An increase in the concentration of plasma (PRL) occurs in response to increased daylength in seasonally breeding animals irrespective of whether they are short or long day breeders. In contrast to the wealth of information on the photoperiodic control of PRL secretion in mammals very little information is available in birds. The objective of this thesis is to increase understanding of the photoperiodic control of PRL secretion in birds. The critical daylength (CDL) required to induce PRL secretion in bantam cockerels reared on short days (8h light/day) was between 11 and 12h light/day. Photoperiods of more than 14h of light were maximally photostimulatory. The CDL was dependent on photoperiodic history because in birds reared on 20h light/day transfer to photoperiods of 14h or less resulted in decreased PRL secretion. Transfer from 8h light/day to a single 20h long day and back to short days induced an increase in PRL secretion 20 to 22h after dawn. This increase persisted as a "carry over effect" for 4 days. Changes in ambient temperature or fasting up to 24h did not affect PRL secretion. This eliminated the possibility that photoperiodically induced changes in PRL secretion could be explained by these factors. Prolonged exposure of intact male and female turkeys or bantams to 20h light/day resulted in a depression in plasma PRL. The depression in plasma PRL appeared to be a direct consequence of the development of photorefractoriness and was not the result of a decrease in plasma gonadal steroids due to ageing or the development of reproductive photorefractoriness. This was deduced from the observation that prolonged exposure of castrated bantam cockerels to 20, 18, 16 and 14 but not 12h photoperiods also depressed plasma prolactin indicating the development of photorefractoriness. A 4h increase in photoperiod did not stimulate PRL release in castrates held for a prolonged period on a 16h photoperiod but did so in castrates held on 12h photoperiod. Exposure of photorefractory castrates to short days for 5 weeks dissipated refractoriness and restored the photoperiodic response. The age at which the PRL photoperiodic response first appeared was determined in prepubertal intact male and female bantams reared on 8h light/day. Sexual maturation occurred at 18 to 20 weeks of age. An increase in PRL secretion was observed after photostimulation in both sexes at 4, 8, 12 and 16 weeks.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:662334 |
| Date | January 1997 |
| Creators | Sreekumar, Kannoth Panicker |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/14471 |
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