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J. Cosmet. Sci., 64, 111–117 (March/April 2013) 111 Cytokine expression correlates with differential sensory perception between lye and no-lye relaxers ROBERT N. TACKEY, HAROLD BRYANT, and FELICIA M. PARKS, L’Oreal Institute for Ethnic Hair and Skin Research, Chicago, IL 20616. Accepted for publication July 23, 2012 Synopsis Differences in perceived sensory scalp discomfort between guanidine carbonate/calcium hydroxide (no-lye) and sodium hydroxide (lye) relaxer technologies have been reported by users for decades. However, the bio- chemical processes responsible for the perceived differences have not been fully studied. We have used an in vitro three-dimensional skin model with well-developed epidermis to explore the expression of cytokines that may partially explain the biological response resulting in differences in sensory perception. The cytokines selected were prostaglandin E2 (PGE2), interleukin-1α (IL-1α), and IL-1 receptor antagonist (IL-1ra) because they have been shown to participate in irritant-induced discomfort. We show that lye relaxer induced over 350% increase in PGE2 expression over untreated control compared to 200% by no-lye in the early phase (4 h) postexposure epidermal response. Expression of IL-1α in the early phase showed no signifi cant difference between lye and no-lye however, no-lye induced higher levels (p 0.0001) in 24 and 48 h. Concomitantly, no-lye induced increased expression of IL-1ra compared to lye at all time points. Given the association of PGE2 with nociceptive activation, these fi ndings suggest that the perceived variation in sensory discomfort reported by consumers between lye and no-lye relaxers may be associated with differ- ences in induced PGE2 expression. INTRODUCTION Hair relaxers are complex cosmetic formulations consisting of many ingredients and are designed to permanently straighten curly hair. The active ingredient in these emulsions is the hydroxide ion, which can be quantifi ed by measuring the pH of the formulation. Lye and no-lye are the two main types of relaxers used in the United States to straighten curly hair. Lye relaxers contain sodium hydroxide as the source of hydroxide ion. No-lye relaxers contain calcium hydroxide and guanidine carbonate, which when combined pro- vides the source of hydroxide ions. Hair relaxers are not designed to be in direct contact with the scalp, but through the process of use, contact may occur. Although measures including, but not limited to, the application of petrolatum (basing) to the scalp are Address all correspondence to Robert N. Tackey at RTackey@rd.us.loreal.com