Effects of melatonin infused into the III ventricle on prolactin, beta-endorphin and luteotropin secretion in ewes during the different stages of the reproductive cycle.

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