Opioid-mediated changes in nociceptive threshold during pregnancy and parturition in the sow

Abstract

This study aimed to investigate if pregnancy-induced hypoalgesia occurs 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 tested in straw bedded pens (2.5 x 2.5 m) during weeks 4, 8 and 12 of pregnancy and over the farrowing period. Testing involved thermal stimulation of eight areas on the rear-quarters 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 injection (i.m.) of an opioid antagonist naloxone (N) (1 mg kg(-1) body weight) with the remainder receiving a control dose of saline (S). Responses were recorded 15 and 30 min post-injection. There was no significant difference between response times over weeks 4, 8 and 12 of pregnancy (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 sows had stopped responding within 16 s (P < 0.001). Response times fell over days 1, 2 and 7 post-partum. After administration of naloxone response times fell compared to control animals at 15 min (P < 0.001) and 30 min (P < 0.01) post-injection. These results suggest that nociceptive threshold increases during late pregnancy in the sow, perhaps as an endogenous defence against labour pain, and that during parturition this change in nociceptive threshold is, at least in part, opioid-mediated. Oxytocin is known to be inhibited by endogenous opioids at parturition, thus future research should consider the potential role of increased nociception at birth as a negative feedback to oxytocin release.

The effect of labor on sevoflurane requirements during cesarean delivery

“…Sensitivity to pain decreases during pregnancy due to the activation of endogenous pain inhibitory systems and systemic or intrathecal administration of opioid antagonists abolishes pregnancy-induced antinociceptive effects. [25][26][27] However, few studies have explored this phenomenon in humans, and the results have been inconsistent; a limited number report decreased sensitivity to pressure, cold and heat during pregnancy. [28][29][30][31][32][33][34] Plasma levels of b-endorphin increase during labor and remain elevated during the early postpartum period.…”

Identifying and monitoring pain in farm animals: a review

“…The reflex is frequently used to measure the response to a controlled painful stimulus in order to test the efficiency of analgesic drugs or the influence of the physiological state of the animal. An example is the latency to the occurrence of a withdrawal or kicking movement when a limited area of the leg, shoulder or rump is subjected to a painful stimulus, for instance, by heating the skin locally with a laser beam (pigs: Jarvis et al, 1997;calves: Veissier et al, 2000;cows: Herskin et al, 2003). Avoidance and defensive behaviours, including leg and body movements as if animals were trying to avoid or escape the painful stimuli, were observed during castration (Marx et al, 2003), teeth resection and tail docking in young piglets (Noonan et al, 1994;Torrey et al, 2009).…”

Development of a mechanical stimulator and force measurement system for the assessment of nociceptive thresholds in pigs

“…The use of thermal stimulation (CO 2 infra-red laser) has been reported in previous studies on pigs to determine nociceptive thresholds in pregnant sows (Jarvis et al, 1997), however, this method was considered to be unworkable under the test conditions employed in this study, and those likely to be encountered in future studies. Thermal stimulation was evaluated using a hand-held radiant heat device, previously used in studies evaluating the effects of castration on hyperalgesia in calves (Molony et al, unpublished results-web link http://www.link.vet.ed.ac.uk/ animalpain/Pages/Moviepages/Moviethermalstimcalf.htm).…”