NALOXONE PREVENTS INTERRUPTION OF PARTURITION AND INCREASES PLASMA OXYTOCIN FOLLOWING ENVIRONMENTAL DISTURBANCE IN PARTURIENT SOWS

Experients in rodents have suggested that environmental disturbance can disrupt parturition through an opioid-mediated inhibition of oxytocin secretion. To test this hypothesis in a large animal model, 14 primiparous female pigs were allowed to commence parturition in a strawed pen. Five of these gilts were allowed to continue parturition undisturbed in this pen, while the remainder were moved to a farrowing crate immediately after the birth of the first piglet. At this time, pigs were injected subcutaneously with either the opioid antagonist naloxone (n = 4; dose 1 mg/kg body weight) or saline (n = 5). Whereas the undisturbed pigs all gave birth to a second piglet within 53 min, in three of the five disturbed and saline-treated pigs no further births occurred for 2 h, at which time oxytocin was administered subcutaneously to restart parturition. By contrast, all of the naloxone-treated pigs gave birth spontaneously within 2 h, although mean interbirth intervals were still prolonged compared to undisturbed pigs. In a second experiment, nine primiparous female pigs with chronic catheters preplaced in the external jugular vein were similarly moved after the birth of their first piglet and either injected with naloxone (n = 5) or saline (n = 4). Again, parturition was interrupted in three out of four saline-treated animals for at least 2.5 h, but resumed promptly when exogenous oxytocin was administered. Plasma concentrations of oxytocin in these pigs were significantly lower than in naloxone-treated pigs, five out of six of which gave birth spontaneously to one or more piglets within 2.5 h. One naloxone-treated animal did not give birth spontaneously; in this exceptional animal oxytocin augmentation was ineffective, and the pig was presumed dystocic. Saline-treated gilts showed higher increments in cortisol than naloxone animals in the postinjection period. In the first hour, postinjection saline-treated animals also stood more often than naloxone animals, although this difference had disappeared by the second hour. The results of the present study suggest that environmental disturbance can disrupt parturition in the pig, that this disruption is accompanied by depression of circulating oxytocin, and that naloxone administration results in an augmentation of oxytocin secretion and a partial reversal of the inhibitory effects of disturbance upon the progress of parturition. In the case of the pig, future research must consider the implications of these findings for sows farrowing in closely confining farrowing environments.