DESCRIPTION (provided by candidate): The proposed experiments are designed to understand how social interactions produce functional changes in the central nervous system. As a model system for studying this work, I will use an African cichlid fish species, Haplochromis burtoni, in which previous work has shown that the size of gonadotropin releasing hormone (GnRH) neurons in the preoptic area (POA) in males depends on social status. Dominant males have large GnRH neurons with elaborated processes while non-dominant animals have small neurons with reduced processes. The size and neural activity of these GnRH neurons corresponds directly with the individual's reproductive potential, namely dominant males can reproduce while non-dominant animals cannot. However, the neural mechanisms through which social and environmental conditions regulate GnRH neurons and hence reproduction are not known. In many species, changes in levels of the neurotransmitters serotonin (5-HT) and dopamine (DA) are related to changes in behavior and social (e.g. rank and reproductive) status. Moreover, 5-HT cells and fibers have been found in close proximity to GnRH neurons in the preoptic area (POA) as has also been shown for dopamine immunoreactive neurons in teleost fish, amphibians and birds. Recently, dopamine has been shown to influence the electrical activity of GnRH cells in the POA of Haplochromis burtoni. The experiments proposed here are designed to understand the role of serotonin and dopamine in regulating behavior, social rank and GnRH neuron size and activity. The expected results will contribute to our understanding of how neural plasticity both control and depend on social behavior.