Extraocular control of seasonal reproduction in female tree sparrows (Spizella arborea)

The work reported here explored extraocular control of seasonal reproduction in a photoperiodic species. Photosensitive female tree sparrows (Spizella arborea) were subjected to bilateral or unilateral ocular enucleation or to sham operation and exposed to a photoregimen designed to stimulate luteinizing hormone (LH) secretion, to induce photorefractoriness, and to restore photosensitivity. As revealed by plasma LH profiles constructed from measurements taken over 181 days, neither bilateral nor unilateral ocular enucleation had a substantive effect on photoinduced LH release, on the development of photorefractoriness, or on the recovery of photosensitivity. In a related experiment, photorefractory sighted females were implanted epicranially with miniature self-powered lights (SPLs) or with non-emitting helium blanks (HBs) and transferred to short days. After 8 weeks, SPLs and HBs were removed, and birds were challenged with long days. As indicated by plasma LH and ovarian responses to the challenge, previously HB-implanted birds had regained photosensitivity, but previously SPL-implanted birds remained photorefractory. When interpreted within the context of the effects of blinding, these findings suggest that an extraocular encephalic mechanism maintains photorefractoriness. Collectively, the data support the hypothesis that photoinduced LH release and the biannual transitions between photosensitivity and photorefractoriness are controlled by an extraocular mechanism(s).