Reducing social stress elicits emotional contagion of pain in mouse and human strangers.

Empathy for another's physical pain has been demonstrated in humans [1] and mice [2]; in both species, empathy is stronger between familiars. Stress levels in stranger dyads are higher than in cagemate dyads or isolated mice [2, 3], suggesting that stress might be responsible for the absence of empathy for the pain of strangers. We show here that blockade of glucocorticoid synthesis or receptors for adrenal stress hormones elicits the expression of emotional contagion (a form of empathy) in strangers of both species. Mice and undergraduates were tested for sensitivity to noxious stimulation alone and/or together (dyads). In familiar, but not stranger, pairs, dyadic testing was associated with increased pain behaviors or ratings compared to isolated testing. Pharmacological blockade of glucocorticoid synthesis or glucocorticoid and mineralocorticoid receptors enabled the expression of emotional contagion of pain in mouse and human stranger dyads, as did a shared gaming experience (the video game Rock Band) in human strangers. Our results demonstrate that emotional contagion is prevented, in an evolutionarily conserved manner, by the stress of a social interaction with an unfamiliar conspecific and can be evoked by blocking the endocrine stress response.

Olfactory exposure to males, including men, causes stress and related analgesia in rodents.

We found that exposure of mice and rats to male but not female experimenters produces pain inhibition. Male-related stimuli induced a robust physiological stress response that results in stress-induced analgesia. This effect could be replicated with T-shirts worn by men, bedding material from gonadally intact and unfamiliar male mammals, and presentation of compounds secreted from the human axilla. Experimenter sex can thus affect apparent baseline responses in behavioral testing.

Pain sensitivity and vasopressin analgesia are mediated by a gene-sex-environment interaction.

Quantitative trait locus mapping of chemical/inflammatory pain in the mouse identified the Avpr1a gene, which encodes the vasopressin-1A receptor (V1AR), as being responsible for strain-dependent pain sensitivity to formalin and capsaicin. A genetic association study in humans revealed the influence of a single nucleotide polymorphism (rs10877969) in AVPR1A on capsaicin pain levels, but only in male subjects reporting stress at the time of testing. The analgesic efficacy of the vasopressin analog desmopressin revealed a similar interaction between the drug and acute stress, as desmopressin inhibition of capsaicin pain was only observed in nonstressed subjects. Additional experiments in mice confirmed the male-specific interaction of V1AR and stress, leading to the conclusion that vasopressin activates endogenous analgesia mechanisms unless they have already been activated by stress. These findings represent, to the best of our knowledge, the first explicit demonstration of analgesic efficacy depending on the emotional state of the recipient, and illustrate the heuristic power of a bench-to-bedside-to-bench translational strategy.

Varying perceived social threat modulates pain behavior in male mice.

UNLABELLED: We previously demonstrated that male mice display significantly reduced pain behavior on the acetic acid abdominal constriction test when confined in close proximity to a stranger male mouse. We show here the testosterone-dependence (via castration and testosterone propionate replacement) of this phenomenon, likely a form of (social) stress-induced analgesia. However, when similar male dyads are separated by vertical metal bars, allowing only partial physical contact, we find that the mice exhibit hyperalgesia, not analgesia, in response to both acetic acid injection and noxious radiant heat, relative to testing in isolation. This finding is specific to same-sex male dyads, and no change in nociceptive sensitivity is observed when males are tested in the presence of a female conspecific. We propose that pain sensitivity varies with respect to the severity of the social threat: mild social threat produces hyperalgesia and more severe social threat produces analgesia. PERSPECTIVE: This work highlights the importance of social threat in modulating pain behavior in a sex-specific manner. The findings add to a growing body of evidence that social factors affect pain behavior in mice, thus allowing the study of the mechanistic underpinnings of social modulation of pain in humans.

Progesterone rapidly recruits female-typical opioid-induced hyperalgesic mechanisms.

Continuous morphine treatment can paradoxically increase nociception (i.e. hyperalgesia) in male and female mice, but sex differences have been reported. Here, we studied progesterone modulation of these differences by assessing nociception on the tail-withdrawal test in male and female mice rendered hyperalgesic during continuous infusion of two different morphine doses (1.6 and 40.0mg/kg/24h). Although the lower morphine infusion dose increased nociception in both sexes by infusion Day 4, this hyperalgesia dissipated by Day 6 in males and ovariectomized females, but not gonadally intact females. A single subcutaneous progesterone (0.0016mg/kg) injection to males and ovariectomized females on Day 6 caused hyperalgesia to recur within 30min and to persist for a minimum of 120min. The larger morphine infusion dose also increased nociception in both sexes on Days 4 and 6. However, the NMDA receptor antagonist MK-801 (0.05mg/kg) reversed hyperalgesia in males and ovariectomized females but not gonadally intact females on infusion Day 6. Subcutaneous progesterone (0.0016mg/kg) injection inhibited this reversal in male and ovariectomized female mice but had no effect on nociception in saline-infused mice of either sex. These data confirm our previous findings that male and female mice utilize distinct hyperalgesic mechanisms, and show for the first time that a single progesterone bolus dose can recruit female-typical hyperalgesia in ovariectomized females and males.

Social approach to pain in laboratory mice.

It has been recently demonstrated that pain behavior in the mouse can be modulated by the presence of a conspecific, but what remains unclear is whether such pain behavior can serve the function of soliciting social approach. Using a novel social approach paradigm, we tested mice in various dyadic or triadic conditions, including "jailed" mice-some in pain via intraperitoneal injection of 0.9% acetic acid-and test mice free to approach or avoid the jailed mice. We observed a sex-specific effect whereby female, but not male, test mice approached a familiar same-sex conspecific in pain more frequently than an unaffected familiar or unfamiliar, but affected, conspecific. Despite a substantial literature emphasizing oxytocin's role in affiliative and pair-bonding behavior, this effect was also observed in female mice lacking the oxytocin receptor, suggesting that pain-related social approach may not be mediated by oxytocin. Furthermore, we found that the frequency of contact by the test mouse was negatively correlated with the pain behavior of the jailed mouse, suggesting that proximity of a familiar unaffected conspecific may have analgesic properties.

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.

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

UNLABELLED: This study demonstrates that reliable sex differences in nociceptive and antinociceptive mechanisms are present in even very young subjects. Sex differences were observed in mice tested either on the day of birth or 1 week later on basal tail-flick latency and morphine analgesic magnitude. Female mice had longer tail-withdrawal latencies; male mice demonstrated stronger analgesic responses to morphine. In addition, basal pain behavior and analgesic responsiveness differed between day-old and week-old animals on the hot plate, with day-old mice showing enhanced pain behavior and reduced morphine antinociception compared to week-old subjects. These findings further support the competence of pain processing circuitry in even very young subjects and highlight the early development of nociceptive and antinociceptive mechanisms. PERSPECTIVE: This study highlights the competence of nociceptive circuitry and the analgesic efficacy of morphine as early as the day of birth in mice, reinforcing the importance of evaluating and treating pain in even the youngest subjects. Sex differences were present, suggesting infant sex as one of several potential factors that predict the experience of procedural or pathological pain and analgesic requirement.

Acute progesterone can recruit sex-specific neurochemical mechanisms mediating swim stress-induced and kappa-opioid analgesia in mice.

There is a qualitative sex difference in the neurochemical mediation of stress-induced and kappa-opioid analgesia; these phenomena are dependent on N-methyl-d-aspartic acid (NMDA) receptors in males but not females. Progesterone modulation of this sex difference was examined in mice. Analgesia against thermal nociception was produced by forced cold water swim or by systemic administration of the kappa-opioid agonist, U50,488. As seen previously, the NMDA receptor antagonist MK-801 blocked both forms of analgesia in male but not female mice. Also as in previous studies, this sex difference was found to be dependent on ovarian hormones such that ovariectomy induced female mice to "switch" to the male-like, NMDAergic system. We now demonstrate that a single injection of progesterone (50 microg), systemically administered 30 min before analgesia assessment, is sufficient to restore female-specific mediation of analgesia (i.e., insensitivity to MK-801 blockade) in ovariectomized female mice. The rapidity of this neurochemical "switching" action of progesterone suggests mediation via cell surface receptors or the action of neuroactive steroid metabolites of progesterone.

Qualitative sex differences in kappa-opioid analgesia in mice are dependent on age.

The effects of aging on sex differences in analgesia from the kappa-opioid agonist, U50,488H (U50), were examined in C57BL/6J mice. U50 analgesia can be blocked by the N-methyl-d-aspartate receptor antagonist, MK-801 (MK), in male rodents and gonadectomized females, but not hormonally intact or estrogen-replaced females, suggesting the existence of alternate neurochemical mediation in females. We now report that MK antagonism of U50 analgesia is age-dependent in females. That is, reproductively senescent females display MK-sensitive U50 analgesia qualitatively similar to that displayed by males or hormonally deprived young females. Age-related reductions in U50 analgesic magnitude were also observed in females. Thus, age and gender are likely to alter the clinical efficacy of analgesic drugs active at kappa-opioid receptors.

Effects of gestational stress and neonatal handling on pain, analgesia, and stress behavior of adult mice.

Stressors presented during the late prenatal and early postnatal periods can have long-term effects on offspring behavior, due to the sensitive periods in the formation of brain circuitry associated with early development. This study investigated the long-term effects of prenatal (restraint during the last week of gestation) and postnatal (daily handling for 14 days postnatal) stress, alone and in combination, on adulthood pain behavior, analgesic responses to stress and morphine, and on behavioral indices of stress reactivity. We found that all of the adult responses measured were altered by perinatal manipulations. Nociceptive thresholds were increased by prenatal or by postnatal stress in males and females; application of both stressors in combination negated these effects. Elevations in morphine analgesia were also observed in animals undergoing either perinatal stressor, but not in those who received both stressors. Behavioral and analgesic responses to stress were consistent with previous observations of reduced stress responsiveness following neonatal handling, with some sex-specific findings. Male and female handled subjects exhibited decreases in stress behavior, and both groups of female handled subjects (regardless of prenatal stress [PS] condition) exhibited decreases in stress-induced analgesia (SIA). Males, on the other hand, exhibited decreases in SIA only if they were prenatally stressed (regardless of handling condition). Thus, prenatal and postnatal stressors have differing effects on the neural circuitry underlying pain, pain inhibition, and stress behavior.

Neonatal testosterone exposure influences neurochemistry of non-opioid swim stress-induced analgesia in adult mice.

The effects of neonatal hormone manipulations on swim stress-induced analgesia (SSIA) magnitude and neurochemical quality were examined in Swiss-Webster mice of both sexes. Previous research has indicated that non-opioid SSIA mechanisms in adult Swiss-Webster mice are sexually dimorphic. Male mice exhibit non-opioid SSIA following a 3-min swim in cold (15 degrees C) water that is antagonized by the non-competitive NMDA antagonist MK-801 (dizocilpine; 0.075 mg/kg), whereas female mice do not display NMDA-mediated analgesia in the presence of estrogen. Since male and female mice show equipotent magnitudes of SSIA, it was concluded that female mice display a neurochemically distinct, estrogen-dependent SSIA mechanism specific to their gender. In the present study, female mice exposed to testosterone during the neonatal period display NMDA-mediated analgesia even in the presence of estrogen in adulthood. Thus, expression of the female-specific, estrogen-dependent SSIA mechanism previously described may be dependent on the absence of testosterone during early ontogeny.

One or two genetic loci mediate high opiate analgesia in selectively bred mice.

The analgesic responses of humans and laboratory animals are characterized by substantial individual differences. The genetic basis of this variability can be studied experimentally in rodents using a program of selective breeding. One such program selected for high (HA) and low (LA) swim stress-induced analgesia (SSIA) on the hot-plate (56 degrees C) test in Swiss-Webster mice. These lines, which have been selectively bred for more than 25 generations, display markedly divergent opioid-mediated SSIA (3-min swims in 38 degrees C water), morphine analgesia (10 mg/kg, i.p.), and analgesia to the kappa-receptor agonist, U-50,488H (30 mg/kg, i.p.). The present study investigated the mode of inheritance of these opioid analgesias in HA and LA mice, using Mendelian genetic analyses. We report that the differential sensitivity of HA and LA mice to each of these analgesic manipulations appears to be determined oligogenically, by one or at the most two major genetic loci. The loci associated with each type of analgesia do not co-segregate, however, indicating that three distinct oligogenic effects have been identified. These findings suggest that the genetic determination of analgesic mechanisms may have simple components and as such may be amenable to further analysis using molecular genetic techniques.

Sex differences in the antagonism of swim stress-induced analgesia: effects of gonadectomy and estrogen replacement.

Sex differences in the neurochemical mediation of swim stress-induced analgesia (SSIA) were examined in Swiss-Webster mice. Intact and gonadectomized adult mice of both sexes were tested for their analgesic response (hot-plate test) to 3 min of forced swimming in 15 degrees C and 20 degrees C water. SSIA resulting from 15 degrees C swim was previously shown to be naloxone-insensitive (i.e., non-opioid) whereas SSIA resulting from 20 degrees C swim produced an analgesia that was partially reversible by naloxone (i.e., mixed opioid/non-opioid). The non-opioid components of these SSIA paradigms were attenuated by the N-methyl-D-aspartate (NMDA) receptor antagonist, dizocilpine (MK-801). We now report that in males, but not females, dizocilpine (0.075 mg/kg, i.p.) and naloxone (10 mg/kg, i.p.) antagonized the non-opioid and opioid components of SSIA, respectively. After ovariectomy, females displayed a pattern of antagonism similar to males such that dizocilpine attenuated non-opioid SSIA, although naloxone remained ineffective in antagonizing 20 degrees C SSIA. Thus, SSIA in intact females was neither opioid- nor NMDA-mediated, yet it was of similar magnitude to the SSIA displayed by intact males. In separate experiments, estrogen replacement (estrogen benzoate; 5.0 micrograms/day, i.p.) administered to ovariectomized mice over a 6-8 day period reinstated the dizocilpine-insensitivity of 15 degrees C SSIA characteristic of intact females. However, a similar estrogen regimen administered to both intact and castrated males did not compromise the sensitivity to dizocilpine previously noted in male mice.(ABSTRACT TRUNCATED AT 250 WORDS)