Clonidine: new research in psychotropic drug pharmacology

The analgesic effect of metformin on paclitaxel-induced neuropathic pain model in rats: By considering pathological results

Background and Objective

Metformin (MET) has been used as an antidiabetic agent for type II diabetes. At the same time, recent researches have shown that the clinical improvement of MET is useful for nerve damage. In this study, we investigated the analgesic effect of MET in paclitaxel (PAC)-induced neuropathic pain.

Materials and Methods

Forty-two adult, female rats, Wistar strain weighing 220 ± 10 g were randomly divided into 5 experimental groups. PAC was intraperitoneally (IP) administered (2.0 mg/kg) for 4 groups every other day (0, 2, 4, and 6 days). By the 30^th day, MET (100, 200, and 400 mg/kg) was administered to 4 groups. Before and after treatment, basal pain threshold values were measured with Randall-Selitto analgesiometer test. At the end of experiment, pathological values were measured in selected regions including brain (motor cortex, M1), spinal cord (L4-L5), sciatic nerve, and muscle.

Results

According to our results, PAC-induced neuropathic pain reached to highest level at 14^th day. Four hundred milligram/kilogram concentration of MET remarkably decreased PAC-induced neuropathic pain. On the other hand, pathologic features have shown that PAC had significant pathological change in the brain and spinal cord while in the peripheral nerves and muscles had not shown any pathological change.

Conclusion

The pathological results of the current study for the first time demonstrated that MET beside of its antidiabetic effects reversed neuropathic pain induced by PAC. Consequently, this research can be promising for cancer patients that suffering from neuropathic pain induced by anticancer drugs.

Antinociception induced by amitriptyline and imipramine is mediated by alpha2A-adrenoceptors

The involvement of alpha2-adrenoceptors in the antinociception induced by the tricyclic antidepressants amitriptyline and imipramine was investigated in mice by using the hot-plate and abdominal constriction tests. The antinociception produced by amitriptyline (15 mg/kg, i.p.) and imipramine (15 mg/kg, i.p.) was prevented by reserpine (2 mg/kg, i.p.) and yohimbine (3-10 mg/kg, i.p.) but not by naloxone (1 mg/kg, i.p.), atropine (5 mg/kg, i.p.), CGP 35348 (100 mg/kg, i.p.) and prazosin (1 mg/kg, i.p.). On the basis of the above data, it can be postulated that amitriptyline and imipramine exerted their antinociceptive effect by activation of alpha2-adrenoceptors. Administration of the alpha2A-adrenoceptor antagonist BRL 44408 (1 mg/kg, i.p.) prevented amitriptyline and imipramine antinociception, whereas the alpha2B/C-adrenoceptor antagonist ARC 239 (10 mg/kg, i.p.) was ineffective. These data indicate that the enhancement of the pain threshold produced by amitriptyline and imipramine is mediated by activation of alpha2A-adrenoceptors. Neither tricyclic antidepressants nor the antagonists used impaired mouse performance evaluated by the rota-rod and hole-board tests.

Role of potassium channels in the antinociception induced by agonists of alpha2-adrenoceptors

1. The effect of the administration of pertussis toxin (PTX) as well as modulators of different subtypes of K+ channels on the antinociception induced by clonidine and guanabenz was evaluated in the mouse hot plate test. 2. Pretreatment with pertussis toxin (0.25 microg per mouse i.c.v.) 7 days before the hot-plate test, prevented the antinociception induced by both clonidine (0.08-0.2 mg kg(-1), s.c.) and guanabenz (0.1-0.5 mg kg(-1), s.c.). 3. The administration of the K(ATP) channel openers minoxidil (10 microg per mouse, i.c.v.), pinacidil (25 microg per mouse, i.c.v.) and diazoxide (100 mg kg(-1), p.o.) potentiated the antinociception produced by clonidine and guanabenz whereas the K(ATP) channel blocker gliquidone (6 microg per mouse, i.c.v.) prevented the alpha2 adrenoceptor agonist-induced analgesia. 4. Pretreatment with an antisense oligonucleotide (aODN) to mKv1.1, a voltage-gated K+ channel, at the dose of 2.0 nmol per single i.c.v. injection, prevented the antinociception induced by both clonidine and guanabenz in comparison with degenerate oligonucleotide (dODN)-treated mice. 5. The administration of the Ca2+-gated K+ channel blocker apamin (0.5-2.0 ng per mouse, i.c.v.) never modified clonidine and guanabenz analgesia. 6. At the highest effective doses, none of the drugs used modified animals' gross behaviour nor impaired motor coordination, as revealed by the rota-rod test. 7. The present data demonstrate that both K(ATP) and mKv1.1 K+ channels represent an important step in the transduction mechanism underlying central antinociception induced by activation of alpha2 adrenoceptors.

Dexamethasone modifies the behavioral effects induced by clonidine in mice

The present study examines the influence of dexamethasone on behavioral effects induced by clonidine in mice. 2. The behavior elements considered were locomoter activity, rota rod, catalepsy and stereotyped behavior (rearing, grooming, social response test, crossing, smelling, washing face, scratching and bar holding). 3. Clonidine (0.1-0.5-1.0 mg/kg, IP) induced a significant reduction of all behavioral elements studied when compared to the saline treated group: the behavioral reduction was significant 10 min after administration and lasted for the entire recording period (120 min). 4. Dexamethasone (0.1-0.5-1.0 mg/kg, IP) per se did not induce significant changes in the behavior elements recorded. 5. Dexamethasone (0.1-0.5 mg/kg, IP) dod not affect behavioral effects induced by the 3 doses of clonidine, whereas the high dose (1 mg/kg) of the steroid significantly reduced its behavioral inhibition. 6. The results of the present study suggest that dexamethasone induces significant effects on clonidine-induced behavioral effects and that this may be related to an interference with the monoaminergic system.

The effects of clonidine and tizanidine on responses of nociceptive neurons in nucleus ventralis posterolateralis of the cat thalamus

The effects of intravenous clonidine and tizanidine on nociceptive neurons in the nucleus ventralis posterolateralis (VPL) of the thalamus, a key station in the lateral system of ascending pain pathways, were evaluated in urethane-chloralose anesthetized cats. Intravenous clonidine and tizanidine produced a dose-dependent (5 and 10 micrograms/kg, and 25 and 50 micrograms/kg, respectively) suppression of responses of nociceptive specific (NS) and wide dynamic range (WDR) neurons in the VPL to high threshold splanchnic input. In contrast, the responses of both NS and WDR units to electrical stimulation of spinothalamic tract fibers in the ventrolateral funiculus (VLF) were little affected. We conclude that a site of suppressive action of the alpha 2-adrenoceptor agonists, as observed in nociceptive VPL neurons, is at the level of the spinal dorsal horn rather than in the VPL itself.

Central alpha-2 adrenoceptors are responsible for a clonidine-induced cue in a rat drug discrimination paradigm

Clonidine produces an interoceptive discriminative stimulus or "cue" in rat drug discrimination studies. This cue may be mediated by its alpha-2 adrenoceptor agonist properties and/or its affinity for the non-adrenoceptor imidazoline preferring receptor. Six rats were trained to respond differentially after receiving clonidine (0.02 mg kg-1, IP) or a saline vehicle. The alpha-2 adrenoceptor agonists clonidine, UK14, 304 and rilmenidine, which bind to the imidazoline preferring receptor, and guanabenz which does not, dose-dependently substituted for (i.e. > 80% total responding was clonidine associated) the clonidine-induced cue in doses up to 0.02, 0.16, 1.25 and 0.32 mg kg-1, respectively. Furthermore, the cue was blocked when clonidine was given in combination with 30-min pretreatments of the highly selective alpha-2 adrenoceptor antagonists RX811059 (2.5 mg kg-1) and fluparoxan (3 mg kg-1). Since the clonidine-induced cue was substituted for by guanabenz, which does not act at the imidazoline-preferring receptor, and antagonised by RX811059 and fluparoxan it appears to be mediated by alpha-2 adrenoceptors. Moreover, abolition of the clonidine-induced cue did not occur with the peripherally acting alpha-2 adrenoceptor antagonist L659, 066 suggesting it involves central as opposed to peripheral sites.

Anxiolytic-like effects of alpha-2-adrenoceptor agonists on conflict behavior in the rat: pre- versus postsynaptic receptor mechanisms

The effects of acute pretest administration and chronic posttest administration of clonidine or the selective alpha 2-adrenoceptor agonist UK-14,304 on conflict behavior were investigated. In daily 10-min sessions, water-deprived rats were trained to drink water from a tube that was occasionally electrified (0.25 mA); electrification was signaled by a tone. Prior to treatment, subjects accepted 25-30 shocks/session (punished responding) and consumed approximately 12-15 ml/session (unpunished responding). Acute pretest administration of clonidine or UK-14,304 did not increase punished responding. In contrast, chronic posttest clonidine administration (40 micrograms/kg, IP, twice daily for 8 weeks) resulted in a robust and time-dependent increase in punished responding (60-70 shocks/session) relative to saline-treated controls. Moreover, the selective alpha 2-adrenoceptor agonist UK-14,304 also increased punished responding when administered chronically (1.0 mg/kg, BID). Administration of the noradrenergic neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine HCl (DSP4, 65 mg/kg, IP) significantly decreased punished responding in control conditioned suppression of drinking sessions. The anticonflict effect associated with chronic posttest clonidine treatment was not altered by DSP4 pretreatment. These findings suggest that chronic posttest alpha 2-adrenoceptor agonist treatment produces an anticonflict effect independent of its actions at presynaptic alpha 2-adrenoceptors.

Corneal and conjunctival/scleral penetration of p-aminoclonidine, AGN 190342, and clonidine in rabbit eyes

The ocular penetration pathways of three alpha 2-adrenergic agents (p-aminoclonidine, AGN 190342, and clonidine) were investigated in rabbits both in vitro and in vivo. The corneal permeabilities of the compounds correlated positively with their octanol/water distribution coefficients. The ocular drug absorption via corneal and conjunctival/scleral penetration routes was evaluated separately after drug perfusion in vivo. In most cases, the corneal route was the major pathway for the intraocular drug absorption. However, the conjunctival/scleral penetration pathway was the predominant pathway for the delivery of p-aminoclonidine, the least lipophilic compound among the three drugs, to the ciliary body. The drug concentration in the iris was contributed mainly by the corneal route and correlated well with drug lipophilicity.

Associative influences on tolerance to decreased fixed-interval responding by clonidine

Responding of rats was maintained under a 5-min fixed-interval schedule of food presentation. One group of animals (n = 5) received the alpha-2-agonist clonidine (0.1 mg/kg/day) before experimental sessions for 16 weeks. Additional animals (n = 5) also received 0.1 mg/kg/day for 16 weeks but experienced drug administration after sessions for 4 weeks, before sessions for 4 weeks, after sessions for 4 weeks, and then finally, before sessions for 4 weeks. Animals receiving clonidine before daily experimental sessions for the entire period developed tolerance to decreased responding within 3 weeks, and their responding remained near control levels except when clonidine was occasionally preceded by the alpha-2-antagonist yohimbine. Animals receiving clonidine after sessions did not develop tolerance, and responding was markedly suppressed during the first exposure to presession clonidine. When these animals subsequently received clonidine again after sessions, responding was disrupted (increased) in spite of continued drug administration as if animals were "dependent" on clonidine in specific circumstances. When these animals again received clonidine before sessions, responding was partially suppressed in spite of uninterrupted drug administration as if animals had "lost" tolerance in specific circumstances. Tolerance to the behavioral effects of clonidine on fixed-interval responding was not determined by the presence of drug alone, but by the associative influence of drug-related effects in the presence of specific environmental stimuli.

Prenatal exposure to antiadrenergic antihypertensive drugs: effects on neurobehavioral development and the behavioral consequences of enriched rearing

One of the beta blockers propranolol (5 or 20 mg/kg) or atenolol (5 or 20 mg/kg) or the alpha-2 agonist clonidine (20 or 100 micrograms/kg) was administered in the drinking water to pregnant Wistar rats over gestation days 1 to 21. Maternal weight gain, litter size or litter weight were unaffected as were a number of measures of physical and behavioral development. The rats were raised after weaning in either enriched or impoverished housing. At maturity, male but not female rats that had been prenatally exposed to either of the three drugs weighed less than the controls. Testing for cognitive function using an automated version of the Hebb-Williams maze indicated impaired spatial maze problem solving ability for female rats that had been exposed to propranolol (20 mg/kg) and raised in impoverishment. Generally, enriched-reared rats committed fewer errors on the maze problems. Prenatal exposure to the drugs did not alter the beneficial effects of enriched rearing on maze solving. However, clonidine-exposed (100 micrograms/kg) female rats that were raised in enrichment showed a significantly increased mortality rate. Thus, prenatal exposure to all of these drugs exerted effects which were latent, appearing in adulthood, and which were to some extent precipitated by environmental factors. We conclude that these drugs may be physical and behavioral teratogens for the rat.

Noradrenergic receptor interactions in feeding elicited by stimulation of the para-ventricular hypothalamus

Food intake and feeding behaviour were examined after the administration of noradrenaline (NA) or clonidine to the para-ventricular nucleus of the hypothalamus, or after systemic administration of clonidine. In 20-hr deprived animals all treatments dose-dependently reduced pellet consumption; however, at a low dose (2-5 micrograms) central clonidine increased eating time and bout length. In 4-hr deprived animals all treatments increased sucrose consumption. Clonidine (peripheral and central) increased feeding time but did not alter feeding rate; NA did not alter feeding time, but did increase feeding rate; NA also increased activity and decreased resting. The effects of NA on feeding rate, activity and resting were blocked by propranolol; however, the propranolol-NA combination increased feeding time. Thus, NA and clonidine increased feeding by different mechanisms, but after propranolol pretreatment the effects of NA were similar to those of clonidine. It is concluded that clonidine enhances feeding by inhibiting satiety and that the feeding stimulant effect of NA is mediated by a complex interaction of alpha- and beta-receptors.

Alpha 2-adrenoceptor antagonist activity may account for the effects of buspirone in an anticonflict test in the rat

The anxiolytic effects of buspirone, its metabolite, 1-(2-pyrimidyl)piperazine (1-PP) and several alpha 2-adrenoceptor antagonists have been compared in an anticonflict (shock-induced suppression of drinking) paradigm in rats. Idazoxan, WY 26392 and yohimbine had anticonflict effects comparable to those of buspirone and 1-PP, and enhanced the release of suppressed responding induced by buspirone. The response to buspirone was antagonised by the alpha 2-adrenoceptor agonist clonidine. In tests of clonidine-induced mydriasis, the antagonist potencies of buspirone, 1-PP, idazoxan, WY 26392 and yohimbine corresponded closely to the doses of the compounds active in the anticonflict test. Clonidine-induced hypolocomotion was also antagonised by 1-PP, although this response was potentiated by buspirone. The results suggest that the anticonflict effects of buspirone involve an alpha 2-adrenoceptor mechanism.

Dose-related effects of detomidine on autonomic responses in the horse

1. Detomidine is a novel veterinary sedative analgesic which is thought to act by stimulation of alpha 2 adrenoreceptors. The present study was undertaken to determine the direction, time course, and dose-response relationship of detomidine on specific autonomic responses in the unanaesthetized horse. 2. Detomidine was administered intravenously to eight adult thoroughbred racehorses at doses of 0.010-0.040 mg kg-1, according to a double-blind Latin square crossover design. Cardiac and respiratory rates, pupil diameter and rectal temperature were monitored for 180 min postinjection. 3. Detomidine produced prolonged dose-related bradycardia and bradypnoea. This was accompanied by a briefer period of dose-related mydriasis. Response duration, rather than peak was consistently increased as a function of dose. 4. Rectal temperature was not altered in a dose-dependent fashion. Low doses of detomidine produced late onset hypothermia, while high doses produced early and late onset hyperthermia. 5. These data indicate the uniqueness in autonomic response by the horse to alpha 2 adrenoreceptor stimulation.

Objective assessment of detomidine-induced analgesia and sedation in the horse

The effects of detomidine, a veterinary sedative analgesic, were studied in the horse. Novel objective techniques were employed to assess the analgesic and sedative potency of this compound. Intravenous doses of 0.010, 0.020 and 0.040 mg/kg were administered to eight adult Thoroughbred racehorses according to a double-blind crossover design. Analgesia was measured by determining the latency to onset of the skin twitch and hoof withdrawal reflexes following noxious thermal stimulation of the withers and fetlock, respectively. Sedation was assessed by quantifying spontaneous locomotor activity in horses confined to an activity chamber, and by electronic measurement of head ptosis. Linear, dose-related increases in the intensity and duration of analgesia were observed. However, dose-related increases in duration, rather than intensity of sedation were observed. In conclusion, detomidine is an effective analgesic in the horse, but it is more potent in producing sedation than analgesia.

Modulation of p,p'-DDT-induced tremor by catecholaminergic agents

p,p'-DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane; 75 mg/kg) or corn oil was administered po to male Fischer 344N rats. Tremor was quantified 8 hr later by spectral analysis of whole body movements. The effect of sc injection of pharmacological challenges on the spectral profile of body movements was determined. The alpha antagonist phenoxybenzamine decreased the intensity of movements over most of the power spectra in animals exposed to DDT, but decreased spectral power only at lower frequencies in control subjects. The alpha-1 antagonist prazosin had similar effects in animals given DDT. The alpha-2 antagonist yohimbine and the beta antagonist propranolol produced lethality and increased the intensity of movements in animals administered DDT, without significantly affecting control animals. The alpha-2 agonist clonidine decreased the spectral profile over a wide range of frequencies in animals exposed to DDT, while depressing the spectral power of control animals only at higher frequencies. The dopamine antagonist haloperidol increased the intensity of movements in DDT-treated animals, without altering the spectral profile of controls. The dopamine agonist apomorphine induced stereotypy in control animals, but failed to significantly alter the power spectra in subjects given DDT. These data suggest a facilitatory and inhibitory role, respectively, for alpha-1 and alpha-2 receptors in the modulation of DDT-induced tremor. Dopamine and beta receptors may be involved in the tonic inhibition of tremor produced by DDT.

Pharmacological modification of tremor and enhanced acoustic startle by chlordecone and p,p'-DDT

Pretreatment of rats with phenoxybenzamine (5 mg/kg; SC), an alpha adrenergic antagonist, decreased the peak tremor power and startle magnitude of rats subsequently given DDT (75 mg/kg; PO) or chlordecone (60 mg/kg; IP), without having a significant effect on control animals. Pretreatment with an intracerebroventricular injection of calcium (3.75 microM in 5 microliters NaCl) decreased the peak tremor power due to subsequently administered DDT, while increasing the tremor response in rats later dosed with chlordecone. The effects of phenoxybenzamine are postulated to be due to a blockade of an excitatory influence of the adrenergic system. Calcium may decrease DDT-induced tremor by acting as a neuronal stabilizer. Potentiation of the tremorigenic effect of chlordecone by calcium may be due to increased levels of intracellular calcium, resulting in augmented release of neurotransmitters in chlordecone-exposed animals.

Selective antagonism by clonidine of the stereotyped and non-stereotyped motor activity elicited by atropine

The effects of clonidine, an indirectly-acting cholinergic antagonist, on 5 behaviors elicited by atropine (locomotion, rearing, sniffing, grooming and gnawing) were studied in rats. Clonidine did not alter the prevalence or magnitude of atropine-elicited locomotion and rearing. In contrast, clonidine suppressed the occurrence and degree of 3 stereotyped behaviors, namely, sniffing, grooming and gnawing. This selectivity of clonidine suggests differences in the neural pathways subserving the various stereotyped motor activities.

Alpha 2 adrenoceptors modulate the nociceptive jaw-opening reflex in rats

The jaw-opening reflex in lightly anesthetized rats induced by intrapulpal (left maxillary) electrical tooth pulp stimulation and quantified by the electromyograms (threshold values) recorded from the ipsilateral digastric muscle was used as the experimental pain index. The threshold for the jaw-opening reflex was significantly elevated by clonidine (12.5 to 50 micrograms/kg, i.v.) and was inversely correlated with the frequency of stimulation. The analgesia elicited by clonidine was antagonized by pretreatment with the alpha 2-adrenoceptor antagonist yohimbine (1 mg/kg, i.v.). All doses of clonidine produced an initial transient pressor response followed by a sustained hypotension and bradycardia. However, there was no direct correlation between the antinociceptive and cardiovascular effects of clonidine. It is proposed that clonidine modulates jaw-opening reflex analgesia by specifically activating alpha 2-adrenoceptors.

Overview of drugs used in treating drug-induced dependence: a treatise interrelating existing hypotheses in order to attain maximal therapeutic benefits

Despite the fact that we do not yet completely understand the physiology of physical dependence, many treatment modalities are available. In this overview, an attempt is made to discuss available treatments and mechanisms to possibly attain a better understanding of the complex interactions that occur between systems. An attempt is also made to understand interrelations between drug misuse and mental disorders as one aspect of the treatment process--the ultimate goal being more efficacious and safer therapy.

Chronic clonidine induces functional down-regulation of presynaptic alpha 2-adrenoceptors regulating [3H]noradrenaline and [3H]5-hydroxytryptamine release in the rat brain

Clonidine inhibited, through the activation of alpha 2 presynaptic receptors, the release of [3H]noradrenaline (pIC30 = 7.47) and [3H]5-hydroxytryptamine (pIC30 = 6.47) evoked by 15 mM KCl from superfused rat cerebral cortex synaptosomes. The natural agonist noradrenaline inhibited the K+-evoked release of the two [3H]amines less effectively after long-term (12 days) treatment with clonidine than after acute treatment. It can be concluded that chronic clonidine administration can induce down-regulation of both the alpha 2 presynaptic autoreceptors located on noradrenaline terminals and the alpha 2 presynaptic heteroreceptors located on serotonin terminals.

Possible involvement of central adrenergic and histaminergic systems in clonidine-induced inhibition of pinna reflex

Clonidine produced dose-dependent inhibition of pinna reflex in rats, the ED50 being 1.20 +/- 0.28 mg/kg. The effect of agents affecting adrenergic, tryptaminergic and histaminergic systems were evaluated on clonidine-induced inhibition of pinna reflex. All these agents had no effect on pinna reflex. Phentolamine, phenoxybenzamine, prazosin, propranolol, haloperidol, cyproheptadine and mepyramine did not affect significantly the action of clonidine. However, reserpine, alpha-methyl-p-tyrosine, alpha-methyldopa, diethyldithiocarbamate, 6-hydroxydopamine, yohimbine and cimetidine significantly blocked clonidine-induced inhibition of pinna reflex, indicating the involvement of central adrenergic-histaminergic systems.

Morphine and specific changes in the sensitivity of noradrenergic receptors within the "limbic" part of the feline caudate nucleus: a behaviour study

The present study describes the behaviour effects of intracerebral injections of the noradrenergic (NE) agonist oxymetazoline and the NE antagonist phentolamine into the "limbic" part of the caudate nucleus of cats primed 24 hr earlier and/or treated acutely with morphine (5 mg/kg, IP). Drug-induced changes in the morphine-specific behaviour served as dependent variables. Experiments were performed during two different periods of the year, each of them marked by a characteristic sensitivity of alpha-like NE receptors to NE agents, viz. the so-called NE "antagonist" period during which the NE receptors were sensitive to the NE antagonist phentolamine and the so-called NE "agonist" period during which the NE receptors were sensitive to NE and the NE agonist oxymetazoline. The present study demonstrates that morphine reversed the initial sensitivity to oxymetazoline respectively insensitivity to phentolamine in animals tested in the NE "agonist" period. In animals tested in the NE "antagonist" period morphine did not reverse the initial insensitivity to oxymetazoline resp. sensitivity to phentolamine. Furthermore, evidence is provided that the initial sensitivity to NE agents did not conspicuously determine the animal's response to the acute administration of morphine. The data are discussed in view of the concept that the firing rate of NE fibres determines the actual sensitivity of presynaptic and postsynaptic NE receptors to NE agonists and antagonists.

Effects of single and repeated daily injections of morphine, clonidine, and l-nantradol on avoidance responding of rats

Interruption of a photobeam by rats was maintained under a continuous shock avoidance schedule, and moderate response rates were maintained at low shock frequencies. Responding decreased, and shock frequency increased, in a dose-dependent manner after acute injections of the narcotic morphine, the antihypertensive l-nantradol clonidine, and the cannabinoid l-nantradol. Clonidine and l-nantradol were about 100 times more potent than morphine for decreasing overall responding, and l-nantradol was about 3 times more potent than clonidine for decreasing escape responding. When drugs were given repeatedly prior to daily experimental sessions, tolerance developed to response rate decreases of morphine and l-nantradol within seven to ten sessions, but tolerance did not develop to rate decreases of clonidine for up to 30 sessions. Continued decreased responding by clonidine was antagonized by yohimbine, but not by prazosin or naltrexone. These results extend observations for the acute effects of l-nantradol and clonidine to operant responding under a schedule of continuous shock avoidance. Different potencies for drugs in the present and previous experiments suggest important effects of response topography on dose effects.

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

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

Supersensitivity of analgesic responses to alpha 2-adrenergic agonists in genetically hypertensive rats

This study compared the responsivity of alpha 2-adrenoceptors to agonists in normotensive (WKY) and genetically hypertensive (SH) rats. Clonidine produced a greater degree of analgesia in SH as compared to WKY rats. This analgesia was antagonized by yohimbine. Neither naphazoline nor 4-hydroxy-clonidine produced analgesia in SH or WKY rats. Our results suggest that the analgesic effects of clonidine in SH rats are probably mediated by supersensitive central alpha 2-adrenoceptors.

Modification of alpha-adrenoceptor agonist antinociceptive activity by nisoxetine: a selective inhibitor of noradrenergic uptake

The alpha-adrenoceptor agonists, clonidine and xylazine, produced a dose-dependent antinociceptive effect in the mouse writhing assay as does morphine. Fluoxetine, a highly-specific inhibitor of serotonin uptake, enhanced the antinociceptive effect of morphine in this test but not that of clonidine or xylazine. In contrast, nisoxetine, a selective inhibitor of noradrenergic uptake, significantly potentiated the antinociceptive activity of morphine, clonidine, and xylazine. These findings strengthen the evidence for an involvement of a noradrenergic mechanism in the antinociceptive effects of alpha-adrenoceptor agonists.

Antinociceptive activity of N-(4-hydroxyphenacetyl)-4-aminoclonidine, a novel analog of clonidine: role of opioid receptors and alpha-adrenoceptors

N-(4-hydroxyphenacetyl)-4-aminoclonidine, a derivative of the alpha-adrenoceptor agonist p-aminoclonidine, was found to exhibit dose-dependent antinociceptive activity in the mouse writhing assay. In this measure of antinociceptive activity it was less potent than clonidine or xylazine. Naloxone, an opioid receptor antagonist, at a dose sufficient to abolish the antinociceptive activity of morphine, did not affect the antinociceptive activity of N-(4-hydroxyphenacetyl)-4-aminoclonidine, clonidine or xylazine. In contrast, yohimbine, a alpha-adrenoceptor antagonist, reduced the antinociceptive activity of N-(4-hydroxyphenacetyl)-4-aminoclonidine, clonidine and xylazine, but not morphine. The affinity of N-(4-hydroxyphenacetyl)-4-aminoclonidine, clonidine and xylazine for alpha-adrenoceptors in rat aorta was correlated highly with the relative potency for writhing inhibition. These results suggest that the antinociceptive activity of N-(4-hydroxyphenacetyl)-4-aminoclonidine is mediated by alpha-adrenoceptors.

Spinal clonidine inhibits neural firing in locus coeruleus

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

Pharmacological suppression of REM sleep prior to weaning counteracts the effectiveness of subsequent environmental enrichment on cortical growth in rats

Male Wistar rat pups were deprived of REM sleep by means of daily injections of clonidine between 8 and 21 days after birth. From day 28 they were reared under either 'enriched' or 'standard' environmental conditions. At 75 days of age the animals were sacrificed, and the regional brain weights were compared with two (differentially reared) control groups. Whereas cortical weight was greater in the enriched than in the standard control rats, no differences were found between the corresponding REM sleep-deprived (i.e. clonidine-treated) groups. These results suggest that REM sleep deprivation and/or disturbances of central noradrenergic function during early development can counteract growth responses of the brain to environmental stimulation later in life.

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.

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.

Antinociceptive action of sodium valproate in the mouse

The antinociceptive action of sodium valproate (VPA) was examined using male NMRI mice. Using the hot-plate assay at 60 degrees C, orally-administered VPA (50-400 mg/kg) produced antinociceptive effects; the ED50 was about 160 mg/kg. Oral doses of VPA (6.3-400 mg/kg) decreased the writhing response elicited by intraperitoneally-injected acetic acid. The antinociceptive effect of VPA, as determined with the writhing test, exhibited complex characteristics, the most pronounced effect occurring at doses of 12.5-50 mg/kg. The antinociceptive effect of VPA in the writhing test was not antagonized by bicuculline or by naloxone. VPA, like other agents which enhance central GABA-ergic mechanisms, might possess analgesic activity.

Impairment of environmental effects on brain weight by adrenergic drugs in rats

Injection of clonidine or methyldopa in rats immediately after daily two-hour exposure to a complex environment (EC) reduced the increase in occipital cortical weight normally associated with such exposure in comparison with animals kept in isolation. Drug injection four hours after EC exposure had no effect. Drug induced suppression of REM sleep during the first four hours after EC exposure was suggested to be the critical variable.