T-4 DEPRESSES OLFACTORY RESPONSES TO L-ALANINE AND PLASMA T-3 AND T-3PRODUCTION IN SMOLTIFYING ATLANTIC SALMON

During natural or induced smoltification, Atlantic salmon exhibit spri ng elevations in olfactory activity and plasma L-thyroxine (T-4). To d etermine whether T-4 influences olfactory activity, we administered T- 4 to late parr in early April and measured olfactory bulb electroencep halogram and olfactory epithelium electroolfactorogram responses to L- alanine nasal stimulation, plasma T-4 and 3,5,3' -triiodo-L-thyronine (T-3) levels, and hepatic and brain monodeiodination. T-4 treatment ra ised plasma T-4 to 15 ng/ml, simulating the smelt T-4 surge, and depre ssed plasma T-3 and electroencephalogram responses, without modifying electroolfactorogram responses. Decreased plasma T-3 may be explained by inhibition of hepatic T-4 outer-ring deiodination, generating T-3, and stimulation of inner-ring deiodination, degrading T-4 and T-3. Bra in T-4 outer-ring deiodination was also strongly inhibited. In conclus ion, creation of a high plasma T-4 concentration simulating the natura l smelt peak depressed olfactory bulb responses to L-alanine due to th e high T-4 concentration or the depressed T-3 availability in brain in duced by T-4. T-4 may terminate the period of heightened olfactory res ponsiveness during smoltification.