Electromyography and Fos immunostaining study establish a possible functional link between trigeminal proprioception and the oculomotor system in rats

The objective of this study was to explore whether there was a functional link between trigeminal proprioception and the oculomotor system mediated through jaw muscle afferents. Electromyography (EMG) was undertaken of the levator palpebrae (LP) and superior rectus (SR), and Fos expression was detected in the brainstem following consecutive down-stretching of the lower jaw at 2-4 Hz in rats. Retrograde tracing was undertaken of the interstitial nucleus of Cajal and Darkschewitsch nucleus (INC/DN) pre-oculomotor neurons. EMG-like responses were recorded from the LP/SR during down-stretching of the lower jaw at 2-4 Hz in 3 out of 11 rats. Fos expression was induced by consecutive down-stretching of the lower jaw at 2-4 Hz for 20-30 seconds. Interestingly, Fos expression was distributed mainly in the bilateral INC/DN area. We also examined Fos-like immunoreactivity in central mesencephalic and paramedian pontine reticular formation that harbors premotor neurons controlling horizontal eye movement, but no Fos-like staining was observed therein. By injection of retrograde tracers into the oculomotor nucleus combined with Fos immunostaining, double labeled pre-oculomotor neurons were visualized to distribute in the INC/DN. In conclusions, there may exist a trigeminal proprioceptive - oculomotor system neural circuit through jaw muscle afferents in rats. Judging from Fos distribution pattern, this pathway might be related to vertical and torsional eye movements.