To study the effect of PRL deficiency on the onset of puberty, PRL release was chronically inhibited by treating immature female rats with the dopaminergic receptor agonist, bromoergocriptine (CB-154). The resulting alterations in the time of puberty onset, and in other associated parameters, such as serum levels of pituitary hormones, ovarian responsiveness to gonadotropins and ovarian hCG receptor content were then evaluated. CB-154 was provided in the drinking water from day 22 onward at the concentration of 20 and 100 µg/ml. The treatment resulted in almost complete suppression of serum PRL levels throughout the entire period studied (day 22 to first diestrus). In contrast, serum GH, FSH, and LH levels were not depressed. Likewise, pulsatile release of FSH was not affected and only a subtle alteration in pulsatile LH release was apparent. The onset of puberty, as determined by the age at vaginal opening, and at first diestrus after the first estrus and by the presence of corpora lutea at sacrifice (first diestrus), was markedly delayed in the hypoprolactinemic (HPO) rats. This inhibitory effect of CB-154 was completely prevented by concomitant administration of PRL. Ovarian weight was significantly decreased in HPO rats at the three ages studied (27, 32, and 36 days of age). By day 36, 50% of the control animals had already ovulated, as compared with only 9% of the HPO rats. Microscopic examination of ovaries from HPO rats revealed a retarded follicular development. In vitro ovarian progesterone response to hCG studied at day 32 and 36 of age was reduced in the HPO rats. Uterine growth was also depressed in HPO rats, the ovaries of which, when incubated in vitro, failed to show the prepubertal increase in estrogen response to hCG seen in control rats between day 32 and 36. Aromatase activity, as measured by the in vitro release of estradiol from ovaries incubated in the presence of an excess of androgen substrate, was depressed in HPO rats. hCG receptor content in the ovaries from HPO rats (counts per min [l25I]hCG bound per µg DNA) was also lower than that of control animals at day 32 and 34 but not at day 36. However, at this later time the hCG receptor content per milligram of ovary was still significantly reduced in HPO rats. The results support the view that PRL plays an important role in the process of ovarian development that leads to the onset of puberty in the female rat and that this effect is, at least in part, exerted through a positive influence of PRL on ovarian LH receptor content.
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