Locomotor activity in morphine-dependent and post-dependent rats

The effects of morphine and naloxone were compared on the locomotor activity of nondependent, morphine-dependent, and post-dependent rats. Dependence was induced and maintained for 30 weeks by scheduled access to 0.05% morphine solution for 10 min every 6 hr. Locomotor activity in nondependent and dependent animals was increased by low doses of morphine and reduced by higher doses. Both components were antagonized by naloxone. Chronic morphine treatment produced marked tolerance to the depressant effect of high morphine doses, but not to the stimulant effect of low doses. Post-dependent animals remained tolerant to the depressant effect of high doses of morphine. The development of tolerance to the depressant but not to the stimulant effect of morphine in dependent and post-dependent animals suggests that different neuronal substrates mediate morphine-induced stimulation and depression of locomotor activity. Abrupt or naloxone-precipitated withdrawal generally disrupted locomotor activity in dependent rats. Naloxone alone also decreased activity in post-dependent animals. Thus, chronic morphine administration produces long-lasting changes in the sensitivity of dependent and post-dependent rats to the effects of morphine and naloxone on locomotor activity.     

Differential classical conditioning of abstinence syndrome in morphine-dependent rats

Independent groups of addicted rats were placed repeatedly in a conditioning procedure: pairings of non-rotatable activity wheels with periods of either 0–11 or 12–23 h of morphine deprivation. Controls consisted of a saline-injected group exposed to the same training stimuli and conditioning procedure, and a morphine addicted group not receiving such training. Subsequently, morphine-addicted animals were withdrawn until primary abstinence signs disappeared, and all groups were tested in unlocked wheels for differential activity resulting from reexposure to previous training environments.Experimental groups not only were found to exhibit significantly increased wheel turning over controls, but subjects having greater morphine deprivation during training were more resistant to extinction than a low deprivation group. Motivational interpretations and methodological artifacts in designs of this nature are discussed.A lengthy review of the literature encompassing the area of the present study, as well as a more thorough discussion of its theoretical aspects, were included in a Ph. D. thesis submitted to the Department of Psychology at the University of Denver, 1969, by the author.     

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.     

Chronic morphine administration: Plasma levels and withdrawal syndrome in rats

Morphine, administered to Sprague-Dawley rats over a period of 65 hr either by the simultaneous implantation of two 75 mg pellets, or by a series of twice daily 20 or 30 mg/kg injections, produced dependence as indicated by the precipitation of the abstinence syndrome with the antagonist, naloxone. Plasma morphine levels, analyzed fluorometrically at various times during the treatment procedures, revealed peak concentrations that were 3 or 4 fold higher for injected animals than the maximum steady-state level established in the pellet-implanted animals. The calculated plasma concentration of the drug over time was not statistically different for the groups. It is noted that although the 2 methods of morphine administration produce a qualitatively identical dependent state, the pellet implantation technique causes greater weight loss and a higher incidence of jumping and wet-dog shakes during withdrawal.     

GHB ameliorates naloxone-induced conditioned place aversion and physical aspects of morphine withdrawal in mice

Rationale Gamma-hydroxybutyric acid (GHB) is a naturally occurring substance in the brain, the administration of which has proved useful in the treatment of the opiate withdrawal symptoms in humans.Objectives The aim of the present work was to validate this beneficial effect on the physical and motivational aspects of morphine withdrawal in mice.Methods In a first experiment, animals rendered morphine-dependent were conditioned to develop a place aversion (CPA) to the compartment paired with naloxone administration in a two-chamber apparatus. The conditioning phase consisted of three pairings of either naloxone (0.250 mg/kg) or vehicle in one compartment, both with similar time allotments during the preconditioning test. During the testing phase, mice were again allowed to explore the entire apparatus. GHB (6, 12.5, 25, and 50 mg/kg) was administered during either the acquisition or expression phase of this conditioning. In a second experiment, the capacity of GHB to ameliorate the intensity of physical signs of morphine withdrawal was evaluated.Results GHB blocked CPA in both phases: administered during acquisition (from 12.5 mg/kg and higher) as well as in the expression phase (from 6 mg/kg, except for 25 mg/kg). It also decreased the intensity of physical signs of morphine withdrawal to near control levels measured by the modified Gellert–Holtzman scale (25 mg/kg and higher). Decreases in jumping, body shakes, and paw tremor were also observed.Conclusions Our results support the idea that GHB ameliorates both aspects of morphine withdrawal, physical as well as motivational signs.     

Noradrenergic and behavioural effects of naloxone injected in the locus coeruleus of morphine-dependent rats and their control by clonidine

Naloxone HCl (10 g/0.5 ml) was injected in the locus coeruleus (LC) of morphine-dependent rats and the behavioural manifestations of morphine withdrawal and the cortical levels of 3-methoxy-4-hydroxyphenylethyleneglycol sulfate (MHPG-SO₄) were measured 30 min later. Naloxone precipitated a withdrawal syndrome and raised cortical MHPG-SO₄ in animals made dependent by ascending doses of morphine for 11 days. An injection of clonidine intraperitoneally (200 g/kg) or in the LC (5 g/0.5 l) blocked most aspects of the withdrawal syndrome except jumping and had no effect on the naloxone-induced rise in cortical MHPG-SO₄. The findings confirm the hypothesis that the LC is one of the sites where naloxone and clonidine, respectively, precipitate and reduce the narcotic withdrawal syndrome but argue against a role of noradrenergic neurons originating in the LC and innervating the cortex in the ability of clonidine to suppress some aspects of withdrawal syndrome precipitated by naloxone in morphine-dependent animals. Visiting scientists from: Department of Pharmacodynamie, Medical Academy, 20-081, Lublin, Poland     

Antagonist-precipitated and discontinuation-induced withdrawal in morphine-dependent rhesus monkeys

Rationale  Upon discontinuation of chronic opioid treatment, withdrawal typically peaks in 1–3 days and decreases markedly within 1 week; however, persistent physiological changes have been reported long after other signs have waned. Objective  The goal of this study was to compare the discriminative stimulus, directly observable signs, and physiological effects of withdrawal in morphine-treated monkeys. Materials and methods  Monkeys received 5.6 mg/kg/12 h morphine and discriminated 0.0178 mg/kg naltrexone while responding under a fixed-ratio 5 schedule of stimulus–shock termination. Drug discrimination, behavioral observation, and telemetry were used to monitor the emergence of withdrawal, as well as any persistent changes, following discontinuation of morphine treatment. Results  Naltrexone dose (0.001–0.032 mg/kg, s.c.) was positively related with indices of withdrawal. In the discrimination study, monkeys responded on the naltrexone lever 1–5 days following discontinuation of treatment; thereafter, they responded exclusively on the saline lever. After discontinuation of morphine, the frequency of observable signs peaked within 2–3 days and most were not significantly increased after 5 days. In contrast, increased heart rate and body temperature persisted for 14 days, returning to values obtained prior to discontinuation by 21 days. Conclusions  To the extent that discriminative stimulus effects of withdrawal in nonhumans are predictive of subjective reports of withdrawal in humans, these data indicate that effective treatments for opioid dependence must address not only the short-term subjective components of withdrawal but also, and perhaps more importantly, lingering behavioral and physiological effects that might contribute to relapse long after chronic drug use is discontinued. This project was supported by USPHS Grants DA05018 and DA17918 from the National Institute on Drug Abuse. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute on Drug Abuse or the National Institutes of Health.     

Suppression of the drug-induced morphine withdrawal syndrome by cyproheptadine

In rats treated with gradually increasing amounts of morphine hydrochloride until they tolerated fatal doses, levallorphan precipitated acute body weight loss and elicited a variety of other typical withdrawal symptoms. Cyproheptadine markedly reduced this b.w. loss and abolished the drug-induced withdrawal syndrome. Fenfluramine also suppressed the major signs of the levallorphan-induced morphine withdrawal; however the combination of the three drugs proved to be very toxic. Since both agents interfere with different hypothalamic feeding mechanisms these results are accordant with the hypothesis of Kerr and Pozuelo (1971) that morphine dependence and tolerance are due to a functional disorganization of the hypothalamic centers concerned wit the regulation of food intake.     

Chronic infusion of clonidine does not alleviate spontaneous morphine withdrawal symptoms in rats

Opiate withdrawal is associated with behavioural symptoms and a sympathetic hyperactivity, the latter being sensitive to clonidine. The central question is whether behavioural symptoms would be also sensitive to clonidine. A rat model was used in which the locomotor activity was measured 24 h a day during the morphine withdrawal phase. Spontaneous withdrawal of morphine reduced strongly the high nocturnal locomotor activity, concomitently decreasing food intake and body weight. Chronic infusion of clonidine (30–120 µg/kg/day) using osmotic minipumps had no effect on the withdrawal symptoms. Higher dosages (250–1000 µg/kg/day) potentiated rather than alleviated the withdrawal symptoms, suggesting an ₁-adrenergic effect of clonidine rather than an ₂-action. Therefore, we studied the action of a more specific ₂-agonist UK-14.304. UK-14.304 was less potent than clonidine in naive animals. It slightly alleviates the decrease of nocturnal activity during spontaneous morphine withdrawal. Furthermore, we have tested whether the effects of high dosages of clonidine could be altered by a specific ₁-antagonist doxazosine. Doxazosine reduced only slightly the potentiation in the decrease in food intake by clonidine during morphine withdrawal. For the other symptoms no interaction between doxazosine and clonidine was found. The data suggest that the use of clonidine in the detoxification of opiate dependent people is based on the suppression of the sympathetic hyperactivity rather than on symptoms with a more behavioural character.     

Butorphanol precipitates abstinence in morphine dependent rats

Butorphanol precipitated a withdrawal syndrome in rats receiving continuous intracerebroventricular infusions of morphine for three days. However, the potency of butorphanol to induce defecation, urination, teeth chattering and escape behavior was one to two orders of magnitude less on a molar basis than that of naloxone. Wet shake behavior, a prominent feature of naloxone precipitated withdrawal, was absent in morphine dependent animals challenged with butorphanol. These data indicate qualitative as well as quantitative differences in the withdrawal syndromes induced by butorphanol and naloxone.     

Evidence of a preferential role of brain serotonin in the mechanisms leading to naloxone-precipitated compulsive jumping in morphine-dependent rats

Various drugs acting on brain serotonin or catecholamines were administered concurrently with morphine during the development of dependence or before naloxoneprecipitated withdrawal syndrome. Of the various drugs only cyproheptadine, a serotonin antagonist, and piribedil, a dopamine agonist, reduced the frequency of jumping (but not of diarrhea or ptosis) when administered with morphine during development of dependence. When administered before naloxone, d-fenfluramine, a serotonin releaser, markedly reduced jumping, but not diarrhea and ptosis, and clonidine blocked these latter signs without affecting the frequency of jumping. Of the other drugs examined only phenoxybenzamine reduced diarrhea in morphine-abstinent rats. It is suggested that serotonin is involved in the mechanisms which lead to compulsive jumping during naloxoneprecipitated withdrawal, whereas adrenergic sites on which clonidine acts are mainly involved in the expression of signs, such as ptosis and diarrhea. No clear evidence was obtained of a role for dopamine in the withdrawal signs studied.     

Nonspecific excitatory effects of morphine: Reverse-order precipitated withdrawal and dose-dose interactions

We recently reported that, in naive mice pretreated with naloxone, morphine can cause withdrawal-like signs that seemingly are not mediated by opiate receptors. Such results were confirmed and extended here with another mouse strain. Repetitive vertical jumping could occur irrespective of injection sequence and depended on dose and dose ratio of the two drugs.     

Conditioning processes contribute to severity of naloxone-precipitated withdrawal from acute opioid dependence

Rationale Single injections with morphine can induce a state of acute opioid dependence in humans and animals, typically measured as precipitated withdrawal when an antagonist such as naloxone is administered 4–24 h after morphine. Repeated treatment with morphine results in further increases in naloxone potency, and prior work has shown that this progressive shift in naloxone potency requires repeated naloxone experience under some but not all experimental conditions.Objective The current study sought to further characterize the experimental conditions that support naloxone experience-dependent and experience-independent potentiation of precipitated suppression of operant responding in morphine pretreated rats, and to assess more directly whether conditioning mechanisms may contribute to the former process.Methods Rats trained on an FR15 schedule for food received a total of five vehicle or morphine injections (5.6 mg/kg SC) at 4, 8, or 22 h prior to an operant session in which a cumulative dose-effect function for naloxone-induced suppression of responding was determined. Separate groups of animals at each interval between morphine and naloxone received cumulative naloxone dosing after all morphine pretreatments (NAL ALL DAYS) or after just the first and last morphine pretreatment (NAL FIRST/LAST). Additional groups of rats at the 4 h MOR–NAL interval received most of their naloxone cumulative dose-effect experience in either the home cage or in the operant context with levers retracted.Results Vehicle-pretreated (Morphine-Naive) rats showed little change in the naloxone dose-effect function even after five cumulative dose-effect determinations. With a single morphine pretreatment, naloxone potency was increased at 4 or 8 h post-morphine, but not at 22 h. With repeated morphine treatment, all MOR–NAL intervals resulted in significant shifts in naloxone potency across treatment days even when naloxone was administered only after the first and last morphine pretreatment. However, much greater shifts in naloxone potency were observed at 4-h and 8-h intervals when naloxone was administered on all treatment days. At the 22 h MOR–NAL interval, there was no further potentiation in naloxone potency with additional naloxone experience provided on the intermediate days. Finally, when the repeated naloxone experience occurred in the home cage at the 4-h interval, naloxone potency was identical to that seen after limited naloxone experience (NAL FIRST/LAST), and significantly less than naloxone potency in groups receiving repeated naloxone experience in the operant context.Conclusions The results suggest that conditioned withdrawal mechanisms may play a significant role in the initial development of opioid dependence.     

The influence of chronic treatment with clonidine, yohimbine and idazoxan on morphine withdrawal

Numerous previous attempts have been made to study the involvement of α₂-adrenoceptors in the expression of morphine withdrawal by studying the effects of selective α₂-agonists and antagonists administered immediately before precipitation of withdrawal by an opioid antagonist such as naloxone. In the present investigation, we examined the effects of chronic treatment with clonidine (α₂-agonist), idazoxan and yohimbine (α₂-antagonists), concomitantly administered with morphine, on the expression of the withdrawal signs. In contrast to their acute effects, clonidine potentiated, while yohimbine and idazoxan attenuated the withdrawal signs precipitated by naloxone in morphine-dependent mice. In addition, mice chronically treated only with yohimbine displayed withdrawal signs similar to those reported with morphine withdrawal and these signs were not influenced by naloxone administration. Mice chronically treated with clonidine displayed withdrawal signs similar to those reported with morphine withdrawal and these signs were further potentiated by naloxone administration. The results suggest that down-regulation of α₂-adrenoceptors by morphine is a major adaptation contributing to development of dependence on opioids and also point the way to more effective treatment of narcotic dependence. This suggestion was based on the hypothesis that the suppression of noradrenergic system during chronic morphine treatment by α₂-antagonists might diminish noradrenergic hyperactivity and consequently the development of dependence and withdrawal signs. Received: 5 October 1996/Final version: 28 January 1997     

Comparative effects of synthetic enkephalinamides and morphine on abstinence responses in morphine-dependent mice

The effects of intracerebroventricular (ICV) administration of two synthetic enkephalinamides, D-Ala²-Met⁵-enkephalinamide and D-Met²-Pro⁵-enkephalinamide, were compared with those of morphine on abstinence responses in morphine dependent mice. Mice were rendered dependent by morphine pellet implantation for three days. When administered six hours after pellet removal, both enkephalinamides and morphine reversed the naloxone-induced abstinence jumping response. On a molar basis, D-Met²-Pro⁵-enkephalinamide was the most potent compound and morphine was the least potent. Thus, substitution of D-Methionine in the two-position and prolineamide in five-position of the naturally occurring enkephalins, methionine- and leucine-enkephalin resulted in potent compound. The enkephalinamides and morphine also prevented hypothermia when injected immediately after or six hours after morphine withdrawal (pellet removal).     

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.     

Simple method for producing an alcohol withdrawal syndrome in rats.

Rats received intragastric intubations of ethanol at 8 hr intervals for 1, 7, 15 or 30 days. The dosage for each animal was one which produced observable signs of intoxication 1 hr after the intubation. All of the rats in the experimental groups developed a tolerance to ethanol as indicated by the increasing dose required to induce intoxication, but the degree of tolerance was related to the duration of the ethanol administration. During the withdrawal period the incidence of hyperreactivity, convulsive symptoms, and the susceptibility to audiogenic seizures was determined for all 4 groups. Although every experimental animal displayed withdrawal symptoms, the incidence of these symptoms was found to be an increasing, negatively accelerated function of the duration of ethanol exposure. For situations where voluntary consumption of alcohol is not necessary this method is a simple, controlled, reliable, way of inducing ethanol tolerance and physical dependence in rats.     

Effects of ondansetron administration on opioid withdrawal syndrome observed in rats

This study tested whether a 5-HT₃ receptor antagonist could reverse the signs of precipitated opioid withdrawal. Rats were treated with either saline or morphine for 4 days. After the four days, half of the rats in each group received naloxone and half received saline. Each animal also received one of four doses of ondansetron (0, 1, 2 and 4 mg/kg i.p.). Administration of ondansetron to rats receiving naloxone after chronic morphine decreased the intensity of withdrawal signs such as increased defecation, jumping and wet-dog shakes, elevated the nociceptive threshold values which were decreased by precipitated withdrawal, but produced no change in urination, rectal temperature or salivation. The effects exhibited by ondansetron administration may be explained through interference of its 5-HT₃ receptor antagonist activity with serotoninergic mechanisms involved in the regulation of these withdrawal symptoms. The use of this drug is thus suggested as a possible treatment of opioid withdrawal signs in heroin addicts.     

Opiate withdrawal signs precipitated by naloxone following a single exposure to morphine: potentiation with a second morphine exposure

Recent studies in humans with no prior history of opiate abuse indicated that naloxone-precipitated signs of opiate withdrawal could be observed after a single exposure to morphine, and that the severity of withdrawal was enhanced following a second morphine exposure 24 h later. The current study was conducted to establish a paradigm in rodents that resembled these conditions described in humans. To that end, naloxone-precipitated (0.03–3.0 mg/kg) suppression of operant response rates and somatic signs of withdrawal following single or repeated treatments with morphine (5.0 mg/kg) were assessed in previously opiate-naive rats. In one group of rats, naloxone was administered 4 h after both the first and second morphine pretreatment, while in a separate group of rats naloxone was administered 4h after the second morphine pretreatment only. A single morphine pretreatment significantly increased naloxone’s potency to suppress operant response rates, and resulted in the precipitation by naloxone of certain somatic signs of withdrawal. The effects of naloxone on both dependent measures (operant response rates and somatic signs) were potentiated following a second morphine pretreatment, regardless of whether naloxone was administered following both morphine exposures or only following the second morphine exposure. Thus, repeated morphine administration appears to be the critical factor underlying the progressive increase in antagonist potency, whereas prior experience with naloxone is not a necessary factor. The results provide additional support for the hypothesis that the development of dependence on opiates is a progressive phenomenon that may begin with a single dosing. Received: 30 May 1996/Final version: 5 August 1996     

Development of physical dependence on morphine in respect to time and dosage and quantification of the precipitated withdrawal syndrome in rats

In rats implanted subcutaneously with morphine containing pellets different degrees of dependence were induced by varying the dosage, frequency of implantation and duration of exposure to morphine. Withdrawal was precipitated by intraperitoneal injection of morphine antagonists, mostly levallorphan. The absorption of morphine from the subcutaneous depots was estimated chemically.When withdrawal was precipitated with a constant dose of antagonist the frequency of occurrence of various counted signs and the presence of some checked signs were studied in respect to varying degrees of dependence. The results were compared to those obtained after administration of increasing doses of antagonist in groups of animals that had developed a constant degree of dependence.In both types of experiments the results were rather similar. Some signs became progressively more pronounced when dependence got stronger or the dose of the antagonist was increased. In contrast, other signs showed a maximal frequency at the lower degrees of dependence or after administration of the lower doses of antagonist and decreased or even disappeared when the degree of dependence was higher or the dose of antagonist further increased. Obviously, in withdrawal the intensity of recessive signs like writhing and wet dog shaking declines when dominant signs like jumping, flying (a vigorous kind of jumping) and teeth chattering increase. An inverse relationship between the occurrence of various signs could also be shown within the 30 min observation period. Changes in the integrative mechanisms controlling behaviour during withdrawal are supposed to be the reason for this shift of signs.In other experiments in which the interval between each morphine implantation was prolonged the frequency of some signs like jumping and teeth chattering tented to plateau. This finding seems to be correlated to some kind of steady state on resorption of morphine from the subcutaneous depots, as was found in chemical analysis.