The role of the cholinergic system in the development of increased naloxone potency in mice

Pretreatment of mice with the anticholinesterase (anti ChE) drugs tacrine or physostigmine augmented the antinociceptive potency of morphine given 3 h later, but had no effect on the antagonist potency of naloxone. Pretreatment with either of these anti ChE drugs together with morphine not only augmented the potency of a subsequent dose of morphine, but also enhanced the antagonist potency of naloxone to a greater extent than after pretreating with morphine only. Neostigmine did not affect the potency of either morphine or naloxone, suggesting that this phenomenon involved central cholinergic mechanisms. Atropine prevented the increase in naloxone potency caused by morphine pretreatment, and greatly reduced the effect of morphine plus the anti ChE drugs. The effects of these various pretreatments on the development of “acute dependence” to morphine was also studied. None of the three anti ChE drugs caused any change in this phenomenon, as tested by naloxone-precipitated jumping, although this was significantly increased by pretreatment with either atropine sulphate or atropine methyl nitrate. It is concluded that the increase in naloxone potency following morphine pretreatment involves both a cholinergic mechanism plus narcotic analgesic action. This phenomenon does not seem to be related to the development of either acute tolerance or acute dependence.     

Increased antagonist potency of naloxone caused by morphine pretreatment in mice

Using the writhing test in mice, it was shown that pretreatment with a single dose of morphine hydrochloride given 3 h previously caused a marked increase in the antagonistic effect of naloxone without any change in the antinociceptive action of morphine itself. It was shown that when mice were pretreated with different doses of either morphine alone, or in combination with naloxone, so that each treatment produced the same antinociceptive effect, the increase in naloxone potency was proportional only to the antinociceptive effect of the pretreatment and not to the total dose of morphine present. It was also found that the concurrent administration of naloxone plus morphine prevented the development of "acute dependence" to morphine, as measured by the jumping reaction after challenge with naloxone.     

Some relations between tolerance and physical dependence to morphine in mice

Tolerance to morphine analgesia and precipatated physical dependence were studied in mice under different conditions. There was a gradual loss of tolerance during the continous absorption of morphine from a pellet. Tolerance was decreased by nalorphine during morphine absorption. An attenuated physical dependence was observed two or four days after a single dose of morphine. In animals previously treated with pellets of morphine, single doses of morphine induced less tolerance than in mice that had never been implanted with pellets; in both cases cycloheximide prevented development of tolerance. Tolerance persisted for more than 20 days after absorption of the morphine pellet. These results reinforce the hypothesis that tolerance and physical dependence are produced by a similar mechanism and that an inhibitory process of tolerance exists.     

Physical dependence and tolerance development after chronic exposure to low levels of morphine in the rat

A sustained-release delivery system containing ¹⁴C-morphine was implanted subcutaneously in rats. Measurement of urinary excretion of ¹⁴C suggested a steady state release of approximately 640 μg ¹⁴C morphine/day during a 10-day test period. Tolerance developed rapidly to the analgetic effects produced by an injected ED₉₅ dose of morphine sulfate in implanted rats tested on the hot-plate. Physical dependence, determined by naloxone-precipitated abstinence behavior, was evident in rats at 24 hr. Morphine dosage was estimated to be as low as 2.5 mg/kg/day. Peak abstinence behavior was observed on Day 4. However, naloxone-precipitated withdrawal signs were markedly diminished by Day 6 and essentially absent by Day 8. These results are discussed with reference to the suggestion that metabolic changes, occuring during chronic exposure to morphine, may explain the lack of abstinence behavior during a time when maximal concentrations of urinary morphine were observed and a high degree of tolerance was manifest.     

Opiate dependence following acute injections of morphine and naloxone: The assessment of various withdrawal signs

The injection of high dose of naloxone 15 minutes after a single injection of morphine in mice was found to produce a jumping response which was behaviorly similar to the jumping response observed during the withdrawal from chronic morphine administration. In addition the jumping response following the acute administration of morphine-naloxone was increased by the injection of atropine and attenuated by oxotremorine. These data are consistent with the reports of effect of these cholinergic drugs on the jumping response which occurred during withdrawal after chronic morphine administration. However, other symptoms associated with opiate withdrawal (hypothermia, weight loss and diarrhea) were not produced by the acute injection of morphine-naloxone. It is therefore suggested that this single injection paradigm is particular to the jumping response than a demonstration of the rapid development of opiate dependence.     

The effect of stress and adrenalectomy on morphine analgesia and naloxone potency in mice.

Pretreatment of mice with a single injection of corticosterone increased the potency of naloxone in antagonising the antinociceptive effect of morphine measured 3 h later. Pretreatment with morphine also induced a dose-dependent increase in naloxone potency. Co-administration of corticosterone with a low dose of morphine further augmented the potency of naloxone, but in combination with a higher dose of morphine it failed to cause any further potentiation of naloxone potency. Restraint stress for 1 h also caused an increased in naloxone potency when tested 3 h later, and this was further enhanced when tacrine 5.0 mg/kg was administered immediately after stress. Administration of atropine sulphate 2.0 mg/kg s.c. immediately before restraint abolished the effect of stress in potentiating the naloxone potency. Adrenalectomy sensitised mice to the antinociceptive effect of morphine. Although adrenalectomy only partially interfered with the development of increased naloxone potency after pretreatment with morphine, the augmenting effect of tacrine on naloxone potency was completely abolished. Adrenalectomy did not alter the incidence of jumping precipitated by naloxone in morphine-pretreated mice. Both restraint and corticosterone reduced the withdrawal jumping. It is concluded that the stress component of morphine is only partly responsible for the induction of increase in naloxone potency. There is an apparent interaction between central cholinergic mechanisms and stress in the induction of this increased naloxone potency. The phenomenon of increased naloxone antagonism may not be related to the development of acute dependence on morphine.     

The effect of biogenic amine modifiers on morphine analgesia and its antagonism by naloxone.

The stimulation of dopaminergic receptors, inhibition of serotonin synthesis or blockade of muscarinic receptors by various modifiers led to inhibition of morphine analgesia in mice. Blockade of dopaminergic receptors or the increase in serotonergic or cholinergic activity resulted in the enhancement of morphine analgesia. Serotonergic and cholinergic systems are proposed as positive and the dopaminergic system as negative modulators of morphine analgesia. The modulation of naloxone antagonism was much more complicated than that of morphine analgesia and often the effect of biogenic amine modifiers on antagonism differed from that on analgesia. The fact than biogenic amine modifiers do not affect morphine analgesia and naloxone antagonism by a similar pattern suggests that interaction of narcotics and narcotic antagonists with analgesic receptors may not be exactly the same.     

Effect of morphine and naloxone on intestinal transit in mice

Morphine caused a dose-dependent slowing of the rate of intestinal transit in mice. This inhibitory effect of morphine was antagonised by naloxone administration. Pretreatment with a single dose of morphine did not induce any detectable tolerance to the inhibitory effect of a second dose of morphine given 5 h later. However, naloxone was more effective in antagonising this inhibitory effect of morphine-pretreated mice than in saline-pretreated animals. Molecular sieve morphine pellet implantation for 24 h induced detectable tolerance to the inhibitory effect of morphine administered 3 h after removal of the pellet. In addition, the antagonistic effect of naloxone was also augmented when compared with blank pellet-implanted control animals. The present study has shown that the enhanced naloxone potency against the inhibitory effect of morphine on intestinal transit was observable before the development of overt tolerance, and that tolerance to the effect of morphine on the small intestine could be induced by implantation of a molecular sieve morphine pellet for 24 h.     

Behavioral effects of morphine and naloxone following chronic morphine administration

The effects of morphine, naloxone, and combinations of these drugs were examined in squirrel monkeys under shock-postponement schedules. In the absence of a lever press, shocks could be presented every 4s, and each response postponed shock for 20s. Acutely, morphine (0.10–3.00 mg/kg) produced not only overall response-rate decreases, but also increases in the number of shocks, whereas naloxone (0.10–30.00 mg/kg) had little effect on responding. When given in combination with morphine, several doses of naloxone antagonized the rate-reducing and shock-increasing effects of morphine. Daily administration of morphine resulted in a substantial decrease in the number of shocks received and a moderate attenuation of the rate-decreasing effects of morphine (tolerance). Lower doses substituted for the fixed daily dose resulted in a smaller effect on behavior than under acute administration. Naloxone given in combination with the daily morphine dose or substituted for the daily administration of morphine, produced effects similar to those seen prior to chronic drugging. Thus, behavioral effects of naloxone were not altered even though tolerance to morphine was observed. Larger doses of naloxone continued to antagonize the effects of morphine for at least 24h. No signs of physical dependence were noted when naloxone was administered or when administration of morphine ended.     

Effect of propranolol on antinociceptive, tolerance- and dependence-producing properties of morphine in rodents and monkeys.

Since an abstinence syndrome may accompany the injection of opioids in addicts pretreated with propranolol the morphine antagonistic properties of this compound were investigated. Racemic propranolol did not significantly affect the antinociceptive ED50 of morphine in rodents and neither precipitated abstinence in morphine-dependent monkeys nor exacerbated the syndrome in 24 hr withdrawn monkeys. Multiple doses of propranolol did not alter the development of physical dependence on morphine in monkeys. Clinical narcotic antagonism would not be predicted from this profile. Evidence for a possible propranolol-morphine interaction came from studies using the mouse tail flick test. Thus, after 8 injections of propranolol (over 4 days) mice were tolerant to normally effective doses of morphine. Concurrent injections of naloxone antagonised this effect. When propranolol and morphine were administered concurrently the morphine ED50 (on day 5) was twice that of the group receiving morphine alone. Similar results were obtained with d-propranolol; practolol had a neutral effect.     

The effects of divalent ions on morphine analgesia and abstinence syndrome in morphine-tolerant and-dependent mice

Intracerebral administration of copper sulfate potentiated morphine analgesia in morphinetolerant and-dependent mice. The stereotyped jumping response, precipitated by naloxone, was inhibited by copper in morphine-dependent mice, but copper failed to affect other abstinence signs. When abstinence was precipitated with a partial antagonist, nalorphine, stereotyped jumping was not inhibited by either calcium or copper. These modification of narcotic effects by copper were produced without alterations in the brain disposition of morphine. Total radioactivity in the brain following radioactive naloxone administration was also not altered.     

The effects of naltrexone on the development of physical dependence on morphine

A single i.p. injection of naltrexone (20 mg/kg) partially inhibited the development of physical dependence upon morphine in mice rendered dependent on morphine by implantation of a pellet containing 75 mg of morphine free base for three days. This was evidenced by an increase in the dose of naloxone (ED50) required to precipitate withdrawal jumping response. The increase in naloxone ED50 was much more pronounced when naltrexone was given prior to and during the course of pellet implantation. Inhibition was also observed when naltrexone was administered one day after the morphine pellet implantation, i.e., after some dependence had already developed. Naltrexone administration prior to and during the development of dependence also inhibited, but only partially, the loss of body weight and hypothermic response observed during abrupt withdrawal of morphine in morphine-dependent mice. The inhibitory effect of naltrexone on morphine dependence development was not associated with changes in brain morphine concentration.     

Allosteric modulation by leucine-enkephalin of [3H]naloxone binding in rat brain

The interaction of morphine and leucine-enkephalin with the binding site labeled by [3H]naloxone in the presence of sodium was compared. The effect of fixed concentrations of morphine and leucine enkephalin on the saturation binding of [3H]naloxone demonstrated that whereas morphine was a competitive inhibitor, leucine enkephalin caused a dose-dependent masking of binding sites. From these data we conclude that the enkephalin receptor is allosterically coupled to the morphine receptor.     

Acute tolerance to the discriminative stimulus properties of morphine

Two pigeons were trained to discriminate intramuscular injections of 1.0 mg/kg morphine from water by presenting food after keypeck responses on one key when morphine was administered and after responses on a second key when water was administered. Following training, close to 100% of responses occurred on the appropriate key following administration of 1.0 mg/kg morphine or water. Morphine (0.1–5.6 mg/kg) produced dose-dependent increases in the percentage of morphine-appropriate responses (discriminative stimulus properties) and decreases in the rate of responding. A shift to the right of the morphine dose-effect curve for the discriminative stimulus properties of morphine resulted from a single injection of morphine (10.0 mg/kg) 24 hrs prior to testing (i.e., acute tolerance) but not from a single injection of pentobarbital (17.0 mg/kg). Tolerance to the discriminative stimulus properties of morphine was reversible within five days of the single injection. Tolerance did not develop to the effects of morphine on response rate. Naloxone antagonized both the discriminative stimulus properties and the response rate-decreasing effects of morphine. Thus, a single administration of morphine can alter morphine discriminability without affecting other aspects of behavior.     

The production of morphine tolerance and physical dependence by the oral route in the rat

A method for the chronical administration of morphine by the oral route is discussed and compared with the production of physical dependence to morphine by injection. The method recommends the administration of Morphine HCl dissolved in a 45% sucrose syrup and given orally for 4 weeks. The initial concentration of morphine in the syrup was 1 mg/ml and was increased weekly up to 4 mg/ml at the end of the experiment. This procedure rendered the animals physically dependent on morphine as observed by drug withdrawal, when abstinence symptoms were easily identified.     

The development of tolerance to morphine in the rat

Tolerance to various effects of morphine in the rat can be quantified by means of a shift of semilogarithmic dose-response curves. Tolerance to analgesia (hot plate, acetic acid writhing), catalepsy, and the tilted plane develops in a closely similar manner. Also, the stimulating effects of about 1 mg/kg morphine-HCl tested in an open-field procedure are somewhat less pronounced in chronically treated rats than in naive ones. There is no correlation between tolerance development and the acute ED₅₀ of different tests.     

A preliminary description of acute physical dependence on morphine in the vervet monkey

Previous animal studies have suggested the rapid development of opiate dependency in 24 hours or less. However, the development of dependence on opioids within twenty-four hours has yet to be demonstrated in previously opiate-free human or nonhuman primate subjects. Following naloxone administration, cable-restrained monkeys which received intravenous morphine hourly for only six hours exhibited a behavioral syndrome characteristic of opioid withdrawal in this particular species. These data indicate that acute physical dependence on morphine may be induced after six hours in a primate species.     

Characterization of neonatal rat morphine tolerance and dependence

The administration of morphine and fentanyl by continuous intravenous infusion has been shown to produce analgesic tolerance and physical dependence in human neonates. In animals, daily repeated morphine bolus injections is a common method of inducing neonatal rat tolerance and dependence. Yet this method differs from the intravenous route reported to affect human neonates. Alzet osmotic minipumps were implanted in postnatal day 14 rats to provide a continuous morphine infusion more closely mimicking the clinical picture. Rats remained naive or were infused with saline or morphine (0.7 mg/kg/h) for 72 h. Morphine's antinociceptive potency was similar between naive and saline-infused animals, while morphine-infused animals were tolerant. Gender did not contribute to the degree of tolerance observed. Naloxone precipitated withdrawal in the morphine pump-implanted rats was similar to that reported by others. Thus, minipumps provide a useful model for assessing the tolerance and dependence liability of different opioids.     

利鲁唑对吗啡镇痛、耐受和依赖作用的影响(英文)

目的　研究利鲁唑对阿片镇痛、耐受及躯体功能的调节。方法　采用冰醋酸扭体 ,5 5℃热板法和热辐射甩尾法观察利鲁唑对小鼠痛阈及吗啡镇痛效应的影响 ;采用小鼠急性和慢性吗啡耐受模型及小鼠吗啡依赖模型 ,观察利鲁唑对吗啡耐受和依赖的作用。结果　单独皮下注射利鲁唑 2 .5～ 10mg·kg- 1在以上 3种模型无镇痛作用 ,然而能剂量依赖性地增强吗啡镇痛效应。利鲁唑 2 .5～ 10mg·kg- 1剂量依赖性地对抗吗啡引起的急性和慢性耐受。在小鼠吗啡依赖模型中 ,利鲁唑 2 .5～ 10mg·kg- 1剂量依赖性地抑制吗啡戒断症状的产生。结论　利鲁唑自身无镇痛作用 ,但能显著增强吗啡镇痛效应 ,并能预防吗啡所引起的耐受和依赖     

Inhibition of naloxone-induced withdrawal in morphine dependent mice by l-trans-Δ9-tetrahydrocannabinol

The effects of various doses of l-trans-Δ⁹-tetrahydrocannabinol (Δ⁹-THC) on naloxone-induced withdrawal were studied in mice rendered dependent on morphine by the pellet implantation procedure. When administered i.p., 30 min prior to naloxone, Δ⁹-THC, inhibited the naloxone-induced withdrawal jumping response. Two other signs of morphine withdrawal (defecation and rearing behavior) were also suppressed by Δ⁹-THC. It is suggested that Δ⁹-THC or some of its derivatives may have potential use in narcotic detoxification.