Long‐Term Effects of Pinealectomy on Testicular Function, Luteinizing Hormone‐Releasing Hormone Hypothalamic System, and Plasma Prolactin Levels in the Mink, a Short‐Day Breeder

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Abstract
Experiments in minks, as in a number of other seasonal breeders, clearly demonstrate that the pineal gland is essential for the photoperiodic control of reproduction. While maintenance of pineal‐intact minks under natural photoperiods results in a set of seasonally appropriate changes in testicular activity, pinealectomized minks undergo none of these changes but rather remain sexually inactive as under long‐day conditions. Thus, the consequences of pinealectomy differ from one photoperiodic species to another, but the unifying feature is the organism's need for the pineal gland to respond appropriately to changes in day length.Although the precise mechanism by which the pineal regulates hypothalamic‐pituitary gonadal function remains unknown, the results of the present study indicate that, in the mink, luteinizing hormone‐releasing hormone axonal transport is affected by pinealectomy.Furthermore, our results suggest that the pineal does not act exclusively upon the neuroendocrine‐gonadal system but also acts on other functions that are influenced by photoperiod. Pinealectomized minks left in natural conditions cannot adjust their prolactin secretion in response to either long or short photoperiods. Operated animals continued to have plasma prolactin variations but at irregular intervals and with no apparent relation to the time of the year. The data strengthen the hypothesis that melatonin may act at some point on the hypothalamic neuroendocrine systems, which regulate the two functions differently, and that melatonin is not an anti‐ or progonadal substance but rather a seasonal transducer.