Insulin Receptor Signaling in the GnRH Neuron Plays a Role in the Abnormal GnRH Pulsatility of Obese Female Mice

<div><p>Infertility associated with obesity is characterized by abnormal hormone release from reproductive tissues in the hypothalamus, pituitary, and ovary. These tissues maintain insulin sensitivity upon peripheral insulin resistance. Insulin receptor signaling may play a role in the dysregulation of gonadotropin-releasing hormone (GnRH) secretion in obesity, but the interdependence of hormone secretion in the reproductive axis and the multi-hormone and tissue dysfunction in obesity hinders investigations of putative contributing factors to the disrupted GnRH secretion. To determine the role of GnRH insulin receptor signaling in the dysregulation of GnRH secretion in obesity, we created murine models of diet-induced obesity (DIO) with and without intact insulin signaling in the GnRH neuron. Obese control female mice were infertile with higher luteinizing hormone levels and higher GnRH pulse amplitude and total pulsatile secretion compared to lean control mice. In contrast, DIO mice with a GnRH specific knockout of insulin receptor had improved fertility, luteinizing hormone levels approaching lean mice, and GnRH pulse amplitude and total secretion similar to lean mice. Pituitary responsiveness was similar between genotypes. These results suggest that in the obese state, insulin receptor signaling in GnRH neurons increases GnRH pulsatile secretion and consequent LH secretion, contributing to reproductive dysfunction.</p></div

Impaired Estrogen Feedback and Infertility in Female Mice with Pituitary-Specific Deletion of Estrogen Receptor Alpha (ESR1)1

Mice lacking estrogen receptor alpha in the pituitary gonadotroph (PitEsr1KO) were generated to determine the physiologic role of pituitary estrogen signaling in the reproductive axis. PitEsr1KO female mice are subfertile or infertile and have elevated levels of serum luteinizing hormone (LH) and LH beta subunit gene expression, reflecting a lack of estrogen negative feedback effect on the gonadotroph. While serum LH values are elevated in PitEsr1KO mice, the degree of elevation is much less than that observed in ESR1-null mice, indicating that the hypothalamus must also have an important role in estrogen negative feedback. PitEsr1KO mice also demonstrate a defect in estrogen positive feedback, as surge LH values and estrous cyclicity are absent in these mice. Although sex steroid feedback in the reproductive axis is thought to involve discrete anatomic regions that mediate either a positive or negative estrogen effect, PitEsr1KO mice demonstrate novel evidence that localizes both estrogen positive feedback and estrogen negative feedback to the gonadotroph, which suggests that they may be mechanistically related

Metabolic function is similar between control and GnIRKO mice.

<p>(A) Body weight in lean mice maintained on regular chow and DIO mice introduced high fat diet at 8 weeks of life. (B) Serum insulin and (C) serum leptin after a 6 hour fast in mice fed high fat diet for 12 weeks. (D) Glucose tolerance test. Values are mean ± SEM. n = 6–8 each group *, p <0.05.</p

Impaired fertility of obese female mice is improved by deletion of IR in GnRH neurons.

<p>(A) Matrix of the breeding study. Each row represents a female, with each bar representing an individual pairing with the male of the column. A black bar indicates a resultant pregnancy, whereas a white bar indicates no pregnancy. (B) Calculated mating success rate under lean and DIO conditions. There is no statistical difference between groups with the same letters. (C) Resultant litter size and (D) Time to birth with observed pregnancies. Data are mean ± SEM.</p

Impaired ovulation is improved by deletion of IR in GnRH neurons.

<p>(A). Graphic representation of the estrous cycle in control DIO (left panel) and GnIRKO (right panel) determined by vaginal cytology for 13 days. (B) Time in each estrous cycle phase. (C) Total corpora lutea counts for control and GnIRKO DIO mice. Values are mean ± SEM. n = 4–6 each group. * p<0.05. (E) Representative control DIO ovary section. (F) GnIRKO ovary section. Asterisk denotes corpora luteum.</p

GnRH secretion is amplified in control DIO state.

<p>Hypothalami from mice were excised, incubated in <i>ex vivo</i> culture, and GnRH determined by RIA. Graphical representation of GnRH release from hypothalami of female (A) control lean (B) control DIO, (C) GnIRKO lean and (D) GnIRKO DIO. Asterisks denote significant peaks as determined by Cluster pulse analysis. Dotted line denotes the lower limit of the assay (1 pg/mL), (E) area under the curve (F) pulse amplitude, (G) pulse frequency, and (H) pulse interval for each group. Values are mean ± SEM. n = 7–8 mice per group. *, p< 0.05.</p

Basal LH is higher in control DIO animals.

<p>(A). Basal Luteinizing hormone (LH) levels across groups. (B) Basal testosterone. (C) LH and (D) FSH were determined at baseline and 40 minutes after injection of insulin. (E) LH and (F) FSH was determined at baseline and 10 minutes after injection of GnRH. Values are mean ±SEM. There is no statistical difference between groups with the same letters. n = 10–20 each group.</p