Photoperiodic Control of TSH‐β Expression in the Mammalian Pars Tuberalis has Different Impacts on the Induction and Suppression of the Hypothalamo‐Hypopysial Gonadal Axis

Nov 17, 2009Journal of neuroendocrinology

Day Length Controls TSH-beta Levels in the Brain Region Regulating Reproductive Hormones Differently for Activating and Suppressing the Reproductive System

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Abstract

TSH-beta and Dio2 expression were induced on day 8 after transferring hamsters from short- to long-day conditions.

Long-day conditions trigger the expression of TSH-beta in the pars tuberalis, which is a key factor for gonadal activation.
Thyroid-stimulating hormone (TSH) activates type 2 deiodinase (Dio2) in the ependymal cell layer, further stimulating the gonadal axis.
Injections of melatonin, simulating short-day conditions, suppressed Dio2 expression within one day, while TSH-beta suppression occurred on day 10.
The findings indicate that TSH-beta and Dio2 are closely linked in the process of activating the gonadal axis.
Different mechanisms may mediate the suppression of the gonadal axis in mammals compared to its activation.

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Seasonal reproduction depends on photoperiod-regulated activation or suppression of the gonadal axis. Recent studies in quail have identified long-day induced TSH-beta expression in the pars tuberalis (PT) as a rapid trigger of gonadal activation. Thyroid-stimulating hormone (TSH) induces type 2 deiodinase (Dio2) in the ependymal cell layer (EC) of the infundibular recess to stimulate the gonadal axis. A similar mechanism is proposed in sheep and mice, but the experimental data on the temporal patterns of induction and suppression of TSH-beta and Dio2 expression are incomplete. In the present study, we examined the expression of TSH-beta and Dio2 in hamsters transferred from short- to long-day conditions for 9 days, and demonstrate the induction of TSH-beta and Dio2 on day 8 after transition. These data demonstrate the close relationship between TSH-beta and Dio2 expression in the inductive pathway. The temporal expression of TSH-beta and Dio2 in the suppressive pathway was also examined by s.c. melatonin injection, which mimics the transition from long to short days. Importantly, Dio2 expression in the EC is suppressed on day 1 after the onset of injection, whereas TSH-beta expression in the PT was not suppressed until day 10. These data suggest that regulated transcription of TSH-beta is involved in the induction of the gonadal axis in mammals, whereas the suppression of this axis is mediated by different mechanisms.

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Photoperiodic Control of TSH‐β Expression in the Mammalian Pars Tuberalis has Different Impacts on the Induction and Suppression of the Hypothalamo‐Hypopysial Gonadal Axis

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