Enhancement of tolerance development to morphine in rats prenatally exposed to morphine,methadone, and buprenorphine

Background  

Abuse of addictive substances is a serious problem that has a significant impact on areas such as health, the economy, and public safety. Heroin use among young women of reproductive age has drawn much attention around the world. However, there is a lack of information on effects of prenatal exposure to opioids on their offspring. In this study, an animal model was established to study effects of prenatal exposure to opioids on offspring.     

Self-administration of heroin, acetylmethadol, morphine, and methadone in rhesus monkeys.

Heroin, α-$\text{l}$-acetylmethadol (LAAM), morphine, and methadone each maintained self-administration in rhesus monkeys. The order of relative potency was heroin ≥ LAAM > morphine ≥ methadone. Total daily drug intake increased as dose per injection increased; maximum daily intake was inversely related to relative potency. At high doses, self-injection of methadone and LAAM caused stupor and/or respiratory failure in some monkeys. These toxic effects were partly or completely reversible by naloxone.     

CSF distribution of morphine, methadone and sucrose after intrathecal injection

The lumbar to cisternal CSF distribution of morphine and methadone were compared to C-14 sucrose, a standard marker of CSF bulk flow, after lumbar subarachnoid injections in a sheep preparation. Morphine appeared and peaked simultaneously with C-14 sucrose in cisternal CSF at 90 to 190 minutes. The mean peak cisternal CSF morphine concentrations were sustained for 30-40 minutes, and averaged 148 ng/ml, representing 0.3% of the administered dose. Methadone was not detectable in cisternal CSF up to 240-300 minutes after lumbar subarachnoid administration. The C-14 sucrose/morphine ratio was increased an average of 6.7 times in cisternal CSF as compared to the ratio of the two compounds injected into the lumbar subarachnoid space. These studies demonstrate that morphine, a hydrophilic opioid, given intrathecally moves rostrally and appears in cisternal CSF by bulk flow. Furthermore the rostral redistribution of morphine is associated with the clearance of morphine from CSF. Methadone, a lipophilic opioid, appears to be completely cleared from CSF before it reaches the cisterna magna. These pharmacokinetic studies support a contribution of supraspinal sites to the analgesic and adverse effects produced by morphine given by spinal routes of administration. In contrast methadone appears to exert its effects predominantly at spinal sites.     

Enhanced sensitivity to methadone in adult rats perinatally exposed to methadone

Adult rats, maternally exposed to methadone during gestation and/or lactation, were evaluated for thermoregulatory and nociceptive responsiveness following a challenge with 5 mg/kg (i.p.) methadone. Prior to drug administration, female rats in the gestation and gestation-lactation groups and all male rats perinatally exposed to methadone were subnormal in body temperature. One hour after acute methadone injection, male control rats were hypothermic. All groups of methadone-treated offspring exhibited a marked lowering in body temperature with respect to their pre-injection levels, as well as in regard to values of methadone-administered control animals. Prior to drug administration, male rats of the gestation-lactation group and female rats of the gestation and lactation groups had elevated nociceptive thresholds. Except for male rats in the lactation group, animals treated with methadone perinatally had longer latencies in response to the hot-plate relative to their pre-injection values, as well as to levels of methadone-injected controls. Three days after acute methadone administration, some groups of rats subjected to this drug during gestation and/or lactation were found to be hypothermic and hypalgesic in respect to their pre-injection values, and also relative to control rats. These results suggest that exposure to methadone early in life can have a profound influence on drug response in adulthood.     

Reversal of morphine, methadone and heroin induced effects in mice by naloxone methiodide

Opioid overdose, which is commonly associated with opioid induced respiratory depression, is a problem with both therapeutic and illicit opioid use. While the central mechanisms involved in the effects of opioids are well described, it has also been suggested that a peripheral component may contribute to the effects observed. This study aimed to further characterise the effects of the peripherally acting naloxone methiodide on the respiratory, analgesic and withdrawal effects produced by various opioid agonists. A comparison of the respiratory and analgesic effects of morphine, methadone and heroin in male Swiss-Albino mice was conducted and respiratory depressive ED(80) doses of each opioid determined. These doses (morphine 9 mg/kg i.p., methadone 7 mg/kg i.p., and heroin 17 mg/kg i.p.) were then used to show that both naloxone (3 mg/kg i.p.) and naloxone methiodide (30-100 mg/kg i.p.) could reverse the respiratory and analgesic effects of these opioid agonists, but only naloxone precipitated withdrawal. Further investigation in female C57BL/6J mice using barometric plethysmography found that both opioid antagonists could reverse methadone induced decreases in respiratory rate and increases in tidal volume. Its effects do not appear to be strain or sex dependent. It was concluded that naloxone methiodide can reverse the respiratory and analgesic actions of a variety of opioid agonists, without inducing opioid withdrawal.     

Reduced dosing and liability in methadone maintenance treatment by targeting oestrogen signal for morphine addiction

Methadone maintenance treatment (MMT) is the major tapering therapy for morphine addictive patients. There have gender differences reported in response to MMT. This study discovered that the estrogen‐response element single nucleotide polymorphism (ERE‐SNP; rs16974799, C/T) of cytochrome 2B6 gene (cyp2b6; methadone catabolic enzyme) responded differently to MMT dosing. Oestradiol was associated with high MMT dosing, high enantiomer (R‐ or S‐) of 2‐ethylidene‐1,5‐dimethyl‐3,3‐dipheny‐pyrrolidine (EDDP; methadone metabolite) to methadone ratio and increased drug‐seeking behaviour, implicating oestradiol‐CYP‐EDDP/methadone axis decreasing MMT efficacy. In mouse model, oestrogen mitigates methadone antinociceptive response, facilitates methadone catabolism and up‐regulates methadone‐associated metabolizing enzymes. Oestrogen also ablates chronic methadone administration‐induced rewarding response. Mechanism dissection revealed the CC genotype of CYP2B6‐ERE‐SNP exerts higher ERE sequence alignment score, higher estrogenic response as compared to TT genotype. At last, preclinical study via targeting estrogen signal that tamoxifen (TMX; selective estrogen receptor modulator, SERM) could facilitate the tolerance phase rewarding response of methadone. Strikingly, TMX also reduces tapering/abstinence phases methadone liability in mice. In conclusion, this study demonstrates altering methadone metabolism through targeting estrogen signals might be able to free morphine addictive patients from the addiction of opioid replacement therapy. Therefore, the add‐on therapy clinical trial introducing SERM in MMT regimen is suggested.     

Relationship of venous PO2 to muscle PO2 during hypoxemia

Anesthetized mechanically ventilated rabbits were subjected to progressive hypoxemia (n = 7) to determine the relationship of venous PO2 (PvO2) to skeletal muscle PO2 (PtiO2). Measures of arterial PO2 (PaO2), right atrial PO2 [(PvO2)RA], and hindlimb PO2 [(PvO2)limb], were obtained from the carotid artery, right atrium, and inferior vena cava, just above the level of the iliac bifurcation. Biceps femoris muscle PtiO2 was measured with a surface O2 microelectrode having eight measuring points. PaO2 was decreased from 90.3 +/- 5.4 to 26.8 +/- 0.8 Torr in five consecutive steps, followed by reoxygenation to 105.6 +/- 10.5 (SE) Torr. Measurements were obtained after each decrement in PaO2. A total of 128 measures of PtiO2 were obtained per experimental stage. The mean and distribution of the muscle PtiO2 histogram were determined. Measurements were compared with analysis of variance and the Newman-Keuls post hoc method. (PvO2)limb had similar values as the average muscle PtiO2 (PtiO2) for PaO2 values greater than 52.1 +/- 4.3 Torr, where (PvO2)limb became greater than PtiO2 (P less than 0.05). The lowest measures of (PvO2)limb and PtiO2 were 15.9 +/- 0.7 and 4.0 +/- 0.1 Torr, respectively (P less than 0.01). The PtiO2 histograms showed no evidence of increased microvascular heterogeneity with hypoxemia. We conclude that in hypoxemia PvO2 is greater than muscle PtiO2. This difference may be related to the establishment of significant physicochemical O2 gradients from erythrocyte to tissue cell.     

Enhanced development of dispositional tolerance to methadone by desipramine given together with methadone

Rats given 2-day oral administration of methadone (15 mg/kg, twice on day 1 and once on day 2) by gastric tube developed dispositional tolerance to methadone analgesia as demonstrated by a decrease in analgesic response and by an increase in methadone metabolism. The increased metabolism of methadone was evidenced by a decrease in brain concentration of 14C-methadone and increases in the percentages of total 14C in liver or urine as 14C-water-soluble metabolites (14C-WSM) after the rats were challenged with a test dose of 14C-methadone. Two-day pretreatment with a combination of desipramine (DMI) (10 mg/kg, ip) and methadone (15 mg/kg, po) enhanced the development of dispositional tolerance to methadone analgesia which was evidenced by a greater decrease in the brain concentration of methadone and a greater increase in methadone metabolism as compared to those changes in rats pretreated with only methadone. Repeated treatment with DMI alone neither decreased the analgesic effect of methadone nor stimulated methadone metabolism. It is suggested that DMI given together with methadone promoted the induction of methadone metabolism in the liver by prolonging the enzyme-stimulating state of methadone, thus enhancing the development of dispositional tolerance to methadone.     

Amino acid auxotrophy increases sensitivity of Saccharomyces cerevisiae to a quaternary ammonium salt IM.

Relationships between amino acid auxotrophy and N-dodecyloxy-carboxy-methyl-N-N-N-trimethyl ammonium chloride (IM) sensitivity have been investigated in isogenic yeast strains Saccharomyces cerevisiae and their meiotic segregants. It has been found, that auxotrophy increases the level of sensitivity to this salt markedly. A gene conferring resistance to that drug cancels the auxotrophy-dependent sensitivity.     

Reduction of the sevoflurane minimum alveolar concentration induced by methadone, tramadol, butorphanol and morphine in rats

This study aimed to estimate the reduction in the minimum alveolar concentration (MAC) of sevoflurane induced by low and high doses of methadone (5 and 10 mg/kg), tramadol (25 and 50 mg/kg), butorphanol (5 and 10 mg/kg) or morphine (5 and 10 mg/kg) in the rat. A control group received normal saline. Sixty-three adult male Sprague-Dawley rats were anaesthetized with sevoflurane (n = 7 per group). Sevoflurane MAC was then determined before and after intraperitoneal administration of the opioids or saline. The duration of the sevoflurane MAC reduction and basic cardiovascular and respiratory measurements were also recorded. The baseline MAC was 2.5 (0.3) vol%. Methadone, tramadol and morphine reduced the sevoflurane MAC (low dose: 31 ± 10, 38 ± 15 and 30 ± 13% respectively; high dose: 100 ± 0, 83 ± 17 and 77 ± 25%, respectively) in a dose-dependent manner. The low and high doses of butorphanol reduced the sevoflurane MAC to a similar extent (33 ± 7 and 31 ± 4%, low and high doses, respectively). Two rats developed apnoea following administration of high-dose butorphanol and methadone. These anaesthetic-sparing effects are clinically relevant and may reduce the adverse effects associated with higher doses of inhalational anaesthetics.