123 Nerve growth factor-induced ovulation in llamas: Evidence of hypothalamic refractoriness to nerve growth factor during the declining phase of the luteinising hormone surge

Abstract

Nerve growth factor (NGF) in semen is responsible for triggering ovulation after copulation in camelids. Interaction of NGF with its cognate receptors results in a preovulatory luteinising hormone (LH) surge that leads to ovulation, but the pharmacokinetics of NGF and the mechanism by which it mediates LH release are unknown. In an effort to elucidate the site and mechanism of action involved, the objective of this study was to determine whether the decline in the LH surge occurs as a result of pituitary depletion (i.e. diminished response to gonadotrophin-releasing hormone (GnRH)) or as an obtunded response at the level of the hypothalamus (diminished GnRH). Adult nonpregnant, nonlactating llamas (n=18) were synchronized by the administration of an ovulatory dose of a GnRH analogue (100μg IM; Fertiline, Vetoquinol). At 10 to 14 days later, the ovaries were examined to confirm the presence of a dominant follicle measuring ≥ 7mm, and a jugular catheter was put in place. The following day, a pretreatment blood sample was taken, and llamas were treated intravenously with 1mg of purified NGF from llama seminal plasma. Blood samples were taken every 30min for 7h from the time of NGF treatment. After the last blood sample was taken, llamas were treated with NGF (n=6; 1mg IV), GnRH (n=5; 100μg IV), or saline (Sal; IV; n=6), and blood samples were taken every 30min for another 7h. The ovaries were examined 48h after initial NGF treatment, via transrectal ultrasonography, to detect ovulation. Plasma was harvested and stored for analysis of LH concentration by radioimmunoassay. Data were compared using ANOVA for repeated measures, and single-point data were analysed using paired t-tests. As expected, most llamas ovulated in response to the initial NGF treatment (5 out of 5 in NGF-GnRH; 5 of 6 in NGF-NGF; 5 of 6 in NGF-Sal). Compared with pretreatment values, all llamas showed a 4-fold increase in plasma LH concentrations within 2h of the initial NGF treatment (P<0.05). Plasma LH concentrations peaked at 3h after initial NGF treatment and began to decline 4.5 to 5h after treatment (P<0.05). Plasma LH concentrations continued to decline following the second dose of NGF or Sal, whereas a transient elevation of LH was detected in llamas treated with GnRH (P<0.05). The LH concentration returned to basal levels (pretreatment) 8, 12, and 13h after NGF treatment in llamas treated with NGF-Sal, NGF-GnRH, and NGF-NGF, respectively. We conclude that the lack of LH response to the second dose of NGF is not because of pituitary depletion but rather due to diminished GnRH. The latter may be attributed to either a downregulation of NGF receptors within the hypothalamus or to temporary depletion of GnRH at the nerve terminals within the median eminence. Research was supported by the Natural Sciences and Engineering Research Council of Canada.