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.

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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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1. The ability of a moult-inducing procedure to restore high levels of sperm production was assessed, in two experiments, using cockerels with reduced sperm production. The moulting procedure consisted of a period of food and light restriction for 6 weeks. The moulted birds were compared with control birds for 20 weeks. 2. Moult induction resulted in decreased daily sperm output (DSO) and plasma testosterone concentration, from weeks 3 to 7. In the first experiment, plasma luteinising hormone (LH) concentration in moulted birds was reduced on week 5. 3. No change in pituitary sensitivity to chicken luteinising hormone-releasing hormone-I (cLHRH-I) was detected at week 3 in moulted cockerels. In moulted birds, a transient increase in plasma thyroxine concentration was detected. 4. After the end of moult induction, testosterone concentrations increased, plasma LH showed a rebound at week 7 and the pituitary sensitivity to LHRH was increased at week 9. 5. This increased activity of the pituitary-testicular axis resulted for a short time in an increase in DSO of moulted birds compared with that of controls. Although amelioration was moderate, this result indicates the possibility of improving sperm production in the cockerel by using an induced moulting procedure.

1. The effect of thyroxine (T4) on reproductive function in the adult cockerel was followed for 11 weeks. Broiler cockerels aged 96 weeks were fed on diets containing either 0, 2 or 5 mg T4/kg for 4 weeks. 2. Daily sperm output (DSO) was significantly reduced (P < 0.01) in the T4-treated groups compared with that of controls at weeks 5 and 7. In the group given 5 mg T4/kg, plasma testosterone concentration was significantly reduced (P < 0.01) compared with that in controls during the T4 treatment, in spite of the fact that there was a decrease in concentration in both control and experimental birds. Plasma luteinising hormone (LH) concentration was significantly decreased (P < 0.05) in both of the groups given T4 treatments after 3 weeks. 3. Plasma testosterone concentrations and DSO had returned to control values at weeks 5 and 11 respectively, while plasma LH showed a transient but significant (P < 0.001) rebound after removal of thyroxine from the food. 4. In contrast to other variables, the pituitary responsivity to cLHRH-I injections, was not decreased during the feeding of the T4 diet but was, on the contrary, significantly increased (P < 0.05) during treatment with 5 mg T4/kg diet, and after the end of the treatment with 2 mg T4/kg diet. 5. These results provide some evidence for an inhibitory effect of large doses of T4 on the reproductive function in the adult cockerel. Although the possibility of a direct effect of T4 on the testes cannot be excluded, T4 is likely to act, at least in part, at the hypothalamo-pituitary level, and not through a reduction in the pituitary sensitivity to LHRH.

Summary ― An in vitro system for the incubation of mediobasal hypothalami (MBH) of cockerels and a radioimmunoassay for chicken luteinising hormone-releasing hormone-I (cLHRH-1) were developed. The size of the hypothalamic fragment (MBH including the median eminence) and the incubation conditions used (40°C, under constant shaking and gassing) preserved the physiological properties of the tissue. It was possible to maintain the MBH in vitro and to study the LHRH release for several hours. The assay proved sensitive enough (ED BO = 0.794 pmol/tube, ie 4.59 pg/ml) and sufficiently precise (within-assay coefficient of variation = 4.4% and between-assay coefficient of variation = 10.2%) to measure the amounts of peptide released in the incubation medium. The use of this incubation system provided the first evidence of the stimulating effect of the excitatory amino acids glutamate, NMDA and kainate on the secretion of cLHRH-1 in birds. Our results suggest that the effect on the NMDA receptor is predominant.

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