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VJ Choy, AJ Nixon, and AJ Pearson

Prolactin is believed to mediate seasonal cues entraining seasonal reproductive and hair follicle growth cycles. Prolactin receptor binding activity and prolactin receptor gene expression in mammalian skin have recently been described. In this report, prolactin receptor immunoreactivity is identified in sheep skin using a monoclonal antibody against the rat liver prolactin receptor. Western blotting analysis of microsomal membrane proteins from skin showed major bands corresponding to molecular weights of 87 and 71 kDa and minor bands at 101 and 21 kDa. RNase protection analysis revealed the presence of mRNA species coding for long and short forms of the prolactin receptor. Formalin-fixed sections, exposed to the monoclonal antibody and stained by an immunogold method, revealed prolactin receptor-immunoreactivity in the dermal papilla, germinal matrix, outer root sheath, lower regions of the inner root sheath and connective tissue sheath of wool follicles. Staining was absent from keratinised cell populations. In all samples, the interfollicular epidermis, sebaceous and sweat glands were positively stained. The distribution of prolactin receptor is described in both growing and inactive wool follicles and related to postulated cycle-specific actions of circulating prolactin in the control of seasonal fibre growth.

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AJ Nixon, CA Ford, JE Wildermoth, AJ Craven, MG Ashby, and AJ Pearson

Seasonal patterns of hair growth are governed, at least in part, by levels of prolactin in circulation, and although receptors for prolactin (PRLR) have been demonstrated in hair follicles, little is known of their regulation in relation to follicular cycles. In this study, a photoperiod-generated increase in prolactin was used to induce a wool follicle cycle during which changes in PRLR expression in sheep skin were determined by ribonuclease protection assay and in situ hybridisation. mRNA for prolactin and both isoforms of PRLR were also detected in skin by reverse transcription and polymerase chain reaction. As circulating prolactin began to rise from low levels, PRLR mRNA in the skin initially fell. These changes immediately preceded the catagen (regressive) phase of the hair cycle. Further increase in prolactin resulted in up-regulation of PRLR during telogen (dormancy), particularly in the epithelial hair germ, to reach a peak during proanagen (reactivation). In anagen (when follicle growth was fully re-established), PRLR mRNA returned to levels similar to those observed before the induced cycle. Hence, this longer term rise and fall of PRLR expression followed that of plasma prolactin concentration with a lag of 12-14 days. PRLR mRNA was most abundant in the dermal papilla, outer root sheath, hair germ, skin glands and epidermis. Location of PRLR in the dermal papilla and outer root sheath indicates action of prolactin on the growth-controlling centres within wool follicles. These cycle-related patterns of PRLR expression suggest dynamic regulation of PRLR by prolactin, thereby modulating hormonal responsiveness of seasonally growing hair follicles.