CHANGES IN SERUM-FREE THYROXINE, PROLACTIN, AND OLFACTORY ACTIVITY DURING INDUCED SMOLTIFICATION IN ATLANTIC SALMON (SALMO-SALAR)

Serum levels of free thyroxine (FT4) and prolactin (PRL) and olfactory activity were studied in juvenile Atlantic salmon (Salmo salar) induced to undergo premature smoltification by a regime of increasing temperature and photoperiod. The olfactory epithelium was stimulated with L-alanine or taurocholate (10(-8)-10(-4) M) while electroencephalographic responses were recorded from the rostrolateral and rostromedial surfaces of the olfactory bulb. Two peaks of olfactory activity occurred during the 8-wk test period for three combinations of recording site and odorant, lateral-alanine, lateral-taurocholate, and medial-taurocholate, but not for the medial-alanine combination. The first peak occurred at the beginning of smoltification (the ''acute phase'') coinciding with the previously described sensitive period for olfactory imprinting. Whereas this peak of olfactory activity was not associated with a rise in serum FT4, the second peak, occurring after the acute phase of smoltification, coincided with a FT4 peak. PRL remained constant throughout the acute phase of smoltification but increased 1 wk after the second peak of olfactory activity. It is concluded that olfactory activity is at least partly related to thyroid activity during induced smoltification.