Suppression of puberty in rats by exercise: effects on hormone levels and reversal with GnRH infusion.

In an effort to better understand the effects of prolonged exercise on the hypothalamic-pituitary-ovarian axis, we made a variety of comparisons between young female rats in two treatment groups: 1) prolonged exercise, in which growth and reproductive development were arrested at a peripubertal stage by requiring rats to run for long periods of time in order to obtain food; and 2) voluntary exercise, in which same-aged control rats were fed ad libitum and given free access to a running wheel. The pulsatile secretion of luteinizing hormone (LH) and growth hormone were completely suppressed by the prolonged exercise treatment. Mean levels of follicle-stimulating hormone, prolactin, and thyroid-stimulating hormone were not affected. Prolonged exercise elevated corticosterone titers, and the secretory pattern of this steroid was changed out of phase with running activity. Tissue levels of gonadotropin-releasing hormone (GnRH) in the hypothalamus and LH in the pituitary were enhanced, not suppressed, by prolonged exercise. Most importantly, pulsatile infusions of GnRH reinstated normal pubertal development and ovulation in rats still growth restricted by the prolonged exercise treatment. The results of this study indicate that the suppressive effects of prolonged exercise somehow affect the production of the hypothalamic GnRH-pulse generator signal.