Differential radiosensitivity of hypothalamo-pituitary function in the young adult rat.

Cranial irradiation in children and adults often results in irreversible hypopituitarism. The earliest and most common endocrine abnormality is GH deficiency, often followed by other pituitary hormone deficits. We investigated whether a similar pattern of progressive hypopituitarism could be reproduced in an animal model. Different doses of cranial irradiation were delivered to the hypothalamo-pituitary region of normal adult male rats, and the effects on their subsequent growth, pituitary weight and hormone contents were studied. Animals received cranial irradiation with 300 kV X-rays at doses of 0, 20, 22 or 24 Gy (n=15 per group) and five animals from each group were killed at 8, 14 or 20 weeks after irradiation. Their anterior pituitary glands were weighed and assayed for GH, LH, TSH, ACTH and prolactin (PRL) content. All three doses of irradiation reduced body weight compared with that in non-irradiated controls and compromised growth between 8 and 20 weeks. Pituitary weight increased between 8 and 20 weeks in control rats, whereas it decreased significantly in the irradiated animals. Irradiation induced time- and dose-dependent changes in pituitary hormone contents. GH and PRL were most sensitive and decreased by more than 90% after irradiation; TSH contents were unaffected 8 weeks after the lowest dose of irradiation, but were reduced at 14 and 20 weeks. LH and ACTH were the slowest to be affected, and only at the greater doses of radiation. Thus progressive multiple pituitary endocrine deficits can be induced differentially in rats by increasing doses of cranial irradiation. This model should prove useful for defining the sites and mechanisms by which cranial irradiation induces neuroendocrine dysfunction.