In animals with complex societies, social signals can profoundly influence behavior, physiology, and the nervous system.  Naked mole-rats exhibit a strict hierarchical social organization in which breeders are dominant to all non-breeders.  Recently, a change in social status has been linked to what appears to be late differentiation of spinal motoneurons.  Naked mole-rats live in large colonies consisting of a single breeding female (the queen), one to three breeding males, and a large cohort of reproductively suppressed subordinates.  Over 95% of subordinates will remain reproductively inactive for their entire lives.  A subordinate can become reproductive, however, if a breeding animal dies or if it is removed from the colony and paired with an opposite-sex mate.  This change in social status causes an increase in the number of motoneurons innervating perineal muscles.  This project will: 1) determine which factors trigger the changes observed in the spinal cord by independently manipulating gonadal hormones and social status, 2) establish the neurochemical identity of cells within the spinal cord that become large motoneurons in breeders using immunocytochemistry for cell-type markers, and 3) determine the targets of the large motoneurons recruited in breeders by retrograde tract-tracing.  The unique social structure and life history of naked mole-rats give us the opportunity to examine neural plasticity in an adult mammal, including the late differentiation or re-specification of neuron subtype in the spinal cord.  This information could be used to develop treatments for degenerative diseases characterized by loss or atrophy of motoneurons, such as amyotrophic lateral sclerosis (ALS). [ unreadable] [unreadable] [unreadable]