The melanocortin-1 receptor gene mediates female-specific mechanisms of analgesia in mice and humans

Sex differences in the effects of amiloride on formalin test nociception in mice

Amiloride is a nonspecific blocker of acid-sensing ion channels (ASICs) that have been recently implicated in the mediation of mechanical and chemical/inflammatory nociception. Preliminary data using a transgenic model are suggestive of sex differences in the role of ASICs. We report here that systemic administration of amiloride (10-70 mg/kg ip) produces a robust, dose-dependent blockade of late/tonic phase nociceptive behavior on the mouse formalin test (5%; 20 microl) in female but not male mice, completely abolishing the known sex difference in formalin test response. Adult gonadectomy produced a "switching" of sex differences in amiloride efficacy, with castrated males displaying an amiloride blockade and ovariectomized females rendered less sensitive to amiloride. Gonadectomized mice could be switched back to their intact status using chronic estrogen benzoate or testosterone propionate replacement via osmotic minipump (6 microg/day or 250 microg/day, respectively). It is unclear whether this striking sex difference is due to sex-specific involvement of ASICs in pain processing, but the present data represent one of the first demonstrations of pain-related sex differences with no obvious opioid involvement.

Symbiotic relationship of pharmacogenetics and drugs of abuse

Pharmacogenetics/pharmacogenomics is the study of how genetic variation affects pharmacology, the use of drugs to treat disease. When drug responses are predicted in advance, it is easier to tailor medications to different diseases and individuals. Pharmacogenetics provides the tools required to identify genetic predictors of probable drug response, drug efficacy, and drug-induced adverse events-identifications that would ideally precede treatment decisions. Drug abuse and addiction genetic data have advanced the field of pharmacogenetics in general. Although major findings have emerged, pharmacotherapy remains hindered by issues such as adverse events, time lag to drug efficacy, and heterogeneity of the disorders being treated. The sequencing of the human genome and high-throughput technologies are enabling pharmacogenetics to have greater influence on treatment approaches. This review highlights key studies and identifies important genes in drug abuse pharmacogenetics that provide a basis for better diagnosis and treatment of drug abuse disorders.

Development of cyclic gamma-MSH analogues with selective hMC3R agonist and hMC3R/hMC5R antagonist activities

A series of cyclic lactam analogues of gamma-MSH (H-Tyr1-Val2-Met3-Gly4-His5-Phe6-Arg7-Trp8-Asp9-Arg10-Phe11-Gly12-OH) with a bulky hydrophobic residue in the direct proximity to the pharmacophore (Xaa-D-Phe/D-Nal(2')-Arg-Trp) were designed and synthesized by solid-phase methods. A variety of amino acids with a broad range of hydrophobic/hydrophilic properties was introduced in position 5 to further explore their complementary role in receptor selectivity. Biological evaluation of these peptides revealed several analogues with potent hMC3R agonist and hMC3R/hMC5R antagonist activities, and good receptor selectivity. Analogue 4, c[Nle-Arg-D-Phe-Arg-Trp-Glu]-NH2, was found to be a very potent and selective hMC3R agonist (EC50=1.2 nM, 112% act). In addition, analogue 13, c[Nle-Val-D-Nal(2')-Arg-Trp-Glu]-NH2, was identified as an hMC3R/hMC5R antagonist with the best selectivity against the hMC4R in this series (pA2(hMC3R)=8.4; pA2(hMC5R)=8.7). These results indicate the significance of steric factors in melanocortin receptor selectivity and suggest that introduction of bulky residues in the direct proximity to the melanocortin pharmacophore is an effective approach to design of novel hMC3R and hMC5R selective ligands.

Dissociation between sex differences in the immunological, behavioral, and physiological effects of kappa- and delta-opioids in Fischer rats

RATIONALE

The sex of the individual can have a profound effect on sensitivity to the effects of opioids. Recently, our laboratory provided the first evidence that females may be more sensitive to the immune-altering effects of mu-opioids than males. However, it remains unknown whether kappa- and delta-opioids produce sexually dimorphic effects on immune responses.

OBJECTIVE

The present study sought to determine whether kappa- and delta-opioids produce differential immunological effects in males and females using the memory-T-cell-dependent in vivo inflammatory response contact hypersensitivity (CHS). As sex differences in the magnitude of opioid effects can be outcome-specific, additional experiments were conducted to compare the immunological effects of kappa- and delta-opioids with other behavioral and physiological effects.

MATERIALS AND METHODS

Contact hypersensitivity was induced in male and female Fischer rats. Prior to elicitation of CHS, animals were administered selected doses of the kappa-opioid spiradoline (0.2-20 mg/kg), delta-opioid SNC80 (1-10 mg/kg), or vehicle. The antinociceptive and diuretic effects of spiradoline were also assessed in males and females, as were the locomotor effects of SNC80.

RESULTS

Spiradoline produced significantly greater enhancement of CHS in females than males, but produced comparable antinociceptive and diuretic effects in both sexes. By contrast, SNC80 did not significantly affect the course of CHS in either sex, but females were significantly more sensitive to its locomotor stimulatory effects.

CONCLUSIONS

These results demonstrate that females are more sensitive than males to the CHS-altering effects of spiradoline and that sex differences in the magnitude and direction of opioid-induced sex differences are outcome dependent.

Potentiation of morphine antinociception by pentobarbital in female vs. male rats

It has been shown previously that female rats are more sensitive than males to barbiturate anesthesia, whereas males may be more sensitive than females to opioid antinociception. The aim of the present study was to determine whether enhancement of morphine antinociception by pentobarbital, previously demonstrated in male animals and humans, occurs similarly in females. Pentobarbital (50 mg/kg i.p.) produced longer-lasting anesthetic effects (loss of muscle tone, righting reflex) in gonadally intact female rats than in males, but greater antinociceptive effects in males at some time points post-injection. There were no significant sex differences in morphine-induced anesthesia or antinociception; however, 50 mg/kg pentobarbital produced greater leftward shifts in the morphine antinociceptive dose-effect curve in gonadally intact females than males, whether pentobarbital was administered 30 vs. 120 min before morphine (times at which there were no sex differences vs. sex differences, respectively, in pentobarbital's effects when administered alone). Dose-addition analysis confirmed that pentobarbital enhancement of morphine antinociception was supra-additive in both sexes; morphine also significantly enhanced pentobarbital-induced anesthesia in both sexes. In gonadectomized males, testosterone did not significantly alter pentobarbital enhancement of morphine antinociception; in contrast, in gonadectomized females, estradiol significantly attenuated the drug interaction. Estradiol did not significantly alter the effects of pentobarbital alone or morphine alone, indicating that the attenuation of the pentobarbital's potentiation of morphine antinociception in estradiol-treated rats is specific to the drug interaction. These results suggest that barbiturate potentiation of opioid antinociception may be greater in females - particularly those in low ovarian hormone states - than in males.

Increased cutaneous nerve fibers in female specimens

BACKGROUND

The control of pain is critical to performing surgery. All surgical procedures cause some degree of pain, and the ability to minimize pain often affects a patient's perception of surgical outcome. Although the development of surgery was boosted by the advent of anesthesia, inadequate pain control continues to plague modern medicine. The mechanism of pain induction is an important area of research in the health care industry. To date, few studies have demonstrated increased perception of pain and lower tolerance for pain in female patients when compared with male patients. The authors hypothesized about whether these differences were related to increased density of nerve fibers in female as compared with male patients.

METHODS

The density of nerve fibers at a specific location (the skin directly overlying the infraorbital nerve foramen) was measured to test this hypothesis. Twenty cadaver skin specimens (1 cm2) were harvested, prepared using immunohistochemistry (S-100 polyclonal antibody), and counted using 45x high-powered microscopy.

RESULTS

Female specimens (n = 10) demonstrated increased nerve fiber density (34 +/- 19 fibers/cm skin) when compared with male specimens (n = 10; 17 +/- 8 fibers/cm skin; p = 0.038).

CONCLUSION

Although preliminary and limited in scope, these findings favor a physical (organic) rather than a psychosocial explanation for more pronounced pain perception in female patients.

Low efficacy opioids: implications for sex differences in opioid antinociception

It is becoming increasingly evident that the sex of an organism is a critical determinant of responsiveness to opioid analgesics. However, the factors that determine the magnitude and direction of sex differences in opioid antinociception have not been fully elucidated. One factor that has received attention is the relative efficacy of the opioid. This review summarizes recent findings in which opioid efficacy was systematically manipulated as an independent variable to probe underlying sex differences in opioid system function. Overall, in rodents and nonhuman primates, mu and kappa opioids are generally more potent and effective in males than in females. The data indicate that although sex differences in the potency of high efficacy opioids such as morphine are generally less than 3.0-fold, sex differences with lower efficacy opioids can be greater than 90-fold. Moreover, that these drugs can function as full agonists in males while functioning as antagonists in females under identical conditions suggests some fundamental sex difference in opioid system function. In addition to efficacy, a number of other variables can affect the outcomes of these studies, including the drug history, genotype, and nociceptive stimulus modality, duration, and intensity. These factors may interact with opioid efficacy to determine the specific conditions under which sex differences are observed. The testing of low efficacy opioids by other laboratories and under other experimental conditions will determine the extent to which this variable affords a strategic research tool. The potential utility of low efficacy opioids in other domains of behavioral pharmacology is also discussed.

New Treatments for Neuropathic Pain

Existing treatments for neuropathic pain deliver inadequate pain relief, unacceptable side effects, or both. The unmet medical need for more effective treatment is driving a large volume of research to discover new drugs. Most existing treatments are drugs introduced to treat other pain conditions or other medical conditions, such as antidepressants and anticonvulsants, which were found empirically to be effective for neuropathic pain. Only recently have drug discovery efforts have become mechanistically driven, addressing targets identified by a molecular neurobiological approach to the pathophysiology of neuropathic states.

Women with red hair report a slightly increased rate of bruising but have normal coagulation tests

There is an anecdotal impression that redheads experience more perioperative bleeding complications than do people with other hair colors. We, therefore, tested the hypothesis that perceived problems with hemostasis could be detected with commonly used coagulation tests. We studied healthy female Caucasian volunteers, 18 to 40 yr of age, comparable in terms of height, weight, and age, with natural bright red (n = 25) or black or dark brown (n = 26) hair. Volunteers were questioned about their bleeding history and the following tests were performed: complete blood count, prothrombin time/international normalized ratio, activated partial thromboplastin time, platelet function analysis, and platelet aggregation using standard turbidimetric methodology. Agonists for aggregation were adenosine diphosphate, arachidonic acid, collagen, epinephrine, and two concentrations of ristocetin. The red-haired volunteers reported significantly more bruising, but there were no significant differences between the red-haired and dark-haired groups in hemoglobin concentration, platelet numbers, prothrombin time/international normalized ratio, or activated partial thromboplastin time. Furthermore, no significant differences in platelet function, as measured by platelet function analysis or platelet aggregometry, were observed. We conclude that if redheads have hemostasis abnormalities, they are subtle.

A window on the genetics of evolution: MC1R and plumage colouration in birds

Melanins are a ubiquitous component of plumage colouration in birds and serve a wide variety of functions. Although the genetic control of melanism has been studied in chickens and other domestic species, little was known about the molecular genetics of melanin distribution in wild birds until recently. Studies have now revealed that a single locus, the melanocortin-1 receptor (MC1R) locus, is responsible for melanic polymorphisms in at least three unrelated species: the bananaquit, the snow goose and the arctic skua. Results show that melanism was a derived trait and allow other evolutionary inferences about the history of melanism to be made. The role of MC1R in plumage patterning is surprisingly diverse among different species. The conserved molecular basis for the evolution of melanism in birds and several other vertebrates is probably related to low pleiotropic effects at the MC1R.

Interface of physical and emotional stress regulation through the endogenous opioid system and mu-opioid receptors

Unraveling the pathways and neurobiological mechanisms that underlie the regulation of physical and emotional stress responses in humans is of critical importance to understand vulnerability and resiliency factors to the development of a number of complex physical and psychopathological states. Dysregulation of central stress response circuits have been implicated in the establishment of conditions as diverse as persistent pain, mood and personality disorders and substance abuse and dependence. The present review examines the contribution of the endogenous opioid system and mu-opioid receptors to the modulation and adaptation of the organism to challenges, such as sustained pain and negative emotional states, which threaten its internal homeostasis. Data accumulated in animal models, and more recently in humans, point to this neurotransmitter system as a critical modulator of the transition from acute (warning signals) to sustained (stressor) environmental adversity. The existence of pathways and regulatory mechanisms common to the regulation of both physical and emotional states transcend classical categorical disease classifications, and point to the need to utilize dimensional, "symptom"-related approximations to their study. Possible future areas of study at the interface of "mind" (cognitive-emotional) and "body" (physical) functions are delineated in this context.

Gene polymorphisms and their effects in the melanocortin system

In addition to its role in human pigmentation, components of the melanocortin system regulate appetite, energy homeostasis and hormone production. Recent studies have suggested possible roles of this system in immunity, transmission of pain signals, and reproductive potential. A number of polymorphisms have been identified in genes of the melanocortin system and are associated with pigmentation in humans, as well as being causative of disorders of adrenal hormone production and obesity. This review gives an outline of these polymorphisms, their functional significance and possible application to or impact on diagnosis and pharmacotherapy based on melanocortin pathways.

The magnitude of mechanical allodynia in a rodent model of lumbar radiculopathy is dependent on strain and sex

STUDY DESIGN

This study examined the differences in tactile hypersensitivity across 6 different strains of male mice, and between male and female rats of 3 different strains in a rodent model of low back pain associated with lumbar radiculopathy.

OBJECTIVE

We investigated the possibility that differences in tactile allodynia following the same nerve root injury are affected by genotype and sex in rodents.

SUMMARY OF BACKGROUND DATA

Low back pain associated with radiculopathy affects countless people throughout the world, encompassing a wide range of individual pain susceptibility. The roles of genetics and sex on differences in nociceptive sensitivities following lumbar nerve root injury have yet to be fully characterized.

METHODS

Six strains of mice (BALB/cJ, CBA/J, C57BL/6J, 129P3/J, C3H/HeJ, and C58/J; all males) and male and female Sprague Dawley, Holtzman, and Long-Evans rats underwent a lumbar nerve root injury followed by assessment of tactile allodynia.

RESULTS

The most sensitive mouse strains following nerve root injury were: 129P3/J, C58/J, and BALB/cJ; and the less sensitive strains were: C57BL/6J, C3H/HeJ, and CBA/J. Female Sprague Dawley and Long-Evans rats displayed increased hypersensitivity following nerve root injury compared to males. No sex differences were observed in Holtzman rats.

CONCLUSIONS

Different mouse strains, and male and female rats that are exposed to identical nerve root injuries have diverse levels of tactile hypersensitivity, supporting the hypothesis that genetic factors and sex play a key role in radicular pain. Our results correlate with data compiled in identical mouse and rat strains after L5-L6 nerve ligation, suggesting that the precise nature of the injury is not relevant to the inheritance of neuropathic symptom sensitivity.

The A118G single nucleotide polymorphism of the mu-opioid receptor gene (OPRM1) is associated with pressure pain sensitivity in humans

Responses to painful stimuli are characterized by tremendous interindividual variability, and genetic factors likely account for some proportion of this variability. However, few studies have identified genetic contributions to experimental pain perception in humans. This experiment investigated whether the A118G single nucleotide polymorphism of the mu-opioid receptor gene ( OPRM1 ) was associated with responses to three different experimental pain modalities in a sample of 167 healthy volunteers (96 female, 71 male). Responses to thermal, mechanical, and ischemic pain were assessed in all subjects, and genotyping of OPRM1 was performed, which revealed that the rare A118G allele occurred in 24 females (25%) and 12 males (17%). Statistical analyses indicated that subjects with a rare allele had significantly higher pressure pain thresholds than those homozygous for the common allele. Also, a sex by genotype interaction emerged for heat pain ratings at 49 degrees C, such that the rare allele was associated with lower pain ratings among men but higher pain ratings among women. These data indicate an association of a common single nucleotide polymorphism of OPRM1 with mechanical pain responses and that this genotype may be associated with heat pain perception in a sex-dependent manner. This study examines the association of the A118G SNP of OPRM1 to experimental pain sensitivity. The results indicate that the rare allele is associated with higher pressure pain thresholds. These results support previous contentions that OPRM1 may be a pain-relevant gene; however, replication of these findings is needed.

Gender is a confounding factor in pain trials: women report more pain than men after arthroscopic surgery

A gender difference in the incidence of acute pain may be a confounder in analgesic trials. We have tested the hypothesis that the incidence of acute pain after knee arthroscopic procedures is greater in women than men. We performed three RCTs on intra-articular analgesics in which no postoperative analgesia was given until the need for such treatment was documented by scoring moderate-to-severe pain on a verbal rating scale (VRS 0-4; n=219), and a 0-100 mm visual analogue pain scale (VAS) within 2 h postoperatively. All trials were performed with an intra-articular catheter technique. The design allowed us to study the natural course of pain after arthroscopic surgery until analgesia was required. Women reported more pain of at least moderate intensity than men (84 vs 57%; P<0.0001), indicating that being female is a risk factor for early postoperative pain (RR 1.47, 95% confidence interval from 1.23 to 1.74). The VAS score corresponding to moderate and severe pain is similar in men and women. Only short acting anaesthetics were given in order to minimise carry-over effects. Since previous trials on arthroscopic analgesics neither measured baseline pain nor stratified for gender, a difference between treatment groups could result from an uneven distribution regarding these factors. Our findings have major implications for the interpretation of previously published trials on intra-articular analgesia.

Individual responder analyses for pain: does one pain scale fit all?

The outcomes of clinical trials are based on the mean responses of large numbers of subjects but fail to address inter-individual differences. The molecular mechanisms that underlie pain vary among individuals over time and among different types of pain to produce wide inter-individual variations in pain perception and response. Gender, ethnicity, temperament and genetic factors also contribute to individual variation in pain sensitivity and responses to analgesics. Pain measurement scales can be used differently across individuals based on the past pain experiences of individuals. We propose that individual responder analyses could be used in clinical trials to better detect analgesic activity across patient groups and within sub-groups, and to identify molecular-genetic mechanisms that contribute to individual variation.

Expression of proopiomelanocortin and proenkephalin mRNA in sexually dimorphic brain regions are altered in adult male and female rats treated prenatally with morphine

The present study demonstrates that prenatal morphine exposure on gestation days 11-18 differentially alters proopiomelanocortin (POMC) and proenkephalin (pENK) mRNA in the hypothalamus and limbic system of adult male and female rats. In adult, prenatally morphine-exposed male rats POMC mRNA levels are decreased in the arcuate nucleus of the hypothalamus (ARC), while the pENK mRNA levels are increased in the paraventricular nucleus of the hypothalamus (PVN) and in the ventrolateral subdivision of the ventromedial nucleus of the hypothalamus (VMH), specifically in the ventrolateral subdivision of the VMH. In adult, prenatally morphine-exposed female rats, POMC mRNA levels in the ARC are increased in ovariectomized (OVX) but not in OVX, estradiol benzoate- (EB) or EB- and progesterone- (P) treated females. In contrast, pENK mRNA levels are decreased in the VMH of morphine-exposed, OVX females and increased in EB-treated females. Further, prenatal morphine exposure decreases pENK mRNA in the ARC and increases it in the medial pre-optic area independently of female gonadal hormones. Finally, POMC mRNA levels are increased in the ARC of saline-exposed, EB- or EB- and P-treated females but not in OVX females. Thus, the present study suggests that prenatal morphine exposure sex and brain region specifically alters the level of POMC and pENK mRNA.

Genome-wide linkage identifies novel modifier loci of aganglionosis in the Sox10Dom model of Hirschsprung disease

Hirschsprung disease (HSCR) is a complex disorder that exhibits incomplete penetrance and variable expressivity due to interactions among multiple susceptibility genes. Studies in HSCR families have identified RET-dependent modifiers for short-segment HSCR (S-HSCR), but epistatic effects in long-segment (L-HSCR) and syndromic cases have not been fully explained. SOX10 mutations contribute to syndromic HSCR cases and Sox10 alleles in mice exhibit aganglionosis and pigmentary anomalies typical of a subset of HSCR patients categorized as Waardenburg-Shah syndrome (WS4, OMIM 277580). Sox10 mutant alleles in mice exhibit strain-dependent variation in penetrance and expressivity of aganglionic megacolon analogous to the variation observed in patients with aganglionosis. In this study, we focused on enteric ganglia deficits in Sox10Dom mice and defined aganglionosis as a quantitative trait in Sox10Dom intercross progeny to investigate the contribution of strain background to variation in enteric nervous system deficits. We observe that the phenotype of Sox10Dom/+ mutants ranges over a continuum from severe aganglionosis to no detectable phenotype in the gut. To systematically identify genes that modulate Sox10-dependent aganglionosis, we performed a single nucleotide polymorphism-based genome scan in Sox10Dom/+ F1 intercross progeny. Our analysis reveals modifier loci on mouse chromosomes 3, 5, 8, 11 and 14 with distinct effects on penetrance and severity of aganglionosis. Three of these loci on chromosomes 3, 8 and 11 do not coincide with previously known aganglionosis susceptibility genes or modifier loci and offer new avenues for elucidating the genetic network that modulates this complex neurocristopathy.

Increased sensitivity to thermal pain and reduced subcutaneous lidocaine efficacy in redheads

BACKGROUND

Anesthetic requirement in redheads is exaggerated, suggesting that redheads may be especially sensitive to pain. Therefore, the authors tested the hypotheses that women with natural red hair are more sensitive to pain and that redheads are resistant to topical and subcutaneous lidocaine.

METHODS

The authors evaluated pain sensitivity in red-haired (n = 30) or dark-haired (n = 30) women by determining the electrical current perception threshold, pain perception, and maximum pain tolerance with a Neurometer CPT/C (Neurotron, Inc., Baltimore, MD). They evaluated the analogous warm and cold temperature thresholds with the TSA-II Neurosensory Analyzer (Medoc Ltd., Minneapolis, MN). Volunteers were tested with both devices at baseline and with the Neurometer after 1-h exposure to 4% liposomal lidocaine and after subcutaneous injection of 1% lidocaine. Data are presented as medians (interquartile ranges).

RESULTS

Current perception, pain perception, and pain tolerance thresholds were similar in the red-haired and dark-haired women at 2,000, 250, and 5 Hz. In contrast, redheads were more sensitive to cold pain perception (22.6 [15.1-26.1] vs. 12.6 [0-20] degrees C; P = 0.004), cold pain tolerance (6.0 [0-9.7] vs. 0.0 [0.0-2.0] degrees C; P = 0.001), and heat pain (46.3 [45.7-47.5] vs. 47.7 [46.6-48.7] degrees C; P = 0.009). Subcutaneous lidocaine was significantly less effective in redheads (e.g., pain tolerance threshold at 2,000-Hz stimulation in redheads was 11.0 [8.5-16.5] vs. > 20.0 (14.5 to > 20) mA in others; P = 0.005).

CONCLUSION

Red hair is the phenotype for mutations of the melanocortin-1 receptor. Results indicate that redheads are more sensitive to thermal pain and are resistant to the analgesic effects of subcutaneous lidocaine. Mutations of the melanocortin-1 receptor, or a consequence thereof, thus modulate pain sensitivity.

Gonadal hormone modulation of mu, kappa, and delta opioid antinociception in male and female rats

Previous studies suggest that sex differences in morphine antinociception in rodents might be attributed to the activational effects of gonadal hormones. The present study determined whether hormonal modulation of opioid antinociception in adult rats extends to opioids other than the prototypic mu agonist morphine. Male and female rats were sham-gonadectomized (sham-GDX) or gonadectomized (GDX) and replaced with no hormone, estradiol (E2, females), progesterone (P4, females), E2+P4 (females), or testosterone (males). Approximately 28 days later, nociception was evaluated on the 50 degrees C hot plate and warm water tail withdrawal tests before and after subcutaneous administration of hydromorphone, buprenorphine, U50,488, or SNC 80. In sham-GDX (gonadally intact) rats, the mu agonists and U50,488 were less effective in females than in males in at least one nociceptive test, and the delta agonist SNC 80 was less effective in males than in females. In males, gonadectomy tended to decrease, and testosterone tended to increase antinociception produced by 3 of the 4 agonists. In females, gonadectomy and hormone treatment had more variable effects, although E2 tended to decrease mu opioid antinociception. The present results suggest that activational effects of gonadal hormones are relatively modest and somewhat inconsistent on antinociception produced by various opioid agonists in the adult rat.

PERSPECTIVE

This study demonstrates that reproductive hormones such as testosterone in males and estradiol in females do not consistently modulate sensitivity to the analgesic effects of opioids in the adult organism.

Sex-related psychological predictors of baseline pain perception and analgesic responses to pentazocine

Sex differences in pain perception and analgesic responses have garnered increasing attention in recent years. We examined the association of psychological factors to baseline pain perception and pentazocine analgesia among 49 healthy women and 39 men. Subjects completed psychological questionnaires measuring positive and negative affect as well as catastrophizing. Subsequently, responses to experimental pain were assessed before and after double-blind administration of intravenous pentazocine (0.5mg/kg). In correlational analyses, positive affect predicted lower pain sensitivity among men but not women. Negative affect predicted lower baseline pain tolerances among both sexes but predicted poorer analgesia only among men. Catastrophizing was associated with greater pain sensitivity and less analgesia more consistently in men than women. Regression models revealed that positive affect predicted lower overall pain sensitivity and catastrophizing predicted poorer overall analgesic responses among men, while no significant predictors of overall pain or analgesia emerged for women. Moreover, positive affect and catastrophizing were negatively and positively correlated, respectively, with side effects from the medication, but only among men. These findings indicate sex-dependent associations of psychological factors with baseline pain perception, analgesic responses, and medication side effects.

The role of endogenous opioids in the placebo effect in post-traumatic stress disorder

The concept of the placebo effect has received a considerable attention over the past several decades. The placebo effect has been observed in different psychiatric disorders, including post-traumatic stress disorder (PTSD), a chronic and severe disorder precipitated by exposure to a psychologically distressing event. The placebo response rates in patients with PTSD range from 19% to 62%. A considerable number of research publications suggest that endogenous opioids are involved in the mechanisms of the placebo effect. Endogenous opioid peptides play an important role in stress response and in the pathophysiology of PTSD. Therefore, endogenous opioids may be involved in the neurobiology of the placebo effect in PTSD. Possibly, the endogenous opioid system mediates the effect of placebo on all 3 PTSD symptom clusters (re-experiencing symptoms, avoidance and numbing, and physiologic arousal). The placebo effect-related activation of the endogenous opioid system may result in an improvement in intrusive symptomatology and symptoms of increased arousal because the administration of exogenous opioids improve these symptoms. The placebo effect-related activation of the endogenous opioid system may have a mood-enhancing effect, and, consequently, diminish avoidance and numbing. Multiple neurotransmitter and neuroendocrine pathways may be involved in the mechanisms of the placebo effect in PTSD. Further studies of the neurobiology of the placebo effect on patients with PTSD and other psychiatric disorders may produce interesting and important results.

Psychiatric comorbidity in fibromyalgia

This review examines the current literature regarding psychiatric comorbidities associated with fibromyalgia. The aim of this review is to enhance understanding of psychiatric disorders that, alone or in combination with other physiologic (eg, neuroendocrine dysfunction) and psychosocial factors (eg, poor coping skills), may contribute to abnormal pain sensitivity and other illness behaviors of individuals with fibromyalgia. The review first identifies the psychiatric comorbidities that are associated most often with fibromyalgia and tend to aggregate within families of individuals with this disorder. It then examines the literature regarding the extent to which psychiatric illness, environmental stressors, or other psychosocial factors may contribute to the development of fibromyalgia. The review also presents recent findings concerning the extent to which psychosocial factors may contribute to treatment-related outcomes in pain and other health status variables among patients with fibromyalgia.

Complex trait analysis of gene expression uncovers polygenic and pleiotropic networks that modulate nervous system function

Patterns of gene expression in the central nervous system are highly variable and heritable. This genetic variation among normal individuals leads to considerable structural, functional and behavioral differences. We devised a general approach to dissect genetic networks systematically across biological scale, from base pairs to behavior, using a reference population of recombinant inbred strains. We profiled gene expression using Affymetrix oligonucleotide arrays in the BXD recombinant inbred strains, for which we have extensive SNP and haplotype data. We integrated a complementary database comprising 25 years of legacy phenotypic data on these strains. Covariance among gene expression and pharmacological and behavioral traits is often highly significant, corroborates known functional relations and is often generated by common quantitative trait loci. We found that a small number of major-effect quantitative trait loci jointly modulated large sets of transcripts and classical neural phenotypes in patterns specific to each tissue. We developed new analytic and graph theoretical approaches to study shared genetic modulation of networks of traits using gene sets involved in neural synapse function as an example. We built these tools into an open web resource called WebQTL that can be used to test a broad array of hypotheses.

Endogenous and exogenous melanocortin antagonists induce anti-allodynic effects in a model of rat neuropathic pain

A number of studies suggest melanocortin (MC) system involvement in nociceptive modulation. Although the mechanism through which this occurs is still unknown, experimental evidence would suggest a primary role of MC4 receptors. To further investigate the implication of this MC receptor subtype in chronic pain, we have studied the effects of several MC antagonists on spinal nerve ligation-induced nociceptive behavior in rats. The intrathecal injection of synthetic antagonists with different selectivity to MC4 receptor and of an endogenous antagonist (Agouti related protein; AgRP) reduced mechanical allodynia in neuropathic rats, as measured by von Frey hair test. Treatments produced an anti-allodynic effect at the dose of 1.5 nmol (25-30% maximum possible effect, MPE, P<0.05). To further investigate the possible physiological role of AgRP in pain modulation we studied its expression in both sham and neuropathic rat spinal cord and dorsal root ganglia (DRG) by quantitative real time PCR and immunohistochemistry. AgRP was present in both spinal cord and DRG, and its expression, was unchanged in neuropathic animals. In conclusion MC4 receptor antagonists with different selectivity profile, induce anti-allodynic effects in one of the most relevant neuropathic pain model. In addition the expression of AgRP in spinal cord and DRG suggests an endogenous tonic inhibitory control on MC system activity. In pathological conditions this steady control could be insufficient to cope with an over activated MC system leading to increase in nociception. These data suggest that targeting MC4 with synthetic antagonists could restore the balance and hence reduce nociception.

Pharmacokinetics of the plant-derived κ-opioid hallucinogen salvinorin A in nonhuman primates

Salvinorin A, a potent hallucinogen isolated from the leaves of Salvia divinorum, has gained popularity among adolescents in the USA. No detailed study of the pharmacokinetics has been conducted in vivo. The present study investigates the in vivo pharmacokinetics of salvinorin A (0.032 mg/kg, i.v. bolus) in rhesus monkeys (n=4, 2 male, 2 female). The elimination t(1/2) was rapid (56.6+/-24.8 min) for all subjects. Pharmacokinetic differences (distribution t(1/2), elimination t(1/2), and AUC) were observed between males and females, suggesting potential sex differences in its pharmacologic effects. Salvinorin B, the presumed major metabolite, is observed to accumulate ex vivo; however, in this study it never reached the limit of detection.

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.

Sex differences in opioid analgesia: clinical and experimental findings

Sex differences in analgesic responses to opioids have received increasing attention in recent years. This article examines the literature on sex differences in opioid analgesia, including the results of studies from the authors' own laboratories. In general, nonhuman animal studies suggest more robust opioid analgesic responses in males relative to females; however, the human studies completed to date seem to indicate greater opioid analgesia among females. The most consistent evidence of sex differences in analgesia comes from studies of kappa-agonist-antagonists administered to patients following oral surgery. These data indicate more robust analgesia in females, and dose-response characteristics suggest that these agents possess both analgesic and antianalgesic properties, and the agonists may produce these effects in different proportions for women versus men. In contrast, the data from laboratory pain models in humans suggest greater analgesic effects in women in response mu-opioid agonists but not kappa-agonist-antagonists. Multiple mechanisms may explain sex differences in opioid analgesia, including gonadal hormonal effects, pharmacokinetics and pharmacodynamics, genetic influences, balance of analgesic/antianalgesic processes, and psychological factors. However, the disparity of results obtained from different pain models--animals versus humans and clinical pain versus experimental pain in humans--suggests that the models themselves are mechanistically different. Additional investigation is warranted in order to further explicate the nature of sex differences in opioid analgesia and to elucidate the underlying mechanisms.

Estrogen attenuates antinociception produced by stimulation of Kölliker-Fuse nucleus in the rat

This is the first demonstration of sex-related differences in the alpha2-adrenoceptor-mediated antinociceptive effects produced by stimulation of an endogenous noradrenergic pathway. Electrical or chemical (substance P) stimulation of Kölliker-Fuse nucleus (KF, A7) is known to produce antinociception mediated by alpha2-adrenoceptors in the spinal cord. KF stimulation has also been shown to inhibit the responses of nociceptive neurons in the dorsal horn of the medulla and the spinal cord. We investigated whether KF stimulation produces sex-specific modulation of trigeminal nociception. The N-methyl-D-aspartic acid (NMDA)-induced nociceptive behavior was employed as an index of nociception. Microinjection of NMDA (2 nmol/10 microL) in the trigeminal region produced nociceptive scratching behavior that was confined to the orofacial region. Male and ovariectomized (OVX) Sprague-Dawley rats were implanted with a guide cannula dorsal to the KF nucleus and a PE-10 cannula in the trigeminal region dorsal to obex. Nociceptive testing was conducted after 5-7 days of recovery. A group of ovariectomized rats (OVX+E) was treated with estradiol benzoate 48 h prior to nociceptive testing. There were no significant differences in the number of NMDA-induced scratches or duration between the male, OVX and OVX+E groups. Microinjection of substance P (3.7 pmol/0.5 microL) in the KF significantly reduced the number of NMDA-induced scratches and their duration in male and OVX groups; these were restored to control levels by yohimbine (30 microg/15 microL), an alpha2-adrenoceptor antagonist. However, KF stimulation failed to inhibit the NMDA-induced scratching behavior in the OVX+E group. We conclude that stimulation of KF produces estrogen-dependent modulation of nociception.

Sexual dimorphism in the antinociception mediated by kappa opioid receptors in the rat temporomandibular joint

This study assessed the effect of the kappa opioid receptor agonist U50,488 administered into the rat temporomandibular joint (TMJ) on nociceptive behavioral responses evoked by formalin injected into the same site. Groups consisted of females, stratified into proestrus and diestrus phases of the estrous cycle, and males. Intra-TMJ formalin induced significantly different dose-dependent responses among the three groups, with diestrus females showing greater responses than males or proestrus females; therefore, equi-nociceptive formalin doses were chosen to test the effects of U50,488. U50,488 significantly reduced formalin-induced nociceptive behavior in all groups, but the reduction was significantly greater in females, especially those in diestrus. Pre-injection of the selective kappa opioid receptor antagonist nor-binaltorphimine (nor-BNI) into the same site significantly attenuated the effect of U50488; U50,488 injection into the contralateral TMJ failed to reduce nociceptive behavior. These findings support a role for kappa opioid receptors local to the site of inflammation to modulate inflammatory pain. Furthermore, since plasma levels of ovarian hormones are low during diestrus, these findings are consistent with the suggestion that sex hormones may play an antagonistic role in these peripheral kappa-mediated effects.

Genetic basis for individual variations in pain perception and the development of a chronic pain condition

Pain sensitivity varies substantially among humans. A significant part of the human population develops chronic pain conditions that are characterized by heightened pain sensitivity. We identified three genetic variants (haplotypes) of the gene encoding catecholamine-O-methyltransferase (COMT) that we designated as low pain sensitivity (LPS), average pain sensitivity (APS) and high pain sensitivity (HPS). We show that these haplotypes encompass 96% of the human population, and five combinations of these haplotypes are strongly associated (P=0.0004) with variation in the sensitivity to experimental pain. The presence of even a single LPS haplotype diminishes, by as much as 2.3 times, the risk of developing myogenous temporomandibular joint disorder (TMD), a common musculoskeletal pain condition. The LPS haplotype produces much higher levels of COMT enzymatic activity when compared with the APS or HPS haplotypes. Inhibition of COMT in the rat results in a profound increase in pain sensitivity. Thus, COMT activity substantially influences pain sensitivity, and the three major haplotypes determine COMT activity in humans that inversely correlates with pain sensitivity and the risk of developing TMD.

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.

The genetics of sun sensitivity in humans

Humans vary >100-fold in their sensitivity to the harmful effects of ultraviolet radiation. The main determinants of sensitivity are melanin pigmentation and less-well-characterized differences in skin inflammation and repair processes. Pigmentation has a high heritability, but susceptibility to cancers of the skin, a key marker of sun sensitivity, is less heritable. Despite a large number of murine coat-color mutations, only one gene in humans, the melanocortin 1 receptor (MC1R), is known to account for substantial variation in skin and hair color and in skin cancer incidence. MC1R encodes a 317-amino acid G-coupled receptor that controls the relative amounts of the two major melanin classes, eumelanin and pheomelanin. Most persons with red hair are homozygous for alleles of the MC1R gene that show varying degrees of diminished function. More than 65 human MC1R alleles with nonsynonymous changes have been identified, and current evidence suggests that many of them vary in their physiological activity, such that a graded series of responses can be achieved on the basis of (i) dosage effects (of one or two alleles) and (ii) individual differences in the pharmacological profile in response to ligand. Thus, a single locus, identified within a Mendelian framework, can contribute significantly to human pigmentary variation.

Modulation of morphine analgesia by site-specific N-methyl-D-aspartate receptor antagonists: dependence on sex, site of antagonism, morphine dose, and time

Pharmacological blockade of N-methyl-D-aspartate (NMDA) receptors can modulate morphine analgesia in experimental animals and humans. However, this literature is highly inconsistent, with NMDA receptor antagonists variously shown to potentiate, attenuate or produce no effect on morphine analgesic magnitude. A number of factors influencing this modulation have been proposed, but no one has examined such factors simultaneously, and all existing studies in mice were conducted exclusively in male subjects. Thus, the influence of systemic administration of site-specific NMDA receptor antagonists-including dextromethorphan, dextrorphan, MK-801, LY235959, L-701,324, and Ro 25-6981-on morphine analgesia (15-45 mg/kg; 15, 30 and 60 min post-injection) was studied in male and female mice using the 49 degrees C tail-withdrawal test. We found that oral and intraperitoneal dextromethorphan, a low-affinity non-competitive antagonist, dose-dependently potentiated low-dose morphine analgesia but attenuated high-dose morphine analgesia. Dextrorphan and MK-801 were found to potentiate low- but not high-dose morphine analgesia. The competitive glutamate-site antagonist, LY235959, and glycine-site antagonist, L-701,324, potentiated morphine analgesia at all doses. In contrast, the polyamine (NR2B) site antagonist, Ro 25-6981, attenuated morphine analgesia at all doses. Strikingly, the non-competitive antagonists produced no modulation of morphine analgesia whatsoever in female mice, whereas no sex differences were observed using competitive or NR2B antagonists. These findings indicate that NMDA modulation of morphine analgesia is critically influenced by sex, site of antagonism, morphine dose and time after injection. Our data suggest that NMDA antagonism via competitive or glycine site antagonism might result in more reliable clinical effects on morphine analgesia in both sexes.

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.

Anesthetic requirement is increased in redheads

BACKGROUND

Age and body temperature alter inhalational anesthetic requirement; however, no human genotype is associated with inhalational anesthetic requirement. There is an anecdotal impression that anesthetic requirement is increased in redheads. Furthermore, red hair results from distinct mutations of the melanocortin-1 receptor. Therefore, the authors tested the hypothesis that the requirement for the volatile anesthetic desflurane is greater in natural redheaded than in dark-haired women.

METHODS

The authors studied healthy women with bright red (n = 10) or dark (n = 10) hair. Blood was sampled for subsequent analyses of melanocortin-1 receptor alleles. Anesthesia was induced with sevoflurane and maintained with desflurane randomly set at an end-tidal concentration between 5.5 and 7.5%. After an equilibration period, a noxious electrical stimulation (100 Hz, 70 mA) was transmitted through bilateral intradermal needles. If the volunteer moved in response to stimulation, desflurane was increased by 0.5%; otherwise, it was decreased by 0.5%. This was continued until volunteers "crossed over" from movement to nonmovement (or vice versa) four times. Individual logistic regression curves were used to determine desflurane requirement (P50). Desflurane requirements in the two groups were compared using Mann-Whitney nonparametric two-sample test; P < 0.05 was considered statistically significant.

RESULTS

The desflurane requirement in redheads (6.2 vol% [95% CI, 5.9-6.5]) was significantly greater than in dark-haired women (5.2 vol% [4.9-5.5]; P = 0.0004). Nine of 10 redheads were either homozygous or compound heterozygotes for mutations on the melanocortin-1 receptor gene.

CONCLUSIONS

Red hair seems to be a distinct phenotype linked to anesthetic requirement in humans that can also be traced to a specific genotype.

The use of “as-needed” range orders for opioid analgesics in the management of acute pain: a consensus statement of the American Society for Pain Management Nursing and the American Pain Society

The use of "as needed" or "PRN" range orders for opioid analgesics in the management of acute pain is a common clinical practice. This approach provides flexibility in dosing to meet individual patients' unique analgesic requirements. Range orders enable necessary and safe dose adjustments based on an individual's response to treatment. The purpose of this paper is to present the consensus statement of the American Society for Pain Management Nursing and the American Pain Society on the use of "as-needed" range orders for opioid analgesics in the management of acute pain. The implementation of this statement should promote quality pain management through safe medication practices and the appropriate use of range orders for opioid analgesics in acute pain management.

Paclitaxel-induced neuropathic hypersensitivity in mice: Responses in 10 inbred mouse strains

Mechanical allodynia, or hypersensitivity to tactile stimuli, is a frequent clinical symptom of neuropathy. Large interindividual differences have been observed in neuropathic pain, both in susceptibility to its development and in its severity. Identification of genetic factors relevant to this variability would be of obvious utility. Although many animal models of neuropathic pain following peripheral nerve injury have been developed, most involve intricate surgeries and are thus poorly suited for large-scale linkage mapping investigations in the mouse. Recently, a schedule of intraperitoneal injections of the chemotherapeutic agent, paclitaxel (Taxol(R)), has been shown to produce a long-lasting, bilateral neuropathy in the rat, featuring hypersensitivity to mechanical, thermal and cold stimuli. We present here a survey of the responses of 10 inbred mouse strains to paclitaxel injections. Virtually all strains developed statistically significant mechanical allodynia, with one strain, DBA/2J, exhibiting especially robust changes. Strain sensitivities to paclitaxel-induced mechanical allodynia were similar to those obtained previously using a surgical model of neuropathic pain, supporting our contention that genetic sensitivity to mechanical allodynia is independent of the precise mode of induction. Using sensitive DBA/2 mice and a resistant strain, C57BL/6J, for comparison, we further characterized the paclitaxel model in mice by examining cold allodynia and thermal hyperalgesia. Both strains displayed equivalent cold allodynia but neither strain developed thermal hyperalgesia. The present data confirm a genetic component in mechanical allodynia using this model, while dissociating mechanical hypersensitivity from other pain modalities.

Kappa opioid receptors in rat spinal cord: sex-linked distribution differences

Activation of kappa opioid receptors (KORs) in the spinal cord can diminish nociception. Humans and rodents show sex differences in the analgesia produced by KOR agonists, and female rats show fluctuations in KOR density and sensitivity across the estrous cycle. However, it is unclear whether there are sex differences in the amount and/or distribution of spinal KORs. In the present study, immunocytochemically labeled KORs were examined in laminae I and II of the lumbosacral spinal dorsal horn of male and normally cycling female Sprague-Dawley rats. The basic pattern of KOR labeling was determined in both sexes using qualitative electron microscopy (EM), and sex-linked differences in the density and subcellular distribution of KOR immunoreactivity were determined with quantitative EM and light microscopy. KOR labeling was visualized with immunoperoxidase for optimally sensitive detection, or with immunogold for precise subcellular localization. By EM, the general pattern of KOR immunoreactivity was similar in males and females. KOR immunoreactivity was common in dendrites, axons, and axon terminals, and was in a few glia and neuronal somata. Most KOR-immunoreactive (-ir) axons were fine-diameter and unmyelinated. Most KOR-ir terminals were small or medium-sized, and a minority formed asymmetric or symmetric synapses with unlabeled dendrites. KOR immunoreactivity was associated both with the plasma membrane and with cytoplasmic organelles, notably including dense core vesicles in terminals. Light microscopic densitometry revealed that KOR immunoreactivity was significantly denser in estrus and proestrus females than in males. By EM, the distribution of KOR-immunogold labeling within axon terminals differed, with a greater proportion of cytoplasmic KOR labeling in estrus females compared with males. In contrast, the abundance and types of KOR-immunoperoxidase-labeled profiles did not show sex-linked differences. We conclude that in both sexes, KORs are positioned to influence both pre- and postsynaptic neurotransmission and are present in morphologically heterogeneous neuron populations. These findings are consistent with complex consequences of KOR activation in the spinal cord. In addition, the presence of increased KOR density and proportionally elevated intracellular KORs in proestrus/estrus females suggests a basis for sex-linked differences in KOR-mediated antinociception.

Do hyporesponsive genetic variants of the melanocortin 1 receptor contribute to the etiology of multiple sclerosis?

Hyporesponsive genetic variants of the melanocortin 1 receptor result in pigmentary phenotypes exhibiting light skin and light color hair, including red hair. These variants are common in populations with high rates of multiple sclerosis, while rare in populations with low rates. Alpha-melanocyte stimulating hormone, the major ligand for this receptor, is responsible for phenotype determination, but is also known for its anti-inflammatory and immune modulating effects, including inhibition of factors implicated in multiple sclerosis pathology. As the melanocortin 1 receptor is expressed on various cell types involved in immune response, it is possible that carriers of hyporesponsive variants of this receptor lack the full anti-inflammatory and immune modulating effects of alpha-melanocyte stimulating hormone. It is proposed that these variants are part of a spectrum of genes involved in the etiology of multiple sclerosis. Related aspects of multiple sclerosis epidemiology are examined.

Internal and External Factors Affecting the Development of Neuropathic Pain in Rodents. Is It All About Pain?

It is important to know the factors that will influence animal models of neuropathic pain. A good reproducibility and predictability in different strains of animals for a given test increases the clinical relevance and possible targeting. An obligatory requirement for enabling comparisons of results of different origin is a meticulous definition of the specific sensitivities of a model for neuropathic pain and a description of the test conditions. Factors influencing neuropathic pain behavior can be subdivided in external and internal factors. The most important external factors are; timing of the measurement of pain after induction of neuropathy, circadian rhythms, seasonal influences, air humidity, influence of order of testing, diet, social variables, housing and manipulation, cage density, sexual activity, external stress factors, and influences of the experimenter. The internal factors are related to the type of animal, its genetic background, gender, age, and the presence of homeostatic adaptation mechanisms to specific situations or stress. In practice, the behavioral presentations to pain depend on the combination of genetic and environmental factors such as accepted social behavior. It also depends on the use of genetic manipulation of the animals such as in transgenic animals. These make the interpretation of data even more difficult. Differences of pain behavior between in- and outbred animals will be better understood by using modern analysis techniques. Substrains of animals with a high likelihood for developing neuropathic pain make the unraveling of specific pathophysiological mechanisms possible. Concerning the effect of stress on pain, it is important to differentiate between external and internal stress such as social coping behavior. The individual dealing with this stress is species sensitive, and depends on the genotype and the social learning. In the future, histo-immunological and genetic analysis will highlight similarities of the different pathophysiological mechanisms of pain between different species and human subjects. The final objective for the study of pain is to describe the genetics of the eliciting pain mechanisms in humans and to look for correlations with the knowledge from basic research. Therefore, it is necessary to know the genetic evolution of the different mechanisms in chronic pain. In order to be able to control the clinical predictability of a putative treatment the evolutionary pharmacogenomic structure of specific transmitters and receptors must be clarified.

Pharmacogenomics of multigenic diseases: sex-specific differences in disease and treatment outcome

Numerous genetic variations have been shown to affect disease susceptibility and drug response. Pharmacogenomics aims at improving therapy on the basis of genetic information for each individual patient. Furthermore, sex chromosomes broadly determine biological differences between males and females. Consequently, substantial sex differences exist in phenotypic manifestation of disease and treatment response. This review discusses the role of sex in coronary artery disease, schizophrenia, and depression--complex multigenic disorders with considerable sex differences in frequency and presentation. Moreover, genetic factors underlying disease and drug response appear to differ between male and female patients. This appears to result at least in part from different physiological effects exerted by sex hormones such that polymorphisms in susceptibility genes may have physiological relevance only in males or females. However, few examples have been discovered to play a role in complex multigenic diseases, and the mechanistic basis of genetic variants as sex-dependent susceptibility factors has yet to be explored. Therefore, pharmacogenomic studies must consider sex differences in an effort to optimize individual drug therapy.