Persistent hindlimb inflammation induces changes in activation properties of hyperpolarization-activated current (Ih) in rat C-fiber nociceptors in vivo.

Abstract A hallmark of chronic inflammation is hypersensitivity to noxious and innocuous stimuli. This inflammatory pain hypersensitivity results partly from hyperexcitability of nociceptive dorsal root ganglion (DRG) neurons innervating inflamed tissue, although the underlying ionic mechanisms are not fully understood. However, we have previously shown that the nociceptor hyperexcitability is associated with increased expression of hyperpolarization-activated cyclic nucleotide-gated channel 2 (HCN2) protein and hyperpolarization-activated current ( I h ) in C-nociceptors. Here we used in vivo voltage-clamp and current-clamp recordings, in deeply anesthetized rats, to determine whether activation properties of I h in these C-nociceptors also change following persistent (not acute) hindlimb inflammation induced by complete Freund’s adjuvant (CFA). Recordings were made from lumbar (L4/L5) C-nociceptive DRG neurons. Behavioral sensory testing was performed 5–7 days after CFA treatment, and all the CFA-treated group showed significant behavioral signs of mechanical and heat hypersensitivity, but not spontaneous pain. Compared with control, C-nociceptors recorded 5–7 days after CFA showed: (a) a significant increase in the incidence of spontaneous activity (from ∼5% to 26%) albeit at low rate (0.14 ± 0.08 Hz (Mean ± SEM); range, 0.01–0.29 Hz), (b) a significant increase in the percentage of neurons expressing I h (from 35%, n  = 43–84%, n  = 50) based on the presence of voltage “sag” of >10%, and (c) a significant increase in the conductance (G h ) of the somatic channels conducting I h along with the corresponding I h, I h, activation rate, but not voltage dependence, in C-nociceptors. Given that activation of I h depolarizes the neuronal membrane toward the threshold of action potential generation, these changes in I h kinetics in CFA C-nociceptors may contribute to their hyperexcitability and thus to pain hypersensitivity associated with persistent inflammation.