Chronic Oestradiol Reduces the Dendritic Spine Density of KNDy (Kisspeptin/Neurokinin B/Dynorphin) Neurones in the Arcuate Nucleus of Ovariectomised Tac2‐Enhanced Green Fluorescent Protein Transgenic Mice

Neurones in the arcuate nucleus that express neurokinin B (NKB), kisspeptin and dynorphin (KNDy) play an important role in the reproductive axis. Oestradiol modulates the gene expression and somatic size of these neurones, although there is limited information available about whether their dendritic structure, a correlate of cellular plasticity, is altered by oestrogens. In the present study, we investigated the morphology of KNDy neurones by filling fluorescent neurones in the arcuate nucleus of Tac2‐enhanced green fluorescent protein (EGFP) transgenic mice with biocytin. Filled neurones from ovariectomised (OVX) or OVX plus 17β‐oestradiol (E₂)‐treated mice were visualised with anti‐biotin immunohistochemistry and reconstructed in three dimensions with computer‐assisted microscopy. KNDy neurones exhibited two primary dendrites, each with a few branches confined to the arcuate nucleus. Quantitative analysis revealed that E₂ treatment of OVX mice decreased the cell size and dendritic spine density of KNDy neurones. The axons of KNDy neurones originated from the cell body or proximal dendrite and gave rise to local branches that appeared to terminate within the arcuate nucleus. Numerous terminal boutons were also visualised within the ependymal layer of the third ventricle adjacent to the arcuate nucleus. Axonal branches also projected to the adjacent median eminence and exited the arcuate nucleus. Confocal microscopy revealed close apposition of EGFP and gonadotrophin‐releasing hormone‐immunoreactive fibres within the median eminence and confirmed the presence of KNDy axon terminals in the ependymal layer of the third ventricle. The axonal branching pattern of KNDy neurones suggests that a single KNDy neurone could influence multiple arcuate neurones, tanycytes in the wall of the third ventricle, axon terminals in the median eminence and numerous areas outside of the arcuate nucleus. In parallel with its inhibitory effects on electrical excitability, E₂ treatment of OVX Tac2‐EGFP mice induces structural changes in the somata and dendrites of KNDy neurones.