Increased β2-Adrenergic Receptor Activity by Thyroid Hormone Possibly Leads to Differentiation and Maturation of Astrocytes in Culture

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Abstract
(1) Our earlier studies indicate a downsteam regulatory role of the β-adrenergic receptor (β-AR) system in thyroid hormone induced differentiation and maturation of astrocytes. In the present study we have investigated the contributions of the subtypes of β-AR in the above phenomenon. (2) Primary astrocyte cultures were grown under thyroid hormone deficient as well as under euthyroid conditions. [125I]Pindolol ([125I]PIN) binding studies showed a gradual increase in the specific binding to β2-AR when observed at 5, 10, 15, and 20 days under both cultural conditions. Thyroid hormone caused an increase in binding of [125I]PIN to β2-AR compared to thyroid hormone deficient controls at all ages of astrocyte culture. (3) Saturation studies using [125I]PIN in astrocyte membranes prepared from 20-day-old cultures showed a significant increase in the affinity of the receptors (K D) in the thyroid hormone treated cells without any change in receptor number (B max). (4) β2-AR mRNA levels were measured by real-time PCR during ontogenic development as well as during exposure of 10-day-old hypothyroid cultures to normal levels of thyroid hormone for 2, 6, 12, and 24 h. None of the conditions caused any significant change in the β2-adrenergic receptor mRNA levels when compared with corresponding hypothyroid controls. (5) Over expression of β2-AR cDNA in hypothyroid astrocytes caused morphological transformation in spite of the absence of thyroid hormone in the medium. (6) Taken together, results suggest thyroid hormone causes a selective increase in [125I]PIN binding to β2-AR due to increase in receptor affinity, which may lead to maturation of astrocytes.