On the mechanism of action of clonidine: effects on single central neurones

Centrally acting antihypertensives change the psychogenic cardiovascular reactivity

Clonidine (CL) and Rilmenidine (RI) are among the most frequently prescribed centrally acting antihypertensives. Here, we compared CL and RI effects on psychogenic cardiovascular reactivity to sonant, luminous, motosensory, and vibrotactile stimuli during neurogenic hypertension. The femoral artery and vein of Wistar (WT - normotensive) and spontaneously hypertensive rats (SHR) were catheterized before (24 h interval) i.p. injection of vehicle (NaCl 0.9%, control - CT group), CL (10 µg/kg), or RI (10 µg/kg) and acute exposure to luminous (5000 lm), sonant (75 dB sudden tap), motor (180° cage twist), and air-jet (10 L/min - restraint and vibrotactile). Findings showed that: (i) CL or RI reduced the arterial pressure of SHR, without affecting basal heart rate in WT and SHR; (ii) different stimuli evoked pressor and tachycardic responses; (iii) CL and RI reduced pressor response to sound; (iv) CL or RI reduced pressor responses to luminous stimulus without a change in peak tachycardia in SHR; (v) cage twist increased blood pressure in SHR, which was attenuated by CL or RI; (vi) air-jet increased pressure and heart rate; (vii) CL or RI attenuated the pressor responses to air-jet in SHR while RI reduced the chronotropic reactivity in both strains. Altogether, both antihypertensives relieved the psychogenic cardiovascular responses to different stimuli. The RI elicited higher cardioprotective effects through a reduction in air-jet-induced tachycardia.

Clonidine, an α2 adrenergic receptor agonist, disrupts reconsolidation of a cocaine-paired environmental memory

Environmental cues can elicit robust cocaine reward memories, contributing to relapse to cocaine abuse. Memories can be manipulated pharmacologically by interfering with reconsolidation after reactivation. Clonidine, an α2 noradrenergic receptor agonist, was tested for its ability to block reconsolidation of cocaine environmental-paired memory. Male Sprague-Dawley rats completed an 8-day cocaine place conditioning procedure to establish a cocaine place preference. Cocaine memory was reactivated by exposure to the cocaine-paired environment in a drug-free state, followed immediately by administration of clonidine (10 or 50 µg/kg) or vehicle. Cocaine place preference was retested 24 h and 1 week later. Clonidine significantly attenuated the previously established cocaine place preference when tested 1 or 7 days later. To investigate the generalizability of this effect to other drug classes, morphine conditioned place preference was tested. Clonidine administration after morphine memory reactivation did not significantly alter the expression of morphine place preference. These results suggest that clonidine can interfere with reconsolidation of cocaine memory and may be a useful approach to reduce relapse.

Xylazine-evoked depression of rat cerebral cortical neurons: a pharmacological study

1. Iontophoretic application of the veterinary tranquilizer xylazine suppressed the spontaneous firing of 134/137 neurons tested, an effect not antagonized by adenosine receptor and alpha-adrenergic receptor blocking agents. 2. Depressions in neuronal activity evoked by norepinephrine, gamma-aminobutyric acid or adenosine failed to be potentiated by xylazine. 3. However, the excitatory effects of acetylcholine and glutamate were both inhibited by xylazine. 4. Thus, the depressant effect of locally applied xylazine may involve a decrease in the excitability of the neuronal membrane.

The effect of microelectrophoretically applied clonidine on single cerebral cortical neurones in the rat. Evidence for interaction with alpha 1-adrenoceptors

The technique of microelectrophoresis was used in order to examine the effects of clonidine on single neurones in the somatosensory cortex of the rat, and to compare its actions with those of noradrenaline and phenylephrine. Clonidine evoked only excitatory responses on cortical neurones. The clonidine-sensitive neurones were also excited by noradrenaline and phenylephrine. Clonidine had a consistently lower apparent potency than either noradrenaline or phenylephrine. Responses to clonidine had a slower time-course than responses to the other two adrenoceptor agonists, both the latencies to onset and the recovery times being longer for responses to clonidine than for responses to noradrenaline and phenylephrine. When the mobilities of clonidine and phenylephrine were compared using an in vitro method, no significant difference was found between the mobilities of the two ionic species, suggesting that they have similar transport numbers. Thus the difference between the potencies and time-courses of responses to clonidine and phenylephrine are presumably of biological origin. Responses to clonidine were antagonised by microelectrophoretically applied prazosin; responses to phenylephrine were equally antagonised, while responses to acetylcholine were not affected. Clonidine could reversibly antagonise excitatory responses to both noradrenaline and phenylephrine, without affecting responses to acetylcholine. The results suggest that clonidine may act as a partial agonist at excitatory alpha 1-adrenoceptors on cortical neurones.

Is clonidine an adenosine antagonist?

Clonidine has been reported to possess the ability to block the depressant actions of purine compounds on the firing of rat cerebral cortical neurons. In studies on rats anaesthetized either with nitrous oxide and methoxyflurane or urethane, it was not possible to confirm this observation. Rather clonidine enhanced the depressant actions of iontophoretically applied adenosine and adenosine 5'-monophosphate (5'-AMP).

Effect of 6-hydroxydopamine treatment on TSH secretion in basal and cold-stimulated conditions in the rat

6-Hydroxydopamine (6-OHDA) (two doses of 200 micrograms each, administered intraventricularly at a 48 h interval) caused a marked decrease of hypothalamic noradrenaline content and blocked the TSH rise elicited by cold exposure. Clonidine (0.4 mg/kg i.p.), a noradrenaline receptor agonist, was able to reverse the 6-OHDA of cold-induced TSH surge. The plasma TSH levels after cold stress in rats treated with 6-OHDA + clonidine were significantly higher than in vehicle + clonidine-injected animals, thus suggesting the presence of noradrenaline receptor supersensitivity in 6-OHDA-pretreated rats. 6-OHDA did not modify the basal concentrations of TSH but the administration of clonidine to 6-OHDA-injected animals caused a significant increase in thyrotropin secretion when compared with the vehicle + clonidine group.

Catecholaminergic depressant effects on bulbar respiratory mechanisms

On the basis of histochemical and pharmacological studies, catecholamines have been implicated in central mechanisms controlling respiration. This hypothesis was tested in iontophoretic studies on neurones located in bulbar respiratory centres. Adrenaline and noradrenaline had a predominantly depressant effect on respiratory as well as on closely situated non-respiratory units. These depressions were mimicked by the application of isoproterenol and clonidine; acetylcholine and serotonin had inconsistent effects on these neurones. In control experiments, microinjections, using a Hamilton syringe, were made in the area of bulbar respiratory centres: noradrenaline, but not serotonin, depressed the central respiratory activity reflected in the phrenic nerve discharge. These results suggest that specific adrenergic and noradrenergic depressant mechanisms could affect both respiratory and other physiological centres at the bulbar level.

Antagonism by clonidine of neuronal depressant responses to adenosine, adenosine-5'-monophosphate and adenosine triphosphate

1. Adenosine and its nucleotides adenosine-5'-monophosphate (AMP) and adenosine triphosphate (ATP) have been applied by microiontophoresis to neurones in the cerebral cortex of rats anesthetized with urethane. The firing rate of most neurones was depressed, though two cells were encountered which showed biphasic responses to ATP consisting of an initial excitation succeeded by depression. 2. The application of clonidine with iontophoretic currents of less than 25 nA resulted in blockade of the depressant responses to the purines, without affecting responses to noradrenaline, 5-hydroxytryptamine or gamma-aminobutyric acid (GABA). At much higher doses of clonidine, direct depression of cell firing occurred and occasional interaction with noradrenaline was noted. 3. In the case of the biphasic responses to ATP, clonidine seemed to block only the depressant phase. Reduction of the excitatory component paralleled changes of background firing. 4. It is concluded that, in common with some other 2-substituted imidazoline derivatives, clonidine possesses the ability to block responses to purine compounds.

CNS site of clonidine induced hypotension: a microiontophoretic study of bulbar cardiovascular neurons

To localize the central site and mechanism of clonidine induced hypotension, the drug was applied by the technique of microiontophoresis to neurons of the bulbar cardiovascular center in decerebrate cats. The excitatory and inhibitory cardiovascular neurons (CVN) were identified by their response to an increase in the arterial blood pressure induced by intravenous injections of small doses of norepinephrine (NE). Clonidine had an inhibitory effect on the spontaneous firing rate of excitatory CVN but had no effect on the firing rate of inhibitory CVN. At the same doses, it had no effect on the firing rate of NCVN recorded from the same area. Furthermore, clonidine had a blocking action on the excitatory response of CVN induced by microiontophoretic application of NE. It is concluded that clonidine produces its hypotensive response by acting on alpha-adrenergic receptors of bulbar CVN.

On spinal noradrenaline receptor supersensitivity: correlation between nerve terminal densities and flexor reflexes various times after intracisternal 6-hydroxydopamine

The noradrenaline (NA)-dependent hindlimb flexor reflex that can be elicited by pinching the foot of acutely spinalized rats given nialamide-DOPA or clonidine was evaluated different time intervals (14 days-6 months) after intracisternal injections of 6-OH-dopamine (6-OH-DA) and correlated to the degree of bulbospinal catecholamine (CA) denervation as seen by Falck-Hillarp fluorescence histochemistry. Six and 14 days after 6-OH-DA, when almost all NA nerve terminals of the spinal cord had degenerated, the NA receptors where supersensitive to stimulation with clonidine as evidenced by an increased flexor reflex. This supersensitivity gradually disappeared as new nerve terminals were formed in the grey matter of the spinal cord during the following 3-6 months. The supersensitivity phenomenon 14 days after 6-OH-DA could also be demonstrated by L-DOPA given to animals pretreated with 100 mg/kg nialamide. Using this relatively low dose of nialamide, almost no reflex response was seen in the control group. Using a higher degree of monoaminoxidase inhibition (nialamide 200 mg/kg) also non-supersensitive, NA receptors became maximally stimulated. Therefore, 6-OH-DA treated rats now showed a weaker reflex than controls, the reflex response being directly correlated to the number of nerve terminals present that could form NA from the precursor. Using 5,6-dihydroxytryptamine, which selectively destroys 5-hydroxytryptamine (5-HT) nerves, it was shown that the flexor reflex changes were specifically related to the NA nerves and unchanged by the simultaneous presence or absence of 5-HT nerve terminals. This was further supported by the finding of a correlation between amount of nerve terminals and flexor reflex responses in individual animals, especially at longer survival times both in the clonidine and the nialamide-DOPA experiments.

Three main factors in rat shuttle behavior: their pharmacology and sequential entry in operation during a two-way avoidance session

The effects of eserine (0.1 mg/kg), nicotine (0.2 mg/kg), atropine (2 mg/kg), methylatropine (5 mg/kg), clonidine (0.2 mg/kg), phenoxybenzamine (10 mg/kg), apomorphine (0.5 mg/kg), and haloperidol (0.5 mg/kg), i.p., on shuttle responses to a buzzer (SBs) were studied on four different behavioral paradigms in rats: (a) D test: 50 buzzers and 25 shocks at random intervals and in random order; (b) DP test: 50 buzzers paired on all trials with shocks irrespective of the performance of SBs (Pavlovian conditioning); (c) DC test: 50 buzzers followed at a randomly variable interval by shocks unless there was an SB; (d) DPC test: 50 buzzer-shock trials omitting shocks every time there was an SB (two-way avoidance). Shock-induced drive was assumed to equally pervade all four situations; stimulus contiguity ('pairing') was present only in the DP and DPC tests; and the avoidance 'contingency' was present only in the DC and DPC paradigms. An analysis of the distribution of SB performance in control animals over the 10 successive blocks of 5 buzzers of each session revealed that the response level was similar for all tests during the first 2 blocks; that of the DC and DPC groups increased above the level of the other two from the third block on; and from the fifth block on, SB performance was higher in the DPC than in the DC group and in the DP over the D group. At all blocks the sum of SBs obtained in the D test, plus DP-D, plus DC-D, gave a value quite close to that experimentally determined in the DPC group. This was interpreted as showing that during the first 10 buzzers drive was the main (or the only) factor influencing SB performance in all groups; after the third block of 5 buzzers 'contingency' became a factor on its own; and 'pairing' assumed some control over SB behavior only from the fifth block on. Eserine depressed SBs in the D test, starting from the first block of buzzers; its effect was antagonized by atropine and by methylatropine. Clonidine depressed responding in the DP and DPC paradigms, and its effect was blocked by phenoxybenzamine. Nicotine, eserine, and apomorphine increased, and atropine, methylatropine, and haloperidol decreased SB performance in both the DC and the DPC test; the effect of the two former substances could be antagonized by any of the two anticholinergic agents, and haloperidol antagonized that of apomorphine. The possibilities are discussed of: (a) a peripheral cholinergic mechanism which inhibits drive; (b) a similar mechanism which favors operation of the 'contingency' factor; (c) a dopaminergic mechanism in 'contingency'; (d) a central adrenergic inhibitory mechanism in 'pairing'.

Interaction of clonidine with pre- and post-synaptic adrenergic receptors of rat brain: effects on cyclic AMP-generating systems

The locus and mechanism of interaction of clonidine with catecholamine-elicited accumulations of cyclic AMP has been investigated in brain slices from control and 6-hydroxydopamine-treated rats of the F-344 and Sprague-Daeley strains. The inhibitory effects of clonidine on the norepinephrine-stimulated accumulation of cyclic AMP and the potentiative effects of clonidine on the isoproterenol-stimulated accumulation of cyclic AMP are present to the same extent in cerebral cortical slices from control and 6-hydroxydopamine-treated Sprague-Dawley rats. Clonidine, at concentrations between 0.0001 and 100 muM has no intrinsic stimulatory activity on cortical cyclic AMP-generating systems from either control or 6-hydroxydopamine-treated rats. In F-344 rats phenoxybenzamine (100 muM) elicits a significant accumulation of cyclic AMP in cerebral cortical slices which can be abolished by pretreatment of rats with 6-hydroxydopamine, or by incubation of tissue slices with either clonidine or sotalol. The hyperresponsiveness to catecholamines usually observed following central administration of 6-hydroxydopamine failed to develop in the F-344 rat. Concentrations of phenoxybenzamine which have no significant stimulatory effects on cyclic AMP accumulation are capable of abolishing the potentiative effects of clonidine on the isoproterenol-stimulated formation of cyclic AMP. The beta-antagonist, sotalol, is a less effective antagonist of isoproterenol-stimulated accumulation of cyclic AMP in the presence of clonidine. Clonidine has no significant effect on the accumulation of cyclic AMP elicited by submaximal concentrations of isoproterenol in cerebellar slices. In toto, the data are consistent with the hypothesis that both the inhibitory effects on norepinephrine-stimulated accumulation of cyclic AMP and the stimulatory effects of clonidine on isoproterenol-elicited accumulation of cyclic AMP are exerted at postsynaptic alpha-adrenoceptors. No evidence was found for a presynaptic generation of cyclic AMP, although clonidine does reverse the stimulatory effects of phenoxybenzamine on cyclic AMP accumulation, presumably by interaction with a presynaptic site controlling norepinephrine release.

Clonidine antinociceptive activity: effects of drugs influencing central monoaminergic and cholinergic mechanisms in the rat

Clonidine is able to increase the threshold for vocalisation during stimulation and the threshold for vocalisation after withdrawal of stimulus (vocalisation afterdischarge). These effects of clonidine were investigated after treatment of rats with drugs influencing central monoaminergic and cholinergic mechanisms. Chlorpromazine, atropine and p-chlorophenylalanine increased the activity of clonidine at both thresholds while phenoxybenzamine and reserpine pretreatment increased the activity at the thresholds for vocalisation only. Yohimbine decreased clonidine activity at both thresholds while 5-HTP and alpha-methyl-p-tyrosine decreased the effects at the threshold for vocalisation afterdischarge. Naloxone did not change the activity of clonidine at either pain response studied. It is concluded from the present findings that influence from several neuronal systems modulate the antinociceptive action of clonidine. The inhibition of the medullary nociceptive response after clonidine might be connected to a decreased activity of noradrenergic neurons. Endogenous noradrenaline seems to be of minor importance in mediating this effect. It is moreover shown that decreased cholinergic receptor activity enhances clonidine antinociceptive action on both medullary and diencephalic-rhinencephalic pain responses. The possible involvement of serotonin these functional responses after clonidine is also discussed.

Inhibition of both noradrenergic and serotonergic neurons in brain by the alpha-adrenergic agonist clonidine

By means of single unit recording techniques it was found that a small systemically administered (intravenous) dose of the alpha-adrenergic agonist clonidine inhibited the spontaneous firing of brain norepinephrine (NE)-containing neurons in the locus coeruleus. In addition, the NE neurons were consistently inhibited by the direct (microiontophoretic) application of minute amounts of NE or clonidine. Intravenous clonidine also inhibited the firing of the great majority of (5-HT) neurons in the midbrain dorsal raphe nucleus. However, this action does not appearto be a direct one since clonidine (and NE) had relatively weak or variable effects when applied microiontophoretically to raphe neurons. The clonidine-induced depression of raphe firing may be secondary to an impairment in adrenergic transmission since (1) the depression could be reversed by the NE-releasing agents D- and L-amphetamine, (2) high doses of clonidine itself (which have been reported to have postsynaptic alpha-agonistic activity) reversed the depression produced by a low dose of clonidine and (3) prior destruction of NE neurons by 6-hydroxydopamine (7-12 days) rendered raphe neurons insensitive to the depressant effect of i.v. clonidine. Dopaminergic (substantia nigra, zona compacta) neurons did not respond to either low or high doses of clonidine. These results are consistent with previous data showing that clonidine decreases NE and 5-HT but not dopamine turnover. We conclude that systemically administered clonidine inhibits the firing of brain NE neurons by acting directly upon adrenergic receptors located on or near the soma of these neurons but that the concomitant inhibition of 5-HT neurons is an indirect effect (possibly secondary to an impairment in noracrenergic transmission).