Spinal hyperpolarization-activated cyclic nucleotide-gated cation channels at primary afferent terminals contribute to chronic pain

Hyperpolarization-activated cyclic nucleotide-gated cation channels (HCN channels) have large influences upon neuronal excitability. However, the participation of spinal HCN channels in chronic pain states, where pathological conditions are related to altered neuronal excitability, has not been clarified. Intraperitoneally (i.p.) or intrathecally (i.t.) administered ZD7288, a selective blocker of Ih channels, reduced thermal and mechanical hypersensitivity in mice under neuropathic conditions induced by partial ligation of the sciatic nerve, while no analgesic effect was observed in naive animals. Moreover, in the mouse formalin test, ZD7288 (i.p. and i.t.) reduced licking/biting behavior observed during the second phase without affecting the first phase. To further explore the pain-modulatory action of spinal HCN channels, whole-cell patch clamp recordings were made from visually identified substantia gelatinosa neurons in adult mouse spinal cord slices with an attached dorsal root, and A-fiber-and/or C-fiber-mediated monosynaptic excitatory postsynaptic currents (EPSCs) were evoked by electrical stimulation of the L4 or L5 dorsal root using a suction electrode. Bath-applied ZD7288 reduced A-fiber-and C-fiber-mediated monosynaptic EPSCs more preferentially in slices prepared from mice after peripheral nerve injury. In addition, ZD7288 reduced the frequency of miniature EPSCs without affecting their amplitude in cells receiving monosynaptic afferent inputs, indicating that it inhibits EPSCs via presynaptic mechanisms. The present behavioral and electrophysiological data suggest that spinal HCN channels, most likely at primary afferent terminals, contribute to the maintenance of chronic pain. (C) 2010 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved.