Endocrinology, Vol 129, 1635-1643, Copyright © 1991 by Endocrine Society

ARTICLES

Role of the thyroid gland in seasonal reproduction. III. Thyroidectomy blocks seasonal suppression of gonadotropin-releasing hormone secretion in sheep

JR Webster, SM Moenter, GK Barrell, MN Lehman and FJ Karsch
Reproductive Sciences Program, University of Michigan, Ann Arbor 48109- 0404.

Previous studies demonstrated that the thyroid gland is required for the changes in LH secretion that lead to the end of the breeding season of female sheep, a reproductive transition generated within the ewe by an endogenous annual rhythm. The present study tested the hypothesis that this role of the thyroid is mediated via the GnRH neurosecretory system. The pulsatile secretion of GnRH into hypophyseal portal blood and LH into peripheral blood, as well as the neuroanatomical distribution of GnRH neurons and their light microscopic morphology, were compared among anestrous ewes and thyroidectomized (THX) ewes that failed to make the transition to the anestrous season. The study was conducted under photoperiodic conditions in which the transition to anestrus was generated by the endogenous rhythm. Each animal was ovariectomized and treated sc with a constant release implant of estradiol to normalize gonadal steroid status among thyroid-intact and THX ewes. High frequency pulses of GnRH and LH were evident in THX ewes that failed to make the transition to anestrus, whereas pulsatile secretion of GnRH and LH was generally not observed in thyroid-intact controls that had entered anestrus. This marked effect of thyroidectomy on GnRH secretion was not associated with widespread changes in the total number, distribution, or light microscopic morphology of GnRH neurons in the hypothalamus and preoptic area. Our finding that pulsatile secretion of GnRH is elevated in THX ewes that fail to make the transition to anestrus supports the hypothesis that the thyroid gland is required for the endogenously generated switch in function of the GnRH neurosecretory system that leads to the end of the breeding season of the ewe.

This article has been cited by other articles:

				G. M. Anderson, S. L. Hardy, M. Valent, H. J. Billings, J. M. Connors, and R. L. Goodman Evidence that Thyroid Hormones Act in the Ventromedial Preoptic Area and the Premammillary Region of the Brain to Allow the Termination of the Breeding Season in the Ewe Endocrinology, July 1, 2003; 144(7): 2892 - 2901. [Abstract] [Full Text] [PDF]

				C. Ko, N. A. Grieshaber, I. Ji, and T. H. Ji Follicle-Stimulating Hormone Suppresses Cytosolic 3,5,3'-Triiodothyronine-Binding Protein Messenger Ribonucleic Acid Expression in Rat Granulosa Cells Endocrinology, June 1, 2003; 144(6): 2360 - 2367. [Abstract] [Full Text] [PDF]

				C. S. Nunemaker, M. Straume, R. A. DeFazio, and S. M. Moenter Gonadotropin-Releasing Hormone Neurons Generate Interacting Rhythms in Multiple Time Domains Endocrinology, March 1, 2003; 144(3): 823 - 831. [Abstract] [Full Text] [PDF]

				N. Vasudevan, S. Ogawa, and D. Pfaff Estrogen and Thyroid Hormone Receptor Interactions: Physiological Flexibility by Molecular Specificity Physiol Rev, October 1, 2002; 82(4): 923 - 944. [Abstract] [Full Text] [PDF]

				G. M. Anderson, J. M. Connors, S. L. Hardy, M. Valent, and R. L. Goodman Thyroid Hormones Mediate Steroid-Independent Seasonal Changes in Luteinizing Hormone Pulsatility in the Ewe Biol Reprod, March 1, 2002; 66(3): 701 - 706. [Abstract] [Full Text] [PDF]

				G. R. Pitts, C. S. Nunemaker, and S. M. Moenter Cycles of Transcription and Translation Do Not Comprise the Gonadotropin-Releasing Hormone Pulse Generator in GT1 Cells Endocrinology, May 1, 2001; 142(5): 1858 - 1864. [Abstract] [Full Text]

				I. S. Parhar, T. Soga, and Y. Sakuma Thyroid Hormone and Estrogen Regulate Brain Region-Specific Messenger Ribonucleic Acids Encoding Three Gonadotropin-Releasing Hormone Genes in Sexually Immature Male Fish, Oreochromis niloticus Endocrinology, May 1, 2000; 141(5): 1618 - 1626. [Abstract] [Full Text] [PDF]

				S. Cecconi, N. Rucci, M. L. Scaldaferri, M. P. Masciulli, G. Rossi, C. Moretti, M. D’Armiento, and S. Ulisse Thyroid Hormone Effects on Mouse Oocyte Maturation and Granulosa Cell Aromatase Activity Endocrinology, April 1, 1999; 140(4): 1783 - 1788. [Abstract] [Full Text]

				C. Viguié, D. F. Battaglia, H. B. Krasa, L. A. Thrun, and F. J. Karsch Thyroid Hormones Act Primarily within the Brain to Promote the Seasonal Inhibition of Luteinizing Hormone Secretion in the Ewe Endocrinology, March 1, 1999; 140(3): 1111 - 1117. [Abstract] [Full Text]

				G. M. Anderson and G. K. Barrell Pulsatile Luteinizing Hormone Secretion in the Ovariectomized, Thyroidectomized Red Deer Hind Following Treatment with Dopaminergic and Opioidergic Agonists and Antagonists Biol Reprod, October 1, 1998; 59(4): 960 - 968. [Abstract] [Full Text]

				H. T. Jansen, L. S. Lubbers, E. Macchia, L. J. DeGroot, and M. N. Lehman Thyroid Hormone Receptor ({alpha}) Distribution in Hamster and Sheep Brain: Colocalization in Gonadotropin-Releasing Hormone and Other Identified Neurons Endocrinology, November 1, 1997; 138(11): 5039 - 5047. [Abstract] [Full Text] [PDF]

				L. A. Thrun, G. E. Dahl, N. P. Evans, and F. J. Karsch A Critical Period for Thyroid Hormone Action on Seasonal Changes in Reproductive Neuroendocrine Function in the Ewe Endocrinology, August 1, 1997; 138(8): 3402 - 3409. [Abstract] [Full Text] [PDF]

				J.-J. Xiong, F. J. Karsch, and M. N. Lehman Evidence for Seasonal Plasticity in the Gonadotropin-Releasing Hormone (GnRH) System of the Ewe: Changes in Synaptic Inputs onto GnRH Neurons Endocrinology, March 1, 1997; 138(3): 1240 - 1250. [Abstract] [Full Text] [PDF]

				H. I'anson, S. K. Terry, M. N. Lehman, and D. L. Foster Regional Differences in the Distribution of Gonadotropin-Releasing Hormone Cells between Rapidly Growing and Growth-Restricted Prepubertal Female Sheep Endocrinology, January 1, 1997; 138(1): 230 - 236. [Abstract] [Full Text] [PDF]