Local inflammation in rat dorsal root ganglion alters excitability and ion currents in small-diameter sensory neurons.
BACKGROUND
Chronic pain conditions may result from peripheral nerve injury, chronic peripheral inflammation, or sensory ganglia inflammation. However, inflammatory processes may also contribute to peripheral nerve injury responses. To isolate the contribution of local inflammation of sensory ganglia to chronic pain states, the authors previously developed a rat model in which long-lasting pain is induced by inflaming sensory ganglia without injuring the neurons. This results in prolonged mechanical pain, local increases in proinflammatory cytokines, increased neuronal hyperexcitability, and abnormal spontaneous activity.


METHODS
The authors used whole cell patch clamp in acutely isolated small-diameter neurons to determine how localized inflammation (3-5 days) of L4 and L5 ganglia altered voltage-gated K and Na currents.


RESULTS
Tetrodotoxin-sensitive Na currents increased twofold to threefold in neurons from inflamed ganglia. Tetrodotoxin-resistant Na currents increased more than twofold, but only in cells that bound isolectin B4. These increases occurred without shifts in voltage dependence of activation and inactivation. Similar results are seen in models of peripheral inflammation, except for the large magnitudes. Unlike most pain models, localized inflammation increased rather than decreased voltage-gated K currents, due to increased amplitudes of the sustained (delayed rectifier) and fast-inactivating transient components. The overall effect in current clamp experiments was an increase in excitability as indicated by decreased rheobase and lower action potential threshold.


CONCLUSIONS
Neuronal inflammation per se, in the absence of nerve injury, causes large increases in Na channel density and enhanced excitability. The unusual finding of increased K current may reflect regulation of excitability in the face of such large increases in Na current.
Pain Research Center, Department of Anesthesiology, University of Cincinnati College of Medicine, Cincinnati, Ohio, USA.
Jun-Gang
JG
Wang
Judith A
JA
Strong
Wenrui
W
Xie
Jun-Ming
JM
Zhang
D000200:Action Potentials / Q000187:drug effects; D000779:Anesthetics, Local / Q000008:administration & dosage; D000818:Animals; D002478:Cells, Cultured; D004195:Disease Models, Animal; D004553:Electric Conductivity; D004594:Electrophysiology / Q000379:methods; D005260:Female; D005727:Ganglia, Spinal / Q000166:cytology / Q000503:physiopathology*; D007249:Inflammation / Q000503:physiopathology*; D015640:Ion Channel Gating; D008564:Membrane Potentials / Q000187:drug effects; D009431:Neural Conduction / Q000187:drug effects; D009475:Neurons, Afferent / Q000187:drug effects / Q000378:metabolism*; D018408:Patch-Clamp Techniques / Q000379:methods; D024642:Potassium Channels, Voltage-Gated / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D015222:Sodium Channels / Q000187:drug effects / Q000378:metabolism*; D013779:Tetrodotoxin / Q000008:administration & dosage
D000779:Anesthetics, Local; D024642:Potassium Channels, Voltage-Gated; D015222:Sodium Channels; D013779:Tetrodotoxin