Plasma and pituitary luteinizing hormone in intact and castrated tree sparrows ( Spizella arborea) during a photoinduced gonadal cycle

Abstract

Plasma and pituitary levels of immunoreactive luteinizing hormone (LH) were measured in female, male, and castrated male tree sparrows during a photoinduced gonadal cycle. Our aims were to compare patterns of LH secretion, particularly during spontaneous gonadal regression, and to determine the testes' influence on the secretory pattern of males. Seven days after females were transferred from 8- to 20-hr daily photoperiods, the concentration of plasma LH had increased 11-fold. Two weeks later, it had declined to a level only 3-fold greater than that in short-day controls, where it remained until day 56, when ovarian growth culminated. The onset of ovarian regression between days 56 and 77 was accompanied by a reduction in plasma LH to the prestimulation level (ca. 0.5 ng/ml) and by a similar reduction in pituitary LH content, which, by contrast, had increased steadily through day 28. The plasma cycle of males was quantitatively similar, but it reflected the transition from photosensitivity to photorefractoriness less incisively. Pituitary cycles of males and females were qualitatively similar. The concentration of plasma LH in castrates, compared with that in intact males, was elevated through day 56. A 4-fold increase in pituitary LH preceded a reduction in the circulating level that paralleled the onset of testicular and ovarian regression. Qualitative similarity among plasma cycles plus the eventual reduction of plasma LH in castrates to the low level characteristic of chronically photostimulated birds of both sexes argues that the LH-release mechanism becomes refractory in castrates as in males and females. We, therefore, conclude that testicular steroids are not obligatory determinants of the temporal pattern of circulating LH in photostimulated males, yet they apparently inhibit the photoresponsive hypothalamohypophysial axis. An elevated plasma level in short-day castrates suggests that a functional relationship between the hypothalamohypophysial axis and the testes exists also in nonphotostimulated males.