Hypothyroid effects on astrocytes and microglia in the adult rat hippocampus

BACKGROUND: Thyroid hormones modulate proliferation of astrocytes and microglia depending on maturation stage and localization. Studies have demonstrated that triiodothyronine treatment or thyroidectomy during developmental stages results in morphological alterations and changes in the number of astrocytes and microglia. Little is known about the effects of hypothyroidism on astrocytes and microglia in adults. OBJECTIVE: To investigate the effects of hypothyroidism on morphology and number of astrocytes and microglia in the adult rat hippocampus. DESIGN, TIME AND SETTING: A randomized, controlled, neuroendocrinological, animal study was performed at the College of Medicine, Hallym University, South Korea between May 2008 and April 2009. MATERIALS: Methimazole, rabbit anti-glial fibrillary acidic protein (GFAP) antiserum, and rabbit anti-lba-1 antiserum were purchased from Sigma, USA. Rabbit anti-GFAP polyclonal antibody was provided by Chemicon, USA. Rabbit anti-lba-1 polyclonal antibody was purchased from Wako, Japan. Terminal deoxynucleotidyl transferase dUTP-biotin nick-end-labeling (TUNEL) kit was provided by Roche Molecular Biochemicals, Mannheim, Germany. METHODS: Hypothyroidism was induced in Wistar rats via methimazole administration (0.025%) in drinking water for 5 weeks, starting at 6 months of age. MAIN OUTCOME MEASURES: Following methimazole treatment, hippocampal neuronal death was determined using TUNEL staining. The morphology and number of GFAP and lba-1 immunoreactive cells were detected by immunohistochemistry. Hippocampal GFAP and lba-1 protein levels were detected by Western blot analysis. Serum-free triiodothyronine and thyroxine levels were quantified. RESULTS: TUNEL-positive neurons were not observed in the hippocampus of euthyroid and hypothyroid rats. Compared with the euthyroid rats, the number of GFAP immunoreactive astrocytes was decreased, and serum triiodothyronine and thyroxine levels were significantly decreased. In contrast, the number of lba-1 immunoreactive microglia was significantly increased in the hypothyroid rats (P < 0.05). In addition, GFAP immunoreactive astrocytes were morphologically at a resting state, and lba-1 immunoreactive microglia were morphologically hypertrophic. GFAP and lBa-1 protein changes in the hippocampus of euthyroid and hypothyroid rats were in accordance with immunohistochemical data. CONCLUSION: Although methimazole-induced hypothyroidism did not induce neuronal injury in the adult rat hippocampus, it did result in decreased astrocyte numbers and increased microglial hypertrophy.