Membrane properties of mouse anteroventral cochlear nucleus neurons in vitro.

Intracellular recordings were made from neurons of the mouse anteroventral cochlear nucleus (AVCN) in vitro. The whole cochlear nucleus was dissected out and maintained submerged in rapidly flowing artificial CSF. This preparation has the advantages of maintaining cell-to-cell connections and dendritic trees whereas slices and enzymatically separated preparations do not. Recordings were made using current clamp technique in the presence and absence of the ion channel blockers, tetrodotoxin (TTX 1 microM), tetraethylammonium (TEA 20 mM), 4-aminopyridine (4-AP 5 mM) or verapamil (150 microM). Two distinct types of neurons were observed when tested with depolarizing current pulses: one which fired only a single action potential at the onset of stimulation followed by a relative depolarization for the remainder of the stimulus period, and the other which fired a sustained train of action potentials, each followed by a large undershoot, then a rapid recovery phase and the slower depolarization to threshold. The single spike cells (n = 24) had resting membrane potentials of -63.4 +/- 4.7 mV, resistance of 48.4 +/- 29.6 M omega, time constant of 3.47 +/- 3.1 ms, capacitance of 0.081 +/- 0.079 nF. The I/V plot was non-linear above the resting membrane potential and linear below. Spike train cells (n = 24) had resting membrane potentials of -64.2 +/- 4.54 mV, resistance of 69.8 +/- 28.9 M omega, time constant of 6.51 +/- 3.09 ms, capacitance of 0.11 +/- 0.087 nF. The I/V plot was linear both below resting membrane potential and up to threshold for spike firing. TTX abolished spike firing in both cell types. TEA significantly increased the spike duration in both cell types. 4-AP increased the spike duration in spike train cells but not in single spike cells. Verapamil had no effect on the firing properties of both cells but it significantly increased the spike duration of spike train cells. The single spike cells are known to fire rapidly and repetitively in vivo. Injection of sine wave currents caused rapid and repetitive firing suggesting that these cells require hyperpolarization to allow for removal of inactivation. There was a linear relationship between injected depolarizing current and the rate of action potential firing in spike train cells.