Unequal opiate cross-tolerance to morphine in the locomotor-activation model in the mouse

Fentanyl depression of respiration: Comparison with heroin and morphine

BACKGROUND AND PURPOSE

Fentanyl overdose deaths have reached "epidemic" levels in North America. Death in opioid overdose invariably results from respiratory depression. In the present work, we have characterized how fentanyl depresses respiration, and by comparing fentanyl with heroin and morphine, the active breakdown product of heroin, we have sought to determine the factors, in addition to high potency, that contribute to the lethality of fentanyl.

EXPERIMENTAL APPROACH

Respiration (rate and tidal volume) was measured in awake, freely moving mice by whole body plethysmography.

KEY RESULTS

Intravenously administered fentanyl produced more rapid depression of respiration than equipotent doses of heroin or morphine. Fentanyl depressed both respiratory rate and tidal volume. Fentanyl did not depress respiration in μ-opioid receptor knockout mice. Naloxone, the opioid antagonist widely used to treat opioid overdose, reversed the depression of respiration by morphine more readily than that by fentanyl, whereas diprenorphine, a more lipophilic antagonist, was equipotent in reversing fentanyl and morphine depression of respiration. Prolonged treatment with morphine induced tolerance to respiratory depression, but the degree of cross tolerance to fentanyl was less than the tolerance to morphine itself.

CONCLUSION AND IMPLICATIONS

We propose that several factors (potency, rate of onset, lowered sensitivity to naloxone, and lowered cross tolerance to heroin) combine to make fentanyl more likely to cause opioid overdose deaths than other commonly abused opioids. Lipophilic antagonists such as diprenorphine may be better antidotes than naloxone to treat fentanyl overdose.

Pharmacokinetic-pharmacodynamic modeling of mood and withdrawal symptoms in relation to plasma concentrations of methadone in patients undergoing methadone maintenance treatment

The aims of the present study were to characterize the relationship between plasma racemic methadone and its enantiomers' concentrations with respect to their pharmacodynamic effects and to investigate the influence of potential covariates on the pharmacodynamic parameters in patients on methadone maintenance treatment (MMT). Eighty-eight regular subjects at the Sheffield Care Trust Substance Misuse Services were studied. Samples of blood and urine were collected before the daily dose of methadone. Blood samples were taken up to 5 hours after dose. Total plasma concentrations of (RS)-methadone and total and unbound plasma concentrations of both enantiomers were measured by liquid chromatography-mass spectrometry. The Total Mood Disturbance Score (TMDS), the Objective Opioid Withdrawal Scale (OOWS), and the Subjective Opioid Withdrawal Scale (SOWS) were used as measures of mood and withdrawal. Population pharmacokinetic/pharmacodynamic analysis and subsequent multiple regression analysis were used to determine the factors influencing the pharmacodynamic effects of methadone. Significant decreases (P ≤ 0.04) were observed in the scores for the TMDS, SOWS, and OOWS for 5 hours after methadone dosage. The TMDS had returned to baseline by 10 hours after dose (P = 0.98), at which time the SOWS remained significantly below baseline (P = 0.001). Multiple regression analysis revealed that 33% of the overall variation in unbound (R)-methadone EC50 was explained by 3 variables, namely CYP3A activity (9%), age (16%), and sex (8%). Age also accounted for 8% and 9% of the variation in total (rac)- and (R)-methadone EC50. The present study has confirmed that the duration of mood change in the present study was shorter than the effect of methadone in stabilizing withdrawal symptoms. Thus, it is likely that a once-daily dose of methadone, albeit effective for preventing withdrawal, may not be sufficient to improve mood in some patients. Finally, it was established that CYP3A activity, years of dependent use, sex, and age are major determinants of methadone EC50 with respect to TMDS.

Lack of crosstolerance between morphine and morphine-6-glucuronide as revealed by locomotor activity

Morphine-6beta-glucuronide is a major metabolite of morphine. We wanted to examine whether the effects related to opiate CNS stimulation could be mediated by different receptors for morphine and M6G by studying the development of crosstolerance between these two drugs. The effect studied was locomotor activity in C57BL/6JBom mice. We observed a dose-dependent development of tolerance to daily injections of morphine, with 20 micromol/kg giving the most rapid development of tolerance, apparent already on the second day of treatment. This was also observed for the same dose of M6G. Crosstolerance to M6G was measured both after 1 day pretreatment and 7 days pretreatment with morphine 20 micromol/kg, while the crosstolerance to morphine was tested only after 1 day pretreatment with M6G (20 micromol/kg). Lack of crosstolerance towards M6G after 1 day of morphine pretreatment was observed, whereas crosstolerance to M6G was observed after 7 days of exposure to morphine pretreatment. Crosstolerance after M6G pretreatment to morphine was observed. It was concluded that the main part of the effect caused by M6G was mediated through a specific M6G receptor.

Sensitivity of patients with painful temporomandibular disorders to experimentally evoked pain: evidence for altered temporal summation of pain

Temporomandibular disorders (TMD) represent a group of chronic painful conditions involving the muscles of mastication and the temporomandibular joint. Several studies have reported that TMD is associated with enhanced sensitivity to experimental pain. Twenty-three TMD subjects and 24 pain-free matched control subjects participated in a set of studies which were designed to evaluate whether the temporal integrative aspects of thermal pain perception are altered in TMD patients compared with control subjects. Specifically, we have examined in both TMD patients and in age- and gender-matched control subjects: (1) the time-course and magnitude of perceived pain evoked by the application of sustained 7-s noxious thermal stimuli (45-48 degrees C) to the face and forearm, (2) the central summation of C-fiber-mediated pain produced by applying brief trains of noxious heat pulses to the skin overlying the ventral aspect of the right palm and (3) the ability to discriminate small increments in noxious heat applied to facial and volar forearm skin. Data collected from these studies indicate that TMD patients show enhanced temporal integration of thermal pain compared with control subjects. TMD patients show greater thermal C-fiber-mediated temporal summation than pain-free subjects and they report a greater magnitude of sustained noxious heat pulses applied to either the face or the forearm than control subjects. In contrast to these findings, TMD and pain-free subjects are equally able to discriminate and detect small increments of heat applied to noxious adapting temperatures. These findings suggest that the augmented temporal integration of noxious stimuli may result from alterations in central nervous system processes which contribute to the enhanced pain sensitivity observed in TMD patients.

Tolerance and cross tolerance to the accuracy- and rate-decreasing effects of mu opioids in rats responding under a fixed-consecutive-number schedule

The purpose of the present investigation was to examine the development of tolerance to the effects of morphine and other mu opioids in rats responding under a fixed-consecutive-number (FCN) schedule of food presentation. To this end, five rats were trained under an FCN schedule and subsequently tested with a variety of mu opioids both before and during chronic exposure to a 0.4 mg/ml morphine drinking solution. Morphine, fentanyl, buprenorphine, butorphanol and nalbuphine produced dose-dependent decreases in both accuracy and response rate when tested prior to the chronic regimen. In most instances, doses of these drugs that decreased accuracy also decreased response rate. During chronic treatment, tolerance developed to the effects of morphine and cross-tolerance was conferred to the effects of fentanyl, buprenorphine and butorphanol. A greater degree of tolerance was conferred to the effects of butorphanol than to the other opioids examined, and the degree of tolerance conferred to butorphanol's rate-decreasing effects was greater than the degree of tolerance conferred to its accuracy-decreasing effects. Doses of naloxone that had no effect prior to morphine treatment produced large decreases in accuracy and response rate when tested during the chronic regimen. In contrast to the other opioids examined, the potency of nalbuphine was not altered by chronic morphine administration. These data emphasize the importance of both pharmacological and procedural variables in the development of tolerance and cross tolerance to the behavioral effects of opioids.

Tolerance and withdrawal to chronic morphine treatment in the week-old rat pup

Chronic treatment of adult animals with morphine results in tolerance but there are fewer reports on the effects of chronic opiates during ontogeny. The present experiments assessed the development of morphine-induced tolerance and withdrawal in infant rats. Pups were injected with morphine twice daily from ages 1-7 days and then tested on day 7 for morphine-induced analgesia in a hot-water immersion test, and separation-induced ultrasonic vocalizations in response to isolation from the dam and littermates at 7 and 10 days of age. Tolerance occurred to the analgesic effects of morphine but not to its suppression of ultrasonic vocalizations. Separation-induced vocalizations were greatly increased in chronic morphine-treated pups following naltrexone-precipitated withdrawal at 7 days of age. The increase in ultrasonic vocalizations following naltrexone treatment in morphine exposed pups may be a developmentally unique sign of opiate withdrawal.

Evaluation of receptor mechanism mediating fentanyl analgesia and toxicity

In the present study the antagonism of fentanyl pharmacodynamics was studied in the mouse and the receptor populations mediating the analgesic and lethal effects of fentanyl were examined. Both 1 and 8 days following implantation (s.c.) of a 15 mg naltrexone pellet there was a significant shift to the right of the fentanyl dose-response curves for analgesia and lethality. The analgesia dose-response curves were shifted significantly more (80- to 264-fold) than the lethality curves (13- to 16-fold) in the presence of naltrexone. In addition, acute naloxone (0.1 mg/kg s.c.), antagonized fentanyl analgesia more than lethality. Consequently, the relative safety ratio of fentanyl (LD50/ED50) was decreased in the presence of opioid antagonists. Pretreatment with naloxonazine (35 mg/kg s.c.) 24 h prior to testing effectively inhibited fentanyl-induced analgesia, but not fentanyl-induced lethality. However, pretreatment with beta-funaltrexamine (beta-FNA) (20 mg/kg, s.c.) 24 h prior to testing inhibited both fentanyl-induced analgesia and lethality. Implantation (s.c.) of a 75 mg morphine pellet for 72 h resulted in cross-tolerance to both fentanyl analgesia and lethality. However, the degree of the cross-tolerance was 1.8-fold for analgesia and 4.5-fold for lethality. Displacement studies of [3H]naltrexone by fentanyl in mouse brain homogenate indicated two populations of binding sites. Taken together, the pharmacodynamic studies and the binding studies suggest that fentanyl exerts its analgesic and lethal effects through different receptor populations.

Is intracellular sodium involved in the mechanism of tolerance to opioid drugs?

Recent studies indicate that one of the more likely mechanisms of opioid tolerance could involve a decrease in the efficiency with which agonists can induce coupling of their specific binding sites in neuronal membranes to the activation (or deactivation) of an effector system. Reports of sodium-induced decreases in opioid receptor agonist binding and in the size of ligand/receptor complexes, as well as modulation of opioid activity by manipulation of sodium in vivo, indicate that sodium might play a physiological role in modulating opioid receptor function. Reports of morphine-induced systemic sodium retention in animals, as well as morphine-induced increases in brain intracellular sodium and decreases in brain Na+, K(+)-ATPase activity, indicate that the development of tolerance may be accompanied by changes in the disposition of sodium. The direction of these sodium- and morphine-induced changes is consistent with the hypothesis that an increase in intracellular sodium could participate in the mechanism(s) of opioid tolerance development.

Tolerance to the respiratory actions of opiates: withdrawal tolerance and asymmetrical cross-tolerance

Cross-tolerance to the respiratory depression induced by i.c.v. [D-Ala2,D-Leu5]enkephalin (DADLE) and by s.c., i.c.v. and i.v. morphine was studied in anesthetized rats that had been rendered tolerant to s.c. sufentanil (4 micrograms/h per 7 days). Tolerance induced by i.c.v. sufentanil was also compared during withdrawal and its absence. Tolerance was evaluated by estimation of the changes in both the maximum attained reduction of respiratory frequency (Emax) and the area under the time-course curve for the effects (AUC). In contrast to the failure to develop tolerance, as estimated with the Emax, after i.c.v. sufentanil in the absence of abstinence (tolerance index: 1.1), rats displayed significant degrees of cross-tolerance to i.c.v. morphine (3.6), i.c.v. DADLE (4.3) and s.c. morphine (6.6). No tolerance, however, was obtained to i.v. morphine. Tolerance to i.c.v. sufentanil in non-abstinent rats was not present when the Emax was considered but was when AUC was considered. During withdrawal animals showed tolerance as estimated with both the Emax and AUC. These results complement previous findings and show that the appearance of dissimilar degrees of cross-tolerance to different agonists depends not only on their intrinsic activities but also on their pharmacokinetic properties. These observations, along with the critical influence of withdrawal upon the manifestation of tolerance, support the need to distinguish between the development of the tolerance state and the degree of tolerance that can be expressed under specific conditions.

Differences in efficacies between morphine and methadone demonstrated in the guinea pig ileum: a possible explanation for previous observations on incomplete opioid cross-tolerance

We have compared the effectiveness of morphine and methadone as agonists in untreated guinea-pig longitudinal muscle preparations and after their treatment with the specific opioid receptor alkylating agent beta-chloronaltrexamine. In untreated ilea, the naloxone pA2 was 8.5 to both morphine and methadone, and their dose-response curves were parallel. After alkylation, the dose-response curve for morphine was shifted to the right with a decreased maximal effect. It was found that about 70% of the receptors were inactivated by the treatment, corresponding to a morphine receptor occupancy at IC50 of about 24%. The maximal effect of methadone was not decreased by the same beta-chloronaltrexamine treatment, indicating a much higher efficacy or receptor reserve for the drug. The possibility that differences in efficacies could account for previously reported heterogeneities in actions of opioids, such as assymmetries in cross-tolerance towards morphine and methadone in experimental animal and man, is discussed.

Endogenous opiates: 1986

This is the ninth installment of our annual review of research involving the endogenous opiate peptides. It is restricted to the non-analgesic and behavioral studies of the opiate peptides published in 1986. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic processes; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; activity; sex, pregnancy, and development; and some other behaviors.