Studies in many species, including sheep, hamsters, mink, and ground squirrels [105,106], show that long daylight hours initiate short periods of daily melatonin secretion by the pineal gland and summer coat development, while short (winter) day-length increases melatonin secretion and stimulates a longer, warmer pelage [7,103]. The pineal gland acts as a neuroendocrine transducer converting nerve impulses stimulated by daylight to reduced secretion of melatonin, normally secreted in the dark. Melatonin signals are generally translated to the follicle by the hypothalamus-pituitary route; for example, melatonin administration into the sheep hypothalamus stimulates short day responses [107]. However, although disconnecting the hypothalamus and pituitary removes seasonal changes in body weight and the wool’s normal cycling pattern, long days stimulate a minor moult [103]. Prolactin levels continue to cycle, suggesting melatonin also acts directly on the pituitary prolactin secretion. Since both growth hormone and IGF-1 levels are also reduced, this may prevent prolactin’s full effect as IGF-1 receptors are present in goat follicles [108] and IGF-1 can stimulate human hair growth in vitro [109].
There is strong evidence for prolactin’s involvement in seasonal coat changes in Djun – garian hamsters [106], goats [108], mink [110], sheep [110,111], and deer [112]. Increased prolactin levels in long daylight correspond to low summer growth and low prolactin concentrations during short days with increased winter growth; moulting occurred in sheep after maximal prolactin levels [103]. Prolactin infusion inhibits goat hair growth locally [113] and prolactin receptors are located in rodent [114,115] and mink [116] skin and the dermal papilla and epithelial compartments of sheep follicles [117]. Interestingly, sheep [111], mink [116], and non-seasonal laboratory rodent [115] follicles also express prolactin mRNA.
Other hormones implicated in regulating mammalian hair growth cycles include the sex steroids, oestradiol and testosterone, and the adrenal steroids; these delay anagen in
rats [7,118], while gonadectomy in rats and adrenalectomy in rats and mink [7,118,119] advance it. Topical application of 17^-oestradiol to mice skin inhibits hair growth and accelerates catagen, while antioestrogens promote early anagen [120-124]. Rat dermal papillae take up oestradiol [125] and both oestrogen receptors a (ERa) and в (ERfl) are detected in human follicles [126] and cultured dermal papilla cells [127]. Testosterone also delays seasonal hair growth in badgers [128], while urinary cortisol levels are negatively correlated with hair loss in rhesus macaque monkeys [129]. In contrast, thyroid hormones advance anagen while thyroidectomy or propythiouracil delay it [7,118]. How these circulating hormones interact is still unclear, but the main drivers in seasonal coat changes are light, melatonin, and prolactin.