Characterization of ionic channels underlying the specific firing pattern of a novel neuronal subtype in the rat prepositus hypoglossi nucleus.

In our previous study on the prepositus hypoglossi nucleus (PHN), we found a neuronal subtype exhibiting a specific firing pattern in which the first interspike interval (ISI) was longer than that of the second, designated FIL (first interspike interval long) neurons. In the present study, we explored the ionic mechanisms underlying this firing pattern using whole-cell recordings of rat brainstem slice preparations. In addition to a longer first ISI, FIL neurons showed properties such as increased slow afterhyperpolarization (AHP) of the first spike relative to the second spike. The application of 4-aminopyridine (4-AP) shortened the longer first ISI and reduced the larger AHP of the first spike, but α-dendrotoxin affected neither the ISI nor the AHP. A voltage clamp study revealed that FIL neurons express transient outward currents with slow decay kinetics. When T-type Ca(2+) currents alone or T-type Ca(2+) plus persistent Na(+) currents were blocked, the FIL firing pattern changed to one with transient hyperpolarization and delayed spike generation characteristic of late-spiking neurons. These findings indicate that A-type K(+) currents showing slow decay, T-type Ca(2+) currents, and persistent Na(+) currents all contribute to the specific firing pattern of FIL neurons.