Search

Reduced pregnancy rates often occur in ruminants grazing endophyte-infected (EI) tall fescue. The objectives were to characterize basal and oxytocin-induced PGF2a concentrations in serum and reproductive function in ewes fed tall fescue seed and to determine whether addition of fish meal (FM) to a diet of EI fescue would alter PGF2a production. Ewes were fed a diet with novel or non-toxic endophyte-infected (NE) or EI tall fescue seed containing either corn gluten meal (CG) or FM. Serum concentrations of prolactin, a measure of severity of fescue toxins, were reduced in ewes fed EI compared with NE forage seed (forageday, Pb0.02) and were greater in NEFM than NECG-fed ewes (Pb0.03). Size and number of corpora lutea (CL), determined by trans-rectal ultrasonography, were similar between diets (PN0.10). Serum concentrations of progesterone were reduced in ewes with two CL fed EI compared with NE seed (forageCL numberday, Pb0.001). Oxytocin-induced PGFM concentrations during the luteal phase were determined as a measure of uterine function. On the day of oxytocin administration, peak plasma concentrations of PGFM were reduced in EI compared with NE-fed ewes (foragetime, Pb0.003), but FM did not influence PGFM concentrations. Estrous cycle length was more variable in EI than NE-fed ewes. There appears to be some asynchrony between NE and EI-fed ewes leading to changes in uterine responsiveness to oxytocin. Inclusion of FM did not alter uterine responsiveness to oxytocin. D 2006 Elsevier B.V. All rights reserved. more...

An experiment was conducted to determine the effect of prostaglandin F2alpha (PGF2alpha) on luteal synthesis of progesterone (P4) and related progestins. Sixteen beef heifers were assigned in equal numbers to four groups in a 2x2 factorial arrangement of treatments. The experiment consisted of two levels of PGF2alpha analog (0 and 500 microg) and two levels of time (4 and 24 h after injection) of corpus luteum collection. All heifers were injected intravenously with saline (2 ml) or PGF2alpha (cloprostenol) on day 8 of the estrous cycle (estrus=day 0). Jugular blood was collected at 0, 1, 2, 3, 4 and 20, 21, 22, 23, and 24 h after injection. Resulting sera were analyzed for P4 by use of radioimmunoassay. Luteal tissue was analyzed by gas chromatography/mass spectrometry for P4, 20beta-hydroxyprogesterone, pregnenolone, and allopregnanolone (3beta-hydroxy-5alpha-pregnan-20-one). Treatment with PGF2alpha reduced serum concentrations of P4 as early as 1 h after injection (P0.005) and steroid levels remained low over 24 h. Similarly, administration of PGF2alpha caused a decline in luteal P4 (P0.005), 20beta-hydroxyprogesterone (P0.10), and pregnenolone (P0.05). In contrast, treatment with PGF(2alpha) caused an increase in luteal allopregnanolone over time (time x treatment interaction; P0.05). These data are interpreted to suggest that PGF2alpha promotes conversion of P4 to the metabolite allopregnanolone. more...

The male-oriented ram is a unique and valuable animal model for the study of hormonal, developmental and genetic contributions to sexual partner preference. Unlike most other mammalian models that are in use currently, variations in sexual attraction occur spontaneously in domestic ram populations. It is estimated that as many as 8-10% of rams exhibit a sexual partner preference for other males, classifying them as male-oriented rams. Studies have failed to identify any compelling social factors that can predict or explain the variations in sexual partner preferences of rams. Nor is there consensus on the endocrine and sensory responsiveness of male-oriented rams to other rams. However, a number of studies have reported differences in brain structure and function between male-oriented and female-oriented rams, suggesting that sexual partner preferences are neurologically hard-wired. Recently, we identified a sexually dimorphic nucleus (oSDN) in the sheep preoptic area-anterior hypothalamus. The oSDN is larger in female-oriented rams than in male-oriented rams and similar in size in male-oriented rams and ewes. In addition, mRNA levels for aromatase in the oSDN were higher in males than in females and were higher in female-oriented rams than in male-oriented rams. These results suggest a relationship between steroid hormones, specifically estrogens and oSDN morphology. In this review, we provide an overview of sexual behavior in rams and discuss the multiple factors that may contribute to the development and adult expression of same-sex partner preferences in rams. more...

The bovine corpus luteum contains a myristoylated alanine-rich C kinase substrate (MARCKS) protein known to crosslink actin filaments in the cytoskeletal cortex associated with the plasma membrane. We conducted experiments to determine whether concentrations of MARCKS mRNA and protein in the bovine corpus luteum varied during the estrous cycle. Using Northern blots probed with a MARCKS cDNA, we found that luteal concentrations of MARCKS mRNA were greatest on d 4, 8, and 12 and markedly reduced on d 16 of the cycle (p0.08). Similarly, Western blot analysis of luteal proteins revealed that concentrations of MARCKS protein were greatest on d 8 and least on d 16 of the cycle (p0.01). Exposure of slices from a d 8 corpus luteum to prostaglandin F2alpha (PGF2alpha) during a 10-min incubation in the presence of [32P]-ortho-phosphate resulted in enhanced phosphorylation of MARCKS in membrane and cytosolic fractions compared to that of controls. We therefore concluded that expression of the luteal MARCKS protein gene may be regulated and that PGF2alpha-induced phosphorylation of this protein is attributable to activation of protein kinase C. more...

This study tested the hypothesis that aromatization of testosterone to estradiol is necessary for sexual differentiation of the sheep brain. Pregnant ewes (n = 10) were treated with the aromatase inhibitor 1,4,6- androstatriene-3,17-dione (ATD) during the period of gestation when the sheep brain is maximally sensitive to the behavior-modifying effects of exogenous testosterone (embryonic d 50-80; 147 d is term). Control (n = 10) ewes received vehicle injections. Fifteen control lambs (7 males and 8 females) and 17 ATD-exposed lambs (7 males and 10 females) were evaluated for sexually dimorphic behavioral and neuroendocrine traits as adults. Prenatal ATD exposure had no significant effect on serum concentrations of androgen at birth, growth rates, expression of juvenile play behaviors, or the onset of puberty in male and female lambs. Rams exposed to ATD prenatally exhibited a modest, but significant, decrease in mounting behavior at 18 mo of age. However, prenatal ATD exposure did not interfere with defeminization of adult sexual partner preferences, receptive behavior, or the LH surge mechanism. In summary, our results indicate that aromatization is necessary for complete behavioral masculinization in sheep. However, before we can conclude that aromatization does not play a role in defeminization of the sheep brain, it will be necessary to evaluate whether intrauterine exposure of male fetuses to higher doses of ATD for a more extended period of time can disrupt normal neuroendocrine and behavioral development. more...

A review of the current state of knowledge of oxytocin production by the preovulatory follicle and corpus luteum is presented. Corpora lutea of a number of mammalian species have been found to synthesize oxytocin. However, the synthesis and secretion of this nanopeptide by the corpus luteum of the ruminant has been most extensively studied because of the potential role of this peptide in facilitating luteal regression. While much information exists relative to various biochemical and endocrine factors that impact on oxytocin gene expression, this aspect about luteal synthesis of this peptide hormone remains enigmatic. Prostaglandin F-2α (PGF-2α) has been shown to be a primary endogenous hormone responsible for triggering luteal secretion of oxytocin. Details are provided regarding the PGF-2α-induced intracellular signal transduction pathway that ultimately results in exocytosis of luteal oxytocin. Evidence is also presented for potential autocrine/paracrine actions of oxytocin in regulating progesterone production by luteal and granulosa cells. Concluding remarks highlight aspects about luteal oxytocin production that require further research. more...

An experiment was conducted to evaluate the effects of diet on fur colour and quality of bromocriptineand melatonin-induced winter pelage in mink. At weaning, 90 standard dark female kit mink were assigned randomly to one of two dietary regimens. Diet 1 (no. = 30) was a basal ranch mink diet, and diet 2 (no. = 60) was the basal diet, supplemented with liver and eggs. On 26 June 1986, mink given each diet were assigned to treatment (no. = 15) and control groups (no. = 15). Animals receiving diet 1 were treated with 120 mg melatonin implants (silastic) while those given diet 2 were treated with 120 mg melatonin implants, 60 mg bromocriptine pellets, or 60 mg bromocriptine pellets plus 18·6 mg melatonin implants (silastic). Control groups on each diet received no implants. Subsequently, fur growth was measured at biweekly intervals. Mink treated with 120 mg melatonin, 60 mg bromocriptine, and 60 mg bromocriptine plus 18·6 mg melatonin all moulted 1 month earlier than controls and exhibited significantly greater fur growth during the months of August and September (P < 0·05). Mink implanted with 120 mg melatonin, and bromocriptine plus melatonin were considered to be in fully prime pelage by mid October and were pelted by October 23. Mink receiving bromocriptine alone were not considered to be in prime pelage in October and were pelted in December with controls. Neither diet supplementation with liver and eggs, nor treatment to induce early fur growth had an effect on fur colour or quality. It was also concluded that treatment of mink with bromocriptine may not completely mimic the effect of exogenous melatonin on winter fur growth. more...

A study was conducted to determine the effect of the milk-ejection reflex on exogenous gonadotropin releasing hormone (GnRH)-induced release of luteinizing hormone (LH) after shortterm calf removal. Twenty-four postpartum multiparous beef cows were assigned randomly to groups arranged in a 23 factorial arrangement. Factors consisted of two levels of suckling [suckled (S) or nonsuckled (NS)], treatment with GnRH [saline(C) or 200 pg GnRH] and days postpartum (d 1 and 14). Dams were isolated from their calves for 4 h on d 1 and 14 postpartum. Atthe end of 4 h dams were reunited with their calves in S+C and S+GnRH groups, while dams of calves in NS+C and NS+GnRH groups remained separated an additional 2 h. Cows were injected iv with saline or GnRH following the 4-h isolation period, 5 min after calves had begun suckling or nuzzling the udder. Sera from jugular blood samples collected 15 min prior to the end of the 4-h isolation period, immediately prior to injection (0 h) and at 15 min intervals thereafter for 120 min were analyzed for LH. Serum concentrations of LH in control cows did not differ due to suckling or stage of the postpartum period and averaged 2.3 s .1 ng/ml. Pituitary response to GnRH was determined by computing the rate of LH release. Rate of LH release (rig LH.ml -I -rain -1 ) in response to GnRH on d 14 was greater (P more...

The relationship of glucocorticoid secretion to artificial light-induced, early winter priming of pelage in ranch mink was investigated. On December 13, 1977, 16 mature female mink were exposed to natural light. On June 26, 1978, the animals were assigned randomly to one of two groups. Animals in Group 1 were housed in a light-control facility and subjected to 6 hr of artificial light daily and ambient temperatures until November 27, 1978. Animals in Group 2 served as controls and were exposed to natural light. Samples of blood and measurements of the fur and vulva were taken biweekly throughout the year. Concentrations of total glucocorticoids in serum were determined by competitive protein-binding assay. The pelage of mink reared under reduced artificial light was fully prime by October 31, while that of control animals did not become prime until November 27 (time x light regimen interaction, P less than .05). A seasonal effect (P less than .01) on total glucocorticoid concentrations in serum of control mink was revealed by a pattern of three peaks over the 12-month period. The pattern of glucocorticoid secretion was not affected by exposure of mink to reduced artificial light. Maximum concentrations of total glucocorticoids in serum occurred on the same date (September 5) in both groups, although levels were significantly higher in the light-treated mink (44.5 +/- 10.4 ng/ml) than in the controls 26.9 +/- 5.7 ng/ml). Fur growth was positively correlated with total glucocorticoid concentrations in serum during the period from June to December (overall r = .14, P less than .05). Both length and width of the vulva increased (P less than .01) during February and March in animals reared under natural light. These data suggest that the increased secretion of glucocorticoids induced by exposure of mink to reduced artificial light may be involved in promoting early priming of the winter pelage of these animals. more...