The properties of the clonidine withdrawal response of guinea-pig isolated ileum

The effects of simultaneous administration of alpha(2) -adrenergic agents with L-NAME or L-arginine on the development and expression of morphine dependence in mice

Both alpha(2)-adrenoceptors and the L-arginine/nitric oxide (NO) pathway have been implicated in the modulation of morphine dependence. This study examined the effects of simultaneous administration of the alpha(2)-adrenoceptor agonist clonidine or the antagonist yohimbine together with the NO precursor L-arginine or the NO synthase (NOS) inhibitor NG-nitro-L-arginine methyl ester (L-NAME) on the induction and expression of morphine dependence as assessed by naloxone-precipitated withdrawal jumping and diarrhoea. Male NMRI mice weighing 20-30 g were used. In the induction phase, clonidine (0.01-0.1 mg/kg) intensified and yohimbine (0.5-2 mg/kg) attenuated the degree of morphine dependence. Yohimbine reversed the effect of clonidine. L-NAME (5 and 10 mg/kg) did not affect the development of morphine dependence, but significantly potentiated the effects of both subeffective (0.01 mg/kg) and effective (0.03 mg/kg) doses of clonidine. L-Arginine did not alter morphine dependence but inhibited the effect of clonidine. The effects of yohimbine in the induction phase were attenuated by L-NAME, but were not significantly affected by L-arginine. In the expression phase, clonidine attenuated and yohimbine intensified the signs of dependence. The effect of clonidine was inhibited by yohimbine. In the expression phase, L-NAME attenuated the withdrawal syndrome at 10 mg/kg and showed potentiation with clonidine in suppressing withdrawal signs. L-Arginine did not alter morphine dependence, but at 20 mg/kg inhibited and at 100 mg/kg potentiated the attenuating effect of clonidine on the expression of withdrawal syndrome. The effect of yohimbine on the expression phase was also attenuated by L-NAME, but was not significantly affected by L-arginine. In conclusion, alpha(2)-adrenergic and NO pathways seem to be functionally linked in the modulation of opioid dependence.

Lithium inhibits the development of physical dependence to clonidine in mice

Based on our previous finding that chronic lithium treatment reduced naloxone-precipitated withdrawal syndrome in morphine-treated mice, the effect of chronic lithium treatment was evaluated on the development of dependence to clonidine. Dependence was induced by injection of either morphine (50, 50 and 75 mg/kg, intraperitoneally with 3 hr interval for 3 consecutive days), or clonidine (2 mg/kg/day, intraperitoneally for 10 days). Naloxone (4 mg/kg, intraperitoneally) precipitated withdrawal signs in both morphine- and clonidine-treated mice. Yohimbine (5 mg/kg, intraperitoneally) precipitated withdrawal signs in the clonidine-treated mice, similar to morphine withdrawal signs; but failed to precipitate any significant sign in the morphine-treated mice. Coadministration of lithium was carried out by adding lithium chloride to drinking water (600 mg/l for 20 days; 10 days before the beginning of clonidine administration and 17 days before the administration of morphine to allow the lithium concentration to reach steady-state). The results indicated that chronic lithium administration significantly attenuated the withdrawal signs, precipitated either by yohimbine or naloxone, in clonidine-treated mice. As a conclusion, clonidine withdrawal signs are very similar to opioid withdrawal signs, and lithium is able to prevent the development of physical dependence to clonidine.

The effect of clonidine on the naltrexone-induced withdrawal response in morphine-treated guinea-pigs

Rapid opioid withdrawal induced by naltrexone is now used as a treatment for heroin addiction. The alpha2-adrenoceptor agonist, clonidine, is currently used in clinical practice to reduce opioid withdrawal in humans. However, few studies have been reported on its effectiveness for this purpose. Guinea-pigs were made dependent and tolerant to morphine using a 3-day chronic morphine regimen (total 410 mg kg(-1) morphine base), and injected with either clonidine (0.1 mg kg(-1), s.c.) or saline, 1 h before induction of withdrawal with naltrexone (15 mg kg(-1), s.c.). Withdrawal behaviours were measured for 90 min and animals were then euthanased and the brains removed. The presence of the immediate early gene protein product, c-Fos, was detected using immunohistochemical techniques. Clonidine reduced the number of head/body shakes, but had no effect on the total withdrawal behaviour score. In the CNS, clonidine increased the number of Fos-LI neurons in the central amygdala. In conclusion, the modest effect of clonidine in the present experiments suggests that the efficacy of clonidine in humans undergoing naltrexone-induced opioid withdrawal requires further investigation.

Interactions between cholecystokinin and opioids in the isolated guinea-pig ileum

1. Although cholecystokinin octapeptide sulphate (CCK-8) activates the opioid system of isolated guinea-pig ileum (GPI) whether it activates the mu- or kappa-system, or both, remains unclear. Neither is it known whether CCK-8 influences the withdrawal responses in GPI preparations briefly exposed to opioid agonists. This study was designed to clarify whether CCK-8 activates mu- or kappa-opioid systems or both; and to investigate its effect on the withdrawal contractures in GPI exposed to mu- or kappa-agonists and on the development of tolerance to the withdrawal response. 2. In GPI exposed to CCK-8, the selective kappa-antagonist nor-binaltorphimine elicited contractile responses that were concentration-related to CCK-8 whereas the selective mu-antagonist cyprodime did not. 3. In GPI preparations briefly exposed to the selective mu-agonist, dermorphin, or the selective kappa-agonist, U-50, 488H, and then challenged with naloxone, CCK-8 strongly enhanced the withdrawal contractures. 4. During repeated opioid agonist/CCK-8/opioid antagonist tests tolerance to opioid-induced withdrawal responses did not develop. 5. These results show that CCK-8 preferentially activates the GPI kappa-opioid system and antagonizes the mechanism(s) that control the expression of acute dependence in the GPI.

Selective potentiation by ouabain of naloxone-induced withdrawal contractions of isolated guinea-pig ileum following acute exposure to morphine

1. Ouabain, an inhibitor of Na+/K+ ATPase induces the release of acetylcholine from central and myenteric cholinergic neurones principally due to partial depolarization of the cell membrane. The effect of ouabain has been examined on neurogenic contractions in the guinea-pig ileum arising from either electrical field stimulation or from naloxone in morphine-exposed preparations. 2. Guinea-pig isolated ileum preparations were stimulated transmurally (0.1 Hz, 0.3 ms, 200 mA) to elicit contractions of the myenteric plexus-longitudinal smooth muscle. 3. Incubation with morphine (0.3 microM, 60 min) was followed by naloxone (1 microM) which produced withdrawal contractions in 16/26 preparations (median of 10.7 [2.2-40.0]% of a maximal contracture to KCl (60 mM)). 4. In parallel experiments, ouabain (1 microM) was added to the tissue before exposure to morphine (0.3 microM, 60 min). Naloxone (1 microM) subsequently displayed a withdrawal contraction in all 26/26 tissues (57.9 [30.5-151.7]% of a maximal contracture to KCl (60 mM). 5. Ouabain neither affected the concentration-dependent contractions of guinea-pig ileum produced by carbachol nor the inhibition of electrically-evoked contraction produced by morphine (0.3 microM). 6. The muscarinic antagonist atropine (0.1 microM) antagonized control naloxone withdrawal responses. The atropine resistant component, evident in ouabain-treated tissues, was blocked by SR140333((S)1-[2-[3-(3,4-dichlorophenyl)-1-(3-isopropoxyphenyla cetyl)piperidin-3-yl]ethyl]-4-phenyl-1-azoniabicyclo[2.2. 2]-octane, chloride), a substance P antagonist. 7. Clonidine (alpha2-adrenoceptor agonist) inhibited electrically-evoked contractions. Exposure to the alpha2-adrenoceptor antagonist RX811059 (2-(2-ethoxy-1,4-benzodioxan-2-yl)-2-imidazoline), resulted in a contracture which was not significantly enhanced by ouabain (1 microM). 8. Ouabain selectively potentiates the naloxone-induced withdrawal contraction following acute exposure to morphine the major components of which are mediated by both acetylcholine and substance P.

Effects of tizanidine administration on precipitated opioid withdrawal signs in rats

An opioid withdrawal syndrome was precipitated by naloxone administration in rats treated with morphine. The withdrawal caused alteration of several physiological signs. The aim of the study was to investigate whether the altered physiological profiles were modified by utilising tizanidine, an alpha 2 adrenergic receptor agonist which is capable of affecting faecal and urinary excretion, rectal temperature, pain threshold levels and salivation. To induce an opioid withdrawal syndrome, morphine was administered in three daily intraperitoneal injections for four days at doses of 9, 16 and 25 mg/kg (1st day), 25, 25 and 50 mg/kg (2nd day), 50, 50 and 50 mg/kg (3rd day) and 50, 50 and 100 mg/kg (4th day): naloxone was injected (30 mg/kg) i.p. 180 min after the last morphine injection. Tizanidine was administered orally at 0.17, 0.35 and 0.7 mg/kg, 60 min after the last morphine injection. Signs such as faecal and urine excretion, rectal temperature and latency times to thermal stimulus, salivation, jumping and wet dog shakes were affected in different ways by morphine, naloxone, tizanidine and by the combination of these agents. Notably, the administration of tizanidine in rats receiving morphine and naloxone decreased the intensity of certain withdrawal symptoms, including altered excretion of faeces and urine, salivation and wet dog shake behavior. Body temperature levels and nociceptive threshold values were also modified. The effects caused by tizanidine administration may be due to its alpha 2 receptor agonist activity interfering with a mechanism involved in the regulation of these previously mentioned withdrawal symptoms. Thus, the use of this drug may be indicated as a possible control of the acute phase of opioid withdrawal in heroin addicts.

Inhibitory effect of mitragynine, an alkaloid with analgesic effect from Thai medicinal plant Mitragyna speciosa, on electrically stimulated contraction of isolated guinea-pig ileum through the opioid receptor

Effect of mitragynine, an indole alkaloid isolated from Thai medicinal plant kratom (Mitragyna speciosa), on electrically stimulated contraction was studied in the guinea-pig ileum. Mitragynine (1 nM-3 microM) inhibited the ileum contraction elicited by electrical stimulation, and its pD2 value was 6.91 +/- 0.04 (n = 5). Morphine (1 nM-1 microM) also inhibited the electrically stimulated contraction in a concentration-dependent manner (pD2 7.68 +/- 0.11; n = 5). Mitragynine was 10 fold less potent than morphine. Mitragynine (3-10 microM) did not show any effect on the smooth muscle contraction induced by acetylcholine or histamine. Naloxone (10-300 nM) reversed the inhibitory effect of mitragynine on electrically stimulated contraction. Furthermore, naloxone showed a shift of concentration-response curve of mitragynine to the right. There was no significant difference in the affinity of naloxone (i.e. pA2) in the presence of mitragynine or morphine. Mitragynine (3-10 microM) inhibited the naloxone-precipitated withdrawal contraction following a brief (5 min) exposure of the ileum to morphine. Tetrodotoxin (1 microM) and atropine (1 microM) inhibited the withdrawal contraction. The present results suggest that mitragynine inhibits the electrically stimulated contraction of guinea-pig ileum through the opioid receptor.

Tachykinins in the gut. Part I. Expression, release and motor function

The preprotachykinin-A gene-derived peptides substance P and neurokinin (NK) A are expressed in distinct neural pathways of the mammalian gut. When released from intrinsic enteric or extrinsic primary afferent neurons, tachykinins have the potential to influence both nerve and muscle by way of interaction with three different types of tachykinin receptor, termed NK1, NK2 and NK3 receptors. Most prominent among the effects of tachykinins is their excitatory action on gastrointestinal motor activity, which is seen in virtually all regions and layers of the mammalian gut. This action depends not only on a direct activation of the muscle through NK1 and/or NK2 receptors, but also on stimulation of excitatory enteric motor pathways through NK3 and/or NK1 receptors. In addition, tachykinins can inhibit motor activity by stimulating either inhibitory neuronal pathways or interrupting excitatory relays. A synopsis of the available data indicates that endogenous substance P and NKA interact with other enteric transmitters in the physiological control of gastrointestinal motor activity. Derangement of the regulatory roles of tachykinins may be a factor in the gastrointestinal dysmotility associated with infection, inflammation, stress and pain. In a therapeutic perspective, it would seem conceivable, therefore, that tachykinin agonists and antagonists are adjuncts to the treatment of motor disorders that involve pathological disturbances of the gastrointestinal tachykinin system.

Role of L-type calcium channels on yohimbine-precipitated clonidine withdrawal in vivo and in vitro

This study was designed to elucidate the possible participation of L-type calcium channels in the expression of clonidine-withdrawal precipitated by yohimbine in clonidine-dependent animals. Mice implanted for 5 days with osmotic minipumps containing the alpha 2-adrenoceptor agonist clonidine showed symptoms of a withdrawal syndrome (jerks, headshakes, defecations and weight loss) when yohimbine, an alpha 2-adrenoceptor antagonist, was injected. Similarly, isolated rat ilea incubated with clonidine in vitro showed a withdrawal contracture when yohimbine was added to the organ bath. The effects of L-type calcium channel blockers (verapamil and diltiazem) and the stimulant Bay K 8644 on these two different types of withdrawal responses were evaluated. A dose-dependent decrease in yohimbine-precipitated clonidine withdrawal in vivo was observed when verapamil (10-40 mg/kg, s.c. and 120 micrograms/mouse, i.c.v.) or diltiazem (5-20 mg/kg, s.c. and 160 micrograms/mouse, i.c.v.) were administered to mice dependent on clonidine. No effect was found after Bay K 8644 (0.5-5 mg/kg, s.c. and 1-5 micrograms/mouse) was injected under these conditions. In vitro, both verapamil (0.1-5 microM) and D-cis-diltiazem (1-50 microM) concentration-dependently reduced the height of the yohimbine-precipitated withdrawal contracture in rat ileum incubated with clonidine. Furthermore, the effect of diltiazem was stereospecific, as D-cis-diltiazem 10 microM markedly inhibited clonidine withdrawal, whereas the same concentration of L-cis-diltiazem had no effect. In contrast, the calcium channel stimulant Bay K 8644 (0.1-1 microM) increased the height of the ileum withdrawal contracture.(ABSTRACT TRUNCATED AT 250 WORDS)

Contractile response of guinea pig ileum by repetitive application of morphine

1. The repetitive application of morphine gradually induced a contracture in the isolated guinea pig ileum. 2. The optimum conditions for induction of the contracture were as follows: the concentration, incubation time and washout time of morphine were 1 or 10 microM, 2 and 3 min, respectively. 3. Preincubation with naloxone or TTX blocked this morphine-induced contracture. 4. Among twitch-inhibiting drugs, only clonidine induced a contracture similar to that induced by morphine, while tetrodotoxin (TTX) and adenosine did not. 5. The contracture was also observed in the longitudinal muscle-myenteric plexus preparations. 6. These findings indicate that morphine has a dual inhibitory and excitatory action on the guinea pig ileum and that its repetitive application preferentially diminish the inhibitory one.

Inhibition of the morphine withdrawal syndrome by a novel muscarinic antagonist (4-DAMP)

It has been recognized for many years that central cholinergic neurons are susceptible to inhibition by opiates and that during withdrawal their firing rates are enhanced. Nevertheless, classical nonselective muscarinic receptor antagonists have not been demonstrated to provide consistent inhibition of withdrawal symptoms in humans or in animal models. The purpose of this study was to determine whether selective blockade of central M1 or M2 muscarinic receptor subtypes could provide inhibition of naloxone precipitated withdrawal symptoms in morphine dependent rats. As with earlier human studies, both cardiovascular and behavioral measures of withdrawal were quantitated. The selective M2 receptor antagonist 4-DAMP was significantly more effective than the M1 antagonist pirenzepine in reducing both cardiovascular and behavioral symptoms. These results are consistent with a role for cholinergic neurons in the expression of certain morphine withdrawal symptoms and suggest that future therapies might be targeted towards central M2 receptors.

Effects of putative neurotransmitters and related drugs on withdrawal contractures of guinea-pig isolated ileum following brief contact with [Met5]enkephalin

1. Brief exposure for 2 min of guinea-pig isolated ileum to [Met5]enkephalin (MEnk) and noradrenaline has been shown previously to produce withdrawal contractures on washout of the agonist or addition of naloxone (MEnk) or phentolamine (noradrenaline). 2. The present study was undertaken to investigate firstly, whether other putative neurotransmitters and/or related drugs which inhibit transmitter release also produced withdrawal responses following 2 min contact with the ileum and secondly, whether they affected the opioid withdrawal response. 3. Adenosine (1-5 microM), but not U-50,488H (1-5 microM), somatostatin (0.01-5 microM), ocreotide (1-5 microM), baclofen (1-25 microM) or dopamine (5, 50 microM), produced a contracture on washout following 2 min contact with the ileum. The adenosine (5 microM) washout contracture, in common with MEnk and noradrenaline washout contractures, was inhibited by the substance P antagonist, spantide (10 microM). 4. Added 30 s before washout at a concentration of 5 microM, noradrenaline, U-50,488H, adenosine, somatostatin and ocreotide significantly inhibited the washout withdrawal response following 2 min contact of the ileum with MEnk, 1 microM. A higher concentration of baclofen, 250 microM, also inhibited this response. 5. The naloxone (1 microM)-precipitated withdrawal response following contact of the ileum with MEnk, 1 microM, for 2 min, was inhibited only by noradrenaline (5 microM) and U-50,488H (5 microM). 6. It is concluded that during naloxone-precipitated opioid withdrawal an additional population of enteric motor neurones is recruited which is not involved in the washout withdrawal response and these neurones have less diversity of presynaptic receptors mediating inhibition of transmitter release than cholinergic motor neurones.

Some pharmacological characteristics of the guinea pig ileum opioid system activated by cholecystokinin

Naloxone, added after contractions induced by CCK-8 on the guinea pig ileum preparation, elicited a contraction attributed to the release of endogenous opioid which could inhibit the excitatory action of the peptide. With large concentrations of CCK-8, the preparation gave reproducible responses with time. Naloxone, added before the peptide, protracted the excitatory response to CCK-8, but not its height. Morphine decreased the response to CCK-8 but simultaneously raised the response to naloxone. The latter effect appeared very similar to the withdrawal contraction observed after brief exposure of the opioid in the guinea pig ileum to opioids. Clonidine, and alpha-2 adrenoceptor agonist, and nifedipine, a calcium channel antagonist, both known to interfere with tolerance and physical dependence, affected the excitatory response to CCK-8 and the subsequent response to naloxone in a different way.

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.