AJP - Regulatory, Integrative and Comparative Physiology, Vol 269, Issue 6 1434-R1440, Copyright © 1995 by American Physiological Society
T4 depresses olfactory responses to L-alanine and plasma T3 and T3 production in smoltifying Atlantic salmon
P. P. Morin, T. J. Hara and J. G. Eales
Department of Zoology, University of Manitoba, Winnipeg, Canada.
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.
