Rhythmic bursting patterns induced in neurons of the rat nucleus tractus solitarii, in vitro, in response to N-methyl-D-aspartate.

The activity of nucleus tractus solitarii (NTS) neurons was recorded extracellularly on rat brainstem slices. Depending on the neuron, bath application of N-methyl-D-aspartate (NMDA; 30-120 microM) elicited either a pattern of rhythmic bursting or repetitive firing. The rhythmic bursting pattern was characterized by trains of action potentials occurring at a rate of 0.36-2 Hz. With most of the neurons, the mean burst duration and the mean discharge frequency ranged between 200 and 800 ms and between 20 and 40 Hz, respectively. Both the repetitive and the rhythmic bursting patterns were reversibly blocked when DL-2-amino-5-phosphonovalerate (80 microM) was applied. Application of quisqualate (30-60 microM) or glutamate (300-1200 microM) on NTS neurons induced only repetitive firing even in neurons exhibiting a rhythmic bursting pattern under NMDA. The present findings show that rhythmic bursting patterns can be generated within the isolated NTS under activation of NMDA receptors. The rhythmic bursting resulted probably from local NTS mechanisms (synaptic interactions or neuronal intrinsic properties) which might be involved in physiological rhythmic activities organized at the NTS level, such as swallowing.