Combination of hypothyroidism and stress abolishes early LTP in the CA1 but not dentate gyrus of hippocampus of adult rats.

Clinical experience suggests that both hypothyroidism and stress interfere with mental concentration and memory. This electrophysiological study examined the effect of hypothyroidism and stress, separately or combined, on long-term potentiation (LTP), a widely accepted cellular model for learning and memory. Measurements of early LTP (E-LTP) were carried out in the hippocampus of urethane-anesthetized adult Wistar rats. Hypothyroidism was achieved by thyroidectomy, and the 'intruder' stress was used as a model of chronic psychosocial stress. Stimulating electrodes were placed in the left CA3 region and right angular bundle and a recording electrode was placed in the right CA1 or the dentate gyrus (DG). The results showed that in the CA1 region of the hippocampus, hypothyroid or stress partially blocked E-LTP. However, when hypothyroidism and stress were combined, they eliminated E-LTP. In contrast, no significant change in E-LTP was seen in the DG of the three groups of rats. These results suggest that impaired memory because of hypothyroidism or stress may be related to impairment of the E-LTP in the Schaffer collateral synapses but not of that of the perforant path synapses.