Suprachiasmatic Nucleus and Photic Entrainment of Circannual Rhythms in Ground Squirrels

Lee, Theresa; Zucker, Irving H.

doi:10.1177/074873049100600403

Cited by 55 publications

(25 citation statements)

References 34 publications

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“…Evidence is mounting that the long photoperiods of spring and summer provide crucial time cues for circannual events, and that synchronization of the entire rhythm occurs at this time of year. This evidence includes observations on circannual rhythms of reproductive activity in rainbow trout [42]; emergence from hibernation, and reproduction and body weight in golden-mantled ground squirrels [44]; and a variety of circannual phenomena in migratory birds [3,5]. Our present findings provide complementary evidence for sheep, in that the most effective synchronizing signals were those mimicking the longer photoperiods perceived during spring and summer.…”

Section: Discussionsupporting

confidence: 60%

“…For example, a given photoperiodic signal may either synchronize the rhythm (causing it to advance or delay) or have no effect depending on when during the rhythm the cue is experienced. Rhythm stage has been suggested to be an important determinant of the photoperiodic response in a number of seasonal breeders, including sheep [40][41][42][43][44]. In the present study, the effects of photoperiod were not separated from any potential effect of rhythm stage.…”

Section: Discussioncontrasting

confidence: 60%

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Photoperiodic Synchronization of a Circannual Reproductive Rhythm in Sheep: Identification of Season-Specific Time Cues1

Woodfill¹,

Wayne²,

Moenter³

et al. 1994

Biology of Reproduction

132

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Seasonal reproduction in the ewe is generated by an endogenous circannual rhythm of reproductive neuroendocrine activity. Exposure to as few as 70 days of photoperiodic information a year is sufficient to synchronize the rhythm. The present study was conducted to identify which portions of the photoperiodic cycle are utilized for synchronization. For this purpose, we used pinealectomized ewes that could not respond reproductively to changes in day length. Selected photoperiodic information was provided via infusion of melatonin, a hormone that provides the neuroendocrine code for day length in this species. Melatonin was delivered according to circadian patterns. The infusion patterns were tailored to mimic those of melatonin secretion in pineal-intact ewes during one of the four seasons: winter, spring, summer, or autumn. The infusions were provided for 90 days a year during each of the three years following pinealectomy. The ewes were ovariectomized and treated with constant-release Silastic capsules containing estradiol; reproductive neuroendocrine activity was monitored by measurement of serum concentrations of LH. In the absence of exogenous melatonin, most (19 of 24) pinealectomized controls exhibited circannual LH cycles that were not in synchrony, indicating that the rhythm was free-running. Melatonin synchronized the rhythm (such that the period was 365 days and the stages of the rhythm were both concurrent among animals and in appropriate phase with the geophysical year), but not all melatonin patterns were equally effective in this regard. The most effective melatonin patterns mimicked those of secretion during summer. Spring and autumn melatonin patterns were less effective, and winter melatonin patterns were ineffective. These results support the concept that there is a seasonal specificity with regard to the photoperiodic cues that synchronize the circannual rhythm of reproductive neuroendocrine activity in the ewe. The rhythm is synchronized most effectively by long-day photoperiodic cues perceived on or around the summer solstice.

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Section: Discussionsupporting

confidence: 60%

Section: Discussioncontrasting

confidence: 60%

Photoperiodic Synchronization of a Circannual Reproductive Rhythm in Sheep: Identification of Season-Specific Time Cues1

Woodfill¹,

Wayne²,

Moenter³

et al. 1994

Biology of Reproduction

132

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“…These models are not mutually exclusive; entrainment may require a specific photoperiod during a particular rhythm stage. Consistent with a role for stage are findings that a given photoperiod can differentially affect seasonal cycles of fish, birds, and mammals (including sheep) depending on the season of exposure [8,[28][29][30]. However, those studies, unlike the present one, utilized animals that had intact photoperiodic response systems.…”

Section: Discussionsupporting

confidence: 71%

Importance of Photoperiodic Signal Quality to Entrainment of the Circannual Reproductive Rhythm of the Ewe1

Barrell

Thrun

Brown

et al. 2000

Biology of Reproduction

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An endogenous circannual rhythm drives the seasonal reproductive cycle of a broad spectrum of species. This rhythm is synchronized to the seasons (i.e., entrained) by photoperiod, which acts by regulating the circadian pattern of melatonin secretion from the pineal gland. Prior work has revealed that melatonin patterns secreted in spring/summer entrain the circannual rhythm of reproductive neuroendocrine activity in sheep, whereas secretions in winter do not. The goal of this study was to determine if inability of the winter-melatonin pattern to entrain the rhythm is due to the specific melatonin pattern secreted in winter or to the stage of the circannual rhythm at that time of year. Either a summer- or a winter-melatonin pattern was infused for 70 days into pinealectomized ewes, centered around the summer solstice, when an effective stimulus readily entrains the rhythm. The ewes were ovariectomized and treated with constant-release estradiol implants, and circannual cycles of reproductive neuroendocrine activity were monitored by serum LH concentrations. Only the summer-melatonin pattern entrained the circannual reproductive rhythm. The inability of the winter pattern to do so indicates that the mere presence of a circadian melatonin pattern, in itself, is insufficient for entrainment. Rather, the characteristics of the melatonin pattern, in particular a pattern that mimics the photoperiodic signals of summer, determines entrainment of the circannual rhythm of reproductive neuroendocrine activity in the ewe.

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“…The first mechanism, typically observed in large long-lived mammals, employs an endogenous circannual clock with a period of approximately a year, even in organisms deprived of annual time cues. When animals are exposed to the natural environment, circannual rhythms adopt the period of the sidereal year through entrainment to the annual photoperiodic cycle [46,54,127,130]. The second type of rhythm, typically employed by small short-lived mammals, is similar to that described previously in birds, incorporating alternating periods of photosensitivity and photorefractoriness.…”

Section: Seasonal Reproduction and Gnih In Mammalsmentioning

confidence: 95%

Seasonal control of gonadotropin-inhibitory hormone (GnIH) in birds and mammals

Kriegsfeld¹,

Ubuka²,

Bentley³

et al. 2015

Frontiers in Neuroendocrinology

106

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Animals inhabiting temperate and boreal latitudes experience marked seasonal changes in the quality of their environments and maximize reproductive success by phasing breeding activities with the most favorable time of year. Whereas the specific mechanisms driving seasonal changes in reproductive function vary across species, converging lines of evidence suggest gonadotropin-inhibitory hormone (GnIH) serves as a key component of the neuroendocrine circuitry driving seasonal changes in reproduction and sexual motivation in some species. In addition to anticipating environmental change through transduction of photoperiodic information and modifying reproductive state accordingly, GnIH is also positioned to regulate acute changes in reproductive status should unpredictable conditions manifest throughout the year. The present overview summarizes the role of GnIH in avian and mammalian seasonal breeding while considering the similarities and disparities that have emerged from broad investigations across reproductively photoperiodic species.

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Suprachiasmatic Nucleus and Photic Entrainment of Circannual Rhythms in Ground Squirrels

Abstract: The efficacy of photoperiod as a zeitgeber for entrainment of circannual body weight and estrous rhythms was tested in female golden-mantled ground squirrels maintained for 3 or more years in either a simulated natural photoperiod (SNP)

Cited by 55 publications

References 34 publications

Photoperiodic Synchronization of a Circannual Reproductive Rhythm in Sheep: Identification of Season-Specific Time Cues1

Photoperiodic Synchronization of a Circannual Reproductive Rhythm in Sheep: Identification of Season-Specific Time Cues1

Importance of Photoperiodic Signal Quality to Entrainment of the Circannual Reproductive Rhythm of the Ewe1

Seasonal control of gonadotropin-inhibitory hormone (GnIH) in birds and mammals

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