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) induc ed to undergo premature smoltification by a regime of increasing tempe rature and photoperiod. The olfactory epithelium was stimulated with L -alanine or taurocholate (10(-8)-10(-4) M) while electroencephalograph ic responses were recorded from the rostrolateral and rostromedial sur faces of the olfactory bulb. Two peaks of olfactory activity occurred during the 8-wk test period for three combinations of recording site a nd odorant, lateral-alanine, lateral-taurocholate, and medial-taurocho late, but not for the medial-alanine combination. The first peak occur red at the beginning of smoltification (the ''acute phase'') coincidin g with the previously described sensitive period for olfactory imprint ing. Whereas this peak of olfactory activity was not associated with a rise in serum FT4, the second peak, occurring after the acute phase o f smoltification, coincided with a FT4 peak. PRL remained constant thr oughout the acute phase of smoltification but increased 1 wk after the second peak of olfactory activity. It is concluded that olfactory act ivity is at least partly related to thyroid activity during induced sm oltification.