Gonadotropin releasing hormone-1 expression in incisors of mice

GnRH-1 is a decapeptide hormone that regulates gonadal maturation and fertility. In brain, GnRH-1 is secreted by neurons residing mainly in the preoptic/hypothalamic area. These neurons arise from cells in the nasal placode during embryonic development. GnRH-1 mRNA and peptide in the nonhypothalamic region have been described, suggesting other functions of GnRH-1. This paper describes for the first time the expression of GnRH-1 in developing incisors in mice. At embryonic day (E) 12.5, GnRH-1 mRNA and peptide were localized in cells in oral and dental epithelia. At postnatal day (P) 6, before incisor eruption, GnRH-1 was expressed in cells in dental epithelial-derived structures that include the papillary layer, outer dental epithelium, stellate reticulum, stratum intermedium, and enamel-secreting ameloblast cell layer. GnRH-1 expression correlated with cell maturity, becoming stronger in cells farther away from the proliferative zone. From E12.5 through P6, GnRH-1 expression was not detected in neural crest-derived dental mesenchyme or in mesenchyme-derived structures that include dental papilla, dental follicle, and dentin-secreting odontoblast. In addition, GnRH-1 expression was not detected in molars, indicating that expression of GnRH-1 is differentially regulated in incisor vs. molars, with only the former exhibiting continuous growth in this species. In homozygous hypogonadal mice at P1, GnRH-1 peptide expression was not detected, yet incisors were present. However, morphological changes in cells between dental follicle and ameloblast cell layer were noted. Taken together, our results indicate that GnRH-1 expression, although not essential for initiation and formation of incisors, may be important in maturation and/or maintenance of these placodally derived structures.