SAEVA Proceedings 2014 | Page 57

46TH  ANNUAL  CONGRESS  OF  THE  SAEVA        SKUKUZA      16-­‐20  FEBRUARY  2014   57     Introduction Horses have a natural breeding season that extends from April to September in the northern hemisphere, a time coinciding with longer day length, grass growth and milder weather [1]. Melatonin is a pineal hormone prod uced during the hours of darkness, its pattern of secretion mimicking the light/dark patterns that occur during each 24 h diurnal cycle. It acts as the daily decoder of seasonal changes in day length and regulates the circaannual reproductive cycles of seasonally breeding mammals [2,3]. The onset of the equine breeding season is triggered by increasing day length and concomitant reductions in melatonin secretion [4]. The mechanism of melatonin suppression has been well documented: light signals received by the retina and processed primarily by the photopigment melanopsin [5], located within the intrinsically photosensitive retinal ganglion cells, travel along the retino-hypothalmic tract to the hypothalamic suprachiasmatic nucleus [6]. The suprachiasmatic nucleus is connected to the pineal gland via the superior cervical ganglia and photic signals inhibit the release of the neurotransmitter norepinephrine, which therefore fails to stimulate the synthesis of melatonin [7,8]. During long days the inhibitory action of melatonin on the mare’s reproductive axis is lifted and increased gonadotropin releasing hormone pulse frequency stimulates the anterior pituitary to release follicle stimulating hormone and luteinising hormone, which act in concert to promote the growth, development and ovulation of ovarian follicles [9–11]. The northern hemisphere specifies a universal birthday for Thoroughbred racehorses of 1 January. Breeders therefore desire foals born early in the year to produce mature yearlings and precocious 2-year-old racehorses. Studies show that annual earnings are significantly higher for horses born in January– February than for those born in April–June [12,13]. For breeders to satisfy industry timelines, it is necessary that they manipulate the mares’ reproductively active period to meet the official start date of the breeding season on 15 February. Beginning 1 December, it has become standard industry practice to extend day length for 8–10 weeks by exposing mares to artificial light until 23.00 h, allowing natural dawn to occur during the most sensitive phase of the 24 h cycle [4]. An artificial photoperiod of 16 h light: 8 h dark is facilitated using light from a 100 W light bulb in a 3.6 °— 3.6 m stall [3,14], loosely described as ‘enough light to read a newspaper’. Light therapy such as this can advance the breeding season by as much as 3 months [15]. Recent research indicates that light within the short-wavelength spectrum (465–485 nm) is most effective at inhibiting melatonin [16] as melanopsin is particularly sensitive to short wavelength, blue light [17,18]. A recent study in Thoroughbred mares has shown that melatonin can be successfully inhibited using low-level blue light (465 nm) from a light emitting diode (LED) source directed at a single eye [19]. We aim to determine if timed, low-intensity, blue light administered to a single eye from head worn light masks is as effective as indoor maintenance under white light at advancing the breeding season in mares. Materials and methods Animals All experimental procedures were approved by the University College Dublin Animal Research Ethics Committee and the University of Kentucky Institutional   57