THE EFFECT OF ENVIRONMENT ON PLASMA-CORTISOL AND BETA-ENDORPHIN IN THE PARTURIENT PIG AND THE INVOLVEMENT OF ENDOGENOUS OPIOIDS

Previous work has indicated that plasma cortisol increases during farr owing in the pig suggesting increasing physiological stress. The aim o f this study was to determine changes in plasma cortisol and beta-endo rphin over farrowing in the pig to obtain a more detailed profile of p ituitary and adrenal release at this time and also to investigate the involvement of endogenous opioids in the mediation of the HPA axis. In dwelling jugular catheters were implanted, under general anaesthesia, in 31 Large White x Landrace gilts approximately 15 days before the ex pected parturition day (EPD). Gilts were moved into either a farrowing crate, without straw (n = 15), or a straw-bedded pen (n = 16) 5 days before the EPD. Samples were taken during the pre-farrowing period and then during farrowing itself. At 7.5 min after the birth of the first piglet (BFP), gilts either received naloxone, an opioid antagonist, ( 1 mg kg(-1) body weight, i.v.) or a control dose of saline. Plasma bet a-endorphin increased following the BFP but remained fairly constant o ver the third and fourth hour of farrowing. Plasma cortisol continued to increase over the 4 h following the BFP. Changes seen in these horm ones were generally insensitive to the environment and there was littl e evidence of opioid mediation of the HPA axis at parturition. From th ese results it is suggested that certain aspect(s) of parturition itse lf stimulate the HPA axis. However it is unknown if the rise in plasma cortisol is a result of some stress-inducing factor of the parturitio n process or whether it reflects a metabolic function. The study also demonstrates the lack of any inhibitory mediation of the HPA axis by e ndogenous opioids at parturition. (C) 1998 Elsevier Science B.V. All r ights reserved.