Inflammatory mediators increase the activity of tetrodotoxin-resistant (TTX-R) voltage-gated sodium channels (VGSCs) in dorsal root ganglion (DRG) neurones by a protein kinase A (PKA)-dependent mechanism (England et al. 1996). In this study we have investigated whether the activity of TTX-sensitive (TTX-S) VGSCs like that of TTX-R, is changed by activation of PKA with dibutyryl cAMP and whether this treatment affects the sensitivity of the channels (either TTX-S or TTX-R) to lidocaine.

Adult rats were killed humanely and DRG neurones were isolated and maintained in culture for up to 2 days. DRG neurones were voltage-clamped using the whole-cell patch-clamp technique. The extracellular solution contained 10 µM La³⁺ to block Ca²⁺ currents. Data were obtained from control cells or cells that were pretreated with dibutyryl cAMP (2 mM) for 2-6 h prior to recording. VGSCs were evoked during 30 ms steps to potentials between -110 and +40 mV from V_h = -90 mV. TTX-R currents were recorded in the presence of 0.5 µM TTX. TTX-S currents were identified by their relatively fast kinetics and this was confirmed by addition of 0.5 µM TTX at the end of each experiment. Statistical comparisons were made using Student’s unpaired t test. Data are expressed as means ± S.E.M.

The activation curve for TTX-R was shifted in neurones pretreated with dibutyryl cAMP such that the V_0.5 for activation was -7.3 ± 2.4 mV in control (n = 9) and -16.8 ± 3.7 mV in pretreated cells (n = 10, P = 0.026). The activation curve for TTX-S was unchanged by dibutyryl cAMP (V_0.5 = -6.9 ± 3.7 mV in control (n = 9) and -10.6 ± 4.2 mV in pre-treated cells (n = 6, P = 0.258)). There was no change in the voltage dependence of inactivation that occurred for either TTX-R or TTX-S during 30 ms pre-pulses to potentials between -110 and +40 mV. In cells that were pretreated with dibutyryl cAMP the EC₅₀ for block by lidocaine of TTX-R was 140 ± 38 µM (n = 8) compared with 433 ± 156 µM (n = 7) in control cells (P = 0.037). This suggests that lidocaine (and potentially other state-dependent blockers of VGSCs) will block activity in sensitized cells more effectively than in normal cells. The sensitivity of TTX-S sodium channels to lidocaine was unaffected by dibutyryl cAMP (718 ± 169 µM (n = 6) and 855 ± 341 µM (n = 6) in control and test groups, respectively; P = 0.363). The data underline the importance of TTX-R VGSCs as a drug target for analgesics and suggest that changes of the phosphorylation state of the channels such as occur during inflammation can influence their pharmacology.This work was supported by the Special Trustees of St Thomas’ Hospital.

England, S., Bevan, S. & Docherty, R.J. (1996). J. Physiol. 495, 429-440. abstract