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

Abstract

To determine whether there is a difference between a transient K + current ( I A) and a sustained K + current ( I K) 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, ( I A) and ( I K), were identified, and two different components (59.5% and 96.3%) of I K to the total K + current were observed at a + 50 mV step-pulse. The IC 50 value for 4-aminoprydine (4-AP, 0.05–50 mM), which inhibited the I A by 50%, was 0.7 mM. That for tetraethylammonium (TEA, 0.02–20 mM) to inhibit 50% of I K 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 IC 50 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 I A and I K had independent effects regulating the intrinsic firing properties of the action potential number and timing, respectively, in adult rat TTX-R TG neurons.