Gonadotropin-Releasing Hormone (GnRH) Agonists and GnRH Antagonists Do Not Alter Endogenous GnRH Secretion in Short-Term Castrated Rams

Abstract

The administration of GnRH agonists and antagonists suppresses pituitary LH secretion. However little is known about their effects on endogenous GnRH secretion. To determine if GnRH analogs act on GnRH secretion through a short or ultrashort loop feedback mechanism, experiments were performed to analyze GnRH secretion in hypophyseal portal blood of conscious short-term castrated rams under both agonist or antagonist treatment. In Study 1, six rams were castrated and surgically prepared for portal blood collection on day -7. Portal and peripheral blood were collected simultaneously every 10 min for 14-15 h on day 0. Five hours after the beginning of the portal blood collection, animals were injected im with 5 mg potent GnRH antagonist (Nal-Glu). In Study 2, six rams were treated daily from day -11 to day 0 with the GnRH agonist D-Trp6 GnRH (0.5 mg im). Castration and surgical preparation for portal blood collection were performed on day -7. On day 0 portal and peripheral blood were collected simultaneously every 10 min for 10-11 h. In both studies, to determine whether an increase in GnRH concentration in hypophyseal portal blood can overcome the inhibitory effect of the GnRH analogs, between 5 and 5.5 h after the injection of the analogs, endogenous GnRH secretion was stimulated by Naloxone administration (3 x 100 mg, iv, at 30-min intervals) followed by a bolus of exogenous GnRH (2 x 10 micrograms, iv at 30-min intervals). In Study 1, Nal-Glu administration led to a rapid cessation of pulsatile LH secretion for the duration of blood collection while GnRH pulse frequency and amplitude were not affected. GnRH and LH pulse frequency before and after Nal-Glu administration were, 6.2 +/- 0.6 vs. 5.7 +/- 0.8 (NS) and 5.3 +/- 0.3 vs. 0.3 +/- 0.2 pulses/6 h (P less than 0.001) respectively. In Study 2, peripheral LH secretion was completely suppressed while GnRH secretion (portal blood) remained pulsatile. GnRH pulses frequency and pulse amplitude were 4.3 +/- 0.3 pulses/6 h and 43.0 +/- 4.7 pg/ml, respectively. In both experiments, neither stimulation of endogenous GnRH secretion by naloxone nor administration of exogenous GnRH allowed reinitiation of LH secretion. However, additional studies in two animals of each treatment group (study-III) showed that this was clearly a dose related effect in antagonist treated but not in agonist-treated animals since higher doses of exogenous GnRH (i.e. 100 micrograms or 1000 micrograms) can increase significantly LH levels.(ABSTRACT TRUNCATED AT 400 WORDS)

On the Role of Gonadotropin-Releasing Hormone (GnRH) in the Operation of the GnRH Pulse Generator in the Rhesus Monkey

“…They are also in agreement with the report that neither a GnRH agonist (D-Trp6 GnRH, 0.5 mg intramuscularly (IM) daily for 12 days) nor a GnRH antagonist (Nal-Glu, 5 mg IM) affected the frequency (or amplitude) of GnRH pulses in the portal blood of short term castrated rams although pituitary LH secretion was inhibited [20]. Similarly, continuous intravenous (IV) infusions of GnRH to ovariectomized rats at rates ranging from 12.5 to 100 ng/h for 2 h failed to affect the frequency of LH pulses despite significantly stimulating basal LH secretion [31], On the other hand, the administration of a GnRH antagonist to ewes has been reported to accelerate GnRH pulse generator activity [17] suggesting an inhibitory ac tion of the decapeptidc in a physiological setting.…”

“…The role of GnRH is further confounded by the observation that GnRH antagonists have no effect on basal pulsatility in GT1-7 cells [18] and in some [15,20], albeit not all [17], in vivo experiments in sheep sug gesting that GnRH may not play a significant role in regu lating its own pulsatile secretion [15,18,20],…”

Evidence for Short or Ultrashort Loop Negative Feedback of Gonadotropin-Releasing Hormone Secretion

“…Note also the differences in GnRH and LH scales between different animals. Our results in the ewe, however, differ from those pre viously reported in male sheep [10], where no increase in GnRH secretion was seen following administration of Nal-Glu to gonadectomized rams. The discrepancy in the results of the two studies could be attributed to either a sex difference or to the inherently high endogenous pulse frequency in short-term castrate rams, which could have masked an effect of Nal-Glu in the previous study.…”

“…However, those studies were conducted in anesthetized hypophysectomized rats and involved limited sampling periods. More recent studies, in which long-term GnRH secretory dy namics were monitored in conscious sheep with an intact hypothalamopituitary axis, failed to support a role for a short or ultrashort loop feedback mechanism in the regula tion of GnRH secretion [10]. In those studies, administra tion of both GnRH agonists and antagonists to short-term castrate rams did not alter any aspect of GnRH secretion.…”

Does Gonadotropin-Releasing Hormone in the Cerebrospinal Fluid Modulate Luteinizing Hormone Release?

“…Such a cellular ultrashort-loop feedback system for GnRH could explain a recent report in ewes that intravenous administration of the GnRH antagonist, Nal-Glu, induced a rapid increase in GnRH secretion into the hypophysial portal system [35]. That study conflicted with an earlier investigation on short-term castrated rams in which intravenously administered Nal-Glu had no effect on portal GnRH concentrations [36], but this apparent discrepancy may be due to the high endogenous GnRH pulse frequency in these rams masking any effect of Nal-Glu [35]. Thus, whether a biologically active ultrashort-loop feedback system is operative at a cellular level in sheep remains unclear, but it should be noted that, unlike the rat, there is no anatomical evidence for direct contact between GnRH neurones in this species [37].…”