When the time is right: The characterization of gonadotropin-inhibitory hormone (GnIH) and its role in fine-tuning reproduction in the white-crowned sparrow (Zonotrichia leucophrys)

All organisms must time breeding to reproduce during times that are most beneficial to offspring survival, thereby ensuring reproductive success. Most vertebrates rely on gonadotropin-releasing hormone (GnRH) to begin the cascade of reproductive events stimulating the hypothalamic-pituitary-gonadal axis. In seasonally breeding vertebrates, GnRH synthesis, storage, and release varies seasonally, identifying GnRH as an important factor in regulating timing of reproduction. Until recently there was no known hypothalamic antagonist for GnRH. The newly discovered hypothalamic neuropeptide, gonadotropin-inhibitory hormone (GnIH), has demonstrated its ability to decrease GnRH-mediated increases in gonadotropins, such as luteinizing hormone (LH), released from the pituitary. In white-crowned sparrows (WCSP) (Zonotrichia leucophrys), GnIH rapidly inhibits plasma LH in castrated, as well as in intact, GnRH-treated birds. GnIH shows seasonal changes in neuronal attributes and protein content implying changes in storage and expression. Thus, there is evidence of potential roles for GnIH in the timing of breeding. In order to characterize GnIH's role in influencing reproduction in WCSP, I examined changes in the response to GnIH challenges, changes in GnIH protein content, and changes in GnIH expression across a season, and among a sampling of WCSP subspecies (Z. l. gambelii, Z. l. pugetensis, and Z. l. nuttalli). I explored a variety of reproductive scenarios: (1) GnIH-injections of cloned and synthesized sparrow-GnIH given to wild breeding birds with naturally high levels of LH across the breeding season to examine changes in the seasonal response to GnIH. (2) Castrated birds implanted with testosterone to elucidate the role of sex steroid feedback on the GnIH system. (3) Birds implanted with GnIH mini-pumps to uncover the role of long-term GnIH exposure on reproductive development. (4) Seasonal collections of brain tissue to examine changes in GnIH expression throughout the year. (5) Comparisons of GnIH influence on three subspecies of WCSP with different breeding season lengths to highlight phylogenetic and ecological differences in mechanisms influencing the timing of breeding. Overall these results demonstrate that GnIH may play an important role in the regulation of reproductive function and help to illuminate the ecological factors and reproductive mechanisms that influence the fine-tuning of seasonal breeding.