Varadaraj Chandrashekar

Consequences of growth hormone (GH) overexpression and GH resistance.

Development of transgenic mice overexpressing GH and GHR-KO mice with GH resistance provided novel animal models for study of the somatotropic axis and for identifying GH actions that may be relevant to its current and contemplated use in medicine and agriculture. Studies of phenotypic characteristics of these animals revealed previously unsuspected actions of GH and IGF-I on neuroendocrine functions related to reproduction and to the release of stress hormones (glucocorticoids and prolactin). These studies also provided novel and still-disputed evidence for involvement of somatotropic axis in the control of aging and life span and in mediating the actions of longevity genes.

The Consequences of Altered Somatotropic System on Reproduction

Abstract Although the primary control of gonadotropin secretion is by the hypothalamic GnRH and the gonadal function is controlled by the pituitary gonadotropins and prolactin, the emerging evidence suggests a vital role of the somatotropic axis, growth hormone (GH), and insulin-like growth factor-I (IGF‐I) in the control of the pituitary and gonadal functions. It has been shown that GH deficiency, GH resistance, and experimental alterations in IGF‐I secretion modify folliculogenesis, ovarian maturation, ovulation, and pregnancy, and in the male, GH/IGF‐I plays an important role in spermatogenesis and the Leydig cell function. The primary focus of this review is to examine the role of GH/ IGF‐I on the onset of puberty, fertility, pituitary, and gonadal endocrine functions. A number of studies have revealed that fertility is affected in GH-deficient dwarf and in IGF‐I gene-ablated mice, possibly due to subnormal function of either the pituitary gland or the gonads. In the female GH receptor gene knockout (GHR-KO) mice, there was impairment in follicular development, ovulation rate, sexual maturation, production of and responsiveness to pheromonal signals, and the corpus luteum function. In IGF‐I-deficient male GHR-KO mice, puberty is delayed, spermatogenesis is affected, and neuroendocrine-gonadal function is attenuated. Similarly, in some of the human Laron syndrome patients, puberty is delayed due to GH resistance. These data suggest that, in addition to GnRH and gonadotropins, GH/IGF‐I influences the pituitary and gonadal functions in animals and humans.

Epithelial defect in prostates of Stat5a-null mice.

The transcription factor Stat5a critically mediates prolactin (PRL)-induced mammary gland development and lactogenesis. PRL also stimulates growth and differentiation of prostate tissue. Specifically, hyperprolactinemia gives rise to prostate hyperplasia, and prostate size is reduced in PRL-deficient mice. We therefore investigated the importance of Stat5a for prostate development and function by examining Stat5a-null mice. The absence of Stat5a in mice was associated with a distinct prostate morphology characterized by an increased prevalence of local disorganization within acinar epithelium of ventral prostates. Affected acini were typically filled with desquamated, granular epithelial cells that had become embedded in dense, coagulated secretory material. These features were reminiscent of acinar cyst formation and degeneration frequently observed in human benign prostate hyperplasia, however, cystic changes in prostate acini of Stat5a-deficient mice were not associated with increased prostate size or morphologic hallmarks of epithelial hyperplasia. Instead, immunohistochemistry of the prostate-specific secretory marker, probasin, suggested that hypersecretory function of the epithelium could underlie local congestion and cyst formation in prostates of Stat5a-null mice. Serum testosterone and PRL levels were normal in Stat5a knockout mice, but prostate PRL receptor expression was reduced as determined by immunohistochemistry. Expression levels or activation states of other PRL signal transduction proteins, including Stat5b, Stat3, Stat1, ERK1, and ERK2 were not altered. The present study offers the first evidence for a direct role of Stat5a in the maintenance of normal tissue architecture and function of the mouse prostate.

Influence of endogenous prolactin on the luteinizing hormone stimulation of testicular steroidogenesis and the role of prolactin in adult male rats

The role of endogenous prolactin (PRL) in the control of testosterone (T) secretion and T responses to LH treatment was evaluated in adult male rats. Rats were actively immunized three times against ovine PRL in Freunds adjuvant-saline mixture (PRL-IMM rats), and control rats were treated with adjuvant-saline mixture (ADJ-CON rats). On day 110 after initial immunization, rats in each of these two groups were divided into three subgroups. Rats in subgroups 1 and 2 were injected with saline while those in subgroup 3 received 200 micrograms ovine PRL in saline, twice a day for a total of 7 injections. On day 113, the seventh injection was given 3 h before the termination of the experiment. On the same day, 2.5 h before the rats were sacrificed, rats in subgroups 1 and 3 were treated with saline; animals in subgroup 2 received 25 micrograms ovine LH in saline. Blood samples were obtained throughout the study, and sera were used for measurement of PRL antibodies, gonadotropins, progesterone (P), and T. PRL antibodies were detected in the sera of all rats actively immunized with PRL. Administration of PRL increased serum T levels in ADJ-CON rats, and this effect was eliminated in rats actively immunized against PRL. LH treatment significantly increased serum T levels in ADJ-CON rats. In PRL-IMM rats, this increase was attenuated while circulating P concentrations were elevated. These data demonstrate that PRL treatment can increase T secretion and that endogenous PRL is required for the complete expression of the stimulatory action of LH on T secretion in adult male rats.

Seasonal differences in testicular receptors and steroidogenesis

Gonadal function in most animal species exhibits considerable annual fluctuations, with gametogenesis and fertility often being confined to a short and rigidly controlled breeding season. In males, production of androgenic steroids by the testis is usually maximal immediately before and during the breeding season. In the golden hamster, seasonal regression of the testes is associated with decrease in the total content of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin (PRL) receptors, and similar findings have been reported for other mammalian species. However, the concentration of LH and FSH receptors per unit of testis weight is typically elevated rather than suppressed during testicular regression. Reduction in the number of testicular LH and PRL receptors in adult golden hamsters exposed to short photoperiod is due primarily to suppression of pituitary PRL release under these circumstances. Regulation of seasonal changes in testicular FSH binding, as well as regulation of the levels of LH, PRL and FSH receptors in other seasonally breeding species remain to be elucidated. Reduction in the content of LH receptors in the testes is accompanied by reduced capacity to produce androgens in response to LH stimulation. Although these events are likely to be causally related, other mechanisms are also involved. In particular, seasonal regression is accompanied by reduced capacity of the testes to convert C21 steroid precursors into biologically active androgens. Seasonal loss of FSH receptors was reported to be accompanied by increased rather than reduced responsiveness of the Sertoli cells to FSH, thus resembling the situation in immature animals. It can be concluded that alterations in the ability of the testes to bind pituitary gonadotropins and to respond to gonadotropic stimulation are among the mechanisms responsible for seasonal shifts between gonadal activity and quiescence.

Influence of Diabetes on the Gonadotropin Response to the Negative Feedback Effect of Testosterone and Hypothalamic Neurotransmitter Turnover in Adult Male Rats

The influence of diabetes on the gonadotropin response to the negative feedback effect of testosterone (T) and hypothalamic neurotransmitter turnover rates in adult male rats was evaluated. Adult male Sprague-Dawley rats were made diabetic by an intraperitoneal injection of streptozotocin (STZ; 5 mg/100 g body weight) in citrate buffer. Vehicle-injected rats served as controls. On day 9, all rats were bilaterally castrated and treated subcutaneously on alternate days with either peanut oil or T propionate (TP) in peanut oil (100 micrograms/rat). Plasma follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin (PRL), and T concentrations were measured by specific radioimmunoassays from blood samples collected on day 1 (before castration) and 2, 4, 6, and 7 days after castration. On day 7 after castration (day 15 after vehicle or STZ treatment), 1 h before autopsy, the rats were injected intraperitoneally with saline or a tyrosine hydroxylase inhibitor, alpha-methyl-p-tyrosine (25 mg/100 g BW), for the measurement of norepinephrine (NE) and dopamine turnover in median eminence and medial basal hypothalamus (MBH). Circulating FSH, LH, PRL, and T levels were significantly lower (FSH and T: p less than 0.001; LH and PRL: p less than 0.05) in gonad-intact rats treated with STZ than in vehicle-injected animals. The castration-induced increase in plasma LH levels was attenuated in diabetic rats. The suppressive effect of T on LH secretion was significantly greater (p less than 0.001) in STZ-treated rats relative to TP-treated nondiabetic controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Hyperprolactinemia Disrupts Neuroendocrine Responses of Male Rats to Female Conspecifics

An increase in plasma luteinizing hormone levels which occurs in the male rat in response to the presence of a receptive female was associated with increased norepinephrine metabolism in several regions of the hypothalamus and reduced norepinephrine metabolism in preoptic area and olfactory bulbs. Hyperprolactinemia induced by grafting three anterior pituitary glands beneath the kidney capsule blocked female-induced changes in norepinephrine metabolism and attenuated the increase in luteinizing hormone release. These results suggest that endocrine and behavioral responses of male rats to the presence of a female are mediated by changes in catecholamine metabolism in several brain regions. The ability of hyperprolactinemia to induce deficits in male sexual behavior may be due to the blockade of these CNS responses to exteroceptive stimuli originating from the female.

Growth Factors in Leydig Cell Function

Leydig cell function is predominantly controlled by gonadotropins. However, a number of intratesticular factors are known to influence Leydig cell steroidogenic function. The presence of growth factors within the testis, their effects on androgen release by the isolated Leydig cells, subnormal reproductive function in animals with disrupted growth hormone/insulin-like growth factor-I secretion, and in acromegalics as well as in Laron syndrome patients clearly indicate that growth factors play an important role in reproduction. It is possible that there might be a concerted effect of intratesticularly produced factors on testosterone secretion. Evidence presented in this chapter indicates that pituitary luteinizing hormone is absolutely essential for Leydig cell endocrine function, but growth factors are required for the full effect of luteinizing hormone on androgen secretion. Also, there are indications that a number of growth factors modify gonadotropin synthesis and release from the pituitary gland. Thus, growth factors can exert endocrine as well as paracrine/autocrine effects in controlling the pituitary and testicular functions in mammals.

Effects of photoperiod on testicular inhibin-α and androgen binding protein mRNA expression during postnatal development in siberian hamsters, Phodopus sungorus

The effects of short photoperiod (SPP) on testicular inhibin-alpha and androgen binding protein (ABP) mRNA expression were investigated in Siberian hamsters during postnatal development. Hamsters were raised in either long photoperiod (LPP; 16L:8D) or in a SPP (6L:18D). Hamsters were sacrificed at the following developmental ages: preweaning (18-20 days), immature (28-30 days), pubertal (38-40 days), postpubertal (48-50 days) and adult (60-62 days; n = 7-12 animals per group). Body and testicular weights were determined and plasma FSH levels were measured by validated RIA. As expected, testicular weight and circulating FSH levels were significantly lower in hamsters raised in SPP than in LPP animals. Photoperiod had no significant effect on body weight except during postpubertal age. The northern analyses of inhibin-alpha and ABP expression in the testes revealed that the corresponding relative mRNA levels were higher in animals exposed to SPP than in animals from LPP. These findings suggest that the testes of hamsters exposed to a SPP can synthesize inhibin-alpha and ABP, and we speculate that the release of these proteins may be regulated by photoperiod and the role of FSH in the control of inhibin-alpha and ABP gene expression in male Siberian hamsters may be minimal.

Assessment of the effects of a synthetic gonadotropin-releasing hormone associated peptide on hormone release from the in situ and ectopic pituitaries in adult male rats☆

The in vivo effects of synthetic human gonadotropin-releasing hormone associated peptide (GAP) were evaluated in adult male rats. In normal rats, intracerebroventricular (III ventricle) injection of 5 ng GAP significantly increased plasma LH levels after 60 minutes. Intracerebroventricular administration of 5, 25 or 125 ng of GAP elevated circulating LH levels also at 120 minutes of injection but did not alter plasma FSH, prolactin or testosterone concentrations. In hypophysectomized-pituitary-grafted rats injection of 125 ng GAP directly into the ectopic pituitary induced no changes in peripheral hormone levels. However, intrapituitary graft injection of 25 ng of GnRH significantly elevated circulating levels of LH and testosterone. These results indicate that the ectopic pituitary graft can respond to acute exogenous GnRH stimulation and that the commercially available synthetic GAP fails to inhibit prolactin release in adult male rats.