Ovarian Involvement in the Suppression of Luteinizing Hormone in the Incubating Turkey (&lt;i&gt;Meleagris gallopavo&lt;/i&gt;)

Although molecular mechanisms underlying steroid effects on GnRH and dopamine receptors are well documented in mammals, little is known in fish. Herein, we describe the expression of pituitary GnRH and dopamine receptors relative to gonadotropin expression and release. We exposed female tilapia to graded doses of estradiol or 17alpha,20beta-dihydroxy-4-pregnen-3-one (DHP) in vitro, and of estradiol in vivo, and determined mRNA levels of gnrhr1, gnrhr3, drd2, lhb, and fshb by real-time PCR. We also determined gonadotropin levels using specific ELISAs. Exposure to low doses of estradiol caused increased gnrhr3 mRNA levels in vivo and in vitro, probably related to positive feedback on FSH release. Increasing concentrations of estradiol resulted in increased drd2 mRNA levels in vivo and in vitro, inhibition of LH and FSH release, and inhibition of lhb mRNA levels in vivo, possibly related to negative feedback. At high doses of estradiol, FSH release increased in preparation for a new generation of follicles. Exposure to nanomolar doses of DHP resulted in increased drd2 mRNA levels, probably related to negative feedback on LH release. A decrease in drd2 levels at the micromolar range of DHP (concomitant with increased gnrhr3 and fshb mRNA levels) may be related to the recruitment of a new generation of oocytes. Exposure to DHP also resulted in increased lhb mRNA levels toward final oocyte maturation. Salmon GnRH analog (sGnRHa) increased mRNA levels of gnrh1and gnrh3; when combined with DHP, sGnRHa synergistically increased expression of gnrh3 only. These results emphasize the role of sex steroids on positive and negative feedbacks controlling the reproductive cycle.

Section: Discussionmentioning

confidence: 99%

“…Studies have been conducted to elucidate the mechanisms of steroid feedback regulation of gonadotropin secretion in mammals and birds [46][47][48]. However, these mechanisms are poorly understood in lower vertebrates, including fish.…”

Section: Discussionmentioning

confidence: 99%

Sex Steroids Are Involved in the Regulation of Gonadotropin-Releasing Hormone and Dopamine D2 Receptors in Female Tilapia Pituitary1

Levavi‐Sivan¹,

Biran²,

Fireman³

2006

“…Gonadal steroids have been shown to exert negative and/ or positive effects on gonadotropin secretion in vertebrates [1][2][3][4][5][6]. The type of influence varies with the gonadal development stage of the individuals.…”

Section: Introductionmentioning

confidence: 99%

Gonadal Stage-Dependent Effects of Gonadal Steroids on Gonadotropin II Secretion in the Atlantic Croaker (Micropogonias undulatus)1

Khan

Hawkins

Thomas

1999

Involvement of gonadal steroids in the control of gonadotropin II (GTH II) (homologous to LH) secretion was investigated in the Atlantic croaker (Micropogonias undulatus) using gonadectomy (Gx) and steroid replacement paradigms. Gonadectomy in males and females during the late gonadal recrudescence phase elicited significant increases in the gonadotropin response to stimulation by an LHRH analog (LHRHa), without altering basal GTH II secretion. Slow-release silicone elastomer implants of testosterone or estradiol significantly inhibited LHRHa-induced GTH II secretion in gonad-intact and Gx males, and in Gx females, whereas 5alpha-dihydrotestosterone, a nonaromatizable androgen, was ineffective. Pretreatment of fish with an aromatase inhibitor, 1,4, 6-androstatrien-3,17-dione, 2 days before the administration of testosterone implants, completely blocked the negative effect of testosterone on LHRHa-induced GTH II secretion in males, but only partially restored it in females. This suggests that the negative feedback of testosterone in males is primarily mediated by its conversion to estradiol at the level of the hypothalamus and/or pituitary gland, while in females the androgen may also exert a direct inhibitory effect on GTH II secretion, probably mediated via an androgen receptor. In addition, estradiol and testosterone exerted positive effects on basal and LHRHa-induced GTH II secretion during the early-recrudescence phase of the gonadal cycle. The steroids switched to a negative effect on LHRHa-induced GTH II secretion once the fish had fully developed gonads, possibly as a mechanism that prevents a precocious surge in GTH II secretion and final gamete maturation until gametogenesis is complete and the environmental conditions are appropriate for spawning.

“…VIP induces hyperprolactinemia, which is associated with incubation behavior. Immunocytochemical studies have shown that increases in hypothalamic VIP immunoreactivity (VIP-IR) within the infundibular nuclear complex of incubating hens parallel increases in circulating PRL [10,11]; the number of these neurons decreases following nest deprivation, a procedure shown to reduce the elevated circulating PRL and PRL mRNA contents of incubating turkeys and chickens [7,12].…”

Section: Introductionmentioning

confidence: 99%

Increased Egg Production by Active Immunization against Vasoactive Intestinal Peptide in the Turkey (Meleagris gallopavo)1

Halawani

Silsby

Rozenboim

et al. 1995

Self Cite

Vasoactive intestinal peptide (VIP) is a hypothalamic prolactin (PRL)-releasing factor in the turkey. The hypothesis in the present study was that active immunization of turkeys with VIP would neutralize endogenous VIP, decrease circulating PRI, and consequently prevent the expression of incubation behavior. Large white female turkeys were divided into three experimental groups comprising untreated controls, control turkeys immunized with keyhole limpet hemocyanin (KLH), and turkeys immunized with synthetic chicken VIP conjugate (KLH-cVIP). Each turkey received four injections at 4-wk intervals, starting on the day of photostimulation. The immune response, measured by the percentage binding of monoiodinated chicken VIP (cVIP) to plasma at a dilution of 1:1000, averaged 11.8 +/- 2.5% during the reproductive life cycle. Immunization against KLH-cVIP prevented the normal increases of PRL that are associated with the photo-induced reproductive cycle. Over a 21-wk period beginning at photostimulation, KLH-cVIP-immunized birds exhibited a maximal plasma PRL level of 82.2 +/- 29.5 ng/ml, compared to 367.7 +/- 66.6 ng/ml and 227.5 +/- 51.7 ng/ml for non- and KLH-immunized turkeys, respectively. The total number of nest visits per hen during the 147-day experimental period decreased from 320.0 +/- 48.2 in the nonimmunized controls to 180.7 +/- 53.7 and 149.4 +/- 13.1 visits in KLH- and KLH-cVIP conjugate-immunized turkeys. Turkeys that showed an immune response to KLH-cVIP immunization did not exhibit incubation behavior, whereas 54% and 33% of non- and KLH-immunized hens incubated their eggs.(ABSTRACT TRUNCATED AT 250 WORDS)