Endocrine Abstracts

Animals living outside the tropics use changes in daylength to adapt to seasonal changes in environment, but the molecular and endocrine mechanisms underlying seasonal reproduction are not fully understood. The Japanese quail is a robust model for the study of these mechanisms because of its rapid and dramatic response to changes in photoperiod. In the previous study, we have demonstrated that local thyroid hormone catabolism within the mediobasal hypothalamus (MBH) by thyroid hormone-activating enzyme (type 2 deiodinase: DIO2) regulates the seasonal reproduction. Rapid induction of DIO2 gene expression in the ependymal cells (EC) lining ventrolateral walls of third ventricle of the MBH is the earliest event yet recorded in the photoperiodic signal transduction pathway. To address the identity of the photoperiodic transduction pathway, we have dissected the molecular dynamics of gene expression regulating photoinduced thyroid hormone metabolism using a chicken high-density oligonucleotide microarray. We identified two waves of gene expression. The first was initiated ~14 h after dawn of the first long day and included increased TSH β subunit expression in the pars tuberalis of the pituitary gland; the second occurred ~4 h later in the EC and included increased DIO2 expression. TSH receptor was found in the EC of the MBH and intracerebroventricular administration of TSH to short day quail stimulated gonadal growth, and expression of DIO2. This TSH induced expression of DIO2 was shown to be mediated through a thyrotropin receptor-cAMP signalling pathway by the promoter analysis. Increased pars tuberalis TSH therefore appeared to trigger long day photoinduced seasonal breeding. In addition, we have also demonstrated the involvement of TSH signalling pathway in mammals by using the TSH receptor null mice.