Target sites for melatonin in the sheep with particular reference to the photoperiodic control of reproduction
The aim of this study was to identify the potential target sites for the action of melatonin in sheep and to relate the findings to the photoperiodic control of reproductive activity in the female. It has been proposed that melatonin may have a direct luteotrophic role in several species, therefore the first experiment in this thesis was designed to investigate if melatonin acts directly on the ovine ovary to enhance progesterone production. The oral administration of 3mg melatonin, daily at 1500h, to anoestrous Blackface ewes advanced the onset of oestrous cyclicity and was associated with an increase in ovulation rate, at the first overt oestrous, compared to naturally ovulating control ewes. Peripheral prolactin concentrations were significantly suppressed within 7 days of the start of melatonin treatment. The concentration of progesterone was determined in samples collected directly from the utero-ovarian vein, and compared to the pulsatile pattern of luteinizing hormone (LH) secretion determined from samples collected from the jugular vein. The pattern of progesterone secretion in the utero-ovarian vein was episodic. The majority of progesterone pulses occurred independently of the pulsatile pattern of LH secretion, although each pulse of LH was followed by an increase in progesterone concentration. During the luteal phase of the first overt oestrous cycle, the pulsatile pattern of LH secretion and the progesterone concentrations in the utero-ovarian venous plasma were similar for both melatonin-treated and naturally ovulating control ewes. This suggests that melatonin does not have a direct luteotrophic action on the ovine ovary. The experiment described in Chapter 6 was designed to investigate if the ovine foetus is sensitive to photoperiodic information while in utero and also to assess the effect of prenatal photoperiod experience on the endocrine status and timing of puberty of ewe lambs. Three groups of female lambs were studied. Groups A and B were raised on a common postnatal photoperiod consisting of a 10 week block of long days (18L:6D) from birth followed by short days (6L:18D) until the end of the experiment at 38 weeks of age. These two groups of lambs differed only in the photoperiod experienced by their mothers during gestation; those in Group A were exposed to short days and those in Group B were exposed to long days from day 25 of gestation to parturition. Lambs in Group C experienced long days prenatally and short days from birth until the end of the experiment. Peripheral prolactin concentrations of lambs on the day of birth were dependent on the photoperiod to which their mothers were exposed during gestation. For the first 10 weeks after birth plasma prolactin profiles of the lambs raised under long days (Groups A and B) were influenced by their prenatal photoperiodic experience. The number of female lambs which exhibited puberty following a decrease in day length from 18L:6D to 6D:18L at 10 weeks of age was increased by maintaining their mothers on long days as opposed to short days during gestation. Puberty was delayed in the majority of female lambs which were exposed to long days in utero followed by short days from birth (Group C). The results suggest that lambs are sensitive to photoperiodic information prior to birth and that female lambs must be exposed to a minimum number of long days prior to a decrease in daylength to time the onset of puberty.