The role of hot plate and general environmental stimuli in morphine analgesic tolerance

Rats given one injection of morphine (5 mg/kg) paired with a hot plate test displayed greater analgesic tolerance than rats given nine injections of morphine paired with a distinct room in which the hot plate apparatus was located. Hot plate stimuli, rather than general environmental stimuli, are prepotent in the acquisition of morphine analgesic tolerance assessed by the hot plate procedure.     

The effect of morphine pretreatment on hypothermia induced by morphine in mice.

Morphine caused an apparently dose-dependent hypothermia in mice. Co-administration of naloxone antagonised this effect. Pretreatment with a single dose of morphine induced detectable tolerance to the hypothermic effect of a second dose of morphine given 3 h later and naloxone was more effective in antagonising the hypothermic effect of morphine in morphine-pretreated mice than in saline-pretreated animals. The present study has shown that morphine pretreatment can augment the antagonistic effect of naloxone towards the hypothermic action of morphine.     

Naloxone reversal of morphine catationia: role of caudate and periaqueductal gray

The narcotic antagonist, naloxone, was microinjected into the head of the caudate nucleus (HC), periaqueductal gray matter (PG), and cerebellar white matter (CB) of rats to counteract the catatonia induced by systemic morphine. Naloxone produced a loss of the catatonic response when administered into HC or PG, but not when microinjected into CB. Isotonic saline in HC, PG, and CB did not counteract the catatonic effects of morphine. The reversal of catatonia was similar for naloxone injections in HC and PG. Both these areas have high concentrations of opiate receptors while CB has few opiate receptors. It is suggested that the HC and PG are involved in the reversal of the catatonic effects of morphine via the high concentrations of opiate receptors found there.     

Hyperglycemic suppression of morphine withdrawal signs in the rat

Male Sprague-Dawley rats maintained under controlled lighting and temperature conditions were used in this experiment. Morphine dependency was induced by giving increasing doses of morphine by intraperitoneal injection (IP group) or by the ingestion of morphine through drinking water (PO group). Animals were injected with 10, 20, 30 and 50 mg/kg morphine sulfate at days 1, 2, 3 and 4, respectively. Another group of animals received increasing concentrations of morphine through drinking water from 0.1, 0.2, 0.3 to 0.4 mg/ml at 48 h intervals. Morphine dependent animals were given naloxone by the intraperitoneal route to precipitate withdrawal. Glucose (3 g/kg or 10 g/kg) was given 10 min prior to the administration of naloxone to the respective groups. Another two groups of animals were made diabetic by the administration of streptozotocin. In one group, animals received increasing concentrations of 10, 20, 30 and 50 mg/kg morphine sulfate by the IP route at days 1, 2, 3 and 4, while the other group was not treated with morphine but was assessed for withdrawal signs to serve as the control. Withdrawal signs were assessed by observing the presence of diarrhea, tremor, piloerection, hunchbacked posture, teeth chattering, salivation, erection, restless activity, territorial exploring, irritability to handling, vocalization and jumping. Results obtained indicate that glucose administration at 10 g/kg abolished most of the withdrawal signs, and we were unable to induce the same degree of morphine dependency in diabetic animals as compared to the non-diabetic groups. It was concluded from this study that hyperglycemia could suppress morphine withdrawal signs.     

Enduring effects of morphine pellets revealed by conditioned taste aversion

Morphine pellets (75 mg) were implanted subcutaneously in albino rats. Three days later, following 24 h without water, these rats (Group MSN) were given access to a saccharin solution for 30 min, then injected with naloxone hydrochloride. The classical abstinence signs, including wet dog shakes and weight loss, were noted in these subjects, but not in controls given placebo pellets and/or saline injections. In addition, when given an opportunity to drink either saccharine solution or water 24 h later, Group MSN rats drank significantly less saccharin than any of the control groups. Similar drinking patterns were found even when naloxone injection was delayed as long as 3 weeks after pellet implantation, when none of the classical abstinence signs were seen and serum levels of morphine and its metabolites were 100 times lower according to radioimmunoassay. This simple and objective technique is thus more sensitive as a measure of prior morphine exposure than any of the commonly used indices. The continued utility of a dependent-nondependent dichotomy is also examined in light of these and other findings.     

Development and loss of tolerance to morphine in the rat

The development of a differential tolerance to morphine was investigated with respect to the mean effective dose, the threshold dose of tolerance, the degree of tolerance after a fixed dose, and the speed of tolerance loss. The mean effective doses, the threshold doses of tolerance, and the degree of tolerance differed considerably from effect to effect, whereas in all tests tolerance loss remained the same. The mean effective doses were not correlated to threshold doses of tolerance, degree of tolerance, or to the loss of tolerance, but a strong correlation exists between threshold doses of tolerance and degree of tolerance to all effects measured. Consequences of these results upon current theories of tolerance are discussed.     

Methysergide and metergoline reduce morphine analgesia with no effect on the development of tolerance in rats

A single subcutaneous injection of 5 mg/kg metergoline or 10 mg/kg methysergide, two serotonin antagonists, or 1 mg/kg naloxone, significantly reduced the effect of a subcutaneous dose of 3 mg/kg morphine in the tail immersion test in rats. The same drugs and doses were administered concurrently with 10 mg/kg morphine twice daily for 3 days and nociceptive responses were measured 96 h later. Tolerance to the effect of 3 mg/kg morphine was comparable in animals which had received vehicle+morphine or serotonin antagonists+morphine, whereas naloxone completely prevented the development of tolerance. The results argue against a role of serotonin in the development of tolerance to the antinociceptive effect of morphine and suggest it may be possible to dissociate morphine analgesia from tolerance development, at least in the conditions used in the present study.     

自噬在雷帕霉素逆转脊髓吗啡耐受形成机制研究

目的研究鞘内注射雷帕霉素对大鼠吗啡耐受形成的作用。方法选择鞘内置管成功的成年雄性SD大鼠32只,随机分为M组、C组、MR组和R组(n=8),M组2次/d,连续7 d鞘内注射吗啡20μg;C组2次/d,连续7 d鞘内注射生理盐水;M R组2次/d,连续7 d鞘内注射吗啡20μg,并于第3天第2次注射吗啡同时鞘内注射雷帕霉素2.3μg,连续3 d;R组2次/d,连续7 d鞘内注射生理盐水,并于第3天第2次注射生理盐水前鞘内注射雷帕霉素2.3μg,连续3 d。于鞘内注射前及第1、3、5、7天,第2次鞘内注药后30 min,采用电子Von Frey测痛仪测定机械缩足反射阈值(M WT),最后一次M WT测定结束后,随机取4只大鼠L4~6脊髓背角,采用Western blot测定自噬标记蛋白LC3Ⅱ(LC3Ⅱ/LC3Ⅰ)及自噬调节信号相关蛋白Beclin-1的表达。结果与M组比较,MR组鞘内注射第3、5、7天后,MWT均升高(P<0.05),虽然MR组MWT有降低趋势,但在第7天M WT仅比第1天下降43%,表明第3天后雷帕霉素与吗啡合用能部分逆转吗啡耐受的形成。Western blot结果:与C组比较,M组、R组和MR组第7天脊髓背角Beclin-1、LC3Ⅱ与LC3Ⅱ/LC3Ⅰ表达均上调(P<0.05)。与M组比较,R组和MR组第7天脊髓背角Beclin-1、LC3Ⅱ与LC3Ⅱ/LC3Ⅰ表达显著上调(P<0.01)。免疫组织化学结果:与M组比较,第7天MR组脊髓背角Beclin-1表达平均光密度值明显升高(P<0.01)。结论吗啡耐受开始形成时合用雷帕霉素可以部分逆转脊髓吗啡耐受的形成。     

Impaired development of tolerance to morphine analgesia in rats with hereditary diabetes insipidus

Recently it was reported that vasopressin facilitates the development of resistance to the analgesic action of morphine. Therefore, the development of tolerance to daily administration of morphine-HCl (10 mg/kg i.p.) was studied in a series of trials on a hot plate using rats with hereditary diabetes insipidus (DI), which lack the ability to synthesize vasopressin. In contrast to heterozygous DI rats, who developed full tolerance after the fifth injection, homozygous DI rats showed a delayed development of tolerance. Substitution of HO-DI rats with either arginine-8-vasopressin (3 g/rat, s.c. daily) or the endocrinologically inert fragment of vasopressin desglycinamide lysine-8-vasopressin (5 g/100 g, s.c. daily) restored the impaired development of tolerance towards normal. The data support the notion that vasopressin is important to the development of tolerance to narcotic analgesics and that its mechanism of action is dissociated from its endocrine effect but rather resembles that of its known influence on memory consolidation.     

Inhibition of mating by naloxone or morphine in recently castrated, but not intact male rats

Administration of naloxone (SC 5 mg/kg) significantly reduced ejaculation and mounting in male rats in the weeks following castration. A similar effect was obtained by injecting morphine (SC 1 or 5 mg/kg). In contrast, the same dosages of naloxone or morphine did not affect the sexual performance of gonadally intact males. Opioid peptides may contribute to the temporary persistence of sexual behavior in testosterone-deficient male mammals, in which incentive qualities of the female partner are an important determinant of sexual arousal.     

Development of tolerance to the analgesic effect of clonidine in rats cross-tolerance to morphine

Summary The site of action of naloxone to induce jumping in morphine tolerant/dependent rats appears to be dissociated from structures where apomorphine initiates its action to reinduce jumping in previously withdrawn animals. These findings suggest that dopaminergic pathways do not directly affect the neuronal circuit involved in withdrawal jumping behavior, but instead exert a facilitatory influence on neurons that become supersensitive during the state of withdrawal. Thus, an increased response to apomorphine during naloxone-precipitated opiate withdrawal does not necessarily imply a specific supersensitivity of the dopaminergic system.     

Tolerance to effects of morphine without cross tolerance to effects of clonidine on schedule-controlled behavior of pigeons

Schedule-controlled responding was maintained under a multiple fixed-interval, fixed-ratio schedule in pigeons. Dose-related decreases in response rates were produced by clonidine (0.001–0.1 mg/kg) and morphine (0.3–5.6 mg/kg). Chronic administration of morphine produced (1) tolerance to effects of morphine, as evidenced by a decrease in potency of morphine and (2) sensitivity to opioid antagonists, as evidenced by an increase in potency of naloxone. Dose-effect curves for clonidine were not appreciably altered by chronic morphine administration. Offprint requests to: J.L. Katz     

Effect of a calcium chelator on morphine tolerance development

To assess the effects of calcium ion chelation on narcotic tolerance development, adult Sprague-Dawley rats were implanted with i.c.v. cannulae connected to osmotic minipumps for continuous infusions of saline or EGTA (2 mumol/24 h) during chronic morphinization by s.c. pellet implantation. Additional rats receiving the same morphine pellet treatment but without minipumps served as non-surgical controls. After 64 h the minipumps and pellets were removed and narcotic tolerance was assessed by the tail-flick technique. A positive analgetic response to an 8.50 mg/kg s.c. dose of morphine sulfate was noted in 6/12 no-minipump controls and 4/9 saline-infused controls. In placebo-treated rats caused no alteration of either baseline tail-flick latencies or in analgetic responsiveness to lower doses of morphine. It is concluded that the enhancement of morphine tolerance by EGTA (a calcium-specific chelator) results from a facilitation of certain adaptive changes of calcium ion disposition that are related to the neurochemical mechanisms of narcotic tolerance and dependence development.     

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.     

Absence of environment-specificity in morphine tolerance acquired in non-distinctive environments: habituation or stimulus overshadowing?

The experiments reported here investigated the mechanisms of drug tolerance acquisition in environments differing in distinctiveness. Specifically, they examined the hypothesis that tolerance acquired in non-distinctive environments might involve habituation while tolerance acquired in distinctive environments involves a classical conditioning or associative learning mechanism. In Experiment 1, rats pre-exposed to injection-ritual cues (placebo injections) prior to acquisition of tolerance to morphine analgesia in distinctive or non-distinctive environments showed a typically attenuated tolerance response on an environment-change test. The magnitude of the attentuation was not affected by the distinctiveness of the acquisition environment. In Experiment 2, rats acquiring tolerance in distinctive or non-distinctive environments, but without prior injection-ritual pre-exposure, did not demonstrate an attenuation of tolerance on an environment-change test. Tolerance acquired in either environment was unaffected by a subsequent rest period in the colony room, but was attenuated by a subsequent period of daily placebo injections in the colony room. It is argued that failure to observe environment-specific tolerance, as in Experiment 2 and in previous reports in the literature, may reflect overshadowing of environmental stimuli by injection-ritual stimuli, and are not indicative of a fundamental difference between the mechanisms of tolerance acquisition in environments varying in distinctiveness.     

慢性神经痛大鼠鞘内联合应用吗啡和氯胺酮在脊髓上水平的抗伤害性作用及对吗啡耐受的影响

目的研究鞘内联合应用吗啡和氯胺酮对慢性神经痛大鼠的抗伤害作用机制及对吗啡耐受的影响。方法 40只Wistar大鼠,体质量220～260 g,制备坐骨神经结扎模型并进行鞘内置管,随机分为5组(n=8):B组为空白对照组;C组鞘内注射0.9%盐水10μL;K组鞘内注射氯胺酮50μg;M组鞘内注射吗啡20μg;KM组鞘内注射吗啡10μg和氯胺酮25μg。坐骨神经结扎术后第4天开始鞘内给药,每日1次,连续7 d。用药7d后处死大鼠,取大脑皮质、海马、脑干组织,硝酸还原酶法测定NO浓度和NOS活性。结果与B组比较,C和K组脑干NO浓度升高,M组皮质、海马、脑干中NO浓度均升高;与C组比较,M组皮质、海马NO浓度升高,KM组海马、脑干中NO浓度降低;与M组比较,KM组皮质、海马、脑干NO浓度均降低(P<0.05或0.01)。与B组比较,C、K和M组皮质、海马、脑干中NOS活性均升高;与C和M组比较,KM组皮质、海马、脑干中NOS活性均降低(P<0.05或0.01)。结论神经病理性痛大鼠鞘内联合应用吗啡和氯胺酮可在脊髓上水平产生抗伤害性作用,并可抑制吗啡耐受的形成,这可能与大脑皮质、海马、脑干的NO水平和NOS活性有关。     

Exposure to a nonfunctional hot plate as a factor in the assessment of morphine-induced analgesia and analgesic tolerance in rats.

Rats not exposed to a hot plate with or without morphine and later tested on the functional hot plate with or without morphine, displayed increased paw lick latency relative to same-injected rats given pretest hot plate exposure. This analgesic effect, was termed behavioral analgesia since it, unlike morphine-induced analgesia, was not reversed by naloxone (Experiment 2). Behavioral tolerance was evident in animals exposed to the nonfunctional hot plate regardless of drug treatment and was dissociated from pharmacological tolerance. Behavioral analgesia and tolerance reported here may involve habituation to novel distractive stimuli associated with the hot plate test environment.     

The action of morphine and naloxone on acid secretion by the rat isolated stomach

The action of morphine and naloxone on acid secretion by the rat isolated stomach has been studied. Morphine (10^?7 to 10^?4 M) had no effect upon spontaneous acid secretion. Morphine (10^?6 M) did not modify the acid output in response to sub-maximal stimulation by pentagastrin, histamine, bethanechol or isoprenaline. Naloxone (10^?6 M) was without effect on the response to pentagastrin or histamine. Our results suggest that opiate receptors do not modify acid secretion in this preparation.     

Induction of tolerance and withdrawal in rats receiving morphine in the spinal subarachnoid space.

Rats implanted with chronic catheters in the spinal subarachnoid space were given twice daily injections for 7 days of morphine sulfate, either intrathecally into the lumbar subarachnoid space (15 or 50 microng) or i.p. (20 mg/kg). The development of tolerance, as manifested in a reduction of the analgetic efficacy of these injections on the hot plate and tail flick, occurred in a dose dependent fashion over a period of 7 days. At this time, injections of i.p. morphine into animals which had received spinal morphine and vice versa revealed the existence of a two way cross tolerance between spinal and systemically administered morphine. Injection of naloxone into the spinal cord of animals exposed to i.p. morphine or conversely, i.p. naloxone in animals tolerant to intrathecal morphine, yielded a hyperreflexia and extreme sensitivity to handling. Other signs commonly observed in percipitated withdrawal, however, such as wet shakes and weight loss, were not observed.     

Arginine vasopressin enhances retention of morphine tolerance

The hot plate method was used to assess tolerance in rats following daily injections of morphine. Following analgesia assessment, or a time equated rest period, rats were injected with either saline or a pituitary peptide. Arginine vasopressin, but not ACTH 4-10, prolonged the retention of morphine tolerance when assessed five weeks after the last injection. Neither the rate nor the degree of tolerance development were influenced by either peptide. These hormones had no effect on retention of tolerance development were influenced by either peptide. These hormones had no effect on retention of tolerance in rats not assessed for analgesia during the period of tolerance development. The effects of pituitary peptides on morphine tolerance are analogous to the effects they have on learning and memory processes, suggesting that similar adaptational processes are occurring in both phenomena.