Circadian Rhythm in Mammary Cytoplasmic Estrogen Receptor Content of Balb/C Female Mice with and Without Pituitary Isografts.

Cytoplasmic estrogen receptors were determined by the dextran-coated charcoal method in inguinal breast tissue of three groups of Balb/C female mice 6-8 weeks following subcutaneous implantation into the intact animals of three pituitary glands and three pieces of skeletal muscle (group I), three pituitary glands and three segments of hypothalamic tissue (group II), or three pieces of skeletal muscle (group III) obtained from animals of the same inbred strain as control. A circadian rhythm in estrogen receptor content was statistically quantified by cosinor analysis in the muscle implanted control and the pituitary and hypothalamic implant groups. In the pituitary and muscle implant group the circadian rhythm is of borderline significance with a P-value between 0.05 and 0.10. The timing (acrophase) and extent of change (amplitude) are similar in all three treatment groups. The average receptor content (MESOR) in the two pituitary-implanted groups, which in previous studies were shown to have an increased breast cancer incidence is about twice that of the control group. The reduction in the pituitary induced breast cancer rate by hypothalamic tissue addition to a cancer incidence between the animals with pituitary and muscle isograft and the mice carrying no pituitary at all has also been shown previously in this strain of mice and is not reflected in receptor content.