The functional difference between transient and sustained K+ currents on the action potentials in tetrodotoxin-resistant adult rat trigeminal ganglion neurons

To determine whether there is a difference between a transient K+ current (IA) and a sustained K+ current (IK) regarding the neuronal function in small-diameter adult rat trigeminal ganglion (TG) neurons, which were insensitive to tetrodotoxin (TTX, 1 μM), we performed two different types of experiments. Primary cultures of dissociated TG neurons were prepared, and electrophysiological recordings were performed with the whole-cell configuration using the patch-clamp technique. In the voltage-clamp mode, two distinct K+ current components, (IA) and (IK), were identified, and two different components (59.5% and 96.3%) of IK to the total K+ current were observed at a + 50 mV step-pulse. The IC50 value for 4-aminoprydine (4-AP, 0.05–50 mM), which inhibited the IA by 50%, was 0.7 mM. That for tetraethylammonium (TEA, 0.02–20 mM) to inhibit 50% of IK was 1.5 mM. In the current-clamp mode, we used 0.5 mM 4-AP and 2 mM TEA at each concentration nearly equal to the IC50 value. Irrespective of the absence or presence of TEA (2 mM), 0.5 mM 4-AP application increased the number of action potentials due to the decreased duration of the depolarization phase (DDP). TEA in the presence and absence of 4-AP prolonged the duration of action potentials as well as the duration of repolarization phase (DRP). These results suggested that IA and IK had independent effects regulating the intrinsic firing properties of the action potential number and timing, respectively, in adult rat TTX-R TG neurons.