Rat whisker movement after facial nerve lesion: Evidence for autonomic contraction of skeletal muscle

- Denervated rat whisker pad muscles spontaneously acquire autonomic innervation.
- Parasympathetic axons to whisker pad muscles travel in the infraorbital nerve.
- Parasympathetic contraction of skeletal muscle can be blocked by hexamethonium.
- Eliciting a vasodilation reflex potentiates autonomic-mediated muscle contraction.
- This is the first report of spontaneous autonomic reinnervation of skeletal muscle.

Vibrissal whisking is often employed to track facial nerve regeneration in rats; however, we have observed similar degrees of whisking recovery after facial nerve transection with or without repair. We hypothesized that the source of non-facial nerve-mediated whisker movement after chronic denervation was from autonomic, cholinergic axons traveling within the infraorbital branch of the trigeminal nerve (ION). Rats underwent unilateral facial nerve transection with repair (N = 7) or resection without repair (N = 11). Post-operative whisking amplitude was measured weekly across 10 weeks, and during intraoperative stimulation of the ION and facial nerves at ⩾18 weeks. Whisking was also measured after subsequent ION transection (N = 6) or pharmacologic blocking of the autonomic ganglia using hexamethonium (N = 3), and after snout cooling intended to elicit a vasodilation reflex (N = 3). Whisking recovered more quickly and with greater amplitude in rats that underwent facial nerve repair compared to resection (P < 0.05), but individual rats overlapped in whisking amplitude across both groups. In the resected rats, non-facial-nerve-mediated whisking was elicited by electrical stimulation of the ION, temporarily diminished following hexamethonium injection, abolished by transection of the ION, and rapidly and significantly (P < 0.05) increased by snout cooling. Moreover, fibrillation-related whisker movements decreased in all rats during the initial recovery period (indicative of reinnervation), but re-appeared in the resected rats after undergoing ION transection (indicative of motor denervation). Cholinergic, parasympathetic axons traveling within the ION innervate whisker pad vasculature, and immunohistochemistry for vasoactive intestinal peptide revealed these axons branching extensively over whisker pad muscles and contacting neuromuscular junctions after facial nerve resection. This study provides the first behavioral and anatomical evidence of spontaneous autonomic innervation of skeletal muscle after motor nerve lesion, which not only has implications for interpreting facial nerve reinnervation results, but also calls into question whether autonomic-mediated innervation of striated muscle occurs naturally in other forms of neuropathy.