46TH
ANNUAL
CONGRESS
OF
THE
SAEVA
SKUKUZA
16-‐20
FEBRUARY
2014
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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
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