The effects of chronic environmental stress on parturition and on oxytocin and vasopressin secretion in the pig

Previous work has suggested that an acute behavioural confinement in mid-partum can inhibit oxytocin secretion and prolong delivery in the pig, an effect that is opioid mediated. The present experiment investigated the effect of longer-term (chronic) behavioural confinement, that has previously been shown to elevate total plasma cortisol, on speed of delivery and on plasma oxytocin and lysine vasopressin concentrations during the peri-parturient period in primiparous pigs (gilts). Five days before their expected parturition (farrowing) date, gilts with preplaced jugular catheters were either confined to farrowing crates that severely restricted maternal behaviour, or housed in pens that permitted free movement and maternal behaviour (e.g. nest building). Blood samples were taken continuously from 24 h before the birth of the first piglet (BFP) to 6 h post-BFP, and for oxytocin on Days 1, 2, and 7 following parturition (Days P1, P2, P7). Both oxytocin and vasopressin were strongly influenced by parturition (P<0.001). There was no overall effect of chronic crating on either hormone, but crated and penned gilts did show significant differences with respect to the pattern of both oxytocin and vasopressin concentrations over time (P<0.05 in both cases). Oxytocin and vasopressin first increased in crated and penned gilts from 3 h pre-BFP (P<0.05). Crated gilts subsequently showed greater increases in both oxytocin and vasopressin over the first hour of delivery than penned gilts (mean oxytocin (pmol 1⁻¹): 53.3±8.5 vs. 39.7±5.0 for crated vs. penned gilts; mean vasopressin (pmol l⁻¹):4.4±0.7 vs. 2.0±04 for crated vs. penned gilts; both P<0.05). For oxytocin, crated gilts then showed subsequent declining concentrations relative to penned gilts (P<0.05). For vasopressin, penned gilts reached similar concentrations as crated gilts in the third hour post-BFP before vasopressin concentrations in both groups declined. Crated gilts also gave birth to piglets faster in the early stages of delivery (e.g. mean interval between Piglets 2 and 3 (min): 9.6±2.5 vs. 25.6±8.54 for crated and penned gilts, respectively: P<0.02). We conclude that confinement of gilts to a farrowing crate for 5 days neither adversely affects the progress of delivery in the primiparous pig nor the secretion of posterior pituitary hormones involved in parturition.