hyperactivity and consequently the development of dependence and withdrawal signs. Received: 5 October 1996/Final version: 28 January 1997     

Morphine-like effects of clonidine on the EEG,slow wave sleep and behavior in the dog

EEG, behavioral and autonomic effects of morphine and clonidine were compared in the unrestrained beagle dog placed in a dimly-lit, sound-attenuated chamber equipped with video monitors. Intravenous (i.v.) morphine (0.5, 1 and 2 mg/kg) and clonidine (11, 33 and 100 μg/kg) caused parallel dose-related increases in NREM sleep with clonidine being 43 times more potent than morphine. Other similar effects were: an initial transient EEG-behavioral dissociation, increases in spectral power (8–16 Hz); decreases in temperature and heart and respiratory rates; emesis, miosis and salivation. Intraventricular (i.v.t.) morphine (33, 100 and 300 μg) and clonidine (10 and 30 μg) caused qualitatively similar EEG, sleep and behavioral effects. Naloxone (30 μg/kg i.v.) prevented EEG synchrony and behavioral effects of i.v. morphine (1 mg/kg) but not those of i.v. clonidine (100 μg/kg). Yohimbine pretreatment (0.1 mg/kg i.v.) was more effective in antagonizing clonidine than morphine. These results suggest that morphine and clonidine induce similar EEG, behavioral and autonomic effects through actions upon different receptors on the same or parallel neural pathways. The results further emphasize the importance of an α₂-adrenergic-opiod interaction to regulate sleep in the dog.