The neuronal potassium current I_A is a potential target for pain during chronic inflammation

Voltage-gated ion channels play a key role in the action potential (AP) initiation and its propagation in sensory neurons. Modulation of their activity during chronic inflammation creates a persistent pain state. In this study, we sought to determine how peripheral inflammation caused by complete Freund's adjuvant (CFA) alters the fast sodium (I_Na ), L-type calcium (I_CaL ), and potassium (I_K ) currents in primary afferent fibers to increase nociception. In our model, intraplantar administration of CFA induced mechanical allodynia and thermal hyperalgesia at day 14 post-injection. Using whole-cell patch-clamp recording in dissociated small (C), medium (Aδ), and large-sized (Aβ) rat dorsal root ganglion (DRG) neurons, we found that CFA prolonged the AP duration and increased the amplitude of the tetrodotoxin-resistant (TTX-r) I_Na in Aβ fibers. In addition, CFA accelerated the recovery of I_Na from inactivation in C and Aδ nociceptive fibers but enhanced the late sodium current (I_NaL ) only in Aδ and Aβ neurons. Inflammation similarly reduced the amplitude of I_CaL in each neuronal cell type. Fourteen days after injection, CFA reduced both components of I_K (I_Kdr and I_A ) in Aδ fibers. We also found that I_A was significantly larger in C and Aδ neurons in normal conditions and during chronic inflammation. Our data, therefore, suggest that targeting the transient potassium current I_A represents an efficient way to shift the balance toward antinociception during inflammation, since its activation will selectively decrease the AP duration in nociceptive fibers. Altogether, our data indicate that complex interactions between I_K , I_Na , and I_CaL reduce pain threshold by concomitantly enhancing the activity of nociceptive neurons and reducing the inhibitory action of Aβ fibers during chronic inflammation.