Temporal dynamics of brain and hormone changes during parr-smolt transformation in salmon

Abstract: This thesis describes a series of studies designed to elucidate brain circuitry changes during parr-smolt transformation (smoltification) in salmon. The studies revealed sequential orderly chemical and structural changes of the brain. The first study showed sequential changes in concentration of mu-and kappa-opiate receptors in select cell groups of the brain during various stages of smoltification. Another study, using GAP-43 immunoreactivity, localized brain regions undergoing brain circuitry changes and showed that the timing of appearance of the transient immunoreactive cells and fibers preceded the principal plasma thyroid hormone surge. Moreover, the appearance of transient serotonin-immunoreactive neurons in the preoptic and habenular region of the brain coincide with the transient GAP-43 immunoreactivity. Those results led to investigations on the role of hormones in the observed neural plasticity. These studies led to the development of the method to measure free plasma thyroxine that gives a new perspective on the significance of the observed surges of thyroid hormones in plasma. The research also revealed relationships of thyroid hormones to growth hormone and melatonin. Other studies showed the relationship between daily and seasonal variation in plasma levels of thyroid hormones and growth hormones. A final study showed the effectiveness of propylthiouracil (PTU) in inducing hypothyroidism, reducing both thyroid hormone and growth hormone levels at the time of smoltification in salmon. Overall the studies of this thesis suggest that smoltification in salmon may be an important model for research on neural plasticity. The studies also provided new insights into the complex interaction of hormones during smoltification and their role in modification of the brain. The studies also showed that the brain likely plays important roles in the parr-smolt transformation.