Changes in pain threshold during pregnancy and parturition in the sow and the involvement of opioids

Abstract

Endogenous opioids are known to inhibit oxytocin release from the neurohypopysis (Bicknell et al, 1982). This mechanism has been suggested to be involved in the timing of parturition and delivery of young (Leng et al, 1985). Environmental disturbance in the rat (Leng et al, 1988) and pig (Lawrence et al, 1992) during parturition resulted in a naloxone reversible increase in birth interval, and an increase in plasma oxytocin concentration after naloxone treatment suggesting stress-induced opioid inhibition of oxytocin. Pain, which is associated with parturition, results in increased opioid activity and may be involved in the inhibition of oxytocin. Pregnancy-induced analgesia has been shown in the human (Whipple et al, 1990) and the rat (Gintzler, 1980), with the latter suggesting involvement of an opioid mechanism. This study aimed to determine whether an increase in pain threshold occurred over pregnancy and parturition in the pig and whether this was opioid-mediated.