The secretory pattern of GH secretion is markedly sexually dimorphic in the adult rat. The patterning of GH secretion is determined by the coordinated activity of somatostatin (SS)- and GH-releasing hormone (GHRH)-containing neurosecretory cells located in the hypothalamus. In this study we examined whether there is sexual dimorphism in the expression of the SS and GHRH genes and, if so, at what developmental stage this becomes evident. To address these questions, we measured SS messenger RNA (mRNA) levels in neurons of the periventricular nucleus and GHRH mRNA levels in the arcuate nucleus and ventromedial nucleus of the hypothalamus in male and female rats at 10, 25, 35, and 75 days of age. Using in situ hybridization and a computerized image analysis system, we measured SS mRNA and GHRH mRNA signal levels in individual neurons and compared these levels among the different age groups. We found that male animals had significantly higher levels of SS mRNA than females at every age. Similarly, males had higher GHRH mRNA levels than females; however, this difference was statistically significant only at 10 and 75 days of age. Developmental changes in GHRH mRNA levels were similar for both sexes, with GHRH message levels increasing gradually over the course of maturation. SS mRNA signal levels also changed over the course of development in both male and female animals. In the male rat, SS mRNA levels increased significantly between 10 and 25 days of age and declined significantly between 35 and 75 days of age. In the female rat, SS mRNA levels increased gradually between 10 and 35 days of age, then, as in the male, declined significantly between days 35 and 75. We conclude that sex differences and age-dependent changes in the expression of the SS and GHRH genes may subserve the sexual dimorphism and developmental alterations in the pattern of GH secretion in the rat.
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