The impact of the GH-IGF-I axis on gonadotropin secretion: inferences from animal models

The Role of Growth Hormone on Ovarian Functioning and Ovarian Angiogenesis

Although not yet well-understood, today it is clear that Growth Hormone (GH) exerts a relevant role in the regulation of ovulation and fertility; in fact, fertility is lower in women with GH deficiency (GHD), and GH receptors (GHR) and GH mRNA have been found in the ovary since the onset of follicular development in humans. However, despite the strong evidence of GH in the regulation of fertility, many aspects of GH actions at this level are still not well-established, and it is likely that some controversial data depend on the species analyzed, the dose of the hormone and the duration of use of GH. Folliculogenesis, ovulation, and corpus luteum formation and maintenance are processes that are critically dependent on angiogenesis. In the ovary, new blood vessel formation facilitates oxygen, nutrients, and hormone substrate delivery, and also secures transfer of different hormones to targeted cells. Some growth factors and hormones overlap their actions in order to control the angiogenic process for fertility. However, we still know very little about the factors that play a critical role in the vascular changes that occur during folliculogenesis or luteal regression. To promote and maintain the production of VEGF-A in granulosa cells, the effects of local factors such as IGF-I and steroids are needed; that VEGF-A-inducing effect cannot be induced by luteinizing hormone (LH) or chorionic gonadotropin (CG) alone. As a result of the influences that GH exerts on the hypothalamic-pituitary-gonadal axis, facilitating the release of gonadotropins, and given the relationship between GH and local ovarian factors such as VEGF-A, FGF-2, IGF-1, or production of sex steroids, we assume that GH has to be a necessary factor in ovarian angiogenesis, as it happens in other vascular beds. In this review we will discuss the actions of GH in the ovary, most of them likely due to the local production of the hormone and its mediators.

Effects of growth hormone administration on luteinizing hormone secretion in healthy older men and women

The known interactions between the somatotropic and hypothalamic-pituitary-gonadal (HPG) axes have not been well delineated in older individuals. Aging-associated decline in insulin like growth factor-1 (IGF-1) levels has been proposed to play a role in reproductive senescence in animals. However, the effects of GH on LH secretion are unknown in older individuals. Our objective was to determine whether GH modulates LH secretion or levels of sex steroids (SS) in healthy older (ages 65-88 years) men (n = 24) and women (n = 24) with low-normal plasma IGF-1 levels. In a double-masked, placebo-controlled (n = 24), randomized study, we evaluated the effects of GH (n = 24, 20 μg/kg sc 3×/week) for 26 weeks on nocturnal LH secretory dynamics [(8 pm to 8 am, Q20) min sampling and analyzed by multiparameter deconvolution algorithm]. Indices of LH secretion [frequency, mass per burst, pulsatile production rate, and approximate entropy (ApEn)] and fasting serum IGF-1, SHBG, and SS (TT, fT, or E2) were measured. At baseline, all indices of LH secretion (frequency, mass per burst, pulsatile production rate) were inversely (P < 0.05) related to IGF-1, but not to mean nocturnal serum GH concentrations. GH administration for 26 weeks increased serum IGF-1, but exerted no significant effects on LH secretory dynamics, or concentrations of SSs (TT, fT, or E2) or SHBG in older women or men. These data suggest that GH-mediated increases in IGF-1 do not modulate the HPG axis in older individuals.

Influence of testicular hormones on the somatostatin-GH system during the growth promoted transition to puberty in sheep

The aim of the present study was to investigate whether the growth promoted transition to puberty in lambs involved changes in the effects of testicular hormones on somatostatin in hypothalamic neurons and GH secretion. The study was performed in infants (9-week-old) testis-intact (TEI) and orchidectomized (ORCHX) at the sixth week of age, and pubertal lambs (16-week-old) TEI and ORCHX at the 12th week of age (n = 20). In TEI lambs, the changes included a pubertal increase in immunoreactive somatostatin in the periventricular nucleus and median eminence with simultaneous neuropeptide depletion in the median eminence, and a decrease in the percentage of the hypophyseal area (PA) occupied by GH-immunoreactive cells (P < 0.05). The mean concentration of GH in the peripheral blood plasma was greater (P < 0.001) in early infancy (5 wk), because of the greater (P < 0.0001) pulse amplitude, and then uniformly low until puberty. The postnatal increase in the body weight (BW) was prominent (P < 0.01) in middle-late infancy (9-12 wk) because of the large daily live-weight gain. After orchidectomy somatostatin was abundant. This effect on nerve terminals in the median eminence was greater (P < 0.01) in infancy and lesser (P < 0.05) in puberty. Conversely, the PA occupied by GH cells was lower in the ORCHX pubertal lambs compared to TEI lambs (P < 0.05). The GH concentration and pulse characteristics were less (P < 0.05) in the infantile and pubertal ORCHX lambs compared to the TEI lambs. However, this effect was weak (P < 0.05) until middle infancy because of no influence on the GH basal concentration, and strong (P < 0.001) after late infancy. The BW did not differ (P > 0.05) between TEI and ORCHX lambs. Findings suggest activation of GH negative autofeedback loop in middle infancy. Testicular factors may play an inhibitory role in regulating somatostatin accumulation and a stimulatory role in GH secretion until puberty. The start of puberty is related to an attenuation in the stimulatory role of gonadal factors in regulating somatostatin depletion in nerve terminals associated with an intensification of the stimulatory role of gonadal factors in regulating GH secretion. From a somatic perspective of growth rate, these mechanisms do not seem to be important. Thus, testicular factors modulate mechanisms within the somatostatin-GH system to integrate somatotropic and gonadotropic functions at the time of growth-promoted sexual maturation in sheep.

Hypothalamic expression of human growth hormone induces post-pubertal hypergonadotrophism in male transgenic growth retarded rats

Growth hormone (GH) is known to regulate peripheral components of the hypothalamo-pituitary gonadal (HPG) axis, but it remains unclear whether GH exerts a significant influence on the activity of the hypothalamo-pituitary components of the HPG axis. In this study, we investigated the development of HPG axis function in the male transgenic growth retarded (Tgr) rat, a model of moderate systemic GH deficiency caused by hypothalamic expression of human (h)GH. Impaired postnatal somatotroph expansion and moderate GH deficiency in male Tgr rats were accompanied by a two- to three-fold increase in pituitary gonadotrophin content, but without a significant change in the pituitary gonadotroph population. A three- to nine-fold elevation in basal circulating luteinising hormone concentration was seen in postpubertal Tgr rats, with a smaller increase in follicle-stimulating hormone. Despite this hypergonadotrophism, there was no corresponding increase in steroidogenic (circulating testosterone and seminal vesicle weights) or gametogenic (spermatozoa counts in seminiferous tubules) activity in the postpubertal Tgr testis. Following puberty, the plasma leptin concentration also became progressively elevated in Tgr males. Circulating gonadotrophin and leptin levels were normalised in Tgr rats by peripheral physiological replacement of rat GH, but plasma testosterone concentration was unaffected. These results confirm that hGH exerts a positive influence on the central control of gonadotrophin secretion in the Tgr rat, but the absence of a corresponding elevation in the steroidogenic or gametogenic function of the Tgr testis implies that the peripheral GH/insulin-like growth factor I axis may also exert a permissive influence on testicular function. The relative contribution of somatogenic and lactogenic mechanisms and the potential influence of elevated leptin and decreased sensitivity to androgen feedback to the development of postpubertal hypergonadotrophism in Tgr males remain to be determined.

Synergistic interaction of estradiol and insulin-like growth factor-I in the activation of PI3K/Akt signaling in the adult rat hypothalamus

Estradiol and insulin-like growth factor-I (IGF-I) interact in the hypothalamus to regulate neuronal function, synaptic plasticity and neuroendocrine events. However, the molecular mechanisms involved in these interactions are still unknown. In the present study, the effect of estradiol on the signaling pathways of IGF-I receptor has been assessed in the hypothalamus of young adult ovariectomized rats, using specific antibodies for the phosphorylated forms of extracellular-signal regulated kinase (ERK) 1 and ERK2 and Akt/protein kinase B (Akt/PKB). Estradiol treatment resulted, between 6 and 24 h after systemic administration, in dose-dependent effects on the phosphorylation of ERK and Akt/PKB. Estradiol did not modify the level of ERK phosphorylation induced by intracerebroventricular administration of IGF-I. However, both hormones had a synergistic effect on the phosphorylation of Akt/PKB. These findings suggest that estrogen effects in the hypothalamus may be mediated in part by the activation of the signaling pathways of the IGF-I receptor.