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.
It has been demonstrated that an endogenous mechanism is involved in photoperiodic time measurement in the mink, a short-day-breeding mannal. A study of testicular activity (testicular volume, plasma testosterone concentration) and plasma prolactin level was carried out in sexually resting minks (the experiment began in November). Groups of minks were kept in the natural photoperiod or subjected to different resonance light-dark (LD) cycles (LD 4:8, LD 4:20, LD 4:32, LD 4:44); an additional group of animals was reared in an ahemeral photoperiod (LD 4:16). A rapid increase of testicular activity was observed in control animals or those kept in LD 4:20 (T 24) and LD 4:44 (T 48). In the other groups of animals, those kept in LD 4:8 (T 12), LD 4:32 (T 36), and LD 4:16 (T 20), testicular function remained at rest. Prolactin secretion was, in contrast, stimulated in the groups kept in LD 4:8 (T 12). LD 4:32 (T 36), and LD 4:16 (T 20), and remained low in the groups kept in LD 4:20 (T 24) and LD 4:44 (T 48). These results show that the effects of the different photoperiodic regimens do not depend on the duration of the photophase, but rather on the period of the LD cycles. The LD cycles that allow an increase of testicular function are those that are inhibitory to reproduction in birds and long-day-breeding mammals. To explain these results, it is suggested that in the mink exposure to light during the circadian photosensitive phase induces inhibition of testicular activity and stimulation of prolactin secretion. To explain the opposite effects of a single photoperiod on testicular function and secretion of prolactin, the hypothesis has been advanced that, in the mink, long days might simultaneously inhibit hypothalamic luteinizing-hormone-releasing hormone (LH-RH) activity and prolactin-inhibiting factor (PIF) activity.
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