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Distribution of Corticotrophin‐Releasing Factor Type 1 Receptor‐Like Immunoreactivity in the Rat Pituitary

Corticotrophin‐releasing factor (CRF) regulates the hypothalamic‐pituitary‐adrenal axis response to stress through its type 1 receptor (CRF1) in the corticotrophs of the anterior pituitary. Although CRF1 mRNA expression has been confirmed in the rat pituitary, the distribution pattern of CRF1 protein in the pituitary has not been reported. Therefore, we generated an antiserum against the amino acid fragment corresponding to the 177–188 sequence of the first extracellular loop of the rat CRF1. Using the antiserum, CRF1‐like immunoreactivity (CRF1‐LI) was detected in the anterior lobe cells of the rat pituitary where some of them expressed intense signals. CRF1‐LI also appeared in the intermediate lobe cells and on the fibre‐like elements of the posterior lobe of the pituitary. Dual immunofluorescence labelling showed that corticotrophs exhibited the highest percentage of CRF1 (male: 27.1 ± 3.0%, female: 18.0 ± 3.0%), followed by lactotrophs (male: 6.7 ± 3.0%, female: 12.1 ± 1.3%), gonadotrophs (male: 2.6 ± 1.0%, female: 7.5 ± 0.5%), thyrotrophs (male: 2.9 ± 0.1%, female: 5.3 ± 1.2%) and somatotrophs (male: 1.1 ± 0.3%, female: 1.2 ± 0.5%). The percentage of CRF1‐LI‐positive cells that were corticotrophs was significantly higher in male rats than in female rats, whereas CRF1‐LI‐positive lactotrophs and gonadotrophs were significantly higher in female rats than in male rats. Almost all of the melanotrophs were positive for CRF1 in the intermediate lobe (98.9 ± 0.2%). CRF1‐LI and the percentage of CRF1‐LI in corticotrophs were decreased in the anterior pituitary, and the distribution patterns were altered from a diffuse to punctate one by adrenalectomy; the changes were restored by treatment with dexamethasone (100 μg/kg bw). These results suggest that CRF1 is involved in the modulation of the functions of the pituitary; moreover, protein expression and the distribution patterns of CRF1 are regulated by glucocorticoids in the rat anterior pituitary.