Thyroidectomy and PTU-Induced Hypothyroidism: Effect of L-Thyroxine on Suppression of Spatial and Non-Spatial Memory Related Signaling Molecules

Abstract

Background: The calcium/calmodulin protein kinase II (CaMKII) signaling cascade is crucial for hippocampus-dependent learning and memory. Hypothyroidism impairs hippocampus- dependent learning and memory in adult rats, which can be prevented by simple replacement therapy with L-thyroxine (thyroxine, T4) treatment. In this study, we compared animal models of hypothyroidism induced by thyroidectomy and treatment with propylthiouracil (PTU) in terms of synaptic plasticity and the effect on underlying molecular mechanisms of spatial and non-spatial types of memory. Methods: Hypothyroidism was induced using thyroidectomy or treatment with propylthiouracil (PTU). L-thyroxin was used as replacement therapy. Synaptic plasticity was evaluated using in vivo electrophysiological recording. Training in the radial arm water maze (RAWM), where rats had to locate a hidden platform, generated spatial and non-spatial learning and memory. Western blotting measured signaling molecules in the hippocampal area CA1 area. Results: Our findings show that thyroidectomy and PTU models are equally effective, as indicated by the identical plasma levels of thyroid stimulating hormone (TSH) and T4. The two models produced an identical degree of inhibition of synaptic plasticity as indicated by depression of long-term potentiation (LTP). For non-spatial memory, rats were trained to swim to a visible platform in an open swim field. Analysis of hippocampal area CA1 revealed that training, on both mazes, of control and thyroxine-treated hypothyroid rats, produced significant increases in the P-calcium calmodulin kinase II (P-CaMKII), protein kinase-C (PKC), calcineurin and calmodulin protein levels, but the training failed to induce such increases in untreated thyroidectomized rats. Conclusion: Thyroxine therapy prevented the deleterious effects of hypothyroidism at the molecular level.