Modulation of TTX-RI_Naby PKC and PKA and Their Role in PGE₂-Induced Sensitization of Rat Sensory NeuronsIn Vitro

Abstract

A tetrodotoxin-resistant voltage-gated Na ϩ current (TTX-R I Na ) appears to be the current primarily responsible for action potential generation in the cell body and terminals of nociceptive afferents. Although other voltage-gated Na ϩ currents are modulated by the activation of protein kinase C (PKC), protein kinase A (PKA), or both, the second messenger pathways involved in the modulation of TTX-R I Na are still being defined. We have examined the modulation of TTX-R I Na in isolated sensory neurons with whole-cell voltage-clamp recording. Activation of either PKC or PKA increased TTX-R I Na . PKA activation also produced a leftward shift in the conductancevoltage relationship of TTX-R I Na and an increase in the rates of current activation, deactivation, and inactivation. Inhibitors of PKC decreased TTX-R I Na , whereas inhibitors of PKA had no effect on the current. Investigating the interaction between PKC and PKA revealed that although inhibitors of PKA had little effect on PKC-induced modulation of TTX-R I Na , inhibitors of PKC significantly attenuated PKA-induced modulation of the current. Finally, although PGE 2 -induced modulation of TTX-R I Na was more similar to PKA-induced modulation of the current than to PKC-induced modulation, PGE 2 -induced effects were inhibited by inhibitors of both PKC and PKA. Thus, although TTX-R I Na is a common target for cellular processes involving the activation of either PKA or PKC, PKC activity is necessary to enable subsequent PKA-mediated modulation of TTX-R I Na . Key words: dorsal root ganglion; inflammatory mediator; nociception; pain; primary afferent; second-messengerStudies of voltage-gated sodium currents (VGSC s) indicate that VGSC isoforms may be differentially modulated by protein kinase C (PKC) and protein kinase A (PK A). For example, a VGSC from brain tissue is decreased by the concurrent activation of PKC and PK A (Gershon et al., 1992;Li et al., 1992; Cantrell et al., 1996 Cantrell et al., , 1997, and a VGSC from cardiac muscle is decreased by PKC activation (Qu et al., 1994) and increased by PK A activation (Frohnwieser et al., 1995(Frohnwieser et al., , 1997. That these changes in VGSCs are physiologically relevant is suggested by the observations that receptor-mediated changes in cellular excitability reflect, at least in part, PKC -and /or PK A-mediated changes in VGSCs.We and others have recently demonstrated that a tetrodotoxinresistant voltage-gated Na ϩ current (TTX-R I Na ), expressed primarily in nociceptive afferents, is modulated by hyperalgesic inflammatory mediators in a manner that is likely to enhance nociceptor excitability (England et al., 1996;Gold et al., 1996b; Cardenas et al., 1997). Although there is evidence both for (England et al., 1996) and against (C ardenas et al., 1997) a role for protein kinase A in the modulation of TTX-R I Na , the contribution of PKC has yet to be investigated.Using agents that activate or inhibit PKC and PK A, we have tested the hypothesis that these kinases are involved in the modulation of T...

The Voltage-Gated Sodium Channel Nav1.9 Is an Effector of Peripheral Inflammatory Pain Hypersensitivity

“…EP4 is coupled to G s , some splice variants of EP3 to G s and some to G i , whereas EP1 is coupled to G q /G 11 (Narumiya et al, 1999). PGE 2 , BK, and 5-HT all increase slow inactivating TTXr currents in DRG neurons with an increase in amplitude and rates of activation and inactivation (England et al, 1996;Gold et al, 1998). PKA potentiates Na v 1.8 and depresses Na v 1.7 currents, and PKC produces shifts in the steady-state activation of both channels in a depolarizing direction (Saab et al, 2003;Vijayaragavan et al, 2004).…”

“…The high threshold of the transducers are reduced by posttranslational processing in response to activation by the inflammatory mediators of multiple intracellular signaling pathways [PKA, PKC, extracellular signalregulated kinase, phosphatidylinositol 3 (PI3) kinase, and phospholipase C] (Cesare et al, 1999;Aley et al, 2001;Bautista et al, 2006). Prostaglandin E 2 (PGE 2 ), 5-HT, and bradykinin (BK) increase the Na v 1.8-like TTXr current in DRG neurons, with an increase in amplitude and rates of activation and inactivation (Gold et al, 1996(Gold et al, , 1998England et al, 2001). Inflammatory pain sensitivity is, moreover, delayed in Na v 1.8 null mutant mice (Akopian et al, 1999).…”

Roles of Tetrodotoxin (TTX)-Sensitive Na+Current, TTX-Resistant Na+Current, and Ca2+Current in the Action Potentials of Nociceptive Sensory Neurons

“…Because both TTX-R sodium channels and calcium channels can be modulated by various transmitters, the amount of calcium entry during the action potential could be controlled by directly modulating calcium channels or by modifying TTX-R current and thus changing the shape of the shoulder. Most transmitters that affect calcium current produce an inhibitory effect, whereas current through TTX-R channels is enhanced by prostaglandin E 2 and serotonin, hyperalgesic agents (England et al, 1996;Gold et al, 1996Gold et al, , 1998. Thus, enhancement of TTX-R current could enhance calcium entry by enhancing the shoulder of the action potential.…”

Effects of PKC and PKA Inhibitors on the cAMP-Stimulant-Induced Enhancement of Tetrodotoxin-Resistant Na+ (Nav1.8) Currents

“…The VGSC into the skeletal and cardiac muscles is not significantly affected by the activation of PKA [33,34]. However, there are increasing evidences that the enhancement of Nav1.8 currents is seen after the application of hyperalgesic inflammation mediators, such as PGE 2 and serotonin, in sensory neurons [7,[35][36][37]. In fact, PGE 2 application can enhance the responsiveness of primary nociceptive neurons to bradykinin and/or capsaicin [38].…”

“…In fact, PGE 2 application can enhance the responsiveness of primary nociceptive neurons to bradykinin and/or capsaicin [38]. Additionally, in neurons expressing sensory nociceptive neurospecific Nav1.8 channels, PGE 2 shigts the activation curve of the Na + current to more negative potentials and potentiates the amplitude of the current, and such effects are mimicked by the application of drugs, which regulate cAMP-dependent PKA phosphorylation of the channel [7,24,36]. The PKA phosphorylation of Nav1.8 channels is therefore to be an important mechanism underlying the hyperalgesic responses in sensory neurons.…”