Effects of active immunization against gonadotropin releasing hormone on serum luteinizing hormone,progesterone levels and estrous cycles in the guinea pig

Mature female guinea pigs that had been observed to undergo three consecutive periods of estrus at approximately 16-day intervals were immunized with either 100 μg gonadotropin releasing hormone (GnRH) conjugated to 100 μg bovine serum albumin (BSA) or 100 μg BSA alone during diestrus (day 5–10) of the fourth cycle. Booster immunizations were administered 32 days after the first injection. Animals were bled by cardiac puncture at the time of first injection and at 16, 32, 48 and 64 days. Animals were necropsied at 64 days after first treatment.Daily observation indicated that vaginal manifestation of estrus was not apparent after a period equal to one estrous cycle in seven of ten GnRH immunized guinea pigs and after two cycles in the remaining three GnRH immunized guinea pigs. Estrous cycles persisted in BSA treated females throughout the experiment.Serum luteinizing hormone (LH) declined significantly by 32 days after the first immunization against GnRH and remained lower than both pretreatment values and levels in control animals at the same bleeding times throughout the experiment. Serum progesterone levels were significantly lower in the GnRH immunized group than in the control group at 48 and 64 days.At necropsy the weight of the ovaries of GnRH immunized guinea pigs was significantly lower than that of controls. Corpora lutea and antral follicles were present in both GnRH treated and control females. The presence of serum progesterone levels and of antral follicles in the GnRH immunized females suggests that a low level of gonadotropic support may have persisted to 64 days after initiation of treatment.Results indicate that immunization against GnRH can reduce LH and progesterone levels and induce cessation of estrous cycles in the guinea pig.