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All these findings can help us to elucidate the mechanisms for the fish to detect changing osmotic conditions and transform signals to osmoregulatory responses.In the first part of the study, PRL and PRL-releasing peptide (PrRP) cDNAs have been isolated from euryhaline silver sea bream. The PRL cDNA consists of 1360 bp encoding 212 amino acids whereas the PrRP cDNA contains 631 bp encoding prepro-PrRP with 122 amino acids. PRL mRNA was uniquely expressed in sea bream pituitary but PrRP mRNA was expressed in a variety of tissues. Expression levels of both PRL and PrRP mRNA have been examined in sea bream adapted to different salinities (0, 6, 12, 33 and 50 ppt). In pituitary, both PRL and PrRP mRNA were synchronized in their expression, being significantly higher in fish adapted to low salinities (0 and 6 ppt), but the expression profile of hypothalamic PrRP in different salinities was different. These data suggested that PrRP may possibly act as a local modulator in pituitary rather than a hypothalamic factor for regulating pituitary PRL expression in silver sea bream.In the second part of the study, silver sea bream abruptly transferred from 33 to 6 ppt exhibited a remarkable pituitary PRL secretion following closely with the temporal changes in serum osmolality and ion levels. In order to investigate the direct effect of extracellular osmolality to pituitary PRL secretion, sea bream pituitary cells were dispersed and exposed to a medium with reduced ion levels and osmolality. PRL released from pituitary cells was found to be significantly elevated. In hyposmotic exposed anterior pituitary cells, cell volume exhibited a 20% increase when exposed to a medium with a 20% decrease in osmolality. These enlarged pituitary cells did not shrink until the surrounding hyposmotic medium was replaced, a phenomenon suggesting an osmosensing ability of silver sea bream PRL cells for PRL secretion in response to a change in extracellular osmotic pressure.In the third part, olfactory rosette in the nasal cavity was surgically removed from silver sea bream adapted to 6 ppt and 33 ppt and mRNA expression of PRL and PrRP in silver sea bream were measured. The elevated pituitary PRL and PrRP mRNA expression levels as seen in 6 ppt-adapted fish were abolished by this olfactory lamellectomy. On the other hand, hypothalamic PrRP mRNA expression in 6 ppt-adapted fish did not change but those in 33 ppt-adapted fish increase significantly after olfactory lamellectomy. These data suggest a possible osmosensing role of the olfactory system for regulation of PRL expression during hypo-osmotic acclimation of the fish. Besides, calcium-sensing receptor (CaSR) was cloned and its mRNA expression in olfactory system, as shown in other fish species previously, was investigated. However, no CaSR expression could be detected in olfactory rosette and nerve but its expression was demonstrated in osmoregulatory tissues and brain. There was no significant difference in CaSR mRNA expression in pituitary, kidney and anterior intestine of fish adapted to different salinities. These studies could not provide conclusive evidence to correlate CaSR with osmosensing in silver sea bream.The present study used silver sea bream (Sparus sarba ) as a euryhaline fish model to investigate the regulation of prolactin (PRL) expression and secretion in fish adapted to different salinities.Kwong, Ka Yee.Adviser: Norman Y. S. Woo.Source: Dissertation Abstracts International, Volume: 70-06, Section: B, page: 3248.Thesis (Ph.D.)--Chinese University of Hong Kong, 2008.Includes bibliographical references (leaves 154-184).Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web.Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web.Abstracts in English and Chinese.School code: 1307