A gain-of-function mutation in TRPA1 causes familial episodic pain syndrome.

Human monogenic pain syndromes have provided important insights into the molecular mechanisms that underlie normal and pathological pain states. We describe an autosomal-dominant familial episodic pain syndrome characterized by episodes of debilitating upper body pain, triggered by fasting and physical stress. Linkage and haplotype analysis mapped this phenotype to a 25 cM region on chromosome 8q12-8q13. Candidate gene sequencing identified a point mutation (N855S) in the S4 transmembrane segment of TRPA1, a key sensor for environmental irritants. The mutant channel showed a normal pharmacological profile but altered biophysical properties, with a 5-fold increase in inward current on activation at normal resting potentials. Quantitative sensory testing demonstrated normal baseline sensory thresholds but an enhanced secondary hyperalgesia to punctate stimuli on treatment with mustard oil. TRPA1 antagonists inhibit the mutant channel, promising a useful therapy for this disorder. Our findings provide evidence that variation in the TRPA1 gene can alter pain perception in humans.

Pain channelopathies.

Pain remains a major clinical challenge, severely afflicting around 6% of the population at any one time. Channelopathies that underlie monogenic human pain syndromes are of great clinical relevance, as cell surface ion channels are tractable drug targets. The recent discovery that loss-of-function mutations in the sodium channel Nav1.7 underlie a recessive pain-free state in otherwise normal people is particularly significant. Deletion of channel-encoding genes in mice has also provided insights into mammalian pain mechanisms. Ion channels expressed by immune system cells (e.g. P2X7) have been shown to play a pivotal role in changing pain thresholds, whilst channels involved in sensory transduction (e.g. TRPV1), the regulation of neuronal excitability (potassium channels), action potential propagation (sodium channels) and neurotransmitter release (calcium channels) have all been shown to be potentially selective analgesic drug targets in some animal pain models. Migraine and visceral pain have also been associated with voltage-gated ion channel mutations. Insights into such channelopathies thus provide us with a number of potential targets to control pain.

Regulation of firing frequency in nociceptive neurons by pro-inflammatory mediators.

Nociceptive neurons generate trains of action potentials in response to painful stimuli, and the frequency of firing signals the intensity of the pain. Pro-inflammatory mediators such as prostaglandin E2 (PGE2) enhance the sensation of pain by increasing the frequency of action potential firing in response to a given level of painful stimulus. The mechanism by which the firing frequency is enhanced is discussed in the present review. One hypothesis proposes that the threshold for action potential initiation is lowered because the activation curve of a nociceptor-specific voltage-activated Na current, Na(V)1.8, is shifted to more negative values by PGE2. Recent measurements in our lab show, however, that the action potential threshold in fact changes little when AP firing is accelerated by PGE2. The enhanced firing is, however, abolished by a blocker of an inward current activated by hyperpolarisation, called I(h). The voltage sensitivity of I(h) shifts in the positive direction in small nociceptive neurons when they are exposed to pro-inflammatory mediators, such as PGE2, which activate adenylate cyclase and therefore increase levels of cAMP. By this mechanism the inward current between the resting membrane potential and the threshold for firing of action potentials is enhanced, and the rate of depolarisation in the interval between action potentials is therefore increased. We conclude that the major mechanism responsible for increasing action potential firing following tissue damage or metabolic stress is the hyperpolarisation-activated inward current, I(h), and that other mechanisms play at most a minor role.

Sensory neuron voltage-gated sodium channels as analgesic drug targets.

Voltage-gated sodium channels are crucial determinants of neuronal excitability and signalling; some specific channel subtypes have been implicated in a number of chronic pain conditions. Human genetic studies show gain-of-function or loss-of-function mutations in Na(V)1.7 lead to an enhancement or lack of pain, respectively, whilst transgenic mouse and knockdown studies have implicated Na(V)1.3, Na(V)1.8 and Na(V)1.9 in peripheral pain pathways. The development of subtype-specific sodium channel blockers, though clearly desirable, has been technically challenging. Recent advances exploiting both natural products and small molecule selective channel blockers have demonstrated that this approach to pain control is feasible. These observations provide a rationale for the development of new analgesics without the side effect profile of broad spectrum sodium channel blockers.

Role of the hyperpolarization-activated current Ih in somatosensory neurons.

The hyperpolarization-activated current (I(h)) is an inward current activated by hyperpolarization from the resting potential and is an important modulator of action potential firing frequency in many excitable cells. Four hyperpolarization-activated, cyclic nucleotide-modulated subunits, HCN1-4, can form I(h) ion channels. In the present study we investigated the function of I(h) in primary somatosensory neurons. Neuronal firing in response to current injection was promoted by elevating intracellular cAMP levels and inhibited by blockers of I(h), suggesting that I(h) plays a critical role in modulating firing frequency. The properties of I(h) in three size classes of sensory neurons were next investigated. In large neurons I(h) was fast activating and insensitive to elevations in cAMP, consistent with expression of HCN1. I(h) was ablated in most large neurons in HCN1(-/-) mice. In small neurons a slower activating, cAMP-sensitive I(h) was observed, as expected for expression of HCN2 and/or HCN4. Consistent with this, I(h) in small neurons was unchanged in HCN1(-/-) mice. In a neuropathic pain model HCN1(-/-) mice exhibited substantially less cold allodynia than wild-type littermates, suggesting an important role for HCN1 in neuropathic pain. This work shows that I(h) is an important modulator of action potential generation in somatosensory neurons.