Stress, Hair Growth Control, and the Neuro-Endocrine-Immune Connection

Abstract

“Stress” has long been associated with disturbances in hair growth and hair pigmentation. Yet, little if any evidence has been provided so far that this real ly is more than a coincidental association. How ever, today, a wide range of experimental data sug gests that skin nerves can indeed modulate hair fol licle (HF) development, growth and/or cycling via the release of neurotransmitters, neuropeptides and/or even of neurotrophins. It is therefore con ceivable that stress-induced changes in the release of these agents from perifollicular sensory and autonomic nerve fibers can alter hair growth. In ad dition, most classical mediators of systemic stress responses (e.g., substance P, ACTH, CRH, prolactin, catecholamines) are now also appreciated as hair growth modulators. Moreover, the HF itself is a potent source of these stress mediators, and expresses cognate receptors for many of them. Final ly, mast cells, with their “central switchboard” function in neurogenic inflammation, have recent ly surfaced as hair growth modulators. Stress mediators, as well as skin neuropeptids and neurotransmitters, may, thus, impact hair growth in directly via the modulation of mast cell activities. As a prominent source of neurotrophins, the hair follicle may influence its own innervation as well as neurotrophin-dependent mast cell functions. On this background, the present essay examines the available evidence that there are neural mechanisms of hair growth control, reports preliminary evi dence that stress actually can inhibit hair growth in mice, and discusses potential pathways by which stress may affect hair growth in the context of de fined neuro-endocrine-immune circuits.