Leptin Signaling in GABA Neurons, But Not Glutamate Neurons, Is Required for Reproductive Function

Abstract

The adipocyte-derived hormone leptin acts in the brain to modulate the central driver of fertility: the gonadotropin releasing hormone (GnRH) neuronal system. This effect is indirect, as GnRH neurons do not express leptin receptors (LEPRs). Here we test whether GABAergic or glutamatergic neurons provide the intermediate pathway between the site of leptin action and the GnRH neurons. Leptin receptors were deleted from GABA and glutamate neurons using Cre-Lox transgenics, and the downstream effects on puberty onset and reproduction were examined. Both mouse lines displayed the expected increase in body weight and region-specific loss of leptin signaling in the hypothalamus. The GABA neuron-specific LEPR knock-out females and males showed significantly delayed puberty onset. Adult fertility observations revealed that these knock-out animals have decreased fecundity. In contrast, glutamate neuron-specific LEPR knock-out mice displayed normal fertility. Assessment of the estrogenic hypothalamic-pituitary-gonadal axis regulation in females showed that leptin action on GABA neurons is not necessary for estradiol-mediated suppression of tonic luteinizing hormone secretion (an indirect measure of GnRH neuron activity) but is required for regulation of a full preovulatory-like luteinizing hormone surge. In conclusion, leptin signaling in GABAergic (but not glutamatergic neurons) plays a critical role in the timing of puberty onset and is involved in fertility regulation throughout adulthood in both sexes. These results form an important step in explaining the role of central leptin signaling in the reproductive system. Limiting the leptin-to-GnRH mediators to GABAergic cells will enable future research to focus on a few specific types of neurons.

Leptin-dependent neuronal NO signaling in the preoptic hypothalamus facilitates reproduction

“…Since most of the nNOS neurons in the preoptic region express the NMDA receptor (26), which is involved in the onset of puberty (49) and regulates nNOS activity (19,26,50,51), it is tempting to speculate that glutamatergic neurons of the PMv morphologically and functionally interact with nNOS neurons of the preoptic region to regulate the activity of GnRH neurons, thus synchronizing the effects of leptin in the 2 regions. Curiously, cre-mediated excision of the LepR in glutamatergic neurons results in no striking metabolic or reproductive phenotype (52,53), suggesting that leptin signaling through glutamate is not required for fertility. Yet, the reactivation of the LepR in mice otherwise null for the LepR within the PMv, which houses predominantly glutamatergic neurons, is sufficient to rescue fertility (25).…”

Leptin-Responsive GABAergic Neurons Regulate Fertility through Pathways That Result in Reduced Kisspeptinergic Tone

“…Mice were anesthetized with isoflurane anesthesia and received an intracerebroventricular injection (Furuta et al, 2001) of 5 l kisspeptin-10 (Kp10, 1 nmol) or vehicle (0.9% NaCl; Gottsch et al, 2004;Castellano et al, 2005). Blood samples (200 l) were collected by retro-orbital bleeding (van Herck et al, 1998) 20 min postinjection. The animals were injected with Kp10 or vehicle in a crossover experimental design with an interval of 1 week.…”

“…We report that the ablation of leptin receptors in GABAergic neurons, but not glutamatergic neurons, recapitulates the ob/ob and db/db infertility phenotype in females (Swerdloff et al, 1978;Batt et al, 1982;de Luca et al, 2005). Cre/ϩ were generated by insertion of an internal ribosome entry site (IRES)-Cre recombinase cassette downstream of the stop codon of endogenous genes Vgat (vesicular GABA transporter) and Vglut2 (synaptic vesicular glutamate transporter), respectively (Vong et al, 2011). The Lepr lox/ϩ mice were generated by the insertion of two loxP sites and an frt site flanking the coding exon 17 of the leptin receptor gene (Balthasar et al, 2004;McMinn et al, 2004).…”

“…Food deprivation can delay, and even prevent, sexual maturation and disrupt fertility across species (Frisch, 1985;Ahima et al, 1996). In humans, starvation is associated with menstrual cycle disruption in females, mediated by a decrease in luteinizing hormone (LH) secretion (Welt et al, 2004) that reflects reduced activity of gonadotropin-releasing hormone (GnRH) neurons in response to metabolic cues (Mantzoros et al, 2011). Circulating LH levels return to normal once dietary needs are addressed (Schreihofer et al, 1993;Mantzoros et al, 2011).…”

Leptin receptor null mice with reexpression of LepR in GnRHR expressing cells display elevated FSH levels but remain in a prepubertal state

“…Expression of LepR only in the brain of mice otherwise null for LepR is sufficient to restore reproductive function in males and females (16). Vice versa, deletion of LepR from hypothalamic neurons precludes sexual maturation and reproductive function (11,40,55). These observations initially diminished the interest in the physiological actions of leptin outside of the brain.…”