Nociception and antinociception during the first week of life in mice: Sex differences and test dependence

Influences of Gender on Intravenous Nalbuphine Actions After Major Abdominal Surgery: A Multicenter Study

INTRODUCTION

Nalbuphine, a synthetic kappa-opioid receptor (KOR) agonist and a partial µ-opioid receptor (MOR) antagonist, has been used for years as an effective analgesic. It has been shown to have a better safety profile than morphine. Considering the long history of use of this drug, it is interesting that only a limited amount of information exists on how gender differences influence nalbuphine responses. In this randomized double-blind comparative trial after major abdominal surgery, the analgesic effects of two doses of continuous intravenous infusion of nalbuphine were evaluated based on gender.

METHODS

Enrolled patients were divided into four groups (two females and two males with 32 patients in each group). Two of them (groups A1 and A2), one male and one female, received postoperative continuous intravenous infusions of nalbuphine at 2 mg/h via patient-controlled analgesia (PCA). Each patient had the potential of receiving a rescue bolus of 1 mg of nalbuphine with a lock out time of 15 min. The other two groups (groups B1 and B2) received half the infusion dose, 1 mg/h, and half the nalbuphine rescue dose with the PCA pump, 0.5 mg maximum every 15 min as needed. Patients' vital signs, numerical pain rating scores, rescue nalbuphine, and incidence of side effects were assessed immediately after the operation, and every 3 h during the first 12 h.

RESULTS

Nalbuphine 2 mg/h dosing led to significantly lower pain scores amongst females compared to males at 6, 9, and 12 h; while the 1 mg/h infusion pain scores were only lower at the 9-h time period. Females receiving the nalbuphine 2 mg dose at 6 h, and the 1 mg dose at 6, 9, and 12-h measurements needed significantly lower doses of rescue nalbuphine. Females on the 1 mg dose experienced significantly more nausea, vomiting, and sedation at the 6-, 9-, and 12-h measurement times. In the multivariate analysis, female gender was a negative predictor at all measurement times.

CONCLUSIONS

The current study supports the hypothesis that although nalbuphine was found to be an effective and well-tolerated analgesic after major abdominal surgery, females were statistically more responsive than males.

TRIAL REGISTRATION

The study was registered at the Pan African Clinical trials Registry PACTR201304000486309, and approved for the Ethical aspects.

Methods Used and Application of the Mouse Grimace Scale in Biomedical Research 10 Years on: A Scoping Review

The Mouse Grimace Scale (MGS) was developed 10 years ago as a method for assessing pain through the characterisation of changes in five facial features or action units. The strength of the technique is that it is proposed to be a measure of spontaneous or non-evoked pain. The time is opportune to map all of the research into the MGS, with a particular focus on the methods used and the technique's utility across a range of mouse models. A comprehensive scoping review of the academic literature was performed. A total of 48 articles met our inclusion criteria and were included in this review. The MGS has been employed mainly in the evaluation of acute pain, particularly in the pain and neuroscience research fields. There has, however, been use of the technique in a wide range of fields, and based on limited study it does appear to have utility for pain assessment across a spectrum of animal models. Use of the method allows the detection of pain of a longer duration, up to a month post initial insult. There has been less use of the technique using real-time methods and this is an area in need of further research.

Neonatal morphine exposure and maternal deprivation alter nociceptive response and central biomarkers' levels throughout the life of rats

In the present study, we investigated the effect of repeated neonatal morphine exposure and/or maternal deprivation(MD) on the nociceptive response and central biomarkers' BDNF, IL-1β, and IL-4 levels at postnatal days 16(PND16), 30(PND30), and 60(PND60). At birth, the litters were standardized to contain 8 pups/dam (n = 58). From PND1 to PND10, the pups of the deprived groups were separated daily from their mothers for 3 h and divided into 5 groups: control(C), saline(S), morphine(M), deprived-saline(DS), and deprived-morphine(DM). The pups received subcutaneous injections of saline/morphine (5 μg) in the mid-scapular area between PND8 and PND14. Nociceptive responses were assessed by hot plate(HP) and tail-flick(TFL) tests and biomarker levels by ELISA. Thermal hyperalgesia(HP) was found in all assessments for the M, DS, and DM groups, and a decrease in nociceptive threshold(TFL) was found in the DS group at PND16; M and DM groups at PND30; and M, DS, and DM groups at PND60. There were interactions between treatment/deprivation/timepoint in all central biomarkers' levels. The current study indicates that neonatal exposure to morphine and MD, which occurs in the pediatric ICU, can alter the nociceptive and neuroinflammatory responses.

Adult brain activation in response to pain is changed by neonatal painful stimulation according to sex: A manganese-enhanced MRI study

Although it is known that nociceptive stimulation in the first postnatal week in rats is useful to model preterm pain, resulting in activation of specific brain areas, as assessed in vivo using manganese-enhanced magnetic resonance imaging (MEMRI), little is known about its long-term effects and sex specificity. Here we aimed to investigate whether inflammatory pain induced in male and female adult rats modify the pattern of brain activation between animals subjected or not to neonatal pain. For this, Complete Freund's adjuvant (CFA) was injected into the left hind paw of rat pups on postnatal day 1 (P1) or P8 to induce inflammatory response. During adulthood, CFA-treated and control animals were injected with CFA 1 hr prior MRI. MEMRI has the ability to enhance the contrast of selective brain structures in response to a specific stimulus, as the pain. MEMRI responses were consistent with activation of nociceptive pathways and these responses were reduced in animals treated with CFA on P1, but increased in animals treated on P8, mainly in the female group. In agreement, P8 female group showed exacerbated responses in the thermal nociceptive test. Using MEMRI, we conclude that the natural ability of adult rats to recognize and react to pain exposition is modified by neonatal painful exposition, mainly among females.

The Duration of Nerve Block from Local Anesthetic Formulations in Male and Female Rats

PURPOSE

It is unknown whether there are sex differences in response to free or encapsulated local anesthetics.

METHODS

We examined nerve block duration and toxicity following peripheral nerve blockade in male and female rats. We studied the local anesthetic bupivacaine (free or encapsulated) as well as tetrodotoxin, which acts on a different site of the same voltage-gated channel.

RESULTS

Sensory nerve blockade was 158.5 [139-190] minutes (median [interquartile range]) (males) compared to 173 [134-171] minutes (females) (p = 0.702) following bupivacaine injection, N = 8 male, 8 female. Motor nerve blockade was 157 [141-171] minutes (males) compared to 172 [146-320] minutes (females) (p = 0.2786). Micellar bupivacaine (N = 8 male, 8 female) resulted in sensory nerve blockade of 266 [227-320] minutes (males) compared to 285 [239-344] minutes (females) (p = 0.6427). Motor nerve blockade was 264 [251-264] minutes (males) compared to 287 [262-287] minutes (females) (p = 0.3823). Liposomal bupivacaine (N = 8 male, 8 female) resulted in sensory nerve blockade of 240 [207-277] minutes (males) compared to 289 [204-348] minutes (females) (p = 0.1654). Motor nerve blockade was 266 [237-372] minutes (males) compared to 317 [251-356] minutes (females) (p = 0.6671). Following tetrodotoxin injection (N = 12 male,12 female) sensory nerve blockade was 54.8 [5-117] minutes (males) compared to 54 [14-71] minutes (females) (p = 0.6422). Motor nerve blockade was 72 [40-112] minutes (males) compared to 64 [32-143] minutes (females) (p = 0.971).

CONCLUSIONS

We found no statistically significant sex differences associated with the formulations tested. In both sexes, durations of nerve block were similar between micellar and liposomal bupivacaine formulations, despite the micellar formulation containing less drug.

GABAB receptors modulate morphine antinociception: Pharmacological and genetic approaches

Previous studies in our laboratory showed an interaction between the GABAergic and opioid systems involved in the analgesic effect of baclofen (BAC). Furthermore, it is known that sex differences exist regarding various pharmacological responses of morphine (MOR) and they are related to an increased sensitivity to MOR effects in males. The aims of the present study were to evaluate the possible involvement of the GABA_B receptors in the antinociceptive responses induced by MOR (1, 3 and 9 mg/kg, s.c.) administration using both pharmacological (BAC 2 mg/kg, i.p.; and 2-OH-saclofen, SAC 0.3 mg/kg, intra cisterna magna) and genetic approaches (GABA_B1 knockout mice; GABA_B1 KO) in mice of both sexes. In addition, we explored the alterations in c-Fos expression of different brain areas involved in the antinociceptive effect of MOR using both approaches. The pharmacological approach showed a higher dose-dependent antinociceptive effect of MOR in male mice compared to female mice. BAC and SAC pretreatment potentiated and attenuated the antinociceptive effect of MOR, respectively, in both sexes. The genetic approach revealed a dose-dependent antinociceptive effect of MOR in the wild type mice, but not in the GABA_B1 KO mice and no sex differences were observed. Additionally, BAC and SAC pretreatment and the lack of GABA_B1 subunit of the GABA_B receptor prevented the changes observed in c-Fos expression in the cingulate cortex and nucleus accumbens of male mice. Our results suggest that the GABA_B receptors are involved in the MOR antinociceptive effect of both male and female mice.

Redundant and nonredundant organismal functions of EPS15 and EPS15L1

This study unveils a redundant function for the endocytic proteins Eps15 and Eps15L1 in mouse embryo development and erythropoiesis, and a unique nonredundant role for Eps15L1 in the nervous system.

EPS15 and its homologous EPS15L1 are endocytic accessory proteins. Studies in mammalian cell lines suggested that EPS15 and EPS15L1 regulate endocytosis in a redundant manner. However, at the organismal level, it is not known to which extent the functions of the two proteins overlap. Here, by exploiting various constitutive and conditional null mice, we report redundant and nonredundant functions of the two proteins. EPS15L1 displays a unique nonredundant role in the nervous system, whereas both proteins are fundamental during embryo development as shown by the embryonic lethality of -Eps15/Eps15L1-double KO mice. At the cellular level, the major process redundantly regulated by EPS15 and EPS15L1 is the endocytosis of the transferrin receptor, a pathway that sustains the development of red blood cells and controls iron homeostasis. Consequently, hematopoietic-specific conditional Eps15/Eps15L1-double KO mice display traits of microcytic hypochromic anemia, due to a cell-autonomous defect in iron internalization.

Impact of Demographic and Obstetric Factors on Infant Brain Volumes: A Population Neuroscience Study

Individual differences in neuroanatomy are associated with intellectual ability and psychiatric risk. Factors responsible for this variability remain poorly understood. We tested whether 17 major demographic and obstetric variables were associated with individual differences in brain volumes in 756 neonates assessed with MRI. Gestational age at MRI, sex, gestational age at birth, and birthweight were the most significant predictors, explaining 31% to 59% of variance. Unexpectedly, earlier born babies had larger brains than later born babies after adjusting for other predictors. Our results suggest earlier born children experience accelerated brain growth, either as a consequence of the richer sensory environment they experience outside the womb or in response to other factors associated with delivery. In the full sample, maternal and paternal education, maternal ethnicity, maternal smoking, and maternal psychiatric history showed marginal associations with brain volumes, whereas maternal age, paternal age, paternal ethnicity, paternal psychiatric history, and income did not. Effects of parental education and maternal ethnicity are partially mediated by differences in birthweight. Remaining effects may reflect differences in genetic variation or cultural capital. In particular late initiation of prenatal care could negatively impact brain development. Findings could inform public health policy aimed at optimizing child development.

Effects of creatine supplementation on nociception in young male and female mice

BACKGROUND

The objective of this study was to evaluate creatine as an anti-nociceptive compound in an animal model of thermal and inflammatory pain. Creatine has the structural potential to interact with acid-sensing ion channels (ASIC), which have been involved in pain sensation modulation. The hypothesis evaluated in this study was that creatine will interact with ASICs leading to decreased nociception.

METHODS

Male and female C57BL/6J mice were fed with either a control diet or the control diet supplemented with creatine (6.25 g/kg diet). After one week on the diet, the mice were tested for thermal hyperalgesia and inflammatory pain response.

RESULTS

The latency to withdraw the tail during the thermal hyperalgesia test was unaffected by sex or diet. During the formalin test, males and females responded differently to the stimulus, and the female mice supplemented with creatine seemed to recover faster than the controls. To determine whether ASICs mediate the action of creatine, GMQ, an ASIC3 agonist, was injected in one paw and pain response was quantified. Females responded more strongly to GMQ injections, and all mice fed creatine had a decreased response to GMQ.

CONCLUSIONS

These preliminary data suggest a potential effect of creatine on inflammation-based nociception that may be mediated via ASIC3. While preliminary, this study warrants further research on the potential of creatine as an analgesic and can serve as a stepping stone for the development of ASIC-based therapeutics.

Interactions between glia, the immune system and pain processes during early development

Pain is a serious problem for infants and children and treatment options are limited. Moreover, infants born prematurely or hospitalized for illness likely have concurrent infection that activates the immune system. It is now recognized that the immune system in general and glia in particular influence neurotransmission and that the neural bases of pain are intimately connected to immune function. We know that injuries that induce pain activate immune function and suppressing the immune system alleviates pain. Despite this advance in our understanding, virtually nothing is known of the role that the immune system plays in pain processing in infants and children, even though pain is a serious clinical issue in pediatric medicine. This brief review summarizes the existing data on immune-neural interactions in infants, providing evidence for the immaturity of these interactions.

In vivo and in vitro comparison of female and male nociceptors

While it is generally accepted that women have lower pain thresholds for diverse forms of noxious stimuli, the mechanistic basis for this sexual dimorphism in nociceptive pain remains to be elucidated. We confirmed, in the rat, that females have lower cutaneous mechanical nociceptive thresholds and established a similar sexual dimorphism in muscle. To determine if a peripheral mechanism underlies this sexual dimorphism in pain threshold, we compared biophysical properties of cultured dorsal root ganglion (DRG) neurons that innervated the gastrocnemius muscle in female and male rats. DRG neurons from female rats, which innervated the gastrocnemius muscle, had a more hyperpolarized resting membrane potential. To determine if this was associated with a higher mechanical nociceptive threshold, in contradiction to our working hypothesis, we compared the function, in vivo, of nociceptive afferents innervating the gastrocnemius muscle in male and female rats. C-fiber nociceptors innervating muscle in female rats had higher mechanical thresholds than those in males. Other response characteristics of these nociceptors were not significantly different. Thus, both in vitro and in vivo electrophysiology experiments support the idea that lower mechanical nociceptive threshold in females may be due to sexual dimorphism in central nervous system mechanisms, a difference large enough to overcome an opposing difference in peripheral pain mechanisms.

PERSPECTIVE

This article unifies in vivo and in vitro electrophysiology with behavioral data examining the differences in mechanical nociceptive threshold between male and female rats. The data provide a novel perspective on the peripheral and behavioral outcomes of noxious mechanical stimulation.

The impact of prenatal stress on basal nociception and evoked responses to tail-docking and inflammatory challenge in juvenile pigs

The consequences of tail-docking (at 2-4 days) and prenatal stress (maternal social stress during the 2nd third of pregnancy) on baseline nociceptive thresholds and responses to acute inflammatory challenge were investigated in juvenile pigs in two studies. Nociceptive thresholds were assessed on the tail root and on the hind foot using noxious mechanical and cold stimulation before and after acute inflammatory challenge by intradermal injection of 30 μg capsaicin (study 1) or 3% carrageenan (study 2) into the tail root. Four groups of 8 (study 1, n=14-16 pigs/treatment) or 5 (study 2, n=6 pigs/treatment/sex) week-old pigs were exposed to the main factors: maternal stress and treatment (docked vs. intact tails). In study 1, tail docking did not significantly alter thresholds to noxious mechanical stimulation, whilst prenatally stressed pigs had significantly higher baseline thresholds to noxious mechanical stimulation on the tail root and on the hind foot than unstressed pigs, whether tail-docked or intact. Capsaicin injection induced localised mechanical allodynia around the tail root in all treatment groups, but had no effect on noxious plantar mechanical responses; however prenatally stressed offspring exhibited significantly attenuated response thresholds to capsaicin compared to controls. In study 2 tail docking did not alter thresholds to either mechanical or noxious cold stimulation. Baseline response durations to noxious cold stimulation of the tail root were significantly shorter in both sexes of prenatally stressed pigs, whilst male but not female prenatally stressed pigs exhibited significantly higher baseline thresholds to mechanical stimulation than controls, although results in female pigs tended towards significance. Carrageenan injection into the tail root induced localised mechanical and cold allodynia in all treatment groups, effects that were attenuated in prenatally stressed pigs. Collectively, these findings indicate that prenatal stress can induce long-term alterations in nociceptive responses, manifest as a reduced sensitivity to noxious mechanical and cold stimulation and evoked inflammatory allodynia. Neonatal tail-docking does not lead to long-term alterations in nociception in pigs.

Sexual differentiation of rat reproductive versus opioid antinociceptive systems

BACKGROUND

It has been suggested that sexual differentiation of opioid analgesic sensitivity may parallel sexual differentiation in reproductive systems.

OBJECTIVE

The present study compared organizational and activational roles of testosterone in sexual differentiation in reproductive versus opioid antinociceptive systems in the rat, to assess whether both systems were similarly testosterone dependent.

METHODS

Male rat pups (Sprague-Dawley and Fisher 344 [F344]) were either handled or castrated on postnatal day (PND) 1, and female pups were injected with testosterone propionate (100 or 1000 microg) on PND 2. In adulthood, all rats were gonadectomized (or simply anesthetized) and implanted with either testosterone filled or blank capsules (one 10-mm capsule/100 g of body weight).

RESULTS

Two hundred one Sprague-Dawley rats and 178 F344 rats were used. In gonadally intact adults of both rat strains, the antinociceptive potency of subcutaneously injected morphine was significantly greater in males than in females (P < or = 0.05). These sex differences were eliminated by neonatal castration in males or by neonatal androgenization in females. However, adult testosterone treatment reversed the effects of neonatal castration in males. Masculinization and defeminization of sexual behavior, ovary weight, and body weight generally met conventional expectations. Compared with male controls, neonatally castrated males gained less body weight, and displayed more lordosis behavior and compromised male sexual behaviors. Compared with female controls, neonatally androgenized females gained more body weight, developed smaller ovaries, and presented less lordosis behavior and more male sexual behaviors. Overall, neonatal testosterone manipulations sufficient to masculinize or defeminize rats in terms of reproductive behavior and physiology also masculinized or defeminized morphine antinociceptive sensitivity. The effects of neonatal castration were reversed by adult testosterone treatment, indicating that sexual differentiation of opioid antinociceptive systems begins before PND 1.

CONCLUSIONS

Sensitivity to opioid antinociception begins to diverge between males and females early in life. The relationship between gonadal hormone-mediated sexual differentiation of the reproductive and the opioid antinociceptive systems suggests that the 2 systems may be functionally linked. This finding has implications for the treatment of pain and analgesia in women and men.

Prenatal endotoxin exposure alters behavioural pain responses to lipopolysaccharide in adult offspring

Evidence suggests that exposure to bacterial endotoxin in early life can alter the production of pro-inflammatory cytokines in later life. This phenomenon may have significant consequences for pain and pain related behaviours as pro-inflammatory cytokines heighten pain sensitivity. This association has yet to be examined. As such, the aim of the present study was to characterize pain behaviours in adult rat offspring following prenatal endotoxin (PE) exposure. Pregnant F344 rats received endotoxin (200microg/kg, s.c.) or saline on gestational days 16, 18 and 20. Pain thresholds were assessed in the adult PE offspring (n=23) and control offspring (n=24) prior to and 4h following administration of lipopolysaccharide (LPS; 100microg/kg, s.c.). Three assays of pain were employed - the hot plate, tail immersion and von Frey tests. Results demonstrated sex-specific effects of prenatal endotoxin on the offspring, with PE males displaying unaltered pain thresholds on the von Frey test post-LPS administration (p<0.01), while male control offspring (n=24) displayed the expected hyperalgesia. Male PE offspring also displayed increased pain thresholds on the tail immersion test (p<0.01), while no change in pain sensitivity was observed in control males following LPS exposure. No difference in response was observed between the female PE and control offspring on the von Frey test, however PE female offspring displayed increased thresholds on the tail immersion test compared to baseline - an effect not observed in the control female offspring. Pain sensitivity on the hot plate test was unaffected by prenatal exposure to endotoxin. These data suggest that prenatal exposure to products associated with bacterial infection have the capacity to alter pain responses, which are evident in the adult offspring.

Analysis of physiological and behavioural parameters in mice after toe clipping as newborns

In this study, we have investigated the short- and long-term impact of toe clipping, a commonly used method for marking and simultaneously taking biopsies of pups, which is controversially discussed because of its potentially negative impact on animals. Furthermore, we have analysed animal welfare aspects such as health, behaviour, development, stress and detrimental effects in young animals and in adults after toe clipping at postnatal days 3 (P3) and 7 (P7). Our findings indicate that for both P3 and P7 pups amputations at the second phalange of one toe of each paw do not have any negative effects on growth and physical development and that the clipped pups do not suffer from rejection by their mother. Our data indicate that even though at both ages no abnormalities have been detected in histology, clipping at P7 is the preferable age for an adequate marking mostly because of the small size of the toes at P3. This was also confirmed by grip tests at the age of 12 weeks where P3 animals had lower grip strength than control animals, whereas P7 pups did not show any impairment. Hotplate tests indicated that toe clipping performed at P3 and P7 did not cause hyperalgesia at the amputation stump. Serum corticosterone analysis directly performed on P7 pups after clipping indicated that major stress was provoked mainly through the handling and not because of the clipping itself. Taken together, these data lead to the conclusion that toe clipping is from a morphological, physiological and welfare point of view an acceptable method for marking and genotyping newborn mice.

Sex, gender, and pain: an overview of a complex field

Traditionally, biomedical research in the field of pain has been conducted with male animals and subjects. Over the past 20-30 yr, it has been increasingly recognized that this narrow approach has missed an important variable: sex. An ever-increasing number of studies have established sex differences in response to pain and analgesics. These studies have demonstrated that the differences between the sexes appear to have a biological and psychological basis. We will provide brief review of the epidemiology, rodent, and human experimental findings. The controversies and widespread disagreement in the literature highlight the need for a progressive approach to the questions involving collaborative efforts between those trained in the basic and clinical biomedical sciences and those in the epidemiological and social sciences. In order for patients suffering from acute and/or chronic pain to benefit from this work, the approach has to involve the use or development of clinically relevant models of nociception or pain to answer the basic, but complex, question. The present state of the literature allows no translation of the work to our clinical decision-making.

Sex chromosome complement affects nociception and analgesia in newborn mice

In animal studies of nociception, females are often more sensitive to painful stimuli, whereas males are often more sensitive to analgesia induced by mu-agonists. Sex differences are found even at birth, and in adulthood are likely caused, at least in part, by differences in levels of gonadal hormones. In this report, we investigate nociception and analgesia in neonatal mice and assess the contribution of the direct action of sex chromosome genes in hotplate and tail withdrawal tests. We used the 4 core genotypes mouse model, in which gonadal sex is independent of the complement of sex chromosomes (XX vs XY). Mice were tested at baseline and then injected with mu-opioid agonist morphine (10 mg/kg) or with the kappa-opioid agonist U50,488H (U50, 12.5 mg/kg) with or without the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (0.1 mg/kg). On the day of birth, XX mice showed faster baseline latencies than XY in tail withdrawal, irrespective of their gonadal type. Gonadal males showed greater effects of morphine than gonadal females in the hotplate test, irrespective of their sex chromosome complement. U50 and morphine were effective analgesics in both tests, but MK-801 did not block the U50 effect. The results suggest that sex chromosome complement and gonadal secretions both contribute to sex differences in nociception and analgesia by the day of birth.

PERSPECTIVE

Sex differences in pain may stem not only from the action of gonadal hormones on pain circuits but from the sex-specific action of X and Y genes. Identification of sex chromosome genes causing sex differences could contribute to better pain therapy in females and males.

Studying sex and gender differences in pain and analgesia: a consensus report

In September 2006, members of the Sex, Gender and Pain Special Interest Group of the International Association for the Study of Pain met to discuss the following: (1) what is known about sex and gender differences in pain and analgesia; (2) what are the "best practice" guidelines for pain research with respect to sex and gender; and (3) what are the crucial questions to address in the near future? The resulting consensus presented herein includes input from basic science, clinical and psychosocial pain researchers, as well as from recognized experts in sexual differentiation and reproductive endocrinology. We intend this document to serve as a utilitarian and thought-provoking guide for future research on sex and gender differences in pain and analgesia, both for those currently working in this field as well as those still wondering, "Do I really need to study females?"

Sex differences in formalin-induced pain in prenatally stressed infant rats

The aim of this work was to study the effects of prenatal stress on nociceptive responses in the formalin test in female and male infant (7-day-old) Long-Evans hooded rats. Prenatally stressed infant rats displayed biphasic flinching+ shaking behavior whereas non-stressed animals showed only a weak second phase. Pain sensitivity in prenatally stressed males was significantly greater than that of prenatally non-stressed males during the second phase only; there were no differences in pain sensitivity between prenatally stressed and non-stressed females. Moreover prenatally stressed male rats pups demonstrated that the second phase of the response to formalin was enhanced relative to the second phase in stressed females. The current and previous data [Butkevich IP, Barr GA, Mikhailenko VA, Otellin VA. Increased formalin-induced pain and expression of fos neurons in the lumbar spinal cord of prenatally stressed infants rats. Neurosci Lett 2006a;403:222-226] show increased tonic pain in prenatally stressed infant rats and a large increase in the number of formalin-induced fos-like immunoreactivity in the spinal cord dorsal horn. There is a concomitant decrease in serotonin-like immunoreactivity in the lumbar spinal cord dorsal horn [Butkevich IP, Barr GA, Otellin VA. Effect of prenatal stress on behavioral and neural indices of formalin-induced pain in infant rats. Abstracts, 35th Annual Meeting of Soc. For Neurosci. 2005a. Program No. 512.4 Washington, DC: Society for Neuroscience]. Given the decreased level of perinatal testosterone in prenatally stressed rats to which infant males are more sensitive than females, we suggest that these hormonal, behavioral and neuronal indices are strongly interrelated in prenatally stressed 7-day-old rat pups and that the decreased surge of testosterone may contribute to the increased behavioral response in the second phase in male rat pups. Mechanisms underlying the behavioral pain response induced by inflammation in prenatally stressed rat pups are characterized by sexual dimorphism even prior to the activational effects of sex hormones.

Acute and chronic morphine alters formalin pain in neonatal rats

The present study tested the hypothesis that morphine exposure during the human developmental equivalent of the third trimester would alter inflammatory pain. This study examined whether acute or continuous opioid exposure in the neonatal rat alters formalin-induced nociception after 4 days of abstinence. Rats were exposed to a single acute administration of morphine on postnatal day 7 or 72 h of opioid infusion from postnatal days 5-7 via osmotic pump. When challenged with intraplantar formalin on postnatal day 11, rats exposed to acute or chronic morphine had increased phase II pain-associated behaviors. These findings suggest that neonatal morphine exposure may have unintended consequences on inflammatory pain.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Long-term effects of neonatal surgery on adulthood pain behavior

The long-term consequences of neonatal noxious stimulation on adulthood pain behavior were investigated in male and female mice. On the day of birth, mouse pups were exposed to a laparotomy under cold anesthesia followed by an analgesic dose of morphine (10 mg/kg) post-operatively, or a saline control. An additional group of subjects was exposed to the non-noxious aspects of the surgical procedure (cold exposure, separation from the dam, injection) comprising a 'sham' surgery control group, whereas another group of control subjects was administered an injection of saline or morphine, but was otherwise undisturbed. Behavioral observations of the pups immediately following the procedure indicated that the laparotomy produced increased distress vocalizations in the ultrasonic range (40 kHz) compared to both groups of control subjects. During 90 min observations periods following the surgery and 1-week later, maternal care did not vary among treatment conditions. In adulthood, offspring were tested for nociceptive sensitivity on the hot-plate (HP; 53 degrees C), tail-withdrawal (TW; 50 degrees C) and acetic acid abdominal constriction test (AC). On both the TW and the AC tests, neonatal surgery decreased pain behavior relative to both groups of control subjects, an effect that was reversed by post-operative morphine treatment. On the HP test, both groups of subjects exposed to the stressful aspects of neonatal surgery (laparotomy or sham surgery) exhibited decreased pain behavior in adulthood. These findings suggest that early exposure to noxious and/or stressful stimuli may induce long-lasting changes in pain behavior, perhaps mediated by alterations in the stress-axis and antinociceptive circuitry.