Importance of testicular hormones in maintaining the annual pattern of LH secretion in the male horse.

Serum LH was measured by radioimmunoassay in 5 long-term (greater than or equal to 3 years) castrated male horses bled at 30-min intervals for 2 h twice a month from March to February. There was no significant effect of month on LH levels; however, mean levels were significantly lower in spring than autumn (P less than 0.05). By contrast, stallions in the same environment showed a markedly seasonal pattern of LH secretion, with LH rising at the onset of the breeding season to reach levels in late spring 3-4 times those in early winter. Despite differences in seasonal patterns of secretion, annual mean LH levels in geldings (19.6 ng/ml) and stallions (20.4 ng/ml) were similar. In mid-summer, LH was measured in 6 geldings castrated 4-6 weeks previously, 25 geldings castrated 3-25 years previously, and 8 stallions; mean +/- s.e.m. LH values were 62.2 +/- 8.0, 14.0 +/- 1.6, and 13.0 +/- 2.2 ng/ml respectively. In the long-term geldings, LH was not affected by age, indicating that, after the rapid 4-fold post-castration rise, LH falls to within the normal range of pre-castration values and stabilizes. These results show that testicular hormones are necessary to maintain the normal seasonal pattern of LH secretion in the male horse, including the increase in LH at the onset of the breeding season. It is suggested that the effects of testicular hormones on LH secretion include (1) a negative feedback at the hypothalamus/pituitary as observed in other species, and (2) a positive component in the pathway by which LH is increased at the onset of the breeding season.