+ Site Statistics
+ Search Articles
+ PDF Full Text Service
How our service works
Request PDF Full Text
+ Follow Us
Follow on Facebook
Follow on Twitter
Follow on LinkedIn
+ Subscribe to Site Feeds
Most Shared
PDF Full Text
+ Translate
+ Recently Requested

Clock genes and the long-term regulation of prolactin secretion: Evidence for a photoperiod/circannual timer in the pars tuberalis

Clock genes and the long-term regulation of prolactin secretion: Evidence for a photoperiod/circannual timer in the pars tuberalis

Journal of Neuroendocrinology 15(4): 390-397

Prolactin secretion is regulated by photoperiod through changes in the 24-h melatonin profile and displays circannual rhythmicity under constant photoperiod. These two processes appear to occur principally within the pituitary gland, controlled by the pars tuberalis. This is evident because: (i) hypothalamic-pituitary disconnected (HPD) sheep show marked changes in prolactin secretion in response to switches in photoperiod and manipulations of melatonin, similar to brain-intact controls; (ii) HPD sheep also show photoperiod-specific, long-term cycles in prolactin secretion under constant long or short days, with the timing maintained even when prolactin secretion is blocked for 2-3 months; and (iii) pars tuberalis cells, but not lactotrophs, express high concentrations of melatonin (MT1) receptor, and exhibit a duration-sensitive, cAMP-dependant, inhibitory response to physiological concentrations of melatonin. This suggests the existence of an intrinsic, reversible photoperiod-circannual timer in pars tuberalis cells. A full complement of clock genes (Bmal1, Clock, Per1, Per2, Cry1 and Cry2) are expressed in the ovine pars tuberalis, and undergo 24-h cyclical expression as observed in a cell autonomous, circadian clock. Activation of Per genes occurs in the early day (melatonin off-set), while activation of Cry genes occurs in the early night (melatonin on-set). This temporal association is evident under both long and short days, thus the Per-Cry interval varies directly with photoperiod. Because, PER : CRY, protein : protein interactions affect stability, nuclear entry and gene transcription based on rodent data, the change in phasing of Per/Cry expression provides a potential mechanism for decoding the long day/short day melatonin signal. A speculative, but testable, extension of this hypothesis is that intrinsically regulated changes in the phase of Per/Cry rhythms, regulates both photorefractoriness and the generation of circannual rhythms in prolactin secretion.

Please choose payment method:

(PDF emailed within 0-6 h: $19.90)

Accession: 010334197

Download citation: RISBibTeXText

PMID: 12622839

DOI: 10.1046/j.1365-2826.2003.00990.x

Related references

Temporal expression of seven clock genes in the suprachiasmatic nucleus and the pars tuberalis of the sheep: evidence for an internal coincidence timer. Proceedings of the National Academy of Sciences of the United States of America 99(21): 13890-5, 2002

The pars tuberalis: The missing link in the photoperiodic regulation of prolactin secretion?. Journal of Neuroendocrinology 12(4): 287-295, 2000

Cells co-expressing luteinising hormone and thyroid-stimulating hormone are present in the ovine pituitary pars distalis but not the pars tuberalis: implications for the control of endogenous circannual rhythms of prolactin. Neuroendocrinology 97(4): 355-362, 2013

Evidence for regulation of basic fibroblast growth factor gene expression by photoperiod and melatonin in the ovine pars tuberalis. Molecular & Cellular Endocrinology 156(1-2): 45-53, 1999

Effects of placing micro-implants of melatonin in the pars tuberalis, pars distalis and the lateral septum of the forebrain on the secretion of FSH and prolactin, and testicular size in rams. Journal of Endocrinology 142(2): 267-276, 1994

Evidence that melatonin binding sites in the pars tuberalis do not mediate the photoperiodic actions of melatonin on LH and prolactin secretion in ewes. Journal of Reproduction and Fertility 101(3): 625-632, 1994

Regulation of melatonin receptors in the pars tuberalis of Syrian hamsters transferred from long to short photoperiod: Implication of melatonin and testosterone. Journal of Neuroendocrinology 10(4): 303-308, 1998

Circannual cycles of luteinizing hormone and prolactin secretion in ewes during prolonged exposure to a fixed photoperiod: evidence for an endogenous reproductive rhythm. Biology of Reproduction 41(6): 1034-1046, 1989

Endogenous circannual cycles of ovarian activity and changes in prolactin and melatonin secretion in wild and domestic female sheep maintained under a long-day photoperiod. Biology of Reproduction 78(3): 552-562, 2008

Identification of marker genes for pars tuberalis morphogenesis in chick embryo: expression of Cytokine-like 1 and Gap junction protein alpha 5 in pars tuberalis. Cell and Tissue Research 366(3): 721-731, 2016

Cell organization of the rat pars tuberalis. Evidence for open communication between pars tuberalis cells, cerebrospinal fluid and tanycytes. Cell and Tissue Research 339(2): 359-381, 2010

Expression of tuberalin II, alpha-subunit of glycoprotein hormones and beta-thyrotropin hormone in the pars tuberalis of the rat: immunocytochemical evidence for pars tuberalis-specific cell types. Neuroendocrinology 90(3): 269-282, 2009