Chloride Channels in Nociceptors

Abstract

Pain may be induced by activation of various ion channels expressed in primary afferent neurons. These channels function as molecular sensors that detect noxious chemical, temperature, or tactile stimuli and transduce them into nociceptor electrical signals. Transient receptor potential channels are good examples because they are activated by chemicals, heat, cold, and acid in nociceptors. Anion channels were little studied in nociception because of the notion that anion channels might induce hyperpolarization of nociceptors on opening. In contrast, opening of Cl- channels in dorsal root ganglion (DRG) neurons depolarizes sensory neurons, resulting in excitation of nociceptors, thereby inducing pain. Anoctamin 1(ANO1)/TMEM16A is a Ca2+-activated Cl- channel expressed mainly in small DRG neurons, suggesting a nociception role. ANO1 is a heat sensor that detects heat over 44°C. Ano1-deficient mice elicit less nocifensive behaviors to hot temperatures. In addition, mechanical allodynia and hyperalgesia induced by inflammation or nerve injury are alleviated in Ano1 -/- mice. More important, Ano1 transcripts are increased in chronic pain models. Bestrophin 1 (Best1) is another Ca2+-activated Cl- channel expressed in nociceptors. Best1 is increased in axotomized DRG neurons. The role of Best1 in nociception is not clear. GABAA receptors are in the central process of DRG neurons; GABA depolarizes the primary afferents. This depolarization consists of primary afferent depolarization essential for inhibiting nociceptive input to second-order neurons in the spinal cord, regulating pain signals to the brain. Thus, although Cl- channels in nociceptors are not as numerous as TRP channels, their role in nociception is distinct and significant.