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 (PSA-NCAM) have been observed. Whether seasonal changes in a specific subset(s) of GnRH inputs occur, or if glial cells specifically play a role in this remodeling is not clear. We therefore examined GnRH neurons of breeding season (BS) and non-breeding season (anestrus, AN) ewes and tested the hypotheses that, a) specific (i.e. GABA, catecholamine, NPY, or -Endorphin (-EN)) inputs to GnRH neurons change seasonally, and b) concomitant with any changes in neural inputs is a change in glial apposition.

Using triple-label immunofluorescent visualization of GnRH, glial acidic fibrillary protein (GFAP), and neuromodulator/neural terminal markers combined with confocal microscopy and optical sectioning techniques, we confirm that total numbers of neural inputs to GnRH neurons vary with season and demonstrate that specific inputs contribute to these overall changes. Specifically, NPY and GABA inputs to GnRH neurons increased during BS while -EN inputs were either greater during AN (GnRH somas) or greater during BS (GnRH dendrites). Associated with the changes in GnRH inputs were seasonal changes in glial apposition, in GFAP density, and in 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.