Photoperiod Entrainment of Testosterone, Luteinizing Hormone, Follicle-Stimulating Hormone, and Prolactin Cycles in Rams in Relation to Testis Size and Semen Quality1

Langford, G. A.; Ainsworth, L.; Marcus, G. J.; Shrestha, J. N. B.

doi:10.1095/biolreprod37.2.489

Cited by 57 publications

(34 citation statements)

References 37 publications

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“…Different neural tissues may control follicle-stimulating hormone and prolactin secretion, each of which changes markedly from summer to winter under the inf luence of Mel (2). This arrangement would be parsimonious in short day breeders such as sheep in which the seasonal suppression of prolactin secretion coincides with enhanced secretion of the gonadotropins (22,23). Refractoriness of separate Mel target tissues could mediate the transition to the spring phenotype independently for separate traits, e.g., reproduction and pelage.…”

Section: Discussionmentioning

confidence: 99%

Refractoriness to melatonin occurs independently at multiple brain sites in Siberian hamsters

Freeman

Zucker

2001

Proc. Natl. Acad. Sci. U.S.A.

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The mid-winter development of refractoriness to melatonin (Mel) triggers recrudescence of the atrophied reproductive apparatus of rodents. As a consequence, over-wintering animals become reproductively competent just before the onset of spring conditions favorable for breeding. The neural target tissues that cease to respond to winter Mel signals have not been identified. We now report that the suprachiasmatic nucleus of the hypothalamus, which contains the principal circadian clock, and the reuniens and paraventricular nuclei of the thalamus, each independently becomes refractory to melatonin. Small implants of Mel that were left in place for 40 wk and that act locally on these brain nuclei, induced testicular regression within 6 wk in male Siberian hamsters; 12 wk later Mel implants no longer suppressed reproduction and gonadal recrudescence ensued. Hamsters that were then given a systemic Mel infusion s.c. immediately initiated a second gonadal regression, implying that neurons at each site become refractory to Mel without compromising responsiveness of other Mel target tissues. Refractoriness occurs locally and independently at each neural target tissue, rather than in a separate ''refractoriness'' substrate. Restricted, target-specific actions of Mel are consistent with the independent regulation by day length of the several behavioral and physiological traits that vary seasonally in mammals.

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Section: Discussionmentioning

confidence: 99%

Refractoriness to melatonin occurs independently at multiple brain sites in Siberian hamsters

Freeman

Zucker

2001

Proc. Natl. Acad. Sci. U.S.A.

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“…In contrast, the majority of gonadal-intact ewes maintained under a constant equatorial photoperiod (12L:12D), also without exposure to rams, did not express a circannual rhythm of reproductive activity [11]. The reason for the apparent differences in the outcome of these two studies is not clear, but it may relate to hormonal mileau [12], environmental conditions [13,14], or length of photoperiod [15][16][17].…”

Section: Introductionmentioning

confidence: 85%

Circannual Rhythms in the Ewe: Patterns of Ovarian Cycles and Prolactin Secretion under Two Different Constant Photoperiods1

Jansen¹,

Jackson²

1993

Biology of Reproduction

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The purpose of this experiment was to determine whether circannual rhythms of reproductive activity and prolactin secretion are expressed differently in ewes housed under two different constant photoperiods and restricted temperatures. Eleven ovary-intact ewes housed previously under constant 12L:12D were used. One group (n = 6) was switched to 8L:16D while the other group (n = 5) remained on 12L:12D. Ovarian cycles and prolactin concentrations were monitored for more than 3 yr. The switch from 12L:12D to 8L:16D caused a change from cycles of varied length to regular 17-day cycles within 25 +/- 3 days in all 6 ewes. Subsequently, one complete circannual cycle (300.8 +/- 10.2 days) of breeding and nonbreeding activity was expressed in all ewes, but it was repeated in only 2 ewes. Ewes maintained on 12L:12D cycled at irregular intervals, and only 2 exhibited a distinct circannual rhythm of anestrus and breeding cycles. Episodic variations in ovarian cycle density were observed in most ewes in both groups. The period of this rhythm was similar between groups (290 +/- 19 and 278 +/- 16 days) and differed from 365 days (p < 0.05). Plasma prolactin exhibited a circannual rhythm under both 8L:16D and 12L:12D with a periodicity different from 365 days. We conclude that photoperiod length does not influence the expression of circannual reproductive and prolactin rhythms in the ewe. The absence of a circannual rhythm of breeding and nonbreeding activity, in the presence of circannual changes in ovarian cycle density, suggests that expression of an underlying endogenous oscillator controlling LH secretion was masked or damped.

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“…In sika deer, 12L is the only photoperiod in which the circannual rhythm in antler growth is not expressed (Goss 1969b;Goss 1984). Rams exhibit a robust circannual rhythm of prolactin secretion under long photoperiods, but rhythmicity is weak or absent in short photoperiods (Howles et al 1982;Langford et al 1987;Lincoln & Clarke 2000). In contrast, obligate circannual species such as the golden-mantled ground squirrel display circannual cycles under all photoperiodic conditions tested, including constant light and constant darkness (Pengelley & Asmundson 1970;Zucker & Boshes 1982;Zucker et al 1983).…”

Section: A Possible Synthesis: Are Circannual Clocks Constructed Frommentioning

confidence: 98%

Tracking the seasons: the internal calendars of vertebrates

Paul

Zucker

Schwartz

2007

Phil. Trans. R. Soc. B

175

152

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Animals have evolved many season-specific behavioural and physiological adaptations that allow them to both cope with and exploit the cyclic annual environment. Two classes of endogenous annual timekeeping mechanisms enable animals to track, anticipate and prepare for the seasons: a timer that measures an interval of several months and a clock that oscillates with a period of approximately a year. Here, we discuss the basic properties and biological substrates of these timekeeping mechanisms, as well as their reliance on, and encoding of environmental cues to accurately time seasonal events. While the separate classification of interval timers and circannual clocks has elucidated important differences in their underlying properties, comparative physiological investigations, especially those regarding seasonal prolactin secretions, hint at the possibility of common substrates.Keywords: seasonality; photoperiodism; circannual; interval timers While the Earth remaineth, seedtime and harvest, and cold and heat, and summer and winter, and day and night shall not cease.Genesis 8: 22, King James Version (1611) As the Earth makes its yearly orbit around the Sun, the planet's 23.58 axial tilt leads to the cyclical environmental changes that we call the seasons. Recurrent challenges to the survival of organisms and their offspring arise with the dawning of each new season, and animals have evolved seasonally induced changes in phenotype that adapt them to these predictable events. For example, in many temperate environments, winter is generally characterized by severe decreases in ambient temperature and food availability, leading to increased energetic demands at a time when resources are diminished. Because energy requirements of female mammals typically increase markedly during lactation (Bronson 1989) and newly weaned offspring are particularly vulnerable to environmental perturbations (Hill 1992), raising offspring during this time of year is often futile. Thus, many temperate species have evolved winter-specific behaviours and physiology to conserve energy (e.g. hibernation, a more insulative pelage, huddling) or provide for escape (e.g. migration). Many species also increase the chances of offspring survival by timing their mating behaviours so that parturition occurs during energetically favourable times of year.

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Photoperiod Entrainment of Testosterone, Luteinizing Hormone, Follicle-Stimulating Hormone, and Prolactin Cycles in Rams in Relation to Testis Size and Semen Quality1

Cited by 57 publications

References 37 publications

Refractoriness to melatonin occurs independently at multiple brain sites in Siberian hamsters

Refractoriness to melatonin occurs independently at multiple brain sites in Siberian hamsters

Circannual Rhythms in the Ewe: Patterns of Ovarian Cycles and Prolactin Secretion under Two Different Constant Photoperiods1

Tracking the seasons: the internal calendars of vertebrates

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