Insulin Action on γ-Aminobutyric Acid (GABA) Neurons is Not Required for Puberty Onset or Reproductive Competency in Mice

Program: Abstracts - Orals, Featured Poster Presentations, and Posters
Session: MON 112-141-Hypothalamus-Pituitary Development & Biology
Basic/Clinical
Monday, June 17, 2013: 1:45 PM-3:45 PM
Expo Halls ABC (Moscone Center)

Poster Board MON-130
Maggie Corr* and Greg M Anderson
University of Otago School of Medical Sciences, Dunedin, New Zealand
Insulin signaling in the brain plays a critical role in the central regulation of fertility, such that mice exhibiting neuron-specific deletion of insulin receptors (InsR) display hypothalamic hypogonadism [1]. However, the specific neurons mediating insulin’s central effects on fertility remain unidentified. Evidence suggests insulin does not signal directly on the gonadotropin-releasing hormone (GnRH) neurons, as mice lacking InsR specifically in GnRH neurons exhibit normal reproductive function [2]. The neurotransmitter GABA is an important upstream modulator of GnRH neurons [3], and GABAergic neurons are widely distributed in the hypothalamus. We therefore hypothesized that insulin’s central effects on fertility are mediated indirectly via insulin signaling on GABAergic neurons. We used the Cre-loxP system to generate mice with a selective inactivation of the InsR gene from GABAergic (Vgat+) cells by crossing InsR-flox mice with Vgat-Cre mice. Multiple reproductive parameters were then compared between knockout (KO) mice (InsR-flox/Vgat-Cre+) and their control littermates (InsR-flox/Vgat-Cre-). Surprisingly, given the widespread nature of GABA expression in the hypothalamus, KO mice (11 males, 8 females) exhibited normal reproductive function compared to controls (7 males, 12 females). No difference in the age of puberty onset (determined in males by backdating 20 days from when they first sired a litter, and at first estrus in females) was observed between KOs and controls (males: 50.9 ± 2.8 vs 49.9 ± 6.0 days, P = 0.617; females: 36.9 ± 3.6 vs 36.0 ± 2.6 days, P = 0.603). Mean estrous cycle length (average estrus-to-estrus interval) did not differ between female KOs and controls (5.3 ± 1.1 vs 5.4 ± 1.4 days, P = 0.818). Lastly, KO mice exhibited normal fertility compared with controls, which was assessed by individually pairing experimental animals with a wild-type mate for at least 100 days to determine mean litter size (males: 7.1 ± 1.4 vs 8.5 ± 2.9 pups per litter, P = 0.208; females: 8.4 ± 1.3 vs 8.9 ± 1.3 pups per litter, P = 0.464) and mean inter-litter interval (males: 27.5 ± 8.3 vs 28.7 ± 13.7 days, P = 0.812; females: 22.9 ± 2.4 vs 23.9 ± 4.6 days, P = 0.117). We conclude that GABAergic cells do not mediate insulin’s central effects on puberty onset or reproductive competency in mice. Future studies are required to determine the mechanism whereby insulin exerts its central effects on the reproductive axis.

(1) Bruning, J.C., et al., Role of brain insulin receptor in control of body weight and reproduction. Science, 2000. 289(5487): p. 2122-5. (2) Divall, S.A., et al., Divergent roles of growth factors in the GnRH regulation of puberty in mice. The Journal of Clinical Investigation, 2010. 120(8): p. 2900-9. (3) Sullivan, S.D. and S.M. Moenter, Gamma-aminobutyric acid neurons integrate and rapidly transmit permissive and inhibitory metabolic cues to gonadotropin-releasing hormone neurons. Endocrinology, 2004. 145(3): p. 1194-202.

Nothing to Disclose: MC, GMA

*Please take note of The Endocrine Society's News Embargo Policy at http://www.endo-society.org/endo2013/media.cfm

Sources of Research Support: The Health Research Council of New Zealand