SAT-425 NKB Signaling in the Posterodorsal Medial Amygdala Stimulates Gonadotropin Release in a Kisspeptin-Independent Manner in Female Mice

Abstract

Female reproduction is regulated by a complex signaling network, the function of which is dependent on the level of circulating estrogens. Neurokinin B (NKB) colocalizes with kisspeptin in Kiss1 neurons of the arcuate nucleus (ARC). The autosynaptic action of NKB via the neurokinin 3 receptor (NK3R) in these neurons induces kisspeptin release in the presence of sex steroids. This has led us to believe that NKB stimulates LH secretion in a kisspeptin-dependent manner despite the latter mechanism having been investigated in the persistent hypogonadal state [e.g. Kiss1r knock out (KO) mice and agonadal monkeys]. Here, we aimed to assess whether in the presence of estradiol, senktide, a NK3R specific agonist, can stimulate LH release in a kisspeptin-independent manner. Adult WT female mice were ovariectomized (OVX) or OVX and estradiol-treated (OVX+E2) and studied in parallel to Kiss1KO or Kiss1KO+E2 littermates (n=10/group). Intracerebroventricular (icv) injections of senktide (600pmol) were performed and blood samples were collected before and 25 min after icv injection. Interestingly, the presence of E2 in Kiss1KO females resulted in LH increase like that observed in WT animals. Subsequently, we aimed to locate the brain area which senktide may be acting to stimulate LH release in Kiss1KO+E2 mice. We mapped the distribution of NK3R in the mouse hypothalamus and moreover investigated the anatomical relationship of NK3R and GnRH expression with dual-label immunohistochemistry as a likely pathway of the kisspeptin-independent stimulation of LH release exerted by NKB. We observed no instances of co-expression between NK3R and GnRH cell bodies (>100 cells analyzed from a total of 16 mice) and only minimal close appositions between the two proteins in the ARC and medial septum. Thus, the kisspeptin-independent action of NKB cannot be attributed to direct stimulation of NK3R located on GnRH neurons. Interestingly, we observed that NK3R immunopositive cells in the posterodorsal medial amygdala (MePD), unlike the ARC, were upregulated in the presence of E2. Next, WT OVX+E2 and Kiss1KO+E2 females were injected with senktide (600pmol; n=5/group) in the ARC, the AVPV/PeN or the MePD; areas found to contain NK3R-expressing neurons, and blood samples were collected before and then every 15 min for 45 min. Senktide significantly stimulated LH secretion in all three areas in WT OVX+E2, but only in the MePD of Kiss1KO+E2 mice. Conversely, in the absence of E2, Kiss1KO females injected with senktide in the MePD did not show any alteration in LH, mimicking the results we obtained after icv administration. These data demonstrate that activation of NK3R in the amygdala stimulates LH release in a kisspeptin independent manner in the presence of estrogen. This novel pathway for LH release, that bypasses the kisspeptin neuron, may play an important role in linking amygdala regulated social cues with gonadotropin release.