Abolition of clonidine's effects on ventricular refractoriness by naloxone in the conscious dog

The interaction between opiate and adrenergic receptors on cardiac electrophysiologic function in the conscious dog was addressed in our study. We examined the effects of opiate receptor blockade with naloxone on clonidine-induced changes in refractoriness of the cardiac ventricle. Nine dogs were chronically instrumented for recording mean arterial blood pressure, administration of drugs and for measurement of effective refractory period of the ventricle. Clonidine (10 micrograms/kg, i.v.) significantly (p less than 0.05) decreased heart rate to 72 +/- 5 beats/minute from 108 +/- 8 beats/minute; mean arterial pressure decreased significantly (p less than 0.05) to 83 +/- 3 mmHg from 91 +/- 4 mmHg. Ventricular refractoriness was increased significantly (p less than 0.05) at current levels of 7 and 10 mA and pacing rates 180 and 200 beats/minute. Naloxone (3-10 mg/kg, i.v.) abolished clonidine's effects on heart rate, mean arterial pressure and ventricular refractoriness. We conclude that ventricular refractoriness may be regulated in part by interactions between central adrenergic and opioidergic systems.

Effects of pertussis toxin on the behavioural and ECoG spectrum changes induced by clonidine and yohimbine after their microinfusion into the locus coeruleus

1. Pertussis toxin, a substance which interferes selectively with receptor-mediated signal transduction mechanisms, was injected into the locus coeruleus of rats 1, 2, 3, 6 or 10 days before the microinjection of clonidine or yohimbine into the same site. 2. Clonidine produced in control rats typical behavioural sedation and/or sleep and ECoG synchronization while yohimbine produced behavioural arousal and ECoG desynchronization. 3. The behavioural and ECoG effects of both compounds were blocked in animals pretreated with pertussis toxin. This activity was more marked from 2 to 6 days after pertussis toxin pretreatment and was restored 10 days after toxin administration. In addition, the behavioural and ECoG slow-wave sleep observed after intraperitoneal administration of clonidine (0.2 mumol kg-1) was significantly reduced by prior (3 days) microinfusion of pertussis toxin into the locus coeruleus. 4. These results are consistent with the hypothesis that the behavioural and ECoG effects of clonidine and yohimbine are mediated via a guanine regulatory protein which is affected by pertussis toxin.

[Mechanism of action of clonidine in the forced-swimming test in mice]

Clonidine displays immobility-reducing effects in the mouse swimming model at doses (0.06-16 mg/kg IP) which decrease spontaneous motility. Tricyclic antidepressants evoke a similar dissociation in motor activity. The immobility-reducing effect of clonidine (1 mg/kg at 30 min pretesting) was reversed by yohimbine (4 mg/kg) but was unaffected by prazosin (2 mg/kg) or alpha-methyl-paratyrosine (100 mg/kg), and was enhanced by reserpine (2.5 mg/kg). Mediation by alpha-2 postjunctional receptors was thus suggested. However, two 5-HT receptor blockers--methysergide (2 mg/kg) and ketanserin (8 mg/kg)--increased this effect of clonidine whereas the non selective agonist 5-MeODMT (1 mg/kg) reduced clonidine action. Conversely, pretreatment with a subthreshold dose of clonidine (0.06 mg/kg at 45 min pretesting) made effective subthreshold doses of three 5-HT uptake inhibitors (citalopram 2 mg/kg, indalpine and fluvoxamine 4 mg/kg) and of the 5-HT1 receptor agonist 8-OH-DPAT (0.5 mg/kg). According to these data, the mouse swimming model would trigger functional relationships between central alpha-noradrenergic and serotonergic mechanisms.

Amphetamine antagonizes the presynaptic inhibitory effect of clonidine through an interaction at the level of the alpha 2-adrenoceptors

1. In the whole rat vas deferens 20 microM noradrenaline (NA) and 0.011 microM clonidine decreased (36 +/- 7.4% and 80 +/- 6.0% respectively) the motor response induced by hypogastric nerve stimulation. These effects were reverted by 1 microM yohimbine. Amphetamine 5.4 microM failed to antagonize the inhibitory effect of NA and attenuated clonidine effect. 2. The effect of amphetamine was not altered by preincubation with either cocaine 1 microM, (-)-propranolol 0.3 microM or cocaine plus prazosin 0.028 microM. 3. In reserpine pretreated animals amphetamine 5.4 microM shifted to the right the concentration-response curve (CRC) to clonidine 0.62 +/- 0.05 log units with a KB value of 1.83 +/- 0.30 microM. 4. Binding of [3H]clonidine and [3H]prazosin were inhibited by amphetamine. Amphetamine was 90 times more potent to inhibit [3H]clonidine binding. 5. The results obtained suggest a possible direct interaction between clonidine and amphetamine on alpha-adrenoceptor.

Interactions between noradrenergic and cholinergic mechanisms involved in spinal nociceptive processing

Antinociceptive effects have been demonstrated after systemic and spinal administration of the adrenoceptor agonist clonidine and cholinomimetic drugs in animals and human. The present investigation was undertaken in rats to study the possible interactions between spinal noradrenergic and cholinergic mechanisms in modulating the reaction to nociceptive stimuli. Using the tail immersion test, an additive antinociceptive effect was found between intrathecal (IT) clonidine (10 micrograms) and physostigmine (15 micrograms, IT). The effect of clonidine was attenuated by atropine (15 micrograms, IT). Physostigmine (15 micrograms, IT) antinociception, which was of short duration was abolished by atropine (15 micrograms, IT) and attenuated by phentolamine (20 micrograms, IT). Neostigmine (5 micrograms, IT) produced a prolonged antinociceptive response. In animals pretreated with 6-hydroxydopamine IT, leading to a selective depletion of spinal cord noradrenaline, physostigmine (15 micrograms, IT) was ineffective in altering the nociceptive test response. Neither clonidine, nor physostigmine produced changes in latency times in the hot plate test (58 degrees C) in the doses employed. In conclusion, a clear-cut interaction exists between spinal noradrenergic and cholinergic systems for antinociception. To explain the interactions, several possible mechanisms may be considered, including cholinomimetic effects produced by clonidine, and the presence of muscarinic receptors in the dorsal horn of the spinal cord.

Alpha noradrenergic agonists promote catalepsy in the mouse

Alpha adrenergic receptors are important in motor control. Adrenergic drugs reportedly modulate the catalepsy caused by other agents, such as opiates and neuroleptics. We tested a variety of adrenergic agonists and blockers in a nondrug, mouse model of catalepsy. A major cataleptic effect was produced by both alpha agonists, phenylephrine (0.5 to 3 mg/kg, IP) and clonidine (0.5 to 2 mg/kg), and this catalepsy was antagonized by pretreatment with both of the respective antagonists, phenoxybenzamine (alpha-1) and yohimbine (alpha-2) (p less than 0.0001). The mixed beta agonist, isoproterenol (2 mg/kg), appeared to have a minor cataleptic effect, and this was abolished by beta antagonist. Open-field locomotor scores during the same treatments revealed that catalepsy and hypokinesia are dissociable.

Chronic stress attenuation of alpha 2-adrenoceptor reactivity is reversed by naltrexone

Low doses of clonidine (50-100 micrograms/kg IP) evoke a clear dose-dependent hypoactivity response. Seven daily immobilization sessions prevented the motor activity decrease induced by clonidine. On the contrary, a single stress failed to modify clonidine response. Pretreatment with naltrexone (2 mg/kg IP) fully antagonized the attenuating effect induced by chronic stress on clonidine sedative action. These evidences suggest that chronic but not acute stress reduces the reactivity of alpha 2-adrenoceptors involved in clonidine-induced sedation. In addition, a regulatory mechanism of endogenous opioids seems to participate on alpha 2-adrenoceptors adaptative changes.

Pertussis toxin blocks the effects of alpha 2-agonists and antagonists on locus coeruleus activity in vivo

This study assessed the effects of pertussis toxin, which is known to inactivate G proteins and therefore to block receptors linked to G proteins, on electrophysiological activity of the locus coeruleus in vivo. Pertussis toxin was injected into the lateral cerebral ventricle of rats, and locus coeruleus activity was then recorded. Compared to vehicle-injected control animals, pretreatment with pertussis toxin markedly increased the spontaneous firing rate of locus coeruleus neurons. In addition, the alpha 2-antagonist idazoxan was no longer able to augment either spontaneous or evoked locus coeruleus activity after pretreatment with pertussis toxin. Finally, pretreatment with pertussis toxin made locus coeruleus neurons resistant to inhibition by the alpha 2-agonist clonidine. These results are consistent with the view that pertussis toxin blocks alpha 2-receptors, receptors linked to G proteins, in vivo.

Are opioid receptors involved in the bradycardiac and hypotensive action of clonidine?

Experimental evidence that supports or rejects an involvement of opioid receptors in the cardiovascular effects of clonidine was sought. In anesthetized rabbits, clonidine decreased blood pressure and heart rate. These effects were related to the clonidine dose. High doses of clonidine produced a biphasic response in blood pressure without resulting in a significant baroreflex. However, it was not difficult to produce hypotensive action of clonidine without the biphasic action using low doses of clonidine. A pretreatment of the animals with naloxone (1 mg/1 kg) prevented the hypotensive action of clonidine at low dose. This antagonistic action of naloxone against clonidine was also evident in other cardiac parameters such as heart rate and efferent sympathetic renal nerve activity. The antagonistic action was not demonstrably significant with the high doses of clonidine. The present study in normotensive anesthetized rabbits suggests that specific central interactions between opioid and adrenergic receptors can be demonstrated under defined conditions, although such interactions were reported in pathophysiologic conditions such as essential hypertension.

Desipramine antagonizes clonidine-induced suppression of dominance in rats: possible involvement of amygdaloid nuclei

Bilateral microinjections of clonidine into the medial or basal amygdaloid nuclei suppressed the dominance behavior in rats competing for food. Desipramine given in five repeated daily doses antagonized clonidine effect. On the other hand suppression of dominance induced by clonidine given bilaterally into the cortical amygdaloid nucleus was resistant to desipramine treatment. The result suggests that the former nuclei are important target areas for clonidine-desipramine interaction at social interaction behavioral level.

Antagonism of clonidine-induced hypothermia by alpha adrenoceptor antagonists in electrically stimulated mice

The involvement of receptor subtypes in clonidine-induced hypothermia in electrically stimulated mice was studied using various alpha-adrenoceptor antagonists. Yohimbine, the selective alpha 2-adrenoceptor antagonist significantly blocked the action of low dose (50 micrograms/kg i.p.) of clonidine pretreated with Minimal Threshold Shock (12 mA, 0.2 Sec), while prazosin, selective alpha 1-adrenoceptor antagonist partially blocked the action, suggesting predominant involvement of alpha 2 and partial involvement of alpha 1-adrenoceptors. Under similar conditions when clonidine was used in higher dose (500 micrograms/kg i.p.), its action was blocked by prazosin, phenoxybenzamine and yohimbine suggesting involvement of both alpha 1 and alpha 2-adrenoceptors. In case of animals pretreated with Maximal Threshold Shock (36 mA, 0.2 Sec) the hypothermic action of clonidine in lower dose was blocked by all the three antagonists viz. prazosin, phenoxybenzamine and yohimbine, suggesting involvement of both alpha 1 and alpha 2-adrenoceptors, however, when clonidine was used in the higher dose, the action was significantly antagonised by prazosin and partially by yohimbine, suggesting predominant involvement of alpha 1 and partial involvement of alpha 2-adrenoceptors.

Prevention of clonidine-stimulated hyperglycemia by thyrotropin-releasing hormone

Central injection of thyrotropin-releasing hormone (TRH) prevented the rise in plasma glucose due to clonidine challenge in mice. This antihyperglycemic action was dose-related, dependent upon the structural integrity of the peptide, dissociated from the peptide's hypophysiotropic influences, and coupled to reversal of clonidine's suppressive action of insulin release. TRH was effective in preventing the rise in plasma glucose when given at different times before clonidine (up to two hours), and it also reversed the hyperglycemia when administered 30 min after clonidine, when plasma glucose was already exceedingly high. The results suggest that TRH is able to physiologically oppose clonidine-induced hyperglycemia by acting in a specific and durable manner upon central mechanisms which modulate insulin secretion.

Blockade of alpha 2-adrenoceptors by 1-(2-pyrimidinyl)-piperazine (PmP) in vivo and its relation to the activity of buspirone

The effect of 1-(2-pyrimidinyl)-piperazine (PmP) and the parent drug, buspirone in counteracting the behavioral and biochemical effects of clonidine were evaluated in the rat. Intraperitoneal or oral administration of PmP, buspirone and yohimbine, but not of prazosin, antagonized the slowing of gastrointestinal motility induced by subcutaneous clonidine (0.1 mg/kg). The doses that inhibited the effect of clonidine on the transit time by 50% were 0.5 mg/kg i.p. and 0.7 mg/kg p.o. for PmP, 7 mg/kg i.p. and 9 mg/kg p.o. for buspirone and 0.5 mg/kg i.p. for yohimbine. PmP (0.5 mg/kg) did not block the antitransit effect of clonidine when administered by intracerebroventricular injection. The antitransit effect of a low dose of morphine (0.05 mg/kg i.p.) was not blocked by PmP (2 mg/kg i.p.). The prolongation of the hexobarbital-induced loss of the righting reflex that occurs after clonidine (0.25 mg/kg i.p.) administration was inhibited by pretreatment with PmP (0.1-2 mg/kg p.o.) or yohimbine (1 mg/kg i.p.) but not by pretreatment with prazosin (2 mg/kg i.p.). Buspirone (1-20 mg/kg) also reduced the effect of clonidine after oral administration, with a maximal effect at 5 mg/kg, whereas the same dose administered i.v. had less effect. PmP (2 mg/kg) and buspirone (15 mg/kg) raised the levels of total 3-methoxy-4-hydroxyphenylgycol (MHPG) in rat cerebral cortex, and prevented the decrease in MHPG induced by clonidine. These findings show that buspirone, in doses at which it is active as an anxiolytic, suppresses the central and peripheral effects of clonidine. This action occurs through alpha 2-adrenoceptors and is mediated primarily by the metabolite, PmP.

An evidence for presynaptic excitatory alpha 2-adrenergic receptors in frog myocardium

This study was undertaken to determine the effects of clonidine on sympathetic neurotransmission in frog myocardium. In the electrically driven ventricular strips of frog heart, clonidine was found to be ineffective on contractility. However, clonidine increased the positive inotropic responses to transient additional stimulations. This effect of clonidine was antagonized by yohimbine, an alpha 2-adrenergic receptor antagonist. Clonidine did not change the positive inotropic effects of exogenously applied noradrenaline. These results suggest that clonidine facilitates sympathetic neurotransmission in frog myocardium via an action on alpha 2-adrenergic receptors located on sympathetic nerve terminals.

Peripheral analgesic action of clonidine: mediation by release of endogenous enkephalin-like substances

Clonidine analgesia was tested on the hyperalgesia induced by intraplantar injection of prostaglandin E2 or carrageenin. The antinociceptive effect of clonidine was dose-dependent and was abolished by local administration of the selective alpha 2-adrenoceptor blocker, yohimbine or of the opioid antagonists naloxone or quaternary nalorphine. St-91, a clonidine analog which does not cross the blood-brain barrier also promoted significant antinociception. Repeated administration of drugs possessing a central mechanism of analgesic action leads to the development of tolerance in this test. Significant analgesic tolerance was observed following repeated (5 days) morphine (8 mg/kg) or high doses of clonidine (0.5 mg/kg). In contrast, no tolerance was detected to the analgesic effect of low doses of clonidine (0.15 mg/kg) or of St-91 (0.5 mg/kg). These results suggest that, in addition to its central analgesic action, clonidine can induce peripheral antinociception by an alpha 2-adrenoceptor-mediated local release of enkephalin-like substances.

A specific antiserum recognizes clonidine-displacing substance: implications for the structure of the brain's own clonidine

A polyclonal antiserum was raised in rabbit against the clonidine analog p-aminoclonidine (PAC) coupled to hemocyanin. The antiserum (anti-PAC3) exhibited high affinity for unconjugated [3H]PAC (Kd 0.32 +/- 0.07 nM) in a rapid-filtration radioimmunoassay. Competition experiments showed that PAC, clonidine, and naphazoline cross-reacted with the anti-PAC3 antiserum, whereas a number of other structurally related compounds did not. An endogenous clonidine-displacing substance (CDS) partially purified from bovine brain also inhibited specific [3H]PAC binding to anti-PAC3 in a dose-dependent manner. Thus, (a) anti-PAC3 antiserum is specific for clonidine and closely related compounds, and (b) CDS may structurally resemble clonidine since it is recognized by this highly specific antiserum.

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.

A low molecular weight brain substance interacts, similarly to clonidine, with alpha 2-adrenoceptors of human platelets

In the present study, we explored the effects of a clonidine-displacing substance (CDS) which was isolated and partially purified from bovine brain. The low molecular weight brain substance competes with clonidine and rauwolscine in rat brain membranes, and mimics clonidine's inhibitory action in rat vas deferens. We find that CDS competes with [3H]rauwolscine-labeled alpha 2-adrenoceptors in human platelets. Further characterization of CDS in human platelets reveals that, like clonidine, it inhibits the epinephrine-induced aggregation, potentiates the ADP- and the collagen-induced aggregation however, by itself, CDS is unable to induce aggregation. Unlike clonidine, CDS does not affect the prostacyclin (PGI2)-stimulated cAMP accumulation in intact platelets. The presence of CDS in human plasma, as we have recently shown, implies a possible role of CDS in the regulation of platelet action.

Opiate receptors and the endorphin-mediated cardiovascular effects of clonidine in rats: evidence for hypertension-induced mu-subtype to delta-subtype changes

Effects of opiate receptor antagonists on centrally mediated cardiovascular responses to clonidine and beta-endorphin were studied in urethane-anesthetized spontaneously hypertensive Okamoto-Aoki rats (SHR), normotensive Sprague-Dawley rats, and Sprague-Dawley rats made hypertensive with deoxycorticosterone pivalate/salt. Microinjection of 270 pmol of naloxone into the nucleus tractus solitarii (NTS) significantly inhibited the hypotensive and bradycardic response to 5 nmol of similarly administered clonidine in both SHR and normotensive Sprague-Dawley rats. In SHR, a similar inhibition was observed after the delta-opiate receptor antagonist ICI 174864, but not after the mu-receptor antagonist beta-funaltrexamine (both at 270 pmol, intra-NTS), whereas in normotensive Sprague-Dawley rats, beta-funaltrexamine, but not ICI 174864, was an effective inhibitor. The same pattern of differential inhibition was seen when clonidine was given i.v. and the opiate antagonists were given intracisternally in SHR and Sprague-Dawley rats. Intra-NTS microinjection of 280 fmol of beta-endorphin caused hypotension and bradycardia, and these effects were similarly inhibited by ICI 174864 in SHR and by beta-funaltrexamine in Sprague-Dawley rats. In Sprague-Dawley rats made hypertensive by chronic administration of deoxycorticosterone pivalate and salt, the hypotensive and bradycardic effects of intra-NTS clonidine were inhibited by ICI 174864, but not by beta-funaltrexamine, a pattern similar to that in SHR, but different from that in normotensive Sprague-Dawley rats. These results support the hypothesis that beta-endorphin release and subsequent stimulation of opiate receptors in the NTS are involved in the cardiovascular effects of clonidine in rats. These results further suggest, however, that hypertension regulates the subtype of opiate receptors mediating these effects.

Raised levels of an endogenous nonadrenergic substance in the serum of pregnancy-induced hypertension patients

A clonidine-displacing substance (CDS) was isolated from bovine brain and shown to mimic clonidine in peripheral tissues. CDS acts--contrary to clonidine--by increasing the mean arterial pressure in the CNS. Methanolic extract from sera of patients with pregnancy-induced hypertension (PIH) showed a significant increase in CDS levels as compared with sera of either normal or chronic hypertensive pregnancy. Although small amounts of CDS were found in the placenta, no positive correlation between placental levels of CDS and serum CDS levels was observed. The pharmacological properties of CDS and its higher levels in sera of PIH patients suggest that CDS is a sympathetic agent involved in the pathophysiology of PIH.

Alpha 2 adrenergic agonists stimulate Na+-H+ antiport activity in the rabbit renal proximal tubule

The role of adrenergic agents in augmenting proximal tubular salt and water flux, was studied in a preparation of freshly isolated rabbit renal proximal tubular cells in suspension. Norepinephrine (NE, 10(-5) M) increased sodium influx (JNa) 60 +/- 5% above control value. The alpha adrenergic antagonist, phentolamine (10(-5) M), inhibited the NE-induced enhanced JNa by 90 +/- 2%, while the beta adrenergic antagonist, propranolol, had a minimal inhibitory effect (10 +/- 2%). The alpha adrenergic subtype was further defined. Yohimbine (10(-5) M), an alpha2 adrenergic antagonist but not prazosin (10(-5) M), an alpha1 adrenergic antagonist completely blocked the NE induced increase in JNa. Clonidine, a partial alpha2 adrenergic agonist, increased JNa by 58 +/- 2% comparable to that observed with NE (10(-5) M). Yohimbine, but not prazosin, inhibited the clonidine-induced increase in JNa, confirming that alpha2 adrenergic receptors were involved. Additional alpha2 adrenergic agents, notably p-amino clonidine and alpha-methyl-norepinephrine, imparted a similar increase in JNa. The clonidine-induced increase in JNa could be completely blocked by the amiloride analogue, ethylisopropyl amiloride (EIPA, 10(-5) M). The transport pathway blocked by EIPA was partially inhibited by Li and cis H+, but stimulated by trans H+, consistent with Na+-H+ antiport. Radioligand binding studies using [3H]prazosin (alpha1 adrenergic antagonist) and [3H]rauwolscine (alpha2 adrenergic antagonist) were performed to complement the flux studies. Binding of [3H]prazosin to the cells was negligible. In contrast, [3H]rauwolscine showed saturable binding to a single class of sites, with Bmax 1678 +/- 143 binding sites/cell and KD 5.4 +/- 1.4 nM. In summary, in the isolated rabbit renal proximal tubular cell preparation, alpha2 adrenergic receptors are the predominant expression of alpha adreno-receptors, and in the absence of organic Na+-cotransported solutes, alpha2 adrenergic agonists enhance 22Na influx into the cell by stimulating the brush border membrane Na+-H+ exchange pathway.

Central mechanisms of ethanol-induced adrenocortical response in selectively bred lines of mice

Selectively bred long-sleep (LS) and short-sleep (SS) mice differ markedly in ethanol-induced adrenocortical response. Intracerebroventricular injections of saline elicited a 'stress-induced' adrenocortical response in both lines of mice, and intracerebroventricular infusions of noradrenergic and cholinergic compounds modulated ethanol-induced and stress-induced adrenocortical responses differentially in these mice. Clonidine, an alpha 2-adrenergic agonist, blocked ethanol-induced elevations in plasma corticosterone in a dose-dependent manner (1 and 10 micrograms) in LS mice; however, only the 10-micrograms dose of clonidine effectively antagonized this response in SS mice. Clonidine was less effective in blocking adrenocortical activity induced by stress than that induced by ethanol. Yohimbine, an alpha 2-adrenergic antagonist, induced a marked elevation in plasma corticosterone in LS mice but not in SS mice; however, this compound did not alter ethanol-induced adrenocortical responses in either line of mice. Yohimbine reversed the inhibitory effect of clonidine in ethanol-treated LS and SS mice. Phentolamine, a nonspecific alpha-adrenergic antagonist, and propranolol, a beta-adrenergic antagonist at high doses (10 micrograms), produced slight increases in plasma corticosterone in LS mice only. Neither these compounds nor methoxamine, a nonspecific alpha-adrenergic agonist, altered the effect of ethanol on adrenocortical activity in LS or SS mice. Carbachol, a mixed muscarinic/nicotinic agonist, significantly increased adrenocortical response in both LS and SS mice and potentiated ethanol-induced elevation in plasma corticosterone in both lines of mice. However, atropine, a nonspecific muscarinic antagonist, or hexamethonium, a nicotinic antagonist, did not modify ethanol-induced elevations in plasma corticosterone in LS and SS mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Antagonism by E. coli endotoxin of some cardiovascular effects induced in the rat by two alpha 2-adrenoceptor agonists

E. coli endotoxin (0.01, 0.1 and 1 microgram/kg i.v.) 1 h before alpha 2-adrenoceptor agonists B-HT 933 and clonidine (i.v.) antagonized their bradycardiac and hypotensive effects in intact rats. This antagonism seems not to depend on the presence of the adrenal glands since similar results were obtained in adrenalectomized rats. Endotoxin at higher doses (1 and 10 micrograms/kg) suppressed the hypotensive and reduced the bradycardiac effect of clonidine injected i.c.v. (5 micrograms/kg). In contrast, endotoxin (up to 100 micrograms/kg) did neither increase arterial pressure nor heart rate in the pithed rat. This suggests the participation of a central site of action for endotoxin. However, E. coli endotoxin (1, 10 or 100 micrograms/kg) did not decrease the inhibition by clonidine of the tachycardia induced by stimulation of the cardioacceleratory nerve. This excludes peripheral presynaptic alpha 2-adrenoceptor blockade by endotoxin. Only 100 micrograms/kg decreased the pressor response to clonidine in the pithed rat. These results show that E. coli endotoxin is a potent modificator of the cardiovascular regulation in the rat. It antagonized central alpha 2-adrenoceptor mediated cardiovascular effects at doses lower than those acting on postsynaptic peripheral alpha-adrenoceptors.

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.

Possible catecholaminergic-opioidergic control of blood pressure during muscular contraction

The effects of an alpha 2 adrenoceptor blocker, yohimbine, and an alpha 1 adrenoceptor blocker, phenoxybenzamine, and the central alpha 2 adrenoceptor agonist, clonidine, on changes in arterial blood pressure and heart rate were studied during fatiguing muscular contractions to determine whether an adrenergic-opioidergic system might be involved in the mediation of cardiovascular function. Fatiguing contractions of the gastrocnemius and plantaris muscles of cats caused an increase in mean arterial blood pressure to 150-170 mmHg from resting values of 110-120 mmHg. Injection of clonidine into the cerebral aqueduct eliminated the increase in blood pressure; this effect was dose dependent. Naloxone antagonised the effects of the highest dose of clonidine (5 micrograms). Injections of yohimbine (1 microgram) into the cerebral aqueduct had no significant effect on this pressor response. Yohimbine (1 microgram) effectively counteracted the antipressor effects of clonidine when the two drugs were injected together until higher doses of clonidine (2-5 micrograms) were used. Phenoxybenzamine had no effect on the pressor response itself but unlike yohimbine was able to attenuate the effects of clonidine only when injected together. These data suggest that activation of muscle ergoreceptor afferent nerve fibres (group III and IV fibres) during muscular contractions may cause an increase in arterial blood pressure by interfering with an inhibitory adrenergic-endorphinergic pathway in the medullary region of the brainstem.

[Quantified electroencephalogram: value in the evaluation of therapy of cerebral aging]

Computerized quantification of electroencephalograms enables pharmacological compounds designed to counteract pathological cerebral aging to be evaluated. The results obtained by this method when substances which sedate or stimulate the central nervous system are used suggest the existence of a negative correlation between neuronal activity and electroencephalographic power. More precisely, the application of this method to pharmacology suggests a relation with the activity of one or several neurotransmission systems. By extension, studies of cerebral aging may benefit from this new technique of cerebral exploration because it allows the demonstration of age related changes in the EEG effect of drugs studied in physiological conditions. Thus we have demonstrated an interaction between the almitrine-raubasine combination and the noradrenergic system in rats. In clinical pharmacology, the evaluation of these two compounds is still liable to numerous experimental biases. The almitrine-raubasine combination was tested in controlled trials in elderly people complaining of intellectual deterioration; the results obtained were fairly similar to those observed in old rats, thus demonstrating an effect on the central nervous system and perhaps also an action on the noradrenergic system in human.

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.

Central alpha 2-adrenergic stimulation increases neurointermediate lobe immunoreactive beta-endorphin in spontaneously hypertensive rats

A possible influence of the central alpha 2-adrenergic system on beta-endorphin was examined in rat anterior pituitary, neurointermediate lobe, and plasma. The concentration of beta-endorphin in anterior pituitary, neurointermediate lobe, and plasma was determined by radioimmunoassay 15 minutes after subcutaneous injection of clonidine in 14-week-old spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). Clonidine reduced the concentration of the plasma beta-endorphinlike immunoreactivity in SHR and to a lesser extent in WKY. No significant changes in the concentration of beta-endorphinlike immunoreactivity were observed in anterior pituitary. Clonidine increased the concentration of neurointermediate lobe beta-endorphinlike immunoreactivity in SHR in a dose-related manner but did not affect the concentration in WKY. Administration of yohimbine (1 mg/kg) completely blocked the clonidine-induced increase of neurointermediate lobe beta-endorphinlike immunoreactivity in SHR, while prazosin (1 mg/kg) had no effect. These data suggest that the central alpha 2-adrenergic activation increases the neurointermediate lobe concentration of beta-endorphinlike immunoreactivity in SHR by suppressing beta-endorphin release from the neurointermediate lobe into the circulation.

Effects of DJ-7141, a new peripherally acting alpha-2 adrenoceptor agonist, on blood pressure and gastric acid output in rats: evidence for the heterogeneity of alpha-2 adrenoceptors in different organs

Effects of intravenously administered clonidine and DJ-7141 on blood pressure and gastric acid output were studied in anesthetized rats. Both drugs caused a dose-dependent increase in blood pressure. Clonidine-induced increases in the blood pressure were attenuated by yohimbine and prazosin. DJ-7141-induced increases in blood pressure were also attenuated by yohimbine, but were little affected even by a large dose of prazosin. Splanchnic nerve stimulation-induced increases in blood pressure were attenuated by clonidine and DJ-7141. The inhibitory effects of DJ-7141 exceeded those of clonidine. On the other hand, these agonists had no effect on exogenously applied norepinephrine-induced increases in blood pressure. Vagally stimulated gastric acid output was inhibited by small doses of clonidine and large doses of DJ-7141. Clonidine-induced inhibition of acid output was abolished by yohimbine (5 mg/kg), but not by prazosin (5 mg/kg). In contrast, the DJ-7141-induced inhibition of acid output was not affected by yohimbine (5 mg/kg), but was attenuated by a small dose of prazosin (0.1 mg/kg). These results indicate that DJ-7141 had no effect on alpha-2 adrenoceptors in gastric parasympathetic nerves. Furthermore, the alpha-2 adrenoceptors on parasympathetic nerves in the gastric wall are probably pharmacologically different from those on sympathetic nerves and blood vessels in rats.

Pertussis toxin or 8-bromo-cAMP block inhibition of the acoustic startle response by the alpha 2-adrenergic agonist ST-91

Stimulation of supraspinal alpha 2-adrenergic receptors by intraventricular infusion of the alpha 2-adrenergic agonist ST-91 depresses a simple vertebrate behavior, the acoustic startle response. Intraventricular pretreatment with pertussis toxin, an agent known to inactivate the inhibitory guanine nucleotide binding protein (Gi) which can inhibit adenylate cyclase, completely prevented the depressant behavioral effect of ST-91. In contrast, pertussis toxin did not alter the depressant effect of intraventricular infusion of the 5-HT 1B agonist 1-m-chlorophenylpiperazine (mCPP). Intraventricular infusion of the cyclic adenosine monophosphate (cAMP) analog 8-bromo-cAMP also reversed the depressant effect of ST-91 without altering the effect of mCPP. These data suggest that inhibition of adenylate cyclase may be involved in the effect of activation of central alpha 2-adrenergic receptors.

Pharmacological evidence for the involvement of 1-(2-pyridinyl)-piperazine (1-PmP) in the interaction of buspirone or gepirone with noradrenergic systems

We investigated in mice the effects of one of the principal metabolites of buspirone and gepirone, 1-(2-pyridinyl)-piperazine (1-PmP), on hypothermia and reduced locomotion induced by clonidine (0.25 and 0.06 mg/kg, respectively), tests related to brain alpha-adrenergic function. Both effects were antagonized dose dependently by 1-PmP (1-16 mg/kg i.p.). Moreover, pretreatment with proadifen (50 mg/kg) prevented the reversal by buspirone and gepirone of clonidine-induced hypothermia. This suggests that 1-PmP could be responsible for some of the apparent noradrenergic effects of buspirone and gepirone.

Prazosin partly blocks clonidine-induced hypotension in patients with essential hypertension

Prazosin has been reported to reduce the hypotensive and/or bradycardic effect of clonidine in various animal models. Investigations in humans have given conflicting conclusions about the effectiveness of the combination of clonidine and prazosin. In patients with essential hypertension prazosin significantly reduced the hypotensive effect of intravenous clonidine, but it failed to affect the clonidine-induced bradycardia. This finding means that the combination of prazosin and clonidine is inappropriate in antihypertensive therapy.

Neurotransmitters in "the smoke reflex" in rabbits

An acute form of "stress-analgesia" is evoked by allowing the smoke of a cigarette to envelope the nostrils of unanaesthetized rabbits. The response consists of an immediate and generalized arrest of spontaneous movements, including respiration and expiration, reduced muscular tone, and unresponsiveness to pinching. This motor "paralysis" is accompanied by a profound bradycardia. Attempts have been made to identify the neurotransmitters involved in "the smoke reflex" by the intervention of antagonists and psychopharmaca. The bradycardia was selectively blocked by atropine, leaving the somatomotor inhibition unaltered. All components of the response were abolished by approximately 60% by clonidine and by 40% by the tricyclic antidepressant amitriptyline, both of which are known to attenuate the release of noradrenaline as agonsits of alpha 2-adrenoceptors. Yohimbine blocked the clonidine effect. Naloxone (1-2 mg/kg), p-chlorphenylalanine and dexamethasone failed to influence the reflex response, suggesting that opiate, serotonergic and ACTH-systems do not play a critical role. The same applied to the benzodiazepine chlordiapoxide. The results suggest that this acute stress-induced analgesia is mediated via a noradrenergic system. The relationship of the smoke reflex to "the fear paralysis reflex", a possible trigger mechanism for the sudden infant death syndrome, is discussed.

A study on the antagonism of the intestinal inhibitory effects of morphine and clonidine by yohimbine in mice

In mice, clonidine administered subcutaneously caused a dose-dependent inhibition of the intestinal motility as assayed by the movement of a charcoal meal. This inhibitory effect of clonidine was antagonized dose-dependently by prior subcutaneous or intracisternal administration of yohimbine. However, yohimbine given intracerebroventricularly was ineffective in antagonising the intestinal inhibitory action of clonidine. Clonidine administered centrally, either intracisternally or intracerebroventricularly, caused a dose-dependent inhibition of intestinal motility. Clonidine given by the intracisternal route appeared to be more effective than by the intracerebroventricular route. Centrally administered clonidine was antagonized by prior subcutaneous administration of yohimbine. The antagonism was related to the doses of yohimbine given. Subcutaneously administered morphine caused a dose-dependent inhibition of intestinal motility and this effect was antagonized by prior subcutaneous administration of yohimbine. However, administration of yohimbine centrally, either intracisternally or intracerebroventricularly, did not affect the intestinal inhibitory action of morphine. On the other hand, morphine injected centrally, either intracisternally or intracerebroventricularly, dose-dependently inhibited the motility of the intestine; such inhibitory action was antagonized by prior subcutaneous administration of yohimbine. The present study suggests that clonidine inhibits intestinal motility at both central and peripheral sites through alpha 2-adrenoceptors. Morphine also inhibits intestinal motility by both central and peripheral mechanisms but it appears that yohimbine only modifies the peripheral aspect of morphine's action.

Central alpha 2-adrenergic control of the pattern of small intestinal motility in rats

The effects of central and peripheral administration of alpha 2-adrenoceptor agonists and antagonists on small intestinal motility were examined in conscious rats chronically fitted with electrodes implanted in the duodenojejunal wall and a cannula placed in a cerebral lateral ventricle. In fasted rats, intracerebroventricular or intraperitoneal administration of clonidine (5 micrograms) immediately disrupted the migrating myoelectric complex pattern with a total inhibition of spiking activity during the first hour, followed by a period of irregular spiking activity for 2 h. The inhibition was abolished by previous intramuscular administration of yohimbine (600 micrograms), and the period of irregular activity was suppressed by intracerebroventricular yohimbine (30 micrograms). Naphazoline, an alpha 2-agonist that poorly crosses the blood-brain barrier, only inhibited spiking activity when administered intraperitoneally (1 microgram) and induced only a period of irregular spiking activity when administered intracerebroventricularly at the same dose. In fed rats, intracerebroventricular administration of yohimbine or phentolamine (30 micrograms), and to a lesser extent prazosin, restores a migrating myoelectric complex pattern typical of the fasted state. Peripheral administration of these three antagonists at a dose 20 times higher was ineffective. Finally, both feeding and central administration of alpha 2-agonists disrupt the migrating myoelectric complex pattern. Such pharmacologic data suggest a possible role of central alpha 2-adrenoceptors in the regulation of intestinal motility in rats.

[Pharmacological studies of MO-8282, a new antidepressant]

We investigated the pharmacological characteristics of MO-8282 as a novel antidepressant. MO-8282 inhibited the specific binding of 3H-clonidine to cerebro-cortical membrane fractions from rats about five times more potently than mianserin, and it competed with clonidine in the twitch response of the isolated guinea-pig ileum under field stimulation. The results indicated that MO-8282 possessed alpha 2-adrenergic receptor blocking activity. MO-8282 in a dose of 30 mg/kg (p.o.) showed no inhibition against the uptake of noradrenaline, dopamine and serotonin in the rat brain, whereas mianserin inhibited the uptake of serotonin specifically. MO-8282, similar to mianserin, had no effect on spontaneous release of 3H-noradrenaline and slightly stimulated the release of 3H-serotonin from the rat cerebrocortical synaptosome. The turnover rate of noradrenaline in rat brain was accelerated by administration of MO-8282 (30 mg/kg) for 15 days; however, that of dopamine and serotonin was not affected. The above findings indicate that MO-8282, unlike tricyclic antidepressants, mainly exerts alpha 2-adrenoceptor blocking action on the central noradrenergic system, similar to mianserin. In addition, the fact that MO-8282 unlike mianserin showed no inhibition against uptake of serotonin in brain suggests that the alpha 2-adrenoceptor blocking of MO-8282 is more specific and potent than that of mianserin.

Reversal of clonidine toxicity by naloxone

Clonidine is a centrally acting antihypertensive agent used in the management of essential hypertension. Oral clonidine loading is now used frequently in the management of hypertensive urgencies (ie, increases in arterial pressure not associated with acute, life-threatening end-organ injury). We report the case of a patient with an acute inferior myocardial infarction associated with blunt chest trauma who developed an abrupt and unexplained increase in arterial pressure 24 hours after admission and who was treated with oral clonidine (0.5 mg in divided doses over two hours). Drug therapy was followed by prolonged (four hours) systemic arterial hypotension (mean arterial pressure less than 70 mm Hg). Four milligrams of naloxone in two divided doses was given. Each naloxone bolus was followed by a 15-mm-Hg increase in mean arterial pressure and a return to values that were normal for this patient. Naloxone may be of value in reversing clonidine toxicity when clonidine is given to treat an acute rise in arterial pressure.

Further validation of in vivo and in vitro pharmacological procedures for assessing the alpha 2/alpha 1-selectivity of test compounds: (1). Alpha-adrenoceptor antagonists

Twenty one chemically dissimilar alpha-adrenoceptor antagonists were assessed in 7 different tests. The potencies of the compounds to protect rats from norepinephrine lethality were closely correlated with their potencies for inhibition of [3H]WB4101, but not of [3H]clonidine, [3H]idazoxan, [3H]rauwolscine or [3H]yohimbine binding to rat brain homogenates. The reverse was found for the potencies of the compounds to prevent the antidiarrheal effect of clonidine. The in vivo anti-norepinephrine/anti-clonidine potency ratios were highly correlated with the ratios between the in vitro potency to inhibit [3H]WB4101 binding on one hand and [3H]clonidine, [3H]idazoxan, [3H]rauwolscine or [3H]yohimbine binding on the other. The reliability of the above models for measuring alpha 1- and alpha 2-adrenoceptor antagonistic activity and selectivity is discussed.

The potentiation of morphine-withdrawal jumping by clonidine is antagonized by m-chlorophenylpiperazine and not by haloperidol

Administration of naloxone (0.5 mg/kg i.p.) to morphine-dependent rats induced a strong withdrawal syndrome consisting of body shakes, escape jumping and various autonomic signs. Clonidine (750 micrograms/kg s.c.) had a dual action on naloxone-precipitated withdrawal symptoms in rats: a suppressive action on body shakes and a potentiating action on jumping. Both actions were found to be mediated by alpha 2-receptors which generally are responsible for sedative effects. Therefore, the action of alpha 2-agonists such as clonidine and azepexole was studied more extensively. Since both serotonergic and dopaminergic systems have been suggested to be involved in morphine-withdrawal jumping, the interaction of clonidine and azepexole with serotonergic and dopaminergic drugs was studied. Neither haloperidol (0.3 mg/kg i.p.) nor the benzamides sulpiride (40 mg/kg p.o.) and metoclopramide (8 mg/kg i.p.) affected the jumping potentiated by the alpha 2-agonists. However, the serotonin-agonist m-chlorophenylpiperazine (2.5 mg/kg i.p.) suppressed the effects of clonidine. Precipitation of jumping in morphine-dependent animals was more effective using bremazocine (1.0 mg/kg i.p.). This effect again could be potentiated by clonidine (750 micrograms/kg s.c.). A low dose of m-chlorophenylpiperazine (0.03 mg/kg i.p.) antagonized the clonidine-effect without affecting the basal jumping response. The data suggest that clonidine potentiates naloxone-precipitated jumping by decreasing serotonergic output while the administration of m-chlorophenylpiperazine can counteract this lack of serotonergic activity.

Comparative studies on antidepressant action of alprazolam in different animal models

Alprazolam, a new benzodiazepine from triazolobenzodiazepine group, produced anxiolytic action in the conflict test with potency similar to that of diazepam. The myorelaxant activity of the drug was relatively weak. Unlike desipramine, alprazolam failed to reduce the immobility of rats in the forced swim test and was unable to prevent clonidine-induced hypothermia. Alprazolam, unlike desipramine, failed also to potentiate behavioral effect of noradrenaline injected into the hippocampus. Alprazolam after acute but not chronic administration antagonized the synchronizing effect of clonidine on EEG pattern. On the other hand, alprazolam similarly to tricyclic antidepressants, prevented the suppression of dominance behavior by clonidine in rats competing for food. The results indicate that alprazolam acts only weakly upon noradrenergic mechanisms related to depression and to antidepressant action of drugs.

Effect of methoxy substitution on the adrenergic activity of three structurally related alpha 2-adrenoreceptor antagonists

We have recently reported the synthesis and alpha 2-antagonist activity of the methoxy derivative 2 [2-(2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline] and described the enhanced potency of this compound over the parent 1,4-benzodioxan, idazoxan, in reversing the inhibition caused by alpha 2-adrenoreceptor agonists of the electrically induced twitch in the rat or mouse vas deferens. It was of interest to us to discover whether a similar substitution in the structurally related alpha 2-adrenoreceptor antagonists piperoxan, prosympal, and fenmetazole would similarly enhance potency. We subsequently discovered that this was not so and potency was decreased markedly. In particular, that of the methoxy derivative of piperoxan was ca. 220 times less than the parent structure.

Effects of para-substitution on tissue levels and alpha-adrenoceptor antagonist properties of tolazoline

Tolazoline and a series of para-substituted analogues were examined in mice to determine the effects of para substitution on alpha-adrenoceptor antagonist potency and tissue disposition. alpha-Adrenoceptor antagonism was measured as abilities to attenuate the hypothermic or sedative actions of clonidine. In general, para substitution by electron-withdrawing metabolically stable groups (Cl, F) resulted in increased or unchanged brain levels of drug relative to tolazoline. The para substitution by electron-donating metabolically labile groups (CH3, OCH3) leads to reduced brain levels. Effects on alpha-adrenoceptor antagonist properties did not follow a similar pattern. Thus, increased or decreased antagonism of clonidine effects by para-chloro- or para-methyl-tolazoline, respectively, could be attributed solely to increased or decreased brain levels of drug. para-Fluorotolazoline did not antagonize clonidine but was present in brain at levels equivalent to those of tolazoline. para-Methoxytolazoline on the other hand could not be detected in any tissue but antagonised hypothermia more readily than sedation. These data indicate that the factors governing the disposition or alpha-adrenoceptor antagonist properties of tolazoline analogues are different and independent of each other.

The St 587-induced flexor reflex in pithed rats: a model to evaluate central alpha 1-receptor blocking properties

A combination of reserpine, nialamide and the specific alpha 1-adrenoceptor agonist St 587 [2-(2-chloro-5-trifluoromethylphenylimino)-imidazolidine] induced a marked increase of the electrically stimulated flexor reflex in pithed rats. This increased reflex - which lasted for more than 3 h - could be inhibited by the alpha 1-receptor antagonist prazosin but not by the alpha 2-adrenoceptor antagonist idazoxan (RX 781094). A range of different drugs was tested, including 17 neuroleptics. Most of the neuroleptics showed marked inhibition of the St 587-induced reflex. Bromperidol, butaclamol and fluperlapine were weak inhibitors while pimozide was without effect. The results showed close correlation to alpha 1-receptor affinities in vitro (r = 0.84, P less than 0.001) while no correlation was found to either D-2 receptor affinities or to 5-HT2 receptor affinities in vitro (r = 0.43, P less than 0.2 and r = 0.41, P less than 0.05, respectively). It is concluded that inhibition of the St 587-induced flexor reflex in pithed rats is an in vivo test model for central alpha 1-receptor blocking properties.

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.

An endogenous, non-catecholamine clonidine antagonist increases mean arterial blood pressure

We report here that topical application of clonidine displacing substance (CDS), an endogenous brain extract, directly in the nucleus reticularis lateralis (NRL) region of anaesthetized cats regularly produced hypertension. CDS (5 units) increased the mean blood pressure by 40 +/- 8%. Pretreatment of anaesthetized rabbits with intracisternal CDS (500 units) shifted to the right the dose-response curve obtained with clonidine alone injected the same way. This brain extract might be considered as an endogenous antagonist for the hypotensive effects of clonidine at least in the NRL region.

Suppression of the growth hormone (GH) response to clonidine and GH-releasing hormone by exogenous GH

GH release in response to clonidine and human GH-releasing hormone-(1-44) (hGHRH-44) was assessed in 11 boys (aged 7-14 yr) with short stature, who had normal GH secretion. The response to these 2 provocative stimuli was repeated after, respectively, 2 and 3 days of treatment with human GH (0.1 U/kg, im). Exogenous GH significantly blunted the response to both clonidine [the mean 2-h integrated serum GH concentration falling from 1050 +/- 350 (+/- SEM) to 749 +/- 297 ng/ml X min; P = 0.03] and hGHRH-44, the 2-h integrated GH concentration falling from 1553 +/- 358 to 547 +/- 202 ng/ml X min; (P = 0.03). Plasma insulin-like growth factor (IGF-II) concentrations did not change after GH administration. In contrast, plasma IGF-I (somatomedin-C) concentrations increased from 97 +/- 16 ng/ml before administration of GH to 142 +/- 32 ng/ml (P = 0.05) after two days and 149 +/- 23 ng/ml (P less than 0.01) after the third treatment day. However, no correlation was found between the changes in response to clonidine or hGHRH-44 and changes in circulating levels of IGF-I. Our data confirm the existence of GH-dependent feedback inhibition of GH release during childhood and suggest that this inhibition operates, at least in part, at the level of the pituitary. While participation of the IGFs/somatomedins in this feedback loop cannot be excluded, the inhibitory effects of exogenous GH do not depend directly on circulating plasma IGF-I or IGF-II levels.