9) Properties of Synaptic Transmission from the Reticular Formation Dorsal to the Facial Nucleus to Jaw-Closing and Jaw- Opening Motoneurons in Rats

We previously reported that electrical and chemical stimulation of the reticular formation dorsal to the facial nucleus (RdVII) elicited excitatory masseter responses, suggesting that excitatory premotor neurons targeting the trigeminal motor nucleus (MoV) are likely located in the RdVII. In the present study we examined properties of synaptic transmission from the RdVII to jaw-closing motoneurons in horizontal brainstem slices from P0-12 Wistar rats. Electrical stimulation of the RdVII evoked optical responses in the MoV of P1-12 rats. Combined bath application of CNQX, APV, bicuculline and strychnine reduced the optical responses. Subsequently, we performed whole-cell patch recordings from P1-12 masseter motoneurons which were identified by retrograde labeling with tetramethyl rhodamine. Electrical stimulation of the RdVII evoked PSCs in masseter motoneurons, and the PSCs were abolished by bath application of CNQX, CPP, strychnine and SR95531. In juvenile rats, electrical stimulation of the RdVII also evoked PSCs. The rate of reduction in PSCs in juvenile rats was not significantly different from that in neonates. To further confirm that RdVII neurons send synaptic inputs to the MoV, we systematically photostimulated every 50μm in and around the RdVII with caged-glutamate, and the focal points of which photostimulation produced the strongest PSCs compared with surrounding areas were found in the RdVII. These results suggest that premotor neurons targeting the MoV were located in the RdVII and that these neurons sent glutamatergic, glycinergic and GABAergic inputs to motoneurons. RdVII neurons may play an important role in the control of masseter muscle activity during feeding behavior.