Bursting discharges evoked in vitro, by solitary tract stimulation or application of N-methyl-D-aspartate, in neurons of the rat nucleus tractus solitarii.

Extracellular recordings of the activity of neurons in the isolated nucleus tractus solitarii (NTS) showed that repetitive stimulation of the tractus solitarius (TS) elicited either long-lasting discharges at low frequency (type A neurons) or short-duration bursting discharges at low or high frequency (type B and C neurons, respectively). Bath application of N-methyl-D-aspartate (NMDA; 60-120 microM; 4-5 min duration) elicited a pattern of rhythmic bursting in most type B (21/24) and in all type C neurons and only a repetitive firing in type A neurons, even with applications of longer duration (6-10 min). The rhythmic bursting pattern was characterized by trains of action potentials occurring at a regular rate with each neuron (mean = 0.9 +/- 0.45 Hz). The present findings suggest that local mechanisms within the NTS (synaptic interactions or neuronal intrinsic properties) are involved in the burst firing elicited either by TS stimulation or NMDA application. The data are discussed in terms of the generation of swallowing taking into account the key role of NTS neurons in the organization of this motor pattern.