Estrogen-Regulated Lateral Septal Kisspeptin Neurons Abundantly Project to GnRH Neurons and the Hypothalamic...

While hypothalamic kisspeptin (KP) neurons play well-established roles in the estrogen-dependent regulation of reproduction, little is known about extrahypothalamic KP-producing (KP_LS) neurons of the lateral septum. As established previously, Kiss1 expression in this region is low and regulated by estrogen receptor- and GABA_B receptor-dependent mechanisms. Our present experiments on Kiss1-Cre/ZsGreen knock-in mice revealed that transgene expression in the LS begins at Postnatal Day (P)33-36 in females and P40-45 in males and is stimulated by estrogen receptor signaling. Fluorescent cell numbers continue to increase in adulthood and are higher in females. Viral tracing uncovered that the bulk of KP_LS fibers joins the medial forebrain bundle and terminates in the hypothalamic supramammillary nucleus. Smaller subsets innervate the medial amygdala or project to other limbic structures. One-quarter of gonadotropin-releasing hormone (GnRH)-immunoreactive perikarya in the preoptic area and their dendrites receive appositions from KP_LS axons. OVX adult Kiss1-Cre/ZsGreen mice treated for 4 d with 17β-estradiol or vehicle were used for RNA sequencing studies of laser-microdissected KP_LS neurons. The transcriptome included markers of GABAergic and neuropeptidergic (Penk, Cartpt, Vgf) cotransmission and 571 estrogen-regulated transcripts. Estrogen treatment upregulated the acetylcholine receptor transcript Chrm2 and, in slice electrophysiology experiments, caused enhanced muscarinic inhibition of KP_LS neurons. Finally, we provided immunohistochemical evidence for homologous neurons in the postmortem human brain, suggesting that KP_LS neurons may contribute to evolutionarily conserved regulatory mechanisms. Future studies will need to investigate the putative roles of KP_LS neurons in the estrogen-dependent control of GnRH neurons and/or various hypothalamic/limbic functions.