Related Experiment Video
Updated: Jun 20, 2026
Implantation of Electroencephalogram and Electrocardiogram Telemetry Devices in Neonatal Rabbit Kits
Published on: February 28, 2025
S-Methadone augments R-methadone induced respiratory depression in the neonatal guinea pig
Daniel A N Silverman1, Rosemary T Nettleton, Katherine B Spencer
1Department of Physiology & Pharmacology, L334, School of Medicine, Oregon Health & Science University, 3181 SW Sam Jackson Park Road, Portland, OR 97239-3098, USA.
Abstract:
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.
Insights
Racemic methadone
Area of Science:
- Neonatal pharmacology
- Respiratory physiology
- Pain management
Background:
- Methadone, used for pain relief, is a racemic mixture.
- Its therapeutic and adverse effects are primarily linked to the R-isomer acting on mu-opioid receptors.
- Neonatal respiratory responses to methadone isomers are not well understood.
Purpose of the Study:
- To investigate the individual and combined effects of R- and S-methadone isomers on neonatal respiratory function.
- To determine if S-methadone augments the respiratory depression caused by R-methadone in developing guinea pigs.
Main Methods:
- Longitudinal study involving 3-, 7-, and 14-day-old guinea pigs.
- Administration of saline, R-methadone, S-methadone, or racemic methadone.
- Respiratory parameters measured using plethysmography under normal air and hypercapnic (5% CO2) conditions.
Main Results:
- S-Methadone alone did not affect respiratory parameters.
- Racemic methadone induced greater respiratory depression and increased inspiratory time compared to R-methadone alone in the youngest animals.
- These effects suggest a synergistic interaction between methadone isomers during early development.
Conclusions:
- S-Methadone isomer potentiates the respiratory depressant effects of R-methadone in neonatal guinea pigs.
- This interaction is most pronounced in the youngest animals studied.
- Findings highlight the importance of isomer-specific effects in neonatal methadone exposure.
