Meeting Abstract
69.1 Thursday, Jan. 6 Thyroid Hormones and Seasonality: The DIO2 Conundrum BENTLEY, GE; UC Berkeley gb7@berkeley.edu
Transfer of photoperiodic birds from short to long day lengths causes a rapid (within hours) release of gonadotropins. The earliest photoperiodic response measured to date is activation of the gene for type 2 iodothyronine deiodinase (DIO2), an enzyme which activates thyroid hormone. Long day lengths induce DIO2 expression in the pars tuberalis and mediobasal hypothalamus, therefore causing a local increase in T3. This increase in T3 is thought to be conveyed via an unknown pathway to gonadotropin-releasing hormone (GnRH) neurons in the pre-optic area. GnRH is then thought to be released to the pituitary gland and gonadotropins are released into the bloodstream to cause gonadal activation. This proposed mechanism for the transduction of changing day length into an endocrine response has been hailed as a conserved mechanism. It has been elegantly mapped out and is highly attractive, yet there are some key questions still remaining. For example, why do thyroidectomized animals still exhibit photoperiodic responses when there is no substrate present upon which DIO2 can act? Why does DIO2 remain upregulated on long days when reproduction is terminated? Why does chemical inhibition of DIO2 only partially inhibit gonadal growth? How can long-day-induced DIO2 be involved in activation of the reproductive system in species that begin to grow their gonads during short days, in preparation for spring breeding? What happens to DIO2 in female birds, which do not ovulate for some weeks after initial photostimulation? These questions imply that although DIO2 might well be involved in rapid activation of the reproductive axis in some species/sexes under specific laboratory conditions, there are likely to be other physiological mechanisms in place that regulate activation/deactivation of GnRH gene expression and GnRH release. These data will be presented with the hope of promoting further research into this intriguing conundrum.
