Characterization of persistent TTX-R Na+ currents in physiological concentration of sodium in rat visceral afferents

Abstract

Persistent tetrodotoxin-resistant (TTX-R) Na(+) (Na(v)1.9/SCN11A) currents are not normally recorded in vagal afferent neurons (VANs) with 50 mM of extracellular Na(+) although the functional expression of this current was observed in the presence of PGE(2) or forskolin. However, it is uncertain whether this current can be seen under physiological condition (150 mM Na(+)). Using the whole-cell patch-clamp technique, we showed that persistent TTX-R Na(+) currents were expressed in 9 out of 38 VANs bathed in 150 mM Na(+). The current density, but not the whole-cell capacitance, was significantly enhanced in the VANs expressing Nav1.9. Persistent TTX-R Na(+) channels were activated at a more hyperpolarized membrane potential near -60 mV, compared with TTX-sensitive (TTX-S at -40 mV) and TTX-R Na(+) channels (at -20 mV). This indicates that persistent TTX-R Na(+) channels provide a wider activation window than TTX-S and TTX-R Na channels to up-regulate neuronal excitability. These results suggest that the persistent TTX-R Na(+) currents may be involved in the neuronal excitability by setting a lower pressure-discharge threshold and higher discharge frequency of VANs, especially the unique subset and gender-specific distribution of myelinated Ah-type VANs, including Ah-type aortic baroreceptor neurons, identified in our previous study.

Keywords: Ion channel; Patch technique; Sodium; Tetrodotoxin; Visceral afferent.

Fig 1

The representative recordings of voltage-gated Na⁺ currents in the extracellular solution contained 150 mM Na⁺ in vagal afferent neurons (VANs) isolated from adult rats. A: Tetrodotoxin-sensitive (TTX-S) Na⁺ currents (Na_v1.7) from a VAN in which TTX-S Na⁺ is the only Na⁺ channel expressed; B: Tetrodotoxin-resistant (TTX-R, Na_v1.8) and persistent TTX-R Na⁺ currents (Na_v1.9) in the presence of 1.0 µM TTX from the VAN that co-expresses Na_v1.7, Na_v1.8, and Na_v1.9; C: TTX-R Na⁺ currents (Na_v1.8) in the presence of 1.0 µM TTX from the VAN that co-expresses Na_v1.7 and Na_v1.8.

Fig 2

The current density of TTX-S and TTX-R Na⁺ channels functionally expressed in low extracellular Na⁺ (50 mM Na⁺) and physiological extracellular Na⁺ environment. TTX-S Na⁺ currents (n = 7 for 50 mM Na⁺ and n = 8 for 150 mM Na⁺) were recorded from the VAN expressed TTX-S Na⁺ channel only; TTX-R Na⁺ currents (n = 34 for 50 mM Na⁺ and n = 21 for 150 mM Na⁺) were recorded from the VAN co-expressed TTX-S and TTX-R Na⁺ channel; and persistent TTX-R Na⁺ currents (n = 9) were recorded from the VAN co-expressed TTX-S, TTX-R, and persistent TTX-R Na⁺. TTX-R Na⁺ currents were separated by 1.0 µM TTX applied. Data are expressed as mean ± SD, *P < 0.05 and **P < 0.01 vs TTX-S in 50 mM Na⁺, ^## P < 0.01 vs TTX-R in 150 mM Na⁺.

Fig 3

Comparison of normalized current-voltage (A.) relationship and voltage-dependent activation (B.). TTX-S, TTX-R, and persistent TTX-R Na⁺ currents were recorded in VANs expressed Na_v1.7 only, Na_v1.7 + Na_v1.8, and Na_v1.7 + Na_v1.8 + Na_v1.9, respectively. TTX-R Na⁺ currents were separated in the presence of 1.0 µM TTX.