Alpha-2 adrenergic receptor sensitivity and the mechanism of action of antidepressant therapy. The effect of long-term amitriptyline treatment

It has been hypothesized that the mechanism of action of antidepressant treatments is related to their ability to decrease the sensitivity of the alpha-2 adrenergic autoreceptor. In order to assess alpha-adrenergic autoreceptor sensitivity, the effects of clonidine, the alpha-2 adrenergic receptor agonist, on plasma levels of the norepinephrine metabolite 3-methoxy-4-hydroxyphenethyleneglycol (MHPG), blood pressure (BP) and patient-rated sedation were measured in nine depressed patients before and during amitripytline treatment. Postsynaptic alpha-2 adrenergic receptor sensitivity was assessed by determining the growth hormone (GH) response to clonidine before and during treatment. Amitriptyline significantly attenuated the effects of clonidine on plasma MHPG, standing systolic BP, and sedation, indicating that alpha-2 adrenergic autoreceptors had become subsensitive. In addition, baseline plasma MHPG levels were significantly reduced. Amitriptyline had no effect on the GH response to clonidine.

Failure of naloxone to reduce clonidine-induced changes in blood pressure, heart rate and sympathetic nerve firing in cats

An endogenous opiate mechanism may be involved in mediating the hypotensive effects of clonidine. In the cat anesthetized with alpha-chloralose, the effect of naloxone pretreatment on the lowering of blood pressure, heart rate and sympathetic nerve activity produced by increasing bolus i.c.v. injections of clonidine was studied. Central injections of clonidine (1-32 micrograms at 30-min intervals) decreased blood pressure, heart rate and renal nerve discharge in a dose-related manner. Effects on carotid sinus nerve activity were variable. Pretreatment with naloxone (3.2 mg x kg-1 i.v. or 1 mg i.c.v.) did not prevent the clonidine-induced reduction in blood pressure, heart rate and renal nerve activity. In some instances, the average blood pressure lowering responses to clonidine were greater in cats pretreated with naloxone compared with saline-pretreated animals. Changes in carotid sinus nerve activity were variable after clonidine in cats pretreated with i.v. naloxone. In contrast, sinus nerve activity decreased significantly after clonidine in cats pretreated with i.c.v. naloxone. Additional postclonidine naloxone injections (3.2 mg x kg-1 i.v. in all cats followed by 100 micrograms i.c.v. in the saline- and i.v. naloxone-pretreatment groups) also failed to consistently reverse the clonidine-induced changes in blood pressure, heart rate and sympathetic nerve activity. The results suggest that clonidine reduces blood pressure, heart rate and efferent sympathetic nerve firing in anesthetized normotensive cats by a mechanism independent of an opiate receptor interaction within the central nervous system.

Selective antagonism of the antinociceptive effect of intrathecally applied alpha adrenergic agonists by intrathecal prazosin and intrathecal yohimbine

We have attempted to define the alpha adrenoceptor subtype(s) on which intrathecally applied alpha adrenergic agonists act to produce their antinociceptive effect. The potencies of intrathecal (i.t.) prazosin or i.t. yohimbine to antagonize the elevations of thermal nociceptive threshold induced by i.t. 2-[2,6-diethylphenylamino]-2-imidazoline (ST-91), methoxamine or norepinephrine (NE) were determined in the rat. Tail-flick and hot plate tests were used to determine thermal nociceptive threshold. At the ID50 level, the alpha-2 selective antagonist yohimbine was significantly more potent than the alpha-1 selective antagonist prazosin at blocking the analgesia produced by the alpha-2 selective agonist ST-91, whereas prazosin was significantly more potent than yohimbine at antagonizing the analgesia produced by the alpha-1 selective agonist methoxamine or by the nonselective alpha agonist NE. Analgesic doses of methoxamine evoked a readily observable disturbance of motor and autonomic function, whereas such effects were not observed after analgesic doses of ST-91 or NE. Both i.t. methoxamine and i.t. ST-91 elevated thermal nociceptive threshold in rats depleted of lumbar spinal cord NE by pretreatment 7 days before with i.t. 6-hydroxydopamine. Our results suggest that stimulation of either one of two separate populations of postsynaptic spinal alpha adrenoceptors will inhibit spinal nociceptive transmission. One of these populations appears to be composed of alpha-2 adrenoceptors. The subtype classification of the alpha adrenoceptors composing the other population remains unclear.

Reversal of the central hypotensive effect of clonidine by intracisternal curare-like agents

Administration of tubocurarine, pancuronium, gallamine, or decamethonium into the cisterna magna of chloralosed dogs induced a rise in blood pressure. Clonidine (3 micrograms/kg) administered into the cisterna magna after tubocurarine, pancuronium, or gallamine significantly increased blood pressure; no significant change was found after decamethonium. The pressor response to clonidine after tubocurarine was antagonized by injection of the alpha 1-adrenoceptor-blocking agents AR-C 239 or prazosin into the cisterna magna at low doses prior to injection of clonidine. Yohimbine, a preferential alpha 2-adrenoceptor-blocking agent was ineffective. It is suggested that the pressor response to intracisternal clonidine after intracisternal tubocurarine is due to stimulation of alpha 1-adrenoceptor stimulation.

CGS 7525A, a new, centrally active alpha 2 adrenoceptor antagonist

CGS 7525A, a new tetracyclic compound, was evaluated for alpha 2 adrenoceptor antagonism in receptor binding assays and in behavioral and electrophysiological tests. 3H-Clonidine, but not 3H-prazosin, binding was potently inhibited in vitro by CGS 7525A. In vivo, CGS 7525A attenuated the suppressant action of clonidine on phenylquinone-induced writhing and on locus coeruleus neuronal firing rate. Mianserin was nearly equipotent with CGS 7525A in the 3H-clonidine binding assay, but considerably less potent in the measures of alpha 2 adrenoceptor antagonism in vivo. Both CGS 7525A and mianserin displaced 3H-spiroperidol binding from frontal cortex 5-HT2 binding sites. Although yohimbine resembled CGS 7525A in most respects, its activity at 5-HT2 binding sites was relatively low, CGS 7525A was not associated with any appreciable blockade of norepinephrine or serotonin uptake in vitro. Thus, CGS 7525A appears to be a promising new pharmacological tool for investigating the behavioral function of brain alpha 2 adrenoceptors.

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

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

Discriminative stimuli produced by clonidine: an investigation of the possible relationship to adrenoceptor stimulation and hypotension

In a leverpressing operant procedure, male rats were trained to respond for food reinforcement on one lever after an injection of clonidine (0.04 mg/kg) and to respond on an alternate lever for food reinforcement after an injection of saline. All 36 rats learned to discriminate the drug reliably from saline, thereby indicating that clonidine produces discriminative interoceptive stimuli. The discriminative stimulus was both dose- and time-dependent, with an ED50 of 0.018 mg/kg and an optimum time of action occurring from 15 to 60 min after injection. Although clonidine produced a reduction in response rate, this was not the basis of the discriminative stimulus as other drugs with similar depressant action did not generalize. The clonidine stimulus was dose-dependently antagonized by the alpha-2 adrenergic antagonist, yohimbine, whereas receptor antagonists of alpha-1 adrenergic, beta adrenergic, dopaminergic, serotonergic, cholinergic or opioid systems were ineffective in blocking the interoceptive stimulus produced by clonidine Lofexidine, guanabenz and methyldopa, all centrally acting hypotensive drugs that act through alpha-2 adrenoceptor mechanisms dose-dependently generalized to the clonidine cue, whereas hydralazine, minoxidil, propranolol and prazosin, hypotensive drugs acting through other mechanisms, did not generalize. These results suggest that clonidine produces interoceptive stimuli that are discriminable by rats and mediated through central alpha-2 adrenoceptor stimulation.

The effect of desipramine upon central adrenergic function in depressed patients

Eleven drug free patients meeting Research Diagnostic Criteria for Major Depressive Disorder have been treated with desipramine and given a clonidine infusion after 0, 1 and 3 weeks of treatment. The sedative and hypotensive effects of clonidine were significantly inhibited after three weeks of treatment with desipramine: a similar interaction was seen after one week of treatment although this just failed to reach statistical significance. The growth hormone (GH) response to clonidine was initially impaired, but increased significantly after one week of treatment. A significant reduction in the GH response occurred during the second and third weeks of treatment with desipramine. This last finding is interpreted as evidence of adaptive change of alpha 2 adrenoceptors: the other changes can be explained by the known ability of desipramine to block the re-uptake of noradrenaline.

Reduction in halothane anesthetic requirement by clonidine, an alpha-adrenergic agonist

The effects of clonidine, a potent central alpha-adrenergic agonist, and of tolazoline, an alpha-adrenergic antagonist, on the minimal anesthetic concentration (MAC) of halothane were studied in male mongrel dogs. Control halothane MAC was 0.8 +/- 0.04 vol% (determined in each dog by gas chromatography of arterial blood, n = 30). Clonidine, 5 microgram/kg (n = 10) and 20 microgram/kg (n = 10), give slowly intravenously, maximally reduced MAC by 42% (at 2.3 hours after clonidine) and 48% (at 2.6 hours after clonidine) for each dose. In another set of animals (n = 5) an alpha-adrenergic antagonist, tolazoline, 5 mg/kg IV, reversed the clonidine-induced reduction in halothane MAC rapidly and completely. Tolazoline alone, 5 mg/kg, (n = 5) had no significant effect on halothane MAC. Thus, the administration of the central alpha-adrenergic agonist clonidine decreased the required anesthetic concentration of halothane, as defined by MAC, by almost half. This effect, as it is reversed by tolazoline, is likely to be mediated through a central alpha-adrenergic receptor mechanism.

Relative potencies of alpha-1 and alpha-2 antagonists in the locus ceruleus, dorsal raphe and dorsal lateral geniculate nuclei: an electrophysiological study

The actions of some adrenoceptor antagonists were examined in single unit studies at alpha-1 adrenoceptors of the dorsal raphe nucleus and the dorsal lateral geniculate nucleus and alpha-2 adrenoceptors of the nucleus locus ceruleus. Prazosin, WB-4101 and corynanthine were found to be effective central alpha-1 adrenoceptor antagonists and piperoxane, yohimbine and rauwolscine were found to be effective central alpha-2 adrenoceptor antagonists. Prazosin, WB-4101 and corynanthine had no activity at alpha-2 adrenoceptors. However, piperoxane, yohimbine and rauwolscine in addition to their alpha-2 adrenoceptor actions, also displayed significant alpha-1 adrenoceptor antagonist properties. Thus, the latter drugs are not as selective for central alpha-2 adrenoceptors as in some other systems. This supports the suggestion that alpha adrenoceptor subtypes are not identical across all species and tissues.

Reversal of clonidine induced miosis by the alpha 2-adrenoreceptor antagonist RX 781094

Clonidine given i.v. at a dose of 0.1 mg and 0.2 mg was found to cause miosis in a placebo controlled double-blind study in six healthy volunteers. In a further single-blind placebo controlled study in three of these volunteers, the alpha 2-adrenoreceptor antagonist RX 781094 at a dose of 0.1 mg/kg i.v. reversed the miosis induced by i.v. clonidine 0.2 mg. At a dose of 0.05 mg/kg the miosis was partially reversed.

Clonidine and lidamidine (WHR-1142) stimulate sodium and chloride absorption in the rabbit intestine

The effects of clonidine and lidamidine on ion transport in the intestine of the rabbit were determined. In the ileum both clonidine (10(-6) M) and lidamidine (10(-3) M) (a) decreased the short circuit current (-1.9 +/- 0.3 and -2.0 +/- 0.4 muEq/h . cm2, respectively) and potential difference; (b) increased net sodium absorption (2.0 +/- 0.6 and 1.8 +/- 0.4 muEq/h . cm2) and chloride absorption (3.4 +/- 0.5 and 3.4 +/- 0.6 muEq/h . cm2); and (c) increased tissue conductance (8.7 +/- 1.7 and 10.0 +/- 1.6 mmho/cm2). The increase in net sodium and chloride absorption was primarily due to an increase in mucosal-to-serosal movement of the ions and a decrease in serosal-to-mucosal movement of chloride. The action of clonidine on the short circuit current was quantitatively similar to the action of epinephrine. Both were readily reversed by yohimbine, a specific alpha 2-adrenergic antagonist. Further, methoxamine, an alpha 1-adrenergic agonist has no effect on the short circuit current up to the concentration of 10(-5) M; and prazosin, an alpha 1-adrenergic antagonist, did not affect the change of the short circuit current induced by epinephrine. The results indicate the presence of alpha 2-adrenergic receptors on the intestine and suggest that alpha 2-adrenergic stimulation may account for the effect of epinephrine on ion transport. Lidamidine was studied because it is structurally related to clonidine and has many similar actions. Yohimbine transiently reversed the effect of lidamidine. alpha 1-Adrenergic or dopaminergic antagonists did not reverse the effect of lidamidine, suggesting that it may affect alpha 2-adrenergic receptors. The results indicate that both clonidine and lidamidine stimulate electrolyte absorption and may be clinically useful.

[Morphine antagonizes the central hypotensive effect of clonidine in the anesthetized cat]

A dose of 1 microgram/kg of clonidine injected intracerebroventricularly (I.C.V.) in the normotensive anaesthetized Cat elicits a significant and long lasting drop in the mean arterial blood pressure (23 +/- 5 mmHg) and the heart rate (34 +/- 5 beats/min.). A pretreatment with morphine (100 micrograms/kg, I.C.V.) completely prevents the central hypotensive effect of clonidine, whereas the bradycardia is only weakly influenced. Administered after clonidine, morphine reverses the hypotensive effect of clonidine whereas morphine does not affect its bradycardic action. These data demonstrate that morphine antagonizes the central hypotensive effect of clonidine.

Presynaptic effect of clonidine antagonized by the tetramine disulphide, benextramine

1 The presynaptic alpha-adrenoceptor blocking activity of the newly synthesized alpha-adrenoceptor blocking drug, benextramine, was evaluated in the isolated left atrium of the guinea-pig heart. 2 High-voltage stimulation increased the force of contraction of electrically driven atrial strips, presumably by releasing noradrenaline from sympathetic nerve endings. Like phentolamine, benextramine increased the effect of high-voltage stimulation, presumably by blocking presynaptic alpha-adrenoceptors. 3 Clonidine reduced the effect of high-voltage stimulation, presumably by stimulating presynaptic alpha-adrenoceptors. The inhibitory effect of clonidine was antagonized noncompetitively by benextramine and competitively by phentolamine. 4 Combined administration of benextramine and phentolamine only resulted in the competitive phentolamine antagonism. Thus phentolamine protected the presynaptic alpha-adrenoceptors against benextramine blockade.

Inhibition by D 600 of norepinephrine- and clonidine-induced responses of the aortae from normotensive (WKY) and spontaneously hypertensive rats (SHR)

Comparison was made of the inhibition by D 600 of the norepinephrine (NE)- and clonidine-induced responses of aortic strips from normotensive (WKY) and spontaneously hypertensive rats (SHR). Clonidine-induced responses of aortae from both strains of rats were markedly more susceptible to inhibition by D 600 than those of NE. Omission of calcium from the physiological solution resulted in a greater reduction in response to clonidine. Aortae from SHR were less susceptible to inhibition by D 600 but showed no difference from WKY rats in their responses in nominally calcium-free physiological solutions. Our results clearly indicate a greater dependence on extracellular calcium by clonidine-induced responses of the aortae relative to those of NE.

Chlorpromazine hyperalgesia antagonizes clonidine analgesia, but enhances morphine analgesia in rats tested in a hot-water tail-flick paradigm

Seventy-six male Sprague-Dawley rats were tested in a hot-water (55 degrees +/- 0.5 degrees C) tail-flick paradigm. Tail-flick latencies (TFL) were obtained at 30 and 15 min before intraperitoneal injection of either morphine (2.5, 5.0 and 10.0 mg/kg) clonidine (25, 50, 100 and 200 microgram/kg), chlorpromazine (CPZ, 2.5 and 5.0 mg/kg), dual injections of these drug combinations, or a saline control injection. Further TFL measures were taken immediately following drug administration and thereafter at 15 min intervals. The mean of the pre-drug TFL's served as each rat's baseline. All other TFL's were calculated as percentage changes from that baseline. Mean changes were determined for each treatment group and differences between groups, at each test time, were analysed. Our results demonstrated morphine and clonidine analgesia but CPZ hyperalgesia. The drug interaction studies revealed that morphine analgesia is enhanced by co-administration of either clinidine or CPZ but that clonidine analgesia is antagonized by chlorpromazine. These data suggest that morphine and clonidine exert their analgesic effects through different neurochemical mechanisms. It is particularly interesting that the clonidine-CPZ combination should result in TFL's similar to baseline levels, even though both drugs are sedatives. The investigation emphasizes the value of chlorpromazine as a pharmacological tool in analgesic research because of its ability to induce hyperalgesia even though it is a sedating agent.

Clonidine inhibition of pancreatic secretion in rats: a possible central site of action

The effects of clonidine on pancreatic secretion were studied in rats fitted with chronic or acute fistulas. Subcutaneous and intracerebroventricular injections of clonidine in conscious rats induced a dose-dependent inhibition of basal pancreatic secretion involving volume, bicarbonate output and protein output with an ED50 of about 10 micrograms/kg. Clonidine inhibition of pancreatic secretion was not dependent on the associated inhibition of gastric acid output. In conscious rats, the pancreatic inhibitory effect of clonidine was completely antagonized by yohimbine and slightly by piperoxane and prazosin. Propranolol, mianserin, naloxone and cimetidine did not antagonize the clonidine effect. Clonidine decreased the basal pancreatic secretion in anaesthetized rats and this action was completely reversed by yohimbine. Clonidine inhibited the pancreatic secretion stimulated by 2-deoxyglucose. This effect was reversed by yohimbine, while prazosin had no effect. Clonidine did not inhibit the pancreatic secretion induced by electrical stimulation of the vagus nerves. These results suggest that clonidine inhibition of pancreatic secretion is mediated through alpha 2-adrenergic receptors, and at least in part by a central nervous system mechanism. Yohimbine alone increased basal pancreatic secretion in conscious rats. This suggests that alpha 2-adrenergic receptors might be involved in the physiological nerve tone to the pancreas.

Influence of salbutamol and other beta-agonists on hypothermia induced by clonidine, apomorphine and reserpine in mice

Clonidine administered to mice induced hypothermia and this effect was antagonized by a single dose of the following beta-agonists: salbutamol, isoprenaline and terbutaline. The effect of these beta-agonists was completely antagonized by propranolol. The effect of terbutaline on reserpine-and apomorphine-induced hypothermia was also studied; it causing a slight but significant reversal of hypothermia. This effect was also blocked by propranolol. The results obtained here showed these beta-agonists behave in a manner similar to other antidepressant drugs. The data reinforces the view that beta-stimulation is of great importance in antidepressant action.

Lack of antagonism in the antihypertensive effects of clonidine and prazosin in man

1. In four patients, whose hypertension was not controlled (supine blood pressure greater than 150--90 mmHg) by addition of 5 mg/day of prazosin to previous treatment (diuretics and beta-blockers), clonidine (150--450 microgram/day) led to control of hypertension in three cases. 2. In ten patients whose hypertension was not controlled by a treatment including clonidine (300--450 microgram/day), the progressive addition of prazosin to a maximal dose of 4--22.5 mg/day led to control of hypertension in nine cases (mean supine blood pressure decreased from 174--102 to 144--88 mmHg). 3. In 13 hospitalized patients, clonidine was abruptly interrupted whereas prazosin was continued. Mean blood pressure (+/- SEM) rose from 145 +/- 1/85 +/- 2 to 169 +/- 7/104 +/- 5 mmHg 48 h after clonidine withdrawal whereas plasma noradrenaline and adrenaline rose from 328 to 758 pg/ml and from 75 to 137 pg/ml. No subjective symptoms were noted. There was a negative correlation between blood pressure increase and the dose of prazosin. 4. It is concluded that (a) there is no antagonism between clonidine and prazosin in their antihypertensive action in man when used at usual clinical doses, (b) a protective effect of prazosin against an eventual clonidine withdrawal syndrome is suggested by the negative correlation between blood pressure increase and the dose of prazosin.

Pre- and postsynaptic a-adrenoceptor blockade by (imidazolinyl-2)-2-benzodioxane 1-4 (170 150): antagonistic action on the central effects of clonidine

While the specificities of alpha 1-adrenoceptor blocking agents are considered as satisfactory, those of alpha 2-adrenoceptor blocking agents are only weak. Therefore, a more selective alpha 2-adrenoceptor blocking agent is needed. In anaesthetized dogs and rats, (imidazolinyl-2)-2-benzodioxane 1-4 (170 150), a benzodioxane derivative, antagonized the pressor effects induced by adrenaline, noradrenaline and phenylephrine, but did not modify the effects of acetylcholine, histamine and isoprenaline. Therefore, its selectivity is satisfactory. In the anaesthetized dog, 170 150 (0.1 mg.kg-1) increased the tachycardia induced by electrical stimulation of the cardiac sympathetic nerve and antagonized the inhibitory effects of clonidine. In addition, 170 150 blocked and reversed some centrally mediated effects of clonidine such as the fall in blood pressure, bradycardia and reduction of splanchnic discharges in the dog, and the loss of the righting reflex induced by clonidine in the chicken. The centrally mediated effects of clonidine are attributed to stimulation of alpha 2-adrenoceptors; therefore, 170 150 appears to antagonize these alpha-adrenoceptors. Moreover, 170 150 appears to be more potent than piperoxan.

[Blocking by naloxone of the antihypertensive effect of clonidine in hypertensive and normotensive animals]

Antagonism between cardiovascular effects of clonidin (10-30 micrograms/kg) and naloxon (0.15-1.0 mg/kg) was studied on anesthetized spontaneously hypertensive rats (SHR) as compared to Wistar Kyoto (WKY) rats and on awake cats. Naloxon was shown to completely block the clonidin hypotensive effect in SHR and cats and to partially block it in WKY rats. In some experiments, naloxon was administered after the blockade of peripheral M-cholinoreceptors by metacin (2 mg/kg). In these experiments naloxon evoked elevation of blood pressure, cardiac output and heart rate (in cats). It is concluded that the opioid link participates in the development of clonidin-evoked bradycardia and hypotension both in hypertensive and in normotensive animals.

Pre- and postsynaptic alpha adrenoceptor selectivity studies with yohimbine and its two diastereoisomers rauwolscine and corynanthine in the anesthetized dog

The selectivity of yohimbine and its two diastereoisomers rauwolscine and corynanthine for pre- and postsynaptic alpha adrenoceptors has been investigated in the anesthetized dog. Antagonism of the inhibitory effect of clonidine on the tachycardia produced by electrical stimulation of the ansa subclavia was used as a measure of presynaptic alpha-2 adrenoceptor blockade. Inhibition of the diastolic pressor response to phenylephrine in ganglion and beta blocked dogs was used as a measure of postsynaptic alpha-1 adrenoceptor blockade. All three of the isomers reduced, and at higher doses reversed, the inhibitory effect of clonidine Yohimbine and rauwolscine were equipotent in this respect and were approximately 100-fold more potent than corynanthine. However, all the isomers were equipotent as antagonists of the diastolic pressure response to phenylephrine. Yohimbine and rauwolscine were approximately 30 times more potent as alpha-2 adrenoceptor than alpha-1 adrenoceptor antagonists, whereas corynanthine was 10-fold more potent at alpha-1 adrenoceptors than at alpha-2 adrenoceptors. These results are in broad agreement with those previously reported from in vitro experiments showing yohimbine and rauwolscine to be preferential alpha-2 adrenoceptor antagonists and corynanthine to be a preferential alpha-1 adrenoceptor antagonist. It is concluded that the high affinity of the antagonists yohimbine and rauwolscine for alpha-2 adrenoceptors is responsible for their selectivity because at the level of blockade of postsynaptic alpha-1 adrenoceptors both isomers were equipotent with corynanthine.

Analysis of pupillary dilation produced by analogs of clonidine

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

Inhibition of sympathetic preganglionic neurons by catecholamines and clonidine: mediation by an alpha-adrenergic receptor

The neuropil surrounding sympathetic preganglionic neurons (SPNs) receives an abundant catecholaminergic innervation originating from the brain stem. The effect of catecholamines (CA) released at this spinal level on the activity of SPNs is still controversial as is the extent to which this particular CA transmission is affected by central antihypertensive drugs, such as clonidine and alpha-methyldopa. The present study was initiated, therefore, to determine the effects of iontophoretic applications of CAs and clonidine on the discharges of identified SPNs and to determine the type of receptor mediating the action of these compounds. Extracellular recordings were made with five- or six-barrel electrodes in 20 pigeons anesthetized with urethane, artificially ventilated, and immobilized. Data were obtained on 83 SPNs localized in the three first thoracic segments and identified on the basis of constancy of antidromic activation latency and collision. All of the cells sampled were inhibited by the application of low iontophoretic currents of clonidine and by a series of CAs, including alpha-methylnorepinephrine, epinephrine, and phenylephrine. For each compound, the amount of charge necessary to decrease the level of cell firing to 50% of control was calculated. Using this value as an index of drug potency, the following rank order could be determined: clonidine greater than alpha-methylnorepinephrine greater than epinephrine greater than norepinephrine greater than phenylephrine. The inhibitory effects of both clonidine and norepinephrine were antagonized by iontophoretic applications of the alpha antagonists, yohimbine, piperoxan, and phentolamine. In contrast, the beta antagonist, sotalol, and the alpha 1 antagonist, prazosin, were found ineffective when similarly applied. It is concluded that CAs and clonidine are inhibitory to the maintained activity of SPNs and that an alpha2-adrenergic receptor may be involved in the action of these compounds.

Characterization of alpha-adrenoceptors participating in the central hypotensive and sedative effects of clonidine using yohimbine, rauwolscine and corynanthine

The central alpha-adrenoceptors responsible for mediating the clonidine-induced central hypotension in anaesthetized cats and sedation in mice have been characterized according to their sensitivities to the alpha-adrenoceptor antagonist yohimbine and its two diastereomeric congeners rauwolscine and corynanthine. Yohimbine and rauwolscine (1-10 microgram/kg) dose-dependently antagonized the central hypotensive response to clonidine (1 microgram/kg) applied 15 min later. Greater amounts of corynanthine (30-100 micrograms/kg) had to be administered to diminish the central depressor effect of clonidine. In these studies the drugs were infused via the left vertebral artery. The prolongation of the hexobarbitone-induced loss of the righting reflex in mice by clonidine (0.3 mg/kg, i.p.) was inhibited by previous treatment with yohimbine and rauwolscine (0.04-5 mg/kg, i.p.) in a dose-dependent manner, but not by corynanthine. Binding experiments with rat isolated cerebral membranes demonstrated the higher affinity of yohimbine and rauwolscine for the [3H] clonidine- than for the [3H]prazosin-specific binding sites. The reverse was found for corynanthine. The relative potencies of yohimbine, rauwolscine and corynanthine in inhibiting these central effects of clonidine are comparable to their order of efficacies in blocking peripheral alpha 2-adrenoceptors. Accordingly, clonidine-induced central hypotension and sedation are mediated by alpha 2-adrenoceptors.

[Effect of clofeline and apressin on systemic arterial pressure and the tone of cerebral and peripheral vessels]

Resistography was used to demonstrate in acute experiments on anesthetized cats that intravenous injection of clofelin (2,6 and 10 microgram/kg) and apressin (1, 2.5 and 5 microgram/kg) induced a dose-dependent decline in the arterial blood pressure which was not infrequently preceded by short-lived hypertension. Clofelin induced an increase in the tone of cerebral vessels while apressin reduced it. As regards femoral vessels both the drugs produced diverse (sometimes phasic) reactions, with vasodilation being predominant.

A pilot study of the mechanism of action of desipramine

To test the hypothesis the desipramine alters alpha adrenoceptor function in depressed patients, the effects of clonidine upon growth hormone sedation and blood pressure have been measured in depressed patients before and after treatment with desipramine. After three weeks of treatment the hypotensive and sedative effects of clonidine were inhibited in all patients even though plasma desipramine concentrations at this time varied from 42 to 560 micrograms/l. Growth hormone responses to clonidine were enhanced in five of the six patients but this effect was not statistically significant. These findings are consistent with the hypothesis that in these patients desipramine altered alpha adrenoceptor function: other explanation are discussed.

Pharmacological characterizations of adrenergic receptors in human adipocytes

Three types of adrenergic receptors, beta, alpha-1, and alpha-2, were identified in human adipocytes, isolated from properitoneal adipose tissue, using both the binding of radioactive ligands and the effects of adrenergic agents on receptor-specific biochemical responses. Adrenergic binding studies showed the following results: [(3)H]dihydroalprenolol binding (beta adrenergic) B(max) 280 fmol/mg protein, K(D) 0.38 nM; [(3)H]para-aminoclonidine binding (alpha-2 adrenergic) B(max) 166 fmol/mg protein, K(D) 0.49 nM; [(3)H]WB 4101 binding (alpha-1 adrenergic) B(max) 303 fmol/mg protein, K(D) 0.86 nM. In adipocytes from subcutaneous adipose tissue, [(3)H]dihydroergocryptine binding indicated the presence of alpha-2 but not alpha-1 receptors. Beta and alpha-2 adrenergic receptors appeared to be positively and negatively coupled to adenylate cyclase, respectively. Cells or cell membranes were incubated with epinephrine (10 muM) alone and in combination with the antagonists yohimbine (alpha-2) and prazosin (alpha-1). Epinephrine alone prompted a modest increase in adenylate cyclase activity, cyclic AMP, and glycerol release, an index of lipolysis. Yohimbine (0.1 muM) greatly enhanced these actions whereas prazosin was without effect. The beta agonist, isoproterenol, stimulated glycerol release, whereas the alpha-2 agonist, clonidine, inhibited lipolysis and cyclic AMP accumulation. To assess further alpha-1 receptors, cells were incubated with [(32)P]phosphate and epinephrine (10 muM) alone and in combination with prazosin and yohimbine. Epinephrine alone caused a three- to fourfold increase in (32)P incorporation into phosphatidylinositol. Prazosin (0.1 muM) blocked this action whereas yohimbine (0.1 muM) was without effect. Thus, in a homogeneous cell preparation, the human adipocyte appears to have three different adrenergic receptors, each of which is coupled to a distinct biochemical response.

Influence of analgesic antipyretics on the central cardiovascular effects of clonidine in rats

The centrally acting antihypertensive drug clonidine has been found to stimulate the synthesis of PGF2 alpha in the brain. Centrally administered PGF2 alpha, in turn, induces rises of blood pressure and heart rate. We therefore studied the influence of inhibitors of prostaglandin (PG) synthesis on the cardiovascular effects of clonidine in urethane-anaesthetised rats. Pretreatment with indomethacin or paracetamol (100 micrograms/rat into the fourth cerebral ventricle) antagonised the central hypotensive effect of clonidine (0.125-16.0 micrograms/rat into the fourth cerebral ventricle). The bradycardic effect of centrally administered clonidine was, however, enhanced by pretreatment with paracetamol but not influenced by indomethacin pretreatment. Sodium meclofenamate (100 micrograms/rat into the fourth cerebral ventricle) did not significantly affect the clonidine-induced changes in blood pressure and heart rate. These results suggest that the clonidine-induced hypotension on one hand and bradycardia on the other hand may be mediated by partly different mechanisms. An interference of the formation of PGF2 alpha with the cardiovascular effects of clonidine cannot be completely excluded since paracetamol pretreatment potentiated the bradycardic effect of clonidine. However, inhibitors of PG synthesis did not enhance but antagonised the hypotensive effect of clonidine. Therefore it is likely that the synthesis of PGF2 alpha does not interfere with the hypotensive effect of clonidine. Moreover, the antagonism of the hypotensive effect by inhibitors of PG synthesis suggests that some hypotensive metabolite of arachidonic acid in the brain could be involved in the central hypotensive effect of clonidine.

The locomotor effect of clonidine and its interaction with alpha-flupenthixol or haloperidol in the developing rat

The interaction of clonidine with alpha-flupenthixol and haloperidol on the locomotion was investigation in the rat during postnatal development. A subcutaneous injection of clonidine 0.039--3.9 mu mol/kg produced a marked hypermotility in infant animals between day 1 and day 7 but hypomotility in animals older than 20 days. Pretreatment with alpha-flupenthixol and haloperidol significantly reduced clonidine-hypermotility in infants. In adult rats, clonidine-hypomotility were increased by a preceding administration of alpha-flupenthixol. It is suggested that intact function of both alpha-adrenoceptors and dopamine receptors is involved in the control of locomotor activity in developing rats and that there is the complex interaction of noradrenergic and dopaminergic system in the control of locomotor activity.

Central cardiovascular actions of d-tubocurarine and inhibition of the hypotensive effect of clonidine

Intracisternal (i. cist.) administration of d-tubocurarine (0.025-0. 100 mg. kg-1) in alpha-chloralose-anaesthetized dogs caused a dose-related increase in blood pressure associated with seizures. When injected in dogs pretreated with guanethidine (15 mg. i.v.), d-tubocurarine elicited the same effect while bilateral adrenalectomy abolished the pressor response. The hypotensive effect of clonidine (0.003 mg. kg-1 i. cist.) was reversed when clonidine was given after the alkaloid. These results suggest possbile interactions of the drugs at central GABA-ergic, glycinergic or cholinergic synapses.

Reversal of clonidine-induced hypotension by beta-adrenoceptor blocking drugs

The interaction between clonidine, propranolol and sotalol was investigated using conscious normotensive and spontaneously hypertensive rabbits as well as conscious normotensive and spontaneously hypertensive rats. Clonidine (20 micrograms/kg) administration into the marginal ear vein of rabbits produced persistent hypotension and bradycardia. In propranolol (0.5 mg/kg, i.v.)-pretreated animals, clonidine-induced hypotension was prevented. In rats, daily oral or subcutaneous clonidine as well as propranolol produced hypotension and bradycardia. Significant antagonism of the observed hypotensive effects resulted when clonidine was given to propranolol-pretreated animals or when propranolol was added to the treatment regimen of animals being maintained on clonidine. No antagonism between sotalol and clonidine was demonstrable. In view of the known central site of action of clonidine, and the failure of sotalol to antagonize clonidine-induced hypotension it would appear that the central nervous system is a possible site of the observed drug interaction.

[Influence of antidepressants on the hypotensive effect of clofeline in rats]

It has been shown in experiments on unanesthesized rats with normal arterial blood pressure that the antidepressants azaphen, pyrazidol and inkasan as well as imipramine and amitryptyline reduce the hypotensive effect of clofelin. With the dosage build-up the antidepressants themselves start to exert the hypotensive effect. However, the antagonism to clofelin in relation to the arterial blood pressure is maintained. It is suggested that such an effect is consequent on the impairment of noradrenaline neuronal uptake induced by the antidepressants.

Inhibition of neuronal uptake reduces the presynaptic effects of clonidine but not of alpha-methylnoradrenaline on the stimulation-evoked release of 3H-noradrenaline from rat occipital cortex slices

The iminoimidazolidine clonidine reduced concentration-dependently the release of 3H-noradrenaline evoked by electrical stimulation from the rate cerebral cortex. Exposure to the neuronal uptake inhibitors cocaine (10 micro M), desipramine (0.1 to 1 micro M) and amphetamine (1 micro M) significantly increased the stimulation-evoked overflow of tritium. These uptake inhibitors antagonized the effects of clonidine on stimulation evoked 3H-noradrenaline release but failed to modify the inhibition induced by the catecholamine alpha-methylnoradrenaline. Inhibition of monoamine oxidase by preincubation of cerebral cortex slices with 0.5 mM pargyline significantly increased the stimulation-evoked overflow of tritium, but clonidine was as effective as in the controls in inhibiting 3H-noradrenaline overflow. The antagonism by desipramine of the clonidine-induced inhibition of neurotransmission could not be attributed to a blockade of presynaptic alpha-adrenoceptors because: (1) the facilitating effect of phentolamine on 3H-noradrenaline overflow was not modified in the presence of desipramine; (2) the magnitude of the inhibition of the stimulation-evoked 3H-noradrenaline release elicited by alpha-methylnoradrenaline was the same in the presence of cocaine or desipramine; (3) exposure to desipramine in the presence of cocaine did not further increase the stimulation-evoked release of 3H-transmitter. Since the catecholamine alpha-methylnoradrenaline inhibited neurotransmission in the presence of desipramine or cocaine, we can conclude that inhibition of neuronal uptake of noradrenaline antagonized selectively the presynaptic inhibitory effects of imidazolines on alpha 2-adrenoceptors. The influence of the inhibition of neuronal uptake on the presynaptic effects of imidazolines and catecholamines should be taken into account when the relative order of potencies of various alpha 2-adrenoceptors agonists is determined.

Copulation-induced abbreviation of estrus in the female guinea pig: block by clonidine

In the female guinea pig, copulation produces a rapid inhibition of subsequent sexual receptivity. Injection of the alpha adrenergic agonist clonidine before mating blocked the early inhibitory effects of coital stimulation, while the serotonergic antagonist methysergide, and the dopaminergic antagonist pimozide were without effect. The effects of clonidine were prevented by pretreatment with the alpha adrenergic antagonist yohimbine. These results suggest that an adrenergic system is involved in the copulation-induced inhibition of receptivity and provide further evidence that such a system has an important facilitative role in the control of female sexual behavior in the guinea pig.

Blockade by burimamide of the effects of clonidine on cardiac contractility phosphorylase activation and cyclic adenosine monophosphate in isolated guinea pig heart

Clonidine in a dose-range of 2.5 microgram to 80 microgram caused positive inotropic effect, which was accompanied by increase in the cyclic AMP levels and phosphorylase-activation of the isolated perfused guinea pig heart. Clonidine-induced biochemical and mechanical effects were blocked by burimamide, an H2-receptor antagonist Propranolol (1 x 10(-6)M), phentolamine (1 x 10(-6)M) or reserpine pretreatment, did not affect the clonidine responses on the perfused guinea pig heart. Clonidine reduced the 4-methyl-histamine (H2-agonist) responses of guinea pig heart. Our data suggest that the cardiac effects of clonidine may be due to stimulation of H2-type of receptors.

[Interaction between clonidine and alpha-blockers (author's transl)]

The inhibitory effects of clonidine on the heart seem to be due to stimulation of alpha 2 adrenoceptors. alpha-Adrenoceptor blocking agents with selective affinity for alpha 2-adrenoceptors antogonize these effects. Since the alpha-adrenoceptor blocking drugs which, at the usual dosage level, antagonize the hypotensive action of clonidine are unknown in man, caution should be exercised for such an association clonidine-alpha-blocker.

Serotoninergic mechanisms of beta-endorphin- and clonidine-induced analgesia in rats

Both beta-endorphin and clonidine proved to have statistically significant analgesic activity (increase in latency to hind-paw lick in hot plate test) in rats. Furthermore, the pain inhibition induced by beta-endorphin and clonidine could be antagonized by prior treatment of animals with either naloxone (a narcotic antagonist) or the depletors of central serotonin pathways such as 5,6-dihydroxytryptamine, 5,7-dihydroxytryptamine and p-chlorophenylalanine have not effect on latency to hind-paw lick. The data indicate that serotoninergic activity in the brain plays a role in the elaboration or modulation of beta-endorphin and clonidine analgesia in rats.

Pharmacology of centrally acting hypotensive drugs

1. A survey is given of the various aspects of centrally acting hypotensive drugs. The majority of centrally acting hypotensive agents act by way of central alpha-adrenoceptors, probably located in the pontomedullary region of the brain. These central receptors are stimulated by clonidine, guanfacine and various related compounds, and also by alpha-methylnoradrenaline, generated in vivo upon biotransformation of alpha-methyldopa within the brain. The stimulation of the alpha-adrenoceptors induces a decrease in peripheral sympathetic tone and thus a fall in arterial blood pressure and bradycardia. 2. The possibility that presynaptic alpha-adrenoceptors in the brain are involved in the central hypotensive action of clonidine, guanfacine and related compounds is discussed. Also, the possible involvement of central histaminergic and cholinergic receptors in central hypotensive effects is reviewed. 3. Various experimental compounds with a central hypotensive effect different from that of clonidine and related drugs are mentioned. 4. Finally, it is pointed out that the hypotensive effects of various beta-sympatholytic drugs and of prazosin are probably not of central origin.

Naloxone reverses the antihypertensive effect of clonidine

In unanesthetized, spontaneously hypertensive rats the decrease in blood pressure and heart rate produced by intravenous clonidine, 5 to 20 micrograms/kg, was inhibited or reversed by nalozone, 0.2 to 2 mg/kg. The hypotensive effect of 100 mg/kg alpha-methyldopa was also partially reversed by naloxone. Naloxone alone did not affect either blood pressure or heart rate. In brain membranes from spontaneously hypertensive rats clonidine, 10(-8) to 10(-5) M, did not influence stereoselective binding of [3H]-naloxone (8 nM), and naloxone, 10(-8) to 10(-4) M, did not influence clonidine-suppressible binding of [3H]-dihydroergocryptine (1 nM). These findings indicate that in spontaneously hypertensive rats the effects of central alpha-adrenoceptor stimulation involve activation of opiate receptors. As naloxone and clonidine do not appear to interact with the same receptor site, the observed functional antagonism suggests the release of an endogenous opiate by clonidine or alpha-methyldopa and the possible role of the opiate in the central control of sympathetic tone.

The effect of prazosin of pre- and post-synaptic alpha-receptors in the pithed rat

1. Prazosin had a marked inhibitory effect on post-synaptic alpha-receptors in the pithed rat, as measured by the blood pressure response to the cumulative administration of clonidine. 2. Prazosin had no inhibitory effect, however, on the pre-synaptic alpha-receptor, as measured by the clonidine-induced reduction in the tachycardia produced by stimulation of the cardiac nerves. 3. These results suggest that prazosin has a selective antagonistic affinity for post-synaptic alpha-receptors.