Influence of oral melatonin on natural and gonadotropin-induced ovarian function in the domestic cat.

Ovarian hyperstimulation after exogenous gonadotropin stimulation is believed to be a cause of poor success after artificial insemination (AI) in felids. The objectives of this study were to assess the effect of oral melatonin on endogenous ovarian activity in the domestic cat and subsequent eCG/hCG-induced ovarian activity. Serum melatonin concentrations peaked approximately 1h after a single oral dose of 30 mg melatonin and remained elevated above endogenous day-time concentrations for >8h. The calculated circulating half-life (mean +/- S.E.M) of oral melatonin was 45.4+/-3.5 min, and the elimination rate constant (k(10)) was 55.2+/-4.2 min(-1). Oral melatonin (30 mg per day) administered 3h before lights-off effectively and reversibly suppressed estrous elevations in fecal estrogens after 25 days of treatment. There was a progressive decrease in baseline estrogen concentrations from inter-estrous concentrations after 25 days of treatment to below inter-estrous concentrations after 35 days of treatment. Oral melatonin treatment (30 mg per day for 30 days) prior to eCG/hCG administration only marginally reduced ancillary follicle development and had no significant effect on the quantity or quality of embryos produced by AI. Thus, oral melatonin effectively inhibited endogenous ovarian activity and had no adverse impact on embryo quality after AI in the domestic cat; however, this treatment was only marginally effective in minimizing eCG/hCG-induced ovarian hyperstimulation.