Experimental and modeling study of Na+ current heterogeneity in rat nodose neurons and its impact on neuronal discharge.

This paper is a combined experimental and modeling study of two fundamental questions surrounding the functional characteristics of Na+ currents in nodose sensory neurons. First, when distinctly different classes of Na+ currents are expressed in the same neuron, is there a significant difference in the intrinsic biological variability associated with the voltage- and time-dependent properties of these currents? Second, in what manner can such variability in functional properties impact the discharge characteristics of these neurons? Here, we recorded the whole cell Na+ currents in acutely dissociated rat nodose sensory neurons using the patch-clamp technique. Two general populations of neurons were observed. A-type neurons (n = 20) expressed a single rapidly inactivating tetrodotoxin-sensitive (TTX-S) Na+ current. C-type neurons (n = 87) coexpressed this TTX-S current along with a slowly inactivating TTX-resistant (TTX-R) Na+ current. The TTX-S currents in both cell types had submillisecond rates of activation at room temperature with thresholds near -50 mV. The TTX-R current exhibited about the same rates of activation but required potentials 20-30 mV more depolarized to reach threshold. Over the same clamp voltages the rates of inactivation for the TTX-R current were three to nine times slower than those for the TTX-S current. However, the TTX-R current recovered from complete inactivation at a rate 10-20 times faster than the TTX-S current (10 ms as compared with 100-200 ms). Across the population of neurons studied the TTX-S data formed a relatively tight statistical distribution, exhibiting low standard deviations across all measured voltage- and time-dependent properties. In contrast, the same pooled measurements on the TTX-R data exhibited standard deviations that were 3-10 times larger. The statistical profiles of the voltage- and time-dependent properties of these currents then were used as a physiological guide to adjust the relevant parameters of a mathematical model of nodose sensory neurons previously developed by our group (). Here, we show how the relative expression of TTX-S and TTX-R Na+ currents and the differences in their apparent biological variability can shape the regenerative discharge characteristics and action potential waveshapes of sensory neurons. We propose that the spectrum of variability robust reactivation characteristics of the TTX-R current are important determinants in establishing the heterogeneous stimulus-response characteristics often observed across the general population of C-type sensory neurons.