Secretion of all the pituitary hormones undergoes marked circadian and seasonal changes.The rhythmicity of these changes is controled by the circadian pacemaker system and the pineal gland transmitting daylength informatoin to the neuroendocrine axis via the secretion of melatonin.This article presents data on the effects of the short-term melatonin administratoin into the third brain ventricle on prolactin, beta-endorphin and luteotropin secretion in ewes kept under the increasing and decreasing daylenght conditions.Additional emphasis is given to dopamine and LHRH release in the madiobasal hypothalmus under the malatonin treatment by the push-pull method.The long-term and short-term actions of melatonin on the hormonal status in ewes is also discussed.
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