Dissecting the Involvement of Arcuate Nucleus Kisspeptin Neurons in Puberty Onset and LH Secretion

Abstract

Puberty is a crucial period of transition to adulthood, marked by an increased activation of gonadotropin-releasing hormone (GnRH) neurons that drives increased pulsatile secretion of pituitary luteinizing hormone (LH). The mechanisms governing GnRH neuron activation at puberty remain unclear but are likely due to enhanced signaling from upstream neuron populations, including kisspeptin neurons. Kisspeptin (encoded by Kiss1) directly stimulates GnRH neurons to drive GnRH release and downstream LH secretion. Humans and animals with Kiss1 mutations fail to reach puberty, demonstrating kisspeptin’s importance in puberty onset. Nonetheless, the specific brain area(s) from where kisspeptin signaling arises to trigger puberty remain undetermined. Kisspeptin is primarily expressed in two hypothalamic areas, the arcuate nucleus (ARC) and anteroventral periventricular (AVPV) region. ARC Kiss1 neurons are known to drive pulsatile GnRH/LH secretion in both sexes whereas AVPV Kiss1 neurons are sexually dimorphic and mediate the preovulatory GnRH/LH surge in females. We previously showed that Kiss1 gene expression increases in both the ARC and AVPV across the peri-pubertal period, yet it still remains to be determined whether just one or both of these populations is essential for proper pubertal timing. Indeed, the relative involvement of either ARC or AVPV Kiss1 neurons in the pubertal process still remains unknown. Here, we hypothesized that ARC Kiss1 neurons are required for normal puberty timing in both sexes and, conversely, AVPV Kiss1 neurons are not sufficient on their own to trigger normal puberty. To test this hypothesis, we used transgenic mice expressing diphtheria toxin receptor (DTR) exclusively in Kiss1 cells (Kiss Cre/iDTR flox) and took advantage of the differential ontogeny of ARC and AVPV Kiss1 neurons to selectively ablate ARC kisspeptin neurons before puberty, while leaving AVPV neurons intact. We found that targeted deletion of just ARC Kiss1 neurons during the juvenile period (which does not alter AVPV Kiss1 cell number) significantly delays puberty onset in both sexes, as measured by vaginal opening, first estrous, and preputial separation. In addition, these mice also exhibit decreased basal and pulsatile LH secretion in adulthood, further supporting a role for ARC kisspeptin neurons in GnRH pulse generation. By contrast, females with ablated ARC Kiss1 cells still exhibit full estradiol-induced LH surges, ruling out a necessary role of ARC kisspeptin neurons in that process and further supporting AVPV kisspeptin as the primary regulator of the surge. Collectively, our findings demonstrate that ARC Kiss1 neurons are required for both properly timed activation of the reproductive axis during puberty and proper pulsatile LH secretion in adulthood, while AVPV Kiss1 neurons are not sufficient to drive normal puberty onset but are sufficient for the preovulatory LH surge.