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Institute of Pharmacology, National Yang-Ming University, Taipei, Taiwan, Republic of China.

1. We characterized in rat brain slices the discharge patterns of spontaneously active neurons in the caudal region of the nucleus tractus solitarii (cNTS) and the neuromodulatory role of GABA and glutamate, via GABAA and NMDA receptors. 2. Spontaneous action potentials recorded intracellularly from cNTS neurons manifested either a regular or an irregular discharge pattern, alongside characteristic waveforms of the action potentials. These discharge patterns were interchangeable, and were highly sensitive to fluctuations in membrane potentials. In addition, the repolarizing rate of the after-hyperpolarization (AHP) in cNTS neurons that exhibited a regular discharge pattern was significantly higher than that of neurons that displayed irregular discharges. 3. cNTS neurons that manifested a regular discharge pattern were converted to irregular discharges upon superfusion with GABA (200 microM). This was accompanied by a reduction in the repolarizing rate of the AHP of both spontaneous and evoked action potentials. Conversion of discharge patterns in the opposite direction was elicited by superfusion with NMDA (6.8 microM). 4. The irregular discharges of spontaneous or evoked cNTS neurons were converted to a regular discharge pattern by bicuculline (200 microM). Subsequent application of D(-)-2-amino-5-phosphonopentanoic acid (250 microM) essentially led the neuronal discharges to revert to an irregular pattern. 5. Our results support the presence of two interchangeable modes of electrophysiological manifestations from the same cNTS neuronal population. They also showed that GABA and glutamate, via GABAA and NMDA receptors, may provide a novel form of neuromodulation at the cNTS by switching the patterns of neuronal discharges.

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