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Circannual prolactin rhythms: calendar-like timer revealed in the pituitary gland

https://doi.org/10.1016/j.tem.2007.07.001Get rights and content

Although photoperiodic regulation of annual rhythms in reproduction and other functions has been well characterized, the basis for the endogenous generation of circannual rhythms during exposure to constant conditions has not been elucidated. Lincoln and colleagues have recently reported that circannual prolactin rhythms in rams persist after hypothalamo–pituitary disconnection, but not after pinealectomy. Does the pars tuberalis, the site of pituitary gland melatonin receptors, generate circannual rhythms and integrate photoperiodic signals mediated by melatonin?

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Annual rhythms: regulation by photoperiod

Many mammals and birds exhibit annual rhythms in physiological and behavioral parameters, including reproduction, molting, body weight, hibernation and migration, that are regulated by changes in hormone secretion. The timing of these annual rhythms is regulated by changes in daylength, or photoperiod, which provide a reliable and predictive indicator of seasonal changes in environmental conditions [1]. In mammals, perception of photoperiodic information begins with light reception by the

Endogenous generation of circannual rhythms

In some long-lived mammalian species, such as sheep, and in many avian species, annual rhythms in molting and/or reproduction have been shown to persist under constant conditions of temperature, lighting and food availability, demonstrating that they are endogenously generated 5, 6, 7. These endogenously generated annual cycles have been termed circannual, reminiscent of the term circadian that designates endogenously generated daily (approximately 24 h) rhythms. Also in a similar manner to

The circannual rhythm in prolactin levels in sheep

Many mammals that exhibit photoperiodically induced seasonal changes in reproduction also exhibit photoperiodically induced seasonal changes in circulating levels of prolactin, which not only regulate lactation, but also modulate other functions, including gonadal activity and seasonal molts 10, 11. In sheep, pinealectomy blocks photoperiodically induced synchronization of the circannual prolactin rhythm, such that the rhythms among individual rams become variable and asynchronous [12].

Hypothalamo–hypophyseal disconnection: a unique model system for investigation of the anatomical substrate for circannual rhythms

Gerald Lincoln and his colleagues used a unique experimental model system, hypothalamo–hypophyseal disconnection (HPD) in the ram, to investigate the anatomical basis of endogenous circannual rhythms. In this model, prolactin levels continue to exhibit photoperiodically induced seasonal changes, but reproduction does not [11]. The surgical disconnection of the pituitary gland, with the pars tuberalis intact, from the hypothalamus allows examination of the circannual rhythm of prolactin

Is pineal gland melatonin secretion necessary for circannual rhythms?

Because photoperiodic effects on reproduction and hormone levels depend upon melatonin secretion from the pineal gland, the potential role of the pineal gland in mediating endogenous circannual rhythms in prolactin levels was also investigated [15]. HPD sheep were subjected to destruction of the SCG to block melatonin secretion. These ganglionectomized sheep, in contrast to HPD sheep with intact SCG, failed to exhibit either a rise in prolactin levels after transfer to a long-day photoperiod or

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  • Binary Switching of Calendar Cells in the Pituitary Defines the Phase of the Circannual Cycle in Mammals

    2015, Current Biology
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    Immunohistochemical studies revealed a dramatic reduction in the number of cells expressing both proteins in the PT, the extent of which relates to the history of prolactin secretion in individual photo-refractory animals. Thus, using prolactin as an endocrine marker for the underlying circannual phase [7–10], our data support a model in which the photoperiodic-input mechanism within the PT is involved in the generation of the circannual cycle. The PT is also directly involved in seasonal prolactin regulation via an intra-pituitary mechanism involving the PT production of a paracrine signal independently of neural input [8, 16, 37], the nature of which is known to change spontaneously in photo-refractory animals [38].

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    In healthy subjects, the circadian rhythm of cortisol levels is similar in women as compared to men, whereas a sex difference for prolactin and melatonin has not been investigated [15]. Recently the pituitary gland has been suggested to regulate prolactin rhythms as integration of photoperiodic signals mediated by melatonin [16]. Proinflammatory cytokines exhibit a peculiar rhythmicity, in particular serum TNF and serum IL-6 (Fig. 1D).

  • Seasonal reproduction in the male

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