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

Placentophagia and the Tao of POEF

Placentophagia, ingestion of placenta and amniotic fluid, usually during parturition, is a behavioral feature of nearly all nonaquatic, placental mammals, and is a nexus for several interlocking behavioral phenomena. Placentophagia has not been typical of human cultures, but in recent years, some women in affluent societies have engaged in it, thereby bringing publicity to the behavior. First, we summarized benefits of placentophagia for nonhuman mammals, which include increased attractiveness of neonates, enhanced onset of maternal behavior, suppression of pseudopregnancy, and enhancement of opioid hypoalgesia by Placental Opioid-Enhancing Factor (POEF), a benefit that may extend well outside the context of parturition. The research on POEF in animals was discussed in detail. Then we discussed placentophagia (placentophagy) in humans, and whether there is validity to the claims of various benefits reported primarily in the pro-placentophagy literature, and, although human afterbirth shows POEF activity, the POEF effect has not yet been tested in humans. Finally, we discussed the general possible implications, for the management of pain and addiction, of isolating and characterizing POEF.

Investigating the effects of jute nesting material and enriched piglet mats on sow welfare and piglet survival

Domesticated sows are motivated to perform nesting behavior prior to farrowing, and nesting material can reduce piglet cold stress. However, nesting material may not be practical in most production systems due to the potential for clogging slurry systems. Therefore, the study objectives were to assess an alternative nesting material provided prior to farrowing on sow welfare and piglet survival, and to investigate the effect of the entire nesting environment on piglet survival and growth performance. We hypothesized that the provision of jute nesting material would decrease sow stress and farrowing duration, and that nesting mat provision would allow piglets to remain euthermic and improve survival and growth. Sows (N = 20) were randomly assigned to one of two treatments: a farrowing crate with three pieces of 40.6 × 21.6 cm jute nesting material (Nest; n = 10) and two enriched piglet mats made from an acrylic board (28.0 × 86.4 cm) covered with a microfiber material, or a farrowing crate without nesting material (Control; n = 10) and one standard plastic piglet mat (28.0 × 86.4 cm). Jute pieces were attached to the front of the crate to prevent substrate from falling through the slatted floors. Saliva samples were collected to measure cortisol and immunoglobulin A (IgA), on days -1, 0, 1, and 2 relative to farrowing, and a final sample was collected at weaning (day 16.9 ± 0.18). Blood was collected from four piglets per litter to measure immunoglobulin G (IgG) at 48 h, day 7, and weaning. Piglet skin temperature (TS) was measured on two piglets per litter using an infrared camera for 3 d after birth at 0800, 1200, 1600, and 2000 h. One piglet was randomly chosen from the heat lamp and nonheat lamp side of the crate to measure TS. Video was continuously coded for observations of jute- and crate-directed interactions. Data were analyzed as a mixed model analysis of variance in SAS 9.4. Nest sows performed less crate-directed behavior than Control sows (P = 0.02). Cortisol tended to be reduced in Nest sows (P = 0.08) when compared to Controls, but no differences in IgA concentrations (P > 0.40) were detected. Nest piglets tended to be heavier on day 7 (P < 0.10), had greater IgG concentrations (P = 0.03), and had greater TS (P = 0.02) versus Controls. No farrowing duration or number of stillbirth differences were observed (P > 0.70). The jute material and piglet nests positively impacted sow welfare and piglet measures but did not translate into improved piglet survival.

Mechanisms Underlying Lumbopelvic Pain During Pregnancy: A Proposed Model

Up to 86% of pregnant women will have lumbopelvic pain during the 3rd trimester of pregnancy and women with lumbopelvic pain experience lower health-related quality of life during pregnancy than women without lumbopelvic pain. Several risk factors for pregnancy-related lumbopelvic pain have been identified and include history of low back pain, previous trauma to the back or pelvis and previous pregnancy-related pelvic girdle pain. During pregnancy, women go through several hormonal and biomechanical changes as well as neuromuscular adaptations which could explain the development of lumbopelvic pain, but this remains unclear. The aim of this article is to review the potential pregnancy-related changes and adaptations (hormonal, biomechanical and neuromuscular) that may play a role in the development of lumbopelvic pain during pregnancy. This narrative review presents different mechanisms that may explain the development of lumbopelvic pain in pregnant women. A hypotheses-driven model on how these various physiological changes potentially interact in the development of lumbopelvic pain in pregnant women is also presented. Pregnancy-related hormonal changes, characterized by an increase in relaxin, estrogen and progesterone levels, are potentially linked to ligament hyperlaxity and joint instability, thus contributing to lumbopelvic pain. In addition, biomechanical changes induced by the growing fetus, can modify posture, load sharing and mechanical stress in the lumbar and pelvic structures. Finally, neuromuscular adaptations during pregnancy include an increase in the activation of lumbopelvic muscles and a decrease in endurance of the pelvic floor muscles. Whether or not a causal link between these changes and lumbopelvic pain exists remains to be determined. This model provides a better understanding of the mechanisms behind the development of lumbopelvic pain during pregnancy to guide future research. It should allow clinicians and researchers to consider the multifactorial nature of lumbopelvic pain while taking into account the various changes and adaptations during pregnancy.

The Programmed Cell Death Ligand-1/Programmed Cell Death-1 Pathway Mediates Pregnancy-Induced Analgesia via Regulating Spinal Inflammatory Cytokines

The maternal pain threshold gradually increases during pregnancy, especially in late pregnancy. A series of mechanisms underlying pregnancy-induced analgesia have been reported. However, these mechanisms are still not completely clear, and the underlying molecular mechanisms need further investigation. We examined the relationship between the antinociceptive effect and the expression level of programmed cell death ligand-1 (PD-L1) during pregnancy and further observed the changes in pain thresholds and expression levels of cytokines in late-pregnant mice before and after blockade of PD-L1 or programmed cell death-1 (PD-1).

Parturition in Mammals: Animal Models, Pain and Distress

Parturition is a complex physiological process and involves many hormonal, morphological, physiological, and behavioural changes. Labour is a crucial moment for numerous species and is usually the most painful experience in females. Contrary to the extensive research in humans, there are limited pain studies associated with the birth process in domestic animals. Nonetheless, awareness of parturition has increased among the public, owners, and the scientific community during recent years. Dystocia is a significant factor that increases the level of parturition pain. It is considered less common in polytocous species because newborns' number and small size might lead to the belief that the parturition process is less painful than in monotocous animal species and humans. This review aims to provide elements of the current knowledge about human labour pain (monotocous species), the relevant contribution of the rat model to human labour pain, and the current clinical and experimental knowledge of parturition pain mechanisms in domestic animals that support the fact that domestic polytocous species also experience pain. Moreover, both for women and domestic animal species, parturition's pain represents a potential welfare concern, and information on pain indicators and the appropriate analgesic therapy are discussed.

Meloxicam and Dexamethasone Administration as Anti-Inflammatory Compounds to Sows Prior to Farrowing Does Not Improve Lactation Performance

Simple Summary

Sows may experience pain and discomfort whilst giving birth. Additionally, the birthing process is accompanied by an inflammation response. Administering anti-inflammatory compounds prior to birth may provide an opportunity to improve piglet survival and growth. The aim of this experiment was to assess the efficacy of both a non-steroidal (NSAID; meloxicam) and steroidal (SAID; dexamethasone) anti-inflammatory drug for improving farrowing house performance of sows. In younger sows, there was no impact of treatment; however, older sows from the NSAID treatment gave birth to fewer live piglets. Postnatal mortality was unaffected by treatment and no improvement in piglet growth was observed. Feed intake of both NSAID and SAID sows was improved when compared with the control group; however, there was a tendency for a delayed oestrus in the NSAID group. Administering NSAID to sows prior to farrowing is not recommended as it reduces piglet survival and subsequent reproduction.

Abstract

The aim of this experiment was to determine whether administration of an anti-inflammatory compound to sows prior to farrowing would, via reduced pain and inflammation, increase piglet survival and growth. At day 114 of gestation, multiparous sows were randomly allocated to one of the following treatments: Control (n = 43), which received 10 mL saline, NSAID (n = 55) which received 0.4 mg/kg meloxicam and SAID (n = 54) which received 0.1 mg/kg dexamethasone. Treatments were applied again on day 116 if farrowing had not occurred. There was no treatment effect on piglets born alive or dead from parity two to four sows but in those of parity five and older, NSAID administration reduced the number of piglets born alive and increased the number of piglets born dead (p < 0.05). Sow rectal temperature and incidence of mastitis were unaffected by treatment (p > 0.05). Lactation day two plasma concentrations of cortisol, prostaglandin F2 alpha metabolite and haptoglobin did not differ among treatments (p > 0.05). Treatment effects were not observed in liveborn piglet mortality at any age, or litter weight at day 21 (p > 0.05). Average feed intake during lactation was increased by both NSAID and SAID treatments (p = 0.001). The use of meloxicam prior to farrowing should be avoided as it reduced the number of piglets born alive and did not improve piglet survival and growth.

The effect of hessian and straw as nesting materials on sow behaviour and piglet survival and growth to weaning

Sows are strongly driven to build a nest prior to farrowing, and the performance of this behaviour is linked to the environment in which the animal is housed. The aim of this study was to investigate the impact of two nest-building materials, hessian and straw, on peri-parturient sow behaviour, plasma cortisol concentration and piglet survival and performance in farrowing crates. In the first experiment, sows (parity 1.7 ± 0.1) were allocated to four treatments: (n = 15), straw provided in the lead up to farrowing in an open farrowing pen, with the pen closed after farrowing (STRAW OPEN); (n = 14), straw provided in the lead up to farrowing in a closed farrowing pen (STRAW CLOSED); (n = 15), a closed farrowing pen with hessian sacks provided in the lead up to farrowing (HESSIAN) and; (n = 13), a closed farrowing pen with no nesting materials provided (CONTROL). A second experiment was performed on a separate farm to assess the effect of the same four treatments were applied to sows (parity 2.9 ± 0.1): SRAW OPEN (n = 68), STRAW CLOSED (n = 64), HESSIAN (n = 66) and CONTROL (n = 66), at a commercial level. The first experiment revealed that providing conventionally housed sows with straw or hessian in the lead up to parturition stimulated sows to perform nest-building behaviours similar to sows housed in an open pen with access to straw (nosing events; 16 ± 11 (CONTROL); 169 ± 36 (HESSIAN); 118 ± 29 (STRAW CLOSED); 199 ± 53 (STRAW OPEN); P < 0.05). Additionally, crated sows provided with straw had reduced cortisol levels immediately after farrowing compared to all other treatments (21.9 ± 6.1 ng/ml vs CONTROL; 49.3 ± 8.6 ng/ml; P < 0.01). Piglets born to STRAW CLOSED sows displayed the highest colostrum intake levels (404.8 ± 22.7 g vs CONTROL 361.9 ± 21.9 g; P < 0.01). The second experiment demonstrated a reduced incidence of piglet mortality both prior to fostering (0.7 ± 0.2; P = 0.001) and after fostering (0.7 ± 0.2; P = 0.001) in litters born to sows which were housed in conventional farrowing crates and provided with straw compared to CONTROL (prior to fostering 1.3 ± 0.2, and postfostering 1.1 ± 0.2). In conclusion, straw and hessian sacks are a suitable substrate for stimulating sows to exhibit nest-building behaviour under crated conditions. However, only the provision of straw in the crate environment improved piglet survival and positively affected sow welfare.

Does muscular or mental fatigue have an influence on the nociceptive flexion reflex? A randomized cross-over study in healthy people

BACKGROUND

The nociceptive flexion reflex (NFR) is a spinally-mediated withdrawal reflex occurring in response to noxious stimuli and is used as an electrophysiological marker of spinal nociception. Although it is well-documented that the NFR is subject to powerful modulation of several personal factors, the effects of experimentally induced fatigue on the NFR have not yet been examined. Hence, this study aimed to characterize if and how fatigue affects spinal nociception in healthy adults.

METHODS

The NFR of 58 healthy people was measured prior to and following rest and two fatiguing tasks performed in randomized order. The NFR was elicited by transcutaneous electrical stimulation of the sural nerve and objectified by electromyographic recordings from the biceps femoris muscle. An isokinetic fatiguing protocol was used to induce localized muscle fatigue of the hamstrings. The modified incongruent Stroop-word task was used to provoke mental fatigue. A linear mixed model analysis was performed to assess the influence of fatigue on the NFR.

RESULTS

Low-to-moderate levels experimentally induced localized muscle and mental fatigue did not affect the NFR in healthy adults. These results suggest that descending pain inhibitory processes to dampen spinal nociception are resistant to the effects of localized muscle and mental fatigue.

CONCLUSIONS

The relative robustness of the NFR to fatigue may be beneficial in both clinical and research settings where the influence of confounders complicates interpretation. Furthermore, the findings possibly help enhance our understanding on why even demanding cognitive/physical exercise-based treatment programs form effective treatment strategies for patients with chronic pain.

SIGNIFICANCE

The present study unraveled that low-to-moderate levels experimentally induced localized muscle and mental fatigue did not affect the NFR. These results suggest that descending pain inhibitory processes to dampen spinal nociception are resistant to the effects of localized muscle and mental fatigue. This relative robustness of the NFR may be beneficial in a clinical setting in which the evaluation of spinal nociception that is unaffected by clinical symptoms of fatigue may be useful (e.g. chronic fatigue syndrome, cancer-related fatigue, etc.).

The challenge of large litters on the immune system of the sow and the piglets

The use of hyperprolific sow lines has increased litter size considerably in the last three decades. Nowadays, in some countries litters can reach up to 18-20 piglets being a major challenge for the sow's physiology during pregnancy, parturition and lactation. The increased number of piglets born per litter prolongs sensibly the duration of farrowing, decreases the piglets' average weight at birth and their vitality, increases the competition for colostrum intake and can affect negatively piglets' survival. This review aims to describe how large litters can affect the immune system of the sow and the piglets and proposes measures to improve this condition.

Stress effects on the regulation of parturition in different domestic animal species

This review summarizes current knowledge on stress-like responses in parturient animals and their role for the onset and fine-tuning of parturition. The antepartum maternal cortisol increase is part of the endocrine changes that initiate parturition but a further increase in cortisol release during labor indicates a stress response. During the last minutes of delivery, sinus arrhythmias occur in 80% of foaling mares and 60% of calving cows. Expulsion of the neonate is thus characterized by parasympathetic dominance. In late-pregnant cows transported by road, cortisol concentrations increased but relations between transport stress and abortion remain unclear. In mares, transport not only elicited a stress response but also advanced the time of foaling. Transferring parturient rats, mice and pigs after birth of the first pup or piglet, respectively, to a stressful environment prolonged the time until delivery of the next littermate. In rats and pigs, this was caused by an increased opioidergic tone that restrained oxytocin release. In mice, a stress-induced delay of subsequent deliveries was caused by increased sympathoadrenal activity. When foaling mares were transferred to an uncomfortable stable at fetal membrane rupture, time until complete birth of the foal was doubled. As in mice, increased sympathetic activity was the mechanism delaying the progress of foaling. An increased sympathetic activity is also present in parturient cows disturbed during an early stage of calving. In equine and bovine neonates, the immediate postnatal period is characterised by high sympathetic activity and an increase in cortisol concentration, indicating a pronounced stress-like response.

"Luteal Analgesia": Progesterone Dissociates Pain Intensity and Unpleasantness by Influencing Emotion Regulation Networks

Background: Pregnancy-induced analgesia is known to occur in association with the very high levels of estradiol and progesterone circulating during pregnancy. In women with natural ovulatory menstrual cycles, more modest rises in these hormones occur on a monthly basis. We therefore hypothesized that the high estradiol high progesterone state indicative of ovulation would be associated with a reduction in the pain experience. Methods: We used fMRI and a noxious thermal stimulus to explore the relationship between sex steroid hormones and the pain experience. Specifically, we assessed the relationship with stimulus-related activity in key regions of networks involved in emotion regulation, and functional connectivity between these regions. Results: We demonstrate that physiologically high progesterone levels are associated with a reduction in the affective component of the pain experience and a dissociation between pain intensity and unpleasantness. This dissociation is related to decreased functional connectivity between the inferior frontal gyrus and amygdala. Moreover, we have shown that in the pre-ovulatory state, the traditionally "male" sex hormone, testosterone, is the strongest hormonal regulator of pain-related activity and connectivity within the emotional regulation network. However, following ovulation the traditionally "female" sex hormones, estradiol and progesterone, appear to dominate. Conclusions: We propose that a phenomenon of "luteal analgesia" exists with potential reproductive advantages.

The Association Between Progesterone, Estradiol, and Oxytocin and Heat Pain Measures in Pregnancy: An Observational Cohort Study

BACKGROUND

Hormonal action has been implicated as a possible mechanism for pregnancy-induced analgesia. Previous investigators have reported an increase in heat pain tolerance during labor compared with nonpregnant controls and postulated it was because of the hormonal changes during pregnancy. However, these previous reports did not include measurement of hormonal values. The purpose of our study was to quantitatively test if changes in pregnancy hormone concentrations correlated with changes in temperature ratings.

METHODS

This was a prospective cohort study consisting of 32 women scheduled for elective cesarean delivery at term between July 2010 and January 2013. Heat pain threshold and tolerance, estrogen, progesterone, and oxytocin levels were measured twice in each patient at term and again 4 to 8 weeks postpartum.

RESULTS

All hormone levels decreased significantly between term pregnancy and the postpartum visit (all P values < 0.029). However, there were no statistically significant differences between term and postpartum heat pain measurements. The mean baseline heat pain threshold was 40.9°C at term compared with 40.3°C °postpartum (P = 0.47; mean change, -0.6°C; 95% confidence interval of change, -1.8°C to +0.7°C). The mean baseline heat pain tolerance was 46.1°C at term and 46.0°C postpartum (P = 0.59; mean change, -0.1°C; 95% confidence interval of change, -0.8°C° to +0.6°C).

CONCLUSIONS

Our findings show that amounts of estradiol and progesterone changed significantly between the term and the postpartum visit; however, the thermal pain tolerance did not significantly change. In summary, we did not observe an association between hormonal changes and changes in pain threshold measures. This finding argues against the concept of simple progesterone- or estrogen-induced analgesia in humans.

Pregnancy suppresses neuropathic pain induced by chronic constriction injury in rats through the inhibition of TNF-α

Purpose

Pregnancy-induced analgesia develops during late pregnancy, but it is unclear whether this analgesia is effective against neuropathic pain. The detailed molecular mechanisms underlying pregnancy-induced analgesia have not been investigated. We examined the antinociceptive effect of pregnancy-induced analgesia in a neuropathic pain model and the expression of tumor necrosis factor (TNF)-α, glial fibrillary acidic protein (GFAP), Iba-1, and c-Fos in the spinal dorsal horn just before parturition.

Materials and methods

Female Sprague Dawley rats (200–250 g) were randomly assigned to one of four groups (pregnant + chronic constriction injury [CCI]; pregnant + sham injury; not pregnant + CCI; and not pregnant + sham injury). Separate groups were used for the behavioral and tissue analyses. CCI of the left sciatic nerve was surgically induced 3 days after confirming pregnancy in the pregnancy group or on day 3 in the not pregnant group. The spinal cord was extracted 18 days after CCI. TNF-α, GFAP, Iba-1, and c-Fos expression levels in the spinal dorsal horn were measured by Western blot analysis. Mechanical threshold was tested using von Frey filaments.

Results

The lowered mechanical threshold induced by CCI was significantly attenuated within 1 day before parturition and decreased after delivery. TNF-α expression in CCI rats was decreased within 1 day before parturition. Further, GFAP, Iba-1, and c-Fos expression in the spinal dorsal horn was reduced in the pregnant rats. Serum TNF-α in all groups was below measurable limits.

Conclusion

Our findings indicate that pregnancy-induced analgesia suppresses neuropathic pain through reducing spinal levels of TNF-α, GFAP, Iba-1, and c-Fos in a rat model of CCI.

A Review of Pain Assessment in Pigs

There is a moral obligation to minimize pain in pigs used for human benefit. In livestock production, pigs experience pain caused by management procedures, e.g., castration and tail docking, injuries from fighting or poor housing conditions, “management diseases” like mastitis or streptococcal meningitis, and at parturition. Pigs used in biomedical research undergo procedures that are regarded as painful in humans, but do not receive similar levels of analgesia, and pet pigs also experience potentially painful conditions. In all contexts, accurate pain assessment is a prerequisite in (a) the estimation of the welfare consequences of noxious interventions and (b) the development of more effective pain mitigation strategies. This narrative review identifies the sources of pain in pigs, discusses the various assessment measures currently available, and proposes directions for future investigation.

The identification of potential behavioural indicators of pain in periparturient sows

Periparturient pain is a welfare concern and could contribute to piglet losses. This has led to studies investigating post-farrowing analgesia. A clear reduction in pain has not been demonstrated, partly due to a lack of pain indicators. This study quantified behaviours as potential pain indicators (PPIn) in sows: i) before, during and after farrowing, and ii) 2min before and after piglet births. Twenty-five sows were observed during and after, and ten pre-farrowing. Behaviour recorded included: 1) back leg forward (back leg pulled forward and/or in); 2) tremble (movement as if shivering); 3) back arch (leg(s) stretch forming an arched back); 4) paw (leg scraped in pawing motion); and 5) tail flick (tail moved rapidly up and down). Behaviours were analysed using generalized linear models and Spearman's rank correlations. All PPIn were rare or absent pre-farrowing, highest during farrowing, and back leg forward, tremble and back arch were greater in the early post-farrowing period. Several significant positive correlations between PPIn during and post-farrowing were found. Back arch, tail flick and paw were higher before than after a piglet birth, and were more frequent earlier in the birth order. Back leg forward and tremble did not differ before and after births, and tremble increased with birth order. These behaviours, which were absent or rare pre-farrowing, present during farrowing and were lower afterwards, and showed consistent individual variation, may be quantitatively associated with pain. Spontaneous behaviours could be used to test the efficacy of analgesics or identify sows that may benefit.

The effect of social isolation, gender and familiarity with the experimental procedure on tests of porcine nociceptive thresholds

OBJECTIVE

To investigate the effects of habituation and isolation on mechanical nociceptive thresholds in pigs at the pelvic limbs and at the tail.

STUDY DESIGN

Prospective randomized multifactorial study.

ANIMALS

Thirty-two healthy castrated male (experiment 1), and 12 castrated male and 12 female (experiment 2) Danish Landrace×Yorkshire pigs, weighing 63.5±0.8 kg and 55.4±0.6 kg (the mean±SD, experiment 1 and 2, respectively).

METHODS

Mechanical nociceptive thresholds were quantified with a von Frey anesthesiometer applied to two distinct anatomical regions (tail and pelvic limbs). Pigs receiving the mechanical challenge in the pelvic limbs were tested inside a cage, whereas pigs exposed to stimuli at the tail region were tested in an open arena. For both experiments, the effect of familiarity to the procedure was evaluated by comparing thresholds of nociception in habituated versus naïve pigs. The presence of a companion animal was also evaluated in pigs receiving stimuli at the pelvic limbs.

RESULTS

Pigs tested inside the cage were affected by the habituation to the procedure as indicated by the increase in willingness and time spent by the animals in the test cage. This effect was reflected in the lower mechanical nociceptive thresholds (medians with 25-75 percentiles) recorded for familiar pigs compared with naïve animals [495 g (302-675) versus 745 g (479-1000), respectively; p=0.026]. Mechanical nociceptive thresholds measured at the tail of the pigs in the open arena were not affected by the familiarity of the animals with the experimental procedure.

CONCLUSIONS AND CLINICAL RELEVANCE

The current results reiterate the value of habituation in research involving animal behaviour. Further characterization of the methodology is needed to allow its application in the evaluation of clinical conditions in pigs.

Effects of age and reproductive status on postoperative pain after routine ovariohysterectomy in cats

A prospective clinical trial to compare the effects of age and reproductive status on postoperative pain was conducted in 145 female cats undergoing ovariohysterectomy using injectable anaesthesia. The cats were grouped appropriately: 60 kittens <4 months old (K), 85 adults >4 months old (A) and, within the adult group, 57 normal adults (nA) and 28 adults who were either pregnant or in oestrus (rA). Pain was assessed using a simple descriptive scale (SDS; 0–3), a dynamic and interactive visual scale (DIVAS; 0–100 mm) and mechanical nociceptive thresholds (MNT; N, 2 mm diameter probe) pre-operatively and at 4 and 24 h postoperatively. Kittens had lower DIVAS areas under the time curve and SDS than adults ( P <0.05), but similar MNT (K: 3.3 ± 2.6, A: 4.3 ± 2.5 N at 4 h, P >0.05). Data from nA and rA were not different ( P >0.05). Kittens had similar wound tenderness, but less affective pain than adults, and reproductive status had no effect.

Nociceptive responses to thermal and mechanical stimulations in awake pigs

BACKGROUND

Porcine skin exhibits a high degree of homology to human skin, and the pig has recently been used as a cutaneous pain model. However, before the full potential of this novel in vivo cutaneous pain model can be achieved, several methodological aspects related to the management of awake animal studies in a large species require further examination. This manuscript describes the initial development of a porcine model of cutaneous nociception and focuses on interactions between the sensory modality, body size and the anatomical location of the stimulation site.

METHODS

Pigs of different body sizes (30 and 60 kg) were exposed to thermal (CO2 laser) and mechanical (pressure application measurement device) stimulations to the flank and the hind legs in a balanced order. The median response latency and the type of behavioural response were recorded.

RESULTS

Small pigs exhibited significantly lower pain thresholds (shorter latency to response) than large pigs to thermal and mechanical stimulations. Stimulations at the two anatomical locations elicited very distinct sets of behavioural responses, with different levels of sensitivity between the flank and the hind legs. Furthermore, small animals exhibited lower levels of individual variability between single stimulations.

CONCLUSIONS

Our data indicate that this experimental approach may be valuable for use in studies that focus on porcine cutaneous nociception.

Pregnancy-induced analgesia: a combined psychophysical and neurophysiological study

BACKGROUND

To investigate changes in heat pain threshold and modifications in heat pain processing during pregnancy and labour, seventy-six nulliparous pregnant women were enrolled in two studies.

METHODS

In the first study (psychophysical), 60 pregnant women underwent a quantitative sensory testing (QST) investigating heat perception in two body areas (right forearm and T10 dermatome) according to these groups: 32-33 gestational weeks (GW), 39-40 GW, early stage of active labour and 24 h after the delivery. In the other study (neurophysiological), contact heat-evoked potentials (CHEPs) were recorded in other 16 women at the 32nd GW and in 11 of these also at the 40th GW.

RESULTS

The psychophysical study showed that heat pain threshold was significantly increased at the forearm at 32-33 GW (median ± IQR: 39.6 ± 0.7 °C), at 39-40 GW (40.6 ± 1.1 °C) and at early stage of active labour (40.8 ± 1.5 °C) as compared to 20 non-pregnant controls (p < 0.001). Heat pain threshold tested at T10 level was significantly increased at 32-33 GW (41.0 ± 1.6 °C), at 39-40 GW (42.1 ± 1.8 °C), and at early stage of active labour (42.3 ± 1.3 °C) as compared to the non-pregnant women (p < 0.001). The N2-P2 CHEP amplitude (main negative N2 and positive P2 components of the vertex biphasic potential) recorded from the pregnant women was significantly lower at the 40th than at the 32nd GW, after stimulation of both the forearm (p < 0.001) and the abdomen (p < 0.001).

CONCLUSIONS

In pregnant women, there is a progressive increase of heat pain threshold and a reduction of the CHEP amplitude, suggesting that a general inhibitory mechanism may be involved.

Effect of neonatal nociceptin or nocistatin imprinting on the brain concentration of biogenic amines and their metabolites

Noradrenaline (NA), dopamine (DA), homovanillic acid (HA), serotonin (5HT) and 5-hydroxyindole acetic acid (5HIAA) content of five brain regions (hypothalamus, hippocampus, brainstem, striatum and frontal cortex) and the cerebrospinal fluid (CSF) was measured in adult (three months old) male and female rats treated neonatally with a single dose of 10 microg nociceptin (NC) or 10 microg nocistatin (NS) for hormonal imprinting. The biogenic amine and metabolite content of cerebrospinal fluid was also determined. In NC treated animals the serotonergic, dopaminergic as well as noradrenergic systems were influenced by the imprinting. The 5HT level increased in hypothalamus, the 5HIAA tissue levels were found increased in hypothalamus. Hippocampus and striatum and the HVA levels increased highly significantly in brainstem. Dopamine level decreased significantly in striatum, however in frontal cortex both noradrenalin and 5HIAA level decreased. Nevertheless, in NS-treated rats decreased NA tissue levels were found in hypothalamus, brainstem and frontal cortex. Decreased DA levels were found in the hypothalamus, brainstem and striatum. NS imprinting resulted in decreased HVA level, but increased one in the brainstem. The 5HT levels decreased in the hypothalamus, brainstem, striatum and frontal cortex, while 5HIAA content of CSF, and frontal cortex decreased, and that of hypothalamus, hippocampus and striatum increased. There was no significant difference between genders except in the 5HT tissue levels of NC treated rats. Data presented show that neonatal imprinting both by NC and NS have long-lasting and brain area specific effects. In earlier experiments endorphin imprinting also influenced the serotonergic system suggesting that during labour release of pain-related substances may durably affect the serotonergic (dopaminergic, adrenergic) system which can impress the animals' later behavior.

Gender differences and pain medication

Subtle genetic and psychological variations are increasingly recognized to contribute to pain and analgesic efficacy and safety. The influence of sex on this relationship remains poorly understood, particularly in humans. The issue is complicated by the overlay of gender onto physical sex, and its associated stereotypes and expectations. Women appear to use more pain-relieving medications than men; however, it remains unclear whether these observations represent true differences in analgesic usage patterns, or reporting bias. Differences in analgesic efficacy relating to body composition, metabolism and hormonal profiles have been demonstrated. Psychological and social elements of gender have also been associated with altered pain experiences and analgesic use profiles, albeit with significant individual variations. Intra-group differences may ultimately prove more important than sex differences. Further research may unravel the various threads linking gender and sex effects on analgesia with the aim of individualizing analgesia to optimize pain relief.

Experimental Heat Pain for Detecting Pregnancy-Induced Analgesia in Humans

Animal studies suggest that increased circulating estrogen and progesterone, and activation of the endorphin system cause prenancy-induced antinociceptive effects. Human studies have provided inconsistent results and have often lacked a nonpregnant control group. In this study, we compared sensitivity to experimental heat and cold pain in pregnant and nonpregnant women. Nineteen healthy nonpregnant female volunteers and 20 pregnant women at term were enrolled. Pain threshold and tolerance were examined using experimental heat-induced pain and cold pressor pain models. Subjects were evaluated pre- and 1-2 days post-delivery (pregnant), or on consecutive days (nonpregnant). Heat pain tolerance was significantly increased in the pregnant women during pre and postdelivery when compared with nonpregnant controls (50.0 +/- 1.0 vs 49.0 +/- 1.2 and 50.1 +/- 0.7 vs 49.2 +/- 1.2 degrees C; mean +/- sd). However, pain induced by the cold pressor test was endured for a similar amount of time by both study groups. Pregnancy-induced analgesic effects at term can be detected in a model of experimental heat pain. These effects persist during the first 24-48 h after delivery. Experimental heat pain is a suitable modality for further characterizing the phenomenon of pregnancy-induced analgesia in humans.

[Plasma levels of cortisol and opioid peptide beta-endorphin during spontaneous vaginal delivery]

INTRODUCTION

Labor pain is very frequent in clinical practice, but the underlying mechanisms as well as numerous neuroendocrine responses activated by such pain have not been fully explained yet.

OBJECTIVE

The objective of the study was to determine the influence of labor pain on plasma levels of cortisol and opioid peptide beta-endorphin.

METHOD

Cortisol and beta-endorphin levels were measured in blood plasma of: health, non-pregnant women (group 1, n=8), health pregnant women (group 2, n=8) and in parturitions, through fourth ages (group 3, n=8). Plasma level ofcortisol was measured by radioimmunoassay, and beta-endorphin by enzyme immunoassay. Data were expressed as mean +/- standard error of mean and were analyzed by Student's t test and Mann Whitney test.

RESULTS

Plasma level of cortisol in group 2 was significantly increased compared to the group 1. During labor progression, plasma level of cortisol was rising till the third labor age. Plasma level of cortisol in fourth labor age was not significantly different from the age one and group 1. Plasma level of beta-endorphin was (ng/L): in group 1: 64 +/- 20, group 2: 70 +/- 22, group 3: the first labor age: 75 +/- 15, the second labor age: 193 +/- 54, the third labor age: 346 +/- 97 and the fourth labor age: 114 +/- 31.

CONCLUSION

These results indicate that both beta-endorphin and cortisol are involved in regulation and modulation of labor pain and stress.

Increased innervation and ripening of the prepartum murine cervix

OBJECTIVE

Ripening of the cervix before birth is coincident with reduced collagen content and leukocyte immigration, characteristics that are analogous to a neurogenic inflammatory-like process. We sought to assess the morphologic relationship between innervation and remodeling of the peripartum cervix.

METHODS

Cervix was obtained from C3H/HeN mice on days 15 and 18 of pregnancy, 1 day postpartum, and from non-pregnant controls. Tissues were immersion-fixed, paraffin-embedded, and some sections stained with Picrosirius red to assess collagen content and complexity of organization. By image analysis of optical density, collagen content and structure were significantly decreased by the day before birth. Other sections were processed to visualize nerve fibers by immunohistochemistry with antibodies against neuron-specific epitopes, PGP9.5, peripherin, as well as brain nitric oxide synthase (bNOS), calcitonin gene-related peptide (CGRP), and other neuropeptides. Fiber density was assessed stereologically and normalized to cell density in non-pregnant cervix to correct for tissue hypertrophy due to reproductive status.

RESULTS

In groups of non-pregnant, day 15 pregnant, and postpartum mice, cervix contained nerve fibers that were immunoreactive for the pan-neural markers PGP9.5 and peripherin. Punctate and beaded varicosities were sparsely distributed in stroma, subepithelium, and in proximity to vascular structures. By day 18 of pregnancy, 1 day before birth, fiber density was increased fourfold or more compared to other groups. bNOS fibers and, to a lesser extent, CGRP accounted for most of the increased innervation of the murine cervix by the day before birth, a period when macrophage numbers are enhanced.

CONCLUSIONS

The findings suggest that increased bNOS and CGRP innervation contribute to early inflammatory-like processes that ripen the cervix before birth.

Pain Detection and Amelioration in Animals on the Farm: Issues and Options

Pain in nonhuman animals is a difficult concept to identify and measure. This article briefly describes the consequences of pain in animals on the farm and explains the reasons for the minimal use of analgesics in farmed animals. Pain can have implications for both animal welfare and economics. The reasons for a low use of analgesics in farmed animals include the lack of recognition of animal pain owing to the apparent lack of anthropomorphically identifiable behavioral changes, concern over human food safety, and lack of research efforts to develop safe analgesics for farm use. Treatment cost relative to the benefits expected is another hindering factor. Interventions to minimize pain must begin with developing objective and practical measures for pain identification and measurement at the farm level. A suggested use of a combination of different behavioral and physiological indicators would help to identify pain in animals. To facilitate continued usage of the methodologies on the farm it also is necessary to evaluate the economic implication of the pain alleviation intervention.

Douleur et analgésie obstétricales

GOAL AND METHODS

Labor pain is of major concern since most parturients experience significant pain of extremely severe intensity for many. The purpose of this review was to provide an overview of the mechanisms and pathways of labor pain (including new insights on integration of the nociceptive signal) and to emphasize the need of effective labor pain relief.

RESULTS

Labor pain can have deleterious effects on the mother, on the fetus and on labor outcome itself. Among the current methods of obstetric analgesia, regional analgesia (the most widespread technique being epidural analgesia) offers the best effectiveness/safety ratio thanks to pharmacological innovations. Systemic analgesia (parenteral opioids, nonopioid painkillers and inhaled anesthetic agents) provides an alternative to regional analgesia but remains less effective and more hazardous. Non-drug approaches (namely psychoprophylaxis and physical methods) may be effective when used with epidural analgesia but are often not potent enough when used alone.

CONCLUSION

Despite its complex pathophysiology, labor pain can be efficiently managed. Thanks to multidisciplinary care, obstetric analgesia (mainly epidural analgesia) prevents deleterious effects of labor pain on the mother and fetus.

A laser-based method to measure thermal nociception in dairy cows: short-term repeatability and effects of power output and skin condition

To validate a laser-based method to measure thermal nociception in dairy cows (e.g., for the use in studies on stress-induced analgesia), we performed three experiments to observe the behavioral responses to a computer-controlled CO2 laser beam applied to the skin on the caudal aspect of the metatarsus. In Exp. 1, effects of power output (0, 1.3, 1.8, 2.2, 2.4 and 2.6 W) on nociceptive responses were examined using 18 dairy cows kept and tested in tie stalls. Increasing the power output affected the latencies to respond (decreasing latencies, P < or = 0.01), types of response (less nonresponding and more kicking, P < 0.0001), and behavior during (increasing frequency of tail flicking, P = 0.003) and between single laser exposures (increasing frequency of kicking, P = 0.02). Therefore, behavioral responses to a laser stimulus seem to be a valid measure of nociception in dairy cows. Repeatability within 15 min was investigated in Exp. 2 using n = 36 dairy cows kept and tested in tie stalls and a power output of 1.8 W. The variables' latency to move the exposed leg and frequency of tail flicking during laser exposure showed the highest level of repeatability (0.50 and 0.38, respectively). However, retesting at t = 15 min led to increased responses in terms of shorter latencies to respond (P < 0.05), increased kicking (P = 0.05), and tail flicking (P = 0.02), which probably can be explained by sensitization. Effects of power output (1.0 vs. 1.8 W) and skin condition (naked vs. intact) were examined in Exp. 3 on 11 group-housed dairy cows, tested just outside their home pen. Increasing the power output and shaving off hair led to increased responses as seen by shorter latencies to respond (P < 0.0001), less nonresponding (P < 0.0001), and increased kicking (P = 0.0003), as well as reduced intra- and interindividual variability (P < or = 0.04). In conclusion, the results of these experiments suggest that behavioral responses to laser stimulation are a valid and reliable measure of nociception in dairy cows, especially when applied on naked skin, both in the home environment and just outside a group pen. The fact that repeated testing in itself at t = 15 min led to increased responses means that the test will be a conservative measure of stress-induced analgesia.

Influence of ovarian sex steroids on spinal methionine-enkephalin release: comparison with dynorphin reveals asymmetrical regulation

The concomitant activation of spinal kappa- and delta-opioid systems is a prerequisite for the antinociception of gestation and its hormonal simulation [via 17 beta-estradiol and progesterone administration; hormone-simulated pregnancy (HSP)]. However, it is not known whether the release of kappa- and delta-opioids is also concomitantly regulated. This study investigates whether the release of methionine-enkephalin and modulation thereof is altered during HSP, as has been reported for dynorphin. K+-stimulated release of spinal methionine-enkephalin from lumbar spinal tissue obtained from control animals is negatively modulated by nociceptin (orphanin FQ; N/OFQ) in a dose-dependent manner, but not by opioids. Conversely, selective blockade of spinal N/OFQ, but not opioid receptors, augments the K+-induced increase in methionine-enkephalin release, indicating that endogenous N/OFQ also functions as a negative modulator of methionine-enkephalin release. The magnitude of K+-evoked methionine-enkephalin release from spinal tissue obtained from ovarian steroid-treated animals remains unchanged, consistent with the insensitivity of its modulation by N/OFQ to the ovarian sex steroid milieu. These characteristics of methionine-enkephalin release stand in sharp contrast to those previously reported for the evoked release of spinal dynorphin. Dynorphin release is subject to negative modulation by opioid (predominantly delta) as well as N/OFQ, both of which are offset during HSP, resulting in an approximately 2-fold increase in the magnitude of its release. These observations reveal that regulation of spinal dynorphin/kappa- and methionine-enkephalin/delta-spinal opioid antinociceptive systems is independent, divergent, and not symmetrical and support the formulation that spinal methionine-enkephalin/delta-opioid tone acts in a permissive/facilitative capacity to accentuate spinal dynorphin/kappa-activity.

The maternal spinal cord: biochemical and physiological correlates of steroid-activated antinociceptive processes

Physiological gestation, as well as the simulation of the associated changes in estrogen and progesterone, is associated with significant elevations in nociceptive response thresholds. This is mediated by spinal cord kappa- and delta-opIoid systems. The predominant spinal mu-opioid system does not appear to participate. One hallmark of pregnancy- and hormonally-induced antinociception is the multiplicative interaction among its components. Approximately 40% results from spinal kappa/delta analgesic synergy on which is superimposed an additional increment (approximately 60%) of synergy that results from the interaction between descending spinal alpha 2-noradrenergic and spinal kappa/delta activities. An intact hypogastric nerve is required for the spinal alpha 2-noradrenergic component. This would explain the requirement for an intact hypogastric nerve in order for the antinociception of pregnancy and its hormonal simulation to be fully manifest. The predominant means by which spinal dynorphin-containing neurons adjust to increased demand is increased post-translational processing of dynorphin precursor intermediates which are present at approximately 10x the concentration of mature dynorphin peptides (1-17 and 1-8). This is indicated by the concomitant decline (approximately 50%) in the spinal cord content of dynorphin precursors and increase (approximately 87%) in the content of prohormone convertase 2, a processing enzyme sufficient to generate mature dynorphin peptides from prodynorphin. The presence of 'high gain' multiplicative spinal opioid antinociceptive pathways that can be activated by estrogen and progesterone has hyperalgesic implications as well, i.e. it could result in disproportionately increased pain responsiveness. This might explain, in part, findings that women are more prone to recurrent pain and pain of greater duration and intensity than men. The underlying mechanisms of gestational antinociception could point the way to pain pharmacotherapies that are gender-based.

Brain preparations for maternity--adaptive changes in behavioral and neuroendocrine systems during pregnancy and lactation. An overview

Pregnancy, parturition and lactation comprise a continuum of adaptive changes necessary for the development and maintenance of the offspring. The endocrine changes that are driven by the conceptus and are essential for the maintenance of pregnancy and are involved in the preparations for motherhood are outlined. These changes include large increases in the secretion of sex steroid hormones, and the secretion of peptide hormones that are unique to pregnancy. The ability of these pregnancy hormones to alter several aspects of brain function in pregnancy is considered, and the adaptive importance of some of these changes is discussed, for example in metabolic and body fluid adjustments, and the induction of maternal behavior. The importance of sex steroids in determining the timing of the various adaptive changes in preparing for parturition and maternal behavior is emphasized, and the concept that the actions of prolactin and oxytocin, quintessential mammalian motherhood neuropeptides, can serve to coordinate a spectrum of adaptive changes is discussed. The part played by oxytocin neurons and their regulatory mechanisms is reviewed to illustrate how neural systems involved in maternity are prepared in pregnancy via changes in phenotype, synaptic organization and in the relative importance of their different inputs, to function optimally when needed. For oxytocin neurons secreting from the posterior pituitary, important in parturition and essential in lactation, these changes include mechanisms to restrain their premature activation, and adaptations to support synchronized burst firing for pulsatile oxytocin secretion in response to stimulation via afferents from the birth canal, olfactory system or suckled nipples. Within the brain, expression of oxytocin receptors permits centrally released oxytocin to facilitate the expression of maternal behavior. Changes in other neuroendocrine systems are similarly extensive, leading to lactation, suppression of ovulation, reduced stress responses and increased appetite; these changes in lactation are driven by the suckling stimulus. The possible link between these adaptations and changes in cognition and mood in pregnancy and post partum are considered, as well as the dysfunctions that lead to common problems of depression and puerperal psychoses.

PGF(2alpha)-induced nest building and choice behaviour in female domestic pigs

The domestic pig, Sus scrofa, builds a maternal nest in the day before parturition. A model for porcine nest building has been established, in which exogenously administered prostaglandin (PG)F(2alpha) is used to induce nesting behaviour in cyclic, pseudopregnant and pregnant pigs. This experiment was designed to examine the effect of PGF(2alpha) on the preferences of non-pregnant gilts for pens bedded with straw compared with bare pens. Ten 6-month-old nulliparous female pigs (gilts) were tested in an arena, which consisted of four pens (1.8mx1.7m), a neutral area (1.5mx3.4m) and a start area (1.5mx3.4m). Two of the pens contained 2kg of fresh straw and the remainder of the testing arena was devoid of straw. On the first day of testing half of the pigs were given a control intramuscular injection of 3ml 0.9% saline and the remainder were given an intramuscular injection of 15mg PGF(2alpha) and their behaviour scored for 1h after treatment. On the following day the treatments were reversed, such that each pig was given both treatments (saline or PGF(2alpha)). There was no significant effect of the order of treatment on behaviour. After saline-treatment the pigs spent most of their time in the pens containing straw (59%) and the least amount of time in bare pens (5%). In the straw pens, saline-treatment induced bouts of oronasal contact with straw of a relatively long duration (11-100s), which we interpret as foraging. In the hour after PGF(2alpha)-treatment the pigs also spent most of their time in the pens containing straw (44%) and the least amount of time in bare pens (10%), but they interacted with the straw in a markedly different way. PGF(2alpha)-treated pigs displayed bouts of oronasal contact with straw of a relatively short duration (2-10s) which, together with high frequencies of pawing at straw, lifting and carrying straw in the mouth, we interpret as nest building behaviour. Superimposed on this is the finding that gilts spend more time in the neutral areas after PGF(2alpha)-treatment than they did after saline-treatment. PGF(2alpha)-treated pigs spent most of their time engaged in nesting behaviour within the straw pens but they also gathered and deposited straw in different areas of the test arena (neutral and start areas); behaviours not seen after saline-treatment. We conclude that pigs generally prefer a pen containing straw bedding to a bare pen but that PGF(2alpha) alters the way they interact with straw, inducing behaviour similar to prepartum nest building.

Reduced pain tolerance during and after pregnancy in women suffering from fear of labor

Pain tolerance in women suffering and not suffering from fear of labor during and after pregnancy were compared. Twenty women with labor fear and 20 control women were subjected to a cold pressor test (CPT) on average 1 month before delivery and 9 months later. Half the volunteers were nulliparous and half parous. Pain endurance time (PET) and intensity of pain (Visual Analogue Scale, VAS) during CPT was assessed. Patients in the fear group tolerated CPT for a significantly shorter time than did women without fear both in pregnancy (154.8+/-109.9 s vs. 282.5+/-60.1 s (mean+/-SD), P<0.001), and in the postpartum period (128.6+/-111.7 s vs. 279.6 +/-60.3 s, P=0.002). Those with labor fear experienced CPT as significantly more painful than did the controls both during pregnancy (VAS 6.68+/-1.9 vs. 3.78+/-2.0 (mean+/-SD), P<0.001) and after (VAS 7.73+/-1.5 vs. 5.92+/-3.0, P=0.04). PET and VAS values correlated during pregnancy (r=-0.62, P<0.001), but not after pregnancy (r=-0.30, not significant). Parity was not associated with either PET or VAS scores. As a rule, pain in all women during CPT was regarded to be lower during pregnancy than after pregnancy (VAS 4.87+/-2.4 vs. 6.60+/-2.6, P=0.001). Patients with fear of labor were characterized by pain intolerance also in circumstances other than labor. This fact may indicate enhanced sensitivity to pain-causing mechanisms in women who develop fear of labor.

Effects of human placental extract on chemical and thermal nociception in mice

Several reports indicate that pregnancy and parturition are associated with elevated maternal pain thresholds to noxious stimuli. The objective of this study was to examine whether the human placental extract, a clinically used preparation, can inhibit experimental nociception. Nociception was assessed in mice using acetic acid-induced writhing and hot-plate tests. The human placental extract (200 and 400 mg/kg, i.p.) elicited dose-related antinociception in the acetic acid-induced writhing test. Furthermore, it (200 mg/kg, i.p.) potentiated the morphine-induced antinociception (1.25 mg/kg, s.c.). In the hot-plate test, the human placental extract (100, 200 and 400 mg/kg, i.p.) per se, displayed no significant antinociception but potentiated the duration of morphine (10 mg/kg, s.c.) analgesia. The potentiation by the extract of the morphine-induced antinociception in both acetic acid and hot-plate tests was, however, found to be naloxone sensitive. Mice treated with the extract (400 mg/kg, i.p.) neither manifested any overt behavioural change in the open-field test nor demonstrated significant influence on pentobarbital sleeping time, suggesting that it has no central depressant or sedative activity. The data provide evidence to show that the human placental extract has a peripheral analgesic property possibly mediated by an opioid mechanism.

Sex-related hormonal influences on pain and analgesic responses

Considerable evidence indicates sex-related differences in pain responses and in the effectiveness of various analgesic agents. Specifically, females are at greater risk for experiencing many forms of clinical pain and are more sensitive to experimentally induced pain relative to males. Regarding analgesic responses, nonhuman animal studies indicate greater opioid analgesia for males, while a limited human literature suggests the opposite. Though the mechanisms underlying these effects remain unclear, the influence of gonadal hormones on nociceptive processing represents one plausible pathway whereby such sex differences could emerge. The present article reviews the complex literature concerning sex steroid effects on pain responses and analgesia. First, nonhuman animal research related to hormonal effects on nociceptive sensitivity and analgesic responses is presented. Next, human studies regarding gonadal hormonal influences on experimental pain responses are reviewed. Several potential mechanisms underlying hormonal effects on nociceptive processing are discussed, including hormonal effects to both peripheral and central nervous system pathways involved in pain transmission. Finally, based on these findings we draw several conclusions and make specific recommendations that will guide future research as it attempts to elucidate the magnitude and importance of sex-related hormonal effects on the experience of pain.