Box 3. Altered Pain States Clinical problems associated with the pain system occur when pain thresholds are dramatically altered so that noxious insults became extremely painful (hyperalgesia) or non-noxious stimuli, such as light touch or mild cold, also become very painful (allodynia). Hyperalgesia is commonly found in inflammatory pain conditions where damaged tissue is infiltrated with immune system cells, and a variety of soluble mediators that act on peripheral neurons are released. Allodynia may occur when nerves themselves are damaged—this is neuropathic pain, which occurs in situations such as diabetic neuropathy, HIV-associated neurop- athy, or following physical trauma. Studies have identified a number of critical regulators of altered pain states, including genes not normally expressed in the nervous system, but present in cells of the immune system that also are activated in situations of tissue damage. For example, P2X7 (P2RX7), an ATP receptor absent from neurons, is essential for altered inflammatory or neuropathic pain [82]. This receptor is found on macrophages and microglia together with P2X4, and is a likely source of the soluble mediators that affect neuronal excitability in peripheral and central neurons during neuropathic pain. Damaged peripheral neurons recruit activated microglia to the dorsal horn, probably through the same mechanisms that act on peripheral immune cells, with unfortunate consequences for central pain processing [83]. A comprehensive pain database that deals with all pain-related knock-outs can be found at http://paingeneticslab.ca/4105/06_02_pain_genetics_da- tabase.asp. Many inflammatory mediators (e.g., IL-1 and PGE2) act directly on sensory neurons and through cascades of second messengers and kinases, altering the sensitivity of both primary transducing receptors and sodium channels required for action potential transmission [84]. A pivotal role for TRPA1 in detecting environ- mental irritants has recently been identified [56,57]. Genetics of Pain in Man Substantial variations in both pain sensitivity and susceptibility to chronic painful conditions occur between individuals in both human and animal populations [34,23]. For example, human pain ratings of a given thermal stimulus can encompass the entire range of the visual-analogue scale (VAS) [34], from ‘‘almost no pain’’ to ‘‘the worst pain imaginable.’’ Pain hypersensitivity may reduce quality of life and is associated with increased susceptibility to chronic pain [35]. High pain thresholds may appear desirable, but may limit protective behaviour in response to injury or hinder clinical intervention in disease. These natural variations in propensity to pain result from a combination of environmental and genetic influences on pain- sensing systems. Environmental factors, such as early exposure to acute painful stimuli, can have long-term effects on nociceptive thresholds in both animals and humans. Perinatal painful events such as circumcision without anaesthesia, for example, have been reported to increase sensitivity to pain in later life [36], and are therefore likely to account for a proportion of the natural variability in pain perception. As discussed earlier in this review, loss- or gain-of-function genetic mutations can result in complete insensitivity to painful stimuli, or in spontaneous pain disorders (e.g., [7,8]). It therefore appears plausible that more subtle, quantitative differences in pain sensitivity may also have a genetic basis. Gender differences in pain sensitivity have been widely reported and are discussed in Box 4. Human Heritable Pain Conditions Heritability studies using sensory testing of twins can identify the importance of genetic contributions to pain traits, and SNP Box 4. Gender Differences in Pain Perception Gender differences in pain perception have been reported by many studies, in both animals and humans. For example, women are more likely to suffer from a variety of chronic pain disorders, including fibromyalgia, complex regional pain syndrome, and trigeminal neuralgia. Experimentally, pain thresholds for pressure pain and electrical stimulation have been shown to be lower for females than for males, while less variation has been observed for thermal pain stimuli. For a detailed treatment of gender differences in pain perception, see the recent and comprehensive review from Greenspan et al. [85]. A proportion of these variations may result from genetic differences at loci on the sex chromo- somes. Gonadal factors such as testosterone and estradiol modulate sensitivity to pain and analgesia [86], resulting in gender differences in pain perception. Mogil et al. [87] reported that certain sex differences in pain and analgesia appear genetically mediated. The investigation of genetically linked factors affecting pain sensitivity, however, is confounded by contributions of gender to disease processes, and by societal influences. Nevertheless, several interesting findings have been reported, including greater opioid-induced analgesia in males than females [88,89]. association studies have correlated a number of genes with altered pain behaviour (see Table 1). In contrast, some rare recessive conditions found in societies that practice consanguineous marriage lead to alterations in pain thresholds, and the genes that underlie such conditions are of major interest. Selective cell loss of peripheral neurons is a characteristic of many pain insensitivity syndromes [37]. The hereditary sensory and autonomic neurop- athies HSAN1-4 are all examples of syndromes where pain behaviour is diminished, and some sensory loss also occurs. Table 2 summarises currently known heritable pain conditions. Despite the fact that most of these syndromes involve cell death, and are thus unappealing in terms of drug development, they have provided important insights into aspects of pain signalling. For example, HSAN-3 demonstrates the essential role of NGF- dependent sensory neurons in the pain process, whilst the recent discovery of the NaV1.7-encoding SCN9A gene as an essential component of human pain provides an exciting new analgesic drug target. Contribution of Genetics to Pain Thresholds Several studies have attempted to evaluate the relative contribu- tions of genetics and environment to variations in pain sensitivity, in both animals and humans. Comparisons of pain thresholds (over 12 modalities) within and between 11 inbred laboratory mouse strains revealed genetic contributions of between 30% and 76% [23–25]. The experimental power of these studies was high due to the lack of environmental variability and the relatively large genetic differences between strains, meaning that these results are likely to represent the maximum contribution of genetic factors to pain perception. This contribution, however, appears to be dependent on both severity and modality of stimulus, with three major clusters of nociception identified [23,24]. These clusters were defined as ‘‘assays of baseline thermal nociception,’’ ‘‘spontaneously-emitted responses to chemical stimuli,’’ and ‘‘baseline mechanical sensitivity and cutaneous hypersensitivity.’’ Surprisingly, acute thermal and mechanical nociception were found to be strongly negatively correlated [24]. While there is a body of literature supporting distinct pathways for these modalities, including different primary afferent and central neurons, differential opioid modulation, and possibly different spinal cord lamina neuron location, this would be expected to result in a PLoS Genetics | www.plosgenetics.org 5 July 2008 | Volume 4 | Issue 7 | e1000086