Inhibitory Effect of Central LHRH on LH Secretion in the Ovariectomized Ewe

Abstract

The role of central luteinizing hormone releasing hormone (LHRH) in the control of pulsatile LHRH and luteinizing hormone (LH) secretion was investigated in ovariectomized adult ewes. Injection of LHRH (2.1–21 pmol) into the third cerebral ventricle caused a delayed but sustained inhibition of LH secretion. Pulse frequency, pulse amplitude and mean LH levels were reduced significantly when compared with the responses to the control injection of saline (50 µl). The inhibitory effect of centrally administered LHRH was not accompanied by a reduction in the pituitary responsiveness to intravenous LHRH. In contrast to the effect on LH, plasma levels of follicle-stimulating hormone (FSH) and prolactin were unaffected by central LHRH. The inhibitory action of LHRH was antagonized by prior injection of an LHRH antagonist ([N-Ac-D-Nal(2)¹ , D-p-Cl-Phe², D-Trp³, D-hArg (Et₂)⁶, D-Ala¹⁰] LHRH, 69 pmol) into the third ventricle. Central injection of the LHRH antagonist alone (at the same concentration) did not influence any characteristic of pulsatile LH secretion. In conclusion, these data indicate that exogenous administration of LHRH into the brain exerts a dose-related and receptor-mediated inhibition of LHRH pulse generator activity. However, the physiological significance of endogenous LHRH in the regulation of the LHRH pulse generator remains unresolved.

Microinjection of LHRH and Its Antagonistic Analog into the Medial Preoptic Area Does Not Affect Pulsatile Secretion of LH in Ovariectomized Rats.

“…The results in OVX rats are quite different from those of other researchers who have shown decreases in LH secretion following intracerebroventricular injections of LHRH in OVX rats [1,2,3] and ewes [4]. The difference in the results is probably related to the difference in the injection site.…”

“…Taking these results together with our previous report, the present study indicates that 1) endogenous LHRH in the MPO is involved in the ultra-short feedback regulation of LHRH release and 2) the ultra-short positive feedback mechanism in the MPO acts at the time of the proestrous LH surge but not under a hormonal milieu as in ovariectomized rats. Key words; LHRH, Preoptic area, Feedback, LH, Ovariectomy, LHRH antagonist (Endocrine Journal 41: 559-563,1994) IT HAS BEEN reported that intraventricular injection of LHRH or LHRH agonist reduces the secretion of LHRH [1] and thus the secretion of LH is decreased in ovariectomized (OVX) rats [2, 3] and ewes [4], implying the existence of an ultra-short negative feedback mechanism for the control of LHRH secretion. In addition, in vitro studies have suggested that the neural structure that is involved in the mechanism is in the mediobasal hypothalamus including the median eminence [5,6].…”

“…IT HAS BEEN reported that intraventricular injection of LHRH or LHRH agonist reduces the secretion of LHRH [1] and thus the secretion of LH is decreased in ovariectomized (OVX) rats [2,3] and ewes [4], implying the existence of an ultra-short negative feedback mechanism for the control of LHRH secretion. In addition, in vitro studies have suggested that the neural structure that is involved in the mechanism is in the mediobasal hypothalamus including the median eminence [5,6].…”

Naloxone does not Affect the Luteinizing Hormone‐Releasing Hormone‐Induced Inhibition of Luteinizing Hormone Secretion in Sheep

“…The physiological significance of endogenous LHRH in regulating the release of LHRH ('ultrashort'-loop feedback) has been examined using a median eminence preparation in vitro (6). In the presence of an LHRH antagonist, basal secretion of LHRH increased in a dose-related manner suggesting that endogenous LHRH may exert a tonic inhibitory influence on its own release.In the sheep, the inhibitory action of exogenous LHRH may be reversed by the central administration of an LHRH antagonist (1) suggesting that a specific interaction with an LHRH receptor, rather than a non-specific effect, is involved. Although an interaction with a central LHRH receptor is postulated, the LHRHinduced inhibition was delayed (1 to 3 h following injection) and sustained (up to 20 h following injection).…”

“…In the sheep, the inhibitory action of exogenous LHRH may be reversed by the central administration of an LHRH antagonist (1) suggesting that a specific interaction with an LHRH receptor, rather than a non-specific effect, is involved. Although an interaction with a central LHRH receptor is postulated, the LHRHinduced inhibition was delayed (1 to 3 h following injection) and sustained (up to 20 h following injection).…”