Dynamic changes in the gonadotrope cell subpopulations during an estradiol-induced surge in the ewe.
Whether estradiol targets a subpopulation of gonadotrope cells was investigated in this study. Ovariectomized ewes (OVX) or OVX ewes immunized against GnRH and treated with hourly pulses of GnRH analogue (OVX-IMG) were killed at 6, 12, 16, and 24 h after administration of 50 microg of 17beta-estradiol (E(2)). Control ewes received no E(2) treatment. In OVX or OVX-IMG ewes killed 6 h after E(2) injection, a decrease in gonadotropin plasma levels was observed compared with non-E(2)-treated ewes. In contrast, a surge in gonadotropin plasma concentrations occurred in ewes killed 16 h after injection. The percentage of total immunoreactive gonadotrope cells among the pituitary cells was lower in E(2)-treated ewes compared with nontreated animals. The proportion of monohormonal LH cells was constant throughout the experiment, except at the surge peak, where it was enhanced. In the OVX ewes, the proportion of bihormonal LH/FSH cells was lower in the E(2)-treated ewes compared to the nontreated ewes (P: < 0.001), with a more pronounced decrease 16 h after E(2) injection. A slight increase occurred 12 h after E(2) injection compared with 6 h after injection (P: < 0.05). A similar pattern was observed in the OVX-IMG ewes, except at 12 h after E(2) injection, when no increase occurred. In both OVX and OVX-IMG ewes, injection of E(2) decreased FSHbeta mRNA expression but did not alter the relative levels of LHbeta mRNA. These data suggest that the negative feedback of E(2) on LH and FSH secretion mainly targets the bihormonal cells and occurs, at least in part, directly at the pituitary level. During the gonadotropin surge, the sustained FSH release from the bihormonal cells would induce a switch from bihormonal cells to monohormonal LH cells by depleting these cells of FSH.
Molter-Gérard
C
C
Institut National de la Recherche Agronomique/Unité de Recherche Associée CNRS 1291, Station de Physiologie de la Reproduction des Mammifères Domestiques, 37380 Nouzilly, France.
Caraty
A
A
Guérin
S
S
Fontaine
J
J
Taragnat
C
C
D000818:Animals; D004958:Estradiol / Q000276:immunology / Q000494:pharmacology*; D005260:Female; D005640:Follicle Stimulating Hormone / Q000097:blood / Q000235:genetics / Q000378:metabolism*; D005786:Gene Expression Regulation / Q000187:drug effects; D007987:Gonadotropin-Releasing Hormone / Q000378:metabolism*; D007116:Immunization, Passive; D007986:Luteinizing Hormone / Q000097:blood / Q000235:genetics / Q000378:metabolism*; D008597:Menstrual Cycle / Q000187:drug effects*; D010052:Ovariectomy; D010902:Pituitary Gland / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism; D012756:Sheep
D007987:Gonadotropin-Releasing Hormone; D004958:Estradiol; D007986:Luteinizing Hormone; D005640:Follicle Stimulating Hormone