Document Detail

Characterization of the short day-induced decrease in median eminence tyrosine hydroxylase activity in the ewe: temporal relationship to the changes in luteinizing hormone and prolactin secretion and short day-like effect of melatonin.
MedLine Citation:
PMID:  8977440     Owner:  NLM     Status:  MEDLINE    
In the ewe, photoperiod modulates LH and PRL secretion as well as median eminence (ME) dopaminergic activity. The studies reported here were designed to characterize the functional significance of this photoperiodic modulation of ME dopaminergic neuron activity in relation to the regulation of LH and PRL secretion. The aim of the first experiment was to assess whether photoperiodic changes in hypothalamic dopaminergic activity were temporally linked to changes in either PRL or LH secretion. The purpose of the second experiment was to determine whether melatonin mimicked the effects of photoperiod on ME dopaminergic activity. In the first experiment, LH and PRL secretion, hypothalamic tyrosine hydroxylase (TH) activity, and catecholamine contents were determined in ovariectomized estradiol-treated ewes either during long days (LD; control group) or after 5, 25, and 76 short days (SD). SD were associated with a stimulation of LH secretion and a decrease in ME TH activity, which were both expressed only in the 76 SD group. In contrast, the SD-induced inhibition of PRL secretion was already maximal in the 25 SD group. In the second experiment, LH secretion and hypothalamic dopaminergic activity were studied in ovariectomized estradiol-treated ewes kept in LD and then treated for 0 (control), 25, or 77 days with melatonin implants producing a SD-like effect on LH secretion. Melatonin induced a decrease in PRL secretion (observed after 25 days of treatment), as well as a stimulation of LH secretion and a decrease in ME TH activity and dopamine content (observed only after 77 days of treatment). In conclusion, the decrease in ME dopaminergic activity associated with SD exposure or the SD-like effect of melatonin appears unrelated to the regulation of PRL secretion. The SD-like effect of melatonin on ME dopaminergic activity suggests that melatonin mediates the effect of SD on this activity. The regulation of ME dopaminergic activity can thus be considered a probable step in the photoperiodic regulation of LH secretion.
C Viguié; J Thibault; J C Thiéry; Y Tillet; B Malpaux
Publication Detail:
Type:  Journal Article; Research Support, Non-U.S. Gov't    
Journal Detail:
Title:  Endocrinology     Volume:  138     ISSN:  0013-7227     ISO Abbreviation:  Endocrinology     Publication Date:  1997 Jan 
Date Detail:
Created Date:  1997-01-23     Completed Date:  1997-01-23     Revised Date:  2006-11-15    
Medline Journal Info:
Nlm Unique ID:  0375040     Medline TA:  Endocrinology     Country:  UNITED STATES    
Other Details:
Languages:  eng     Pagination:  499-506     Citation Subset:  AIM; IM    
Institut National de la Recherche Agronomique, Physiologie de la Reproduction des Mammiferes Domestiques, Laboratoire de Neuroendocrinologie Sexuelle, Nouzilly, France.
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MeSH Terms
Catecholamines / analysis
Dopamine / physiology*
Luteinizing Hormone / secretion*
Median Eminence / enzymology*
Melatonin / pharmacology*
Prolactin / secretion*
Tyrosine 3-Monooxygenase / metabolism*
Reg. No./Substance:
0/Catecholamines; 73-31-4/Melatonin; 9002-62-4/Prolactin; 9002-67-9/Luteinizing Hormone; EC 3-Monooxygenase

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