Figure 6. A combination of peripheral diffusion and intraparenchymal dissemination causes the rapid spreading of TeNT activity among brainstem neurons.

(A) Scheme showing the bilateral innervation of LAL muscles by the posterior auricularis branch of the facial nerve connecting LAL NMJ motoneuron somas residing in the FN. (B) Scheme of facial nerve innervation of the dermomuscular system of the mouse head; each facial nerve exits at the level of the stylomastoid foramen (gray circle) and splits into distinct branches. (C) TeNT injection (1 ng/kg in 5 μL) between the 2 LAL muscles causes the appearance of cl-VAMP (red) in both left and right LALs; AChR labeling (green) with fluorescent α-bungarotoxin shows the NMJs. (D) TeNT injection between the 2 LAL muscles causes the bilateral appearance of cl-VAMP (red) in both FN; at day 1, the signal is restricted to the medial portions and then spreads distally, affecting the entire FN at day 5. (E) Scheme of facial motor subnuclei with efferent to specific muscles of the head. (F) Scheme of right facial nerve transection (red bars) showing the disconnection of all facial nerve branches except posterior auricular and digastric. (G and H) TeNT injection between the 2 LAL muscles with partial transection of the right facial nerve causes at day 5 a different distribution of cl-VAMP (red) in the FN (G) and PGRN (H) (central panels, scale bar: 500 μm): the nonaxotomized FN and PGRN display a signal diffused throughout the entire nucleus; cl-VAMP in the axotomized FN mainly affects the auricular and digastric subnuclei. Cl-VAMP also appears in distal FN subnuclei and PGRN like puncta around neuron somas, as shown via the progressive magnifications (scale bars: #1 = 50 μm; #2 = 20 μm). Images are representative of 1 of 3 animals.