Cold stress stimulates the release of both ACTH and TSH from the pituitary. More striking changes in ACTH content have been seen in the intermediate lobe after cold stress. Therefore, this study was designed to test responses of individual anterior lobe corticotropes to cold exposure. Male rats were exposed to either 30 min of cold (+3-5 C), 30 min of a novel, temperate environment (+24 C) or were unstressed (+24 C). Pituitaries were fixed and embedded in preparation for immu- nolabeling for ACTH or TSH-β at the light (semithin sections) and electron microscopic levels. The semithin sections were used to measure areas of corticotropes and thyrotropes with Bioquant image analysis equipment. Separate groups of pituitaries were dissociated and the cells were cultured for 2 or 15 h. Then the cells were stimulated for 5-10 min with biotinylated analogs of corticotropin-releasing hormone (bio-CRH) or arginine vasopressin (bio-AVP) to detect the target cells cytochemically. A third group of dissociated cells were fixed for immunolabeling for ACTH, 16K fragment of pro-opiomelanocortin, β-endorphin, or TSH-β. Cold exposure resulted in a 1-4-fold increase in the levels of servm ACTH over that of unstressed rats. This was correlated with a 40% increase in the percentage of cells that contained 16K fragment and a 30-40% increase in percentages of cells storing ACTH or β-endorphin. Cold stress also increased the percentage of cells that bound bio-CRH or bio-AVP by 45%. Analyses of semithin sections showed that areas of corticotropes increased by 21% following cold stress. The number of rows of immunolabeled (ACTH) secretion granules also increased in corticotropes from cold-stressed rats. Exposure to a novel environment for 30 min resulted in no significant increase in serum ACTH over that of unstressed rats. There was, however, a 20% increase in percentages of cells that stored 16K fragment, β- endorphin, or target cells that bound bio-CRH. However, the corticotropes were not significantly larger. Many of the cells exhibited reduced numbers of immunolabeled secretory granules. Other corticotropes resembled those from cold-stressed rats. When TSH cells were studied, their percentages increased from 8 ± 3% to 15.8 ± 4% and their areas increased by 22% following exposure to cold. After exposure to a novel environment, percentages of cells that stored TSH-β increased to 11 ± 2%, however, no changes in areas of TSH cells were measured. These studies demonstrated that the anterior lobe corticotrope is clearly activated by exposure to both cold and novel environment. Furthermore, the thyrotropes enlarged after cold stress, and became more numerous after both types of stresses. These additional corticotropes and thyrotropes may have been derived from a previously silent population of stem, or reserve cells.
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