Is Marcus Gunn jaw winking a primitive reflex? Rat neuroanatomy

Abstract

Aim: To investigate a possible trigeminal proprioceptive-oculomotor neural pathway and explore possible synaptic connections between neurons in this pathway. Attempt to bring a new insight to mechanism of Marcus Gunn syndrome (MGS).

Methods: Anterograde and retrograde tract tracing was applied and combined with immunofluorescent stain in rats. After electrophysiological identifying mesencephalic trigeminal nucleus (Vme) neurons, intracellular injection of tracer was performed to trace axon trajectory.

Results: Following injections of anterograde tracers into the Vme, labeled terminals were observed ipsilateral in oculomotor and trochlear nuclei (III/IV), as well as in their premotor neurons in interstitial nucleus of Cajal and Darkschewitsch nucleus (INC/DN). Combining with choline acetyltransferase (ChAT) immunofluorescent stain, it showed that Vme projecting terminals contact upon ChAT positive III/IV motoneurons under confocal microscope. By retrograde labeling premotor neurons of the III, it showed that Vme neuronal terminals contact with retrogradely labeled pre-oculomotor neurons in the INC/DN. Axons of intracellularly labeled Vme neurons that respond to electric stimuli of the masseter nerve traveled into the ipsilateral III.

Conclusion: There may exist a trigeminal proprioceptive-oculomotor system neural circuit in the rat, which is probably related to vertical-torsional eye movements. Possible association of this pathway with MGS etiology was discussed.

Keywords: anterograde and retrograde tract tracing; interstitial nucleus of Cajal and Darkschewitsch nucleus; intracellular tract tracing; mesencephalic trigeminal nucleus; oculomotor and trochlear nuclei.

Figure 1. Projections of Vme neurons to III/IV and INC/DN

A: Vme injection site; B: PHA-L labeled terminals in ipsilateral III; C: Magnified frame area in B, showing contacts of labeled terminals on Nissl stained motoneurons (arrowheads); D: Labeling in IV; E: Vme afferent terminals ended upon both motoneurons (arrowheads) and interneurons (arrow); F: Distribution of labeled terminals in INC and DN; G: An enlarged frame area in F, showing contacts of labeled terminals on Nissl stained INC (arrowheads) neurons (arrowheads); H: The other magnified frame area in F. 3V: 3^rd ventricle; 4V: 4^th ventricle; DN: Darkschewitsch nucleus; INC: Interstitial nucleus of Cajal; LC: Locus ceoruleus; mlf: Medial longitudinal fasciculus; MPB: Medial parabrachial nucleus; PAG: Periaqueductal gray; scp: Superior cerebellar peduncle; Vme: Mesencephalic trigeminal nucleus; Vmo: Trigeminal motor nucleus. Scale bars: 300 µm in B, D; 50 µm in C, E; 400 µm in F and 25 µm in G, H.

Figure 2. Potential synaptic contacts between Vme neuronal terminals and III/IV motoneurons

A: BDA injection site; B: Overlapping of BDA labeling with ChAT positive motoneurons in III; C: Overlapping in IV; D: Confocal microscopic revealing of contacts of BDA terminals on ChAT motoneurons in III; E: Contacts in IV; F: Magnified frame area in D, showing those contacts (arrowheads) in III; G: Enlarged frame area in E, showing contacts in IV (arrowheads). Aq: Aqueduct; Vp: Principal trigeminal sensory nucleus. Scale bars: 400 µm in A-C; 100 µm in D, E; 25 µm in F, G.

Figure 3. Potential synaptic contacts between Vme neuronal terminals and INC/DN pre-oculomotor neurons

A: BDA injection site in caudal Vme; B: CTB injection site in contralateral III; C: Overlapping of BDA labeled fibers and terminals with CTB labeled pre-oculomotor neurons in the INC/DN, ipsilateral to the BDA injection and contralateral to the CTB delivery; D-G: Confocal microscopic viewing of contacts (arrowheads) between BDA labeled boutons and CTB labeled pre-oculomotor somata or dendrite in the INC (D, E and F) and DN (G). Scale bars: 400 µm in A, B; 200 µm in C; 25 µm in D-G.

Figure 4. Jaw muscle afferent Vme neurons sending axons to the ipsilateral III

A: Camera Lucida reconstructed intracellularly labeled Vme soma and axons in midbrain levels of the brain stem, this Vme neuron responded to electric stimulation of ipsilateral masseter nerve (bottom right); B: Intracellular labeled Vme neuronal axons (arrowheads) traveling through the border area of the III in a neutral-red counterstained section. Scale bar =100 µm.