Evidence that synaptic plasticity of glutamatergic inputs onto KNDy neurones during the ovine follicular phase is dependent on increasing levels of oestradiol

Neurones in the arcuate nucleus co‐expressing kisspeptin, neurokinin B (NKB) and dynorphin (KNDy) play a critical role in the control of gonadotrophin‐releasing hormone (GnRH) and luteinising hormone (LH) secretion. In sheep, KNDy neurones mediate both steroid‐negative‐ and ‐positive‐feedback during pulsatile and preovulatory surge secretions of GnRH/LH, respectively. In addition, KNDy neurones receive glutamatergic inputs expressing vGlut2, a glutamate transporter that serves as a marker for those terminals, from both KNDy neurones and other populations of glutamatergic neurones. Previous work reported higher numbers of vGlut2‐positive axonal inputs onto KNDy neurones during the LH surge than in luteal phase ewes. In the present study, we further examined the effects of the ovarian steroids progesterone (P) and oestradiol (E2) on glutamatergic inputs to KNDy neurones. Ovariectomised (OVX) ewes received either no further treatment (OVX) or steroid treatments that mimicked the luteal phase (low E2 + P), and early (low E2) or late follicular (high E2) phases of the oestrous cycle (n = 4 or 5 per group). Brain sections were processed for triple‐label immunofluorescent detection of NKB/vGlut2/synaptophysin and analysed using confocal microscopy. We found higher numbers of vGlut2 inputs onto KNDy neurones in high E2 compared to the other three treatment groups. These results suggest that synaptic plasticity of glutamatergic inputs onto KNDy neurones during the ovine follicular phase depend on increasing levels of E2 required for the preovulatory GnRH/surge. These synaptic changes likely contribute to the positive‐feedback action of oestrogen on GnRH/LH secretion and thus the generation of the preovulatory surge in the sheep. Kisspeptin/neurokin B/dynorphin (KNDy) cells play a critical role in the control of gonadotrophin‐releasing hormone (GnRH) and luteinising hormone (LH) secretion in sheep and other mammals. We previously demonstrated an increase in the number of glutamatergic synaptic inputs to KNDy neurones during the follicular phase of the ovine oestrous cycle. Here, we investigated whether this plasticity is a result of changing hormone levels, and also show that the change depends on increasing levels of oestradiol seen during the follicular phase, suggesting that this aspect of synaptic plasticity may play a role in the generation of the preovulatory GnRH/LH surge.