Impact of L-thyroxine on hippocampal activity, morphology, and behavioral performance in thyroidectomized rats

Thyroid hormones, particularly L-thyroxine, are integral to cognitive processes like learning and memory. However, electrophysiological and morphological evidence connecting thyroid hormone activity to hippocampal function remains limited. This study aims to investigate the electrophysiological, morphological, and behavioral effects of L-thyroxine treatment in thyroidectomized (TX) rats, addressing the complex interplay between thyroid hormone regulation and hippocampal function. Adult male and female Wistar rats were assigned to three groups: TX, TX + L-thyroxine (10 µg/100 g/day, i.p., administered one week post-surgery for 4 weeks), and vehicle-control. Behavioral assessments were conducted prior to electrophysiological and morphological recordings. Under urethane anesthesia, hippocampal electrical activity was recorded following entorhinal cortex stimulation, while morphological changes were assessed in the capillary networks of both the hippocampus and the paraventricular nucleus. Results revealed significant changes in hippocampal electrophysiological responses in the L-thyroxine-treated group, including enhanced synaptic plasticity, increased excitatory activity, and more frequent, synchronized neuronal firing in the CA1 and CA3 regions. Additionally, L-thyroxine treatment led to changes in capillary morphology. This integrative study bridges electrophysiological, morphological, and behavioral approaches, enhancing our understanding of thyroid hormone influence on hippocampal function. The findings suggest that the observed electrophysiological and morphological changes could contribute to the cognitive and memory-related symptoms reported by individuals with thyroid dysfunction.