Drug-seeking behavior in the dog: lack of effect of prior passive dependence on morphine

Twelve dogs were made dependent on morphine (20 mg/kg per 24 hours) by intravenous passive administration of the drug. Six were gradually withdrawn and six were withdrawn abruptly. Subsequent tests for self-administration of morphine began within 8 to 18 weeks after morphine was last administered passively. Morphine was available for self-administration at several unit dose levels for 8 weeks. In comparison with control dogs administered with saline, there was no evidence that prior dependence on morphine influences subsequent self-administration of morphine, or that morphine is a primary reinforcer for dogs. Possible implications of these and related observations are discussed.     

Morphine Withdrawal Dramatically Reduces Lymphocytes in Morphine-Dependent Macaques

The immune effects of chronic opiate exposure and/or opiate withdrawal are not well understood. The results of human studies with opiate abusers are variable and may not be able to control for important factors such as subjects' drug histories, health and nutritional status. Nonhuman primate models are necessary to control these important factors. A model of opiate dependence in macaques was developed to study the effects of opiate dependence and withdrawal on measures of immune function. Four pigtailed macaques drank a mixture of morphine (20 mg/kg/session) and orange-flavored drink every 6 h for several months. During stable morphine dependence, absolute numbers of neutrophils, monocytes and lymphocytes did not change relative to pre-morphine levels. However, there was a significant decrease in the absolute number and percentage of natural killer (NK) cells in morphine dependence. Either precipitated withdrawal or abstinence for 24 h resulted in behavioral withdrawal signs in all animals. Absolute lymphocyte counts decreased and absolute netrophil counts increased significantly in withdrawal, relative to levels during morphine dependence. Lymphocyte subset (CD4+, CD8+, CD20+) cells were also decreased in absolute numbers with little change in their percentage distributions. There was, however, a significant increase in the percentage of NK cells in withdrawal relative to levels during morphine dependence. This study demonstrates the usefulness of voluntary oral self-dosing procedures for maintaining morphine dependence in nonhuman primates and demonstrates that the morphine withdrawal syndrome includes large alterations in blood parameters of immune system function, including nearly 50% reduction in numbers of CD4+, CD8+ and CD20+ cells.     

Characteristics of precipitated withdrawal from spinal morphine: changes in [Met5]enkephalin levels.

This investigation was carried out to study the development of physical dependence on spinally administered morphine, and it was determined if this phenomenon is associated with altered levels of [Met5]enkephalin. Morphine was infused continuously into the intrathecal space of rats for three or six days. In morphine-dependent animals, an intrathecal naloxone challenge produced increased reaction to nociceptive stimuli, hypertension, hyperthermia, decreased urinary output, and loss of body weight. Chronic spinal infusion of morphine alone failed to alter levels of [Met5]enkephalin in sacral and lumbar spinal cord. However, 24 h after the naloxone challenge, there was a significant increase in spinal enkephalin levels in morphine-dependent animals. It is concluded that spinal morphine treatment leads to the development of physical dependence. Certain characteristics of this phenomenon, as reflected in the naloxone-precipitated withdrawal signs, differ from those associated with dependence on systemic morphine.     

Indomethacin facilitates acute tolerance to and dependence upon morphine as measured by changes in fixed-ratio behavior and rectal temperature in rats

The effects of indomethacin, a prostaglandin (PG) synthetase inhibitor, on acute tolerance to and dependence on morphine were investigated. Twelve mature, male Long-Evans rats were trained to lever press for food reinforcement on a fixed-ratio 30 schedule (FR 30 behavior) and have their rectal temperature taken. The experimental protocol began with taking the rat's temperature followed by a 30 minute behavioral session. Immediately after this session the animal was injected with indomethacin or its vehicle. Two-and-a-half hours later this procedure was repeated, except that morphine or saline was administered. After an additional 2.5 hours had elapsed, a 60 minute behavioral session occurred. Half-way through the session the rat was injected with morphine (tolerance), naloxone (dependence), or saline. Immediately after the session the rat's temperature was recorded. Indomethacin potentiated the acute tolerance to the behavioral suppressant and hyperthermic effects of morphine. Indomethacin pretreatment also greatly enhanced the capacity of naloxone to decrease temperature and suppress FR 30 behavior in morphine-treated rats. These effects were not due to indomethacin altering the acute effects of morphine or the amount of morphine in the brain. These data suggest that indomethacin is inhibiting synthesis of PGs which are important in morphine tolerance and dependence.     

Rapid induction and quantitation of morphine dependence in the rat by pellet implantation

Four schedules of subcutaneous morphine pellet implantation were developed to render rats rapidly physically dependent on morphine. The schedules included implantation of four morphine pellets over a 3-day period (schedule 1), six morphine pellets over a 3-day period (schedule 2), six pellets over a 7-day period (schedule 3), and ten pellets over a 10-day period (schedule 4). Each morphine pellet contained 75 mg of morphine base. The degree of morphine dependence was quantitated by determining the median effective dose (ED₅₀) of naloxone required to induce the stereotyped jumping response. Hypothermia and weight loss, during abrupt and naloxone-induced withdrawal, were also measured. Rats on schedule 4 exhibited a high degree of dependence on morphine as evidenced by an extremely low naloxone ED₅₀ for the precipitated withdrawal jumping response, whereas schedules 1 and 2 produced a low degree of dependence as shown by high naloxone ED₅₀'s. Further evidence for a high degree of physical dependence on morphine is schedule 4 rats was indicated by their greater loss in body weight and greater hypothermic response after abrupt and after naloxone precipitated withdrawal compared with these responses in the rats in the other three schedules. A correlation was found to exist between naloxone ED₅₀ for the jumping response, body weight loss, and hypothermia observed during naloxone-induced withdrawal in morphine-dependent rats. These studies suggest that the implantation of four morphine pellets in the rat produces a mild degree of dependence and that caution should be exercised when making generalized conclusions about the biochemical correlations involved when four or less number of pellets, each containing 75 mg of morphine base, are used to induce morphine dependence in the rat.     

Influence of hemicholinium (HC-3) on morphine analgesia, tolerance, physical dependence and on brain acetylcholine

Administration of HC-3 intracerebrally at a dose which reduced brain acetylcholine (ACh) without any change in choline (Ch) levels antagonized morphine antinociception slightly as measured by inhibition of the tail-flick response, in both naive mice and in mice rendered tolerant to and dependent on morphine by pellet implantation. However, the development of tolerance to morphine and of dependence on morphine were not affected by HC-3. Although naloxone precipitated withdrawal jumping was enhanced irrespective of whether the HC-3 was administered before or after the dependence on morphine had developed, body weight loss during abrupt withdrawal was unaffected by HC-3. The results indicate that although some of the acute and withdrawal effects of morphine may be associated either directly or indirectly with acetylcholine, these actions do not appear to be the primary process responsible for initiating the development of tolerance and dependence.     

Attenuation of morphine tolerance development by electroconvulsive shock in mice

The development of tolerance to morphine has been proposed as being analogous to learning or memory, since the presentation of a novel stimulus (morphine) to an organism results in an altered response on subsequent presentations of the same stimulus. Electroconvulsive shock has been widely used to disrupt memory in animals and its effect on the development of tolerance to morphine was examined. Pretreatment of mice with six, progressively increasing intraperitoneal doses of morphine induced tolerance to an excitant action of morphine, shown by a marked reduction in the locomotor activity elicited by a subsequent intraperitoneal test dose of morphine. In three separate experiments, the administration of eleetroconvulsive shock 2–3 hr after each pretreatment with morphine attenuated the degree of tolerance developed. The eleetroconvulsive shock had no significant effect on the locomotor activity of non-tolerant mice tested under similar conditions in each experiment. In a single pilot study, electroconvulsive shock was found to reduce the frequency of jumping precipitated by an intraperitoneal dose of naloxone, suggesting a possible decrease in the degree of dependence. The present results are interpreted as supporting the view that the mechanisms of morphine tolerance may be similar to those involved in learning or memory.     

Extracellular signal-regulated kinase (ERK) inhibition does not prevent the development or expression of tolerance to and dependence on morphine in the mouse

The clinical use of opioids is limited by the development of tolerance and physical dependence. Opioid tolerance and dependence are believed to result from complex adaptations in the CNS, representing a form of neural plasticity. Extracellular signal-regulated kinases (ERKs) are involved in many forms of neural plasticity, and therefore could also be involved in the development of opioid tolerance and dependence. In this study, we investigated the effect of a systemically bioavailable MEK (ERK kinase) inhibitor, SL327, upon the development and the expression of tolerance to and dependence on morphine in mice. In tolerance and dependence development studies, two strains of mice were treated daily for 8 or 9 days with 5mg/kg morphine s.c. Tolerance development was assessed by tail flick latency. Withdrawal was then precipitated by subcutaneous injection of 2mg/kg naloxone s.c. and signs recorded. Co-administration of 50mg/kg SL327 i.p. prior to morphine administration had no effect on the development of tolerance or withdrawal signs. To study possible effects of ERK inhibition on the expression of tolerance and dependence, mice were implanted with 75mg morphine pellets s.c. Tolerance and dependence were assessed as previously described. An acute i.p. injection of 50mg/kg SL327 after 4 days of morphine exposure had no effect on the expression of either morphine tolerance or physical dependence. To verify that this dose of SL327 inhibited morphine-induced ERK modulation, mice received an acute i.p. injection of 50mg/kg SL327 prior to morphine administration, and sacrificed 30min later. Western blots demonstrated that SL327 did inhibit morphine-induced ERK modulation. Taken together, these data suggest that unlike many other observed forms of neural plasticity, the ERK signaling cascade is not involved in the development or expression of opioid tolerance and dependence.     

Eating pattern of morphine dependent rats

To analyze the drug ingestion patterns of rats in the course of dependence development while on the drug-admixed food (DAF) method, an automatic food intake measuring apparatus was developed. Rats were put on morphine-admixed food, and the food ingestion patterns were recorded with the apparatus in the course of dependence development, during drug withdrawal and at the time of challenge with levallorphan. The naive rats ate the regular diet intermittently at night, and the eating time of morphine-treated rats was longer than that of naive rats. The treated rats also exhibited a frequent eating behavior after 4--5 days on the morphine treatment. During morphine withdrawal, the animal gradually ate the regular diet at about 1-hour intervals, even after evolvement of abstinence signs. When the morphine-dependent rats were given levallorphan, they neither ate nor approached to the food for the first 2--3 hours, but after this time, showed abrupt increases in these activities. Thus, the drug intake pattern of rats in the course of morphine dependence development suggests a correlation between the stage of development of physical dependence and the stage when the animals frequently eat the drug-admixed food.     

High affinity binding of 2-chloroadenosine to rat brain synaptic membranes

Intracerebroventicular (i.c.v.) administration of subanalgesic doses of β- and leucibe-endorphin in mice 15 min before subcutaneous morphine injection, significantly enhanced the analgesic effects of the morphine as measured by the tail-flick assay. A similar effect was seen with levorphanol analgesia but no enhancement of leucine- or methionine-enkephalin analgesia by β-endorphin was observed. The same doses of the endorphins did not affect the development of single-dose morphine tolerance and dependence. Leucine-enkephalin failed to affect morphine analgesia, tolerance or dependence development, while low doses of methionine-enkephalin administered i.c.v. were observed to have an antagonistic effect on morphine analgesia without affecting tolerance or dependence development. Neither enkephalin had any effect on β-endorphin analgesia.     

Development of narcotic tolerance and physical dependence: effects of Pro-Leu-Gly-NH2 and cyclo (Leu-Gly)

Administration of Pro-Leu-Gly-NH2 (MIF) and cyclo (Leu-Gly) blocked the development of tolerance to and physical dependence on morphine, induced by the pellet implanation procedure in mice. Inhibition of tolerance development by peptides was evidenced by the presence of an analgesic response (increase in jump threshold) as determined by measuring the jump threshold to an increasing electric current, after a challenge dose of morphine (40 mg/kg). The same dose of morphine did not alter the jump threshold in morphine tolerant mice which were injected with saline prior to pellet implantation. The inhibition of the development of physical dependence on morphine by these peptides was evidenced by the antagonism of the hypothermic response which occurs during abrupt or naloxone-induced withdrawal. The naloxone-induced withdrawal jumping response was unaffected by these peptides. Dose-response experiments indicated that cyclo (leu-Gly) was much more potent than MIF in these tests. These peptides, when given after the development of tolerance and dependence, did not modify either the analgesic response to morphine or the symptoms of abrupt and naloxone-precipitated withdrawal. The inhibition of development of analgesic tolerance and physical dependence was not associated with changes in brain morphine concentration. The data indicate that these peptides do not interfere withe the morphine-morphine receptor complex formation but alter a subsequent step in the genesis of some aspects of tolerance and dependence processes.     

Evidence for the dissociation of morphine analgesia, tolerance and dependence

A single large dose of morphine produced profound analgesia accompanied by the development of tolerance and physical dependence. The tolerance developed acutely within 24 h and was further intensified, reaching a peak on the 5th day, then gradually disappeared. Partial or complete masking of morphine analgesia by naloxone inhibited the development of the acute, but could not prevent the development of the delayed, tolerance. These results suggest there are two kinds of tolerance and that the analgesic effect is separate from tolerance. Similarly, treatment with morphine produced physical dependence which was precipitated by naloxone. Unlike tolerance, dependence did not develop when morphine analgesia was completely masked by naloxone. The findings provide for the dissociation or morphine analgesia, tolerance and dependence.     

Development of morphine dependence in rats: Lack of effect of previous ingestion of other drugs

It has often been observed that periods of abuse of non-narcotic drugs precede the development of dependence on opiates, but this correlation does not necessarily imply that there is a simple causal relationship. We have previously described a procedure for inducing rats to drink solutions of morphine in preference to water; self-administration of the drug in this way seems to be a valid model of dependence. For the first 46 days of the experiment described here, solutions of alcohol, amylobarbitone, chlordiazepoxide, cocaine or dexamphetamine were made available to different groups of rats. Most of these drugs were ingested in substantial doses, had clear effects on behaviour and produced characteristic patterns of drinking over the repeated trials. However, there was no unequivocal evidence of dependence, and indeed the rats learned to reject the solutions of dexamphetamine. Ingestion of these drugs did not affect the eventual development of dependence when solutions of morphine were substituted at a later stage, although the avoidance of dexamphetamine seemed to temporarily transfer to morphine. Further studies using other methods and species are needed before inferences can be made about escalation to dependence on opioids in man.     

Effects of naltrexone pellet implantation on morphine tolerance and physical dependence in the rat

• 1.1. The effect of naltrexone pellets containing either 10 or 30 mg of naltrexone base on the development of tolerance and physical dependence on morphine was assessed in male Sprague-Dawley rats. Tolerance-dependence on morphine was induced by s.c. implantation of six morphine pellets, each containing 75 mg morphine base for 7 days.
• 2.2. Naltrexone pellet implantation blocked the development of tolerance to the analgesic and hyperthermic effects of morphine. Similarly, naltrexone pellet implantation reversed morphine withdrawal-induced body weight loss. The effect of pellets containing 10 and 30 mg naltrexone did not differ.
• 3.3. The effect of naltrexone (10 mg) pellet implantation on various signs of naltrexone-precipitated withdrawal such as body weight loss, hypothermia and increases in urinary and fecal output was investigated. Naltrexone pellet implantation did not alter the naltrexone-precipitated withdrawal-induced body weight loss. Concurrent naltrexone pellet implantation blocked the naltrexone-precipitated withdrawal-induced hypothermia, increased fecal and urinary output in morphine-dependent rats.
• 4.4. These results indicate that a single pellet of 10 mg of naltrexone can effectively block morphine tolerance and physical dependence in the rat. Such a procedure may be useful in studying biochemical, endocrinological and immunological mechanisms involved in opioid addiction processes.

     

Attenuation of morphine tolerance and withdrawal syndrome by coadministration of nalbuphine

Morphine has been used widely on the treatment of many types of chronic pain. However the development of tolerance to and dependence on morphine by repeat application is a major problem in pain therapy. The purpose of the present study was to investigate whether combined administration of nalbuphine with morphine affects the development of tolerance to and dependence on morphine. We hypothesize that the use of nalbuphine, κ-agonist may prove to be useful adjunct therapy to prevent morphine-induced undesirable effects in the management of some forms of chronic pain. Morphine (10 mg/kg) was injected to rats intraperitoneally for 5 day. The variable dose of nalbuphine (0.1, 1.0 and 5.0 mg/kg) was administered (i.p.) in combination with morphine injection. The development of morphine tolerance was assessed by measuring the antinociceptive effect with the Randall-Selitto apparatus. The development of dependence on morphine was determined by the scoring the precipitated withdrawal signs for 30 min after injection of naloxone (10 mg/kg, i.p.). Nalbuphine did not attenuate antinociceptive effect of morphine in rats. Interestingly, combined administration of morphine with nalbuphine (10∶1) significantly attenuated the development of dependence on morphine. The elevation of [³H]MK-801 binding in frontal cortex, dentate gyrus, and cerebellum after chronic morphine infusion was suppressed by the coadministration of nalbuphine. In addition, the elevation of NR1 expression by morphine was decreased by the coadministration of nalbuphine in rat cortex. These results suggest that the coadministration of nalbuphine with morphine in chronic pain treatment can be one of therapies to reduce the development of tolerance to and dependence on morphine.     

Antagonism by naloxone of tolerance and dependence in mice given a single dose of morphine

The effect of naloxone given at various times after morphine administration on the development of tolerance to and dependence on a single dose of morphine was studied. Naloxone antagonized the analgesic effect of morphine and the development of tolerance to and dependence on morphine, dose dependently. The time course of the development of tolerance to a single dose of morphine almost paralleled that of dependence on morphine but the time course of the disappearance of tolerance did not coincide with that of dependence. When start of the duration of action of morphine was blocked by naloxone for various time intervals, the degree of tolerance to and dependence on morphine was antagonized, time dependently. When the end of the duration of action of morphine was antagonized by naloxone for various time intervals, tolerance and dependence which developed up to that time was completely antagonized by naloxone.     

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).     

Antagonizing effect of aspartic acid on the development of physical dependence on and tolerance to morphine in the rat

As free amino acids in the brain have a role in the development of physical dependence on and tolerance to morphine, and in the mechanism of action of some drugs, the effects of aspartic acid which antagonizes some effects of the single dose of morphine were studied during the development of the physical dependence on morphine and after the withdrawal of morphine. 108 rats were given morphine and aspartic acid in different combinations in drinking water for 30 days. Every tenth day the dose of morphine was increased: At the end of this period some of them in each group continued or began to receive aspartic acid depending on the experimental conditions after the withdrawal of morphine. During the experiments body weight, spontaneous motor activity and analgesic threshold were determined. Aspartic acid prevented the alterations induced by morphine during the development of physical dependence and tolerance. Furthermore the rats that received aspartic acid after the withdrawal showed no body weight loss.     

THE EFFECT OF ADRENALECTOMY ON THE DEVELOPMENT OF MORPHINE TOLERANCE AND PHYSICAL DEPENDENCE IN MICE

A molecular sieve morphine pellet implanted for 24 h induced measurable tolerance and physical dependence in mice. 2 Adrenalectomy sensitized the animals to the antinociceptive effect of morphine. However, the degree of tolerance induced by morphine pellet implantation was not significantly affected. 3 Quantitative assessment of naloxone-precipitated withdrawal symptoms showed that adrenalectomy slightly enhanced the development of physical dependence. 4 These results indicate that adrenalectomy has no effect on the rate of development of morphine tolerance but may be involved in the development of physical dependence.     

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.