Thyroid Activity Concomitant with Olfactory Learning and Heart Rate Changes in Atlantic Salmon Salmo salar, during Smoltification

Abstract

Thyroid-histological (epithelial cell height, follicular eccentricity) and thyroid-radiochemical (thyroxine, triiodothyronine) activities were examined in Atlantic salmon, Salmo salar, undergoing smoltification in relation to cardiac conditioning to L-cysteine (olfactory learning). Changes in plasma levels of thyroxine were also examined along with those of resting heart rate, during and after smoltification. In a related study, we reported greater learning ability in age-groups 3 (612–619 d since birth) and 6 (642–649 d) as well as a greater long-term olfactory memory in age-group 3. In the present study, thyroid-histological activity was correlated with olfactory learning during smoltification. Higher histological values occurred concomitantly with greater learning in age-groups 3 and 6. During smoltification, changes in thyroid-histological activity were different from those of radiochemical activity. Apparently different plasma–tissue fluxes of thyroid hormones occurred between age-groups 3 and 6. Plasma thyroxine was correlated with resting heart rate. Our results suggested that thyroid hormones play a role in olfactory learning and imprinting in anadromous salmonids.