The effect of opioid antagonism and environmental restriction on plasma oxytocin and vasopressin concentrations in parturient gilts

Abstract

Oxytocin plays an important role at parturition due to its involvement in uterine contractions, foetal expulsion and the onset of maternal behaviour. The role of the related neurohypophysial hormone, vasopressin, is less clear; however, there is some evidence that it is also involved in maternal behaviour and its role in osmotic regulation is well established. The aim of this study was to investigate the inhibitory effects of endogenous opioids on these hormones during the expulsive phase of parturition in the pig, and to examine how opioid restraint interacts with environmental restriction.The subjects of this study were 31 Large White Landrace primiparous sows (gilts). An indwelling jugular catheter was implanted under general anaesthesia at 12 days before the expected parturition day (EPD). From 5 days before the EPD 15 of the gilts were individually housed in a restrictive parturition crate without straw and 16 were individually housed in a straw-bedded pen. Blood samples were taken with increasing frequency towards and during parturition through a catheter extension to reduce disturbance. At 7·5 min after the birth of the first piglet half of the gilts in each environment received a dose of the opioid receptor antagonist naloxone (1 mg/kg, i.v.) with the remaining gilts receiving saline as a control.Overall, there was no effect of environment on either circulating oxytocin or vasopressin. However, both oxytocin and vasopressin were inhibited by endogenous opioids during the expulsive phase. The inhibitory effects of opioids on these hormones did not appear to have any adverse effects on the progress of parturition as judged by cumulative piglet birth intervals.The regulation of the opioid inhibition of oxytocin and vasopressin during parturition is discussed in relation to other neurotransmitters and whether opioid inhibition of these neurohypophysial hormones is part of the 'normal' physiological response to parturition or whether it is stress-induced.

6. Applied animal behaviour science and animal welfare: seeking the best balance between our science and its application

“…However when we analysed blood samples from sows farrowing in different environments, we found no effect of environment on circulating oxytocin and vasopressin (Lawrence et al, 1995). We did however find that the opioid antagonist naloxone increased circulating levels of both oxytocin and vasopressin, suggesting that a degree of opioid inhibition of these reproductive hormones is part of a normal parturition (Jarvis et al, 2000) (Figure 6.3). However our funding at the time, which was more directed at applied questions, did not allow us to investigate the more basic question of the function of this opioid-inhibition (e.g.…”

“…BFP = birth of the first piglet. There was a significant increase in oxytocin in the early stages of parturition in sows receiving naloxone (redrawn from Jarvis et al, 2000).…”