Sodium currents in vagotomized primary afferent neurones of the rat

1. Nodose ganglion neurones (NGNs) become less excitable following section of the vagus; nerve. To determine the role of sodium currents (I-Na) in the se changes, standard patch-clamp recording techniques were used to measure I-Na in rat NGNs maintained in vivo for 5-6 days following vagotomy, and th en in vitro for 2-9 h. 2. Total I-Na and I-Na density in vagotomized NGNs were similar to control values. However, steady-state I-Na inactivation in vagotomized NGNs was shi fted -9 mV relative to control values (V-u2, -74 +/- 2 vs. -65 +/- 2 mV, P < 0.01) and I-Na activation was shifted by -7 mV (V, -21 +/- 2 vs. -14 +/- 2 mV, P < 0.006). recovery from inactivation was also slower in vagotomized NGNs (fast time constant, 2.8 +/- 0.4 vs. 1.6 +/- 0.3 ms, P < 0.02). 3. The fraction of I-Na resistant to 1 mum tetrodotoxin (TTX-R) was halved in vagotomized NGNs (21 +/- 8 vs. 56 +/- 8 % of total I-Na, P < 0.05). This change from TTX-R I-Na to TTX-sensitive (TTX-S) I-Na may explain altered I -Na activation, inactivation and repriming in vagotomized NGNs. 4. The contribution of alterations in I-Na to NGN firing patterns was asses sed by measuring I-Na evoked by a series of action potential (AP) waveforms . In general, control NGNs produced large, repetitive TTX-R while vagotomiz ed NGNs produced smaller TTX-S I-Na that rapidly inactivated during AP disc harge. We conclude that TTX-R I-Na. is important for sustained AP discharge in NGNs, and that its diminution underlies the decreased AP discharge of v agotomized NGNs.