Evidence for cAMP-independent thyrotropin effects on astroglial cells.

Thyroid hormones are essential for normal brain development and function. Brain astroglial cells express type II iodothyronine 5'-deiodinase which converts thyroxine into 3,5,3'-triiodothyronine. This type II deiodinase is regulated through various signalling pathways, allowing probably for the local adaptation of the level of 3,5,3'-triiodothyronine. Our results demonstrated that thyrotropin was able to induce type II deiodinase activity in astrocytes. A thyrotropin receptor was demonstrated. It was not coupled, as in thyroid, to adenylyl cyclase and phospholipase C, but it stimulated cytosolic phospholipase A2. The stimulation by thyrotropin of both thyroxine synthesis in thyroid and its local activation in astrocytes, could protect the brain from variations in the level of 3,5,3'-triiodothyronine.