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

Oxytocin plays an important role at parturition due to its involvement in u terine contractions, foetal expulsion and the onset of maternal behaviour. The role of the related neurohypophysial hormone, vasopressin, is less clea r; however, there is some evidence that it is also involved in maternal beh aviour and its role in osmotic regulation is well established. The aim of t his study was to investigate the inhibitory effects of endogenous opioids o n 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 x Landrace primiparous sows (gilts). An indwelling jugular catheter was implanted under general anaesth esia at 12 days before the expected parturition day (EPD). From 5 days befo re the EPD 15 of the gilts were individually housed in a restrictive partur ition 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 en dogenous opioids during the expulsive phase. The inhibitory effects of opio ids on these hormones did not appear to have any adverse effects on the pro gress 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 'norma l' physiological response to parturition or whether it is stress-induced.