Differential dependency of unmyelinated and A delta epidermal and upper dermal innervation on neurotrophins, trk receptors, and p75LNGFR.

The study examined the role of neurotrophins and their receptors in the innervation of the mystacial pads of mice, focusing on unmyelinated and thinly myelinated sensory and autonomic fibers. It was found that NGF/trkA signaling was crucial for sensory axon outgrowth and proliferation, while NT-3/trkA was important for sensory ending formation. TrkC was essential for certain trkA-dependent sensory innervations, and trkB, influenced by BDNF and NT-4, suppressed most trkA-dependent sensory innervations. Sympathetic innervation to hair follicles relied on NGF/trkA and p75, with NT-3 being detrimental to sympathetic terminations. TrkB signaling, mediated by BDNF, suppressed sympathetic terminations, while a sparse innervation set depended on NT-3 and trkC. The study concluded that innervation in these areas was regulated by a balance of promoting and suppressing effects of neurotrophins.