Active Immunization of Female Rats with Luteinizing Hormone Releasing Hormone (LHRH)

Abstract

Cycling rats were immunized against LHRH. In most of these rats, antibody to LHRH became detectable 20 days after the first immunizing injection and reached a maximum level about 40 days later. The elevation of antibody titers, accompanied by the lengthening of the estrous cycle, resulted in continuous anestrus approximately 50 days after the first injection. Peripheral serum LH declined to nondetectable levels but prolactin was not significantly altered throughout the experimental period. The presence of circulating antibody to LHRH was confirmed by the blockade of ovulation in cycling hamsters after i.v. injection of serum from the immunized rats. About 60 days after the establishment of anestrus, the ovaries of the actively immunized rats weighed one-fifth those of the normal cyding rats, but were significantly heavier than the ovaries of rats hypophysectomized for 2 months. The ovaries of immunized anestrous rats consisted mostly of small- to medium-sized follicles; occasionally follicles with small antra were encountered. In immunized rats LHRH content of the hypothalami remained at the level of normal cycling rats; however, LH concentration of the pituitary was lower than that in normal rats. In the immunized rats an i.v. injection of a large dose of LHRH caused a small rise in LH but did not induce ovulation. Ovulation was induced by injection of PMS followed by LH. Cervical stimulation after ovulation evoked an elevation of peripheral progesterone comparable to that in normal pseudopregnant rats until Day 5. These results suggest that rats actively immunized with LHRH provide a model which has low levels of LH and relatively unimpaired levels of prolactin.