Galanin gene expression in radiothyroidectomy-induced thyrotroph adenomas

Galanin gene expression is markedly increased in the anterior pituitary glands of estrogen-treated rats (lactotroph hyperplasia) as well as human growth hormone-releasing hormone transgenic mice (somatotroph hyperplasia). The objective of this study was to examine galanin in a mouse model of thyrotroph adenoma formation. Male mice were radiothyroidectomized by use of iodine-131 (131I), and galanin peptide levels were assessed in the hypothalamic-pituitary axis. Immunoreactive galanin concentrations in the anterior pituitaries of 131I-treated mice were decreased 80% at 3, 6, 9, and 12 mo after radiothyroidectomy. Galanin peptide levels in the hypothalamus were decreased 20-25% at these times. Treatment with either estradiol or 3,3',5-triiodo-L-thyronine increased galanin peptide concentrations in the anterior pituitaries of 131I-treated mice, but neither treatment restored galanin concentrations. Galanin mRNA levels were decreased > 80% 1 yr after radiothyroidectomy. We conclude that, unlike animal models of lactotroph and somatotroph hyperplasia, galanin gene expression is suppressed throughout the development of thyrotroph adenomas, suggesting that galanin does not have a stimulatory role in the proliferation of thyrotrophs. Moreover, these data show that thyroid hormones are important positive regulators of galanin gene expression in the mouse and that estrogen may stimulate galanin gene expression in the absence of thyroid hormones.