Maternal hypothyroidism decreases progesterone receptor expression in the cortical subplate of foetal rat brain

Steroid hormones exert profound effects on the development of brain areas controlling complex cognitive function in adulthood. One class, progestins, may contribute by acting on the progestin receptor (PR), which is transiently expressed in a critical layer of developing cortex: the subplate. PR expression in the subplate coincides with the establishment of ongoing cortical connectivity and may play an important organisational role. Identification of the factor(s) that regulate the precise timing of PR expression within subplate may help elucidate the function of PR. Thyroid hormone may interact with hormone response elements within the PR gene. The present study examined the effects of maternal hypothyroidism on levels of PR immunoreactivity (PR-IR) within the foetal subplate. Pregnant rats were made hypothyroid by the administration of methimazole and potassium perchlorate in drinking water. Maternal hypothyroidism significantly decreased PR-IR within the foetal subplate. Using the incorporation of 5-bromo-2'-deoxyuridine (BrDU) during subplate cell neurogenesis (embryonic day 13.5) to determine subplate cell survival in hypothyroid animals, we found that decreases in PR-IR cannot be attributed to significant subplate cell loss but are more likely the result of altered PR expression. Gestational thyroxine replacement to hypothyroid dams prevented the decrease in PR-IR within the subplate. These results identify thyroid hormone as a potential factor in the regulation of PR expression in the developing brain. These results are consistent with the idea that endocrine cross-talk between progesterone and thyroid hormone may be one mechanism by which maternal hypothyroidism alters normal cortical development.