Cardiovascular effects of alpha-adrenergic drugs: differences between clonidine and guanabenz

Guanabenz induced a pressor effect in pithed rats through postsynaptic alpha 2-adrenoceptors whereas clonidine activated both vascular alpha 1 and alpha 2-adrenoceptors. Previous treatment with prazosin, an alpha 1-antagonist, or depletion of the noradrenergic stores by reserpine produced supersensitivity to the pressor response to clonidine only, probably through postsynaptic alpha 1-adrenoceptors. The hypotension and bradycardia developed in normotensive rats after intravenous guanabenz administration were abolished by prazosin, whereas the central effects of clonidine were antagonized by both prazosin and yohimbine. Selective destruction of central noradrenergic neurons by [N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine] (DSP 4) or reserpine plus blockade of catecholamine synthesis by alpha-methyl-p-tyrosine abolished the hypotension and bradycardia produced by guanabenz but merely reduced the bradycardia from clonidine. The present results suggest that, in rats, guanabenz is a selective stimulant of central alpha-autoadrenoceptors antagonized by prazosin whereas at a vascular level guanabenz preferentially activates alpha-adrenoceptors antagonized by yohimbine. The differences observed between the mechanisms by which guanabenz and clonidine produce their central cardiovascular responses might be attributed to their acting on different nuclei.

Effect of nicorandil, N-(2-hydroxyethyl)nicotinamide nitrate, a new anti-anginal agent, on contractile responses to alpha-1- and alpha-2-adrenoceptor agonists in isolated rabbit aorta

Effects of nicorandil on contractile responses to alpha 1- and alpha 2-adrenoceptor agonists were examined in isolated rabbit aorta. Nicorandil (10(-6) or 10(-5) M) inhibited contractile responses to clonidine (CL) and BHT-920 in a concentration-dependent manner, but had no effect on the response to methoxamine (MO). Nifedipine (10(-6) and 10(-5) M) had no significant effect on responses to CL and MO, but it had a noticeable inhibitory effect on the response to BHT-920. In tissues pretreated with phenoxybenzamine, nicorandil (10(-5) M) inhibited the residual response to MO, and nifedipine (10(-5) M) inhibited responses to MO and CL. The relationship between maximum contraction and percent receptor occupancy was found to be nonlinear for MO, but was near linear for CL and BHT-920. The inhibitory effect of prazosin (pA2 of about 9) on MO and CL was much greater than that of yohimbine (pA2 of about 6). Nicorandil had no apparent or slight inhibitory effect on responses to potassium and Ca2+, and this inhibitory effect was much less than that of nifedipine. These results indicate that the responses induced by MO, CL, and BHT-920 in the rabbit aorta are due to activation of alpha 1-adrenoceptors. It is also suggested that nicorandil minimally affects voltage-dependent Ca2+ influx and that differential effects of nicorandil on the responses to alpha 1 and alpha 2 agonists may be the result of differences in the amount of receptor reserve that exist for MO, CL, and BHT-920 in this blood vessel.

Clonidine--induced behavioural despair in mice: reversal by antidepressants

The effects of various alpha 2 adrenoceptor agonists on forced swimming-induced despair behaviour were studied in mice. Clonidine, B-HT 920 and guanfacine significantly prolonged the total immobility duration. Clonidine-induced behavioural despair was antagonized by prior treatment with yohimbine. The tricyclic antidepressants imipramine, desipramine, trimipramine, amitriptyline, nortriptyline and doxepin, the MAO inhibitor tranylcypromine, and the antimanic agent lithium reversed clonidine-induced behavioural despair. Chronic treatment with imipramine evoked more pronounced reversal as compared to acute treatment. Amphetamine, a psychostimulant, inhibited clonidine-induced enhancement of immobility duration but diazepam, a skeletal muscle relaxant was without any effect. On the other hand, adenosine showed potentiation of the submaximal response of clonidine. These observations suggest that clonidine-induced behavioural despair is probably mediated through its presynaptic action on alpha 2 adrenoceptors, resulting in reduced central noradrenergic outflow. The present data proposes a simple test system to induce depression-like syndrome in animals, sensitive to antidepressant therapy.

Imidazole has similar behavioural effects to yohimbine

A number of animal behavioural models were used to study the activity of imidazole (IMID) on the central nervous system. IMID antagonized in a dose-related fashion penile erections (PE) as well as stretching and yawning (SY) elicited in male rats by B-HT 920, an alpha 2 and dopamine (DA) autoreceptor agonist. Inhibition of B-HT 920-induced PE and SY was also exhibited by haloperidol, a DA receptor blocker, and yohimbine, but not by prazosin, alpha 2 and alpha 1 receptor antagonists respectively. Moreover IMID behaved similarly to yohimbine in: 1) counteracting clonidine-induced hypothermia in mice; 2) antagonizing sedation and sleep induced by clonidine and B-HT 920 in chicks, while haloperidol was ineffective. When administered to sexually active rats before the copulatory test, IMID at low doses, significantly altered some aspects of mating, a result which is interpretable in terms of enhanced sexual arousal and resembling the aphrodisiac effect reported for yohimbine. The neurochemical mechanisms involved in these effects are discussed.

Discriminative stimulus properties of clonidine: substitution by ergot derivatives

Bromocriptine, lergotrile and lisuride but not apomorphine, ergonovine or lysergic acid diethylamide (LSD) mimicked clonidine in rats trained to discriminate this compound (0.02 mg/kg) from saline. BC 105, ketanserin and haloperidol failed to block the clonidine cue. Yohimbine was an effective antagonist of clonidine but not of the lisuride substitution for clonidine. Since dopamine does not appear to be involved in the stimulus effects of clonidine, the behavioral similarities between clonidine and some ergots may be related to alpha-adrenoceptor stimulation; however, the role of blood pressure changes in the stimulus effects of all of these compounds should not be overlooked.

Role of brain cholinergic system in the antihypertensive effect of clonidine in different models of rat hypertension

Acetylcholine in the central nervous system appears to be involved in some aspects of hypertension. Clonidine and methyldopa may inhibit acetylcholine (Ach) release in several brain areas. The present study was therefore designed to determine whether a depletion of brain Ach could modify the antihypertensive effect of clonidine in freely moving spontaneously hypertensive (SHR), Grollman hypertensive (GHR) and DOCA-salt hypertensive (DHR) rats. Intravenous injection of clonidine (15, 30 and 75 micrograms/kg) reduced mean arterial pressure (MAP) and heart rate (HR) in all hypertensive animals. The hypotensive effect was more marked in SHR than in DHR and GHR. The effect was also reproducible when the drug dose was repeated 3 h later in rats pretreated with saline (5 microliters) in the lateral cerebral ventricle (i.c.v.). When clonidine administration was repeated in hypertensive animals 3 h after i.c.v. hemicholinium-3 (20 micrograms/5 microliters), the decrease in MAP and HR was significantly reduced compared with that observed in the same animals after the first injection. The data suggest that the antihypertensive effect of clonidine depends partially upon the integrity of central cholinergic neurons.

Effects of nipradilol (K-351) on alpha-adrenoceptor mediated responses in various isolated tissues

Nipradilol competitively antagonized norepinephrine- or phenylephrine-induced contractile responses of guinea-pig thoracic aorta. These actions of nipradilol were about 6 times less potent than those of phentolamine. Nitroglycerin showed a non-competitive antagonistic action on norepinephrine-induced contractions of aorta. Furthermore, nipradilol competitively inhibited norepinephrine-induced contractions of rat vas deferens and dose-dependently reduced the phenylephrine-induced inhibitory responses in rabbit ileum. These antagonistic actions of nipradilol were 30 to 100 times less potent than those of phentolamine. Nitroglycerin did not appreciably affect these alpha-adrenoceptor mediated responses in rat vas deferens and rabbit ileum. The inhibitory action of clonidine on the twitch contraction of rat vas deferens produced by intramural stimulation was only slightly antagonized by nipradilol (pA2 = 5.4). Nipradilol and nitroglycerin showed a non-competitive antagonistic action on clonidine-induced contractions of canine saphenous vein after the exposure to phenoxybenzamine, while phentolamine competitively inhibited the clonidine responses. These results suggest that nipradilol possesses an alpha 1-adrenoceptor blocking action; it possesses very weak or practically no presynaptic alpha 2-blocking activity but shows a non-competitive antagonistic action on postsynaptic alpha 2-adrenoceptor mediated contractile responses.

Antagonistic effects of S9871 or (imidazolinyl-2)-2-dihydro 2,3 benzofurane and its stereoisomers on some central and peripheral actions of alpha 2-agonists

(+/-) and (+), but not (-) S9871 are new alpha 2-adrenoceptor selective antagonists. The effect of the racemic mixture and of the stereoisomers on cardiovascular and sedative responses to clonidine have been studied in rats and chickens, respectively. Blockade of central alpha 2-adrenoceptors was also measured as a recovery of the sympathoinhibitory effect induced by intravenous administration of B-HT 933 (azepexole). The potency profiles of these agents established in the central nervous system were confirmed in studies using the vas deferens in situ in the pithed rat. (+/-) and (+) S9871 blocked and antagonized some centrally mediated effects of clonidine such as the depressor response to both intravenous and intracerebroventricular administration. However, the return of arterial pressure to the control value, after intravenous administration of (-) S9871, does not result from an antagonistic action on alpha 2-adrenoceptors, since the depressor effects of clonidine were not blocked, but could be explained by alpha-agonistic properties of (-) S9871. (+/-) and (+) S9871 also blocked and antagonized the hypotensive and bradycardic action induced by intravenous administration of B-HT 933. The loss of the righting reflex induced by clonidine in the chicken was prevented by (+/-) and (+) S9871, as shown by a shift of the dose-response curve to clonidine to the right by both agents; on the contrary, (-) S9871 potentiated the sedation induced by clonidine. In the pithed rat, intravenously administered (+/-) and (+) S9871 fully antagonized the inhibitory effects of clonidine on the electrically induced contractions of the vas deferens. These observations are consistent with a selective alpha 2-adrenoceptors antagonistic effect of (+/-) and (+) S9871 at central and peripheral alpha 2-adrenoceptors.

Reduction of centrally-stimulated gastric acid secretion by tizanidine, a new imidazoline derivative, in anesthetized rats

The effect of a novel imidazoline derivative (tizanidine) on stimulated gastric acid secretion was studied in the perfused stomach of anesthetized rats. Tizanidine, which did not prevent peripherally-stimulated gastric acid secretion, inhibited 2DG- or TRH-stimulated gastric acid secretion. Yohimbine and phentolamine reduced the inhibition of TRH-stimulated acid secretion by tizanidine. Clonidine was found to have similar effects to tizanidine at a lower dose. These results indicate that tizanidine may inhibit gastric acid secretion via the central alpha-adrenergic system similar to clonidine in anesthetized rats.

Behavioural pharmacology of imidazole, a potential antidepressant agent

Imidazole (IMID) inhibited (+/-) N-n-propylnorapomorphine (NPA) and BHT-920 induced penile erections (PE) and stretching and yawning (SY) in rats as well as apomorphine (APO) induced hypothermia in mice, enhanced shock-elicited aggressiveness in rats and antagonized sleep induced by clonidine in chicks. IMID moreover displayed activity in behavioural tests used in specific screening for antidepressants, potentiating yohimbine toxicity in mice and antagonizing immobility time in the despair test, with a potency in some cases equal to imipramine. IMID per se, depressed motor activity in both mice and rats. The possible mechanism of action and receptors involved are briefly discussed as well as IMID's profile as an antidepressant drug.

Differential blocking actions of idazoxan against the inhibitory effects of 6-fluoronoradrenaline and clonidine in the rat vas deferens

The prejunctional inhibitory effects of clonidine and 6-fluoronoradrenaline (6-FNA) have been evaluated in the isolated prostatic segment of the rat vas deferens, against the twitch response evoked by low frequency (0.1 Hz) field stimulation. The inhibitory potency of 6FNA was significantly increased in the presence of cocaine (1 microM) or pargyline (10 microM), but was not modified in the vas deferens from rats pretreated with reserpine when the endogenous levels of noradrenaline (NA) were decreased by 97%. Clonidine was significantly more potent than 6-FNA as an inhibitory agonist, and the potency of clonidine was not modified after cocaine, pargyline or reserpine. The alpha 2-adrenoceptor blocking agent idazoxan, was a competitive antagonist against the inhibitory effects of clonidine under all experimental conditions. In contrast, the only antagonism shown by idazoxan against the inhibitory effects of 6-FNA was in the presence of cocaine (1 microM), and this antagonist effect of idazoxan was not concentration-related. Low concentrations of 6-FNA caused concentration-dependent facilitatory effects on the twitch response, which were significantly greater after treatment with idazoxan (1 microM) in reserpine-treated vas deferens. These facilitatory effects of 6-FNA were always observed in the presence of prazosin (300 nM) and also after treatment of the preparations with phenoxybenzamine (10 microM), a concentration which abolished the inhibitory actions of both clonidine and 6-FNA. The facilitatory effects on the twitch response induced by low concentrations of 6-FNA are therefore unlikely to be due to either alpha 1- or alpha 2-adrenoceptor stimulation. In conclusion, the failure of idazoxan to block the inhibitory effects of 6-FNA, while exerting a potent competitive antagonism of clonidine-induced inhibitory effects, supports the proposal that alpha 2-adrenoceptors may in fact be subdivided into two subclasses, involving imidazoline and phenylethylamine recognition sites.

Antinociceptive action of tizanidine in mice and rats

The antinociceptive action of tizanidine [5-chloro-(2-imidazolin-2-yl-amino)-2,1,3-benzothiadiazole], a centrally acting muscle relaxant, was evaluated after subcutaneous or peroral administration in mice and rats. Tizanidine strongly inhibited the writhing response induced by acetic acid, phenyl-p-benzoquinone and acetylcholine in mice, and its potency was found to be greater than that of morphine. Tizanidine showed antinociceptive action like morphine not only in tail pressure and electrical stimulation tests in mice but also in tail-flick tests in mice and rats. The antinociceptive action of tizanidine was unaffected by pretreatment with naloxone. These findings suggest that tizanidine develops relatively strong antinociceptive action by a nonopioid mechanism.

Effect of clonidine and naloxone on the pressor response during contraction of cat hind-limb muscles

The possible involvement of an adrenergic-endorphin system in the mediation of the pressor response to isometric muscular contraction was studied in cats. Fatiguing contractions of the gastrocnemius and plantaris muscles caused an increase in the mean arterial blood pressure by 35 to 70 mmHg. Intravenous infusion (30 micrograms X kg-1) as well as intracisternal injection (2.5 micrograms) of clonidine-HCl eliminated the pressor response to muscular contraction. In both sets of experiments, the mean blood pressure remained at the resting level throughout the duration of the isometric contraction. Injection of naloxone (0.5 mumol X litre-1) into the cisterna magna did not alter the resting blood pressure and did not affect the rise in mean arterial pressure during muscle contractions. Intracisternal injection of naloxone (0.5 mumol X litre-1) prior to an intracisternal injection of clonidine (2.5 micrograms) did not alter the resting blood pressure but effectively antagonised the anti-pressor effects of clonidine during fatiguing isometric contractions. These data may indicate that activation of muscle "ergoreceptor" afferents (group III and IV fibres) during muscular contraction may cause an increase in the arterial blood pressure by interfering with an inhibitory adrenergic-endorphinergic pathway in the brainstem.

Adrenoreceptor interactions of the enantiomers and metabolites of mianserin: are they responsible for the antidepressant effect?

Mianserin is a tetracyclic antidepressant whose postulated mechanism of action involves release of noradrenaline mediated via cortical alpha 2-adrenergic autoreceptor blockade. This property resides stereoselectively in the S(+)-enantiomer of mianserin, which is also more potent in behavioural tests indicative of antidepressant activity, and in the reversal of clonidine-induced effects. Cortical receptor binding studies have indicated that although a similar stereoselectivity prevails for the inhibition of both alpha 2-binding (clonidine) and alpha 1-binding (prazosin), mianserin and its enantiomers are more potent antagonists at alpha 2- than at alpha 1-binding sites. However, no stereoselectivity is apparent for the antagonism of cortical alpha 2-heteroreceptors controlling serotonin release. Following chronic administration, (+/-)- and S(+)-mianserin, but not the R(-)-enantiomer, produce functional supersensitivity at alpha 2-autoreceptors which is unaccompanied by changes in clonidine binding. Neither mianserin nor its enantiomers alter the sensitivity of alpha 2-heteroreceptors following chronic administration. Like mianserin and its S(+)-enantiomer, but unlike R(-)-mianserin and the 8-hydroxy metabolite, the desmethyl metabolite inhibits noradrenaline uptake in vitro. 8-hydroxymianserin and, to a lesser extent, desmethylmianserin release noradrenaline from cortical slices via alpha 2-autoreceptor antagonism, but only the 8-hydroxy metabolite blocks alpha 2-autoreceptors and alpha 2-heteroreceptors in synaptosomal preparations. It is likely that S(+)-mianserin, desmethylmianserin, and 8-hydroxymianserin contribute substantially to the overall facilitating effect of mianserin on noradrenergic transmission in vivo. As yet it is unclear whether this effect is exclusively responsible for the antidepressant activity of mianserin or whether the stereoselectivity also shown by the mianserin enantiomers towards serotonin receptors plays a complementary role.

The interaction of cimetidine with various antihypertensive agents in the spontaneously hypertensive rat

The H2-receptor antagonist cimetidine (250 micrograms) administered intracerebroventricularly (i.c.v.) 15 and 30 min before clonidine (25 micrograms kg-1 i.v.), significantly antagonized clonidine-induced hypotension in anaesthetized spontaneously hypertensive rats. The hypertensive response of cimetidine was correlated with the inhibition of clonidine-induced hypotension. In addition, cimetidine (250 micrograms i.c.v.) counteracted the hypotensive effects of pentolinium (5.0 mg kg-1 i.v.), guanethidine (5.0 mg kg-1 i.v.) and minoxidil (1.0 mg kg-1 i.v.) These data do not support previous suggestions that the hypotensive action of clonidine is caused by stimulation of the H2-receptor, but suggest that central administration of cimetidine causes peripheral vasoconstriction and this may offer resistance to the hypotensive action of different antihypertensive agents.

A difference in the affinity of some selective alpha 2-adrenoceptor antagonists when compared on isolated vasa deferentia of rat and rabbit

The alpha 2-adrenoceptor antagonist potencies of the benzoquinolizines (Wy 26 703, Wy 25 309, Wy 26 392), the benzodioxans (RX 781 094, RS 21 361), yohimbine and rauwolscine have been compared at presynaptic alpha 2-adrenoceptors in the isolated vasa deferentia of the rat and rabbit. Yohimbine and rauwolscine are of equal potency as antagonists in both the rat and rabbit against the agonists clonidine or UK 14304. The benzoquinolizines and benzodioxans are very weak antagonists of clonidine or UK 14304 at the presynaptic alpha 2-adrenoceptors of the rabbit vas deferens when compared to their potency at the presynaptic alpha 2-adrenoceptors of the rat vas deferens. This suggests that the presynaptic alpha 2-adrenoceptors present in the rat vasa deferentia may be different from those present in the rabbit vasa deferentia.

Stereospecific reduction by narcotic antagonists of clonidine-induced food intake

The present study examined the effect of opiate antagonists on clonidine-induced feeding in rabbits. The change in food intake induced by clonidine was blocked by naltrexone. The active (-)-isomer of the antagonist 5,9 alpha-diethyl-2-(3-furylmethyl)-2'-hydroxy-6,7-benzomorphan had an effect similar to naltrexone. Similar doses of the (+)-isomer were inactive, except at the highest dose used in the study. The results suggest that opiate antagonists block feeding elicited by a specific noradrenoreceptor agonist and that this inhibition is due to a direct interaction with opiate systems.

Pharmacological analysis of alpha-2 adrenergic mechanisms in nociception and ataxia

In order to assess the involvement of alpha-2 adrenergic receptors in nociception, the in vitro potencies of seven alpha-2 adrenergic agonists (clonidine, guanabenz, guanfacine, BHT-920, ICI 106270, xylazine and lofexidine) were compared with their ability to prevent the writhing response elicited by i.p. administration of phenyl-p-guinone. Administration of each compound elicited antinociception, and this effect was attenuated by pretreatment with the alpha-2 adrenergic antagonists, yohimbine. The potency of these compounds to cause antinociception was correlated with their potency to displace [3H]clonidine from its binding site on brain membranes and with their ability to inhibit the twitch of the electrically stimulated vas deferens, suggesting an alpha-2 involvement in the antinociceptive action. In addition to causing antinociception, administration of these agonists also impaired rotorod performance in mice. These agonists were 2.5 to 72 times more potent in inhibiting writhing than in impairing rotorod performance, and, except for ICI 106270, there was a correlation between antinociceptive and ataxic potency. ICI 106270 was a notable exception to this correlation, however, producing only minimal ataxia, which unlike the other agonists was not reversed by yohimbine. These results indicate that alpha-2 adrenergic agonists can produce antinociception and further suggest that this may be dissociable from the ataxia.

Naloxone reverses the antinociceptive action of clonidine in spontaneously hypertensive rats

Earlier studies have shown that the antihypertensive action of clonidine is reversed by naloxone in hypertensive (SHR), but not in normotensive rats (WKY). We investigated the effects of clonidine and naloxone on pain sensitivity of SHR and WKY by using the formalin test (FT) and the tail-flick test (TFT). Using the FT, basal pain sensitivity was similar in SHR and WKY. Clonidine produced dose-dependent analgesia (0.03-0.15 mg/kg i.p.), and it was more potent in SHR than in WKY. The effect of clonidine was partially antagonized by naloxone (2 mg/kg i.p.) in SHR, but not in WKY. Naloxone alone caused moderate analgesia in SHR and no effect in WKY. Using the TFT, SHR displayed a naloxone-reversible decrease in basal pain sensitivity, when compared to WKY. Clonidine was ineffective (WKY) or caused moderate hyperalgesia (SHR). These results indicate that the two pain tests activate different pain controlling mechanisms, with different sensitivity to the antinociceptive action of clonidine. In SHR, this action seems to involve the release of endogenous opioids.

Cirazoline, an alpha 2-adrenoceptor antagonist in guinea-pig ileum

Prejunctional effects of cirazoline have been investigated in guinea pig ileum. At high concentrations cirazoline has been shown to have antimuscarinic activity, pA2 5.25 +/- 0.29. At concentrations below those producing blockade of acetylcholine, cirazoline blocks the prejunctional alpha 2-adrenoceptor activity of clonidine, pA2 6.81 +/- 0.22, and alpha-methylnoradrenaline. Results are discussed in the light of controversial evidence for the activity of cirazoline on alpha-adrenoceptors.

Thyrotropin releasing hormone and naloxone attenuate the antihypertensive action of central alpha 2-adrenoceptor stimulation through different mechanisms

In various forms of shock, TRH is equivalent to naloxone in reversing the hypotension and improving the survival rate. The present findings indicate that in spontaneously hypertensive rats (SHR), TRH has another naloxone-like effect in antagonizing the antihypertensive response to clonidine and alpha-methyldopa. When given during the hypotensive response to alpha-methyldopa, both naloxone and TRH produce a pressor response. While this effect of naloxone is blocked by prazosin, the effect of TRH is not influenced by prazosin or hexamethonium but is inhibited by a vasopressin pressor antagonist. This suggests that the pressor response to naloxone is mediated by the sympathetic nervous system, whereas the similar action of TRH is independent of sympatho-adrenomedullary functions and it is mediated by vasopressin.

Calcium influx-dependent and -independent alpha 1-adrenoceptor-mediated processes of vasoconstriction in vivo do not operate via different alpha 1-adrenoceptor subtypes

In pithed rats, the selective alpha 1-adrenoceptor agonists St 587 and cirazoline show preponderant calcium influx-dependent and -independent vasoconstriction, respectively. By using these agonists, selective (competitive) antagonists for either process of vasoconstriction were sought. For this purpose, antagonism was analyzed for eight structurally different antagonists (prazosin, BE 2254, AR-C239, R 28935, corynanthine, phentolamine, sulpiride, and chlorpromazine) opposing the pressor responses evoked by cirazoline and St 587. Where pA2 values (-log dose antagonist evoking a twofold shift for the agonist dose-response curve) could be calculated, no significantly different pA2 values against either agonist resulted. However, with respect to the slopes of the Schild plots, deviations from unity were found for prazosin, R 28935, AR-C239, sulpiride, and chlorpromazine, but not uniformly against both agonists. Following treatment with phenoxybenzamine (PB) (30 micrograms/kg) and nifedipine (1 mg/kg), which produced calcium influx-sensitive and -insensitive vasoconstriction to cirazoline, respectively, Schild plots were constructed for BE 2254, prazosin, and chlorpromazine. Using cirazoline as an agonist, unity slopes were now obtained for prazosin and chlorpromazine. The Schild plots of BE 2254 versus cirazoline after PB or nifedipine administration, however, exhibited a slope deviating from unity. For prazosin and chlorpromazine, identical pA2 values still resulted against both processes of vasoconstriction to cirazoline. The results are compatible with the view that alpha 1-adrenoceptors mediating calcium influx-dependent and -independent vasoconstriction in vivo are not distinctly different entities, but are separate recognition sites of the same receptor.

Effects of clonidine and guanfacine in essential hypertension

Daily doses of 0.3 mg clonidine and 3 mg guanfacine were equiactive in decreasing blood pressure and heart rate in 17 subjects with essential hypertension. Clonidine decreased cardiac output and guanfacine decreased total peripheral resistance, while clonidine had no effect on stroke volume but guanfacine increased it. Both clonidine and guanfacine decreased plasma renin activity. Naloxone, 0.4 mg iv, reversed the antihypertensive effect of clonidine but was ineffective even at higher doses (1.6 mg iv) when subjects were treated with placebo or guanfacine. It is suggested that the hemodynamic differences between the two centrally acting alpha 2-adrenoceptor agonist antihypertensive drugs may at least in part result from the involvement of opioid mechanisms only in the action of clonidine.

Evidence for a peripheral component in the sympatholytic effect of clonidine in rats

In an attempt to assess separately the peripheral and central effects of clonidine on cardiovascular parameters and plasma catecholamine levels, the selective alpha 2-adrenoceptor antagonist idazoxan (RX 781094) was given either intravenously (i.v.) or intracerebroventricularly (i.c.v.) to anaesthetized rats before administration of intravenous clonidine. Plasma noradrenaline and plasma growth hormone concentrations were used as indices of peripheral sympathetic nervous activity and central alpha-adrenoceptor stimulation, respectively. Peripheral and central administration of idazoxan antagonized the cardiovascular responses to i.v. clonidine, 5 micrograms kg-1. However, idazoxan was more effective against the hypotension than the bradycardia induced by clonidine. Idazoxan 300 micrograms kg-1 i.v. and 50 micrograms i.c.v. prevented clonidine-induced falls in plasma noradrenaline and adrenaline. The results suggest that 50 micrograms idazoxan i.c.v. caused some blockade of peripheral as well as central alpha 2-adrenoceptors. Idazoxan, 10 micrograms i.c.v., caused similar inhibition of the hypotensive response to clonidine as 300 micrograms kg-1 i.v. and 50 micrograms i.c.v. but did not significantly inhibit the clonidine-induced fall in plasma noradrenaline concentration. Animals pretreated with i.v. or i.c.v. idazoxan had significantly lower levels of plasma growth hormone than vehicle-treated rats. Idazoxan 10 micrograms and 50 micrograms i.c.v. suppressed growth hormone secretion to the same extent. These results suggest that stimulation of peripheral, prejunctional alpha 2-adrenoceptors in anaesthetized rats may contribute to the fall in plasma catecholamines produced by i.v. clonidine, and confirm that the hypotensive effect is centrally mediated.

High doses of clonidine elicit jumping behavior in the mouse

Neuronal mechanisms involved in the jumping produced by high doses of clonidine were studied in adult mice. Intraperitoneal administrations of clonidine at doses ranging from 10 to 40 mg/kg elicited jumping behavior in dose-dependent manner, the highest frequency of jumping being observed at a dose of 30 mg/kg. The jumping behavior induced by clonidine (30 mg/kg) was decreased by pretreatment with physostigmine (0.2 mg/kg IP), apomorphine (2 mg/kg IP) and phentolamine (10 mg/kg IP), and conversely increased by atropine (5 mg/kg IP), apomorphine (0.25 mg/kg), haloperidol (2 mg/kg IP) and propranolol (10 mg/kg IP). The jumping behavior was unaffected by methylatropine (5 mg/kg IP) and mecamylamine (0.1 mg/kg IP). In addition, N6-cyclohexyl adenosine (0.2 mg/kg IP) or diazepam (2 mg/kg IP) did not affect, either. The result suggests that high doses of clonidine elicit jumping in which alpha-adrenoceptor activation, dopaminergic and cholinergic inhibition are concomitantly involved.

Adrenoceptor and tetrabenazine antagonism activities of some pyridinyltetrahydropyridines

A series of pyridinyltetrahydropyridine derivatives was synthesized and evaluated as adrenoceptor and tetrabenazine antagonists. 4-(3-Fluoro-2-pyridinyl)-1,2,5,6-tetrahydropyridine proved to be the most potent and selective alpha 2-adrenoceptor antagonist of the series as measured in vitro by displacement of [3H]clonidine and [3H]prazosin from membrane binding sites of calf cerebral cortex and by antagonism of the effects of clonidine and methoxamine in the rat isolated, field-stimulated vas deferens. In addition, this compound, and the corresponding desfluoro derivative, blocked tetrabenazine-induced ptosis in the mouse.

Treatment with clorgyline and pargyline differentially decreases clonidine-induced hypotension and bradycardia

A study was made of the effects of acute and chronic treatment with monoamine-oxidase (MAO) inhibitors on the peripheral and central cardiovascular response induced by clonidine in anaesthetized normotensive rats. Clonidine (30 nmoles X kg-1 i.v.) produced a biphasic change in mean blood pressure; an initial transient increase was followed by a prolonged hypotensive effect, coinciding with the maximal bradycardia. Twenty-four hours after acute (single) or chronic (daily for 7 days) administration of MAO inhibitors (pargyline 10 mg X kg-1 SC or clorgyline 0.3 mg X kg-1 SC) there was no effect either on the basal cardiovascular parameters or on the initial pressor response induced by clonidine. Chronic but not acute treatment with clorgyline, an inhibitor of type A MAO, greatly decreased the hypotension and bradycardia induced by clonidine for as long as 5 days after its discontinuation. On the other hand, after chronic administration of pargyline (10 mg X kg-1), a preferential type B MAO inhibitor, the hypotension and bradycardia caused by clonidine were differently affected. There was a reduction in the bradycardia up to the third day following the discontinuation of pargyline, whereas the hypotensive response induced by clonidine was only attenuated for 24 h and unaffected with a lower dose of pargyline (0.3 mg X kg-1). It is concluded that chronic administration of the type A MAO inhibitor, clorgyline, attenuates the central responses to clonidine through the reduction in sensitivity of brain alpha-adrenoceptors. Pargyline, that preferentially inhibits type B MAO, reduces only the bradycardia induced by clonidine. This result may indicate a different modulation of the receptors involved in this response to clonidine.

Simple behavioral measures of central alpha-adrenoceptor blocking activity of drugs

Behavior of mice exposed to open field was investigated after treatment with various alpha-adrenoceptor antagonists. In addition, the interaction with clonidine and exploratory response in rats were tested. Preferential alpha-2 antagonists were found to change characteristically the open-field behavior in mice: they consistently increased grooming and decreased rearing activity. Yohimbine, rauwolscine and piperoxane produced bell-shaped effects on grooming which were compatible with their alpha-1 antagonistic activity at higher doses. Similarly, these drugs antagonized the clonidine-induced hypoactivity at lower dose-ranges and decreased the exploration in rats after higher doses. Selective alpha-1 antagonists showed only depressant effects on various behavioral measures. The results of this study indicate that the selectivity of alpha-1 or alpha-2 receptor blockade in mice and rats can be reliably determined by using behavioral parameters.

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.

Antagonism of the antidiarrhoeal effect of clonidine and the lethal effect of noradrenaline in rats: a reliable procedure to evaluate the in-vivo alpha 1- and alpha 2-blocking activity of drugs?

Eight compounds with alpha-adrenergic blocking activity were tested for their ability to antagonize the antidiarrhoeal effect of clonidine (clonidine test) and the lethal effect of noradrenaline (noradrenaline test). Six of the compounds studied are alpha-adrenergic blocking agents with known alpha 2/alpha 1 selectivity. Two compounds, ketanserin (R 41 468) and butanserin (R 53 393), are 5-hydroxytryptamine S2-antagonists. The ED50-values (mg kg-1) obtained in the clonidine test were: phentolamine (0.34), RX781094 (0.34), yohimbine (0.51), piperoxan (9.36), butanserin (greater than 5.0), prazosin (greater than 10.0), phenoxybenzamine (greater than 40.0), and ketanserin (greater than 80.0). In the noradrenaline test the ED50's (mg kg-1) were: butanserin (0.014), prazosin (0.032), phentolamine (0.59), phenoxybenzamine (1.02), ketanserin (4.69), RX781094 (12.4), piperoxan (21.5), and yohimbine (25.0). The selectivity alpha 2/alpha 1-ratios (ED50 clonidine/ED50 noradrenaline were: yohimbine (0.020), RX781094 (0.027), piperoxan (0.44), phentolamine (0.58), ketanserin (greater than 39), prazosin (greater than 312), and butanserin (greater than 357). These results show that yohimbine and RX781094 are equipotent and relatively selective alpha 2-antagonists; piperoxan and phentolamine block both alpha 1- and alpha 2-receptors at closely related doses; ketanserin, prazosin and butanserin are selective blockers of alpha 1-receptors, ketanserin being very weak, prazosin and butanserin being very potent compounds in this respect. The potent and selective alpha 1-blocking activity of butanserin, combined to its 5-HT S2-antagonism makes butanserin a very interesting experimental drug in view of earlier reported data concerning the amplifying effects between 5-hydroxytryptaminergic and noradrenergic vascular mechanisms.

Effects of desipramine treatment upon central adrenoceptor function in normal subjects

Six normal subjects were given clonidine infusions after 0, 1 and 3 weeks of treatment with desipramine (2 mg/kgm) and at 1 and 3 weeks after withdrawal from desipramine. The sedative and hypotensive effects of clonidine were inhibited after one and three weeks of desipramine treatment, and returned to normal after stopping treatment without any rebound increase. Such a time-course can be explained in terms of the acute effects of the drug, no adaptive changes at receptors need be invoked. By contrast, the growth hormone response to clonidine tended to be increased after one week of desipramine, reduced after three weeks of treatment, and further reduced after discontinuation. Such a time-course is consistent with an adaptive down regulation at alpha 2 adrenoceptors in response to their acute stimulation, due to noradrenaline re-uptake blockade.

Endogenous opioid peptides: critical care implications

The recent evolution of our understanding of endogenous OPs has led to important new insights into the pathophysiology of many disease states. Opiate antagonism may provide the critical care physician with yet another lifesaving weapon. Opiate antagonists are not approved for human use in the various conditions discussed in this article; their use is strictly experimental and should be restricted to controlled trials. We look forward to continued research and clinical trials involving these agents.

Isolation of an endogenous clonidine-displacing substance from rat brain

An endogenous substance which specifically displaces clonidine, yohimbine and rauwolscine from rat brain alpha 2-adrenergic receptors, has been isolated. The new compound, designed clonidine-displacing-substance (CDS), has been partially purified by ion exchange chromatography, zone electrophoresis and high performance liquid chromatography (HPLC). CDS binds specifically to alpha 2-adrenergic receptors by competing with either alpha 2-adrenergic agonists or alpha 2-antagonists, but has no effect on the specific binding of [3H]prazosin to alpha 1-adrenergic receptors in rat brain membranes. In the course of isolation, CDS was shown to be neither the endogenous neurotransmitter (-)norepinephrine (NE) nor the guanyl nucleotide GTP which lowers the specific binding of alpha 2-agonists to the alpha 2-adrenergic receptors.

Pertussis toxin blocks the action of morphine, norepinephrine and clonidine on isolated guinea-pig ileum

Administration of pertussis toxin (60 micrograms/kg i.p.) to guinea-pigs blocked the ability of morphine, norepinephrine and clonidine to inhibit electrically stimulated contractions in the isolated ileum. The toxin reached its maximum effect 6 days after its administration. The effect of the toxin was reversible; a slow but full recovery of the response to morphine was observed in ilea from guinea-pigs treated with toxin 18 days before the experiment. It is suggested, based on the known action of pertussis toxin, that inhibition of adenylate cyclase through Ni (guanine-nucleotide regulatory protein) is involved in the acute action of morphine, norepinephrine and clonidine in the motoneurons of the myenteric plexus of the ileum.

Comparative effects of RX 781094, mianserin, yohimbine, rauwolscine and prazosin in reversing clonidine inhibition of MIX-stimulated lipolysis in hamster isolated white fat cells

Different alpha-adrenergic receptor antagonists: RX 781094, yohimbine and rauwolscine (selective alpha 2-adrenergic antagonists); mianserin (tetracyclic anti-depressant, antagonist at alpha 2-presynaptic autoreceptors) and prazosin (selective alpha 1-adrenergic antagonist) were used at concentrations ranging from 10(-7) to 10(-4) M, to reverse clonidine inhibition of MIX-stimulated lipolysis in the hamster fat cell. In this adipose tissue (like human but unlike rat) there co-exist prolipolytic beta-adrenoceptors and antilipolytic alpha-adrenoceptors. Although no effects were observed with prazosin, RX 781094 was ten times more potent than yohimbine or its isomer rauwolscine in reversing clonidine inhibition of the MIX-stimulated lipolysis. Mianserin was an effective blocker only from a concentration of 10(-4) M, consistent with its relative lack of specificity for alpha 2-adrenoceptors cited elsewhere. Overall these results confirm the utility of this model for testing compounds presumed to act at alpha 2-adrenoceptors.

Bupropion does not antagonize cardiovascular actions of clonidine in normal subjects and spontaneously hypertensive rats

Tricyclic antidepressants (TADs) are known to antagonize the hypotensive and sedative actions of clonidine. We compared the effects of bupropion and imipramine pretreatment on the acute hypotensive and sedative actions of clonidine in eight normotensive male subjects in a randomized, double-blind crossover study. Pretreatment with bupropion, 100 mg by mouth three times a day for 9 days, had no effect on baseline supine blood pressure (BP) and heart rate (HR) and did not modify the hypotensive, bradycardic, and sedative actions of clonidine. Imipramine, 25 mg by mouth three times a day for 9 days, increased supine and standing HR and decreased standing systolic BP. In half the subjects the hypotensive action of clonidine was reduced 40% to 50% by imipramine. The specific binding of 3H-yohimbine to alpha 2-receptors of platelet membranes was not affected by pretreatment with either antidepressant. In spontaneously hypertensive rats, 16 days of bupropion, 25 mg/kg subcutaneously, had no effect on baseline BP and HR and did not antagonize the hypotensive and bradycardic effects of clonidine, 5 mg/kg iv. Pretreatment with desipramine, 5 mg/kg subcutaneously for 16 days, accelerated baseline HR and reduced cardiovascular actions of clonidine. These observations suggest that not all antidepressants antagonize the effects of clonidine. If the negative interaction between TADs and clonidine is a result of sensitivity of alpha 2-receptors, these receptor changes are not the common denominator of antidepressant activity and may only be seen with TADs.

Changes in the effects of clonidine on left atrium and hindlimb vasculature of rats in various thyroid states. A study of the responsiveness of alpha 2-adrenoceptors in the cardiovascular system

Postsynaptic alpha 2-adrenoceptors have been reported to exist in various tissues, including vascular smooth muscle. In order to investigate the possibility of their mediating a positive inotropic change and clarify the influence of the thyroid on their responsiveness, we examined the effects of clonidine, a known alpha 2-agonist, on the isolated left atria and femoral vascular beds of rats which were made hypo-, hyper- or euthyroid. Clonidine caused a dose-dependent positive inotropic change in the hypothyroid rat atrium, which was thought to be due to its alpha 1-stimulating action because of the antagonistic effect exerted by either phentolamine (10(-6) M) or prazosin (10(-7) M), but not by yohimbine (10(-7) M) or cimetidine (10(-5) M). In the hyperthyroid rat atrium, clonidine exerted a negative inotropic effect at high concentrations, which was thought to be due to its direct action on the cardiac muscle. Clonidine did not cause any inotropic change in the euthyroid rat atrium. Thus, an inotropic change mediated by the postsynaptic alpha 2-adrenoceptors could not be demonstrated in the rat heart. In the experiment involving hindlimb perfusion, clonidine caused vasoconstriction which was antagonized by yohimbine (10 micrograms/min). This effect was significantly augmented in the hypothyroid rats but not changed in the hyperthyroid ones. The vasoconstrictive effect of phenylephrine was found to be reduced in both hypo- and hyperthyroid rats. These results suggest that, in the peripheral vascular system, thyroid function also influences the postsynaptic alpha 2-adrenoceptors, but not in the same way as it affects the alpha 1-adrenoceptors.

Effect of clonidine on neuromuscular transmission and the nicotinic acetylcholine receptor

The effects of clonidine on neuromuscular transmission were investigated in the mouse phrenic nerve-diaphragms and chicken biventer cervicis. Clonidine inhibited the indirect twitch response dose-dependently and reversibly without an effect on the direct response of the muscles to electrical stimulation and KCl. This effect was antagonized effectively by diaminopyridine but not by yohimbine, phentolamine or physostigmine. The quantal content was not affected although the amplitudes of end-plate potential (epp) and spontaneous miniature epp (mepp) were markedly depressed. Clonidine also decreased the slope of the ACh dose-response curve and maximal response in denervated mouse diaphragms as well as the carbachol response in the chinck muscle. In the latter, ACh response was not depressed by clonidine probably because of its inherent anticholinesterase activity. Clonidine facilitated the fading of ACh-contracture either in mouse or chick muscle. It is concluded that clonidine impairs the neuromuscular transmission by a noncompetitive blockade of ACh receptors, most likely affecting the ACh channel but not the recognition site of the ACh receptor. Its inhibitory effect is not mediated by alpha 2-adrenoceptor, suggesting that there is no alpha 2-adrenoceptor on the motor nerve terminal to modulate the transmitter release.

Partial blockade by naloxone of clonidine-induced increase in plasma growth hormone in hypertensive patients

Studies in animals and man indicate a functional interaction between the adrenergic and the opiate systems. In the present study the effect of the opiate receptor blocker naloxone on the increase in plasma GH induced by clonidine was investigated in eight patients with essential hypertension. In a randomized order the patients received a bolus dose of naloxone (10 micrograms/kg) or physiological saline followed by a slow infusion of naloxone (5 micrograms/kg X h) or saline, respectively. Fifteen minutes after the bolus dose of saline or naloxone, clonidine (3 micrograms/kg) was infused for 10 min. Clonidine induced a significant increase in plasma GH. Naloxone pretreatment resulted in a significantly reduced GH increase after the clonidine infusion. These results indicate that the clonidine-induced increase in plasma GH in hypertensive man is partly mediated via activation of opiate receptors which can be blocked by naloxone.

A study of the effects of clonidine on the EEG in rats treated with single and multiple doses of antidepressants

The influence of repeated and single administrations of desipramine, amitryptiline, and mianserin on the EEG effects of clonidine has been investigated in rats implanted with chronic cortical electrodes. Clonidine induced a dose-dependent EEG synchronization in control animals. Signs of behavioral depression occurred after administration of moderate (0.1 mg/kg) and higher (0.2 mg/kg) doses of clonidine. Single doses of desipramine and amitryptiline attenuated the clonidine effect, while mianserine potentiated clonidine-induced synchronization. Antidepressants given once daily for 14 days completely (desipramine and amitryptiline) or partially (mianserin) reduced the effect of clonidine. Antidepressants alone produced only a slight effect on cortical EEG pattern.

Differential blocking effects of prazosin and yohimbine on vasopressor responses to sympathetic nerve stimulation and intravenous norepinephrine in the pithed rat

The effects of prazosin and yohimbine on the vasopressor response to sympathetic nerve stimulation, and to i.v. administration of norepinephrine were studied in the pithed rat to ascertain whether prazosin and yohimbine would preferentially block pressure responses due to exogenous versus endogenous alpha-adrenergic receptor activation. Prazosin (3 and 10 micrograms/kg, i.v.) was more effective in blocking the response due to sympathetic nerve stimulation than that due to i.v. norepinephrine. On the other hand, yohimbine (0.3 and 1 mg/kg, i.v.) produced greater inhibition of the i.v. norepinephrine response than the sympathetic nerve stimulation response. Yohimbine at the 0.1 mg/kg dose enhanced the nerve stimulation response while at higher doses the response was either unchanged (at 0.3 mg/kg) or substantially reduced (at 1 mg/kg). In this model, prazosin and yohimbine showed dose-related blocking effects on the pressor response of phenylephrine and clonidine, respectively. The results suggest that prazosin and yohimbine preferentially block the pressor responses of postsynaptic alpha-adrenergic receptor activation due to endogenous and exogenous norepinephrine, respectively. The diverse effects of yohimbine on the sympathetic nerve stimulation response may be due to an action on both the presynaptic (low dose) and postsynaptic (high dose) alpha-2 adrenergic receptors in vascular smooth muscle.

Sgd 101/75 is distinguished from other selective alpha 1-adrenoceptor agonists by the inhibition of its pressor responses by calcium entry blockade and vasodilatation in pithed rats and cats

The vasopressor effects of the selective alpha 1-adrenoceptor agonist Sgd 101/75 (2-[2-methylindazol-4-imino]-imidazolidine HCl) were analyzed in pithed rats and cats. Vasodilatation by the beta 2-adrenoceptor agonist salbutamol (1 mg/kg i.v.) or by the converting enzyme inhibitor captopril (5 mg/kg i.v.) antagonized the vasoconstriction by Sgd 101/75 in pithed rats. The effect of salbutamol was abolished by restoration of the baseline diastolic pressure by infusion of vasopressin. Calcium entry blockade by nifedipine (0.1-3 mg/kg i.v.) and (-)-verapamil (0.3 and 1 mg/kg i.v.) dose dependently inhibited the rise in the diastolic pressure induced by Sgd 101/75 pithed rats. This inhibition could not be attenuated by an infusion of vasopressin. In pithed cats, nifedipine most effectively antagonized the pressor effects of Sgd 101/75. In this respect, Sgd 101/75 is different from other alpha 1-adrenoceptor agonists, which are known to elicit a vasoconstriction which is virtually insensitive to vasodilatory measures and calcium entry blockade. These findings may be explained on the basis of a further subdivision of vascular postjunctional alpha 1-adrenoceptors.

Pyridinylpiperazines, a new class of selective alpha 2-adrenoceptor antagonists

A series of 1-(2-pyridinyl)piperazine derivatives was synthesized and evaluated for adrenergic activity. In vitro activity was assessed through the antagonism of clonidine's effect in the rat, isolated, field-stimulated vas deferens and by the displacement of [3H]clonidine from membrane binding sites of calf cerebral cortex. Antagonism of clonidine-induced mydriasis in the rat was used as an in vivo assay. Several members of the series proved to be potent, selective alpha 2-adrenoceptor antagonists. 1-(3-Fluoro-2-pyridinyl)piperazine was more potent than either yohimbine or rauwolscine in displacement of [3H]clonidine and had a higher affinity for this binding site (alpha 2) than for the [3H]prazosin site (alpha 1). In vivo, the 3-F derivative was more potent than the reference standards in reversing clonidine-induced mydriasis. None of the members of this series was more selective or potent than rauwolscine in antagonizing clonidine in the rat vas deferens.

Failure of naloxone to reverse clonidine toxic effect

Acute ingestions of imidazoline compounds, including clonidine hydrochloride, are a recognized clinical entity. The signs and symptoms of this overdose superficially resemble those of opiate intoxication, leading to attempts to reverse this poisoning with naloxone hydrochloride. Despite previous descriptions of success using naloxone in acute clonidine poisoning, five cases of acute pediatric ingestions of clonidine ranging from mild to severe occurred in which naloxone hydrochloride in doses up to 0.1 mg/kg was unsuccessful in reversing the signs and symptoms of this intoxicant. Although naloxone can be safely administered to children who ingest clonidine to distinguish them from those who are intoxicated by opiates, it is not therapeutically useful to use naloxone to reverse poisoning with this class of agents.

Spinal clonidine inhibits neural firing in locus coeruleus

Intrathecally-administered clonidine inhibited the spontaneous firing of single neurons in the pontine nucleus locus coeruleus of rats. Such inhibition of neuron firing was not observed when the non-lipophilic alpha 2 adrenoceptor agonist (oxymetazoline) was administered intrathecally. It is concluded that lipophilic drugs like clonidine, when administered intrathecally, can have profound supraspinal actions and thus caution should be exercised in interpreting the sites of action of such drugs.

Failure of naloxone to reduce the clonidine induced reduction of blood pressure and plasma noradrenaline in patients with essential hypertension

Studies in animals and man indicate a functional interaction between the adrenergic and the opiate systems. In the present study the effect of the opiate receptor blocker naloxone on the reduction of blood pressure and plasma noradrenaline induced by the alpha 2-agonist clonidine was investigated in nine patients with essential hypertension. In a randomised manner the patients received a bolus dose of naloxone (10 micrograms/kg) or physiological saline followed by a slow infusion of naloxone (5 micrograms/kg/h) or saline, respectively. Fifteen minutes after the respective bolus dose, clonidine (3 micrograms/kg) was infused over 10 minutes. Naloxone had no effect on the clonidine induced hypotension and reduction of plasma noradrenaline. Accordingly, there is no evidence that the clonidine induced reduction of blood pressure and plasma noradrenaline involves opiate receptors that can be blocked by naloxone. Plasma adrenaline increased significantly during the early phase of naloxone infusion.

Neurochemical lesion of the locus coeruleus of the rat does not suppress the sedative effect of clonidine

The locus coeruleus of male rats was destroyed bilaterally by injection of 6-hydroxydopamine. Rats injected with the vehicle and normal rats served as controls. Starting 20 days after the lesion, the locomotor activity of all rats was measured for 5 min every day. For the first 6 days, the lesioned rats were significantly less active than control rats; from the 7th to the 15th day, on the other hand, the locomotor activity of the two groups of rats was the same. From the 16th day onwards, the sedative effect of small doses of clonidine (2.5-100 micrograms/kg) was measured in lesioned and control animals. In spite of an almost total loss of noradrenaline in the cerebral cortex and hippocampus and a 33% loss of noradrenaline in the brain-stem of the lesioned rats, the sedative effect of clonidine was the same as in the control rats. This result suggests that the sedation produced by clonidine is not dependent on presynaptically located alpha 2-adrenoceptors.

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.

Naloxone does not antagonize the antihypertensive effect of clonidine in essential hypertension

Reports on spontaneously hypertensive rats suggested that naloxone blocked the antihypertensive effects of clonidine. We compared the effects of an 8-hr intravenous naloxone infusion (6 micrograms/kg/hr) or 5% dextrose in water (D5/W) begun 2 hr before single oral doses of clonidine (0.3 mg) in six men with mild to moderate essential hypertension (EHT). Supine and standing (after 5 min) blood pressure (BP) and heart rate (HR) were measured every 20 min. Initial treatment with naloxone or placebo (D5/W) infusion was randomly allocated, with the alternate treatment given 1 wk later. Naloxone did not modify either supine or standing BP or HR. Clonidine induced a gradual, sustained reduction in both supine and standing systolic and diastolic BP and in supine HR, and there was an increase in standing HR. Naloxone did not modify the onset, maximal effect, or recovery of the hypotensive and HR effects of clonidine in both the supine and standing positions. Our data indicate that hypotensive and bradycardiac effects of clonidine in EHT are not mediated by naloxone-sensitive opioid receptors. They also suggest that opioid receptors play no role in the maintenance of hypertension nor in the BP and HR adjustments induced by postural changes in EHT.

Effect of naloxone on the hypotensive action of clonidine in the conscious, normotensive goat

The possible involvement of opioid receptors in the effects of clonidine was studied in conscious, normotensive goats. Clonidine was infused intravenously followed by an intravenous injection of naloxone. Blood pressure was recorded through an intraarterial catheter. All doses of clonidine (0.2-7 micrograms/kg) significantly lowered blood pressure and decreased heart rate. Hypotension after 0.2-0.7 micrograms/kg of clonidine was significantly reduced by naloxone (1 mg/kg), while naloxone did not antagonize hypotension induced by the larger doses of clonidine. The bradycardiac effect of 0.7-2 micrograms/kg of clonidine was reduced by naloxone while that of 0.2 and 7 micrograms/kg was not. Clonidine also induced sedation and respiratory irregularity, but these changes were not modified by naloxone. Opioid mechanisms possibly participate in the hypotensive action of clonidine, but less in the normotensive animal than in spontaneously hypertensive animals.