Effects of melatonin and thyrotropin releasing hormone on mares during the nonbreeding season.

Two hormonal treatments, chosen for their effectiveness in other seasonally breeding species, were tested in mares during the nonbreeding season to determine if they could induce ovarian activity and estrus during the winter. Of 15 functionally anestrous (anovulatory) mares, five received intravaginal, polyurethane sponges containing .75 g of melatonin on December 16; fresh sponges containing melatonin were inserted weekly until February 3. These mares also received daily injections of saline. Five other mares received daily im injections of 100 micrograms of thyrotropin-releasing hormone (TRH) and control sponges during the treatment period. The remaining five mares were given control injections and control sponges throughout the experiment. Intravaginal sponges containing melatonin increased (P less than .05) concentrations of melatonin in systemic plasma for at least 7 d to levels at least 10-fold higher than those expected during the nighttime hours. The TRH significantly increased concentrations of thyroid-stimulating hormone within 60 min after injection, whereas there was no detectable increase in concentrations of prolactin after TRH. Ovarian size in all three groups of mares was increased (P less than .05) shortly after the onset of the treatment regimens. Moreover, there were surges in concentrations of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in plasma closely associated with sponge insertion and(or) injection of TRH or saline in mares of all groups. Due to the temporal correlation of gonadotropin surges and sponge insertion, we suspect that placement of intravaginal sponges may have caused the release of LH and FSH, perhaps through a neuroendocrine reflex. These surges in gonadotropins may have mediated the ovarian response. Alternatively, ovarian activity may have been stimulated by an unknown environmental factor, a possibility that was not examined in this study. Melatonin or TRH did not augment or inhibit this nonspecific response.