Somatostatin Receptor 2 Antagonist, CYN154806, Stimulates Steroid Independent Episodic LH Secretion in Ewes
Presentation Number: SUN 473
Date of Presentation: April 2nd, 2017
Richard B. McCosh*, Brett M Szeligo, Michelle N. Bedenbaugh, Justin A. Lopez, Steven L. Hardy, Stanley M. Hileman, John M Connors and Robert L. Goodman
West Virginia University School of Medicine, Morgantown, WV
Administration of somatostatin (SST) or a SST receptor 2 (SSTR2) agonist inhibits pulsatile LH secretion in sheep and humans, respectively, and we have recently demonstrated, using an SSTR2 antagonist, that endogenous SST suppresses episodic LH secretion in estradiol and progesterone treated ewes. To determine if this effect is steroid dependent, we administered the SSTR2 antagonist, CYN154806 (CYN), to ovary intact anestrous ewes (presence of estradiol only) and to ovariectomized (OVX) ewes. Chronic cannulae were placed into the 3rd cerebral ventricle of ewes early in anestrus. Ewes (n=8) received a 60nmol injection of CYN or saline (SAL); jugular blood samples were collected every 12 min for 2 hrs pre- and 4 hrs post- injection. This protocol was repeated 4 days later using a cross-over design. CYN significantly (P = 0.006) increased mean LH concentrations during the first 2 hrs after injection from 1.5 ± 0.2 ng/mL to 5.4 ± 1.1 ng/mL, while SAL did not (pre:1.3 ± 0.1 ng/mL; post: 2.9 ± 0.6ng/mL). To test the effects of CYN in the absence of gonadal hormones, the same protocol was performed in OVX ewes (n=6) in anestrus with similar 3rd ventricle cannulae. CYN caused a shorter (P = 0.015) LH inter-pulse interval (40.3 ± 6.9 min) during the 4hrs after injection compared to SAL (71.7 ± 9.6 min). The first pulse after CYN injection tended (P=0.06) to occur sooner (32 ± 6.7 min) than the first pulse after SAL injection (64 ± 14.4 min). To identify possible neuroanatomical substrates for the inhibitory effect of SST on LH we quantified c-Fos co-localization within subpopulations of GnRH and kisspeptin cells. Intact anestrus ewes were euthanized 2 hrs after injection with CYN (n = 4) or SAL (n = 4). CYN treatment caused an increase (P=0.01) in the percentage of GnRH cells within the mediobasal hypothalamus (MBH) that contained c-Fos (CYN: 25 ± 3.6%; SAL: 7.5 ± 1.8%), but did not alter the percentage of GnRH cells that contained c-Fos within other areas, or the total number of GnRH cells in any area. CYN administration caused a significant (P = 0.02), though modest, increase in the percentage of kisspeptin cells that contained c-Fos within the caudal arcuate nucleus (ARC; CYN: 9.5 ± 2.7%; SAL: 1.2 ± 0.5%), but not in the rostral or middle regions of ARC, or the total number of kisspeptin cells in any portion of ARC. These data demonstrate that endogenous SST acts, at least in part, through SSTR2 to suppress pulsatile LH secretion independent of gonadal steroid status. SSTR2 blockade activated the same population of GnRH cells that are activated when pulsatile LH secretion is increased by other methods (Boukhliq et al., ENDO140:5929-36, 1999). Together these data support the hypothesis that endogenous SST suppresses pulsatile LH secretion via inhibition of GnRH, and possibly kisspeptin, cells within the MBH, and raise the possibility that SST neurons are involved in the steroid-independent actions of inhibitory photoperiod in ewes.
Nothing to Disclose: RBM, BMS, MNB, JAL, SLH, SMH, JMC, RLG