**9. Kisspeptin expression is induced by glucagon-like peptide-1 in rHypoE8 cells and GT1-7 cells**

clearly increased by treatment with kisspeptin. Therefore, we could conclude that kisspeptin can stimulate GnRH synthesis in GnRH-expressing neurons in vivo. However, kisspeptin increased GnRH mRNA expression only up to about 1.5-fold [31]. In addition, we have found that GnRH stimulation increased the expression of the Kiss1 gene as well as that of the GnRH gene and also found that kisspeptin itself increased the expression of the Kiss1 gene. We postulate that GnRH neurons reversibly interact with kisspeptin neurons and also form an

**Figure 2.** Schematic summary of the regulation of Kiss1 mRNA and GnRH mRNA expression in primary cultures of fetal rat brain and the proposed interaction between kisspeptin neurons and GnRH neurons. In experiments using primary cultures of fetal rat neuronal cells, Kiss1 mRNA, but not GnRH mRNA expression, was upregulated by estradiol (E2). GnRH mRNA expression was clearly increased by treatment with kisspeptin. GnRH stimulation increased the expression of both Kiss1 and GnRH mRNAs, and kisspeptin itself was found to increase the expression of the Kiss1 gene. We postulate that GnRH neurons reversibly interact with kisspeptin neurons and also form an autocrine interaction with

Observations from the studies using GT1-7 cells and primary cultures of fetal rat brain imply that kisspeptin could affect GnRH neurons and increase GnRH expression. In addition, kisspeptin may change the GT1-7 cells' expression levels of Kiss1R. GnRH synthesis is not only regulated by kisspeptin, but several experimental reagents can modify the GnRH synthesis in GnRH-producing cells. Trichostatin A (TSA), a selective inhibitor of histone deacetylase, is an experimental reagent that modifies gene expression by opening chromatin structure through hyperacetylation of histones [32]. The structural change in chromatin allows transcription factors to bind DNA to modify gene expression. In GnRH-producing GT1-7 cells, TSA significantly reduced GnRH expression, with a concomitant increase in the gene encoding retinaldehyde dehydrogenase, which catalyzes the oxidation of retinol to retinoic acid [33]. Because retinoic acid also reduces GnRH expression in these cells, epigenetic mechanisms modified through retinaldehyde dehydrogenase, and retinoic acid might have an inhibitory effect on

autocrine interaction with kisspeptin neurons (**Figure 2**).

GnRH production (**Figure 1**).

kisspeptin neurons.

22 Hypothalamus in Health and Diseases

**7. Trichostatin A, a selective inhibitor of mammalian histone** 

**deacetylase, reduces GnRH expression in GT1-7 cells**

As described above, we used GT1-7 cells as a model for GnRH-producing neurons; however, GT1-7 cells also express the Kiss1 gene, which encodes kisspeptin [40]. rHypoE8 cells, another hypothalamic model that was developed from rat embryonic hypothalamic primary cultures, express the Kiss1 gene, and they also express the GnRH gene [22]. Because both rHypoE8 and GT1-7 are immortalized cell lines derived from heterogeneous hypothalamic cell populations, they express several types of neuropeptides. Using these hypothalamic cell models, we found that Kiss1 mRNA was regulated by several metabolic factors. Glucagon-like peptide-1 (GLP-1) is a gastrointestinal hormone produced by the small intestine and colon in response to food intake [41]. GLP-1 is also expressed in the central nervous system, and its expression in the brain is altered during fasting or feeding [42], suggesting that GLP-1 plays a role as a satiety factor. We found that GLP-1 increased the expression of Kiss1 mRNA in rHypoE8 cells as well as GT1-7 cells [43]. Moreover, leptin, which is an anorexigenic factor that is released from adipocytes, can also stimulate Kiss1 mRNA expression in these cells (**Figure 3**). These observations suggest that the levels of metabolic factors such as GLP-1 or leptin, which change during a state of starvation or negative energy balance, can critically influence the HPG axis by changing kisspeptin expression.

**Declaration of interest**

**Author details**

Japan

**References**

the impartiality of the review reported.

The authors declare that there is no conflict of interest that could be perceived as prejudicing

Studies on the Character of Hypothalamic GnRH Neurons and Kisspeptin Neurons Using…

http://dx.doi.org/10.5772/intechopen.73128

25

Department of Obstetrics and Gynecology, Shimane University Faculty of Medicine, Izumo,

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Haruhiko Kanasaki\*, Aki Oride, Tuvshintugs Tumurbaatar and Satoru Kyo

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**Figure 3.** The metabolic factors glucagon-like peptide-1 (GLP-1) and leptin can affect the HPG axis via kisspeptin neurons. From experiments using the hypothalamic cell models rHypoE8 and GT1-7 cells, we found that metabolic factors such as GLP-1 and leptin had the ability to stimulate Kiss1 gene expression. These metabolic factors may affect the HPG axis by modulating the synthesis and release of kisspeptin.

## **10. Conclusion**

Within the past decade, our understanding of the hypothalamic control of female reproductive function has matured considerably. The identification of hypothalamic kisspeptin, a regulator of GnRH, has provided us decisive insight into previously unanswerable questions. Kisspeptin neurons within the hypothalamus play a pivotal role in the control of the HPG axis, but it is still not entirely clear how kisspeptin release and expression are regulated in the brain during the reproductive cycle. Furthermore, the precise biology of kisspeptin and GnRH neurons remains unknown because of the difficulty of isolation of these neurons from heterogeneous neuronal populations of the hypothalamus.

In this review, we described our observations concerning the regulation of kisspeptin and GnRH neurons using hypothalamic cell models. Because we believe these cell models may reflect the original character of genuine kisspeptin and GnRH neurons, future studies using these cells are likely to contribute to our understanding of the mechanisms of regulation of the HPG axis.
