Objective: 

Adult-onset hyperthyroidism is known to induce cognitive deficits in learning and memory. However, studies evaluating memory with hyperthyroidism are controversial and scarce in number. We aimed to evaluate short-term spatial working memory (by Y maze) and synaptic plasticity in hyperthyroid adult rats.

Methods: 

The experiments were carried out on adult rats after receiving approval from Erciyes University, Ethics Committee. Rats were randomly assigned into 2 groups: control and hyperthyroid groups. To induce hyperthyroidism, L-thyroxine (0.2 mg/kg/day ip) was given. Y-maze was used to test the alternation behavior. The in vivo electrophysiological recordings were taken from the dentate gyrus of hippocampus. A pair of pulses of equal intensity was administered at different inter-pulse intervals to analyze the paired-pulse index.

Results: 

The level of spontaneous alternation was different within the groups (p<0.01). In the hyperthyroid rats the percentage of alternation was significantly lower than the control (p<0.05). During the analysis of paired pulse response similar pattern was observed in both groups with a significantly less facilitation in hyperthyroid group than the control (p<0.05).

Conclusions: 

We have shown that elevation of thyroid hormones could impair short-term spatial working memory of adult rats. Our results also suggest that the changes in thyroid hormone metabolism can disturb synaptic plasticity in the dentate gyrus which denotes spatial short-term memory. Since almost all studies focused on the effect of low levels of thyroxine on learning and memory, the worse performance of the hyperthyroid rats on the short-term memory measures seems to be an interesting finding.