The relationship among circulating levels of follicle-stimulating hormone (FSH), luteinizing hormone (LH), immunoreactive (ir) inhibin, progesterone, testosterone and estradiol-17β were investigated in fillies from birth to 6 months of age. Six hundred ninety-four Thoroughbred female foals were used for evaluating the changes in these hormones. Blood samples were collected by jugular venipuncture for hormonal assays. The results show that after birth, progesterone, testosterone and estradiol-17β levels rapidly drop within 48 hr, and remain at this lower level until six months of age. On the other hand, plasma concentrations of ir-inhibin begin to decrease at 3 days after birth, falling by more than half in the first 20 days after birth, and then continue to slowly decline until six months of age. Circulating levels of FSH show a small increase within 48 hr after birth, then decline to the lowest levels at 30 days after birth, followed by a rise to more or less stable levels at six months of age, whereas LH concentrations rapidly decrease after birth, dropping to a lower level at 10 days after birth, then remain at the lower value until six months of age. These results demonstrate that, after birth, in a filly, there is a notable decline in steroid hormones which then remain at these relatively stable lower levels until six months of age. The steady increase in basal FSH negatively correlates with the decline in circulating inhibin. This slow increase in the basal levels of FSH may stimulate follicular development during the early stages of puberty in a filly.

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To elucidate the function of inhibin/activin subunits in developmental fetal adrenal glands, we investigated immunolocalization of inhibin α and inhibin/activin (β_A and β_B) subunits protein in the equine fetal adrenal glands during mid to late gestation. Fetal adrenal glands of three equines were obtained at 217, 225 and 235 days of gestation. The sections of fetal adrenal glands were immunostained by the avidin-biotin-peroxidase complex method (ABC) using polyclonal antisera raised against porcine inhibin α, inhibin/activin β_A and inhibin/activin β_B. Inhibin/activin β_B was identified in cortex cells of fetal adrenal glands throughout 217-235 days of gestation, but inhibin α and inhibin/activin β_A were not observed in fetal adrenal glands in this period. These results suggest that equine fetal adrenal glands have the ability to synthesize activin B but not inhibins and activin A. Also, inhibin/activin β_B may play a regulatory role in developing equine fetal adrenal glands.

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The purpose of this investigation was to evaluate the clinical usefulness of the total radiographic bone aluminum equivalency (RBAE) method for the measurement of bone mineral content (BMC) in the third metacarpal bone (McIII) of the young Thoroughbred by examining: a) changes in BMC and bone metabolism marker levels in young Thoroughbreds being trained in the growth period; b) correlation of BMC and bone metabolism marker levels; and c) relationships of BMC and bone metabolism marker levels with the state of occurrence of bone disorders. The total RBAE method by the ortho system was chosen for this study. Ninety-one 2-year-old Thoroughbreds that showed no abnormalities on physical examination before the study were evaluated for BMC, and 39 were selected at random for evaluation of the bone metabolism marker levels. BMC of the normal horses showed no change between Days 0 and 90 of the study, but significantly increased on Day 180 (p<0.01). BMC showed no correlations with bone metabolism markers. In addition, there was a high incidence of bone disorders in Days 0-90 of the study when BMC did not change though training increased. These results suggest that the measurement of BMC by the total RBAE method in young Thoroughbreds is useful for providing information about skeletal development, and also for obtaining information on the predisposing period for bone disorders.

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The changes in testicular and pituitary functions in response to human chorionic gonadotropin (hCG) in the breeding and non-breeding season were investigated and compared in the Thoroughbred stallion. Five mature Thoroughbred stallions ranging in ages from 7 to 21 years were injected in May (breeding season) and October (non-breeding season). All animals received an intramuscular injection of 5,000 IU hCG in the experiments. Peripheral blood samples were collected in heparinized tubes from the jugular vein for hormonal assays just before the injection (Day 0) and at a daily interval for five days following the injections (Days 1, 2, 3, 4 and 5). Basal levels of immunoreactive (ir) inhibin, testosterone, estradiol-17β, FSH and LH were higher in the breeding season than in the non-breeding season. There were significant differences in estradiol-17β, FSH and LH between the two seasons. Ir-inhibin exhibited significant increases on Day 2 after the hCG treatment in the non-breeding season, though there was no change in the breeding season. Plasma levels of testosterone showed remarkable increases after the hCG injection in both seasons. The peak levels of plasma testosterone were observed on Days 2 and 3 in the breeding and non-breeding seasons. Plasma estradiol-17β was significantly higher than the basal level on Day 3 in the non-breeding season, whereas there was no change in the breeding season. Plasma levels of FSH declined from Day 1 to Day 3, then recovered to the basal level (Day 0) on Day 4 after hCG treatment in the breeding season, whereas there was no change in the non-breeding season. Circulating LH showed a significant decrease on Day 3 compared to the based level in the breeding season, but no significant change in the non-breeding season after a treatment of hCG. A significant negative correlation was observed between testosterone and FSH in the breeding season. In conclusion, hCG treatment stimulates secretion of testicular hormones and these testicular hormones temporarily suppress secretion of gonadotropins from the pituitary gland. These results also suggested that Leydig cells of thoroughbred stallion testes have LH receptors in the non-breeding season, the same as in the breeding season.

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A new equine influenza vaccine, which contains A/equine/La Plata/93 (American lineage virus) and A/equine/Avesta/93 (European lineage virus), became commercially available in 2004. To investigate the antibody responses of racehorses inoculated with the new vaccine, hemagglutination inhibition (HI) antibody titer levels against A/equine/La Plata/93 and A/equine/Avesta/93 were measured using sera samples collected from racehorses at the Ritto Training Center from 2004 to 2005. Three-year-old and four-year-old horses inoculated with the new vaccine once or twice as booster immunization possessed comparatively high level HI antibody titers against A/equine/La Plata/93, however, they possessed low level HI antibody titers against A/equine/Avesta/93. Two-year-old horses inoculated with the new vaccine from the beginning of their vaccination history possessed comparatively high levels of HI antibody titers against both strains.

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