Firing pattern of neurons in the nucleus tractus solitarius: modulation by membrane hyperpolarization

Neurons in the ventral region of the nucleus tractus solitarius (NTS) of guinea pigs were studied using an in vitro brainstem slice preparation. One group of neurons was characterized electrophysiologically by a delay between the onset of a depolarizing stimulus and the first spike. This delay could be as large as 760 ms and was modulated by the membrane potential level preceding the stimulus. The firing rate during the depolarizing stimulus was also modulated by the preceding membrane potential level. A fast transient outward current, similar to A-current in molluscan neurons, appeared to be responsible for the delay in firing while a slower calcium-activated potassium current affected the firing rate. These data suggest that intrinsic membrane properties may play an important role in determining the firing pattern of NTS neurons. In vivo, inhibitory synaptic inputs could modulate the expression of these intrinsic properties during subsequent excitation.