The role of prostaglandins and the hypothalamic and hippocampal histamine in the clonidine-induced pituitary-adrenocortical response.

Involvement of prostaglandins (PGs) and histamine in the hypothalamus and hippocampus in the clonidine-induced pituitary-adrenocortical response was investigated in conscious rats. The hypothalamic-pituitary adrenocortical (HPA) activity was assessed indirectly by measuring corticosterone secretion. Clonidine, an alpha 2-adrenergic agonist, given intracerebroventricularly (10 micrograms icv), considerably increased the serum corticosterone and hypothalamic histamine levels and markedly elevated the hippocampal histamine levels. Systemic or icv pretreatment with indomethacin (2 mg/kg or 10 micrograms), an inhibitor of prostaglandin synthesis, significantly reduced the clonidine-induced corticosterone response and abolished the increase in the hypothalamic and hippocampal histamine levels elicited by clonidine. Indomethacin in the doses used did not substantially change the resting serum corticosterone or hypothalamic and hippocampal histamine levels. These results indicate that prostaglandins and hypothalamic histamine are considerably involved in the HPA response to alpha 2-adrenergic receptor stimulation. They also suggest involvement of prostaglandins and histamine of the hippocampus in the clonidine-induced HPA response.

Analysis of the mechanisms underlying the antinociceptive effect of the extracts of plants from the genus Phyllanthus.

1. We examine some of the mechanisms underlying the analgesic effects of the hydroalcoholic extracts (HE) of Phyllanthus urinaria and P. niruri against formalin-induced nociception in mice. In addition, we also investigate the action of both HEs against capsaicin-mediated pain. 2. Both prazosin and yohimbine (0.15 mg/kg, i.p.) induced a marked inhibition of the analgesic effect caused by phenylephrine (10 mg/kg, i.p.) and clonidine (0.1 mg/kg, i.p.), respectively, but had no effect on the antinociceptive action caused by HE of P. urinaria (10 mg/kg, i.p.) or P. niruri (30 mg/kg, i.p.). 3. NG-nitro-L-arginine (L-NOARG, 75 mg/kg, i.p.) caused marked analgesic effect against the second phase of formalin-induced pain. Treatment of animals with L-arginine (600 mg/kg) completely antagonized the antinociceptive effect of L-NOARG but had no significant effect against the HE of P. urinaria (10 mg/kg, i.p.) or P. niruri (30 mg/kg. i.p.) analgesic properties. 4. The antinociceptive effects caused by the HEs of P. urinaria (10 mg/kg, i.p.) and P. niruri (30 mg/kg, i.p.) were unaffected by methysergide (5 mg/kg, i.p.), p-chloro-phenylalanine-methyl-ester (100 mg/kg, i.p., once a day for 4 consecutive days) or after previous adrenalectomy of animals. 5. The HE of P. urinaria and P. niruri given either intraperitoneally (1-30 mg/kg) or orally (25-200 mg/kg) caused marked and dose-related inhibition of capsaicin-induced pain with ID50 of 2.1 and 6.1 mg/kg given intraperitoneally and 39 and 35 mg/kg given orally, respectively.

Modulation of central noradrenergic function by RS-15385-197.

1. RS-15385-197, a highly potent and selective alpha 2-adrenoceptor antagonist, was examined in a variety of in vitro and in vivo functional tests to assess the selectivity of its interaction with central noradrenergic neurones in the rat. 2. In hypothalamic slices, RS-15385-197 was potent in augmenting K(+)-evoked release of [3H]-noradrenaline, with an EC50 of 9 nM. Idazoxan and yohimbine showed 100 fold less activity. This was due to its antagonist action at presynaptic alpha 2-adrenoceptors, as RS-15385-197 (10 microM), did not directly release [3H]-noradrenaline from cortical slices unlike reserpine (10 microM), and did not inhibit noradrenaline re-uptake into cortical synaptosomes. 3. In vivo, RS-15385-197 (0.5 mg kg-1, p.o.) increased levels of 3-methoxy-4-hydroxy-phenylglycol (MHPG) in the cerebral cortex without modifying levels of 5-hydroxyindoleacetic acid (5-HIAA). This dose, but not a lower dose (0.1 mg kg-1, p.o.) caused beta-adrenoceptor down-regulation in the cortex when administered once daily for 14 days whereas 5-HT2 receptor number was unaltered, indicating a selective effect on noradrenergic transmission. 4. Selective depletion of cortical 5-HT by administration of p-chlorophenylalanine (PCPA; 100 mg kg-1, i.p. for 14 days) or 5,7-dihydroxytryptamine (5,7-DHT; 150 micrograms i.c.v.) prevented the beta-adrenoceptor down-regulation caused by RS-15385-197, indicating that a tonic 5-hydroxytryptaminergic input was required for it to elicit beta-adrenoceptor down-regulation. It was not possible to prevent the loss of activity of RS-15385-197 in these 5-HT-depleted animals by co-administration with the 5-HT1A partial agonist, 8-hydroxy-n-dipropyl aminotetralin (8-OH-DPAT, 0.3 mg kg-1, i.p. twice daily for final 3 days).5. At a dose (1 mg kg-1, p.o.) which completely prevented the hypoactivity produced by clonidine(0.1 mgkg-1, p.o.), RS-15385-197 did not affect behavioural stereotypy induced by 8-OH-DPAT(0.3 mg kg-1, s.c.). Similarly, following chronic dosing with the racemate, RS-15385-196 (3 mg kg-1,p.o., once daily for 14 days), there was no effect on the behavioural and hypothermic response to 8-OH-DPAT (0.5 mg kg-1, s.c.). Therefore, RS-1 5385-197 was selective for central alpha2-adrenoceptors over 5-HT1A receptors in in vivo functional tests.6. Thus, RS-15385-197 was highly selective in interacting with central noradrenergic neurones in the rat in vitro and in vivo. It is therefore currently the agent of choice for investigations of the role of alpha 2-adrenoceptors in the CNS.

Clonidine poisoning in young children.

We reviewed 47 consecutive inpatient records to determine the clinical course, role of supportive measures, and response to naloxone in children with clonidine poisoning. Severity of illness was assigned by means of the "pediatric risk of mortality" (PRISM) score. The children's ages ranged from 9 to 84 months. Central nervous system effects were noted in 44 patients; bradycardia occurred in 25, and apnea or depressed respiration was seen in 18. Thirty-four patients had symptoms within 1 hour of presentation, but no patient had further clinical deterioration more than 4 hours after presentation. Six patients required endotracheal intubation and mechanical ventilation. There was no difference in PRISM score or duration of symptoms between those patients who received naloxone and those who did not. More patients receiving naloxone required intubation, and only three patients had definite improvement after naloxone administration. We conclude that (1) young children who ingest clonidine have a wide spectrum of serious findings, (2) delayed progression of symptoms after clonidine poisoning is unlikely in a young child with normal renal function, and (3) naloxone is an inconsistent antidote for clonidine poisoning.

Clonidine suppresses digitalis-induced ventricular tachycardia in cats.

Effects of intravenous (IV) and intravertebral arterial (IA) administrations of alpha-2 adrenoceptor agonist, clonidine (CLO) and its antagonist, yohimbine (YOH, 0.05 mg/kg, IA), on the ventricular tachycardia (VT) induced with IV acetyl strophanthidin (AS) were studied in cats anesthetized with intraperitoneal chloralose. IVAS does-dependently produced cardiac arrhythmias including complete atrioventricular conduction block (118 +/- 14 micrograms/kg), junctional tachycardia (128 +/- 20 micrograms/kg), multiform ventricular premature beats (157 +/- 21 micrograms/kg) and sustained VT (220 +/- 23 micrograms/kg). IACLO dose for terminating VT induced with IVAS was about one fifth of IVCLO dose. IACLO dose for terminating VT induced with IVAS + IVYOH was significantly higher than that in VT induced with IVAS (14.8 +/- 3.7 vs 5.8 +/- 1.0 micrograms/kg, p less than 0.005). These experiments clearly demonstrated that IAYOH specifically antagonized the antiarrhythmic effect of IA CLO on the AS-induced VT. Since small dose of IA administration of the drugs acts mainly on the central nervous system, we suggest that the antiarrhythmic effect of CLO on AS-induced VT is likely through the central alpha-2 adrenoceptor.

Pharmacological evaluation of in vivo tests for alpha 2-adrenoceptor blockade in the central nervous system and the effects of the enantiomers of mianserin and its aza-analog ORG 3770.

A series of compounds with actions on the central nervous system was tested for antagonism of clonidine-induced sleep in chicks and clonidine-induced mydriasis in rats for the purpose of evaluating these methods as tests for demonstrating an in vivo alpha 2-adrenoceptor blocking effect of a novel compound. Clonidine-induced mydriasis was found to be the most selective method. The order of potency for compounds fully antagonizing clonidine-induced mydriasis was MSD 26 greater than physostigmine = idazoxan greater than aptazapine greater than piperoxan greater than yohimbine greater than mianserin greater than tolazoline. Partial antagonism was found for quipazine and sulpiride. Misleading results can arise from the involvement of cholinergic mechanisms in the control of the pupil diameter. The order of potency for compounds antagonizing clonidine-induced sleep in chicks was apomorphine greater than yohimbine greater than idazoxan greater than aptazapine = MSD 26 greater than quipazine greater than methysergide greater than piperoxan = mianserin = bepridil = metergoline = cyproheptadine = desipramine greater than tolazoline greater than dexchlorpheniramine, although antagonism was not complete for all of these compounds. Misleading results can arise from effects on arousal of the chicks but cholinergic mechanisms do not play a disturbing role so that the method with chicks can be a useful supplement to the mydriasis method. The enantiomers of mianserin and of a compound related to mianserin, Org 3770, were tested in the 2 methods and the alpha 2-blocking effect of these compounds was found to be residing in the S(+)-enantiomers.

Rostral hypothalamus: a new neuroanatomical site of neurochemically-induced emesis in the cat.

The localized effect of noradrenergic agonists administered directly in the anterior hypothalamic preoptic area (AH/POA) in inducing emesis in the cat was investigated. Of the noradrenergic agonists tested, which included norepinephrine, clonidine, phenylephrine and methoxamine, only clonidine in doses of 5.0-50.0 micrograms was found to evoke emesis consistently when micro-injected in a volume of 1.0 microliter into AH/POA of the unrestrained cat. The emetic response to clonidine was short-lasting, generally dose-dependent in terms of latency and frequency, and occurred in bouts of one to three episodes. The sequence of the vomiting response, beginning with licking and retching, functionally resembled a normal pattern of an emetic response. The clonidine-induced emesis was not antagonized by the following antagonists micro-injected in AH/POA just prior to clonidine: alpha-adrenergic blocking agents, yohimbine, RX 781094 and phentolamine; the antimuscarinic drug, atropine; the serotonin antagonist, methysergide; the opioid antagonist, naloxone; and the dopamine antagonist, chlorpromazine. Therefore, it would appear that clonidine-induced emesis is not mediated by alpha noradrenergic, serotonergic, dopaminergic, muscarinic and opiate receptor systems within the AH/POA of the cat. Finally, the obtained results show that apart from the area postrema and a circumscribed zone of the brain-stem reticular formation, the hypothalamus is now implicated as a neuroanatomical site in the central nervous system mechanism underlying neurochemically-induced emesis.

Effects of calcium antagonists on alpha-adrenoceptor mediated vasoconstrictions of the canine intermediate auricular artery.

The effects of calcium antagonists, diltiazem (DI) and verapamil (VE), on alpha-adrenoceptor-mediated contractile responses of the isolated and perfused canine intermediate auricular artery were examined by use of the stainless steel cannula inserting method. KCI (3 mg)-induced contractile responses were dose-dependently inhibited by DI and VE. Norepinephrine (0.1 microgram)-induced vasoconstrictions were also dose-dependently inhibited by both DI and VE. DI and VE shifted dose-response curves for phenylephrine and clonidine to the right with suppression of maximal responses. At large doses, DI and VE suppressed clonidine-induced contractions more than phenylephrine-induced ones, but not significantly. It is concluded that not only alpha 1- but also alpha 2-adrenoceptor mediated vasoconstrictions of the canine intermediate auricular artery are dependent on the influx of extracellular calcium ions, the present results may provide in vitro proof for the effectiveness of calcium antagonists on skin circulatory disturbances such as Raynaud's phenomenon and the isolated and perfused canine intermediate auricular artery would be a good in vitro model for investigating responsiveness of the skin supplying artery.

Cardiovascular effects of perfusion of the rostral rat hypothalamus with clonidine: differential interactions with prazosin and yohimbine.

The present studies examined the role of alpha 1- versus alpha 2-adrenoceptors in the cardiovascular effects of clonidine administered into the anterior hypothalamic/pre-optic (AH/PO) region of the forebrain by the push-pull perfusion technique. Push-pull cannulas were placed bilaterally into the AH/PO region of anesthetized, paralyzed and ventilated rats. Perfusion of this area with artificial CSF (0.015 ml/min), yohimbine (5 or 50 microM) for 30 min did not affect mean arterial blood pressure or heart rate. Perfusion of the AH/PO region with clonidine (0.55-5.50 mM) resulted in a concentration-dependent reduction of mean arterial pressure and heart rate. The hypotensive effects of clonidine were found to be greater than the bradycardic effects, when expressed as a percent of pre-infusion baseline values. Co-perfusion with yohimbine (5, 50 microM) significantly attenuated the hypotensive, but not the bradycardic, effects of a single concentration (1.75 mM) of clonidine; this selective antagonism of the hypotensive effect of clonidine by yohimbine was concentration dependent. In contrast to yohimbine, co-perfusion with 5 microM prazosin did not significantly affect either the clonidine-induced hypotension or bradycardia. Co-perfusion with the higher concentration (50 microM) of prazosin significantly reversed the bradycardic, but not the hypotensive, effects of 1.75 mM clonidine. These results suggest that AH/PO clonidine perfusion depresses both mean arterial pressure and heart rate, and that the clonidine-induced hypotension is due to alpha 2-adrenoceptor activation, while the clonidine-induced bradycardia is due to alpha 1-adrenoceptor activation.

The effect of antidepressant drugs on dominance behavior in rats competing for food.

Wistar male rats were maintained on a regimen of restricted access to food. The animals were randomly paired and placed in a special apparatus containing the feeder constructed in such a manner that only one rat at a time could drink sweetened milk. The dominance-subordination (d-s) relationship developed in about 80% of the pairs. This was manifested by the dominant rat eating for a longer period of time than the submissive rat. Clonidine (0.1 mg/kg ip) given to the dominant rats abolished the stable d-s hierarchies. Tricyclic antidepressants and mianserin given for 5 consecutive days prevented the clonidine action. Yohimbine, the antagonist of alpha 2 adrenoceptors produced a similar effect. Drugs belonging to other groups (e.g. neuroleptics and anxiolytics) had no effect on the clonidine-induced suppression of dominance.

Self-stimulation responses are altered following long-term but not short-term treatment with clorgyline.

Clorgyline (a selective monoamine oxidase-inhibiting antidepressant) given chronically facilitated hypothalamic self-stimulation in rats, while acute treatment was without effect. Furthermore, long-term but not short-term clorgyline treatment significantly attenuated the suppressive effect of the selective alpha 2-adrenergic agonist clonidine on this behavior. These findings suggest that adaptative alterations in the modulation of rewarded behavior by inhibitory presynaptic noradrenergic receptors may be involved in antidepressant efficacy.

Antagonism of behavioural depression produced by clonidine in the Mongolian gerbil: a potential screening test for antidepressant drugs.

The effects of various antidepressant drugs and some other therapeutic agents on the depression of locomotion and exploratory activity induced by clonidine (0.1 mg/kg IP) were investigated in the Mongolian gerbil (Meriones unquiculatus). In parallel experiments, the effect of yohimbine on clonidine-induced sedation was observed. The following behavioral components were analysed: ambulation, rearing and novel object investigation. Yohimbine antagonized the effects of clonidine in a dose-dependent manner. All antidepressants similarly antagonized the effect of clonidine on ambulation but they differed to a greater extent in their potency in counteracting the clonidine action on exploration, particularly the novel object investigation. On the other hand diazepam and neuroleptic agents such as pimozide and flupentixol failed to antagonize the clonidine effects. The antagonism of clonidine-induced behavioral depression might be used in the selection of antidepressants.

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.

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.