Facilitation by Progesterone of Ovulating Hormone Release in Sodium Pentobarbital-Blocked Proestrous Rats

Abstract

SynopsisIn order to elucidate the facilitatory action of progesterone on the CNS mechanism(s) regulating ovulating hormone (OH) release, the steroid (5mg/rat) dissolved in sesame oil was injected subcutaneously to proestrous rats which had been treated with the minimal dose of sodium pentobarbital (PB, 25 or 22.5mg/kg, ip) to inhibit spontaneous OH release. Low (20-30%) incidence of ovulation (No. of rats ovulated against No. of rats treated) was obtained on the following day in the rats given PB alone. When progesterone was injected simultaneously with PB or until 21:00 proestrus, 4hr after PB injection, incidence of ovulation markedly increased to attain about 80 %. In these rats, ovulation was observed from 10:00 to 11:00 on the morning of estrus regardless of the timing of progesterone injection given between 17:00 and 21:00 proestrus. The rats given progesterone at 22:00 or later on proestrus, however, failed to show any increase in the incidence of ovulation. In the rats pretreated with 30mg/kg or more of PB, progesterone was ineffective to increase ovulation incidence. It is likely, therefore, that progesterone exerts its action on the CNS to prolong and possibly heighten the activity to induce OH release. This also suggests that endogenous progesterone, increased by the initial part of released OH on the afternoon of proestrus, acts, in turn, on the CNS ovulating mechanism(s) to accomplish normal OH release.It is well known that progesterone has a biphasic (stimulatory and inhibitory) effect on ovulating hormone (OH) release and ovulation (see reviews for references: Everett, 1964 and1969;Schwartz, 1969 and. The stimulatory effect of progesterone has been demonstrated under various conditions, e.g., in persistent estrous rats (Everett, 1940), in normal 5-day cyclic rats (Everett, 1948;Brown-Grant, 1967) and in immature rats primed with pregnant mare's serum gonadotropin Meyer, 1963 and1965). In addition, the condition for a facilitatory action of progesterone exists on the day of proestrus in normal cyclic rats and the release of OH can be advanced by a few hr by the injection of exogenous progesterone (Zeilmaker, 1966;Redmond, 1968).In the previous studies, we also elucidated the facilitation of OH release in 4-day cyclic rats either by injecting progesterone subcutaneously or by implanting the steroid into the median eminence-arcuate region of the hypothalamus on the morning or early afternoon of proestrus (Kobayashi et al., 1970). It was further presumed in the rats given acute hypothalamic deafferentation with a small knife that the facilitation by progesterone of OH release may be due to the increase in the responsiveness of the medial basal hypothalamus to the ovulatory stimulus coming from the higher CNS (Kobayashi and Miyake, 1971). Uchida et al. (1969bUchida et al. ( , 1970Uchida et al. ( and 1972 of our laboratory also demonstrated that progesterone given on the morning of proestrus causes a few hr advancement along with a

Opioidergic modulation of the oestradiol-induced LH surge in the rat: roles of ovarian steroids

“…Specifi¬ cally, progesterone administered on dioestrus is known to delay the genesis of that phenomenon by 24-48 h (Zeilmaker, 1966;Kobayashi, Hara & Miyake, 1969a;Caligaris, Astrada & Taleisnik, 1971; Beattie & Corbin, 1975;Taya, Terranova & Greenwald, 1981), while progesterone administration on the morning of pro-oestrus will advance the timing of the onset of the LH surge by a few hours (BrownGrant & Naftolin, 1972;Kobayashi, Hara & Miyake, 1973;Kalra & McCann, 1974;DePaolo & Barraclough, 1979a;Barraclough, Camp, Wieland & Akabori, 1986). In addition, in the ovariectomized, oestradiol-primed rat model, progesterone also markedly augments LH surge amplitude (Aiyer, Sood & Brown-Grant, 1976;DePaolo & Barraclough, 19796;Wise, Grossman & Barraclough, 1981a).…”

Effect of Serotonin and Progesterone on Induced Ovulation in Immature Rats

“…Brown-Grant (1969) has suggested that the pre-ovulatory release of pro¬ gesterone may act as a ' fail-safe ' mechanism in stimulating LH release via a positive feedback mechanism. Recently Kobayashi, Hara & Miyake (1973) have shown that in pro-oestrous rats treated with sodium pentobarbitone, progesterone administered after the critical period significantly increases the incidence of ovulation.…”