Serotonin Receptor 1A (HTR1A), a Novel Regulator of GnRH Neuronal Migration in Chick Embryo.

The hypothalamic GnRH neurons are a small group of cells that regulate the reproductive axis. These neurons are specified within the olfactory placode, delaminate from this structure, and then migrate to enter the forebrain before populating the hypothalamus. We have used microarray technology to analyze the transcriptome of the olfactory placode at a number of key time points for GnRH ontogeny using the chick embryo. This resulted in the identification of a large number of genes whose expression levels change significantly over this period. This repertoire includes those genes that are known to be important for GnRH neuronal development as well as many novel genes, such as the serotonin receptor 1A, HTR1A. We find that HTR1A is expressed in the region of the olfactory placode that generates GnRH neurons. We further show that when this receptor is inactivated using a selective HTR1A antagonist as well as a gene knockdown approach using RNAi, this resulted in delayed migration causing the GnRH neurons to stall just outside the forebrain. These findings implicate HTR1A as being important for GnRH neuronal migration from the olfactory placode to the forebrain. Our study thus extends the repertoire of genes involved in GnRH neuron biology and thus identifies new candidate genes that can be screened for in patients who do not show mutations in any of the previously identified hypogonadotrophic hypogonadism/Kallmann syndrome genes.