Early effects of streptozotocin-induced diabetes on insulin-like growth factor-I in the kidneys of growth hormone-transgenic and growth hormone-deficient dwarf mice

BACKGROUND

The renal growth and hyperfiltration observed in humans and animals with early diabetes might be dependent on growth hormone (GH) and insulin-like growth factor (IGF)-I. The aim of this study was to investigate the early changes in kidney IGF-I in experimental diabetes in mice transgenic for bovine GH and in genetically GH-deficient Ames dwarf mice.

METHODS

At 2, 4 and 8 days after a single intraperitoneal injection with streptozotocin, animals were weighted, bled and killed; plasma was analyzed for glucose and IGF-I. IGF-I levels were determined in tissue from snap-frozen kidney and liver.

RESULTS

Body weight decreased significantly after the induction of diabetes. Kidney weight increased significantly in GH-transgenic, but not in normal or dwarf mice. Plasma IGF-I was significantly decreased at day 2 in GH-transgenic and normal mice, while liver IGF-I was increased at day 4 in all mice. Kidney IGF-I increased significantly in normal and GH-transgenic mice and was increased more than 3-fold at day 4 in GH-transgenic mice. In dwarf mice, no kidney IGF-I was detectable.

CONCLUSION

The diabetes-induced increase in renal IGF-I is dependent on the presence of GH. GH deficiency may protect diabetic animals from early changes in the kidney.

Inhibitory avoidance and appetitive learning in aged normal mice: comparison with transgenic mice having elevated plasma growth hormone levels

Groups of 25-month-old ("old") B6C3 hybrid male mice, 6-month-old ("young") normal males, and their age-matched transgenic (TG) siblings overexpressing the bovine growth hormone gene were given an inhibitory avoidance training trial (0.20-mA electric shock, 1.0-s duration). The old B6C3 hybrids and the young TG mice displayed poorer retention (shorter latencies to enter the shock compartment) 24 h and 42 days after training than did the young normal mice. In a subsequent multiple-trial acquisition test, young TG and old normal mice required more trials to reach the criterion of complete inhibition of step-through responding for 300 s than did young normal mice. Young normal and young TG mice did not differ in trials to extinction, but TG mice met the extinction criterion sooner than did old normal mice, suggesting poorer longterm retention. In tests of T-maze appetitive learning, young normal, old normal, and young TG mice did not differ in acquisition or 24-h retention. Contrary to expectation, TG mice acquired T-maze reversal learning in fewer trials than did young normal or old normal mice. The TG and young normal mice did not differ in retention when retested 44 days after initial training, but old normal mice showed poorer retention than did the young normals. Results of locomotor activity and shock response tests suggested that learning impairments were not due to differences in locomotor activity or shock response thresholds in these animals. Tests in an elevated plus maze indicated that young TG mice were less anxious in a novel environment than their normal siblings, which may contribute to their impaired inhibitory avoidance learning. These findings suggest that 6-month-old TG mice overexpressing the bovine growth hormone gene display alterations in inhibitory avoidance (but not appetitive) learning similar to those occurring in 25-month-old normal mice. The neurobiological mechanisms mediating inhibitory avoidance and T-maze appetitive learning in these animals may be largely dissociated.

Modulation of the testicular steroidogenic pathway by cyclosporin A in adult rats pretreated with diethylstilbestrol

Treatment of male Fischer-344 rats with diethylstilbestrol (DES) induces hyperprolactinemia and alterations in testicular steroidogenesis. Cyclosporin A (CsA), an immunosuppressive drug, is believed to act by opposing the effects of prolactin (PRL) and was reported to influence testicular function. We have examined the effect of CsA on gonadal function in rats pre-treated with DES. Male rats were implanted with DES-containing silastic capsules. After 19 weeks, the capsules were removed, and, starting 2.5 weeks later, rats were treated for 1 week with CsA. At sacrifice, plasma and testes were collected. Testis fragments were incubated with or without 12.5 mlU of human chorionic gonadotropin (hCG)/ml at 32 degrees C for 4 hr. As expected, plasma PRL levels were increased in DES exposed rats, while testicular, seminal vesicle, and prostate weights were reduced. Cyclosporin A treatment did not further modify these parameters. Treatment with CsA and/or DES decreased circulating levels of testosterone, while testosterone content in the testes was not modified. Although CsA did not affect in vitro release of testosterone, it reduced the stimulatory influence of DES pretreatment on testicular responses to hCG in vitro. Plasma and testicular content of progesterone (P) was increased by DES administration but was not affected by CsA. Both CsA and DES administration decreased plasma 17-OH-Progesterone (17-OH-P) levels, but only CsA decreased testicular 17-OH-P contents. Combined exposure to CsA and DES enhanced the stimulatory effect of hCG on the accumulation of 17-OH-P in the media. Media estradiol levels were not affected by treatment with either CsA or DES. The present results indicate that CsA may interfere with the enhancement of the conversion of P to testosterone in DES-treated rats. As CsA did not change plasma PRL or gonadotropin levels, a direct effect of the drug on the testicular function is suspected.

Copulatory behavior and fertility in transgenic male mice expressing human placental growth hormone gene

Male transgenic (TG) mice overexpressing the human placental growth hormone (GH) variant gene (hGH-V) exhibit reproductive deficits in spite of normal testosterone levels and normal sperm counts. To evaluate the relationship of copulatory behaviors to fertility, we first measured mount, intromission, and ejaculation indices in 2-5-month-old mice (10 TG and 10 normal litter mate controls) during 1 hour tests with ovariectomized, estrogen-, and progesterone-primed females. After eight tests, each male was housed with three intact females for 27 consecutive days. Females were checked daily for vaginal plugs and sacrificed 14 days after insemination to determine the numbers of corpora lutea and live and dead fetuses. Relative to their normal siblings, TG mice mounted less often and intromitted sooner after the initial mount, made marginally more intromissions (with and without ejaculation), and were slower to ejaculate. In subsequent fertility tests, TG males inseminated fewer females and sired fewer live fetuses per insemination than non-TG controls. Across TG and normal males, the length of interval between initial mount and initial intromission was inversely correlated with the number of live offspring sired. This suggests that reduced fertility in hGH-V transgenic male mice may be related to altered copulatory behavior, including a rapid progression from first mount to first intromission.

Concentration of free growth hormone-binding protein in the serum of mice is not regulated by growth hormone

Ames dwarf mice that do not express growth hormone (GH) or prolactin (PRL) genes were used to study the effects of GH deficiency on the presence and the characteristics of GH-binding protein (GHBP) in serum. Chromatographic techniques were used to allow characterization of biological rather than immunological activity of GHBP. Two GH-binding fractions were found in dwarf mice serum, one with low affinity and high capacity (GHBPI) and one with high affinity, low capacity and lower molecular mass (GHBPII). Serum concentration of the high-affinity GHBP was 0.73 +/- 0.03 nM with a Kd of 6.3 +/- 1.7 nM. Since Ames dwarf mice have no GH in the circulation, all the GHBP is free. Interestingly, the concentration of GHBP in dwarf mice was similar to the levels of free GHBP measured in normal mice from the same line. Moreover, this value (0.7 nM) closely resembles the concentration of free GHBP in the serum of transgenic mice overexpressing GH, in which peripheral GH levels are grossly elevated. These observations can be interpreted as evidence that the levels of free GHBP in mouse serum are independent of GH concentration, and that GH influences only the levels of bound GHBP in peripheral circulation.

The detection of thyrotropin-releasing hormone (TRH) and TRH receptor gene expression in Siberian hamster testes

Thyrotropin-releasing hormone (TRH) from the hypothalamus is the major regulator of TSH synthesis and secretion. Most recently, TRH and TRH receptors (TRH-R), as well as their mRNAs, have been identified in rat testis. To expand our knowledge on the testicular TRH and TRH receptor gene expression in different species, in the present study the mRNA levels of testicular TRH and TRH-R were investigated in Siberian hamsters. To further localize the cellular sites of the gene expression, the animal model was treated with a single injection of ethylene dimethane sulfonate (EDS) (i.p., 80 mg/kg body weight), a compound known as to specifically eliminate testicular Leydig cells. The elimination of Leydig cells induced by EDS treatment was confirmed by histological studies of the testis sections and by serum hormonal analyses, which showed a dramatic reduction of serum testosterone (T) levels and significantly elevated serum LH concentrations. Messenger RNA levels of TRH and TRH-R in the testes were determined by Northern blot analyses quantitated with densitometry scanning. The results showed that specific TRH-R mRNA, 3.8 kb in size, was identified in Siberian hamster testes and the mRNA levels were significantly elevated in the EDS-treated testes compared to the controls (p < 0.01). Testicular TRH mRNA was also detected; however, no significant differences in TRH mRNA levels were found between EDS-treated and control groups. The size of TRH mRNA was characterized as about 1.2 kb in hamster testes, which was smaller than that observed in the rat hypothalamus (1.6 kb) and in the rat testis (2.0 kb). Further studies by RNase H digestion revealed the presence of smaller TRH transcripts in the hamster testes than those in the rat testis. No hybridization signal for TRH mRNA was detected by RNase protection assay, when a rat TRH riboprobe was applied to hamster testis RNA, suggesting the limited homology of TRH gene sequences between these two species. Our results demonstrate that both TRH and TRH-R genes are expressed in Siberian hamster testes, and a significant increase of TRH-R mRNA levels occurs in the Leydig cell eliminated hamster testes. Unlike the rat testicular TRH mRNA mainly detected in Leydig cells, in hamster TRH mRNA could also be detected in other testicular compartment.

Neurohypophyseal vasopressin in the Syrian hamster: response to short photoperiod, pinealectomy, melatonin treatment, or osmotic stimulation

In the present study, the effect of photoperiod on vasopressin content in the pituitary neurointermediate lobe (NIL), as well as the ability of pinealectomy to prevent and melatonin to mimic the short photoperiod-induced changes in NIL vasopressin were studied in male Syrian hamsters. The ability of melatonin to modify the hyperosmotically stimulated vasopressin release was also determined. Exposure to short photoperiod (SD) for 4 or 10 weeks increased vasopressin content in the hamster NIL. In long photoperiod (LD)-exposed hamsters, pinealectomy induced a decrease in NIL vasopressin content, whereas no effect of melatonin injections on vasopressin storage in the NIL was detected. In SD-exposed animals, pineal removal failed to alter vasopressin content in the NIL. Hypertonic saline administration led to the expected decrease in vasopressin content in the NIL both in vehicle- and melatonin-treated animals. The hyperosmotically stimulated release of vasopressin was not modified by previous treatment with melatonin. The data from the present study show that, in male Syrian hamsters, exposure of animals to SD increases the vasopressin content in the posterior pituitary, but these changes appear not to be mediated by SD-induced changes in melatonin secretion. Furthermore, the exposure of animals to SD prevents the pinealectomy-induced changes in NIL vasopressin content. Melatonin does not modify the hyperosmotically stimulated vasopressin release in the male Syrian hamster.

Overexpression of bovine growth hormone in transgenic mice is associated with changes in hepatic insulin receptors and in their kinase activity

Transgenic mice expressing a hybrid gene produced by linking the promoter regulatory region of phosphoenolpyruvate carboxykinase (PEPCK) gene to the bovine growth hormone (bGH) gene, were used to investigate the effects of GH on insulin binding and insulin dependent tyrosine kinase activity of hepatic insulin receptors. Transgenic mice had normal levels of blood glucose, despite hyperinsulinemia, indicating that these animals were insulin resistant. The number of insulin receptors in the liver of transgenic mice was significantly decreased in both the particulate fraction (25%) and the solubilized membranes (40%) indicating that expressed (functional) and non-expressed (cryptic) receptors were affected. Scatchard analysis of competitive binding curves for insulin indicated that the affinity of the receptor did not differ between transgenic and normal mice. Insulin dependent tyrosine kinase activity in insulin receptors partially purified by wheat germ agglutin (WGA) agarose chromatography from solubilized liver membranes, was measured. The stimulatory action of insulin on phosphorylation of the synthetic substrate (a copolymer Glu-Tyr, 4:1) was increased 100% in transgenic, as compared to normal mice, using the same binding activity. Since transgenic mice are hyperinsulinemic, it is likely that the decreased insulin binding in this group reflects down regulation of the expressed and non-expressed insulin receptors, and the increased kinase activity represents a compensatory mechanism. We conclude that alterations in the insulin receptor number and in the tyrosine kinase activity develop in response to changes in insulin levels. Thus, insulin resistance detected in the liver of transgenic mice overexpressing GH may be due to post receptor defects.

Systemically administered gonadotrophin-releasing hormone enhances copulatory behaviour in castrated, testosterone-treated hyperprolactinaemic male rats

Hyperprolactinaemic male rats exhibit deficits in copulatory behaviour which can be reversed by a single injection of GnRH. We tested whether systemically administered GnRH can stimulate copulatory behaviour independently of LH-mediated increases in plasma testosterone levels. Gonadectomized, pituitary-grafted adult male Fischer 344 rats bearing implants of 5, 10 or 20 mm capsules of testosterone were administered a single injection of 500 ng GnRH or saline s.c., 30 min prior to copulation tests. Pituitary-grafted castrates displayed copulatory deficits, relative to sham-operated castrates with identical levels of testosterone replacement. Administration of 500 ng GnRH to pituitary-grafted castrates bearing 10 mm testosterone implants significantly increased the proportion of rats that mounted, intromitted and ejaculated during a 30 min test. This treatment also reduced significantly the latency of intromission and ejaculation, and increased significantly the frequency of intromission. The copulatory behaviour of the sexually unresponsive, pituitary-grafted castrates bearing 5 mm testosterone implants, or of the more sexually responsive castrates bearing 20 mm testosterone implants, was not altered significantly by GnRH injections. These results support the hypothesis that copulatory deficits in moderately hyperprolactinaemic rats are due in part to reduced hypothalamic GnRH release, and suggest that GnRH can stimulate sexual behaviour in these animals via mechanisms that are independent of luteinizing hormone-induced testosterone release. However, a threshold level of testosterone (achieved with 10 mm implants) appears to be required for GnRH to elicit this effect.

Oxytocin and prolactin release after hypertonic saline administration in melatonin-treated male Syrian hamsters

The aim of the present investigations was to examine the effects of melatonin (Mel) on oxytocin (OT) release under conditions of osmotic stimulation, brought about by hypertonic saline administration, as well as to determine whether osmotically stimulated OT release in Mel-treated Syrian hamster is associated with alterations in the release of prolactin (PRL) and in norepinephrine (NE) and dopamine (DA) content in the hypothalamus. In both Mel- and vehicle-treated hamsters, injection of hypertonic saline was followed by a significant decrease in OT content in the pituitary neurointermediate lobe (NIL) and elevation of plasma OT and PRL levels. Melatonin injections had no significant affect on NIL OT content in either isotonic- or hypertonic-saline treated animals. Pretreatment with Mel did not alter plasma OT or PRL levels in isotonic saline-injected animals. However, Mel facilitated the release of OT, but prevented the release of PRL after hypertonic saline administration. Melatonin treatment reduced hypothalamic NE content (but not that of DA) in isotonic-saline treated animals. After osmotic stimulation, hypothalamic content of NE and DA was significantly lower in Mel-treated than in vehicle-treated animals. Data from the present study suggest that the osmotically-stimulated release of OT and PRL seems to be related to the activation of noradrenergic rather than dopaminergic transmission. Both dopaminergic and noradrenergic transmission may be, however, involved in mediating the effects of Mel on the osmotically-activated OT and PRL release.

Developmental aspects of prolactin receptor gene expression in fetal and neonatal mice

The prolactin receptor (PRL-R), a member of the hematopoietin cytokine receptor superfamily, is widely distributed among mammalian tissues. To understand better the potential sites of action and onset of potential PRL responsiveness, the developmental distribution pattern of PRL-R mRNA expression in fetal and neonatal mice was examined. Fetal mouse tissues were collected at distinct stages from timed pregnancies. Following extraction of total RNA, onset of gene expression was evaluated via reverse transcription-polymerase chain reaction (RT-PCR) and Southern hybridization was employed for verification. Expression of PRL-R mRNA was first observed on day 14 in the liver and cranium and on day 15 in the kidney, lung and thymus gland. Pituitary and adrenal glands were positive for PRL-R at day 18 of gestation through to day 1 of postnatal life. Neither whole fetuses prior to day 14 (days 10-13) of gestation nor skin and bladder tissues from 2-day-old mice generated detectable RT-PCR signals for PRL-R. The presence of PRL-R mRNA in fetal thymus and spleen tissues suggests a possible role for PRL in the development of the immune system. Prolactin may act directly on the pituitary to influence its own secretion and/or that of other pituitary-derived factors, as evidenced by the presence of PRL-R mRNA in the pituitary glands of fetal and 1-day-old mice. These data are the first to show the presence of PRL-R gene expression in various organ systems in fetal mice and suggest that PRL is among several factors necessary to coordinate developmental activities.

Genetic determination of coat color affects testicular steroidogenesis in the Mustela vison

Coat color genes in mammals are known to be developmental genes with wide pleiotropic effects. The present study was undertaken to study testicular steroidogenesis in American Mink (Mustela vison) of various coat color phenotypes. No differences in testicular steroid levels were observed between fertile and infertile mink with the standard phenotype and genotype (BB jj MM PP). Mink with the opaline phenotype and genotype (bb mm pp), were found to have in their testes, 20-40% higher levels of progesterone, five times higher levels of 17-hydroxyprogesterone, and eight times higher levels of testosterone, than the corresponding values in other mink. No other differences were observed among the different types of mink. Since the genotype of the opaline mink differs from the other mink studied, only in their combination at the pastel (b) and moyle (m) loci, their bb mm genotype could be assumed to be responsible for the increase in testicular steroids.

Testicular gamma-aminobutyric acid and circulating androgens in Syrian and Djungarian hamsters during sexual development

Several factors, besides luteinizing hormone (LH), participate in the modulation of testicular function. A number of neurotransmitters are reported to be involved in this process, including a stimulatory action of gamma-aminobutyric acid (GABA) on steroidogenesis in the rat testis. The purpose of this study was to investigate the testicular pattern of GABA and glutamic acid, one of its main precursors, during sexual maturation in two seasonally breeding species: Syrian (golden) and Djungarian hamsters. Plasma androgen levels were also measured. The animals were maintained under long-day photoperiod (16:8, L:D) and were killed at 23, 30, 36, 46, 60, and 90 days of age. A different pattern of developmental changes in body and testicular weight was observed in these two species. GABA was present in the testes at all ages studied. GABA concentration and content showed a sharp elevation in the prepubertal period in golden as well as Djungarian hamsters. However, glutamic acid concentrations remained nearly constant during development in both species. Glutamic acid content increased gradually with age in the golden hamster, while a marked peak at 36 days of age was detected in the Djungarian hamster. Plasma testosterone and dihydrotestosterone levels were maximal at pubertal age in both species. The plasma levels of 5 alpha-androstane-3 alpha, 17 beta-diol increased significantly at 30 days of age in the golden hamster while in Djungarian hamsters this steroid remained unchanged. These results suggest that glutamic acid may serve as a precursor for GABA biosynthesis in the testis. In addition, changes in testicular GABA and plasma androgen levels might reflect a modulatory effect of this neurotransmitter in the acquisition of steroidogenic capability during development.

The effects of short photoperiod, pinealectomy, and melatonin treatment on oxytocin synthesis and release in the male syrian hamster

The pineal gland has been shown to affect plasma oxytocin (OT) levels, but the mechanism of this action is not apparent. In the present study, the ability of the photoperiod to affect plasma OT levels, neurointermediate lobe (NIL) OT content, and hypothalamic OT mRNA levels was studied in male Syrian hamsters. In addition, the ability of pinealectomy to prevent and melatonin (MEL) to mimic the short photoperiod-induced changes were also determined. Exposure to short days (SD) led to the expected decrease in testes weight and plasma PRL levels, but plasma OT levels were unchanged. However, NIL OT content was increased in the SD-exposed animals. Hypothalamic OT mRNA levels were not significantly altered by SD exposure. Pinealectomy blocked the effects of SD on testes weight, whereas afternoon MEL injections mimicked the effects of SD. In long day (LD)-exposed hamsters, pinealectomy induced a decrease in NIL OT content without altering hypothalamic OT mRNA levels. In SD-exposed animals, NIL OT content was not affected by pinealectomy. Melatonin injections had no significant effect on NIL OT content or hypothalamic OT mRNA levels. The data from the present study suggest that exposure of male Syrian hamsters to short photoperiods influences some aspects of OT synthesis and/or transport to produce its increased accumulation in the NIL, but does not affect OT release. These changes are apparently not the result of SD-induced changes in MEL secretion, but conceivably could be related to the previously documented effects of SD on hypothalamic catecholamine turnover.

Increased plasma corticosterone levels in bovine growth hormone (bGH) transgenic mice: effects of ACTH, GH and IGF-I on in vitro adrenal corticosterone production

Previous work from our laboratory provided evidence for increased plasma corticosterone levels in mice transgenic for human and bovine growth hormone (GH). Corticosterone was elevated in both sexes, under both basal and ether-induced stress conditions. The objectives of the present study were to investigate the in vitro adrenal sensitivity to ACTH, GH and/or IGF-I in normal and bGH transgenic mice, to examine plasma corticosterone levels at different times of the day, and to determine plasma levels of ACTH in these animals. For the measurement of plasma corticosterone and ACTH levels, transgenic and normal siblings were housed 2 per cage and decapitated simultaneously within 20 seconds of the first disturbance of the cage. The corticosterone production by in vitro adrenal incubations did not differ between adrenals from normal and transgenic mice at the basal level or in the presence of different doses of ACTH. Growth hormone or IGF-I did not have any effect on corticosterone production in vitro when given alone, and did not modify the effects of ACTH on the accumulation of corticosterone production in vitro when alone, and did not modify the effects of ACTH on the accumulation of corticosterone in the media. Plasma corticosterone concentrations were higher in transgenic than in normal animals in both morning and evening. Plasma concentrations of ACTH in animals killed in the morning were sharply increased in transgenic males as compared with their normal siblings. The results indicate that increased circulating levels of corticosterone in transgenic mice are not due to a potentiation of ACTH actions by GH or IGF-I, but rather to a chronic increase in plasma ACTH levels. The increase in ACTH is presumably a reflection of GH actions in the hypothalamic-pituitary system.

Influence of hypothalamus and ovary on pituitary function in transgenic mice expressing the bovine growth hormone gene and in growth hormone-deficient Ames dwarf mice

Female transgenic mice expressing the bovine (b) growth hormone (GH) gene exhibit reduced fertility, and GH-deficient dwarf mice are sterile. However, in these animals, the influence of the hypothalamus and ovary on pituitary functions is poorly understood. To address this issue, female Ames dwarf mice received twice-daily injections of either vehicle or bGH (25 micrograms/injection per mouse) for 12 days. Vehicle-injected normal female siblings served as controls. On Day 7, these mice and adult female transgenic mice expressing the bGH gene with mouse metallothionein-I promoter, as well as their normal littermates, were ovariectomized. Two experiments were conducted with each group of mice: on Day 8 after ovariectomy of transgenic mice and their normal siblings, and on Day 4 after ovariectomy of dwarf mice and their normal siblings. In experiment I, mice were primed with 0.5 micrograms estradiol benzoate (EB) in oil and 24 h later received injections of 5 micrograms EB/100 g BW. The next day, these mice received injections of either saline or GnRH (1 ng/g BW), and 15 min later blood was obtained via heart puncture under anesthesia. In experiment II, ovariectomized mice either received injections of oil or were primed with 0.5 micrograms EB and 24 h later treated with 10 micrograms EB/100 g BW in oil. On the next day, blood was obtained as in experiment I. In transgenic mice and in their normal littermates, plasma bGH, FSH, LH, and prolactin levels were measured. In dwarf mice and in their normal siblings, plasma LH and insulin-like growth factor (IGF)-I levels were determined. Bovine GH was secreted in transgenic mice, but this hormone was not detectable in normal siblings. Expression of the GH gene resulted in attenuated (p < 0.001) FSH response to GnRH treatment, while LH response was similar to that in controls. The ovariectomy-induced increases in plasma FSH (p < 0.001) and LH (p < 0.05) levels were decreased in transgenic mice. The absolute circulating FSH levels were also decreased (p < 0.001) in estrogen-treated transgenic mice, while plasma LH level were similar in normal mice and in mice bearing the bGH gene. The ovariectomized transgenic mice were hyperprolactinemic. In ovariectomized dwarf mice, IGF-I was not detectable, and GH treatment induced its secretion. In these mice, plasma LH concentrations were reduced (p < 0.05), and GH treatment increased (p < 0.01) LH secretion. In oil-injected ovariectomized dwarf mice, the plasma LH response to GnRH treatment was reduced (p < 0.001), and GH administration normalized this response. The negative feedback effect of estrogen on LH secretion was decreased (p < 0.025) in dwarf mice. These indicate that the alterations in gonadotropin secretion are dependent on the duration of exposure of neuroendocrine system to bGH and that bGH alters the effects of GnRH and estrogen on gonadotropin secretion in these two widely divergent experimental mouse models.

Effects of photoperiod on hypothalamic and anterior pituitary tachykinins of male Siberian hamsters during development

In previous reports from our laboratory we showed that the hypothalamus and especially the anterior pituitary gland of the Siberian hamster contain very high concentrations of substance P- and neurokinin A-like immunoreactive substances, as compared with other common laboratory rodents. It was thought, therefore, that a study of the developmental changes of these two tachykinins in the hypothalamus and anterior pituitary of male Siberian hamsters would be of interest. In addition, as this species is very sensitive to changes in environmental light, these studies were carried out in animals kept under short or long daily photoperiods. The results of this investigation show that in male Siberian hamsters, photoperiod did not markedly influence the hypothalamic concentrations of substance P- and neurokinin A-like immunoreactive substances, which steadily increased from prepuberty up to 40-50 days of age. The influence of photoperiod, however, was much more marked on tachykinin levels in the anterior pituitary gland, because the developmental increase of tachykinins in this gland was significantly more evident in animals kept under short daily photoperiods than in animals under long daily exposure to light. These results are similar to those previously obtained in female Siberian hamsters, in spite of the fact that estradiol and testosterone seem to have opposite effects on anterior pituitary tachykinin concentrations. It seems, therefore, that, in addition to gonadal steroids, other factors may be important mediators of the tachykinin response to photoperiods in the anterior pituitary of the male Siberian hamster.

Effects of growth hormone on neuroendocrine function

Although the role of growth hormone (GH) in the control of reproductive functions is not well understood, there is considerable evidence that the states of both GH deficiency and GH excess are typically associated with reproductive deficits. To identify the possible involvement of functional alterations in the hypothalamic-pituitary system in producing these deficits, we are studying neuroendocrine function related to reproduction in transgenic animals overexpressing GH, in animals with congenital GH deficiency, and in animals with selective immunoneutralization of GH. The results indicate that GH acts on the hypothalamus to alter dopaminergic and noradrenergic control of prolactin and gonadotropin release. Life-long elevation of GH levels outside the physiological ranges disrupts feedback control of luteinizing hormone (LH) release by gonadal streroids. Plasma LH and follicle-stimulating hormone (FSH) levels and feedback control of LH release are also abnormal in GH-deficient animals indicating that physiological levels of endogenous GH are normally involved in the control of gonadotropin release. Differences between the effects of bovine vs. human GH in transgenic mice and differential effects of GH deficiency in mice and rats should facilitate identification of the mechanisms involved in the actions of GH on the hypothalamic-pituitary system.

Development of a specific radioimmunoassay for neuropeptide K: its application to Siberian hamster tissues

In this report we describe the development of a radioimmunoassay for neuropeptide K (NPK) that is not influenced by the presence of free neurokinin A (NKA) or neuropeptide gamma (NPG). The antisera for the radioimmunoassay were obtained by immunizing rabbits with beta-preprotachykinin A fragment 69-91. This fragment contains the sequence 1-20 of NPK and has only the sequence 18-20 (Gly-His-Gly) in common with NPG. Two antisera were obtained, neither cross-reacting with NKA or NPG. The specificity of these antisera was confirmed by the finding of a major immunoreactive peak in the eluates obtained from HPLC separation, corresponding to NPK. This radioimmunoassay was applied to the determination of NPK in hypothalamic and anterior pituitary extracts from Siberian hamsters. Hypothalami from male and female hamsters contained similar concentrations of NPK, although slightly higher in males. NPK levels in the hypothalamus of female Siberian hamsters showed few fluctuations during the estrous cycle, with the levels measured at estrus being significantly lower than at proestrus, diestrus I, and diestrus II. In anterior pituitaries from Siberian hamsters the concentrations of NPK were significantly lower in females than in males. This radioimmunoassay has demonstrated the presence of NPK in the anterior pituitary gland of the Siberian hamster, and it should be very useful for future studies on mechanisms of regulation of NPK secretion in different tissues.

Age-related effects of ectopic pituitary transplants on the activation of Ames dwarf mouse lymphocytes in vitro

Prolactin (PRL), one of the anterior pituitary hormones, has been implicated in the development and maintenance of immune system function. The following experiments were conducted to evaluate age-related effects of PRL on immune system activity in a PRL-deficient mouse model, the Ames dwarf. Two- and ten-month-old dwarf and phenotypically normal male mice were used. Six dwarf mice from each age group received a surgically implanted pituitary (under the kidney capsule) from a normal donor female mouse. Six different dwarfs were similarly operated but had no pituitary graft inserted and will be referred to as sham-operated dwarfs. Three weeks following surgery, spleens were removed from the dwarfs and age-matched normal mice and splenocytes isolated. The two age groups will subsequently be referred to as "3-month-old" and "11-month-old", respectively, The splenocytes were used in mitogen-induced (concanavalin A (Con A); phytohemagglutinin (PHA); lipopolysaccharide (LPS)) proliferation assays, and histopaque isolated lymphocytes were used for T cell surface marker determination. Pituitary grafting increased body weights of dwarf mice although the weights were less than control mice independent of age. A similar pattern was observed for total number of splenocytes per spleen. In young animals, the relative number of splenocytes per gram of body weight increased in pituitary grafted dwarfs reaching values of control mice, whereas much smaller differences were observed in older animals. The relative percentage of CD4+ cells was reduced (P<0.01) in 3-month-old pituitary-grafted mice, compared with sham-operated dwarf mice, while no differences were observed between sham- operated dwarf and normal mice. Pituitary grafting did not affect the numbers of CD8+ cells. In 11-month-old animals, the relative percentages of CD4+ and CD8+ cells were greater (P<0.05) in sham-operated dwarf than in normal mice but not affected by pituitary grafting. At 3 months of age, proliferation of splenocytes in response to Con A was increased (P<0.05) in sham-operated dwarfs and reduced (P<0.05) in pituitary-grafted dwarfs compared with normal mice. In contrast, PHA stimulation of splenocytes was decreased (P<0.05) in sham-operated compared with normal, and was still lower in mice receiving an ectopic pituitary. Substantially different responses were obtained in 11-month-old animals. In response to Con A, splenocytes from sham-operated dwarfs exhibited a reduced (P<0.05) proliferative capacity as compared with normals, and proliferation was increased in pituitary-grafted as compared with sham-operated dwarfs. In the presence of PHA, the proliferative capacity of splenocytes was greater (P<0.05) in sham-operated dwarfs as compared to normals and pituitary grafting normalized this parameter. These results demonstrate differential effects of PRL deficiency (sham-operated dwarfs versus normal) and of PRL replacement (pituitary-grafted versus sham-operated) in young as compared with old dwarf mice on immune system activation.

Melatonin inhibits oxytocin and vasopressin release from the neurointermediate lobe of the hamster pituitary

The aim of this investigation was to study whether melatonin affects the release of oxytocin and vasopressin by the pituitary neurointermediate lobe of the Syrian hamster in vitro. The effect of melatonin was studied on the unstimulated (pre- and post-K+ -stimulated) release of oxytocin and vasopressin and on the response to K+ stimulation. Melatonin significantly inhibited unstimulated release of these hormones in all concentrations (10(-11) M, 10(-9) M and 10(-7) M) studied. K+ -stimulated release of oxytocin and vasopressin was significantly decreased by the 10(-9) M dose of melatonin. It is concluded that melatonin is active in modifying the release of these peptides in the Syrian hamster neurointermediate lobe, as it has been previously demonstrated in the rat hypothalamus.

In vivo and in vitro effects of neuropeptide K and neuropeptide gamma on the release of growth hormone

There is anatomical and experimental evidence suggesting that tachykinins have a role in the regulation of secretion of anterior pituitary hormones. In this investigation, the effects of neuropeptide K and neuropeptide gamma on the secretion of GH were studied in vivo and in vitro. Injections of neuropeptide K into the third ventricle of freely moving, ovariectomized rats resulted in a significant increase of plasma GH, but neuropeptide gamma induced no significant changes in these levels, although it did induce a significant increase in GH pulse height. In vitro, neither neuropeptide K nor neuropeptide gamma had any apparent effect on GH release from hemipituitaries incubated for 2 h. These results suggest that neuropeptide K may have a modulatory function in the regulation of GH secretion from the anterior pituitary, through an action exerted at the hypothalamic level, and the effects of neuropeptide gamma seem to be more marginal.

The Ames dwarf gene is required for Pit-1 gene activation

The tissue-specific POU domain transcription factor Pit-1 is required for phenotypic specification of three cell types in the anterior pituitary gland of mammals: somatotropes, thyrotropes, and lactotropes. Mutations in the Pit-1 gene, as first described in Snell and Jackson dwarf (dw) mice, led to pituitary hypoplasia due to lack of these three cell types, as well as hypothyroidism and dwarfism because of deficiency of thyroid-stimulating hormone and growth hormone, respectively. The Ames dwarf (df) exhibits a phenotype identical to that of Pit-1 mutated mice but the defective gene has been mapped to a locus on mouse chromosome 11, distinct from the Pit-1 gene located on mouse chromosome 6. Our studies indicate that initial activation of the Pit-1 gene is deficient in the Ames dwarf. This suggests that the df gene is required for activation of the Pit-1 gene and provides evidence for a hierarchy of tissue-specific factors required for cellular commitment in the anterior pituitary gland.

Developmental expression of the prolactin receptor gene in rat gonads

The prolactin receptor (PRLR) is a member of the cytokine/prolactin/GH receptor family, and it is widely expressed in various mammalian tissues. Expression of the two different forms of PRLR, differing in the length of their cytoplasmic domains, was studied in rat gonads during fetal and postnatal development. The two forms of PRLR mRNA were analyzed by reverse transcription (RT)-PCR using primer pairs specific for the different forms. The specificity of the cDNA species generated by RT-PCR was verified by Southern hybridization using nested 32P-labeled oligonucleotides. The results indicated that both forms of PRLR mRNA are expressed in the rat testis and ovary, which is in agreement with previous reports. The onset of expression of the two PRLR forms occurs on day 14.5 of fetal life in rat testis. In the ovary, the long form of PRLR mRNA appears 1 day before the short form, i.e. these forms begin to be expressed on fetal days 14.5 and 15.5 respectively. In situ hybridization with antisense cRNA probes specific to each form of the PRLR mRNAs demonstrated specific hybridization of both forms, localized in Leydig cells from day 18.5 of fetal life and at the postnatal ages studied. Compared with our previous findings concerning the ontogeny of LH receptor gene expression, PRLR gene expression starts earlier in development and exhibits no sexual dimorphism. The presence of two forms of PRLR mRNA in the fetal gonads suggest that they might play differential roles in gonadal development and function.