Associative morphine tolerance in the rat: examinations of compensatory responding and cross-tolerance with stress-induced analgesia

The results of three experiments designed to examine the processes subserving associative tolerance to morphine's analgesic effects, as assessed by the tail-flick test, are reported. The experiments indicated that associative tolerance in male Holtzman rats was not subserved by conditioned compensatory responses but was cross-tolerant with an apparently nonopioid form of stress-induced analgesia (SIA). Experiment 1 showed that rats tested for morphine analgesia in a distinctive context that had been paired previously with morphine injections (5 mg/kg) were more tolerant than animals that had had this context explicitly unpaired with a series of morphine injections or animals that were drug-naive. There was no evidence of a conditioned compensatory response of hyperalgesia in animals given a saline injection in the presence of environmental stimuli that had been previously paired with morphine, even under test conditions designed to minimize the possibility of floor effects that might have obscured the detection of drug-compensatory hyperalgesia. Experiment 2 demonstrated that the footshock procedures used for stress induction produced an SIA that was not attenuated by naloxone (10 mg/kg). Experiment 3 replicated the associative tolerance phenomenon of Experiment 1 and again failed to find evidence of conditioned compensatory responses. It was found that animals exposed to footshock in the context that had been associated with morphine administration developed significantly less SIA than control animals. The relevance of these findings for associative models of drug tolerance is discussed.     

毒扁豆碱对吗啡导致的大鼠行为敏感化的抑制作用

药物滥用导致的行为敏感化被认为与成瘾过程密切相关。本实验探讨吗啡导致的大鼠行为敏感化与神经递质乙酰胆碱的关系。实验动物分为3组,分别进行生理盐水、吗啡（10.0mg/kg）、吗啡（10.0mg/kg）＋胆碱酯酶抑制剂毒扁豆碱（0.2mg/kg）前处理,36小时腹腔注射4次。前处理结束1周所有动物注射小剂量吗啡（4.0mg/kg）;使用生理盐水前处理的动物,第2周注射毒扁豆碱（0.2mg/kg）;使用吗啡前处理的动物,第2周注射小剂量吗啡＋毒扁豆碱（0.2mg/kg）,第3周再次注射小剂量吗啡。动物每次接受注射后立即记录其在两小时内的活动量(10分钟为一个记录单元)。结果表明,毒扁豆碱既能够抑制吗啡诱导的行为敏感化,也能够阻断小剂量吗啡对行为敏感化的“点燃”作用。由此推论,吗啡导致的行为敏感化与其抑制乙酰胆碱分泌有关。     

吗啡行为及条件性行为敏感化效应及其个体差异

目的：考察吗啡处理下,大鼠行为敏感化及条件性行为敏感化效应及其个体差异性表现。方法：根据大鼠在初次抵达的新颖环境中水平活动量的高低,将大鼠划分为高反应大鼠（High responder, HR）和低反应大鼠（Low responder, LR）,应用自动监测大鼠活动箱,分别考察HR和LR大鼠在行为及条件性行为敏感化效应表达上的差异。结果：（1）连续5天吗啡给药,LR大鼠活动量显著升高,HR大鼠无此效应;（2）条件测试日（第6天）,给药与环境匹配大鼠,活动量较给药与环境非匹配组动物和对照组动物显著为高;此效应在HR和LR大鼠同时存在;（3）从给予吗啡到给予盐水,发现LR大鼠活动量显著下降,而HR大鼠活动量无显著改变。结论：在连续给药下,LR大鼠较HR大鼠,在行为敏感化效应的形成中,具有更为显著的效应,此效应为LR动物对吗啡更高的药物效应,而非条件效应所致,同时HR和LR大鼠都可以对吗啡条件性线索产生应答,产生条件性行为敏感化效应。     

Effects of daily morphine administration and deprivation on choice and demand for remifentanil and cocaine in rhesus monkeys

Choice procedures have indicated that the relative reinforcing effectiveness of opioid drugs increases during opioid withdrawal. The demand curve, an absolute measure of reinforcer value, has not been applied to this question. The present study assessed whether mild morphine withdrawal would increase demand for or choice of remifentanil or cocaine. Four rhesus monkeys chose between remifentanil and cocaine during daily sessions. Demand curves for both drugs were subsequently obtained. The effects of daily injections of 3.2 mg/kg morphine on both choice and demand for these drugs was assayed 3 and 20.5 hr after each morphine injection, and then during a postmorphine period. Three hours following morphine injections, choice of remifentanil over cocaine decreased and demand for remifentanil--but not cocaine--became more elastic. During morphine withdrawal (20.5 hr postinjection), choice of remifentanil increased and remifentanil demand became more inelastic in 3 of 4 monkeys. Cocaine demand also became more inelastic during this period. Four to five weeks following the morphine regimen, demand for both drugs was more inelastic relative to the initial determination. The results suggest that both the relative and absolute reinforcing effectiveness of remifentanil decreased following morphine administration and increased during morphine withdrawal. The absolute reinforcing effectiveness of cocaine also increased during morphine withdrawal. In addition, extended exposure to drug self-administration and/or exposure to the morphine regimen produced long-term increases in demand for both drugs.     

Conditioned increases in locomotor activity produced with morphine as an unconditioned stimulus, and the relation of conditioning to acute morphine effect and tolerance

Rats administered 5 mg/kg morphine SO4, through subcutaneously implanted catheters, during each of several daily sessions in an open field showed a progressive increase in locomotor activity measured in the open field prior to each morphine administration. Since the increases in activity were not observed in rats given morphine in a different environment (home cage) and saline in the open field, it is concluded that the increases were due to conditioning. In addition, the increases in activity were retained over a 7-day rest period; they were also produced when a second opiate (5 microgram/kg etorphine HCl) was substituted for morphine, were not seen when 2 mg/kg naloxone HCl (ip) was administered during treatment, and were present in rats showing tolerance to opiate-produced hypoactivity. Morphine's direct effect on activity is believed to have a biphasic dose-response curve; therefore, the relation of dose to conditioning was also studied. Increases in activity were the only conditioned behaviors observed; they were present only at the higher doses (16, 4, and 1 vs. .25, .065, and 0 mg/kg), and they occurred whether or not the dose was associated with unconditioned hypoactivity. The discussion deals with the relation of conditioning and morphine tolerance, the question of whether the unconditioned stimulus of morphine conditioning is a compensatory or a direct effect of morphine, and the similarity of conditioned increases in activity produced by morphine and by other stimuli that are reinforcing.     

Interoceptive conditioning through repeated suppression of morphine-abstinence

Experimental evaluation of Wikler’s interoceptive conditioning hypothesis of relapse to opioid use in ex-addicts requires a preliminary study of the degree of physical dependence produced by two methods of drug administration. Wistar rats were made physically dependent on morphine by single daily intravenous injections or by a continuous i.v. infusion. Rats received the same total daily dose regardless of administration schedule. The initial daily morphine dose was 20 mg/kg, and was increased every fourth day by 20 mg/kg, until a dose of 200 mg/kg per day was reached. The rats were maintained at the highest dose level for 18 days, at which time morphine was discontinued. Body weight and water intake were the primary variables measured during addiction, maintenance, and abstinence phases of the study. Equivalent and parallel changes in mean weight and water intake in injection and infusion rats indicate equivalent degrees of physical dependence were developed. This finding allows separation of the contribution of conditioning factors and of protracted abstinence in facilitating opioid self-administration in formerly-dependent organisms.     

东莨菪碱对吗啡诱导的大鼠行为敏感化的影响

药物重复处理导致的行为敏感化与成瘾过程密切相关。本实验检验东莨菪碱对吗啡诱导的大鼠行为敏感化发展和转化的影响。实验一动物分为3组,分别进行生理盐水（对照组）、吗啡（10mg/kg,吗啡组）、吗啡（10mg/kg）＋东莨菪碱（3mg/kg,吗啡-东莨菪碱组）前处理,36小时腹腔注射4次（第1~2天）。自然戒断7天（第3~9天）。第10天,所有动物均使用吗啡（4mg/kg）激发,记录动物的自发活动量;第24天,吗啡组和吗啡-东莨菪碱组动物重复第10天的操作。实验二动物分为3组,分别接受生理盐水（对照组）、吗啡（10mg/kg,吗啡组）、吗啡（10mg/kg,吗啡-东莨菪碱组）处理,36小时腹腔注射4次（第1~3天）;间隔12小时后,3组动物分别接受生理盐水、生理盐水和东莨菪碱（3mg/kg）处理,仍为36小时腹腔注射4次（第3 ~5天）。第6~9天不进行药物处理。第10天和第17天,分别使用吗啡（4mg/kg）激发,记录动物的活动量。记录时间均为两小时(10分钟为一个记录单元)。结果表明,东莨菪碱能够抑制吗啡诱导的行为敏感化的发展,一定程度上也能够延缓行为敏感化的转化但没有阻断这种转化     

Some effects of exposure to a heat stressor upon the rat's subsequent reactions to that stressor

In six experiments, rats were placed on a heated plate on two occasions (test and retest), and the latencies with which they licked their paws were taken as an index of their sensitivity to nociceptive stimulation. Experiment 1 provided evidence that rats were selectively analgesic on retest depending upon the intensity of the heat experienced on the initial test. Experiment 2 showed that this analgesia was recruited rapidly and was long-lasting, as it was observed when retest was scheduled as early as 0.2 and as late as 48 h after the initial exposure to the hot-plate. Experiment 3 documented a role for conditioning processes, as this analgesia was removed by an extinction-like procedure conducted between test and retest. Experiments 4 and 5 provided evidence for a non-opioid mechanism of pain control, because the analgesia was not diminished by the opioid antagonist, naloxone, nor by a history of morphine injections. These experiments also revealed that the analgesia observed on retest was enhanced and reduced when rats were given naloxone and morphine, respectively, on the initial test. Finally, Experiment 6 showed that the analgesia on retest summated with that produced by morphine. The results were taken to mean that the hot-plate assay for analgesia can itself activate endogenous mechanisms of pain control, and some speculations were offered as to how this occurred.     

Excessive grooming induced by the administration of codeine and morphine

Previous studies have shown that peripheral injections of the codeinone RX 336-M can induce excessive grooming in rats in an age-dependent fashion. The present experiment demonstrates that codeine, itself, also induces excessive grooming but with apparent equal effectiveness at all ages tested. Because of the structural similarity between codeine and morphine, the effects of intraperitoneally administered morphine were examined as well. Morphine was found to produce a temporal course of excessive grooming quite different from that produced by codeine. Morphine did not affect grooming in the first half (30 min) of the observation period, but accentuated it, briefly, for the next 30-45 min. Intraventricular administration of codeine at doses of 0.3 or 1.0 microgram/3 microliter had no effect on grooming in animals previously shown to demonstrate excessive grooming in response to either dose.     

MK-801与环境线索交互作用对吗啡行为敏感化的影响

为探讨MK-801与环境线索交互作用对吗啡诱导行为敏感化的影响,将42只大鼠随机分为对照组,MK-801组,吗啡组（匹配和非匹配）和MK-801+吗啡组（匹配和非匹配）。行为敏感化模型建立分为发展期,戒断期和表达期三个阶段。实验结果发现：同时给予MK-801（0.1mg·kg-1）会促进吗啡（5mg·kg-1）诱导大鼠行为敏感化的发展,而且会使MK-801成为大鼠行为敏感化表达所必需的条件性刺激。MK-801的内部线索作用过强,从而削弱了环境的外部线索作用。研究结果表明：MK-801对吗啡诱导行为敏感化的影响存在状态依赖（state-dependency）现象,提示在NMDA受体与行为敏感化关系的研究中应考虑选择刺激特性更小的药物     

冲动性对吗啡诱导的条件性位置偏爱及行为敏感化的影响

许多研究显示, 冲动性与尼古丁、可卡因、海洛因等药物成瘾显著相关, 但它对吗啡成瘾的影响尚未见到报道。本实验考察冲动性对吗啡诱导的条件性位置偏爱及行为敏感化的影响。实验采用延迟奖赏模型将大鼠的冲动性区分为高、中、低三个水平, 每个水平设置吗啡处理组和盐水处理组。结果发现：动物对吗啡诱导的条件性位置偏爱的程度表现为低冲动组>中冲动组>高冲动组, 并且中、低冲动组动物形成了条件性位置偏爱而高冲动组没有形成这种偏爱; 在行为敏感化表达期, 与相应的盐水组比较, 高、中冲动组动物的吗啡运动激活效应显著, 而低冲动组没有达到显著水平。这些结果提示, 条件性位置偏爱任务中, 动物的冲动性越高, 吗啡的强化效应越低; 行为敏感化任务中, 动物的冲动性越高, 吗啡的运动激活效应越高。由此可见, 动物的冲动性对吗啡诱导的条件性位置偏爱及行为敏感化的影响存在差异。     

Attenuation of Morphine-Induced Behavioral Changes in Rodents byd-andl-Glucose

Administration ofd-glucose enhances learning and memory in several tasks and also attenuates memory impairments and other behavioral effects of several drugs, including morphine. The present experiment compared the effects of peripherally administeredd-glucose with those ofl-glucose, a stereoisomer ofd-glucose that is not metabolized and does not readily cross the blood–brain barrier. Liked-glucose, though at somewhat different doses, peripherally administeredl-glucose attenuated morphine-induced deficits in spontaneous alternation performance in rats and mice and attenuated morphine-induced hyperactivity in mice.l-Glucose did not raise circulating levels of plasmad-glucose, suggesting that the effects ofl-glucose are not secondary to increased availability ofd-glucose. Using direct injections ofd- andl-glucose and morphine into the medial septum of rats, the findings indicate thatd-glucose but notl-glucose attenuated morphine-induced deficits in spontaneous alternation performance; indeed, intraseptal injections ofl-glucose alone impaired spontaneous alternation performance. These findings suggest that peripherall-glucose antagonizes morphine-induced behavioral effects by a peripheral signaling mechanism, one distinct from the mechanisms that mediate at least some of the effects ofd-glucose on brain function.     

Voluntary exercise ameliorates cognitive deficits in morphine dependent rats: the role of hippocampal brain-derived neurotrophic factor

Chronic exposure to opiates impairs spatial learning and memory. Given the well-known beneficial effects of voluntary exercise on cognitive functions, we investigated whether voluntary exercise would ameliorate the cognitive deficits that are induced by morphine dependence. If an effect of exercise was observed, we aimed to investigate the possible role of hippocampal brain-derived neurotrophic factor (BDNF) in the exercise-induced enhancement of learning and memory in morphine-dependent rats. The rats were injected with bi-daily doses (10mg/kg, at 12h intervals) of morphine over a period of 10 days of voluntary exercise. Following these injections, a water maze task was performed twice a day for five consecutive days, followed by a probe trial 2 days later. A specific BDNF inhibitor (TrkB-IgG chimera) was used to block the hippocampal BDNF action during the 10 days of voluntary exercise. We found that voluntary exercise blocked the ability of chronic morphine to impair spatial memory retention. A blockade of the BDNF action blunted the exercise-induced improvement of spatial memory in the dependent rats. Moreover, the voluntary exercise diminished the severity of the rats' dependency on morphine. This study demonstrates that voluntary exercise ameliorates, via a TrkB-mediated mechanism, the cognitive deficits that are induced by chronic morphine. Thus, voluntary exercise might be a potential method to ameliorate some of the deleterious behavioral consequences of the abuse of morphine and other opiates.     

Effects of morphine and naloxone upon the reactions of rats to a heat stressor

Five experiments used rats to examine the conditioned hypoalgesia induced by exposure to a heated floor. Experiments 1 and 2 demonstrated that this hypoalgesia is mediated by non-opioid mechanisms of pain control, as evidenced by insensitivity to the opioid antagonist naloxone and by the absence of cross-tolerance with the opioid agonist morphine. Although non-opioid in nature, the acquisition of conditioned hypoalgesia was facilitated by naloxone and impaired by morphine (Experiments 3 and 4). These effects did not appear to be due to an opioid regulation of pain. (1) Pairing morphine with the heated floor attenuated acquisition in drug-tolerant rats. (2) This attenuation by morphine was removed when naloxone was given after exposure to the heated floor. (3) Conditioning was facilitated when naloxone was given after exposure to the heated floor (Experiment 5). The results were discussed in terms of an opioid regulation of (a) surprise, (b) arousal of an aversive motivational system, and (c) the affective component of pain.     

Hippocampal injections of amyloid beta-peptide 1-40 impair subsequent one-trial/day reward learning

The injection of amyloid beta-peptide (Abeta) into rat CNS has been reported to induce cellular neuropathology. The present study investigated whether multiple intrahippocampal injections of Abeta 1-40 would impair one-trial/day reward learning 14 days later. Twenty-four male Sprague-Dawley rats, 3-4 months old, were injected with either Abeta 1-40 or distilled water into seven hippocampal sites bilaterally. Ten rats received 3 nmol Abeta 1-40 in 2 microl of distilled water per injection site, while 14 rats received distilled water alone. Following a 9-day recovery period, rats were gradually food deprived to 82% of their initial body weight. Fourteen days after the intrahippocampal injection, all rats received an initial training trial and three subsequent daily retention trials. Rats receiving Abeta 1-40 were significantly impaired on the second retention trial in terms of accuracy (number of unbaited alleys entered) and on the second and third retention trials in terms of speed (reciprocal of latency to reward). Histological analysis showed that Abeta 1-40 injections produced significant neuronal loss and gliosis. Abeta 1-40 immunoreactivity persisted locally at the injection site and in macrophages 2 weeks following the hippocampal injections. These effects appear to be sequence-specific; rats receiving Abeta 1-42 with a scrambled peptide sequence did not differ significantly from rats receiving distilled water alone in retention of the learning task or degree of histological damage.     

小剂量吗啡对大鼠活动性的影响

腹腔注射不同剂量吗啡,观察各组大鼠在给药后不同时间的活动性（ｌｏｃｏｍｏｔｏｒａｃｔｉｖｉｔｙ,ＬＡ）,连续给药８天,每天给药后９５ｍｉｎ内,每间隔１５ｍｉｎ,记录大鼠５ｍｉｎ内在限定空间中所走格数。结果表明：随吗啡给药剂量或次数增加,ＬＡ呈升高趋势;使大鼠ＬＡ明显兴夯的适宜低剂量为４ｍｇ／ｋｇ／ｄａｙ,该剂量下每天药后１５－２０ｍｉｎＬＡ为峰值,而且此时段ＬＡ逐日升高,至第８日出现下降趋势,此毕     

The effects of clozapine on delayed spatial alternation deficits in rats with hippocampal damage

Clozapine is an atypical antipsychotic drug that has been shown to improve spatial memory in some animal models; however its efficacy in reversing spatial memory impairment in rats with hippocampal lesions is unknown. To address this issue, we tested the effects of clozapine on delayed spatial alternation deficits in rats with hippocampal damage in three separate experiments. In each experiment, adult male rats received sham surgery or direct stereotaxic infusions of the excitotoxin, NMDA, into the hippocampus. In the first study, seven days after surgery, the sham control animals received daily saline injections while the lesioned animals were split into two groups that received daily saline or clozapine (2.0 mg/kg, sc) injections. During the fifth week of injections, all animals were tested in a food-motivated delayed spatial alternation task. Saline-treated rats with excitotoxic hippocampal damage displayed significant deficits in delayed spatial alternation. Daily clozapine injections completely reversed this deficit. In a second experiment, it was found that clozapine treatment limited to the testing days only did not improve alternation performance in lesioned rats. Finally, in a third experiment, chronic clozapine treatment did not improve alternation performance in lesioned rats that were pre-trained in the alternation task prior to surgery. These results suggest that chronic, but not acute, clozapine treatment enables rats with hippocampal damage to develop new spatial learning, but can not rescue old spatial learning established prior to damage. These results may have implications for the treatment of cognitive deficits caused by hippocampal dysfunction in disorders such as schizophrenia, Alzheimer's disease, and others.     

石杉碱甲对应激诱导的吗啡行为敏感化表达的影响

为探讨石杉碱甲对应激诱导的吗啡行为敏感化表达的影响, 将40只动物随机分为5组: 盐水组、石杉碱甲组、吗啡组、应激组、石杉碱甲+应激组。实验发现, 急性/慢性空瓶应激都能够激发吗啡行为敏感化的表达, 而石杉碱甲能够显著抑制空瓶应激的这种激发作用。经过吗啡点燃后, 急性空瓶应激并不能显著影响动物的行为敏感化, 提示与成瘾性药物的再现相比, 空瓶应激对动物的影响力度较小。研究结果表明, 石杉碱甲能够抑制急性/慢性空瓶应激诱导的吗啡行为敏感化表达, 表明这类胆碱酯酶抑制剂有可能成为治疗药物依赖的潜在药物。     

The effects of beta-noradrenergic receptor blockade on acquisition of eyeblink conditioning in 3-month-old F344 rats

There is evidence that blocking beta-noradrenergic receptors will cause deficits in some forms of learning. We investigated the effects of systemic injections of 1, 5, and 10 mg/kg doses of propranolol on acquisition of delay eyeblink conditioning in 3-month-old Fischer 344 rats. We presented a 3-kHz, 90-dB tone as a conditioning stimulus and a 6 psi airpuff as our unconditioned stimulus to freely moving rats. We monitored eyelid activity using EMG signals. The treatment subjects were injected with either propranolol or saline 0.5 h prior to daily training sessions. Two groups of control subjects, one receiving injections of saline and one receiving injections of 5 mg/kg propranolol, received daily training sessions with unpaired and randomized presentation of the tone and airpuff. Each daily training session for the treatment groups consisted of 27 paired training trials and 3 conditioned stimulus-alone training trials. Rats injected with saline vehicle or with 1 mg/kg propranolol achieved a 60% or better learned response rate within two training sessions. Rats injected with 5 or 10 mg/kg propranolol never achieved a response rate significantly different from animals that received unpaired, random presentations of the tone and airpuff stimuli. These results agree with prior studies from our lab that have shown a dose-dependent effect of beta-noradrenergic receptor blockade on learning in rabbit eyeblink conditioning as well as in a runway, motor learning paradigm. We believe that the beta-noradrenergic system plays an important role in learning and memory in more than one cerebellar-dependent learning paradigm.     

Opioid regulation of spinal cord plasticity: evidence the kappa-2 opioid receptor agonist GR89696 inhibits learning within the rat spinal cord

Spinal cord neurons can support a simple form of instrumental learning. In this paradigm, rats completely transected at the second thoracic vertebra learn to minimize shock exposure by maintaining a hindlimb in a flexed position. Prior exposure to uncontrollable shock (shock independent of leg position) disrupts this learning. This learning deficit lasts for at least 24h and depends on the NMDA receptor. Intrathecal application of an opioid antagonist blocks the expression, but not the induction, of the learning deficit. A comparison of selective opioid antagonists implicated the kappa-opioid receptor. The present experiments further explore how opioids affect spinal instrumental learning using selective opioid agonists. Male Sprague-Dawley rats were given an intrathecal injection (30 nmol) of a kappa-1 (U69593), a kappa-2 (GR89696), a mu (DAMGO), or a delta opioid receptor agonist (DPDPE) 10 min prior to instrumental testing. Only the kappa-2 opioid receptor agonist GR89696 inhibited acquisition (Experiment 1). GR89696 inhibited learning in a dose-dependent fashion (Experiment 2), but had no effect on instrumental performance in previously trained subjects (Experiment 3). Pretreatment with an opioid antagonist (naltrexone) blocked the GR89696-induced learning deficit (Experiment 4). Administration of GR89696 did not produce a lasting impairment (Experiment 5) and a moderate dose of GR89696 (6 nmol) reduced the adverse consequences of uncontrollable nociceptive stimulation (Experiment 6). The results suggest that a kappa-2 opioid agonist inhibits neural modifications within the spinal cord.