SHORT COMMUNICATION Waiting for the Sun: the circannual programme of reindeer is delayed by the recurrence of rhythmical melatonin secretion after the arctic night David Hazlerigg 1, *, Arnoldus Schytte Blix 1,2 and Karl-Arne Stokkan 1, * ABSTRACT At temperate latitudes, the annual cycle of day length synchronizes circannual rhythms, and, in mammals, this is mediated via nocturnal production of the pineal hormone melatonin, proportional to the length of the night. Here, we studied circannual synchronization in an arctic species, the reindeer (Rangifer tarandus tarandus), which ceases to produce a rhythmic melatonin signal when it is exposed to extended periods of continuous midwinter darkness and continuous midsummer light. Using food intake, antler growth and moult as endpoints, we demonstrate that when animals living at 70°N are transferred from natural photoperiods in late autumn to either continuous light or continuous darkness, they undergo a conspicuous acceleration of the circannual programme. We conclude that rhythmical melatonin secretion, recommencing when the Sun reappears late in January, is required for proper timing of spring physiological responses, through a delaying effect on the circannual programme set in motion during the preceding autumn. KEY WORDS: Rangifer, Photoperiod, Pars tuberalis, Pineal gland, Circadian rhythms INTRODUCTION Seasonal rhythms in physiological responses depend upon innate long-term (circannual) timers, the phase of which is determined by photoperiodic synchronization. Circannual timers are synchronized to the annual environmental cycle through a process known as photoperiodic entrainment. This involves measurement of day length, which, in mammals, is transduced into a nocturnal rhythm of melatonin secretion by the pineal gland. Without the pineal gland, mammals either cease to express circannual rhythms or express disorganized rhythms that are no longer entrained to the solar year (Hazlerigg and Simonneaux, 2014). In rodents and in sheep, the mechanisms through which changes in melatonin signal duration are transduced rely upon a circadian- based ‘coincidence timer’ residing in the pars tuberalis (PT) of the anterior pituitary (Dardente et al., 2010; Matsumoto et al., 2010). This timer converts changes in nightly duration of melatonin exposure into changes in the amplitude of expression of a transcription coactivator and developmental switch, Eya3. Eya3 promotes the expression of differentiated thyrotrophs in the PT, in turn governing hypothalamic function through thyroid hormone- dependent mechanisms (Dardente et al., 2014). According to the current model (Dardente et al., 2010; Matsumoto et al., 2010; Dardente et al., 2014), shorter melatonin signals generate large amplitude daily oscillations of Eya3 RNA expression in the PT, while long-duration signals suppress the oscillation via a direct suppressive effect of melatonin in a critical time window occurring approximately 12 h after dark onset. Hence, induction of a physiological programme typical of spring, such as antler growth and moult in reindeer (Rangifer tarandus), is seen as a consequence of an increasing amplitude of a melatonin-dependent circadian rhythm of PT gene expression, induced by exposure to long days. The importance of the nightly rhythm of melatonin secretion is supported by experiments in pinealectomized rodents and sheep, in which the absence of melatonin or melatonin replacement as a continuous infusion failed to synchronize or maintain circannual rhythms in reproduction, body mass or prolactin secretion (Bartness and Goldman, 1989; Bartness et al., 1993; Lincoln et al., 2006). The capacity of arctic species to maintain photoperiodic synchronization of circannual rhythms challenges this model, because for long periods of the year surrounding the summer and winter solstices, the rhythmicity of melatonin secretion disappears (Stokkan et al., 1994). This suggests either that circannual entrainment relies upon narrow time windows around the equinoxes or that in species adapted to high latitudes, periods of continuous presence or absence of melatonin can act as circannual synchronizers. Experiments to distinguish between these possibilities have not been undertaken. If autumnal day length exposure is sufficient to set the phase of the circannual programme that runs during the winter, then subsequent exposure to continuous darkness or continuous light should not affect the timing of the spring programme of physiological change. Here, we tested this prediction in reindeer raised under semi-natural conditions at 70°N. Our data demonstrate that rhythmical melatonin secretion, which reappears with the return of the Sun in late January (Stokkan et al., 1994), is an essential requirement for proper phasing of the circannual rhythm. MATERIALS AND METHODS Animals A total of 15 semi-domesticated male Eurasian reindeer, Rangifer tarandus (Linnaeus 1758), aged 17 months were used for the described photoperiod treatments. Experimental procedures underwent local ethical review before the study commenced. Protocol Space limitations permitted up to two indoor photoperiod treatments to be conducted at any one time. Received 27 May 2017; Accepted 30 August 2017 1 Department of Arctic and Marine Biology, UiT – The Arctic University of Norway, Tromsø NO-9037, Norway. 2 St Catharine’s College, Cambridge CB2 1RL, UK. *Authors for correspondence (david.hazlerigg@uit.no; kst002@post.uit.no) D.H., 0000-0003-4884-8409 3869 © 2017. Published by The Company of Biologists Ltd | Journal of Experimental Biology (2017) 220, 3869-3872 doi:10.1242/jeb.163741 Journal of Experimental Biology