Photostimulated Expression of Type 2 Iodothyronine Deiodinase mRNA is Greatly Attenuated in the Rostral Tuberal Hypothalamus of the Photorefractory Turkey Hen

Abstract

For many temperate-zone avian species, termination of breeding occurs when individuals no longer respond to previously stimulatory day lengths, a condition called photorefractoriness. Long day lengths induce significantly greater expression of c-fos and fos-related antigens (FRAs) in the tuberal hypothalamus of the photosensitive hen than that of the photorefractory hen. The tuber is also a site of photoinducible glial expression of type 2 iodothyronine deiodinase (Dio2), which converts thyroxine into its active form, triiodothyronine (T3). T3 induces withdrawal of glial processes from gonadotrophin-releasing hormone (GnRH) I nerve terminals, which is believed to permit the efficient release of GnRH I into the associated portal vasculature. Using a riboprobe, we tested whether long days induce Dio2 mRNA expression in the turkey tuber and, if so, whether this expression is reduced in photorefractory hens. Long days significantly induced rostral and caudal tuberal hypothalamic Dio2 expression in photosensitive hens. Photorefractory hens had reduced expression of Dio2 with most subjects expressing no detectable mRNA in the rostral tuber and variably attenuated amounts throughout the medial and caudal tuber. We also performed double-label immunohistochemistry to identify co-localisation between FRAs and glial fibrillary acidic protein, a glial marker. FRAs were present in the nuclei of a few astrocytes in the median eminence and infundibular nucleus of the tuber. The temporal and spatial coincidence between FRA and Dio2 expression, their mutual association with glia, and the attenuation of their response during photorefractoriness suggests that the two events are linked and that photorefractoriness involves a reduced capacity for photo-inducible gene expression within glia of the tuberal hypothalamus.

GABAergic Neurotransmission in the Premammillary Nucleus of the Turkey Hypothalamus Regulates Reproductive Seasonality and the Onset of Photorefractoriness

“…Thyroid hormones have been known to be related to photoperiodic responses in avian species and play an important role in the development of photorefractoriness [43,44]. The decreases in type 2 iodothyronine deiodinase expression and fos-like-ir in the tuberal hypothalamus of photorefractory turkey hens coincide with the results of the present study [45,46]. Photoperiodic induction of c-fos expression in the PMM was minimized after GABA receptor agonist administration, indicating a lack of PMM DA-Mel neuronal activation which results in the loss of downstream neuroendocrine cascade output, causing the development of photorefractoriness.…”

Tuberal hypothalamic expression of the glial intermediate filaments, glial fibrillary acidic protein and vimentin across the turkey hen (Meleagris gallopavo) reproductive cycle: Further evidence for a role of glial structural plasticity in seasonal reproduction

“…Upon post-mortem inspection, PR hens were observed to have completely regressed ovaries. This protocol is the same as has been previously used to test photoinducibilty of turkey hen hypothalamic regions in our lab (Millam et al, 2003; Steinman et al, 2008). Hens were euthanized by an intravenous overdose of chloral hydrate (20%) in water, and then underwent perfusion fixation via the carotid artery by exsanguinating with sodium phosphate-buffered saline (PBS) and heparin and then fixing with 10 ml of cold 4% paraformaldehyde.…”

“…As with many other avian photoperiodic breeders, female turkeys (Siopes, 1994) develop a state of photorefractoriness in which previously stimulatory day lengths become insufficient to maintain reproductive activity and the HPG-axis shuts down in anticipation of unfavorable environmental conditions (Nicholls et al, 1988); exposure to short days for several weeks is necessary to reestablish photosensitivity (Siopes, 1989). In sharp contrast to reproductively inactive short-day but photosensitive hens, as well as photorefractory hens, the rostral and central ME of reproductively active long-day turkey hens express high levels of type 2-iodothyronine deiodinase, a glial derived enzyme that increases thyroid hormone activity (Steinman et al, 2008). Thyroid hormone in turn has been reported to induce tanycyte retraction from GnRH terminals in Japanese quail (Yamamura et al, 2006).…”

“…Thyroid hormone in turn has been reported to induce tanycyte retraction from GnRH terminals in Japanese quail (Yamamura et al, 2006). Likewise long day turkeys show high expression of residual proteins marking recent expression of c-fos, a transcription factor and indirect marker of cellular activation (Morgan and Curran, 1991), in the anterior portion of the ME, while short day and photorefractory hens show little evidence of c-fos immunoreactivity (Millam et al, 2003; Steinman et al, 2008). Many of the c-fos-immunoreactive (ir) nuclei in the LD turkey appear inside stellate astrocytes (Steinman et al, 2008).…”

“…Specifically, we predicted that reduced GFAP expression would be accompanied by elevated vimentin expression in the ME of photostimulated long day hens as compared to photosensitive short day and photorefractory turkeys. We also examined expression of these two intermediate filaments in the infundibular area (IA), another photoresponsive portion of the tuberal hypothalamaus dorsal to the ME (Meddle and Follett, 1995; Millam et al, 2003; Steinman et al, 2008). Results indicated that GFAP expression was in fact reduced in the central portion of the ME of long day hens, but surprisingly, the effect was most pronounced in the inner portion (ME-IZ).…”