Almeida, Osborne Francisco Xavier

2018-05-14T10:11:49Z

2018-05-14T10:11:49Z

1982

The studies described in this thesis were undertaken to investigate the role of melatonin (MEL), a secretion of the pineal gland, in the photoperiodic regulation of testicular activity in rams of the Soay breed. For this, rams were exposed to a variety of artificial light-dark (LD) cycles and their reproductive responses and temporal patterns of MEL in the blood were monitored.

Exposure of intact rams to short daylengths induced testicular development, whereas exposure to long daylengths resulted in testicular regression. While MEL was secreted maximally during periods of darkness in intact rams kept under short and long daylenghts, temporal differences, related to the duration of light and darkness, were observed in the profiles obtained under each photoperiod. There were no differences in the absolute amounts of MEL secreted.

Rams held under constant light or constant darkness for up to lOd maintained a 24h rhythm in their blood levels of MEL, suggesting that this rhythm was an endogenous circadian rhythm, with the LD cycle simply serving to entrain it.

Evidence that the blood MEL rhythm is a circadian one was obtained in a study in which rams were exposed to a photoperiod consisting of 8hL:40hD (48h cycle) or one of 8hL:28hD (36h cycle). In the 48h cycle, the 8h light period occurred at the same circadian time in each cycle, whereas in the 36h cycle it occurred at alternating circadian times. This protocol would test whether rams measure daylength by a circadian mechanism or by counting up the hours of light and darkness in each cycle. A consistent 24h MEL rhythm was found in the rams exposed to the 48h cycle, but not in those exposed to the 36h cycle, i.e. entrainment of the 24h MEL rhythm apparently depends on a LD cue occurring at 24h intervals or multiples thereof.

In the above experiment, the testes of the rams under the 48h cycle became fully developed as though the rams were exposed to ordinary short daylengths whereas those of the rams under the 36h cycle became only partially developed. Thus a circadian mechanism also appears to underly the photoperiodic regulation of reproduction, and there may be a correlation between blood patterns of MEL, photoperiod and reproductive status.

Correlations between blood patterns of MEL and reproductive state were also obtained in an experiment in which rams were exposed to a prolonged period (96 weeks) of either long or short daylengths. The animals eventually became refractory to the normal reproductive effects of each of these photoperiods, showing gonadal involution and recrudescence despite the daylengths they were exposed to. The 24h pattern in the blood levels of MEL were found to become disrupted concomitantly with the onset of photo-refractoriness.

Correlations between blood patterns of MEL and reproductive state were also obtained in an experiment in which rams were exposed to a prolonged period (96 weeks) of either long or short daylengths. The animals eventually became refractory to the normal reproductive effects of each of these photoperiods, showing gonadal involution and recrudescence despite the daylengths they were exposed to. The 24h pattern in the blood levels of MEL were found to become disrupted concomitantly with the onset of photo-refractoriness.

A central site of action is presumed in a model for the way in which MEL might be involved in the photoperiodic regulation of reproduction in the ram. It is proposed that its principal role is to relay information about daylength to the hypothalamic-pituitary-gonad axis. The patterns of MEL secretion closely reflect the LD cycle through a circadian mechanism. In addition, there is a circadian rhythm in brain sensitivity to MEL. Interpretation of the photoperiod, and thus gonadal activation or involution, is then based upon the phase relationships of the rhythms in MEL release and the sensitivity of the brain to MEL.

http://hdl.handle.net/1842/29693

The University of Edinburgh

Annexe Thesis Digitisation Project 2018 Block 18

Already catalogued

Melatonin and the control of seasonal breeding in the Soay ram

Thesis or Dissertation

Doctoral

PhD Doctor of Philosophy