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Blockade of Tyrosine Hydroxylase Activity in the Median Eminence Partially Reverses the Long Day‐Induced Inhibition of Pulsatile LH Secretion in the Ewe
Author(s) -
Viguié Catherine,
Picard Sophie,
Thiéry JeanClaude,
Malpaux Benoît
Publication year - 1998
Publication title -
journal of neuroendocrinology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.062
H-Index - 116
eISSN - 1365-2826
pISSN - 0953-8194
DOI - 10.1046/j.1365-2826.1998.00237.x
Subject(s) - median eminence , endocrinology , medicine , blockade , pulsatile flow , tyrosine hydroxylase , tyrosine 3 monooxygenase , secretion , chemistry , biology , dopamine , hypothalamus , receptor
The photoperiod‐induced stimulation of LH secretion is associated with a decrease in dopamine content, as well as in the activity of its rate limiting enzyme, tyrosine hydroxylase (TH), in the median eminence (ME) of the ewe. We therefore hypothesize that ME‐TH activity can constitute a limiting factor of photoperiod‐induced inhibition of LH pulsatile secretion. To test this hypothesis, we studied whether the inhibition of ME‐TH activity can reverse the long day‐induced inhibition of LH. Using microdialysis, a 3 mM solution of alpha methyl‐p‐tyrosine ( α MPT; a competitive inhibitor of TH), was administered in the ME of ovariectomized ewes bearing a 0.5 cm oestradiol implant at the beginning of a LD‐induced inhibition of LH secretion. The vehicle solution was infused for 4 h followed by a 3 mM α MPT solution infused for an additional 4 h. LH pulsatile secretory patterns within the same animal were compared between the control period and the α MPT period. α MPT infusion in the ME was associated with an increase in LH pulse frequency whereas it did not affect prolactin secretion. In conclusion, our results suggest that the inhibition of TH activity in the ME causes a stimulation of LH secretion in long‐day inhibited ewes.