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      Microinjection of Dihydrotestosterone into the Medial Preoptic Area Produces Male-Like Pattern of Growth Hormone Secretion in Ovariectomized Female Rats

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          The pattern of growth hormone (GH) secretion is sexually dimorphic in rats. We have previously shown that the secretory pattern in adult ovariectomized (OVX) female rats is masculinized by the administration of a single dose of dihydrotestosterone (DHT), a nonaromatizable androgen. To investigate the primary site of action of DHT in the brain, a small amount of DHT was injected directly into a defined area of the brain, and the blood GH profile was observed for 18 h in conscious adult OVX female rats. The bilateral direct injection of 1 µg DHT into the medial preoptic area (MPA) produced a male-like secretory pattern of GH in OVX rats. The masculinizing effects became apparent at 9 h after injection, from which time the episodic GH secretion was produced regularly at intervals of about 150 min, the amplitude of the peak increased and baseline levels were lowered. These parameters, analyzed during 9–18 h after DHT injection, were not different from those in adult male rats. On the contrary, microinjection of DHT into the bed nucleus of the stria terminalis, the hypothalamic periventricular nucleus, or the hypothalamic arcuate-ventromedial nucleus did not affect the secretory pattern of GH. The data indicate that DHT primarily acts on cells in the MPA through androgen receptors and modulates the secretion of somatostatin and/or GH-releasing hormone secondarily to masculinize the GH secretory pattern in OVX rats.

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          Most cited references 8

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          Nuclear Receptor Coregulators: Cellular and Molecular Biology

           N McKenna (1999)
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            Emerging diversities in the mechanism of action of steroid hormones.

            The classical genomic action of steroid hormones acting through intracellular receptors is well recognized. Within this concept of action, questions regarding the ultimate fate of the hormone and lack of a tight correlation between tissue uptake and biological activity with receptor binding remain unanswered. Evidence has accumulated that steroid hormones can exert non-classical action that is characterized by rapid effect of short duration. In most of these cases, the hormone effects occurs at the membrane level and is not associated with entry into the cell. The possible mechanisms for these non-classical actions are: (a) changes in membrane fluidity; (b) steroid hormone acting on receptors on plasma membranes; (c) steroid hormones regulating GABAA receptors on plasma membranes; and (d) activation of steroid receptors by factors such as EGF, IGF-1 and dopamine. Data have also been obtained indicating that receptor-mediated insertion of steroid hormones into DNA may take place with the steroid acting as a transcription factor. These new proposed mechanism of action of steroid hormones should not be viewed as a challenge to the classical mechanism. These diverse modes of action provide for an integrated action of hormones which may be rapid and of short duration or prolonged to address the physiological needs of the individual.
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              Growth hormone-releasing hormone receptor (GHRH-R) and growth hormone secretagogue receptor (GHS-R) mRNA levels during postnatal development in male and female rats.

              Experimental evidence suggests that differential pituitary sensitivity to hypothalamic signals exerts a role in mediating both age and sex dependent patterns of growth hormone (GH) release and synthesis. One mechanism by which pituitary sensitivity to hypothalamic GH regulators could be modified is by the differential synthesis of their pituitary receptors. In the present report we therefore studied the age and sex dependency of the expression of receptors for two known stimulators of GH release, growth hormone-releasing hormone (GHRH) and the synthetic peptidyl and non-peptidyl GH secretagogues (GHSs). Pituitary GHRH receptor (GHRH-R) and GHS receptor (GHS-R) mRNA levels were measured by reverse transcriptase-polymerase chain reaction (RT-PCR) in male and female rats at postnatal day 1, 10, 30 and 75. We also examined the age- and sex-dependent expression of the GHS-R in whole hypothalamic extracts, since the GHS-R is also expressed in a variety of nuclei within the hypothalamus and has been linked to central regulation of the GH-axis. Pituitary GHRH-R mRNA concentrations were age-dependent; the highest levels were observed in d1 pituitaries and then declined with age, reaching a nadir by d30. These results are in concordance with the age-related decline in pituitary GHRH sensitivity. In contrast, the ontogenic pattern of GHS-R expression was bimodal; GHS-R mRNA concentrations in dl and d30 pituitaries were approximately twice those at d10 and d75. These results mirror the transient increase in GHS sensitivity observed around the onset of puberty, suggesting that gonadal steroids mediate GHS-R expression. GHRH-R mRNA levels were comparable in males and females within each age while GHS-R mRNA levels were gender dependent. At d30, male GHS-R mRNA levels were 30% greater than in their female counterparts. This was reversed at d75, when females had 89% more GHS-R mRNA per pituitary and 65% more per somatotrope than did age-matched males. These sexual differences further support a role for gonadal steroids in the modulation of pituitary GHS-R synthesis. The ontogenic and gender-specific pattern of hypothalamic GHS-R expression differed from that observed for the pituitary. Hypothalamic GHS-R mRNA levels increased with age but exhibited no significant sex difference at each age tested. Taken together, these data demonstrate that changes in the levels of pituitary GHS-R mRNA, but not GHRH-R mRNA, are associated with changes in the gonadal steroid environment, thereby implicating the GHS/GHS-R signalling system as a control point in the establishment and maintenance of sexually dimorphic patterns of GH secretion.

                Author and article information

                S. Karger AG
                June 2002
                07 June 2002
                : 75
                : 6
                : 384-391
                Department of Bioregulation, Institute of Gerontology, Nippon Medical School, Kawasaki, Japan
                59435 Neuroendocrinology 2002;75:384–391
                © 2002 S. Karger AG, Basel

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                Page count
                Figures: 4, Tables: 1, References: 48, Pages: 8
                Hypothalamic and Pituitary Actions of Gonadal Steroids


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