Work outlined in this proposal will uncover the neuronal mechanisms and hypothalamic structures involved in photoperiodic regulation of reproduction in turkey hens. Reproduction in turkeys is initiated by increased daylength and becomes limited by the onset of broodiness, eventually resulting in the termination of egg laying. Hens lose their sensitivity to the reproductively stimulating effects of long days, even under ever increasing daylengths. Loss of productivity is costly, as birds must cared for until coming back into production. In birds, photoperiodic information is sensed directly in the brain by specialized photoreceptors. Although the site(s) of these photoreceptors are unknown, several brain areas have been suggested to house these photoreceptors. The PIs will use immunohistological, autoradiography and MRI techniques to: 1) Determine if opsin expression in the brain of the turkey breeder hen changes under photosensitive, egg laying and photorefractory conditions. 2) Determine what areas of the brain are functionally active during photoactivation of the avian reproductive system, during egg laying and during photorefractoriness. 3) Determine if the axon fibers connecting different structures of the brain of the avian change in number or thickness depending upon reproductive state.The PIs will use this information in order to understand how the brains of turkey hens change at different stages of reproduction to regulate the reproductive cycle in turkeys. Our findings will permit the development of strategies for alleviating production inefficiency due to the limited lay period resulting from photoperiodic insensitivity.