<EM>PHOX2B</EM>-EXPRESSING NEURONS IN THE NUCLEUS OF THE SOLITARY TRACT GENERATE RHYTHMIC JAW MOVEMENTS VIA THE PARVOCELLULAR RETICULAR FORMATION

The transcription factor Phox2b is essential for autonomic nervous system development, and Phox2b⁺ neurons are proposed to regulate feeding-related movements. We investigated the involvement of Phox2b⁺ cells in the nucleus of the solitary tract (NTS) in jaw movement regulation. In transgenic rats expressing channelrhodopsin variant ChRFR(C167A) in Phox2b⁺ cells, optogenetic stimulation of the NTS (470 nm, 1-2 mW/mm², 1 s) consistently induced rhythmic jaw movements. Electromyographic (EMG) recordings from masticatory and digastric muscles revealed burst activity with an average frequency of 5.3 Hz. Both masseter and digastric EMG bursts had a duration of 0.11 s, although the peak amplitude of digastric bursts was approximately three times larger than that of masseter bursts. This rhythmic activity began with a latency of <1 s from stimulation onset and persisted for approximately 10 s.
To identify the downstream circuitry, we chemogenetically inhibited the reticular formation dorsal to the trigeminal motor nucleus (RdV) or the parvocellular reticular formation (PCRt) using AAV-hM4D(Gi). In rats expressing hM4D(Gi) in PCRt neurons, CNO administration (1 mg/kg, i.p.) significantly reduced the number of NTS-induced EMG bursts to 40% of the baseline. In contrast, no significant change was observed in rats with RdV inhibition. These findings demonstrate that Phox2b⁺ NTS neurons trigger rhythmic jaw movements through a pathway involving the PCRt, highlighting the existence of a neural circuit involved in the feeding-related motor control.