The role of supersensitivity to acetyl- choline in the production of tolerance to morphine in stimulated guinea-pig ileum

Effects of tizanidine on morphine physical dependence: attenuation and intensification

It has previously been shown that subchronic and acute administration of L-asparaginase and glutaminase inhibitors D-Aspartic acid (D-ASP) and prolyl-leucyl-glycinamide (PLG) intensifies and attenuates morphine (M) physical dependence, respectively, by the inhibition of ASP and glutamic acid (GLU) production, and subsequently their normal releases. Tizanidine (TIZ) has long been known to be an alpha 2-adrenoceptor agonist and inhibitor of ASP and GLU release. Therefore, in this study TIZ has been administered subchronically during the development of M physical dependence to rats in which M-containing pellets had been implanted or acutely 30 min before naloxone (NL)-induced abstinence syndrome. The subchronic administration of TIZ intensified NL-precipitated abstinence syndrome whereas its acute administration attenuated it, as did D-ASP and PLG. On the other hand, TIZ added into the medium prevented the in vitro M-dependent-made guinea pig ileum from contracting following NL application. Furthermore, TIZ stopped the already started contraction by NL of the M-dependent ileum, which completely relaxed later. These effects of TIZ on M-dependent ileum were antagonized by the alpha 2-adrenoceptor antagonist yohimbine. The intensification by subchronic TIZ administration of abstinence syndrome was attributed to the lesser release of ASP and GLU, which resulted in the larger blockade of M of ASPergic/GLUergic receptors due to the lesser release of their endogenous agonist ASP and GLU and consequently the higher upregulation of the receptors. The attenuation by acute TIZ administration of NL-precipitated abstinence syndrome was explained with lesser release of ASP and GLU and concomitantly the lesser stimulation of the receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of chronic morphine administration to mice on the contractile activity in vitro of the vas deferens without and with morphine

Chronic treatment of mice with morphine enhances maximal responses to noradrenaline in isolated vasa deferentia. Addition of morphine to the bath further increases the responses to noradrenaline (facilitatory effect). Vasa deferentia from reserpine-pretreated mice showed increased responses to noradrenaline, but upon addition of morphine to the bath no further facilitatory response to noradrenaline. The facilitatory response was not inhibited by naloxone and it was not elicited by narcotic analgesics other than morphine. High calcium in the medium (3.8 mM) suppressed the enhanced maximal responses to noradrenaline of vasa deferentia from mice chronically treated with morphine and also suppressed the facilitatory response to noradrenaline. Low calcium in the medium (1.43 mM) had the opposite effects. Reserpine-induced changes of noradrenaline responses were not affected by alteration of the calcium concentration in the medium. In the K-depolarized vas deferens, a reduction of the depressant effect of morphine on the calcium-induced contractions was observed after chronic administration of morphine.

Pharmacological characterization of opiate physical dependence in the isolated ileum of the guinea-pig

1 Physical dependence was produced in ilea from naive guinea-pigs by exposure of the tissue to different opiates for logarithmically-spaced periods of time (20-320 min). The responsiveness of the tissue to naloxone, as indicated by a strong contracture of the ileum, was enhanced in contrast to that found in intestines not exposed to opiates. 2 The dose-response curves to naloxone obtained in tissues individually exposed to different opiates showed that their relative potency in increasing sensitivity to naloxone was as follows: levorphan greater than morphine greater than Met-enkephalin greater than nalorphine greater than pentazocine. 3 The naloxone-induced response was dose-dependent and was directly related to the opiate concentration and length of exposure. 4 Dextrorphan the inactive isomer of levorphan, did not increase the responsiveness of the tissues to the narcotic antagonist, indicating that the phenomenon is stereospecific. 5 The naloxone-induced contraction in ilea exposed for 320 min to morphine (1 x 10(-6)M) was not prevented or suppressed by the administration of a large dose of morphine (1 x 10(-5)M) before or immediately after the naloxone challenge. 6 The evidence presented here shows that a phenomenon resembling in vivo opiate physical dependence can be acutely produced in vitro with pharmacological characteristics similar to other naloxone-induced abstinence effects.

Changes in the sensitivity of smooth muscle to neurotransmitter during acute morphine treatment and its association with analgesia

Acute in vivo administration of morphine to mice produced dose- and time-dependent subsensitivity to acetylcholine in ileum and supersensitivity to norepinephrine in vas deferens. These responses were specific to the agonist as potassium chloride-induced responses were not altered. A correlation was observed between the analgesia and the sensitivity changes produced by morphine. The sensitivity changes were mediated through opiate receptors as naloxone antagonised the effect of morphine.

Log dose/response curve flattening in rats after daily injection of opiates

Rats injected (IP) daily with 0, 20, and 200 mg/kg morphine-SO4 for 25-49 days experienced log dose/response (LDR) curve flattening (decrease in slope and/or maximum response) for analgesia (tail immersion test) produced by etorphine-HCl injected IP or intracerebroventricularly (ICV), and for latency to maximum rectal temperature increase produced by IP etorphine. Rats treated similarly with 0, 50, and 500 micrograms/kg etorphine-HCl for 32 days exhibited LDR-curve flattening for analgesia produced by etorphine and morphine (IP). In addition, a profound body weight loss produced by high-dose morphine treatment (200 mg/kg) was found not to be involved in flattening, since similar body weight decreases produced by food restriction in 0 and 20 mg/kg rats did not have this effect. Flattening, however, may be due to a rapidly acquired and rapidly lost within-session (acute) tolerance. When flattening was not seen at short intervals after IP or ICV test etorphine doses, flattening was seen when rats were retested at longer test intervals. Forty-eight hours after cessation of chronic etorphine treatment, flattening of the etorphine analgesia LDR curve was lost, but parallel shift was unaffected. Similarly, 200 mg/kg morphine-treated rats lost morphine tolerance more rapidly than 20 mg/kg-treated rats during the first 12 days after the last treatment injection. Subsequently, however, levels of the analgesia and the amounts of tolerance loss were comparable in both chronically treated groups. The data support the notion that chronic tolerance reflects an enhancement or prolongation of acute tolerance.

Sensitivity of mouse vas deferens to neurotransmitters: changes after morphine treatment

1. The pharmacological responses of the isolated vas deferens of the mouse were investigated after acute and chronic treatment with morphine. 2. The addition of morphine to the bath did not alter the responses of the vas deferens to exogenous noradrenaline, adrenaline or dopamine. 3. Low doses of morphine depressed the responses to acetylcholine. Very high concentrations of the opioid (8.5 x 10(-4) M) completely abolished, in about 50% of the preparations, the responses to exogenous acetylcholine, while in the other 50% a potentiation of the responses to low concentrations of acetylcholine was observed. 4. The vas deferens of mice chronically treated with morphine showed increased sensitivity to exogenous noradrenaline, but decreased sensitivity to acetylcholine. 5. A fresh amount of morphine added to the bath enhanced the responses of morphine-tolerant preparations to noradrenaline but not to dopamine or acetylcholine. The specificity of this phenomenon was demonstrated by the use of pentobarbitone instead of the opioid. 6. These results are in agreement with the theory that tolerance could result from a form of disuse supersensitivity.

Nature of seasonal variation in development of acute tolerance to morphine

Morphine inhibits contractions of coaxially stimulated guinea pig ileum. Acute tolerance to this effect occurs on continuous perfusion with morphine for 1.5 h. This is associated with an increase in the sensitivity of the tissue to acetylcholine. Acute tolerance does not always develop in spite of identical treatment, but shows a distinct seasonal incidence. It is significantly higher in the summer (May--Nov.) than winter (Dec.--April). This seasonal variation in development of tolerance does not appear to be related to changes in diet, environmental light or temperature, or to the sensitivity of the ileum to morphine. In the winter, the acetylcholine release from stimulated ilea is lower than in the summer. In addition to inhibiting acetylcholine release, morphine also has a non-specific blocking effect in the winter. This postjunctional effect could prevent the development of supersensitivity to acetylcholine during continuous morphine treatment so that acute tolerance cannot occur.

Supersensitivity of cortical neurones of the rat to acetylcholine and L-glutamate following chronic morphine treatment

the effect of microelectrophoretically applied L-glutamate and acetylcholine on discharge activity of cortical neurones was studied in naive and in morphine-tolerant/dependent rats. The thresholds for increase in discharge activity elicited by these 2 putative neurotransmitters were 3 times lower in the tolerant/dependent rats than in the naive rats, indicating the development of supersensitivity.