Correlative Surges of LHRH, LH and FSH in Pituitary Stalk Plasma and Systemic Plasma of Rat during Proestrus

Abstract

An effective recovery procedure has been utilized for determining luteinizing hormone-releasing hormone (LHRH) concentrations in rat pituitary stalk plasma. With this new recovery protocol it was revealed that stalk plasma immunoreactive LHRH concentrations increased 5-fold to 206 pg/ml during proestrus (p < 0.001) and decreased to diestrous levels during estrus. In contrast, the LHRH concentration in systemic plasma extracts remained unchanged throughout the estrous cycle and did not exceed 4 pg/ml. The stalk plasma:systemic plasma ratio increased from 12:1 at diestrus to 76:1 at proestrus; it then decreased to 22:1 at estrus (p < 0.001). Correlative luteinizing hormone (LH) and follicle-stimulating hormone (FSH) surges of statistically significant magnitudes were observed in the systemic plasma of nonanesthetized cardiac-catheterized rats during proestrus. When administered before the onset of the critical period, Althesin and other anesthetic agents suppressed but did not completely inhibit the peripheral LH surge in cardiac-catheterized rats.The FSH surge was suppressed also but to a lesser degree. These results indicate that the brain triggers the preovulatory surge of LH and FSH via massive secretion of LHRHinto the pituitary portal circulation. They also reveal that, aside from any dosage considerations, the type of anesthetic used and the time of administration in relation to the critical period, can significantly affect the magnitude of pituitary LH and FSH secretion.

In vitro basal and GnRH-stimulated secretion of gonadotrophins reflects long-lasting modulatory effects, and peripheral levels are not predicted by pituitary responsiveness to GnRH

“…The role of GnRH in the generation of FSH profiles has been debated for some time. The rise in serum FSH at the same time as that of LH is consistent with GnRH being primarily responsible for the first phase of the FSH rise (27,45,46). At the end of prooestrus, however, serum FSH was still raised, as previously observed in several studies (1)(2)(3), and in contrast to the fall in LH.…”

“…Basal release had a temporal pattern similar to that for peripheral LH levels. GnRH increases have been observed in the portal blood in parallel with peripheral LH (26,27) and it is believed that increased GnRH secretion is the important factor in eliciting the LH surge (28). In the present study, GnRH-stimulated release from the pituitary in vitro followed a similar pattern to basal release.…”

“…Thus, lower basal LH secretory activity (as observed after the LH peak), even accompanied by high responsiveness, is an impediment to high in vivo levels. There is some debate over the profile of GnRH during the declining phase of the LH surge, some noting an early decline after peak levels are reached (26), whereas others have observed a plateau of retained high GnRH levels (27). However, whatever the levels of GnRH which are delivered via the portal blood system to the pituitary, the control of basal secretory levels by the modulatory processes in vivo is apparently an important component of the regulation of the gonadotrophin serum profile.…”

Gonadotropin-Releasing Hormone Neurons Express KATPChannels That Are Regulated by Estrogen and Responsive to Glucose and Metabolic Inhibition

“…Gonadotropinreleasing hormone (GnRH), the primary neural signal involved in activation of luteinizing hormone (LH) release from the pituitary, is synthesized and secreted episodically by hypothalamic GnRH neurons into the hypothalamo-hypophysial portal system Ramirez, 1980, 1982;Sarkar and Fink, 1980;Ching, 1982;Moenter et al, 1992). Estrogen appears to be the primary humoral signal responsible for the preovulatory LH and GnRH surges as well as negative feedback to suppress their secretion (Legan et al, 1975;Terasawa et al, 1979;Caraty et al, 1989;Terasawa, 1994).…”

The refinement of the pulsatile Luteinizing Hormone (LH) profile following pubertal maturation: a potential mechanism for enhanced recruitment of ovarian follicles for improved fertility

“…The release of GnRH into hypopheal portal system from nerve terminals located in the median eminence occurs in a pulsatile fashion, which creates the pattern of episodic LH release [118,119]. Fluctuations in the pattern of GnRH release combined with alterations in the secretory capacity of the pituitary gonadotrophs generate the marked changes in LH secretion profile observed over the course of the ovarian cycle [120][121][122]. LH and FSH secreted by the anterior lobe of pituitary gland are produced in a pulsatile fashion with differential frequencies and amplitude regulated by gonadotropin releasing hormone (GnRH) [123].…”