An investigation into opioid systems in the pregnant rat

The role of opioid peptides during pregnancy and parturition has not been defined. They may function as natural analgesics during parturition. Pain thresholds were tested during pregnancy but, in contrast with a previous study, were found not to change. Plasma Beta-endorphin-like-immunoreactivity (B-ELI) was measured and towards the end of pregnancy a significant increase was recorded. Peptide levels returned to the pre-gestational concentration just prior to parturition. Pituitary B-ELI was also measured and was only significantly raised on the day before expected parturition. These changes add to the evidence that opioid peptides have a physiological function during pregnancy and parturition.