Role of sex steroids in the initiation of puberty

Abstract

Earlier work from our laboratory indicated that in the immature female rat there was evidence of oestrogen secretion by day 32 of life and an increased sensitivity of the pituitary to LHRH by day 34. These data suggested that in addition to the increased release of LHRH during puberty, a steroid induced alteration in the pituitary's responsiveness to LHRH may also be a significant contributory factor in the increase in secretion of gonadotropins at puberty. The direct effect of oestradiol in enhancing the pituitary's sensitivity to LHRH was confirmed in the castrated female rat in which the pituitary stalk was cut surgically and vascular regeneration prevented by the use of aluminium foil. The direct effect of oestradiol and testosterone on the pituitary's sensitivity to LHRH was also confirmed in the male stalk-sectioned adult rat. The stimulatory effect appeared to be related both to the quantity of steroid and the challenging dose of LHRH. With the documentation of the direct effect of testosterone on the pituitary's sensitivity to LHRH, the effects of small doses of testosterone were investigated in the immature male rat. For this purpose immature rats were castrated and implanted with a testosterone filled Silastic capsule. The capsule size was such that the secondary organ weights were maintained slightly above intact levels. A dramatic increase in sensitivity to LHRH in the release of LH as judged by the 10min response occurred in comparison with the response in intact animals. The administration of 5α-dihydrotestosterone or 5α-androstane-3α, 17β-diol 12 h before the LHRH challenge brought about an increased release in FSH as well. The steroid potentiation effect was not present in pubertal rats. At this stage of development, however, a significant self-priming effect of LHRH on the responsiveness of the pituitary gland was found. Such an effect was not present in the immature rat. These studies show that in addition to changes in sensitivity at the level of the hypothalamus, the CNS and gonads, steroid and LHRH modulation of the response of the pituitary gland, are important events in the onset of puberty.