OBJECTIVE During the critical period of brain development, insufficiency of thyroid hormone results in severe mental retardation and learning deficit. This study was designed to investigate the effects of hypothyroidism on apoptosis and the expression of Bcl-2 and Bax gene in the developing rat hippocampus neurons and to explore the mechanism of brain development regulated by thyroid hormone. METHOD Hypothyroidism was induced by administration of propylthiouracil (PTU, 50 mg/d) solution to the dams from gestational day 15 by gavage. Pups from both hypothyroid and control groups were harvested at postnatal day 1 (P1), P5, P10 and P15, respectively. Blood samples were collected at the time of death for the determination of thyroid hormone. Serum free tri-iodothyronine (FT(3)) and free thyroxine (FT(4)) were measured by using chemoluminescence. Hippocampus collected from the control and hypothyroid pups were examined under light and transmissional electron microscopy. Measurement of DNA fragmentation was carried out by agarose gel electrophoresis. The expression of Bcl-2 and Bax protein in the developing rat hippocampus neurons was performed by Western blotting. RESULTS Significantly lower circulating FT(4) and FT(3) levels confirmed the hypothyroid status of the experimental pups. The shrunken and contracted degenerations increased in hippocampus neurons of hypothyroid pups under light microscopy. Enhanced apoptotic cells were found in hippocampus neurons of hypothyroid pups under transmission electron microscopy, especially at P10 and P15. Extensive DNA fragmentation was seen throughout development in hippocampus of hypothyroid pups, but not in the euthyroid controls except for basal level at P10. The expression of Bcl-2 in the hippocampus neurons of hypothyroid pups was significantly lower than that of euthyroid controls at all stages of development (P1: 1.95 +/- 0.27 vs. 2.59 +/- 0.19, P < 0.05, P5: 1.86 +/- 0.24 vs. 2.47 +/- 0.17, P < 0.05, P10: 1.29 +/- 0.22 vs. 1.86 +/- 0.28, P < 0.05 and P15: 1.21 +/- 0.27 vs. 2.18 +/- 0.17, P < 0.01, respectively). The relative amount of expression varied significantly with age in the control pups. The level of Bcl-2 was high in hippocampus neurons of euthyroid at P1, P5, and decreased significantly at P10, and showed a trend of recovery at P15. Similar age-related variation in the expression of Bcl-2 gene was observed in the hypothyroid group at P1, P5 and P10, but the level was maintained low at P15. The expression of Bax in the hippocampus neurons of hypothyroid pups was significantly higher than that of control pups at all stages of development (P1: 1.69 +/- 0.14 vs. 1.24 +/- 0.23, P < 0.05, P5: 1.78 +/- 0.16 vs. 1.29 +/- 0.17, P < 0.05, P10: 1.92 +/- 0.18 vs. 1.45 +/- 0.14, P < 0.05 and P15: 1.86 +/- 0.14 vs. 1.51 +/- 0.12, P < 0.05, respectively). The ratio of Bcl-2/Bax in hippocampus neurons of hypothyroid pups was lower than that of age-matched controls (P1: 1.16 +/- 0.17 vs. 2.12 +/- 0.35, P < 0.05, P5: 1.05 +/- 0.16 vs. 1.94 +/- 0.36, P < 0.05, P10: 0.68 +/- 0.17 vs. 1.29 +/- 0.16, P < 0.05 and P15: 0.67 +/- 0.19 vs. 1.45 +/- 0.22, P < 0.01, respectively). CONCLUSION Thyroid hormone significantly prevents apoptosis of hippocampus neurons. Congenital hypothyroidism increases not only the extent but also the duration of apoptosis by down-regulation of the anti-apoptotic gene Bcl-2 and maintaining a high level of the pro-apoptotic gene Bax.