Unmyelinated innervation of sinus hair follicles in rats

Abstract

Unmyelinated nerve fibres comprise approximately one third of the innervation of rodent sinus hair follicles but their function is unknown. They may play a role as high-threshold sensory fibres, or may be autonomic efferents controlling the vascular sinus. In the present experiments capsaicin and surgical sympathectomy were used to establish whether these unmyelinated fibres are afferent fibres or autonomic efferents. The deep vibrissal nerves of mystacial follicles (C₁ and C₄) and a non-mystacial follicle (the postero-orbital, PO) were assessed in normal adult animals (n = 6) and compared with those treated with neonatal capsaicin (n = 6) or bilateral superior cervical ganglionectomy (n = 7). In capsaicin-treated animals, counts of fibres in the deep vibrissal nerves from all follicles showed normal numbers of myelinated axons, but approximately 80% reduction in unmyelinated fibres (normal mean ± SD: C₁ 94± 10, C₄ 89 ± 9, PO 85 ± 6; after neonatal capsaicin: C₁ 17 ± 8, C₄ 16 ± 6, PO 18 ± 6; n = 6, P < 0.001 for all follicles). After sympathectomy there was no significant reduction in myelinated or unmyelinated fibre numbers. Labelling of PO follicles with WGA-HRP showed minimal numbers of labelled cells (0–10) within the superior cervical ganglion, also suggesting minimal sympathetic innervation. This sparse sympathetic supply to the follicle was further demonstrated by a lack of tyrosine hydroxylase reactivity within the follicle complex; tissues outside the dermal capsule showed reactivity. It is concluded that most of the unmyelinated fibres entering sinus hair follicles are sensory in function. Moreover, the sparse autonomic innervation suggests minimal efferent control of the vascular sinus. Changes in vascular pressure are therefore unlikely to be a mechanism for regulating follicle sensitivity.