Abstract
Obesity is a significant public health concern due to its high prevalence and numerous comorbidities. It is associated with hypogonadism in males, characterized by low testosterone and sperm number. Previous studies determined that these stem from dysregulation of hypothalamic circuitry that regulates reproduction, by unknown mechanisms, since studies are confounded by chronic nature of this condition that leads to synaptic changes and alterations in neuron responsiveness to stimuli. Herein, we used mice fed chronic high-fat diet, that mimics human obesity, to determine mechanisms of impairment at the level of the hypothalamus, in particular gonadotropin-releasing hormone (GnRH) neurons that regulate luteinizing hormone levels (LH), which in turn regulates testosterone synthesis. Consistent with obese humans, we demonstrated lower LH, and lower pulse frequency of LH secretion, but unchanged pituitary responsiveness to GnRH. Pulse frequency of LH is regulated by pulsatile GnRH secretion, which is controlled by kisspeptin. Peripheral and central kisspeptin injections, and DREADD-mediated activation of kisspeptin neurons, demonstrated that kisspeptin neurons were suppressed in obese mice. Thus, we investigated regulators of kisspeptin secretion. We determined that LH response to NMDA was lower in obese mice, corresponding to fewer glutamate receptors in kisspeptin neurons, which may be critical for kisspeptin synchronization. Given that kisspeptin neurons also interact with POMC neurons, which regulate satiety and are particularly affected by obesity, we examined their crosstalk, and determined that LH response to either DREADD-mediated activation of POMC neurons or central injection of αMSH, a product of POMC, is abolished in obese mice. This was accompanied by diminished levels of αMSH receptor, MC4R, in kisspeptin neurons. The reason may be that chronically higher activity of POMC neurons in response to obesity, downregulates αMSH receptors on target neurons, including kisspeptin. This may lead to suppression of kisspeptin neurons, and their inability to regulate pulsatile secretion of GnRH. Together, our studies determined that obesity leads to downregulation of receptors that regulate kisspeptin neurons, which is associated with lower LH pulse frequency, leading to lower LH and hypogonadism.