Proceedings of The Physiological Society
University of Manchester (2010) Proc Physiol Soc 19, PC237
Inflammation-induced increase in hyperpolarization-activated current (Ih) in C-fibre nociceptive dorsal root ganglion neurons in rats in vivo
X. Weng1, L. Djouhri1
1. Pharmacology, Univeristy of Liverpool, Liverpool, United Kingdom.
Hypersensitivity to painful stimuli (hyperalgesia) and/or normally non-painful stimuli (allodynia) is a hallmark of inflammation. This hypersensitivity is partly due to peripheral sensitization, in which nociceptive dorsal root ganglion (DRG) neurons exhibit increased spontaneous activity (SA) and decreased activation threshold (Woolf & Ma, 2007). The underlying mechanisms of SA generation in these sensory neurons are poorly understood. However, this sensory neuron hyperexcitability is likely to result from alterations in expression and/or activation properties of certain ion channels during chronic inflammatory states. Hyperpolarization-activated cyclic nucleotide gated (HCN) channels, which mediate Ih, are possible candidates, because they act to produce an inward current that slowly depolarizes the membrane potential influencing the threshold for action potential generation. Therefore, we examined whether after tissue inflammation, expression and/or activation properties of Ih changed in C-fibre nociceptive neurons, which exhibit SA after both nerve injury and tissue inflammation (Djouhri et al. 2006). Hindlimb inflammation was induced, under isoflurane anaesthesia (3% with O2 and N2O set at 2 l/min each), by a 150μl intradermal injection of complete Freund’s adjuvant (CFA) into the plantar surface of the left hindpaws of female Wistar rats (160-200g). Three to five days after CFA treatment, discontinuous current-clamp (DCC) and discontinuous single electrode voltage clamp (dSEVC) were performed in untreated (control) rats (n=24) and CFA treated rats (n=21) deeply anaesthetized with sodium pentobarbitone (60 mg/kg, i.p). C-fibre nociceptors were identified on the basis of their dorsal root conduction velocities (<1.0 m/s) and their responses to noxious mechanical and thermal stimuli. Ih was assessed with 1 second hyperpolarizing voltage steps from resting potential to -130 mV and was identified in vivo on the basis of its activation properties and reversal potential and by the presence of time-dependant rectification “sag” elicited by a series of depolarizing current pulses (200 ms duration). Ih was considered to be present in a C-neuron if its magnitude was >50 pA. Interestingly, not only did a significantly higher proportion of C-nociceptors in CFA rats express Ih compared to control (76% (38/50) vs. 48% (21/43), p<0.01, Chi2 test), but Ih was ≥ 100 pA in 64% of the C-nociceptors in CFA rats compared to 26% in control. Furthermore, CFA inflammation induced, in these neurons, significant increases in the mean Ih density (3.31 ± 0.65 nA (control) vs. 4.43 ± 0.72 (CFA), P<0.01, Mann Whitney test) and rate of Ih activation. These results suggest that Ih/HCN channels are involved in the hyperexcitability of C-nocicetive DRG neurons and the hypersensitivity associated with tissue inflammation.
Where applicable, experiments conform with Society ethical requirements