alpha 2-adrenoceptors modulating diarrhoea in morphine-dependent rats

The effect of clonidine on gastrointestinal side effects associated with ultra-rapid opioid detoxification

IMPLICATIONS

Clonidine, an alpha(2)-adrenergic agonist, is used to minimize withdrawal symptoms related to ultra-rapid opioid detoxification procedures. These preliminary data suggest that clonidine possesses dose-related antidiarrheal activity.

Clonidine infusions into the locus coeruleus attenuate behavioral and neurochemical changes associated with naloxone-precipitated withdrawal

Clonidine, an alpha-2-adrenergic agonist, suppresses signs of opiate withdrawal in animals and in man. Electrical or chemical stimulation of the nucleus locus coeruleus (LC) increases noradrenergic activity and brain concentration of the noradrenergic metabolite MHPG, and produces many signs of opiate withdrawal. Thus, clonidine's ability to attenuate withdrawal might be due to the reduction of noradrenergic neuronal activity originating in the LC, but additional alpha-2-adrenergic receptors throughout the body and other mechanisms may also play a role. The present study explored the neuroanatomical and pharmacological selectivity of alpha-2-adrenergic receptors of the LC in the anti-withdrawal action of clonidine. Experiment 1 tested the hypothesis that behavioral and biochemical measures of naloxone-precipitated withdrawal from morphine would be blocked by infusions of clonidine (0.6 or 2.4 micrograms/microliters) into the LC. Significant reductions were observed in the occurrence of diarrhea, ptosis, weight loss and wet-dog shakes. Clonidine also reversed the naloxone-precipitated increase in hippocampus MHPG concentration. In experiment 2 subjects received an LC infusion or IP injection of a non-lipophilic alpha-2-agonist (ST-91), which does not penetrate the blood-brain barrier, or of clonidine into the dorsal parabrachial nucleus (DPB) to test the selectivity of the effects of clonidine infusions into the LC. ST-91 infusions into the LC reduced several of the observed withdrawal signs and increased others (e.g., jumping). Although peripheral injections of ST-91 attenuated some of the checked signs associated with naloxone-precipitated withdrawal, the frequency of wet-dog shakes was not reduced. ST-91 infusions into the LC, but not systemic ST-91 administration, prevented the withdrawal-induced increase in hippocampus MHPG concentration. Clonidine infused lateral to the LC into the DPB did not significantly attenuate withdrawal or reduce hippocampus MHPG levels. These results provide behavioral and biochemical evidence to support the suggestion that clonidine significantly attenuates naloxone-precipitated withdrawal through an interaction with noradrenergic neurons located in the vicinity of the LC.

Insights into opioid action in the intestinal tract

Opioids have been used for centuries as antidiarrhoeal drugs. In recent years, their mechanism and sites of action in exerting their antidiarrhoeal effect have been studied intensely. Attempts have been made to propose their general mode of action. Whilst there are numerous similarities in their general effects on motility, fluid secretion, and neuroeffector transmission, the differences between species, in some cases, can be remarkable. We highlight and contrast the similarities and differences in the commonly examined species and compare them to humans. Insights into mechanisms of opioid antidiarrhoeal action now also provide some new perspectives of opioid action in the intestine.

The effect of some alpha 2-adrenoceptor agonists and antagonists on gastrointestinal transit in mice: influence of morphine, castor oil and glucose

1. The effects of graded doses of the alpha 2-adrenoceptor agonists clonidine, tizanidine and BHT-920, and the alpha 2-adrenoceptor antagonists yohimbine and idazoxan, on gastrointestinal transit were investigated in mice using the charcoal meal test. 2. The agonists produced significant and dose-dependent decreases in gastrointestinal transit, and the antagonists produced the opposite effect. In affecting the gastrointestinal transit, clonidine (1 mg/kg) was as effective as tizanidine (12 mg/kg) and BHT-920 (40 mg/kg), while yohimbine (2 mg/kg) was as effective as idazoxan (1 mg/kg). 3. Morphine (2, 4 and 8 mg/kg) significantly inhibited gastrointestinal transit. This effect was significantly reversed by the co-administration of yohimbine (2 mg/kg) and idazoxan (1 mg/kg). 4. The acute administration of glucose (5.04 g/kg, i.p.) potentiated the inhibition of gastrointestinal transit produced by clonidine (1 mg/kg) and BHT-920 (40 mg/kg). Glucose treatment, however, had no significant effect on the increase in gastrointestinal transit induced by yohimbine (2 mg/kg) or idazoxan (1 mg/kg). 5. Castor oil (0.25 mL/mouse, orally) induced diarrhoea in saline-treated animals within about 45 min. Clonidine (1 mg/kg), tizanidine (12 mg/kg) and BHT-920 (40 mg/kg) delayed the occurrence of diarrhoea to 2.1, 1.2 and 1.4 h, respectively.

A paradoxical stimulatory effect of berberine on guinea-pig ileum contractility: possible involvement of acetylcholine release from the postganglionic parasympathetic nerve and cholinesterase inhibition

The effects of berberine on guinea-pig ileum contractility were studied in both transmurally-stimulated and unstimulated preparations. Transmural stimulation (80 V, 0.5 ms, 0.05 Hz) of the guinea-pig ileal segments produced a twitch response. Berberine (10(-8)-10(-5) M) enhanced this response dose-dependently. Atropine (10(-7) M), but not mecamylamine (10(-5) M), abolished this response. Acetylcholine (3 x 10(-9) and 10(-8) M) also enhanced the response to transmural stimulation. Pretreatment with hemicholinium (3 x 10(-5) M) antagonized the effect of berberine but failed to change that of acetylcholine. Berberine (10(-5) M) also antagonized the alpha 2-adrenoceptor agonist xylazine (10(-8) and 10(-7) M)-induced inhibition of the twitch response to transmural stimulation. In unstimulated ileal preparations, berberine (10(-5) M) produced contractile responses. In these preparations, atropine (10(-6) M), but not mecamylamine (10(-4) M), abolished the response to berberine. Furthermore, berberine (10(-6)-10(-4) M) inhibited dose-dependently the cholinesterase activity of the guinea-pig blood. The results suggest that berberine increases ileal contractility by: 1) increasing acetylcholine release from the postganglionic parasympathetic nerve terminal, 2) increasing acetylcholine retention through an inhibition of cholinesterase activity, and 3) blocking alpha 2-adrenoceptors, possibly in the postganglionic parasympathetic nerve.

Effect of adenosine analogues on the expression of opiate withdrawal in rats

The aim of this study was to test whether convergent dependence occurs in vivo. The adenosine A1 receptor agonist N6-[(R)-1-methyl-2-phenylethyl]adenosine (R-PIA), the A2 agonist 2-(phenylamino)adenosine (CV-1808), the nonselective A1, A2 agonist (adenosine-5'-ethylcarboxamide (NECA), and the alpha 2-adrenoceptor agonist clonidine were screened (each at 30, 100, and 300 micrograms/kg, SC) for their ability to alter naloxine-precipitated withdrawal signs in morphine-dependent rats. The results indicate that there is convergent dependence involving opioid and adenosine A1 receptors on those effects expressed by withdrawal diarrhoea, paw-shakes, teeth-chattering, body-shakes, and jumping. Further, dependence expressed by body-shakes involves convergence involving A1 receptors, as well as alpha 2-adrenoceptors; while A1 receptors are involved in dependence expressed by jumping, stimulation of alpha 2-adrenoceptors augments this sign. Adenosine analogues may be of clinical value for detoxification of opiate addicts.

Reproducible withdrawal contractions of isolated guinea-pig ileum after brief morphine exposure: effects of clonidine and nifedipine

Guinea-pig ileum stored for 30 min in Krebs solution and then mounted in Tyrode solution gave reproducible contracture responses to naloxone after brief exposure to morphine. The preparation lasted for several hours and a variety of pharmacological tests could be made. Clonidine, an alpha 2-adrenoceptor agonist, and nifedipine, a calcium channel antagonist, both known to interfere with tolerance and physical dependence, inhibited naloxone withdrawal contractures in a dose related way. Their action seemed to be receptor-mediated since yohimbine and Bay k 8644, respectively, reversed their inhibitory effect.

Clonidine attenuates increased brain glucose metabolism during naloxone-precipitated morphine withdrawal

The effect of two doses of clonidine on regional cerebral metabolic rates for glucose were measured during morphine withdrawal in rats. In the first study, 0 or 200 micrograms/kg clonidine was administered to rats subjected to naloxone-precipitated morphine withdrawal (naloxone, 0.5 mg/kg, s.c.), and to non-dependent control rats. In a second study of similar design, 0 or 20 micrograms/kg clonidine were administered. Withdrawal signs in rats subjected to naloxone-precipitated morphine withdrawal and receiving 0, 20 or 200 micrograms/kg clonidine were also assessed. Naloxone-precipitated morphine withdrawal stimulated regional cerebral metabolic rates for glucose (59 of 83 regions in study no. 1; 73 of 83 regions in study no. 2). At 200 micrograms/kg, clonidine attenuated this effect (33 of 59 regions). Although 200 micrograms/kg clonidine directly suppressed regional cerebral metabolic rates for glucose in many regions (significant main effect of clonidine), it attenuated the naloxone-precipitated morphine withdrawal effect specifically in the lateral septal nucleus, medial habenula, subiculum and gracile nucleus (significant interactions between clonidine and morphine withdrawal). The 20 micrograms/kg dose of clonidine had no statistically significant effect. In behavioral experiments, both doses of clonidine diminished withdrawal in that there was no diarrhea, fewer wet-dog shakes and less abnormal posturing. However, locomotion, grooming and jumping were increased by clonidine. Most of these effects were statistically significant only with the 200 micrograms/kg dose. The results of these studies show that clonidine reduces morphine withdrawal-induced increases in regional cerebral metabolic rates for glucose in many brain regions, irrespective of the distribution of alpha 2-adrenoceptors. Although clonidine has been thought to ameliorate morphine withdrawal by actions primarily at the locus coeruleus and central amygdala, it may play a major role in other regions as well.

Adenosine triphosphate blocks opiate withdrawal symptoms in rats and mice

The effect of adenosine triphosphate (ATP) on the expression of opiate withdrawal was examined using a chronic model of morphine-dependence. ATP was studied for its ability to modify or block jumping in morphine-abstinent mice. In mice administered 2 mg/kg ATP intravenously, the naloxone ED50 for withdrawal jumping increased by 11-fold in comparison to saline-treated mice. Nalaxone-precipitated morphine-withdrawal in the rats, has been shown to induce a specific pattern of intestinal hypermyoelectric activity and to increase the arterial blood pressure. Administration of ATP at dose of 1 and 2 mg/kg intravenously inhibited the induction of hypermyoelectric activity pattern in 80 and 100% of animals tested respectively. ATP also blocked the increase in mean arterial blood pressure seen during withdrawal in a dose-dependent fashion. Investigations were carried out to determine if blocking of the alpha 2-adrenoreceptors with yohimbine would result in an alteration in antiwithdrawal action of ATP. Yohimbine reversed the effect of ATP in blocking naloxone-precipitated withdrawal on the myoelectric activity of jejunum and colon, however, it failed to antagonize the effect of ATP on withdrawal jumping and to block the effect of ATP on the pressor response produced by naloxone in morphine-dependent animals.

Sites of clonidine action to inhibit gut propulsion in mice: demonstration of a central component

The role of central (supraspinal and spinal) and peripheral alpha-adrenoceptors in the regulation of gastrointestinal propulsion in the mouse was studied using clonidine, an alpha 2-adrenoceptor agonist. Clonidine produced a dose-dependent inhibition of propulsion when given intracerebroventricularly, intrathecally, or subcutaneously, but was most potent when given intracerebroventricularly. The antitransit effects of centrally given clonidine were antagonized by intracerebroventricular (i.c.v.) yohimbine, but higher doses were required when this antagonist was given peripherally. Whereas i.c.v. and s.c. administration of clonidine were effective in inhibiting gut transit in spinally transected mice, intrathecal (i.th.) administration of this agonist was not. A supraspinal site of clonidine action is suggested based upon (a) the higher central to peripheral potency of clonidine; (b) the greater potency of i.c.v., compared with s.c., administration of yohimbine in blocking i.c.v. clonidine; (c) the lack of effect of i.th. administration of clonidine in spinally transected mice; and (d) the reduced potency of i.c.v., but not s.c., administration of clonidine in spinally transected mice. Additionally, a peripheral site of clonidine action is suggested by (a) the lower potency of i.c.v. yohimbine in blocking s.c., compared with i.c.v., clonidine; (b) the lower potency of i.c.v. yohimbine in blocking i.c.v. clonidine in transected mice (compared with normal mice); (c) the equal potency of s.c. clonidine in slowing propulsion in normal and spinally transected mice; and (d) the equal potency of i.c.v. yohimbine in blocking s.c. clonidine in normal and spinally transected mice. These data in mice would thus support the concept that normal (peripheral) therapeutic administration of clonidine would affect gut motor function by interactions within the brain and directly at the level of the gut.

Changes in locomotor-activity patterns as a measure of spontaneous morphine withdrawal: no effect of clonidine

The anti-withdrawal action of clonidine was studied in a model of spontaneous morphine withdrawal in rats. After withdrawal the behaviour of the animals was registered continuously for several days. In the initial phase of induction of dependence the locomotor activity was enhanced during daytime. Partial tolerance to this increase developed in the course of 3 weeks. In morphine withdrawn animals the activity decreased strongly at night, and this effect was maximal on the second night after removal of morphine. After four nights the nightly activity was restored. Treatment with clonidine (100 micrograms/kg s.c. twice daily) changed neither the observed decrease in nightly locomotor activity nor other withdrawal symptoms such as a decrease in food intake and loss of body weight. In non-dependent animals clonidine induced a biphasic effect in locomotor activity, i.e. a decrease in the first few hours of the night and an increase in the second part of the night. The latter can be interpreted as a rebound phenomenon occurring after only three injections. It was concluded that clonidine was not effective as an anti-withdrawal compound in a model for spontaneous morphine abstinence. The low incidence of symptoms relating to a hyperactive sympathetic system during spontaneous withdrawal may be an explanation for the absence of an effect of clonidine.

Clonidine in neuropsychiatric disorders: a review

We describe the basic pharmacological principles underlying the activity of the alpha 2-adrenergic receptor agonist clonidine, including its interactions with the cholinergic, histaminergic, serotoninergic, endorphinergic and possibly dopaminergic systems. The use of clonidine for the therapy of various neuropsychiatric indications in which it appears to be beneficial is described. These conditions include migraine, Korsakoff's psychosis, Tourette's syndrome, withdrawal states, tardive dyskinesia, essential tremor, neuroleptic-induced akathisia, neurogenic bladder, idiopathic orthostatic hypotension, paroxysmal localised hyperhydrosis, diabetic neuropathy and stiff-man syndrome. The need for long term evaluation of this agent in some of these diseases is stressed.

Antidiarrheal therapy. Prospects for new agents

Successful treatment of severe diarrhea has traditionally relied upon opiates or opiate derivatives. Recent advances in our understanding of intestinal fluid and electrolyte absorption have provided the opportunity to develop therapeutic agents specific for various points in the secretory and absorptive process. Present and proposed antidiarrheal agents, in addition to antimotility activity, will be capable of stimulating intestinal fluid absorption, inhibiting intestinal fluid secretion, or both. The mechanism(s) of action and clinical implications for proposed antidiarrheal agents are reviewed.

Dopaminergic and alpha 1-adrenergic properties of B-HT920 revealed in morphine-dependent rats

B-HT920 is known to be a selective alpha 2-adrenoceptor agonist, and has been used in a study on morphine-withdrawal in rats. In accordance with other alpha 2-agonists B-HT920 was found to potentiate "jumping" and to reduce "body shakes." However, B-HT920 did not suppress body weight loss. Furthermore, it induced strong salivation and prevented ptosis (described for the alpha 1-adrenergic agonist ST-587). Rearing and locomotor activity appeared to be enhanced, an effect shared by dopamine-agonist lisurid. The effects of B-HT920 have been specified using the alpha-adrenergic antagonists yohimbine and prazosin and the dopamine antagonist haloperidol. Yohimbine could not antagonize any of the actions of B-HT920. However the increase in rearing and locomotion was blocked by haloperidol. The induction of salivation was prevented by prazosin. Pretreatment with prazosin showed a decrease in the loss of body weight caused by B-HT920, while pretreatment with yohimbine showed that B-HT920 induced an increased loss in body weight. These data suggest that B-HT920 under certain conditions exerts dopamine-agonistic actions in stimulating locomotor activity and alpha 1-adrenergic actions in inducing salivation and enhanced loss of body weight.

Further validation of in vivo and in vitro pharmacological procedures for assessing the alpha 2/alpha 1-selectivity of test compounds: (2). Alpha-adrenoceptor agonists

Eighteen chemically dissimilar alpha-adrenoceptor agonists were assessed in 6 different tests. The antidiarrheal activity of the compounds was highly correlated with the diuretic activity. Both activities were highly correlated with the inhibition of [3H]clonidine or [3H]idazoxan binding; the degree of correlation with inhibition of [3H]WB4101 binding or with antiptotic activity was much less. Antiptotic activity was better correlated, although still poorly, with inhibition of [3H]WB4101 binding than with inhibition of [3H]clonidine or [3H]idazoxan binding. The in vivo antiptotic/diuretic or antiptotic/antidiarrheal potency ratios both reflected the known alpha 2/alpha 1-selectivity of the compounds tested and were significantly correlated with the in vitro potency ratios between inhibition of [3H]WB4101 binding on one hand and of [3H]clonidine or [3H]idazoxan binding on the other. The reliability of the above models for measuring alpha 1- and alpha 2-adrenoceptor agonistic activity and selectivity is discussed.

Further validation of in vivo and in vitro pharmacological procedures for assessing the alpha 2/alpha 1-selectivity of test compounds: (1). Alpha-adrenoceptor antagonists

Twenty one chemically dissimilar alpha-adrenoceptor antagonists were assessed in 7 different tests. The potencies of the compounds to protect rats from norepinephrine lethality were closely correlated with their potencies for inhibition of [3H]WB4101, but not of [3H]clonidine, [3H]idazoxan, [3H]rauwolscine or [3H]yohimbine binding to rat brain homogenates. The reverse was found for the potencies of the compounds to prevent the antidiarrheal effect of clonidine. The in vivo anti-norepinephrine/anti-clonidine potency ratios were highly correlated with the ratios between the in vitro potency to inhibit [3H]WB4101 binding on one hand and [3H]clonidine, [3H]idazoxan, [3H]rauwolscine or [3H]yohimbine binding on the other. The reliability of the above models for measuring alpha 1- and alpha 2-adrenoceptor antagonistic activity and selectivity is discussed.

Effects of alpha 2-agonists on morphine withdrawal behaviour: potentiation of jumping mediated by alpha 2-receptors

Clonidine has a dual action on naloxone-precipitated morphine withdrawal symptoms in rats: a suppressive action on body shakes and body weight loss and a potentiating action on jumping and aggression. It has been suggested that this potentiating, excitatory action is mediated by alpha 1-receptors. More specific alpha 2-agonists therefore should have a less excitatory effect on the latter symptoms. This hypothesis has been studied in rats dependent on morphine. Withdrawal was precipitated using naloxone. Prior to naloxone the alpha 2-agonists clonidine, guanfacine, azepexole, BHT-920, UK 14304 or the centrally acting alpha 1-agonist ST 587 were administered. All alpha 2-agonists but not the alpha 1-agonist potentiated the jumping and decreased body shakes and body weight loss. The effects of clonidine and azepexole were characterized pharmacologically using the alpha-antagonists yohimbine and prazosin. Jumping potentiated by clonidine was antagonized by yohimbine whereas prazosin had no effect. Azepexole induced jumping was decreased by yohimbine both with respect to incidence and frequency, whereas prazosin only lowered the frequency. The suppressive actions of clonidine and azepexole on body shakes were reversed by yohimbine and not by prazosin. The data indicate that the potentiation of jumping by alpha 2-agonists as well as the suppression of body shakes in morphine withdrawal behaviour is mediated by alpha 2-receptors.

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.

Yohimbine exacerbates and clonidine attenuates acute morphine withdrawal in rats

The involvement of alpha 2-noradrenergic receptors in the expression of opiate withdrawal was studied using an operant behavioral model of acute morphine dependence. Clonidine, an alpha 2-agonist, attenuated and yohimbine, an alpha 2-antagonist, exacerbated the naloxone-induced suppression of fixed ratio 15 responding in rats pretreated several hours earlier with a single, moderate dose of morphine. These data indicate that the alpha 2-agonist action of clonidine is responsible for its amelioration of withdrawal symptoms. It also validates the acute dependence model for studying the pharmacology of opiates and associated adaptive processes.

Further support for the central origin of the gastric antisecretory properties of clonidine in conscious rats

The effects of clonidine (CL) on interdigestive gastric acid secretion were studied in chronic gastric fistula rats and compared with those of St 91, a CL-like drug which does not cross the blood-brain barrier. CL induced a dose-dependent inhibition of gastric acid output when administered s.c. (ED50 13.5 micrograms X kg-1), in the lateral ventricle of the brain (ED50 5.80 micrograms X kg-1) or in the cisterna magna (ED50 0.49 micrograms X kg-1). St 91 and several alpha-adrenergic antagonists administered subcutaneously also inhibited gastric acid secretion dose dependently. Decreasing activities were prazosin greater than St 91 greater than phentolamine greater than yohimbine greater than piperoxan; phenoxybenzamine was a poor inhibitor. The anti-secretory properties of CL on basal secretion were antagonized by yohimbine and piperoxan. Other drugs either did not antagonize (phentolamine) or potentiated (phenoxybenzamine, prazosin) the CL effect, while yohimbine and phenoxybenzamine potentiated the response to St. 91. CL had no activity on methacholine while it drastically inhibited 2-DG-, histamine- and pentagastrin-stimulated gastric secretion. The inhibitory activity of CL on histamine stimulation was significantly decreased by yohimbine but not by phentolamine. These results confirm that the CL inhibition of the gastric acid secretion in unanesthetized rats is mediated at least partly through the stimulation of alpha 2-adrenergic receptors. Most of the data favour a major role of the CNS in the antisecretory activity of CL and suggest the possibility that most of the receptors involved are located in the medulla oblongata.

Xylazine-induced delay of small intestinal transit in mice

Subcutaneous injection of xylazine (0.1-3.0 mg/kg) produced a dose-dependent delay of small intestinal transit without affecting gastric emptying in the conscious mice. The xylazine-induced delay of small intestinal transit was antagonized by alpha 2-adrenoceptor antagonists, e.g., yohimbine, piperoxan and tolazoline. The antagonism of xylazine activity by yohimbine was dose-dependent, and the maximal antagonistic effect was seen at 1 mg/kg. Other adrenoceptor antagonists with only alpha 1-blocking activity, e.g., thymoxamine, prazosin and phenoxybenzamine at the doses studied did not reduce the depressant effect of xylazine on small intestinal transit. A beta-adrenergic antagonist, propranolol was not effective in reducing xylazine activity. The opioid antagonist, naloxone did not reduce the effective of xylazine, nor did yohimbine antagonize the morphine-induced delay of small intestinal transit. The xylazine-induced delay of small intestinal transit was not altered by atropine, hexamethonium, haloperidol, methysergide, chlorpheniramine or cimetidine. Pretreatment of mice with reserpine and alpha-methyl-p-tyrosine or 6-hydroxydopamine failed to reduce the intestinal effect of xylazine. These results suggest that xylazine-induced delay of small intestinal transit is mediated by postjunctional alpha 2-adrenoceptors and appears not to involve activation of opioid, cholinergic, dopaminergic, histaminergic, or serotonergic receptors.