T4 depresses olfactory responses to L-alanine and plasma T3 and T3 production in smoltifying Atlantic salmon.

Abstract

During natural or induced smoltification, Atlantic salmon exhibit spring elevations in olfactory activity and plasma L-thyroxine (T4). To determine whether T4 influences olfactory activity, we administered T4 to late parr in early April and measured olfactory bulb electroencephalogram and olfactory epithelium electroolfactorogram responses to L-alanine nasal stimulation, plasma T4 and 3,5,3'-triiodo-L-thyronine (T3) levels, and hepatic and brain monodeiodination. T4 treatment raised plasma T4 to 15 ng/ml, simulating the smolt T4 surge, and depressed plasma T3 and electroencephalogram responses, without modifying electroolfactorogram responses. Decreased plasma T3 may be explained by inhibition of hepatic T4 outer-ring deiodination, generating T3, and stimulation of inner-ring deiodination, degrading T4 and T3. Brain T4 outer-ring deiodination was also strongly inhibited. In conclusion, creation of a high plasma T4 concentration simulating the natural smolt peak depressed olfactory bulb responses to L-alanine due to the high T4 concentration or the depressed T3 availability in brain induced by T4. T4 may terminate the period of heightened olfactory responsiveness during smoltification.