Background: The hypothalamic neuropeptides kisspeptin and neurokinin B (NKB) are both obligate for normal gonadotrophin secretion. Studies in patients with loss-of-function mutations in NKB signalling suggest that kisspeptin is functionally upstream of NKB, but this hierarchy is unexplored in healthy men and women. We hypothesised that kisspeptin augmentation of estrogen-induced mid-cycle LH secretion will not be abrogated by pharmacological blockade of NKB.
Methods: Ten healthy women were administered NKBR antagonist AZD4901 (AstraZeneca) 40 mg po bd (or no treatment controls, n=9) from cycle day 45 for 6 days; transdermal estradiol (200 μg/day) was applied after 5 days and induced LH secretion 48 h later (controls: 0 h: 4.4±0.6; 24 h: 3.1±0.3; 48 h: 8.6±1.0; 72 h 9.1±2.8 IU/l, P<0.0001). At 24 h estradiol treatment, women were randomised to 7 h kisspeptin-10 (4 μg/kg per hr) or saline infusion, returning in a subsequent cycle for the alternate infusion. LH was analysed by t-test and ANOVA with Bonferroni multiple comparison post hoc analysis.
Results: NKBR antagonist did not affect estrogen-induced LH secretion (48 h: 9.7±2.2; 72 h: 6.5±1.2 IU/l vs estrogen only, ns). During estrogen treatment, kisspeptin-10 stimulated LH secretion, from 3.8±0.6 to 17.3±4.3 IU/l (P<0.0001) at the end of infusion; LH remained significantly elevated at 48 and 72 h (15.3±4.0 and 8.1±1.3 IU/l, both P<0.01 vs start of infusion). NKBR antagonist did not affect the immediate response to kisspeptin-10 (21.6±5.6 vs 17.3±4.3 IU/l, ns). However, NKBR antagonist blunted the duration of the response, with LH being significantly lower at 48 h (7.5±1.5 vs 15.3±4.0, P<0.05).
Conclusions: Kisspeptin-10 advanced estrogen-induced LH secretion. Treatment with the NKB antagonist did not affect the positive feedback response to estrogen, and while the immediate LH response to kisspeptin was maintained, the duration was shortened. These data suggest neurokinin B is predominantly proximal to kisspeptin in the regulation of GnRH, but indicate a complex interaction between these neuropeptides.