Unraveling a masticatory - oculomotor neural pathway in rat: Implications for a pathophysiological neural circuit in human?

Anterograde tracers were injected into the mesencephalic trigeminal nucleus (Vme) in pons, labeled axons and terminals were observed in ipsilateral oculomotor (III) and trochlear (IV) nuclei, as well as in interstitial nucleus of Cajar and Darkschewitsch nucleus (INC/DN), the well-known premotor nuclei to the III/IV, but not in abducens nucleus and central mesencephalic and paramedian pontine reticular formation (CMRF/PPRF). Retrogradely labeled INC/DN neurons do ensue from injection of tracers into the III. Confocal microscopy revealed labeled Vme axonal terminals contact with labeled pre-oculomotor neurons in the INC/DN. In response to electrical stimulation of trigeminal nerve root (TR) jaw muscle branches, which contains peripheral processes of the jaw muscle spindle, extracellular unit discharges were recorded in the ipsilateral III/IV and INC/DN. Electromyography (EMG) was also recorded from superior rectus (SR) and levator palpebrae (LP) following electrical stimulation of the TR. Moreover, stimulation of the TR induced Fos expression in the INC/DN pre-oculomotor neurons, but not in CMRF/PPRF that harbors horizontal eye moving premotor neurons. By injection of retrograde tracers into the III combined with Fos immunostain, double labeled pre-oculomotor neurons were observed in the INC/DN. About 80% of retrogradely labeled III premotor neurons express Fos. These results suggest a neural pathway from the masticatory Vme neurons to the oculomotor system that is probably involved exclusively in vertical and torsional eye movement as well as eyelid retraction. The potential relationship between this pathway and Marcus Gunn Syndrome (MGS), a congenital jaw-winking syndrome, was discussed.