Membrane resonance and subthreshold membrane oscillations in mesencephalic V neurons: Participants in burst generation

Trigeminal mesencephalic (Mes V) neurons are critical components of the circuits controlling oral-motor activity. The possibility that they can function as interneurons necessitates a detailed understanding of the factors controlling their soma excitability. Using whole-cell patch-clamp recording, in vitro, we investigated the development of the ionic mechanisms responsible for the previously described subthreshold membrane oscillations and rhythmical burst discharge in Mes V neurons from rats ages postnatal day (P) 2-12. We found that the oscillation amplitude and frequency increased during development, whereas bursting emerged after P6. Furthermore, when bursting was initiated, the spike frequency was largely determined by the oscillation frequency. Frequency domain analysis indicated that these oscillations emerged from the voltage-dependent resonant properties of Mes V neurons. Low doses of 4-aminopyridine (<100 M) reduced the oscillations and abolished resonance in most neurons, suggesting that the resonant current is a steady-state K+ current (I4-AP). Sodium ion replacement or TTX reduced substantially the oscillations and peak amplitude of the resonance, suggesting the presence of a persistent Na+ current (I-NaP) that functions to amplify the resonance and facilitate the emergence of subthreshold oscillations and bursting.