Estrogen receptor expression in GnRH neurons: back to the future†

Estrogen feedback within the hypothalamic-pituitary-gonadal axis is mediated primarily by upstream estrogen-sensitive neuronal populations, most notably kisspeptin neurons, which translate circulating sex steroid levels into excitatory or inhibitory input to gonadotropin-releasing hormone (GnRH) neurons, the final neural output controlling pituitary gonadotropin secretion. Despite extensive research, the expression and role of estrogen receptors in GnRH neurons remain poorly understood. To systematically evaluate what can be inferred from transcriptomic data alone, we conducted a comprehensive analysis of open-source single-cell and single-nucleus RNA-sequencing (sc/snRNA-seq) datasets encompassing mouse embryogenesis through adulthood, supplemented by cross-species datasets from rat, squirrel, pig, and human hypothalami. In adult mice, Esr1 transcripts were detected in a distinct subset of Gnrh1+ cells more frequently than Esr2 transcripts. Notably, this expression was dynamic, with an increased proportion of Esr1+Gnrh1+ cells emerging during maturation, particularly after puberty. In contrast, Esr2 detection remained minimal across developmental stages. Pseudotime analysis showed Esr1+Gnrh1+ cells at late pseudotime, coinciding with advanced maturation states. Cross-species comparisons further revealed species-specific patterns, with Esr1+Gnrh1+ cells consistently more prevalent than Esr2+Gnrh1+ cells in rodents, while ESR2+GNRH1+ cells were slightly more detectable in pigs and humans. These results support a model where estrogen signaling within GnRH neurons is heterogeneous, developmentally regulated, and often mediated indirectly through upstream estrogen-sensitive circuits, such as kisspeptin and nitric oxide synthase neurons. This study provides a comprehensive, unbiased transcriptomic framework for understanding estrogen receptor expression in GnRH neurons, offering new insights into how estrogen might influence reproductive function through both direct and indirect signaling pathways.