Wendy F. Sternberg

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.

Sex differences in thermal nociception and morphine antinociception in rodents depend on genotype

It has been appreciated for some time that the sexes can differ in their sensitivity to pain and its inhibition. Both the human and rodent literatures remain quite contentious, with many investigators failing to observe sex differences that others document clearly. Recent data from our laboratory have pointed to an interaction between sex and genotype in rodents, such that sex differences are observed in some strains but not others. However, these studies employed inbred mouse strains and are thus not directly relevant to existing data. We presently examined whether the observation of statistically significant sex differences in nociception and morphine antinociception might depend on the particular outbred rodent population chosen for study. Rats of both sexes and three common outbred strains were obtained from three suppliers (Long Evans, Simonsen; Sprague Dawley, Harlan; Wistar Kyoto, Taconic) and tested for nociceptive sensitivity on the 49 degrees C tail-withdrawal assay, and antinociception following morphine (1-10mg/kg, i.p.). In further studies, three outbred populations of mice (CD-1, Harlan; Swiss Webster, Harlan; Swiss Webster, Simonsen) were bred in our vivarium for several generations and tested for tail-withdrawal sensitivity and morphine antinociception (1-20male, and no significant difference. In a separate study in which the estrous cycle was tracked in female mice, we found evidence for an interaction between genotype and estrous phase relevant to morphine antinociception. However, estrous cyclicity did not explain the observed sex differences. These data are discussed with respect to the existing sex difference and pain literature, and also as they pertain to future investigations of these phenomena.

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

&NA; 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°C and 20°C water. SSIA resulting from 15°C swim was previously shown to be naloxone‐insensitive (i.e., non‐opioid) whereas SSIA resulting from 20°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°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 &mgr;g/day, i.p.) administered to ovariectomized mice over a 6–8 day period reinstated the dizocilpine‐insensitivity of 15°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. These findings suggest the existence of a novel female‐specific estrogen‐dependent mechanism of SSIA and highlight the need to consider gender as a factor in basic and clinical research in this area.

The NMDA receptor antagonist MK-801 prevents long-lasting non-associative morphine tolerance in the rat

Several studies have demonstrated that the N-methyl-D-aspartate (NMDA) antagonist MK-801 attenuates the development of morphine tolerance and withdrawal. These studies employed repeated morphine injections to induce tolerance, a procedure in which learning has been suggested to play a significant role in tolerance development. MK-801 has been reported to block some types of learning, and it is unclear, therefore, whether the effect of MK-801 on tolerance development is due to its antagonism of associative (learning) or non-associative factors. Moreover, previous studies have tested the effects of MK-801 on morphine tolerance only up to 48 h after its induction; yet morphine tolerance can persist for many months, and it is not known whether MK-801 can block long-lasting tolerance. In the present study, therefore, we adopted a model of morphine tolerance in which the involvement of learning is minimized by using a single injection of morphine in a sustained-release preparation, and we tested tolerance for up to 56 days. In the first experiment, simultaneously administering MK-801 (0.2 mg/kg, s.c.) and morphine (60 mg/kg, s.c.), each in a sustained-release preparation, abolished tolerance that lasted at least 12 days. Analgesia was measured in the hot-plate test following a test dose of morphine (15 mg/kg, i.p.). In the second experiment, delivering MK-801 and morphine as before, the duration of morphine-induced catalepsy and analgesia was prolonged. Nevertheless, 24 h later one symptom of naloxone-precipitated withdrawal was significantly attenuated in these same animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Opioid and Nonopioid Swim Stress-Induced Analgesia: A Parametric Analysis in Mice

Environmental stress causes the activation of two types of endogenous pain inhibitory systems in animals: opioid analgesia is antagonized by opiate receptor blockers (e.g., naloxone and naltrexone), whereas analgesia produced by nonopioid systems is insensitive to such antagonism. A large literature documents that the parameters of the laboratory stressor will determine the neurochemical identity of the resultant analgesia. In rats, low severity stressors produce opioid analgesia and higher severity stressors produce nonopioid analgesia. A recent parametric analysis of swim stress-induced analgesia (SSIA) in the female Quackenbush mouse, however, observed the opposite pattern. The present study is a parametric analysis of SSIA using a range of swim temperatures (15-38 degrees C), swim durations (45 s to 7 min), and genetic models [male Swiss-Webster mice, and mice selectively bred from this outbred strain for high (HA), low (LA), or control SSIA]. We find that in nonselected mice low severity swims (i.e., warm temperature, short duration) produce naloxone-sensitive opioid SSIA, whereas high severity swims (i.e., cold temperature, long duration) produce nonopioid SSIA. This pattern is also seen in HA mice displaying very high analgesic magnitudes, but not in LA mice displaying minimal SSIA. In the selectively bred mice, analgesia and hypothermia from forced swimming are positively correlated, but can be dissociated both genetically and neurochemically. Furthermore, swimming in body temperature (38 degrees C) water produces analgesia without concommitant hypothermia, and the increased magnitude of 38 degrees C SSIA displayed by HA mice over control levels is entirely opioid.

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.

N-methyl-D-aspartic acid (NMDA) receptor antagonist MK-801 blocks non-opioid stress induced analgesia. II. Comparison across three swim stress paradigms in selectively bred mice.

The effects of the specific N-methyl-D-aspartic acid (NMDA) receptor antagonist MK-801 (dizocilpine, 0.075 mg/kg, i.p.) on swim-stress-induced analgesia (SSIA) were studied in control (C) mice and in mice selectively bred for high (HA) or low (LA) SSIA. In three consecutive experiments, animals were subjected to forced swimming at water temperature of 20 degrees C, 32 degrees C and 15 degrees C and the resulting analgesia (hot-plate test) was found to be mixed opioid/non-opioid, opioid and non-opioid, respectively, as a function of the degree of antagonism by naloxone (10 mg/kg, i.p.). The major finding of this study is that MK-801 attenuated 15 degrees C SSIA, against which naloxone was ineffective, but had no effect on 32 degrees C SSIA, which naloxone blocked completely. A combination of naloxone and MK-801 significantly attenuated 20 degrees C SSIA in C and HA mice and in HA mice this attenuation was significantly larger than that produced by either drug alone. Morphine analgesia (10 mg/kg, i.p.) was unaffected by MK-801. It is concluded that low doses of MK-801 selectively block non-opioid mechanisms of SSIA.

Reducing Social Stress Elicits Emotional Contagion of Pain in Mouse and Human Strangers.

Empathy for anothers 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.

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

&NA; 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°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.

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 °C), tail‐withdrawal (TW; 50 °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.