Effects of medullary lesions on conditional pacemaker activity of neonatal rat hypoglossal motoneurons in vitro

N-methyl-d-aspartate (NMDA) has been demonstrated to induce rhythmic activity in various neurons, including hypoglossal motoneurons (XIIms) and converts them to conditional pacemakers. Using whole-cell patch clamp recording in a slice preparation from neonatal rats, we confirmed that some XIIms act as conditional pacemakers, with TTX-insensitivity and a burst period that is voltage-dependent during NMDA application. Other XIIms in this study only fired tonically with NMDA application. Effects of medullary structures on conditional pacemaker XIIms were assessed using lesioned preparations. As a result, NMDA-induced rhythm (NIR) in the XIIm was observed with ventral lesions (excluding inspiratory neurons) and with dorsal lesions (excluding the swallowing center located in the nucleus of the solitary tract). The NIR was also observed with lateral lesions, but with a significantly decreased burst period. These data suggest that NMDA receptor activation selects a subset of XIIms and changes them to pacemakers whose properties can be altered by their excitability. The data also demonstrate that structures fundamental to the NIR are located within the area near the XII nucleus, indicating that the NIR is distinct from inspiratory and swallowing activities. The lateral medulla is considered to be a source of modulation of the excitability of XIIms.