CALCITONIN-LIKE PEPTIDE-CONTAINING GONADOTROPHS ARE JUXTAPOSED TO CUP-SHAPED LACTOTROPHS

Calcitonin (CT) is known to inhibit basal and TRH-stimulated prolactin release in cultured anterior pituitary cells in vitro and pituitary C T-like peptide (pit-CT) is synthesized and released by isolated anteri or pituitary cells, However, the specific cell type containing pit-CT has not been identified. To determine this, double label immunohistoch emistry was performed on pituitary sections from male rats using antis era for specific marker peptides of gonadotrophs, thyrotrophs, lactotr ophs, somatotrophs, corticotrophs, and folliculo-stellate cells. CT wa s only colocalized with gonadotroph-specific markers and the distribut ion of pit-CT immunoreactive (IR) cells followed the patterns of gonad otroph distribution in male and female rats. Double and triple label i mmunohistochemistry using antiserum for CT, FSH, and PRL showed an app osition of calcitonin-like peptide containing gonadotrophs to cup-shap ed lactotrophs. To examine whether pit-CT IR was altered, similarly to gonadotrophs, with known changes in PRL serum levels, studies were ex tended to ovariectomized, pregnant, and lactating rats. The area cover ed by pit-CT immunoreactivity and the tissue content of pit-CT signifi cantly differed between physiological states and the pit-CT level was inversely related to the known PRL status. Pit-CT containing gonadotro phs were in all cases apposed to cup-shaped lactotrophs. These results provide histological support for previous studies proposing that pit- CT serves as a paracrine inhibitor of PRL release.