Methadone-induced respiratory depression in the neonatal guinea pig

Robustness of arterial blood gas analysis for assessment of respiratory safety pharmacology in rats

Whole body plethysmography using unrestrained animals is a common technique for assessing the respiratory risk of new drugs in safety pharmacology studies in rats. However, wide variations in experimental technique make cross laboratory comparison of data difficult and raise concerns that non-appropriate conditions may mask the deleterious effects of test compounds - in particular with suspected respiratory depressants. Therefore, the objective of this study was to evaluate the robustness of arterial blood gas analysis as an alternative to plethysmography in rats. We sought to do this by assessing the effect of different vehicles and times post-surgical catheterization on blood gas measurements, in addition to determining sensitivity to multiple opioids. Furthermore, we determined intra-lab variability from multiple datasets utilizing morphine and generated within a single lab and lastly, inter-lab variability was measured by comparing datasets generated in two separate labs. Overall, our data show that arterial blood gas analysis is a measure that is both flexible in terms of experimental conditions and highly sensitive to respiratory depressants, two key limitations when using plethysmography. As such, our data strongly advocate the adoption of arterial blood gas analysis as an investigative approach to reliably examine the respiratory depressant effects of opioids.

OPRM1 and CYP2B6 gene variants as risk factors in methadone-related deaths

Methadone is a medication valued for its effectiveness in the treatment of heroin addiction; however, many fatal poisonings associated with its use have been reported over the years. We have examined the association between CYP2B6 and micro-opioid receptor (OPRM1) gene variations and apparent susceptibility to methadone poisoning. Genomic DNA was extracted from postmortem whole blood of 40 individuals whose deaths were attributed to methadone poisoning. The presence of CYP2B6*4,*9, and *6 alleles and the OPRM1 A118G variant was determined by SNP genotyping. CYP2B6 *4, *9, and *6 alleles were found to be associated with higher postmortem methadone concentrations in blood (P < or = 0.05). OPRM1 A118G was also associated with higher postmortem methadone concentrations in blood but not to a level of statistical significance (P = 0.39). In these methadone-related deaths, OPRM1 118GA was associated with higher postmortem benzodiazepine concentrations (P = 0.04), a finding not associated with morphine-related deaths. The risk of a methadone-related fatality during treatment may be evaluated in part by screening for CYP2B6*6 and A118G.

Substance use and women's health

Scientific findings show that substance abuse in women yields a higher risk of a variety of health problems than substance abuse in men. Research suggests that women experience addiction telescoping when they abuse alcohol, tobacco, specific stimulants, and possibly opioids. Medical side effects also develop more rapidly in women than men when they abuse many substances. Cancer and cardiac complications, specifically, pose a significant threat for women who abuse almost all types of substances. However, the physical consequences are not the only ones women suffer when they engage in substance abuse. Research on substance abuse in women ties opioids to mood and anxiety disorders, heroin to neurological deficiencies, cocaine to immune system suppression, and alcohol to intimate partner abuse. Additionally, female substance abusers, on average, have a lower level of education and lower rates of employment. In light of these gender-specific concerns, physicians should give particular consideration to detecting substance abuse in women.

Chronic in utero buprenorphine exposure causes prolonged respiratory effects in the guinea pig neonate

Our laboratory studies the effects of in utero opioid exposure on the neonate. In this work we test the effects of chronic in utero exposure to buprenorphine on the neonate. Buprenorphine is a promising candidate for treatment of opioid addiction during pregnancy and it has been suggested to decrease the neonatal abstinence syndrome in human infants. In our guinea pig model, we focused not only on the respiratory effects of in utero exposure on the neonate, but also studied withdrawal signs in the neonate, a major concern of all opioid treatment during pregnancy. Pregnant guinea pigs were treated with daily subcutaneous injections of 0.1mg/kg buprenorphine during the second half of gestation. We measured weight, locomotor activity and respiratory function in pups of ages 3 to 14 days. Respiratory response was recorded using a two-chamber plethysmograph, while pups were breathing either room air or 5% CO(2). Our results show that chronic in utero exposure to buprenorphine induces respiratory effects up to day 14 after birth, while earlier studies have shown that effects of either in utero methadone or morphine only persist in the first week after birth in the guinea pig model. These data provide important information for clinical trials of buprenorphine treatment suggesting that duration and severity of respiratory effects of in utero buprenorphine exposure should be monitored.

S-Methadone augments R-methadone induced respiratory depression in the neonatal guinea pig

Methadone is administered as a racemic mixture, although its analgesic and respiratory effects are attributed to R-isomer activity at the mu opioid receptor (MOP). Recently, we observed a four-fold increase in inspiratory time in 3-day-old guinea pigs following an injection of racemic methadone. We hypothesized that this effect was due to augmentation of R-methadone induced respiratory depression by the S-methadone isomer. In the current longitudinal study, we injected 3-, 7-, and 14-day-old neonatal guinea pigs with saline, R-methadone, S-methadone, or R- plus S-methadone in order to characterize the roles of the individual isomers, as well as the synergistic effects of co-administration. Using plethysmography, we measured respiratory parameters while breathing room air and during a 5% CO(2) challenge. S-Methadone alone had no respiratory effects. However, the R- plus S-methadone group showed greater respiratory depression and increased inspiratory time than the R-methadone group in the youngest animals, suggesting that the respiratory effects of R-methadone are augmented by S-methadone in early development.

Endogenous opiates and behavior: 2007

This paper is the thirtieth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2007 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Respiratory effects of chronic in utero methadone or morphine exposure in the neonatal guinea pig

This study uses a neonatal guinea pig model to compare the effects of in utero methadone or morphine exposure upon breathing control. We hypothesize that in utero methadone exposure will result in similar respiratory disturbances to those seen in morphine exposed neonates, but that the onset will be slower and the duration longer, due to methadone's longer elimination half-life. Pregnant Dunkin-Hartley guinea pigs received once-daily injections of methadone, morphine, or vehicle (saline) during the last half of gestation and pups were studied 3, 7, or 14 days after birth. In utero methadone or morphine exposure resulted in decreased birth weight compared to vehicle, and pups experienced a withdrawal syndrome which included increased locomotor activity and respiratory disturbances but no change in rectal temperature. Both opioid exposures increased inspiratory minute ventilation during CO(2) challenge at 3 days after birth, but only in morphine exposed pups was this withdrawal effect still present on day 7. Surprisingly, only morphine exposure increased inspiratory minute ventilation during room air breathing. We conclude that in utero methadone exposure is not equivalent to in utero morphine exposure. With respect to neonatal respiratory control, methadone-induced changes in respiration are only apparent during hypercapnia.