Activation of N-methyl-d-aspartate Receptors Induces Endogenous Rhythmic Bursting Activities in Nucleus Tractus Solitarii Neurons: An Intracellular Study on Adult Rat Brainstem Slices

A brainstem slice preparation and intracellular recording techniques were used to examine the effects of N-methyl-d-aspartate (NMDA) application on neurons within the swallowing area of the nucleus tractus solitarii (NTS). According to their cellular properties, NTS neurons were classified into type I and type II neurons. The most striking difference was the occurrence of delayed excitation in type I but not in type II neurons, when they were depolarized from membrane potentials more negative than -60 mV. Bath application of NMDA (30 - 60 microM) elicited depolarization and triggered stable repetitive firing in all the NTS neurons but one. During the NMDA-induced depolarization, hyperpolarization below -60 mV elicited, in some type I neurons, a rhythmic bursting pattern. The duration of the bursts (300 - 1000 ms) and their frequency (0.5 - 2 Hz) depended on the membrane potential. With hyperpolarizations below -75 mV, rhythmic bursting was converted into rhythmic single discharges, a pattern elicited directly in the other type I neurons. In all cases, rhythmic patterns were superimposed on cyclic depolarizations of the membrane potential characterized by an initial ramp-shaped phase. In type II neurons, rhythmic bursting discharges, superimposed on rhythmic oscillations of the membrane potential, were also obtained upon hyperpolarization during the NMDA-induced depolarization. In all type I neurons tested, NMDA-induced cyclic ramp-shaped depolarizations continued after addition of tetrodotoxin to the medium. Rhythmic bursting was not elicited by bath application of kainate (10 - 20 microM). Application of d-2-amino-5-phosphonovalerate (50 microM) blocked NMDA-induced depolarizations without modifying those elicited by kainate, which were selectively depressed by 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM). Moreover, removal of Mg2+ from the medium suppressed NMDA-induced cyclic depolarizations. Results demonstrate that both NMDA and non-NMDA receptors are present in NTS neurons and that selective activation of NMDA receptors induced rhythmic bursting and/or rhythmic single discharges. Rhythmic patterns were not driven by synaptic mechanisms but originated from endogenous properties of NTS neurons activated by NMDA. Thus, NTS neurons can be considered as conditional pacemakers. According to the location of the neurons, the conditional properties shown in these in vitro experiments might be involved in vivo in the generation of rhythmic motor activities set up at the NTS level, such as swallowing.