Search PubMed⌕ Search

Biomedical subjects

R T Gladwell

Publications and source records attributed to R T Gladwell.

At least 19 recordsLinked to original sources
PubMed2003

Ovarian follicle development in the laying hen is accompanied by divergent changes in inhibin A, inhibin B, activin A and follistatin production in granulosa and theca layers.

To study the potential involvement of inhibin A (inhA), inhibin B (inhB), activin A (actA) and follistatin (FS) in the recruitment of follicles into the preovulatory hierarchy, growing follicles (ranging from 1 mm to the largest designated F1) and the three most recent postovulatory follicles (POFs) were recovered from laying hens (n=11). With the exception of <4 mm follicles and POFs, follicle walls were dissected into separate granulosa (G) and theca (T) layers before extraction. Contents of inhA, inhB, actA and FS in tissue extracts were assayed using specific two-site ELISAs and results are expressed per mg DNA. InhB content of both G and T followed a similar developmental pattern, although the content was >4-fold higher in G than in T at all stages. InhB content was very low in follicles <4 mm but increased ~50-fold (P<0.0001) to peak in 7-9 mm follicles, before falling steadily as follicles entered and moved up the follicular hierarchy (40-fold; 8 mm vs F2). In stark contrast, inhA remained very low in prehierarchical follicles (< or =9 mm) but then increased progressively as follicles moved up the preovulatory hierarchy to peak in F1 (approximately 100-fold increase; P<0.0001); In F1 >97% of inhA was confined to the G layer whereas in 5-9 mm follicles inhA was only detected in the T layer. Both inhA and inhB contents of POFs were significantly reduced compared with F1. Follicular actA was mainly confined to the T layer although detectable levels were present in G from 9 mm; actA was low between 1 and 9 mm but increased sharply as follicles entered the preovulatory hierarchy (approximately 6-fold higher in F4; P<0.0001); levels then fell approximately 2-fold as the follicle progressed to F1. Like actA, FS predominated in the T although significant amounts were also present in the G of prehierarchical follicles (4-9 mm), in contrast to actA, which was absent from the G. The FS content of T rose approximately 3-fold from 6 mm to a plateau which was sustained until F1. In contrast, the FS content of G was greatest in prehierarchical follicles and fell approximately 4-fold in F4-F1 follicles. ActA and FS contents of POFs were reduced compared with F1. In vitro studies on follicle wall explants confirmed the striking divergence in the secretion of inhA and inhB during follicle development. These findings of marked stage-dependent differences in the expression of inhA, inhB, actA and FS proteins imply a significant functional role for these peptides in the recruitment and ordered progression of follicles within the avian ovary.

Activins↗
PubMed2002

Hox gene expression in adult tissues with particular reference to the adrenal gland.

In comparison to the embryo, very little work has been carried out on the expression and role of Hox genes in the adult animal. An expression profile of all 39 vertebrate Hox genes on a select panel of adult human tissues reveals that in fact these genes are widely expressed throughout the adult human and a colinear pattern of expression is displayed similar to that of the developing embryo. Of particular interest is the abundance of Hox genes that are expressed within the adult adrenal gland. Adrenal cortical cells are continuously renewed to sustain production of zonal steroids. Cell proliferation occurs at the periphery of the cortex and cells are then displaced centripetally, phenotypically switching as they migrate through the gland before undergoing apoptosis at the zona reticularis/medullary boundary. It is still unclear which mechanisms cause the cells to differentiate as they cross the zonal boundaries and we hypothesise that Hox genes may be involved in the phenotypic switching of the adrenocortical cells. In situ hybridisation experiments were carried out on adult rat adrenal gland sections and Hox gene expression was localized within the zonal borders, coinciding with the localization of cells that undergo phenotypic differentiation, and thus supporting our hypothesis that Hox genes may be involved in the phenotypic switching of the adrenocortical cells. As in the developing embryo, the genes display colinear expression with the 3' Hox genes being expressed within the outer gland and the 5' genes within the inner zones.

Adrenal Glands↗
PubMed2002

Modulatory effects of gonadotrophins and insulin-like growth factor on the secretion of inhibin A and progesterone by granulosa cells from chicken preovulatory (F1-F3) follicles.

The aim of this study was to compare the actions and interactions of gonadotrophins (LH and FSH) and an analogue of insulin-like growth factor I (LR3-IGF-I) on the secretion of inhibin A, inhibin B and progesterone by cultured chicken granulosa cells derived from the three largest (F1--F3) follicles of the preovulatory hierarchy. Treatment with LH or FSH promoted marked dose-(P < 0.0001) and time- (P < 0.0001) dependent increases in both inhibin A and progesterone secretion, with the magnitude of response (< 15-fold compared with basal) increasing over time in culture. Concentrations of inhibin B were below the detection limit in all samples. Initially, F1 cells were more LH-responsive than were F3 cells in terms of progesterone secretion (P < 0.02) but this difference between follicles decreased over time in culture. In contrast, LH-induced inhibin A secretion tended to be highest from F3 cells, although this was not significant. Cells from F3 follicles were consistently more FSH-responsive than F1 cells in terms of both progesterone (P < 0.01) and inhibin A (P < 0.02) secretion. Initially, F1 cells were more responsive to LR3-IGF-I than were F3 cells in terms of progesterone secretion (P < 0.001) but were less responsive in terms of inhibin A secretion (P < 0.001). Again, these inter-follicle differences decreased over time in culture (not significant on day 3 of treatment). Co-treatment experiments showed that LR3-IGF-I enhanced both LH- and FSH-induced secretion of inhibin A and progesterone in a time- (P < 0.001) and follicle- (P < 0.001) dependent way. Initially, F1 cells showed highest LR3-IGF-I enhancement of LH-induced inhibin A and progesterone secretion; in contrast, F3 cells showed the highest LR3- IGF-1 enhancement of FSH-induced inhibin A and progesterone secretion. These inter-follicle differences persisted over time in the case of FSH-induced hormone responses but not in the case of LH-induced responses, even though the relative degree of LR3-IGF-I enhancement increased markedly over time. Collectively, these data support a positive role for IGF-I, presumably of thecal origin, as an amplifier of gonadotrophin action on granulosa cell inhibin A and progesterone production by preovulatory chicken follicles.

Animals↗
PubMed2002

Differential effects of activin A on basal and gonadotrophin-induced secretion of inhibin A and progesterone by granulosa cells from preovulatory (F1-F3) chicken follicles.

Previous work has shown that activin A is expressed selectively within the theca rather than the granulosa layer of preovulatory chicken follicles. In the present study, this finding was verified and the potential paracrine actions of activin A on basal and gonadotrophin-induced secretion of inhibin A, inhibin B and progesterone by granulosa cells from the three largest preovulatory follicles (F1-F3) were investigated. Treatment with activin A (0, 0.25, 2.5 and 25 ng ml(-1)) alone increased inhibin A secretion markedly in a follicle- and time-dependent manner, with the greatest response (up to 15-fold increase; P < 0.0001) in F1 follicles after 3 days of treatment. In contrast, activin A alone had no effect on progesterone output at any time. Cells from F3 follicles were more responsive to FSH than were F1 cells in terms of both inhibin A (P < 0.02) and progesterone (P < 0.01) secretion. Furthermore, activin A greatly enhanced FSH-induced secretion of both inhibin A (up to tenfold; P < 0.0001) and progesterone (up to sixfold; P < 0.0001). In terms of LH-induced inhibin A and progesterone secretion, cells from F1, F2 and F3 follicles showed similar responses. Co-treatment with activin A enhanced LH-induced secretion of inhibin A markedly (up to ninefold; P < 0.0001) but had only a marginal effect on LH-induced progesterone secretion (up to twofold; P < 0.001). The presence of activin receptor subtypes IA, IB, IIA and IIB in cultured granulosa cells from F1, F2 and F3 follicles was demonstrated using immunocytochemistry. These findings support the hypothesis that activin A secreted by the theca layers of avian preovulatory follicles exerts a local paracrine action on granulosa cells to modulate 'basal' inhibin A secretion and to upregulate gonadotrophin-induced secretion of both inhibin A and progesterone. However, the extent to which this local role of activin A contributes to the generation of the preovulatory LH-progesterone surge remains to be established.

Activin Receptors↗
PubMed2001

Characterization of a serine protease that cleaves pro-gamma-melanotropin at the adrenal to stimulate growth.

The adrenal gland requires stimuli from peptides derived from the ACTH precursor, pro-opiomelanocortin (POMC), to maintain its tonic state. Studies have proposed that a specific postsecretional cleavage of the nonmitogenic N-terminal 16 kDa fragment, also known as pro-gamma-melanotropin (pro-gamma-MSH), is required, releasing shorter fragments that promote adrenal growth. Here, we provide evidence for this hypothesis by the cloning and characterization of a serine protease that is upregulated during growth of the adrenal cortex. It is expressed exclusively in the outer adrenal cortex, the site of cell proliferation, and in the Y1 adrenal cell line. We also show that it is required for growth of Y1 cells, remains bound to the cell surface, and cleaves its substrate, pro-gamma-MSH, at a specific bond.

Adrenal Cortex↗
PubMed2001

Changes in plasma inhibin A levels during sexual maturation in the female chicken and the effects of active immunization against inhibin alpha-subunit on reproductive hormone profiles and ovarian function.

Inhibins and activins are firmly implicated in the control of pituitary FSH secretion and ovarian follicular development in mammals. As in mammals, inhibin A and activin A are expressed in the preovulatory follicles of birds, and a defined ovulation cycle for inhibin A has recently been demonstrated in the laying hen. To investigate further the role of inhibin-related proteins in developing pullets, circulating concentrations of inhibin A, inhibin B, total immunoreactive inhibin alpha-subunit (ir-alpha), activin A, LH, FSH, and progesterone were measured from the juvenile state through to sexual maturity in 22 birds. In the 11 birds assigned to control groups, plasma inhibin A levels were low from 7 to 13 wk of age rising about threefold to a peak at Week 19 after which levels fell slightly to a plateau level characteristic of adult hens. Plasma inhibin A levels were negatively correlated with FSH (r = -0. 33; P: < 0.001) and positively correlated with progesterone (r = 0. 67; P: < 0.001) and ir-alpha (r = 0.53; P: < 0.001). Plasma ir-alpha levels were much higher than inhibin A levels although the relative differences varied with age. Plasma levels of inhibin B and activin A were below assay detection limits at all times. The remaining group of 11 birds was actively immunized (IMM) against a synthetic chicken inhibin alpha-subunit peptide (amino acids 1-26). The IMM generated circulating antibodies that bound native bovine inhibin A but altered neither plasma FSH nor progesterone levels relative to control birds at any stage of development nor the timing of first oviposition in week 19. Apart from a transient decline 1 wk after primary IMM, plasma LH concentrations did not differ from controls. Comparison of the numbers and size-class distribution of ovarian follicles at 29 wk showed an approximate twofold increase in the number of 8- to 9.9-mm-diameter follicles (control; 1.82 +/- 0.44 vs. IMM; 3.91 +/- 0.89; P: < 0.05), a size class that corresponds to follicles that have just joined the preovulatory hierarchy. The numbers of growing follicles in other size-classes and the sizes of hierarchical F(1)-F(7) follicles were not altered by IMM. However, the number of postovulatory follicles increased (control 3.73 +/- 0. 20 vs. IMM 5.55 +/- 0.28; P: < 0.01), and significantly more (P: < 0. 02) immunized hens laid two eggs within a 24-h period on at least one occasion (control 1 of 11 vs. IMM 9 of 11). The IMM increased (P: < 0.05) activin A content of F(1) and F(2) theca layers and decreased (P: < 0.05) activin A content in F(3) and F(4) granulosa layers, raising the possibility of a local intraovarian role of activin in mediating the response to IMM. These findings support a role for inhibin A in regulating the entry of follicles into the preovulatory hierarchy in the chicken, although further studies are required to establish the mechanism by which inhibin IMM increases the rate of follicle selection and ovulation without raising plasma FSH.

Activins↗
PubMed2000

Excessive placental secretion of neurokinin B during the third trimester causes pre-eclampsia.

Pre-eclampsia is a principal cause of maternal morbidity and mortality, affecting 5-10% of first pregnancies worldwide. Manifestations include increased blood pressure, proteinuria, coagulopathy and peripheral and cerebral oedema. Although the aetiology and pathogenesis remain to be elucidated, the placenta is undoubtedly involved, as termination of pregnancy eradicates the disease. Here we have cloned a complementary DNA from human placental messenger RNA encoding a precursor protein of 121 amino acids which gives rise to a mature peptide identical to the neuropeptide neurokinin B (NKB) of other mammalian species. In female rats, concentrations of NKB several-fold above that of an animal 20 days into pregnancy caused substantial pressor activity. In human pregnancy, the expression of NKB was confined to the outer syncytiotrophoblast of the placenta, significant concentrations of NKB could be detected in plasma as early as week 9, and plasma concentrations of NKB were grossly elevated in pregnancy-induced hypertension and pre-eclampsia. We conclude that elevated levels of NKB in early pregnancy may be an indicator of hypertension and pre-eclampsia, and that treatment with certain neurokinin receptor antagonists may be useful in alleviating the symptoms.

Animals↗
PubMed2000

Agouti related protein in the rat adrenal cortex: implications for novel autocrine mechanisms modulating the actions of pro-opiomelanocortin peptides.

Agouti related protein (AgRP) is a recently discovered melanocortin receptors (MCR) antagonist implicated in the control of feeding behaviour. Expression of AgRP has been shown to be localized by in situ hybridization to the arcuate nucleus and median eminence of the brain, where it acts as an antagonist to the MC3 and MC4 receptors, while in the periphery the only significant expression was located in the adrenal medulla. As AgRP is only a weak antagonist of the MC2 and MC5 receptors, which are expressed principally by adipocytes and in the adrenal cortex, the question arizes as to the function of peripheral AgRP. In this study, we investigated the expression of AgRP in the rat adrenal and suggest that it is expressed in the adrenal cortex and not as previously described in the medulla. We also show that AgRP mRNA expression is upregulated in the adrenal during fasting and in the contralateral gland following unilateral adrenalectomy but not during chronic stress. These results indicate an as yet undefined role for AgRP in the periphery and are supportive of the suggestion that a further melanocortin receptor exists.

Adrenal Cortex↗
PubMed2000

Measurement of dimeric inhibins and effects of active immunization against inhibin alpha-subunit on plasma hormones and testis morphology in the developing cockerel.

Inhibins and activins are implicated as endocrine regulators of follicle-stimulating hormone production and of testicular steroidogenesis and spermatogenesis in mammals. The potential involvement of these proteins in cockerels was investigated by measurement of circulating inhibin A, inhibin B, total inhibin alpha-subunit immunoreactivity (ir-alpha), activin A, LH, FSH, and testosterone from the juvenile state through to sexual maturity. Plasma inhibin A remained low between 6 to 12 wk of age and increased approximately threefold (P < 0.05) to a prepubertal peak between Weeks 14 to 18, followed by a gradual decline to the end of the study (Week 24). Although plasma FSH levels were not correlated to inhibin A before Week 16 (r = -0.17), they were negatively correlated from Week 18 (r = -0.49; P < 0.005). Inhibin B levels were below the assay detection limit until 16 wk of age but thereafter rose steadily in parallel with FSH (r = 0.27; P < 0.02) and testosterone (r = 0.35; P < 0.005). Thus, inhibins A and B showed divergent profiles during sexual maturation. Plasma ir-alpha levels were much higher than dimeric inhibin levels throughout, although the relative difference varied with age. Plasma activin A levels were below the assay detection at all times. Juvenile cockerels were actively immunized against a synthetic chicken inhibin alpha-subunit peptide conjugate to determine effects on plasma hormones and on testicular weight, morphology, and activin A content. Immunization generated circulating antibodies that bound (125)I-bovine 32-kDa inhibin but did not affect plasma FSH or testosterone levels at any stage of development. However, immunization reduced postpubertal plasma LH levels (P < 0.05) and promoted increased testicular weight (24%; P < 0.01) and total testicular activin A content (42%; P < 0.001) at 24 wk. Testis weight of immunized birds was positively correlated with inhibin antibody titer (r = 0.61; P < 0.05). Live weight gain was not affected by immunization. Morphometric analysis of testis sections showed that inhibin immunization had no effect on the fractional volume of the seminiferous tubule wall, seminiferous tubule lumen, or interstitial tissue area. Likewise, seminiferous tubule surface area and surface area:volume ratios were not different from controls. These findings support differential roles for inhibins A and B in regulating the pituitary-testicular axis during sexual maturation in the cockerel but highlight the need for more detailed studies to distinguish between potential endocrine and local intragonadal roles of inhibin-related peptides and to elucidate the mechanism by which immunization against inhibin alpha-subunit promotes testis enlargement without raising plasma FSH.

Activins↗
PubMed2000

Circulating concentrations of inhibin-related proteins during the ovulatory cycle of the domestic fowl (Gallus domesticus) and after induced cessation of egg laying.

Circulating inhibin A, inhibin B, activin A, total immunoreactive inhibin alpha-subunit (ir-alpha inhibin), LH, FSH and progesterone concentrations were measured throughout the normal ovulatory cycle and after cessation of egg laying induced by feed restriction to investigate the potential involvement of inhibins and activins in the ovulatory cycle of the domestic hen. Plasma inhibin A varied significantly (P < 0.05) during the ovulatory cycle; the concentration was highest at the preovulatory LH surge and reached a nadir 10 h later, at about the time the F(2) follicle makes the transition to become the new F(1) follicle. Plasma FSH concentrations did not change significantly throughout the cycle and showed no correlation with inhibin A. Total ir-alpha inhibin concentrations were much higher than those of inhibin A at all stages of the ovulatory cycle and showed no correlation with inhibin A or FSH. Plasma concentrations of inhibin B and of activin A were below the detection limit of the assays in all plasma samples analysed. In the feed restriction study, plasma inhibin A and total ir-alpha inhibin showed little change until the last day of oviposition (day 0) after which they fell significantly (P < 0.05) and remained low to the end of the experiment (approximately 70-78% decrease relative to day -4). Conversely, plasma FSH increased after cessation of laying and was significantly higher (P < 0.05) from day 3 to the end of the study (approximately 50% increase on day 6 relative to day -4). Plasma FSH values were negatively correlated with inhibin A (r = -0.39; P < 0.005) and total ir-alpha inhibin (r = -0.36; P < 0.005). Plasma LH and progesterone also decreased (P < 0.05) during feed restriction. The decrease in LH preceded the terminal oviposition and the associated fall in inhibin A by 2 days; there was a positive correlation between LH and inhibin A (r = 0.35; P < 0.005). Taken together these findings support (i) a role for LH in promoting inhibin A secretion by preovulatory follicles and (ii) an endocrine role for inhibin A secreted by preovulatory follicles in the maintenance of tonic FSH secretion in laying hens.

Activins↗
PubMed1998

Differential changes in inhibin A, activin A, and total alpha-subunit levels in granulosa and thecal layers of developing preovulatory follicles in the chicken.

Accumulating evidence implicates inhibins and activins as endocrine and local regulators of follicular development in mammals, and it was recently confirmed that inhibin/activin alpha and betaA genes are also expressed in the avian ovary. To investigate the potential involvement of these proteins in the chicken ovary, thecal and granulosa layers of the four largest follicles (F1-F4) and the most recent postovulatory follicle were collected from hens (10/group) killed 4, 12, and 20 h before the expected time of F1 ovulation. Inhibin A and activin A concentrations of tissue extracts (expressed per mg DNA) were measured using validated two-site enzyme-linked immunosorbent assays; total immunoreactive inhibin alpha-subunit (ir-alpha) was also measured by heterologous RIA (Monash assay). Inhibin A and ir-alpha were largely confined to the granulosa layer, whereas activin A was much more abundant in the thecal layer. Granulosa inhibin A contents were similar in F4 and F3, but increased approximately 40-fold from F3-F1 (P < 0.0001). As such, the F1 granulosa layer was by far the richest source of inhibin A in the chicken ovary, but contained very little activin A. Total ir-alpha in granulosa was much more abundant than inhibin A and increased only 3-fold from F4-F1 (P < 0.001). Activin A in both granulosa and theca showed little variation between F1 and F4 follicles (by ANOVA, P > 0.05). The inhibin A content of F1 granulosa was maximal 12 h before ovulation and had fallen approximately 6-fold (P < 0.0001) within 8 h, suggesting an inhibitory effect of the preovulatory LH surge on the F1 capacity to synthesize inhibin A. Inhibin A, activin A, and ir-alpha were all less in the postovulatory follicle compared with F1 before ovulation (P < 0.0001). In conclusion, application of the present two-site enzyme-linked immunosorbent assays to the chicken ovary revealed 1) divergent tissue distribution of inhibin A and activin A within preovulatory follicles, and 2) differential regulation of granulosa cell production of inhibin A and activin A dimers during preovulatory follicular development. These findings of dynamic changes in inhibin A, activin A, and total ir-alpha support the hypothesis that these proteins subserve regulatory roles during preovulatory follicular development in the hen.

Activins↗
PubMed1991

Diurnal changes in the plasma concentrations of LH and hypothalamic contents of LHRH-I and LHRH-II in the domestic hen.

Diurnal changes of LH secretion in sexually immature hens of 9, 11, 13 and 15 weeks of age consisted of 25-40% increases in the mean concentrations of LH in plasma between 15.00 and 18.00 h, i.e. between 2 h before and 1 h after the onset of darkness. During this time there was a tendency for the mean contents of LHRH-I in the anterior hypothalamus and posterior hypothalamus to increase by 21-74% and 20-56% respectively. In hens of 9 and 15 weeks, diurnal changes in the plasma concentration of LH closely paralleled those of LHRH-I content in the posterior hypothalamus. In contrast, the diurnal rhythm of LH secretion in hens of 11 and 13 weeks was more marked and plasma concentrations of LH continued to rise steeply between 18.00 and 21.00 h, i.e. between 1 and 4 h after the onset of darkness. At 11 weeks, this was associated with a reduction (P less than 0.01) in the contents of LHRH-I and LHRH-II, particularly in the anterior hypothalamus. In laying hens, a diurnal decline (P less than 0.01) in the plasma concentration of LH between 1 and 4 h after the onset of darkness was preceded by a fall (P less than 0.05) in the content of LHRH-I in the posterior hypothalamus and in the total hypothalamic content of LHRH-II (P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Differential responses of hypothalamic LHRH-I and -II to castration and gonadal steroid or tamoxifen treatment in cockerels.

Changes in the hypothalamic contents of LHRH-I and LHRH-II were determined in intact and castrated cockerels injected i.m. with gonadal steroids or tamoxifen. An increase in the plasma concentration of LH after castration was accompained by a significant increase in the content of LHRH-I in the posterior hypothalamus (including the mediobasal hypothalamus and median eminence) which was reversed by oestradiol benzoate given on days 14 and 15 after castration. Under similar circumstances, testosterone propionate did not modify the hypothalamic content of LHRH-I, even though both steroids reduced the plasma concentrations of LH to levels below those of intact cockerels. Treatment of intact cockerels with oestradiol benzoate significantly increased the content of LHRH-I in the posterior hypothalamus, whilst testosterone propionate was again without effect. Tamoxifen significantly raised the plasma concentration of LH in intact cockerels and partially antagonized the suppressive effect of oestradiol benzoate and testosterone on LH secretion in castrated cockerels. However, an anti-oestrogenic effect of tamoxifen on the hypothalamic content of LHRH-I was not demonstrated. There was no evidence of any changes in the hypothalamic content of LHRH-II after castration, with or without gonadal steroid replacement. A change in the hypothalamic content of LHRH-I in response to manipulation of the steroid environment would imply an involvement of this peptide in the mechanism by which gonadal steroids regulate the release of LH. The absence of changes in the hypothalamic content of LHRH-II in the same circumstances suggest that it is not directly involved in the control of LH secretion by the gonadal steroid negative feedback loop.

Animals↗
PubMed1990

Changes in the hypothalamic contents of LHRH-I and -II and in pituitary responsiveness to synthetic chicken LHRH-I and -II during the progesterone-induced surge of LH in the laying hen.

The contents of LHRH-I and -II in the anterior hypothalamus and posterior hypothalamus (including the mediobasal hypothalamus and median eminence) were measured at 90, 180 and 360 min after the i.m. injection of laying hens with progesterone. Whilst no changes were observed in the content of LHRH-I in the anterior hypothalamus, LHRH-I in the posterior hypothalamus tended to fall at 90 and 180 min after injection of progesterone in hens maintained on 16 h light:8 h darkness (16L:8D) and 8L:16D respectively. Pretreatment of laying hens with tamoxifen significantly increased the hypothalamic contents of LHRH-I and -II, raised the basal plasma concentration of LH and modified the LH response to progesterone injection. In hens in which tamoxifen prevented an increase in the plasma concentration of LH after progesterone injection, the content of LHRH-I in the posterior hypothalamus remained unchanged. In contrast, in hens in which progesterone stimulated a steep increase in LH within 90 min, there was a pronounced and significant fall in LHRH-I content of the posterior hypothalamus. No change in the hypothalamic content of LHRH-II was observed during the progesterone-induced surge of LH until plasma concentrations had attained maximal values or started to decline. Then, in hens maintained on 16L:8D, a significant fall in the content of LHRH-II in the anterior hypothalamus was found at both 180 and 360 min after injection with progesterone. Tests in vitro and in vivo of the responsiveness of the pituitary gland to synthetic LHRH-I and -II revealed no change at 90 min after injection of laying hens with progesterone, when plasma concentrations of LH were increasing, but a pronounced reduction when plasma LH concentrations were maximal or falling. These results suggest that LHRH-I mediates in the progesterone-induced increase in the plasma concentration of LH. Although the subsequent decline in plasma LH was associated with a reduced responsiveness of the pituitary gland to LHRH, a significant correlation between the contents of LHRH-I and -II in the anterior hypothalamus and a fall in the hypothalamic content of LHRH-II when plasma LH was maximal or declining allows the possibility of an involvement of this peptide in the neuroendocrine events preceding ovulation.

Animals↗
PubMed1989

Maturational changes in the LH response of domestic fowl to synthetic chicken LHRH-I and -II.

Treatment of chickens at different stages of sexual development with a single i.v. injection of synthetic chicken LHRH (cLHRH)-I or -II stimulated a rise in the plasma concentration of LH within 1 min. The activity of cLHRH-II was 1.3- to 2.7-fold greater than that of cLHRH-I in sexually immature cockerels and hens as determined by the changes in the plasma concentration of LH during the 5 or 10 min after injection. This could be attributed to both a greater effectiveness of cLHRH-II to stimulate LH release and to a more prolonged action. Thus, LH concentrations in plasma were maximal within 1-2 min of injection of all doses of cLHRH-I but within 2-5 min of injection at the higher doses of cLHRH-II. The responsiveness of the pituitary gland to cLHRH-I and -II was substantially greater in the sexually immature cockerel than in the hen and diminished during sexual development of the hen. Coincident with the onset of egg laying, the characteristics of the LH response to cLHRH-II changed to consist of an initial rise during the first 2 min, followed by a more sustained increase with LH concentrations still rising 10 min after injection. In contrast, after injection with cLHRH-I, plasma concentrations of LH rose to a peak at 2 min and thereafter declined gradually. Treatment of the sexually immature hen with oestradiol, progesterone or a combination of both steroids did not enable the expression of a laying hen-type response to the injection of cLHRH-II.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1985

Release of LHRH in vitro and anterior pituitary responsiveness to LHRH in vivo during sexual maturation in pullets (Gallus domesticus).

An in-vitro superfusion technique was used to study basal and depolarization-induced (32 mmol K+/l) release of LHRH from the mediobasal hypothalamus (MBH) of pullets at 8-25 weeks of age. Plasma LH concentrations and the incremental change (delta LH) after an i.v. injection of 1 or 15 micrograms synthetic ovine LHRH/kg body weight were also determined. Between 8 and 25 weeks of age, significant (P less than 0.01) increases in basal and depolarization-induced release of LHRH (93 and 330%, respectively) were accompanied by a significant (P less than 0.01) rise in the residual LHRH content of MBH tissue (152%), observations which suggest that the ability of the hypothalamus to synthesize and secrete LHRH increases as sexual maturation proceeds. However, plasma LH, which reached a maximum concentration of 2.05 +/- 0.43 micrograms/l at 15 weeks, fell significantly (P less than 0.05) to 1.14 +/- 0.05 micrograms/l at 25 weeks. Since delta LH in response to exogenous LHRH showed a marked and progressive decline between 12 and 20 weeks of age, the low plasma concentration of LH typical of the mature hen is probably attributable to a direct negative-feedback action of ovarian steroids on the anterior pituitary gland rather than to an impaired secretion of LHRH from the median eminence. It is suggested that a dramatic increase in the responsiveness of LHRH nerve terminals in the MBH to depolarization by 32 mmol K+/l between 20 and 25 weeks of age (mean age at onset of lay 21.9 weeks; range 19-25 weeks) may reflect the development of hypothalamic responsiveness to the positive feedback action of progesterone.

Animals↗