OPIOID-MEDIATED CHANGES IN NOCICEPTIVE THRESHOLD DURING PREGNANCY ANDPARTURITION IN THE SOW

This study aimed to investigate if pregnancy-induced hypoalgesia occur s in the sow, and to examine the role of endogenous opioids which are known to be released in response to nociception. Sixteen Large White x Landrace multiparous sows were rested in straw bedded pens (2.5 x 2.5 m) during weeks 4, 8 and 12 of pregnancy and over the farrowing perio d. Testing involved thermal stimulation of eight areas on the rear-qua rters of the sows with a CO2 infra-red laser until a physical response was seen (tail flick, leg move or muscle twitch) or for a maximum of 16 s, Over the farrowing period testing was more frequent, and at 3.75 h after the birth of the first piglet, half the sows received an inje ction (i.m.) of an opioid antagonist naloxone (N) (1 mg kg(-1) body we ight) with the remainder receiving a control dose of saline (S). Respo nses were recorded 15 and 30 min post-injection, There was no signific ant difference between response times over weeks 4, 8 and 12 of pregna ncy (P = 0.152), however a significant rise was seen from week 12 to 5 days before parturition (P = 0.002). Response times continued to rise until the birth of the first piglet by which time the majority of sow s had stopped responding within 16 s (P < 0.001). Response times fell over days 1, 2 and 7 post-partum. After administration of naloxone res ponse times fell compared to control animals at 15 min (P < 0.001) and 30 min (P < 0.01) post-injection. These results suggest that nocicept ive threshold increases during late pregnancy in the sow, perhaps as a n endogenous defence against labour pain, and that during parturition this change in nociceptive threshold is, at least in part, opioid-medi ated. Oxytocin is known to be inhibited by endogenous opioids at partu rition, thus future research should consider the potential role of inc reased nociception at birth as a negative feedback to oxytocin release . (C) 1997 International Association for the Study of Pain. Published by Elsevier Science B.V.