Modulation of gender-specific effects upon swim analgesia in gonadectomized rats

Restraint stress-induced antinociceptive effects in acute pain: Involvement of orexinergic system in the nucleus accumbens

The complicated relevance between stress and pain has been identified. Neurotransmitters and neuropeptides of various brain areas play a role in this communication. Pain inhibitory response is known as stress-induced analgesia (SIA). The studies demonstrated that the nucleus accumbens (NAc) is critical in modulating pain. As a neuropeptide, orexin is crucially involved in initiating behavioral and physiological responses to threatening and unfeeling stimuli. However, the role of the orexin receptors of the NAc area after exposure to restraint stress (RS) as acute physical stress in the modulation of acute pain is unclear. One hundered twenty adult male albino Wistar rats (230-250 g) were used. Animals were unilaterally implanted with cannulae above the NAc. The SB334867 and TCS OX2 29 were used as antagonists for OX1r and OX2r, respectively. Different doses of the antagonists (1, 3, 10, and 30 nmol/0.5 µl DMSO) were microinjected intra-NAc five minutes before exposure to RS (3 hours). Then, the tail-flick test as a model of acute pain was performed, and the nociceptive threshold (Tail-flick latency; TFL) was measured in 60-minute time set intervals. According to this study's findings, the antinociceptive effects of RS in the tail-flick test were blocked during intra-NAc administration of SB334867 or TCS OX2 29. The RS as acute stress increased TFL and deceased pain-like behavior responses. The 50 % effective dose values of the OX1r and OX2r antagonists were 12.82 and 21.64 nmol, respectively. The result demonstrated contribution of the OX1r into the NAc was more remarkable than that of the OX2r on antinociceptive responses induced by the RS. Besides, in the absence of RS, the TFL was attenuated. The current study's data indicated that OX1r and OX2r into the NAc induced pain modulation responses during RS in acute pain. In conclusion, the findings revealed the involvement of intra-NAc orexin receptors in improving SIA.

Restraint stress induced the antinociceptive responses via the dopamine receptors within the hippocampal CA1 area in animal model of persistent inflammatory pain

It has been declared that dopamine receptors within the hippocampal formation are involved in emotion, memory, and pain processing. Remarkably, both CA1 and dentate gyrus (DG) areas of the hippocampal formation are involved in persistent peripheral nociceptive perception. A prior study showed that dopamine receptors within the hippocampal DG have a critical role in antinociception induced by forced swim stress (FSS), as a physical stressor, in the presence of formalin irritation. The present experiments were designed to assess whether dopaminergic receptors within the CA1 have any role in antinociceptive responses induced by restraint stress (RS) as a psychological stressor after applying the formalin test as an animal model of persistent inflammatory pain. The D1- and D2-like dopamine receptor antagonists, SCH23390 and Sulpiride (0.25, 1, and 4 μg/0.5 μl), were injected into the CA1 areas of ninety-six male albino Wistar rats 5 min before a 3-h period of restraint stress. Ten min after stress termination, a 50-μl formalin 2.5 % was subcutaneously injected into the plantar surface of the rat's hind paw to induce persistent inflammatory pain. Nociceptive behaviors in both phases of the formalin test were analyzed in the 5-min blocks for a 60-min period. The obtained results demonstrate that although RS could induce an antinociceptive response in both phases of the formalin test, microinjection of D1- and D2-like dopamine receptors, antagonists attenuated RS-induced analgesia. These results support the hypothesis that acute restraint stress could induce analgesia via dopaminergic projection to the CA1 region of the hippocampal formation.

The stress-induced antinociceptive responses to the persistent inflammatory pain involve the orexin receptors in the nucleus accumbens

Stress suppresses the sense of pain, a physiological phenomenon known as stress-induced analgesia (SIA). Brain orexin peptides regulate many physiological functions, including wakefulness and nociception. The contribution of the orexinergic system within the nucleus accumbens (NAc) in the modulation of antinociception induced by forced swim stress (FSS) remains unclear. The present study addressed the role of intra-accumbal orexin receptors in the antinociceptive responses induced by FSS during the persistent inflammatory pain model in the rat. Stereotaxic surgery was performed unilaterally on 106 adult male Wistar rats weighing 250-305 g. Different doses (1, 3, 10, and 30 nmol/ 0.5 μl DMSO) of orexin-1 receptor (OX1r) antagonist (SB334867) or OX2 receptor antagonist (TCS OX2 29) were administered into the NAc five minutes before exposure to FSS for a 6-min period. The formalin test was carried out using formalin injection (50 μl; 2.5%) into the rat's hind paw plantar surface, which induces biphasic pain-related responses. The first phase begins immediately after formalin infusion and takes 3-5 min. Subsequently, the late phase begins 15-20 min after formalin injection and takes 20-40 min. The findings demonstrated that intra-accumbal microinjection of SB334867 or TCS OX2 29 attenuated the FSS-induced antinociception in both phases of the formalin test, with the TCS OX2 29 showing higher potency. Moreover, the effect of TCS OX2 29 was more significant during the early phase of the formalin test. The results suggest that OX1 and OX2 receptors in the NAc might modulate the antinociceptive responses induced by the FSS.

Interactive Mechanisms of Supraspinal Sites of Opioid Analgesic Action: A Festschrift to Dr. Gavril W. Pasternak

Almost a half century of research has elaborated the discoveries of the central mechanisms governing the analgesic responses of opiates, including their receptors, endogenous peptides, genes and their putative spinal and supraspinal sites of action. One of the central tenets of "gate-control theories of pain" was the activation of descending supraspinal sites by opiate drugs and opioid peptides thereby controlling further noxious input. This review in the Special Issue dedicated to the research of Dr. Gavril Pasternak indicates his contributions to the understanding of supraspinal mediation of opioid analgesic action within the context of the large body of work over this period. This review will examine (a) the relevant supraspinal sites mediating opioid analgesia, (b) the opioid receptor subtypes and opioid peptides involved, (c) supraspinal site analgesic interactions and their underlying neurophysiology, (d) molecular (particularly AS) tools identifying opioid receptor actions, and (e) relevant physiological variables affecting site-specific opioid analgesia. This review will build on classic initial studies, specify the contributions that Gavril Pasternak and his colleagues did in this specific area, and follow through with studies up to the present.

Neutrophil-Mediated Endogenous Analgesia Contributes to Sex Differences in Oral Cancer Pain

The incidence of oral cancer in the United States is increasing, especially in young people and women. Patients with oral cancer report severe functional pain. Using a patient cohort accrued through the New York University Oral Cancer Center and immune-competent mouse models, we identify a sex difference in the prevalence and severity of oral cancer pain. A neutrophil-mediated endogenous analgesic mechanism is present in male mice with oral cancer. Local naloxone treatment potentiates cancer mediator-induced orofacial nociceptive behavior in male mice only. Tongues from male mice with oral cancer have significantly more infiltrating neutrophils compared to female mice with oral cancer. Neutrophils isolated from the cancer-induced inflammatory microenvironment express beta-endorphin and met-enkephalin. Furthermore, neutrophil depletion results in nociceptive behavior in male mice. These data suggest a role for sex-specific, immune cell-mediated endogenous analgesia in the treatment of oral cancer pain.

Gender differences in acute and chronic pain in the emergency department: results of the 2014 Academic Emergency Medicine consensus conference pain section

Pain is a leading public health problem in the United States, with an annual economic burden of more than $630 billion, and is one of the most common reasons that individuals seek emergency department (ED) care. There is a paucity of data regarding sex differences in the assessment and treatment of acute and chronic pain conditions in the ED. The Academic Emergency Medicine consensus conference convened in Dallas, Texas, in May 2014 to develop a research agenda to address this issue among others related to sex differences in the ED. Prior to the conference, experts and stakeholders from emergency medicine and the pain research field reviewed the current literature and identified eight candidate priority areas. At the conference, these eight areas were reviewed and all eight were ratified using a nominal group technique to build consensus. These priority areas were: 1) gender differences in the pharmacological and nonpharmacological interventions for pain, including differences in opioid tolerance, side effects, or misuse; 2) gender differences in pain severity perceptions, clinically meaningful differences in acute pain, and pain treatment preferences; 3) gender differences in pain outcomes of ED patients across the life span; 4) gender differences in the relationship between acute pain and acute psychological responses; 5) the influence of physician-patient gender differences and characteristics on the assessment and treatment of pain; 6) gender differences in the influence of acute stress and chronic stress on acute pain responses; 7) gender differences in biological mechanisms and molecular pathways mediating acute pain in ED populations; and 8) gender differences in biological mechanisms and molecular pathways mediating chronic pain development after trauma, stress, or acute illness exposure. These areas represent priority areas for future scientific inquiry, and gaining understanding in these will be essential to improving our understanding of sex and gender differences in the assessment and treatment of pain conditions in emergency care settings.

Sex differences in animal models of psychiatric disorders

Psychiatric disorders are characterized by sex differences in their prevalence, symptomatology and treatment response. Animal models have been widely employed for the investigation of the neurobiology of such disorders and the discovery of new treatments. However, mostly male animals have been used in preclinical pharmacological studies. In this review, we highlight the need for the inclusion of both male and female animals in experimental studies aiming at gender-oriented prevention, diagnosis and treatment of psychiatric disorders. We present behavioural findings on sex differences from animal models of depression, anxiety, post-traumatic stress disorder, substance-related disorders, obsessive-compulsive disorder, schizophrenia, bipolar disorder and autism. Moreover, when available, we include studies conducted across different stages of the oestrous cycle. By inspection of the relevant literature, it is obvious that robust sex differences exist in models of all psychiatric disorders. However, many times results are conflicting, and no clear conclusion regarding the direction of sex differences and the effect of the oestrous cycle is drawn. Moreover, there is a lack of considerable amount of studies using psychiatric drugs in both male and female animals, in order to evaluate the differential response between the two sexes. Notably, while in most cases animal models successfully mimic drug response in both sexes, test parameters and treatment-sensitive behavioural indices are not always the same for male and female rodents. Thus, there is an increasing need to validate animal models for both sexes and use standard procedures across different laboratories.

Visceral analgesia induced by acute and repeated water avoidance stress in rats: sex difference in opioid involvement

BACKGROUND

Chronic psychological stress-induced alterations in visceral sensitivity have been predominantly assessed in male rodents. We investigated the effect of acute and repeated water avoidance stress (WAS) on the visceromotor response (VMR) to colorectal distension (CRD) and the role of opioids in male and cycling female Wistar rats using a novel non-invasive manometric technique.

METHODS

After a baseline VMR (1st CRD, day 0), rats were exposed to WAS (1 h day(-1) ) either once or for four consecutive days, without injection or with naloxone (1 mg kg(-1) ) or saline injected subcutaneously before each WAS session.

KEY RESULTS

The VMR to CRD recorded on day 1 or 4 immediately after the last WAS was reduced in both females and males. The visceral analgesia was mainly naloxone-dependent in females, but naloxone-independent in males. In non-injected animals, on days 2 and 5, VMR was not significantly different from baseline in males whereas females exhibited a significant VMR increase at 60 mmHg on day 5. Basal CRD and CRD on days 1, 2, and 5 in both sexes without WAS induced similar VMR.

CONCLUSIONS & INFERENCES

  When monitored non-invasively, psychological stress induces an immediate poststress visceral analgesia mediated by an opiate signaling system in females while naloxone-independent in males, and hyperalgesia at 24 h after repeated stress only in females. These data highlight the importance of sex-specific interventions to modulate visceral pain response to stress.

Stress-induced analgesia

For over 30 years, scientists have been investigating the phenomenon of pain suppression upon exposure to unconditioned or conditioned stressful stimuli, commonly known as stress-induced analgesia. These studies have revealed that individual sensitivity to stress-induced analgesia can vary greatly and that this sensitivity is coupled to many different phenotypes including the degree of opioid sensitivity and startle response. Furthermore, stress-induced analgesia is influenced by age, gender, and prior experience to stressful, painful, or other environmental stimuli. Stress-induced analgesia is mediated by activation of the descending inhibitory pain pathway. Pharmacological and neurochemical studies have demonstrated involvement of a large number of neurotransmitters and neuropeptides. In particular, there are key roles for the endogenous opioid, monoamine, cannabinoid, gamma-aminobutyric acid and glutamate systems. The study of stress-induced analgesia has enhanced our understanding of the fundamental physiology of pain and stress and can be a useful approach for uncovering new therapeutic targets for the treatment of pain and stress-related disorders.

Sex differences in learning processes of classical and operant conditioning

Males and females learn and remember differently at different times in their lives. These differences occur in most species, from invertebrates to humans. We review here sex differences as they occur in laboratory rodent species. We focus on classical and operant conditioning paradigms, including classical eyeblink conditioning, fear-conditioning, active avoidance and conditioned taste aversion. Sex differences have been reported during acquisition, retention and extinction in most of these paradigms. In general, females perform better than males in the classical eyeblink conditioning, in fear-potentiated startle and in most operant conditioning tasks, such as the active avoidance test. However, in the classical fear-conditioning paradigm, in certain lever-pressing paradigms and in the conditioned taste aversion, males outperform females or are more resistant to extinction. Most sex differences in conditioning are dependent on organizational effects of gonadal hormones during early development of the brain, in addition to modulation by activational effects during puberty and adulthood. Critically, sex differences in performance account for some of the reported effects on learning and these are discussed throughout the review. Because so many mental disorders are more prevalent in one sex than the other, it is important to consider sex differences in learning when applying animal models of learning for these disorders. Finally, we discuss how sex differences in learning continue to alter the brain throughout the lifespan. Thus, sex differences in learning are not only mediated by sex differences in the brain, but also contribute to them.

Study of morphine-induced dependence in gonadectomized male and female mice

In this study we evaluated the effects of sex difference and also sex hormones on the naloxone-precipitated morphine withdrawal in both orchidectomized (ORC) male and ovariectomized (OVX) female mice. Morphine (50, 50 and 75 mg/kg/day for 4 days, s.c.) was administered to animals and at 5th day naloxone (4 mg/kg, i.p.)-precipitated morphine withdrawal signs, jumpings and the percentage of weight loss, were measured. There was no significant alteration in withdrawal jumpings between male and female mice, though weight loss was significantly higher in male ones. Jumpings was significantly lower in both OVX and ORC mice and percentage of weight loss was significantly higher in OVX mice than corresponding non-operated or sham animals. In OVX mice, E(2)V (10 mg/kg, s.c.) increased number of jumpings and decreased percentage of weight loss. Progesterone (25 mg/kg, s.c.) had no effect on jumpings, whereas it decreased weight loss in OVX mice. Testosterone (2.5 mg/kg, s.c.) increased jumpings in ORC mice while it had no effect on percentage of weight loss. Our results demonstrated that sex hormones could play a role in the morphine withdrawal syndrome in both ORC male and OVX female 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.

Females do not express learned helplessness like males do

Women are more likely than men to suffer from stress-related mental disorders, such as depression. In the present experiments, we identified sex differences in one of the most common animal models of depression, that of learned helplessness. Male and female rats were trained to escape a mild footshock each day for 7 days (controllable stress). Each rat was yoked to another rat that could not escape (uncontrollable stress), but was exposed to the same amount of shock. One day later, all stressed rats and unstressed controls were tested on a more difficult escape task in a different context. Most males exposed to uncontrollable stress did not learn to escape and were therefore helpless. In contrast, most females did learn to escape on the more difficult escape task, irrespective of whether they had been exposed to controllable or uncontrollable stress. The sex differences in helplessness behavior were not dependent on the presence of sex hormones in adulthood, because neither ovariectomy of females nor castration of males abolished them. The absence of helplessness in females was neither dependent on organizational effects of testosterone during the day of birth, because masculinized females did not express helplessness as adults. Thus, sex differences in helplessness behavior are independent of gonadal hormones in adulthood and testosterone exposure during perinatal development. Learned helplessness may not constitute a valid model for depressive behavior in women, at least as reflected by the response of female rats to operant conditioning procedures after stressful experience.

Genetic influences on behavioral and neuroendocrine responses to predator-odor stress in rats

The exposure of animals to a variety of stressful events can induce behavioral and physiological responses, which can be modulated by anxiety levels. It is well recognized that genetic factors play a substantial role in both anxiety and stress reactivity. The present study examined the effect of exposure to 2,4,5-trimethylthiazoline (TMT), a component of fox feces, on nociception and corticosterone levels in Lewis (LEW) and Spontaneously Hypertensive (SHR) inbred rat strains (which display genetic differences in anxiety models such as the elevated plus-maze and open-field). The influence of two quantitative trait loci (QTL), named Ofil1 and Ofil2, which are known to affect emotionality in LEW versus SHR intercrosses on the responses to TMT was also investigated. LEW and SHR rats of both sexes displayed similar levels of behavioral and neuroendocrine responses after TMT exposure. As expected, TMT odor stress produced analgesia and enhanced corticosterone levels. Ofil1 on chromosome 4 affected stress-induced analgesia in males only. Ofil2 on chromosome 7 had no effect. The results suggest that behaviors measured in classical models of generalized anxiety and reactivity to stress produced by predator odors can be genetically dissociated. Finding a locus with an effect on the behavioral responses to stress represents the starting point in the search for genes responsible for stress-related traits.

Testosterone modulation of reproductive indices vs. morphine antinociception in male rats

The purpose of this study was to determine whether testosterone (T) concurrently modulates reproductive and nociceptive systems in the adult male. Male Sprague-Dawley rats were orchidectomized, and then 28 days later implanted with capsules containing T or nothing (blanks). After 2, 7, 14 or 28 days' exposure to T-filled or blank capsules, rats were tested for male sexual and nociceptive behaviors in a counter-balanced design. As the duration of T exposure lengthened, the percentage of rats showing male sexual behaviors and the weights of steroid-sensitive organs systematically increased, and latencies to show sexual behaviors decreased. T treatment did not affect basal nociception on either the hotplate or tail withdrawal tests, but significantly increased morphine's antinociceptive potency on the tail withdrawal test -- however, this effect was small, and independent of duration of T exposure. Thus, T treatment that altered male sexual behavior and reproductive physiology in a systematic, duration-dependent manner did not similarly alter basal nociception or morphine antinociception. These findings suggest that in adult male rats, although T may modulate both male sexual behaviors and opioid antinociceptive sensitivity, these T effects do not occur in concert.

Neurotensin and pain modulation

Neurotensin (NT) can produce a profound analgesia or enhance pain responses, depending on the circumstances. Recent evidence suggests that this may be due to a dose-dependent recruitment of distinct populations of pain modulatory neurons. NT knockout mice display defects in both basal nociceptive responses and stress-induced analgesia. Stress-induced antinociception is absent in these mice and instead stress induces a hyperalgesic response, suggesting that NT plays a key role in the stress-induced suppression of pain. Cold water swim stress results in increased NT mRNA expression in hypothalamic regions known to project to periaqueductal gray, a key region involved in pain modulation. Thus, stress-induced increases in NT signaling in pain modulatory regions may be responsible for the transition from pain facilitation to analgesia. This review focuses on recent advances that have provided insights into the role of NT in pain modulation.

Strain and sex differences in the expression of nociceptive behavior and stress-induced analgesia in rats

Evidence indicates that genetic, gender, and emotional/attentional aspects modulate the pain sensation. The present study examined the effect of swim-stress on nociceptive responses in Lewis (LEW) and spontaneously hypertensive (SHR) inbred rats (contrasting for anxiety-related behaviors), as well as in Wistar (WIS) rats of both sexes. Furthermore, we explored possible neurochemical mechanisms involved. In addition, we investigated whether habituation in the hot-plate apparatus could modify the hypoalgesic phenotype of SHR. Male and female LEW, SHR, and WIS rats were tested immediately before and 2 min after a 3-min swim in 15 degrees C water. The swim-stress induced analgesia in LEW and WIS, but not in SHR male rats. The same stressor induced analgesia in females of all three strains. In WIS female rats, the stress-induced analgesia (SIA) seems to involve, at least partially, a nonopioid N-methyl-d-aspartate (NMDA) analgesic system. Moreover, five brief exposures (90 s; 10-min intertrial interval) to the unheated hot-plate apparatus completely abolished the differences in basal hot-plate latencies observed in SHR compared with LEW and WIS strains. The present results demonstrate genetic and gender differences in nociceptive sensitivity and in the activation of endogenous analgesic systems in rats and highlight the influence of emotional reactivity. The SHR's hypoalgesic phenotype seems to involve central cognitive processes. Therefore, the LEW and SHR inbred strains may provide an important tool for study of the molecular bases underlying nociception and its modulation and the relationship with emotional/attentional processes.

Sex differences in pain and analgesia: the role of gonadal hormones

There is now strong evidence for sex differences in pain and analgesia. These differences imply that gonadal steroid hormones such as estradiol and testosterone modulate sensitivity to pain and analgesia. The goal of this review is to present an overview of gonadal steroid modulation of pain and analgesia in animals and humans, and to describe mechanisms by which males' and females' biology may differentially predispose them to pain and to analgesic effects of drugs and stress. Evidence is presented to demonstrate that sex differences in pain and analgesia may be both quantitative and qualitative in nature. Current research suggests that sex-specific management of clinical pain will be a reality in the not-so-distant future.

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.

Sex and estrus cycle differences in the modulatory effects of morphine on seizure susceptibility in mice

PURPOSE

To evaluate the effects of sex and estrus cycle on biphasic anticonvulsant and proconvulsant modulation of seizure threshold by morphine.

METHODS

The threshold for the clonic seizures (CST) induced by acute intravenous administration of gamma-aminobutyric acid (GABA)-antagonist pentylenetetrazole (PTZ) was assessed in male and female mice. Estrus cycle was assessed by vaginal smears. The effect of removing circulating sex hormones was assessed by gonadectomy.

RESULTS

At baseline, diestrus females had a higher CST compared with males and estrus females. Morphine at lower doses (0.5-3 mg/kg) had a significant anticonvulsant effect in males and estrus females compared with that in vehicle-treated controls, whereas female mice in diestrus phase showed a relative subsensitivity to this effect. Morphine at higher doses (30 and 60 mg/kg) significantly decreased CST in males and diestrus females, with less relative effect in estrus mice. In both phases, morphine exerted stronger effects in males compared with females. Ovariectomy brought the baseline CST to the male level and resulted in significant expression of both phases of morphine effect but did not abolish the sex difference in responsiveness to morphine.

CONCLUSIONS

The biphasic modulation of seizure threshold is subject to both constitutive sex differences in sensitivity to morphine and hormonal fluctuations during the estrus cycle.

Endogenous neurotensin facilitates visceral nociception and is required for stress-induced antinociception in mice and rats

Central neurotensin (NT) administration can both facilitate and inhibit somatic and visceral nociception, depending on the dose and administration site. NT microinjection in the rostroventral medulla facilitates nociception at low doses, while NT antagonist microinjection can markedly attenuate nociception, supporting the hypothesis that endogenous NT facilitates nociception. However, higher doses of NT produce a mu-opioid receptor-independent analgesia, similar to that resulting from various intense stressors. Furthermore, intense stress results in increased NT expression in several hypothalamic nuclei that have been implicated in stress-induced antinociception (SIAN); however, there is little direct evidence that endogenous NT is required for SIAN. We have investigated the role of endogenous NT in both basal visceral nociception and SIAN using both NT knockout mice and pharmacological approaches in rats. Visceral nociception was monitored by measuring visceromotor responses during colorectal distension both prior to and following water avoidance stress. Visceral nociception was significantly attenuated in both NT knockout mice and rats pre-treated with the NT antagonist SR 48692. Disruption of NT signaling also blocked SIAN, revealing a novel stress-induced hyperalgesic response that was significantly greater in female than in male rats. NT was also required for acetic acid-induced hyperalgesia. These results indicate that endogenous NT normally facilitates visceral pain responses, is required for irritant-induced hyperalgesia, and plays a critical role in SIAN.

Gonadal steroid hormone modulation of nociception, morphine antinociception and reproductive indices in male and female rats

The purpose of this study was to examine how gonadal steroid hormones modulate basal nociception and morphine antinociception relative to regulating reproduction in the adult rat. Male and female Sprague-Dawley rats were either gonadectomized (GDX) or sham-gonadectomized (sham); GDX males were implanted subcutaneously with capsules containing testosterone (T), estradiol (E2), dihydrotestosterone (DHT), E2 and DHT, or nothing (0). GDX females received E2, T, or empty (0) capsules immediately after surgery, and vehicle or progesterone (P4) injections at 4-day intervals. Basal nociception and morphine antinociception were tested 28 days after surgery on 50 degrees C and 54 degrees C hotplate tests, and reproductive behavior and physiology were assessed shortly thereafter. There were no significant differences in baseline hotplate latencies among the male treatment groups, but morphine was significantly more potent in sham and GDX+T males than in GDX+0 males. The ability of T to increase morphine's potency was approximated by its major metabolites E2 and DHT, given together but not alone. Baseline hotplate latencies were higher in sham females tested during diestrus than in those tested during estrus. Morphine was significantly more potent in sham females tested during proestrus and diestrus than in those tested during estrus. Baseline hotplate latencies were significantly higher, and morphine was significantly less potent in GDX+E2, GDX+E2/P4 and GDX+T females than in GDX+0 females. All group differences in basal nociception and morphine antinociception observed on the 50 degrees C hotplate test were smaller and generally non-significant on the 54 degrees C hotplate test. Steroid manipulations produced the expected changes in reproductive behaviors and steroid-sensitive organs. These results demonstrate that in adult rats, gonadal steroid manipulations, that are physiologically relevant, modulate (1) basal nociception in females but not males, and (2) morphine's antinociceptive potency in both males and females.

Sex differences in drug- and non-drug-induced analgesia

Historically, biomedical research has been conducted almost exclusively with male subjects. A growing number of studies now demonstrate sex differences in analgesia produced both by drugs and by environmental stimuli. This review summarizes the current literature on sex differences in analgesia produced by opioids, cholinergics and other drugs, and by stress, exercise and other environmental manipulations. A brief overview of the physiological mechanisms underlying sex differences in analgesia is provided, as well as suggestions for future research. It is not yet known whether the development of sex-specific analgesia treatment guidelines is warranted.

Influence of testosterone on autotomy in castrated male rats

Sex-related differences exist in nociception and gonadal steroids influence the analgesic response in animals and humans. As we have shown previously, estrogen could modify autotomy in female rats using the sciatic nerve transection model. To further characterize the relationship between gonadal steroid and nociception, the role of testosterone on autotomy in sciatic nerve sectioned rats was investigated. Twenty male rats were subjected to orchiectomy (ORX). Then ten rats received subcutaneous sesame oil and the other ten were treated with testosterone propionate in sesame oil (TP; 500 microg/day/rat). All the rats underwent sciatic nerve resection in left hind limb. Degree of self-mutilation was measured daily for 8 weeks. TP reinstatement resulted in significantly lower autotomy scores in orchiectomized rats. The results demonstrated that testosterone could modify the autotomy behavior, an indicator of neuropathic pain, in rats after nerve injury.

Group differences in pain modulation: pain-free women compared to pain-free men and to women with TMD

Previously reported differences in sensitivity to experimental pain stimuli between the sexes, as well as between temporomandibular disorder (TMD) patients and healthy control subjects, may be attributable in part to group differences in two pain modulatory mechanisms: the baroreceptor reflex arc and the endogenous opioid system. Twenty-two pain-free (PF) men, 20 PF women and 20 women with TMD underwent two testing sessions in which heat pain and ischemic arm pain threshold and tolerance were measured during both sessions, but followed relaxation during one session and laboratory stress tasks during the other. Blood pressure (BP) and plasma -endorphin (E) concentration were measured during a baseline rest and during the stress or relaxation periods. PF men's threshold and tolerance for heat pain, but not for ischemic pain, exceeded that of PF women's during both sessions. PF women and TMD women did not differ in sensitivity to either pain modality; however, significantly lower ischemic pain threshold (IPTh) was linked to oral contraceptive use in PF women but not TMD patients. In the men alone, higher baseline systolic BP (SBP) was correlated with higher heat pain threshold on both days and heat pain tolerance on the stress day. Conversely, in TMD women, higher baseline SBP was correlated with lower ischemic pain tolerance (IPTol) on both days; BP and pain sensitivity were not related in PF women. In men, but not in PF or TMD women, stress systolic and diastolic BP were positively correlated with heat pain threshold and tolerance and higher diastolic reactivity to stress were correlated with higher heat pain and IPTh and tolerance. On the stress day, higher baseline E level was strongly associated with higher IPTol in PF women but marginally associated with lower IPTol in TMD women. Thus, it appears that a BP-related analgesic mechanism (probably baroreceptor-mediated) predominates in PF men, while an endogenous opioid mechanism predominates in PF women. Stress enhances the expression of these central mechanisms. Female TMDs appear unable to effectively engage normal pain-inhibitory systems; opioid receptor desensitization and/or downregulation are probably implicated, because TMDs' production of E appears normal.

Reversal of sex differences in morphine analgesia elicited from the ventrolateral periaqueductal gray in rats by neonatal hormone manipulations

Male rats display significantly greater analgesic responses than female rats following systemic, ventricular and intracerebral morphine administration into either the ventrolateral periaqueductal gray (vlPAG) or the rostral ventromedial medulla, and following beta-endorphin administration into the vlPAG. Although adult gonadectomy severely reduces nonopioid forms of swim stress-induced analgesia, the marked sex differences in morphine analgesia were minimally affected by either male or female adult gonadectomy. Since very little is known about neonatal effects of gonadal hormones upon sex differences in morphine analgesia elicited from the vlPAG, the present study evaluated the effects of neonatal (within 1 day of birth) castration in male rat pups relative to sham-operated controls, and systemic androgenization with testosterone propionate in female rat pups relative to vehicle-injected controls upon baseline nociceptive thresholds and morphine analgesia elicited from the vlPAG in rats tested as adults. Significant sex differences in morphine analgesia elicited from the vlPAG were observed with adult males receiving neonatal sham surgeries displaying significantly greater morphine analgesia on two nociceptive measures than adult females tested during the estrous phase and receiving neonatal vehicle injections. Neonatal gonadectomy essentially reversed the pattern of sex difference effects upon morphine analgesia elicited from the vlPAG. Neonatally-castrated male rats tested in adulthood displayed dramatic reductions in morphine analgesia elicited from the vlPAG on both the tail-flick (approximately 15-fold rightward shift) and jump (6-fold rightward shift) tests relative to sham-operated males, and essentially mirrored those of vehicle-treated females. Conversely, neonatally-androgenized female rats tested in adulthood displayed dramatic increases in morphine analgesia elicited from the vlPAG on the tail-flick (5-fold leftward shift) and jump (12-fold leftward shift) tests relative to vehicle-treated females, and approximated those observed in sham-operated males. The potent differences between neonatally-castrated and sham-operated male rats and between neonatally-androgenized and vehicle-treated female rats suggest a possible 'organizational' role of gonadal hormones in mediating sex differences in morphine analgesia elicited from the vlPAG.

Estrous cycle modulation of nociceptive behaviors elicited by electrical stimulation and formalin

The impact of circulating ovarian hormones on nociceptive behaviors elicited by phasic and tonic stimuli was evaluated in rats using two behavioral tests: an operant escape task and the formalin test. The operant escape task was structured to separately evaluate hindlimb flexion reflexes, the latency of escape, and the amplitude of peak vocalization to a series of phasic electrocutaneous stimuli (0.05-0.8 mA), whereas the formalin test evaluated nociceptive behaviors elicited by tonic stimulation following a subcutaneous injection of dilute formalin (1%). Hindlimb reflex amplitude, escape latency, and peak vocalization varied across the estrous cycle, such that rats were most sensitive to electrical stimuli during proestrus (reflex and escape latency) and diestrus (vocalization). Furthermore, morphine-induced (3 mg/kg sc) attenuation of hindlimb reflex amplitude was sensitive to estrous cycling. During proestrus, morphine produced less attenuation of hindlimb reflex amplitude than during nonproestrus phases. However, estrous cycling did not alter nociceptive behaviors elicited by 1% formalin. These data support the notion that circulating ovarian hormones may differentially modulate behaviors associated with phasic and tonic pain.

Modulation of morphine-induced antinociception by palatable solutions in male and female rats

The analgesic potency of opioid drugs varies as a function of gender, and can be modified by the intake of palatable sweet-tasting solutions. To determine if gender interacts with diet-induced changes in antinociceptive responses, male and female Long-Evans rats were fed laboratory chow and water alone, or chow, water and either a 32% w/v sucrose solution or a 0.15% w/v saccharin solution, and tested in two analgesic paradigms, the tail-flick test and the hot-plate test. For both tests, antinociceptive responses of male and female rats were tested following administration of cumulative doses (1.25, 2.5, 5.0, and 10.0 mg/kg, SC) of morphine sulfate. On the tail-flick test, morphine produced dose-related increases in antinociceptive responses. In addition, relative to both the chow only and saccharin conditions, chronic intake of the sucrose solution access significantly augmented morphine's antinociceptive properties. On the hot-plate test, when the plate was heated to 51 degrees C, morphine led to significant dose-related increases in antinociceptive responses, but diet did not affected antinociceptive responses. When the temperature of the hot plate was increased to 53 degrees C, there was a trend for animals given sucrose to have greater antinociceptive responses than those given either chow alone or saccharin. No differences in baseline pain sensitivity or morphine-induced analgesia were observed as a function of gender.

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.

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.

The effects of acute and repeated restraint stress on the nociceptive response in rats

The effects of acute and repeated restraint stress on nociception, as measured by the tail-flick latency, were studied in adult male and female rats. After the exposure to a single restraint session, both male and female rats presented an increased latency in the tail-flick test. On the other hand, chronically stressed females presented a performance similar to the control group, whereas chronically stressed male rats responded to restraint with a decrease in the tail-flick latency. This response could be determined by the chronic treatment itself or by the restraint done just before the measurement. Thus, the effect of chronic stress upon basal tail-flick latency was evaluated. In male rats, this latency was significantly decreased in the stressed animals compared with the control group. In female rats, no difference between those groups was observed. Therefore, the results suggest that: (a) acute restraint stress induces an analgesic response in both male and female rats, and (b) there is a gender-specific nociceptive response induced by repeated restraint stress with a hyperalgesic effect in response to stress only in males.

Blood pressure, gender, and parental hypertension are factors in baseline and poststress pain sensitivity in normotensive adults

We studied 38 men and 36 women to learn whether a brief speech stressor reduced normotensive humans' thermal pain sensitivity, whether baseline and poststress pain threshold and tolerance varied with blood pressure (BP) and hemodynamic measures, and whether these relations differed by gender and parental hypertension (PH). PH-women with low-resting BPs had lower baseline pain tolerance than did all the other groups (ps <.05), and this group alone exhibited stress-induced analgesia (p = .008). In women, pre- and poststress pain tolerance varied directly with rest and stress BP (ps <.05).

Suckling-induced changes in responsivity to the hypoalgesic effect of morphine

The hypoalgesic effect of morphine in lactating rats was assessed using the hot-plate test. At midlactation (days 12 and 18 postpartum) females nursing litters of 8 pups were less responsive to the hypoalgesic effect of morphine than ovariectomised or cycling females and females on days 6 or 24 of lactation. Subsequent studies showed that the hypoalgesic response to morphine was inhibited in lactating rats at a number of time points after drug administration and across a variety of doses. This effect was not dependent on milk delivery but was dependent on the hormonal state of the female since separation of dams and their litters for 96 h was sufficient to reinstate the response to morphine if it resulted in a reappearance of vaginal estrus.

Naloxone facilitates spatial learning in a water-maze task in female, but not male, adult nonbreeding meadow voles

The present study examined the effects of the opiate antagonist naloxone on spatial acquisition and retention in a water-maze task by adult, nonbreeding, male and female meadow voles (Microtus pennsylvanicus). Voles were required to learn the position of a hidden, submerged platform using distal visual cues. There were four trials per day for 6 days. Daily pretraining (15 min before first trial) systemic administrations of naloxone (1.0 mg/kg, IP) significantly facilitated spatial acquisition in female, but not in male, voles in a water-maze task on days 2, 3, and 4. There were two probe tasks given 1 day and 1 week after the last training trial. All groups acquired the spatial task by the end of the fifth day with no significant effects of naloxone on retention of the spatial task. There were also no significant sex differences in acquisition of the spatial task and task retention in control, nonbreeding adult voles. It is suggested that the lack of sex differences in basal spatial performance may be related to the low levels of testosterone in male nonbreeding voles. The obtained sex differences in the effects of naloxone on spatial acquisition are considered in relation to sex differences in stress, opiate responses, and gonadal steroid levels.

Interactions among aging, gender, and gonadectomy effects upon naloxone hypophagia in rats

The present study examined the dose-dependent (0.25-5 mg/kg) effects of systemic naloxone upon deprivation-induced intake and high-fat intake as functions of age (4, 8, 14, and 20 months), gender, and gonadectomy in rats. Significant increases in body weight were observed as functions of age and gonadectomy. Whereas aging significantly reduced basal deprivation-induced intake, it generally failed to alter basal high-fat intake. Whereas age, gender, and gonadectomy failed to alter the decreases in deprivation-induced intake following low (0.25-2.5 mg/kg) naloxone doses, sham males displayed significantly greater age-related and gender-related inhibition following the 5 mg/kg dose of naloxone. Young gonadectomized rats displayed significant increases in naloxone's inhibition of deprivation-induced intake as well. More dramatic changes occurred in naloxone's inhibition of high-fat intake. Naloxone's potency increased in sham female rats as a function of age, and decreased in sham males and ovariectomized females as a function of age. Whereas sham males and ovariectomized females were most sensitive to naloxone's inhibition of high-fat intake at young ages, sham females were most sensitive at older ages. These data indicate that effects of age, gender, and gonadectomy upon naloxone-induced hypophagia dissociate as a function of the type of intake. Because selective opioid antagonist studies demonstrate that deprivation-induced intake is mediated by the mu1 receptor and high-fat intake is mediated by kappa and mu2 receptors, it is postulated that the differential effects of aging, gender, and gonadectomy variables upon opioid mediation of the two forms of intake may reflect their interaction with different opioid receptor subtypes.

Interactions among aging, gender, and gonadectomy effects upon morphine antinociception in rats

In addition to age-related deficits in morphine antinociception in female rats, gender and gonadectomy differences have also been observed, with male rats displaying greater magnitudes of effects than females and castrated males. Since there are little data indicating how aging, gender, and gonadectomy interact in modulating morphine antinociception, the present study evaluated alterations in this response as functions of age (6, 12, 18, and 24 months), gender, and gonadal status (intact, gonadectomized) across a dose range (1-10 mg/kg) and time course (0.5-2 h) on the tail-flick test. The maximal percentage effect (MPE) of morphine (1 mg/kg) was significantly increased in castrated males (18 months), sham females (18 and 24 months), and ovariectomized females (18 months) relative to 6-month-old groups. Increases in the MPE of morphine (1 mg/kg) occurred in sham females (24 months) relative to corresponding sham males and ovariectomized females. The MPE of morphine (2.5 mg/kg) was significantly increased in sham males (18 months) and decreased in sham females (12 months). Decreases in the MPE of morphine (2.5 mg/kg) occurred in castrated males (18 and 24 months) as well as sham (18 months) and ovariectomized (18 and 24 months) females relative to sham males. Whereas the MPE of morphine (5 mg/kg) was unchanged by these variables, the MPE of morphine (10 mg/kg) was significantly decreased in sham females (18 and 24 months) relative to females aged 6 months, as well as males and ovariectomized females aged 24 months.(ABSTRACT TRUNCATED AT 250 WORDS)

Diet and estrous cycle influence pain sensitivity in rats

Effects of estrous cycle and acute and chronic access to palatable fluids on tail-flick latency and opiate-induced analgesia were assessed in 124 female Long Evans rats. Following three consistent cycles, rats were water deprived for 8 h and then given ad lib access to 20 ml of either water, a 32% sucrose solution, or corn oil for 5 h. Nociceptive testing was conducted immediately preceding and 30, 60, and 90 min following an SC injection of morphine sulfate (7.5 mg/kg). Diestrus rats had prolonged premorphine tail-flick latencies compared to rats in proestrus. Rats that consumed corn oil had longer tail-flick latencies preceding and 30 min following morphine injections than rats that drank water or the sucrose solution. Rats were retested after they had ad lib access to the same fluid for 3 weeks. No estrous cycle differences were noted following chronic consumption. Rats with chronic access to sucrose showed increased baseline pain sensitivity and increased morphine-induced analgesia at 30, 60, and 90 min postinjection. These data support the notion that palatable fluid consumption attenuates estrous cycle-dependent differences in pain sensitivity.

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)

Sex differences in the effects of neuropeptide FF and IgG from neuropeptide FF on morphine- and stress-induced analgesia

There is evidence suggesting that the endogenous mammalian octapeptide FLFQPQRFamide (F8Fa or neuropeptide FF, NPFF) has modulatory effects on opioid-mediated analgesia in rodents. There is also substantial evidence for sex differences in opioid analgesia, whereby male rats and mice display greater levels of opioid-mediated analgesia than females. In the present study, determinations were made of the effects of NPFF and IgG from antiserum against NPFF on morphine- and restraint stress-induced opioid analgesia in male and female deer mice. Intracerebroventricular (ICV) administrations of NPFF (0.10-10 micrograms) reduced in a dose-dependent manner morphine- and stress-induced analgesia in both male and female mice, with NPFF having markedly greater antagonistic effects in the male than female mice. Additionally, ICV administrations of NPFF-IgG increased the levels of morphine- and stress-induced analgesia and significantly reduced basal nociceptive sensitivity in male mice, whereas, in female mice, NPFF-IgG had no significant effects on either opioid-mediated analgesia or nociceptive sensitivity. These results indicate that there are sex differences in the modulatory effects of NPFF on opioid-mediated analgesia.

Roles of gender and gonadectomy in pilocarpine and clonidine analgesia in rats

Central and systemic morphine analgesia as well as both opioid and nonopioid forms of swim analgesia display gender differences with male rats showing greater magnitudes of analgesia than female rats. Since nonopioid swim analgesia is dependent upon muscarinic cholinergic and alpha 2-noradrenergic mechanisms, the present study evaluated in rats whether gender, adult gonadectomy or estrous phase altered analgesia induced by either the muscarinic cholinergic receptor agonist, pilocarpine or the alpha 2-noradrenergic receptor agonist, clonidine. Pilocarpine (1-10 mg/kg) analgesia was significantly greater in male rats. Female rats displayed 7-fold and 3-fold rightward shifts in peak analgesia on the tail-flick and jump tests respectively. Clonidine (100-500 micrograms/kg) analgesia was significantly greater on both nociceptive tests in males, but only produced a 2-fold rightward shift in peak analgesia in females on the jump test. Whereas castration failed to shift either dose-response curve, ovariectomy mitigated the gender differences in pilocarpine and clonidine analgesia. Both pilocarpine and clonidine analgesia were not altered by estrous phase changes. These data indicate that gender differences in analgesia are not specific to opioid systems.

Gender effects and central opioid analgesia

Central morphine analgesia is significantly greater in male than in female rats. Since mu and delta opioid receptor subtypes have been implicated in supraspinal analgesia, the present study evaluated whether gender or adult gonadectomy altered (a) analgesia on the tail-flick and jump tests following central administration of the mu-selective agonist, [D-Ala2, Me-Phe4, Gly(ol)5] enkephalin (DAMGO) and the delta-selective agonist, [D-Ser2,Leu5] enkephalin-Thr6 (DSLET) and (b) mu1, mu2 and delta opioid receptor binding. Sham-operated male rats displayed significantly greater magnitudes of peak and total analgesia than sham-operated females on the tail-flick test following DAMGO, but not DSLET. Gender differences were not observed for DAMGO and DSLET analgesia on the jump test. Gonadectomy failed to consistently affect either DAMGO or DSLET analgesia. Regression analyses failed to reflect significant shifts in the dose-response functions for either agonist on either measure. Gender differences were not observed for mu1, mu2, or delta binding in hypothalamus or cortex. These data are compared with analgesic responses sensitive to gender differences.

Sex-dependent effects of inescapable shock administration on shuttlebox-escape performance and elevated plus-maze behavior

The present experiment investigated the effects of exposure to inescapable shocks (IS) on subsequent behavior in an elevated plus-maze and on shuttlebox-escape performance in male and female rats. In the elevated plus-maze, exposure to IS resulted in suppression of "total number of arm entries" and "rearings" in males but not in females. In addition "time on open arms" was reduced in both sexes after exposure to IS, but this effect seemed stronger in males than in females. Exposure to IS disrupted shuttlebox-escape performance of males, whereas escape performance of females was unaffected. Affected escape performance in males was transient and limited to the initial phase of escape training. A sex difference in emotional reaction to stress might contribute to the observed sex difference in the acquisition of an escape response. The strong passive avoidance tendency observed in males, which may be strengthened by IS, strongly interferes with the acquisition of the escape response by this sex, resulting in sex differences in susceptibility to behavioral disturbances induced by IS.

Developmental changes in opiate-induced analgesia in deer mice: sex and population differences

We examined developmental changes in nociception and mu (morphine) and kappa (U-50,488) opiate-induced analgesia in male and female deer mice of two different populations; Peromyscus maniculatus artemisiae from a mainland region and P. m. angustus from a small island. Both populations displayed significant developmental changes in nociception and morphine (10 mg/kg) and U-50,488 (10 mg/kg)-induced analgesia. Basal thermal response latencies (nociceptive responses) and the levels of mu and kappa opiate-induced analgesia increased over 14-35 days of age, with maximum analgesic responses in adults (35+ days of age). In both of the populations, young (neonatal-weaning) male mice displayed significantly higher thermal response latencies and greater levels of naloxone (1.0 mg/kg) antagonized opiate-induced analgesia than young females. There were also population differences in the levels of analgesia, the insular mice displaying greater mu and lower kappa opiate-induced analgesic responses than the mainland animals. The population differences in mu and kappa opiate-induced analgesia were evident in young and adult mice of both sexes. These results show that there are significant sex and population differences in nociception and opiate-induced analgesia in young (neonatal-weaning) and adult deer mice.

Reduction in opioid and non-opioid forms of swim analgesia by 5-HT2 receptor antagonists

Acute exposure to continuous (CCWS) or intermittent (ICWS) cold-water swims elicits non-opioid and opioid forms of analgesia respectively. Intrathecal administration of methysergide blocks ICWS, but not CCWS analgesia. The present study evaluated the role of serotonin (5-HT) receptor subtypes in the mediation of CCWS and ICWS analgesia on the tail-flick and jump tests following administration of methysergide, a non-specific 5-HT antagonist and pirenpirone and ketanserin, two 5-HT2 receptor subtype antagonists. Systemic methysergide was more effective in reducing CCWS analgesia (50-58%, 0.1-1.0 mg/kg) than ICWS analgesia (21%, 5 mg/kg) on both pain tests. Systemic pirenpirone (0.04-0.2 mg/kg) and ketanserin (1-5 mg/kg) were also more effective in reducing CCWS analgesia (43-57%) on both tests than ICWS analgesia (pirenpirone: 0.4 mg/kg, 34%; ketanserin: 5 mg/kg, 21%) on the tail-flick test. Indeed, both 5-HT2 receptor antagonists potentiated ICWS analgesia on the jump test. While serotonin antagonist effects upon hypothermia could not account for CCWS analgesia effects, similar potentiations in ICWS analgesia and hypothermia were observed following pirenpirone and ketanserin. Finally, both 5-HT2 receptor antagonists differentially reduced CCWS hypothermia and potentiated ICWS hypothermia. These data suggest differential serotonergic modulation of the two forms of swim analgesia with opioid-mediated ICWS analgesia acting through spinal 5-HT1 receptors and non-opioid-mediated CCWS analgesia acting through supraspinal 5-HT2 receptors.

Long-term effects of parity on opioid and nonopioid behavioral and endocrine responses

Parity (number of parturitions) affects the endogenous opioid system. Multiparous lactating rats are less sensitive to the effects of morphine (MOR) on maternal behavior (MB) and analgesia than primiparous lactating rats. In order to determine whether these changes in opiate sensitivity persist beyond the lactational state, the present study compared the sensitivity of ovariectomized nulliparous and nonlactating primiparous rats to MOR's effects on MB (Experiment 1), analgesia (Experiment 2) and prolactin release (Experiment 3) in addition to stress-induced analgesia (Experiment 2). In Experiments 1 and 2 primiparous rats were allowed to give birth and remain with their litter (culled to 6 pups) until weaning. At that time the pups were removed and the dams and age-matched nulliparous rats were ovariectomized. Four weeks later animals were exposed to foster pups daily in order to induce MB (Experiment 1). On day 5 or 6 of full MB the primiparous and nulliparous rats received either saline or one of four doses of MOR (0.625, 1.25, 2.5, or 5.0 mg/kg, SC) and 60 min later MB was assessed. MOR, at the 2.5 mg/kg dose, disrupted MB in a significantly greater percentage of nulliparous as compared to primiparous animals (100% vs. 55%, respectively). In Experiment 2, nulliparous and nonlactating primiparous animals received 2.5 mg/kg of MOR four weeks after ovariectomy. Analgesia was assessed on a tail-flick apparatus 30, 60, 90, 120 and 150 min postinjection. One week later the same animals were exposed to cold-water swims (CWS, 2 degrees C, 3.5 min) and tail-flick latencies were again recorded.(ABSTRACT TRUNCATED AT 250 WORDS)

Roles of gender, gonadectomy and estrous phase in the analgesic effects of intracerebroventricular morphine in rats

Gender and gonadal function have previously been shown to influence the magnitude of analgesia following systemic morphine and opioid and nonopioid forms of swim analgesia with male rats displaying greater analgesia than female rats and gonadectomy reducing analgesic magnitude in both genders. These effects have been presumed to be centrally mediated. The present study evaluated the roles of gender, gonadectomy and estrous phase upon dose-response and time-response functions of analgesia following intracerebroventricular administration of morphine as measured by the tail-flick and jump tests. Sham-operated male rats displayed significantly greater magnitudes of peak and total analgesia following central morphine than sham-operated female rats on both nociceptive measures. This striking effect was reflected both in terms of magnitude and ED50; while male rats displayed near-maximal analgesia at a 5 micrograms dose of morphine, female rats displayed moderate analgesia at doses as high as 40 micrograms of morphine. Castration produced small, but significant reductions in the magnitude of central morphine analgesia; the ED50 of morphine analgesia, however, was not changed. Although female rats in either proestrous or estrous displayed significantly greater magnitude of analgesia than ovariectomized rats or rats in a combined met-/di-estrous phase at some doses, the ED50 of morphine analgesia was not significantly altered as functions of estrous phase or ovariectomy. The interaction of opiate receptors and gonadal steroid receptors is considered as one possible determinant of gender differences observed in the magnitude and potency of central morphine analgesia.

Post-natal morphine differentially affects opiate and stress analgesia in adult rats

Alterations in nociceptive reactivity, opiate receptor binding, and other behavioral responses occur in rats exposed to morphine either in utero or post-natally. The present study examined whether post-natal morphine (0, 1 or 20 micrograms, days 1-7) altered analgesia on the tail-flick and jump tests induced by nonopioid-mediated continuous cold-water swims (CCWS), opioid-mediated intermittent cold-water swims (ICWS) or morphine (2.5 and 5.0 mg/kg, SC) in adult male and female rats. Changes in body weight, developmental signs (e.g., eye opening), basal pain thresholds, and both CCWS and ICWS hypothermia were also assessed. Previously-reported gender differences occurred for all forms of analgesia in control rats. Post-natal morphine treatment transiently increased ICWS analgesia and hypothermia, and transiently decreased CCWS analgesia and hypothermia, suggesting that these effects were not specific to pain inhibition. Post-natal morphine treatment significantly increased the magnitude of morphine analgesia on both tests in females, and significantly decreased the magnitude of morphine analgesia on both tests in males, thereby acting to vitiate the observed gender differences in morphine analgesia. Such effects could not be explained by concomitant changes in other measures. These data indicate that post-natal morphine treatment exerts highly selective effects upon specific analgesic responses which are gender sensitive.