Role of the pineal gland in the photoperiodic control of reproductive and non-reproductive functions in mink (Mustela vison).

Mink are long-lived mammals that exhibit seasonal changes in body weight, gonadal activity, pelage and plasma prolactin levels. Mating in February-March is followed by an obligatory delay in implantation during which the corpora lutea stay quiescent. All these events are controlled by annual variations in daylength. The role of the pineal gland and its product, melatonin, in conveying photic information to the target organs has been studied. Pineal denervation by bilateral ablation of the cervical superior ganglia rendered the mink unresponsive to artificial manipulations of the daily photoperiod: prolactin and progesterone secretion and the spring moult were no longer stimulated by long days or inhibited by short days in pregnant females; in the same way the increase in body weight in late summer and the autumn moult were no longer advanced by artificial shortening of the photoperiod. Pinealectomy seemed to desynchronize body weight, prolactin and moulting cycles from those in intact mink. Melatonin injections reproduced the effects of short days on hormonal secretion during the delayed implantation period. Melatonin capsules given to males during the phase of testicular activity delayed the decrease in body weight, testicular regression and onset of the spring moult exactly as did short days. In contrast, melatonin administered during the phase of testicular inactivity triggered an increase in body weight, the onset of the spring moult and testicular recrudescence in this short-day breeder. These results support the hypothesis that in mink all photoperiodic signals are conveyed by the pineal gland. But although the pineal seems essential for the seasonal timing of the cycles, it does not modify the events themselves once they are initiated.