Neuroendocrine Control of Fertility by Leptin and Insulin via Glutamate Neurons

Abstract

Obesity is a growing problem in the western world, and this is accompanied by an increase in infertility. In fact, the link between nutrition and fertility occurs at both ends of the body weight spectrum: severely obese or undernourished individuals have altered puberty onset and impaired fertility.

The pathways used by hormones known to signal metabolic status to modulate fertility are currently unknown. There is evidence that leptin and insulin act in the brain to affect the GnRH (gonadotropin releasing hormone) neurons that control the HPG (hypothalamic-pituitary-gonadal) axis. However, it is now known that they do not target GnRH neurons directly, but through intermediate neurons. These intermediate neurons have not yet been determined, although a number of neuronal subtypes have been tested.

A key area thought to be involved in the nutritional control of fertility is the PMv (ventral premammillary nucleus). This nucleus responds to leptin, and has been shown to contain neurons that innervate sites of GnRH neurons. The PMv is also known to contain an abundance of glutamate neurons. This has led to the hypothesis that leptin (and possibly insulin) acts through glutamatergic neurons in the PMv to control fertility.

The hypothesis that leptin and insulin modulate fertility via glutamatergic neurons was tested using a conditional knock-out mouse model, where insulin or leptin receptors were knocked out of glutamate neurons using the Cre-Lox system. This was confirmed using a GFP-reporter mouse, to verify that the Cre recombinase used to induce the knock-out had occurred in brain regions known to express glutamate.

The fertility of these mice was then examined. The onset of puberty was measured in females by determining the age at vaginal opening and first estrus, and in males by the date of the first successful mating. There was no effect on puberty onset when leptin or insulin receptors were knocked out. Estrous cycles were then examined in the female mice, but there was no difference when compared to control animals.

A major part of this study was to examine the fertility and fecundity of the knock-out mice. The mice were paired with a wild-type mate, and the dates of birth of the litters used to determine litter-to-litter interval, and therefore the fertility, and the litter size was used to determine fecundity. However, there were no fertility defects in either the leptin or insulin receptor knock-out mice when compared to the controls.

The conclusions from this study are therefore that leptin and insulin do not target glutamate neurons (such as those in the PMv) in the control of fertility. This result has made it easier to narrow down the targets used, since a large group of neurons can now be excluded. Concurrent research in our laboratory has highlighted GABAergic neurons as a likely alternate candidate for mediating leptin’s effects on fertility. If the targets of metabolic hormones in the control of fertility can be determined this may have important implications for designing novel fertility treatments.