Gonadotropin-inhibitory hormone in seasonally-breeding songbirds: Neuroanatomy and functional biology

The discovery of gonadotropin-inhibitory hormone (GnIH) in Japanese Quail Coturnix japonica has given us a new perspective on the neuroendocrine control of reproductive physiology and behavior. Using highly photoperiodic passerine songbirds as our model system, we have been able to investigate GnIH neuroanatomy and functional biology. The distribution of GnIH peptide is similar among the galliform and passerine species studied so far. We have identified GnIH cDNA and localized its transcript in Gambel's White-crowned Sparrow Zonotrichia leucophrys gambelii. There is a high degree of homology between the galliform and passerine GnIH cDNA sequences and peptides. Based on transcript localization, GnIH precursor polypeptide is only produced in the paraventricular nucleus, yet GnIH-immunoreactive fibers project to multiple brain areas, suggesting multiple physiological and behavioral functions. GnIH immunoreactive neurons appear to contact gonadotropin-releasing hormone (GnRH)-I and -II neurons, raising the possibility of direct regulation of GnRH by GnIH. In a photoperiod experiment, GnIH neurons increased in area during the termination of reproduction in Song Sparrows, Melospiza melodia, implying temporal photoperiodic regulation of this neuropeptide. Using peripheral administration of GnIH in vivo, we demonstrated rapid inhibition of LH release in laboratory and field experiments. Furthermore, we sought to determine the effects of central infusions of GnIH. Centrally-administered GnIH rapidly reduced circulating LH and significantly reduced the number of copulation solicitations performed in response to song playback, with no reduction of locomotor activity. Central infusion of rhodaminated GnIH elucidated putative GnIH binding sites in the median eminence close to GnRH-I fiber terminals, and in the midbrain on or close to GnRH-II neurons. These data demonstrate direct effects of GnIH upon reproductive physiology and behavior, possibly at multiple physiological levels and over different time-frames. Thus, the discovery of GnIH has unearthed a complex and interesting system for the regulation of avian reproductive biology.