The role of melatonin and the pineal gland in the photoperiodic control of reproduction and smoltification in Salmonid fish
The timing of seasonal events in salmonids is thought to be controlled by endogenous circannual rhythm(s) which are entrained by the seasonally-changing daylength. This thesis investigates the role of the pineal gland in the perception of the photoperiodic zeitgeber and the subsequent transmission o...
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| Format: | Doctoral or Postdoctoral Thesis |
| Language: | English |
| Published: |
University of Stirling
1996
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| Online Access: | http://hdl.handle.net/1893/26676 http://dspace.stir.ac.uk/bitstream/1893/26676/1/porter-thesis.pdf |
| Summary: | The timing of seasonal events in salmonids is thought to be controlled by endogenous circannual rhythm(s) which are entrained by the seasonally-changing daylength. This thesis investigates the role of the pineal gland in the perception of the photoperiodic zeitgeber and the subsequent transmission of this information to the brain through neural or hormonal pathways. Melatonin biosynthesis by isolated rainbow trout pineal glands was shown to exhibit a differential response to graded photic or thermal stimuli. In vitro experiments were carried out at 10±0.50 C as this provided optimum melatonin levels for radioimmunoassay analysis together with a pineal longevity of up to 14 days. By incorporating a variety of light intensities into the light/dark cycle, the salmonid pineal gland was shown to synthesise significantly different levels of melatonin even when light levels varied by only 0.5 lux. Early work on the salmonid pineal suggested it was unresponsive to red light, having a spectral sensitivity which peaks between 500 and 550 nm, this study has revealed that the pineal is also capable of responding to wavelengths between 660 to 800 nm, at which pineal reception was previously thought to be severely limited. No endogenous rhythm of melatonin secretion was observed within the isolated rainbow' trout pineal gland. Both Atlantic salmon and Atlantic halibut pineals exhibited elevated levels- of melatonin in response to the dark phase, however, they also appeared capable of maintaining this rhythm in the absence of external stimuli. This provides the first evidence that the daily rhythm of melatonin production in these species is controlled by an endogenous circadian oscillator located within the pineal II gland. The pinealectomy technique developed during the course of this thesis successfully abolished the diel rhythm of melatonin secretion and, together with an enucleation procedure, enabled the pineal to be identified as the predominant source of the dark phase melatonin in Atlantic salmon and rainbow trout. ... |
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