Chronic inflammatory pain is associated with increased excitability and hyperpolarization-activated current (Ih) in C- but not Aδ-nociceptors

Inflammatory pain hypersensitivity results partly from hyperexcitability of nociceptive (damage-sensing) dorsal root ganglion (DRG) neurons innervating inflamed tissue. However, most of the evidence for this is derived from experiments using acute inflammatory states. Herein, we used several approaches to examine the impact of chronic or persistent inflammation on the excitability of nociceptive DRG neurons and on their expression of Ih and the underlying hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, which regulate neuronal excitability. Using in vivo intracellular recordings of somatic action potentials from L4/L5 DRG neurons in normal rats and rats with hindlimb inflammation induced by complete Freund's adjuvant (CFA), we demonstrate increased excitability of C- but not Aδ-nociceptors, 5 to 7 days after CFA. This included an afterdischarge response to noxious pinch, which may contribute to inflammatory mechanohyperalgesia, and increased incidence of spontaneous activity (SA) and decreased electrical thresholds, which are likely to contribute to spontaneous pain and nociceptor sensitization, respectively. We also show, using voltage clamp in vivo, immunohistochemistry and behavioral assays that (1) the inflammation-induced nociceptor hyperexcitability is associated, in C- but not Aδ-nociceptors, with increases in the mean Ih amplitude/density and in the proportion of Ih expressing neurons, (2) increased proportion of small DRG neurons (mainly IB4-negative) expressing HCN2 but not HCN1 or HCN3 channel protein, (3) increased HCN2- immunoreactivity in the spinal dorsal horn, and (4) attenuation of inflammatory mechanoallodynia with the selective Ih antagonist, ZD7288. Taken together, the findings suggest that C- but not Aδ-nociceptors sustain chronic inflammatory pain and that Ih/HCN2 channels contribute to inflammation-induced C-nociceptor hyperexcitability.