Oscillatory membrane potential activity in the soma of a primary afferent neuron

In the present report, we provide evidence that mesencephalic trigeminal (M es-V) sensory neurons, a peculiar type of primary afferent cell with its ce ll body located within the CNS, may operate in different functional modes d epending on the degree of their membrane polarization. Using intracellular recording techniques in the slice preparation of the adult rat brain stem, we demonstrate that when these neurons are depolarized, they exhibit sustai ned, high-frequency, amplitude-modulated membrane potential oscillations. U nder these conditions, the cells discharge high-frequency trains of spikes. Oscillations occur at membrane potential levels more depolarized than -53 +/- 2.3 mV (mean +/- SD). The amplitude of these oscillations increases wit h increasing levels of membrane depolarization. The peak-to-peak amplitude of these waves is similar to 3 mV when the cells are depolarized to levels near threshold for repetitive firing. The frequency of oscillations is simi lar in different neurons (108.9 +/- 15.5 Hz) and was not modified, in any i ndividual neuron, by changes in the membrane potential level. These oscilla tions are abolished by hyperpolarization and by TTX, whereas blockers of vo ltage-dependent K+ currents slow the frequency of oscillations but do not a bolish the activity. These data indicate that the oscillations are generate d by the activation of inward Na+ current/s and shaped by voltage-dependent K+ outward currents. The oscillatory activity is not modified by perfusion with low-calcium, high-magnesium, or cobalt-containing solutions nor is it modified in the presence of cadmium or Apamin. These results indicate that a calcium-dependent K+ current does not play a significant role in this ac tivity. We postulate that the membrane oscillatory activity in Mes-V neuron s is synchronized in adjoining electrotonically coupled cells and that this activity may be modulated in the behaving animal by synaptic influences.