Sexual dimorphism and testosterone-dependent regulation of somatostatin gene expression in the periventricular nucleus of the rat brain

Gender differences in hypothalamic somatostatin (SS) secretion may account in part for the sexually dimorphic patterns of GH secretion in rats. Since males have lower baseline serum GH levels than females, and SS inhibits GH secretion, we hypothesized that the SS neurons in the periventricular nucleus (PeN) of the male rat would have greater biosynthetic activity than those of the female. We tested this hypothesis by measuring SS mRNA in cells in the PeN of intact male and proestrous female rats. Using in situ hybridization and a computerized image analysis system, we measured SS mRNA content in individual cells in the PeN and compared signal levels (autoradiographic grains per cell) between male and proestrous female animals. The signal level of SS mRNA in cells of the PeN was significantly greater in males than in proestrous females (males, 210 +/- 7 grains/cell; females, 158 +/- 5 grains/cell; P less than 0.0005), whereas no difference was observed in SS cells of the frontal cortex (males, 100 +/- 0.8 grains/cell; females, 99 +/- 5.9 grains/cell). This difference in SS mRNA levels is likely to be the result of different hormonal environments exerting an influence on neurons of the hypothalamus. To test the hypothesis that testosterone stimulates SS gene expression in neurons of the PeN, adult male rats were castrated and immediately implanted with either empty (sham; n = 3) or testosterone-containing (n = 3) Silastic implants of a size that would deliver physiological levels of testosterone (3.6 +/- 1.5 ng/ml). We observed that castrated animals had significantly lower levels of SS mRNA signal in neurons of the PeN compared with intact animals (intact, 195 +/- 3 grains/cell; castrated, 159 +/- 6 grains/cell; P less than 0.003) and that physiological levels of testosterone prevent this reduction in SS mRNA levels (castrated testosterone-replaced, 182 +/- 4 grains/cell; castrated, 159 +/- 6 grains/cell; P less than 0.003). Furthermore, testosterone-treated castrates had SS mRNA signal levels indistinguishable from those of intact controls (intact, 195 +/- 3 grains/cell; castrated testosterone-replaced, 182 +/- 4 grains/cell). There was no significant difference in SS mRNA levels in neurons of the frontal cortex (intact, 98 +/- 2 grains/cell; castrated, 98 +/- 3 grains/cell; castrated testosterone-replaced, 102 +/- 2 grains/cell).(ABSTRACT TRUNCATED AT 400 WORDS

Sexual dimorphism and testosterone-dependent regulation of somatostatin gene expression in the periventricular nucleus of the rat brain

Gender differences in hypothalamic somatostatin (SS) secretion may account in part for the sexually dimorphic patterns of GH secretion in rats. Since males have lower baseline serum GH levels than females, and SS inhibits GH secretion, we hypothesized that the SS neurons in the periventricular nucleus (PeN) of the male rat would have greater biosynthetic activity than those of the female. We tested this hypothesis by measuring SS mRNA in cells in the PeN of intact male and proestrous female rats. Using in situ hybridization and a computerized image analysis system, we measured SS mRNA content in individual cells in the PeN and compared signal levels (autoradiographic grains per cell) between male and proestrous female animals. The signal level of SS mRNA in cells of the PeN was significantly greater in males than in proestrous females (males, 210 +/- 7 grains/cell; females, 158 +/- 5 grains/cell; P less than 0.0005), whereas no difference was observed in SS cells of the frontal cortex (males, 100 +/- 0.8 grains/cell; females, 99 +/- 5.9 grains/cell). This difference in SS mRNA levels is likely to be the result of different hormonal environments exerting an influence on neurons of the hypothalamus. To test the hypothesis that testosterone stimulates SS gene expression in neurons of the PeN, adult male rats were castrated and immediately implanted with either empty (sham; n = 3) or testosterone-containing (n = 3) Silastic implants of a size that would deliver physiological levels of testosterone (3.6 +/- 1.5 ng/ml). We observed that castrated animals had significantly lower levels of SS mRNA signal in neurons of the PeN compared with intact animals (intact, 195 +/- 3 grains/cell; castrated, 159 +/- 6 grains/cell; P less than 0.003) and that physiological levels of testosterone prevent this reduction in SS mRNA levels (castrated testosterone-replaced, 182 +/- 4 grains/cell; castrated, 159 +/- 6 grains/cell; P less than 0.003). Furthermore, testosterone-treated castrates had SS mRNA signal levels indistinguishable from those of intact controls (intact, 195 +/- 3 grains/cell; castrated testosterone-replaced, 182 +/- 4 grains/cell). There was no significant difference in SS mRNA levels in neurons of the frontal cortex (intact, 98 +/- 2 grains/cell; castrated, 98 +/- 3 grains/cell; castrated testosterone-replaced, 102 +/- 2 grains/cell).(ABSTRACT TRUNCATED AT 400 WORDS)

Growth hormone-releasing hormone messenger ribonucleic acid in the hypothalamus of the adult male rat is increased by testosterone

Since intact adult male rats have higher GH pulse amplitude than do castrated animals and since GH-releasing hormone (GHRH) secretion is predominantly responsible for the production of these GH pulses, we hypothesized that testosterone stimulates GHRH synthesis in neurons of the hypothalamus. To test this hypothesis, we compared GHRH mRNA content in individual neurons of the arcuate (ARC) and ventromedial (VMH) nuclei among groups of intact (n = 3), castrated (n = 5), and castrated testosterone-replaced (n = 5) adult male rats. Cellular GHRH mRNA content was measured by using semiquantitative in situ hybridization with an 35S-labeled cRNA probe complementary to the coding sequence of rat GHRH mRNA. Castration resulted in an approximately 35% decline in GHRH mRNA signal relative to that in intact animals in both the ARC (P less than 0.005) and VMH (P less than 0.005). Replacement with testosterone at the time of castration completely prevented the decline in both areas. Testosterone can exert effects either through activation of the androgen receptor directly or through aromatization to estradiol; therefore, we also examined the effects on GHRH mRNA of replacement with 17 beta-estradiol (n = 5) or dihydrotestosterone (DHT), a nonaromatizable androgen (n = 4). Estradiol had no effect on the castration-induced decline in GHRH mRNA in either the ARC or VMH. In contrast, DHT partially prevented the postcastration decline in GHRH in the ARC (P less than 0.005), while having no statistically significant effect on GHRH mRNA in the VMH. These results clearly indicate that testosterone stimulates expression of GHRH mRNA in neurons of the hypothalamus. Furthermore, the failure of estradiol to substitute for testosterone and the ability of DHT to substantially support GHRH mRNA suggest that testosterone exerts its effects on GHRH gene expression predominantly through direct activation of the androgen receptor

Regulation of somatostatin and growth hormone-releasing hormone gene expression in the rat brain

We have studied the regulation of somatostatin (SS) and growth hormone-releasing hormone (GHRH) gene expression in the brain of the laboratory rat. We report that hypophysectomy in the adult male reduces SS mRNA in cells of the periventricular nucleus (PeN), while GH reverses this effect. We demonstrate that cellular levels of SS mRNA in the PeN are higher in male compared to female animals. We report that castration reduces cellular levels of GHRH mRNA and SS mRNA in the arcuate nucleus and PeN, respectively, and that testosterone reverses this effect through an androgen receptor-dependent mechanism. Finally, we present a theoretical model to explain the generation of the ultradian rhythm in GH secretion, which implicates the reciprocal interaction between GH feedback and the transcriptional regulation of the SS and GHRH genes and the kinetics of these relationships