Calretinin in the rat pituitary: colocalization with thyroid-stimulating hormone

The purpose of this study was to examine the distribution of calretinin immunoreactivity (CR) in the male rat pituitary gland by immunofluorescence microscopy. CR was found in cells of the anterior pituitary and in granules in the posterior pituitary. In the intermediate lobe, nerve fibers in close proximity to the melanotropes were CR-immunoreactive (CR-ir). Fine CR-ir varicose fibers were also observed in the anterior and posterior pituitary. Colocalization studies revealed that the majority of the CR-containing cells of the anterior pituitary also contained thyroid-stimulating hormone (TSH). These CR/TSH cells represented about 32% of the thyrotrope population. Following thyroidectomy, a massive increase in both the number of CR-ir cells and in the expression of CR mRNA was observed in the anterior pituitary. Thyroxine treatment, however, resulted in a reduction in the number and size of the CR-ir cells in the same lobe. In the intermediate lobe, CR-ir was colocalized with tyrosine hydroxylase (TH) immunoreactive dopaminergic fibers. These intermediate lobe fibers disappeared following pituitary stalk section, as did the CR/TH fibers and the CR-ir granular material in the posterior pituitary. The findings in the anterior pituitary suggest that consideration be given to the idea that CR might function in the synthesis and/or release mechanism of TSH in thyrotropes and that its expression is modulated by the hypothalamo-pituitary-thyroid axis.