The synthesis rate and turnover time of 5-hydroxy-tryptamine in brains of rats treated chronically with morphine.

1. Four schedules of subcutaneous pellet implantation were used to induce tolerance to and physical dependence on morphine in Sprague-Dawley rats. 2. The schedules included implantation of four morphine pellets (each containing 75 mg of morphine free base) during a 3 day period (schedule 1); six pellets during 3 days (schedule 2); six pellets during 7 days (schedule 3) and ten pellets during a 10 day period (schedule 4). 3. A high degree of tolerance and dependence on morphine, comparable to that induced in mouse by implantation of a single morphine pellet for 3 days, was produced with schedule 4. 4. Brain 5-hydroxytryptamine (5-HT) turnover rates as measured by rate of accumulation of 5-HT after monoamine oxidase inhibition by pargyline were not different in rats rendered tolerant to and dependent on morphine according to schedules 1 to 4 when compared with corresponding placebo pellet-implanted rats. 5. The turnover rates of 5-HT in brain of morphine-and placebo pellet-implanted rats (schedule 4) from which the pellets had been removed for 24 h were also similar. 6. It is concluded that tolerance to, and physical dependence upon morphine in the rat is not associated with changes in brain 5-HT dynamics.     

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.     

Development of antinociceptive tolerance and physical dependence following morphine i.c.v. infusion in mice

The chronic i.c.v. infusion of morphine has been reported for rats but not for mice. In the current report, the antinociceptive tolerance to both i.c.v. morphine infusion and s.c. implantation of morphine pellets in mice was compared. Physical dependence after i.c.v. morphine infusion was also evaluated. Osmotic minipumps were filled with morphine (50 mM), connected to i.c.v. cannulae, and implanted s.c. to deliver 50 nmol/h for 3 days (i.e., 3.6 micromol total). Robust jumping precipitated by naloxone (1 mg/kg, s.c.) indicated the development of physical dependence. Tolerance to i.c.v., i.t., and i.v. morphine (6.3-, 2.0-, and 4.4-fold, respectively) was observed using the tail flick test. Mice implanted with pellets containing 75 mg morphine for 3 days (i.e., approximately 260 micromol total) were also tolerant to morphine (6.5-, 7.5- and 18-fold, respectively). Thus, the tolerance developed using the two methods was not identical. These results allow comparison of morphine tested by 3 different routes (i.c.v., i.t., and i.v.) after chronic morphine treatment by two routes (i.c.v. and s.c.) in a single study.     

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.

     

Continuous morphine produces more tolerance than intermittent or acute treatment

Dosing protocol and analgesic efficacy have been proposed to be important determinants of the magnitude of opioid tolerance. The present study examined the effect of acute, intermittent and continuous treatment with the low analgesic efficacy agonist morphine on analgesic tolerance. Mice were implanted s.c. with a 25 mg morphine pellet for 1-7 days. Other mice were implanted s.c. with two 25 mg, or one 75 mg morphine pellet for 7 days. The release of morphine from subcutaneous implanted pellets was quantitated using a spectrophotometric assay. In other studies, mice were injected with morphine once (18.5-185 mg/kg/day; ≈ 10-100 times ED₅₀ for morphine analgesia) or once/day for 7 days. Controls were implanted with a placebo pellet or injected with saline. Analysis of drug release from a 25 mg pellet indicated that release was greatest during the first 24 h, declined and then remained relatively constant. The amount of morphine released over 7 days by a 75 mg pellet (23.9 mg) was more than that of a single 25 mg pellet (15.4 mg) but less than two 25 mg pellets (30.8 mg). Following treatment, morphine cumulative dose-response studies were conducted (tailflick). Continuous treatment with morphine using pellet implantation produced a dose-dependent shift in the morphine ED₅₀ by 3.3, 5.8 and 8.5 fold for one 25 mg pellet, one 75 mg pellet and two 25 mg pellets, respectively. Acute and intermittent morphine administration produced substantially less analgesic tolerance than continuous release of morphine by implant pellets. The maximum shift in the ED₅₀ was 1.6 for acute treatment and 2.7 for 7 day intermittent treatment; despite a larger total daily dose. The present results indicate that continuous treatment with morphine results in greater analgesic tolerance than acute or intermittent morphine treatment even at comparable daily doses. These results are consistent with the suggestion that intermittent dosing has reduced risk of producing opioid tolerance.     

Pharmacodynamics and kinetics of loss of tolerance and physical dependence on morphine induced by pellet implantation in the rat.

The decay characteristics of tolerance and physical dependence on morphine induced by a pellet implantation procedure were determined in male Sprague-Dawley rats. Rats were implanted subcutaneously with 6 morphine pellets during a 7-day period. The pellets were removed, and at various times thereafter tolerance to the analgesic and hyperthermic effects of morphine was measured by determining the response in rats implanted with morphine and placebo pellets. Similarly, the physical dependence was assessed by monitoring withdrawal signs following an injection of naloxone. A high degree of tolerance developed to the analgesic and hyperthermic effects of morphine. Similarly, a high degree of physical dependence also developed as evidenced by a high incidence of jumping response, teeth chattering and production of fecal boli induced by injections of naloxone. In addition, loss of body weight and body temperature also occurred. The analgesic and hyperthermic response to morphine recovered very gradually. There was no significant difference in the analgesic and hyperthermic responses to morphine on day 4 after the pellet removal in rats implanted with morphine and placebo pellets. The decay of tolerance was linear with time for the analgesic effect (r = 0.98) and for the hyperthermic effect (r = 0.93). The change in symptoms of physical dependence on morphine with time depended on the specific symptom monitored. The average number of jumps and teeth chattering decreased with time in a linear fashion with r values of 0.98 and 0.99, respectively. However, the number of fecal boli and wet dog shakes increased linearly with time (r = 0.97). The recovery of loss of body weight was also linear with time. Thus, it is clear that fecal boli and wet dog shakes, which increase in number as the dependence decays, are signs of a low degree of dependence. The results suggest that different central or peripheral mechanisms may be operating in different withdrawal symptoms. These studies may prove to be useful when studying the mechanisms involved in the induction and reversibility of tolerance and dependence processes, and in long-term effects of opiates at a time when the tolerance and physical dependence is no longer evident.     

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.     

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

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

Tolerance-dependence and serum elimination of morphine in rats implanted with morphine pellets.

1. In order to determine whether the degree of tolerance to and physical dependence on morphine induced by pellet implantation procedure in the rat depends on the dose used and the kinetic parameters, the effect of implantation of different number of pellets on tolerance-dependence and elimination kinetics of morphine from serum was determined. 2. Male Sprague-Dawley rats were implanted subcutaneously with pellets. Each pellet contained 75 mg of morphine free base. Three schedules of implantation were used. They included 2 pellets during a 3-day period (2/3), 4 pellets during a 3-day period (4/3) and 6 pellets during a 7-day period (6/7). Placebo pellets which did not contain morphine were implanted in rats which served as controls. 3. The degree of tolerance to and physical dependence on morphine increased as the number of morphine pellets implanted increased. 4. In separate groups of rats implanted with pellets, elimination kinetics of morphine was studied using radioimmunoassay. The kinetic parameters were: area under serum morphine concentration time curve (AUC0----infinity), serum concentration of morphine extrapolated to time zero (Cmax), half-life (t1/2), elimination rate constant (k), mean residence time (MRT) and total body clearance (Clt). 5. The AUC0----infinity and Cmax increased in proportion to the number of pellets implanted. The t1/2, k, MRT and Clt values for 2/3 and 4/3 schedules did not differ, but for 6/7 schedule were significantly different from the other two schedules. The degree of tolerance to and physical dependence on morphine was directly related to the AUC0----infinity and Cmax. The longer t1/2 and MRT and lower Clt and k values in 6/7 schedule may reflect a saturation of glucuronic acid transferase, the main enzyme metabolizing morphine in the liver, and may account for the greater degree of tolerance and physical dependence.     

Studies on the possible role of pharmacokinetics in the development of tolerance to morphine in the rat.

1. The possible role of pharmacokinetics of morphine in the development of tolerance to the analgesic and hyperthermic effects of morphine was studied in the rat. 2. Male Sprague-Dawley rats were made tolerant to morphine by implanting 6 morphine pellets each containing 75 mg of morphine base for 7 days. The assessment of the degree of tolerance to morphine and pharmacokinetic parameters were done 72 hr after pellet removal. 3. Tolerance developed to both the analgesic and hyperthermic effects of morphine as evidenced by decreased responses to morphine in morphine pellet implanted rats compared with placebo pellet implanted rats. 4. The pharmacokinetic parameters, AUC0-->infinity, Cmax, t1/2, k, MRT, Vss and Clt were determined after injecting 5 and 10 mg/kg doses of morphine intravenously to placebo and morphine pellet implanted rats and using a highly sensitive and specific RIA method to quantitate serum levels of morphine. For a 5 mg/kg dose of morphine, the AUC0-->infinity and t1/2 in morphine pellet implanted rats were significantly higher than in placebo pellet implanted rats, but the k value was lower. The other pharmacokinetic parameters for morphine in the two treatment groups did not differ. For 10 mg/kg dose, the only change was an increase in the MRT in morphine tolerant rats when compared to nontolerant rats. 5. The results establish that the development of tolerance to the analgesic and hyperthermic effects of morphine is not related to pharmacokinetics of morphine in serum but may be related to modification of receptor systems in the central nervous system.     

Cerebral cortical 5-HT1 and 5-HT2 receptors of morphine tolerant-dependent rats

The effect of chronic administration of morphine to rats on 5-HT₁ and 5-HT₂ receptors in the cerebral cortex was determined. Male Sprague-Dawley rats were implanted subcutaneously with 6 pellets of morphine (each containing 75 mg of morphine free base) during a 7 day period. Animals which served as controls were implanted with placebo pellets. The procedure for implantation of pellets produced a high degree of tolerance to and physical dependence on morphine in the rat. The tolerance to the analgesic and hyperthermic effects of morphine was demonstrated by decreased responses in the rats implanted with morphine pellet in comparison to the placebo-treated controls. The physical dependence was shown by the greater weight loss after removal of the pellet in the rats implanted with morphine pellets when compared to rats implanted with placebo pellets. The pellets were removed (withdrawn) and, after 6–8 h, the rats were sacrificed and the cerebral cortex was isolated. In another experiment the pellets were left in place (tolerant-dependent rats). The 5-HT₁ and 5-HT₂ receptors were characterized by using [^3H]5-HT and [^3H]spiroperidol as the ligands and unlabelled 5-HT and ketanserin, respectively, to determine non-specific binding. The [^3H]5-HT bound to 5-HT₁ receptors on membranes from the cerebral cortex of rats implanted with placebo pellets, at a single high affinity site, with a B_max of 102 ± 10 fmol/mg protein and a K_d of 6.02 ± 0.98 nM. Implantation of morphine pellets, followed by removal of the pellets resulted in a 50% increase in the B_max value of [^3H]5-HT but the K_d values did not change. In rats from which the pellets were not removed, the B_max and K_d values of [^3H]5-HT in placebo- and morphine-treated groups did not differ. [^3H]Spiroperidol bound to 5-HT₂ receptors on cortical membranes of rats implanted with placebo pellet at a single high affinity site with B_max and K_d values of 131 ± 5 fmol/mg protein and 0.22 ± 0.01 nM, respectively. The implantation of pellets of morphine followed by removal of the pellets did not alter the characteristics of 5-HT₂, receptors, however in rats with the pellets in place, the B_max for 5-HT₂ receptors in placebo- and morphine-treated groups did not differ but the K_d values were much smaller in morphine-treated rats compared to rats implanted with placebo pellets. It is concluded that the development of tolerance to, and physical dependence on, morphine by implantation of pellets results in up-regulation of 5-HT₂ receptors whereas in morphine-abstinent rats there is a selective up-regulation of 5-HT₁ receptors on the membranes in the cerebral cortex.     

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.     

Role of nitric oxide in the induction and expression of morphine tolerance and dependence in mice.

1. The possible involvement of nitric oxide (NO) in the induction and expression of morphine tolerance and dependence was studied in mice. A two-day repeated injection regimen was used to induce morphine tolerance and dependence. Tolerance was assessed by the tail flick test and physical dependence by naloxone challenge, on the third day. 2. Two days pretreatment with L-arginine (20 mg kg-1, twice daily) or D-NG-nitro arginine methyl ester (D-NAME, 20 mg kg-1, twice daily) alone had no effect on subsequent morphine antinociception. L-NG-monomethyl arginine (L-NMMA, 10 mg kg-1, twice daily) for two days led to a slight increase (not statistically significant) in morphine antinociception; while L-NG-nitro arginine methyl ester (L-NAME, 10 mg kg-1, twice daily) for two days led to attenuation of morphine analgesia. None of the animals treated with these drugs alone showed signs characteristic of the opioid withdrawal syndrome upon naloxone challenge. 3. Induction phase L-arginine slowed the development of opioid tolerance and physical dependence, while L-NAME and L-NMMA led to a higher degree of tolerance but had no effect on the development of physical dependence. 4. L-Arginine and D-NAME had no effect on the expression of morphine tolerance and physical dependence. Expression phase L-NAME and L-NMMA, on the other hand, attenuated morphine tolerance and reduced the incidence of withdrawal signs. 5. NO may, therefore, play a role in both phases of morphine tolerance and dependence: elevation of NO levels during the induction phase delays the development of opioid tolerance/dependence, while inhibition of NO synthase accelerates the development of tolerance. Inhibition of NO attenuates the expression of both tolerance and physical dependence.     

PRELIMINARY STUDIES ON INCREASED NALOXONE POTENCY AND ITS RELATIONSHIP TO MORPHINE TOLERANCE AND DEPENDENCE IN MICE

1. Morphine pretreatment (8 0 mg/kg s.c.) induced no overt tolerance to its antinociceptive effect in mice 4 h later, but enhanced the antagonistic potency of naloxone. 2. A molecular sieve morphine pellet implanted for 24 h induced measurable tolerance, but the relative potency of naloxone was not significantly different from that observed after single-dose morphine pretreatment. The development of tolerance and increased naloxone potency do not, therefore, run parallel. 3. Naloxone-precipitated withdrawal symptoms were observed after single-dose morphine and after pellet implantation. However, molecular sieve morphine pellet implantation induced a higher degree of dependence as compared with single dose morphine pretreatment. 4. These results indicate that the rate of development of increased naloxone potency and of morphine tolerance and dependence do not run parallel. This implies that caution must be exercised in regarding increased naloxone potency as a sensitive indicator of the initiation and development of tolerance and dependence to morphine.     

Morphine preference in rats previously morphine dependent

Morphine preference and tendency to relapse to morphine tolerance and dependence were studied in rats which were previously made morphine dependent. Tolerance to, and physical dependence on, morphine were initially produced by administration of increasing concentrations of morphine sulphate in 5% sucrose solution for 3 weeks. A test for drinking preference was performed 4 days after the rats had been successfully detoxified and showed no significant signs of morphine dependence. It was found that, while control animals drank only negligible amounts of morphine solution, previously morphine-dependent rats consumed significantly larger volumes of morphine solution and had recurrence of morphine tolerance and dependence. The present findings show that chronic administration of morphine in drinking fluid produces tolerance and physical dependence as well as addiction in rats; the latter definition is exemplified by these animals having a high tendency to relapse after successful drug withdrawal.     

An improved implantation pellet for rapid induction of morphine dependence in mice.

A new type of morphine implantation pellet for the rapid induction of physical dependence in mice can be prepared by absorbing 7 mg morphine sulphate onto molecular sieves Type 4A (BDH). The small cylindrical pellets can be implanted subcutaneously without trauma and the need for anaesthesia, and are easily removed at any time from the animals. The peak of physical dependence is reached 24 h after implantation, and mortality is negligible. Withdrawal symptoms can be precipitated by intraperitoneal injection of naloxone, without removal of the pellet, and up to 70% of a group of mice show the characteristic urge to jump off a raised platform. This type of pellet has definite advantages over some other sustained-release preparations used in studies on morphine addiction in small animals.     

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.     

Facilitation of morphine-induced tolerance and physical dependence by prolyl-leucyl-glycinamide

The pretreatment of mice with 30 mg/kg morphine s.c. did not alter the analgesic effect of morphine in mice pretreated with saline but decreased the analgesic effect of morphine in mice pretreated with prolyl-leucyl-glycinamide (PLG). Tolerance was evaluated by the effect of PLG on morphine-induced enhancement of naloxone potency which is a measure of the capacity of naloxone to antagonize morphine-induced analgesia and is postulated to be an indicator of tolerance development. The naloxone potency of PLG-treated mice was 2-fold greater than that of control mice. PLG did not alter the whole brain levels of morphine, nor did it alter the naloxone potency in mice which were not pretreated with 30 mg/kg morphine. In mice treated with 100 mg/kg morphine or implanted with 50 mg morphine pellets for 24 or 72 h, the amount of naloxone required to induce jumping was not altered by PLG. However, PLG treatment did increase the hypothermia and body weight loss seen after naloxone-induced withdrawal. Administration of PLG to morphine-dependent mice 1 h prior to naloxone did not modify the resultant hypothermia or body weight loss. These results indicate that PLG facilitated the development of morphine tolerance and dependence.     

Tolerance, dependence and lethality in morphine-dependent mice after repeated oral administration of methadone

Mice were rendered tolerant to and dependent on morphine via a morphine pellet implantation. Three days later methadone hydrochloride was administered at a dose of 100mg/kg per os 3 hours after pellet removal and then daily for a total of 5--6 days. This dose of methadone was shown to exhibit a high efficacy for the blockade of morphine abrupt withdrawal jumping and only minimal toxicity. Under these conditions, the level of analgetic tolerance with respect to morphine and methadone and the level of dependence as measured by the naloxone ED50 were initially elevated by the morphine treatment. However, upon substitution with oral methadone these levels declined with time at a rate which did not differ from that of a group of mice receiving only water after morphine pellet removal. Despite these findings, the methadone treatment was associated with an increasing tolerance to methadone lethality during the administration of this narcotic which was nearly double that of a similarly treated water control group by the sixth day. This observation could not be explained by an elevation in the level of cellular tolerance rendered by the methadone treatment since the morphine LD50 was not elevated following identical treatment with morphine and then methadone. The significance of these results is discussed with respect to the role of methadone administration and its metabolism in the modification of tolerance and dependence.     

The effect of aspartic acid on the intensity of physical dependence in morphine dependent mice

The mice (Balb/C strain) given 2% aspartic acid in 5% saccharose solution or only saccharose solution (p. os) starting 12 hr before the sc morphine pellet implantation until the removal of pellet were rendered physically morphine dependent. During the development of morphine dependence and after the removal of pellets (on 3-d day) spontaneous motor activity and analgesic threshold were measured as reliable abrupt withdrawal signs. Aspartic acid prevented to some extent the appearance of symptoms of physical morphine dependence.