**AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** 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 IDENTIFY THE BRAIN AREAS RESPONSIBLE FOR LIGHT SENSING AND THE NEURONAL PATHWAYS THAT ARE ACTIVATED DURING PHOTOSTIMULATION TO CARRY PHOTIC INFORMATION TOTHE PITUITARY. THE PIS WILL ALSO RECORD ELECTRICAL ACTIVITY FROM NEURONS IN THE BRAINS OF AWAKE BIRDS IN ORDER TO UNDERSTAND THE TYPE OF NEURONAL SIGNALS PRODUCED AND THE NEURONS INVOLVED IN THEIR PRODUCTION IN ORDER TO PROVIDE A TARGET FOR FUTURE MODULATION OF REPRODUCTIVE ACTIVITY. THE PIS WILL ALSO COMPARE THESE BRAIN AREAS PRIOR TO PHOTOPERIODIC ACTIVATION, DURING EGG LAYING, AND DURING BROODINESS IN ORDER TO UNDERSTAND HOW THE BRAINS OF TURKEY HENS CHANGE 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.