SUN-257 Evidence that Urocortin 2 Contributes to the Suppressive Effect of Metabolic Stress on LH Secretion in Female Mice

Abstract

Pulsatile luteinizing hormone (LH) secretion is disrupted by numerous stimuli including metabolic stress. Insulin-induced hypoglycemia is a model of metabolic stress that suppresses LH secretion in numerous species including mice. Our recent work provides evidence that this inhibition of LH secretion occurs via suppression of neurons that contain kisspeptin (Kiss1), neurokinin B (NKB) and dynorphin (Dyn) in the arcuate (ARC) nucleus (KNDy cells). Thus, our current objective is to identify the neural components responsible for the suppression of KNDy cells during metabolic stress. Several lines of evidence support the hypothesis that the neuropeptide urocortin 2 (UCN2) has a key role in the inhibition of LH during stress in rats. First, ICV injection of UCN2 suppresses LH secretion. Second, an antagonist to the receptor for UCN2 reverses the suppression of LH during metabolic stress. Finally, restraint and osmotic stress increase UCN2 mRNA abundance in the paraventricular nucleus (PVN). To determine if UCN2 neurons in the PVN are activated during metabolic stress we performed immunohistochemistry for UCN2 and c-Fos in tissue collected 120 min after saline or insulin (0.75mU/kg) injection (n = 2/group, ovariectomized, adult, C57/BL6). Insulin significantly increased both the number of UCN2 cells (saline: 109.0 ± 8.5, insulin: 156.3 ± 10.8 cells) and the percentage of UCN2 cells that expressed c-Fos (saline: 13.1 ± 2.5%, insulin: 31.2 ± 0.8%). Next, we administered UCN2 (7.23nmol) via ICV injection to determine if this molecule suppresses LH secretion and/or mRNA abundance of KNDy genes. LH was measured in serial blood samples collected from 60 min prior to and 30-90 min following injection. Tissue was collected 3 h after ICV injection to confirm injection site and quantify mRNA abundance in ARC micropunches. In saline-treated mice (n = 5 successful injections), mean LH concentration and the number of LH pulses did not differ across sampling periods (mean: 6.4 ± 0.4 ng/mL vs. 6.0 ± 0.4 ng/mL; pulses: 2.6 ± 0.2 vs. 3.0 ± 0.3, pre vs. post). In contrast, in mice with successful UCN2 injections (n = 4) there was a significant reduction in both mean LH and the number of LH pulses following UCN2 (mean: 5.0 ± 0.3 ng/mL vs. 1.4 ± 0.2 ng/mL; pulses: 3.0 ± 0.0 vs. 0.25 ± 0.25, pre vs post). UCN2-treated animals had a significant reduction in the abundance of mRNAs encoding Kiss1 (~35%) and NKB (~40%) compared to saline-treated animals; the abundance of Dyn mRNA did not differ between treatments. These data demonstrate that PVN UCN2 cells are activated during metabolic stress and that UCN2 is sufficient to suppress LH secretion and the expression of genes involved in stimulating LH pulses. These data support the hypothesis that UCN2 released from neurons in the PVN impairs KNDy cell function and LH secretion during acute stress.