Effects of morphine,naloxone, buprenorphine,butorphanol, haloperidol and imipramine on morphine withdrawal signs in cynomolgus monkeys

This study was conducted to characterize the opiate dependence potential of a number of opiate and non-opiate psychoactive drugs in morphine-dependent cynomolgus monkeys (Macaca fascicularis). In addition, the agonist/antagonist profiles of buprenorphine and butorphanol were directly compared. Six male cynomolgus monkeys were maintained on morphine (3.0 mg/kg, q.i.d.) for 6 months. On evaluation days, monkeys were scored for opiate withdrawal signs 18 h after the last dose of morphine. Single dose suppression studies were conducted by giving subcutaneous injections of morphine (3 or 6 mg/kg), naloxone (0.01, 0.05 or 0.1 mg/kg), buprenorphine (1, 3, 10, 30 or 100 µg/kg), butorphanol (0.01, 0.1, 1.0 or 3.2 mg/kg), haloperidol (0.01, 0.05 or 0.1 mg/kg), imipramine (2, 5 or 10 mg/kg) or saline and measuring the number and frequency of withdrawal signs that appeared over a 2-h period. In a separate precipitation experiment either saline or 0.01 or 0.1 mg/kg butorphanol was administered 2 h after the last maintenance dose of morphine. In the single dose suppression test, morphine completely suppressed most withdrawal signs while naloxone increased the severity of withdrawal. All doses of buprenorphine increased many withdrawal signs such as backward gait, rearing, chafing face, chain biting, vomiting, masturbation, and vocalizations after intimidation. Only a few signs were reduced, but the overall withdrawal scores were not significantly increased. Low doses of butorphanol suppressed some signs while the highest dose almost completely eliminated all withdrawal signs. Butorphanol also failed to precipitate opiate withdrawal, and actually reversed the signs present in the saline-treated monkeys. Imipramine and haloperidol had little effect on the morphine withdrawal syndrome. These data suggest buprenorphine exerts only opioid antagonistic activity and butorphanol exerts only opioid agonistic activity over a wide range of doses studied in morphine-dependent cynomolgus monkeys.     

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.     

Nitroglycerin inhibits the development of morphine tolerance and dependence in rats

The development of tolerance to and physical dependence on opioids remains a significant barrier to their clinical use. N-Methyl-D-aspartate (NMDA) receptor antagonists inhibit tolerance and dependence. However, many NMDA antagonists have undesirable side effects. It has been shown that nitroglycerin (NTG) can antagonize NMDA receptor activity. This study was designed to determine whether NTG could inhibit the development of morphine tolerance and dependence. Rats were anesthetized and implanted with either morphine or placebo pellets, and pumps infusing vehicle or NTG (doses from 0.1 microg/kg/day to 10 mg/kg/day). Tolerance development was assessed by tail-flick latency (TFL). After 6 days, withdrawal was precipitated by subcutaneous injection of 2 mg/kg naloxone. Withdrawal signs were observed for 15 min. Placebo-pelleted rats showed no changes in TFL over the course of the study and no withdrawal signs. Morphine-pelleted rats developed tolerance. The 0.1 mg/kg/day NTG dose significantly attenuated tolerance development, while the other doses had no significant effect. The 0.1 mg/kg/day dose also attenuated some withdrawal signs. Higher or lower doses were not effective, possibly because of competing biochemical effects.     

Acute morphine dependence in the hamster

Two experiments investigated naloxone-precipitated withdrawal following a brief course of morphine administration in hamsters. In Experiment 1, observable withdrawal symptoms (e.g., wet-dog shakes) were elicited by two doses of naloxone (0.4 and 1.0 mg/kg) following four and eight daily injections of morphine (15 mg/kg), a regimen that replicated previous studies in our laboratory using a locomotor activity paradigm. At the lower dose of naloxone, the frequency of withdrawal signs was greater after eight than after four morphine injections. In Experiment 2, observable withdrawal symptoms were elicited by the same two doses of naloxone, 70 min after a single morphine injection. These results suggest that acute dependence in the hamster, as in other species, beings to develop with the first morphine exposure.     

Tolerance to morphine analgesia: evidence for stimulus intensity as a key factor and complete reversal by a glycine site-specific NMDA antagonist

N-methyl-D-aspartate (NMDA) receptors are widely involved in opioid tolerance. However, it is less clear whether NMDA receptor antagonists reverse already-established tolerance and whether the intensity of the nociceptive stimulus influences morphine tolerance. Three days after implantation of morphine or control pellets the effects of i.v. morphine and pre-administration of saline or (+)-HA966 (a glycine site-specific NMDA receptor antagonist), were studied on the C-fibre reflex elicited by a wide range of stimulus intensities. Morphine both increased the threshold and decreased the slope of the recruitment curve in the "non-tolerant" group of animals. In the "morphine-tolerant" group, the threshold did not change but the gain of the stimulus-response curve decreased. The expression of tolerance to morphine depended on the intensity of the stimulus, being maximal when threshold stimulus intensities were used but considerably less with supra-threshold stimulation. As expected, a single treatment with (+)-HA966, potentiated morphine antinociception in "non-tolerant" rats. However, in "morphine-tolerant" rats (+)-HA966 reversed established morphine tolerance and increased the antinociceptive effects of morphine. These results suggest that (+)-HA966 interfered with expression of morphine tolerance, and offered an encouraging therapeutic approach for pain management in opioid abusers.     

The glycine site-specific NMDA antagonist (+)-HA966 enhances the effect of morphine and reverses morphine tolerance via a spinal mechanism

Using the C-fibre reflex as a nociceptive response elicited by a wide range of stimulus intensities in the rat, we recently reported that a single treatment with (+)-HA966, a glycine site-specific NMDA receptor antagonist: (1) potentiates morphine antinociception; and (2) reverses an established morphine tolerance. We presently aimed at determining whether our observation was likely to result from a direct effect on the spinal cord or an indirect effect of supraspinal origin. In a 2x2x2 experimental design, we compared the effects of 5 mg/kg morphine in: (1) sham-operated rats or animals whose brainstems had been transected at the level of the obex; (2) rats that were implanted with pellets, either 150 mg morphine or placebo; and (3) animals injected either with saline or 10 mg/kg (+)-HA966. The control C-fibre reflexes were similar in all groups of animals. As compared to "non-tolerant" rats, the depressive effect of morphine was weaker in "morphine-tolerant" animals where the threshold did not change following morphine but the gain of the stimulus-response curve decreased, albeit to a significantly lesser extent than in the "non-tolerant" group. Whether in "non-tolerant" or "tolerant" groups, the effects of morphine were stronger in "obex-transected" than in "sham-operated" animals. In all groups, the effects of morphine were potentiated by the preliminary administration of (+)-HA966. However, in the "morphine-tolerant" group, the preliminary administration of (+)-HA966 was more potent in the "sham-operated" than in the "obex-transected" groups. Since overall effects were very similar in "sham-operated" and "obex-transected" animals, we concluded for our model that the critical site for the expression of the neuronal plastic changes associated with morphine tolerance lies in the spinal cord.     

A method of reducing the opioid withdrawal intensity using progressively increasing doses of naloxone

We assessed the withdrawal intensity in acutely morphine-dependent mice using a pretreatment with escalating doses of naloxone. All animals received a single dose of morphine (100 mg/kg) for the induction of acute opioid dependency. Group 1 (control) received three injections of normal saline and then naloxone 0.8 mg/kg. Group 2 received increasing pretreatment doses of naloxone (0.1, 0.2, and 0.4 mg/kg) and a challenge dose of 0.8 mg/kg. Group 3 received three injections of naloxone 0.1 mg/kg and a challenge dose of 0.8 mg/kg. Groups 4 and 5 were used to verify whether ED(50) found in previous studies was comparable with values obtained in the current experiments. The withdrawal intensity was determined by the number of jumps. The mice of group 1 exhibited significantly more jumps after 0.8 mg/kg of naloxone as compared with group 2. The number of jumps in response to naloxone between groups 1 and 2 and groups 2 and 3 was not significantly different. The results show that pretreatment with increasing naloxone doses significantly reduced the withdrawal intensity as compared with the control group; whereas pretreatment with repeated low antagonist did not reduce it significantly.     

Morphine tolerance and dependence in mice with history of repeated exposures to NMDA receptor channel blockers.

Mice were subjected to two successive treatment protocols: first with NMDA receptor channel blockers (14 days, once a day) and second with morphine (5 mg/kg, 8 days, once a day). Treatment with the higher doses of dizocilpine (1 mg/kg), memantine (30 mg/kg), and MRZ 2/576 (30 mg/kg) upon discontinuation revealed only minor behavioral abnormalities attributable to the state of withdrawal. Following repeated administration of low-dose morphine, tolerance to morphine analgesia developed in mice preexposed to dizocilpine (1 mg/kg but not 0.3 mg/kg) but not memantine (10 and 30 mg/kg), MRZ 2/579 (10 and 30 mg/kg), or saline. There were no signs of morphine dependence in any treatment group. Overall, the present study found only minor effects of the subchronic administration of high doses of NMDA receptor channel blockers, suggesting that clinical use of NMDA receptor channel blockers such as memantine will not be accompanied by increased propensity to induction of morphine tolerance and dependence.     

NMDA antagonist modulation of morphine antinociception in female vs. male rats

NMDA antagonists may be useful for their potential to increase or prolong opioid analgesia while attenuating the development of opioid tolerance and dependence. The purpose of this study was to determine whether there are sex differences in NMDA antagonist modulation of morphine antinociception. Adult female and male Sprague–Dawley rats were injected s.c. with saline or one dose of MK-801 (0.005, 0.01, 0.02, or 0.04 mg/kg), dextromethorphan (5, 10, or 20 mg/kg), or LY235959 (0.5, 1.0, or 2.0 mg/kg) in combination with saline or one dose of morphine (1.8, 3.2, or 5.6 mg/kg), and tested on the 50 °C hotplate and tail withdrawal assays 15–120 min post-injection. At the doses examined, only LY235959 produced any antinociception when administered alone. MK-801 attenuated morphine antinociception on both assays, but only at sporadic (inconsistent) dose-combinations. Dextromethorphan increased morphine antinociception on the hotplate but not tail withdrawal assay, at all three morphine doses in males, but only the higher morphine doses in females. In contrast, LY235959 modulated morphine antinociception on both assays; the lowest dose attenuated, and higher doses enhanced morphine antinociception, but the particular morphine doses and assay in which these effects occurred depended on the sex of the subject. Thus, all three NMDA antagonists modulated morphine antinociception in female and male rats, but the direction of this modulation depended on the particular antagonist examined, the nociceptive test, the dose of antagonist and of morphine, and time post-injection.     

Repeated experience with naloxone facilitates acute morphine withdrawal: potential role for conditioning processes in acute opioid dependence

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 at 24-h intervals can result in a progressive shift in potency of naloxone to produce such acute withdrawal signs, including suppression of operant responding for food reward. The current study characterized fully both morphine and naloxone dose-effect functions in an effort to establish the relative contributions of repeated morphine vs. repeated naloxone (Nal) experience to these potency shifts. Rats trained on an FR15 schedule for food received four vehicle or morphine injections (0.56-5.6 mg/kg sc), spaced 24 h apart. Four hours after each morphine pretreatment (Repeat Nal), or 4 h after the fourth and final morphine pretreatment only (Single Nal), a cumulative dose-effect function for naloxone-induced suppression of responding was determined. Vehicle-pretreated (Morphine Naive) rats showed little change in the naloxone dose effect function even after four cumulative dose-effect determinations. By contrast, a progressive increase in naloxone potency was observed following successive pretreatments with morphine under Repeat Nal conditions, and the magnitude of naloxone potency shift was morphine dose dependent. At a morphine dose of 5.6 mg/kg, repeated naloxone experience in the presence of morphine was not an absolute requirement to produce an increase in naloxone potency across days, but repeated naloxone could potentiate the magnitude of the observed shift, indicating both experience-independent and experience-dependent processes at work. At lower doses of morphine (1.0 and 3.3 mg/kg) no shift in naloxone potency was observed across days of morphine treatment in the absence of repeated naloxone experience (Single Nal conditions), indicating an increasing contribution of naloxone experience-dependent processes as dose of morphine was decreased. It is argued that these experience-dependent processes in the progressive shift of naloxone potency observed in the current study may reflect an important role of conditioning in the early development of opioid dependence.     

Effect of p-chloroamphetamine on morphine withdrawal syndrome

Adult male rats were injected four times a day with increasing doses of morphine sulfate until a dose of 405 mg/kg/day was tolerated. After 5 days of maintenance at that dose, withdrawal signs were observed at 24, 48, and 72 hr of withdrawal. Given 4 or 18 hr after last morphine injection, a single administration (3–12 mg/kg) of p-chloroamphetamine (PCA) blocked withdrawal “wet shakes” and ptosis during the following withdrawal period. Dose-dependent reduction in withdrawal hypothermia was observed only at 24 hr of withdrawal. Writhing was blocked only at higher doses. Morphine-withdrawal aggression measured at 72 hr of withdrawal and loss in body weight were not blocked by any delayed action of PCA. However, withdrawal aggression was blocked by PCA when given 1 hr before measurement at a dose of 3 mg/kg. Mice that were made morphine dependent by repeated injections of morphine given in large doses, exhibited withdrawal jumping either after cessation of morphine injections, or on injection of naloxone. PCA (3–12 mg/kg) given 2 hr before naloxone blocked this jumping in a dose-dependent manner. It also blocked withdrawal jumping in morphine-deprived mice when injected 2 hr, but not 14 or 17 hr before measurement. Jumping induced by α-naphthyloxyacetic acid (α-NOAA) was not blocked. The effect of PCA on different signs of morphine withdrawal is time dependent and may be related to its similarly time-dependent effect on brain serotonin and dopamine.     

Effects of NMDA receptor antagonists on acute mu-opioid analgesia in the rat

Mixed research findings have led to a debate regarding the effect of N-methyl-D-aspartate (NMDA) receptor antagonists on opiate analgesia. NMDA antagonists have been found in various studies to enhance, to inhibit, or to have no effect on opiate analgesia. The present research used a single protocol to explore the effects of six NMDA receptor antagonists on acute morphine (3.0 mg/kg s.c.) and fentanyl (0.05 mg/kg s.c.) analgesia in adult male Sprague-Dawley rats. NMDA receptor antagonists were selected based on their abilities to block various sites on the NMDA receptor complex, including the noncompetitive antagonists MK-801 (0.1 and 0.3 mg/kg i.p.), dextromethorphan (10.0 and 30.0 mg/kg i.p.), and memantine (3.0 and 10.0 mg/kg i.p.), a glycine site antagonist, (+)-HA-966 (10.0 and 30.0 mg/kg i.p.), a competitive antagonist, LY235959 (1.0 and 3.0 mg/kg i.p.), and a polyamine site antagonist, ifenprodil (1.0 and 3.0 mg/kg i.p.). Analgesia was assessed using the tail-flick test. A single dose of each opiate was used. The low doses of the antagonists, which are known to produce significant neural and behavioral actions at NMDA receptors, had no effect on morphine or fentanyl analgesia. At the higher doses, morphine analgesia was significantly enhanced by LY235959 (3.0 mg/kg), and fentanyl analgesia was significantly enhanced by LY235959 (3.0 mg/kg), dextromethorphan (30.0 mg/kg), and (+)-HA-966 (30.0 mg/kg). Enhancement of analgesia occurred without any apparent adverse side effects. None of the NMDA antagonists affected tail-flick responses on their own, except the higher dose of LY235959 (3.0 mg/kg), which produced a mild analgesic effect. Because no consistent effects were observed, the data suggest that NMDA receptors are not involved in acute mu-opioid analgesia. The mechanisms underlying the enhancement of opiate analgesia by selected NMDA antagonists, such as LY235959, dextromethorphan, and (+)-HA-966, remain to be determined.     

Opioid physical dependence development in humans: effect of time between agonist pretreatments

We examined the effect of morphine pretreatment spacing on intensity of subsequent precipitated withdrawal response to test the hypothesis that withdrawal intensity would be inversely related to pretreatment spacing. Subjects were ten nondependent male volunteers who reported using opioids an average of 2.5 times per week. Three IM morphine injections (each 18 mg/70 kg) were administered during each of four experimental conditions. The experimental conditions involved spacing injections at 12-, 24-, 48- or 72-h intervals. Naloxone (10 mg/70 kg IM) was administered 24 h after the last morphine exposure. A comparison condition was included in which a naloxone challenge was given at 24 h following a single IM morphine pretreatment (18 mg/70 kg). Subject rated measures of symptoms and observer-rated measures of signs indicated that withdrawal intensity was inversely related to the morphine spacing interval and in particular that intensity of withdrawal precipitated after three pretreatments spaced at 12-h intervals was greater than that precipitated after a single morphine pretreatment. Physiological data supported a more intense withdrawal response in the 12-h spacing condition and provided evidence of overshoot on blood pressure and skin temperature measures. These findings are pertinent to the transition between acute and chronic physical dependence; they suggest that there is a temporal window during which repeated opioid administrations result in escalation of physical dependence but that dependence levels after widely spaced multiple exposures may be no greater than after a single exposure.This research was supported by USPHS Grant DA-04011 and Research Training Grant T32 DA-07209 from the National Institute on Drug Abuse     

Clinically available NMDA antagonist, memantine, attenuates tolerance to analgesic effects of morphine in a mouse tail flick test

Converging lines of evidence indicate that N-methyl-D-aspartate (NMDA) receptor antagonists attenuate the development of morphine tolerance tested in antinociception assays in rodents. The present study extends these findings to the effects of clinically available NMDA receptor antagonist, memantine. Male Albino Swiss mice were tested for analgesia using the tail-flick apparatus. Preliminary experiment was designed to find out the optimal dose of morphine and the number of injections that would produce tolerance to its analgesic effects. In the main experiment, during the development of tolerance period (6 days), mice received 10 mg/kg sc b.i.d. morphine injections in the animal room (non-associative tolerance). This treatment resulted in 5.8 fold rightward shift of morphine cumulative dose-response effect from 3.39 mg/kg on day 1 to 16.19 mg/kg on day 8 of the experiment. Memantine pretreatment (5 and 10 mg/kg, but not 2.5 mg/kg), given 30 min prior to each morphine dose during the development of tolerance period, inhibited the rightward shift of morphine cumulative dose-response curve. Thus, pretreatment with memantine at doses of 2.5, 5 and 10 mg/kg resulted in ED50 values of 12.13, 4.74 and 1.95 mg/kg, respectively, corresponding to 3.35, 1.02 and 0.94 fold changes. These data indicate that low affinity, clinically available NMDA receptor antagonist, memantine, may be used to inhibit the development of morphine tolerance.     

Attenuation of morphine dependence and withdrawal by glycine B site antagonists in rats

Numerous data indicate that noncompetitive and competitive N-methyl-D-aspartate (NMDA) receptor antagonists inhibit the development of physical dependence on opioids when these substances are administered together, and NMDA receptor antagonists are used at lower range of doses. Higher doses of these antagonists can enhance some opioid-induced effects. The present study extends these findings to the effects of NMDA/glycine (glycine(B)) site antagonists. Wistar rats were rendered dependent on morphine by implantation of morphine pellets. Both of the glycine(B) site antagonists used, 7-chloro-4-hydroxy-3-(3-phenoxy)-phenyl-2(H)-quinolone (L-701,324; 2.5 and 5.0 mg/kg) and 5,7-dichlorokynurenic acid (5,7-DCKA; 25, 50, and 100 mg/kg), suppressed the expression of morphine withdrawal syndrome estimated as wet dog shakes. Furthermore, L-701,324 (2.5 and 5 mg/kg), given twice a day during the development of morphine dependence, attenuated the development of morphine dependence, and the results were comparable to those obtained after administration of noncompetitive NMDA receptor antagonist - MK801 (0.1 mg/kg). Our data suggest that glycine(B) site antagonists may attenuate wet dog shakes (withdrawal) and the development of dependence, both being induced by chronic morphine administration in rats.     

Brain reward deficits accompany naloxone-precipitated withdrawal from acute opioid dependence

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 a progressive shift in potency of naloxone to produce such acute withdrawal signs. The current study examined alterations in brain reward thresholds after acute and repeated treatment with morphine (5.6 mg/kg) using a discrete-trial current-intensity brain-stimulation reward procedure. Rats with stimulation electrodes aimed at the medial forebrain bundle at the level of the lateral hypothalamus were tested in twice daily sessions separated by 4 h. Separate groups of rats received treatment with morphine immediately after the first daily test session, and one of several doses of naloxone (0.10, 0.33, 1.0 mg/kg) 4 h later and immediately before the second session; these morphine and naloxone treatments were repeated for four consecutive days (Morphine-Repeat NAL). Additional groups examined the independent contribution of repeated morphine or repeated naloxone. One control group (Morphine-Vehicle) received morphine on all four treatment days, but vehicle before the second test session. A second group (Morphine-Single NAL) also received morphine on all four treatment days, but received 1.0 mg/kg only once after the final morphine pretreatment. A final control group received no morphine at all but received the 1.0-mg/kg dose of naloxone four times (Vehicle-Repeat NAL) before the second daily test session. Repeated naloxone alone (Vehicle-Repeat NAL) produced no changes in brain reward thresholds. Repeated morphine alone (Morphine-Vehicle) failed to alter reward thresholds measured 4 h postmorphine, but produced a slight increase in thresholds in the test sessions that occurred before morphine treatment on Days 3 and 4 (and hence 23.5 h after the previous day's morphine injection). This suggested the development of a modest spontaneous withdrawal-induced reward deficit measurable at 23.5 but not 4 h postmorphine. Naloxone dose-dependently increased brain reward thresholds 4 h after a single morphine pretreatment, with a further shift to the left in the naloxone dose-effect function resulting from repeated morphine and naloxone administration (Morphine-Repeat NAL). However, when the highest dose of naloxone was tested only after the final morphine pretreatment (Morphine-Single NAL), its potency was no different than when administered after the first morphine pretreatment. The results indicate that neuroadaptation within brain reward circuitry results in significant reward deficits after a single morphine pretreatment, and this deficit increases rapidly with repeated morphine and naloxone-induced withdrawal experience.     

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.     

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.     

Evidence that physical dependence on morphine is mediated by the ventral midbrain

Rats were given daily injections of increasing doses of morphine sulfate (40-100 mg/kg, s.c.), for 4 days. Twenty hours after the last injection of morphine, the animals received bilateral injections of naloxone (1-10 micrograms) into the substantia nigra, ventral tegmental area or sites 2 mm rostral, caudal or dorsal to the site in the nigra. Withdrawal signs were monitored for 20 min after the intracerebral injection. Naloxone administered into the nigra in morphine-dependent rats produced dose-dependent significant increases in wet dog shakes, irritability to touch, teeth chattering, diarrhea and locomotion, compared to morphine-dependent animals that received injections of saline into the nigra. The injection of naloxone (3 micrograms) into the ventral tegmental area of morphine-dependent animals, produced irritability to touch and diarrhea, compared to morphine-dependent controls that received saline in this region of the brain. Significant differences in withdrawal signs were observed between morphine-dependent animals, that received injections of naloxone (3 micrograms) into the nigra and those that received naloxone (3 micrograms) into the ventral tegmental area or rostral or caudal sites. No differences between the substantia nigra and the dorsal sites were observed. However, withdrawal symptoms were produced by injections of naloxone into the substantia nigra and ventral tegmental area, even when the guide cannulae were angled to avoid penetration of sites dorsal to these regions of the brain. Naloxone, injected into the ventral midbrain of non-dependent animals, produced no signs of withdrawal. These studies suggest that the ventral midbrain mediates physical dependence on morphine.     

Buprenorphine self-administration by rhesus monkey

Intravenous injections of buprenorphine, a partial opiate agonist and antagonist, maintained operant responding under second order schedule control (FR 3 VR 16:S) across a dose range of 0.005 to 0.10 mg/kg/inj. A drug naive monkey and four monkeys with a history of morphine self-administration all self-administered buprenorphine at all doses studied. Four monkeys showed dose-related increases in the total amount of buprenorphine (mg/kg) self-administered each day as the available dose increased from 0.01 to 0.10 mg/kg/inj. Injections per day remained equivalent to the number of injections at the lowest dose studied or increased significantly (p<0.05, 0.01), as the dose per injection increased in three monkeys. Even at high buprenorphine doses, there was no evidence of sedation. Monkeys consistently self-administered significantly more buprenorphine than saline in control studies (p<0.01). Buprenorphine's agonistic effects appear to persist for 24 to 48 hours. When saline and buprenorphine were available on alternate days, monkeys did not distinguish between them, but when 3 days of saline were alternated with 1 day of buprenorphine (0.03 mg/kg/inj), monkeys took significantly more buprenorphine than saline (p<0.02?0.001). Abrupt discontinuation of buprenorphine (0.10 mg/kg/inj) did not result in discernible withdrawal signs. The effects of buprenorphine on food intake were inconsistent, but there were no significant changes in body weight as a function of chronic buprenorphine self-administration or withdrawal. These data indicate that buprenorphine is a positive reinforcer in rhesus monkeys and maintains behavior leading to its administration on second order schedules over a wide dose range. Despite its opiate agonist properties, there was no evidence of physical dependence.