Effect of clonidine on sucrose intake and water intake varies as a function of dose, deprivation state, and duration of exposure

Lick frequency was monitored in five-minute intervals over a one-hour period in rats given access to 8% sucrose (Experiment 1) or water (Experiment 2). Prior to the session, the rats were administered either isotonic saline or clonidine (6.24, 12.5, or 25 micrograms/kg). In deprived rats (82%) clonidine led to a dose-related increase in consummatory behavior. Water intake in deprived rats was depressed by clonidine. In rats maintained on a free-feeding schedule, the higher clonidine doses led to a decrement in sucrose intake over the first 15 minutes of access; whereas the 6.25 micrograms/kg dose stimulated consummatory behavior, but only during the first five minutes of access. There were no reliable effects of clonidine on sucrose intake late in the access period for the free-feeding rats. Water intake in free-feeding rats tended to be enhanced by the low dose of clonidine, particularly late in the access period. In general, deprivation enhanced sucrose intake and depressed water intake and clonidine exaggerated both of these trends.     

Satietin: route of injection, dose response, effect on food and water intake and on running-wheel activity in the rat

Semipurified satietin significantly (p less than 0.05) reduced food intake when injected subcutaneously at 10, 15, 20 mg/kg into 48 hr fasted rats with no indication of a dose response. When infused intracerebroventricularly (ICV) at 12.5, 25 and 50 micrograms/rat (10 microliter vol) into ad lib fed rats at the end of the light period there was no effect on food intake for the first hour but 24 hr food intake was (p less than 0.001) reduced at all doses. The ICV dose response curve was shallow, with similar suppression at both 12.5 and 25 micrograms doses, but a (p less than 0.05) greater suppression with the 50 micrograms dose. An ICV threshold between 6.25 micrograms and 12.5 micrograms appears to exist since no suppression occurred after a dose of 6.25 micrograms. Four consecutive daily ICV infusions of satietin (25 micrograms/rat) in two rats progressively suppressed food intake to low levels, suggesting a cumulative effect. Following termination of satietin treatment daily food intake slowly returned towards normal without evidence of rebound feeding. In other ad lib fed rats, four ICV infusions of semipurified satietin, on days alternated with no infusion, reduced food intake (p less than 0.001), water intake (p less than 0.003) and running wheel activity (p less than 0.001) on the first day of injection but not on subsequent injection days. Suppression of activity approached significance on the second injection day. Highly purified satietin infused ICV produced similar responses. These findings may indicate a general disruption of behavior by satietin, thus, it may not play a physiological role in feeding behavior because of its apparent non-specificity.     

Biphasic effects of clonidine on conflict behavior: involvement of different alpha-adrenoceptors

The effect of the alpha-2-adrenoceptor agonist clonidine on anxiety-related behavior was investigated using two different rat anxiety models: a modified Vogel's drinking conflict model and Montgomery's elevated plus-maze. In both models biphasic dose-response curves were obtained; in a narrow low-dose range (6.25-10.0 micrograms/kg) the drug produced anxiolytic-like effects, while anxiogenic-like properties were found after higher doses (12.5-80.0 micrograms/kg). Attempts to block the effects obtained were made in Montgomery's elevated plus-maze. The specific alpha-2-adrenoceptor antagonist idazoxan blocked the anxiolytic-like effect but did not influence the anxiogenic-like activity. Conversely, the specific alpha-1-adrenoceptor antagonist prazosin blocked the anxiogenic-like effect but did not alter the anxiolytic-like activity. These findings may suggest that alpha-1- and alpha-2-adrenergic receptor mechanisms are reciprocally involved in anxiety-related behavior.     

Influence of clonidine on the ACTH-induced behavioral syndrome

In male rats, clonidine in a dose range of 1-3000 micrograms/kg i.p. antagonized the stretching-yawning syndrome induced by the intraventricular injection of ACTH-(1-24) (3 micrograms/rat) dose-dependently. On the other hand, the effect of clonidine on ACTH-induced penile erections was potentiation at low doses (5 and 10 micrograms/kg) and inhibition at the highest doses (1000 and 3000 micrograms/kg), the intermediate doses (50 and 100 micrograms/kg) being without effect. There was no relationship between these behavioral effects and the effect on arterial blood pressure.     

Acute tolerance to clonidine hypotension and bradycardia in normotensive and hypertensive rats

Tolerance to the antihypertensive effect of clonidine has been observed in humans but there are few studies showing either acute or chronic tolerance to clonidine hypotension in experimental animals. We determined whether tachyphylaxis to acute clonidine hypotension could be demonstrated in normotensive Sprague Dawley rats by i.v. injections of 2 repetitive clonidine doses of either 1.5, 3.0, 5.0, or 10.0 micrograms/kg, or in SHR rats by repeated intracerebroventricular injections of 1.5 or 3.0 micrograms/kg, while monitoring arterial blood pressure. Second doses of clonidine were given 30 minutes after recovery of arterial pressure from the first dose. At 3.0 micrograms/kg and greater, the hypotension produced by the second dose was at least 50% less than that elicited by the first dose. By contrast, no acute tachyphylaxis was observed to the decrease in tension caused by clonidine in the transmurally stimulated isolated guinea pig ileum after 2 repeated exposures to clonidine, with intervening wash, at concentrations between 5 X 10(-10) M to 5 X 10(-8) M. Also, no tachyphylaxis occurred to the initial hypertensive phase of clonidine in Sprague Dawley rats.     

Modulation of the behavioral effects of amphetamine in rats by clonidine

Clonidine (0.01, 0.05, 0.5 mg/kg) dramatically reduced the locomotor response to amphetamine (2 mg/kg) in a dose related fashion. In contrast, the same doses of clonidine had no effect on locomotions produced by a higher dose of amphetamine (6 mg/kg). Clonidine also had no effect on stereotyped head movements or the duration of the behavioral response to amphetamine. The lower dose of clonidine reduced amphetamine induced licking/biting while the two higher doses potentiated amphetamine induced licking/biting. Thus the behavioral effects of clonidine vary depending upon the dose of amphetamine and the particular behavior selected for study. The known neurochemical effects of clonidine do not account for this phenomenon.     

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

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

Neonatal 6-hydroxydopamine potentiates clonidine's locomotor effects throughout maturation in the rat

Newborn rats were administered 6-hydroxydopamine (6-OHDA) systemically (2 X 100 micrograms/g) or vehicle and subsequently tested for their locomotor response to one of three doses of clonidine (10, 100 or 1000 micrograms/kg) at either 10, 20, 30 or 50 days of age. Clonidine caused a dose-related increase in activity in 10-day-old rats and this effect was potentiated in the 6-OHDA-treated rats. At 20 days, clonidine did not affect activity in the vehicle rats but at the highest dose, increased that of the 6-OHDA rats. At 30 days, clonidine again did not affect activity in the vehicle rats but the 1000 micrograms/kg (when compared to the 100 micrograms/kg) dose, significantly increased activity of the 6-OHDA rats. A biphasic effect of clonidine was apparent at 50 days. At this age the 6-OHDA treatment exaggerated the depressant effect of the 100 micrograms/kg dose. Thus, neonatal-OHDA generally potentiated the locomotor response to clonidine despite the fact that the effect of the drug changed with age. It is concluded that the locomotor effects of clonidine are mediated through alpha 2 adrenoceptors which are not located on noradrenergic terminals but rather are postsynaptic to them. With maturation, the "functional wiring" of these terminals appears to be altered.     

Pharmacokinetics of clonidine in the rat and cat

To investigate the pharmacokinetic behavior of clonidine, rats were given clonidine intravenously at 125, 250, and 500 micrograms/kg and blood clonidine concentrations were followed for 6 hr. The disposition of clonidine in two brain regions was studied in rats after an i.v. dose of 500 micrograms/kg. The liver clearance in rats was investigated by liver perfusion techniques. The results obtained indicate that the disposition characteristics of clonidine can be described by a two-compartment open model in both rats and cats. The penetration of clonidine into tissues is rapid, and brain levels in rats were about 1.7 times higher than blood levels. Brain tissues were found to be an indistinguisible part of the central (blood) compartment. Dose-dependent pharmacokinetic behavior was found for clonidine in rats at the doses used. This was demonstrated by a decrease of both the rate constant of distribution to the peripheral compartment and the overall elimination rate constant from the body, with increase in dose. As a consequence, the volume of distribution and the clearance both decreased with increasing dose. Possible explanations for the dose-dependent behavior of clonidine are discussed.     

Interaction between central alpha 1- and alpha 2-adrenoceptors on sympathetic tone in rats

The interaction between piperoxan and alpha 2-agonists on sympathetic tone was studied in rats. The sympatho-inhibitory effect of alpha 2-agonists (clonidine, guanfacine, B-HT 933) was assessed by recording heart rate in normotensive bilaterally-vagotomized rats. Clonidine (3 micrograms/kg, i.c.v.) and B-HT 933 (100 micrograms/kg, i.c.v.) induced a bradycardia which was fully reversed by piperoxan (30 micrograms/kg, i.c.v.). However, in rats treated with guanfacine, piperoxan induced a partial recovery of the bradycardic effect. The injection of a small dose of the specific alpha 1-adrenoceptor blocking drug, AR-C 239 (10 micrograms/kg, i.c.v.) which, by itself did not modify heart rate, completely inhibited the reversal effect of piperoxan in rats treated with clonidine, B-HT 933 or guanfacine. In rat brainstem membranes, B-HT 933 was found to bind to both alpha 1- and alpha 2-adrenoceptors and was as potent as clonidine in competing for alpha 1-sites bound by [3H]prazosin. On the other hand, in bilaterally vagotomized rats, piperoxan (30 micrograms/kg, i.c.v.) induced an increase in blood pressure and heart rate which was inhibited by previous administration of AR-C 239 (10 micrograms/kg, i.c.v.). These data suggest that, by inhibiting central alpha 2-adrenoceptors, piperoxan unmasks central alpha 1-adrenoceptor stimulation by endogenous catecholamines leading to an increase in the sympathetic tone, but a full recovery in heart rate could be observed only with the mixed alpha 1- and alpha 2-adrenoceptor agonists, clonidine and B-HT 933. In addition, these data further indicate that alpha 1-adrenoceptors are implicated in a tonic control of the sympathetic nerve activity in normotensive rats.     

Effect of time and dose on angiotensin converting enzyme during captopril treatment in the rat

The effect of treatment time and dose of captopril with regard to angiotensin converting enzyme (ACE) in serum, lungs and kidneys of the rat were studied. Normotensive Wistar rats were treated with a constant dose of captopril (0.2 mg/ml) during various time periods. In a second study rats were treated with different captopril doses (6.25 micrograms, 12.5 micrograms, 25 micrograms, 50 micrograms, and 200 micrograms/ml water) during three weeks. Serum ACE activity and pulmonary and kidney plasma membrane ACE concentrations were measured in both studies. Captopril treatment resulted in a rapid decrease of ACE in pulmonary and kidney plasma membranes and a simultaneously increase of serum ACE activity during the first day of treatment. This was followed by increased membrane concentrations of ACE in the lungs and return to normal ACE concentrations in membranes of kidneys, presumably due to increased ACE biosynthesis. Serum ACE activity continued to increase during the whole study. Serum ACE activity increased in a dose dependent manner during treatment with different captopril doses. Increased plasma membrane ACE concentrations were not observed in the rats treated with captopril at doses below 200 micrograms/ml water.     

Effects of subchronic soman on avoidance-escape behavior and cholinesterase activities

Male Sprague-Dawley rats were trained on a discriminated avoidance-escape task. They were administered subchronically saline, 12.7 micrograms/kg (0.125 LD50) soman, or 25.5 micrograms/kg (0.25 LD50) soman. Injections were given 5 days per week for 4 weeks. Injections were given subcutaneously immediately following the avoidance behavior test session. Soman produced a reduction in avoidance behavior efficiency in a dose dependent manner. When soman was discontinued, the rats recovered their pre-soman control baselines. Untrained rats given soman according to the same soman regimen were used to measure acetylcholine in brain and cholinesterase activities in brain, blood, and diaphragm. After 18 soman injections at 12.7 and 25.5 micrograms/kg acetylcholine was reduced significantly only in the amygdala. Blood cholinesterase was inhibited as much as 57% after 12.7 micrograms/kg soman and 74% after 25.5 micrograms/kg. Plasma cholinesterase was inhibited to 24% by the 12.7 micrograms/kg dose of soman and to 38% by the 25.5 micrograms/kg dose. Plasma cholinesterase recovered to control levels 11 days after cessation of soman, and whole blood cholinesterase recovered 25 days after cessation of the higher soman dose. Cholinesterase was inhibited significantly in the hippocampus and amygdala in a dose dependent manner. The cholinesterase activities appear to parallel the soman induced decrement in avoidance behavior and the subsequent recovery to control levels following withdrawal of soman.     

Guanfacine, but not clonidine, improves planning and working memory performance in humans.

The present study compares, using a double-blind, placebo controlled design the effects of two alpha 2-agonists, clonidine (0.5, 2, and 5 micrograms/kg) and guanfacine (7 and 29 micrograms/kg) on spatial working memory, planning and attentional set-shifting, functions thought to be dependent on the "central executive" of the prefrontal cortex. Blood pressure and the subjective feeling of sedation were affected equally by clonidine and guanfacine. The 0.5 microgram/kg and 5 micrograms/kg doses of clonidine disrupted spatial working memory, but the medium dose had no effect. The 0.5 and 2 micrograms/kg doses of clonidine increased impulsive responding in the planning test. The 5 micrograms/kg dose of clonidine slowed responding at effortful levels of planning and attentional set-shifting tests. The 29 micrograms/kg dose of guanfacine improved spatial working memory and planning. Guanfacine had no effect on attentional set-shifting. These data indicate that guanfacine improved planning and spatial working memory, but clonidine dose-dependently disrupted performance. It is possible that the greater selectivity of guanfacine for alpha 2A-adrenoceptor subtype may underlie its differences from clonidine.     

An analysis of the effects of systemically administered clonidine on the food and water intake of rats.

1 It is known that intracerebral injections of clonidine can induce eating in rats but it has not been clear whether systemic administration can produce similar effects. 2 Subcutaneous injections of clonidine (0.01, 0.03, 0.1 mg/kg) increased food and water intake during the 6 h period following injection in non-deprived male rats. 3 Pretreatment with a dose of yohimbine (1.0 mg/kg) shifted the clonidine dose-response curves to the right, suggesting competitive antagonism. 4 A dose of naloxone (0.1 mg/kg) produced a lowering of the clonidine dose-response curve but statistical analysis suggested that the opiate antagonist did not produce a competitive antagonism of the effect of clonidine. 5 The results are consistent with a role for alpha 2-adrenoceptors in appetite regulation.     

Clonidine-induced hypoactivity and mydriasis in mice are respectively mediated via pre- and postsynaptic alpha 2-adrenoceptors in the brain

Since brain alpha 2-adrenoceptors occur both pre- and postsynaptically, experiments were carried out to determine the synaptic locations of those receptors mediating clonidine-induced hypoactivity and mydriasis. Intraperitoneal (i.p.) injection of clonidine (1-3000 micrograms/kg) to mice dose dependently induced these two responses and also decreased brain concentrations of 3-methoxy-4-hydroxyphenylglycol (MHPG). The ED50 values were: 120 micrograms/kg for hypoactivity (95% confidence limits 103-140 micrograms/kg), 54 micrograms/kg for mydriasis (95% confidence limits 40-74 micrograms/kg) and 18 micrograms/kg for MHPG reduction (95% confidence limits 8-36 micrograms/kg) suggesting that these responses could all be presynaptically mediated. However, methamphetamine which increases noradrenaline turnover was found to dose dependently produce mydriasis, but not hypoactivity, after peripheral (0.1-5 mg/kg i.p.) or central (0.5-10 micrograms i.c.v.) injection. The mydriasis produced by methamphetamine (0.5 mg/kg i.p.) was abolished by i.c.v. injection of 1 micrograms idazoxan or yohimbine, but not 2.5 micrograms prazosin or pindolol, showing this effect was mediated by central alpha 2-adrenoceptors. Methamphetamine (1-10 micrograms i.c.v.) potentiated the mydriasis induced by clonidine (50 micrograms/kg i.p.) suggesting this was a postsynaptic alpha 2-adrenoceptor response. By contrast, methamphetamine (1-10 micrograms i.c.v.) dose dependently reversed clonidine (100 micrograms/kg i.p.) hypoactivity indicating this response was mediated by presynaptic alpha 2-adrenoceptors. These hypotheses were confirmed by destruction of noradrenergic neurones using DSP-4 (100 mg/kg i.p. x 2). This treatment prevented the mydriasis response to methamphetamine (0.5 mg/kg i.p.), but not clonidine (100 micrograms/kg i.p.) and markedly attenuated clonidine (100 micrograms/kg i.p.) hypoactivity.     

Cross-tolerance between morphine and clonidine: a study on motility in rats

The effects of clonidine on motility were determined in non-dependent, morphine-dependent (a 20 mg/kg dose i.p. for 26 days) and post-dependent rats. In naive animals, clonidine (30-100 micrograms/kg) produced a dose-related suppression of motility. However, when the drug was administered in morphine-dependent rats, it induced slight hyperactivity at small doses and a decrease of activity only at the largest dose (100 micrograms/kg). Thus, tolerance to morphine conferred cross-tolerance to clonidine. On the contrary, the effects of clonidine on motility in post-dependent animals did not differ from those observed in control animals. The results are discussed in terms of similarities and differences between the depressant and excitatory effects of morphine and clonidine.     

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

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

Possible mechanisms underlying the hypertensive response to clonidine in freely moving, normotensive rats

Possible mechanisms underlying the hypertensive response to intracerebroventricular (i.c.v.) or intravenous (i.v.) injection of clonidine were investigated in freely moving, normotensive rats. In conscious rats, clonidine (2-20 micrograms) injected i.c.v. caused a dose-dependent and long-lasting pressor response associated with bradycardia. A similarly long-lasting pressor response was induced following an initial rapid rise in mean blood pressure after i.v. bolus injections of clonidine (5-50 micrograms/kg). In pentobarbital-anesthetized rats, the prolonged pressor responses to i.v. and i.c.v. injected clonidine at high doses were significantly smaller than those in conscious rats. Low doses of clonidine caused only depressor responses which developed gradually. No significant changes in concentrations of plasma norepinephrine and epinephrine were found during the pressor period after i.c.v. injection of clonidine (20 micrograms). Systemic (2 mg/kg, i.v.) or central (100 micrograms, i.c.v.) pretreatment with phentolamine abolished only the prolonged pressor response to both i.c.v. (20 micrograms) and i.v. (50 micrograms/kg) injected clonidine. The prolonged pressor response to clonidine (20 micrograms, i.c.v.) was enhanced by pretreatment with hexamethonium (25 mg/kg, i.v.), methylatropine (1 mg/kg, i.v.) or atropine (1 mg/kg, i.v.) and it was not affected by pretreatment with saralasin (300 micrograms/kg and 25 micrograms/kg/min, i.v.), d(CH2)5Tyr(Me)-arginine-vasopressin, a vasopressin antagonist (50 micrograms/kg, i.v.) or naloxone (1 mg/kg, i.v.). Neither adrenalectomy nor adrenal demedullation had an effect on the pressor response to clonidine (20 micrograms, i.c.v.). In adrenalectomized rats, systemic pretreatment with hexamethonium (25 mg/kg, i.v.) caused a potentiation of the pressor response to clonidine (20 micrograms, i.c.v.). These results suggest that clonidine induces the pressor response through activation of central alpha-adrenoceptors, probably the alpha 2 subtype, without an increase in sympatho-adrenomedullary activity. It is speculated that the response may be mediated by vasoactive humoral substance(s).     

Chronic antidepressant and clonidine treatment effects on conflict behavior in the rat

The present studies examined the effects of chronic treatment with several antidepressants and clonidine on conflict behavior. In daily ten-minute sessions, water-deprived rats were trained to drink from a tube which was occasionally electrified (0.25 or 0.5 mA). Electrification was signalled by a tone. Chronic desipramine (5 mg/kg, IP, b.i.d.) or clonidine (40 micrograms/kg, b.i.d.) treatment resulted in time-dependent anticonflict effects, with a latency to onset of approximately 3-4 weeks. In contrast, chronic buproprion (up to 10 mg/kg, IP, b.i.d.), mianserin (up to 10 mg/kg, IP, b.i.d.) or trazodone (up to 40 mg/kg, IP, b.i.d.) treatment resulted in at best only a weak anticonflict effect. The efficacy of these antidepressants and clonidine to increase punished responding when administered chronically correlates well with their efficacy as antipanic agents in man.