SEX STEROID EFFECTS ON THE DEVELOPMENT AND FUNCTIONING OF THE GROWTH-HORMONE AXIS

1. The secretory pattern of growth hormone (GH) is sexually dimorphic in the adult rat, However, this difference between the sexes does not become apparent until after the onset of puberty, suggesting that pube rtal sex steroids play an important role in the manifestation of this phenomenon. 2. We have addressed the question as to whether there exis ts a sexual dimorphism in the hypothalamic neuropeptides that regulate GH release from the anterior pituitary, i.e., somatostatin (SS) and g rowth hormone-releasing hormone (GHRH), In addition, we have investiga ted whether the developmental changes in the GH secretory pattern are correlated with changes in these neuropeptides. The effect of testoste rone treatment on SS and GHRH neurons during both the neonatal period and adulthood have also been studied. 3. We have found that the synthe tic capacity, as reflected in relative messenger RNA (mRNA) levels, of both SS and GHRH neurons changes throughout development in both male and female rats, These mRNA levels are sexually dimorphic at certain t imes during maturation and can be modulated by changes in testosterone levels, suggesting that sex steroid modulation of these two neuropept ide systems could at least partially account for the sexual dimorphism seen in the adult GH secretory pattern. 4. The neonatal steroid envir onment has also been suggested to be involved in the generation of the final adult GH secretory pattern, although the mechanisms underlying this effect are even less well understood. In support of the hypothesi s that the neonatal steroid environment plays an important role in org anizing the GH axis, we have found that the number of GHRH neurons in the adult brain, as well as their sensitivity to adult steroids, is mo dulated by neotatal testosterone treatment. The number of SS neurons i n the periventricular and paraventricular nuclei were not modulated by neonatal steroids; however, the synthetic capacity of these neurons d oes appear to be influenced by the neonatal steroid environment. 5. Th ese studies suggest that both the neonatal and adult sex steroid envir onments influence the adult GH secretory pattern by modulating GHRH an d SS neurons.