Comparison of simultaneous administration of lithium with L-NAME or L-arginine on morphine withdrawal syndrome in mice

Due to the claim that chronic administration of lithium or L-N(G)-nitroarginine methyl ester (L-NAME), a nitric oxide synthase (NOS) inhibitor reduces morphine withdrawal syndrome, the effects of chronic administration of lithium, L-NAME, or L-arginine (L-Arg), a precursor of NO, alone or co-administration of lithium with L-Arg or L-NAME, on naloxone-precipitated withdrawal syndrome and physical dependence development to morphine in mice chronically treated with morphine, were evaluated. Morphine dependency was induced by the intraperitoneal injection (i.p.) of morphine (10 mg/kg), once daily for 7 days. Physical dependence to morphine was observed by precipitating an abstinence syndrome with naloxone (2 mg/kg, i.p.). Chronic administration of L-NAME (10 mg/kg, i.p., once daily, for 7 days after 10 days of receiving only tap water and food prior to naloxone), decreased all withdrawal signs significantly, while L-Arg (200 mg/kg, as above) increased only some withdrawal signs significantly in morphine-dependent mice. Chronic administration of lithium (600 mg/kg, in drinking water) alone or co-administration of lithium (as above) with L-NAME (10 mg/kg) or L-Arg (200 mg/kg, i.p., once daily) for 7 days after 10 days of receiving only lithium (as above) and food, decreased all withdrawal signs and physical dependence significantly in morphine-dependent mice. The results obtained indicate that co-administration of L-NAME with lithium increases the effect of lithium or L-NAME alone, on withdrawal signs, but this increase is not significantly different as compared to chronic lithium or L-NAME administration alone; while co-administration of L-Arg with lithium decreases the effects of lithium on withdrawal signs and this decrease is not significant as compared to chronic lithium administration alone. These findings indicate that nitric oxide may be involved in modulation of naloxone-induced withdrawal syndrome, and treatment with lithium could have some effect on this system. Copyright 2000 John Wiley & Sons, Ltd.     

Chronic variable stress or chronic morphine facilitates immobility in a forced swim test: reversal by naloxone

The behaviors displayed in a forced swim test were investigated in rats previously exposed to a chronic variable stress treatment or chronic administration of morphine. In addition, to further explore the participation of an endogenous opiate mechanism in these behavioral effects, naloxone was either administered during the chronic treatment (prior to each stress or morphine exposure) or immediately prior to the forced swim test. Animals were submitted daily to a different stressor for 1 week or injected with morphine (10 mg/kg, IP) for 6 days, whereas controls were unmanipulated except for the injection process. On the day following the last stressor, control and stressed animals were administered saline or naloxone (2 mg/kg, IP) 15 min prior to the forced swim test. Morphine treated animals were similarly tested on the third day following the last morphine injection. In a separate group of rats, naloxone (2 mg/kg, IP) was administered daily 10 min prior to each stressor of the chronic stress regime or each daily morphine injection. A significant increase in the time spent in immobility was observed in stressed animals as well as in rats chronically treated with morphine. In both groups, this potentiated immobility was attenuated by naloxone pretreatment prior to the forced swim test or when given before each daily stressor or morphine injection. In addition, the concurrent exposure to stress or morphine along with naloxone administration enhanced struggling in the first 5 min of the forced swim test. Taken together, the results of these experiments support the conclusion that the increase in immobility seen following chronic variable stress or repeated morphine exposure is modulated by the activation of an endogenous opiate mechanism, given that this effect is attenuated by naloxone administration.     

Clonidine suppresses methylxanthine induced quasi-morphine withdrawal syndrome

The effects of the alpha-2 adrenergic agonist, clonidine, on the "quasi-morphine withdrawal syndrome" (QMWS) were examined in drug naive rats. The QMWS was induced by combined systemic administration of iso-butyl-methylxanthine (IBMX: 15 mg/kg, IP) and naloxone (1 mg/kg, IP). Pretreatment with clonidine (50 micrograms/kg, IP) significantly decreased the incidence of 11 out of 16 withdrawal signs. Since clonidine suppresses signs and symptoms of true morphine withdrawal, the suppression of methylxanthine effects demonstrates an additional similarity of the QMWS to true morphine withdrawal. These results suggest that a significant common neural mechanism of both the QMWS and true morphine withdrawal is affected by clonidine.     

Influence of prazosin and clonidine on morphine analgesia,tolerance and withdrawal in mice

Rapid development of tolerance and dependence limits the usefulness of morphine in long-term treatment. We examined the effects of clonidine (₂-adrenoceptor agonist) and prazosin (₁-adrenoceptor antagonist) on morphine analgesia, tolerance and withdrawal. Morphine tolerance was induced using a 3-day cumulative twice-daily dosing regimen with s.c. doses up to 120 mg/kg. Tolerance was assessed on day 4, as loss of the antinociceptive effect of a test dose of morphine (5 mg/kg). After 10 h, morphine withdrawal was precipitated with naloxone (1 mg/kg). Prazosin had no analgesic effect alone but dose-dependently potentiated morphine analgesia in morphine-naive mice. Another ₁-adrenoceptor antagonist, corynanthine, had similar effects. Prazosin also increased the analgesic potency of the morphine test dose in morphine-tolerant mice. Naloxone-precipitated vertical jumping was not affected, but weight loss was reduced by prazosin. Acutely administered clonidine potentiated morphine analgesia and alleviated opioid withdrawal signs, as expected. We conclude that in addition to the already established involvement of ₂-adrenoceptors in opioid actions, also ₁-adrenoceptors have significant modulatory role in opioid analgesia and withdrawal.     

The effect of intrathecal administration of the neuronal N-type calcium channels antagonist, ω-conotoxin MVIIA,on attenuating the spontaneous and naloxone-precipitated morphine withdrawal in rats

The effect of ω-conotoxin MVIIA (Ziconotide or SNX-111) as an inhibitor of the neuronal N-type calcium channels on the signs of morphine withdrawal was investigated. The animals were rendered to physical dependence by the continuous intraperitoneal (IP) injections of gradually enhancing the doses of morphine (10–50?mg/kg) during 7 days. The withdrawal was induced in animals through the IP administration of naloxone with the dose of 2?mg/kg or it was elicited spontaneously. The single and chronic intrathecal injections of ω-conotoxin following the naloxone-precipitated withdrawal diminished the number of jumpings (p?<?0.05). Moreover, both acute and chronic administrations of the drug alleviated the intensity of ptosis, piloerection, teeth chattering, diarrhea, irritability and genital grooming (p?<?0.01) during the two models of the experiment. It can be concluded that the class of N-type calcium channel blockers ameliorate the severity of morphine withdrawal syndrome.     

Effects of clonidine and morphine on opioid withdrawal in rhesus monkeys

Rhesus monkeys undergoing opioid withdrawal either due to withholding morphine administration for 14 h or due to administration of naloxone, were treated with either morphine or clonidine. Morphine eliminated all of the withdrawal signs that developed when morphine was withheld for 14 h. Clonidine also eliminated some but not all signs that developed when morphine was withheld. The frequencies of individual signs prior to drug administration were directly related to the minimal doses necessary to eliminate signs for morphine but not for clonidine. Morphine also eliminated most of the signs precipitated by naloxone, whereas clonidine did not eliminate as many of the naloxone-precipitated signs. Additionally, some of the naloxone-precipitated signs that were eliminated by clonidine were not eliminated by morphine. The present results are consistent with clinical findings indicating an efficacy of clonidine in the treatment of opioid withdrawal through a non-opioid mechanism.     

Modification of morphine-induced analgesia, tolerance and dependence by bromocriptine

The effect of two doses of bromocriptine, a dopamine agonist, on morphine-induced analgesia, tolerance and dependence was investigated in mice. Bromocriptine at doses of 0.04 and 0.08 mg/kg did not affect the baseline tail flick latency of mice but potentiated the morphine analgesia. Pretreatment of mice with 5 mg/kg of sulpiride, a D-2 antagonist, not only blocked the effect of 0.08 mg/kg of bromocriptine but also antagonized the morphine analgesia. Control animals given daily injections of 10 mg/kg of morphine rapidly developed tolerance to the analgesic effect. A combined treatment of bromocriptine with morphine given daily suppressed the development of tolerance to morphine analgesia. However, development of tolerance to morphine analgesia was not significantly modified in the animals treated daily with bromocriptine (0.08 mg/kg) plus sulpiride (5 mg/kg). Acute dependence was induced by the administration of 100 mg/kg of morphine. The administration of bromocriptine 30 min before naloxone significantly decreased the ED₅₀ value for naloxone for inducing jumping in mice. Coadministration of sulpiride and bromocriptine attenuated the ability of bromocriptine to potentiate the withdrawal syndrome of morphine dependence. The results indicate that bromocriptine potentiates morphine analgesia, suppresses the development of tolerance to morphine analgesia but exacerbates opiate withdrawal signs in morphine-dependent mice. These effects of bromocriptine appear to be mediated via D-2 receptors.     

Study of morphine-induced dependence in gonadectomized male and female mice

In this study we evaluated the effects of sex difference and also sex hormones on the naloxone-precipitated morphine withdrawal in both orchidectomized (ORC) male and ovariectomized (OVX) female mice. Morphine (50, 50 and 75 mg/kg/day for 4 days, s.c.) was administered to animals and at 5th day naloxone (4 mg/kg, i.p.)-precipitated morphine withdrawal signs, jumpings and the percentage of weight loss, were measured. There was no significant alteration in withdrawal jumpings between male and female mice, though weight loss was significantly higher in male ones. Jumpings was significantly lower in both OVX and ORC mice and percentage of weight loss was significantly higher in OVX mice than corresponding non-operated or sham animals. In OVX mice, E₂V (10 mg/kg, s.c.) increased number of jumpings and decreased percentage of weight loss. Progesterone (25 mg/kg, s.c.) had no effect on jumpings, whereas it decreased weight loss in OVX mice. Testosterone (2.5 mg/kg, s.c.) increased jumpings in ORC mice while it had no effect on percentage of weight loss. Our results demonstrated that sex hormones could play a role in the morphine withdrawal syndrome in both ORC male and OVX female mice.     

The acoustic startle response as a measure of behavioral dependence in rats

A series of experiments was conducted to assess the sensitivity of the acoustic startle response to chronic morphine administration and naloxone-precipitated withdrawal. Rats were implanted with two subcutaneous pellets containing either 75 mg each of morphine or containing only placebo. In experiment 1, withdrawal induced by 0.05–0.2 mg/kg naloxone dose-dependently decreased the magnitude of the startle response. Physical dependence was confirmed by a naloxone-induced acute weight loss seen in morphine-implanted rats, but naloxone had no effect on startle or body weight in nondependent animals. In experiment 2, a modified procedure with fewer trials per session and fewer test days was employed. Naloxone (0.2 mg/kg) given 4–5 days after implantation induced large startle-response decreases in morphine-dependent rats while having no effect in placebo-implanted rats. Post-naloxone saline tests revealed no significant differences in startle between morphine and placebo groups. Startle scores were significantly higher in morphine-implanted rats than in placebo rats during a saline test given 3 days following pellet implantation. In a separate group of animals, however, acute IP injections of morphine from 0.3–10 mg/kg had no significant effect on startle amplitude. The effect of repeated pairings of withdrawal with the startle environment was assessed in experiment 3. Morphine-dependent rats startled significantly less if naloxone injections were given before the startle session than if they were administered 4 h later. Conditioned withdrawal effects, expressed during a final test session when all rats received saline, were observed for the body-weight measure but not for the startle response. These results suggest that the acoustic startle response may be a useful objective measure in evaluating physical dependence produced by substances of abuse.     

Conditioned place aversion is a highly sensitive index of acute opioid dependence and withdrawal

Rationale Conditioned place aversion (CPA) is known to be a sensitive measure of the aversive motivational state produced by opioid withdrawal in rats made chronically dependent on opioids. Objective The purpose of the present study was to examine the sensitivity of the CPA model in detecting a possible aversive state associated with naloxone-precipitated withdrawal from acute treatment with morphine. Methods Doses of morphine and naloxone, as well as number of conditioning trials, were systematically varied to determine the minimum conditions that would result in a detectable CPA in male Wistar rats. Naloxone (0.003–16.7 mg/kg) was administered 4 h after an injection of vehicle or morphine (1.0, 3.3, or 5.6 mg/kg) and immediately prior to confinement to one compartment of the conditioning apparatus; rats received either one or two such naloxone-conditioning trials (separate by 48 h). Results Morphine (5.6 mg/kg) followed 4 h later by vehicle produced no significant preference or aversion. In morphine-naive rats, 10 mg/kg naloxone was required to produce a significant CPA with two cycles of conditioning. When increasing doses of morphine were administered (1.0, 3.3, 5.6 mg/kg), significant increases in naloxone potency to elicit a CPA were observed (16-, 211-, and 1018-fold potency shifts, respectively). Naloxone potency after two pretreatments with 5.6 mg/kg morphine was comparable to its potency to elicit a CPA after chronic exposure to morphine. Although naloxone was still effective in producing a CPA after a single conditioning cycle (and hence a single morphine exposure), its effects were dramatically reduced relative to those seen with two conditioning cycles. Conclusions CPA is a reliable and sensitive index of the aversive motivational state accompanying withdrawal from acute opioid dependence.     

Effect of propranolol on antinociceptive and withdrawal characteristics of morphine

Propranolol at a dose (10 mg/kg) which did not alter tail-flick latency by itself, did not alter the ED50 of morphine when given 10 min prior to the narcotic. Propranolol at doses of 10 and 25 mg/kg given 10 min prior to naloxone challenge did not significantly alter the frequency of naloxone induced jumping 72 hr after morphine pellet implantation. The ED50 of naloxone in morphine pelleted mice was not altered by treatment with propranolol at 0, 24, and 48 hr after pellet implantation. Naloxone caused hyperactivity in mice when administered 72 hr after morphine pellet implantation. An injection of 25 mg/kg propranolol 10 min prior to naloxone did not block this hyperactivity. In addition, administration of 10 mg/kg of propranolol every 8 hr to rats during withdrawal from morphine failed to alleviate the withdrawal syndrome as evidenced by changes in either body weight or water intake. These data suggest that the beta-adrenergic blocking agent, propranolol, does not alter the antinociceptive activity or lessen the withdrawal syndrome of morphine in rodents.     

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     

Morphine and naloxone's effects on sexual behavior of the female golden hamster

The effects of morphine and naloxone were observed after administration to female golden hamsters (Mesocricetus auratus). Large doses of morphine, 80 mg/kg, consistently produced sedation and behavioral depression of responses to nociceptive stimuli. Smaller doses of morphine (e.g., 10 mg/kg), that produced few other behavioral changes, suppressed a measure of female sexual responding. The suppressive effects on sexual behavior were reversed by 4 mg/kg of naloxone. Morphine administered intracerebroventricularly had little effect on sexual responding, even at doses which produced other side effects. Doses of 4 and 8 mg/kg of naloxone in opioid-naive subjects did not reliably alter sexual responding up to 2 hr after administration. These observations lead to the suggestion that morphine produces effects which are incompatible with full sexual functioning in female hamsters.     

Withdrawal from acute morphine dependence is accompanied by increased anxiety-like behavior in the elevated plus maze

Pretreatment with a single moderate dose of morphine (e.g. 5.6-10 mg/kg) 4-24 hr prior to challenge with an opioid antagonist such as naloxone results in reliable expression of behaviors that resemble aversive or emotional consequences of withdrawal from chronic opioid exposure, including suppression of operant responding, elevations in brain reward thresholds, and conditioned place aversion. Repeated daily or weekly treatment with these same morphine doses results in a progressive increase in naloxone potency to elicit these withdrawal signs. The current study sought to determine whether increased anxiety-like behavior during withdrawal from chronic opioid dependence is also seen after acute morphine exposure, and progresses with repeated intermittent treatment. Male Wistar rats were handled and injected with either vehicle or morphine for 4 consecutive days. Three injection regimens were employed: Morphine Naive (4 vehicle injections), Acute Morphine (3 vehicle injections, 4th injection 5.6 or 10 mg/kg morphine), or Repeat Morphine (all 4 injections with 5.6 or 10 mg/kg morphine). Acute pretreatment with 5.6 mg/kg or 10 mg/kg morphine resulted in time-dependent increases in exploration of the open arms of the plus maze in naloxone-naive rats when tested at 2, 4 or 8 hr after the final pretreatment injection, with the effects at the higher dose appearing later (4 hr) than after the lower dose (2 hr). This pattern of results, in combination with a separate study which confirmed a significant anxiolytic-like effect of a low dose of morphine (0.56 mg/kg) administered 15 min prior to test, suggested that low residual morphine levels remaining in plasma at 2-4 hr after 5.6 and 10 mg/kg morphine may be sufficient to elicit anxiolytic-like effects. Repeat treatment with either dose of morphine resulted in a further increase in the magnitude and duration of this anxiolytic-like effect. These effects had dissipated by 8 hr post-morphine, and therefore precipitation of withdrawal by one of several doses of naloxone (0.10-3.3 mg/kg) was assessed in separate cohorts of rats 8 hr after the final pretreatment under Morphine Naïve, Acute Morphine, or Repeat Morphine conditions. Naloxone resulted in a significant dose-dependent expression of anxiety-like behavior with no effects on general activity after Acute Morphine pretreatment at either 5.6 or 10 mg/kg morphine. A further significant shift in naloxone potency was observed after Repeat Morphine pretreatment at the 10 mg/kg but not the 5.6 mg/kg dose. Thus, anxiety-like behavior is a prominent feature of the negative emotional consequences of naloxone-precipitated withdrawal from acute opioid dependence.     

Lithium inhibits the development of physical dependence to clonidine in mice

Based on our previous finding that chronic lithium treatment reduced naloxone-precipitated withdrawal syndrome in morphine-treated mice, the effect of chronic lithium treatment was evaluated on the development of dependence to clonidine. Dependence was induced by injection of either morphine (50, 50 and 75 mg/kg, intraperitoneally with 3 hr interval for 3 consecutive days), or clonidine (2 mg/kg/day, intraperitoneally for 10 days). Naloxone (4 mg/kg, intraperitoneally) precipitated withdrawal signs in both morphine- and clonidine-treated mice. Yohimbine (5 mg/kg, intraperitoneally) precipitated withdrawal signs in the clonidine-treated mice, similar to morphine withdrawal signs; but failed to precipitate any significant sign in the morphine-treated mice. Coadministration of lithium was carried out by adding lithium chloride to drinking water (600 mg/l for 20 days; 10 days before the beginning of clonidine administration and 17 days before the administration of morphine to allow the lithium concentration to reach steady-state). The results indicated that chronic lithium administration significantly attenuated the withdrawal signs, precipitated either by yohimbine or naloxone, in clonidine-treated mice. As a conclusion, clonidine withdrawal signs are very similar to opioid withdrawal signs, and lithium is able to prevent the development of physical dependence to clonidine.     

Generalization of [DAla2]-enkephalinamide but not of substance P to the morphine cue

Rats were trained to discriminate morphine (7.5 mg/kg, IP) from saline in a two bar positively reinforced lever pressing paradigm on a FR4 schedule. Morphine (IP) showed a naloxone reversible dose-related generalization to the training dose. [DAla²]-Methionine enkephalinamide (DAE) at 1 mg/kg and Substance P (SP) at 0.1 and 0.25 mg/kg showed vehicle appropriate responding after IP injection. DAE (5 mg/kg) disrupted responding completely; SP (0.5 and 0.1 mg/kg) disrupted responding in 50% of the rats. The disruption caused by IP injection of DAE was not naloxone reversible. Intraventricular injection of morphine (5 μg/rat) and DAE (5 μ/rat) produced generalization to the opiate cue. The effect of DAE was reversed by naloxone (1 mg/kg, SC). SP (500 and 750 ng/rat, IVT) produced saline-like responding; 1 μg/rat disrupted responding completely. These data demonstrate that morphine and enkephalin, but not Substance P, share similar discriminative properties.     

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.     

Dynorphin A (2–17) attenuates the unconditioned but not the conditioned effects of opiate withdrawal in the rat

OBJECTIVES: An unbiased place preference conditioning procedure was used to examine the influence of the non-opioid peptide, dynorphin A 2-17 (DYN 2-17), upon the conditioned and unconditioned effects of opiate withdrawal in the rat. METHODS: Rats were implanted SC with two pellets containing 75 mg morphine or placebo. Single-trial place conditioning sessions with saline and the opioid receptor antagonist naloxone (0.1-1.0 mg/kg; SC) commenced 4 days later. Ten minutes before SC injections, animals received an IV infusion of saline or DYN 2-17 (0.1-5.0 mg/kg). Additional groups of placebo- and morphine-pelleted animals were conditioned with saline and DYN 2-17. During each 30-min conditioning session, somatic signs of withdrawal were quantified. Tests of place conditioning were conducted in pelleted animals 24 h later. RESULTS: Naloxone produced wet-dog shakes, body weight loss, ptosis and diarrhea in morphine-pelleted animals. Morphine-pelleted animals also exhibited significant aversions for an environment previously associated with the administration of naloxone. These effects were not observed in placebo-pelleted animals. DYN 2-17 pretreatment resulted in a dose-related attenuation of somatic withdrawal signs. However, conditioned place aversions were still observed in morphine-pelleted animals that had received DYN 2-17 in combination with naloxone. Furthermore, the magnitude of this effect did not differ from control animals. CONCLUSIONS: These data demonstrate that the administration of DYN 2-17 attenuates the somatic, but not the conditioned aversive effects of antagonist-precipitated withdrawal from morphine in the rat. Differential effects of this peptide in modulating the conditioned and unconditioned effects of opiate withdrawal are suggested.     

Acute effects of morphine on Substance P concentrations in microdissected regions of guinea-pig brain

The present study investigated the effects of acute morphine treatment and of naloxone-induced morphine withdrawal on Substance P (SP) concentrations in microdissected regions of the guinea-pig brain. Guinea-pigs, which were treated with a single dose of morphine sulphate (15mg/kg s.c.), received naloxone hydrochloride (15mg/kg s.c.) after 2h. Control animals received injections of saline, saline and naloxone, or morphine and saline. Locomotor and behavioural activities were measured throughout the experiments. Animals were killed 0.5h after naloxone administration, brains were removed and SP-like immunoreactivity (SP-LI) was measured in microdissected regions using radioimmunoassay. Morphine significantly increased the concentration of SP-LI in the central nucleus of the amygdala, but reduced SP-LI overall in the mesencephalon. Guinea-pigs pretreated with morphine and then given naloxone to precipitate withdrawal showed no change in SP-LI concentrations in any brain region, compared with control animals, despite an increase in locomotor activity and other behaviours characteristic of opioid withdrawal in guinea-pigs.     

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.