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Dive into the research topics where I. J. Clarke is active.

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Featured researches published by I. J. Clarke.


Journal of Neuroendocrinology | 2007

Changes in insulin, glucose and ketone bodies, but not leptin or body fat content precede restoration of luteinising hormone secretion in ewes.

L.A. Szymanski; J.E. Schneider; M.I. Friedman; H. Ji; Y. Kurose; Dominique Blache; Alexandra Rao; F. R. Dunshea; I. J. Clarke

The reproductive system, including pulsatile luteinising hormone (LH) secretion, is inhibited by deficits in energy availability and restored by energy surfeits. Plasma LH, insulin, leptin, ghrelin, glucose, ketone body, and nonesterified fatty acid concentrations were measured in ovariectomised, food‐restricted ewes before and after return to ad libitum feeding to determine the factors that change in time to account for the restoration of pulsatile LH secretion. At 07.00u2003h, blood was sampled every 10u2003min for 5u2003h from ovariectomised, hypogonadotrophic, chronically food‐restricted and ad libitum‐fed ewes (Fed). At 12.00u2003h, four of the food‐restricted sheep were given ad libitum access to food (Re‐Fed), while three ewes continued to be food restricted (Restricted). Sampling continued for 5u2003h and resumed again on the mornings of days 2, 4, and 9. A pulse of LH was seen within 1u2003h of re‐feeding in all Re‐Fed ewes, and interpulse interval (IPI) was significantly shorter in Re‐Fed compared to Restricted ewes and longer than in Fed ewes during the period after re‐feeding. Re‐Fed LH IPI was not restored to that of Fed ewes until sometime between days 4 and 9. The first pulse occurred within minutes, whereas restoration of IPI occurred after 4–8u2003days. Prior to the initial LH pulses seen in Re‐Fed ewes, plasma ketone bodies first fell and then rose to levels significantly above those in Restricted ewes. Significant changes in circulating insulin, ghrelin, glucose, and total ketone body concentrations, daily food intake and lean body mass preceded restoration of Re‐Fed LH IPI some time between days 4 and 9, but there were no significant changes in adiposity or circulating leptin concentrations, consistent with the hypothesis that LH pulses are reinitiated by changes in the availability of oxidisable metabolic fuels and possibly insulin, but not leptin concentrations.


Hormones and Behavior | 1993

Treatment of Young Rams with an Agonist of GnRH Delays Reproductive Development

Alan J. Tilbrook; D. B. Galloway; A.H. Williams; I. J. Clarke

The objective of this study was to determine if the continuous treatment of young rams with an agonist of gonadotropin-releasing hormone (GnRH) in the period immediately prior to puberty would delay the onset of adult sexual behavior and retard testicular development. In the first experiment the GnRH agonist was shown to be effective in suppressing the plasma concentrations of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in adult wethers (neonatally castrated rams) when administered by either a biocompatible slow release implant (implant) or a mini osmotic pump (minipump) that released the agonist for 4 weeks. The minipumps were more effective than the implants in suppressing the secretion of LH and FSH. In a second experiment, administration of the GnRH agonist by implant or minipump to prepubertal rams for 16 weeks immediately prior to puberty inhibited the development of sexual behavior, reduced the plasma concentrations of testosterone, retarded testicular and epididymal development, and inhibited growth rates. The effects on sexual behavior were clearly reversible but testicular and epididymal weights were still reduced in treated rams 8 weeks after the end of treatment. These results indicate that the reproductive function of rams is sensitive to gonadotropins and testicular hormones immediately prior to puberty. The agonist of GnRH was successfully delivered to the rams in a biocompatible implant which may offer a practical means of manipulating reproductive function in young rams.


Biology of Reproduction | 2001

Influence of the Degree of Stimulation of the Pituitary by Gonadotropin-Releasing Hormone on the Action of Inhibin and Testosterone to Suppress the Secretion of the Gonadotropins in Rams

Alan J. Tilbrook; David M. de Kretser; I. J. Clarke

Abstract This experiment determined if the degree of stimulation of the pituitary gland by GnRH affects the suppressive actions of inhibin and testosterone on gonadotropin secretion in rams. Two groups (n = 5) of castrated adult rams underwent hypothalamopituitary disconnection and were given two i.v. injections of vehicle or 0.64 μg/kg of recombinant human inhibin A (rh-inhibin) 6 h apart when treated with i.m. injections of oil and testosterone propionate every 12 h for at least 7 days. Each treatment was administered when the rams were infused i.v. with 125 ng of GnRH every 4 h (i.e., slow-pulse frequency) and 125 ng of GnRH every hour (i.e., fast-pulse frequency). The FSH concentrations and LH pulse amplitude were lower and the LH concentrations higher during the fast GnRH pulse frequency. The GnRH pulse frequency did not influence the ability of rh-inhibin and testosterone to suppress FSH secretion. Testosterone did not affect LH secretion. Following rh-inhibin treatment, LH pulse amplitude decreased at the slow, but not at the fast, GnRH pulse frequency, and LH concentrations decreased at both GnRH pulse frequencies. We conclude that the degree of stimulation of the pituitary by GnRH does not influence the ability of inhibin or testosterone to suppress FSH secretion in rams. Inhibin may be capable of suppressing LH secretion under conditions of low GnRH.


Australian Journal of Experimental Agriculture | 1995

Lupin and cowpea supplements for growth, wool production, and reproduction in rams

C. C. Pomares; D. B. Galloway; J. H. G. Holmes; I. J. Clarke; Alan J. Tilbrook

Lupins and cowpeas were fed at 0.0, 0.5, 1.0, and 2.0% liveweight (LW) to groups of 6 or 7 South Australian Merino rams for 11 weeks as supplements to a maintenance diet of grass hay. Productive and reproductive parameters were examined. Supplementation at all levels of either grain increased (P<0.001) LW, condition score, backfat thickness, and wool growth, with rams showing similar responses when given cowpeas and lupins. The weights of the pancreas of rams decreased (P<0.01) as the levels of supplementation increased. Semen characteristics (volume, concentration, motility, etc.) and response to freezing did not differ between the diets. Feeding either legume increased (P<0.001) testicular size, lupins giving a slightly greater response than cowpeas. Sperm production per g testicular tissue was not significantly altered. Increasing levels of either supplement increased (P<0.001) the weights of the seminal vesicles, more (P<0.01) so with lupins. The same effects occurred with the weight of epididymides except for rams fed cowpeas at the highest level. Histologically, the testes revealed an increase (P<0.001) in the number of round spermatogenic cell nuclei and the cross-sectional diameter of stage 8 tubules in rams after legume grain supplementation at 2.0% LW compared with controls. The response was higher in lupin-fed rams (P<0.05). Plasma luteinising hormone (LH) and follicle stimulating hormone (FSH) were measured in rams fed at maintenance or with lupins or cowpeas at 2.0% LW. Rams responded equally to all legumes. Concentrations of LH in peripheral blood increased (P<0.001) by 140% and FSH concentrations were elevated (P<0.01) 4-fold. Cowpeas, a tropical grain legume similar in nutrient composition to lupins, have similar potential when used as a supplement to improve productive and reproductive capabilities of Merino rams.


Reviews of Reproduction | 2000

Effects of stress on reproduction in non-rodent mammals: the role of glucocorticoids and sex differences

Alan J. Tilbrook; Anne I. Turner; I. J. Clarke


Endocrinology | 1991

The Negative Feedback Effects of Testicular Steroids Are Predominantly at the Hypothalamus in the Ram

Alan J. Tilbrook; David M. de Kretser; James T. Cummins; I. J. Clarke


Journal of Endocrinology | 2004

Characterisation of the rapid release of activin A following acute lipopolysaccharide challenge in the ewe

Kristian Lee Jones; David M. de Kretser; I. J. Clarke; J-P Y. Scheerlinck; David J. Phillips


Proceedings of the National Academy of Sciences of the United States of America | 2004

Reconstruction of in vivo time-evolving neuroendocrine dose-response properties unveils admixed deterministic and stochastic elements

Daniel M. Keenan; Susan L. Alexander; Clifford Irvine; I. J. Clarke; Christopher Scott; Anne I. Turner; Alan J. Tilbrook; Benedict J. Canny; Johannes D. Veldhuis


Journal of Endocrinology | 1995

IGF feedback effects on growth hormone secretion in ewes: evidence for action at the pituitary but not the hypothalamic level.

T. P. Fletcher; G. B. Thomas; F. R. Dunshea; L G Moore; I. J. Clarke


Journal of Endocrinology | 1986

Prolonged secretion of prolactin in response to thyrotrophin-releasing hormone after hypothalamo-pituitary disconnection in the ewe.

G. B. Thomas; J. T. Cummins; J. M. Hammond; R. J. E. Horton; I. J. Clarke

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Alan J. Tilbrook

South Australian Research and Development Institute

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David M. de Kretser

Hudson Institute of Medical Research

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Ben Canny

Prince Henry's Institute of Medical Research

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Fahri Fahri

University of Melbourne

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