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Seasonal Plasticity within the Gonadotropin-Releasing Hormone (GnRH) System of the Ewe: Changes in Identified GnRH Inputs and Glial Association

Department of Veterinary and Comparative Anatomy, Pharmacology, and Physiology and Center for Reproductive Biology (H.T.J.), Washington State University College of Veterinary Medicine, Pullman, Washington 99164-6520; Department of Cell Biology, Neurobiology, and Anatomy (C.C., M.N.L.), University of Cincinnati College of Medicine, Cincinnati, Ohio 45267-0521; and Department of Physiology (S.H., R.L.G.), West Virginia University Health Sciences Center, Morgantown, West Virginia 26506-9229

Address all correspondence and requests for reprints to: Heiko T. Jansen, Department of Veterinary and Comparative Anatomy, Pharmacology, and Physiology, Programs in Neuroscience and Center for Reproductive Biology, Washington State University, 205 Wegner Hall, Pullman, Washington 99164-6520. E-mail: heiko{at}vetmed.wsu.edu.

The annual reproductive cycle in sheep may reflect a functional remodeling within the GnRH system. Specifically, changes in total synaptic input and association with the polysialylated form of neural cell adhesion molecule have been observed. Whether seasonal changes in a specific subset(s) of GnRH inputs occur or whether glial cells specifically play a role in this remodeling is not clear. We therefore examined GnRH neurons of breeding season (BS) and nonbreeding season (anestrus) ewes and tested the hypotheses that specific (i.e. -aminobutyric acid, catecholamine, neuropeptide Y, or ß-endorphin) inputs to GnRH neurons change seasonally, and concomitant with any changes in neural inputs is a change in glial apposition. Using triple-label immunofluorescent visualization of GnRH, glial acidic fibrillary protein and neuromodulator/neural terminal markers combined with confocal microscopy and optical sectioning techniques, we confirmed that total numbers of neural inputs to GnRH neurons vary with season and demonstrated that specific inputs contribute to these overall changes. Specifically, neuropeptide Y and -aminobutyric acid inputs to GnRH neurons increased during BS and ß-endorphin inputs were greater during either anestrus (GnRH somas) or BS (GnRH dendrites). Associated with the changes in GnRH inputs were seasonal changes in glial apposition, glial acidic fibrillary protein density, and the thickness of glial fibrils. These findings are interpreted to suggest an increase in net stimulatory inputs to GnRH neurons during the BS contributes to the seasonal changes in GnRH neurosecretion and that this increased innervation is perhaps stabilized by glial processes.

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