Ontogeny of gonadotropin-releasing hormone (GnRH) gene expression reveals a distinct origin for GnRH-containing neurons in the midbrain.

In the teleost fish, Haplochromis burtoni, three gonadotropin-releasing hormone (GnRH) peptides and their corresponding cDNA sequences and full-length genes have previously been reported. Here we describe the ontogeny of mRNA expression for these three GnRH forms in H. burtoni. Each of the three forms has been shown to have a distinct spatial expression pattern in the adult brain. ¿Ser8¿GnRH (the releasing form) is expressed exclusively in the hypothalamus, ¿His5Trp7Tyr8¿GnRH is expressed in the midbrain mesencephalon, and ¿Trp7Leu8¿GnRH is expressed in the terminal nerve area of the telencephalon. Previous work in other animals has shown that GnRH-containing neurons in the preoptic area arise from the olfactory placode and that these cells migrate into their final positions in the brain during early development. By using molecular probes to identify the cell types expressing distinct GnRH forms, our data are consistent with the migration of both ¿Ser8¿GnRH and ¿Trp7Leu8¿GnRH neurons from the placode to their appropriate adult locations in the brain. In contrast, we show that ¿His5Trp7Tyr8¿GnRH neurons arise from the germinal zone of the third ventricle. By using in situ hybridization with digoxigenin-labeled cRNA probes, ¿His5Trp7Tyr8¿GnRH mRNA was first evident at day 4, ¿Trp7Leu8¿GnRH mRNA at day 8, and ¿Ser8¿GnRH mRNA at day 14. However, by using the reverse-transcriptase polymerase chain reaction (RT-PCR), all three GnRH mRNAs were found in whole embryos at day 4 of the 14 days of embryogenesis. This striking difference may be due to the greater sensitivity of RT-PCR compared with in situ hybridization. Alternatively, it is possible that ¿Ser8¿GnRH and ¿Trp7Leu8¿GnRH are expressed outside the brain during early development and only later inside the brain.