The aim of this work was to study the interaction of melatonin and prolactin in the control of seasonal breeding and coat development in goats. British Saanen goats (n=5) were maintained for 8 weeks in long days (20L: 40) followed by 8 weeks exogenous melatonin. Group treatment was staggered over the year in an attempt to dissociate breeding season advance from advanced coat growth. To assess the effect of prolactin suppression, the 20L:4D treatment from January was repeated in consecutive years but followed in March by melatonin (Year 1) or bromocriptine (Year 2). To study further the interaction of melatonin and prolactin on coat development and reproductive advance, goats (n=5) were maintained as follows: 1) natural photoperiod and temperature, Jul-Juni 2) 16L: 8D, natural temperature, Jul-Deci 3) 16L:8D, 17°C, Jul-Decj 4) 8L:16D, 8°C, Dec-Jun. To study melatonin suppression of prolactin, the secretagogues arginine vasopressin, serotonin and thyrotropin releasing hormone were given to goats (n=6) maintained in sequence under 8L:16D, 20L:4D and simulated short days (20L:4D+melatonin). Depending on long-day Imelatonin treatment onset, the breeding season was advanced from November to May, August or September. With 20L:4D followed by melatonin or bromocriptine the spring prolactin rise was delayed. A corresponding delay in winter coat moulting was observed only under melatonin. However, a winter secondary to primary (SIP) follicle ratio was retained with both treatments. Prolactin may determine coat structure, whilst melatonin influences the duration of fibre retention. Under conditions of summer or winter solstice hold, the seasonal prolactin variation was inhibited and initial SIP follicle ratios were retained. A tendency for a spontaneous change in prolactin secretion suggested existence of an endogenous rhythm. Maintained seasonal temperature did not influence melatonin secretion. Dusk and dawn prolactin peaks corresponded with the rise and fall in nocturnal melatonin. Basal prolactin concentrations and peak responses to the secretagogues altered with photoperiod, being low under 8L:16D and increased under 20L:4D. Exogenous melatonin under 20L:4D did not suppress prolactin to short-day concentrations. Invol vement of another factor mediating photoperiodic changes in prolactin secretion is suggested.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:334717 |
| Date | January 1993 |
| Creators | Gebbie, Fiona E. |
| Publisher | University of Surrey |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://epubs.surrey.ac.uk/2747/ |
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