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. One hypothesis to explain the increase proposes that the threshold for action potential initiation is lowered because the activation curve of a nociceptor-specific voltage-activated Na current, NaV1.8, is shifted to more negative values by PGE₂ (Akopian et al., 1996; England et al., 1996). Recent measurements in our lab show, however, that the action potential threshold in fact changes little when AP firing is accelerated by PGE₂ (Momin et al., 2008). 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 I_h is a major mechanism responsible for increasing the frequency of action potential firing following tissue damage or metabolic stress. The HCN isoforms responsible for I_h in sensory neurons have also been investigated. Genetic deletion of HCN1 ablates the fast-activating I_h which is seen in large neurons and in a small sub-population of cold-sensitive neurons, but leaves unaffected the slowly-activating I_h seen in small neurons. Interestingly, the cold allodynia seen in a neuropathic pain model is also found to be reduced in HCN1^-/- mice. We are now investigating the effects of deletion of other HCN isoforms on the I_h current and on pain thresholds.